**Biological**

[41] Pernak J, Chwała P. Synthesis and anti‐microbial activities of choline‐like quaternary ammonium chlorides. European Journal of Medicinal Chemistry. 2003; 38: 1035–1042.

[42] Busetti A, Crawford DE, Earle MJ, Gilea MA, Gilmore BF, Gorman SP, Laverty G,

[43] Iwai N, Nakayama K, Kitazume T. Antibacterial activities of imidazolium, pyrrolidi‐ nium andpiperidinium salts. Bioorganic & Medicinal ChemistryLetters. 2011; 21: 1728–

[44] Brycki B, Kowalczyk I, Koziróg A. Synthesis, molecular structure, spectral properties

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[46] Carson L, Chau PKW, Earle MJ, MA, Gilmore BF, Gorman SP, McCann MT, Seddon KR. Antibiofilm activities of 1‐alkyl‐3‐methylimidazolium chloride ionic liquids. Green

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Lowry AF, McLaughlin

10.1039/B919872E

72 Progress and Developments in Ionic Liquids

**Chapter 4**

**Provisional chapter**

**The Role of Ionic Liquids in Protein Folding/Unfolding**

Ionic liquids (ILs) have emerged as novel solvent medium for several biotechnological processes *in vitro*. The use of ILs starts from protein extraction to catalysis to folding/ unfolding studies. ILs are becoming the most favorite non-aqueous medium for protein studies due to their unique ionic combinations (cation + anion) and tunable physical properties. In this context, several research results have been published that use of pure or aqueous IL solutions as stabilizer for proteins. Hence, herein, in this chapter, we present a collection of research work that focuses on the importance of ILs (and their mixture) in protein stabilities. In addition, we have also reviewed the unique properties of ILs as counteracting solvents for cold-induced denaturation and also their refolding properties. This report will definitely generate a new understanding for the ILs, their importance

**Keywords:** ionic liquids (ILs), proteins, stability, biocompatibility, counteraction

In recent years, various solvents have been used for numerous processes in academia and industries. Nevertheless, because of new environmental regulations, the challenges of using non-harmful solvents have prompted a great development of innovative products [1, 2]. In this regard, ionic liquids (ILs) emerged as new and novel class of solvents that are now considered to reduce both economic and environmental pollution [2–4]. The term ILs describe a popular class of organic salts that melt below ~100°C and have an appreciable liquid range [1–4]. IL is entirely composed of positive and negative ions [5, 6]. ILs typically consist of organic nitrogencontaining heterocyclic cations and inorganic anions [7]. Historically, the organic compound

**The Role of Ionic Liquids in Protein Folding/Unfolding** 

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution,

© 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

and reproduction in any medium, provided the original work is properly cited.

Awanish Kumar, Meena Bisht, Indrani Jha and

Awanish Kumar, Meena Bisht, Indrani Jha

Additional information is available at the end of the chapter

and applicability in protein folding studies.

**1. Overview of the structure and importance of ILs**

Additional information is available at the end of the chapter

**Studies**

**Studies**

Pannuru Venkatesu

**Abstract**

and Pannuru Venkatesu

http://dx.doi.org/10.5772/65924

#### **The Role of Ionic Liquids in Protein Folding/Unfolding Studies The Role of Ionic Liquids in Protein Folding/Unfolding Studies**

Awanish Kumar, Meena Bisht, Indrani Jha and Pannuru Venkatesu Awanish Kumar, Meena Bisht, Indrani Jha and Pannuru Venkatesu

Additional information is available at the end of the chapter Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/65924

#### **Abstract**

Ionic liquids (ILs) have emerged as novel solvent medium for several biotechnological processes *in vitro*. The use of ILs starts from protein extraction to catalysis to folding/ unfolding studies. ILs are becoming the most favorite non-aqueous medium for protein studies due to their unique ionic combinations (cation + anion) and tunable physical properties. In this context, several research results have been published that use of pure or aqueous IL solutions as stabilizer for proteins. Hence, herein, in this chapter, we present a collection of research work that focuses on the importance of ILs (and their mixture) in protein stabilities. In addition, we have also reviewed the unique properties of ILs as counteracting solvents for cold-induced denaturation and also their refolding properties. This report will definitely generate a new understanding for the ILs, their importance and applicability in protein folding studies.

**Keywords:** ionic liquids (ILs), proteins, stability, biocompatibility, counteraction

### **1. Overview of the structure and importance of ILs**

In recent years, various solvents have been used for numerous processes in academia and industries. Nevertheless, because of new environmental regulations, the challenges of using non-harmful solvents have prompted a great development of innovative products [1, 2]. In this regard, ionic liquids (ILs) emerged as new and novel class of solvents that are now considered to reduce both economic and environmental pollution [2–4]. The term ILs describe a popular class of organic salts that melt below ~100°C and have an appreciable liquid range [1–4]. IL is entirely composed of positive and negative ions [5, 6]. ILs typically consist of organic nitrogencontaining heterocyclic cations and inorganic anions [7]. Historically, the organic compound

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

that is now considered as the first IL is ethylammonium nitrate ([C<sup>2</sup> H5 NH3 ][NO3 ], EAN) and was prepared by Paul Walden in 1914 and has a melting point of 12°C [8]. After a long gap, the major studies of room temperature molten salts of pyridinium halides with aluminum chloride (AlCl3 ) were made in the 1940s by a group led by Frank Hurley and Tom Weir at Rice University [9]. Later, alkylimidazolium salts (Cnmim)+ were also reported in the early 1980s [10]. An excellent short history of the birth of ILs, which covers the crucial moments of this area, is presented by John S. Wilkes [11].

of the ions are small and they also tend to melt at lower temperatures than their APIL analogs. The obvious difference of the PILs compared with APILs is the reversible hydrogen transfer between the acid and the base [50, 51]. This implies that for PILs where the transfer is weak, the properties are closer to the corresponding binary liquid, whereas the aprotic ILs keep their

The Role of Ionic Liquids in Protein Folding/Unfolding Studies

http://dx.doi.org/10.5772/65924

77

ILs in green chemistry mean that it should be applied to all the aspects of the product life cycle that begins from its invention to the disposal. Broadly speaking, it should be recycled easily from the environment [53]. Tremendous amount of recent research has focused on the physical properties of ILs and more recently relationship and cross-linking between the chemical properties, the toxicity and biological properties of ILs have been one of the most highly debated topics in this field [31, 54]. Pham et al. have reviewed the toxic effect of ILs on the environment and biological systems in a comprehensive way [55]. It was shown that with increasing hydrophobicity of the cation, the IL gets more toxic [56]. Literature reveals that increased alkyl chain

The advantages of using ILs in enzymatic biocatalysis, as compared to volatile organic compounds (VOCs), are the enhancement in the solubility of substrates or products without inactivation of the enzymes, high conversion rates and high activity and stability [35, 53]. These unique properties of ILs make them very attractive nonaqueous solvents for protein stability studies. It is revealed from various studies that physicochemical properties of ILs can play a pivotal role in altering the structure, stability and activity of proteins/enzymes [44]. Moreover, ILs offer new possibilities of application of solvent engineering to enzymatic reactions. Biocatalysis with ILs as reaction medium was first showed in the beginning of 2000 [58–60]. Review on ILs as cosolvents in aqueous biocatalytic reactions reveals that these ILs help to dissolve nonpolar substrates while avoiding enzyme inactivation like water-miscible organic solvents, as dimethyl sulfoxide (DMSO) or acetonitrile [61]. During the last decade, ILs have increased their attention as reaction media for enzymes in aqueous media with some

In the present situation, the stability of proteins in ILs has been an area for active research because of their biological and pharmaceutical applications. The first report on the protein stability in the presence of ILs came in the year 2000 by Summers and Flowers [64]. Later, in 2004, Iborra and coworkers [65] studied the stabilizing ability of 1-ethyl-3-methylimidazo-

bility of CT in this IL was compared with water, 3 M sorbitol and 1-propanol. Subsequently, lots of works have been reported on the stability of various proteins in various ILs. Among all the solvents, the IL was found to be a strong stabilizer for CT structure than with other solvent

] on α-chymotrypsin (CT) and the sta-

length in the cation of ILs showed higher toxicities on biological systems [57].

ionic character until decomposition [52].

**3. Toxicity in ILs and its environmental impact**

**4. Protein stability in the presence of ILs**

lium bis[(trifluoromethyl)sulfonyl]imide [Emim][NTf<sup>2</sup>

remarkable results [62, 63].

In its initial revolutionary stage, ILs were vastly considered in analytical chemistry based on their unique and tunable physical properties. Since then, synthesis of a large number of multifunctional ILs has been a prime interest for synthetic chemists [12, 13]. In this context, varieties of task-specific ILs have been synthesized and the advantages of their physical properties have been reported in an open literature [14]. Common ILs include ammonium, phosphonium, sulfonium, guanidinium, pyridinium, imidazolium and pyrrolidinium cations. The most common anions are chloride, bromide, tetrafluoroborate, hexafluorophosphate, trifluoromethanesulfonyl, bis(trifluoromethanesulfonyl)imide, dicyanamide and alkyl sulfate anions. These ILs have some unique properties such as negligible vapor pressure, good thermal stability, tunable viscosity and miscibility with water, a wide electrochemical window, high conductivity and high heat capacity [15–17]. These physical properties make IL a promising material in numerous fields, for example, their use in electrochemical devices and replacements for several organic reactions [18, 19]. Gordon [20], Parvulescu and Hardacre [21] and Crowhurst et al. [22] pointed out that there is an obvious advantage in performing many reactions in ILs due to the improvement in reaction activity, selectivity and yield. An in-depth literature survey reveals that there tremendously exist a large number of scholarly articles as well as elegant reviews that explicitly elucidate the various scientific applications of ILs [23–45].

### **2. Classification of ILs**

ILs are composed solely of ions and their bulk and interfacial behavior is complex, governed by Coulombic, van der Waals, dipole-dipole, hydrogen-bonding and solvophobic forces [22, 46]. When an IL is formed by mixing a strong acid with a strong base, the proton is generally assumed to be located very strongly on the base. In this situation, the IL is most likely composed entirely of ions; however, ion complexation and aggregate formation may also occur [47]. A majority of the ILs with various combinations of the cation as well as the anions have been classified as protic ILs (PILs) and aprotic ILs (APILs) based on their respective physical properties to protonate/deprotonate in aqueous media [48]. The reason for this distinction is that PILs are volatile by their nature because the acidic proton can be abstracted by the basic anion at ambient temperature. The acid-base equilibrium for the abstraction reaction allows the formation of neutral molecular species that readily evaporate [49].

The potential environmental impact of PILs is expected to be smaller than the impact of APILs, due to their simpler structure. These PILs can be easily produced through the combination of a Brønsted base and a Brønsted acid [36]. A comparison with the APILs reveals that PILs often have higher conductivities and fluidities than the APILs. On the other hand, in PILs, the sizes of the ions are small and they also tend to melt at lower temperatures than their APIL analogs. The obvious difference of the PILs compared with APILs is the reversible hydrogen transfer between the acid and the base [50, 51]. This implies that for PILs where the transfer is weak, the properties are closer to the corresponding binary liquid, whereas the aprotic ILs keep their ionic character until decomposition [52].

### **3. Toxicity in ILs and its environmental impact**

that is now considered as the first IL is ethylammonium nitrate ([C<sup>2</sup>

University [9]. Later, alkylimidazolium salts (Cnmim)+

area, is presented by John S. Wilkes [11].

76 Progress and Developments in Ionic Liquids

**2. Classification of ILs**

chloride (AlCl3

was prepared by Paul Walden in 1914 and has a melting point of 12°C [8]. After a long gap, the major studies of room temperature molten salts of pyridinium halides with aluminum

[10]. An excellent short history of the birth of ILs, which covers the crucial moments of this

In its initial revolutionary stage, ILs were vastly considered in analytical chemistry based on their unique and tunable physical properties. Since then, synthesis of a large number of multifunctional ILs has been a prime interest for synthetic chemists [12, 13]. In this context, varieties of task-specific ILs have been synthesized and the advantages of their physical properties have been reported in an open literature [14]. Common ILs include ammonium, phosphonium, sulfonium, guanidinium, pyridinium, imidazolium and pyrrolidinium cations. The most common anions are chloride, bromide, tetrafluoroborate, hexafluorophosphate, trifluoromethanesulfonyl, bis(trifluoromethanesulfonyl)imide, dicyanamide and alkyl sulfate anions. These ILs have some unique properties such as negligible vapor pressure, good thermal stability, tunable viscosity and miscibility with water, a wide electrochemical window, high conductivity and high heat capacity [15–17]. These physical properties make IL a promising material in numerous fields, for example, their use in electrochemical devices and replacements for several organic reactions [18, 19]. Gordon [20], Parvulescu and Hardacre [21] and Crowhurst et al. [22] pointed out that there is an obvious advantage in performing many reactions in ILs due to the improvement in reaction activity, selectivity and yield. An in-depth literature survey reveals that there tremendously exist a large number of scholarly articles as well as elegant

reviews that explicitly elucidate the various scientific applications of ILs [23–45].

the formation of neutral molecular species that readily evaporate [49].

ILs are composed solely of ions and their bulk and interfacial behavior is complex, governed by Coulombic, van der Waals, dipole-dipole, hydrogen-bonding and solvophobic forces [22, 46]. When an IL is formed by mixing a strong acid with a strong base, the proton is generally assumed to be located very strongly on the base. In this situation, the IL is most likely composed entirely of ions; however, ion complexation and aggregate formation may also occur [47]. A majority of the ILs with various combinations of the cation as well as the anions have been classified as protic ILs (PILs) and aprotic ILs (APILs) based on their respective physical properties to protonate/deprotonate in aqueous media [48]. The reason for this distinction is that PILs are volatile by their nature because the acidic proton can be abstracted by the basic anion at ambient temperature. The acid-base equilibrium for the abstraction reaction allows

The potential environmental impact of PILs is expected to be smaller than the impact of APILs, due to their simpler structure. These PILs can be easily produced through the combination of a Brønsted base and a Brønsted acid [36]. A comparison with the APILs reveals that PILs often have higher conductivities and fluidities than the APILs. On the other hand, in PILs, the sizes

) were made in the 1940s by a group led by Frank Hurley and Tom Weir at Rice

H5 NH3

][NO3

were also reported in the early 1980s

], EAN) and

ILs in green chemistry mean that it should be applied to all the aspects of the product life cycle that begins from its invention to the disposal. Broadly speaking, it should be recycled easily from the environment [53]. Tremendous amount of recent research has focused on the physical properties of ILs and more recently relationship and cross-linking between the chemical properties, the toxicity and biological properties of ILs have been one of the most highly debated topics in this field [31, 54]. Pham et al. have reviewed the toxic effect of ILs on the environment and biological systems in a comprehensive way [55]. It was shown that with increasing hydrophobicity of the cation, the IL gets more toxic [56]. Literature reveals that increased alkyl chain length in the cation of ILs showed higher toxicities on biological systems [57].

### **4. Protein stability in the presence of ILs**

The advantages of using ILs in enzymatic biocatalysis, as compared to volatile organic compounds (VOCs), are the enhancement in the solubility of substrates or products without inactivation of the enzymes, high conversion rates and high activity and stability [35, 53]. These unique properties of ILs make them very attractive nonaqueous solvents for protein stability studies. It is revealed from various studies that physicochemical properties of ILs can play a pivotal role in altering the structure, stability and activity of proteins/enzymes [44]. Moreover, ILs offer new possibilities of application of solvent engineering to enzymatic reactions. Biocatalysis with ILs as reaction medium was first showed in the beginning of 2000 [58–60]. Review on ILs as cosolvents in aqueous biocatalytic reactions reveals that these ILs help to dissolve nonpolar substrates while avoiding enzyme inactivation like water-miscible organic solvents, as dimethyl sulfoxide (DMSO) or acetonitrile [61]. During the last decade, ILs have increased their attention as reaction media for enzymes in aqueous media with some remarkable results [62, 63].

In the present situation, the stability of proteins in ILs has been an area for active research because of their biological and pharmaceutical applications. The first report on the protein stability in the presence of ILs came in the year 2000 by Summers and Flowers [64]. Later, in 2004, Iborra and coworkers [65] studied the stabilizing ability of 1-ethyl-3-methylimidazolium bis[(trifluoromethyl)sulfonyl]imide [Emim][NTf<sup>2</sup> ] on α-chymotrypsin (CT) and the stability of CT in this IL was compared with water, 3 M sorbitol and 1-propanol. Subsequently, lots of works have been reported on the stability of various proteins in various ILs. Among all the solvents, the IL was found to be a strong stabilizer for CT structure than with other solvent media [65]. Studies carried out by various groups using CT suggest that the physical properties of ILs such as polarity and hydrophobicity play a major role in their stabilizing behavior toward CT [18, 52, 53]. Among various families of ILs, ammonium-based ILs are identified to display their wide range application in biochemical processes [44].

Interestingly, the stability of lysozyme in imidazolium-based ILs was observed to decrease significantly as the alkyl chain of the ILs increased from ethyl to butyl to hexyl. However, all

The Role of Ionic Liquids in Protein Folding/Unfolding Studies

http://dx.doi.org/10.5772/65924

79

Interestingly, experimental results reveal that the partitioning of bovine serum albumin (BSA) is predominated by the hydrophobic interactions between the protein and imidazolium cation of the ILs in the aqueous system, which can be improved or modulated by changing the cation chain length of the ILs or modifying the surface of the BSA [74, 75]. The study indicated that the secondary structure of BSA was stabilized at low IL concentration (<3 mM) and the protein was denatured at higher IL concentration (>3 mM) [76]. Rawat and Bohidar [77, 78] reported that the interaction between imidazolium-based ILs and BSA is obviously dependent on the concentrations of the ILs. They observed that the BSA retained the second-

0–0.2% w/v. Moving above this concentration range, the BSA structure was denatured significantly that is most likely caused due to intercalation of alkyl chain of the imidazolium cation

In the earlier sections, the protein stabilization studies of most commonly used proteins have been delineated. Even though, the literature is still scattered, which deals with the stability of proteins in the presence of ILs. Hence, in this section, we have tried together all these research reports [79–114] under the same roof, so that it is easily available to the readers and also to expose its importance to the scientific world. An overview of the literature reveals stability studies of proteins such as amyloglucosidase [79], thyroglobulin [79], glutamate dehydrogenase [79], lactate dehydrogenase [79], glucose dehydrogenase [80], formate dehydrogenase [94], glycosidase (α and β) [81], monellin [82], β-galactosidase [83], glucose oxidase [106], lactate oxidase [109], oxidoreductases [107], subtilisin Carlsberg [84], Amano protease P6 [85], pepsin [86], papain [87, 100] esterases from *Bacillus subtilis* and *Bacillus stearothermophilus* [87], *Penicillium expansum* lipase [88], mushroom tyrosinase [88], chloroperoxidase [89], porcine pancreatic lipase [91], α-helical protein Im7 [92], pepsin [93], adenosine deaminase [95], α-amylases [96], xylanase II (GH11 enzyme, from *Trichoderma longibrachiatum* [97], lactoferrin [99], α-synuclein [101, 103], ribonuclease A [102], casein [104, 105], epoxide hydrolase [110], avidin [111], Abeta [1–40] peptide [112], zein [113] and firefly luciferase (*Photinus pyralis* luciferase) in the ILs. Among them some of the proteins have been stabilized [79–91], while some

mim][Cl]) of

ILs acted as refolding enhancers for the completely denatured lysozyme structure.

ary structure at low concentrations of 1-octyl-3-methylimidazolium chloride ([C8

**5. Structure and stability of some other proteins in different ILs**

other proteins have shown to be destabilized in the presence of ILs [92–97].

Apart from the studies related to protein folding/unfolding, there are other research articles which have recognized the use of ILs as two-phase systems which are used for protein preservation, protein separation, purification, partitioning of proteins and many more purposes. For example, the extraction efficiency of papain was increased to 98.3% in the biphasic mixtures containing ILs [100]. On the other hand, Ebrahimi et al. [114] delineated the activity and stability of *P. pyralis* luciferase in two tetramethylguanidine-based [TMG][Lac] and [TMG] [Pro]. The authors found that the luciferase activity increased up to 0.25 M of [TMG][Lac]

mim][Cl] IL into the hydrophobic interior of the protein [77].

of [C8

Many key studies related to ammonium-based IL interactions with lysozyme have been carried out, focusing on the role of these ILs. A study by Summers and Flowers [64] on lysozyme showed that ethylammonium nitrate (EAN) stabilized the lysozyme against irreversible thermal denaturation. Byrne et al. [66] reported the thermal refolding as well as extended period stabilization of lysozyme with concentration >200 mg/mL. To achieve the refolded fraction of the protein, EAN was used. Triethylammonium methanesulfonate [TEA][MS] was able to refold 97% of thermally denatured lysozyme [66]. Mann et al. [67] observed that ammonium-based ILs such as ethylammonium formate (EAF), propylammonium formate (PAF), 2-methoxyethylammonium formate (MOEAF) and ethanolammonium formate (EtAF) not only acted as good stabilizers for the lysozyme native structure but also protected the protein against thermal unfolding. Ammonium-based ILs such as EAN, triethylammonium triflate (TEATF) and triethylammonium mesylate (TEAMS) were observed to be acting as solvents for solubilizing the aggregates (amyloids) of denatured lysozyme structure. Interestingly, after solubilization in the ILs, the activity of aggregated lysozyme was observed to reappear up to 80% in the presence of EAN and more than 50% in the rest of the ILs [68]. Many studies, related to the role of ammonium-based ILs on lysozyme, suggest the role of ammonium family ILs as a refolding additive, fibrillizing agent, precipitating agent, additives for protein crystallization, prevention of aggregation and renaturing agent, as well as stabilizers against thermal unfolding [66–70]. Also, based on the above experimental results, EAN can be termed as a refolding additive, from the thermally as well as chemically denatured lysozyme. Furthermore, talking particularly about the cations having various hydrogen bond donor sites result in more effective coordination to the protein, thereby stabilizing the biomolecule structure in a more efficient manner. The stability of lysozyme in imidazolium-based ILs is an interesting aspect that helps us to understand the interactions that are responsible for stabilizing/destabilizing the protein structure in various ILs. The thermal stability of lysozyme crystals was obtained using imidazolium-based ILs such as 1-butyl-3-methylimidazolium tetrafluoroborate ([Bmim][BF<sup>4</sup> ]), 1-butyl-3-methylimidazolium chloride ([Bmim][Cl]), 1-butyl-3-methylimidazolium bromide ([Bmim][Br]) and 1,3-dimethylimidazolium iodine([Mmim] [I]), as additives during lysozyme crystallization [71].

The stability of lysozyme in imidazolium-based IL is observed to vary with the concentrations of the IL. In this regard, Takekiyo et al. [72] observed the structural change of lysozyme in aqueous 1-butyl-3-methylimidazolium nitrate ([Bmim][NO3 ]) solutions by using Fourier transform infrared (FTIR), circular dichroism (CD) spectra and small-angle X-ray scattering (SAXS) methods. The results illustrated that the structure of the protein significantly varied with changes in the structure and concentration of the ILs. In the first view, the authors observed that the increase in the [Bmim][NO3 ] concentration completely disrupted the tertiary structure of lysozyme at 5 M of IL. Lange et al. [73] reported that the imidazoliumbased ILs can also be considered as refolding agents. They tested the refolding of lysozyme in the set of imidazolium ILs, [Emim]+ , [Bmim]+ and [Hexmim]+ cations with a fixed anion Cl− . Interestingly, the stability of lysozyme in imidazolium-based ILs was observed to decrease significantly as the alkyl chain of the ILs increased from ethyl to butyl to hexyl. However, all ILs acted as refolding enhancers for the completely denatured lysozyme structure.

media [65]. Studies carried out by various groups using CT suggest that the physical properties of ILs such as polarity and hydrophobicity play a major role in their stabilizing behavior toward CT [18, 52, 53]. Among various families of ILs, ammonium-based ILs are identified to

Many key studies related to ammonium-based IL interactions with lysozyme have been carried out, focusing on the role of these ILs. A study by Summers and Flowers [64] on lysozyme showed that ethylammonium nitrate (EAN) stabilized the lysozyme against irreversible thermal denaturation. Byrne et al. [66] reported the thermal refolding as well as extended period stabilization of lysozyme with concentration >200 mg/mL. To achieve the refolded fraction of the protein, EAN was used. Triethylammonium methanesulfonate [TEA][MS] was able to refold 97% of thermally denatured lysozyme [66]. Mann et al. [67] observed that ammonium-based ILs such as ethylammonium formate (EAF), propylammonium formate (PAF), 2-methoxyethylammonium formate (MOEAF) and ethanolammonium formate (EtAF) not only acted as good stabilizers for the lysozyme native structure but also protected the protein against thermal unfolding. Ammonium-based ILs such as EAN, triethylammonium triflate (TEATF) and triethylammonium mesylate (TEAMS) were observed to be acting as solvents for solubilizing the aggregates (amyloids) of denatured lysozyme structure. Interestingly, after solubilization in the ILs, the activity of aggregated lysozyme was observed to reappear up to 80% in the presence of EAN and more than 50% in the rest of the ILs [68]. Many studies, related to the role of ammonium-based ILs on lysozyme, suggest the role of ammonium family ILs as a refolding additive, fibrillizing agent, precipitating agent, additives for protein crystallization, prevention of aggregation and renaturing agent, as well as stabilizers against thermal unfolding [66–70]. Also, based on the above experimental results, EAN can be termed as a refolding additive, from the thermally as well as chemically denatured lysozyme. Furthermore, talking particularly about the cations having various hydrogen bond donor sites result in more effective coordination to the protein, thereby stabilizing the biomolecule structure in a more efficient manner. The stability of lysozyme in imidazolium-based ILs is an interesting aspect that helps us to understand the interactions that are responsible for stabilizing/destabilizing the protein structure in various ILs. The thermal stability of lysozyme crystals was obtained using imidazolium-based ILs such as 1-butyl-3-methylimidazolium tet-

]), 1-butyl-3-methylimidazolium chloride ([Bmim][Cl]), 1-butyl-

]) solutions by using Fourier

cations with a fixed anion Cl−

.

] concentration completely disrupted the ter-

3-methylimidazolium bromide ([Bmim][Br]) and 1,3-dimethylimidazolium iodine([Mmim]

The stability of lysozyme in imidazolium-based IL is observed to vary with the concentrations of the IL. In this regard, Takekiyo et al. [72] observed the structural change of lysozyme

transform infrared (FTIR), circular dichroism (CD) spectra and small-angle X-ray scattering (SAXS) methods. The results illustrated that the structure of the protein significantly varied with changes in the structure and concentration of the ILs. In the first view, the authors

tiary structure of lysozyme at 5 M of IL. Lange et al. [73] reported that the imidazoliumbased ILs can also be considered as refolding agents. They tested the refolding of lysozyme in

and [Hexmim]+

, [Bmim]+

display their wide range application in biochemical processes [44].

rafluoroborate ([Bmim][BF<sup>4</sup>

78 Progress and Developments in Ionic Liquids

[I]), as additives during lysozyme crystallization [71].

observed that the increase in the [Bmim][NO3

the set of imidazolium ILs, [Emim]+

in aqueous 1-butyl-3-methylimidazolium nitrate ([Bmim][NO3

Interestingly, experimental results reveal that the partitioning of bovine serum albumin (BSA) is predominated by the hydrophobic interactions between the protein and imidazolium cation of the ILs in the aqueous system, which can be improved or modulated by changing the cation chain length of the ILs or modifying the surface of the BSA [74, 75]. The study indicated that the secondary structure of BSA was stabilized at low IL concentration (<3 mM) and the protein was denatured at higher IL concentration (>3 mM) [76]. Rawat and Bohidar [77, 78] reported that the interaction between imidazolium-based ILs and BSA is obviously dependent on the concentrations of the ILs. They observed that the BSA retained the secondary structure at low concentrations of 1-octyl-3-methylimidazolium chloride ([C8 mim][Cl]) of 0–0.2% w/v. Moving above this concentration range, the BSA structure was denatured significantly that is most likely caused due to intercalation of alkyl chain of the imidazolium cation of [C8 mim][Cl] IL into the hydrophobic interior of the protein [77].

### **5. Structure and stability of some other proteins in different ILs**

In the earlier sections, the protein stabilization studies of most commonly used proteins have been delineated. Even though, the literature is still scattered, which deals with the stability of proteins in the presence of ILs. Hence, in this section, we have tried together all these research reports [79–114] under the same roof, so that it is easily available to the readers and also to expose its importance to the scientific world. An overview of the literature reveals stability studies of proteins such as amyloglucosidase [79], thyroglobulin [79], glutamate dehydrogenase [79], lactate dehydrogenase [79], glucose dehydrogenase [80], formate dehydrogenase [94], glycosidase (α and β) [81], monellin [82], β-galactosidase [83], glucose oxidase [106], lactate oxidase [109], oxidoreductases [107], subtilisin Carlsberg [84], Amano protease P6 [85], pepsin [86], papain [87, 100] esterases from *Bacillus subtilis* and *Bacillus stearothermophilus* [87], *Penicillium expansum* lipase [88], mushroom tyrosinase [88], chloroperoxidase [89], porcine pancreatic lipase [91], α-helical protein Im7 [92], pepsin [93], adenosine deaminase [95], α-amylases [96], xylanase II (GH11 enzyme, from *Trichoderma longibrachiatum* [97], lactoferrin [99], α-synuclein [101, 103], ribonuclease A [102], casein [104, 105], epoxide hydrolase [110], avidin [111], Abeta [1–40] peptide [112], zein [113] and firefly luciferase (*Photinus pyralis* luciferase) in the ILs. Among them some of the proteins have been stabilized [79–91], while some other proteins have shown to be destabilized in the presence of ILs [92–97].

Apart from the studies related to protein folding/unfolding, there are other research articles which have recognized the use of ILs as two-phase systems which are used for protein preservation, protein separation, purification, partitioning of proteins and many more purposes. For example, the extraction efficiency of papain was increased to 98.3% in the biphasic mixtures containing ILs [100]. On the other hand, Ebrahimi et al. [114] delineated the activity and stability of *P. pyralis* luciferase in two tetramethylguanidine-based [TMG][Lac] and [TMG] [Pro]. The authors found that the luciferase activity increased up to 0.25 M of [TMG][Lac] conversely the activity diminished in the presence of similar concentrations of [TMG][Pro]. Further, thermal stability studies show more stability of luciferase only in the presence of [TMG][Lac], whereas thermal stability was not improved in [TMG][Pro] [114]. Similar effects were observed in the stability of insulin in the presence of a set of imidazolium-based ILs such as [Bmim][Cl], [Bmim][Br], 1-butyl-3-methylimidazolium thiocyanate ([Bmim][SCN]), 1-butyl-3-methylimidazolium hydrogen sulfate ([Bmim][HSO4 ]), 1-butyl-3-methylimidazolium acetate ([Bmim][CH3 COO]) and 1-butyl-3-methylimidazolium iodide ([Bmim][I]) [115]. The experimental findings reveal that [Bmim][Br] and [Bmim][Cl] ILs stabilized the native state of insulin, while the rest of the [Bmim] ILs with anions such as SCN<sup>−</sup> , HSO4 − , CH3 COO<sup>−</sup> and I<sup>−</sup> were destabilizers for the native form of insulin. Moreover, imidazolium-based ILs were also found to enhance the aggregated structure in insulin [115]. In support, Bae [101] and Hwang et al. [103] investigated the effect of imidazolium-based ILs on the aggregation properties of α-synuclein. Their results indicated the increase in the aggregated structure of the protein due to the ILs [101, 103].

[C16mim][Tf2

[Emim][BF4

color) (Ref. [126]).

] and [C16mim][Tf2

N] [125]. When Lee et al. [125] used the binary mixtures of these two ILs as addi-

N] were 10-fold and 14-fold greater than in silica gel without

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*max*) for CT in buffer

*max* for the CT

tives, the optimal immobilized lipase showed both high activity and stability. The hydrolysis and esterification activities of lipase co-immobilized with the mixture of 1:1 at molar ratio of

ILs, respectively. Therefore, the binary mixtures of these ILs as additives were used to obtain

Very recently, our group also showed the influence of mixture of two ILs on the structure and the stability of CT [126]. Evidently, the fluorescence and CD spectral results demonstrated that [Bmim][Br] alone acts as a stabilizer at low concentrations, while it acts as a destabilizer at high concentrations for the native structure of CT. On the other hand, [Bmim][I] is a destabilizer at all the concentrations. Nevertheless, the denaturing ability of [Bmim][I] was compensated by the [Bmim][Br]. Further, to offset the action of [Bmim][Br] on deleterious action of [Bmim][I] is more pronounced at lower concentration (0.025 M) than at higher con-

was 73.6 a.u., in 0.025 M [Bmim][Br] was 71.5 a.u. and in 0.2 M [Bmim][I] was 39.7 a.u. [126].

Interestingly, after addition of 0.025 M [Bmim][Br] to 0.2 M [Bmim][I], it is noticeable how

**Figure 1.** Fluorescence intensity changes for α-chymotrypsin (CT) in Tris-HCl buffer (black color line), 0.025 M [Bmim] [Br] (red color line), 0.2 M [Bmim][I] (magenta color line) and 0.025 M [Bmim][Br] + 0.2 M [Bmim][I] mixture (dark cyan

in [Bmim][I], although the intensity was lower in the presence of [Bmim][Br] + [Bmim][I] than that of the native protein, which is, however, certainly larger than that of [Bmim][I].

the fluorescence spectra of the CT were clearly modified compared with the *I*

the optimal immobilized lipase which shows both high activity and stability [125].

centrations. As shown in **Figure 1**, the fluorescence intensity maximum (*I*

### **6. Influence of mixture of ILs on the structure and stability of proteins**

It is now obvious after examining the effect of various ILs (from different families of ILs) that some of the industrially important ILs acted as destabilizers for the proteins. Therefore, the search to offset the negative effects of ILs on proteins came into limelight. Therefore, the maintenance of ILs as the "green solvent medium" is a great challenge for a chemists or biochemists [116]. There are reports available in the literature that projects the negative effects of the ILs on the proteins. Klähn et al. [117, 118] reported the destabilization of *Candida antarctica* lipase B (CALB) in imidazolium- or guanidinium-based ILs through MD simulations. Further, very recently our research group has shown the destabilization of *heme* proteins in the presence of ammonium-based ILs [119]. We stress that all these results can be considered as an alarm for a chemist and biochemist to search for the novel method of counteraction for the denaturation action of ILs on the biomolecules.

In this context, the mixtures of ILs have been of continuing interest, of great fundamental practical importance and increasingly received a lot of attention from both academia and industry [120–123]. Recently, mixtures of ILs exhibit interesting and increased scope to the access of the properties, which are not readily apparent from those of the individual IL. Keeping this in mind, Lozano and coworkers [124] showed that [Bmim][Cl] behaved as a powerful enzyme-deactivating agent for cellulase. On the other hand, hydrophobic IL butyltrimethylammonium bis(trifluoromethylsulfonyl)imide ([N1114][NTf2 ]) clearly enhanced the enzyme thermal stability. Apparently, the mixture of [N1114][NTf2 ] and [Bmim][Cl] greatly improved the thermal stability of cellulase with respect to [Bmim][Cl] alone. By increasing the hydrophobic IL concentration, the deactivation effect of [Bmim][Cl] was reduced, which could be attributed to the preservation of the essential water molecules around the protein [124].

Similarly, the highest hydrolytic activity of immobilized lipase was obtained when the hydrophilic IL, 1-ethyl-3-methylimidazolium tetrafluoroborate ([Emim][BF<sup>4</sup> ]), was used as an additive, while the highest stability of immobilized lipase was obtained by using hydrophobic IL, [C16mim][Tf2 N] [125]. When Lee et al. [125] used the binary mixtures of these two ILs as additives, the optimal immobilized lipase showed both high activity and stability. The hydrolysis and esterification activities of lipase co-immobilized with the mixture of 1:1 at molar ratio of [Emim][BF4 ] and [C16mim][Tf2 N] were 10-fold and 14-fold greater than in silica gel without ILs, respectively. Therefore, the binary mixtures of these ILs as additives were used to obtain the optimal immobilized lipase which shows both high activity and stability [125].

conversely the activity diminished in the presence of similar concentrations of [TMG][Pro]. Further, thermal stability studies show more stability of luciferase only in the presence of [TMG][Lac], whereas thermal stability was not improved in [TMG][Pro] [114]. Similar effects were observed in the stability of insulin in the presence of a set of imidazolium-based ILs such as [Bmim][Cl], [Bmim][Br], 1-butyl-3-methylimidazolium thiocyanate ([Bmim][SCN]),

The experimental findings reveal that [Bmim][Br] and [Bmim][Cl] ILs stabilized the native

**6. Influence of mixture of ILs on the structure and stability of proteins**

It is now obvious after examining the effect of various ILs (from different families of ILs) that some of the industrially important ILs acted as destabilizers for the proteins. Therefore, the search to offset the negative effects of ILs on proteins came into limelight. Therefore, the maintenance of ILs as the "green solvent medium" is a great challenge for a chemists or biochemists [116]. There are reports available in the literature that projects the negative effects of the ILs on the proteins. Klähn et al. [117, 118] reported the destabilization of *Candida antarctica* lipase B (CALB) in imidazolium- or guanidinium-based ILs through MD simulations. Further, very recently our research group has shown the destabilization of *heme* proteins in the presence of ammonium-based ILs [119]. We stress that all these results can be considered as an alarm for a chemist and biochemist to search for the novel method of counteraction for

In this context, the mixtures of ILs have been of continuing interest, of great fundamental practical importance and increasingly received a lot of attention from both academia and industry [120–123]. Recently, mixtures of ILs exhibit interesting and increased scope to the access of the properties, which are not readily apparent from those of the individual IL. Keeping this in mind, Lozano and coworkers [124] showed that [Bmim][Cl] behaved as a powerful enzyme-deactivating agent for cellulase. On the other hand, hydrophobic IL but-

greatly improved the thermal stability of cellulase with respect to [Bmim][Cl] alone. By increasing the hydrophobic IL concentration, the deactivation effect of [Bmim][Cl] was reduced, which could be attributed to the preservation of the essential water molecules

Similarly, the highest hydrolytic activity of immobilized lipase was obtained when the hydro-

tive, while the highest stability of immobilized lipase was obtained by using hydrophobic IL,

yltrimethylammonium bis(trifluoromethylsulfonyl)imide ([N1114][NTf2

philic IL, 1-ethyl-3-methylimidazolium tetrafluoroborate ([Emim][BF<sup>4</sup>

the enzyme thermal stability. Apparently, the mixture of [N1114][NTf2

 were destabilizers for the native form of insulin. Moreover, imidazolium-based ILs were also found to enhance the aggregated structure in insulin [115]. In support, Bae [101] and Hwang et al. [103] investigated the effect of imidazolium-based ILs on the aggregation properties of α-synuclein. Their results indicated the increase in the aggregated structure of

COO]) and 1-butyl-3-methylimidazolium iodide ([Bmim][I]) [115].

]), 1-butyl-3-methylimidazo-

, HSO4 − , CH3

]) clearly enhanced

]), was used as an addi-

] and [Bmim][Cl]

COO<sup>−</sup>

1-butyl-3-methylimidazolium hydrogen sulfate ([Bmim][HSO4

state of insulin, while the rest of the [Bmim] ILs with anions such as SCN<sup>−</sup>

lium acetate ([Bmim][CH3

80 Progress and Developments in Ionic Liquids

the protein due to the ILs [101, 103].

the denaturation action of ILs on the biomolecules.

around the protein [124].

and I<sup>−</sup>

Very recently, our group also showed the influence of mixture of two ILs on the structure and the stability of CT [126]. Evidently, the fluorescence and CD spectral results demonstrated that [Bmim][Br] alone acts as a stabilizer at low concentrations, while it acts as a destabilizer at high concentrations for the native structure of CT. On the other hand, [Bmim][I] is a destabilizer at all the concentrations. Nevertheless, the denaturing ability of [Bmim][I] was compensated by the [Bmim][Br]. Further, to offset the action of [Bmim][Br] on deleterious action of [Bmim][I] is more pronounced at lower concentration (0.025 M) than at higher concentrations. As shown in **Figure 1**, the fluorescence intensity maximum (*I max*) for CT in buffer was 73.6 a.u., in 0.025 M [Bmim][Br] was 71.5 a.u. and in 0.2 M [Bmim][I] was 39.7 a.u. [126].

**Figure 1.** Fluorescence intensity changes for α-chymotrypsin (CT) in Tris-HCl buffer (black color line), 0.025 M [Bmim] [Br] (red color line), 0.2 M [Bmim][I] (magenta color line) and 0.025 M [Bmim][Br] + 0.2 M [Bmim][I] mixture (dark cyan color) (Ref. [126]).

Interestingly, after addition of 0.025 M [Bmim][Br] to 0.2 M [Bmim][I], it is noticeable how the fluorescence spectra of the CT were clearly modified compared with the *I max* for the CT in [Bmim][I], although the intensity was lower in the presence of [Bmim][Br] + [Bmim][I] than that of the native protein, which is, however, certainly larger than that of [Bmim][I]. The main reason behind the enhancement of intensity with the addition of [Bmim][Br] into the protein solution having [Bmim][I] is the movement of Trp toward a more hydrophobic environment and therefore high fluorescence intensity is observed due to higher quantum yield. Obviously, the mixture of the ILs may improve the stability of CT structure [126]. This phenomenon is schematically shown in **Scheme 1**.

different ILs may be a very useful method to make new IL solvent media for the structure and stability of biomolecules. These results improve knowledge of the excellent properties of IL mixtures as stabilizers for the native conformation of protein, since IL mixtures are able to stabilize enzymes and are suitable as reaction media for enzymatic biotransformations of industrial interest. Currently, these discoveries have opened new opportunities for obtaining

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better activity and improvement of stability of proteins in the mixtures of ILs.

**7. The significant and specific contribution of ILs on protein stability**

Practically, the native conformations of the globular proteins have adapted to environmental stresses that normally denature the proteins and the nature has provided a mechanism of adaptation that some of the cosolvents protect against denatured proteins [133, 134]. In fact, the protecting cosolvents stabilize the proteins against denaturing stresses and their presence in the cell does not alter protein functional activity [135]. The protein aggregation and its cold and thermal denaturation have been recognized as a major manifestation of instability that can severely affect a protein's functionality. As a group of novel green solvents, ILs have attracted extensive attention and gained popularity to overcome these physiological stresses. As discussed in earlier sections, ILs are potential cosolvent media for preservation of biomolecules because of their high stability and unusual solvent properties. We saw some of the ILs acted as efficient additives for the suppression of protein aggregation. Also, in the previous sections, we observed that the ILs behaved differently with various proteins. From the obtained results, we concluded that the stability of the proteins in ILs is completely dependent on the interactions of the ions of ILs with AA residues at the surface of the proteins.

In this section, we support the concept of novel behavior of ILs and explain the prevention of the self-aggregation of proteins, refolding of thermally and chemically perturbed proteins and also counteracting effects of ILs against the thermally and cold denaturation of proteins. Therefore, mechanistic insight into the effects of ILs on preventing all the deleterious effects on the proteins is desired for the understanding and designing of protein processes in bio-

As mention before, in 2000, Summers and Flowers [64] have observed that EAN has the ability to prevent lysozyme aggregation and is an efficient refolding additive for a completely denatured protein. Further, EAN has been utilized as a precipitating agent for the crystallization of lysozyme, providing crystals with good diffraction [135]. In another study, ammonium-based ILs such as EAF, PAF, 2-methoxyethylammonium formate (MOEAF) and Ethanolammonium formate (EtAF) not only acted as good stabilizers for the lysozyme native structure but also they protected the protein against thermal unfolding [67]. Subsequently, EAN IL not only stabilized the lysozyme native structure for long term rather it acted as a refolding agent preventing the lysozyme from aggregation. Bisht et al. [136] noticed that the presence of 1% v/v ammonium-based ILs can increase the activity of lysozyme up to 13% also refolded the ureainduced unfolded lysozyme structure. Interestingly, EAN has been observed to be possessing multicharacter in protein stability [64, 66, 67, 70]. We believe that information will surely help in understanding the microscopic mechanism existing between protein and ILs, particularly EAN.

physical chemistry and biotechnology.

**Scheme 1.** The presence of [Bmim][Br] counteracts the strong denaturation action of [Bmim][I] (Ref. [126]).

In 2006, the Lee group published a pioneering work [127] that showed that the effect of chloride impurity on the activity and stability of lipase in 1-octyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([Omim][Tf<sup>2</sup> N]) and [Omim][Cl] ILs. In result, the activity of enzyme exponentially decreased with increasing Cl<sup>−</sup> content in [Omim][Tf2 N] and the activity of lipase in [Omim][Tf2 N] mixture containing 2% [Omim][Cl] was only about 2% of the activity in pure [Omim][Tf2 N]. The reason provided by the authors for the decrease in the activity of the enzyme in Cl<sup>−</sup> ILs is due to the denaturation of the enzyme in these ILs. As mentioned before, the activity of the enzyme linearly decreased at about 5% for every 1% increase in [Omim][Cl] with there being no activity in [Omim][Tf2 N] containing about 20% [Omim][Cl] [127]. In another work of the Lee group [128], they systematically showed that the highest lipase activity was obtained in water-miscible [Bmim][TfO] which can dissolve a high concentration of glucose, while the highest stability of lipase was shown in hydrophobic [Bmim][Tf2 N]. The optimal activity and stability of lipase could be obtained in the mixture of [Bmim][TfO] and [Bmim][Tf2 N]. Therefore, the productivity obtained by using IL mixtures was higher than those in pure ILs [128]. A later study by this same research group reported that higher enzyme activity was achieved under ultrasound irradiation on lipasecatalyzed esterification of fructose in the mixture of [Bmim][TfO] and [Omim][Tf<sup>2</sup> N] [129]. These results show that enzymatic reaction in ILs mixture under ultrasound irradiation is an effective method for enzyme activity and stability resulting in economic competitiveness of green process [129]. It is well documented that 100% conversion of cellulose to useful biochemical process in the presence of the mixture of ILs at low temperature, which overcomes the long intrinsic phase problem in the conversion of biomass to chemicals [130, 131]. Yao et al. [132] reported the activity and stability of *Candida rugosa* lipase in binary ILs.

Mixing of two different ILs, which show different physicochemical properties, can easily make new ILs because hydrophobic ILs and hydrophilic ILs are generally miscible. The mixing of different ILs may be a very useful method to make new IL solvent media for the structure and stability of biomolecules. These results improve knowledge of the excellent properties of IL mixtures as stabilizers for the native conformation of protein, since IL mixtures are able to stabilize enzymes and are suitable as reaction media for enzymatic biotransformations of industrial interest. Currently, these discoveries have opened new opportunities for obtaining better activity and improvement of stability of proteins in the mixtures of ILs.

The main reason behind the enhancement of intensity with the addition of [Bmim][Br] into the protein solution having [Bmim][I] is the movement of Trp toward a more hydrophobic environment and therefore high fluorescence intensity is observed due to higher quantum yield. Obviously, the mixture of the ILs may improve the stability of CT structure [126]. This

In 2006, the Lee group published a pioneering work [127] that showed that the effect of chloride impurity on the activity and stability of lipase in 1-octyl-3-methylimidazolium

**Scheme 1.** The presence of [Bmim][Br] counteracts the strong denaturation action of [Bmim][I] (Ref. [126]).

As mentioned before, the activity of the enzyme linearly decreased at about 5% for every 1%

[Omim][Cl] [127]. In another work of the Lee group [128], they systematically showed that the highest lipase activity was obtained in water-miscible [Bmim][TfO] which can dissolve a high concentration of glucose, while the highest stability of lipase was shown in hydro-

mixtures was higher than those in pure ILs [128]. A later study by this same research group reported that higher enzyme activity was achieved under ultrasound irradiation on lipase-

These results show that enzymatic reaction in ILs mixture under ultrasound irradiation is an effective method for enzyme activity and stability resulting in economic competitiveness of green process [129]. It is well documented that 100% conversion of cellulose to useful biochemical process in the presence of the mixture of ILs at low temperature, which overcomes the long intrinsic phase problem in the conversion of biomass to chemicals [130, 131]. Yao et

Mixing of two different ILs, which show different physicochemical properties, can easily make new ILs because hydrophobic ILs and hydrophilic ILs are generally miscible. The mixing of

catalyzed esterification of fructose in the mixture of [Bmim][TfO] and [Omim][Tf<sup>2</sup>

al. [132] reported the activity and stability of *Candida rugosa* lipase in binary ILs.

N]) and [Omim][Cl] ILs. In result, the activ-

content in [Omim][Tf2

N] mixture containing 2% [Omim][Cl] was only about 2% of

N]. The reason provided by the authors for the decrease in

ILs is due to the denaturation of the enzyme in these ILs.

N]. Therefore, the productivity obtained by using IL

N]. The optimal activity and stability of lipase could be obtained in the

N] and the

N] [129].

N] containing about 20%

phenomenon is schematically shown in **Scheme 1**.

82 Progress and Developments in Ionic Liquids

bis(trifluoromethylsulfonyl)imide ([Omim][Tf<sup>2</sup>

activity of lipase in [Omim][Tf2

the activity in pure [Omim][Tf2

phobic [Bmim][Tf2

the activity of the enzyme in Cl<sup>−</sup>

mixture of [Bmim][TfO] and [Bmim][Tf2

ity of enzyme exponentially decreased with increasing Cl<sup>−</sup>

increase in [Omim][Cl] with there being no activity in [Omim][Tf2

### **7. The significant and specific contribution of ILs on protein stability**

Practically, the native conformations of the globular proteins have adapted to environmental stresses that normally denature the proteins and the nature has provided a mechanism of adaptation that some of the cosolvents protect against denatured proteins [133, 134]. In fact, the protecting cosolvents stabilize the proteins against denaturing stresses and their presence in the cell does not alter protein functional activity [135]. The protein aggregation and its cold and thermal denaturation have been recognized as a major manifestation of instability that can severely affect a protein's functionality. As a group of novel green solvents, ILs have attracted extensive attention and gained popularity to overcome these physiological stresses. As discussed in earlier sections, ILs are potential cosolvent media for preservation of biomolecules because of their high stability and unusual solvent properties. We saw some of the ILs acted as efficient additives for the suppression of protein aggregation. Also, in the previous sections, we observed that the ILs behaved differently with various proteins. From the obtained results, we concluded that the stability of the proteins in ILs is completely dependent on the interactions of the ions of ILs with AA residues at the surface of the proteins.

In this section, we support the concept of novel behavior of ILs and explain the prevention of the self-aggregation of proteins, refolding of thermally and chemically perturbed proteins and also counteracting effects of ILs against the thermally and cold denaturation of proteins. Therefore, mechanistic insight into the effects of ILs on preventing all the deleterious effects on the proteins is desired for the understanding and designing of protein processes in biophysical chemistry and biotechnology.

As mention before, in 2000, Summers and Flowers [64] have observed that EAN has the ability to prevent lysozyme aggregation and is an efficient refolding additive for a completely denatured protein. Further, EAN has been utilized as a precipitating agent for the crystallization of lysozyme, providing crystals with good diffraction [135]. In another study, ammonium-based ILs such as EAF, PAF, 2-methoxyethylammonium formate (MOEAF) and Ethanolammonium formate (EtAF) not only acted as good stabilizers for the lysozyme native structure but also they protected the protein against thermal unfolding [67]. Subsequently, EAN IL not only stabilized the lysozyme native structure for long term rather it acted as a refolding agent preventing the lysozyme from aggregation. Bisht et al. [136] noticed that the presence of 1% v/v ammonium-based ILs can increase the activity of lysozyme up to 13% also refolded the ureainduced unfolded lysozyme structure. Interestingly, EAN has been observed to be possessing multicharacter in protein stability [64, 66, 67, 70]. We believe that information will surely help in understanding the microscopic mechanism existing between protein and ILs, particularly EAN.

Apparently, Lange et al. [73] explicitly elucidated that the set of imidazolium-based ILs such as [Emim]+ , [Bmim]+ and [Hexmim]+ cations with a fixed anion Cl− can also be considered as refolding agents. The stability of lysozyme in these ILs was observed to decrease significantly as the alkyl chain of the ILs increased from ethyl, butyl to hexyl. Nonetheless, all ILs acted as refolding enhancers for the completely denatured lysozyme structure. A later study found that a series of the ILs such as [Mmim][Cl], [Emim][Cl], [Pmim][Cl], [Bmim][Cl], [Penmim] [Cl], [Hexmim][Cl], [Hepmim][Cl], [Omim][Cl], [Ddmim][Cl], [i-Bmim][Cl] and [Bemim][[Cl] were applied to the denatured lysozyme structure and a significant refolding of the protein was observed. Among the ILs, in the presence of [Bmim][Cl], the refolding yield reached up to maximum of 84% [137]. The less hydrophobic ILs such as N-alkylpyridinium chlorides [EtPy][Cl], [BPy][Cl] and [HexPy][Cl] were effective in enhancing the refolding agents for the lysozyme structure and yielded up to 46–69% refolding [76]. As a consequence, the results conclude that more hydrophobic ILs behaved as a denaturants for lysozyme while the same ILs acted as refolding agents for the denatured lysozyme structure. In support to the abovementioned facts, Takekiyo et al. [72] observed that the secondary structure of lysozyme was refolded in the 6–10 M concentration range of imidazolium-based IL, whereas the tertiary structure breaks down. Upon increase in the concentration more than 10 M of [Bmim][NO3 ], the secondary structure of the protein was still observed to be in a partially refolded state, while the tertiary structure was completely disrupted.

NMR results confirmed that TEAA strongly counteracted the deleterious actions of wellknown denaturant, urea, on the CT structure [144]. It was demonstrated that TEAA and urea mixture substantially increased the Tm values which showed the counterbalance of the ureainduced denaturation of CT. Most importantly, deleterious action 5 M urea on CT was coun-

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It has been already noted that a protic IL triethylammonium phosphate (TEAP) acted as a refolding additive for the urea-induced chemical denaturated state of the two enzymes, CT and succinylated Concanavalin A (S Con A) [22]. In one of the studies by Attri and Venkatesu, TEAP was shown to be acting as an efficient refolding agent for thermally denatured S Con A [145]. In 2013, Attri and Choi [146] showed that TEAP strongly attenuated the detrimental action of atmospheric pressure plasma jet (APPJ) on CT. This ammonium-based IL TEAP is able to maintain the structural integrity as well as activity of CT even after the exposure of APPJ [147]. The results show that one can use both enzyme and plasma simultaneously without affecting the enzyme structure and activity on the material surface, which can prove to be applicable in

Recent studies on ammonium-based ILs offer some valuable information to prevent the selfaggregation of the proteins. In this regard, Awanish and Venkatesu [148] for the first time showed ammonium-based ILs as a novel solvent for offsetting self-aggregation of insulin in the presence of TMAS, TEAS, TMAP, TEAP and TMAA. Therefore, the native structure of insulin was found to be stabilized in the presence of ammonium-ILs by unfavorable interactions with the surface of protein. The stability studies of insulin in ILs have opened a new way that can lead us to overcome the aggregation properties of insulin. This will not only increase the shelf life of insulin, whereas suitable formulations of insulin in biocompatible ILs can lead to safe and durable insulin formulations in pharmaceutical products. The ammonium-based ILs such as ethylammonium mesylate (EaM), diethylammonium mesylate (DeaM)-stabilized *tobacco mosaic virus*, whereas triethylammonium mesylate (TeaM) caused a change in the sec-

From the literature and from our own experience, it can be suggested that ammonium-based ILs are more biocompatible as compared to the imidazolium-based ILs [44, 140–155]. Yu et al. [150] explored the stability of laccase in the presence of both ammonium- and imidazolium-based ILs such as [TMA][TfO], [Bmim][TfO] and [Bmpyr][TfO]. They found that only ammonium-based ILs [TMA][TfO] stabilized laccase, while [Bmim][TfO] and [Bmpyr] [TfO] destabilized it. The contrasting nature of ammonium family ILs is also consistent with Rodrigues et al. [151], where among different families of ILs only ammonium-containing IL shows higher activity as compared to imidazolium-based ILs for *Thermomyces lanuginosus* lipase (TlL). **Scheme 2** shows the difference between the biocompability behaviors of ammo-

On the other hand, Jha et al. [152] have explored the influence of a of imidazolium-based IL, 1-allyl-3-methylimidazolium chloride ([Amim][Cl]), on the stability of Hb. Unprecedented improvement in the stability of Hb in the presence of [Amim][Cl] at the lower concentration of [Amim][Cl] was observed by the authors [153]. Furthermore, the effect of [Amim][Cl] on bromelain stability and activity was investigated in another work. They observed that at low concentrations (0.01−0.10 M)

of [Amim][Cl], there is ostensible only change in the stability and activity of BM.

teracted by only 1 M TEAA.

ondary structure of the virus [149].

nium- and imidazolium-based ILs.

various fields.

Very recently, at low concentration of [Bmim][Cl], the cyt c starts to unfold and again starts refolding with increasing concentration of the IL [138]. These results suggest a partial refolding of the secondary structure of cyt c in [Bmim][Cl]. The [Emim][Cl] was a very efficient in promoting refolding of the recombinant plasminogen activator (rPA) [139]. The delicate balance of favorable interactions with side chains and unfavorable interactions with the peptide backbone provides a molecular explanation of how this IL suppresses protein aggregation and simultaneously promotes refolding. Nonetheless, the protein denatured at high concentrations of [Emim][Cl] which indicates strong favorable interactions between AA side chains and ions of the IL [139].

The effect of EAN on renaturation of cyt c has been shown by Jaganathan et al. [140] and the results show that EAN in the range of 10−4 M helps in refolding of the protein from urea (8 M)-induced denaturation of cyt c. On the other hand, it was observed that at moderate concentrations (50–150 mM) [Emim][CH3 COO] did not induce any significant effect over Mb structure, however, [Bmim][BF4 ], at the same concentrations significantly destabilized the Mb [141]. Further, there was minimal variation in the structure of Mb, when the mixture of 1.4 M GdnHCl and 150 mM of [Emim][CH3 COO] IL was used as a cosolvent. That is, the protein is not completely unfolded. While in the presence of 150 mM [Bmim][BF4 ] + 1 M of GdnHCl, the Mb completely unfolds. Thus, if compared, a combination of imidazolium cation with [CH3 COO] anion in an IL is more biocompatible and giving protection against GdnHCl denaturation action on Mb native structure than those in ILs containing [BF4 ] anion. Moreover, it is interesting to note that some of the imidazolium-based ILs could be used as anticancer solvents [142].

IL such as triethylammonium acetate (TEAA) was found to be an efficient refolding additive for a thermally unfolded CT structure [143]. The refolding ability of TEAA against thermally denatured CT structure was monitored using 1 H NMR. In addition, the fluorescence, CD and NMR results confirmed that TEAA strongly counteracted the deleterious actions of wellknown denaturant, urea, on the CT structure [144]. It was demonstrated that TEAA and urea mixture substantially increased the Tm values which showed the counterbalance of the ureainduced denaturation of CT. Most importantly, deleterious action 5 M urea on CT was counteracted by only 1 M TEAA.

Apparently, Lange et al. [73] explicitly elucidated that the set of imidazolium-based ILs such

refolding agents. The stability of lysozyme in these ILs was observed to decrease significantly as the alkyl chain of the ILs increased from ethyl, butyl to hexyl. Nonetheless, all ILs acted as refolding enhancers for the completely denatured lysozyme structure. A later study found that a series of the ILs such as [Mmim][Cl], [Emim][Cl], [Pmim][Cl], [Bmim][Cl], [Penmim] [Cl], [Hexmim][Cl], [Hepmim][Cl], [Omim][Cl], [Ddmim][Cl], [i-Bmim][Cl] and [Bemim][[Cl] were applied to the denatured lysozyme structure and a significant refolding of the protein was observed. Among the ILs, in the presence of [Bmim][Cl], the refolding yield reached up to maximum of 84% [137]. The less hydrophobic ILs such as N-alkylpyridinium chlorides [EtPy][Cl], [BPy][Cl] and [HexPy][Cl] were effective in enhancing the refolding agents for the lysozyme structure and yielded up to 46–69% refolding [76]. As a consequence, the results conclude that more hydrophobic ILs behaved as a denaturants for lysozyme while the same ILs acted as refolding agents for the denatured lysozyme structure. In support to the abovementioned facts, Takekiyo et al. [72] observed that the secondary structure of lysozyme was refolded in the 6–10 M concentration range of imidazolium-based IL, whereas the tertiary structure breaks down. Upon increase in the concentration more than 10 M of [Bmim][NO3

the secondary structure of the protein was still observed to be in a partially refolded state,

Very recently, at low concentration of [Bmim][Cl], the cyt c starts to unfold and again starts refolding with increasing concentration of the IL [138]. These results suggest a partial refolding of the secondary structure of cyt c in [Bmim][Cl]. The [Emim][Cl] was a very efficient in promoting refolding of the recombinant plasminogen activator (rPA) [139]. The delicate balance of favorable interactions with side chains and unfavorable interactions with the peptide backbone provides a molecular explanation of how this IL suppresses protein aggregation and simultaneously promotes refolding. Nonetheless, the protein denatured at high concentrations of [Emim][Cl] which indicates strong favorable interactions between AA side chains

The effect of EAN on renaturation of cyt c has been shown by Jaganathan et al. [140] and the results show that EAN in the range of 10−4 M helps in refolding of the protein from urea (8 M)-induced denaturation of cyt c. On the other hand, it was observed that at moderate con-

Further, there was minimal variation in the structure of Mb, when the mixture of 1.4 M GdnHCl

in an IL is more biocompatible and giving protection against GdnHCl denaturation action on

IL such as triethylammonium acetate (TEAA) was found to be an efficient refolding additive for a thermally unfolded CT structure [143]. The refolding ability of TEAA against thermally

pletely unfolds. Thus, if compared, a combination of imidazolium cation with [CH3

that some of the imidazolium-based ILs could be used as anticancer solvents [142].

COO] did not induce any significant effect over Mb struc-

1 M of GdnHCl, the Mb com-

] anion. Moreover, it is interesting to note

H NMR. In addition, the fluorescence, CD and

COO] anion

], at the same concentrations significantly destabilized the Mb [141].

COO] IL was used as a cosolvent. That is, the protein is not com-

] +

cations with a fixed anion Cl−

can also be considered as

],

as [Emim]+

, [Bmim]+

84 Progress and Developments in Ionic Liquids

and ions of the IL [139].

ture, however, [Bmim][BF4

and 150 mM of [Emim][CH3

centrations (50–150 mM) [Emim][CH3

pletely unfolded. While in the presence of 150 mM [Bmim][BF4

Mb native structure than those in ILs containing [BF4

denatured CT structure was monitored using 1

and [Hexmim]+

while the tertiary structure was completely disrupted.

It has been already noted that a protic IL triethylammonium phosphate (TEAP) acted as a refolding additive for the urea-induced chemical denaturated state of the two enzymes, CT and succinylated Concanavalin A (S Con A) [22]. In one of the studies by Attri and Venkatesu, TEAP was shown to be acting as an efficient refolding agent for thermally denatured S Con A [145]. In 2013, Attri and Choi [146] showed that TEAP strongly attenuated the detrimental action of atmospheric pressure plasma jet (APPJ) on CT. This ammonium-based IL TEAP is able to maintain the structural integrity as well as activity of CT even after the exposure of APPJ [147]. The results show that one can use both enzyme and plasma simultaneously without affecting the enzyme structure and activity on the material surface, which can prove to be applicable in various fields.

Recent studies on ammonium-based ILs offer some valuable information to prevent the selfaggregation of the proteins. In this regard, Awanish and Venkatesu [148] for the first time showed ammonium-based ILs as a novel solvent for offsetting self-aggregation of insulin in the presence of TMAS, TEAS, TMAP, TEAP and TMAA. Therefore, the native structure of insulin was found to be stabilized in the presence of ammonium-ILs by unfavorable interactions with the surface of protein. The stability studies of insulin in ILs have opened a new way that can lead us to overcome the aggregation properties of insulin. This will not only increase the shelf life of insulin, whereas suitable formulations of insulin in biocompatible ILs can lead to safe and durable insulin formulations in pharmaceutical products. The ammonium-based ILs such as ethylammonium mesylate (EaM), diethylammonium mesylate (DeaM)-stabilized *tobacco mosaic virus*, whereas triethylammonium mesylate (TeaM) caused a change in the secondary structure of the virus [149].

From the literature and from our own experience, it can be suggested that ammonium-based ILs are more biocompatible as compared to the imidazolium-based ILs [44, 140–155]. Yu et al. [150] explored the stability of laccase in the presence of both ammonium- and imidazolium-based ILs such as [TMA][TfO], [Bmim][TfO] and [Bmpyr][TfO]. They found that only ammonium-based ILs [TMA][TfO] stabilized laccase, while [Bmim][TfO] and [Bmpyr] [TfO] destabilized it. The contrasting nature of ammonium family ILs is also consistent with Rodrigues et al. [151], where among different families of ILs only ammonium-containing IL shows higher activity as compared to imidazolium-based ILs for *Thermomyces lanuginosus* lipase (TlL). **Scheme 2** shows the difference between the biocompability behaviors of ammonium- and imidazolium-based ILs.

On the other hand, Jha et al. [152] have explored the influence of a of imidazolium-based IL, 1-allyl-3-methylimidazolium chloride ([Amim][Cl]), on the stability of Hb. Unprecedented improvement in the stability of Hb in the presence of [Amim][Cl] at the lower concentration of [Amim][Cl] was observed by the authors [153]. Furthermore, the effect of [Amim][Cl] on bromelain stability and activity was investigated in another work. They observed that at low concentrations (0.01−0.10 M) of [Amim][Cl], there is ostensible only change in the stability and activity of BM.

**Scheme 3.** Schematic representation of two-state unfolding transitions in a protein with temperature (Ref. [157]).

The Role of Ionic Liquids in Protein Folding/Unfolding Studies

http://dx.doi.org/10.5772/65924

87

**Scheme 4.** The ability of the ILs to offset the cold-induced unfolding of proteins.

**Scheme 2.** The biocompatible behavior of ammonium-based ILs as compared to imidazolium-based ILs for proteins (Ref. [44]).

However, we cannot overlook the wide applications and uses of imidazolium-based ILs in various fields. Therefore, it is very important to emphasize the role of these ILs in biomedical applications, for example, counteraction of cold-induced unfolding of Mb and CT structures.

Cold denaturation is a fundamental fact in aqueous solutions where the native structure of globular protein disorders on extreme cooling [156]. Unlike thermal denaturation, whereby a native protein is disrupted at high temperature, cold denaturation is accompanied by decreases in both the system entropy and enthalpy [150]. As shown in **Scheme 3**, very recently we experimentally observed in one of our recent studies that the cold-induced unfolding of Mb and CT approaches closely a two-state folding mechanism similar to that experienced in the thermal denaturation of proteins [157].

Interestingly, for the first time, ILs having CH<sup>3</sup> COO<sup>−</sup> or Br<sup>−</sup> with [Bmim]+ proved to counteract the cold-induced unfolding of Mb and CT structures. Nevertheless, ILs containing Cl<sup>−</sup> , HSO4 − and SCN<sup>−</sup> with [Bmim]+ failed to prevent the Mb and CT structures against cold denaturation. These findings are concluded through **Scheme 4**.

**Scheme 3.** Schematic representation of two-state unfolding transitions in a protein with temperature (Ref. [157]).

**Scheme 4.** The ability of the ILs to offset the cold-induced unfolding of proteins.

However, we cannot overlook the wide applications and uses of imidazolium-based ILs in various fields. Therefore, it is very important to emphasize the role of these ILs in biomedical applications, for example, counteraction of cold-induced unfolding of Mb and CT

**Scheme 2.** The biocompatible behavior of ammonium-based ILs as compared to imidazolium-based ILs for proteins

Cold denaturation is a fundamental fact in aqueous solutions where the native structure of globular protein disorders on extreme cooling [156]. Unlike thermal denaturation, whereby a native protein is disrupted at high temperature, cold denaturation is accompanied by decreases in both the system entropy and enthalpy [150]. As shown in **Scheme 3**, very recently we experimentally observed in one of our recent studies that the cold-induced unfolding of Mb and CT approaches closely a two-state folding mechanism similar to that experienced in

COO<sup>−</sup>

the cold-induced unfolding of Mb and CT structures. Nevertheless, ILs containing Cl<sup>−</sup>

or Br<sup>−</sup>

failed to prevent the Mb and CT structures against cold denaturation.

with [Bmim]+

proved to counteract

, HSO4 −

structures.

(Ref. [44]).

86 Progress and Developments in Ionic Liquids

and SCN<sup>−</sup>

the thermal denaturation of proteins [157].

with [Bmim]+

Interestingly, for the first time, ILs having CH<sup>3</sup>

These findings are concluded through **Scheme 4**.

An advantage of studying cold denaturation of globular proteins in ILs will certainly help us to encounter the reversible unfolding of proteins. Apparently, both cold and heat effects can lead to the unfolding of the proteins, which causes several human diseases. Evidently, the novel character of ILs offsets both the deleterious actions on protein keeping it in a proper folded conformation. It is hoped that the results obtained from these studies will be useful in recommending tailor-made ILs for various applications in biological systems as well as novel pharmaceutical applications.

**Author details**

Awanish Kumar1

**References**

Cambridge, UK; 2010.

, Meena Bisht2

\*Address all correspondence to: venkatesup@hotmail.com

2 Department of Chemistry, University of Delhi, Delhi, India

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\*

The Role of Ionic Liquids in Protein Folding/Unfolding Studies

http://dx.doi.org/10.5772/65924

89

Obviously, full access to the cold- and thermal-induced unfolding of proteins in ILs is still lacking. The interaction of the ions with protein surface is a complex result of the ability of the ions to enhance and disrupt water structure and internal protein residue interactions that contribute to the overall protein stability. Therefore, it is not necessary that only the IL is responsible for stabilization or destabilization of a protein under varying conditions, rather the solvent environment is equally responsible which creates variations in the interactions of the ILs with the protein surface. Apparently, ILs may stabilize or destabilize the proteins which are solely dependent on the molecular environment in the protein's surroundings. However, based on the literature survey and our experience, these studies might be inapplicable to all proteins in general (other than investigated proteins). Therefore, a lot of studies in this regard are essentially required to build a universal conclusion on ILs to protect the protein against various external stresses.

#### **8. Conclusions**

The extents of stabilization ability of ILs on the proteins vary and usually depend upon the combination of ions (both cation and anion). The activity and stability of protein in ILs that depend not only on the nature of the ions of IL but also on the functional groups of AAs sequences arrangement of the protein. Therefore, interactions of ions of ILs with proteins are important for understanding the effects shown by them on proteins whether in stabilization or destabilization. The results based on experiments revealed that the concentration of the ILs can play a major role in stabilizing/destabilizing a particular protein and also the alkyl chain length of the cation. Some of the novel characters in protein stability by ILs have been highlighted in this chapter. Prevention of self-aggregation and counteraction against extreme heat, cold and chemicals by ILs have been systematically presented. Cold counteraction is a new approach to stabilize proteins in ILs. This will help in increasing the stability of protein-based pharmaceutical products, which sometimes become inactive due to cold-induced denaturation of the proteins when stored at low temperature. Similarly, self-aggregation in proteins is also an issue which we believe can be controlled using biocompatible ILs. However, a very little amount of literature is available in this field of research.

### **Acknowledgments**

We gratefully acknowledge Physical Chemistry Chemical Physics and Royal Society of Chemistry for reusing our figures such as **Figure 1** and **Schemes 1**–**3**.

### **Author details**

An advantage of studying cold denaturation of globular proteins in ILs will certainly help us to encounter the reversible unfolding of proteins. Apparently, both cold and heat effects can lead to the unfolding of the proteins, which causes several human diseases. Evidently, the novel character of ILs offsets both the deleterious actions on protein keeping it in a proper folded conformation. It is hoped that the results obtained from these studies will be useful in recommending tailor-made ILs for various applications in biological systems as well as novel

Obviously, full access to the cold- and thermal-induced unfolding of proteins in ILs is still lacking. The interaction of the ions with protein surface is a complex result of the ability of the ions to enhance and disrupt water structure and internal protein residue interactions that contribute to the overall protein stability. Therefore, it is not necessary that only the IL is responsible for stabilization or destabilization of a protein under varying conditions, rather the solvent environment is equally responsible which creates variations in the interactions of the ILs with the protein surface. Apparently, ILs may stabilize or destabilize the proteins which are solely dependent on the molecular environment in the protein's surroundings. However, based on the literature survey and our experience, these studies might be inapplicable to all proteins in general (other than investigated proteins). Therefore, a lot of studies in this regard are essentially required to build a universal conclusion on ILs to protect the protein against various external stresses.

The extents of stabilization ability of ILs on the proteins vary and usually depend upon the combination of ions (both cation and anion). The activity and stability of protein in ILs that depend not only on the nature of the ions of IL but also on the functional groups of AAs sequences arrangement of the protein. Therefore, interactions of ions of ILs with proteins are important for understanding the effects shown by them on proteins whether in stabilization or destabilization. The results based on experiments revealed that the concentration of the ILs can play a major role in stabilizing/destabilizing a particular protein and also the alkyl chain length of the cation. Some of the novel characters in protein stability by ILs have been highlighted in this chapter. Prevention of self-aggregation and counteraction against extreme heat, cold and chemicals by ILs have been systematically presented. Cold counteraction is a new approach to stabilize proteins in ILs. This will help in increasing the stability of protein-based pharmaceutical products, which sometimes become inactive due to cold-induced denaturation of the proteins when stored at low temperature. Similarly, self-aggregation in proteins is also an issue which we believe can be controlled using biocompatible ILs. However, a very

We gratefully acknowledge Physical Chemistry Chemical Physics and Royal Society of

little amount of literature is available in this field of research.

Chemistry for reusing our figures such as **Figure 1** and **Schemes 1**–**3**.

pharmaceutical applications.

88 Progress and Developments in Ionic Liquids

**8. Conclusions**

**Acknowledgments**

Awanish Kumar1 , Meena Bisht2 , Indrani Jha2 and Pannuru Venkatesu2 \*

\*Address all correspondence to: venkatesup@hotmail.com

1 Department of Chemistry, Massachusetts Institute of Technology, Cambridge, MA, USA

2 Department of Chemistry, University of Delhi, Delhi, India

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**Chapter 5**

**Provisional chapter**

**Ionic Liquid-Induced Unique Structural Transitions**

The structural transitions of proteins in aqueous solutions of various ionic liquids (ILs) over a wide concentration range (*x* (mol% IL) = 0–30) were investigated using Fouriertransform infrared and near-UV circular dichroism spectroscopy combined with small-angle X-ray scattering. The proteins in the aqueous IL solutions showed two structural transition patterns: (i) the folded state → unfolded state → partial globular state (α-helical formation disrupted tertiary structure) and (ii) the folded state → unfolded state → aggregation (amyloid-like aggregation or disordered aggregation). We found that the helical formation of proteins in the condensed IL solutions was strongly related to the competition between the low polarity and denaturation effect of ions. Moreover, the amyloid-like aggregate formation correlated with the competition between the size of the confined water assemblies in the IL layer and the IL-amino acid residue interactions. On the basis of these results, we discussed the future applications of ILs, including their use as cryoprotectants for proteins and as agents for the suppression of amyloid

**Keywords:** protein, aqueous ionic liquid solution, aggregation, helix formation,

**Ionic Liquid-Induced Unique Structural Transitions** 

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution,

© 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

and reproduction in any medium, provided the original work is properly cited.

Aqueous mixtures of proteins and ionic liquids (ILs), which comprise organic cations and anions and remain in the liquid state below 373 K, are employed in protein engineering applications, such as protein storage media, biocatalysts, and buffers [1–3]. Although these applications are based on the unique solvent properties of these mixtures, such as their solubility in water and solution structure [2, 4], the detailed relationship between the proteins and aqueous IL solutions at the molecular level is unclear. Thus, to realize the protein engineering

Takahiro Takekiyo and Yukihiro Yoshimura

Additional information is available at the end of the chapter

Takahiro Takekiyo and Yukihiro Yoshimura

Additional information is available at the end of the chapter

**of Proteins**

**of Proteins**

http://dx.doi.org/10.5772/65886

**Abstract**

formation.

**1. Introduction**

optical spectroscopy

**Provisional chapter**

### **Ionic Liquid-Induced Unique Structural Transitions of Proteins Ionic Liquid-Induced Unique Structural Transitions of Proteins**

Takahiro Takekiyo and Yukihiro Yoshimura Takahiro Takekiyo and Yukihiro Yoshimura Additional information is available at the end of the chapter

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/65886

#### **Abstract**

The structural transitions of proteins in aqueous solutions of various ionic liquids (ILs) over a wide concentration range (*x* (mol% IL) = 0–30) were investigated using Fouriertransform infrared and near-UV circular dichroism spectroscopy combined with small-angle X-ray scattering. The proteins in the aqueous IL solutions showed two structural transition patterns: (i) the folded state → unfolded state → partial globular state (α-helical formation disrupted tertiary structure) and (ii) the folded state → unfolded state → aggregation (amyloid-like aggregation or disordered aggregation). We found that the helical formation of proteins in the condensed IL solutions was strongly related to the competition between the low polarity and denaturation effect of ions. Moreover, the amyloid-like aggregate formation correlated with the competition between the size of the confined water assemblies in the IL layer and the IL-amino acid residue interactions. On the basis of these results, we discussed the future applications of ILs, including their use as cryoprotectants for proteins and as agents for the suppression of amyloid formation.

**Keywords:** protein, aqueous ionic liquid solution, aggregation, helix formation, optical spectroscopy

### **1. Introduction**

Aqueous mixtures of proteins and ionic liquids (ILs), which comprise organic cations and anions and remain in the liquid state below 373 K, are employed in protein engineering applications, such as protein storage media, biocatalysts, and buffers [1–3]. Although these applications are based on the unique solvent properties of these mixtures, such as their solubility in water and solution structure [2, 4], the detailed relationship between the proteins and aqueous IL solutions at the molecular level is unclear. Thus, to realize the protein engineering

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

applications of ILs, numerous studies have been conducted on the structural stability and activity of proteins in aqueous IL solutions [2–8].

**2. Experimental methodology**

chloride ([bmim][Cl]) (Kanto Chemical Co.), [bmim][NO<sup>3</sup>

were prepared by mixing the required amount of the IL and D2

analyses were performed using the GRAMS software (Galactic Software).

1-ethyl-3-methylimidazolium nitrate ([emim][NO<sup>3</sup>

were subtracted from the protein solution spectra.

Chicken lysozyme, bovine ribonuclease A (RNase A) and bovine β-lactoglobulin (β-LG), horse cytochrome *c*, bovine myoglobin, bovine rhodanese, and bovine insulin were purchased from Sigma and were used without further purification. The ILs 1-butyl-3-methylimidazolium

(MAN) (Iolitec), ethylammonium nitrate (EAN) (Iolitec), and propylammonium nitrate (PAN) (Iolitec) were used in this study. All aqueous mixtures with concentrations of *x* (mol% IL)

room temperature. The prepared concentrations of the aqueous IL solutions were *x* = 0–30 because of the overlap of the cations with proteins in the Fourier-transform infrared (FTIR) spectra. The concentrations of the proteins were adjusted to 20 mg mL−1, which does not result in protein aggregation in water (*x* = 0). Samples were loaded into a transmission cell with CaF2 windows and a Teflon spacer (50 μm) for FTIR and near-UV circular dichroism (CD) spectral measurement under the same protein conditions as used in the condensed ILs. These spectral

The amide I′ vibrational mode (deuterated peptide groups) in FTIR spectra is highly sensitive to the secondary structure of proteins, and thus it serves as an indicator of α-helix and β-sheet formation [20]. FTIR spectra were recorded using a Nicolet 6700 FTIR spectrometer equipped with a mercury-cadmium-telluride liquid-nitrogen detector. Typically, 512 interferograms were collected to obtain spectra with a resolution of 4 cm−1. Solvent spectra were also measured under the same conditions as those used for the protein solution measurements and

Near-UV CD spectra in the range of 250–300 nm are sensitive to the presence of specific rigid packing interactions between aromatic side chains, indicating changes in the tertiary structure [21]. CD spectra were measured over a wavelength range of 250–300 nm on a JASCO J-820 spectropolarimeter. Typically, spectra were accumulated at a scan rate of 20 nm min−1 in 0.1 nm steps. Five scans were averaged for each spectrum. The obtained spectra were converted into mean residue ellipticity units using [*θ*] = *θ*obs/(10*ncl*), where *θ*obs is the observed ellipticity, *l* is the path length, *c* is the protein concentration, and *n* is the

SAXS is a powerful technique for investigating protein size and the presence of protein aggregation [22]. SAXS experiments were conducted using a Kratky camera system (BioSAXS-1000, Rigaku Co.) at a brilliance of 56.0 kW mm−2. CuKα radiation (*λ* = 0.1542 nm) was selected and

] (Sigma), [bmim][SCN] (Sigma),

O (Kanto Chemical Co.) at

http://dx.doi.org/10.5772/65886

99

]) (Iolitec), methylammonium nitrate

Ionic Liquid-Induced Unique Structural Transitions of Proteins

**2.1. Materials**

**2.2. FTIR spectroscopy**

**2.3. CD spectroscopy**

number of residues.

**2.4. SAXS**

For instance, Lange et al. [6] demonstrated that the addition of imidazolium-based ILs (up to 4 M) to renaturation buffers caused high protein renaturation without protein aggregation, whereas addition to the folded protein induced a decrease in the structural stability. Many aqueous IL solutions are found to degrade the structural stability and activity of proteins at *x* (mol% IL) < 6 [2, 3, 7, 8]. This decrease in protein stability in dilute aqueous IL solutions can be explained by the Hofmeister series [7, 8]. This series ranks the relative influence of ions on the physical behavior of a wide variety of aqueous processes ranging from colloidal assembly to protein folding. Originally, it was assumed that the influence of ions on protein folding was caused at least in part by "making" and "breaking" bulk water structures [9]. However, these investigations of protein stability in dilute aqueous IL solutions have not sufficiently considered essential properties of ILs. Thus, it is necessary to obtain basic information on protein stability over a wide IL concentration range to realize protein engineering using ILs.

Recently, intriguing phenomena such as protein refolding and aggregate formation have been observed in condensed IL solutions (*x* > 10) or pure IL. Imidazolium-based or alkylammoniumbased ILs at a concentration of *x* > 10 induce the formation of α-helical structures of protein, such as human interleukin-2 [10] and succinylated concanavalin A [11]. Moreover, Hwang et al. [12] showed that imidazolium- and pyridinium-based ILs promoted amyloid formation in α-synuclein and α-lactalbumin. Similarly, Debeljuh et al. [13] demonstrated that addition of protic ILs, such as triethylammonium-based ILs, to Aβ1–40 peptide promotes amyloid aggregation. These intriguing protein-refolding/amyloid-formation phenomena in condensed ILs or pure ILs may be related to the essential properties of ILs and cannot be explained by Hofmeiester series, as in the case of protein unfolding in dilute aqueous IL solutions.

Related to these phenomena, the solvent properties of ILs, such as viscosity and dielectric constant, drastically change at a certain IL concentration [2]. These changes depend on the amount of water in the mixture. In addition, it is known that IL solutions adopt a nanoheterogeneous structure with a polar domain, i.e., the ionic parts of the cations and anions, and a nonpolar domain, i.e., the alkyl chain of the cations [14–16]. In binary solutions under waterrich conditions [17], IL-water mixtures adopt IL-aggregated structures that are surrounded by bulk water molecules; therefore, the nanoheterogeneity of these systems is relatively low. However, under IL-rich conditions wherein the mixtures exhibit molten-salt-like behavior, the water molecules are scattered in the polar domain and self-assemble in the ILs. The water molecules in this state are termed "confined water" [18, 19] and the nanoheterogeneity of these systems is higher. As mentioned earlier, these solvent properties may contribute to unique structural transitions of the proteins. Thus, detailed information on protein stability over a wide range of IL concentrations is valuable, and this information would facilitate the use of ILs in protein engineering applications.

This manuscript aims to determine the structural stabilities of various model proteins over a wide concentration range of ILs using optical spectroscopy combined with small-angle X-ray scattering (SAXS). The origin of the structural transitions of proteins in condensed aqueous IL solutions has been discussed.

### **2. Experimental methodology**

#### **2.1. Materials**

applications of ILs, numerous studies have been conducted on the structural stability and

For instance, Lange et al. [6] demonstrated that the addition of imidazolium-based ILs (up to 4 M) to renaturation buffers caused high protein renaturation without protein aggregation, whereas addition to the folded protein induced a decrease in the structural stability. Many aqueous IL solutions are found to degrade the structural stability and activity of proteins at *x* (mol% IL) < 6 [2, 3, 7, 8]. This decrease in protein stability in dilute aqueous IL solutions can be explained by the Hofmeister series [7, 8]. This series ranks the relative influence of ions on the physical behavior of a wide variety of aqueous processes ranging from colloidal assembly to protein folding. Originally, it was assumed that the influence of ions on protein folding was caused at least in part by "making" and "breaking" bulk water structures [9]. However, these investigations of protein stability in dilute aqueous IL solutions have not sufficiently considered essential properties of ILs. Thus, it is necessary to obtain basic information on protein stability over a wide IL concentration range to realize protein engineering using ILs. Recently, intriguing phenomena such as protein refolding and aggregate formation have been observed in condensed IL solutions (*x* > 10) or pure IL. Imidazolium-based or alkylammoniumbased ILs at a concentration of *x* > 10 induce the formation of α-helical structures of protein, such as human interleukin-2 [10] and succinylated concanavalin A [11]. Moreover, Hwang et al. [12] showed that imidazolium- and pyridinium-based ILs promoted amyloid formation in α-synuclein and α-lactalbumin. Similarly, Debeljuh et al. [13] demonstrated that addition of protic ILs, such as triethylammonium-based ILs, to Aβ1–40 peptide promotes amyloid aggregation. These intriguing protein-refolding/amyloid-formation phenomena in condensed ILs or pure ILs may be related to the essential properties of ILs and cannot be explained by

Hofmeiester series, as in the case of protein unfolding in dilute aqueous IL solutions.

Related to these phenomena, the solvent properties of ILs, such as viscosity and dielectric constant, drastically change at a certain IL concentration [2]. These changes depend on the amount of water in the mixture. In addition, it is known that IL solutions adopt a nanoheterogeneous structure with a polar domain, i.e., the ionic parts of the cations and anions, and a nonpolar domain, i.e., the alkyl chain of the cations [14–16]. In binary solutions under waterrich conditions [17], IL-water mixtures adopt IL-aggregated structures that are surrounded by bulk water molecules; therefore, the nanoheterogeneity of these systems is relatively low. However, under IL-rich conditions wherein the mixtures exhibit molten-salt-like behavior, the water molecules are scattered in the polar domain and self-assemble in the ILs. The water molecules in this state are termed "confined water" [18, 19] and the nanoheterogeneity of these systems is higher. As mentioned earlier, these solvent properties may contribute to unique structural transitions of the proteins. Thus, detailed information on protein stability over a wide range of IL concentrations is valuable, and this information would facilitate the

This manuscript aims to determine the structural stabilities of various model proteins over a wide concentration range of ILs using optical spectroscopy combined with small-angle X-ray scattering (SAXS). The origin of the structural transitions of proteins in condensed aqueous IL

activity of proteins in aqueous IL solutions [2–8].

98 Progress and Developments in Ionic Liquids

use of ILs in protein engineering applications.

solutions has been discussed.

Chicken lysozyme, bovine ribonuclease A (RNase A) and bovine β-lactoglobulin (β-LG), horse cytochrome *c*, bovine myoglobin, bovine rhodanese, and bovine insulin were purchased from Sigma and were used without further purification. The ILs 1-butyl-3-methylimidazolium chloride ([bmim][Cl]) (Kanto Chemical Co.), [bmim][NO<sup>3</sup> ] (Sigma), [bmim][SCN] (Sigma), 1-ethyl-3-methylimidazolium nitrate ([emim][NO<sup>3</sup> ]) (Iolitec), methylammonium nitrate (MAN) (Iolitec), ethylammonium nitrate (EAN) (Iolitec), and propylammonium nitrate (PAN) (Iolitec) were used in this study. All aqueous mixtures with concentrations of *x* (mol% IL) were prepared by mixing the required amount of the IL and D2 O (Kanto Chemical Co.) at room temperature. The prepared concentrations of the aqueous IL solutions were *x* = 0–30 because of the overlap of the cations with proteins in the Fourier-transform infrared (FTIR) spectra. The concentrations of the proteins were adjusted to 20 mg mL−1, which does not result in protein aggregation in water (*x* = 0). Samples were loaded into a transmission cell with CaF2 windows and a Teflon spacer (50 μm) for FTIR and near-UV circular dichroism (CD) spectral measurement under the same protein conditions as used in the condensed ILs. These spectral analyses were performed using the GRAMS software (Galactic Software).

#### **2.2. FTIR spectroscopy**

The amide I′ vibrational mode (deuterated peptide groups) in FTIR spectra is highly sensitive to the secondary structure of proteins, and thus it serves as an indicator of α-helix and β-sheet formation [20]. FTIR spectra were recorded using a Nicolet 6700 FTIR spectrometer equipped with a mercury-cadmium-telluride liquid-nitrogen detector. Typically, 512 interferograms were collected to obtain spectra with a resolution of 4 cm−1. Solvent spectra were also measured under the same conditions as those used for the protein solution measurements and were subtracted from the protein solution spectra.

#### **2.3. CD spectroscopy**

Near-UV CD spectra in the range of 250–300 nm are sensitive to the presence of specific rigid packing interactions between aromatic side chains, indicating changes in the tertiary structure [21]. CD spectra were measured over a wavelength range of 250–300 nm on a JASCO J-820 spectropolarimeter. Typically, spectra were accumulated at a scan rate of 20 nm min−1 in 0.1 nm steps. Five scans were averaged for each spectrum. The obtained spectra were converted into mean residue ellipticity units using [*θ*] = *θ*obs/(10*ncl*), where *θ*obs is the observed ellipticity, *l* is the path length, *c* is the protein concentration, and *n* is the number of residues.

#### **2.4. SAXS**

SAXS is a powerful technique for investigating protein size and the presence of protein aggregation [22]. SAXS experiments were conducted using a Kratky camera system (BioSAXS-1000, Rigaku Co.) at a brilliance of 56.0 kW mm−2. CuKα radiation (*λ* = 0.1542 nm) was selected and collimated using a parabolic multilayer mirror. The beam was focused by a converting optical tool (CBO-*f*, Rigaku Co.). The beam size was 0.5 mm (V) × 0.1 mm (H) at the sample position, and the camera distance was 500 mm. The combination of the 2D Kratky block and focusing optics can achieve a wide *q* range. Here the scattering vector *q* is defined as 4*π*sin*θ*/*λ* (nm−1). A 2D detector (PILATUS 100 K/R) was used. Samples were put into quartz capillaries with a diameter of 1.0 mm and a thickness of 0.1 mm. The scattering of the aqueous IL solutions was subtracted, and the final scattering curve was obtained using the program *PRIMUS*.

Guinier plots is observed. In aqueous [bmim][NO<sup>3</sup>

does not completely unfold without aggregation.

= 13.8 Å) and unfolded state (*R*<sup>g</sup>

were obtained in the same manner. The *R*<sup>g</sup>

for *x* = 5, and 15.0 Å for *x* = 20. The *R*<sup>g</sup>

the folded state (*R*<sup>g</sup>

speculated above, the *R*<sup>g</sup>

The addition of [bmim][NO<sup>3</sup>

*c* (Abs1655cm−1) as a function of [bmim][NO<sup>3</sup>

at several [bmim][NO<sup>3</sup>

*c* in aqueous [bmim][NO<sup>3</sup>

[NO<sup>3</sup>

] solutions, the *R*<sup>g</sup>

value of cytochrome *c* at *x* = 20 is larger than that at *x* = 0 and smaller

than that at *x* = 5. Thus, cytochrome *c* at *x* = 20 takes a more compact structure than at *x* = 5 and

A similar result is also obtained using Kratky plots (**Figure 2b**). Kratky plots provide insight into the compactness of a protein, i.e., a bell shape in the plot indicates a globular protein, whereas a plateau, seen in the high *q* region, suggests that the protein is unfolded.

**Figure 1.** FTIR spectra in the amide I' region of (a) myoglobin and (b) cytochrome *c* in aqueous [bmim][NO<sup>3</sup>

] concentrations, is not completely unfolded. Combination with FTIR and SAXS

cating that the size of cytochrome *c* increases with increasing [bmim][NO<sup>3</sup>

] solutions at *x* = 0, 5, and 10, respectively.

This implies that cytochrome *c* in aqueous [bmim][NO<sup>3</sup>

] shifts the peak of the bell shape to a smaller *q* region, indi-

] concentration. Inset figure shows the second derivative spectra of cytochrome

] concentrations. Changes in absorbance of (c) myoglobin (Abs1651cm−1) and (d) cytochrome

] concentration.

] solutions

] solutions, even at high [bmim]

values of cytochrome *c* are 13.4 Å for *x* = 0, 25.0 Å

Ionic Liquid-Induced Unique Structural Transitions of Proteins

= 24.0 Å) reported by Cinelli et al. [24]. As

values at *x* = 0 and 5 are in good agreement with those of

values of cytochrome *c*

http://dx.doi.org/10.5772/65886

101

### **3. Results and discussion**

#### **3.1. Structural transition of proteins in aqueous solutions with [bmim]-based ILs**

As representative results, **Figure 1a** and **b** shows the FTIR amide I' spectra of myoglobin and cytochrome *c* in aqueous solutions under concentrations of [bmim][NO<sup>3</sup> ] up to *x* = 30. The amide I' spectra of both proteins change significantly as a function of [bmim][NO<sup>3</sup> ] concentration. The peaks at ca. 1615 and ca. 1690 cm−1, which are due to the intermolecular β-sheet structure and indicate myoglobin aggregation [23], are observed above *x* = 5. However, no such peaks appear for cytochrome *c* over the studied [bmim][NO<sup>3</sup> ] concentrations.

To further investigate the changes in the secondary structure of both proteins, we plotted the changes in the maximum absorbance (Abs) values of these two proteins against the [bmim] [NO<sup>3</sup> ] concentrations, as shown in **Figure 1c** and **d**. For myoglobin, the first decrease in the Abs value, indicating myoglobin unfolding, is observed in the region *x* = 1–5. The second decrease in the Abs value, indicating the formation of an intermolecular β-sheet structure, is observed at *x* > 7. Thus, the addition of [bmim][NO<sup>3</sup> ] to myoglobin causes the fold → unfold → intermolecular β-sheet transition.

Conversely, a drastic decrease in Abs for cytochrome *c* is observed up to *x* = 7, and it is noteworthy that the Abs value increases at *x* > 7. From the second-derivative analysis of the FTIR spectra, the peak at ca. 1645 cm−1 indicates an increase in the disordered structure up to *x* = 7, and further addition causes the increase in the peak at ca. 1655 cm−1, which indicates an α-helical structure (inset in **Figure 1d**). Based on these FTIR spectra, the increase in Abs at *x* > 7 is due to the partial refolding of the secondary structure of cytochrome *c*. However, the partial refolding at *x* > 7 is not sufficiently confirmed by only the FTIR result. If the partial refolding of cytochrome *c* occurs at *x* > 7, the whole cytochrome *c* size would be smaller than in the unfolded state and larger than in the folded state. Therefore, to further investigate the state of cytochrome *c* in aqueous [bmim][NO<sup>3</sup> ] solutions, we performed SAXS measurements.

**Figure 2a** shows a Guinier plot of cytochrome *c* in aqueous [bmim][NO<sup>3</sup> ] solutions where the radius of gyration (*R*<sup>g</sup> ) was estimated. The Guinier equation is defined as follows:

$$I(q) = I(0) \exp(-R\_{\ll}{}^2 q^2) \tag{1}$$

where *I*(0) is the intensity at *q* = 0. The Guinier equation is valid in the range of *R*<sup>g</sup> *q* < 1. In this study, despite the fact that the range 0 < *R*<sup>g</sup> *q* < 2 was employed, a linear relationship in the Guinier plots is observed. In aqueous [bmim][NO<sup>3</sup> ] solutions, the *R*<sup>g</sup> values of cytochrome *c* were obtained in the same manner. The *R*<sup>g</sup> values of cytochrome *c* are 13.4 Å for *x* = 0, 25.0 Å for *x* = 5, and 15.0 Å for *x* = 20. The *R*<sup>g</sup> values at *x* = 0 and 5 are in good agreement with those of the folded state (*R*<sup>g</sup> = 13.8 Å) and unfolded state (*R*<sup>g</sup> = 24.0 Å) reported by Cinelli et al. [24]. As speculated above, the *R*<sup>g</sup> value of cytochrome *c* at *x* = 20 is larger than that at *x* = 0 and smaller than that at *x* = 5. Thus, cytochrome *c* at *x* = 20 takes a more compact structure than at *x* = 5 and does not completely unfold without aggregation.

collimated using a parabolic multilayer mirror. The beam was focused by a converting optical tool (CBO-*f*, Rigaku Co.). The beam size was 0.5 mm (V) × 0.1 mm (H) at the sample position, and the camera distance was 500 mm. The combination of the 2D Kratky block and focusing optics can achieve a wide *q* range. Here the scattering vector *q* is defined as 4*π*sin*θ*/*λ* (nm−1). A 2D detector (PILATUS 100 K/R) was used. Samples were put into quartz capillaries with a diameter of 1.0 mm and a thickness of 0.1 mm. The scattering of the aqueous IL solutions was

subtracted, and the final scattering curve was obtained using the program *PRIMUS*.

**3.1. Structural transition of proteins in aqueous solutions with [bmim]-based ILs**

amide I' spectra of both proteins change significantly as a function of [bmim][NO<sup>3</sup>

cytochrome *c* in aqueous solutions under concentrations of [bmim][NO<sup>3</sup>

such peaks appear for cytochrome *c* over the studied [bmim][NO<sup>3</sup>

observed at *x* > 7. Thus, the addition of [bmim][NO<sup>3</sup>

→ intermolecular β-sheet transition.

*c* in aqueous [bmim][NO<sup>3</sup>

study, despite the fact that the range 0 < *R*<sup>g</sup>

radius of gyration (*R*<sup>g</sup>

As representative results, **Figure 1a** and **b** shows the FTIR amide I' spectra of myoglobin and

tration. The peaks at ca. 1615 and ca. 1690 cm−1, which are due to the intermolecular β-sheet structure and indicate myoglobin aggregation [23], are observed above *x* = 5. However, no

To further investigate the changes in the secondary structure of both proteins, we plotted the changes in the maximum absorbance (Abs) values of these two proteins against the [bmim]

Conversely, a drastic decrease in Abs for cytochrome *c* is observed up to *x* = 7, and it is noteworthy that the Abs value increases at *x* > 7. From the second-derivative analysis of the FTIR spectra, the peak at ca. 1645 cm−1 indicates an increase in the disordered structure up to *x* = 7, and further addition causes the increase in the peak at ca. 1655 cm−1, which indicates an α-helical structure (inset in **Figure 1d**). Based on these FTIR spectra, the increase in Abs at *x* > 7 is due to the partial refolding of the secondary structure of cytochrome *c*. However, the partial refolding at *x* > 7 is not sufficiently confirmed by only the FTIR result. If the partial refolding of cytochrome *c* occurs at *x* > 7, the whole cytochrome *c* size would be smaller than in the unfolded state and larger than in the folded state. Therefore, to further investigate the state of cytochrome

] solutions, we performed SAXS measurements.

) was estimated. The Guinier equation is defined as follows:

2 2 ( ) (0)exp( ) *<sup>g</sup> Iq I R q* = − (1)

*q* < 2 was employed, a linear relationship in the

**Figure 2a** shows a Guinier plot of cytochrome *c* in aqueous [bmim][NO<sup>3</sup>

where *I*(0) is the intensity at *q* = 0. The Guinier equation is valid in the range of *R*<sup>g</sup>

] concentrations, as shown in **Figure 1c** and **d**. For myoglobin, the first decrease in the Abs value, indicating myoglobin unfolding, is observed in the region *x* = 1–5. The second decrease in the Abs value, indicating the formation of an intermolecular β-sheet structure, is

] up to *x* = 30. The

] solutions where the

*q* < 1. In this

] concentrations.

] to myoglobin causes the fold → unfold

] concen-

**3. Results and discussion**

100 Progress and Developments in Ionic Liquids

[NO<sup>3</sup>

**Figure 1.** FTIR spectra in the amide I' region of (a) myoglobin and (b) cytochrome *c* in aqueous [bmim][NO<sup>3</sup> ] solutions at several [bmim][NO<sup>3</sup> ] concentrations. Changes in absorbance of (c) myoglobin (Abs1651cm−1) and (d) cytochrome *c* (Abs1655cm−1) as a function of [bmim][NO<sup>3</sup> ] concentration. Inset figure shows the second derivative spectra of cytochrome *c* in aqueous [bmim][NO<sup>3</sup> ] solutions at *x* = 0, 5, and 10, respectively.

A similar result is also obtained using Kratky plots (**Figure 2b**). Kratky plots provide insight into the compactness of a protein, i.e., a bell shape in the plot indicates a globular protein, whereas a plateau, seen in the high *q* region, suggests that the protein is unfolded. The addition of [bmim][NO<sup>3</sup> ] shifts the peak of the bell shape to a smaller *q* region, indicating that the size of cytochrome *c* increases with increasing [bmim][NO<sup>3</sup> ] concentration. This implies that cytochrome *c* in aqueous [bmim][NO<sup>3</sup> ] solutions, even at high [bmim] [NO<sup>3</sup> ] concentrations, is not completely unfolded. Combination with FTIR and SAXS results indicate that the aqueous [bmim][NO<sup>3</sup> ] solution at *x* = 20 causes cytochrome *c* to partially refold without aggregation.

**Figure 2.** (a) Guinier plots and (b) Kratky plots of cytochrome *c* in aqueous [bmim][NO<sup>3</sup> ] solutions at *x* = 0, 5, and 20.

Next, we measured the changes in the tertiary structure of both proteins induced by [bmim] [NO<sup>3</sup> ] using near-UV CD spectroscopy (**Figure 3a** and **b**). Although the negative CD intensity at 290 nm for myoglobin and 288 nm for cytochrome *c* due to the aromatic residues drastically decreases at [bmim][NO<sup>3</sup> ] concentrations of up to *x* = 7, no increase in the negative CD intensity occurs above *x* = 7 (**Figure 3c** and **d**). This indicates that an increase in the [bmim][NO<sup>3</sup> ] concentration completely disrupts the tertiary structure of both proteins.

The results from the FTIR, SAXS, and near-UV CD analyses show that aqueous [bmim][NO<sup>3</sup> ] solutions of up to *x* = 5–7 cause myoglobin and cytochrome *c* to unfold. Further addition of [bmim][NO<sup>3</sup> ] induces formation of the intermolecular β-sheet for myoglobin and the partially globular (PG) state, which is the α-helical formation disrupted tertiary structure, for cytochrome *c*. Consequently, changes in the concentration of [bmim][NO<sup>3</sup> ] induce structural transitions of the folded state → unfolded state → intermolecular β-sheet aggregation for myoglobin, and the folded state → unfolded state → PG state for cytochrome *c*. Similar structural transitions are observed in other proteins (β-LG, lysozyme, and RNase A) in aqueous solutions with other [bmim]-based ILs ([bmim][Cl] and [bmim][SCN]), except for cytochrome *c* in condensed [bmim][SCN] solutions (cytochrome *c* in this media takes the disordered-rich aggregate) [24–28]. **Figure 4** summarizes the structural transitions of proteins in aqueous solutions over a wide IL concentration range.

The most remarkable result is that condensed solutions with [bmim]-based ILs cause the formation of an α-helical structure, and intermolecular β-sheets or disordered-rich aggregation. From the previous results, the former state is similar to the intermediate in the on- or offpathway for the protein folding process [29], and the latter state is similar to the amyloid structure associated with neurodegenerative conditions such as Parkinson's disease [30, 31] and the structure of the inclusion body in expression proteins [30, 32]. Thus, it is important to reveal the origin of the structural formation of proteins in condensed aqueous IL solutions in

**Figure 4.** Summary of structural transition of proteins in aqueous ILs solutions.

solutions at several [bmim][NO<sup>3</sup>

([θ]288) as a function of [bmim][NO<sup>3</sup>

**Figure 3.** Near-UV CD spectra in the amide I' region of (a) myoglobin and (b) cytochrome *c* in aqueous [bmim][NO<sup>3</sup>

] concentration.

] concentrations. Changes in ellipticity of (c) myoglobin ([θ]290) and (d) cytochrome *c*

Ionic Liquid-Induced Unique Structural Transitions of Proteins

http://dx.doi.org/10.5772/65886

103

]

results indicate that the aqueous [bmim][NO<sup>3</sup>

Next, we measured the changes in the tertiary structure of both proteins induced by [bmim]

sity occurs above *x* = 7 (**Figure 3c** and **d**). This indicates that an increase in the [bmim][NO<sup>3</sup>

The results from the FTIR, SAXS, and near-UV CD analyses show that aqueous [bmim][NO<sup>3</sup>

solutions of up to *x* = 5–7 cause myoglobin and cytochrome *c* to unfold. Further addition of

tially globular (PG) state, which is the α-helical formation disrupted tertiary structure, for

transitions of the folded state → unfolded state → intermolecular β-sheet aggregation for myoglobin, and the folded state → unfolded state → PG state for cytochrome *c*. Similar structural transitions are observed in other proteins (β-LG, lysozyme, and RNase A) in aqueous solutions with other [bmim]-based ILs ([bmim][Cl] and [bmim][SCN]), except for cytochrome *c* in condensed [bmim][SCN] solutions (cytochrome *c* in this media takes the disordered-rich aggregate) [24–28]. **Figure 4** summarizes the structural transitions of proteins in aqueous

The most remarkable result is that condensed solutions with [bmim]-based ILs cause the formation of an α-helical structure, and intermolecular β-sheets or disordered-rich aggregation. From the previous results, the former state is similar to the intermediate in the on- or offpathway for the protein folding process [29], and the latter state is similar to the amyloid structure associated with neurodegenerative conditions such as Parkinson's disease [30, 31] and the structure of the inclusion body in expression proteins [30, 32]. Thus, it is important to reveal the origin of the structural formation of proteins in condensed aqueous IL solutions in

] induces formation of the intermolecular β-sheet for myoglobin and the par-

concentration completely disrupts the tertiary structure of both proteins.

**Figure 2.** (a) Guinier plots and (b) Kratky plots of cytochrome *c* in aqueous [bmim][NO<sup>3</sup>

cytochrome *c*. Consequently, changes in the concentration of [bmim][NO<sup>3</sup>

solutions over a wide IL concentration range.

] using near-UV CD spectroscopy (**Figure 3a** and **b**). Although the negative CD intensity at 290 nm for myoglobin and 288 nm for cytochrome *c* due to the aromatic residues drastically

] concentrations of up to *x* = 7, no increase in the negative CD inten-

]

]

] induce structural

] solutions at *x* = 0, 5, and 20.

partially refold without aggregation.

102 Progress and Developments in Ionic Liquids

[NO<sup>3</sup>

[bmim][NO<sup>3</sup>

decreases at [bmim][NO<sup>3</sup>

] solution at *x* = 20 causes cytochrome *c* to

**Figure 3.** Near-UV CD spectra in the amide I' region of (a) myoglobin and (b) cytochrome *c* in aqueous [bmim][NO<sup>3</sup> ] solutions at several [bmim][NO<sup>3</sup> ] concentrations. Changes in ellipticity of (c) myoglobin ([θ]290) and (d) cytochrome *c* ([θ]288) as a function of [bmim][NO<sup>3</sup> ] concentration.

**Figure 4.** Summary of structural transition of proteins in aqueous ILs solutions.

view of protein engineering application using ILs. In the following sections, we discuss the preferential formation of the α-helical structure (PG state) in Section 3.2, and intermolecular β-sheet aggregation in Section 3.3.

to that seen in alcohols (32.6 for methanol [37] and 27 for TFE [38]). The results suggest that [bmim]-based ILs cause an enhancement of the intramolecular hydrogen bonding in proteins by removing water molecules from their proximity. Accordingly, the similar solvent polarity of aqueous [bmim]-based ILs and aqueous alcohol solutions likely causes the structural changes observed for both proteins and stabilizes their α-helix structure. While the condensed [bmim][SCN] solutions, which is a strong denaturant, did not show the helix formation for β-LG and RNase A. Thus, the helix-forming ability of [bmim]-based ILs depended on the anionic species, and is related to the competition between the low polarity of condensed IL

Ionic Liquid-Induced Unique Structural Transitions of Proteins

http://dx.doi.org/10.5772/65886

105

and denaturant effect of anions showing the anion-protein interaction.

**Figure 5.** FTIR (a and b) and second derivative (c and d) spectra of β-LG and RNase A in aqueous [bmim][NO<sup>3</sup>

at several [bmim][NO<sup>3</sup>

] concentrations.

] solutions

### **3.2. Helix formation ability of ILs for proteins**

We found that condensed aqueous solutions with [bmim]-based ILs induce the helical formation disrupted tertiary structure (PG state) for some proteins. Generally, it is well known that β-LG and RNase A, having substantial β-sheet contents, take non-native helical formations in aqueous alcohol solutions, such as 2,2,2-trifluoroetahnol [33–35]. This is termed alcohol denaturation. The PG state in condensed IL solutions structurally resembles that from alcohol denaturation. Here, we focused on the details of helical formation ability of ILs for β-LG and RNase A.

**Figure 5a** and **b** shows the FTIR spectra of β-LG and RNase A in aqueous [bmim][NO<sup>3</sup> ] solutions of several concentrations. On the whole, the absorbance of both proteins at ca. 1635 cm−1, indicating intramolecular β-sheet structure, decreases, and that at ca. 1656 cm−1, indicating the α-helix structure, increases with [bmim][NO<sup>3</sup> ] concentration (**Figure 5a** and **c**). Both proteins undergo helix formation at high [bmim][NO<sup>3</sup> ] concentrations, though β-LG forms an intermolecular β-sheet structure in addition to an α-helical structure. Characterization of the helix formation in condensed aqueous [bmim][NO<sup>3</sup> ] solutions reveals that it is similar to that seen for alcohol denaturation, which results in a direct β-α transition. However, it is intriguing whether the condensed [bmim][NO<sup>3</sup> ] solutions induce direct β-α transition, as in the case of alcohol denaturation.

We assessed the second-derivative FTIR spectra of β-LG and RNase A in aqueous [bmim][NO<sup>3</sup> ] solutions at different concentrations of [bmim][NO<sup>3</sup> ] (**Figure 5c** and **d**). Increasing the [bmim] [NO<sup>3</sup> ] concentration up to *x* = 1–5 causes a decrease in the intramolecular β-sheet structure and an increase in the disordered structure of both proteins. With further addition of [bmim] [NO<sup>3</sup> ] (up to *x* = 30), the disordered structure decreases and the α-helical content increases. Conversely, our previous FTIR spectra showed that the intramolecular β-sheet structure of β-LG in the aqueous TFE solution drastically decreases with increasing TFE concentration, and the α-helical content increases without the appearance of the disordered structure [26]. Importantly, the metastable intermediate, i.e., native β-sheet → disordered structure → nonnative α-helix, in the β-α transition process, can be observed in aqueous [bmim][NO<sup>3</sup> ] solutions. Unlike alcohol denaturation (direct β-α transition), the aqueous [bmim][NO<sup>3</sup> ] solutions cause helix formation in β-LG and RNase A through an intermediate disordered structure. Although similar helix formation ability is observed in [bmim][Cl], its ability is weaker than [bmim][NO<sup>3</sup> ]. Besides, [bmim][SCN], representing a strong denaturant, does not show helix formation ability.

Here we discuss the origin of helix-forming ability of [bmim]-based ILs for β-LG and RNase A. Generally, alcohol denaturation is thought to arise from solvent properties such as low polarity. Low solvent polarity weakens the hydrophobic interactions that stabilize the compact native structure of proteins while simultaneously strengthening the intramolecular electrostatic interactions, such as hydrogen bonds, and stabilizing secondary structures, particularly the α-helix. The dielectric constant (*ε*) of the [bmim]-based ILs is low (*ε* = 10–20) [36], similar to that seen in alcohols (32.6 for methanol [37] and 27 for TFE [38]). The results suggest that [bmim]-based ILs cause an enhancement of the intramolecular hydrogen bonding in proteins by removing water molecules from their proximity. Accordingly, the similar solvent polarity of aqueous [bmim]-based ILs and aqueous alcohol solutions likely causes the structural changes observed for both proteins and stabilizes their α-helix structure. While the condensed [bmim][SCN] solutions, which is a strong denaturant, did not show the helix formation for β-LG and RNase A. Thus, the helix-forming ability of [bmim]-based ILs depended on the anionic species, and is related to the competition between the low polarity of condensed IL and denaturant effect of anions showing the anion-protein interaction.

view of protein engineering application using ILs. In the following sections, we discuss the preferential formation of the α-helical structure (PG state) in Section 3.2, and intermolecular

We found that condensed aqueous solutions with [bmim]-based ILs induce the helical formation disrupted tertiary structure (PG state) for some proteins. Generally, it is well known that β-LG and RNase A, having substantial β-sheet contents, take non-native helical formations in aqueous alcohol solutions, such as 2,2,2-trifluoroetahnol [33–35]. This is termed alcohol denaturation. The PG state in condensed IL solutions structurally resembles that from alcohol denaturation. Here, we focused on the details of helical formation ability of ILs for β-LG

of several concentrations. On the whole, the absorbance of both proteins at ca. 1635 cm−1, indicating intramolecular β-sheet structure, decreases, and that at ca. 1656 cm−1, indicating the α-helix

β-sheet structure in addition to an α-helical structure. Characterization of the helix formation in

turation, which results in a direct β-α transition. However, it is intriguing whether the condensed

We assessed the second-derivative FTIR spectra of β-LG and RNase A in aqueous [bmim][NO<sup>3</sup>

tions. Unlike alcohol denaturation (direct β-α transition), the aqueous [bmim][NO<sup>3</sup>

cause helix formation in β-LG and RNase A through an intermediate disordered structure. Although similar helix formation ability is observed in [bmim][Cl], its ability is weaker than

Here we discuss the origin of helix-forming ability of [bmim]-based ILs for β-LG and RNase A. Generally, alcohol denaturation is thought to arise from solvent properties such as low polarity. Low solvent polarity weakens the hydrophobic interactions that stabilize the compact native structure of proteins while simultaneously strengthening the intramolecular electrostatic interactions, such as hydrogen bonds, and stabilizing secondary structures, particularly the α-helix. The dielectric constant (*ε*) of the [bmim]-based ILs is low (*ε* = 10–20) [36], similar

]. Besides, [bmim][SCN], representing a strong denaturant, does not show helix

] solutions induce direct β-α transition, as in the case of alcohol denaturation.

] concentration up to *x* = 1–5 causes a decrease in the intramolecular β-sheet structure and an increase in the disordered structure of both proteins. With further addition of [bmim]

] (up to *x* = 30), the disordered structure decreases and the α-helical content increases. Conversely, our previous FTIR spectra showed that the intramolecular β-sheet structure of β-LG in the aqueous TFE solution drastically decreases with increasing TFE concentration, and the α-helical content increases without the appearance of the disordered structure [26]. Importantly, the metastable intermediate, i.e., native β-sheet → disordered structure → nonnative α-helix, in the β-α transition process, can be observed in aqueous [bmim][NO<sup>3</sup>

] concentration (**Figure 5a** and **c**). Both proteins undergo

] concentrations, though β-LG forms an intermolecular

] (**Figure 5c** and **d**). Increasing the [bmim]

] solutions reveals that it is similar to that seen for alcohol dena-

] solutions

]

] solu-

] solutions

**Figure 5a** and **b** shows the FTIR spectra of β-LG and RNase A in aqueous [bmim][NO<sup>3</sup>

β-sheet aggregation in Section 3.3.

104 Progress and Developments in Ionic Liquids

structure, increases with [bmim][NO<sup>3</sup>

helix formation at high [bmim][NO<sup>3</sup>

solutions at different concentrations of [bmim][NO<sup>3</sup>

condensed aqueous [bmim][NO<sup>3</sup>

and RNase A.

[bmim][NO<sup>3</sup>

[bmim][NO<sup>3</sup>

formation ability.

[NO<sup>3</sup>

[NO<sup>3</sup>

**3.2. Helix formation ability of ILs for proteins**

**Figure 5.** FTIR (a and b) and second derivative (c and d) spectra of β-LG and RNase A in aqueous [bmim][NO<sup>3</sup> ] solutions at several [bmim][NO<sup>3</sup> ] concentrations.

Next we discuss the generality of helix formation of ILs with NO<sup>3</sup> − . The helix-forming ability of [bmim][NO<sup>3</sup> ] for proteins has been connected to its low polarity; however, similar solution properties are also observed in other ILs with NO<sup>3</sup> − anion. To elucidate the generality of helix formation of β-sheet-rich proteins in ILs with NO<sup>3</sup> − , we compared two imidazolium-based ILs ([bmim][NO<sup>3</sup> ] and 1-ethyl-3-methylimidazolium nitrate ([emim][NO<sup>3</sup> ])) and the three alkylammonium nitrates (RAN-ILs): MAN, EAN, and PAN.

amyloid-like aggregation is related to neurodegenerative diseases such as Parkinson's disease and the structure of the inclusion body [30–32]. The origin of amyloid-like aggregate formation in ILs is related to the suppression of protein aggregation. Consequently, we focused on amyloid-like aggregate formation in condensed aqueous solutions of [bmim][NO<sup>3</sup>

model proteins, which have different secondary structures and sizes in condensed aqueous

globin, and clearly present peaks at ca. 1615 and ca. 1690 cm−1, are indicating amyloid-like aggregation. In contrast, the four remaining proteins (lysozyme, RNase A, cytochrome *c*, and insulin) do not present these two peaks. Similar FTIR spectra are also obtained form an aqueous [bmim][Cl] solution at *x* = 20. To investigate whether the four proteins aggregate, their SAXS profiles were recorded [39]. The SAXS curves show a drastic increase in *I*(*q*) below *q* = 0.03 nm−1 for rhodanese, α-chymotrypsin, β-LG, myoglobin, indicating protein aggregation. However, the other four proteins do not aggregate, as indicated by their SAXS

**Figure 7.** FTIR spectra of various proteins ((A) rhodanese, (B) α-chymotrypsin, (C) β-LG, (D) myoglobin, (E) lysozyme,

and (c) EAN) solutions (*x* = 20). The solid and dashed-dotted lines represent aggregated and nonaggregated proteins,

(F) RNase A, (G) cytochrome *c*, and (H) insulin) in water (dotted line) and in ILs ((a) [bmim][NO<sup>3</sup>

] solutions (*x* = 20). The FTIR spectra of rhodanese, α-chymotrypsin, β-LG, myo-

[bmim][SCN].

[bmim][NO<sup>3</sup>

respectively.

First, we address the case of [bmim][NO<sup>3</sup>

] and

107

], (b) [emim][NO<sup>3</sup>

],

]. **Figure 7a** shows the FTIR spectra of the eight

Ionic Liquid-Induced Unique Structural Transitions of Proteins

http://dx.doi.org/10.5772/65886

As a representative result, the FTIR spectra of RNase A in condensed aqueous solutions with various ILs at *x* = 30 are shown in **Figure 6a**. Although the FTIR spectral shape of RNase A in MAN shows slight RNase A unfolding, those with other ILs show a decrease in the native sheet structure and an increase in the helix structure. The amounts of α-helix structures in RNase A and β-LG were determined using a curve-fitting method (**Figure 6b**). Notably, the amounts of α-helix structures in RNase A (○) and β-LG (●) with the ILs (except for MAN) are essentially the same (within estimated experimental errors). This indicates that the effect of IL cations on the secondary structure of both proteins is negligible. The orders of solventinduced helix formation of RNase A and β-LG are D2 O << MAN << [emim][NO<sup>3</sup> ] < EAN ~ PAN ~ [bmim][NO<sup>3</sup> ] (the value of α-helical content of β-LG in [bmim][NO<sup>3</sup> ] is slightly lower than those in EAN, PAN, and [bmim][NO<sup>3</sup> ] by the formation of intermolecular β-sheets). The present results indicate that condensed aqueous ILs with NO<sup>3</sup> − solutions show a high helix-forming ability for β-sheet-rich proteins, such as RNase A and β-LG.

On the basis of these results, we can conclude that the helix-forming ability of IL depended on the anionic species rather than the cationic species. Besides, this ability is strongly related to the competition between the low polarity and denaturation effect of anions.

**Figure 6.** (a) FTIR spectra in the amide I' region of RNase A at various ILs with NO<sup>3</sup> − (*x* = 30). (b) Contents of the α-helical structures of RNase A and β-LG at various ILs with NO<sup>3</sup> − (*x* = 30).

#### **3.3. Ionic liquid-induced amyloid-like aggregation**

Another intriguing phenomena associated with condensed IL solutions are the formation of intermolecular β-sheet structures (i.e., amyloid-like aggregation). As mentioned in Section 3.1, amyloid-like aggregation is related to neurodegenerative diseases such as Parkinson's disease and the structure of the inclusion body [30–32]. The origin of amyloid-like aggregate formation in ILs is related to the suppression of protein aggregation. Consequently, we focused on amyloid-like aggregate formation in condensed aqueous solutions of [bmim][NO<sup>3</sup> ] and [bmim][SCN].

Next we discuss the generality of helix formation of ILs with NO<sup>3</sup>

properties are also observed in other ILs with NO<sup>3</sup>

formation of β-sheet-rich proteins in ILs with NO<sup>3</sup>

alkylammonium nitrates (RAN-ILs): MAN, EAN, and PAN.

induced helix formation of RNase A and β-LG are D2

**3.3. Ionic liquid-induced amyloid-like aggregation**

structures of RNase A and β-LG at various ILs with NO<sup>3</sup>

**Figure 6.** (a) FTIR spectra in the amide I' region of RNase A at various ILs with NO<sup>3</sup>

The present results indicate that condensed aqueous ILs with NO<sup>3</sup>

helix-forming ability for β-sheet-rich proteins, such as RNase A and β-LG.

the competition between the low polarity and denaturation effect of anions.

than those in EAN, PAN, and [bmim][NO<sup>3</sup>

of [bmim][NO<sup>3</sup>

106 Progress and Developments in Ionic Liquids

ILs ([bmim][NO<sup>3</sup>

PAN ~ [bmim][NO<sup>3</sup>

−

anion. To elucidate the generality of helix

O << MAN << [emim][NO<sup>3</sup>

] by the formation of intermolecular β-sheets).

−

−

, we compared two imidazolium-based

] for proteins has been connected to its low polarity; however, similar solution

−

] and 1-ethyl-3-methylimidazolium nitrate ([emim][NO<sup>3</sup>

As a representative result, the FTIR spectra of RNase A in condensed aqueous solutions with various ILs at *x* = 30 are shown in **Figure 6a**. Although the FTIR spectral shape of RNase A in MAN shows slight RNase A unfolding, those with other ILs show a decrease in the native sheet structure and an increase in the helix structure. The amounts of α-helix structures in RNase A and β-LG were determined using a curve-fitting method (**Figure 6b**). Notably, the amounts of α-helix structures in RNase A (○) and β-LG (●) with the ILs (except for MAN) are essentially the same (within estimated experimental errors). This indicates that the effect of IL cations on the secondary structure of both proteins is negligible. The orders of solvent-

] (the value of α-helical content of β-LG in [bmim][NO<sup>3</sup>

On the basis of these results, we can conclude that the helix-forming ability of IL depended on the anionic species rather than the cationic species. Besides, this ability is strongly related to

Another intriguing phenomena associated with condensed IL solutions are the formation of intermolecular β-sheet structures (i.e., amyloid-like aggregation). As mentioned in Section 3.1,

− (*x* = 30). −

. The helix-forming ability

])) and the three

] < EAN ~

] is slightly lower

solutions show a high

(*x* = 30). (b) Contents of the α-helical

First, we address the case of [bmim][NO<sup>3</sup> ]. **Figure 7a** shows the FTIR spectra of the eight model proteins, which have different secondary structures and sizes in condensed aqueous [bmim][NO<sup>3</sup> ] solutions (*x* = 20). The FTIR spectra of rhodanese, α-chymotrypsin, β-LG, myoglobin, and clearly present peaks at ca. 1615 and ca. 1690 cm−1, are indicating amyloid-like aggregation. In contrast, the four remaining proteins (lysozyme, RNase A, cytochrome *c*, and insulin) do not present these two peaks. Similar FTIR spectra are also obtained form an aqueous [bmim][Cl] solution at *x* = 20. To investigate whether the four proteins aggregate, their SAXS profiles were recorded [39]. The SAXS curves show a drastic increase in *I*(*q*) below *q* = 0.03 nm−1 for rhodanese, α-chymotrypsin, β-LG, myoglobin, indicating protein aggregation. However, the other four proteins do not aggregate, as indicated by their SAXS

**Figure 7.** FTIR spectra of various proteins ((A) rhodanese, (B) α-chymotrypsin, (C) β-LG, (D) myoglobin, (E) lysozyme, (F) RNase A, (G) cytochrome *c*, and (H) insulin) in water (dotted line) and in ILs ((a) [bmim][NO<sup>3</sup> ], (b) [emim][NO<sup>3</sup> ], and (c) EAN) solutions (*x* = 20). The solid and dashed-dotted lines represent aggregated and nonaggregated proteins, respectively.

curves, which do not show a significant increase in *I*(*q*) below *q* = 0.03 nm−1. These results are consistent with those from the FTIR experiments. The FTIR and SAXS results indicate that aqueous [bmim][NO<sup>3</sup> ] solutions at *x* = 20 promote the formation of amyloid-like aggregates in rhodanese, α-chymotrypsin, β-LG, and myoglobin, whereas the same solutions inhibit aggregation for lysozyme, RNase A, cytochrome *c*, and insulin.

Next, we discuss amyloid-like aggregation in aqueous [bmim][SCN] solutions showing the strong denaturant. **Figure 8** shows the FTIR spectra of five of the investigated proteins (cytochrome *c*, myoglobin, lysozyme, RNase A, and β-LG) at *x* = 30. Remarkably, the spectra of four of the proteins (with the spectrum of cytochrome *c* being the exception) clearly present

To further investigate the changes in the secondary structures of the proteins, we determined their intermolecular-β-sheet contents (β%) using curve-fitting analysis. The β% values at *x* = 30 are 0% for cytochrome *c*, 11 ± 1.6% for myoglobin, 34 ± 3.5% for lysozyme, 21 ± 4.0% for

aggregation patterns do not correlate with the protein size or with their secondary structures. Thus, the origin of the amyloid-like aggregation in aqueous [bmim][SCN] solutions is an interesting topic. When structural transitions of proteins occur in aqueous solutions with salts and ILs, the cations and anions interact directly with specific amino acid residues on the proteins [43, 44], and this interaction is enhanced in condensed IL solutions. Consequently, we focused on the relationship between the secondary-structure content of the proteins in the

**Figure 8.** (a) Curve-fitted FTIR spectra in the amide I' region of (A) cytochrome *c*, (B) myoglobin, (C) lysozyme, (D) RNase A, and (E) β-LG in aqueous [bmim][SCN] solutions at *x* = 30. (b) Relationship between the β% and amino acid

residues. Closed and open circles represent Lys and Gln residues, respectively.

], the amyloid-like

http://dx.doi.org/10.5772/65886

109

Ionic Liquid-Induced Unique Structural Transitions of Proteins

RNase A, and 17 ± 3.2% for β-LG. Compared with the case of [bmim][NO<sup>3</sup>

peaks attributable to intermolecular β-sheet structures.

condensed ILs and their amino acid residue contents.

Here, to investigate the influence of the imidazolium cation alkyl chain length on the decrease in amyloid-like aggregation, FTIR spectra were recorded for proteins in aqueous solutions (*x* = 20) of [emim][NO<sup>3</sup> ], which has a shorter alkyl chain length (**Figure 7b**). Intriguingly, the two peaks indicating an intermolecular β-sheet structure are observed for rhodanese and α-chymotrypsin. Conversely, the spectra of the other proteins do not contain these peaks, and the six remaining proteins form an α-helical structure, as in the case of [bmim][NO<sup>3</sup> ]. The protein size at which aggregation occurs in aqueous [emim][NO<sup>3</sup> ] solutions is larger than that in aqueous [bmim][NO<sup>3</sup> ] solutions. Thus, the formation of amyloid-like aggregates depends on the alkyl chain length of the cation.

In order to gain insight into the amyloid-like aggregate formation, we have focused on the solution structure and protein size. As discussed in Section 1, it has been suggested that the structural changes of proteins in aqueous IL solutions are strongly related to the solution structures of these media. 1-Alkyl-3-methylimdazolium-based ILs form nanoheterogeneous structures containing polar and nonpolar domains. The solutions exhibit molten-salt-like behavior, and the water molecules are scattered in the polar domain and self-assemble into confined-water-type domains under IL-rich conditions. The SAXS and small-angle neutron scattering (SANS) results imply that confined water exists in aqueous [bmim][NO<sup>3</sup> ] solutions at *x* = 20 [18, 19]. Moreover, an increase in alkyl chain length results in an enhancement of nanoheterogeneity [40]. Thus, in the condensed aqueous IL solutions, a decrease in the alkyl chain length of the cation may induce an increase in the size of the confined water domains. In terms of the relationship between the protein structure and the solution structure of the aqueous IL solutions, it has been suggested that proteins in condensed IL solutions are hydrated with water molecules in IL layers [3, 41, 42]. Jaganathan et al. [41] demonstrated the organization of ILs around hydrated cytochrome *c* in high-concentration ILs using molecular dynamics simulations. Similarly, according to the analysis of transfer free energy (Δ*G*<sup>t</sup> ) in cyclic dipeptides from water to aqueous IL solutions conducted by Attri and Venkatesu [42], ILs interact unfavorably with protein surfaces, thus promoting the formation of hydration layers around the proteins.

On the basis of these results, we propose that aggregated proteins in aqueous [bmim][NO<sup>3</sup> ] or [emim][NO<sup>3</sup> ] solutions at *x* = 20 are even less sufficiently hydrated than the small-sized proteins; therefore, protein-protein interactions are enhanced. However, the nonaggregated proteins selectively interact with water molecules at aggregated water sites in the polar domains. Consequently, the formation of amyloid-like aggregates is strongly related to the size of the confined water domains in the IL layer. Unfortunately, further elucidation of the direct correlation between the protein size and confined water in aqueous IL solutions is difficult. Further experimental studies, such as investigations into the influence of aggregated water in aqueous IL solutions with/without proteins using SAXS and SANS methods, are required.

Next, we discuss amyloid-like aggregation in aqueous [bmim][SCN] solutions showing the strong denaturant. **Figure 8** shows the FTIR spectra of five of the investigated proteins (cytochrome *c*, myoglobin, lysozyme, RNase A, and β-LG) at *x* = 30. Remarkably, the spectra of four of the proteins (with the spectrum of cytochrome *c* being the exception) clearly present peaks attributable to intermolecular β-sheet structures.

curves, which do not show a significant increase in *I*(*q*) below *q* = 0.03 nm−1. These results are consistent with those from the FTIR experiments. The FTIR and SAXS results indicate that

in rhodanese, α-chymotrypsin, β-LG, and myoglobin, whereas the same solutions inhibit

Here, to investigate the influence of the imidazolium cation alkyl chain length on the decrease in amyloid-like aggregation, FTIR spectra were recorded for proteins in aqueous solutions

the two peaks indicating an intermolecular β-sheet structure are observed for rhodanese and α-chymotrypsin. Conversely, the spectra of the other proteins do not contain these peaks, and the six remaining proteins form an α-helical structure, as in the case of [bmim][NO<sup>3</sup>

In order to gain insight into the amyloid-like aggregate formation, we have focused on the solution structure and protein size. As discussed in Section 1, it has been suggested that the structural changes of proteins in aqueous IL solutions are strongly related to the solution structures of these media. 1-Alkyl-3-methylimdazolium-based ILs form nanoheterogeneous structures containing polar and nonpolar domains. The solutions exhibit molten-salt-like behavior, and the water molecules are scattered in the polar domain and self-assemble into confined-water-type domains under IL-rich conditions. The SAXS and small-angle neutron

scattering (SANS) results imply that confined water exists in aqueous [bmim][NO<sup>3</sup>

tions at *x* = 20 [18, 19]. Moreover, an increase in alkyl chain length results in an enhancement of nanoheterogeneity [40]. Thus, in the condensed aqueous IL solutions, a decrease in the alkyl chain length of the cation may induce an increase in the size of the confined water domains. In terms of the relationship between the protein structure and the solution structure of the aqueous IL solutions, it has been suggested that proteins in condensed IL solutions are hydrated with water molecules in IL layers [3, 41, 42]. Jaganathan et al. [41] demonstrated the organization of ILs around hydrated cytochrome *c* in high-concentration ILs using molecular dynamics simulations. Similarly, according to the analysis of transfer free energy (Δ*G*<sup>t</sup>

cyclic dipeptides from water to aqueous IL solutions conducted by Attri and Venkatesu [42], ILs interact unfavorably with protein surfaces, thus promoting the formation of hydration

On the basis of these results, we propose that aggregated proteins in aqueous [bmim][NO<sup>3</sup>

proteins; therefore, protein-protein interactions are enhanced. However, the nonaggregated proteins selectively interact with water molecules at aggregated water sites in the polar domains. Consequently, the formation of amyloid-like aggregates is strongly related to the size of the confined water domains in the IL layer. Unfortunately, further elucidation of the direct correlation between the protein size and confined water in aqueous IL solutions is difficult. Further experimental studies, such as investigations into the influence of aggregated water in aqueous IL solutions with/without proteins using SAXS and SANS methods, are required.

] solutions at *x* = 20 are even less sufficiently hydrated than the small-sized

aggregation for lysozyme, RNase A, cytochrome *c*, and insulin.

The protein size at which aggregation occurs in aqueous [emim][NO<sup>3</sup>

] solutions at *x* = 20 promote the formation of amyloid-like aggregates

], which has a shorter alkyl chain length (**Figure 7b**). Intriguingly,

] solutions. Thus, the formation of amyloid-like aggregates

].

] solu-

) in

]

] solutions is larger

aqueous [bmim][NO<sup>3</sup>

108 Progress and Developments in Ionic Liquids

(*x* = 20) of [emim][NO<sup>3</sup>

than that in aqueous [bmim][NO<sup>3</sup>

layers around the proteins.

or [emim][NO<sup>3</sup>

depends on the alkyl chain length of the cation.

To further investigate the changes in the secondary structures of the proteins, we determined their intermolecular-β-sheet contents (β%) using curve-fitting analysis. The β% values at *x* = 30 are 0% for cytochrome *c*, 11 ± 1.6% for myoglobin, 34 ± 3.5% for lysozyme, 21 ± 4.0% for RNase A, and 17 ± 3.2% for β-LG. Compared with the case of [bmim][NO<sup>3</sup> ], the amyloid-like aggregation patterns do not correlate with the protein size or with their secondary structures. Thus, the origin of the amyloid-like aggregation in aqueous [bmim][SCN] solutions is an interesting topic. When structural transitions of proteins occur in aqueous solutions with salts and ILs, the cations and anions interact directly with specific amino acid residues on the proteins [43, 44], and this interaction is enhanced in condensed IL solutions. Consequently, we focused on the relationship between the secondary-structure content of the proteins in the condensed ILs and their amino acid residue contents.

**Figure 8.** (a) Curve-fitted FTIR spectra in the amide I' region of (A) cytochrome *c*, (B) myoglobin, (C) lysozyme, (D) RNase A, and (E) β-LG in aqueous [bmim][SCN] solutions at *x* = 30. (b) Relationship between the β% and amino acid residues. Closed and open circles represent Lys and Gln residues, respectively.

To investigate the correlation between the β% values and the occurrence of 20 amino acid residues in the five investigated proteins, we determined the correlation coefficient (*R*<sup>2</sup> ) of the relationship between β% and the 20 amino acid residues of the proteins using the slope of **Figure 8b**. **Figure 9a** shows *R*<sup>2</sup> value between amino acid residues of protein and β%. The value of β% appears to be dependent on the presence of hydrophilic amino acid residues, such as those on Lys, Arg, and Glu residues, rather than on the presence of hydrophobic or aromatic amino acid residues, such as those on Ile, Tyr, and Phe residues. The amino acid residues with *R*<sup>2</sup> > 0.7 are those on Lys, Arg, and Glu residues. Because Lys and Arg residues contain an amino group and Asn and Glu residues contain a carboxyl group, the former tend to interact with SCN− ions while the latter tend to interact with [bmim]<sup>+</sup> ions. An important result is that the Lys residues exhibit the highest *R*<sup>2</sup> value. A straightforward interpretation of this result is that SCN− ions bind mainly to the Lys residues of proteins in aqueous solutions. The present result is consistent with the previous X-ray diffraction studies that the SCN− ions weakly bind to the Lys and Arg residues of proteins in the crystalline state [45, 46].

residue interactions. The SCN−

**3.4. Future application of protein engineering**

residue interactions.

formation.

ous [bmim][NO<sup>3</sup>

protein solutions.

for the suppression of amyloid aggregation.

anion is a stronger denaturant than the NO<sup>3</sup>

denaturation effect of the anions becomes stronger, the origin of amyloid-like aggregation in condensed IL solutions changes from solution structural properties to the IL-amino acid

We have discussed the unique structural transitions of proteins in aqueous solutions with [bmim]-based ILs. Aqueous [bmim]-based IL solutions induce two structural transition patterns: the folded state → unfolded state → intermolecular β-sheet aggregation for myoglobin, and the folded state → unfolded state → partial globular state. These transitions are strongly related to solution properties, such as the presence of confined water around the IL layers (i.e., the nanoheterogeneity), a low polarity, and IL-amino acid residue interaction. On the basis of these results, we propose that future applications of ILs in protein engineering may be as cryoprotectants for proteins and as agents for the suppression of amyloid

The inhibition of ice-nucleation and a high structural reversibility of proteins without protein aggregation are important criteria for a protein cryoprotectant. Recently, Yoshimura et al. reported that the aqueous IL solutions in the wide IL concentration range exhibit glassy formation at 77 K [47]. In addition, the present study shows that condensed IL solutions cause the helical formation for some proteins without protein aggregation. Related to these results, we found that low temperatures (77 K) induce structural reversibility for lysozyme in aque-

sition without aggregation. Similar results were obtained in the case of RNase A in aqueous solutions of choline dihydrogen phosphate [49]. Thus, condensed IL solutions forming the glassy state at 77 K that induce helical formation without protein aggregation may be applicable as cryoprotectants for proteins, specifically as cryopreservation agents for recombinant proteins. However, in order to use ILs as cryoprotectants, it is necessary to investigate the enzyme activity in condensed IL solutions after cooling and removal of the IL from aqueous

As mentioned in Section 3.3, we have demonstrated that specific IL-amino acid residue interactions in condensed IL solutions cause inhibition of amyloid-like aggregation (i.e., intermolecular β-sheet structures). Related to this, we found that [bmim][SCN], EAN, and PAN ILs suppress thermally induced insulin amyloid formation [50]. Furthermore, condensed solutions of EAN or PAN demonstrate a high protectant ability for the structure of monomeric insulin. The affinity between ILs and specific amino acid residues in insulin is the main cause of the suppression of insulin amyloid formation. Thus, ILs can potentially be used as agents

We proposed the applications of ILs in protein engineering as cryoprotectants for proteins and as agents for the suppression of amyloid formation using properties of the condensed IL solutions. In addition to these, the solution properties of condensed IL solution (the presence of confined water around the IL layers, a low polarity, and IL-amino acid residue interaction)

will have a wide potential for applications of ILs in protein engineering in the future.

] solutions [48]. After cooling, the lysozyme structure shows reversible tran-

−

http://dx.doi.org/10.5772/65886

Ionic Liquid-Induced Unique Structural Transitions of Proteins

anion. As the

111

**Figure 9.** (a) Correlation coefficients (*R*<sup>2</sup> ) between amino acid residues of proteins and the content of intermolecular β-sheet structure (β%). (b) FTIR spectra of PLL in aqueous [bmim][SCN] solutions at *x* = 0 and 30.

Here, we can speculate that a Lys-rich polypeptide does not form an intermolecular β-sheet structure in aqueous [bmim][SCN] solutions if Lys residues are directly related to the formation of intermolecular β-sheet structures. To confirm this speculation, we measured the FTIR spectra of poly-L-lysine (PLL) (Lys = 100%) in aqueous [bmim][SCN] solutions at *x* = 0 and 30. They are significantly different from the spectra of the model proteins, in which PLL does not present the two peaks associated with intermolecular β-sheet formation. Accordingly, we propose that the SCN− ions bind primarily to Lys residues in the proteins, and that Lys-rich proteins do not undergo intermolecular β-sheet formation in the presence of [bmim][SCN]. Thus, the formation of amyloid-like aggregates by the addition of [bmim][SCN] is related to IL-amino acid residue interactions.

These results indicate that the origin of amyloid-like aggregate formation in [bmim][NO<sup>3</sup> ] is different from that in [bmim][SCN]. The former is due to the relationship between the protein size and the confined water size in the IL layer, while the latter is due to IL-amino acid residue interactions. The SCN− anion is a stronger denaturant than the NO<sup>3</sup> − anion. As the denaturation effect of the anions becomes stronger, the origin of amyloid-like aggregation in condensed IL solutions changes from solution structural properties to the IL-amino acid residue interactions.

#### **3.4. Future application of protein engineering**

To investigate the correlation between the β% values and the occurrence of 20 amino acid residues in the five investigated proteins, we determined the correlation coefficient (*R*<sup>2</sup>

the relationship between β% and the 20 amino acid residues of the proteins using the slope

value of β% appears to be dependent on the presence of hydrophilic amino acid residues, such as those on Lys, Arg, and Glu residues, rather than on the presence of hydrophobic or aromatic amino acid residues, such as those on Ile, Tyr, and Phe residues. The amino acid

contain an amino group and Asn and Glu residues contain a carboxyl group, the former tend

ions while the latter tend to interact with [bmim]<sup>+</sup>

Here, we can speculate that a Lys-rich polypeptide does not form an intermolecular β-sheet structure in aqueous [bmim][SCN] solutions if Lys residues are directly related to the formation of intermolecular β-sheet structures. To confirm this speculation, we measured the FTIR spectra of poly-L-lysine (PLL) (Lys = 100%) in aqueous [bmim][SCN] solutions at *x* = 0 and 30. They are significantly different from the spectra of the model proteins, in which PLL does not present the two peaks associated with intermolecular β-sheet formation. Accordingly, we

β-sheet structure (β%). (b) FTIR spectra of PLL in aqueous [bmim][SCN] solutions at *x* = 0 and 30.

proteins do not undergo intermolecular β-sheet formation in the presence of [bmim][SCN]. Thus, the formation of amyloid-like aggregates by the addition of [bmim][SCN] is related to

These results indicate that the origin of amyloid-like aggregate formation in [bmim][NO<sup>3</sup>

different from that in [bmim][SCN]. The former is due to the relationship between the protein size and the confined water size in the IL layer, while the latter is due to IL-amino acid

ions bind primarily to Lys residues in the proteins, and that Lys-rich

) between amino acid residues of proteins and the content of intermolecular

The present result is consistent with the previous X-ray diffraction studies that the SCN−

weakly bind to the Lys and Arg residues of proteins in the crystalline state [45, 46].

> 0.7 are those on Lys, Arg, and Glu residues. Because Lys and Arg residues

ions bind mainly to the Lys residues of proteins in aqueous solutions.

value between amino acid residues of protein and β%. The

of **Figure 8b**. **Figure 9a** shows *R*<sup>2</sup>

110 Progress and Developments in Ionic Liquids

result is that the Lys residues exhibit the highest *R*<sup>2</sup>

residues with *R*<sup>2</sup>

to interact with SCN−

this result is that SCN−

propose that the SCN−

IL-amino acid residue interactions.

**Figure 9.** (a) Correlation coefficients (*R*<sup>2</sup>

) of

ions

] is

ions. An important

value. A straightforward interpretation of

We have discussed the unique structural transitions of proteins in aqueous solutions with [bmim]-based ILs. Aqueous [bmim]-based IL solutions induce two structural transition patterns: the folded state → unfolded state → intermolecular β-sheet aggregation for myoglobin, and the folded state → unfolded state → partial globular state. These transitions are strongly related to solution properties, such as the presence of confined water around the IL layers (i.e., the nanoheterogeneity), a low polarity, and IL-amino acid residue interaction. On the basis of these results, we propose that future applications of ILs in protein engineering may be as cryoprotectants for proteins and as agents for the suppression of amyloid formation.

The inhibition of ice-nucleation and a high structural reversibility of proteins without protein aggregation are important criteria for a protein cryoprotectant. Recently, Yoshimura et al. reported that the aqueous IL solutions in the wide IL concentration range exhibit glassy formation at 77 K [47]. In addition, the present study shows that condensed IL solutions cause the helical formation for some proteins without protein aggregation. Related to these results, we found that low temperatures (77 K) induce structural reversibility for lysozyme in aqueous [bmim][NO<sup>3</sup> ] solutions [48]. After cooling, the lysozyme structure shows reversible transition without aggregation. Similar results were obtained in the case of RNase A in aqueous solutions of choline dihydrogen phosphate [49]. Thus, condensed IL solutions forming the glassy state at 77 K that induce helical formation without protein aggregation may be applicable as cryoprotectants for proteins, specifically as cryopreservation agents for recombinant proteins. However, in order to use ILs as cryoprotectants, it is necessary to investigate the enzyme activity in condensed IL solutions after cooling and removal of the IL from aqueous protein solutions.

As mentioned in Section 3.3, we have demonstrated that specific IL-amino acid residue interactions in condensed IL solutions cause inhibition of amyloid-like aggregation (i.e., intermolecular β-sheet structures). Related to this, we found that [bmim][SCN], EAN, and PAN ILs suppress thermally induced insulin amyloid formation [50]. Furthermore, condensed solutions of EAN or PAN demonstrate a high protectant ability for the structure of monomeric insulin. The affinity between ILs and specific amino acid residues in insulin is the main cause of the suppression of insulin amyloid formation. Thus, ILs can potentially be used as agents for the suppression of amyloid aggregation.

We proposed the applications of ILs in protein engineering as cryoprotectants for proteins and as agents for the suppression of amyloid formation using properties of the condensed IL solutions. In addition to these, the solution properties of condensed IL solution (the presence of confined water around the IL layers, a low polarity, and IL-amino acid residue interaction) will have a wide potential for applications of ILs in protein engineering in the future.

### **4. Conclusion**

We have investigated the structural transition of proteins in aqueous solutions of ILs over a wide concentration range using FTIR and near-UV CD spectroscopy combined with SAXS. Aqueous IL solutions induced two structural transition patterns; (i) the folded state → unfolded state → partial globular state (α-helical formation disrupted tertiary structure), and (ii) the folded state → unfolded state → aggregation (amyloid-like aggregation or disordered aggregation). These transition patterns are strongly related to the condensed IL solution properties, such as the presence of confined water in the IL layers, low polarity, denaturant effect of anions, and IL-amino acid residue interactions. On the basis of these results, we proposed the new application of ILs as novel cryoprotectants and amyloid suppression agents. The present results will be basic information for the design of ILs for protein engineering applications. Although we have fully investigated the structural properties of proteins in aqueous IL solutions, detailed information on enzyme activity and methods for the removal of ILs from these media is still required to use the protein engineering applications.

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[18] Abe H, Takekiyo T, Shigemi M, Yoshimura Y, Tsuge S, Hanasaki T, Onishi K, Takata S, Suzuki J. Direct evidence of confined water in room-temperature ionic liquids by complementary use of small-angle X-ray and neutron scattering. J. Phys. Chem. Lett. 2014;

[19] Abe H, Takekiyo T, Yoshimura Y, Saihara K, Shimizu A. Anomalous freezing of nanoconfined water in room temperature ionic liquid, 1-butyl-3-methylimidazolium nitrate.

tion in ionic liquid/water mixtures. J. Phys. Chem. B. 2007; 111: 4812–4818.

aggregation in ionic liquids. Phys. Chem. Chem. Phys. 2010; 12: 1756–1763.

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from α-synuclein. Anal. Biochem. 2009; 386: 293–295.

ionic liquids. J. Phys. Chem. B. 2006; 111: 4641–4644.

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5: 1175–1180.

### **Author details**

Takahiro Takekiyo\* and Yukihiro Yoshimura

\*Address all correspondence to: take214@nda.ac.jp

Department of Applied Chemistry, National Defense Academy, Hashirimizu, Yokosuka, Japan

### **References**


[6] Lange C, Patil G, Rudolph R. Ionic liquids as refolding additives: N'-alkyl and N'-(ωhydroxyalkyl) N-methylimidazolium chlorides. Protein Sci. 2005; 14: 2693–2701.

**4. Conclusion**

112 Progress and Developments in Ionic Liquids

applications.

Japan

**References**

2757–2785.

**Author details**

Takahiro Takekiyo\* and Yukihiro Yoshimura

\*Address all correspondence to: take214@nda.ac.jp

proteins. Phys. Chem. Chem. Phys. 2012; 14: 415–426.

nium nitrate. Protein Sci. 2000; 9: 2001–2008.

We have investigated the structural transition of proteins in aqueous solutions of ILs over a wide concentration range using FTIR and near-UV CD spectroscopy combined with SAXS. Aqueous IL solutions induced two structural transition patterns; (i) the folded state → unfolded state → partial globular state (α-helical formation disrupted tertiary structure), and (ii) the folded state → unfolded state → aggregation (amyloid-like aggregation or disordered aggregation). These transition patterns are strongly related to the condensed IL solution properties, such as the presence of confined water in the IL layers, low polarity, denaturant effect of anions, and IL-amino acid residue interactions. On the basis of these results, we proposed the new application of ILs as novel cryoprotectants and amyloid suppression agents. The present results will be basic information for the design of ILs for protein engineering applications. Although we have fully investigated the structural properties of proteins in aqueous IL solutions, detailed information on enzyme activity and methods for the removal of ILs from these media is still required to use the protein engineering

Department of Applied Chemistry, National Defense Academy, Hashirimizu, Yokosuka,

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**Chapter 6**

**Provisional chapter**

**Green Composites from Ionic Liquid-Assisted**

**of Sustainable Resources: A Brief Overview**

Hamayoun Mahmood, Muhammad Moniruzzaman,

Hamayoun Mahmood, Muhammad Moniruzzaman,

Suzana Yusup and Hazizan Md. Akil

Suzana Yusup and Hazizan Md. Akil

http://dx.doi.org/10.5772/65796

**Abstract**

Additional information is available at the end of the chapter

composites have been gradually realized.

tion, biopolymer, lignocellulose

**1. Introduction**

Additional information is available at the end of the chapter

**Processing of Sustainable Resources: A Brief Overview**

The massive use of synthetic, petroleum-based polymeric composites has disturbed the fragile environmental equilibrium of our planet. Composites made solely from polysaccharides can offer unique intrinsic properties such as renewability, biodegradability, easy availability, eco-friendliness, facile processing, flexibility, and exciting physico-mechanical characteristics. The development of green processing of lignocellulosic materials and bio-based polymers such as cellulose, starch, chitin, and chitosan, the most abundant biorenewable materials on earth, is urgent from the perspectives of both environmental protection and sustainability in materials industries. Recently, the enormous potential of ionic liquids (ILs) as an alternative to ecologically harmful conventional organic solvents has been well recognized. Presently, a wide range of pronounced approaches have been explored to further improve the performance of ionic liquid-based processing of polysaccharides for green composite manufacturing. This review presents recent technological developments in which the advantages of ionic liquids as a dissolution medium for polysaccharides for production of plethora of green

**Keywords:** ionic liquids, polysaccharides, biocomposites, biofilms, biofiber, plasticiza-

Ionic liquids (ILs) are termed as "liquid salts" and entirely composed of ions. Most of these materials are liquids at ambient or far below ambient temperature and have been widely used as a potential alternative to toxic, hazardous, volatile, and highly flammable organic solvents

and reproduction in any medium, provided the original work is properly cited.

© 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution,

© 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

**Green Composites from Ionic Liquid-Assisted Processing**


#### **Abstract**

The massive use of synthetic, petroleum-based polymeric composites has disturbed the fragile environmental equilibrium of our planet. Composites made solely from polysaccharides can unique intrinsic properties such as renewability, biodegradphysico-mechanical characteristics. The development of green processing of lignocellulosic materials and bio-based polymers such as cellulose, starch, chitin, and chitosan, the most abundant biorenewable materials on earth, is urgent from the perspectives of both environmental protection and sustainability in materials industries. Recently, the enormous potential of ionic liquids (ILs) as an alternative to ecologically harmful conventional organic solvents has been well recognized. Presently, a wide range of pronounced approaches have been explored to furtherimprove the performance ofionic liquid-based processing of polysaccharides for green composite manufacturing. This review presents recent technological developments in which the advantages of ionic liquids as a dissolution medium for polysaccharides for production of plethora of green composites have been gradually realized.

**Keywords:** ionic liquids, polysaccharides, biocomposites, tion, biopolymer, lignocellulose

### **1. Introduction**

Ionic liquids (ILs) are termed as "liquid salts" and entirely composed of ions. Most of these materials are liquids at ambient or far below ambient temperature and have been widely used

[1–3]. Various unique and attractive physicochemical properties of ILs, such as remarkable thermal and chemical stability [4, 5], extremely low vapor pressure [6], high solvation interactions with inorganic and organic compounds [7], broad electrochemical window, and sharp ionic conductivity, make ILs promising candidates for the replacement of volatile organic compounds (VOCs) for polysaccharide dissolution and modification [8]. ILs have been accredited as "designer solvents" as their properties can be tailored by appropriate combinations of anions and cations [9]. The combination of all these unique properties has triggered the use of ILs as environmentally benign dissolution media for lignocellulose and various biopolymers for the manufacturing of different composite products [10].

**2. Ionic liquids and their properties**

been successfully applied to cope with higher viscosities [28].

**polysaccharide dissolution and modification**

summarized as follows:

conventional organic solvents [29].

**3. A comparison of ILs with conventional organic solvents for**

in the alkyl‐substituted imidazolium‐based ILs was reported [30, 31].

In addition to the greener aspects and the excellent physicochemical properties, some impor‐ tant merits of the ILs over conventional organic solvents for bio‐based polymers could be

**•** Ionic liquids are tunable solvents, and hence they can be designed by appropriate selection of cations and anions for particular application which is generally not possible using

**•** ILs can dissolve biopolymers under relatively mild conditions of temperature and time and at normal atmospheric pressure which offer remarkable benefit to ILs in comparison with other molecular solvents. The dissolution of cellulose at the temperature of 45 °C for 30 min

**•** ILs have also been proved to be highly effective solvents for lignocellulosic materials under solid biomass loadings of as high as 50 % in a continuous pretreatment reactor [32]. Thus, the feasibility of high‐throughput continuous pretreatment could enhance the potential for use of IL‐based pretreatment as a cost‐effective and highly invaluable technology for

fabrication of sustainable composite materials from polysaccharide raw materials.

**4. Ionic liquid-assisted processing of polysaccharides for biocomposites**

Composites are engineered materials fabricated from two or more components usually referred as reinforcement and matrix. However, for the composites fabricated from polysac‐

Typical ionic liquids consist of an organic cation with an inorganic anion with melting point usually below 100°C, and they persist in liquid state for a wide temperature range (typically <400 °C). ILs could be capable of having a broad range of intermolecular interactions with biopolymers including hydrogen bonding, dispersive, ionic, and dipolar [23]. Some ILs are considered as highly polar solvents due to their excellent solvation properties. A number of techniques could be used to predict the polarity of ILs, such as solvatochromic dyes [24], partition [25], and fluorescence probe methods [26]. Generally, ILs are immiscible with most of the organic solvents like hexane and ether but miscible with most of the polar solvents, such as ketones, lower alcohols, and dichloromethane [27]. Furthermore, ILs can also be classified into two categories: hydrophilic and hydrophobic based on their solubility in water. Usually, ILs exhibit higher viscosities in comparison with ordinary molecular solvents which signifi‐ cantly impede the dissolution of polysaccharide materials in ILs. Different organic cosolvents such as dimethylformamide, dimethylsulfoxide, and 1, 3‐dimethyl‐2‐imidazolidinone have

Green Composites from Ionic Liquid-Assisted Processing of Sustainable Resources: A Brief Overview

http://dx.doi.org/10.5772/65796

119

The extensive use of petroleum-based polymers and composites and their existing anti-natural processing methods has disturbed the fragile environmental equilibrium and exhausting limited petroleum reserves [11]. A fierce public debate regarding the future of the earth and the need for transition toward a CO2 neutral bio-based economy was emphasized in the UN conference on climate change [12]. To this end, polymeric carbohydrates, e.g., starch, cellulose, chitin, inulin, chitosan, lignin, etc., are natural polymers found abundantly in nature as structural building elements and could be potential alternatives for petroleum-based nonbiodegradable polymers [13].

Manufacturing of sustainable composites demands not only the assortment of renewable or biodegradable resources for their manufacturing but also the utilization of mild pretreatment methods that avoid the use and production of hazardous by products [14, 15]. The strong interand intramolecular hydrogen bonding and the highly recalcitrant nature of the biopolymers and lignocellulose offer a critical challenge to extend the novel applications of these materials in composite industries [16, 17]. In this regard, various pretreatment technologies were developed to reduce the recalcitrance of lignocellulosic polymers, which apply chemical or hydrothermal treatments after mechanical comminution. However, most of the current pretreatment methods exhibit several drawbacks. Some pretreatments have to be tailored to the specific biopolymer material and or may cause decomposition of biopolymer constituents to side products, which can severely hinder downstream processing of these materials [18]. Further, some pretreatment technologies require strong acids or bases and extreme conditions of temperatures and pressures for which special equipment are necessary. Therefore, the development of alternative processing techniques for widespread potential applications of biobased polymeric carbohydrates and lignocellulosic agricultural waste for fabrication of biocomposite material still remains challenging [19, 20].

Ionic liquids have attracted numerous interest as a new and highly effective solvent for a plethora of biodegradable polymers and lignocellulosic materials. Thus, the technological utilization of such materials for biocomposite manufacturing could be enhanced remarkably by their dissolution in ILs rather than the use of conventional organic solvents [5, 21]. Various reports on dissolution of a wide variety of polysaccharides in ILs over the past 10–15 years suggested that by using ILs, efficient selective extraction of the components is also feasible [22]. The main purpose of the present work is to depict a short overview of the state of the art on the current role of ILs as dissolution medium to explore polysaccharide-based sustainable raw materials for engineered green materials applications.

### **2. Ionic liquids and their properties**

[1–3]. Various unique and attractive physicochemical properties of ILs, such as remarkable thermal and chemical stability [4, 5], extremely low vapor pressure [6], high solvation interactions with inorganic and organic compounds [7], broad electrochemical window, and sharp ionic conductivity, make ILs promising candidates for the replacement of volatile organic compounds (VOCs) for polysaccharide dissolution and modification [8]. ILs have been accredited as "designer solvents" as their properties can be tailored by appropriate combinations of anions and cations [9]. The combination of all these unique properties has triggered the use of ILs as environmentally benign dissolution media for lignocellulose and various

The extensive use of petroleum-based polymers and composites and their existing anti-natural processing methods has disturbed the fragile environmental equilibrium and exhausting limited petroleum reserves [11]. A fierce public debate regarding the future of the earth and the need for transition toward a CO2 neutral bio-based economy was emphasized in the UN conference on climate change [12]. To this end, polymeric carbohydrates, e.g., starch, cellulose, chitin, inulin, chitosan, lignin, etc., are natural polymers found abundantly in nature as structural building elements and could be potential alternatives for petroleum-based nonbio-

Manufacturing of sustainable composites demands not only the assortment of renewable or biodegradable resources for their manufacturing but also the utilization of mild pretreatment methods that avoid the use and production of hazardous by products [14, 15]. The strong interand intramolecular hydrogen bonding and the highly recalcitrant nature of the biopolymers and lignocellulose offer a critical challenge to extend the novel applications of these materials in composite industries [16, 17]. In this regard, various pretreatment technologies were developed to reduce the recalcitrance of lignocellulosic polymers, which apply chemical or hydrothermal treatments after mechanical comminution. However, most of the current pretreatment methods exhibit several drawbacks. Some pretreatments have to be tailored to the specific biopolymer material and or may cause decomposition of biopolymer constituents to side products, which can severely hinder downstream processing of these materials [18]. Further, some pretreatment technologies require strong acids or bases and extreme conditions of temperatures and pressures for which special equipment are necessary. Therefore, the development of alternative processing techniques for widespread potential applications of biobased polymeric carbohydrates and lignocellulosic agricultural waste for fabrication of

Ionic liquids have attracted numerous interest as a new and highly effective solvent for a plethora of biodegradable polymers and lignocellulosic materials. Thus, the technological utilization of such materials for biocomposite manufacturing could be enhanced remarkably by their dissolution in ILs rather than the use of conventional organic solvents [5, 21]. Various reports on dissolution of a wide variety of polysaccharides in ILs over the past 10–15 years suggested that by using ILs, efficient selective extraction of the components is also feasible [22]. The main purpose of the present work is to depict a short overview of the state of the art on the current role of ILs as dissolution medium to explore polysaccharide-based sustainable raw

biopolymers for the manufacturing of different composite products [10].

biocomposite material still remains challenging [19, 20].

materials for engineered green materials applications.

degradable polymers [13].

118 Progress and Developments in Ionic Liquids

Typical ionic liquids consist of an organic cation with an inorganic anion with melting point usually below 100°C, and they persist in liquid state for a wide temperature range (typically <400 °C). ILs could be capable of having a broad range of intermolecular interactions with biopolymers including hydrogen bonding, dispersive, ionic, and dipolar [23]. Some ILs are considered as highly polar solvents due to their excellent solvation properties. A number of techniques could be used to predict the polarity of ILs, such as solvatochromic dyes [24], partition [25], and fluorescence probe methods [26]. Generally, ILs are immiscible with most of the organic solvents like hexane and ether but miscible with most of the polar solvents, such as ketones, lower alcohols, and dichloromethane [27]. Furthermore, ILs can also be classified into two categories: hydrophilic and hydrophobic based on their solubility in water. Usually, ILs exhibit higher viscosities in comparison with ordinary molecular solvents which signifi‐ cantly impede the dissolution of polysaccharide materials in ILs. Different organic cosolvents such as dimethylformamide, dimethylsulfoxide, and 1, 3‐dimethyl‐2‐imidazolidinone have been successfully applied to cope with higher viscosities [28].

### **3. A comparison of ILs with conventional organic solvents for polysaccharide dissolution and modification**

In addition to the greener aspects and the excellent physicochemical properties, some impor‐ tant merits of the ILs over conventional organic solvents for bio‐based polymers could be summarized as follows:


### **4. Ionic liquid-assisted processing of polysaccharides for biocomposites**

Composites are engineered materials fabricated from two or more components usually referred as reinforcement and matrix. However, for the composites fabricated from polysac‐ charide materials, the end-product properties could be fitted without strict distinction for reinforcement and matrix. The present section briefly describes the IL-based preparation of different composite products from polysaccharides or their sources. Keeping in view the ethical standards, the sources may include plant cell walls or some sort of living species.

as biocompatibility, biodegradability, and significant specificity [5]. With the aim to replace commonly used noxious solvents, ILs have been investigated as new, nonvolatile, and nonflammable media for the electrospinning of biopolymers. A typical electrospinning apparatus based on IL-assisted dissolution and regeneration of cellulose is shown in **Figure 2**.

Green Composites from Ionic Liquid-Assisted Processing of Sustainable Resources: A Brief Overview

http://dx.doi.org/10.5772/65796

121

**Figure 2.** A typical electrospinning apparatus for IL-based processing of biofibers.

heparin solution in the IL by using electrospinning technique [40].

Polaskova et al. [36] dissolved raw pine wood in IL [emim][OAc] and utilized wet electrospinning technique to transform it into microfibers (1–4 μm). It was noted that 5% wood loading in IL was the most appropriate concentration for electrospinning, and further increase in the biomass loading up to 10 % could complicate the process due to significant increase in the viscosity of solution. Similarly, electrospinning technology was utilized to obtain nonwoven nanoscale fibers from regenerated cellulose in the IL [bmim][Cl] [37]. The influence of the viscosity of biopolymer solution IL on the structure and size of the resulting biofiber was explored. Besides, Qin et al. [38] noted that high-molecular-weight and high purity chitin powder could be recovered after complete dissolution of raw crustacean shells in IL [emim] [OAc] (**Figure 3**). The direct fabrication of chitin fibers and films from the extract solution was also reported. The conversion of cellulose and starch into fibrous material by making their homogeneous solution in IL [bmim][Cl] was described [39]. As explained in **Figure 4**, the fine linear material was obtained by raising the viscous biopolymer mixture with the help of a spatula and subsequently soaked into acetone to remove IL and then vacuum dried. The fabricated fiber showed the compatabilized fibrous structure of ca. 100–200 μm with higher thermal stability than that of gel made from the same biopolymers in [bmim][Cl]. Branched fibers with the size range of micro- to nanometer were extruded from 10 % (w/w) of cellulose-

#### **4.1. Biofilms and biofibers**

Although petroleum-based synthetic polymer products have offered excellent services to modern society, their extensive use has become a serious threat to the environment. Therefore, interest has been focused for the exploitation of natural biopolymers. Cellulose is a linear polysaccharide, which exhibited outstanding characteristics and broad range of applications as engineering material. Generally, it does not melt or dissolve in ordinary solvents, which makes its processing extremely difficult. Recently, the capabilities of ILs to dissolve cellulose have significantly impacted its processing for fabrication of biodegradable plastic films [9, 22].

Dissolution and regeneration of cellulosic biofilms from IL [bmim][Cl] by using cotton pulp as raw cellulose source was reported by Liu et al. [33]. It was observed that solubility of cellulose could reach up to 13 wt% at 90°C in 7 h. Takegawa et al. [34] fabricated the bicomponent biopolymer film with cellulose and chitin each dissolved separately in the ILs [amim][Br] and [bmim][Cl], respectively, at 100 °C. The biofilms became more elastic by decreasing the relative ratio of chitin to cellulose in the final product. Further, **Figure 1** depicts the scheme of successful dissolution and regeneration of the native skin collagen in IL [bmim][Cl] [35]. The possible mechanism of dissolution of collagen in IL was also suggested which was based mainly on the hydrogen bond breaking.

**Figure 1.** Schematic representation for preparation of collagen/cellulose composite materials using IL [bmim][Cl].

Electrospinning of polymer solution has turned up as a dominant technology for the preparation of fibrous materials with high specific surface area, controllable compositions, and high porosities for various applications. Particularly, the electrospinning of biopolymers for fabrication of biofiber has attracted numerous interests not only because of the renewable resources but also due to the advantageous characteristics of these biomacromolecules such as biocompatibility, biodegradability, and significant specificity [5]. With the aim to replace commonly used noxious solvents, ILs have been investigated as new, nonvolatile, and nonflammable media for the electrospinning of biopolymers. A typical electrospinning apparatus based on IL-assisted dissolution and regeneration of cellulose is shown in **Figure 2**.

**Figure 2.** A typical electrospinning apparatus for IL-based processing of biofibers.

charide materials, the end-product properties could be fitted without strict distinction for reinforcement and matrix. The present section briefly describes the IL-based preparation of different composite products from polysaccharides or their sources. Keeping in view the ethical standards, the sources may include plant cell walls or some sort of living species.

Although petroleum-based synthetic polymer products have offered excellent services to modern society, their extensive use has become a serious threat to the environment. Therefore, interest has been focused for the exploitation of natural biopolymers. Cellulose is a linear polysaccharide, which exhibited outstanding characteristics and broad range of applications as engineering material. Generally, it does not melt or dissolve in ordinary solvents, which makes its processing extremely difficult. Recently, the capabilities of ILs to dissolve cellulose have significantly impacted its processing for fabrication of biodegradable plastic films [9,

Dissolution and regeneration of cellulosic biofilms from IL [bmim][Cl] by using cotton pulp as raw cellulose source was reported by Liu et al. [33]. It was observed that solubility of cellulose could reach up to 13 wt% at 90°C in 7 h. Takegawa et al. [34] fabricated the bicomponent biopolymer film with cellulose and chitin each dissolved separately in the ILs [amim][Br] and [bmim][Cl], respectively, at 100 °C. The biofilms became more elastic by decreasing the relative ratio of chitin to cellulose in the final product. Further, **Figure 1** depicts the scheme of successful dissolution and regeneration of the native skin collagen in IL [bmim][Cl] [35]. The possible mechanism of dissolution of collagen in IL was also suggested which was based mainly on the

**Figure 1.** Schematic representation for preparation of collagen/cellulose composite materials using IL [bmim][Cl].

Electrospinning of polymer solution has turned up as a dominant technology for the preparation of fibrous materials with high specific surface area, controllable compositions, and high porosities for various applications. Particularly, the electrospinning of biopolymers for fabrication of biofiber has attracted numerous interests not only because of the renewable resources but also due to the advantageous characteristics of these biomacromolecules such

**4.1. Biofilms and biofibers**

120 Progress and Developments in Ionic Liquids

hydrogen bond breaking.

22].

Polaskova et al. [36] dissolved raw pine wood in IL [emim][OAc] and utilized wet electrospinning technique to transform it into microfibers (1–4 μm). It was noted that 5% wood loading in IL was the most appropriate concentration for electrospinning, and further increase in the biomass loading up to 10 % could complicate the process due to significant increase in the viscosity of solution. Similarly, electrospinning technology was utilized to obtain nonwoven nanoscale fibers from regenerated cellulose in the IL [bmim][Cl] [37]. The influence of the viscosity of biopolymer solution IL on the structure and size of the resulting biofiber was explored. Besides, Qin et al. [38] noted that high-molecular-weight and high purity chitin powder could be recovered after complete dissolution of raw crustacean shells in IL [emim] [OAc] (**Figure 3**). The direct fabrication of chitin fibers and films from the extract solution was also reported. The conversion of cellulose and starch into fibrous material by making their homogeneous solution in IL [bmim][Cl] was described [39]. As explained in **Figure 4**, the fine linear material was obtained by raising the viscous biopolymer mixture with the help of a spatula and subsequently soaked into acetone to remove IL and then vacuum dried. The fabricated fiber showed the compatabilized fibrous structure of ca. 100–200 μm with higher thermal stability than that of gel made from the same biopolymers in [bmim][Cl]. Branched fibers with the size range of micro- to nanometer were extruded from 10 % (w/w) of celluloseheparin solution in the IL by using electrospinning technique [40].

During dissolution of lignocellulose particle in IL, some bonds between major biopolymer components are broken down leading to the swelling of plant cell wall. Separation of hydrogen and oxygen atoms occurs as a result of interaction of IL with lignocellulose which causes it to dissolve due to the disruption in the intermolecular and intramolecular hydrogen bonds. A fraction of hemicellulose and lignin is decreased in the pretreated material due to partial removal of these components. The results of lignocellulosic characterization of OPF samples before and after pretreatment showed that untreated OPF contained 26.4 %, 47.6 %, and 26 % of cellulose, hemicellulose, and lignin, respectively. This composition was changed to, respectively, 48%, 38%, 14% for pretreatment with IL [bmim][Cl], and 41% cellulose, 49% hemicel-

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Investigation of the thermal properties of lignocellulosic materials is important to explore their suitability for thermo-mechanical processing of biocomposite where the glass transition temperature of certain thermoplastic polymers is above 200°C. The thermal decomposition profiles obtained by TGA for untreated IL-treated OPF fibers are sketched in **Figure 5a**. These measurements clearly indicated that the thermal stability of the fiber was increased after pretreatment with IL. Indeed, treatment with both ILs [bmim][Cl] and [emim][dep] had a significant impact on the thermal decomposition profiles of the OPF fibers, raising the temperature at which the thermal degradation was initiated. This increase in the thermal stability of lignocellulosic fiber could be due to the removal of some constituents with lower thermal stability than cellulose. Besides, the thermal decomposition profiles of the composites fabricated from both treated and untreated fibers are provided in **Figure 5b**. It could be noted that the modifications occurred in the thermal properties of the fibers as a result of ILs pretreatment embarked a positive influence on the thermal properties of the fabricated composite as implied by their relatively higher thermal stability. It was observed that IL pretreatment increased the 10 % loss temperature (T10) from 206°C to 225°C and 223°C for biocomposites manufactured from ILs [bmim][Cl] and [emim][dep] treated fibers, respectively. In fact, the improved thermal properties of the IL-treated biocomposites indicated that IL pretreatment was capable to increase the interfacial adhesion of the OPF fiber with polymer binder so that a higher amount of thermal energy was essential to break these fiber-binder

Furthermore, mechanical testing of the fabricated biocomposite panels before and after IL pretreatment in the flexural mode was also conducted to find the properties such as bending strength and bending modulus. **Figure 6** schematically depicts the details of the bending test. The obtained results plainly indicated that pretreatment of OPF fiber with both ILs [bmim][Cl] and [emim][dep] had a noteworthy impact on the bending strength and bending modulus. The bending strength of the untreated composite was found to be 4.9 MPa, which was increased to 8.3 and 8.9 MPa after the pretreatment with ILs [emim][dep] and [bmim][Cl], respectively. It could be possible that IL pretreatment reconstituted the lignocellulose structure by providing a more accessible interfacial area for thermoplastic binder flow during the thermo-molding step. This could lead to the improved fiber-matrix interfacial adhesion which consequently

lulose, and 10% lignin were achieved for IL [emim][dep]-treated fiber [44, 45].

linkages [44].

increased the bending properties [44].

**Figure 3.** Preparation of chitin fiber from IL solution of crustacean shells.

**Figure 4.** Cellulose-starch composite gel (a) and fiber (b).

### **4.2. Role of ionic liquids to provide alternative raw materials for wood composite industry**

Although woody biomass has been the most promising raw material for production of composite panels, the excessive deforestation and at the same time increasing demand for wood composite panels has evoked a critical raw material issue in the wood composite industry [41]. Therefore, extensive research has been focused for the possibilities of using lignocellulosic residues of agro-industries as a direct substitute for wood fiber for the manufacturing of products such as fiberboard, particleboard, plywood, and so on [42]. Every year, about 184.6 million tons of lignocellulosic solid waste is being generated worldwide only from the oil palm industry [43]. The effective utilization of these lignocellulosic materials for manufacturing of industrial products would be highly helpful for development of agriculturalbased economy in the rural areas.

Recently, we have reported the pretreatment of oil palm biomass with imidazolium-based ILs to produce cellulose-rich fiber (CRF) which was subsequently compounded with thermoplastic starch biopolymer binder to fabricate thermo-molded "green" composite board [44, 45]. Oil palm frond (OPF) samples were ground into particle size below 250 μm and pretreated with IL 1-butyl-3-methylimidazolium ([bmim][Cl]) or 1-ethyl-3-methylimidazolium diethyl phosphate ([emim][dep]) prior to mix with thermoplastic starch biopolymer. Finally, the compounded mixture was hot-pressed at 170°C and 25 MPa in a 30 ton Carver Laboratory Machine (CARVER, Inc., USA) [44].

During dissolution of lignocellulose particle in IL, some bonds between major biopolymer components are broken down leading to the swelling of plant cell wall. Separation of hydrogen and oxygen atoms occurs as a result of interaction of IL with lignocellulose which causes it to dissolve due to the disruption in the intermolecular and intramolecular hydrogen bonds. A fraction of hemicellulose and lignin is decreased in the pretreated material due to partial removal of these components. The results of lignocellulosic characterization of OPF samples before and after pretreatment showed that untreated OPF contained 26.4 %, 47.6 %, and 26 % of cellulose, hemicellulose, and lignin, respectively. This composition was changed to, respectively, 48%, 38%, 14% for pretreatment with IL [bmim][Cl], and 41% cellulose, 49% hemicellulose, and 10% lignin were achieved for IL [emim][dep]-treated fiber [44, 45].

**Figure 3.** Preparation of chitin fiber from IL solution of crustacean shells.

122 Progress and Developments in Ionic Liquids

**Figure 4.** Cellulose-starch composite gel (a) and fiber (b).

based economy in the rural areas.

Machine (CARVER, Inc., USA) [44].

**4.2. Role of ionic liquids to provide alternative raw materials for wood composite industry**

Although woody biomass has been the most promising raw material for production of composite panels, the excessive deforestation and at the same time increasing demand for wood composite panels has evoked a critical raw material issue in the wood composite industry [41]. Therefore, extensive research has been focused for the possibilities of using lignocellulosic residues of agro-industries as a direct substitute for wood fiber for the manufacturing of products such as fiberboard, particleboard, plywood, and so on [42]. Every year, about 184.6 million tons of lignocellulosic solid waste is being generated worldwide only from the oil palm industry [43]. The effective utilization of these lignocellulosic materials for manufacturing of industrial products would be highly helpful for development of agricultural-

Recently, we have reported the pretreatment of oil palm biomass with imidazolium-based ILs to produce cellulose-rich fiber (CRF) which was subsequently compounded with thermoplastic starch biopolymer binder to fabricate thermo-molded "green" composite board [44, 45]. Oil palm frond (OPF) samples were ground into particle size below 250 μm and pretreated with IL 1-butyl-3-methylimidazolium ([bmim][Cl]) or 1-ethyl-3-methylimidazolium diethyl phosphate ([emim][dep]) prior to mix with thermoplastic starch biopolymer. Finally, the compounded mixture was hot-pressed at 170°C and 25 MPa in a 30 ton Carver Laboratory Investigation of the thermal properties of lignocellulosic materials is important to explore their suitability for thermo-mechanical processing of biocomposite where the glass transition temperature of certain thermoplastic polymers is above 200°C. The thermal decomposition profiles obtained by TGA for untreated IL-treated OPF fibers are sketched in **Figure 5a**. These measurements clearly indicated that the thermal stability of the fiber was increased after pretreatment with IL. Indeed, treatment with both ILs [bmim][Cl] and [emim][dep] had a significant impact on the thermal decomposition profiles of the OPF fibers, raising the temperature at which the thermal degradation was initiated. This increase in the thermal stability of lignocellulosic fiber could be due to the removal of some constituents with lower thermal stability than cellulose. Besides, the thermal decomposition profiles of the composites fabricated from both treated and untreated fibers are provided in **Figure 5b**. It could be noted that the modifications occurred in the thermal properties of the fibers as a result of ILs pretreatment embarked a positive influence on the thermal properties of the fabricated composite as implied by their relatively higher thermal stability. It was observed that IL pretreatment increased the 10 % loss temperature (T10) from 206°C to 225°C and 223°C for biocomposites manufactured from ILs [bmim][Cl] and [emim][dep] treated fibers, respectively. In fact, the improved thermal properties of the IL-treated biocomposites indicated that IL pretreatment was capable to increase the interfacial adhesion of the OPF fiber with polymer binder so that a higher amount of thermal energy was essential to break these fiber-binder linkages [44].

Furthermore, mechanical testing of the fabricated biocomposite panels before and after IL pretreatment in the flexural mode was also conducted to find the properties such as bending strength and bending modulus. **Figure 6** schematically depicts the details of the bending test. The obtained results plainly indicated that pretreatment of OPF fiber with both ILs [bmim][Cl] and [emim][dep] had a noteworthy impact on the bending strength and bending modulus. The bending strength of the untreated composite was found to be 4.9 MPa, which was increased to 8.3 and 8.9 MPa after the pretreatment with ILs [emim][dep] and [bmim][Cl], respectively. It could be possible that IL pretreatment reconstituted the lignocellulose structure by providing a more accessible interfacial area for thermoplastic binder flow during the thermo-molding step. This could lead to the improved fiber-matrix interfacial adhesion which consequently increased the bending properties [44].

could be highly promising and green alternative route for efficient utilization of lignocellulosic

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Processing of natural fiber reinforcements and a wide variety of biopolymer materials with IL for manufacturing of various biocomposite products would achieve commercial success only if the advantages of ILs outweigh its limitations, the most important of which is prices of ILs as compared to the value of the material processed [46]. In general, the design of a certain pretreatment process and selection of appropriate conditions for efficient and economical processing of biocomposite materials would need an adjustment between conflicting objectives which could be restrain by the use multi-objective optimization tool in design and selection of

Process modeling for the pretreatment operation could be highly beneficial to estimate thermal energy consumption for dissolution step and the associated cost so that the comparison with other conventional pretreatment technologies would be made. Additionally, molecular-level simulation could be helpful to better understand the interaction mechanism of ILs with biological molecules [48, 49]. Furthermore, the physicochemical characteristics of IL may also be tuned by optimized selection of both cation and anion, and the careful selection of processing conditions may allow the more efficient utilization of ILs in the production of green composite materials from lignocellulosic and biopolymer-based sustainable raw materials [50].

**Table 1** provides a cost comparison for ILs with conventional organic solvents. Although the cost of ILs has always been one of the major issues, all ILs are not expensive, particularly, when considered at large scale [51]. Recently, the ILs derived from "low-cost" feedstock or from renewable raw materials may open new pathways for synthesis of cost-effective and competitive ILs for pretreatment of natural fibers and polymers for biocomposite manufac-

Ionic liquid [bmim]Cl 0.262

Solvent MeOH 0.041

**Pretreatment solvent Price (\$/g)**

[emim]Cl 0.325 [bmim][OAc] 0.696 [amim]Cl 6.250

DMSO 0.453 NMMO 2.010 DMF 0.094 H2SO4 0.574

**Table 1.** Price comparison of some ILs used for lignocellulose dissolution with ordinary biomass pretreatment solvents.

biomass in the wood composite industries.

particular pretreatment process [47].

**5. Future prospects**

turing [52, 53].

**Figure 5.** TGA profiles for untreated and IL-treated OPF fibers (a) and the fabricated composites (b).

**Figure 6.** Schematics of the flexural test of composite board. P = peak load, P1 = load at proportional limit, y1 = deflection at proportional limit, and L, b, and d = length, width, and depth of specimen.

Thus, ionic liquid-treated composite panels exhibited superior mechanical and thermal properties because of partial removal of noncellulosic impurities from the lignocellulosic fiber after IL pretreatment. These studies demonstrated that IL-facilitated pretreatment technology could be highly promising and green alternative route for efficient utilization of lignocellulosic biomass in the wood composite industries.

### **5. Future prospects**

**Figure 5.** TGA profiles for untreated and IL-treated OPF fibers (a) and the fabricated composites (b).

124 Progress and Developments in Ionic Liquids

**Figure 6.** Schematics of the flexural test of composite board. P = peak load, P1 = load at proportional limit, y1 = deflec-

Thus, ionic liquid-treated composite panels exhibited superior mechanical and thermal properties because of partial removal of noncellulosic impurities from the lignocellulosic fiber after IL pretreatment. These studies demonstrated that IL-facilitated pretreatment technology

tion at proportional limit, and L, b, and d = length, width, and depth of specimen.

Processing of natural fiber reinforcements and a wide variety of biopolymer materials with IL for manufacturing of various biocomposite products would achieve commercial success only if the advantages of ILs outweigh its limitations, the most important of which is prices of ILs as compared to the value of the material processed [46]. In general, the design of a certain pretreatment process and selection of appropriate conditions for efficient and economical processing of biocomposite materials would need an adjustment between conflicting objectives which could be restrain by the use multi-objective optimization tool in design and selection of particular pretreatment process [47].

Process modeling for the pretreatment operation could be highly beneficial to estimate thermal energy consumption for dissolution step and the associated cost so that the comparison with other conventional pretreatment technologies would be made. Additionally, molecular-level simulation could be helpful to better understand the interaction mechanism of ILs with biological molecules [48, 49]. Furthermore, the physicochemical characteristics of IL may also be tuned by optimized selection of both cation and anion, and the careful selection of processing conditions may allow the more efficient utilization of ILs in the production of green composite materials from lignocellulosic and biopolymer-based sustainable raw materials [50].

**Table 1** provides a cost comparison for ILs with conventional organic solvents. Although the cost of ILs has always been one of the major issues, all ILs are not expensive, particularly, when considered at large scale [51]. Recently, the ILs derived from "low-cost" feedstock or from renewable raw materials may open new pathways for synthesis of cost-effective and competitive ILs for pretreatment of natural fibers and polymers for biocomposite manufacturing [52, 53].


**Table 1.** Price comparison of some ILs used for lignocellulose dissolution with ordinary biomass pretreatment solvents.

The major aspects that should be focused for the future research work in IL-based processing of various biofibers and biopolymers could be summarized as:

pretreatment for cellulosic biofuel production," *Biotechnology for Biofuels*, vol. 7, p. 1,

Green Composites from Ionic Liquid-Assisted Processing of Sustainable Resources: A Brief Overview

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### **Acknowledgements**

The authors would like to acknowledge Universiti Teknologi PETRONAS for University Research Internal Fund (URIF) under grant 0153AA-B80 and Universiti Sains Malaysia for research grant 203.PBAHAN.6071337 for this research work.

### **Author details**

Hamayoun Mahmood1 , Muhammad Moniruzzaman1\*, Suzana Yusup1 and Hazizan Md. Akil2

\*Address all correspondence to: m.moniruzzaman@petronas.com.my

1 Department of Chemical Engineering, Universiti Teknologi Petronas, Perak Darul Ridzuan, Malaysia

2 School of Materials and Minerals Resources Engineering, Universiti Sains Malaysia, Penang, Malaysia

### **References**


pretreatment for cellulosic biofuel production," *Biotechnology for Biofuels*, vol. 7, p. 1, 2014.

The major aspects that should be focused for the future research work in IL-based processing

The authors would like to acknowledge Universiti Teknologi PETRONAS for University Research Internal Fund (URIF) under grant 0153AA-B80 and Universiti Sains Malaysia for

, Muhammad Moniruzzaman1\*, Suzana Yusup1

1 Department of Chemical Engineering, Universiti Teknologi Petronas, Perak Darul Ridzuan,

[1] M. Moniruzzaman, H. Mahmood, M. F. Ibrahim, S. Yusup, and Y. Uemura, "Effects of pressure and temperature on the dissolution of cellulose in ionic liquids," *Advanced*

[2] N. M. Konda, J. Shi, S. Singh, H. W. Blanch, B. A. Simmons, and D. Klein-Marcuschamer, "Understanding cost drivers and economic potential of two variants of ionic liquid

2 School of Materials and Minerals Resources Engineering, Universiti Sains Malaysia,

and

of various biofibers and biopolymers could be summarized as:

• "Dry" pretreatment under high biomass loading

• Applicability to a wide range of raw materials

• Optimization of IL recycling to reduce losses

• "Greenness" of ILs (environmental and health impact)

research grant 203.PBAHAN.6071337 for this research work.

\*Address all correspondence to: m.moniruzzaman@petronas.com.my

*Materials Research*, vol. 1133, pp. 588-592, 2016.

• Development of low-cost ILs

126 Progress and Developments in Ionic Liquids

• Minimum dissolution time

**Acknowledgements**

**Author details**

Hazizan Md. Akil2

Penang, Malaysia

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**Section 3**

**Electrochemistry**

**Section 3**

**Electrochemistry**

**Chapter 7**

**Provisional chapter**

**Ionic Liquids as Electrodeposition Additives**

**Ionic Liquids as Electrodeposition Additives** 

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution,

© 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

and reproduction in any medium, provided the original work is properly cited.

Ionic liquids (ILs) are molten salts with a melting point of 100°C or below and solely consist of cations and anions. As a kind of novel green solvent, ILs have been obtained broad and deep investigations, and enormous progresses in various fields have been made during the recent 20 years. Despite the fact that the application studies of ILs have been proposed in various fields, no processes have yet been developed to an industrial scale. However, the main interests are still focused on their industrial applications. In this chapter, two perspective applications of ILs in electrochemical fields including additives for metal electrodeposition and inhibitors for metal anti-corrosion were introduced.

**Keywords:** ionic liquids, additives, metal electrodeposition, corrosion inhibitors,

Additives are widely used in electrodeposition of metals and alloys due to their special functions in the deposition process. These additives are found to affect both the deposition and crystal-building processes through their adsorbates at the electrode surface [1]. Traditional colloidal and some organic additives have gained wide industrial use and achieved good additive effect, even though they can be decomposed easily and are not environmentally friendly due to their disadvantages, such as thermal stability, bad chemical and high toxicity. Consequently, there is a continuing search for better additives that combine good stability, high efficiency and environmentally friendliness. The effect of corrosion inhibitors is similar to that of electrodeposition additives to some extent, as their excellent corrosion resistance performance on metals is attributed to their adsorption on the metal surface, which protects the metal from attack by the acidic solutions [2]. However, most commercially available

**and Corrosion Inhibitors**

**and Corrosion Inhibitors**

Additional information is available at the end of the chapter

Additional information is available at the end of the chapter

Zhang Qibo and Hua Yixin

Zhang Qibo and Hua Yixin

http://dx.doi.org/10.5772/65807

**Abstract**

adsorption

**1. Introduction**

### **Ionic Liquids as Electrodeposition Additives and Corrosion Inhibitors Ionic Liquids as Electrodeposition Additives and Corrosion Inhibitors**

Zhang Qibo and Hua Yixin Zhang Qibo and Hua Yixin

Additional information is available at the end of the chapter Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/65807

#### **Abstract**

Ionic liquids (ILs) are molten salts with a melting point of 100°C or below and solely consist of cations and anions. As a kind of novel green solvent, ILs have been obtained broad and deep investigations, and enormous progresses in various fields have been made during the recent 20 years. Despite the fact that the application studies of ILs have been proposed in various fields, no processes have yet been developed to an industrial scale. However, the main interests are still focused on their industrial applications. In this chapter, two perspective applications of ILs in electrochemical fields including additives for metal electrodeposition and inhibitors for metal anti-corrosion were introduced.

**Keywords:** ionic liquids, additives, metal electrodeposition, corrosion inhibitors, adsorption

### **1. Introduction**

Additives are widely used in electrodeposition of metals and alloys due to their special functions in the deposition process. These additives are found to affect both the deposition and crystal-building processes through their adsorbates at the electrode surface [1]. Traditional colloidal and some organic additives have gained wide industrial use and achieved good additive effect, even though they can be decomposed easily and are not environmentally friendly due to their disadvantages, such as thermal stability, bad chemical and high toxicity. Consequently, there is a continuing search for better additives that combine good stability, high efficiency and environmentally friendliness. The effect of corrosion inhibitors is similar to that of electrodeposition additives to some extent, as their excellent corrosion resistance performance on metals is attributed to their adsorption on the metal surface, which protects the metal from attack by the acidic solutions [2]. However, most commercially available

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

picking inhibitors are toxic compounds that should be replaced by new environmentallyfriendly ones. Research studies in the field of 'green' corrosion inhibitors have been aimed at using cheap, effective molecules with low or 'zero' environmental impact [3].

reproducible operation in these ILs requires either a strictly controlled inert gas atmosphere with extremely low water concentration or at least closed vessel conditions with limited contamination [49], the applications of ILs in electrodeposition additives and corrosion inhibitors are not only cost accepted with the trace amount of consumption, but also have more

Ionic Liquids as Electrodeposition Additives and Corrosion Inhibitors

http://dx.doi.org/10.5772/65807

137

On the basis of these discussions, it is accepted that the applications of ILs as additives for metal electrodeposition and corrosion inhibitors are with favourable industrial application prospect, as in our opinion they are very likely to initiatively realize the zero breakthrough of

In modern electrodeposition and electrowinning practice, it is well known that the introduction of one or more inorganic or organic additives in the electrolyte leads to produce smooth, free of voids and compacted metallic deposits at the cathode. The quantity of additives required is always considerably small, but their action is often specific [50]. Although the number of these additives studied in electrodeposition is very high, their action mechanism can be divided into two main categories, which are levelling additive and brightening additive. Levelling additive [51] has been defined as the additive with the ability to produce deposit relatively thinner on small protrusions and then decrease in depth on height of the small surface irregularities. While brightening additive [52] can be defined as the ability to obtain fine deposits with the crystallites smaller than the wavelengths of visible light. In this section, we reported the use of a series of novel 1-alkyl-3-methylimidazolium hydrosulphate

and [OMIM]HSO4

sition from the acidic sulphate electrolyte. Furthermore, effects of ILs on copper electrodepo-

Zinc electrodeposition is very sensitive when it comes into contact with some types of impurities in augmenting simultaneous evolution of hydrogen during zinc ion electroreduction from aqueous solutions. Apart from occluding hydrogen into the zinc deposit, hydrogen evolution will increase specific electric energy consumption and decrease current efficiency (CE), bringing about an increment in the level of internal stress to produce pitted deposits [19]. To counteract the harmful effect of metallic impurities, achieve high CE and produce a smooth, levelled and dense cathodic deposit, additives such as glue and gum arabic are most often used. In addition, enormous organic additives [53–62] have been proposed for use as additives and some authorities have reported them to work better than glue or gum arabic. We have reported that 1-alkyl-3-methylimidazolium hydro-

inhibiting effect on Zn2+ electroreduction and all were efficient as levelling agents for zinc

as additives in zinc electrodepo-

and [OMIM] HSO4 [19]) had a pronounced

practical operability.

industrial applications of ILs.

ILs, namely, [BMIM]HSO4

electrodeposition.

**3.1. Additives for zinc electrodeposition**

sulphate ILs ([BMIM]HSO4 [18], [HMIM]HSO4

**3. Additives for metal electrodeposition**

, [HMIM]HSO4

sition from acidic sulphate electrolyte were also introduced.

Ionic liquids (ILs) are organic salts that are liquids at ambient temperature and comprised entirely of organic cations and organic/inorganic anions. Due to the unique structure characteristics, ILs have many attractive properties and attract a great deal of interest in various fields [4–17]. Some of the most important prosperities of ILs are their thermal stability and avirulence, which make them potential additives for metal electrodeposition and green inhibitors for metal anti-corrosion. In our previous studies, alkylimidazolium, alkylpyridinium and quaternary ammonium-based ionic liquids were observed to be an excellent levelling agent in zinc [18–22] and copper [23–25] electrodeposition and showed favourable corrosion resistant on metals such as aluminium [26], copper [27] and mild steel [2].

### **2. Why use ILs as electrodeposition additives and corrosion inhibitors?**

As mentioned above, the effect mechanism of electrodeposition additives and corrosion inhibitors is quite similar. Both are dependent on their surface adsorbability to achieve the expected additive effect. The main difference from each other could be the use of additives is under electric field and their electrode surface adsorption behaviour will be influenced by the electric field distribution, while the corrosion inhibitors are used without galvanization and their adsorption mainly depends on certain physico-chemical properties of the inhibitor group, such as electron density at the donor atom, *π*-orbital character and the electronic structure of the molecule [28].

ILs are composed entirely of organic cations and organic/inorganic anions that are liquid at low temperature. Their relatively high cationic configuration makes them readily adsorb on the cathode surface under the electric field. In addition, some functional groups such as –C=N– group and electronegative nitrogen in the molecule of imidazolium ILs enables them spontaneous adsorption on the metal surface due to the specific interaction between these functional groups and the metal surface [2]. Furthermore, the high thermal stability, negligible vapour pressure and environmentally benign characteristics of ILs allow them to be considered as very promising replacements for the traditional volatile organic solvents. Therefore, it is hopeful to overcome these defects of traditional additives and toxic organic corrosion inhibitors, and help to realize additives with good stability and inhibitors with avirulence by using ILs as metal electrodeposition additives and corrosion inhibitors, respectively.

The application studies of ILs have been proposed varying from precious metal processing [29–40] to mineral leaching [41–44]; however, very few have come to practical fruition although several are at a pilot scale [45–47]. Except some technical difficulties that are hard to solve at our present state of knowledge [48], the relatively prohibitive high cost of ILs is also a main reason for delaying their industrial application [44]. In comparing to the use ILs as electrolytes for metal electrodeposition and rechargeable batteries, where the reproducible operation in these ILs requires either a strictly controlled inert gas atmosphere with extremely low water concentration or at least closed vessel conditions with limited contamination [49], the applications of ILs in electrodeposition additives and corrosion inhibitors are not only cost accepted with the trace amount of consumption, but also have more practical operability.

On the basis of these discussions, it is accepted that the applications of ILs as additives for metal electrodeposition and corrosion inhibitors are with favourable industrial application prospect, as in our opinion they are very likely to initiatively realize the zero breakthrough of industrial applications of ILs.

### **3. Additives for metal electrodeposition**

picking inhibitors are toxic compounds that should be replaced by new environmentallyfriendly ones. Research studies in the field of 'green' corrosion inhibitors have been aimed at

Ionic liquids (ILs) are organic salts that are liquids at ambient temperature and comprised entirely of organic cations and organic/inorganic anions. Due to the unique structure characteristics, ILs have many attractive properties and attract a great deal of interest in various fields [4–17]. Some of the most important prosperities of ILs are their thermal stability and avirulence, which make them potential additives for metal electrodeposition and green inhibitors for metal anti-corrosion. In our previous studies, alkylimidazolium, alkylpyridinium and quaternary ammonium-based ionic liquids were observed to be an excellent levelling agent in zinc [18–22] and copper [23–25] electrodeposition and showed favourable corrosion

**2. Why use ILs as electrodeposition additives and corrosion inhibitors?**

As mentioned above, the effect mechanism of electrodeposition additives and corrosion inhibitors is quite similar. Both are dependent on their surface adsorbability to achieve the expected additive effect. The main difference from each other could be the use of additives is under electric field and their electrode surface adsorption behaviour will be influenced by the electric field distribution, while the corrosion inhibitors are used without galvanization and their adsorption mainly depends on certain physico-chemical properties of the inhibitor group, such as electron density at the donor atom, *π*-orbital character and the electronic

ILs are composed entirely of organic cations and organic/inorganic anions that are liquid at low temperature. Their relatively high cationic configuration makes them readily adsorb on the cathode surface under the electric field. In addition, some functional groups such as –C=N– group and electronegative nitrogen in the molecule of imidazolium ILs enables them spontaneous adsorption on the metal surface due to the specific interaction between these functional groups and the metal surface [2]. Furthermore, the high thermal stability, negligible vapour pressure and environmentally benign characteristics of ILs allow them to be considered as very promising replacements for the traditional volatile organic solvents. Therefore, it is hopeful to overcome these defects of traditional additives and toxic organic corrosion inhibitors, and help to realize additives with good stability and inhibitors with avirulence by using ILs as metal electrodeposition additives and corrosion inhibitors,

The application studies of ILs have been proposed varying from precious metal processing [29–40] to mineral leaching [41–44]; however, very few have come to practical fruition although several are at a pilot scale [45–47]. Except some technical difficulties that are hard to solve at our present state of knowledge [48], the relatively prohibitive high cost of ILs is also a main reason for delaying their industrial application [44]. In comparing to the use ILs as electrolytes for metal electrodeposition and rechargeable batteries, where the

using cheap, effective molecules with low or 'zero' environmental impact [3].

resistant on metals such as aluminium [26], copper [27] and mild steel [2].

structure of the molecule [28].

136 Progress and Developments in Ionic Liquids

respectively.

In modern electrodeposition and electrowinning practice, it is well known that the introduction of one or more inorganic or organic additives in the electrolyte leads to produce smooth, free of voids and compacted metallic deposits at the cathode. The quantity of additives required is always considerably small, but their action is often specific [50]. Although the number of these additives studied in electrodeposition is very high, their action mechanism can be divided into two main categories, which are levelling additive and brightening additive. Levelling additive [51] has been defined as the additive with the ability to produce deposit relatively thinner on small protrusions and then decrease in depth on height of the small surface irregularities. While brightening additive [52] can be defined as the ability to obtain fine deposits with the crystallites smaller than the wavelengths of visible light. In this section, we reported the use of a series of novel 1-alkyl-3-methylimidazolium hydrosulphate ILs, namely, [BMIM]HSO4 , [HMIM]HSO4 and [OMIM]HSO4 as additives in zinc electrodeposition from the acidic sulphate electrolyte. Furthermore, effects of ILs on copper electrodeposition from acidic sulphate electrolyte were also introduced.

#### **3.1. Additives for zinc electrodeposition**

Zinc electrodeposition is very sensitive when it comes into contact with some types of impurities in augmenting simultaneous evolution of hydrogen during zinc ion electroreduction from aqueous solutions. Apart from occluding hydrogen into the zinc deposit, hydrogen evolution will increase specific electric energy consumption and decrease current efficiency (CE), bringing about an increment in the level of internal stress to produce pitted deposits [19]. To counteract the harmful effect of metallic impurities, achieve high CE and produce a smooth, levelled and dense cathodic deposit, additives such as glue and gum arabic are most often used. In addition, enormous organic additives [53–62] have been proposed for use as additives and some authorities have reported them to work better than glue or gum arabic. We have reported that 1-alkyl-3-methylimidazolium hydrosulphate ILs ([BMIM]HSO4 [18], [HMIM]HSO4 and [OMIM] HSO4 [19]) had a pronounced inhibiting effect on Zn2+ electroreduction and all were efficient as levelling agents for zinc electrodeposition.

**Figure 1(a)** shows the effect of [BMIM]HSO4 on the CE during zinc electrodeposition. The CE increases with the initial addition of [BMIM]HSO4 and steadily decreases at higher concentrations. Without additives, the CE is ~89%, whereas at 5 mg dm−3 [BMIM]HSO4 the CE was ~92.7% and then fell to 87.8% at 50 mg dm−3. The trend observed in CE with increasing [HMIM] HSO4 and [OMIM]HSO4 concentration is similar to that for [BMIM]HSO4 , where we obtained a CE of 92.9–92.2% with the addition of 2 mg·dm−3 [HMIM]HSO4 and 1 mg dm−3 [OMIM] HSO4 , respectively, but at higher concentrations there is a reduction. The CE decreases with increasing additive concentrations in the order [OMIM]HSO4 > [HMIM]HSO4 > [BMIM]HSO4 , which reflects increasing absorbability at the electrode surface with increasing molecular size and hence molecular mass [62, 63].

The investigated additives significantly changed the morphology of the zinc deposits as compared with those obtained from solutions without additives, as shown in **Figure 2**. The zinc deposit obtained from addition-free electrolyte is bright but not smooth and consists of hexagonal platelets of moderate size (**Figure 2a**). Introducing the additives into the solution did not affect the shape of the crystals but improved the quality of deposits by reducing the platelet sizes and giving smooth and compact deposits (**Figure 2b** and **d**) with changing the

impurities, such as copper, iron, cobalt, nickel and lead, to some extent and led to a reduction of the impurity content in the zinc deposits and improved the CE and the quality of

to have a catalytic effect on oxygen evolution by stimulating the reaction rate constant.

thermal stabilities compared with traditional industrial additives, gelatine and gum arabic. The inhibition effects of gelatine and gum arabic on the zinc electrocrystallization weakened significantly under observation because of their partial degradation after 12-h long-time successive electrolysis and high temperature (90°C) treatments. In contrast, 24-h long-term successive electrolysis and high-temperature treatments have no effect on the

the cathodic deposits [65]. Considering the anodic reaction, [BMIM]HSO4

was observed to relieve the harmful effect of

Ionic Liquids as Electrodeposition Additives and Corrosion Inhibitors

can markedly reduce the oxygen evolution charge

for zinc electrodeposition was also found in a typical

can induce the for-

) [22], in which [BMIM]HSO4

manifested superior chemical and

was also found

http://dx.doi.org/10.5772/65807

139

preferred crystal orientations [18, 19].

Moreover, the addition of [BMIM]HSO4

Introduction of 5 mg dm−3 [BMIM]HSO4

The inhibition effect of [BMIM]HSO4

+ 0.5 M Na<sup>2</sup>

–5 mg·dm−3, (c) [HMIM]HSO4 –10 mg·dm−3 and (d) [OMIM]HSO4 –10 mg·dm−3 [18, 19].

SO4

mation of finer-grained deposits by the adsorption of additive in the first stages of deposition

**Figure 2.** Scanning electron micrographs of zinc deposits in the presence of different additives. (a) Blank, (b) [BMIM]

plating solution (0.1 M ZnSO4

HSO4

activity of [BMIM]HSO4

transfer resistance by at least 50% [66]. [BMIM]HSO4

[67].

The cyclic voltammograms recorded for zinc electrodeposition from acidic sulphate solution in the absence and presence of ILs additive [BMIM]HSO4 are presented in **Figure 1(b)**. The voltammograms were initiated at point '*A*' (−0.70 V versus SCE), scanned in the negative direction and reversed at −1.30 V in the positive direction. The nucleation overpotential (NOP) for zinc deposition on an aluminium substrate is defined as the potential difference between the electroreduction potential of zinc ions at '*B*' and the crossover potential at '*D*', which is regarded as an indicator of the extent of polarization of a cathode [64]. It is clear that the addition of additives has a significant effect on the zinc electrocrystallization process as shown in **Figures 1(b)** and **2**–**4**, where the NOP values increase substantially, along with the reduction of the cathodic process area. The strong adsorption of the additives on the electrode surface is usually held to be responsible for this. The extent of absorption appears to be in the order [OMIM]HSO4 > [HMIM]HSO4 > [BMIM]HSO4 , which shows the adsorbability of the studied additives and reflects their effect on the process of deposition. The analysis of the kinetic parameters [18, 19] indicates that the presence of ILs does not have any significant effect on the Tafel slopes and transfer coefficients, suggesting that they do not change the zinc electrodeposition mechanism in the absence of additives.

**Figure 1.** (a) Effect of additives on current efficiency during zinc electrodeposition. () [BMIM]HSO4 , (●) [HMIM]HSO4 and () [OMIM]HSO4 . (b) Cyclic voltammograms of acidic zinc sulphate solutions in the absence and presence of 5 mg dm−3 different additives. (1) Blank, (2) [BMIM]HSO4 , (3) [HMIM]HSO4 and (4) [OMIM]HSO4 [18, 19].

The investigated additives significantly changed the morphology of the zinc deposits as compared with those obtained from solutions without additives, as shown in **Figure 2**. The zinc deposit obtained from addition-free electrolyte is bright but not smooth and consists of hexagonal platelets of moderate size (**Figure 2a**). Introducing the additives into the solution did not affect the shape of the crystals but improved the quality of deposits by reducing the platelet sizes and giving smooth and compact deposits (**Figure 2b** and **d**) with changing the preferred crystal orientations [18, 19].

**Figure 1(a)** shows the effect of [BMIM]HSO4

and [OMIM]HSO4

138 Progress and Developments in Ionic Liquids

and hence molecular mass [62, 63].

the order [OMIM]HSO4

and () [OMIM]HSO4

5 mg dm−3 different additives. (1) Blank, (2) [BMIM]HSO4

HSO4

HSO4

increases with the initial addition of [BMIM]HSO4

trations. Without additives, the CE is ~89%, whereas at 5 mg dm−3 [BMIM]HSO4

a CE of 92.9–92.2% with the addition of 2 mg·dm−3 [HMIM]HSO4

increasing additive concentrations in the order [OMIM]HSO4

tion in the absence and presence of ILs additive [BMIM]HSO4

> [HMIM]HSO4

**Figure 1.** (a) Effect of additives on current efficiency during zinc electrodeposition. () [BMIM]HSO4

. (b) Cyclic voltammograms of acidic zinc sulphate solutions in the absence and presence of

, (3) [HMIM]HSO4 and (4) [OMIM]HSO4

electrodeposition mechanism in the absence of additives.

~92.7% and then fell to 87.8% at 50 mg dm−3. The trend observed in CE with increasing [HMIM]

which reflects increasing absorbability at the electrode surface with increasing molecular size

The cyclic voltammograms recorded for zinc electrodeposition from acidic sulphate solu-

The voltammograms were initiated at point '*A*' (−0.70 V versus SCE), scanned in the negative direction and reversed at −1.30 V in the positive direction. The nucleation overpotential (NOP) for zinc deposition on an aluminium substrate is defined as the potential difference between the electroreduction potential of zinc ions at '*B*' and the crossover potential at '*D*', which is regarded as an indicator of the extent of polarization of a cathode [64]. It is clear that the addition of additives has a significant effect on the zinc electrocrystallization process as shown in **Figures 1(b)** and **2**–**4**, where the NOP values increase substantially, along with the reduction of the cathodic process area. The strong adsorption of the additives on the electrode surface is usually held to be responsible for this. The extent of absorption appears to be in

> [BMIM]HSO4

the studied additives and reflects their effect on the process of deposition. The analysis of the kinetic parameters [18, 19] indicates that the presence of ILs does not have any significant effect on the Tafel slopes and transfer coefficients, suggesting that they do not change the zinc

concentration is similar to that for [BMIM]HSO4

, respectively, but at higher concentrations there is a reduction. The CE decreases with

on the CE during zinc electrodeposition. The CE

> [HMIM]HSO4

and steadily decreases at higher concen-

the CE was

, where we obtained

> [BMIM]HSO4

, (●) [HMIM]HSO4

[18, 19].

,

and 1 mg dm−3 [OMIM]

are presented in **Figure 1(b)**.

, which shows the adsorbability of

Moreover, the addition of [BMIM]HSO4 was observed to relieve the harmful effect of impurities, such as copper, iron, cobalt, nickel and lead, to some extent and led to a reduction of the impurity content in the zinc deposits and improved the CE and the quality of the cathodic deposits [65]. Considering the anodic reaction, [BMIM]HSO4 was also found to have a catalytic effect on oxygen evolution by stimulating the reaction rate constant. Introduction of 5 mg dm−3 [BMIM]HSO4 can markedly reduce the oxygen evolution charge transfer resistance by at least 50% [66]. [BMIM]HSO4 manifested superior chemical and thermal stabilities compared with traditional industrial additives, gelatine and gum arabic. The inhibition effects of gelatine and gum arabic on the zinc electrocrystallization weakened significantly under observation because of their partial degradation after 12-h long-time successive electrolysis and high temperature (90°C) treatments. In contrast, 24-h long-term successive electrolysis and high-temperature treatments have no effect on the activity of [BMIM]HSO4 [67].

