**State of the Art Applications**

[39] Wang X, Han M, Wan H, Yang C, Guan G. Study on extraction of thiophene from model gasoline with bronsted acidic ionic liquids. Frontiers of Chemical Science and Engineer-

[40] Zhang S, Zhang Q, Zhang ZC. Extractive desulfurization and denitrogenation of fuels using ionic liquids. Industrial & Engineering Chemistry Research. 2004;43:614-622

ing. 2011;5:107-112

120 Recent Advances in Ionic Liquids

**Chapter 7**

**Provisional chapter**

**Applications of Ionic Liquids in Elastomeric**

**Applications of Ionic Liquids in Elastomeric** 

DOI: 10.5772/intechopen.76978

Ionic liquids (ILs) are organic salts that are liquid at ambient temperatures. ILs are considered a versatile class of chemicals because their properties can be easily tailored for specific applications. Due to their negligible vapor pressure, non-flammability, and thermal stability in the temperature range for preparation and processing of elastomeric composites, ILs are being used increasingly in the field of elastomer science and technology. In this review, the advantages of ILs as functional additives for elastomeric composites are discussed, with special emphasis on their use as dispersing agents for fillers, components of conducting rubber composites, crosslinkers or components of crosslinking

**Keywords:** ionic liquids, elastomer, composites, dispersion degree, conductive

ILs are generally defined as salts with melting temperatures lower than 100°C [1]. Recently, IL research has been one of the most rapidly growing fields in chemistry and in industry, mainly due to many unique properties of ionic liquids. ILs can solvate a large variety of organic polar and nonpolar compounds, and they show potentially "environmentally friendly" characteristics due to their negligible vapor pressure and non-flammability [2, 3]. Therefore, ILs are used as "green" solvents for numerous applications [4]. Moreover, their chemical and physical properties can be tuned for a wide range of potential applications by varying their cation and anion components. The anti-electrostatic properties of ILs have also been described [5]. Recently, ILs have not only been employed as solvents for various types of polymerizations [6–8] but they

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

© 2018 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.

Anna Sowinska and Magdalena Maciejewska

Anna Sowinska and Magdalena Maciejewska

Additional information is available at the end of the chapter

Additional information is available at the end of the chapter

**Composites: A Review**

**Composites: A Review**

http://dx.doi.org/10.5772/intechopen.76978

**Abstract**

systems.

composites

**1. Introduction**

#### **Applications of Ionic Liquids in Elastomeric Composites: A Review Applications of Ionic Liquids in Elastomeric Composites: A Review**

DOI: 10.5772/intechopen.76978

Anna Sowinska and Magdalena Maciejewska Anna Sowinska and Magdalena Maciejewska

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

http://dx.doi.org/10.5772/intechopen.76978

#### **Abstract**

Ionic liquids (ILs) are organic salts that are liquid at ambient temperatures. ILs are considered a versatile class of chemicals because their properties can be easily tailored for specific applications. Due to their negligible vapor pressure, non-flammability, and thermal stability in the temperature range for preparation and processing of elastomeric composites, ILs are being used increasingly in the field of elastomer science and technology. In this review, the advantages of ILs as functional additives for elastomeric composites are discussed, with special emphasis on their use as dispersing agents for fillers, components of conducting rubber composites, crosslinkers or components of crosslinking systems.

**Keywords:** ionic liquids, elastomer, composites, dispersion degree, conductive composites

#### **1. Introduction**

ILs are generally defined as salts with melting temperatures lower than 100°C [1]. Recently, IL research has been one of the most rapidly growing fields in chemistry and in industry, mainly due to many unique properties of ionic liquids. ILs can solvate a large variety of organic polar and nonpolar compounds, and they show potentially "environmentally friendly" characteristics due to their negligible vapor pressure and non-flammability [2, 3]. Therefore, ILs are used as "green" solvents for numerous applications [4]. Moreover, their chemical and physical properties can be tuned for a wide range of potential applications by varying their cation and anion components. The anti-electrostatic properties of ILs have also been described [5]. Recently, ILs have not only been employed as solvents for various types of polymerizations [6–8] but they

