Specific Ionic Liquids for Industrial Applications

## **Chapter 4**

## Perspective Chapter: Applications of Novel Ionic Liquids as Catalyst

*Ganesan Kilivelu*

## **Abstract**

Ionic liquids have much interesting attention in the area of biomedical and it's an alternative to traditional organic solvents owing to their unique chemical, physical properties and environmentally eco-friendly catalytic responses. Ionic liquids have distinct properties like tunability that allows their physical and chemical behaviors to be changed as desired by changing the organic cations with inorganic anions or inorganic cations with organic anions or both cation and anion from organic moieties. Most of the organic reactions are carried out with assistance of catalyst, usually commercially available catalyst are very expensive, more hydroscopic in nature, thermally unstable and very difficult to recycle them but ionic liquids are acted as potential Lewis acidic behaviors, thermally stable, easily recycle inexpensive compared to commercial catalyst and easy to prepare electrically neutral organic cation which are loosely bind with inorganic anions, and organic (pyridinium/imidazolium/ piperidinium) cation could be easily accelerate (or) activate the functional group for most of the organic reactions. Hence, ionic liquids plays a vital role in modern organic synthetic field and may be inevitable in future research.

**Keywords:** substituted dimeric imidazolium cation, trimeric imidazolium salt, catalyst regenerations, pyrimidine derivative, one pot preparation

## **1. Introduction**

Ionic liquids have attracted increasing interest from chemist in the last few decades because of their distinguishable properties including chemical stability, nonflammability, non-volatility and high thermal stability. Commercially available catalysts are very expensive, more hydroscopic in nature, thermally unstable and very difficult to recycle, whereas ionic liquids are thermally stable, easily recyclable, inexpensive compared to commercial catalyst, easy to prepare and have potential Lewis acidic behavior and so, nowadays it plays a vital role in catalysis. Ionic liquids are electrically neutral molecule consist of organic cations which are loosely bind with inorganic anions, hence organic pyridinium/imidazolium/piperidinium cation can easily accelerate (or) activate the functional group in most of the organic reactions. Most of the organic reactions are carried out with the assistance of catalyst [1, 2]. for example, in one pot preparation of 3,4-dihydro-3-substituted oxazine derivatives at room temperature, 1-ethylimidazolium sulphonate is used as a catalyst [3]. Acid based bifunctional pyridinium salt used as a catalyst in Knoevenagal condensation reaction with and without solvent [4]. *N*-Methyl pyridinium bromide acted as highly efficient reagent for aliphatic nucleophilic substitution reaction of sulphate aryl substituted aliphatic ether [5]. 1,4-Diazobicyclo[2.2.2]octane based quarternary ammonium bromide which is recyclable, cheaper and environmentally friendly are used in the preparation of bisnapththolmethane [6]. Chiral biaryl aluminate anion with imidazolium salt showed excellent catalytic response for asymmetric Baeyer-villiger oxidation with higher percentage of conversion [7]. Preparation of hydroxyl methyl substituted furfural in higher yield with shorter reaction time is achieved in the presence of chiral dimeric ionic liquids [8]. Bronsted acidic ionic liquid is used as a catalyst in the preparation of pyridylimidazopyridine [9].

Lewis acid accepts the pair of electrons to attain the octet electronic configuration. Acid is the electron deficient positive charged species that accepts the electrons to form a covalent bond. Base is the electron rich negative charged species that donates to electron deficient ions to form a covalent bond. Organic molecule containing electro positive nitrogen are acted as second-generation Lewis acid. Lewis acid such as ZnCl2, BF3, SnCl3, AlCl3 and CH3AlCl2 catalyze the reaction between electron rich diene and electron deficient dienophile and also catalyze the inverse electron demand Diels Alder reaction. In most of the organic reactions like Aldol condensation reaction, Friedel craft alkylation and acylation, carbon- carbon (or) carbon-nitrogen/carbonsulfur/carbon–oxygen bond formation can be catalyzed by Lewis acid. Most of the organic reactions are carried out with polar/non-polar organic solvents rather than water, because the reactants are insoluble/sparingly soluble or unstable/decompose in water. Now a days, despite few disadvantages, water plays a crucial role as a solvent for organic reactions due to its more polar nature, environment friendly, non-toxic, moderate boiling solvent, easily available and more abundant solvent in the earth. In recent years, Ionic liquids became a very interesting area of research in catalysis, drug delivery system and electro chemical aspects. ionic liquids are excellent alternate for the toxic organic solvents because of its low volatile nature, thermally stable and nontoxic nature. In this book chapter, we will discuss about the catalytic importance and plausible organic reaction mechanism of novel mono, di and trimeric imidazolium and pyridinium salts (ionic liquid) as a catalyst in Aldol condensation, Biginelli reaction, Erlenmeyer reaction, Mannich reaction and Pechmann reaction.

#### **2. Novel 6,6**<sup>0</sup> **-(butane-1,4-diylbis(oxy))bis(methylene)bis (2,4-dimethyl-3-nitro-1-(4-nitrobenzyl)pyridinium) bromide used as a catalyst in Aldol condensation reaction**

In Aldol condensation reaction, β-hydroxy derivatives are formed from α-hydrogen containing aldehyde (or) ketone in the presence of strong alkali. In this reaction, strong alkali abstracts the proton from α-hydrogen containing carbonyl compound and carbanion will attack another carbonyl group to form β-keto derivatives. There are two types of Aldol condensation reaction (simple and mixed Aldol condensation reactions). If both the reacting substrate are similar, then it is called simple Aldol reaction and if two different α-hydrogen containing carbon compounds are involved, then it is called mixed Aldol condensation reaction (**Figure 1**). α-Hydrogen containing aliphatic aldehydes are more reactive than the α–hydrogen containing ketones. In mixed Aldol condensation reaction, one α-hydrogen containing aliphatic aldehyde can react with or without α-hydrogen containing carbonyl compounds. The presence of

**Figure 1.** *Mixed Aldol Condensation reaction.*

Lewis acid enhances the carbanion attack with (or) without α-hydrogen carbonyl compound in Aldol condensation reaction.

In the above reaction, a very expensive chiral catalysts are used to prepare αhydroxy derivatives. Simple Aldol condensation reaction is more advantageous than the mixed Aldol condensation [10, 11].

Lewis acid facilitates the carbonyl compound to attack enolate nucleophile. Hongxin Liu and co-workers used very expensive catalyst which consumes longer reaction time and lesser β-hydroxy derivates [12]. The literature shows that catalyst such as TiCl4, AlCl3, BF3, AlCl3 or ZnCl2 are used in Aldol condensation reaction. These catalysts activate only one carbonyl compound to facilitate the enolate attack (**Figure 2**) whereas, dimeric pyridinium salt in a very low concentration i.e. one equivalence of pyridinium salt activates two equivalence of carbonyl compounds (**Figure 3**). After completion of inter (or) intra-molecular Aldol condensation reaction, the catalyst is easily recovered, recycled and used upto four cycles [13].

**Figure 2.** *Feasibility of nucleophilic attack.*

**Figure 3.** *Aldol condensation catalyzed by dimeric ionic liquid 1.*

**Figure 4.** *Plausible mechanism for benzoxazole formation with trimeric imidazolium salt 2.*

#### **2.1 Novel 3,3**<sup>0</sup> **-(2,4,6-trimethyl-5-((2-methyl-5-nitro-1H-imidazol-1-ium-3-yl)methyl)- 1,3-phenylene)bis(methylene)bis(1,2-dimethyl-5-nitro-1H-imidazol-3-ium) bromide as a catalyst in the preparation of benzoxazole**

Synthesis of dimeric and trimeric substituted imidazolium cation with different anion is carried out using easily available starting materials under conventional as well as solvent free solid supported method [14]. Benzoxazole and its derivatives are prepared using very low concentration of catalyst (dimeric/trimeric substituted imidazolium salts). Trimeric imidazolium salt showed excellent catalytic response than the dimeric substituted imidazolium salts. 0.33 equivalent of trimeric substituted imidazolium salt is sufficient to accelerate the benzoxazole formation where as other catalyst requires equal molar ratio. Benzoxazole and its derivatives are prepared by reaction between substituted aryl aldehyde and *o*-amino phenol (or) *o*-amino thiol in the presence/absence of solvents. With the required equivalence of starting materials, in the absence of catalyst for 10 hours gives only 48% of yield whereas addition of catalyst {trimeric imidazolium salt (2.2458 � <sup>10</sup>�<sup>4</sup> mmol)} in CH3CN solvent under refluxing condition for 30 min. gives 89% of benzoxazole derivatives. From the above reaction, trimeric substituted imidazolium salts activate three equivalence of substituted aryl aldehyde and then *o-*phenol is more facile for cyclization (**Figure 4**). Same benzoxazole and its derivatives are prepared using trimeric substituted imidazolium salts in the absence of solvent and reaction is completed in shorter reaction time with the higher yield and with the easy purification process. It is environment friendly due to the absence of solvent. Benzoxazole is prepared with optimum concentration of trimeric substituted imidazolium salts in the presence of polar and moderately polar solvents such as CHCl3, THF, Acetone, C2H5OH and DMSO. Among these solvents, DMSO showed higher percentage of benzoxazole formation in shorter reaction time [15].

#### **2.2 Biginelli reaction catalyzed by novel 2,2**0 **-(butane-1,4-diylbis(oxy))bis (1-(4-nitrobenzyl)pyridinium) bromide**

Preparation of pyrimidone derivatives from one pot multi component reaction using ethyl acetoacetate, diamide and simple/substituted aryl aldehyde with the assistance of Lewis acid. The literature shows that, the Biginelli reaction required longer reaction time, expensive catalyst and gives very low percentage of yield [16–19].

Preparation of pyrimidine derivatives from conventional method [6] takes 24 hours to complete the reaction, whereas flexible dimeric pyridinium cation as catalyst showed excellent catalytic activity even at very low concentration by

**Figure 5.** *Pyrimidone formation under Biginelli reaction.*

activating the substituted benzaldehyde even in half the equivalence of dimeric pyridinium salts (**Figure 5**). In this reaction (**Figure 6**), various counter anions such as Br, BF4 �, PF6 � and CF3SO3 � are used. Among these, bromide counter anion containing flexible dimeric pyridinium cation showed effective catalysis. Here, the size of the counter anion plays a crucial role in catalytic response. Bromide counter anion containing dimeric pyridinium cations are freely available, which in turn easily activate the simple/substituted aryl aldehyde when compared with other counter anions [20].

#### **2.3 Novel 1,1**<sup>0</sup> **,1″-(2,4,6-trimethylbenzene-1,3,5-triyl)tris(methylene)tris (4-(4-nitrophenyl)pyridinium) bromide as a catalyst Erlenmeyer reaction**

Preparation of benzylidene oxazolone by Erlenmeyer reaction is carried out between aryl aldehyde and hippuric acid in the presence of anhydrous K2CO3 and acetic acid without any catalyst at room temperature for 5 hours. There is no interesting findings are observed. Some of the recent report states that, benzylidene oxazolone derivatives are also prepared using special reaction setup with higher concentration of ionic liquids (20%) at high temperature using expensive catalyst [21–23]. Hence, 4-Nitro benzyl substituted monomeric, dimeric and trimeric pyridinium bromides are tried as a catalyst for Erlenmeyer reaction. 4-Nitro benzyl substituted pyridinium salts showed excellent catalytic response and suitable for Erlenmeyer reaction when compared with other literature catalyst due to inexpensive starting material and stability. 5.7 � <sup>10</sup>�<sup>5</sup> mmol. concentration of 4-nitro benzyl substituted pyridinium salt is sufficient to complete Erlenmeyer reaction in short time at room temperature with higher yield. 4-Nitro benzyl substituted trimeric pyridinium cation showed excellent catalytic activity than the 4-nitro benzyl substituted dimeric pyridinium salt. Dimeric pyridinium salt showed good catalytic response than the monomeric substituted pyridinium salt. One equivalence of 4 nitro benzyl substituted pyridinium cation will activate three equivalence of carbonyl compounds (**Figure 7**), hence 0.33 equivalence of 4-nitro substituted trimeric pyridinium cation is sufficient for Erlenmeyer reaction [24].

## **2.4 Novel 1-benzyl-2-methoxypyridinium bromide and 1-benzyl-2,6-dimethoxypyridinium bromide as a catalyst in Mannich reaction**

Aliphatic triaryl amine is one of the most important functional groups in the active pharmaceutical ingredients because of its interaction *via* H-bond donor/acceptor with the target binding site. Hence, β-amino carbonyl compound containing triaryl amine plays a crucial role in medicinal chemistry. One pot preparation of oxirane derivatives from easily available aryl amine, phenol and paraformaldehyde (**Figure 8**).

2–Methoxy benzyl substituted pyridinium salts showed excellent catalytic properties than the 2, 6–dimethoxy benzyl substituted pyridinium salts. In the catalyst, methoxy group acted as an electron donating group. If electron withdrawing group is more in the Lewis acid, then the activation of carbonyl group may be less or inactive. So, 2, 6–dimethoxy benzyl substituted pyridinium salts catalytic activity is lesser than the 2–methoxy benzyl substituted pyridinium salts. Different inorganic counter anions such as Br�, BF4 �, PF6 � and CF3SO3 � containing 2–methoxy benzyl substituted pyridinium salts plays a crucial role in the activation of carbonyl group because of its bulkier (or) less electro negative nature. The pyridinium carbon is freely

**Figure 7.** *Plausible mechanism for Erlenmeyer reaction.*

**Figure 8.** *Mannich reaction catalyzed by monomeric pyridinium ionic liquid 4.*

available and its easily bind and activate the carbonyl compounds. The Mannich reaction is tried with 2–methoxy benzyl substituted pyridinium cation with Br, BF4 , PF6 and CF3SO3 . Among the catalyst, bromide counter anion containing catalyst showed excellent catalytic response than the BF4 , PF6 and CF3SO3 . The catalytic efficiency depends on electron deficient and freely available pyridinium cation [25]. One pot multi component Mannich reaction is carried out with the assistance of flexible longer alkyl chain containing substituted dimeric imidazolium cation with various counter anions (Br, BF4 , PF6 and CF3SO3 ). Flexible dimeric substituted imidazolium cation acted as a potential catalyst when compared with 2–methoxy benzyl substituted pyridinium cation. One equivalence of flexible dimeric substituted imidazolium cation catalyst activates two equivalences of carbonyl compound. The catalytic efficiency of flexible dimeric substituted imidazolium cation with bromide anion showed excellent catalytic response than the others. 1.66 <sup>10</sup><sup>4</sup> mmol. (optimum concentration) of catalyst is sufficient to form Mannich product with shorter reaction time. Reuse of flexible dimeric substituted imidazolium salts showed same efficiency even after fourth cycle [26]. Substituted oxazine derivatives are acted as an important candidate in the area of medicinal industries [27, 28]. One pot multi component preparation of oxazine derivative of naphtho hetrocyclic substituted compounds using various Lewis acids and Ionic liquids showed poor yield and longer reaction time [29–31]. 1,2-Dimethyl benzyl substituted imidazolium salt is used as a third generation Lewis catalyst for the preparation of naphtho heterocyclic substituted oxazine derivatives instead of expensive/inexpensive catalyst which gives poor yield with longer reaction time. 1,2-Dimethyl benzyl substituted imidazolium cation showed shorter reaction time and higher yield due to its effective activation of carbonyl compound (**Figure 9**). The efficiency of the recycled catalyst is also the same even after the fourth cycle [32].