**Figure 2.** Scanning electron micrographs of zinc deposits in the presence of different additives. (a) Blank, (b) [BMIM] HSO4 –5 mg·dm−3, (c) [HMIM]HSO4 –10 mg·dm−3 and (d) [OMIM]HSO4 –10 mg·dm−3 [18, 19].

The inhibition effect of [BMIM]HSO4 for zinc electrodeposition was also found in a typical plating solution (0.1 M ZnSO4 + 0.5 M Na<sup>2</sup> SO4 ) [22], in which [BMIM]HSO4 can induce the formation of finer-grained deposits by the adsorption of additive in the first stages of deposition (**Figure 3a** and **d**). The corrosion behaviour of Q235 steel with coating by a thin layer of zinc in the presence of [BMIM]HSO4 exhibited more excellent protection of the base metal in comparison to the additive-free one in 3.5% NaCl solution (**Figure 3e**).

be explained as follows: cathodic surface can effectively adsorb [BMIM]HSO4

in the electrolyte, the additive molecules adsorb at the cathodic surface and

<sup>2</sup> Cu e Cu +− + + → (1)

Ionic Liquids as Electrodeposition Additives and Corrosion Inhibitors

Cu e Cu + − + ↔ (2)

+ + + ↔− (3)

on the complex-plane impedance diagrams is illustrated in **Figure 4**.

4 ads <sup>4</sup> [Cu [BMIM]HSO ] e Cu [BMIM]HSO + − <sup>−</sup> +→ + (4)

ions produced from Eq. (1) to form a copper-[BMIM]HSO4

Cu [BMIM]HSO [Cu [BMIM]HSO ] <sup>4</sup> 4 ads

We attribute the inhibition effect of [BMIM]HSO4 on the process of copper electroreduction to the adsorption of the complex at active sites, where it may receive an electron from the cathode and discharge copper atoms which are embedded at the active sites (Eq. (4)). The [BMIM]

The complex-plane impedance spectra obtained from the additive-free solution exhibit two capacitive features at high frequencies followed by an inductive loop at low-frequency values (**Figure 4a**). On the other hand, two intermediate-frequency capacitive features, far more separated at high concentration of additive (**Figure 4b** and **c**) in the presence of [BMIM]HSO4

are obtained; that implies that two adsorbed species play a role in the process of copper electrodeposition and the addition of [BMIM]HSO4 brought about a change in the copper electro-

**Figure 5** shows the SEM images of copper deposits obtained by small-scale electrolysis from

**Figure 4.** Impedance plots for copper electrodeposition at *E*= −0.15 V in the absence (a) and presence of [BMIM]HSO4

**Figure 5(b** and **c)**, introducing [BMIM]HSO4 has brought a notable effect on the surface quality of the copper deposits as compared with those gained from the additive-free bath

the sulphate electrolyte in the absence and presence of [BMIM]HSO4

will be released and can then become a complex.

[BMIM]HSO4

HSO4

interact with the Cu+

Eq. (3) by these equations:

The effect of [BMIM]HSO4

deposition mechanism.

(b) 10 mg·dm−3, (c) 50 mg·dm−3 [21].

. When we add

http://dx.doi.org/10.5772/65807

complex

141

,

. As it can be seen from

**Figure 3.** SEM micrographs of zinc electrodeposits produced on GC electrode from solution 0.1 M ZnSO4 + 0.5 M NaSO4 , pH 2.6 in the absence and presence of [BMIM]HSO4 . (a) Blank, (b) 100 mg dm−3, (c) 250 mg dm−3 and (d) 500 mg dm−3. The deposition potential was −1.30 V and *Q* = 1.00 ± 0.01 C (e). Potentiodynamic polarization curves of Q235 steel substrate free and coated by a thin layer of zinc from the baths in the presence of different concentrations of [BMIM]HSO4 in 3.5% NaCl solutions. The deposition charges, *Qd*, were approximately 1.00 ± 0.01 C [22].

#### **3.2. Additives for copper electrodeposition**

Copper electrodeposition from acidic sulphate electrolyte with different small amounts of certain additives has been investigated extensively and it is well known that they lead to significant changes in the properties and orientation of the deposit [68]. Appropriate amounts of additives are necessary for the formation of fine-grained, smooth and compact deposits. Additives, such as thiourea [69–71], gelatine [72, 73] and animal glue [74], are commonly used as levelling and brightening agents in copper electrodeposition and electrowinning in order to produce smooth, free of voids and porosity copper deposits. Although advances have been made, in many cases the use of these additives is still carried out in an empirical way, and there are still many unknown aspects concerning the mechanism of action of additives.

Recently, we have investigated the effect of [BMIM]HSO4 on copper electrodeposition [21]. Its effects on the morphology of cathodic deposits and the kinetic parameters of the cathodic process were deeply studied. Similar to the case for zinc electrodeposition, the addition of [BMIM]HSO4 was found to have a strong inhibiting effect on the electroreduction process, and the effect is more pronounced at higher additive concentrations.

The kinetic parameters obtained show that the presence of [BMIM]HSO4 has an inhibiting effect on the kinetics of the copper discharge process with slight changes in the copper electrodeposition reaction pathway, indicated by the changes in Tafel slopes and the corresponding charge transfer coefficient [21]. A possible mechanism of the action of this additive may be explained as follows: cathodic surface can effectively adsorb [BMIM]HSO4 . When we add [BMIM]HSO4 in the electrolyte, the additive molecules adsorb at the cathodic surface and interact with the Cu+ ions produced from Eq. (1) to form a copper-[BMIM]HSO4 complex Eq. (3) by these equations:

(**Figure 3a** and **d**). The corrosion behaviour of Q235 steel with coating by a thin layer of zinc

Copper electrodeposition from acidic sulphate electrolyte with different small amounts of certain additives has been investigated extensively and it is well known that they lead to significant changes in the properties and orientation of the deposit [68]. Appropriate amounts of additives are necessary for the formation of fine-grained, smooth and compact deposits. Additives, such as thiourea [69–71], gelatine [72, 73] and animal glue [74], are commonly used as levelling and brightening agents in copper electrodeposition and electrowinning in order to produce smooth, free of voids and porosity copper deposits. Although advances have been made, in many cases the use of these additives is still carried out in an empirical way, and there are still many unknown aspects concerning the mechanism of action of

deposition potential was −1.30 V and *Q* = 1.00 ± 0.01 C (e). Potentiodynamic polarization curves of Q235 steel substrate free and coated by a thin layer of zinc from the baths in the presence of different concentrations of [BMIM]HSO4

**Figure 3.** SEM micrographs of zinc electrodeposits produced on GC electrode from solution 0.1 M ZnSO4

NaCl solutions. The deposition charges, *Qd*, were approximately 1.00 ± 0.01 C [22].

Its effects on the morphology of cathodic deposits and the kinetic parameters of the cathodic process were deeply studied. Similar to the case for zinc electrodeposition, the addition of [BMIM]HSO4 was found to have a strong inhibiting effect on the electroreduction process, and

effect on the kinetics of the copper discharge process with slight changes in the copper electrodeposition reaction pathway, indicated by the changes in Tafel slopes and the corresponding charge transfer coefficient [21]. A possible mechanism of the action of this additive may

parison to the additive-free one in 3.5% NaCl solution (**Figure 3e**).

exhibited more excellent protection of the base metal in com-

. (a) Blank, (b) 100 mg dm−3, (c) 250 mg dm−3 and (d) 500 mg dm−3. The

on copper electrodeposition [21].

has an inhibiting

+ 0.5 M NaSO4

,

in 3.5%

in the presence of [BMIM]HSO4

140 Progress and Developments in Ionic Liquids

**3.2. Additives for copper electrodeposition**

pH 2.6 in the absence and presence of [BMIM]HSO4

Recently, we have investigated the effect of [BMIM]HSO4

the effect is more pronounced at higher additive concentrations.

The kinetic parameters obtained show that the presence of [BMIM]HSO4

additives.

$$\text{Cu}^{2+} + \text{e}^- \rightarrow \text{Cu}^\* \tag{1}$$

$$\text{Cu}^+ + \text{e}^- \leftrightarrow \text{Cu} \tag{2}$$

$$\text{Cu}^+ + \text{[BMIM]HSO}\_4 \leftrightarrow \text{[Cu}-\text{[BMIM]HSO}\_4\text{]}\_{\text{ads}}^\* \tag{3}$$

$$\text{[Cu} - \text{[BMIIM]HSO}\_4\text{]}\_{\text{ads}}^\ast + \text{e}^- \rightarrow \text{Cu} + \text{[BMIIM]HSO}\_4 \tag{4}$$

We attribute the inhibition effect of [BMIM]HSO4 on the process of copper electroreduction to the adsorption of the complex at active sites, where it may receive an electron from the cathode and discharge copper atoms which are embedded at the active sites (Eq. (4)). The [BMIM] HSO4 will be released and can then become a complex.

The effect of [BMIM]HSO4 on the complex-plane impedance diagrams is illustrated in **Figure 4**. The complex-plane impedance spectra obtained from the additive-free solution exhibit two capacitive features at high frequencies followed by an inductive loop at low-frequency values (**Figure 4a**). On the other hand, two intermediate-frequency capacitive features, far more separated at high concentration of additive (**Figure 4b** and **c**) in the presence of [BMIM]HSO4 , are obtained; that implies that two adsorbed species play a role in the process of copper electrodeposition and the addition of [BMIM]HSO4 brought about a change in the copper electrodeposition mechanism.

**Figure 4.** Impedance plots for copper electrodeposition at *E*= −0.15 V in the absence (a) and presence of [BMIM]HSO4 (b) 10 mg·dm−3, (c) 50 mg·dm−3 [21].

**Figure 5** shows the SEM images of copper deposits obtained by small-scale electrolysis from the sulphate electrolyte in the absence and presence of [BMIM]HSO4 . As it can be seen from **Figure 5(b** and **c)**, introducing [BMIM]HSO4 has brought a notable effect on the surface quality of the copper deposits as compared with those gained from the additive-free bath (**Figure 5a**) consisting of comparatively large, coarse grains. The size of the copper grain is smaller and continuously decreases with increasing additive concentrations in the presence of [BMIM]HSO4 (**Figure 5b** and **c**). The fact that there was a blockage of the electrocrystallization process is indicated by the results. Blockage of the crystal growth process is the action of [BMIM]HSO4 , which induces a relative improvement in the process of nucleation. This results in a finer grained deposit. It is also noteworthy that the copper deposits' morphology remains essentially unchanged, irrespective of the additive concentration. The influence of [BMIM] HSO4 on the crystallographic structure of the deposits is presented in **Figure 5(d)**. The copper deposit consists of (111), (200), (220), (311) and (222) crystal orientations without additives. The addition of 10 mg·dm−3 [BMIM]HSO4 inhibited the growth in the direction of the (111), (311), (222) planes and promoted the growth of the (220) plane.

cathodic surface and block the kinetics of the Cu2+ reduction process. A higher inhibition effect

For further nucleation investigation [23], the initial stages of the process of copper electrodeposition take place through a three-dimensional instantaneous nucleation with diffusion-controlled growth of the nuclei. We changed the practically instantaneous nucleation mechanism observed in the additive-free solution to become more progressive for the additives in the solution. The blocking effect of alklpyridinium hydrosulphate-based ILs on the copper electrocrystallization process through its cathodic adsorption on the active sites of the electrode surface brought about this change in the nucleation mechanism and, as a consequence, caused a decrease in the nucleation and growth rate of these nuclei and induced the formation of

To be distinguished from alkylimidazolium and alkylpyridinium-based ILs, for which their additive effects were found to come from surface adsorption together with complexing action, quaternary ammonium-based ILs feature their action simply through specific adsorption [25]. The typically feasible cathodic adsorption of quaternary ammonium-based ILs, including

forces between dissolved quaternary ammonium cations and the electrically charged surface of the cathode, which can be schematically described in **Figure 6(c)**. Furthermore, the possible difference applied during the process of electrodeposition makes the charge on the cathode surface more negative and enhances the adsorption of positively charged quaternary ammonium ions. At a lower additive concentration, the alkyl chain of these quaternary ammonium cations may be oriented in the direction of the electrolyte. Nevertheless, a horizontal arrangement to the cathode can also be made of them. As there is an increment in the concentration of added ILs, the alkyl chain may trend for a vertical arrangement to improve absorption [75].

HSO4

, which may be due to HpyHSO4

Ionic Liquids as Electrodeposition Additives and Corrosion Inhibitors

.

's stronger adsorb-

143

http://dx.doi.org/10.5772/65807

, **Figure 6a**) and tetrabutyl-ammonium

, **Figure 6b**), may have been caused by electrostatic attractive

HSO4

, (b) NBu4

HSO4

and (c) the proposed surface

than by BpyHSO4

ability and complexation in comparison with BpyHSO4

levelled and finer grained copper electrodeposits.

tetraethylammonium hydrogen sulphate (NEt4

HSO4

**Figure 6.** Structures of quaternary ammonium-based ILs used. (a) NEt4

adsorption of NBu4+ at different additive concentrations on the cathode [25].

hydrogen sulphate (NBu4

is offered by HpyHSO4

**Figure 5.** Scanning electron micrographs of copper deposits in the absence and presence of [BMIM]HSO4 . (a) Blank, (b) 10 mg·dm−3 and (c) 50 mg·dm−3. (Insets: local magnified graphs). (d) XRD patterns for the copper deposits in the absence and presence of [BMIM]HSO4 (1) Blank, (2) 10 mg·dm−3 [21].

Pyridinium-based ionic liquids were also found to be readily adsorbed on the metallic surface similar to that of imidazolium-based ILs [21], which provide a larger potential group to be researched as novel metal electrodeposition additives. We have previously studied the effects of two alkylpyridiniumILs (py-iLs), including N-butylpyridinium hydrogen sulphate (BpyHSO4 ) and N-hexylpyridinium hydrogen sulphate (HpyHSO4 ), on copper electrodeposition from acidic sulphate electrolyte [22]. BpyHSO4 and HpyHSO4 both turn out to be efficient levelling additives in copper electrodeposition, which leads to more levelled and fine-grained cathodic deposits. Copper electrodeposition is associated with the growth process and a nucleation. The addition of py-iLs has a blocking effect on copper electrodeposition, which causes a blockage of the nuclei growth process and some improvement in the process of nucleation. Both additives increase the cathodic polarization of copper through their adsorption on the cathodic surface and block the kinetics of the Cu2+ reduction process. A higher inhibition effect is offered by HpyHSO4 than by BpyHSO4 , which may be due to HpyHSO4 's stronger adsorbability and complexation in comparison with BpyHSO4 .

(**Figure 5a**) consisting of comparatively large, coarse grains. The size of the copper grain is smaller and continuously decreases with increasing additive concentrations in the presence

tion process is indicated by the results. Blockage of the crystal growth process is the action of

in a finer grained deposit. It is also noteworthy that the copper deposits' morphology remains essentially unchanged, irrespective of the additive concentration. The influence of [BMIM]

Pyridinium-based ionic liquids were also found to be readily adsorbed on the metallic surface similar to that of imidazolium-based ILs [21], which provide a larger potential group to be researched as novel metal electrodeposition additives. We have previously studied the effects of two alkylpyridiniumILs (py-iLs), including N-butylpyridinium hydrogen sulphate

(b) 10 mg·dm−3 and (c) 50 mg·dm−3. (Insets: local magnified graphs). (d) XRD patterns for the copper deposits in the

**Figure 5.** Scanning electron micrographs of copper deposits in the absence and presence of [BMIM]HSO4

(1) Blank, (2) 10 mg·dm−3 [21].

levelling additives in copper electrodeposition, which leads to more levelled and fine-grained cathodic deposits. Copper electrodeposition is associated with the growth process and a nucleation. The addition of py-iLs has a blocking effect on copper electrodeposition, which causes a blockage of the nuclei growth process and some improvement in the process of nucleation. Both additives increase the cathodic polarization of copper through their adsorption on the

) and N-hexylpyridinium hydrogen sulphate (HpyHSO4

tion from acidic sulphate electrolyte [22]. BpyHSO4

 on the crystallographic structure of the deposits is presented in **Figure 5(d)**. The copper deposit consists of (111), (200), (220), (311) and (222) crystal orientations without additives.

(**Figure 5b** and **c**). The fact that there was a blockage of the electrocrystalliza-

, which induces a relative improvement in the process of nucleation. This results

inhibited the growth in the direction of the (111),

), on copper electrodeposi-

. (a) Blank,

and HpyHSO4 both turn out to be efficient

of [BMIM]HSO4

142 Progress and Developments in Ionic Liquids

The addition of 10 mg·dm−3 [BMIM]HSO4

(311), (222) planes and promoted the growth of the (220) plane.

[BMIM]HSO4

HSO4

(BpyHSO4

absence and presence of [BMIM]HSO4

For further nucleation investigation [23], the initial stages of the process of copper electrodeposition take place through a three-dimensional instantaneous nucleation with diffusion-controlled growth of the nuclei. We changed the practically instantaneous nucleation mechanism observed in the additive-free solution to become more progressive for the additives in the solution. The blocking effect of alklpyridinium hydrosulphate-based ILs on the copper electrocrystallization process through its cathodic adsorption on the active sites of the electrode surface brought about this change in the nucleation mechanism and, as a consequence, caused a decrease in the nucleation and growth rate of these nuclei and induced the formation of levelled and finer grained copper electrodeposits.

To be distinguished from alkylimidazolium and alkylpyridinium-based ILs, for which their additive effects were found to come from surface adsorption together with complexing action, quaternary ammonium-based ILs feature their action simply through specific adsorption [25]. The typically feasible cathodic adsorption of quaternary ammonium-based ILs, including tetraethylammonium hydrogen sulphate (NEt4 HSO4 , **Figure 6a**) and tetrabutyl-ammonium hydrogen sulphate (NBu4 HSO4 , **Figure 6b**), may have been caused by electrostatic attractive forces between dissolved quaternary ammonium cations and the electrically charged surface of the cathode, which can be schematically described in **Figure 6(c)**. Furthermore, the possible difference applied during the process of electrodeposition makes the charge on the cathode surface more negative and enhances the adsorption of positively charged quaternary ammonium ions. At a lower additive concentration, the alkyl chain of these quaternary ammonium cations may be oriented in the direction of the electrolyte. Nevertheless, a horizontal arrangement to the cathode can also be made of them. As there is an increment in the concentration of added ILs, the alkyl chain may trend for a vertical arrangement to improve absorption [75].

**Figure 6.** Structures of quaternary ammonium-based ILs used. (a) NEt4 HSO4 , (b) NBu4 HSO4 and (c) the proposed surface adsorption of NBu4+ at different additive concentrations on the cathode [25].

In this manner, more quaternary ammonium cations can adsorb at the cathode surface and act together to form a layer above the head group; this will block the cathode surface and bring about an increment in the inhibition effect on the approach of the Cu2+ species and the resulting electroreduction reaction. Longer alkyl chain will result in greater surface adsorbability of the ILs cations because of the alkyl group's electron releasing ability [76], which is found to improve with any increment in the alkyl chain. Therefore, NBu4 HSO4 provides a higher inhibition effect than NEt4 HSO4 .

of the Nyquist plots observed in the absence and presence of the inhibitors indicates that the addition of inhibitors does not change the mechanism for the dissolution of mild steel in HCl.

The corrosion mechanism of iron in hydrochloric acid was proposed [97, 98] as follows.

**Figure 7.** Nyquist plots for mild steel in 1 M HCl solution in the absence (□) and presence of 5 × 10−4 (), 1 × 10−3 (),

[2].

Ionic Liquids as Electrodeposition Additives and Corrosion Inhibitors

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145

metal surface. Then the inhibitor molecules can be adsorbed via electrostatic interactions between the negatively charged metal surface and the positively charged molecules. These

layers (by forming a complex) on the surface of the steel; it protects the surface of the steel

(bd). In contrast, in competition with hydrogen ions reducing hydrogen evolution (eg),

Meanwhile, the presence of the electron-donating groups on the imidazolium base structure, such as Cl and S, increases the electron density on the nitrogen of the –C=N– group due to their ability of offer free electrons. In particular, the ability of S atom is more excellent than

is more effective than BMIC in inhibiting the corrosion

anion is onto the positively charged

)ads species to form monomolecular

)ads as seen in steps

We assume that columbic attraction first adsorbed Cl−

5 × 10−3 () and 1 × 10−2 M () inhibitor at 303 K: (a) BMIC, (b) [BMIM]HSO4

adsorbed inhibitor molecules come together with (FeCl<sup>−</sup>

These are the steps followed by the anodic dissolution of iron:

The cathodic hydrogen evolution follows the steps:

a. ads Fe Cl (FeCl ) − − + ↔

**b.**  ads ads (FeCl ) (FeCl) e − − ↔ +

**c.**  ads (FeCl) (FeCl ) e → + + − **d.**  <sup>2</sup> (FeCl ) Fe Cl + +− ↔ +

**e.**  ads Fe H (FeH ) + + + ↔

of mild steel in HCl.

**f.**  ads ads (FeH ) e (FeH) + − + →

g. ads <sup>2</sup> (FeH) H e Fe H + − + +→+

that of Cl [99]. Therefore, [BMIM]HSO4

from attack by chloride ions and prevents the oxidation reaction of (FeCl−

the protonated imidazolium molecules are also adsorbed at cathodic sites.

### **4. Corrosion inhibitors**

Because of the general aggressiveness of acid solutions, inhibitors are commonly used to reduce the corrosive attack on metallic materials. Numerous possible inhibitors have been investigated. Amongst them there are inorganic inhibitors [77], but in much greater numbers there are organic compounds. Most of the effective organic inhibitors used contain heteroatoms such as oxygen, nitrogen, sulphur, phosphorous and multiple bonds in the organic compound molecules through which they can adsorbed on the metals surface [78–83]. The adsorption behaviour could include two main modes [1]. They are chemisorption (involving chemical combination between the metal and the adsorbate where electrons are shared and/ or transferred, usually leading to the formation of covalent bonds) and physisorption (involving physical force such as van der Waals and pure electrostatic attraction between the charged metal and the charged inhibitor molecules). The former may occur if the inhibitor contains lone pairs of electrons, multiple bonds or conjugated p-type bond system. And there is no electron transfer and no electron sharing in the later adsorption mode. In this section, we reported the effect of some alkylimidazolium ILs on the corrosion inhibition of metals such as mild steel, aluminium and copper in acid solution.

#### **4.1. Mild steel**

Acid solutions are widely used in industry, such as acid pickling, industrial acid cleaning, acid descaling and oil well acidizing. However, due to their general aggressively, inhibitors are generally used in these processes to control metal dissolution. There are various organic inhibitors that tend to decrease the corrosion rate of steel and iron in acidic solutions [28, 83–86]. ILs with imidazolium [87–92] and pyridinium cations [93, 94] have showed excellent corrosion inhibition performance on mild steel in an acidic environment. We have first investigated the acid corrosion inhibition process of mild steel [2] in 1 M HCl by 1-butyl-3-methyl-imidazolium chlorides (BMIC) and 1-butyl-3-methyl-imidazolium hydrogen sulphate ([BMIM]HSO4 ) and found that the studied inhibitors are mixed type inhibitors. For both inhibitors, the inhibition efficiency increased with an increase in the concentration of the inhibitor and the effectiveness of the two inhibitors are in the order [BMIM]HSO4 > BMIC.

**Figure 7** shows the Nyquist plot diagram for mild steel in 1 M HCl solution in the absence and presence of BMIC and [BMIM]HSO4 . It is clear from these figures that the impedance spectra obtained yield a semi-circular shape, suggesting that the corrosion of the mild steel in 1 M HCl solution is mainly controlled by a charge transfer process [95, 96]. A similar profile of the Nyquist plots observed in the absence and presence of the inhibitors indicates that the addition of inhibitors does not change the mechanism for the dissolution of mild steel in HCl.

**Figure 7.** Nyquist plots for mild steel in 1 M HCl solution in the absence (□) and presence of 5 × 10−4 (), 1 × 10−3 (), 5 × 10−3 () and 1 × 10−2 M () inhibitor at 303 K: (a) BMIC, (b) [BMIM]HSO4 [2].

The corrosion mechanism of iron in hydrochloric acid was proposed [97, 98] as follows. We assume that columbic attraction first adsorbed Cl− anion is onto the positively charged metal surface. Then the inhibitor molecules can be adsorbed via electrostatic interactions between the negatively charged metal surface and the positively charged molecules. These adsorbed inhibitor molecules come together with (FeCl<sup>−</sup> ) ads species to form monomolecular layers (by forming a complex) on the surface of the steel; it protects the surface of the steel from attack by chloride ions and prevents the oxidation reaction of (FeCl− )ads as seen in steps (bd). In contrast, in competition with hydrogen ions reducing hydrogen evolution (eg), the protonated imidazolium molecules are also adsorbed at cathodic sites.

These are the steps followed by the anodic dissolution of iron:

a. ads Fe Cl (FeCl ) − − + ↔

In this manner, more quaternary ammonium cations can adsorb at the cathode surface and act together to form a layer above the head group; this will block the cathode surface and bring about an increment in the inhibition effect on the approach of the Cu2+ species and the resulting electroreduction reaction. Longer alkyl chain will result in greater surface adsorbability of the ILs cations because of the alkyl group's electron releasing ability [76], which is found

Because of the general aggressiveness of acid solutions, inhibitors are commonly used to reduce the corrosive attack on metallic materials. Numerous possible inhibitors have been investigated. Amongst them there are inorganic inhibitors [77], but in much greater numbers there are organic compounds. Most of the effective organic inhibitors used contain heteroatoms such as oxygen, nitrogen, sulphur, phosphorous and multiple bonds in the organic compound molecules through which they can adsorbed on the metals surface [78–83]. The adsorption behaviour could include two main modes [1]. They are chemisorption (involving chemical combination between the metal and the adsorbate where electrons are shared and/ or transferred, usually leading to the formation of covalent bonds) and physisorption (involving physical force such as van der Waals and pure electrostatic attraction between the charged metal and the charged inhibitor molecules). The former may occur if the inhibitor contains lone pairs of electrons, multiple bonds or conjugated p-type bond system. And there is no electron transfer and no electron sharing in the later adsorption mode. In this section, we reported the effect of some alkylimidazolium ILs on the corrosion inhibition of metals such as

Acid solutions are widely used in industry, such as acid pickling, industrial acid cleaning, acid descaling and oil well acidizing. However, due to their general aggressively, inhibitors are generally used in these processes to control metal dissolution. There are various organic inhibitors that tend to decrease the corrosion rate of steel and iron in acidic solutions [28, 83–86]. ILs with imidazolium [87–92] and pyridinium cations [93, 94] have showed excellent corrosion inhibition performance on mild steel in an acidic environment. We have first investigated the acid corrosion inhibition process of mild steel [2] in 1 M HCl by 1-butyl-3-methyl-imidazolium chlorides (BMIC) and 1-butyl-3-methyl-imidazolium hydrogen sul-

both inhibitors, the inhibition efficiency increased with an increase in the concentration of the

**Figure 7** shows the Nyquist plot diagram for mild steel in 1 M HCl solution in the absence

spectra obtained yield a semi-circular shape, suggesting that the corrosion of the mild steel in 1 M HCl solution is mainly controlled by a charge transfer process [95, 96]. A similar profile

inhibitor and the effectiveness of the two inhibitors are in the order [BMIM]HSO4

) and found that the studied inhibitors are mixed type inhibitors. For

. It is clear from these figures that the impedance

HSO4

provides a higher

> BMIC.

to improve with any increment in the alkyl chain. Therefore, NBu4

HSO4 .

mild steel, aluminium and copper in acid solution.

inhibition effect than NEt4

144 Progress and Developments in Ionic Liquids

**4. Corrosion inhibitors**

**4.1. Mild steel**

phate ([BMIM]HSO4

and presence of BMIC and [BMIM]HSO4


The cathodic hydrogen evolution follows the steps:


Meanwhile, the presence of the electron-donating groups on the imidazolium base structure, such as Cl and S, increases the electron density on the nitrogen of the –C=N– group due to their ability of offer free electrons. In particular, the ability of S atom is more excellent than that of Cl [99]. Therefore, [BMIM]HSO4 is more effective than BMIC in inhibiting the corrosion of mild steel in HCl.

The characteristics of adsorption of the imidazolium base inhibitors on the mild steel in 1.0 M HCl solution follow Langmuir's adsorption isotherm. The analysis of thermodynamic parameters [2] such as equilibrium constant and standard free energy indicate that the inhibitors are physically adsorbed on the metal surface and the adsorption of inhibitor molecule with the corroding mild steel surface is a spontaneous and exothermic process [100].

<sup>2</sup> [AlOH] Cl [AlOHCl] + − <sup>+</sup> + → (8)

Ionic Liquids as Electrodeposition Additives and Corrosion Inhibitors

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147

The water molecules originally adsorbed on the surface are partly displaced by the adsorption of imidazolium compounds on the aluminium surface; this blocks the formation of AlOHads (Eq. (5)). Thus, we can prevent both the oxidation reaction of AlOHads to Al3+ as shown by step (Eq. (6)) and the complexation reaction between the hydrated cation [AlOH]2+ species that are formed in step (Eq. (7)) and chloride ions (Eq. (8)). Moreover, these protonated molecules also compete with the hydrogen ions, which will curtail the evolution of hydrogen. The presence of the electron-donating group (Cl) on the imidazolium base structure is observed to increase the electron density on the nitrogen of the –C=N– group and to result in high inhibition efficiency. In particular, this effect appears more pronounced with the increase in the chain length of the alkyl connecting with N(3) of the imidazolium ring. Therefore, the effectiveness

Copper and its alloys have been found widespread applications in many industrial processes such as industrial equipment, building construction, electricity, electronics, coinages and ornamental parts due to their electrical, thermal, mechanical and corrosion resistance properties [112]. However, the presence of aggressive ions such as chlorides, sulphates or nitrates creates extensive localized attack [113]. One effective approach to protect metals against the general aggression of acid solutions is the use of organic inhibitors, which can effectively control the metal dissolution and eliminate the undesirable acid consumption. Many organic compounds including triazole, imidazole, thiazole, tetrazole, indole and its derivatives [114, 115] have been developed as corrosion inhibitors to inhibit copper corrosion in aggressive environ-

**Figure 8.** Effect of inhibitors on the impedance response of aluminum in 1.0 M HCl solution in the absence (□) and

) have proved to be excellent inhibitors for the corrosion inhibition of copper

, [HMIM]HSO4

and

ments. As an example of ILs, alkylimidazolium-based ILs ([BMIM]HSO4

presence of 5 × 10−4 M () BMIC, () HMIC, () OMIC at 303 K [26].

of these inhibitors with the sequence of OMIC > HMIC > BMIC is obtained.

**4.3. Copper**

[OMIM]HSO4

#### **4.2. Aluminium**

A rapidly formed compact, strongly adherent and continuous oxide film may be responsible for the resistance of aluminium against corrosion in aqueous media [101, 102]. Consequently, many industries, such as reaction vessels, pipes, machinery and chemical batteries, have come to rely heavily on aluminium and its alloys. Hydrochloric acid solutions are employed for pickling, chemical and electrochemical etching of aluminium [103]. Having aggressive ions such as chloride, however, creates a huge localized attack [104]. The corrosion inhibition of aluminium in an acid medium has been reported while using hydrazone [105], anionic surfactants [106] and amino acid [107] as inhibitors. We have studied the corrosion inhibition performance of three alkylimidazolium ILs namely 1-butyl-3-methylimidazolium chlorides, 1-hexyl-3-methylimidazolium chlorides (HMIC) and 1-octyl-3-methyl-imidazolium chlorides (OMIC) on the corrosion of aluminium in 1.0 M HCl [22]. All the inhibitors studied could be classified as mixed type inhibitors with the obvious inhibition effect on both cathodic and anodic reactions of the corrosion process. The inhibition efficiency increased with an increase in the concentration of inhibitor and the effectiveness of these inhibitors was in the order of OMIC > HMIC > BMIC. Similar to the adsorption behaviour of imidazolium base inhibitors on the mild steel, the adsorption of these inhibitors on the aluminium surface also obeyed Langmuir adsorption isotherm and had a physical mechanism involving a spontaneous and exothermic process.

The Nyquist plots (**Figure 8**) for aluminium in 1.0 M HCl solution showed that the impedance spectra are made up of a large capacitive loop at high frequencies, which is followed by a small inductive one at low-frequency values. There is a relationship between the high-frequency capacitive loop and the charge transfer of the corrosion process and the double layer behaviour, and the inductive loop could be interpreted in terms of the relaxation processes in the oxide film covering the electrode surface [108] or attributed to the stabilization of the layer by the adsorbed intermediate products of the corrosion reaction on the electrode surface involving inhibitor molecules as well as reactive products [109].

A general mechanism for the dissolution of aluminium in the presence of chloride ions would be similar to that reported in the literature [110, 111].

$$\text{Al}\_{\text{(s)}} + \text{H}\_{2}\text{O} \leftrightarrow \text{AlOH}\_{\text{ads}} + \text{H}^{\*} + \text{e}^{-}\tag{5}$$

$$\text{AlOH}\_{\text{ads}} + \text{H}^\* \leftrightarrow \text{Al}^{3+} + \text{H}\_2\text{O} + 2\text{e}^- \tag{6}$$

$$\text{AlOH}\_{\text{ads}} + \text{H}^\* \leftrightarrow \text{Al}^{3+} + \text{H}\_2\text{O} + 2\text{e}^- \tag{7}$$

$$\text{[AlOH]}^{2+} + \text{Cl}^{-} \rightarrow \text{[AlOHCl]}^{\*} \tag{8}$$

The water molecules originally adsorbed on the surface are partly displaced by the adsorption of imidazolium compounds on the aluminium surface; this blocks the formation of AlOHads (Eq. (5)). Thus, we can prevent both the oxidation reaction of AlOHads to Al3+ as shown by step (Eq. (6)) and the complexation reaction between the hydrated cation [AlOH]2+ species that are formed in step (Eq. (7)) and chloride ions (Eq. (8)). Moreover, these protonated molecules also compete with the hydrogen ions, which will curtail the evolution of hydrogen. The presence of the electron-donating group (Cl) on the imidazolium base structure is observed to increase the electron density on the nitrogen of the –C=N– group and to result in high inhibition efficiency. In particular, this effect appears more pronounced with the increase in the chain length of the alkyl connecting with N(3) of the imidazolium ring. Therefore, the effectiveness of these inhibitors with the sequence of OMIC > HMIC > BMIC is obtained.

**Figure 8.** Effect of inhibitors on the impedance response of aluminum in 1.0 M HCl solution in the absence (□) and presence of 5 × 10−4 M () BMIC, () HMIC, () OMIC at 303 K [26].

#### **4.3. Copper**

The characteristics of adsorption of the imidazolium base inhibitors on the mild steel in 1.0 M HCl solution follow Langmuir's adsorption isotherm. The analysis of thermodynamic parameters [2] such as equilibrium constant and standard free energy indicate that the inhibitors are physically adsorbed on the metal surface and the adsorption of inhibitor molecule with the

A rapidly formed compact, strongly adherent and continuous oxide film may be responsible for the resistance of aluminium against corrosion in aqueous media [101, 102]. Consequently, many industries, such as reaction vessels, pipes, machinery and chemical batteries, have come to rely heavily on aluminium and its alloys. Hydrochloric acid solutions are employed for pickling, chemical and electrochemical etching of aluminium [103]. Having aggressive ions such as chloride, however, creates a huge localized attack [104]. The corrosion inhibition of aluminium in an acid medium has been reported while using hydrazone [105], anionic surfactants [106] and amino acid [107] as inhibitors. We have studied the corrosion inhibition performance of three alkylimidazolium ILs namely 1-butyl-3-methylimidazolium chlorides, 1-hexyl-3-methylimidazolium chlorides (HMIC) and 1-octyl-3-methyl-imidazolium chlorides (OMIC) on the corrosion of aluminium in 1.0 M HCl [22]. All the inhibitors studied could be classified as mixed type inhibitors with the obvious inhibition effect on both cathodic and anodic reactions of the corrosion process. The inhibition efficiency increased with an increase in the concentration of inhibitor and the effectiveness of these inhibitors was in the order of OMIC > HMIC > BMIC. Similar to the adsorption behaviour of imidazolium base inhibitors on the mild steel, the adsorption of these inhibitors on the aluminium surface also obeyed Langmuir adsorption isotherm and had a physical mechanism involving a spontaneous and

The Nyquist plots (**Figure 8**) for aluminium in 1.0 M HCl solution showed that the impedance spectra are made up of a large capacitive loop at high frequencies, which is followed by a small inductive one at low-frequency values. There is a relationship between the high-frequency capacitive loop and the charge transfer of the corrosion process and the double layer behaviour, and the inductive loop could be interpreted in terms of the relaxation processes in the oxide film covering the electrode surface [108] or attributed to the stabilization of the layer by the adsorbed intermediate products of the corrosion reaction on the electrode surface

A general mechanism for the dissolution of aluminium in the presence of chloride ions would

Al H O AlOH H e (s) 2 ads

<sup>3</sup> AlOH H Al H O 2e ads <sup>2</sup>

<sup>3</sup> AlOH H Al H O 2e ads <sup>2</sup>

+ − + ↔ ++ (5)

++ − +↔ + + (6)

++ − +↔ + + (7)

involving inhibitor molecules as well as reactive products [109].

be similar to that reported in the literature [110, 111].

corroding mild steel surface is a spontaneous and exothermic process [100].

**4.2. Aluminium**

146 Progress and Developments in Ionic Liquids

exothermic process.

Copper and its alloys have been found widespread applications in many industrial processes such as industrial equipment, building construction, electricity, electronics, coinages and ornamental parts due to their electrical, thermal, mechanical and corrosion resistance properties [112]. However, the presence of aggressive ions such as chlorides, sulphates or nitrates creates extensive localized attack [113]. One effective approach to protect metals against the general aggression of acid solutions is the use of organic inhibitors, which can effectively control the metal dissolution and eliminate the undesirable acid consumption. Many organic compounds including triazole, imidazole, thiazole, tetrazole, indole and its derivatives [114, 115] have been developed as corrosion inhibitors to inhibit copper corrosion in aggressive environments. As an example of ILs, alkylimidazolium-based ILs ([BMIM]HSO4 , [HMIM]HSO4 and [OMIM]HSO4 ) have proved to be excellent inhibitors for the corrosion inhibition of copper in 0.5 mol·L−1 H2 SO4 solution [27], which behave as mixed type inhibitors with a pre-dominantly cathodic action. The corresponding electrochemical impedance results suggested that these imidazolium-based molecules act by adsorbing at the copper/solution interface. The adsorption of these imidazolium-based compounds on the copper surface in an acidic solution is found to fit the Langmuir adsorption isotherm and occurs by a physisorption-based mechanism involving a spontaneous process.

**Author details**

**References**

Zhang Qibo\* and Hua Yixin

Sci., 2007, 49(2): 373.

Soc., 2003, 150(4): A499.

Polym. Sci., 2004, 29(1): 3.

Chem., 2006, 78(9): 2892.

2004, 43(38): 4988.

74(1–2): 157.

\*Address all correspondence to: qibozhang@kmust.edu.cn

Kunming University of Science and Technology, Kunming, Yunnan, China

& Sons, Inc., Publication. Hoboken, New Jersey, 2006, 168.

uids in hydrochloric acid. Electrochim. Acta, 2009, 54(6): 1881.

parabolic trough systems. J. Solar Energy Eng., 2003, 125(1): 112.

capacitors. Electrochem. Commun., 2004, 6(6): 566.

Key Lab of Ionic Liquids Metallurgy, Faculty of Metallurgical and Energy Engineering,

Ionic Liquids as Electrodeposition Additives and Corrosion Inhibitors

http://dx.doi.org/10.5772/65807

149

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The anodic dissolution of copper proceeds via a two-step reaction mechanism and can be described as follows: [116]


where (CuSO4 − ) ads is an adsorbed species at the copper surface and does not diffuse into the bulk solution [52]. Consequently, the mass transport has little influence on dissolution of copper.

It is assumed that the negative sulphated ions are first adsorbed onto the positively charged metal surface by columbic attraction. Since the imidazolium group as well as nitrogen atom in the heteroaromatic ring of imidazolium compounds can be protonated in acidic solutions [117]. The protonated inhibitor molecules can be adsorbed through electrostatic interactions between the positively charged molecules and the negatively charged metal surface [2]. These adsorbed imidazolium compound molecules will interact with (CuSO4 − )ads ions generated from (a) to form a protective layer (by forming a complex) at active sites, which hiders both mass and charge transfers and blocks the further oxidation reaction of (CuSO4 − ) ads to Cu2+ as shown in step (b).

### **5. Conclusions**

In this chapter, two perspective application studies of ILs in using as additives for metal electrodeposition and inhibitors for metal anti-corrosion were summarized. It was shown that ILs had some intrinsic advantages over traditional organic agents. Due to their stability, high conductivity, low vapour pressure and environmental friendly nature, ILs were excellent levelling agents for both zinc and copper electrodeposition, which were superior to traditional additives, and showed favourable corrosion resistant performance on mild steel, aluminium and copper in acidic solutions.

It is apparent that the future for ILs-based technology in these two aspects is extremely bright; however, more fundamental aspects issues have to be solved so that their applications will become a practical reality instead of laboratorial studies. What is the adsorption mechanism of ILs on the metal surface? What is the effect of ILs on the nucleation and growth of metal? And how is the influence of ILs on the structures of electric double layer, etc.? All these issues are the critical problems for further study.

### **Author details**

in 0.5 mol·L−1 H2

where (CuSO4

copper.

described as follows: [116]

a. <sup>2</sup> Cu SO e (CuSO ) <sup>4</sup> 4 ads − − <sup>−</sup> + −↔ b. 2 2 4 ads <sup>4</sup> (CuSO ) e Cu SO − − +− −↔ +

> − )

to Cu2+ as shown in step (b).

and copper in acidic solutions.

are the critical problems for further study.

**5. Conclusions**

SO4

148 Progress and Developments in Ionic Liquids

mechanism involving a spontaneous process.

solution [27], which behave as mixed type inhibitors with a pre-domi-

ads is an adsorbed species at the copper surface and does not diffuse into the

− ) ads ions

> − ) ads

nantly cathodic action. The corresponding electrochemical impedance results suggested that these imidazolium-based molecules act by adsorbing at the copper/solution interface. The adsorption of these imidazolium-based compounds on the copper surface in an acidic solution is found to fit the Langmuir adsorption isotherm and occurs by a physisorption-based

The anodic dissolution of copper proceeds via a two-step reaction mechanism and can be

bulk solution [52]. Consequently, the mass transport has little influence on dissolution of

It is assumed that the negative sulphated ions are first adsorbed onto the positively charged metal surface by columbic attraction. Since the imidazolium group as well as nitrogen atom in the heteroaromatic ring of imidazolium compounds can be protonated in acidic solutions [117]. The protonated inhibitor molecules can be adsorbed through electrostatic interactions between the positively charged molecules and the negatively charged metal sur-

generated from (a) to form a protective layer (by forming a complex) at active sites, which hiders both mass and charge transfers and blocks the further oxidation reaction of (CuSO4

In this chapter, two perspective application studies of ILs in using as additives for metal electrodeposition and inhibitors for metal anti-corrosion were summarized. It was shown that ILs had some intrinsic advantages over traditional organic agents. Due to their stability, high conductivity, low vapour pressure and environmental friendly nature, ILs were excellent levelling agents for both zinc and copper electrodeposition, which were superior to traditional additives, and showed favourable corrosion resistant performance on mild steel, aluminium

It is apparent that the future for ILs-based technology in these two aspects is extremely bright; however, more fundamental aspects issues have to be solved so that their applications will become a practical reality instead of laboratorial studies. What is the adsorption mechanism of ILs on the metal surface? What is the effect of ILs on the nucleation and growth of metal? And how is the influence of ILs on the structures of electric double layer, etc.? All these issues

face [2]. These adsorbed imidazolium compound molecules will interact with (CuSO4

Zhang Qibo\* and Hua Yixin

\*Address all correspondence to: qibozhang@kmust.edu.cn

Key Lab of Ionic Liquids Metallurgy, Faculty of Metallurgical and Energy Engineering, Kunming University of Science and Technology, Kunming, Yunnan, China

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Ionic Liquids as Electrodeposition Additives and Corrosion Inhibitors

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**Chapter 8**

**Provisional chapter**

**Ionic Liquid Enhancement of Polymer Electrolyte**

**Ionic Liquid Enhancement of Polymer Electrolyte** 

**Electrochemical Devices**

**Electrochemical Devices**

Abdul Kariem Arof

**Abstract**

**1. Introduction**

Abdul Kariem Arof

http://dx.doi.org/10.5772/65752

supercapacitors, fuel cells

Siti Nor Farhana Yusuf, Rosiyah Yahya and

Siti Nor Farhana Yusuf, Rosiyah Yahya and

Additional information is available at the end of the chapter

Additional information is available at the end of the chapter

**Conductivity and their Effects on the Performance of**

Ionic liquids (ILs) are molten salts at ambient temperature and consist of poorly coordinating cations and anions. They have good electrical conductivity with a wide voltage window and high thermal stability, but negligible vapor pressure. ILs can enhance ionic conductivity when added to polymer electrolytes. Conductivity enhancement is due to the additional ions supplied by the IL, the plasticizing nature of the IL and the low viscosity that facilitates ion mobility. The plasticizing nature of ILs softens the polymer chain giving rise to easier polymer segmental motion. Increase in polymer segmental motion implies that IL can increase amorphousness of a polymer electrolyte (PE). This article discusses the involvement of ionic liquid as electrolytes in selected devices, namely dye

sensitized photovoltaics, batteries, fuel cells and supercapacitors.

473–573 K, wide potential windows and high electrical conductivity [2].

this article. These are listed at the end of the article.

**Keywords:** polymer electrolyte, ionic liquid, dye-sensitized photovoltaic,

**Conductivity and their Effects on the Performance of** 

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution,

© 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

and reproduction in any medium, provided the original work is properly cited.

An ionic liquid (IL) is a molten salt at room temperature (RT). They consist of cations and anions that are weakly coordinating [1]. Ionic liquids (ILs) have thermal stability as high as

ILs have low melting temperature and negligible vapor pressure. They are therefore green solvents making them suitable for applications in industries [3]. The negligible vapor pressure prevents loss of solvent to the environment. To economize space, abbreviations are used in


**Provisional chapter**

### **Ionic Liquid Enhancement of Polymer Electrolyte Conductivity and their Effects on the Performance of Electrochemical Devices Ionic Liquid Enhancement of Polymer Electrolyte Conductivity and their Effects on the Performance of Electrochemical Devices**

Siti Nor Farhana Yusuf, Rosiyah Yahya and Abdul Kariem Arof Abdul Kariem Arof Additional information is available at the end of the chapter

Siti Nor Farhana Yusuf, Rosiyah Yahya and

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/65752

#### **Abstract**

[104] El-Etre A Y. Inhibition of aluminum corrosion using Opuntia extract. Corros. Sci., 2003,

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[108]Al-Kharafi F M, Badawy W A. Phosphoric acid passivated niobium and tantalum

[109] Yurt A, Ulutas S, Dal H. Electrochemical and theoretical investigation on the corrosion of aluminium in acidic solution containing some Schiff bases. Appl. Surf. Sci., 2006, 253(2): 919.

[110] Ford F P, Burstein G T, Hoar T P. Bare surface reaction rates and their relation to environment controlled cracking of aluminum alloys. J. Electrochem. Soc., 1980, 127(6): 1325.

[111] Nguyen T H, Foley R T. The chemical nature of aluminum corrosion. J. Electrochem.

[112] Tavakoli H, Shahrabi T, Hosseini M G. Synergistic effect on corrosion inhibition of copper by sodium dodecylbenzenesulphonate (SDBS) and 2-mercaptobenzoxazole. Mater.

[113] Zucchi F, Grassi V, Frignani A, Trabanelli G. Inhibition of copper corrosion by silane

[114] Zhao Y S, Pang Z Z. The action mechanism of MMI as corrosion inhibitor of copper for

[115] Scendo M, Poddebniak D, Malyszko J. Indole and 5-chloroindole as inhibitors of anodic dissolution and cathodic deposition of copper in acidic chloride solutions. J. Appl.

[116] Smyrl W H, Bockris J O M, Conway B E, Yeager E, White R E. Comprehensive Treatise

[117] Quraishi M A, Rafiquee M Z A, Khan S, Saxena N. Corrosion inhibition of aluminium in acid solutions by some imidazoline derivatives. J. Appl. Electrochem., 2007, 37(10):

hydrochloric acid pickling. Acta Phys. Chim. Sin., 2003, 19(5): 419–422.

of Electrochemistry. Plenum Press, New York, 1981, Vol. 4.

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EIS-comparative study. Electrochim. Acta, 1995, 40(16): 1811.

45(11): 2485.

156 Progress and Developments in Ionic Liquids

Soc., 1982, 129(3): 464.

Chem. Phys., 2008, 109(2): 281.

coatings. Corros. Sci., 2004, 46(11): 2853.

Electrochem., 2003, 33(3–4): 287–293.

1153–1162.

Ionic liquids (ILs) are molten salts at ambient temperature and consist of poorly coordinating cations and anions. They have good electrical conductivity with a wide voltage window and high thermal stability, but negligible vapor pressure. ILs can enhance ionic conductivity when added to polymer electrolytes. Conductivity enhancement is due to the additional ions supplied by the IL, the plasticizing nature of the IL and the low viscosity that facilitates ion mobility. The plasticizing nature of ILs softens the polymer chain giving rise to easier polymer segmental motion. Increase in polymer segmental motion implies that IL can increase amorphousness of a polymer electrolyte (PE). This article discusses the involvement of ionic liquid as electrolytes in selected devices, namely dye sensitized photovoltaics, batteries, fuel cells and supercapacitors.

**Keywords:** polymer electrolyte, ionic liquid, dye-sensitized photovoltaic, supercapacitors, fuel cells

### **1. Introduction**

An ionic liquid (IL) is a molten salt at room temperature (RT). They consist of cations and anions that are weakly coordinating [1]. Ionic liquids (ILs) have thermal stability as high as 473–573 K, wide potential windows and high electrical conductivity [2].

ILs have low melting temperature and negligible vapor pressure. They are therefore green solvents making them suitable for applications in industries [3]. The negligible vapor pressure prevents loss of solvent to the environment. To economize space, abbreviations are used in this article. These are listed at the end of the article.

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

### **2. ILs in polymer electrolytes (PEs)**

ILs have been introduced in PEs for conductivity enhancement. Kumar and Hashmi [4] have introduced EMImTf, in PVdF-HFP-NaCF3 SO3 PE system. EMImTf is a room temperature ionic liquid (RTIL). The PVdF-HFP with EMImTf PE system gave an electrical conductivity of the order 10−1 S m−1 at room temperature (RT), good thermal stability and wide potential window.

Another example of an IL-PE system consisted of PVA, CH3

temperatures. The addition of BMImPF6

**3. ILs in dye-sensitized photovoltaic systems**

−

conduction band (TiO2

/I3 −

also improve thermal stability.

mediator, for example, I/I3

ions. The I<sup>−</sup>

For a DSSC with I<sup>−</sup>

nano-TiO2

into I<sup>−</sup>

TiO2

COONH4

into corn starch–based electrolyte with LiPF6

is normally used as part of the photoanode). After leaving the

ions then return the electrons to the oxidized dye molecules. The dye molecules

−

ions of the redox couple

ions is important. IL

[10]. The IL increased the amorphousness of the polymer electrolyte and this enhanced ionic mobility. The plasticizing nature of the IL contributed to the softening of the polymer backbone that increased segmental motion. The segmental motion will bring a cation coordinated to the electron donor atom of the PVA near to another possible site and on compression of the chain, the cation can jump to the vacant site with low, but sufficient activation energy and will then be transported away from its former site as the segmental motion moves forward.

Ionic Liquid Enhancement of Polymer Electrolyte Conductivity and their Effects on the Performance...

The 49 wt. % PEMA-21 wt. % (PVdF-HFP)-30 wt. % LiTf exhibited an increase in RT maximum conductivity on addition of 60 wt. % BMImTf from 2.9 × 10−5 to 8.59 × 10−3 S m−1. At low BMImTf content, the number density of mobile ions was observed to influence the ionic conductivity, but the mobility and diffusion coefficient were more dominant at higher BMImTf contents. Incorporation of BMImTf also increased thermal stability. This is advantageous as the high decomposition temperature enables the polymer electrolyte to be used at elevated

increased ionic conductivity [11]. This was attributed to the amorphousness of the sample.

We have shown that IL can enhance ion transport in an electrolyte. IL acts as a plasticizer, increased the amorphousness of a PE and the segmental motion of the polymeric chain. IL can

Dye-sensitized solar cell (DSSC) is one of the third generation photovoltaic technologies that have been developed. It uses cheap materials that do not require tedious purifications, easy to fabricate and has high efficiency [12]. A DSSC consists of three main parts, namely (i) photoactive electrode or photoanode, (ii) Pt counter electrode and (iii) electrolyte with a redox

The DSSC mimics the process of photosynthesis. The sensitizer absorbs solar energy that excites the dye molecules and oxidizes them. The electrons released are injected into the mesoporous

are regenerated, the circuit is completed and current flows. The electrolyte is a key DSSC component. The high device photon to current efficiency is also associated with electrolyte conductivity.

redox couple, the contribution to conductivity by the I<sup>−</sup>

An EMImI ionic liquid has been employed in a DSSC. However, EMImI can easily crystallize and this will prevent them from entering the pores of the nanocrystalline semiconducting

 films. In order to inhibit EMImI from crystallizing, another type of IL, TIPIL can be added to EMImI electrolyte [13]. Hence, without TIPIL and if EMImI crystallizes, the photon to electricity conversion efficiency will be low [14]. The electrolyte containing TIPIL had

can be used to increase the iodide mobility, which may result in high DSSC efficiency.

as shown in **Figure 1**.

photoanode, the electrons move to the cathode where they reduce the I3

BMImTf was incorporated into a PEMA/PVdF-HFP blend with LiTf being the Li+

and BmImBr ionic liquid

http://dx.doi.org/10.5772/65752

source [1].

159

salt also

EMImBF4 [5] has also been shown to increase ionic conductivity of PVdF-HFP-LiBF4 PE system from 4.4 × 10−5 to 2.0 × 10−3 S m−1. The incorporation of EMImBF4 introduced additional mobile ions namely EMIm+ and BF4 − . This increased the concentration of mobile ions that is one of the parameters that govern ionic conductivity. The additional charge carriers also interacted with the electron donors of the polymer, which reduced the polymer-polymer intermolecular interaction. This will soften the polymer chains resulting in increased polymer segmental motion. Increased polymer segmental motion implied increased structural disorder or amorphousness of the PE system. Ion conduction only takes place in the amorphous region of the PE, hence with increased amorphousness, conductivity of the PE also increased.

The PP24TFSI ionic liquid has been added with LiTFSI to impart Li+ ion transport [6]. The electrolyte displayed ionic conductivity of the order 10−2 S m−1 and potential stability window is 2.7 V versus Li+ /Li. GPEs have also been successfully prepared by incorporating the Li+ -IL mixture into PVdF-HFP. The GPE containing 80 wt. % IL solution attained an ionic conductivity of the order 1 S m−1 at 383 K.

The BMImBF4 ionic liquid [7] conductivity was expected to decrease on addition of PVdF-HFP to the IL. This is because, on adding polymer to the IL will increase the IL viscosity. This should decrease the ionic mobility, and hence conductivity. However, the sample with 10 wt. % polymer and 90 wt. % IL exhibited RT conductivity which is higher than the pristine BMImBF4 . This observation can be clarified successfully using the polymer breathing model. The model assumes that the polymer chains can open and fold. Chain opening is analogous to "inhaling" and chain folding to "exhaling". This inhaling-exhaling process of the polymeric chain led to volume and local pressure changes that either dissociate ion pairs and increased number density of mobile ions or reduce viscosity and increased ionic mobility, both of which, will result in conductivity enhancement [8]. However, at high polymer content, viscosity effect would dominate.

One of the possible ways to determine the choice of an IL is by considering the type of alkyl chain. Different alkyl chains have different chain lengths. As an example, consider the three SPE samples chitosan/[C2 mim][CnSO3 ], chitosan/[C2 mim][CnSO4 ] and chitosan/[C2 mim][diCnPO4 ]). [C2 mim]+ is the 1-ethyl-3-methylimidazolium cation [9] with sulfonate, sulfate and dialkylphosphate anions. The alkyl chains are determined by n. For the sulfonate anion, the subscript n values were 1, 2 and 4; for the sulfate anion, n values were 1 and 2 and n also had values of 1 and 2 for the dialkylphosphate anions. The highest conducting chitosan/[C2 mim][C1 SO3 ] sample exhibited a conductivity of 7.78 S m−1 at 298 K and 0.75 S m−1 at 373 K. Shorter alkyl chain length ILs exhibited higher conductivity due to their lower viscosity that facilitates ion mobility, hence higher conductivity. This can therefore be a method of choosing the appropriate IL for conductivity enhancement.

Another example of an IL-PE system consisted of PVA, CH3 COONH4 and BmImBr ionic liquid [10]. The IL increased the amorphousness of the polymer electrolyte and this enhanced ionic mobility. The plasticizing nature of the IL contributed to the softening of the polymer backbone that increased segmental motion. The segmental motion will bring a cation coordinated to the electron donor atom of the PVA near to another possible site and on compression of the chain, the cation can jump to the vacant site with low, but sufficient activation energy and will then be transported away from its former site as the segmental motion moves forward.

BMImTf was incorporated into a PEMA/PVdF-HFP blend with LiTf being the Li+ source [1]. The 49 wt. % PEMA-21 wt. % (PVdF-HFP)-30 wt. % LiTf exhibited an increase in RT maximum conductivity on addition of 60 wt. % BMImTf from 2.9 × 10−5 to 8.59 × 10−3 S m−1. At low BMImTf content, the number density of mobile ions was observed to influence the ionic conductivity, but the mobility and diffusion coefficient were more dominant at higher BMImTf contents. Incorporation of BMImTf also increased thermal stability. This is advantageous as the high decomposition temperature enables the polymer electrolyte to be used at elevated temperatures. The addition of BMImPF6 into corn starch–based electrolyte with LiPF6 salt also increased ionic conductivity [11]. This was attributed to the amorphousness of the sample.

We have shown that IL can enhance ion transport in an electrolyte. IL acts as a plasticizer, increased the amorphousness of a PE and the segmental motion of the polymeric chain. IL can also improve thermal stability.

### **3. ILs in dye-sensitized photovoltaic systems**

**2. ILs in polymer electrolytes (PEs)**

introduced EMImTf, in PVdF-HFP-NaCF3

mobile ions namely EMIm+

158 Progress and Developments in Ionic Liquids

is 2.7 V versus Li+

The BMImBF4

BMImBF4

ity of the order 1 S m−1 at 383 K.

tent, viscosity effect would dominate.

ate IL for conductivity enhancement.

mim][CnSO3

samples chitosan/[C2

[C2 mim]+

EMImBF4

ILs have been introduced in PEs for conductivity enhancement. Kumar and Hashmi [4] have

liquid (RTIL). The PVdF-HFP with EMImTf PE system gave an electrical conductivity of the order 10−1 S m−1 at room temperature (RT), good thermal stability and wide potential window.

[5] has also been shown to increase ionic conductivity of PVdF-HFP-LiBF4

is one of the parameters that govern ionic conductivity. The additional charge carriers also interacted with the electron donors of the polymer, which reduced the polymer-polymer intermolecular interaction. This will soften the polymer chains resulting in increased polymer segmental motion. Increased polymer segmental motion implied increased structural disorder or amorphousness of the PE system. Ion conduction only takes place in the amorphous region of the PE, hence with increased amorphousness, conductivity of the PE also increased.

electrolyte displayed ionic conductivity of the order 10−2 S m−1 and potential stability window

mixture into PVdF-HFP. The GPE containing 80 wt. % IL solution attained an ionic conductiv-

HFP to the IL. This is because, on adding polymer to the IL will increase the IL viscosity. This should decrease the ionic mobility, and hence conductivity. However, the sample with 10 wt. % polymer and 90 wt. % IL exhibited RT conductivity which is higher than the pristine

The model assumes that the polymer chains can open and fold. Chain opening is analogous to "inhaling" and chain folding to "exhaling". This inhaling-exhaling process of the polymeric chain led to volume and local pressure changes that either dissociate ion pairs and increased number density of mobile ions or reduce viscosity and increased ionic mobility, both of which, will result in conductivity enhancement [8]. However, at high polymer con-

One of the possible ways to determine the choice of an IL is by considering the type of alkyl chain. Different alkyl chains have different chain lengths. As an example, consider the three SPE

phosphate anions. The alkyl chains are determined by n. For the sulfonate anion, the subscript n values were 1, 2 and 4; for the sulfate anion, n values were 1 and 2 and n also had values

sample exhibited a conductivity of 7.78 S m−1 at 298 K and 0.75 S m−1 at 373 K. Shorter alkyl chain length ILs exhibited higher conductivity due to their lower viscosity that facilitates ion mobility, hence higher conductivity. This can therefore be a method of choosing the appropri-

], chitosan/[C2

of 1 and 2 for the dialkylphosphate anions. The highest conducting chitosan/[C2

/Li. GPEs have also been successfully prepared by incorporating the Li+

ionic liquid [7] conductivity was expected to decrease on addition of PVdF-

. This observation can be clarified successfully using the polymer breathing model.

mim][CnSO4

is the 1-ethyl-3-methylimidazolium cation [9] with sulfonate, sulfate and dialkyl-

] and chitosan/[C2

PE system. EMImTf is a room temperature ionic

. This increased the concentration of mobile ions that

PE sys-


introduced additional

ion transport [6]. The

mim][diCnPO4

mim][C1

]).

SO3 ]

SO3

tem from 4.4 × 10−5 to 2.0 × 10−3 S m−1. The incorporation of EMImBF4

The PP24TFSI ionic liquid has been added with LiTFSI to impart Li+

−

and BF4

Dye-sensitized solar cell (DSSC) is one of the third generation photovoltaic technologies that have been developed. It uses cheap materials that do not require tedious purifications, easy to fabricate and has high efficiency [12]. A DSSC consists of three main parts, namely (i) photoactive electrode or photoanode, (ii) Pt counter electrode and (iii) electrolyte with a redox mediator, for example, I/I3 − as shown in **Figure 1**.

The DSSC mimics the process of photosynthesis. The sensitizer absorbs solar energy that excites the dye molecules and oxidizes them. The electrons released are injected into the mesoporous nano-TiO2 conduction band (TiO2 is normally used as part of the photoanode). After leaving the photoanode, the electrons move to the cathode where they reduce the I3 − ions of the redox couple into I<sup>−</sup> ions. The I<sup>−</sup> ions then return the electrons to the oxidized dye molecules. The dye molecules are regenerated, the circuit is completed and current flows. The electrolyte is a key DSSC component. The high device photon to current efficiency is also associated with electrolyte conductivity. For a DSSC with I<sup>−</sup> /I3 − redox couple, the contribution to conductivity by the I<sup>−</sup> ions is important. IL can be used to increase the iodide mobility, which may result in high DSSC efficiency.

An EMImI ionic liquid has been employed in a DSSC. However, EMImI can easily crystallize and this will prevent them from entering the pores of the nanocrystalline semiconducting TiO2 films. In order to inhibit EMImI from crystallizing, another type of IL, TIPIL can be added to EMImI electrolyte [13]. Hence, without TIPIL and if EMImI crystallizes, the photon to electricity conversion efficiency will be low [14]. The electrolyte containing TIPIL had

**Figure 1.** Configuration of DSSC.

improved the photovoltaic efficiency of the DSSC to 5.37%, which was about an 18% increase in efficiency compared to the DSSC without TIPIL. The inhibiting effect toward EMImI crystal growth decreased surface tension between EMImI and the dyed-TiO2 films and improved interfacial wetting. The stability of the cell was also improved and thus able to maintain more than 90% of the initial efficiency after the aging test. The device using EMImI with I− /I3 − redox mediator electrolyte without TIPIL exhibited a decrease of durability in less than 10 days. Hence the use of a crystallizing inhibiting IL can help to maintain stability of the DSSC.

Imidazolium iodide ILs have been used widely as a solvent in electrolytes containing I<sup>−</sup> /I3 − redox mediator for the fabrication of DSSCs [11–15]. However, the high viscosity of pure imidazolium iodide IL can obstruct diffusion of the I− and I3 − ions of the mediator, thus limiting solar cell performance [15]. Also, if the I<sup>−</sup> concentration is large, the dye molecules that have been excited (D\*) can be reduced by the iodide ions, forming reduced dye molecules (D<sup>−</sup> ) and iodine radicals I• following the reaction [16]

$$\rm D^{\prime} + I^{-} \rightarrow \rm D^{-} + I^{\*}.\tag{1}$$

This is a recombination reaction that will result in electron loss and is "parasitic" to the DSSC performance because it can result in a decrease in photocurrent. If this happens, it implies that recombination reaction is dominant over electron injection and cell efficiency will be reduced. Hence, to reduce I-ion concentration and viscosity of the electrolyte, binary or double ILs

Ionic Liquid Enhancement of Polymer Electrolyte Conductivity and their Effects on the Performance...

Since iodine has a negative effect on the efficiency when used in large concentrations, liquid electrolytes free from iodine had to be prepared [17]. Iodine in the electrolyte gives rise to

> − I2 <sup>↔</sup> I5 −

content can also result in enhanced light absorption by the carrier medi-

anion and the redox couple is generated.

increase. Polyiodides can assist electron recombination and also aid in solar cell increasing

ator. This will make the dye molecules harvest less light and led to the decrease in *V*OC and *J*

DSSCs with configuration FTO (fluorine tin oxide)/photoanode/IL-CB/Pt/FTO and FTO/pho-

stands for carbon black and PACB for polyaniline-loaded carbon black. FTO or fluoride tin oxide was the current collector. The IL was either BMImI or MPImI. With CB, the efficiencies exhibited were 4.38% (MPImI/CB) and 3.68% (BMImI/CB). The MPImI/PACB containing cell

DSSC using PACB has been investigated at RT and at 343 K. Its performance was superior to

DMPImI that has been gelled [22]. The DSSC employing the GPE achieved a photon conver-

As IL is considered a "green" solvent, it has also been applied in the DSSCs with natural dyes as sensitizer [23]. The electrolyte was 2.33 wt. % functionalized alkoxide precursor + 47.24 wt. % sulfolane + 23.62 wt. % 3-methoxypropionitrile + 10.87 wt. % AcOH + 3.54 wt. % LiI + 3.54 wt. % MPImI + 1.77 wt. % I2 + 6.02 wt. % TBP + 1.06 wt. % GuSCN. TBP and GuSCN were

IL is often used in devices as electrolyte, but fabrication of devices using liquid electrolytes (LEs) is difficult. LEs are volatile and may leak [24, 25]. This will cause the electrode to corrode

anion that under illumination

. (4)

http://dx.doi.org/10.5772/65752

ion was formed by iodide oxidation. The IL in the





matrix will also



SC.

161

were used in electrolyte preparation [17–20] and this can avoid efficiency loss.

according to the equation:

At high iodine content, polyiodide concentration in the mesoporous dyed-TiO2

toanode/IL-PACB/Pt/FTO have been fabricated [21]. The electrolytes were I2

exhibited an efficiency of 5.81% under 1 Sun illumination. The working of the I<sup>2</sup>

−

SC as well as *V*OC.

−

containing DSSC, but the I3

in the fabrication of DSSCs for performance improvement.

3 I<sup>−</sup> + 2 dye+ → I3

anion can be oxidized to I3

a DSSC using organic solvent electrolyte.

Another type of gel electrolyte that is I2

sion efficiency of 6.44%. The I<sup>2</sup>

introduced to help increase *J*

and solar cell dark current. The I2

polyiodides such as I3

dark current. A high I2

similar to the I2

Hence, I<sup>−</sup>

electrolyte can provide sufficient I−

− or I5 −

I<sup>−</sup> + I2 ↔ I3

The reduced dye molecules can either inject the electrons into the TiO2 following the equation

$$\rm D^{-} \rightarrow \rm D + e \text{(TiO}\_{2}\text{)}\tag{2}$$

and this leads to photocurrent which will be benefit the device performance. The reduced dye can also react with an I3 − ion according to the equation below.

$$\rm D^{-} + \rm I\_{3}^{-} \rightarrow \rm D + \rm I\_{2}^{-} + \rm I^{-}. \tag{3}$$

This is a recombination reaction that will result in electron loss and is "parasitic" to the DSSC performance because it can result in a decrease in photocurrent. If this happens, it implies that recombination reaction is dominant over electron injection and cell efficiency will be reduced. Hence, to reduce I-ion concentration and viscosity of the electrolyte, binary or double ILs were used in electrolyte preparation [17–20] and this can avoid efficiency loss.

Since iodine has a negative effect on the efficiency when used in large concentrations, liquid electrolytes free from iodine had to be prepared [17]. Iodine in the electrolyte gives rise to polyiodides such as I3 − or I5 − according to the equation:

$$\mathbf{I}^- \mathbf{+} \mathbf{I}\_2 \leftrightarrow \mathbf{I}\_3^- \star \mathbf{\hat{I}}\_3^- \, \mathrm{I}^- \tag{4}$$

At high iodine content, polyiodide concentration in the mesoporous dyed-TiO2 matrix will also increase. Polyiodides can assist electron recombination and also aid in solar cell increasing dark current. A high I2 content can also result in enhanced light absorption by the carrier mediator. This will make the dye molecules harvest less light and led to the decrease in *V*OC and *J* SC.

DSSCs with configuration FTO (fluorine tin oxide)/photoanode/IL-CB/Pt/FTO and FTO/photoanode/IL-PACB/Pt/FTO have been fabricated [21]. The electrolytes were I2 -free. Here CB stands for carbon black and PACB for polyaniline-loaded carbon black. FTO or fluoride tin oxide was the current collector. The IL was either BMImI or MPImI. With CB, the efficiencies exhibited were 4.38% (MPImI/CB) and 3.68% (BMImI/CB). The MPImI/PACB containing cell exhibited an efficiency of 5.81% under 1 Sun illumination. The working of the I<sup>2</sup> -free DSSC is similar to the I2 containing DSSC, but the I3 − ion was formed by iodide oxidation. The IL in the electrolyte can provide sufficient I− anion that under illumination

$$\text{3 I'} + \text{2 dye}^\* \rightarrow \text{I}\_\text{j} + \text{2dye}.\tag{5}$$

Hence, I<sup>−</sup> anion can be oxidized to I3 − anion and the redox couple is generated.

improved the photovoltaic efficiency of the DSSC to 5.37%, which was about an 18% increase in efficiency compared to the DSSC without TIPIL. The inhibiting effect toward EMImI crystal

interfacial wetting. The stability of the cell was also improved and thus able to maintain more

mediator electrolyte without TIPIL exhibited a decrease of durability in less than 10 days. Hence the use of a crystallizing inhibiting IL can help to maintain stability of the DSSC.

redox mediator for the fabrication of DSSCs [11–15]. However, the high viscosity of pure

have been excited (D\*) can be reduced by the iodide ions, forming reduced dye molecules (D<sup>−</sup>

D\* + I<sup>−</sup> → D<sup>−</sup> + I•. (1)

D<sup>−</sup> → D + e(Ti O2), (2)

and this leads to photocurrent which will be benefit the device performance. The reduced dye

<sup>−</sup> → D + I2

<sup>−</sup> + I<sup>−</sup>

ion according to the equation below.

 and I3 −

Imidazolium iodide ILs have been used widely as a solvent in electrolytes containing I<sup>−</sup>

than 90% of the initial efficiency after the aging test. The device using EMImI with I−

films and improved

ions of the mediator, thus limit-

. (3)

following the equation

concentration is large, the dye molecules that

/I3 − redox

> /I3 −

> > )

growth decreased surface tension between EMImI and the dyed-TiO2

The reduced dye molecules can either inject the electrons into the TiO2

imidazolium iodide IL can obstruct diffusion of the I−

ing solar cell performance [15]. Also, if the I<sup>−</sup>

and iodine radicals I• following the reaction [16]

−

D<sup>−</sup> + I3

can also react with an I3

**Figure 1.** Configuration of DSSC.

160 Progress and Developments in Ionic Liquids

DSSC using PACB has been investigated at RT and at 343 K. Its performance was superior to a DSSC using organic solvent electrolyte.

Another type of gel electrolyte that is I2 -free has been prepared using a mixture of KI and DMPImI that has been gelled [22]. The DSSC employing the GPE achieved a photon conversion efficiency of 6.44%. The I<sup>2</sup> -free gel electrolyte was able to reduce electron recombination and solar cell dark current. The I2 -free electrolyte system demonstrated a method to be used in the fabrication of DSSCs for performance improvement.

As IL is considered a "green" solvent, it has also been applied in the DSSCs with natural dyes as sensitizer [23]. The electrolyte was 2.33 wt. % functionalized alkoxide precursor + 47.24 wt. % sulfolane + 23.62 wt. % 3-methoxypropionitrile + 10.87 wt. % AcOH + 3.54 wt. % LiI + 3.54 wt. % MPImI + 1.77 wt. % I2 + 6.02 wt. % TBP + 1.06 wt. % GuSCN. TBP and GuSCN were introduced to help increase *J* SC as well as *V*OC.

IL is often used in devices as electrolyte, but fabrication of devices using liquid electrolytes (LEs) is difficult. LEs are volatile and may leak [24, 25]. This will cause the electrode to corrode and the dye to decompose in the medium [26, 27]. On top of these, sealing and stability problems are also associated with LEs [28, 29]. The use of IL, though beneficial is not leakage-proof due to its liquid state. To overcome this, IL can be added into polymers [30–35], gelators [24, 36] and nanoparticles [37] to form gel or quasi-solid-state electrolytes. The use of gel electrolytes solves electrolyte leakage problem, enhances robustness and increases stability [38]. According to Ref. [39], a gel-type membrane is obtained when an IL solution is immobilized in a polymer matrix. The solvents trapped in the host polymer exhibit good electrolyte conductivity. Electrolyte-photoanode and electrolyte-cathode contacts were also improved. This will also contribute to improve efficiency of the DSSCs [25, 40]. Gel electrolyte is also an alternative to replace solid electrolytes in DSSC. This is because solid electrolyte cannot fill the pores of the nanosemiconducting mesoporous TiO2 . This causes dye regeneration problems, recombination enhancement and efficiency lowering [2].

Huo et. al [36] investigated quasi–solid-state DSSC (QS-DSSCs) prepared using the low

extended alkyl chain length can delay electron recombination, enhanced *J*SC and improves IPCE. The DSSC with MHImI exhibited the highest efficiency of 3.25%. All DSSCs based on

Electrolyte solidification may solve problems associated with leakage of organic solvents. However, issues associated with evaporation and toxicity still limit the DSSCs long-term operation. To overcome problems due to toxicity and evaporation, the use of low viscosity ILs were suggested. ILs can also dissolve polysaccharides and biomacromolecules. It has been shown [38]

exhibited efficiency of ~4%. Electrolyte solidification can also be done using synthetic polymers. GPEs are electrolytes that contain mobile cations and anions. Thus the electrolyte can be polarized. For example, since in DSSCs with iodide/triiodide redox mediators require iodide ion conductivity, it would be useful to "immobilize" the cations and increase the anion transport or transference number [35]. This has led to the synthesis of an electrolyte in which, the cation is constrained to the polymer backbone. This is known as a PIL. A PIL with the cation constrained to the polymer backbone is therefore a single ion conductor. A DSSC using PMAPII as the PIL-based GPE and PEDOT-NF as the counter electrode has been reported in Ref. [45]. The transference number of the imidazolium cations is reduced due to their being immobilized, but the iodide anions can easily migrate. The PIL formed a stable GPE with

transference number. The most conducting electrolyte (0.49 S m−1) contained 16

wt. % PMAPII. The DSSC with the PIL GPE and PEDOT-NF electrode showed 8.12% efficiency. The performance was better than the liquid electrolyte DSSC and Pt counter electrode

An IL-imbibed polymer gel electrolyte has been designed [46] using an IL solvent, an IL iodide source and a polymer. The solvent was BMImCl. The iodide source was MPImI ionic liquid and poly(HEMA/GR), a hydrophilic and lipophilic host matrix. The host matrix possesses unique microporosity, extraordinary absorption and good electrolyte loading and retention to form a stable gel. The interconnected poly(HEMA/GR) framework stores the IL electrolyte. The GPE had high room temperature conductivity and good stability. The DSSC exhibited 7.15% efficiency. This was higher than the 6% efficiency exhibited by DSSC employing acetonitrile-based electrolyte-imbibed in the same polymer host. The *V*OC of the DSSC with the

MPD was 71.1 W m−2. The electrolyte concept of imbibing IL into a polymer host indicates the

imbibed into poly(AA/GR) and poly(AA/CTAB) to form stable ion conducting medium in gel

The ILs studied were MPImI, MBImI and MHImI and were gelled with C18H36O3

conduction band shift and distribution of surface states. The ILGE containing Im+

is commonly used as gelling agent for organic solvents.

, electron recombination or transport, charge diffusion, TiO<sup>2</sup>

<sup>2</sup> + BMImI + EMiDCA + TBP-based solidified electrolyte

SC was 0.014 A. The product of *V*OC and *J*SC is 106.2 W m−2. The

and the iodide ion supplier, MPImI, have been

cation alkyl chain lengths influenced adsorption

, LiI and N-MBI in MPImI, MBImI and MHImI ionic liquids, respectively,

Ionic Liquid Enhancement of Polymer Electrolyte Conductivity and their Effects on the Performance...

. The electro-

163

http://dx.doi.org/10.5772/65752

cation with

. C18H36O3

and Li+

to form ILGEs. The ILGEs with different Im<sup>+</sup>

ILGEs exhibited stable photovoltaic performance.

that a DSSC using a solvent free LiI + I

that exhibited an efficiency of 7.20%.

IL-imbibed GPE was 0.76 V and *J*

potential application in DSSCs.

As another example, the solvent BMImNO3

molecular mass C18H36O3

competition between Im+

lytes contained I2

increased I<sup>−</sup>

The PEO:KI:I2 membrane has been added with EMImTFSI. The IL has low viscosity of 0.034 Pa S at 298 K. The IL increased ionic conductivity of the membrane and improved efficiency of DSSC [41]. The conductivity increase can be accredited to the charge carriers supplied by the IL. The IL also increased amorphousness of the electrolyte. Adding another IL of low viscosity, namely, (EMImTf) into a PEO:NaI:I2 system [42] reduced crystallinity and enhanced the ionic conductivity. As for the DSSC performance, the addition of EMImTf enhanced *J* SC by about four times and the solar cell efficiency by more than three times. This work seems to show that mobility of the iodide ions and efficiency are related. By adding EMImDCN into a PEG plasticized PEO:NaI:I2 system, the performance of DSSC improved from 0.74% with no IL to 3.02% at 1 Sun illumination. This again showed conductivity-efficiency relationship and the role of IL as plasticizer [43].

PAN-based GPEs have been prepared using three salts, namely, LiI, Pr4 NI and BMImI IL. The highest RT conductivity was 0.393 S m−1 [28]. When used in DSSC with mesoporous TiO2 , the cell exhibited *J*SC of 206 A m−2 and efficiency 5.41%. The efficiency, *V*OC and *J* SC varied with electrolyte composition. These can be explained based on Li+ and Pr4 N+ adsorption on the mesoporous TiO2 surface that shifted the Fermi level of the semiconductor to a more positive or negative energy level that would certainly affect electron injection into the TiO<sup>2</sup> conduction band and *J* SC and the *V*OC, since the distance between the Fermi and the redox mediator levels can be shortened or lengthened. The ternary iodide electrolyte that includes LiI, Pr4 NI and BMImI ionic liquid uncovers the advantage of a ternary system that has been shown to increase the DSSC efficiency by 30%.

The DSSC [44] with GPE composed of P(VP-co-VAc), ethylene and propylene carbonates, KI and I2 also showed efficiency enhancement when MPImI ionic liquid was added. Again IL played the role as a plasticizing agent that increased the amorphousness of the electrolyte and eventuated in conductivity enhancement. The IL softened the polymer backbone, increased segmental mobility and provided new pathways for ionic motion. The large MPIm+ cation enabled easier I<sup>−</sup> anion dissociation that helped to increase *J*SC as the MPImI concentration increased. The smaller K+ cation used in the same electrolyte reduced electron movement through the mesoporous nano-TiO2 layer in the photoanode causing a shift in the TiO2 conduction band toward the redox potential. This led to increased *J* SC and efficiency of the DSSCs.

Huo et. al [36] investigated quasi–solid-state DSSC (QS-DSSCs) prepared using the low molecular mass C18H36O3 . C18H36O3 is commonly used as gelling agent for organic solvents. The ILs studied were MPImI, MBImI and MHImI and were gelled with C18H36O3 . The electrolytes contained I2 , LiI and N-MBI in MPImI, MBImI and MHImI ionic liquids, respectively, to form ILGEs. The ILGEs with different Im<sup>+</sup> cation alkyl chain lengths influenced adsorption competition between Im+ and Li+ , electron recombination or transport, charge diffusion, TiO<sup>2</sup> conduction band shift and distribution of surface states. The ILGE containing Im+ cation with extended alkyl chain length can delay electron recombination, enhanced *J*SC and improves IPCE. The DSSC with MHImI exhibited the highest efficiency of 3.25%. All DSSCs based on ILGEs exhibited stable photovoltaic performance.

and the dye to decompose in the medium [26, 27]. On top of these, sealing and stability problems are also associated with LEs [28, 29]. The use of IL, though beneficial is not leakage-proof due to its liquid state. To overcome this, IL can be added into polymers [30–35], gelators [24, 36] and nanoparticles [37] to form gel or quasi-solid-state electrolytes. The use of gel electrolytes solves electrolyte leakage problem, enhances robustness and increases stability [38]. According to Ref. [39], a gel-type membrane is obtained when an IL solution is immobilized in a polymer matrix. The solvents trapped in the host polymer exhibit good electrolyte conductivity. Electrolyte-photoanode and electrolyte-cathode contacts were also improved. This will also contribute to improve efficiency of the DSSCs [25, 40]. Gel electrolyte is also an alternative to replace solid electrolytes in DSSC. This is because solid electrolyte cannot fill the

membrane has been added with EMImTFSI. The IL has low viscosity of

SC by about four times and the solar cell efficiency by more than three times. This

surface that shifted the Fermi level of the semiconductor to a more positive

SC and the *V*OC, since the distance between the Fermi and the redox mediator

anion dissociation that helped to increase *J*SC as the MPImI concentration

cation used in the same electrolyte reduced electron movement

layer in the photoanode causing a shift in the TiO2

0.034 Pa S at 298 K. The IL increased ionic conductivity of the membrane and improved efficiency of DSSC [41]. The conductivity increase can be accredited to the charge carriers supplied by the IL. The IL also increased amorphousness of the electrolyte. Adding another

and enhanced the ionic conductivity. As for the DSSC performance, the addition of EMImTf

work seems to show that mobility of the iodide ions and efficiency are related. By adding

from 0.74% with no IL to 3.02% at 1 Sun illumination. This again showed conductivity-effi-

The highest RT conductivity was 0.393 S m−1 [28]. When used in DSSC with mesoporous TiO2

. This causes dye regeneration problems,

system [42] reduced crystallinity

NI and BMImI IL.

adsorption on the

SC varied with

conduc-

NI

con-

SC and efficiency of the DSSCs.

,

system, the performance of DSSC improved

and Pr4

N+

pores of the nanosemiconducting mesoporous TiO2

The PEO:KI:I2

162 Progress and Developments in Ionic Liquids

enhanced *J*

mesoporous TiO2

tion band and *J*

enabled easier I<sup>−</sup>

increased. The smaller K+

through the mesoporous nano-TiO2

and I2

increase the DSSC efficiency by 30%.

recombination enhancement and efficiency lowering [2].

IL of low viscosity, namely, (EMImTf) into a PEO:NaI:I2

ciency relationship and the role of IL as plasticizer [43].

electrolyte composition. These can be explained based on Li+

duction band toward the redox potential. This led to increased *J*

PAN-based GPEs have been prepared using three salts, namely, LiI, Pr4

the cell exhibited *J*SC of 206 A m−2 and efficiency 5.41%. The efficiency, *V*OC and *J*

or negative energy level that would certainly affect electron injection into the TiO<sup>2</sup>

levels can be shortened or lengthened. The ternary iodide electrolyte that includes LiI, Pr4

and BMImI ionic liquid uncovers the advantage of a ternary system that has been shown to

The DSSC [44] with GPE composed of P(VP-co-VAc), ethylene and propylene carbonates, KI

 also showed efficiency enhancement when MPImI ionic liquid was added. Again IL played the role as a plasticizing agent that increased the amorphousness of the electrolyte and eventuated in conductivity enhancement. The IL softened the polymer backbone, increased segmental mobility and provided new pathways for ionic motion. The large MPIm+ cation

EMImDCN into a PEG plasticized PEO:NaI:I2

Electrolyte solidification may solve problems associated with leakage of organic solvents. However, issues associated with evaporation and toxicity still limit the DSSCs long-term operation. To overcome problems due to toxicity and evaporation, the use of low viscosity ILs were suggested. ILs can also dissolve polysaccharides and biomacromolecules. It has been shown [38] that a DSSC using a solvent free LiI + I <sup>2</sup> + BMImI + EMiDCA + TBP-based solidified electrolyte exhibited efficiency of ~4%. Electrolyte solidification can also be done using synthetic polymers.

GPEs are electrolytes that contain mobile cations and anions. Thus the electrolyte can be polarized. For example, since in DSSCs with iodide/triiodide redox mediators require iodide ion conductivity, it would be useful to "immobilize" the cations and increase the anion transport or transference number [35]. This has led to the synthesis of an electrolyte in which, the cation is constrained to the polymer backbone. This is known as a PIL. A PIL with the cation constrained to the polymer backbone is therefore a single ion conductor. A DSSC using PMAPII as the PIL-based GPE and PEDOT-NF as the counter electrode has been reported in Ref. [45]. The transference number of the imidazolium cations is reduced due to their being immobilized, but the iodide anions can easily migrate. The PIL formed a stable GPE with increased I<sup>−</sup> transference number. The most conducting electrolyte (0.49 S m−1) contained 16 wt. % PMAPII. The DSSC with the PIL GPE and PEDOT-NF electrode showed 8.12% efficiency. The performance was better than the liquid electrolyte DSSC and Pt counter electrode that exhibited an efficiency of 7.20%.

An IL-imbibed polymer gel electrolyte has been designed [46] using an IL solvent, an IL iodide source and a polymer. The solvent was BMImCl. The iodide source was MPImI ionic liquid and poly(HEMA/GR), a hydrophilic and lipophilic host matrix. The host matrix possesses unique microporosity, extraordinary absorption and good electrolyte loading and retention to form a stable gel. The interconnected poly(HEMA/GR) framework stores the IL electrolyte. The GPE had high room temperature conductivity and good stability. The DSSC exhibited 7.15% efficiency. This was higher than the 6% efficiency exhibited by DSSC employing acetonitrile-based electrolyte-imbibed in the same polymer host. The *V*OC of the DSSC with the IL-imbibed GPE was 0.76 V and *J* SC was 0.014 A. The product of *V*OC and *J*SC is 106.2 W m−2. The MPD was 71.1 W m−2. The electrolyte concept of imbibing IL into a polymer host indicates the potential application in DSSCs.

As another example, the solvent BMImNO3 and the iodide ion supplier, MPImI, have been imbibed into poly(AA/GR) and poly(AA/CTAB) to form stable ion conducting medium in gel form [47]. These polymer are also "hydrophilic and lipophilic" or amphiphilic. The imbibed poly(AA/GR) GPE exhibited RT electrical conductivity of 1.78 S m−1 and that of IL imbibed poly(AA/CTAB) GPE was 1.84 S m−1. The efficiencies were 7.19% and 7.15%, respectively. The DSSCs employing acetonitrile-based GPEs with the same matrices exhibited lower efficiencies of less than 7%.

imidazolium IL of [EmIm]HSO4

increased to 650 W m−2. Fuel cell with [N1114]HSO4

and [Epdy]+

est adsorption energy of 249 kJ mol−1 followed by [Epdy]+

IL molecules compared to [N1114]+

ing the surface of the Pt catalyst. The calculations showed that [EMIm]+

olium IL, [Epdy]HSO4

performance of [N1114]+

cell performance with [N1114]+

ized ILGO has enhanced the H+

PO4

ILGO bonds with H3

attributed to the higher H<sup>+</sup>

tial for application in AFCs.

[EMIm]+

had H+

*J*

Novel OH<sup>−</sup>

was used as an electrolyte. The maximum current density

can work at 400 A m−2 with voltage higher

, calculations using Gaussian 03 was

http://dx.doi.org/10.5772/65752

(200 kJ mol−1) and [N1114]+

and [EMIm]+

ILs. These results explain why fuel

transport. Hence, the ILGO is able

fuel cell with [VBI]Br:styrene

fuel cell was 1160 W m−2 at

exhibited the high-

ILs.

IL

165

that

, the current density increased to 700 A m−2. When [N1114]HSO4

Ionic Liquid Enhancement of Polymer Electrolyte Conductivity and their Effects on the Performance...

was only 100 A m−2. The MPD was 16 W m−2. When the IL was replaced with non-imidaz-

was employed, the current density drastically increased to around 1750 A m−2 and the MPD

than 0.65 V for nearly 1 h without significant decrease in performance. To verify the better

ILs over [EMIm]+

carried out to determine the adsorption energy of these cations on protonated platinum cluster surface. Higher adsorption energy reflects more cation adsorption on the Pt surface. The higher adsorption energy also implies that many of a particular cation species are occupy-

exhibited only 7.5 kJ mol−1. Hence the electrochemical active surface area was reduced by the

Graphite oxide (GO) has been successfully functionalized using 3-aminopropyltriethoxysilane IL [52]. The fuctionalized GO was used in high temperature PEMFCs as an additive in a polybenzimidazole (PBI) anion exchange membrane (AEM). The composite membrane

imbibed with low concentrations of phosphoric acid (PA). The acidic groups in the 3-aminopropyltriethoxysilane IL functionalized GO such as carboxyl and epoxy oxygen groups, could facilitate proton hopping. This enables the membrane to be used in PEMFC. The functional-

in enhancing the PA imbibed PBI conductivity signaled its suitability for use in PEMFC. The

to enhance conductivity without requiring high PA loading. This indicates that the ILGO-PBI film has potential for application in fuel cells. When tested in PEMFC, the ILGO-PBI-PA membranes showed overwhelming performance over a PBI-PA membrane. The MPDs of the fuel cells employing PBI and ILGO-PBI with the oxygen group located in GO were 2.6 W m−2 and 3.2 W m−2 with the higher MPD for 3.5 per repeat unit. The enhanced performance has been

Fang et al. [53] have copolymerized [VMI]I and [VBI]Br with styrene for alkaline fuel cell (AFC) application. The ionic conductivity of these [VBI]Br:styrene and [VMI]I:styrene AEM was 2.26 S m−1 and 1.32 S m−1 at 303 K, respectively. The water uptake and ion exchange capacity are 56.8% and 1.26 mmol g−1, respectively. The membrane was chemically stable even after treat-

SC of 2300 A m−2 at 333 K. These results demonstrate bright prospect of AEMs for AFC.

ion has been synthesized for AFC [54]. The anionic conductivity of the electrolyte film reached 4.19 S m−1 at 353 K. This work showed that AEMs based on imidazolium cations have poten-

AEMs with IL consisting of butanediyl-1,4-bis(N-dodecylimidazole bromide) cat-

and provides pathways for fast H+

conductivity.

ment in high NaOH concentrations at 333 K for 120 h. The H2

of molar ratio 10:18 exhibited *V*OC of 1.07 V. The MPD of the H2

was higher than that with [Epdy]+

and [Epdy]+

conductivity of 3.5 and 2.5 S m−1 at 418 K for 3.5 and 2.0 per repeat units of PBI when

conduction even with low PA loading. The ILGO performance

/O2

/O2

Besides imidazolium, other ILs based on sulfonium and ammonium have also been used in DSSCs. Sulfonium ion–based ILs mostly had higher conductivity than ammonium ion–based ILs due to their lower viscosity. DSSCs employing IL with asymmetric diethylalkylsulfonium cation, [Et2 (n-C4 H9 )S]+ , showed efficiency of 4.61% while DSSC using IL based on quaternary ammonium-based cations, [Et3 (n-C8 H17)N]+ , showed 3.95% efficiency [15]. This is due to the high charge-transfer conductivity.

The above examples illustrate how ILs have been used to improve conductivity and enhance DSSC performance.

### **4. ILs in fuel cells**

Fossil fuels cannot be renewed, pollute the environment and are the primary cause of global warming. To minimize some of these problems, it is necessary to produce energy using approaches that do not "destroy" the environment. One approach is to use hydrogen fuel cells in which water is a product besides electricity. Fuel cells are chemical to electrical energy converters. They are classed according to their operating temperature and electrolyte.

PEMFCs use hydrogen or methanol as the fuels. PEMFCs are used in transportation and also portable applications. PEMFCs have shown high electrical efficiency. They do not produce pollutant and they are easy to install. For application in PEMFC, the membranes should have high proton conductivity, good mechanical strength, dimensionally stable and good chemical, electrochemical and thermal stabilities [48].

Polymer electrolytes with addition of IL have been employed in fuel cells. Imidazolium type aprotic IL has been introduced into sulfonated poly (ether ketone) or SPEK for short. The electrolyte was prepared by solution casting [49]. The electrical conductivity was two orders of magnitude higher than the IL-free SPEK membrane even under anhydrous condition. The membrane was able to operate successfully under anhydrous condition between 313 and 413 K. The plasticizing effect of the ILs has made the electrolyte membrane flexible. The ability of the membrane to operate at 413 K shows its potential as a candidate for PEMFC. The same group has also prepared anhydrous H+ conducting composite membranes with BMImTf ionic liquid [50]. Under anhydrous condition, the H+ conductivity of the membranes was of the order 10−1 S m−1 between 303 and 413 K. The H+ conductivity increased with temperature and IL content. The Tg was observed to decrease implying that the IL has penetrated the polymer chains and enhanced segmental motion of the SPEK/ethylene glycol polymer.

The effects of IL cation on the PEMFC performance have been investigated by Gao et. al [51] using [N1114]+ , [Epdy]+ and [EMIm]+ cations. The fuel cell worked poorly when the imidazolium IL of [EmIm]HSO4 was used as an electrolyte. The maximum current density was only 100 A m−2. The MPD was 16 W m−2. When the IL was replaced with non-imidazolium IL, [Epdy]HSO4 , the current density increased to 700 A m−2. When [N1114]HSO4 IL was employed, the current density drastically increased to around 1750 A m−2 and the MPD increased to 650 W m−2. Fuel cell with [N1114]HSO4 can work at 400 A m−2 with voltage higher than 0.65 V for nearly 1 h without significant decrease in performance. To verify the better performance of [N1114]+ and [Epdy]+ ILs over [EMIm]+ , calculations using Gaussian 03 was carried out to determine the adsorption energy of these cations on protonated platinum cluster surface. Higher adsorption energy reflects more cation adsorption on the Pt surface. The higher adsorption energy also implies that many of a particular cation species are occupying the surface of the Pt catalyst. The calculations showed that [EMIm]+ exhibited the highest adsorption energy of 249 kJ mol−1 followed by [Epdy]+ (200 kJ mol−1) and [N1114]+ that exhibited only 7.5 kJ mol−1. Hence the electrochemical active surface area was reduced by the [EMIm]+ IL molecules compared to [N1114]+ and [Epdy]+ ILs. These results explain why fuel cell performance with [N1114]+ was higher than that with [Epdy]+ and [EMIm]+ ILs.

form [47]. These polymer are also "hydrophilic and lipophilic" or amphiphilic. The imbibed poly(AA/GR) GPE exhibited RT electrical conductivity of 1.78 S m−1 and that of IL imbibed poly(AA/CTAB) GPE was 1.84 S m−1. The efficiencies were 7.19% and 7.15%, respectively. The DSSCs employing acetonitrile-based GPEs with the same matrices exhibited lower efficiencies

Besides imidazolium, other ILs based on sulfonium and ammonium have also been used in DSSCs. Sulfonium ion–based ILs mostly had higher conductivity than ammonium ion–based ILs due to their lower viscosity. DSSCs employing IL with asymmetric diethylalkylsulfonium

The above examples illustrate how ILs have been used to improve conductivity and enhance

Fossil fuels cannot be renewed, pollute the environment and are the primary cause of global warming. To minimize some of these problems, it is necessary to produce energy using approaches that do not "destroy" the environment. One approach is to use hydrogen fuel cells in which water is a product besides electricity. Fuel cells are chemical to electrical energy

PEMFCs use hydrogen or methanol as the fuels. PEMFCs are used in transportation and also portable applications. PEMFCs have shown high electrical efficiency. They do not produce pollutant and they are easy to install. For application in PEMFC, the membranes should have high proton conductivity, good mechanical strength, dimensionally stable and good chemi-

Polymer electrolytes with addition of IL have been employed in fuel cells. Imidazolium type aprotic IL has been introduced into sulfonated poly (ether ketone) or SPEK for short. The electrolyte was prepared by solution casting [49]. The electrical conductivity was two orders of magnitude higher than the IL-free SPEK membrane even under anhydrous condition. The membrane was able to operate successfully under anhydrous condition between 313 and 413 K. The plasticizing effect of the ILs has made the electrolyte membrane flexible. The ability of the membrane to operate at 413 K shows its potential as a candidate for PEMFC. The

mer chains and enhanced segmental motion of the SPEK/ethylene glycol polymer.

and [EMIm]+

The effects of IL cation on the PEMFC performance have been investigated by Gao et. al

converters. They are classed according to their operating temperature and electrolyte.

(n-C8

H17)N]+

, showed efficiency of 4.61% while DSSC using IL based on quaternary

, showed 3.95% efficiency [15]. This is due to the

conducting composite membranes with BMImTf

cations. The fuel cell worked poorly when the

was observed to decrease implying that the IL has penetrated the poly-

conductivity of the membranes was of

conductivity increased with temperature

of less than 7%.

cation, [Et2

(n-C4 H9 )S]+

164 Progress and Developments in Ionic Liquids

DSSC performance.

**4. ILs in fuel cells**

ammonium-based cations, [Et3

high charge-transfer conductivity.

cal, electrochemical and thermal stabilities [48].

same group has also prepared anhydrous H+

and IL content. The Tg

[51] using [N1114]+

ionic liquid [50]. Under anhydrous condition, the H+

the order 10−1 S m−1 between 303 and 413 K. The H+

, [Epdy]+

Graphite oxide (GO) has been successfully functionalized using 3-aminopropyltriethoxysilane IL [52]. The fuctionalized GO was used in high temperature PEMFCs as an additive in a polybenzimidazole (PBI) anion exchange membrane (AEM). The composite membrane had H+ conductivity of 3.5 and 2.5 S m−1 at 418 K for 3.5 and 2.0 per repeat units of PBI when imbibed with low concentrations of phosphoric acid (PA). The acidic groups in the 3-aminopropyltriethoxysilane IL functionalized GO such as carboxyl and epoxy oxygen groups, could facilitate proton hopping. This enables the membrane to be used in PEMFC. The functionalized ILGO has enhanced the H+ conduction even with low PA loading. The ILGO performance in enhancing the PA imbibed PBI conductivity signaled its suitability for use in PEMFC. The ILGO bonds with H3 PO4 and provides pathways for fast H+ transport. Hence, the ILGO is able to enhance conductivity without requiring high PA loading. This indicates that the ILGO-PBI film has potential for application in fuel cells. When tested in PEMFC, the ILGO-PBI-PA membranes showed overwhelming performance over a PBI-PA membrane. The MPDs of the fuel cells employing PBI and ILGO-PBI with the oxygen group located in GO were 2.6 W m−2 and 3.2 W m−2 with the higher MPD for 3.5 per repeat unit. The enhanced performance has been attributed to the higher H<sup>+</sup> conductivity.

Fang et al. [53] have copolymerized [VMI]I and [VBI]Br with styrene for alkaline fuel cell (AFC) application. The ionic conductivity of these [VBI]Br:styrene and [VMI]I:styrene AEM was 2.26 S m−1 and 1.32 S m−1 at 303 K, respectively. The water uptake and ion exchange capacity are 56.8% and 1.26 mmol g−1, respectively. The membrane was chemically stable even after treatment in high NaOH concentrations at 333 K for 120 h. The H2 /O2 fuel cell with [VBI]Br:styrene of molar ratio 10:18 exhibited *V*OC of 1.07 V. The MPD of the H2 /O2 fuel cell was 1160 W m−2 at *J* SC of 2300 A m−2 at 333 K. These results demonstrate bright prospect of AEMs for AFC.

Novel OH<sup>−</sup> AEMs with IL consisting of butanediyl-1,4-bis(N-dodecylimidazole bromide) cation has been synthesized for AFC [54]. The anionic conductivity of the electrolyte film reached 4.19 S m−1 at 353 K. This work showed that AEMs based on imidazolium cations have potential for application in AFCs.

Ortiz-Martínez et al. [55] have embedded a polymer-IL membrane into a carbon cathode for application in microbial fuel cells (MFCs). The IL was methyltrioctylammonium chloride. The MFC works when organic matter is oxidized by microbes and the electrons produced are transferred to the carbon cathode. The cell achieved a volumetric MPD of 0.613 W m−3, which is about 10 times higher than the output of typical membranes. Coulombic efficiency also increased from 19.18 to 64.96%. The results obtained by these researchers indicate the potential application of embedded polymer IL membrane-cathode assembly in MFCs. This approach also improved contact between the separator and cathode.

assembled Li/PIL-IL-LiTFSI-SiO2

stability (5.8 V versus Fc/Fc+

specific capacities with LiFePO<sup>4</sup>

/quaternary PE/Li4

high capacity and good cyclability.

exhibited a maximum Li+

The ionic liquid BMImBF4

good cyclability.

The Li/50% SN/LiFePO4

great potential in LIBs.

Li+

PE can also be safely used with LiMn2

[61]. The LiMn2

LiFePO4

of LiFePO4

LiMn2 O4

the cell was still 134 Ah kg−1 even after 50 cycles.

/LiFePO4

at 368 K. The IL-GPE electrochemical stability was ~4.8 V versus Li/Li<sup>+</sup>

lyte interfacial contact and is a promising electrolyte for flexible LIBs.

A SPE system comprising PVdF host, an ionic liquid MePrPipNTf2

polymer batteries at elevated temperatures. The Li<sup>+</sup>

O4

O4

Yang et al. [62] have used poly(VdF-co-HFP)-based GPEs with LiTFSI salt and B4

for application in LIB. The ILGPE 66.7 wt. % (PVdF-HFP/LiTFSI)-33.3 wt. % B4

[63]. The GPE was microporous. After addition of IL into the microporous GPE, Tg

observed to decrease. However the IL uptake into the membrane increased. The reason was that the IL that interacted with the polymer disrupted some of the PVDF-HFP crystalline

PIL-plastic crystal CPE consisting of P(DADMA)TFSI PIL, succinonitrile (SN) and LiTFSI as the

298 K and has good capacity retention. The findings illustrated the fact that the electrolyte has

 ion source [64]. The 40%PIL-40%SN-20%LiTFSI (designated as 50% SN) electrolyte exhibited a high ambient conductivity, a wide potential window and good mechanical strength.

cell discharged at C/10 rate delivered a capacity of ~150 Ah kg−1 at

Ti5

phases and increased amorphousness that decreased Tg

also helped to expand the amorphous zone. The LiFePO4

the electrode and exhibited discharge capacity of ~136 Ah kg−1 at 0.1 C. The cell recorded capacity fading of 0.075 Ah kg−1 per cycle for 40 cycles. The specific discharge capacity of

ILGPE has been prepared by photocuring a mixture containing MPPipTFSI ionic liquid, LiTFSI and ethoxylated bisphenol A diacrylate, a long chain monomer [59]. MPPipTFSI had a sufficiently high conductivity to be applied in electrochemical devices and its wide voltage

IL-GPE conductivity was 6.4 × 10−3 S m−1 at 298 K and increased with temperature to 0.48 S m−1

capacity at C/20 rate and 298 K. This showed that the IL-GPE provides good electrode/electro-

been prepared by Swiderska-Mocek [60]. VC is an additive to help form the solid electrolyte interphase (SEI) layer. The conductivity of the elastic membrane was 0.44 S m−1 at 298 K. The activation energy was 12.4 kJ mol−1. The IL-incorporated membrane decomposed at 583 K. The high decomposition temperature signaled the safe use of the membrane in lithium

PVdF + LiTFSI + MePrPipNTf2 + VC can display good cyclability and safely deliver high

as cathode in LIBs.

ionic conductivity of 0.20 S m−1. The Li/ILGPE/LiFePO4

was introduced into PVDF-HFP/PMMA-LiClO4

cathode. The researchers also used LiMn2

cathode also exhibited good specific capacity. The full

O12 cell displayed good cycling performance indicating that the

displayed stable cyclability with capacity that is 4% less than the LiFePO4

) provided good compatibility with the LiFePO4

Ionic Liquid Enhancement of Polymer Electrolyte Conductivity and their Effects on the Performance...

cell showed that the IL was able to penetrate into

cathode. The

167

theoretical

. The cell Li/IL-GPE/

http://dx.doi.org/10.5772/65752

, LiTFSI salt and VC have

O4

in place

MePyTFSI

MePyTFSI

was

cell showed

blend membrane

cell with the quarternary electrolyte

of the GPE. The addition of LiClO4

/GPE/Li coin-type cell exhibited

SILMs also have potential for use in MFCs as proton exchange membranes. SILMs have been evaluated by Hernández-Fernández et al. [56]. The ILs used were [BMIm+ ] and [OMIM+ ] and [MTOA+ ] cation combined with Cl<sup>−</sup> , BF4 − , PF6 − and TFSI<sup>−</sup> anions. The voltage of the MFC with different SILMs was monitored continuously for more than 160 h. The fuel cell using the supported [OMIM+ ][PF6 − ] film attained the highest voltage of ~0.14 V. It is unfortunate that the IL in the film has the highest toxicity. The highest chemical oxygen demand (COD) removal for the MFCs has been achieved using [MTOA+ ][Cl<sup>−</sup> ]. Its value was 89.1%. Its nonecotoxicity is comparable to the commercial membranes such as Ultrex® and Nafion® with 88.3% and 90.7%, respectively.

In another study Ref. [57] [MTOA+ ][Cl<sup>−</sup> ], [OMIM+ ][PF6 − ], [P4,4,4,1+ ][MeSO4 − ] and [P4,4,4,1+ ] [TOS<sup>−</sup> ] IL-based membranes were evaluated. The highest COD removal value was still achieved by the [MTOA+ ][Cl<sup>−</sup> ]-based membrane while the [P4,4,4,1+ ][TOS<sup>−</sup> ] and Nafion®117 membranes exhibited slightly lower CODs. In terms of volumetric power density (VPD), [P4,4,4,1+ ][TOS<sup>−</sup> ] membrane offered higher VPD (0.795 W m−3) compared to Nafion©117 (0.756 W m−3). The fuel cell based on [MTOA+ ][Cl<sup>−</sup> ] membrane exhibited 0.237 W m−3. Overall performance among the ILs applied, [P4,4,4,1+ ][TOS<sup>−</sup> ] showed better performance than Nafion®117 in terms of VPD and COD removal.

### **5. IL in lithium batteries**

Due to their high energy density, lithium ion batteries (LIBs) have become attractive power sources for many applications such as mobiles, cameras and laptops. The specific energy in Wh kg−1 that can be delivered by LIBs, Ni-MH, Ni-Cd and lead acid batteries is 160, 90, 45 and 40, respectively. Hence lead acid batteries can only deliver 25% of the energy that can be supplied by LIBs.

Li et al. [58] have prepared a novel CPE with a PIL, namely, poly((4-vinylbenzyl) trimethylammonium bis(trifluoromethanesulfonyl)imide) as the host, DEME-TFSI ionic liquid, LiTFSI salt and nanosilica filler. PIL-based electrolytes with IL could effectively improve the electrode/electrolyte interface, which signaled a direction for the application of PIL electrolytes in LIBs. The compatible PIL-IL combination provides stable PEs with minimized phase separation and leakage. The electrical conductivity increased with IL content attributed to the increased amorphousness brought about by the plasticizing effect of DEME-TFSI. At 60% IL content, the electrolyte conductivity was 7.58 × 10−2 S m−1 at 333 K. The assembled Li/PIL-IL-LiTFSI-SiO2 /LiFePO4 cell showed that the IL was able to penetrate into the electrode and exhibited discharge capacity of ~136 Ah kg−1 at 0.1 C. The cell recorded capacity fading of 0.075 Ah kg−1 per cycle for 40 cycles. The specific discharge capacity of the cell was still 134 Ah kg−1 even after 50 cycles.

Ortiz-Martínez et al. [55] have embedded a polymer-IL membrane into a carbon cathode for application in microbial fuel cells (MFCs). The IL was methyltrioctylammonium chloride. The MFC works when organic matter is oxidized by microbes and the electrons produced are transferred to the carbon cathode. The cell achieved a volumetric MPD of 0.613 W m−3, which is about 10 times higher than the output of typical membranes. Coulombic efficiency also increased from 19.18 to 64.96%. The results obtained by these researchers indicate the potential application of embedded polymer IL membrane-cathode assembly in MFCs. This

SILMs also have potential for use in MFCs as proton exchange membranes. SILMs have been

different SILMs was monitored continuously for more than 160 h. The fuel cell using the sup-

in the film has the highest toxicity. The highest chemical oxygen demand (COD) removal for

][Cl<sup>−</sup>

comparable to the commercial membranes such as Ultrex® and Nafion® with 88.3% and 90.7%,

], [OMIM+

membrane offered higher VPD (0.795 W m−3) compared to Nafion©117 (0.756 W m−3). The fuel

Due to their high energy density, lithium ion batteries (LIBs) have become attractive power sources for many applications such as mobiles, cameras and laptops. The specific energy in Wh kg−1 that can be delivered by LIBs, Ni-MH, Ni-Cd and lead acid batteries is 160, 90, 45 and 40, respectively. Hence lead acid batteries can only deliver 25% of the energy that can be

Li et al. [58] have prepared a novel CPE with a PIL, namely, poly((4-vinylbenzyl) trimethylammonium bis(trifluoromethanesulfonyl)imide) as the host, DEME-TFSI ionic liquid, LiTFSI salt and nanosilica filler. PIL-based electrolytes with IL could effectively improve the electrode/electrolyte interface, which signaled a direction for the application of PIL electrolytes in LIBs. The compatible PIL-IL combination provides stable PEs with minimized phase separation and leakage. The electrical conductivity increased with IL content attributed to the increased amorphousness brought about by the plasticizing effect of DEME-TFSI. At 60% IL content, the electrolyte conductivity was 7.58 × 10−2 S m−1 at 333 K. The

exhibited slightly lower CODs. In terms of volumetric power density (VPD), [P4,4,4,1+

and TFSI<sup>−</sup>

] film attained the highest voltage of ~0.14 V. It is unfortunate that the IL

], [P4,4,4,1+

][TOS<sup>−</sup>

] membrane exhibited 0.237 W m−3. Overall performance among the

] showed better performance than Nafion®117 in terms of VPD

][PF6 −

] IL-based membranes were evaluated. The highest COD removal value was still achieved

] and [OMIM+

] and [P4,4,4,1+

] and Nafion®117 membranes

anions. The voltage of the MFC with

]. Its value was 89.1%. Its nonecotoxicity is

][MeSO4 − ] and

]

][TOS<sup>−</sup> ]

approach also improved contact between the separator and cathode.

] cation combined with Cl<sup>−</sup>

the MFCs has been achieved using [MTOA+

][Cl<sup>−</sup>

][TOS<sup>−</sup>

][PF6 −

166 Progress and Developments in Ionic Liquids

In another study Ref. [57] [MTOA+

][Cl<sup>−</sup>

[MTOA+

ported [OMIM+

respectively.

by the [MTOA+

cell based on [MTOA+

ILs applied, [P4,4,4,1+

**5. IL in lithium batteries**

and COD removal.

supplied by LIBs.

[TOS<sup>−</sup>

evaluated by Hernández-Fernández et al. [56]. The ILs used were [BMIm+

, BF4 − , PF6 −

][Cl<sup>−</sup>

]-based membrane while the [P4,4,4,1+

ILGPE has been prepared by photocuring a mixture containing MPPipTFSI ionic liquid, LiTFSI and ethoxylated bisphenol A diacrylate, a long chain monomer [59]. MPPipTFSI had a sufficiently high conductivity to be applied in electrochemical devices and its wide voltage stability (5.8 V versus Fc/Fc+ ) provided good compatibility with the LiFePO4 cathode. The IL-GPE conductivity was 6.4 × 10−3 S m−1 at 298 K and increased with temperature to 0.48 S m−1 at 368 K. The IL-GPE electrochemical stability was ~4.8 V versus Li/Li<sup>+</sup> . The cell Li/IL-GPE/ LiFePO4 displayed stable cyclability with capacity that is 4% less than the LiFePO4 theoretical capacity at C/20 rate and 298 K. This showed that the IL-GPE provides good electrode/electrolyte interfacial contact and is a promising electrolyte for flexible LIBs.

A SPE system comprising PVdF host, an ionic liquid MePrPipNTf2 , LiTFSI salt and VC have been prepared by Swiderska-Mocek [60]. VC is an additive to help form the solid electrolyte interphase (SEI) layer. The conductivity of the elastic membrane was 0.44 S m−1 at 298 K. The activation energy was 12.4 kJ mol−1. The IL-incorporated membrane decomposed at 583 K. The high decomposition temperature signaled the safe use of the membrane in lithium polymer batteries at elevated temperatures. The Li<sup>+</sup> cell with the quarternary electrolyte PVdF + LiTFSI + MePrPipNTf2 + VC can display good cyclability and safely deliver high specific capacities with LiFePO<sup>4</sup> cathode. The researchers also used LiMn2 O4 in place of LiFePO4 [61]. The LiMn2 O4 cathode also exhibited good specific capacity. The full LiMn2 O4 /quaternary PE/Li4 Ti5 O12 cell displayed good cycling performance indicating that the PE can also be safely used with LiMn2 O4 as cathode in LIBs.

Yang et al. [62] have used poly(VdF-co-HFP)-based GPEs with LiTFSI salt and B4 MePyTFSI for application in LIB. The ILGPE 66.7 wt. % (PVdF-HFP/LiTFSI)-33.3 wt. % B4 MePyTFSI exhibited a maximum Li+ ionic conductivity of 0.20 S m−1. The Li/ILGPE/LiFePO4 cell showed high capacity and good cyclability.

The ionic liquid BMImBF4 was introduced into PVDF-HFP/PMMA-LiClO4 blend membrane [63]. The GPE was microporous. After addition of IL into the microporous GPE, Tg was observed to decrease. However the IL uptake into the membrane increased. The reason was that the IL that interacted with the polymer disrupted some of the PVDF-HFP crystalline phases and increased amorphousness that decreased Tg of the GPE. The addition of LiClO4 also helped to expand the amorphous zone. The LiFePO4 /GPE/Li coin-type cell exhibited good cyclability.

PIL-plastic crystal CPE consisting of P(DADMA)TFSI PIL, succinonitrile (SN) and LiTFSI as the Li+ ion source [64]. The 40%PIL-40%SN-20%LiTFSI (designated as 50% SN) electrolyte exhibited a high ambient conductivity, a wide potential window and good mechanical strength. The Li/50% SN/LiFePO4 cell discharged at C/10 rate delivered a capacity of ~150 Ah kg−1 at 298 K and has good capacity retention. The findings illustrated the fact that the electrolyte has great potential in LIBs.

### **6. IL in supercapacitors**

The application of supercapacitors for energy storage can help to minimize dependence on fossil fuels. According to Mysyk et al. [65], supercapacitors have energy density higher than electrolytic capacitors. They also have power density higher than rechargeable batteries. Supercapacitors bridge the gap between electrolytic capacitors and rechargeable batteries. Supercapacitors can be divided into three general classes. There are electrical double layer capacitors (EDLCs), pseudocapacitors and hybrid capacitors. Each class is characterized by its charge storage mechanism. These mechanism are non-Faradaic, Faradaic and a combination of both. Non-Faradaic mechanism can be likened to electrostatic charge storage. Faradaic reaction is a heterogenous charge transfer reaction that occurs on the surface of an electrode.

Decrease in viscosity led to the increase in anion and cation self-diffusion. This implied that

Ionic Liquid Enhancement of Polymer Electrolyte Conductivity and their Effects on the Performance...

A carbon-based supercapacitor has been fabricated using protic ionic liquid (PrIL) as novel electrolyte [65]. PrILs are created when one proton is transferred between a Brönsted acid and base. When compared to aprotic ILs, PrILs are cheaper and easier to synthesize [73]. Cyclic voltammograms of PrILs exhibit reversible redox peaks. This indicated pseudo-Fara-

resulted in enhanced capacitance that is attributed to pseudo-Faradaic contribution. PrILbased capacitors show a wider potential window than capacitors using aqueous H2

AC-PrIL combination shows promise for supercapacitors with good energy characteristics.

able to operate at 2.5 V. At water concentration of 200 ppm the electrolyte was considered dry. The voltage of AC-PrIL-based supercapacitors will be affected if the electrolyte contains a significant amount of water. The cell voltage will then not be able to exceed 1.2 V, that is, the potential window of water. Brandt et. al [75] have studied various carbon-based super-

NHTFSI, Me3

for long-term cycling stability. They reported that the supercapacitor employing PrIL-based supercapacitors exhibited stability even after 30,000 cycles. The supercapacitors can also per-

Timperman et al. [67] have described the use of eutectic PrIL mixtures of PYRNO3

The performance of IL electrolytes consisting of various cations such as EMIm+

PYRTFSI as ion conducting medium for application in AC-based supercapacitors. The capaci-

the capacitor showed more than 95% efficiency at 1.5 V for at least 80 h and at 2.0 V for 110 h.

excellent electrolyte behavior for supercapacitor with graphene nanosheet (GNS) electrodes. The supercapacitor delivered 235 F g−1 specific capacitance, energy density of 88 Wh kg−1 and specific power 17.5 kW kg−1 at 298 K. IL electrolytes perform better at high temperatures. This is because their conductivity increased with temperature and their viscosity decreased. The decrease in viscosity led to enhancement in ionic mobility. This led to increased conductivity.

One of the challenges facing IL supercapacitors is to improve electrolyte accessibility to the nanocarbon surface. According to Trigueiro et al. [77], the wettability of the electrodes with an IL electrolyte can be facilitated by the poly(IL) molecules electrostatically linked to reduced graphene oxide (RGO) surface electrode, producing high specific capacitance. As an example, a [MPPy][TFSI]-modified RGO electrode (PIL:RGO) exhibited a drop in specific capacitance of only 20% as scan rate increased from 10 to 30 mV s−1 at 298 K, while an RGO capacitor lost 42% of its specific capacitance under similar conditions. The effective intercalation and distribution of the poly(IL) molecules in the RGO nanosheets contributed to the improved

temperatures down to 250 K. The supercapacitor containing the protic IL Et3

tors showed good capacity retention. Using electrolyte with [PYR]/[NO3

HTFSI PrIL has been used as an ion conducting medium in supercapacitors with AC elec-

treatment of AC augmented surface performance and

O in NEt3

NHTFSI and PYRNO3

anions have been investigated [76]. BMP-DCA ionic liquid showed

NHTFSI mixed electrolyte can be used even at

SO4 . 169

H TFSI electrolyte are

http://dx.doi.org/10.5772/65752

as electrolytes

NHTFSI showed

] ratio of 0.72 at 298 K,

and

and BMP+

ion dissociation and ionic mobility have increased at elevated temperatures.

daic charge transfer. Oxidative HNO3

capacitors using various PrILs such as Et3

stable performance from 283 to 333 K.

and DCA<sup>−</sup>

Stability was also maintained at elevated temperatures.

form over wide temperature range. The PC-Me3

trodes [74]. Supercapacitors with less than 200 ppm of H2

Et3

and TFSI<sup>−</sup>

, BF4<sup>−</sup>

Supercapacitors comprise an anode, electrolyte and a cathode [66]. The potential stability window of the electrolyte determines the supercapacitor voltage. The electrode and electrolyte also determine the power density and cyclability of the supercapacitor. Hence, it is important to choose the proper electrolyte [67].

A lot of attention has been given to carbon-based capacitors. This is because these carbonaceous materials possess diversified morphologies, excellent cycling stability, high power capability and conductivity [68, 69]. EMImBF4 was introduced as an ion conducting medium in an EDLC of AC nanofibers. Propylene carbonate (PC) that can dissolve EMImBF<sup>4</sup> was also added into the electrolyte. Without PC, the EDLC delivered a decreasing capacitance from ~78 to ~60 F g−1. On addition of PC, the capacitance delivered maintained at ~80 Fg−1. This showed that PC helped to recover the lost capacitance by breaking the interaction between the carbon micropore wall and EMIm+ cation and also releasing the immobile ions in the micropores during the cycling. By incorporating Cu(II) into EMImBF4 [70], the electrochemical capacitors exhibited good stability with only ~9% capacitance loss after 500 cycles. The capacitance is higher than that obtained from an EDLC using pure EMImBF4 as electrolyte. The average specific capacitance is 108 F g−1 for EMImBF4 and 225 F g−1 for EMImBF4 with Cu(II). The increase indicated that the Faradaic pseudocapacitance associated with the Cu species has contributed considerably to the total capacitance of the EDLC.

The ionic liquid DEME-BF4 , with wide potential window of 6 V also showed high potential for electrochemical capacitors. Its conductivity at 25°C is considerably high, 4.8 mS cm−1 [71]. Aliphatic quaternary ammonium-based ILs are expected to show higher cathodic stability since its melting point is higher compared to aromatic ILs. EDLC with showed little gas release and smaller capacity fade at 373 K. This implies that AC electrodes and DEME-BF4 electrolyte combination contributed to cell stability compared to the EDLC using TEA-BF4 / PC, which exhibited a large capacity fade at 373 K. The DEME-BF4 EDLC retained 85% of its initial capacity even after 500 cycles.

According to Yuyama et al. [72], ILs with BF4 − anion exhibited better stability and performance compared to PF6 − and TFSI<sup>−</sup> anions when used in EDLCs. The equivalent series resistance (ESR) and direct current (dc) resistance at 298 K and 243 K decreased accordingly as TFSI<sup>−</sup> > PF6 <sup>−</sup> > BF4 − . According to Fletcher et. al [69], dc resistance and ESR decreased with increasing temperature and this can be accredited to the decreasing IL electrolyte viscosity. Decrease in viscosity led to the increase in anion and cation self-diffusion. This implied that ion dissociation and ionic mobility have increased at elevated temperatures.

**6. IL in supercapacitors**

168 Progress and Developments in Ionic Liquids

important to choose the proper electrolyte [67].

capability and conductivity [68, 69]. EMImBF4

cific capacitance is 108 F g−1 for EMImBF4

considerably to the total capacitance of the EDLC.

during the cycling. By incorporating Cu(II) into EMImBF4

higher than that obtained from an EDLC using pure EMImBF4

PC, which exhibited a large capacity fade at 373 K. The DEME-BF4

micropore wall and EMIm+

The ionic liquid DEME-BF4

initial capacity even after 500 cycles.

mance compared to PF6

<sup>−</sup> > BF4 −

TFSI<sup>−</sup> > PF6

According to Yuyama et al. [72], ILs with BF4

−

and TFSI<sup>−</sup>

The application of supercapacitors for energy storage can help to minimize dependence on fossil fuels. According to Mysyk et al. [65], supercapacitors have energy density higher than electrolytic capacitors. They also have power density higher than rechargeable batteries. Supercapacitors bridge the gap between electrolytic capacitors and rechargeable batteries. Supercapacitors can be divided into three general classes. There are electrical double layer capacitors (EDLCs), pseudocapacitors and hybrid capacitors. Each class is characterized by its charge storage mechanism. These mechanism are non-Faradaic, Faradaic and a combination of both. Non-Faradaic mechanism can be likened to electrostatic charge storage. Faradaic reaction is a heterogenous charge transfer reaction that occurs on the surface of an electrode. Supercapacitors comprise an anode, electrolyte and a cathode [66]. The potential stability window of the electrolyte determines the supercapacitor voltage. The electrode and electrolyte also determine the power density and cyclability of the supercapacitor. Hence, it is

A lot of attention has been given to carbon-based capacitors. This is because these carbonaceous materials possess diversified morphologies, excellent cycling stability, high power

added into the electrolyte. Without PC, the EDLC delivered a decreasing capacitance from ~78 to ~60 F g−1. On addition of PC, the capacitance delivered maintained at ~80 Fg−1. This showed that PC helped to recover the lost capacitance by breaking the interaction between the carbon

exhibited good stability with only ~9% capacitance loss after 500 cycles. The capacitance is

indicated that the Faradaic pseudocapacitance associated with the Cu species has contributed

tial for electrochemical capacitors. Its conductivity at 25°C is considerably high, 4.8 mS cm−1 [71]. Aliphatic quaternary ammonium-based ILs are expected to show higher cathodic stability since its melting point is higher compared to aromatic ILs. EDLC with showed little gas release and smaller capacity fade at 373 K. This implies that AC electrodes and DEME-BF4 electrolyte combination contributed to cell stability compared to the EDLC using TEA-BF4

−

tance (ESR) and direct current (dc) resistance at 298 K and 243 K decreased accordingly as

increasing temperature and this can be accredited to the decreasing IL electrolyte viscosity.

and 225 F g−1 for EMImBF4

in an EDLC of AC nanofibers. Propylene carbonate (PC) that can dissolve EMImBF<sup>4</sup>

was introduced as an ion conducting medium

[70], the electrochemical capacitors

as electrolyte. The average spe-

with Cu(II). The increase

EDLC retained 85% of its

anion exhibited better stability and perfor-

anions when used in EDLCs. The equivalent series resis-

. According to Fletcher et. al [69], dc resistance and ESR decreased with

cation and also releasing the immobile ions in the micropores

, with wide potential window of 6 V also showed high poten-

was also

/

A carbon-based supercapacitor has been fabricated using protic ionic liquid (PrIL) as novel electrolyte [65]. PrILs are created when one proton is transferred between a Brönsted acid and base. When compared to aprotic ILs, PrILs are cheaper and easier to synthesize [73]. Cyclic voltammograms of PrILs exhibit reversible redox peaks. This indicated pseudo-Faradaic charge transfer. Oxidative HNO3 treatment of AC augmented surface performance and resulted in enhanced capacitance that is attributed to pseudo-Faradaic contribution. PrILbased capacitors show a wider potential window than capacitors using aqueous H2 SO4 . AC-PrIL combination shows promise for supercapacitors with good energy characteristics. Et3 HTFSI PrIL has been used as an ion conducting medium in supercapacitors with AC electrodes [74]. Supercapacitors with less than 200 ppm of H2 O in NEt3 H TFSI electrolyte are able to operate at 2.5 V. At water concentration of 200 ppm the electrolyte was considered dry. The voltage of AC-PrIL-based supercapacitors will be affected if the electrolyte contains a significant amount of water. The cell voltage will then not be able to exceed 1.2 V, that is, the potential window of water. Brandt et. al [75] have studied various carbon-based supercapacitors using various PrILs such as Et3 NHTFSI, Me3 NHTFSI and PYRNO3 as electrolytes for long-term cycling stability. They reported that the supercapacitor employing PrIL-based supercapacitors exhibited stability even after 30,000 cycles. The supercapacitors can also perform over wide temperature range. The PC-Me3 NHTFSI mixed electrolyte can be used even at temperatures down to 250 K. The supercapacitor containing the protic IL Et3 NHTFSI showed stable performance from 283 to 333 K.

Timperman et al. [67] have described the use of eutectic PrIL mixtures of PYRNO3 and PYRTFSI as ion conducting medium for application in AC-based supercapacitors. The capacitors showed good capacity retention. Using electrolyte with [PYR]/[NO3 ] ratio of 0.72 at 298 K, the capacitor showed more than 95% efficiency at 1.5 V for at least 80 h and at 2.0 V for 110 h.

The performance of IL electrolytes consisting of various cations such as EMIm+ and BMP+ and TFSI<sup>−</sup> , BF4<sup>−</sup> and DCA<sup>−</sup> anions have been investigated [76]. BMP-DCA ionic liquid showed excellent electrolyte behavior for supercapacitor with graphene nanosheet (GNS) electrodes. The supercapacitor delivered 235 F g−1 specific capacitance, energy density of 88 Wh kg−1 and specific power 17.5 kW kg−1 at 298 K. IL electrolytes perform better at high temperatures. This is because their conductivity increased with temperature and their viscosity decreased. The decrease in viscosity led to enhancement in ionic mobility. This led to increased conductivity. Stability was also maintained at elevated temperatures.

One of the challenges facing IL supercapacitors is to improve electrolyte accessibility to the nanocarbon surface. According to Trigueiro et al. [77], the wettability of the electrodes with an IL electrolyte can be facilitated by the poly(IL) molecules electrostatically linked to reduced graphene oxide (RGO) surface electrode, producing high specific capacitance. As an example, a [MPPy][TFSI]-modified RGO electrode (PIL:RGO) exhibited a drop in specific capacitance of only 20% as scan rate increased from 10 to 30 mV s−1 at 298 K, while an RGO capacitor lost 42% of its specific capacitance under similar conditions. The effective intercalation and distribution of the poly(IL) molecules in the RGO nanosheets contributed to the improved wettability as well as interaction with the IL. These results implied that 80% capacity is retained for PIL:RGO and 58% RGO capacitors. At 333 K, 70% of the capacity was retained for PIL:RGO and 61% capacity retention for RGO-based supercapacitors.

The ionic liquid, BmImBr has been introduced into a PVA/CH3

COONH4

was obtained with better electrochemical characteristics.

. The primary cause of reduction in Tg

Addition of BMImTf ionic liquid in PVA/CH3

for 250 cycles.

The BdMImBF4

of BdMImBF4

**7. Summary**

21 W kg−1, respectively.

ductivity of and PEO:NaI:I2

Tg

BmImCl to the PVA/CH3

cation in EDLC [81]. The EDLC delivered a capacitance of 21.89 F g−1 and continued to do so

Ionic Liquid Enhancement of Polymer Electrolyte Conductivity and their Effects on the Performance...

An electrical conductivity of 0.73 S m−1 was achieved at 393 K upon adulteration of 50 wt. %

softened the polymer backbone, increased segmental motion thus helped in facilitating ionic transport. On addition of the IL containing ion conducting medium, the amorphousness of the membrane was observed to increase. It was also observed that the polymer electrolyte exhibited a wider potential stability range. The presence of IL has also improved the electrochemical behavior of the EDLC delivering 2.39 Wh kg−1 of specific energy and 19.79 W kg−1 of specific power with Coulombic efficiency above 90%. The specific capacitance of 28.36 F g−1

COONH4

ionic liquid was added into a gel matrix that comprised a blended host of

was added. The calculated specific energy and power were 14 Wh kg−1 and

electrolyte exhibiting strong adhesive properties. Thus, better electrode-electrolyte interfacial contact can be provided. The specific capacitance of the EDLC obtained was 2.02 from 0.14 F g−1 without IL. This implied that the IL-based polymer electrolytes possessed higher dielectric constant compared to the IL-free polymer electrolyte. The addition of IL has also

PVP/PVdF-HFP and magnesium triflate salt. The gel was used in an EDLC [83]. The interaction between the IL and the polar groups of the polymer chain increased amorphousness of the GPE. The ionic conductivity achieved a maximum value of 0.29 S m−1 after 7.5 wt. %

ILs consist of poorly coordinating cations and anions and are molten salts at room temperature. They have high thermal stability, wide potential window and high electrical conductivity. ILs are green solvents with negligible vapor pressure. IL such as EMImTf, EMImBF4

PP24TFSI, to name a few can enhance ionic conductivity when added to PEs. Conductivity enhancement is attributed to the additional ions supplied by the IL, the plasticizing nature of the IL and its low viscosity that facilitates ion mobility. The plasticizing nature of ILs softens the polymer backbone resulting in the increase in polymer segmental motion. This implies that IL can increase amorphousness of a PE. ILs with short alkyl chains are less viscous and

chain length can delay electron recombination, enhance *J*SC and improves IPCE in DSSCs. EMImTFSI, EMImTf and EMImDCN are low viscosity ILs that are able to increase ionic con-

have higher conductivity than ammonium ion-based ILs due to the lower viscosity of the

systems. It is also to be noted that most sulfonium ion-based ILs

extended the electrolyte voltage window and improved thermal stability.

hence suitable for facilitating ion mobility. However, IL containing Im+

COONH4

was the ability of the IL to plasticize. Plasticization

PE system [82] resulted in the polymer

and

cation with long alkyl

PE system [80]. This is accompanied with the lowering of

PE system for appli-

171

http://dx.doi.org/10.5772/65752

Another approach to obtain stable polymer electrolytes thermally and electrochemically is by incorporating RTIL into polymer electrolytes [78]. Pandey et. al [79] has compared the EDLC performance using electrolyte in the solid-state and ionic liquid incorporated polymer electrolytes. The electrodes of EDLCs comprised multiwalled carbon nanotube (MWCNT). The polymer electrolytes consisted of PEO, triflate salts of magnesium and lithium and EMImTf ionic liquid. The ambient conductivity was ~10−2 S m−1. The addition of IL increased the polymer backbone flexibility and enhanced segmental motion. The use of the triflate salts and IL introduced Li+ /Mg2+, EMIm+ and CF3 SO3 − free ions. The presence of many types of mobile ionic species increased the possibility of formation of more double layers where energy was stored. The enhanced flexibility is also useful for proper electrode-electrolyte contact. Incorporating EMImTf resulted in a substantial increment of PEO-Mg(Tf)2 electrolyte conductivity. MWCNT with Li-based and Mg-based electrolyte could deliver between 1.7 and 2.1 F g−1 and between 2.6 and 3.0 F g−1, specific capacitance, respectively. Without IL incorporation the capacitance per unit mass were ~0.03 F g−1 and ~0.01 F g−1 for the Mg- and Li-system, respectively. This showed IL contribution on EDLC capacitance.

Ayalneh Tiruye et al. [78] have assembled supercapacitors containing two parts of a pyrrolidinium-based PIL, that is, pDADMTFSI and three parts PYR14TFSI ionic liquid. A supercapacitor with PIL-based PE delivered specific capacitance and specific energy of 100 F g−1 and 32 Wh kg−1, respectively. At 333 K, specific energy increased to 42 Wh kg−1 at discharge current density 1 mA cm−2. These values are a little less than that using pristine PYR14TFS, but are higher than that of supercapacitors using conventional polymer electrolytes. The IL-based polymer electrolytes have a wide potential window that allowed operation of the all solidstate supercapacitors at voltages of 3.5 V.

The addition of BmImCl into the PVA/CH3 COONH4 electrolyte enhanced the ionic conductivity due to its strong plasticizing effect, environmental friendly nature and high ion content [80]. In the presence of IL, the polymer chains become more flexible, thus the polymer segmental motion increased. This assisted ionic transport that conferred the high ionic conductivity. This result agreed with the differential scanning calorimetric (DSC) analysis as subambient Tg was observed on addition of IL. At these sub-ambient temperatures, the polymer electrolyte is in the rubbery state as the surrounding temperature is much higher than Tg . The molecules in the polymer matrix are allowed to undergo orientation and conformational changes in the rubbery phase. Moreover, the physicochemistry of IL such as viscosity and dielectric constant also contribute to ionic conductivity enhancement. EDLC fabricated with the most conducting ion conducting polymer can be charged up to 4.8 V. By doping IL into the polymer electrolyte also increased the capacitive nature of EDLC as the specific capacitance of 28.36 F g−1 was achieved. The inclusion of IL not only improved the electrode-electrolyte interfacial contact, but also improved the electrolyte and EDLC properties. Therefore, this is also a good prospect for improving the electrochemical performance of an EDLC.

The ionic liquid, BmImBr has been introduced into a PVA/CH3 COONH4 PE system for application in EDLC [81]. The EDLC delivered a capacitance of 21.89 F g−1 and continued to do so for 250 cycles.

An electrical conductivity of 0.73 S m−1 was achieved at 393 K upon adulteration of 50 wt. % BmImCl to the PVA/CH3 COONH4 PE system [80]. This is accompanied with the lowering of Tg . The primary cause of reduction in Tg was the ability of the IL to plasticize. Plasticization softened the polymer backbone, increased segmental motion thus helped in facilitating ionic transport. On addition of the IL containing ion conducting medium, the amorphousness of the membrane was observed to increase. It was also observed that the polymer electrolyte exhibited a wider potential stability range. The presence of IL has also improved the electrochemical behavior of the EDLC delivering 2.39 Wh kg−1 of specific energy and 19.79 W kg−1 of specific power with Coulombic efficiency above 90%. The specific capacitance of 28.36 F g−1 was obtained with better electrochemical characteristics.

Addition of BMImTf ionic liquid in PVA/CH3 COONH4 PE system [82] resulted in the polymer electrolyte exhibiting strong adhesive properties. Thus, better electrode-electrolyte interfacial contact can be provided. The specific capacitance of the EDLC obtained was 2.02 from 0.14 F g−1 without IL. This implied that the IL-based polymer electrolytes possessed higher dielectric constant compared to the IL-free polymer electrolyte. The addition of IL has also extended the electrolyte voltage window and improved thermal stability.

The BdMImBF4 ionic liquid was added into a gel matrix that comprised a blended host of PVP/PVdF-HFP and magnesium triflate salt. The gel was used in an EDLC [83]. The interaction between the IL and the polar groups of the polymer chain increased amorphousness of the GPE. The ionic conductivity achieved a maximum value of 0.29 S m−1 after 7.5 wt. % of BdMImBF4 was added. The calculated specific energy and power were 14 Wh kg−1 and 21 W kg−1, respectively.

### **7. Summary**

wettability as well as interaction with the IL. These results implied that 80% capacity is retained for PIL:RGO and 58% RGO capacitors. At 333 K, 70% of the capacity was retained for

Another approach to obtain stable polymer electrolytes thermally and electrochemically is by incorporating RTIL into polymer electrolytes [78]. Pandey et. al [79] has compared the EDLC performance using electrolyte in the solid-state and ionic liquid incorporated polymer electrolytes. The electrodes of EDLCs comprised multiwalled carbon nanotube (MWCNT). The polymer electrolytes consisted of PEO, triflate salts of magnesium and lithium and EMImTf ionic liquid. The ambient conductivity was ~10−2 S m−1. The addition of IL increased the polymer backbone flexibility and enhanced segmental motion. The use of the triflate salts and

ionic species increased the possibility of formation of more double layers where energy was stored. The enhanced flexibility is also useful for proper electrode-electrolyte contact.

ductivity. MWCNT with Li-based and Mg-based electrolyte could deliver between 1.7 and 2.1 F g−1 and between 2.6 and 3.0 F g−1, specific capacitance, respectively. Without IL incorporation the capacitance per unit mass were ~0.03 F g−1 and ~0.01 F g−1 for the Mg- and Li-system,

Ayalneh Tiruye et al. [78] have assembled supercapacitors containing two parts of a pyrrolidinium-based PIL, that is, pDADMTFSI and three parts PYR14TFSI ionic liquid. A supercapacitor with PIL-based PE delivered specific capacitance and specific energy of 100 F g−1 and 32 Wh kg−1, respectively. At 333 K, specific energy increased to 42 Wh kg−1 at discharge current density 1 mA cm−2. These values are a little less than that using pristine PYR14TFS, but are higher than that of supercapacitors using conventional polymer electrolytes. The IL-based polymer electrolytes have a wide potential window that allowed operation of the all solid-

COONH4

was observed on addition of IL. At these sub-ambient temperatures, the

ductivity due to its strong plasticizing effect, environmental friendly nature and high ion content [80]. In the presence of IL, the polymer chains become more flexible, thus the polymer segmental motion increased. This assisted ionic transport that conferred the high ionic conductivity. This result agreed with the differential scanning calorimetric (DSC) analysis

polymer electrolyte is in the rubbery state as the surrounding temperature is much higher

. The molecules in the polymer matrix are allowed to undergo orientation and conformational changes in the rubbery phase. Moreover, the physicochemistry of IL such as viscosity and dielectric constant also contribute to ionic conductivity enhancement. EDLC fabricated with the most conducting ion conducting polymer can be charged up to 4.8 V. By doping IL into the polymer electrolyte also increased the capacitive nature of EDLC as the specific capacitance of 28.36 F g−1 was achieved. The inclusion of IL not only improved the electrode-electrolyte interfacial contact, but also improved the electrolyte and EDLC properties. Therefore, this is also a good prospect for improving the electrochemical performance

free ions. The presence of many types of mobile

electrolyte enhanced the ionic con-

electrolyte con-

PIL:RGO and 61% capacity retention for RGO-based supercapacitors.

and CF3

respectively. This showed IL contribution on EDLC capacitance.

Incorporating EMImTf resulted in a substantial increment of PEO-Mg(Tf)2

SO3 −

IL introduced Li+

170 Progress and Developments in Ionic Liquids

as subambient Tg

than Tg

of an EDLC.

/Mg2+, EMIm+

state supercapacitors at voltages of 3.5 V.

The addition of BmImCl into the PVA/CH3

ILs consist of poorly coordinating cations and anions and are molten salts at room temperature. They have high thermal stability, wide potential window and high electrical conductivity. ILs are green solvents with negligible vapor pressure. IL such as EMImTf, EMImBF4 and PP24TFSI, to name a few can enhance ionic conductivity when added to PEs. Conductivity enhancement is attributed to the additional ions supplied by the IL, the plasticizing nature of the IL and its low viscosity that facilitates ion mobility. The plasticizing nature of ILs softens the polymer backbone resulting in the increase in polymer segmental motion. This implies that IL can increase amorphousness of a PE. ILs with short alkyl chains are less viscous and hence suitable for facilitating ion mobility. However, IL containing Im+ cation with long alkyl chain length can delay electron recombination, enhance *J*SC and improves IPCE in DSSCs. EMImTFSI, EMImTf and EMImDCN are low viscosity ILs that are able to increase ionic conductivity of and PEO:NaI:I2 systems. It is also to be noted that most sulfonium ion-based ILs have higher conductivity than ammonium ion-based ILs due to the lower viscosity of the former. Incorporation of ILs, for example, BMImTf can increase thermal stability of a PE such as in 49 wt. % PEMA-21 wt. % (PVdF-HFP)-30 wt. % LiTf sample. DMPImI is an ionic liquid that has been added to a PEO-PEG-KI system to produce an I2 -free GPE that was able to reduce electron recombination and dark current in a DSSC. Imidazolium type aprotic IL when added into sulfonated poly (ether ketone) or SPEK not only increased ionic conductivity, but also enabled the membrane to operate under anhydrous condition between at elevated temperatures. The ability of the membrane to operate at 413 K shows its potential as a candidate for PEMFC. The MePrPipNTf2 incorporated into a PVdF + LiTFSI + VC electrolyte system decomposed at a high temperature of ~583K. This signaled the safe use of the membrane in lithium polymer batteries at elevated temperatures. The ILGPE 66.7 wt. % (PVdF-HFP/ LiTFSI)-33.3 wt. % B4 MePyTFSI exhibited a maximum Li+ ionic conductivity of 0.20 S m−1. The Li/ILGPE/LiFePO4 cell showed high capacity and good cyclability. The addition of BmImCl to the PVA/CH3 COONH4 PE system is accompanied with the lowering of Tg , wider potential stability range and improved EDLC electrochemical behavior.

BF4

Et3

<sup>−</sup> Tetrafluoroborate anion

BMImBF4 1-Butyl-3-methylimidazolium tetrafluoroborate

BMImPF6 1-Butyl-3-methylimidazolium hexafluorophosphate BMImTf 1-Butyl-3-methylimidazolium trifluoromethanesulfonate

DEME-BF4 N,N-diethyl-N-methyl-N-(2-methoxyethyl)ammonium tetrafluoroborate

Ionic Liquid Enhancement of Polymer Electrolyte Conductivity and their Effects on the Performance...

http://dx.doi.org/10.5772/65752

173

DEME-TFSI N,N-diethyl-N-methyl-N-(2-methoxyethyl) ammonium bis (trifluoromethanesulfonyl) imide

BmImBr 1-Butyl-3-methylimidazolium bromide BMImCl 1-Butyl-3-methylimidazolium chloride BMImI 1-Butyl-3-methylimidazolium iodide BMImNO3 1-Butyl-3-methylimidazolium nitrate

BMP+ N-Butyl-N-methylpyrrolidinium cation

DMPImI 1,2-Dimethyl-3-propylimidazolium iodide

EMImBF4 1-Ethyl-3-methylimidazolium tetrafluoroborate EMImDCN 1-Ethyl-3-methylimidazolium dicyanamide EMImI 1-Ethyl-3-methylimidazolium iodide

EMImTf 1-Ethyl-3-methylimidazolium trifluoromethanesulfonate

NHTFSI Triethylammonium bis(tetrafluoromethylsulfonyl)imide

IPCE Incident photon-to-electron conversion efficiency

EMImTFSI 1-Ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide

C18H36O3 12-Hydroxystearic acid CPE Composite polymer electrolyte

DCA<sup>−</sup> Dicyanamide anion

DSSC Dye-sensitized solar cell EMiDCA 1-Ethyl-3-methyl dicyanamide EMIm+ 1-Ethyl-3-methylimidazolium cation

Epdy+ Ethyl pyridinium cation

GPE Gel polymer electrolyte GuSCN Guanidine thiocyanate

ILGE Ionic liquid gel electrolyte ILGO Ionic liquid graphite oxide

*J*SC Short-circuit current density LGPE Ionic liquid gel polymer electrolyte LiPF6 Lithium hexafluorophosphate LiTf Lithium trifluoromethanesulfonate

FF Fill factor

IL Ionic liquid

### **Abbreviations**



former. Incorporation of ILs, for example, BMImTf can increase thermal stability of a PE such as in 49 wt. % PEMA-21 wt. % (PVdF-HFP)-30 wt. % LiTf sample. DMPImI is an ionic liquid

reduce electron recombination and dark current in a DSSC. Imidazolium type aprotic IL when added into sulfonated poly (ether ketone) or SPEK not only increased ionic conductivity, but also enabled the membrane to operate under anhydrous condition between at elevated temperatures. The ability of the membrane to operate at 413 K shows its potential as a candidate

decomposed at a high temperature of ~583K. This signaled the safe use of the membrane in lithium polymer batteries at elevated temperatures. The ILGPE 66.7 wt. % (PVdF-HFP/

MePyTFSI exhibited a maximum Li+

incorporated into a PVdF + LiTFSI + VC electrolyte system

cell showed high capacity and good cyclability. The addition of BmImCl

PE system is accompanied with the lowering of Tg

] 1-Ethyl-3-methylimidazolium sulfonate; (subscript n had values n = 1, 2, 4 as

] 1-Ethyl-3-methylimidazolium phosphate (subscript n had values n = 1, 2 as dimethanephosphate and diethanephosphate, respectively)

] 1-Ethyl-3-methylimidazolium sulfate (subscript n had values n = 1, 2 as methanesulfonate and ethanesulfonate, respectively)

[MPPy][TFSI] Poly(1-methyl-1-propylpyrrolidinium bis(trifluoromethylsulfonyl)imide

] 1-Octyl-3-methylimidazolium hexafluorophosphate

] Triisobutyl(methyl)phosphonium tosylate,

] Tributylmethyl phosphonium methylsulfate

MePyTFSI 1-Butyl-4-methylpyridinium bis(trifluoromethanesulfonyl)imide

BdMImBF4 1-Butyl-2,3-dimethylimidazolium tetrafluoroborate

methanesulfonate, ethanesulfonate and butanesulfonate, respectively)


ionic conductivity of 0.20 S m−1. The

, wider potential

that has been added to a PEO-PEG-KI system to produce an I2

stability range and improved EDLC electrochemical behavior.

[EmIm]HSO4 Ethylmethyl imidazolium hydrosulfate [Epdy]HSO4 Ethyl pyridinium hydrosulphate

] Methyl trioctylammonium cation

[N1114]HSO4 Trimethylethyl amide hydrosulphate

[VBI]Br 1-Vinyl-3-butylimidazolium bromide [VMI]I 1-Vinyl-3-methylimidazolium iodide

AC Activated carbon AFC Alkaline fuel cell

[N1114]+ Trimethylethyl amide cation

] Methyl trioctyl ammonium chloride

for PEMFC. The MePrPipNTf2

172 Progress and Developments in Ionic Liquids

COONH4

LiTFSI)-33.3 wt. % B4

Li/ILGPE/LiFePO4

**Abbreviations**

mim][CnSO3

mim][CnSO4

mim][diCnPO4

][Cl<sup>−</sup>

][PF6<sup>−</sup>

] [TOS<sup>−</sup>

][MeSO4 −

[C2

[C2

[C2

[MTOA+

[MTOA+

[OMIM+

[P4,4,4,1+

[P4,4,4,1+

B4

to the PVA/CH3

#### 174 Progress and Developments in Ionic Liquids


PMMA Poly(methyl methacrylate)

PYRNO3 Pyrrolidinium nitrate

RT Room temperature

TBP Tert-butylpyridine

VC Vinylene carbonate *V*OC Open circuit voltage

**Acknowledgements**

**Author details**

Lumpur, Malaysia

**References**

TEA-BF4

RTIL Room temperature ionic liquid SILM Supported IL membrane SPE Solid polymer electrolyte SPEK Sulfonated poly (ether ketone)

TFSI<sup>−</sup> Bis(trifluoromethylsulfonyl)imide anion

TIPIL Three-armed imidazolium phenoxy ionic liquid

Siti Nor Farhana Yusuf, Rosiyah Yahya and Abdul Kariem Arof\*

\*Address all correspondence to: akarof@um.edu.my

Tg Glass transition temperature

PYR14TFSI (N-butyl-N-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide

Ionic Liquid Enhancement of Polymer Electrolyte Conductivity and their Effects on the Performance...

http://dx.doi.org/10.5772/65752

175

/PC Tetraethylammonium tetrafluoroborate/propylene carbonate

Authors would like to thank University of Malaya for the UMRG Grant no. RP003-13AFR.

Department of Physics, Centre for Ionics University Malaya, University of Malaya, Kuala

[1] Sim LN, Majid SR, and Arof AK. Effects of 1-butyl-3-methyl imidazolium trifluoromethanesulfonate ionic liquid in poly(ethyl methacrylate)/poly(vinylidenefluoride-cohexafluoropropylene) blend based polymer electrolyte system. Electrochimica Acta.

2014;123:190–197. http://dx.doi.org/10.1016/j.electacta.2014.01.017

PYRTFSI Pyrrolidinium bis(trifluoromethanesulfonyl)imide


### **Acknowledgements**

Authors would like to thank University of Malaya for the UMRG Grant no. RP003-13AFR.

### **Author details**

LiTFSI Lithium bis(tetrafluoromethylsulfonyl)imide

MEMP-BF4 N-(2-methoxyethyl)-N-methylpyrrolidinium MePrPip+ N-methyl-N-propylpiperidinium cation

NHTFSI Trimethylammonium bis(tetrafluoromethylsulfonyl)imide

MePrPipNTf2 N-methyl-N-propylpiperidinium bis(trifluoromethanesulfonyl)imide

MPPipTFSI N-methyl-N-propylpiperidinium bis(trifluoromethanesulfonyl)imide

pDADMTFSI Poly(diallyldimethylammonium) bis(trifluoromethanesulfonyl)imide

PMAPII Poly(1-methyl 3-(2-acryloyloxypropyl) imidazolium iodide) Poly(AA/CTAB) Poly(acrylic acid/cetyltrimethylammonium bromide)

PP24TFSI N-butyl-N-ethylpiperidinium N,N-bis(trifluoromethane)sulfonimide

PVP/PVdF-HFP Poly(vinyl pyrrolidone)/poly(vinylidene fluoride-co-hexafluoropropylene)

O12 Spinel lithium titanate

174 Progress and Developments in Ionic Liquids

MBImI 1-Methyl-3-butylimidazolium iodide

MHImI 1-Methyl-3-hexylimidazolium iodide

MPImI 1-Methyl-3-propylimidazolium iodide

<sup>−</sup> Bis(trifluoromethanesulfonyl)imide anion P[VP-co-VAc] Poly[1-vinylpyrrolidone-co-vinyl acetate]

PEDOT-NF Poly(3,4-ethylenedioxythiophene) nanofibers

PEMFC Proton exchange membrane fuel cells

Poly(HEMA/GR) Poly(hydroxyethyl methacrylate/glycerol)

PVdF-HFP Poly(vinylidene fluoride-co-hexafluoropropylene)

NI Tetrapropyl ammonium iodide

MPD Maximum power density

N-MBI N-methylbenzimidazole

PAN Polyacrylonitrile PBI Polybenzimidazole

PE Polymer electrolyte

PEG Polyethylene glycol PEMA Poly(ethyl methacrylate)

PEO Poly(ethylene oxide) PIL Polymeric ionic liquid

Poly(AA/GR) Poly(acrylic acid/gelatin)

PrIL Protic ionic liquid PVA Polyvinyl alcohol

Li4 Ti5

Me3

NTf2

Pr4

Siti Nor Farhana Yusuf, Rosiyah Yahya and Abdul Kariem Arof\*

\*Address all correspondence to: akarof@um.edu.my

Department of Physics, Centre for Ionics University Malaya, University of Malaya, Kuala Lumpur, Malaysia

### **References**

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**Chapter 9**

**Provisional chapter**

**Recent Advances in Electrocatalytic Applications of**

Ionic liquids have emerged as an environmentally friendly alternative to the volatile organic solvents. Being designer solvents, they can be modulated to suit the reaction conditions, therefore earning the name "task-specific ionic liquids." Though primarily used as solvents, they are now finding applications in various fields such as catalysis, electrochemistry, spectroscopy, and material science to mention a few. The goal of this chapter is focused on the electrocatalytic applications of ionic liquids, which can be used as catalysts and catalytic supports in electrochemistry. Their scope has marched beyond academic research laboratories to industries where their practical applications have been leading to various sustainable technologies. Flexibility to modulate properties by changing design endows freedom to a chemist to design an ionic liquid according to one's own requirement. To conclude, it can be said that the field of ionic liquid

**Keywords:** ionic liquid, electrochemistry, electrocatalysis, sensors, electrodeposition,

Ionic liquids (ILs) are organic salts remaining liquid even under ambient temperatures [1]. They consist of organic cations (e.g., imidazolium, pyridinium, pyrrolidinium, phosphonium, ammonium) and organic/inorganic anions with side chains of alkyl or different functional groups and aromatic moieties (e.g., trifluoromethanesulfonate, bis(trifluoromethyl)sulfonyl imide) [2, 3]. Characteristic interesting properties of these liquid salts include good thermal stability, wide liquid temperature range, considerable ionic conductivity, a broad electrochem-

and reproduction in any medium, provided the original work is properly cited.

© 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution,

© 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

**Recent Advances in Electrocatalytic Applications of**

**Ionic Liquids**

**Ionic Liquids**

http://dx.doi.org/10.5772/65808

electroredox, application

**1. Introduction**

**Abstract**

Additional information is available at the end of the chapter

electrocatalysis holds enormous possibilities to be explored.

Additional information is available at the end of the chapter

Yu Lin Hu

Yu Lin Hu

#### **Recent Advances in Electrocatalytic Applications of Ionic Liquids Recent Advances in Electrocatalytic Applications of Ionic Liquids**

#### Yu Lin Hu Yu Lin Hu

Additional information is available at the end of the chapter Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/65808

#### **Abstract**

Ionic liquids have emerged as an environmentally friendly alternative to the volatile organic solvents. Being designer solvents, they can be modulated to suit the reaction conditions, therefore earning the name "task-specific ionic liquids." Though primarily used as solvents, they are now finding applications in various fields such as catalysis, electrochemistry, spectroscopy, and material science to mention a few. The goal of this chapter is focused on the electrocatalytic applications of ionic liquids, which can be used as catalysts and catalytic supports in electrochemistry. Their scope has marched beyond academic research laboratories to industries where their practical applications have been leading to various sustainable technologies. Flexibility to modulate properties by changing design endows freedom to a chemist to design an ionic liquid according to one's own requirement. To conclude, it can be said that the field of ionic liquid electrocatalysis holds enormous possibilities to be explored.

**Keywords:** ionic liquid, electrochemistry, electrocatalysis, sensors, electrodeposition, electroredox, application

### **1. Introduction**

Ionic liquids (ILs) are organic salts remaining liquid even under ambient temperatures [1]. They consist of organic cations (e.g., imidazolium, pyridinium, pyrrolidinium, phosphonium, ammonium) and organic/inorganic anions with side chains of alkyl or different functional groups and aromatic moieties (e.g., trifluoromethanesulfonate, bis(trifluoromethyl)sulfonyl imide) [2, 3]. Characteristic interesting properties of these liquid salts include good thermal stability, wide liquid temperature range, considerable ionic conductivity, a broad electrochem-

© 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

ical window, and a wide solubility and miscibility range [4, 5]. Due to these advantages, they have been considered as environmental benign solvents compared to volatile organic solvents and "green designer solvents." As a result, ILs are widely used in various chemical transfor‐ mations such as electrocatalysis, electrosynthesis, electrodeposition, electrochemical capacitor, and lithium batteries (**Figure 1**) [6]. Nowadays, ionic liquids are of immensely growing importance for the electrochemical technology [7–9]. The applications of ILs reveal new perspectives in electrocatalysis and other branches of applied electrochemistry [10–12].

composites which exhibit the synergistic effects [17]) for the fabrication of sensors due to their performance of achieving electron transfer (DET) directly. The modified electrodes in electro‐ analysis offer several advantages. They can not only lower the overpotential but also increase the reaction rate and sensitivity and improve selectivity. These advantages have been evi‐

Rahman et al. [18] reported the synthesis of ionic liquid [ClPr]NTf2 by simple metathesis reaction at room temperature (**Figure 2**). The [ClPr]NTf2 can fabricate the chemical sensor with conducting coating binders onto glassy carbon electrodes, which showed high selective and sensitivity on sensing applications. The performances of the fabricated sensors are excellent in terms of selectivity, sensitivity, detection limit, etc. This novel approach was introduced a well‐organized route of efficient chemical sensor development for environmental pollutants

> **H2O r.t., 2hrs**

**Preparation of ClPrNTf2 ionic liquid**

denced by numerous experiments.

and health‐care fields in broad scales.

**S N**

**Cl**

**N**

**+**

**HCl**

**ClPrCl LiNTf2**

**GCE CIPrNTf2/GCE**

**F3C S N**

**F3C S O O**

**O O**

**Li**

**WE 3-Methoxy**

posed adsorption mechanisms of 3‐methoxy phenol detection in presence of ClPrNTf2 onto GCE.

determination of EP in human urine samples with good recovery results.

**phenol**

**Figure 2.** Preparation of ClPrNTf2 and the mechanism of sensor development. (a) ClPrNTf2 coated GCE with conduct‐ ing coating binders, (b) detection I‐V method (theo‐retical), (c) observed I‐V responses by ClPrNTf2/GCE, and (d) pro‐

Atta et al. [19] fabricated reduced cyclodextrin/ionic liquid crystal/graphene composite electrode for the determination of some neurotransmitters such as dopamine (DA), epinephr‐ ine (EP), and norepinephrine (NEP) (**Figure 3**). Besides the pre‐concentrating effect of CD, this electrode expected large graphene surface area, good electron mobility, and high ionic stability and conductivity of ionic liquid. Optimization of the sensor performance was presented and resulted in a better current signal. The sensor was sensitive and successfully applied for direct

**Target Analytes**

**S NH**

**Cl**

**N**

**ClPrNTf2**

**F3C S N**

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187

**F3C S O O**

**O O**

**Bindes + CIPrNTf2**

**(b)**

**Figure 1.** Schematic representation of the general properties of ILs, electrochemical techniques, and electrochemical ap‐ plications of ILs.

Electrocatalysis is a special type of catalysis that speeds up the rate of an electrochemical reaction occurring on electrode surfaces or at liquid/solid interfaces [13]. In this chapter, we mainly overviewed recent advances in electrocatalytic applications of ionic liquids such as electrosensing, electrodeposition, electroredox, and electropolymerization.

### **2. Electrochemical applications of ionic liquids**

#### **2.1. Electrochemical sensors**

Nonenzymatic amperometric sensor for direct determination of some electroactive molecules is an attractive alternative technique to solve the disadvantages of enzymatic biosensors [14]. They have received continuously increasing interest in the recent years [15, 16]. ILs are used primarily as modifying materials of electrodes (IL can interact with other materials to get composites which exhibit the synergistic effects [17]) for the fabrication of sensors due to their performance of achieving electron transfer (DET) directly. The modified electrodes in electro‐ analysis offer several advantages. They can not only lower the overpotential but also increase the reaction rate and sensitivity and improve selectivity. These advantages have been evi‐ denced by numerous experiments.

ical window, and a wide solubility and miscibility range [4, 5]. Due to these advantages, they have been considered as environmental benign solvents compared to volatile organic solvents and "green designer solvents." As a result, ILs are widely used in various chemical transfor‐ mations such as electrocatalysis, electrosynthesis, electrodeposition, electrochemical capacitor, and lithium batteries (**Figure 1**) [6]. Nowadays, ionic liquids are of immensely growing importance for the electrochemical technology [7–9]. The applications of ILs reveal new perspectives in electrocatalysis and other branches of applied electrochemistry [10–12].

**Figure 1.** Schematic representation of the general properties of ILs, electrochemical techniques, and electrochemical ap‐

Electrocatalysis is a special type of catalysis that speeds up the rate of an electrochemical reaction occurring on electrode surfaces or at liquid/solid interfaces [13]. In this chapter, we mainly overviewed recent advances in electrocatalytic applications of ionic liquids such as

Nonenzymatic amperometric sensor for direct determination of some electroactive molecules is an attractive alternative technique to solve the disadvantages of enzymatic biosensors [14]. They have received continuously increasing interest in the recent years [15, 16]. ILs are used primarily as modifying materials of electrodes (IL can interact with other materials to get

electrosensing, electrodeposition, electroredox, and electropolymerization.

**2. Electrochemical applications of ionic liquids**

plications of ILs.

**2.1. Electrochemical sensors**

186 Progress and Developments in Ionic Liquids

Rahman et al. [18] reported the synthesis of ionic liquid [ClPr]NTf2 by simple metathesis reaction at room temperature (**Figure 2**). The [ClPr]NTf2 can fabricate the chemical sensor with conducting coating binders onto glassy carbon electrodes, which showed high selective and sensitivity on sensing applications. The performances of the fabricated sensors are excellent in terms of selectivity, sensitivity, detection limit, etc. This novel approach was introduced a well‐organized route of efficient chemical sensor development for environmental pollutants and health‐care fields in broad scales.

**Figure 2.** Preparation of ClPrNTf2 and the mechanism of sensor development. (a) ClPrNTf2 coated GCE with conduct‐ ing coating binders, (b) detection I‐V method (theo‐retical), (c) observed I‐V responses by ClPrNTf2/GCE, and (d) pro‐ posed adsorption mechanisms of 3‐methoxy phenol detection in presence of ClPrNTf2 onto GCE.

Atta et al. [19] fabricated reduced cyclodextrin/ionic liquid crystal/graphene composite electrode for the determination of some neurotransmitters such as dopamine (DA), epinephr‐ ine (EP), and norepinephrine (NEP) (**Figure 3**). Besides the pre‐concentrating effect of CD, this electrode expected large graphene surface area, good electron mobility, and high ionic stability and conductivity of ionic liquid. Optimization of the sensor performance was presented and resulted in a better current signal. The sensor was sensitive and successfully applied for direct determination of EP in human urine samples with good recovery results.

**Figure 3.** Schematic representation of the proposed sensor with the inclusion complex between the studied compounds and CD.

Yu et al. [20] fabricated an ionic liquid‐Fe3O4 nanoparticle‐graphite composite electrode (IL‐ Fe3O4NPs‐GP) (**Figure 4**). A hydrophobic ionic liquid BMP‐TFSA was used to combine Fe3O4NPs and graphene paper (GP) and to substitute paraffin oil that is conventionally used as the organic binder for preparing carbon‐paste electrodes. The electrode showed good stability and the synergistic effect from the combination of IL and Fe3O4NPs, exhibiting a high sensitivity but a narrower dynamic range with a detection limit of 0.5 lM.

**Figure 5.** Preparation process of PtAu alloy nanoparticle decorated graphene‐CNT‐IL/GP.

**Figure 6.** Illustration of IL/PU/PEDOT:PSS composite actuator.

as flexible and soft electrodes, which showed excellent bending toward anode.

Okuzaki et al. [22] synthesized the transparent ionic liquid/polyurethane (IL/PU) gels firstly (**Figure 6**). They observed that with increasing the IL content from 0 wt% to 40 wt%, both ionic conductivity and electric‐double‐layer capacitance increased, respectively, while the compres‐ sion modulus slightly decreased. After that, the workers fabricated the IL/PU/PEDOT:PSS composites by sandwiching the IL/PU gel between two conductive polymer films (PEDOT:PSS)

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189

**Figure 4.** IL‐Fe3O4NPs‐GP electrode for detection of H2O2.

He et al. [21] reported the synthetic method for the free‐standing GP‐supported graphene‐ CNT‐IL nanocomposite graphene‐CNT‐IL/GP (**Figure 5**), which was formulated by blending three‐dimensional porous graphene‐carbon nanotube (CNT) assembly with ionic liquid on two‐dimensional graphene paper (GP). The results showed that the graphene‐CNT‐IL/GP exhibited excellent sensing characteristics in terms of selectivity, reproducibility, and sensitiv‐ ity in electrochemical detection of glucose.

**Figure 5.** Preparation process of PtAu alloy nanoparticle decorated graphene‐CNT‐IL/GP.

**Figure 3.** Schematic representation of the proposed sensor with the inclusion complex between the studied compounds

Yu et al. [20] fabricated an ionic liquid‐Fe3O4 nanoparticle‐graphite composite electrode (IL‐ Fe3O4NPs‐GP) (**Figure 4**). A hydrophobic ionic liquid BMP‐TFSA was used to combine Fe3O4NPs and graphene paper (GP) and to substitute paraffin oil that is conventionally used as the organic binder for preparing carbon‐paste electrodes. The electrode showed good stability and the synergistic effect from the combination of IL and Fe3O4NPs, exhibiting a high

**OH + OH- (or H2O + OH-**

**H2O2 (or H2O2 + H+)**

**: Fe3O4 : Graphite powder**

He et al. [21] reported the synthetic method for the free‐standing GP‐supported graphene‐ CNT‐IL nanocomposite graphene‐CNT‐IL/GP (**Figure 5**), which was formulated by blending three‐dimensional porous graphene‐carbon nanotube (CNT) assembly with ionic liquid on two‐dimensional graphene paper (GP). The results showed that the graphene‐CNT‐IL/GP exhibited excellent sensing characteristics in terms of selectivity, reproducibility, and sensitiv‐

**)**

sensitivity but a narrower dynamic range with a detection limit of 0.5 lM.

**Fe3O4 , ox (Fe3+)**

e- **Fe3O4 , red (Fe2+)**

**Figure 4.** IL‐Fe3O4NPs‐GP electrode for detection of H2O2.

ity in electrochemical detection of glucose.

and CD.

188 Progress and Developments in Ionic Liquids

Okuzaki et al. [22] synthesized the transparent ionic liquid/polyurethane (IL/PU) gels firstly (**Figure 6**). They observed that with increasing the IL content from 0 wt% to 40 wt%, both ionic conductivity and electric‐double‐layer capacitance increased, respectively, while the compres‐ sion modulus slightly decreased. After that, the workers fabricated the IL/PU/PEDOT:PSS composites by sandwiching the IL/PU gel between two conductive polymer films (PEDOT:PSS) as flexible and soft electrodes, which showed excellent bending toward anode.

**Figure 6.** Illustration of IL/PU/PEDOT:PSS composite actuator.

Xia et al. [23] fabricated the molecularly imprinted electrochemical biosensor based on chitosan/ionic liquid‐graphene composites modified electrode (CS/IL‐GR/GCE) for determi‐ nation of bovine serum albumin (BSA) (**Figure 7**). The synergistic effects of chitosan, ionic liquid, and graphene nanocomposites improved the electrochemical response and the sensitivity of the sensor. The fabricated sensor possessed a high selectivity, good reproduci‐ bility, excellent stability, and acceptable recovery, which indicated the potential application in clinical field.

**Figure 8.** Illustration of the construction of the 2,6‐DAP‐imprinted core‐shell nanoparticles (DICSNs).

**N**

**N**

**CH3**

**CH3 N-**

**O F O F F**

**S S O O**

**<sup>C</sup> N- <sup>C</sup> <sup>N</sup> <sup>N</sup>**

**Figure 9.** (a) High‐resolution TEM micrograph of synthesized Pd nanoparticle means sampling depth is low and (b)

**F <sup>F</sup> <sup>F</sup>**

**BMP–TFSI**

**BMP–DCA**

**H3C**

**a b**

**H3C**

schematic of constituent ion orientation in ILs based on different underlying supports.

interfering gases.

Wang et al. [25] synthesized the GNS/Pd nanocomposites by the incorporation of different kinds of ionic liquid (IL) to increase the electrode sensing current toward different analytes (**Figure 9**). They found that BMP‐TFSI IL is beneficial for glucose detection, whereas the electrode with BMP‐DCA IL shows high sensitivity toward ascorbic acid (AA). Angle‐resolved X‐ray photoelectron spectroscopy analyses indicate that GNSs can create an aligned cation/ anion orientation in the adsorbed IL film, with the anions preferentially occupying the topmost surface. As a result, the electrode sensitivity and selectivity are mainly determined by the IL constituent anions. Chen et al. [26] reported the ultra‐sensitive gaseous NH3 sensor (ECL sensor) based on ionic liquid mediated for directly detecting gaseous NH3 (**Figure 10**). The NH3ECL sensor has a very high sensitivity and an excellent selectivity against common

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191

**Figure 7.** Schematic diagram of the preparation procedure of the molecular imprinted electrochemical sensor.

Zhao and Hao [24] constructed an analytical approach for detecting diaminopyridine deriva‐ tives using a molecular imprinting‐electrochemical sensor synthesized with 6‐aminouracil and 2,6‐diaminopyridine (**Figure 8**). Ionic liquid and graphene can assist 2,6‐diaminopyridine‐ imprinted core‐shell nanoparticles in electrochemical reaction by increasing conductivity. This proposed method has been demonstrated appropriate sensitivity and selectivity, with a linear range of 0.0500–35.0 mg kg−1 and a detection limit as low as 0.0275 mg kg−1.

**Figure 8.** Illustration of the construction of the 2,6‐DAP‐imprinted core‐shell nanoparticles (DICSNs).

Xia et al. [23] fabricated the molecularly imprinted electrochemical biosensor based on chitosan/ionic liquid‐graphene composites modified electrode (CS/IL‐GR/GCE) for determi‐ nation of bovine serum albumin (BSA) (**Figure 7**). The synergistic effects of chitosan, ionic liquid, and graphene nanocomposites improved the electrochemical response and the sensitivity of the sensor. The fabricated sensor possessed a high selectivity, good reproduci‐ bility, excellent stability, and acceptable recovery, which indicated the potential application in

**Figure 7.** Schematic diagram of the preparation procedure of the molecular imprinted electrochemical sensor.

range of 0.0500–35.0 mg kg−1 and a detection limit as low as 0.0275 mg kg−1.

Zhao and Hao [24] constructed an analytical approach for detecting diaminopyridine deriva‐ tives using a molecular imprinting‐electrochemical sensor synthesized with 6‐aminouracil and 2,6‐diaminopyridine (**Figure 8**). Ionic liquid and graphene can assist 2,6‐diaminopyridine‐ imprinted core‐shell nanoparticles in electrochemical reaction by increasing conductivity. This proposed method has been demonstrated appropriate sensitivity and selectivity, with a linear

clinical field.

190 Progress and Developments in Ionic Liquids

Wang et al. [25] synthesized the GNS/Pd nanocomposites by the incorporation of different kinds of ionic liquid (IL) to increase the electrode sensing current toward different analytes (**Figure 9**). They found that BMP‐TFSI IL is beneficial for glucose detection, whereas the electrode with BMP‐DCA IL shows high sensitivity toward ascorbic acid (AA). Angle‐resolved X‐ray photoelectron spectroscopy analyses indicate that GNSs can create an aligned cation/ anion orientation in the adsorbed IL film, with the anions preferentially occupying the topmost surface. As a result, the electrode sensitivity and selectivity are mainly determined by the IL constituent anions. Chen et al. [26] reported the ultra‐sensitive gaseous NH3 sensor (ECL sensor) based on ionic liquid mediated for directly detecting gaseous NH3 (**Figure 10**). The NH3ECL sensor has a very high sensitivity and an excellent selectivity against common interfering gases.

**Figure 9.** (a) High‐resolution TEM micrograph of synthesized Pd nanoparticle means sampling depth is low and (b) schematic of constituent ion orientation in ILs based on different underlying supports.

**Figure 11.** SEM images of silver electrodeposited onto a GC electrode from EAN containing 0.1 M Ag+

and (g and h) Edep = −0.2 V, *t* = 300 s.

with pristine MWCNTs.

and (c) Ni/P‐MWCNTs.

deposition parameters. (a and b) Edep = −0.2 V, *t* = 30 s; (c and d) Edep = −0.6 V, *t* = 30 s; (e and f) Edep = −0.2 V, *t* = 60 s;

Martis and coworkers [32] investigated the nickel‐multiwalled carbon nanotube (Ni/MWCNT) composites electrodeposited in choline chloride/urea‐based deep eutectic solvent on a copper substrate (**Figure 12**). Electrodeposition of Ni/MWCNT composites could be easily achieved due to the excellent dispersion stability of MWCNTs in DES nickel chloride solution. Different morphologies and high surface roughness of MWCNTs to the coating were observed through the uniform distribution in the nickel matrix. The results showed that coating with oxygen‐ functionalized MWCNTs exhibited better corrosion resistance and higher stability than that

**Figure 12.** A: Cyclic voltammograms for a copper electrode (a) DES + 0.3 M NiCl2, (b) O‐MWCNT + 0.3 M NiCl2, and (c) P‐MWCNT in DES + 0.3 M NiCl2. SR = 0.02V/s. B: Cyclic voltammograms for (a) bare nickel, (b) Ni/O‐MWCNTs,

with different

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193

**Figure 10.** Illustrative ECL sensor for directly detecting gaseous NH3.

#### **2.2. Electrodeposition**

Electrodeposition is an important process, which has been widely applied in industry from functional and decorative anticorrosion to wear‐resistant coatings [27]. Aqueous electrolytes are traditionally used in the electrodeposition, and they suffer from limitations such as gas evolution, narrow potential windows, environmental hazards, and necessity for complexing agents [28–30]. Consequently, to overcome these shortcomings, several alternative electrolytes have been developed to substitute for aqueous solvent, although organic solvents have a wider electrochemical potential window; however, their toxicity, volatility, and handling are the major problems for their industrial applications [30]. Ionic liquids (ILs) are green and impor‐ tant electrolytes in electrode position of metals and can circumvent these limitations. Many studies have demonstrated IL electrolytes are favorable for electrode position of nanocrystal‐ line metals, while in aqueous media, pulsed electrode position and addition of additives are required, which often complicates the reaction mechanisms significantly [31].

Suryanto et al. [27] achieved the electrode position of silver onto glassy carbon, gold, and indium tin oxide‐coated glass substrates from three room‐temperature protic ionic liquids (PILs) (**Figure 11**). The results showed that the electrode position took place through a progressive nucleation and diffusion controlled 3D growth reaction mechanism. The silver micro‐/nanoparticles were then employed as electrocatalysts in oxygen reduction reaction and had excellent catalytic activity. This research provided promise for using protic ionic liquids as alternative electrolytes for the electrode position of metals and nanostructured electrocata‐ lysts.

**Figure 11.** SEM images of silver electrodeposited onto a GC electrode from EAN containing 0.1 M Ag+ with different deposition parameters. (a and b) Edep = −0.2 V, *t* = 30 s; (c and d) Edep = −0.6 V, *t* = 30 s; (e and f) Edep = −0.2 V, *t* = 60 s; and (g and h) Edep = −0.2 V, *t* = 300 s.

Martis and coworkers [32] investigated the nickel‐multiwalled carbon nanotube (Ni/MWCNT) composites electrodeposited in choline chloride/urea‐based deep eutectic solvent on a copper substrate (**Figure 12**). Electrodeposition of Ni/MWCNT composites could be easily achieved due to the excellent dispersion stability of MWCNTs in DES nickel chloride solution. Different morphologies and high surface roughness of MWCNTs to the coating were observed through the uniform distribution in the nickel matrix. The results showed that coating with oxygen‐ functionalized MWCNTs exhibited better corrosion resistance and higher stability than that with pristine MWCNTs.

**Figure 10.** Illustrative ECL sensor for directly detecting gaseous NH3.

Electrodeposition is an important process, which has been widely applied in industry from functional and decorative anticorrosion to wear‐resistant coatings [27]. Aqueous electrolytes are traditionally used in the electrodeposition, and they suffer from limitations such as gas evolution, narrow potential windows, environmental hazards, and necessity for complexing agents [28–30]. Consequently, to overcome these shortcomings, several alternative electrolytes have been developed to substitute for aqueous solvent, although organic solvents have a wider electrochemical potential window; however, their toxicity, volatility, and handling are the major problems for their industrial applications [30]. Ionic liquids (ILs) are green and impor‐ tant electrolytes in electrode position of metals and can circumvent these limitations. Many studies have demonstrated IL electrolytes are favorable for electrode position of nanocrystal‐ line metals, while in aqueous media, pulsed electrode position and addition of additives are

Suryanto et al. [27] achieved the electrode position of silver onto glassy carbon, gold, and indium tin oxide‐coated glass substrates from three room‐temperature protic ionic liquids (PILs) (**Figure 11**). The results showed that the electrode position took place through a progressive nucleation and diffusion controlled 3D growth reaction mechanism. The silver micro‐/nanoparticles were then employed as electrocatalysts in oxygen reduction reaction and had excellent catalytic activity. This research provided promise for using protic ionic liquids as alternative electrolytes for the electrode position of metals and nanostructured electrocata‐

required, which often complicates the reaction mechanisms significantly [31].

**2.2. Electrodeposition**

192 Progress and Developments in Ionic Liquids

lysts.

**Figure 12.** A: Cyclic voltammograms for a copper electrode (a) DES + 0.3 M NiCl2, (b) O‐MWCNT + 0.3 M NiCl2, and (c) P‐MWCNT in DES + 0.3 M NiCl2. SR = 0.02V/s. B: Cyclic voltammograms for (a) bare nickel, (b) Ni/O‐MWCNTs, and (c) Ni/P‐MWCNTs.

Mascia et al. [33] studied the electrochemical deposition of Cu/Nb composites by using 1‐ butyl‐1‐methylpyrrolidinium bis(trifluoromethylsulfonyl)imide as solvent (**Figure 13**). Structural and chemical analyses indicated that the obtained deposits cover uniformly the electrode surface and exhibit individual layers with a characteristic size ranging between 50 and 100 nm.

Serrà et al. [35] demonstrated a method for grow mesoporous films of Pt‐poor alloys (Co3Pt and CoPt3) through electrode position in ionic liquid‐water microemulsions (**Figure 15**). The electrolytic aqueous solution in the IL/W system favors a significant deposition rate. The mesoporous alloys exhibit excellent durability in acidic and alkaline media, maintaining their peculiar morphology, and this prepared catalysts efficient for the methanol electrooxidation

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195

**Figure 15.** Illustrative approach to grow mesoporous films of Pt‐poor alloys (Co3Pt and CoPt3), based on electrodeposi‐

Bakkar and Neubert [36] conducted the electrode position of aluminum in air, after preparation of ionic liquids in a glove box and covering them by a nonwater‐absorbable layer of particular organic compound (**Figure 16**). The functional aluminum layers were successfully deposited from a first‐generation ionic liquid AlCl3/[EMIm]Cl (60/40 mol%) on low‐carbon steel. SEM/EDX assessments showed that uniform, dense, and adherent Al layers were obtained. Furthermore, adherence of Al to the steel substrate was improved via in situ electrochemical

**Figure 16.** (a) Photograph of a beaker containing AlCl3/[EMIm]Cl ionic liquid insulated from air by a floating layer of decane, in addition to Al electroplated steel strip. (b) SEM micrograph of electrodeposited Al onto low‐carbon steel substrate from AlCl3/[EMIm]Cl (60/40 mol%) ionic liquid protected from air by a layer of decane at a potential of −500

Hekmata et al. [37] arranged the multiwall carbon nanotubes (MWCNTs) in nanochannels of anodic aluminum oxide template (AAO) by electrophoretic deposition (EPD) to make a vertically aligned carbon nanotube (VA‐CNT)‐based electrode (**Figure 17**). The stabilized CNTs in a water‐soluble room‐temperature ionic liquid (1‐methyl‐3‐octadecylimidazolium bromide)

in alkaline media.

etching.

mV at ambient atmosphere.

tion in ionic liquid‐in‐water (IL/W) microemulsions.

**Figure 13.** SEM micrograph and elemental maps of the Cu/Nb electrodeposit obtained in dual bathmode: first deposit of copper prepared at −0.75 V for 1800 s, followed by niobium deposit obtained at −1.5 V for 3600 s. The procedure was repeated twice.

Izgorodin et al. [34] synthesized a novel material based on manganese oxide catalyst, which is sensitized and stabilized through a surface phosphorylation reaction in the ionic liquid electrode position reaction (**Figure 14**). The results showed that the surface of the MnOx contained phosphorous at a P/Mn ratio of 1/2, indicating that the surface layer contained phosphate and oxide characteristics. The stability of the catalyst was enhanced and more than 25 h of continuous water oxidation is demonstrated.

**Figure 14.** SEM images of the manganese dioxide substrate before (left) and after (right) surface phosphorylation treat‐ ment.

Serrà et al. [35] demonstrated a method for grow mesoporous films of Pt‐poor alloys (Co3Pt and CoPt3) through electrode position in ionic liquid‐water microemulsions (**Figure 15**). The electrolytic aqueous solution in the IL/W system favors a significant deposition rate. The mesoporous alloys exhibit excellent durability in acidic and alkaline media, maintaining their peculiar morphology, and this prepared catalysts efficient for the methanol electrooxidation in alkaline media.

Mascia et al. [33] studied the electrochemical deposition of Cu/Nb composites by using 1‐ butyl‐1‐methylpyrrolidinium bis(trifluoromethylsulfonyl)imide as solvent (**Figure 13**). Structural and chemical analyses indicated that the obtained deposits cover uniformly the electrode surface and exhibit individual layers with a characteristic size ranging between 50

**Figure 13.** SEM micrograph and elemental maps of the Cu/Nb electrodeposit obtained in dual bathmode: first deposit of copper prepared at −0.75 V for 1800 s, followed by niobium deposit obtained at −1.5 V for 3600 s. The procedure was

Izgorodin et al. [34] synthesized a novel material based on manganese oxide catalyst, which is sensitized and stabilized through a surface phosphorylation reaction in the ionic liquid electrode position reaction (**Figure 14**). The results showed that the surface of the MnOx contained phosphorous at a P/Mn ratio of 1/2, indicating that the surface layer contained phosphate and oxide characteristics. The stability of the catalyst was enhanced and more than

**Figure 14.** SEM images of the manganese dioxide substrate before (left) and after (right) surface phosphorylation treat‐

25 h of continuous water oxidation is demonstrated.

and 100 nm.

194 Progress and Developments in Ionic Liquids

repeated twice.

ment.

**Figure 15.** Illustrative approach to grow mesoporous films of Pt‐poor alloys (Co3Pt and CoPt3), based on electrodeposi‐ tion in ionic liquid‐in‐water (IL/W) microemulsions.

Bakkar and Neubert [36] conducted the electrode position of aluminum in air, after preparation of ionic liquids in a glove box and covering them by a nonwater‐absorbable layer of particular organic compound (**Figure 16**). The functional aluminum layers were successfully deposited from a first‐generation ionic liquid AlCl3/[EMIm]Cl (60/40 mol%) on low‐carbon steel. SEM/EDX assessments showed that uniform, dense, and adherent Al layers were obtained. Furthermore, adherence of Al to the steel substrate was improved via in situ electrochemical etching.

**Figure 16.** (a) Photograph of a beaker containing AlCl3/[EMIm]Cl ionic liquid insulated from air by a floating layer of decane, in addition to Al electroplated steel strip. (b) SEM micrograph of electrodeposited Al onto low‐carbon steel substrate from AlCl3/[EMIm]Cl (60/40 mol%) ionic liquid protected from air by a layer of decane at a potential of −500 mV at ambient atmosphere.

Hekmata et al. [37] arranged the multiwall carbon nanotubes (MWCNTs) in nanochannels of anodic aluminum oxide template (AAO) by electrophoretic deposition (EPD) to make a vertically aligned carbon nanotube (VA‐CNT)‐based electrode (**Figure 17**). The stabilized CNTs in a water‐soluble room‐temperature ionic liquid (1‐methyl‐3‐octadecylimidazolium bromide) were deposited in the pores of AAO templates which were conductive by deposition of Ni nanoparticles in the bottom of pores. The capacitive performance of prepared electrodes was analyzed with a maximum value of 50 Fg−1 at the scan rate of 20 mV s−1 that was achieved for the specific capacitance.

Abebe et al. [39] prepared 4,4‐bipyridinium‐based ionic liquids which exhibited good metal coordinating abilities and were able to dissolve metal salts at high concentrations (**Figure 19**). The ionic liquid ([C4Bipyr][Tf2N]) exhibited a large liquidus range and a wide electrochemical window. Moreover, the successful electrodeposition of Cu (II) to Cu (0) from a solution of Cu(NO3)2 in [C4Bipyr][Tf2N] showed the potential of this new type of ionic liquids for electro‐

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197

**Figure 18.** (a) SEM of silver coatings deposited from "wet" (A and B) and "dry" (C and D) AgTf2N:[bmim][Tf2N] 1:2 molar ratio solution. (b) SEM of copper layers deposited from "wet" Cu(Tf2N)2: [bmim][Tf2N] 1:2 molar ratio solution as a function of the applied potential. (E) −0.55 V, (F) −0.70 V, (G) −0.85 V, and (H) −0.90 V. (c) SEM of copper layers deposited from "wet" Co(Tf2N)2: [bmim][Tf2N] 1:2 molar ratio solution. (I) −1.30 V "dry" solution and (J) −1.30 V "wet" solution. (d) SEM of zinc layers deposited from "wet" Zn(Tf2N)2: [bmim][Tf2N] 1:2 molar ratio solution. (K) −1.30 V

**C4 N N Br -**

**Figure 19.** Cyclic voltammogram for the neat [C4Bipyr][Tf2N] (black trace) and the solution of Cu(NO3)2 in [C4Bipyr]

deposition.

"dry" solution and (L) −1.30 V "wet" solution.

**4,4-bipyridine [C4BiPyr]Br**

**Synthetic route for the preparation of [C4Bipyr][Tf2N]**

**Li(Tf2N) Water 60**

**bromobutane 1,4 dioxane (reflux)**

**N N**

**[C4BiPyr][Tf2N]**

[Tf2N] (red trace).

**C4 <sup>N</sup> <sup>N</sup> [Tf2N]-**

**Figure 17.** Schematic diagram of an EPD cell for electrophoretic deposition of well‐dispersed CNTs on the cathode (as prepared AAO template).

Caporali et al. [38] assessed the feasibility of the use of highly concentrated solutions of ionic liquids to achieve high rates of metal electrodeposition (**Figure 18**). Different ionic liquids containing five transition metals were obtained by dissolving Tf2N salts of the metals (Ag, Cu, Co, Ni, and zinc) in [bmim][Tf2N] in a 1:2 molar ratio. The experimental results showed that with the exception of the Ni system, for all the ILs, it was possible to achieve the electroreduc‐ tion of the metal operating in a normal air atmosphere.

Abebe et al. [39] prepared 4,4‐bipyridinium‐based ionic liquids which exhibited good metal coordinating abilities and were able to dissolve metal salts at high concentrations (**Figure 19**). The ionic liquid ([C4Bipyr][Tf2N]) exhibited a large liquidus range and a wide electrochemical window. Moreover, the successful electrodeposition of Cu (II) to Cu (0) from a solution of Cu(NO3)2 in [C4Bipyr][Tf2N] showed the potential of this new type of ionic liquids for electro‐ deposition.

were deposited in the pores of AAO templates which were conductive by deposition of Ni nanoparticles in the bottom of pores. The capacitive performance of prepared electrodes was analyzed with a maximum value of 50 Fg−1 at the scan rate of 20 mV s−1 that was achieved for

**Figure 17.** Schematic diagram of an EPD cell for electrophoretic deposition of well‐dispersed CNTs on the cathode (as

Caporali et al. [38] assessed the feasibility of the use of highly concentrated solutions of ionic liquids to achieve high rates of metal electrodeposition (**Figure 18**). Different ionic liquids containing five transition metals were obtained by dissolving Tf2N salts of the metals (Ag, Cu, Co, Ni, and zinc) in [bmim][Tf2N] in a 1:2 molar ratio. The experimental results showed that with the exception of the Ni system, for all the ILs, it was possible to achieve the electroreduc‐

the specific capacitance.

196 Progress and Developments in Ionic Liquids

prepared AAO template).

tion of the metal operating in a normal air atmosphere.

**Figure 18.** (a) SEM of silver coatings deposited from "wet" (A and B) and "dry" (C and D) AgTf2N:[bmim][Tf2N] 1:2 molar ratio solution. (b) SEM of copper layers deposited from "wet" Cu(Tf2N)2: [bmim][Tf2N] 1:2 molar ratio solution as a function of the applied potential. (E) −0.55 V, (F) −0.70 V, (G) −0.85 V, and (H) −0.90 V. (c) SEM of copper layers deposited from "wet" Co(Tf2N)2: [bmim][Tf2N] 1:2 molar ratio solution. (I) −1.30 V "dry" solution and (J) −1.30 V "wet" solution. (d) SEM of zinc layers deposited from "wet" Zn(Tf2N)2: [bmim][Tf2N] 1:2 molar ratio solution. (K) −1.30 V "dry" solution and (L) −1.30 V "wet" solution.

**Figure 19.** Cyclic voltammogram for the neat [C4Bipyr][Tf2N] (black trace) and the solution of Cu(NO3)2 in [C4Bipyr] [Tf2N] (red trace).

Kosta et al. [40] proposed an electrochemical route to obtain CuI films (**Figure 20**). The approach was based on the electrochemical reduction of I2 in a solution of copper bis(trifluoromethanesulfonyl)imide salt in 1-butyl-1-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide room-temperature ionic liquid. The mechanisms pointed out that the CuI formation occurred from the chemical reaction between the Cu2+ and I− generated from the I2 reduction. The researchers also investigated the electrodeposition from organic solvent-based media and found that the solvent nature affected strongly to the nanocrystal packing density. The electrodeposition of ZnO/CuI heterostructures, with high transmittance in the visible range (i.e., >75%), was also reported. The current density-voltage characteristic of the resulting device exhibited clear rectifying behavior with a rectification of ~ 2 × 103 at *V* = ±1.5 V.

temperature operation. The continuum‐based cell level simulation results showed that the battery performance can be improved significantly by increasing operating temperature. Simulation results also revealed that by increasing the operating temperature, the specific capacity can be improved significantly for high load current density. They also investigated the effect of different temperatures on the performance of Li‐air battery, and the results showed that the transport limitation of oxygen and lithium ions could be alleviated at higher temper‐

**O**

**Cl**

**+**

**O**

**3**

**T**

= 1 <sup>6</sup> during the continuous scans. The

**IP1**

**IP2**

**+ \_**

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**36h DMF 70°C** **S(CF2)2SO3 -**

**Li+ LiO**

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199

**S(CF2)2SO3 - Li<sup>+</sup>**

**O**

**O**

**O**

**S(CF2)2SO3 -**

**S(CF2)2SO3 -**

**Figure 21.** Synthesis route to obtain redox cationic and anionic species for the biredox ILs named IP1 and IP2. AQ and

Ueda et al. [43] investigated the electrochemical stability of a fullerene (C60) thin film supported on Au(1 1 1) in an ionic liquid electrolyte [C4mpyrr][Tf2N] (**Figure 23**). The obtained result indicated that the dissolution of the C60 film was caused by cation insertion and accelerated by

redox states of C60 in [C4mpyrr][Tf2N] could be clearly controlled by the scan rate and tem‐

**N N N**

> **N N T**

atures.

**N O**

**NaH 3h Acetone** **N O**

**O**

**Br**

**N N**

**H2O, RT, 2h -LiBr**

**H2O, RT, 2h -LiBr**

T denote the anthraquinone and TEMPO moiety, respectively.

the generation of multiple redox states of C60

**RT Br Br**

**<sup>N</sup> <sup>N</sup> O N <sup>O</sup> <sup>N</sup> Br -**

**QA O**

**QA O**

**24h 45°C** **O N O**

**OH**

**TMSBr 3h RT**

> **N O**

**Br**

**24h 45°C**

**N**

**1 + 3**

**1 + 2**

**2 1**

**N N**

**<sup>N</sup> <sup>O</sup>**

perature.

**Figure 20.** FESEM micrographs of the top view and cross section of films obtained from PYR14TFSI (a and b) and isopropanol (c and d)-based media.

#### **2.3. Electroredox**

Mourada et al. [41] described the biredox ionic liquid electrolytes in which both anion and cation are functionalized with anthraquinone and 2,2,6,6-tetramethylpiperidinyl-1-oxyl (TEMPO) groups, respectively (**Figure 21**). At the same time, they carried out the in-depth investigations based on crossed experimental and theoretical studies to elucidate how the bulkiness of ions bearing a redox moiety impacted electron and mass transfers and accordingly the efficiency of electrochemical devices. In such redox species, the electron transfer was not governed by the overall size of the solvated redox species, which took preferential orientation toward the surface.

Yoo et al. [42] investigated the electrical performance of a Li-air cell with ionic liquid electrolytes operating at high temperature. A continuum-based model was used to quantify the performance of the Li-air cell, with an ionic liquid (MPPY-TFSI) electrolyte as a function of operating temperature (**Figure 22**). The molecular dynamics (MD) simulations indicated that oxygen solubility in ionic liquid increases with temperature, which is very favorable for hightemperature operation. The continuum‐based cell level simulation results showed that the battery performance can be improved significantly by increasing operating temperature. Simulation results also revealed that by increasing the operating temperature, the specific capacity can be improved significantly for high load current density. They also investigated the effect of different temperatures on the performance of Li‐air battery, and the results showed that the transport limitation of oxygen and lithium ions could be alleviated at higher temper‐ atures.

Kosta et al. [40] proposed an electrochemical route to obtain CuI films (**Figure 20**). The approach was based on the electrochemical reduction of I2 in a solution of copper bis(trifluoromethanesulfonyl)imide salt in 1-butyl-1-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide room-temperature ionic liquid. The mechanisms pointed out that the CuI formation

The researchers also investigated the electrodeposition from organic solvent-based media and found that the solvent nature affected strongly to the nanocrystal packing density. The electrodeposition of ZnO/CuI heterostructures, with high transmittance in the visible range (i.e., >75%), was also reported. The current density-voltage characteristic of the resulting device

**Figure 20.** FESEM micrographs of the top view and cross section of films obtained from PYR14TFSI (a and b) and iso-

Mourada et al. [41] described the biredox ionic liquid electrolytes in which both anion and cation are functionalized with anthraquinone and 2,2,6,6-tetramethylpiperidinyl-1-oxyl (TEMPO) groups, respectively (**Figure 21**). At the same time, they carried out the in-depth investigations based on crossed experimental and theoretical studies to elucidate how the bulkiness of ions bearing a redox moiety impacted electron and mass transfers and accordingly the efficiency of electrochemical devices. In such redox species, the electron transfer was not governed by the overall size of the solvated redox species, which took preferential orientation

Yoo et al. [42] investigated the electrical performance of a Li-air cell with ionic liquid electrolytes operating at high temperature. A continuum-based model was used to quantify the performance of the Li-air cell, with an ionic liquid (MPPY-TFSI) electrolyte as a function of operating temperature (**Figure 22**). The molecular dynamics (MD) simulations indicated that oxygen solubility in ionic liquid increases with temperature, which is very favorable for high-

generated from the I2 reduction.

at *V* = ±1.5 V.

occurred from the chemical reaction between the Cu2+ and I−

exhibited clear rectifying behavior with a rectification of ~ 2 × 103

propanol (c and d)-based media.

198 Progress and Developments in Ionic Liquids

**2.3. Electroredox**

toward the surface.

**Figure 21.** Synthesis route to obtain redox cationic and anionic species for the biredox ILs named IP1 and IP2. AQ and T denote the anthraquinone and TEMPO moiety, respectively.

Ueda et al. [43] investigated the electrochemical stability of a fullerene (C60) thin film supported on Au(1 1 1) in an ionic liquid electrolyte [C4mpyrr][Tf2N] (**Figure 23**). The obtained result indicated that the dissolution of the C60 film was caused by cation insertion and accelerated by the generation of multiple redox states of C60 = 1 <sup>6</sup> during the continuous scans. The redox states of C60 in [C4mpyrr][Tf2N] could be clearly controlled by the scan rate and tem‐ perature.

**Figure 22.** Illustration of Li‐air battery with corresponding electrochemical reactions at anode and cathode side.

**Figure 24.** Chemical events between rGO and FeIL leading to the intercalation of Fe at defect positions (shown in red)

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201

**Figure 25.** Schemes of the proton transport mechanism for (a) PA‐PBI and (b) PPDC3X membranes.

Ueda et al. [46] investigated the control of the redox states of highly charged C60 anions in two ionic liquids (ILs) (**Figure 26**). The results showed that highly charged reduced states of C60

interface between a multilayered C60 adlayer on a Au(1 1 1) electrode and N‐butyl‐N‐methyl‐

6 − could be produced and detected at room temperature by using an electrochemical

5 −

stabilized by a ferrocene/ferrocenyl configuration.

and C60

**Figure 23.** (a) AFM image (1.5 × 1.5 μm2 ) and (b) high‐resolution STM image (10 × 10 nm2 ) of C60‐deposited Au/mica surface. Note that STM image was obtained under electrochemical condition in 0.1 M HClO4.

Sonkaria et al. [44] carried out the ionic liquid‐induced synthesis of a graphene intercalated ferrocene nanocatalyst (**Figure 24**), which was the first demonstration approach for the assembly of ferrocene in ionic liquids. Hooshyari et al. [45] prepared two types of innovative composite membranes based on polybenzimidazole (PBI) containing dicationic ionic liquid PDC3 and monocationic ionic liquid PMC6 as electrolyte for high‐temperature fuel cell applications under anhydrous conditions (**Figure 25**). The analyses of results displayed high proton conductivity and thermal stability. Moreover, the fuel cell performance of PA doped PDC3 composite membranes was enhanced at high temperatures.

Recent Advances in Electrocatalytic Applications of Ionic Liquids http://dx.doi.org/10.5772/65808 201

**Figure 24.** Chemical events between rGO and FeIL leading to the intercalation of Fe at defect positions (shown in red) stabilized by a ferrocene/ferrocenyl configuration.

**Figure 22.** Illustration of Li‐air battery with corresponding electrochemical reactions at anode and cathode side.

) and (b) high‐resolution STM image (10 × 10 nm2

Sonkaria et al. [44] carried out the ionic liquid‐induced synthesis of a graphene intercalated ferrocene nanocatalyst (**Figure 24**), which was the first demonstration approach for the assembly of ferrocene in ionic liquids. Hooshyari et al. [45] prepared two types of innovative composite membranes based on polybenzimidazole (PBI) containing dicationic ionic liquid PDC3 and monocationic ionic liquid PMC6 as electrolyte for high‐temperature fuel cell applications under anhydrous conditions (**Figure 25**). The analyses of results displayed high proton conductivity and thermal stability. Moreover, the fuel cell performance of PA doped

surface. Note that STM image was obtained under electrochemical condition in 0.1 M HClO4.

PDC3 composite membranes was enhanced at high temperatures.

) of C60‐deposited Au/mica

**Figure 23.** (a) AFM image (1.5 × 1.5 μm2

200 Progress and Developments in Ionic Liquids

**Figure 25.** Schemes of the proton transport mechanism for (a) PA‐PBI and (b) PPDC3X membranes.

Ueda et al. [46] investigated the control of the redox states of highly charged C60 anions in two ionic liquids (ILs) (**Figure 26**). The results showed that highly charged reduced states of C60 5 − and C60 6 − could be produced and detected at room temperature by using an electrochemical interface between a multilayered C60 adlayer on a Au(1 1 1) electrode and N‐butyl‐N‐methyl‐ pyrrolidinium‐based IL, whereas a tributylmethylammonium‐based IL provided less than four redox waves of C60. The results of the present study suggested that both a wide potential window and the interaction between C60 anions and IL cations are important for controlling the multiple redox states of C60 at room temperature.

**OCH3 O**

**Figure 28.** (a) Schematic setup of sulfur in the electrochemical cell and (b) possible equilibriums of different phases of

Viaua et al. [49] studied the electropolymerization of pyrrole films in three room‐temperature ionic liquids: bmim‐PF6, emim‐TFSA, and bmp‐TFSA (**Figure 29**). The experimental results showed that the difference of activity from one polymer film to the other was mainly attributed to the difference of viscosity between the solvents used. Li et al. [50] prepared the composite films of poly(3,4‐ethylenedioxythiophene) (PEDOT) doped with various functional ionic liquids by electropolymerization process on indium‐doped tin oxide (ITO) conducting glasses (**Figure 30**). The ITO glasses coated with the composite films were used as the counter electrodes in dye‐sensitized solar cells, and various imidazolium cations with different alkyl chains and anions were used as the ionic liquids. Palombi et al. [51] investigated the double electrosynthesis of 3‐((4S)‐benzyl‐2‐oxo‐oxazolidin‐3‐carbonyl)‐heptane‐2,6‐dione (2a) at the cathodic and anodic compartments of a divided glass cell in ionic liquid [Emim]BF4 (**Figure 31**). In this electrolysis, ionic liquid played the role of solvent/electrolyte system for the cathodically initiated reaction and electrolyte/pre‐catalyst for the anodic one. Dong et al. [52] synthesized the monodisperse poly(ionic liquid) particles for use as high‐performance

**S**

**O**

**O**

**S S S**

**O**

**Electron Donors**

**<sup>n</sup> <sup>S</sup>**

**S S S**

**F**

**O O**

**n**

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203

**O**

**PBDTTT-C PTB7**

**O**

**OCH3 O**

**Electron Acceptors**

**PC61BM PC71BM**

**Figure 27.** Chemical structures of BenMeIm‐Cl IL and BHJ materials.

sulfur in [EMIm]TFSI.

**2.4. Other applications**

**<sup>S</sup> <sup>n</sup> Cl-**

**P3HT**

**N N**

**BenMelm-Cl**

**Interfacial Material**

**Figure 26.** Typical cyclic voltammograms of a clean Au(1 1 1) electrode (gray line) and a C60‐modified Au(1 1 1) elec‐ trode obtained in [TFSI]− ‐based ILs with (a) [N1,4,4,4] <sup>+</sup> and (b) [C4mpyrr]+ , respectively, at 25 °C. The blue dotted and red solid lines indicate the first and second scans, respectively, of the C60‐modified Au(1 1 1) single‐crystal electrode. The scan rate was 0.5 V s−1.

Fu et al. [47] presented a spontaneous vertical phase separation (SVPS) self‐assembled bilayers structure with BenMeIm‐Cl ionic liquid (IL) interfacial bottom layer and a photoactive top layer via a single spin‐coated step of BenMeIm‐Cl IL and organic donor‐acceptor composite and achieved a PCE as high as 8% based on IPSCs with PTB7 as the donor (**Figure 27**). The presence of BenMeIm‐Cl IL reduced the work function of ITO and led to a better energy‐level matching for efficient charge transfer. The driving force of SVPS self‐assembled structure was from the relative surface energy difference between organic materials and BenMeIm‐Cl ILs, together with their interactions with the substrates. This self‐assembled process procedure paved the way to simplify the manufacturing of low‐cost and large‐area organic electronic devices. Chen et al. [48] studied the metal binary sulfides (TiS2, FeS2), in either powder or thin film forms, Li insertion electrodes for rechargeable lithium batteries. They exploited the equilibrium solubility of molecular sulfur into ionic liquids at its melting point (120°C), to prepare thin films of both Co9S8 and FeS*x* (**Figure 28**). The researchers demonstrated that the growth of Co9S8 films involved the reaction of soluble sulfur with the electrodeposited Co metallic layer.

**Figure 27.** Chemical structures of BenMeIm‐Cl IL and BHJ materials.

**Figure 28.** (a) Schematic setup of sulfur in the electrochemical cell and (b) possible equilibriums of different phases of sulfur in [EMIm]TFSI.

#### **2.4. Other applications**

pyrrolidinium‐based IL, whereas a tributylmethylammonium‐based IL provided less than four redox waves of C60. The results of the present study suggested that both a wide potential window and the interaction between C60 anions and IL cations are important for controlling

**Figure 26.** Typical cyclic voltammograms of a clean Au(1 1 1) electrode (gray line) and a C60‐modified Au(1 1 1) elec‐

solid lines indicate the first and second scans, respectively, of the C60‐modified Au(1 1 1) single‐crystal electrode. The

Fu et al. [47] presented a spontaneous vertical phase separation (SVPS) self‐assembled bilayers structure with BenMeIm‐Cl ionic liquid (IL) interfacial bottom layer and a photoactive top layer via a single spin‐coated step of BenMeIm‐Cl IL and organic donor‐acceptor composite and achieved a PCE as high as 8% based on IPSCs with PTB7 as the donor (**Figure 27**). The presence of BenMeIm‐Cl IL reduced the work function of ITO and led to a better energy‐level matching for efficient charge transfer. The driving force of SVPS self‐assembled structure was from the relative surface energy difference between organic materials and BenMeIm‐Cl ILs, together with their interactions with the substrates. This self‐assembled process procedure paved the way to simplify the manufacturing of low‐cost and large‐area organic electronic devices. Chen et al. [48] studied the metal binary sulfides (TiS2, FeS2), in either powder or thin film forms, Li insertion electrodes for rechargeable lithium batteries. They exploited the equilibrium solubility of molecular sulfur into ionic liquids at its melting point (120°C), to prepare thin films of both Co9S8 and FeS*x* (**Figure 28**). The researchers demonstrated that the growth of Co9S8 films involved the reaction of soluble sulfur with the electrodeposited Co

<sup>+</sup> and (b) [C4mpyrr]+

, respectively, at 25 °C. The blue dotted and red

the multiple redox states of C60 at room temperature.

202 Progress and Developments in Ionic Liquids

‐based ILs with (a) [N1,4,4,4]

trode obtained in [TFSI]−

scan rate was 0.5 V s−1.

metallic layer.

Viaua et al. [49] studied the electropolymerization of pyrrole films in three room‐temperature ionic liquids: bmim‐PF6, emim‐TFSA, and bmp‐TFSA (**Figure 29**). The experimental results showed that the difference of activity from one polymer film to the other was mainly attributed to the difference of viscosity between the solvents used. Li et al. [50] prepared the composite films of poly(3,4‐ethylenedioxythiophene) (PEDOT) doped with various functional ionic liquids by electropolymerization process on indium‐doped tin oxide (ITO) conducting glasses (**Figure 30**). The ITO glasses coated with the composite films were used as the counter electrodes in dye‐sensitized solar cells, and various imidazolium cations with different alkyl chains and anions were used as the ionic liquids. Palombi et al. [51] investigated the double electrosynthesis of 3‐((4S)‐benzyl‐2‐oxo‐oxazolidin‐3‐carbonyl)‐heptane‐2,6‐dione (2a) at the cathodic and anodic compartments of a divided glass cell in ionic liquid [Emim]BF4 (**Figure 31**). In this electrolysis, ionic liquid played the role of solvent/electrolyte system for the cathodically initiated reaction and electrolyte/pre‐catalyst for the anodic one. Dong et al. [52] synthesized the monodisperse poly(ionic liquid) particles for use as high‐performance anhydrous polyelectrolyte‐based smart electrorheological materials (**Figure 32**). The results showed that the ionic liquid particles possessed strong electrorheological effect in dry state and the electrorheological effect depended on the size of cation/anion parts.

**Pt Anode**

**BF4**

**4G-glass septum**

**EMImBF4**

**O O O**

**O O**

**Xc**

**Figure 32.** The SEM images of PIL particles: (a) poly(VBTMA+

poly(VBTEA+

(CF3SO2)2N−

).