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 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.

have also been used to dissolve polymers (cellulose [9], silk fibroin [10] and starch [11]) to create new polymer composites [12], as plasticizers for many kinds of polymers [13] or for the preparation of polymer gels [14] and finally as effective wood preservatives [15]. The applications of ILs as novel electrolytes for electrochemical polymerization have also been reviewed [16]. ILs, due to their high-boiling points (300–450°C, which are also their decomposition temperatures), their low vapor pressure, their low volatility, their ability to create nonexplosive mixtures of their vapors with air, and their high flash-point, can be used successfully in elastomer technologies.

the MWCNT in the CR elastomer, and as a consequence, increased the filler network, which was supported by strain sweep measurements. The optimum MWCNT/BmiTFSI ratio was reported to be 1:5. Higher loading of BmiTFSI caused deterioration of the composites mechanical properties. It was concluded that the use of an imidazolium ionic liquid resulted in increased electrical conductivity, as well as a reinforcing effect of the nanotubes in the CR elastomer. Addition of BmiTFSI-modified MWCNTs increased the tensile modulus and hardness of the CR compos-

Applications of Ionic Liquids in Elastomeric Composites: A Review

http://dx.doi.org/10.5772/intechopen.76978

125

Das et al. [21] confirmed that the addition of ILs ensures better interactions between rubber and the MWCNTs, and therefore, improves the compatibility between the nanotubes and the elastomer and, consequently, their dispersibility in the elastomeric matrix. ILs, such as 1-allyl-3-methylimidazolium chloride (AMICl), were used to blend the styrene-butadiene (S-SBR) and polybutadiene rubber (BR) that contained MWCNTs. Using 3 phr, AMICl-functionalized MWCNTs caused a three-fold increase in the composites' tensile strength. Despite such small quantities of MWCNTs, the sample was stretched up to 456% without mechanical failure. Therefore, it was clear that an AMICl with a double bond in the structure produced the best reinforcing activity for composites containing 3 phr of MWCNTs. Moreover, it was suggested that AMICl acted as a coupling agent between the CNTs and the rubber chains and, hence, that the twisted structure of the CNTs held a certain amount of rubber and enhanced the three-dimensional interactions of the CNTs with the elastomeric matrix. A specific interaction between the MWCNTs and the rubber matrices in the presence of AMICl was postulated. The double bond of AMICl was presumably chemically linked with the double bond of the diene rubber chains by sulfur bridges, which then interacted with the π-electrons of the CNTs due to delocalization of the π-electrons in the imidazolium cation of AMICl. Transmission electron microscopic images confirmed the improved dispersion of MWCNTs that were functionalized with AMICl in the rubber matrix. Moreover, this ionic liquid promoted the formation of MWCNT clusters with a cellular structure in the elastomeric matrix that confirmed the strong adhesion of carbon nanotubes to the rubber phase and a special bound rubber aggregation in the reported composites.

One of the most effective reinforcing fillers for rubber compounds is carbon black (CB) with the surface of graphitic structures, which possess delocalized π-electrons. Particles of CB can interact with ILs in a manner similar to CNTs. Kreyenschulte et al. [22] investigated the interactions of AMICl with different types of carbon black and demonstrated strong interactions between AMICl and CB, as well as an improvement of the filler's dispersion in the elastomers. CB formed a bucky gel with AMICl, and an increase in the glass transition temperature (T<sup>g</sup>

of ionic liquid was observed. This effect was more apparent when the CB was graphitized; thus, a larger fraction of the CB surface was coated with graphitic crystals that can interact with AMICl via cation π-interactions. AMICl-modified carbon blacks were applied as fillers for S-SBR/BR composites and ethylene-propylene-diene rubber (EPDM). AMICl was premixed with CB, using ethanol and an ultrasonic treatment, and then dried before mixing with elastomers. As mentioned above, Fukushima et al. have reported that CNTs are able to build socalled bucky gels by physical crosslinking of nanotube bundles by mediating the molecular ordering of imidazolium-based ILs [23]. To study this behavior, which can be caused by CB or by its mixture with AMICl, analysis of the frequency dependence of the storage modulus and the loss modulus was performed which revealed significant differences. Hydrodynamic

)

ites, which clearly promoted dispersion of the nanotubes in the elastomeric matrix.