## **2.5 Novel 5-methyl-2-nitro-3-(4-nitrobenzyl)-1H-imidazol-3-ium bromide as a catalyst in Pechmann reaction**

2-Methyl-5-nitro substituted imidazolium salts are prepared by conventional and solid supported muffle furnace method. Quartinization of 2-methyl-5-nitro imidazole with benzyl bromide/4-nitro benzyl bromide under reflexing condition requires 13 hours for completion, whereas solvent free Muffle furnace method requires only 1/4th of reaction period. Electron Donating/electron withdrawing substituent containing phenol (**Figure 10**) is treated with ethyl acetoacetate in the presence of optimum concentration of 2.0128x10<sup>4</sup> mmol. afforded substituted chromenone derivatives. 4-chloro phenol is treated with ethyl acetate along with 2.0128 <sup>10</sup><sup>4</sup> mmol. of

**Figure 10.** *Reactive probability of substituted phenols.*

2-methyl-5-nitro substituted imidazolium type of ionic liquids as a catalyst and gives 6 chloro-4-methylchromenone derivative in 80% of yield in 30 minutes. Whereas, same optimized concentration of 2-methyl-5-nitro substituted imidazolium type of ionic liquid is used in the reaction between 4-nitro phenol and ethyl acetoacetate and afforded 75% of Pechmann product (**Figure 11**) in longer reaction time [33].

Pechmann reaction is tried with various concentrations such as 8.385 � <sup>10</sup>�<sup>5</sup> mmol. 1.3418 � <sup>10</sup>�<sup>4</sup> mmol., 2.012 � <sup>10</sup>�<sup>4</sup> mmol. and 2.683 � <sup>10</sup>�<sup>4</sup> mmol. Among these concentrations, 2.012 � <sup>10</sup>�<sup>4</sup> mmol. of catalyst showed higher percentage of conversion with shorter reaction time. There is no appreciable progress in both reaction time and percentage of yield, when increasing the concentration of the catalyst to 2.683 � <sup>10</sup>�<sup>4</sup> mmol. so the optimum concentration of the catalyst is 2.012 � <sup>10</sup>�<sup>4</sup> mmol. for Pechmann reaction. Preparation of 6-chloro-4-methyl chromonone derivative from required equivalence of starting materials in various solvents such as THF, acetone, ethanol, methanol and DMSO. Among these solvents, DMSO showed higher yield with shorter reaction time compared with other solvents.

#### **2.6 Preparation of Xanthene and its derivatives using novel 3,3**0 **,3″-(2,4, 6-trimethylbenzene-1,3,5-triyl)tris(methylene)tris(1,2-dimethyl-1H-imidazol-3-ium) as a catalyst**

Xanthene and its derivatives play a crucial role in pharmaceutical applications as analgesic, antiviral, antibacterial and anti-inflammatory drugs [34–36]. One pot preparation of benzoxanthene and its derivatives in the presence of various expensive and nano-particle supported catalyst [37–41]; and it has some limitations such as very high toxic halogenated solvents, high reaction temperature, tedious purification procedure and very low percentage of conversion. 2:1 ratio of β-naphthol and simple/substituted benzaldehyde with or without catalyst in the presence or absence of solvent afforded aryl substituted benzoxanthene and its derivatives [34–36]. The reaction is tried with electron donating/electron withdrawing substituted containing aryl aldehyde with two equivalence of β-naphthol. In these reactions, the electron withdrawing substituent containing aryl aldehyde reacts faster than the electron donating substituent because of the carbanion in β-naphthol which easily attack more electro positive carbonyl carbon of aryl aldehyde. The β-naphthol carbanion is much more facile to attack, if the catalyst is trimeric substituted imidazolium salts. In benzoxanthene and its derivates preparation, various polar and non-polar solvents are tried under conventional method. Dimethyl sulphoxide (DMSO) solvent showed excellent response,

**Figure 12.** *Xanthene derivative formation catalyzed by trimeric imidazolium salt 8.*

such as higher yield in shorter reaction time when compared with other solvents such as THF, ethyl alcohol, chloroform and acetone [32]. In β-naphthol, hydroxy group is an electron donating group, hence, it is acted as *o* and *p*-directing group for aromatic electrophilic substitution reaction. In this regard, *para* position is already substituted, only two possible positions are available to make C-C bond formation with aryl aldehyde. The C-C bond formation is mostly at the first position of β-naphthol (**Figures 12** and **13**).

## **3. Conclusions**

Mono, di and trimeric imidazolium and pyridinium salts are more useful in catalysis to prepare biologically important intermediate and target molecules from novel synthetic methodology. We have synthesized novel mono, di and trimeric imidazolium and pyridinium salts (ionic liquids) and used these ionic liquids as a catalyst in the following reactions. Oxazolone derivatives which has antibacterial, anti-inflammatory, anti-fungal and immunomodulatory properties are prepared by Erlenmeyer reaction with the assistance of trimeric nitro substituted pyridinium salts as catalyst with higher yields. Inexpensive flexible dimeric nitro substituted dimeric pyridinium salts are used in the preparation of biologically active β-hydroxy substituted alkanal in excellent yield. In one pot multi component reaction, oxirane and its derivatives are prepared by Mannich reaction in the presence of very low concentration of recyclable 2-methoxy benzyl substituted pyridinium salts under normal reaction condition. In general, Biginelli reaction requires longer reaction time and gives moderate yield even in the presence of expensive catalyst. Whereas, inexpensive, recyclable flexible longer alkyl chain linked dimeric pyridinium bromide gives excellent response. Flexible longer alkyl chain linked methyl substituted imidazolium bromide acted as an excellent Lewis catalyst for Mannich reaction in the formation of xanthene and its derivatives. 1/3rd of 2.245 <sup>10</sup><sup>4</sup> mmol. concentration of trimeric mesityl core connected imidazolium bromide is sufficient to prepare benzoxazole and its derivatives. In Pechmann reaction, minimum concentration of monomeric, dimeric and trimeric substituted imidazolium/pyridinium salts showed excellent catalytic response.

## **Acknowledgements**

Author thanks Mrs. Revathi Ganesan, Dr. P. Ganapathi, Dr. C. Manikandan, Dr. R. Tamilarasan, Dr. R. Sundaram, Dr. N. Arunagirinathan, Dr. N. Vijayakanth, Dr. A. Aravind, Mr. R. Naveenkumar, Mrs. R. Rajalakshmi, Mr. Senthilnathan Govindaraj, Mr. Sadaiyan Govindaraj for their great support, hard work and co-operation.

## **Conflict of interest**

"The authors declare no conflict of interest."

## **Author details**

Ganesan Kilivelu PG and Research Department of Chemistry, Presidency College, Chennai, India

\*Address all correspondence to: kiliveluganesan@yahoo.co.in

© 2022 The Author(s). Licensee IntechOpen. 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.

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## **Chapter 5**

## Iron-Based Ionic Liquids for Magnetic Resonance Imaging Application

*Praveen Singh Gehlot and Arvind Kumar*

## **Abstract**

In the biomedical treatment, identification of diseases and their diagnosis are running with help of many biomedical techniques including imaging such as magnetic resonance imaging (MRI). MRI technique requires an identification of targeted cell or lesion area which can be achieved by contrast agent. For clinical use, T1 positive MRI contrast agents and T2 negative MRI contrast agents are being used. However, these contrast agents have several drawbacks such as toxic effect of metal centre, poor resolution, weak contrast, low intensity image and short signal for long-term in vivo measurement. Therefore, development of new contrast agents is imperative. Ionic liquids with their unique properties have been tried as novel contrasting materials. Particularly, iron-containing amino-acid-based ionic liquids or amino-acid-based paramagnetic ionic liquids (PMILs) have been reported and demonstrated as MRI contrast agents. These PMILs have shown superior features over reported contrast agents such as dual-mode contrast, biofriendly nature, involvement of non-toxic magnetic centre (Fe), stable aqueous solution, better image intensity at low concentration level and easy to synthesis. PMILs have been characterized well and studied with animal DNA using various techniques. The result revealed that animal DNA is remain safe and stable structurally up to 5 mmol.l�<sup>1</sup> . These cost-effective PMILs opened the greater opportunity in the field of contrast-based biomedical applications.

**Keywords:** ionic liquids, paramagnetic, contrast agents, magnetic resonance imaging, MRI, relaxitivity, DNA

## **1. Introduction**

It is well known that, among various biomedical imaging modalities, magnetic resonance imaging (MRI) is one of the most powerful, ionizing-radiation-free and non-invasive imaging technique, and principally it resembles NMR (Nuclear Magnetic Resonance) technique. Image is formed by spatially encoding NMR signals received from the proton relaxation of the molecules under applied magnetic field [1, 2]. During imaging, the identification of targeted cell or lesion can be visualized by using some of tracking or sensing agent. In the X-ray techniques, BaSO4 will be used to enhance the contrasting level and make difference in brightness of background to

target or lesions area. Similarly, in the magnetic resonance imaging (MRI) technique, MRI contrasting agents are widely used to boost up the image sensitivity and achieve anatomical differentiation or detection accuracy by enhancing the contrast of the image. According to the nature of generating contrast, MRI contrast agents used clinically are T1 MRI contrast agents and T2 MRI contrast agents. T1 contrast (positive contrast) enhances brightness in T1-weighted images; however, T2 contrast (negative contrast) enhances darkness in T2-weighted images [2]. Clinically used contrast agent is mostly composed of gadolinium metal ion. These contrast agents are metal–ligand structure. Gadolinium-based contrast agents (GBCAs) show T1 positive contrast during imaging process [3]. Most commonly, a contrast agent exhibits either T1 or T2 contrasting nature in domination, but recently dual-contrasting nature in single agent with significant T1 and T2 relaxivity values has been reported. Most of these dualcontrasting agents are either multi-layered core-shell nanoparticle or nanoparticle-Gd-chelate complexes which need a highly precise multistep and sophisticated synthesis procedure [4, 5]. But at present, none of the dual (T1 and T2) contrasting agents are commercially available for MRI diagnosis. For GBCAs, it is reported that a sever nephrogenic systemic fibrosis (NSF) complication is recently recognized. Deposited Gd metal ion can induce critical clinical problems such as chronic kidney disease (CKD) and acute kidney injury, etc. to the patient [6]. Thus, in short, there are adverse effects of such contrast agents which are dominant and need to remove or overcome these obstacles either in performance or heath issues. For sake of knowledge, the contrast agents can be grouped broadly into three categories depending upon their function and nature of contrast. T1 contrast agent: gadolinium (Gd)-chelate-based contrast agents (GBCAs) such as Magnevist®, Dotarem® and Omniscan™ are commonly used in daily present clinical practice for T1 contrast in most of clinical aspects [5]. The Magnevist is ionic contrast agent and has Nmethylglucamine counter positive ion. It is used for the visualization of abnormal vascularity. However, these GBCAs have higher osmolality values [7]. Theoretically, the origin of T1 response is related to the longitudinal relaxivity (r1) of aqueous solutions metal–ligand-bearing contrast agent. The main parameters such as the number of water molecules in the first coordination sphere of the metal ion(q), their residence time in the first coordination sphere (tM) and the molecular tumbling time (tR) are optimized to determine the value of r1. These parameters are related to water exchanging process between the first coordination sphere of paramagnetic metal ionsligand moiety and the surrounding water [8]. Most of GBCAs for which q value falls in the range of 1–3 have sufficient thermodynamic and kinetic stability [9, 10]. T2 contrast agents: superparamagnetic iron oxide nanoparticles (SPIONs)-based contrast agents such as Resovist®, Feridex® and Gastromark™ are commercially available and clinically approved T2 contrasting agents. They have higher relaxivity value and considered as negative contrast agent due to enhancement of darkness in T2 images [11, 12]. To prevent agglomeration, either each of these particles is covered with a core-shell or magnetic crystallite embedded in a coating. For example, ferumoxide is made from dextran, whereas ferumoxsil is made of siloxanes. Size of the core determines the relaxivity property of particle. Here also, parameters such as to r1 are governed transverse relaxivity (r2), which is further related to T2 contrast. The relaxation induced by superparamagnetic particles can be explained by the classic outersphere relaxation theory [13]. According to this theory, the relaxation rates of water protons diffusing nearby the unpaired electrons present in paramagnetic ions are responsible for the particle's magnetization [14]. Enhancement in T2 relaxation increases with the particle size [15, 16]. Therefore, SPIOs were firstly developed as T2

## *Iron-Based Ionic Liquids for Magnetic Resonance Imaging Application DOI: http://dx.doi.org/10.5772/intechopen.107948*

contrast agents due to their larger size [17]. According to the overall size of the particles, superparamagnetic iron oxides are classified [7]. Ultra-small superparamagnetic iron oxide (USPIO) nanoparticles [18] have a diameter less than 50 nm, whereas small superparamagnetic iron oxide (SSPIO) nanoparticles have size between 1 mm and 50 nm. Micron-sized particles of iron oxide (MPIO) nanoparticles are large particles with a diameter of several microns. Since T1- and T2-weighted contrast agents exhibit great response and possess unique qualities, but there are some reports which have described their limitations. Therefore, synergic integration of these two functions (T1 and T2) for MRI is expected to get more comprehensive and cooperative diagnostic information over the single T1 or T2 contrast agent [19–21]. The development of dual-mode contrast agent of MRI in a single instrumental system could proficiently eliminate certain difficulties. It could also improve the diagnostic accuracy for most of diseases. It is reported that some functionalized or mixed nanomaterials exhibit intrinsic dual-contrast effects in magnetic resonance imaging. The FeCo nanoparticles (NPs) reported by H. Dai show high T1/T2 contrast effects, but there is a lack of understanding of the dual-mode contrast mechanism [22]. Many researchers have reported the Gd3+-containing magnetite (Fe3O4) NPs, MnOcontaining nanoparticles and SPIONs as dual-MRI contrast agents [4, 5, 23–29]. A recent patent shows pH-sensitive nano-formulates (PMNs) contrast agent comprising extremely small iron oxide nanoparticles (ESIONs) [30]. Songjun Zeng and Jianhua Hao have been used a hybrid lanthanide nanoparticle as a dual-mode contrast agent for imaging-directed tumor diagnosis [31]. The iron oxide nanoparticles coated with Gd-DTPA and fibrin-binding peptides have also been reported by Xu et al. for the detection and localization of thrombosis [32]. Similarly, the cyclic RGD functionalized liposomes (cRGD@MLP-Gd) encapsulated with gadolinium diethylenetriamine penta-acetic acid (Gd-DTPA) and superparamagnetic iron oxide (SPIO) are prepared by Fang Yang and Chun-Jian Li and used for thrombus-targeted imaging activity [33].