**O**

**Figure 31.** Synthesis of Michael adduct 2a by paired electrolysis in BF4‐based ionic liquid.

**O Xc**

**2a**

**cathodic current yield 7.3 mol of 2a/Faraday**

**Xc = O N**

**O O**

**0.1F/mol**

**1a**

**,**

**O**

**Xc**

**Bn**

CF3SO3 −

Kaminski et al. [53] presented an innovative methodology for a liquid‐liquid extraction process based on an electrically induced emulsion of ionic liquid 1‐[bmim][MeSO4] as the extracting solvent dispersed in an organic mixture (**Figure 33**), and this liquid‐liquid extraction provided an environmentally friendly process as an alternative to azeotropic distillation. Martínez et al. [54] studied the performance of terracotta separators modified with the same ionic liquid [Emim][NTf2], neat, and also mixed with PTFE binder (**Figure 34**). They found the operational limitations when the IL was integrated in the ceramic separator, there was a significant enhancement of the MFC performance when added as part of the activated layer mixture of the cathode. Najafabadi and Gyenge [55] reported the simultaneous anodic and cathodic GN

), (b) poly (VBTMA+

(CF3SO2)2N−

), and (c)

**O O**

**overrall isolated yield: 71%**

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205

**anodic current yield 6.8 mol of 2a/Faraday**

Recent Advances in Electrocatalytic Applications of Ionic Liquids

**EMImBF4**

**- BF3**

**Pt cathode**

**Xc**

**Figure 29.** AFM images of PPy‐bmim‐PF6 (a), PPy‐emim‐TFSA (b), and PPy‐bmp‐TFSA (c). SEM images of PPy‐bmim‐ PF6 (d), PPy‐emim‐TFSA (e), and PPy‐bmp‐TFSA (f).

**Figure 30.** Illustrative electropolymerization process for the PEDOT counter electrodes (a) doped with various alkyl chains in imidazolium cations and (b) doped with different anions.

**Figure 31.** Synthesis of Michael adduct 2a by paired electrolysis in BF4‐based ionic liquid.

anhydrous polyelectrolyte‐based smart electrorheological materials (**Figure 32**). The results showed that the ionic liquid particles possessed strong electrorheological effect in dry state

**N**

**bmpTFSA**

**thickness**

**(d)**

**Figure 29.** AFM images of PPy‐bmim‐PF6 (a), PPy‐emim‐TFSA (b), and PPy‐bmp‐TFSA (c). SEM images of PPy‐bmim‐

**CH3**

**BF4 -**

**C10H21**

**NH(SO2CF3)2**

**(b)**

**Electropolymerization**

**Electropolymerization**

**adhesion**

**doping**

**(e)**

**(c)**

**O O**

**:**

**:**

**:**

**X=BF4 -**

**O O**

**:**

**:**

**n**

**S**

**X-**

**:**

**: :**

**S**

**O O**

**:**

**:**

**:**

**O O**

**:**

**:**

**n**

**S**

**X-**

**:**

**: :**

**X=BF4 - , PF6 - , SO3CF3 - , TFSI-**

**S**

**(f)**

and the electrorheological effect depended on the size of cation/anion parts.

**NH(SO2CF3)2**

**(a)**

**CH3**

**BF4 - <sup>N</sup> N**

> **N N**

**C6H13**

**4.HMIPF6**

**CH3**

**O**

**O**

**O**

**O**

**Figure 30.** Illustrative electropolymerization process for the PEDOT counter electrodes (a) doped with various alkyl

**O CF3 F3C S N S CF3**

**N N**

**C6H13 6.HMITFSI**

**CH3**

**PF6 -**

**C6H13**

**morphology**

**N N**

**emimTFSA**

**electrochemical activity**

**N N**

**bmimPF6**

204 Progress and Developments in Ionic Liquids

**O O**

**+**

**S**

**O O**

**+**

**S EDOT** **PF6**

PF6 (d), PPy‐emim‐TFSA (e), and PPy‐bmp‐TFSA (f).

**N N**

**C2H5**

**N N**

**C6H13**

**2.HMIBF4**

**CH3**

**BF4 -**

**N N**

**C6H13 5.HMISO3CF3**

**CH3**

**O S O**

chains in imidazolium cations and (b) doped with different anions.

**CH3**

**BF4 - <sup>N</sup> N**

**EDOT 1.EMIBF4 2.HMIBF4 3.DMIBF4**

**Tunable electrodeposited polymer film properties using different RTILS as solvent**

**Figure 32.** The SEM images of PIL particles: (a) poly(VBTMA+ CF3SO3 − ), (b) poly (VBTMA+ (CF3SO2)2N− ), and (c) poly(VBTEA+ (CF3SO2)2N− ).

Kaminski et al. [53] presented an innovative methodology for a liquid‐liquid extraction process based on an electrically induced emulsion of ionic liquid 1‐[bmim][MeSO4] as the extracting solvent dispersed in an organic mixture (**Figure 33**), and this liquid‐liquid extraction provided an environmentally friendly process as an alternative to azeotropic distillation. Martínez et al. [54] studied the performance of terracotta separators modified with the same ionic liquid [Emim][NTf2], neat, and also mixed with PTFE binder (**Figure 34**). They found the operational limitations when the IL was integrated in the ceramic separator, there was a significant enhancement of the MFC performance when added as part of the activated layer mixture of the cathode. Najafabadi and Gyenge [55] reported the simultaneous anodic and cathodic GN

production in two types of electrochemical cells in aprotic ionic liquid electrolytes (**Fig‐ ure 35**). They demonstrated a synergistic exfoliation effect when the iso‐molded graphite anode and cathode were subjected to a constant cell potential, generating up to three times higher exfoliation yields than single electrode studied on each side.

**Figure 35.** Electro‐exfoliation of graphitic anodes and cathodes in ionic liquids.

molten salt, will play a greater role in electrocatalytic system.

Address all correspondence to: huyulin1982@163.com

In summary, a remarkable amount of progress has been made in recent years in the field of electrocatalysis in the presence of ionic liquids. Despite the impressive progress, a number of challenges still remain: the current studies mainly focus on the applications of ILs while lacking of systematicness in its theoretical research. There is absolutely a need for the search of inherent structures, purity, distribution, and orientation of ILs at electrode interface. We do believe that ionic liquids as a green solvent, which is expected to replace the water, organic solvents, and

Recent Advances in Electrocatalytic Applications of Ionic Liquids

http://dx.doi.org/10.5772/65808

207

College of Materials and Chemical Engineering, China Three Gorges University, Yichang, PR

[1] H. Karkhanechi, S. Salmani, and M. Asghari, "A Review on Gas Separation Applications of Supported Ionic Liquid Membranes," *ChemBioEng Reviews*, vol. 2, pp. 290–302, 2015.

**3. Conclusions**

**Author details**

Yu Lin Hu

China

**References**

**Figure 33.** Scheme of the general experimental arrangement.

**Figure 34.** Configuration of the cathodes and the separators.

**Figure 35.** Electro‐exfoliation of graphitic anodes and cathodes in ionic liquids.

### **3. Conclusions**

production in two types of electrochemical cells in aprotic ionic liquid electrolytes (**Fig‐ ure 35**). They demonstrated a synergistic exfoliation effect when the iso‐molded graphite anode and cathode were subjected to a constant cell potential, generating up to three times

higher exfoliation yields than single electrode studied on each side.

206 Progress and Developments in Ionic Liquids

**Figure 33.** Scheme of the general experimental arrangement.

**Figure 34.** Configuration of the cathodes and the separators.

In summary, a remarkable amount of progress has been made in recent years in the field of electrocatalysis in the presence of ionic liquids. Despite the impressive progress, a number of challenges still remain: the current studies mainly focus on the applications of ILs while lacking of systematicness in its theoretical research. There is absolutely a need for the search of inherent structures, purity, distribution, and orientation of ILs at electrode interface. We do believe that ionic liquids as a green solvent, which is expected to replace the water, organic solvents, and molten salt, will play a greater role in electrocatalytic system.

### **Author details**

Yu Lin Hu

Address all correspondence to: huyulin1982@163.com

College of Materials and Chemical Engineering, China Three Gorges University, Yichang, PR China

#### **References**

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Liquid) Particles with Different Size of Cation/Anion Parts," *Polymer*, vol. 97, pp. 408– 417, 2016.

**Chapter 10**

**Provisional chapter**

**Purification of Rare Earth Amide Salts by**

**Purification of Rare Earth Amide Salts by**

**Metals Using Ionic Liquids**

**Metals Using Ionic Liquids**

Additional information is available at the end of the chapter

2− + 3e− → Nd(0) + 5[TFSA]−

Additional information is available at the end of the chapter

Masahiko Matsumiya

Masahiko Matsumiya

http://dx.doi.org/10.5772/66300

**Abstract**

(TFSA)5]

**Hydrometallurgy and Electrodeposition of Rare Earth**

This paper reports a novel bench-scale hydrometallurgical procedure and electrodeposition using triethyl-pentyl-phosphonium bis(trifluoromethyl-sulfonyl)amide ([P2225][TFSA]) ionic liquids (ILs) for the recovery of rare earth (RE) metals from spent Nd-Fe-B magnets. The hydrometallurgical treatments were carried out at bench scale to produce RE amide salts of high purity. In the leaching process employing 1.7 kg of oxidized Nd-Fe-B fine powder and 14.2 L of an acid medium of 1,1,1-trifluoro-*N*- [(trifluoromethyl)sulfonyl]methanesulfonamide (H[TFSA]), selective leaching of RE ions (85.7±5.8% Nd) was performed at bench scale. Then, Fe (<99.9%) was successfully separated from RE ions in the deironization process. The total amount of the recovered amide salts through the evaporation treatment using a spray dryer was 3.57 kg. From the CV/EQCM measurements for Nd(III) at 373 K, a clear cathodic peak with the mass increased, and the *ηρ* decreased was observed at −2.79 V. Considering our previous investigations, the reduction of Nd(III)/Nd(0) was indicated as [Nd(III)

−2.49 V ~ −2.94 V was 46.8 g mol−1, which was close to the theoretical value for the electrodeposition reaction of Nd(III)/Nd(0), 48.1 g mol−1. Moreover, the electrodeposition of Nd(0) was carried out under the condition of −3.20 V versus Fc/Fc+ at 373 K. The electrodeposits were identified with the metallic Nd in the middle layer investigated by X-ray diffraction and X-ray photoelectron spectroscopy. Finally, we demonstrated that the novel recovery process consisted of hydrometallurgy and

electrodeposition using ILs was effective by calculating material flow.

**Keywords:** electrodeposition, hydrometallurgy, ionic liquids, neodymium metal

and reproduction in any medium, provided the original work is properly cited.

. In addition, the *M*app value in the range of

© 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution,

© 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

**Hydrometallurgy and Electrodeposition of Rare Earth**


### **Purification of Rare Earth Amide Salts by Hydrometallurgy and Electrodeposition of Rare Earth Metals Using Ionic Liquids Purification of Rare Earth Amide Salts by Hydrometallurgy and Electrodeposition of Rare Earth Metals Using Ionic Liquids**

### Masahiko Matsumiya Masahiko Matsumiya

Liquid) Particles with Different Size of Cation/Anion Parts," *Polymer*, vol. 97, pp. 408–

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417, 2016.

212 Progress and Developments in Ionic Liquids

317–324, 2016.

Additional information is available at the end of the chapter Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/66300

#### **Abstract**

This paper reports a novel bench-scale hydrometallurgical procedure and electrodeposition using triethyl-pentyl-phosphonium bis(trifluoromethyl-sulfonyl)amide ([P2225][TFSA]) ionic liquids (ILs) for the recovery of rare earth (RE) metals from spent Nd-Fe-B magnets. The hydrometallurgical treatments were carried out at bench scale to produce RE amide salts of high purity. In the leaching process employing 1.7 kg of oxidized Nd-Fe-B fine powder and 14.2 L of an acid medium of 1,1,1-trifluoro-*N*- [(trifluoromethyl)sulfonyl]methanesulfonamide (H[TFSA]), selective leaching of RE ions (85.7±5.8% Nd) was performed at bench scale. Then, Fe (<99.9%) was successfully separated from RE ions in the deironization process. The total amount of the recovered amide salts through the evaporation treatment using a spray dryer was 3.57 kg. From the CV/EQCM measurements for Nd(III) at 373 K, a clear cathodic peak with the mass increased, and the *ηρ* decreased was observed at −2.79 V. Considering our previous investigations, the reduction of Nd(III)/Nd(0) was indicated as [Nd(III) (TFSA)5] 2− + 3e− → Nd(0) + 5[TFSA]− . In addition, the *M*app value in the range of −2.49 V ~ −2.94 V was 46.8 g mol−1, which was close to the theoretical value for the electrodeposition reaction of Nd(III)/Nd(0), 48.1 g mol−1. Moreover, the electrodeposition of Nd(0) was carried out under the condition of −3.20 V versus Fc/Fc+ at 373 K. The electrodeposits were identified with the metallic Nd in the middle layer investigated by X-ray diffraction and X-ray photoelectron spectroscopy. Finally, we demonstrated that the novel recovery process consisted of hydrometallurgy and electrodeposition using ILs was effective by calculating material flow.

**Keywords:** electrodeposition, hydrometallurgy, ionic liquids, neodymium metal

© 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

### **1. Introduction**

Rare earth (RE) elements are currently regarded to be the most critical elements necessary for sustainable applications, and their RE groups play an important role in the development of future high-tech industries. Significant price fluctuations and high demand have raised their potential recovery from end-of-life products [1]. It is important to recover them from urban mining and secondary products containing permanent magnets.

ical window, low vapor pressure, and incombustibility [12]. The electrochemical behaviors and the electrodepositions of RE metals such as La, Sm, Eu, and Yb in ILs are reported [13, 14]. We have already demonstrated the electrochemical behaviors and the electrodepositions of Nd [15–18] and Dy metals [19, 20] using hydrophobic ILs, such as triethyl-pentyl-phosphonium bis(trifluoromethyl-sulfonyl) amide [P2225][TFSA] and 2-hydroxyethyl-trimethyl-ammonium bis(trifluoromethyl-sulfonyl) amide [N1112OH][TFSA]. In addition, we have developed the wet separation processes such as the solvent extraction using the hydrophobic ILs [21–23] and the precipitation separation [8, 9]. The wet separation process was combined with the electrodeposition for the recovery of the Nd and Dy metals from the practical wastes of Nd-Fe-B

Purification of Rare Earth Amide Salts by Hydrometallurgy and Electrodeposition of Rare Earth Metals...

http://dx.doi.org/10.5772/66300

215

For the purpose of the analysis of reduction behavior of Nd(III) in ILs, in situ investigation using an electrochemical quartz crystal microbalance (EQCM) was conducted in this study. The EQCM technique is based on the piezoelectric properties of a quartz crystal and can detect a nano-level mass change in a quartz crystal electrode during electrochemical experiments from the resonance frequency of the quartz [24]. Although a conventional oscillator technique, namely self-excited or active technique EQCM, was inoperative in some ILs due to their high viscosity, the possibility of applying EQCM measurements with the use of separately excited or passive technique in highly viscous fluids including ILs was recently demonstrated and the cases of successful application have been reported [25–30]. On the EQCM measurement, the resonance resistance can also be measured and reflects a product of the viscosity and the density of the media near the quartz crystal electrode [28] and the viscoelasticity of the electrodeposits [26, 27, 31, 32]. Thus, the change in the mass and the viscoelasticity of the electrodeposits on the electrode and the product of the viscosity and the density of the electrolyte near the electrode, relating to the concentration of a soluble species particularly in the case of ILs [25, 26] and corresponding to the electrochemical behavior, can simultaneously be observed by using the EQCM measurements, and very useful information for the specific

In this study as a new attempt, the electrochemical behavior of Nd(III) in ILs was analyzed by EQCM measurement at elevated temperatures because it is desirable to decrease the overpotential of the electrodeposition and to increase the diffusion rate of the metallic species by lowering the viscosity by means of elevating the temperature. There were a few reports about the theoretical equations: the Sauerbrey equation [24] and Kanazawa-Gordon [33]. However, these equations were not applied at elevated temperatures [25, 34]; therefore, we have demonstrated that the applicability of EQCM method in a medium temperature range around 373 K was revealed in the previous study [35]. In addition, we discuss the electroreduction

Considering the fundamental electrochemical investigations, the electrodeposition on the condition of constant potential was carried out on a relatively large scale. The Nd metal recovered by electrodeposition is applicable in the production of new Nd-Fe-B magnets because Nd metal of high purity is obtained by polishing the oxide layer after electrodeposition using ILs. Finally, we demonstrated the effectiveness of hydrometallurgy and electrodeposi-

examination of the electrode reaction can be acquired.

tion process through the material flow.

behavior of Nd(III) in ILs on the potentiostatic condition in this study.

magnets.

Hydrometallurgical treatment is widely applied as an effective method for extracting RE components from primary sources and is potential in reclaiming these elements [2]. As listed in **Table 1**, the various techniques effective for the recovery of RE elements, such as chemical vapor transport [3], solvent extraction [4], electrolytic method [5], and hydrometallurgical processes [6, 7], have been listed, although a number of recovery methods for RE elements on the laboratory scale have not been so widespread. There was almost no information about the plants and processing paths. From the above situation, this study focused on a bench-scale hydrometallurgy to separate and recover RE components from spent Nd-Fe-B magnet. The preliminary research in our procedure is reported in the previous publications [8, 9].


**Table 1.** Various recovery techniques for rare earths.

Pyrometallurgical treatment using high-temperature molten salts (HTMSs) is generally known as a conventional method for the recovery of RE metals. However, the HTMSs such as fluorides [10, 11] consumes a large amount of thermal energy owing to the high melting points of molten salts; thus, the recovery method of RE metals from HTMS baths is inappropriate as a nextgeneration technique. From the standpoint of saving energy, the development of a recovery process for RE metals with reduced energy consumption is hopeful in the near future. For this purpose, we have proposed an electrowinning-recovery method of the REs using ILs, which have unique physicochemical properties, such as high ionic conductivity, a wide electrochemical window, low vapor pressure, and incombustibility [12]. The electrochemical behaviors and the electrodepositions of RE metals such as La, Sm, Eu, and Yb in ILs are reported [13, 14]. We have already demonstrated the electrochemical behaviors and the electrodepositions of Nd [15–18] and Dy metals [19, 20] using hydrophobic ILs, such as triethyl-pentyl-phosphonium bis(trifluoromethyl-sulfonyl) amide [P2225][TFSA] and 2-hydroxyethyl-trimethyl-ammonium bis(trifluoromethyl-sulfonyl) amide [N1112OH][TFSA]. In addition, we have developed the wet separation processes such as the solvent extraction using the hydrophobic ILs [21–23] and the precipitation separation [8, 9]. The wet separation process was combined with the electrodeposition for the recovery of the Nd and Dy metals from the practical wastes of Nd-Fe-B magnets.

**1. Introduction**

214 Progress and Developments in Ionic Liquids

Solvent extraction and liquid-membrane transport

Rare earth (RE) elements are currently regarded to be the most critical elements necessary for sustainable applications, and their RE groups play an important role in the development of future high-tech industries. Significant price fluctuations and high demand have raised their potential recovery from end-of-life products [1]. It is important to recover them from urban

Hydrometallurgical treatment is widely applied as an effective method for extracting RE components from primary sources and is potential in reclaiming these elements [2]. As listed in **Table 1**, the various techniques effective for the recovery of RE elements, such as chemical vapor transport [3], solvent extraction [4], electrolytic method [5], and hydrometallurgical processes [6, 7], have been listed, although a number of recovery methods for RE elements on the laboratory scale have not been so widespread. There was almost no information about the plants and processing paths. From the above situation, this study focused on a bench-scale hydrometallurgy to separate and recover RE components from spent Nd-Fe-B magnet. The

preliminary research in our procedure is reported in the previous publications [8, 9].

scrap of RE intermetallic materials.

recover Nd and Dy metals in fused Na2SO4 from Nd and Dy oxides

indicated as 121 from RE-Ni alloys

69.7% Nd and 51% Dy from magnetic waste sludge

Pyrometallurgical treatment using high-temperature molten salts (HTMSs) is generally known as a conventional method for the recovery of RE metals. However, the HTMSs such as fluorides [10, 11] consumes a large amount of thermal energy owing to the high melting points of molten salts; thus, the recovery method of RE metals from HTMS baths is inappropriate as a nextgeneration technique. From the standpoint of saving energy, the development of a recovery process for RE metals with reduced energy consumption is hopeful in the near future. For this purpose, we have proposed an electrowinning-recovery method of the REs using ILs, which have unique physicochemical properties, such as high ionic conductivity, a wide electrochem-

The selective permeation of Nd and Dy by IL based supported liquid-membrane using *N,N*-dioctyldiglycolamic acid.

**Methods Remarks Reference**

[3]

[4]

[5]

[6]

[7]

mining and secondary products containing permanent magnets.

Chemical vapor transport 59% Nd and 68% Dy were recovered from

Electrolysis Polarization on Pt electrode led to

Electrochemical process Mass ratio between Nd and Dy was

**Table 1.** Various recovery techniques for rare earths.

Hydrometallurgical process The recovery efficiencies were indicated as

For the purpose of the analysis of reduction behavior of Nd(III) in ILs, in situ investigation using an electrochemical quartz crystal microbalance (EQCM) was conducted in this study. The EQCM technique is based on the piezoelectric properties of a quartz crystal and can detect a nano-level mass change in a quartz crystal electrode during electrochemical experiments from the resonance frequency of the quartz [24]. Although a conventional oscillator technique, namely self-excited or active technique EQCM, was inoperative in some ILs due to their high viscosity, the possibility of applying EQCM measurements with the use of separately excited or passive technique in highly viscous fluids including ILs was recently demonstrated and the cases of successful application have been reported [25–30]. On the EQCM measurement, the resonance resistance can also be measured and reflects a product of the viscosity and the density of the media near the quartz crystal electrode [28] and the viscoelasticity of the electrodeposits [26, 27, 31, 32]. Thus, the change in the mass and the viscoelasticity of the electrodeposits on the electrode and the product of the viscosity and the density of the electrolyte near the electrode, relating to the concentration of a soluble species particularly in the case of ILs [25, 26] and corresponding to the electrochemical behavior, can simultaneously be observed by using the EQCM measurements, and very useful information for the specific examination of the electrode reaction can be acquired.

In this study as a new attempt, the electrochemical behavior of Nd(III) in ILs was analyzed by EQCM measurement at elevated temperatures because it is desirable to decrease the overpotential of the electrodeposition and to increase the diffusion rate of the metallic species by lowering the viscosity by means of elevating the temperature. There were a few reports about the theoretical equations: the Sauerbrey equation [24] and Kanazawa-Gordon [33]. However, these equations were not applied at elevated temperatures [25, 34]; therefore, we have demonstrated that the applicability of EQCM method in a medium temperature range around 373 K was revealed in the previous study [35]. In addition, we discuss the electroreduction behavior of Nd(III) in ILs on the potentiostatic condition in this study.

Considering the fundamental electrochemical investigations, the electrodeposition on the condition of constant potential was carried out on a relatively large scale. The Nd metal recovered by electrodeposition is applicable in the production of new Nd-Fe-B magnets because Nd metal of high purity is obtained by polishing the oxide layer after electrodeposition using ILs. Finally, we demonstrated the effectiveness of hydrometallurgy and electrodeposition process through the material flow.

### **2. Experimental**

#### **2.1. Pretreatment process**

The spent Nd-Fe-B magnets were recovered from voice coil motors (VCMs) that were heated in an electric furnace at 623 K for 3 h for the demagnetization treatment. After demagnetization, the magnetic flux density was measured using a digital TESLA meter. The residual magnetic force field of this sample was almost zero, and the percentage of demagnetization was >99.9%. Then, the Ni-Cu-Ni triple layer on the Nd-Fe-B sample was removed by a grinding machine. After the stripping of the layer, fragments of Nd-Fe-B sample were pulverized using a stamp mill. The fine powders obtained were sieved to less than 150 μm and heated at 90 K h−1 to 1133 K, which was kept for 3 h in an electric furnace in order to oxidize the Nd-Fe-B components. After the roasting process, these fine powders were reground again by the automatic grinder. The surface area and the particle size (*D*50) of oxidized Nd-Fe-B sample measured by the Brunauer-Emmett-Teller (BET) method were 0.630 m2 g−1 and 59.43 μm, respectively.

measured. The best terminal point of the precipitation reaction for [Fe(OH)*x*]

components in the M(TFSA)*n* salts was measured by ICP-AES analysis.

formation of [Fe(OH)*x*]

3−*x*

**2.4. Electrochemical analysis**

revealed in the previous study [35].

a ferrocene (Fc)/ferrocenium (Fc+

[Fe(OH)*x*]

important to develop a rapid solid-liquid separation technique because [Fe(OH)*x*]

be pH = 4.92–4.93 because no iron component was detected in the leaching solution. It was

Purification of Rare Earth Amide Salts by Hydrometallurgy and Electrodeposition of Rare Earth Metals...

tates are colloidal and the separation from colloidal precipitates is very difficult. Therefore, hematite (*α*-Fe2O3) containing oxidized Nd-Fe-B sample was applied as a seed crystal in the

particle size greatly contributed to the filtration in solid-liquid separation. Finally, the

After the deironization treatment, a turquoise filtrate was obtained; the color of the solution was based on the RE components. The evaporation treatment of the filtrate was carried out by a spray dryer (SD-1000, EYELA Co., Ltd.). In the operation of the spray dryer, the inlet temperature was maintained at 473 ± 1.6 K and dried nitrogen gas was introduced through the evaporation part at 105 ± 5.0 kPa. Then, the H[TFSA] filtrate was introduced using a roller pump at 150 mL h−1. In order to recover the dried M(TFSA)*n* salts, the recovery part was heated at 413 ± 1.5 K with a heating mantle. The amount of M(TFSA)*<sup>n</sup>* salts for one batch and the total amount of M(TFSA)*n* salts were >300 g and 3.57 kg, respectively. The amount of metallic

The resistance of a quartz oscillator and resonance frequency were observed using an EQCM system, (Seiko EG&G, QCA922) applying AT-cut platinum-coated [9 MHz, *ϕ* = 5.0 mm, Seiko EG&G, QA-A9M-PT(P)] with a well-type cell (Seiko EG&G, QA-CL4PK) as shown in **Figure 1**. The employed O-rings (Seiko EG&G, P-S75B) had a high resistance for heat and low expansibility. The temperature of the EQCM system was elevated using a heating mantle controlled by a thermostat with a proportional-integral-derivative (PID) controller. The temperature was slowly increased at a rate of 1.0–1.5 K min−1 to prevent the strain occurring in the crystal structure of the quartz. The bath temperature was measured using a K-type thermocouple (*ϕ* = 1.6 mm). The cell covered with the heating mantle was connected to the EQCM system with an extension cord (Seiko EG&G, QCA922-10-EX10). In terms of the functionality of EQCM technique at elevated temperatures, the relationship between the viscosity and the density of Nd(III) samples, *ηρ* values, the shifts of the resonance frequency, and the resistance before and after contacting the samples with the quartz have been already

The voltammetric measurements were carried out using an electrochemical analyzer (ALS-440A, BAS Inc.,) with the EQCM system employing the Pt-coated quartz oscillator as a working electrode. Two Pt wires with 0.5 mm inside diameter were used as a counter and a quasi-reference electrode (QRE). The counter electrode was surrounded by a Vycor glass filter at the bottom in order to prevent the diffusion of decomposition components from the anode into the electrolyte. The Pt QRE showed a high stability and a good reproducibility of the potential at elevated temperatures. The potential was compensated for the IL standard using

the dissolved oxygen was removed from the electrolytes by bubbling Ar gas for 30 min, and

) redox couple. Before all the electrochemical measurements,

3−*<sup>x</sup>* precipitates. The formation of [Fe(OH)*x*]

precipitates were completely removed from the leaching solution.

3−*<sup>x</sup>* was found to

3−*<sup>x</sup>* precipitates with a large

http://dx.doi.org/10.5772/66300

3−*<sup>x</sup>* precipi-

217

#### **2.2. Bench-scale leaching**

The fine powders of oxidized Nd-Fe-B sample (1.7 kg) were leached in 14.2 L of a 1.0 M aqueous solution of 1,1,1-trifluoro-*N*-[(trifluoromethyl)sulfonyl]methanesulfonamide (HN(SO2CF3)2, H[TFSA]). The leaching solution was heated at 323 K, stirring at 500 rpm. The leaching behavior was researched from the potential (*E*)-pH diagram for Fe-H2O and Nd-H2O systems. Therefore, the oxidation-reduction potential (ORP) and pH in the leaching solution were measured by a high-precision digital meter (MM-60R, DKK-TOA Corp.). The quantification of Fe2+ was available for the complexation of [Fe(phen)3] 2+ using 1,10-phenanthroline. The concentration of Fe2+ was measured by an ultraviolet-visible-near-infrared (UV-Vis-NIR) spectrometer (Perkin Elmer, Lambda750) at 508 nm [36, 37]. The concentration of Fe3+ was calculated from the total amount of Fe2+ and Fe3+ ions obtained from inductively coupled plasma atomic emission spectroscopy (ICP-AES) analysis (ICPE-9000, Shimadzu Co.). All of the precise concentrations for metallic species were quantitatively determined from ICP-AES analysis.

#### **2.3. Bench-scale deironization and preparation of RE salts**

Dried oxygen gas was introduced into the leaching solution with a flow rate of 5.0 L min−1 after the leaching process. The oxidizing agent for Fe2+ was effective in the leaching solution at pH > 3.2. Some kinds of alkali metal hydroxides [8, 9] were acted as precipitation agents in the previous study, and the perfect removal of the iron components was successfully carried out at laboratory scale. The deironization treatment without precipitation agents is important as a further development, because the additive materials contaminated the final RE salts. Considering the leaching process, reuse of the oxidized Nd-Fe-B powder as a precipitation agent is desirable, because RE2O3 in the oxidized Nd-Fe-B sample was selectively leached in the H[TFSA] solution. The oxidized Nd-Fe-B fine powder was also available for sediment formation of [Fe(OH)*x*] 3−*x* precipitates in this study. A small amount of the oxide Nd-Fe-B powder was introduced carefully into leaching solution while the alternation in pH was measured. The best terminal point of the precipitation reaction for [Fe(OH)*x*] 3−*<sup>x</sup>* was found to be pH = 4.92–4.93 because no iron component was detected in the leaching solution. It was important to develop a rapid solid-liquid separation technique because [Fe(OH)*x*] 3−*<sup>x</sup>* precipitates are colloidal and the separation from colloidal precipitates is very difficult. Therefore, hematite (*α*-Fe2O3) containing oxidized Nd-Fe-B sample was applied as a seed crystal in the formation of [Fe(OH)*x*] 3−*<sup>x</sup>* precipitates. The formation of [Fe(OH)*x*] 3−*<sup>x</sup>* precipitates with a large particle size greatly contributed to the filtration in solid-liquid separation. Finally, the [Fe(OH)*x*] 3−*x* precipitates were completely removed from the leaching solution.

After the deironization treatment, a turquoise filtrate was obtained; the color of the solution was based on the RE components. The evaporation treatment of the filtrate was carried out by a spray dryer (SD-1000, EYELA Co., Ltd.). In the operation of the spray dryer, the inlet temperature was maintained at 473 ± 1.6 K and dried nitrogen gas was introduced through the evaporation part at 105 ± 5.0 kPa. Then, the H[TFSA] filtrate was introduced using a roller pump at 150 mL h−1. In order to recover the dried M(TFSA)*n* salts, the recovery part was heated at 413 ± 1.5 K with a heating mantle. The amount of M(TFSA)*<sup>n</sup>* salts for one batch and the total amount of M(TFSA)*n* salts were >300 g and 3.57 kg, respectively. The amount of metallic components in the M(TFSA)*n* salts was measured by ICP-AES analysis.

#### **2.4. Electrochemical analysis**

**2. Experimental**

**2.1. Pretreatment process**

216 Progress and Developments in Ionic Liquids

**2.2. Bench-scale leaching**

formation of [Fe(OH)*x*]

the Brunauer-Emmett-Teller (BET) method were 0.630 m2

available for the complexation of [Fe(phen)3]

**2.3. Bench-scale deironization and preparation of RE salts**

3−*x*

The spent Nd-Fe-B magnets were recovered from voice coil motors (VCMs) that were heated in an electric furnace at 623 K for 3 h for the demagnetization treatment. After demagnetization, the magnetic flux density was measured using a digital TESLA meter. The residual magnetic force field of this sample was almost zero, and the percentage of demagnetization was >99.9%. Then, the Ni-Cu-Ni triple layer on the Nd-Fe-B sample was removed by a grinding machine. After the stripping of the layer, fragments of Nd-Fe-B sample were pulverized using a stamp mill. The fine powders obtained were sieved to less than 150 μm and heated at 90 K h−1 to 1133 K, which was kept for 3 h in an electric furnace in order to oxidize the Nd-Fe-B components. After the roasting process, these fine powders were reground again by the automatic grinder. The surface area and the particle size (*D*50) of oxidized Nd-Fe-B sample measured by

The fine powders of oxidized Nd-Fe-B sample (1.7 kg) were leached in 14.2 L of a 1.0 M aqueous solution of 1,1,1-trifluoro-*N*-[(trifluoromethyl)sulfonyl]methanesulfonamide (HN(SO2CF3)2, H[TFSA]). The leaching solution was heated at 323 K, stirring at 500 rpm. The leaching behavior was researched from the potential (*E*)-pH diagram for Fe-H2O and Nd-H2O systems. Therefore, the oxidation-reduction potential (ORP) and pH in the leaching solution were measured by a high-precision digital meter (MM-60R, DKK-TOA Corp.). The quantification of Fe2+ was

of Fe2+ was measured by an ultraviolet-visible-near-infrared (UV-Vis-NIR) spectrometer (Perkin Elmer, Lambda750) at 508 nm [36, 37]. The concentration of Fe3+ was calculated from the total amount of Fe2+ and Fe3+ ions obtained from inductively coupled plasma atomic emission spectroscopy (ICP-AES) analysis (ICPE-9000, Shimadzu Co.). All of the precise concentrations for metallic species were quantitatively determined from ICP-AES analysis.

Dried oxygen gas was introduced into the leaching solution with a flow rate of 5.0 L min−1 after the leaching process. The oxidizing agent for Fe2+ was effective in the leaching solution at pH > 3.2. Some kinds of alkali metal hydroxides [8, 9] were acted as precipitation agents in the previous study, and the perfect removal of the iron components was successfully carried out at laboratory scale. The deironization treatment without precipitation agents is important as a further development, because the additive materials contaminated the final RE salts. Considering the leaching process, reuse of the oxidized Nd-Fe-B powder as a precipitation agent is desirable, because RE2O3 in the oxidized Nd-Fe-B sample was selectively leached in the H[TFSA] solution. The oxidized Nd-Fe-B fine powder was also available for sediment

powder was introduced carefully into leaching solution while the alternation in pH was

precipitates in this study. A small amount of the oxide Nd-Fe-B

g−1 and 59.43 μm, respectively.

2+ using 1,10-phenanthroline. The concentration

The resistance of a quartz oscillator and resonance frequency were observed using an EQCM system, (Seiko EG&G, QCA922) applying AT-cut platinum-coated [9 MHz, *ϕ* = 5.0 mm, Seiko EG&G, QA-A9M-PT(P)] with a well-type cell (Seiko EG&G, QA-CL4PK) as shown in **Figure 1**. The employed O-rings (Seiko EG&G, P-S75B) had a high resistance for heat and low expansibility. The temperature of the EQCM system was elevated using a heating mantle controlled by a thermostat with a proportional-integral-derivative (PID) controller. The temperature was slowly increased at a rate of 1.0–1.5 K min−1 to prevent the strain occurring in the crystal structure of the quartz. The bath temperature was measured using a K-type thermocouple (*ϕ* = 1.6 mm). The cell covered with the heating mantle was connected to the EQCM system with an extension cord (Seiko EG&G, QCA922-10-EX10). In terms of the functionality of EQCM technique at elevated temperatures, the relationship between the viscosity and the density of Nd(III) samples, *ηρ* values, the shifts of the resonance frequency, and the resistance before and after contacting the samples with the quartz have been already revealed in the previous study [35].

The voltammetric measurements were carried out using an electrochemical analyzer (ALS-440A, BAS Inc.,) with the EQCM system employing the Pt-coated quartz oscillator as a working electrode. Two Pt wires with 0.5 mm inside diameter were used as a counter and a quasi-reference electrode (QRE). The counter electrode was surrounded by a Vycor glass filter at the bottom in order to prevent the diffusion of decomposition components from the anode into the electrolyte. The Pt QRE showed a high stability and a good reproducibility of the potential at elevated temperatures. The potential was compensated for the IL standard using a ferrocene (Fc)/ferrocenium (Fc+ ) redox couple. Before all the electrochemical measurements, the dissolved oxygen was removed from the electrolytes by bubbling Ar gas for 30 min, and

when the bath was not stirred. The overpotential was constant at −3.20 V versus Fc/Fc+

PHI, Inc) and X-ray diffraction (XRD) (RINT-2500, Rigaku Co.), respectively.

during potentiostatic electrodeposition in a glovebox. After electrodeposition, the electrodeposits were leached into a super-dehydrated acetone (>99.5%, Wako Pure Chemical Industries, Ltd., water content <10 ppm) in a glovebox to remove the electrolyte thoroughly. The surface morphology of the electrodeposits was observed by scanning electron microscopy (SEM) and the composition of the electrodeposits was analyzed by energy dispersive X-ray analysis (EDX) (JSM-6510LA, JED-2300, JEOL, Ltd.). The metallic state and the crystallinity of the electrodeposits were evaluated by X-ray photoelectron spectroscopy (XPS) (Quantera SXM, ULVAC-

Purification of Rare Earth Amide Salts by Hydrometallurgy and Electrodeposition of Rare Earth Metals...

**Figure 2.** The schematic illustration of the electrodeposition cell with three-electrode system, (a) Fe rod (anode), (b) Teflon cap, (c) heat insulator, (d) soda-lime glass tube, (e) [P2225][TFSA], (f) Vycor glass filter, (g) K-type thermocouple, (h) Pt wire (Q.R.E.), (i) Cu substrate (cathode), (j) heating mantle, (k) stirrer, and (l) [P2225][TFSA] including M(TFSA)*n* or

The leaching reactions of the oxidized Nd-Fe-B sample were represented as follows:

[ ] ( ) <sup>3</sup> RE O 6H TFSA 2RE 6TFSA 3H O RE Pr, Nd, Dy 2 3 <sup>2</sup>

<sup>+</sup> - + ®+ + = (1)

Nd(TFSA)3.

**3. Results and discussion**

**3.1. Leaching behavior at bench scale**

at 373 K

219

http://dx.doi.org/10.5772/66300

**Figure 1.** The schematic illustration of EQCM cell with separate system: (a) Ar flow, (b) K-type thermocouple– 1 (for recording by a data logger), (c) Pt wire (counter electrode), (d) heating mantle, (e) separated electrolyte, (f) Vycor glass, (g) quartz oscillator coated with Pt (working electrode), (h) K-type thermocouple– 2 (for controlling temperature by PID), (i) Pt wire (quasi-reference electrode), (j) main electrolyte, (k) heat insulator.

the measurements were conducted under flowing Ar gas in the cell with a rate of 20 ml min−1. Cyclic voltammetry (CV) of 0.01 M Fc in [P2225][TFSA] was carried out at 298 K with a sweep rate of 1.0 mV s−1 after *iR* compensation (RC constant: 977.3) in order to confirm EQCM behavior while there was an outer-sphere electron-transfer reaction of Fc/Fc+ . The electrochemical behavior of Nd(III) in [P2225][TFSA] was investigated by CV measurements of 0.05 M Nd(III) in [P2225][TFSA] at 373 K with a sweep rate of 2.0 mV s−1. For the analysis of the electrodeposition behavior of Nd(0) from Nd(III) in [P2225][TFSA], the controlled potential electrolysis (CPE) under −3.20 V vs. Fc/Fc+ was performed at 373 K using 0.05 and 0.10 M Nd(III) in [P2225][TFSA] as electrolytes.

#### **2.5. Electrodeposition**

A schematic illustration of the electrodeposition cell is shown in **Figure 2**. In the electrodeposition with the three-electrode system, a copper substrate with a surface area of 2.0 × 10−2 m2 and a platinum were employed as the working electrode and quasi-reference electrode, respectively. Fe rod was employed as a counter electrode and was surrounded with a glass tube via a Vycor glass filter at the bottom to prevent the diffusion of dissolved [Fe(TFSA)3] − complexes from the anode into the electrolyte. The electrolytic bath was stirred at 500 rpm, because the current during electrodeposition decreased immediately to the limiting current when the bath was not stirred. The overpotential was constant at −3.20 V versus Fc/Fc+ at 373 K during potentiostatic electrodeposition in a glovebox. After electrodeposition, the electrodeposits were leached into a super-dehydrated acetone (>99.5%, Wako Pure Chemical Industries, Ltd., water content <10 ppm) in a glovebox to remove the electrolyte thoroughly. The surface morphology of the electrodeposits was observed by scanning electron microscopy (SEM) and the composition of the electrodeposits was analyzed by energy dispersive X-ray analysis (EDX) (JSM-6510LA, JED-2300, JEOL, Ltd.). The metallic state and the crystallinity of the electrodeposits were evaluated by X-ray photoelectron spectroscopy (XPS) (Quantera SXM, ULVAC-PHI, Inc) and X-ray diffraction (XRD) (RINT-2500, Rigaku Co.), respectively.

**Figure 2.** The schematic illustration of the electrodeposition cell with three-electrode system, (a) Fe rod (anode), (b) Teflon cap, (c) heat insulator, (d) soda-lime glass tube, (e) [P2225][TFSA], (f) Vycor glass filter, (g) K-type thermocouple, (h) Pt wire (Q.R.E.), (i) Cu substrate (cathode), (j) heating mantle, (k) stirrer, and (l) [P2225][TFSA] including M(TFSA)*n* or Nd(TFSA)3.

### **3. Results and discussion**

the measurements were conducted under flowing Ar gas in the cell with a rate of 20 ml min−1. Cyclic voltammetry (CV) of 0.01 M Fc in [P2225][TFSA] was carried out at 298 K with a sweep rate of 1.0 mV s−1 after *iR* compensation (RC constant: 977.3) in order to confirm EQCM behavior

recording by a data logger), (c) Pt wire (counter electrode), (d) heating mantle, (e) separated electrolyte, (f) Vycor glass,

**Figure 1.** The schematic illustration of EQCM cell with separate system: (a) Ar flow, (b) K-type thermocouple–

behavior of Nd(III) in [P2225][TFSA] was investigated by CV measurements of 0.05 M Nd(III) in [P2225][TFSA] at 373 K with a sweep rate of 2.0 mV s−1. For the analysis of the electrodeposition behavior of Nd(0) from Nd(III) in [P2225][TFSA], the controlled potential electrolysis (CPE) under −3.20 V vs. Fc/Fc+ was performed at 373 K using 0.05 and 0.10 M Nd(III) in [P2225][TFSA]

A schematic illustration of the electrodeposition cell is shown in **Figure 2**. In the electrodeposition with the three-electrode system, a copper substrate with a surface area of 2.0 × 10−2 m2 and a platinum were employed as the working electrode and quasi-reference electrode, respectively. Fe rod was employed as a counter electrode and was surrounded with a glass tube via a Vycor glass filter at the bottom to prevent the diffusion of dissolved [Fe(TFSA)3]

complexes from the anode into the electrolyte. The electrolytic bath was stirred at 500 rpm, because the current during electrodeposition decreased immediately to the limiting current

. The electrochemical

2 (for controlling temperature by

1 (for

−

while there was an outer-sphere electron-transfer reaction of Fc/Fc+

(g) quartz oscillator coated with Pt (working electrode), (h) K-type thermocouple–

PID), (i) Pt wire (quasi-reference electrode), (j) main electrolyte, (k) heat insulator.

as electrolytes.

**2.5. Electrodeposition**

218 Progress and Developments in Ionic Liquids

#### **3.1. Leaching behavior at bench scale**

The leaching reactions of the oxidized Nd-Fe-B sample were represented as follows:

$$\text{R}\text{R}\text{E}\_2\text{O}\_3 + 6\text{H}\text{[TFSA]} \rightarrow 2\text{RE}^{3+} + 6\text{TFSA}^- + 3\text{H}\_2\text{O} \text{ (RE}=\text{Pr, Nd, Dy)}\tag{1}$$

$$\rm Fe\_3O\_4 + \rm 8H [TFSA] \rightarrow Fe^{2+} + \rm 2Fe^{3+} + \rm 8TFSA^- + \rm 4H\_2O \tag{2}$$

**3.2. Deironization and purification of RE amide salts at bench scale**

**Blower flow rate/m3**

**Table 2.** The refining condition of leaching solution by spray dryer.

 **min−1**

 2.5 0.35–0.50 150 35.10 82.4 2.5 0.35–0.50 300 73.25 84.7 2.5 0.35–0.50 450 110.81 88.1 2.7 0.35–0.50 600 154.13 89.8 2.7 0.15–0.35 750 201.97 91.8 2.7 0.15–0.35 1050 273.85 91.2 2.9 0.15–0.35 1200 314.59 91.6 2.9 0.15–0.35 1200 320.62 93.7 2.9 0.15–0.35 1200 321.73 93.5 2.9 0.15–0.35 1200 325.64 94.4 2.9 0.15–0.35 1200 329.89 95.3 2.9 0.15–0.35 1200 328.74 94.6 2.9 0.15–0.35 1200 387.83 95.8 2.9 0.15–0.35 1200 393.32 96.8

from Fe2+ to Fe3+ by oxygen bubbling, because [Fe(OH)*x*]

expressed as follows:

3−*x*

**Run no. Flow rate /mL min−1**

[Fe(OH)*x*]

The effective treatment for deironization is precipitation separation through the oxidation

pH conditions [8, 9]. The precipitation reaction using the oxidized Nd-Fe-B sample was

( ) ( ) <sup>3</sup> <sup>3</sup> <sup>3</sup> 23 2 2Fe RE O 3H O 2 Fe OH 2RE RE Pr, Nd, Dy - <sup>+</sup> <sup>+</sup> é ù <sup>ë</sup> <sup>+</sup> <sup>û</sup> + ® += *<sup>x</sup>*

H[TFSA] solution. Therefore, we applied a seed crystal method, and hematite (*α*-Fe2O3) in oxidized Nd-Fe-B sample was employed as a seed crystal for the formation of [Fe(OH)*x*]

precipitates. The precipitate with a large particle size greatly facilitated the filtration in the solid-liquid separation treatment. From the ICP-AES analysis, the separation percentage of Fe component was confirmed to be >99.9%. This result allowed us to conclude that the oxidized Nd-Fe-B sample was applicable for perfect removal of iron component as a seed crystal.

After the solid-liquid separation, the evaporation treatment of filtrate was performed using an improved spray dryer to effectively recover dried M(TFSA)*n* (M = Pr, Nd, Dy, B, Al, K, and trace elements) salts. The refining conditions of the H[TFSA] solution for the oxidized Nd-Fe-B sample are listed in **Table 2**. The total amount of recovered M(TFSA)*<sup>n</sup>* and the average

> **Volume of H[TFSA] aq./mL**

3−*x*

Purification of Rare Earth Amide Salts by Hydrometallurgy and Electrodeposition of Rare Earth Metals...

is generally a colloidal precipitate, and it is very difficult to separate from the

precipitates are formed under acidic

http://dx.doi.org/10.5772/66300

3−*x*

221

**Recovery Yield/%**

Total: 3571.5 g Ave.: 91.7%

*<sup>x</sup>* (3)

**Amount of M(TFSA)***n***/g**

The leaching behavior in H[TFSA] solution using the oxidized Nd-Fe-B fine powder as a precipitation agent is shown in **Figure 3**. The leaching percentage of Nd and Fe for 66h in the H[TFSA] solution were 85.7±5.8% and 5.8±0.1%, respectively. A drastic increase in the pH value was observed at the initial stage of the leaching process, which indicates that leaching reaction Eq. (1) mainly occurred in this system. The leaching behavior accounted for the potential (*E*) pH diagrams of Fe-H2O and Nd-H2O systems as shown in **Figure 4**. The actual measurement data in the bench scale is also plotted in the potential (*E*)-pH diagram. From the *E*-pH diagram, at *E* = ~0.75 and pH < 1.0, the most stable states of Fe and Nd components were found to be solid Fe2O3 and Nd3+ ion, respectively. This result indicated that the selective leaching of Nd3+ (leaching percentage > 90%) was carried out at bench scale.

**Figure 3.** The leaching behavior of each metallic component in H[TFSA] solution at bench scale.

**Figure 4.** Potential(*E*)-pH diagram of Fe-H2O and Nd-H2O systems. Leaching time ○:3 h, ●:15 h, □:18 h, ■:21 h.

#### **3.2. Deironization and purification of RE amide salts at bench scale**

[ ] <sup>2</sup> <sup>3</sup> Fe O 8H TFSA Fe 2Fe 8TFSA 4H O 3 4 <sup>2</sup>

(leaching percentage > 90%) was carried out at bench scale.

220 Progress and Developments in Ionic Liquids

**Figure 3.** The leaching behavior of each metallic component in H[TFSA] solution at bench scale.

**Figure 4.** Potential(*E*)-pH diagram of Fe-H2O and Nd-H2O systems. Leaching time ○:3 h, ●:15 h, □:18 h, ■:21 h.

The leaching behavior in H[TFSA] solution using the oxidized Nd-Fe-B fine powder as a precipitation agent is shown in **Figure 3**. The leaching percentage of Nd and Fe for 66h in the H[TFSA] solution were 85.7±5.8% and 5.8±0.1%, respectively. A drastic increase in the pH value was observed at the initial stage of the leaching process, which indicates that leaching reaction Eq. (1) mainly occurred in this system. The leaching behavior accounted for the potential (*E*) pH diagrams of Fe-H2O and Nd-H2O systems as shown in **Figure 4**. The actual measurement data in the bench scale is also plotted in the potential (*E*)-pH diagram. From the *E*-pH diagram, at *E* = ~0.75 and pH < 1.0, the most stable states of Fe and Nd components were found to be solid Fe2O3 and Nd3+ ion, respectively. This result indicated that the selective leaching of Nd3+

<sup>+</sup> <sup>+</sup> - + ®+ + + (2)

The effective treatment for deironization is precipitation separation through the oxidation from Fe2+ to Fe3+ by oxygen bubbling, because [Fe(OH)*x*] 3−*x* precipitates are formed under acidic pH conditions [8, 9]. The precipitation reaction using the oxidized Nd-Fe-B sample was expressed as follows:

$$2\text{Fe}^{3+} + \text{RE}\_2\text{O}\_3 + 3\text{H}\_2\text{O} \rightarrow 2\left[\text{Fe}\left(\text{OH}\right)\_x\right]^{3-x} + 2\text{RE}^{3+}\left(\text{RE} = \text{Pr, Nd, Dy}\right) \tag{3}$$

[Fe(OH)*x*] 3−*x* is generally a colloidal precipitate, and it is very difficult to separate from the H[TFSA] solution. Therefore, we applied a seed crystal method, and hematite (*α*-Fe2O3) in oxidized Nd-Fe-B sample was employed as a seed crystal for the formation of [Fe(OH)*x*] 3−*x* precipitates. The precipitate with a large particle size greatly facilitated the filtration in the solid-liquid separation treatment. From the ICP-AES analysis, the separation percentage of Fe component was confirmed to be >99.9%. This result allowed us to conclude that the oxidized Nd-Fe-B sample was applicable for perfect removal of iron component as a seed crystal.

After the solid-liquid separation, the evaporation treatment of filtrate was performed using an improved spray dryer to effectively recover dried M(TFSA)*n* (M = Pr, Nd, Dy, B, Al, K, and trace elements) salts. The refining conditions of the H[TFSA] solution for the oxidized Nd-Fe-B sample are listed in **Table 2**. The total amount of recovered M(TFSA)*<sup>n</sup>* and the average


**Table 2.** The refining condition of leaching solution by spray dryer.

recovery yield were 3571.5 g and 91.7%, respectively. The obtained M(TFSA)*<sup>n</sup>* salt was a fine pale purple powder, and the water content in the M(TFSA)*n* salt was less than 10 ppm. The composition of the recovered M(TFSA)*n* salts is tabulated in **Table 3**. The stoichiometric number of M(TFSA)*n* and average molecular weight were 3.00 and 943.68, respectively. In addition, the average RE component in M(TFSA)*<sup>n</sup>* was 96.81%. This series of results allowed us to conclude that the separation factor of Fe in the deironization treatment was >99.9% and the recovery yield of M(TFSA)*n* salts was 82.4–96.8% using a spray dryer at bench scale.

( )

where *f*0 is the fundamental resonance frequency, *A* is the surface area of the electrode

of the product of the liquid viscosity and density, (*ηρ*)1/2 demonstrated by Kanazawa-Gordon

q q

1/2

hr

pm r

Although a frequency shift by the aqueous solution contacting the quartz is small, in the case of employing ILs as the electrolyte, it is necessary to consider the influence of Δ*fηρ* due to the exceedingly high viscosity of ILs. In the EQCM measurements, the resonance resistance (*R*)

( )

*k* p hr

0 2 *A f* 2

where *k* is an electromechanical coupling factor and often used when the electrical model of the quartz crystal oscillator is converted to a mechanical model. The *k* value was estimated from the shifts in the resonance frequency and the resistances before and after the liquid sample came into contact with the quartz according to Eqs. (6) and (7), respectively, for each measurement in this study. Δ*fm* is isolated from the total shift of frequency (Δ*f*) by using Eqs. (4), (6), and (7). In addition, the apparent molar mass, *M*app, of the electrodeposited species can be calculated by using Δ*m* estimated from Δ*fm*, the electrical charge *Q* passed during electrode-

The theoretical value of *M*app based on the reduction reaction of Nd(III) + 3e− → Nd(0) was 48.1 g mol−1. The theoretical equations of the EQCM measurements related to the frequency response are based on the analysis of an admittance spectrum of the quartz crystal near its resonance frequency [39]. The responses of the resonance frequency and the resistance to (*ηρ*)1/2 of the adjacent liquid to the quartz crystal are derived from the solutions of the equation, describing the steady-state shear waves in the AT-cut quartz oscillator under the condition that the transverse velocity of the surface of the quartz oscillator is identical with that of the adjacent liquid and that the force exerted by the liquid on the quartz is equal and opposite to the force

æ ö = - ç ÷ ç ÷ è ø

3 2 0

Δ*f f* hr

can simultaneously be measured, and (*ηρ*)1/2 is also estimated from *R* value [38].

*R*

position, and the Faraday constant *F*.

), *μq* is the shear modulus of quartz (2.95 × 1010 kg m−1 s−2 at 298 K), and *ρq* is the density

Purification of Rare Earth Amide Salts by Hydrometallurgy and Electrodeposition of Rare Earth Metals...

kg m−3 at 298 K). On the other hand, Δ*fηρ* is proportional to the square root

2 0 1/2 q q

<sup>D</sup> <sup>=</sup> (5)

http://dx.doi.org/10.5772/66300

= (7)

app = (8)

(6)

223

<sup>2</sup> <sup>Δ</sup> *<sup>m</sup> f m <sup>f</sup> A* m r

(0.196 cm2

[33].

of quartz (2.65 × 103


\* RE indicates the summation of the composition for Pr, Nd, and Dy. \*\**n* indicates the stoichiometric number of M(TFSA)*n* salts.

**Table 3.** The composition (wt.%) of recovered M(TFSA)*n* salts on the condition of **Table 1**.

#### **3.3. Theory**

A frequency shift (Δ*f*) observed on the EQCM analyzer includes effects relating to the mass change (Δ*m*) in the quartz crystal electrode (Δ*fm*) and the viscosity (*η*) and the density (*ρ*) of the liquid adjacent to the quartz (Δ*fηρ*).

$$
\Delta f = \Delta f\_m + \Delta f\_{\eta \rho} \tag{4}
$$

The relationship between Δ*fm* and Δ*m* is expressed by the Sauerbrey equation [24].

Purification of Rare Earth Amide Salts by Hydrometallurgy and Electrodeposition of Rare Earth Metals... http://dx.doi.org/10.5772/66300 223

$$
\Delta f\_w = \frac{2f\_0^2 \Delta m}{A \left(\mu\_q \rho\_q\right)^{1/2}} \tag{5}
$$

where *f*0 is the fundamental resonance frequency, *A* is the surface area of the electrode (0.196 cm2 ), *μq* is the shear modulus of quartz (2.95 × 1010 kg m−1 s−2 at 298 K), and *ρq* is the density of quartz (2.65 × 103 kg m−3 at 298 K). On the other hand, Δ*fηρ* is proportional to the square root of the product of the liquid viscosity and density, (*ηρ*)1/2 demonstrated by Kanazawa-Gordon [33].

recovery yield were 3571.5 g and 91.7%, respectively. The obtained M(TFSA)*<sup>n</sup>* salt was a fine pale purple powder, and the water content in the M(TFSA)*n* salt was less than 10 ppm. The composition of the recovered M(TFSA)*n* salts is tabulated in **Table 3**. The stoichiometric number of M(TFSA)*n* and average molecular weight were 3.00 and 943.68, respectively. In addition, the average RE component in M(TFSA)*<sup>n</sup>* was 96.81%. This series of results allowed us to conclude that the separation factor of Fe in the deironization treatment was >99.9% and the recovery

 20.81 72.87 2.48 96.16 0.00 3.84 938.15 3.00 20.76 73.55 2.47 96.78 0.00 3.22 943.46 3.00 19.80 74.96 2.06 96.82 0.00 3.18 943.77 3.00 19.39 74.78 2.46 96.63 0.00 3.37 942.11 3.00 19.83 74.58 2.46 96.87 0.00 3.13 944.26 3.00 19.80 74.77 2.40 96.96 0.00 3.04 944.26 3.00 19.53 74.85 2.53 96.91 0.00 3.09 944.71 3.00 19.86 74.87 2.05 96.78 0.00 3.22 943.42 3.00 20.01 74.80 2.00 96.82 0.00 3.18 943.78 3.00 20.07 74.84 1.99 96.90 0.00 3.10 944.52 3.00 21.05 73.93 1.97 96.95 0.00 3.05 944.98 3.00 20.10 74.77 2.13 96.99 0.00 3.01 945.40 3.00 19.75 74.99 2.16 96.89 0.00 3.11 944.45 3.00 20.01 74.73 2.12 96.87 0.00 3.13 944.23 3.00 Ave. 20.06 74.52 2.23 96.81 0.00 3.19 943.68 3.00

 **Fe B Molecular weight**

**of M(TFSA)***<sup>n</sup>*

*n* **\*\***

yield of M(TFSA)*n* salts was 82.4–96.8% using a spray dryer at bench scale.

**Run no. Pr Nd Dy RE\***

222 Progress and Developments in Ionic Liquids

RE indicates the summation of the composition for Pr, Nd, and Dy. \*\**n* indicates the stoichiometric number of M(TFSA)*n* salts.

**Table 3.** The composition (wt.%) of recovered M(TFSA)*n* salts on the condition of **Table 1**.

A frequency shift (Δ*f*) observed on the EQCM analyzer includes effects relating to the mass change (Δ*m*) in the quartz crystal electrode (Δ*fm*) and the viscosity (*η*) and the density (*ρ*) of the

hr

(4)

*<sup>m</sup> ff f* D =D +D

The relationship between Δ*fm* and Δ*m* is expressed by the Sauerbrey equation [24].

\*

**3.3. Theory**

liquid adjacent to the quartz (Δ*fηρ*).

$$
\Delta f\_{\eta \rho} = -f\_0^{\frac{3}{2}} \left( \frac{\eta \rho}{\pi \mu\_q \rho\_q} \right) \tag{6}
$$

Although a frequency shift by the aqueous solution contacting the quartz is small, in the case of employing ILs as the electrolyte, it is necessary to consider the influence of Δ*fηρ* due to the exceedingly high viscosity of ILs. In the EQCM measurements, the resonance resistance (*R*) can simultaneously be measured, and (*ηρ*)1/2 is also estimated from *R* value [38].

$$R = \frac{A\left(2\pi f\_0 \eta \rho\right)^{\mathbb{I}^2}}{k^2} \tag{7}$$

where *k* is an electromechanical coupling factor and often used when the electrical model of the quartz crystal oscillator is converted to a mechanical model. The *k* value was estimated from the shifts in the resonance frequency and the resistances before and after the liquid sample came into contact with the quartz according to Eqs. (6) and (7), respectively, for each measurement in this study. Δ*fm* is isolated from the total shift of frequency (Δ*f*) by using Eqs. (4), (6), and (7). In addition, the apparent molar mass, *M*app, of the electrodeposited species can be calculated by using Δ*m* estimated from Δ*fm*, the electrical charge *Q* passed during electrodeposition, and the Faraday constant *F*.

$$\mathcal{M}\_{\text{app}} = \frac{F \Delta m}{\Delta Q} \tag{8}$$

The theoretical value of *M*app based on the reduction reaction of Nd(III) + 3e− → Nd(0) was 48.1 g mol−1. The theoretical equations of the EQCM measurements related to the frequency response are based on the analysis of an admittance spectrum of the quartz crystal near its resonance frequency [39]. The responses of the resonance frequency and the resistance to (*ηρ*)1/2 of the adjacent liquid to the quartz crystal are derived from the solutions of the equation, describing the steady-state shear waves in the AT-cut quartz oscillator under the condition that the transverse velocity of the surface of the quartz oscillator is identical with that of the adjacent liquid and that the force exerted by the liquid on the quartz is equal and opposite to the force exerted by the quartz on the liquid [33, 38]. Strictly speaking, the estimation of Δ*m* from Δ*fm* by Eq. (4) is valid for thin and rigid films coated on the quartz. Moreover, Δ*fηρ* and *R* reflect not only the viscosity and the density of contacting liquid with the quartz but also the roughness, the viscoelastic properties, and the films [31, 32]. By considering these parameters in combination, it is possible to discuss detailed states on the surface of the quartz electrode accompanied by the electrochemical behaviors.

#### **3.4. Electrochemical analysis**

For the investigation of the reduction behavior for Nd(III) in [P2225][TFSA], cyclic voltammetry with EQCM measurements (CV/EQCM) were carried out at elevated temperatures. The EQCM behavior was confirmed in advance by CV/EQCM at 298 K measuring Fc/Fc+ redox couple in [P2225][TFSA]. The potential difference, Δ*E*p, between the anode and cathode peak was 67 mV after *iR* compensation. The Δ*E*p value was close to the theoretical value; 59 mV in a reversible and one-electron reaction at 298 K and thus the observation of the electrochemical behavior was confirmed with high precision. Moreover, there were no significant alternations for the mass and *ηρ* values in this reversible reaction. The results indicated that no changes of Δ*m* and Δ*ηρ* were detected during the outer-sphere electron-transfer reaction and consistent with the Ref. [26].

The CV/EQCM results for Nd(III) in [P2225][TFSA] at 373 K were shown in **Figure 5**. A clear cathodic peak with the mass increased and the *ηρ* decreased was observed at −2.79 V. Considering our previous investigations [15–18], the reduction of Nd(III)/Nd(0) was indicated as follows:

$$\left[\text{Nd}^{\text{(III)}}(\text{TFSA})\_{\circ}\right]^{2} + \text{3c}^{\bullet} \rightarrow \text{Nd}(0) + \text{S[TFSA]}^{\bullet} \tag{9}$$

**Figure 5.** CV/EQCM analysis for 0.05 M Nd(III) in [P2225][TFSA] at 373 K with 2.0 mV s−1; (a) voltammogram, (b) the

Purification of Rare Earth Amide Salts by Hydrometallurgy and Electrodeposition of Rare Earth Metals...

http://dx.doi.org/10.5772/66300

225

The controlled potential electrolysis with EQCM measurements (CPE/EQCM) at −3.20 V was conducted with 0.05 and 0.10 M Nd(III) in [P2225][TFSA] at 373 K. The parameters of Δ*m* and Δ*ηρ* were plotted as a function of the charge density *Qd* passed during CPE as shown in **Figure 6**. The calculated values of *M*app in 0.05 and 0.10 M Nd(III) at the initial stage of CPE/ EQCM analysis were 49.1 and 49.6 g mol−1, respectively, which were close to the theoretical value when the reduction of Nd(III)/Nd(0) occurred, 48.1 g mol−1, so that the electrodeposition of Nd(0) metal was also proved from the CPE/EQCM analysis. The total values of *M*app in 0.10 M solution, 18.4 ~ 29.8 g mol−1, were larger than those in 0.05 M solution, 7.7 ~ 18.6 g mol−1. These results indicated that the competition reaction for Nd(III)/Nd(0) reduction and IL decomposition would occur and depend on the Nd(III) concentration. The value of *ηρ* after 0.45 C cm−2 increased and the IL decomposition was also deduced from this result because the *ηρ* change implied that the quantity of the soluble species increase near the electrode and/or the viscoelastic film might be formed on the electrode surface [27, 31, 32, 40] by the IL decomposition.

mass change, and (c) the change in *ηρ* values.

The *M*app value in the range of −2.49 V ~ −2.94 V calculated from the mass change was 46.8 g mol−1, which was close to the theoretical value for the electrodeposition reaction of Nd(III)/Nd(0), 48.1 g mol−1. Moreover, the observed decrease of the *ηρ* value indicated that the concentration of Nd(III) near the electrode was locally decreased by consuming Nd(III) in the electrodeposition reduction of Nd(III)/Nd(0). This is based on the *ηρ* change in the IL system that largely depends on the concentration of the metal ion. Therefore, these results are an evidence for the electrodeposition reaction of Nd(III)/Nd(0). At a more negative potential than −2.79 V, the mass and *ηρ* increased with the comparatively low values of *M*app for 0.05 M Nd(III) in [P2225][TFSA]. This behavior indicated that the cathodic decomposition products from IL were generated on the electrode surface. A part of side chain of [P2225] + would be decomposed on the cathode considering the *M*app value estimated in this potential range (−2.96 V ~ −3.30 V). In addition, in the range of 0 ~ −2.0 V the mass unexpectedly increased during slight current and no change in the *ηρ* value for 0.05 M Nd(III) in [P2225] [TFSA]. There was no observation of mass increase in the EQCM analysis for neat [P2225] [TFSA] in this potential. Therefore, it was deduced that the reaction would be the formation of divalent species; Nd(II) [Nd(III) + e− → Nd(II)]. The related disproportionation reaction [2Nd(III) + Nd(0) → 3Nd(II)] would occur in the IL system.

Purification of Rare Earth Amide Salts by Hydrometallurgy and Electrodeposition of Rare Earth Metals... http://dx.doi.org/10.5772/66300 225

exerted by the quartz on the liquid [33, 38]. Strictly speaking, the estimation of Δ*m* from Δ*fm* by Eq. (4) is valid for thin and rigid films coated on the quartz. Moreover, Δ*fηρ* and *R* reflect not only the viscosity and the density of contacting liquid with the quartz but also the roughness, the viscoelastic properties, and the films [31, 32]. By considering these parameters in combination, it is possible to discuss detailed states on the surface of the quartz electrode accompa-

For the investigation of the reduction behavior for Nd(III) in [P2225][TFSA], cyclic voltammetry with EQCM measurements (CV/EQCM) were carried out at elevated temperatures. The EQCM

[P2225][TFSA]. The potential difference, Δ*E*p, between the anode and cathode peak was 67 mV after *iR* compensation. The Δ*E*p value was close to the theoretical value; 59 mV in a reversible and one-electron reaction at 298 K and thus the observation of the electrochemical behavior was confirmed with high precision. Moreover, there were no significant alternations for the mass and *ηρ* values in this reversible reaction. The results indicated that no changes of Δ*m* and Δ*ηρ* were detected during the outer-sphere electron-transfer reaction and consistent with the

The CV/EQCM results for Nd(III) in [P2225][TFSA] at 373 K were shown in **Figure 5**. A clear cathodic peak with the mass increased and the *ηρ* decreased was observed at −2.79 V. Considering our previous investigations [15–18], the reduction of Nd(III)/Nd(0) was indi-

The *M*app value in the range of −2.49 V ~ −2.94 V calculated from the mass change was 46.8 g mol−1, which was close to the theoretical value for the electrodeposition reaction of Nd(III)/Nd(0), 48.1 g mol−1. Moreover, the observed decrease of the *ηρ* value indicated that the concentration of Nd(III) near the electrode was locally decreased by consuming Nd(III) in the electrodeposition reduction of Nd(III)/Nd(0). This is based on the *ηρ* change in the IL system that largely depends on the concentration of the metal ion. Therefore, these results are an evidence for the electrodeposition reaction of Nd(III)/Nd(0). At a more negative potential than −2.79 V, the mass and *ηρ* increased with the comparatively low values of *M*app for 0.05 M Nd(III) in [P2225][TFSA]. This behavior indicated that the cathodic decomposition products from IL were generated on the electrode surface. A part of side chain of [P2225]

would be decomposed on the cathode considering the *M*app value estimated in this potential range (−2.96 V ~ −3.30 V). In addition, in the range of 0 ~ −2.0 V the mass unexpectedly increased during slight current and no change in the *ηρ* value for 0.05 M Nd(III) in [P2225] [TFSA]. There was no observation of mass increase in the EQCM analysis for neat [P2225] [TFSA] in this potential. Therefore, it was deduced that the reaction would be the formation of divalent species; Nd(II) [Nd(III) + e− → Nd(II)]. The related disproportionation reaction

[2Nd(III) + Nd(0) → 3Nd(II)] would occur in the IL system.

redox couple in

(9)

+

behavior was confirmed in advance by CV/EQCM at 298 K measuring Fc/Fc+

nied by the electrochemical behaviors.

**3.4. Electrochemical analysis**

224 Progress and Developments in Ionic Liquids

Ref. [26].

cated as follows:

**Figure 5.** CV/EQCM analysis for 0.05 M Nd(III) in [P2225][TFSA] at 373 K with 2.0 mV s−1; (a) voltammogram, (b) the mass change, and (c) the change in *ηρ* values.

The controlled potential electrolysis with EQCM measurements (CPE/EQCM) at −3.20 V was conducted with 0.05 and 0.10 M Nd(III) in [P2225][TFSA] at 373 K. The parameters of Δ*m* and Δ*ηρ* were plotted as a function of the charge density *Qd* passed during CPE as shown in **Figure 6**. The calculated values of *M*app in 0.05 and 0.10 M Nd(III) at the initial stage of CPE/ EQCM analysis were 49.1 and 49.6 g mol−1, respectively, which were close to the theoretical value when the reduction of Nd(III)/Nd(0) occurred, 48.1 g mol−1, so that the electrodeposition of Nd(0) metal was also proved from the CPE/EQCM analysis. The total values of *M*app in 0.10 M solution, 18.4 ~ 29.8 g mol−1, were larger than those in 0.05 M solution, 7.7 ~ 18.6 g mol−1. These results indicated that the competition reaction for Nd(III)/Nd(0) reduction and IL decomposition would occur and depend on the Nd(III) concentration. The value of *ηρ* after 0.45 C cm−2 increased and the IL decomposition was also deduced from this result because the *ηρ* change implied that the quantity of the soluble species increase near the electrode and/or the viscoelastic film might be formed on the electrode surface [27, 31, 32, 40] by the IL decomposition.

**3.5. Electrodeposition**

Considering the above fundamental electrochemical behavior of Nd(III), the electrodeposition of Nd(0) was carried out, and the condition was listed in **Table 4**. The electrodeposition was smoothly performed on the high anodic current efficiency. The current slowly decreased to the limiting current during electrodeposition. After the electrodeposition, the blackish electrodeposits were obtained on the Cu substrate. The electrodeposits observed by SEM had a granular morphology with a nonuniform size distribution. This morphology is considered to be explained from the fact that the initial stage of nucleation and growth occurred according to the progressive nucleation model [15]. The quantitative analysis using EDX for the electrode-

Purification of Rare Earth Amide Salts by Hydrometallurgy and Electrodeposition of Rare Earth Metals...

The results indicated that the electrodeposits on Cu substrate comprised mainly Nd component. However, a small amount of O component was also included in the electrodeposits, suggesting that the oxidizing Nd metal with O atoms would occur. In order to investigate the chemical bond state of Nd, XPS analysis with Al-Kα was carried out on the electrodeposits. The metallic and oxide components for Nd correspond to the binding energies of Nd3d5/2 at 980.5–981.0 eV and 981.7–982.3 eV, respectively, in the case of monochromated Al-Kα line [41]. The Nd3d5/2 spectra for the middle layer and top surface of electrodeposits are shown in **Figure 7**. For an in-depth analysis of the middle layer, the oxide layer of the electrodeposits was perfectly removed with an Ar ion beam, that is, (a) 0.42 μm under the electrodeposits. The peak maximum in the Nd3d5/2 spectrum acquired for the layers below 0.42 μm was at 980.77 eV. Hence, the electrodeposits obtained through electrodeposition using [P2225][TFSA] with M(TFSA)3 were identified as Nd metal and partial oxide mixtures. This result indicated that metallic Nd would have been electrodeposited on the Cu substrate and subsequently oxidized by O in the electrolyte, that is, residual water or dissolved oxygen. The XRD profile of the electrodeposits is shown in **Figure 8** with the profile from Ref. [42] for Nd metal. The position of 2*θ* of the electrodeposits was nearly identical to that of Nd metal. Therefore, the electrode-

is summarized in **Table 5**.

http://dx.doi.org/10.5772/66300

227

0.42 μm, (b) top surface.

posits obtained from the electrodeposition at −3.20 V versus Fc/Fc+

posits were identified to be crystalline Nd metal.

**Figure 7.** XPS spectra for Nd3d5/2 region of electrodeposits (a) middle layer at −

**Figure 6.** CPE/EQCM analysis for 0.05 and 0.10 M Nd(III) in [P2225][TFSA] induced on − 3.20 V at 373 K; (a) the mass change and (b) the change in *ηρ* values.


**Table 4.** The potentiostatic electrodeposition condition using [P2225][TFSA] including M(TFSA)3 at 373 K.


**Table 5.** The composition of Nd electrodeposits analyzed by EDX.

#### **3.5. Electrodeposition**

**Figure 6.** CPE/EQCM analysis for 0.05 and 0.10 M Nd(III) in [P2225][TFSA] induced on −

**Transported charge,** *Q***/C**

1 −3.20 6054.6 Anode:−1.694 Anode:96.7

2 −3.20 6634.8 Anode:−1.794 Anode:93.4

3 −3.20 7618.2 Anode:−2.094 Anode:95.0

**Table 4.** The potentiostatic electrodeposition condition using [P2225][TFSA] including M(TFSA)3 at 373 K.

1 5.52 0.84 12.38 0.46 0.31 0.96 0.02 15.82 63.69 2 3.62 0.43 9.62 0.32 0.18 0.68 0.00 13.64 71.51 3 4.82 0.68 10.23 0.36 0.23 0.82 0.00 12.46 70.40

**Weight change,**

**Δ***w***/g**

**C N O F P S Fe Cu Nd**

change and (b) the change in *ηρ* values.

226 Progress and Developments in Ionic Liquids

*η***/V vs. Fc/Fc+**

**Run no. Composition/wt.%**

**Table 5.** The composition of Nd electrodeposits analyzed by EDX.

**Run no. Overpotential,**

3.20 V at 373 K; (a) the mass

**Current efficiency,**

*ε***/%**

Cathode:+2.184 Cathode:72.4

Cathode:+2.541 Cathode:76.9

Cathode:+2.832 Cathode:74.6

Considering the above fundamental electrochemical behavior of Nd(III), the electrodeposition of Nd(0) was carried out, and the condition was listed in **Table 4**. The electrodeposition was smoothly performed on the high anodic current efficiency. The current slowly decreased to the limiting current during electrodeposition. After the electrodeposition, the blackish electrodeposits were obtained on the Cu substrate. The electrodeposits observed by SEM had a granular morphology with a nonuniform size distribution. This morphology is considered to be explained from the fact that the initial stage of nucleation and growth occurred according to the progressive nucleation model [15]. The quantitative analysis using EDX for the electrodeposits obtained from the electrodeposition at −3.20 V versus Fc/Fc+ is summarized in **Table 5**. The results indicated that the electrodeposits on Cu substrate comprised mainly Nd component. However, a small amount of O component was also included in the electrodeposits, suggesting that the oxidizing Nd metal with O atoms would occur. In order to investigate the chemical bond state of Nd, XPS analysis with Al-Kα was carried out on the electrodeposits. The metallic and oxide components for Nd correspond to the binding energies of Nd3d5/2 at 980.5–981.0 eV and 981.7–982.3 eV, respectively, in the case of monochromated Al-Kα line [41]. The Nd3d5/2 spectra for the middle layer and top surface of electrodeposits are shown in **Figure 7**. For an in-depth analysis of the middle layer, the oxide layer of the electrodeposits was perfectly removed with an Ar ion beam, that is, (a) 0.42 μm under the electrodeposits. The peak maximum in the Nd3d5/2 spectrum acquired for the layers below 0.42 μm was at 980.77 eV. Hence, the electrodeposits obtained through electrodeposition using [P2225][TFSA] with M(TFSA)3 were identified as Nd metal and partial oxide mixtures. This result indicated that metallic Nd would have been electrodeposited on the Cu substrate and subsequently oxidized by O in the electrolyte, that is, residual water or dissolved oxygen. The XRD profile of the electrodeposits is shown in **Figure 8** with the profile from Ref. [42] for Nd metal. The position of 2*θ* of the electrodeposits was nearly identical to that of Nd metal. Therefore, the electrodeposits were identified to be crystalline Nd metal.

**Figure 7.** XPS spectra for Nd3d5/2 region of electrodeposits (a) middle layer at − 0.42 μm, (b) top surface.

H[TFSA] is currently slightly expensive compared with mineral acids. However, H[TFSA] can be prepared through an ion-exchange reaction of residual components. Studies to further

Purification of Rare Earth Amide Salts by Hydrometallurgy and Electrodeposition of Rare Earth Metals...

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229

improve the recovery process are now in progress.

**Figure 9.** Material flow of hydrometallurgy and electrodeposition using [P2225][TFSA].

Hydrometallurgical process based on leaching, deironization, and purification of rare earth (RE) amide salts were carried out at bench scale. In the leaching process using 1.7 kg of oxidized Nd-Fe-B sample and 14.2 L of an aqueous solution of 1,1,1-trifluoro-*N*-[(trifluoromethyl)

**4. Conclusion**

**Figure 8.** XRD profile of the Nd electrodeposits obtained from [P2225][TFSA].

#### **3.6. Material flow of VCM recycling**

As described earlier, it is worthwhile to evaluate the material flow from a series of processes such as pretreatment, hydrometallurgy, and electrodeposition using [P2225][TFSA] melts. The whole material flow is shown in **Figure 9,** and the recovery target in this material flow was based on the oxidized Nd-Fe-B wastes after the roasting process. As the first step of hydrometallurgy, the selective leaching of RE components (85.7 ± 5.8% Nd and 5.8 ± 0.1% Fe in 66 h) was performed in the leaching process. Then, the deironization treatment was carried out using precipitation formation of [Fe(OH)*x*] 3−*x* , and the residual Fe component was perfectly removed in this process. After the deironization process, the M(TFSA)*n* salts with high purity were obtained from the evaporation by a spray dryer, and the yield of M(TFSA)*n* salts was as high as 91.7%. Scaling up for the vaporization treatment is relatively simple because a large-scale spray dryer can be available through cooperation with an associated company. A series of hydrometallurgy indicates that 78.6% (356.0 g/453.0 g × 100) Nd and 77.9% (10.9 g/14.0 g) Dy can be recovered as purified M(TFSA)*n* salts from the initial oxidized Nd-Fe-B powder. After the hydrometallurgical process, M(TFSA)*n* salts were available as an electrolytic bath in the electrodeposition process. In terms of material flow, the induced overpotential (*E* = −3.20 V vs. Fc/Fc+ ) and the cathodic current efficiency (*ε* = 74.6%) were determined from the actual electrodeposition results described above. Assuming that the total transported charge is *Q* = 7618.2 × 102 C (100 times at laboratory scale) under proper conditions based on the scalingup electrolytic bath, 283.2 g of Nd metal can be recovered during the electrodeposition process, and the recovery yield calculated from the starting material (453.0 g Nd) was estimated to be 62.5%. Therefore, the recovery process based on hydrometallurgy and electrodeposition using [P2225][TFSA] was applicable for practical Nd-Fe-B wastes. From the economic point of view, H[TFSA] is currently slightly expensive compared with mineral acids. However, H[TFSA] can be prepared through an ion-exchange reaction of residual components. Studies to further improve the recovery process are now in progress.

**Figure 9.** Material flow of hydrometallurgy and electrodeposition using [P2225][TFSA].

### **4. Conclusion**

**Figure 8.** XRD profile of the Nd electrodeposits obtained from [P2225][TFSA].

As described earlier, it is worthwhile to evaluate the material flow from a series of processes such as pretreatment, hydrometallurgy, and electrodeposition using [P2225][TFSA] melts. The whole material flow is shown in **Figure 9,** and the recovery target in this material flow was based on the oxidized Nd-Fe-B wastes after the roasting process. As the first step of hydrometallurgy, the selective leaching of RE components (85.7 ± 5.8% Nd and 5.8 ± 0.1% Fe in 66 h) was performed in the leaching process. Then, the deironization treatment was carried out using

in this process. After the deironization process, the M(TFSA)*n* salts with high purity were obtained from the evaporation by a spray dryer, and the yield of M(TFSA)*n* salts was as high as 91.7%. Scaling up for the vaporization treatment is relatively simple because a large-scale spray dryer can be available through cooperation with an associated company. A series of hydrometallurgy indicates that 78.6% (356.0 g/453.0 g × 100) Nd and 77.9% (10.9 g/14.0 g) Dy can be recovered as purified M(TFSA)*n* salts from the initial oxidized Nd-Fe-B powder. After the hydrometallurgical process, M(TFSA)*n* salts were available as an electrolytic bath in the electrodeposition process. In terms of material flow, the induced overpotential (*E* = −3.20 V vs.

) and the cathodic current efficiency (*ε* = 74.6%) were determined from the actual electrodeposition results described above. Assuming that the total transported charge is

up electrolytic bath, 283.2 g of Nd metal can be recovered during the electrodeposition process, and the recovery yield calculated from the starting material (453.0 g Nd) was estimated to be 62.5%. Therefore, the recovery process based on hydrometallurgy and electrodeposition using [P2225][TFSA] was applicable for practical Nd-Fe-B wastes. From the economic point of view,

C (100 times at laboratory scale) under proper conditions based on the scaling-

, and the residual Fe component was perfectly removed

3−*x*

**3.6. Material flow of VCM recycling**

228 Progress and Developments in Ionic Liquids

precipitation formation of [Fe(OH)*x*]

Fc/Fc+

*Q* = 7618.2 × 102

Hydrometallurgical process based on leaching, deironization, and purification of rare earth (RE) amide salts were carried out at bench scale. In the leaching process using 1.7 kg of oxidized Nd-Fe-B sample and 14.2 L of an aqueous solution of 1,1,1-trifluoro-*N*-[(trifluoromethyl) sulfonyl]methanesulfonamide (HN(SO2CF3)2, H[TFSA]), The leaching percentage of Nd and Fe for 66h in the H[TFSA] solution were 85.7±5.8% and 5.8±0.1%, respectively. Moreover, the oxidized Nd-Fe-B sample applied as a precipitation agent in the deironization process, and >99.9% Fe component was successfully separated from RE components. Finally, 3.57 kg of purified amide salts (M(TFSA)3, M = Pr, Nd, Dy, B, Al, and trace elements) were recovered through the evaporation process using an improved spray dryer and the percentage of RE components for amide salts was 96.81%.

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The cyclic voltammetry (CV) with electrochemical quartz crystal microbalance (EQCM) was applied at elevated temperatures in this study. CV/EQCM measurements for the investigation of the reduction behavior related to Nd(III) in triethyl-pentyl-phosphonium bis(trifluoromethyl-sulfonyl)amide ([P2225][TFSA]) were conducted at 373 K. At the potential of −2.79 V, the objective electrodeposition, Nd(III)/Nd(0) was confirmed because a clear cathodic peak was observed and the apparent molar mass, *M*app was calculated to be 46.8 g mol−1, and this value was consistent with the theoretical value for Nd(III)/Nd(0), 48.1 g mol−1. At more negative potential than −2.79 V, the mass and *ηρ* increased with the comparatively low values of *M*app for 0.05 M Nd(III) in [P2225][TFSA]. This behavior indicated that the cathodic decomposition reaction of IL occurred on the electrode surface. Moreover, from the controlled potential electrolysis with EQCM (CPE/EQCM) measurements at −3.20 V versus Fc/Fc+ in the solutions of 0.05 and 0.10 M Nd(III) in [P2225][TFSA] at 373 K, the electrodeposits of Nd(0) metal were confirmed at the initial stage considering the *M*app values (49.1 and 49.6 g mol−1). The electrodeposition of Nd was carried out under potentiostatic conditions of −3.20 V versus Fc/Fc+ at 373 K. The electrodeposits in the middle layer 0.15 μm below the surface were identified to be Nd metal from the analysis of SEM/EDX, XPS, and XRD. Finally, the material flow of whole process allowed us to conclude that the novel recovery process was effective for practical use.

### **Acknowledgements**

This study was partly supported by the Grant-in-Aid for Scientific Research (No. 15H02848) from the Ministry of Education, Culture, Sports, Science, and Technology, Japan.

### **Author details**

Masahiko Matsumiya

Address all correspondence to: mmatsumi@ynu.ac.jp

Graduate School of Environment and Information Sciences, Yokohama National University, Hodogaya-ku, Yokohama, Japan

### **References**

sulfonyl]methanesulfonamide (HN(SO2CF3)2, H[TFSA]), The leaching percentage of Nd and Fe for 66h in the H[TFSA] solution were 85.7±5.8% and 5.8±0.1%, respectively. Moreover, the oxidized Nd-Fe-B sample applied as a precipitation agent in the deironization process, and >99.9% Fe component was successfully separated from RE components. Finally, 3.57 kg of purified amide salts (M(TFSA)3, M = Pr, Nd, Dy, B, Al, and trace elements) were recovered through the evaporation process using an improved spray dryer and the percentage of RE

The cyclic voltammetry (CV) with electrochemical quartz crystal microbalance (EQCM) was applied at elevated temperatures in this study. CV/EQCM measurements for the investigation of the reduction behavior related to Nd(III) in triethyl-pentyl-phosphonium bis(trifluoromethyl-sulfonyl)amide ([P2225][TFSA]) were conducted at 373 K. At the potential of −2.79 V, the objective electrodeposition, Nd(III)/Nd(0) was confirmed because a clear cathodic peak was observed and the apparent molar mass, *M*app was calculated to be 46.8 g mol−1, and this value was consistent with the theoretical value for Nd(III)/Nd(0), 48.1 g mol−1. At more negative potential than −2.79 V, the mass and *ηρ* increased with the comparatively low values of *M*app for 0.05 M Nd(III) in [P2225][TFSA]. This behavior indicated that the cathodic decomposition reaction of IL occurred on the electrode surface. Moreover, from the controlled potential electrolysis with EQCM (CPE/EQCM) measurements at −3.20 V versus Fc/Fc+

solutions of 0.05 and 0.10 M Nd(III) in [P2225][TFSA] at 373 K, the electrodeposits of Nd(0) metal were confirmed at the initial stage considering the *M*app values (49.1 and 49.6 g mol−1). The electrodeposition of Nd was carried out under potentiostatic conditions of −3.20 V ver-

identified to be Nd metal from the analysis of SEM/EDX, XPS, and XRD. Finally, the material flow of whole process allowed us to conclude that the novel recovery process was effec-

This study was partly supported by the Grant-in-Aid for Scientific Research (No. 15H02848)

Graduate School of Environment and Information Sciences, Yokohama National University,

from the Ministry of Education, Culture, Sports, Science, and Technology, Japan.

at 373 K. The electrodeposits in the middle layer 0.15 μm below the surface were

in the

components for amide salts was 96.81%.

230 Progress and Developments in Ionic Liquids

sus Fc/Fc+

tive for practical use.

**Acknowledgements**

**Author details**

Masahiko Matsumiya

Hodogaya-ku, Yokohama, Japan

Address all correspondence to: mmatsumi@ynu.ac.jp


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**Chapter 11**

**Provisional chapter**

**Electrodeposition from Deep Eutectic Solvents**

Deep eutectic solvents constitute a class of compounds sharing many similarities with properly named ionic liquids. The accepted definition of ionic liquid is a fluid (liquid for T<100 °C) consisting of ions, while DES are eutectic mixtures of Lewis or Brønsted acids and bases. Their most attractive properties are the wide potential windows and the chemical properties largely different from aqueous solutions. In the last few decades, the possibility to electrodeposit decorative and functional coatings employing deep eutectic solvents as electrolytes has been widely investigated. A large number of the deposition procedures described in literature, however, cannot find application in the industrial practice due to competition with existing processes, cost or difficult scalability. From one side, there is the real potential to replace existing plating protocols and to find niche applications for high added-value productions; to the other one, this paves the path towards the electrodeposition of metals and alloys thermodynamically impossible to be obtained via usual aqueous solution processes. The main aim of this chapter is therefore the critical discussion of the applicability of deep eutectic solvents to the electrodeposition of metals and alloys, with a particular attention to the industrial and applicative point of

**Keywords:** electrodeposition, deep eutectic solvents, metals, alloys

The electrodeposition of metals for industrial surface finishing is nowadays a well-established industrial practice. Many processes are available to obtain a wide variety of coatings on most of

and reproduction in any medium, provided the original work is properly cited.

© 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution,

© 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

**Electrodeposition from Deep Eutectic Solvents**

R. Bernasconi, G. Panzeri, A. Accogli, F. Liberale,

R. Bernasconi, G. Panzeri, A. Accogli, F. Liberale,

Additional information is available at the end of the chapter

Additional information is available at the end of the chapter

L. Nobili and L. Magagnin

L. Nobili and L. Magagnin

http://dx.doi.org/10.5772/64935

**Abstract**

view.

**1. Introduction**


#### **Electrodeposition from Deep Eutectic Solvents Electrodeposition from Deep Eutectic Solvents**

R. Bernasconi, G. Panzeri, A. Accogli, F. Liberale, R. Bernasconi, G. Panzeri, A. Accogli, F. Liberale, L. Nobili and L. Magagnin

L. Nobili and L. Magagnin

[40] K. Naoi, M. Mori, Y. Shinagawa, Study of deposition and dissolution processes of lithium in carbonate based solutions by means of the quartz crystal microbalance, J.

[41] J. F. Moulder, W. F. Stickle, P. E. Sobol, K. D. Bomben, Handbook of X-ray Photoelectron

[42] G. C. Che, J. Liang, Y. Yi, Crystal structure of X-ray diffraction properties, J. Metall. 22

Electrochem. Soc. 143(8) (1996) 2517-2522.

(1986) B206-B211.

234 Progress and Developments in Ionic Liquids

Spectroscopy, Perkin-Elmer Corp., Eden Prairie, MN (1992).

Additional information is available at the end of the chapter Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/64935

#### **Abstract**

Deep eutectic solvents constitute a class of compounds sharing many similarities with properly named ionic liquids. The accepted definition of ionic liquid is a fluid (liquid for T<100 °C) consisting of ions, while DES are eutectic mixtures of Lewis or Brønsted acids and bases. Their most attractive properties are the wide potential windows and the chemical properties largely different from aqueous solutions. In the last few decades, the possibility to electrodeposit decorative and functional coatings employing deep eutectic solvents as electrolytes has been widely investigated. A large number of the deposition procedures described in literature, however, cannot find application in the industrial practice due to competition with existing processes, cost or difficult scalability. From one side, there is the real potential to replace existing plating protocols and to find niche applications for high added-value productions; to the other one, this paves the path towards the electrodeposition of metals and alloys thermodynamically impossible to be obtained via usual aqueous solution processes. The main aim of this chapter is therefore the critical discussion of the applicability of deep eutectic solvents to the electrodeposition of metals and alloys, with a particular attention to the industrial and applicative point of view.

**Keywords:** electrodeposition, deep eutectic solvents, metals, alloys

### **1. Introduction**

The electrodeposition of metals for industrial surface finishing is nowadays a well-established industrial practice. Many processes are available to obtain a wide variety of coatings on most of

© 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

the substrates used in manufacturing. There are however some important limitations, as not all the metal/substrate combinations are possible in the current state of the art. This is connected to the nature of the electrolytes used for the vast majority of the plating processes, which are water based. Water is the most obvious choice to formulate a plating electrolyte, and in the majority of the cases, it is also the most convenient from the point of view of the results obtained. This solvent however presents some limitations: narrow potential window, reactivity towards specific metals, high hydrogen evolution in specific conditions, etc. If metal plating is limited to aqueous solutions, many possibilities are therefore precluded.

The word eutectic comes from a Greek word that means "easily melted" and indicates the temperature where the phases simultaneously crystallize from molten solution [2]. DESs includes a wide range of liquids close to the eutectic composition of the mixtures; differently from ionic liquids, which are formed primarily of one type of discrete anion and cation, they can contain a variety of anionic and cationic species [1]. They are mainly obtained by the complexation of a quaternary ammonium salt with a metal salt or an hydrogen bond donor

Brönsted acid. Depending on the nature of the complexing agent used, four types of DESs can

X− *z*MCl*<sup>x</sup>* M = Zn, Sn, Fe, Al, Ga, In

X− *z*RZ Z = CONH2, COOH, OH

and quaternary ammonium species [2]. As in traditional ionic liquid systems, the strongest depression of the freezing point is encountered when imidazolium-based DESs are used, due the intense interaction between the anion and the complexing agent. The simple handling and manufacture of DESs is one reason for their success. Mixing the two components, a mild endothermic reaction occurs, requiring only a gentle heating and stirring [2]. Moreover, they are water insensitive, which make it unnecessary to work in a glovebox with a controlled

Type I DESs formed from MClx and quaternary ammonium salt are the analogues of the wellstudied metal halide/imidazolium salt systems [1]. Despite the fact they include a wide range of eutectic mixtures, from famous chloroaluminate/imidazolium to less common EMIC with different metal halides (FeCl2, AgCl, CuCl, LiCl, CdCl2, CuCl2, SnCl2, ZnCl2, LaCl3, YCl3, SnCl4, etc. [3]), the number of non-hydrated metal halides with a suitable low melting point is limited. Among them, the only ones which form ionic liquids with pyridinium, imidazolium and quaternary ammonium halides are FeCl3, ZnCl2, SnCl2, CuCl, InCl3, CdCl2, AuCl3 [2]. ZnCl2-based DESs have been studied deeper in detail with respect to other systems: it was

depending on the ionic liquid composition [2, 5]. They differ in dimensions and charge density one from the other and thus have peculiar electrostatic interactions with the cation. For

·RZ + MCl*x+*1− M = Al, Zn and Z = CONH2, OH

species studied have been based on pyridinium, imidazolium

species are present in the liquid, with proportions

X− *z*MCl*xy*H2O M = Cr, Co, Cu, Ni, Fe

X-

Electrodeposition from Deep Eutectic Solvents

http://dx.doi.org/10.5772/64935

and z molecules of Y, either a Lewis or

zY, where Cat+

is a Lewis base, generally

is in

237

(HBD) and can be schematically expressed by the general formula Cat+

principle any ammonium, phosphonium or sulfonium cation and X-

**Type General formula Examples of terms**

be categorized [1]. Such classification is presented in **Table 1**.

a halide anion [1]. The interaction occurs between X-

Type I Cat+

Type II Cat+

Type III Cat+

atmosphere.

**2.1. Type I eutectics**

found that ZnCl3


and Zn3Cl7


Type IV MCl*x* + RZ = MCl*x‐1*<sup>+</sup>

**Table 1.** Classification of DESs and examples [1].

Up to now, the most widely Cat+

A possible way to extend the range of coating/substrate combinations is the use of non-aqueous solvents, characterized by extended potential windows and improved chemical inertness. A notable amount of scientific literature is available on a high number of non-aqueous systems. Deep eutectic solvents (DESs) are a particular class of such systems, and in the last few decades, they are finding application in the electrodeposition of many metals and alloys. This chapter is intended to be a review of the current state of art for electrodeposition from DESs and a critical discussion of the realistic applicability of DESs with particular attention to the industrial point of view.

#### **2. Chemistry of DESs**

Despite the fact that the physical properties of DESs are similar to those of other ionic liquids (ILs), their chemical properties exhibit peculiarities, which make them suitable for specific and different applications [1]. Melting point of two components strongly depends upon their reciprocal interaction: when considering a binary mixture of A + B, the difference in the freezing point at the eutectic composition compared to that of a theoretical ideal mixture is directly proportional to the interaction between the two single components A and B. The stronger the interaction, the larger will be the depression of the mixture melting point [1, 2]. This effect is schematically shown in the phase diagram presented in **Figure 1**.

**Figure 1.** Eutectic formation in a two components phase diagram [1].

The word eutectic comes from a Greek word that means "easily melted" and indicates the temperature where the phases simultaneously crystallize from molten solution [2]. DESs includes a wide range of liquids close to the eutectic composition of the mixtures; differently from ionic liquids, which are formed primarily of one type of discrete anion and cation, they can contain a variety of anionic and cationic species [1]. They are mainly obtained by the complexation of a quaternary ammonium salt with a metal salt or an hydrogen bond donor (HBD) and can be schematically expressed by the general formula Cat+ XzY, where Cat+ is in principle any ammonium, phosphonium or sulfonium cation and X is a Lewis base, generally a halide anion [1]. The interaction occurs between X and z molecules of Y, either a Lewis or Brönsted acid. Depending on the nature of the complexing agent used, four types of DESs can be categorized [1]. Such classification is presented in **Table 1**.


**Table 1.** Classification of DESs and examples [1].

Up to now, the most widely Cat+ species studied have been based on pyridinium, imidazolium and quaternary ammonium species [2]. As in traditional ionic liquid systems, the strongest depression of the freezing point is encountered when imidazolium-based DESs are used, due the intense interaction between the anion and the complexing agent. The simple handling and manufacture of DESs is one reason for their success. Mixing the two components, a mild endothermic reaction occurs, requiring only a gentle heating and stirring [2]. Moreover, they are water insensitive, which make it unnecessary to work in a glovebox with a controlled atmosphere.

#### **2.1. Type I eutectics**

the substrates used in manufacturing. There are however some important limitations, as not all the metal/substrate combinations are possible in the current state of the art. This is connected to the nature of the electrolytes used for the vast majority of the plating processes, which are water based. Water is the most obvious choice to formulate a plating electrolyte, and in the majority of the cases, it is also the most convenient from the point of view of the results obtained. This solvent however presents some limitations: narrow potential window, reactivity towards specific metals, high hydrogen evolution in specific conditions, etc. If metal plating is limited to

A possible way to extend the range of coating/substrate combinations is the use of non-aqueous solvents, characterized by extended potential windows and improved chemical inertness. A notable amount of scientific literature is available on a high number of non-aqueous systems. Deep eutectic solvents (DESs) are a particular class of such systems, and in the last few decades, they are finding application in the electrodeposition of many metals and alloys. This chapter is intended to be a review of the current state of art for electrodeposition from DESs and a critical discussion of the realistic applicability of DESs with particular attention to the industrial

Despite the fact that the physical properties of DESs are similar to those of other ionic liquids (ILs), their chemical properties exhibit peculiarities, which make them suitable for specific and different applications [1]. Melting point of two components strongly depends upon their reciprocal interaction: when considering a binary mixture of A + B, the difference in the freezing point at the eutectic composition compared to that of a theoretical ideal mixture is directly proportional to the interaction between the two single components A and B. The stronger the interaction, the larger will be the depression of the mixture melting point [1, 2].

This effect is schematically shown in the phase diagram presented in **Figure 1**.

**Figure 1.** Eutectic formation in a two components phase diagram [1].

aqueous solutions, many possibilities are therefore precluded.

point of view.

**2. Chemistry of DESs**

236 Progress and Developments in Ionic Liquids

Type I DESs formed from MClx and quaternary ammonium salt are the analogues of the wellstudied metal halide/imidazolium salt systems [1]. Despite the fact they include a wide range of eutectic mixtures, from famous chloroaluminate/imidazolium to less common EMIC with different metal halides (FeCl2, AgCl, CuCl, LiCl, CdCl2, CuCl2, SnCl2, ZnCl2, LaCl3, YCl3, SnCl4, etc. [3]), the number of non-hydrated metal halides with a suitable low melting point is limited. Among them, the only ones which form ionic liquids with pyridinium, imidazolium and quaternary ammonium halides are FeCl3, ZnCl2, SnCl2, CuCl, InCl3, CdCl2, AuCl3 [2]. ZnCl2-based DESs have been studied deeper in detail with respect to other systems: it was found that ZnCl3 - , Zn2Cl5 and Zn3Cl7 species are present in the liquid, with proportions depending on the ionic liquid composition [2, 5]. They differ in dimensions and charge density one from the other and thus have peculiar electrostatic interactions with the cation. For example, since ZnCl3 ions are smaller and have stronger electrostatic interactions, the freezing point increases [2]. It is difficult to model the potential energy between the ions due to the complex nature of the anion and the non-centrosymmetric charge distribution on the cation. However, considering simultaneously the freezing point difference between the quaternary ammonium salts and the complexed metal salt and the potential energy, a correlation can be found: the variation of interionic potential energy and of the freezing temperature are indeed strictly related [2]. It was found that is possible to extrapolate the phase behaviour from simple ionic size considerations and that the symmetry has a minimum effect on the depression of the freezing point. Endres et al. show that the cation has a little effect on the freezing point of the eutectic-based ionic liquids: smaller cations depress the freezing point more because the correspondent halide salts have a higher freezing point [2]. Considering different cation dimensions, the two effects are to some extent compensated and the freezing point does not almost change. The ions size strictly influences both the conductivity and the viscosity of the type I eutectics: anhydrous zinc and iron salts based on DESs show lower conductivity and higher viscosity with respect to corresponding aluminium ionic liquids because of the larger ion size. In general, imidazolium-based liquids have lower viscosity and higher conductivity than the pyridinium or quaternary ammonium eutectics formed under the analogous conditions.

the freezing point is extremely high, even higher than 200°C for the oxalic acid-zinc chloride

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239

A metal chloride hydrate and a hydrogen bond donor mainly form these eutectic mixtures. Type IV eutectics are very interesting due to the production of cationic metal complexes, guaranteeing a high metal ion concentration close to the electrode surface [7]. A noteworthy example is ZnCl2, which forms eutectic compounds with different substances such as urea,

Viscosity is one of the most important properties of DESs due to its practical and industrial relevance. Although numerous works are present in scientific literature, there is a lack of information on DESs' behaviour at pressures different from ambient one and for related applications [8]. Typical DESs viscosity ranges from tens to hundreds of mPa s at temperatures around ambient one or slightly higher [1, 2, 8]. Some exceptions occur, for example, in the case of compounds containing ChCl + ZnCl2 in a 1:2 molar ratio, which can give rise to viscosity of 85,000 mPa s at room temperature [9]. The most remarkable DESs for practical purposes are in general those with low viscosities, close to the widely used organic solvents or water-based solutions. It is important to note that differences in experimental methods, sample preparation

The presence of impurities is the variable that more deeply affects the rheological behaviour of these systems. Moreover, some of DESs are non-Newtonian fluids: this feature can influence the viscosity analysis methods for this kind of system. The hygroscopic nature of many DESs strongly influences their viscosity, which can change of even two orders of magnitude from dehydrated to hydrated eutectics [8]. An example is that of ChCl and oxalic acid equimolar mixtures, whose viscosity passes from 5363 to 44.49 mPa s when water is absorbed [9]: water addition could be an easy method for tuning the viscosity and it is indeed applied in different cases. However, because water may compete with CO2 in absorption sites occupation in systems tailored for CO2 absorption, the efficiency could be depressed affecting the obtained result [10, 11]. Moreover, the presence of water could be detrimental for some electrodeposition

The temperature-viscosity dependence has been deeply investigated in literature following Arrhenius equation or Vogel-Fulcher-Tammann one [7]. As intuitively predictable, viscosity decreases with increasing temperature. In particular, using an approach based on a temperature–viscosity fitting through an Arrhenius model, it is possible to calculate the activation energy Ea and therefore the strength of intermolecular forces in DESs [2, 12, 13]. Low-viscosity DESs have low Ea values, whereas more viscous systems show higher activation energies.

system [6].

**3.1. Viscosity**

procedures [1, 2, 8].

**2.4. Type IV eutectics**

acetamide, ethylene glycol and 1,6-hexanediol [1].

and impurities can affect significantly the measure obtained [8].

**3. Physical properties of DESs**

#### **2.2. Type II eutectics**

The number of metals that can be incorporated into an ionic liquid can be increased by using hydrated metal halides and choline chloride (ChCl) [1]. The presence of water decreases the melting point since the lattice energy is decreased too. Therefore, hydrated salts are more prone to form mixtures with quaternary ammonium salts that are liquid at ambient temperature with respect to anhydrous salts [2]. Examples of metal salts mixed with ChCl are CrCl3·6H2O, MgCl2·6H2O, CoCl2·6H2O, LaCl3·6H2O, CuCl2·2H2O [1]. Studies on the first mixture showed how the main charge carrying species are [Choline]+ and [Cl3H2O]- : this is consistent with the highest conductivity of these liquid compared to anhydrous salt mixtures [1]. The addition of Li+ ions from LiCl does not cause the expected increase of the conductivity, probably due to the high ion solvation or to its high association with the anion. These systems are more temperature sensitive with respect to anhydrous metal salts. Although different metal salts can be theoretically deposited from these type of eutectic mixtures, up to now only Cr and Co have been deposited. Since hydrated systems are being considered, the deposition of Al and other low reduction potential metals is not possible due to the limited potential window [2].

#### **2.3. Type III eutectics**

These eutectics are formed from choline chloride and hydrogen bond donors [1]. They are able to solvate different metal species, including chlorides and oxides. Since the number of hydrogen bond donors is high, they constitute a group of easily tuneable liquids, adjustable for specific peculiar applications. Hydrogen bond donors (HBDs) include amides, carboxylic acids and alcohols [1]. These eutectic mixtures are easy to prepare and handle, as they are almost unreactive with water and biodegradable. For some of these eutectics, depression of the freezing point is extremely high, even higher than 200°C for the oxalic acid-zinc chloride system [6].

#### **2.4. Type IV eutectics**

example, since ZnCl3 ions are smaller and have stronger electrostatic interactions, the freezing point increases [2]. It is difficult to model the potential energy between the ions due to the complex nature of the anion and the non-centrosymmetric charge distribution on the cation. However, considering simultaneously the freezing point difference between the quaternary ammonium salts and the complexed metal salt and the potential energy, a correlation can be found: the variation of interionic potential energy and of the freezing temperature are indeed strictly related [2]. It was found that is possible to extrapolate the phase behaviour from simple ionic size considerations and that the symmetry has a minimum effect on the depression of the freezing point. Endres et al. show that the cation has a little effect on the freezing point of the eutectic-based ionic liquids: smaller cations depress the freezing point more because the correspondent halide salts have a higher freezing point [2]. Considering different cation dimensions, the two effects are to some extent compensated and the freezing point does not almost change. The ions size strictly influences both the conductivity and the viscosity of the type I eutectics: anhydrous zinc and iron salts based on DESs show lower conductivity and higher viscosity with respect to corresponding aluminium ionic liquids because of the larger ion size. In general, imidazolium-based liquids have lower viscosity and higher conductivity than the pyridinium or quaternary ammonium eutectics formed under the analogous condi-

The number of metals that can be incorporated into an ionic liquid can be increased by using hydrated metal halides and choline chloride (ChCl) [1]. The presence of water decreases the melting point since the lattice energy is decreased too. Therefore, hydrated salts are more prone to form mixtures with quaternary ammonium salts that are liquid at ambient temperature with respect to anhydrous salts [2]. Examples of metal salts mixed with ChCl are CrCl3·6H2O, MgCl2·6H2O, CoCl2·6H2O, LaCl3·6H2O, CuCl2·2H2O [1]. Studies on the first mixture showed

highest conductivity of these liquid compared to anhydrous salt mixtures [1]. The addition of

These eutectics are formed from choline chloride and hydrogen bond donors [1]. They are able to solvate different metal species, including chlorides and oxides. Since the number of hydrogen bond donors is high, they constitute a group of easily tuneable liquids, adjustable for specific peculiar applications. Hydrogen bond donors (HBDs) include amides, carboxylic acids and alcohols [1]. These eutectic mixtures are easy to prepare and handle, as they are almost unreactive with water and biodegradable. For some of these eutectics, depression of

 ions from LiCl does not cause the expected increase of the conductivity, probably due to the high ion solvation or to its high association with the anion. These systems are more temperature sensitive with respect to anhydrous metal salts. Although different metal salts can be theoretically deposited from these type of eutectic mixtures, up to now only Cr and Co have been deposited. Since hydrated systems are being considered, the deposition of Al and other low reduction potential metals is not possible due to the limited potential window [2].

and [Cl3H2O]-

: this is consistent with the

tions.

Li+

**2.2. Type II eutectics**

238 Progress and Developments in Ionic Liquids

**2.3. Type III eutectics**

how the main charge carrying species are [Choline]+

A metal chloride hydrate and a hydrogen bond donor mainly form these eutectic mixtures. Type IV eutectics are very interesting due to the production of cationic metal complexes, guaranteeing a high metal ion concentration close to the electrode surface [7]. A noteworthy example is ZnCl2, which forms eutectic compounds with different substances such as urea, acetamide, ethylene glycol and 1,6-hexanediol [1].

### **3. Physical properties of DESs**

#### **3.1. Viscosity**

Viscosity is one of the most important properties of DESs due to its practical and industrial relevance. Although numerous works are present in scientific literature, there is a lack of information on DESs' behaviour at pressures different from ambient one and for related applications [8]. Typical DESs viscosity ranges from tens to hundreds of mPa s at temperatures around ambient one or slightly higher [1, 2, 8]. Some exceptions occur, for example, in the case of compounds containing ChCl + ZnCl2 in a 1:2 molar ratio, which can give rise to viscosity of 85,000 mPa s at room temperature [9]. The most remarkable DESs for practical purposes are in general those with low viscosities, close to the widely used organic solvents or water-based solutions. It is important to note that differences in experimental methods, sample preparation and impurities can affect significantly the measure obtained [8].

The presence of impurities is the variable that more deeply affects the rheological behaviour of these systems. Moreover, some of DESs are non-Newtonian fluids: this feature can influence the viscosity analysis methods for this kind of system. The hygroscopic nature of many DESs strongly influences their viscosity, which can change of even two orders of magnitude from dehydrated to hydrated eutectics [8]. An example is that of ChCl and oxalic acid equimolar mixtures, whose viscosity passes from 5363 to 44.49 mPa s when water is absorbed [9]: water addition could be an easy method for tuning the viscosity and it is indeed applied in different cases. However, because water may compete with CO2 in absorption sites occupation in systems tailored for CO2 absorption, the efficiency could be depressed affecting the obtained result [10, 11]. Moreover, the presence of water could be detrimental for some electrodeposition procedures [1, 2, 8].

The temperature-viscosity dependence has been deeply investigated in literature following Arrhenius equation or Vogel-Fulcher-Tammann one [7]. As intuitively predictable, viscosity decreases with increasing temperature. In particular, using an approach based on a temperature–viscosity fitting through an Arrhenius model, it is possible to calculate the activation energy Ea and therefore the strength of intermolecular forces in DESs [2, 12, 13]. Low-viscosity DESs have low Ea values, whereas more viscous systems show higher activation energies. Abbot and co-workers [8], following the same approach used to relate melting temperature and activation energy of molten salts, found out an almost linear relationship also for DESs: larger melting points are indeed associated with larger Ea (and subsequently larger viscosity) (**Figure 2**).

approach for determining conductivity can be followed using the hole-theory approach [15, 16]. Because of both holes availability and kind of bonds between ions and HBDs have to be taken into account, achievement of a univocal trend in all the analysed systems is difficult to obtain. During conductivity modelling, many aspects from chemical to hydrodynamic ones have to be taken into account. The typical trend implies an increase in conductivity when higher quantities of salts are used [16], but this is not always true, since both the type of salt and HBD and their interaction have to be considered. Some DESs indeed manifest deviation from the general trend: for example, ChCl:ethylene glycol-based DESs show a maximum in conductivity, which then decreases after a certain salt concentration [8]. The conductivity dependence on temperature is well described by an Arrhenius model, through which analogous computation to those used for viscosity can lead to activation energy evaluation [8].

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241

Viscosity and conductivity are usually plotted together in the so-called Walden plot, where molar conductivity and the inverse of viscosity, that is fluidity, are represented in a logarithmic scale [8]. The obtained data are compared with an ideal line passing through the origin for a potassium chloride solution. The deviation from ideality is higher in the case of DESs with respect to common ILs, since in the first case ions and HBDs complexes have to be

The deviation from the ideal line is associated with the systems ionicity: low viscous liquids have lower ionicity i.e. high deviation, whereas high DESs show the opposite behavior [15]. Ionicity is influenced by both salt nature and interaction between ions and different HBDs [8, 15]. Nowadays, although good correlation between effective experimentally measured conductivity and theoretically predicted one has been proved with some models for a set of

considered [16]. An example of Walden plot is reported in **Figure 3** [8].

**Figure 3.** Walden plots for a series of DESs [13].

**Figure 2.** The almost linear relationship between activation energy and temperature [13].

In the case of DESs, ions have a relatively large size if compared with the voids between the ions themselves. Such voids represent the free space between the different moieties that constitute the DES and are generally called holes. Relating viscosity data in the light of holetheory approach [14–16], for which viscosity and electrical conductivity are correlated with availability of holes in the fluid, it is possible to evince that DESs with larger holes lead to less viscous fluids. Deviation from this general trend is shown when very viscous systems are taken into account.

#### **3.2. Conductivity**

Conductivity cannot be considered separately from viscosity, since the two are strictly related. Generally, a low viscosity is usually associated with a high conductivity and vice versa [8]. Low viscosity DESs have conductivity in the order of even tens of mS cm-1 [16], while for most common high density DESs an order of magnitude lower is usually observed. A theoretical approach for determining conductivity can be followed using the hole-theory approach [15, 16]. Because of both holes availability and kind of bonds between ions and HBDs have to be taken into account, achievement of a univocal trend in all the analysed systems is difficult to obtain. During conductivity modelling, many aspects from chemical to hydrodynamic ones have to be taken into account. The typical trend implies an increase in conductivity when higher quantities of salts are used [16], but this is not always true, since both the type of salt and HBD and their interaction have to be considered. Some DESs indeed manifest deviation from the general trend: for example, ChCl:ethylene glycol-based DESs show a maximum in conductivity, which then decreases after a certain salt concentration [8]. The conductivity dependence on temperature is well described by an Arrhenius model, through which analogous computation to those used for viscosity can lead to activation energy evaluation [8].

Viscosity and conductivity are usually plotted together in the so-called Walden plot, where molar conductivity and the inverse of viscosity, that is fluidity, are represented in a logarithmic scale [8]. The obtained data are compared with an ideal line passing through the origin for a potassium chloride solution. The deviation from ideality is higher in the case of DESs with respect to common ILs, since in the first case ions and HBDs complexes have to be considered [16]. An example of Walden plot is reported in **Figure 3** [8].

**Figure 3.** Walden plots for a series of DESs [13].

Abbot and co-workers [8], following the same approach used to relate melting temperature and activation energy of molten salts, found out an almost linear relationship also for DESs: larger melting points are indeed associated with larger Ea (and subsequently larger viscosity)

**Figure 2.** The almost linear relationship between activation energy and temperature [13].

In the case of DESs, ions have a relatively large size if compared with the voids between the ions themselves. Such voids represent the free space between the different moieties that constitute the DES and are generally called holes. Relating viscosity data in the light of holetheory approach [14–16], for which viscosity and electrical conductivity are correlated with availability of holes in the fluid, it is possible to evince that DESs with larger holes lead to less viscous fluids. Deviation from this general trend is shown when very viscous systems are taken

Conductivity cannot be considered separately from viscosity, since the two are strictly related. Generally, a low viscosity is usually associated with a high conductivity and vice versa [8]. Low viscosity DESs have conductivity in the order of even tens of mS cm-1 [16], while for most common high density DESs an order of magnitude lower is usually observed. A theoretical

(**Figure 2**).

240 Progress and Developments in Ionic Liquids

into account.

**3.2. Conductivity**

The deviation from the ideal line is associated with the systems ionicity: low viscous liquids have lower ionicity i.e. high deviation, whereas high DESs show the opposite behavior [15]. Ionicity is influenced by both salt nature and interaction between ions and different HBDs [8, 15]. Nowadays, although good correlation between effective experimentally measured conductivity and theoretically predicted one has been proved with some models for a set of DESs, a unique broadly applicable method for conductivity is still not available or, at least, not reliable [8]. An important factor which governs both viscosity and conductivity is the diffusion coefficient of molecules present in ILs [17, 18] influenced by both hole size distribution and intermolecular forces [8].

carried out using several solvents, due to the well-known baths chemistry and handling, electrodeposition of metals and alloys is performed usually from aqueous solutions. The number of metallic coatings obtaining from aqueous baths is strictly related to those having redox potential higher than the water one: this narrow potential window entails gas evolution with subsequent hydrogen embrittlement phenomena and the formation of insoluble oxides and/or hydroxides on the electrode surface (passivation) which hinder the deposition of thick metallic coatings. Moreover, many of the traditional aqueous solutions are based on toxic components and show low current efficiency [e.g. Cr(VI) plating]. For these reasons and especially for the possibility to electrodeposit metals having Nernst potential well below water decomposition one, for example titanium, aluminium, tungsten suitable for many industrial applications (e.g. anti-corrosion, batteries) that cannot be plated starting from aqueous solutions, electrodeposition based on deep eutectic solvents (DESs) could be the alternative [21]. DESs show high solubility for metal and metal oxides and hydroxides allowing the possibility to avoid passivation phenomena during electrodeposition or electropolishing, to plate thicker metal layers and to use them in electrochemical processes such as metal recovery and metal separation. Furthermore, the wider potential window, the absence of water (no embrittlement phenomena), the relatively high conductivity compared to other non-aqueous solvents, the relatively low cost, simply preparation and biodegradability make them suitable for industrial electrodeposition processes, although these are not economically competitive with respect to the existing ones. However, the instability of DES baths, while the electrodeposition proceeds, has to be solved prior to a possible industrial transfer [2]. Electrodeposition from DESs could be the way to easy circumvent legislative restriction related to aqueous precursor toxicity used in current technological electroplating systems (Ni, Cr, Co) known to be carcinogenic and the related high disposal costs [1, 21]. In case of DESs deposition, morphology and adhesion of the growing metal coatings are dependent on applied current density, DES composition and presence of additive. As in case of aqueous-based baths, the plating process can be carried out under constant current or constant voltage regimes [2]. In this dissertation, electrodeposition of the most popular metallic coatings starting from DESs made

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243

of ChCl and either ethylene glycol or glycerol or urea are shown.

Nickel metal coatings can be successfully obtained by dissolving nickel chloride dehydrate salts both in ChCl:urea and ChCl:ethylene glycol [16, 22]: coatings morphology obtained from DESs baths is completely different compared to the aqueous plated Ni, due to the different thermodynamics and kinetics of the two processes [23]. Recent works demonstrate that the addition of different components into DESs-based electrolyte can induce changes in terms of microstructure, morphology and redox behavior of the Ni(II) ion. Let us consider the case of nicotinic acid [23] and ethylenediamine [24], whose addition in a ChCl: urea bath affects the morphology and the microstructure of the metal coatings and allows obtaining high uniform and shiny deposits. The effect of these species on the electrodeposition process can be ascribed to the formation of Ni(II) complexes and their subsequent absorption on the electrode surface. These phenomena decrease the nickel deposition current and its nucleation lowering the particles size. The effect of process temperature on the coating quality was investigated by Gu

**4.1. Nickel**

#### **3.3. Electrochemical reactions**

Due to their practical application, DESs electrochemical properties are very important: they are evaluated by means of cyclic voltammetries and electrochemical impedance spectroscopy [19]. The material used as working electrode has obviously a relevant influence, and peculiarities are revealed for each material selected. Many studies have been performed on mixtures, revealing some differences with respect to high-temperature molten salts [4]: the differential capacitance indeed increases with temperature for all the electrodes studied with the exception of Au [1]. For example, when different HBDs are used for the same ChCl system, specificities are encountered related to the HBD type: it has indeed an influence on the negative limit of polarization [1]. Moreover, an increase in temperature of DESs results in narrower electrochemical windows and in larger capacitance currents [1]. Although the electrochemical window is generally smaller than pure ILs one, it is suitable for a wide range of electrochemical purposes [1, 8]. In the cathodic part of CV curves, again for ChCl-based DESs, a layer of HBDs molecules adheres to the electrode surface, while choline cations are in contact with the electrolyte [1]. When passing to less negative potentials, the absorption of anions occurs replacing partially positive choline ions [1]. The Helmholtz type layer model of the double layer at large negative potential is validated from very similar results obtained by replacing choline with other cations, such as acetylcholine [1]. Because of DESs ionic character is dependent on their chemical-physical properties, the comprehension of metal ions behaviour in the liquid is really important. Many efforts have been made by Abbott's group to determine, through equilibrium electrochemical measurements, the activity coefficients for metal salts in DESs and how they vary with concentration [16]. Understanding the activity of a solute in solution is indeed fundamental for using the full potential of a reactive species. Comparing the redox potentials of metal couples in an established DES mixture with those in an aqueous medium allows understanding the potentialities of a system for electrodeposition purposes and other applications. However, since there is plenty of factors influencing them, a univocally valid model cannot be found.

### **4. Electrodeposition of metals from DESs**

Due to their good properties as electrolytes, electrodeposition is an obvious possible application for DESs. Nowadays, this technique is one of the most used surface finishing methods in industrial applications. It consists mainly in the formation of a solid metal coating on the electrode surface starting from metal cations dispersed in the electrolyte, reduced on the cathode under an electrical potential. This electrochemical process is used to functionalize surface to obtain required properties, for example hardness, corrosion and wear resistances, abrasion, brightness, magnetism, electrocatalysis [20]. Although electrodeposition can be carried out using several solvents, due to the well-known baths chemistry and handling, electrodeposition of metals and alloys is performed usually from aqueous solutions. The number of metallic coatings obtaining from aqueous baths is strictly related to those having redox potential higher than the water one: this narrow potential window entails gas evolution with subsequent hydrogen embrittlement phenomena and the formation of insoluble oxides and/or hydroxides on the electrode surface (passivation) which hinder the deposition of thick metallic coatings. Moreover, many of the traditional aqueous solutions are based on toxic components and show low current efficiency [e.g. Cr(VI) plating]. For these reasons and especially for the possibility to electrodeposit metals having Nernst potential well below water decomposition one, for example titanium, aluminium, tungsten suitable for many industrial applications (e.g. anti-corrosion, batteries) that cannot be plated starting from aqueous solutions, electrodeposition based on deep eutectic solvents (DESs) could be the alternative [21]. DESs show high solubility for metal and metal oxides and hydroxides allowing the possibility to avoid passivation phenomena during electrodeposition or electropolishing, to plate thicker metal layers and to use them in electrochemical processes such as metal recovery and metal separation. Furthermore, the wider potential window, the absence of water (no embrittlement phenomena), the relatively high conductivity compared to other non-aqueous solvents, the relatively low cost, simply preparation and biodegradability make them suitable for industrial electrodeposition processes, although these are not economically competitive with respect to the existing ones. However, the instability of DES baths, while the electrodeposition proceeds, has to be solved prior to a possible industrial transfer [2]. Electrodeposition from DESs could be the way to easy circumvent legislative restriction related to aqueous precursor toxicity used in current technological electroplating systems (Ni, Cr, Co) known to be carcinogenic and the related high disposal costs [1, 21]. In case of DESs deposition, morphology and adhesion of the growing metal coatings are dependent on applied current density, DES composition and presence of additive. As in case of aqueous-based baths, the plating process can be carried out under constant current or constant voltage regimes [2]. In this dissertation, electrodeposition of the most popular metallic coatings starting from DESs made of ChCl and either ethylene glycol or glycerol or urea are shown.

#### **4.1. Nickel**

DESs, a unique broadly applicable method for conductivity is still not available or, at least, not reliable [8]. An important factor which governs both viscosity and conductivity is the diffusion coefficient of molecules present in ILs [17, 18] influenced by both hole size distribution and

Due to their practical application, DESs electrochemical properties are very important: they are evaluated by means of cyclic voltammetries and electrochemical impedance spectroscopy [19]. The material used as working electrode has obviously a relevant influence, and peculiarities are revealed for each material selected. Many studies have been performed on mixtures, revealing some differences with respect to high-temperature molten salts [4]: the differential capacitance indeed increases with temperature for all the electrodes studied with the exception of Au [1]. For example, when different HBDs are used for the same ChCl system, specificities are encountered related to the HBD type: it has indeed an influence on the negative limit of polarization [1]. Moreover, an increase in temperature of DESs results in narrower electrochemical windows and in larger capacitance currents [1]. Although the electrochemical window is generally smaller than pure ILs one, it is suitable for a wide range of electrochemical purposes [1, 8]. In the cathodic part of CV curves, again for ChCl-based DESs, a layer of HBDs molecules adheres to the electrode surface, while choline cations are in contact with the electrolyte [1]. When passing to less negative potentials, the absorption of anions occurs replacing partially positive choline ions [1]. The Helmholtz type layer model of the double layer at large negative potential is validated from very similar results obtained by replacing choline with other cations, such as acetylcholine [1]. Because of DESs ionic character is dependent on their chemical-physical properties, the comprehension of metal ions behaviour in the liquid is really important. Many efforts have been made by Abbott's group to determine, through equilibrium electrochemical measurements, the activity coefficients for metal salts in DESs and how they vary with concentration [16]. Understanding the activity of a solute in solution is indeed fundamental for using the full potential of a reactive species. Comparing the redox potentials of metal couples in an established DES mixture with those in an aqueous medium allows understanding the potentialities of a system for electrodeposition purposes and other applications. However, since there is plenty of factors influencing them, a univocally

Due to their good properties as electrolytes, electrodeposition is an obvious possible application for DESs. Nowadays, this technique is one of the most used surface finishing methods in industrial applications. It consists mainly in the formation of a solid metal coating on the electrode surface starting from metal cations dispersed in the electrolyte, reduced on the cathode under an electrical potential. This electrochemical process is used to functionalize surface to obtain required properties, for example hardness, corrosion and wear resistances, abrasion, brightness, magnetism, electrocatalysis [20]. Although electrodeposition can be

intermolecular forces [8].

**3.3. Electrochemical reactions**

242 Progress and Developments in Ionic Liquids

valid model cannot be found.

**4. Electrodeposition of metals from DESs**

Nickel metal coatings can be successfully obtained by dissolving nickel chloride dehydrate salts both in ChCl:urea and ChCl:ethylene glycol [16, 22]: coatings morphology obtained from DESs baths is completely different compared to the aqueous plated Ni, due to the different thermodynamics and kinetics of the two processes [23]. Recent works demonstrate that the addition of different components into DESs-based electrolyte can induce changes in terms of microstructure, morphology and redox behavior of the Ni(II) ion. Let us consider the case of nicotinic acid [23] and ethylenediamine [24], whose addition in a ChCl: urea bath affects the morphology and the microstructure of the metal coatings and allows obtaining high uniform and shiny deposits. The effect of these species on the electrodeposition process can be ascribed to the formation of Ni(II) complexes and their subsequent absorption on the electrode surface. These phenomena decrease the nickel deposition current and its nucleation lowering the particles size. The effect of process temperature on the coating quality was investigated by Gu et al. [25] performing electrodeposition at ambient temperature and at 90°C using a ChCl:ethylene glycol bath on a brass foil. This work demonstrates that the surface roughness increases with temperature due to the formation of nanosheets with a thickness of 10–20 nm and grains size of about 10–50 nm. This phenomenon is related to a decrease of DES viscosity causing an increase of ion species mobility, enhancing in this way the deposition process and the nucleation of Ni. The obtained coating shows also low corrosion potential. Abbott et al. [22] have also studied the Ni electrodeposition starting from the ChCl:ethylene glycol DES bath, obtaining a dark grey deposit: the addition of ethylenediamine and acetylacetamide in this kind of electrolyte induces the suppression of the Ni underpotential deposition. Due to the growing industrial interest on composite materials, several studies have been done in order to develop electrodeposition process starting from DESs. Using Ni as matrix, compact Ni-multiwalled carbon nanotubes (MWCNTs) were deposited on copper substrate. In order to have a high efficient deposition process, a homogeneous dispersion of MWCNTs into the electrolyte has to be guaranteed. This has been obtained dispersing MWCNTs into ChCl:urea DES before the addition of soluble Ni chloride salt. Morphology, crystallinity and roughness of Ni coating were affected by the presence of MWCNTs [26].

aqueous solution is inexpensive and produces very good coatings, the study of electroplating from DESs is mostly used for Zn alloys production [1]. However, several studies have been done in order to understand the Zn deposition process using different DESs, in particular ChCl:urea [31] and ChCl:ethylene glycol [32]. Compared to aqueous electrodeposited Zn having a dendritic structure, those ones plated with DES shows very different morphologies. Using ChCl:ethylene glycol bath, deposited Zn shows very thin platelets with the planar face perpendicular to the electrode surface, according to progressive nucleation mechanism. In case of ChCl:urea, zinc coating has a rice-grain morphology, consistent with a rapid nucleation mechanism. The effects induced by the addition of some chelating agents such as acetonitrile, ammonia and ethylenediamine on the electrochemical process were studied [31]: these additives act as brightener but at the same time affect the electrodeposition process and lead to the formation of macro-crystalline deposit like in water. This phenomenon can be ascribed to the adsorption inhibition of chloride on the electrode surface induced by the presence of ammonia and ethylenediamine. Because of the addition of complexing agent to ChCl:urea and ChCl:ethyleneglycole baths does not change the metal species present in the baths ([ZnCl4]

it is possible to deduce that the morphology variation after their addition is caused by the chemical processes taking place in the diffusion layer or on the cathodic surface rather than in the bulk [16, 33]. Another important study was performed by Bakkar et al., in which the electrodeposition of Zn using ChCl based electrolytes on magnesium substrates [34]. Mg and its alloys are difficult to be electroplated and so cannot be used as substrate in aqueous solution due to their water-sensitiveness, their tendency to form MgO and/or Mg(OH)2 film inhibiting adhesion of the electrodeposited coating and their high reactivity inducing formation of loose immersion layers on the surface by replacement that stops the following electrodeposition. Furthermore, magnesium suffers of microgalvanic deterioration if it is in contact with cathodic metals in a wet environment: this means that Mg is subjected to strong corrosion attack by aqueous electrolytes. Developing an electrodeposition process based on DESs could be an important turning point for a successful application of protective coatings on magnesium largely used in automobiles, aerospace and electronics industries. The authors have compared the efficiency of several ChCl-based DESs, in particular 1 ChCl:2 urea, 1 ChCl:2 ethylene glycol, 1 ChCl:1 malonic acid, 1 ChCl:2 glycerol and 1 ChCl:2 ZnCl2. From this study, it is possible to evince that Mg has the lowest corrosion rate in 1 ChCl:2 urea, the most feasible mixture for successful Zn electrodeposition, whereas the other mixtures produce powdery deposits or Mg corrosion. In particular, in case of 1 ChCl:2 glycerol pitting phenomena on Mg surface occur. The application of pulsed cathodic current during the electroplating process helps for the production of smooth, uniform and corrosion-resistant Zn coating, similar to that of pure zinc

Aluminium is one of the metals that cannot be plated in aqueous media because of its Nernst potential, well below the water decomposition one. Moreover, the high stability of aluminium oxide makes it high resistant to corrosion, but unfortunately this means that it cannot be electroplated from aqueous electrolytes [21]. In any case, aluminium electrodeposition is an important technological target due to its application in electronic, energy storage and anti-

[34].

**4.4. Aluminium**

2-),

245

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#### **4.2. Chromium**

For several decades, hard chromium plating has been the most used metallic coating employed for several applications, especially to protect components operating in high wear and corrosion environment. This was due to its high hardness and its natural ability to inhibit corrosion. Due to its significant use in the industry, chromium electroplating based on Cr(VI) aqueous electrolyte is the highest optimized process. However, a series of issues, with most important the extremely negative environmental impact of the hard chromium plating process, due to the use of the carcinogenic hexavalent chromium, has led to a number of directives and legislation related to the restriction of this method [27]. This favours the necessity of finding less hazardous method replacing the present one. Up to now, some Cr(III) processes are on the market, but the obtained results are not repeatable. As demonstrated by recent scientific studies, the Cr electrodeposition from DESs could be the way to achieve metallic coatings having same properties of Cr(VI) ones. In particular, Abbott et al. have reported DESs obtained mixing ChCl with trivalent chromium chloride salts [28, 29]: in this electrolyte, electrodeposition process with a very high current efficiency (>90%) leads to a formation of a very thick and adherent chromium layer. Modifying the Cr(III)-based electrolyte is possible to plate coatings with different morphologies: soft but not microcracked (dull black), hard chromium (hardness > 700 HV, increased up to 1500 HV after thermal treatment) and very thin layer with a mirror appearance. The same research group demonstrated that the addition of LiCl into the bath promotes the formation of nanocrystalline crack-free black chromium deposit, suitable for decorative applications, with good corrosion resistance [30].

#### **4.3. Zinc**

Due to its peculiar features such as low cost and protection against corrosion, zinc has a paramount importance into the metal finishing industry. Because of Zn electrodeposition from aqueous solution is inexpensive and produces very good coatings, the study of electroplating from DESs is mostly used for Zn alloys production [1]. However, several studies have been done in order to understand the Zn deposition process using different DESs, in particular ChCl:urea [31] and ChCl:ethylene glycol [32]. Compared to aqueous electrodeposited Zn having a dendritic structure, those ones plated with DES shows very different morphologies. Using ChCl:ethylene glycol bath, deposited Zn shows very thin platelets with the planar face perpendicular to the electrode surface, according to progressive nucleation mechanism. In case of ChCl:urea, zinc coating has a rice-grain morphology, consistent with a rapid nucleation mechanism. The effects induced by the addition of some chelating agents such as acetonitrile, ammonia and ethylenediamine on the electrochemical process were studied [31]: these additives act as brightener but at the same time affect the electrodeposition process and lead to the formation of macro-crystalline deposit like in water. This phenomenon can be ascribed to the adsorption inhibition of chloride on the electrode surface induced by the presence of ammonia and ethylenediamine. Because of the addition of complexing agent to ChCl:urea and ChCl:ethyleneglycole baths does not change the metal species present in the baths ([ZnCl4] 2-), it is possible to deduce that the morphology variation after their addition is caused by the chemical processes taking place in the diffusion layer or on the cathodic surface rather than in the bulk [16, 33]. Another important study was performed by Bakkar et al., in which the electrodeposition of Zn using ChCl based electrolytes on magnesium substrates [34]. Mg and its alloys are difficult to be electroplated and so cannot be used as substrate in aqueous solution due to their water-sensitiveness, their tendency to form MgO and/or Mg(OH)2 film inhibiting adhesion of the electrodeposited coating and their high reactivity inducing formation of loose immersion layers on the surface by replacement that stops the following electrodeposition. Furthermore, magnesium suffers of microgalvanic deterioration if it is in contact with cathodic metals in a wet environment: this means that Mg is subjected to strong corrosion attack by aqueous electrolytes. Developing an electrodeposition process based on DESs could be an important turning point for a successful application of protective coatings on magnesium largely used in automobiles, aerospace and electronics industries. The authors have compared the efficiency of several ChCl-based DESs, in particular 1 ChCl:2 urea, 1 ChCl:2 ethylene glycol, 1 ChCl:1 malonic acid, 1 ChCl:2 glycerol and 1 ChCl:2 ZnCl2. From this study, it is possible to evince that Mg has the lowest corrosion rate in 1 ChCl:2 urea, the most feasible mixture for successful Zn electrodeposition, whereas the other mixtures produce powdery deposits or Mg corrosion. In particular, in case of 1 ChCl:2 glycerol pitting phenomena on Mg surface occur. The application of pulsed cathodic current during the electroplating process helps for the production of smooth, uniform and corrosion-resistant Zn coating, similar to that of pure zinc [34].

#### **4.4. Aluminium**

et al. [25] performing electrodeposition at ambient temperature and at 90°C using a ChCl:ethylene glycol bath on a brass foil. This work demonstrates that the surface roughness increases with temperature due to the formation of nanosheets with a thickness of 10–20 nm and grains size of about 10–50 nm. This phenomenon is related to a decrease of DES viscosity causing an increase of ion species mobility, enhancing in this way the deposition process and the nucleation of Ni. The obtained coating shows also low corrosion potential. Abbott et al. [22] have also studied the Ni electrodeposition starting from the ChCl:ethylene glycol DES bath, obtaining a dark grey deposit: the addition of ethylenediamine and acetylacetamide in this kind of electrolyte induces the suppression of the Ni underpotential deposition. Due to the growing industrial interest on composite materials, several studies have been done in order to develop electrodeposition process starting from DESs. Using Ni as matrix, compact Ni-multiwalled carbon nanotubes (MWCNTs) were deposited on copper substrate. In order to have a high efficient deposition process, a homogeneous dispersion of MWCNTs into the electrolyte has to be guaranteed. This has been obtained dispersing MWCNTs into ChCl:urea DES before the addition of soluble Ni chloride salt. Morphology, crystallinity and roughness of Ni coating

For several decades, hard chromium plating has been the most used metallic coating employed for several applications, especially to protect components operating in high wear and corrosion environment. This was due to its high hardness and its natural ability to inhibit corrosion. Due to its significant use in the industry, chromium electroplating based on Cr(VI) aqueous electrolyte is the highest optimized process. However, a series of issues, with most important the extremely negative environmental impact of the hard chromium plating process, due to the use of the carcinogenic hexavalent chromium, has led to a number of directives and legislation related to the restriction of this method [27]. This favours the necessity of finding less hazardous method replacing the present one. Up to now, some Cr(III) processes are on the market, but the obtained results are not repeatable. As demonstrated by recent scientific studies, the Cr electrodeposition from DESs could be the way to achieve metallic coatings having same properties of Cr(VI) ones. In particular, Abbott et al. have reported DESs obtained mixing ChCl with trivalent chromium chloride salts [28, 29]: in this electrolyte, electrodeposition process with a very high current efficiency (>90%) leads to a formation of a very thick and adherent chromium layer. Modifying the Cr(III)-based electrolyte is possible to plate coatings with different morphologies: soft but not microcracked (dull black), hard chromium (hardness > 700 HV, increased up to 1500 HV after thermal treatment) and very thin layer with a mirror appearance. The same research group demonstrated that the addition of LiCl into the bath promotes the formation of nanocrystalline crack-free black chromium deposit, suitable

Due to its peculiar features such as low cost and protection against corrosion, zinc has a paramount importance into the metal finishing industry. Because of Zn electrodeposition from

were affected by the presence of MWCNTs [26].

for decorative applications, with good corrosion resistance [30].

**4.2. Chromium**

244 Progress and Developments in Ionic Liquids

**4.3. Zinc**

Aluminium is one of the metals that cannot be plated in aqueous media because of its Nernst potential, well below the water decomposition one. Moreover, the high stability of aluminium oxide makes it high resistant to corrosion, but unfortunately this means that it cannot be electroplated from aqueous electrolytes [21]. In any case, aluminium electrodeposition is an important technological target due to its application in electronic, energy storage and anticorrosion fields. During the last years, many efforts have been put on aluminium electrodeposition from pure ionic liquids (ILs). However, the high hygroscopic nature of AlCl3-based ILs obligates to prepare and handle them under inert gas atmosphere, delaying progresses in this kind of process [35]. Several kinds of electroplating processes for Al deposition have been developed starting from DESs, both I type and IV type. Although the anodic reaction of this process is still low and the deposition rate has to be increased, the simple addition of acetamide to AlCl3 allows obtaining an electrolyte suitable for Al electrodeposition, relatively insensitive to water. Electrolytes characterization has shown that both cationic and anionic aluminium species, namely [AlCl2] + and [AlCl4] - , are present [1].

glycol DES, the Ag coating morphology is not affected by the particles presence. The most important point is that the hardness of the Ag coating from DESs after the incorporation of the particles is higher with respect to the same deposit obtained from aqueous electrolytes. Moreover, this value is not dependent on the reinforcing particles size. Al2O3 reinforced coatings show a low friction coefficient. Because of the lithium salts modify the mechanism of metal nucleation, LiF was added during the Ag deposition, inducing higher hardness leaving the structure unchanged [41, 42]. Magnesium metal can be plated using dimethylformamide and magnesium chloride hexahydrate-based DES. Cyclic voltammetry analyses indicate that the deposition of this metal is irreversible whereas from X-ray diffraction analyses, it is possible to evince that the plated coatings are Mg2Cu and MgO, revealing that an alloy with Cu substrate is formed [43]. Rahman et al. have studied indium electrodeposition starting from 1ChCl: 2urea DES at which indium sulphate is added [44]. The electrodeposition was performed on Au and Mo. To determine the type of nucleation, the current-time transients were normalized and compared to the theoretical dimensionless current-time transients obtained from Scharifker-Hills model. The work reveals that indium electrodeposition on molybdenum electrode proceeds via instantaneous nucleation with diffusion-controlled growth, whereas on the Au electrode, the deposition proceeds via progressive nucleation. The surface roughness decreases

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A significant amount of scientific work is present on the electrodeposition of pure metals from DESs. The interest in such systems is even greater considering the electrodeposition of alloys; the use of aprotic liquids (e.g. DES) avoids the limited potential window and hydrogen evolution issues of aqueous solutions [1, 45]. Thus, the number of possible metallic elements combination is largely increased. The study on systems already affirmed in the galvanic industry increases the amount of useful information on DES without claiming new possible industrial solution. On the contrary, for example, the possibility to deposit alloys containing rare earth elements with relatively ease may represent a unique and valid solution also outside

The demand of corrosion resistant coating is one of the highest in the surface finishing market, consequently great efforts are put in finding new solutions to improve nowadays processes. DES may avoid problems related to bath toxicity of existing process or for improving process efficiency [1, 43]. One example is the electrodeposition of Zn alloy, largely employed in the galvanic industry, for example ZnNi, ZnFe, ZnMn. The main advantage over the aqueous solutions is related to the reduction of hydrogen evolution, causing substrate embrittlement and poor deposit quality [45]. In addition, the relatively high redox potential difference between Zn and the correspondent alloying element complicates the deposition process, and anomalous deposition is observed. Employing DES system, chlorometallate ions are formed in the solution shifting the redox potential; higher compositional control of the deposits is

at higher current density [44].

academics [1, 45].

**5.1. Corrosion resistant alloys**

**5. Electrodeposition of alloys from DESs**

#### **4.5. Copper**

Since Cu electrodeposition is very common in surface finishing industry, many studies involving DESs copper plating have been done. In particular, Popescu et al. have studied Cu electrodeposition from ChCl combined with urea, malonic acid, oxalic acid end ethylene glycol, using as CuCl as copper source [36]. From those studies, it is possible to evince that the better deposit can be obtained from ChCl:oxalic acid and ChCl:ethylene glycol: the coating was fine, homogeneous and adherent. Furthermore, Abbott et al. have produced different kind of Cu deposits depending on Cu concentration into ChCl:ethylene glycol DES: bright nanostructured deposit corresponding to progressive nucleation of Cu was obtained for concentrations 0.01–0.1 M; for concentration lower than 0.01 M, black deposit was formed corresponding to a spontaneous nucleation of Cu [37]. Using chronoamperommetry, impedance spectroscopy and cyclic voltammetry (CV) analyses into ChCl:ethylene glycol DES, Murtomaki et al. have studied the electron transfer kinetics of Cu+ /Cu2+: the reaction was found to be quasi reversible and the [38]. In some cases electrodeposition from DESs not containing chloride ions can be advantageous. This is valid in general when a metallic layer is deposited on thin layers of metals sensitive to chloride corrosion, like nickel or iron. In the case of copper, deposition from a chloride free DES was studied for instance by Bernasconi et al [39] using a choline chloride dihydrogencitrate:ethylene glycol mixture with anhydrous copper sulfate as metal source. Abbott et al. have studied the electrodeposition of composite Cu coating with SiC and Al2O3. The addition of these particles does not affect the morphology and the size of Cu particles. The amount of particles dispersed into the DESs is directly related to the composition of the composite materials [37].

#### **4.6. Silver and other metals**

Silver electrodeposition from ChCl:ethylene glycol DES was performed using Cu as substrate: Ag not only oxidizes the copper substrate, but it is deposited on the electrode surface on the same time, forming a very shiny nanocrystalline coating. Gomez et al. have evaluated the chloride anion function during the electrodeposition process in 2 urea: choline chloride DES by means of voltammetric and chronoamperometric analyses [40]. The work reveals that Ag deposition takes place as 3D nucleation and growth process under diffusion control. The best hypothesis is that chloride induces the formation of rounded grains at short deposition times hindering uncontrolled directional growth. Adding SiC and Al2O3 particles to ChCl:ethylene glycol DES, the Ag coating morphology is not affected by the particles presence. The most important point is that the hardness of the Ag coating from DESs after the incorporation of the particles is higher with respect to the same deposit obtained from aqueous electrolytes. Moreover, this value is not dependent on the reinforcing particles size. Al2O3 reinforced coatings show a low friction coefficient. Because of the lithium salts modify the mechanism of metal nucleation, LiF was added during the Ag deposition, inducing higher hardness leaving the structure unchanged [41, 42]. Magnesium metal can be plated using dimethylformamide and magnesium chloride hexahydrate-based DES. Cyclic voltammetry analyses indicate that the deposition of this metal is irreversible whereas from X-ray diffraction analyses, it is possible to evince that the plated coatings are Mg2Cu and MgO, revealing that an alloy with Cu substrate is formed [43]. Rahman et al. have studied indium electrodeposition starting from 1ChCl: 2urea DES at which indium sulphate is added [44]. The electrodeposition was performed on Au and Mo. To determine the type of nucleation, the current-time transients were normalized and compared to the theoretical dimensionless current-time transients obtained from Scharifker-Hills model. The work reveals that indium electrodeposition on molybdenum electrode proceeds via instantaneous nucleation with diffusion-controlled growth, whereas on the Au electrode, the deposition proceeds via progressive nucleation. The surface roughness decreases at higher current density [44].

### **5. Electrodeposition of alloys from DESs**

A significant amount of scientific work is present on the electrodeposition of pure metals from DESs. The interest in such systems is even greater considering the electrodeposition of alloys; the use of aprotic liquids (e.g. DES) avoids the limited potential window and hydrogen evolution issues of aqueous solutions [1, 45]. Thus, the number of possible metallic elements combination is largely increased. The study on systems already affirmed in the galvanic industry increases the amount of useful information on DES without claiming new possible industrial solution. On the contrary, for example, the possibility to deposit alloys containing rare earth elements with relatively ease may represent a unique and valid solution also outside academics [1, 45].

#### **5.1. Corrosion resistant alloys**

corrosion fields. During the last years, many efforts have been put on aluminium electrodeposition from pure ionic liquids (ILs). However, the high hygroscopic nature of AlCl3-based ILs obligates to prepare and handle them under inert gas atmosphere, delaying progresses in this kind of process [35]. Several kinds of electroplating processes for Al deposition have been developed starting from DESs, both I type and IV type. Although the anodic reaction of this process is still low and the deposition rate has to be increased, the simple addition of acetamide to AlCl3 allows obtaining an electrolyte suitable for Al electrodeposition, relatively insensitive to water. Electrolytes characterization has shown that both cationic and anionic aluminium

, are present [1].

Since Cu electrodeposition is very common in surface finishing industry, many studies involving DESs copper plating have been done. In particular, Popescu et al. have studied Cu electrodeposition from ChCl combined with urea, malonic acid, oxalic acid end ethylene glycol, using as CuCl as copper source [36]. From those studies, it is possible to evince that the better deposit can be obtained from ChCl:oxalic acid and ChCl:ethylene glycol: the coating was fine, homogeneous and adherent. Furthermore, Abbott et al. have produced different kind of Cu deposits depending on Cu concentration into ChCl:ethylene glycol DES: bright nanostructured deposit corresponding to progressive nucleation of Cu was obtained for concentrations 0.01–0.1 M; for concentration lower than 0.01 M, black deposit was formed corresponding to a spontaneous nucleation of Cu [37]. Using chronoamperommetry, impedance spectroscopy and cyclic voltammetry (CV) analyses into ChCl:ethylene glycol DES,

to be quasi reversible and the [38]. In some cases electrodeposition from DESs not containing chloride ions can be advantageous. This is valid in general when a metallic layer is deposited on thin layers of metals sensitive to chloride corrosion, like nickel or iron. In the case of copper, deposition from a chloride free DES was studied for instance by Bernasconi et al [39] using a choline chloride dihydrogencitrate:ethylene glycol mixture with anhydrous copper sulfate as metal source. Abbott et al. have studied the electrodeposition of composite Cu coating with SiC and Al2O3. The addition of these particles does not affect the morphology and the size of Cu particles. The amount of particles dispersed into the DESs is directly related to the com-

Silver electrodeposition from ChCl:ethylene glycol DES was performed using Cu as substrate: Ag not only oxidizes the copper substrate, but it is deposited on the electrode surface on the same time, forming a very shiny nanocrystalline coating. Gomez et al. have evaluated the chloride anion function during the electrodeposition process in 2 urea: choline chloride DES by means of voltammetric and chronoamperometric analyses [40]. The work reveals that Ag deposition takes place as 3D nucleation and growth process under diffusion control. The best hypothesis is that chloride induces the formation of rounded grains at short deposition times hindering uncontrolled directional growth. Adding SiC and Al2O3 particles to ChCl:ethylene

/Cu2+: the reaction was found

species, namely [AlCl2]

246 Progress and Developments in Ionic Liquids

**4.5. Copper**

+

and [AlCl4]

Murtomaki et al. have studied the electron transfer kinetics of Cu+

position of the composite materials [37].

**4.6. Silver and other metals**


The demand of corrosion resistant coating is one of the highest in the surface finishing market, consequently great efforts are put in finding new solutions to improve nowadays processes. DES may avoid problems related to bath toxicity of existing process or for improving process efficiency [1, 43]. One example is the electrodeposition of Zn alloy, largely employed in the galvanic industry, for example ZnNi, ZnFe, ZnMn. The main advantage over the aqueous solutions is related to the reduction of hydrogen evolution, causing substrate embrittlement and poor deposit quality [45]. In addition, the relatively high redox potential difference between Zn and the correspondent alloying element complicates the deposition process, and anomalous deposition is observed. Employing DES system, chlorometallate ions are formed in the solution shifting the redox potential; higher compositional control of the deposits is obtained. Fashu et al. show a significant dependence of composition and surface morphology of ZnNi deposits, obtained from choline chloride-urea solution, on electrolyte concentration, temperature and voltage applied; alloy composition can thus be tuned significantly with different process parameters [46]. Abbot et al. reported the electrodeposition of ZnSn alloys, candidates for replacing Cd, from ethylene glycol-based solution; ChCl:urea and ChCl:ethylene glycol DESs containing 0.5 M ZnCl2:0.05 M SnCl2 are studied in terms of deposit composition and morphology [47]. In view of corrosion resistance coatings, different alloys have been reported in literature.

**5.3. Semiconductors and photovoltaic alloys**

the same DES solution [60].

[54].

**5.4. Alloys for electrocatalysis**

With the emerging field of thin films solar cells, electrodeposition process started to gain importance also in the photovoltaic field. Unlike common vacuum techniques employed in the semiconductors industries, electrodeposition is a low-temperature atmospheric process allowing the fabrication of small features with high precision and control [57]. Among the active materials, CdTe is the most extensively studied chalcogenide material in the field of electrodeposition; the process is affirmed, and efforts are paid to bring the process to high volume production. On the other hand, Cu(In,Ga)Se2 (CIGS) is the material having the best absorbing properties [56]. Studies on aqueous solution show the difficulties in controlling the composition, for example non-linearity between the metal concentration in solution and in the deposit; moreover, the process is characterized by very low faradaic efficiency (5%) due to Ga low reduction potential [58]. Malaquias et al. proposed a Mo/Cu/InGa metal stack subsequently selenized to obtain CIGS [58]; InGa deposition has been successfully carried out from1ChCl:1Urea system [59]. Steichen et al. proposed a similar route for the fabrication of CuGaSe2(CGS) active material; controlled electrodeposition of CuGa alloy is proposed using

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Electrocatalytic processes are largely employed in the industry in different area of interest such as hydrogen production, energy conversion/production, electroplating. The employment of a catalyst is in fact a fundamental pillar of all the electrochemical processes. For example, nickel alloys have assumed an important role in water electrolysis process, both for the hydrogen and oxygen evolution reaction [61]. Vijayakumar et al. successfully deposited NiCoSn alloy from 1ChCl:2EG system and evaluated the improvement with respect to NiSn and CoSn alloys, obtained from the same solution, in terms of catalytic activity in 1 M KOH alkaline solution [59]. PtCo alloys are characterized by high electrocatalytic activities as well, and Guillamat et al. successfully electrodeposited alloys with different composition between 54 and 90% wt. Pt

A series of concerns must be considered when evaluating the realistic applicability of DESs electrodeposition in the industry. The main driving force for the development of new surface finishing processes is of course the need to improve existing industrial processes or to introduce brand new ones. For this reason, also electrodeposition from deep eutectic solvents must open new possibilities with respect to the current state of the art in order to be implemented as an industrial finishing process. Almost all the plating technologies used nowadays in the galvanic field use aqueous-based solutions. Each new DES-based process is therefore in competition with its homologous water-based counterpart, and possible advantages and disadvantages of its substitution must be carefully weighted. Aqueous solutions are widely used, the know-how for their employment is well established and in most cases their cost is

**6. Industrial applicability and implemented processes**

You et al. carried out a study on NiCo alloys from ChCl:ethylene glycol; Ni-rich alloys with 4– 40 wt.% Co showed an important improvement with respect to pure Ni deposit in the corrosion potential [48]. In a similar DES system, Saravanan et al. replace Ni with Cr evaluating both microstructure and potentiodynamic polarization behaviour in the composition interval 65– 81 wt.% Co of CoCr deposits [49]. The same research group obtained NiFeCr deposits containing approximately 53–61% Fe, 34–41% Ni and 4–15% Cr from ChCl:ethylene glycol [50]. Zhang et al. successfully deposited CrP coating from a Cr(III) solution by adding ammonium hypophosphite (NH4H2PO2) to a ChCl:ethylene glycol deep eutectic solvent containing CoCl2 salts [51]. Employing the same system, You et al. firstly deposited NiP alloy coatings at room temperature; deposits with 8 wt.% P have been evaluated from the corrosion point of view [48].

#### **5.2. Magnetic alloys**

With progressive improvements and miniaturization in the microelectronic industries, the ability to obtain functional thin films is fundamental. Magnetic alloys have assumed a crucial role in a huge number of microelectromechanical system (MEMS), for example actuators, sensors [52]. In addition, the huge potential in biomedical applications of wirelessly controlled microrobots has attracted the attention of many researchers [53]. Nowadays, electrodeposition from aqueous solution is largely employed for the fabrication of magnetic films, for example FeCo, FeCoNi and CoPt, while few studies are present employing DES solvent. In the field of MEMS application, Guillamat et al. successfully obtain hard-magnetic CoPt without the need of subsequent heat treatments [54], avoiding a critical fabrication step for microelectromechanical system with layered and complex architecture. Yanai et al. showed the suitability of ChCl:ethylene glycol-based DES for FeNi alloys where deposits composition has shown to be easily controlled varying the reagent in the bath [55].

On the other hand, complex systems are commercially available for high performances magnets: SmCo, AlNiCo and NeFeB alloys. The correspondent metallic products are mainly fabricated through sintering or casting; the presence of rare earth metals or elements having high negative reduction potential limits the suitability of electrodeposition processes. Gomez, Cojocaru and co-workers firstly succeeded in the electrodeposition of SmCo alloys employing a choline chloride–urea solution [40, 56]; the relative ease, low cost and precision of this process may represent an alternative to the metallurgical ones.

#### **5.3. Semiconductors and photovoltaic alloys**

obtained. Fashu et al. show a significant dependence of composition and surface morphology of ZnNi deposits, obtained from choline chloride-urea solution, on electrolyte concentration, temperature and voltage applied; alloy composition can thus be tuned significantly with different process parameters [46]. Abbot et al. reported the electrodeposition of ZnSn alloys, candidates for replacing Cd, from ethylene glycol-based solution; ChCl:urea and ChCl:ethylene glycol DESs containing 0.5 M ZnCl2:0.05 M SnCl2 are studied in terms of deposit composition and morphology [47]. In view of corrosion resistance coatings, different alloys have been

You et al. carried out a study on NiCo alloys from ChCl:ethylene glycol; Ni-rich alloys with 4– 40 wt.% Co showed an important improvement with respect to pure Ni deposit in the corrosion potential [48]. In a similar DES system, Saravanan et al. replace Ni with Cr evaluating both microstructure and potentiodynamic polarization behaviour in the composition interval 65– 81 wt.% Co of CoCr deposits [49]. The same research group obtained NiFeCr deposits containing approximately 53–61% Fe, 34–41% Ni and 4–15% Cr from ChCl:ethylene glycol [50]. Zhang et al. successfully deposited CrP coating from a Cr(III) solution by adding ammonium hypophosphite (NH4H2PO2) to a ChCl:ethylene glycol deep eutectic solvent containing CoCl2 salts [51]. Employing the same system, You et al. firstly deposited NiP alloy coatings at room temperature; deposits with 8 wt.% P have been evaluated from the corrosion point of view [48].

With progressive improvements and miniaturization in the microelectronic industries, the ability to obtain functional thin films is fundamental. Magnetic alloys have assumed a crucial role in a huge number of microelectromechanical system (MEMS), for example actuators, sensors [52]. In addition, the huge potential in biomedical applications of wirelessly controlled microrobots has attracted the attention of many researchers [53]. Nowadays, electrodeposition from aqueous solution is largely employed for the fabrication of magnetic films, for example FeCo, FeCoNi and CoPt, while few studies are present employing DES solvent. In the field of MEMS application, Guillamat et al. successfully obtain hard-magnetic CoPt without the need of subsequent heat treatments [54], avoiding a critical fabrication step for microelectromechanical system with layered and complex architecture. Yanai et al. showed the suitability of ChCl:ethylene glycol-based DES for FeNi alloys where deposits composition has shown to be

On the other hand, complex systems are commercially available for high performances magnets: SmCo, AlNiCo and NeFeB alloys. The correspondent metallic products are mainly fabricated through sintering or casting; the presence of rare earth metals or elements having high negative reduction potential limits the suitability of electrodeposition processes. Gomez, Cojocaru and co-workers firstly succeeded in the electrodeposition of SmCo alloys employing a choline chloride–urea solution [40, 56]; the relative ease, low cost and precision of this process

reported in literature.

248 Progress and Developments in Ionic Liquids

**5.2. Magnetic alloys**

easily controlled varying the reagent in the bath [55].

may represent an alternative to the metallurgical ones.

With the emerging field of thin films solar cells, electrodeposition process started to gain importance also in the photovoltaic field. Unlike common vacuum techniques employed in the semiconductors industries, electrodeposition is a low-temperature atmospheric process allowing the fabrication of small features with high precision and control [57]. Among the active materials, CdTe is the most extensively studied chalcogenide material in the field of electrodeposition; the process is affirmed, and efforts are paid to bring the process to high volume production. On the other hand, Cu(In,Ga)Se2 (CIGS) is the material having the best absorbing properties [56]. Studies on aqueous solution show the difficulties in controlling the composition, for example non-linearity between the metal concentration in solution and in the deposit; moreover, the process is characterized by very low faradaic efficiency (5%) due to Ga low reduction potential [58]. Malaquias et al. proposed a Mo/Cu/InGa metal stack subsequently selenized to obtain CIGS [58]; InGa deposition has been successfully carried out from1ChCl:1Urea system [59]. Steichen et al. proposed a similar route for the fabrication of CuGaSe2(CGS) active material; controlled electrodeposition of CuGa alloy is proposed using the same DES solution [60].

#### **5.4. Alloys for electrocatalysis**

Electrocatalytic processes are largely employed in the industry in different area of interest such as hydrogen production, energy conversion/production, electroplating. The employment of a catalyst is in fact a fundamental pillar of all the electrochemical processes. For example, nickel alloys have assumed an important role in water electrolysis process, both for the hydrogen and oxygen evolution reaction [61]. Vijayakumar et al. successfully deposited NiCoSn alloy from 1ChCl:2EG system and evaluated the improvement with respect to NiSn and CoSn alloys, obtained from the same solution, in terms of catalytic activity in 1 M KOH alkaline solution [59]. PtCo alloys are characterized by high electrocatalytic activities as well, and Guillamat et al. successfully electrodeposited alloys with different composition between 54 and 90% wt. Pt [54].

### **6. Industrial applicability and implemented processes**

A series of concerns must be considered when evaluating the realistic applicability of DESs electrodeposition in the industry. The main driving force for the development of new surface finishing processes is of course the need to improve existing industrial processes or to introduce brand new ones. For this reason, also electrodeposition from deep eutectic solvents must open new possibilities with respect to the current state of the art in order to be implemented as an industrial finishing process. Almost all the plating technologies used nowadays in the galvanic field use aqueous-based solutions. Each new DES-based process is therefore in competition with its homologous water-based counterpart, and possible advantages and disadvantages of its substitution must be carefully weighted. Aqueous solutions are widely used, the know-how for their employment is well established and in most cases their cost is

highly competitive. For this reason, in order to be accepted as realistic alternatives for waterbased treatments, DESs must present significant advantages. This part of the chapter is intended to be an analysis of the realistic possibility to see large-scale applications for DESbased electroplating in the next few decades, as anticipated by some existing reviews [35]. Some pilot projects, already used on smaller scales, are presented as well to give a perspective and to demonstrate that, if the application is well calibrated, DESs can find application in industry.

A notable case in which it may be preferable to use DESs on an easy to plate substrate like steel is zinc deposition. Zinc is efficiently deposited from aqueous solutions, but, in this case, a considerable amount of the current supplied is lost for hydrogen evolution. Besides the low cathodic efficiency, the hydrogen produced can cause many problems to high-strength steels, inducing an embrittlement of the material [45, 69]. It can be thus beneficial to use DES-based

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electrolytes to plate zinc due to their low hydrogen evolution.

**Figure 4.** A chrome plating pilot plant part of the IONMET consortium activities [80].

may include Ag [73] and Pd [74].

Another possible application is the electrodeposition of some metals that are difficult or costly to electrodeposit from aqueous solutions. These may include highly reactive metals like samarium [40, 56] or inert metals such as indium [44]. In addition, the electrodeposition of metals which reduction potential falls outside the electrochemical window of water, like Al [70], can be in principle achieved. Their plating however must be performed in inert atmosphere with anhydrous solutions and a coating quality lower than imidazolium-based ionic liquids is in general achieved [71, 72]. For this reason, DESs do not offer real advantages with respect to other ionic liquids. Silver and other precious metals, characterized by high reduction potentials, can present some advantages when electrodeposited from DESs. Typically, these metals require strongly complexed electrolytes or the presence of intermediate plating steps to avoid undesired displacement deposition (resulting in poor adhesion) [62]. DESs strongly complex metallic ions, and for this reason, their use can result in good quality layers. Examples

A particular case is represented by chromium plating. Functional chromium coatings are usually obtained from Cr(VI) containing solutions, but Cr(VI) is a recognized carcinogenic and environmentally hazardous chemical [75]. For this reason, it will be progressively banned from the industrial practice. EU is currently eliminating Cr(VI) as a chromium source in the case of decorative coatings, since Cr(III)-based aqueous solutions constitute a good alternative [76].

#### **6.1. Industrial applicability of metal deposition from DESs**

As reported previously in this chapter, in principle DESs can be used to electrodeposit the main commodities metals: copper, nickel and zinc. From the practical point of view, however, their application to the deposition of coatings on most of the metals used in the industry (iron, copper, brass, nickel and their alloys) to produce functional parts is not realistic. These substrates are in general easy to plate after suitable pretreatments, and a wide variety of aqueous-based solutions is available for the most common metals [20]. A good example is copper, which can be deposited from alkaline, nearly neutral or acidic solutions reaching high thicknesses, excellent surface finishing and cathodic efficiencies [62]. The same is true also for nickel or zinc [62]. Moreover, deposits from DES-based solutions present in general a lower quality with respect to the ones from aqueous solutions. This is not always true, as Abbott et al. demonstrated in the case of nickel [63]. In this case, however the comparison is done between non-additive aqueous nickel baths and a choline chloride-based DES, while in industrial practice, solutions are most of the times modified with suitable additives to improve properties of the deposit (hardness, …) or surface finishing.

There are however some applications where plating commodities metals from a non-aqueous solution are preferable, like in the case of substrates that are not easy to metalize (e.g. aluminium and magnesium). These metals are widely used, in general as alloys, for their low weight and good mechanical properties. The application of metallic layers on their surface is not trivial, since they tend to quickly passivate (or corrode, according to the pH of the electrolyte) upon exposure to the plating solution [64–66]. As already previously exposed, Mg alloys are in particular the most difficult due to their tendency to form MgO and/or Mg(OH)2 film inhibiting adhesion of the electrodeposited coating and their high reactivity inducing formation of loose immersion layers on the surface by replacement that stop the successive electrodeposition. Furthermore, Mg is subjected to strong corrosion attack by aqueous electrolytes. The use of DES-based electrolytes can allow the deposition on aluminium or magnesium without specific pretreatments. This was demonstrated by Abbott et al. [22], Florea et al. [67] and Bernasconi et al. [68] in the case of nickel on aluminum and by Bakkar et al. [34] in the case of zinc on magnesium. Compact metallic layers can be applied on Al 1100 alloys and Mg-RE alloys by using a 1:2 mixture of choline chloride and urea additivated with metal ions and, in the case of nickel, a suitable complexing agent (ethylenediamine). The resulting coatings present good adhesion and corrosion behaviour. Other metals, such as chromium, can be plated on the resulting nickel if it is used as adhesion layer, producing thus multilayers [68].

A notable case in which it may be preferable to use DESs on an easy to plate substrate like steel is zinc deposition. Zinc is efficiently deposited from aqueous solutions, but, in this case, a considerable amount of the current supplied is lost for hydrogen evolution. Besides the low cathodic efficiency, the hydrogen produced can cause many problems to high-strength steels, inducing an embrittlement of the material [45, 69]. It can be thus beneficial to use DES-based electrolytes to plate zinc due to their low hydrogen evolution.

**Figure 4.** A chrome plating pilot plant part of the IONMET consortium activities [80].

highly competitive. For this reason, in order to be accepted as realistic alternatives for waterbased treatments, DESs must present significant advantages. This part of the chapter is intended to be an analysis of the realistic possibility to see large-scale applications for DESbased electroplating in the next few decades, as anticipated by some existing reviews [35]. Some pilot projects, already used on smaller scales, are presented as well to give a perspective and to demonstrate that, if the application is well calibrated, DESs can find application in

As reported previously in this chapter, in principle DESs can be used to electrodeposit the main commodities metals: copper, nickel and zinc. From the practical point of view, however, their application to the deposition of coatings on most of the metals used in the industry (iron, copper, brass, nickel and their alloys) to produce functional parts is not realistic. These substrates are in general easy to plate after suitable pretreatments, and a wide variety of aqueous-based solutions is available for the most common metals [20]. A good example is copper, which can be deposited from alkaline, nearly neutral or acidic solutions reaching high thicknesses, excellent surface finishing and cathodic efficiencies [62]. The same is true also for nickel or zinc [62]. Moreover, deposits from DES-based solutions present in general a lower quality with respect to the ones from aqueous solutions. This is not always true, as Abbott et al. demonstrated in the case of nickel [63]. In this case, however the comparison is done between non-additive aqueous nickel baths and a choline chloride-based DES, while in industrial practice, solutions are most of the times modified with suitable additives to improve properties

There are however some applications where plating commodities metals from a non-aqueous solution are preferable, like in the case of substrates that are not easy to metalize (e.g. aluminium and magnesium). These metals are widely used, in general as alloys, for their low weight and good mechanical properties. The application of metallic layers on their surface is not trivial, since they tend to quickly passivate (or corrode, according to the pH of the electrolyte) upon exposure to the plating solution [64–66]. As already previously exposed, Mg alloys are in particular the most difficult due to their tendency to form MgO and/or Mg(OH)2 film inhibiting adhesion of the electrodeposited coating and their high reactivity inducing formation of loose immersion layers on the surface by replacement that stop the successive electrodeposition. Furthermore, Mg is subjected to strong corrosion attack by aqueous electrolytes. The use of DES-based electrolytes can allow the deposition on aluminium or magnesium without specific pretreatments. This was demonstrated by Abbott et al. [22], Florea et al. [67] and Bernasconi et al. [68] in the case of nickel on aluminum and by Bakkar et al. [34] in the case of zinc on magnesium. Compact metallic layers can be applied on Al 1100 alloys and Mg-RE alloys by using a 1:2 mixture of choline chloride and urea additivated with metal ions and, in the case of nickel, a suitable complexing agent (ethylenediamine). The resulting coatings present good adhesion and corrosion behaviour. Other metals, such as chromium, can be plated on the resulting nickel if it is used as adhesion layer, producing thus multilayers [68].

**6.1. Industrial applicability of metal deposition from DESs**

of the deposit (hardness, …) or surface finishing.

industry.

250 Progress and Developments in Ionic Liquids

Another possible application is the electrodeposition of some metals that are difficult or costly to electrodeposit from aqueous solutions. These may include highly reactive metals like samarium [40, 56] or inert metals such as indium [44]. In addition, the electrodeposition of metals which reduction potential falls outside the electrochemical window of water, like Al [70], can be in principle achieved. Their plating however must be performed in inert atmosphere with anhydrous solutions and a coating quality lower than imidazolium-based ionic liquids is in general achieved [71, 72]. For this reason, DESs do not offer real advantages with respect to other ionic liquids. Silver and other precious metals, characterized by high reduction potentials, can present some advantages when electrodeposited from DESs. Typically, these metals require strongly complexed electrolytes or the presence of intermediate plating steps to avoid undesired displacement deposition (resulting in poor adhesion) [62]. DESs strongly complex metallic ions, and for this reason, their use can result in good quality layers. Examples may include Ag [73] and Pd [74].

A particular case is represented by chromium plating. Functional chromium coatings are usually obtained from Cr(VI) containing solutions, but Cr(VI) is a recognized carcinogenic and environmentally hazardous chemical [75]. For this reason, it will be progressively banned from the industrial practice. EU is currently eliminating Cr(VI) as a chromium source in the case of decorative coatings, since Cr(III)-based aqueous solutions constitute a good alternative [76]. This is unfortunately not true in the case of functional hard chromium and aqueous solutions are not able nowadays to provide properties comparable to those coming from Cr(VI)-based electrolytes. DES-based Cr(III) solutions on the contrary showed promising results. In particular, cracked and crack-free thick coatings have been obtained from CrCl3/choline chloride mixtures with cathodic current efficiencies higher than Cr(VI)-based electrolytes [29, 30, 45].

present however interesting properties, suitable for magnetic recording devices. The use of DESs for their electrochemical deposition can remove many obstacles with respect to aqueous plating. This was demonstrated by Gomez et al. [40, 56], which obtained high coercivity layers

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From the applicative point of view, for alloys the IONMET consortium is currently scaling up laboratory scale ZnSn deposition to perform barrel plate on industrial scale [78]. This is so far the only notable example of industrial application for alloys plating from DESs. Also in the case of alloys deposition, like in the case of pure metals, some patents for industrial processes

In this part are treated some processes not strictly related to electrodeposition but of some industrial interest. In the case of electroless and displacement deposition, few processes are available and almost none of them can be applied in current industrial practice. Some notable

Water-based solutions for immersion plating of PCB contacts are currently used for corrosion protection of electrical contact, but some critical issues have been reported from their use [35]. Novel DES-based processes have been developed in the case of silver application on copper [45, 94] or copper application on aluminium [78]. **Figure 5** represents a PCB treated with

**6.3. Industrial applicability of electroless and displacement deposition from DESs**

exceptions are the displacement deposition of some noble metals for PCB technology.

**Figure 5.** A PCB presenting copper tracks immersion plated with Ag from DESs [79].

ENIG-like processes have been developed as well, with gold deposited on nickel-plated contacts [94, 95]. The neutral nature of the DES can help avoiding the "black pads" effect,

from a choline chloride/urea DES.

immersion silver deposition.

are available: CrNi [91], CIGS [92] and AuSn [93].

Not many examples of plants using DES-based electrodeposition processes are available nowadays, and all of them are pilot or semi-industrial projects. The Abbott group [77] at theUniversity of Leicester is the most active in the industrialization of DES-based processes with its collaboration in the IONMET consortium [78], created under the 6th EU Framework Programme of Research. IONMET, in collaboration with Scionix [79], realized the ionic liquid demonstrator (ILD), which is a multi-application pilot plant facility to showcase the application of ionic liquids. Industrial electroplating of chromium and nickel was demonstrated at ILD. The IONMET consortium itself is scaling up many laboratory processes [78, 80], the most interesting one being hard chromium plating from DESs. Scionix is the largest world manufacturer of ionic liquids for industrial applications and of DESs in particular (**Figure 4**) [79]. Some patents are available as well for general metal deposition [81] or for specific applications: Ag [82], Au [83], superhydrophobic Ni layers [84], Fe [85], Zn and Ni [86] and Ga [87].

Some patents are available as well for general metal deposition [78] or for specific applications: Ag [79], Au [80], superhydrophobic Ni layers [81], Fe [82], Zn and Ni [83] and Ga [84].

#### **6.2. Industrial applicability of alloys deposition from DESs**

The considerations exposed in the case of metals deposition can be adapted to the plating of alloys. In particular, many different alloys are difficult to electrodeposit from aqueous solutions. The reasons are mainly correlated to the properties of some metals, which present high reactivity or refractoriness in water.

ZnMn alloys are difficult to plate in aqueous mediums due to the low reduction potential of manganese and to its reactivity [88]. These alloys, useful for corrosion protection, can be however obtained from choline chloride/urea mixtures [89] with good coating properties. On the contrary, Mo-based alloys are difficult to obtain in a wide composition range due to the inertness of molybdenum. The use of DES to obtain these alloys, also in this case promising for corrosion protection, can extend the compositional range with respect to water solutions (like in the case of NiMo and CoMo [90]). Always in the field of Zn-based materials, ZnSn is a particular case of alloy that can be deposited in water but present some advantages when plated in DESs. This material, a good option to replace cadmium in anticorrosion coatings, is plated from aqueous solutions [47], but composition control is easier when the deposition is performed in strongly complexing electrolytes like DESs [47]. Plating with DESs also avoids excessive hydrogen evolution, which may result in embrittlement of the coating. Similar considerations can be done for ZnNi and ZnFe.

Due to their negative standard reduction potential and high reactivity, almost all the rare earthbased alloys are not easy to plate in water-based solutions. Many of these alloys, like SmCo, present however interesting properties, suitable for magnetic recording devices. The use of DESs for their electrochemical deposition can remove many obstacles with respect to aqueous plating. This was demonstrated by Gomez et al. [40, 56], which obtained high coercivity layers from a choline chloride/urea DES.

This is unfortunately not true in the case of functional hard chromium and aqueous solutions are not able nowadays to provide properties comparable to those coming from Cr(VI)-based electrolytes. DES-based Cr(III) solutions on the contrary showed promising results. In particular, cracked and crack-free thick coatings have been obtained from CrCl3/choline chloride mixtures with cathodic current efficiencies higher than Cr(VI)-based electrolytes [29,

Not many examples of plants using DES-based electrodeposition processes are available nowadays, and all of them are pilot or semi-industrial projects. The Abbott group [77] at theUniversity of Leicester is the most active in the industrialization of DES-based processes with its collaboration in the IONMET consortium [78], created under the 6th EU Framework Programme of Research. IONMET, in collaboration with Scionix [79], realized the ionic liquid demonstrator (ILD), which is a multi-application pilot plant facility to showcase the application of ionic liquids. Industrial electroplating of chromium and nickel was demonstrated at ILD. The IONMET consortium itself is scaling up many laboratory processes [78, 80], the most interesting one being hard chromium plating from DESs. Scionix is the largest world manufacturer of ionic liquids for industrial applications and of DESs in particular (**Figure 4**) [79]. Some patents are available as well for general metal deposition [81] or for specific applications: Ag [82], Au [83], superhydrophobic Ni layers [84], Fe [85], Zn and Ni [86] and Ga [87].

Some patents are available as well for general metal deposition [78] or for specific applications: Ag [79], Au [80], superhydrophobic Ni layers [81], Fe [82], Zn and Ni [83] and Ga [84].

The considerations exposed in the case of metals deposition can be adapted to the plating of alloys. In particular, many different alloys are difficult to electrodeposit from aqueous solutions. The reasons are mainly correlated to the properties of some metals, which present

ZnMn alloys are difficult to plate in aqueous mediums due to the low reduction potential of manganese and to its reactivity [88]. These alloys, useful for corrosion protection, can be however obtained from choline chloride/urea mixtures [89] with good coating properties. On the contrary, Mo-based alloys are difficult to obtain in a wide composition range due to the inertness of molybdenum. The use of DES to obtain these alloys, also in this case promising for corrosion protection, can extend the compositional range with respect to water solutions (like in the case of NiMo and CoMo [90]). Always in the field of Zn-based materials, ZnSn is a particular case of alloy that can be deposited in water but present some advantages when plated in DESs. This material, a good option to replace cadmium in anticorrosion coatings, is plated from aqueous solutions [47], but composition control is easier when the deposition is performed in strongly complexing electrolytes like DESs [47]. Plating with DESs also avoids excessive hydrogen evolution, which may result in embrittlement of the coating. Similar

Due to their negative standard reduction potential and high reactivity, almost all the rare earthbased alloys are not easy to plate in water-based solutions. Many of these alloys, like SmCo,

**6.2. Industrial applicability of alloys deposition from DESs**

high reactivity or refractoriness in water.

considerations can be done for ZnNi and ZnFe.

30, 45].

252 Progress and Developments in Ionic Liquids

From the applicative point of view, for alloys the IONMET consortium is currently scaling up laboratory scale ZnSn deposition to perform barrel plate on industrial scale [78]. This is so far the only notable example of industrial application for alloys plating from DESs. Also in the case of alloys deposition, like in the case of pure metals, some patents for industrial processes are available: CrNi [91], CIGS [92] and AuSn [93].

### **6.3. Industrial applicability of electroless and displacement deposition from DESs**

In this part are treated some processes not strictly related to electrodeposition but of some industrial interest. In the case of electroless and displacement deposition, few processes are available and almost none of them can be applied in current industrial practice. Some notable exceptions are the displacement deposition of some noble metals for PCB technology.

Water-based solutions for immersion plating of PCB contacts are currently used for corrosion protection of electrical contact, but some critical issues have been reported from their use [35]. Novel DES-based processes have been developed in the case of silver application on copper [45, 94] or copper application on aluminium [78]. **Figure 5** represents a PCB treated with immersion silver deposition.

**Figure 5.** A PCB presenting copper tracks immersion plated with Ag from DESs [79].

ENIG-like processes have been developed as well, with gold deposited on nickel-plated contacts [94, 95]. The neutral nature of the DES can help avoiding the "black pads" effect, induced by an excessive corrosion of Ni in the Au electrolyte. This advantage can increase the industrial attractiveness of such processes.

[6] Abbott AP, Boothby D, Capper G, Davies DL, Rasheed RK. Deep eutectic solvents formed between choline chloride and carboxylic acids: versatile alternatives to ionic

Electrodeposition from Deep Eutectic Solvents

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[7] Abbott AP, Al-Barzinjy AA, Abbott PD, Frisch G, Harris RC, Hartley J, et al. Speciation, physical and electrolytic properties of eutectic mixtures based on CrCl 3 6H 2 O and

[8] Abbott AP, Capper G, Davies DL, Rasheed R. Ionic liquids based upon metal halide/ substituted quaternary ammonium salt mixtures. Inorg Chem 2004;43(11):3447–3452.

[9] Florindo C, Oliveira F, Rebelo L, Fernandes AM, Marrucho I. Insights into the synthesis and properties of deep eutectic solvents based on cholinium chloride and carboxylic

[10] Su WC, Wong DSH, Li MH. Effect of water on solubility of carbon dioxide in (aminomethanamide 2-hydroxy-N, N, N-trimethylethanaminium chloride). Journal of

[11] Aparicio S, Atilhan M. Water effect on CO 2 absorption for hydroxylammonium based ionic liquids: A molecular dynamics study. Chem Phys 2012;400:118–125.

[12] Emi T, Bockris JO. Semiempirical calculation of 3.7 RTm term in the heat of activation

[13] García G, Aparicio S, Ullah R, Atilhan M. Deep eutectic solvents: Physicochemical properties and gas separation applications. Energy Fuels 2015;29(4):2616–2644.

[14] Abbott AP, Capper G, Gray S. Design of improved deep eutectic solvents using hole

[15] Abbott AP, Harris RC, Ryder KS. Application of hole theory to define ionic liquids by their transport properties. The Journal of Physical Chemistry B 2007;111(18):4910–4913.

[16] Zhang Q, Vigier KDO, Royer S, Jérôme F. Deep eutectic solvents: syntheses, properties

[17] Abbott AP, Harris RC, Ryder KS, D'Agostino C, Gladden LF, Mantle MD. Glycerol

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[20] Gamburg YD, Zangari G. Theory and practice of metal electrodeposition. : Springer

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for viscous flow of ionic liquid. J Phys Chem 1970;74(1):159–163.

urea. Physical Chemistry Chemical Physics 2014;16(19):9047–9055.

acids. ACS Sustainable Chemistry & Engineering 2014;2(10):2416–2425.

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theory. ChemPhysChem 2006;7(4):803–806.

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Science & Business Media; 2011.

21383–21391.

and applications. Chem Soc Rev 2012;41(21):7108–7146.

### **7. Conclusions**

Deep eutectic solvents have the real potential to widen the panorama of modern galvanic techniques. Their application on large-scale production is however subordinated to the challenge of competing with well-established technologies and industrial systems not always easy to adapt to new techniques. As demonstrated by the examples presented, deep eutectic solvents are slowly leaving the laboratories to approach a real usage in plating plants. The next few decades will be the reference timeframe to understand whether these electrolytes have the possibility to be accepted as complements to existing water-based processes.

### **Author details**

R. Bernasconi, G. Panzeri, A. Accogli, F. Liberale, L. Nobili and L. Magagnin\*

\*Address all correspondence to: luca.magagnin@polimi.it

Department of Chemistry, Materials and Chemical Engineering "Giulio Natta", Politecnico di Milano, Milano, Italy

### **References**


[6] Abbott AP, Boothby D, Capper G, Davies DL, Rasheed RK. Deep eutectic solvents formed between choline chloride and carboxylic acids: versatile alternatives to ionic liquids. J Am Chem Soc 2004;126(29):9142–9147.

induced by an excessive corrosion of Ni in the Au electrolyte. This advantage can increase the

Deep eutectic solvents have the real potential to widen the panorama of modern galvanic techniques. Their application on large-scale production is however subordinated to the challenge of competing with well-established technologies and industrial systems not always easy to adapt to new techniques. As demonstrated by the examples presented, deep eutectic solvents are slowly leaving the laboratories to approach a real usage in plating plants. The next few decades will be the reference timeframe to understand whether these electrolytes have the

Department of Chemistry, Materials and Chemical Engineering "Giulio Natta", Politecnico di

[1] Smith EL, Abbott AP, Ryder KS. Deep eutectic solvents (DESs) and their applications.

[2] Endres F, MacFarlane D, Abbott A. Electrodeposition from ionic liquids. : John Wiley

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[4] Seventh International Conference on Molten Salts. : The Electrochemical Society

[5] Lecocq V, Graille A, Santini CC, Baudouin A, Chauvin Y, Basset JM, et al. Synthesis and characterization of ionic liquids based upon 1-butyl-2, 3-dimethylimidazolium

chloride/ZnCl 2. New journal of chemistry 2005;29(5):700–706.

possibility to be accepted as complements to existing water-based processes.

R. Bernasconi, G. Panzeri, A. Accogli, F. Liberale, L. Nobili and L. Magagnin\*

\*Address all correspondence to: luca.magagnin@polimi.it

Chem Rev 2014;114(21):11060–11082.

Montreal, Quebec, Canada; 1990.

industrial attractiveness of such processes.

254 Progress and Developments in Ionic Liquids

**7. Conclusions**

**Author details**

Milano, Milano, Italy

& Sons; 2008.

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**Chapter 12**

**Provisional chapter**

**Electrodeposition of Zn, Cu, and Zn-Cu Alloys**

**Electrodeposition of Zn, Cu, and Zn-Cu Alloys** 

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution,

© 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

and reproduction in any medium, provided the original work is properly cited.

**Keywords:** electrodeposition, deep eutectic solvents, metal oxides, cyclic voltammetry,

The electrodeposition of Zn, Cu, and Zn-Cu alloys for corrosion-resistant coatings and electrochemical applications such as electrocatalysis and electronic devices has received considerable attention in recent years [1–7]. The electrodeposition of Zn, Cu, and Zn-Cu alloys

Deep eutectic solvents (DESs) comprising choline chloride (ChCl) with either urea or ethylene glycol (EG) have been successfully used as powerful and potential electrolytes for extracting metals from their corresponding metal oxide precursors. In this work, for electrodeposition of Zn and Zn-Cu alloys, ChCl/urea-based DES was employed. Cyclic voltammetry study demonstrates that the reduction of Zn(II) to Zn is a diffusioncontrolled quasi-reversible, one-step, two electrons transfer process. Micro-/nanostructured Zn and Zn-Cu alloys films have been electrodeposited directly from their metal oxide precursors in DES, and the Zn and Zn-Cu alloy films exhibit homogeneous morphologies with controlled particle sizes. Besides, the electrodeposition of Cu from CuO in the eutectics based on ChCl with urea and EG has been investigated, respectively. The higher coordinated Cu species in the ChCl/urea-based DES are obviously more difficult to reduce, and higher overpotential is needed to drive the nucleation process compared with the lower coordinated Cu species in the ChCl/EG-based DES. The surface morphology of the Cu electrodeposits is significantly affected by the type of DES and the electrodeposition potentials. Furthermore, the Cu electrodeposits obtained in the ChCl/urea-based DES possess more dense microstructures than those produced in

**from Deep Eutectic Solvents**

**from Deep Eutectic Solvents**

Xingli Zou, Xionggang Lu and Xueliang Xie

Xingli Zou, Xionggang Lu and Xueliang Xie

Additional information is available at the end of the chapter

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/65883

the ChCl/EG-based DES.

morphology

**1. Introduction**

**Abstract**

### **Electrodeposition of Zn, Cu, and Zn-Cu Alloys from Deep Eutectic Solvents Electrodeposition of Zn, Cu, and Zn-Cu Alloys from Deep Eutectic Solvents**

Xingli Zou, Xionggang Lu and Xueliang Xie Xingli Zou, Xionggang Lu and Xueliang Xie Additional information is available at the end of the chapter

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/65883

#### **Abstract**

Deep eutectic solvents (DESs) comprising choline chloride (ChCl) with either urea or ethylene glycol (EG) have been successfully used as powerful and potential electrolytes for extracting metals from their corresponding metal oxide precursors. In this work, for electrodeposition of Zn and Zn-Cu alloys, ChCl/urea-based DES was employed. Cyclic voltammetry study demonstrates that the reduction of Zn(II) to Zn is a diffusioncontrolled quasi-reversible, one-step, two electrons transfer process. Micro-/nanostructured Zn and Zn-Cu alloys films have been electrodeposited directly from their metal oxide precursors in DES, and the Zn and Zn-Cu alloy films exhibit homogeneous morphologies with controlled particle sizes. Besides, the electrodeposition of Cu from CuO in the eutectics based on ChCl with urea and EG has been investigated, respectively. The higher coordinated Cu species in the ChCl/urea-based DES are obviously more difficult to reduce, and higher overpotential is needed to drive the nucleation process compared with the lower coordinated Cu species in the ChCl/EG-based DES. The surface morphology of the Cu electrodeposits is significantly affected by the type of DES and the electrodeposition potentials. Furthermore, the Cu electrodeposits obtained in the ChCl/urea-based DES possess more dense microstructures than those produced in the ChCl/EG-based DES.

**Keywords:** electrodeposition, deep eutectic solvents, metal oxides, cyclic voltammetry, morphology

### **1. Introduction**

The electrodeposition of Zn, Cu, and Zn-Cu alloys for corrosion-resistant coatings and electrochemical applications such as electrocatalysis and electronic devices has received considerable attention in recent years [1–7]. The electrodeposition of Zn, Cu, and Zn-Cu alloys

and reproduction in any medium, provided the original work is properly cited.

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, © 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

is commonly performed in aqueous electrolyte solutions. Traditional Zn plating is mainly carried out in sulfuric acidic aqueous baths [8, 9]. Generally, Zn is extracted from zinc sulfide ore. The ore is mined from the earth's crust and beneficiated by flotation methods that produce zinc sulfide concentrates. The mineral concentrates are oxidized to metal oxides during high temperature roasting and then the metal oxides are leached with sulfuric acid to produce zinc sulfate solution. Finally, the Zn electrodeposits are obtained from the zinc sulfate electrolyte under constant current electrolysis [10, 11]. However, the traditional Zn electrodeposition process is very sensitive to impurities and requires effective purification methods to produce pure Zn [12]. Copper electrodeposition is mostly performed in aqueous solutions, which possess high solubility of copper salts (e.g., CuCl<sup>2</sup> and CuSO4 ) and high electrolyte conductivity. However, the acid- and cyanide-based aqueous electrolytes are corrosive and the inevitable hydrogen evolution reaction commonly occurs during the electrolysis process [13]. Consequently, finding new alternative electrolytes for the electrodeposition of Zn, Cu, and Zn-Cu alloy films at low temperature has become the focus in recent years.

ideal electrolyte for copper salts such as nitrates, halides, and sulfates because of their poor solubilities [36]. Chen et al. [37] attempted to introduce the cuprous ions into the ionic liquid by the anodic dissolution of a Cu electrode, however, it is a time-consuming process. Therefore, searching for suitable ionic liquid-metal precursors systems for the electrodeposition of Zn,

Electrodeposition of Zn, Cu, and Zn-Cu Alloys from Deep Eutectic Solvents

DESs, which are made from quaternary ammonium salts mixed with hydrogen bond donors, such as carboxylic acids, alcohols, and amides, are promising electrolytes for electrodeposition application [38, 39]. The DESs are simple to prepare, relatively stable in air and moisture, many are biodegradable, and are relatively low cost [40–47]. Moreover, the DESs show considerable selective solubilities for many metal oxides, which may provide a new route for preparation of metals from metal oxide precursors in the DESs [48, 49]. In comparison

used directly as new promising precursors for the electrodeposition of Zn, Cu, and Zn-Cu alloys films in DESs without chloridization pretreatment. ChCl/urea-based DES has been used as a potential solvent for Zn recovery from waste oxide residues [50]. Yang and Reddy [41, 42] studied the electrodeposition of Zn and Pb films from their oxide precursors in the ChCl/urea-based DES due to their relatively high solubilities in the DES. Tsuda et al.

found that micro-/nanostructured Zn and Cu films can be electrodeposited from ZnO and CuO precursors in DESs, respectively [44, 45]. Zhang et al. [46] also demonstrated that the

urea-based DES. This previous work generally showed that zinc and copper oxides have the potential to be used as the precursors for the direct electrodeposition of Zn, Cu, and Zn-Cu

The blank CV curve obtained from a platinum working electrode in ChCl/urea-based DES at 333 K is illustrated in **Figure 1(a)**. The cathodic limit is approximately –1.2 V and the anodic limit is about 1.2 V. It can be seen from the blank CV that the electrochemical window of the ChCl/urea-based DES is approximately 2.4 V. **Figure 1(b)** shows the CV curve obtained from a platinum working electrode in the ChCl/urea-based DES dissolved with 0.1 M ZnO at 333 K and potential range of –1.3 to –0.1 V. As evidenced from this figure, the single cathodic current peak observed at about –1.1 V is attributed to the reduction of Zn2+ to the metal Zn, the anodic current peak occurred at approximately –0.95 V and is due to the stripping of the

In order to further investigate the electrochemical behavior of Zn, CV experiments using a platinum electrode as working electrode at different scan rates in ChCl/urea–ZnO (0.1 M) were also performed systematically, and the CV curves are shown in **Figure 2(a)**.

O, and CuO have potential to be

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265

O in the same DES, and they found

O can be achieved in ChCl/

O precursor. More recently, we have also

, CuCl, and CuCl2 precursors [22–26], ZnO, Cu2

[43] investigated the electrochemical behavior of Cu2

electrochemical synthesis of uniform Cu nanoparticles from Cu<sup>2</sup>

**2. Electrodeposition of Zn from ZnO in ChCl/urea-based DES**

that Cu can be directly electrodeposited from Cu<sup>2</sup>

Cu, and Zn-Cu alloys is extremely needed.

with ZnCl<sup>2</sup>

alloys films in DESs.

electrodeposited Zn.

**2.1. Cyclic voltammetry (CV) analysis**

More recently, the direct electrochemical reduction of metal oxides/compounds to metals/ alloys in molten salts has been extensively studied because of its low energy consumption and environmental compatibility [14, 15]. This previous innovative work shows that the production of metallic/coating materials directly from their metal oxide precursors in liquid salt is a promising route. Low-temperature electrolytic production of iron film from iron oxide in alkaline solution has been studied in our previous work [16], which showed that metal oxide has the potential to be used as a precursor for the electrodeposition at low temperature; the electrochemical process can be controlled effectively and the electrodeposition process generally exhibits acceptable current efficiency. However, in comparison with aqueous solutions, room temperature ionic liquids (RTILs) have attracted much interest as promising electrolyte candidates for metal electrodeposition due to their remarkable characteristics, such as high thermal and chemical stability, negligible vapor pressure, wide electrochemical windows, high ionic conductivity, simplicity of handing, and good solubility for quite a lot of metal salts [17–19]. The hydrogen embrittlement and hydrogen evolution reactions occurring in aqueous solutions can be avoided by using ionic liquids as electrolytes. The electrodeposition of Zn has been investigated in many ionic liquids, particularly in AlCl<sup>3</sup> -based ionic liquids [20] and chlorozincate ionic liquids [21]. In addition, the electrodeposition of Zn from ZnCl2 precursor has also been studied in choline chloride (ChCl)-based deep eutectic solvents (DESs) [22–26]. Liu et al. [27] illustrated the electrodeposition of Zn films from zinc triflate (Zn(TfO)2 ) in 1-butyl-1-methylpyrrolidinium trifluoromethylsulfonate ([Py1,4]TfO) and 1-ethyl-3-methylimidazolium trifluoromethylsulfonate ([EMIm]TfO) ionic liquids. Zheng et al. [28] investigated the electrodeposition of Zn films from ZnO in imidazolium chloride/urea ionic liquid, which suggested that ZnO has appreciable solubility in the electrolyte. Besides, the electrochemical behavior of copper species (e.g., CuCl and CuCl<sup>2</sup> ) has been investigated in a range of ionic liquids such as Lewis acidic, basic chloroaluminate ionic liquids, and air-/water-stable ionic liquids based on [BF<sup>4</sup> ] – , [N(CN)2 ] – , and [Tf2 N]– [29–34]. Although these ionic liquids show many advantages in electrodeposition of Cu, issues such as toxicity, cost, and tedious synthesis procedures may limit their realistic applications [35]. Furthermore, many air-/water-stable ionic liquids are not ideal electrolyte for copper salts such as nitrates, halides, and sulfates because of their poor solubilities [36]. Chen et al. [37] attempted to introduce the cuprous ions into the ionic liquid by the anodic dissolution of a Cu electrode, however, it is a time-consuming process. Therefore, searching for suitable ionic liquid-metal precursors systems for the electrodeposition of Zn, Cu, and Zn-Cu alloys is extremely needed.

DESs, which are made from quaternary ammonium salts mixed with hydrogen bond donors, such as carboxylic acids, alcohols, and amides, are promising electrolytes for electrodeposition application [38, 39]. The DESs are simple to prepare, relatively stable in air and moisture, many are biodegradable, and are relatively low cost [40–47]. Moreover, the DESs show considerable selective solubilities for many metal oxides, which may provide a new route for preparation of metals from metal oxide precursors in the DESs [48, 49]. In comparison with ZnCl<sup>2</sup> , CuCl, and CuCl2 precursors [22–26], ZnO, Cu2 O, and CuO have potential to be used directly as new promising precursors for the electrodeposition of Zn, Cu, and Zn-Cu alloys films in DESs without chloridization pretreatment. ChCl/urea-based DES has been used as a potential solvent for Zn recovery from waste oxide residues [50]. Yang and Reddy [41, 42] studied the electrodeposition of Zn and Pb films from their oxide precursors in the ChCl/urea-based DES due to their relatively high solubilities in the DES. Tsuda et al. [43] investigated the electrochemical behavior of Cu2 O in the same DES, and they found that Cu can be directly electrodeposited from Cu<sup>2</sup> O precursor. More recently, we have also found that micro-/nanostructured Zn and Cu films can be electrodeposited from ZnO and CuO precursors in DESs, respectively [44, 45]. Zhang et al. [46] also demonstrated that the electrochemical synthesis of uniform Cu nanoparticles from Cu<sup>2</sup> O can be achieved in ChCl/ urea-based DES. This previous work generally showed that zinc and copper oxides have the potential to be used as the precursors for the direct electrodeposition of Zn, Cu, and Zn-Cu alloys films in DESs.

### **2. Electrodeposition of Zn from ZnO in ChCl/urea-based DES**

#### **2.1. Cyclic voltammetry (CV) analysis**

is commonly performed in aqueous electrolyte solutions. Traditional Zn plating is mainly carried out in sulfuric acidic aqueous baths [8, 9]. Generally, Zn is extracted from zinc sulfide ore. The ore is mined from the earth's crust and beneficiated by flotation methods that produce zinc sulfide concentrates. The mineral concentrates are oxidized to metal oxides during high temperature roasting and then the metal oxides are leached with sulfuric acid to produce zinc sulfate solution. Finally, the Zn electrodeposits are obtained from the zinc sulfate electrolyte under constant current electrolysis [10, 11]. However, the traditional Zn electrodeposition process is very sensitive to impurities and requires effective purification methods to produce pure Zn [12]. Copper electrodeposition is mostly performed in aque-

high electrolyte conductivity. However, the acid- and cyanide-based aqueous electrolytes are corrosive and the inevitable hydrogen evolution reaction commonly occurs during the electrolysis process [13]. Consequently, finding new alternative electrolytes for the electrodeposition of Zn, Cu, and Zn-Cu alloy films at low temperature has become the focus in

More recently, the direct electrochemical reduction of metal oxides/compounds to metals/ alloys in molten salts has been extensively studied because of its low energy consumption and environmental compatibility [14, 15]. This previous innovative work shows that the production of metallic/coating materials directly from their metal oxide precursors in liquid salt is a promising route. Low-temperature electrolytic production of iron film from iron oxide in alkaline solution has been studied in our previous work [16], which showed that metal oxide has the potential to be used as a precursor for the electrodeposition at low temperature; the electrochemical process can be controlled effectively and the electrodeposition process generally exhibits acceptable current efficiency. However, in comparison with aqueous solutions, room temperature ionic liquids (RTILs) have attracted much interest as promising electrolyte candidates for metal electrodeposition due to their remarkable characteristics, such as high thermal and chemical stability, negligible vapor pressure, wide electrochemical windows, high ionic conductivity, simplicity of handing, and good solubility for quite a lot of metal salts [17–19]. The hydrogen embrittlement and hydrogen evolution reactions occurring in aqueous solutions can be avoided by using ionic liquids as electrolytes. The electrodeposition of Zn has been

and CuSO4


) has been investigated in a range of ionic liquids such as

[29–34]. Although these ionic liquids show many advantages in

precursor has also

) in 1-butyl-1-meth-

) and

ous solutions, which possess high solubility of copper salts (e.g., CuCl<sup>2</sup>

investigated in many ionic liquids, particularly in AlCl<sup>3</sup>

copper species (e.g., CuCl and CuCl<sup>2</sup>

, and [Tf2

N]–

] –

[BF<sup>4</sup> ] –

, [N(CN)2

incate ionic liquids [21]. In addition, the electrodeposition of Zn from ZnCl2

[27] illustrated the electrodeposition of Zn films from zinc triflate (Zn(TfO)2

been studied in choline chloride (ChCl)-based deep eutectic solvents (DESs) [22–26]. Liu et al.

ylpyrrolidinium trifluoromethylsulfonate ([Py1,4]TfO) and 1-ethyl-3-methylimidazolium trifluoromethylsulfonate ([EMIm]TfO) ionic liquids. Zheng et al. [28] investigated the electrodeposition of Zn films from ZnO in imidazolium chloride/urea ionic liquid, which suggested that ZnO has appreciable solubility in the electrolyte. Besides, the electrochemical behavior of

Lewis acidic, basic chloroaluminate ionic liquids, and air-/water-stable ionic liquids based on

electrodeposition of Cu, issues such as toxicity, cost, and tedious synthesis procedures may limit their realistic applications [35]. Furthermore, many air-/water-stable ionic liquids are not

recent years.

264 Progress and Developments in Ionic Liquids

The blank CV curve obtained from a platinum working electrode in ChCl/urea-based DES at 333 K is illustrated in **Figure 1(a)**. The cathodic limit is approximately –1.2 V and the anodic limit is about 1.2 V. It can be seen from the blank CV that the electrochemical window of the ChCl/urea-based DES is approximately 2.4 V. **Figure 1(b)** shows the CV curve obtained from a platinum working electrode in the ChCl/urea-based DES dissolved with 0.1 M ZnO at 333 K and potential range of –1.3 to –0.1 V. As evidenced from this figure, the single cathodic current peak observed at about –1.1 V is attributed to the reduction of Zn2+ to the metal Zn, the anodic current peak occurred at approximately –0.95 V and is due to the stripping of the electrodeposited Zn.

In order to further investigate the electrochemical behavior of Zn, CV experiments using a platinum electrode as working electrode at different scan rates in ChCl/urea–ZnO (0.1 M) were also performed systematically, and the CV curves are shown in **Figure 2(a)**.

**Figure 1.** (a) CV curve of a Pt electrode in pure ChCl/urea at 333 K with a scan rate of 10 mV s–1. (b) CV curve of a Pt electrode in ChCl/urea–ZnO (0.1 M) at 333 K with a scan rate of 10 mV s–1.

**Figure 2.** (a) CV curve of a Pt electrode in ChCl/urea–ZnO (0.1 M) at 333 K with different scan rates. The scan rates were

Electrodeposition of Zn, Cu, and Zn-Cu Alloys from Deep Eutectic Solvents

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267

pc) and square root of

5, 10, 15, 20, and 25 mV s–1, respectively. (b) Relationship between cathodic peak current density (*j*

scan rate (*ν*1/2) calculated from (a).

**Figure 2.** (a) CV curve of a Pt electrode in ChCl/urea–ZnO (0.1 M) at 333 K with different scan rates. The scan rates were 5, 10, 15, 20, and 25 mV s–1, respectively. (b) Relationship between cathodic peak current density (*j* pc) and square root of scan rate (*ν*1/2) calculated from (a).

**Figure 1.** (a) CV curve of a Pt electrode in pure ChCl/urea at 333 K with a scan rate of 10 mV s–1. (b) CV curve of a

Pt electrode in ChCl/urea–ZnO (0.1 M) at 333 K with a scan rate of 10 mV s–1.

266 Progress and Developments in Ionic Liquids

The cathodic and anodic peak's current densities increase with the increase of scan rate, and the cathodic and anodic peak potentials shift to more negative and positive sides, respectively. In **Figure 2(b)**, the cathodic peak current density (*j* pc) vary linearly as a function of the square root of scan rate (*ν*1/2), implying the reduction process of Zn(II) is mainly diffusioncontrolled. Besides, the cathodic peak and half-peak potentials |*E*pc – *E*pc/2| increase with the increase of scan rate. At the lowest scan rate, the difference in the value of 42 mV is larger than the value for the reversible process (31 mV at 333 K). All of these results suggest that the reduction of Zn(II) to Zn in ChCl/urea-based DES is a diffusion-controlled quasi-reversible process [51].

For a quasi-reversible charge transfer process, the diffusion coefficient of Zn(II) can be determined by the irreversible Randles-Sevick equation (1) [51], which is applicable to the quasi-reversible systems as well [52, 53],

$$j\_{\rm pc} = 0.4958 n FAC\_{\rm Zn(l)} D\_{\rm Zn(l)}^{1/2} \left(\frac{\alpha n\_a F \nu}{RT}\right)^{1/2} \tag{1}$$

where *j* pc is the cathodic peak current density, *F* is the Faraday constant, *n* is the number of exchanged electrons, *CZn II* ( ) is the Zn(II) species concentration, *A* is the electrode area, *DZn II* ( )

is the diffusion coefficient of Zn(II) species, *ν* is the scan rate, *α* is the transfer coefficient, *nα* is the electron transfer number in the rate determining step, *R* is the gas constant, and *T* is the absolute temperature. The value *α* can be obtained from Eq. (2) [51]:

$$\left| E\_{\rm pc} - E\_{\rm pc/2} \right| = 1.857 \,\text{RT/} \omega n\_a \text{F} \tag{2}$$

also increase gradually with the growth of the particles, which result in the Zn films become porous. The Zn electrodeposits with hexagonal structure (**Figure 3a**) continue to further nucleate and grow to form polygonal Zn plates (**Figure 3b**) and then transform to multilayer structure (**Figure 3c**–**d**). The Zn particles gradually change from dispersive nanoparticle to multilayer microparticle with irregular shapes. It should be noted that the morphology change during the electrodeposition process is mainly attributed to the increased electrodeposition rate and the electrodeposition temperature. It is worth noting that the morphology of the electrodeposited Zn can be influenced by the electrodeposi-

**Figure 3.** SEM images of the Zn electrodeposits obtained from ChCl/urea–ZnO (0.1 M) at –1.15 V on a Cu substrate

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269

at different temperatures: (a) 333 K, (b) 343 K, (c) 353 K, and (d) 363 K for 2 h.

X-ray diffraction (XRD) pattern of the Zn electrodeposits on a Cu substrate obtained from 0. 1 M ZnO in ChCl/urea-based DES is shown in **Figure 4**. It can be seen that only two metallic phases, Zn and Cu (substrate), are identified. It is obvious that the electrodeposit is composed

tion temperature.

of high purity Zn.

where *E*pc and *E*pc/2 are the cathodic peak potential and half-peak potential, respectively. According to Eq. (2) and the data obtained from **Figure 2(a)**, the average transfer coefficient can be calculated as 0.53. Substituting this and other parameters in Eq. (1), the diffusion coefficient of Zn(II) in ChCl/urea-based DES is determined to be 6.21 × 10–9 cm<sup>2</sup> s–1 at 333 K, which is smaller than that of Zn(II) in Bu<sup>3</sup> MeN-TFSI (1.6 × 10–7 cm<sup>2</sup> s–1 [54]) and AlCl3 -EMIC (2.6 × 10–6 cm<sup>2</sup> s–1 [55]) ionic liquids. The relatively low mobility of Zn(II) species may be ascribed to the formation of large, sterically hindered Zn-complex anions when ZnO is dissolved in the DES [44] and high viscosity of the ChCl/urea-based DES [56].

#### **2.2. Morphology and phase composition analyses of the electrodeposited Zn films**

The SEM images of the Zn films electrodeposited at different electrodeposition temperatures are shown in **Figure 3**. It is obvious that the microstructure of the Zn films changes gradually with increasing the electrodeposition temperature. As illustrated in **Figure 3(a)**, compact Zn films with particle size approximately 400 nm can be obtained at 333 K. The particle size of the Zn electrodeposits apparently increases with increasing electrodeposition temperature (**Figure 3b**–**d**). In addition, the interspaces between the Zn particles

The cathodic and anodic peak's current densities increase with the increase of scan rate, and the cathodic and anodic peak potentials shift to more negative and positive sides, respec-

square root of scan rate (*ν*1/2), implying the reduction process of Zn(II) is mainly diffusioncontrolled. Besides, the cathodic peak and half-peak potentials |*E*pc – *E*pc/2| increase with the increase of scan rate. At the lowest scan rate, the difference in the value of 42 mV is larger than the value for the reversible process (31 mV at 333 K). All of these results suggest that the reduction of Zn(II) to Zn in ChCl/urea-based DES is a diffusion-controlled quasi-reversible

For a quasi-reversible charge transfer process, the diffusion coefficient of Zn(II) can be determined by the irreversible Randles-Sevick equation (1) [51], which is applicable to the

<sup>=</sup>

exchanged electrons, *CZn II* ( ) is the Zn(II) species concentration, *A* is the electrode area, *DZn II* ( ) is the diffusion coefficient of Zn(II) species, *ν* is the scan rate, *α* is the transfer coefficient, *nα* is the electron transfer number in the rate determining step, *R* is the gas constant, and *T* is the

pc pc/2 *E E RT n F* 1.857 /

where *E*pc and *E*pc/2 are the cathodic peak potential and half-peak potential, respectively. According to Eq. (2) and the data obtained from **Figure 2(a)**, the average transfer coefficient can be calculated as 0.53. Substituting this and other parameters in Eq. (1), the diffusion coefficient

ionic liquids. The relatively low mobility of Zn(II) species may be ascribed to the formation of large, sterically hindered Zn-complex anions when ZnO is dissolved in the DES [44] and high

The SEM images of the Zn films electrodeposited at different electrodeposition temperatures are shown in **Figure 3**. It is obvious that the microstructure of the Zn films changes gradually with increasing the electrodeposition temperature. As illustrated in **Figure 3(a)**, compact Zn films with particle size approximately 400 nm can be obtained at 333 K. The particle size of the Zn electrodeposits apparently increases with increasing electrodeposition temperature (**Figure 3b**–**d**). In addition, the interspaces between the Zn particles

**2.2. Morphology and phase composition analyses of the electrodeposited Zn films**

absolute temperature. The value *α* can be obtained from Eq. (2) [51]:

of Zn(II) in ChCl/urea-based DES is determined to be 6.21 × 10–9 cm<sup>2</sup>

MeN-TFSI (1.6 × 10–7 cm<sup>2</sup>

pc Zn(II) Zn(II) 0.4958 *n F j nFAC D RT*

1/2

pc is the cathodic peak current density, *F* is the Faraday constant, *n* is the number of

α ν α

α α

s–1 [54]) and AlCl3

pc) vary linearly as a function of the

(1)

s–1 at 333 K, which is smaller

s–1 [55])


1/2

− = (2)

tively. In **Figure 2(b)**, the cathodic peak current density (*j*

quasi-reversible systems as well [52, 53],

268 Progress and Developments in Ionic Liquids

process [51].

where *j*

than that of Zn(II) in Bu<sup>3</sup>

viscosity of the ChCl/urea-based DES [56].