This chapter presents an overview of different applications of ILs in elastomeric composites. The main applications of ILs in elastomer technology include the improvement of fillers and the dispersion of other solids in the elastomer matrix, the preparation of conducting rubber composites, or using ILs as crosslinkers or other components of crosslinking systems (e.g., activators or vulcanization accelerators).

#### **2. Use of ILs to improve the dispersion of solids in elastomers**

The basic requirement for the reinforcement effect is to produce a homogeneous dispersion of the filler particles in the elastomeric matrix that results in good interphase adhesion [17]. However, most fillers exhibit a high degree of agglomeration, which makes it technologically difficult to obtain a homogenous dispersion in the elastomer. The unique chemical structure of ILs promotes the dispersion of nanoparticles in polymers owing to the surface modification of the nanoparticles with the ILs, which increases the interactions between filler particles and the elastomer matrices, especially in the case of silica, clays, carbon black and carbon nanotubes (CNTs). Because ILs are nonvolatile and nonflammable, the process of carbon nanotube modification and incorporation into an elastomeric matrix could be environmentally friendly. Moreover, ILs act as a lubricant; so, the process of modification is not accompanied by structural disruption of CNTs [18].

In 2007, Fukushima and co-workers reported a novel use for ILs as modifiers for carbon nanotubes, which enabled the production of soft composite materials that contained CNTs [19]. The use of ILs allowed for the noncovalent and covalent modifications of CNTs and for the formation of polymer composites with improved physical properties. In the presence of ILs, CNT bundles were reduced and unrolled. The result was fine bundles of CNTs that formed a network structure in the elastomeric matrix. This was related to the possible specific interactions between the imidazolium cation of ILs and the π-electronic carbon nanotube surface. The use of ILs during the processing of CNTs did not require solvents and did not alter the structure of the π-conjugated nanotube.

Subramaniam et al. reported an effect of the 1-butyl-3-methylimidazolium bis(trifluoromethy lsulfonyl)imide (BmiTFSI) on the properties of polychloroprene rubber (CR) composites that contained multi-walled carbon nanotubes (MWCNTs) and suggested an optimum weight ratio (MWCNTs:BmiTFSI) to achieve an appropriate balance between mechanical and electrical properties of the composites [20]. After the modification process, the structure of the carbon nanotubes was well maintained, indicating a physical interaction between π-electrons on the MWCNT surface and the imidazolium cation of the BmiTFSI. This improved the dispersion of the MWCNT in the CR elastomer, and as a consequence, increased the filler network, which was supported by strain sweep measurements. The optimum MWCNT/BmiTFSI ratio was reported to be 1:5. Higher loading of BmiTFSI caused deterioration of the composites mechanical properties. It was concluded that the use of an imidazolium ionic liquid resulted in increased electrical conductivity, as well as a reinforcing effect of the nanotubes in the CR elastomer. Addition of BmiTFSI-modified MWCNTs increased the tensile modulus and hardness of the CR composites, which clearly promoted dispersion of the nanotubes in the elastomeric matrix.

have also been used to dissolve polymers (cellulose [9], silk fibroin [10] and starch [11]) to create new polymer composites [12], as plasticizers for many kinds of polymers [13] or for the preparation of polymer gels [14] and finally as effective wood preservatives [15]. The applications of ILs as novel electrolytes for electrochemical polymerization have also been reviewed [16]. ILs, due to their high-boiling points (300–450°C, which are also their decomposition temperatures), their low vapor pressure, their low volatility, their ability to create nonexplosive mixtures of their vapors with air, and their high flash-point, can be used successfully in elastomer technologies. This chapter presents an overview of different applications of ILs in elastomeric composites. The main applications of ILs in elastomer technology include the improvement of fillers and the dispersion of other solids in the elastomer matrix, the preparation of conducting rubber composites, or using ILs as crosslinkers or other components of crosslinking systems (e.g.,

**2. Use of ILs to improve the dispersion of solids in elastomers**

the process of modification is not accompanied by structural disruption of CNTs [18].