Each and every researcher is familiar with Ionic Liquids (ILs) and its functionality. Due to its tuneable nature, ionic liquids have gained special attention. Ionic liquids are compounds comprising entirely of ions where at least one ion should be asymmetric organic ion and melts below the 100°C [34]. It is already reported that the physicochemical properties of ILs depend on the nature of cation, anion and alkyl substitutions [35], and properties can be altered by varying the nature of ions, thus making them taskspecific [36]. Task-specific ionic liquids with characteristic physicochemical properties produced many advantages and have been used in various applications widely [37–40]. Researchers have introduced the inherent magnetic properties by using transition metal at molecular level. First time paramagnetic magnetic ionic liquid (PMIL) has been reported by Hayashi Satoshi et al. and their magnetic property explained. These ionic liquids are composed of iron metal halide (FeCl4 � ion) [41]. Due to inherent paramagnetic character, these ionic liquids were termed as paramagnetic ionic liquids (PMILs). PMILs are made up of a distinct group with versatile properties such as magnetic character and widely used in a various applied fields. For example, the PMILs have been used in desulfurizations [42], organic synthesis [43], microextraction [44, 45], electrocatalysis [46], probe for vesicles [47], self-assembling media for surfactants [48], acidic catalysis [49, 50], density measurements [51], paramagnetic polymer synthesis [52, 53], microemulsion formulation [54], synthesis of chitosan supported magnetic ionic liquid based catalysis [55], CO2 separation [56], application in analytical science [57] and other various applications [58]. Kumar et al. have reported paramagnetic surface-active ionic liquids (PMSAILs), another class of PMILs. Paramagnetic surface-active ionic liquids (PMSAILs) are long chain bearing those ionic liquids which have amphiphilic nature and


#### **Table 1.**

*Name of amino acid and chemical structures of used in the study as contrast agents.*

have ability to form nano-aggregates such as micelle and vesicles in their solution. The PMSAILs are demonstrated as contrast agent in aqueous solution for MRI application and also studied with animal DNA to check its structural stability [59]. Many ionic liquids have been prepared using amino acid, and their biocompatible and biofriendly nature [60, 61] have been checked. Since L-amino acid is biological monomer that is the building block of proteins. L-amino-acid-based chiral PMILs have been synthesized and studied by Isiah M. Warner et al. [62]. Here, the iron-containing amino-acid-based PMILs are studied and first time explored the application as contrast agents which more promisingly interact with essential biological molecules and surprisingly enhance the contrast intensity with retention time. These PMILs-based contrast agents were made of biofriendly amino acid and iron halide. Authors have studied broadly and investigated its interaction with DNA through various techniques including CD, fluorescence, ITC, zeta and gel electrophoresis. MRI property of these PMILs is also investigated and claimed their superior contrast activity. Since these are made of amino acid and iron moiety, therefore, they are non-hazardous, toxic-metal-free and biofriendly contrast agent (**Table 1**) [63]. This work is patented in Indian patent office [64].

## **2. Paramagnetic ionic liquids as contrast agents**

As it has been already mentioned in introduction, ionic liquids (ILs) have gained special attention due their flexibility and versality. Thus, ionic liquids have been used in various applications, and ionic liquid is still being used. In the continuation of the applications, Carla I. Daniel et al. have mentioned the possibility of low-toxic magnetic ionic liquid as contrast agent at end of conclusion. They have conducted a proton nuclear magnetic relaxation dispersion <sup>1</sup> H NMRD study of the molecular dynamics in mixtures of phosphonium-based magnetic ionic liquid[P66614][FeCl4] with [P66614] [Cl] ionic liquid and mixtures of [P66614][FeCl4] with dimethyl sulfoxide (DMSO). The enhancement in r1 relaxation rate of MIL in mixture of ILs + [P66614][FeCl4]) with DMSO solvent [65]. The relaxation enhancement is directly linked with contrast property as we know. The proton spin–lattice relaxation dispersion rate (r1), was measured for both these systems, and the rate indicates a much larger paramagnetic relaxation enhancement for [P66614][FeCl4] with [P66614][Cl], in comparison with that *Iron-Based Ionic Liquids for Magnetic Resonance Imaging Application DOI: http://dx.doi.org/10.5772/intechopen.107948*

**Figure 1.**

*Representative UV and Raman spectrum of PMIL (ProC1[FeCl4]). Reproduced from Ref. [63] with permission from Royal Society of Chemistry respectively.*

observed for the mixtures of [P66614][FeCl4] with DMSO. This difference has reflected that in this mixture, the proton spin–lattice relaxation does not depend on the concentration of paramagnetic ions [FeCl4] � linearly. The paramagnetic ions [FeCl4] � seem to disturb the local molecular organization, molecular order, dynamics and packing in the [P66614][Cl] ionic medium as compared with non-ionic organic solvent DMSO. Thus, here, aqueous solution of iron-containing PMILs is examined for imaging, and their relaxivity rate at various concentrations is studied.

Kumar et al. have synthesized amino-acid-based PMILs [63]. These amino acids are used in two forms—esterified and without esterified. Esterified amino acids are prepared as reported earlier in the literature [61, 66]. Chloro halide salt of amino acid is simply mixed with ferric chloride in equimolar ratio in the ethanol solvent. Dark brown PMILs are obtained at end of process after solvent evaporation. So, these PMILs are easy to synthesize and can be cost-effective. These PMILs are characterized well, and structural verification of [FeCl4] � ion is done by UV and Raman shift using Shimadzu UV-2700 UV–VIS spectrophotometer, Japan, and LabRAM HR Evolution Horiba Jobin Yvon Raman spectrometer, Japan, at 298.15 K (**Figure 1**). The Raman shift value for [FeCl4] � ion should be near 334 cm�<sup>1</sup> [67, 68]. The paramagnetic nature of PMILs is confirmed by the EPR spectrum using mt-MiniScope MS5000 ESRStudio by Freiberg instrument at 298.15 K. The EPR spectrum of PMILs in solution phase indicates a single isotropic EPR line which appeared due to mixed <sup>6</sup> S1 state. EPR spectrum of magnetic centre, here Fe3+ ion, strongly depends on its tetrahedral environment [69]. The amount of Fe in each PMILs is measured and calculated using Perkin Elmer ICP optima 2000 DV ICP-OES (Inductively Coupled Plasma–Optical Emission Spectroscopy) analyzer. In the terms of Fe, aqueous solutions from 0.1 to 5 mmol.l�<sup>1</sup> are prepared for further experiments including interaction with DNA and magnetic resonance imaging.

## **3. Physiochemical properties of PMILs**

Degradation and glass transition temperature of PMILs was investigated using NETZSCH TG 209 F1 Libra TGA and NETZCH DSC 204 F1 Phoenix DSC, respectively. A presentative differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) thermograms are given in **Figure 2**. Combinedly, these are thermal analyses, and it is the technique in which physical properties of a substance or a mixture of substances are measured against either of temperature or time, wherein the substances are subjected to a controlled temperature programme system. If the

**Figure 2.**

*Representative DSC and TGA thermogram (ProC1[FeCl4]). Reproduced from Ref. [63] with permission from Royal Society of Chemistry respectively.*

physical property is weight, then it is termed as a TGA. In thermogravimetric analysis (TGA), weight changes or % of the mass loss of PMILs is measured as a function of temperature. Thermogravimetric curves (graph between % mass loss versus temperature) allow an evaluation of thermal stabilities. Degradation temperature (Tg) was marked where maximum mass changes occur [70–73]. In the figure, initial loss in mass is due to moisture or water elimination at near 120–200°C. After that maximum loss in mass was observed at 200–300°C. For the TGA thermogram, onset temperature (Tonset) is the intersection of the baseline of weight (after the loss of the water) and the tangent of the weight versus temperature curve or simply, a temperature at which the sample loses weight with fastest rate. The start temperature (Tstart) is beginning of decomposition [72]. For ionic liquid, glass transition temperature is measured due to its amorphous nature. Glass transition temperature does not have sharp changes like melting point. Glass transition temperatures (Tg) for PMILs are found below 100°C. There is assumption that lowering the melting point of ionic liquids is achieved due to distortion in the lattice of crystal, and these disturbances generate low lattice enthalpy and weak ionic attraction between asymmetric ions. However, like branching or enlargement of substituent is majorly responsible of disruption in crystal packing [74]. In DSC, the sample and reference are kept at the same temperature, and the energy d(Δq)/dt required to preserve zero temperature differential (ΔT = 0) between the sample and the reference is measured on the function of temperature during a thermal event in the sample. As a result, the endothermic peak indicates absorption of heat, and the exothermic peak will rise when heat is released. Depending on the nature of peaks, glass transition temperature (Tg), melting point (Tm), crystallization transition (Tc) and heat capacity can be calculated [71, 75–77]. Since, Tg values are less than 100°C and fulfilled the criteria of an IL [78], therefore our product will be called as paramagnetic ionic liquids (PMILs).

## **4. Interaction of PMILs with animal DNA**

To explore the biofriendly nature and structural stability of animal DNA, the physical interaction of this PMILs has been investigated. For this whole investigation, 90–92 ng ul�<sup>1</sup> concentration of DNA was prepared in buffer solution using NanoDrop® Spectrophotometer ND-1000. After that, PMILs are studied with animal (salmon fish) double-stranded β-DNA (ds-β-DNA) and studied the conformational and structural stability of DNA in aqueous medium. For this, PMILs were examined

with DNA using various techniques including circular dichroism (CD), fluorescence, isothermal thermal calorimetry (ITC), zeta potential and Agarose gel-electrophoresis, and the concentration regime where DNA remains safe in native form is identified. It is found that long-chain-bearing PMILs undergo complex formation (DNA-PMILs) in the form of precipitate at higher concentration [59].

## **4.1 Circular dichroism (CD) and fluorescence**

To investigate the structural and conformational stability, CD (Jasco J-815 CD spectrometer under the N2 environment at temperature 298.15 K) and fluorescence spectra of DNA are recorded in the presence of aqueous solution of PMILs, and the spectra are shown in **Figure 3**. It is well known that the presence of a positive band at about 276 nm and a negative band near 245 nm with crossover point at nearby 258 nm collectively indicates the existence of native pure DNA in buffer solution. These values indicate that used pure DNA is the fully hydrated doublehelix β form and dextrorotatory in nature [79]. The secondary structure of DNA remained same as native DNA within the concentration range (0.1–5 mmol.l�<sup>1</sup> Fe); after that, at higher concentrations (above 3 mmol.l�<sup>1</sup> Fe), PMIL underwent interaction process with negative sites of DNA, and consequently, bands are distorted. Thus, at low concentration of PMILs, CD bands of DNA unchanged and are likely to superimpose on native bands representing the structural and conformational stability of DNA in these concentration ranges. For tertiary structure confirmation, interaction of PMILs with DNA was observed by Ethidium Bromide (EB) exclusion assay using a Fluorolog horiba Jobin Yvon fluorescence spectrophotometer. Fluorescence intensity of intercalating dye EB abruptly enhanced when

#### **Figure 3.**

*Representative CD spectra, fluorescence spectra of ED-DNA complex, ITC binding enthalpogram and zeta potential at various concentrations (ProC1[FeCl4]). Reproduced from Ref. [63] with permission from Royal Society of Chemistry respectively.*

dye intercalate at minor grooves of DNA with order of 20–25 times with respect to alone EB in buffer medium. Here, water is responsible which acts as a strong quencher [80–83]. From **Figure 3**, it can observed that intensity peak height is similar to EB-Native DNA complex at low concentration (0.1–3 mmol.l�<sup>1</sup> ) that means PMILs did not bind to the EB-DNA complex efficiently. But at higher concentration, intensity peak height reduces due to involvement of cationic counterpart to the EB-DNA complex. Positive counter ion of PMILs started to interact with the minor negative groove of DNA via strong electrostatic interaction after complete removal of the spine of hydration, and consequently, EB dislocates effectively from its hydrophobic environment [84, 85]. It is assumed that more hydrated small cation-containing PMILs are incompetent to dislocate the EB efficiently due to weak electrostatic interactions, but these interactions become dominant when concentration is increased. In the case of long-chain-containing PMILs, DNA showed compaction phenomenon at higher concentration [59]. DNA compaction is confirmed by distortion in intensity and shifting in band position due to formation of cationic surfactant complex (lipoplex) [86]. So, from figure it can be judged that CD spectra and fluorescence spectra confirmed conformational and structural stability of DAN, which remains similar to native DNA at low concentration of PMILs.

#### **4.2 Isothermal titration calorimeter and zeta potential**

To investigate the interaction of PMILs with DNA thermodynamically, binding isothermal enthalpogram is measured from Isothermal Titration Calorimetry (ITC) experiments using MicroCal ITC200 microcalorimeter instrument with controlled Hamiltonian syringe. Measured enthalpy in the ITC experiment is a combination of overall heat produced from various phenomena involving the binding of PMILs on DNA through the electrostatic and hydrophobic interactions, hydration of PMILs and the change in the conformation of hydrated DNA [87]. From **Figure 3**, it is observed that variation in enthalpy (ΔH) of DNA-PMILs interaction process is less at low concentration and shows negligible DNA-PMILs binding. But, at higher concentration, a characteristic large enthalpic peak appeared which revealed significant DNA-PMILs interactions or can say complex formation [59, 88], which means PMILs have a significant impact on DNA after certain critical concentration. In the continuation, the effect of PMILs on DNA negative surface is observed by measuring zeta potential (ζ) using a Zetasizer Nano ZS light scattering apparatus (Malvern Instruments, U.K.) with a He-Ne laser (633 nm, 100 mW) at 298.15 K. The trend of changes in zeta potential with respect to concentration of PMILs is negative to positive. Initially it is negative due to surface charge of DNA alone or less binding of PMILs at low concentration. After that, it is passed through neutral point to positive value at higher concentration. It can be understood like, initially a few molecules of PMILs interact with DNA surface and at this time, some hidden negative charges of core area of the DNA chains become exposed outside; consequently, zeta value initially decreases [89]. After that, more positive counter ions of PMILs started to bind with exposed negative surface of DNA when overall exposed negative surface of DNA was neutralized, and then, subsequently, zeta value shifted to its positive values and then finally reached its maximum positive value. The neutralized negative surface of DNA makes PMIL-DNA complex formation, and such a complex occurs when all negative phosphate groups bind with available positive cations [90–93]. This observation also concedes with ITC enthalpogram.