**Figure 3.** SEM images of the Zn electrodeposits obtained from ChCl/urea–ZnO (0.1 M) at –1.15 V on a Cu substrate at different temperatures: (a) 333 K, (b) 343 K, (c) 353 K, and (d) 363 K for 2 h.

also increase gradually with the growth of the particles, which result in the Zn films become porous. The Zn electrodeposits with hexagonal structure (**Figure 3a**) continue to further nucleate and grow to form polygonal Zn plates (**Figure 3b**) and then transform to multilayer structure (**Figure 3c**–**d**). The Zn particles gradually change from dispersive nanoparticle to multilayer microparticle with irregular shapes. It should be noted that the morphology change during the electrodeposition process is mainly attributed to the increased electrodeposition rate and the electrodeposition temperature. It is worth noting that the morphology of the electrodeposited Zn can be influenced by the electrodeposition temperature.

X-ray diffraction (XRD) pattern of the Zn electrodeposits on a Cu substrate obtained from 0. 1 M ZnO in ChCl/urea-based DES is shown in **Figure 4**. It can be seen that only two metallic phases, Zn and Cu (substrate), are identified. It is obvious that the electrodeposit is composed of high purity Zn.

**Figure 4.** XRD pattern of the Zn electrodeposits obtained from ChCl/urea–ZnO (0.1 M) on a Cu substrate at –1.15 V and 333 K.

### **3. Electrodeposition of Cu from CuO in ChCl/EG and ChCl/urea-based DESs**

#### **3.1. CV study**

In order to investigate the electrochemical behavior of Cu, CV analysis was carried out in ChCl/EG–CuO (0.01 M)- and ChCl/urea–CuO (0.01 M)-based DESs, and the CV curves are shown in **Figure 5**. For comparison, **Figure 5(a)** and **(b)** illustrates the voltammetric behavior of Cu(II) in the ChCl/EG–CuO (0.01 M) and ChCl/urea–CuO (0.01 M) electrolytes, respectively, and the CVs were recorded at 353 K with a scan rate of 10 mV s–1. Two redox couples (c1 /a1 and c<sup>2</sup> /a2 ) are occurred in each CV curve. The redox couple (c<sup>1</sup> /a1 ) is assigned to the reaction of Cu(II) + e– ↔ Cu(I), and the redox couple (c<sup>2</sup> /a2 ) is attributed to the reaction of Cu(I) + e– ↔ Cu. As shown in **Figure 5(a)** and **(b)**, the peak current density for the reduction of Cu(I)/ Cu(0) in the ChCl/EG system is higher than that in the ChCl/urea system. Moreover, the redox potentials of the Cu(II)/Cu(I) and Cu(I)/Cu(0) couples in the ChCl/urea system occur at potentials more negative than those observed in the ChCl/EG system. The different redox potentials

**Figure 5.** CV curves of a Fe electrode in (a) ChCl/EG–CuO (0.01 M) and (b) ChCl/urea–CuO (0.01 M) at 353 K with a scan

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rate of 10 mV s–1.

**3. Electrodeposition of Cu from CuO in ChCl/EG and ChCl/urea-based**

) are occurred in each CV curve. The redox couple (c<sup>1</sup>

tion of Cu(II) + e– ↔ Cu(I), and the redox couple (c<sup>2</sup>

In order to investigate the electrochemical behavior of Cu, CV analysis was carried out in ChCl/EG–CuO (0.01 M)- and ChCl/urea–CuO (0.01 M)-based DESs, and the CV curves are shown in **Figure 5**. For comparison, **Figure 5(a)** and **(b)** illustrates the voltammetric behavior of Cu(II) in the ChCl/EG–CuO (0.01 M) and ChCl/urea–CuO (0.01 M) electrolytes, respectively, and the CVs were recorded at 353 K with a scan rate of 10 mV s–1. Two redox couples

**Figure 4.** XRD pattern of the Zn electrodeposits obtained from ChCl/urea–ZnO (0.1 M) on a Cu substrate at –1.15 V

e– ↔ Cu. As shown in **Figure 5(a)** and **(b)**, the peak current density for the reduction of Cu(I)/ Cu(0) in the ChCl/EG system is higher than that in the ChCl/urea system. Moreover, the redox potentials of the Cu(II)/Cu(I) and Cu(I)/Cu(0) couples in the ChCl/urea system occur at potentials more negative than those observed in the ChCl/EG system. The different redox potentials

/a2

/a1

) is attributed to the reaction of Cu(I) +

) is assigned to the reac-

**DESs**

and 333 K.

270 Progress and Developments in Ionic Liquids

(c1 /a1 and c<sup>2</sup> /a2

**3.1. CV study**

**Figure 5.** CV curves of a Fe electrode in (a) ChCl/EG–CuO (0.01 M) and (b) ChCl/urea–CuO (0.01 M) at 353 K with a scan rate of 10 mV s–1.

may be attributed to the differences in ligand activity between the two DESs. There is a strong coordination between the chloride ions and urea, which can effectively decrease the activity of chloride compared with EG [25]. These results can be ascribed to the lower viscosity of the ChCl/EG system and the facile charge-transfer kinetics in the ChCl/EG system compared to that of the ChCl/urea system [41]. Therefore, the lower coordinated Cu species in the ChCl/ EG ionic liquid are obviously more easier to be reduced, whereas the higher coordinated Cu species in the ChCl/urea ionic liquid are more difficult to be reduced.

It is indicated that the electrodeposition potential has significant influences on the mor-

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The SEM images of the Cu films electrodeposited in ChCl/urea ionic liquid at different electrodeposition potentials are shown in **Figure 7**. As evidenced in **Figure 7(a)**, the uniform, dense, and compact electrodeposits are formed at −0.90 V. As the electrodeposition potential increases to –0.95 and –1.00 V, some spherical nodular electrodeposits are formed and become more porous (**Figure 7b** and **c**). **Figure 7(d)** shows the EDS spectra of the Cu film corresponding to **Figure 7(b)**. Only two elements Cu and Fe (substrate) are determined from the EDS spectra. The differences in morphology are probably due to the different Cu species formed in the electrolyte, the higher coordinated and lower superficial diffusion of Cu species in ChCl/ urea ionic liquid results in a homogenous distribution of particles with particle size smaller

XRD patterns of the Cu electrodeposits on a Fe substrate obtained from 0.01 M CuO in ChCl/ EG and ChCl/urea ionic liquids are shown in **Figure 8(a)** and **(b)**. It can be seen that only two metallic phases Cu and Fe (substrate) are identified in both media. It is evidenced that the

**Figure 7.** SEM images of the Cu electrodeposits obtained on a Fe substrate in ChCl/urea–CuO (0.01 M) at (a) –0.90 V, (b) –0.95 V, (c) –1.00 V and 353 K for 3 h, and (d) EDS spectra of the electrodeposited Cu film corresponding to (b).

phology of Cu electrodeposits.

than those observed in ChCl/EG ionic liquid.

electrodeposit is composed of high purity Cu.

#### **3.2. Characterization of the Cu electrodeposits**

The SEM images of the Cu films electrodeposited in ChCl/EG ionic liquid at different electrodeposition potentials are shown in **Figure 6**. In **Figure 6(a)**, a porous and nonuniform electrodeposit is formed. As the electrodeposition potential was made progressively negative to –0.80 V, the agglomeration of Cu particles occurred. The Cu particles generated at high cathodic potential are composed of some larger agglomerates with fine nanoscale particles (**Figure 6c**). **Figure 6(d)** shows the EDS spectra of the Cu film corresponding to **Figure 6(b)**. Only two elements Cu and Fe (substrate) are determined from the EDS spectra.

**Figure 6.** SEM images of the Cu electrodeposits obtained on a Fe substrate in ChCl/EG–CuO (0.01 M) at (a) –0.75 V, (b) –0.80 V, and (c) –0.85 V and 353 K for 3 h, and (d) EDS spectra of the electrodeposited Cu film corresponding to (b).

It is indicated that the electrodeposition potential has significant influences on the morphology of Cu electrodeposits.

may be attributed to the differences in ligand activity between the two DESs. There is a strong coordination between the chloride ions and urea, which can effectively decrease the activity of chloride compared with EG [25]. These results can be ascribed to the lower viscosity of the ChCl/EG system and the facile charge-transfer kinetics in the ChCl/EG system compared to that of the ChCl/urea system [41]. Therefore, the lower coordinated Cu species in the ChCl/ EG ionic liquid are obviously more easier to be reduced, whereas the higher coordinated Cu

The SEM images of the Cu films electrodeposited in ChCl/EG ionic liquid at different electrodeposition potentials are shown in **Figure 6**. In **Figure 6(a)**, a porous and nonuniform electrodeposit is formed. As the electrodeposition potential was made progressively negative to –0.80 V, the agglomeration of Cu particles occurred. The Cu particles generated at high cathodic potential are composed of some larger agglomerates with fine nanoscale particles (**Figure 6c**). **Figure 6(d)** shows the EDS spectra of the Cu film corresponding to **Figure 6(b)**. Only two elements Cu and Fe (substrate) are determined from the EDS spectra.

**Figure 6.** SEM images of the Cu electrodeposits obtained on a Fe substrate in ChCl/EG–CuO (0.01 M) at (a) –0.75 V, (b) –0.80 V, and (c) –0.85 V and 353 K for 3 h, and (d) EDS spectra of the electrodeposited Cu film corresponding to (b).

species in the ChCl/urea ionic liquid are more difficult to be reduced.

**3.2. Characterization of the Cu electrodeposits**

272 Progress and Developments in Ionic Liquids

The SEM images of the Cu films electrodeposited in ChCl/urea ionic liquid at different electrodeposition potentials are shown in **Figure 7**. As evidenced in **Figure 7(a)**, the uniform, dense, and compact electrodeposits are formed at −0.90 V. As the electrodeposition potential increases to –0.95 and –1.00 V, some spherical nodular electrodeposits are formed and become more porous (**Figure 7b** and **c**). **Figure 7(d)** shows the EDS spectra of the Cu film corresponding to **Figure 7(b)**. Only two elements Cu and Fe (substrate) are determined from the EDS spectra. The differences in morphology are probably due to the different Cu species formed in the electrolyte, the higher coordinated and lower superficial diffusion of Cu species in ChCl/ urea ionic liquid results in a homogenous distribution of particles with particle size smaller than those observed in ChCl/EG ionic liquid.

XRD patterns of the Cu electrodeposits on a Fe substrate obtained from 0.01 M CuO in ChCl/ EG and ChCl/urea ionic liquids are shown in **Figure 8(a)** and **(b)**. It can be seen that only two metallic phases Cu and Fe (substrate) are identified in both media. It is evidenced that the electrodeposit is composed of high purity Cu.

**Figure 7.** SEM images of the Cu electrodeposits obtained on a Fe substrate in ChCl/urea–CuO (0.01 M) at (a) –0.90 V, (b) –0.95 V, (c) –1.00 V and 353 K for 3 h, and (d) EDS spectra of the electrodeposited Cu film corresponding to (b).

**4. Electrodeposition of Zn-Cu alloys in ChCl/urea-based DES**

, a<sup>2</sup> , a<sup>3</sup>

are at about 0.32, –0.30, and –1.05 V, respectively, and the oxidation peak (a3

potential range of –1.10 to 0.50 V. The oxidation peaks for pure Cu (a1

**Figure 9** shows the CV curve of the ChCl/urea ionic liquid dissolved with 0.01 M CuO and 0.1 M ZnO on a Fe electrode at 343 K with a scan rate of 10 mV s–1. There are three reduction peaks on the cathodic branch of the voltammogram. The two cathodic reduction peaks

to Cu(I) reduction process and the Cu(I) to Cu reduction process, respectively. The reduction

, and a4

 and a4 ).

**Figure 9.** CV curve of a Fe electrode in ChCl/urea ionic liquid containing 0.1 M ZnO and 0.01 M CuO at 343 K with a

) and –0.85 V (c2

) can be ascribed to the Zn(II) to Zn reduction process. When the scan

Electrodeposition of Zn, Cu, and Zn-Cu Alloys from Deep Eutectic Solvents

) are attributed to the Cu(II)

http://dx.doi.org/10.5772/65883

) and pure Zn (a4

) is for the Zn-Cu

)

275

), as shown in **Figure 9**, are observed in the

and a<sup>2</sup>

**4.1. CV analysis**

peak at about –1.15 V (c3

scan rate of 10 mV s–1.

is reversed, four oxidation peaks (a<sup>1</sup>

observed at approximately 0.25 V (labeled as c1

electrodeposits between the two potentials (a<sup>2</sup>

**Figure 8.** XRD patterns of the Cu electrodeposits obtained on a Fe substrate in (a) ChCl/EG–CuO (0.01 M) at –0.80 V and 353 K for 3 h and (b) ChCl/urea–CuO (0.01 M) at –0.95 V and 353 K for 3 h.

### **4. Electrodeposition of Zn-Cu alloys in ChCl/urea-based DES**

#### **4.1. CV analysis**

**Figure 8.** XRD patterns of the Cu electrodeposits obtained on a Fe substrate in (a) ChCl/EG–CuO (0.01 M) at –0.80 V and

353 K for 3 h and (b) ChCl/urea–CuO (0.01 M) at –0.95 V and 353 K for 3 h.

274 Progress and Developments in Ionic Liquids

**Figure 9** shows the CV curve of the ChCl/urea ionic liquid dissolved with 0.01 M CuO and 0.1 M ZnO on a Fe electrode at 343 K with a scan rate of 10 mV s–1. There are three reduction peaks on the cathodic branch of the voltammogram. The two cathodic reduction peaks observed at approximately 0.25 V (labeled as c1 ) and –0.85 V (c2 ) are attributed to the Cu(II) to Cu(I) reduction process and the Cu(I) to Cu reduction process, respectively. The reduction peak at about –1.15 V (c3 ) can be ascribed to the Zn(II) to Zn reduction process. When the scan is reversed, four oxidation peaks (a<sup>1</sup> , a<sup>2</sup> , a<sup>3</sup> , and a4 ), as shown in **Figure 9**, are observed in the potential range of –1.10 to 0.50 V. The oxidation peaks for pure Cu (a1 and a<sup>2</sup> ) and pure Zn (a4 ) are at about 0.32, –0.30, and –1.05 V, respectively, and the oxidation peak (a3 ) is for the Zn-Cu electrodeposits between the two potentials (a<sup>2</sup> and a4 ).

**Figure 9.** CV curve of a Fe electrode in ChCl/urea ionic liquid containing 0.1 M ZnO and 0.01 M CuO at 343 K with a scan rate of 10 mV s–1.

#### **4.2. Morphology and phase composition analyses of the electrodeposited Zn-Cu films**

**5. Conclusions**

**Acknowledgements**

**Author details**

Shanghai, China

**References**

Xingli Zou\*, Xionggang Lu\* and Xueliang Xie

coefficient of Zn(II) was estimated to be 6.21 × 10–9 cm<sup>2</sup>

The electrodeposition of Zn, Cu, and Zn-Cu alloys from ZnO/CuO precursors has been investigated in DESs. Electrochemical measurements showed that the Zn electrodeposition is a diffusion-controlled quasi-reversible, one-step, two electrons transfer process. The diffusion

Zn electrodeposits can form under suitable electrodeposition potentials and lower temperatures. Besides, the electrodeposition of Cu from CuO in the eutectics based on ChCl with urea and EG has been respectively investigated and compared. The voltammetric measurements show the electrodeposition of Cu in ChCl/EG and ChCl/urea systems through a two-step process. The higher coordinated Cu species in the ChCl/urea ionic liquid are more difficult to be reduced. The surface morphology of the Cu electrodeposits can be significantly affected by the ionic liquids and the electrodeposition potential. Furthermore, the Cu electrodeposits obtained in the ChCl/urea ionic liquid possess more homogenous microstructures than those produced in the ChCl/EG ionic liquid. In addition, the Zn-Cu alloy films have also been electrodeposited directly from their metal oxide precursors in ChCl/urea-based DES, the phase composition of the Zn-Cu alloys depends on the electrodeposition potential. These results may have implications on the electrodeposition of other alloy films from oxide precursors in DESs system.

The authors thank China National Funds for Distinguished Young Scientists (No. 51225401), the National Natural Science Foundation of China (Nos. 51304132 and 51574164), the National Basic Research Program of China (No. 2014CB643403), the Science and Technology Commissions of Shanghai Municipality (No. 14JC1491400), and the Young Teacher Training Program of Shanghai Municipal Education Commission for financial support. The authors also thank the Instrumental

State Key Laboratory of Advanced Special Steel & Shanghai Key Laboratory of Advanced Ferrometallurgy & School of Materials Science and Engineering, Shanghai University,

[1] Liu H, Szunerits S, Xu WX. Preparation of superhydrophobic coatings on zinc as effective corrosion barriers. ACS Appl. Mat. Interfaces. 2009;**1**:1150–1153. DOI: 10.1021/am900100q

Analysis and Research Center of Shanghai University for materials characterization.

\*Address all correspondence to: xlzou@shu.edu.cn and luxg@shu.edu.cn

s–1 at 333 K. Uniform, dense, and compact

http://dx.doi.org/10.5772/65883

277

Electrodeposition of Zn, Cu, and Zn-Cu Alloys from Deep Eutectic Solvents

The XRD patterns of the Zn-Cu alloys electrodeposited on a Fe substrate in the ChCl/urea ionic liquid containing 0.1 M ZnO and 0.01 M CuO at different potentials and 343 K for 3 h are shown in **Figure 10(a)**. It can be seen that the dominate phases of the Zn-Cu films electrodeposited at –1.05 V are Cu5 Zn<sup>8</sup> and Cu (substrate). As the electrodeposition potential increases to –1.10 V, two new phases (CuZn5 , Zn) are observed and the Cu<sup>5</sup> Zn<sup>8</sup> phase is disappeared. It is mainly the result of increasing the Zn electrodeposition rate under more negative potential. **Figure 10(b)** and **(c)** shows the SEM images of the Zn-Cu alloys electrodeposited in the ChCl/ urea ionic liquid containing 0.1 M ZnO and 0.01 M CuO at different cathodic potentials and 343 K for 3 h. The Zn-Cu electrodeposit obtained at –1.05 V is composed of spherical clusters with some void space between the particles (**Figure 10b**). When the potential changes from –1.05 to –1.10 V, the agglomeration of Zn-Cu particles is observed and the electrodeposits are porous and nonuniform (**Figure 10c**).

**Figure 10.** (a) XRD patterns of the Cu–Zn alloys electrodeposited on a Fe substrate in the ChCl/urea ionic liquid containing 0.1 M ZnO and 0.01 M CuO at different cathodic potentials and 343 K for 3 h. (b) and (c) SEM images of the Cu–Zn electrodeposits obtained on a Fe substrate in the 12CU ionic liquid containing 0.1 M ZnO and 0.01 M CuO at 343 K for 3 h, (b) –1.05 V and (c) –1.10 V.

### **5. Conclusions**

**4.2. Morphology and phase composition analyses of the electrodeposited Zn-Cu films**

posited at –1.05 V are Cu5

276 Progress and Developments in Ionic Liquids

to –1.10 V, two new phases (CuZn5

porous and nonuniform (**Figure 10c**).

at 343 K for 3 h, (b) –1.05 V and (c) –1.10 V.

Zn<sup>8</sup>

The XRD patterns of the Zn-Cu alloys electrodeposited on a Fe substrate in the ChCl/urea ionic liquid containing 0.1 M ZnO and 0.01 M CuO at different potentials and 343 K for 3 h are shown in **Figure 10(a)**. It can be seen that the dominate phases of the Zn-Cu films electrode-

, Zn) are observed and the Cu<sup>5</sup>

mainly the result of increasing the Zn electrodeposition rate under more negative potential. **Figure 10(b)** and **(c)** shows the SEM images of the Zn-Cu alloys electrodeposited in the ChCl/ urea ionic liquid containing 0.1 M ZnO and 0.01 M CuO at different cathodic potentials and 343 K for 3 h. The Zn-Cu electrodeposit obtained at –1.05 V is composed of spherical clusters with some void space between the particles (**Figure 10b**). When the potential changes from –1.05 to –1.10 V, the agglomeration of Zn-Cu particles is observed and the electrodeposits are

**Figure 10.** (a) XRD patterns of the Cu–Zn alloys electrodeposited on a Fe substrate in the ChCl/urea ionic liquid containing 0.1 M ZnO and 0.01 M CuO at different cathodic potentials and 343 K for 3 h. (b) and (c) SEM images of the Cu–Zn electrodeposits obtained on a Fe substrate in the 12CU ionic liquid containing 0.1 M ZnO and 0.01 M CuO

and Cu (substrate). As the electrodeposition potential increases

Zn<sup>8</sup>

phase is disappeared. It is

The electrodeposition of Zn, Cu, and Zn-Cu alloys from ZnO/CuO precursors has been investigated in DESs. Electrochemical measurements showed that the Zn electrodeposition is a diffusion-controlled quasi-reversible, one-step, two electrons transfer process. The diffusion coefficient of Zn(II) was estimated to be 6.21 × 10–9 cm<sup>2</sup> s–1 at 333 K. Uniform, dense, and compact Zn electrodeposits can form under suitable electrodeposition potentials and lower temperatures. Besides, the electrodeposition of Cu from CuO in the eutectics based on ChCl with urea and EG has been respectively investigated and compared. The voltammetric measurements show the electrodeposition of Cu in ChCl/EG and ChCl/urea systems through a two-step process. The higher coordinated Cu species in the ChCl/urea ionic liquid are more difficult to be reduced. The surface morphology of the Cu electrodeposits can be significantly affected by the ionic liquids and the electrodeposition potential. Furthermore, the Cu electrodeposits obtained in the ChCl/urea ionic liquid possess more homogenous microstructures than those produced in the ChCl/EG ionic liquid. In addition, the Zn-Cu alloy films have also been electrodeposited directly from their metal oxide precursors in ChCl/urea-based DES, the phase composition of the Zn-Cu alloys depends on the electrodeposition potential. These results may have implications on the electrodeposition of other alloy films from oxide precursors in DESs system.

### **Acknowledgements**

The authors thank China National Funds for Distinguished Young Scientists (No. 51225401), the National Natural Science Foundation of China (Nos. 51304132 and 51574164), the National Basic Research Program of China (No. 2014CB643403), the Science and Technology Commissions of Shanghai Municipality (No. 14JC1491400), and the Young Teacher Training Program of Shanghai Municipal Education Commission for financial support. The authors also thank the Instrumental Analysis and Research Center of Shanghai University for materials characterization.

### **Author details**

Xingli Zou\*, Xionggang Lu\* and Xueliang Xie

\*Address all correspondence to: xlzou@shu.edu.cn and luxg@shu.edu.cn

State Key Laboratory of Advanced Special Steel & Shanghai Key Laboratory of Advanced Ferrometallurgy & School of Materials Science and Engineering, Shanghai University, Shanghai, China

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2010;**157**:F96–F103. DOI: 10.1149/1.3377117

2007;**52**:2746–2754. DOI: 10.1016/j.electacta.2006.08.064


**Section 4**

**Liquid Crystals**

**Section 4**

## **Liquid Crystals**

**Chapter 13**

Provisional chapter

**Ionic Liquid Crystals Based on Pyridinium Salts**

This chapter describes the liquid crystalline properties of the ionic liquid crystals (ILC) based on pyridinium salts as well as their metal-containing compounds with an emphasis on the recent systems described in literature. The main factors that influence the liquid crystalline properties of pyridinium ILC are discussed. Selected thermal data are given according to mesogenic group employed and its position (either N-substitution or pyridinium ring substitution) and the number of structural cationic units (mono-, di-, or

Ionic liquid crystals (ILCs) are extensively studied nowadays due to their unique properties resulting from the combination of liquid crystal (LC) and ionic liquid (IL) properties. Several reviews covering this topic were published in the recent years [1–3]. The field of ILCs is continuously growing as many recent applications were found: solar cells, membranes, battery materials, electrochemical sensors, or electroluminescent switches. Different factors are responsible for governing the nature of ILC phases, such as the molecular shape, location, and size of ionic groups, intermolecular interactions, and microphase segregation. Thus, the hydrophobic interactions between the long alkyl chain groups, ionic, dipole-dipole, anion– cation hydrogen bonding, and cation-π interactions together with the π-π stacking of the aromatic rings, all have a contribution to the stabilization of the liquid crystalline phase. For instance, there is a strong tendency to stabilize lamellar (smectic) phases, with SmA the most common phase for ILC, due to electrostatic interactions and ion-ion stacking in ILC. The combination of all these factors leads to LC behavior ranges from typical calamitic materials to discotic. The imidazolium or pyridinium derivatives, one of the most studied classes of ILC,

), are well known for their high thermal and electrochemical stabilities. It is worth to

© The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons

© 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and eproduction in any medium, provided the original work is properly cited.

−

) and hexafluorophosphate

Ionic Liquid Crystals Based on Pyridinium Salts

Additional information is available at the end of the chapter

Keywords: ionic liquids, liquid crystals, pyridinium salts

with weakly coordinating anions, such as tetrafluoroborate (BF4

Additional information is available at the end of the chapter

Viorel Cîrcu

Viorel Cîrcu

http://dx.doi.org/10.5772/65757

polycationic pyridinium ILC).

Abstract

1. Introduction

(PF6 −

#### **Ionic Liquid Crystals Based on Pyridinium Salts** Ionic Liquid Crystals Based on Pyridinium Salts

Viorel Cîrcu Viorel Cîrcu

Additional information is available at the end of the chapter Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/65757

#### Abstract

This chapter describes the liquid crystalline properties of the ionic liquid crystals (ILC) based on pyridinium salts as well as their metal-containing compounds with an emphasis on the recent systems described in literature. The main factors that influence the liquid crystalline properties of pyridinium ILC are discussed. Selected thermal data are given according to mesogenic group employed and its position (either N-substitution or pyridinium ring substitution) and the number of structural cationic units (mono-, di-, or polycationic pyridinium ILC).

Keywords: ionic liquids, liquid crystals, pyridinium salts

### 1. Introduction

Ionic liquid crystals (ILCs) are extensively studied nowadays due to their unique properties resulting from the combination of liquid crystal (LC) and ionic liquid (IL) properties. Several reviews covering this topic were published in the recent years [1–3]. The field of ILCs is continuously growing as many recent applications were found: solar cells, membranes, battery materials, electrochemical sensors, or electroluminescent switches. Different factors are responsible for governing the nature of ILC phases, such as the molecular shape, location, and size of ionic groups, intermolecular interactions, and microphase segregation. Thus, the hydrophobic interactions between the long alkyl chain groups, ionic, dipole-dipole, anion– cation hydrogen bonding, and cation-π interactions together with the π-π stacking of the aromatic rings, all have a contribution to the stabilization of the liquid crystalline phase. For instance, there is a strong tendency to stabilize lamellar (smectic) phases, with SmA the most common phase for ILC, due to electrostatic interactions and ion-ion stacking in ILC. The combination of all these factors leads to LC behavior ranges from typical calamitic materials to discotic. The imidazolium or pyridinium derivatives, one of the most studied classes of ILC, with weakly coordinating anions, such as tetrafluoroborate (BF4 − ) and hexafluorophosphate (PF6 − ), are well known for their high thermal and electrochemical stabilities. It is worth to

© The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, © 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

distribution, and eproduction in any medium, provided the original work is properly cited.

mention here that the pyridinium-based ILC has been known for a long time, displaying very similar properties with the related imidazolium-based ILC.

The most common types of LC phases displayed by ILCs are represented in Figure 1. Their identification relies upon three characterization techniques: polarizing optical microscopy (POM), differential scanning calorimetry (DSC), and powder X-ray diffraction (XRD). The usual textures seen by POM for different mesophases are presented in Figure 2. It is very common to see the SmA phase of ionic mesogens as a so-called oily streak texture by POM, in particular during heating runs (Figure 2). When cooling from the isotropic state, very often the SmA phase can develop spontaneous homeotropic alignment due to interactions developed between the cationic groups and the glass substrate surface (the samples are sandwiched between two microscope slides), and further orientation of the mesogenic groups. As a result, the microscopy image contains large dark regions corresponding to these homeotropic alignments.

The POM technique is an important tool for mesophase identification, but an ultimate technique that confirms unequivocally the phase type is the XRD method. The later one can give also important information regarding the internal organization within the mesophase resulting from the different molecular packing related to the chemical structure of the ionic mesogens and the nature of interactions between them.

The thermal behavior as well as the mesophase type of pyridinium-based ILCs depends on several factors: the position and the nature of the mesogenic group attached to the pyridinium ring and the counterion employed. These structural factors will be discussed further and selected examples will be presented to illustrate their influence on the mesomorphic behavior. The pyridinium-based ILC classification was made according to mesogenic group used and its position (either N-substitution or pyridinium ring substitution) and the number of cationic units contained in their structure (mono-, di-, or

Figure 2. Typical textures for LC phases: marbled texture of a nematic phase (a), oily-streaks texture for a SmA phase on heating run (b), focal conical fan-shaped texture for SmA phase (c), and fan-shaped texture of a hexagonal columnar

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287

The simplest 4-alkyl substitution protonated pyridinium salts with halide as counterions (1) show no liquid crystalline properties as they melt straight to the isotropic phase. Such products have been synthesized by reaction between the corresponding hydrogen halide with their

polycationic pyridinium ILC).

phase Colh (d).

2.1. Simple protonated pyridinium salts

corresponding 4-alkyl pyridine compounds [4].

2. Monocationic pyridinium ionic liquid crystals

Figure 1. Schematic representation of the most common liquid crystalline phases encountered for ionic liquid crystals.

Figure 2. Typical textures for LC phases: marbled texture of a nematic phase (a), oily-streaks texture for a SmA phase on heating run (b), focal conical fan-shaped texture for SmA phase (c), and fan-shaped texture of a hexagonal columnar phase Colh (d).

The thermal behavior as well as the mesophase type of pyridinium-based ILCs depends on several factors: the position and the nature of the mesogenic group attached to the pyridinium ring and the counterion employed. These structural factors will be discussed further and selected examples will be presented to illustrate their influence on the mesomorphic behavior. The pyridinium-based ILC classification was made according to mesogenic group used and its position (either N-substitution or pyridinium ring substitution) and the number of cationic units contained in their structure (mono-, di-, or polycationic pyridinium ILC).

### 2. Monocationic pyridinium ionic liquid crystals

#### 2.1. Simple protonated pyridinium salts

mention here that the pyridinium-based ILC has been known for a long time, displaying very

The most common types of LC phases displayed by ILCs are represented in Figure 1. Their identification relies upon three characterization techniques: polarizing optical microscopy (POM), differential scanning calorimetry (DSC), and powder X-ray diffraction (XRD). The usual textures seen by POM for different mesophases are presented in Figure 2. It is very common to see the SmA phase of ionic mesogens as a so-called oily streak texture by POM, in particular during heating runs (Figure 2). When cooling from the isotropic state, very often the SmA phase can develop spontaneous homeotropic alignment due to interactions developed between the cationic groups and the glass substrate surface (the samples are sandwiched between two microscope slides), and further orientation of the mesogenic groups. As a result, the microscopy image contains large dark regions corresponding to these

The POM technique is an important tool for mesophase identification, but an ultimate technique that confirms unequivocally the phase type is the XRD method. The later one can give also important information regarding the internal organization within the mesophase resulting from the different molecular packing related to the chemical structure of the ionic mesogens

Figure 1. Schematic representation of the most common liquid crystalline phases encountered for ionic liquid crystals.

similar properties with the related imidazolium-based ILC.

homeotropic alignments.

286 Progress and Developments in Ionic Liquids

and the nature of interactions between them.

The simplest 4-alkyl substitution protonated pyridinium salts with halide as counterions (1) show no liquid crystalline properties as they melt straight to the isotropic phase. Such products have been synthesized by reaction between the corresponding hydrogen halide with their corresponding 4-alkyl pyridine compounds [4].

By changing the alkyl group with an alkoxy group, the resulting protonated chloride pyridinium salts show a SmA phase, with decomposition near the isotropization process. The melting and isotropization temperatures of these products were very insensitive to the chain lengths, suggesting that the hydrophobic interactions are the least significant factor in the thermal behavior of such protonated pyridinium salts [5].

A series of more elaborated protonated pyridinium salts have been prepared by the reaction between pyridine derivatives and phosphoric acid with the aim of studying their anhydrous proton conduction. X-ray diffraction measurements suggested that the pyridinium salts formed a bilayer structure with head-to-head configuration in the SmA phase [6].

The protonated pyridinium cation was employed to generate liquid crystalline materials having as counterions a series of wedge-shaped benzenesulfonate mesogens 4–6 [7, 8]. Compound 4, having a rather unusual substitution pattern, 2,3,4-tris(dodecyloxy)benzenesulfonate, exhibits a reversible transition from a columnar disordered phase into an ordered columnar during the slow heating and cooling cycles. The thermal behavior of pyridinium salts 5 and 6 was compared to thermal properties of their corresponding benzenesulfonic acid and its sodium salts. It was found that 5 and 6 exhibit much lower transition temperatures than the sulfonic acids (the corresponding sulfonic acid of 6 was not stable upon drying) and sodium salts, respectively.

When the protonated pyridinium moiety is part of a α-diketone compound, the resulting βdiketone pyridinium chloride salts are not mesomorphic. Replacement of chloride anion with the tetrachlorozincate ion produced mesomorphic ionic salts displaying a SmA phases over a

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By changing the alkyl group with an alkoxy group, the resulting protonated chloride pyridinium salts show a SmA phase, with decomposition near the isotropization process. The melting and isotropization temperatures of these products were very insensitive to the chain lengths, suggesting that the hydrophobic interactions are the least significant factor in the

A series of more elaborated protonated pyridinium salts have been prepared by the reaction between pyridine derivatives and phosphoric acid with the aim of studying their anhydrous proton conduction. X-ray diffraction measurements suggested that the pyridinium salts

The protonated pyridinium cation was employed to generate liquid crystalline materials having as counterions a series of wedge-shaped benzenesulfonate mesogens 4–6 [7, 8]. Compound 4, having a rather unusual substitution pattern, 2,3,4-tris(dodecyloxy)benzenesulfonate, exhibits a reversible transition from a columnar disordered phase into an ordered columnar during the slow heating and cooling cycles. The thermal behavior of pyridinium salts 5 and 6 was compared to thermal properties of their corresponding benzenesulfonic acid and its sodium salts. It was found that 5 and 6 exhibit much lower transition temperatures than the sulfonic acids (the corresponding sulfonic acid of 6 was not stable upon drying) and sodium salts,

formed a bilayer structure with head-to-head configuration in the SmA phase [6].

thermal behavior of such protonated pyridinium salts [5].

288 Progress and Developments in Ionic Liquids

respectively.

When the protonated pyridinium moiety is part of a α-diketone compound, the resulting βdiketone pyridinium chloride salts are not mesomorphic. Replacement of chloride anion with the tetrachlorozincate ion produced mesomorphic ionic salts displaying a SmA phases over a broad temperature range [9]. The same authors reported the thermal behavior and photophysical properties of the metal-free β-diketone pyridinium ligands, and their allylpalladium(II) complexes (8, 9) [10, 11]. Interestingly, an equilibrium between 8 and 9 exists in solution. Another peculiar aspect about these complexes is that normally, the β-diketone ligands are first deprotonated before coordination to the metal center. Pd(II) metallomesogens with tetradecyloxy groups display a SmC phase, with reduced mesomorphic range for the protonated compound. The emission properties of Pd(II) complexes were preserved in the LC state.

#### 2.2. N-alkylated pyridinium salts

#### 2.2.1. N-methyl pyridinium salts

A clear trend for izotropization temperatures was seen for N-methyl pyridinium salts, meaning that, based on strong dependence of these temperatures on the alkyl chain length, longer alkyl chain length in position 4 of the pyridinium ring lead to higher transition temperatures. Moreover, it was observed that the 1-methyl-4-alkoxycarbonylpyridinium iodides salts have thermochromic properties. Color changes were observed on heating at both crystal to crystal and crystal to mesophase transitions, while such an observation could not be made for the 1 methyl-4-alkylpyridinium iodides salts [4, 12, 13].

Importantly, these salts (12) exhibit a SmA phase with relatively large mesophase ranges that become relatively independent of the chain length for alkyl chain length greater than 14 carbon atoms. On the contrary, for the tetrachlorometalate(II) salts, the mesophase stability increases monotonically with an increase in chain length. While initially, the n-dodecyl derivative has been found to show only a SmA phase, later it was reported that this compound showed cubic and smectic A phases on the heating run, and columnar (16–46°C), cubic (46–66°C), and smectic A (46–143°C) phases on the cooling run. The authors found that the phase transition temperatures were strongly influenced by a glass surface and on the

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Replacement of chloride anion with bulkier hexafluorophosphate anion led to mesophase destabilization for pyridinium salts with alkyl chain shorter than 16 carbon atoms. Extensive studies were performed for N-alkyl pyridinium salts with alkylsulfates as counterion, all of them displaying a typical SmA phase, the mesophase temperature range depending on the combination of chain lengths of both N-alkyl and the aliphatic chain connected to the sulfate

The tetrachlorocuprate pyridinium salts show a very interesting and rich polymorphism for alkyl chain longer than or equal to 12 carbon atoms, with hexagonal columnar, cubic, and SmA

phases appearing in the order of increasing the chain length and/or temperature [18].

thermal history [15].

group [16, 17].

#### 2.2.2. Other N-alkylated pyridinium salts

The simple N-alkylated chloride pyridinium salts 12 were obtained by reacting the corresponding alkyl chloride and pyridine, while the tetrachlorometalates, where M = Co(II) or Ni(II), were prepared by the reaction between these chloride pyridinium salts and anhydrous metallic chlorides [14]. The N-alkylated pyridinium chloride salts with a chain length below 12 units do not display liquid-crystal behavior.

broad temperature range [9]. The same authors reported the thermal behavior and photophysical properties of the metal-free β-diketone pyridinium ligands, and their allylpalladium(II) complexes (8, 9) [10, 11]. Interestingly, an equilibrium between 8 and 9 exists in solution. Another peculiar aspect about these complexes is that normally, the β-diketone ligands are first deprotonated before coordination to the metal center. Pd(II) metallomesogens with tetradecyloxy groups display a SmC phase, with reduced mesomorphic range for the protonated compound. The emission properties of Pd(II) complexes were preserved in the LC

A clear trend for izotropization temperatures was seen for N-methyl pyridinium salts, meaning that, based on strong dependence of these temperatures on the alkyl chain length, longer alkyl chain length in position 4 of the pyridinium ring lead to higher transition temperatures. Moreover, it was observed that the 1-methyl-4-alkoxycarbonylpyridinium iodides salts have thermochromic properties. Color changes were observed on heating at both crystal to crystal and crystal to mesophase transitions, while such an observation could not be made for the 1-

The simple N-alkylated chloride pyridinium salts 12 were obtained by reacting the corresponding alkyl chloride and pyridine, while the tetrachlorometalates, where M = Co(II) or Ni(II), were prepared by the reaction between these chloride pyridinium salts and anhydrous metallic chlorides [14]. The N-alkylated pyridinium chloride salts with a chain length

state.

2.2. N-alkylated pyridinium salts

methyl-4-alkylpyridinium iodides salts [4, 12, 13].

2.2.2. Other N-alkylated pyridinium salts

below 12 units do not display liquid-crystal behavior.

2.2.1. N-methyl pyridinium salts

290 Progress and Developments in Ionic Liquids

Importantly, these salts (12) exhibit a SmA phase with relatively large mesophase ranges that become relatively independent of the chain length for alkyl chain length greater than 14 carbon atoms. On the contrary, for the tetrachlorometalate(II) salts, the mesophase stability increases monotonically with an increase in chain length. While initially, the n-dodecyl derivative has been found to show only a SmA phase, later it was reported that this compound showed cubic and smectic A phases on the heating run, and columnar (16–46°C), cubic (46–66°C), and smectic A (46–143°C) phases on the cooling run. The authors found that the phase transition temperatures were strongly influenced by a glass surface and on the thermal history [15].

Replacement of chloride anion with bulkier hexafluorophosphate anion led to mesophase destabilization for pyridinium salts with alkyl chain shorter than 16 carbon atoms. Extensive studies were performed for N-alkyl pyridinium salts with alkylsulfates as counterion, all of them displaying a typical SmA phase, the mesophase temperature range depending on the combination of chain lengths of both N-alkyl and the aliphatic chain connected to the sulfate group [16, 17].

The tetrachlorocuprate pyridinium salts show a very interesting and rich polymorphism for alkyl chain longer than or equal to 12 carbon atoms, with hexagonal columnar, cubic, and SmA phases appearing in the order of increasing the chain length and/or temperature [18].

By exchanging the chloride anion with biologically active picrate, dodecylbenzenesulfonate, or cholate anions, different thermal behavior was found [19]. Thus, only the compound with dodecylbenzenesulfonate anion displayed a SmA phase stable up to 152°C, while the other two showed no liquid crystalline properties. The absence of mesomorphic properties in the case of picrate anion was attributed to the formation of interlayer 3-D hydrogen bond network between pyridinium and picrate ions.

considered to pack in an interdigitated fashion, with the anions sandwiched between the

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A series of N-alkylated pyridinium salts, having an alkoxy group in 4-position of pyridinium ring, were synthesized by the reaction of either N-alkyl-4-pyridones or 4-alkoxypyridines with corresponding alkyl bromides. Further, hexafluorophosphate and tetrafluoroborate pyridinium

These salts exhibit smectic A phase. The transition temperatures, both melting and clearing, as well as the mesophase stability were influenced by the alkyl chain length and the counterion

<sup>−</sup> or PF6

, produced smaller mesophase ranges, followed by

, when compared to bromide pyridinium salts as a consequence

<sup>−</sup> than for the Br<sup>−</sup> [5].

−

salts could be prepared by a metathesis reaction with the ammonium salts.

type. The hexafluorophosphate anion, PF6

of a weaker cation-anion interaction for the BF4

−

tetrafluoroborate anion, BF4

pyridinium rings.

#### 2.2.3. Chiral N-alkylated pyridinium salts

Recently, a series of N-alkylated pyridinium salts containing a chiral center in the four positions with respect to nitrogen atoms have been prepared and investigated by Laschat et al. [20, 21]. Their thermal properties were compared to their imidazolium counterparts. Generally, these salts show a SmA phase, with melting and isotropization temperatures lower than the temperatures of their corresponding imidazolium counterparts. The mesophase stability range depends on the alkyl group length as well as on the counterion and it was found to decrease in the following order: Br<sup>−</sup> > OAc<sup>−</sup> > BF4 <sup>−</sup> > I<sup>−</sup> > SCN<sup>−</sup> . The pyridinium salts with hexafluorophosphate anion show no liquid crystalline properties.

For chiral 1-citronellylpyridinium bromide salt 14, only a glass transition was detected and no mesomorphism was evidenced.

#### 2.2.4. N-alkylated pyridinium salts derived from picoline

The pyridinium salts derived from either 4- or 3-picoline, 16 and 17, have been extensively studied [12, 22–24]. Their mesophase stability (SmA phase) depends strongly on the alkyl chain length as well as on the position of methyl group on the pyridinium ring. The mesophase temperature range increases significantly with increasing the alkyl chain length, while the alkyl substitution at the 3- and 4-positions on the pyridinium ring leads to a decrease in the melting point compared with the corresponding unsubstitution pyridinium salts. The effect of pyridinium ring substitution was also seen on the mesophase broadness, the pyridinium salts derived from 4-picoline having a much broader mesomorphic range compared to those derived from 3-picoline. Regarding the internal mesophase structure, the molecules are considered to pack in an interdigitated fashion, with the anions sandwiched between the pyridinium rings.

By exchanging the chloride anion with biologically active picrate, dodecylbenzenesulfonate, or cholate anions, different thermal behavior was found [19]. Thus, only the compound with dodecylbenzenesulfonate anion displayed a SmA phase stable up to 152°C, while the other two showed no liquid crystalline properties. The absence of mesomorphic properties in the case of picrate anion was attributed to the formation of interlayer 3-D hydrogen bond network

Recently, a series of N-alkylated pyridinium salts containing a chiral center in the four positions with respect to nitrogen atoms have been prepared and investigated by Laschat et al. [20, 21]. Their thermal properties were compared to their imidazolium counterparts. Generally, these salts show a SmA phase, with melting and isotropization temperatures lower than the temperatures of their corresponding imidazolium counterparts. The mesophase stability range depends on the alkyl group length as well as on the counterion and it was found to decrease in

<sup>−</sup> > I<sup>−</sup> > SCN<sup>−</sup>

For chiral 1-citronellylpyridinium bromide salt 14, only a glass transition was detected and no

The pyridinium salts derived from either 4- or 3-picoline, 16 and 17, have been extensively studied [12, 22–24]. Their mesophase stability (SmA phase) depends strongly on the alkyl chain length as well as on the position of methyl group on the pyridinium ring. The mesophase temperature range increases significantly with increasing the alkyl chain length, while the alkyl substitution at the 3- and 4-positions on the pyridinium ring leads to a decrease in the melting point compared with the corresponding unsubstitution pyridinium salts. The effect of pyridinium ring substitution was also seen on the mesophase broadness, the pyridinium salts derived from 4-picoline having a much broader mesomorphic range compared to those derived from 3-picoline. Regarding the internal mesophase structure, the molecules are

. The pyridinium salts with hexafluoro-

between pyridinium and picrate ions.

292 Progress and Developments in Ionic Liquids

2.2.3. Chiral N-alkylated pyridinium salts

the following order: Br<sup>−</sup> > OAc<sup>−</sup> > BF4

mesomorphism was evidenced.

2.2.4. N-alkylated pyridinium salts derived from picoline

phosphate anion show no liquid crystalline properties.

A series of N-alkylated pyridinium salts, having an alkoxy group in 4-position of pyridinium ring, were synthesized by the reaction of either N-alkyl-4-pyridones or 4-alkoxypyridines with corresponding alkyl bromides. Further, hexafluorophosphate and tetrafluoroborate pyridinium salts could be prepared by a metathesis reaction with the ammonium salts.

These salts exhibit smectic A phase. The transition temperatures, both melting and clearing, as well as the mesophase stability were influenced by the alkyl chain length and the counterion type. The hexafluorophosphate anion, PF6 − , produced smaller mesophase ranges, followed by tetrafluoroborate anion, BF4 − , when compared to bromide pyridinium salts as a consequence of a weaker cation-anion interaction for the BF4 <sup>−</sup> or PF6 <sup>−</sup> than for the Br<sup>−</sup> [5].

Pyridinium salts with a 1,3-dioxane ring attached in 4-position were prepared and their liquid crystalline properties were investigated [25, 26]. A very wide range for the SmA mesophase (between −24 and 150°C) was found for ionic thermotropic liquid crystal system having two rings in its central core (19) reported by Haramoto et al. [25]. Based on the same design, chiral pyridinium liquid crystals 20 show an interesting thermal behavior [26].

a mesophase was made based on polarizing optical microscopy when a planar texture with bright oily streaks was identified for such compounds. No additional XRD studies were

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Several other pyridinium ILCs having a second aromatic ring connected to the 4-position of the pyridinium ring via a linking group (azo, 22, acetylene, 23, or imine, 24) were designed and investigated [28, 29]. Their thermal properties are influenced by the alkyl chain length, the nature of linking groups, while the dominant factor for 24 was the size of perfluorinated

N-alkyl-stilbazolium ILC, having different groups at the 4'-position of the stilbazolium head unit, were investigated by several authors [23, 30, 31]. The 4'-substitution at the stilbazolium unit has a great influence on the LC properties. For instance, the compounds with NO2 or CN groups and bromide as counterion show a relatively narrow temperature range of the LC phase [30]. By introduction of chloride anions, the LC phase stability increases and the compounds decompose before reaching the isotropic phase. The use of dialkylamino group at the 4'-position in the stilbazolium core led to a significant decrease of the temperature range of the mesophase. For compounds 25a-f, the substituents at the 4'-position of stilbazolium core increase the stability of the SmA phase in the following order: OCH3 > OH > H, meaning that the mesomorphic behavior is also influenced by dipolar interactions resulting from OH and OCH3 groups [23]. All these salts show an increase of the SmA phase range with increasing the alkyl chain length. On the other hand, the stability of the SmA phase is influenced by the size

undertaken to characterize this phase.

2.2.5. N-Alkyl-4'-substitution-stilbazolium halides

ring [29].

First, LC phases were observed only for long alkyl chains on both sides of the molecule. Furthermore, the authors found that the enantiomeric purity influences significantly the nature of the mesophase (chiral nematic phase N\* for pure single enantiomeric compounds and nematic and smectic phases for racemic mixture) and only slightly the transition temperatures between solid state and LC phase and LC phase and isotropic state. Moreover, the LC phase stability depends on the counterion size (higher stability for halide and tetrafluoroborate anions) due to the contribution of bigger anions to decreasing the packing of the mesomorphic cationic units. Yousif et al. found an enantiotropic cholesteric phase (N\*) for a series of quaternized cholesteryl isonicotinates with tosylate counterions [27]. The assignment of such

a mesophase was made based on polarizing optical microscopy when a planar texture with bright oily streaks was identified for such compounds. No additional XRD studies were undertaken to characterize this phase.

Several other pyridinium ILCs having a second aromatic ring connected to the 4-position of the pyridinium ring via a linking group (azo, 22, acetylene, 23, or imine, 24) were designed and investigated [28, 29]. Their thermal properties are influenced by the alkyl chain length, the nature of linking groups, while the dominant factor for 24 was the size of perfluorinated ring [29].

#### 2.2.5. N-Alkyl-4'-substitution-stilbazolium halides

Pyridinium salts with a 1,3-dioxane ring attached in 4-position were prepared and their liquid crystalline properties were investigated [25, 26]. A very wide range for the SmA mesophase (between −24 and 150°C) was found for ionic thermotropic liquid crystal system having two rings in its central core (19) reported by Haramoto et al. [25]. Based on the same design, chiral

First, LC phases were observed only for long alkyl chains on both sides of the molecule. Furthermore, the authors found that the enantiomeric purity influences significantly the nature of the mesophase (chiral nematic phase N\* for pure single enantiomeric compounds and nematic and smectic phases for racemic mixture) and only slightly the transition temperatures between solid state and LC phase and LC phase and isotropic state. Moreover, the LC phase stability depends on the counterion size (higher stability for halide and tetrafluoroborate anions) due to the contribution of bigger anions to decreasing the packing of the mesomorphic cationic units. Yousif et al. found an enantiotropic cholesteric phase (N\*) for a series of quaternized cholesteryl isonicotinates with tosylate counterions [27]. The assignment of such

pyridinium liquid crystals 20 show an interesting thermal behavior [26].

294 Progress and Developments in Ionic Liquids

N-alkyl-stilbazolium ILC, having different groups at the 4'-position of the stilbazolium head unit, were investigated by several authors [23, 30, 31]. The 4'-substitution at the stilbazolium unit has a great influence on the LC properties. For instance, the compounds with NO2 or CN groups and bromide as counterion show a relatively narrow temperature range of the LC phase [30]. By introduction of chloride anions, the LC phase stability increases and the compounds decompose before reaching the isotropic phase. The use of dialkylamino group at the 4'-position in the stilbazolium core led to a significant decrease of the temperature range of the mesophase. For compounds 25a-f, the substituents at the 4'-position of stilbazolium core increase the stability of the SmA phase in the following order: OCH3 > OH > H, meaning that the mesomorphic behavior is also influenced by dipolar interactions resulting from OH and OCH3 groups [23]. All these salts show an increase of the SmA phase range with increasing the alkyl chain length. On the other hand, the stability of the SmA phase is influenced by the size of the counterion; lower melting and clearing temperatures were seen for iodide compared to bromide stilbazolium salts.

alkyl sulfate ions, a significant depression of clearing temperature was achieved (about 40°C) [32]. On the other hand, a total suppression of liquid crystalline phase was observed when the bis(2-ethylhexyl)-sulfosuccinate ion (DOSS) was employed. Interestingly, the pyridinium bromide salts display thermochromic properties [33]. Thus, the crystal to SmA transition of such compounds was accompanied with a color change from pale yellow to bright red that is almost fully reversible upon cooling. Such thermochromic effect was not observed on changing the bromide anion for alkylsulfate or DOSS ions. The origin of the thermochromic properties was attributed by authors to a charge-transfer couple formation between the bromide and the

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More recently, a systematic study of liquid crystalline and photophysical properties of a series of pyridinium salts was reported [34, 35], where the 1,3,4-oxadiazole unit connects one

The N-decyl pyridinium salts exhibit SmA mesomorphism, regardless of the counterion, while their N-methyl counterparts show decomposition and no liquid crystalline properties for

decompose before reaching the isotropic phase from the previous SmA phase, suggesting that the thermal properties (melting point, mesophase range, and clearing or decomposition temperature) are sensitive to counterion exchange. Moreover, the two pyridinium salts with DS as counterion are similar in terms of thermal behavior, and this was explained by XRD studies. These compounds have a monolayered SmA phase, instead of a bilayered SmA phase found for the others compounds of the series, and the same molecular lengths (due to the presence of the same counterion), and such a different mesophase structure could account for their similar

− , ClO4 −

, and DS), the salts

iodine and nitrate anions, and, for the remaining anions (BF4

pyridinium ring and one mono- or di-substitution phenyl ring with alkoxy groups.

pyridinium ring.

behavior.

#### 2.2.6. Monocationic oxadiazole pyridinium salts

The pyridinium bromide salts 26a-d, all of them show a SmA phase with a slight increase of clearing temperatures on increasing the alkyl chain length. By replacing the bromide ion with

alkyl sulfate ions, a significant depression of clearing temperature was achieved (about 40°C) [32]. On the other hand, a total suppression of liquid crystalline phase was observed when the bis(2-ethylhexyl)-sulfosuccinate ion (DOSS) was employed. Interestingly, the pyridinium bromide salts display thermochromic properties [33]. Thus, the crystal to SmA transition of such compounds was accompanied with a color change from pale yellow to bright red that is almost fully reversible upon cooling. Such thermochromic effect was not observed on changing the bromide anion for alkylsulfate or DOSS ions. The origin of the thermochromic properties was attributed by authors to a charge-transfer couple formation between the bromide and the pyridinium ring.

of the counterion; lower melting and clearing temperatures were seen for iodide compared to

The pyridinium bromide salts 26a-d, all of them show a SmA phase with a slight increase of clearing temperatures on increasing the alkyl chain length. By replacing the bromide ion with

bromide stilbazolium salts.

296 Progress and Developments in Ionic Liquids

2.2.6. Monocationic oxadiazole pyridinium salts

More recently, a systematic study of liquid crystalline and photophysical properties of a series of pyridinium salts was reported [34, 35], where the 1,3,4-oxadiazole unit connects one pyridinium ring and one mono- or di-substitution phenyl ring with alkoxy groups.

The N-decyl pyridinium salts exhibit SmA mesomorphism, regardless of the counterion, while their N-methyl counterparts show decomposition and no liquid crystalline properties for iodine and nitrate anions, and, for the remaining anions (BF4 − , ClO4 − , and DS), the salts decompose before reaching the isotropic phase from the previous SmA phase, suggesting that the thermal properties (melting point, mesophase range, and clearing or decomposition temperature) are sensitive to counterion exchange. Moreover, the two pyridinium salts with DS as counterion are similar in terms of thermal behavior, and this was explained by XRD studies. These compounds have a monolayered SmA phase, instead of a bilayered SmA phase found for the others compounds of the series, and the same molecular lengths (due to the presence of the same counterion), and such a different mesophase structure could account for their similar behavior.

An interesting example showing how the 3'- or 4'-substitution pattern of the pyridinium ring affects the thermal behavior is represented by the iodides and trifluoromethanesulfonates salts derived from perfluoroalkylated 1,2,4-oxadiazolylpyridines 28–31 [36]. Thus, the 3'-substitution derivatives exhibited liquid crystalline properties (SmX phase) on a narrow temperature range, while the corresponding 4'-substitution derivatives were found to pass from the crystalline state straight to the isotropic phase. Such a behavior was attributed to a charge delocalization over the entire structure, including the oxadiazole ring, which makes the cation/anion electrostatic interactions weaker and, thus, leading to the mesophase detabilization for 4' substitution compounds.

The pyridinium bromide salt shows one enantiotropic columnar mesophase and one additional monotropic columnar phase at lower temperatures. The size of counterion has a significant contribution to the LC phase stability. Surprisingly, when the bromide ion (Br−) was

liquid crystalline behavior. The photoluminescent properties of these pyridinium salts were investigated both in solution and solid state and it has been shown that their emission is only

The classical triphenylene unit was connected to pyridinium ring via a flexible methylene flexible chain to give discotic liquid crystals [38]. The mesophase was identified to be a columnar phase based on microscopy observations of optical textures. The stability of this columnar phase was found to depend both on the peripheral alkyl chain length as well as the methylene spacer length. Thus, the stability of columnar phase increases by increasing the number of carbon atoms on the peripheral chains of the triphenylene core, while longer spacer

Interesting results were obtained for pyridinium bromides salts containing a biphenyl core and alkyl chains of different lengths 34 [39]. The substitution of the pyridine ring greatly influences the thermal behavior of these salts. While the unsubstitution pyridinium groups promote

connecting the triphenylene unit with the pyridine ring destabilized the mesophase.

, and PF6

−

), the resulting products showed no

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− , BF4 −

replaced with bulkier counterions (NO3

slightly influenced by the nature of counterion employed.

The fact that only the iodide salts showed liquid crystalline properties was explained by the greater coordinating ability of the iodide anions with respect to the trifluoromethanesulfonates anions. Moreover, all iodide salts showed thermochromism phenomena suggesting prospective applications in optoelectronics.

#### 2.3. Monocationic pyridinium salts with mesogenic groups attached to nitrogen atom

A series of pyridinium salts with the 3,4,5-tridodecyloxybenzyl moiety attached to nitrogen atom and different counterions (bromide, nitrate, tetrafluoroborate, and hexafluorophosphate) 32 were prepared starting from 4-hydroxypyridine [37].

The pyridinium bromide salt shows one enantiotropic columnar mesophase and one additional monotropic columnar phase at lower temperatures. The size of counterion has a significant contribution to the LC phase stability. Surprisingly, when the bromide ion (Br−) was replaced with bulkier counterions (NO3 − , BF4 − , and PF6 − ), the resulting products showed no liquid crystalline behavior. The photoluminescent properties of these pyridinium salts were investigated both in solution and solid state and it has been shown that their emission is only slightly influenced by the nature of counterion employed.

An interesting example showing how the 3'- or 4'-substitution pattern of the pyridinium ring affects the thermal behavior is represented by the iodides and trifluoromethanesulfonates salts derived from perfluoroalkylated 1,2,4-oxadiazolylpyridines 28–31 [36]. Thus, the 3'-substitution derivatives exhibited liquid crystalline properties (SmX phase) on a narrow temperature range, while the corresponding 4'-substitution derivatives were found to pass from the crystalline state straight to the isotropic phase. Such a behavior was attributed to a charge delocalization over the entire structure, including the oxadiazole ring, which makes the cation/anion electrostatic interactions weaker and, thus, leading to the mesophase detabilization for 4'-

The fact that only the iodide salts showed liquid crystalline properties was explained by the greater coordinating ability of the iodide anions with respect to the trifluoromethanesulfonates anions. Moreover, all iodide salts showed thermochromism phenomena suggesting prospec-

2.3. Monocationic pyridinium salts with mesogenic groups attached to nitrogen atom

A series of pyridinium salts with the 3,4,5-tridodecyloxybenzyl moiety attached to nitrogen atom and different counterions (bromide, nitrate, tetrafluoroborate, and hexafluorophosphate)

substitution compounds.

298 Progress and Developments in Ionic Liquids

tive applications in optoelectronics.

32 were prepared starting from 4-hydroxypyridine [37].

The classical triphenylene unit was connected to pyridinium ring via a flexible methylene flexible chain to give discotic liquid crystals [38]. The mesophase was identified to be a columnar phase based on microscopy observations of optical textures. The stability of this columnar phase was found to depend both on the peripheral alkyl chain length as well as the methylene spacer length. Thus, the stability of columnar phase increases by increasing the number of carbon atoms on the peripheral chains of the triphenylene core, while longer spacer connecting the triphenylene unit with the pyridine ring destabilized the mesophase.

Interesting results were obtained for pyridinium bromides salts containing a biphenyl core and alkyl chains of different lengths 34 [39]. The substitution of the pyridine ring greatly influences the thermal behavior of these salts. While the unsubstitution pyridinium groups promote mesomorphism (SmA and SmC phases), the 2- and 4-ethyl-substitution pyridinium groups give rise to liquid crystalline phases only with sufficiently long alkyl chains (decyl chains on both sides of the biphenyl core). The salts having a 3,5-dimethyl substitution pyridine ring do not show any liquid crystalline properties. Moreover, the substitution pattern at the pyridinium groups resulted in different types of smectic phases (SmA, SmC, and SmE).

2.3.2. Nematic pyridinium ionic liquid crystals

spacer length between the ionic core and the mesogenic group.

Starting from 4-hydroxypyridine, it is possible to attach two cyanobiphenyl mesogenic units, via flexible alkyl spacer, on both sides of the pyridinium ring giving rise to a series of nematic pyridinium liquid crystals [42]. The nematic phase has numerous technological applications due to its highest fluidity of all LC phases and hence the possibility to align it by applying an external electric/magnetic field. Moreover, the nematic phase is commonly used in electrooptical devices. The nematic phase is quite rare in the case of ILCs, several examples have been reported so far for ammonium [43, 44], imidazolium [45–48], pyridinium-based ILC [49], or miscellaneous type of ILC [50–54]. Generally, as the smectic phases are the most common phases for ILCs, especially due to electrostatic interactions, the nematic phase can be seen rather as an exception. It has to be reminded here the cholesteryl-containing compounds 21ag that display a cholesteric phase (chiral nematic phase N\*) at relatively high temperatures and on a narrow temperature range. Compounds 39, all of them with the exception of 39d and 39e, displayed a monotropic nematic phase on cooling from the isotropic state. This is the first example of a series of ILC that exhibit a long range nematic phase, no matter the counterion or

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The nematic phase was identified by using the combination of the three techniques: POM, DSC, and XRD, as well as, miscibility studies with 5 CB and doping with a chiral compound. A

When an additional ring was added, the new pyridinium bromide salts 35 with a triphenylene moiety displayed tilted smectic mesophases (SmC) with molecules stacked in a double layer morphology, as indicated by XRD studies [40]. The tilt angle of these compounds shows a great temperature dependence, and, in fact, at lower temperature being closed to 0<sup>o</sup> .

The XRD studies together with molecular modeling indicated that π-π interactions counterbalance the strong ionic forces leading to a full segregation of molecular parts in the smectic structures.

#### 2.3.1. Dendritic pyridinium ionic liquid crystals

Pyridinium chloride salts having dendritic building blocks connected to the nitrogen atom have been investigated by Percec et al. [41]. Depending on the number of peripheral alkoxy groups, these molecules were found to form columns or spheres, leading to 2D hexagonal columnar phases or a 3D cubic phase, respectively.

#### 2.3.2. Nematic pyridinium ionic liquid crystals

mesomorphism (SmA and SmC phases), the 2- and 4-ethyl-substitution pyridinium groups give rise to liquid crystalline phases only with sufficiently long alkyl chains (decyl chains on both sides of the biphenyl core). The salts having a 3,5-dimethyl substitution pyridine ring do not show any liquid crystalline properties. Moreover, the substitution pattern at the pyridinium groups resulted in different types of smectic phases (SmA, SmC, and SmE).

When an additional ring was added, the new pyridinium bromide salts 35 with a triphenylene moiety displayed tilted smectic mesophases (SmC) with molecules stacked in a double layer morphology, as indicated by XRD studies [40]. The tilt angle of these compounds shows a

The XRD studies together with molecular modeling indicated that π-π interactions counterbalance the strong ionic forces leading to a full segregation of molecular parts in the smectic

Pyridinium chloride salts having dendritic building blocks connected to the nitrogen atom have been investigated by Percec et al. [41]. Depending on the number of peripheral alkoxy groups, these molecules were found to form columns or spheres, leading to 2D hexagonal

structures.

2.3.1. Dendritic pyridinium ionic liquid crystals

300 Progress and Developments in Ionic Liquids

columnar phases or a 3D cubic phase, respectively.

.

great temperature dependence, and, in fact, at lower temperature being closed to 0<sup>o</sup>

Starting from 4-hydroxypyridine, it is possible to attach two cyanobiphenyl mesogenic units, via flexible alkyl spacer, on both sides of the pyridinium ring giving rise to a series of nematic pyridinium liquid crystals [42]. The nematic phase has numerous technological applications due to its highest fluidity of all LC phases and hence the possibility to align it by applying an external electric/magnetic field. Moreover, the nematic phase is commonly used in electrooptical devices. The nematic phase is quite rare in the case of ILCs, several examples have been reported so far for ammonium [43, 44], imidazolium [45–48], pyridinium-based ILC [49], or miscellaneous type of ILC [50–54]. Generally, as the smectic phases are the most common phases for ILCs, especially due to electrostatic interactions, the nematic phase can be seen rather as an exception. It has to be reminded here the cholesteryl-containing compounds 21ag that display a cholesteric phase (chiral nematic phase N\*) at relatively high temperatures and on a narrow temperature range. Compounds 39, all of them with the exception of 39d and 39e, displayed a monotropic nematic phase on cooling from the isotropic state. This is the first example of a series of ILC that exhibit a long range nematic phase, no matter the counterion or spacer length between the ionic core and the mesogenic group.

The nematic phase was identified by using the combination of the three techniques: POM, DSC, and XRD, as well as, miscibility studies with 5 CB and doping with a chiral compound. A marbled texture or thread-like texture could be seen by POM, which flashed brightly under pressure, while several samples also exhibited regions with not well developed Schlieren texture. Previous examples of pyridinium ILC showing a Schlieren texture were assigned to a SmC phase [39]. For this reason, additional XRD studies were undertaken to rule out the possibility of misinterpretation of experimental data. Indeed, the diffractograms showed no sharp peaks in the low-angle region, but just a broad signal centered at 4.5 Å assigned to the average intermolecular separation, close to the typical value for liquid crystalline phases, confirming the nematic phase nature. Additional confirmation came from miscibility studies with common nematic LC, a mixture of 39i 40% wt. and 5 CB displayed the N-Iso transition at 54°C. Furthermore, doping 39i with a chiral dopant, a typical fingerprint texture could be seen by POM, a good indication of nematic phase. It has been shown that the temperature range of the liquid crystalline phase is greatly influenced by the spacer length and the nature of counterion employed. In fact, the very bulky PF6 <sup>−</sup> and OTf<sup>−</sup> anions have a great contribution to the destabilization of nematic phase.

range of stability, from about 0°C up to above 140°C. More structural changes on these 4,4' bipyridinium systems, in general, give rise to more ordered smectic phases. 4,4'-bipyridine group can also be employed to prepare polymeric liquid crystals 41, and one example bearing a pentaphenylene connecting group via flexible chain length is presented below [60]. This polymeric ionic liquid crystal shows single layer smectic-type mesophases, both tilted (SmC)

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Columnar phases could be obtained for liquid crystals bearing two 3,4,5-tris(alkoxy) benzyl units attached to the 4,4'-bipyridinium moiety [61]. The viologen product bearing six octyloxy chains shows a hexagonal columnar (Colh) phase, while the analogues with longer alkoxy chains (12 or 16) show rectangular columnar (Colr) phases. For these products, the clearing temperatures could not be measured due to thermal degradation before reaching the isotropic state. Different molecular packing (Colh or Colr) was explained by formation of more elliptical molecular structures for compounds with elongated terminal alkoxy chains that may prefer to

and orthogonal (SmA) phases.

form the Colr phases.

The NMR data are strongly dependent on the anion-cation interaction specific to ILC, and they can be related to the presence of hydrogen-bonding interactions that cause downfield chemical shift of the related H atoms for imidazolium- and pyridinium-based ILC. Stronger interactions with anions lead to further downfield chemical shifts, in particular for C-H adjacent to nitrogen atom. The chemical shifts of these protons, from the <sup>1</sup> H NMR spectra of pyridinium salts 39a-s, were found to follow the order: Br<sup>−</sup> > SCN<sup>−</sup> ~NO3 − > OTf <sup>−</sup> > BF4 <sup>−</sup> > PF6 − .

### 3. Dicationic bis(pyridinium) and polycationic pyridinium salts

#### 3.1. 4,4'-Bipyridinium-based ionic liquid crystals

The 4,4'-bipyridinium salts or viologens are a special class of materials that show interesting properties, such as electrochromism and electrical conductivity, which were successfully employed in producing liquid crystalline materials. The reports dealing with liquid crystals based on 4,4' bipyridinium salts significantly grew after 2000. It is important to mention that such materials can be part of a two steps reduction process, and this process is reversible, as depicted below.

Additionally, it is of interest to note here that many dicationic and tetracationic ILC based on the 4,4'-bipyridinium rigid core (viologen-based ILC) have been investigated so far, displaying mesomorphic properties typically of calamitic and discotic materials [55–59].

The mesogenic properties of compounds based on asymmetric viologen salts of bis(trifluoromethanesulfonyl)amide ([NTf2] − ) 40 were influenced by their symmetry. It was found that the strongly asymmetric system does not display mesomorphic behavior, having low melting points (below 40°C). The less asymmetric products exhibit a smectic phase (SmB) with a wide range of stability, from about 0°C up to above 140°C. More structural changes on these 4,4' bipyridinium systems, in general, give rise to more ordered smectic phases. 4,4'-bipyridine group can also be employed to prepare polymeric liquid crystals 41, and one example bearing a pentaphenylene connecting group via flexible chain length is presented below [60]. This polymeric ionic liquid crystal shows single layer smectic-type mesophases, both tilted (SmC) and orthogonal (SmA) phases.

marbled texture or thread-like texture could be seen by POM, which flashed brightly under pressure, while several samples also exhibited regions with not well developed Schlieren texture. Previous examples of pyridinium ILC showing a Schlieren texture were assigned to a SmC phase [39]. For this reason, additional XRD studies were undertaken to rule out the possibility of misinterpretation of experimental data. Indeed, the diffractograms showed no sharp peaks in the low-angle region, but just a broad signal centered at 4.5 Å assigned to the average intermolecular separation, close to the typical value for liquid crystalline phases, confirming the nematic phase nature. Additional confirmation came from miscibility studies with common nematic LC, a mixture of 39i 40% wt. and 5 CB displayed the N-Iso transition at 54°C. Furthermore, doping 39i with a chiral dopant, a typical fingerprint texture could be seen by POM, a good indication of nematic phase. It has been shown that the temperature range of the liquid crystalline phase is greatly influenced by the spacer length and the nature of

The NMR data are strongly dependent on the anion-cation interaction specific to ILC, and they can be related to the presence of hydrogen-bonding interactions that cause downfield chemical shift of the related H atoms for imidazolium- and pyridinium-based ILC. Stronger interactions with anions lead to further downfield chemical shifts, in particular for C-H adjacent to nitro-

> SCN<sup>−</sup>

The 4,4'-bipyridinium salts or viologens are a special class of materials that show interesting properties, such as electrochromism and electrical conductivity, which were successfully employed in producing liquid crystalline materials. The reports dealing with liquid crystals based on 4,4' bipyridinium salts significantly grew after 2000. It is important to mention that such materials can

Additionally, it is of interest to note here that many dicationic and tetracationic ILC based on the 4,4'-bipyridinium rigid core (viologen-based ILC) have been investigated so far, displaying

The mesogenic properties of compounds based on asymmetric viologen salts of bis(trifluor-

strongly asymmetric system does not display mesomorphic behavior, having low melting points (below 40°C). The less asymmetric products exhibit a smectic phase (SmB) with a wide

be part of a two steps reduction process, and this process is reversible, as depicted below.

mesomorphic properties typically of calamitic and discotic materials [55–59].

−

3. Dicationic bis(pyridinium) and polycationic pyridinium salts

~NO3 − > OTf <sup>−</sup>

<sup>−</sup> and OTf<sup>−</sup> anions have a great contribution

> BF4

) 40 were influenced by their symmetry. It was found that the

H NMR spectra of pyridinium salts

<sup>−</sup> > PF6 − .

counterion employed. In fact, the very bulky PF6

gen atom. The chemical shifts of these protons, from the <sup>1</sup>

to the destabilization of nematic phase.

302 Progress and Developments in Ionic Liquids

39a-s, were found to follow the order: Br<sup>−</sup>

omethanesulfonyl)amide ([NTf2]

3.1. 4,4'-Bipyridinium-based ionic liquid crystals

Columnar phases could be obtained for liquid crystals bearing two 3,4,5-tris(alkoxy) benzyl units attached to the 4,4'-bipyridinium moiety [61]. The viologen product bearing six octyloxy chains shows a hexagonal columnar (Colh) phase, while the analogues with longer alkoxy chains (12 or 16) show rectangular columnar (Colr) phases. For these products, the clearing temperatures could not be measured due to thermal degradation before reaching the isotropic state. Different molecular packing (Colh or Colr) was explained by formation of more elliptical molecular structures for compounds with elongated terminal alkoxy chains that may prefer to form the Colr phases.

#### 3.2. Dicationic bis(pyridinium) salts with flexible linker

Bis(pyridinium) salts with flexible spacers 43 derived from 4-hydroxypyridine containing mesogenic 3,4,5-tris(alkyloxy)benzyl moieties (alkyl = dodecyl or tetradecyl) on each side and various counterions, such as bromide (Br<sup>−</sup> ), hexafluorophosphate (PF6 − ), tetrafluoroborate (BF4 − ), and triflate (OTf<sup>−</sup> ), were reported recently [62]. While there are numerous examples of dicationic pyridinium ILC, this one represents the first example of this type. These dicationic pyridinium salts display an enantiotropic liquid crystalline behavior with a hexagonal columnar (Colh) phase assigned on the basis of their characteristic texture, pseudo-focal conic and spherulitic textures, when observed by POM and XRD studies. The temperature range of the hexagonal columnar phase is greatly influenced by the terminal chain length (12 or 14) and the nature of counterion employed. For example, the triflate salt with 12 carbon atoms in the terminal chains, 43d, has the lowest melting point, 15°C, below ambient temperature and does not show any mesogenic behavior. It was found that the thermal behavior resembles the general trend found for pyridinium ILC, where transition temperatures show a decreasing tendency with an increase in the size of anions. Furthermore, the compounds with higher number of carbon atoms have higher melting points and isotropization temperatures with broader mesophase ranges (31°C for 43a and 40°C for 43e).

3.3. Dicationic bis(pyridinium) salts with a rigid core

ophosphate salts) rectangular columnar phases.

3.4. Tripodal pyridinium ILC

in solution [65].

crystalline properties and stimulus responsive luminescence [63].

Dicationic pyridinium salts with an anthracene moiety connecting two pyridine rings substitution with tris(alkoxy)benzyl groups 44 were designed and investigated for their liquid

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The anthracene moiety act as a fluorophore in the center of molecules, and the solvate of pyridiniums salts 44, n = 1, show piezoluminescence by grinding. The anion has an influence on the LC phase stability and thus, the long alkyl chain derivatives 44a-f show either enantiotropic (for bromide and tetrafluoroborate salts) or monotropic (in the case of hexafluor-

Photoluminescent tripodal pyridinium-based ionic liquid crystals were reported by Kato et al. [64, 65]. These tricationic pyridinium salts show thermotropic hexagonal or rectangular columnar phases or cubic phases with a large temperature ranges. The columnar or cubic phase stability is given by the difference in the anion size. For small-size anions (bromide), the cationic pyridinium cores should be packed more closely through electrostatic interactions and hence, the molecules prefer to self-assemble in cubic structures at higher temperatures (45a). Upon photoirradiation with UV light, these pyridinium salts emit blue-green or green light with photoluminescence quantum yields rather low, 3–6%


The study of emission properties of these luminescent bis(pyridinium) salts revealed a weak emission in dichloromethane solutions at room temperature, with quantum yields up to 4.4%. Their solid-state emission is significantly red shifted by comparison to solution emission spectra recorded in dichloromethane, suggesting a more complex emission mechanism, probably an aggregate-type emission in solid state [62].

#### 3.3. Dicationic bis(pyridinium) salts with a rigid core

3.2. Dicationic bis(pyridinium) salts with flexible linker

broader mesophase ranges (31°C for 43a and 40°C for 43e).

ably an aggregate-type emission in solid state [62].

various counterions, such as bromide (Br<sup>−</sup>

), and triflate (OTf<sup>−</sup>

304 Progress and Developments in Ionic Liquids

(BF4 −

Bis(pyridinium) salts with flexible spacers 43 derived from 4-hydroxypyridine containing mesogenic 3,4,5-tris(alkyloxy)benzyl moieties (alkyl = dodecyl or tetradecyl) on each side and

dicationic pyridinium ILC, this one represents the first example of this type. These dicationic pyridinium salts display an enantiotropic liquid crystalline behavior with a hexagonal columnar (Colh) phase assigned on the basis of their characteristic texture, pseudo-focal conic and spherulitic textures, when observed by POM and XRD studies. The temperature range of the hexagonal columnar phase is greatly influenced by the terminal chain length (12 or 14) and the nature of counterion employed. For example, the triflate salt with 12 carbon atoms in the terminal chains, 43d, has the lowest melting point, 15°C, below ambient temperature and does not show any mesogenic behavior. It was found that the thermal behavior resembles the general trend found for pyridinium ILC, where transition temperatures show a decreasing tendency with an increase in the size of anions. Furthermore, the compounds with higher number of carbon atoms have higher melting points and isotropization temperatures with

The study of emission properties of these luminescent bis(pyridinium) salts revealed a weak emission in dichloromethane solutions at room temperature, with quantum yields up to 4.4%. Their solid-state emission is significantly red shifted by comparison to solution emission spectra recorded in dichloromethane, suggesting a more complex emission mechanism, prob-

), hexafluorophosphate (PF6

), were reported recently [62]. While there are numerous examples of

−

), tetrafluoroborate

Dicationic pyridinium salts with an anthracene moiety connecting two pyridine rings substitution with tris(alkoxy)benzyl groups 44 were designed and investigated for their liquid crystalline properties and stimulus responsive luminescence [63].

The anthracene moiety act as a fluorophore in the center of molecules, and the solvate of pyridiniums salts 44, n = 1, show piezoluminescence by grinding. The anion has an influence on the LC phase stability and thus, the long alkyl chain derivatives 44a-f show either enantiotropic (for bromide and tetrafluoroborate salts) or monotropic (in the case of hexafluorophosphate salts) rectangular columnar phases.

#### 3.4. Tripodal pyridinium ILC

Photoluminescent tripodal pyridinium-based ionic liquid crystals were reported by Kato et al. [64, 65]. These tricationic pyridinium salts show thermotropic hexagonal or rectangular columnar phases or cubic phases with a large temperature ranges. The columnar or cubic phase stability is given by the difference in the anion size. For small-size anions (bromide), the cationic pyridinium cores should be packed more closely through electrostatic interactions and hence, the molecules prefer to self-assemble in cubic structures at higher temperatures (45a). Upon photoirradiation with UV light, these pyridinium salts emit blue-green or green light with photoluminescence quantum yields rather low, 3–6% in solution [65].

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### Acknowledgements

This work was supported by a grant of the Romanian Authority for Scientific Research, CNCS-UEFISCDI, and project number PN-II-ID-PCE-2011-3-0384.

### Author details

Viorel Cîrcu

Address all correspondence to: viorel.circu@chimie.unibuc.ro

Inorganic Chemistry Department, University of Bucharest, Bucharest, Romania

### References


Acknowledgements

306 Progress and Developments in Ionic Liquids

Author details

Viorel Cîrcu

References

cr0400919

DOI: 10.3390/ma4010206

This work was supported by a grant of the Romanian Authority for Scientific Research, CNCS-

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Inorganic Chemistry Department, University of Bucharest, Bucharest, Romania


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**Chapter 14**

**Provisional chapter**

**Ionic Liquids/Ionic Liquid Crystals for Safe and**

Ionic liquid crystals are organic salts having synergistic properties of ionic liquids and liquid crystalline materials endowed with non-covalently bound delocalised ion pairs of large organic cations and anions. They can undergo stimulus-responsive anisotropic phase change, followed by enhancement in ionic diffusion and conductivity, which makes them ideal candidates as electrolyte in energy storage systems. Our goal in this chapter is to survey the key developments in the field of ionic liquid crystalline electrolytes and to generate curiosity in the wider research community in tackling challenges in the electrolyte materials for sustainable energy related devices, such as supercapacitors,

**Keywords:** ionic liquids, ionic liquid crystals, energy storage, supercapacitors, batteries

Research and development, in the arena of sustainable energy, is receiving overwhelming interest due to the rapid proliferation of portable nano-electronic devices and also evolution in lifestyle. Due to the depletion of petroleum-based energy resources, researchers are turning towards the energy production from wind, solar, tidal, hydro and geothermal energy sources. But these resources are intermittent with time and season. Safe, sustainable and clean strategies for energy storage, such as supercapacitors, batteries, fuel cells, etc. have been explored tremendously in recent years to store energy in a sustainable way. Electrodes and electrolytes are the main components present in the energy storage devices. Electrolytes are the materials through which the transport of ions takes place from anode to cathode and vice versa, during charge-discharge processes. Transport and diffusion of ions through the electrolyte are largely influenced by various material parameters, such as viscosity, porosity and ionic conductivity

**Ionic Liquids/Ionic Liquid Crystals for Safe** 

**and Sustainable Energy Storage Systems**

Li batteries, fuel cells and dye-sensitized solar cells (DSSCs).

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution,

© 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

and reproduction in any medium, provided the original work is properly cited.

**Sustainable Energy Storage Systems**

Additional information is available at the end of the chapter

Additional information is available at the end of the chapter

Sudha J. Devaki and Renjith Sasi

Sudha J. Devaki and Renjith Sasi

http://dx.doi.org/10.5772/65888

**Abstract**

**1. Introduction**

**Provisional chapter**

### **Ionic Liquids/Ionic Liquid Crystals for Safe and Sustainable Energy Storage Systems Ionic Liquids/Ionic Liquid Crystals for Safe and Sustainable Energy Storage Systems**

Sudha J. Devaki and Renjith Sasi Sudha J. Devaki and Renjith Sasi

Additional information is available at the end of the chapter Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/65888

#### **Abstract**

Ionic liquid crystals are organic salts having synergistic properties of ionic liquids and liquid crystalline materials endowed with non-covalently bound delocalised ion pairs of large organic cations and anions. They can undergo stimulus-responsive anisotropic phase change, followed by enhancement in ionic diffusion and conductivity, which makes them ideal candidates as electrolyte in energy storage systems. Our goal in this chapter is to survey the key developments in the field of ionic liquid crystalline electrolytes and to generate curiosity in the wider research community in tackling challenges in the electrolyte materials for sustainable energy related devices, such as supercapacitors, Li batteries, fuel cells and dye-sensitized solar cells (DSSCs).

**Keywords:** ionic liquids, ionic liquid crystals, energy storage, supercapacitors, batteries

### **1. Introduction**

Research and development, in the arena of sustainable energy, is receiving overwhelming interest due to the rapid proliferation of portable nano-electronic devices and also evolution in lifestyle. Due to the depletion of petroleum-based energy resources, researchers are turning towards the energy production from wind, solar, tidal, hydro and geothermal energy sources. But these resources are intermittent with time and season. Safe, sustainable and clean strategies for energy storage, such as supercapacitors, batteries, fuel cells, etc. have been explored tremendously in recent years to store energy in a sustainable way. Electrodes and electrolytes are the main components present in the energy storage devices. Electrolytes are the materials through which the transport of ions takes place from anode to cathode and vice versa, during charge-discharge processes. Transport and diffusion of ions through the electrolyte are largely influenced by various material parameters, such as viscosity, porosity and ionic conductivity

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

of the electrolytes. Among the presently used liquid electrolytes, ionic liquids (ILs) are delivering excellent performance due to their high ionic conductivity, wide electrochemical stability window, good thermal stability, wide liquidity range, non-volatility and non-toxicity. Being composed of ion pairs bound by non-covalent interactions, they are bestowed with innumerable fascinating properties suitable for various applications [1, 2]. Generally, ionic liquids contain the cationic centres, such as nitrogen, oxygen, sulphur, phosphorous atoms and counter anions, such as halides, hexafluorophosphate (PF<sup>6</sup> − ), tetrafluoroborate (BF<sup>4</sup> − ), bistriflimide [(CF<sup>3</sup> SO<sup>2</sup> ) 2 N]<sup>−</sup> , etc. as shown in **Figure 1** [3–10].

structural variation by changing cations, anions and the substituents [13–17]. They exhibit high degree of miscibility with a wide array of organic and inorganic materials, since they contain both organic cations and inorganic anions or vice versa. ILs is found to have higher affinity

reducing the greenhouse effect [18–22]. The main criteria for exploiting a compound as an electrolyte for electrochemical energy storage systems are high ionic conductivity, non-volatility, non-flammability, high thermal stability and good electrochemical window. Electrochemical window can be defined as the limiting potential range, in which, the electrolyte is electrochemically stable and is free from any unwanted redox reactions. As the electrochemical window of electrolyte is directly related to the energy density, high EC window is necessary for electrolytes for energy storage systems. In comparison with the commercially used electrolytes, ILs has greater ionic conductivities and better potential windows to draw superior energy storage performances. Unlike aqueous electrolytes with limited potential window less than 1 V, IL-based electrolytes have very high window of 3–4 V, very high ionic conductivity, very good liquidity range and thermal stability for efficient energy storage applications, such as electrochemical supercapacitors, Li-ion batteries, Na- or Mg-batteries, Fuel cells, etc. [23].

Compared to ILs, ionic liquid crystals (ILCs) are receiving importance in flexible electronics as safe and efficient electrolytes. They exhibit synergistic properties of both ionic liquids and liquid crystals. Just like ILs, ILCs are also solely composed of ions bound with ionic interactions in different extents. Liquid crystals are often regarded as the fourth state of matter, with ordering in-between highly rigid crystalline lattices and disordered liquid state. Mesophase formed between solid and liquid phases exhibit impressive properties due to positional order and flow properties. Rotating isotropic spherical molecules are responsible for the plastic crystal formation, whereas, the non-covalent interactions are behind the anisotropic behaviour of liquid crystals. Ionic interactions are often responsible for strong crystalline ordering, whereas, other non-covalent interactions like hydrogen bonding, van der Waals forces and layer-by-layer self-assembly are responsible for liquid crystalline mesophase formation. In addition to the cationic and anionic centres, long alkyl side-chains and functional groups are present in ILCs for modulating the mesophase self-assembly. They are the real smart materials for realising multi-functional applications, such as stimuli-responsive conductors for energy storage devices, electrochromic supercapacitors, flexible batteries, etc. Stimuliresponsive phase transition of ILCs enables to modulate the direction of ionic conduction in them as shown in **Figure 2**. Based on the order and texture of mesogens, mesophases exhibited by ILCs are mainly of two types: nematic phase and smectic phase [24]. Mesogenic molecules tend to align along the molecular axis in the nematic phase (ID ordered). Smectic phase is more ordered where the molecules are arranged in layers (2-dimensionally ordered). Based on the extent of ordering of mesogens within the layers, smectic phase is subdivided into Sm A, Sm B, Sm C, Sm D, Sm F, Sm G, etc. Sm A is the least ordered one, among all the smectic mesophases [25–28]. Tschierske defined the LC state as a condensed matter state, in which, there is orientational and/or positional long-range order in at least one direction and no fixed position for individual molecules [25]. Columnar and cubic phases are other major mesophases observed in ionic liquid crystals. Based on the lattice in which columns are arranged,

columnar phase is divided into hexagonal columnar (Colh), rectangular (Col<sup>r</sup>

ing moisture or poisonous gases from the mixtures and also helping in CO<sup>2</sup>

and water vapour. This could be explored in devising gas separators for remov-

Ionic Liquids/Ionic Liquid Crystals for Safe and Sustainable Energy Storage Systems

sequestration and

315

http://dx.doi.org/10.5772/65888

) and oblique

towards CO<sup>2</sup>

**Figure 1.** Common cations and anions of ionic liquids.

The structural design of ionic liquids is playing a very important role as it is decisive for controlling the temperature limits, self-assembly and associate functional properties. Selfassembly processes are mainly controlled by the type of cations, anions and side-chains. In ILs, only delocalised charge centres are present which makes the intra-molecular interactions less intense to gift them their exquisite properties. The organic cations and anions of ILs are often stabilised by resonance with highly delocalised charges. Ions with lower symmetry or without symmetry are another major feature of ILs, which restricts the molecular packing into a low-energy crystal structure, ultimately lowering the melting point [11, 12]. Their physical and functional properties could be well tuned, since they are capable for infinite number of structural variation by changing cations, anions and the substituents [13–17]. They exhibit high degree of miscibility with a wide array of organic and inorganic materials, since they contain both organic cations and inorganic anions or vice versa. ILs is found to have higher affinity towards CO<sup>2</sup> and water vapour. This could be explored in devising gas separators for removing moisture or poisonous gases from the mixtures and also helping in CO<sup>2</sup> sequestration and reducing the greenhouse effect [18–22]. The main criteria for exploiting a compound as an electrolyte for electrochemical energy storage systems are high ionic conductivity, non-volatility, non-flammability, high thermal stability and good electrochemical window. Electrochemical window can be defined as the limiting potential range, in which, the electrolyte is electrochemically stable and is free from any unwanted redox reactions. As the electrochemical window of electrolyte is directly related to the energy density, high EC window is necessary for electrolytes for energy storage systems. In comparison with the commercially used electrolytes, ILs has greater ionic conductivities and better potential windows to draw superior energy storage performances. Unlike aqueous electrolytes with limited potential window less than 1 V, IL-based electrolytes have very high window of 3–4 V, very high ionic conductivity, very good liquidity range and thermal stability for efficient energy storage applications, such as electrochemical supercapacitors, Li-ion batteries, Na- or Mg-batteries, Fuel cells, etc. [23].

of the electrolytes. Among the presently used liquid electrolytes, ionic liquids (ILs) are delivering excellent performance due to their high ionic conductivity, wide electrochemical stability window, good thermal stability, wide liquidity range, non-volatility and non-toxicity. Being composed of ion pairs bound by non-covalent interactions, they are bestowed with innumerable fascinating properties suitable for various applications [1, 2]. Generally, ionic liquids contain the cationic centres, such as nitrogen, oxygen, sulphur, phosphorous atoms

The structural design of ionic liquids is playing a very important role as it is decisive for controlling the temperature limits, self-assembly and associate functional properties. Selfassembly processes are mainly controlled by the type of cations, anions and side-chains. In ILs, only delocalised charge centres are present which makes the intra-molecular interactions less intense to gift them their exquisite properties. The organic cations and anions of ILs are often stabilised by resonance with highly delocalised charges. Ions with lower symmetry or without symmetry are another major feature of ILs, which restricts the molecular packing into a low-energy crystal structure, ultimately lowering the melting point [11, 12]. Their physical and functional properties could be well tuned, since they are capable for infinite number of

−

), tetrafluoroborate (BF<sup>4</sup>

− ), bis-

and counter anions, such as halides, hexafluorophosphate (PF<sup>6</sup>

, etc. as shown in **Figure 1** [3–10].

triflimide [(CF<sup>3</sup>

SO<sup>2</sup> )2 N]<sup>−</sup>

314 Progress and Developments in Ionic Liquids

**Figure 1.** Common cations and anions of ionic liquids.

Compared to ILs, ionic liquid crystals (ILCs) are receiving importance in flexible electronics as safe and efficient electrolytes. They exhibit synergistic properties of both ionic liquids and liquid crystals. Just like ILs, ILCs are also solely composed of ions bound with ionic interactions in different extents. Liquid crystals are often regarded as the fourth state of matter, with ordering in-between highly rigid crystalline lattices and disordered liquid state. Mesophase formed between solid and liquid phases exhibit impressive properties due to positional order and flow properties. Rotating isotropic spherical molecules are responsible for the plastic crystal formation, whereas, the non-covalent interactions are behind the anisotropic behaviour of liquid crystals. Ionic interactions are often responsible for strong crystalline ordering, whereas, other non-covalent interactions like hydrogen bonding, van der Waals forces and layer-by-layer self-assembly are responsible for liquid crystalline mesophase formation. In addition to the cationic and anionic centres, long alkyl side-chains and functional groups are present in ILCs for modulating the mesophase self-assembly. They are the real smart materials for realising multi-functional applications, such as stimuli-responsive conductors for energy storage devices, electrochromic supercapacitors, flexible batteries, etc. Stimuliresponsive phase transition of ILCs enables to modulate the direction of ionic conduction in them as shown in **Figure 2**. Based on the order and texture of mesogens, mesophases exhibited by ILCs are mainly of two types: nematic phase and smectic phase [24]. Mesogenic molecules tend to align along the molecular axis in the nematic phase (ID ordered). Smectic phase is more ordered where the molecules are arranged in layers (2-dimensionally ordered). Based on the extent of ordering of mesogens within the layers, smectic phase is subdivided into Sm A, Sm B, Sm C, Sm D, Sm F, Sm G, etc. Sm A is the least ordered one, among all the smectic mesophases [25–28]. Tschierske defined the LC state as a condensed matter state, in which, there is orientational and/or positional long-range order in at least one direction and no fixed position for individual molecules [25]. Columnar and cubic phases are other major mesophases observed in ionic liquid crystals. Based on the lattice in which columns are arranged, columnar phase is divided into hexagonal columnar (Colh), rectangular (Col<sup>r</sup> ) and oblique columnar (Col<sup>o</sup> ) phases [29, 30]. Molecules with bent-core or banana shape displayed unique mesophases different from the normal mesophases [30–34].

nano-sized ionic channels in 1-, 2-, or 3-dimensions, due to the controlled self-assembly. These ionic channels are able to conduct charge carriers across the mesophase. The dimensions and directions of the nano-channels are controlled by the extent of self-assembly, and hence, the stimuli responsive mesophase formation can indirectly affect ionic conduction too [36, 37]. This photo-responsive ILC contains an azobenzene-core, which undergoes Cis-trans isomerisation when subjected to UV irradiation. Cis-trans isomerisation will shift the orientation of charge carriers, which are aligned in perpendicular direction to the molecular axis with concomitant mesophase transition from homogeneous to homeotropic smectic phases. Ordered charge carriers can be tuned and oriented with respect to external stimuli, which can

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Liquid-crystalline phase formation of a quaternary ammonium ionic liquid crystal (QSs) in response to temperature or concentration by the controlled self-assembly of mesogens was studied from Sasi et al., recently [37]. Studies showed that the nature of the mesophase formed relies on the surface alignment of self-assemblies, topographical defects and molecular ordering, which are governed by various non-covalent interactions among the molecules, temperature and polarity of the solvent. Investigation of the LC phase formation, under PLM, reveals polymorphic textures including crystalline, plastic crystalline, nematic, hexagonal columnar phases, randomly oriented molecular layers as shown in **Figure 3**. In the nematic phase, molecular layers are uniaxially oriented and the plane of molecular layers may be perpendicular to the director. However, columnar layers are biaxially oriented and the molecular layers in this phase are perpendicular to the layer planes. A slight tilt in the director angle is responsible for the observation of various textures arising from the difference in the nature of defects.

Generally, at room temperature, QSs are three -dimensionally ordered crystalline solids with highly birefringent spherulitic textures. During cooling from melt, they undergo transition to the gyroid-type of 1-dimensionally ordered nematic phase, further to hexagonal columnar phase by gaining the positional order. Further cooling may lead to the formation of lamellar and crystalline phases. Formations of conducting mesophase, in response to external stimuli, are mainly

**Figure 3.** Mesophase formation by the self-assembly of mesogens and corresponding textural variation. Reproduced

be used as the electrolyte for various applications.

with permission from [37].

**Figure 2.** Conducting channels generated by the controlled self-assembly of ILCs. Columnar and lamellar phases with their polarised light microscopic (PLM) images.

Generally, for such mesophases, functional properties like refractive index, dielectric permittivity, magnetic susceptibility, conductivity, elasticity, etc. are measured to have different values in different directions (anisotropy). Since the molecular kinetic energy is high for the LC phase, it is capable to undergo self-adjustments in response to the external stimuli. For thermotropic ILCs, crystalline phase changes to mesophase on heating and then to isotropic phase on further heating. On cooling from isotropic melt, mesophase is revisited and finally crystalline phase on further cooling. If the mesophase is observed both in the heating as well as cooling ramp, such ILCs are known as enantiotropic ILCs. In some occasions, the mesophase is observed only during cooling from the isotropic phase. This type of ILCs belongs to the category of monotropic ILCs. Liquid-crystalline phase arises, due to the introduction of solvent or by varying concentration, which is regarded as lyotropic ILCs. On cooling the ionic liquid crystalline molecule from isotropic phase, the relative variation of intermolecular interactions and molecular kinetic energy leads to the formation of liquid crystalline mesophases. The alignment of charge carriers along the mesophase boundaries will produce conducting nano-channels for improving the ionic conduction [27, 28]. Mesophases exhibit characteristic birefringent textures, when examined under PLM.

LC phase in ILCs can be induced in response to thermal, concentration, potential, magnetic, light as well as mechanic stimuli. Unlike other stimuli, light could make the shift in conduction direction without making any physical contact with the mesophase, avoiding mechanical disintegration to the phase. Soberats et al. reported light-induced re-orientation of an imidazolium ionic liquid crystal effecting variation in the ionic conduction directions, transverse and longitudinal [35]. Mesophase formation of ILCs is accompanied with the formation of nano-sized ionic channels in 1-, 2-, or 3-dimensions, due to the controlled self-assembly. These ionic channels are able to conduct charge carriers across the mesophase. The dimensions and directions of the nano-channels are controlled by the extent of self-assembly, and hence, the stimuli responsive mesophase formation can indirectly affect ionic conduction too [36, 37]. This photo-responsive ILC contains an azobenzene-core, which undergoes Cis-trans isomerisation when subjected to UV irradiation. Cis-trans isomerisation will shift the orientation of charge carriers, which are aligned in perpendicular direction to the molecular axis with concomitant mesophase transition from homogeneous to homeotropic smectic phases. Ordered charge carriers can be tuned and oriented with respect to external stimuli, which can be used as the electrolyte for various applications.

columnar (Col<sup>o</sup>

316 Progress and Developments in Ionic Liquids

mesophases different from the normal mesophases [30–34].

their polarised light microscopic (PLM) images.

) phases [29, 30]. Molecules with bent-core or banana shape displayed unique

Generally, for such mesophases, functional properties like refractive index, dielectric permittivity, magnetic susceptibility, conductivity, elasticity, etc. are measured to have different values in different directions (anisotropy). Since the molecular kinetic energy is high for the LC phase, it is capable to undergo self-adjustments in response to the external stimuli. For thermotropic ILCs, crystalline phase changes to mesophase on heating and then to isotropic phase on further heating. On cooling from isotropic melt, mesophase is revisited and finally crystalline phase on further cooling. If the mesophase is observed both in the heating as well as cooling ramp, such ILCs are known as enantiotropic ILCs. In some occasions, the mesophase is observed only during cooling from the isotropic phase. This type of ILCs belongs to the category of monotropic ILCs. Liquid-crystalline phase arises, due to the introduction of solvent or by varying concentration, which is regarded as lyotropic ILCs. On cooling the ionic liquid crystalline molecule from isotropic phase, the relative variation of intermolecular interactions and molecular kinetic energy leads to the formation of liquid crystalline mesophases. The alignment of charge carriers along the mesophase boundaries will produce conducting nano-channels for improving the ionic conduction [27, 28]. Mesophases exhibit characteristic birefringent textures, when examined under PLM. LC phase in ILCs can be induced in response to thermal, concentration, potential, magnetic, light as well as mechanic stimuli. Unlike other stimuli, light could make the shift in conduction direction without making any physical contact with the mesophase, avoiding mechanical disintegration to the phase. Soberats et al. reported light-induced re-orientation of an imidazolium ionic liquid crystal effecting variation in the ionic conduction directions, transverse and longitudinal [35]. Mesophase formation of ILCs is accompanied with the formation of

**Figure 2.** Conducting channels generated by the controlled self-assembly of ILCs. Columnar and lamellar phases with

Liquid-crystalline phase formation of a quaternary ammonium ionic liquid crystal (QSs) in response to temperature or concentration by the controlled self-assembly of mesogens was studied from Sasi et al., recently [37]. Studies showed that the nature of the mesophase formed relies on the surface alignment of self-assemblies, topographical defects and molecular ordering, which are governed by various non-covalent interactions among the molecules, temperature and polarity of the solvent. Investigation of the LC phase formation, under PLM, reveals polymorphic textures including crystalline, plastic crystalline, nematic, hexagonal columnar phases, randomly oriented molecular layers as shown in **Figure 3**. In the nematic phase, molecular layers are uniaxially oriented and the plane of molecular layers may be perpendicular to the director. However, columnar layers are biaxially oriented and the molecular layers in this phase are perpendicular to the layer planes. A slight tilt in the director angle is responsible for the observation of various textures arising from the difference in the nature of defects.

**Figure 3.** Mesophase formation by the self-assembly of mesogens and corresponding textural variation. Reproduced with permission from [37].

Generally, at room temperature, QSs are three -dimensionally ordered crystalline solids with highly birefringent spherulitic textures. During cooling from melt, they undergo transition to the gyroid-type of 1-dimensionally ordered nematic phase, further to hexagonal columnar phase by gaining the positional order. Further cooling may lead to the formation of lamellar and crystalline phases. Formations of conducting mesophase, in response to external stimuli, are mainly attributed to the variation of molecular kinetic energy and the extent of intermolecular interactions. In the crystalline phase, molecules are tightly-packed together by means of very strong intermolecular interactions and consequently, they will exhibit the least amount of molecular kinetic energy. On the contrary, isotropic liquid is characterised by lower intermolecular interactions and very high molecular kinetic energy. Crystalline state has 3-dimensional orientational, positional and translational orders. But in the mesophase, the crystalline order is reduced and the degree of freedom of molecules gets enhanced so that a fluidic phase is resulted.

**2.1. Supercapacitors**

Supercapacitors are widely employed as electrochemical energy storage devices as a replacement or complementary to batteries. High power density, faster charge-discharge rates, better cycle stability and service life of supercapacitors make them far superior to other energy storage systems [51]. They found to be more common in industrial and automobile applications rather than consumer electronics. Unlike batteries, which need considerable time for initial recharge, supercapacitors possess faster charge and discharge to facilitate regenerative pulse and peak assists necessary for starting the engine. Their capacity to generate intense high power discharge in short periods could be suitable to produce flash lamps for aerospace applications. Faster responses of supercapacitors are attributed to fast surface processes like

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319

Based on the charging mechanism, supercapacitors are divided into two categories, such as electrical double-layer capacitors (EDLC) and pseudo-capacitors. In EDLC, the electrical energy is stored by the formation of Helmholtz double layers at the electrode-electrolyte interface. The width of the charge separation influenced the capacitance of the device to a great extent. In EDLC, the storage mechanism mainly involves the adsorption and desorption of charge carriers on the porous organic electrodes. Various porous carbon derivatives, such as graphite, mesoporous activated carbon, CNTs and conducting polymers were frequently employed for performing the function of porous electrodes. Capacitive action of pseudocapacitors involves the charge transfer in-between the electrodes by means of faradaic reactions. Normally transition metal oxide derived electrodes which can provide smooth electron transfer were deployed for generating high-performing pseudo-capacitors. Symmetric and asymmetric supercapacitors are differed by the nature of electrodes used for fabricating them. Two electrodes with identical capacitances are used for fabricating symmetric supercapacitors, but two distinctly different electrodes are used for asymmetric supercapacitors [52].

Device structure of supercapacitors constituted by two same or different electrodes, are separated by an ion-transparent dielectric membrane wetted with ionic electrolytes. On charging, the electrodes get polarised, so that, the ions from the electrolyte will migrate towards oppositely charged electrodes and gets adsorbed over the electrode surface resulting in the formation of double layers. Each electrode will associate with a layer of diffused charges from the electrolyte, and hence, the whole device will function as two capacitors separated by a resistor. Double-layer capacitance (C) is directly related to the specific surface area (S) of the

So, activated carbon electrodes with very high surface area are tenable to deliver extremely high capacitive behaviour. If C1 and C2 are the individual capacitances, then the total capaci-

> *<sup>C</sup>* <sup>=</sup> \_\_\_1 *C*1 + \_\_\_1 *C*2

*<sup>d</sup>* (1)

(2)

electrodes and inversely related to the distance (d) between ions and electrodes.

*<sup>C</sup>* <sup>=</sup> *<sup>ε</sup>*<sup>0</sup> *<sup>ε</sup><sup>r</sup> <sup>S</sup>* \_\_\_\_

tance of the entire device can be given as,

\_\_1

adsorption or desorption of charge carriers at the electrode-electrolyte interface.

Ionic conductivity exhibited by ILCs is receiving significant importance and finding applications in electronic devices and drug delivery systems [38–46]. As the temperature increases, the 3-dimensionally ordered crystalline phase decreases and transforms into 2- or 1-dimensionally ordered LC phases, where there is more freedom for the transport of charge carriers, leading to higher conductivity. This conductivity enhancement with temperature is attributed to the thermotropic phase transitions that can be further envisaged from the temperature-dependent variation of charge diffusion coefficient (D) values. Similarly, lyotropic liquid-crystalline phase formation of the QSs is studied by means of conductivity and rheological analysis. Storage modulus increases with self-assembly and finally transformed into highly organized gel phase.

#### **2. As electrolytes for energy storage systems**

For any electrochemical energy storage systems (EESs), electrodes, electrolytes and separators are the main components [47, 48]. Electrolytes are considered as the heart of an EES, being the charge transport medium, which facilitates the ionic charge carriers for maintaining the device reactions alive. Electrolyte with good ionic conductivity and electrochemical window is necessary for exploiting maximum device performance [49]. Electrolyte plays an important role in controlling the cycle stability, rate capability, specific capacity and safety of the EESs. Choice of electrolyte is crucial in deriving the maximum output, since it could eliminate the possible parasitic side reactions effectively. Non-aqueous electrolytes have extended electrochemical windows to extract high energy and power densities. Conventional non-aqueous electrolytes used for energy systems are composed of ionic salts dissolved in volatile organic solvents. This is not at all safe, since there is a possibility of generating electric sparks when the system is charged at high rates, which can lead to the explosion of the device by burning easy flammable electrolytes. Also, the leakage of poisonous electrolytes while crimping the cell or on standing for long will also create serious environmental problems. ILs and ILCs with high ionic conductivity, high EC window and environmentally benign nature prove to be better alternates for conventional electrolytes in realising high performing energy storage devices [50]. Solid electrolytes like gel-polymer electrolytes and ionogels electrolytes, which are derived from ILs, are also found to be suitable for powering energy storage systems. Ion-conducting polymeric films can also be prepared by cross-linking the polymerizable side-chains in the self-assembled systems including liquid electrolytes, plasticisers and polymerizable mesogens. Also, ILCs with ordered ion channels were found to be extremely advantageous over conventional I− /I3 − couple, which shuttles the energy transport in dye-sensitized solar cells (DSSCs). This book chapter reviews various ILs, ILCs and their derived electrolyte systems employed for various energy storage systems and their advantages over conventional electrolytes.

#### **2.1. Supercapacitors**

attributed to the variation of molecular kinetic energy and the extent of intermolecular interactions. In the crystalline phase, molecules are tightly-packed together by means of very strong intermolecular interactions and consequently, they will exhibit the least amount of molecular kinetic energy. On the contrary, isotropic liquid is characterised by lower intermolecular interactions and very high molecular kinetic energy. Crystalline state has 3-dimensional orientational, positional and translational orders. But in the mesophase, the crystalline order is reduced and

Ionic conductivity exhibited by ILCs is receiving significant importance and finding applications in electronic devices and drug delivery systems [38–46]. As the temperature increases, the 3-dimensionally ordered crystalline phase decreases and transforms into 2- or 1-dimensionally ordered LC phases, where there is more freedom for the transport of charge carriers, leading to higher conductivity. This conductivity enhancement with temperature is attributed to the thermotropic phase transitions that can be further envisaged from the temperature-dependent variation of charge diffusion coefficient (D) values. Similarly, lyotropic liquid-crystalline phase formation of the QSs is studied by means of conductivity and rheological analysis. Storage modulus increases with self-assembly and finally transformed into highly organized gel phase.

For any electrochemical energy storage systems (EESs), electrodes, electrolytes and separators are the main components [47, 48]. Electrolytes are considered as the heart of an EES, being the charge transport medium, which facilitates the ionic charge carriers for maintaining the device reactions alive. Electrolyte with good ionic conductivity and electrochemical window is necessary for exploiting maximum device performance [49]. Electrolyte plays an important role in controlling the cycle stability, rate capability, specific capacity and safety of the EESs. Choice of electrolyte is crucial in deriving the maximum output, since it could eliminate the possible parasitic side reactions effectively. Non-aqueous electrolytes have extended electrochemical windows to extract high energy and power densities. Conventional non-aqueous electrolytes used for energy systems are composed of ionic salts dissolved in volatile organic solvents. This is not at all safe, since there is a possibility of generating electric sparks when the system is charged at high rates, which can lead to the explosion of the device by burning easy flammable electrolytes. Also, the leakage of poisonous electrolytes while crimping the cell or on standing for long will also create serious environmental problems. ILs and ILCs with high ionic conductivity, high EC window and environmentally benign nature prove to be better alternates for conventional electrolytes in realising high performing energy storage devices [50]. Solid electrolytes like gel-polymer electrolytes and ionogels electrolytes, which are derived from ILs, are also found to be suitable for powering energy storage systems. Ion-conducting polymeric films can also be prepared by cross-linking the polymerizable side-chains in the self-assembled systems including liquid electrolytes, plasticisers and polymerizable mesogens. Also, ILCs with ordered ion channels were found to be extremely advantageous over conventional

 couple, which shuttles the energy transport in dye-sensitized solar cells (DSSCs). This book chapter reviews various ILs, ILCs and their derived electrolyte systems employed for

various energy storage systems and their advantages over conventional electrolytes.

the degree of freedom of molecules gets enhanced so that a fluidic phase is resulted.

**2. As electrolytes for energy storage systems**

318 Progress and Developments in Ionic Liquids

I− /I3 − Supercapacitors are widely employed as electrochemical energy storage devices as a replacement or complementary to batteries. High power density, faster charge-discharge rates, better cycle stability and service life of supercapacitors make them far superior to other energy storage systems [51]. They found to be more common in industrial and automobile applications rather than consumer electronics. Unlike batteries, which need considerable time for initial recharge, supercapacitors possess faster charge and discharge to facilitate regenerative pulse and peak assists necessary for starting the engine. Their capacity to generate intense high power discharge in short periods could be suitable to produce flash lamps for aerospace applications. Faster responses of supercapacitors are attributed to fast surface processes like adsorption or desorption of charge carriers at the electrode-electrolyte interface.

Based on the charging mechanism, supercapacitors are divided into two categories, such as electrical double-layer capacitors (EDLC) and pseudo-capacitors. In EDLC, the electrical energy is stored by the formation of Helmholtz double layers at the electrode-electrolyte interface. The width of the charge separation influenced the capacitance of the device to a great extent. In EDLC, the storage mechanism mainly involves the adsorption and desorption of charge carriers on the porous organic electrodes. Various porous carbon derivatives, such as graphite, mesoporous activated carbon, CNTs and conducting polymers were frequently employed for performing the function of porous electrodes. Capacitive action of pseudocapacitors involves the charge transfer in-between the electrodes by means of faradaic reactions. Normally transition metal oxide derived electrodes which can provide smooth electron transfer were deployed for generating high-performing pseudo-capacitors. Symmetric and asymmetric supercapacitors are differed by the nature of electrodes used for fabricating them. Two electrodes with identical capacitances are used for fabricating symmetric supercapacitors, but two distinctly different electrodes are used for asymmetric supercapacitors [52].

Device structure of supercapacitors constituted by two same or different electrodes, are separated by an ion-transparent dielectric membrane wetted with ionic electrolytes. On charging, the electrodes get polarised, so that, the ions from the electrolyte will migrate towards oppositely charged electrodes and gets adsorbed over the electrode surface resulting in the formation of double layers. Each electrode will associate with a layer of diffused charges from the electrolyte, and hence, the whole device will function as two capacitors separated by a resistor. Double-layer capacitance (C) is directly related to the specific surface area (S) of the electrodes and inversely related to the distance (d) between ions and electrodes.

$$\mathbf{C} = \frac{\varepsilon\_0 \varepsilon\_r S}{d} \tag{1}$$

So, activated carbon electrodes with very high surface area are tenable to deliver extremely high capacitive behaviour. If C1 and C2 are the individual capacitances, then the total capacitance of the entire device can be given as,

$$\frac{1}{C} = \frac{1}{C\_1} + \frac{1}{C\_2} \tag{2}$$

The storable energy of a capacitor quadratically related to the potential window of the electrolyte is ,

$$E = \frac{1}{2}C\,V^2 = \frac{1}{2}QV\tag{3}$$

Lin et al. reported the use of a mixture of piperidinium and pyrrolinium ILs for preparing the electrolyte for supercapacitors for temperatures ranging from −50 to 100°C [55]. It delivered an outstanding capacitance of 180 F/g, which was attributed to a wide EC window of 3.5 V. Similarly, Ruiz et al. demonstrated a mixture of pyrrolidinium IL in butyronitrile as power electrolyte for supercapacitors for temperatures ranging from −20 to 80°C [56]. Low-viscous ILs has higher conductivity and capacitive performance than high-viscous ILs, due to better ionic mobility. Likewise, variation of anionic or cationic sizes also has a prominent effect

other anions, because their ionic size is complementary to the pores of the mesoporous carbon electrodes, which is widely employed in supercapacitors. Clever structural design of ILs could also improve the device performance. Rennie et al. extensively studied on this aspect and found that the ILs with ether-bonded alkyl side-chains possesses higher capacitance in comparison with others. They found that the presence of ether bond could induce significant flexibility to the structural design. It could lower the viscosity and further enhance the conductivity, which in turn improves the capacitive performance [57]. Xu et al. explored a new kind of ILs with fluorohydrogenate anions with comparatively lower viscosity and better ionic conductivities to be used as conducting medium in a wide range of applications, mainly

Like ILs, their solid analogues and secondary derivatives, such as ILCs, ionogels, etc. are also receiving technological importance in energy storage systems [59]. Presence of well-ordered conducting channels provide them strong propensity to transport and store charges. ILCs could be explored as electrolytes either in the solution state in suitable solvents or in their conducting liquid crystalline mesophase. Thermotropic mesophase formation improves the molecular conductivity to extract better capacitance at high temperatures. We have exploited the lyotropic mesophase formation of an imidazolium ionic liquid crystal in acetonitrile to derive a high-conducting electrolyte for supercapacitors. Unlike pure solution, where the ion density is very low, liquid-crystalline phase have high ionic density and weight-specific performance to yield superior performance. Not only the ionic concentration, but also the ionic mobility is important in governing the storage performance. Lyotropic columnar phase of ILC found to be adequate with conductivity and lower viscosity to be employed in supercapacitors as electrolytes. In mesophase, self-assembly of mesogens provide adequate nano-oriented pathways to effect charge diffusion with ease for developing double layers in response to the subjected potential. Lyotropic columnar mesophase yielded a specific capacitance of 134 F/g with 80% capacitance retention, after 2000 cycles [60]. Schematic diagram showing the capacitive behaviour of ionic liquid crystalline electrolyte in powering high-performance

supercapacitors and corresponding liquid crystalline phases are given in **Figure 4**.

In the LC phase, the charge carriers are aligned by the molecular self-assembly and have considerable degree of freedom to migrate to corresponding electrodes on charging, forming electrical double layers. While on discharge, when the electrodes are depolarised, ions move away from the electrodes to the centre depleting the double layers. These processes are highly reversible to achieve better cycle stability. Solid ionic electrolytes have also been obtained by mixing ionic liquids with polymer binders or carbon materials to generate safe energy storage systems. Zhang et al. prepared a flexible gel-polymer electrolyte containing

anions have better capacitive performance over

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321

Ionic Liquids/Ionic Liquid Crystals for Safe and Sustainable Energy Storage Systems

− or BF<sup>4</sup> −

supercapacitors and other energy storage systems [58].

on the performance. ILs with PF<sup>6</sup>

where, V is the potential window and Q is the charge stored in the capacitor. The potential window is limited by the decomposition potential of the electrolyte used. In the case of aqueous electrolytes, it is not possible to go beyond 1 V, whereas for non-aqueous electrolytes, the decomposition potential is quite high to deliver very good energy density. Acetonitrile-based systems could be used up to 2.8 V and propylene carbonate-based systems can go around 3 V. Potential-driven break down of dielectric material will also affect device performance, which will limit application of very high potentials also. As pointed out earlier, de-merits of conventional electrolytes pave the way for new non-conventional safety electrolytes for powering supercapacitors [53]. Initially, ILs was used as high-conducting electrolytes for supercapacitors by dissolving in suitable solvents. Even though they have extra-high conductivity and performed exceedingly well with porous carbon electrodes, the energy density is limited by the decomposition potential of the solvent used. ILs with excellent electrochemical window, generally greater than 3 V, could be used alone as high-conducting electrolytes. Anions of ILs are oxidised only at high potentials and the reduction of cations also require very low potentials, giving them very high electrochemical windows and rendering very high energy densities. In comparison with ILs with halide anions, the ones with fluorine containing coordinate anions displayed higher EC windows. Apart from the EC window, ion size, ion type, interaction of ions with solvent and electrodes will also influence the overall performance of the devices [54]. Equivalent series resistance (ESR), an important quality parameter of supercapacitors, is strongly influenced by the ion type, conductivity as well as the viscosity of the electrolyte. If the ions are too heavy, less diffusible or too viscous, it will considerably drag down the electrolyte conductivity and increase the ESR. This will affect the storage efficiency, as the ESR is inversely related to the power density of the device as,

$$P = \frac{V^2}{4 \cdot ESR \cdot m} \tag{4}$$

where V represents the potential window and m is the active mass of the electrodes. Also, the interaction of the electrolytes with solvents or electrodes also has a strong role to play in deciphering the cycle stability of the system. High temperature stability of ILs was found to be promising in exploiting them as electrolytes for powering supercapacitors for a wide range of temperatures. Delivering power at extreme condition seems to be difficult with conventional electrolytes, as they may be either in frozen or gaseous state in which the charge conduction is impossible. Inception of ILs with wide liquid range helped to solve this difficulty to a greater extent. This will be suitable in producing high-power short-pulse arc lamps for polar regions, space stations and in mines. It was found that the viscosity of ILs could be reduced by suitable formulation of eutectic mixture of two or more ILs with identical anions, which can also be applied to a wide range of temperatures due to the depression in freezing points of ILs.

Lin et al. reported the use of a mixture of piperidinium and pyrrolinium ILs for preparing the electrolyte for supercapacitors for temperatures ranging from −50 to 100°C [55]. It delivered an outstanding capacitance of 180 F/g, which was attributed to a wide EC window of 3.5 V. Similarly, Ruiz et al. demonstrated a mixture of pyrrolidinium IL in butyronitrile as power electrolyte for supercapacitors for temperatures ranging from −20 to 80°C [56]. Low-viscous ILs has higher conductivity and capacitive performance than high-viscous ILs, due to better ionic mobility. Likewise, variation of anionic or cationic sizes also has a prominent effect on the performance. ILs with PF<sup>6</sup> − or BF<sup>4</sup> − anions have better capacitive performance over other anions, because their ionic size is complementary to the pores of the mesoporous carbon electrodes, which is widely employed in supercapacitors. Clever structural design of ILs could also improve the device performance. Rennie et al. extensively studied on this aspect and found that the ILs with ether-bonded alkyl side-chains possesses higher capacitance in comparison with others. They found that the presence of ether bond could induce significant flexibility to the structural design. It could lower the viscosity and further enhance the conductivity, which in turn improves the capacitive performance [57]. Xu et al. explored a new kind of ILs with fluorohydrogenate anions with comparatively lower viscosity and better ionic conductivities to be used as conducting medium in a wide range of applications, mainly supercapacitors and other energy storage systems [58].

The storable energy of a capacitor quadratically related to the potential window of the

<sup>2</sup> *<sup>C</sup> <sup>V</sup>*<sup>2</sup> <sup>=</sup> \_\_1

where, V is the potential window and Q is the charge stored in the capacitor. The potential window is limited by the decomposition potential of the electrolyte used. In the case of aqueous electrolytes, it is not possible to go beyond 1 V, whereas for non-aqueous electrolytes, the decomposition potential is quite high to deliver very good energy density. Acetonitrile-based systems could be used up to 2.8 V and propylene carbonate-based systems can go around 3 V. Potential-driven break down of dielectric material will also affect device performance, which will limit application of very high potentials also. As pointed out earlier, de-merits of conventional electrolytes pave the way for new non-conventional safety electrolytes for powering supercapacitors [53]. Initially, ILs was used as high-conducting electrolytes for supercapacitors by dissolving in suitable solvents. Even though they have extra-high conductivity and performed exceedingly well with porous carbon electrodes, the energy density is limited by the decomposition potential of the solvent used. ILs with excellent electrochemical window, generally greater than 3 V, could be used alone as high-conducting electrolytes. Anions of ILs are oxidised only at high potentials and the reduction of cations also require very low potentials, giving them very high electrochemical windows and rendering very high energy densities. In comparison with ILs with halide anions, the ones with fluorine containing coordinate anions displayed higher EC windows. Apart from the EC window, ion size, ion type, interaction of ions with solvent and electrodes will also influence the overall performance of the devices [54]. Equivalent series resistance (ESR), an important quality parameter of supercapacitors, is strongly influenced by the ion type, conductivity as well as the viscosity of the electrolyte. If the ions are too heavy, less diffusible or too viscous, it will considerably drag down the electrolyte conductivity and increase the ESR. This will affect the storage efficiency,

as the ESR is inversely related to the power density of the device as,

where V represents the potential window and m is the active mass of the electrodes. Also, the interaction of the electrolytes with solvents or electrodes also has a strong role to play in deciphering the cycle stability of the system. High temperature stability of ILs was found to be promising in exploiting them as electrolytes for powering supercapacitors for a wide range of temperatures. Delivering power at extreme condition seems to be difficult with conventional electrolytes, as they may be either in frozen or gaseous state in which the charge conduction is impossible. Inception of ILs with wide liquid range helped to solve this difficulty to a greater extent. This will be suitable in producing high-power short-pulse arc lamps for polar regions, space stations and in mines. It was found that the viscosity of ILs could be reduced by suitable formulation of eutectic mixture of two or more ILs with identical anions, which can also be applied to a wide range of temperatures due to the depression in freezing

*P* = *<sup>V</sup>*<sup>2</sup> \_\_\_\_\_\_\_\_\_\_

<sup>2</sup> *QV* (3)

<sup>4</sup> <sup>⋅</sup> *ESR* <sup>⋅</sup> *<sup>m</sup>* (4)

electrolyte is ,

320 Progress and Developments in Ionic Liquids

points of ILs.

*E* = \_\_1

Like ILs, their solid analogues and secondary derivatives, such as ILCs, ionogels, etc. are also receiving technological importance in energy storage systems [59]. Presence of well-ordered conducting channels provide them strong propensity to transport and store charges. ILCs could be explored as electrolytes either in the solution state in suitable solvents or in their conducting liquid crystalline mesophase. Thermotropic mesophase formation improves the molecular conductivity to extract better capacitance at high temperatures. We have exploited the lyotropic mesophase formation of an imidazolium ionic liquid crystal in acetonitrile to derive a high-conducting electrolyte for supercapacitors. Unlike pure solution, where the ion density is very low, liquid-crystalline phase have high ionic density and weight-specific performance to yield superior performance. Not only the ionic concentration, but also the ionic mobility is important in governing the storage performance. Lyotropic columnar phase of ILC found to be adequate with conductivity and lower viscosity to be employed in supercapacitors as electrolytes. In mesophase, self-assembly of mesogens provide adequate nano-oriented pathways to effect charge diffusion with ease for developing double layers in response to the subjected potential. Lyotropic columnar mesophase yielded a specific capacitance of 134 F/g with 80% capacitance retention, after 2000 cycles [60]. Schematic diagram showing the capacitive behaviour of ionic liquid crystalline electrolyte in powering high-performance supercapacitors and corresponding liquid crystalline phases are given in **Figure 4**.

In the LC phase, the charge carriers are aligned by the molecular self-assembly and have considerable degree of freedom to migrate to corresponding electrodes on charging, forming electrical double layers. While on discharge, when the electrodes are depolarised, ions move away from the electrodes to the centre depleting the double layers. These processes are highly reversible to achieve better cycle stability. Solid ionic electrolytes have also been obtained by mixing ionic liquids with polymer binders or carbon materials to generate safe energy storage systems. Zhang et al. prepared a flexible gel-polymer electrolyte containing 1-butyl-3-methylimidazolium chloride and Li<sup>2</sup> SO<sup>4</sup> in a matrix of poly(vinyl alcohol) to be used as flexible safe electrolyte for supercapacitors [61]. The high ionic conductivity and a high fracture strain of the electrolyte is beneficiated by delivering a specific capacitance of 136 F/g, with a maximum energy density of 10.6 Wh/kg and a power density of 3400 W/kg. The conductivity and capacitive behaviour of supercapacitors remains with negligible fade, even after the repeated bending cycles illustrated the flexibility of electrolyte and its application in future plastic electronics. Similarly, many reports are there in utilising ILs or ILCs in combinations with PEO, PVDF, PHEMA, etc. to materialise highly conducting and flexible ionogels for powering supercapacitors.

It was observed that bulkier cations of ILs, sometimes, form inhibiting layers on the surface of carbonaceous anodes hindering cell reactions. Fluorinated anions are able to remove such unwanted layer formation and rule out the need for secondary additives for this purpose. IL-based electrolytes demonstrated their excellent performance with variety of electrode materials. High redox stability of ILs will help to explore new horizon of electrodes with highvoltage stabilities, so that the energy and power densities could be improved [66]. The main factor, which needs to be addressed while choosing a solvent for Li batteries, is the handling of dissolved Li-ions and its influence on charge transport, while cell reactions are on. Structural design of ILs by incorporating anionic or cationic functionality to interact with dissolved ions or the usage of external additives or diluents were also tried to eliminate filament formation and other undesirable reactions by the dissolved Li-ions. IL structural design has a special role to play on the formation of protective film (solid electrolyte interface, SEI) and also to modulate ion transport, charge transfer and electrolyte stability [67]. Each ILs has definite reaction pathways for modulating the Li-ion transfer, depending on the association or other interaction of the constituting ions. Considering the needs, such as safety, sustainability, strength, cycle stability and high temperature operability, new designs and methodologies were introduced. Inception of thin and flexible batteries further added much interest towards IL-based electrolytes. Even though many studies have invested to analyse the energy storage performance of IL-based electrolyte in combination with a large array of electrode materials, the commercialisation of them is still uncertain. High-cost of ILs, in comparison with conventional carbonate solvents, may be one of the major hindering factors. In order to solve the situation, efforts were taken to reduce the cost of ILs by introducing new low-cost cations or anions with simple

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**Figure 5.** Design of Li-ion battery incorporating ionic liquid crystal membrane separator.

**Figure 4.** Bio-based ionic liquid crystal as electrolyte for supercapacitors. Reproduced with permission from [60].

#### **2.2. Lithium-ion batteries**

Like supercapacitors, or comparatively to a higher extent, Li-ion batteries attracted the scientific interest particularly because of its higher energy density and superior performance [62, 63]. In Li-ion batteries, device structure is more or less similar to that of supercapacitors with a separator soaked with electrolyte, sandwiched in-between a pair of electrodes (cathode and anode). A typical device structure of the Li-ion battery is depicted in **Figure 5**. Similar to pseudo-capacitors, the energy storage is realised by the Li-ion transport across the permeable separator. Leakage and flammability issues associated with conventional volatile organic electrolytes lead to the inception of safe, cost-effective and eco-friendly electrolytes, mainly ILs [64]. Low evaporation power and less flammable nature make them more safe and convenient. Initially, quaternary ammonium or phosphonium salts with triflate anions were explored for electrolyte applications. Less viscous and cathodically stable ILs are necessary for effective lithium cycling. Fluorinated anions, such as BF<sup>4</sup> − , PF<sup>6</sup> − , TFSI− , etc. were found to be more efficient in realising better Li intercalation reactions. As the size of anion significantly affects the ionic transport, Matsumoto et al showed high-rate cycling of Li/LiCoO<sup>2</sup> cell with smaller sulphonamide anions [65].

**Figure 5.** Design of Li-ion battery incorporating ionic liquid crystal membrane separator.

1-butyl-3-methylimidazolium chloride and Li<sup>2</sup>

for powering supercapacitors.

322 Progress and Developments in Ionic Liquids

**2.2. Lithium-ion batteries**

anions, such as BF<sup>4</sup>

− , PF<sup>6</sup> − , TFSI−

showed high-rate cycling of Li/LiCoO<sup>2</sup>

SO<sup>4</sup>

as flexible safe electrolyte for supercapacitors [61]. The high ionic conductivity and a high fracture strain of the electrolyte is beneficiated by delivering a specific capacitance of 136 F/g, with a maximum energy density of 10.6 Wh/kg and a power density of 3400 W/kg. The conductivity and capacitive behaviour of supercapacitors remains with negligible fade, even after the repeated bending cycles illustrated the flexibility of electrolyte and its application in future plastic electronics. Similarly, many reports are there in utilising ILs or ILCs in combinations with PEO, PVDF, PHEMA, etc. to materialise highly conducting and flexible ionogels

Like supercapacitors, or comparatively to a higher extent, Li-ion batteries attracted the scientific interest particularly because of its higher energy density and superior performance [62, 63]. In Li-ion batteries, device structure is more or less similar to that of supercapacitors with a separator soaked with electrolyte, sandwiched in-between a pair of electrodes (cathode and anode). A typical device structure of the Li-ion battery is depicted in **Figure 5**. Similar to pseudo-capacitors, the energy storage is realised by the Li-ion transport across the permeable separator. Leakage and flammability issues associated with conventional volatile organic electrolytes lead to the inception of safe, cost-effective and eco-friendly electrolytes, mainly ILs [64]. Low evaporation power and less flammable nature make them more safe and convenient. Initially, quaternary ammonium or phosphonium salts with triflate anions were explored for electrolyte applications. Less viscous and cathodically stable ILs are necessary for effective lithium cycling. Fluorinated

**Figure 4.** Bio-based ionic liquid crystal as electrolyte for supercapacitors. Reproduced with permission from [60].

calation reactions. As the size of anion significantly affects the ionic transport, Matsumoto et al

, etc. were found to be more efficient in realising better Li inter-

cell with smaller sulphonamide anions [65].

in a matrix of poly(vinyl alcohol) to be used

It was observed that bulkier cations of ILs, sometimes, form inhibiting layers on the surface of carbonaceous anodes hindering cell reactions. Fluorinated anions are able to remove such unwanted layer formation and rule out the need for secondary additives for this purpose. IL-based electrolytes demonstrated their excellent performance with variety of electrode materials. High redox stability of ILs will help to explore new horizon of electrodes with highvoltage stabilities, so that the energy and power densities could be improved [66]. The main factor, which needs to be addressed while choosing a solvent for Li batteries, is the handling of dissolved Li-ions and its influence on charge transport, while cell reactions are on. Structural design of ILs by incorporating anionic or cationic functionality to interact with dissolved ions or the usage of external additives or diluents were also tried to eliminate filament formation and other undesirable reactions by the dissolved Li-ions. IL structural design has a special role to play on the formation of protective film (solid electrolyte interface, SEI) and also to modulate ion transport, charge transfer and electrolyte stability [67]. Each ILs has definite reaction pathways for modulating the Li-ion transfer, depending on the association or other interaction of the constituting ions. Considering the needs, such as safety, sustainability, strength, cycle stability and high temperature operability, new designs and methodologies were introduced. Inception of thin and flexible batteries further added much interest towards IL-based electrolytes. Even though many studies have invested to analyse the energy storage performance of IL-based electrolyte in combination with a large array of electrode materials, the commercialisation of them is still uncertain. High-cost of ILs, in comparison with conventional carbonate solvents, may be one of the major hindering factors. In order to solve the situation, efforts were taken to reduce the cost of ILs by introducing new low-cost cations or anions with simple strategies. Bio-based ILs/ILCs with negligible production cost and comparable performance could be prepared by suitable modification of industrial waste by-products like cashew nut shell liquid (CNSL). They can be used for powering energy storage systems either as such or in combination with solvents or as solid films or gels with suitable polymeric binder. Since the conversion of waste to value-added electrolytes seems to be a double benefit to the society, it can be a remedy for pollution as well as a sustainable energy storage pathway. Similarly, dicyanamide [N(CN)<sup>2</sup> ] with highly delocalised negative charge proves to be a better alternate for fluorinated anions to produce low-cost ionic liquids. Their highly delocalised ionic charge is responsible for the extremely low freezing points, which they have [68].

rines to power the arc lamps and to make use of by-product water for drinking purpose. The major working elements of fuel cells are bipolar plates, diffusion layers, electrodes (anodes and cathodes) and the electrolytes. Mainly, aqueous solutions with acidic or basic nature have been used as the electrolyte for conventional fuel cells. Even though fuel cell technology possesses an eventful 150-year-old history, no one had succeeded in using proton carriers other than hydronium ions or hydroxide ions in a fuel cell, until last few decades. Obviously, it confined the fuel cell science within the aqueous electrolytes of acid or basic character. The effective potential range also limited below 1 V, due to the use of aqueous electrolyte alone. In addition, the operating temperature range also limited to below 100°C, when aqueous electrolytes are employed. In proton exchange membrane (PEM) fuel cell, the core is called the membrane electrode assembly (MEA), which comprises the PEM placed between two electrodes. Different functions of PEMs are separating the gaseous reactants, conducting protons from the anode to the cathode, electrically insulating the electrons and supporting the catalyst. The necessary requirements an exchange membrane should have to function exquisitely are excellent proton conductivity in dry and wet states, unflinching mechanical strength and dimensional stability, chemical, electrochemical and thermal stability under the working conditions, low fuel and oxygen crossover, easy conformation to form a membrane electrode assembly

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Nafion (perfluorosulfonic acid) ionomer is one of the most widely used membranes in PEMFC devices because of its chemical inertness, high ionic conductivity and better mechanical strength. But, the conductivity of the Nafion usually reduces when the temperature goes beyond 100°C, as the evaporation of water will hinder the ionic mobility considerably. Even then, elevated temperatures are advantageous because the contamination tolerance of the catalyst is improved and even low purity hydrogen could be beneficiated at higher temperatures. It would also eradicate both carbon monoxide poisoning and noble metal catalyst corrosion frequently associated with aqueous electrolytes. In addition, electrode reaction kinetics also improved at elevated temperatures. All these issues could be solved by introducing room temperature ionic liquids as the electrolytes either by gelling ILs with polymers or polymerizing polymerizable ionic liquids. Protic ionic liquids could be generally considered as a salt of a Bronsted acid and a Bronsted base. The transfer of a proton from an acid to a base

**Figure 6.** Application of protic ILs in fuel cells as electrolytes. Reproduced with permission from [73].

and low-cost **Figure 6** [74, 75].

Thermo-mechanical stability of ILs/ILCs is utilised for generating stable batteries with different categories of electrodes and with good cycling stability. Rate performance of the cells also found to be improved, due to the better conductivity, tuneable viscosity and reversibility of IL-based electrolytes. But, in some cases, rate capability is significantly affected by the ionassociation with ILs during cell cycling, which enhances the viscosity and thereby hinders cycling efficiency. In contrary to this, fluorosulfonamide anion containing ILs displayed highrate performance with higher Li-ion concentrations, but lacked the device stability. Mixture electrolytes containing ILs and conventional solvents, found a better strategy to cope up with the desired conductivity and device stability. Recently, newer electrode combinations such as Li-air, Li-S, Na, Mg and Al batteries have formulated to overcome the shortcomings of Li-ion batteries [69]. In all these new-generation batteries also, ILs seems to be a better alternate for ionic shuttling. Their wide operation window and liquid range will be applicable in many more upcoming storage systems.

Solid-state electrolytes, which could function as both electrolyte and separator, could be generated by integrating ILs/ILCs with polymeric binders [69]. Schematic diagram showing the separator composed of ILC-based hybrid membrane that functions as both separator and electrolyte is given in **Figure 5**. High conductivity, porosity, better Li-ion transport, high electrolyte affinity and uptake, mechanical stability and thermal stability are the main requisites for solid electrolytes [70]. Kato et al. developed thermotropic liquid crystalline (LC) electrolytes for lithium-ion batteries for the first time by the self-assembly of rod-like LC molecule having a cyclic carbonate moiety with lithium salts and is used to form self-assembled two-dimensional ion-conductive pathways. Electrochemical and thermal stability and efficient ionic conduction is observed for the liquid crystal and is employed as an electrolyte in lithium-ion batteries with highly reversible charge–discharge for both positive and negative electrodes [71]. Presence of ambhiphilic ILs/ILCs will definitely improve the electrolyte affinity and further ionic transport upheld the reaction kinetics and improves efficiency. Flexible, wearable storage batteries are the future directives of new age storage systems.

#### **2.3. Fuel cells**

Fuel cells are unanimously agreed to be one of the green technologies for sustainable energy, due to their high energetic efficiency and low environmental impact. These devices directly convert chemical energy to electrical energy with water as by-product, thereby omitting toxic emissions prevalent with fossil fuels [72]. They have been explored in space crafts and submarines to power the arc lamps and to make use of by-product water for drinking purpose. The major working elements of fuel cells are bipolar plates, diffusion layers, electrodes (anodes and cathodes) and the electrolytes. Mainly, aqueous solutions with acidic or basic nature have been used as the electrolyte for conventional fuel cells. Even though fuel cell technology possesses an eventful 150-year-old history, no one had succeeded in using proton carriers other than hydronium ions or hydroxide ions in a fuel cell, until last few decades. Obviously, it confined the fuel cell science within the aqueous electrolytes of acid or basic character. The effective potential range also limited below 1 V, due to the use of aqueous electrolyte alone. In addition, the operating temperature range also limited to below 100°C, when aqueous electrolytes are employed. In proton exchange membrane (PEM) fuel cell, the core is called the membrane electrode assembly (MEA), which comprises the PEM placed between two electrodes. Different functions of PEMs are separating the gaseous reactants, conducting protons from the anode to the cathode, electrically insulating the electrons and supporting the catalyst. The necessary requirements an exchange membrane should have to function exquisitely are excellent proton conductivity in dry and wet states, unflinching mechanical strength and dimensional stability, chemical, electrochemical and thermal stability under the working conditions, low fuel and oxygen crossover, easy conformation to form a membrane electrode assembly and low-cost **Figure 6** [74, 75].

strategies. Bio-based ILs/ILCs with negligible production cost and comparable performance could be prepared by suitable modification of industrial waste by-products like cashew nut shell liquid (CNSL). They can be used for powering energy storage systems either as such or in combination with solvents or as solid films or gels with suitable polymeric binder. Since the conversion of waste to value-added electrolytes seems to be a double benefit to the society, it can be a remedy for pollution as well as a sustainable energy storage pathway. Similarly,

for fluorinated anions to produce low-cost ionic liquids. Their highly delocalised ionic charge

Thermo-mechanical stability of ILs/ILCs is utilised for generating stable batteries with different categories of electrodes and with good cycling stability. Rate performance of the cells also found to be improved, due to the better conductivity, tuneable viscosity and reversibility of IL-based electrolytes. But, in some cases, rate capability is significantly affected by the ionassociation with ILs during cell cycling, which enhances the viscosity and thereby hinders cycling efficiency. In contrary to this, fluorosulfonamide anion containing ILs displayed highrate performance with higher Li-ion concentrations, but lacked the device stability. Mixture electrolytes containing ILs and conventional solvents, found a better strategy to cope up with the desired conductivity and device stability. Recently, newer electrode combinations such as Li-air, Li-S, Na, Mg and Al batteries have formulated to overcome the shortcomings of Li-ion batteries [69]. In all these new-generation batteries also, ILs seems to be a better alternate for ionic shuttling. Their wide operation window and liquid range will be applicable in many

Solid-state electrolytes, which could function as both electrolyte and separator, could be generated by integrating ILs/ILCs with polymeric binders [69]. Schematic diagram showing the separator composed of ILC-based hybrid membrane that functions as both separator and electrolyte is given in **Figure 5**. High conductivity, porosity, better Li-ion transport, high electrolyte affinity and uptake, mechanical stability and thermal stability are the main requisites for solid electrolytes [70]. Kato et al. developed thermotropic liquid crystalline (LC) electrolytes for lithium-ion batteries for the first time by the self-assembly of rod-like LC molecule having a cyclic carbonate moiety with lithium salts and is used to form self-assembled two-dimensional ion-conductive pathways. Electrochemical and thermal stability and efficient ionic conduction is observed for the liquid crystal and is employed as an electrolyte in lithium-ion batteries with highly reversible charge–discharge for both positive and negative electrodes [71]. Presence of ambhiphilic ILs/ILCs will definitely improve the electrolyte affinity and further ionic transport upheld the reaction kinetics and improves efficiency. Flexible,

wearable storage batteries are the future directives of new age storage systems.

Fuel cells are unanimously agreed to be one of the green technologies for sustainable energy, due to their high energetic efficiency and low environmental impact. These devices directly convert chemical energy to electrical energy with water as by-product, thereby omitting toxic emissions prevalent with fossil fuels [72]. They have been explored in space crafts and subma-

is responsible for the extremely low freezing points, which they have [68].

with highly delocalised negative charge proves to be a better alternate

dicyanamide [N(CN)<sup>2</sup>

324 Progress and Developments in Ionic Liquids

more upcoming storage systems.

**2.3. Fuel cells**

] -

**Figure 6.** Application of protic ILs in fuel cells as electrolytes. Reproduced with permission from [73].

Nafion (perfluorosulfonic acid) ionomer is one of the most widely used membranes in PEMFC devices because of its chemical inertness, high ionic conductivity and better mechanical strength. But, the conductivity of the Nafion usually reduces when the temperature goes beyond 100°C, as the evaporation of water will hinder the ionic mobility considerably. Even then, elevated temperatures are advantageous because the contamination tolerance of the catalyst is improved and even low purity hydrogen could be beneficiated at higher temperatures. It would also eradicate both carbon monoxide poisoning and noble metal catalyst corrosion frequently associated with aqueous electrolytes. In addition, electrode reaction kinetics also improved at elevated temperatures. All these issues could be solved by introducing room temperature ionic liquids as the electrolytes either by gelling ILs with polymers or polymerizing polymerizable ionic liquids. Protic ionic liquids could be generally considered as a salt of a Bronsted acid and a Bronsted base. The transfer of a proton from an acid to a base leads to the formation of hydrogen-bonded networks between proton donor and acceptors. Protic ILs with more fluidity found to have more conductivity also underlines that the conduction follows vehicular mechanism. IL-based solid electrolyte membranes have exquisite conductivity and thermo-mechanical stability and are also free from water. Mainly, azoles like imidazole, pyrazole, triazole, benzimidazole, etc. or amines are the proton relay molecules in IL-based electrolytes with a wide pH range, which opens a new horizon to design alternate low-cost electrode materials, with an aim to replace expensive and scarce platinum electrodes. Protonated imidazole derivatives are found to have an excellent conductivity and it conducts like water through Grotthuss conduction mechanism [76–78].

but simple and convenient device structure, easy fabrication and comparatively low cost active materials made DSSC so popular. As we pointed out electrolyte is one of the major components of DSSC, the main function of which is to facilitate a conducting pathway to keep the electron transfer mechanism alive by regenerating the oxidised dye by providing

Volatile solvent and toxic additives make the use of this electrolyte undesirable even if they gave very good efficiency. Repeated exposure to the solar heating may evaporate the solvents so that the device efficiency lowered due to poor charge transport. Quest for alternate electrolytes found ILs/ILCs as one among possible candidates. Combination of redox active salts with ILs is often employed for generating the ionic couple which can produce faster charge transport. Viscosity and conductivity of ILs are the determining factors for improving charge transport. High-viscous ILs often fails to harvest good efficiencies owing to poor ionic diffusion and transport. Eutectic mixtures of ILs were employed by many groups as a low-viscous high-conducting electrolyte system with good efficiencies. It was also found that addition

to improve the electron injection between dye and TiO2

more basic anions like dicyanamide will also influence the conduction levels to modify the Voc of DSSC and thereby efficiency. Better redox couple diffusion, fast electron transfer, slow recombination due to high electron diffusion lengths, excellent electrochemical reversibility are the main advantages of using IL based electrolytes over conventional ones. Even then, the paradigm remains in finding appropriate ILs with energy levels closer to the dye for its fast regeneration, but at the same time it should be in a level so that Voc remain unaltered. Cobalt based redox couples have used very recently in harnessing very high efficiencies; a positive sign. Cobalt based systems in conjugation with ILs may lead to much better performances. Since the efficiency of DSSC is influenced by a large number of factors optimisation of each one with IL electrolytes is essential. Different photosensitizers like porphyrins, Ru-based inorganic dyes, organic molecules, etc. have been studied for solar energy conversion in combination with IL electrolytes. A porphyrin based dye recorded a 4.9% efficiency with [C<sup>2</sup>

ILs and ILCs are more suitable for flexible DSSCs since they could be easily formed in the form of thin films with the combination of some polymeric binders. Large scale batch to batch production of thin film electrolytes will definitely revise the face of modern solar technology. Easy miscibility of ILs with conventional polymers and non-volatility made them highly desirable for safe, flexible and durable energy conversions. Moisture sensitivity and atmospheric stability of the electrolyte membranes should also be optimum. Instead of easy corrodible ITO/FTO electrodes new stable substrates have to be designed and developed for working with IL based electrolytes. Semiconducting metal nano-particles, carbon derivatives

Quasi-solidified electrolytes obtained by gelling ILs with polymers or ionogels can also utilised for DSSCs. It will reduce leakage problems, improve connectivity, flexibility, etc. Use of ILCs as electrolytes is another possible option. Being solid-state analogues of ILs,

couple in acetonitrile is used as electrolyte in DSSCs to regenerate the dyes.

Ionic Liquids/Ionic Liquid Crystals for Safe and Sustainable Energy Storage Systems

based photoanode could lower the con-

[86]. Similarly

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327

mim]

electrons.

Normally I<sup>−</sup>

/I3 −

duction band of TiO2

[B(CN)<sup>4</sup>

of Lithium salt with IL in combination with TiO2

]-based IL electrolytes [87–90].

and polymers can also function as electrode materials.

The main challenge in employing ILs in PEMs is choosing the suitable molecular design of the ion pair, so that the whole requirements for the membrane electrolyte in a fuel cell including mechanical stability without compromising the proton transport activity are attained. Lee et al. fabricated a SPI/[dema][TfO] membrane for PEMFCs under non-humidified conditions, which fetch a peak power density of 100 mW cm−2 and maximum current density of 400 mA cm−2 at 120°C [73]. However, this membrane has an unstable three-phase boundary in the catalyst layer formed by leaked [dema][TfO]. Lee et al. reported many fuel cell varieties with the ILs provided by Ohno et al. [79]. But, the mechanical integrity of the solid electrolytes needs to be improved. Relatively, a new approach for fabricating mechanically stable and high-conducting solid electrolytes is the polymerization of ionic liquid monomers. Ionic liquid monomer [HSO<sup>3</sup> − BVIm][TfO] was polymerized by Díaz et al. and employed in a PEMFC at 25°C under anhydrous conditions, measuring a maximum current density of 154 mA cm−2 and a peak power density of 33.1 mW cm−2 [80]. These membranes were thermally stable up to 300°C. Utilisation of organic plastic crystals or ionic liquid crystals as high-conducting solid safety electrolytes is another intriguing strategy. Membranes based on the cellulose acetate supported with [choline][DHP] doped with various acids were studied by Rana et al. [81]. Belieres et al. recently identified that the small cation NH<sup>4</sup> + could be useful in replacement of H3 O+ [82]. Also, IL-based electrolytes with nitrate anion prove to have narrow ranges of current density, in which, there appears to be almost no energy barrier to oxygen reduction. The reasons for this are still not clarified, though they are presumably related to some instability of the nitrate anion. Hagiwara et al. proposed the application of fluorohydrogenate ionic liquid in fuel cells [83, 84]. In principle, the FHFC needs no humidification, since the fluorohydrogenate ion conduction does not require the presence of water. They have also demonstrated composite membranes consisting of 2-hydroxyethyl methacrylate (HEMA) and EMPyr(FH)1.7F and achieved a maximum power density of 200 mW cm−2.

#### **2.4. Dye-sensitized solar cells (DSSCs)**

DSSC is a third-generation photovoltaic device, which can convert solar energy into electrical energy in a more convenient and cost-effective way than the conventional semi-conducting photovoltaics [85]. They can be also designed as flexible and multi-coloured forms. The device structure of DSSCs include a photosensitizer (often organic dyes that is why the name dyesensitized), a photo anode comprised of semiconducting nano-particles with well-adjusted conduction band to accept electrons, a counter electrode (mainly Pt based) and electrolyte. The theoretical maximum efficiency that could be extracted from a DSSC is around 20% only, but simple and convenient device structure, easy fabrication and comparatively low cost active materials made DSSC so popular. As we pointed out electrolyte is one of the major components of DSSC, the main function of which is to facilitate a conducting pathway to keep the electron transfer mechanism alive by regenerating the oxidised dye by providing electrons.

leads to the formation of hydrogen-bonded networks between proton donor and acceptors. Protic ILs with more fluidity found to have more conductivity also underlines that the conduction follows vehicular mechanism. IL-based solid electrolyte membranes have exquisite conductivity and thermo-mechanical stability and are also free from water. Mainly, azoles like imidazole, pyrazole, triazole, benzimidazole, etc. or amines are the proton relay molecules in IL-based electrolytes with a wide pH range, which opens a new horizon to design alternate low-cost electrode materials, with an aim to replace expensive and scarce platinum electrodes. Protonated imidazole derivatives are found to have an excellent conductivity and

The main challenge in employing ILs in PEMs is choosing the suitable molecular design of the ion pair, so that the whole requirements for the membrane electrolyte in a fuel cell including mechanical stability without compromising the proton transport activity are attained. Lee et al. fabricated a SPI/[dema][TfO] membrane for PEMFCs under non-humidified conditions, which fetch a peak power density of 100 mW cm−2 and maximum current density of 400 mA cm−2 at 120°C [73]. However, this membrane has an unstable three-phase boundary in the catalyst layer formed by leaked [dema][TfO]. Lee et al. reported many fuel cell varieties with the ILs provided by Ohno et al. [79]. But, the mechanical integrity of the solid electrolytes needs to be improved. Relatively, a new approach for fabricating mechanically stable and high-conducting solid electrolytes is the polymerization of ionic liquid monomers. Ionic liq-

at 25°C under anhydrous conditions, measuring a maximum current density of 154 mA cm−2 and a peak power density of 33.1 mW cm−2 [80]. These membranes were thermally stable up to 300°C. Utilisation of organic plastic crystals or ionic liquid crystals as high-conducting solid safety electrolytes is another intriguing strategy. Membranes based on the cellulose acetate supported with [choline][DHP] doped with various acids were studied by Rana et al. [81].

[82]. Also, IL-based electrolytes with nitrate anion prove to have narrow ranges of

current density, in which, there appears to be almost no energy barrier to oxygen reduction. The reasons for this are still not clarified, though they are presumably related to some instability of the nitrate anion. Hagiwara et al. proposed the application of fluorohydrogenate ionic liquid in fuel cells [83, 84]. In principle, the FHFC needs no humidification, since the fluorohydrogenate ion conduction does not require the presence of water. They have also demonstrated composite membranes consisting of 2-hydroxyethyl methacrylate (HEMA) and

DSSC is a third-generation photovoltaic device, which can convert solar energy into electrical energy in a more convenient and cost-effective way than the conventional semi-conducting photovoltaics [85]. They can be also designed as flexible and multi-coloured forms. The device structure of DSSCs include a photosensitizer (often organic dyes that is why the name dyesensitized), a photo anode comprised of semiconducting nano-particles with well-adjusted conduction band to accept electrons, a counter electrode (mainly Pt based) and electrolyte. The theoretical maximum efficiency that could be extracted from a DSSC is around 20% only,

BVIm][TfO] was polymerized by Díaz et al. and employed in a PEMFC

+

could be useful in replacement

it conducts like water through Grotthuss conduction mechanism [76–78].

uid monomer [HSO<sup>3</sup>

326 Progress and Developments in Ionic Liquids

of H3 O+ −

**2.4. Dye-sensitized solar cells (DSSCs)**

Belieres et al. recently identified that the small cation NH<sup>4</sup>

EMPyr(FH)1.7F and achieved a maximum power density of 200 mW cm−2.

Normally I<sup>−</sup> /I3 − couple in acetonitrile is used as electrolyte in DSSCs to regenerate the dyes. Volatile solvent and toxic additives make the use of this electrolyte undesirable even if they gave very good efficiency. Repeated exposure to the solar heating may evaporate the solvents so that the device efficiency lowered due to poor charge transport. Quest for alternate electrolytes found ILs/ILCs as one among possible candidates. Combination of redox active salts with ILs is often employed for generating the ionic couple which can produce faster charge transport. Viscosity and conductivity of ILs are the determining factors for improving charge transport. High-viscous ILs often fails to harvest good efficiencies owing to poor ionic diffusion and transport. Eutectic mixtures of ILs were employed by many groups as a low-viscous high-conducting electrolyte system with good efficiencies. It was also found that addition of Lithium salt with IL in combination with TiO2 based photoanode could lower the conduction band of TiO2 to improve the electron injection between dye and TiO2 [86]. Similarly more basic anions like dicyanamide will also influence the conduction levels to modify the Voc of DSSC and thereby efficiency. Better redox couple diffusion, fast electron transfer, slow recombination due to high electron diffusion lengths, excellent electrochemical reversibility are the main advantages of using IL based electrolytes over conventional ones. Even then, the paradigm remains in finding appropriate ILs with energy levels closer to the dye for its fast regeneration, but at the same time it should be in a level so that Voc remain unaltered. Cobalt based redox couples have used very recently in harnessing very high efficiencies; a positive sign. Cobalt based systems in conjugation with ILs may lead to much better performances. Since the efficiency of DSSC is influenced by a large number of factors optimisation of each one with IL electrolytes is essential. Different photosensitizers like porphyrins, Ru-based inorganic dyes, organic molecules, etc. have been studied for solar energy conversion in combination with IL electrolytes. A porphyrin based dye recorded a 4.9% efficiency with [C<sup>2</sup> mim] [B(CN)<sup>4</sup> ]-based IL electrolytes [87–90].

ILs and ILCs are more suitable for flexible DSSCs since they could be easily formed in the form of thin films with the combination of some polymeric binders. Large scale batch to batch production of thin film electrolytes will definitely revise the face of modern solar technology. Easy miscibility of ILs with conventional polymers and non-volatility made them highly desirable for safe, flexible and durable energy conversions. Moisture sensitivity and atmospheric stability of the electrolyte membranes should also be optimum. Instead of easy corrodible ITO/FTO electrodes new stable substrates have to be designed and developed for working with IL based electrolytes. Semiconducting metal nano-particles, carbon derivatives and polymers can also function as electrode materials.

Quasi-solidified electrolytes obtained by gelling ILs with polymers or ionogels can also utilised for DSSCs. It will reduce leakage problems, improve connectivity, flexibility, etc. Use of ILCs as electrolytes is another possible option. Being solid-state analogues of ILs, ILCs have ions with a definite structural order to impart excellent solid-state conductivities. They could be further exploited for stimuli responsive conduction or temperature dependent storage, etc. Modern device structures for performing under dim light or extreme conditions will be in reality when combined IL/ILC electrolytes. It will also be suitable for wearable or stretchable device architectures to harness energy in a more sustainable manner.

**2.5. Other applications**

**3. Conclusions**

**Author details**

**References**

9228–9250.

J. Chem. 2010; 49A: 635–648.

Rev. 2015; 115: 6357−6426.

supercapacitors, fuel cells and batteries.

\*Address all correspondence to: sudhajd2001@yahoo.co.in

chains. Bull. Korean Chem. Soc. 2006; 27:847–852.

Chemical Science and Technology Division, CSIR-NIIST, Trivandrum, Kerala, India

cesses for energy production. Chem. Soc. Rev. 2014; 43 :7838–7869.

[1] S. Zhang, J. Sun, X. Zhang, J. Xin, Q. Miao and J. Wang. Ionic liquid-based green pro-

[2] M. Smiglak, J. M. Pringle, X. Lu, L. Han, S. Zhang, H. Gao, D. R. MacFarlane and R. D. Rogers. Ionic liquids for energy, materials, and medicine. Chem. Commun. 2014; 50:

[3] N. D. Khupse and A. Kumar. Ionic liquids: New materials with wide applications. Indian

[4] R. Hayes, G. G. Warr and R. Atkin. Structure and nanostructure in ionic liquids. Chem.

[5] G.-H. Min, T. Yim, H. Y. Lee, D. H. Huh, E. Lee, J. Mun, S. M. Oh and Y. G. Kim. Synthesis and properties of ionic liquids: Imidazolium tetrafluoroborates with unsaturated side

Sudha J. Devaki\* and Renjith Sasi

Apart from the above discussed energy applications, ionic liquids and ionic liquid crystals were also exploited in systems like thin film transistors, thermo-electrochemical cells, hydrogen storage applications, etc. Application of ILs/ILCs with more eco-friendly nature will generate safe and sustainable energy storage for the future. Application of ILs as gate electrolytes

Ionic Liquids/Ionic Liquid Crystals for Safe and Sustainable Energy Storage Systems

http://dx.doi.org/10.5772/65888

329

The present survey on the electrolyte systems used in various energy devices suggests that ionic liquid crystals are demonstrated as efficient electrolyte systems because of their anisotropic phase induced exciting chattels, such as high ionic conductivity and diffusion along with excellent thermal stability, low dimensionally ordered phase, low vapour pressure and non-toxicity. They can be considered as prospectable and sustainable materials for bringing safe and affordable electronic devices. Their ability to formulate solid electrolyte films, in combination with polymers, can make revolution in the field of flexible, wearable, bendable

in thin film transistors has pave way to the modern flexible electronics [96–99].

Quasi-solid electrolytes were also developed by dissolving poly (ionic liquid) in room temperature IL harvested a good PCE of 5.92% with excellent long-term stability. The superior performance is attributed to the improved conductivity owing to the extended charge transport networks via the π-π stacking of imidazolium rings [91]. Gunko et.al prepared new electrolytes by suspending smaller amounts of graphene flakes in RTILs to extract more than 25 times improved photovoltaic performance [92]. Kuang et al. used carbon black- IL mixture as better performing electrolyte without adding iodine to yield 6.37% [93]. Jang et al. used an efficient ionic plastic crystal derived from imidazolium iodide salt as an excellent electrolyte for solid-state DSSCs with high PCE of 7.8% [94].

The plastic crystal electrolyte membrane was fabricated over the dye-adsorbed TiO2 by melt processing. Wang et al. developed ionogels with silica nano-particles and ILs is to used as high-performing solid DSSC electrolytes [95]. Schematic diagram showing the electronic transfer between silica-ionic liquid quasi-solid electrolytes to the dye to re-generate it on photo-oxidation in solid-state DSSC is given in **Figure 7**. Polymer electrolytes composed of conventional polymer binders and ionic liquids could also employed for solidstate DSSC applications. Recently, Kato and co-workers developed high-temperature stable liquid crystalline electrolytes for DSSCs. The self-assembly of a carbonate mesogen and an IL forms 2D ion channels for the smooth passage of I− /I3 − couple, harvesting till date better performance up to 120°C. Utilisation of ionic liquid crystalline electrolytes helps in fabricating quasi-solid-state DSSCs with excellent power conversion efficiencies over a wide temperature range.

**Figure 7.** Quasi-solid-state electrolytes based on ionic liquids for DSSCs. Reproduced with permission from [95].

#### **2.5. Other applications**

ILCs have ions with a definite structural order to impart excellent solid-state conductivities. They could be further exploited for stimuli responsive conduction or temperature dependent storage, etc. Modern device structures for performing under dim light or extreme conditions will be in reality when combined IL/ILC electrolytes. It will also be suitable for wearable or stretchable device architectures to harness energy in a more sus-

Quasi-solid electrolytes were also developed by dissolving poly (ionic liquid) in room temperature IL harvested a good PCE of 5.92% with excellent long-term stability. The superior performance is attributed to the improved conductivity owing to the extended charge transport networks via the π-π stacking of imidazolium rings [91]. Gunko et.al prepared new electrolytes by suspending smaller amounts of graphene flakes in RTILs to extract more than 25 times improved photovoltaic performance [92]. Kuang et al. used carbon black- IL mixture as better performing electrolyte without adding iodine to yield 6.37% [93]. Jang et al. used an efficient ionic plastic crystal derived from imidazolium iodide salt as an excellent electrolyte

The plastic crystal electrolyte membrane was fabricated over the dye-adsorbed TiO2

melt processing. Wang et al. developed ionogels with silica nano-particles and ILs is to used as high-performing solid DSSC electrolytes [95]. Schematic diagram showing the electronic transfer between silica-ionic liquid quasi-solid electrolytes to the dye to re-generate it on photo-oxidation in solid-state DSSC is given in **Figure 7**. Polymer electrolytes composed of conventional polymer binders and ionic liquids could also employed for solidstate DSSC applications. Recently, Kato and co-workers developed high-temperature stable liquid crystalline electrolytes for DSSCs. The self-assembly of a carbonate mesogen and an

performance up to 120°C. Utilisation of ionic liquid crystalline electrolytes helps in fabricating quasi-solid-state DSSCs with excellent power conversion efficiencies over a wide

**Figure 7.** Quasi-solid-state electrolytes based on ionic liquids for DSSCs. Reproduced with permission from [95].

/I3 −

couple, harvesting till date better

tainable manner.

328 Progress and Developments in Ionic Liquids

temperature range.

for solid-state DSSCs with high PCE of 7.8% [94].

IL forms 2D ion channels for the smooth passage of I−

Apart from the above discussed energy applications, ionic liquids and ionic liquid crystals were also exploited in systems like thin film transistors, thermo-electrochemical cells, hydrogen storage applications, etc. Application of ILs/ILCs with more eco-friendly nature will generate safe and sustainable energy storage for the future. Application of ILs as gate electrolytes in thin film transistors has pave way to the modern flexible electronics [96–99].

### **3. Conclusions**

The present survey on the electrolyte systems used in various energy devices suggests that ionic liquid crystals are demonstrated as efficient electrolyte systems because of their anisotropic phase induced exciting chattels, such as high ionic conductivity and diffusion along with excellent thermal stability, low dimensionally ordered phase, low vapour pressure and non-toxicity. They can be considered as prospectable and sustainable materials for bringing safe and affordable electronic devices. Their ability to formulate solid electrolyte films, in combination with polymers, can make revolution in the field of flexible, wearable, bendable supercapacitors, fuel cells and batteries.

### **Author details**

by

Sudha J. Devaki\* and Renjith Sasi

\*Address all correspondence to: sudhajd2001@yahoo.co.in

Chemical Science and Technology Division, CSIR-NIIST, Trivandrum, Kerala, India

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**Section 5**

**Physical Properties**


**Section 5**

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336 Progress and Developments in Ionic Liquids

**Chapter 15**

**Provisional chapter**

**Predicting Density and Refractive Index of Ionic Liquids**

The determination of the physicochemical properties of ionic liquids (ILs), such as density and refractive index, is essential for the design of processes that involve ILs. Density has been widely studied in ILs because of its importance whereas refractive index has received less attention even though its determination is rapid, highly accurate and needs a small amount of sample in most techniques. Due to the large number of possible cation and anion combinations, it is not practical to use trial and error methods to find a suitable ionic liquid for a given function. It would be preferable to predict physical properties of ILs from their structure. We compile in this work different methods to predict density and refractive index of ILs from literature. Especially, we describe the method developed by the authors in a previous work for predicting density of ILs through their molecular volume. We also correlate our experimental measure‐ ments of density and refractive index of ILs in order to predict one of the parameters knowing the other one as a function of temperature. As the measurement of refractive index is very fast and needs only a drop of the ionic liquid, this is also a very useful

**Keywords:** ionic liquids, density, refractive index, ionic volume, molecular volume,

Ionic liquids (ILs) are organic salts with low melting point so most of them are liquid at room temperature. During the last years, scientists and engineers have shown a huge interest for ILs in research and industrial fields because of their capacity for being chemical and biochem‐ ical reaction media. ILs are also of interest because they can be considered a new group of

and reproduction in any medium, provided the original work is properly cited.

© 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution,

© 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

**Predicting Density and Refractive Index of Ionic Liquids**

Mercedes G. Montalbán, Mar Collado-González,

Mercedes G. Montalbán, Mar Collado-González,

F. Guillermo Díaz-Baños and Gloria Víllora

F. Guillermo Díaz-Baños and Gloria Víllora

Additional information is available at the end of the chapter

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/65790

**Abstract**

approach.

**1. Introduction**

prediction, correlation

#### **Predicting Density and Refractive Index of Ionic Liquids Predicting Density and Refractive Index of Ionic Liquids**

Mercedes G. Montalbán, Mar Collado-González, F. Guillermo Díaz-Baños and Gloria Víllora Mercedes G. Montalbán, Mar Collado-González, F. Guillermo Díaz-Baños and Gloria Víllora

Additional information is available at the end of the chapter Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/65790

#### **Abstract**

The determination of the physicochemical properties of ionic liquids (ILs), such as density and refractive index, is essential for the design of processes that involve ILs. Density has been widely studied in ILs because of its importance whereas refractive index has received less attention even though its determination is rapid, highly accurate and needs a small amount of sample in most techniques. Due to the large number of possible cation and anion combinations, it is not practical to use trial and error methods to find a suitable ionic liquid for a given function. It would be preferable to predict physical properties of ILs from their structure. We compile in this work different methods to predict density and refractive index of ILs from literature. Especially, we describe the method developed by the authors in a previous work for predicting density of ILs through their molecular volume. We also correlate our experimental measure‐ ments of density and refractive index of ILs in order to predict one of the parameters knowing the other one as a function of temperature. As the measurement of refractive index is very fast and needs only a drop of the ionic liquid, this is also a very useful approach.

**Keywords:** ionic liquids, density, refractive index, ionic volume, molecular volume, prediction, correlation

### **1. Introduction**

Ionic liquids (ILs) are organic salts with low melting point so most of them are liquid at room temperature. During the last years, scientists and engineers have shown a huge interest for ILs in research and industrial fields because of their capacity for being chemical and biochem‐ ical reaction media. ILs are also of interest because they can be considered a new group of

© 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

polar and nonaqueous solvents. Their most important advantage as solvents is that they possess negligible vapor pressure [1]. For that, they are well known as "green solvents" compared to conventional volatile organic compounds (VOCs). Other relevant properties of the ILs are that they are highly stable from the chemical and thermal points of view. However, ILs are mainly valued because of the possibility of modulating their physical and chemical properties, such as melting point, viscosity, density, hydrophobicity and polarity by selecting the appropriate anion and cation and, in this way, they can be used for a specific application. For this reason, some authors have called them "designer solvents." Hence the number of different combinations of anions and cations that can be chosen to form potential ILs is enormous [2]. Some years ago, Álvarez‐Guerra and Irabien [3] claimed that "more than 106 different ILs may be synthesized, with 1012 binary combinations and 1018 ternary systems possible," while the number of traditional solvents widely used in industry is around a few hundred. The excellent properties of the ILs permit their application in many different fields such as synthesis, catalysis, electrochemistry, separation technology, analytical chemistry and nanotechnology [4].

the density of all the feasible ILs. In addition, it is not worthwhile to use trial and error methods to find the suitable IL for a specific application. Therefore, developing reliable predictive methods and correlations to estimate the density of ILs in a wide range of temperature is essential. Furthermore, this kind of method allows a better understanding of the influence of

Predicting Density and Refractive Index of Ionic Liquids

http://dx.doi.org/10.5772/65790

341

During the last years, authors have developed different methods to estimate density of ILs. We have compiled in this chapter published studies concerning methods to predict or estimate density of ILs according to the classification established by Paduszyński and Domańska [7]. Briefly, some authors have used methods based on quantitative structure‐property relation‐ ships (QSPRs) [11–15] and on artificial neural networks (ANNs) [16–18]. Other authors have developed estimation methods for density of ILs by adopting equation of state (EoS) [19–29]. There are also some studies in which the estimation of density of ILs is carried out by group contribution methods (GCMs) [6, 7, 10, 30–37] and correlations between density and other properties such as refractive index, molar refraction or surface tension [6, 38–42]. In order to improve the predicting capabilities, it is usual to find in the literature a combination of methods

A QSPR model is a mathematical model that links the structure‐derived features of a chemical compound to a physicochemical property. They are based on quantum chemistry calculations. This is their great advantage and, at the same time, their main drawback. While virtually any imagined compound can be studied with no previous experimental knowledge, usually the calculations are not easy and only can be developed by very specialized research groups.

In the literature, we can find several QSPR models to predict density of ILs. Trohalaki et al. [13] developed a QSPR model by the use of CODESSA software. They use three types of descriptors (electrostatic, quantum mechanical and thermodynamic) in order to predict the density of triazolium‐based ILs. Palomar et al. [15] determined the density of 40 imidazolium‐based ILs using COSMO‐RS. In this model, thermodynamic data are obtained from the molecular surface polarity of the individual compounds of the mixture. A year later, they combined COSMO‐RS with ANN to get a computational approach with a new descriptor which was useful to simulate the density of 45 imidazolium‐based ILs [14]. Interestingly, this approach allows them to propose a design strategy which introduces the desired IL properties as input into inverse neural networks to obtain a selection of counterions. Lazzús [18] used a QSPR model with 11 descriptors based on semiempirical calculations to estimate the density of ILs as a function of temperature and pressure. Specifically, the range of temperature and pressure was 258–393 K and 0.09–207 MPa, respectively. Finally, El‐Harbawi et al. [11] proposed a new QSPR model using MATLABTM software for the development of the algorithm and the same molecular descriptors used by Shen et al. [24]. The code was written based on a combination of multiple

the structure of the ILs on the density and on other physicochemical properties [7].

included in different categories of this classification.

linear regression and polynomial equation.

**2.1. Estimation by quantitative structure-property relationships (QSPRs)**

In order to design processes involving ILs, it is essential to determine and understand some of their physicochemical properties, such as density and refractive index [5]. On the one hand, density of ILs is related to the mechanics and engineering components of a process and is usually used to determine parameters like rates of liquid‐liquid phase separation, mass transfer, power requirements of mixing and pumping and equipment sizing [6]. In fact, density of ILs is necessary to solve material or energy balance equations of chemical processes in industry [7]. On the other hand, refractive index of ILs is more linked to specific chemical properties like the structuredness, polarity and relative hydrogen bonding donating and accepting ability which help to determine solubilities, partition constants and reaction rates [8]. In general, refractive index of a compound is especially used to verify a material and check its purity, or to determine the concentration of a mixture. It is also related to the forces between molecules or their behavior in solution [9] and can be easily correlated with certain properties of the material such as the dielectric constant, density and surface tension by means of thermodynamic equations [10].

We compile in this chapter different methods to predict density and refractive index of ILs from the literature. Especially, we describe the method developed by the authors in a previous work for predicting density of ionic liquids through their molecular volume. We also correlate our experimental measurements of density and refractive index for a set of ionic liquids in order to predict one of the parameters knowing the other one as a function of temperature [6].

### **2. Prediction methods of density of ionic liquids**

Density (*ρ*) is defined as the mass (*m*) per volume unit (*V*), *ρ* = *m*/*V*, and is one of the most relevant physical properties of a chemical compound.

As it has been mentioned before, nowadays the possible number of combinations of cation/ anion to form ILs is huge. For this reason, it is almost impossible to measure experimentally the density of all the feasible ILs. In addition, it is not worthwhile to use trial and error methods to find the suitable IL for a specific application. Therefore, developing reliable predictive methods and correlations to estimate the density of ILs in a wide range of temperature is essential. Furthermore, this kind of method allows a better understanding of the influence of the structure of the ILs on the density and on other physicochemical properties [7].

polar and nonaqueous solvents. Their most important advantage as solvents is that they possess negligible vapor pressure [1]. For that, they are well known as "green solvents" compared to conventional volatile organic compounds (VOCs). Other relevant properties of the ILs are that they are highly stable from the chemical and thermal points of view. However, ILs are mainly valued because of the possibility of modulating their physical and chemical properties, such as melting point, viscosity, density, hydrophobicity and polarity by selecting the appropriate anion and cation and, in this way, they can be used for a specific application. For this reason, some authors have called them "designer solvents." Hence the number of different combinations of anions and cations that can be chosen to form potential ILs is enormous [2]. Some years ago, Álvarez‐Guerra and Irabien [3] claimed that "more than 106 different ILs may be synthesized, with 1012 binary combinations and 1018 ternary systems possible," while the number of traditional solvents widely used in industry is around a few hundred. The excellent properties of the ILs permit their application in many different fields such as synthesis, catalysis, electrochemistry, separation technology, analytical chemistry and

In order to design processes involving ILs, it is essential to determine and understand some of their physicochemical properties, such as density and refractive index [5]. On the one hand, density of ILs is related to the mechanics and engineering components of a process and is usually used to determine parameters like rates of liquid‐liquid phase separation, mass transfer, power requirements of mixing and pumping and equipment sizing [6]. In fact, density of ILs is necessary to solve material or energy balance equations of chemical processes in industry [7]. On the other hand, refractive index of ILs is more linked to specific chemical properties like the structuredness, polarity and relative hydrogen bonding donating and accepting ability which help to determine solubilities, partition constants and reaction rates [8]. In general, refractive index of a compound is especially used to verify a material and check its purity, or to determine the concentration of a mixture. It is also related to the forces between molecules or their behavior in solution [9] and can be easily correlated with certain properties of the material such as the dielectric constant, density and surface tension by means of

We compile in this chapter different methods to predict density and refractive index of ILs from the literature. Especially, we describe the method developed by the authors in a previous work for predicting density of ionic liquids through their molecular volume. We also correlate our experimental measurements of density and refractive index for a set of ionic liquids in order to predict one of the parameters knowing the other one as a function of temperature [6].

Density (*ρ*) is defined as the mass (*m*) per volume unit (*V*), *ρ* = *m*/*V*, and is one of the most

As it has been mentioned before, nowadays the possible number of combinations of cation/ anion to form ILs is huge. For this reason, it is almost impossible to measure experimentally

nanotechnology [4].

340 Progress and Developments in Ionic Liquids

thermodynamic equations [10].

**2. Prediction methods of density of ionic liquids**

relevant physical properties of a chemical compound.

During the last years, authors have developed different methods to estimate density of ILs. We have compiled in this chapter published studies concerning methods to predict or estimate density of ILs according to the classification established by Paduszyński and Domańska [7]. Briefly, some authors have used methods based on quantitative structure‐property relation‐ ships (QSPRs) [11–15] and on artificial neural networks (ANNs) [16–18]. Other authors have developed estimation methods for density of ILs by adopting equation of state (EoS) [19–29]. There are also some studies in which the estimation of density of ILs is carried out by group contribution methods (GCMs) [6, 7, 10, 30–37] and correlations between density and other properties such as refractive index, molar refraction or surface tension [6, 38–42]. In order to improve the predicting capabilities, it is usual to find in the literature a combination of methods included in different categories of this classification.

#### **2.1. Estimation by quantitative structure-property relationships (QSPRs)**

A QSPR model is a mathematical model that links the structure‐derived features of a chemical compound to a physicochemical property. They are based on quantum chemistry calculations. This is their great advantage and, at the same time, their main drawback. While virtually any imagined compound can be studied with no previous experimental knowledge, usually the calculations are not easy and only can be developed by very specialized research groups.

In the literature, we can find several QSPR models to predict density of ILs. Trohalaki et al. [13] developed a QSPR model by the use of CODESSA software. They use three types of descriptors (electrostatic, quantum mechanical and thermodynamic) in order to predict the density of triazolium‐based ILs. Palomar et al. [15] determined the density of 40 imidazolium‐based ILs using COSMO‐RS. In this model, thermodynamic data are obtained from the molecular surface polarity of the individual compounds of the mixture. A year later, they combined COSMO‐RS with ANN to get a computational approach with a new descriptor which was useful to simulate the density of 45 imidazolium‐based ILs [14]. Interestingly, this approach allows them to propose a design strategy which introduces the desired IL properties as input into inverse neural networks to obtain a selection of counterions. Lazzús [18] used a QSPR model with 11 descriptors based on semiempirical calculations to estimate the density of ILs as a function of temperature and pressure. Specifically, the range of temperature and pressure was 258–393 K and 0.09–207 MPa, respectively. Finally, El‐Harbawi et al. [11] proposed a new QSPR model using MATLABTM software for the development of the algorithm and the same molecular descriptors used by Shen et al. [24]. The code was written based on a combination of multiple linear regression and polynomial equation.

#### **2.2. Estimation by artificial neural networks (ANNs)**

ANN is an especially efficient computer algorithm to approximate any function with a finite number of discontinuities by learning the relationships between input and output vectors [43]. ANNs are usually suitable to model chemical properties whose behavior is highly nonlinear because nonlinear relationships are well described with ANN. The predicting capabilities of this method depend on the quality of the algorithm for learning and very importantly on the quality, quantity and nature of experimental (or calculated) data used for the learning process. Some authors have developed approaches to estimate density of ILs based on ANN combined with a GCM.

to obtain the density of the desired compound usually some previous experimental (or calculated) data of certain critical properties (temperature, pressure, volume, …) are needed. Goharshadi and Moosavi [19] proposed a simple EoS (called GMA EoS) to predict the density of phosphonium and imidazolium‐based ILs at different temperatures and pressures. Valder‐ rama and Zarricueta [21] used ten expressions based on the corresponding state principle to estimate the density of ILs and analyzed one of them in depth to obtain a new generalized, accurate and simple model using only the critical properties and the molecular weight of the ionic liquid. As several authors have claimed that the dependence between density and temperature is linear, the proposal of Valderrama and Zarricueta was to linearize the Valder‐ rama and Abu‐Sharkh model [47] to end up with a simple and totally predictive model, the

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343

linear generalized model, to estimate the density of ILs with acceptable accuracy.

spherical approximation (MSA) and they improved the results.

results than these two equations especially near the critical areas.

densities of ILs at elevated pressures.

density of ILs.

Ji and Adidharma [26] modeled the density of three families of ILs ([Cnmim][Tf2N], [Cnmim] [BF4] and [Cnmim][PF6]) in a wide temperature and pressure range by the usage of a model published in previous works [48–50]. Specifically, this model uses a heterosegmented statistical associating fluid (hetero‐SAFT) equation of state which can predict the properties of an ionic liquid based on the information of its alkyl substituent, cation head and anion. However, this model does not taken into account the electrostatic interactions. On the contrary, Wang et al. [23, 27] included an electrostatic term to residual Helmholtz energy expressed by mean

Abildskow et al. [22] used the statistical mechanical fluctuation solution theory to develop two models which provide a direct connection between integrals of the molecular direct correlation function and isothermal derivatives of pressure and density. In this way, they can predict

Shen et al. [24] extended the Valderrama and Robles [51] group contribution model for the critical properties to the estimation of densities of ILs at different temperatures and pressures representing the critical properties by the modified the Lydersen‐Joback‐Reid group contri‐

Patel and Joshipura [25] used a simple correlation presented by Nasrifar and Moshfeghian [52] in conjunction with the predictive‐Soave‐Redlich‐Kwong (PSRK) equation of state to estimate

More recently, Mahboub and Farrokhpour [28] developed a molecular modeling of ILs incorporating the perturbed thermodynamic linear Yukawa isotherm regularity (LYIR) equation of state, which is derived based on an effective nearest neighboring pair attractive interaction of the Yukawa potential. They used this model to predict the densities of ILs up to high pressures (35 MPa) and in the temperature range 293.15–393.15 K. To use the LYIR for ILs, a simple molecular model was proposed to describe their molecular structure, in which they were considered as a liquid consisting of the ion pairs moving together in the fluid, and each ion pair was assumed to be a one‐center spherical united atom. Farzi and Esmaeilzadeh [29] used the Esmaeilzadeh‐Roshanfekr EoS obtained in a previous paper [53] to predict density of ILs. This EoS is based on the Patel‐Teja EoS and Peng‐Robinson EoS and offer better

bution method and predicting the density by the Patel‐Teja (PT) equation of state.

Properties of chemical compounds like boiling point, critical temperature, critical pressure, vapor pressure, heat capacity, enthalpy of sublimation, heat of vaporization, density, refractive index, surface tension, viscosity, thermal conductivity, and acentric factor have been analyzed by ANN in the literature [44]. For instance, Valderrama et al. [17] proposed a combined GCM + ANN method which divides the molecule in defined groups but instead of determining the value of the group contributions, an ANN is used to get the relationship between the dependent and independent variables. In this way, the relationship between density and molecular structure was determined. Lazzús [16] also proposed in a first paper another combined (GCM + ANN) method, which uses a feedforward backpropagation neural network. It is shown that this network is very efficient in representing nonlinear relationships among properties. Specifically, the network, which was programmed with the software MATLAB, consists of a multilayer network, in which the flow of information spreads forward through the layers while the propagation of the error is back. In this way, he predicted the density of 72 ILs as a function of temperature and pressure. Later, Lazzús [18] published the estimation results obtained with another similar model replacing standard backpropagation with particle swarm optimization (PSO) because some authors had shown that PSO‐based ANN led to a better training per‐ formance and predicting capacity and a faster convergence rate than the standard backpro‐ pagation algorithm. Briefly, PSO is a population‐based optimization tool, where the system is initialized with a population of random particles and the algorithm searches for optima by updating generations [45]. In a PSO system, each particle is "flown" through the multidimen‐ sional search space, adjusting its position in search space according to its own experience and that of neighboring particles. The particle therefore makes use of the best position encountered by itself and that of its neighbors to position itself toward an optimal solution. The performance of each particle is evaluated using a predefined fitness function, which encapsulates the characteristics of the optimization problem [46].

#### **2.3. Estimation by equation of state (EoS)**

Equations of state are well known for determining the relationship between pressure, tem‐ perature, volume and composition of components providing a theoretical way to calculate some physical properties such as density. Therefore, some equations of state have been used to estimate density of ILs in the literature. Usually the proposed EoS contains some constants whose values have been obtained by previous fitting of the available experimental results. The main advantage is the simplicity in the use of these methods but a possible drawback is that to obtain the density of the desired compound usually some previous experimental (or calculated) data of certain critical properties (temperature, pressure, volume, …) are needed.

**2.2. Estimation by artificial neural networks (ANNs)**

342 Progress and Developments in Ionic Liquids

characteristics of the optimization problem [46].

**2.3. Estimation by equation of state (EoS)**

with a GCM.

ANN is an especially efficient computer algorithm to approximate any function with a finite number of discontinuities by learning the relationships between input and output vectors [43]. ANNs are usually suitable to model chemical properties whose behavior is highly nonlinear because nonlinear relationships are well described with ANN. The predicting capabilities of this method depend on the quality of the algorithm for learning and very importantly on the quality, quantity and nature of experimental (or calculated) data used for the learning process. Some authors have developed approaches to estimate density of ILs based on ANN combined

Properties of chemical compounds like boiling point, critical temperature, critical pressure, vapor pressure, heat capacity, enthalpy of sublimation, heat of vaporization, density, refractive index, surface tension, viscosity, thermal conductivity, and acentric factor have been analyzed by ANN in the literature [44]. For instance, Valderrama et al. [17] proposed a combined GCM + ANN method which divides the molecule in defined groups but instead of determining the value of the group contributions, an ANN is used to get the relationship between the dependent and independent variables. In this way, the relationship between density and molecular structure was determined. Lazzús [16] also proposed in a first paper another combined (GCM + ANN) method, which uses a feedforward backpropagation neural network. It is shown that this network is very efficient in representing nonlinear relationships among properties. Specifically, the network, which was programmed with the software MATLAB, consists of a multilayer network, in which the flow of information spreads forward through the layers while the propagation of the error is back. In this way, he predicted the density of 72 ILs as a function of temperature and pressure. Later, Lazzús [18] published the estimation results obtained with another similar model replacing standard backpropagation with particle swarm optimization (PSO) because some authors had shown that PSO‐based ANN led to a better training per‐ formance and predicting capacity and a faster convergence rate than the standard backpro‐ pagation algorithm. Briefly, PSO is a population‐based optimization tool, where the system is initialized with a population of random particles and the algorithm searches for optima by updating generations [45]. In a PSO system, each particle is "flown" through the multidimen‐ sional search space, adjusting its position in search space according to its own experience and that of neighboring particles. The particle therefore makes use of the best position encountered by itself and that of its neighbors to position itself toward an optimal solution. The performance of each particle is evaluated using a predefined fitness function, which encapsulates the

Equations of state are well known for determining the relationship between pressure, tem‐ perature, volume and composition of components providing a theoretical way to calculate some physical properties such as density. Therefore, some equations of state have been used to estimate density of ILs in the literature. Usually the proposed EoS contains some constants whose values have been obtained by previous fitting of the available experimental results. The main advantage is the simplicity in the use of these methods but a possible drawback is that Goharshadi and Moosavi [19] proposed a simple EoS (called GMA EoS) to predict the density of phosphonium and imidazolium‐based ILs at different temperatures and pressures. Valder‐ rama and Zarricueta [21] used ten expressions based on the corresponding state principle to estimate the density of ILs and analyzed one of them in depth to obtain a new generalized, accurate and simple model using only the critical properties and the molecular weight of the ionic liquid. As several authors have claimed that the dependence between density and temperature is linear, the proposal of Valderrama and Zarricueta was to linearize the Valder‐ rama and Abu‐Sharkh model [47] to end up with a simple and totally predictive model, the linear generalized model, to estimate the density of ILs with acceptable accuracy.

Ji and Adidharma [26] modeled the density of three families of ILs ([Cnmim][Tf2N], [Cnmim] [BF4] and [Cnmim][PF6]) in a wide temperature and pressure range by the usage of a model published in previous works [48–50]. Specifically, this model uses a heterosegmented statistical associating fluid (hetero‐SAFT) equation of state which can predict the properties of an ionic liquid based on the information of its alkyl substituent, cation head and anion. However, this model does not taken into account the electrostatic interactions. On the contrary, Wang et al. [23, 27] included an electrostatic term to residual Helmholtz energy expressed by mean spherical approximation (MSA) and they improved the results.

Abildskow et al. [22] used the statistical mechanical fluctuation solution theory to develop two models which provide a direct connection between integrals of the molecular direct correlation function and isothermal derivatives of pressure and density. In this way, they can predict densities of ILs at elevated pressures.

Shen et al. [24] extended the Valderrama and Robles [51] group contribution model for the critical properties to the estimation of densities of ILs at different temperatures and pressures representing the critical properties by the modified the Lydersen‐Joback‐Reid group contri‐ bution method and predicting the density by the Patel‐Teja (PT) equation of state.

Patel and Joshipura [25] used a simple correlation presented by Nasrifar and Moshfeghian [52] in conjunction with the predictive‐Soave‐Redlich‐Kwong (PSRK) equation of state to estimate density of ILs.

More recently, Mahboub and Farrokhpour [28] developed a molecular modeling of ILs incorporating the perturbed thermodynamic linear Yukawa isotherm regularity (LYIR) equation of state, which is derived based on an effective nearest neighboring pair attractive interaction of the Yukawa potential. They used this model to predict the densities of ILs up to high pressures (35 MPa) and in the temperature range 293.15–393.15 K. To use the LYIR for ILs, a simple molecular model was proposed to describe their molecular structure, in which they were considered as a liquid consisting of the ion pairs moving together in the fluid, and each ion pair was assumed to be a one‐center spherical united atom. Farzi and Esmaeilzadeh [29] used the Esmaeilzadeh‐Roshanfekr EoS obtained in a previous paper [53] to predict density of ILs. This EoS is based on the Patel‐Teja EoS and Peng‐Robinson EoS and offer better results than these two equations especially near the critical areas.

#### **2.4. Estimation by group contribution methods (GCMs)**

Other authors have preferred to develop GCMs to estimate the density of ILs. First of all, we will revise briefly the published studies in the literature and after that we will detail our own predictive method based on a GCM. In these methods, the desired property is obtained as individual contribution of each of the components of the final compound. Then, predictions are obtained using available simple equations. The main drawback of these methods is that their predictive capability depends on previous experimental (or calculated) data. With some exceptions, it is impossible to predict density if no data are available for both of the components of an ionic liquid. On the other hand, usually good predictions can be obtained very easily for any combination of known counterions in a few minutes.

*T***/K** *Vm***/nm3** *αp* **× 10−4/K−1** *Vm***/nm3** *αp* **× 10−4/K−1** *V***m/nm3** *αp* **× 10−4/K−1**

293.15 0.2561 5.93 0.3119 5.96 0.3676 6.06

293.15 0.4234 6.14 0.3112 6.08 0.3691 6.19

293.15 0.5401 6.09 0.3442 6.13 0.3996 6.14

293.15 0.4556 6.18 0.4264 6.65 0.4834 6.72

293.15 0.5397 6.71 0.5959 6.78 0.3160 5.47

303.15 0.2577 0.3134 0.3698 313.15 0.2592 0.3152 0.3721 323.15 0.2607 0.3171 0.3744 333.15 0.2623 0.3190 0.3766 343.15 0.2638 0.3209 0.3789

303.15 0.4260 0.3131 0.3714 313.15 0.4286 0.3150 0.3737 323.15 0.4313 0.3169 0.3760 333.15 0.4339 0.3189 0.3784 343.15 0.4366 0.3208 0.3808

**] [bmim+**

**] [emim+**

**] [omim+**

temperatures *T* ranging from 293.15 to 343.15 K at pressure *p* = 0.1 MPa.

303.15 0.5434 0.5999 0.3177 313.15 0.5470 0.6040 0.3195 323.15 0.5507 0.6081 0.3212 333.15 0.5544 0.6122 0.3230 343.15 0.5581 0.6164 0.3248

303.15 0.5434 0.3463 0.4021 313.15 0.5467 0.3485 0.4045 323.15 0.5500 0.3506 0.4070 333.15 0.5534 0.3528 0.4096 343.15 0.5568 0.3549 0.4121

303.15 0.4584 0.4292 0.4866 313.15 0.4612 0.4321 0.4903 323.15 0.4641 0.4349 0.4933 333.15 0.4670 0.4378 0.4966 343.15 0.4699 0.4408 0.5003

**][BF4 −**

**][TfO−**

**][PF6 −**

**][NTf2 −**

**][NTf2 −**

**Table 1.** Experimental values of molecular volume, *Vm*, and thermal expansion coefficient, *αp*, of the studied ILs at

**] [hmim+**

**] [bmim+**

**] [hmim+**

**] [bmim+**

**] [emim+**

**][BF4 − ]**

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345

**][TfO− ]**

**][PF6 − ]**

**][NTf2 − ]**

**][EtSO4 − ]**

**] [bmim+**

**] [emim+**

**[emim+**

**[omim+**

**[bmim+**

**[omim+**

**[hmim+**

**][NTf2 −**

**][PF6 −**

**][OcSO4 −**

**][BF4 −**

**][BF4 −**

Slattery et al. [32] supported that a strong relationship exists between molecular volumes and density. Thus they described this relationship and predict the density of ILs only from their molecular volumes and an anion‐dependent correlation. They considered the molecular volume as the sum of the individual contributions of the anion and the cation. Ye and Shreeve [36] widened a predictive method proposed by Jenkins et al. [54] for estimation of thermo‐ chemical radius and closed packed volume of single ions and used it as GCM for ILs. The method provided good density prediction results but, unfortunately, is limited to room temperature and ambient pressure. To solve this disadvantage, Gardas and Coutinho [31] extended this method estimating three coefficients independent of the ionic liquid which take into account the influence of temperature and pressure on the molecular volume. In this way, they can predict the density of ILs in a wide range of temperature (273.15–393.15 K) and pressure (0.10–100 MPa). Jacquemin et al. [34, 35] studied the suitability of GCMs to predict density of ILs. The study was based on an assumption proposed by Rebelo et al. [55, 56] that the molar volume of the ILs can be obtained from the effective molar volumes of the ions considering the ionic liquid as an "ideal" mixture. Qiao et al. [30] introduced the interaction between several substitutes on the same center in the partition of groups. For that, the same group structure attached to different substitutes may have different group values. Paduszyński and Domańnska [7] proposed a simple and generalized correlation to estimate the density of ILs as a function of temperature and pressure. They divided the ILs in 177 functional groups which are classified in three subgroups: cation cores, anion cores and substituents. In this way, they obtained the group contributions to molar volume for each of the functional groups and the universal coefficients describing the relationship pressure‐density‐temperature. Lately, Keshavarz et al. [37] provided a simple correlation to predict density of ILs based on their size, structure and types of cations and anions. They introduced two correcting terms which take into account the effect that some specific cations and anion may have in the ionic packing leading to its increase or decrease and hence affect to the density values. Finally, Kermanioryani et al. [33] found new group contribution parameters using the Gardas and Coutinho model [31] to predict the density of ILs more accurately.

Recently, we have proposed our own method to predict density of ILs [6]. It can be considered as a GCM and it is based on the prediction of the molecular volume of ILs from the ionic volume. It will be described in detail in the following paragraphs.


**2.4. Estimation by group contribution methods (GCMs)**

344 Progress and Developments in Ionic Liquids

any combination of known counterions in a few minutes.

[31] to predict the density of ILs more accurately.

volume. It will be described in detail in the following paragraphs.

Other authors have preferred to develop GCMs to estimate the density of ILs. First of all, we will revise briefly the published studies in the literature and after that we will detail our own predictive method based on a GCM. In these methods, the desired property is obtained as individual contribution of each of the components of the final compound. Then, predictions are obtained using available simple equations. The main drawback of these methods is that their predictive capability depends on previous experimental (or calculated) data. With some exceptions, it is impossible to predict density if no data are available for both of the components of an ionic liquid. On the other hand, usually good predictions can be obtained very easily for

Slattery et al. [32] supported that a strong relationship exists between molecular volumes and density. Thus they described this relationship and predict the density of ILs only from their molecular volumes and an anion‐dependent correlation. They considered the molecular volume as the sum of the individual contributions of the anion and the cation. Ye and Shreeve [36] widened a predictive method proposed by Jenkins et al. [54] for estimation of thermo‐ chemical radius and closed packed volume of single ions and used it as GCM for ILs. The method provided good density prediction results but, unfortunately, is limited to room temperature and ambient pressure. To solve this disadvantage, Gardas and Coutinho [31] extended this method estimating three coefficients independent of the ionic liquid which take into account the influence of temperature and pressure on the molecular volume. In this way, they can predict the density of ILs in a wide range of temperature (273.15–393.15 K) and pressure (0.10–100 MPa). Jacquemin et al. [34, 35] studied the suitability of GCMs to predict density of ILs. The study was based on an assumption proposed by Rebelo et al. [55, 56] that the molar volume of the ILs can be obtained from the effective molar volumes of the ions considering the ionic liquid as an "ideal" mixture. Qiao et al. [30] introduced the interaction between several substitutes on the same center in the partition of groups. For that, the same group structure attached to different substitutes may have different group values. Paduszyński and Domańnska [7] proposed a simple and generalized correlation to estimate the density of ILs as a function of temperature and pressure. They divided the ILs in 177 functional groups which are classified in three subgroups: cation cores, anion cores and substituents. In this way, they obtained the group contributions to molar volume for each of the functional groups and the universal coefficients describing the relationship pressure‐density‐temperature. Lately, Keshavarz et al. [37] provided a simple correlation to predict density of ILs based on their size, structure and types of cations and anions. They introduced two correcting terms which take into account the effect that some specific cations and anion may have in the ionic packing leading to its increase or decrease and hence affect to the density values. Finally, Kermanioryani et al. [33] found new group contribution parameters using the Gardas and Coutinho model

Recently, we have proposed our own method to predict density of ILs [6]. It can be considered as a GCM and it is based on the prediction of the molecular volume of ILs from the ionic

**Table 1.** Experimental values of molecular volume, *Vm*, and thermal expansion coefficient, *αp*, of the studied ILs at temperatures *T* ranging from 293.15 to 343.15 K at pressure *p* = 0.1 MPa.

It is known that volume is more informative about structure and packing efficiency than density, and the conclusions and predictions obtained for volume can be immediately trans‐ lated to density values. The molecular volume *Vm* (or formula‐unit volume) of a salt is a physical observable and is defined as the sum of the ionic volumes, *Vion*, of the constituent ions [32]. For a binary ionic liquid, *Vm* is given by:

$$V\_m = \; V\_{cat} + \; V\_{an} \tag{1}$$

of the ionic liquid, and, as a consequence, the volume that the cation occupies varies linearly with the number of carbons. Then, we can conclude that the length of the chain does not change

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347

significantly the interaction between ions or the packing efficiency of cations.

[emim+

[bmim+

[hmim+

[omim+

[bmim+ ][PF6 −

[hmim+

[omim+ ][PF6 −

[emim+

[bmim+

[emim+ ][BF4 −

[bmim+ ][BF4 −

[hmim+

[omim+ ][BF4 −

[emim+

[bmim+

][NTf2 −

][NTf2 −

][NTf2 −

][NTf2 −

][PF6 −

][TfO−

][TfO−

][BF4 −

][EtSO4 −

][OcSO4 −

**IL** *MW***/g·mol−1** *MWanion***/g·mol−1** *Van***/nm3** *ρan/***g·mol−1·nm−3**

] 391.31 280.133 0.270 1035.995

] 419.37 0.287 976.073

] 447.43 0.298 940.991

] 475.47 0.308 909.818

] 284.18 144.961 0.148 978.144

] 312.24 0.158 919.803

] 340.29 0.168 864.922

] 260.23 149.062 0.155 960.451

] 288.29 0.173 861.132

] 197.97 86.802 0.100 867.153

] 226.02 0.116 748.939

] 254.08 0.126 691.099

] 282.13 0.135 641.078

] 236.29 125.126 0.160 782.037

] 348.50 209.280 0.344 608.372

**Table 2.** The molecular weight of the ionic liquid (*Mw*), molecular weight of the anion (*MWanion*), volume (*Van*), and density of the molecular volume not occupied by the cation (*ρan*) for all investigated ILs obtained from the method

We can study the effect of the volume of the ion in the global *Vm*. The ionic volume is a measure of the size of an ion, equally valid for symmetrical and nonsymmetrical ions. But, to obtain quantitatively the volume of the constituents in an ionic liquid is not a trivial task. One way is to define the contribution of one of the ions to the molecular volume and, from Eq. (1), to obtain the other ionic volume. To do so, Slattery et al. [32] have proposed to derive the volume of the cation from crystal structures (e.g., the CCDC Database) [59] containing the ion of interest in combination with a reference ion of known volume or predicted by the contribution methods proposed by Rebelo et al. [55] and Jacquemin et al. [34, 35]. From the cationic volumes obtained by Slattery et al. [32] and our experimental molecular volume for each temperature, it is possible to assess the fraction of the molecular volume not occupied by the cation, *Van*, and the density of it, *ρan* (*ρan = MWanion/Van*). The values of *Vm*, *Mw*, *MWanion*, *Van* and *ρan* are reported in **Table 2**. We think that this is not a good approximation. Ionic volume assigns a certain fraction of the total molecular volume to one of the ions. When doing so, the ionic volume is a measure not only of the volume of the actual molecular structure of the ion, but also of the interionic separation. This interionic separation must be the consequence of the interactions between

proposed by Slattery et al. [32] for a temperature (*T*) of 293.15 K at pressure *p* = 0.1 MPa.

If we define the molecular volume (*Vm*) as

$$N\_m = \left(M\_w \mid \mathcal{P}\right) / N\_A \tag{2}$$

In Eq. (2), where *Mw* is the molecular weight and NA is Avogadro's constant, we can study the influence in *Vm* (see **Table 1** for numerical values) of the characteristics of the ions.

**Figure 1.** Dependence of the ILs' molecular volume *Vm* at 293.15 K on the alkyl chain length of the cation. [NTf2 − ], linear fitting: *Vm* = 0.3702 + 0.02823 *NC*, *r*<sup>2</sup> = 0.99998; [TfO− ], linear fitting: *Vm* = 0.2533 + 0.02895 *NC*, *r*<sup>2</sup> = 1; [PF6 − ], linear fitting: *Vm* = 0.2327 + 0.02785 *NC*, *r*<sup>2</sup> = 0.99999; [BF4 − ], linear fitting: *Vm* = 0.20035 + 0.02788*NC*, *r*<sup>2</sup> = 0.99999.

Regarding the cations, our data (see **Figure 1** and **Table 1**) reveal a linear increase of *V*m with increasing alkyl chain length, and so, with the volume of the cation. From the slope of the linear fit to d*V*m/d*NC*, with *NC* being the number of carbons of the imidazolium alkyl chain, the volume of one methylene group (–CH2–) is calculated to be 0.0281 ± 0.0004 nm3 (between 0.0279 and 0.0289 nm3 ) at 293.15 K, corresponding to a molar volume of 17 cm3 mol−1. These values agree well with the calculations of Kolbeck et al. [57] (0.0283 nm3 ), Glasser [58] (0.0272–0.0282 nm3 ) and Tariq et al. [39] (17 cm3 mol−1) at 298.15 K. Therefore, according to our results the increase in *Vm* per added methylene group is always the same, irrespective of the nature of the anion


of the ionic liquid, and, as a consequence, the volume that the cation occupies varies linearly with the number of carbons. Then, we can conclude that the length of the chain does not change significantly the interaction between ions or the packing efficiency of cations.

It is known that volume is more informative about structure and packing efficiency than density, and the conclusions and predictions obtained for volume can be immediately trans‐ lated to density values. The molecular volume *Vm* (or formula‐unit volume) of a salt is a physical observable and is defined as the sum of the ionic volumes, *Vion*, of the constituent ions [32]. For

> ( ) / / *VM N mw A* = r

influence in *Vm* (see **Table 1** for numerical values) of the characteristics of the ions.

In Eq. (2), where *Mw* is the molecular weight and NA is Avogadro's constant, we can study the

**Figure 1.** Dependence of the ILs' molecular volume *Vm* at 293.15 K on the alkyl chain length of the cation. [NTf2

−

Regarding the cations, our data (see **Figure 1** and **Table 1**) reveal a linear increase of *V*m with increasing alkyl chain length, and so, with the volume of the cation. From the slope of the linear fit to d*V*m/d*NC*, with *NC* being the number of carbons of the imidazolium alkyl chain, the volume

in *Vm* per added methylene group is always the same, irrespective of the nature of the anion

= 0.99998; [TfO−

= 0.99999; [BF4

of one methylene group (–CH2–) is calculated to be 0.0281 ± 0.0004 nm3

well with the calculations of Kolbeck et al. [57] (0.0283 nm3

) at 293.15 K, corresponding to a molar volume of 17 cm3

*VV V m cat an* = + (1)

(2)

], linear fitting: *Vm* = 0.2533 + 0.02895 *NC*, *r*<sup>2</sup>

], linear fitting: *Vm* = 0.20035 + 0.02788*NC*, *r*<sup>2</sup>

mol−1) at 298.15 K. Therefore, according to our results the increase

− ],

− ],

)

= 1; [PF6

= 0.99999.

(between 0.0279 and

mol−1. These values agree

), Glasser [58] (0.0272–0.0282 nm3

a binary ionic liquid, *Vm* is given by:

346 Progress and Developments in Ionic Liquids

If we define the molecular volume (*Vm*) as

linear fitting: *Vm* = 0.3702 + 0.02823 *NC*, *r*<sup>2</sup>

linear fitting: *Vm* = 0.2327 + 0.02785 *NC*, *r*<sup>2</sup>

and Tariq et al. [39] (17 cm3

0.0289 nm3

**Table 2.** The molecular weight of the ionic liquid (*Mw*), molecular weight of the anion (*MWanion*), volume (*Van*), and density of the molecular volume not occupied by the cation (*ρan*) for all investigated ILs obtained from the method proposed by Slattery et al. [32] for a temperature (*T*) of 293.15 K at pressure *p* = 0.1 MPa.

We can study the effect of the volume of the ion in the global *Vm*. The ionic volume is a measure of the size of an ion, equally valid for symmetrical and nonsymmetrical ions. But, to obtain quantitatively the volume of the constituents in an ionic liquid is not a trivial task. One way is to define the contribution of one of the ions to the molecular volume and, from Eq. (1), to obtain the other ionic volume. To do so, Slattery et al. [32] have proposed to derive the volume of the cation from crystal structures (e.g., the CCDC Database) [59] containing the ion of interest in combination with a reference ion of known volume or predicted by the contribution methods proposed by Rebelo et al. [55] and Jacquemin et al. [34, 35]. From the cationic volumes obtained by Slattery et al. [32] and our experimental molecular volume for each temperature, it is possible to assess the fraction of the molecular volume not occupied by the cation, *Van*, and the density of it, *ρan* (*ρan = MWanion/Van*). The values of *Vm*, *Mw*, *MWanion*, *Van* and *ρan* are reported in **Table 2**. We think that this is not a good approximation. Ionic volume assigns a certain fraction of the total molecular volume to one of the ions. When doing so, the ionic volume is a measure not only of the volume of the actual molecular structure of the ion, but also of the interionic separation. This interionic separation must be the consequence of the interactions between ions, mainly due to electrostatic attraction, but also of geometry of their molecular structures, their polarizability, their ability to establish some other type of interactions (i.e., hydrogen bond) and other factors. Then, it is not plausible to assume that the volume of the cation is not affected by the anion in front (like it is proposed in this method), and, at the same time, to assume that the volume of the anion changes with the cation in a very significant amount (i.e., from **Table 2**, for [BF4 − ] we find a 35% increase from [emim+ ] to [omim+ ]). Another question is that in this treatment the effect of temperature on the ionic volumes is not clear. Clearly, if we assume that molecular structure (i.e., covalent bonds) are temperature‐independent in a certain temperature range, but an increment in *T* decreases density (and increases *Vm*), then the increment in ionic volumes should mean that the interionic distances increase. Following our argument, if there is no change in molecular structure that justifies a significant change in the geometry of packing, the main reason for the increase in the interionic distance should be a decrease in the strength of interaction, what should have other known consequences as the decrease in viscosity.

For comparison purposes, **Table 3** includes ionic volume for the cations used in this work proposed by Slattery et al. [32]. Their values are systematically lower than ours. In addition

While Slattery et al. [32] proposed a constant absolute increment of ionic volume, we propose

Using Eq. (1), we can obtain the ionic volume occupied by the anions at 293.15 K (see **Table 4**).

 **(theoretical) Ionic volume/nm3**

] 0.05420 0.0634 ± 0.0008 17

] 0.07135 0.0952 ± 0.0007 33

] 0.08520 0.1201 ± 0.0010 41

] 0.09639 0.1242 29

] 0.15671 0.2349 ± 0.0002 50

] 0.19820 0.2919 47

**Table 4.** van der Waals volume of molecular structure from theoretical calculation (structural volume) and ionic volume proposed in this work for anions present in the ILs studied at 293.15 K at pressure *p* = 0.1 MPa.

We find various advantages in this procedure. First, we are able to obtain a plausible ionic volume for the anions participating in the ionic liquid studied. Our results show that the ionic volume of the anion is not affected significantly by the length of the alkyl chain of the cation.

The second consequence of our approximation is that there is a very good correlation between the molecular volume with both, volume of the cation and also with the volume of the anion.

Indeed, in **Figure 2**, we observe that volume of the anion is the fundamental factor to deter‐ mine the molecular volume (the higher the volume of the anion, the higher *Vm*). According

ometry of the anion, distribution of charge,…) have some influence in the global volume, but according to our results their importance is smaller than the volume of the anion. These

Using our treatment, we can obtain the increment of anionic volume with respect to the theoretical volume of the molecular structure (**Table 4**). This increment takes into account interionic distances and effect of temperature and could be taken as a measure of the packing quality. If we assume this idea, we conclude that packing quality is higher for small molecules, but when the size of the anion becomes similar to the cation, the packing quality decreases,

]. As we mentioned before, it is very likely that some other factors (ge‐

to theoretical calculations (**Table 4**), the order of volumes is: [OcSO4

data explain the trend observed in the experimental values of the density.

reaches a certain level and probably other factors than volume become significant.

a constant percentual increment of ionic volume with alkyl chain length.