In 2007, Fukushima and co-workers reported a novel use for ILs as modifiers for carbon nanotubes, which enabled the production of soft composite materials that contained CNTs [19]. The use of ILs allowed for the noncovalent and covalent modifications of CNTs and for the formation of polymer composites with improved physical properties. In the presence of ILs, CNT bundles were reduced and unrolled. The result was fine bundles of CNTs that formed a network structure in the elastomeric matrix. This was related to the possible specific interactions between the imidazolium cation of ILs and the π-electronic carbon nanotube surface. The use of ILs during the processing of CNTs did not require solvents and did not alter the

Subramaniam et al. reported an effect of the 1-butyl-3-methylimidazolium bis(trifluoromethy lsulfonyl)imide (BmiTFSI) on the properties of polychloroprene rubber (CR) composites that contained multi-walled carbon nanotubes (MWCNTs) and suggested an optimum weight ratio (MWCNTs:BmiTFSI) to achieve an appropriate balance between mechanical and electrical properties of the composites [20]. After the modification process, the structure of the carbon nanotubes was well maintained, indicating a physical interaction between π-electrons on the MWCNT surface and the imidazolium cation of the BmiTFSI. This improved the dispersion of

The basic requirement for the reinforcement effect is to produce a homogeneous dispersion of the filler particles in the elastomeric matrix that results in good interphase adhesion [17]. However, most fillers exhibit a high degree of agglomeration, which makes it technologically difficult to obtain a homogenous dispersion in the elastomer. The unique chemical structure of ILs promotes the dispersion of nanoparticles in polymers owing to the surface modification of the nanoparticles with the ILs, which increases the interactions between filler particles and the elastomer matrices, especially in the case of silica, clays, carbon black and carbon nanotubes (CNTs). Because ILs are nonvolatile and nonflammable, the process of carbon nanotube modification and incorporation into an elastomeric matrix could be environmentally friendly. Moreover, ILs act as a lubricant; so,

activators or vulcanization accelerators).

124 Recent Advances in Ionic Liquids

structure of the π-conjugated nanotube.

Das et al. [21] confirmed that the addition of ILs ensures better interactions between rubber and the MWCNTs, and therefore, improves the compatibility between the nanotubes and the elastomer and, consequently, their dispersibility in the elastomeric matrix. ILs, such as 1-allyl-3-methylimidazolium chloride (AMICl), were used to blend the styrene-butadiene (S-SBR) and polybutadiene rubber (BR) that contained MWCNTs. Using 3 phr, AMICl-functionalized MWCNTs caused a three-fold increase in the composites' tensile strength. Despite such small quantities of MWCNTs, the sample was stretched up to 456% without mechanical failure. Therefore, it was clear that an AMICl with a double bond in the structure produced the best reinforcing activity for composites containing 3 phr of MWCNTs. Moreover, it was suggested that AMICl acted as a coupling agent between the CNTs and the rubber chains and, hence, that the twisted structure of the CNTs held a certain amount of rubber and enhanced the three-dimensional interactions of the CNTs with the elastomeric matrix. A specific interaction between the MWCNTs and the rubber matrices in the presence of AMICl was postulated. The double bond of AMICl was presumably chemically linked with the double bond of the diene rubber chains by sulfur bridges, which then interacted with the π-electrons of the CNTs due to delocalization of the π-electrons in the imidazolium cation of AMICl. Transmission electron microscopic images confirmed the improved dispersion of MWCNTs that were functionalized with AMICl in the rubber matrix. Moreover, this ionic liquid promoted the formation of MWCNT clusters with a cellular structure in the elastomeric matrix that confirmed the strong adhesion of carbon nanotubes to the rubber phase and a special bound rubber aggregation in the reported composites.