*Iron-Based Ionic Liquids for Magnetic Resonance Imaging Application DOI: http://dx.doi.org/10.5772/intechopen.107948*

#### **Figure 4.**

*Representative agarose gel-electrophoresis electrophoresis pattern of DNA in the presence of PMILs at various concentrations (ProC1[FeCl4]). Reproduced from Ref. [63] with permission from Royal Society of Chemistry respectively.*

#### **4.3 Agarose gel electrophoresis**

Whether DNA breaks down or not in the presence of PMILs, with this objective, the agarose gel electrophoresis experiment was performed using Electrophoresis Power supply BGPS 300/400. In **Figure 4**, initial bright bands are similar to pure DNA at low concentration (up to 5 mM Fe) which indicates the presence of unbound DNA molecules. It was ensured that DNA degradation does not happen in lower concentration regimes after that the band becomes remarkably vague at higher concentration; further these illuminated vague bands disappear when concentration increases. Disappearance of band tells that all DNA molecules have been bound with PMILs, and there are no free DNA molecules available. Moreover, the absence of multiple bands like a ladder confirmed that DNA did not degrade or break in the presence of PMILs molecules [94].

Therefore, collectively it can be summed that these PMILs are safer in terms of biofriendly nature, structural stability and degradation of DNA at certain range of concentration. These PMILs are safe and free from any adverse effect on animal DNA. This concentration range is selected for magnetic resonance imaging experiment.

## **5. Relaxation study of PMILs and its outcomes**

Magnetic resonance imaging and relaxation study has been carried to demonstrate the utility and effectiveness of PMILs as an MRI contrast agent (CA). MR imaging experiment and relaxivity measurement were carried out using 11.7 Tesla MRI instrument (Brücker Advance 500 MHz proton NMR) using a micro-imaging probe and Paravision imaging software at CSIR CSMCRI, India. T1- and T2-weighted MRI images of aqueous phantoms of each PMIL at different concentration (0.3, 0.5, 0.7 and 1.0 mM Fe) are measured using the spin-echo pulse sequences (RAREVTR and MSME) with acquisition parameters FOV = 0.4 cm, TR = 350 ms, TE = 8 ms, 128 128 matrix and FOV = 0.4 cm, TR = 2000 ms, TE = 36, 128 128 matrix respectively for PMSAILs. T1 and T2 relaxation times of aqueous phantoms are measured using Bruker RAREVTR (FOV = 0.4 or 0.5 cm, TE = 8 ms, TR = 250 to 2850 ms and 128 128 matrix) and MSME (FOV = 0.4 or 0.5 cm, TE = 12 to 72 ms, TR = 2000 ms and 128 128 matrix) MRI pulse sequences respectively. T1 and T2 relaxivity values were calculated through Eq. (1) by linear curve fitting of relaxation rate (1/T1 and 1/T2) versus Fe concentration using Paravision software.

The study has explored its utility and estimated the effectiveness of the PMILs as MRI contrast agents and can be used for diagnosis. Generally, the intensity of MRI image depends on the population of <sup>1</sup> H nuclei of water molecules present in the biological tissue or cell or solvent, relaxation time and their relaxation rate also. Relaxation rate belongs to spin–spin relaxation and spin–lattice relaxation. Relaxation rate greatly influenced by environment of water molecules that are going to exchange process between magnetic centre or surrounding. Thus, relaxation rate (r) can vary with the variation of the local magnetic field and magnetic field inhomogeneity around the <sup>1</sup> H nuclei of the sample. Incorporation of a magnetic entity such as gadolinium chelates and superparamagnetic nanoparticles of Gd, Fe, Mn, into the samples will bring changes in relaxation rate via generating a variation in the local magnetic field and magnetic field inhomogeneity around the <sup>1</sup> H nuclei of the concerned sample.

The relaxation rate of PMILs obeys a linear relationship with Fe concentration and can be represented mathematically by the following expression [95].

$$\frac{1}{T\_{i,C}} = \frac{1}{T\_{i,0}} + r\_i C \tag{1}$$

Ti,C and Ti,0 (i = 1 or 2) are relaxation time of sample at C concentration and absence of contrast reagent respectively, and ri is the relaxivity of the contrast agent. T1- and T2-weighted images are recorded via using spin echo (SE) MRI pulse sequence, and the signal intensity for SE pulse sequence can be expressed as [2].

$$I = I\_0 \left(\mathbf{1} - e^{-TR/T\_1}\right) \left(e^{-TE/T\_2}\right) \tag{2}$$

Intensity of T1- and T2-weighted image is totally T1- and T2-dependent; T1 and T2 weighting can be achieved by eliminating one term in the presence of other. For T1 weighted image, T2 term should be eliminated and the same for T2-weighted image, T1 term should be eliminated with the selection of the appropriate combination of TE (time of echo) and TR (time of repetition) values. Further, it can be seen from Eqs (1) and (2) that the intensity increases in T1-weighted image but decreases in T2-weighted images with the increase of Fe concentration and vice versa too. Local magnetic field inhomogeneity generated by Fe constituent also affects the transverse relaxation time (T2), and due to this involvement of local field inhomogeneity, T2 converts into T2\*. T2\* relaxation time is a combination of true T2 relaxation time with relaxation rate generated due to local magnetic field inhomogeneity, and its value is always larger than T2 relaxation time and can be mathematically expressed as [96].

$$\frac{1}{T\_{2,C}^{\*}} = \frac{1}{T\_{2,C}} + \chi \Delta B \tag{3}$$

where γ is the gyromagnetic ratio, ΔB is magnetic field inhomogeneity across the voxel and 1/T2 is the relaxation rate contribution of magnetic field inhomogeneity. T2\*-weighted MRI images are obtained with gradient echo pulse sequence by choosing appropriate values of TE, TR and flip angle (α) of the excitation pulse to minimize the T1 effect in the images. Signal intensity under this pulse sequence can be illustrated as

$$I = I\_0 \frac{\left(\mathbf{1} - e^{-TR/T\_1}\right) \sin a}{\left(\mathbf{1} - e^{-TR/T\_1}\right) \cos a} e^{-TE/T\_2^\*} \tag{4}$$

## *Iron-Based Ionic Liquids for Magnetic Resonance Imaging Application DOI: http://dx.doi.org/10.5772/intechopen.107948*

In order to investigate the contrast property of PMILs, five different concentrations (0.0, 0.3, 0.5, 0.7 and 1.0 mM Fe) were prepared, and T1, T2 and T2\*-weighted MRI images of aqueous phantoms were obtained. Their relaxation times were determined by using Eq. (2) for T1, T2, and Eq. (4) for T2\*. T1 and T2-weighted MRI images of PMILs are shown in **Figure 2**. An intensity reduction in T2 and T2\*-weighted images and simultaneously an intensity enhancement in T1-weighted image were observed along with Fe concentration. This intensity variation found in MRI images with respect to Fe concentration reveals contrast property of aqueous PMILs qualitatively and further suggests that these PMILs have negative as well as positive contrast behavior or can say PMLs can be used as T1 and T2 contrast agents.

**Figure 5** represent a linear relationship of relaxation rate (r) with Fe concentration of PMILs which is found similar as reported in the literature available for various contrast agents [97]. This contrast property of PMILs is determined quantitatively for PMILs; r1, r2 and r2\* relaxivity measured by a linear curve fitting of their corresponding curves (Eq. (1)). The measured r1 and r2 values are given in **Table 2**.

It is also found that these PMILs are able to generate dual contrast with intermediate r2/r1 value like metal nanoparticles. Low value of r2/r1 represents the positive contrast, whereas high value indicates negative, but intermediate value of this represents dual nature of contrast agent.

Tegafaw et al. reported that the r2/r1 value for Gd-Dy oxide made hybrid nanoparticles is near 6 and claimed its dual nature [5]. Fe2O3 + Fe3O4 nanoparticles (Resovist) also have r2/r1 value near 5.9 [3]. Similarly, r2/r1 value for these PMILs is also observed near 5.2 and which is greater than Gd-BOPTA (r2/r1 ≈ 1.1). Therefore, it can be summarized that the synthesized nanoparticle-free and biofriendly PMILs are potential T2 and T1 dual-mode contrast agents. These PMILs can take position in the list of newly discovered or clinically used contrast agents.

**Figure 5.**

*T1- and T2-weighted MR images and relaxivity pattern of amino-acid-based PMILs at various Fe concentrations. Reproduced from Ref. [63] with permission from Royal Society of Chemistry respectively.*


## **Table 2.**

*Osmolality, relaxivity values (r1 and r2) and ratio of r2 and r1 (r2/r1) for PMILs and Gd-BOPTA.*

## **6. Conclusion and future prospect**

PMILs has been explored as a dual-responsive (T1 and T2) contrast agents for magnetic resonance imaging, and such dual-behavior contrast agents are reported by rare researchers [4, 5]. PMILs have green nature (use of biofriendly amino acid moiety) and magnetic property (use of biocompatible non-toxic Fe (III) ion). Use of Fe eliminated the adverse effect of toxic metal ion like Gd(III) on cell functionality [98] or any Gd metal concerned diseases [99]. However, it is also reported that metallic nanoparticles used for single as well as dual contrast agents have also showed the harmful effect on cell and its physiology [100–102]. For the sake of knowledge, Maureen R. Gwinn and Val Vallyathan have reviewed and reported the toxic effect of nanoparticles (NPs)-based contrast agents which similarly act as air-borne UFPs (Ultra-Fine Particles) and cause diseases with long latency [102]. Similarly, Meng Tang et al. have also reviewed the various adverse effects of NPs on cell organelles and reported that NPs create mitochondrial dysfunction, endoplasmic reticulum stress and lysosomal rupture [103]. Even clinically used Gd-based contrast agents (GBCAs) are also under the question mark with their health-related issues and lengthy synthesis. The report from FDA drug safety newsletter also has criticized the GBCAs and explained the drastic effect of Gd on kidney-related issues such as fibrosing disease, NSF (Nephrogenic Systemic Fibrosis) with acute or chronic severe renal (kidney) insufficiency and renal dysfunction. Some other reports are also available where researchers have reported the accumulation of gadolinium in the renal tissue of patients suffering from NSF [98]. In this study, PMILs have been studied, and their possible use in imaging techniques such as MRI contrast agent with some better advantage has been revealed. Most frequently used Gd metal has been replaced with Fe as a magnetic source in PMILs for imaging due to kidney-related issues. The efficacy of these PMILs is also compared with clinically approved ongoing Gd-based contrast agent. If we compare, PMILs neither contain any metallic nanoparticles nor metal with bulky ligand complex. These PMILs are found to be free from the issue of metal leaching, and any adverse or side effect which is common for nanomaterials when they are used as contrast agents. These PMILs are easy to synthesize and cost-effective, and at certain concentration, they are safe in reference to animal DNA. However, for real effect on cell or health, there is research still ongoing. So, targeted and specific uses of these PMILs need further deep investigations with living cells or organisms.

## *Iron-Based Ionic Liquids for Magnetic Resonance Imaging Application DOI: http://dx.doi.org/10.5772/intechopen.107948*

The comparative study of PMILs with commercially available and FDA-approved Gd-based contrast agents is also completed to examine the comparative performance. Gadobentae Diglumine (Gd-BOPTA) is used as a model contrast agent for this studied. All the experiments were carried out under the similar conditions, and it was found that PMILs-based contrast agents have remarkable properties with significant imaging responses and relaxivity values. The relaxivity values (r1 and r2) for Gd-BOPTA are much less compared with PMILs and wherein the ratio of *r2/r1* is found around 1.2 for Gd-BOPTA which makes it positive contrast agent (T1 mode) only. The relaxivity values (r1 and r2), ratio of *r2/r1* and osmolality value of PMILs with Gd-BOPTA are given in **Table 2**. The measured osmolarity values are found in range of 3 Osmol.Kg�<sup>1</sup> for PMILs which is comparable to Gd-BOTA contrast agent. Low osmolality helps to reduce the pain and other contrary effect during injection [104]. From the results discussed above, it can be concluded that PMILs have several advantages over existing contrast agents (NPs and GBCAs). The PMILs have following advantages:


It can be said that any biological moiety including proteins, vitamin, nitrogen contain sugar moiety, nitrogen-containing heterocycles or drug molecules or any molecule which can be turned into positive ions may be used to synthesize the PMILs via given synthetic procedure in the literature [63]. This study opens other new potential applications of PMILs in the fields of medical, pharma and analytical science. Moreover, such study also triggered further investigations into cytotoxicity, effect on biological process, cancer cell growth, tracking of drug molecules in body and its other uses in physical constant verification and measurement. In the field of imaging, PMILs may become most potential and promising contrast agent with suitable modification. However, research is still ongoing to explore new molecules with better advantages of such iron (III)-containing ionic-liquid-based contrast agents.

## **Acknowledgements**

Praveen Singh Gehlot gratefully acknowledges the Commissioner of Higher Education Department, Government of Gujarat, India, and his former fellowship (JRF & SRF) granted by University Grant Commission, India. Authors also acknowledge financial support through Department of Science and Technology (DST), India. Authors are thankful to CSIR-CSMCRI, India, for providing the place for research work and additional assistance.

## **Conflict of interest**

The authors declare no conflict of interest.

## **Author details**

Praveen Singh Gehlot<sup>1</sup> and Arvind Kumar2,3\*

1 Department of Chemistry, Government Science College, Pardi, Gujarat, India

2 CSIR-Central Salt and Marine Chemicals Research Institute (CSIR-CSMCRI), Bhavnagar, Gujarat, India

3 Academy of Scientific and Innovative Research, Ghaziabad, India

\*Address all correspondence to: arvind@csmcri.res.in

© 2022 The Author(s). Licensee IntechOpen. 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.

*Iron-Based Ionic Liquids for Magnetic Resonance Imaging Application DOI: http://dx.doi.org/10.5772/intechopen.107948*

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## **Chapter 6**

## Compatibility of Filter Materials Used with Ionic Liquids' Uses in Hydraulic Drive Control Systems and a Filterability Test

*Darko Lovrec and Vito Tič*

## **Abstract**

Developments in the field of ionic liquids have led to industrial applications within various industrial processes, as they can be tailored to a specific purpose of use. Due to certain excellent physico-chemical properties, the first industrial applications also appear in the field of hydraulic drives. In these cases, efficient filtration of the hydraulic fluid is extremely important, as the safe, reliable, as well as long-lasting and economical operation of a heavily loaded hydraulic system depends on the efficiency of the filter and the cleanliness of the fluid. In the case of ionic hydraulic fluids, the question of compatibility with the materials of hydraulic components, including filters, arises. The chapter addresses the issue of compatibility of ionic hydraulic fluids with all filter materials, including the filter material that does the actual filtering. At the forefront of the discussion is the issue of incompatibility with cellulose-based filter material, which is considered the most effective when filtering conventional hydraulic fluids. The discussion is also related to off-line filter devices and the standardised filterability test, which prescribes cellulose filter membranes, and the resulting problems of practical and credible implementation of the standardised filterability test.

**Keywords:** ionic liquids, hydraulic drives, filter materials, compatibility, filterability test

## **1. Introduction**

Filtering technical fluids used in various processes on a variety of machines and devices is very important, if not a key task that we must address, both before and during the operation of the machine or device. Only an appropriate clean fluid ensures the integrity of a particular process and the long service life of both the fluid itself and the individual components and the entire device. This is especially true for hydraulic fluids, which are one of the most loaded technical fluids – liquid lubricants and working media at the same time. In addition to the transmission of forces and the movement of actuators as basic tasks, hydraulic fluid performs several other important tasks. These include cooling all hydraulic components and flushing wear particles from the gaps between the moving parts of components and transporting them to the filter, where they are trapped and removed from the system. In this way, we ensure and maintain a limited, permissible amount and size of the particles. This ensures low component wear and, consequently, minimal leakage losses, as well as unforeseen machine and device downtime due to sudden component failure [1, 2].