, lower than ours and others found in the literature (see above).

 **(this work) % increment**

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349

−

] > [NTf2

−

] > [EtSO4

− ] >

they propose a *V–CH*2*–* –= 0.023 nm3

**Anion Structural volume/nm3**

[BF4 −

[PF6 −

[TfO−

[EtSO4 −

[NTf2 −

[OcSO4 −

[TfO−

] > [PF6 − ] > [BF4 −

We now propose an alternative way to assign the ionic volume, and so to explain the experi‐ mental results of density. Volume of a given chemical structure can be theoretically calculated. We used the web page chemicalize.org [60] to obtain the van der Waals volume of the ions studied. This volume is not comparable with the ionic volume, because it only takes into account the isolated molecular structure and it is independent of the temperature. We propose below a way to assign the ionic volumes from the theoretically calculated volumes of the components and the experimental measurements of density, which takes into account the effect of temperature.


**Table 3.** Volume of molecular structure from theoretical calculation (structure volume), ionic volume proposed in this work and ionic volume proposed by Slattery et al. [32] for the alkyl methylimidazolium cations present in the ILs studied in this work at 293.15 K at pressure *p* = 0.1 MPa.

From the values of the theoretical van der Waals volume obtained for the set of alkyl methyl‐ imidazolium cations used in this work (see **Table 3**), it is straightforward to assign a molecular volume to one methylene group (–CH2–), *V(CH*2*)Theo* = 0.01697 ± 0.00001 nm3 . If we compare with the value obtained from experimental values of density at 293.15 K, *V(CH*2*)Density* = 0.0281 ± 0.0004 nm3 , the increment is 65.7%. In our hypothesis, this increment includes the fraction corre‐ sponding to the cation of interionic distance. Given that this increment is the same regardless of the length of the alkyl chain of the cation or the anion in front, it is reasonable to assume that this increment is approximately the same for the theoretically calculated volume of the whole molecule, which allows us to assign a ionic volume of the cation.

For comparison purposes, **Table 3** includes ionic volume for the cations used in this work proposed by Slattery et al. [32]. Their values are systematically lower than ours. In addition they propose a *V–CH*2*–* –= 0.023 nm3 , lower than ours and others found in the literature (see above). While Slattery et al. [32] proposed a constant absolute increment of ionic volume, we propose a constant percentual increment of ionic volume with alkyl chain length.

ions, mainly due to electrostatic attraction, but also of geometry of their molecular structures, their polarizability, their ability to establish some other type of interactions (i.e., hydrogen bond) and other factors. Then, it is not plausible to assume that the volume of the cation is not affected by the anion in front (like it is proposed in this method), and, at the same time, to assume that the volume of the anion changes with the cation in a very significant amount (i.e.,

that in this treatment the effect of temperature on the ionic volumes is not clear. Clearly, if we assume that molecular structure (i.e., covalent bonds) are temperature‐independent in a certain temperature range, but an increment in *T* decreases density (and increases *Vm*), then the increment in ionic volumes should mean that the interionic distances increase. Following our argument, if there is no change in molecular structure that justifies a significant change in the geometry of packing, the main reason for the increase in the interionic distance should be a decrease in the strength of interaction, what should have other known consequences as the

We now propose an alternative way to assign the ionic volume, and so to explain the experi‐ mental results of density. Volume of a given chemical structure can be theoretically calculated. We used the web page chemicalize.org [60] to obtain the van der Waals volume of the ions studied. This volume is not comparable with the ionic volume, because it only takes into account the isolated molecular structure and it is independent of the temperature. We propose below a way to assign the ionic volumes from the theoretically calculated volumes of the components and the experimental measurements of density, which takes into account the effect

 **(theoretical) Ionic volume/nm3**

**Table 3.** Volume of molecular structure from theoretical calculation (structure volume), ionic volume proposed in this work and ionic volume proposed by Slattery et al. [32] for the alkyl methylimidazolium cations present in the ILs

From the values of the theoretical van der Waals volume obtained for the set of alkyl methyl‐ imidazolium cations used in this work (see **Table 3**), it is straightforward to assign a molecular

the value obtained from experimental values of density at 293.15 K, *V(CH*2*)Density* = 0.0281 ± 0.0004

, the increment is 65.7%. In our hypothesis, this increment includes the fraction corre‐ sponding to the cation of interionic distance. Given that this increment is the same regardless of the length of the alkyl chain of the cation or the anion in front, it is reasonable to assume that this increment is approximately the same for the theoretically calculated volume of the

] 0.11553 0.19142 0.156

] 0.14945 0.24762 0.196

] 0.18341 0.30388 0.242

] 0.21735 0.36012 0.288

volume to one methylene group (–CH2–), *V(CH*2*)Theo* = 0.01697 ± 0.00001 nm3

whole molecule, which allows us to assign a ionic volume of the cation.

] to [omim+

 **(this work) Ionic volume/nm3**

]). Another question is

 **(Slattery et al. [32])**

. If we compare with

] we find a 35% increase from [emim+

from **Table 2**, for [BF4

348 Progress and Developments in Ionic Liquids

decrease in viscosity.

of temperature.

[emim+

[bmim+

[hmim+

[omim+

nm3

**Cation Structural volume/nm3**

studied in this work at 293.15 K at pressure *p* = 0.1 MPa.

−

Using Eq. (1), we can obtain the ionic volume occupied by the anions at 293.15 K (see **Table 4**).


**Table 4.** van der Waals volume of molecular structure from theoretical calculation (structural volume) and ionic volume proposed in this work for anions present in the ILs studied at 293.15 K at pressure *p* = 0.1 MPa.

We find various advantages in this procedure. First, we are able to obtain a plausible ionic volume for the anions participating in the ionic liquid studied. Our results show that the ionic volume of the anion is not affected significantly by the length of the alkyl chain of the cation.

The second consequence of our approximation is that there is a very good correlation between the molecular volume with both, volume of the cation and also with the volume of the anion.

Indeed, in **Figure 2**, we observe that volume of the anion is the fundamental factor to deter‐ mine the molecular volume (the higher the volume of the anion, the higher *Vm*). According to theoretical calculations (**Table 4**), the order of volumes is: [OcSO4 − ] > [NTf2 − ] > [EtSO4 − ] > [TfO− ] > [PF6 − ] > [BF4 − ]. As we mentioned before, it is very likely that some other factors (ge‐ ometry of the anion, distribution of charge,…) have some influence in the global volume, but according to our results their importance is smaller than the volume of the anion. These data explain the trend observed in the experimental values of the density.

Using our treatment, we can obtain the increment of anionic volume with respect to the theoretical volume of the molecular structure (**Table 4**). This increment takes into account interionic distances and effect of temperature and could be taken as a measure of the packing quality. If we assume this idea, we conclude that packing quality is higher for small molecules, but when the size of the anion becomes similar to the cation, the packing quality decreases, reaches a certain level and probably other factors than volume become significant.

Finally, this treatment allows us to predict with high accuracy the densities of all the combi‐ nations at all temperatures of these anions and cations by obtaining molecular volumes of ILs after assigning proper ionic volumes to its constituents. Indeed, following the method we have used for *T* = 293.15 K for the rest of temperatures studied, we obtain (see **Figure 3**) a linear

(*r*<sup>2</sup>

*T***/K Ionic volume/nm3 S.D. % increase**

**Table 5.** Effect of temperature (*T*) on ionic volume of –CH2– and % of increase over theoretically calculated van der

( ) <sup>3</sup> 1.328 1.120 10 *V V al imid al imid theo T*-

( ) ( ) 3 0.08159 0.01697 1.328 1.120 10 *V NT al imid <sup>C</sup>*

In Eq. (3), *Val-imid* is the ionic volume of any alkyl imidazolium cation and *Val-imid-theo* is the

and *T* is the temperature in Kelvin. Although the limits of validity should be established it is

Regarding the dependence of the ionic volumes of the anions studied in this work on temper‐ ature, in all cases we find a very good linear dependence, with a tendency to higher slopes for higher ionic volumes (see **Figure 4** and **Table 6** for numerical values). From our results, the

Knowing this quantitative dependence the ionic volume of alkyl imidazolium cations with different alkyl chain lengths can be properly assigned for any temperature. We propose the




. In Eq. (4), *NC* is the number of carbons

293.15 0.0281 0.0004 65.7 303.15 0.0283 0.0004 66.8 313.15 0.0285 0.0005 67.8 323.15 0.0287 0.0004 69.1 333.15 0.0289 0.0005 70.1 343.15 0.0291 0.0005 71.3

= 0.9996) of the ionic volume of CH2 with

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351

dependence *VCH*<sup>2</sup> *=* 1.922 × 10−5 *T +* 0.02247 nm3

= 0.9990.

calculated theoretical van der Waals volume, both in nm3

likely that the range of application of these equations is rather broad.

ionic volume of these ions can be predicted for any temperature:

temperature (numerical values in **Table 5**).

% increase = 32.83 + 0.112 *T*; *r*<sup>2</sup>

following equation:

Or in terms of number of carbons

Waals volume.

**Figure 2.** Molecular volume at 293.15 K of ILs based on [bmim+ ] versus volume of anion. [OcSO4 − ]; [NTf2 − ]; [EtSO4 − ]; [TfO− ]; [PF6 − ]; [BF4 − ]. *Vm* for [bmim+ ][EtSO4 − ] is not measured but predicted. Full symbols are for van der Waals theoretically calculated anionic volume. Empty symbols are for our proposed ionic volume obtained from averages over experimental measurements of density (**Table 4**). In this plot, standard deviations are smaller than sym‐ bols. Fitting results: ‐ ‐ ‐ theoretical volume *Vm =* 1.592 *Van +* 0.2279 nm3 , *r*<sup>2</sup> = 0.9956. — — — ionic volumes *Vm =* 0.9980 *Van +* 0.2489 nm3 , *r*<sup>2</sup> = 0.99999.

**Figure 3.** Dependence of the ionic volume of –CH2– ionic (obtained from density experimental values) on temperature. All numerical data can be found in **Table 6**. Linear fit: *V–CH*2 *–*= 0.02247 + 1.922 × 10−5 *T; r*<sup>2</sup> = 0.9996.

Finally, this treatment allows us to predict with high accuracy the densities of all the combi‐ nations at all temperatures of these anions and cations by obtaining molecular volumes of ILs after assigning proper ionic volumes to its constituents. Indeed, following the method we have used for *T* = 293.15 K for the rest of temperatures studied, we obtain (see **Figure 3**) a linear dependence *VCH*<sup>2</sup> *=* 1.922 × 10−5 *T +* 0.02247 nm3 (*r*<sup>2</sup> = 0.9996) of the ionic volume of CH2 with temperature (numerical values in **Table 5**).


**Table 5.** Effect of temperature (*T*) on ionic volume of –CH2– and % of increase over theoretically calculated van der Waals volume.

Knowing this quantitative dependence the ionic volume of alkyl imidazolium cations with different alkyl chain lengths can be properly assigned for any temperature. We propose the following equation:

$$V\_{al\text{ }-limit} = V\_{al\text{ }-limit\text{ }-thos} \left( 1.328 + 1.120 \times 10^{-3} T \right) \tag{3}$$

Or in terms of number of carbons

**Figure 2.** Molecular volume at 293.15 K of ILs based on [bmim+

bols. Fitting results: ‐ ‐ ‐ theoretical volume *Vm =* 1.592 *Van +* 0.2279 nm3

]. *Vm* for [bmim+

][EtSO4 −

der Waals theoretically calculated anionic volume. Empty symbols are for our proposed ionic volume obtained from averages over experimental measurements of density (**Table 4**). In this plot, standard deviations are smaller than sym‐

**Figure 3.** Dependence of the ionic volume of –CH2– ionic (obtained from density experimental values) on temperature.

All numerical data can be found in **Table 6**. Linear fit: *V–CH*2 *–*= 0.02247 + 1.922 × 10−5 *T; r*<sup>2</sup>

]; [PF6 − ]; [BF4 −

= 0.99999.

, *r*<sup>2</sup>

350 Progress and Developments in Ionic Liquids

[EtSO4 − ]; [TfO−

*Van +* 0.2489 nm3

] versus volume of anion. [OcSO4

, *r*<sup>2</sup>

] is not measured but predicted. Full symbols are for van

= 0.9996.

= 0.9956. — — — ionic volumes *Vm =* 0.9980

− ]; [NTf2 − ];

$$V\_{al-mid} = \begin{pmatrix} 0.08159 + 0.01697 N\_C \end{pmatrix} \begin{pmatrix} 1.328 + 1.120 \times 10^{-3} T \end{pmatrix} \tag{4}$$

In Eq. (3), *Val-imid* is the ionic volume of any alkyl imidazolium cation and *Val-imid-theo* is the calculated theoretical van der Waals volume, both in nm3 . In Eq. (4), *NC* is the number of carbons and *T* is the temperature in Kelvin. Although the limits of validity should be established it is likely that the range of application of these equations is rather broad.

Regarding the dependence of the ionic volumes of the anions studied in this work on temper‐ ature, in all cases we find a very good linear dependence, with a tendency to higher slopes for higher ionic volumes (see **Figure 4** and **Table 6** for numerical values). From our results, the ionic volume of these ions can be predicted for any temperature:

$$V\_{BF4} = V\_{BF4\text{4ho}} \left( 1.083 + 0.365 \times 10^{-3} T \right) \\
= 0.0587 + 1.98 \times 10^{-5} T \tag{5}$$

*T***/K [emim+**

**] [bmim+**

**Table 6.** Dependence of the ionic volumes (in nm3

(a) [NTf2 − ]; (b) [PF6 − ]; (c) [TfO−

symbol.

parameters to equation *Vion = aT + b* are also included.

**] [hmim+**

**] [omim+**

293.15 0.1914 0.2476 0.3039 0.3601 0.0640 0.2356 0.1206 0.1246 0.0959 0.2925 303.15 0.1927 0.2492 0.3059 0.3625 0.0642 0.2372 0.1213 0.1250 0.0964 0.2942 313.15 0.1939 0.2508 0.3078 0.3648 0.0644 0.2390 0.1220 0.1256 0.0969 0.2959 323.15 0.1953 0.2527 0.3101 0.3674 0.0645 0.2404 0.1225 0.1259 0.0972 0.2973 333.15 0.1966 0.2543 0.3121 0.3698 0.0648 0.2421 0.1232 0.1264 0.0978 0.2991 343.15 0.1980 0.2506 0.3143 0.3724 0.0649 0.2437 0.1238 0.1268 0.0980 0.3007 *a* **× 10 <sup>3</sup> /nm <sup>3</sup> ·K −1** 0.1309 0.1693 0.2078 0.2462 0.0198 0.1617 0.0629 0.0451 0.0425 0.1644 *b***/nm <sup>3</sup>** 0.1530 0.1979 0.2829 0.2878 0.0587 0.1882 0.1022 0.1114 0.0835 0.2443 *r***<sup>2</sup>** 0.9996 0.9996 0.9996 0.9996 0.9828 0.9994 0.9979 0.9962 0.9924 0.9997

**Figure 5.** Density at 313.15 K of different ILs versus number of carbons of the alkyl chain of the cation. The anions are:

values); (— — —) predicted from Eqs. (5–10). For experimental values, the error bars are smaller than the size of the

]. ○ Experimental (averaged from different sources, see **Table 8** for numerical

]; (d) [BF4 − **] [BF4 − ] [NTf2 − ] [TFO−**

**] [EtSO4 − ] [PF6 − ] [OctSO4 − ]**

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353

) of all the ions studied in this work on the temperature (*T*). Fitting

$$V\_{PF6} = V\_{PF60\text{iso}} \left( 1.171 + 0.596 \times 10^{-3} T \right) \tag{6}$$

$$V\_{PF} = 0.0835 + 4.25 \times 10^{-5} T \tag{6}$$

$$V\_{NTf2} = V\_{NTf2\text{-}hho} \left( 1.201 + 1.032 \times 10^{-3} T \right) \\ = 0.1882 + 16.17 \times 10^{-5} T \tag{7}$$

$$V\_{TFO} = V\_{TFOho} \left( 1.200 + 0.738 \times 10^{-3} T \right) \\
= 0.1022 + 6.29 \times 10^{-5} T \tag{8}$$

$$V\_{ESO4} = V\_{ESO4\text{abs}} (1.156 + 0.468 \times 10^{-3} T) = 0.1114 + 4.51 \times 10^{-5} T \tag{9}$$

3 5 4 4 (1.233 0.829 10 ) 0.0835 16.44 10 *V V OctSO OctSO theo T T* - - = + ´ = +´ (10)

**Figure 4.** Ionic volume (*Vion*) versus temperature (*T*). This figure shows the linear dependence of ionic volume on tem‐ perature, which is higher for bigger structures. All numerical data (including fitting parameters) can be found in **Table 6**. [emim+ ]; [bmim+ ]; [hmim+ ]; [omim+ ]; [BF4 − ]; [NTf2 − ]; [TfO− ]; [EtSO4 − ]; [PF6 − ]; [OcSO4 − ].

In Eqs. (5–10), *Vanion* is the ionic volume of each anion and *Vanion-theo* is the calculated theoretical van der Waals volume, both in nm3 . *T* is the temperature in Kelvin.

To check our procedure we have compared the calculated densities from Eqs. (1–10) with our experimental values and some others found in the literature [34, 39, 40, 61–68]. As an illustration, some results are plotted in **Figure 5** (see also **Tables 7** and **8** for numerical val‐ ues).


( ) 3 5

( ) 3 5

( ) 3 5 2 2 1.201 1.032 10 0.1882 16.17 10 *V V NTf NTf theo T T* - - = +´ = +´ (7)

4 4 1.083 0.365 10 0.0587 1.98 10 *V V BF BF theo T T* - - = + ´ = +´ (5)

6 6 1.171 0.596 10 0.0835 4.25 10 *V V PF PF theo T T* - - = + ´ = +´ (6)

( ) 3 5 1.200 0.738 10 0.1022 6.29 10 *V V TFO TFOtheo T T* - - = + ´ = +´ (8)

4 4 (1.233 0.829 10 ) 0.0835 16.44 10 *V V OctSO OctSO theo T T* - - = + ´ = +´ (10)

**Figure 4.** Ionic volume (*Vion*) versus temperature (*T*). This figure shows the linear dependence of ionic volume on tem‐ perature, which is higher for bigger structures. All numerical data (including fitting parameters) can be found in **Ta-**

In Eqs. (5–10), *Vanion* is the ionic volume of each anion and *Vanion-theo* is the calculated theoretical

To check our procedure we have compared the calculated densities from Eqs. (1–10) with our experimental values and some others found in the literature [34, 39, 40, 61–68]. As an illustration, some results are plotted in **Figure 5** (see also **Tables 7** and **8** for numerical val‐

. *T* is the temperature in Kelvin.

]; [BF4 − ]; [NTf2 − ]; [TfO−

**ble 6**. [emim+

ues).

]; [bmim+

352 Progress and Developments in Ionic Liquids

van der Waals volume, both in nm3

]; [hmim+

]; [omim+

3 5 4 4 (1.156 0.468 10 ) 0.1114 4.51 10 *V V EtSO EtSO theo T T* - - = + ´ = +´ (9)

3 5

]; [EtSO4 − ]; [PF6 − ]; [OcSO4 − ].

**Table 6.** Dependence of the ionic volumes (in nm3 ) of all the ions studied in this work on the temperature (*T*). Fitting parameters to equation *Vion = aT + b* are also included.

**Figure 5.** Density at 313.15 K of different ILs versus number of carbons of the alkyl chain of the cation. The anions are: (a) [NTf2 − ]; (b) [PF6 − ]; (c) [TfO− ]; (d) [BF4 − ]. ○ Experimental (averaged from different sources, see **Table 8** for numerical values); (— — —) predicted from Eqs. (5–10). For experimental values, the error bars are smaller than the size of the symbol.


*ρ***/(g/cm3 ) Experimental (different sources) Exp. Average ± σ** *Predicted* **Error\* Error+**

] 1.50371 1.49962 1.50293 1.50343 1.50384 1.5027 0.0018 1.5005 −0.0022 −0.15

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355

] 1.42021 1.42162 1.42306 1.41985 1.42145 1.42353 1.42123 1.4215 0.0015 1.4210 −0.0005 −0.04

] 1.35801 1.35722 1.35693 1.35706 1.3573 0.0005 1.3581 0.0008 0.06

] 1.30701 1.30552 1.30703 1.30564 1.3063 0.0008 1.3070 0.0007 0.05

] 1.26532 1.26533 1.2653 0.0000 1.2646 −0.0007 −0.06

] 1.23182 1.2318 1.2289 −0.0029 −0.23

] 1.20062 1.2006 1.1985 −0.0021 −0.17

] 1.35401 1.35132 1.35333 1.35183 1.35345 1.35287 1.3528 0.0010 1.3562 0.0034 0.25

] 1.28151 1.28042 1.2810 0.0008 1.2801 −0.0009 −0.07

] 1.22501 1.22242 1.2237 0.0018 1.2227 −0.0010 −0.08

] 1.26811 1.2681 1.2721 0.0040 0.31

] 1.19051 1.18872 1.19013 1.18893 1.19086 1.19078 1.1900 0.0009 1.1898 −0.0002 −0.01

] 1.13371 1.1337 1.1328 −0.0009 −0.08

] 1.09291 1.0929 1.0908 −0.0021 −0.19

] 1.37161 1.3716 1.3673 −0.0043 −0.31

] 1.28081 1.28772 1.28563 1.29247 1.2866 0.0048 1.2832 −0.0034 −0.26

**Table 8.** Comparison of predicted (this work) and experimental (different sources) densities (*ρ*) for different ILs at

Tariq et al. [39] showed the experimental density of some ILs composed with combinations of cations and anions not used for our research group. A further test for our method is to predict volumes of these ions. For *T* = 293.15 K, we obtain the volume of methylsulfate ([MeSO4

; VOAc = 0.0640 nm3

− ]) and

. We can see that

]

. The volume of the trihexyl(tetradec‐

]) can be obtained from data of three different ILs: [P6 6 6 14+

]. We obtain V[P6 6 6 14+] = 0.945 ± 0.002 nm3

[NTf2

[emim+

[bmim+

[hmim+

[omim+

[c10mim+

[c12mim+

[c14mim+

[bmim+

[hmim+

[omim+

[emim+

[bmim+

[hmim+

[omim+

[emim+

[bmim+

1 This work.

2

3

4

5

6

7

8

\*

+ Error, in %.

[TfO−

Tariq et al. [39].

Gardas et al. [63].

Troncoso et al. [64].

Soriano et al. [40].

Error, in g/cm3

acetate ([OAc−

]; [P6 6 6 14+

[NTf2 −

Iglesias‐Otero et al. [62].

Gomes de Azevedo et al. [61].

.

temperature *T* = 313.15 K at pressure *p* = 0.1 MPa.

yl)phosphonium cation ([P6 6 6 14+

][TfO−

]): VMeSO4 = 0.0956 nm3

]; [P6 6 6 14+

][OAc−

Jacquemin et al. [34].

]

 [BF4 − ]

 [PF6 − ]

− ]

**Table 7.** Comparison of predicted (this work) and experimental (different sources) densities (*ρ*) for different ILs at temperature *T* = 293.15 K at pressure *p* = 0.1 MPa.


*ρ***/(g/cm3 ) Experimental (different sources) Exp. Average ± σ** *Predicted* **Error\* Error+**

] 1.52381 1.51972 1.52293 1.52343 1.52604 1.5232 0.0023 1.5215 −0.0016 −0.11

] 1.44031 1.44082 1.44273 1.44023 1.43895 1.44055 1.4406 0.0012 1.4408 0.0003 0.02

] 1.37631 1.37552 1.37513 1.37546 1.37506 1.3755 0.0005 1.3770 0.0015 0.11

] 1.32481 1.32342 1.32453 1.32814 1.3252 0.0020 1.3251 −0.0001 −0.01

] 1.28282 1.28243 1.2826 0.0003 1.2821 −0.0005 −0.04

] 1.24902 1.2490 1.2459 −0.0031 −0.25

] 1.37071 1.36792 1.36983 1.36813 1.37045 1.37167,8 1.3698 0.0015 1.3734 0.0036 0.26

] 1.29731 1.29642 1.29797,8 1.2972 0.0008 1.2964 −0.0008 −0.06

] 1.24021 1.23782 1.23967,8 1.2392 0.0012 1.2385 −0.0007 −0.06

] 1.28331 1.2833 1.2869 0.0036 0.28

] 1.20311 1.20292 1.20493,9 1.20383 1.2037 0.0009 1.2041 0.0004 0.04

] 1.14751 1.1475 1.1467 −0.0008 −0.07

] 1.10641 1.1064 1.1044 −0.0020 −0.18

] 1.38841 1.3884 1.3848 −0.0036 −0.26

] 1.2966x 1.30352 1.30133 1.290010 1.2979 0.0060 1.2998 0.0019 0.15

**Table 7.** Comparison of predicted (this work) and experimental (different sources) densities (*ρ*) for different ILs at

[NTf2

[emim+

[bmim+

[hmim+

[omim+

[c10mim+

[c12mim+

 [PF6 − ]

[bmim+

[hmim+

[omim+

[emim+

[bmim+

[hmim+

[omim+

[emim+

[bmim+

Tariq et al. [39].

Gardas et al. [63].

Troncoso et al. [64].

Pereiro and Rodríguez [66];

Pereiro and Rodríguez [67].

.

temperature *T* = 293.15 K at pressure *p* = 0.1 MPa.

Iglesias‐Otero et al. [62]. 10Bonhôte et al. [68].

Łachwa et al. [65].

Error, in g/cm3

Error, in %.

Jacquemin et al. [34].

1 This work.

2

3

4

5

6

7

8

9

\*

+

[TfO−

]

 [BF4 − ]

− ]

354 Progress and Developments in Ionic Liquids

**Table 8.** Comparison of predicted (this work) and experimental (different sources) densities (*ρ*) for different ILs at temperature *T* = 313.15 K at pressure *p* = 0.1 MPa.

Tariq et al. [39] showed the experimental density of some ILs composed with combinations of cations and anions not used for our research group. A further test for our method is to predict volumes of these ions. For *T* = 293.15 K, we obtain the volume of methylsulfate ([MeSO4 − ]) and acetate ([OAc− ]): VMeSO4 = 0.0956 nm3 ; VOAc = 0.0640 nm3 . The volume of the trihexyl(tetradec‐ yl)phosphonium cation ([P6 6 6 14+ ]) can be obtained from data of three different ILs: [P6 6 6 14+ ] [NTf2 − ]; [P6 6 6 14+ ][TfO− ]; [P6 6 6 14+ ][OAc− ]. We obtain V[P6 6 6 14+] = 0.945 ± 0.002 nm3 . We can see that although [P6 6 6 14+ ][OAc− ] is formed by two anions not used to obtain the equations proposed in this work, the result obtained is very close to the ones obtained from the other ILs.

( ) ( )( ) 2 2 *f n=n n+* -1/ 2 (12)

( ) ( ) ( ) <sup>2</sup> *f n = n n+* -1 / 0.4 (14)

( ) ( )( ) 2 22 *f n=n n+ n* -1 2 1/ (15)

**System L-L D-G Eykman Oster A-B Newton**

] 0.1948 0.3220 0.4289 1.9418 1.1130 0.7752

] 0.2125 0.3524 0.4687 2.1276 1.1966 0.8518

] 0.2251 0.3738 0.4969 2.2585 1.2586 0.9055

] 0.2359 0.3923 0.5212 2.3719 1.3102 0.9523

] 0.1881 0.3128 0.4155 1.8914 1.0442 0.7594

] 0.2027 0.3375 0.4481 2.0415 1.1208 0.8204

] 0.1813 0.2996 0.3992 1.8059 1.0404 0.7203

] 0.1947 0.3224 0.4291 1.9450 1.1052 0.7775

] 0.2061 0.3418 0.4546 2.0638 1.1607 0.8263

] 0.1679 0.2784 0.3703 1.6809 0.9457 0.6729

] 0.1789 0.2969 0.3947 1.7933 1.0031 0.7186

] 0.1888 0.3137 0.4168 1.8959 1.0526 0.7605

] 0.1969 0.3275 0.4350 1.9797 1.0952 0.7946

] 0.2286 0.3853 0.5088 2.3446 1.2020 0.9525

All these equations assume that the relation between density and refractive index is temperature-independent. But, from our results (see **Tables 1** and **9**) it can be deduced that *k* in Eq. (11) has a certain dependence on *T* (as it is illustrated below in **Figure 5** in the case of Eq. (16)).

·g−1 for the different empirical equations from Eq. (6) for the equations proposed by Lorentz-

**S.D.** 0.0032 0.0020 0.0025 0.0016 0.0099 0.0018

Lorenz (L-L), Dale-Gladstone (D-G), Eykman, Oster, Arago-Biot (A-B) and Newton.

] 0.2620 0.4407 0.5825 2.6791 1.3885 1.0865

[emim+ ][BF4 −

[bmim+ ][BF4 −

[hmim+

[omim+ ][BF4 −

[emim+

[bmim+

[bmim+

[bmim+ ][PF6 −

[hmim+

[omim+ ][PF6 −

[emim+

[bmim+

[hmim+

[omim+

[emim+

][BF4 −

][TfO−

][TfO−

][OcSO4 −

][PF6 −

][NTf2 −

][NTf2 −

][NTf2 −

][NTf2 −

][EtSO4 −

**Table 9.** Parameters *k*/cm3

*f n =n* ( ) (16)

( ) <sup>2</sup> *f n =n* -1 (17)

*f n=n* () ( ) -1 (13)

Predicting Density and Refractive Index of Ionic Liquids

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357

To have an idea of the quality of the predictions we can use the average deviation from experimental values. Then, we can compare our values (see **Tables 7** and **8**) with those obtained by Tariq et al. [39]. Those average deviations are between σave = 0.21 and σave = 1.29. Our method gives σave = 0.13.

As a summary, with our results we should be able to predict, prior to synthesis, the density and molecular volumes for IL with alkyl imidazolium cations and with different anions (and very likely combinations of ions of similar chemical structure) in a wide range of temperatures from easily accessible theoretical calculations of the structure of the proposed ions.

#### **2.5. Estimation by correlations between density and other properties**

The last approach to estimate density is through correlations between density and other properties. Obviously, no prediction can be obtained if these correlations have not been previously established. But these methods can be especially interesting if the measurement of the property used is easier or cheaper than the measurement of the density. A good example is the refractive index.

There are some authors who have studied the relationship between density and refractive index in order to predict one of the properties from the other one due to the experimental measurement of refractive index only needs a drop of the ionic liquid and it is very fast. However, in the case of density the determination requires higher volumes of ionic liquid and it takes more time. Tariq et al. [39] and Soriano et al. [38, 40] used different empirical models (Lorentz-Lorenz, Dale-Gladstone, Eykman, Oster, Arago-Biot and Newton equations) to predict density of ILs from their refractive index. Other authors such as Deetlefs et al. [41] and Gardas et al. [42] used the parachor to estimate density. Parachor is a surface-tension-weighted molar volume, which constitutes a link between the structure, density and surface tension of the ILs. Deetlefs et al. [41] also used the refraction molar to estimate density.

We detail in the following paragraphs our prediction of the density from the refractive index.

It is known that the density and refractive index (*n*) were correlated using several empirical equations of the form:

$$f\left(n\right) = k\,\rho\tag{11}$$

where *f(n)* is a function of the refractive index, *k* is an empirical constant that depends on the liquid and the wavelength at which the refractive index is measured and *ρ* is the density of the liquid. The *f(n)* function is associated with several empirical equations. In this work, we have used the most common equations, those of Lorentz-Lorenz (Eq. (12)), Dale-Gladstone (Eq. (13)), Eykman (Eq. (14)), Oster (Eq. (15)), Arago-Biot (Eq. (16)) and Newton (Eq. (17)), which have also been used by different authors to correlate the density with the refractive index.

#### Predicting Density and Refractive Index of Ionic Liquids http://dx.doi.org/10.5772/65790 357

$$f\left(n\right) = \left(n^2 - 1\right) / \left(n^2 + 2\right) \tag{12}$$

although [P6 6 6 14+

356 Progress and Developments in Ionic Liquids

gives σave = 0.13.

is the refractive index.

equations of the form:

][OAc−

] is formed by two anions not used to obtain the equations proposed

in this work, the result obtained is very close to the ones obtained from the other ILs.

from easily accessible theoretical calculations of the structure of the proposed ions.

**2.5. Estimation by correlations between density and other properties**

the ILs. Deetlefs et al. [41] also used the refraction molar to estimate density.

To have an idea of the quality of the predictions we can use the average deviation from experimental values. Then, we can compare our values (see **Tables 7** and **8**) with those obtained by Tariq et al. [39]. Those average deviations are between σave = 0.21 and σave = 1.29. Our method

As a summary, with our results we should be able to predict, prior to synthesis, the density and molecular volumes for IL with alkyl imidazolium cations and with different anions (and very likely combinations of ions of similar chemical structure) in a wide range of temperatures

The last approach to estimate density is through correlations between density and other properties. Obviously, no prediction can be obtained if these correlations have not been previously established. But these methods can be especially interesting if the measurement of the property used is easier or cheaper than the measurement of the density. A good example

There are some authors who have studied the relationship between density and refractive index in order to predict one of the properties from the other one due to the experimental measurement of refractive index only needs a drop of the ionic liquid and it is very fast. However, in the case of density the determination requires higher volumes of ionic liquid and it takes more time. Tariq et al. [39] and Soriano et al. [38, 40] used different empirical models (Lorentz-Lorenz, Dale-Gladstone, Eykman, Oster, Arago-Biot and Newton equations) to predict density of ILs from their refractive index. Other authors such as Deetlefs et al. [41] and Gardas et al. [42] used the parachor to estimate density. Parachor is a surface-tension-weighted molar volume, which constitutes a link between the structure, density and surface tension of

We detail in the following paragraphs our prediction of the density from the refractive index.

It is known that the density and refractive index (*n*) were correlated using several empirical

r

where *f(n)* is a function of the refractive index, *k* is an empirical constant that depends on the liquid and the wavelength at which the refractive index is measured and *ρ* is the density of the liquid. The *f(n)* function is associated with several empirical equations. In this work, we have used the most common equations, those of Lorentz-Lorenz (Eq. (12)), Dale-Gladstone (Eq. (13)), Eykman (Eq. (14)), Oster (Eq. (15)), Arago-Biot (Eq. (16)) and Newton (Eq. (17)), which have

(11)

*fn k* ( ) =

also been used by different authors to correlate the density with the refractive index.

$$f\begin{pmatrix} n \end{pmatrix} = \begin{pmatrix} n-1 \end{pmatrix} \tag{13}$$

$$f\left(n\right) = \left(n^2 - 1\right) / \left(n + 0.4\right) \tag{14}$$

$$f\left(n\right) = \left(n^2 - 1\right)\left(2n^2 + 1\right) / n^2 \tag{15}$$

$$f\left(n\right) = n \tag{16}$$

$$f\left(n\right) = n^2 - 1\tag{17}$$


**Table 9.** Parameters *k*/cm3 ·g−1 for the different empirical equations from Eq. (6) for the equations proposed by Lorentz-Lorenz (L-L), Dale-Gladstone (D-G), Eykman, Oster, Arago-Biot (A-B) and Newton.

All these equations assume that the relation between density and refractive index is temperature-independent. But, from our results (see **Tables 1** and **9**) it can be deduced that *k* in Eq. (11) has a certain dependence on *T* (as it is illustrated below in **Figure 5** in the case of Eq. (16)). Nevertheless, as a first approximation, and assuming that the dependence is small, it is worth finding the equation that best fits our experimental results. In this approximation we shall assume that *k = f(n)ρ* only depends on the nature of ionic liquid. The value for each ionic liquid is obtained as the average for all the temperatures. The standard deviation is obtained as described in Eq. (18). From these calculations (**Table 9**), we conclude that the Oster equation is the one that best fitted our experimental results.

( ) <sup>1</sup>

2 2

**ILs** *k***0 × 104**

[emim+ ][BF4 −

[bmim+ ][BF4 −

[hmim+

[omim+ ][BF4 −

[emim+

[bmim+

[bmim+

[bmim+ ][PF6 −

[hmim+

[omim+ ][PF6 −

[emim+

[bmim+

[hmim+

[omim+

[emim+

][BF4 −

][TfO−

][TfO−

][OcSO4 −

][PF6 −

][NTf2 −

][NTf2 −

][NTf2 −

][NTf2 −

][EtSO4 − **/cm3**

**·g−1·K−1** *k***1***/***cm3**

] 4.709 0.9632 0.9882 0.0009

] 5.632 1.0174 0.9992 0.0003

] 5.829 1.0731 0.9994 0.0002

] 6.139 1.1149 0.9999 0.0001

] 4.712 0.8942 0.9996 0.0002

] 5.140 0.9572 0.9971 0.0005

] 4.717 0.8904 0.9998 0.0001

] 4.888 0.9496 0.9997 0.0001

] 5.221 0.9946 0.9997 0.0001

] 4.622 0.7986 1.0000 0.00003

] 4.695 0.8538 0.9981 0.0003

] 5.123 0.8896 0.9999 0.0001

] 5.351 0.9250 0.9997 0.0002

] 4.831 1.0482 0.9952 0.0006

**Table 10.** Fitting parameters for *k = n/ρ* (*n*, refractive index, *ρ* density) of ILs studied to a linear equation (*k = k*0*T + k*1).

As a consequence, we think that a very good and simple description of the correlation between density and the refraction index can be obtained by including the dependence on *T*. If we compare the S.D. results with those in **Table 9** for any of the equations proposed, a substantial improvement in the quality of the fitting can be observed. Given the quality of the data, once the quantitative relation between density and refractive index is known in a range of temper‐ atures, it is possible just to measure one of these properties at any temperature (probably even

The refractive index (*n*) can be defined as the ratio of the speed of light in vacuum to that in a given medium [69]. Research studies focused on the modeling and even on the measurement of the refractive index of ILs are scarce in the literature even though its measurement is very simple and fast [9]. However, due to the high number of potential ILs we think it is substantially

Some of the studies related to the estimation of refractive index are listed here. The first pre‐ diction model in the literature was developed by Deetlefs et al. [41]. They calculated the mo‐

relevant to develop methods to estimate the refractive index of ILs prior to synthesis.

outside the range) to know the value of the other with a high degree of precision.

**3. Prediction methods of refractive index of ionic liquids**

] 6.189 1.1916 0.9991 0.0003

**·g−1** *r***<sup>2</sup> S.D.**

Predicting Density and Refractive Index of Ionic Liquids

http://dx.doi.org/10.5772/65790

359

**Average** 0.0003

**Figure 6.** Values of *k* = *n*/*ρ* of different ILs for different temperatures. Parameters for linear fitting can be found in **Table 10**.

As mentioned above, a small dependence of *k* on *T* can be observed from our experimental results, and, as a consequence, we used another approach, defining *k* = *n*/*ρ* and fitting the results to a linear equation in the form *k = k*0*T + k*1. **Figure 6** (numerical values can be found in **Table 10**) shows the good quality of the fittings.


Nevertheless, as a first approximation, and assuming that the dependence is small, it is worth finding the equation that best fits our experimental results. In this approximation we shall assume that *k = f(n)ρ* only depends on the nature of ionic liquid. The value for each ionic liquid is obtained as the average for all the temperatures. The standard deviation is obtained as described in Eq. (18). From these calculations (**Table 9**), we conclude that the Oster equation

( ) <sup>1</sup>

exp . *calc z z S D= <sup>n</sup>*

**Figure 6.** Values of *k* = *n*/*ρ* of different ILs for different temperatures. Parameters for linear fitting can be found in

As mentioned above, a small dependence of *k* on *T* can be observed from our experimental results, and, as a consequence, we used another approach, defining *k* = *n*/*ρ* and fitting the results to a linear equation in the form *k = k*0*T + k*1. **Figure 6** (numerical values can be found in

**Table 10**.

**Table 10**) shows the good quality of the fittings.

é ù - ê ú ê ú ë û

2 2

å (18)

is the one that best fitted our experimental results.

358 Progress and Developments in Ionic Liquids

**Table 10.** Fitting parameters for *k = n/ρ* (*n*, refractive index, *ρ* density) of ILs studied to a linear equation (*k = k*0*T + k*1).

As a consequence, we think that a very good and simple description of the correlation between density and the refraction index can be obtained by including the dependence on *T*. If we compare the S.D. results with those in **Table 9** for any of the equations proposed, a substantial improvement in the quality of the fitting can be observed. Given the quality of the data, once the quantitative relation between density and refractive index is known in a range of temper‐ atures, it is possible just to measure one of these properties at any temperature (probably even outside the range) to know the value of the other with a high degree of precision.

### **3. Prediction methods of refractive index of ionic liquids**

The refractive index (*n*) can be defined as the ratio of the speed of light in vacuum to that in a given medium [69]. Research studies focused on the modeling and even on the measurement of the refractive index of ILs are scarce in the literature even though its measurement is very simple and fast [9]. However, due to the high number of potential ILs we think it is substantially relevant to develop methods to estimate the refractive index of ILs prior to synthesis.

Some of the studies related to the estimation of refractive index are listed here. The first pre‐ diction model in the literature was developed by Deetlefs et al. [41]. They calculated the mo‐ lar refraction to predict the refraction indices of ILs from their surface tension. However, all the parameters involved in this model should be experimentally measured or correlated with other experimental properties; thus, it is not suitable to predict refractive indices of new ILs. In addition, it was developed only from 9 ILs so it is far away of being a universal method. The next model was published by Gardas and Coutinho [10] who developed a GCM to predict refractive index of ILs from 245 experimental values of refractive index of ILs in the temperature range from 283.15 to 363.15 K. All the ILs used to model the refrac‐ tive index were based on imidazolium salts (with different anions) thus this model is not appropriate to estimate the refractive index of ILs with another cation. The same method was used by Freire et al. [70] and Soriano et al. [38] who broadened the method by incorpo‐ rating some new ILs. Xu et al. [71] synthesized a new ionic liquid ([C3mim][Val]) and meas‐ ured its refractive index in the temperature range of 298.15–333.15 K. After that, they estimated the refractive index for the homologue ILs ([C*n*mim][Val]; *n* = 2, 4, 5, 6) following the same procedure than Deetlefs et al. [41]. This model is also very limited because it was developed only for a specific family of ILs. Finally, Sattari et al. [9] tried to solve the limita‐ tions of the rest of the predictive models of refractive index of ILs developing a widely ap‐ plicable model based on a QSPR method using genetic function approximation (GFA). They used experimental data of 82 ILs with a great variety of structures and developed a 9‐pa‐ rameter model with very good prediction results.

usually some previous experimental (or calculated) data of certain critical properties (temper‐

Predicting Density and Refractive Index of Ionic Liquids

http://dx.doi.org/10.5772/65790

361

In the GCM methods, the desired property is obtained as individual contribution of each of the components of the final compound. Then, predictions are obtained using available simple equations. The main drawback of these methods is that their predictive capability depends on previous experimental (or calculated) data. With some exceptions, it is impossible to predict density if no data are available for both of the components of an ionic liquid. On the other hand, usually good predictions can be obtained very easily for any combination of known

Estimating the density of ILs by correlations between this property and other physical properties is especially interesting if the measurement of the property used is easier or cheaper than the measurement of the density like in the case of refractive index. Nevertheless, the main drawback is that no prediction can be obtained if previous correlations have not been properly

Our method provides a fast and simple way to predict the density of ILs with alkyl imidazo‐ lium cations and with different anions (and very likely combinations of ions of similar chemical structure) based on the molecular volume in a wide range of temperatures from simple

The correlation between density and the refraction index is usually described in the literature by equations which are assumed to be temperature‐independent; we have shown that a substantial improvement can be obtained if the dependence on temperature is included.

Prediction of refractive index of ILs has not been so widely studied in the literature like in the case of density because the relationships between this property and the ion constituents of the ILs are not so direct and accessible. For this reason, more studies in this field are

We support the idea that ILs can be designed with adjustable properties (at least density and refractive index, but very probably some others) based on the structure of the cation and anion chosen. Our method may provide valuable contributions for the design and study of present

This work was partially supported by FEDER/ERDF funds from the European Commission, the Spanish Ministry of Economy and Competitiveness (MINECO) (Ref. CTQ2014‐57467‐R) and the research support programme of the Seneca Foundation of Science and Technology of Murcia, Spain (Ref. 19499/PI/14). Mercedes G. Montalbán acknowledges support from

theoretical calculations of the structure of the proposed ions.

ature, pressure, volume,…) are needed.

counterions in a few minutes.

established.

required.

and future ILs.

**Acknowledgements**

MINECO (FPI grant, BES‐2012‐053267).

### **4. Conclusions**

Many efforts have been done during the last years to develop a great variety of prediction methods of density of ILs. Thus they have reached a high degree of perfection. Prediction methods of density of ILs can be classified in five categories although the methods are sometimes a combination of more than one category: i) QSPR, ii) ANN, iii) EoS, iv) GCM, and v) correlations between density and other properties.

QSPR models are based on quantum chemistry calculations and this is their great advantage and, at the same time, their main drawback. While virtually any imagined compound can be studied with no previous experimental knowledge, usually the calculations are not easy and only can be developed by very specialized research groups.

ANN is an especially efficient computer algorithm whose main advantage is that it is usually suitable to model chemical properties whose behavior is highly nonlinear because nonlinear relationships are well described with ANN. However, the predicting capabilities of this method depend on the quality of the algorithm for learning and very importantly on the quality, quantity and nature of experimental (or calculated) data used for the learning process.

Equations of state are well known for determining the relationship between pressure, tem‐ perature, volume and composition of components providing a theoretical way to calculate some physical properties such as density. The main advantage of this method is the simplicity in their use but a possible drawback is that to obtain the density of the desired compound usually some previous experimental (or calculated) data of certain critical properties (temper‐ ature, pressure, volume,…) are needed.

In the GCM methods, the desired property is obtained as individual contribution of each of the components of the final compound. Then, predictions are obtained using available simple equations. The main drawback of these methods is that their predictive capability depends on previous experimental (or calculated) data. With some exceptions, it is impossible to predict density if no data are available for both of the components of an ionic liquid. On the other hand, usually good predictions can be obtained very easily for any combination of known counterions in a few minutes.

Estimating the density of ILs by correlations between this property and other physical properties is especially interesting if the measurement of the property used is easier or cheaper than the measurement of the density like in the case of refractive index. Nevertheless, the main drawback is that no prediction can be obtained if previous correlations have not been properly established.

Our method provides a fast and simple way to predict the density of ILs with alkyl imidazo‐ lium cations and with different anions (and very likely combinations of ions of similar chemical structure) based on the molecular volume in a wide range of temperatures from simple theoretical calculations of the structure of the proposed ions.

The correlation between density and the refraction index is usually described in the literature by equations which are assumed to be temperature‐independent; we have shown that a substantial improvement can be obtained if the dependence on temperature is included.

Prediction of refractive index of ILs has not been so widely studied in the literature like in the case of density because the relationships between this property and the ion constituents of the ILs are not so direct and accessible. For this reason, more studies in this field are required.

We support the idea that ILs can be designed with adjustable properties (at least density and refractive index, but very probably some others) based on the structure of the cation and anion chosen. Our method may provide valuable contributions for the design and study of present and future ILs.

### **Acknowledgements**

lar refraction to predict the refraction indices of ILs from their surface tension. However, all the parameters involved in this model should be experimentally measured or correlated with other experimental properties; thus, it is not suitable to predict refractive indices of new ILs. In addition, it was developed only from 9 ILs so it is far away of being a universal method. The next model was published by Gardas and Coutinho [10] who developed a GCM to predict refractive index of ILs from 245 experimental values of refractive index of ILs in the temperature range from 283.15 to 363.15 K. All the ILs used to model the refrac‐ tive index were based on imidazolium salts (with different anions) thus this model is not appropriate to estimate the refractive index of ILs with another cation. The same method was used by Freire et al. [70] and Soriano et al. [38] who broadened the method by incorpo‐ rating some new ILs. Xu et al. [71] synthesized a new ionic liquid ([C3mim][Val]) and meas‐ ured its refractive index in the temperature range of 298.15–333.15 K. After that, they estimated the refractive index for the homologue ILs ([C*n*mim][Val]; *n* = 2, 4, 5, 6) following the same procedure than Deetlefs et al. [41]. This model is also very limited because it was developed only for a specific family of ILs. Finally, Sattari et al. [9] tried to solve the limita‐ tions of the rest of the predictive models of refractive index of ILs developing a widely ap‐ plicable model based on a QSPR method using genetic function approximation (GFA). They used experimental data of 82 ILs with a great variety of structures and developed a 9‐pa‐

Many efforts have been done during the last years to develop a great variety of prediction methods of density of ILs. Thus they have reached a high degree of perfection. Prediction methods of density of ILs can be classified in five categories although the methods are sometimes a combination of more than one category: i) QSPR, ii) ANN, iii) EoS, iv) GCM, and

QSPR models are based on quantum chemistry calculations and this is their great advantage and, at the same time, their main drawback. While virtually any imagined compound can be studied with no previous experimental knowledge, usually the calculations are not easy and

ANN is an especially efficient computer algorithm whose main advantage is that it is usually suitable to model chemical properties whose behavior is highly nonlinear because nonlinear relationships are well described with ANN. However, the predicting capabilities of this method depend on the quality of the algorithm for learning and very importantly on the quality, quantity and nature of experimental (or calculated) data used for the learning process.

Equations of state are well known for determining the relationship between pressure, tem‐ perature, volume and composition of components providing a theoretical way to calculate some physical properties such as density. The main advantage of this method is the simplicity in their use but a possible drawback is that to obtain the density of the desired compound

rameter model with very good prediction results.

v) correlations between density and other properties.

only can be developed by very specialized research groups.

**4. Conclusions**

360 Progress and Developments in Ionic Liquids

This work was partially supported by FEDER/ERDF funds from the European Commission, the Spanish Ministry of Economy and Competitiveness (MINECO) (Ref. CTQ2014‐57467‐R) and the research support programme of the Seneca Foundation of Science and Technology of Murcia, Spain (Ref. 19499/PI/14). Mercedes G. Montalbán acknowledges support from MINECO (FPI grant, BES‐2012‐053267).

### **Author details**

Mercedes G. Montalbán1 , Mar Collado‐González2 , F. Guillermo Díaz‐Baños2 and Gloria Víllora1\*

\*Address all correspondence to: gvillora@um.es

1 Department of Chemical Engineering, Faculty of Chemistry, Regional Campus of International Excellence "Campus Mare Nostrum," University of Murcia, Murcia, Spain

2 Department of Physical Chemistry, Faculty of Chemistry, Regional Campus of Internation‐ al Excellence "Campus Mare Nostrum," University of Murcia, Murcia, Spain

ionic liquids incorporating the imidazolium cation. Green Chem 2001;3:156–164. DOI:

Predicting Density and Refractive Index of Ionic Liquids

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363

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**Author details**

Gloria Víllora1\*

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**Chapter 16**

py]

**Provisional chapter**

**Thermodynamic Properties of Ionic Liquids**

**Thermodynamic Properties of Ionic Liquids**

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution,

© 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

and reproduction in any medium, provided the original work is properly cited.

Ionic liquids (ILs) are salts while can exist as liquid at room temperature or near room temperature, which are completely composed of ions [1–3]. Compared with traditional organic solvents, ILs have exhibited outstanding properties, such as negligible vapor pressures, nonflammable, wide electrochemical window, high electrical conductivity, adjustable acidity, high dissolving

Basic physicochemical properties were discussed at different temperatures for 18 hydrophobic ionic liquids (ILs) which containing imidazolium and pyridinium as cations, separately. The ILs include 1-ethyl-3-methylimizazolium tris(pentafluoroethyl)trifluorophosphate

]), 1-acetonitrile-3-ethylimimdazolium bis(trifluoromethylsulfonyl)

]), 1-(cyanopropyl)-3-methylimidazolium bis[(trifluoromethyl)sulfonyl]

], 1-ethanol-3-ethylimimdazolium bis(trifluoromethylsulfonyl)imide

]), N-alkylpyridinium bis(trifluoromethylsulfonyl)imide {[C*<sup>n</sup>*

] (*n* = 2, 4, 6)}. The molar volume, standard molar entropy, and lattice

]), 1-butylamide-3-ethylimimdazolium bis(trifluoromethylsulfonyl)-imide

] (*n* = 3, 4, 6)}, and N-alkyl-4-methylpyridinium bis(trifluoromethylsulfonyl)

] (*n* = 2, 3, 4, 5, 6)}, N-alkyl-3-methylpyridinium bis(trifluoromethyl-sulfonyl)imide

energy were estimated by the empirical and semiempirical equations. The dependences of density, dynamic viscosity, and electrical conductivity on temperature are discussed in the measured temperature range. It is found that with the increasing temperature, the density and dynamic viscosity decreased, while the electrical conductivity increases. The influences of microstructures of ILs, such as the introduction of the methylene, methyl, and functional

**Keywords:** ionic liquid, density, surface tension, dynamic viscosity, electrical conductivity

groups on cations, on their basic physicochemical properties are discussed.

Liu Qingshan, Mou Lin, Zheng Qige and Xia Quan

Additional information is available at the end of the chapter

Additional information is available at the end of the chapter

Liu Qingshan, Mou Lin, Zheng Qige and

http://dx.doi.org/10.5772/65792

mim][PF3

([EOHMIM][NTf<sup>2</sup>

3Mpy][NTf<sup>2</sup>

imide ([MCNMIM][NTf<sup>2</sup>

imide [PCNMIM][NTf<sup>2</sup>

(CF<sup>2</sup> CF3 )3

CONHBuEIM][NTf<sup>2</sup>

4Mpy][NTf<sup>2</sup>

**Abstract**

([C<sup>2</sup>

([CH<sup>2</sup>

[NTf<sup>2</sup>

imide {[C*<sup>n</sup>*

{[C*<sup>n</sup>*

**1. Introduction**

Xia Quan


### **Thermodynamic Properties of Ionic Liquids**

**Thermodynamic Properties of Ionic Liquids**

Liu Qingshan, Mou Lin, Zheng Qige and Xia Quan Xia Quan

Additional information is available at the end of the chapter Additional information is available at the end of the chapter

Liu Qingshan, Mou Lin, Zheng Qige and

http://dx.doi.org/10.5772/65792

#### **Abstract**

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[70] Freire MG, Teles ARR, Rocha MAA, Schröder B, Neves CMSS, Carvalho PJ, Evtuguin DV, Santos LMNBF, Coutinho JAP. Thermophysical characterization of ionic liquids able to dissolve biomass. J Chem Eng Data 2011;56:4813–4822. DOI: 10.1021/je200790q

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Chem Res 2012;51:4105–4111. DOI: 10.1021/ie201530b

b513451j

368 Progress and Developments in Ionic Liquids

Basic physicochemical properties were discussed at different temperatures for 18 hydrophobic ionic liquids (ILs) which containing imidazolium and pyridinium as cations, separately. The ILs include 1-ethyl-3-methylimizazolium tris(pentafluoroethyl)trifluorophosphate ([C<sup>2</sup> mim][PF3 (CF<sup>2</sup> CF3 )3 ]), 1-acetonitrile-3-ethylimimdazolium bis(trifluoromethylsulfonyl) imide ([MCNMIM][NTf<sup>2</sup> ]), 1-(cyanopropyl)-3-methylimidazolium bis[(trifluoromethyl)sulfonyl] imide [PCNMIM][NTf<sup>2</sup> ], 1-ethanol-3-ethylimimdazolium bis(trifluoromethylsulfonyl)imide ([EOHMIM][NTf<sup>2</sup> ]), 1-butylamide-3-ethylimimdazolium bis(trifluoromethylsulfonyl)-imide ([CH<sup>2</sup> CONHBuEIM][NTf<sup>2</sup> ]), N-alkylpyridinium bis(trifluoromethylsulfonyl)imide {[C*<sup>n</sup>* py] [NTf<sup>2</sup> ] (*n* = 2, 3, 4, 5, 6)}, N-alkyl-3-methylpyridinium bis(trifluoromethyl-sulfonyl)imide {[C*<sup>n</sup>* 3Mpy][NTf<sup>2</sup> ] (*n* = 3, 4, 6)}, and N-alkyl-4-methylpyridinium bis(trifluoromethylsulfonyl) imide {[C*<sup>n</sup>* 4Mpy][NTf<sup>2</sup> ] (*n* = 2, 4, 6)}. The molar volume, standard molar entropy, and lattice energy were estimated by the empirical and semiempirical equations. The dependences of density, dynamic viscosity, and electrical conductivity on temperature are discussed in the measured temperature range. It is found that with the increasing temperature, the density and dynamic viscosity decreased, while the electrical conductivity increases. The influences of microstructures of ILs, such as the introduction of the methylene, methyl, and functional groups on cations, on their basic physicochemical properties are discussed.

**Keywords:** ionic liquid, density, surface tension, dynamic viscosity, electrical conductivity

### **1. Introduction**

Ionic liquids (ILs) are salts while can exist as liquid at room temperature or near room temperature, which are completely composed of ions [1–3]. Compared with traditional organic solvents, ILs have exhibited outstanding properties, such as negligible vapor pressures, nonflammable, wide electrochemical window, high electrical conductivity, adjustable acidity, high dissolving

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

capacity for inorganic and organic compounds or polymers and can be recycled, etc. Moreover, ILs can be designed through the introduction of functional groups on anion or cation to modify their physicochemical properties. As the new designed and functional solvents, ILs have been used in fields of synthesis, extraction, catalysis, electrochemistry, etc. The basic physicochemical properties of ILs are of great importance for their design and applications; however, related data are very deficient. Therefore, IL's properties and related theoretical studies have received increasing attention.

before and after the measurement of properties. The water mass fractions of the ILs are lower than 300 × 10−6 and 500 × 10−6 for before and after the property determination, respectively.

Thermodynamic Properties of Ionic Liquids http://dx.doi.org/10.5772/65792 371

The densities of ILs were measured by a Westphal balance (or an automated SVM3000 Anton Paar rotational Stabinger viscometer-densimeter with a cylinder geometry) in the temperature range of *T* = (283.15 to 338.15) ± 0.05 K. The density values were recorded at every 5 K. For the

Using the tensiometer (DP-AW type produced by Sang Li Electronic Co.) of the forced bubble method, the surface tension of the ILs was measured with the experimental error that is ± 0.1 mJ⋅m−2. The temperature was controlled by a thermostat. The uncertainties of the

The dynamic viscosity of the ILs was measured using an Ostwald viscometer (or an automated SVM3000 Anton Paar rotational Stabinger viscometer-densimeter with a cylinder geometry, the principle is based on a modified Couette according to a rapidly rotating outer tube and a relatively slow rotating inner measuring bob). The values were recorded at every 5 K. The uncertain-

The electrical conductivity of the ILs was carried out using a MP522 conductivity instrument with the cell constants of 1 cm−1 (the cell was calibrated with the aqueous KCl solution). The uncertainty was reckoned to less than ± 1%. The temperature was regulated by a thermostat with a precision of ± 0.05 K. The experimental data were reported per 5 K after 30 min thermal

A straight line can be obtained according to plot ln*ρ* against *T*/K. And the ln*ρ* against *T*/K can be

ln*ρ* / g · cm–<sup>3</sup> = *b*–*αT* / K (1)

Westphal balance method, the sample was placed in a cell with a jacket.

measurement are in the range of ± 0.2 mJ⋅m−2.

**4. Property measurement**

**4.1. Density**

**4.2. Surface tension**

**4.3. Dynamic viscosity**

ties were estimated to be ± 1%.

fitted by the following empirical equation:

**4.4. Electrical conductivity**

equilibrium time.

**5. Formulas**

**5.1. Density**

### **2. Preparation of ionic liquids**

All ionic liquids (ILs) were synthesized according to the reported method [4] except [CH<sup>2</sup> CON HBuEIM][NTf<sup>2</sup> ] and [C<sup>2</sup> mim][PF3 (CF<sup>2</sup> CF3 )3 ].

The chloride (or bromide) type compounds were synthesized by the N-alkylation reaction. A slight excess of halide was added dropwise into N-alkyl compounds by stirring at 353 K for 24 h. The products were recrystallized from acetonitrile/ethyl acetate solution several times. The products were dried under high vacuum for 48 h at 353 K before the synthesis of the target products. The [C<sup>2</sup> mim][PF3 (CF<sup>2</sup> CF3 ) 3 ] was supplied by Merck Co. (batch: S9588301). The compound [CH<sup>2</sup> CONHBuEIM][NTf<sup>2</sup> ] was synthesized according to the reported method [5]. Chloroacetyl chloride was added to n-butylamine drop by drop in the same molar ratio in an ice bath. After completion of the reaction, the organic layer was separated then washed with ω(HCl) = 5% or ω (NaHCO3 ) = 5% until the water layer became neutral. The product (chloroacetyl-n-butylamine) was dried under vacuum conditions. Acetonitrile was chosen as a solvent, ethylimidazole was added to chloroacetyl-n-butylamine in a small excess molar ratio to allow the complete reaction of chloroacetyl-n-butylamine at 85°C for 18 h. The product was recrystallized twice with acetonitrile and ethyl acetate ester. The resulting product was vacuum dried to obtain pure [EimCH<sup>2</sup> CONHBu]Cl.

The hydrophobic ILs were synthesized in the distilled water by the traditional ion exchange reaction. The chloride (or bromide) type compounds were placed in a flask and dissolved with distilled water, an equivalent amount of lithium bis[(trifluoromethyl)sulfonyl]imide (LiNTf<sup>2</sup> ) salt was also dissolved in distilled water and added to the flask. The solution was stirred vigorously for 3 h. The bottom liquid was washed with distilled water until no halogen as detected by AgNO3 / HNO3 solution (the mass fraction of halogen was reckoned to be less than 50 ppm). The products were finally dried on vacuum drying line at 353 K before the determination of the thermodynamic properties. The final products were characterized by 1 H NMR spectra.

### **3. Water content**

The impurity of the water is the most serious influence factor to the properties of ILs. Since the residual water cannot be removed by conventional methods in the ILs. The mass fraction of the residual water was determined by a Cou-Lo Aquamax Karl Fischer moisture meter (v.10.06) before and after the measurement of properties. The water mass fractions of the ILs are lower than 300 × 10−6 and 500 × 10−6 for before and after the property determination, respectively.

### **4. Property measurement**

#### **4.1. Density**

CON

) salt was

/

capacity for inorganic and organic compounds or polymers and can be recycled, etc. Moreover, ILs can be designed through the introduction of functional groups on anion or cation to modify their physicochemical properties. As the new designed and functional solvents, ILs have been used in fields of synthesis, extraction, catalysis, electrochemistry, etc. The basic physicochemical properties of ILs are of great importance for their design and applications; however, related data are very deficient. Therefore, IL's properties and related theoretical studies have received

All ionic liquids (ILs) were synthesized according to the reported method [4] except [CH<sup>2</sup>

The chloride (or bromide) type compounds were synthesized by the N-alkylation reaction. A slight excess of halide was added dropwise into N-alkyl compounds by stirring at 353 K for 24 h. The products were recrystallized from acetonitrile/ethyl acetate solution several times. The products were dried under high vacuum for 48 h at 353 K before the synthesis of the tar-

Chloroacetyl chloride was added to n-butylamine drop by drop in the same molar ratio in an ice bath. After completion of the reaction, the organic layer was separated then washed with

acetyl-n-butylamine) was dried under vacuum conditions. Acetonitrile was chosen as a solvent, ethylimidazole was added to chloroacetyl-n-butylamine in a small excess molar ratio to allow the complete reaction of chloroacetyl-n-butylamine at 85°C for 18 h. The product was recrystallized twice with acetonitrile and ethyl acetate ester. The resulting product was vacuum dried to

The hydrophobic ILs were synthesized in the distilled water by the traditional ion exchange reaction. The chloride (or bromide) type compounds were placed in a flask and dissolved with dis-

also dissolved in distilled water and added to the flask. The solution was stirred vigorously for 3 h. The bottom liquid was washed with distilled water until no halogen as detected by AgNO3

The impurity of the water is the most serious influence factor to the properties of ILs. Since the residual water cannot be removed by conventional methods in the ILs. The mass fraction of the residual water was determined by a Cou-Lo Aquamax Karl Fischer moisture meter (v.10.06)

 solution (the mass fraction of halogen was reckoned to be less than 50 ppm). The products were finally dried on vacuum drying line at 353 K before the determination of the thermodynamic

tilled water, an equivalent amount of lithium bis[(trifluoromethyl)sulfonyl]imide (LiNTf<sup>2</sup>

] was supplied by Merck Co. (batch: S9588301). The

] was synthesized according to the reported method [5].

) = 5% until the water layer became neutral. The product (chloro-

H NMR spectra.

increasing attention.

370 Progress and Developments in Ionic Liquids

HBuEIM][NTf<sup>2</sup>

get products. The [C<sup>2</sup>

obtain pure [EimCH<sup>2</sup>

**3. Water content**

HNO3

ω(HCl) = 5% or ω (NaHCO3

compound [CH<sup>2</sup>

**2. Preparation of ionic liquids**

] and [C<sup>2</sup>

mim][PF3

mim][PF3

CONHBuEIM][NTf<sup>2</sup>

CONHBu]Cl.

properties. The final products were characterized by 1

(CF<sup>2</sup> CF3 ) 3 ].

(CF<sup>2</sup> CF3 ) 3 The densities of ILs were measured by a Westphal balance (or an automated SVM3000 Anton Paar rotational Stabinger viscometer-densimeter with a cylinder geometry) in the temperature range of *T* = (283.15 to 338.15) ± 0.05 K. The density values were recorded at every 5 K. For the Westphal balance method, the sample was placed in a cell with a jacket.

#### **4.2. Surface tension**

Using the tensiometer (DP-AW type produced by Sang Li Electronic Co.) of the forced bubble method, the surface tension of the ILs was measured with the experimental error that is ± 0.1 mJ⋅m−2. The temperature was controlled by a thermostat. The uncertainties of the measurement are in the range of ± 0.2 mJ⋅m−2.

#### **4.3. Dynamic viscosity**

The dynamic viscosity of the ILs was measured using an Ostwald viscometer (or an automated SVM3000 Anton Paar rotational Stabinger viscometer-densimeter with a cylinder geometry, the principle is based on a modified Couette according to a rapidly rotating outer tube and a relatively slow rotating inner measuring bob). The values were recorded at every 5 K. The uncertainties were estimated to be ± 1%.

#### **4.4. Electrical conductivity**

The electrical conductivity of the ILs was carried out using a MP522 conductivity instrument with the cell constants of 1 cm−1 (the cell was calibrated with the aqueous KCl solution). The uncertainty was reckoned to less than ± 1%. The temperature was regulated by a thermostat with a precision of ± 0.05 K. The experimental data were reported per 5 K after 30 min thermal equilibrium time.

### **5. Formulas**

#### **5.1. Density**

A straight line can be obtained according to plot ln*ρ* against *T*/K. And the ln*ρ* against *T*/K can be fitted by the following empirical equation:

$$
\ln \rho / \mathbf{g} \cdot \mathbf{cm}^{\circ} = b \text{--} aT / \mathbf{K} \tag{1}
$$

where *b* is an empirical constant and *α* is the thermal expansion coefficient.

At 298.15 K, the molecular volume, *V*, standard molar entropy, *S*<sup>0</sup> , and lattice energy, *U*POT, of the ILs can be obtained from the experimental density by the following equations:

$$V = \mathbf{M} / (\mathbf{N} \cdot \boldsymbol{\rho}) \tag{2}$$

According to Yang et al., the molar volume of ILs is composed of the volume of inherent and interstices; herein, the molar volume of the interstice is, ∑*ν* = 2*Nν*, the molar volume of the ILs

At 298.15 K, Yang et al. pointed out that the expansion volume of ILs only results from the interstices expansion following the temperature increase. Then, the thermal expansion coefficient, *α*,

The temperature dependence of the dynamic viscosity for ILs can be fitted using the Vogel-

can be estimated by the interstice model theory by the following equation:

, *B*, and *T*<sup>0</sup>

Usually, the Arrhenius equation was used to fit the dynamic viscosity and the equation is:

+ 2*Nν* (9)

Thermodynamic Properties of Ionic Liquids http://dx.doi.org/10.5772/65792 373

*<sup>p</sup>* = 3*Nν* / *VT* (10)

· exp(*B* / (*T*– *T*0)) (11)

 / (*kB T*)) (12)

 / (*kB*(*T*– *T*0)) (13)

· exp(–*B* / (*T*– *T*0)) (14)

is the inherent volume, and 2*Nv* is the interstice volume.

are the fitting parameters.

/*k*B. The activation energy of dynamic viscosity was introduced in the

are fitting parameters.

is the activation energy for dynamic viscosity, *η*∞ is the maximum dynamic viscosity,

According to Vila et al. [12], the VFT equation for dynamic viscosity was related to the Arrhenius

Usually, the VFT is also used for the fitting of temperature dependence on electrical conductivity. Herein, the temperature dependence of electrical conductivity of the ILs was also fitted according

, *B*, and *T*<sup>0</sup>

final version VFT equation. The final version of the VFT equation can be expressed as follows:

can be calculated by the following equation:

herein, *V* is the molar volume, *V*<sup>i</sup>

**5.3. Dynamic viscosity**

where *E*<sup>a</sup>

equation, *η*<sup>0</sup>

Fulcher-Tammann (VFT) equation:

where *η* is the dynamic viscosity; *η*<sup>0</sup>

and *k*B is the Boltzmann constant.

**5.4. Electrical conductivity**

to the following VFT equation:

*σ* = *σ*<sup>0</sup>

here *σ* is the electrical conductivity; *σ*<sup>0</sup>

= *η*∞ and *B* = *E<sup>η</sup>*

*η* = *η*<sup>0</sup>

*η* = *η*∞ · exp(*E<sup>η</sup>*

*η* = *η*∞ · exp(*E<sup>η</sup>*

*V* = *V*<sup>i</sup>

*α* = (1 / *V*)(∂ *V* / ∂ *T*)

$$S^0 = \ 1246.5 \cdot (\mathcal{V}) + 29.5\tag{3}$$

$$\mathcal{U}\_{\rm corr} = \ 1981.2 \cdot \left(\rho/\mathcal{M}\right)^{13} + 103.8 \tag{4}$$

where *M* is molar mass, *ρ* is the density, and *N* is the Avogadro's constant.

#### **5.2. Surface tension**

The surface tension, *γ*, has the relationship with the temperature in terms of the Eötvös equation:

$$
\gamma \, V^{23} = k \left( T\_c - T \right) \tag{5}
$$

where *V* is the molar volume of the liquid, *T*<sup>c</sup> , is the critical temperature, and *k*, is an empirical constant.

The parachor, *P*, was estimated by the following equation:

$$P(\text{298.15 K}) = (M \, \gamma^{1/4})/\rho \tag{6}$$

where *M* is molar mass, *ρ* is the density, and *γ* is surface tension.

The molar enthalpy of vaporization, Δ <sup>l</sup> g *H*<sup>m</sup> 0 , was estimated by the following equation:

$$
\Delta\_{\text{l}} \, ^\circ H\_{\text{m}} \, ^\circ \text{(298.15K)} / \text{kJ} \cdot \text{mol}^{-1} = 0.01121 \text{[} \, ^\circ V^{23} \, ^\circ \text{N}^{13} \text{]} + 2.4 \tag{7}
$$

where *V* is molar volume, *γ* is surface tension, and *N* is Avogadro's constant.

At 298.15 K, according to the literature studies [6, 7], the interstice volume, *v*, can be estimated by interstice model theory:

$$v = \, 0.6791 \, (k\_{\flat} T/\gamma \,)^{32} \tag{8}$$

herein, *kb* is the Boltzmann constant, *T* is thermodynamic temperature, and *γ* is the surface tension of ILs.

According to Yang et al., the molar volume of ILs is composed of the volume of inherent and interstices; herein, the molar volume of the interstice is, ∑*ν* = 2*Nν*, the molar volume of the ILs can be calculated by the following equation:

$$V = V\_i + \text{2N}\nu \tag{9}$$

herein, *V* is the molar volume, *V*<sup>i</sup> is the inherent volume, and 2*Nv* is the interstice volume.

At 298.15 K, Yang et al. pointed out that the expansion volume of ILs only results from the interstices expansion following the temperature increase. Then, the thermal expansion coefficient, *α*, can be estimated by the interstice model theory by the following equation:

$$a = \text{(1/V)(\partial V/\partial T)}\_{\rho} = \text{3Nv/VT} \tag{10}$$

#### **5.3. Dynamic viscosity**

where *b* is an empirical constant and *α* is the thermal expansion coefficient.

where *M* is molar mass, *ρ* is the density, and *N* is the Avogadro's constant.

ILs can be obtained from the experimental density by the following equations:

*V* = M / (N · *ρ*) (2)

*S*<sup>0</sup> = 1246.5 · (*V*) + 29.5 (3)

The surface tension, *γ*, has the relationship with the temperature in terms of the Eötvös

*γ V*<sup>2</sup>/3 = *k*(*Tc* − *T*) (5)

*P*(298.15 K ) = (*M γ*1/4 ) / *ρ* (6)

At 298.15 K, according to the literature studies [6, 7], the interstice volume, *v*, can be estimated by

*v* = 0.6791 (*kb T* / *γ* )3/<sup>2</sup> (8)

is the Boltzmann constant, *T* is thermodynamic temperature, and *γ* is the surface ten-

1/3

, and lattice energy, *U*POT, of the

+ 103.8 (4)

, is the critical temperature, and *k*, is an empirical

, was estimated by the following equation:

0(298.15K) / kJ ⋅ mol−1 = 0.01121(*γ V*<sup>2</sup>/3 *N*1/3) + 2.4 (7)

At 298.15 K, the molecular volume, *V*, standard molar entropy, *S*<sup>0</sup>

*U*POT = 1981.2 · (*ρ* / M)

where *V* is the molar volume of the liquid, *T*<sup>c</sup>

The molar enthalpy of vaporization, Δ <sup>l</sup>

*<sup>g</sup> H*<sup>m</sup>

*Δ*<sup>1</sup>

interstice model theory:

herein, *kb*

sion of ILs.

The parachor, *P*, was estimated by the following equation:

where *M* is molar mass, *ρ* is the density, and *γ* is surface tension.

g *H*<sup>m</sup> 0

where *V* is molar volume, *γ* is surface tension, and *N* is Avogadro's constant.

**5.2. Surface tension**

372 Progress and Developments in Ionic Liquids

equation:

constant.

The temperature dependence of the dynamic viscosity for ILs can be fitted using the Vogel-Fulcher-Tammann (VFT) equation:

$$\eta = \eta\_0 \cdot \exp\{B/(T - T\_a)\}\tag{11}$$

where *η* is the dynamic viscosity; *η*<sup>0</sup> , *B*, and *T*<sup>0</sup> are the fitting parameters.

Usually, the Arrhenius equation was used to fit the dynamic viscosity and the equation is:

$$\eta = \eta\_w \cdot \exp\left(\mathbb{E}\_{\eta}/(k\_{\mathfrak{s}}\, T)\right) \tag{12}$$

where *E*<sup>a</sup> is the activation energy for dynamic viscosity, *η*∞ is the maximum dynamic viscosity, and *k*B is the Boltzmann constant.

According to Vila et al. [12], the VFT equation for dynamic viscosity was related to the Arrhenius equation, *η*<sup>0</sup> = *η*∞ and *B* = *E<sup>η</sup>* /*k*B. The activation energy of dynamic viscosity was introduced in the final version VFT equation. The final version of the VFT equation can be expressed as follows:

$$\eta = \eta\_{\omega} \cdot \exp(\mathbf{E}\_{\eta}/(k\_{\rm g}(T - T\_{\rm o})) \tag{13}$$

#### **5.4. Electrical conductivity**

Usually, the VFT is also used for the fitting of temperature dependence on electrical conductivity. Herein, the temperature dependence of electrical conductivity of the ILs was also fitted according to the following VFT equation:

$$
\sigma = \sigma\_0 \cdot \exp\left(-\mathbb{B}/\left(T - T\_o\right)\right) \tag{14}
$$

here *σ* is the electrical conductivity; *σ*<sup>0</sup> , *B*, and *T*<sup>0</sup> are fitting parameters. Sometimes, the Arrhenius equation is also used to fit the electrical conductivity:

$$
\sigma = \sigma\_{\omega} \cdot \exp\left(-E\_{o}/(k\_{\text{g}}T)\right) \tag{15}
$$

where *E*σ is the activation energy, which indicates the energy needed for the ion to hop into a free hole, *σ*∞ is the maximum electrical conductivity, and *k*B is the Boltzmann constant.

According to the discussion, Vila et al. [8] have introduced the activation energy of electrical conductivity in the VFT equation by establishing the fitting parameters of the VFT equation with the Arrhenius equation: *σ*<sup>0</sup> = *σ*∞ and *B* = *E<sup>σ</sup>* /*k*B. The final version of the VFT equation can be expressed as follows:

$$
\sigma = \sigma\_{\text{as}} \cdot \exp(-E\_{\sigma}/(k\_{\text{p}}(T - T\_{\text{o}})) \tag{16}
$$

#### **5.5. Walden rule**

The classical Walden rule was usually used for the assessing of the ionicity of ILs [9, 10]. The ionic mobilities (represented by the equivalent conductivity Λ = *F*Σ*μ*<sup>i</sup> Zi ) and the fluidity *φ* (*φ* = *η*−1) of the medium can be related to the Walden rule through the ions move. On the basis of this fact, the relationship of the molar electrical conductivity and dynamic viscosity for ILs can be described by the following equation:

$$
\Lambda \mathfrak{v} = k \tag{17}
$$

By plotting ln *ρ* against (*T* − 298.15) K, a straight line can be obtained (see **Figure 2**) according to Eq. (1). According to Eq. (1), the correlation coefficient is *R* = 0.9999, the standard deviation *s* = 3.0 × 10−5 g cm−3, *b* = 0.53603, the thermal expansion coefficient of the IL is *α* = 6.96 × 10−4 K−1

mim][PF3

(CF<sup>2</sup> CF3 )3

] from 283.15 to 338.15 K.

Thermodynamic Properties of Ionic Liquids http://dx.doi.org/10.5772/65792 375

at 298.15 K, respectively.

**Figure 2.** Plot of ln *ρ* vs. *T*/K − 298.15 of IL [C<sup>2</sup>

mim][PF3

(CF<sup>2</sup> CF3 ) 3 ].

**Figure 1.** The structure of IL ([C<sup>2</sup>

mim][PF3

**Table 1.** Experimental values of density, *ρ*, and surface tension, *γ*, of IL [C<sup>2</sup>

(CF<sup>2</sup> CF3 )3 ]).

*T*/K 283.15 288.15 293.15 298.15 303.15 308.15 *ρ*/g cm−3 1.72705 1.72113 1.71517 1.70926 1.70332 1.69740 *γ*/mJ m−2 35.3 35.1 34.9 34.8 34.6 34.4 *T*/K 313.15 318.15 323.15 328.15 333.15 338.15 *ρ*/g cm−3 1.69150 1.68562 1.67975 1.67388 1.66804 1.66221 *γ*/mJ m−2 34.2 34.1 34.0 33.8 33.6 33.4

where *Λ* is the molar electrical conductivity, *η* is the dynamic viscosity, and *k* is a temperature dependent constant. The Walden's product (in [S·cm<sup>2</sup> ·mol−1][mP·s]) can be calculated at 298.15 K.

#### **6. Density and surface tension of ionic liquid [C2 mim][PF3 (CF2 CF3 ) 3 ] and prediction of properties [C***<sup>n</sup>* **mim][PF3 (CF2 CF3 ) 3 ] (***n* **= 1, 3, 4, 5, 6)**

As organic salts, the ionic liquids (ILs) have shown many excellent properties, such as the low melting temperature, good solvation, and nonvolatility. So, the industrial and scientific communities have applied ILs in a broad range as the green organic solvents. In particular, the air- and water-stable hydrophobic ILs have been used in some special fields as the stable ILs. Actually, the most ILs are hydrophilic, so, 1-alkyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (C*<sup>n</sup>* mimNTf<sup>2</sup> ) has attracted serious concern as an air- and water-stable hydrophobic compound. And the properties were reporting in succession when the air- and water-stable hydrophobic compounds were synthesized. As another air- and water-stable hydrophobic type IL 1-ethyl-3-methylimizazolium tris(pentafluoroethyl)trifluorophosphate [C<sup>2</sup> mim][PF3 (CF<sup>2</sup> CF3 ) 3 ] was provided by Merck Co. This is also the air- and water-stable hydrophobic IL. So, the study on the properties of this type ILs is significant in many concerned fields.

The structure of [C<sup>2</sup> mim][PF3 (CF<sup>2</sup> CF3 ) 3 ] is shown in **Figure 1**.

The experimental measured values of density and surface tension of IL [C<sup>2</sup> mim][PF3 (CF<sup>2</sup> CF3 ) 3 ] are listed in **Table 1** [11].

**Figure 1.** The structure of IL ([C<sup>2</sup> mim][PF3 (CF<sup>2</sup> CF3 )3 ]).

Sometimes, the Arrhenius equation is also used to fit the electrical conductivity:

hole, *σ*∞ is the maximum electrical conductivity, and *k*B is the Boltzmann constant.

where *E*σ is the activation energy, which indicates the energy needed for the ion to hop into a free

According to the discussion, Vila et al. [8] have introduced the activation energy of electrical conductivity in the VFT equation by establishing the fitting parameters of the VFT equation with the

The classical Walden rule was usually used for the assessing of the ionicity of ILs [9, 10]. The ionic

the medium can be related to the Walden rule through the ions move. On the basis of this fact, the relationship of the molar electrical conductivity and dynamic viscosity for ILs can be described by

 *Λη* = *k* (17) where *Λ* is the molar electrical conductivity, *η* is the dynamic viscosity, and *k* is a temperature

> **(CF2 CF3 ) 3**

As organic salts, the ionic liquids (ILs) have shown many excellent properties, such as the low melting temperature, good solvation, and nonvolatility. So, the industrial and scientific communities have applied ILs in a broad range as the green organic solvents. In particular, the air- and water-stable hydrophobic ILs have been used in some special fields as the stable ILs. Actually, the most ILs are hydrophilic, so, 1-alkyl-3-methylimidazolium

water-stable hydrophobic compound. And the properties were reporting in succession when the air- and water-stable hydrophobic compounds were synthesized. As another air- and water-stable hydrophobic type IL 1-ethyl-3-methylimizazolium tris(pentafluoroethyl)triflu-

water-stable hydrophobic IL. So, the study on the properties of this type ILs is significant in

] is shown in **Figure 1**.

mimNTf<sup>2</sup>

 / (*kB T*)) (15)

 / (*kB*(*T* − *T*0)) (16)

·mol−1][mP·s]) can be calculated at 298.15 K.

**(CF2 CF3 ) 3 ] and** 

) has attracted serious concern as an air- and

mim][PF3

(CF<sup>2</sup> CF3 ) 3 ]

) and the fluidity *φ* (*φ* = *η*−1) of

/*k*B. The final version of the VFT equation can be expressed

Zi

**mim][PF3**

**] (***n* **= 1, 3, 4, 5, 6)**

] was provided by Merck Co. This is also the air- and

*σ* = *σ*∞ · exp(– *E<sup>σ</sup>*

*σ* = *σ*∞ · exp(– *E<sup>σ</sup>*

dependent constant. The Walden's product (in [S·cm<sup>2</sup>

**6. Density and surface tension of ionic liquid [C2**

**mim][PF3**

= *σ*∞ and *B* = *E<sup>σ</sup>*

mobilities (represented by the equivalent conductivity Λ = *F*Σ*μ*<sup>i</sup>

Arrhenius equation: *σ*<sup>0</sup>

374 Progress and Developments in Ionic Liquids

as follows:

**5.5. Walden rule**

the following equation:

**prediction of properties [C***<sup>n</sup>*

bis(trifluoromethylsulfonyl)imide (C*<sup>n</sup>*

mim][PF3

mim][PF3

(CF<sup>2</sup> CF3 )3

(CF<sup>2</sup> CF3 ) 3

The experimental measured values of density and surface tension of IL [C<sup>2</sup>

orophosphate [C<sup>2</sup>

The structure of [C<sup>2</sup>

many concerned fields.

are listed in **Table 1** [11].


**Table 1.** Experimental values of density, *ρ*, and surface tension, *γ*, of IL [C<sup>2</sup> mim][PF3 (CF<sup>2</sup> CF3 )3 ] from 283.15 to 338.15 K.

By plotting ln *ρ* against (*T* − 298.15) K, a straight line can be obtained (see **Figure 2**) according to Eq. (1). According to Eq. (1), the correlation coefficient is *R* = 0.9999, the standard deviation *s* = 3.0 × 10−5 g cm−3, *b* = 0.53603, the thermal expansion coefficient of the IL is *α* = 6.96 × 10−4 K−1 at 298.15 K, respectively.

**Figure 2.** Plot of ln *ρ* vs. *T*/K − 298.15 of IL [C<sup>2</sup> mim][PF3 (CF<sup>2</sup> CF3 ) 3 ].

The experiment values of *γ* against (*T* − 298.15) K can be fitted according to the linear equation (see **Figure 3**). According to the linear equation, the correlation coefficient and standard deviation can be obtained and the values are 0.998 and 0.04 mJ∙m−2, respectively. In **Figure 3**, the surface entropy, *S*<sup>a</sup> = − (∂*γ*/∂(*T* − 298.15))p, can also be obtained and the value is 33.4 × 10−3 mJ∙K−1·m−2 at 298.15 K. At 298.15 K, the surface energy can be calculated from the surface tension value by the imperial equation, *E*<sup>a</sup> = *γ* − *T*(∂*γ*/∂(*T* − 298.15))p, and the value is 44.8 mJ∙m−2. Compared with surface energies of the fused salts and organic liquids, the value of the IL [C<sup>2</sup> mim][PF3 (CF<sup>2</sup> CF3 ) 3 ] is close to the organic liquids, even less than some organic liquids, for example, 146 mJ∙m−2 for NaNO3 , 67 mJ∙m−2 for benzene, and 51.1 mJ∙m−2 for octane. This fact shows that the interaction energy between ions in [C<sup>2</sup> mim][PF3 (CF<sup>2</sup> CF3 ) 3 ] is less than that in fused salts. The physicochemical properties (molecular volume, *V*m, parachor, *P*, thermal expansion coefficient, α, standard entropy, *S*<sup>0</sup> , lattice energy, *U*pot, and molar enthalpy of vaporization, Δ<sup>l</sup> g *H*<sup>m</sup> 0 ) were estimated by using the experimental data of density and surface tension according to Eqs. (1)–(7).

According to the literature [12, 13], the contribution per methylene (**−**CH<sup>2</sup>

be calculated to be 34.3. So, the average value can be used to predict the parachor of the ILs

can be obtained from the predicted density and surface tension. The data are listed in **Table 2**. According to the predicted values of density and surface tension, the other properties can be

According to the interstice model and Eqs. (8)–(10) [6, 7], the interstice volume, *v*, the molar vol-

molar volume of the interstice, ∑*ν* = 2*Nν*, the thermal expansion coefficient, α, can be predicted from the interstice model at 298.15 K. All of the data obtained from estimation and prediction are

**Table 2** shows the comparison of the predicted and experimental thermal expansion coefficients

predicted values of the expansion coefficient can be as the reference data when lack of reliable

lated from the predicted density and parachor. The molar enthalpy of vaporization, Δ<sup>l</sup>

*M*/g⋅mol−1 542.15 556.18 570.20 584.23 598.26 612.29 *V*m/nm3 0.5130 0.5405 0.5680 0.5955 0.6230 0.6505 *ρ/*g⋅cm−3 1.75552 1.70926 <sup>m</sup> 1.66756 1.62962 1.59516 1.56356

/J⋅K−1⋅mol−1 669.0 703.3 737.5 771.8 806.1 840.3 *U*pot/kJ⋅mol−1 397 392 387 383 379 375 *V*/cm−3⋅mol−1 308.8 325.4 341.9 358.5 375.0 391.6 *p* 757.8 792.1 826.4 860.7 895.0 929.3

1024 *v*/cm3 25.93 27.63 28.48 29.65 30.75 31.78 ∑*v*/cm3 31.22 33.33 34.29 35.70 37.03 38.26

∑*v/V* 10.11 10.22 10.29 9.96 9.87 9.77

*α*/K−1 5.10 5.14 5.04 5.00 4.96 4.91

/N⋅m−1 36.3 34.8 <sup>m</sup> 34.1 33.2 32.4 31.7

161.9 157.8 160.3 161.1 162.0 163.1

] and 37.5 for [C*<sup>n</sup>*

**Table 2.** Estimated and predicted values of physicochemical properties of IL [C*<sup>n</sup>*

Notes: <sup>m</sup> measurement value; <sup>p</sup> data in the column were predicted values; <sup>e</sup>

predicted and the values are also listed in **Table 2**.

ume of ionic liquids, *V*, consists of the inherent volume, *V*<sup>i</sup>

31.1 for [C*<sup>n</sup>*

at 298.15 K.

mim][PF3

**Properties [C1**

*S*0

Δl g *H*<sup>m</sup> 0 / kJ⋅mol−1

10<sup>2</sup>

104

104

*γ* 103

**mim] [PF3 (CF2 CF3 ) 3 ] p [C2 mim] [PF3 (CF2 CF3 )3 ] e [C3 mim] [PF3 (CF2 CF3 ) 3 ] p [C4 mim] [PF3 (CF2 CF3 ) 3 ] p [C5 mim] [PF3 (CF2 CF3 ) 3 ] p [C6 mim] [PF3 (CF2 CF3 ) 3 ] p**

listed in **Table 2**.

mim][PF3

experimental values.

(CF<sup>2</sup> CF3 ) 3

of [C<sup>2</sup>

[C*<sup>n</sup>*

mim][AlCl4

*α*/K−1 6.96 <sup>m</sup>

(CF<sup>2</sup> CF3 )3 **−**) to parachor is

] (*n* = 1, 2, 3, 4, 5, 6)

g *H*<sup>m</sup> 0 ,

mim][Ala]. An average value of the contribution can

mim][PF3

, and the volume of the interstices; the

data in the column were estimated values.

Thermodynamic Properties of Ionic Liquids http://dx.doi.org/10.5772/65792 377

(CF<sup>2</sup> CF3 )3

] (*n* = 1, 3, 4, 5, 6). According to Eq. (6), the surface tension can be calcu-

] at 298.15 K. The difference of the two values is about 26%. So, the

**Figure 3.** Plot of *γ* vs. *T*/K − 298.15 of IL [C<sup>2</sup> mim][PF3 (CF<sup>2</sup> CF3 )3 ].

The contribution of the methylene (**−**CH<sup>2</sup> **−**) group to the molecular volumes can be obtained according to the literature studies [12, 13]. The values of the ILs [C*<sup>n</sup>* mim][BF4 ], [C*<sup>n</sup>* mim][NTf<sup>2</sup> ], [C*<sup>n</sup>* mim][AlCl4 ], and [C*<sup>n</sup>* mim][Ala] are 0.0272, 0.0282, 0.0270 and 0.0278 nm3 , respectively. From the above values, the contribution of the methylene to the molecular volume can be considered to be similar. So, the mean value of the contribution can be calculated to be 0.0275 nm3 , the physicochemical properties (density, standard entropy, lattice energy) of the homologues of [C*<sup>n</sup>* mim][PF3 (CF<sup>2</sup> CF3 )3 ] (*n* = 1, 3, 4, 5, 6) can be predicted. All of the predicted data are listed in **Table 2**.


The experiment values of *γ* against (*T* − 298.15) K can be fitted according to the linear equation (see **Figure 3**). According to the linear equation, the correlation coefficient and standard deviation can be obtained and the values are 0.998 and 0.04 mJ∙m−2, respectively. In **Figure 3**,

mJ∙K−1·m−2 at 298.15 K. At 298.15 K, the surface energy can be calculated from the surface ten-

Compared with surface energies of the fused salts and organic liquids, the value of the IL

in fused salts. The physicochemical properties (molecular volume, *V*m, parachor, *P*, thermal

= − (∂*γ*/∂(*T* − 298.15))p, can also be obtained and the value is 33.4 × 10−3

] is close to the organic liquids, even less than some organic liquids,

) were estimated by using the experimental data of density and surface

= *γ* − *T*(∂*γ*/∂(*T* − 298.15))p, and the value is 44.8 mJ∙m−2.

, 67 mJ∙m−2 for benzene, and 51.1 mJ∙m−2 for octane. This

(CF<sup>2</sup> CF3 ) 3

**−**) group to the molecular volumes can be obtained

] (*n* = 1, 3, 4, 5, 6) can be predicted. All of the predicted data

mim][Ala] are 0.0272, 0.0282, 0.0270 and 0.0278 nm3

From the above values, the contribution of the methylene to the molecular volume can be considered to be similar. So, the mean value of the contribution can be calculated to be 0.0275

, the physicochemical properties (density, standard entropy, lattice energy) of the homo-

mim][BF4

], [C*<sup>n</sup>*

mim][NTf<sup>2</sup>

, respectively.

],

, lattice energy, *U*pot, and molar enthalpy of

] is less than that

mim][PF3

the surface entropy, *S*<sup>a</sup>

(CF<sup>2</sup> CF3 ) 3

376 Progress and Developments in Ionic Liquids

mim][PF3

vaporization, Δ<sup>l</sup>

[C<sup>2</sup>

[C*<sup>n</sup>*

nm3

mim][AlCl4

logues of [C*<sup>n</sup>*

are listed in **Table 2**.

sion value by the imperial equation, *E*<sup>a</sup>

for example, 146 mJ∙m−2 for NaNO3

g *H*<sup>m</sup> 0

tension according to Eqs. (1)–(7).

fact shows that the interaction energy between ions in [C<sup>2</sup>

expansion coefficient, α, standard entropy, *S*<sup>0</sup>

The contribution of the methylene (**−**CH<sup>2</sup>

**Figure 3.** Plot of *γ* vs. *T*/K − 298.15 of IL [C<sup>2</sup>

], and [C*<sup>n</sup>*

mim][PF3

(CF<sup>2</sup> CF3 ) 3

according to the literature studies [12, 13]. The values of the ILs [C*<sup>n</sup>*

mim][PF3

(CF<sup>2</sup> CF3 )3 ]. Notes: <sup>m</sup> measurement value; <sup>p</sup> data in the column were predicted values; <sup>e</sup> data in the column were estimated values.

**Table 2.** Estimated and predicted values of physicochemical properties of IL [C*<sup>n</sup>* mim][PF3 (CF<sup>2</sup> CF3 )3 ] (*n* = 1, 2, 3, 4, 5, 6) at 298.15 K.

According to the literature [12, 13], the contribution per methylene (**−**CH<sup>2</sup> **−**) to parachor is 31.1 for [C*<sup>n</sup>* mim][AlCl4 ] and 37.5 for [C*<sup>n</sup>* mim][Ala]. An average value of the contribution can be calculated to be 34.3. So, the average value can be used to predict the parachor of the ILs [C*<sup>n</sup>* mim][PF3 (CF<sup>2</sup> CF3 )3 ] (*n* = 1, 3, 4, 5, 6). According to Eq. (6), the surface tension can be calculated from the predicted density and parachor. The molar enthalpy of vaporization, Δ<sup>l</sup> g *H*<sup>m</sup> 0 , can be obtained from the predicted density and surface tension. The data are listed in **Table 2**.

According to the predicted values of density and surface tension, the other properties can be predicted and the values are also listed in **Table 2**.

According to the interstice model and Eqs. (8)–(10) [6, 7], the interstice volume, *v*, the molar volume of ionic liquids, *V*, consists of the inherent volume, *V*<sup>i</sup> , and the volume of the interstices; the molar volume of the interstice, ∑*ν* = 2*Nν*, the thermal expansion coefficient, α, can be predicted from the interstice model at 298.15 K. All of the data obtained from estimation and prediction are listed in **Table 2**.

**Table 2** shows the comparison of the predicted and experimental thermal expansion coefficients of [C<sup>2</sup> mim][PF3 (CF<sup>2</sup> CF3 ) 3 ] at 298.15 K. The difference of the two values is about 26%. So, the predicted values of the expansion coefficient can be as the reference data when lack of reliable experimental values.

For the majority materials, the volume expansions are in the range of 10−15% from the solid state to the liquid state. From **Table 2**, the estimated and predicted interstice fractions are in the range of 9−11% for the serious ILs [C*<sup>n</sup>* mim][PF3 (CF<sup>2</sup> CF3 ) 3 ] (*n* = 1, 2, 3, 4, 5, 6) at 298.15 K. The values are in good agreement with the reported values. Therefore, the interstice model theory can be used for calculation of the thermal expansion coefficient.

geous properties. As the solvent, it can be applied as a suitable reaction media and ligands in catalytic reactions, as an electrolyte in lithium batteries, as a solvent for extraction of metals and

Although the FILs have been applied in some areas, the physicochemical properties are not enough for the application [20–22]. In this chapter, the properties of 1-acetonitrile-3-ethylimim-

CONHBuEIM][NTf<sup>2</sup>

], and [CH<sup>2</sup>

The density, dynamic viscosity, and electrical conductivity of the four FILs are listed in

], (b) [BMIM][NTf<sup>2</sup>

], and (h) [CH<sup>2</sup>

In order to compare the influences of methylene and functional group on the properties of ILs, the values of density, dynamic viscosity, and electrical conductivity for ILs are listed in

] are plotted in **Figure 5**.

], (c) [EMMIM][NTf<sup>2</sup>

CONHBuEIM][NTf<sup>2</sup>

], [BMIM][NTf<sup>2</sup>

], [EOHMIM][NTf<sup>2</sup>

The temperature dependences on the density of the FILs [MCNMIM][NTf<sup>2</sup>

], (g) [EOHMIM][NTf<sup>2</sup>

], [PCNMIM][NTf<sup>2</sup>

CONHBuEIM][NTf<sup>2</sup>

], [EMMIM][NTf<sup>2</sup>

CONHBuEIM][NTf<sup>2</sup>

], 1-(cyanopropyl)-3-methylimid-

Thermodynamic Properties of Ionic Liquids http://dx.doi.org/10.5772/65792

], 1-ethanol-3-ethylimimdazolium

] were compared with the traditional

MMIM][NTf<sup>2</sup>

], (*n* = 2, 4) and

], [MCNMIM]

] is shown in **Figure 4**.

], [PCNMIM][NTf<sup>2</sup>

], [BMMIM]

], (e)[MCNMIM]

CONHBuEIM]

], [EMMIM][NTf<sup>2</sup>

], (d) [BMMIM][NTf<sup>2</sup>

].

], and [CH<sup>2</sup>

],

], (*n* = 2, 4).

379

], 1-butylamide-3-ethylimimdazolium

MIM][NTf<sup>2</sup>

], [BMMIM][NTf<sup>2</sup>

dazolium bis(trifluoromethylsulfonyl)imide [MCNMIM][NTf<sup>2</sup>

azolium bis[(trifluoromethyl)sulfonyl]imide [PCNMIM][NTf<sup>2</sup>

ILs 1-alkyl-3-mthylimimdazolium bis(trifluoromethylsulfonyl)imide [C*<sup>n</sup>*

1-alkyl-2,3-dimthylimimdazolium bis(trifluoromethylsulfonyl)imide [C*<sup>n</sup>*

], [EOHMIM][NTf<sup>2</sup>

], [BMIM][NTf<sup>2</sup>

bis(trifluoromethylsulfonyl)imide [EOHMIM][NTf<sup>2</sup>

bis(trifluoromethylsulfonyl)-imide [CH<sup>2</sup>

The structure of [EMIM][NTf<sup>2</sup>

], [PCNMIM][NTf<sup>2</sup>

**Tables 3**–**5** [23–25].

], (f) [PCNMIM][NTf<sup>2</sup>

[EOHMIM][NTf<sup>2</sup>

[NTf<sup>2</sup>

[NTf<sup>2</sup>

[NTf<sup>2</sup>

], and [CH<sup>2</sup>

**Figure 4.** The structures of the (a) [EMIM][NTf<sup>2</sup>

**Table 6** at 298.15 K. The ILs are [EMIM][NTf<sup>2</sup>

], [MCNMIM][NTf<sup>2</sup>

], respectively.

[NTf<sup>2</sup>

dissolution of cellulose.

#### **Conclusion**

In this section, the density and surface tension of the imidazolium-type hydrophobic IL [C<sup>2</sup> mim][PF3 (CF<sup>2</sup> CF3 )3 ] (*n* = 1, 2, 3, 4, 5, 6) were determined and predicted in the temperature range of 283.15–338.15 K. According to the estimated equations and interstice model theory, the thermodynamic properties of the serious ILs were calculated by the empirical and semiempirical equations at 298.15 K. The effect of the methylene on molecular volume and parachor was discussed and used for the prediction of the thermodynamic properties of ILs. From the predicted values of thermal expansion coefficient, the other predicted values can be used as the reference data when lack the reliable experimental values.

### **7. Density, dynamic viscosity, and electrical conductivity of imidazoliumtype hydrophobic functional ionic liquids**

Ionic liquids (ILs) have exhibited outstanding physicochemical properties, such as good solvation, negligible vapor pressure, good thermal stability, and designability. ILs have been used as the green solvents in industrial and scientific areas. The functional ionic liquids (FILs) have been paid much more attention because of the designability [14–24]. The physicochemical properties can be designed according to the introduction of the functional groups, such as −CN, −OH, and −CH<sup>2</sup> -O-CH3 .

Egashira et al. [14–16] have introduced the cyano group on the imidazolium FILs and quaternary ammonium FILs, respectively. The FILs have been applied in the lithium batteries as electrolyte components. The FILs have showed an improved cycle behavior compared with the electrolyte based on a tetraalkylammonium ionic liquid without a cyano group. The quaternary ammonium-based FILs containing a cyano group showed the better stability of the cathodic than the imidazolium-based FILs. Hardacre et al. [17, 18] have also synthesized two series pyridinium type FILs. The effect of electron-withdrawing groups on the properties was discussed according to the presence of the nitrile or trifluoromethyl in this type FILs. The introduction of the two functional groups leads to the increasing of the melting temperature compared the traditional ILs. On the basis of this fact, the authors have observed the liquid charge-transfer complexes form upon contacting electron-rich aromatics with an electron withdrawing group appended 1-alkyl-4-cyanopyridinium ionic liquids. Zhang et al. [19] have studied the solubilities of C<sup>2</sup> H4 and CO<sup>2</sup> in the cyano-type imidazolium FILs using the gas chromatography. Compared with the 1,3-dialkylimidazolium-type ILs, the cyano-type FILs result in a remarkable decrease of the interactions of hydrocarbons. The cyano-type ILs have exhibited the advantageous properties. As the solvent, it can be applied as a suitable reaction media and ligands in catalytic reactions, as an electrolyte in lithium batteries, as a solvent for extraction of metals and dissolution of cellulose.

For the majority materials, the volume expansions are in the range of 10−15% from the solid state to the liquid state. From **Table 2**, the estimated and predicted interstice fractions are in the range

in good agreement with the reported values. Therefore, the interstice model theory can be used

In this section, the density and surface tension of the imidazolium-type hydrophobic IL

ture range of 283.15–338.15 K. According to the estimated equations and interstice model theory, the thermodynamic properties of the serious ILs were calculated by the empirical and semiempirical equations at 298.15 K. The effect of the methylene on molecular volume and parachor was discussed and used for the prediction of the thermodynamic properties of ILs. From the predicted values of thermal expansion coefficient, the other predicted values can be

**7. Density, dynamic viscosity, and electrical conductivity of imidazolium-**

Ionic liquids (ILs) have exhibited outstanding physicochemical properties, such as good solvation, negligible vapor pressure, good thermal stability, and designability. ILs have been used as the green solvents in industrial and scientific areas. The functional ionic liquids (FILs) have been paid much more attention because of the designability [14–24]. The physicochemical properties can be designed according to the introduction of the functional groups, such as −CN, −OH, and

Egashira et al. [14–16] have introduced the cyano group on the imidazolium FILs and quaternary ammonium FILs, respectively. The FILs have been applied in the lithium batteries as electrolyte components. The FILs have showed an improved cycle behavior compared with the electrolyte based on a tetraalkylammonium ionic liquid without a cyano group. The quaternary ammonium-based FILs containing a cyano group showed the better stability of the cathodic than the imidazolium-based FILs. Hardacre et al. [17, 18] have also synthesized two series pyridinium type FILs. The effect of electron-withdrawing groups on the properties was discussed according to the presence of the nitrile or trifluoromethyl in this type FILs. The introduction of the two functional groups leads to the increasing of the melting temperature compared the traditional ILs. On the basis of this fact, the authors have observed the liquid charge-transfer complexes form upon contacting electron-rich aromatics with an electron withdrawing group appended 1-alkyl-4-cyanopyridinium ionic liquids. Zhang et al. [19] have studied the solu-

Compared with the 1,3-dialkylimidazolium-type ILs, the cyano-type FILs result in a remarkable decrease of the interactions of hydrocarbons. The cyano-type ILs have exhibited the advanta-

in the cyano-type imidazolium FILs using the gas chromatography.

] (*n* = 1, 2, 3, 4, 5, 6) were determined and predicted in the tempera-

] (*n* = 1, 2, 3, 4, 5, 6) at 298.15 K. The values are

(CF<sup>2</sup> CF3 ) 3

mim][PF3

used as the reference data when lack the reliable experimental values.

of 9−11% for the serious ILs [C*<sup>n</sup>*

378 Progress and Developments in Ionic Liquids

(CF<sup>2</sup> CF3 )3

**Conclusion**

mim][PF3

[C<sup>2</sup>

−CH<sup>2</sup>


bilities of C<sup>2</sup>

H4

and CO<sup>2</sup>

for calculation of the thermal expansion coefficient.

**type hydrophobic functional ionic liquids**

Although the FILs have been applied in some areas, the physicochemical properties are not enough for the application [20–22]. In this chapter, the properties of 1-acetonitrile-3-ethylimimdazolium bis(trifluoromethylsulfonyl)imide [MCNMIM][NTf<sup>2</sup> ], 1-(cyanopropyl)-3-methylimidazolium bis[(trifluoromethyl)sulfonyl]imide [PCNMIM][NTf<sup>2</sup> ], 1-ethanol-3-ethylimimdazolium bis(trifluoromethylsulfonyl)imide [EOHMIM][NTf<sup>2</sup> ], 1-butylamide-3-ethylimimdazolium bis(trifluoromethylsulfonyl)-imide [CH<sup>2</sup> CONHBuEIM][NTf<sup>2</sup> ] were compared with the traditional ILs 1-alkyl-3-mthylimimdazolium bis(trifluoromethylsulfonyl)imide [C*<sup>n</sup>* MIM][NTf<sup>2</sup> ], (*n* = 2, 4) and 1-alkyl-2,3-dimthylimimdazolium bis(trifluoromethylsulfonyl)imide [C*<sup>n</sup>* MMIM][NTf<sup>2</sup> ], (*n* = 2, 4).

The structure of [EMIM][NTf<sup>2</sup> ], [BMIM][NTf<sup>2</sup> ], [EMMIM][NTf<sup>2</sup> ], [BMMIM][NTf<sup>2</sup> ], [MCNMIM] [NTf<sup>2</sup> ], [PCNMIM][NTf<sup>2</sup> ], [EOHMIM][NTf<sup>2</sup> ], and [CH<sup>2</sup> CONHBuEIM][NTf<sup>2</sup> ] is shown in **Figure 4**.

**Figure 4.** The structures of the (a) [EMIM][NTf<sup>2</sup> ], (b) [BMIM][NTf<sup>2</sup> ], (c) [EMMIM][NTf<sup>2</sup> ], (d) [BMMIM][NTf<sup>2</sup> ], (e)[MCNMIM] [NTf<sup>2</sup> ], (f) [PCNMIM][NTf<sup>2</sup> ], (g) [EOHMIM][NTf<sup>2</sup> ], and (h) [CH<sup>2</sup> CONHBuEIM][NTf<sup>2</sup> ].

The density, dynamic viscosity, and electrical conductivity of the four FILs are listed in **Tables 3**–**5** [23–25].

The temperature dependences on the density of the FILs [MCNMIM][NTf<sup>2</sup> ], [PCNMIM][NTf<sup>2</sup> ], [EOHMIM][NTf<sup>2</sup> ], and [CH<sup>2</sup> CONHBuEIM][NTf<sup>2</sup> ] are plotted in **Figure 5**.

In order to compare the influences of methylene and functional group on the properties of ILs, the values of density, dynamic viscosity, and electrical conductivity for ILs are listed in **Table 6** at 298.15 K. The ILs are [EMIM][NTf<sup>2</sup> ], [BMIM][NTf<sup>2</sup> ], [EMMIM][NTf<sup>2</sup> ], [BMMIM] [NTf<sup>2</sup> ], [MCNMIM][NTf<sup>2</sup> ], [PCNMIM][NTf<sup>2</sup> ], [EOHMIM][NTf<sup>2</sup> ], and [CH<sup>2</sup> CONHBuEIM] [NTf<sup>2</sup> ], respectively.


**[MCNMIM] [NTf2**

343.15 7.56 6.66 14.47 348.15 8.78 7.60 16.26 353.15 10.12 18.15

[NTf<sup>2</sup>

], and [CH<sup>2</sup>

HBuEIM][NTf<sup>2</sup>

].

CONHBuEIM][NTf<sup>2</sup>

**Figure 5.** Density vs. temperature plots for FILs [MCNMIM][NTf<sup>2</sup>

**Table 5.** Experimental values of electrical conductivity, *σ*/mS⋅cm−1, of [MCNMIM][NTf<sup>2</sup>

] from 283.15 to 353.15 K at atmosphere pressure.

], [PCNMIM][NTf<sup>2</sup>

], [EOHMIM][NTf<sup>2</sup>

], and [CH<sup>2</sup>

CON

**] [PCNMIM] [NTf2**

283.15 0.283 1.319 0.0606 288.15 0.439 0.612 1.794 0.0985 293.15 0.648 0.841 2.37 0.1527 298.15 0.919 1.125 3.05 0.233 303.15 1.294 1.473 3.87 0.332 308.15 1.727 1.874 4.81 0.467 313.15 2.25 2.33 5.89 0.639 318.15 2.85 2.88 7.04 0.854 323.15 3.60 3.49 8.30 1.106 328.15 4.41 4.17 9.67 1.414 333.15 5.36 4.94 11.12 1.774 338.15 6.44 5.78 12.77 2.19

**] [EOHMIM] [NTf2**

**] [CH2**

], [PCNMIM][NTf<sup>2</sup>

], [EOHMIM]

**[NTf2 ]**

Thermodynamic Properties of Ionic Liquids http://dx.doi.org/10.5772/65792

**CONHBuEIM]** 

381

**Table 3.** Experimental values of density, *ρ*/kg⋅m-3, of [MCNMIM][NTf<sup>2</sup> ], [PCNMIM][NTf<sup>2</sup> ], [EOHMIM][NTf<sup>2</sup> ], and [CH<sup>2</sup> CONHBuEIM][NTf<sup>2</sup> ] from 283.15 to 353.15 K at atmosphere pressure.


**Table 4.** Experimental values of dynamic viscosity, *η*/mPa⋅s, of [MCNMIM][NTf<sup>2</sup> ], [PCNMIM][NTf<sup>2</sup> ], [EOHMIM][NTf<sup>2</sup> ], and [CH<sup>2</sup> CONHBuEIM][NTf<sup>2</sup> ] from 283.15 to 353.15 K at atmosphere pressure.


**[MCNMIM] [NTf2**

380 Progress and Developments in Ionic Liquids

**] [PCNMIM] [NTf2**

343.15 1.5654 1.4737 1.5302 348.15 1.5606 1.4692 1.5255 353.15 1.5558 1.4648 1.5207

**Table 3.** Experimental values of density, *ρ*/kg⋅m-3, of [MCNMIM][NTf<sup>2</sup>

283.15 1140 612.68 213.4 288.15 708.6 419.66 153.7 293.15 463.4 303.31 114.1

**Table 4.** Experimental values of dynamic viscosity, *η*/mPa⋅s, of [MCNMIM][NTf<sup>2</sup>

] from 283.15 to 353.15 K at atmosphere pressure.

] from 283.15 to 353.15 K at atmosphere pressure.

**] [PCNMIM] [NTf2**

298.15 315.5 222.35 86.89 777.5 303.15 222.8 166.86 67.66 517.3 308.15 162.4 131.14 53.88 359.0 313.15 121.7 99.090 43.52 255.2 318.15 93.56 78.438 35.85 187.9 323.15 73.61 63.791 29.83 138.9 328.15 58.99 55.023 25.20 106.0 333.15 47.93 44.728 21.52 82.1 338.15 39.70 37.515 18.46 64.4 343.15 33.27 29.223 16.08 53.3 348.15 28.11 26.418 14.06 43.5 353.15 24.06 23.573 12.44 35.8

[CH<sup>2</sup>

and [CH<sup>2</sup>

CONHBuEIM][NTf<sup>2</sup>

CONHBuEIM][NTf<sup>2</sup>

**[MCNMIM] [NTf2**

283.15 1.6259 1.5290 1.5886 1.4310 288.15 1.6205 1.5239 1.5836 1.4270 293.15 1.6150 1.5191 1.5786 1.4227 298.15 1.6097 1.5143 1.5737 1.4180 303.15 1.6046 1.5097 1.5688 1.4135 308.15 1.5996 1.5051 1.5639 1.4090 313.15 1.5946 1.5006 1.5591 1.4044 318.15 1.5898 1.4961 1.5542 1.3997 323.15 1.5848 1.4916 1.5494 1.3954 328.15 1.5799 1.4870 1.5446 1.3910 333.15 1.5751 1.4826 1.5398 1.3867 338.15 1.5702 1.4781 1.5350 1.3823

**] [EOHMIM] [NTf2**

**] [CH2**

], [PCNMIM][NTf<sup>2</sup>

**] [CH2**

], [PCNMIM][NTf<sup>2</sup>

**] [EOHMIM] [NTf2**

], [EOHMIM][NTf<sup>2</sup>

**CONHBuEIM] [NTf2**

], [EOHMIM][NTf<sup>2</sup>

],

], and

**]**

**CONHBuEIM] [NTf2**

**]**

**Table 5.** Experimental values of electrical conductivity, *σ*/mS⋅cm−1, of [MCNMIM][NTf<sup>2</sup> ], [PCNMIM][NTf<sup>2</sup> ], [EOHMIM] [NTf<sup>2</sup> ], and [CH<sup>2</sup> CONHBuEIM][NTf<sup>2</sup> ] from 283.15 to 353.15 K at atmosphere pressure.

**Figure 5.** Density vs. temperature plots for FILs [MCNMIM][NTf<sup>2</sup> ], [PCNMIM][NTf<sup>2</sup> ], [EOHMIM][NTf<sup>2</sup> ], and [CH<sup>2</sup> CON HBuEIM][NTf<sup>2</sup> ].


As indicated in **Table 6**, the density and dynamic viscosity of the FILs are higher than the nonfunctional ILs, and the electrical conductivity is lower than the nonfunctional ILs after

leads to the increasing of Van der Waals force between the cation and the anion relative to the nonfunctional ILs. The order of the effect of the group to the thermodynamic properties

According to Eqs. (1)–(4), the calculated values of the thermal expansion coefficient molecular volume, standard molar entropy, and lattice energy are calculated and listed in **Table 7**, respectively.

**] [PCNMIM] [NTf2**

) 0.4151 0.4721 0.4299 0.5746

MW/(g mol−1) 402.29 430.34 407.30 490.47

*α*/(K−1) 6.26 6.09 6.23 6.36 *V*m/(cm−3 mol−1) 249.9 284.18 258.8 345.89

/(J K−1 mol−1) 547.0 617.9 565.4 745.7 *U*pot/(kJ mol−1) 418 405 414 613

From **Table 7**, the contribution of the methylene to the molecular volume is 0.0285 nm3

] and [PCNMIM][NTf<sup>2</sup>

] at 288.15–338.15 K under atmospheric pressure.

=2, 4, 6) [27] at 298.15 K. The lattice energies of the FILs are much lower than traditional salts, such as *U*POT, CsI = 613 kJ mol−1 [28]. Usually, the ILs exhibit the low lattice energy [4, 23–27]. And it is the reason that the ILs having the relatively low melting temperature can exist in the

Usually, the Vogel-Fulcher-Tammann (VFT) is used for the fitting of temperature dependence on dynamic viscosity. The temperature dependences on dynamic viscosity of the FILs

], [EOHMIM][NTf<sup>2</sup>

empirical Eq. (11). From **Table 8**, the obtained correlation coefficient values, *R*, are better than 0.9999. The results indicate that the experimental dynamic viscosity of the FILs [MCNMIM]

According to Eq. (12), the 1000/*T* dependence of ln *η* was plotted for four FILs [MCNMIM]

], and [CH<sup>2</sup>

], and [CH<sup>2</sup>

py][NTf<sup>2</sup>

], and [CH<sup>2</sup>

, and the correlation coefficient, *R*, are listed in **Table 8** from the

CONHBuEIM][NTf<sup>2</sup>

CONHBuEIM][NTf<sup>2</sup>

] (*n* =3, 4, 6) [27], and 0.0289 nm3

for ILs [C*<sup>n</sup>*

OH functional group on the imidazolium ring, this result

**] [EOHMIM] [NTf2**

for the

], [EOHMIM]

mim]

] (*n*

] are

for [C*<sup>n</sup>*

**CONHBuEIM] [NTf2**

**]**

383

4mpy][NTf<sup>2</sup>

] can be fitted by

] (see **Figure 7**).

]. The value is in good agreement

], [PCNMIM][NTf<sup>2</sup>

CONHBuEIM][NTf<sup>2</sup>

for [C*<sup>n</sup>*

] [4, 26], 0.0282 nm3

**] [CH2**

Thermodynamic Properties of Ionic Liquids http://dx.doi.org/10.5772/65792

the introduction of the −CN or −CH<sup>2</sup>

**Property [MCNMIM] [NTf2**

cyano-type FILs [MCNMIM][NTf<sup>2</sup>

liquid state at room temperature.

], 0.0277 nm3

[MCNMIM][NTf<sup>2</sup>

plotted in **Figure 6**.

the VFT equation.

The best fitted values of *η*<sup>0</sup>

], [PCNMIM][NTf<sup>2</sup>

], [PCNMIM][NTf<sup>2</sup>

[NTf<sup>2</sup>

*V*/(nm3

104

*S*0

[NTf<sup>2</sup>

], and [CH<sup>2</sup>

[NTf<sup>2</sup>

[NTf<sup>2</sup>

with the reported values of 0.0280 nm3

CONHBuEIM][NTf<sup>2</sup>

for [C*<sup>n</sup>*

3mpy][NTf<sup>2</sup>

**Table 7.** Calculated values of thermodynamic properties of FILs [MCNMIM][NTf<sup>2</sup>

], [PCNMIM][NTf<sup>2</sup>

, *B, T*<sup>0</sup>

], [EOHMIM][NTf<sup>2</sup>

], [EOHMIM][NTf<sup>2</sup>

OH > −CH3

.

is: −CN > −CH<sup>2</sup>

**Table 6.** Comparison of density, *ρ*, dynamic viscosity, *η*, and electrical conductivity, *σ*, of [EMIM][NTf<sup>2</sup> ], [BMIM][NTf<sup>2</sup> ], [EMMIM][NTf<sup>2</sup> ], [BMMIM][NTf<sup>2</sup> ], [MCNMIM][NTf<sup>2</sup> ], [PCNMIM][NTf<sup>2</sup> ], [EOHMIM][NTf<sup>2</sup> ], and [CH<sup>2</sup> CONHBuEIM] [NTf<sup>2</sup> ] at 298.15 K at atmosphere pressure.

From **Table 6**, based on the same anion, at 298.15 K, the density follows the order of ILs [CH<sup>2</sup> CONHBuEIM][NTf<sup>2</sup> ] < [BMMIM][NTf<sup>2</sup> ] < [BMIM][NTf<sup>2</sup> ] < [EMMIM][NTf<sup>2</sup> ] < [PCNMIM] [NTf<sup>2</sup> ] < [EMIM][NTf<sup>2</sup> ] < [EOHMIM][NTf<sup>2</sup> ] < [MCNMIM][NTf<sup>2</sup> ].

The dynamic viscosity follows the order of ILs [EMIM][NTf<sup>2</sup> ] < [BMIM][NTf<sup>2</sup> ] < [EMMIM] [NTf<sup>2</sup> ] < [EOHMIM][NTf<sup>2</sup> ] < [BMMIM][NTf<sup>2</sup> ] < [PCNMIM][NTf<sup>2</sup> ] < [MCNMIM][NTf<sup>2</sup> ] < [CH<sup>2</sup> CONHBuEIM][NTf<sup>2</sup> ].

The electrical conductivity follows the order of ILs [CH<sup>2</sup> CONHBuEIM][NTf<sup>2</sup> ] < [MCNMIM] [NTf<sup>2</sup> ] < [PCNMIM][NTf<sup>2</sup> ] < [BMMIM][NTf<sup>2</sup> ] < [EOHMIM][NTf<sup>2</sup> ] < [EMMIM][NTf<sup>2</sup> ] < [BMIM] [NTf<sup>2</sup> ] < [EMIM][NTf<sup>2</sup> ].

As shown in **Table 6**, the three series ILs have exhibited the same tendency for density after the introduction of methylene on the alkyl side chain. Usually, for dynamic viscosity and electrical conductivity, the introduction of methylene leads to the dynamic viscosity increase and electrical conductivity decrease, such as [EMIM][NTf<sup>2</sup> ] and [BMIM][NTf<sup>2</sup> ]; [EMMIM][NTf<sup>2</sup> ] and [BMMIM][NTf<sup>2</sup> ]. However, for the FILs, the values exhibited the contrary tendency with the traditional ILs. The dynamic viscosity values of FIL [PCNMIM][NTf<sup>2</sup> ] are lower than FIL [MCNMIM][NTf<sup>2</sup> ] and the electrical conductivity values of FIL [PCNMIM][NTf<sup>2</sup> ] are higher than FIL [MCNMIM][NTf<sup>2</sup> ] in the temperature range. The abnormal results have been also discovered for traditional pyridinium-type ILs from our group (see here). For the pyridiniumtype ILs, the electrical conductivity values increase when the methyl group is introduced on position 4. The dynamic viscosity values decrease when the methyl group is introduced on position 4. We believed that the electron-withdrawing and electron-donating groups play the important role to the effect of the properties. −CN is the electron-withdrawing group, −CH<sup>2</sup> − and −CH3 are the electron-donating group. For the two series ILs, the presence of the −CN and −CH3 leads to the cations that have the relatively symmetry structure after the introduction of −CH<sup>2</sup> −. Then, the cation and anion have the relatively far away and the force of them becomes weak. So, these two types of ILs exhibited the high fluidity after the introduction of −CH<sup>2</sup> −.

As indicated in **Table 6**, the density and dynamic viscosity of the FILs are higher than the nonfunctional ILs, and the electrical conductivity is lower than the nonfunctional ILs after the introduction of the −CN or −CH<sup>2</sup> OH functional group on the imidazolium ring, this result leads to the increasing of Van der Waals force between the cation and the anion relative to the nonfunctional ILs. The order of the effect of the group to the thermodynamic properties is: −CN > −CH<sup>2</sup> OH > −CH3 .

According to Eqs. (1)–(4), the calculated values of the thermal expansion coefficient molecular volume, standard molar entropy, and lattice energy are calculated and listed in **Table 7**, respectively.


From **Table 6**, based on the same anion, at 298.15 K, the density follows the order of ILs

] 391.31 257.75 1.5182<sup>a</sup> 32.0<sup>a</sup> 8.96<sup>a</sup>

] 419.36 291.91 1.4366<sup>a</sup> 51.7<sup>a</sup> 3.98<sup>a</sup>

] 405.33 271.48 1.4931<sup>a</sup> 72.2<sup>a</sup> 3.89<sup>a</sup>

] 433.38 304.70 1.4224<sup>a</sup> 101.6<sup>a</sup> 2.12<sup>a</sup>

] 402.29 249.92 1.6097<sup>b</sup> 315.5<sup>b</sup> 0.919<sup>b</sup>

] 430.34 284.18 1.5143<sup>c</sup> 222.35<sup>c</sup> 1.125<sup>c</sup>

] 407.30 258.8 1.5737 86.89 3.05

], [PCNMIM][NTf<sup>2</sup>

As shown in **Table 6**, the three series ILs have exhibited the same tendency for density after the introduction of methylene on the alkyl side chain. Usually, for dynamic viscosity and electrical conductivity, the introduction of methylene leads to the dynamic viscosity increase and

] and the electrical conductivity values of FIL [PCNMIM][NTf<sup>2</sup>

are the electron-donating group. For the two series ILs, the presence of the −CN and

leads to the cations that have the relatively symmetry structure after the introduction of

−. Then, the cation and anion have the relatively far away and the force of them becomes

weak. So, these two types of ILs exhibited the high fluidity after the introduction of −CH<sup>2</sup>

discovered for traditional pyridinium-type ILs from our group (see here). For the pyridiniumtype ILs, the electrical conductivity values increase when the methyl group is introduced on position 4. The dynamic viscosity values decrease when the methyl group is introduced on position 4. We believed that the electron-withdrawing and electron-donating groups play the important role to the effect of the properties. −CN is the electron-withdrawing group, −CH<sup>2</sup>

] < [BMIM][NTf<sup>2</sup>

] 490.47 345.89 1.4180 777.5 0.233

] < [MCNMIM][NTf<sup>2</sup>

] < [EOHMIM][NTf<sup>2</sup>

] < [PCNMIM][NTf<sup>2</sup>

]. However, for the FILs, the values exhibited the contrary tendency with

] in the temperature range. The abnormal results have been also

] < [EMMIM][NTf<sup>2</sup>

] < [BMIM][NTf<sup>2</sup>

] < [EMMIM][NTf<sup>2</sup>

CONHBuEIM][NTf<sup>2</sup>

] and [BMIM][NTf<sup>2</sup>

].

], [EOHMIM][NTf<sup>2</sup>

 **mol−1** *ρ* **kg m−3** *η* **mPa s** *σ* **mS cm−1**

] < [PCNMIM]

], [BMIM][NTf<sup>2</sup>

CONHBuEIM]

] < [EMMIM]

] < [MCNMIM]

]; [EMMIM][NTf<sup>2</sup>

] are lower than FIL

] are higher

] < [BMIM]

] <

],

]

−

−.

] < [MCNMIM][NTf<sup>2</sup>

], and [CH<sup>2</sup>

] < [BMMIM][NTf<sup>2</sup>

], [MCNMIM][NTf<sup>2</sup>

**Table 6.** Comparison of density, *ρ*, dynamic viscosity, *η*, and electrical conductivity, *σ*, of [EMIM][NTf<sup>2</sup>

] < [BMMIM][NTf<sup>2</sup>

] < [BMMIM][NTf<sup>2</sup>

the traditional ILs. The dynamic viscosity values of FIL [PCNMIM][NTf<sup>2</sup>

] < [EOHMIM][NTf<sup>2</sup>

**MW g mol−1** *V* **cm3**

The dynamic viscosity follows the order of ILs [EMIM][NTf<sup>2</sup>

].

].

The electrical conductivity follows the order of ILs [CH<sup>2</sup>

electrical conductivity decrease, such as [EMIM][NTf<sup>2</sup>

[CH<sup>2</sup>

[NTf<sup>2</sup>

[CH<sup>2</sup>

[EMIM][NTf<sup>2</sup>

[BMIM][NTf<sup>2</sup>

[EMMIM][NTf<sup>2</sup>

[BMMIM][NTf<sup>2</sup>

[MCNMIM][NTf<sup>2</sup>

[PCNMIM][NTf<sup>2</sup>

[EOHMIM][NTf<sup>2</sup>

[EMMIM][NTf<sup>2</sup>

CONHBuEIM][NTf<sup>2</sup>

382 Progress and Developments in Ionic Liquids

[NTf<sup>2</sup>

[NTf<sup>2</sup>

[CH<sup>2</sup>

[NTf<sup>2</sup>

[NTf<sup>2</sup>

CONHBuEIM][NTf<sup>2</sup>

], [BMMIM][NTf<sup>2</sup>

] at 298.15 K at atmosphere pressure.

] < [EOHMIM][NTf<sup>2</sup>

CONHBuEIM][NTf<sup>2</sup>

] < [PCNMIM][NTf<sup>2</sup>

] < [EMIM][NTf<sup>2</sup>

and [BMMIM][NTf<sup>2</sup>

[MCNMIM][NTf<sup>2</sup>

and −CH3

−CH3

−CH<sup>2</sup>

than FIL [MCNMIM][NTf<sup>2</sup>

] < [EMIM][NTf<sup>2</sup>

**Table 7.** Calculated values of thermodynamic properties of FILs [MCNMIM][NTf<sup>2</sup> ], [PCNMIM][NTf<sup>2</sup> ], [EOHMIM] [NTf<sup>2</sup> ], and [CH<sup>2</sup> CONHBuEIM][NTf<sup>2</sup> ] at 288.15–338.15 K under atmospheric pressure.

From **Table 7**, the contribution of the methylene to the molecular volume is 0.0285 nm3 for the cyano-type FILs [MCNMIM][NTf<sup>2</sup> ] and [PCNMIM][NTf<sup>2</sup> ]. The value is in good agreement with the reported values of 0.0280 nm3 for ILs [C*<sup>n</sup>* py][NTf<sup>2</sup> ] [4, 26], 0.0282 nm3 for [C*<sup>n</sup>* mim] [NTf<sup>2</sup> ], 0.0277 nm3 for [C*<sup>n</sup>* 3mpy][NTf<sup>2</sup> ] (*n* =3, 4, 6) [27], and 0.0289 nm3 for [C*<sup>n</sup>* 4mpy][NTf<sup>2</sup> ] (*n* =2, 4, 6) [27] at 298.15 K. The lattice energies of the FILs are much lower than traditional salts, such as *U*POT, CsI = 613 kJ mol−1 [28]. Usually, the ILs exhibit the low lattice energy [4, 23–27]. And it is the reason that the ILs having the relatively low melting temperature can exist in the liquid state at room temperature.

Usually, the Vogel-Fulcher-Tammann (VFT) is used for the fitting of temperature dependence on dynamic viscosity. The temperature dependences on dynamic viscosity of the FILs [MCNMIM][NTf<sup>2</sup> ], [PCNMIM][NTf<sup>2</sup> ], [EOHMIM][NTf<sup>2</sup> ], and [CH<sup>2</sup> CONHBuEIM][NTf<sup>2</sup> ] are plotted in **Figure 6**.

The best fitted values of *η*<sup>0</sup> , *B, T*<sup>0</sup> , and the correlation coefficient, *R*, are listed in **Table 8** from the empirical Eq. (11). From **Table 8**, the obtained correlation coefficient values, *R*, are better than 0.9999. The results indicate that the experimental dynamic viscosity of the FILs [MCNMIM] [NTf<sup>2</sup> ], [PCNMIM][NTf<sup>2</sup> ], [EOHMIM][NTf<sup>2</sup> ], and [CH<sup>2</sup> CONHBuEIM][NTf<sup>2</sup> ] can be fitted by the VFT equation.

According to Eq. (12), the 1000/*T* dependence of ln *η* was plotted for four FILs [MCNMIM] [NTf<sup>2</sup> ], [PCNMIM][NTf<sup>2</sup> ], [EOHMIM][NTf<sup>2</sup> ], and [CH<sup>2</sup> CONHBuEIM][NTf<sup>2</sup> ] (see **Figure 7**).

From **Table 5**, the temperature dependences on electrical conductivity of the FILs [MCNMIM]

], and [CH<sup>2</sup>

], [PCNMIM][NTf<sup>2</sup>

CONHBuEIM][NTf<sup>2</sup>

], [EOHMIM][NTf<sup>2</sup>

], [PCNMIM][NTf<sup>2</sup>

], [EOHMIM][NTf<sup>2</sup>

],

], and [CH<sup>2</sup>

CONHBuEIM]

] are plotted in

385

Thermodynamic Properties of Ionic Liquids http://dx.doi.org/10.5772/65792

], [EOHMIM][NTf<sup>2</sup>

[NTf<sup>2</sup>

[NTf<sup>2</sup> ].

and [CH<sup>2</sup>

CONHBuEIM][NTf<sup>2</sup>

**Figure 8**.

], [PCNMIM][NTf<sup>2</sup>

**Figure 7.** Plot of ln*η* vs. 1000/*T* for FILs [MCNMIM][NTf<sup>2</sup>

**Figure 8.** Electrical conductivity vs. temperature plots for FILs [MCNMIM][NTf<sup>2</sup>

].

**Figure 6.** Dynamic viscosity vs. temperature plots for FILs [MCNMIM][NTf<sup>2</sup> ], [PCNMIM][NTf<sup>2</sup> ], [EOHMIM][NTf<sup>2</sup> ], and [CH<sup>2</sup> CONHBuEIM][NTf<sup>2</sup> ].


**Table 8.** Fitted parameter values of *η*<sup>0</sup> , *B, T*<sup>0</sup> , and correlation coefficient, *R*, and *E<sup>η</sup>* .

The 1000/*T* dependences on ln *η* of the four FILs [MCNMIM][NTf<sup>2</sup> ], [PCNMIM][NTf<sup>2</sup> ], [EOHMIM][NTf<sup>2</sup> ], and [CH<sup>2</sup> CONHBuEIM][NTf<sup>2</sup> ] are also fitted in the temperature range (**Figure 7**). The values of the correlation coefficient, *R*, are 0.9878, 0.9937, 0.9926, and 0.9943, respectively. The values are obvious lower than the correlation coefficient values, *R* = 0.99999, 0.99989, 0.99999, and 0.99998, which obtained according to the VFT equation. So, the same result indicated that the dynamic viscosity values of [MCNMIM][NTf2], [PCNMIM][NTf2], [EOHMIM][NTf2], and [CH2CONHBuEIM][NTf2] cannot be fitted according to the Arrhenius Eq. (12). From **Figure 7**, it can be obviously observed that the measurement points lie far away from the fitted straight lines.

According to Eq. (13), the activation energies of dynamic viscosity for the FILs [MCNMIM] [NTf<sup>2</sup> ], [PCNMIM][NTf<sup>2</sup> ], [EOHMIM][NTf<sup>2</sup> ], and [CH<sup>2</sup> CONHBuEIM][NTf<sup>2</sup> ] were calculated and are listed in **Table 8**.

From **Table 5**, the temperature dependences on electrical conductivity of the FILs [MCNMIM] [NTf<sup>2</sup> ], [PCNMIM][NTf<sup>2</sup> ], [EOHMIM][NTf<sup>2</sup> ], and [CH<sup>2</sup> CONHBuEIM][NTf<sup>2</sup> ] are plotted in **Figure 8**.

**Figure 7.** Plot of ln*η* vs. 1000/*T* for FILs [MCNMIM][NTf<sup>2</sup> ], [PCNMIM][NTf<sup>2</sup> ], [EOHMIM][NTf<sup>2</sup> ], and [CH<sup>2</sup> CONHBuEIM] [NTf<sup>2</sup> ].

The 1000/*T* dependences on ln *η* of the four FILs [MCNMIM][NTf<sup>2</sup>

, *B, T*<sup>0</sup>

**] [PCNMIM] [NTf2**

/(mPa s) 0.2166 0.1600 0.2144 0.0820 *B*/K 728.8 882.1 692.3 990.9

/eV 62.9 76.1 59.7 85.5

/K 198.1 176.2 182.9 189.9 *R* 0.99999 0.99989 0.99999 0.99998

**Figure 6.** Dynamic viscosity vs. temperature plots for FILs [MCNMIM][NTf<sup>2</sup>

], [EOHMIM][NTf<sup>2</sup>

CONHBuEIM][NTf<sup>2</sup>

(**Figure 7**). The values of the correlation coefficient, *R*, are 0.9878, 0.9937, 0.9926, and 0.9943, respectively. The values are obvious lower than the correlation coefficient values, *R* = 0.99999, 0.99989, 0.99999, and 0.99998, which obtained according to the VFT equation. So, the same result indicated that the dynamic viscosity values of [MCNMIM][NTf2], [PCNMIM][NTf2], [EOHMIM][NTf2], and [CH2CONHBuEIM][NTf2] cannot be fitted according to the Arrhenius Eq. (12). From **Figure 7**, it can be obviously observed that the measurement points lie far away

, and correlation coefficient, *R*, and *E<sup>η</sup>*

According to Eq. (13), the activation energies of dynamic viscosity for the FILs [MCNMIM]

], and [CH<sup>2</sup>

], and [CH<sup>2</sup>

[EOHMIM][NTf<sup>2</sup>

[CH<sup>2</sup>

*η*0

103 *Eη*

*T*0

CONHBuEIM][NTf<sup>2</sup>

**Property [MCNMIM] [NTf2**

384 Progress and Developments in Ionic Liquids

**Table 8.** Fitted parameter values of *η*<sup>0</sup>

].

[NTf<sup>2</sup>

from the fitted straight lines.

], [PCNMIM][NTf<sup>2</sup>

and are listed in **Table 8**.

], [PCNMIM][NTf<sup>2</sup>

], [EOHMIM][NTf<sup>2</sup>

**CONHBuEIM] [NTf2**

], and

**]**

] were calculated

] are also fitted in the temperature range

.

], [PCNMIM][NTf<sup>2</sup>

**] [CH2**

**] [EOHMIM] [NTf2**

CONHBuEIM][NTf<sup>2</sup>

],

**Figure 8.** Electrical conductivity vs. temperature plots for FILs [MCNMIM][NTf<sup>2</sup> ], [PCNMIM][NTf<sup>2</sup> ], [EOHMIM][NTf<sup>2</sup> ], and [CH<sup>2</sup> CONHBuEIM][NTf<sup>2</sup> ].

The best fitted values of *σ*<sup>0</sup> , *B, T*<sup>0</sup> , and the correlation coefficient, *R*, are listed in **Table 9** from the empirical Eq. (14). From **Table 9**, the obtained correlation coefficient, *R*, is better than 0.9999. The results indicate that the measurement electrical conductivity of the FILs [MCNMIM] [NTf<sup>2</sup> ], [PCNMIM][NTf<sup>2</sup> ], [EOHMIM][NTf<sup>2</sup> ], and [CH<sup>2</sup> CONHBuEIM][NTf<sup>2</sup> ] can be fitted by the VFT equation.

In **Figure 9**, the 1000/*T* dependences on ln *σ* of the four FILs [MCNMIM][NTf<sup>2</sup>

(see **Table 9**). So, the measurement electrical conductivity of [MCNMIM][NTf<sup>2</sup>

The activation energies of electrical conductivity for four FILs [MCNMIM][NTf<sup>2</sup>

], and [CH<sup>2</sup>

CONHBuEIM][NTf<sup>2</sup>

CONHBuEIM][NTf<sup>2</sup>

CONHBuEIM][NTf<sup>2</sup>

According to Eq. (17), the log*Λ* dependences on log*η*−1 are illustrated in **Figure 10** for the four

], [EOHMIM][NTf<sup>2</sup>

Eq. (15). From **Figure 9**, it can be obviously seen that the measurement points lie far away

The values of the correlation coefficient, *R*, are 0.9878, 0.9908, 0.9905, and, 0.9930, respectively. The same results can be obtained that the values are much lower than the correlation coefficient values, *R* = 0.99997, 0.99998, 0.99998, and 0.99999, which obtained by the VFT equation

], and [CH<sup>2</sup>

], and [CH<sup>2</sup>

], and [CH<sup>2</sup>

ing to Eq. (17), and the values are 73, 71, 69, and 63, respectively.

], [PCNMIM][NTf<sup>2</sup>

**Figure 10.** Plot of lg*Λ* vs. lg*η*−1 for the four FILs [MCNMIM][NTf<sup>2</sup>

[NTf<sup>2</sup>

[NTf<sup>2</sup>

[NTf<sup>2</sup>

[CH<sup>2</sup>

CONHBuEIM][NTf<sup>2</sup>

], [EOHMIM][NTf<sup>2</sup>

], [EOHMIM][NTf<sup>2</sup>

from the fitted straight lines.

], [EOHMIM][NTf<sup>2</sup>

At 298.15 K, the Walden's product (in [S·cm<sup>2</sup>

], [EOHMIM][NTf<sup>2</sup>

are listed in **Table 9**.

[PCNMIM][NTf<sup>2</sup>

FILs [MCNMIM][NTf<sup>2</sup>

from 283.15 to 353.15 K.

], [PCNMIM]

387

], [PCNMIM]

], [PCNMIM]

],

]

] are fitted in the temperature range.

Thermodynamic Properties of Ionic Liquids http://dx.doi.org/10.5772/65792

] does not well follow the Arrhenius

] were also calculated by Eq. (16) and

] can be determined accord-

CONHBuEIM][NTf<sup>2</sup>

·mol−1][mP·s]) for the four FILs [MCNMIM][NTf<sup>2</sup>

], and [CH<sup>2</sup>

], [PCNMIM][NTf<sup>2</sup>

] from 283.15 to 353.15 K. The solid straight line is the ideal line for aqueous KCl solutions.

], [EOHMIM][NTf<sup>2</sup>

], and

CONHBuEIM][NTf<sup>2</sup>

According to Eq. (15), the 1000/*T* dependence on ln *σ* was plotted of four FILs [MCNMIM] [NTf<sup>2</sup> ], [PCNMIM][NTf<sup>2</sup> ], [EOHMIM][NTf<sup>2</sup> ], and [CH<sup>2</sup> CONHBuEIM][NTf<sup>2</sup> ] (see **Figure 9**).


**Table 9.** Fitted parameter values of *σ*<sup>0</sup> , *B, T*<sup>0</sup> , correlation coefficient, *R* and *E<sup>σ</sup>* .

**Figure 9.** Plot of ln *σ* and ln*σ* vs. 1000/*T* for FILs [MCNMIM][NTf<sup>2</sup> ], [PCNMIM][NTf<sup>2</sup> ], [EOHMIM][NTf<sup>2</sup> ], and [CH<sup>2</sup> CONHBuEIM][NTf<sup>2</sup> ].

In **Figure 9**, the 1000/*T* dependences on ln *σ* of the four FILs [MCNMIM][NTf<sup>2</sup> ], [PCNMIM] [NTf<sup>2</sup> ], [EOHMIM][NTf<sup>2</sup> ], and [CH<sup>2</sup> CONHBuEIM][NTf<sup>2</sup> ] are fitted in the temperature range. The values of the correlation coefficient, *R*, are 0.9878, 0.9908, 0.9905, and, 0.9930, respectively. The same results can be obtained that the values are much lower than the correlation coefficient values, *R* = 0.99997, 0.99998, 0.99998, and 0.99999, which obtained by the VFT equation (see **Table 9**). So, the measurement electrical conductivity of [MCNMIM][NTf<sup>2</sup> ], [PCNMIM] [NTf<sup>2</sup> ], [EOHMIM][NTf<sup>2</sup> ], and [CH<sup>2</sup> CONHBuEIM][NTf<sup>2</sup> ] does not well follow the Arrhenius Eq. (15). From **Figure 9**, it can be obviously seen that the measurement points lie far away from the fitted straight lines.

The best fitted values of *σ*<sup>0</sup>

386 Progress and Developments in Ionic Liquids

the VFT equation.

], [PCNMIM][NTf<sup>2</sup>

], [PCNMIM][NTf<sup>2</sup>

**Property [MCNMIM] [NTf2**

**Table 9.** Fitted parameter values of *σ*<sup>0</sup>

[NTf<sup>2</sup>

[NTf<sup>2</sup>

*σ*0

103 *Eσ*

*T*0

[CH<sup>2</sup>

CONHBuEIM][NTf<sup>2</sup>

].

, *B, T*<sup>0</sup>

], [EOHMIM][NTf<sup>2</sup>

], [EOHMIM][NTf<sup>2</sup>

**] [PCNMIM] [NTf2**

/eV 53.6 57.1 47.6 74.3

/K 203.0 189.3 192.0 192.9 *R* 0.99997 0.99997 0.99998 0.99999

, *B, T*<sup>0</sup>

**Figure 9.** Plot of ln *σ* and ln*σ* vs. 1000/*T* for FILs [MCNMIM][NTf<sup>2</sup>

/(S⋅cm−1) 0.64 0.49 0.56 0.82 *B*/K 621.3 661.8 551.9 860.5

, and the correlation coefficient, *R*, are listed in **Table 9** from the

CONHBuEIM][NTf<sup>2</sup>

CONHBuEIM][NTf<sup>2</sup>

**] [CH2**

] can be fitted by

] (see **Figure 9**).

**CONHBuEIM] [NTf2**

**]**

empirical Eq. (14). From **Table 9**, the obtained correlation coefficient, *R*, is better than 0.9999. The results indicate that the measurement electrical conductivity of the FILs [MCNMIM]

According to Eq. (15), the 1000/*T* dependence on ln *σ* was plotted of four FILs [MCNMIM]

, correlation coefficient, *R* and *E<sup>σ</sup>*

], and [CH<sup>2</sup>

], and [CH<sup>2</sup>

**] [EOHMIM] [NTf2**

.

], [PCNMIM][NTf<sup>2</sup>

], [EOHMIM][NTf<sup>2</sup>

], and

The activation energies of electrical conductivity for four FILs [MCNMIM][NTf<sup>2</sup> ], [PCNMIM] [NTf<sup>2</sup> ], [EOHMIM][NTf<sup>2</sup> ], and [CH<sup>2</sup> CONHBuEIM][NTf<sup>2</sup> ] were also calculated by Eq. (16) and are listed in **Table 9**.

At 298.15 K, the Walden's product (in [S·cm<sup>2</sup> ·mol−1][mP·s]) for the four FILs [MCNMIM][NTf<sup>2</sup> ], [PCNMIM][NTf<sup>2</sup> ], [EOHMIM][NTf<sup>2</sup> ], and [CH<sup>2</sup> CONHBuEIM][NTf<sup>2</sup> ] can be determined according to Eq. (17), and the values are 73, 71, 69, and 63, respectively.

According to Eq. (17), the log*Λ* dependences on log*η*−1 are illustrated in **Figure 10** for the four FILs [MCNMIM][NTf<sup>2</sup> ], [PCNMIM][NTf<sup>2</sup> ], [EOHMIM][NTf<sup>2</sup> ], and [CH<sup>2</sup> CONHBuEIM][NTf<sup>2</sup> ] from 283.15 to 353.15 K.

**Figure 10.** Plot of lg*Λ* vs. lg*η*−1 for the four FILs [MCNMIM][NTf<sup>2</sup> ], [PCNMIM][NTf<sup>2</sup> ], [EOHMIM][NTf<sup>2</sup> ], and [CH<sup>2</sup> CONHBuEIM][NTf<sup>2</sup> ] from 283.15 to 353.15 K. The solid straight line is the ideal line for aqueous KCl solutions.

Usually, the Walden rule can be used for the presentation of the independent ions of the liquid. If the Walden points close to the ideal line, the liquid can be considered as a relative ideal liquid. The ideal line position was determined according to the aqueous KCl solution at high dilution. As an ideal line, the slop of the ideal line should be unity and not have any interaction of the ions [10, 29, 30]. In **Figure 10**, it can be seen that the approximately straight lines can be obtained according to the experimental points. The results indicate that the FILs [MCNMIM][NTf<sup>2</sup> ], [PCNMIM][NTf<sup>2</sup> ], [EOHMIM][NTf<sup>2</sup> ], and [CH<sup>2</sup> CONHBuEIM][NTf<sup>2</sup> ] follow the Walden rule to some extent. The slopes of the lines for the four FILs [MCNMIM] [NTf<sup>2</sup> ], [PCNMIM][NTf<sup>2</sup> ], [EOHMIM][NTf<sup>2</sup> ], and [CH<sup>2</sup> CONHBuEIM][NTf<sup>2</sup> ] are 0.941, 0.927, 0.939, and 0.913, respectively. The lines for the two FILs below are close to the ideal KCl line, as shown in **Figure 10**. Most of the reported traditional ILs [9, 10, 29, 31] and our previous studied ILs [24–27, 32, 33] have the same trend. From the result, the FILs [MCNMIM][NTf<sup>2</sup> ], [PCNMIM][NTf<sup>2</sup> ], [EOHMIM][NTf<sup>2</sup> ], and [CH<sup>2</sup> CONHBuEIM][NTf<sup>2</sup> ] can be named "subionic" [34].

enough [41]. The systematical research on the properties including density, dynamic viscosity, and electrical conductivity is still scarce which can provide the well information of the

In this section, the basic physicochemical properties of three serious Ils N-alkylpyridinium

was introduced on positions 3 and 4 of the pyridinium ring, respectively. The basic physicochemical properties, including density, dynamic viscosity, and electrical conductivity, were measured by the traditional methods. The other physicochemical properties, including molecular volume, standard molar entropy, lattice energy, were estimated in terms of empirical and semiempirical equations on the basis of the experimental value. The effect of the methylene and methyl groups

] (*n* = 2, 3, 4, 5, 6)}, N-alkyl-3-methylpyridinium

3Mpy][NTf<sup>2</sup>

] (*n* = 3, 4, 6)}, and N-alkyl-4-methylpyridinium

Thermodynamic Properties of Ionic Liquids http://dx.doi.org/10.5772/65792

] (*n* = 2, 4, 6)} were discussed. The methyl group

] (*n* = 3, 4, 6), and [C*<sup>n</sup>*

4Mpy]

389

py][NTf<sup>2</sup>

3Mpy][NTf<sup>2</sup>

] (*n* = 2, 3, 4, 5, 6), [C*<sup>n</sup>*

4Mpy][NTf<sup>2</sup>

suitable IL for a specific purpose.

bis(trifluoromethylsulfonyl)imide {[C*<sup>n</sup>*

bis(trifluoromethyl-sulfonyl)imide {[C*<sup>n</sup>*

bis(trifluoromethylsulfonyl)imide {[C*<sup>n</sup>*

on the properties is discussed at 298.15 K.

] (*n* = 2, 4, 6) are shown in **Figure 11**.

py][NTf<sup>2</sup>

The structures of ILs [C*<sup>n</sup>*

**Figure 11.** The structure of ILs [C*<sup>n</sup>*

py][NTf<sup>2</sup>

] (*n* = 2, 3, 4, 5, 6), [C*<sup>n</sup>*

3Mpy][NTf<sup>2</sup>

] (n = 3, 4, 6), and [C*<sup>n</sup>*

4Mpy][NTf<sup>2</sup>

] (*n* = 2, 4, 6).

[NTf<sup>2</sup>

#### **Conclusion**

The density, dynamic viscosity, and electrical conductivity of the FILs [MCNMIM][NTf<sup>2</sup> ], [PCNMIM][NTf<sup>2</sup> ], [EOHMIM][NTf<sup>2</sup> ], and [CH<sup>2</sup> CONHBuEIM][NTf<sup>2</sup> ] were measured at the temperature from 283 to 353 K. The others thermodynamic properties of the FILs, like thermal expansion coefficient, molecular volume, standard molar entropy, and lattice energy, were estimated according to the classical empirical equations. The introduction of the methylene group on the −CN (electron-withdrawing group) type series FILs leads to a different change in the dynamic viscosity, and electrical conductivity with the traditional ILs. The dynamic viscosity values of FIL [PCNMIM][NTf<sup>2</sup> ] are lower than FIL [MCNMIM][NTf<sup>2</sup> ] and the electrical conductivity values of FIL [PCNMIM][NTf<sup>2</sup> ] are higher than FIL [MCNMIM] [NTf<sup>2</sup> ] in the temperature range. The temperature dependences on the dynamic viscosity and electrical conductivity values of the ILs can be satisfactorily fitted by the VFT equation. However, the experimental values do not follow the Arrhenius behavior described by the Arrhenius equation.

### **8. Density, dynamic viscosity, and electrical conductivity of pyridinium-based hydrophobic ionic liquids**

Actually, most of the studied ILs are hydrophilic-type ILs. The hydrophobic ILs have been paid much more attention in many fields as a special functional ILs. The bis(trifluoromethylsulfonyl)imide [NTf<sup>2</sup> ]− as an air- and water-stable anion has been applied in many fields [35–37]. These types of anion ILs have exhibited a relatively wide liquid range, higher electrical conductivity, and thermal stability than the hydrophilic-type ILs. However, the study of thermodynamic properties of the [NTf<sup>2</sup> ]-type ILs mainly focuses on the imidazolium-type cation ILs [38–40]. The study of the pyridinium type cation-based ILs is still not enough [41]. The systematical research on the properties including density, dynamic viscosity, and electrical conductivity is still scarce which can provide the well information of the suitable IL for a specific purpose.

Usually, the Walden rule can be used for the presentation of the independent ions of the liquid. If the Walden points close to the ideal line, the liquid can be considered as a relative ideal liquid. The ideal line position was determined according to the aqueous KCl solution at high dilution. As an ideal line, the slop of the ideal line should be unity and not have any interaction of the ions [10, 29, 30]. In **Figure 10**, it can be seen that the approximately straight lines can be obtained according to the experimental points. The results indicate that the FILs

], [EOHMIM][NTf<sup>2</sup>

low the Walden rule to some extent. The slopes of the lines for the four FILs [MCNMIM]

0.939, and 0.913, respectively. The lines for the two FILs below are close to the ideal KCl line, as shown in **Figure 10**. Most of the reported traditional ILs [9, 10, 29, 31] and our previous studied ILs [24–27, 32, 33] have the same trend. From the result, the FILs [MCNMIM][NTf<sup>2</sup>

The density, dynamic viscosity, and electrical conductivity of the FILs [MCNMIM][NTf<sup>2</sup>

temperature from 283 to 353 K. The others thermodynamic properties of the FILs, like thermal expansion coefficient, molecular volume, standard molar entropy, and lattice energy, were estimated according to the classical empirical equations. The introduction of the methylene group on the −CN (electron-withdrawing group) type series FILs leads to a different change in the dynamic viscosity, and electrical conductivity with the traditional ILs. The

] in the temperature range. The temperature dependences on the dynamic viscosity and electrical conductivity values of the ILs can be satisfactorily fitted by the VFT equation. However, the experimental values do not follow the Arrhenius behavior described by the

Actually, most of the studied ILs are hydrophilic-type ILs. The hydrophobic ILs have been paid much more attention in many fields as a special functional ILs. The

in many fields [35–37]. These types of anion ILs have exhibited a relatively wide liquid range, higher electrical conductivity, and thermal stability than the hydrophilic-type ILs. However,

olium-type cation ILs [38–40]. The study of the pyridinium type cation-based ILs is still not

]−

], and [CH<sup>2</sup>

], and [CH<sup>2</sup>

], and [CH<sup>2</sup>

], and [CH<sup>2</sup>

CONHBuEIM][NTf<sup>2</sup>

CONHBuEIM][NTf<sup>2</sup>

CONHBuEIM][NTf<sup>2</sup>

] are lower than FIL [MCNMIM][NTf<sup>2</sup>

as an air- and water-stable anion has been applied

]-type ILs mainly focuses on the imidaz-

] are higher than FIL [MCNMIM]

CONHBuEIM][NTf<sup>2</sup>

] fol-

],

],

] and

] are 0.941, 0.927,

] can be named "subi-

] were measured at the

[MCNMIM][NTf<sup>2</sup>

[PCNMIM][NTf<sup>2</sup>

[PCNMIM][NTf<sup>2</sup>

Arrhenius equation.

onic" [34].

**Conclusion**

[NTf<sup>2</sup>

], [PCNMIM][NTf<sup>2</sup>

388 Progress and Developments in Ionic Liquids

[NTf<sup>2</sup>

], [PCNMIM][NTf<sup>2</sup>

], [EOHMIM][NTf<sup>2</sup>

], [EOHMIM][NTf<sup>2</sup>

dynamic viscosity values of FIL [PCNMIM][NTf<sup>2</sup>

the electrical conductivity values of FIL [PCNMIM][NTf<sup>2</sup>

**pyridinium-based hydrophobic ionic liquids**

the study of thermodynamic properties of the [NTf<sup>2</sup>

bis(trifluoromethylsulfonyl)imide [NTf<sup>2</sup>

**8. Density, dynamic viscosity, and electrical conductivity of** 

], [EOHMIM][NTf<sup>2</sup>

In this section, the basic physicochemical properties of three serious Ils N-alkylpyridinium bis(trifluoromethylsulfonyl)imide {[C*<sup>n</sup>* py][NTf<sup>2</sup> ] (*n* = 2, 3, 4, 5, 6)}, N-alkyl-3-methylpyridinium bis(trifluoromethyl-sulfonyl)imide {[C*<sup>n</sup>* 3Mpy][NTf<sup>2</sup> ] (*n* = 3, 4, 6)}, and N-alkyl-4-methylpyridinium bis(trifluoromethylsulfonyl)imide {[C*<sup>n</sup>* 4Mpy][NTf<sup>2</sup> ] (*n* = 2, 4, 6)} were discussed. The methyl group was introduced on positions 3 and 4 of the pyridinium ring, respectively. The basic physicochemical properties, including density, dynamic viscosity, and electrical conductivity, were measured by the traditional methods. The other physicochemical properties, including molecular volume, standard molar entropy, lattice energy, were estimated in terms of empirical and semiempirical equations on the basis of the experimental value. The effect of the methylene and methyl groups on the properties is discussed at 298.15 K.

The structures of ILs [C*<sup>n</sup>* py][NTf<sup>2</sup> ] (*n* = 2, 3, 4, 5, 6), [C*<sup>n</sup>* 3Mpy][NTf<sup>2</sup> ] (*n* = 3, 4, 6), and [C*<sup>n</sup>* 4Mpy] [NTf<sup>2</sup> ] (*n* = 2, 4, 6) are shown in **Figure 11**.

**Figure 11.** The structure of ILs [C*<sup>n</sup>* py][NTf<sup>2</sup> ] (*n* = 2, 3, 4, 5, 6), [C*<sup>n</sup>* 3Mpy][NTf<sup>2</sup> ] (n = 3, 4, 6), and [C*<sup>n</sup>* 4Mpy][NTf<sup>2</sup> ] (*n* = 2, 4, 6).

#### **8.1. N-Alkyl type pyridinium type ionic liquids**

The results of the density, surface tension, dynamic viscosity, and electrical conductivity of the ILs [C*<sup>n</sup>* py][NTf<sup>2</sup> ] (*n* = 2, 3, 4, 5, 6) are listed in **Tables 10**–**13** [4, 31, 33].


**Table 10.** Experimental values of density of ILs [C*<sup>n</sup>* py][NTf<sup>2</sup> ] (*n* = 2, 3, 4, 5, 6).


From **Tables 10**–**13**, it can be concluded that the density and electrical conductivity decrease as the alkyl side chain length of the cation increases for the N-alkyl type pyridinium ILs. The dynamic viscosity increases with the extension of the alkyl side chain of the cation for the three

py][NTf<sup>2</sup>

] (*n* = 2, 3, 4, 5, 6).

According to Eqs. (1)–(10), the thermodynamic properties are calculated and listed in **Table 14**,

series of N-alkyl type pyridinium ILs.

**Table 13.** Experimental values of electrical conductivity of ILs [C*<sup>n</sup>*

**[C2 py][NTf2**

**] [C3**

**Table 12.** Experimental values of dynamic viscosity of ILs [C*<sup>n</sup>*

**] [C3**

**py][NTf2**

283.15 1.67 1.04 0.76 288.15 1.91 1.37 1.00 293.15 5.01 2.50 1.77 1.30 298.15 5.99 3.21 2.22 1.66 303.15 7.06 3.95 2.75 2.08 308.15 8.24 4.73 3.36 2.57 313.15 9.53 7.19 5.63 4.03 3.11 318.15 10.90 8.30 6.70 4.81 3.87 323.15 12.33 9.55 7.79 5.63 4.46 328.15 14.28 10.83 8.96 6.53 5.38 333.15 16.09 12.23 10.19 7.42 6.21 338.15 17.84 13.65 11.47 8.24 7.12

**] [C4**

**[C2 py][NTf2** **py][NTf2**

298.15 39.4 58.3 71.9 84.5 303.15 32.5 46.7 57.1 66.4 308.15 27.1 33.0 38.0 45.6 53.2 313.15 23.2 27.6 31.4 37.2 43.1 318.15 20.0 23.3 26.4 30.6 35.2 323.15 17.3 19.8 22.4 25.6 29.1 328.15 15.2 17.1 19.2 21.7 24.5 333.15 13.5 14.9 16.6 18.4 20.9 338.15 11.9 13.5 14.4 15.8 17.9

**] [C4**

**py][NTf2**

py][NTf<sup>2</sup>

**py][NTf2**

**] [C5**

] (*n* = 2, 3, 4, 5, 6).

**] [C5**

**py][NTf2**

**] [C6**

**py][NTf2 ]**

**py][NTf2**

**] [C6**

Thermodynamic Properties of Ionic Liquids http://dx.doi.org/10.5772/65792

> **py][NTf2 ]**

391

respectively.

**Table 11.** Experimental values of surface tension of ILs [C*<sup>n</sup>* py][NTf<sup>2</sup> ] (*n* = 2, 3, 4, 5, 6).


**Table 12.** Experimental values of dynamic viscosity of ILs [C*<sup>n</sup>* py][NTf<sup>2</sup> ] (*n* = 2, 3, 4, 5, 6).

**8.1. N-Alkyl type pyridinium type ionic liquids**

**] [C3**

ILs [C*<sup>n</sup>*

py][NTf<sup>2</sup>

390 Progress and Developments in Ionic Liquids

**[C2 py][NTf2**

**Table 10.** Experimental values of density of ILs [C*<sup>n</sup>*

**Table 11.** Experimental values of surface tension of ILs [C*<sup>n</sup>*

**] [C3**

**py][NTf2**

283.15 33.1 32.5 288.15 37.7 32.8 32.2 293.15 37.6 32.7 32.0 298.15 37.4 33.4 32.5 31.7 303.15 37.1 33.2 32.2 31.6 308.15 36.9 32.9 32.0 31.4 313.15 36.7 32.8 31.8 31.2 318.15 36.6 32.4 31.5 31.0 323.15 36.4 32.1 31.3 30.8 328.15 36.1 32.0 31.1 30.6 333.15 35.9 31.8 30.8 30.3 338.15 35.6 31.5 30.5 30.1

**[C2 py][NTf2**

The results of the density, surface tension, dynamic viscosity, and electrical conductivity of the

**] [C4**

**py][NTf2**

**] [C5**

**py][NTf2**

**] [C6**

**py][NTf2 ]**

] (*n* = 2, 3, 4, 5, 6) are listed in **Tables 10**–**13** [4, 31, 33].

283.15 1.4331 1.4008 288.15 1.5457 1.4296 1.3966 293.15 1.5414 1.4259 1.3923 298.15 1.5375 1.4547 1.4214 1.3877 303.15 1.5332 1.4506 1.4169 1.3831 308.15 1.5291 1.4845 1.4462 1.4128 1.3789 313.15 1.5249 1.4800 1.4417 1.4083 1.3744 318.15 1.5205 1.4757 1.4372 1.4038 1.3699 323.15 1.5164 1.4710 1.4332 1.3989 1.3655 328.15 1.5122 1.4667 1.4291 1.3942 1.3615 333.15 1.5078 1.4623 1.4245 1.3893 1.3569 338.15 1.5037 1.4574 1.4205 1.3851 1.3525

py][NTf<sup>2</sup>

**] [C4**

] (*n* = 2, 3, 4, 5, 6).

**py][NTf2**

py][NTf<sup>2</sup>

] (*n* = 2, 3, 4, 5, 6).

**] [C5**

**py][NTf2**

**] [C6**

**py][NTf2 ]**

**py][NTf2**


**Table 13.** Experimental values of electrical conductivity of ILs [C*<sup>n</sup>* py][NTf<sup>2</sup> ] (*n* = 2, 3, 4, 5, 6).

From **Tables 10**–**13**, it can be concluded that the density and electrical conductivity decrease as the alkyl side chain length of the cation increases for the N-alkyl type pyridinium ILs. The dynamic viscosity increases with the extension of the alkyl side chain of the cation for the three series of N-alkyl type pyridinium ILs.

According to Eqs. (1)–(10), the thermodynamic properties are calculated and listed in **Table 14**, respectively.


**Table 14.** Estimated values of physicochemical properties of [C*<sup>n</sup>* py][NTf<sup>2</sup> ] at 298.15 K.

From **Table 14**, the thermal expansion coefficients are 5.63 × 10−4, 5.99 × 10−4, 6.30 × 10−4, and 6.40 × 10−4 K−1 for [C<sup>2</sup> py][NTf<sup>2</sup> ], [C4 py][NTf<sup>2</sup> ], [C5 py][NTf<sup>2</sup> ], and [C6 py][NTf<sup>2</sup> ], respectively. The values are in good agreement in the range of 5 × 10−4 to 7 × 10−4 K−1 obtained by Jacquemin et al. [40]. From **Table 14**, compared with the fused salts and organic liquids, for example, *E*<sup>a</sup> = 146 mJ∙m−2 for NaNO3 , *E*<sup>a</sup> = 67 mJ∙m−2 for benzene and *E*<sup>a</sup> = 51.1 mJ∙m−2 for octane. The values of surface energy of the samples are close to the organic liquids. This fact shows that the interaction energy between ions in the samples is less than fused salts. The values of the molecular volume are 0.4196 nm3 for [C<sup>2</sup> py][NTf<sup>2</sup> ], 0.4754 nm3 for [C4 py][NTf<sup>2</sup> ], 0.5030 nm3 for [C5 py][NTf<sup>2</sup> ], and 0.5320 nm3 for [C6 py][NTf<sup>2</sup> ] at 298.15 K. The plotting *V*m against the number of the carbons, *n*, in the alkyl chain of the samples ([C*<sup>n</sup>* py][NTf<sup>2</sup> ]) can be described (see **Figure 4**). The average contribution of the methylene to the molecular volume can be obtained by the slop of the fitting equation. The value is 0.0280 nm3 at 298.15 K. The value is close to the values of 0.0272 nm3 for imidazolium-type ILs [C*<sup>n</sup>* mim][BF4 ] and 0.0282 nm3 for imidazolium-type [C*<sup>n</sup>* mim][NTf<sup>2</sup> ].

According to Eq. (5), by plotting the *γV*<sup>m</sup>

**Table 15.** Estimated volumetric properties of the ILs [C*<sup>n</sup>*

reported that the normal boiling point, *T*<sup>b</sup>

py][NTf<sup>2</sup>

fractions, ∑*v/V*, are in the range of 11−13% for the ILs [C*<sup>n</sup>*

thermal expansion coefficient of pyridinium-based ILs.

py][NTf<sup>2</sup>

liquids and fused salts in terms of the value of the *k*.

boiling point, *T*<sup>b</sup>

[C<sup>2</sup> py][NTf<sup>2</sup>

[C4 py][NTf<sup>2</sup>

[C5 py][NTf<sup>2</sup>

[C6 py][NTf<sup>2</sup>

It indicated that ILs [C*<sup>n</sup>*

liquid state. For the ILs [C*<sup>n</sup>*

From the plot, the value of empirical constant (*k*) and critical temperature (*T*<sup>c</sup>

**Figure 12.** Plot of *V*m vs. *n* at 298.15 K, the correlation coefficient is *R* = 0.9999 for ILs [C*<sup>n</sup>*

*ρ***/g**⋅**cm−3** *V***m/nm3** *M***<sup>+</sup>** *V***<sup>+</sup>**

] 1.5375 0.4196 108.16 0.2164 0.372

] 1.4547 0.4754 136.22 0.2722 0.402

] 1.4214 0.5030 150.24 0.2998 0.415

] 1.3877 0.5320 164.27 0.3288 0.428

py][NTf<sup>2</sup>

] *n* = 2, 4, 5, 6 at 298.15 K.

according to the fitting equation and the values are listed in **Table 16**. Rebelo et al. [44] have

of organic liquids, *k* ≈ 2.1 × 10−7 J∙K−1, but for fused salts, *k* = 0.4 × 10−7 J∙K−1 for fused NaCl [7].

From **Table 14**, the values of estimation of the thermal expansion coefficient are in good agreement with experimental values. It also can be seen that the estimation values of interstice

majority materials, the volume expansions are in the range of 10−15% from the solid state to

the reported values. Therefore, the interstice model theory can be used for calculation of the

, is approximately 0.6*T*<sup>c</sup>

, can be calculated and the values are also listed in **Table 16**. For the majority

2/3 against *T*, straight lines were obtained (see **Figure 13**).

py][NTf<sup>2</sup>

**/nm3** *r***<sup>+</sup>**

**/nm**

393

Thermodynamic Properties of Ionic Liquids http://dx.doi.org/10.5772/65792

] (*n* = 2, 4, 5, 6) have the medium polarity between organic

py][NTf<sup>2</sup>

] (*n* = 2, 4, 5, 6), the values are in good agreement with

) can be obtained

for ILs. Herein, the normal

] (*n* = 2, 4, 5, 6).

] (*n* = 2, 4, 5, 6). For the

The molar masses of cations of the ILs [C*<sup>n</sup>* py][NTf<sup>2</sup> ] (*n* = 2, 4, 5, 6) are listed in **Table 15**. The plotting *V*m against the molar mass of cations of the samples ([C*<sup>n</sup>* py][NTf<sup>2</sup> ]) can be obtained (see **Figure 12**). The intercept of the linear regression can be approximately regarded the volume of the anion, NTf<sup>2</sup> − , the value is 0.2032 nm3 . The volume value of NTf<sup>2</sup> − is much higher than 0.1390 nm3 for AlCl4 − [42] and 0.1548 nm3 GaCl4 − [43]. The volume values of the cations and the radii are listed in **Table 15**.


**Table 15.** Estimated volumetric properties of the ILs [C*<sup>n</sup>* py][NTf<sup>2</sup> ] *n* = 2, 4, 5, 6 at 298.15 K.

**Figure 12.** Plot of *V*m vs. *n* at 298.15 K, the correlation coefficient is *R* = 0.9999 for ILs [C*<sup>n</sup>* py][NTf<sup>2</sup> ] (*n* = 2, 4, 5, 6).

From **Table 14**, the thermal expansion coefficients are 5.63 × 10−4, 5.99 × 10−4, 6.30 × 10−4, and

values are in good agreement in the range of 5 × 10−4 to 7 × 10−4 K−1 obtained by Jacquemin et al. [40]. From **Table 14**, compared with the fused salts and organic liquids, for example, *E*<sup>a</sup>

face energy of the samples are close to the organic liquids. This fact shows that the interaction energy between ions in the samples is less than fused salts. The values of the molecular vol-

contribution of the methylene to the molecular volume can be obtained by the slop of the fit-

] and 0.0282 nm3

py][NTf<sup>2</sup>

(see **Figure 12**). The intercept of the linear regression can be approximately regarded the vol-

 GaCl4 −

], 0.4754 nm3

py][NTf<sup>2</sup>

py][NTf<sup>2</sup>

py][NTf<sup>2</sup>

for [C4

], and [C6

] at 298.15 K.

py][NTf<sup>2</sup>

] at 298.15 K. The plotting *V*m against the number of the carbons,

. The volume value of NTf<sup>2</sup>

at 298.15 K. The value is close to the values of 0.0272 nm3

for imidazolium-type [C*<sup>n</sup>*

py][NTf<sup>2</sup>

], 0.5030 nm3

]) can be described (see **Figure 4**). The average

] (*n* = 2, 4, 5, 6) are listed in **Table 15**. The

−

[43]. The volume values of the cations

py][NTf<sup>2</sup>

= 51.1 mJ∙m−2 for octane. The values of sur-

], respectively. The

for [C5

= 146

],

].

py][NTf<sup>2</sup>

mim][NTf<sup>2</sup>

]) can be obtained

is much higher

], [C5

6.40 × 10−4 K−1 for [C<sup>2</sup>

**Property [C2**

392 Progress and Developments in Ionic Liquids

103 *S*a

*E*a

*S*0

107

*T*c

*T*b

Δl g *H*<sup>m</sup> 0

10<sup>2</sup>

104

104

**py][NTf2**

**] [C4**

**py][NTf2**

/ mJ⋅K−1⋅m−2 41.6 47.9 46.2 41.8

/ mJ⋅m−2 49.8 47.7 46.3 44.1 *V*m/nm3 0.4196 0.4754 0.5030 0.5320

/J⋅K−1⋅mol−1 552.5 622.1 656.4 692.6

*k*/J∙K−1 1.131 1.590 1.503 1.316

/K 1618 1212 1271 1429

/K 970 727 763 857 *U*pot/kJ⋅mol−1 417 404 399 393 *V*/cm−3⋅mol−1 252.6 286.2 302.8 320.2 *p* 625.0 688.9 723.5 759.9

/kJ⋅mol−1 143.8 139.1 140.9 142.9 1024 *v*/cm3 24.80 29.38 30.67 31.77 ∑*v*/cm3 29.86 35.38 36.92 38.25

∑*v/V* 11.82 12.36 12.19 11.95

*α*/K−1 (exp.) 5.63 5.99 6.30 6.40

*α*/K−1 (cal.) 5.95 6.22 6.13 6.01

**] [C5**

**py][NTf2**

**] [C6**

**py][NTf2 ]**

mJ∙m−2 for NaNO3

ume are 0.4196 nm3

and 0.5320 nm3

py][NTf<sup>2</sup>

**Table 14.** Estimated values of physicochemical properties of [C*<sup>n</sup>*

, *E*<sup>a</sup>

for [C6

*n*, in the alkyl chain of the samples ([C*<sup>n</sup>*

ting equation. The value is 0.0280 nm3

The molar masses of cations of the ILs [C*<sup>n</sup>*

for AlCl4

and the radii are listed in **Table 15**.

−

−

for imidazolium-type ILs [C*<sup>n</sup>*

ume of the anion, NTf<sup>2</sup>

than 0.1390 nm3

for [C<sup>2</sup>

py][NTf<sup>2</sup>

], [C4

py][NTf<sup>2</sup>

mim][BF4

plotting *V*m against the molar mass of cations of the samples ([C*<sup>n</sup>*

, the value is 0.2032 nm3

[42] and 0.1548 nm3

py][NTf<sup>2</sup>

= 67 mJ∙m−2 for benzene and *E*<sup>a</sup>

According to Eq. (5), by plotting the *γV*<sup>m</sup> 2/3 against *T*, straight lines were obtained (see **Figure 13**). From the plot, the value of empirical constant (*k*) and critical temperature (*T*<sup>c</sup> ) can be obtained according to the fitting equation and the values are listed in **Table 16**. Rebelo et al. [44] have reported that the normal boiling point, *T*<sup>b</sup> , is approximately 0.6*T*<sup>c</sup> for ILs. Herein, the normal boiling point, *T*<sup>b</sup> , can be calculated and the values are also listed in **Table 16**. For the majority of organic liquids, *k* ≈ 2.1 × 10−7 J∙K−1, but for fused salts, *k* = 0.4 × 10−7 J∙K−1 for fused NaCl [7]. It indicated that ILs [C*<sup>n</sup>* py][NTf<sup>2</sup> ] (*n* = 2, 4, 5, 6) have the medium polarity between organic liquids and fused salts in terms of the value of the *k*.

From **Table 14**, the values of estimation of the thermal expansion coefficient are in good agreement with experimental values. It also can be seen that the estimation values of interstice fractions, ∑*v/V*, are in the range of 11−13% for the ILs [C*<sup>n</sup>* py][NTf<sup>2</sup> ] (*n* = 2, 4, 5, 6). For the majority materials, the volume expansions are in the range of 10−15% from the solid state to liquid state. For the ILs [C*<sup>n</sup>* py][NTf<sup>2</sup> ] (*n* = 2, 4, 5, 6), the values are in good agreement with the reported values. Therefore, the interstice model theory can be used for calculation of the thermal expansion coefficient of pyridinium-based ILs.

0.9977, 0.9984, and 0.9983, respectively. The values are obviously lower than the values (all of the values are 0.9999) which obtained by the empirical VFT equation. So, the measure-

(12). From **Figure 15**, it can also be obviously seen that the measurement points lie far away

According to **Table 13** and Eq. (14), the temperature dependences of electrical conductivity

listed in **Table 18**. From **Table 18**, the obtained values of the correlation coefficient, *R*, are 0.9996, which indicates that the VFT equation can be used for fitting the experimental electri-

, and the corresponding correlation coefficient, *R*, are

] (*n* = 2, 3, 4, 5, 6).

**py][NTf2 ]**

values of the ILs can also be fitted using the VFT Eq. (14), see **Figure 16**.

, *B, E<sup>η</sup>* , *T*<sup>0</sup>

, *B, T*<sup>0</sup>

The best-fitting parameters of *σ*<sup>0</sup>

**Table 17.** Fitted values of dynamic viscosity of *η*<sup>0</sup>

cal conductivity.

**Property [C2**

*η***0**

103 *Eη*

*T*0

] have also exhibited the same result.

py][NTf<sup>2</sup>

**] [C6**

, and *R* according to Eqs. (11) and (13).

**py][NTf2**

] (*n* = 2, 3, 4, 5, 6) does not follow the Arrhenius Eq.

], [PCNMIM][NTf<sup>2</sup>

Thermodynamic Properties of Ionic Liquids http://dx.doi.org/10.5772/65792

], [EOHMIM]

395

py][NTf<sup>2</sup>

from the fitted straight lines. The FILs [MCNMIM][NTf<sup>2</sup>

CONHBuEIM][NTf<sup>2</sup>

**Figure 14.** Plot of dynamic viscosity, *η*, vs. temperature, *T*, of ILs [C*<sup>n</sup>*

**] [C4**

**py][NTf2**

**/(mPa**⋅**s)** 0.3907 0.2021 0.0758 0.0645 *B*/K 531.7 695.1 966.2 1036.0

/eV 45.9 60.0 83.4 89.4

/K 182.9 175.4 157.2 153.8 *R* 0.9999 0.9999 0.9999 0.9999

**] [C5**

**py][NTf2**

ment dynamic viscosity of [C*<sup>n</sup>*

], and [CH<sup>2</sup>

[NTf<sup>2</sup>

**Figure 13.** Plot of *γV*<sup>m</sup> 2/3 vs. *T*(K) for ILs[C*<sup>n</sup>* py][NTf<sup>2</sup> ] (*n* = 2, 4, 5, 6).


**Table 16.** The estimated values of *k, T*<sup>b</sup> , and *T*<sup>c</sup> for ILs [C*<sup>n</sup>* py][NTf<sup>2</sup> ] *n* = 2, 4, 5, 6.

According to **Table 12** and Eq. (11), the temperature dependence on dynamic viscosity values of the ILs can also be fitted using the VFT Eq. (11), see **Figure 14**.

The best fitting parameters of *η*<sup>0</sup> , *B, T*<sup>0</sup> , and the corresponding correlation coefficient, *R*, are listed in **Table 17**. From **Table 17**, the obtained values of the correlation coefficient, *R*, are 0.9999, which indicates that the VFT equation can be used for fitting the experimental dynamic viscosity.

According to Eq. (13), the activation energies of dynamic viscosity for [C*<sup>n</sup>* py][NTf<sup>2</sup> ] (*n* = 2, 4, 5, 6) were calculated by Eq. (13) and are listed in **Table 17**.

According to Eq. (12), the 1000/*T* dependence of ln *η* was plotted for [C*<sup>n</sup>* py][NTf<sup>2</sup> ] (*n* = 2, 3, 4, 5, 6) (see **Figure 15**).

The 1000/*T* dependences on ln *η* of the for [C*<sup>n</sup>* py][NTf<sup>2</sup> ] (*n* = 2, 3, 4, 5, 6) were fitted in the temperature range (see **Figure 15**). The values of the correlation coefficient, *R*, are 0.9976, 0.9965, 0.9977, 0.9984, and 0.9983, respectively. The values are obviously lower than the values (all of the values are 0.9999) which obtained by the empirical VFT equation. So, the measurement dynamic viscosity of [C*<sup>n</sup>* py][NTf<sup>2</sup> ] (*n* = 2, 3, 4, 5, 6) does not follow the Arrhenius Eq. (12). From **Figure 15**, it can also be obviously seen that the measurement points lie far away from the fitted straight lines. The FILs [MCNMIM][NTf<sup>2</sup> ], [PCNMIM][NTf<sup>2</sup> ], [EOHMIM] [NTf<sup>2</sup> ], and [CH<sup>2</sup> CONHBuEIM][NTf<sup>2</sup> ] have also exhibited the same result.

**Figure 14.** Plot of dynamic viscosity, *η*, vs. temperature, *T*, of ILs [C*<sup>n</sup>* py][NTf<sup>2</sup> ] (*n* = 2, 3, 4, 5, 6).

According to **Table 12** and Eq. (11), the temperature dependence on dynamic viscosity values of

py][NTf<sup>2</sup>

] (*n* = 2, 4, 5, 6).

**] [C5**

] *n* = 2, 4, 5, 6.

**py][NTf2**

**] [C6**

**py][NTf2 ]**

**py][NTf2**

*K*/J∙K−1 1.131 1.559 1.503 1.316

/K 1618 1230 1815 1429

/K 971 738 1089 857

for ILs [C*<sup>n</sup>*

are listed in **Table 17**. From **Table 17**, the obtained values of the correlation coefficient, *R*, are 0.9999, which indicates that the VFT equation can be used for fitting the experimental

py][NTf<sup>2</sup>

perature range (see **Figure 15**). The values of the correlation coefficient, *R*, are 0.9976, 0.9965,

, and the corresponding correlation coefficient, *R*,

py][NTf<sup>2</sup>

py][NTf<sup>2</sup>

] (*n* = 2, 3, 4, 5, 6) were fitted in the tem-

] (*n* = 2, 4, 5, 6)

] (*n* = 2, 3, 4, 5, 6)

the ILs can also be fitted using the VFT Eq. (11), see **Figure 14**.

2/3 vs. *T*(K) for ILs[C*<sup>n</sup>*

**[C2 py][NTf2**

were calculated by Eq. (13) and are listed in **Table 17**.

The 1000/*T* dependences on ln *η* of the for [C*<sup>n</sup>*

, *B, T*<sup>0</sup>

py][NTf<sup>2</sup>

**] [C4**

, and *T*<sup>c</sup>

According to Eq. (13), the activation energies of dynamic viscosity for [C*<sup>n</sup>*

According to Eq. (12), the 1000/*T* dependence of ln *η* was plotted for [C*<sup>n</sup>*

The best fitting parameters of *η*<sup>0</sup>

**Table 16.** The estimated values of *k, T*<sup>b</sup>

394 Progress and Developments in Ionic Liquids

dynamic viscosity.

**Figure 13.** Plot of *γV*<sup>m</sup>

107

*T*b

*T*c

(see **Figure 15**).


**Table 17.** Fitted values of dynamic viscosity of *η*<sup>0</sup> , *B, E<sup>η</sup>* , *T*<sup>0</sup> , and *R* according to Eqs. (11) and (13).

According to **Table 13** and Eq. (14), the temperature dependences of electrical conductivity values of the ILs can also be fitted using the VFT Eq. (14), see **Figure 16**.

The best-fitting parameters of *σ*<sup>0</sup> , *B, T*<sup>0</sup> , and the corresponding correlation coefficient, *R*, are listed in **Table 18**. From **Table 18**, the obtained values of the correlation coefficient, *R*, are 0.9996, which indicates that the VFT equation can be used for fitting the experimental electrical conductivity.

According to Eq. (16), the activation energies of dynamic viscosity for [C*<sup>n</sup>*

**] [C4**

**py][NTf2**

, *B, E<sup>σ</sup>* , *T*<sup>0</sup>

/(S⋅cm−1) 2.39 0.47 0.23 0.81 *B*/K 1067.5 577.7 472.9 803.1

/eV 92.1 49.9 40.8 69.3

/K 119.9 182.3 196.5 168.4 *R* 0.9996 0.9996 0.9996 0.9996

**] [C5**

**py][NTf2**

, and *R* according to Eqs. (**15**) and (**17**).

**] [C6**

**py][NTf2 ]**

Thermodynamic Properties of Ionic Liquids http://dx.doi.org/10.5772/65792

According to Eq. (15), the 1000/*T* dependence on ln *σ* was also plotted for the [C*<sup>n</sup>*

surement points lie far away from the fitted straight lines. The FILs [MCNMIM][NTf<sup>2</sup>

py][NTf<sup>2</sup>

] (*n* = 2, 3, 4, 5, 6).

CONHBuEIM][NTf<sup>2</sup>

temperature range. The values of the correlation coefficient, *R*, are 0.9990, 0.9990, 0.9939, and 0.9970, respectively. The values are also lower than the values (all of the values of *R* = 0.9996) which obtained by the empirical VFT equation (see **Table 18**). So, the measurement electrical conductivity does not follow the Arrhenius Eq. (15). In **Figure 17**, it can also be seen that the mea-

py][NTf<sup>2</sup>

were calculated by Eq. (16) and are listed in **Table 18**.

**Table 18.** Fitted values of electrical conductivity of *σ*<sup>0</sup>

**py][NTf2**

In **Figure 17**, the 1000/*T* dependence on ln *σ* of the [C*<sup>n</sup>*

], and [CH<sup>2</sup>

4, 5, 6) (see **Figure 17**).

**Property [C2**

*σ*0

103 *Eσ*

*T*0

], [EOHMIM][NTf<sup>2</sup>

**Figure 17.** Plot of Ln *σ* vs. 1000/*T* of ILs [C*<sup>n</sup>*

[NTf<sup>2</sup>

py][NTf<sup>2</sup>

] (*n* = 2, 3, 4, 5, 6) was fitted in the

] have also exhibited the same result.

] (*n* = 2, 4, 5, 6)

], [PCNMIM]

] (*n* = 2,

397

py][NTf<sup>2</sup>

**Figure 15.** Plot of dynamic viscosity, *η*, vs. temperature, *T*, of [C*<sup>n</sup>* py][NTf<sup>2</sup> ] (*n* = 2, 3, 4, 5, 6).

**Figure 16.** Plot of electrical conductivity, *σ*, vs. temperature, *T*, of [C*<sup>n</sup>* py][NTf<sup>2</sup> ] (*n* = 2, 3, 4, 5, 6).


**Table 18.** Fitted values of electrical conductivity of *σ*<sup>0</sup> , *B, E<sup>σ</sup>* , *T*<sup>0</sup> , and *R* according to Eqs. (**15**) and (**17**).

According to Eq. (16), the activation energies of dynamic viscosity for [C*<sup>n</sup>* py][NTf<sup>2</sup> ] (*n* = 2, 4, 5, 6) were calculated by Eq. (16) and are listed in **Table 18**.

According to Eq. (15), the 1000/*T* dependence on ln *σ* was also plotted for the [C*<sup>n</sup>* py][NTf<sup>2</sup> ] (*n* = 2, 4, 5, 6) (see **Figure 17**).

In **Figure 17**, the 1000/*T* dependence on ln *σ* of the [C*<sup>n</sup>* py][NTf<sup>2</sup> ] (*n* = 2, 3, 4, 5, 6) was fitted in the temperature range. The values of the correlation coefficient, *R*, are 0.9990, 0.9990, 0.9939, and 0.9970, respectively. The values are also lower than the values (all of the values of *R* = 0.9996) which obtained by the empirical VFT equation (see **Table 18**). So, the measurement electrical conductivity does not follow the Arrhenius Eq. (15). In **Figure 17**, it can also be seen that the measurement points lie far away from the fitted straight lines. The FILs [MCNMIM][NTf<sup>2</sup> ], [PCNMIM] [NTf<sup>2</sup> ], [EOHMIM][NTf<sup>2</sup> ], and [CH<sup>2</sup> CONHBuEIM][NTf<sup>2</sup> ] have also exhibited the same result.

**Figure 17.** Plot of Ln *σ* vs. 1000/*T* of ILs [C*<sup>n</sup>* py][NTf<sup>2</sup> ] (*n* = 2, 3, 4, 5, 6).

**Figure 16.** Plot of electrical conductivity, *σ*, vs. temperature, *T*, of [C*<sup>n</sup>*

**Figure 15.** Plot of dynamic viscosity, *η*, vs. temperature, *T*, of [C*<sup>n</sup>*

396 Progress and Developments in Ionic Liquids

py][NTf<sup>2</sup>

py][NTf<sup>2</sup>

] (*n* = 2, 3, 4, 5, 6).

] (*n* = 2, 3, 4, 5, 6).

The Walden's product (in [S·cm<sup>2</sup> ·mol−1][mP·s]) can be calculated according to Eq. (17). The values are 60, 54, 48, 45 for [C<sup>2</sup> py][NTf<sup>2</sup> ], [C4 py][NTf<sup>2</sup> ], [C5 py][NTf<sup>2</sup> ], and [C6 py][NTf<sup>2</sup> ] at 298.15 K, respectively. From the results, the values are decrease with the methylene introduced.

[NTf<sup>2</sup>

Two series ILs [C*<sup>n</sup>*

ILs are listed in **Table 22**.

**3mpy] [NTf2**

**] [C4**

**3mpy] [NTf2**

**] [C6**

[27, 33, 45].

*T***/K [C3**

Note: <sup>c</sup>

calculated values.

at pressure *p* = 0.1 MPa.

**Table 19.** Experimental values of density, *ρ*, of two series ILs [C*<sup>n</sup>*

] (*n* = 2, 3, 4, 5, 6) can be called "subionic." It means that the ILs [C*<sup>n</sup>*

**8.2. N-Alkyl-3-methyl or N-alkyl-4-methyl type pyridinium-type ionic liquids**

] (n = 3, 4, 6) and [C*<sup>n</sup>*

the proton transfer is incomplete. The behavior of the ILs [C*<sup>n</sup>*

dinium ring and the positions are position 3 and 4, respectively.

3Mpy][NTf<sup>2</sup>

4, 5, 6) did not yield the expected conductivity from the high fluidities because on average

there is only a small population of ions and the "ionicity" of the ILs is therefore reduced [34].

in this section. From **Figure 11**, the two series ILs introduced the methyl group on the pyri-

The values of the density, dynamic viscosity, and electrical conductivity are listed in **Tables 19**–**21**

In order to compare the density, dynamic viscosity, and electrical conductivity with the N-alkyl type pyridinium-type ILs at 298.15 K, the values of the three series pyridinium-based

**3mpy] [NTf2**

278.15 1.4685<sup>c</sup> 1.4399 1.3781<sup>c</sup> 1.5100<sup>c</sup> 1.4373<sup>c</sup> 1.3695<sup>c</sup> 283.15 1.4640<sup>c</sup> 1.4357 1.3736 1.5052 1.4328 1.3653 288.15 1.4596 1.4315 1.3697 1.5010 1.4284 1.3608 293.15 1.4556 1.4271 1.3653 1.4961 1.4234 1.3563 298.15 1.4514 1.4226 1.3615 1.4920 1.4187 1.3518 303.15 1.4471 1.4183 1.3570 1.4877 1.4140 1.3474 308.15 1.4426 1.4142 1.3529 1.4830 1.4093 1.3429 313.15 1.4380 1.4098 1.3487 1.4783 1.4047 1.3385 318.15 1.4332 1.4051 1.3447 1.4734 1.4002 1.3341 323.15 1.4287 1.4007 1.3403 1.4688 1.3958 1.3296 328.15 1.4245 1.3964 1.3360 1.4644 1.3913 1.3252 333.15 1.4203 1.3921 1.3317 1.4599 1.3869 1.3208 338.15 1.4160 1.3876 1.3280 1.4551 1.3825 1.3165 343.15 1.4113<sup>c</sup> 1.3832<sup>c</sup> 1.3237<sup>c</sup> 1.4506<sup>c</sup> 1.3777<sup>c</sup> 1.3121 348.15 1.4069<sup>c</sup> 1.3787<sup>c</sup> 1.3195<sup>c</sup> 1.4460<sup>c</sup> 1.3731<sup>c</sup> 1.3078 353.15 1.4025<sup>c</sup> 1.3742<sup>c</sup> 1.3154<sup>c</sup> 1.4414<sup>c</sup> 1.3686<sup>c</sup> 1.3034

**] [C2**

3Mpy][NTf<sup>2</sup>

] (*n* = 3, 4, 6) and [C*<sup>n</sup>*

4Mpy][NTf<sup>2</sup>

] (*n* = 2, 4, 6)

**4mpy] [NTf2**

**] [C4**

**4mpy] [NTf2**

**] [C6**

**4mpy] [NTf2**

**]**

4Mpy][NTf<sup>2</sup>

py][NTf<sup>2</sup>

Thermodynamic Properties of Ionic Liquids http://dx.doi.org/10.5772/65792

] (*n* = 2, 4, 6) were synthesized

] (*n* = 2, 3, 4, 5, 6) as if

py][NTf<sup>2</sup>

] (*n* = 2, 3,

399

According to Eq. (17), the log*Λ* dependence on log*η*−1 is illustrated in **Figure 18** for the [C*<sup>n</sup>* py] [NTf<sup>2</sup> ] (*n* = 2, 3, 4, 5, 6) from 283.15 to 338.15 K.

**Figure 18.** Walden plots for samples at temperature from 283.15 to 338.15 K for [C*<sup>n</sup>* py][NTf<sup>2</sup> ] (*n* = 2, 3, 4, 5, 6).

Usually, the Walden rule can be used for the presentation of the independent ions of the liquid. If the Walden points close to the ideal line, the liquid can be considered as a relative ideal liquid. The ideal line position was determined according to the aqueous KCl solution at high dilution. As an ideal line, the slop of the ideal line should be unity and not have any interaction of the ions [10, 29, 30]. In **Figure 10**, it can be seen that the approximately straight lines can be obtained according to the experimental points.

Like the FILs, the Walden rule can also be used for the presentation of the independent ions of the ILs [C*<sup>n</sup>* py][NTf<sup>2</sup> ] (*n* = 2, 3, 4, 5, 6). From **Figure 18**, it can be seen that the approximately straight lines can be obtained according to the experimental values. The fitted slopes of the lines for the ILs [C*<sup>n</sup>* py][NTf<sup>2</sup> ] (*n* = 2, 3, 4, 5, 6) are 0.941, 0.906, 0.935, 0.893, and 0.959, respectively. The lines for the ILs below are close to the ideal KCl line from **Figure 18**. Most of the reported traditional ILs [9, 10, 29–31] have the same tendency. Herein, the ILs [C*<sup>n</sup>* py]

[NTf<sup>2</sup> ] (*n* = 2, 3, 4, 5, 6) can be called "subionic." It means that the ILs [C*<sup>n</sup>* py][NTf<sup>2</sup> ] (*n* = 2, 3, 4, 5, 6) did not yield the expected conductivity from the high fluidities because on average the proton transfer is incomplete. The behavior of the ILs [C*<sup>n</sup>* py][NTf<sup>2</sup> ] (*n* = 2, 3, 4, 5, 6) as if there is only a small population of ions and the "ionicity" of the ILs is therefore reduced [34].

#### **8.2. N-Alkyl-3-methyl or N-alkyl-4-methyl type pyridinium-type ionic liquids**

The Walden's product (in [S·cm<sup>2</sup>

398 Progress and Developments in Ionic Liquids

according to the experimental points.

py][NTf<sup>2</sup>

py][NTf<sup>2</sup>

**Figure 18.** Walden plots for samples at temperature from 283.15 to 338.15 K for [C*<sup>n</sup>*

of the lines for the ILs [C*<sup>n</sup>*

of the ILs [C*<sup>n</sup>*

py][NTf<sup>2</sup>

] (*n* = 2, 3, 4, 5, 6) from 283.15 to 338.15 K.

], [C4

py][NTf<sup>2</sup>

According to Eq. (17), the log*Λ* dependence on log*η*−1 is illustrated in **Figure 18** for the [C*<sup>n</sup>*

Usually, the Walden rule can be used for the presentation of the independent ions of the liquid. If the Walden points close to the ideal line, the liquid can be considered as a relative ideal liquid. The ideal line position was determined according to the aqueous KCl solution at high dilution. As an ideal line, the slop of the ideal line should be unity and not have any interaction of the ions [10, 29, 30]. In **Figure 10**, it can be seen that the approximately straight lines can be obtained

Like the FILs, the Walden rule can also be used for the presentation of the independent ions

mately straight lines can be obtained according to the experimental values. The fitted slopes

respectively. The lines for the ILs below are close to the ideal KCl line from **Figure 18**. Most of the reported traditional ILs [9, 10, 29–31] have the same tendency. Herein, the ILs [C*<sup>n</sup>*

] (*n* = 2, 3, 4, 5, 6). From **Figure 18**, it can be seen that the approxi-

] (*n* = 2, 3, 4, 5, 6) are 0.941, 0.906, 0.935, 0.893, and 0.959,

py][NTf<sup>2</sup>

] (*n* = 2, 3, 4, 5, 6).

respectively. From the results, the values are decrease with the methylene introduced.

], [C5

ues are 60, 54, 48, 45 for [C<sup>2</sup>

[NTf<sup>2</sup>

·mol−1][mP·s]) can be calculated according to Eq. (17). The val-

], and [C6

py][NTf<sup>2</sup>

] at 298.15 K,

py]

py]

py][NTf<sup>2</sup>

Two series ILs [C*<sup>n</sup>* 3Mpy][NTf<sup>2</sup> ] (n = 3, 4, 6) and [C*<sup>n</sup>* 4Mpy][NTf<sup>2</sup> ] (*n* = 2, 4, 6) were synthesized in this section. From **Figure 11**, the two series ILs introduced the methyl group on the pyridinium ring and the positions are position 3 and 4, respectively.

The values of the density, dynamic viscosity, and electrical conductivity are listed in **Tables 19**–**21** [27, 33, 45].

In order to compare the density, dynamic viscosity, and electrical conductivity with the N-alkyl type pyridinium-type ILs at 298.15 K, the values of the three series pyridinium-based ILs are listed in **Table 22**.


**Table 19.** Experimental values of density, *ρ*, of two series ILs [C*<sup>n</sup>* 3Mpy][NTf<sup>2</sup> ] (*n* = 3, 4, 6) and [C*<sup>n</sup>* 4Mpy][NTf<sup>2</sup> ] (*n* = 2, 4, 6) at pressure *p* = 0.1 MPa.


*T***/K [C3**

Note: <sup>c</sup>

[NTf<sup>2</sup>

[C<sup>2</sup> py][NTf<sup>2</sup>

[C4 py][NTf<sup>2</sup>

[C5 py][NTf<sup>2</sup>

[C6 py][NTf<sup>2</sup>

[C3

[C4

[C6

[C<sup>2</sup>

[C4

[C6

3mpy][NTf<sup>2</sup>

3mpy][NTf<sup>2</sup>

3mpy][NTf<sup>2</sup>

4mpy][NTf<sup>2</sup>

4mpy][NTf<sup>2</sup>

4mpy][NTf<sup>2</sup>

calculated values.

] (*n* = 2, 4, 6) at pressure *p* = 0.1 MPa.

based ILs at 298.15 K and pressure *p* = 0.1 MPa.

**3mpy] [NTf2**

**] [C4**

**3mpy] [NTf2**

**Table 21.** Experimental values of electrical conductivity, *σ*, of two series ILs [C*<sup>n</sup>*

**] [C6**

**3mpy] [NTf2**

278.15 1.524 1.00 0.495 3.39<sup>c</sup> 1.24 0.592<sup>c</sup> 283.15 1.990 1.32 0.670 4.24 1.66 0.839 288.15 2.53 1.72 0.885 5.25 2.15 1.101 293.15 3.18 2.17 1.157 6.38 2.73 1.447 298.15 3.93 2.72 1.480 7.63 3.41 1.852 303.15 4.76 3.35 1.869 9.01 4.19 2.31 308.15 5.71 4.09 2.31 10.50 5.09 2.83 313.15 6.71 4.90 2.83 12.08 6.00 3.46 318.15 7.86 5.76 3.42 13.78 7.02 4.15 323.15 9.04 6.90 4.06 15.54 8.17 4.87 328.15 10.36 7.96 4.77 17.50 9.41 5.71 333.15 11.79 9.10 5.53 19.55 10.75 6.52 338.15 13.29 10.31 6.38 21.7 12.07 7.56 343.15 14.85 11.60 7.28 23.9<sup>c</sup> 13.56<sup>c</sup> 8.61 348.15 16.50 12.96 8.22 26.2<sup>c</sup> 15.10<sup>c</sup> 9.68 353.15 18.28 14.37 9.20 28.5<sup>c</sup> 16.70<sup>c</sup> 10.75

**] [C2**

**4mpy] [NTf2**

**] [C4**

3Mpy][NTf<sup>2</sup>

*ρ***/(g**⋅**cm−3)** *η***/(mPa**⋅**s)** *σ***/(mS**⋅**cm−1)**

] 1.5375 39.4 5.99

] 1.4547 58.3 3.21

] 1.4214 71.9 2.22

] 1.3877 84.5 1.66

] 1.4514 54.12 3.93

] 1.4226 63.2 2.72

] 1.3615 86.59 1.480

] 1.4920 32.75 7.63

] 1.4187 55.14 3.41

] 1.3518 77.989 1.852

**Table 22.** The comparison of density, *ρ*, dynamic viscosity, *η*, and electrical conductivity, *σ*, of three series pyridinium-

] (*n* = 3, 4, 6) and [C*<sup>n</sup>*

4Mpy]

**4mpy] [NTf2**

Thermodynamic Properties of Ionic Liquids http://dx.doi.org/10.5772/65792

**] [C6**

**4mpy] [NTf2 ]**

401

**Table 20.** Experimental values of dynamic viscosity, *η*, of two series ILs [C*<sup>n</sup>* 3Mpy][NTf<sup>2</sup> ] (*n* = 3, 4, 6) and [C*<sup>n</sup>* 4Mpy][NTf<sup>2</sup> ] (*n* = 2, 4, 6) at pressure *p* = 0.1 MPa.

From **Table 22**, the density of the three series of pyridinium-type ILs decreases with the introduction of the methylene group on the alkyl side chain of the pyridinium-type ILs. The result is the same with the imidazolium-type ILs [12, 13]. The introduction of the methyl group on the pyridinium-type ILs leads the apparent decrease of the density. However, the degree of decreasing is different on the position 3 and 4 of the pyridinium ring. The introduction of the methyl group on position 4 leads the more decrease than position 3 on the pyridinium ring. The order is as follows: [C<sup>2</sup> py][NTf<sup>2</sup> ] > [C<sup>2</sup> 4mpy][NTf<sup>2</sup> ]; [C4 py][NTf<sup>2</sup> ] > [C4 3mpy][NTf<sup>2</sup> ] > [C4 4mpy][NTf<sup>2</sup> ]; [C6 py][NTf<sup>2</sup> ] > [C6 3mpy][NTf<sup>2</sup> ] > [C6 4mpy][NTf<sup>2</sup> ].

As shown in **Table 22**, the electrical conductivity of the three series pyridinium-type ILs decreases with the introduction of the methylene group on the alkyl side chain of the pyridinium-type ILs. But, the introduction of the methyl group on the ring leads to the different change tendency for electrical conductivity. For density, the values are decrease with the introduction of the methyl group on positions 3 and 4 of the pyridinium ring. However, the electrical conductivity decreases after the introduction of methyl group on position 3 and increases after the introduction of the methyl group on position 4 of the pyridinium ring. The tendency is just the reverse and the order is as follow: [C<sup>2</sup> py][NTf<sup>2</sup> ] < [C<sup>2</sup> 4mpy][NTf<sup>2</sup> ]; [C4 3mpy][NTf<sup>2</sup> ] < [C4 py][NTf<sup>2</sup> ] < [C4 4mpy][NTf<sup>2</sup> ]; [C6 3mpy][NTf<sup>2</sup> ] < [C6 py][NTf<sup>2</sup> ] < [C6 4mpy][NTf<sup>2</sup> ].


**Table 21.** Experimental values of electrical conductivity, *σ*, of two series ILs [C*<sup>n</sup>* 3Mpy][NTf<sup>2</sup> ] (*n* = 3, 4, 6) and [C*<sup>n</sup>* 4Mpy] [NTf<sup>2</sup> ] (*n* = 2, 4, 6) at pressure *p* = 0.1 MPa.

From **Table 22**, the density of the three series of pyridinium-type ILs decreases with the introduction of the methylene group on the alkyl side chain of the pyridinium-type ILs. The result is the same with the imidazolium-type ILs [12, 13]. The introduction of the methyl group on the pyridinium-type ILs leads the apparent decrease of the density. However, the degree of decreasing is different on the position 3 and 4 of the pyridinium ring. The introduction of the methyl group on position 4 leads the more decrease than position 3 on the pyridinium ring. The order is as follows:

] > [C4

]; [C4

].

4mpy][NTf<sup>2</sup>

As shown in **Table 22**, the electrical conductivity of the three series pyridinium-type ILs decreases with the introduction of the methylene group on the alkyl side chain of the pyridinium-type ILs. But, the introduction of the methyl group on the ring leads to the different change tendency for electrical conductivity. For density, the values are decrease with the introduction of the methyl group on positions 3 and 4 of the pyridinium ring. However, the electrical conductivity decreases after the introduction of methyl group on position 3 and increases after the introduction of the methyl group on position 4 of the pyridinium ring. The tendency is just the reverse and the order

3mpy][NTf<sup>2</sup>

3mpy][NTf<sup>2</sup>

] > [C4

3Mpy][NTf<sup>2</sup>

] < [C4

py][NTf<sup>2</sup>

] < [C4

4mpy][NTf<sup>2</sup>

] (*n* = 3, 4, 6) and [C*<sup>n</sup>*

]; [C6

py][NTf<sup>2</sup>

4Mpy][NTf<sup>2</sup>

]

4mpy][NTf<sup>2</sup>

] >

];

[C<sup>2</sup>

Note: <sup>c</sup>

[C6

[C6

py][NTf<sup>2</sup>

*T***/K [C3**

**3mpy] [NTf2**

400 Progress and Developments in Ionic Liquids

**] [C4**

**3mpy] [NTf2**

**] [C6**

**3mpy] [NTf2**

278.15 160.80<sup>c</sup> 177.0<sup>c</sup> 276.16<sup>c</sup> 79.45<sup>c</sup> 159.65<sup>c</sup> 253.47<sup>c</sup> 283.15 118.12<sup>c</sup> 133.3<sup>c</sup> 199.61<sup>c</sup> 61.98<sup>c</sup> 118.57<sup>c</sup> 181.62 288.15 89.08<sup>c</sup> 102.3<sup>c</sup> 147.89<sup>c</sup> 49.30<sup>c</sup> 90.07<sup>c</sup> 133.92 293.15 68.73<sup>c</sup> 79.9<sup>c</sup> 112.00<sup>c</sup> 39.89<sup>c</sup> 69.82<sup>c</sup> 101.07 298.15 54.12 63.2 86.59 32.75 55.14 77.989 303.15 43.46 51.1 67.83 27.31 44.20 61.301 308.15 35.27 41.4 54.23 23.10 35.85 49.031 313.15 29.18 34.0 44.24 19.69 29.61 39.755 318.15 24.32 28.4 36.21 16.95 24.77 32.729 323.15 20.87 23.9 29.97 14.66 20.85 27.381 328.15 17.94 20.3 25.08 12.85 17.86 23.111 333.15 15.15 17.5 21.31 11.34 15.37 19.709 338.15 13.40 15.2 18.28 10.10 13.42 16.968 343.15 11.74 13.0 15.88 9.07 11.65 14.729 348.15 10.37 11.3 13.71 8.19 10.15 12.866 353.15 9.19 10.0 11.99 7.43 8.89 11.324

**] [C2**

**4mpy] [NTf2**

**] [C4**

**4mpy] [NTf2**

**] [C6**

**4mpy] [NTf2**

**]**

3mpy][NTf<sup>2</sup>

is as follow: [C<sup>2</sup>

3mpy][NTf<sup>2</sup>

] > [C<sup>2</sup>

calculated values.

(*n* = 2, 4, 6) at pressure *p* = 0.1 MPa.

] > [C6

py][NTf<sup>2</sup>

] < [C6

4mpy][NTf<sup>2</sup>

4mpy][NTf<sup>2</sup>

] < [C<sup>2</sup>

py][NTf<sup>2</sup>

]; [C4

].

358.15 6.76 7.94 363.15 6.19 7.20<sup>c</sup>

**Table 20.** Experimental values of dynamic viscosity, *η*, of two series ILs [C*<sup>n</sup>*

py][NTf<sup>2</sup>

4mpy][NTf<sup>2</sup>

] < [C6


**Table 22.** The comparison of density, *ρ*, dynamic viscosity, *η*, and electrical conductivity, *σ*, of three series pyridiniumbased ILs at 298.15 K and pressure *p* = 0.1 MPa.

For the dynamic viscosity, the values of the three series pyridinium-type ILs increase with the introduction of the methylene group on the alkyl side chain of pyridinium-type cation ILs. Like the electrical conductivity, the dynamic viscosity also exhibited the difference tendency with the density after the introduction of the methyl group on the pyridinium ring. However, the tendency is in contrast to the electrical conductivity. For dynamic viscosity, the values increase with the introduction of the methyl group on position 3 of the pyridinium ring and decrease with the introduction of the methyl group on position 4 of the pyridinium ring with the nonsubstituting pyridinium ring. The order is as follows: [C<sup>2</sup> py][NTf<sup>2</sup> ] > [C<sup>2</sup> 4mpy][NTf<sup>2</sup> ]; [C4 3mpy] [NTf<sup>2</sup> ] > [C4 py][NTf<sup>2</sup> ] > [C4 4mpy][NTf<sup>2</sup> ]; [C6 3mpy][NTf<sup>2</sup> ] > [C6 py][NTf<sup>2</sup> ] > [C6 4mpy][NTf<sup>2</sup> ].