Wear particles and other contaminants that occur in the hydraulic system can be of various origins. Solid particles in the form of metal parts, abrasive dust, welding residues, sand, etc. already occur during the manufacture and installation of the device. Even fresh oil is not clean. The number of contaminants in new, freshly supplied hydraulic fluid is often significantly higher than allowed during operation. During operation, dust, fine sand particles and water or moisture from the surroundings can enter the hydraulic fluid through the filling opening on the tank. Particles of wear on metal parts, as well as seals, also occur in the hydraulic system. Chemical changes in the fluid due to temperature, pressure and shear stresses lead to ageing products such as sediments and acids.

To achieve the cleanest possible hydraulic fluid, and the required cleanliness level of the fluid defined by the requirements of each installed component, filters are installed in different places on the hydraulic device. The locations of the various filters in the hydraulic system are shown in **Figure 1**.

Optionally installed suction filters protect the hydraulic pump from possible dirt in the tank, and pressure filters protect the high-quality, high-performance control valves. Filtration of the fluid returned to the tank is the most used variant. These return line filters are installed either on the return line or on the reservoir cover. Due to the low pressure on the return line, these filters are quite affordable, easy to maintain and include cleaning of the entire amount of fluid returned to the tank. Depending on whether we filter only a part, or the entire flow supplied by the pump, we distinguish between part-flow and main-flow filters. In addition, a separate filter circuit is also possible in combination with a fluid cooler.

Depending on the purpose of the filter, the place of its installation and the associated height of the present pressure, the filters are designed differently and made of different

**Figure 1.** *Location of various types of filters in the hydraulic system.*

*Compatibility of Filter Materials Used with Ionic Liquids' Uses in Hydraulic… DOI: http://dx.doi.org/10.5772/intechopen.107962*

materials. In the present case, we will focus in more detail on the filters of the total pump flow – i.e. filtering the entire fluid flow. At the forefront will be low-pressure return filters, which we consider a mandatory, indispensable part of every hydraulic device.

In the case of using conventional types of hydraulic fluids, designers and users of hydraulic devices do not prioritise the compatibility of the materials installed in the filter or in the filter component, but they only choose it for a specific type of fluid, usually hydraulic mineral-based oil. Thus, the filter is chosen mainly according to the required degree of cleanliness of the hydraulic fluid and the flow rate.

In the case of new or special hydraulic fluids, before using them, it is necessary to pay a lot of attention to the compatibility of the fluid with all the built-in materials in the filter. This is the task of both the filter manufacturers and later also the users, who must follow the given guidelines and recommendations of the manufacturer to select the appropriate filter with the appropriate materials installed. This dilemma certainly arises in the case of ionic liquids, which are now also used as hydraulic fluids – ionic hydraulic fluids. The first step in this direction is certainly a good knowledge of the filter structure with the various materials incorporated, as well as knowledge of the physico-chemical properties of the used fluid.

## **2. Filter structure and materials**

When it comes to the basic properties of the hydraulic filter, such as type and size of filter, its compressive strength, pressure drop across the filter, filter cartridge collapse pressure, filter efficiency in removing and retaining solid contaminants, the compatibility of all filter materials with the hydraulic fluid used is very important. The latter requirement applies not only to the actual filter material but also all materials built into the filter cartridge and filter housing. A schematic illustration of the flow and filtration process, as well the cross-section of a hydraulic filter and typical structure of a filter cartridge, is shown in **Figure 2**.

**Figure 2.** *Flow through filter (left) and typical structure of a filter cartridge (right).*

#### **Figure 3.**

*Principle of the operation of the surface (left) and depth filter (right).*

In addition, it is necessary to mention the design of the filter element, which determines the principle of filtering. In the case of hydraulic filters, we distinguish two principles of filter operation and the resulting types of filters: surface filters (mesh and sieve) and depth filters (with a fibrous filter insert) [3–5].

Surface filters are mainly meshing, with pores that are smaller than the particles we want to eliminate – usually pore sizes from 25 μm to 100 μm. The surface filter material is usually a mesh of metal or plastic. To increase the surface area and, thus, increase the filtering effect, the filter mesh folds into a star-like, pleated shape.

Depth filters differ from surface filters in that the deposition and accumulation of particles takes place inside it. Depth filters consist of fibrous materials, metal fibres or plastics in the form of wool (fleece or yarn). This group also includes deep pressure filters made of sintered or glued metal balls. All depth filters have a high capacity to accumulate particles, cause a small pressure drop, are relatively affordable and able to retain longer parts inside, e.g. fibres. The principle of the operation of both types of filters is shown in **Figure 3**.

A whole range of different materials are used, depending on the type of filter and its structure, as well as the type of medium we want to filter.

A complete filter, consisting of a housing and a filter cartridge, is made of different types of materials and not just a material that performs the actual filtering function. When we mention the compatibility of a filter or filter cartridge, we usually think only of the material we use for the actual filtering and pay too little attention to other builtin materials. This includes the inner and outer support material of the filter cartridge, both cartridge covers, the seals on the cartridge and the housing where the filter cartridge is inserted [6–8].

#### **2.1 Materials for filter housing and filter media support**

The hydraulic filter housing body is the outer part of the filter. The material for a hydraulic filter housing is a critical part that needs to be considered when selecting a hydraulic filter housing. Proper filter operation depends on the ability of the housing material to withstand fluid operational pressure. Also, the material needs to withstand external factors, such as the environment in which it is operating, as well the type of filtered media inside the housing.

Filter manufacturers most commonly offer three different types of material used for the filter housing. Most hydraulic filter housing bodies are made of copper, carbon steel or stainless steel. All housing materials used must have adequate strength and be compatible with the filtered fluid present inside the filter housing.

At both ends of the filter element are circular caps – end discs. The end discs in a hydraulic filter element are available in different forms and shapes and are of plastic or metal material. These caps play an essential role in squeezing the other parts of the fluid power element into position. In the long run, it offers support to these parts, which ensures that they can withstand the fluid pressure (**Figure 2**).

*Compatibility of Filter Materials Used with Ionic Liquids' Uses in Hydraulic… DOI: http://dx.doi.org/10.5772/intechopen.107962*

The protecting cage is an outer section of the filter cartridge and is usually perforated. The cage protects integral parts of the hydraulic filter element from damage and has two main functions. First, it allows for continuous flow of the fluid, which enables filtration to take place; it also offers mechanical support to the entire filter element against external damage. The protective cages are available in different shapes, sizes and materials.

On the innermost part of the filter element is a perforated support body which is usually either of a metallic or plastic material. The function of this body is to offer support and strength to the filter element.

The supporting tube holds or supports the filter media, either from inside or outside. In many cases, the supporting tube is made of stainless steel. Basically, it should support the weight of the filter media and should withstand high pressure and fluctuating temperature. A supporting tube is perforated and allows flow of the hydraulic fluid from the filtration material and basically represents the skeleton of the hydraulic filter element. It is usually made of stainless or corrosion-resistant material.

Seals on the filter cartridge prevent fluid from flowing past the cartridge. A seal can be fixed, but, in certain cases, it is replaceable and can be selected according to the type of liquid to be filtered. NBR (Nitrile/Perbunan/Buna-N) and FKM (Viton) are most used as sealing materials. When selecting sealing material you should, besides the filtration requirements of the application, consider the resistance of the material to breaking or bursting (the burst strength of the sealing material), the flexibility of the fabric you choose, the tensile strength of the material to withstand the mounting temperature and pressure, the outgassing behaviour of the material and the radiation resistance of the material. The chemical compatibility of the material with the type of hydraulic fluid and resistance of the material to corrosion from corrosive agents should also be considered. For these purposes, in the case of new hydraulic fluids or new formulations of existing ones, it makes sense (i.e. urgently) to carry out tests to verify these properties.

#### **2.2 Filter media material**

When the fluid passes through the filter media, the actual filtration is performed by retaining contaminants and allowing the cleaner fluid to pass. For example, a filter media of different large pores can filter particles in the range of 3–50 microns. The filter media is mainly a pleated material which can either be plastic, fibreglass, (e.g. Polyester, Nylon, Dacron … ) paper, fleece, cellulose, phenolic-impregnated cellulose or stainless steel mesh [9, 10].

Stainless steel wire mesh filter media is also a popular material in hydraulic filter. This is a type of metal screen with stainless or standard steel. It is woven to make a mesh with different pore sizes. It is also resistant to corrosion. Stainless steel filter media is also used in extreme conditions such as high temperature and pressure.

Fibreglass/micro glass is one of the best media for hydraulic filter elements because it has uniform pores. Manufacturers often choose a co-pleat material, because it is more durable than paper. You are likely to find hydraulic filters having co-pleats with other types of materials, such as polymer mesh, stainless steel wire clothes or annealed epoxy-coated steel wire.

Activated carbon as a filter media material is identified and used due to its ability to remove certain contaminants in very effective way. One gram of the activated carbon has a wide surface area of about 500–3000 square metres. The massive and wide surface area will allow active carbon to have better efficacy in absorbing

contaminants. As the hydraulic fluids flow through the activated carbon, the contaminants such as chemicals stick to the carbon. It removes certain types of 'stubborn' contaminants actively, e.g. dissolved solids that include salts, minerals and metals, most of the microbial contaminants in the hydraulic fluid and inorganic contaminants, including lead. However, activated carbon can only last for a short period of time; therefore, you need to check it every week. Due to this aspect, it is less common in hydraulic systems, as it requires much more attention and control than the other mentioned filter materials.

Paper as a filter media is suitable for disposable hydraulic filter elements. Paper is cheap material though good at absorbing water in hydraulic fluid. Manufacturers use polytetrafluoroethylene (PTFE) or polyester fibre in making the hydraulic filter paper. Also, the pore size and thickness vary, depending on the application.

### **2.3 Cellulose filter media**

Cellulose filters (usually in the form of a filter paper) are generally considered to be versatile and diverse microfiltration agents that work by trapping particles in a random matrix of cellulosic fibres. According to the manufacturers, cellulose filters are generally excellent in terms of separation effect, and they are considered economical filters because they can be removed in an environmentally friendly way; they are harmless from a medical point of view and, thus, to the user, and they are reliable and very effective [11].

Experts dealing with filtration efficiency often recommend the use of cellulose depth filters to achieve efficiency. Cellulose depth filtration is a relatively old but reliable technology. The 'depth' fibre matrix of cellulose-based material is used to trap suspended particles, separating them from their filtered fluid. Working by adsorption and absorption, it has the unique filtration ability to trap particles and moisture.

Deep filtration is most used in applications where exceptional lubricant cleanliness is required. For this purpose, and to achieve the best results, certain filter manufacturers recommend the use of deep cellulose type filters, which, with proper operation and design of the filter, can achieve a filtration rate of up to 3-micron particles. In the case of hydraulic drive technology, this level of cleanliness is suitable for high-tech hydraulic servo drive systems.

Cellulose filter paper is also used as a filter paper to determine the approximate degree of cleanliness level of a hydraulic fluid using on-site testing kits for real-time contamination results or bottle sampling options for ISO counts, wear metals and fluid property determination. The simple process is based on manual sampling of the fluid and filtering through a membrane filter disc. The filter membrane is made of mixed cellulose esters (MCE).

## **3. Ionic liquids and compatibility of filter materials**

The materials used in the filter are very compatible with common types of hydraulic fluids, especially in the case of the most used hydraulic mineral-based oil. In the case of fire-resistant glycol-based hydraulic fluids – HFC fluid (water glycol hydraulic fluid), care must be taken in the selection of seal materials and other materials. Namely, there are some application limitations due to compatibility when using water glycol fluids. Regarding metals, the HFC fluid is corrosive to zinc, cadmium and non-anodised aluminium, and the reaction with these metals causes rapid

deterioration of the fluid. The synthetic rubber seal and gasket compatibility is good; however, polyurethane, leather or cork materials should be avoided. Typical paints will soften in the presence of water glycols; therefore, painted surfaces should be painted with epoxy resin paints.

In the case of a completely new type of hydraulic fluid such as ionic hydraulic fluids, the compatibility of the individual material in the filter must be checked individually with each type of fluid.

Recently, (about the last 10 years), ionic liquids have been moving increasingly from the development pilot phases to real industrial, commercial use. Industrial applications cover many diverse technical fields and even megatrends such as mobility, health and the green economy.

Ionic liquid applications are implemented based on publicly available data and approved personal communications of the author with the industry in almost every technical field, e.g. solvents, energy, catalysts, electrolytes, Nanotechnology, chemicals, electronics, paper and pulp, textiles, pharmaceuticals, biotech, nutrition, health, personal care, metal processing, oil and gas, the automobile industry and all the way to the area of hydraulic drives andsystems [12, 13].

Ionic liquids applied in the fields of Lubricants, Heat Transfer and Storage Fluids, Heating, Ventilation, Air Conditioning (HVAC), sealing fluids, cutting and drilling fluids, pressure transmission fluids (hydraulics) and generally as operating liquids in process machines are summarised as ionic engineering fluids. Thus, it is evident that ionic liquids, due to their excellent lubricating and other properties, are also used as lubricants and as hydraulic fluids – ionic hydraulic fluids [14–18].

In most of the mentioned cases, especially in the case of using ionic liquid as a lubricant or as a working fluid within hydraulic drive control systems, fluid filtration is required for the purpose of maintaining the prescribed cleanliness level of the fluid. In this case, the use of effective filters is crucial. This raises the question of the compatibility of filter materials with ionic hydraulic fluids, especially, because several different types of materials are used in the filter (see Section 2).

Methods and standards related to the determination and testing of the material properties of lubricants and the compatibility of materials relate mostly to petroleumbased lubricants. This includes hydraulic fluids, the most commonly used mineralbased hydraulic oils, as well as hydraulic fluids of other types, e.g. fire-resistant hydraulic fluids of the HF type or biodegradable, faster degradable hydraulic fluids of the HE type.

The compatibility of metallic and non-metallic filter materials with ionic hydraulic fluids has been tested in various ways. In cases where there are standards with precisely described procedures and testing conditions, these have been consistently followed. In cases where there is no standard available for testing a particular material or component and only recommendations are given, these have been taken into account. In the case when there are no recommendations or guidelines, a practical, non-standard test was performed. In this case, the conditions present on the real device were taken into account, e.g. applied paint as in the case of a real component, a tested part completely or partially immersed in liquid and similar. However, in cases where the components' part is exposed to the surrounding environment, we have come close to these conditions (there are minor deviations between the industrial environment and the laboratory environment, but these do not significantly affect the main conclusions and findings).