From **Table 20**, the temperature dependence on dynamic viscosity can be fitted according to VFT

**3mpy] [NTf2**

) 0.4765 0.5025 0.5593 0.4479 0.5039 0.5633

**] [C2**

**4mpy] [NTf2**

**] [C4**

**4mpy] [NTf2**

Thermodynamic Properties of Ionic Liquids http://dx.doi.org/10.5772/65792

**] [C6**

**4mpy] [NTf2 ]**

403

**] [C6**

MW/(g⋅mol−1) 416.35 430.38 458.43 402.33 430.38 458.43

*α*/(K−1) 6.12 6.20 6.19 6.17 6.52 6.62 *V*/(cm−3⋅mol−1) 286.9 302.5 336.7 269.7 303.4 339.1

/(J⋅K−1⋅mol−1) 623.5 655.9 726.7 587.8 657.6 731.7 *U*pot/(kJ⋅mol−1) 404 399 389 410 399 388

**Table 23.** Estimated values of physicochemical properties of two series pyridinium-based ILs at 298.15 K.

Eq. (11), see **Figure 20**.

**3mpy] [NTf2**

**] [C4**

**3mpy] [NTf2**

**Property [C3**

*V*m/(nm3

104

*S*0

The best-fitting parameters of *η*<sup>0</sup>

mental dynamic viscosity.

[NTf<sup>2</sup>

] (*n* = 2, 4, 6).

, *B, T*<sup>0</sup>

**Figure 20.** Plot of dynamic viscosity, *η*, vs. temperature, *T*, for two series ILs [C*<sup>n</sup>*

listed in **Table 24**. From **Table 24**, the obtained values of the correlation coefficient, *R*, are higher than 0.9999, which indicates that the VFT equation can be used for fitting the experi-

, and the corresponding correlation coefficient, *R*, are

3Mpy][NTf<sup>2</sup>

] (*n* = 3, 4, 6) and [C*<sup>n</sup>*

4Mpy]

According to **Table 19**, the temperature dependence of the density values can be plotted and fitted according to the linear equation (**Figure 19**).

**Figure 19.** Plot of density, *ρ*, vs. temperature, *T*, for two series ILs [C*<sup>n</sup>* 3Mpy][NTf<sup>2</sup> ] (*n* = 3, 4, 6) and [C*<sup>n</sup>* 4Mpy][NTf<sup>2</sup> ] (*n* = 2, 4, 6).

The thermal expansion coefficient, *α*, molecular volume, *V*m, standard molar entropy, *S*<sup>0</sup> , and lattice energy, *U*POT, were calculated from experimental density using the empirical Eqs. (1)–(4). The obtained data from the empirical equations are listed in **Table 23**.

From **Table 23**, the thermal expansion coefficients are 6.12 × 10−4 K−1 for [C3 3mpy][NTf<sup>2</sup> ], 6.20 × 10−4 K−1 for [C4 3mpy][NTf<sup>2</sup> ], 6.19 × 10−4 K−1 for [C6 3mpy][NTf<sup>2</sup> ], 6.17 × 10−4 K−1 for [C<sup>2</sup> 4mpy][NTf<sup>2</sup> ], 6.52 × 10−4 K−1 for [C4 4mpy][NTf<sup>2</sup> ], and 6.62 × 10−4 K−1 for [C6 4mpy][NTf<sup>2</sup> ], respectively. The values are in good agreement with the range of 5 × 10−4 to 7 × 10−4 K−1 obtained by Jacquemin et al. 44]. According to **Table 23**, the mean contributions of the methylene to the molecular volume are 0.0277 nm3 for [C*<sup>n</sup>* 3mpy][NTf<sup>2</sup> ] (*n* = 3, 4, 6) and 0.0289 nm3 for [C*<sup>n</sup>* 4mpy][NTf<sup>2</sup> ] (*n* = 2, 4, 6) at 298.15 K. The values are in good agreement with the values of 0.0280 nm3 for ILs [C*<sup>n</sup>* py][NTf<sup>2</sup> ] [4, 26].

From **Table 20**, the temperature dependence on dynamic viscosity can be fitted according to VFT Eq. (11), see **Figure 20**.

For the dynamic viscosity, the values of the three series pyridinium-type ILs increase with the introduction of the methylene group on the alkyl side chain of pyridinium-type cation ILs. Like the electrical conductivity, the dynamic viscosity also exhibited the difference tendency with the density after the introduction of the methyl group on the pyridinium ring. However, the tendency is in contrast to the electrical conductivity. For dynamic viscosity, the values increase with the introduction of the methyl group on position 3 of the pyridinium ring and decrease with the introduction of the methyl group on position 4 of the pyridinium ring with the non-

3mpy][NTf<sup>2</sup>

According to **Table 19**, the temperature dependence of the density values can be plotted and fit-

py][NTf<sup>2</sup>

] > [C6

] > [C<sup>2</sup>

py][NTf<sup>2</sup>

4mpy][NTf<sup>2</sup>

] > [C6

]; [C4

4mpy][NTf<sup>2</sup>

3mpy]

].

, and lat-

] (*n* = 2, 4, 6).

], 6.20 ×

],

4mpy][NTf<sup>2</sup>

] [4, 26].

3mpy][NTf<sup>2</sup>

4Mpy][NTf<sup>2</sup>

py][NTf<sup>2</sup>

], respectively. The values

] (*n* = 2, 4, 6) at 298.15

], 6.17 × 10−4 K−1 for [C<sup>2</sup>

] (*n* = 3, 4, 6) and [C*<sup>n</sup>*

4mpy][NTf<sup>2</sup>

for ILs [C*<sup>n</sup>*

4mpy][NTf<sup>2</sup>

for [C*<sup>n</sup>*

substituting pyridinium ring. The order is as follows: [C<sup>2</sup>

4mpy][NTf<sup>2</sup>

]; [C6

The thermal expansion coefficient, *α*, molecular volume, *V*m, standard molar entropy, *S*<sup>0</sup>

], and 6.62 × 10−4 K−1 for [C6

] (*n* = 3, 4, 6) and 0.0289 nm3

obtained data from the empirical equations are listed in **Table 23**.

K. The values are in good agreement with the values of 0.0280 nm3

From **Table 23**, the thermal expansion coefficients are 6.12 × 10−4 K−1 for [C3

], 6.19 × 10−4 K−1 for [C6

tice energy, *U*POT, were calculated from experimental density using the empirical Eqs. (1)–(4). The

are in good agreement with the range of 5 × 10−4 to 7 × 10−4 K−1 obtained by Jacquemin et al. 44]. According to **Table 23**, the mean contributions of the methylene to the molecular volume are

3mpy][NTf<sup>2</sup>

3Mpy][NTf<sup>2</sup>

] > [C4

ted according to the linear equation (**Figure 19**).

[NTf<sup>2</sup>

] > [C4

10−4 K−1 for [C4

0.0277 nm3

6.52 × 10−4 K−1 for [C4

for [C*<sup>n</sup>*

3mpy][NTf<sup>2</sup>

4mpy][NTf<sup>2</sup>

**Figure 19.** Plot of density, *ρ*, vs. temperature, *T*, for two series ILs [C*<sup>n</sup>*

3mpy][NTf<sup>2</sup>

py][NTf<sup>2</sup>

402 Progress and Developments in Ionic Liquids


**Table 23.** Estimated values of physicochemical properties of two series pyridinium-based ILs at 298.15 K.

**Figure 20.** Plot of dynamic viscosity, *η*, vs. temperature, *T*, for two series ILs [C*<sup>n</sup>* 3Mpy][NTf<sup>2</sup> ] (*n* = 3, 4, 6) and [C*<sup>n</sup>* 4Mpy] [NTf<sup>2</sup> ] (*n* = 2, 4, 6).

The best-fitting parameters of *η*<sup>0</sup> , *B, T*<sup>0</sup> , and the corresponding correlation coefficient, *R*, are listed in **Table 24**. From **Table 24**, the obtained values of the correlation coefficient, *R*, are higher than 0.9999, which indicates that the VFT equation can be used for fitting the experimental dynamic viscosity.

According to Eq. (13), the activation energies of dynamic viscosity for the two series ILs [C*<sup>n</sup>* 3Mpy] [NTf<sup>2</sup> ] (*n* = 3, 4, 6) and [C*<sup>n</sup>* 4Mpy][NTf<sup>2</sup> ] (*n* = 2, 4, 6) were calculated and are listed in **Table 24**.

In **Figure 21**, the 1000/*T* dependences on ln *η* of the two ILs [C*<sup>n</sup>*

], [PCNMIM][NTf<sup>2</sup>

[C*<sup>n</sup>*

4Mpy][NTf<sup>2</sup>

[MCNMIM][NTf<sup>2</sup>

also exhibited the same result.

The best-fitting parameters of *σ*<sup>0</sup>

[NTf<sup>2</sup>

] (*n* = 2, 4, 6).

mental electrical conductivity.

, *B, T*<sup>0</sup>

**Figure 22.** Plot of electrical conductivity, *σ*, vs. temperature, *T*, of two ILs [C*<sup>n</sup>*

listed in **Table 25**. From **Table 25**, the obtained values of the correlation coefficient, *R*, are higher than 0.9999, which indicates that the VFT equation can be used for fitting the experi-

VFT Eq. (14), see **Figure 22**.

3Mpy][NTf<sup>2</sup>

CONHBuEIM][NTf<sup>2</sup>

Thermodynamic Properties of Ionic Liquids http://dx.doi.org/10.5772/65792

] (*n* = 2, 4, 6) were fitted in the temperature range. The values of the correlation

], and [CH<sup>2</sup>

, and the corresponding correlation coefficient, *R*, are

3Mpy][NTf<sup>2</sup>

] (*n* = 3, 4, 6) and [C*<sup>n</sup>*

4Mpy]

coefficient, *R*, are 0.9972, 0.9987, 0.9978, 0.9966, 0.9981, and 0.9947, respectively. The values are much lower than the values (all of the values are higher than *R* = 0.9999) which fitted according to the empirical VFT equation (see **Table 24**). So, the measurement dynamic viscosity cannot be well fitted with the Arrhenius Eq. (12). From **Figure 21**, it can also be obviously obtained that the measurement points lie far away from the straight fitting lines. The FILs

], [EOHMIM][NTf<sup>2</sup>

From **Table 21**, the temperature dependence on electrical conductivity can be fitted according to

] (*n* = 3, 4, 6) and

] have

405

The 1000 *T*−1 dependence on Ln *η* was plotted for the ILs [C*<sup>n</sup>* 3Mpy][NTf<sup>2</sup> ] (*n* = 3, 4, 6) and [C*<sup>n</sup>* 4Mpy][NTf<sup>2</sup> ] (*n* = 2, 4, 6) (see **Figure 21**) according to Eq. (12).


**Table 24.** Fitted parameter values of *η*<sup>0</sup> , *B, T*<sup>0</sup> , and correlation coefficient, *R*<sup>2</sup> , by Eq. (**11**) and *Eη* by Eq. (**13**).

**Figure 21.** Plot of Ln *η* vs. 1000/*T* of ILs [C*<sup>n</sup>* 3Mpy][NTf<sup>2</sup> ] (*n* = 3, 4, 6) and [C*<sup>n</sup>* 4Mpy][NTf<sup>2</sup> ] (*n* = 2, 4, 6).

In **Figure 21**, the 1000/*T* dependences on ln *η* of the two ILs [C*<sup>n</sup>* 3Mpy][NTf<sup>2</sup> ] (*n* = 3, 4, 6) and [C*<sup>n</sup>* 4Mpy][NTf<sup>2</sup> ] (*n* = 2, 4, 6) were fitted in the temperature range. The values of the correlation coefficient, *R*, are 0.9972, 0.9987, 0.9978, 0.9966, 0.9981, and 0.9947, respectively. The values are much lower than the values (all of the values are higher than *R* = 0.9999) which fitted according to the empirical VFT equation (see **Table 24**). So, the measurement dynamic viscosity cannot be well fitted with the Arrhenius Eq. (12). From **Figure 21**, it can also be obviously obtained that the measurement points lie far away from the straight fitting lines. The FILs [MCNMIM][NTf<sup>2</sup> ], [PCNMIM][NTf<sup>2</sup> ], [EOHMIM][NTf<sup>2</sup> ], and [CH<sup>2</sup> CONHBuEIM][NTf<sup>2</sup> ] have also exhibited the same result.

According to Eq. (13), the activation energies of dynamic viscosity for the two series ILs [C*<sup>n</sup>*

**3mpy] [NTf2**

/(mPa⋅s) 0.1523 0.0474 0.0749 0.1768 0.0859 0.1170 *B*/K 751.0 1151.7 997.9 720.6 914.6 847.2

/eV 64.8 99.4 86.1 62.2 79.9 73.1

/K 170.3 138.1 156.6 160.2 156.6 167.9 *R*<sup>2</sup> 0.99999 0.9999 0.99999 0.99997 0.99996 0.99999

, and correlation coefficient, *R*<sup>2</sup>

**] [C2**

**4mpy] [NTf2**

] (*n* = 2, 4, 6) (see **Figure 21**) according to Eq. (12).

**] [C6**

] (*n* = 2, 4, 6) were calculated and are listed in **Table 24**.

3Mpy][NTf<sup>2</sup>

**4mpy] [NTf2**

, by Eq. (**11**) and *Eη* by Eq. (**13**).

**] [C4**

4Mpy][NTf<sup>2</sup>

The 1000 *T*−1 dependence on Ln *η* was plotted for the ILs [C*<sup>n</sup>*

**3mpy] [NTf2**

, *B, T*<sup>0</sup>

[NTf<sup>2</sup>

[C*<sup>n</sup>*

*η*0

103 *Eη*

*T*0

] (*n* = 3, 4, 6) and [C*<sup>n</sup>*

404 Progress and Developments in Ionic Liquids

**3mpy] [NTf2**

**Table 24.** Fitted parameter values of *η*<sup>0</sup>

**Figure 21.** Plot of Ln *η* vs. 1000/*T* of ILs [C*<sup>n</sup>*

3Mpy][NTf<sup>2</sup>

] (*n* = 3, 4, 6) and [C*<sup>n</sup>*

4Mpy][NTf<sup>2</sup>

] (*n* = 2, 4, 6).

**] [C4**

4Mpy][NTf<sup>2</sup>

**Property [C3**

3Mpy]

] (*n* = 3, 4, 6) and

**4mpy] [NTf2**

**]**

**] [C6**

From **Table 21**, the temperature dependence on electrical conductivity can be fitted according to VFT Eq. (14), see **Figure 22**.

**Figure 22.** Plot of electrical conductivity, *σ*, vs. temperature, *T*, of two ILs [C*<sup>n</sup>* 3Mpy][NTf<sup>2</sup> ] (*n* = 3, 4, 6) and [C*<sup>n</sup>* 4Mpy] [NTf<sup>2</sup> ] (*n* = 2, 4, 6).

The best-fitting parameters of *σ*<sup>0</sup> , *B, T*<sup>0</sup> , and the corresponding correlation coefficient, *R*, are listed in **Table 25**. From **Table 25**, the obtained values of the correlation coefficient, *R*, are higher than 0.9999, which indicates that the VFT equation can be used for fitting the experimental electrical conductivity.

The activation energies of electrical conductivity for the two series ILs [C*<sup>n</sup>* 3Mpy][NTf<sup>2</sup> ] (*n* = 3, 4, 6) and [C*<sup>n</sup>* 4Mpy][NTf<sup>2</sup> ] (*n* = 2, 4, 6) were calculated by Eq. (16) and are listed in **Table 25**.

In **Figure 23**, the 1000/*T* dependence on ln *σ* of the two ILs [C*<sup>n</sup>*

], [EOHMIM][NTf<sup>2</sup>

According to Eq. (17), the Walden products (in [S·cm<sup>2</sup>

Log Λ dependence on log *η*−1 was plotted for the ILs [C*<sup>n</sup>*

**Figure 24.** Walden plots for samples at temperature from 273.15 to 353.15 K.

], 52 for [C4

], and 49 for [C6

] (*n* = 2, 4, 6) from 273.15 to 353.15 K according to Eq. (17) (see **Figure 24**).

3mpy][NTf<sup>2</sup>

the N-alkyl pyridinium-type ILs.

4mpy][NTf<sup>2</sup>

[C*<sup>n</sup>*

4Mpy][NTf<sup>2</sup>

[PCNMIM][NTf<sup>2</sup>

same result.

are 61 for [C3

], 53 for [C4

[NTf<sup>2</sup>

[NTf<sup>2</sup>

3Mpy][NTf<sup>2</sup>

Thermodynamic Properties of Ionic Liquids http://dx.doi.org/10.5772/65792

·mol−1][cP]) are calculated and the values

] at 298.15 K. From the results we

] (*n* = 3, 4, 6) and [C*<sup>n</sup>*

3mpy][NTf<sup>2</sup>

] (*n* = 2, 4, 6) was fitted in the temperature range. The values of the correla-

CONHBuEIM][NTf<sup>2</sup>

], 43 for [C6

3mpy][NTf<sup>2</sup>

4mpy][NTf<sup>2</sup>

tion coefficient, *R*, are 0.9957, 0.9935, 0.9933, 0.9943, 0.9958, and 0.9936, respectively. The values are much lower than the values (all of the values are higher than *R* = 0.9999) which fitted by the empirical VFT equation (see **Table 25**). So, the measurement values cannot be well fitted with the Arrhenius Eq. (15). From **Figure 23**, the measurement points can also be obviously shown far away from the straight fitting lines. The FILs [MCNMIM][NTf<sup>2</sup>

], and [CH<sup>2</sup>

3mpy][NTf<sup>2</sup>

found that the values are also decrease with the introduction of the methylene group such as

] (*n* = 3, 4, 6) and

] have also exhibited the

], 67 for [C<sup>2</sup>

],

407

4mpy]

4mpy]

The 1000/*T* dependence on ln *σ* was plotted for the two series ILs [C*<sup>n</sup>* 3Mpy][NTf<sup>2</sup> ] (*n* = 3, 4, 6) and [C*<sup>n</sup>* 4Mpy][NTf<sup>2</sup> ] (*n* = 2, 4, 6) (see **Figure 23**) by Eq. (15).


**Table 25.** Fitted parameter values of *σ*<sup>0</sup> , *B, T*<sup>0</sup> , and correlation coefficient, *R*<sup>2</sup> , by Eq. (**15**) and *E<sup>σ</sup>* by Eq. (**17**).

**Figure 23.** Plot of Ln *σ* vs. 1000/*T* of ILs for the two series ILs [C*<sup>n</sup>* 3Mpy][NTf<sup>2</sup> ] (*n* = 3, 4, 6) and [C*<sup>n</sup>* 4Mpy][NTf<sup>2</sup> ] (*n* = 2, 4, 6).

In **Figure 23**, the 1000/*T* dependence on ln *σ* of the two ILs [C*<sup>n</sup>* 3Mpy][NTf<sup>2</sup> ] (*n* = 3, 4, 6) and [C*<sup>n</sup>* 4Mpy][NTf<sup>2</sup> ] (*n* = 2, 4, 6) was fitted in the temperature range. The values of the correlation coefficient, *R*, are 0.9957, 0.9935, 0.9933, 0.9943, 0.9958, and 0.9936, respectively. The values are much lower than the values (all of the values are higher than *R* = 0.9999) which fitted by the empirical VFT equation (see **Table 25**). So, the measurement values cannot be well fitted with the Arrhenius Eq. (15). From **Figure 23**, the measurement points can also be obviously shown far away from the straight fitting lines. The FILs [MCNMIM][NTf<sup>2</sup> ], [PCNMIM][NTf<sup>2</sup> ], [EOHMIM][NTf<sup>2</sup> ], and [CH<sup>2</sup> CONHBuEIM][NTf<sup>2</sup> ] have also exhibited the same result.

The activation energies of electrical conductivity for the two series ILs [C*<sup>n</sup>*

The 1000/*T* dependence on ln *σ* was plotted for the two series ILs [C*<sup>n</sup>*

**3mpy] [NTf2**

, *B, T*<sup>0</sup>

] (*n* = 2, 4, 6) (see **Figure 23**) by Eq. (15).

**] [C6**

**3mpy] [NTf2**

/(S⋅cm−1) 0.64 0.49 0.47 0.64 0.50 0.46 *B*/K 650.2 608.4 684.6 574.6 586.5 647.3

/eV 56.1 52.4 59.0 49.5 50.6 55.9

/K 170.7 181.0 179.3 168.5 180.4 181.0 *R*<sup>2</sup> 0.99996 0.9999 0.99992 0.99997 0.99996 0.99999

, and correlation coefficient, *R*<sup>2</sup>

] (*n* = 2, 4, 6) were calculated by Eq. (16) and are listed in **Table 25**.

**] [C2**

3Mpy][NTf<sup>2</sup>

] (*n* = 3, 4, 6) and [C*<sup>n</sup>*

4Mpy][NTf<sup>2</sup>

] (*n* = 2, 4, 6).

**4mpy] [NTf2**

, by Eq. (**15**) and *E<sup>σ</sup>*

**] [C4**

and [C*<sup>n</sup>*

and [C*<sup>n</sup>*

*σ*0

103 *Eσ*

*T*0

**Property [C3**

4Mpy][NTf<sup>2</sup>

406 Progress and Developments in Ionic Liquids

4Mpy][NTf<sup>2</sup>

**3mpy] [NTf2**

**Table 25.** Fitted parameter values of *σ*<sup>0</sup>

**] [C4**

**Figure 23.** Plot of Ln *σ* vs. 1000/*T* of ILs for the two series ILs [C*<sup>n</sup>*

3Mpy][NTf<sup>2</sup>

3Mpy][NTf<sup>2</sup>

**4mpy] [NTf2**

by Eq. (**17**).

] (*n* = 3, 4, 6)

] (*n* = 3, 4, 6)

**4mpy] [NTf2 ]**

**] [C6**

According to Eq. (17), the Walden products (in [S·cm<sup>2</sup> ·mol−1][cP]) are calculated and the values are 61 for [C3 3mpy][NTf<sup>2</sup> ], 52 for [C4 3mpy][NTf<sup>2</sup> ], 43 for [C6 3mpy][NTf<sup>2</sup> ], 67 for [C<sup>2</sup> 4mpy] [NTf<sup>2</sup> ], 53 for [C4 4mpy][NTf<sup>2</sup> ], and 49 for [C6 4mpy][NTf<sup>2</sup> ] at 298.15 K. From the results we found that the values are also decrease with the introduction of the methylene group such as the N-alkyl pyridinium-type ILs.

Log Λ dependence on log *η*−1 was plotted for the ILs [C*<sup>n</sup>* 3mpy][NTf<sup>2</sup> ] (*n* = 3, 4, 6) and [C*<sup>n</sup>* 4mpy] [NTf<sup>2</sup> ] (*n* = 2, 4, 6) from 273.15 to 353.15 K according to Eq. (17) (see **Figure 24**).

**Figure 24.** Walden plots for samples at temperature from 273.15 to 353.15 K.

From **Figure 24**, it can be observed that the curves are approximately straight lines. The slopes of the lines for the ILs [C*<sup>n</sup>* 3mpy][NTf<sup>2</sup> ] (*n* = 3, 4, 6) and [C*<sup>n</sup>* 4mpy][NTf<sup>2</sup> ] (*n* = 2, 4, 6) are 0.887, 0.949, 0.954, 0.915, 0.912, and 0.946, respectively. The position of the ideal line was established using aqueous KCl solutions at high dilution. The lines of the ILs [C*<sup>n</sup>* 3mpy] [NTf<sup>2</sup> ] (*n* = 3, 4, 6) and [C*<sup>n</sup>* 4mpy][NTf<sup>2</sup> ] (*n* = 2, 4, 6) lie below and closely to the ideal KCl line. The ILs [C*<sup>n</sup>* 3mpy][NTf<sup>2</sup> ] (*n* = 3, 4, 6) and [C*<sup>n</sup>* 4mpy][NTf<sup>2</sup> ] (*n* = 2, 4, 6) are "subionic" [38].

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] and Prediction of Properties [C*<sup>n</sup>*

Fragilities. J. Phys. Chem. B. 2003;107(25):6170−6178. DOI: 10.1021/jp0275894

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mim][PF3

mim][Ala] (*n* = 2, 3, 4, 5, 6). J. Phys. Chem. B.

(CF<sup>2</sup> CF3 ) 3

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py][NTf<sup>2</sup>

2012;14(6):1721−1727. DOI: 10.1039/C2GC16560K

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Conductivity, and the Relevance of ΔpK<sup>a</sup>

(CF<sup>2</sup> CF3 ) 3

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DOI: 10.1021/ic951325x

DOI: 10.1021/je100507n

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fluid.2006.01.022

Liquid [C<sup>2</sup>

mim][PF3

jpowsour.2004.06.022

Tension of Ionic Liquids [C*<sup>n</sup>*

#### **Conclusion**

The density, surface tension, dynamic viscosity, and electrical conductivity of the three series hydrophobic pyridinium-type ILs [C*<sup>n</sup>* py][NTf<sup>2</sup> ] (*n* = 2, 3, 4, 5, 6), [C*<sup>n</sup>* 3Mpy][NTf<sup>2</sup> ] (*n* = 3, 4, 6), and [C*<sup>n</sup>* 4Mpy][NTf<sup>2</sup> ] (*n* = 2, 4, 6) were determined at atmospheric pressure in the temperature range of 278–363 K. The thermal expansion coefficient, molecular volume, standard molar entropy, and lattice energy of the samples were estimated in terms of empirical and semiempirical equations. The density and electrical conductivity decrease with the introduction of the methylene group on the alkyl side chain of the pyridinium type. However, the dynamic viscosity exhibited the inverse tendency. Compared with the methylene group, the introduction of the methyl group on the pyridinium ring exhibited the irregular tendency to the density, dynamic viscosity and electrical conductivity. The order is as follows: [C<sup>2</sup> 4mpy][NTf<sup>2</sup> ] < [C<sup>2</sup> py][NTf<sup>2</sup> ], [C4 4mpy][NTf<sup>2</sup> ] < [C4 3mpy][NTf<sup>2</sup> ] < [C4 py][NTf<sup>2</sup> ], and [C6 4mpy][NTf<sup>2</sup> ] < [C6 3mpy][NTf<sup>2</sup> ] < [C6 py][NTf<sup>2</sup> ] for density; [C<sup>2</sup> 4mpy][NTf<sup>2</sup> ] < [C<sup>2</sup> py] [NTf<sup>2</sup> ], [C4 4mpy][NTf<sup>2</sup> ] < [C4 py][NTf<sup>2</sup> ] < [C4 3mpy][NTf<sup>2</sup> ], and [C6 4mpy][NTf<sup>2</sup> ] < [C6 py][NTf<sup>2</sup> ] < [C6 3mpy][NTf<sup>2</sup> ] for dynamic viscosity; [C<sup>2</sup> 4mpy][NTf<sup>2</sup> ] > [C<sup>2</sup> py][NTf<sup>2</sup> ], [C4 4mpy][NTf<sup>2</sup> ] > [C4 py][NTf<sup>2</sup> ] > [C4 3mpy][NTf<sup>2</sup> ], and [C6 4mpy][NTf<sup>2</sup> ] > [C6 py][NTf<sup>2</sup> ] > [C6 3mpy][NTf<sup>2</sup> ] for electrical conductivity. According to the correlation coefficients, the empirical equation can be satisfactorily used for the fitting of the dynamic viscosity and electrical conductivity of the pyridinium-type ILs. However, the Arrhenius equation cannot be used for the fitting of the measurement values.

### **Author details**

Liu Qingshan\*, Mou Lin, Zheng Qige and Xia Quan

\*Address all correspondence to: 13478787524@163.com

Shenyang Agriculture University, College of Science, Shenyang, Liaoning, P. R. China

### **References**

[1] J. Fuller, R. T. Carlin. Structural and Electrochemical Characterization of 1,3-bis-(4-methylphenyl)imidazolium Chloride. J. Chem. Crystallogr. 1994;24(8):489−493. DOI: 10.1007/ BF01666725

[2] C. M. Gordon, J. D. Holbrey, A. R. Kennedy, K. R. Seddon. Ionic Liquid Crystals: Hexafluorophosphate Salts. J. Mater. Chem. 1998;8(12):2627−2636. DOI: 10.1039/A806169F

From **Figure 24**, it can be observed that the curves are approximately straight lines. The

are 0.887, 0.949, 0.954, 0.915, 0.912, and 0.946, respectively. The position of the ideal line

The density, surface tension, dynamic viscosity, and electrical conductivity of the three

temperature range of 278–363 K. The thermal expansion coefficient, molecular volume, standard molar entropy, and lattice energy of the samples were estimated in terms of empirical and semiempirical equations. The density and electrical conductivity decrease with the introduction of the methylene group on the alkyl side chain of the pyridinium type. However, the dynamic viscosity exhibited the inverse tendency. Compared with the methylene group, the introduction of the methyl group on the pyridinium ring exhibited the irregular tendency to the density, dynamic viscosity and electrical conductivity. The order is

py][NTf<sup>2</sup>

4mpy][NTf<sup>2</sup>

py][NTf<sup>2</sup>

3mpy][NTf<sup>2</sup>

4mpy][NTf<sup>2</sup>

electrical conductivity. According to the correlation coefficients, the empirical equation can be satisfactorily used for the fitting of the dynamic viscosity and electrical conductivity of the pyridinium-type ILs. However, the Arrhenius equation cannot be used for the fitting of

4mpy][NTf<sup>2</sup>

] (*n* = 3, 4, 6) and [C*<sup>n</sup>*

4mpy][NTf<sup>2</sup>

4mpy][NTf<sup>2</sup>

] (*n* = 2, 4, 6) are "subionic" [38].

] (*n* = 2, 4, 6) lie below and closely to the ideal KCl

] (*n* = 2, 3, 4, 5, 6), [C*<sup>n</sup>*

] (*n* = 2, 4, 6) were determined at atmospheric pressure in the

] < [C4

], and [C6

] > [C<sup>2</sup>

] > [C6

] for density; [C<sup>2</sup>

py][NTf<sup>2</sup>

3mpy][NTf<sup>2</sup>

4mpy][NTf<sup>2</sup>

] > [C6

py][NTf<sup>2</sup>

] < [C4

] < [C6

4mpy][NTf<sup>2</sup>

], [C4

py][NTf<sup>2</sup>

py][NTf<sup>2</sup>

] < [C<sup>2</sup>

4mpy][NTf<sup>2</sup>

3mpy][NTf<sup>2</sup>

],

]

] >

] for

py]

] (*n* = 2, 4, 6)

3Mpy][NTf<sup>2</sup>

3mpy]

] (*n*

3mpy][NTf<sup>2</sup>

] (*n* = 3, 4, 6) and [C*<sup>n</sup>*

4mpy][NTf<sup>2</sup>

was established using aqueous KCl solutions at high dilution. The lines of the ILs [C*<sup>n</sup>*

slopes of the lines for the ILs [C*<sup>n</sup>*

408 Progress and Developments in Ionic Liquids

] (*n* = 3, 4, 6) and [C*<sup>n</sup>*

3mpy][NTf<sup>2</sup>

series hydrophobic pyridinium-type ILs [C*<sup>n</sup>*

4mpy][NTf<sup>2</sup>

] < [C6

] < [C4

3mpy][NTf<sup>2</sup>

Liu Qingshan\*, Mou Lin, Zheng Qige and Xia Quan

\*Address all correspondence to: 13478787524@163.com

] < [C<sup>2</sup>

3mpy][NTf<sup>2</sup>

] for dynamic viscosity; [C<sup>2</sup>

py][NTf<sup>2</sup>

py][NTf<sup>2</sup>

], and [C6

], [C4

] < [C6

Shenyang Agriculture University, College of Science, Shenyang, Liaoning, P. R. China

[1] J. Fuller, R. T. Carlin. Structural and Electrochemical Characterization of 1,3-bis-(4-methylphenyl)imidazolium Chloride. J. Chem. Crystallogr. 1994;24(8):489−493. DOI: 10.1007/

] < [C4

4Mpy][NTf<sup>2</sup>

[NTf<sup>2</sup>

line. The ILs [C*<sup>n</sup>*

= 3, 4, 6), and [C*<sup>n</sup>*

as follows: [C<sup>2</sup>

], [C4

py][NTf<sup>2</sup>

**Author details**

**References**

BF01666725

3mpy][NTf<sup>2</sup>

the measurement values.

4mpy][NTf<sup>2</sup>

4mpy][NTf<sup>2</sup>

] > [C4

and [C6

[NTf<sup>2</sup>

< [C6

[C4

**Conclusion**


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ISBN: 0−8493−0484−9

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jp067589u


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[16] M. Egashira, M. Nakagawa, I. Watanabe, S. Okada, J. I. Yamaki. Cyano-Containing Quaternary Ammonium-Based Ionic Liquid as a 'Co-Solvent' for Lithium Battery Electrolyte. J. Power Sources. 2005;146(1−2):685−688. DOI: 10.1016/j.jpowsour.2005.03.069 [17] C. Hardacre, J. D. Holbrey, C. L. Mullan, M. Nieuwenhuyzen, W. M. Reichert, K. R. Seddon, S. J. Teat. Ionic Liquid Characteristics of 1-Alkyl-N-Cyanopyridinium and 1-Alkyl-N-(Trifluoromethyl)pyridinium Salts. New J. Chem. 2008;32(11):1953−1967.

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[19] J. Zhang, Q. Zhang, B. Qiao, Y. Deng. Solubilities of the Gaseous and Liquid Solutes and Their Thermodynamics of Solubilization in the Novel Room-Temperature Ionic Liquids at Infinite Dilution by Gas Chromatography. J. Chem. Eng. Data. 2007;52(6):2277−2283.

[21] Y. Cai, Y. Peng. Amino-Functionalized Ionic Liquid as an Efficient and Recyclable Catalyst for Knoevenagel Reactions in Water. Catal. Lett. 2006;109(1):61−64. DOI: 10.1007/

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py][NTf<sup>2</sup>

] and [C6

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Capture by a Task-Specific Ionic

CONHBu][NTf<sup>2</sup>

]. J. Chem. Eng.

]. J.

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B921160H


[40] J. Jacquemin, P. Husson, A. A. H. Padua, V. Majer. Density and Viscosity of Several Pure and Water-Saturated Ionic Liquids. Green Chem. 2006;8(2):172−180. DOI: 10.1039/B513231B

**Chapter 17**

**Provisional chapter**

**Behavior of Ionic Liquids Under Nanoconfinement**

Ionic liquids (ILs) are organic salts consisting of anions and cations that exist as liquids at room temperature. ILs exhibit many attractive properties such as negligible volatility, low flammability, and relatively high thermal stability. These properties can be varied in a controlled fashion through systematic changes in the molecular structure of their constituent ions. Some recent studies have aimed to use ILs as new lubricant materials. However, the behavior of ILs as lubricants on the sliding interfaces has not been elucidated. In this chapter, we describe the nano- and macrolubrication properties of some ILs with different types of anions using resonance shear measurement (RSM) and conventional ball-on-plate-type tribotests, respectively. This study reveals that the properties observed by RSM for nanoscale systems can provide important insights for the study of the friction

**Behavior of Ionic Liquids Under Nanoconfinement**

**Greatly Affects Actual Friction**

**Greatly Affects Actual Friction**

Additional information is available at the end of the chapter

Additional information is available at the end of the chapter

Takaya Sato and Kazue Kurihara

Takaya Sato and Kazue Kurihara

http://dx.doi.org/10.5772/65758

**Abstract**

**1. Introduction**

Toshio Kamijo, Hiroyuki Arafune, Takashi Morinaga,

coefficients (macrolubrication properties) obtained by tribotests.

details of the lubrication mechanism of ILs are not clearly understood.

**Keywords:** tribology, lubricant, nanolubrication, confinement, friction coefficient

Ionic liquids (ILs) are expected to be promising candidate materials for new lubricants [1–5]. In particular, their stability under severe conditions, such as ultrahigh vacuum [6, 7] and high temperatures [8], has attracted increasing interest. To choose ILs suitable for use as lubricants, it is important to understand the characteristics of the target materials. However, currently, the

The tribological properties of ILs have been studied using a macroscopic tribotester. Most previous research has focused on the lubricating behavior of ILs in the boundary lubrication

and reproduction in any medium, provided the original work is properly cited.

© 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution,

© 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Toshio Kamijo, Hiroyuki Arafune, Takashi Morinaga,


#### **Behavior of Ionic Liquids Under Nanoconfinement Greatly Affects Actual Friction Behavior of Ionic Liquids Under Nanoconfinement Greatly Affects Actual Friction**

Toshio Kamijo, Hiroyuki Arafune, Takashi Morinaga, Takaya Sato and Kazue Kurihara Toshio Kamijo, Hiroyuki Arafune, Takashi Morinaga, Takaya Sato and Kazue Kurihara

Additional information is available at the end of the chapter Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/65758

#### **Abstract**

[40] J. Jacquemin, P. Husson, A. A. H. Padua, V. Majer. Density and Viscosity of Several Pure and Water-Saturated Ionic Liquids. Green Chem. 2006;8(2):172−180. DOI: 10.1039/B513231B

[41] A. Seduraman, P. Wu, M. Klähn. Extraction of Tryptophan with Ionic Liquids Studied with Molecular Dynamics Simulations. J. Phys. Chem. B. 2012;116(1):296−304. DOI: 10.1021/

[42] J. Tong, M. Hong, W. Guan, J. B. Li, J. Z. Yang. Studies on the Thermodynamic Properties of New Ionic Liquids: 1-Methyl-3-Pentylimidazolium Salts Containing Metal of Group III.

[43] J. Tong, Q. Liu, W. Guan, J. Yang. Estimation of Physicochemical Properties of Ionic Liquid

[44] L. P. N. Rebelo, J. N. C. Lopes, J. M. S. S. Esperança, E. Filipe. On the Critical Temperature, Normal Boiling Point, and Vapor Pressure of Ionic Liquids. J. Phys. Chem. B.

[45] Q. G. Zhang, Y. Wei, S. S. Sun, C. Wang, M. Yang, Q. S. Liu, Y. A. Gao. Study on Thermodynamic Properties of Ionic Liquid N-Butyl-3-Methylpyridinium Bis(trifluoromethylsulfonyl)imide.

Using Surface Tension and Density. J. Phys. Chem. B. 2007;111(12):3197−200.

J. Chem. Thermodyn. 2006;38(11):1416−1421. DOI: 10.1016/j.jct.2006.01.017

jp206748z

412 Progress and Developments in Ionic Liquids

C6

MIGaCl4

DOI: 10.1021/jp068793k

2005;109(13):6040−6043. DOI: 10.1021/jp050430h

J. Chem. Eng. Data. 2012;57(8):2185−2190. DOI: 10.1021/je300153f

Ionic liquids (ILs) are organic salts consisting of anions and cations that exist as liquids at room temperature. ILs exhibit many attractive properties such as negligible volatility, low flammability, and relatively high thermal stability. These properties can be varied in a controlled fashion through systematic changes in the molecular structure of their constituent ions. Some recent studies have aimed to use ILs as new lubricant materials. However, the behavior of ILs as lubricants on the sliding interfaces has not been elucidated. In this chapter, we describe the nano- and macrolubrication properties of some ILs with different types of anions using resonance shear measurement (RSM) and conventional ball-on-plate-type tribotests, respectively. This study reveals that the properties observed by RSM for nanoscale systems can provide important insights for the study of the friction coefficients (macrolubrication properties) obtained by tribotests.

**Keywords:** tribology, lubricant, nanolubrication, confinement, friction coefficient

### **1. Introduction**

Ionic liquids (ILs) are expected to be promising candidate materials for new lubricants [1–5]. In particular, their stability under severe conditions, such as ultrahigh vacuum [6, 7] and high temperatures [8], has attracted increasing interest. To choose ILs suitable for use as lubricants, it is important to understand the characteristics of the target materials. However, currently, the details of the lubrication mechanism of ILs are not clearly understood.

The tribological properties of ILs have been studied using a macroscopic tribotester. Most previous research has focused on the lubricating behavior of ILs in the boundary lubrication

© 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

regime at high loads of several GPa and on their tribochemical reactions with solid surfaces [9– 13]. The formation of tribochemical layers on metal surfaces from ILs containing a halogen such as fluorine under sliding conditions has been studied by X-ray photoelectron spectroscopy (XPS) [9–12], scanning electron microscope with energy dispersive X-ray spectroscopy (SEM-EDS) [9–11], and time-of-flight secondary ion mass spectrometry (TOF-SIMS) [12]. Therefore, ILs have also been used as additives for the formation of a tribochemical layer under high loads of several GPa [14–16]. When used, these layers have been considered to contribute to the reduction of friction in the system.

**2. Nanolubrication properties**

silica gel in the sample chamber.

**Figure 1.** Schematic of the RSM system. The surfaces are in the crossed-cylinder geometry.

The RSM system measured the surface force and resonance shear response by continuously changing the thickness of the liquid film confined between two solid surfaces with a nanometer resolution. The liquid thickness was controlled and determined using interferometric methods in the surface force apparatus. The shear response via resonance method provided a sensitive method for detecting the tiny changes in the liquid properties between the substrates, allowing

× 5 μm2

**2.1. RSM for nanoscale properties of ILs**

RSM was performed using an in-house resonance shear system based on an SFA [22], as shown schematically in **Figure 1**. The experimental setup and procedures for RSM are described in detail in a previous publication [21]. Silica sheets used as samples were prepared following the procedure reported by Horn et al. [24]. The root mean square (RMS) roughness value measured by AFM (Toyo Corporation, Agilent 5100 AFM/SPM Microscope) over an area of 5

molecularly smooth silica sheets was measured across IL films at a surface separation *D* with a resolution of 0.1 nm. The value of *D* was determined from the fringes of equal chromatic order (FECO) analysis. In brief, two back-silvered silica sheets (thickness of ca. 2–4 μm) were glued onto cylindrical quartz lenses (with a radius of curvature (*R*) of ca. 20 mm) and mounted onto the RSM system. The RSM system was composed of the upper surface unit suspended by a pair of vertical leaf springs and the lower surface unit mounted on a horizontal leaf spring. The upper surface unit was connected to a four-section piezo tube. In this case, it can be laterally oscillated at various frequencies (*ω*) by an application of a sinusoidal input voltage (*U*in). The deflection (Δ*x*) of the leaf spring was detected as an output voltage (*U*out) by a capacitance probe (Microsense 4830, Japan ADE Ltd.). Then, resonance curves were recorded at various *D* values as the normalized amplitude (*U*out/*U*in) as a function of the frequency *ω*. RSM was conducted at room temperature (295 ± 0.5 K) and at a humidity of less than 25% maintained by placing

for the silica sheets was 0.31 nm. Using RSM, the resonance curve between the

Behavior of Ionic Liquids Under Nanoconfinement Greatly Affects Actual Friction

http://dx.doi.org/10.5772/65758

415

On the other hand, due to the interest in applying ILs as lubricants, the properties of ILs confined in a nanoscale space have also been studied by atomic force microscopy (AFM) [17–19] and the surface force apparatus (SFA) [20, 21]. The oscillating forces observed by both AFM and SFA demonstrated the layered structure of ILs in narrow gaps. However, the relationship between these nanoscale properties and the macrolubrication properties is still not fully understood even for the same surface and ILs. Motivated by this problem, we have recently used resonance shear measurement (RSM) to show that some ILs form a layered structure in the nanoscale space created by the sliding surface [21, 22]. We also revealed that a nanostructure consisting of only several IL layers had a large influence on macroscale friction.

In this chapter, we describe the nano- and macrolubrication properties of some ILs (**Table 1**) with different anions by using RSM and a conventional ball-on-plate-type tribotester, respectively. This study reveals that ILs with different structures form different nanolayered structures and that their nanoscale behaviors are correlated with their macroscale tribology. In addition to providing information related to the lubrication mechanism of ILs, we also describe the principles for choosing an IL as a lubricant.


**Table 1.** Name, abbreviation, structure, molecular weight (MW), density (*ρ*), molecular volume (MV), ion pair diameter (*D*m), and viscosity (*η*) of ILs. *D*m is determined from (*ρ*) assuming a cubic packing geometry according to the method described by Horn et al. [23].

### **2. Nanolubrication properties**

regime at high loads of several GPa and on their tribochemical reactions with solid surfaces [9– 13]. The formation of tribochemical layers on metal surfaces from ILs containing a halogen such as fluorine under sliding conditions has been studied by X-ray photoelectron spectroscopy (XPS) [9–12], scanning electron microscope with energy dispersive X-ray spectroscopy (SEM-EDS) [9–11], and time-of-flight secondary ion mass spectrometry (TOF-SIMS) [12]. Therefore, ILs have also been used as additives for the formation of a tribochemical layer under high loads of several GPa [14–16]. When used, these layers have been considered to contribute

On the other hand, due to the interest in applying ILs as lubricants, the properties of ILs confined in a nanoscale space have also been studied by atomic force microscopy (AFM) [17–19] and the surface force apparatus (SFA) [20, 21]. The oscillating forces observed by both AFM and SFA demonstrated the layered structure of ILs in narrow gaps. However, the relationship between these nanoscale properties and the macrolubrication properties is still not fully understood even for the same surface and ILs. Motivated by this problem, we have recently used resonance shear measurement (RSM) to show that some ILs form a layered structure in the nanoscale space created by the sliding surface [21, 22]. We also revealed that a nanostructure

In this chapter, we describe the nano- and macrolubrication properties of some ILs (**Table 1**) with different anions by using RSM and a conventional ball-on-plate-type tribotester, respectively. This study reveals that ILs with different structures form different nanolayered structures and that their nanoscale behaviors are correlated with their macroscale tribology. In addition to providing information related to the lubrication mechanism of ILs, we also describe

**Table 1.** Name, abbreviation, structure, molecular weight (MW), density (*ρ*), molecular volume (MV), ion pair diameter (*D*m), and viscosity (*η*) of ILs. *D*m is determined from (*ρ*) assuming a cubic packing geometry according to the

**cm−3**

**MV/nm3** *D***m/nm Viscosity/**

426.40 1.42 0.49 0.79 77 [22]

233.06 1.17 0.33 0.69 400 [22]

419.36 1.44 0.48 0.78 58.4 [21]

226.02 1.20 0.31 0.68 124.6 [21]

**mPa s**

consisting of only several IL layers had a large influence on macroscale friction.

to the reduction of friction in the system.

414 Progress and Developments in Ionic Liquids

the principles for choosing an IL as a lubricant.

*N*,*N*-diethyl-*N*-methyl-*N*-(2 methoxyethyl) ammonium bis (trifluoromethane sulfonyl) imide

*N*,*N*-diethyl-*N*-methyl-*N*-(2 -methoxyethyl) ammonium tetrafluoroborate

1-Butyl-3-methylimidazolium bis(trifluoromethyl sulfonyl)

1-Butyl-3-methylimidazolium

method described by Horn et al. [23].

tetrafluoroborate

imide

**Ionic liquid Abbreviation Structure MW/g**  *ρ***/g**

[DEME][TFSI]

[DEME][BF4]

[BMIM][TFSI]

[BMIM][BF4]

#### **2.1. RSM for nanoscale properties of ILs**

RSM was performed using an in-house resonance shear system based on an SFA [22], as shown schematically in **Figure 1**. The experimental setup and procedures for RSM are described in detail in a previous publication [21]. Silica sheets used as samples were prepared following the procedure reported by Horn et al. [24]. The root mean square (RMS) roughness value measured by AFM (Toyo Corporation, Agilent 5100 AFM/SPM Microscope) over an area of 5 × 5 μm2 for the silica sheets was 0.31 nm. Using RSM, the resonance curve between the molecularly smooth silica sheets was measured across IL films at a surface separation *D* with a resolution of 0.1 nm. The value of *D* was determined from the fringes of equal chromatic order (FECO) analysis. In brief, two back-silvered silica sheets (thickness of ca. 2–4 μm) were glued onto cylindrical quartz lenses (with a radius of curvature (*R*) of ca. 20 mm) and mounted onto the RSM system. The RSM system was composed of the upper surface unit suspended by a pair of vertical leaf springs and the lower surface unit mounted on a horizontal leaf spring. The upper surface unit was connected to a four-section piezo tube. In this case, it can be laterally oscillated at various frequencies (*ω*) by an application of a sinusoidal input voltage (*U*in). The deflection (Δ*x*) of the leaf spring was detected as an output voltage (*U*out) by a capacitance probe (Microsense 4830, Japan ADE Ltd.). Then, resonance curves were recorded at various *D* values as the normalized amplitude (*U*out/*U*in) as a function of the frequency *ω*. RSM was conducted at room temperature (295 ± 0.5 K) and at a humidity of less than 25% maintained by placing silica gel in the sample chamber.

**Figure 1.** Schematic of the RSM system. The surfaces are in the crossed-cylinder geometry.

The RSM system measured the surface force and resonance shear response by continuously changing the thickness of the liquid film confined between two solid surfaces with a nanometer resolution. The liquid thickness was controlled and determined using interferometric methods in the surface force apparatus. The shear response via resonance method provided a sensitive method for detecting the tiny changes in the liquid properties between the substrates, allowing us to evaluate the viscosity change associated with liquid structuring, frictional/lubricational property, as well as other properties, by simply changing the liquid film thickness.

The RSM system described above was used to study the nanolubrication properties of ILs between smooth silica surfaces. **Figure 3(a)** and (**b**) shows the resonance curves for [DEME] [TFSI] and [DEME][BF4] ILs confined between silica surfaces at various separation distances. For reference, the resonance curves for AS and SC were measured prior to the resonance shear measurement of the ILs (**Figure 3**). In the absence of the ILs, the system parameters (mass, damping parameter, and spring constant for the apparatus) determined the AS and SC resonance curves. In the presence of [DEME][TFSI], the resonance curve at *D* = 228.1 nm showed a peak at a frequency of 185 rad s−1 that was almost the same as the AS peak. The peak intensity *U*out/*U*in was lower than the AS peak intensity, corresponding to the energy dissipation due to the bulk viscosity of the IL. With decreasing *D* value, the amplitude of the resonance peaks started to decrease at 10.2 nm and disappeared at *D* = 4.9 nm. In the range from *D* = 4.9 to 2.5 nm, broad resonance curves were observed at intermediate frequencies between the AS and SC frequencies, and the peak frequency started to shift toward the SC peak because of the weak coupling of the upper and lower surfaces mediated by the confined [DEME][TFSI] [21]. When the applied load (*N*) was increased further, the surface separation did not change and remained at 2.5 nm (**Figure 4**); however, the peak frequency shifted further toward that of the SC peak and the amplitude increased. This means that [DEME][TFSI] remained between the silica surfaces at the separation of 2.5 nm, and the coupling of the upper and lower surfaces

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**Figure 3.** Resonance curves for (a) [DEME][TFSI] and (b) [DEME][BF4] confined between the silica surfaces at various separation distances under an applied load *N*. Resonance curves for AS and SC are also shown [22]. *Solid lines* denote

The resonance shear behavior of [DEME][BF4] (**Figure 3(a)**) was similar to that of [DEME] [TFSI], except in the small *D* region below 2.5 nm (**Figure 3(b)**). The resonance curves at 186 rad s−1 did not change in the *D* range from 220.6 to 95.5 nm, and their amplitudes started to decrease at *D* = 20.8 nm. The resonance frequencies of these curves were almost the same as the frequency of the AS peak. The amplitude of the resonance peaks gradually decreased with decreasing *D* from 20.8 to 11.4 nm and disappeared at *D* = 9.4 nm. This distance is higher than the disappearance distance (*D* = 4.9 nm) for [DEME][TFSI]. In the range from *D* = 9.4 to 2.1 nm, broad resonance curves were observed at intermediate frequencies, and the peak

became stronger with the increased applied load.

the best fit curves to a physical model [27].

**Figure 2** shows the typical resonance shear curves for different surface separations. The resonance curves for the two reference states of separation in air (AS) and for silica-silica contact (SC) were measured prior to the RSM of a liquid (**Figure 2**). In the absence of a liquid, the system parameters (mass, damping parameter, and spring constant for the apparatus) determined the AS and SC resonance curves. In the presence of a liquid, the resonance curve at long distances showed a peak at a low frequency that was almost the same as the AS peak. The peak intensity *U*out/*U*in was lower than the AS peak intensity, corresponding to the energy dissipation due to the bulk viscosity of the liquid. With decreasing *D* value, the amplitude of the resonance peaks first decreased, and then, the peak disappeared with a further decrease in the distance. For a further decrease in the surface separation, broad resonance curves were observed at intermediate frequencies between the AS and SC frequencies, and the peak frequencies started to shift toward the SC peak because of the weak coupling of the upper and lower surfaces mediated by the confined liquid [25]. When the applied load (*N*) was further increased by decreasing the surface separation, the peak frequency shifted further toward the SC peak and the amplitude increased, until finally, for a surface separation of 0 nm, the amplitude was almost identical to that of the SC peak. This means that the liquid was completely removed from the gap between the silica surfaces [26]. Similarly, when the applied load (*N*) was further increased while the surface separation was fixed at a certain distance, the peak frequency also shifted further toward the SC peak frequency, while the amplitude increased. This means that the liquid remained between the silica surfaces at a certain surface separation due to the stronger coupling of the upper and lower surfaces with the increasing applied load.

**Figure 2.** Typical resonance shear curves with different surface separations.

The RSM system described above was used to study the nanolubrication properties of ILs between smooth silica surfaces. **Figure 3(a)** and (**b**) shows the resonance curves for [DEME] [TFSI] and [DEME][BF4] ILs confined between silica surfaces at various separation distances. For reference, the resonance curves for AS and SC were measured prior to the resonance shear measurement of the ILs (**Figure 3**). In the absence of the ILs, the system parameters (mass, damping parameter, and spring constant for the apparatus) determined the AS and SC resonance curves. In the presence of [DEME][TFSI], the resonance curve at *D* = 228.1 nm showed a peak at a frequency of 185 rad s−1 that was almost the same as the AS peak. The peak intensity *U*out/*U*in was lower than the AS peak intensity, corresponding to the energy dissipation due to the bulk viscosity of the IL. With decreasing *D* value, the amplitude of the resonance peaks started to decrease at 10.2 nm and disappeared at *D* = 4.9 nm. In the range from *D* = 4.9 to 2.5 nm, broad resonance curves were observed at intermediate frequencies between the AS and SC frequencies, and the peak frequency started to shift toward the SC peak because of the weak coupling of the upper and lower surfaces mediated by the confined [DEME][TFSI] [21]. When the applied load (*N*) was increased further, the surface separation did not change and remained at 2.5 nm (**Figure 4**); however, the peak frequency shifted further toward that of the SC peak and the amplitude increased. This means that [DEME][TFSI] remained between the silica surfaces at the separation of 2.5 nm, and the coupling of the upper and lower surfaces became stronger with the increased applied load.

us to evaluate the viscosity change associated with liquid structuring, frictional/lubricational

**Figure 2** shows the typical resonance shear curves for different surface separations. The resonance curves for the two reference states of separation in air (AS) and for silica-silica contact (SC) were measured prior to the RSM of a liquid (**Figure 2**). In the absence of a liquid, the system parameters (mass, damping parameter, and spring constant for the apparatus) determined the AS and SC resonance curves. In the presence of a liquid, the resonance curve at long distances showed a peak at a low frequency that was almost the same as the AS peak. The peak intensity *U*out/*U*in was lower than the AS peak intensity, corresponding to the energy dissipation due to the bulk viscosity of the liquid. With decreasing *D* value, the amplitude of the resonance peaks first decreased, and then, the peak disappeared with a further decrease in the distance. For a further decrease in the surface separation, broad resonance curves were observed at intermediate frequencies between the AS and SC frequencies, and the peak frequencies started to shift toward the SC peak because of the weak coupling of the upper and lower surfaces mediated by the confined liquid [25]. When the applied load (*N*) was further increased by decreasing the surface separation, the peak frequency shifted further toward the SC peak and the amplitude increased, until finally, for a surface separation of 0 nm, the amplitude was almost identical to that of the SC peak. This means that the liquid was completely removed from the gap between the silica surfaces [26]. Similarly, when the applied load (*N*) was further increased while the surface separation was fixed at a certain distance, the peak frequency also shifted further toward the SC peak frequency, while the amplitude increased. This means that the liquid remained between the silica surfaces at a certain surface separation due to the stronger coupling of the upper and lower surfaces with the increasing

property, as well as other properties, by simply changing the liquid film thickness.

applied load.

416 Progress and Developments in Ionic Liquids

**Figure 2.** Typical resonance shear curves with different surface separations.

**Figure 3.** Resonance curves for (a) [DEME][TFSI] and (b) [DEME][BF4] confined between the silica surfaces at various separation distances under an applied load *N*. Resonance curves for AS and SC are also shown [22]. *Solid lines* denote the best fit curves to a physical model [27].

The resonance shear behavior of [DEME][BF4] (**Figure 3(a)**) was similar to that of [DEME] [TFSI], except in the small *D* region below 2.5 nm (**Figure 3(b)**). The resonance curves at 186 rad s−1 did not change in the *D* range from 220.6 to 95.5 nm, and their amplitudes started to decrease at *D* = 20.8 nm. The resonance frequencies of these curves were almost the same as the frequency of the AS peak. The amplitude of the resonance peaks gradually decreased with decreasing *D* from 20.8 to 11.4 nm and disappeared at *D* = 9.4 nm. This distance is higher than the disappearance distance (*D* = 4.9 nm) for [DEME][TFSI]. In the range from *D* = 9.4 to 2.1 nm, broad resonance curves were observed at intermediate frequencies, and the peak frequencies started to shift toward the frequency of the SC peak because of the weak coupling of the upper and lower surfaces mediated by the confined [DEME][BF4]. When the applied load (*N*) was further increased, the surface separation remained at 2.1 nm (**Figure 4**), indicating that [DEME][BF4] remained trapped between the silica surfaces.

ces. The *η*eff values at larger separations were constant and essentially identical to the bulk IL viscosity of 400 for [DEME][BF4] and 77 mPa s for [DEME][TFSI]. However, the *η*eff value increased sharply with decreasing *D* below 11.4 nm for [DEME][TFSI] and below 10.4 nm for [DEME][BF4]. The *η*eff value for [DEME][TFSI] confined in the nanospace (*D* ≦ 4.9 nm) was one to two orders of magnitude higher than that at larger separations (*D* ≧ 10.2), and the *η*eff value for [DEME][BF4] confined in the nanospace (*D* ≦ 9.4 nm) was about one order of magnitude higher than that at larger separations (*D* ≧ 11.4). Note that in the nanospace, the *η*eff value of [DEME][TFSI] was higher than that of [DEME][BF4], whereas in the bulk, the opposite was the case and the viscosity of [DEME][BF4] was higher than that of [DEME][TFSI]. This behavior is similar to the trend shown by the ILs [BMIM][TFSI] and [BMIM][BF4] confined between the silica surfaces [21], even though the cations used in this study are different from those used in

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**Figure 5.** Relative intensity (peak intensity confined ILs/SC peak intensity) versus the applied load for [DEME][TFSI]

**Figure 6.** Effective viscosity *η*eff versus the separation distance for [DEME][TFSI] (●) and [DEME][BF4] (□) [22].

Ref. [21].

(●) and [DEME][BF4] (□) [22].

**Figure 4.** Profiles of normal force normalized by surface curvature radius (*F*/*R*) as a function of surface separation *D* between silica surfaces in [DEME][TFSI] (●) and [DEME][BF4] (□) [22].

#### **2.2. Dynamics of ILs by changing the load applied to the two friction surfaces**

**Figure 5** plots the relative intensity (*I*r) of the resonance peak, that is, the ratio of the peak intensity for confined ILs is divided by the SC peak intensity as a function of applied load. This intensity ratio is a measure of the lubrication behavior of the confined ILs, with smaller values indicating better lubrication behavior. With an increase in the applied load from 0.21 to 0.33 mN, the *I*r values of [DEME][TFSI] clearly increased from ca. 0.09 to 0.16 and then plateaued when the load was increased to 0.40 mN. The *I*r of [DEME][BF4] showed a behavior similar to that of [DEME][TFSI], gradually increasing from ca. 0.07 to 0.12 as the applied load increased from 0.19 to 0.50 mN, and reaching a plateau when the load was increased to 0.70 mN. The noteworthy difference between the two ILs was observed at higher loads (>0.4 mN), with the 0.18 *I*r value obtained for [DEME][TFSI] being significantly larger than the 0.12 value obtained for [DEME][BF4]. These results indicate that, under higher loads (>0.4 mN), [DEME][BF4] is a better lubricant than [DEME][TFSI]. Additionally, RSM revealed that an IL layer with ca. 2 nm thickness was maintained between the silica surfaces even under high applied loads (>0.4 mN). The plateau of the relative intensity under applied loads (>0.4 mN) indicated that the IL layer confined between the silica surfaces maintained its lubricating properties.

#### **2.3. Effective viscosity (***η***eff) of the confined ILs measured to quantitatively measure lubrication performance**

We analyzed the resonance curves using a previously developed physical model [27] to obtain a quantitative understanding of the properties of the confined ILs. The details of the analytical procedure are described in the literature [21, 27]. **Figure 6** plots the effective viscosity (*η*eff) obtained for the ILs using the model versus the separation distance between the silica surfaces. The *η*eff values at larger separations were constant and essentially identical to the bulk IL viscosity of 400 for [DEME][BF4] and 77 mPa s for [DEME][TFSI]. However, the *η*eff value increased sharply with decreasing *D* below 11.4 nm for [DEME][TFSI] and below 10.4 nm for [DEME][BF4]. The *η*eff value for [DEME][TFSI] confined in the nanospace (*D* ≦ 4.9 nm) was one to two orders of magnitude higher than that at larger separations (*D* ≧ 10.2), and the *η*eff value for [DEME][BF4] confined in the nanospace (*D* ≦ 9.4 nm) was about one order of magnitude higher than that at larger separations (*D* ≧ 11.4). Note that in the nanospace, the *η*eff value of [DEME][TFSI] was higher than that of [DEME][BF4], whereas in the bulk, the opposite was the case and the viscosity of [DEME][BF4] was higher than that of [DEME][TFSI]. This behavior is similar to the trend shown by the ILs [BMIM][TFSI] and [BMIM][BF4] confined between the silica surfaces [21], even though the cations used in this study are different from those used in Ref. [21].

frequencies started to shift toward the frequency of the SC peak because of the weak coupling of the upper and lower surfaces mediated by the confined [DEME][BF4]. When the applied load (*N*) was further increased, the surface separation remained at 2.1 nm (**Figure 4**), indicating

**Figure 4.** Profiles of normal force normalized by surface curvature radius (*F*/*R*) as a function of surface separation *D*

**Figure 5** plots the relative intensity (*I*r) of the resonance peak, that is, the ratio of the peak intensity for confined ILs is divided by the SC peak intensity as a function of applied load. This intensity ratio is a measure of the lubrication behavior of the confined ILs, with smaller values indicating better lubrication behavior. With an increase in the applied load from 0.21 to 0.33 mN, the *I*r values of [DEME][TFSI] clearly increased from ca. 0.09 to 0.16 and then plateaued when the load was increased to 0.40 mN. The *I*r of [DEME][BF4] showed a behavior similar to that of [DEME][TFSI], gradually increasing from ca. 0.07 to 0.12 as the applied load increased from 0.19 to 0.50 mN, and reaching a plateau when the load was increased to 0.70 mN. The noteworthy difference between the two ILs was observed at higher loads (>0.4 mN), with the 0.18 *I*r value obtained for [DEME][TFSI] being significantly larger than the 0.12 value obtained for [DEME][BF4]. These results indicate that, under higher loads (>0.4 mN), [DEME][BF4] is a better lubricant than [DEME][TFSI]. Additionally, RSM revealed that an IL layer with ca. 2 nm thickness was maintained between the silica surfaces even under high applied loads (>0.4 mN). The plateau of the relative intensity under applied loads (>0.4 mN) indicated that the IL layer

**2.2. Dynamics of ILs by changing the load applied to the two friction surfaces**

confined between the silica surfaces maintained its lubricating properties.

**lubrication performance**

**2.3. Effective viscosity (***η***eff) of the confined ILs measured to quantitatively measure**

We analyzed the resonance curves using a previously developed physical model [27] to obtain a quantitative understanding of the properties of the confined ILs. The details of the analytical procedure are described in the literature [21, 27]. **Figure 6** plots the effective viscosity (*η*eff) obtained for the ILs using the model versus the separation distance between the silica surfa-

that [DEME][BF4] remained trapped between the silica surfaces.

418 Progress and Developments in Ionic Liquids

between silica surfaces in [DEME][TFSI] (●) and [DEME][BF4] (□) [22].

**Figure 5.** Relative intensity (peak intensity confined ILs/SC peak intensity) versus the applied load for [DEME][TFSI] (●) and [DEME][BF4] (□) [22].

**Figure 6.** Effective viscosity *η*eff versus the separation distance for [DEME][TFSI] (●) and [DEME][BF4] (□) [22].

We suppose that the rapid increase in the effective viscosity with decreasing distance observed in **Figure 6** corresponds to the distance at which the formation of a solid-like structure due to the confinement is initiated. For [DEME][BF4], the sharp viscosity increase began at a clearly shorter distance than that for [DEME][TFSI], indicating that this ammonium salt restructures more readily than the TFSI salt, or in other words, that the BF4 salt is more easily crystallized. This consideration is also supported by the results of the crystallization temperature measurement using differential thermal calorimetry (DSC) [28]. As indicated by the DSC measurements, [DEME][BF4], for which the viscosity increases at a relatively long distance, shows a distinct crystallization temperature, whereas the TFSI salt only shows a glass transition temperature but not a crystallization temperature. The same trend is found for aromatic ILs. The viscosity of [BMIM][TFSI], which has a specific crystallization temperature, rises rapidly at a large distance, whereas the viscosity of [BMIM][BF4], which does not show a crystallization temperature, rises rapidly only at a short distance [29].

**3. Macroscopic tribological properties**

plate were 9.9 and 1.2 nm over an area of 5 × 5 μm2

**Figure 8.** Schematic of reciprocating-type tribotester.

**3.1. Reciprocating-type tribotests for evaluation of macroscopic properties**

Friction coefficients obtained for at least five trials were recorded and averaged.

Friction measurements were carried out using a conventional reciprocating tribotester, TRIBOGEAR TYPE 38 (Shinto Scientific Co. Ltd., Tokyo), using a glass ball of 10 mmφ and a glass plate. To obtain a clean surface, the glass ball and glass plate were treated in fresh nitric acid at 373 K for 75 min. The RMS roughness values measured by AFM for the glass ball and

is shown in **Figure 8**. The measurements were performed at a movement distance of 10 mm, with a sliding velocity ranging from 5.0 × 10−4 to 3.0 × 10−2 m s−1 under an applied load of 196– 980 mN at room temperature (295 ± 0.5 K). The friction force was measured by an all-in-one load converter from a gauge attached to the sample holder and was recorded as a function of time. The friction coefficient (*μ*) was calculated as the friction force divided by the normal load.

The Stribeck diagram of the friction behavior is used to explain the rubbing phenomena occurring in lubricated contacts [30]. A schematic representation of the Stribeck diagram is shown in **Figure 9**. For high values of *ηV*/*N*, the friction coefficient is linearly ascending due to fluid film lubrication, and the friction is related to the viscous dragging forces in the fluid film. When the load increases or fluid viscosity and/or velocity decreases, the *ηV*/*N* factor falls. Then, the fluid film becomes thinner, and consequently, the friction coefficient decreases down to the minimum value. For even smaller *ηV*/*N* values, the fluid film thickness is further reduced and solid-to-solid contact starts to occur, leading to an increase in the friction coefficient as the *ηV*/*N* factor decreases. Such a rise in the friction coefficient is also related to the fluid viscosity increase in some regions of the contact area under high contact pressure. These phenomena characterize the mixed lubrication regime. Further reduction in the *ηV*/*N* factor strengthens

, respectively. A schematic of the tribotester

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**Figure 7** plots *η*eff versus the applied load. The *η*eff values for [DEME][TFSI] and [DEME][BF4] increased with the applied load and plateaued when the load was increased to 0.4 mN. At higher loads (>0.4 mN), *η*eff of ca. 13,000 for [DEME][TFSI] was larger than the ca. 3000 value obtained for [DEME][BF4].

**Figure 7.** Plots of the effective viscosity *η*eff versus the applied load for [DEME][TFSI] (●) and [DEME][BF4] (□) [22].

In the RSM, the facing silica surfaces were completely separated to avoid partial contact and to allow the analysis of the boundary lubrication property of the confined liquid at a certain separation distance. In this configuration, an increase in the viscosity of the lubricant layer under confinement directly leads to increased friction at the sliding interface. The results obtained from RSM showed that the *η*eff of [DEME][BF4] is lower than that of [DEME][TFSI] at a similar separation distance (ca. 2 nm), meaning that under such confinement, the lubrication performance of [DEME][BF4] is better than that of [DEME][TFSI]. We thus concluded that, for higher loads (>0.4 mN), [DEME][BF4] is a better lubricant than [DEME][TFSI].

### **3. Macroscopic tribological properties**

We suppose that the rapid increase in the effective viscosity with decreasing distance observed in **Figure 6** corresponds to the distance at which the formation of a solid-like structure due to the confinement is initiated. For [DEME][BF4], the sharp viscosity increase began at a clearly shorter distance than that for [DEME][TFSI], indicating that this ammonium salt restructures more readily than the TFSI salt, or in other words, that the BF4 salt is more easily crystallized. This consideration is also supported by the results of the crystallization temperature measurement using differential thermal calorimetry (DSC) [28]. As indicated by the DSC measurements, [DEME][BF4], for which the viscosity increases at a relatively long distance, shows a distinct crystallization temperature, whereas the TFSI salt only shows a glass transition temperature but not a crystallization temperature. The same trend is found for aromatic ILs. The viscosity of [BMIM][TFSI], which has a specific crystallization temperature, rises rapidly at a large distance, whereas the viscosity of [BMIM][BF4], which does not show a crystallization

**Figure 7** plots *η*eff versus the applied load. The *η*eff values for [DEME][TFSI] and [DEME][BF4] increased with the applied load and plateaued when the load was increased to 0.4 mN. At higher loads (>0.4 mN), *η*eff of ca. 13,000 for [DEME][TFSI] was larger than the ca. 3000 value

**Figure 7.** Plots of the effective viscosity *η*eff versus the applied load for [DEME][TFSI] (●) and [DEME][BF4] (□) [22].

higher loads (>0.4 mN), [DEME][BF4] is a better lubricant than [DEME][TFSI].

In the RSM, the facing silica surfaces were completely separated to avoid partial contact and to allow the analysis of the boundary lubrication property of the confined liquid at a certain separation distance. In this configuration, an increase in the viscosity of the lubricant layer under confinement directly leads to increased friction at the sliding interface. The results obtained from RSM showed that the *η*eff of [DEME][BF4] is lower than that of [DEME][TFSI] at a similar separation distance (ca. 2 nm), meaning that under such confinement, the lubrication performance of [DEME][BF4] is better than that of [DEME][TFSI]. We thus concluded that, for

temperature, rises rapidly only at a short distance [29].

obtained for [DEME][BF4].

420 Progress and Developments in Ionic Liquids

### **3.1. Reciprocating-type tribotests for evaluation of macroscopic properties**

Friction measurements were carried out using a conventional reciprocating tribotester, TRIBOGEAR TYPE 38 (Shinto Scientific Co. Ltd., Tokyo), using a glass ball of 10 mmφ and a glass plate. To obtain a clean surface, the glass ball and glass plate were treated in fresh nitric acid at 373 K for 75 min. The RMS roughness values measured by AFM for the glass ball and plate were 9.9 and 1.2 nm over an area of 5 × 5 μm2 , respectively. A schematic of the tribotester is shown in **Figure 8**. The measurements were performed at a movement distance of 10 mm, with a sliding velocity ranging from 5.0 × 10−4 to 3.0 × 10−2 m s−1 under an applied load of 196– 980 mN at room temperature (295 ± 0.5 K). The friction force was measured by an all-in-one load converter from a gauge attached to the sample holder and was recorded as a function of time. The friction coefficient (*μ*) was calculated as the friction force divided by the normal load. Friction coefficients obtained for at least five trials were recorded and averaged.

**Figure 8.** Schematic of reciprocating-type tribotester.

The Stribeck diagram of the friction behavior is used to explain the rubbing phenomena occurring in lubricated contacts [30]. A schematic representation of the Stribeck diagram is shown in **Figure 9**. For high values of *ηV*/*N*, the friction coefficient is linearly ascending due to fluid film lubrication, and the friction is related to the viscous dragging forces in the fluid film. When the load increases or fluid viscosity and/or velocity decreases, the *ηV*/*N* factor falls. Then, the fluid film becomes thinner, and consequently, the friction coefficient decreases down to the minimum value. For even smaller *ηV*/*N* values, the fluid film thickness is further reduced and solid-to-solid contact starts to occur, leading to an increase in the friction coefficient as the *ηV*/*N* factor decreases. Such a rise in the friction coefficient is also related to the fluid viscosity increase in some regions of the contact area under high contact pressure. These phenomena characterize the mixed lubrication regime. Further reduction in the *ηV*/*N* factor strengthens the solid-to-solid contact, and the film thickness becomes smaller than the height of the surface asperities, leading to the transition of the boundary lubrication regime.

lubrication performance of [DEME][BF4] is better than that of [DEME][TFSI] under such confined conditions. The inversion of the order of IL viscosities under confinement from the order of bulk viscosity values appears to be due to the differences in the layering structure of

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**Figure 10.** Friction coefficient between a glass ball and a glass plate in [DEME][TFSI] (●), in [DEME][BF4] (□), by sliding the glass plate over a distance range of 10 mm at sliding velocity of 5.0 × 10−4 m s−1–3.0 × 10−2 m s−1 and under an applied

We now discuss the possible structural origin for the observed viscosity effects. Canova et al. studied the layering structure of ILs ([BMIM][TFSI] and [BMIM][BF4]) confined to a silica contact plane using molecular dynamics simulations. For [BMIM][TFSI], it was found that ion pairs orient alternately to form a checker-board structure. This means that each ion is forced to contact ions with the same charge during shearing [31]. The repulsive force between the ions with the same sign of the charge destabilizes the crystal structure of the IL and triggers an overall restructuring of inner molecular layers to form a more stable configuration, resulting in high friction. In contrast, [BMIM][BF4] formed a layer-by-layer structure where cations and anions did not suffer from such repulsions because each ion layer is sandwiched between the oppositely charged layers, leading to smooth shearing and low friction. Even though the cations used in our study are different from those studied by Canova et al., based on this insight, we ascribe the better boundary lubrication performance of [DEME][BF4] relative to that of [DEME][TFSI] to the differences in the layered structures of these ILs under confine-

The RSM showed that ILs were maintained between the silica surfaces at a surface separation of 2.5 nm for [DEME][TFSI] and 2.1 nm for [DEME][BF4], which are usually referred to as hardwall thicknesses. Assuming a cubic packing geometry and following Horn et al.'s [23] method, the diameters of the ion pairs determined from the density were 0.79 nm for [DEME][TFSI] and 0.69 nm for [DEME][BF4]. Based on these values, we determined that in both systems, three layers of ILs were trapped between the silica surfaces. The dramatic reduction in the friction coefficient in "the boundary lubrication region" with ILs was presumably due to the presence of the three layers of ILs between the silica surfaces under an applied load of 196 mN.

the ILs due to the differences in their anion structure.

load of 196–980 mN at 298 K. Dashed lines are guides to the eyes [22].

ment.

**Figure 9.** Schematic of Stribeck diagram; friction coefficient as a function of the lubrication parameter: *ηV*/*N*. *η*: fluid viscosity, *V*: sliding velocity, *N*: normal load.

We studied the boundary lubrication properties of ILs between a glass ball and a glass plate by using a macroscopic tribotester. **Figure 10** plots the friction coefficients between a glass ball and a glass plate in [DEME][TFSI] or [DEME][BF4] versus the sliding velocity (*V*) divided by the load (*L*) (m s−1 N−1). The measurements were conducted by sliding a glass plate over a distance of 10 mm at various sliding velocities from 5.0 × 10−4 to 3.0 × 10−2 m s−1 and under various applied loads from 196 to 980 mN at 298 K. The friction coefficient for [DEME][BF4] increased from 0.05 to 0.08 as the *V*/*L* parameter decreased from 1.5 × 10−1 to 5.1 × 10−3 and was nearly constant at *μ* ≈ 0.08 over the *V*/*L* range of 5.1 × 10−3 to 1.0 × 10−3. The friction coefficient results for [DEME][TFSI] were similar to those for [DEME][BF4]; however, the friction coefficient observed in the region of *V*/*L* ranging from 1.5 × 10−1 to 1.0 × 10−4 was significantly higher than that of [DEME][BF4]. Such dependence of the friction coefficient on *V*/*L* corresponds to the shift in the lubrication regime of [DEME][TFSI] and [DEME][BF4] from mixed lubrication to boundary lubrication. As a control, the friction coefficient was measured between the glass ball and glass plate without an IL and was found to be ≈0.7 at the sliding velocity of 1.0 × 10−3 m s−1 and under a normal load of 196 mN. Thus, the presence of ILs at the glass-glass interface dramatically reduced the friction coefficient to less than 20% of the value obtained without ILs.

#### **3.2. Comparison of measurement results revealing a correlation between the macroscale friction phenomena and the physicochemical properties of ILs in nanospace**

For a confined IL, boundary lubrication is dominant, and the contribution of hydrodynamic lubrication due to the change in lubricant thickness is not effective. Therefore, the difference in boundary lubrication under the measurement conditions between [DEME][BF4] and [DEME][TFSI] can be explained based on the effective viscosity obtained from RSM. The obtained RSM results showed that the *η*eff of [DEME][BF4] was lower than that of [DEME][TFSI] when these ILs were maintained at a similar separation distance (ca. 2 nm), indicating that the lubrication performance of [DEME][BF4] is better than that of [DEME][TFSI] under such confined conditions. The inversion of the order of IL viscosities under confinement from the order of bulk viscosity values appears to be due to the differences in the layering structure of the ILs due to the differences in their anion structure.

the solid-to-solid contact, and the film thickness becomes smaller than the height of the surface

**Figure 9.** Schematic of Stribeck diagram; friction coefficient as a function of the lubrication parameter: *ηV*/*N*. *η*: fluid

We studied the boundary lubrication properties of ILs between a glass ball and a glass plate by using a macroscopic tribotester. **Figure 10** plots the friction coefficients between a glass ball and a glass plate in [DEME][TFSI] or [DEME][BF4] versus the sliding velocity (*V*) divided by the load (*L*) (m s−1 N−1). The measurements were conducted by sliding a glass plate over a distance of 10 mm at various sliding velocities from 5.0 × 10−4 to 3.0 × 10−2 m s−1 and under various applied loads from 196 to 980 mN at 298 K. The friction coefficient for [DEME][BF4] increased from 0.05 to 0.08 as the *V*/*L* parameter decreased from 1.5 × 10−1 to 5.1 × 10−3 and was nearly constant at *μ* ≈ 0.08 over the *V*/*L* range of 5.1 × 10−3 to 1.0 × 10−3. The friction coefficient results for [DEME][TFSI] were similar to those for [DEME][BF4]; however, the friction coefficient observed in the region of *V*/*L* ranging from 1.5 × 10−1 to 1.0 × 10−4 was significantly higher than that of [DEME][BF4]. Such dependence of the friction coefficient on *V*/*L* corresponds to the shift in the lubrication regime of [DEME][TFSI] and [DEME][BF4] from mixed lubrication to boundary lubrication. As a control, the friction coefficient was measured between the glass ball and glass plate without an IL and was found to be ≈0.7 at the sliding velocity of 1.0 × 10−3 m s−1 and under a normal load of 196 mN. Thus, the presence of ILs at the glass-glass interface dramatically reduced the friction coefficient to less than 20% of the value obtained

**3.2. Comparison of measurement results revealing a correlation between the macroscale**

For a confined IL, boundary lubrication is dominant, and the contribution of hydrodynamic lubrication due to the change in lubricant thickness is not effective. Therefore, the difference in boundary lubrication under the measurement conditions between [DEME][BF4] and [DEME][TFSI] can be explained based on the effective viscosity obtained from RSM. The obtained RSM results showed that the *η*eff of [DEME][BF4] was lower than that of [DEME][TFSI] when these ILs were maintained at a similar separation distance (ca. 2 nm), indicating that the

**friction phenomena and the physicochemical properties of ILs in nanospace**

asperities, leading to the transition of the boundary lubrication regime.

viscosity, *V*: sliding velocity, *N*: normal load.

422 Progress and Developments in Ionic Liquids

without ILs.

**Figure 10.** Friction coefficient between a glass ball and a glass plate in [DEME][TFSI] (●), in [DEME][BF4] (□), by sliding the glass plate over a distance range of 10 mm at sliding velocity of 5.0 × 10−4 m s−1–3.0 × 10−2 m s−1 and under an applied load of 196–980 mN at 298 K. Dashed lines are guides to the eyes [22].

We now discuss the possible structural origin for the observed viscosity effects. Canova et al. studied the layering structure of ILs ([BMIM][TFSI] and [BMIM][BF4]) confined to a silica contact plane using molecular dynamics simulations. For [BMIM][TFSI], it was found that ion pairs orient alternately to form a checker-board structure. This means that each ion is forced to contact ions with the same charge during shearing [31]. The repulsive force between the ions with the same sign of the charge destabilizes the crystal structure of the IL and triggers an overall restructuring of inner molecular layers to form a more stable configuration, resulting in high friction. In contrast, [BMIM][BF4] formed a layer-by-layer structure where cations and anions did not suffer from such repulsions because each ion layer is sandwiched between the oppositely charged layers, leading to smooth shearing and low friction. Even though the cations used in our study are different from those studied by Canova et al., based on this insight, we ascribe the better boundary lubrication performance of [DEME][BF4] relative to that of [DEME][TFSI] to the differences in the layered structures of these ILs under confinement.

The RSM showed that ILs were maintained between the silica surfaces at a surface separation of 2.5 nm for [DEME][TFSI] and 2.1 nm for [DEME][BF4], which are usually referred to as hardwall thicknesses. Assuming a cubic packing geometry and following Horn et al.'s [23] method, the diameters of the ion pairs determined from the density were 0.79 nm for [DEME][TFSI] and 0.69 nm for [DEME][BF4]. Based on these values, we determined that in both systems, three layers of ILs were trapped between the silica surfaces. The dramatic reduction in the friction coefficient in "the boundary lubrication region" with ILs was presumably due to the presence of the three layers of ILs between the silica surfaces under an applied load of 196 mN.

These results indicated that the tribotester macroscopic tribological properties correspond to the nanoscale lubrication properties obtained from nanoscopic RSM.

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### **4. Conclusion**

We performed RSM and reciprocating-type tribotests to evaluate the friction properties of lubrication systems consisting of some types of ILs between the silica surfaces. In the case of [DEME][BF4] and [DEME][TFSI], the RSM results revealed that IL layers with a thickness of ca. 2 nm remained between the silica surfaces under applied loads. For these conditions, the effective viscosity of the IL including BF4 anion was smaller than that of the IL including the TFSI anion. Similarly, in the boundary lubrication regime, the friction coefficient *μ* of [DEME] [BF4] obtained by the tribotests was lower than that of [DEME][TFSI]. Even though the RSM and tribotest measurement were performed under different applied loads, the difference in the friction coefficient between [DEME][TFSI] and [DEME][BF4] in the boundary lubrication regime observed by the tribotest corresponded to their behavior under confinement between the silica surfaces, as observed by RSM. These results indicate that the nanoscale properties observed by RSM can provide important insights for the study of the friction coefficients (macrolubrication properties) obtained by the tribotests.

### **Acknowledgements**

This work was supported in part by the "Green Tribology Innovation Network" Advanced Environmental Materials Area, Green Networks of Excellence (GRENE) program and Grantsin-Aid for Scientific Research (nos. 25810091, 26820034, and 30399258) sponsored by the Ministry of Education, Culture, Sports, Science and Technology (MEXT) in Japan.

### **Author details**

Toshio Kamijo1 , Hiroyuki Arafune1 , Takashi Morinaga1 , Takaya Sato1\* and Kazue Kurihara2,3

\*Address all correspondence to: takayasa@tsuruoka-nct.ac.jp

1 Department of Creative Engineering, National Institute of Technology, Tsuruoka College, Tsuruoka, Japan

2 Institute of Multidisciplinary Research for Advanced Materials (IMRAM), Tohoku University, Aoba-ku, Sendai, Japan

3 WPI-Advanced Institute of Materials Research (WPI-AIMR), Tohoku University, Aoba-ku, Sendai, Japan

### **References**

These results indicated that the tribotester macroscopic tribological properties correspond to

We performed RSM and reciprocating-type tribotests to evaluate the friction properties of lubrication systems consisting of some types of ILs between the silica surfaces. In the case of [DEME][BF4] and [DEME][TFSI], the RSM results revealed that IL layers with a thickness of ca. 2 nm remained between the silica surfaces under applied loads. For these conditions, the effective viscosity of the IL including BF4 anion was smaller than that of the IL including the TFSI anion. Similarly, in the boundary lubrication regime, the friction coefficient *μ* of [DEME] [BF4] obtained by the tribotests was lower than that of [DEME][TFSI]. Even though the RSM and tribotest measurement were performed under different applied loads, the difference in the friction coefficient between [DEME][TFSI] and [DEME][BF4] in the boundary lubrication regime observed by the tribotest corresponded to their behavior under confinement between the silica surfaces, as observed by RSM. These results indicate that the nanoscale properties observed by RSM can provide important insights for the study of the friction coefficients

This work was supported in part by the "Green Tribology Innovation Network" Advanced Environmental Materials Area, Green Networks of Excellence (GRENE) program and Grantsin-Aid for Scientific Research (nos. 25810091, 26820034, and 30399258) sponsored by the

, Takashi Morinaga1

1 Department of Creative Engineering, National Institute of Technology, Tsuruoka College,

2 Institute of Multidisciplinary Research for Advanced Materials (IMRAM), Tohoku Univer-

3 WPI-Advanced Institute of Materials Research (WPI-AIMR), Tohoku University, Aoba-ku,

, Takaya Sato1\* and Kazue Kurihara2,3

Ministry of Education, Culture, Sports, Science and Technology (MEXT) in Japan.

the nanoscale lubrication properties obtained from nanoscopic RSM.

(macrolubrication properties) obtained by the tribotests.

, Hiroyuki Arafune1

\*Address all correspondence to: takayasa@tsuruoka-nct.ac.jp

**4. Conclusion**

424 Progress and Developments in Ionic Liquids

**Acknowledgements**

**Author details**

Toshio Kamijo1

Tsuruoka, Japan

Sendai, Japan

sity, Aoba-ku, Sendai, Japan


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am201646k

426 Progress and Developments in Ionic Liquids


**Chapter 18**

**Provisional chapter**

**Ecotoxicity of Ionic Liquids Towards** *Vibrio fischeri***:**

Ionic liquids (ILs) have gained significant attention within the academic and industrial circle owing to their attractive and unique characters. However, the usual green image of the ionic liquids mainly associated with their low vapour pressure has become increasingly doubtful. Several recent studies have highlighted the underestimated ILs toxicity which has not been adequately addressed. Therefore, improving the understanding of the ionic liquids toxicity towards aquatic organisms will undoubtedly lead to formulation of right solutions to address the toxicity problem hence contributing towards the development of green and sustainable ILs‐based technology. The chapter provides a collective review of studies conducted on the effect of ILs structure on toxicity, specifically focussing on the various types of cations and anions, and the length of the alkyl chain attached. Based on the qualitative outcome from the review, a discussion on the development of statistical modelling on the impact of ILs structural features towards the overall toxicity is presented. The application of quantitative structure activity relationship

(QSAR) for developing the predictive model for toxicity is highlighted.

**Keywords:** ionic liquids, ecotoxicity, *Vibrio fischeri*, structural features, QSAR

**Ecotoxicity of Ionic Liquids Towards Vibrio fischeri:** 

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution,

© 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

and reproduction in any medium, provided the original work is properly cited.

One of the major developments made on green solvents focuses on the design of new and more environmental friendly solvents. From the green chemistry perspective, green solvent should be non‐toxic, readily biodegradable and is synthesized using the environmental friendly synthesis procedure, whilst at the same time able to meet the application target technologically and economically [1, 2]. For several years, ionic liquids (ILs) have been gaining significant attention as the candidate for future 'green solvents' from the scientific and indus-

**Experimental and QSAR Studies**

**Experimental and QSAR Studies**

Additional information is available at the end of the chapter

Additional information is available at the end of the chapter

Mohamed Ibrahim Abdul Mutalib and

Mohamed Ibrahim Abdul Mutalib and

Ouahid Ben Ghanem

Ouahid Ben Ghanem

http://dx.doi.org/10.5772/65795

**Abstract**

**1. Introduction**

**Provisional chapter**

### **Ecotoxicity of Ionic Liquids Towards** *Vibrio fischeri***: Experimental and QSAR Studies Ecotoxicity of Ionic Liquids Towards Vibrio fischeri: Experimental and QSAR Studies**

Mohamed Ibrahim Abdul Mutalib and Ouahid Ben Ghanem Ouahid Ben Ghanem Additional information is available at the end of the chapter

Mohamed Ibrahim Abdul Mutalib and

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/65795

#### **Abstract**

Ionic liquids (ILs) have gained significant attention within the academic and industrial circle owing to their attractive and unique characters. However, the usual green image of the ionic liquids mainly associated with their low vapour pressure has become increasingly doubtful. Several recent studies have highlighted the underestimated ILs toxicity which has not been adequately addressed. Therefore, improving the understanding of the ionic liquids toxicity towards aquatic organisms will undoubtedly lead to formulation of right solutions to address the toxicity problem hence contributing towards the development of green and sustainable ILs‐based technology. The chapter provides a collective review of studies conducted on the effect of ILs structure on toxicity, specifically focussing on the various types of cations and anions, and the length of the alkyl chain attached. Based on the qualitative outcome from the review, a discussion on the development of statistical modelling on the impact of ILs structural features towards the overall toxicity is presented. The application of quantitative structure activity relationship (QSAR) for developing the predictive model for toxicity is highlighted.

**Keywords:** ionic liquids, ecotoxicity, *Vibrio fischeri*, structural features, QSAR

### **1. Introduction**

One of the major developments made on green solvents focuses on the design of new and more environmental friendly solvents. From the green chemistry perspective, green solvent should be non‐toxic, readily biodegradable and is synthesized using the environmental friendly synthesis procedure, whilst at the same time able to meet the application target technologically and economically [1, 2]. For several years, ionic liquids (ILs) have been gaining significant attention as the candidate for future 'green solvents' from the scientific and indus-

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

trial community. It has shown several advantages over the volatile organic solvents (VOCs), among others, covering three major aspects namely:


Whilst most studies have converged opinion on the first two aspects, the third has been increasingly contested. The work presented focuses on addressing the latter in order to provide further clarity pertaining to the issue of ILs toxicity. It has been generally agreed that the unique feature which enables ILs to capture significant interest is the ability to design them for specific application by changing the cation and anion coupling to meet specified physical, chemical and biological properties. To date, significant number of ILs has been developed and most of them are now commercially available. In addition, there are few more millions of possible ILs that could be theoretically synthesized [3].

ILs are mainly designed to be inflammable, non‐volatile, and non‐explosive media with a high thermal stability [4]. Due to their hardly measurable vapour pressure, they are not expected to contribute towards atmospheric pollution. On the contrary, most of them display high aqueous solubility. Even the supposedly most hydrophobic IL was found to exert some degree of solubility hence allowing their possible dispersion into aquatic systems, raising concerns on their subsequent environmental impact [5, 6]. Given the almost

unlimited combinations of possible ILs that could be developed, the toxicity determination could become highly laborious and extremely costly as well as time consuming. Developing predictive methods would require systematic understanding of the complex interactions between the cation and anion pairings leading to the toxicity properties, which

Ecotoxicity of Ionic Liquids Towards *Vibrio fischeri*: Experimental and QSAR Studies

http://dx.doi.org/10.5772/65795

431

On another note, recent studies have pointed towards some possible draw backs on the use of ILs on an industrial scale. One of the major concerns highlighted was from aquatic toxicity studies showing potential drastic impact of some ILs which was considered as being green, on various aquatic organisms. A number of these ILs were found to possess higher toxicity than some of the acute organic solvents. Therefore, evaluating their overall toxicity has become of primary interest to the industries and public at large prior to their bulk application. This necessitates the development of a predictive method to substitute the laborious manual toxicity measurement in light of the increasing interest on ILs

are not easily done.

**Figure 1.** Structures of cations and anions discussed in the study.

applications.

**Figure 1.** Structures of cations and anions discussed in the study.

trial community. It has shown several advantages over the volatile organic solvents (VOCs),

**1.** Extremely low vapour pressure in comparison to the VOCs resulting in insignificant

**3.** Non‐toxic perception due to minute losses through vaporization into the atmosphere com-

Whilst most studies have converged opinion on the first two aspects, the third has been increasingly contested. The work presented focuses on addressing the latter in order to provide further clarity pertaining to the issue of ILs toxicity. It has been generally agreed that the unique feature which enables ILs to capture significant interest is the ability to design them for specific application by changing the cation and anion coupling to meet specified physical, chemical and biological properties. To date, significant number of ILs has been developed and most of them are now commercially available. In addition, there are few more millions of

ILs are mainly designed to be inflammable, non‐volatile, and non‐explosive media with a high thermal stability [4]. Due to their hardly measurable vapour pressure, they are not expected to contribute towards atmospheric pollution. On the contrary, most of them display high aqueous solubility. Even the supposedly most hydrophobic IL was found to exert some degree of solubility hence allowing their possible dispersion into aquatic systems, raising concerns on their subsequent environmental impact [5, 6]. Given the almost

**2.** Inflammable as opposed to the flammable VOCs hence easier to handle and store.

among others, covering three major aspects namely:

possible ILs that could be theoretically synthesized [3].

vaporization losses to the atmosphere.

pared to the VOCs.

430 Progress and Developments in Ionic Liquids

unlimited combinations of possible ILs that could be developed, the toxicity determination could become highly laborious and extremely costly as well as time consuming. Developing predictive methods would require systematic understanding of the complex interactions between the cation and anion pairings leading to the toxicity properties, which are not easily done.

On another note, recent studies have pointed towards some possible draw backs on the use of ILs on an industrial scale. One of the major concerns highlighted was from aquatic toxicity studies showing potential drastic impact of some ILs which was considered as being green, on various aquatic organisms. A number of these ILs were found to possess higher toxicity than some of the acute organic solvents. Therefore, evaluating their overall toxicity has become of primary interest to the industries and public at large prior to their bulk application. This necessitates the development of a predictive method to substitute the laborious manual toxicity measurement in light of the increasing interest on ILs applications.

It has been known for some time that the structural feature of ILs may have different contributions on the ILs overall toxicity. Hence, a systematic study to assess the variation of the ILs structure on its overall toxicity has to be commissioned separately. The review attempted at investigating the influence of the changes in ILs structural features involves: (i) the cation core and the functional group substituent, (ii) the length of the alkyl chain substituent, and (iii) anion nature, on the ILs toxicity using bioluminescent *Vibrio fischeri.* The effective concentration at 50% i.e. EC50 values for 83 ILs were collected from different literature reports in order to configure the effect of changing the functional group and the structure of the ILs on its toxicity. All the various functional group substituents together with their different structures reflected on the cation and anion are shown in **Figure 1**. The study also introduces some further insight into the recent development pertaining to the quantitative structure activity relationship models (QSAR) which are proposed as the approach for developing models for predicting ILs toxicity based on the *V. fischeri*.

**2.1. Effect of the cation core**

group A.

**Group A**

**Group B**

Numerous cations have been used to create ILs such as imidazolium and pyridinium, which have been appearing mostly in past ILs studies particularly for the room temperature ionic liquids (RTILs). In the study, the influences of the cation core on the ILs toxicity were investigated using imidazolium‐, pyridinium‐, pyrrolidinium‐, piperidinium‐ and morpholinium‐based cations. The structure with regards to the chain length variation on the cation core was kept within 1‐butyl‐(1 or 3)‐methyl (cation) bromide ILs [15], as shown in **Table 2**,

mim][Br] 1002

Ecotoxicity of Ionic Liquids Towards *Vibrio fischeri*: Experimental and QSAR Studies

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433

mim][Br] 334

mim][Br] 50.9

mpip][Br] 3958

mpip][Br] 230

mpip][Br] 29.3

mpyrr][Br] 5525

mpyrr][Br] 387

mpyrr][Br] 50.8

mpy][Br] 130.48

mpy][Br] 29.99

mpy][Br] 1.77

mmor][Br] 66,729

mim][BF4] 1846.44

mim][BF4] 801.88

mim][BF4] 331.29

**Ionic liquid names Abbreviation EC50 in mg L<sup>−</sup><sup>1</sup>**

1‐Butyl‐1‐methylimidazolium bromide [C<sup>4</sup>

1‐Hexyl‐1‐methylimidazolium bromide [C<sup>6</sup>

1‐Octyl‐1‐methylimidazolium bromide [C8

1‐Butyl‐1‐methylpiperidinium bromide [C<sup>4</sup>

1‐Hexyl‐1‐methylpiperidinium bromide [C<sup>6</sup>

1‐Octyl‐1‐methylpiperidinium bromide [C8

1‐Butyl‐1‐methylpyrrolidinium bromide [C<sup>4</sup>

1‐Hexyl‐1‐methylpyrrolidinium bromide [C<sup>6</sup>

1‐Octyl‐1‐methylpyrrolidinium bromide [C8

1‐Butyl‐3‐methylpyridinium bromide [C<sup>4</sup>

1‐Hexyl‐3‐methylpyridinium bromide [C<sup>6</sup>

1‐Octyl‐3‐methylpyridinium bromide [C8

1‐Butyl‐1‐methylmorpholinium bromide [C<sup>4</sup>

1‐Propyl‐3‐methylimidazolium tetrafluoroborate [C<sup>3</sup>

1‐Butyl‐3‐methylimidazolium tetrafluoroborate [C<sup>4</sup>

1‐Pentyl‐3‐methylimidazolium tetrafluoroborate [C<sup>5</sup>

1‐Methylimidazole [mim] 2864 1‐Methylmorpholine [mmor] 2328 Pyridine [py] 867 1‐Methylpiperidine [mpip] 700 1‐Methylpyrrolidine [mpyrr] 493 2,3‐Dimethylpyridine [2,3mpy] 238 3,5‐Dimethylpyridine [3,5mpy] 65.9 2,3,5‐Trimethylpyridine [2,3,5mpy] 43

### **2. Ecotoxicity measurement using** *V. fischeri*

The bioluminescent *V. fischeri* is a Gram‐negative, rod‐shaped bacterium that bioluminesces through a population‐dependent mechanism called quorum sensing [7, 8]. The Microtox assay system (MAS) against bioluminescent *V. fischeri* was often chosen as the first sequence in a test battery to evaluate the toxicity of chemicals due to their simple, quick, good sensitivity and cost‐effectiveness as well as a widely acceptable method for ecotoxicity assessments [9, 10]. In addition, *V. fischeri* is also sensitive to a wide variety of toxic substances hence making it a popular proxy method for detecting environmental pollutants for ecotoxicity studies. Furthermore, *V. fischeri* is also considered as a common test organism, well published in the Aquatic Toxicity Information Retrieval database (AQUIRE) produced by the US Environmental Protection Agency (EPA). Several other large environmental‐based organizations have also recommended these species for aquatic toxicity assessment [11, 12]. It was earlier reported that *V. fischeri* assay yielded fairly replicable results which were comparable to those obtained using the standard tests, with an advantage of only requiring about 5% of the actual work involved in the standard procedures. Therefore, it was suggested that the MAS be used as a pre‐screening tool in the hazard assessment of chemicals [13].

In the reported study, the ILs are classified based on their EC50 values according to the hazard ranking as described by Passino and Smith [14], shown in **Table 1**.


**Table 1.** Hazard ranking classification for aquatic organisms.

#### **2.1. Effect of the cation core**

It has been known for some time that the structural feature of ILs may have different contributions on the ILs overall toxicity. Hence, a systematic study to assess the variation of the ILs structure on its overall toxicity has to be commissioned separately. The review attempted at investigating the influence of the changes in ILs structural features involves: (i) the cation core and the functional group substituent, (ii) the length of the alkyl chain substituent, and (iii) anion nature, on the ILs toxicity using bioluminescent *Vibrio fischeri.* The effective concentration at 50% i.e. EC50 values for 83 ILs were collected from different literature reports in order to configure the effect of changing the functional group and the structure of the ILs on its toxicity. All the various functional group substituents together with their different structures reflected on the cation and anion are shown in **Figure 1**. The study also introduces some further insight into the recent development pertaining to the quantitative structure activity relationship models (QSAR) which are proposed as the approach for developing models for

The bioluminescent *V. fischeri* is a Gram‐negative, rod‐shaped bacterium that bioluminesces through a population‐dependent mechanism called quorum sensing [7, 8]. The Microtox assay system (MAS) against bioluminescent *V. fischeri* was often chosen as the first sequence in a test battery to evaluate the toxicity of chemicals due to their simple, quick, good sensitivity and cost‐effectiveness as well as a widely acceptable method for ecotoxicity assessments [9, 10]. In addition, *V. fischeri* is also sensitive to a wide variety of toxic substances hence making it a popular proxy method for detecting environmental pollutants for ecotoxicity studies. Furthermore, *V. fischeri* is also considered as a common test organism, well published in the Aquatic Toxicity Information Retrieval database (AQUIRE) produced by the US Environmental Protection Agency (EPA). Several other large environmental‐based organizations have also recommended these species for aquatic toxicity assessment [11, 12]. It was earlier reported that *V. fischeri* assay yielded fairly replicable results which were comparable to those obtained using the standard tests, with an advantage of only requiring about 5% of the actual work involved in the standard procedures. Therefore, it was suggested that the MAS

In the reported study, the ILs are classified based on their EC50 values according to the hazard

be used as a pre‐screening tool in the hazard assessment of chemicals [13].

ranking as described by Passino and Smith [14], shown in **Table 1**.

**Hazard ranking Concentration of ILs in mg L<sup>−</sup><sup>1</sup>**

Practically harmless 100–1000 Moderately toxic 10–100 Slightly toxic 1–10 Highly toxic 0.1–1

**Table 1.** Hazard ranking classification for aquatic organisms.

predicting ILs toxicity based on the *V. fischeri*.

432 Progress and Developments in Ionic Liquids

**2. Ecotoxicity measurement using** *V. fischeri*

Numerous cations have been used to create ILs such as imidazolium and pyridinium, which have been appearing mostly in past ILs studies particularly for the room temperature ionic liquids (RTILs). In the study, the influences of the cation core on the ILs toxicity were investigated using imidazolium‐, pyridinium‐, pyrrolidinium‐, piperidinium‐ and morpholinium‐based cations. The structure with regards to the chain length variation on the cation core was kept within 1‐butyl‐(1 or 3)‐methyl (cation) bromide ILs [15], as shown in **Table 2**, group A.



**Ionic liquid names Abbreviation EC50 in mg L<sup>−</sup><sup>1</sup>** Cholinium acetate [Chol][Ac] 673.21 Cholinium dihydrogenphosphate [Chol][DHPhosp] 572.72 Cholinium propanoate [Chol][Prop] 487.9 Cholinium chloride [Chol]Cl 469.34 Cholinium salicylate [Chol][Sal] 236.11 Cholinium bitartrate [Chol][Bit] 37.9 Cholinium dihydrogencitrate [Chol][DHCit] 37.23 2‐Hydroxyethanolamine formate [2‐HEA][F] 700 2‐Hydroxyethanolamine butanoate [2‐HEA][B] 2239 2‐Hydroxydiethanolamine formate [2‐HDEA][F] 800 2‐Hydroxydiethanolamine acetate [2‐HDEA][ace] 1750 2‐Hydroxydiethanolamine propionate [2‐HDEA][Pr] 650 2‐Hydroxydiethanolamine butanoate [2‐HDEA][B] 800 2‐Hydroxydiethanolamine isobutanoate [2‐HDEA][iB] 850 2‐Hydroxydiethanolamine pentanoate [2‐HDEA][Pe] 350 2‐Hydroxytriethanolamine butanoate [2‐HTEA][B] 501 2‐Hydroxytriethanolamine pentanoate [2‐HTEA][Pe] 461

mim][Cl] 9213.24

Ecotoxicity of Ionic Liquids Towards *Vibrio fischeri*: Experimental and QSAR Studies

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435

mim][TfO] 1430.13

mim][Ace] 1321.25

OHmim][Gly] 11649.23

OHmim][Ala] 8123.27

OHmim][Ser] 10526

OHmim][Pro] 14509

OHmim][I] 1972.20

] 4250.70

] 2974.71

] 1631.25

] 9.99

] 4896.94

mim][PF<sup>6</sup>

[C<sup>2</sup>

[C<sup>2</sup>

Methanol MeOH 320400 Dimethyl sulfoxide DMSO 98359.84

mim][EtSO<sup>4</sup>

mim][NTf<sup>2</sup>

mim][FeCl<sup>4</sup>

OHmim][NTf<sup>2</sup>

1‐Ethyl‐3‐methylimidazolium chloride [C<sup>2</sup>

1‐Ethyl‐3‐methylimidazolium hexafluorophosphate [C<sup>2</sup>

1‐Ethyl‐3‐methylimidazolium ethylsulphate [C<sup>2</sup>

1‐Ethyl‐3‐methylimidazolium triflate [C<sup>2</sup>

1‐Ethyl‐3‐methylimidazolium acetate [C<sup>2</sup>

1‐Ethyl‐3‐methylimidazolium tetrachloroferrate [C<sup>2</sup>

1‐(2‐Hydroxylethyl)‐3‐methylimidazolium glycinate [C<sup>2</sup>

1‐(2‐Hydroxylethyl)‐3‐methylimidazolium alaninate [C<sup>2</sup>

1‐(2‐Hydroxylethyl)‐3‐methylimidazolium serinate [C<sup>2</sup>

1‐(2‐Hydroxylethyl)‐3‐methylimidazolium prolinate [C<sup>2</sup>

1‐(2‐Hydroxylethyl)‐3‐methylimidazolium iodide [C<sup>2</sup>

1‐(2‐Hydroxylethyl)‐3‐methylimidazolium bis((trifluoromethyl)sulfonyl)amide

1‐Ethyl‐3‐methylimidazolium bis((trifluoromethyl)

sulfonyl)amide

**Group D**


**Ionic liquid names Abbreviation EC50 in mg L<sup>−</sup><sup>1</sup>**

1‐Decyl‐3‐methylimidazolium tetrafluoroborate [C10mim][BF4] 0.20

1‐Decyl‐3‐methylimidazolium chloride [C10mim][Cl] 0.15 1‐Tetradecyl‐3‐methylimidazolium chloride [C14mim][Cl] 0.22 1‐Hexadecyl‐3‐methylimidazolium chloride [C16mim][Cl] 0.58 1‐Octadecyl‐3‐methylimidazolium chloride [C18mim][Cl] 10.46

[C<sup>2</sup>

[C<sup>3</sup>

[C<sup>4</sup>

[C<sup>5</sup>

[C<sup>6</sup>

[C<sup>7</sup>

[C8

Tetramethylammonium bromide [N1111][Br] >15405.0 Tetraethylammonium bromide [N2222][Br] 21016.00 Tetrabutylammonium bromide [N4444][Br] 600.28 Hexyltriethylammonium bromide [N6222][Br] 64.65 Tetrabutylphosphonium bromide [P2222][Br] 174.03 Tributylethylphosphonium diethylphosphate [P4442][Br] 451.71 Trihexyl(tetradecyl)phosphonium bromide [N66614][Br] 1449.09

Cholinium bicarbonate [Chol][Bic] >20000 Cholinium butanoate [Chol][But] 884.1

mim][NTf<sup>2</sup>

mim][NTf<sup>2</sup>

mim][NTf<sup>2</sup>

mim][NTf<sup>2</sup>

mim][NTf<sup>2</sup>

mim][NTf<sup>2</sup>

mim][NTf<sup>2</sup>

mim][BF4] 384.44

mim][BF4] 73.81

mim][BF4] 7.25

mim][BF4] 1.55

mim][Cl] 3134.68

mim][Cl] 515.49

mim][Cl] 164.78

] 330.23

] 240.18

] 141.99

] 46.87

] 22.8

] 19.25

] 6.44

mim][Cl] 2.36

1‐Hexyl‐3‐methylimidazolium tetrafluoroborate [C<sup>6</sup>

434 Progress and Developments in Ionic Liquids

1‐Heptyl‐3‐methylimidazolium tetrafluoroborate [C<sup>7</sup>

1‐Octyl‐3‐methylimidazolium tetrafluoroborate [C8

1‐Nonyl‐3‐methylimidazolium tetrafluoroborate [C<sup>9</sup>

1‐Ethyl‐3‐methylimidazolium chloride [C<sup>2</sup>

1‐Butyl‐3‐methylimidazolium chloride [C<sup>4</sup>

1‐Hexyl‐3‐methylimidazolium chloride [C<sup>6</sup>

1‐Octyl‐3‐methylimidazolium chloride [C8

1‐Ethyl‐3‐methylimidazolium bis((trifluoromethyl)

1‐Propyl‐3‐methylimidazolium bis((trifluoromethyl)

1‐Butyl‐3‐methylimidazolium bis((trifluoromethyl)

1‐Pentyl‐3‐methylimidazolium bis((trifluoromethyl)

1‐Hexyl‐3‐methylimidazolium bis((trifluoromethyl)

1‐Heptyl‐3‐methylimidazolium bis((trifluoromethyl)

1‐Octyl‐3‐methylimidazolium bis((trifluoromethyl)

sulfonyl)amide

sulfonyl)amide

sulfonyl)amide

sulfonyl)amide

sulfonyl)amide

sulfonyl)amide

sulfonyl)amide

**Group C**


Generally, it was extremely difficult to establish sensible and systematic structure‐toxicity relationship with exception to the observation involving relationship between the position and the number of methyl groups attached to the pyridine cation and the effect on toxicity of the ILs as a whole. Also, the result indicates an important general trend which shows that the ILs with the cation structure containing aromatic, are always more toxic than the non‐aromatic ones. According to Ventura et al. [6] and Kurnia et al. [18], the aromatic cation are more soluble in water and therefore capable of directly exhibiting its high toxicity effect on the aqua environment compared to the non‐aromatic‐based ILs which are much less soluble. However, it is worth to note that the toxicity of the non‐cyclic cations such as sulphonium, ammonium and phosphonium has not been rigorously studied. Nevertheless, the present study still takes into account of the toxicity involving few ammonium and phosphonium ILs reported earlier and the results are included.

Ecotoxicity of Ionic Liquids Towards *Vibrio fischeri*: Experimental and QSAR Studies

It is known that the alkyl chain length of the cation has a strong effect on the physical and chemical properties of the ILs. For example, the extension of the cation side resulting from a longer alkyl chain commonly results in lower density and solubility, slower diffusion rate and increases viscosity [19, 20]. In addition, it was also observed that the cation alkyl chain length has a pronounced effect on the ecotoxicity towards microorganisms. Most of the established ecotoxicity data covering not only *V. fischeri* but also other organisms such as cell, green algae, fish, and bacteria employed for investigating the influence of the alkyl chain length on the ionic liquid toxicity. **Table 2** group B provides the EC50 values for different counters to represent the impact of the alkyl chain length associated with different cation type on the ILs toxicity. Viboud et al. [15] have reported the effect of butyl, hexyl and octyl on the tox-

that the EC50 values sharply decreases with the reduction in the alkyl chain length of the cat-

The type of cation also seems to have an important role in changing the toxicity effect of the ILs when its' substituent chain length are further extended. For the shorter alkyl chain (butyl), the pyrrolidinium cation demonstrated lower toxicity compared to piperidinium

this marked a reduction of 109‐fold in the EC50 value, signifying a tremendous increase in toxicity when the side alkyl chain was extended from four to eight carbon atoms. For the piperidinium type, the recorded effect was even more drastic with the toxicity increased by

mpip][Br] to 29.3 mg L-1 for [C8

mim][Br]. The lowest rate of increase in toxicity could be explained by

cations, imidazolium recorded the lowest rate of increase in toxicity with the increase in the

the fact that imidazolium is already possessing highest toxicity even with shorter alkyl chain

A similar study has been conducted for pyridinium‐based ILs by Docherty and Kulpa [21] who studied the effect of butyl, hexyl and octyl substituents on the toxicity of 1‐alkyl‐3‐meth-

alkyl chain length, where the increment was only by 20‐fold from 1002 mg L-1 for [C<sup>4</sup>

mim][Br], 1‐alkyl‐1‐methylpyrrolidinium bromide

mpip][Br]. It was noticeable

http://dx.doi.org/10.5772/65795

437

mpyrr][Br] is 5525 mg L-1 and

mpip][Br]. Between the three

mim][Br]

**2.2. Effect of alkyl chain length**

icity for 1‐alkyl‐1‐methylimidazolium [C*<sup>n</sup>*

ion, indicating clear toxicity relationship.

135‐fold from 3958 mg L-1 for [C<sup>4</sup>

length compared to the others.

to 50.9 mg L-1 for [C8

mpyrr][Br] and 1‐alkyl‐1‐methyl piperidinium bromide [C*<sup>n</sup>*

and imidazolium, respectively. The recorded EC50 value of [C<sup>4</sup>

[C*<sup>n</sup>*

**Table 2.** Ionic liquids ecotoxicity against *Vibrio fischeri* expressed in mg. L-1.

Although the EC50 values for the five ILs fall under the same category i.e. practically harmless, the impact of the cation variations on the overall toxicity are found to be obvious. The result on EC50 values highlighted that the imidazolium‐based ILs exhibited about 4–5 magnitude higher toxicity measurement compared to piperidinium‐ and pyrrolidinium‐based ILs respectively. The most toxic IL is the one based on pyridinium cation where a slightly higher toxicity values were observed compared to the imidazolium analogue. Meanwhile the morpholinium cation demonstrates far less toxicity behaviour than the other counterparts with EC50 value reaching as high as 66,729 mg L-1. The piperidinium and morpholinium exhibited almost similar cationic core structure where the latter can simply be established by replacing the carbon atom located opposite to the amine group in the piperidinium structure, with an oxygen atom. Despite the slight structural differences, the presence of the oxygenated atom in the morpholinium cation led to its significant toxicity reduction in the order of 17 times, compared to the piperidinium‐based IL. This finding augurs well with the earlier work reported by Samorì et al. [16, 17].

The toxicity of some starting reactant for the cation used in the ILs synthesized in the study were tested against *V. fischeri,* with results shown in **Table 2**, group A. The EC50 values for 1‐methylimidazolium, 1‐methylpyrrolidinium, 1‐methylpiperidinium, 1‐methylmorpholinium, pyridinium, 2,3‐dimethylpyridine, 3,5‐dimethylpyridine and 2,3,5‐dimethylpyridine are found to be 2864 mg L-1, 493 mg L-1, 700 mg L-1, 2328 mg L-1, 867 mg L-1, 238 mg L-1, 65.9 mg L-1 and 43 mg L-1, respectively. The reported toxicity of these compounds did not show any clear and logical pattern linking the toxicity to the ILs structure. Hence, there were not any structure‐toxicity relationships that could be established. As an example, the results show that 1‐methylpyrrolidinium and 1‐methylpiperidinium cation‐based ILs displayed 5.8 and 4 times higher toxicity respectively, when compared to 1‐methylimidazolium, which is contrary to the anticipated trend which predicts imidazolium‐based to have higher toxicity than the earlier two ILs.

Generally, it was extremely difficult to establish sensible and systematic structure‐toxicity relationship with exception to the observation involving relationship between the position and the number of methyl groups attached to the pyridine cation and the effect on toxicity of the ILs as a whole. Also, the result indicates an important general trend which shows that the ILs with the cation structure containing aromatic, are always more toxic than the non‐aromatic ones. According to Ventura et al. [6] and Kurnia et al. [18], the aromatic cation are more soluble in water and therefore capable of directly exhibiting its high toxicity effect on the aqua environment compared to the non‐aromatic‐based ILs which are much less soluble. However, it is worth to note that the toxicity of the non‐cyclic cations such as sulphonium, ammonium and phosphonium has not been rigorously studied. Nevertheless, the present study still takes into account of the toxicity involving few ammonium and phosphonium ILs reported earlier and the results are included.

#### **2.2. Effect of alkyl chain length**

Although the EC50 values for the five ILs fall under the same category i.e. practically harmless, the impact of the cation variations on the overall toxicity are found to be obvious. The result on EC50 values highlighted that the imidazolium‐based ILs exhibited about 4–5 magnitude higher toxicity measurement compared to piperidinium‐ and pyrrolidinium‐based ILs respectively. The most toxic IL is the one based on pyridinium cation where a slightly higher toxicity values were observed compared to the imidazolium analogue. Meanwhile the morpholinium cation demonstrates far less toxicity behaviour than the other counterparts with EC50 value reaching as high as 66,729 mg L-1. The piperidinium and morpholinium exhibited almost similar cationic core structure where the latter can simply be established by replacing the carbon atom located opposite to the amine group in the piperidinium structure, with an oxygen atom. Despite the slight structural differences, the presence of the oxygenated atom in the morpholinium cation led to its significant toxicity reduction in the order of 17 times, compared to the piperidinium‐based IL. This finding augurs well with the earlier work

**Table 2.** Ionic liquids ecotoxicity against *Vibrio fischeri* expressed in mg. L-1.

**Ionic liquid names Abbreviation EC50 in mg L<sup>−</sup><sup>1</sup>** Ethanol EtOH 34947.67 2‐Propanol IPA 35389.50 Acetonitrile Acn 24172.03 Acetone Ace 17140.62 N,N‐Dimethylformamide DMAc 20130.66 Dichloromethane DCM 2877.80 Chloroform CHCl<sup>3</sup> 671.32 Cyclohexane CYHEX 226.52 Pyridine py 212.90 Benzene bz 102.97 Toluene TOL 19.70

The toxicity of some starting reactant for the cation used in the ILs synthesized in the study were tested against *V. fischeri,* with results shown in **Table 2**, group A. The EC50 values for 1‐methylimidazolium, 1‐methylpyrrolidinium, 1‐methylpiperidinium, 1‐methylmorpholinium, pyridinium, 2,3‐dimethylpyridine, 3,5‐dimethylpyridine and 2,3,5‐dimethylpyridine are found to be 2864 mg L-1, 493 mg L-1, 700 mg L-1, 2328 mg L-1, 867 mg L-1, 238 mg L-1, 65.9 mg L-1 and 43 mg L-1, respectively. The reported toxicity of these compounds did not show any clear and logical pattern linking the toxicity to the ILs structure. Hence, there were not any structure‐toxicity relationships that could be established. As an example, the results show that 1‐methylpyrrolidinium and 1‐methylpiperidinium cation‐based ILs displayed 5.8 and 4 times higher toxicity respectively, when compared to 1‐methylimidazolium, which is contrary to the anticipated trend which predicts imidazolium‐based to have higher toxicity than

reported by Samorì et al. [16, 17].

436 Progress and Developments in Ionic Liquids

the earlier two ILs.

It is known that the alkyl chain length of the cation has a strong effect on the physical and chemical properties of the ILs. For example, the extension of the cation side resulting from a longer alkyl chain commonly results in lower density and solubility, slower diffusion rate and increases viscosity [19, 20]. In addition, it was also observed that the cation alkyl chain length has a pronounced effect on the ecotoxicity towards microorganisms. Most of the established ecotoxicity data covering not only *V. fischeri* but also other organisms such as cell, green algae, fish, and bacteria employed for investigating the influence of the alkyl chain length on the ionic liquid toxicity. **Table 2** group B provides the EC50 values for different counters to represent the impact of the alkyl chain length associated with different cation type on the ILs toxicity. Viboud et al. [15] have reported the effect of butyl, hexyl and octyl on the toxicity for 1‐alkyl‐1‐methylimidazolium [C*<sup>n</sup>* mim][Br], 1‐alkyl‐1‐methylpyrrolidinium bromide [C*<sup>n</sup>* mpyrr][Br] and 1‐alkyl‐1‐methyl piperidinium bromide [C*<sup>n</sup>* mpip][Br]. It was noticeable that the EC50 values sharply decreases with the reduction in the alkyl chain length of the cation, indicating clear toxicity relationship.

The type of cation also seems to have an important role in changing the toxicity effect of the ILs when its' substituent chain length are further extended. For the shorter alkyl chain (butyl), the pyrrolidinium cation demonstrated lower toxicity compared to piperidinium and imidazolium, respectively. The recorded EC50 value of [C<sup>4</sup> mpyrr][Br] is 5525 mg L-1 and this marked a reduction of 109‐fold in the EC50 value, signifying a tremendous increase in toxicity when the side alkyl chain was extended from four to eight carbon atoms. For the piperidinium type, the recorded effect was even more drastic with the toxicity increased by 135‐fold from 3958 mg L-1 for [C<sup>4</sup> mpip][Br] to 29.3 mg L-1 for [C8 mpip][Br]. Between the three cations, imidazolium recorded the lowest rate of increase in toxicity with the increase in the alkyl chain length, where the increment was only by 20‐fold from 1002 mg L-1 for [C<sup>4</sup> mim][Br] to 50.9 mg L-1 for [C8 mim][Br]. The lowest rate of increase in toxicity could be explained by the fact that imidazolium is already possessing highest toxicity even with shorter alkyl chain length compared to the others.

A similar study has been conducted for pyridinium‐based ILs by Docherty and Kulpa [21] who studied the effect of butyl, hexyl and octyl substituents on the toxicity of 1‐alkyl‐3‐methylpyridinium bromide‐based ILs. A similar increasing trend in the toxicity associated with increasing alkyl chain length was observed. As expected, the least toxic compound of this cation type is the butyl‐based ILs with EC50 value of 130.48 mg L-1. The EC50 value reduces to 29.99 mg L-1 through the addition of two carbon atoms to the alkyl chain (hexyl) resulting in a fourfold increase in toxicity. Switching the hexyl substituent to an octyl causes an increase in toxicity i.e. by 17‐fold compared to the hexyl‐based IL and up to 73‐fold compared to the butyl‐based IL.

The toxicity of hydrophobic [Cnmim][NTf<sup>2</sup>

instance, the EC50 value for [C<sup>2</sup>

] and [C8

[NTf<sup>2</sup>

[C<sup>3</sup>

[C8

mg L-1.

**2.3. Effect of the anions**

than [C<sup>2</sup>

mim][BF<sup>4</sup>

mim][NTf<sup>2</sup>

For this ILs, the alkyl chain is varied from C<sup>2</sup>

mim][Cl]. On the contrary, for the [C8

mim][BF<sup>4</sup>

compounds with a large molecular size [2, 26].

]‐based ILs was also discussed by Ventura et al. [6].

Ecotoxicity of Ionic Liquids Towards *Vibrio fischeri*: Experimental and QSAR Studies

] is 330.23 mg L-1 which is 5.6‐fold more toxic than

] were 2.36 mg L-1, 6.44 mg L-1 and 7.25 mg L-1, respectively.

mim]‐based ILs, much smaller differences in

(see **Table 2** group B). The reported EC50

] and [Cl] anions. For

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mim][ Cl],

439

to C8

] despite having shorter alkyl chain attached to the cation, and 9.5‐fold more toxic

values are found to be comparable and it follows a similar trend observed for the chloride and tetrafluoroborate‐based ILs. The exception was only observed for the short alkyl chain, where

This could be explained based on earlier reported work that the *V. fischeri* organism was less

Phosphonium‐based ILs also showed a similar behaviour as discussed earlier where its toxicity reduces as the alkyl chain length grew longer than decyl. For instance, the reported EC50 value for trihexyl(tetradecyl)phosphonium bromide is 1449.09 mg L-1, eightfold greater than the EC50 value of tetrabutylphosphonium bromide i.e. 174.09 mg L-1 [25], as presented in **Table 2** group B. The phenomenon is well reported in the literature for highly lipophilic substances (log *K*ow > 5) known as the cut‐off effect. For this phenomenon, different explanations were presented based either on insufficient solubility i.e. nominal concentration deviating from real test concentration, or on kinetic aspects i.e. slower uptake due to steric effects for

The increasing trend in toxicity with alkyl chain length was also confirmed for quaternary ammonium‐based ILs [25]. As can be seen from the tabulated data shown in **Table 2** group B, tetramethylammonium bromide and tetraethylammonium bromide show non‐toxic behaviour with EC50 values greater than 5000 mg L-1. The toxicity of this ILs increases i.e. EC50 value reduces to 600.28 mg L-1, for tetrabutylammonium bromide. Further noticeable increase in toxicity was observed for hexyltriethylammonium bromide with EC50 value reduces to 64.65

Generally it can also be seen that the quaternary ammonium‐based ILs exhibits lower toxicity

The anion chemistry has a great impact on the alteration of the ILs properties. Most of ILs properties such as melting point, hydrophobicity, chemical and thermal stabilities, ability to dissolve organic and inorganic solutes and miscibility with organic solvent rely mainly on the type of the anion [27–29]. Although there is no clear pattern that could be drawn for the anion influence on the ILs toxicity, recent studies have given more attention towards the impact of anions type on ILs toxicity. The data reported for the anion effect are tabulated in **Table 2** group C. Ventura et al. [30] investigated the toxicity of 10 ILs with 9 of them comprising the cholinium cation with different anions. Cholinium‐based ILs has received significant attention due to its non‐toxic and biocompatible nature [31–33]. Using cholinium as the cation, the study on the impact of various anions on toxicity of the ILs was conducted. The bicarbonate

against *V. fischeri* than the ILs with cyclic cations (aromatic and non‐aromatic).

]‐based ILs exhibited higher toxicity than their corresponding [BF<sup>4</sup>

EC50 values were reported for the three different anions, where the values for [C8

sensitive to the hydrophobic ILs than other organisms such as Folsomia candida [24].

mim][NTf<sup>2</sup>

Generally, it can be stated that increase in the side chain for the pyrrolidinium‐ and piperidinium‐based ILs produces more pronounced effect than those of the aromatic‐based cation such as imidazolium‐ and pyridinium‐based ILs. Although there were almost zero data reported for much longer alkyl chain except for imidazolium‐type ILs which will be further discussed below, the increasing trend of the toxicity with respect to the increase in the side alkyl chain reveals that longer extension on the alkyl chain beyond C<sup>9</sup> and above, will produce highly toxic ILs and therefore should be avoided.

For the longer alkyl chain, Ranke et al. [22] studied the toxicities for 1‐alkyl‐3‐methylimidazolium tetrafluoroborate with alkyl chain varied from C<sup>3</sup> to C10. Based on their EC50 values, as can be seen from **Table 2** group B, the tetrafluoroborate‐based ILs having alkyl chain length up to heptyl, can still be classified as practically harmless. Further extension of the alkyl chain will lead to more toxic effect on the *V. fischeri*, with the C8 ‐ and C<sup>9</sup> ‐based ILs classified as slightly toxic ILs. A more drastic increase in toxicity was observed when the alkyl chain length reaches decyl with the resultant IL producing a highly toxic one. A similar study has been done by Stolte et al. [23] where they investigated the toxicities for 1‐alkyl‐3‐ methylimidazolium chloride with alkyl chain varied from C<sup>2</sup> to C10 with an increment rate of two carbon atoms at a time, and for C14, C16 and C18. It is clear that chloride‐based ILs showed a slightly more toxic character than their tetrafluoroborate counter parts which is a result of the contribution from the halide anion. In agreement with what have been discussed above, the toxicity of the chloride‐based ILs was found to follow the same trend observed for tetrafluoroborate‐based ILs. The alkyl chain length extension of ethyl, butyl and hexyl are categorized as practically harmless, whereas the octyl is found to be slightly toxic. Increasing the alkyl chain from 8 to 10 carbon atoms causes a reduction of 15‐fold on the EC50 values. The EC50 value reduces from 2.36 mg L-1 for [C8 mim][Cl] to 0.15 mg L-1 for [C10mim][Cl], producing the most toxic IL for [Cnmim][Cl] ILs. Also for the latter, the toxicity hazard impact increases to highly toxic when compared to the slightly toxic octyl. The reported EC50 values of [C14mim][Cl] and [C16mim][Cl] were 0.22 mg L-1 and 0.58 mg L-1, respectively.

Despite the above highlighted findings, a temporary reverse effect was observed for the ILs when the alkyl chain reaches decyl, where a noticeable increase in the EC50 value indicating reduction in the toxicity of the ILs. The observed effect continues until the chain length reaches C17 before reversing back to the earlier trend. As a result, the hazard ranking of [C18mim][Cl] changes back to a slightly toxic effect, similar to the earlier hazard ranking for the ILs when the alkyl chain was at C8 . Hence it can still be concluded that for long alkyl side chains beyond C8 , the dependence between the chain length and toxicity is still valid.

The toxicity of hydrophobic [Cnmim][NTf<sup>2</sup> ]‐based ILs was also discussed by Ventura et al. [6]. For this ILs, the alkyl chain is varied from C<sup>2</sup> to C8 (see **Table 2** group B). The reported EC50 values are found to be comparable and it follows a similar trend observed for the chloride and tetrafluoroborate‐based ILs. The exception was only observed for the short alkyl chain, where [NTf<sup>2</sup> ]‐based ILs exhibited higher toxicity than their corresponding [BF<sup>4</sup> ] and [Cl] anions. For instance, the EC50 value for [C<sup>2</sup> mim][NTf<sup>2</sup> ] is 330.23 mg L-1 which is 5.6‐fold more toxic than [C<sup>3</sup> mim][BF<sup>4</sup> ] despite having shorter alkyl chain attached to the cation, and 9.5‐fold more toxic than [C<sup>2</sup> mim][Cl]. On the contrary, for the [C8 mim]‐based ILs, much smaller differences in EC50 values were reported for the three different anions, where the values for [C8 mim][ Cl], [C8 mim][NTf<sup>2</sup> ] and [C8 mim][BF<sup>4</sup> ] were 2.36 mg L-1, 6.44 mg L-1 and 7.25 mg L-1, respectively. This could be explained based on earlier reported work that the *V. fischeri* organism was less sensitive to the hydrophobic ILs than other organisms such as Folsomia candida [24].

Phosphonium‐based ILs also showed a similar behaviour as discussed earlier where its toxicity reduces as the alkyl chain length grew longer than decyl. For instance, the reported EC50 value for trihexyl(tetradecyl)phosphonium bromide is 1449.09 mg L-1, eightfold greater than the EC50 value of tetrabutylphosphonium bromide i.e. 174.09 mg L-1 [25], as presented in **Table 2** group B. The phenomenon is well reported in the literature for highly lipophilic substances (log *K*ow > 5) known as the cut‐off effect. For this phenomenon, different explanations were presented based either on insufficient solubility i.e. nominal concentration deviating from real test concentration, or on kinetic aspects i.e. slower uptake due to steric effects for compounds with a large molecular size [2, 26].

The increasing trend in toxicity with alkyl chain length was also confirmed for quaternary ammonium‐based ILs [25]. As can be seen from the tabulated data shown in **Table 2** group B, tetramethylammonium bromide and tetraethylammonium bromide show non‐toxic behaviour with EC50 values greater than 5000 mg L-1. The toxicity of this ILs increases i.e. EC50 value reduces to 600.28 mg L-1, for tetrabutylammonium bromide. Further noticeable increase in toxicity was observed for hexyltriethylammonium bromide with EC50 value reduces to 64.65 mg L-1.

Generally it can also be seen that the quaternary ammonium‐based ILs exhibits lower toxicity against *V. fischeri* than the ILs with cyclic cations (aromatic and non‐aromatic).

#### **2.3. Effect of the anions**

ylpyridinium bromide‐based ILs. A similar increasing trend in the toxicity associated with increasing alkyl chain length was observed. As expected, the least toxic compound of this cation type is the butyl‐based ILs with EC50 value of 130.48 mg L-1. The EC50 value reduces to 29.99 mg L-1 through the addition of two carbon atoms to the alkyl chain (hexyl) resulting in a fourfold increase in toxicity. Switching the hexyl substituent to an octyl causes an increase in toxicity i.e. by 17‐fold compared to the hexyl‐based IL and up to 73‐fold compared to the

Generally, it can be stated that increase in the side chain for the pyrrolidinium‐ and piperidinium‐based ILs produces more pronounced effect than those of the aromatic‐based cation such as imidazolium‐ and pyridinium‐based ILs. Although there were almost zero data reported for much longer alkyl chain except for imidazolium‐type ILs which will be further discussed below, the increasing trend of the toxicity with respect to the increase in the side alkyl chain

For the longer alkyl chain, Ranke et al. [22] studied the toxicities for 1‐alkyl‐3‐methylimid-

as can be seen from **Table 2** group B, the tetrafluoroborate‐based ILs having alkyl chain length up to heptyl, can still be classified as practically harmless. Further extension of the

sified as slightly toxic ILs. A more drastic increase in toxicity was observed when the alkyl chain length reaches decyl with the resultant IL producing a highly toxic one. A similar study has been done by Stolte et al. [23] where they investigated the toxicities for 1‐alkyl‐3‐

of two carbon atoms at a time, and for C14, C16 and C18. It is clear that chloride‐based ILs showed a slightly more toxic character than their tetrafluoroborate counter parts which is a result of the contribution from the halide anion. In agreement with what have been discussed above, the toxicity of the chloride‐based ILs was found to follow the same trend observed for tetrafluoroborate‐based ILs. The alkyl chain length extension of ethyl, butyl and hexyl are categorized as practically harmless, whereas the octyl is found to be slightly toxic. Increasing the alkyl chain from 8 to 10 carbon atoms causes a reduction of 15‐fold

for [C10mim][Cl], producing the most toxic IL for [Cnmim][Cl] ILs. Also for the latter, the toxicity hazard impact increases to highly toxic when compared to the slightly toxic octyl. The reported EC50 values of [C14mim][Cl] and [C16mim][Cl] were 0.22 mg L-1 and 0.58 mg

Despite the above highlighted findings, a temporary reverse effect was observed for the ILs when the alkyl chain reaches decyl, where a noticeable increase in the EC50 value indicating reduction in the toxicity of the ILs. The observed effect continues until the chain length reaches C17 before reversing back to the earlier trend. As a result, the hazard ranking of [C18mim][Cl] changes back to a slightly toxic effect, similar to the earlier hazard ranking for the ILs when

. Hence it can still be concluded that for long alkyl side chains beyond

and above, will produce highly

to C10. Based on their EC50 values,

to C10 with an increment rate

mim][Cl] to 0.15 mg L-1

‐based ILs clas-

‐ and C<sup>9</sup>

reveals that longer extension on the alkyl chain beyond C<sup>9</sup>

azolium tetrafluoroborate with alkyl chain varied from C<sup>3</sup>

alkyl chain will lead to more toxic effect on the *V. fischeri*, with the C8

on the EC50 values. The EC50 value reduces from 2.36 mg L-1 for [C8

, the dependence between the chain length and toxicity is still valid.

methylimidazolium chloride with alkyl chain varied from C<sup>2</sup>

toxic ILs and therefore should be avoided.

butyl‐based IL.

438 Progress and Developments in Ionic Liquids

L-1, respectively.

the alkyl chain was at C8

C8

The anion chemistry has a great impact on the alteration of the ILs properties. Most of ILs properties such as melting point, hydrophobicity, chemical and thermal stabilities, ability to dissolve organic and inorganic solutes and miscibility with organic solvent rely mainly on the type of the anion [27–29]. Although there is no clear pattern that could be drawn for the anion influence on the ILs toxicity, recent studies have given more attention towards the impact of anions type on ILs toxicity. The data reported for the anion effect are tabulated in **Table 2** group C. Ventura et al. [30] investigated the toxicity of 10 ILs with 9 of them comprising the cholinium cation with different anions. Cholinium‐based ILs has received significant attention due to its non‐toxic and biocompatible nature [31–33]. Using cholinium as the cation, the study on the impact of various anions on toxicity of the ILs was conducted. The bicarbonate anion was found to be the least toxic whilst the dihydrogen citrate being the most toxic. In fact, there was no EC50 value reported for cholinium bicarbonate due to its unnaturally high value. The maximum luminescence inhibition caused by this IL was as low as 35% at a very high concentration of 20,000 mg L−<sup>1</sup> .

It is worthy to highlight that it may be worthwhile to use some of the carboxylic acid anions such as decanoic and undecanoic acid which have longer alkyl chains, to synthesize novel ILs for undertaking the study to confirm the effect of the alkyl chain of the anion to the ILs toxicity. More recently, Montalbán et al. [35] studied the toxicity of 1‐ethyl‐3 methylimidazolium‐based

, TfO, NTf<sup>2</sup>

> Cl. Contrary to the above finding, Alvarez‐Guerra and Irabien [36] studied the

] found to possess toxicity of 132‐fold higher compared to [Ace] which was the most

alkyl chain used, the six ILs possess very low toxicity and hence all of them were classified as practically harmless. The reduction in toxicity of these ILs follow the trend of Ace > TfO > NTf<sup>2</sup>

toxicity of tetrachloroferrate(III) anion with the same cation and found a very low EC50 value

extreme toxicity effect when compared to the earlier six anions and the alkyl chain length used.

toxic among the former anions. According to the QSAR model developed by Alvarez‐Guerra and Irabien [36], it was similarly suggested that the highly toxic behaviour exerted by the anion was due to the presence of Fe in its structure, exerting a significant influence on the toxicity.

Based on past reported work, introducing amino acid anion to the structure of the ILs can be considered as a convenient approach to reduce its toxicity. Interestingly, the claim has yet to be experimentally proven. In our recent work [37], the variation of the toxicity of 1‐(2‐

amino acid anions namely glycinate, alaninate, serinate and prolinate was studied towards the *V. fischeri*. The reported EC50 for the amino acid‐based ILs was found to be greater than 5000 mg L-1, which highlighted their non‐toxic behaviour. The toxicity of the same cation

[36] in EC50 values was 4896.94 mg L-1 and 1972.20 mg L-1, respectively as shown in **Table 2** group C. This indicates a marked change in toxicity when changing the anion from amino acid type. Nevertheless, there was a recent report by Egorova et al. [38] contravening the role of the amino acid anion in ILs in lowering the ILs toxicity and the fact that they should not be seen as entirely green compounds for initial design. Nonetheless, the comparison between the EC50 values of 1‐(2‐hydroxylethyl)‐3‐methylimidazolium‐based ILs has sufficiently indicated a clear impact of the amino acid anions on lowering the toxicity of the ILs compared to the [I]

Generally, it can be argued that the impact contributed by the anion on the toxicity did not show any strong systematic relationship involving its structure i.e. alkyl chain length as demonstrated in the case of the cation. However, changing the type of the anion would be crucial

Volatile organic solvents (VOCs) are considered as a major source of atmospheric pollution. They exert high vapour pressure hence have high volatility leading to their significant losses to the atmosphere. Their vapour can be highly toxic depending on the type of component

i.e. 9.99 mg L-1 indicating highly toxic behaviour. Clearly, the [FeCl<sup>4</sup>

]‐based ILs. Furthermore, the toxicity of [C<sup>2</sup>

OHmim][I] by 1.6 and 4‐fold, respectively.

as it can change the ILs' toxicity significantly.

**2.4. Toxicity of starting material and organic solvent**

among the amino acid‐based ILs, displayed even lower toxicity than [C<sup>2</sup>

, Cl, Ace and EtSO<sup>4</sup>

Ecotoxicity of Ionic Liquids Towards *Vibrio fischeri*: Experimental and QSAR Studies

OHmim] by pairing it with four different types of

OHmim][Ala] which is the most toxic

OHmim][NTf<sup>2</sup>

] and

] and [I] anions as reported earlier by Alvarez‐Guerra and Irabien

. Due to the short

441

] anion contributes to the

http://dx.doi.org/10.5772/65795

ILs using six different anions including PF<sup>6</sup>

hydroxylethyl)‐3‐methylimidazolium, [C<sup>2</sup>

when paired with [NTf<sup>2</sup>

> EtSO<sup>4</sup>

The [FeCl<sup>4</sup>

and [NTf<sup>2</sup>

[C<sup>2</sup>

> PF<sup>6</sup>

Judging on the anion structure, the butanoate anion which corresponds to the addition of propanoate and acetate anions with one and two methyl group, respectively is expected to possess higher toxicity due to longer alkyl chain on its structure. However, the toxicity reported for the cholinium cation did not show any consequence leading to noticeable toxicity increase with increase in the alkyl chain on the anion side. Except for cholinium bitartrate and cholinium dihydrogencitrate which are classified as moderately toxic with EC50 values lower than 100 mg L-1, the other seven compounds reported fell under practically harmless class. The reported ecotoxicity data also demonstrated that some classic ILs, pairing the imidazolium or pyridinium cation with alkyl chain varied from C<sup>1</sup> to C<sup>6</sup> with similar anions, may possess lower toxicity than the one exerted by the cholinium cation. Hence, contrary to the effect of alkyl chain length on the IL cation towards ILs toxicity, the same effect is proven to be inconclusive for the anion.

Overall, the order of toxicity sequence reported in increasing order is bicarbonate < butanoate < acetate < dihydrogenphosphate < propanoate < chloride < salicylate < bitartrate < dihydrogencitrate.

Another recent work concerning the anion impact on IL toxicity is the study conducted by Peric et al. [34] where they investigated the ecotoxicity of compounds based on substituted amines as the cations including monoethanolamine, [2‐HEA], diethanolamine [2‐HDEA] and triethanolamine [2‐HTEA], paired with organic acids with different numbers of carbon atoms (formic, propionic, butanoic, isobutanoic and pentanoic acid) as the anions. From the reported data, it is apparent that the alkyl chain length did show some influence on the ILs toxicity. The 2‐hydroxydiethanolamine pentanoate [2‐HDEA][Pe] was found to be the most toxic with EC50 values of 350 mg L-1, followed by 2‐hydroxytriethanolamine pentanoate [2‐HTEA][Pe] with EC50 of 461 mg L-1. The two ILs have pentanoic as its anion, indicating the presence of five carbon atoms in the anion side chain. The toxicity exerted by the two ILs demonstrated stronger influence by the cation structure although [2‐HDEA][Pe] displayed a 1.3‐fold higher toxicity than the [2‐HTEA][Pe] despite the latter having a larger cation size. The lower toxicity of [2‐HTEA] is attributed mainly to the presence of three hydroxyl groups (‐OH) in the cation structure as opposed to only two in [2‐HDEA]. As a matter of fact, the observation agrees well with the conclusion stated earlier on the influence of the oxygenated atom within the cation structure which tends to lower the ILs toxicity. Interestingly, this is not always true for the butanoate anion as the toxicity trend displayed different results. The 2‐hydroxydiethanolamine butanoate [2‐HDEA][B] was reported to be 1.6‐fold less toxic than the 2‐hydroxytriethanolamine butanoate (2‐HTEAB) despite having more hydroxyl (‐OH) group attached to it. In this respect, the authors argued that the noticeable increase in the toxicity is due to the more dominant effect contributed from the longer alkyl chain length of the anion. The argument is rather controversial as it was earlier explained that the trend on the effect of the alkyl chain length of the anion on toxicity is inconclusive and lesser compared to the cation. Nevertheless, all the studied ILs were non‐toxic and fell under the class of practically harmless.

It is worthy to highlight that it may be worthwhile to use some of the carboxylic acid anions such as decanoic and undecanoic acid which have longer alkyl chains, to synthesize novel ILs for undertaking the study to confirm the effect of the alkyl chain of the anion to the ILs toxicity.

anion was found to be the least toxic whilst the dihydrogen citrate being the most toxic. In fact, there was no EC50 value reported for cholinium bicarbonate due to its unnaturally high value. The maximum luminescence inhibition caused by this IL was as low as 35% at a very

Judging on the anion structure, the butanoate anion which corresponds to the addition of propanoate and acetate anions with one and two methyl group, respectively is expected to possess higher toxicity due to longer alkyl chain on its structure. However, the toxicity reported for the cholinium cation did not show any consequence leading to noticeable toxicity increase with increase in the alkyl chain on the anion side. Except for cholinium bitartrate and cholinium dihydrogencitrate which are classified as moderately toxic with EC50 values lower than 100 mg L-1, the other seven compounds reported fell under practically harmless class. The reported ecotoxicity data also demonstrated that some classic ILs, pairing the imidazolium or pyridinium

with similar anions, may possess lower toxicity

to C<sup>6</sup>

all the studied ILs were non‐toxic and fell under the class of practically harmless.

than the one exerted by the cholinium cation. Hence, contrary to the effect of alkyl chain length on the IL cation towards ILs toxicity, the same effect is proven to be inconclusive for the anion.

Overall, the order of toxicity sequence reported in increasing order is bicarbonate < butanoate < acetate < dihydrogenphosphate < propanoate < chloride < salicylate < bitartrate <

Another recent work concerning the anion impact on IL toxicity is the study conducted by Peric et al. [34] where they investigated the ecotoxicity of compounds based on substituted amines as the cations including monoethanolamine, [2‐HEA], diethanolamine [2‐HDEA] and triethanolamine [2‐HTEA], paired with organic acids with different numbers of carbon atoms (formic, propionic, butanoic, isobutanoic and pentanoic acid) as the anions. From the reported data, it is apparent that the alkyl chain length did show some influence on the ILs toxicity. The 2‐hydroxydiethanolamine pentanoate [2‐HDEA][Pe] was found to be the most toxic with EC50 values of 350 mg L-1, followed by 2‐hydroxytriethanolamine pentanoate [2‐HTEA][Pe] with EC50 of 461 mg L-1. The two ILs have pentanoic as its anion, indicating the presence of five carbon atoms in the anion side chain. The toxicity exerted by the two ILs demonstrated stronger influence by the cation structure although [2‐HDEA][Pe] displayed a 1.3‐fold higher toxicity than the [2‐HTEA][Pe] despite the latter having a larger cation size. The lower toxicity of [2‐HTEA] is attributed mainly to the presence of three hydroxyl groups (‐OH) in the cation structure as opposed to only two in [2‐HDEA]. As a matter of fact, the observation agrees well with the conclusion stated earlier on the influence of the oxygenated atom within the cation structure which tends to lower the ILs toxicity. Interestingly, this is not always true for the butanoate anion as the toxicity trend displayed different results. The 2‐hydroxydiethanolamine butanoate [2‐HDEA][B] was reported to be 1.6‐fold less toxic than the 2‐hydroxytriethanolamine butanoate (2‐HTEAB) despite having more hydroxyl (‐OH) group attached to it. In this respect, the authors argued that the noticeable increase in the toxicity is due to the more dominant effect contributed from the longer alkyl chain length of the anion. The argument is rather controversial as it was earlier explained that the trend on the effect of the alkyl chain length of the anion on toxicity is inconclusive and lesser compared to the cation. Nevertheless,

.

high concentration of 20,000 mg L−<sup>1</sup>

440 Progress and Developments in Ionic Liquids

cation with alkyl chain varied from C<sup>1</sup>

dihydrogencitrate.

More recently, Montalbán et al. [35] studied the toxicity of 1‐ethyl‐3 methylimidazolium‐based ILs using six different anions including PF<sup>6</sup> , TfO, NTf<sup>2</sup> , Cl, Ace and EtSO<sup>4</sup> . Due to the short alkyl chain used, the six ILs possess very low toxicity and hence all of them were classified as practically harmless. The reduction in toxicity of these ILs follow the trend of Ace > TfO > NTf<sup>2</sup> > EtSO<sup>4</sup> > PF<sup>6</sup> > Cl. Contrary to the above finding, Alvarez‐Guerra and Irabien [36] studied the toxicity of tetrachloroferrate(III) anion with the same cation and found a very low EC50 value i.e. 9.99 mg L-1 indicating highly toxic behaviour. Clearly, the [FeCl<sup>4</sup> ] anion contributes to the extreme toxicity effect when compared to the earlier six anions and the alkyl chain length used. The [FeCl<sup>4</sup> ] found to possess toxicity of 132‐fold higher compared to [Ace] which was the most toxic among the former anions. According to the QSAR model developed by Alvarez‐Guerra and Irabien [36], it was similarly suggested that the highly toxic behaviour exerted by the anion was due to the presence of Fe in its structure, exerting a significant influence on the toxicity.

Based on past reported work, introducing amino acid anion to the structure of the ILs can be considered as a convenient approach to reduce its toxicity. Interestingly, the claim has yet to be experimentally proven. In our recent work [37], the variation of the toxicity of 1‐(2‐ hydroxylethyl)‐3‐methylimidazolium, [C<sup>2</sup> OHmim] by pairing it with four different types of amino acid anions namely glycinate, alaninate, serinate and prolinate was studied towards the *V. fischeri*. The reported EC50 for the amino acid‐based ILs was found to be greater than 5000 mg L-1, which highlighted their non‐toxic behaviour. The toxicity of the same cation when paired with [NTf<sup>2</sup> ] and [I] anions as reported earlier by Alvarez‐Guerra and Irabien [36] in EC50 values was 4896.94 mg L-1 and 1972.20 mg L-1, respectively as shown in **Table 2** group C. This indicates a marked change in toxicity when changing the anion from amino acid type. Nevertheless, there was a recent report by Egorova et al. [38] contravening the role of the amino acid anion in ILs in lowering the ILs toxicity and the fact that they should not be seen as entirely green compounds for initial design. Nonetheless, the comparison between the EC50 values of 1‐(2‐hydroxylethyl)‐3‐methylimidazolium‐based ILs has sufficiently indicated a clear impact of the amino acid anions on lowering the toxicity of the ILs compared to the [I] and [NTf<sup>2</sup> ]‐based ILs. Furthermore, the toxicity of [C<sup>2</sup> OHmim][Ala] which is the most toxic among the amino acid‐based ILs, displayed even lower toxicity than [C<sup>2</sup> OHmim][NTf<sup>2</sup> ] and [C<sup>2</sup> OHmim][I] by 1.6 and 4‐fold, respectively.

Generally, it can be argued that the impact contributed by the anion on the toxicity did not show any strong systematic relationship involving its structure i.e. alkyl chain length as demonstrated in the case of the cation. However, changing the type of the anion would be crucial as it can change the ILs' toxicity significantly.

#### **2.4. Toxicity of starting material and organic solvent**

Volatile organic solvents (VOCs) are considered as a major source of atmospheric pollution. They exert high vapour pressure hence have high volatility leading to their significant losses to the atmosphere. Their vapour can be highly toxic depending on the type of component and its respective concentration thus posing toxic exposure to process operators and the surrounding community. This is where ILs, having significantly low volatility, was promoted by many researchers as the ideal replacement to VOCs for many of the industrial application. However, several studies have reported comparable EC50 values towards *V. fischeri* for some of the common VOCs (**Table 2** group D) compared to several common ILs. Hence, the idea of ILs being a greener alternative to the VOCs has to be carefully evaluated. In fact, most of VOCs displayed lower toxicity to the *V. fischeri* than some of the least toxic ILs discussed earlier.

The evaluation of the model accuracy and stability are highly important. The correlation

and stable. Therefore, model validation is another important step to ascertain the model stability which signifies the ability to display consistently good prediction for ILs especially for data outside the experimental range used during model development stage. In other words, validation of the QSAR models is a crucial issue for judging its ability in predicting similar

Different methods were adopted to validate the QSAR/QSPR models such as internal, external and cross‐validation [42]. Most of the developed models used the multiple linear regression technique (MLR) which was widely employed due to its simplicity, transparency and reproducibility as well as easy interpretability [43]. It also provides useful statistical parameter for evaluating the significance of the selected descriptors (i.e. *P*-value and *t*‐statistic), thus guiding the elimination of insignificant descriptors that have none or insignificant impact on

In most cases reported, the end measure for ecotoxicity of ILs towards *V. fischeri* was expressed

found on QSAR/QSPR model dealing with the toxicity of ILs towards *V. fischeri* was developed by Couling et al. [25]. The model was developed using four descriptors namely E‐state indices, surface area, surface charge density and shadow parameter. This model was able

developed novel QSAR models based on multiple linear regression method to predict the ecotoxicity. In their two studies, they proposed the used of group contribution approach i.e. the functional group, as molecular descriptors, in order to assess the contribution of different structural elements on the overall toxicity of ILs. Supposedly, an ionic liquids structure could be divided into three main components namely the cation, the anion and the substitution group, and each group could further be divided into subgroups based on their toxicity effect. In their earlier model, nine descriptors were used to represent a medium dataset obtained for 43 ILs. In their later model, the number of descriptors was increased to 15 as the dataset was expanded to 96 ILs. Despite the effort of increasing the number of descriptors with a larger pool of ILs, the later multiple linear regression models hardly produced improvement com-

0.924, respectively. This was due to the fact that the group contribution descriptors were used as independent variables for the prediction of a dimensionless toxicity value. Alvarez‐Guerra and Irabien [36] proposed a new approach for estimating the ecotoxicity of ILs by means of partial least square‐discriminant analysis (PLS‐DA) to classify the ecotoxicity for relatively large dataset comprising of 148 ILs. The developed model was able to achieve a high correlation coefficient value of 0.929. The same dataset was used later by Das and Roy [46] to develop their ecotoxicity predictive model. Various two‐dimensional chemical descriptors were used to build the input dataset code including constitutional, topological, connectivity, information indices, extended topo‐chemical atom (ETA) indices, atom‐type E‐state indices and molecular

when tested using external validation i.e. using data set outside the data range used during

pared to the earlier, both displaying only acceptable regression statistics with *R*<sup>2</sup>

properties, using the Dragon™ software. The regression model produced an *R*<sup>2</sup>

properties of new ILs set not included during model development [41].

) was used mostly as the statistical parameter to evaluate the model accuracy.

value (close to 1) does not necessarily mean that the model is reliable

Ecotoxicity of Ionic Liquids Towards *Vibrio fischeri*: Experimental and QSAR Studies

http://dx.doi.org/10.5772/65795

443

. In the extensive literature study conducted, the pioneering work

value of 0.78. Luis et al. [44]and [45] later

= 0*.*925 and

value of 0.739

coefficient (*R*<sup>2</sup>

However, the high *R*<sup>2</sup>

the model performance.

as log EC50 in µmol L−<sup>1</sup>

to predict the toxicity with accuracy producing *R*<sup>2</sup>

### **3. Quantitative structure activity/property relationship (QSAR/QSPR)**

Quantitative structure activity/property relationships (QSAR/QSPR) are models which can be used to predict the relationship between the chemical compound structure and a desired end measure which could refer to any type of physical, chemical or biological activities/properties [39]. Reliable experimental data is crucial at the model development stage in order to produce good prediction model. However, the more extensive the experimental data used, the more time and resources required causing higher cost.

The main aim of the QSAR development is to develop reliable predictive models with minimum possible experimental data thus reducing the time and resources required. The basic principles of the development of the QSAR/QSPR as outlined by Todeschini and Consonni [40] are;


As the QSAR/QSPR modelling involves computational work, it reflects the benefits as below;


In this study, ILs toxicity is the property of interest and an attempt was made to develop efficient relationship with the ILs molecular structure. There have been several QSAR models reported for predicting ILs ecotoxicity against *V. fischeri*. Nevertheless, all the QSAR/QSPR models developed suffer from limitation due to the lack of experimental data involving some specific family of ILs. The main differences between the various QSAR/QSPR models were mainly on the selection of the descriptors used in developing the predictive model, and the algorithm learning methods used to establish the relationships between input descriptor and the identified property of interest.

The evaluation of the model accuracy and stability are highly important. The correlation coefficient (*R*<sup>2</sup> ) was used mostly as the statistical parameter to evaluate the model accuracy. However, the high *R*<sup>2</sup> value (close to 1) does not necessarily mean that the model is reliable and stable. Therefore, model validation is another important step to ascertain the model stability which signifies the ability to display consistently good prediction for ILs especially for data outside the experimental range used during model development stage. In other words, validation of the QSAR models is a crucial issue for judging its ability in predicting similar properties of new ILs set not included during model development [41].

and its respective concentration thus posing toxic exposure to process operators and the surrounding community. This is where ILs, having significantly low volatility, was promoted by many researchers as the ideal replacement to VOCs for many of the industrial application. However, several studies have reported comparable EC50 values towards *V. fischeri* for some of the common VOCs (**Table 2** group D) compared to several common ILs. Hence, the idea of ILs being a greener alternative to the VOCs has to be carefully evaluated. In fact, most of VOCs displayed lower toxicity to the *V. fischeri* than some of the least toxic ILs discussed earlier.

**3. Quantitative structure activity/property relationship (QSAR/QSPR)**

time and resources required causing higher cost.

ship with their molecular structure.

in a similar fashion.

442 Progress and Developments in Ionic Liquids

the identified property of interest.

libraries,

Quantitative structure activity/property relationships (QSAR/QSPR) are models which can be used to predict the relationship between the chemical compound structure and a desired end measure which could refer to any type of physical, chemical or biological activities/properties [39]. Reliable experimental data is crucial at the model development stage in order to produce good prediction model. However, the more extensive the experimental data used, the more

The main aim of the QSAR development is to develop reliable predictive models with minimum possible experimental data thus reducing the time and resources required. The basic principles of the development of the QSAR/QSPR as outlined by Todeschini and Consonni [40] are; **1.** The property of interest of the studied compound must have some strong form of relation-

**2.** Similar compounds judging from the orientation of the molecular structure, must behave

As the QSAR/QSPR modelling involves computational work, it reflects the benefits as below; • low costs and high productivity levels especially dealing with data from large chemical

• more environmental friendly approach leading to reduction in necessary chemical experiments and/or animal testing, which could be further reduced with selection of good

• possibility to predict properties of newly synthesized compounds based on its chemical

In this study, ILs toxicity is the property of interest and an attempt was made to develop efficient relationship with the ILs molecular structure. There have been several QSAR models reported for predicting ILs ecotoxicity against *V. fischeri*. Nevertheless, all the QSAR/QSPR models developed suffer from limitation due to the lack of experimental data involving some specific family of ILs. The main differences between the various QSAR/QSPR models were mainly on the selection of the descriptors used in developing the predictive model, and the algorithm learning methods used to establish the relationships between input descriptor and

descriptor linking the molecular structure to the property of interest,

structure without the need to conduct any experimental or test procedure.

Different methods were adopted to validate the QSAR/QSPR models such as internal, external and cross‐validation [42]. Most of the developed models used the multiple linear regression technique (MLR) which was widely employed due to its simplicity, transparency and reproducibility as well as easy interpretability [43]. It also provides useful statistical parameter for evaluating the significance of the selected descriptors (i.e. *P*-value and *t*‐statistic), thus guiding the elimination of insignificant descriptors that have none or insignificant impact on the model performance.

In most cases reported, the end measure for ecotoxicity of ILs towards *V. fischeri* was expressed as log EC50 in µmol L−<sup>1</sup> . In the extensive literature study conducted, the pioneering work found on QSAR/QSPR model dealing with the toxicity of ILs towards *V. fischeri* was developed by Couling et al. [25]. The model was developed using four descriptors namely E‐state indices, surface area, surface charge density and shadow parameter. This model was able to predict the toxicity with accuracy producing *R*<sup>2</sup> value of 0.78. Luis et al. [44]and [45] later developed novel QSAR models based on multiple linear regression method to predict the ecotoxicity. In their two studies, they proposed the used of group contribution approach i.e. the functional group, as molecular descriptors, in order to assess the contribution of different structural elements on the overall toxicity of ILs. Supposedly, an ionic liquids structure could be divided into three main components namely the cation, the anion and the substitution group, and each group could further be divided into subgroups based on their toxicity effect. In their earlier model, nine descriptors were used to represent a medium dataset obtained for 43 ILs. In their later model, the number of descriptors was increased to 15 as the dataset was expanded to 96 ILs. Despite the effort of increasing the number of descriptors with a larger pool of ILs, the later multiple linear regression models hardly produced improvement compared to the earlier, both displaying only acceptable regression statistics with *R*<sup>2</sup> = 0*.*925 and 0.924, respectively. This was due to the fact that the group contribution descriptors were used as independent variables for the prediction of a dimensionless toxicity value. Alvarez‐Guerra and Irabien [36] proposed a new approach for estimating the ecotoxicity of ILs by means of partial least square‐discriminant analysis (PLS‐DA) to classify the ecotoxicity for relatively large dataset comprising of 148 ILs. The developed model was able to achieve a high correlation coefficient value of 0.929. The same dataset was used later by Das and Roy [46] to develop their ecotoxicity predictive model. Various two‐dimensional chemical descriptors were used to build the input dataset code including constitutional, topological, connectivity, information indices, extended topo‐chemical atom (ETA) indices, atom‐type E‐state indices and molecular properties, using the Dragon™ software. The regression model produced an *R*<sup>2</sup> value of 0.739 when tested using external validation i.e. using data set outside the data range used during


With such capability, the design of ILs for any application could be made to consider its toxicity thus enabling greener ILs developed for industrial application right from the design stage.

Ecotoxicity of Ionic Liquids Towards *Vibrio fischeri*: Experimental and QSAR Studies

The collective study on the relationship between ILs ecotoxicity towards luminescent marine bacterium *V. fischeri* has demonstrated the impact of the ILs structure on the overall toxicity. Although, most of ILs highlighted in the works discussed were practically rated between harmless to moderately toxic towards luminescent marine bacterium *V. fischeri*, few of them were found to be highly toxic with an EC50 values lower than 1 mg L-1*.* These ILs are mainly characterized by the presence of a cyclic cation having long alky chain attachment. The extension of the alkyl side chain leads to increase in the ILs hydrophobicity, and hence increasing

There seems to be consensus from the past literature that the alkyl chain length appeared to be the dominant parameter controlling the ILs toxicity towards different aquatic organisms including *V. fischeri* and other trophic organisms such as cress (*Lepidium sativum*), mammalian cells (IPC‐81), limnic unicellular green algae (*Scenedesmus vacuolatus*), enzymes (acetylcholinesterase), wheat (*Triticum aestivum*) and duckweed (*Lemna minor*). Therefore, the focus on the ILs design for low toxicity should be focussed on the ILs structure including the type of chemical elements attached to it. Two useful guidelines that could be applied to consistently

to the ILs cation, and functional group containing oxygen paired to its atom on the ILs cation side. Any alkyl chain used as extension on the ILs cation should be kept at less than C<sup>4</sup>

the toxicity increases significantly beyond the limit. Past toxicity studies have also displayed that the non‐aromatic cation such as piperizinium, pyrrolidinium and morpholinium, shows lower toxicity compared to the imidazolium and pyridinium cations which contained an aro-

Besides the above factors highlighted on ILs cation affecting its toxicity, the proper selection of the anion moiety could also have impact on controlling its toxicity. With the exception

anion types demonstrated heterogeneous and diverse effect on the ILs toxicity. It was difficult to ascertain an identifiable pattern that could explain the toxicity variation. Even the effect of the side alkyl chain length for the carboxylic acids‐based anions does not show any clear trend relating to the changes in the corresponding ILs toxicity. Also, for some of the anions possessing more than one oxygenated atom in their structure, the expected reduction in toxicity as seen in the cations' effect, was not evidenced. Hence, it can be concluded that although changing the anions' structure and content can alter the chemical and physical properties of

Overall, from the aquatic toxicity point of view, ILs may not seem to necessarily perform better when compared to the organic solvent, which it supposed to replace for many industrial applications. However, considering their negligible impact on the atmosphere as a result of extremely low vapour pressure as well as being non‐flammable, and coupled with the unique

anion which showed very high toxicity behaviour towards *V. fischeri*, the other

to C<sup>4</sup>

http://dx.doi.org/10.5772/65795

445

) attached

since

design low toxicity ILs are the utilisation of shorter alkyl chain varied from (C<sup>1</sup>

the ionic liquids but the effect on ILs toxicity remained uncertain.

**4. Conclusion**

the toxicity drastically.

matic structure.

of the FeCl<sup>4</sup>


b Partial least squares‐discriminant analysis.

c Multifactorial analysis.

**Table 3.** Summary of published QSAR models for predicting the ecotoxicity for *V. fischeri*.

There were two aspects clearly noticeable from the comparison made across all the published models. These are the variation in the dataset size and the number and type of molecular descriptors used. It is worth highlighting that adding new ILs which pair different elements in their structure, not considered in the earlier models, would require new set of descriptors for better molecular representation and good applicability of the QSAR model. For instance, Viboud et al. [15] developed two linear QSAR models for relatively small datasets containing 10 and 19 ILs. The first dataset which comprised pyridinium bromide‐based ILs was expanded by the addition of 9 ILs pairing different cations (imidazolium, pyrrolidinium and piperidinium). Although a single descriptor was used in the model construction, good correlation coefficients was achieved i.e. 0.934 and 0.861, respectively. However, it is clear that expanding dataset size would affect the model accuracy resulting in the accuracy drop. Therefore, many QSAR models developed to cover relatively huge dataset were constructed using higher number of molecular descriptors to cover the significant variation in all the molecular structure. So far, the largest dataset used comprised 157 ILs covering 74 cations and 22 anions, studied by Yan et al. [47]. They used linear regression to propose a predictive model with good correlation coefficient i.e. *R*<sup>2</sup> = 0.908, using large number of topological descriptors i.e. 28.

Overall, it can be concluded that the application of proper method for the selection of molecular descriptor, and the model validation method used, become the key factors in influencing the outcome of the QSAR/QSPR model developed. With the right selection of the molecular descriptors, the accuracy and reliability of the predictive models developed could be enhanced significantly. With such capability, the design of ILs for any application could be made to consider its toxicity thus enabling greener ILs developed for industrial application right from the design stage.

### **4. Conclusion**

the model development stage. The achievement has led to more QSAR models developed to

thermodynamic, and topological descriptors

43 MLR 9 Group contribution 0.925 [44] 96 MLR 15 Group contribution 0.924 [45]

157 MLR 28 Topological index 0.908 [47]

cation

**Descriptor type** *R***<sup>2</sup> Ref**

0.78 [25]

0.934 & 0.861 [15]

[36]

[46]

Te =0.929

Te = 0.757

There were two aspects clearly noticeable from the comparison made across all the published models. These are the variation in the dataset size and the number and type of molecular descriptors used. It is worth highlighting that adding new ILs which pair different elements in their structure, not considered in the earlier models, would require new set of descriptors for better molecular representation and good applicability of the QSAR model. For instance, Viboud et al. [15] developed two linear QSAR models for relatively small datasets containing 10 and 19 ILs. The first dataset which comprised pyridinium bromide‐based ILs was expanded by the addition of 9 ILs pairing different cations (imidazolium, pyrrolidinium and piperidinium). Although a single descriptor was used in the model construction, good correlation coefficients was achieved i.e. 0.934 and 0.861, respectively. However, it is clear that expanding dataset size would affect the model accuracy resulting in the accuracy drop. Therefore, many QSAR models developed to cover relatively huge dataset were constructed using higher number of molecular descriptors to cover the significant variation in all the molecular structure. So far, the largest dataset used comprised 157 ILs covering 74 cations and 22 anions, studied by Yan et al. [47]. They used linear regression to propose a predictive

& number of carbons in the

Overall, it can be concluded that the application of proper method for the selection of molecular descriptor, and the model validation method used, become the key factors in influencing the outcome of the QSAR/QSPR model developed. With the right selection of the molecular descriptors, the accuracy and reliability of the predictive models developed could be enhanced significantly.

= 0.908, using large number of topological

model with good correlation coefficient i.e. *R*<sup>2</sup>

descriptors i.e. 28.

a

b

c

predict ILs toxicity against *V. fischeri*, which are summarized in **Table 3**.

148 PLS‐DAb 94 Negative or positive effect Tr = 0.963

147 MLR 12 DRAGON Tr = 0.936

**Table 3.** Summary of published QSAR models for predicting the ecotoxicity for *V. fischeri*.

**descriptor**

25 GFA<sup>a</sup> 4 Electronic, spatial, structural,

10 & 19 MCA<sup>c</sup> 1 Number of aliphatic carbons

**No of IL Method Molecular** 

444 Progress and Developments in Ionic Liquids

Genetic function approximation.

Multifactorial analysis.

Partial least squares‐discriminant analysis.

The collective study on the relationship between ILs ecotoxicity towards luminescent marine bacterium *V. fischeri* has demonstrated the impact of the ILs structure on the overall toxicity. Although, most of ILs highlighted in the works discussed were practically rated between harmless to moderately toxic towards luminescent marine bacterium *V. fischeri*, few of them were found to be highly toxic with an EC50 values lower than 1 mg L-1*.* These ILs are mainly characterized by the presence of a cyclic cation having long alky chain attachment. The extension of the alkyl side chain leads to increase in the ILs hydrophobicity, and hence increasing the toxicity drastically.

There seems to be consensus from the past literature that the alkyl chain length appeared to be the dominant parameter controlling the ILs toxicity towards different aquatic organisms including *V. fischeri* and other trophic organisms such as cress (*Lepidium sativum*), mammalian cells (IPC‐81), limnic unicellular green algae (*Scenedesmus vacuolatus*), enzymes (acetylcholinesterase), wheat (*Triticum aestivum*) and duckweed (*Lemna minor*). Therefore, the focus on the ILs design for low toxicity should be focussed on the ILs structure including the type of chemical elements attached to it. Two useful guidelines that could be applied to consistently design low toxicity ILs are the utilisation of shorter alkyl chain varied from (C<sup>1</sup> to C<sup>4</sup> ) attached to the ILs cation, and functional group containing oxygen paired to its atom on the ILs cation side. Any alkyl chain used as extension on the ILs cation should be kept at less than C<sup>4</sup> since the toxicity increases significantly beyond the limit. Past toxicity studies have also displayed that the non‐aromatic cation such as piperizinium, pyrrolidinium and morpholinium, shows lower toxicity compared to the imidazolium and pyridinium cations which contained an aromatic structure.

Besides the above factors highlighted on ILs cation affecting its toxicity, the proper selection of the anion moiety could also have impact on controlling its toxicity. With the exception of the FeCl<sup>4</sup> anion which showed very high toxicity behaviour towards *V. fischeri*, the other anion types demonstrated heterogeneous and diverse effect on the ILs toxicity. It was difficult to ascertain an identifiable pattern that could explain the toxicity variation. Even the effect of the side alkyl chain length for the carboxylic acids‐based anions does not show any clear trend relating to the changes in the corresponding ILs toxicity. Also, for some of the anions possessing more than one oxygenated atom in their structure, the expected reduction in toxicity as seen in the cations' effect, was not evidenced. Hence, it can be concluded that although changing the anions' structure and content can alter the chemical and physical properties of the ionic liquids but the effect on ILs toxicity remained uncertain.

Overall, from the aquatic toxicity point of view, ILs may not seem to necessarily perform better when compared to the organic solvent, which it supposed to replace for many industrial applications. However, considering their negligible impact on the atmosphere as a result of extremely low vapour pressure as well as being non‐flammable, and coupled with the unique tuning ability to meet specific industrial requirements, ILs can still be largely considered as promising class of greener material. In view of the need to perform the toxicity assessment to confirm fully green behaviour, the QSAR/QSPR method can be the key towards providing the predictive ability which could guide the design of novel greener ILs for industrial application.

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Ecotoxicity of Ionic Liquids Towards *Vibrio fischeri*: Experimental and QSAR Studies

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### **Author details**

Mohamed Ibrahim Abdul Mutalib\* and Ouahid Ben Ghanem

\*Address all correspondence to: ibrahmat@petronas.com.my

Department of Chemical Engineering, Universiti Teknologi PETRONAS, Bandar Seri Iskandar, Perak, Malaysia

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**Chapter 19**

Provisional chapter

**Dielectric Characteristics of Ionic Liquids and Usage in**

Dielectric Characteristics of Ionic Liquids and Usage

Before the application of ionic liquids, it is important to know their fundamental physical and chemical properties. Practical experience has shown that it is important to look at these materials in the behaviour of the function frequency and temperature. To understand obtained information understanding the molecular-physic bases is needed. Research and application of ionic liquids have attracted an increasing attention in the areas of nuclear industry, oil and gas industry, petrochemical industry, chemical and electrochemical industry. The number of studies dealing with the question is proliferating which opens up new horizons in the field of chemical operations in microwave field with ionic liquids (organic chemical synthesis, catalytic operations, etc.). As a result of the relatively high destroying temperature of ionic liquids, a wider temperature range of operations can be done and it offers environmental friendly solution in the replacement of the toxic solvents with generally low evaporating temperatures. The area of application is becoming more widespread as electrolyte of novel battery cells. Being aware of the physical and chemical properties of ionic liquids is necessary in order to apply them. The main goal of this research was to test the dielectric properties, viscosity and temperature dependence of the electrical conductivity. Based on our results, we can claim that significant temperature dependence of the three properties can be shown in the case of ionic liquids. These findings are crucial for the usability of applications, planning and preparing of production and optimization processes. The significance and importance of these results become even more obvious if we consider the fact that these energy storage cells are exposed to large temperature differences. The present study discusses the

© The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons

© 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and eproduction in any medium, provided the original work is properly cited.

**Advanced Energy Storage Cells**

Additional information is available at the end of the chapter

Additional information is available at the end of the chapter

in Advanced Energy Storage Cells

Attila Göllei

Attila Göllei

http://dx.doi.org/10.5772/66948

Abstract

**Chapter 19**

Provisional chapter

### **Dielectric Characteristics of Ionic Liquids and Usage in**

Dielectric Characteristics of Ionic Liquids and Usage

### **Advanced Energy Storage Cells**

Attila Göllei in Advanced Energy Storage Cells

Additional information is available at the end of the chapter Attila Göllei

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/66948

#### Abstract

Before the application of ionic liquids, it is important to know their fundamental physical and chemical properties. Practical experience has shown that it is important to look at these materials in the behaviour of the function frequency and temperature. To understand obtained information understanding the molecular-physic bases is needed. Research and application of ionic liquids have attracted an increasing attention in the areas of nuclear industry, oil and gas industry, petrochemical industry, chemical and electrochemical industry. The number of studies dealing with the question is proliferating which opens up new horizons in the field of chemical operations in microwave field with ionic liquids (organic chemical synthesis, catalytic operations, etc.). As a result of the relatively high destroying temperature of ionic liquids, a wider temperature range of operations can be done and it offers environmental friendly solution in the replacement of the toxic solvents with generally low evaporating temperatures. The area of application is becoming more widespread as electrolyte of novel battery cells. Being aware of the physical and chemical properties of ionic liquids is necessary in order to apply them. The main goal of this research was to test the dielectric properties, viscosity and temperature dependence of the electrical conductivity. Based on our results, we can claim that significant temperature dependence of the three properties can be shown in the case of ionic liquids. These findings are crucial for the usability of applications, planning and preparing of production and optimization processes. The significance and importance of these results become even more obvious if we consider the fact that these energy storage cells are exposed to large temperature differences. The present study discusses the

© 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

© The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons

Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and eproduction in any medium, provided the original work is properly cited.

sample materials, their usage possibilities and the results of the research from the previous work of the author. In the case of ionic liquids, it is important to know their behaviour in electric field. In lot of cases, there is no fundamental difference between the static and dynamic behaviours. Static state (like in accumulators) is similar to the dynamic. Ionic liquids are well characterized and grouped with their dielectric behaviour. First of all a short summarizing of basics of the electrical permittivity and then a modelling procedure will be shown modelling lots of parameters using dielectric characteristic of material. At the end the practical usage and application will be shown by using ionic liquids as the electrolyte of batteries.

<sup>ε</sup> <sup>¼</sup> <sup>D</sup>

The electrical permittivity of a point in space is a multiplication of the relative constant of the

ε<sup>r</sup> is a value without dimension, the permittivity of the material, the so-called dielectric value of the

Most of the insulation applied in practice have the relative permittivity of insulation value of a digit. Water has particularly high value, due to the highly polar water molecules and high

In most computing procedures, the relative dielectric constant of air can be considered with the

The value of the field of the electric charge can be obtained from the very simple relationship:

The introduction of the displacement vector, which is formally a simple relationship between the dielectric constant and the electric field strength [3]. The situation is complicated when the insulation material is placed in the chamber. This is a natural thing since the smallest building blocks of matter are charged particles, although they usually seem to be neutral in the outing direction. In a small part of the chamber, the same amount of positive and negative charges can be found, but we know that these charges are in influenced by the electric field force and the consequence of the previously neutral dielectric material that creates a force field as well. This effect of the external space will change

<sup>C</sup><sup>0</sup> <sup>¼</sup> <sup>ε</sup><sup>0</sup> � <sup>A</sup>

where C is the capacitance, A is the surface area of opposing arms, d is the distance of armaments and ε is the dielectric constant of material positioned between the plates. If there

vacuum permittivity and the permittivity of room-filling material at the point:

value of 1 because of its difference from one is in the order of 10−<sup>4</sup>

The value of a capacitor can be calculated by the following equation:

1.2. Electric field in the insulating material

where <sup>ε</sup>0≈8, <sup>852</sup> � <sup>10</sup><sup>−</sup><sup>12</sup> As

material.

dipole moments.

the field of force.

1.3. Electrophysiological approach

<sup>E</sup> (3)

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453

ε ¼ ε<sup>0</sup> � ε<sup>r</sup> (4)

.

D ¼ ε0E (5)

<sup>4</sup><sup>π</sup> � <sup>d</sup> (6)

Vm, the permittivity of the vacuum, the so-called dielectric field constant and

Dielectric Characteristics of Ionic Liquids and Usage in Advanced Energy Storage Cells

Keywords: ionic liquids, energy storage cells, conductivity, temperature dependence, viscosity

### 1. The electrical permittivity of ionic liquids: the concept of dielectric constant

#### 1.1. The introduction of the concept of the dielectric constant

Charge carriers in ideally sealants are not able to move. Perfect vacuum is considered to be an ideal insulation for the fluid. In reality, in the practical devices containing the vacuum (such as vacuum switches), the vacuum is always bounded with material that allocates charge carriers in the vacuum; therefore, they cannot be considered as perfect insulators in practice [2].

Dielectric is a solid, liquid or gaseous substance, which acts as an electrically insulating material. Resistivity is greater than 108 Ωm. In dielectric there are practically no free charge carriers. Polarization occurs in the dielectric materials as a result of electric field. It is the permittivity characteristics of the substances that affect the amount of electric field. The dielectric constant is a scalar value.

Electric field strength from the charge by r distance created by a point charge Q from the context of

$$E = \frac{Q}{4\pi \cdot \varepsilon \cdot r^2} \to \varepsilon = \frac{Q}{4\pi \cdot E \cdot r^2} \tag{1}$$

so:

$$\mathbb{E}\left[\varepsilon\right] = \frac{\left[Q\right]}{\left[E\right] \cdot \left[r^2\right]} = \frac{1As}{1\frac{V}{m} \cdot 1 m^2} = 1\frac{As}{Vm} \tag{2}$$

The proportionality factor is between the electrical permittivity of the prevailing electric field of a given electrical offset (D) and electric field (E), which is characterized by filling the space at the point of medium volume and independent of the electric field in the material:

$$
\varepsilon = \frac{D}{E} \tag{3}
$$

The electrical permittivity of a point in space is a multiplication of the relative constant of the vacuum permittivity and the permittivity of room-filling material at the point:

$$
\varepsilon = \varepsilon\_0 \cdot \varepsilon\_r \tag{4}
$$

where <sup>ε</sup>0≈8, <sup>852</sup> � <sup>10</sup><sup>−</sup><sup>12</sup> As Vm, the permittivity of the vacuum, the so-called dielectric field constant and ε<sup>r</sup> is a value without dimension, the permittivity of the material, the so-called dielectric value of the material.

Most of the insulation applied in practice have the relative permittivity of insulation value of a digit. Water has particularly high value, due to the highly polar water molecules and high dipole moments.

In most computing procedures, the relative dielectric constant of air can be considered with the value of 1 because of its difference from one is in the order of 10−<sup>4</sup> .

#### 1.2. Electric field in the insulating material

sample materials, their usage possibilities and the results of the research from the previous work of the author. In the case of ionic liquids, it is important to know their behaviour in electric field. In lot of cases, there is no fundamental difference between the static and dynamic behaviours. Static state (like in accumulators) is similar to the dynamic. Ionic liquids are well characterized and grouped with their dielectric behaviour. First of all a short summarizing of basics of the electrical permittivity and then a modelling procedure will be shown modelling lots of parameters using dielectric characteristic of material. At the end the practical usage and application will be shown by

Keywords: ionic liquids, energy storage cells, conductivity, temperature dependence,

1. The electrical permittivity of ionic liquids: the concept of dielectric

Charge carriers in ideally sealants are not able to move. Perfect vacuum is considered to be an ideal insulation for the fluid. In reality, in the practical devices containing the vacuum (such as vacuum switches), the vacuum is always bounded with material that allocates charge carriers in the vacuum; therefore, they cannot be considered as perfect insulators in practice [2].

Dielectric is a solid, liquid or gaseous substance, which acts as an electrically insulating material. Resistivity is greater than 108 Ωm. In dielectric there are practically no free charge carriers. Polarization occurs in the dielectric materials as a result of electric field. It is the permittivity characteristics of the substances that affect the amount of electric field. The dielec-

Electric field strength from the charge by r distance created by a point charge Q from the

<sup>4</sup><sup>π</sup> � <sup>ε</sup> � <sup>r</sup><sup>2</sup> ! <sup>ε</sup> <sup>¼</sup> <sup>Q</sup>

½ �� <sup>E</sup> <sup>r</sup><sup>2</sup> ½ � <sup>¼</sup> <sup>1</sup>As 1 V

The proportionality factor is between the electrical permittivity of the prevailing electric field of a given electrical offset (D) and electric field (E), which is characterized by filling the space at the point of medium volume and independent of the electric field

<sup>m</sup> � <sup>1</sup>m<sup>2</sup> <sup>¼</sup> <sup>1</sup> As

<sup>4</sup><sup>π</sup> � <sup>E</sup> � <sup>r</sup><sup>2</sup> (1)

Vm (2)

<sup>E</sup> <sup>¼</sup> <sup>Q</sup>

½ �¼ <sup>ε</sup> ½ � <sup>Q</sup>

using ionic liquids as the electrolyte of batteries.

1.1. The introduction of the concept of the dielectric constant

viscosity

452 Progress and Developments in Ionic Liquids

tric constant is a scalar value.

constant

context of

so:

in the material:

The value of the field of the electric charge can be obtained from the very simple relationship:

$$\mathbf{D} = \varepsilon\_0 \mathbf{E} \tag{5}$$

The introduction of the displacement vector, which is formally a simple relationship between the dielectric constant and the electric field strength [3]. The situation is complicated when the insulation material is placed in the chamber. This is a natural thing since the smallest building blocks of matter are charged particles, although they usually seem to be neutral in the outing direction. In a small part of the chamber, the same amount of positive and negative charges can be found, but we know that these charges are in influenced by the electric field force and the consequence of the previously neutral dielectric material that creates a force field as well. This effect of the external space will change the field of force.

#### 1.3. Electrophysiological approach

The value of a capacitor can be calculated by the following equation:

$$C\_0 = \frac{\varepsilon\_0 \cdot A}{4\pi \cdot d} \tag{6}$$

where C is the capacitance, A is the surface area of opposing arms, d is the distance of armaments and ε is the dielectric constant of material positioned between the plates. If there is a vacuum between the condenser armatures, the measured capacity of the condenser is marked C0. If there is electrically insulating material between the armaments, the capacitance increases:

$$\mathbb{C} = \varepsilon\_r \cdot \mathbb{C}\_0 \tag{7}$$

δ ¼ 90°−φ (8)

http://dx.doi.org/10.5772/66948

Dielectric Characteristics of Ionic Liquids and Usage in Advanced Energy Storage Cells

(9)

455

The tan δ is an analytically similar electro-physical constant to ε. From the magnitude of tan δ, even more reliable conclusions can be obtained about the purity of a substance than some cases from the dielectric constants. The tan δ of a capacitor comprising a parallel loss resistance is

<sup>ω</sup> � <sup>C</sup> � <sup>R</sup> <sup>¼</sup> Ga

where Ga is the electrical conductivity of the dielectric, measured together with the dielectric

Therefore ε″ is a value without dimension that characterizes the amount of absorbed energy by

Two groups of the dielectric structure are distinguished regarding the structure of materials.

In the case of non-polar molecules, the molecules of the insulating material are neutral if there is no outward electric field. The positive and negative electric charge centres of gravity are in the same point and they only constitute small dipoles, depending on the outer space's field strength. This group includes, for example, a symmetrical hydrocarbons without permanent dipole moment molecules (methane, acetylene, benzene, naphthalene) and molecules consist of two identical atoms (F2, Cl2) and the noble gases.

In the case of polar molecules, the molecules of the dielectric are situated in an unsettled manner without an electric field. The positive and negative potentials of the centre of gravity in these materials do not coincide and they have dipole moment without an electric field (permanent dipole moment). The electric field handles and deforms these

ω<sup>0</sup> � Ca

tg<sup>δ</sup> <sup>¼</sup> <sup>1</sup>

calculated as follows:

capacitance of Ca.

<sup>ε</sup>′ than ε″ = ε′ � tgδ.

1.5. Molecular physics approach

a. Non-polar molecules

Figure 2. Lossless capacitor.

b. Polar molecules

the material from the electromagnetic space.

So tg<sup>δ</sup> <sup>¼</sup> <sup>ε</sup>″

The relationship shows that the value of the dielectric constant is at least one or greater than one value.

Then <sup>ε</sup><sup>r</sup> <sup>¼</sup> <sup>C</sup> <sup>C</sup><sup>0</sup> or in relative units, <sup>ε</sup><sup>r</sup> <sup>¼</sup> <sup>C</sup>þC<sup>0</sup> C0

If the capacitance change is large(C >> C0), the values of the two previous relationships do not differ significantly. The obtained dielectric constant value is marked ε′ and this metric number characterizes the interaction between the material and the electromagnetic field. The dielectric constant is a number that shows how many times greater the capacity of a given capacitor filled with dielectric material is than in the vacuum.

#### 1.4. Interpretation of the dissipation factor

If an ideal lossless capacitor is coupled in AC circuit, ϕ = 90° phase shift is generated between the voltage and the current of the capacitor (Figure 1).

Figure 1. Phase between current and volt of capacitor.

In this case the capacitor behaves like a lossless impedance (reactive resistance). Accordingly, the capacitor does not absorb energy from the circuit. The context of the AC power load can be seen

```
W = U I cosϕ where ϕ = 90° cosϕ = 0 and so W = 0.
```
If there is a dielectric material between the capacitor armatures, the dielectric loss is created. This can be modelled with an ideal capacitor and a resistor coupled in parallel.

The value of the reciprocal of the resistance is often presented as electrical conductivity. In this case, the phase difference between voltage and current will be less than 90° (Figure 2).

The dielectric material absorbs electromagnetic energy from the space (directly from the circuit) and consequently the movement of the dielectric molecules increases. Part of the electrical energy is converted into heat, so dielectric loss occurs. The magnitude of the losses is called phase angle supplementary angle and it equals to the tangent of the loss angle:

$$
\delta = 90^{\circ} \text{-} \phi \tag{8}
$$

The tan δ is an analytically similar electro-physical constant to ε. From the magnitude of tan δ, even more reliable conclusions can be obtained about the purity of a substance than some cases from the dielectric constants. The tan δ of a capacitor comprising a parallel loss resistance is calculated as follows:

$$
tg \delta = \frac{1}{\omega \cdot \mathbb{C} \cdot R} = \frac{G\_a}{\omega\_0 \cdot \mathbb{C}\_a} \tag{9}
$$

where Ga is the electrical conductivity of the dielectric, measured together with the dielectric capacitance of Ca.

So tg<sup>δ</sup> <sup>¼</sup> <sup>ε</sup>″ <sup>ε</sup>′ than ε″ = ε′ � tgδ.

is a vacuum between the condenser armatures, the measured capacity of the condenser is marked C0. If there is electrically insulating material between the armaments, the capacitance

The relationship shows that the value of the dielectric constant is at least one or greater than

If the capacitance change is large(C >> C0), the values of the two previous relationships do not differ significantly. The obtained dielectric constant value is marked ε′ and this metric number characterizes the interaction between the material and the electromagnetic field. The dielectric constant is a number that shows how many times greater the capacity of a given capacitor

If an ideal lossless capacitor is coupled in AC circuit, ϕ = 90° phase shift is generated between

In this case the capacitor behaves like a lossless impedance (reactive resistance). Accordingly, the capacitor does not absorb energy from the circuit. The context of the AC power load can be

If there is a dielectric material between the capacitor armatures, the dielectric loss is created.

The value of the reciprocal of the resistance is often presented as electrical conductivity. In this

The dielectric material absorbs electromagnetic energy from the space (directly from the circuit) and consequently the movement of the dielectric molecules increases. Part of the electrical energy is converted into heat, so dielectric loss occurs. The magnitude of the losses is called phase angle supplementary angle and it equals to the tangent of the loss angle:

case, the phase difference between voltage and current will be less than 90° (Figure 2).

This can be modelled with an ideal capacitor and a resistor coupled in parallel.

C0

C ¼ ε<sup>r</sup> � C<sup>0</sup> (7)

increases:

454 Progress and Developments in Ionic Liquids

one value.

Then <sup>ε</sup><sup>r</sup> <sup>¼</sup> <sup>C</sup>

seen

<sup>C</sup><sup>0</sup> or in relative units, <sup>ε</sup><sup>r</sup> <sup>¼</sup> <sup>C</sup>þC<sup>0</sup>

filled with dielectric material is than in the vacuum.

the voltage and the current of the capacitor (Figure 1).

W = U I cosϕ where ϕ = 90° cosϕ = 0 and so W = 0.

1.4. Interpretation of the dissipation factor

Figure 1. Phase between current and volt of capacitor.

Therefore ε″ is a value without dimension that characterizes the amount of absorbed energy by the material from the electromagnetic space.

Figure 2. Lossless capacitor.

#### 1.5. Molecular physics approach

Two groups of the dielectric structure are distinguished regarding the structure of materials.

a. Non-polar molecules

In the case of non-polar molecules, the molecules of the insulating material are neutral if there is no outward electric field. The positive and negative electric charge centres of gravity are in the same point and they only constitute small dipoles, depending on the outer space's field strength. This group includes, for example, a symmetrical hydrocarbons without permanent dipole moment molecules (methane, acetylene, benzene, naphthalene) and molecules consist of two identical atoms (F2, Cl2) and the noble gases.

b. Polar molecules

In the case of polar molecules, the molecules of the dielectric are situated in an unsettled manner without an electric field. The positive and negative potentials of the centre of gravity in these materials do not coincide and they have dipole moment without an electric field (permanent dipole moment). The electric field handles and deforms these dipoles of the molecule and the electric field is trying to turn to the direction of the field despite of the heat movement. This group contains molecules that are related together with electrovalent or ion binding (e.g. water, alcohols, compounds containing a carboxyl group, an amino group). So polar molecules and materials can be polarized by the deformation and orientation effects of the outdoor electric space (Figure 3).

polar materials, the temperature increase causes dielectric constant decreasing (incoordination

Dielectric Characteristics of Ionic Liquids and Usage in Advanced Energy Storage Cells

http://dx.doi.org/10.5772/66948

457

The polarization of polar materials, as has been previously mentioned, is made up of displacement and polarization orientation. In the case of polarization orientation, the thermal motion of molecules prevents the orientation effect of the field and prevents molecules from turning

The polarization level depends on the number and strength of the dipole and the intensity of the thermal motion. There are some liquid compound temperature coefficients in Table 1. As shown, the temperature coefficient increases with the increase of the value of

1.7. The effect of frequency changing to the dielectric constant (frequency dependence)

<sup>ε</sup>� <sup>¼</sup> <sup>ε</sup>′

The ε″ is the dielectric power consumption from the electric field, thus referred to as the absorption coefficient or loss numbers sometimes. There is the following relation between ε′,

tg<sup>δ</sup> <sup>¼</sup> <sup>ε</sup>″

The following figure shows the frequency dependence of ε″ and ε′. ε′ retains its value by a certain frequency (approximately 10<sup>8</sup> Hz). Here the electric field and the polarization are in phase and the dielectric constant is maximum (Figure 4).This is called the quasi-static dielectric

−jε″ (10)

<sup>ε</sup>′ (11)

The dielectric constant depends on many other things, mostly the frequency at measurement. This can easily be seen if we introduce the concept of the complex dielectric

This effect is greater when the temperature and the heat movement are higher.

Material Temperature coefficient

Table 1. Temperature coefficient of the dielectric constant in case of liquid compounds.

Benzene 0.00160 Chlorine-benzene 0.00174 Ethylene chloride 0.0553 Nitro-benzene 0.185 Pure water 0.366

is increasing).

polarity.

constant:

ε″ and tgδ:

constant values [4].

into the direction of the field.

Figure 3. Arrangement of non-polar and polar molecules.

The dielectric polarization is composed of several parts:

#### Polarization shift

1. The electron cloud shifts from relative to the nucleus without having to change relative positions of the nuclei inside the molecule.

(elektron polarization, Pe)

2. Atoms or ions are shifted relative to each other.

(atomic or ion polarization, Pa).

Orientation polarization.

The dipoles are arranged in the effect of the outing field

(-Po).

#### 1.6. The effect of temperature on the dielectric constant (temperature dependence)

In non-polar solvents, the resulting dipole molecules are always arranged in the direction of the field and this state is not formed by the heat movement significantly; therefore, the dielectric constant of the non-polar material does not depend on temperature. In the case of polar materials, the temperature increase causes dielectric constant decreasing (incoordination is increasing).

The polarization of polar materials, as has been previously mentioned, is made up of displacement and polarization orientation. In the case of polarization orientation, the thermal motion of molecules prevents the orientation effect of the field and prevents molecules from turning into the direction of the field.

This effect is greater when the temperature and the heat movement are higher.

dipoles of the molecule and the electric field is trying to turn to the direction of the field despite of the heat movement. This group contains molecules that are related together with electrovalent or ion binding (e.g. water, alcohols, compounds containing a carboxyl group, an amino group). So polar molecules and materials can be polarized by the

1. The electron cloud shifts from relative to the nucleus without having to change relative

deformation and orientation effects of the outdoor electric space (Figure 3).

The dielectric polarization is composed of several parts:

Figure 3. Arrangement of non-polar and polar molecules.

positions of the nuclei inside the molecule.

2. Atoms or ions are shifted relative to each other.

The dipoles are arranged in the effect of the outing field

1.6. The effect of temperature on the dielectric constant (temperature dependence)

In non-polar solvents, the resulting dipole molecules are always arranged in the direction of the field and this state is not formed by the heat movement significantly; therefore, the dielectric constant of the non-polar material does not depend on temperature. In the case of

(elektron polarization, Pe)

456 Progress and Developments in Ionic Liquids

Orientation polarization.

(atomic or ion polarization, Pa).

Polarization shift

(-Po).

The polarization level depends on the number and strength of the dipole and the intensity of the thermal motion. There are some liquid compound temperature coefficients in Table 1. As shown, the temperature coefficient increases with the increase of the value of polarity.


Table 1. Temperature coefficient of the dielectric constant in case of liquid compounds.

#### 1.7. The effect of frequency changing to the dielectric constant (frequency dependence)

The dielectric constant depends on many other things, mostly the frequency at measurement. This can easily be seen if we introduce the concept of the complex dielectric constant:

$$
\varepsilon^\* = \varepsilon^\prime \text{-j}\varepsilon^\prime\tag{10}
$$

The ε″ is the dielectric power consumption from the electric field, thus referred to as the absorption coefficient or loss numbers sometimes. There is the following relation between ε′, ε″ and tgδ:

$$
tg \delta = \frac{\varepsilon'}{\varepsilon'} \tag{11}
$$

The following figure shows the frequency dependence of ε″ and ε′. ε′ retains its value by a certain frequency (approximately 10<sup>8</sup> Hz). Here the electric field and the polarization are in phase and the dielectric constant is maximum (Figure 4).This is called the quasi-static dielectric constant values [4].

Figure 4. Frequency dependency of ε″ és ε′ .

With increasing the frequency, the dielectric constant ε′ ω0 is retained until you reach certain frequency areas, areas of anomalous dispersion, where the dielectric constant is reduced to a constant value ε′ ∞ . This value of n is called the refractive index in the next context on the basis of Maxwell's equation:

$$
\varepsilon'\_{\approx} = n^2 \tag{12}
$$

1.8. The Cole-Cole diagram

<sup>∞</sup> and ε′

Where ε′

<sup>ε</sup>′ <sup>¼</sup> <sup>ε</sup>′ <sup>∞</sup> þ ε′ ω0 −ε′ ∞ <sup>1</sup> <sup>þ</sup> <sup>ω</sup><sup>2</sup>τ<sup>2</sup> <sup>ε</sup>″ <sup>¼</sup> <sup>ε</sup>′

used, although they are from different models.

ε′ − ε′ ωο <sup>þ</sup> <sup>ε</sup>′ ∞ 2 <sup>2</sup>

ordinate and ε′ with as abscissa, a semicircle is obtained.

of the differential structure, this is a useful research method.

Figure 5. Cole-Cole diagram, ideal Debye behaviour.

The real and the imaginary part is obtained by applying the following relationships:

ω0 −ε′ ∞ <sup>þ</sup>

<sup>ω</sup><sup>0</sup> are the high frequency and static dielectric constant and τ and ω are the

frequency and relaxation time, which is characterized by the formation and cessation of the polarization ratio. These relationships are derived both from liquids and solids and can also be

The formula above gives the equation of the circle. Accordingly, in the diagram ε″ with as

Figure 5 shows an ideal Cole-Cole diagram, when the behaviour of that material fits the above equations. In this case, the centre of the circle is at the abscissa and a different behaviour lies when the centre of semicircle lies below the abscissa. From the ideal Debye behaviour analysis

<sup>þ</sup> <sup>ε</sup>″ <sup>2</sup> <sup>¼</sup> <sup>ε</sup>′

ωο−ε′ ∞ 2 <sup>2</sup>

Dielectric Characteristics of Ionic Liquids and Usage in Advanced Energy Storage Cells

From the combination of relationships above, we can obtain the following relationships:

ωτ

<sup>1</sup> <sup>þ</sup> <sup>ω</sup><sup>2</sup>τ<sup>2</sup> (15)

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(16)

459

Theoretically, this can be explained by the electrons, atoms and atomic groups and the permanent dipoles can track slow changes of the field in the quasi-static area. Significant change occurs when the frequency of the alternating space is increased to a greater extent. Above a certain frequency, the permanent dipoles cannot follow the changes of space rapidly, so the orientation polarization no longer contributes to the total polarization. The dipole orientation cannot keep up with the force field and suffers delayed phase shift. Then we can talk about the anomalous dispersion of the dielectric constant. According to the Debye's theory of dispersion, this situation depends on the viscosity (η), the molecular radius (r):

$$
\pi = \frac{4\pi \cdot \eta \cdot r^3}{k \cdot T} \tag{13}
$$

τ is the so-called relaxation time τ (τ = 1/ωx), the measured value of the relaxation rate. This is the time period, while the orientation polarization reduces e-fold by switching off the static field afterwards [5].

The absorption coefficient ε″ behaves differently depending on the frequency. The quasi-static and high-frequency field ε″ are almost immeasurably small. Only the dispersion area can be measured and reaches its maximum at a frequency ωx. The theoretical analysis of the frequency dependence of the dielectric constant begins to form detailed in Debye equations:

$$
\varepsilon^\* = \varepsilon\_{\circ}^{'} + \frac{\varepsilon\_{\circ 0}^{'} - \varepsilon\_{\circ}^{'}}{1 + j\omega\tau} \tag{14}
$$

#### 1.8. The Cole-Cole diagram

With increasing the frequency, the dielectric constant ε′

.

this situation depends on the viscosity (η), the molecular radius (r):

constant value ε′

Maxwell's equation:

field afterwards [5].

∞

Figure 4. Frequency dependency of ε″ és ε′

458 Progress and Developments in Ionic Liquids

ω0

. This value of n is called the refractive index in the next context on the basis of

frequency areas, areas of anomalous dispersion, where the dielectric constant is reduced to a

Theoretically, this can be explained by the electrons, atoms and atomic groups and the permanent dipoles can track slow changes of the field in the quasi-static area. Significant change occurs when the frequency of the alternating space is increased to a greater extent. Above a certain frequency, the permanent dipoles cannot follow the changes of space rapidly, so the orientation polarization no longer contributes to the total polarization. The dipole orientation cannot keep up with the force field and suffers delayed phase shift. Then we can talk about the anomalous dispersion of the dielectric constant. According to the Debye's theory of dispersion,

<sup>τ</sup> <sup>¼</sup> <sup>4</sup><sup>π</sup> � <sup>η</sup> � <sup>r</sup><sup>3</sup>

τ is the so-called relaxation time τ (τ = 1/ωx), the measured value of the relaxation rate. This is the time period, while the orientation polarization reduces e-fold by switching off the static

The absorption coefficient ε″ behaves differently depending on the frequency. The quasi-static and high-frequency field ε″ are almost immeasurably small. Only the dispersion area can be measured and reaches its maximum at a frequency ωx. The theoretical analysis of the frequency dependence of the dielectric constant begins to form detailed in Debye equations:

> <sup>ε</sup>� <sup>¼</sup> <sup>ε</sup>′ <sup>∞</sup> þ ε′ ω0 −ε′ ∞

ε′

is retained until you reach certain

<sup>∞</sup> <sup>¼</sup> <sup>n</sup><sup>2</sup> (12)

<sup>k</sup> � <sup>T</sup> (13)

<sup>1</sup> <sup>þ</sup> <sup>j</sup>ωτ (14)

The real and the imaginary part is obtained by applying the following relationships:

$$\boldsymbol{\varepsilon}^{\prime} = \boldsymbol{\varepsilon}^{\prime}\_{\circ \circ} + \frac{\boldsymbol{\varepsilon}^{\prime}\_{\circ \circ \circ} - \boldsymbol{\varepsilon}^{\prime}\_{\circ \circ}}{1 + \boldsymbol{\omega}^{2} \tau^{2}} \qquad \boldsymbol{\varepsilon}^{\prime} = \left(\boldsymbol{\varepsilon}^{\prime}\_{\circ \circ} - \boldsymbol{\varepsilon}^{\prime}\_{\circ \circ}\right) + \frac{\boldsymbol{\omega} \tau}{1 + \boldsymbol{\omega}^{2} \tau^{2}} \tag{15}$$

Where ε′ <sup>∞</sup> and ε′ <sup>ω</sup><sup>0</sup> are the high frequency and static dielectric constant and τ and ω are the frequency and relaxation time, which is characterized by the formation and cessation of the polarization ratio. These relationships are derived both from liquids and solids and can also be used, although they are from different models.

From the combination of relationships above, we can obtain the following relationships:

$$
\left(\varepsilon^{'} - \frac{\varepsilon^{'}\_{\alpha \alpha} + \varepsilon^{'}\_{\alpha}}{2}\right)^{2} + \left(\varepsilon^{\*}\right)^{2} = \left(\frac{\varepsilon^{'}\_{\alpha \alpha} - \varepsilon^{'}\_{\alpha}}{2}\right)^{2} \tag{16}
$$

The formula above gives the equation of the circle. Accordingly, in the diagram ε″ with as ordinate and ε′ with as abscissa, a semicircle is obtained.

Figure 5 shows an ideal Cole-Cole diagram, when the behaviour of that material fits the above equations. In this case, the centre of the circle is at the abscissa and a different behaviour lies when the centre of semicircle lies below the abscissa. From the ideal Debye behaviour analysis of the differential structure, this is a useful research method.

Figure 5. Cole-Cole diagram, ideal Debye behaviour.

### 2. Materials in electromagnetic field

During the microwave treatment, temperature of the sample continuously rises and its dielectric properties also change [6]. From the generator's viewpoint, the value of terminating impedance represented by the transmission line changes accordingly. For the microwave generator, the transmission line acts as an impedance terminator whose value depends on the wavelength and on the geometric properties of the transmission line [7]. The impedance of transmission line also depends on the dielectric properties of the material which are either partially or fully filling the transmission line. Since the temperature of the sample changes due to the energy impact, the value of the impedance terminator represented by parameters the transmission line also changes together with the sample properties. During the energy impact, the varying dielectric properties of the sample change the axial distribution of the microwave energy in the transmission line; therefore, the amount of energy absorbed in the sample also changes.

reflected from the shortcut at the end of the transmission line leaves the line towards the generator. The ratio of forwarding and reflected waves is the standing wave ratio and it depends first of all on the dielectric loss of the medium filling the transmission line. The standing wave ratio is infinity in the ideal case (there is no sample, i.e. loss in the transmission line) if the transmission line is closed with the wave impedance of the line. Since the standing wave ratio is given as the ratio of forwarding and reflected waves, it is greater than one if the transmission line has loss and it equals to the ratio of powers entering and leaving the

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That is, knowing the standing wave ratio r and the generator power PM, it is possible to determine the power entering the transmission line. Since the medium's (sample's) dielectric parameters are functions of the temperature, it is necessary to appear the temperature dependence in the formulae explicitly. Denoting the microwave power entering the transmission line

PAð Þ¼ <sup>T</sup> <sup>1</sup>

A part of this, power will dissipate and heat up the sample placed inside the transmission line. The amount of power absorbed in the sample depends on the sample's dielectric properties: it is directly proportional to the dielectric loss and inversely proportional to the square root of the

The temperature dependence of the dielectric parameters is respected in the above formula. The power absorbed in the sample increases its temperature; the degree of warming depends on the specific heat (Cp) and the density (ρ) of the sample. A sample having greater specific

gives the speed of temperature change; on the other hand, the integral of the formula with respect to time gives the value of the temperature in the sample as a function of time. The temperature dependence of the dielectric parameters (ε′, ε″) of the sample can be measured with a microwave dielectrometer designed by the author of this paper [11]; thus, it is possible to define the relationships ε′(T) and ε″(T) regarding the specific sample material by fitting

ð Þ Tffiffiffiffiffiffiffiffiffiffi ε′

r Tð Þ PM (17)

ð Þ <sup>T</sup> <sup>p</sup> PAð Þ <sup>T</sup> (18)

PDð Þ T (19)

. The formula

by PA and the generator's power by PM, the following formula holds:

dielectric constant. The coefficient 0.5126 is an experimental value [10]:

PDð Þ¼ <sup>T</sup> <sup>0</sup>:<sup>5126</sup> <sup>ε</sup>″

heat or density warms slower. The change of temperature is given as follows:

dT dt <sup>¼</sup> <sup>K</sup> <sup>1</sup> Cpρ

where K is a coefficient regarding the volume of the sample and its unit is 1/cm<sup>3</sup>

polynomials on the resulted data.

transmission line.

Because of the constant microwave energy input, temperature-dependent energy-, impedanceand dielectric conditions are developed. Some of them (e.g. temperature, dielectric property) are measurable; others can only be determined by computation [8]. By constructing a model containing the parameters of the transmission line and the sample placed in the transmission line, it is possible to determine the continuously varying parameters during the heating.

### 3. Modelling of power and temperature dependence of ionic liquids

During treatment and usage, outing energy is transferred to a material sample placed in an applicator of given geometric parameters. As a result of the energy transfer, the sample absorbs energy from the microwave field depending on its dielectric properties. The degree of energy absorption is directly proportional to the dielectric loss and proportional to the square root of the dielectric constant. The temperature of the sample continuously increases due to the energy transfer and the dielectric properties of the sample also change with the rising temperature. Although the microwave energy supply is constant, time- and temperature-dependent energy impedance and dielectric relations are developed. A part of them is measurable, but the other part of them cannot be directly measured; they can only be computed from the previously measured ones. In a closed model which contains the parameters of the sample and the waveguide, the continuously changing parameters can be determined in relation of the temperature. These parameters are as follows: attenuation of the transmission line, temporal change of the sample temperature, dielectric properties of the sample, loss factor of the sample, penetration depth, impedance of the transmission line and standing wave ratio reflection factor. The modelling procedure was invented on the University of Pannonia by the research group. The above parameters can be modelled as a function of the sample's temperature or as a function of time [9].

The power leaving the generator enters and propagates in the transmission line as a wave. The medium in the transmission line—according to its dielectric properties—modifies the portrait of lines of force and takes energy from the electromagnetic field. The electromagnetic wave reflected from the shortcut at the end of the transmission line leaves the line towards the generator. The ratio of forwarding and reflected waves is the standing wave ratio and it depends first of all on the dielectric loss of the medium filling the transmission line. The standing wave ratio is infinity in the ideal case (there is no sample, i.e. loss in the transmission line) if the transmission line is closed with the wave impedance of the line. Since the standing wave ratio is given as the ratio of forwarding and reflected waves, it is greater than one if the transmission line has loss and it equals to the ratio of powers entering and leaving the transmission line.

2. Materials in electromagnetic field

460 Progress and Developments in Ionic Liquids

function of time [9].

therefore, the amount of energy absorbed in the sample also changes.

During the microwave treatment, temperature of the sample continuously rises and its dielectric properties also change [6]. From the generator's viewpoint, the value of terminating impedance represented by the transmission line changes accordingly. For the microwave generator, the transmission line acts as an impedance terminator whose value depends on the wavelength and on the geometric properties of the transmission line [7]. The impedance of transmission line also depends on the dielectric properties of the material which are either partially or fully filling the transmission line. Since the temperature of the sample changes due to the energy impact, the value of the impedance terminator represented by parameters the transmission line also changes together with the sample properties. During the energy impact, the varying dielectric properties of the sample change the axial distribution of the microwave energy in the transmission line;

Because of the constant microwave energy input, temperature-dependent energy-, impedanceand dielectric conditions are developed. Some of them (e.g. temperature, dielectric property) are measurable; others can only be determined by computation [8]. By constructing a model containing the parameters of the transmission line and the sample placed in the transmission line, it is possible to determine the continuously varying parameters during the heating.