### **3.1 Determining the compatibility of metallic filter materials**

When selecting a filter as a complete component and suitable materials from which it is made (if this information is known and if the choice is possible), we must know the properties of the hydraulic fluid to be filtered well. Given that we encounter different types of metallic and non-metallic materials in the filters (Section 2.1), checking the compatibility of the material with the type of fluid used is a primary and extensive task. This is especially important when it comes to a completely new type of hydraulic fluid, such as ionic hydraulic fluid in our case.

Good lubricating properties and good corrosion protection are the basic aspects of any hydraulic fluid. Unlike other hydraulic components (pumps, valves, hydraulic cylinders and hydro motors), lubricating properties are not at the forefront in the case of filters, but the corrosivity of the fluid to different filter metal materials.

Since ionic liquids are salts, the risk of corrosion can be expected to be one of the most difficult aspects to achieve compared with conventional hydraulic fluids, especially mineral-based oils. That was confirmed within extensive pre-studies and laboratory tests, particularly, in a corrosion test in a humid chamber, where most of the tested ionic liquids proved to be considerably worse than the mineral hydraulic oil (more details are given in the literature, e.g. [19]).

In addition to the humid chamber test (According Standard DIN 51386-1), the corrosion protection capacity was determined by the standard method of determination of corrosiveness to copper (according to ASTM D 130-04) and practical method of determination of corrosion in the open air (practical method, in accordance with the real operating atmosphere conditions).

In the latter case, it was a practical method of immersing and exposing characteristic materials for hydraulic components to various ionic hydraulic liquids. One part of the tested component was completely immersed in the liquid, part was just wetted with IL and exposed to the surrounding ambient air (temperature between 20°C and 22°C, humidity between 35% and 40%) - **Figure 4**.

In this case, parts of the different valve housings (made of cast iron GG30, DIN 169, steel 10,718, steel Hyt 60R), a valve control spool (made from steel 10,715, case hardened 0.4 mm and hardened to 58-62 HRC, steel 16MnCr5), springs (made of steel spring wire according to DIN 17223-1), a washer (made of steel DC01), a bolt (steel 10,715 without subsequent thermal treatment), piston housing (made of steel 42CrMo4 without subsequent thermal treatment) were used as test pieces. Similar materials were used in the case of other hydraulic components. Similar materials are also present in other components of the hydraulic system, such as for pumps, hydraulic cylinders, fittings and piping, as well as filters [19].

The possibility of using filter parts made of copper was also mentioned, and in some cases, aluminium parts could also be encountered. Testing the corrosive or oxidative stability of any parts of the aluminium filter can be performed in the same

**Figure 4.** *Hydraulic valve samples during test of compatibility with ionic liquids.*

## *Compatibility of Filter Materials Used with Ionic Liquids' Uses in Hydraulic… DOI: http://dx.doi.org/10.5772/intechopen.107962*

way as the described immersion test, except that, in this case, either aluminium tape or an aluminium part is used. Alternatively, testing is as available for copper test specimens in the form of test strips, this procedure being specified in the ASTM D 130 Standard.

According to this Standard, the results are given as the corrosiveness to Cu with the designation of the corrosion degree determined by comparison with the tested etalon. Based on the results of our own preliminary testing of different ionic liquids, there were no visible colour changes of the Cu strip, meaning that the (tested) ionic liquids were compatible with materials containing copper. We have come to the same conclusion with other tested ILs – corrosiveness to copper is much lower than that of steel and does not represent any problems. For more details, see [19].

According to the results of the compatibility test, corrosion problems are not to be expected in the practical use of most of the ionic liquids used within the hydraulic system, unless water or increased humidity is present in the ionic liquid. The risk of corrosion could occur if the metal wetted with ionic liquid is exposed to surrounding air with an increased level of humidity (e.g. relative humidity above cca. 60%). As the filter housing is completely filled with fluid and is not in contact with the ambient air, no corrosion problems are expected when using ionic liquid. In addition, most filter housings are made of stainless steel, so there is no risk of corrosion in this case.

#### **3.2 Determining the compatibility of the non-metallic materials in the filter**

Regarding the non-metallic materials present in the filter, various plastic materials (for example, both filter cartridge covers, perforated outer protection cover … ) and filter cartridge seals were mentioned. In certain cases, painted parts can also possibly be found. In the case of any protective paint coat inside the filter and its compatibility with the ionic hydraulic fluid used, the problem is like the case of testing the compatibility of the internal protective paint coating of steel hydraulic reservoirs. Only in these cases can the hydraulic fluid be in direct contact with the paint.

Concerning protective coatings, in particular the compatibility of the protective paint regarding the type of hydraulic fluid, there is not a specific standard which would be related to this issue in detail. There are just recommendations or Recommended Practices (RP), linked mostly to related areas, providing guidance on achieving effective corrosion control in storage tanks. They contain information pertinent to the selection of lining materials, surface preparation, lining application, cure and inspection of tank bottom linings for existing and new storage tanks, e.g. the API RP 652 Standard – Linings of Aboveground Petroleum Storage Tank Bottoms. Thus, manufacturers of lubricants are using simple, practical experiments, e.g. testing by continuous contact through the immersion of painted metal samples in the liquid under test, at constant room temperature: 20–25°C.

In our case of testing, the paint coat compatibility was performed with two different types of ionic liquids. The test metal plates were painted with two paints typically used for tank interiors and exteriors. The interior was painted with an epoxy type priming coat, while the exterior was additionally coated with an epoxy-type thicklayer finishing coat. The painted test plates were first completely soaked with the selected ionic liquid and then half immersed in the liquid.

As a result, after a few days of testing (five to six), it turned out that, in the case of one ionic liquid, there were no changes in the paint coat, and in the case of the other, there was wrinkling and swelling and deviation of the paint coat from the metal plate [19]. Given this, each type of ionic hydraulic fluid should be tested separately with the type of paint used before use. This represents additional work that should be done by either the filter manufacturer or the ionic hydraulic fluids manufacturer, but only in the case if the inside of the filter housing is painted. There are no such paint coat compatibility problems with standard filter housing designs or in the case of a stainless-steel filter housing.

The next non-metallic material present in the filter is seals placed between the filter cartridge and the housing. As mentioned in Section 2.1, NBR or Viton seals are the most used for filter cartridge seals. These materials were also used in the compatibility test with various ionic liquids suitable for use as hydraulic fluids.

The test was performed in accordance with the ASTM D 1414 Standard, with the emphasis on the first three of the six tests (volume swell, shrinkage, hardness change, tensile strength change, elongation change, work function change). According to the test procedure, the thermal loading of the seal in the tested fluid, according to the Standard, takes place for different durations of time (in our case 70, 250, and 500 hours) at a temperature of 90°C [20].

After the elapsed time of thermal loading, the parameters of hardness of the tested seal material and change of geometry were checked, and deviations of values in comparison with deviations valid for mineral hydraulic oils were checked (e.g. permissible changes in % for the case of 500 hours of testing: volume swell �20%, shrinkage: �4%, hardness change: +/� 10% in relation to baseline values before testing).

Visible changes in the colour, shape, geometry and hardness of the seal, for the case of two different seal materials and two different ionic liquids (B2002b® suitable for use in hydraulic systems and EMIM EtSO4 'conventional' ionic liquid, both manufactured by proionic), are shown in **Table 1**.

NBR and Viton materials are quite resistant to thermal stress with various ionic liquids, or they are very compatible with them. This is especially true for the NBR,


#### **Table 1.**

*Changes in geometry and hardness in the case of two seal materials and two ionic liquids compared with the values typical of mineral hydraulic oils.*

which shows comparable results compared with conventional hydraulic mineral oils. However, this does not apply to all types of ionic liquids (due to their different chemical structures) and other types of sealing materials. Therefore, in the case of using ionic hydraulic fluid, it is necessary to check its compatibility individually with the seal material used in the filter.

## **3.3 Determining the compatibility of filter media material**

Section 2.2 mentioned the different types of materials used commonly for filtration, and point 2.3 highlighted the cellulose-based filter material that filter manufacturers consider to be very effective. Such cellulose filters in the form of filter membranes are also used in on-site and in laboratory tests to determine the cleanliness class of hydraulic fluid (based on the gravimetric method, with weighing and/or visual evaluation of particle size and quantity). In the following, due to the specificity of cellulosic materials, we will focus on their compatibility with different ionic liquids.

The authors dealing with ionic liquids have already reported on the degradability of cellulose with ionic liquids, and based on this knowledge, ionic liquids were also used for the purpose of cellulose decomposition. For example, Swatloski et al. [21] found that the ILs can dissolve cellulose in a microwave oven as well as in a conventional oven. They reported about initial results that demonstrate that cellulose can be dissolved without activation or pre-treatment in, and regenerated from, 1-butyl-3 methylimidazolium chloride and other hydrophilic ionic liquids. This may enable the application of ionic liquids as alternatives to environmentally undesirable solvents currently used for dissolution of this important bioresource. Ren et al. [22] reported that imidazolium-based chloride ILs can be used to dissolve and regenerate cellulose into a variety of physical forms. They found that 1-allyl-3-methylimidazolium chloride ([AMIM]Cl) showed better capability for dissolving cellulose than [BMIM]Cl. Zhang et al. [23] also found that [AMIM] Cl can be used as solvent for the dissolution and regeneration of cellulose without any pre-treatment or activation. The research results indicated that cellulose with a degree of polymerisation as high as 650 can be dissolved in [AMIM] Cl within 30 min. In these applications, recycling of IL is highly necessary from the points of view of economics, IL disposal and toxicity.

It should be emphasised again that this property of ionic liquids is more or less known to experts in the field of chemistry, to those who deal with ionic liquids. Experts in the field of construction of hydraulic devices, who encounter this type of liquid during their first applications, certainly not.

To test the compatibility of the cellulose-based filter material, a 47 mm diameter membrane white filter paper was used, gridded (to facilitate possible deformation of the filter paper) with a pore size of 0.8 μm and made of mixed cellulose ester (MCE), manufactured by Millipore.

Five different types of ionic liquids were used to test the compatibility of ionic liquids with the cellulose filter material: IL1 (Trioctylmethylammonium dibutyl phosphate,) IL2 (EMIM EtSO4), IL3(B2001®), IL4 (B2002a®) and IL5 (B2002b®). All mentioned ionic liquids are from the manufacturer proionic. For this purpose, the filter paper was covered with 3 ml of ionic liquid, and the effect of the liquid on the filter material was observed after 1 hour and after 2 days (at normal ambient air conditions). The results of the simple test of the compatibility of tested ionic liquids with a cellulose filter, based on visual observation, are shown in **Figure 5**.

In the present case of testing, the compatibility of the ionic liquid with the cellulose-based filter material, in all cases the deformation of the filter paper occurred

**Figure 5.** *Effect of ionic liquid on cellulose-based filter material.*

first, followed a little later by the disintegration of the paper or another type of deformation. As a result of greater or lesser degradation of the cellulose membrane, a part of the cellulose is definitely present in the ionic liquid and was not visible to the naked eye. We did not measure the amount of cellulose in the ionic hydraulic fluid, because in this case the compatibility of the cellulose filter was in the foreground. The resulting deformation and decomposition are a big enough warning that ionic liquids are not compatible with cellulose filters, which is why it is necessary to use another type of filter material.

## **4. Tests related to filters and filterability test**

There are a wide range of fluid filters used in industry, especially in the filtration of fuel, water and lubricants, including hydraulic filters. Hydraulic filters are required to be compliant with the international contamination code of cleanliness for the fluid contamination levels (ISO 4406) and corresponding filtration standards (e.g. ASTM D3948–14). The required filtration efficiency and dirt holding capacity for the removal of solid particles, water droplets and water moisture from lubricants depend on the filtration system used, the required cleanliness levels of the fluid and the fluid properties (e.g. type, viscosity, surface tension, etc.), which influence the fibre wetting process in fluid filtration. There are many Standards defining performance requirements and test methods for fuel and lubricant filtration [24].

For example, methods for characterising the porous structure and integrity of a filter are defined in ISO 2942 and EN 24003; different methods of testing filtration efficiency and retention capacity are defined in the ISO 4548-12, ISO 16889, ISO 19438 and ISO 23369 Standards; the other test standards also include diesel fuel–water separator performance (ISO 16332 and SAE J1488/J1839), performance of fuel filters for diesel engines (for road vehicles) (ISO 4020), an online automatic particle counter calibration test rig (ISO 11943) and single-pass/gravimetric fuel filter test rigs (SAE J1985 and SAE J905).

In addition to the mentioned standardised tests, the filterability test is important for determining the efficiency of the filter material. The filterability of hydraulic oils is a measure of the ability of a clean fluid to pass through a standard filter without clogging or plugging. Solid additives in hydraulic oils affect filterability largely. This is

### *Compatibility of Filter Materials Used with Ionic Liquids' Uses in Hydraulic… DOI: http://dx.doi.org/10.5772/intechopen.107962*

especially important, because modern hydraulic systems with narrow gap tolerances, high operating pressures and ever finer filters place increasing demands on the filterability of hydraulic oils. To determine this precisely, the filterability test has been developed in accordance with ISO 13357 and included in the DIN 51524 Standard for HLP hydraulic oils [25].

Using filterability tests in which oil flows through a filter, often under intensified conditions, the interaction between the filter membrane and oil is examined in more detail. Most 'static' filterability tests are carried out as internal tests, due to the lack of a Standard, using different membrane pore sizes and liquid volumes under a vacuum or pressure conditions. However, the results can vary enormously according to the procedure used and often depend on the individual experiences of the laboratory. Since the early 1980s, scientists have been searching for a test capable of becoming a Standard. This has now been passed as the ISO 13357 filterability test parts 1 and 2 [26, 27].

The ISO 13357 Standard test is intended for fresh hydraulic oils with a viscosity grade up to ISO VG 100. The test is divided into the 'wet' filterability test, in which the oil is replaced by water (ISO 13357, part 1) and the test with 'dry' oil (ISO 13357, part 2). The quantity of 300 ml of hydraulic oil is flowed – filtered through a dried 0.8 μm membrane filter under the pressure specified in the Standard. The filtration volume and time are recorded during this process. The filterability is calculated by determining the volume-time relationships at the start, for a test duration of between 10 and 50 seconds (level I) and between 200 and 300 seconds (level II). The test ends when the required data have been recorded or the filtration time exceeds 2 hours. The filterability is indicated by the letter 'F'. If the value is F > 50, the test is concluded as 'passed' according to the Standard. The closer 'F' is to 100, the better the filterability. If F < 50, the test is deemed to have 'failed'. If the filtration time exceeds 2 hours, the test liquid must be marked as 'unable to filter'.

As already mentioned, Standard ISO 13357-1 and ISO 13357-2 prescribe the use of a cellulose membrane filter with the specified pore size of 0.8 μm, which is not problematic in case of the testing of mineral-based oils and HFD fluids. However, it is not known what may happen in the case of testing the filterability of another type of hydraulic fluid. As shown in Section 3.3, ionic liquids (at least tested), including ionic hydraulic fluids, are not compatible with cellulosic filter materials, as they deform rapidly and, in some cases, degrade completely.