3. Modelling of power and temperature dependence of ionic liquids

During treatment and usage, outing energy is transferred to a material sample placed in an applicator of given geometric parameters. As a result of the energy transfer, the sample absorbs energy from the microwave field depending on its dielectric properties. The degree of energy absorption is directly proportional to the dielectric loss and proportional to the square root of the dielectric constant. The temperature of the sample continuously increases due to the energy transfer and the dielectric properties of the sample also change with the rising temperature. Although the microwave energy supply is constant, time- and temperature-dependent energy impedance and dielectric relations are developed. A part of them is measurable, but the other part of them cannot be directly measured; they can only be computed from the previously measured ones. In a closed model which contains the parameters of the sample and the waveguide, the continuously changing parameters can be determined in relation of the temperature. These parameters are as follows: attenuation of the transmission line, temporal change of the sample temperature, dielectric properties of the sample, loss factor of the sample, penetration depth, impedance of the transmission line and standing wave ratio reflection factor. The modelling procedure was invented on the University of Pannonia by the research group. The above parameters can be modelled as a function of the sample's temperature or as a

The power leaving the generator enters and propagates in the transmission line as a wave. The medium in the transmission line—according to its dielectric properties—modifies the portrait of lines of force and takes energy from the electromagnetic field. The electromagnetic wave That is, knowing the standing wave ratio r and the generator power PM, it is possible to determine the power entering the transmission line. Since the medium's (sample's) dielectric parameters are functions of the temperature, it is necessary to appear the temperature dependence in the formulae explicitly. Denoting the microwave power entering the transmission line by PA and the generator's power by PM, the following formula holds:

$$P\_A(T) = \frac{1}{r(T)} P\_M \tag{17}$$

A part of this, power will dissipate and heat up the sample placed inside the transmission line. The amount of power absorbed in the sample depends on the sample's dielectric properties: it is directly proportional to the dielectric loss and inversely proportional to the square root of the dielectric constant. The coefficient 0.5126 is an experimental value [10]:

$$P\_D(T) = 0.5126 \frac{\varepsilon^\*(T)}{\sqrt{\varepsilon'(T)}} P\_A(T) \tag{18}$$

The temperature dependence of the dielectric parameters is respected in the above formula. The power absorbed in the sample increases its temperature; the degree of warming depends on the specific heat (Cp) and the density (ρ) of the sample. A sample having greater specific heat or density warms slower. The change of temperature is given as follows:

$$\frac{dT}{dt} = K \frac{1}{C\_p \rho} P\_D(T) \tag{19}$$

where K is a coefficient regarding the volume of the sample and its unit is 1/cm<sup>3</sup> . The formula gives the speed of temperature change; on the other hand, the integral of the formula with respect to time gives the value of the temperature in the sample as a function of time. The temperature dependence of the dielectric parameters (ε′, ε″) of the sample can be measured with a microwave dielectrometer designed by the author of this paper [11]; thus, it is possible to define the relationships ε′(T) and ε″(T) regarding the specific sample material by fitting polynomials on the resulted data.

The impedance of the transmission line is a function of its geometric parameters and the dielectric properties of the sample. The characteristic wave impedance Z0<sup>t</sup> of the transmission line depends on the size of the transmission line sizes (in two dimensions) and the wavelength (λg) of the electromagnetic wave propagates inside according to the following formula:

$$Z\_{0t} = \frac{2Z\_0b}{a\sqrt{1 - \left(\frac{\lambda\_r}{2a}\right)^2}} = 754\frac{4,4}{9,4\sqrt{1 - \left(\frac{12,24}{18,8}\right)^2}} = 465\Omega\tag{20}$$

where Z0<sup>l</sup> is the open air wave impedance of the air (377 Ω), is the electromagnetic wavelength a and b are the dimensions of the transmission line (9.4 cm, 4.4 cm), respectively. Note that Z0<sup>t</sup> is independent of the sample properties and depends only on the transmission line geometry and wavelength. From Z0<sup>t</sup> and the dielectric loss, it is possible to calculate the overall impedance (of the transmission line and the sample) as a function of temperature [12]:

$$Z\_T(T) = \frac{Z\_{0t}}{\sqrt{\varepsilon'(T)}} \left( 1 - \frac{3}{8} (\text{tg}\,\delta(T))^2 + j\frac{1}{2}\text{tg}\,\delta(T) \right) \tag{21}$$

From the overall impedance and the wave impedance, one can determine the reflection coefficient of the transmission line:

$$|I(T)| = \frac{Zr(T) - Z\_{0t}}{Z\_T(T) + Z\_{0t}} \tag{22}$$

Figure 6. Overall modelling of the filled cavity.

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Figure 7. The MatLab model for simulation.

Afterwards, it is possible to express the standing wave ratio:

$$r(T) = \frac{1 + |\Gamma(T)|}{1 - |\Gamma(T)|} \tag{23}$$

From the standing wave ratio, it is possible to give the power entering the transmission line knowing the generator power, as it was mentioned before. Now, it is possible to build a model from the above equations, which has the following input parameters: microwave generator power, density of the sample, specific heat of the sample, sample specific dielectric characteristics as a function of temperature, transmission line and wavelength parameters.

#### 3.1. Modelling set-up in filled cavity

Overall we can obtain an overall figure about modelling of microwave cavity filled with ionic liquid or other material (Figure 6).

The model structure has been implemented in MatLab® environment (Figure 7). First the pure water has been investigated. The temperature dependence of dielectric values of water is described by Eq. (24) This equation is from the literature [13] and showed in (Figure 8):

$$
\varepsilon'(T) = 87 \text{--} 0.36T \qquad \qquad \varepsilon''(T) = 283/T \text{--} 1.17 \tag{24}
$$

Dielectric Characteristics of Ionic Liquids and Usage in Advanced Energy Storage Cells http://dx.doi.org/10.5772/66948 463

Figure 6. Overall modelling of the filled cavity.

The impedance of the transmission line is a function of its geometric parameters and the dielectric properties of the sample. The characteristic wave impedance Z0<sup>t</sup> of the transmission line depends on the size of the transmission line sizes (in two dimensions) and the wavelength

9, 4

where Z0<sup>l</sup> is the open air wave impedance of the air (377 Ω), is the electromagnetic wavelength a and b are the dimensions of the transmission line (9.4 cm, 4.4 cm), respectively. Note that Z0<sup>t</sup> is independent of the sample properties and depends only on the transmission line geometry and wavelength. From Z0<sup>t</sup> and the dielectric loss, it is possible to calculate the overall imped-

> 3 8

From the overall impedance and the wave impedance, one can determine the reflection coeffi-

j j¼ <sup>Γ</sup>ð Þ <sup>T</sup> ZTð Þ <sup>T</sup> <sup>−</sup>Z0<sup>t</sup>

r Tð Þ¼ <sup>1</sup> <sup>þ</sup> j j <sup>Γ</sup>ð Þ <sup>T</sup>

From the standing wave ratio, it is possible to give the power entering the transmission line knowing the generator power, as it was mentioned before. Now, it is possible to build a model from the above equations, which has the following input parameters: microwave generator power, density of the sample, specific heat of the sample, sample specific dielectric character-

Overall we can obtain an overall figure about modelling of microwave cavity filled with ionic

The model structure has been implemented in MatLab® environment (Figure 7). First the pure water has been investigated. The temperature dependence of dielectric values of water is described by Eq. (24) This equation is from the literature [13] and showed in (Figure 8):

istics as a function of temperature, transmission line and wavelength parameters.

ð Þ¼ <sup>T</sup> <sup>87</sup>−0:36<sup>T</sup> <sup>ε</sup>″

ZTð Þþ T Z0<sup>t</sup>

ð Þ tgδð Þ <sup>T</sup> <sup>2</sup> <sup>þ</sup> <sup>j</sup>

� �

1 2 tgδð Þ T

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 1− <sup>12</sup>,<sup>24</sup> 18, 8

� �<sup>2</sup> <sup>r</sup> <sup>¼</sup> <sup>465</sup><sup>Ω</sup> (20)

<sup>1</sup>−j j <sup>Γ</sup>ð Þ <sup>T</sup> (23)

ð Þ¼ T 283=T−1:17 (24)

(21)

(22)

(λg) of the electromagnetic wave propagates inside according to the following formula:

� �<sup>2</sup> <sup>r</sup> <sup>¼</sup> <sup>754</sup> <sup>4</sup>, <sup>4</sup>

<sup>Z</sup>0<sup>t</sup> <sup>¼</sup> <sup>2</sup>Z0lb a

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 1− <sup>λ</sup><sup>g</sup> 2a

ance (of the transmission line and the sample) as a function of temperature [12]:

ZTð Þ¼ <sup>T</sup> <sup>Z</sup>0<sup>t</sup> ffiffiffiffiffiffiffiffiffiffiffi ε 0 ð Þ <sup>T</sup> <sup>p</sup> <sup>1</sup><sup>−</sup>

Afterwards, it is possible to express the standing wave ratio:

cient of the transmission line:

462 Progress and Developments in Ionic Liquids

3.1. Modelling set-up in filled cavity

ε′

liquid or other material (Figure 6).

Figure 7. The MatLab model for simulation.

It is important to note that the ionic liquid 1-ethyl-3-methylimidazolium hydrogen sulphate (EMIM-HSO4) similar to many other ionic liquids, such as those based on nitrate and dihydrogenphosphate anions, has also been found to work well in this battery design. Novel batteries are designed using standard cathode materials such as MnO2, PbO2, NiO and AgO and anode materials such as Zn, Sn and Pb. Additionally, by using a solid polymer electrolyte composed of polyvinyl alcohol and anionic liquid, new types of solid-state batteries are demonstrated with discharge voltages ranging up to 1.8 V, depending upon the type of

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465

Ionic liquids like 1-butyl-3-methylimidazolium tetrafluoroborate (IMIM-BF4) or hexafluorophosphate (IMIM-PF6) and 1-butyl-4-methylpyridinium tetrafluoroborate (PyBF4) were mixed with organic solvents such as butyrolactone (BL) and acetonitrile (ACN). A lithium salt (LiBF4 or LiPF6) was added to these mixtures for possible application in the field of energy storage (batteries or supercapacitors). Viscosities, conductivities and electrochemical windows at a Pt electrode of these electrolytes were investigated. All the studied electrolytes are stable towards oxidation and exhibit a vitreous phase transition, which has been determined by application of the conductivity measurements. Mixtures containing the BF4− anion exhibit the lowest viscos-

Aromatic cations, such as 1-ethyl-3-methylimidazolium (EMI), have been used for as the cationic component of the RTILs. The EMI cation is the best cation to form the RTIL, which has a low viscosity and low melting point, with various anions. However, the electrochemical stability as a lithium battery electrolyte was not satisfactory since the cathodic limiting potential is ca. +1.0 V versus Li/Li+ and additives, such as thionyl chloride, were essential for improving the coulombic efficiency for lithium deposition in an RTIL based on EMI [20].

There are several combinations of alkylimidazolium cations and inorganic and organic anions have been investigated to date. A survey of these salts including fluoroanions will be given

Several properties of ionic liquids should be examined before its application in batteries; therefore, we used different measurement methods at our disposal on a number of ionic liquids. The different microwave measurements included processes such as checking the temperature rise in microwave field and measuring microwave dielectric properties at 2.45 GHz frequency, electrical conductivity according to temperature and viscosity changes

We used CEM Discover unit to measure the velocity of temperature rise of ionic liquids. The CEM Discover unit is a widely used and available apparatus in microwave chemistry. The device has a cylindrical operating space and on the cylindrical peripheral surface, there are many slots where microwave energy can enter; this way the high homogeneity of the microwave field is ensured. We measured the temperature at the bottom of the compartment with an infrared thermometer. The amount of the tested samples was 0.5 g, which were placed in an

inner diameter of 12.5 mm of borosilicate cylindrical glass flask.

cathode and anode used [18].

ity and the highest conductivity [19].

in [21].

4.1. Methods

depending on the temperature.

Figure 8. Results from the model. tgδ(T) values of different materials.

### 4. Introduction of battery cell

Dielectric constant and dielectric dissipation factors are the main parameters in the modelling of microwave behaviour of ionic liquids, in addition the parameters characterizing the polarizability and the microwave energy absorption. The static dielectric constant of ionic liquids cannot be approached with traditional measurement methods because they are characterized by high electrical conductivity which results in intense shortcut. Systematic study of these contexts has not been conducted so far because the subject is relatively new; however, the results are very important in the planning and controlling of chemical reactions.

At the end of the 1990s, the discovery of ionic liquids opened new ways of technological applications especially in the area of chemistry and in particular green chemistry. Their special chemical and physical properties make it very beneficial and important since the use of energy storage units, particularly mobile phones, electric vehicles and uninterruptible power supply systems, is widely used [1]. Recently, room temperature ionic liquids (RTILs) have been extensively studied as electrolytes of lithiumion batteries from cellular phones to electric vehicles. RTILs are good options for the electrolyte bases of a safe lithium battery due to their unique properties [14, 15].

RTILs are known for being thermally stable and non-flammable and they might have the capacity to improve the safety of electrochemical devices with aprotic solvents, such as Li batteries and supercapacitors in abuse.

The high ionic conductivity of 1-Ethyl-3-methylimidazolium tetrafluoroborate (EMIM-BF4) is comparable to those of organic solvent electrolytes and its viscosity is low which makes it a good electrolyte for Li batteries. Li battery applications can benefit from these and we also found that a Li/LiCoO2 cell with RTILs as an electrolyte base works reversibly, indicating that QAimide RTILs are quite stable even at the Li reduction potential. We can claim that RTILs improve the safety of Li-ion batteries with carbon-negative electrodes. For improving the safety of Li-metal batteries, RTILs seem to be the most promising and applicable electrolytes, which could also be important for higher energy densities [16].

A set of four imidazolium ionic liquids (solid at room temperature) and one imidazolium ionic solid were screened for their potentials as electrolytes in manganese dioxide-free Leclanché batteries, equipped with a zinc anode and graphite cathode [17].

It is important to note that the ionic liquid 1-ethyl-3-methylimidazolium hydrogen sulphate (EMIM-HSO4) similar to many other ionic liquids, such as those based on nitrate and dihydrogenphosphate anions, has also been found to work well in this battery design. Novel batteries are designed using standard cathode materials such as MnO2, PbO2, NiO and AgO and anode materials such as Zn, Sn and Pb. Additionally, by using a solid polymer electrolyte composed of polyvinyl alcohol and anionic liquid, new types of solid-state batteries are demonstrated with discharge voltages ranging up to 1.8 V, depending upon the type of cathode and anode used [18].

Ionic liquids like 1-butyl-3-methylimidazolium tetrafluoroborate (IMIM-BF4) or hexafluorophosphate (IMIM-PF6) and 1-butyl-4-methylpyridinium tetrafluoroborate (PyBF4) were mixed with organic solvents such as butyrolactone (BL) and acetonitrile (ACN). A lithium salt (LiBF4 or LiPF6) was added to these mixtures for possible application in the field of energy storage (batteries or supercapacitors). Viscosities, conductivities and electrochemical windows at a Pt electrode of these electrolytes were investigated. All the studied electrolytes are stable towards oxidation and exhibit a vitreous phase transition, which has been determined by application of the conductivity measurements. Mixtures containing the BF4− anion exhibit the lowest viscosity and the highest conductivity [19].

Aromatic cations, such as 1-ethyl-3-methylimidazolium (EMI), have been used for as the cationic component of the RTILs. The EMI cation is the best cation to form the RTIL, which has a low viscosity and low melting point, with various anions. However, the electrochemical stability as a lithium battery electrolyte was not satisfactory since the cathodic limiting potential is ca. +1.0 V versus Li/Li+ and additives, such as thionyl chloride, were essential for improving the coulombic efficiency for lithium deposition in an RTIL based on EMI [20].

There are several combinations of alkylimidazolium cations and inorganic and organic anions have been investigated to date. A survey of these salts including fluoroanions will be given in [21].

#### 4.1. Methods

4. Introduction of battery cell

464 Progress and Developments in Ionic Liquids

Figure 8. Results from the model. tgδ(T) values of different materials.

batteries and supercapacitors in abuse.

which could also be important for higher energy densities [16].

batteries, equipped with a zinc anode and graphite cathode [17].

Dielectric constant and dielectric dissipation factors are the main parameters in the modelling of microwave behaviour of ionic liquids, in addition the parameters characterizing the polarizability and the microwave energy absorption. The static dielectric constant of ionic liquids cannot be approached with traditional measurement methods because they are characterized by high electrical conductivity which results in intense shortcut. Systematic study of these contexts has not been conducted so far because the subject is relatively new; however, the

At the end of the 1990s, the discovery of ionic liquids opened new ways of technological applications especially in the area of chemistry and in particular green chemistry. Their special chemical and physical properties make it very beneficial and important since the use of energy storage units, particularly mobile phones, electric vehicles and uninterruptible power supply systems, is widely used [1]. Recently, room temperature ionic liquids (RTILs) have been extensively studied as electrolytes of lithiumion batteries from cellular phones to electric vehicles. RTILs are good options

RTILs are known for being thermally stable and non-flammable and they might have the capacity to improve the safety of electrochemical devices with aprotic solvents, such as Li

The high ionic conductivity of 1-Ethyl-3-methylimidazolium tetrafluoroborate (EMIM-BF4) is comparable to those of organic solvent electrolytes and its viscosity is low which makes it a good electrolyte for Li batteries. Li battery applications can benefit from these and we also found that a Li/LiCoO2 cell with RTILs as an electrolyte base works reversibly, indicating that QAimide RTILs are quite stable even at the Li reduction potential. We can claim that RTILs improve the safety of Li-ion batteries with carbon-negative electrodes. For improving the safety of Li-metal batteries, RTILs seem to be the most promising and applicable electrolytes,

A set of four imidazolium ionic liquids (solid at room temperature) and one imidazolium ionic solid were screened for their potentials as electrolytes in manganese dioxide-free Leclanché

results are very important in the planning and controlling of chemical reactions.

for the electrolyte bases of a safe lithium battery due to their unique properties [14, 15].

Several properties of ionic liquids should be examined before its application in batteries; therefore, we used different measurement methods at our disposal on a number of ionic liquids. The different microwave measurements included processes such as checking the temperature rise in microwave field and measuring microwave dielectric properties at 2.45 GHz frequency, electrical conductivity according to temperature and viscosity changes depending on the temperature.

We used CEM Discover unit to measure the velocity of temperature rise of ionic liquids. The CEM Discover unit is a widely used and available apparatus in microwave chemistry. The device has a cylindrical operating space and on the cylindrical peripheral surface, there are many slots where microwave energy can enter; this way the high homogeneity of the microwave field is ensured. We measured the temperature at the bottom of the compartment with an infrared thermometer. The amount of the tested samples was 0.5 g, which were placed in an inner diameter of 12.5 mm of borosilicate cylindrical glass flask.

It is important to note that during microwave treatment, a conversion process takes place in which the microwave energy interacts with the treated material and the material converts the electric energy into thermal energy according its characteristic of dielectric properties. During this process, the measurable increase of the temperature in the treated material is the macroscopically observable result. The rate of the temperature increase depends on the microwave field and the treated material properties, which is described in the following equation:

$$
\Delta \mathbf{T} / \Delta \mathbf{t} = P\_v / \rho \mathbf{C} \mathbf{p} = \mathbf{j} \mathbf{E}^2 f \varepsilon \mathbf{\hat{z}} / \rho \mathbf{C} \mathbf{p} \tag{25}
$$

The method is based on the compensation of phase change due to the microwave energy absorption of the liquid sample. The short circuit piston situated behind the sample must be actuated for compensation. The energy conditions created by the wave front in the waveguide

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Dielectric constants, dielectric loss factors and the temperature dependence of the dielectric properties of ionic liquids intended to be used in batteries were determined by the abovedescribed self-designed microwave dielectrometric apparatus (Figure 3) at the frequency of 2.45 GHz and at different temperatures (30°C, 40°C, 50°C, 60°C, 70°C, 80°C, 90°C, 100°C, 110° C and 120°C). The speed of the change in temperature depends on the electrical field strength in the material (E), the absorbed microwave power, (Pv) density (ρ), the specific heat capacity

The temperature increase of the new type of ionic liquids was examined, as it is shown in

(Cp) and the dielectric loss factor ε″ and can be given by Eq. (1) [11].

are measured by four diode detectors.

Figure 9. Scheme of the experimental set-up.

4.3. Results and obtained characteristics

Figure 10. The structure of investigated ionic liquids.

Figure 10 and in Table 2.

where the ΔT/Δt is the velocity of temperature rising in Kelvin per second, Pv is the absorbed power in the sample, ρ is the density of the sample, Cp is the specific heat of the sample, E2 is the strength of the electromagnetic field in the sample, f is the frequency of the field and ε″ is the dielectric loss; j means this is a complex value [15]. This formula shows that the temperature of the treated material is influenced by a number of factors. Testing their effects separately is not simple, because ρ, ε Cp features themselves are temperature dependent and it is difficult to measure this value accurately inside the material. In the case of strictly homogeneous series of examined compounds, the situation is more simplified, because some simplifications are permissible.

Assuming ρ and Cp do not change significantly in the function of temperature, the multiplication ρ Cp was almost considered to be constant, so that the rate of temperature rise is determined by E and ε″ only. Further simplifications can be made if the device is single mode with the same volume and shape and the microwave energy is constant during the investigation. The value of E is determined by ε′ and ε′ and ε″ and the rate of temperature rise is essentially determined by ε″ value.

#### 4.2. Measurement set-up of ionic liquids

Figure 9 depicts the schematic representation of the self-designed experimental set-up applied for automatic and online measurement of dielectric properties of ionic liquids in a definite temperature range. It is composed of the following devices and instruments: cylindrical sample holder unit, thermostat, peristaltic pump, waveguide, temperature sensor, displaceable piston, stepper motor, magnetron, detectors, control unit and a PC.

The IL sample, which is placed in the thermostat to keep it at the desired constant temperature, is flown across the waveguide having a length of about 3λ through the sample holder tube with the help of a peristaltic pump. The electric energy is transformed into microwave energy by the magnetron. The stepper motor is controlled by a microprocessor control unit, which contains an Intel 8-bit microcontroller, 12-bit A/D converters for receiving the four diode-detector signals, a stepper motor driver and a RS-232 serial interface to connect it to the PC. The control unit collects the detector signals and the temperature data determined by the temperature sensor and sends them to the PC. Furthermore it controls the position of the short circuit displaceable piston on the basis of algorithm software elaborated for this purpose.

Dielectric Characteristics of Ionic Liquids and Usage in Advanced Energy Storage Cells http://dx.doi.org/10.5772/66948 467

Figure 9. Scheme of the experimental set-up.

It is important to note that during microwave treatment, a conversion process takes place in which the microwave energy interacts with the treated material and the material converts the electric energy into thermal energy according its characteristic of dielectric properties. During this process, the measurable increase of the temperature in the treated material is the macroscopically observable result. The rate of the temperature increase depends on the microwave field and the treated material properties, which is described in the following

<sup>Δ</sup>T=Δ<sup>t</sup> <sup>¼</sup> Pv=ρCp <sup>¼</sup> jE<sup>2</sup>

where the ΔT/Δt is the velocity of temperature rising in Kelvin per second, Pv is the absorbed power in the sample, ρ is the density of the sample, Cp is the specific heat of the sample, E2 is the strength of the electromagnetic field in the sample, f is the frequency of the field and ε″ is the dielectric loss; j means this is a complex value [15]. This formula shows that the temperature of the treated material is influenced by a number of factors. Testing their effects separately is not simple, because ρ, ε Cp features themselves are temperature dependent and it is difficult to measure this value accurately inside the material. In the case of strictly homogeneous series of examined compounds, the situation is more simplified, because some simplifications are

Assuming ρ and Cp do not change significantly in the function of temperature, the multiplication ρ Cp was almost considered to be constant, so that the rate of temperature rise is determined by E and ε″ only. Further simplifications can be made if the device is single mode with the same volume and shape and the microwave energy is constant during the investigation. The value of E is determined by ε′ and ε′ and ε″ and the rate of temperature rise is essentially

Figure 9 depicts the schematic representation of the self-designed experimental set-up applied for automatic and online measurement of dielectric properties of ionic liquids in a definite temperature range. It is composed of the following devices and instruments: cylindrical sample holder unit, thermostat, peristaltic pump, waveguide, temperature sensor, displaceable

The IL sample, which is placed in the thermostat to keep it at the desired constant temperature, is flown across the waveguide having a length of about 3λ through the sample holder tube with the help of a peristaltic pump. The electric energy is transformed into microwave energy by the magnetron. The stepper motor is controlled by a microprocessor control unit, which contains an Intel 8-bit microcontroller, 12-bit A/D converters for receiving the four diode-detector signals, a stepper motor driver and a RS-232 serial interface to connect it to the PC. The control unit collects the detector signals and the temperature data determined by the temperature sensor and sends them to the PC. Furthermore it controls the position of the short circuit displaceable piston on the basis of algorithm software elaborated for this pur-

piston, stepper motor, magnetron, detectors, control unit and a PC.

f ε″

=ρCp (25)

equation:

466 Progress and Developments in Ionic Liquids

permissible.

pose.

determined by ε″ value.

4.2. Measurement set-up of ionic liquids

The method is based on the compensation of phase change due to the microwave energy absorption of the liquid sample. The short circuit piston situated behind the sample must be actuated for compensation. The energy conditions created by the wave front in the waveguide are measured by four diode detectors.

Dielectric constants, dielectric loss factors and the temperature dependence of the dielectric properties of ionic liquids intended to be used in batteries were determined by the abovedescribed self-designed microwave dielectrometric apparatus (Figure 3) at the frequency of 2.45 GHz and at different temperatures (30°C, 40°C, 50°C, 60°C, 70°C, 80°C, 90°C, 100°C, 110° C and 120°C). The speed of the change in temperature depends on the electrical field strength in the material (E), the absorbed microwave power, (Pv) density (ρ), the specific heat capacity (Cp) and the dielectric loss factor ε″ and can be given by Eq. (1) [11].

#### 4.3. Results and obtained characteristics

The temperature increase of the new type of ionic liquids was examined, as it is shown in Figure 10 and in Table 2.

Figure 10. The structure of investigated ionic liquids.


was measured up to 95°C. Water was used as a heat transfer material and it was not possible to achieve 100°C. The measurements were performed in a SV-10 type of vibration viscometer (A&D Ltd. Japan). The following figures show that the viscosity in room temperature is high, typically several hundred Pa\*s and it decreases rapidly with the temperature rising similar to a y=1/x curve. Reaching the 100°C temperature, the value of viscosity can be ten times lower

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The investigation of the viscosity may be important; therefore, it basically influences the mobility of ions in the electrolyte, which can affect the conductivity and thus the internal resistance of the battery cell. By cooling ionic liquids, the viscosity is increasing rapidly. Around freezing temperature the ionic liquids were already similar to a "honey density" mass. Since the energy storage cells are used in a wide temperature range, it is important to know

In Figure 13 five different electrical conductivity plots of ionic liquids in the function of temperature can be seen. It clearly shows that it is not BMIM-BF4 which has the highest conductivity value, but the conductivity value, depending on the temperature of this material,

Results in Figure 14 are both new and impressive. The dielectric constant value of BMIM-BF4 reaches a value of 1 at close to 100°C, similarly to the value of the vacuum and if the temperature is raised further, the value will decrease to the negative range. This outcome suggests that the electrical conductivity is rising greatly with an increase in temperature and this compound is no longer an insulating material but a conductor. Therefore it is important to keep the temperature in an adequate range when batteries are used and especially in the

that the electrical conductivity of the electrolytes is a function of temperature.

increases with the greatest intensity [7].

charging period.

than at the initial value as we can see in Figure 12.

Figure 12. Viscosity of BMIM-BF4 in the function of the temperature.

Table 2. The formulas of investigated ionic liquids.

The effect of R groups was examined in the case of two anions—BF4 (compounds 1–4) and PF6 (compounds 5–8) at 3–5 watts of microwave energy.

Our results show that there is a negative correlation between the R group carbon number and the temperature elevation rate: a decrease in the previous increases the latter. By increasing the microwave power significantly, the temperature speeds converge to each other and sometimes change this order, but this is not typical in Figure 11. Overall, the rate of temperature rising follows the order of ε′ and ε″ suggesting that in a strictly homologous series, the temperature rise of ionic liquids is determined by ε″ [18].

Figure 11. Temperature rising of 1-butyl-3-methylimidazolium tetrafluoroborates at 3 W.

After the temperature rising data rate, we investigated two additional parameters of the ionic liquids used in the advanced batteries, i.e. viscosity and electrical conductivity. Both properties are the functions of temperature and they were examined between 20°C and 100–120°C temperature range. The sample material was heated through a heat exchanger, so the viscosity was measured up to 95°C. Water was used as a heat transfer material and it was not possible to achieve 100°C. The measurements were performed in a SV-10 type of vibration viscometer (A&D Ltd. Japan). The following figures show that the viscosity in room temperature is high, typically several hundred Pa\*s and it decreases rapidly with the temperature rising similar to a y=1/x curve. Reaching the 100°C temperature, the value of viscosity can be ten times lower than at the initial value as we can see in Figure 12.

Figure 12. Viscosity of BMIM-BF4 in the function of the temperature.

The effect of R groups was examined in the case of two anions—BF4 (compounds 1–4) and PF6

Compound R Y Compound R Y 1 CH3 BF4 8 C4H9 PF6 2 C2H5 BF4 9 C2H5 CI 3 C3H7 BF4 10 C2H5 Br 4 C4H9 BF4 11 C2H5 SCN 5 CH3 PF6 12 C2H5 N(CN)2 6 C2H5 PF6 13 C2H5 N(SO2CF3)2

Our results show that there is a negative correlation between the R group carbon number and the temperature elevation rate: a decrease in the previous increases the latter. By increasing the microwave power significantly, the temperature speeds converge to each other and sometimes change this order, but this is not typical in Figure 11. Overall, the rate of temperature rising follows the order of ε′ and ε″ suggesting that in a strictly homologous series, the temperature

After the temperature rising data rate, we investigated two additional parameters of the ionic liquids used in the advanced batteries, i.e. viscosity and electrical conductivity. Both properties are the functions of temperature and they were examined between 20°C and 100–120°C temperature range. The sample material was heated through a heat exchanger, so the viscosity

Figure 11. Temperature rising of 1-butyl-3-methylimidazolium tetrafluoroborates at 3 W.

(compounds 5–8) at 3–5 watts of microwave energy.

7 C3H7 PF6

468 Progress and Developments in Ionic Liquids

Table 2. The formulas of investigated ionic liquids.

rise of ionic liquids is determined by ε″ [18].

The investigation of the viscosity may be important; therefore, it basically influences the mobility of ions in the electrolyte, which can affect the conductivity and thus the internal resistance of the battery cell. By cooling ionic liquids, the viscosity is increasing rapidly. Around freezing temperature the ionic liquids were already similar to a "honey density" mass.

Since the energy storage cells are used in a wide temperature range, it is important to know that the electrical conductivity of the electrolytes is a function of temperature.

In Figure 13 five different electrical conductivity plots of ionic liquids in the function of temperature can be seen. It clearly shows that it is not BMIM-BF4 which has the highest conductivity value, but the conductivity value, depending on the temperature of this material, increases with the greatest intensity [7].

Results in Figure 14 are both new and impressive. The dielectric constant value of BMIM-BF4 reaches a value of 1 at close to 100°C, similarly to the value of the vacuum and if the temperature is raised further, the value will decrease to the negative range. This outcome suggests that the electrical conductivity is rising greatly with an increase in temperature and this compound is no longer an insulating material but a conductor. Therefore it is important to keep the temperature in an adequate range when batteries are used and especially in the charging period.

Figure 13. Conductivity of five compounds in a function of temperature.

Taking into account the dielectric constant, dielectric loss and electrical conductivity results of all the investigated ILs, it can be concluded that the alkyl chain length of the cation and the structure of the anion strongly influence the dielectric properties of the ILs and that the highest G value is exhibited by the [DiEtMeIm][BF4] IL at 30°C temperature; hence, it is the most suitable candidate for battery applications. Furthermore, it can be stated that implementing the knowledge about the connection between the IL structure and the dielectric properties another ILs should be

Dielectric Characteristics of Ionic Liquids and Usage in Advanced Energy Storage Cells

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471

Based on our study, ionic liquids turned out to be excellent candidates for environmentally sound, green electrolytes in batteries due to their useful features, such as wide electrochemical windows, high inherent conductivities, high thermal and electrochemical stability, tuneable physicochemical properties, etc. Before making decisions about their applicability, careful

Although techniques aiming at the determination of the dielectric properties of ILs have been the main focus of several studies, so far no convincing findings can be found in the literature due to the fact that most measurements are based on classical methods. As it was detailed in this study, these methods are doomed to failure because of the high conductivity

Based on the analyses of previous results, it can be concluded that the physical, chemical and electrical parameters of ionic liquids are greatly temperature dependent when used in energy storage cells. In any case, when they are applied, we should specify the range of application temperature. Stepping out of this range of the energy storage cell may not meet the expected specification values, or in the worst case, it may be permanently

studied in order to select the ones most adequate for electrolyte application.

Figure 15. Dielectric constant of ILs containing common [BF4] anion and different cations.

measurements on their dielectric properties should be done.

5. Conclusion

of ILs.

damaged.

Figure 14. Dielectric constant of BMIM-BF4 in a function of the temperature.

The dielectric constant (E1) and dielectric loss factor values of ILs built up of the same [BF4] anion and six previously described cations containing alkyl chains with different lengths ([DiEtMeIm], [DiEtEtIm], [DiEtPrIm] and [DiEtBuIm]) at different temperatures between 30° C and 120°C are shown in Figure 15, respectively.

At the initial measuring temperature of 30°C, all four of the studied ILs have similar dielectric constants around 7. With the increase in temperature, the E1 values for [DiEtMeIm][BF4], [DiEtEtIm][BF4] and [DiEtPrIm][BF4] slightly increase up to 13, while the dielectric constant for [DiEtBuIm][BF4] shows a sudden break at 90°C and at 120°C it reaches the value as high as 27. This could be explained with some sudden changes in the structure of the ILs or in the physicochemical interactions between the anion and the cation. Excluding the results for [DiEtBuIm][BF4], the values at elevated temperature show that the highest E1 value belongs to [DiEtMeIm][BF4], followed by [DiEtEtIm][BF4] and then by the [DiEtPrIm][BF4]; hence, the dielectric constant increases with the decrease in the alkyl chain length of the cation.

Figure 15. Dielectric constant of ILs containing common [BF4] anion and different cations.

Taking into account the dielectric constant, dielectric loss and electrical conductivity results of all the investigated ILs, it can be concluded that the alkyl chain length of the cation and the structure of the anion strongly influence the dielectric properties of the ILs and that the highest G value is exhibited by the [DiEtMeIm][BF4] IL at 30°C temperature; hence, it is the most suitable candidate for battery applications. Furthermore, it can be stated that implementing the knowledge about the connection between the IL structure and the dielectric properties another ILs should be studied in order to select the ones most adequate for electrolyte application.

### 5. Conclusion

The dielectric constant (E1) and dielectric loss factor values of ILs built up of the same [BF4] anion and six previously described cations containing alkyl chains with different lengths ([DiEtMeIm], [DiEtEtIm], [DiEtPrIm] and [DiEtBuIm]) at different temperatures between 30°

At the initial measuring temperature of 30°C, all four of the studied ILs have similar dielectric constants around 7. With the increase in temperature, the E1 values for [DiEtMeIm][BF4], [DiEtEtIm][BF4] and [DiEtPrIm][BF4] slightly increase up to 13, while the dielectric constant for [DiEtBuIm][BF4] shows a sudden break at 90°C and at 120°C it reaches the value as high as 27. This could be explained with some sudden changes in the structure of the ILs or in the physicochemical interactions between the anion and the cation. Excluding the results for [DiEtBuIm][BF4], the values at elevated temperature show that the highest E1 value belongs to [DiEtMeIm][BF4], followed by [DiEtEtIm][BF4] and then by the [DiEtPrIm][BF4]; hence, the

dielectric constant increases with the decrease in the alkyl chain length of the cation.

C and 120°C are shown in Figure 15, respectively.

Figure 14. Dielectric constant of BMIM-BF4 in a function of the temperature.

Figure 13. Conductivity of five compounds in a function of temperature.

470 Progress and Developments in Ionic Liquids

Based on our study, ionic liquids turned out to be excellent candidates for environmentally sound, green electrolytes in batteries due to their useful features, such as wide electrochemical windows, high inherent conductivities, high thermal and electrochemical stability, tuneable physicochemical properties, etc. Before making decisions about their applicability, careful measurements on their dielectric properties should be done.

Although techniques aiming at the determination of the dielectric properties of ILs have been the main focus of several studies, so far no convincing findings can be found in the literature due to the fact that most measurements are based on classical methods. As it was detailed in this study, these methods are doomed to failure because of the high conductivity of ILs.

Based on the analyses of previous results, it can be concluded that the physical, chemical and electrical parameters of ionic liquids are greatly temperature dependent when used in energy storage cells. In any case, when they are applied, we should specify the range of application temperature. Stepping out of this range of the energy storage cell may not meet the expected specification values, or in the worst case, it may be permanently damaged.

### Acknowledgements

The present study discusses the sample materials, their usage possibilities and the results of the research from the previous work of the author.

Research Symposium Proceedings, Cambridge, USA, July 5–8, 2010. pp 23–27.

Dielectric Characteristics of Ionic Liquids and Usage in Advanced Energy Storage Cells

and ε″) of ionic

473

http://dx.doi.org/10.5772/66948

[10] MacDowell J.F.: Microwave heating of nepheline glass-ceramics, American Ceramic Soci-

liquids, Review of Scientific Instruments 80 (2009): 044703; doi:10.1063/1.3117352.

[12] Almássy Gy.: Microwave handbook, XII-3, Technical Publishing House, Budapest, 1973. [Almássy, Gy: Mikrohullámú kézikönyv; XII-3. Műszaki Könyvkiadó, Bp, 1973.]

[13] Kegel, K.: Electric Thermal Engineering Technical Manual, Technical Publishing House, Budapest, 1978. [Kegel, K.: Villamos Hőtechnikai Kézikönyv, Műszaki Könyvkiadó,

[14] Alarco, P.J., Yaser, A.L., Ravet, N., Armand, M.: Lithium conducting pyrazoliumimides plastic crystals: a new solid state electrolyte matrix. Solid State Ionics, 172 (2004): 1–4, 53–

[15] Schiffmann R.F.: Mircowave and dielectric drying, in: Mujamder A.S.(Ed), 2nd., Handbook of Industrial Drying, vol 1, Marcel Dekker, New York, 1995, pp. 345–372.

[16] Hikari S., Hajime M., Kuniaki T.: Application of room temperature ionic liquids to Li

[17] Zhang Z., Gao X., Yang L.: Electrochemical properties of room temperature ionic liquids incorporating BF4 and TFSI anions as green electrolytes. Chinese Science Bulletin 50(18)

[18] Gao, X.H., Zhu, M.J., Zhang, Z.X. et al., Research on room temperature molten salts with alkylated imidazolium salt as electrolytes. ChemicalWorld 45(Supplement I) (2004): 156–

[19] Nishida, T., Tashiro, Y., Yamamoto, M.: Physical and electrochemical properties of 1 alkyl-3-methylimidazolium tetrafluoroborate for electrolyte. Journal of Fluorine Chemis-

[20] Matsumoto H., Sakaebe H., Tatsumi K.: Preparation of room temperature ionic liquids based on aliphatic onium cations and asymmetric amide anions and their electrochemical properties as a lithium battery electrolyte. Journal of Power Sources 146 (2005): 45–50. [21] Hagiwara R., Ito Y.: Room temperature ionic liquids of alkylimidazolium cations and

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Budapest, 1978.]

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(2005): 2005–2009.

157 (in Chinese).

try 120 (2003): 135–141.

### Author details

#### Attila Göllei

Address all correspondence to: gollei.attila@virt.uni-pannon.hu

Department of Electrical Engineering and Information Technology, University of Pannonia, Veszprém, Hungary

### References


Research Symposium Proceedings, Cambridge, USA, July 5–8, 2010. pp 23–27. DOI:10.2529/PIERS091216050152.

[10] MacDowell J.F.: Microwave heating of nepheline glass-ceramics, American Ceramic Society Bulletin 63 (1984): 282–286.

Acknowledgements

472 Progress and Developments in Ionic Liquids

Author details

Veszprém, Hungary

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Attila Göllei

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Address all correspondence to: gollei.attila@virt.uni-pannon.hu

of Industry & Chemistry 41(1) (2013): 11–15.

The present study discusses the sample materials, their usage possibilities and the results of

Department of Electrical Engineering and Information Technology, University of Pannonia,

[1] Göllei A., Magyar A.: Ionic liquids in advanced energy storage cells. Hungarian Journal

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**Section 6**

**Separations**

**Section 6**

## **Separations**

**Chapter 20**

**Provisional chapter**

**Ionic Liquids in Multiphase Systems**

**Ionic Liquids in Multiphase Systems**

Stella Nickerson, Elizabeth Nofen, Denzil Frost and

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution,

© 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

and reproduction in any medium, provided the original work is properly cited.

Ionic liquids (ILs) can be used to replace one or more phases in conventional oil/water emulsions including Pickering emulsions—surfactant-free emulsions which utilize nanoor micron-sized particles to stabilize the immiscible liquid-liquid interface. Due to the extreme tunability of both the ILs and particles used, the study of IL-based Pickering emulsions yields novel emulsion morphologies and insights into the ionic liquid-liquid-particle interactions present. This work discusses extensive experimental work on IL-based Pickering emulsions and IL/liquid interfaces, emphasizing unique phenomena—such as "bridging" between emulsion droplets and spontaneous particle transport across the interface—never observed in more conventional Pickering emulsions. Molecular dynamics (MD) simulations of particles at the IL/liquid interface are also discussed, and fundamental insights from these simulations are used to enhance under-

**Keywords:** ionic liquids, emulsions, Pickering emulsions, particles, colloids, interfaces

The unique properties and rich variety of ionic liquids (ILs) make them promising for a wide range of applications and the potential of pure ILs is multiplied by introducing them to multicomponent systems. Ionic liquids' complex interactions with other materials enable multiphase systems that are both theoretically fascinating and potentially useful. This work focuses in particular on systems involving solid particles and immiscible liquid phases. It explores ionic liquid-liquid interfaces and their role in ionic liquid Pickering emulsions through both experimental and simulation approaches, including several unique and fascinating interface phenomena. **Figure 1** illustrates some of these phenomena—particle self-assembly on an emulsion surface, particle "bridging" between emulsion droplets, and spontaneous transport

standing of the unique interface behavior revealed by experiment.

Additional information is available at the end of the chapter

Additional information is available at the end of the chapter

Stella Nickerson, Elizabeth Nofen, Denzil

Lenore L. Dai

http://dx.doi.org/10.5772/65281

**Abstract**

**1. Introduction**

Frost and Lenore L. Dai

### **Ionic Liquids in Multiphase Systems Ionic Liquids in Multiphase Systems**

Stella Nickerson, Elizabeth Nofen, Denzil Frost and Lenore L. Dai Stella Nickerson, Elizabeth Nofen, Denzil Frost and Lenore L. Dai

Additional information is available at the end of the chapter Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/65281

#### **Abstract**

Ionic liquids (ILs) can be used to replace one or more phases in conventional oil/water emulsions including Pickering emulsions—surfactant-free emulsions which utilize nanoor micron-sized particles to stabilize the immiscible liquid-liquid interface. Due to the extreme tunability of both the ILs and particles used, the study of IL-based Pickering emulsions yields novel emulsion morphologies and insights into the ionic liquid-liquid-particle interactions present. This work discusses extensive experimental work on IL-based Pickering emulsions and IL/liquid interfaces, emphasizing unique phenomena—such as "bridging" between emulsion droplets and spontaneous particle transport across the interface—never observed in more conventional Pickering emulsions. Molecular dynamics (MD) simulations of particles at the IL/liquid interface are also discussed, and fundamental insights from these simulations are used to enhance understanding of the unique interface behavior revealed by experiment.

**Keywords:** ionic liquids, emulsions, Pickering emulsions, particles, colloids, interfaces

#### **1. Introduction**

The unique properties and rich variety of ionic liquids (ILs) make them promising for a wide range of applications and the potential of pure ILs is multiplied by introducing them to multicomponent systems. Ionic liquids' complex interactions with other materials enable multiphase systems that are both theoretically fascinating and potentially useful. This work focuses in particular on systems involving solid particles and immiscible liquid phases. It explores ionic liquid-liquid interfaces and their role in ionic liquid Pickering emulsions through both experimental and simulation approaches, including several unique and fascinating interface phenomena. **Figure 1** illustrates some of these phenomena—particle self-assembly on an emulsion surface, particle "bridging" between emulsion droplets, and spontaneous transport

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

across the liquid-liquid interface—as well as an example of the molecular dynamics (MD) simulations used to further explore the fundamentals of IL interactions with other phases. There continues to be active research in Pickering emulsions [1, 2], immiscible phases [3, 4], and ILs [5–9], and one particularly relevant application is employing ionic liquid-in-water emulsions as "a new class of fluorescent sensors for metal ions" [9].

applications in extraction, catalysis, reaction schemes, etc., and provide even more extensive tunability through the complex interactions of differing choices of particles and ionic liquids. Surfactant-free, solid-stabilized Pickering emulsions can act as templates for these studies in which the ionic liquid subsists as the droplet phase, continuous phase, or both phases in the emulsion. Deviating from the conventional oil-in-water emulsion systems, employing ILs allows for interesting interfacial phenomena due to the charged nature of the IL and other

Our first foray into ionic liquid-based Pickering emulsions was to simply replace the droplet phase in a conventional oil-in-water emulsion with an IL to create an IL-in-water Pickering emulsion [10]. While IL-in-water Pickering emulsions had been created previously, their use of silica nanoparticles prevented easy observation of the resulting particle morphology [11, 12], and thus, fluorescent micron-sized particles were used in this work. The hydrophobic IL

water, was chosen for the droplet phase. In order to observe the effect of particle hydrophobicity and surface charge and learn the resultant particle morphology on the droplets and the partition preference of the particles, 1 μm fluorescent surface-treated polystyrene (PS) particles were employed in conjunction with a confocal laser-scanning microscope. For this

H, AS-PS, green color), or amine (–NH2

S-PS and AS-PS particles were relatively hydrophobic and negatively charged, while the A-PS

**Figure 2** shows the resulting droplet morphologies and partition preferences of the parti-

images on the left of the figure show the resulting surface coverage of the droplet when a single particle type is used, while the two large images on the right, with additional smaller representative images of selected systems in the middle, show the morphology when two differing particle types are mixed in the emulsion at equal concentrations. All of the larger images, whether a single or binary particle type was used, show the aggregated domain morphology in which the IL droplet is nearly fully covered by the particles. In the binary systems, there is no significant partitioning of the particles on the droplet interface, that is to say, the particles are well mixed. For the A-PS and S-PS/A-PS particle systems, some fully covered droplets were observed in addition to the aggregated domain morphology. Full covered droplets were never seen in the S-PS, AS-PS, or S-PS/AS-PS systems, likely suggesting that the hydrophobicity or contact angle plays a role, as the A-PS particles were the only hydrophilic particles used. This contact angle difference could thus allow for closer packing on the IL droplet. This hypothesis is further confirmed by the partition preference of the particles for either the water phase or the ionic liquid phase. For the S-PS, A-PS, and S-PS/A-PS systems, many A-PS particles remain in the water phase, while the S-PS particles prefer the IL phase, even being extracted into the IL droplets (as seen for the cross section of the S-PS/A-PS droplet in the middle of **Figure 2**). The AS-PS particles also show a higher affinity for the IL-water interface, and the S-PS/AS-PS system further shows this behavior with a large amount of both S-PS and AS-PS particles being extracted into the IL phase, as seen in the cross section droplet

]), which is immiscible with

Ionic Liquids in Multiphase Systems http://dx.doi.org/10.5772/65281 479

H, S-PS, blue color), alde-

] IL emulsion droplet suspended in water. The three

, A-PS, green color). The

factors intrinsic to the liquid including ion ordering, interfacial tension, etc.

1-butyl-3-methylimidazolium hexafluorophosphate ([BMIM][PF<sup>6</sup>

work, the various surface chemistries of the PS were sulfate (–SO3

particles were relatively hydrophilic and positively charged.

hyde sulfate (–CHO and –SO3

cles, each image showing a [BMIM][PF<sup>6</sup>

**Figure 1.** Upper left: confocal microscopy image of an IL-in-water emulsion droplet. Upper-middle: confocal microscopy image of IL-in-IL emulsion droplets. Lower left and lower-middle: particle bridging between emulsion droplets. Right: MD simulation of nanoparticles at IL/water interface.

Given the complex and unusual molecular-scale interactions at ionic liquid/liquid interfaces, it is reasonable to suspect that they might behave differently than conventional aqueous/ organic interfaces. Accepting this, the challenge becomes identifying those unique behaviors and developing applications based on them. It is equally important to develop fundamental theories of these interfaces and their interactions with, for example, solid particles. This work outlines the progress made toward these goals.

### **2. Ionic liquid-liquid interfaces and particle interactions**

#### **2.1. Unique morphologies of aqueous and nonaqueous ionic liquid Pickering emulsions**

The extremely high tunability of the material properties of ionic liquids (ILs), caused by the virtually endless combinations of anions and cations, allows ILs to be unique candidates for the study of liquid-liquid interfaces. Additionally, by incorporating particles into these IL-liquid systems, novel and varied behaviors can be achieved. These suggest unique applications in extraction, catalysis, reaction schemes, etc., and provide even more extensive tunability through the complex interactions of differing choices of particles and ionic liquids. Surfactant-free, solid-stabilized Pickering emulsions can act as templates for these studies in which the ionic liquid subsists as the droplet phase, continuous phase, or both phases in the emulsion. Deviating from the conventional oil-in-water emulsion systems, employing ILs allows for interesting interfacial phenomena due to the charged nature of the IL and other factors intrinsic to the liquid including ion ordering, interfacial tension, etc.

across the liquid-liquid interface—as well as an example of the molecular dynamics (MD) simulations used to further explore the fundamentals of IL interactions with other phases. There continues to be active research in Pickering emulsions [1, 2], immiscible phases [3, 4], and ILs [5–9], and one particularly relevant application is employing ionic liquid-in-water

Given the complex and unusual molecular-scale interactions at ionic liquid/liquid interfaces, it is reasonable to suspect that they might behave differently than conventional aqueous/ organic interfaces. Accepting this, the challenge becomes identifying those unique behaviors and developing applications based on them. It is equally important to develop fundamental theories of these interfaces and their interactions with, for example, solid particles. This work

**Figure 1.** Upper left: confocal microscopy image of an IL-in-water emulsion droplet. Upper-middle: confocal microscopy image of IL-in-IL emulsion droplets. Lower left and lower-middle: particle bridging between emulsion droplets. Right:

**2.1. Unique morphologies of aqueous and nonaqueous ionic liquid Pickering emulsions**

The extremely high tunability of the material properties of ionic liquids (ILs), caused by the virtually endless combinations of anions and cations, allows ILs to be unique candidates for the study of liquid-liquid interfaces. Additionally, by incorporating particles into these IL-liquid systems, novel and varied behaviors can be achieved. These suggest unique

outlines the progress made toward these goals.

MD simulation of nanoparticles at IL/water interface.

**2. Ionic liquid-liquid interfaces and particle interactions**

emulsions as "a new class of fluorescent sensors for metal ions" [9].

478 Progress and Developments in Ionic Liquids

Our first foray into ionic liquid-based Pickering emulsions was to simply replace the droplet phase in a conventional oil-in-water emulsion with an IL to create an IL-in-water Pickering emulsion [10]. While IL-in-water Pickering emulsions had been created previously, their use of silica nanoparticles prevented easy observation of the resulting particle morphology [11, 12], and thus, fluorescent micron-sized particles were used in this work. The hydrophobic IL 1-butyl-3-methylimidazolium hexafluorophosphate ([BMIM][PF<sup>6</sup> ]), which is immiscible with water, was chosen for the droplet phase. In order to observe the effect of particle hydrophobicity and surface charge and learn the resultant particle morphology on the droplets and the partition preference of the particles, 1 μm fluorescent surface-treated polystyrene (PS) particles were employed in conjunction with a confocal laser-scanning microscope. For this work, the various surface chemistries of the PS were sulfate (–SO3 H, S-PS, blue color), aldehyde sulfate (–CHO and –SO3 H, AS-PS, green color), or amine (–NH2 , A-PS, green color). The S-PS and AS-PS particles were relatively hydrophobic and negatively charged, while the A-PS particles were relatively hydrophilic and positively charged.

**Figure 2** shows the resulting droplet morphologies and partition preferences of the particles, each image showing a [BMIM][PF<sup>6</sup> ] IL emulsion droplet suspended in water. The three images on the left of the figure show the resulting surface coverage of the droplet when a single particle type is used, while the two large images on the right, with additional smaller representative images of selected systems in the middle, show the morphology when two differing particle types are mixed in the emulsion at equal concentrations. All of the larger images, whether a single or binary particle type was used, show the aggregated domain morphology in which the IL droplet is nearly fully covered by the particles. In the binary systems, there is no significant partitioning of the particles on the droplet interface, that is to say, the particles are well mixed. For the A-PS and S-PS/A-PS particle systems, some fully covered droplets were observed in addition to the aggregated domain morphology. Full covered droplets were never seen in the S-PS, AS-PS, or S-PS/AS-PS systems, likely suggesting that the hydrophobicity or contact angle plays a role, as the A-PS particles were the only hydrophilic particles used. This contact angle difference could thus allow for closer packing on the IL droplet. This hypothesis is further confirmed by the partition preference of the particles for either the water phase or the ionic liquid phase. For the S-PS, A-PS, and S-PS/A-PS systems, many A-PS particles remain in the water phase, while the S-PS particles prefer the IL phase, even being extracted into the IL droplets (as seen for the cross section of the S-PS/A-PS droplet in the middle of **Figure 2**). The AS-PS particles also show a higher affinity for the IL-water interface, and the S-PS/AS-PS system further shows this behavior with a large amount of both S-PS and AS-PS particles being extracted into the IL phase, as seen in the cross section droplet for the S-PS/AS-PS system. As the Gibbs free energy of adhesion to the interface is very high for microparticles, and the particles were originally dispersed in the water phase, this extraction phenomenon is quite interesting as it defies the general thermodynamic thinking. This is likely due to the fact that the S-PS and AS-PS particles are hydrophobic, and thus prefer the hydrophobic IL phase, rather than the aqueous. Thus the surface chemistry of the particles was found to have a significant effect on the resultant droplet morphologies and extraction characteristic of the IL, with the resultant emulsion scheme being tuned by the hydrophobicity of the particle type or types chosen. However, it is important to that such extraction has never been reported in systems involving oil-water interfaces thus the uniqueness of ILs plays a critical role.

The simple inversion of the droplet phase/continuous phase identities in these IL-based Pickering emulsions lead to a very interesting and novel particle morphology not seen in the conventional oil-in-water or IL-in-water emulsions, that of particle bridging. **Figure 3** outlines this phenomenon for both the oil-in-IL and water-in-IL Pickering emulsions studied, focusing on emulsions of S-PS, AS-PS, C-PS, and A-PS particles. In the oil-in-IL emulsions, for the negatively charged particles used (S-PS, AS-PS, and C-PS), a clear bridged morphology was observed in which the particles preferred inter-oil droplet bridges rather than the oil-IL interface, with this liquid-liquid interface being nearly completely devoid of particles. For the A-PS (positively charged) system, some bridges were observed, but the main morphology consisted of the particles sparsely covering the visible oil droplets, with more particles simply dispersed in the continuous IL phase. As both C-PS and A-PS are hydrophilic, this shows that for these systems, particle surface charge rather than hydrophobicity plays a more important role in terms of the bridge formation. Additionally, the bridges formed were monolayers between connected droplets, and the bridges prevented droplet coalescence by hindering drainage of the inter-droplet film. Also, there was no particle transport into the oil droplet phase, which was expected, as there is no particle extraction with conventional oil-in-water

Ionic Liquids in Multiphase Systems http://dx.doi.org/10.5772/65281 481

For the water-in-IL Pickering emulsions, a similar trend was observed in which the negatively charged S-PS, AS-PS, and C-PS showed bridging, while the positively charged A-PS did not, with no particle extraction seen in any of the cases. It is interesting that the bridging phenomenon occurs in these systems, as the particles were originally dispersed in water, thus it was first hypothesized that the water droplets would reabsorb the particles, making bridge

**Figure 3.** Various observed particle morphologies on (upper row) PDMS droplets and (bottom row) water droplets in

] IL continuous phase for a variety of PS particle surface chemistries with 1 μm particle diameters in all

Pickering emulsions [15, 16].

a [BMIM][PF<sup>6</sup>

images. The 25 μm scale bar is valid for all images.

**Figure 2.** Various observed particle morphologies on [BMIM][PF<sup>6</sup> ] IL droplets in water for a variety of PS particle surface chemistries with 1 μm particle diameters in all images. All scale bars are 10 μm.

After studying the effect of having an ionic liquid as the droplet phase of a Pickering emulsion, we then turned to using the IL as the continuous phase, to form oil-in- or water-in-ionic liquid Pickering emulsions [13, 14]. Polydimethylsiloxane (PDMS) oil was used, and the same [BMIM][PF<sup>6</sup> ] IL was used for either oil-in- or water-in-IL emulsions, as this IL is immiscible with both water and the PDMS oil used. For the particle types, 1 μm fluorescent surfacetreated polystyrene particles were again employed with the same surface chemistries as the IL-in-water Pickering emulsion work, including sulfate (–SO3 H, S-PS, blue color), aldehyde sulfate (–CHO and –SO3 H, AS-PS, green color), and amine (–NH2 , A-PS, green color), adding the carboxylate (–COOH, C-PS, red color) for this work. It is worthwhile to note that the S-PS and AS-PS are relatively hydrophobic while the C-PS and A-PS are relatively hydrophilic, with all particles exhibiting a negative surface charge in water, expect for A-PS, which is positively charged.

The simple inversion of the droplet phase/continuous phase identities in these IL-based Pickering emulsions lead to a very interesting and novel particle morphology not seen in the conventional oil-in-water or IL-in-water emulsions, that of particle bridging. **Figure 3** outlines this phenomenon for both the oil-in-IL and water-in-IL Pickering emulsions studied, focusing on emulsions of S-PS, AS-PS, C-PS, and A-PS particles. In the oil-in-IL emulsions, for the negatively charged particles used (S-PS, AS-PS, and C-PS), a clear bridged morphology was observed in which the particles preferred inter-oil droplet bridges rather than the oil-IL interface, with this liquid-liquid interface being nearly completely devoid of particles. For the A-PS (positively charged) system, some bridges were observed, but the main morphology consisted of the particles sparsely covering the visible oil droplets, with more particles simply dispersed in the continuous IL phase. As both C-PS and A-PS are hydrophilic, this shows that for these systems, particle surface charge rather than hydrophobicity plays a more important role in terms of the bridge formation. Additionally, the bridges formed were monolayers between connected droplets, and the bridges prevented droplet coalescence by hindering drainage of the inter-droplet film. Also, there was no particle transport into the oil droplet phase, which was expected, as there is no particle extraction with conventional oil-in-water Pickering emulsions [15, 16].

for the S-PS/AS-PS system. As the Gibbs free energy of adhesion to the interface is very high for microparticles, and the particles were originally dispersed in the water phase, this extraction phenomenon is quite interesting as it defies the general thermodynamic thinking. This is likely due to the fact that the S-PS and AS-PS particles are hydrophobic, and thus prefer the hydrophobic IL phase, rather than the aqueous. Thus the surface chemistry of the particles was found to have a significant effect on the resultant droplet morphologies and extraction characteristic of the IL, with the resultant emulsion scheme being tuned by the hydrophobicity of the particle type or types chosen. However, it is important to that such extraction has never been reported in systems involving oil-water interfaces thus the uniqueness of ILs plays

After studying the effect of having an ionic liquid as the droplet phase of a Pickering emulsion, we then turned to using the IL as the continuous phase, to form oil-in- or water-in-ionic liquid Pickering emulsions [13, 14]. Polydimethylsiloxane (PDMS) oil was used, and the same

with both water and the PDMS oil used. For the particle types, 1 μm fluorescent surfacetreated polystyrene particles were again employed with the same surface chemistries as the

H, AS-PS, green color), and amine (–NH2

the carboxylate (–COOH, C-PS, red color) for this work. It is worthwhile to note that the S-PS and AS-PS are relatively hydrophobic while the C-PS and A-PS are relatively hydrophilic, with all particles exhibiting a negative surface charge in water, expect for A-PS, which is

IL-in-water Pickering emulsion work, including sulfate (–SO3

chemistries with 1 μm particle diameters in all images. All scale bars are 10 μm.

**Figure 2.** Various observed particle morphologies on [BMIM][PF<sup>6</sup>

] IL was used for either oil-in- or water-in-IL emulsions, as this IL is immiscible

H, S-PS, blue color), aldehyde

] IL droplets in water for a variety of PS particle surface

, A-PS, green color), adding

a critical role.

480 Progress and Developments in Ionic Liquids

[BMIM][PF<sup>6</sup>

sulfate (–CHO and –SO3

positively charged.

**Figure 3.** Various observed particle morphologies on (upper row) PDMS droplets and (bottom row) water droplets in a [BMIM][PF<sup>6</sup> ] IL continuous phase for a variety of PS particle surface chemistries with 1 μm particle diameters in all images. The 25 μm scale bar is valid for all images.

For the water-in-IL Pickering emulsions, a similar trend was observed in which the negatively charged S-PS, AS-PS, and C-PS showed bridging, while the positively charged A-PS did not, with no particle extraction seen in any of the cases. It is interesting that the bridging phenomenon occurs in these systems, as the particles were originally dispersed in water, thus it was first hypothesized that the water droplets would reabsorb the particles, making bridge formation impossible. However, the bridging did similarly occur, with more compact structures seen rather than the long chains of droplets observed in the oil-in-IL systems. This occurrence provides an interesting look at the importance of contact angle, as the same particles stabilize the aqueous/nonaqueous emulsions to yield similar bridged morphologies. We may want to add the practical significance—instead of particle stabilization, the bridging caused a creamy layer and made the emulsions less stable.

occur when both phases are ILs. These results additionally show that these morphologies can be tuned by the constituent ILs, not only the particles, and can have important implications

Ionic Liquids in Multiphase Systems http://dx.doi.org/10.5772/65281 483

Further research in this area includes ionic liquid emulsions stabilized by more exotic particles—microgel particles, for example. Work by Monteillet et al., for example, finds that microgels self-assemble on the surface of IL emulsion droplets in a similar manner to the solid particles discussed above. These microgel particles, however, are responsive to various stimuli such as pH and temperature, making the emulsion system itself responsive and open-

**Figure 4.** Confocal laser-scanning fluorescent images of IL-in-IL Pickering emulsions, including [P66614][Phos]-in-EAN

], stabilized by 1 μm particle diameter PS particles. The right schematic shows generated

During our extensive study of ionic liquid-based Pickering emulsions, we noticed the separate and unique phenomenon of microparticle extraction across the liquid-liquid interface [18]. Spontaneous particle transport defies traditional understanding of the thermodynamics of particle-interface interactions and suggests intriguing and novel physics taking place at the

ing up still further potential applications [5].

chemical structures of the IL molecules for visualization purposes.

and [P66614][Phos]-in-[BMIM][PF<sup>6</sup>

**2.3. Spontaneous particle transport across the ionic liquid interface**

for various IL-based applications.

#### **2.2. Ionic liquid-in-ionic liquid Pickering emulsions**

Prior to our work on the subject, ionic liquid-in-ionic liquid Pickering emulsions had not been studied [17]. While many IL systems are miscible due to ion exchange, immiscible ionic liquid pairs do exist, and we chose two systems to study: (1) trihexyltetradecylphosphonium *bis*- (2,2,4-trimethylphentyl)-phosphinate ([P66614][Phos]) and ethylammonium nitrate (EAN) and (2) [P66614][Phos] and [BMIM][PF<sup>6</sup> ] Imidazolium- and phosphonium-based ILs are known to form an immiscible pair, which is thought to be caused by strong hydrogen bonds formed when the imidazolium ions diffuse into the phosphonium IL, increasing the degree of order within the phosphonium IL, resulting in a negative entropy of mixing. However, while it was observed that the [BMIM][PF<sup>6</sup> ] IL slowly gelled the [P66614][Phos] IL over time, the diffusion between the two ILs was slow enough to allow for droplets to remain stable for hours, and thus their subsequent droplet morphology study. For the [P66614][Phos]/EAN system, it was quite stable against mixing as long as EAN was the continuous phase of the emulsion.

The chemical structures of the ILs used can be seen in the right schematic of **Figure 4**. As far as droplet morphologies are concerned, the [P66614][Phos]/EAN system showed very similar results as the [BMIM][PF<sup>6</sup> ]-in-water Pickering emulsions we studied previously, as seen in the leftmost column of images in **Figure 4**. The S-PS and C-PS systems showed aggregated domains of particles at the interface with particle absorption into the droplet. The A-PS system interestingly showed a different morphology, that of droplet bridging, as seen in the oil/waterin-IL emulsions, with no particle absorption, as typical of the bridged systems. Previously, A-PS particles were the only particles that did not form bridges in either water/IL or oil/IL systems, so the presence of bridging in the IL-in-IL system was unexpected. This allows for the overarching hypothesis describing the modulation of particle morphology in these IL-based Pickering emulsion systems that an active surface chemistry is required in order for particles to exclusively form bridges. For a further explanation, the major difference between this system and the bridging systems reported previously is that the continuous phase here is protic. This is significant as in order for A-PS particles to assume a surface charge, the amine groups needed to accept a proton and can do so from the protic EAN. This is further illuminated by observing the [P66614][Phos]-in-[BMIM][PF<sup>6</sup> ] system in which the S-PS and C-PS particle types form bridges, while the A-PS does not. It is hypothesized that the anion [BMIM][PF<sup>6</sup> ] can remove the acidic hydrogens of the sulfate and carboxylate surface chemistries, activating them, and thus allowing for bridging to occur. This driving force does not exist in the [P66614] [Phos]/EAN system, and thus bridging does not occur there for S-PS and C-PS. For the A-PS in [P66614][Phos]/[BMIM][PF<sup>6</sup> ], again there are no protons to activate the surface chemistry, and thus bridging does not occur. Overall, it is clear that the particle self-assembly phenomena is a strong function of the continuous phase/IL type and that phenomena generally unique to IL-based Pickering emulsions, including exclusive bridging and particle absorption, can also occur when both phases are ILs. These results additionally show that these morphologies can be tuned by the constituent ILs, not only the particles, and can have important implications for various IL-based applications.

formation impossible. However, the bridging did similarly occur, with more compact structures seen rather than the long chains of droplets observed in the oil-in-IL systems. This occurrence provides an interesting look at the importance of contact angle, as the same particles stabilize the aqueous/nonaqueous emulsions to yield similar bridged morphologies. We may want to add the practical significance—instead of particle stabilization, the bridging caused a

Prior to our work on the subject, ionic liquid-in-ionic liquid Pickering emulsions had not been studied [17]. While many IL systems are miscible due to ion exchange, immiscible ionic liquid pairs do exist, and we chose two systems to study: (1) trihexyltetradecylphosphonium *bis*- (2,2,4-trimethylphentyl)-phosphinate ([P66614][Phos]) and ethylammonium nitrate (EAN) and

form an immiscible pair, which is thought to be caused by strong hydrogen bonds formed when the imidazolium ions diffuse into the phosphonium IL, increasing the degree of order within the phosphonium IL, resulting in a negative entropy of mixing. However, while it was

between the two ILs was slow enough to allow for droplets to remain stable for hours, and thus their subsequent droplet morphology study. For the [P66614][Phos]/EAN system, it was quite stable against mixing as long as EAN was the continuous phase of the emulsion.

The chemical structures of the ILs used can be seen in the right schematic of **Figure 4**. As far as droplet morphologies are concerned, the [P66614][Phos]/EAN system showed very similar

the leftmost column of images in **Figure 4**. The S-PS and C-PS systems showed aggregated domains of particles at the interface with particle absorption into the droplet. The A-PS system interestingly showed a different morphology, that of droplet bridging, as seen in the oil/waterin-IL emulsions, with no particle absorption, as typical of the bridged systems. Previously, A-PS particles were the only particles that did not form bridges in either water/IL or oil/IL systems, so the presence of bridging in the IL-in-IL system was unexpected. This allows for the overarching hypothesis describing the modulation of particle morphology in these IL-based Pickering emulsion systems that an active surface chemistry is required in order for particles to exclusively form bridges. For a further explanation, the major difference between this system and the bridging systems reported previously is that the continuous phase here is protic. This is significant as in order for A-PS particles to assume a surface charge, the amine groups needed to accept a proton and can do so from the protic EAN. This is further illuminated by

form bridges, while the A-PS does not. It is hypothesized that the anion [BMIM][PF<sup>6</sup>

remove the acidic hydrogens of the sulfate and carboxylate surface chemistries, activating them, and thus allowing for bridging to occur. This driving force does not exist in the [P66614] [Phos]/EAN system, and thus bridging does not occur there for S-PS and C-PS. For the A-PS

thus bridging does not occur. Overall, it is clear that the particle self-assembly phenomena is a strong function of the continuous phase/IL type and that phenomena generally unique to IL-based Pickering emulsions, including exclusive bridging and particle absorption, can also

] Imidazolium- and phosphonium-based ILs are known to

] IL slowly gelled the [P66614][Phos] IL over time, the diffusion

]-in-water Pickering emulsions we studied previously, as seen in

] system in which the S-PS and C-PS particle types

], again there are no protons to activate the surface chemistry, and

] can

creamy layer and made the emulsions less stable.

(2) [P66614][Phos] and [BMIM][PF<sup>6</sup>

482 Progress and Developments in Ionic Liquids

observed that the [BMIM][PF<sup>6</sup>

results as the [BMIM][PF<sup>6</sup>

observing the [P66614][Phos]-in-[BMIM][PF<sup>6</sup>

in [P66614][Phos]/[BMIM][PF<sup>6</sup>

**2.2. Ionic liquid-in-ionic liquid Pickering emulsions**

**Figure 4.** Confocal laser-scanning fluorescent images of IL-in-IL Pickering emulsions, including [P66614][Phos]-in-EAN and [P66614][Phos]-in-[BMIM][PF<sup>6</sup> ], stabilized by 1 μm particle diameter PS particles. The right schematic shows generated chemical structures of the IL molecules for visualization purposes.

Further research in this area includes ionic liquid emulsions stabilized by more exotic particles—microgel particles, for example. Work by Monteillet et al., for example, finds that microgels self-assemble on the surface of IL emulsion droplets in a similar manner to the solid particles discussed above. These microgel particles, however, are responsive to various stimuli such as pH and temperature, making the emulsion system itself responsive and opening up still further potential applications [5].

#### **2.3. Spontaneous particle transport across the ionic liquid interface**

During our extensive study of ionic liquid-based Pickering emulsions, we noticed the separate and unique phenomenon of microparticle extraction across the liquid-liquid interface [18]. Spontaneous particle transport defies traditional understanding of the thermodynamics of particle-interface interactions and suggests intriguing and novel physics taking place at the molecular scale. For this work, 1 μm fluorescent sulfate-treated polystyrene particles were again employed (–SO3 H, S-PS, blue color) and dispersed in the water phase. The water phase was then carefully brought into contact with the particle free IL [P66614][Phos], by placing droplets in contact with one another on a glass slide. The interface was then observed over time with a confocal laser-scanning microscope to observe the resulting particle motion.

and liquid-IL interfaces. Additionally, the discovery of spontaneous transport of microparticles through a liquid-liquid interface was highlighted to show the intrinsic, powerful particle

Ionic Liquids in Multiphase Systems http://dx.doi.org/10.5772/65281 485

The unique nature of ionic liquids is largely due to molecular-scale effects—bulky, asymmetric organic molecules that resist steric packing, ionic charges distributed across each IL molecule, and the endless tunability provided by the ability to mix and match cation/anion pairs. Given the importance of the molecular scale to ILs and their behaviors, it is vital to study IL systems at the level of individual molecules and atoms. Molecular dynamics (MD) simulations provide a valuable tool in this effort. By modeling the forces between atoms and predicting the behavior of molecules, we can gain insight into the fundamental physics of ILs in multiphase systems and are able to form solid theories on the phenomena explored in our

**Figure 5.** Images showing the microparticle transport phenomenon across the IL-water interface for (a) a single 1 μm S-PS particle and (b) a cluster of S-PS particles over time. (c) Schematic of the proposed mechanism for the observed

Our first molecular dynamics study of ionic liquids in multiphase systems used a model IL and a model nanoparticle to examine the behavior of particles at IL/water and IL/oil interfaces [19]. The IL was chosen as 1-butyl-3-methylimidazolium hexafluorophosphate ([BMIM][PF<sup>6</sup>

perhaps the most commonly studied IL, and because it is immiscible with both water and hexane (our model oil). Furthermore, a force field model had been developed for it specifically tuned to surface properties [20]. For the nanoparticle, we took a diamond lattice, cut it off in a roughly spherical shape, and saturated the surface with hydrogen. This model was taken to represent hydrophobic nanoparticles as a class. Two interfaces were simulated—IL/water and

]),

extraction capability of ionic liquids.

**3. Insights from molecular dynamic simulations**

spontaneous particle transport. Adapted from Ref. [18].

**3.1. Particle self-assembly at ionic liquid interfaces at the molecular-scale**

experimental work.

**Figure 5** shows the spontaneous microparticle transport for both (a) a single particle and (b) a cluster of particles from the water phase to the IL phase. The adhesive Gibbs energy for a microparticle at the liquid-liquid interface is in the order of 107 kT, thus prior to this work, spontaneous particle transport was not observed, instead propelling the particles by shear forces or functionalizing them with surfactants was required to see this phenomenon. The single particle moved through the interface onto the IL side at *t* = 0.8 s, and moving past the interface and into the IL, the particle remained attached to the IL/water interface for a time before finally detaching. For the particle cluster, it moved through the interface at *t* = 0.4 s, followed by two more particles at *t* = 4.3 s. The latter event forced the first cluster to detach from the interface, with only one particle of the cluster remaining attached until complete absorption at *t* = 15.6 s.

**Figure 5(c)** shows the proposed mechanism for the microparticle transport across the liquidliquid interface, from the water phase to the IL phase, showing spontaneous particle extraction by the IL. This is due to the association of the ions with the particle surfaces that can be thought of as multiple equilibrium constants driving this association and the subsequent particle motion. Because the ILs removed particles from the continuous phase, we hypothesized that the dissolved IL ions in this phase played a role in the absorption process, as it is known that IL solubility in water depends on both the hydrophobicities of the cation and anion. A series of equilibrium relationship were used to describe the mechanism, as seen in **Figure 5(c)**. *K*1 represents the equilibrium between IL ions in the IL phase and those IL ions dissolved in the aqueous or solvent phase. The dissolved ions can then interact with and cover the particles dispersed in the aqueous phase. This is likely given the previous observations of the particle extraction for S-PS particles in the studied IL-based Pickering emulsions. Similarly, *K*<sup>2</sup> represents the equilibrium between ion-covered particles dispersed in the aqueous phase and the particles extracted into the IL phase. This explanation allows for identification of IL-particle– solvent systems that may be adequate to experience particle absorption. For example, [P66614] [Phos] is minimally soluble in water, thus exhibiting a low *K*<sup>1</sup> , but the constituent ions would exhibit strong binding to the particles, exhibiting a high *K*<sup>2</sup> . This supports the fact that [P66614] [Phos] was one of the most efficient ILs for particle absorption and fits with the results seen for extraction with a myriad of other ILs in our work on the subject [18].

Thus, the study of ionic liquids and their interfaces with other liquids and solid particles reveal many interesting phenomena, including fully covered emulsion droplets, droplet/particle bridging, particle transport into the IL during emulsification and spontaneously, and a combination of these phenomena for the same emulsion system depending on the particle type. These phenomena are unique to IL-based emulsions, not seen in their traditional oil/ water emulsion counterparts, and were likely due to the charged nature of the ionic liquid and additionally due to the behavior of their substituent cations and anions at the particle-IL and liquid-IL interfaces. Additionally, the discovery of spontaneous transport of microparticles through a liquid-liquid interface was highlighted to show the intrinsic, powerful particle extraction capability of ionic liquids.

**Figure 5.** Images showing the microparticle transport phenomenon across the IL-water interface for (a) a single 1 μm S-PS particle and (b) a cluster of S-PS particles over time. (c) Schematic of the proposed mechanism for the observed spontaneous particle transport. Adapted from Ref. [18].

### **3. Insights from molecular dynamic simulations**

molecular scale. For this work, 1 μm fluorescent sulfate-treated polystyrene particles were

was then carefully brought into contact with the particle free IL [P66614][Phos], by placing droplets in contact with one another on a glass slide. The interface was then observed over time

**Figure 5** shows the spontaneous microparticle transport for both (a) a single particle and (b) a cluster of particles from the water phase to the IL phase. The adhesive Gibbs energy for a

spontaneous particle transport was not observed, instead propelling the particles by shear forces or functionalizing them with surfactants was required to see this phenomenon. The single particle moved through the interface onto the IL side at *t* = 0.8 s, and moving past the interface and into the IL, the particle remained attached to the IL/water interface for a time before finally detaching. For the particle cluster, it moved through the interface at *t* = 0.4 s, followed by two more particles at *t* = 4.3 s. The latter event forced the first cluster to detach from the interface, with only one particle of the cluster remaining attached until complete

**Figure 5(c)** shows the proposed mechanism for the microparticle transport across the liquidliquid interface, from the water phase to the IL phase, showing spontaneous particle extraction by the IL. This is due to the association of the ions with the particle surfaces that can be thought of as multiple equilibrium constants driving this association and the subsequent particle motion. Because the ILs removed particles from the continuous phase, we hypothesized that the dissolved IL ions in this phase played a role in the absorption process, as it is known that IL solubility in water depends on both the hydrophobicities of the cation and anion. A series of equilibrium relationship were used to describe the mechanism, as seen in **Figure 5(c)**.

 represents the equilibrium between IL ions in the IL phase and those IL ions dissolved in the aqueous or solvent phase. The dissolved ions can then interact with and cover the particles dispersed in the aqueous phase. This is likely given the previous observations of the particle

sents the equilibrium between ion-covered particles dispersed in the aqueous phase and the particles extracted into the IL phase. This explanation allows for identification of IL-particle– solvent systems that may be adequate to experience particle absorption. For example, [P66614]

[Phos] was one of the most efficient ILs for particle absorption and fits with the results seen

Thus, the study of ionic liquids and their interfaces with other liquids and solid particles reveal many interesting phenomena, including fully covered emulsion droplets, droplet/particle bridging, particle transport into the IL during emulsification and spontaneously, and a combination of these phenomena for the same emulsion system depending on the particle type. These phenomena are unique to IL-based emulsions, not seen in their traditional oil/ water emulsion counterparts, and were likely due to the charged nature of the ionic liquid and additionally due to the behavior of their substituent cations and anions at the particle-IL

extraction for S-PS particles in the studied IL-based Pickering emulsions. Similarly, *K*<sup>2</sup>

[Phos] is minimally soluble in water, thus exhibiting a low *K*<sup>1</sup>

for extraction with a myriad of other ILs in our work on the subject [18].

exhibit strong binding to the particles, exhibiting a high *K*<sup>2</sup>

with a confocal laser-scanning microscope to observe the resulting particle motion.

microparticle at the liquid-liquid interface is in the order of 107

H, S-PS, blue color) and dispersed in the water phase. The water phase

kT, thus prior to this work,

repre-

, but the constituent ions would

. This supports the fact that [P66614]

again employed (–SO3

484 Progress and Developments in Ionic Liquids

absorption at *t* = 15.6 s.

*K*1

The unique nature of ionic liquids is largely due to molecular-scale effects—bulky, asymmetric organic molecules that resist steric packing, ionic charges distributed across each IL molecule, and the endless tunability provided by the ability to mix and match cation/anion pairs. Given the importance of the molecular scale to ILs and their behaviors, it is vital to study IL systems at the level of individual molecules and atoms. Molecular dynamics (MD) simulations provide a valuable tool in this effort. By modeling the forces between atoms and predicting the behavior of molecules, we can gain insight into the fundamental physics of ILs in multiphase systems and are able to form solid theories on the phenomena explored in our experimental work.

#### **3.1. Particle self-assembly at ionic liquid interfaces at the molecular-scale**

Our first molecular dynamics study of ionic liquids in multiphase systems used a model IL and a model nanoparticle to examine the behavior of particles at IL/water and IL/oil interfaces [19]. The IL was chosen as 1-butyl-3-methylimidazolium hexafluorophosphate ([BMIM][PF<sup>6</sup> ]), perhaps the most commonly studied IL, and because it is immiscible with both water and hexane (our model oil). Furthermore, a force field model had been developed for it specifically tuned to surface properties [20]. For the nanoparticle, we took a diamond lattice, cut it off in a roughly spherical shape, and saturated the surface with hydrogen. This model was taken to represent hydrophobic nanoparticles as a class. Two interfaces were simulated—IL/water and IL/hexane—both with and without nanoparticles. The systems simulated with nanoparticles are shown in **Figure 6**.

As shown in that figure, the second immiscible liquid phase makes a dramatic difference to the self-assembly of nanoparticles at the liquid-liquid interface. When the second phase is water, the hydrophobic nanoparticles are partially absorbed into the IL phase. However, when the second phase is hydrophobic oil, the particles prefer to remain on the hexane-side of the interface. This has obvious implications for several potential applications of these systems, including particle extraction from an oil or water phase or the formation of microscale structure from particles adhered to emulsion droplets.

**Figure 6.** Snapshots from MD simulations of particles at the (a) IL/water and (b) IL/hexane interface. [BMIM] is yellow, [PF<sup>6</sup> ] is green, particles are red, water is blue, and hexane is purple. Adapted from Ref. [19].

While the molecules of the liquid phases influence particle behavior, the opposite is equally true. By comparing the simulations of the systems with nanoparticles to those without, we were able to examine the effect of nanoparticles on the interface through various analytical techniques. **Figure 7** illustrates two of those techniques—calculating density profiles and ordering parameters. Mass density profiles are a simple way to illustrate the physical distribution of molecules across the system. The ordering parameter Sz is a measure of how flat the carbon chain on the IL cation (the butyl in 1-butyl-3-methylimidazolium) lies against the plane of the interface. When Sz is zero, there is no particular ordering of ionic liquid atoms they are oriented in all directions. Sz rising indicates that the IL cation is orienting itself flat against the plane of the interface, creating an ordered pattern of molecules. This ordering is similar to that widely observed in molecular dynamics studies of other ionic liquid interfaces including the IL/oil [21], IL/graphite [22], IL/gold [23], and IL/vapor interfaces [24, 25]. Drawing from this wide range of studies, it can confidently be said that ordering of ionic liquid molecules is a fundamental trait of IL interfaces, and it is no surprise that it affects and is affected by particles at the interface. Such ordering has also been confirmed experimentally through spectroscopy [26].

**Figure 7.** Density profiles and ordering parameters for IL/water (a, c, e, g) and IL hexane (b,d,f,h) systems without (a, b, e, f) and with (c, d, g, h) particles. The four figures above (a, b, c, d) show density profiles for water (blue), IL (green), and particles (red). The four figures below (e, f, g, h) compare the IL density profile to the ordering parameter Sz, a measure

Ionic Liquids in Multiphase Systems http://dx.doi.org/10.5772/65281 487

of how flat the IL cation molecules lay against the interface. Adapted from Ref. [19].

IL/hexane—both with and without nanoparticles. The systems simulated with nanoparticles

As shown in that figure, the second immiscible liquid phase makes a dramatic difference to the self-assembly of nanoparticles at the liquid-liquid interface. When the second phase is water, the hydrophobic nanoparticles are partially absorbed into the IL phase. However, when the second phase is hydrophobic oil, the particles prefer to remain on the hexane-side of the interface. This has obvious implications for several potential applications of these systems, including particle extraction from an oil or water phase or the formation of microscale

While the molecules of the liquid phases influence particle behavior, the opposite is equally true. By comparing the simulations of the systems with nanoparticles to those without, we were able to examine the effect of nanoparticles on the interface through various analytical techniques. **Figure 7** illustrates two of those techniques—calculating density profiles and ordering parameters. Mass density profiles are a simple way to illustrate the physical distribution of molecules across the system. The ordering parameter Sz is a measure of how flat the carbon chain on the IL cation (the butyl in 1-butyl-3-methylimidazolium) lies against the plane of the interface. When Sz is zero, there is no particular ordering of ionic liquid atoms they are oriented in all directions. Sz rising indicates that the IL cation is orienting itself flat against the plane of the interface, creating an ordered pattern of molecules. This ordering is similar to that widely observed in molecular dynamics studies of other ionic liquid interfaces including the IL/oil [21], IL/graphite [22], IL/gold [23], and IL/vapor interfaces [24, 25]. Drawing from this wide range of studies, it can confidently be said that ordering of ionic liquid molecules is a fundamental trait of IL interfaces, and it is no surprise that it affects and is affected by particles at the interface. Such ordering has also been confirmed experimentally

**Figure 6.** Snapshots from MD simulations of particles at the (a) IL/water and (b) IL/hexane interface. [BMIM] is yellow,

] is green, particles are red, water is blue, and hexane is purple. Adapted from Ref. [19].

are shown in **Figure 6**.

486 Progress and Developments in Ionic Liquids

through spectroscopy [26].

[PF<sup>6</sup>

structure from particles adhered to emulsion droplets.

**Figure 7.** Density profiles and ordering parameters for IL/water (a, c, e, g) and IL hexane (b,d,f,h) systems without (a, b, e, f) and with (c, d, g, h) particles. The four figures above (a, b, c, d) show density profiles for water (blue), IL (green), and particles (red). The four figures below (e, f, g, h) compare the IL density profile to the ordering parameter Sz, a measure of how flat the IL cation molecules lay against the interface. Adapted from Ref. [19].

**Figure 7** illustrates the impact of the particles on the IL/water interface. In the case of the IL/ water system, the addition of particles broadens the interface, with potential relevance to interface properties such as surface tension. However, this does not occur in the IL/hexane system, likely because the hydrophobic oil is especially repulsive to [BMIM][PF<sup>6</sup> ] molecules. Instead, the density of the IL features a slight bump at the IL/hexane interface. Examining the ordering parameter Sz explains this oddity—Sz spikes up at the interface as [BMIM] molecules lies laterally against the surface. This ordering likely explains, in part, why particles equilibrate on the hexane side of the interface—absorption into the IL would disrupt this ordering.

In Section 3.1, it was concluded that the presence of the particle at the interface affected properties of that interface. In particle, a hydrophobic particle broadened and softened the IL/ water interface, leading to a region of relatively intermingled water and IL molecules. Particle charge was found to have a significant influence on this effect. In the IL/water system, a neutral particle widened the interface significantly more than either negatively or positively charged particles, and the interface tended to narrow as the charge grew strongly positive. The effect on the IL/hexane system showed the opposite trend, with a neutral particle resulting in a narrower interface than either a negative or positive charge. This indicates that particle charge contributes to a complex system of balanced coulombic forces affecting the interactions of all

Another technique that was utilized in the first study (though that data was not described in this work) was the potential of mean force (PMF). A PMF diagram allows researchers to compare the energy effects of physical configurations with a simulation system. In the case of these particle/interface studies, a particle was forcibly dragged from midway in the water or hexane phase to midway within the IL phase. Snapshots were saved of the system with the particles at intervals of 0.2 nm. Then, each of these snapshots were taken as the starting point of a new simulation. This time, however, the particle was frozen in place. The system energies were then recorded as the system moved around the frozen particle and plotted against the

Graphs (a) and (b) in **Figure 8** compare carbon and silica particles. Graphs (c) and (d) compare carbon particles with –4, neutral, and +4 charges. The equilibrium state of each system is at the lowest point of the energy. This allows us to confirm some things that we already know—that carbon particles equilibrate just inside the IL phase in the IL/water system and just inside the hexane phase in the IL/hexane system, that the hydrophilic and hydrophobic particles exhibit

tive interactions with a neutral particle than either a negative or positively charged one. We also see that in the hexane system, the particle tends to equilibrate at the interface regardless of charge. This again shows the powerful influence of the second liquid phase. In a multiphase system, all elements affect all others in profound ways. When one of those phases is an ionic liquid with its complex constituent ions, the interactions become even more elaborate, allowing for the unusual behavior and intriguing phenomena revealed by experiment.

**3.3. Ionic liquid effects: IL cation and anion influence on interface/particle interactions**

model IL since it is widely studied, and a well-tested model is available. However, one of the greatest assets of ionic liquids is their near endless variety, tunability, and adaptability. Different ionic liquids differ from each other in profound ways, and we are inherently unable to make definitive statements on the nature of IL interfaces after only studying one IL. The final study described here aims to address that issue. It compares four ionic liquids: 1-ethyl-

The length of the carbon chain on the cation is directly correlated to the hydrophobicity

]), and 1-butyl-3-methyl bis(trifluoromethylsulfonyl)imide ([BMIM][Tf<sup>2</sup>

] has stronger, more attrac-

Ionic Liquids in Multiphase Systems http://dx.doi.org/10.5772/65281 489

]. This is a solid choice for a

]), 1-butyl-3-methyl-imidazolium

N]).

]), 1-hexyl-3-methylimidazolium hexafluorophosphate

distance of the particle from the interface. The result is shown in **Figure 7**.

roughly opposing trends. From (c) we determine that [BMIM][PF<sup>6</sup>

The previous studies all utilized the ionic liquid [BMIM][PF<sup>6</sup>

3-methylimidazolium hexafluorophosphate ([EMIM][PF<sup>6</sup>

hexafluorophosphate ([BMIM][PF<sup>6</sup>

([HMIM][PF<sup>6</sup>

elements within the interfaces.

This study gave an important insight into the effect of the second, non-IL, liquid phase on the self-assembly of particles at the interface. It also demonstrated that the particles influence the interface as well. These effects occur at the molecular scale because of the interactions between atoms in the IL and other phases, and are vital to fully understanding corresponding behavior at the macroscale.

### **3.2. Particle effects: particle hydrophobicity and charge**

The initial MD study described in Section 3.1 was inherently limited. It only examined one type of ionic liquid and one type of particle. Two further studies allowed us to examine the effect of changing particle properties on self-assembly. This is an extremely important variable to study if we are to derive conclusions about real-world phenomena. In particular, one of the most promising applications for ILs in multiphase systems is the extraction of sand particles from water or oil, and sand is made up of hydrophilic silica that cannot be expected to behave like our model hydrophobic carbon particle. Another important factor is particle charge. Many particles of interest have some charge, which would obviously have an effect on the ions making up ionic liquids. Therefore, we completed two studies. Both simulated the same [BMIM][PF<sup>6</sup> ]/water and [BMIM][PF<sup>6</sup> ]/hexane interfaces as the original study. The first compared two types of particles—the same hydrophobic carbon particle as before, and a new, hydrophilic silica particle meant to behave similarly to, for example, a grain of sand [26]. The second utilized the carbon particle but arbitrarily varied the charge [27].

These systems were analyzed using the same techniques utilized in the first study. Some results were as expected. For example, the hydrophilic silica particle equilibrated on the opposite side of the interface from the hydrophobic carbon particle—on the water side in the IL/ water system, and on the IL side in the IL/hexane system. Though this result was predictable, it is highly encouraging to one of the most exciting potential applications of these systems—oil spill clean-up. If silica particles are naturally absorbed into the ionic liquid phase when it is placed alongside oil, ionic liquids may be able to passively clean oil by absorbing grains of sand.

Somewhat more interestingly, the silica particle did not adhere as strongly to the IL/water interface as the carbon particle did to the IL/hexane interface. In fact, many of the silica particles remained in the water phase throughout the simulation. This indicates that a silica particle, or particles with similar properties, would be less effective at stabilizing Pickering emulsions than more hydrophobic particles. The silica particles also had the fascinating effect of eliminating the "ordering" of cation molecules at the IL/hexane interface. This suggests that the particle intermediates the repulsive forces between the two liquid phases. (This observation in part inspired aspects of another study described in Section 3.3 below.)

In Section 3.1, it was concluded that the presence of the particle at the interface affected properties of that interface. In particle, a hydrophobic particle broadened and softened the IL/ water interface, leading to a region of relatively intermingled water and IL molecules. Particle charge was found to have a significant influence on this effect. In the IL/water system, a neutral particle widened the interface significantly more than either negatively or positively charged particles, and the interface tended to narrow as the charge grew strongly positive. The effect on the IL/hexane system showed the opposite trend, with a neutral particle resulting in a narrower interface than either a negative or positive charge. This indicates that particle charge contributes to a complex system of balanced coulombic forces affecting the interactions of all elements within the interfaces.

**Figure 7** illustrates the impact of the particles on the IL/water interface. In the case of the IL/ water system, the addition of particles broadens the interface, with potential relevance to interface properties such as surface tension. However, this does not occur in the IL/hexane

Instead, the density of the IL features a slight bump at the IL/hexane interface. Examining the ordering parameter Sz explains this oddity—Sz spikes up at the interface as [BMIM] molecules lies laterally against the surface. This ordering likely explains, in part, why particles equilibrate on the hexane side of the interface—absorption into the IL would disrupt this ordering.

This study gave an important insight into the effect of the second, non-IL, liquid phase on the self-assembly of particles at the interface. It also demonstrated that the particles influence the interface as well. These effects occur at the molecular scale because of the interactions between atoms in the IL and other phases, and are vital to fully understanding corresponding behavior

The initial MD study described in Section 3.1 was inherently limited. It only examined one type of ionic liquid and one type of particle. Two further studies allowed us to examine the effect of changing particle properties on self-assembly. This is an extremely important variable to study if we are to derive conclusions about real-world phenomena. In particular, one of the most promising applications for ILs in multiphase systems is the extraction of sand particles from water or oil, and sand is made up of hydrophilic silica that cannot be expected to behave like our model hydrophobic carbon particle. Another important factor is particle charge. Many particles of interest have some charge, which would obviously have an effect on the ions making up ionic liquids. Therefore, we completed two studies. Both simulated

first compared two types of particles—the same hydrophobic carbon particle as before, and a new, hydrophilic silica particle meant to behave similarly to, for example, a grain of sand

These systems were analyzed using the same techniques utilized in the first study. Some results were as expected. For example, the hydrophilic silica particle equilibrated on the opposite side of the interface from the hydrophobic carbon particle—on the water side in the IL/ water system, and on the IL side in the IL/hexane system. Though this result was predictable, it is highly encouraging to one of the most exciting potential applications of these systems—oil spill clean-up. If silica particles are naturally absorbed into the ionic liquid phase when it is placed alongside oil, ionic liquids may be able to passively clean oil by absorbing grains of sand. Somewhat more interestingly, the silica particle did not adhere as strongly to the IL/water interface as the carbon particle did to the IL/hexane interface. In fact, many of the silica particles remained in the water phase throughout the simulation. This indicates that a silica particle, or particles with similar properties, would be less effective at stabilizing Pickering emulsions than more hydrophobic particles. The silica particles also had the fascinating effect of eliminating the "ordering" of cation molecules at the IL/hexane interface. This suggests that the particle intermediates the repulsive forces between the two liquid phases. (This observa-

[26]. The second utilized the carbon particle but arbitrarily varied the charge [27].

tion in part inspired aspects of another study described in Section 3.3 below.)

]/hexane interfaces as the original study. The

] molecules.

system, likely because the hydrophobic oil is especially repulsive to [BMIM][PF<sup>6</sup>

**3.2. Particle effects: particle hydrophobicity and charge**

]/water and [BMIM][PF<sup>6</sup>

at the macroscale.

488 Progress and Developments in Ionic Liquids

the same [BMIM][PF<sup>6</sup>

Another technique that was utilized in the first study (though that data was not described in this work) was the potential of mean force (PMF). A PMF diagram allows researchers to compare the energy effects of physical configurations with a simulation system. In the case of these particle/interface studies, a particle was forcibly dragged from midway in the water or hexane phase to midway within the IL phase. Snapshots were saved of the system with the particles at intervals of 0.2 nm. Then, each of these snapshots were taken as the starting point of a new simulation. This time, however, the particle was frozen in place. The system energies were then recorded as the system moved around the frozen particle and plotted against the distance of the particle from the interface. The result is shown in **Figure 7**.

Graphs (a) and (b) in **Figure 8** compare carbon and silica particles. Graphs (c) and (d) compare carbon particles with –4, neutral, and +4 charges. The equilibrium state of each system is at the lowest point of the energy. This allows us to confirm some things that we already know—that carbon particles equilibrate just inside the IL phase in the IL/water system and just inside the hexane phase in the IL/hexane system, that the hydrophilic and hydrophobic particles exhibit roughly opposing trends. From (c) we determine that [BMIM][PF<sup>6</sup> ] has stronger, more attractive interactions with a neutral particle than either a negative or positively charged one. We also see that in the hexane system, the particle tends to equilibrate at the interface regardless of charge. This again shows the powerful influence of the second liquid phase. In a multiphase system, all elements affect all others in profound ways. When one of those phases is an ionic liquid with its complex constituent ions, the interactions become even more elaborate, allowing for the unusual behavior and intriguing phenomena revealed by experiment.

#### **3.3. Ionic liquid effects: IL cation and anion influence on interface/particle interactions**

The previous studies all utilized the ionic liquid [BMIM][PF<sup>6</sup> ]. This is a solid choice for a model IL since it is widely studied, and a well-tested model is available. However, one of the greatest assets of ionic liquids is their near endless variety, tunability, and adaptability. Different ionic liquids differ from each other in profound ways, and we are inherently unable to make definitive statements on the nature of IL interfaces after only studying one IL. The final study described here aims to address that issue. It compares four ionic liquids: 1-ethyl-3-methylimidazolium hexafluorophosphate ([EMIM][PF<sup>6</sup> ]), 1-butyl-3-methyl-imidazolium hexafluorophosphate ([BMIM][PF<sup>6</sup> ]), 1-hexyl-3-methylimidazolium hexafluorophosphate ([HMIM][PF<sup>6</sup> ]), and 1-butyl-3-methyl bis(trifluoromethylsulfonyl)imide ([BMIM][Tf<sup>2</sup> N]). The length of the carbon chain on the cation is directly correlated to the hydrophobicity of the IL, so adjusting the length makes for a convenient comparison. Tf2 N is both more hydrophobic than PF<sup>6</sup> and allows us to study an IL with an anion of a different shape. We are particularly interested in any insight this might give to the phenomenon of spontaneous particle transport across the interface as described in Section 2.3, which varies widely with different ILs. This study also varied the number of particles, simulating systems of zero, eight, and thirty-two particles. This was done to allow a further study of particle effects on the rest of the system.

hydrophobic nanoparticles from the water phase. However, the [EMIM][PF<sup>6</sup>

have drawn water molecules into the ionic liquid phase with them. [BMIM][PF<sup>6</sup>

shows a sharp interface with very little uptake into the IL. Density profiles confirm these

Given these observations, it would seem that differences in hydrophobicity do not entirely explain the differing behavior of the IL interfaces. Hydrophobicity is certainly a factor—after all, relatively hydrophilic EMIM experiences more interface-broadening than hydrophobic HMIM. However, HMIM does not experience the same IL-crowding at the interface. The dif-

transport exhibited in **Figure 5**, it becomes clear that a major relevant difference between

The results of the thirty-two particle simulations are intriguing for another reason. [EMIM] [PF6], water, and particles form a broad, intermingled interface, with the particles quickly aggregating into a single cluster (this behavior was also observed in the parallel simulation

distinct interfaces, and the nanoparticles are not quickly drawn into a large cluster. In the

confirms prior suspicions that nanoparticles intermediate the repulsive interactions between water and the ionic liquid molecules. This creates a unique energy-favorable region with

] and [BMIM][Tf<sup>2</sup>

N] (no transport) and [P66614][Phos] (strong transport), both highly hydrophobic

] snapshot, the particles can be seen drawing ions out into the water phase. This

ference is not hydrophobicity but the shape of the anion, with Tf2

 **Figure 9.** Snapshots from simulations of four IL interfaces with 32 particles.

of lying flat on the interface, while PF<sup>6</sup>

phosphinate with its branching chains.

runs). Hydrophobic [HMIM][PF<sup>6</sup>

ILs, may be the shape of the anion, with Tf2

[BMIM][Tf<sup>2</sup>

[HMIM][PF<sup>6</sup>

] also seem to be doing this to different extents. The [BMIM][Tf<sup>2</sup>

[PF<sup>6</sup>

suspicions.

] also seems to

491

] and [HMIM]

N] system, however,

Ionic Liquids in Multiphase Systems http://dx.doi.org/10.5772/65281

N being linear and capable

is round. Applying these observations to the particle

N much more linear and much more rigid than

N], on the other hand, maintain relatively

**Figure 8.** Potential of mean force (PMF) diagrams. (a) and (b) compare carbon (blue) and silica (red) particles. (c) and (d) compare carbon particles with −4 (red), neutral (black), and +4 (blue) charges. Adapted from Refs. [26, 27].

Adding particles to these systems reveals how these interface properties affect particle/interface interactions. **Figure 9** shows snapshots from the 32-particle simulations of each interface system. To some extent all four interfaces behave similarly, equilibrating with the particles aggregated at the surface and a thin layer of IL molecules wrapping around, insulating the hydrophobic nanoparticles from the water phase. However, the [EMIM][PF<sup>6</sup> ] also seems to have drawn water molecules into the ionic liquid phase with them. [BMIM][PF<sup>6</sup> ] and [HMIM] [PF<sup>6</sup> ] also seem to be doing this to different extents. The [BMIM][Tf<sup>2</sup> N] system, however, shows a sharp interface with very little uptake into the IL. Density profiles confirm these suspicions.

 **Figure 9.** Snapshots from simulations of four IL interfaces with 32 particles.

of the IL, so adjusting the length makes for a convenient comparison. Tf2

are particularly interested in any insight this might give to the phenomenon of spontaneous particle transport across the interface as described in Section 2.3, which varies widely with different ILs. This study also varied the number of particles, simulating systems of zero, eight, and thirty-two particles. This was done to allow a further study of particle effects on

Adding particles to these systems reveals how these interface properties affect particle/interface interactions. **Figure 9** shows snapshots from the 32-particle simulations of each interface system. To some extent all four interfaces behave similarly, equilibrating with the particles aggregated at the surface and a thin layer of IL molecules wrapping around, insulating the

**Figure 8.** Potential of mean force (PMF) diagrams. (a) and (b) compare carbon (blue) and silica (red) particles. (c) and (d)

compare carbon particles with −4 (red), neutral (black), and +4 (blue) charges. Adapted from Refs. [26, 27].

and allows us to study an IL with an anion of a different shape. We

hydrophobic than PF<sup>6</sup>

490 Progress and Developments in Ionic Liquids

the rest of the system.

N is both more

Given these observations, it would seem that differences in hydrophobicity do not entirely explain the differing behavior of the IL interfaces. Hydrophobicity is certainly a factor—after all, relatively hydrophilic EMIM experiences more interface-broadening than hydrophobic HMIM. However, HMIM does not experience the same IL-crowding at the interface. The difference is not hydrophobicity but the shape of the anion, with Tf2 N being linear and capable of lying flat on the interface, while PF<sup>6</sup> is round. Applying these observations to the particle transport exhibited in **Figure 5**, it becomes clear that a major relevant difference between [BMIM][Tf<sup>2</sup> N] (no transport) and [P66614][Phos] (strong transport), both highly hydrophobic ILs, may be the shape of the anion, with Tf2 N much more linear and much more rigid than phosphinate with its branching chains.

The results of the thirty-two particle simulations are intriguing for another reason. [EMIM] [PF6], water, and particles form a broad, intermingled interface, with the particles quickly aggregating into a single cluster (this behavior was also observed in the parallel simulation runs). Hydrophobic [HMIM][PF<sup>6</sup> ] and [BMIM][Tf<sup>2</sup> N], on the other hand, maintain relatively distinct interfaces, and the nanoparticles are not quickly drawn into a large cluster. In the [HMIM][PF<sup>6</sup> ] snapshot, the particles can be seen drawing ions out into the water phase. This confirms prior suspicions that nanoparticles intermediate the repulsive interactions between water and the ionic liquid molecules. This creates a unique energy-favorable region with water, ionic liquid, and particles commingling. This observation was further confirmed by calculating interaction energies between the ILs and water for system with varying numbers of particles. It was found that changing the number of particles had a profound effect on the repulsive and attractive forces between ionic liquids and water. This may explain phenomena such as spontaneous transport of particles across the IL-water interface—the particles themselves distort and broaden the interface, changing their fundamental nature and the energetic physics which typically cause particles to adhere strongly to the interface.

[2] Tikekar, R.V. Fate of curcumin encapsulated in silica nanoparticle stabilized Pickering emulsion during storage and simulated digestion. *Food Res. Int.* **2013**, *51*, 370–377.

Ionic Liquids in Multiphase Systems http://dx.doi.org/10.5772/65281 493

[3] Zhu, Z.; Chen, D.; Wu, G. Molecular dynamic simulation of asphaltene co-aggregation

[4] Nagarkar, S.; Velankar, S.S. Rheology and morphology of model immiscible polymer blends with monodisperse spherical particles at the interface. *J. Rheol.* **2013**, *57*, 901–926.

[5] Monteillet, H.J.M.; Workamp, M.J.; Li, X.; Schuur, B. Multi-responsive ionic liquid emul-

[6] Luczak, J.; Paszkiewicz, M.; Krukowska, A.; Malankowska, A. Ionic liquids for nanoand microstructures preparation. Part 1: properties and multifunctional role. *Adv. Colloid* 

[7] Federicia Canova, F.; Mizukami, M.; Imamura, T.; Kurihara, K.; Shluger, A.L. Structural stability and polarization of ionic liquid films on silica surfaces. *Phys. Chem. Chem. Phys.*

[8] Anouti, M. Room-temperature molten salts: protic ionic liquids and deep eutectic solvents as media for electrochemical application. Chapter in *Electrochemistry in Ionic* 

[9] Bettoschi, A. Highly stable ionic liquid-in-water emulsions as a new class of fluorescent sensors for metal ions: the case study of Fe3+ sensing. *RSC Adv.* **2015**, *5*, 37385–37391.

[10] Ma, H.; Dai, L. Particle self-assembly in ionic liquid-in-water Pickering emulsions.

[11] Binks, B.P.; Fletcher, P.D.I.; Dyab, A.K.F. Novel emulsions of ionic liquids stabilised

[12] Binks, B.P.; Dyab, A.K.F.; Fletcher, P.D.I. Contact angles in relation to emulsions stabilised solely by silica nanoparticles including systems containing room temperature ionic

[13] Walker, E.M.; Frost, D.S.; Dai, L.L. Particle self-assembly in oil-in-ionic liquid Pickering

[14] Frost, D.S.; Schoepf, J.J.; Nofen, E.M.; Dai, L.L. Understanding droplet bridging in ionic

[15] Tarimala, S.; Dai, L.L. Structure of microparticles in solid-stabilized emulsions. *Langmuir*

[16] Dai, L.L.; Tarimala, S.; Wu, C.; Guttula, S.; Wu, J. The structure and dynamics of mic-

[17] Frost, D.S.; Nofen, E.M.; Dai, L.L. Particle self-assembly at ionic liquid-based interfaces.

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with humic acid during oil spill. *Chemosphere* **2015**, *138*, 412–421.

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**2004**, *20*, 3492–3494.

### **4. Summary and discussion**

The unique properties of ionic liquids lend themselves to a multitude of applications relying on various diverse and intriguing systems. This work has focused on multiphasic systems where IL undergo complex interactions with other materials. In particular, this work explored ionic-liquid-based Pickering emulsions (stabilized by solid particles) and the behavior of particles at ionic liquid/liquid interfaces. These systems exhibit several remarkable phenomena including "bridging" of particles in between emulsion droplets and, perhaps most intriguingly, spontaneous transport of particles across the liquid/liquid interface. The unique behavior and properties of ionic liquids in these systems make them potentially relevant to a variety of applications including oil spill cleanup, drug delivery, and the creation of novel materials. What is more, the unusual nature of ionic liquids makes their study a vital tool in fully understanding the fundamental nature of physical systems such as liquid/liquid interfaces and Pickering emulsions. Molecular dynamics simulations, in particular, reveal how the properties of ionic liquids at the molecular level influence the behaviors of the systems. Further work in this field may take a variety of directions including developing practical extraction techniques based on ionic liquids, developing novel materials utilizing or based on ionic liquid Pickering emulsions, or environmentally responsive emulsions. Regardless, ionic liquids in multiphase systems promise continued relevance to both scientific advancement and practical application in the years to come.

### **Author details**

Stella Nickerson, Elizabeth Nofen, Denzil Frost and Lenore L. Dai\*

\*Address all correspondence to: lenore.dai@asu.edu

School for Engineering of Matter, Transport Energy, Arizona State University, Tempe, Arizona, United States of America

### **References**

[1] Destribats, M.; Rouvet, M.; Gehin-Delval, C.; Schmitt, C.; Binks, B.P. Emulsions stabilized by whey protein microgel particles: towards food-grade Pickering emulsions. *Soft Matter* **2014**, *10*, 6941–6954.

[2] Tikekar, R.V. Fate of curcumin encapsulated in silica nanoparticle stabilized Pickering emulsion during storage and simulated digestion. *Food Res. Int.* **2013**, *51*, 370–377.

water, ionic liquid, and particles commingling. This observation was further confirmed by calculating interaction energies between the ILs and water for system with varying numbers of particles. It was found that changing the number of particles had a profound effect on the repulsive and attractive forces between ionic liquids and water. This may explain phenomena such as spontaneous transport of particles across the IL-water interface—the particles themselves distort and broaden the interface, changing their fundamental nature and the energetic physics which typically cause particles to adhere strongly to the interface.

The unique properties of ionic liquids lend themselves to a multitude of applications relying on various diverse and intriguing systems. This work has focused on multiphasic systems where IL undergo complex interactions with other materials. In particular, this work explored ionic-liquid-based Pickering emulsions (stabilized by solid particles) and the behavior of particles at ionic liquid/liquid interfaces. These systems exhibit several remarkable phenomena including "bridging" of particles in between emulsion droplets and, perhaps most intriguingly, spontaneous transport of particles across the liquid/liquid interface. The unique behavior and properties of ionic liquids in these systems make them potentially relevant to a variety of applications including oil spill cleanup, drug delivery, and the creation of novel materials. What is more, the unusual nature of ionic liquids makes their study a vital tool in fully understanding the fundamental nature of physical systems such as liquid/liquid interfaces and Pickering emulsions. Molecular dynamics simulations, in particular, reveal how the properties of ionic liquids at the molecular level influence the behaviors of the systems. Further work in this field may take a variety of directions including developing practical extraction techniques based on ionic liquids, developing novel materials utilizing or based on ionic liquid Pickering emulsions, or environmentally responsive emulsions. Regardless, ionic liquids in multiphase systems promise continued relevance to both scientific advancement

**4. Summary and discussion**

492 Progress and Developments in Ionic Liquids

and practical application in the years to come.

\*Address all correspondence to: lenore.dai@asu.edu

Arizona, United States of America

*Matter* **2014**, *10*, 6941–6954.

Stella Nickerson, Elizabeth Nofen, Denzil Frost and Lenore L. Dai\*

School for Engineering of Matter, Transport Energy, Arizona State University, Tempe,

[1] Destribats, M.; Rouvet, M.; Gehin-Delval, C.; Schmitt, C.; Binks, B.P. Emulsions stabilized by whey protein microgel particles: towards food-grade Pickering emulsions. *Soft* 

**Author details**

**References**


[18] Frost, D.S.; Ngan, M.; Dai, L.L. Spontaneous transport of microparticles across liquidliquid interfaces. *Langmuir* **2013**, *29*, 9310–9315.

**Chapter 21**

Provisional chapter

**Imidazolium-Based Ionic Liquid Binary Solvent System**

**as an Extraction Medium in Enhancing the Rotenone**

as an Extraction Medium in Enhancing the Rotenone

Derris

Imidazolium-Based Ionic Liquid Binary Solvent System

Rotenone, is a biopesticide which can be isolated from Derris species roots. However, procuring significant amount of rotenone using green alternative solvent rather than harmful organic solvents for commercialization is a challenge to be faced. Therefore, an approach using imidazolium-based ionic liquids (ILs) as an extraction medium was employed in this study. Five different types of binary solvent systems comprising a combination of acetone and five respective ionic liquids (ILs) of (1) [BMIM] Cl; (2) [BMIM] OAc; (3) [BMIM] NTf2; (4) [BMIM] OTf; and (5) [BMPy] Cl were used in the normal soaking extraction (NSE) of rotenone for a 24-hour extraction. The yield of the rotenone, % (w/w), and its concentration (mg/mL) in the dried roots was quantitatively determined by means of the reversed-phase high-performance liquid chromatography (RP-HPLC) and thin-layer chromatography (TLC). The results showed that a binary solvent system of [BMIM] OTf:acetone was the best solvent system combination compared to other solvent systems (p < 0.05). It contributed to the highest rotenone content of 2.69 ± 0.21% (w/w) (4.04 ± 0.34 mg/ml) at the 14th hour of the exhaustive extraction time. In conclusion, a combination of certain ILs with a selective organic solvent has been proven to be able to increase a significant amount of bioactive constituents in the

Keywords: rotenone, Derris sp., binary solvent system, imidazolium-based, ionic

The destructive effects of numerous pests from the time immemorial led to a large decline in crop yield. Through the advent of chemical pesticides, this crisis was resolved to a great extent.

© The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons

© 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and eproduction in any medium, provided the original work is properly cited.

elliptica

Roots

**Yield Extracted from** *Derris elliptica* **Roots**

Zetty Shafiqa Othman, Nur Hasyareeda Hassan and

Additional information is available at the end of the chapter

Additional information is available at the end of the chapter

Zetty Shafiqa Othman, Nur Hasyareeda

Saiful Irwan Zubairi

Abstract

liquids

1. Introduction

http://dx.doi.org/10.5772/66777

Yield Extracted from

Hassan and Saiful Irwan Zubairi

phytochemical extraction process.


### **Imidazolium-Based Ionic Liquid Binary Solvent System as an Extraction Medium in Enhancing the Rotenone Yield Extracted from** *Derris elliptica* **Roots** Provisional chapter Imidazolium-Based Ionic Liquid Binary Solvent System as an Extraction Medium in Enhancing the Rotenone

elliptica

Roots

Derris

Zetty Shafiqa Othman, Nur Hasyareeda Hassan and Saiful Irwan Zubairi Zetty Shafiqa Othman, Nur Hasyareeda

Additional information is available at the end of the chapter Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/66777

Yield Extracted from

Hassan and Saiful Irwan Zubairi

#### Abstract

[18] Frost, D.S.; Ngan, M.; Dai, L.L. Spontaneous transport of microparticles across liquid-

[19] Frost, D.S.; Dai, L.L. Molecular dynamics simulations of nanoparticle self-assembly at ionic liquid-water and ionic liquid-oil interfaces. *Langmuir* **2011**, *27*, 11339–11346. [20] Bhargava, B.L.; Balasubramanian, S. Refined potential model for atomistic simulations

]. *J. Chem. Phys.* **2007**, *127*, 114510.

ionic liquid: a nonlinear vibrational spectroscopy and molecular

] ionic liquid on graphite surfaces: molecular dynamics

]. *J. Am. Chem. Soc.* **2006**, *128*, 10073–10078.

[21] Iwahashi, T.; Ishiyama, T.; Sakai, Y.; Ouchi, Y. Liquid/liquid interface layering of 1-buta-

[22] Maolin, S.; Fuchun, Z.; Guozhong, W.; Haiping, F.; Chunlei, W.; Shimou, C. Yi, Z.; Jun,

[23] Ferreira, E.S.C.; Pereira, C.M.; Cordeiro, M.N.D.S.; dos Santos, D.J.V.A. Molecular dynamics study of the gold/ionic liquids interface. *J. Phys. Chem. B* **2015**, *119*, 9883–9892.

[24] Heggen, B.; Zhao, W.; Leory, F.; Dammers, A.J.; Muller-Plathe, F. Interfacial properties of an ionic liquid by molecular dynamics. *J. Phys. Chem. B* **2010**, *114*, 6954–6961.

[25] Bhargava, B.L.; Balasubramanian, S. Layering at an ionic liquid-vapor interface: a molec-

[26] Frost, D.S.; Machas, M.; Dai, L.L. Molecular dynamics studies on the adaptability of an ionic liquid in the extraction of solid nanoparticles. *Langmuir* **2012**, *28*, 13924–13932. [27] Frost, D.S.; Dai, L.L. Molecular dynamics simulations of charged nanoparticle selfassembly at ionic liquid-water and ionic liquid-oil interfaces. *J. Chem. Phys.* **2012**, *136*,

dynamics simulation study. *Phys. Chem. Chem. Phys.* **2015**, *17*, 24587–24597.

liquid interfaces. *Langmuir* **2013**, *29*, 9310–9315.

of ionic liquid [bmim][PF<sup>6</sup>

H. Ordering layers of [bmim][PF<sup>6</sup>

simulation. *J. Chem. Phys.* **2008**, *128*, 134504.

ular dynamics simulation study of [bmim][PF<sup>6</sup>

nol and [bmim]PF<sup>6</sup>

494 Progress and Developments in Ionic Liquids

084706.

Rotenone, is a biopesticide which can be isolated from Derris species roots. However, procuring significant amount of rotenone using green alternative solvent rather than harmful organic solvents for commercialization is a challenge to be faced. Therefore, an approach using imidazolium-based ionic liquids (ILs) as an extraction medium was employed in this study. Five different types of binary solvent systems comprising a combination of acetone and five respective ionic liquids (ILs) of (1) [BMIM] Cl; (2) [BMIM] OAc; (3) [BMIM] NTf2; (4) [BMIM] OTf; and (5) [BMPy] Cl were used in the normal soaking extraction (NSE) of rotenone for a 24-hour extraction. The yield of the rotenone, % (w/w), and its concentration (mg/mL) in the dried roots was quantitatively determined by means of the reversed-phase high-performance liquid chromatography (RP-HPLC) and thin-layer chromatography (TLC). The results showed that a binary solvent system of [BMIM] OTf:acetone was the best solvent system combination compared to other solvent systems (p < 0.05). It contributed to the highest rotenone content of 2.69 ± 0.21% (w/w) (4.04 ± 0.34 mg/ml) at the 14th hour of the exhaustive extraction time. In conclusion, a combination of certain ILs with a selective organic solvent has been proven to be able to increase a significant amount of bioactive constituents in the phytochemical extraction process.

Keywords: rotenone, Derris sp., binary solvent system, imidazolium-based, ionic liquids

### 1. Introduction

The destructive effects of numerous pests from the time immemorial led to a large decline in crop yield. Through the advent of chemical pesticides, this crisis was resolved to a great extent.

© 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

© The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons

Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and eproduction in any medium, provided the original work is properly cited.

However, an overdose, overdependence on and uncontrolled usage of synthetic pesticides eventually created pest resistance which simultaneously led to frequent applications, application of bulk quantity of pesticide and a high cost [1]. In addition, the violative pesticides' residues had contributed to food safety concern among consumers. Therefore, an eco-friendly alternative is needed to overcome the drawbacks of synthetic pesticides. As agriculturalindustrial tools, biopesticides demonstrate exemplary benefits over chemically synthesized pesticides through harnessing the natural capabilities of organisms and their molecular constituents in minimizing the crop and plant damages from pests, affording the opportunity for protection, maintenance of biodiversity, and commerce-strengthening alternatives for organic farming and safe guarding of human health. Rotenone is one of the biopesticides that can be extracted from Derris and Lonchocarpus plants' roots [2]. It exhibits a strong pesticidal activity due to its strong paralysis action (knock-down effect) on cold-blooded animals. Besides, its high degradability, exceptionally selective and poor absorption across the gut and skin of humans enhances its eco-friendly usage [3–7]. In accordance with that, the extraction process plays a major role in optimizing the extraction of the yield of rotenone compound. Conventionally, organic solvents were used as an extraction medium and the selection of solvent systems largely depended on the specific nature of the bioactive compound from natural products. Rotenone is an isoflavonoid and it does not dissolve in water but it dissolves in organic solvents. According to John and Ron [4], the solubility of pure rotenone in acetone is 0.066 g/ml, ethanol, 0.002 g/ml and chloroform, 0.47 g/ml. However, a research completed by Zubairi [8] showed that acetone extracted more rotenone and other bioactive constituents compared to a high-polarity solvent as it could extract rotenone from from 39.5% up to 72.8% compared to chloroform and ethanol.

biphasic systems) and the organic species have a high solubility in these ILs, making them ideal solvents for bioactive compound extraction from plants and as mobile phase modifiers to improve liquid chromatography separation of bioactive targets. The usage of ILs as plant bioactive constituent extractants has a great impact and potential as they alleviate the environmental pollution and improve the selectivity and extraction yields of interesting compounds in

Imidazolium-Based Ionic Liquid Binary Solvent System as an Extraction Medium in Enhancing the Rotenone Yield...

http://dx.doi.org/10.5772/66777

497

However, due to their charged and asymmetric structures, ILs have a relatively high polarity [15], as they do not have a good affinity with weak-polar compounds, thus causing a reduction in the distribution of weak-polar compounds in the IL phase. The viscosity of ILs increases with longer alkyl chain of ILs in accordance with the strong electrostatic and hydrogen bonding interaction between ions [16, 17]. The high viscosity of ILs will hinder the mixing and transferring of properties in the extraction process by influencing the dissolution of the compounds in ILs. In exchange, the mixture of ILs and polar molecular solvents as an extraction medium could be an effective approach to solve their flaws. Besides creating a wideadjusted range of solvent polarity, hydrophobicity, hydrogen-bond acidity, and basicity [18, 19], the addition of miscible molecular solvents as co-solvents helps to break the microscopic hydrogen-bond network and the aggregation of ILs, which significantly reduces the viscosity and improves the mixing and transferring process in their mediated extraction [20, 21]. The rotenone compound is an acidic isoflavonoid compound that consists of ketonic chemical groups (R−C(=O)−R) [22] which has the potential of interacting with intermediate-polar solvents. It can be easily dissolved in moderate-polar organic solvents (e.g., methanol, chloroform, and acetone) [23] and is sparingly soluble in water [24]. For that reason, a combination of any ILs with a moderate polarity of organic solvents would perhaps increase the chances of extracting a high rotenone content due to its low viscosity and mediate polarity property and the high tendency of interaction between rotenone compound with the anion and cation of ILs. The previous study indicated that solubility of flavonoids and their derivatives can be increased by using ILs as the of flavonoids are greatly anion-dependent [25]. The anionic potency of both organic solvents and ILs in extracting a large amount of bioactive compounds (e.g., rotenone) and moving into solvent systems is significantly undeniable as both chemicals facilitate the extraction process via salvation power and multiple interactions (e.g., hydrogen

sample pretreatment process compared to conventional organic solvents.

bonding, polarity, ionic/charge-to-charge, and π-π, π-n) with the analytes [26, 27].

2. Imidazolium-based ionic liquids as a green extraction medium

Ionic liquids (ILs) are organic salts in the liquid state under ambient temperature that comprise a normally charged stabilized organic cation paired with an organic or inorganic anion. The widely used cations (Figure 1) are ammonium, sulfonium, imidazolium, pyridinium, pyrrolidinium, tetraalkylammonium, phosphonium, picolinium, and the functionalized cations with different substitutions [28, 29]. On the other hand, anions are weakly basic inorganic or organic compounds that have a diffuse or protected negative

−

, or [PF6]

<sup>−</sup> ions are not preferred due to their

2.1. Structural features of ionic liquids

charge [28]. ILs based on halides such as [BF4]

Although conventional organic solvents have been used for so long as the extraction medium, their drawbacks such as volatility, toxicity, and flammability that lead to several human risks and environmental problems, limit its usage as an extractant. Taking all these into account, there have been several studies conducted on the exploration of ionic liquids (ILs) compatibility as green solvents for plant extraction. According to Fu et al. [9], ILs can be used as an alternative green solvent to replace volatile organic solvents. ILs are organic salts in the liquid state under ambient temperature that comprise a normally charge-stabilized organic cation paired with an organic or inorganic anion. They display a wide range of unique properties such as high thermal stability, nonflammability, insignificant vapor pressure, and low chemical reactivity. In addition to that, they also have fine tunable density, viscosity, polarity, and miscibility with other common solvents through the change of the cation and anion [10]. Some of ILs are also immiscible with organic solvents which define their polar alternative with nonaqueous nature for two-phase systems and ILs can be recycled that this enhances their green properties. The viscosity of ILs also plays a crucial role in the extraction and separation of bioactive compounds from plants. Their viscosity is affected by a range of intermolecular interactions such as electrostatic, van der Waals forces, and hydrogen bonding interaction [11]. The increase in temperature and asymmetry of ILs' anions lead to the decrease in their viscosity. For instance, using the imidazolium cationic species, the viscosity can be intensified by increasing the substituted alkyl chain length or branching due to more van der Waals interactions between the ions themselves [11–14]. Some ILs also are immiscible in water (formation of biphasic systems) and the organic species have a high solubility in these ILs, making them ideal solvents for bioactive compound extraction from plants and as mobile phase modifiers to improve liquid chromatography separation of bioactive targets. The usage of ILs as plant bioactive constituent extractants has a great impact and potential as they alleviate the environmental pollution and improve the selectivity and extraction yields of interesting compounds in sample pretreatment process compared to conventional organic solvents.

However, due to their charged and asymmetric structures, ILs have a relatively high polarity [15], as they do not have a good affinity with weak-polar compounds, thus causing a reduction in the distribution of weak-polar compounds in the IL phase. The viscosity of ILs increases with longer alkyl chain of ILs in accordance with the strong electrostatic and hydrogen bonding interaction between ions [16, 17]. The high viscosity of ILs will hinder the mixing and transferring of properties in the extraction process by influencing the dissolution of the compounds in ILs. In exchange, the mixture of ILs and polar molecular solvents as an extraction medium could be an effective approach to solve their flaws. Besides creating a wideadjusted range of solvent polarity, hydrophobicity, hydrogen-bond acidity, and basicity [18, 19], the addition of miscible molecular solvents as co-solvents helps to break the microscopic hydrogen-bond network and the aggregation of ILs, which significantly reduces the viscosity and improves the mixing and transferring process in their mediated extraction [20, 21]. The rotenone compound is an acidic isoflavonoid compound that consists of ketonic chemical groups (R−C(=O)−R) [22] which has the potential of interacting with intermediate-polar solvents. It can be easily dissolved in moderate-polar organic solvents (e.g., methanol, chloroform, and acetone) [23] and is sparingly soluble in water [24]. For that reason, a combination of any ILs with a moderate polarity of organic solvents would perhaps increase the chances of extracting a high rotenone content due to its low viscosity and mediate polarity property and the high tendency of interaction between rotenone compound with the anion and cation of ILs. The previous study indicated that solubility of flavonoids and their derivatives can be increased by using ILs as the of flavonoids are greatly anion-dependent [25]. The anionic potency of both organic solvents and ILs in extracting a large amount of bioactive compounds (e.g., rotenone) and moving into solvent systems is significantly undeniable as both chemicals facilitate the extraction process via salvation power and multiple interactions (e.g., hydrogen bonding, polarity, ionic/charge-to-charge, and π-π, π-n) with the analytes [26, 27].

### 2. Imidazolium-based ionic liquids as a green extraction medium

#### 2.1. Structural features of ionic liquids

However, an overdose, overdependence on and uncontrolled usage of synthetic pesticides eventually created pest resistance which simultaneously led to frequent applications, application of bulk quantity of pesticide and a high cost [1]. In addition, the violative pesticides' residues had contributed to food safety concern among consumers. Therefore, an eco-friendly alternative is needed to overcome the drawbacks of synthetic pesticides. As agriculturalindustrial tools, biopesticides demonstrate exemplary benefits over chemically synthesized pesticides through harnessing the natural capabilities of organisms and their molecular constituents in minimizing the crop and plant damages from pests, affording the opportunity for protection, maintenance of biodiversity, and commerce-strengthening alternatives for organic farming and safe guarding of human health. Rotenone is one of the biopesticides that can be extracted from Derris and Lonchocarpus plants' roots [2]. It exhibits a strong pesticidal activity due to its strong paralysis action (knock-down effect) on cold-blooded animals. Besides, its high degradability, exceptionally selective and poor absorption across the gut and skin of humans enhances its eco-friendly usage [3–7]. In accordance with that, the extraction process plays a major role in optimizing the extraction of the yield of rotenone compound. Conventionally, organic solvents were used as an extraction medium and the selection of solvent systems largely depended on the specific nature of the bioactive compound from natural products. Rotenone is an isoflavonoid and it does not dissolve in water but it dissolves in organic solvents. According to John and Ron [4], the solubility of pure rotenone in acetone is 0.066 g/ml, ethanol, 0.002 g/ml and chloroform, 0.47 g/ml. However, a research completed by Zubairi [8] showed that acetone extracted more rotenone and other bioactive constituents compared to a high-polarity solvent as it could extract rotenone from from 39.5% up to 72.8%

Although conventional organic solvents have been used for so long as the extraction medium, their drawbacks such as volatility, toxicity, and flammability that lead to several human risks and environmental problems, limit its usage as an extractant. Taking all these into account, there have been several studies conducted on the exploration of ionic liquids (ILs) compatibility as green solvents for plant extraction. According to Fu et al. [9], ILs can be used as an alternative green solvent to replace volatile organic solvents. ILs are organic salts in the liquid state under ambient temperature that comprise a normally charge-stabilized organic cation paired with an organic or inorganic anion. They display a wide range of unique properties such as high thermal stability, nonflammability, insignificant vapor pressure, and low chemical reactivity. In addition to that, they also have fine tunable density, viscosity, polarity, and miscibility with other common solvents through the change of the cation and anion [10]. Some of ILs are also immiscible with organic solvents which define their polar alternative with nonaqueous nature for two-phase systems and ILs can be recycled that this enhances their green properties. The viscosity of ILs also plays a crucial role in the extraction and separation of bioactive compounds from plants. Their viscosity is affected by a range of intermolecular interactions such as electrostatic, van der Waals forces, and hydrogen bonding interaction [11]. The increase in temperature and asymmetry of ILs' anions lead to the decrease in their viscosity. For instance, using the imidazolium cationic species, the viscosity can be intensified by increasing the substituted alkyl chain length or branching due to more van der Waals interactions between the ions themselves [11–14]. Some ILs also are immiscible in water (formation of

compared to chloroform and ethanol.

496 Progress and Developments in Ionic Liquids

Ionic liquids (ILs) are organic salts in the liquid state under ambient temperature that comprise a normally charged stabilized organic cation paired with an organic or inorganic anion. The widely used cations (Figure 1) are ammonium, sulfonium, imidazolium, pyridinium, pyrrolidinium, tetraalkylammonium, phosphonium, picolinium, and the functionalized cations with different substitutions [28, 29]. On the other hand, anions are weakly basic inorganic or organic compounds that have a diffuse or protected negative charge [28]. ILs based on halides such as [BF4] − , or [PF6] <sup>−</sup> ions are not preferred due to their unfavorable properties and they are also strongly hygroscopic [26]. The most preferred anions are the ones which are more complex, perfluorated anions such as bis(trifluoromethane sulfonyl) amide or trifluoromethanesulfonate or halogen-free ions such as dicyandiamide, tosylate, or n-alkyl sulfates (Figure 1) [30]. The environment constituted by ionic liquids is completely different from that of polar and nonpolar molecular solvents. In addition to the existing interactions in conventional organic solvents such as hydrogen bonding, dipole-dipole, and van der Waals interactions, ionic liquids have strong electrostatic interactions.

2.3. Structural organization of imidazolium-based ionic liquids

thiosulfate (S2O3

(C2O4 2− 2−

depended on alkyl chain length [37, 39].

), chromate (CrO4

interactions and good distribution of charge in the cation.

2−

2.4. Physicochemical properties of imidazolium-based ionic liquids

), dichromate (Cr2O7

intermolecular structuring. Besides, it was observed that the size of structural heterogeneities

Ionic liquids are constituted exclusively by ions and hence they experience a strong interionic interaction that yield a long-lived association of ions [40]. The nature and types of cation-anion interactions and intermolecular forces in bulk ionic liquids affect their physical and chemical properties and how they interact with other solutes [41]. The examples of conformational heterogeneity of cations and anions are the coexisting trans-trans and trans-gauche conformations of n-butyl chain in 1-butyl-3-methylimidazolium cation and bisimide [Tf2N] anion conformational which forms trans- and cis conformers and this seems to be crucial in lowering their ionic liquid melting point [42]. Ionic liquids which comprise a low symmetry cation possess a low melting point than the one with a higher symmetry due to weak intermolecular

Generally, ionic liquids (ILs) are denser than water. The density (ρ) of ionic liquids (ILs) decreases with the increase in organic cation bulkiness and anion selection affects ILs' density. Normally, the density of ILs varies in the range of 1.05–1.36 g/cm3 at ambient temperature [43, 44]. In terms of ILs' thermal stability, a research conducted had observed that thermal stability was dependent on both the cation and the anion of ILs. Ngo et al. [45] reported that imidazolium-based cations exhibited a higher thermal stability than tetraalkylammonium cations based on thermal gravimetric analysis (TGA) and differential scanning calorimetry

) increase the electrostatic interaction between cation and anion and enhance

2−

), carbonate (CO3

2−

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Long-range coulomb interaction may play a major role in ionic liquids which are composed solely of ions by creating the structure and dynamics that are unique to ionic liquids without being associated with molecular liquids [36]. From macroscopic point of view, ionic liquids can be considered as a continuum system characterized by their macroscopic constants such as boiling point, vapor pressure, density, and surface tension. However, from microscopic point of view, they are a discontinuum system consisting of individual, mutually interacting molecules characterized by molecular properties such as dipole moment, electronic polarizability, hydrogen-bond donor (HBD), and hydrogen bond acceptor (HBA) capability, electron pair donor (EPD) and electron pair acceptor (EPA). The types and degrees of these interactions control and determine the macroscopic properties of ionic liquids and their possibilities for various applications. Specifically, the ionic liquids' structure exhibits a unique spatial heterogeneity due to their inherent polar/nonpolar phase separation. The underlying reason for the microphase segregation resulted from the interplay between electrostatic interaction (between polar imidazolium ring and anion) and van der Waals interaction with the nonpolar alkyl tails of the cation [37]. In fact, charge-charge distribution and anion size affect the nanostructural segregation of ionic liquids. As the ions' size increases, the charge becomes more delocalized and the cation-anion interaction is reduced resulting in less charge ordering and nanostructural segregation [38]. On the other hand, divalent anions such as sulfate (SO4

Imidazolium-Based Ionic Liquid Binary Solvent System as an Extraction Medium in Enhancing the Rotenone Yield...

2− ),

), and oxalate

Figure 1. Some typical cations and anions.

#### 2.2. Imidazolium-based ionic liquid characterization

Among all ionic liquids, the ionic liquid based on imidazolium cation is widely used and studied due to the stability of the imidazolium ring and its excellent liquescency [31] which is resulted from its electronic structure of the aromatic cation. With delocalized 3-centre-4-electron configuration across the N1-C2-N3 moiety, a double bond between C4 and C5 at the opposite side of the ring, and a weak delocalization in the central region (Figure 2) [32], the hydrogen atoms C2-H, C4-H and C5-H carry almost the same charge but carbon C2 is positively charged owing to the electron deficit in the C=N bond, whereas C4 and C5 are practically neutral. The resulting acidic proton or hydrogen on the C2 carbon is the key for understanding the properties of the ionic liquids (ILs) and it is presented that the hydrogen on the C2 carbon (C2-H) binds specifically with solute molecules [33, 34] or its counter ion [35] as a good hydrogen bond donor.

Figure 2. Electronic structure of 1,3-dialkylimidazolium cation.

#### 2.3. Structural organization of imidazolium-based ionic liquids

unfavorable properties and they are also strongly hygroscopic [26]. The most preferred anions are the ones which are more complex, perfluorated anions such as bis(trifluoromethane sulfonyl) amide or trifluoromethanesulfonate or halogen-free ions such as dicyandiamide, tosylate, or n-alkyl sulfates (Figure 1) [30]. The environment constituted by ionic liquids is completely different from that of polar and nonpolar molecular solvents. In addition to the existing interactions in conventional organic solvents such as hydrogen bonding, dipole-dipole, and van der Waals interactions, ionic liquids have strong electro-

Among all ionic liquids, the ionic liquid based on imidazolium cation is widely used and studied due to the stability of the imidazolium ring and its excellent liquescency [31] which is resulted from its electronic structure of the aromatic cation. With delocalized 3-centre-4-electron configuration across the N1-C2-N3 moiety, a double bond between C4 and C5 at the opposite side of the ring, and a weak delocalization in the central region (Figure 2) [32], the hydrogen atoms C2-H, C4-H and C5-H carry almost the same charge but carbon C2 is positively charged owing to the electron deficit in the C=N bond, whereas C4 and C5 are practically neutral. The resulting acidic proton or hydrogen on the C2 carbon is the key for understanding the properties of the ionic liquids (ILs) and it is presented that the hydrogen on the C2 carbon (C2-H) binds specifically with solute molecules [33, 34] or its counter ion [35] as a good

static interactions.

498 Progress and Developments in Ionic Liquids

hydrogen bond donor.

2.2. Imidazolium-based ionic liquid characterization

Figure 2. Electronic structure of 1,3-dialkylimidazolium cation.

Figure 1. Some typical cations and anions.

Long-range coulomb interaction may play a major role in ionic liquids which are composed solely of ions by creating the structure and dynamics that are unique to ionic liquids without being associated with molecular liquids [36]. From macroscopic point of view, ionic liquids can be considered as a continuum system characterized by their macroscopic constants such as boiling point, vapor pressure, density, and surface tension. However, from microscopic point of view, they are a discontinuum system consisting of individual, mutually interacting molecules characterized by molecular properties such as dipole moment, electronic polarizability, hydrogen-bond donor (HBD), and hydrogen bond acceptor (HBA) capability, electron pair donor (EPD) and electron pair acceptor (EPA). The types and degrees of these interactions control and determine the macroscopic properties of ionic liquids and their possibilities for various applications. Specifically, the ionic liquids' structure exhibits a unique spatial heterogeneity due to their inherent polar/nonpolar phase separation. The underlying reason for the microphase segregation resulted from the interplay between electrostatic interaction (between polar imidazolium ring and anion) and van der Waals interaction with the nonpolar alkyl tails of the cation [37]. In fact, charge-charge distribution and anion size affect the nanostructural segregation of ionic liquids. As the ions' size increases, the charge becomes more delocalized and the cation-anion interaction is reduced resulting in less charge ordering and nanostructural segregation [38]. On the other hand, divalent anions such as sulfate (SO4 2− ), thiosulfate (S2O3 2− ), chromate (CrO4 2− ), dichromate (Cr2O7 2− ), carbonate (CO3 2− ), and oxalate (C2O4 2− ) increase the electrostatic interaction between cation and anion and enhance intermolecular structuring. Besides, it was observed that the size of structural heterogeneities depended on alkyl chain length [37, 39].

#### 2.4. Physicochemical properties of imidazolium-based ionic liquids

Ionic liquids are constituted exclusively by ions and hence they experience a strong interionic interaction that yield a long-lived association of ions [40]. The nature and types of cation-anion interactions and intermolecular forces in bulk ionic liquids affect their physical and chemical properties and how they interact with other solutes [41]. The examples of conformational heterogeneity of cations and anions are the coexisting trans-trans and trans-gauche conformations of n-butyl chain in 1-butyl-3-methylimidazolium cation and bisimide [Tf2N] anion conformational which forms trans- and cis conformers and this seems to be crucial in lowering their ionic liquid melting point [42]. Ionic liquids which comprise a low symmetry cation possess a low melting point than the one with a higher symmetry due to weak intermolecular interactions and good distribution of charge in the cation.

Generally, ionic liquids (ILs) are denser than water. The density (ρ) of ionic liquids (ILs) decreases with the increase in organic cation bulkiness and anion selection affects ILs' density. Normally, the density of ILs varies in the range of 1.05–1.36 g/cm3 at ambient temperature [43, 44]. In terms of ILs' thermal stability, a research conducted had observed that thermal stability was dependent on both the cation and the anion of ILs. Ngo et al. [45] reported that imidazolium-based cations exhibited a higher thermal stability than tetraalkylammonium cations based on thermal gravimetric analysis (TGA) and differential scanning calorimetry (DSC). In addition, imidazolium-based ILs have a thermal stability that increases in the following order: [Cl]<sup>−</sup> , [Br]<sup>−</sup> , [I]<sup>−</sup> <[BF4] − <[CF3SO3] − <[NTf2] − <[PF6] − and Ngo et al. [45] also reported that IL-based organic anions have a higher thermal stability than that of those based on inorganic anions.

ILs α ß π\* [OMIM][BF4] [49] 0.62 0.41 0.98 [OMIM][NTf2] 0.60 0.29 0.96 [HMIM][NTf2] [48] 0.65 0.26 0.97 [HMIM][Cl] [49] 0.48 0.94 1.02 [HMIM][Br][49] 0.45 0.74 1.09 [BMIM][BF4] [49] 0.77 0.39 1.04

Imidazolium-Based Ionic Liquid Binary Solvent System as an Extraction Medium in Enhancing the Rotenone Yield...

[BMIM][NTf2] 0.72 0.24 0.90 [47]

[BMIM][(C8)OSO3] [49] 0.69 0.79 0.89 [BMIM][PF6] 0.68 0.21 1.02 [49]

[BMIM][SBF6] 0.62 0.15 1.04 [BMIM][OTf] 0.62 0.49 1.00 [BMIM][MeCO2] 0.57 1.18 0.89 [50]

[BMIM][N(CN)2] 0.54 0.60 1.05 [BMIM][MeSO4] 0.53 0.66 1.06 [BMIM][PrCO2] [51] 0.51 1.23 0.92 [BMIM][(HO)C1CO2] [51] 0.44 0.87 1.12 [BMIM][EtCO2] [51] 0.48 1.16 0.94 [BMIM][Me2PO4] 0.45 1.13 0.98 [BMIM][MeSO3] 0.44 0.77 1.02 [BMIM]O2CCH2CH(OH)CO2] [51] 0.41 1.00 1.10 [BMIM][O2CCH2CH2CO2] [51] 0.39 1.08 1.09 [BMIM][O2CCHCHCO2] [51] 0.34 1.02 1.11 [BM2IM][BF4] 0.39 0.36 1.08 [BM2IM][NTf2] 0.38 0.26 1.02 [EMIM][NTf2] 0.71 0.23 0.98 [48]

[EMIM][(C6)SO4] 0.65 0.71 0.98

0.63 0.37 1.05

0.65 0.25 1.02 0.63 0.24 1.02 [49] 0.63 0.19 1.04

0.43 1.05 1.04 [51] 0.48 1.20 0.96

0.42 0.10 1.02 [47] 0.63 0.23 1.00

0.64 0.25 0.97 [48] 0.61 0.23 0.99

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The viscosity of ILs also plays a crucial role in the extraction and separation of bioactive compounds from plants. Their viscosity is affected by a range of intermolecular interactions such as electrostatic, van der Waals forces, and hydrogen bonding interaction [11]. The increase in temperature and asymmetry of IL anions leads to the decrease in their viscosity. For instance, using imidazolium cationic species, the viscosity can be increased by increasing the substituted alkyl chain length or branching due to higher van der Waals interactions between the ions themselves [12–14]. Ionic liquids also have their own polarity values according to Kamlet-Taft parameters such as dipolarity or polarizability (π\*), hydrogen bond basicity (ß), and hydrogen bond acidity (α) [46]. Although measurement has not been made for a large number of ionic liquids, the general trend suggests that the π\* values for ionic liquids are higher than that of alkyl chain alcohol, while the α values are less than those of water and alkyl chain alcohols and also the magnitude of ß is determined by the anion of ionic liquids. Tables 1 and 2 display the summary of some physicochemical properties of ILs and Kamlet-Taft parameters for some ionic liquids.


Table 1. Summary of some physicochemical properties of ILs at 25°C.

Imidazolium-Based Ionic Liquid Binary Solvent System as an Extraction Medium in Enhancing the Rotenone Yield... http://dx.doi.org/10.5772/66777 501

(DSC). In addition, imidazolium-based ILs have a thermal stability that increases in the fol-

that IL-based organic anions have a higher thermal stability than that of those based on

The viscosity of ILs also plays a crucial role in the extraction and separation of bioactive compounds from plants. Their viscosity is affected by a range of intermolecular interactions such as electrostatic, van der Waals forces, and hydrogen bonding interaction [11]. The increase in temperature and asymmetry of IL anions leads to the decrease in their viscosity. For instance, using imidazolium cationic species, the viscosity can be increased by increasing the substituted alkyl chain length or branching due to higher van der Waals interactions between the ions themselves [12–14]. Ionic liquids also have their own polarity values according to Kamlet-Taft parameters such as dipolarity or polarizability (π\*), hydrogen bond basicity (ß), and hydrogen bond acidity (α) [46]. Although measurement has not been made for a large number of ionic liquids, the general trend suggests that the π\* values for ionic liquids are higher than that of alkyl chain alcohol, while the α values are less than those of water and alkyl chain alcohols and also the magnitude of ß is determined by the anion of ionic liquids. Tables 1 and 2 display the summary of some physicochemical properties of ILs and Kamlet-

Cation Anion Density (g/ml) Viscosity (cP)

<sup>−</sup> 1.248 66

<sup>−</sup> 1.373 450

<sup>−</sup> 1.208 233

<sup>−</sup> 1.373 400 [Br]<sup>−</sup> 1.134 Solid [Cl]<sup>−</sup> 1.12 Solid

<sup>−</sup> 1.29 90

<sup>−</sup> 1.404 48

<sup>−</sup> 1.213 321

<sup>−</sup> 1.075 211

<sup>−</sup> 1.304 800

<sup>−</sup> 1.11 440 [Cl]<sup>−</sup> 1 16,000

<sup>−</sup> 1.44 39


<sup>+</sup> [HCOO]<sup>−</sup> 0.99 11.5

[(CF3SO3)2N]<sup>−</sup> 1.42 52

and Ngo et al. [45] also reported

− <[NTf2] − <[PF6] −

<[CF3SO3]

lowing order: [Cl]<sup>−</sup>

500 Progress and Developments in Ionic Liquids

inorganic anions.

ILs

, [Br]<sup>−</sup>

Taft parameters for some ionic liquids.

[PF6]

[PF6]

[CF3SO3]

[NTf2]

[PF6]

Table 1. Summary of some physicochemical properties of ILs at 25°C.

[EMIM]+ [BF4]

[BMIM]<sup>+</sup> [BF4]

[AMIM]<sup>+</sup> [BF4]

[HMIM]<sup>+</sup> [BF4]

[OMIM]<sup>+</sup> [BF4]

[MPPyr]<sup>+</sup> [NTf2]

[BMPyrrol]<sup>+</sup> [NTf2]

[C2H5NH3]

, [I]<sup>−</sup>

<[BF4] −



hours. Once dried, the roots were ground into smaller particles of the size of approximately 0.86 ± 0.20 mm (Figure 3b). The selected sieved, ground samples were weighed prior to the

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The binary solvent system comprises five selected ILs which are listed as follows: (1) 1-butyl-3-methylimidazolium chloride, [BMIM] Cl; (2) 1-butyl-3-methylimidazolium acetate, [BMIM] OAc; (3) 1-butyl-3-methylimidazolium bis(trifluorosulfonyl)imide, [BMIM] NTf2; (4) 1-butyl-3-methylimidazolium trifluoromethanesulfonate, [BMIM] OTf; and (5) 1-butyl-1 methylpyrrolidinium chloride, [BMPy] Cl. The binary solvent systems were prepared by adding 2 ml of respective ILs into a round bottom flask (with stopper) containing 18 ml of organic solvent (acetone) with the ratio of 1: 9. To avoid any moisture absorption due to the hygroscopic properties of some ILs, the ILs' collection was carried out in the glove box. The mixtures were stirred by using a magnetic stirrer for 5–6 hours to homogenize the combined solvents. The ratio of 1:9 was based on the exploratory experiment results (data not shown). The mixing of the ILs in acetone was considered homogeneous if no apparent residue

The extraction process was conducted at room temperature (28 ± 2°C) by using a combination of five different types of ILs and acetone with a mixing ratio of 1:9. The optimized parameters were utilized in accordance to protocols of Zubairi et al. [52, 53] as presented in Table 3. The extraction process was carried out by soaking 0.50 g of dried roots in 10 ml of the solvent systems for 24 hours with the solvent-to-solid ratio of 10 ml/g (n = 3). The liquid crude extract was collected twice at the 14th hour and 24th hour prior to the reversed-phase high-performance liquid chromatography (RP-HPLC) and thin-layer chromatography (TLC)

normal soaking extraction process (NSE).

3.2. Preparation of binary solvent system

Figure 3. (a) Derris elliptica roots and (b) ground fine roots.

appeared in the flask.

analysis.

3.3. Normal soaking extraction (NSE)

Table 2. Kamlet-Taft parameters for some imidazolium-based ionic liquids.

#### 2.5. Ionic liquid binary solvent system as phytochemical extractant

The usage of ILs as plant bioactive constituents' extractants has a great impact and potential as they alleviate the environmental pollution and improve the selectivity and extraction yields of interesting compounds in the sample pre-treatment process compared to conventional organic solvents. However, ILs have a relatively high polarity due to their charged and asymmetric structures [15], which cause them not to have a good affinity with weak-polar compounds and thus this gives rise to a reduction in the distribution of weak-polar compounds in the IL phase. Although a longer alkyl chain of ILs has a lower polarity, their viscosity is large in accordance with the strong electrostatic and hydrogen bonding interaction between the ions [16, 17]. This drawback impairs the mixing and transferring properties in the extraction process by influencing the dissolution of the compounds in ILs. In exchange, the mixture of ILs and organic molecular solvents as an extraction medium could be an effective approach to solve their flaws. Besides creating a wide-adjusted range of solvent polarity, hydrophobicity, hydrogenbond acidity, and basicity [18, 19], the addition of miscible molecular solvents as cosolvents helps to break the microscopic hydrogen-bond network and the aggregation of ILs, which significantly reduces the viscosity of ILs and improves the mixing and transferring process in their mediated extraction [20, 21].

### 3. Materials and methods

#### 3.1. Sample collection and preparation

Derris elliptica roots were first collected from Ladang 2, Faculty of Agriculture, Universiti Putra Malaysia, UPM, Malaysia. The collected roots (Figure 3a) were cleaned and cut into smaller parts prior to rapid drying. The cleaned parts of the roots were placed in the freezer to maintain their freshness and dried using a vacuum oven at the temperature of 28 ± 2°C for 24 hours. Once dried, the roots were ground into smaller particles of the size of approximately 0.86 ± 0.20 mm (Figure 3b). The selected sieved, ground samples were weighed prior to the normal soaking extraction process (NSE).

Figure 3. (a) Derris elliptica roots and (b) ground fine roots.

#### 3.2. Preparation of binary solvent system

2.5. Ionic liquid binary solvent system as phytochemical extractant

Table 2. Kamlet-Taft parameters for some imidazolium-based ionic liquids.

their mediated extraction [20, 21].

[(HO)3

[(HO)3

[(HO)<sup>3</sup>

[(HO)3

[(HO)<sup>3</sup>

[(HO)<sup>3</sup>

[(HO)3

[(HO)<sup>3</sup>

(HO)2

502 Progress and Developments in Ionic Liquids

(HO)2

(HO)2

(HO)2

(HO)2

3. Materials and methods

3.1. Sample collection and preparation

The usage of ILs as plant bioactive constituents' extractants has a great impact and potential as they alleviate the environmental pollution and improve the selectivity and extraction yields of interesting compounds in the sample pre-treatment process compared to conventional organic solvents. However, ILs have a relatively high polarity due to their charged and asymmetric structures [15], which cause them not to have a good affinity with weak-polar compounds and thus this gives rise to a reduction in the distribution of weak-polar compounds in the IL phase. Although a longer alkyl chain of ILs has a lower polarity, their viscosity is large in accordance with the strong electrostatic and hydrogen bonding interaction between the ions [16, 17]. This drawback impairs the mixing and transferring properties in the extraction process by influencing the dissolution of the compounds in ILs. In exchange, the mixture of ILs and organic molecular solvents as an extraction medium could be an effective approach to solve their flaws. Besides creating a wide-adjusted range of solvent polarity, hydrophobicity, hydrogenbond acidity, and basicity [18, 19], the addition of miscible molecular solvents as cosolvents helps to break the microscopic hydrogen-bond network and the aggregation of ILs, which significantly reduces the viscosity of ILs and improves the mixing and transferring process in

ILs α ß π\* [EMIM][(C8)SO4] 0.65 0.77 0.93 [EMIM][MeCO2] 0.57 1.06 0.97

C3C1IM][Cl] 1.12 0.99 0.82

C3C1IM][N(CN)2] 0.87 0.47 1.17

C3C1IM][NTf2] 1.20 0.13 1.15

C3C1C1IM][N(CN)2] 0.87 0.47 1.17

C3C1C1IM][NTf2] 0.93 0.11 1.14

C3C1IM][MeCO2] 0.51 0.99 1.08

C2C1IM][NTf2] 1.14 0.28 1.08

C2C1IM][MeCO2] 0.53 0.90 1.04

Derris elliptica roots were first collected from Ladang 2, Faculty of Agriculture, Universiti Putra Malaysia, UPM, Malaysia. The collected roots (Figure 3a) were cleaned and cut into smaller parts prior to rapid drying. The cleaned parts of the roots were placed in the freezer to maintain their freshness and dried using a vacuum oven at the temperature of 28 ± 2°C for 24 The binary solvent system comprises five selected ILs which are listed as follows: (1) 1-butyl-3-methylimidazolium chloride, [BMIM] Cl; (2) 1-butyl-3-methylimidazolium acetate, [BMIM] OAc; (3) 1-butyl-3-methylimidazolium bis(trifluorosulfonyl)imide, [BMIM] NTf2; (4) 1-butyl-3-methylimidazolium trifluoromethanesulfonate, [BMIM] OTf; and (5) 1-butyl-1 methylpyrrolidinium chloride, [BMPy] Cl. The binary solvent systems were prepared by adding 2 ml of respective ILs into a round bottom flask (with stopper) containing 18 ml of organic solvent (acetone) with the ratio of 1: 9. To avoid any moisture absorption due to the hygroscopic properties of some ILs, the ILs' collection was carried out in the glove box. The mixtures were stirred by using a magnetic stirrer for 5–6 hours to homogenize the combined solvents. The ratio of 1:9 was based on the exploratory experiment results (data not shown). The mixing of the ILs in acetone was considered homogeneous if no apparent residue appeared in the flask.

#### 3.3. Normal soaking extraction (NSE)

The extraction process was conducted at room temperature (28 ± 2°C) by using a combination of five different types of ILs and acetone with a mixing ratio of 1:9. The optimized parameters were utilized in accordance to protocols of Zubairi et al. [52, 53] as presented in Table 3. The extraction process was carried out by soaking 0.50 g of dried roots in 10 ml of the solvent systems for 24 hours with the solvent-to-solid ratio of 10 ml/g (n = 3). The liquid crude extract was collected twice at the 14th hour and 24th hour prior to the reversed-phase high-performance liquid chromatography (RP-HPLC) and thin-layer chromatography (TLC) analysis.


and 150 mm in length. The physical parameters involved in the RP-HPLC are as follows: (1) 0.7 ml/min flow rate; (2) injection volume of 20 µl; (3) mobile phase of acetonitrile and deionized water with the ratio of 60:40 and (4) photodiode array detector (PDA) wave-

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To enhance the rotenone extraction capacity, several ionic liquids were selected preliminarily. However, there was a drawback with regard to their solubility in organic solvents. The solubility of the selected ILs in acetone was observed by conducting a normal mixing with the ratio of 1:9. An exploratory experiment implemented revealed that the higher the amount of ILs used in the mixture, the higher the tendency of the ILs to produce undissolved solid residue (data not shown). Of the five selected ILs, only 1-butyl-3-methylimidazolium acetate, [BMIM] OAc, 1-butyl-3-methylimidazolium bis(trifluorosulfonyl)imide, [BMIM] NTf2, and 1-butyl-3-methylimidazolium trifluoromethanesulfonate, [BMIM] OTf could be easily homogenized. In contrast, 1-butyl-3 methylimidazolium chloride, [BMIM] Cl and 1-butyl-1-pyrrolidinlium chloride, [BMPy] Cl required a longer time to be homogenized (with the aid of heating at 80°C prior to mixing with acetone) which this could be possibly due to their physical properties (solid form at ambient temperature) and a specific range of organic solvent polarity, so that the solubility of the ILs in the organic solvent could be achieved. Theoretically, ionic liquids are miscible with an organic solvent of a medium to a high dielectric constant («) and they become immiscible with a low dielectric constant («) [54, 55]. Thus, the solubility of all of the selected ILs was considered satisfactory as there was no apparent residue appeared in the flask due to the high dielectric constant of acetone

The qualitative analysis of thin-layer chromatography (TLC) was performed to identify the existence of rotenone in the extracts of all of the binary solvent systems used. Figure 4 displays the images of rotenone markers visualized under the UV light of 254 nm. All samples exhibited the presence of rotenone markers in the extracts. On the other hand, Table 4 shows the migration distance (cm) and retardation factor (Rf) of rotenone in a standard solution and liquid crude extracts. The results indicated that the rotenone's Rf value in a standard solution and all extracts (including control) were determined to be insignificantly different when compared to each other (p > 0.05). However, there were still a lot of impurities (unknown markers left behind rotenone) as presented in Figure 4. For that reason, a purification process of the liquid crude extracts via high-vacuum pressure liquid chromatography (VLC) is highly recommended as to increase the accuracy of

length of 294 nm.

4. Results and discussion

4.1. Solubility of ionic liquids (ILs) in acetone

that aided the solubility of both chemicals.

rotenone and its derivative compounds' identification [54, 56].

4.2. Qualitative analysis of rotenone

Table 3. Processing parameters used in the rotenone extraction process.

#### 3.4. Liquid crude extract collection

The liquid crude extracts were collected at the 14th hour and 24th hour and placed in the labeled vials. There were 18 samples in total of five different types of solvent system used in three replicates (n = 3). Acetone was used as a control solvent. Later, the collected samples were placed in a freezer (−18°C) to prevent any thermal degradation.

#### 3.5. Preparation of fine debris-free liquid crude extract

The collected liquid crude extracts were diluted using analytical grade acetonitrile, Sigma-Aldrich, 95% (v/v) with the dilution factor (DF) of 20. Then, the extracts were filtered by using polytetrafluoroethylene (PTFE—0.45 µm pore size) vacuum filtration to remove any fine debris. A 2-ml vial was used to store the extracts prior to the qualitative and quantitative analyses.

#### 3.6. Qualitative analysis using thin-layer chromatography

MERCK Silica gel 60 F254 TLC aluminum sheet was used as a stationary phase to observe the presence of rotenone in the liquid crude extracts (n = 3). The migrations of rotenone markers were compared with rotenone standard. The markers and their migrated distance were visualized and determined under UV light of 254 and 365 nm wavelengths, respectively. In the development chamber, chloroform and n-hexane were combined and utilized as a mobile phase system with the ratio of 70:30. The retardation factors (Rf) of each extract were calculated by using Eq. (1).

$$\text{Retardation factor} \left( R\_f \right) = \frac{\text{Migration distance of substance}}{\text{Migration distance of solvent front}} \tag{1}$$

#### 3.7. Quantitative analysis using reversed-phase high-performance liquid chromatography

Approximately 21.80 mg of rotenone standard Dr. Ehrenstorfer GmbH, 93.80% (w/w) was diluted with 50 ml of acetonitrile in a volumetric flask. The stock solution was filtered using Whatman™ filter paper no. 2 with 8 µm pore size. The quantitative analysis was completed by using Symmetry® C18 5 µl column, Waters with the internal diameter of 4.6 and 150 mm in length. The physical parameters involved in the RP-HPLC are as follows: (1) 0.7 ml/min flow rate; (2) injection volume of 20 µl; (3) mobile phase of acetonitrile and deionized water with the ratio of 60:40 and (4) photodiode array detector (PDA) wavelength of 294 nm.

### 4. Results and discussion

3.4. Liquid crude extract collection

504 Progress and Developments in Ionic Liquids

analyses.

by using Eq. (1).

placed in a freezer (−18°C) to prevent any thermal degradation.

Solvent-to-solid ratio (mg/ml) 10 Weight of raw material (g) 0.50 Raw material particle size (mm) 0.86 ± 0.20 Temperature (°C) 28 ± 2 ILs-to-acetone ratio 1:9 Extraction time (hour) 14

3.5. Preparation of fine debris-free liquid crude extract

Table 3. Processing parameters used in the rotenone extraction process.

3.6. Qualitative analysis using thin-layer chromatography

The liquid crude extracts were collected at the 14th hour and 24th hour and placed in the labeled vials. There were 18 samples in total of five different types of solvent system used in three replicates (n = 3). Acetone was used as a control solvent. Later, the collected samples were

The collected liquid crude extracts were diluted using analytical grade acetonitrile, Sigma-Aldrich, 95% (v/v) with the dilution factor (DF) of 20. Then, the extracts were filtered by using polytetrafluoroethylene (PTFE—0.45 µm pore size) vacuum filtration to remove any fine debris. A 2-ml vial was used to store the extracts prior to the qualitative and quantitative

MERCK Silica gel 60 F254 TLC aluminum sheet was used as a stationary phase to observe the presence of rotenone in the liquid crude extracts (n = 3). The migrations of rotenone markers were compared with rotenone standard. The markers and their migrated distance were visualized and determined under UV light of 254 and 365 nm wavelengths, respectively. In the development chamber, chloroform and n-hexane were combined and utilized as a mobile phase system with the ratio of 70:30. The retardation factors (Rf) of each extract were calculated

Retardation factor <sup>ð</sup>RfÞ ¼ Migration distance of substance

3.7. Quantitative analysis using reversed-phase high-performance liquid chromatography Approximately 21.80 mg of rotenone standard Dr. Ehrenstorfer GmbH, 93.80% (w/w) was diluted with 50 ml of acetonitrile in a volumetric flask. The stock solution was filtered using Whatman™ filter paper no. 2 with 8 µm pore size. The quantitative analysis was completed by using Symmetry® C18 5 µl column, Waters with the internal diameter of 4.6

Migration distance of solvent front (1)

#### 4.1. Solubility of ionic liquids (ILs) in acetone

To enhance the rotenone extraction capacity, several ionic liquids were selected preliminarily. However, there was a drawback with regard to their solubility in organic solvents. The solubility of the selected ILs in acetone was observed by conducting a normal mixing with the ratio of 1:9. An exploratory experiment implemented revealed that the higher the amount of ILs used in the mixture, the higher the tendency of the ILs to produce undissolved solid residue (data not shown). Of the five selected ILs, only 1-butyl-3-methylimidazolium acetate, [BMIM] OAc, 1-butyl-3-methylimidazolium bis(trifluorosulfonyl)imide, [BMIM] NTf2, and 1-butyl-3-methylimidazolium trifluoromethanesulfonate, [BMIM] OTf could be easily homogenized. In contrast, 1-butyl-3 methylimidazolium chloride, [BMIM] Cl and 1-butyl-1-pyrrolidinlium chloride, [BMPy] Cl required a longer time to be homogenized (with the aid of heating at 80°C prior to mixing with acetone) which this could be possibly due to their physical properties (solid form at ambient temperature) and a specific range of organic solvent polarity, so that the solubility of the ILs in the organic solvent could be achieved. Theoretically, ionic liquids are miscible with an organic solvent of a medium to a high dielectric constant («) and they become immiscible with a low dielectric constant («) [54, 55]. Thus, the solubility of all of the selected ILs was considered satisfactory as there was no apparent residue appeared in the flask due to the high dielectric constant of acetone that aided the solubility of both chemicals.

#### 4.2. Qualitative analysis of rotenone

The qualitative analysis of thin-layer chromatography (TLC) was performed to identify the existence of rotenone in the extracts of all of the binary solvent systems used. Figure 4 displays the images of rotenone markers visualized under the UV light of 254 nm. All samples exhibited the presence of rotenone markers in the extracts. On the other hand, Table 4 shows the migration distance (cm) and retardation factor (Rf) of rotenone in a standard solution and liquid crude extracts. The results indicated that the rotenone's Rf value in a standard solution and all extracts (including control) were determined to be insignificantly different when compared to each other (p > 0.05). However, there were still a lot of impurities (unknown markers left behind rotenone) as presented in Figure 4. For that reason, a purification process of the liquid crude extracts via high-vacuum pressure liquid chromatography (VLC) is highly recommended as to increase the accuracy of rotenone and its derivative compounds' identification [54, 56].

and 24th hour. The dependent variables were calculated based on the external standard method of RP-HPLC. The retention times of the rotenone in the standard solution (8.83 mins) and liquid crude extract (8.82 mins) are shown in Figures 7 and 8, respectively. Overall, it was observed that the binary solvent system of 1-butyl-3-methylimidazolium trifluoromethanesulfonate, [BMIM] OTf produced the highest rotenone concentration of 4.04 ± 0.34 and 4.19 ± 0.48 mg/ml as compared to the others ionic liquids and control solvent (acetone) (p < 0.05) at the 14th hour and 24th hour, respectively. The highest yield of rotenone ((2.69 ± 0.21% (w/w) and 2.03 ± 0.11% (w/w) in dried roots) was also determined in the 1-butyl-3-methylimidazolium trifluoromethanesulfonate, [BMIM] OTf solvent system at the 14th hour and 24th hour, respectively. However, both of the optimized processing parameters and the control extract (acetone) reported in the previous study resulted in only 2.44 ± 0.02% (w/w) [8] and 2.44 ± 0.09% (w/w) with the concentration of 3.65 ± 0.13 mg/ml respectively. The results were approximately 10.25% lower than that of the yield of rotenone extracted using a combination of [BMIM] OTf:

Imidazolium-Based Ionic Liquid Binary Solvent System as an Extraction Medium in Enhancing the Rotenone Yield...

This phenomenon can be explained from the perspective of the types of ILs. Of the five different ILs, four of them have the same cation but different anions ([BMIM] OTf, [BMIM] OAc, [BMIM] NTf2, [BMIM] Cl), and one with the same anion but a different cation ([BMPy] Cl). The increase of the rotenone yield is related to the anion and cation of ILs as ILs have different polarities depending on the anion and cation presence. ILs' polarity is referred to Kamlet-Taft parameters such as polarity (π\*), hydrogen bond basicity (ß) and hydrogen bond acidity (α) [46]. ILs' hydrogen bond basicity (ß) depends on anion, while hydrogen bond acidity (α) depends on the cation. [BMIM] OTf has lower hydrogen bond basicity compared to [BMIM] OAc and [BMIM] Cl, but higher than that of [BMIM]

The rotenone compound is an acidic isoflavonoid compound that consists of ketonic chemical groups (R−C(=O) −R) [22] which has the potential of interacting with intermediate-polar solvents. It can be easily dissolved in moderate-polar organic solvents (e.g., methanol, chloroform, and acetone) [23] and sparingly soluble in water [24]. These are the factors that lead to the increase in the rotenone yield extracted when [BMIM] OTf with mid polarities is used as the extraction medium. The abundant presence of anion [OTf]− helped to attract more hydroxyl groups of rotenone to form more hydrogen bonds [25, 26]. With respect to the impact of the ILS' cation, [BMIM] Cl and [BMPy] Cl on rotenone extraction, it was discovered that the rotenone yield extracted was high when the cation [BMIM]<sup>+</sup> was used. This was due to the presence of an acid proton in the imidazole ring [32], which had the potential to form the hydrogen bond with oxygen of the rotenone compounds. The significant increase in the rotenone content can be explained in relation to several aspects as follows: (1) the trend of functional groups of rotenone toward foreign charge and (2) the capacity of the IL in the vicinity of various charges. For that reason, any combination of ionic liquids and middlepolar organic solvents does not only optimize the absorption of solute into the solvent due to the low viscosity but is also even able to increase the opportunities for extracting higher

) and has hydrogen bond acidity higher than that of

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507

acetone (p < 0.05).

NTf2 ([OAc]<sup>−</sup>

rotenone compounds.

[BMPy] Cl.

>[Cl]<sup>−</sup>

>[OTf]<sup>−</sup>

>[NTf2] −

Figure 4. Visualization of rotenone migration markers of five different types of binary solvent systems used at the 14th hour of extraction. The markers were visualized on the alumina-based TLC plate under the UV light of 254 nm. (a) Acetone; (b) [BMIM] Cl; (c) [BMIM] Oac; (d) [BMIM] NTF2; (e) [BMIM] OTF and (f) [BMP] Cl. The dark circled markers of STD and R (R1, R2, and R3) represent the rotenone standard and replication of each of the binary solvent systems used.


<sup>a</sup> Rotenone standard was prepared in acetone + ILs.

Table 4. Qualitative analysis of rotenone via TLC on varied binary solvent systems.

#### 4.3. Quantitative analysis of rotenone content

Table 5, Figures 5 and 6 show the concentration (mg/ml) and yield of rotenone, % (w/w), respectively, from five different types of binary solvent systems extracted at the 14th hour and 24th hour. The dependent variables were calculated based on the external standard method of RP-HPLC. The retention times of the rotenone in the standard solution (8.83 mins) and liquid crude extract (8.82 mins) are shown in Figures 7 and 8, respectively. Overall, it was observed that the binary solvent system of 1-butyl-3-methylimidazolium trifluoromethanesulfonate, [BMIM] OTf produced the highest rotenone concentration of 4.04 ± 0.34 and 4.19 ± 0.48 mg/ml as compared to the others ionic liquids and control solvent (acetone) (p < 0.05) at the 14th hour and 24th hour, respectively. The highest yield of rotenone ((2.69 ± 0.21% (w/w) and 2.03 ± 0.11% (w/w) in dried roots) was also determined in the 1-butyl-3-methylimidazolium trifluoromethanesulfonate, [BMIM] OTf solvent system at the 14th hour and 24th hour, respectively. However, both of the optimized processing parameters and the control extract (acetone) reported in the previous study resulted in only 2.44 ± 0.02% (w/w) [8] and 2.44 ± 0.09% (w/w) with the concentration of 3.65 ± 0.13 mg/ml respectively. The results were approximately 10.25% lower than that of the yield of rotenone extracted using a combination of [BMIM] OTf: acetone (p < 0.05).

This phenomenon can be explained from the perspective of the types of ILs. Of the five different ILs, four of them have the same cation but different anions ([BMIM] OTf, [BMIM] OAc, [BMIM] NTf2, [BMIM] Cl), and one with the same anion but a different cation ([BMPy] Cl). The increase of the rotenone yield is related to the anion and cation of ILs as ILs have different polarities depending on the anion and cation presence. ILs' polarity is referred to Kamlet-Taft parameters such as polarity (π\*), hydrogen bond basicity (ß) and hydrogen bond acidity (α) [46]. ILs' hydrogen bond basicity (ß) depends on anion, while hydrogen bond acidity (α) depends on the cation. [BMIM] OTf has lower hydrogen bond basicity compared to [BMIM] OAc and [BMIM] Cl, but higher than that of [BMIM] NTf2 ([OAc]<sup>−</sup> >[Cl]<sup>−</sup> >[OTf]<sup>−</sup> >[NTf2] − ) and has hydrogen bond acidity higher than that of [BMPy] Cl.

The rotenone compound is an acidic isoflavonoid compound that consists of ketonic chemical groups (R−C(=O) −R) [22] which has the potential of interacting with intermediate-polar solvents. It can be easily dissolved in moderate-polar organic solvents (e.g., methanol, chloroform, and acetone) [23] and sparingly soluble in water [24]. These are the factors that lead to the increase in the rotenone yield extracted when [BMIM] OTf with mid polarities is used as the extraction medium. The abundant presence of anion [OTf]− helped to attract more hydroxyl groups of rotenone to form more hydrogen bonds [25, 26]. With respect to the impact of the ILS' cation, [BMIM] Cl and [BMPy] Cl on rotenone extraction, it was discovered that the rotenone yield extracted was high when the cation [BMIM]<sup>+</sup> was used. This was due to the presence of an acid proton in the imidazole ring [32], which had the potential to form the hydrogen bond with oxygen of the rotenone compounds. The significant increase in the rotenone content can be explained in relation to several aspects as follows: (1) the trend of functional groups of rotenone toward foreign charge and (2) the capacity of the IL in the vicinity of various charges. For that reason, any combination of ionic liquids and middlepolar organic solvents does not only optimize the absorption of solute into the solvent due to the low viscosity but is also even able to increase the opportunities for extracting higher rotenone compounds.

4.3. Quantitative analysis of rotenone content

Table 4. Qualitative analysis of rotenone via TLC on varied binary solvent systems.

<sup>a</sup> Rotenone standard was prepared in acetone + ILs.

Binary solvent system

506 Progress and Developments in Ionic Liquids

Table 5, Figures 5 and 6 show the concentration (mg/ml) and yield of rotenone, % (w/w), respectively, from five different types of binary solvent systems extracted at the 14th hour

Figure 4. Visualization of rotenone migration markers of five different types of binary solvent systems used at the 14th hour of extraction. The markers were visualized on the alumina-based TLC plate under the UV light of 254 nm. (a) Acetone; (b) [BMIM] Cl; (c) [BMIM] Oac; (d) [BMIM] NTF2; (e) [BMIM] OTF and (f) [BMP] Cl. The dark circled markers of STD and R (R1, R2, and R3) represent the rotenone standard and replication of each of the binary solvent systems used.

Rotenone standard<sup>a</sup> 1.80 ± 0.05 2.60 ± 0.03 0.45 ± 0.04 0.65 ± 0.03 Control (acetone) 2.03 ± 0.06 2.60 ± 0.01 0.51 ± 0.02 0.65 ± 0.04 [BMIM] Cl + acetone 1.93 ± 0.06 2.20 ± 0.02 0.49 ± 0.01 0.55 ± 0.01 [BMIM] Oac + acetone 1.53 ± 0.06 1.40 ± 0.04 0.39 ± 0.01 0.35 ± 0.01 [BMIM] NTF2 + acetone 3.20 ± 0.01 2.17 ± 0.06 0.80 ± 0.01 0.54 ± 0.01 [BMIM] OTF + acetone 3.20 ± 0.08 1.70 ± 0.07 0.80 ± 0.08 0.43 ± 0.06 [BMP] Cl + acetone 3.10 ± 0.00 2.93 ± 0.06 0.78 ± 0.00 0.74 ± 0.01

Rotenone migration distance, Ds (cm) Retardation factor (Rf)

14 hour 24 hour 14 hour 24 hour

The previous study also revealed that the solubility of flavonoids and their derivatives could be increased by using ILs, as the components were greatly an anion-dependent [25]. The anionic potency of both organic solvent and ILs in extracting a large amount of bioactive compounds (e.g., rotenone) and moving into the solvent systems was significantly undeniable as both chemicals facilitated the extraction process via salvation power and multiple interactions (e.g., hydrogen bonding, polarity, ionic charge-to-charge, and π-π, π-n) with the analysis [26, 27].


Table 5. Rotenone quantitative analysis using RP-HPLC using different binary solvent systems at the 14th hour and 24th hour of extraction time.

Figure 6. Yield of rotenone, % (w/w) in dried roots with respect to five different types of binary solvent systems. \*p < 0.05 —[BMIMI] OTf : acetone was the best binary solvent system to procure the highest rotenone content in dried roots (n = 3).

Imidazolium-Based Ionic Liquid Binary Solvent System as an Extraction Medium in Enhancing the Rotenone Yield...

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Figure 7. Chromatogram of rotenone standard.

Figure 5. Concentration of rotenone (mg/ml) with respect to five different types of binary solvent systems. <sup>Ψ</sup>p < 0.05— [BMIMI] OTf: acetone was the best binary solvent system to procure the highest rotenone concentration (mg/ml) (n = 3). \*p < 0.05—[BMIM] NTf2: acetone and acetone extract produced a high concentration as compared to the [BMIM] OAc, [BMPy] Cl and [BMIM] Cl.

Imidazolium-Based Ionic Liquid Binary Solvent System as an Extraction Medium in Enhancing the Rotenone Yield... http://dx.doi.org/10.5772/66777 509

Figure 6. Yield of rotenone, % (w/w) in dried roots with respect to five different types of binary solvent systems. \*p < 0.05 —[BMIMI] OTf : acetone was the best binary solvent system to procure the highest rotenone content in dried roots (n = 3).

Figure 7. Chromatogram of rotenone standard.

The previous study also revealed that the solubility of flavonoids and their derivatives could be increased by using ILs, as the components were greatly an anion-dependent [25]. The anionic potency of both organic solvent and ILs in extracting a large amount of bioactive compounds (e.g., rotenone) and moving into the solvent systems was significantly undeniable as both chemicals facilitated the extraction process via salvation power and multiple interactions (e.g., hydrogen bonding, polarity, ionic charge-to-charge, and π-π, π-n) with the analysis

Concentration (mg/ml) (±SD) Yield (%, w/w) (±SD)

Acetone (control) 3.65 ± 0.13 3.50 ± 0.02 2.44 ± 0.09 1.73 ± 0.09 [BMIM] Cl:Acetone 2.73 ± 0.00 2.57 ± 0.02 2.00 ± 0.06 1.55 ± 0.01 [BMIM] OAc:Acetone 2.51 ± 0.02 2.69 ± 0.14 1.60 ± 0.01 1.18 ± 0.06 [BMIM] NTf2:Acetone 3.70 ± 0.11 3.80 ± 0.03 2.42 ± 0.14 2.07 ± 0..07 [BMIM] OTf:Acetone 4.04 ± 0.34 4.19 ± 0.48 2.69 ± 0.21 2.03 ± 0.11 [BMPy] Cl:Acetone 2.66 ± 0.20 2.37 ± 0.26 1.78 ± 0.08 1.27 ± 0.16

Table 5. Rotenone quantitative analysis using RP-HPLC using different binary solvent systems at the 14th hour and 24th

Figure 5. Concentration of rotenone (mg/ml) with respect to five different types of binary solvent systems. <sup>Ψ</sup>p < 0.05— [BMIMI] OTf: acetone was the best binary solvent system to procure the highest rotenone concentration (mg/ml) (n = 3). \*p < 0.05—[BMIM] NTf2: acetone and acetone extract produced a high concentration as compared to the [BMIM] OAc,

14 hour 24 hour 14 hour 24 hour

[26, 27].

Solvent system type

508 Progress and Developments in Ionic Liquids

hour of extraction time.

[BMPy] Cl and [BMIM] Cl.

Author details

References

408.

Zetty Shafiqa Othman, Nur Hasyareeda Hassan and Saiful Irwan Zubairi\*

Universiti Kebangsaan Malaysia (UKM), Bangi, Selangor, Malaysia

Faculty of Science and Technology, School of Chemical Sciences and Food Technology,

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Figure 8. Chromatogram of [BMIM] OTf: acetone binary solvent system at the 14th hour.

#### 5. Conclusion

In conclusion, the best ionic liquid to assist the organic solvent (acetone) extraction system was 1-butyl-3-methylimidazolium trifluoromethanesulfonate, [BMIM] OTf. The selected binary solvent system had contributed to the highest rotenone content of 2.69 ± 0.21% (w/w) with a concentration of 4.04 ± 0.34 mg/ml at the 14th hour (the time of the exhaustive extraction as reported in the previous study). The rotenone content was 10.25% higher than the optimized parameter of the acetone extract (control) (p < 0.05). Therefore, the addition of certain ionic liquids to the organic solvent will potentially give rise to a significant increase in the amount of bioactive constituent in the phytochemical extraction process. Further study is required to optimize several processing parameters especially on the mixing ratio between the ILs and organic solvent in order to verify the increase in the rotenone content as the solubility problem between both chemicals, is relatively prominent.

#### Acknowledgements

We would like to thank the Ministry of Higher Education (MOHE), Malaysia for providing financial support to this project (FRGS/2/2013/TK04/UKM/03/1 and GGPM-2013-078).

### Author details

Zetty Shafiqa Othman, Nur Hasyareeda Hassan and Saiful Irwan Zubairi\*

\*Address all correspondence to: saiful-z@ukm.edu.my

Faculty of Science and Technology, School of Chemical Sciences and Food Technology, Universiti Kebangsaan Malaysia (UKM), Bangi, Selangor, Malaysia

### References

5. Conclusion

510 Progress and Developments in Ionic Liquids

relatively prominent.

Acknowledgements

2013-078).

In conclusion, the best ionic liquid to assist the organic solvent (acetone) extraction system was 1-butyl-3-methylimidazolium trifluoromethanesulfonate, [BMIM] OTf. The selected binary solvent system had contributed to the highest rotenone content of 2.69 ± 0.21% (w/w) with a concentration of 4.04 ± 0.34 mg/ml at the 14th hour (the time of the exhaustive extraction as reported in the previous study). The rotenone content was 10.25% higher than the optimized parameter of the acetone extract (control) (p < 0.05). Therefore, the addition of certain ionic liquids to the organic solvent will potentially give rise to a significant increase in the amount of bioactive constituent in the phytochemical extraction process. Further study is required to optimize several processing parameters especially on the mixing ratio between the ILs and organic solvent in order to verify the increase in the rotenone content as the solubility problem between both chemicals, is

Figure 8. Chromatogram of [BMIM] OTf: acetone binary solvent system at the 14th hour.

We would like to thank the Ministry of Higher Education (MOHE), Malaysia for providing financial support to this project (FRGS/2/2013/TK04/UKM/03/1 and GGPM-


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514 Progress and Developments in Ionic Liquids


**Chapter 22**

**Provisional chapter**

**Use of Ionic Liquids in Solid-Liquid Separation**

**Use of Ionic Liquids in Solid-Liquid Separation Processes**

This chapter reports the possible use of ionic liquids (ILs) in solid-liquid separation processes by their immobilization in suitable solid supports. This method presents some benefits such as economical one—due to the fact that a smaller quantity of ILs is used and the loss of ILs in the aqueous phase is avoided; and second the efficiency benefit because the advantages of the ILs are combined with the properties of the solid support, and this enhances the removal process of metal ions from aqueous solutions and could be successfully used in the removal processes of metal ions from aqueous solutions containing trace amounts. The type of solid supports used for the immobilization of different ILs, and the methods used for the immobilization were discussed. Also the adsorption efficiency of these ionic liquid immobilized solid supports in the removal process of different metal ions (Cr, Hg, Pt, Au, Pd, Cs, Sr, Tl, etc.) from aqueous solutions were presented. The inorganic materials present a higher efficiency to be used as solid supports for the immobilization of the ILs. It was observed that the physical method of impregnation, especially ultrasonication, has a positive effect on the adsorption

**Keywords:** ionic liquids, heavy metals, impregnation, encapsulation, adsorption

The huge quantities of waste, discharged from various industries and from human activities, and their negative effect on human health and the environment, have led to some stringent regulations. These have driven researchers to find and develop some new efficient methods for the removal and recovery of organic and mineral contaminants from discharged wastes.

Many separation techniques have been proposed especially for the treatment of wastewaters containing heavy metals, such as precipitation [1, 2] ion-exchange [3, 4] liquid-liquid

and reproduction in any medium, provided the original work is properly cited.

© 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution,

© 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Lavinia Lupa , Petru Negrea and Adriana Popa

Lavinia Lupa, Petru Negrea and Adriana Popa

Additional information is available at the end of the chapter

capacities of the materials obtained.

Additional information is available at the end of the chapter

**Processes**

http://dx.doi.org/10.5772/65890

**Abstract**

**1. Introduction**

### **Use of Ionic Liquids in Solid-Liquid Separation Processes Use of Ionic Liquids in Solid-Liquid Separation Processes**

Lavinia Lupa , Petru Negrea and Adriana Popa Lavinia Lupa, Petru Negrea and Adriana Popa Additional information is available at the end of the chapter

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/65890

#### **Abstract**

This chapter reports the possible use of ionic liquids (ILs) in solid-liquid separation processes by their immobilization in suitable solid supports. This method presents some benefits such as economical one—due to the fact that a smaller quantity of ILs is used and the loss of ILs in the aqueous phase is avoided; and second the efficiency benefit because the advantages of the ILs are combined with the properties of the solid support, and this enhances the removal process of metal ions from aqueous solutions and could be successfully used in the removal processes of metal ions from aqueous solutions containing trace amounts. The type of solid supports used for the immobilization of different ILs, and the methods used for the immobilization were discussed. Also the adsorption efficiency of these ionic liquid immobilized solid supports in the removal process of different metal ions (Cr, Hg, Pt, Au, Pd, Cs, Sr, Tl, etc.) from aqueous solutions were presented. The inorganic materials present a higher efficiency to be used as solid supports for the immobilization of the ILs. It was observed that the physical method of impregnation, especially ultrasonication, has a positive effect on the adsorption capacities of the materials obtained.

**Keywords:** ionic liquids, heavy metals, impregnation, encapsulation, adsorption

### **1. Introduction**

The huge quantities of waste, discharged from various industries and from human activities, and their negative effect on human health and the environment, have led to some stringent regulations. These have driven researchers to find and develop some new efficient methods for the removal and recovery of organic and mineral contaminants from discharged wastes.

Many separation techniques have been proposed especially for the treatment of wastewaters containing heavy metals, such as precipitation [1, 2] ion-exchange [3, 4] liquid-liquid

extraction [5, 6] and membrane processes [7]. These processes may encounter some technical, economical and environmental problems, which limit their application [8]. Even if the precipitation processes results in an efficiency of 90%, the disadvantage remains that precipitation agents are needed to reach the maximum admitted value for discharge. Thus, after treatment, huge amounts of highly contaminated sludge and filter backwash liquid remain [8–10]. Ion exchange, the most extensively used method is expensive for large-scale applications [9]. Membrane processes, especially reverse osmosis, also present economic disadvantages [8, 10].

erties are influenced by the used solid support, by the type of the immobilized ILs and by the

A number of different inorganic and organic solid supports have been tested for the immobilization of ILs during the last decade. The surface properties and the specific surface area are the two most relevant criteria used for the selection of a suitable solid support [25, 26]. Some researchers consider that the macroporous organic polymers are suitable to be used as solid supports due to their high surface area and good mechanical stability, faster kinetics for the removal of contaminant even from dilute solutions, ease of regeneration and high adsorption capacity. On the other hand, other researchers consider that inorganic types of solid support present some advantages over organic supports such as higher thermal and chemical stability, well-ordered periodic pore structure and controllable pore diameter [26]. Therefore, in this section, the discussion is about the most frequently used solid supports, the ones used for immobilization of ILs, which are reported in literature, and also the considerations that have

Polymer-supported reagents are widely employed in the separation of metal ions. Amberlite XAD resins were a kind of commercial resin frequently used as a solid support for the immobilization of various ILs [18, 20, 24, 25] with uniform pore size distribution, high surface area and good mechanical and chemical stability. Their surface properties differ in function depending on their structure [27, 28]: XAD-2, XAD-4 are hydrophobic, XAD-7 and XAD-8 are moderate hydrophilic and have a higher polarity compared with the other two. For these reasons, the group of Guibal, Gallardo and Navarro used XAD-7 resin as a solid support for the impregnation of tetraalkyl phosphonium ionic liquid—tetradecyl (trihexyl) phosphonium chloride—Cyphos IL-101 and used the obtained materials in the removal of cadmium, zinc and bismuth, respectively [24, 27, 28]. Yang et al. impregnated the Amberlite XAD-7 resin with Cyphos IL-104 in order for it to be used for Cr(VI) removal from aqueous solutions [20]. Cr(VI) removal was also studied by Saha et al. [29] through adsorption onto Amberlite XAD-7 impregnated with Aliquat 336. Kalidhasan et al. used an ion exchange resin as a solid support because this type of resin has a high degree of metal recovery and selectivity. Their study was made with Dowex 1 x 8, a styrene-divinylbenzene polymeric resin containing quaternary

mL. They impregnated Aliquat 336 ionic liquid onto Dowex [30]. Popa and co-workers used styrene-divinylbenzene as a solid support grafted with different pendant groups (triphenylphosphonium, izo-propylphosphonate and aminoethylaminomethyl) [31]. Styrene-divinylbenzene grafted with aminoethylaminomethyl groups was impregnated with three different ionic liquids: (trihexyl) tetradecyl phosphonium—Cyphos IL-101, 1-octyl-3-methyl imidazolium tetrafluoroborate—OmimBF4; and 1-butyl-3-methyl imidazolium hexafluorophosphate —(BmimPF6). This was done in order to determine the synergistic effect of the functional groups grafted on the polymer and the functional groups from the impregnated ionic liquid

and Sr2+ ions from aqueous solutions [32].

The disadvantage of synthetic resins is the fact that at the end of their life cycle they must be degraded and their thermal degradation produces toxic gaseous compounds. For this reason, an appropriate elimination procedure should be developed. This disadvantage could be

) and a total exchange capacity of 1.2 mequiv/

Use of Ionic Liquids in Solid-Liquid Separation Processes

http://dx.doi.org/10.5772/65890

519

method used for the immobilization of the ILs onto the solid support.

led to the researches focus on them.

ammonium functional groups with chloride (Cl-

in the removal process of Tl+

A widely applied method for the separation of metal ions from an aqueous solution is solvent extraction. Recently, this method has become more attractive because instead of using volatile organic compounds (VOCs) ionic liquids (ILs) or so-called "green extractants" were used, due to their well-known properties [11–13]. Even so, the liquid-liquid extraction method has some drawbacks such as: use of large quantities of ILs [10, 14], the possible loss of IL in the aqueous phase due to incomplete phase separations [15–19], it can only be considered efficient for the treatment of aqueous solutions with high metal concentrations [9, 18–21]. Therefore, there is an increased demand to find alternative technologies to overcome the drawbacks of the liquidliquid extractions, which should be competitive, efficient and environmentally friendly to achieve targets for sustainable growth. An answer to this problem is the use of ionic liquid in solid-liquid separation processes. This can happen if the ILs are immobilized in a suitable solid support. In this way (i) the minimum amount of ILs is consumed, (ii) the extraction and stripping step is combined into a single step, (iii) the immobilization reduces the loss of ILs in the aqueous phase, (iv) the advantages of the ILs are combined with the properties of the solid support, and this enhances the removal process of metal ions from aqueous solutions, (v) the ILs immobilized solid support obtained can be used for the removal of metal ions from dilute and complex solutions with possible use in simple fixed bed columns for commercial applications [14, 22–25].

This chapter reports various aspects of the use of ILs in solid-liquid separation processes by their immobilization in suitable solid support such as:


### **2. Materials used as solid support for ILs immobilization**

The use of ILs immobilized on a suitable solid support represents a link between the solvent extraction and adsorption processes, for the separation of various pollutants from aqueous solutions. The product obtained should develop an increased selectivity, a higher degree of adsorbent-adsorbate interactions, and should have mechanical stability [26]. All these properties are influenced by the used solid support, by the type of the immobilized ILs and by the method used for the immobilization of the ILs onto the solid support.

extraction [5, 6] and membrane processes [7]. These processes may encounter some technical, economical and environmental problems, which limit their application [8]. Even if the precipitation processes results in an efficiency of 90%, the disadvantage remains that precipitation agents are needed to reach the maximum admitted value for discharge. Thus, after treatment, huge amounts of highly contaminated sludge and filter backwash liquid remain [8–10]. Ion exchange, the most extensively used method is expensive for large-scale applications [9]. Membrane processes, especially reverse osmosis, also present economic

A widely applied method for the separation of metal ions from an aqueous solution is solvent extraction. Recently, this method has become more attractive because instead of using volatile organic compounds (VOCs) ionic liquids (ILs) or so-called "green extractants" were used, due to their well-known properties [11–13]. Even so, the liquid-liquid extraction method has some drawbacks such as: use of large quantities of ILs [10, 14], the possible loss of IL in the aqueous phase due to incomplete phase separations [15–19], it can only be considered efficient for the treatment of aqueous solutions with high metal concentrations [9, 18–21]. Therefore, there is an increased demand to find alternative technologies to overcome the drawbacks of the liquidliquid extractions, which should be competitive, efficient and environmentally friendly to achieve targets for sustainable growth. An answer to this problem is the use of ionic liquid in solid-liquid separation processes. This can happen if the ILs are immobilized in a suitable solid support. In this way (i) the minimum amount of ILs is consumed, (ii) the extraction and stripping step is combined into a single step, (iii) the immobilization reduces the loss of ILs in the aqueous phase, (iv) the advantages of the ILs are combined with the properties of the solid support, and this enhances the removal process of metal ions from aqueous solutions, (v) the ILs immobilized solid support obtained can be used for the removal of metal ions from dilute and complex solutions with possible use in simple fixed bed columns for commercial appli-

This chapter reports various aspects of the use of ILs in solid-liquid separation processes by

**2.** The kind of methods used for the immobilization of various ILs onto suitable solid

**3.** The use of solid support immobilized with ILs in the process of removing various metal

The use of ILs immobilized on a suitable solid support represents a link between the solvent extraction and adsorption processes, for the separation of various pollutants from aqueous solutions. The product obtained should develop an increased selectivity, a higher degree of adsorbent-adsorbate interactions, and should have mechanical stability [26]. All these prop-

**1.** The type of materials used as solid supports for the immobilization of various ILs.

ions from aqueous solutions using solid-liquid separation processes.

**2. Materials used as solid support for ILs immobilization**

disadvantages [8, 10].

518 Progress and Developments in Ionic Liquids

cations [14, 22–25].

supports.

their immobilization in suitable solid support such as:

A number of different inorganic and organic solid supports have been tested for the immobilization of ILs during the last decade. The surface properties and the specific surface area are the two most relevant criteria used for the selection of a suitable solid support [25, 26]. Some researchers consider that the macroporous organic polymers are suitable to be used as solid supports due to their high surface area and good mechanical stability, faster kinetics for the removal of contaminant even from dilute solutions, ease of regeneration and high adsorption capacity. On the other hand, other researchers consider that inorganic types of solid support present some advantages over organic supports such as higher thermal and chemical stability, well-ordered periodic pore structure and controllable pore diameter [26]. Therefore, in this section, the discussion is about the most frequently used solid supports, the ones used for immobilization of ILs, which are reported in literature, and also the considerations that have led to the researches focus on them.

Polymer-supported reagents are widely employed in the separation of metal ions. Amberlite XAD resins were a kind of commercial resin frequently used as a solid support for the immobilization of various ILs [18, 20, 24, 25] with uniform pore size distribution, high surface area and good mechanical and chemical stability. Their surface properties differ in function depending on their structure [27, 28]: XAD-2, XAD-4 are hydrophobic, XAD-7 and XAD-8 are moderate hydrophilic and have a higher polarity compared with the other two. For these reasons, the group of Guibal, Gallardo and Navarro used XAD-7 resin as a solid support for the impregnation of tetraalkyl phosphonium ionic liquid—tetradecyl (trihexyl) phosphonium chloride—Cyphos IL-101 and used the obtained materials in the removal of cadmium, zinc and bismuth, respectively [24, 27, 28]. Yang et al. impregnated the Amberlite XAD-7 resin with Cyphos IL-104 in order for it to be used for Cr(VI) removal from aqueous solutions [20]. Cr(VI) removal was also studied by Saha et al. [29] through adsorption onto Amberlite XAD-7 impregnated with Aliquat 336. Kalidhasan et al. used an ion exchange resin as a solid support because this type of resin has a high degree of metal recovery and selectivity. Their study was made with Dowex 1 x 8, a styrene-divinylbenzene polymeric resin containing quaternary ammonium functional groups with chloride (Cl- ) and a total exchange capacity of 1.2 mequiv/ mL. They impregnated Aliquat 336 ionic liquid onto Dowex [30]. Popa and co-workers used styrene-divinylbenzene as a solid support grafted with different pendant groups (triphenylphosphonium, izo-propylphosphonate and aminoethylaminomethyl) [31]. Styrene-divinylbenzene grafted with aminoethylaminomethyl groups was impregnated with three different ionic liquids: (trihexyl) tetradecyl phosphonium—Cyphos IL-101, 1-octyl-3-methyl imidazolium tetrafluoroborate—OmimBF4; and 1-butyl-3-methyl imidazolium hexafluorophosphate —(BmimPF6). This was done in order to determine the synergistic effect of the functional groups grafted on the polymer and the functional groups from the impregnated ionic liquid in the removal process of Tl+ and Sr2+ ions from aqueous solutions [32].

The disadvantage of synthetic resins is the fact that at the end of their life cycle they must be degraded and their thermal degradation produces toxic gaseous compounds. For this reason, an appropriate elimination procedure should be developed. This disadvantage could be eliminated by using some biopolymers (such as alginate, cellulose and chitosan) as a solid support as they have a thermal degradation which is much more environmentally friendly than that of synthetic resins [9, 21]. Using the biopolymers or renewable resources as a solid support for the immobilization of ILs, the process will conform with the principle of sustainable development [10]. The research group of Guibal, Vincent, Campos and other co-workers reported that the Cyphos IL-101 immobilized in biopolymer capsules of alginate and gelatin leads to the obtaining of an efficient sorbent material which was used with success in the processes of recovering of Hg, Pd, Bi, Au and Pt from acidic solutions [9, 10, 21, 33–35].

Carbon-based materials are the most widely used adsorbent materials, due to their attractive properties (large surface area, favourable chemical and thermal stability, ease of surface functionalization and modification). Thus, other researchers used these kinds of materials as solid supports for the immobilization of ionic liquids. Activated carbon obtained from palm shell was impregnated with task-specific ionic liquids (trioctyl methyl ammonium thiosalicylate (TOMATS)) by Ismaiel and his collaborators in order to remove mercury from contaminated water. The modification of palm shell-activated carbon by immobilization of the IL led to an increased efficiency in the removal of Hg(II) in comparison with the efficiency of virginactivated carbon. This is due to the increased number of functional groups (thio groups) present on the surface of the activated carbon [46]. Tokalioglu et al. [47] reported the use of

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Nanosized metal oxides, such as Al2O3, SiO2 and TiO2, have attracted a lot of interest as adsorbents in the solid-liquid separation processes due to their high surface area; crystalline and well-ordered periodic pore structure and chemical, thermal and radiation stability. The main disadvantage of these solids is their lack of selectivity, which can lead to a decrease in adsorption capacity when, in some matrixes there are various interfering ions present with the target metal. This problem can be overcome by chemical or physical modification of the sorbent surface. Therefore, a lot of researchers studied the possible use of these metal oxides as solid supports for the immobilization of ILs [48–58]. Amjadi and Samadi [48] modified the surface of TiO2 by coating it with 1-hexadecyl-3-methyl imidazolium bromide. Sprynskyy et al. [49] reported the adsorption performance of diatomite modified by chemical treatment with 1 ethyl-3-methyl imidazolium chloride solution in the removal of uranium ions from aqueous solutions. The metal oxide most extensively used as a solid support for IL immobilization is SiO2. **Table 1** summarizes literature reports on the types of ILs, which have been immobilized

In addition to studying silica as a possible solid support for the immobilization of ILs, the research group of Lupa, Negrea and co-workers, tested Florisil, which is a magnesium silicate [58–60]. They impregnated onto Florisil trihexyl (tetradecyl) phosphonium chloride—Cyphos IL-101 ionic liquid and used the adsorbent obtained in the removal of Sr2+ [58] and Cs+

from aqueous solutions. Also they determined the adsorption performance of Florisil impregnated with various ILs: Cyphos IL-101, OmimBF4 and BmimPF6 in the removal process of Tl+ ions from aqueous solutions [60]. Another study was carried out by the same group of researchers on the impregnation of Cyphos IL-101 on various organic solid supports (Dowex resin, Amberlite XAD7 and dibenzo-18-Crown-6-crown ether) and inorganic solid supports (Florisil and silica) and their use in the removal of Cd, Pb, Co, Ni, Cr, Cs, Sr, Tl and La. It was demonstrated that the type of solid support used for the immobilization of the ILs affected the adsorption properties of the resulting material [26]. In this study, the highest adsorption capacities were achieved by the inorganic solid supports. This can be explained by the fact that after the impregnation process the particles of the ILs studied were more homogenously distributed and bonded onto the surface of the inorganic solid supports compared to the

[59]

carbon nanospheres as a solid support for IL immobilization.

onto silica and their applications in solid-liquid separation processes.

organic solid supports, as can be seen from the SEM images presented [26].

Zhang et al. [36] proposed the use of poly(vinyl alcohol) (PVA)-alginate matrix gel as a solid support for the immobilization of [A336][MTBA] IL as a new kind of solid-phase extractant for an efficient recovery of Hg(II) from aqueous solutions. PVA is a water-soluble material containing large amounts of hydroxide groups, but it cannot be degraded by microorganism, has a higher mechanical strength and larger durability in high acidic solutions than alginate gel. For this reason, they wanted to design a novel, simple, competitive and environmentally friendly solid-phase sorbent for treatment of wastewaters [36].

Kalidhasan et al. [37] studied the possible adsorption enhancement of cellulose, a natural biodegradabile polysaccharide, by its impregnation with methyl trioctyl ammonium chloride. Through appropriate modification of the cellulose by immobilization with an IL, an adsorbent with good mechanical properties is obtained.

Li et al. [38, 39] reported the adsorption performance of an ionic liquid/chitosan/graphene composite. Both materials chitosan and graphene are recognized as excellent adsorbents due to their high surface area and their active sites: amino and hydroxyl groups for chitosan and carbonyl, carboxylic and alcoholic functional groups for graphene. However, their individual use presents some drawbacks such as possible oxidation in acidic solutions, and they are difficult to recycle due to their small size. These individual problems were eliminated by Li and co-workers through the synthesis of a chitosan-graphene oxide composite (MCGO). The MCGO obtained was used as a platform for the impregnation of IL [38–40].

Other researchers focussed on the use of Fe3O4 magnetic nanoparticles (MNPs) as a solid support for IL immobilization because of their properties such as unique size and magnetic properties. These were also chosen due to other advantages: ease of preparation and surface modification, high adsorption capacity due to their high surface area, and their simple and convenient separation from solution using a magnetic field [41–44]. The immobilization of IL on MNPs is advantageous because it prevents the aggregation and oxidation of the nanoparticles during the solid-liquid separation processes. Mehdinia et al. [41] used the individual MNPs for the immobilization of Aliquat 336 IL; Chen et al. [42] immobilized 1 alkyl-3 vinyl imidazolium chloride onto MNPs coated with SiO2 and Zheng et al. [43] studied the immobilization of 1-vinyl-3-hexyl imidazolium bromide on Fe3O4 co-precipitated with SiO2. Cheng et al. also used a superparamagnetic mesoporous core/shell nanocomposite (Fe3O4@nSiO2@mSiO2) as a solid support for amino ionic liquids (Si-DHIM-NH2). The adsorbent obtained was used for removing dye from aqueous solutions [45].

Carbon-based materials are the most widely used adsorbent materials, due to their attractive properties (large surface area, favourable chemical and thermal stability, ease of surface functionalization and modification). Thus, other researchers used these kinds of materials as solid supports for the immobilization of ionic liquids. Activated carbon obtained from palm shell was impregnated with task-specific ionic liquids (trioctyl methyl ammonium thiosalicylate (TOMATS)) by Ismaiel and his collaborators in order to remove mercury from contaminated water. The modification of palm shell-activated carbon by immobilization of the IL led to an increased efficiency in the removal of Hg(II) in comparison with the efficiency of virginactivated carbon. This is due to the increased number of functional groups (thio groups) present on the surface of the activated carbon [46]. Tokalioglu et al. [47] reported the use of carbon nanospheres as a solid support for IL immobilization.

eliminated by using some biopolymers (such as alginate, cellulose and chitosan) as a solid support as they have a thermal degradation which is much more environmentally friendly than that of synthetic resins [9, 21]. Using the biopolymers or renewable resources as a solid support for the immobilization of ILs, the process will conform with the principle of sustainable development [10]. The research group of Guibal, Vincent, Campos and other co-workers reported that the Cyphos IL-101 immobilized in biopolymer capsules of alginate and gelatin leads to the obtaining of an efficient sorbent material which was used with success in the processes of recovering of Hg, Pd, Bi, Au and Pt from acidic solutions [9, 10, 21, 33–35].

Zhang et al. [36] proposed the use of poly(vinyl alcohol) (PVA)-alginate matrix gel as a solid support for the immobilization of [A336][MTBA] IL as a new kind of solid-phase extractant for an efficient recovery of Hg(II) from aqueous solutions. PVA is a water-soluble material containing large amounts of hydroxide groups, but it cannot be degraded by microorganism, has a higher mechanical strength and larger durability in high acidic solutions than alginate gel. For this reason, they wanted to design a novel, simple, competitive and environmentally

Kalidhasan et al. [37] studied the possible adsorption enhancement of cellulose, a natural biodegradabile polysaccharide, by its impregnation with methyl trioctyl ammonium chloride. Through appropriate modification of the cellulose by immobilization with an IL, an adsorbent

Li et al. [38, 39] reported the adsorption performance of an ionic liquid/chitosan/graphene composite. Both materials chitosan and graphene are recognized as excellent adsorbents due to their high surface area and their active sites: amino and hydroxyl groups for chitosan and carbonyl, carboxylic and alcoholic functional groups for graphene. However, their individual use presents some drawbacks such as possible oxidation in acidic solutions, and they are difficult to recycle due to their small size. These individual problems were eliminated by Li and co-workers through the synthesis of a chitosan-graphene oxide composite (MCGO). The

Other researchers focussed on the use of Fe3O4 magnetic nanoparticles (MNPs) as a solid support for IL immobilization because of their properties such as unique size and magnetic properties. These were also chosen due to other advantages: ease of preparation and surface modification, high adsorption capacity due to their high surface area, and their simple and convenient separation from solution using a magnetic field [41–44]. The immobilization of IL on MNPs is advantageous because it prevents the aggregation and oxidation of the nanoparticles during the solid-liquid separation processes. Mehdinia et al. [41] used the individual MNPs for the immobilization of Aliquat 336 IL; Chen et al. [42] immobilized 1 alkyl-3 vinyl imidazolium chloride onto MNPs coated with SiO2 and Zheng et al. [43] studied the immobilization of 1-vinyl-3-hexyl imidazolium bromide on Fe3O4 co-precipitated with SiO2. Cheng et al. also used a superparamagnetic mesoporous core/shell nanocomposite (Fe3O4@nSiO2@mSiO2) as a solid support for amino ionic liquids (Si-DHIM-NH2). The

MCGO obtained was used as a platform for the impregnation of IL [38–40].

adsorbent obtained was used for removing dye from aqueous solutions [45].

friendly solid-phase sorbent for treatment of wastewaters [36].

with good mechanical properties is obtained.

520 Progress and Developments in Ionic Liquids

Nanosized metal oxides, such as Al2O3, SiO2 and TiO2, have attracted a lot of interest as adsorbents in the solid-liquid separation processes due to their high surface area; crystalline and well-ordered periodic pore structure and chemical, thermal and radiation stability. The main disadvantage of these solids is their lack of selectivity, which can lead to a decrease in adsorption capacity when, in some matrixes there are various interfering ions present with the target metal. This problem can be overcome by chemical or physical modification of the sorbent surface. Therefore, a lot of researchers studied the possible use of these metal oxides as solid supports for the immobilization of ILs [48–58]. Amjadi and Samadi [48] modified the surface of TiO2 by coating it with 1-hexadecyl-3-methyl imidazolium bromide. Sprynskyy et al. [49] reported the adsorption performance of diatomite modified by chemical treatment with 1 ethyl-3-methyl imidazolium chloride solution in the removal of uranium ions from aqueous solutions. The metal oxide most extensively used as a solid support for IL immobilization is SiO2. **Table 1** summarizes literature reports on the types of ILs, which have been immobilized onto silica and their applications in solid-liquid separation processes.

In addition to studying silica as a possible solid support for the immobilization of ILs, the research group of Lupa, Negrea and co-workers, tested Florisil, which is a magnesium silicate [58–60]. They impregnated onto Florisil trihexyl (tetradecyl) phosphonium chloride—Cyphos IL-101 ionic liquid and used the adsorbent obtained in the removal of Sr2+ [58] and Cs+ [59] from aqueous solutions. Also they determined the adsorption performance of Florisil impregnated with various ILs: Cyphos IL-101, OmimBF4 and BmimPF6 in the removal process of Tl+ ions from aqueous solutions [60]. Another study was carried out by the same group of researchers on the impregnation of Cyphos IL-101 on various organic solid supports (Dowex resin, Amberlite XAD7 and dibenzo-18-Crown-6-crown ether) and inorganic solid supports (Florisil and silica) and their use in the removal of Cd, Pb, Co, Ni, Cr, Cs, Sr, Tl and La. It was demonstrated that the type of solid support used for the immobilization of the ILs affected the adsorption properties of the resulting material [26]. In this study, the highest adsorption capacities were achieved by the inorganic solid supports. This can be explained by the fact that after the impregnation process the particles of the ILs studied were more homogenously distributed and bonded onto the surface of the inorganic solid supports compared to the organic solid supports, as can be seen from the SEM images presented [26].