According to the results when testing the filterability in conformity with the ISO 13357 Standard and compatibility with the cellulose filter paper discussed at this point, it can be concluded that the ionic liquids are incompatible with the cellulose filter paper. Therefore, the use of cellulose filter elements in hydraulic systems is not recommended. The filterability in the case of using the filter elements from other materials, for example, glass fibres, should not be problematic; however, they still need to be tested.

The same applies to all other systems that use a certain type of ionic liquid as a liquid which also needs to be filtered. Because there are many types of ionic liquids used for different purposes and in different processes, care must be taken when using cellulose-based materials. It is best to avoid them carefully or, at least, to check the compatibility of the ionic liquid with the cellulose filter before use.

However, regarding standard filterability testing strictly according to the ISO 13357 Standard, it is not feasible, as the use of a cellulose membrane filter is problematic. Other types of filter materials are usually used in today's hydraulic devices. The filterability through filter elements made from other material, for example, glass

fibres, should not be a problem regarding filter material and fluid compatibility. However, the compatibility test must still be established and, of course, to supplement the standard filterability test procedure for these types of fluids.

## **5. Conclusion**

The trend of development of modern hydraulic systems is moving towards more efficient systems with higher energy density and the use of new, high-tech hydraulic fluids. Modern hydraulic components are, thus, made with ever narrower tolerances, smaller gaps between internal moving parts, better materials are used and better surface quality. As a result, higher operating pressures can be used, making hydraulic components smaller, lighter and less space consuming. Due to the smaller tanks, a smaller amount of hydraulic fluid is also used, which is, therefore, much more loaded. As well as in the field of hydraulic fluids, development is taking place in the direction of more energy-efficient fluids, with better lubricating properties, a wider temperature operating range, non-flammability and greater environmental friendliness.

Thus, new types of hydraulic fluids are appearing, which have certain physical and chemical properties that are much better than classic hydraulic fluids. Among them certainly belong ionic liquids suitable for use in hydraulic systems – ionic hydraulic fluids.

Regardless of the type of hydraulic fluid, effective filtering is crucial to achieve adequate operational reliability and a long service life of the components and the entire system. Thus, the average pore size of filters has been reduced from 10 to 20 μm to today's 3 to 12 μm, and new and better filter materials are being used. In the case of new hydraulic fluids, however, the question arises of their compatibility with the materials used commonly in hydraulic components, including filters.

The paper discusses in more detail the issue of compatibility of new ionic hydraulic fluids with the materials used in filters. The emphasis is on return filter materials, which are considered an indispensable component of any hydraulic system. All filter materials are considered, including the filter housing, seals, filter cartridge as a whole, as well as the filter material itself.

Extensive research has shown that no problems can be expected with the metal materials we use for filter housings. Corrosion of steel parts can occur in certain cases only if moisture is present. However, since the filter housing is usually filled with liquid, this problem is not to be expected. However, a problem may arise if the inside of the filter housing is painted with protective paint. In certain cases, softening, wrinkling and peeling of the paint can occur, which can be problematic. It is necessary to individually check the compatibility of a certain type of ionic hydraulic fluid with a certain type of paint before use. If the filter housing and other metal materials of the filter are not painted or are made of other materials (for example, based on copper or stainless steel), these problems do not exist.

We have also not detected any compatibility issues of ionic hydraulic fluids with the plastic materials present on the filter. The same applies to the seals, if they are made of NBR or Viton, the seal material most often found in the filter.

However, it is necessary to be careful with the material of the filter cartridges. In the event that plastic, fibreglass or micro glass (e.g. Polyester, Nylon, Dacron...) is used as the filter material, we did not detect any problems during durability testing on a real device under real operating conditions. However, a compatibility problem arises when using cellulose-based filters. These are considered the most efficient in the field *Compatibility of Filter Materials Used with Ionic Liquids' Uses in Hydraulic… DOI: http://dx.doi.org/10.5772/intechopen.107962*

of hydraulics, but unfortunately ionic hydraulic fluids degrade them (deform and partially decompose).

Cellulose filter membranes are also used in the case of the use of portable off-line devices to determine the cleanliness class of hydraulic fluid. Cellulose-based filter material has proven to be incompatible with ionic liquids.

However, cellulose membranes are also prescribed for determination a very important parameter of filter efficiency, i.e. its filterability. The test is standardised but is mainly intended for testing the filterability of conventional fluids, including hydraulic fluids. Since ionic liquids degrade cellulose-based materials, it will be necessary to update the standardised filterability test for cases where ionic hydraulic fluids are used.

Findings regarding the compatibility of filter materials must be considered, not only when determining the filterability but also when choosing the materials of all filter components. This is true in general and especially in all cases where we do not use classic hydraulic and other technical fluids.

## **Acknowledgements**

This research was supported by the company proionic GmbH from Grambach, Austria, which provided all the samples of ionic liquids and was a very cooperative partner in the IL-selection process, for sharing their wisdom with us. We are also grateful to the company OLMA d.o.o. from Ljubljana, Slovenia, which allowed us to use their equipment and facilities, as well as the personnel to carry out very extensive experimental work. We are thankful to all colleagues in both companies who provided their expertise and skills that assisted this research greatly.

## **Author details**

Darko Lovrec\* and Vito Tič Faculty of Mechanical Engineering, University of Maribor, Maribor, Slovenia

\*Address all correspondence to: darko.lovrec@um.si

© 2022 The Author(s). Licensee IntechOpen. 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.

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[5] Hydac International. Filter Elements for Use in Hydac Filters. TDS No.: E7.200.11/03.12. Sulzbach/Saar, Germany: Hydac International; 2012. pp. 38-46

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[26] ISO 13357-1:2017. Petroleum products - Determination of the filterability of lubricating oils – Part 1; Procedure for oils in the presence of water

[27] ISO 13357-2:2017. Petroleum products – Determination of the filterability of lubricating oils – Part 2: Procedure for dry oils

## **Chapter 7**

## Development of Low-Friction Ion Gels for Industrial Applications

*Toshio Kamijo, Hiroyuki Arafune, Takashi Morinaga and Takaya Sato*

## **Abstract**

Friction reduction is imperative for improving the service life and energy efficiency of mechanical systems. Ion gels using ionic liquids (ILs) as swelling agents are expected to be stable gel lubricants owing to the high thermal stability and negligible volatility of ILs; they can maintain their swollen state even under harsh conditions. Therefore, we investigated two types of ion gels: an IL-substituted double-network gel (DN ion S-gel), in which the water in the DN hydrogel is replaced by the IL 3-ethyl-1-methyl-imidazolium ethylsulfate; and a DN ion gel containing N,Ndiethyl-N-(2-methoxyethyl)-N-methyl-ammonium bis(trifluoromethylsulfonyl) imide (DEME-TFSI), where one of the polymer backbones is a network of poly(N,Ndiethyl-N-(2-methacryloylethyl)-N-methylammonium bis(trifluoromethylsulfonyl) imide), an IL-type polymer based on our previous synthetic study of IL polymer technology. The DN ion S-gel and DN ion gel achieved compression strengths of 25 and 30 MPa, respectively, and were thermally stable until 196°C and 335°C (10% weight-loss temperature), respectively. The coefficient of friction remained stable and low (0.02) after repeated measurements under harsh conditions (high temperature or vacuum conditions), affirming the durability of the DN ion gel.

**Keywords:** double-network gel, ionic liquid, low friction, highly robust, high vacuum

## **1. Introduction**

Low-friction materials are demanded for energy and resource conservation. As 75% of machine failures are attributed to friction wear [1], friction reduction would improve the service life and energy efficiency of mechanical systems [2]. Low-friction materials can be modeled on the human joint, which has a low coefficient of friction (CoF = 10−3) at pressures above 102 atm and remains lubricated for decades [3]. The gel-like structure of human joints, comprising proteoglycans and collagen fibers with a high water content (75–80 wt%), has inspired extensive research on gel lubricants [2–4]. In the early 2000s, researchers in Japan developed hydrogels with high mechanical strength [4–7], promoting the application of gel as biomaterials. Double-network (DN) hydrogels with high mechanical strength, low friction, and biocompatibility are representative gel-lubricant materials [8] and suitable candidates for artificial human joints or cartilages [9]. However, the evaporation of the solvent (water) and

consequent loss of the swollen state have limited the industrial exploitation of gels with these benefits.

Previously, we developed ion gels comprising monodisperse silica nanoparticles and ionic liquids (ILs). In these gels, the silica surface is densely grafted by a welldefined polymer, forming a colloidal self-assembled crystal with a face-centered cubic structure [10]. The ionic conductivity and diffusion coefficients of our gels exceeded those of the bulk polymer. We thus considered the possibility of introducing the highly mobile ions in ion gels to a low-frictional gel surface. To test this idea, we combined ILs with gel lubricants. Owing to their high thermal and oxidative stability, ultralow volatility, and low friction [11–13], IL swelling agents are expected to retain the swelling state of low-friction gels. Whereas DN hydrogels have been targeted as tough hydrogels for biomaterial applications [14], DN ion gels have been mainly studied as gas separation membranes obtained by replacing the water in DN hydrogels with amino-acid-based ILs [15]. As shown in our recent study, IL-based lubrication systems and compatible polymer brushes grafted on Si substrates are efficient and robust lubricants [16]. These lubricants are composed of an IL called N,N-diethyl-N-(2-methoxyethyl)-N-methyl-ammonium bis(trifluoromethylsulfonyl)imide (DEME-TFSI) and high-density IL-type polymer brushes comprising N,N-diethyl-N-(2-methacryloylethyl)-N-methylammonium bis(trifluoromethyl-sulfonyl)imide (DEMM-TFSI), which is a derivative of DEME-TFSI with a polymerizable group. The CoF of the IL-type polymer brushes remained as low as 0.003 over 4,000 cycles.

The present study proposes two types of DN ion gels: an IL-substituted gel in which the water in the DN hydrogel is replaced by an IL [17], and a gel in which one of the polymer backbones of the DN ion gel is a network of poly(DEMM-TFSI) (hereafter, these two gels will be named DN ion S-gel and DN ion gel, respectively). The IL-type polymer in the latter gel is based on our previous synthetic study [18].

The ILs we used, 3-ethyl-1-methylimidazolium ethyl sulfate (EMI-EtSulf) and DEME-TFSI, are commercially readily available and were selected for their compatibility with the gel networks. EMI-EtSulf was also chosen because it can easily substitute water in the hydrogels and has low toxicity. DEME-TFSI has been used for a long time in our material development due to its low viscosity and high electrochemical and thermal stability [13, 16–20]. We are also considering its eventual industrial applications. This IL has been approved by the Japan Chemical Substances Control Law, thereby indicating its potential for industrial mass production and significant cost reductions. Considering the compatibility with this IL, we designed monomers and developed polymerization technology. Therefore, we are developing functional materials by combining DEME-TFSI and poly(DEMM-TFSI) with a maximum consideration of compatibility.

The present study discusses and compares the different tribological properties of DN hydrogels, DN ion S-gels, and DN ion gels. Specifically, it evaluates the gel immobilization techniques of the gels on substrates, the mechanical and thermal stabilities of the ion gels, their lubrication properties in a vacuum, and other required functions for industrial applications.

## **2. Two types of DN ion gels for industrial applications**

### **2.1 IL-substituted DN gel (DN ion S-gel)**

**Figure 1** is a schematic of an IL-substituted DN gel. First, a DN hydrogel was fabricated via sequential photopolymerization [6]. A solution of 2-acrylamido

*Development of Low-Friction Ion Gels for Industrial Applications DOI: http://dx.doi.org/10.5772/intechopen.107942*

**Figure 1.** *Schematic of the IL-substituted DN gel (DN ion S-gel).*

methylpropane sulfonic acid (AMPS) as the first monomer, α-Ketoglutaric acid as the initiator, N,N-methylene bisacrylamide (MBAA) as the cross-linker, and water as the solvent was mixed and transferred to a Schlenk tube. Using an injection syringe, the solution was inserted into a reaction cell formed by sandwiching 1-mm-thick Si rubber between two glass plates. After 18 h of rotation under ultraviolet (UV) irradiation, the first gel was obtained. This gel was polymerized under Ar atmosphere, then immersed in a second gel solution comprising dimethylacrylamide as a second monomer, α-Ketoglutaric acid as the initiator, MBAA as the cross-linker, and water as the solvent. Once swelling equilibrium was reached, the second gel solution (i.e., the swelled first gel) was set in the reaction cell and rotated for 18 h under UV light irradiation. The obtained gel was washed with acetonitrile and EMI-EtSulf (volume ratio 1:1), then vacuum-dried at 70°C for 12 h to obtain the substituted DN ion gel. The gel prepared using this method is called DN ion S-gel. The synthesis is detailed elsewhere [17].

## **2.2 DN ion gel comprising IL and IL polymer, DN ion gel**

**Figure 2** schematizes the DN ion gel comprising an IL and its polymer (DN ion gel). The first and second networks were composed of DEMM-TFSI and poly (methylmethacrylate) (PMMA), respectively. DEMM-TFSI as the IL monomer, IRGACURE 369 as the initiator, triethyleneglycol dimethacrylate (TEGDMA) as the cross-linker, and DEME-TFSI as the solvent were mixed in a Schlenk tube and deoxygenated by Ar bubbling for 5 min. Using an injection syringe, the solution was injected into a reaction cell formed by sandwiching 2-mm-thick Si rubber between a pair of glass plates. The first gel was rotated for 18 h under UV light irradiation to promote polymerization in an Ar atmosphere. The polymerized gel was washed with acetonitrile to remove impurities and then swollen with DEME-TFSI (poly(DEMM-TFSI) gel as a reference material. PMMA gel was prepared similarly by replacing DEMM-TFSI with methyl methacrylate (MMA) at the same weight ratio. The first gel was polymerized and immersed in a second gel solution comprising MMA monomer, benzophenone

**Figure 2.** *Schematic of the DN ion gel composed of an IL and an IL polymer (DN ion gel).*

as the initiator, TEGDMA as the cross-linker, and propylene carbonate as the solvent. After immersion for 2 days to reach swelling equilibrium, the swollen first gel was placed in a reaction cell and rotated for 18 h under UV irradiation. The resulting second gel was washed with an acetonitrile and DEME-TFSI (volume ratio 1:1) to remove the impurities and then vacuum-dried at 70°C for 12 h. The resulting DN ion gel was rich in DEME-TFSI (75 wt%). The synthesis is detailed elsewhere [18].

## **2.3 Gel immobilization technology on substrates**

**Figure 3** is a schematic of the gel immobilization technique. The DN ion gels were covalently immobilized on glass substrates via a silane coupling reaction [21]. Briefly, a mixture of ethanol and 3-(trimethoxysilylpropyl)methacrylate was slowly dropped into mixed ethanol and 25-wt% ammonia solution while stirring. The solution was used to immerse a washed glass substrate for 16 h at room temperature to impart the vinyl moieties that covalently anchor the gel to the glass surface. The resulting substrate coated one side of the reaction cell. During the second gel synthesis, the DN ion gel was covalently bonded to the surface silanol groups of a glass substrate. The same technique can prepare DN hydrogels. It is also applicable to other substrates if their surfaces possess hydroxyl groups that can be modified by a silane coupling reagent with vinyl groups, which can help anchor the gel.