**3. Methods used for the immobilization of ILs on solid supports**

**Figure 1.** The process of IL immobilization in biopolymer capsules through encapsulation.

or ultrasonication method [14, 19, 28].

There are both chemical and physical methods used for the immobilization of ILs onto suitable solid supports. The chemical methods of immobilization include: the encapsulation of the IL into microcapsules, the chemical bonding method and the sol-gel method. The immobilization of the ILs using a physical technique may be achieved through different processes such as: wet method, dry method, impregnation in the presence of a modifying agent, the dynamic method

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The immobilization of ILs through encapsulation is, in most cases, carried out in biopolymers, this technique being intensively studied by the research group of Guibal [10, 21, 33–35]. The schematic flow process for obtaining ILs immobilized in biopolymer capsules through encapsulation is presented in **Figure 1**. In the first step, the IL is mixed with a NaOH solution,

**Table 1.** Type of ILs immobilized onto silica.

### **3. Methods used for the immobilization of ILs on solid supports**

**No. Type of ILs immobilized Method of ILs**

2. N-methyl imidazole Stirring in the presence

1. 1-butyl-3-methyl imidazolium hexafluorophosphate [C4MIM][PF6]

522 Progress and Developments in Ionic Liquids

3. Tropine-type chiral ionic liquid with proline anion

IL-104

4. Trihexyl (tetradecyl) phosphonium bis 2,4,4-trimethyl pentyl phosphopinate—Cyphos

5. Trialkyl methyl ammonium bis 2,4,4-trimethyl pentyl phosphopinate—[A336][C272]

6. Trihexyl (tetradecyl) phosphonium bis 2,4,4-trimethyl pentyl phosphopinate—Cyphos IL-104 mixed with imidazolium

> PF6 -

ionic liquid Cnmim+

(C8mim+ PF6 − ) and trialkyl phosphine oxides

(Cyanex 923)

8. N,N-EPANTf2 which was impregnated onto silica gel activated through chemically bound

method with 3-

(APTMS)

aminopropyltrimethoxysilane

9. Trihexyl (tetradecyl) phosphonium chloride—Cyphos IL-101

10. Trihexyl (tetradecyl) phosphonium chloride—Cyphos IL-101

**Table 1.** Type of ILs immobilized onto silica.

7. 1-octyl-3-methyl imidazolium hexafluorophosphate

**immobilization**

of acetonitrile and

3-chloropropyltriethoxysilane

Stirring method Cadmium

Chemical bonding method Separation of

Sol-gel method Removal of Cr(III,VI) [53]

Sol-gel method Removal of Cr(III,VI) [53]

Sol-gel method Extraction of Yttrium [54]

Sol-gel method Extraction of Yttrium [55]

Stirring method Separation of Zr(IV) [56]

from aqueous solutions

from aqueous solutions

[57]

[58]

Dry method of impregnation Adsorption of Cs+

Dry method of impregnation Adsorption of Sr2+

**Application of the obtained ILs immobilized silica**

pre-concentration

Removal of 12 sulfonylurea herbicides

Cu2+, Fe3+, Mn2+ and Ni2+ Separation of racemic amino acids **References**

[50]

[51]

[52]

There are both chemical and physical methods used for the immobilization of ILs onto suitable solid supports. The chemical methods of immobilization include: the encapsulation of the IL into microcapsules, the chemical bonding method and the sol-gel method. The immobilization of the ILs using a physical technique may be achieved through different processes such as: wet method, dry method, impregnation in the presence of a modifying agent, the dynamic method or ultrasonication method [14, 19, 28].

The immobilization of ILs through encapsulation is, in most cases, carried out in biopolymers, this technique being intensively studied by the research group of Guibal [10, 21, 33–35]. The schematic flow process for obtaining ILs immobilized in biopolymer capsules through encapsulation is presented in **Figure 1**. In the first step, the IL is mixed with a NaOH solution,

**Figure 1.** The process of IL immobilization in biopolymer capsules through encapsulation.

then it is mixed with an aqueous solution of gelatin. To the gelatin-ionic liquid solution is added alginate sodium solution and the suspension is ultrasonicated thus obtaining a slightly viscous white solution. This is extruded through a nozzle into an ionotropic gelling solution of CaCl2. The beads obtained are kept in the coagulation bath overnight before being rinsed with HCl solution. In order to avoid the degradation of the composite biopolymer matrix, the obtained capsules are stored in 0.1 M HCl solution [10, 21, 33–35]. Zhang et al. also used the encapsulation method for the immobilization of ILs. They introduced PVA into the alginate solid support to enhance the strength and durability of the beads. Before the extrusion step, they blended the PVA alginate solution with the IL ([A336][MTBA]) for 6 h at 500 rpm and 30°C [36].

Because the immobilization of the IL by the dry method of impregnation involves a lot of time for contact between the IL and the solid support, some researchers used the stirring method [20, 29, 41, 46–50]. They stirred these two phases (the IL dissolved in a diluent and the solid support), different stirring times are reported, and then filtered, washed and dried the solid support immobilized with the IL in order to evaporate the diluent. The total time taken for the immobilization of the IL on a solid support using the stirring method depends on how much treatment is carried out on the solid support before impregnation, and how many steps of the

**Figure 2.** The obtaining of ionic liquid immobilized on a solid support using the dry method of impregnation.

Negrea et al. studied the influence of the impregnation method on the adsorption capacity of the resulting materials. For the immobilization of Cyphos IL-101 on Florisil and Silica solid supports, the methods they used for impregnation were: the dry method, the stirring method, the dynamic column method and the ultrasound method. The materials obtained were used

various methods they studied were characterized using FTIR, BET, SEM and EDX analysis. It was observed that the method used for the impregnation of the IL onto the solid support influences the surface morphology of the product obtained. This led to the materials having

efficient method of impregnation proved to be the ultrasound method with which the distribution of the ILs particles onto the surface of the studied solid supports was uniform. This

Other researchers used the ultrasonication method for the immobilization of the ionic liquid onto a solid support because it is an economic alternative process, which is not so time-

The research group of Lupa, Negrea and co-workers studied the influence of the ultrasonication conditions (amplitude and time of ultrasonication) upon the immobilization of Cyphos IL-101, OmimBF4 and BmimPF6 onto Florisil. It was observed that in order to obtain a stable and homogenous impregnation of the solid support with the studied IL, it is not necessary to increase the ultrasonication time, but a higher amplitude should be used. Increasing the ultrasonication time leads to the conglomeration of the IL particles on the solid support affecting in this way the reproducibility of the adsorption experiments [63]. Increasing the amplitude of sonication assures an easier transmission of the IL particles through the liquid

ions from aqueous solutions [62]. The materials obtained through the

ions from aqueous solutions. The most

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525

[62].

washing there are before the final product is obtained.

different adsorption capacities in the removal of Cs+

method also resulted in the highest adsorption capacity of Cs+

in the removal of Cs+

consuming [30, 37–40, 45, 60].

Shanthana Lakshmi et al. [61] immobilized Cyphos IL-101 in polymer microspheres using the phase-inversion technique. In the first step a Phase I is obtained, a casting solution to form the microspheres, which contain the polymer and the IL dissolved in N,N-dimethylformamide. The microspheres pass through a second oil phase (dodecane) and then into a coagulation bath (Phase III—a mixture of ethanol and double-distilled water). In the coagulation bath, the phase inversion is induced by exchange across the interface of the solvent from the polymer solution with a non-solvent [61].

Qian et al. immobilized a tropine-type chiral ionic liquid with proline anion on silica gel by a chemical bonding method. In the first step, they activated the surface of the silica gel, mixed the tropine 3-iodopropyl trimethoxysilane and potassium iodide in ethanol, and then mixed all the reagents and left the reaction to perform at 110°C. The compound which resulted was added to a mixture of l-proline and sodium hydroxide solution and stirred continuously for 48 h [52].

Other researchers immobilized ILs onto magnetic nanoparticle surfaces by free radical copolymerization [42, 43]. The reaction is performed under a nitrogen atmosphere and each of the papers mentions different times taken for the reaction (6–24 h) and different temperatures (70–110°C). Liu and co-workers used the sol-gel method for the immobilization of the IL onto silica-based materials [53, 54].

The use of chemical processes to immobilize the IL onto a suitable solid support involves the use of a lot of reagents which limit their application from the economical point of view. Therefore, physical processes are most widely used. The physical method which has been tested most is the dry method of impregnation because researchers consider that through this process the stability of the extractant on the solid support is increased [24, 25, 27, 28, 57–59]. The schematic process of ILs immobilization onto a solid support, using the dry method of impregnation, is presented in **Figure 2**.

Some researchers activate the surface of the solid supports before impregnation [27, 28]. Also the ionic liquid is dissolved in a volatile diluent (ketone, methanol and ethanol). The two components are kept in contact for at least 24 h. Subsequently, the diluent is evaporated and pure ionic liquid remains inside the pores of the support. The evaporation of the diluent is achieved through drying in an oven at 50°C for 24 h or by using roto-vapour.

Because the immobilization of the IL by the dry method of impregnation involves a lot of time for contact between the IL and the solid support, some researchers used the stirring method [20, 29, 41, 46–50]. They stirred these two phases (the IL dissolved in a diluent and the solid support), different stirring times are reported, and then filtered, washed and dried the solid support immobilized with the IL in order to evaporate the diluent. The total time taken for the immobilization of the IL on a solid support using the stirring method depends on how much treatment is carried out on the solid support before impregnation, and how many steps of the washing there are before the final product is obtained.

then it is mixed with an aqueous solution of gelatin. To the gelatin-ionic liquid solution is added alginate sodium solution and the suspension is ultrasonicated thus obtaining a slightly viscous white solution. This is extruded through a nozzle into an ionotropic gelling solution of CaCl2. The beads obtained are kept in the coagulation bath overnight before being rinsed with HCl solution. In order to avoid the degradation of the composite biopolymer matrix, the obtained capsules are stored in 0.1 M HCl solution [10, 21, 33–35]. Zhang et al. also used the encapsulation method for the immobilization of ILs. They introduced PVA into the alginate solid support to enhance the strength and durability of the beads. Before the extrusion step, they blended the PVA alginate solution with the IL ([A336][MTBA]) for 6 h at 500 rpm and

Shanthana Lakshmi et al. [61] immobilized Cyphos IL-101 in polymer microspheres using the phase-inversion technique. In the first step a Phase I is obtained, a casting solution to form the microspheres, which contain the polymer and the IL dissolved in N,N-dimethylformamide. The microspheres pass through a second oil phase (dodecane) and then into a coagulation bath (Phase III—a mixture of ethanol and double-distilled water). In the coagulation bath, the phase inversion is induced by exchange across the interface of the solvent from the polymer solution

Qian et al. immobilized a tropine-type chiral ionic liquid with proline anion on silica gel by a chemical bonding method. In the first step, they activated the surface of the silica gel, mixed the tropine 3-iodopropyl trimethoxysilane and potassium iodide in ethanol, and then mixed all the reagents and left the reaction to perform at 110°C. The compound which resulted was added to a mixture of l-proline and sodium hydroxide solution and stirred continuously for

Other researchers immobilized ILs onto magnetic nanoparticle surfaces by free radical copolymerization [42, 43]. The reaction is performed under a nitrogen atmosphere and each of the papers mentions different times taken for the reaction (6–24 h) and different temperatures (70–110°C). Liu and co-workers used the sol-gel method for the immobilization of the IL

The use of chemical processes to immobilize the IL onto a suitable solid support involves the use of a lot of reagents which limit their application from the economical point of view. Therefore, physical processes are most widely used. The physical method which has been tested most is the dry method of impregnation because researchers consider that through this process the stability of the extractant on the solid support is increased [24, 25, 27, 28, 57–59]. The schematic process of ILs immobilization onto a solid support, using the dry method of

Some researchers activate the surface of the solid supports before impregnation [27, 28]. Also the ionic liquid is dissolved in a volatile diluent (ketone, methanol and ethanol). The two components are kept in contact for at least 24 h. Subsequently, the diluent is evaporated and pure ionic liquid remains inside the pores of the support. The evaporation of the diluent is

achieved through drying in an oven at 50°C for 24 h or by using roto-vapour.

30°C [36].

48 h [52].

with a non-solvent [61].

524 Progress and Developments in Ionic Liquids

onto silica-based materials [53, 54].

impregnation, is presented in **Figure 2**.

**Figure 2.** The obtaining of ionic liquid immobilized on a solid support using the dry method of impregnation.

Negrea et al. studied the influence of the impregnation method on the adsorption capacity of the resulting materials. For the immobilization of Cyphos IL-101 on Florisil and Silica solid supports, the methods they used for impregnation were: the dry method, the stirring method, the dynamic column method and the ultrasound method. The materials obtained were used in the removal of Cs+ ions from aqueous solutions [62]. The materials obtained through the various methods they studied were characterized using FTIR, BET, SEM and EDX analysis. It was observed that the method used for the impregnation of the IL onto the solid support influences the surface morphology of the product obtained. This led to the materials having different adsorption capacities in the removal of Cs+ ions from aqueous solutions. The most efficient method of impregnation proved to be the ultrasound method with which the distribution of the ILs particles onto the surface of the studied solid supports was uniform. This method also resulted in the highest adsorption capacity of Cs+ [62].

Other researchers used the ultrasonication method for the immobilization of the ionic liquid onto a solid support because it is an economic alternative process, which is not so timeconsuming [30, 37–40, 45, 60].

The research group of Lupa, Negrea and co-workers studied the influence of the ultrasonication conditions (amplitude and time of ultrasonication) upon the immobilization of Cyphos IL-101, OmimBF4 and BmimPF6 onto Florisil. It was observed that in order to obtain a stable and homogenous impregnation of the solid support with the studied IL, it is not necessary to increase the ultrasonication time, but a higher amplitude should be used. Increasing the ultrasonication time leads to the conglomeration of the IL particles on the solid support affecting in this way the reproducibility of the adsorption experiments [63]. Increasing the amplitude of sonication assures an easier transmission of the IL particles through the liquid media until it reaches the cavities of the solid support. Through this process, instead of the 24 h of impregnation needed in case of the dry method, the immobilization is achieved in 10 min. After the immobilization of the IL onto the solid support through ultrasonication, the suspension is dried for 24 h at 50°C for solvent evaporation. This time could be minimized if the evaporation of the solvent is carried out using a roto-vapour. The same group of researchers studied the possibility of IL immobilization using the process of pellicular vacuum solvent vaporization using a roto-vapour. They studied the influence of the impregnation conditions: stirring time and temperature in the case of the immobilization of the 1-n-hexyl-3-methyl imidazolium chloride on Florisil. The material obtained was used in the removal of Tl ions from aqueous solutions [66]. It was observed that to improve the adsorption capacity of the obtained ionic-liquid impregnated material, it is not necessary to increase the stirring time during the impregnation process, but it is necessary to increase the temperature. The temperature increase led to a large quantity of ionic liquid being impregnated onto the solid support and to a shorter time for solvent evaporation [66].

### **4. Use of ionic liquid immobilized solid supports for the removal of metal ions from aqueous solutions**

Ionic liquid immobilized solid supports were used for the removal of different metal ions from various samples such as: wastewater, sea water and hydrochloric acid solution. The adsorption efficiency of the studied materials was determined using the mass balance equation:

$$\mathbf{q} = \frac{\left(\mathbf{C}\_0 - \mathbf{C}\_\mathbf{e}\right) \cdot \mathbf{V}}{\mathbf{m}} \tag{1}$$

higher when resin impregnated with Cyphos IL-101 was used in the removal of Zn(II) from acidic solutions. They reported that the impregnated materials could be regenerated after adsorption using Na2SO4, H2SO4 or HNO3. Also sorption and desorption efficiencies were maintained almost constantly over five sorption/desorption cycles [27]. The removal of lead ions was studied by Sun et al. [40] through adsorption onto graphene oxide and magnetic chitosan ionic-liquid and by Tokalioglu et al. [47] using carbon nanospheres coated with an ionic liquid. In the first case, an adsorption capacity of 85 mg/g was obtained, while the carbon

A lot of studies were made on the removal of Cr ions from aqueous solutions. Chromium compounds are widely applied in many industries (pigments, metallurgy, electroplating, leather tanning, stainless steel production, steel, photography) giving rise to huge quantities of wastewater containing chromium [20, 53]. The discharged chromium exists in two important states of oxidation, Cr(III) and Cr(VI). Cr(VI) is more toxic than Cr(III) because of its ability to oxidize other substances. In **Table 2**, there is a summary of the adsorption capacities of different ionic liquid immobilized solid supports in the removal of Cr(VI) from aqueous solutions. It can be observed from the data presented in **Table 2** that the highest adsorption capacity was given by the Dowex 1 x 8 in the removal process of Cr(VI) from aqueous solutions. This could be explained by the fact that Dowex is known to be a good ion exchanger and so it may be that the success in this removal process results from a combination between adsorption and ion

nanospheres coated with ionic liquid gave an adsorption capacity of 50.3 mg/g.

**method**

2 g of IL/1 g of SS

6 mL of IL/1 g of SS

Ultrasonication 1 g IL/1 g SS

Ultrasonication 1 g IL/1 g SS

**Table 2.** The adsorption capacities developed by ionic liquid immobilized solid support in the removal process of

Cellulose MeTOACl Ultrasonication 38.94 S:L = 10 g/L; t = n.m.,

Silica Cyphos IL-104 Sol-gel method 19.31 S:L = 4 g/L; t = 4

Silica [A336][C272] Sol-gel method 15.29 S:L = 4 g/L; t = 4

0.25 g of IL/1 g of SS

**qm, mg/g Work conditions References**

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527

t = 30 min, T = 300 K; pH = 3.5-4

t = 60 min, T = 303 K;

h; T = 298 K; pH = 6

h; T = 298 K; pH = 6

T = room temp.; pH = 2

[19]

[29]

[30]

[37]

[38]

[39]

[53]

[53]

44.85 S:L = 4 g/L; t = 60 min; T = 298 K; pH = not mentioned (n.m.)

50.44 S:L = 1.6 g/L; t = 24 h; T = 298 K; pH = 6

230.9 S:L = 10 g/L;

107.99 S:L = 1g/L;

pH = 2

145.35 S:L = 1g/L; t = 40 min, T = 303 K; pH = 3

**Solid support Ionic liquid Immobilization**

Amberlite XAD-7 Cyphos IL-104 Stirring method

Amberlite XAD-7 Aliquat 336 Stirring method

Dowex 1 x 8 Aliquat 336 Ultrasonication

N-(3-Aminopropyl) imidazole

Tetraoctylammonium

bromide

Magnetic chitosan/ graphene oxide

Magnetic chitosan/ graphene oxide

Cr(VI) from aqueous solutions.

where C0 and Ce represent the initial and equilibrium concentration of the studied metal ion [mg/L]; V is the volume of the aqueous solution sample containing the metal ion [L] and m represents the mass of the used ionic liquid immobilized solid support [g].

This section presents the results of studies on the adsorption efficiency obtained by various ionic liquid immobilized solid supports used in the removal of metal ions from different samples. Identifying the adsorbent with the greatest capacity of adsorption is difficult as the experimental conditions vary from one study to another. However, in most of the adsorption studies, the following were analyzed to find their influence on adsorption capacity: pH, contact time between the adsorbent and adsorbate, and initial concentration of the metal ions. With some exceptions, when more models are tested, the equilibrium data were fitted with Langmuir and Freundlich isotherms and the kinetic data were discussed using the pseudo-first order and pseudo-second order kinetic models.

Gallardo et al. studied zinc (II) extraction from HCl acid solutions using Amberlite XAD7 impregnated with Cyphos IL-101 through the dry method. They studied the influence of the HCl concentration, the content of the Cyphos IL-101 upon the uptake efficiency and sorption isotherms [27]. Compared to the raw Amberlite XAD7, the adsorption capacity was 10 times higher when resin impregnated with Cyphos IL-101 was used in the removal of Zn(II) from acidic solutions. They reported that the impregnated materials could be regenerated after adsorption using Na2SO4, H2SO4 or HNO3. Also sorption and desorption efficiencies were maintained almost constantly over five sorption/desorption cycles [27]. The removal of lead ions was studied by Sun et al. [40] through adsorption onto graphene oxide and magnetic chitosan ionic-liquid and by Tokalioglu et al. [47] using carbon nanospheres coated with an ionic liquid. In the first case, an adsorption capacity of 85 mg/g was obtained, while the carbon nanospheres coated with ionic liquid gave an adsorption capacity of 50.3 mg/g.

media until it reaches the cavities of the solid support. Through this process, instead of the 24 h of impregnation needed in case of the dry method, the immobilization is achieved in 10 min. After the immobilization of the IL onto the solid support through ultrasonication, the suspension is dried for 24 h at 50°C for solvent evaporation. This time could be minimized if the evaporation of the solvent is carried out using a roto-vapour. The same group of researchers studied the possibility of IL immobilization using the process of pellicular vacuum solvent vaporization using a roto-vapour. They studied the influence of the impregnation conditions: stirring time and temperature in the case of the immobilization of the 1-n-hexyl-3-methyl imidazolium chloride on Florisil. The material obtained was used in the removal of Tl ions from aqueous solutions [66]. It was observed that to improve the adsorption capacity of the obtained ionic-liquid impregnated material, it is not necessary to increase the stirring time during the impregnation process, but it is necessary to increase the temperature. The temperature increase led to a large quantity of ionic liquid being impregnated onto the solid support

**4. Use of ionic liquid immobilized solid supports for the removal of metal**

Ionic liquid immobilized solid supports were used for the removal of different metal ions from various samples such as: wastewater, sea water and hydrochloric acid solution. The adsorption

efficiency of the studied materials was determined using the mass balance equation:

q

represents the mass of the used ionic liquid immobilized solid support [g].

( ) CCV 0 e


m

where C0 and Ce represent the initial and equilibrium concentration of the studied metal ion [mg/L]; V is the volume of the aqueous solution sample containing the metal ion [L] and m

This section presents the results of studies on the adsorption efficiency obtained by various ionic liquid immobilized solid supports used in the removal of metal ions from different samples. Identifying the adsorbent with the greatest capacity of adsorption is difficult as the experimental conditions vary from one study to another. However, in most of the adsorption studies, the following were analyzed to find their influence on adsorption capacity: pH, contact time between the adsorbent and adsorbate, and initial concentration of the metal ions. With some exceptions, when more models are tested, the equilibrium data were fitted with Langmuir and Freundlich isotherms and the kinetic data were discussed using the pseudo-first

Gallardo et al. studied zinc (II) extraction from HCl acid solutions using Amberlite XAD7 impregnated with Cyphos IL-101 through the dry method. They studied the influence of the HCl concentration, the content of the Cyphos IL-101 upon the uptake efficiency and sorption isotherms [27]. Compared to the raw Amberlite XAD7, the adsorption capacity was 10 times

and to a shorter time for solvent evaporation [66].

order and pseudo-second order kinetic models.

**ions from aqueous solutions**

526 Progress and Developments in Ionic Liquids

A lot of studies were made on the removal of Cr ions from aqueous solutions. Chromium compounds are widely applied in many industries (pigments, metallurgy, electroplating, leather tanning, stainless steel production, steel, photography) giving rise to huge quantities of wastewater containing chromium [20, 53]. The discharged chromium exists in two important states of oxidation, Cr(III) and Cr(VI). Cr(VI) is more toxic than Cr(III) because of its ability to oxidize other substances. In **Table 2**, there is a summary of the adsorption capacities of different ionic liquid immobilized solid supports in the removal of Cr(VI) from aqueous solutions. It can be observed from the data presented in **Table 2** that the highest adsorption capacity was given by the Dowex 1 x 8 in the removal process of Cr(VI) from aqueous solutions. This could be explained by the fact that Dowex is known to be a good ion exchanger and so it may be that the success in this removal process results from a combination between adsorption and ion


**Table 2.** The adsorption capacities developed by ionic liquid immobilized solid support in the removal process of Cr(VI) from aqueous solutions.

exchange. Also high adsorption capacities were obtained by magnetic chitosan/graphene oxide immobilized with different ionic liquids. The mixture between an organic solid support (chitosan) and an inorganic one (Fe3O4) leads to the enhancement of the adsorption capacity of the obtained product. The lowest adsorption capacity was obtained by the silica impregnated with the studied ILs using the sol-gel method of impregnation. The materials obtained through the impregnation of the IL onto the solid supports by ultrasonication presented the highest efficiency in the removal process of Cr(VI) from aqueous solutions.

Ionic liquid impregnated solid supports were also used as adsorbent materials in the removal of some radionuclides from aqueous solutions. It can be observed from **Table 4** that ionic liquid impregnated solid support can be used for the removal of radionuclides from aqueous solutions containing trace amounts. The materials impregnated with the studied ionic liquid developed a higher efficiency when the ultrasonication is used for the impregnation of the IL onto the solid support. The inorganic materials presented a higher efficiency than the organic

> **Removed metal**

Stirring method U(VI) 88 S:L = 1 g/L;

**qm, mg/g** **Work conditions References**

[49]

[59]

[57]

[63]

[64]

[65]

[58]

[58]

[32]

[32]

[32]

t = n.m.; T = 298 K;

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pH = 4.2

h; T = 298 K; pH = 8

T = 298 K; pH = 8

T = 298 K; pH = 3.5

t = 1 h, T = 298 K; pH = 3.5

t = 1 h, T = 298 K; pH = 3.5

t = 2 h; T = 298 K; pH = 6

t = 2 h; T = 298 K; pH = 6

t = 2 h; T = 298 K; pH = 3.5

t = 2 h; T = 298

t = 2 h; T = 298 K; pH = 3.5

materials as solid support for the impregnation of the IL.

methylimidazolium

chloride

**Ionic liquid Immobilization**

**method**

Florisil Cyphos IL-101 Dry method Cs(I) 3.086 S:L = 4 g/L; t = 2

Silica Cyphos IL-101 Dry method Cs(I) 1.48 S:L = 4 g/L; t = 2 h

Florisil Cyphos IL-101 Ultrasound Cs(I) 2.95 S:L = 4 g/L; t = 1 h,

Florisil OmimBF4 Ultrasound Cs(I) 2.27 S:L = 4 g/L;

Florisil BmimPF6 Ultrasound Cs(I) 1.6 S:L = 4 g/L;

Florisil Cyphos IL-101 Dry method Sr(II) 2.94 S:L = 4 g/L;

Silica Cyphos IL-101 Dry method Sr(II) 3.97 S:L = 4 g/L;

OmimBF4 Ultrasonication Sr(II) 15.6 S:L = 4 g/L;

BmimPF6 Ultrasonication Sr(II) 13.7 S:L = 4 g/L;

Cyphos IL-101 Ultrasonication Sr(II) 8.13 S:L = 4 g/L;

**Solid support**

Functionalized styrene divinylbenzene

Functionalized styrene divinylbenzene

Functionalized styrene

Diatomite 1-ethyl-3-

Ionic liquids immobilized through encapsulation in biopolymer capsules were used in the removal of some precious metals such as: Au, Pd, Hg, Pt. The work conditions and the obtained adsorption capacities of the ionic liquid immobilized biopolymers are presented in **Table 3**. It can be observed that the Cyphos IL-101 immobilized in microcapsules of gelatin and alginate composite were efficient in removing Hg, Pd, Bi, Au, Pt ions from acid solutions. Adsorption capacities of over 100 mg/g were obtained. [A336][MTBA] immobilized through encapsulation in alginate beads functionalized with PVA developed a smaller adsorption capacity for the removal of Hg(II) ions from aqueous solutions. A higher adsorption capacity for the removal of Hg(II) from aqueous solutions resulted from using TOMATS impregnated onto palm shell activated carbon. In this case, equilibrium is achieved after 3 h of shaking.


**Table 3.** The efficiency of the IL immobilized in biopolymer capsules in the removal process of metal ions from aqueous solutions.

Ionic liquid impregnated solid supports were also used as adsorbent materials in the removal of some radionuclides from aqueous solutions. It can be observed from **Table 4** that ionic liquid impregnated solid support can be used for the removal of radionuclides from aqueous solutions containing trace amounts. The materials impregnated with the studied ionic liquid developed a higher efficiency when the ultrasonication is used for the impregnation of the IL onto the solid support. The inorganic materials presented a higher efficiency than the organic materials as solid support for the impregnation of the IL.

exchange. Also high adsorption capacities were obtained by magnetic chitosan/graphene oxide immobilized with different ionic liquids. The mixture between an organic solid support (chitosan) and an inorganic one (Fe3O4) leads to the enhancement of the adsorption capacity of the obtained product. The lowest adsorption capacity was obtained by the silica impregnated with the studied ILs using the sol-gel method of impregnation. The materials obtained through the impregnation of the IL onto the solid supports by ultrasonication presented the

Ionic liquids immobilized through encapsulation in biopolymer capsules were used in the removal of some precious metals such as: Au, Pd, Hg, Pt. The work conditions and the obtained adsorption capacities of the ionic liquid immobilized biopolymers are presented in **Table 3**. It can be observed that the Cyphos IL-101 immobilized in microcapsules of gelatin and alginate composite were efficient in removing Hg, Pd, Bi, Au, Pt ions from acid solutions. Adsorption capacities of over 100 mg/g were obtained. [A336][MTBA] immobilized through encapsulation in alginate beads functionalized with PVA developed a smaller adsorption capacity for the removal of Hg(II) ions from aqueous solutions. A higher adsorption capacity for the removal of Hg(II) from aqueous solutions resulted from using TOMATS impregnated onto palm shell

> **Removed metal**

Cyphos IL-101 Encapsulation Hg 150 S:L = 0.2 g/L;

[A336][MTBA] Encapsulation Hg 49.89 S:L = 1 g/L; t = 24

TOMATS Orbital shacking Hg 83.33 S:L = 20 g/L;

**Table 3.** The efficiency of the IL immobilized in biopolymer capsules in the removal process of metal ions from

**qm, mg/g**

Pd 130-145 S:L = 0.6 g/L; t = 72

Bi 110-130 S:L = 0.25 g/L; t = 96

Au 140 S:L = 0.6 g/L;

Pt 177 S:L = 0.6 g/L;

**Work conditions References**

[10]

[21]

[33]

[34]

[35]

[36]

[46]

t = 96 h; T = 298 K; In the presence of HCl solution

h; T = 298 K; In the presence of HCl solution

h; T = 298 K; In the presence of HCl solution

t = 96h; T = 298 K; In the presence of HCl solution

t = 48–72 h; T = 298 K; In the presence of HCl solution

h; T = 298 K; pH = 6

t = 3 h; T = 308 K; pH = 8

highest efficiency in the removal process of Cr(VI) from aqueous solutions.

activated carbon. In this case, equilibrium is achieved after 3 h of shaking.

**Ionic liquid Immobilization method**

**Solid support**

Composite polymer (gelatin and alginate)

528 Progress and Developments in Ionic Liquids

PVA alginate gel beads

Palm shell activated carbon

aqueous solutions.



tion of ILs, the second one proved to be more advantageous. The materials obtained through the impregnation of the IL onto the solid supports by ultrasonication presented the highest efficiency in the removal process of metal ions from aqueous solutions. Even if the results are very promising, a wider combination of solid supports and ILs can be tested, opening the door

Use of Ionic Liquids in Solid-Liquid Separation Processes

http://dx.doi.org/10.5772/65890

531

This work was supported by a grant of the Romanian National Authority for Scientific

1 Faculty of Industrial Chemistry and Environmental Engineering, Politehnica University of

2 Institute of Chemistry Timisoara of Romanian Academy, Romanian Academy, Timisoara,

[1] Soylak M, Saracoglu S, Divrikli U, Elci L: Coprecipitation of heavy metals with erbium hydroxide for their flame atomic absorption spectrometric determinations in environ-

[2] Munter R: Technology for the removal of radionuclides from natural water and waste management: state of the art. Proceedings of the Estonia Academy of Science. 2013;

[3] Lehto J, Brodkin L, Harjula R: SrTreat: A highly effective ion exchanger for the removal of radioactive strontium from nuclear waste solutions. Radioactive Waste Management

[4] Hamed MM, Attallah MF, Shehata FA: Synthesis, characterization and sorption behaviour of some radionuclides on zirconium tungstate ion-exchanger. Arab Journal

mental samples. Talanta. 2005;66:1098–1102, DOI:10.1016/j.talanta.2005.01.030.

Research, CNCS – UEFISCDI, project number PN-II-RU-TE-2012-3-0198".

and Adriana Popa2

to explore new challenges.

**Acknowledgements**

**Author details**

Romania

**References**

Lavinia Lupa1\*, Petru Negrea1

Timisoara, Timisoara, Romania

\*Address all correspondence to: lavinia.lupa@upt.ro

62:122–132, DOI: 10.3176/proc.2013.2.06.

and Environmental Remediation. ASME. 1997;245–248.

of Nuclear Science and Applications. 2012;45:37–50.

**Table 4.** The adsorption capacities developed by ionic liquid immobilized solid support in the removal process of different radionuclides from aqueous solutions.

### **5. Conclusion**

Ionic liquids could be used in the solid-liquid separation processes by their immobilization in suitable solid supports. In this way, the loss of ILs into the aqueous phase is avoided, and a smaller amount of IL is used. The ionic liquid immobilized solid supports could be efficiently used for metal ions removal from aqueous solutions even from dilute samples. Through the immobilization of IL onto different solid supports, the adsorption performance of the materials is enhanced because of the combination of the advantages of the IL and the properties of the solid support. The type of materials used as solid support for ILs immobilization, and the method used for the immobilization process influences the adsorption efficiency of the product obtained. Regarding to the materials, silica and Amberlite XAD-7 have been the most tested as solid supports. The most immobilized ionic liquids were quaternary phosphonium ionic liquid and imidazolium type. The immobilization of the ILs in biopolymers was performed through encapsulation. Between the chemical and physical methods used for the immobilization of ILs, the second one proved to be more advantageous. The materials obtained through the impregnation of the IL onto the solid supports by ultrasonication presented the highest efficiency in the removal process of metal ions from aqueous solutions. Even if the results are very promising, a wider combination of solid supports and ILs can be tested, opening the door to explore new challenges.

### **Acknowledgements**

**Solid support**

Functionalized styrene divinylbenzene

530 Progress and Developments in Ionic Liquids

Functionalized styrene divinylbenzene

Functionalized styrene divinylbenzene

Florisil 1-hexyl-3-

**5. Conclusion**

methylimidazolium

chloride

different radionuclides from aqueous solutions.

**Ionic liquid Immobilization**

**method**

Florisil OmimBF4 Ultrasonication Tl(I) 8.48 S:L = 2 g/L; t = 2

Florisil BmimPF6 Ultrasonication Tl(I) 7.97 S:L = 2 g/L;

Florisil Cyphos IL-101 Ultrasonication Tl(I) 11.1 S:L = 2 g/L;

Pellicular vacuum solvent vaporization

**Table 4.** The adsorption capacities developed by ionic liquid immobilized solid support in the removal process of

Ionic liquids could be used in the solid-liquid separation processes by their immobilization in suitable solid supports. In this way, the loss of ILs into the aqueous phase is avoided, and a smaller amount of IL is used. The ionic liquid immobilized solid supports could be efficiently used for metal ions removal from aqueous solutions even from dilute samples. Through the immobilization of IL onto different solid supports, the adsorption performance of the materials is enhanced because of the combination of the advantages of the IL and the properties of the solid support. The type of materials used as solid support for ILs immobilization, and the method used for the immobilization process influences the adsorption efficiency of the product obtained. Regarding to the materials, silica and Amberlite XAD-7 have been the most tested as solid supports. The most immobilized ionic liquids were quaternary phosphonium ionic liquid and imidazolium type. The immobilization of the ILs in biopolymers was performed through encapsulation. Between the chemical and physical methods used for the immobiliza-

OmimBF4 Ultrasonication Tl(I) 4.83 S:L = 4 g/L;

BmimPF6 Ultrasonication Tl(I) 3.92 S:L = 4 g/L;

Cyphos IL-101 Ultrasonication Tl(I) 2.94 S:L = 4 g/L; t = 2

divinylbenzene K; pH = 3.5

**Removed metal**

**qm, mg/g** **Work conditions References**

[32]

[32]

[32]

[60]

[60]

[60]

[66]

t = 2 h; T = 298 K; pH = 3.5

t = 2 h; T = 298 K; pH = 3.5

h; T = 298 K; pH = 3.5

t = 2 h; T = 298 K; pH = 3.5

t = 2 h; T = 298 K; pH = 3.5

t = 2 h; T = 298 K; pH = 3.5

Tl(I) 2.95 S:L = 40 g/L;

h; T = 298 K; pH = 3.5

This work was supported by a grant of the Romanian National Authority for Scientific Research, CNCS – UEFISCDI, project number PN-II-RU-TE-2012-3-0198".

### **Author details**

Lavinia Lupa1\*, Petru Negrea1 and Adriana Popa2

\*Address all correspondence to: lavinia.lupa@upt.ro

1 Faculty of Industrial Chemistry and Environmental Engineering, Politehnica University of Timisoara, Timisoara, Romania

2 Institute of Chemistry Timisoara of Romanian Academy, Romanian Academy, Timisoara, Romania

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[56] Marwani HM, Alsafrani AE, Asiri AM, Rahman MM: Silica-gel particles loaded with an ionic liquid for separation of Zr(IV) prior to its determination by ICP-OES. Sensors.

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removal from aqueous solutions through

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2013.04.059.

536 Progress and Developments in Ionic Liquids


**Chapter 23**

**Provisional chapter**

**Supported Ionic Liquid Membranes for Metal**

Metals are widely used in various areas of human life, and their existence in the environ‐ ment at high concentrations has become a cause for concern. Metals can enter the human body and disturb the human metabolic system. Therefore, research to recover metals from their matrix both from industrial wastewater and from ores or scraps containing metals is of great importance. One of the separation techniques proposed to overcome those issues involves using supported ionic liquid membranes (SILMs). This chapter summarizes the recovery of metals using SILM. In SILM, an ionic liquid that acts as an extractant is embedded in small pores of a polymer support. The latest type of physical impregnation of ionic liquid, which is the type most commonly used in metal separation, is called polymer inclusion membrane (PIM). PIMs were prepared by casting a solution containing an ionic liquid, a plasticizer and a base polymer to form a thin, flexible and stable film. A PIM including ionic liquids has a similar configuration to SILM, and it is considered to be a kind of SILM. In this chapter, effects on the stability and selectivity in

**Keywords:** liquid membrane, ionic liquid, metal, separation, supported membrane

The separation of metals has become a hot topic of research in recent years. This is because metals are widely used in many aspects of human life, and their existence in the environ‐ ment at high concentrations is a cause for concern. Although some metals are essential for the human body, their presence in high concentrations will disturb the human metabolic system. Therefore, research on recovering metals from their matrix both from industrial wastewa‐ ter and from ores and ore scrap is of great importance. In hydrometallurgy, several conven‐ tional methods are being used to remove and recover heavy metals from aqueous solutions.

**Supported Ionic Liquid Membranes for Metal** 

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution,

© 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

and reproduction in any medium, provided the original work is properly cited.

Pius Dore Ola and Michiaki Matsumoto

Pius Dore Ola and Michiaki Matsumoto

Additional information is available at the end of the chapter

SILM and PIM for metal separation are reviewed.

Additional information is available at the end of the chapter

**Separation**

**Separation**

http://dx.doi.org/10.5772/65754

**Abstract**

**1. Introduction**

### **Supported Ionic Liquid Membranes for Metal Separation Supported Ionic Liquid Membranes for Metal Separation**

Pius Dore Ola and Michiaki Matsumoto Pius Dore Ola and Michiaki Matsumoto

Additional information is available at the end of the chapter Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/65754

#### **Abstract**

Metals are widely used in various areas of human life, and their existence in the environ‐ ment at high concentrations has become a cause for concern. Metals can enter the human body and disturb the human metabolic system. Therefore, research to recover metals from their matrix both from industrial wastewater and from ores or scraps containing metals is of great importance. One of the separation techniques proposed to overcome those issues involves using supported ionic liquid membranes (SILMs). This chapter summarizes the recovery of metals using SILM. In SILM, an ionic liquid that acts as an extractant is embedded in small pores of a polymer support. The latest type of physical impregnation of ionic liquid, which is the type most commonly used in metal separation, is called polymer inclusion membrane (PIM). PIMs were prepared by casting a solution containing an ionic liquid, a plasticizer and a base polymer to form a thin, flexible and stable film. A PIM including ionic liquids has a similar configuration to SILM, and it is considered to be a kind of SILM. In this chapter, effects on the stability and selectivity in SILM and PIM for metal separation are reviewed.

**Keywords:** liquid membrane, ionic liquid, metal, separation, supported membrane

### **1. Introduction**

The separation of metals has become a hot topic of research in recent years. This is because metals are widely used in many aspects of human life, and their existence in the environ‐ ment at high concentrations is a cause for concern. Although some metals are essential for the human body, their presence in high concentrations will disturb the human metabolic system. Therefore, research on recovering metals from their matrix both from industrial wastewa‐ ter and from ores and ore scrap is of great importance. In hydrometallurgy, several conven‐ tional methods are being used to remove and recover heavy metals from aqueous solutions.

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

These methods include chemical precipitation [1], reverse osmosis [2], adsorption [3], ion exchange [4] and solvent extraction processes [5]. Those techniques have their own inherent limitations such as low efficiency, sensitive operation conditions, production of secondary sludge, high capital and operating costs, and expensive disposal [6]. Hence, more efficient and cost‐ effective removal and recovery methods are sought to overcome these problems.

membrane phase and diffuse through it to enter the aqueous stripping phase. Although it is possible to transport the hydrophobic compound through the hydrophilic membrane, this review concerns the transport of water‐soluble compounds such as metal ions. The mass trans‐ fer in this system takes place due to the difference in the chemical potential across the mem‐

Supported Ionic Liquid Membranes for Metal Separation

http://dx.doi.org/10.5772/65754

541

According to the configuration definition, three groups of liquid membranes are usually con‐ sidered as illustrated in **Figure 1**: bulk (BLM), supported or immobilized (SLM or ILM) and emulsion (ELM) liquid membrane transport [12]. A supported liquid membrane (SLM) is one of the three‐phase liquid membrane systems in which the membrane phase (liquid) is held by capillary force in the pores of microporous polymeric and inorganic film. The immobilized liquid is a membrane phase, and a microporous film serves as a support for the membrane [13]. SLM was reported for the first time by Scholander [14], who used thin cellulose acetate

Successful applications of SLMs are possible due to their advantages compared to other sepa‐ ration methods. The main advantages of SLMs are the small amounts of organic phase and extractant (carrier) used, which allows for use of expensive extractants, one‐step mass trans‐ fer, the possibility of achieving high separation factors, enrichment of extracted compound(s) during separation and low separation costs. Nevertheless, there are some problems limiting the practical application of SLMs. The main problem is the stability of the liquid membrane, caused by leakage and/or losses of membrane phase components during the transport pro‐

**Figure 1.** Liquid membranes: (a) bulk, (b) supported and (c) emulsion. (F: feed, M: membrane, R: receiving).

ILs are organic salts remaining as liquids under ambient temperatures. They normally consist of an organic cation (e.g. imidazolium, pyridinium, pyrrolidinium, phosphonium, ammo‐ nium) and inorganic anion (e.g. tetrafluoroborate, hexafluorophosphate, chloride) or, increas‐ ingly, an organic anion (e.g. trifluoromethylsulphonate, bis[(trifluoromethyl) sulphonyl] imide) [15]. The main advantages of these media are their near‐zero vapour pressure, which means that they will not evaporate easily from the pores of membranes, have good chemical and thermal stability, and have a large temperature range where they are stable. They also

cess. Ionic liquids (ILs) come to solve the problem of membrane instability.

filters impregnated with an aqueous haemoglobin for oxygen transport.

brane as a driving force.

One technique proposed to overcome those issues involves using supported ionic liquid membranes (SILMs). This technique is superior to the above techniques because the advan‐ tages of the liquid membrane and ionic liquid are included in the technique of SILM. The advantages of liquid membranes such as combining the extraction and stripping into a single step and using a very small amount of solvent give SILM very low capital and operating costs and make it easy to scale up. Using ionic liquid as an extractant in this technique increases the efficiency and selectivity of SILM. In addition, the very low vapour pressure and very high viscosity of ionic liquid give the best stability of the liquid membrane, which also absolutely affects the flux and selectivity and is environmentally friendlier.

The properties and applications of SILMs are greatly affected by support types, supporting methods and kind of ionic liquid used. The supporting method could be chemical or physical. The physical immobilization of an ionic liquid can be conducted through simple impregna‐ tion [7], sol‐gel method [8], encapsulation [9], and so on. In chemical immobilization, the ionic liquid is bound to solid support via covalent bonding [10, 11]. The kind of ionic liquids determines the efficiency and selectivity of separation.

This chapter reviews use of supported ionic liquid membranes for separation of metals. At the beginning, it is important to briefly present a summary of supported liquid membrane using ionic liquid and metal separation using this method to ensure the better understanding regarding this topic. Then, we review several articles discussing the effect of related param‐ eters such as type of solid support and supporting method as well as kind of ionic liquids on the stability of the membrane, efficiency and selectivity of metal separation.

### **2. Supported liquid membrane and ionic liquid**

A membrane is a semipermeable barrier between two phases. If membranes are viewed as semipermeable phase separator, then the concept of membranes including solids as polymer or ceramic films can be extended to include liquids, and these are defined as liquid mem‐ branes (LMs). A liquid membrane system involves a liquid that is immiscible with the source (feed) and receiving (product) solutions and serves as a semipermeable and barrier between two liquid and gas phases [12]. Liquid membrane separation combines the solvent extraction and stripping (back‐extraction) in a single step [13]. The great potential for energy saving, low capital and operating costs, and the possibility to use expensive extractants due to the small amount of the membrane phase make SLMs an area attracting the special attention of both researchers and practitioners.

The transport in a membrane is a dynamic and non‐equilibrium process. The transporting compound dissolved in the feed aqueous solution has to dissolve in the organic, hydrophobic membrane phase and diffuse through it to enter the aqueous stripping phase. Although it is possible to transport the hydrophobic compound through the hydrophilic membrane, this review concerns the transport of water‐soluble compounds such as metal ions. The mass trans‐ fer in this system takes place due to the difference in the chemical potential across the mem‐ brane as a driving force.

These methods include chemical precipitation [1], reverse osmosis [2], adsorption [3], ion exchange [4] and solvent extraction processes [5]. Those techniques have their own inherent limitations such as low efficiency, sensitive operation conditions, production of secondary sludge, high capital and operating costs, and expensive disposal [6]. Hence, more efficient and

One technique proposed to overcome those issues involves using supported ionic liquid membranes (SILMs). This technique is superior to the above techniques because the advan‐ tages of the liquid membrane and ionic liquid are included in the technique of SILM. The advantages of liquid membranes such as combining the extraction and stripping into a single step and using a very small amount of solvent give SILM very low capital and operating costs and make it easy to scale up. Using ionic liquid as an extractant in this technique increases the efficiency and selectivity of SILM. In addition, the very low vapour pressure and very high viscosity of ionic liquid give the best stability of the liquid membrane, which also absolutely

The properties and applications of SILMs are greatly affected by support types, supporting methods and kind of ionic liquid used. The supporting method could be chemical or physical. The physical immobilization of an ionic liquid can be conducted through simple impregna‐ tion [7], sol‐gel method [8], encapsulation [9], and so on. In chemical immobilization, the ionic liquid is bound to solid support via covalent bonding [10, 11]. The kind of ionic liquids

This chapter reviews use of supported ionic liquid membranes for separation of metals. At the beginning, it is important to briefly present a summary of supported liquid membrane using ionic liquid and metal separation using this method to ensure the better understanding regarding this topic. Then, we review several articles discussing the effect of related param‐ eters such as type of solid support and supporting method as well as kind of ionic liquids on

A membrane is a semipermeable barrier between two phases. If membranes are viewed as semipermeable phase separator, then the concept of membranes including solids as polymer or ceramic films can be extended to include liquids, and these are defined as liquid mem‐ branes (LMs). A liquid membrane system involves a liquid that is immiscible with the source (feed) and receiving (product) solutions and serves as a semipermeable and barrier between two liquid and gas phases [12]. Liquid membrane separation combines the solvent extraction and stripping (back‐extraction) in a single step [13]. The great potential for energy saving, low capital and operating costs, and the possibility to use expensive extractants due to the small amount of the membrane phase make SLMs an area attracting the special attention of both

The transport in a membrane is a dynamic and non‐equilibrium process. The transporting compound dissolved in the feed aqueous solution has to dissolve in the organic, hydrophobic

the stability of the membrane, efficiency and selectivity of metal separation.

cost‐ effective removal and recovery methods are sought to overcome these problems.

affects the flux and selectivity and is environmentally friendlier.

determines the efficiency and selectivity of separation.

540 Progress and Developments in Ionic Liquids

**2. Supported liquid membrane and ionic liquid**

researchers and practitioners.

According to the configuration definition, three groups of liquid membranes are usually con‐ sidered as illustrated in **Figure 1**: bulk (BLM), supported or immobilized (SLM or ILM) and emulsion (ELM) liquid membrane transport [12]. A supported liquid membrane (SLM) is one of the three‐phase liquid membrane systems in which the membrane phase (liquid) is held by capillary force in the pores of microporous polymeric and inorganic film. The immobilized liquid is a membrane phase, and a microporous film serves as a support for the membrane [13]. SLM was reported for the first time by Scholander [14], who used thin cellulose acetate filters impregnated with an aqueous haemoglobin for oxygen transport.

**Figure 1.** Liquid membranes: (a) bulk, (b) supported and (c) emulsion. (F: feed, M: membrane, R: receiving).

Successful applications of SLMs are possible due to their advantages compared to other sepa‐ ration methods. The main advantages of SLMs are the small amounts of organic phase and extractant (carrier) used, which allows for use of expensive extractants, one‐step mass trans‐ fer, the possibility of achieving high separation factors, enrichment of extracted compound(s) during separation and low separation costs. Nevertheless, there are some problems limiting the practical application of SLMs. The main problem is the stability of the liquid membrane, caused by leakage and/or losses of membrane phase components during the transport pro‐ cess. Ionic liquids (ILs) come to solve the problem of membrane instability.

ILs are organic salts remaining as liquids under ambient temperatures. They normally consist of an organic cation (e.g. imidazolium, pyridinium, pyrrolidinium, phosphonium, ammo‐ nium) and inorganic anion (e.g. tetrafluoroborate, hexafluorophosphate, chloride) or, increas‐ ingly, an organic anion (e.g. trifluoromethylsulphonate, bis[(trifluoromethyl) sulphonyl] imide) [15]. The main advantages of these media are their near‐zero vapour pressure, which means that they will not evaporate easily from the pores of membranes, have good chemical and thermal stability, and have a large temperature range where they are stable. They also will not be replaced easily from the pores of the membrane due to their very high viscos‐ ity. The properties of ILs (hydrophobicity, viscosity, solubility, etc.), which can be varied by altering the substitutive group on the cation or the combined anion [16], make them more widely applicable in many physical and chemical fields. Therefore, they have been considered environmentally benign solvents as compared to volatile organic solvents. The structures of common ILs used in metal separation are shown in **Table 1**. Most ILs show the pronounced miscibility in the aqueous phases due to their polar nature. ILs have to be hydrophobic as shown in **Table 1** to extract the metal ions from the aqueous phases. For this reason, the ILs used in the metal separation were limited to those listed in **Table 1**.

ionic liquid in the membrane. Aqueous solution of metal ions in hydrogen chloride was used as the feed phase in order to form the anionic metal complex, although other forms such as nitrate and sulphate were also used in several investigations. The reactions of metal ions (Mn+)

<sup>n</sup> M (n 1)HCl MCl (n 1)H n 1 + −+ + + <sup>+</sup> + +

membrane because of the anion‐exchange reaction with IL ( Q X+ − ) in the membrane to form

MCl Q X Q MCl X n 1 n 1 − +− + − −

This complex diffuses through the membrane to the membrane‐stripping solution interface. At the interface between the membrane and stripping solution of low chloride concentration (usually electrolyte solution such as sodium sulphate), the reverse reactions of above equations are proceeded to release the metal complex and to form free metal ion in the stripping phase. The transport of metal ion from the feed to the stripping phase by SILM is a coupled transport. In other cases, the metal extractants were dissolved in IL instead of conventional organic sol‐ vent, and the membrane was impregnated with IL containing the extractant as a metal carrier. Permeation mechanism of the metal ions depends on the extractant included in the membrane.

The stability of SILMs is greatly affected by the type and characteristic of solid support used to immobilize the IL as well as the preparation methods of the SILMs. When the SILMs will be used to separate the metals from the metal matrix, impregnated liquid with the conven‐ tional organic solvent in membrane can be released because of two factors: evaporation of the impregnated liquid due to high vapour pressure of volatile organic solvent and dissolving impregnated liquid both in the feed solution and in the stripping solution. Use of ILs with very low vapour pressure as the impregnated liquid resolved the first problem. On the other hand, dissolution of ILs to adjacent phases remains a problem that has to be solved. The stability of SILM is mainly affected by the properties of the support membrane and IL, and

Several groups have prepared SILMs using many types of solid support as well as method of preparation for metal separation. The stability of the membranes yielded was further evalu‐ ated using several techniques. de los Ríos et al. [18] used nylon, a hydrophilic polyamide mem‐ brane, as a solid support to impregnate methyltrioctylammonium chloride. This membrane was used to transport Zn(II), Cd(II), Cu(II) and Fe(III). Physical impregnation was conducted by mixing the solid support and ionic liquid in an ultrafiltration unit and applying nitro‐ gen pressure to force the ionic liquid to enter the pores of the solid support. The membrane

Furthermore, at the feed solution‐membrane interface, the complex of MCln 1

**3.1. Effect of solid support and preparation method on stability of membrane**

(1)

543

<sup>+</sup> enters into the

−

Supported Ionic Liquid Membranes for Metal Separation

http://dx.doi.org/10.5772/65754

+ + + + (2)

and HCl to form the complex are as follows:

the metal complex Q MCln 1 + −+ .

*3.1.1. Supported ionic liquid membrane*

preparation method [17].


b These values were determined using water‐saturated ionic liquid.

**Table 1.** Structures of common ILs used in metals separation.

### **3. Metal separation using supported ionic liquid membrane (SILM)**

An interesting field of application of supported ionic liquid membranes is the removal of metal ions from aqueous solutions. In this review, SLMs with IL as a metal carrier dissolved in the conventional organic solvent were excluded, and SLMs impregnated with IL or the mixture of IL and metal extractant in the absence of conventional solvents were discussed as SILM. In the SILMs impregnated with IL only, metals are slowed to be separated from their matrix if the metal ions are in the form of an anionic complex that reacts with the cation of ionic liquid in the membrane. Aqueous solution of metal ions in hydrogen chloride was used as the feed phase in order to form the anionic metal complex, although other forms such as nitrate and sulphate were also used in several investigations. The reactions of metal ions (Mn+) and HCl to form the complex are as follows:

$$\text{M}^{\text{n}+} + (\text{n} + \text{l})\text{HCl} \rightleftharpoons \text{MCl}\_{\text{n}+\text{l}}^{-} + (\text{n} + \text{l})\text{H}^{+} \tag{1}$$

Furthermore, at the feed solution‐membrane interface, the complex of MCln 1 − <sup>+</sup> enters into the membrane because of the anion‐exchange reaction with IL ( Q X+ − ) in the membrane to form the metal complex Q MCln 1 + −+ .

$$\rm{MCl}^{-}\_{\rm n+l} + \rm{Q}^{+}X^{-} \rightleftharpoons \rm{Q}^{+}\rm{MCl}^{-}\_{\rm n+l} + X^{-} \tag{2}$$

This complex diffuses through the membrane to the membrane‐stripping solution interface. At the interface between the membrane and stripping solution of low chloride concentration (usually electrolyte solution such as sodium sulphate), the reverse reactions of above equations are proceeded to release the metal complex and to form free metal ion in the stripping phase. The transport of metal ion from the feed to the stripping phase by SILM is a coupled transport.

In other cases, the metal extractants were dissolved in IL instead of conventional organic sol‐ vent, and the membrane was impregnated with IL containing the extractant as a metal carrier. Permeation mechanism of the metal ions depends on the extractant included in the membrane.

#### **3.1. Effect of solid support and preparation method on stability of membrane**

#### *3.1.1. Supported ionic liquid membrane*

will not be replaced easily from the pores of the membrane due to their very high viscos‐ ity. The properties of ILs (hydrophobicity, viscosity, solubility, etc.), which can be varied by altering the substitutive group on the cation or the combined anion [16], make them more widely applicable in many physical and chemical fields. Therefore, they have been considered environmentally benign solvents as compared to volatile organic solvents. The structures of common ILs used in metal separation are shown in **Table 1**. Most ILs show the pronounced miscibility in the aqueous phases due to their polar nature. ILs have to be hydrophobic as shown in **Table 1** to extract the metal ions from the aqueous phases. For this reason, the ILs

 **cP Water contenta**

**, wt %**

**3. Metal separation using supported ionic liquid membrane (SILM)**

An interesting field of application of supported ionic liquid membranes is the removal of metal ions from aqueous solutions. In this review, SLMs with IL as a metal carrier dissolved in the conventional organic solvent were excluded, and SLMs impregnated with IL or the mixture of IL and metal extractant in the absence of conventional solvents were discussed as SILM. In the SILMs impregnated with IL only, metals are slowed to be separated from their matrix if the metal ions are in the form of an anionic complex that reacts with the cation of

 Trihexyltetradecylphosphonium bis‐2,4,4‐trimethylpentyl

used in the metal separation were limited to those listed in **Table 1**.

542 Progress and Developments in Ionic Liquids

**Molecular structure Product name Viscositya**

**Chemical name**

*N*‐Methyl‐*N,N,N*‐ trioctylammonium chloride

Trihexyl(tetradecyl) phosphonium chloride

Trihexyl(tetradecyl) phosphonium bromide

phosphinate

These values were determined using water‐saturated ionic liquid. **Table 1.** Structures of common ILs used in metals separation.

a 25°C. b

Aliquat‐336 1450 (79.05)b 4.3 (20.3)b

Cyphos IL‐101 1824 (95.8)b 0.67 (12.8)b

Cyphos IL‐102 2094 (190.4)b 0.002 (6.1)b

Cyphos IL‐104 805.8 Not measured

The stability of SILMs is greatly affected by the type and characteristic of solid support used to immobilize the IL as well as the preparation methods of the SILMs. When the SILMs will be used to separate the metals from the metal matrix, impregnated liquid with the conven‐ tional organic solvent in membrane can be released because of two factors: evaporation of the impregnated liquid due to high vapour pressure of volatile organic solvent and dissolving impregnated liquid both in the feed solution and in the stripping solution. Use of ILs with very low vapour pressure as the impregnated liquid resolved the first problem. On the other hand, dissolution of ILs to adjacent phases remains a problem that has to be solved. The stability of SILM is mainly affected by the properties of the support membrane and IL, and preparation method [17].

Several groups have prepared SILMs using many types of solid support as well as method of preparation for metal separation. The stability of the membranes yielded was further evalu‐ ated using several techniques. de los Ríos et al. [18] used nylon, a hydrophilic polyamide mem‐ brane, as a solid support to impregnate methyltrioctylammonium chloride. This membrane was used to transport Zn(II), Cd(II), Cu(II) and Fe(III). Physical impregnation was conducted by mixing the solid support and ionic liquid in an ultrafiltration unit and applying nitro‐ gen pressure to force the ionic liquid to enter the pores of the solid support. The membrane resulting from this process was still stable after 456 h with the percentage of retained ionic liquid ranging from 75 to 89%, and in most membranes was higher than 80%.

faster than that with IL alone, loss of 1‐dodecanol from the newly developed PIM to the aque‐

Supported Ionic Liquid Membranes for Metal Separation

http://dx.doi.org/10.5772/65754

545

CTA is the most commonly used polymer support. Regel‐Rosocka et al. [26] prepared the PIM using CTA as a support and Cyphos IL 101 as carriers to remove Zn(II) and Fe(III) from chloride solution. Although the highest Zn(II) flux was obtained for membrane without plas‐ ticizer, with the highest Cyphos IL 101 content, the membranes with o‐nitrophenyl octyl ether as a plasticizer have been selected because of their better mechanical properties. Thus, CTA‐ based PIM needed plasticizer to form stable and mechanically strong membrane. Gardner et al. [27] prepared a series of new cellulose‐based PIMs. The ester linkages in the cellulose backbone of the polymer are susceptible to hydrolysis under extremes of pH, especially under alkaline conditions. The durability of the newly prepared PIMs against hydrolysis under alka‐ line and acidic conditions was evaluated. Durability increased with replacement of acetyl substitution on the cellulose polymer with propionyl or butyryl, while they also observed that

ion transport across the membrane decreased as the alkyl chain lengths increased.

Studies were carried out to compare the use of CTA and PVC as base polymer of PIM with Aliquat 336 [28] and Cyphos IL 101 and 104 [29]. Under the optimum condition, in Cr (VI) permeation with PIM including Aliquat 336, the permeation rates of CTA‐ and PVC‐based membranes were comparative, while in Zn (II) permeation with PIM including Cyphos IL 101 or 104, transport abilities of CTA‐based membranes were much better than those of PVC. The

PVDF is one of the most commonly used solid supports for SILM. Guo et al. [30, 31] suc‐ cessfully prepared new PIM including PVDF as a base polymer, 1‐alkyl‐3‐methylimidazolium hexafluorophosphate or tetrafluoroborate as ionic liquid plasticizers and Cyphos IL 104 and modified Aliquat 336 as a carrier and used this for transport of Cr(VI). The permeation rate with PIM including modified Aliquat 336/PVDF was faster than PIMs including Cyphos IL 104/PVDF or original Aliquat 336/PVC. After nine cycles, permeation rates of PIM composed of Cyphos IL 104/PVDF decreased to 69%, while it was found that the permeation rates of similar PIM composed of original Aliquat 336/CTA and modified Aliquat 336/PVDF decreased to 33 and 59% after six cycles, respectively. More recently, Bonggotgetsakul et al. prepared new PIM containing Cyphos IL‐104 as a carrier and poly(vinylidene fluoride‐co‐hexafluoro‐ propene) (PVDF‐HFP) as a base polymer to extract Au(III) from a hydrochloric acid solution [32]. PVDF‐HFP was found to be an excellent base polymer because of its high hydrophobicity, excellent thermal and mechanical properties, higher stability in strong acids and better solu‐ bility in tetrahydrofuran used for preparing membrane casting solutions. Extraction perfor‐ mance of this PIM was decreased to about 70% after 2 h and until 8 h extraction performance

Recently, polymer fibres using electrospinning method were successfully prepared for the metal extraction from the aqueous solutions [33–35]. Electospinning is an innovative technique for the production of polymer fibres with diameter of less than a few micrometres, resulting in a large surface area‐to‐volume ratio and high porosity. A solution is first prepared by dissolv‐ ing PVC and Aliquat 336 in the solvent. Then the solution is electospun to produce the mats consisting of electospun fibres. The role of Aliquat 336 in electospun fibres differed from that in

ous solutions in contact with it was observed.

CTA‐based PIM lost 42% of efficiency after 6 days [28].

remained at the same level.

Polyvinylidene fluoride (PVDF) is another solid support commonly used to immobi‐ lize IL. Baba et al. [19] used this material to impregnate 1‐octyl‐3‐methylimidazolium bis(trifluoromethanesulphonyl)imide ([C8mim][Tf2N]) containing N‐N‐dioctyldiglycol amic acid (DODGAA) to separate Dy(III) and Nd(III) in magnet scrap. The SEM micrograph after immersing PVDF in IL containing DODGAA showed that the porous structure of the liquid membrane appeared was well filled with [C8mim][Tf2N]. The membrane was proved to be stable during more than 140 h of operation.

The new material, which is probably a candidate for SILM, was successfully synthesized by Qian et al. [20]. Silica gel was used as a solid support to chemically bind tropine‐type ionic liq‐ uid, which contained 10% of ionic liquid. Although tropine‐type ionic liquid‐modified silica has not been used in membrane permeation, there is a high probability to use it as an extract‐ ant in the membrane permeation of metals because it is not only stable below 200°C, but it also has 19.36 mg/g of adsorption capacity to the Cu(II) ion.

#### *3.1.2. Polymer inclusion membrane*

The most recent type of physical impregnation, which is also the type most commonly used in metal separation, is called polymer inclusion membrane (PIM) [21]. While SILMs were commonly prepared by immersing IL to the pore of the solid support, PIMs were prepared by casting a solution containing a carrier, a plasticizer and a base polymer such as cellulose triacetate (CTA), poly (vinyl chloride) (PVC) or PVDF to form a thin, flexible and stable film. In many cases, a plasticizer or modifier is additionally incorporated into the mem‐ brane preparation in order to improve the PIM flexibility and the compatibility between the membrane components. It should be noted that an IL acts not only as a carrier but also as a plasticizer. Therefore, in the PIM using IL as a carrier, plasticizers are sometimes not needed. A PIM including ILs has a similar configuration to SILM, and it is considered to be a kind of SILM. It is found that the PIM including ILs became more stable than conventional SILM [22].

PVC is a commonly used base polymer in the preparation of PIM. Stability studies of PVC‐ based PIM containing Aliquat 336 as a carrier have been conducted [23]. They found that the mass loss of the membrane is due to leaching Aliquat 336 from the membrane and is suppressed in the salt solutions. They concluded that, although PIMs are capable of losing some membrane liquid phase when exposed to aqueous solutions, this loss can be minimized or even eliminated by increasing the solution concentration of the counter anion of IL. It was found that membrane of Aliquat 336 content higher than 50% was soft and sticky and mechanically too weak to be used for the metal extraction [24].

Recently, Bonggotgetsakul et al. prepared PIM containing Cyphos IL‐104 as a carrier using PVC support to extract Au(III) from a hydrochloric acid solution [25]. PIMs prepared with Cyphos IL‐104 alone, or with the addition of the modifier 1‐dodecanol, were homogeneous, transparent and flexible. Although the permeation rate with PIM including the modifier was faster than that with IL alone, loss of 1‐dodecanol from the newly developed PIM to the aque‐ ous solutions in contact with it was observed.

resulting from this process was still stable after 456 h with the percentage of retained ionic

Polyvinylidene fluoride (PVDF) is another solid support commonly used to immobi‐ lize IL. Baba et al. [19] used this material to impregnate 1‐octyl‐3‐methylimidazolium bis(trifluoromethanesulphonyl)imide ([C8mim][Tf2N]) containing N‐N‐dioctyldiglycol amic acid (DODGAA) to separate Dy(III) and Nd(III) in magnet scrap. The SEM micrograph after immersing PVDF in IL containing DODGAA showed that the porous structure of the liquid membrane appeared was well filled with [C8mim][Tf2N]. The membrane was proved to be

The new material, which is probably a candidate for SILM, was successfully synthesized by Qian et al. [20]. Silica gel was used as a solid support to chemically bind tropine‐type ionic liq‐ uid, which contained 10% of ionic liquid. Although tropine‐type ionic liquid‐modified silica has not been used in membrane permeation, there is a high probability to use it as an extract‐ ant in the membrane permeation of metals because it is not only stable below 200°C, but it also

The most recent type of physical impregnation, which is also the type most commonly used in metal separation, is called polymer inclusion membrane (PIM) [21]. While SILMs were commonly prepared by immersing IL to the pore of the solid support, PIMs were prepared by casting a solution containing a carrier, a plasticizer and a base polymer such as cellulose triacetate (CTA), poly (vinyl chloride) (PVC) or PVDF to form a thin, flexible and stable film. In many cases, a plasticizer or modifier is additionally incorporated into the mem‐ brane preparation in order to improve the PIM flexibility and the compatibility between the membrane components. It should be noted that an IL acts not only as a carrier but also as a plasticizer. Therefore, in the PIM using IL as a carrier, plasticizers are sometimes not needed. A PIM including ILs has a similar configuration to SILM, and it is considered to be a kind of SILM. It is found that the PIM including ILs became more stable than conventional

PVC is a commonly used base polymer in the preparation of PIM. Stability studies of PVC‐ based PIM containing Aliquat 336 as a carrier have been conducted [23]. They found that the mass loss of the membrane is due to leaching Aliquat 336 from the membrane and is suppressed in the salt solutions. They concluded that, although PIMs are capable of losing some membrane liquid phase when exposed to aqueous solutions, this loss can be minimized or even eliminated by increasing the solution concentration of the counter anion of IL. It was found that membrane of Aliquat 336 content higher than 50% was soft and sticky and

Recently, Bonggotgetsakul et al. prepared PIM containing Cyphos IL‐104 as a carrier using PVC support to extract Au(III) from a hydrochloric acid solution [25]. PIMs prepared with Cyphos IL‐104 alone, or with the addition of the modifier 1‐dodecanol, were homogeneous, transparent and flexible. Although the permeation rate with PIM including the modifier was

liquid ranging from 75 to 89%, and in most membranes was higher than 80%.

stable during more than 140 h of operation.

*3.1.2. Polymer inclusion membrane*

544 Progress and Developments in Ionic Liquids

SILM [22].

has 19.36 mg/g of adsorption capacity to the Cu(II) ion.

mechanically too weak to be used for the metal extraction [24].

CTA is the most commonly used polymer support. Regel‐Rosocka et al. [26] prepared the PIM using CTA as a support and Cyphos IL 101 as carriers to remove Zn(II) and Fe(III) from chloride solution. Although the highest Zn(II) flux was obtained for membrane without plas‐ ticizer, with the highest Cyphos IL 101 content, the membranes with o‐nitrophenyl octyl ether as a plasticizer have been selected because of their better mechanical properties. Thus, CTA‐ based PIM needed plasticizer to form stable and mechanically strong membrane. Gardner et al. [27] prepared a series of new cellulose‐based PIMs. The ester linkages in the cellulose backbone of the polymer are susceptible to hydrolysis under extremes of pH, especially under alkaline conditions. The durability of the newly prepared PIMs against hydrolysis under alka‐ line and acidic conditions was evaluated. Durability increased with replacement of acetyl substitution on the cellulose polymer with propionyl or butyryl, while they also observed that ion transport across the membrane decreased as the alkyl chain lengths increased.

Studies were carried out to compare the use of CTA and PVC as base polymer of PIM with Aliquat 336 [28] and Cyphos IL 101 and 104 [29]. Under the optimum condition, in Cr (VI) permeation with PIM including Aliquat 336, the permeation rates of CTA‐ and PVC‐based membranes were comparative, while in Zn (II) permeation with PIM including Cyphos IL 101 or 104, transport abilities of CTA‐based membranes were much better than those of PVC. The CTA‐based PIM lost 42% of efficiency after 6 days [28].

PVDF is one of the most commonly used solid supports for SILM. Guo et al. [30, 31] suc‐ cessfully prepared new PIM including PVDF as a base polymer, 1‐alkyl‐3‐methylimidazolium hexafluorophosphate or tetrafluoroborate as ionic liquid plasticizers and Cyphos IL 104 and modified Aliquat 336 as a carrier and used this for transport of Cr(VI). The permeation rate with PIM including modified Aliquat 336/PVDF was faster than PIMs including Cyphos IL 104/PVDF or original Aliquat 336/PVC. After nine cycles, permeation rates of PIM composed of Cyphos IL 104/PVDF decreased to 69%, while it was found that the permeation rates of similar PIM composed of original Aliquat 336/CTA and modified Aliquat 336/PVDF decreased to 33 and 59% after six cycles, respectively. More recently, Bonggotgetsakul et al. prepared new PIM containing Cyphos IL‐104 as a carrier and poly(vinylidene fluoride‐co‐hexafluoro‐ propene) (PVDF‐HFP) as a base polymer to extract Au(III) from a hydrochloric acid solution [32]. PVDF‐HFP was found to be an excellent base polymer because of its high hydrophobicity, excellent thermal and mechanical properties, higher stability in strong acids and better solu‐ bility in tetrahydrofuran used for preparing membrane casting solutions. Extraction perfor‐ mance of this PIM was decreased to about 70% after 2 h and until 8 h extraction performance remained at the same level.

Recently, polymer fibres using electrospinning method were successfully prepared for the metal extraction from the aqueous solutions [33–35]. Electospinning is an innovative technique for the production of polymer fibres with diameter of less than a few micrometres, resulting in a large surface area‐to‐volume ratio and high porosity. A solution is first prepared by dissolv‐ ing PVC and Aliquat 336 in the solvent. Then the solution is electospun to produce the mats consisting of electospun fibres. The role of Aliquat 336 in electospun fibres differed from that in PIMs. Electospun fibres were homogeneous and plasticized by Aliquat 336. On the other hand, PIMs composed of PVC and Aliquat 336 were visually transparent but were phase separated. Percolation threshold of electospun fibrous mats was much lower than that of PIMs.

#### **3.2. Effect of ionic liquid on metal separation**

Separation of metal by ionic liquid‐containing membrane is based on the chemical reaction between the ionic liquid and metal ion itself. Therefore, the efficiency and selectivity of metal separation are totally affected by the characteristic and type of ionic liquid in the membrane. This is because each ionic liquid has the specific affinity to the metallic ion as a result of the difference in functional groups bonded by an ionic liquid compound. Stojanovic et al. [36] summarized ammonium and phosphonium‐based ionic liquids as shown in **Table 1** in the extraction process. This section will focus on the efficiency and selectivity of metal separation with ionic liquid itself and using supported ionic liquid membrane.

#### *3.2.1. Quaternary ammonium salts*

Quaternary ammonium salts as anion exchangers have been frequently used to extract metal ions and hence are only effective in the presence of strong anionic ligands as indicated in Eq. (1). It is known that Aliquat 336 dissolved in the organic solvent had the enhanced perfor‐ mance for the removal of metal ions from hydrochloric acid solutions compared to the alkyl amine extractants [37]. Sulphate [38] and thiocyanate [39] as ligands also have been used in the metal solvent extraction system using Aliquat 336. Generally, selectivity of anion exchange reactions is believed to be poor compared to that of chelating reaction. A further approach is to anchor different functional groups onto the anion [36]. By this, it is possible to combine the hydrophobicity of ammonium cation with the affinity of the functional group to desired metal ion. This enhances both efficiency and selectivity of the extractant. Aliquat 336 was modified by exchange the chloride anion of original Aliquat 336 ( Q Cl + − ) with a salicylate anion ( HSal<sup>−</sup> ) for the extraction of Cu (II) and Fe (III) [40]. The following extraction reaction of Fe(III) was proposed.

$$\text{Fe}^{3+} + 2\text{Q}^{+} \text{HSal}^{-} + \text{HSO}\_{4}^{-} \rightleftharpoons \text{Q}^{+} \text{FeSal}\_{2}^{-} + \text{Q}^{+} \text{HSO}\_{4}^{-} + 2\text{H}^{+} \tag{3}$$

There are few examples of SILM using Aliquat 336 in the absence of diluents for metal separa‐ tion. In most cases of SLM, Aliquat 336 was diluted in the organic solvent, and the membrane

de los Ríos et al. [18] studied metal separation with the SILM based on methyltrioctylam‐

the aqueous solutions. To realize the selective permeation in SILM process, they examined

allowed the recovery of Cd(II). These results indicate that high selectivity of metal separation

As described above, Aliquat 336 has been extensively applied as a carrier in PIM and used to extract different metal species (e.g. Co(II), Ni(II), Cd(II), Cu(II), Cr(VI), Au(III), As(V), Pd(II), Zn(II)) [21, 36]. Kebiche‐Senhaji et al. [28] studied Cr(VI) transport from a mixture containing

the composition of the receiving solution. They found that Milli‐Q water and Na2

as the receiving solution allowed the recovery of Zn(II), Fe(III) while the use of NH3

][Cl−] allowed the almost complete removal of Zn(II), Cd(II), Fe(III) and Cu(II) from

][Cl−]. In the solvent extraction, it was found that the ionic liquid

Supported Ionic Liquid Membranes for Metal Separation

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547

CO3

solution

(6 M)

was impregnated with diluted Aliquat 336 solution.

**Figure 2.** Molecular structures of anion part of modified Aliquat 336.

can be reached by changing the receiving phase.

monium chloride, [MTOA<sup>+</sup>

[MTOA<sup>+</sup>

Both extraction efficiency and selectivity of Fe (III) were enhanced compared to those of original one. And thiosalicylate‐exchanged Aliquat 336 was reported to be selective to Cd (II) [41]. Aliquat 336‐based ionic liquids containing bis(2‐ethylhexyl)phosphate ([DEHP]−) or bis(2‐ethylhexyl)diglycolamate ([DGA]−) anions were synthesized. The extraction of Eu (III) over Am (III) was studied, and superior extraction of Eu (III) and excellent separation fac‐ tors were achieved [42]. The following extraction mechanism of Eu(III) with bis(2‐ethylhexyl) phosphate‐exchanged Aliquat 336 was proposed [43].

$$\text{Eu}^{3+} + 3\text{Q}^{+} \text{DEHP}^{-} + 3\text{NO}\_{3}^{-} \rightleftharpoons \text{Eu(NO}\_{3})\_{3} \text{\AA}^{+} \text{DEHP}^{-} \tag{4}$$

In this case, ion association mechanism was proposed instead of the anion exchange mecha‐ nism of Eq. (2) because both the Q+ and [DEHP]− are involved in Eu(III) extraction. The molecu‐ lar structures of counter‐anions of Aliquat 336 described in this chapter are shown in **Figure 2**.

Supported Ionic Liquid Membranes for Metal Separation http://dx.doi.org/10.5772/65754 547

PIMs. Electospun fibres were homogeneous and plasticized by Aliquat 336. On the other hand, PIMs composed of PVC and Aliquat 336 were visually transparent but were phase separated.

Separation of metal by ionic liquid‐containing membrane is based on the chemical reaction between the ionic liquid and metal ion itself. Therefore, the efficiency and selectivity of metal separation are totally affected by the characteristic and type of ionic liquid in the membrane. This is because each ionic liquid has the specific affinity to the metallic ion as a result of the difference in functional groups bonded by an ionic liquid compound. Stojanovic et al. [36] summarized ammonium and phosphonium‐based ionic liquids as shown in **Table 1** in the extraction process. This section will focus on the efficiency and selectivity of metal separation

Quaternary ammonium salts as anion exchangers have been frequently used to extract metal ions and hence are only effective in the presence of strong anionic ligands as indicated in Eq. (1). It is known that Aliquat 336 dissolved in the organic solvent had the enhanced perfor‐ mance for the removal of metal ions from hydrochloric acid solutions compared to the alkyl amine extractants [37]. Sulphate [38] and thiocyanate [39] as ligands also have been used in the metal solvent extraction system using Aliquat 336. Generally, selectivity of anion exchange reactions is believed to be poor compared to that of chelating reaction. A further approach is to anchor different functional groups onto the anion [36]. By this, it is possible to combine the hydrophobicity of ammonium cation with the affinity of the functional group to desired metal ion. This enhances both efficiency and selectivity of the extractant. Aliquat 336 was modified by exchange the chloride anion of original Aliquat 336 ( Q Cl + − ) with a salicylate anion ( HSal<sup>−</sup> ) for the extraction of Cu (II) and Fe (III) [40]. The following extraction reaction of Fe(III) was

Both extraction efficiency and selectivity of Fe (III) were enhanced compared to those of original one. And thiosalicylate‐exchanged Aliquat 336 was reported to be selective to Cd (II) [41]. Aliquat 336‐based ionic liquids containing bis(2‐ethylhexyl)phosphate ([DEHP]−) or bis(2‐ethylhexyl)diglycolamate ([DGA]−) anions were synthesized. The extraction of Eu (III) over Am (III) was studied, and superior extraction of Eu (III) and excellent separation fac‐ tors were achieved [42]. The following extraction mechanism of Eu(III) with bis(2‐ethylhexyl)

In this case, ion association mechanism was proposed instead of the anion exchange mecha‐

lar structures of counter‐anions of Aliquat 336 described in this chapter are shown in **Figure 2**.

<sup>3</sup> Fe 2Q HSal HSO Q FeSal Q HSO 2H 4 24 + + − − + −+ − + + + ++ (3)

<sup>3</sup> Eu 3Q DEHP 3NO Eu(NO ) 3Q DEHP <sup>3</sup> 3 3 ++ − − + − + + (4)

and [DEHP]− are involved in Eu(III) extraction. The molecu‐

Percolation threshold of electospun fibrous mats was much lower than that of PIMs.

with ionic liquid itself and using supported ionic liquid membrane.

phosphate‐exchanged Aliquat 336 was proposed [43].

nism of Eq. (2) because both the Q+

**3.2. Effect of ionic liquid on metal separation**

*3.2.1. Quaternary ammonium salts*

546 Progress and Developments in Ionic Liquids

proposed.

**Figure 2.** Molecular structures of anion part of modified Aliquat 336.

There are few examples of SILM using Aliquat 336 in the absence of diluents for metal separa‐ tion. In most cases of SLM, Aliquat 336 was diluted in the organic solvent, and the membrane was impregnated with diluted Aliquat 336 solution.

de los Ríos et al. [18] studied metal separation with the SILM based on methyltrioctylam‐ monium chloride, [MTOA<sup>+</sup> ][Cl−]. In the solvent extraction, it was found that the ionic liquid [MTOA<sup>+</sup> ][Cl−] allowed the almost complete removal of Zn(II), Cd(II), Fe(III) and Cu(II) from the aqueous solutions. To realize the selective permeation in SILM process, they examined the composition of the receiving solution. They found that Milli‐Q water and Na2 CO3 solution as the receiving solution allowed the recovery of Zn(II), Fe(III) while the use of NH3 (6 M) allowed the recovery of Cd(II). These results indicate that high selectivity of metal separation can be reached by changing the receiving phase.

As described above, Aliquat 336 has been extensively applied as a carrier in PIM and used to extract different metal species (e.g. Co(II), Ni(II), Cd(II), Cu(II), Cr(VI), Au(III), As(V), Pd(II), Zn(II)) [21, 36]. Kebiche‐Senhaji et al. [28] studied Cr(VI) transport from a mixture containing Ni(II), Zn(II), Cd(II) and Cu(II) with the PIM based on Aliquat 336. Because Q<sup>2</sup> CrO4 − complex is predominant form in the membrane, in sulphate media, Ni(II), Zn(II), Cd(II) and Cu(II) were not formed anionic species, which is exchangeable with chloride ion on Aliquat 336. Therefore, Ni(II), Zn(II), Cd(II) and Cu(II) were not transported through the PIM, while Cr(VI) is transported with high efficiency. Pont et al. [44] studied Cd(II) transport with PIM contain‐ ing Aliquat 336 from the chloride solution containing Ni(II). Although the reason of this selec‐ tivity is undescribed in the original paper, this may be because of difference in the stability constants of Cd(II) and Ni(II) of Eq. (1). Similar result was obtained for Cd(II) transport with PIM containing Aliquat 336 from the chloride solution containing Cu(II) [24].

and CTA [60]. High selectivity of Cd(II) over Cu(II) was obtained because of difference in the stability constants of Cd(II) and Cu(II) of Eq. (1). Gold (III) was successfully recovered from the hydrochloric acid solution through a PIM composed of Cyphos IL 104 and PVDF‐HFP to sodium sulphite solution [32]. In the stripping process, Au(III) is reduced to Au(I), which forms a complex with the sulphite anion. Therefore, complete transport of Au(III) from the

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549

In this chapter, we reviewed several articles discussing the effect of related parameters such as type of solid support and supporting method as well as kind of ILs on the stability of the conventional SILM and PIM, efficiency and selectivity of metal separation. Ammonium and phosphonium ILs have been used not only as the metal extractants and carriers but also as the diluents in the solvent extraction and the membrane separation processes. In gas/vapour separation, non‐volatile nature of ILs should allow for significant improvement to current processes and the development of new approaches to gas/vapour separation, while in liquid separation, problems of instability of the membrane still seem to remain despite achieving considerable improvement compared with SLM using conventional organic solvent [61]. The physical dissolution of ILs to adjacent liquid phases is in principle inevitable. On this occa‐ sion, PIM will become an excellent alternative. Although it is expected to be the lower diffu‐ sivity in PIM than in SILM, this disadvantage can be easily offset by creating a much thinner membrane in comparison to its traditional SLM counterpart [62]. So far, base polymers used in PIMs were limited to CTA and PVC. Recently, new base polymers are developed for the metal separation in PIMs. Anion‐exchanged ILs such as Aliquat 336 and Cyphos IL 101 had the different capacity and selectivity of metal extraction compared with the original ones. This

Department of Chemical Engineering and Materials Science, Doshisha University, Kyotanabe,

[1] Fu F., Xie L., Tang B., Wang Q., Jiang S. Application of a novel strategy‐advanced Fenton‐ chemical precipitation to the treatment of strong stability chelated heavy metal contain‐ ing wastewater. Chemical Engineering Journal. 2012;189–190:283–287. DOI: 10.1016/j.

feed to the receiving solution even in the presence of other metal ions was realized.

is suggesting that design of task‐specific ILs is important and possible.

\*Address all correspondence to: mmatsumo@mail.doshisha.ac.jp

**4. Conclusion**

**Author details**

Kyoto, Japan

**References**

cej.2012.02.073

Pius Dore Ola and Michiaki Matsumoto\*

Guo et al. [31] prepared a series of PIMs composed of anion‐exchanged Aliquat 336 and used them for Cr(VI) permeation in the presence of Fe(III), Co(II) Cu(II) and Zn(II). These transition metals were not transported through the PIM composed Aliquat 336 exchanged with acidic phosphorus compounds, while Cr(VI) is selectively transported. Because the optimum pHs of extractions of these transition metal cations with these modified Aliquat 336 extractants were within a range around neutrality, under the acidic conditions, the extraction reactions of these transition metal ions did not proceed.

### *3.2.2. Phosphonium salts*

Tetraalkyophosphonium salts like Cyphos IL 101, 102 and 104 listed in **Table 1** have recently been investigated as potential new IL extractants. Cyphos IL 101 diluted in organic solvent is extensively studied to extract metal ions such as Zn(II), Fe(III), Pd(II) and U(VI) [26, 45–48]. Cyphos IL 104 and 109 ((trihexyl)tetradecylphosphonium bis(trifluoromethylsulphonyl) imide) are also used as the extractants for Zn(II), Fe(III), Cd(II), Cr(VI) and lantanides [26, 49–51] and Au(III) [52].

Cyphos IL101 is diluted and used solely as the extractant, and many of the advantages using ILs are lost. Viscosity of pure Cyphos IL 101 is 24.69 Pa·s at 20°C, and it drops to 11.10 Pa·s by mixing with 1% water. Therefore, undiluted Cyphos IL 101 can be used as both extractant and diluents when contacting with aqueous phase [53]. Undiluted Cyphos IL 101 was used for extractions of Co(II), Fe(III) and rare earths [53–56]. Extraction mechanism of metal ions with undiluted Cyphos IL101 is same as that of Cyphos IL 101 diluted in organic solvent. Cholio‐Gonzalez et al. employed Cyphos IL 101 as diluent and Cyanex 272 as extractant for separation of Co(II) and Ni(II) [57].

Unfortunately, there are no reports on metal separation with SILM with undiluted phospho‐ nium‐based ionic liquids. A number of reports on SLM described the use of diluted ionic liquids. Fe(III) and Zn(II) were transported through a PIM composed of Cyphos IL 101 and CTA or PVC [26, 58]. Sulphuric acid was found to be an effective stripping phase. Cr(VI) was removed from hydrochloric acid through a PIM composed of Cyphos IL 104 and PVDF to sodium hydroxide solution [30]. Pd(II) was permeated through a PIM composed of Cyphos IL 101, 102 or 104 and CTA [59]. The highest values of extraction efficiency were obtained for Cyphos IL 102 and 104 as IL and 3 M HCl as a receiving phase. The efficiency strongly depended on the type of carrier and the receiving phase. Separation of Cd(II) and Cu(II) from hydrochloric acid solution was conducted using PIMs composed of Cyphos IL 101 and 104, and CTA [60]. High selectivity of Cd(II) over Cu(II) was obtained because of difference in the stability constants of Cd(II) and Cu(II) of Eq. (1). Gold (III) was successfully recovered from the hydrochloric acid solution through a PIM composed of Cyphos IL 104 and PVDF‐HFP to sodium sulphite solution [32]. In the stripping process, Au(III) is reduced to Au(I), which forms a complex with the sulphite anion. Therefore, complete transport of Au(III) from the feed to the receiving solution even in the presence of other metal ions was realized.

### **4. Conclusion**

Ni(II), Zn(II), Cd(II) and Cu(II) with the PIM based on Aliquat 336. Because Q<sup>2</sup>

PIM containing Aliquat 336 from the chloride solution containing Cu(II) [24].

transition metal ions did not proceed.

*3.2.2. Phosphonium salts*

548 Progress and Developments in Ionic Liquids

49–51] and Au(III) [52].

separation of Co(II) and Ni(II) [57].

is predominant form in the membrane, in sulphate media, Ni(II), Zn(II), Cd(II) and Cu(II) were not formed anionic species, which is exchangeable with chloride ion on Aliquat 336. Therefore, Ni(II), Zn(II), Cd(II) and Cu(II) were not transported through the PIM, while Cr(VI) is transported with high efficiency. Pont et al. [44] studied Cd(II) transport with PIM contain‐ ing Aliquat 336 from the chloride solution containing Ni(II). Although the reason of this selec‐ tivity is undescribed in the original paper, this may be because of difference in the stability constants of Cd(II) and Ni(II) of Eq. (1). Similar result was obtained for Cd(II) transport with

Guo et al. [31] prepared a series of PIMs composed of anion‐exchanged Aliquat 336 and used them for Cr(VI) permeation in the presence of Fe(III), Co(II) Cu(II) and Zn(II). These transition metals were not transported through the PIM composed Aliquat 336 exchanged with acidic phosphorus compounds, while Cr(VI) is selectively transported. Because the optimum pHs of extractions of these transition metal cations with these modified Aliquat 336 extractants were within a range around neutrality, under the acidic conditions, the extraction reactions of these

Tetraalkyophosphonium salts like Cyphos IL 101, 102 and 104 listed in **Table 1** have recently been investigated as potential new IL extractants. Cyphos IL 101 diluted in organic solvent is extensively studied to extract metal ions such as Zn(II), Fe(III), Pd(II) and U(VI) [26, 45–48]. Cyphos IL 104 and 109 ((trihexyl)tetradecylphosphonium bis(trifluoromethylsulphonyl) imide) are also used as the extractants for Zn(II), Fe(III), Cd(II), Cr(VI) and lantanides [26,

Cyphos IL101 is diluted and used solely as the extractant, and many of the advantages using ILs are lost. Viscosity of pure Cyphos IL 101 is 24.69 Pa·s at 20°C, and it drops to 11.10 Pa·s by mixing with 1% water. Therefore, undiluted Cyphos IL 101 can be used as both extractant and diluents when contacting with aqueous phase [53]. Undiluted Cyphos IL 101 was used for extractions of Co(II), Fe(III) and rare earths [53–56]. Extraction mechanism of metal ions with undiluted Cyphos IL101 is same as that of Cyphos IL 101 diluted in organic solvent. Cholio‐Gonzalez et al. employed Cyphos IL 101 as diluent and Cyanex 272 as extractant for

Unfortunately, there are no reports on metal separation with SILM with undiluted phospho‐ nium‐based ionic liquids. A number of reports on SLM described the use of diluted ionic liquids. Fe(III) and Zn(II) were transported through a PIM composed of Cyphos IL 101 and CTA or PVC [26, 58]. Sulphuric acid was found to be an effective stripping phase. Cr(VI) was removed from hydrochloric acid through a PIM composed of Cyphos IL 104 and PVDF to sodium hydroxide solution [30]. Pd(II) was permeated through a PIM composed of Cyphos IL 101, 102 or 104 and CTA [59]. The highest values of extraction efficiency were obtained for Cyphos IL 102 and 104 as IL and 3 M HCl as a receiving phase. The efficiency strongly depended on the type of carrier and the receiving phase. Separation of Cd(II) and Cu(II) from hydrochloric acid solution was conducted using PIMs composed of Cyphos IL 101 and 104,

CrO4

− complex

In this chapter, we reviewed several articles discussing the effect of related parameters such as type of solid support and supporting method as well as kind of ILs on the stability of the conventional SILM and PIM, efficiency and selectivity of metal separation. Ammonium and phosphonium ILs have been used not only as the metal extractants and carriers but also as the diluents in the solvent extraction and the membrane separation processes. In gas/vapour separation, non‐volatile nature of ILs should allow for significant improvement to current processes and the development of new approaches to gas/vapour separation, while in liquid separation, problems of instability of the membrane still seem to remain despite achieving considerable improvement compared with SLM using conventional organic solvent [61]. The physical dissolution of ILs to adjacent liquid phases is in principle inevitable. On this occa‐ sion, PIM will become an excellent alternative. Although it is expected to be the lower diffu‐ sivity in PIM than in SILM, this disadvantage can be easily offset by creating a much thinner membrane in comparison to its traditional SLM counterpart [62]. So far, base polymers used in PIMs were limited to CTA and PVC. Recently, new base polymers are developed for the metal separation in PIMs. Anion‐exchanged ILs such as Aliquat 336 and Cyphos IL 101 had the different capacity and selectivity of metal extraction compared with the original ones. This is suggesting that design of task‐specific ILs is important and possible.

### **Author details**

Pius Dore Ola and Michiaki Matsumoto\*

\*Address all correspondence to: mmatsumo@mail.doshisha.ac.jp

Department of Chemical Engineering and Materials Science, Doshisha University, Kyotanabe, Kyoto, Japan

### **References**

[1] Fu F., Xie L., Tang B., Wang Q., Jiang S. Application of a novel strategy‐advanced Fenton‐ chemical precipitation to the treatment of strong stability chelated heavy metal contain‐ ing wastewater. Chemical Engineering Journal. 2012;189–190:283–287. DOI: 10.1016/j. cej.2012.02.073

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554 Progress and Developments in Ionic Liquids


**Section 7**

**Supported Ionic Liquids**

**Supported Ionic Liquids**

**Chapter 24**

**Provisional chapter**

**Ionic Liquids Immobilized on Magnetic Nanoparticles**

Ionic liquids (ILs) immobilized on supports are among the most important derivatives of ILs. The immobilization process of ILs can transfer their desired properties to substrates. The combination of the advantages of ILs with those of support materials will derive new performances while retaining the properties of both moieties. As green media in organic catalytic reactions, based on utilizing the ability of ILs to stabilize the catalysts, they have many advantages over free ILs, including avoiding the leaching of ILs, reducing their amount, and improving the recoverability and reusability of both themselves and catalysts. This has critical significance from both environmental and economical points of view. Recently, ionic liquids immobilized on magnetic nanoparti‐ cles (MNPs) have drawn increasing attention in catalytic reactions and separation technologies and achieved substantial progress. The combination of MNPs and ILs gives magnetic‐supported ionic liquids, which exhibit the unique properties of ILs as well as facile separation by an external magnetic field. The excellent efficiency of this kind of immobilized ionic liquids offers a great advantage compared with other sorts of magnetic supports. In this chapter, the green catalytic processes and recent advances in organic synthesis catalyzed by ionic liquids immobilized on magnetic nanoparticles are

**Keywords:** ionic liquids, magnetic nanoparticles, green synthesis, retrievable catalysts

Ionic liquids (ILs) are progressively being studied for targeted chemical tasks due to their unique chemical and physical properties such as nonvolatility, nonflammability, thermal stability and controlled miscibility [1]. In spite of the fact that ILs contained several advantages, but their extensive practical application was still prevented by some drawbacks like high viscosity, difficult recyclability and high cost of ILs in large‐scale utilization [2]. Therefore, in

and reproduction in any medium, provided the original work is properly cited.

© 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution,

© 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

**Ionic Liquids Immobilized on Magnetic Nanoparticles**

Masoud Mokhtary

**Abstract**

highlighted.

**1. Introduction**

Masoud Mokhtary

http://dx.doi.org/10.5772/65794

Additional information is available at the end of the chapter

Additional information is available at the end of the chapter

#### **Ionic Liquids Immobilized on Magnetic Nanoparticles Ionic Liquids Immobilized on Magnetic Nanoparticles**

#### Masoud Mokhtary Masoud Mokhtary

Additional information is available at the end of the chapter Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/65794

#### **Abstract**

Ionic liquids (ILs) immobilized on supports are among the most important derivatives of ILs. The immobilization process of ILs can transfer their desired properties to substrates. The combination of the advantages of ILs with those of support materials will derive new performances while retaining the properties of both moieties. As green media in organic catalytic reactions, based on utilizing the ability of ILs to stabilize the catalysts, they have many advantages over free ILs, including avoiding the leaching of ILs, reducing their amount, and improving the recoverability and reusability of both themselves and catalysts. This has critical significance from both environmental and economical points of view. Recently, ionic liquids immobilized on magnetic nanoparti‐ cles (MNPs) have drawn increasing attention in catalytic reactions and separation technologies and achieved substantial progress. The combination of MNPs and ILs gives magnetic‐supported ionic liquids, which exhibit the unique properties of ILs as well as facile separation by an external magnetic field. The excellent efficiency of this kind of immobilized ionic liquids offers a great advantage compared with other sorts of magnetic supports. In this chapter, the green catalytic processes and recent advances in organic synthesis catalyzed by ionic liquids immobilized on magnetic nanoparticles are highlighted.

**Keywords:** ionic liquids, magnetic nanoparticles, green synthesis, retrievable catalysts

### **1. Introduction**

Ionic liquids (ILs) are progressively being studied for targeted chemical tasks due to their unique chemical and physical properties such as nonvolatility, nonflammability, thermal stability and controlled miscibility [1]. In spite of the fact that ILs contained several advantages, but their extensive practical application was still prevented by some drawbacks like high viscosity, difficult recyclability and high cost of ILs in large‐scale utilization [2]. Therefore, in

© 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

orderto decrease these difficulties, immobilized IL had been prepared as a new heterogeneous catalyst with the useful features of ILs and inorganic acids for catalyzed reactions [3]. Among several usages of ILs in organic chemistry, imidazolium ionic liquid‐type catalysts indicate one of the most successful developments [4]. This chapterfocuses on the recent progress in organic synthesis catalayzed by inoic liquids immobilized on magnetic nanoparticles.

The synthesis of a magnetic nanoparticle‐supported polyoxometalate has been reported, and this nanoparticle has been used as an efficient heterogeneous catalyst to prepare *α*‐amino‐ phosphonates under solvent‐less conditions at room temperature [8]. The catalyst is easily recovered by simple magnetic separation and can be recycled several times with no consider‐

Ionic Liquids Immobilized on Magnetic Nanoparticles

http://dx.doi.org/10.5772/65794

561

In another research, a magnetic‐supported acidic ionic liquid has been prepared and evaluated in one‐pot synthesis of spirooxindoles [9]. Main properties of this approach are simplicity, low price, high efficiency, wide application scope, reusability and easy retrieval of the catalyst by

A chemoselective hydrogenation of *α*,*β*‐unsaturated aldehydes and alkynes was successfully demonstrated by Abu‐Reziq et al. [10]. The Pt nanoparticles were adsorbed on the IL‐func‐ tionalized MNPs via ion exchange with K2PtCl4 following by reduction with hydrazine. The diphenylacetylene was hydrogenated in methanol using this catalyst at 90°C under hydrogen

able loss in its catalytic activity (**Scheme 3**).

an external magnet (**Scheme 4**).

**Scheme 3.** PTA/Si‐imid@Si‐MNPs catalyzed synthesis of *α*‐aminophosphonates.

**Scheme 4.** Different synthesized spirooxindoles catalyzed by MSAIL.

pressure with the selective preparation of *cis*‐alkenes (**Scheme 5**).

### **2. Ionic liquids immobilized on MNPs in multicomponent reactions**

Recent studies represent that magnetic nanoparticles (MNPs) are excellent supports for ILs owing to their good stability, easily preparation and functionalization, high surface area, low toxicity and simple separation by external magnetic attractions [5]. These special features have made MNPs a convenient alternative to catalyst supports. As an example, a magnetically Fe3O4@SiO2 nanoparticle‐immobilized ionic liquid (MNPs@SiO2‐IL) was prepared by Azgomi and Mokhtary [6]. The MNPs@SiO2‐IL was assessed as a recyclable catalyst for the one‐pot synthesis of 1,3‐thiazolidin‐4‐ones with good to great efficiency under solvent‐less conditions. The catalyst could be simply recycled using a magnetic field and reused for 10 times with no considerable loss in its activity (**Scheme 1**).

**Scheme 1.** One‐pot synthesis of 1,3‐thiazolidin‐4‐ones catalyzed using MNPs@SiO2‐IL.

A dicationic ionic liquid immobilized on superparamagnetic iron oxide nanoparticles (SPION‐ACl2) has been used as a green and powerful catalyst to effectively synthesis deriva‐ tives of *β*‐amidoalkylnaphthol with high to excellent yields [7]. This catalyst could be recovered and reused for at least six times with no loss in catalytic activity (**Scheme 2**).

**Scheme 2.** Synthesis of *β*‐amidoalkylnaphthols catalyzed by SPION‐ACl2.

The synthesis of a magnetic nanoparticle‐supported polyoxometalate has been reported, and this nanoparticle has been used as an efficient heterogeneous catalyst to prepare *α*‐amino‐ phosphonates under solvent‐less conditions at room temperature [8]. The catalyst is easily recovered by simple magnetic separation and can be recycled several times with no consider‐ able loss in its catalytic activity (**Scheme 3**).

**Scheme 3.** PTA/Si‐imid@Si‐MNPs catalyzed synthesis of *α*‐aminophosphonates.

orderto decrease these difficulties, immobilized IL had been prepared as a new heterogeneous catalyst with the useful features of ILs and inorganic acids for catalyzed reactions [3]. Among several usages of ILs in organic chemistry, imidazolium ionic liquid‐type catalysts indicate one of the most successful developments [4]. This chapterfocuses on the recent progress in organic

synthesis catalayzed by inoic liquids immobilized on magnetic nanoparticles.

considerable loss in its activity (**Scheme 1**).

560 Progress and Developments in Ionic Liquids

**Scheme 1.** One‐pot synthesis of 1,3‐thiazolidin‐4‐ones catalyzed using MNPs@SiO2‐IL.

**Scheme 2.** Synthesis of *β*‐amidoalkylnaphthols catalyzed by SPION‐ACl2.

and reused for at least six times with no loss in catalytic activity (**Scheme 2**).

**2. Ionic liquids immobilized on MNPs in multicomponent reactions**

Recent studies represent that magnetic nanoparticles (MNPs) are excellent supports for ILs owing to their good stability, easily preparation and functionalization, high surface area, low toxicity and simple separation by external magnetic attractions [5]. These special features have made MNPs a convenient alternative to catalyst supports. As an example, a magnetically Fe3O4@SiO2 nanoparticle‐immobilized ionic liquid (MNPs@SiO2‐IL) was prepared by Azgomi and Mokhtary [6]. The MNPs@SiO2‐IL was assessed as a recyclable catalyst for the one‐pot synthesis of 1,3‐thiazolidin‐4‐ones with good to great efficiency under solvent‐less conditions. The catalyst could be simply recycled using a magnetic field and reused for 10 times with no

A dicationic ionic liquid immobilized on superparamagnetic iron oxide nanoparticles (SPION‐ACl2) has been used as a green and powerful catalyst to effectively synthesis deriva‐ tives of *β*‐amidoalkylnaphthol with high to excellent yields [7]. This catalyst could be recovered In another research, a magnetic‐supported acidic ionic liquid has been prepared and evaluated in one‐pot synthesis of spirooxindoles [9]. Main properties of this approach are simplicity, low price, high efficiency, wide application scope, reusability and easy retrieval of the catalyst by an external magnet (**Scheme 4**).

**Scheme 4.** Different synthesized spirooxindoles catalyzed by MSAIL.

A chemoselective hydrogenation of *α*,*β*‐unsaturated aldehydes and alkynes was successfully demonstrated by Abu‐Reziq et al. [10]. The Pt nanoparticles were adsorbed on the IL‐func‐ tionalized MNPs via ion exchange with K2PtCl4 following by reduction with hydrazine. The diphenylacetylene was hydrogenated in methanol using this catalyst at 90°C under hydrogen pressure with the selective preparation of *cis*‐alkenes (**Scheme 5**).

**Scheme 7.** Synthesis of thiazoloquinolines in the existence of MNPs‐HPW.

**Scheme 8.** Mizoroki–Heck cross‐coupling reaction using Pd‐DABCO‐*γ*‐Fe2O3.

ported basic ionic liquids [14].

Sobhani and Pakdin‐Parizi have improved an efficient heterogeneous catalyst for the Mizor‐ oki–Heck cross‐coupling [13]. In this technique, *γ*‐Fe2O3 magnetic nanoparticles immobilized with palladium‐DABCO complex (Pd‐DABCO‐*γ*‐Fe2O3) were prepared as an recyclable catalyst for Mizoroki–Heck cross‐coupling reaction of aryl halides with olefins whose activity

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The synthesis of hydroxyapatite‐encapsulated magnetic nanoparticles immobilized with diethyl aliphatic amine basic ionic liquids (HAP‐*γ*‐Fe2O3@BILs) was reported, and it was applied as effective magnetic catalysts for Knoevenagel condensation reactions in aqueous medium (**Scheme 9**). The reactants were quantitatively converted under moderate conditions; the catalyst activity showed no significant loss after recovering via suitable magnetic field. The magnetic catalyst showed an excellent efficiency compared with homogeneous basic ionic liquid catalyst and the basic ionic liquid‐modified polystyrene resin catalyst that was attributed to the cooperation between the base sites produced through framework HAP and the sup‐

is not changed even after five times under solvent‐less conditions (**Scheme 8**).

**Scheme 5.** Hydrogenation of diphenylacetylene in the presence of IL‐PtMNPs.

Anchoring AlxCly‐IL onto the silica‐coated *γ*‐Fe2O3 particles afforded AlxCly‐IL‐SiO2@*γ*‐Fe2O3 [11]. The catalyst assessment was performed to synthesis of *β*‐ketoenol ethers (**Scheme 6**). The effectiveness of immobilized catalyst was confirmed, and the products were produced in high with excellent efficiency at ambient temperature. Furthermore, the catalyst could be simply retrieved using an external magnet and reused for six times with no considerable loss in its catalytic activity.

**Scheme 6.** Synthesis of *β*‐keto enol ethers by AlxCly‐IL‐SiO2@γ‐Fe2O3.

The synthesis of Fe3O4@SiO2/salen/Mn/IL/HPW has been performed through attaching H3PW12O40 on magnetite nanoparticles modified with ionic liquid [12]. The catalyst was used for one‐pot synthesis of thiazoloquinolines, in good to great efficiency under solvent‐less conditions (**Scheme 7**). The catalyst could be simply recovered using a magnetic field and reused ten times with no significant loss in its activity.

**Scheme 7.** Synthesis of thiazoloquinolines in the existence of MNPs‐HPW.

**Scheme 5.** Hydrogenation of diphenylacetylene in the presence of IL‐PtMNPs.

**Scheme 6.** Synthesis of *β*‐keto enol ethers by AlxCly‐IL‐SiO2@γ‐Fe2O3.

reused ten times with no significant loss in its activity.

catalytic activity.

562 Progress and Developments in Ionic Liquids

Anchoring AlxCly‐IL onto the silica‐coated *γ*‐Fe2O3 particles afforded AlxCly‐IL‐SiO2@*γ*‐Fe2O3 [11]. The catalyst assessment was performed to synthesis of *β*‐ketoenol ethers (**Scheme 6**). The effectiveness of immobilized catalyst was confirmed, and the products were produced in high with excellent efficiency at ambient temperature. Furthermore, the catalyst could be simply retrieved using an external magnet and reused for six times with no considerable loss in its

The synthesis of Fe3O4@SiO2/salen/Mn/IL/HPW has been performed through attaching H3PW12O40 on magnetite nanoparticles modified with ionic liquid [12]. The catalyst was used for one‐pot synthesis of thiazoloquinolines, in good to great efficiency under solvent‐less conditions (**Scheme 7**). The catalyst could be simply recovered using a magnetic field and Sobhani and Pakdin‐Parizi have improved an efficient heterogeneous catalyst for the Mizor‐ oki–Heck cross‐coupling [13]. In this technique, *γ*‐Fe2O3 magnetic nanoparticles immobilized with palladium‐DABCO complex (Pd‐DABCO‐*γ*‐Fe2O3) were prepared as an recyclable catalyst for Mizoroki–Heck cross‐coupling reaction of aryl halides with olefins whose activity is not changed even after five times under solvent‐less conditions (**Scheme 8**).

**Scheme 8.** Mizoroki–Heck cross‐coupling reaction using Pd‐DABCO‐*γ*‐Fe2O3.

The synthesis of hydroxyapatite‐encapsulated magnetic nanoparticles immobilized with diethyl aliphatic amine basic ionic liquids (HAP‐*γ*‐Fe2O3@BILs) was reported, and it was applied as effective magnetic catalysts for Knoevenagel condensation reactions in aqueous medium (**Scheme 9**). The reactants were quantitatively converted under moderate conditions; the catalyst activity showed no significant loss after recovering via suitable magnetic field. The magnetic catalyst showed an excellent efficiency compared with homogeneous basic ionic liquid catalyst and the basic ionic liquid‐modified polystyrene resin catalyst that was attributed to the cooperation between the base sites produced through framework HAP and the sup‐ ported basic ionic liquids [14].

Also, chiral amine hybrids with magnetic POMs were easily prepared by mixing MNP‐PW

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The MNP‐PW‐immobilized chiral amine catalysts were next applied in typical enamine‐based asymmetric direct aldol condensations. Acetone reacted with different aromatic aldehydes in the presence of 5 mol% of MNP‐1‐PW‐A, to give the desired products with high efficiency and

**Entry R t/h Yieldb (%) ee (%)c**

.

Also, aldol donors such as cyclohexanone and cyclopentanone worked very well in this catalytic system (**Table 3**). Moreover, the outcomes achieved from the similar reactions with POM‐chiral amine hybrid PW‐A given in **Table 3** (entries 1 vs. 2). The magnetic POM‐ immobilized catalyst MNP‐1‐PW‐A showed slightly higher stereoselectivity and enantiose‐ lectivity albeit with a little loss of activity. The resulted noncovalently immobilized catalyst could be reused up to 11 times with essentially no loss of activity and enantioselectivity.

 2‐NO2C6H4 12 83 89 3‐NO2C6H4 12 87 89 4‐CF3C6H4 30 81 90 2‐CNC6H4 30 80 87 2‐ClC6H4 48 72 87 2‐BrC6H4 48 77 88

Reaction conditions: catalyst (5 mol%), acetone (0.20 mL) and aldehyde (0.25 mmol).

and chiral amine via sonication in dry THF (**Scheme 11**).

**Scheme 11.** Magnetic POM supported chiral amine catalyst MNP‐1‐PW‐A.

enantioselectivities (**Table 2**).

a

b

c

Isolated yield.

Determined by chiral HPLC.

**Table 2.** MNP‐1‐PW‐A catalyzed aldol reaction of acetonea

**Scheme 9.** A Knoevenagel condensation reaction catalyzed by HAP‐*γ*‐Fe2O3@BILs.

Magnetic nanoparticles supported with polyoxometalates (POMs) via ionic interaction were acquired through an easy sonication between modified magnetic nanoparticles and polyoxo‐ metalates. This material can be used as a highly active acid catalyst and as a catalyst support for chiral amines [15]. The immobilization of POM on MNPs was obtained by sonication of a mixture of MNPs (MNP‐1 or MNP‐2) and POM (H3PW12O40) in dry THF (**Scheme 10**).

**Scheme 10.** Structure of MNP‐1‐PW and MNP‐2‐PW catalysts.

The Friedel‐Crafts reaction of indole and chalcone was selected to examine the acidic catalytic activity and reusability of the prepared MNP‐1‐PW and MNP‐2‐PW catalysts. As observed in **Table 1**, the reactions between diverse chalcones and indoles continued efficiently to result the desired products with a high efficiency. Both magnetic POMs showed excellent activity and reusability for 12 recycles.


a Reaction conditions: catalyst (5 mol%), indole derivative (0.25 mmol), chalcone derivative (0.20 mmol) and THF (0.2  ml).

b Isolated yield.

**Table 1.** Friedel‐Crafts reactions of indoles and chalconesa . Also, chiral amine hybrids with magnetic POMs were easily prepared by mixing MNP‐PW and chiral amine via sonication in dry THF (**Scheme 11**).

**Scheme 11.** Magnetic POM supported chiral amine catalyst MNP‐1‐PW‐A.

The MNP‐PW‐immobilized chiral amine catalysts were next applied in typical enamine‐based asymmetric direct aldol condensations. Acetone reacted with different aromatic aldehydes in the presence of 5 mol% of MNP‐1‐PW‐A, to give the desired products with high efficiency and enantioselectivities (**Table 2**).


a Reaction conditions: catalyst (5 mol%), acetone (0.20 mL) and aldehyde (0.25 mmol).

b Isolated yield.

**Scheme 9.** A Knoevenagel condensation reaction catalyzed by HAP‐*γ*‐Fe2O3@BILs.

**Scheme 10.** Structure of MNP‐1‐PW and MNP‐2‐PW catalysts.

**Table 1.** Friedel‐Crafts reactions of indoles and chalconesa

reusability for 12 recycles.

564 Progress and Developments in Ionic Liquids

**Entry R1**

a

ml). b

Isolated yield.

Magnetic nanoparticles supported with polyoxometalates (POMs) via ionic interaction were acquired through an easy sonication between modified magnetic nanoparticles and polyoxo‐ metalates. This material can be used as a highly active acid catalyst and as a catalyst support for chiral amines [15]. The immobilization of POM on MNPs was obtained by sonication of a

The Friedel‐Crafts reaction of indole and chalcone was selected to examine the acidic catalytic activity and reusability of the prepared MNP‐1‐PW and MNP‐2‐PW catalysts. As observed in **Table 1**, the reactions between diverse chalcones and indoles continued efficiently to result the desired products with a high efficiency. Both magnetic POMs showed excellent activity and

**t/h Yieldb (%) t/h Yieldb (%)**

Reaction conditions: catalyst (5 mol%), indole derivative (0.25 mmol), chalcone derivative (0.20 mmol) and THF (0.2 

.

 **R2 MNP‐1‐PW MNP‐2‐PW**

 H 5‐MeO 20 93 20 94 H 5‐Me 20 96 20 96 H 5‐Br 7 98 6 97 H 5‐Cl 7 97 6 97 H 5‐I 7 99 6 98 H 6‐Cl 7 98 6 98 4‐Cl H 12 90 12 92 4‐Me H 12 88 12 90

mixture of MNPs (MNP‐1 or MNP‐2) and POM (H3PW12O40) in dry THF (**Scheme 10**).

c Determined by chiral HPLC.

**Table 2.** MNP‐1‐PW‐A catalyzed aldol reaction of acetonea .

Also, aldol donors such as cyclohexanone and cyclopentanone worked very well in this catalytic system (**Table 3**). Moreover, the outcomes achieved from the similar reactions with POM‐chiral amine hybrid PW‐A given in **Table 3** (entries 1 vs. 2). The magnetic POM‐ immobilized catalyst MNP‐1‐PW‐A showed slightly higher stereoselectivity and enantiose‐ lectivity albeit with a little loss of activity. The resulted noncovalently immobilized catalyst could be reused up to 11 times with essentially no loss of activity and enantioselectivity.


*α*‐Fe2O3‐MCM‐41 immobilized with amino acid ionic liquid was prepared as a retrievable catalyst for synthesizing quinazolin‐4(3*H*)‐ones at ambient temperature in short reaction times under oxidant and solvent‐less conditions (**Scheme 13**). It is supposed that the L‐prolinium nitrate in the mesochannels of (*α*‐Fe2O3)‐MCM‐41 might enhance the strength of Brønsted acid

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**Scheme 13.** (*α*‐Fe2O3)‐MCM‐41‐L‐prolinium nitrate for the synthesis of quinazolin‐4(3*H*)‐ones.

Moreover, there is a report on synthesis of magnetic nanoparticles immobilized with Ni2+ ion‐ containing 1‐methyl‐3‐(3‐trimethoxysilylpropyl) imidazolium chloride ionic liquid as a recoverable nanocatalyst for the Heck reaction at 100°C, and it can be applied after washing

γ‐Fe2O3 nanoparticles immobilized with 2‐Hydroxyethylammonium sulphonate (γ‐Fe2O3‐2‐ HEAS) were prepared through the reaction of n‐butylsulfonated γ‐Fe2O3 with ethanolamine [19]. Here, the catalyst effectively increases the condensation of both aliphatic and aromatic

and oxidation power of catalytic system [17].

with no loss in activity (**Scheme 14**) [18].

**Scheme 14.** Heck reaction catalyzed by IL–Ni(II)–MNPs.

a Reaction conditions: catalyst (5 mol%), ketone (0.20 mL) and aldehyde (0.25 mmol).

b Isolated yield.

c Determined by chiral HPLC.

dDetermined by chiral HPLC.

e PW‐A (1 mol%) was employed.

**Table 3.** MNP‐1‐PW‐A catalyzed aldol reaction of different aldol donorsa .

A suitable approach has been improved for preparation of retrievable Pd catalyst using immobilization of palladium nanoparticles on magnetic nanoparticles modified with func‐ tional ionic liquid [16]. The amine functionalized ionic liquid immobilized Pd nanoparticles in the Pd/IL‐NH2/SiO2/Fe3O4 catalyst demonstrates great catalytic activity for a wide diversity of aryl iodides and bromides in the Suzuki coupling reactions at ambient temperature (**Scheme 12**). Furthermore, the catalyst is able to be good distributed in the reaction media, simply retrieved from the reaction mixture by using a magnet, and reused for several times with no significant loss in activity. Because of all these advantages, this procedure is a green and appropriate for other important reactions catalyzed with metal.

**Scheme 12.** The Suzuki coupling reactions of aryl iodides and bromides by Pd/IL‐NH2/SiO2/Fe3O4 catalyst.

*α*‐Fe2O3‐MCM‐41 immobilized with amino acid ionic liquid was prepared as a retrievable catalyst for synthesizing quinazolin‐4(3*H*)‐ones at ambient temperature in short reaction times under oxidant and solvent‐less conditions (**Scheme 13**). It is supposed that the L‐prolinium nitrate in the mesochannels of (*α*‐Fe2O3)‐MCM‐41 might enhance the strength of Brønsted acid and oxidation power of catalytic system [17].

**Scheme 13.** (*α*‐Fe2O3)‐MCM‐41‐L‐prolinium nitrate for the synthesis of quinazolin‐4(3*H*)‐ones.

Moreover, there is a report on synthesis of magnetic nanoparticles immobilized with Ni2+ ion‐ containing 1‐methyl‐3‐(3‐trimethoxysilylpropyl) imidazolium chloride ionic liquid as a recoverable nanocatalyst for the Heck reaction at 100°C, and it can be applied after washing with no loss in activity (**Scheme 14**) [18].

**Scheme 14.** Heck reaction catalyzed by IL–Ni(II)–MNPs.

**Entry N R t/h Yieldb (%) Syn/antic eed (%)** 1 4‐NO2C6H4 5 97 6:94 97 2e 1 4‐NO2C6H4 6 86 23:77 95 1 2‐NO2C6H4 5 97 24:76 98 1 3‐NO2C6H4 5 96 16:84 98 1 4‐CF3C6H4 12 88 17:73 97 1 4‐ClC6H4 48 86 20:80 96 2 2‐NO2C6H4 8 98 13:87 99 2 3‐NO2C6H4 8 97 14:86 98 2 4‐NO2C6H4 6 97 14:86 97 2 4‐CF3C6H4 11 93 13:87 98 2 4‐ClC6H4 48 92 17:83 98

.

A suitable approach has been improved for preparation of retrievable Pd catalyst using immobilization of palladium nanoparticles on magnetic nanoparticles modified with func‐ tional ionic liquid [16]. The amine functionalized ionic liquid immobilized Pd nanoparticles in the Pd/IL‐NH2/SiO2/Fe3O4 catalyst demonstrates great catalytic activity for a wide diversity of aryl iodides and bromides in the Suzuki coupling reactions at ambient temperature (**Scheme 12**). Furthermore, the catalyst is able to be good distributed in the reaction media, simply retrieved from the reaction mixture by using a magnet, and reused for several times with no significant loss in activity. Because of all these advantages, this procedure is a green

Reaction conditions: catalyst (5 mol%), ketone (0.20 mL) and aldehyde (0.25 mmol).

and appropriate for other important reactions catalyzed with metal.

**Scheme 12.** The Suzuki coupling reactions of aryl iodides and bromides by Pd/IL‐NH2/SiO2/Fe3O4 catalyst.

**Table 3.** MNP‐1‐PW‐A catalyzed aldol reaction of different aldol donorsa

a

b

c

e

Isolated yield.

Determined by chiral HPLC. dDetermined by chiral HPLC.

PW‐A (1 mol%) was employed.

566 Progress and Developments in Ionic Liquids

γ‐Fe2O3 nanoparticles immobilized with 2‐Hydroxyethylammonium sulphonate (γ‐Fe2O3‐2‐ HEAS) were prepared through the reaction of n‐butylsulfonated γ‐Fe2O3 with ethanolamine [19]. Here, the catalyst effectively increases the condensation of both aliphatic and aromatic aldehydes and thiols with malononitrile resulting in 2‐amino‐3,5‐dicarbonitrile‐6‐thio‐ pyridines in good to excellent efficiency under solvent‐less conditions (**Scheme 15**). Separating the product and recycling the catalyst are easily performed using a suitable external magnet. The catalyst can be reused for five times with no significant loss in its catalytic activity.

effective heterogeneous acidic IL catalyst from aldehydes, 2‐thiobarbituric acid and ammoni‐ um acetate under moderate condition in excellent efficiencies (**Scheme 17**). This procedure is greener compared with other reported methods because of its moderate conditions of reaction,

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excellent efficiencies and simple recycling of the catalyst [21].

**Scheme 17.** One‐pot synthesis of pyrido[2,3‐d:6,5‐d]dipyrimidines catalyzed by Fe‐MCM‐41‐IL.

ambient temperature with high efficiencies (**Scheme 18**).

**Scheme 18.** Acetylation of alcohols catalyzed by TPPA‐IL‐Fe3O4.

A simple and effective process has been reported for synthesizing 3‐((3‐(trisilyloxy)propyl) propionamide)‐1‐methylimidazolium chloride ionic liquid supported on magnetic nanopar‐ ticles (TPPA‐IL‐Fe3O4) [22]. The TPPA‐IL‐Fe3O4 assessed as a recoverable heterogeneous catalyst for the alcohols acetylation with acetic anhydride under moderate conditions at

The hydroxyl groups were chemoselectively acetylated in the existence of other reactive groups by using the synthesized catalyst. The acetylation of 4‐bormobenzyl alcohol did occur

**Scheme 15.** Synthesis of 2‐amino‐3,5‐dicarbonitrile‐6‐thio‐pyridines catalyzed using γ‐Fe2O3 MNPs.

An effective synthesis of 2,4,5‐trisubstituted imidazoles is achieved by silica‐coated magnetite nanoparticles immobilized with multi‐SO3H functionalized acidic ionic liquid (**Scheme 16**). Because of high performance, recoverability, short reaction times, efficiency of products and operational simplicity, this process is an attractive substitute for the green synthesis of 2,4,5‐ trisubstituted imidazoles as biological and pharmaceutical‐related substances [20].

**Scheme 16.** Synthesis of 2,4,5‐trisubstituted imidazoles catalyzed by Fe3O4@SiO2.HM.SO3H.

An ionic liquid stabilized iron‐containing mesoporous silica nanoparticles (Fe‐MCM‐41‐IL) was synthesized by fixing a triazolium ionic liquid on Fe‐coated MCM. The pyrimidine derivatives were synthesized through one‐pot method in the presence of Fe‐MCM‐41‐IL as an effective heterogeneous acidic IL catalyst from aldehydes, 2‐thiobarbituric acid and ammoni‐ um acetate under moderate condition in excellent efficiencies (**Scheme 17**). This procedure is greener compared with other reported methods because of its moderate conditions of reaction, excellent efficiencies and simple recycling of the catalyst [21].

aldehydes and thiols with malononitrile resulting in 2‐amino‐3,5‐dicarbonitrile‐6‐thio‐ pyridines in good to excellent efficiency under solvent‐less conditions (**Scheme 15**). Separating the product and recycling the catalyst are easily performed using a suitable external magnet. The catalyst can be reused for five times with no significant loss in its catalytic activity.

568 Progress and Developments in Ionic Liquids

**Scheme 15.** Synthesis of 2‐amino‐3,5‐dicarbonitrile‐6‐thio‐pyridines catalyzed using γ‐Fe2O3 MNPs.

trisubstituted imidazoles as biological and pharmaceutical‐related substances [20].

**Scheme 16.** Synthesis of 2,4,5‐trisubstituted imidazoles catalyzed by Fe3O4@SiO2.HM.SO3H.

An ionic liquid stabilized iron‐containing mesoporous silica nanoparticles (Fe‐MCM‐41‐IL) was synthesized by fixing a triazolium ionic liquid on Fe‐coated MCM. The pyrimidine derivatives were synthesized through one‐pot method in the presence of Fe‐MCM‐41‐IL as an

An effective synthesis of 2,4,5‐trisubstituted imidazoles is achieved by silica‐coated magnetite nanoparticles immobilized with multi‐SO3H functionalized acidic ionic liquid (**Scheme 16**). Because of high performance, recoverability, short reaction times, efficiency of products and operational simplicity, this process is an attractive substitute for the green synthesis of 2,4,5‐

**Scheme 17.** One‐pot synthesis of pyrido[2,3‐d:6,5‐d]dipyrimidines catalyzed by Fe‐MCM‐41‐IL.

A simple and effective process has been reported for synthesizing 3‐((3‐(trisilyloxy)propyl) propionamide)‐1‐methylimidazolium chloride ionic liquid supported on magnetic nanopar‐ ticles (TPPA‐IL‐Fe3O4) [22]. The TPPA‐IL‐Fe3O4 assessed as a recoverable heterogeneous catalyst for the alcohols acetylation with acetic anhydride under moderate conditions at ambient temperature with high efficiencies (**Scheme 18**).

**Scheme 18.** Acetylation of alcohols catalyzed by TPPA‐IL‐Fe3O4.

The hydroxyl groups were chemoselectively acetylated in the existence of other reactive groups by using the synthesized catalyst. The acetylation of 4‐bormobenzyl alcohol did occur selectively in the presence of 4‐bromophenol, and the hydroxyl group of phenol was intact during this reaction (**Scheme 19**).

The Fe3O4@MCM‐41‐SO3H@[HMIm][HSO4] efficiently catalyzed the one‐pot three‐component condensation of *α* or *β* naphthol, cyclic 1,3‐diketone and isatin derivatives for the synthesis of spiro[benzoxanthene‐indoline]diones (**Scheme 21**) [24]. This active catalyst was thermally stable, green, recyclable and easy to prepare. In addition, its separation of the reaction mixture is easy and it could be retrieved up to five times with no significant influence on its activity or

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**Scheme 21.** Synthesis of spiro[benzoxanthene‐indoline]diones catalyzed by Fe3O4@MCM‐41‐SO3H@ [HMIm][HSO4].

reapplied for at least seven times with no change in its activity.

**Scheme 22.** Synthesis of triarylpyridines catalyzed by MNP‐[pmim]HSO4

The silica‐coated magnetic particles immobilized with 1‐methyl‐3‐(triethoxysilylpropyl) imidazolium chloride provided the corresponding supported ionic liquid. Substituting the Cl¯ anion through treatment with H2SO4 resulted in Bronsted ionic liquid 1‐methyl‐3‐(triethoxy‐ silylpropyl) imidazolium hydrogensulfate (MNP‐[pmim]HSO4) [25]. The activity of the immobilized ionic liquid was studied as a catalyst for synthesizing the polysubstituted pyridines using condensation of aromatic aldehydes with acetophenones and ammonium acetate in modest to excellent efficiency under solvent‐less conditions (**Scheme 22**). The recycling of the catalyst can be simply performed using an external magnet, and it can be

> − .

the reaction efficiency.

**Scheme 19.** Chemoselectivity of the acetylation hydroxyl groups in the presence of TPPA‐IL‐Fe3O4.

An ecologically friendly technique has been improved for preparing isobenzofuran‐1(3*H*)‐ones in the existence of [HSO3PMIM]OTf‐SiO2@MNPs as a highly retrievable catalyst under solvent‐ less thermal conditions and MW irradiation [23]. Mono‐ and bis‐isobenzofuran‐1(3*H*)‐ones was effectively synthesized in the presence of this catalyst under thermal conditions and MW irradiation. The considerable advantages of this procedure for the synthesis of isobenzofuran‐ 1(3*H*)‐ones are its simplicity, excellent yields, short times of reaction, eco‐friendly and simple recycling of the catalyst (**Scheme 20**).

**Scheme 20.** Synthesis of isobenzofuran‐1(3*H*)‐ones catalyzed using [HSO3PMIM]OTf‐SiO2@ MNPs.

The Fe3O4@MCM‐41‐SO3H@[HMIm][HSO4] efficiently catalyzed the one‐pot three‐component condensation of *α* or *β* naphthol, cyclic 1,3‐diketone and isatin derivatives for the synthesis of spiro[benzoxanthene‐indoline]diones (**Scheme 21**) [24]. This active catalyst was thermally stable, green, recyclable and easy to prepare. In addition, its separation of the reaction mixture is easy and it could be retrieved up to five times with no significant influence on its activity or the reaction efficiency.

selectively in the presence of 4‐bromophenol, and the hydroxyl group of phenol was intact

**Scheme 19.** Chemoselectivity of the acetylation hydroxyl groups in the presence of TPPA‐IL‐Fe3O4.

**Scheme 20.** Synthesis of isobenzofuran‐1(3*H*)‐ones catalyzed using [HSO3PMIM]OTf‐SiO2@ MNPs.

An ecologically friendly technique has been improved for preparing isobenzofuran‐1(3*H*)‐ones in the existence of [HSO3PMIM]OTf‐SiO2@MNPs as a highly retrievable catalyst under solvent‐ less thermal conditions and MW irradiation [23]. Mono‐ and bis‐isobenzofuran‐1(3*H*)‐ones was effectively synthesized in the presence of this catalyst under thermal conditions and MW irradiation. The considerable advantages of this procedure for the synthesis of isobenzofuran‐ 1(3*H*)‐ones are its simplicity, excellent yields, short times of reaction, eco‐friendly and simple

during this reaction (**Scheme 19**).

570 Progress and Developments in Ionic Liquids

recycling of the catalyst (**Scheme 20**).

**Scheme 21.** Synthesis of spiro[benzoxanthene‐indoline]diones catalyzed by Fe3O4@MCM‐41‐SO3H@ [HMIm][HSO4].

The silica‐coated magnetic particles immobilized with 1‐methyl‐3‐(triethoxysilylpropyl) imidazolium chloride provided the corresponding supported ionic liquid. Substituting the Cl¯ anion through treatment with H2SO4 resulted in Bronsted ionic liquid 1‐methyl‐3‐(triethoxy‐ silylpropyl) imidazolium hydrogensulfate (MNP‐[pmim]HSO4) [25]. The activity of the immobilized ionic liquid was studied as a catalyst for synthesizing the polysubstituted pyridines using condensation of aromatic aldehydes with acetophenones and ammonium acetate in modest to excellent efficiency under solvent‐less conditions (**Scheme 22**). The recycling of the catalyst can be simply performed using an external magnet, and it can be reapplied for at least seven times with no change in its activity.

**Scheme 22.** Synthesis of triarylpyridines catalyzed by MNP‐[pmim]HSO4 − .

Silica‐coated Fe3O4 magnetic nanoparticles immobilized with urea‐based ionic liquid [Fe3O4@SiO2@(CH2)3‐Urea‐SO3H/HCl] have been prepared [26]. The catalyst was studied for the synthesis of bis(indolyl)methane derivatives through the reaction between 2‐methylindole and aldehydes at ambient temperature under solvent‐free conditions. In addition, pyrano[2,3‐ d]pyrimidinones were synthesized in the presence of the catalyst through the one‐pot condensation reaction of 1,3‐dimethylbarbituric acid, aldehydes and malononitrile under solvent‐less conditions at 60°C (**Scheme 23**).

**Scheme 25.** The procedure for preparation of co‐p[VRIm][OH]/MCFs.

ethyl cyanoacetate.

The activities of the catalysts were studied for the Knoevenagel condensation reaction of benzaldehyde with ethyl cyanoacetate and for the transesterification of glycerol trioleate (TG) with methanol (**Scheme 26**). The results showed that in contrast to the sample synthesized by

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**Scheme 26.** The transesterification of glycerol trioleate (TG) and the Knoevenagel condensation of benzaldehyde with

Because of the influence of steric hindrance and active sites, g‐p[VDoIm][OH]/MCFs showed a higher catalytic activity than co‐p[VDoIm][OH]/MCFs. The conversion of benzaldehyde was around 97% for g‐p[VDoIm][OH]/MCFs higher than 72.5% for co‐p[VDoIm][OH]/MCFs that was in accordance with the reaction of transesterification. Moreover, *N*‐propyl‐sulfonic acid bonded onto magnetic nanoparticle coated with poly(ionic liquid) (Fe3O4@PIL) catalyst was successfully synthesized through polymerizing functionalized vinylimidazolium in the existence of magnetic nanoparticles with modified surface [28]. The obtained catalyst is indicated to be an effective heterogeneous acidic nanocatalyst for synthesizing 1,1‐diacetal from aldehydes under solvent‐less conditions and ambient temperature in a good efficiency. In addition, the catalyst demonstrates an excellent activity for the deprotection reaction of acetals (**Scheme 27**). The catalyst has an excellent thermal stability and reusability because the

surface of the magnetic nanoparticles is coated with polymer layers.

the copolymerization technique, the catalysts had a good catalytic efficiency.

**Scheme 23.** Synthesis of bis(indolyl)methanes and pyrano[2,3‐d]pyrimidinones catalyzed by MNPs@ILs.

### **3. Polymeric ionic liquids immobilized on magnetic nanoparticles**

The successful synthesis of magnetic CoFe2O4 nanoparticles coated with basic poly(ionic liquids) was carried out, and the catalyst synthesized using the surface grafting technique (g‐ p[VRIm][OH]/MCFs) (**Scheme 24**) had a better stability, greater loading of ionic liquids and good paramagnetism compared with that synthesized through the conventional copolymeri‐ zation technique (co‐p[VRIm][OH]/MCFs) (**Scheme 25**) [27].

**Scheme 24.** The procedure for preparation of g‐p[VRIm][OH]/MCFs.

**Scheme 25.** The procedure for preparation of co‐p[VRIm][OH]/MCFs.

Silica‐coated Fe3O4 magnetic nanoparticles immobilized with urea‐based ionic liquid [Fe3O4@SiO2@(CH2)3‐Urea‐SO3H/HCl] have been prepared [26]. The catalyst was studied for the synthesis of bis(indolyl)methane derivatives through the reaction between 2‐methylindole and aldehydes at ambient temperature under solvent‐free conditions. In addition, pyrano[2,3‐ d]pyrimidinones were synthesized in the presence of the catalyst through the one‐pot condensation reaction of 1,3‐dimethylbarbituric acid, aldehydes and malononitrile under

**Scheme 23.** Synthesis of bis(indolyl)methanes and pyrano[2,3‐d]pyrimidinones catalyzed by MNPs@ILs.

**3. Polymeric ionic liquids immobilized on magnetic nanoparticles**

zation technique (co‐p[VRIm][OH]/MCFs) (**Scheme 25**) [27].

**Scheme 24.** The procedure for preparation of g‐p[VRIm][OH]/MCFs.

The successful synthesis of magnetic CoFe2O4 nanoparticles coated with basic poly(ionic liquids) was carried out, and the catalyst synthesized using the surface grafting technique (g‐ p[VRIm][OH]/MCFs) (**Scheme 24**) had a better stability, greater loading of ionic liquids and good paramagnetism compared with that synthesized through the conventional copolymeri‐

solvent‐less conditions at 60°C (**Scheme 23**).

572 Progress and Developments in Ionic Liquids

The activities of the catalysts were studied for the Knoevenagel condensation reaction of benzaldehyde with ethyl cyanoacetate and for the transesterification of glycerol trioleate (TG) with methanol (**Scheme 26**). The results showed that in contrast to the sample synthesized by the copolymerization technique, the catalysts had a good catalytic efficiency.

**Scheme 26.** The transesterification of glycerol trioleate (TG) and the Knoevenagel condensation of benzaldehyde with ethyl cyanoacetate.

Because of the influence of steric hindrance and active sites, g‐p[VDoIm][OH]/MCFs showed a higher catalytic activity than co‐p[VDoIm][OH]/MCFs. The conversion of benzaldehyde was around 97% for g‐p[VDoIm][OH]/MCFs higher than 72.5% for co‐p[VDoIm][OH]/MCFs that was in accordance with the reaction of transesterification. Moreover, *N*‐propyl‐sulfonic acid bonded onto magnetic nanoparticle coated with poly(ionic liquid) (Fe3O4@PIL) catalyst was successfully synthesized through polymerizing functionalized vinylimidazolium in the existence of magnetic nanoparticles with modified surface [28]. The obtained catalyst is indicated to be an effective heterogeneous acidic nanocatalyst for synthesizing 1,1‐diacetal from aldehydes under solvent‐less conditions and ambient temperature in a good efficiency. In addition, the catalyst demonstrates an excellent activity for the deprotection reaction of acetals (**Scheme 27**). The catalyst has an excellent thermal stability and reusability because the surface of the magnetic nanoparticles is coated with polymer layers.

**Acknowledgements**

acknowledged.

**Author details**

Masoud Mokhtary

**References**

6998–7015.

2014.11.018

10.1016/j.crci.2013.01.019

Address all correspondence to: mmokhtary@iaurasht.ac.ir

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http://dx.doi.org/10.5772/65794

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**Scheme 27.** Acetylation of aldehydes and their deprotection using Fe3O4@PIL as catalyst.

### **4. Summary and outlook**

According to this chapter, there are some interesting new advancements in ionic liquids supported on magnetic nanoparticles. There is a clear procedure including silica coating of magnetic nanoparticle core followed by functionalization using proper alkoxysilane deriva‐ tives. Easy modification of the magnetic iron oxide surfaces with organic ligands increases the adsorption of catalytically active metal nanoparticles, as highlighted with palladium‐mediated C‐C coupling and Pt‐catalyzed hydrogenation reactions. The high dispersity of the MNPs in different solvents is another advantage, when it exposes the surface‐bound active reaction sites for the reactants in an optimized way. This lets diffusion restriction to be dominated, which is generally found in microporous or mesoporous heterogenized solids. Clearly, the unique magnetic properties of the superparamagnetic particles lead to recyclable magnetic nanopar‐ ticles immobilized with ionic liquids for several times using a suitable magnet with no significant loss in their catalytic activity. The sustainable synthesis of magnetically retrievable ionic liquids using readily available reactants will also make this field of research green. Additional interesting improvement is the poly(ionic liquids) stabilized magnetic nanoparti‐ cles as a new group of heterogeneous catalyst that is mainly attractive in organic synthesis practiced in an ecologically friendly way. In the end, future efforts for more efficient protocols will still focus on the stability, sustainability, environmental impact and considerable cost and energy savings due to the growing needs of industry. These attempts will permit a wide diversity of industrial usages for ionic liquids immobilized on magnetic nanoparticles in the future.

### **Acknowledgements**

Financial support by Rasht Branch, Islamic Azad University Grant No. 4.5830 is gratefully acknowledged.

### **Author details**

Masoud Mokhtary

Address all correspondence to: mmokhtary@iaurasht.ac.ir

Department of Chemistry, Rasht Branch, Islamic Azad University, Rasht, Iran

### **References**

**Scheme 27.** Acetylation of aldehydes and their deprotection using Fe3O4@PIL as catalyst.

According to this chapter, there are some interesting new advancements in ionic liquids supported on magnetic nanoparticles. There is a clear procedure including silica coating of magnetic nanoparticle core followed by functionalization using proper alkoxysilane deriva‐ tives. Easy modification of the magnetic iron oxide surfaces with organic ligands increases the adsorption of catalytically active metal nanoparticles, as highlighted with palladium‐mediated C‐C coupling and Pt‐catalyzed hydrogenation reactions. The high dispersity of the MNPs in different solvents is another advantage, when it exposes the surface‐bound active reaction sites for the reactants in an optimized way. This lets diffusion restriction to be dominated, which is generally found in microporous or mesoporous heterogenized solids. Clearly, the unique magnetic properties of the superparamagnetic particles lead to recyclable magnetic nanopar‐ ticles immobilized with ionic liquids for several times using a suitable magnet with no significant loss in their catalytic activity. The sustainable synthesis of magnetically retrievable ionic liquids using readily available reactants will also make this field of research green. Additional interesting improvement is the poly(ionic liquids) stabilized magnetic nanoparti‐ cles as a new group of heterogeneous catalyst that is mainly attractive in organic synthesis practiced in an ecologically friendly way. In the end, future efforts for more efficient protocols will still focus on the stability, sustainability, environmental impact and considerable cost and energy savings due to the growing needs of industry. These attempts will permit a wide diversity of industrial usages for ionic liquids immobilized on magnetic nanoparticles in the

**4. Summary and outlook**

574 Progress and Developments in Ionic Liquids

future.


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**Chapter 25**

**Provisional chapter**

**Preparation of Ionic Liquids Containing Siloxane**

**Preparation of Ionic Liquids Containing Siloxane** 

This chapter deals with our recent researches on the preparation and properties of thermally stable ionic liquids (ILs) containing siloxane frameworks. ILs containing randomly structured oligosilsesquioxanes with quaternary ammonium side-chain groups (**Am-Random-SQ-IL**) and with imidazolium side-chain groups (**Im-Random-SQ-IL**) were successfully prepared by the hydrolytic condensation of the corresponding trifunctional alkoxysilanes in aqueous bis(trifluoromethanesulfonyl)imide (HNTf2) solution. It is also reported that ILs containing cage-like oligosilsesquioxanes (POSSs) with imidazolium side-chain groups (**Im-Cage-SQ-IL**) and with random distribution of quaternary ammonium and imidazolium side-chain groups (**Amim-Cage-SQ-IL**)were obtained, when the similar hydrolytic condensations were performed in a water/methanol (1 : 19 v/v) mixed solution of HNTf2. In addition, we investigated the preparation of ILs containing cyclic oligosiloxanes with various imidazolium side-chain groups (**MeIm-CyS-**

> © 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution,

© 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

, **EtIm-CyS-IL-NTf2**

) by the hydrolytic condensation of the corresponding difunc-

, **PrIm-CyS-IL-NTf2**

and trifluoromethane-

,

and reproduction in any medium, provided the original work is properly cited.

Ionic liquids (ILs), molten salts below 100°C or 150°C, have attracted much attention because of their potential application to green solvents [1–4] and electrolyte materials [5–7]. These

Yoshiro Kaneko, Akiyuki Harada, Takuya Kubo and

, **MeIm-CyS-IL-OTf**, **HIm-CyS-IL-NTf2**

POSS, siloxane, silsesquioxane, superacid

tional alkoxysilanes in the solutions of superacids, such as HNTf2

**Keywords:** alkoxysilane, cyclic oligosiloxane, hydrolytic condensation, ionic liquid,

Additional information is available at the end of the chapter

Additional information is available at the end of the chapter

Yoshiro Kaneko, Akiyuki Harada, Takuya

**Frameworks**

**Frameworks**

Takuhiro Ishii

http://dx.doi.org/10.5772/65892

**Abstract**

**IL-NTf2**

**1. Introduction**

and **BuIm-CyS-IL-NTf2**

sulfonic acid (HOTf).

Kubo and Takuhiro Ishii

#### **Preparation of Ionic Liquids Containing Siloxane Frameworks Preparation of Ionic Liquids Containing Siloxane Frameworks**

Yoshiro Kaneko, Akiyuki Harada, Takuya Kubo and Takuhiro Ishii Yoshiro Kaneko, Akiyuki Harada, Takuya Kubo and Takuhiro Ishii

Additional information is available at the end of the chapter Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/65892

#### **Abstract**

This chapter deals with our recent researches on the preparation and properties of thermally stable ionic liquids (ILs) containing siloxane frameworks. ILs containing randomly structured oligosilsesquioxanes with quaternary ammonium side-chain groups (**Am-Random-SQ-IL**) and with imidazolium side-chain groups (**Im-Random-SQ-IL**) were successfully prepared by the hydrolytic condensation of the corresponding trifunctional alkoxysilanes in aqueous bis(trifluoromethanesulfonyl)imide (HNTf2) solution. It is also reported that ILs containing cage-like oligosilsesquioxanes (POSSs) with imidazolium side-chain groups (**Im-Cage-SQ-IL**) and with random distribution of quaternary ammonium and imidazolium side-chain groups (**Amim-Cage-SQ-IL**)were obtained, when the similar hydrolytic condensations were performed in a water/methanol (1 : 19 v/v) mixed solution of HNTf2. In addition, we investigated the preparation of ILs containing cyclic oligosiloxanes with various imidazolium side-chain groups (**MeIm-CyS-IL-NTf2** , **MeIm-CyS-IL-OTf**, **HIm-CyS-IL-NTf2** , **EtIm-CyS-IL-NTf2** , **PrIm-CyS-IL-NTf2** , and **BuIm-CyS-IL-NTf2** ) by the hydrolytic condensation of the corresponding difunctional alkoxysilanes in the solutions of superacids, such as HNTf2 and trifluoromethanesulfonic acid (HOTf).

**Keywords:** alkoxysilane, cyclic oligosiloxane, hydrolytic condensation, ionic liquid, POSS, siloxane, silsesquioxane, superacid

#### **1. Introduction**

Ionic liquids (ILs), molten salts below 100°C or 150°C, have attracted much attention because of their potential application to green solvents [1–4] and electrolyte materials [5–7]. These

and reproduction in any medium, provided the original work is properly cited.

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, © 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

compounds indicate the negligible vapor pressure, high thermal stability, and high ionic conductivity. Most ILs are regarded as organic compounds because of the presence of large amount of organic components in ILs. On the other hand, ILs with relatively more inorganic components could be applied to a wide range of materials research due to their significantly higher thermostability derived from the inorganic components.

viscous product was heated at 150°C for ca. 10 h. The product (**Am-Random-SQ-IL**) was soluble in dimethyl sulfoxide (DMSO), *N,N*-dimethylformamide (DMF), methanol, acetone, tetrahydrofuran (THF), and ethyl acetate, but insoluble in water, ethanol, 1-propanol, 2-pro-

Preparation of Ionic Liquids Containing Siloxane Frameworks

http://dx.doi.org/10.5772/65892

581

The energy dispersive X-ray (EDX) pattern of **Am-Random-SQ-IL** did not show the peaks due to Cl atom (2.6 and 2.8 keV). In addition, because the Si:S elemental ratio was 1:2.04,

tures. The integrated ratio of these signals was estimated to be ca. 44:56. Although this compound had a relatively high proportion of the silanol groups, it was stable, i.e., without causing condensation and aggregation. The weight-average molecular weight (*M*w) of **Am-Random-SQ-IL** estimated by static light scattering (SLS) measurements in methanol

a randomly structured oligosilsesquioxane containing quaternary ammonium cations and

(−56 to −61 ppm) and *T*<sup>3</sup>

. Based on these results, it was concluded that **Am-Random-SQ-IL** was

calculated to be ca. 1:1. The 29Si NMR spectrum of **Am-Random-SQ-IL** in DMSO-*d*<sup>6</sup>

anion in **Am-Random-SQ-IL** was

(−64 to −70 ppm) struc-

at 60°C

panol, chloroform, diethyl ether, toluene, and *n*-hexane.

the molar ratio of quaternary ammonium cation to NTf2

**Scheme 1.** Preparation of (a) **Am-Random-SQ-IL** and (b) **Am-Cage-SQ**.

indicated two broad signals due to the *T*<sup>2</sup>

was ca. 1.8 × 103

anions.

NTf2

Based on such considerations, some ILs containing inorganic frameworks, such as cagelike oligosilsesquioxanes (polyhedral oligomeric silsesquioxanes: POSSs) have been developed so far. A POSS IL (melting point (*T*m) = 23°C) was first developed by Chujo et al. [8]. This POSS IL had carboxylate anionic side-chain groups and imidazolium counter cations. In other cases, a POSS IL (*T*m = 18°C) containing imidazolium cationic side-chain groups and dodecyl sulfate counter anions was prepared by Feng and coworkers [9]. However, these POSS ILs had relatively lower thermal decomposition (pyrolysis) temperatures (*T*ds < 250°C) because of the large proportion of organic components in their side-chains or counter ions.

In this chapter, we would like to describe our recent work on the preparation of thermally stable ILs containing siloxane frameworks, such as randomly structured oligosilsesquioxanes, POSSs, and cyclic oligosiloxanes, by the hydrolytic condensation of the corresponding triand di-alkoxysilanes using superacid catalysts.

### **2. Preparation of a quaternary ammonium-type ionic liquid containing randomly structured oligosilsesquioxane**

So far, we have prepared ionic siloxane compounds with regular structures, such as POSSs [10–12], ladder-like polysilsesquioxanes [13–19], and cyclic siloxanes [20], by the hydrolytic condensation of tri- and di-alkoxysilanes containing functional organic groups, which can be converted into ionic groups during the reactions. While performing these studies on the preparation of regularly structured ionic siloxane compounds, we fortuitously found a highly thermostable IL containing randomly structured oligosilsesquioxane, which has quaternary ammonium side-chain groups. We first describe the preparation and properties of this IL.

A quaternary ammonium-type IL containing randomly structured oligosilsesquioxane (**Am-Random-SQ-IL**) was successfully prepared by the hydrolytic condensation of the quaternary ammonium salt containing organotrialkoxysilane, trimethyl[3-(triethoxysilyl)propyl] ammonium chloride (TTACl), in aqueous bis(trifluoromethanesulfonyl)imide (HNTf2 ) solution under the following conditions (**Scheme 1a**) [21]: TTACl was stirred in aqueous HNTf2 solution (0.5 mol/L) at room temperature for 2 h. Here, molar ratio of HNTf2 /TTACl (= 1.5) is the important factor. The water-insoluble viscous product was isolated, washed with water, and dried under reduced pressure. Then, the crude product was dissolved in methanol and the resulting solution was heated in an open system until the solvent completely evaporated to remove the small amount of water remaining in the product. In addition, the resulting viscous product was heated at 150°C for ca. 10 h. The product (**Am-Random-SQ-IL**) was soluble in dimethyl sulfoxide (DMSO), *N,N*-dimethylformamide (DMF), methanol, acetone, tetrahydrofuran (THF), and ethyl acetate, but insoluble in water, ethanol, 1-propanol, 2-propanol, chloroform, diethyl ether, toluene, and *n*-hexane.

**Scheme 1.** Preparation of (a) **Am-Random-SQ-IL** and (b) **Am-Cage-SQ**.

compounds indicate the negligible vapor pressure, high thermal stability, and high ionic conductivity. Most ILs are regarded as organic compounds because of the presence of large amount of organic components in ILs. On the other hand, ILs with relatively more inorganic components could be applied to a wide range of materials research due to their significantly

Based on such considerations, some ILs containing inorganic frameworks, such as cagelike oligosilsesquioxanes (polyhedral oligomeric silsesquioxanes: POSSs) have been developed so far. A POSS IL (melting point (*T*m) = 23°C) was first developed by Chujo et al. [8]. This POSS IL had carboxylate anionic side-chain groups and imidazolium counter cations. In other cases, a POSS IL (*T*m = 18°C) containing imidazolium cationic side-chain groups and dodecyl sulfate counter anions was prepared by Feng and coworkers [9]. However, these POSS ILs had relatively lower thermal decomposition (pyrolysis) temperatures (*T*ds < 250°C) because of the large proportion of organic components in their side-chains or

In this chapter, we would like to describe our recent work on the preparation of thermally stable ILs containing siloxane frameworks, such as randomly structured oligosilsesquioxanes, POSSs, and cyclic oligosiloxanes, by the hydrolytic condensation of the corresponding tri-

**2. Preparation of a quaternary ammonium-type ionic liquid containing** 

So far, we have prepared ionic siloxane compounds with regular structures, such as POSSs [10–12], ladder-like polysilsesquioxanes [13–19], and cyclic siloxanes [20], by the hydrolytic condensation of tri- and di-alkoxysilanes containing functional organic groups, which can be converted into ionic groups during the reactions. While performing these studies on the preparation of regularly structured ionic siloxane compounds, we fortuitously found a highly thermostable IL containing randomly structured oligosilsesquioxane, which has quaternary ammonium side-chain groups. We first describe the preparation and properties

A quaternary ammonium-type IL containing randomly structured oligosilsesquioxane (**Am-Random-SQ-IL**) was successfully prepared by the hydrolytic condensation of the quaternary ammonium salt containing organotrialkoxysilane, trimethyl[3-(triethoxysilyl)propyl] ammonium chloride (TTACl), in aqueous bis(trifluoromethanesulfonyl)imide (HNTf2

tion under the following conditions (**Scheme 1a**) [21]: TTACl was stirred in aqueous HNTf2

the important factor. The water-insoluble viscous product was isolated, washed with water, and dried under reduced pressure. Then, the crude product was dissolved in methanol and the resulting solution was heated in an open system until the solvent completely evaporated to remove the small amount of water remaining in the product. In addition, the resulting

solution (0.5 mol/L) at room temperature for 2 h. Here, molar ratio of HNTf2

) solu-

/TTACl (= 1.5) is

higher thermostability derived from the inorganic components.

and di-alkoxysilanes using superacid catalysts.

**randomly structured oligosilsesquioxane**

counter ions.

580 Progress and Developments in Ionic Liquids

of this IL.

The energy dispersive X-ray (EDX) pattern of **Am-Random-SQ-IL** did not show the peaks due to Cl atom (2.6 and 2.8 keV). In addition, because the Si:S elemental ratio was 1:2.04, the molar ratio of quaternary ammonium cation to NTf2 anion in **Am-Random-SQ-IL** was calculated to be ca. 1:1. The 29Si NMR spectrum of **Am-Random-SQ-IL** in DMSO-*d*<sup>6</sup> at 60°C indicated two broad signals due to the *T*<sup>2</sup> (−56 to −61 ppm) and *T*<sup>3</sup> (−64 to −70 ppm) structures. The integrated ratio of these signals was estimated to be ca. 44:56. Although this compound had a relatively high proportion of the silanol groups, it was stable, i.e., without causing condensation and aggregation. The weight-average molecular weight (*M*w) of **Am-Random-SQ-IL** estimated by static light scattering (SLS) measurements in methanol was ca. 1.8 × 103 . Based on these results, it was concluded that **Am-Random-SQ-IL** was a randomly structured oligosilsesquioxane containing quaternary ammonium cations and NTf2 anions.

When the differential scanning calorimetry (DSC) measurement of **Am-Random-SQ-IL** was performed, the baseline shift assigned to the glass-transition point (*T*<sup>g</sup> ) was observed at 15°C (Run 1 in **Table 1**). On the other hand, the endothermic peak due to *T*m could not be detected, indicating that **Am-Random-SQ-IL** is an amorphous compound. So far, ILs without *T*m have been reported, e.g., 1-butyl-3-methylimidazolium tetrafluoroborate [22] and 1-ethyl-3-methylimidazolium phosphonate derivatives [23].

As described above, **Am-Random-SQ-IL** had an amorphous structure and displayed IL nature. Its amorphous structure is probably one of the most important factors for such IL properties. Therefore, to investigate the correlation between the IL nature and the structures of the silsesquioxanes, we investigated the preparation of a POSS compound with crystalline structure using the same reagent and superacid catalyst. When the hydrolytic conden-

(1:19 v/v) instead of the aqueous solution as described above, a powdered POSS compound (**Am-Cage-SQ**) was prepared (**Scheme 1b**) [21]. A visual flow temperature of **Am-Cage-SQ** (~155°C) was much higher than that of **Am-Random-SQ-IL** because of the presence of higher *T*m (172°C), although pyrolysis temperature was notably high (*T*d5 = 420°C) (Run 2 in **Table 1**). Such high *T*ms and flow temperatures of these POSS compounds are probably derived from

**3. Preparation of imidazolium-type ionic liquids containing random-**

the imidazolium-group-containing organotrialkoxysilane using aqueous HNTf2

1-propanol, 2-propanol, chloroform, diethyl ether, toluene, and *n*-hexane.

gosilsesquioxane compound composed of imidazolium cations and NTf2

idity at ~40°C, i.e., it was not a room temperature IL (RT-IL). Generally, imidazolium-type ILs have relatively low *T*m [24]. Therefore, to prepare a RT-IL containing a randomly structured oligosilsesquioxane framework (**Im-Random-SQ-IL**), the hydrolytic condensation of

gated [25]. **Im-Random-SQ-IL** could be prepared from 1-methyl-3-[3-(triethoxysilyl)propyl] imidazolium chloride (MTICl) as a starting material by the same procedure for the preparation of **Am-Random-SQ-IL** as described above (**Scheme 2a**). **Im-Random-SQ-IL** was soluble in DMSO, DMF, methanol, acetone, THF, and ethyl acetate, but insoluble in water, ethanol,

The EDX pattern of **Im-Random-SQ-IL** also indicated the absence of Cl. In addition, the Si:S elemental ratio of **Im-Random-SQ-IL** was estimated to be 1:2.03, indicating that the

aforementioned quaternary ammonium salt-type IL (**Am-Random-SQ-IL**), this compound was also stable, although it had a relatively high proportion of the silanol groups. The *M*<sup>w</sup> of **Im-Random-SQ-IL** estimated by SLS data obtained in methanol was ca. 8.8 × 102

on these results, it was concluded that **Im-Random-SQ-IL** was a randomly structured oli-

The DSC analysis of **Im-Random-SQ-IL** was performed. The baseline shift assigned to *T*<sup>g</sup> was observed at −25°C (Run 3 in **Table 1**). Conversely, the endothermic peak due to *T*m was not detected. The amorphous structure of **Im-Random-SQ-IL** may give rise to poor packing of the ions. The flow temperature of **Im-Random-SQ-IL** was confirmed by the same procedure for **Am-Random-SQ-IL** as described above. Consequently, it showed obvious fluidity at

at 60°C exhibited two broad signals in the *T*<sup>2</sup>

(−64 to −70 ppm) regions with an integrated ratio of ca. 40:60. Similar to the

as a catalyst in water/methanol mixed solvent

Preparation of Ionic Liquids Containing Siloxane Frameworks

http://dx.doi.org/10.5772/65892

583

anions was ca. 1:1. The 29Si NMR spectrum of

anions.

of 15°C and exhibited flu-

was investi-

(−53 to −61

. Based

sation of TTACl was performed using HNTf2

their highly symmetrical and crystalline structures.

molar ratio of imidazolium cations to NTf2

~0°C, i.e., it is a RT-IL (Run 3 in **Table 1**).

**Im-Random-SQ-IL** in DMSO-*d*<sup>6</sup>

ppm) and *T*<sup>3</sup>

**structured and cage-like oligosilsesquioxanes**

As described in the previous section, **Am-Random-SQ-IL** had *T*<sup>g</sup>

The flow temperature of **Am-Random-SQ-IL** was visually confirmed by the following procedure: **Am-Random-SQ-IL** was kept horizontal at 100°C for 15 min in a glass vessel, and the sample in the vessel was cooled to room temperature in the horizontal state. Then, the vessel stood at various temperatures for 15 min with tilting. Accordingly, **Am-Random-SQ-IL** showed obvious fluidity over 40°C (Run 1 in **Table 1**).

The thermal stability of **Am-Random-SQ-IL** on pyrolysis was investigated by thermogravimetric analyses (TGA). The temperatures of 3% (*T*d3), 5% (*T*d5), and 10% (*T*d10) weight losses of **Am-Random-SQ-IL** (411, 417, and 425°C, respectively) (Run 1 in **Table 1**) were higher than those of *N,N,N*-trimethyl-*N*-propylammonium bis(trifluoromethanesulfonyl) imide ([TMPA][NTf2 ]) (392, 400, and 411°C, respectively), which is an IL compound with the structure of the side-chains of **Am-Random-SQ-IL**. These results indicate that the thermal stability of **Am-Random-SQ-IL** was enhanced by connection to the silsesquioxane framework.


a Determined by DSC.

b Determined by visual observation.

c Determined by TGA.

<sup>d</sup> Not detected.

**Table 1.** Properties of ILs containing siloxane frameworks.

As described above, **Am-Random-SQ-IL** had an amorphous structure and displayed IL nature. Its amorphous structure is probably one of the most important factors for such IL properties. Therefore, to investigate the correlation between the IL nature and the structures of the silsesquioxanes, we investigated the preparation of a POSS compound with crystalline structure using the same reagent and superacid catalyst. When the hydrolytic condensation of TTACl was performed using HNTf2 as a catalyst in water/methanol mixed solvent (1:19 v/v) instead of the aqueous solution as described above, a powdered POSS compound (**Am-Cage-SQ**) was prepared (**Scheme 1b**) [21]. A visual flow temperature of **Am-Cage-SQ** (~155°C) was much higher than that of **Am-Random-SQ-IL** because of the presence of higher *T*m (172°C), although pyrolysis temperature was notably high (*T*d5 = 420°C) (Run 2 in **Table 1**). Such high *T*ms and flow temperatures of these POSS compounds are probably derived from their highly symmetrical and crystalline structures.

When the differential scanning calorimetry (DSC) measurement of **Am-Random-SQ-IL** was

(Run 1 in **Table 1**). On the other hand, the endothermic peak due to *T*m could not be detected, indicating that **Am-Random-SQ-IL** is an amorphous compound. So far, ILs without *T*m have been reported, e.g., 1-butyl-3-methylimidazolium tetrafluoroborate [22] and 1-ethyl-3-methy-

The flow temperature of **Am-Random-SQ-IL** was visually confirmed by the following procedure: **Am-Random-SQ-IL** was kept horizontal at 100°C for 15 min in a glass vessel, and the sample in the vessel was cooled to room temperature in the horizontal state. Then, the vessel stood at various temperatures for 15 min with tilting. Accordingly, **Am-Random-SQ-IL**

The thermal stability of **Am-Random-SQ-IL** on pyrolysis was investigated by thermogravimetric analyses (TGA). The temperatures of 3% (*T*d3), 5% (*T*d5), and 10% (*T*d10) weight losses of **Am-Random-SQ-IL** (411, 417, and 425°C, respectively) (Run 1 in **Table 1**) were higher than those of *N,N,N*-trimethyl-*N*-propylammonium bis(trifluoromethanesulfonyl)

the structure of the side-chains of **Am-Random-SQ-IL**. These results indicate that the thermal stability of **Am-Random-SQ-IL** was enhanced by connection to the silsesquioxane

 **Am-Random-SQ-IL** 15 ND<sup>d</sup> ~40 417 **Am-Cage-SQ** ND<sup>d</sup> 172 ~155 420 **Im-Random-SQ-IL** −25 ND<sup>d</sup> ~0 437 **Im-Cage-SQ-IL** −21 106 ~100 436

 **Amim-Cage-SQ-IL** −8 ND<sup>d</sup> ~30 420 **MeIm-CyS-IL-NTf2** −43 ND<sup>d</sup> ~0 415 **MeIm-CyS-IL-OTf** −14 ND<sup>d</sup> ~20 391 **HIm-CyS-IL-NTf2** −38 ND<sup>d</sup> ~0 – **EtIm-CyS-IL-NTf2** −44 ND<sup>d</sup> ~0 – **PrIm-CyS-IL-NTf2** −44 ND<sup>d</sup> ~0 – **BuIm-CyS-IL-NTf2** −45 ND<sup>d</sup> ~0 –

]) (392, 400, and 411°C, respectively), which is an IL compound with

 **(°C)a** *T***m (°C)a Flow temp. (°C)b** *T***d5 (°C)c**

−7 164 ~120 420

) was observed at 15°C

performed, the baseline shift assigned to the glass-transition point (*T*<sup>g</sup>

limidazolium phosphonate derivatives [23].

582 Progress and Developments in Ionic Liquids

**Run IL** *T***<sup>g</sup>**

5 Mixture of **Am-Cage-SQ** and **Im-Cage-SQ-IL**

imide ([TMPA][NTf2

framework.

a

b

c

Determined by DSC.

 Determined by TGA. <sup>d</sup> Not detected.

Determined by visual observation.

**Table 1.** Properties of ILs containing siloxane frameworks.

showed obvious fluidity over 40°C (Run 1 in **Table 1**).

### **3. Preparation of imidazolium-type ionic liquids containing randomstructured and cage-like oligosilsesquioxanes**

As described in the previous section, **Am-Random-SQ-IL** had *T*<sup>g</sup> of 15°C and exhibited fluidity at ~40°C, i.e., it was not a room temperature IL (RT-IL). Generally, imidazolium-type ILs have relatively low *T*m [24]. Therefore, to prepare a RT-IL containing a randomly structured oligosilsesquioxane framework (**Im-Random-SQ-IL**), the hydrolytic condensation of the imidazolium-group-containing organotrialkoxysilane using aqueous HNTf2 was investigated [25]. **Im-Random-SQ-IL** could be prepared from 1-methyl-3-[3-(triethoxysilyl)propyl] imidazolium chloride (MTICl) as a starting material by the same procedure for the preparation of **Am-Random-SQ-IL** as described above (**Scheme 2a**). **Im-Random-SQ-IL** was soluble in DMSO, DMF, methanol, acetone, THF, and ethyl acetate, but insoluble in water, ethanol, 1-propanol, 2-propanol, chloroform, diethyl ether, toluene, and *n*-hexane.

The EDX pattern of **Im-Random-SQ-IL** also indicated the absence of Cl. In addition, the Si:S elemental ratio of **Im-Random-SQ-IL** was estimated to be 1:2.03, indicating that the molar ratio of imidazolium cations to NTf2 anions was ca. 1:1. The 29Si NMR spectrum of **Im-Random-SQ-IL** in DMSO-*d*<sup>6</sup> at 60°C exhibited two broad signals in the *T*<sup>2</sup> (−53 to −61 ppm) and *T*<sup>3</sup> (−64 to −70 ppm) regions with an integrated ratio of ca. 40:60. Similar to the aforementioned quaternary ammonium salt-type IL (**Am-Random-SQ-IL**), this compound was also stable, although it had a relatively high proportion of the silanol groups. The *M*<sup>w</sup> of **Im-Random-SQ-IL** estimated by SLS data obtained in methanol was ca. 8.8 × 102 . Based on these results, it was concluded that **Im-Random-SQ-IL** was a randomly structured oligosilsesquioxane compound composed of imidazolium cations and NTf2 anions.

The DSC analysis of **Im-Random-SQ-IL** was performed. The baseline shift assigned to *T*<sup>g</sup> was observed at −25°C (Run 3 in **Table 1**). Conversely, the endothermic peak due to *T*m was not detected. The amorphous structure of **Im-Random-SQ-IL** may give rise to poor packing of the ions. The flow temperature of **Im-Random-SQ-IL** was confirmed by the same procedure for **Am-Random-SQ-IL** as described above. Consequently, it showed obvious fluidity at ~0°C, i.e., it is a RT-IL (Run 3 in **Table 1**).

The DSC curve for **Im-Cage-SQ-IL** indicated the baseline shift due to *T*<sup>g</sup>

RT-IL, in addition to the types of substituent groups in the silsesquioxanes.

imidazolium bis(trifluoromethylsulfonyl)imide ([C3mim][NTf2

pared by hydrolytic condensation of MTICl using superacid HNTf2

described above.

side-chains and NTf2

**imidazolium side-chain groups**

endothermic peak due to *T*m at 105°C (Run 4 in **Table 1**). In addition, **Im-Cage-SQ-IL** showed fluidity at ~100°C (Run 4 in **Table 1**), confirmed by the same procedure as described above for **Im-Random-SQ-IL**. This indicated that **Im-Cage-SQ-IL** was not a RT-IL. Because **Im-Cage-SQ-IL** is a crystalline compound, its flow temperature was near its *T*m (~100°C). On the other

results suggest that the amorphous structure of **Im-Random-SQ-IL** is essential for achieving

The thermal stabilities of **Im-Random-SQ-IL** and **Im-Cage-SQ-IL** upon pyrolysis were investigated by TGA. The *T*d3, *T*d5, and *T*d10 values for **Im-Random-SQ-IL** were 429, 437, and 447°C, respectively (Run 3 in **Table 1**), while those of **Im-Cage-SQ-IL** were 427, 436, and 446 °C, respectively (Run 4 in **Table 1**). These values were higher than those of 1-methyl-3-propyl-

tively). This compound is an IL with the structure of the side-chains of **Im-Random-SQ-IL** and **Im-Cage-SQ-IL**. These results indicated that the thermal stabilities of **Im-Random-SQ-IL** and **Im-Cage-SQ-IL** were increased by incorporation of the silsesquioxane frameworks. Such a tendency was also observed in a quaternary ammonium-type IL, **Am-Random-SQ-IL**, as

**4. Preparation of ionic liquids containing cage-like oligosilsesquioxane (POSS) with the random distribution of quaternary ammonium and** 

As described in Section 3, a highly thermostable POSS IL containing imidazolium cationic

tion, a quaternary ammonium-type POSS (**Am-Cage-SQ**) could also be prepared from TTACl as a starting material using the same procedure, as described in Section 2. However, visual flow temperatures of these compounds were relatively high (~155°C for **Am-Cage-SQ** and ~100°C for **Im-Cage-SQ-IL**) because of their higher *T*ms (172°C for **Am-Cage-SQ** and 105°C for **Im-Cage-SQ-IL**) (Run 2, 4 in **Table 1**). Such high *T*ms and flow temperatures of these POSS compounds are probably derived from their highly symmetrical and crystalline structures.

The development of POSS RT-ILs with high thermal stabilities is expected for both academic and application reasons because RT-ILs are particularly useful for many applications of green solvents and electrolyte materials. Therefore, to prepare such POSS ILs, we focused on our previous studies on the preparation of low-crystalline POSS [11] and amorphous POSS-linking polymer [12]. Their synthesis was achieved by hydrolytic condensation of a mixture of two types of amino-group-containing organotrialkoxysilanes. The molecular symmetry of the resulting POSS derivatives was low because of the random distribution of the two types of sidechain groups. Consequently, their crystallization was suppressed. In this section, we describe the preparation of a thermally stable POSS RT-IL (**Amim-Cage-SQ-IL**), which contained a

anions as counter ions (**Im-Cage-SQ-IL**) could be successfully pre-

hand, **Im-Random-SQ-IL** with an amorphous structure exhibited fluidity above its *T*<sup>g</sup>

at −22°C and the

http://dx.doi.org/10.5772/65892

Preparation of Ionic Liquids Containing Siloxane Frameworks

]) (366, 380, and 399°C, respec-

as a catalyst. In addi-

. These

585

**Scheme 2.** Preparation of (a) **Im-Random-SQ-IL** and (b) **Im-Cage-SQ-IL**.

We assumed that such IL properties were probably attributed to the amorphous structure. Therefore, as well as the quaternary ammonium-type ILs as described in the previous section, a POSS compound with crystalline structure was prepared. A POSS compound (**Im-Cage-SQ-IL**) was prepared by the hydrolytic condensation of MTICl using HNTf2 as a catalyst in water/methanol (1:19, v/v) mixed solvent (**Scheme 2b**) [25]. **Im-Cage-SQ-IL** was soluble in DMSO, DMF, methanol, acetone, THF, and ethyl acetate, but insoluble in water, ethanol, 1-propanol, 2-propanol, chloroform, diethyl ether, toluene, and *n*-hexane. The 1 H NMR and EDX results for **Im-Cage-SQ-IL** were almost same as those for **Im-Random-SQ-IL**.

The 29Si NMR spectrum of **Im-Cage-SQ-IL** in DMSO-*d*<sup>6</sup> at 40°C showed two signals assigned to the *T*<sup>3</sup> structures at −66.5 ppm (a main signal) and at −68.7 ppm (a minor signal), indicating the absence of silanol groups. These signals were derived from cage-like octasilsesquioxane (*T*<sup>8</sup> ) and cage-like decasilsesquioxane (*T*10), respectively. Because the integrated ratio of these signals was estimated to be 75:25, the molar ratio of *T*<sup>8</sup> :*T*10 was calculated to be 79:21 (= 75/8:25/10). In addition, the MALDI-TOF MS results supported the formation of such POSS structures. Finally, the XRD pattern of **Im-Cage-SQ-IL** showed many sharp diffraction peaks, indicating the formation of a crystalline structure, unlike that of **Im-Random-SQ-IL**, which did not exhibit any diffraction peaks.

The DSC curve for **Im-Cage-SQ-IL** indicated the baseline shift due to *T*<sup>g</sup> at −22°C and the endothermic peak due to *T*m at 105°C (Run 4 in **Table 1**). In addition, **Im-Cage-SQ-IL** showed fluidity at ~100°C (Run 4 in **Table 1**), confirmed by the same procedure as described above for **Im-Random-SQ-IL**. This indicated that **Im-Cage-SQ-IL** was not a RT-IL. Because **Im-Cage-SQ-IL** is a crystalline compound, its flow temperature was near its *T*m (~100°C). On the other hand, **Im-Random-SQ-IL** with an amorphous structure exhibited fluidity above its *T*<sup>g</sup> . These results suggest that the amorphous structure of **Im-Random-SQ-IL** is essential for achieving RT-IL, in addition to the types of substituent groups in the silsesquioxanes.

The thermal stabilities of **Im-Random-SQ-IL** and **Im-Cage-SQ-IL** upon pyrolysis were investigated by TGA. The *T*d3, *T*d5, and *T*d10 values for **Im-Random-SQ-IL** were 429, 437, and 447°C, respectively (Run 3 in **Table 1**), while those of **Im-Cage-SQ-IL** were 427, 436, and 446 °C, respectively (Run 4 in **Table 1**). These values were higher than those of 1-methyl-3-propylimidazolium bis(trifluoromethylsulfonyl)imide ([C3mim][NTf2 ]) (366, 380, and 399°C, respectively). This compound is an IL with the structure of the side-chains of **Im-Random-SQ-IL** and **Im-Cage-SQ-IL**. These results indicated that the thermal stabilities of **Im-Random-SQ-IL** and **Im-Cage-SQ-IL** were increased by incorporation of the silsesquioxane frameworks. Such a tendency was also observed in a quaternary ammonium-type IL, **Am-Random-SQ-IL**, as described above.

### **4. Preparation of ionic liquids containing cage-like oligosilsesquioxane (POSS) with the random distribution of quaternary ammonium and imidazolium side-chain groups**

We assumed that such IL properties were probably attributed to the amorphous structure. Therefore, as well as the quaternary ammonium-type ILs as described in the previous section, a POSS compound with crystalline structure was prepared. A POSS compound (**Im-Cage-SQ-IL**) was prepared by the hydrolytic condensation of MTICl using HNTf2

a catalyst in water/methanol (1:19, v/v) mixed solvent (**Scheme 2b**) [25]. **Im-Cage-SQ-IL** was soluble in DMSO, DMF, methanol, acetone, THF, and ethyl acetate, but insoluble in water, ethanol, 1-propanol, 2-propanol, chloroform, diethyl ether, toluene, and *n*-hex-

ing the absence of silanol groups. These signals were derived from cage-like octasilsesqui-

79:21 (= 75/8:25/10). In addition, the MALDI-TOF MS results supported the formation of such POSS structures. Finally, the XRD pattern of **Im-Cage-SQ-IL** showed many sharp diffraction peaks, indicating the formation of a crystalline structure, unlike that of **Im-Random-SQ-IL**,

H NMR and EDX results for **Im-Cage-SQ-IL** were almost same as those for

structures at −66.5 ppm (a main signal) and at −68.7 ppm (a minor signal), indicat-

) and cage-like decasilsesquioxane (*T*10), respectively. Because the integrated ratio

ane. The 1

to the *T*<sup>3</sup>

oxane (*T*<sup>8</sup>

**Im-Random-SQ-IL**.

584 Progress and Developments in Ionic Liquids

The 29Si NMR spectrum of **Im-Cage-SQ-IL** in DMSO-*d*<sup>6</sup>

**Scheme 2.** Preparation of (a) **Im-Random-SQ-IL** and (b) **Im-Cage-SQ-IL**.

which did not exhibit any diffraction peaks.

of these signals was estimated to be 75:25, the molar ratio of *T*<sup>8</sup>

as

at 40°C showed two signals assigned

:*T*10 was calculated to be

As described in Section 3, a highly thermostable POSS IL containing imidazolium cationic side-chains and NTf2 anions as counter ions (**Im-Cage-SQ-IL**) could be successfully prepared by hydrolytic condensation of MTICl using superacid HNTf2 as a catalyst. In addition, a quaternary ammonium-type POSS (**Am-Cage-SQ**) could also be prepared from TTACl as a starting material using the same procedure, as described in Section 2. However, visual flow temperatures of these compounds were relatively high (~155°C for **Am-Cage-SQ** and ~100°C for **Im-Cage-SQ-IL**) because of their higher *T*ms (172°C for **Am-Cage-SQ** and 105°C for **Im-Cage-SQ-IL**) (Run 2, 4 in **Table 1**). Such high *T*ms and flow temperatures of these POSS compounds are probably derived from their highly symmetrical and crystalline structures.

The development of POSS RT-ILs with high thermal stabilities is expected for both academic and application reasons because RT-ILs are particularly useful for many applications of green solvents and electrolyte materials. Therefore, to prepare such POSS ILs, we focused on our previous studies on the preparation of low-crystalline POSS [11] and amorphous POSS-linking polymer [12]. Their synthesis was achieved by hydrolytic condensation of a mixture of two types of amino-group-containing organotrialkoxysilanes. The molecular symmetry of the resulting POSS derivatives was low because of the random distribution of the two types of sidechain groups. Consequently, their crystallization was suppressed. In this section, we describe the preparation of a thermally stable POSS RT-IL (**Amim-Cage-SQ-IL**), which contained a random distribution of the two types of side-chain groups, by the hydrolytic condensation of a mixture of TTACl and MTICl using HNTf2 as a catalyst in water/methanol mixed solvent [26].

The XRD patterns of **Am-Cage-SQ** and **Im-Cage-SQ-IL** supported that they were crystalline compounds. Therefore, **Am-Cage-SQ** and **Im-Cage-SQ-IL** showed relatively high flow temperatures (~155 and ~100°C, respectively) because of their high crystallinity (Run 2, 4 in **Table 1**). In addition, a mixture of **Am-Cage-SQ** and **Im-Cage-SQ-IL** also maintained crystalline structure, because the endothermic peak due to *T*m was observed at 164°C; it showed

Conversely, the DSC curve of **Amim-Cage-SQ-IL** showed a baseline shift at −8°C due to

The *T*d3, *T*d5, and *T*d10 values estimated by TGA of **Amim-Cage-SQ-IL** were 414°C, 420°C, and 428 °C, respectively (Run 6 in **Table 1**). These values were higher than those of ILs with the

In the previous sections, we described that ILs containing silsesquioxane frameworks, such as randomly structured silsesquioxanes and POSSs, were successfully prepared. In particular, **Am-Random-SQ-IL, Im-Random-SQ-IL**, and **Amim-Cage-SQ-IL** had both relatively low flow temperatures (<~40°C) and high thermal stabilities (*T*d5 > ~400°C). However, they also displayed high viscosities, probably because of the presence of silanol groups for randomly structured silsesquioxane ILs and relatively higher degrees of polymerization (DP) for all silsesquioxane ILs. It is assumed that siloxane-based ILs without silanol groups and with lower DP probably exhibit high thermal stability, low flow temperature, and low viscosity. In this section, therefore, we describe the preparation and properties of ILs containing cyclic

To achieve the preparation of such ILs containing cyclic oligosiloxanes, we referred to our previous study for the facile preparation of cationic cyclotetrasiloxane (this is not an IL) by the hydrolytic condensation of 3-aminopropylmethyltriethoxysilane using the superacid trifluoromethanesulfonic acid (HOTf) [20]. Therefore, when the hydrolytic condensation of 1-[3-(dimethoxymethylsilyl)propyl]-3-methylimidazolium chloride (DSMIC) was performed

Therefore, the phase transition from amorphous solid to fluid occurred above *T*<sup>g</sup>

**5. Preparation of ionic liquids containing cyclic oligosiloxanes**

these results, it was concluded that **Amim-Cage-SQ-IL** had *T*<sup>g</sup>

, whereas an endothermic peak due to *T*m was not detected (Run 6 in **Table 1**), indicating that **Amim-Cage-SQ-IL** is an amorphous compound. The XRD pattern of **Amim-Cage-SQ-IL** did not show any diffraction peaks, supporting the amorphous structure of this compound. **Amim-Cage-SQ-IL** exhibited obvious fluidity at ~30°C (Run 6 in **Table 1**). Because the molecular symmetry of the resulting POSS compound with a random distribution of the two types of side-chain groups was low, its crystallization was suppressed.

. Based on

of −8°C and showed fluidity

] (392, 400, and 411°C, respectively) and [C3mim]

Preparation of Ionic Liquids Containing Siloxane Frameworks

http://dx.doi.org/10.5772/65892

587

and HOTf, we found that imidazolium salt-type

and **MeIm-CyS-IL-OTf**)

fluidity at 120°C (Run 5 in **Table 1**).

at ~30°C, i.e., it is a RT-IL.

side-chain structures of this IL: [TMPA][NTf2

oligosiloxanes as the siloxane frameworks.

using superacid catalysts such as HNTf2

were successfully prepared [27].

ILs containing cyclic oligosiloxane frameworks (**MeIm-CyS-IL-NTf2**

] (366, 380, and 399°C, respectively).

*T*g

[NTf2

**Amim-Cage-SQ-IL** was prepared from a mixture of TTACl and MTICl (1:1 mol/mol) by same procedures for the preparation of **Im-Cage-SQ-IL** and **Am-Cage-SQ** as described above (**Scheme 3**). **Amim-Cage-SQ-IL** was soluble in DMSO, acetonitrile, DMF, methanol, acetone, THF, and ethyl acetate, but insoluble in water, ethanol, 1-propanol, 2-propanol, chloroform, diethyl ether, toluene, and *n*-hexane.

**Scheme 3.** Preparation of **Amim-Cage-SQ-IL**.

The 1 H NMR spectrum of **Amim-Cage-SQ-IL** in DMSO-*d*<sup>6</sup> showed the signals attributable to the side-chain groups of both the *N,N,N*-trimethyl-*N*-propylammonium group and the 1-methyl-3-propylimidazolium group. The average compositional ratio of TTACl to MTICl components in the product was estimated to be ca. 1:1 from the 1 H NMR spectrum. The EDX pattern of **Amim-Cage-SQ-IL** did not indicate any peaks originating from Cl, and the Si:S elemental ratio was estimated to be 1.00:2.03, indicating that the molar ratio of cation species (imidazolium and ammonium) to NTf2 anions was ca. 1:1.

The 29Si NMR spectrum of **Amim-Cage-SQ-IL** in DMSO-*d*<sup>6</sup> at 40°C only showed four sharp signals due to the *T*<sup>3</sup> structure at −66.8, −67.3, −68.8, and −69.3 ppm, indicating the absence of silanol groups. These signals could be attributed to the MTICl and TTACl components of *T*<sup>8</sup> and the MTICl and TTACl components of *T*10, respectively, because these chemical shifts were almost same as those of **Am-Cage-SQ** and **Im-Cage-SQ-IL** as described in the previous sections. Because the integrated ratio of *T*<sup>8</sup> :*T*10 signals was estimated to be 77:23, the molar ratio of *T*<sup>8</sup> :*T*10 was calculated to be 81:19 (= 77/8:23/10), indicating that *T*<sup>8</sup> was the main product. The MALDI-TOF MS analysis of **Amim-Cage-SQ-IL** also supported the 29Si NMR results.

The DSC curves of **Am-Cage-SQ** and **Im-Cage-SQ-IL** (POSS compounds as described in Sections 2 and 3) indicated the endothermic peaks for *T*ms at 172 and 105°C, respectively (Run 2, 4 in **Table 1**), i.e., **Am-Cage-SQ** and **Im-Cage-SQ-IL** are crystalline compounds. The XRD patterns of **Am-Cage-SQ** and **Im-Cage-SQ-IL** supported that they were crystalline compounds. Therefore, **Am-Cage-SQ** and **Im-Cage-SQ-IL** showed relatively high flow temperatures (~155 and ~100°C, respectively) because of their high crystallinity (Run 2, 4 in **Table 1**). In addition, a mixture of **Am-Cage-SQ** and **Im-Cage-SQ-IL** also maintained crystalline structure, because the endothermic peak due to *T*m was observed at 164°C; it showed fluidity at 120°C (Run 5 in **Table 1**).

random distribution of the two types of side-chain groups, by the hydrolytic condensation of a

**Amim-Cage-SQ-IL** was prepared from a mixture of TTACl and MTICl (1:1 mol/mol) by same procedures for the preparation of **Im-Cage-SQ-IL** and **Am-Cage-SQ** as described above (**Scheme 3**). **Amim-Cage-SQ-IL** was soluble in DMSO, acetonitrile, DMF, methanol, acetone, THF, and ethyl acetate, but insoluble in water, ethanol, 1-propanol, 2-propanol, chloroform,

to the side-chain groups of both the *N,N,N*-trimethyl-*N*-propylammonium group and the 1-methyl-3-propylimidazolium group. The average compositional ratio of TTACl to MTICl

pattern of **Amim-Cage-SQ-IL** did not indicate any peaks originating from Cl, and the Si:S elemental ratio was estimated to be 1.00:2.03, indicating that the molar ratio of cation species

silanol groups. These signals could be attributed to the MTICl and TTACl components of *T*<sup>8</sup> and the MTICl and TTACl components of *T*10, respectively, because these chemical shifts were almost same as those of **Am-Cage-SQ** and **Im-Cage-SQ-IL** as described in the previous sec-

The DSC curves of **Am-Cage-SQ** and **Im-Cage-SQ-IL** (POSS compounds as described in Sections 2 and 3) indicated the endothermic peaks for *T*ms at 172 and 105°C, respectively (Run 2, 4 in **Table 1**), i.e., **Am-Cage-SQ** and **Im-Cage-SQ-IL** are crystalline compounds.

MALDI-TOF MS analysis of **Amim-Cage-SQ-IL** also supported the 29Si NMR results.

anions was ca. 1:1.

structure at −66.8, −67.3, −68.8, and −69.3 ppm, indicating the absence of

:*T*10 signals was estimated to be 77:23, the molar ratio

as a catalyst in water/methanol mixed solvent [26].

showed the signals attributable

at 40°C only showed four sharp

was the main product. The

H NMR spectrum. The EDX

mixture of TTACl and MTICl using HNTf2

586 Progress and Developments in Ionic Liquids

diethyl ether, toluene, and *n*-hexane.

H NMR spectrum of **Amim-Cage-SQ-IL** in DMSO-*d*<sup>6</sup>

components in the product was estimated to be ca. 1:1 from the 1

The 29Si NMR spectrum of **Amim-Cage-SQ-IL** in DMSO-*d*<sup>6</sup>

:*T*10 was calculated to be 81:19 (= 77/8:23/10), indicating that *T*<sup>8</sup>

(imidazolium and ammonium) to NTf2

**Scheme 3.** Preparation of **Amim-Cage-SQ-IL**.

tions. Because the integrated ratio of *T*<sup>8</sup>

signals due to the *T*<sup>3</sup>

The 1

of *T*<sup>8</sup>

Conversely, the DSC curve of **Amim-Cage-SQ-IL** showed a baseline shift at −8°C due to *T*g , whereas an endothermic peak due to *T*m was not detected (Run 6 in **Table 1**), indicating that **Amim-Cage-SQ-IL** is an amorphous compound. The XRD pattern of **Amim-Cage-SQ-IL** did not show any diffraction peaks, supporting the amorphous structure of this compound. **Amim-Cage-SQ-IL** exhibited obvious fluidity at ~30°C (Run 6 in **Table 1**). Because the molecular symmetry of the resulting POSS compound with a random distribution of the two types of side-chain groups was low, its crystallization was suppressed. Therefore, the phase transition from amorphous solid to fluid occurred above *T*<sup>g</sup> . Based on these results, it was concluded that **Amim-Cage-SQ-IL** had *T*<sup>g</sup> of −8°C and showed fluidity at ~30°C, i.e., it is a RT-IL.

The *T*d3, *T*d5, and *T*d10 values estimated by TGA of **Amim-Cage-SQ-IL** were 414°C, 420°C, and 428 °C, respectively (Run 6 in **Table 1**). These values were higher than those of ILs with the side-chain structures of this IL: [TMPA][NTf2 ] (392, 400, and 411°C, respectively) and [C3mim] [NTf2 ] (366, 380, and 399°C, respectively).

### **5. Preparation of ionic liquids containing cyclic oligosiloxanes**

In the previous sections, we described that ILs containing silsesquioxane frameworks, such as randomly structured silsesquioxanes and POSSs, were successfully prepared. In particular, **Am-Random-SQ-IL, Im-Random-SQ-IL**, and **Amim-Cage-SQ-IL** had both relatively low flow temperatures (<~40°C) and high thermal stabilities (*T*d5 > ~400°C). However, they also displayed high viscosities, probably because of the presence of silanol groups for randomly structured silsesquioxane ILs and relatively higher degrees of polymerization (DP) for all silsesquioxane ILs. It is assumed that siloxane-based ILs without silanol groups and with lower DP probably exhibit high thermal stability, low flow temperature, and low viscosity. In this section, therefore, we describe the preparation and properties of ILs containing cyclic oligosiloxanes as the siloxane frameworks.

To achieve the preparation of such ILs containing cyclic oligosiloxanes, we referred to our previous study for the facile preparation of cationic cyclotetrasiloxane (this is not an IL) by the hydrolytic condensation of 3-aminopropylmethyltriethoxysilane using the superacid trifluoromethanesulfonic acid (HOTf) [20]. Therefore, when the hydrolytic condensation of 1-[3-(dimethoxymethylsilyl)propyl]-3-methylimidazolium chloride (DSMIC) was performed using superacid catalysts such as HNTf2 and HOTf, we found that imidazolium salt-type ILs containing cyclic oligosiloxane frameworks (**MeIm-CyS-IL-NTf2** and **MeIm-CyS-IL-OTf**) were successfully prepared [27].

**MeIm-CyS-IL-NTf2** was prepared by the following procedure (**Scheme 4a**): DSMIC was stirred in a water/methanol (1:19, v/v) mixed solution of HNTf2 at room temperature. Then, the solvent was evaporated by heating at ~50°C in an open system. The resulting crude product was further heated at 100°C for 2 h, washed with water, and then dried at 150°C for ca. 5 h to obtain **MeIm-CyS-IL-NTf2** . On the other hand, **MeIm-CyS-IL-OTf** was prepared using almost same procedure as that of **MeIm-CyS-IL-NTf2** but using an aqueous HOTf as a catalyst (**Scheme 4b**). The EDX results of **MeIm-CyS-IL-NTf2** and **MeIm-CyS-IL-OTf** indicated the absence of Cl and the molar ratio of imidazolium cations to NTf2 or OTf anions were ca. 1:1.

and **MeIm-CyS-IL-OTf** were RT-ILs. The *T*d3, *T*d5, and *T*d10 values estimated by TGA were 407,

randomly structured oligosilsesquioxane framework, as described in Section 3. Both ILs have same side-chain groups and showed low flow temperatures (~0°C), yet the siloxane frameworks differed between the ILs. **Figure 1** shows the photographs of these two samples

**Im-Random-SQ-IL** did not show fluidity after 10 s. These results indicated that cyclic oligosiloxane frameworks were important factors for the lower viscosity of **MeIm-CyS-IL-NTf2**

For this chapter, we newly investigated the effects of the alkyl chain length in the imidazolium groups of ILs containing cyclic oligosiloxane frameworks. Therefore, imidazolium salt-type ILs containing cyclic oligosiloxane with various lengths of alkyl chains (R

of the corresponding imidazolium-group-containing dimethoxysilanes using the superacid

(R = CH2

mic peaks due to the *T*ms were not detected for all ILs. In addition, all ILs showed obvious fluidity at ~0°C (**Figure 2a–d** inset, Run 9–12 in **Table 1**). On the basis of these results, we concluded that the alkyl chain lengths in imidazolium groups of ILs containing cyclic oli-

gosiloxane frameworks had an insignificant effect on the IL natures, such as *T*<sup>g</sup>

uct) and cyclic pentasiloxanes (minor product), with some stereoisomers, respectively.

 in a water/methanol (1:19, v/v) mixed solvent (**Scheme 5**). Based on the results of the 29Si NMR and MALDI-TOF MS analyses, we determined that the resulting products [**HIm-**

CH3

Further detailed studies for viscosity determination are currently in progress.

(Run 7 in **Table 1**) and 380, 391, and 402°C for **MeIm-**

Preparation of Ionic Liquids Containing Siloxane Frameworks

was lower than that of **Im-Random-SQ-IL** containing

) were prepared by the hydrolytic condensation

(R = (CH2

(**Figure 2c**, Run 11 in **Table 1**),

)2 CH3 ),

s were observed

and flow

), **PrIm-CyS-IL-NTf2**

and (b) **Im-Random-SQ-IL** after 0 and 10 s with tilting at 14°C.

(**Figure 2a**, Run 9 in **Table 1**), −44°C for **EtIm-CyS-IL-NTf2**

(**Figure 2d**, Run 12 in **Table 1**). These values were almost

(−43°C) (Run 7 in **Table 1**). Conversely, the endother-

)] were mixtures of cyclic tetrasiloxanes (main prod-

obviously flowed after 10 s, while

http://dx.doi.org/10.5772/65892

.

589

415, and 427°C for **MeIm-CyS-IL-NTf2**

The viscosity of **MeIm-CyS-IL-NTf2**

after 0 and 10 s, with tilting at 14°C. **MeIm-CyS-IL-NTf2**

**CyS-IL-OTf** (Run 8 in **Table 1**).

= H, CH2

**CyS-IL-NTf2**

temperatures.

HNTf2

CH3

and **BuIm-CyS-IL-NTf2**

at −38°C for **HIm-CyS-IL-NTf2**

and −45°C for **BuIm-CyS-IL-NTf2**

same as that of **MeIm-CyS-IL-NTf2**

, (CH2 )2 CH3

**Figure 1.** Photographs of (a) **MeIm-CyS-IL-NTf2**

, and (CH2

(R = H), **EtIm-CyS-IL-NTf2**

(R = (CH2

(**Figure 2b**, Run 10 in **Table 1**), −44°C for **PrIm-CyS-IL-NTf2**

)3 CH3

)3 CH3

The DSC curves of the resulting ILs showed the baseline shifts assigned to *T*<sup>g</sup>

**Scheme 4.** Preparation of (a) **MeIm-CyS-IL-NTf2** and (b) **MeIm-CyS-IL-OTf**.

In the MALDI-TOF MS analysis of **MeIm-CyS-IL-NTf2** , several peaks assigned to cyclic siloxane tetramer (main peaks) and pentamer (minor peaks) were observed. Furthermore, the 1 H NMR spectrum exhibited multiplet signals due to methyl groups at 0.23 to −0.23 ppm. In addition, the 29Si NMR spectrum of **MeIm-CyS-IL-NTf2** in DMSO-*d*<sup>6</sup> at 40°C also showed two multiplet signals due to the *D*<sup>2</sup> structure (−19.2 to −19.6 ppm for cyclic tetrasiloxane (main signals) and −21.4 to −21.9 ppm for cyclic pentasiloxane (minor signals)). On the other hand, the MALDI-TOF MS results of **MeIm-CyS-IL-OTf** indicated the existence of a mixture of cyclic siloxane tetramer (main product), pentamer (main product), and hexamer (minor product). In addition, **MeIm-CyS-IL-OTf** had some stereoisomers, confirmed by the <sup>1</sup> H NMR spectrum with multiplet signals assigned to the methyl groups at 0.16–−0.23 ppm and the 29Si NMR spectrum with three multiplet signals due to the *D*<sup>2</sup> structure (−19.1 to −19.7 ppm for cyclic tetrasiloxane (main signals), −21.3 to −21.9 ppm for cyclic pentasiloxane (main signals), and −22.2 to −22.5 ppm for cyclic hexasiloxane (minor signals)). These results indicated that **MeIm-CyS-IL-NTf2** was a mixture of cyclic tetrasiloxanes and cyclic pentasiloxanes, while **MeIm-CyS-IL-OTf** was a mixture of cyclic tetrasiloxanes, cyclic pentasiloxanes, and cyclic hexasiloxane, with some stereoisomers.

The DSC curves of the resulting products indicated the baseline shifts assigned to *T*<sup>g</sup> s at −43°C for **MeIm-CyS-IL-NTf2** (Run 7 in **Table 1**) and at −14°C for **MeIm-CyS-IL-OTf** (Run 8 in **Table 1**), respectively. These values were newly estimated using different DSC equipment from that in the original paper [27] and were slightly different from the values in the original paper. Conversely, the endothermic peaks due to *T*m were not detected. In addition, **MeIm-CyS-IL-NTf2** and **MeIm-CyS-IL-OTf** showed obvious fluidity at ~0 and ~20°C, respectively (Run 7, 8 in **Table 1**). On the basis of these results, it was concluded that **MeIm-CyS-IL-NTf2**

and **MeIm-CyS-IL-OTf** were RT-ILs. The *T*d3, *T*d5, and *T*d10 values estimated by TGA were 407, 415, and 427°C for **MeIm-CyS-IL-NTf2** (Run 7 in **Table 1**) and 380, 391, and 402°C for **MeIm-CyS-IL-OTf** (Run 8 in **Table 1**).

**MeIm-CyS-IL-NTf2**

h to obtain **MeIm-CyS-IL-NTf2**

588 Progress and Developments in Ionic Liquids

stirred in a water/methanol (1:19, v/v) mixed solution of HNTf2

absence of Cl and the molar ratio of imidazolium cations to NTf2

almost same procedure as that of **MeIm-CyS-IL-NTf2**

(**Scheme 4b**). The EDX results of **MeIm-CyS-IL-NTf2**

In the MALDI-TOF MS analysis of **MeIm-CyS-IL-NTf2**

addition, the 29Si NMR spectrum of **MeIm-CyS-IL-NTf2**

NMR spectrum with three multiplet signals due to the *D*<sup>2</sup>

multiplet signals due to the *D*<sup>2</sup>

**Scheme 4.** Preparation of (a) **MeIm-CyS-IL-NTf2**

**MeIm-CyS-IL-NTf2**

for **MeIm-CyS-IL-NTf2**

**CyS-IL-NTf2**

hexasiloxane, with some stereoisomers.

was prepared by the following procedure (**Scheme 4a**): DSMIC was

. On the other hand, **MeIm-CyS-IL-OTf** was prepared using

in DMSO-*d*<sup>6</sup>

structure (−19.2 to −19.6 ppm for cyclic tetrasiloxane (main sig-

was a mixture of cyclic tetrasiloxanes and cyclic pentasiloxanes, while

(Run 7 in **Table 1**) and at −14°C for **MeIm-CyS-IL-OTf** (Run 8 in

and **MeIm-CyS-IL-OTf** showed obvious fluidity at ~0 and ~20°C, respectively

the solvent was evaporated by heating at ~50°C in an open system. The resulting crude product was further heated at 100°C for 2 h, washed with water, and then dried at 150°C for ca. 5

ane tetramer (main peaks) and pentamer (minor peaks) were observed. Furthermore, the 1

NMR spectrum exhibited multiplet signals due to methyl groups at 0.23 to −0.23 ppm. In

and (b) **MeIm-CyS-IL-OTf**.

nals) and −21.4 to −21.9 ppm for cyclic pentasiloxane (minor signals)). On the other hand, the MALDI-TOF MS results of **MeIm-CyS-IL-OTf** indicated the existence of a mixture of cyclic siloxane tetramer (main product), pentamer (main product), and hexamer (minor product).

trum with multiplet signals assigned to the methyl groups at 0.16–−0.23 ppm and the 29Si

cyclic tetrasiloxane (main signals), −21.3 to −21.9 ppm for cyclic pentasiloxane (main signals), and −22.2 to −22.5 ppm for cyclic hexasiloxane (minor signals)). These results indicated that

**MeIm-CyS-IL-OTf** was a mixture of cyclic tetrasiloxanes, cyclic pentasiloxanes, and cyclic

**Table 1**), respectively. These values were newly estimated using different DSC equipment from that in the original paper [27] and were slightly different from the values in the original paper. Conversely, the endothermic peaks due to *T*m were not detected. In addition, **MeIm-**

(Run 7, 8 in **Table 1**). On the basis of these results, it was concluded that **MeIm-CyS-IL-NTf2**

The DSC curves of the resulting products indicated the baseline shifts assigned to *T*<sup>g</sup>

In addition, **MeIm-CyS-IL-OTf** had some stereoisomers, confirmed by the <sup>1</sup>

at room temperature. Then,

but using an aqueous HOTf as a catalyst

and **MeIm-CyS-IL-OTf** indicated the

, several peaks assigned to cyclic silox-

structure (−19.1 to −19.7 ppm for

at 40°C also showed two

H NMR spec-

s at −43°C

H

or OTf anions were ca. 1:1.

The viscosity of **MeIm-CyS-IL-NTf2** was lower than that of **Im-Random-SQ-IL** containing randomly structured oligosilsesquioxane framework, as described in Section 3. Both ILs have same side-chain groups and showed low flow temperatures (~0°C), yet the siloxane frameworks differed between the ILs. **Figure 1** shows the photographs of these two samples after 0 and 10 s, with tilting at 14°C. **MeIm-CyS-IL-NTf2** obviously flowed after 10 s, while **Im-Random-SQ-IL** did not show fluidity after 10 s. These results indicated that cyclic oligosiloxane frameworks were important factors for the lower viscosity of **MeIm-CyS-IL-NTf2** . Further detailed studies for viscosity determination are currently in progress.

**Figure 1.** Photographs of (a) **MeIm-CyS-IL-NTf2** and (b) **Im-Random-SQ-IL** after 0 and 10 s with tilting at 14°C.

For this chapter, we newly investigated the effects of the alkyl chain length in the imidazolium groups of ILs containing cyclic oligosiloxane frameworks. Therefore, imidazolium salt-type ILs containing cyclic oligosiloxane with various lengths of alkyl chains (R = H, CH2 CH3 , (CH2 )2 CH3 , and (CH2 )3 CH3 ) were prepared by the hydrolytic condensation of the corresponding imidazolium-group-containing dimethoxysilanes using the superacid HNTf2 in a water/methanol (1:19, v/v) mixed solvent (**Scheme 5**). Based on the results of the 29Si NMR and MALDI-TOF MS analyses, we determined that the resulting products [**HIm-CyS-IL-NTf2** (R = H), **EtIm-CyS-IL-NTf2** (R = CH2 CH3 ), **PrIm-CyS-IL-NTf2** (R = (CH2 )2 CH3 ), and **BuIm-CyS-IL-NTf2** (R = (CH2 )3 CH3 )] were mixtures of cyclic tetrasiloxanes (main product) and cyclic pentasiloxanes (minor product), with some stereoisomers, respectively.

The DSC curves of the resulting ILs showed the baseline shifts assigned to *T*<sup>g</sup> s were observed at −38°C for **HIm-CyS-IL-NTf2** (**Figure 2a**, Run 9 in **Table 1**), −44°C for **EtIm-CyS-IL-NTf2** (**Figure 2b**, Run 10 in **Table 1**), −44°C for **PrIm-CyS-IL-NTf2** (**Figure 2c**, Run 11 in **Table 1**), and −45°C for **BuIm-CyS-IL-NTf2** (**Figure 2d**, Run 12 in **Table 1**). These values were almost same as that of **MeIm-CyS-IL-NTf2** (−43°C) (Run 7 in **Table 1**). Conversely, the endothermic peaks due to the *T*ms were not detected for all ILs. In addition, all ILs showed obvious fluidity at ~0°C (**Figure 2a–d** inset, Run 9–12 in **Table 1**). On the basis of these results, we concluded that the alkyl chain lengths in imidazolium groups of ILs containing cyclic oligosiloxane frameworks had an insignificant effect on the IL natures, such as *T*<sup>g</sup> and flow temperatures.

**6. Conclusions**

**Author details**

**References**

**NTf2**

oligosiloxanes (**MeIm-CyS-IL-NTf2**

of these siloxane-based ILs are found.

Challenging Exploratory Research) Number 15K13711.

Yoshiro Kaneko\*, Akiyuki Harada, Takuya Kubo and Takuhiro Ishii

\*Address all correspondence to: ykaneko@eng.kagoshima-u.ac.jp

Rev. 1999;**99**:2071–2084. DOI: 10.1021/cr980032t

1998;1765–1766. DOI: 10.1039/A803999B

uid/supercritical CO<sup>2</sup>

6360. DOI: 10.1021/cm102263s

anie.200390070

nmat2448

sis. 2. Chem. Rev. 2011;**111**:3508–3576. DOI: 10.1021/cr1003248

, **PrIm-CyS-IL-NTf2**

In this chapter, we described the preparation and properties of thermally stable ILs containing siloxane frameworks, such as randomly structured oligosilsesquioxanes (**Am-Random-SQ-IL** and **Im-Random-SQ-IL**), POSSs (**Im-Cage-SQ-IL** and **Amim-Cage-SQ-IL**), and cyclic

The authors gratefully acknowledge Prof. J. Ohshita (Hiroshima University) and Dr. T. Mizumo (Samsung R & D Institute Japan) for their enthusiastic collaborations. The authors also gratefully give thanks for financial supports from JSPS KAKENHI (Grant-in-Aid for

Graduate School of Science and Engineering, Kagoshima University, Kagoshima, Japan

[1] Welton T. Room-temperature ionic liquids. Solvents for synthesis and catalysis. Chem.

[2] Hallett JP, Welton T. Room-temperature ionic liquids: solvents for synthesis and cataly-

[3] Huddleston JG, Willauer HD, Swatloski RP, Visser AE, Rogers RD. Room temperature ionic liquids as novel media for 'clean' liquid–liquid extraction. Chem. Commun.

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, and **BuIm-CyS-IL-NTf2**

, **MeIm-CyS-IL-OTf, HIm-CyS-IL-NTf2**

, **EtIm-CyS-IL-**

591

http://dx.doi.org/10.5772/65892

). We are expecting that new applications

Preparation of Ionic Liquids Containing Siloxane Frameworks

**Scheme 5.** Preparation of (a) **HIm-CyS-IL-NTf2** , (b) **EtIm-CyS-IL-NTf2** , (c) **PrIm-CyS-IL-NTf2** , and (d) **BuIm-CyS-IL-NTf2** .

**Figure 2.** DSC curves and photographs of (a) **HIm-CyS-IL-NTf2** , (b) **EtIm-CyS-IL-NTf2** , (c) **PrIm-CyS-IL-NTf2** , and (d) **BuIm-CyS-IL-NTf2** .

### **6. Conclusions**

In this chapter, we described the preparation and properties of thermally stable ILs containing siloxane frameworks, such as randomly structured oligosilsesquioxanes (**Am-Random-SQ-IL** and **Im-Random-SQ-IL**), POSSs (**Im-Cage-SQ-IL** and **Amim-Cage-SQ-IL**), and cyclic oligosiloxanes (**MeIm-CyS-IL-NTf2** , **MeIm-CyS-IL-OTf, HIm-CyS-IL-NTf2** , **EtIm-CyS-IL-NTf2** , **PrIm-CyS-IL-NTf2** , and **BuIm-CyS-IL-NTf2** ). We are expecting that new applications of these siloxane-based ILs are found.

The authors gratefully acknowledge Prof. J. Ohshita (Hiroshima University) and Dr. T. Mizumo (Samsung R & D Institute Japan) for their enthusiastic collaborations. The authors also gratefully give thanks for financial supports from JSPS KAKENHI (Grant-in-Aid for Challenging Exploratory Research) Number 15K13711.

### **Author details**

Yoshiro Kaneko\*, Akiyuki Harada, Takuya Kubo and Takuhiro Ishii

\*Address all correspondence to: ykaneko@eng.kagoshima-u.ac.jp

Graduate School of Science and Engineering, Kagoshima University, Kagoshima, Japan

### **References**

**Figure 2.** DSC curves and photographs of (a) **HIm-CyS-IL-NTf2**

**BuIm-CyS-IL-NTf2**

.

**Scheme 5.** Preparation of (a) **HIm-CyS-IL-NTf2**

590 Progress and Developments in Ionic Liquids

, (b) **EtIm-CyS-IL-NTf2**

, (c) **PrIm-CyS-IL-NTf2**

, and (d) **BuIm-CyS-IL-NTf2**

.

, (b) **EtIm-CyS-IL-NTf2**

, (c) **PrIm-CyS-IL-NTf2**

, and (d)


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592 Progress and Developments in Ionic Liquids

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polymer.2012.10.052


### *Edited by Scott Handy*

Ionic liquids, including the newer subcategory of deep eutectic solvents, continue to attract a great deal of research attention in an even increasing number of areas, including traditional areas such as synthesis (organic and materials), electrochemistry, and physical property studies and predictions, as well as less obvious areas such as lubrication and enzymatic transformations. In this volume, recent advances in a number of these different areas are reported and reviewed, thus granting some appreciation for the future that ionic liquid research holds and affording inspiration for those who have not previously considered the application of ionic liquids in their area of interest.

Progress and Developments in Ionic Liquids

Progress and Developments

in Ionic Liquids

*Edited by Scott Handy*

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