## **3. Characterization of DN and DN ion gels**

## **3.1 Mechanical properties from stress-strain curves**

The mechanical strengths of the DN ion gels were evaluated at 25°C and 40% relative humidity using a universal testing machine (Instron 3342, Instron Japan, Kawasaki, Japan). From the stress-strain curves, the compression fracture stresses of the DN

*Development of Low-Friction Ion Gels for Industrial Applications DOI: http://dx.doi.org/10.5772/intechopen.107942*

#### **Figure 3.**

*Schematic of gel immobilization technology.*

#### **Figure 4.**

*Stress-strain curves of DN hydrogel (blue), DN ion S-gel (green), single DEMM-TFSI gel (brown), and DN ion gel (red). Inset is an expansion of the change region of the curves.*

hydrogel and DN ion S-gel were found to be 20 and 25 MPa, respectively (**Figure 4**), which were comparable to those of DN ion gel substituted with tetrabutylphosphonium prolinate [14]. The stress-strain curve of DN ion gel presented a smaller initial slope than that of DN hydrogel; the loss of hydration force softened the DN ion gel. PAMPS is an electrolyte polymer that swells in pure water via hydration of negatively charged AMPS. In addition, its surface charge is easily shielded in ionic salt at high concentration. The DN ion gel shrank after displacement, suggesting that it softened through loss of hydration power caused by the high ionic strength of EMI-EtSulf.

Moreover, the compressive fracture stress of the DN ion gel was 30 MPa at 87% strain (**Figure 4**), which is 10 MPa higher than that of the DN hydrogel (20 MPa) and DN ion S-gel (25 MPa). The compressive fracture stresses of single-network gels were much lower (1 MPa for poly(DEMM-TFSI) and 3 MPa for PMMA), suggesting that the high mechanical strength of the combined materials derives from the formed DN structure. Because conventional DN gels substituted with amino-acid-based

ILs exhibit similarly high mechanical strengths (>25 MPa) [14], we concluded that combining ILs with compatible polymers can boost the mechanical strength without hydration assistance from electrolyte polymers.

## **3.2 Thermal properties of the gels**

The thermal properties of DN ion S-gel, DN ion gel, and DN hydrogel were evaluated using thermogravimetric analysis (TGA), and the results were compared with those of pure ILs and the conventional lubricants poly-α-olefin (PAO) and glycerol. **Figure 5** compares the TGA curves of DN hydrogel (blue), DN ion S-gel (light blue), EMI-EtSulf (green), DN ion gel (red), DEME-TFSI (light green), PAO (gray), and glycerol (black). The initial weight loss (up to 100°C) in the TGA curves of DN ion S-gel and EMI-EtSulf was attributed to release of absorbed moisture. The secondary weight loss at ~150°C indicated pyrolysis of the sulfonate and sulfate portion of PAMPS and EMI-EtSulf [22]. The 10% weight-loss temperatures (T10) of DN ion S-gel and EMI-EtSulf were determined as 196°C and 247°C, respectively. The T10 of DN hydrogel was much lower (39°C), thereby indicating that replacing water with ILs decidedly improves the thermal stability of DN gels (ΔT10, DN ion S-gel = 157°C). The DN ion gel thermally decomposed at nearly 300°C with a weight loss 10% at 335°C. The obtained T10 value of DN ion gel was much higher than that of the DN hydrogel (ΔT10, DN ion gel = 295°C) and 139°C higher than that of the S-gel. The 10% weight loss of DN ion gels occurred at a much higher temperature than that of glycerol (165°C) and PAO (241°C); therefore, DN ion gel can be used as stable and efficient lubricant under high-temperature conditions for industrial applications.

## **4. Tribological properties of the DN ion gel**

## **4.1 Sliding-speed dependence of CoF on gel properties**

For the friction test using a ball-on-plate-type reciprocating tribometer (Shinto Scientific Co., Ltd, Tokyo, Japan), the gel sample was fixed on a sliding table and a glass ball sample (ϕ10 mm) was placed in a ball holder connected to a load cell. To

## **Figure 5.**

*TGA curves of DN hydrogel (blue), DN ion S-gel (right blue), EMI-EtSulf (green line), DN ion gel (red), DEME-TFSI (light green), PAO (gray), and glycerol (black).*

*Development of Low-Friction Ion Gels for Industrial Applications DOI: http://dx.doi.org/10.5772/intechopen.107942*

#### **Figure 6.**

*Sliding-speed dependence of CoF at glass ball/DN hydrogel (light blue circles), glass ball/DN ion S-gel (green circles), and glass ball/DN ion gel (red circles).*

evaluate the lubrication properties, the ball/substrate frictional force was measured at different loads and sliding speeds. **Figure 6** plots the CoFs as functions of sliding speed for the ball/DN hydrogel (light blue circles), glass ball/DN ion S-gel (green circles), and glass ball/DN ion gel (red circles) under an applied load of 0.98 N. The CoF of the glass ball/DN hydrogel remained within 0.01–0.02 over the range of measured sliding speeds. The minimum CoF at 3.0 × 10−2 ms−1 indicated a shift from the elastic lubrication regime to mixed lubrication. By contrast, the CoFs of the glass ball/DN ion S-gel ranged from 0.04 to 0.05 over the sliding-speed range and were minimized at a speed six times slower (5.0 × 10−3 ms−1) than that of the DN hydrogel. These results can be attributed to the following factors: (1) high viscosity of the swelling agent incorporated in the DN gels, and (2) increased polymer adhesion after substituting water with IL. As the viscosities of water and EMI-EtSulf at 25°C are 0.89 and 71 MPa·s, respectively, a fluid film of IL is much thicker than a water film. Therefore, the viscosity change could not explain the disparate lubrication properties of DN ion S-gel and DN hydrogel. Regarding factor (2), PAMPS and the glass surface easily dissociate to produce negative charges in pure water, and the fluid film thickness is preserved by electrostatic repulsion between the glass surface and PAMPS [23, 24]. Electrostatic repulsion is essential for lowering the friction at an electrolyte interface. Using a surface force apparatus, Raviv et al. [23] studied the friction between polymer brush layers adsorbed on a molecularly smooth mica surface in water. The friction induced by the sheer force between the polyelectrolyte brush layers (CoF = 0.0006–0.001) was lower than that induced between neutral brush layers. In the former case, friction was decreased by electrostatic repulsion between the negatively charged tribopaired polymer brushes. Dunlop et al. [24] measured the normal and shear forces between polyelectrolyte brush layers grafted on a mica surface. After examining the contribution of ionic strength to friction, they found that salt at high concentrations shielded the electric double layer, thereby increasing the shear forces. Being fully composed of cations and anions, EMI-EtSulf has a high salt concentration (≈5 molar); therefore, it shielded almost all of the hydroxyl groups on the glass surface and the sulfonate groups on PAMPS. The higher friction of the DN ion gel than of the DN hydrogel was thus attributed to polymer adhesion on the glass surface, enabled by electrostatic shielding of the glass and PAMPS surfaces.

The red circles in **Figure 6** plot the relationship between CoF and sliding speed of the glass ball/DN ion gel under a 0.98-N load. The CoF decreased as the sliding velocity increased to 1.5 × 10−3 ms−1 and increased slowly thereafter, indicating a transition from a mixed (elastic fluid) to a fluid lubrication regime. The higher CoF of the DN ion gel than of the hydrogels and DN ion S-gel at sliding velocities above 5.0 × 10−3 ms−1 may be explained by the smaller elastic deformation (and consequent entry to the fluid lubrication regime) of the synthesized DN ion gel. The CoF of the DN ion gel exceeded those of the DN hydrogel and ion S-gel in the same velocity region. Meanwhile, the effect of surface-to-surface contact becomes significant at velocities below 1.5 × 10−3 ms−1 (in the mixed lubrication regime). The smaller CoF of the DN ion gel than that of the DN ion S-gel in the slow-velocity region means a smaller interaction between the glass surface and DN ion gel than between the glass surface and DN ion S-gel; however, the former interaction exceeded the interaction between the glass surface and DN hydrogel. The CoF of the DN hydrogel was lower at room temperature when water was not volatilized (that is, under the experimental conditions of the present study).

## **4.2 Repeated durability and thermal stability of gels**

**Figure 7** shows the temporal changes in the measured CoFs of glass balls/DN ion S-gel and glass balls/DN ion gel at different temperatures under a 0.98 N load and a sliding speed of 5.0 × 10−3 ms−1. At 25°C, 80°C, and 100°C, the CoF values of the glass ball/DN ion S-gel were 0.067, 0.054, and 0.037, respectively, and those of the glass balls/DN ion gel were 0.025, 0.017, and 0.013, respectively. Both sets of CoFs monotonically decreased with increasing temperature. Higher temperatures promote gel softening and decrease the average contact pressure. They also increase the fluid film thickness and hence the viscous resistance. Therefore, decreased polymer adhesion is considered as the main cause of the friction reduction. At 50°C, the CoF of the DN hydrogels jumped by more than 10-fold during repeated measurements, probably due to thermal aggregation of the dried polymer [18] as the water solvent evaporated. In contrast, the CoF of the DN ion gel remained stable after 500 friction cycles even at 100°C because it did not volatilize, indicating that the DN ion S-gel and DN ion gels were more thermally stable than the DN hydrogel. Therefore, they can potentially be applied as lubricating gels in high-temperature applications that are unsuitable for hydrogels.

## **4.3 Lubricating properties of gels in a vacuum and future development of gel materials**

**Figure 8** shows the stability of the DN ion gel under a high vacuum during repeated friction measurements of glass balls/DN ion gel. The sliding velocity, load, and pressure were 5.0× 10−3 ms−1, 0.20 N, and 2.2×10−4 Pa, respectively. For this test, a custom-made ball-on-plate-type tribometer was installed in a vacuum chamber [25]. The tribopairs of DN ion gel with glass balls and SUJ2 balls showed a stable response (CoF = 0.023) after 500 friction cycles, indicating that the low-friction surface of the DN ion gel remained stable under high-vacuum conditions. Along with the temperature stability results in **Figure 7**, this result affirms that DN ion gels can reduce the friction and improve the energy efficiency of mechanical systems operating under high-temperature and high-vacuum conditions, such as bearings and mechanical seals.

*Development of Low-Friction Ion Gels for Industrial Applications DOI: http://dx.doi.org/10.5772/intechopen.107942*

#### **Figure 7.**

*CoF versus number of friction cycles for glass ball/DN ion S-gel (black: 25°C, red: 80°C, green: 100°C) and glass ball/DN ion gel (gray: 25°C, pink: 80°C, yellow–green: 100°C). All measurements were taken at 5.0 × 10−3 ms−1 under a load of 0.98 N.*

Thus, we combined DN gels and ILs and evolved them into an industrially viable material. For widespread usage of this material, we must halt the metal corrosion of ILs and replace the expensive ILs with cheaper alternatives such as deep eutectic solvents (DES) [26]. In DES compounds, hydrogen-bond donors are mixed with hydrogen-bond acceptors (either or both solid) at a certain ratio, forming a liquid at room temperature. DESs exhibit similar properties to ILs but are inexpensive and their combination possibilities are numerous. We plan to develop new materials based on DESs.

In addition, the DN gel with IL in this study requires a two-step preparation, which is difficult to synthesize in large quantities. This issue must also be resolved in industrial preparations. Currently, we are developing a one-step preparation method for DN gels [27].

#### **Figure 8.**

*CoF versus number of friction cycles for glass ball/DN ion gel (black) and SUJ2 ball/DN ion gel (red), measured at 5.0 × 10−3 ms−1 under a 0.20 N load and 2.2 × 10−4 Pa.*

## **5. Conclusion**

In this study, we investigated gels comprising two types of ILs for industrial applications: DN ion S-gel in which the water in the hydro-DN gel is replaced by an IL EMI-EtSulf, and DN ion gel containing IL DEME-TFSI where one of the polymer backbones of the DN ion gel becomes a network of poly(DEMM-TFSI), an IL-type polymer developed through our previous IL polymer technology.

The obtained DN ion S-gel and DN ion gel achieved high compression strengths (25 and 30 MPa, respectively) and were thermally stable up to 196°C and 335°C (the 10% weight-loss temperature), respectively. The decomposition behavior of the DN ion S-gel and DN ion gel reflects their thermal stabilities after incorporating EMI-EtSulf and DEME-TFSI, respectively. This phenomenon is commonly observed in ion gels. The CoF of the DN ion gel was low (0.02) and stable after repeated measurements at high temperature and under vacuum conditions, confirming the durability of this gel even under harsh conditions.

This study elucidated the fabrication of lubricant gels with high mechanical strength and robustness. The gels are expected to reduce the energy and resource consumption of materials in the temperature range at which conventional hydrogels cannot be used (DN ion S-gel: ΔT10, DN ion S-gel = 157°C, DN ion gel; ΔT10, DN ion gel = 295°C).

## **Acknowledgements**

This study was supported in part by the e "Green Tribology Innovation Network" Advanced Environmental Materials Area, Green Network of Excellence (GRENE) Program, the ACCEL program and Grants-in-Aid for Scientific Research (No.20H02060, No. 25810091, and No. 26820034) sponsored by the Ministry of Education, Culture, Sports, Science and Technology (MEXT) in Japan. The authors would like to thank Enago (www.enago.jp) for the English language review.

## **Author details**

Toshio Kamijo\*, Hiroyuki Arafune, Takashi Morinaga and Takaya Sato Department of Creative Engineering, National Institute of Technology, Tsuruoka College, Tsuruoka, Japan

\*Address all correspondence to: kamijo@tsuruoka-nct.ac.jp

© 2022 The Author(s). Licensee IntechOpen. 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.

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## *Edited by Fabrice Mutelet*

Due to their very low volatility, high thermal stability, and ability to dissolve a wide variety of compounds, ionic liquids appear to meet the rigorous criteria for industrial applications. Among other uses, ionic liquids appear to be efficient for gas capture, biomass pretreatment, separation problems, and electrochemistry. They are also used in electrolytes, as lubricants, catalysts, or as antistatic agents. This book discusses the various uses of ionic liquids. Chapters discuss such topics as the use of ionic liquids in batteries, new mono, di, and trimeric imidazolium and pyridinium ionic liquids as catalysts in organic chemistry, the physico-chemical properties of ionic liquid-substituted double-network gels for industrial applications, the use of paramagnetic ionic liquids in magnetic resonance imaging, the compatibility of filter materials used with ionic liquids, and the development of low-friction ion gels for industrial applications.

Published in London, UK © 2023 IntechOpen © neirfy / iStock

Industrial Applications of Ionic Liquids

Industrial Applications of

Ionic Liquids

*Edited by Fabrice Mutelet*