One of the most effective reinforcing fillers for rubber compounds is carbon black (CB) with the surface of graphitic structures, which possess delocalized π-electrons. Particles of CB can interact with ILs in a manner similar to CNTs. Kreyenschulte et al. [22] investigated the interactions of AMICl with different types of carbon black and demonstrated strong interactions between AMICl and CB, as well as an improvement of the filler's dispersion in the elastomers. CB formed a bucky gel with AMICl, and an increase in the glass transition temperature (T<sup>g</sup> ) of ionic liquid was observed. This effect was more apparent when the CB was graphitized; thus, a larger fraction of the CB surface was coated with graphitic crystals that can interact with AMICl via cation π-interactions. AMICl-modified carbon blacks were applied as fillers for S-SBR/BR composites and ethylene-propylene-diene rubber (EPDM). AMICl was premixed with CB, using ethanol and an ultrasonic treatment, and then dried before mixing with elastomers. As mentioned above, Fukushima et al. have reported that CNTs are able to build socalled bucky gels by physical crosslinking of nanotube bundles by mediating the molecular ordering of imidazolium-based ILs [23]. To study this behavior, which can be caused by CB or by its mixture with AMICl, analysis of the frequency dependence of the storage modulus and the loss modulus was performed which revealed significant differences. Hydrodynamic reinforcement of the elastomers in the presence of CB particles that were modified with ILs could not explain the differences in the frequency dependencies of the storage and the loss modulus. Therefore, it was concluded that particles of CB can form gels in the presence of ILs, similar to CNTs. AMICl appeared to react either with vulcanization systems or with the double bonds of the rubber matrix, especially for EPDM, which decreased the crosslink density and thus affected the mechanical properties of the composites. For S-SBR/BR blends with AMIClmodified CB, the elongation at break was increased slightly, but there was no influence on the rupture strength. For EPDM vulcanizates, AMICl-CB caused a huge increase in the elongation at break and a significant increase in the rapture strength compared to samples without ILs.

Addition of GO intercalated with BmiPF<sup>6</sup>

as the Tg

area of the GO-BmiPF<sup>6</sup>

in proper dispersants [33].

of the DCP during the vulcanization process.

increased the thermal stability of composites as well

Applications of Ionic Liquids in Elastomeric Composites: A Review

http://dx.doi.org/10.5772/intechopen.76978

/

127

of BIIR, due to the attractive interactions of the elastomer chains with the large surface

Silica is widely used as a filler for elastomers because of its excellent mechanical properties, high thermal and chemical stability, a suitable pore structure, the presence of silanol groups on the surface and its high specific surface area [30]. ILs can be used to modify the surface of silica [31], mainly due to the π-π interactions between the cations and anions of the ILs with active sites on the silica's surface [32]. Generally, nanoparticles of silica after modification with ILs are monodispersed and smooth spheres. The structure of the applied ILs significantly influences the surface and properties of modified silica nanoparticles, thereby improving the hydrophobicity to encourage the stability of dispersions and suspensions of this filler

ILs can be used to increase the degree of dispersion of silica fillers and vulcanization activators (zinc oxide) nanoparticles, as was reported by Maciejewska et al. [34]. The addition of ILs (salts of benzylimidazolium, alkylpyridinium, alkylpyrrolidinium, or alkylpiperidinium) produced a homogeneous dispersion of silica and zinc oxide nanoparticles in the SBR rubber. This resulted in reduced time and temperature of vulcanization and increased the crosslink density, thermal

ILs with specific structures can be employed as interfacial modifiers for various silica-filled polymer composites [35]. Lei and Tang reported using 1-methylimidazolium methacrylate (MimMa) as a modifier for SBR to improve the dispersion of silica and increase the interfacial interactions between SBR and the filler [36]. MimMa was polymerized radically and grafted onto rubber chains during vulcanization, forming the graft product poly(SBR-g-MimMa), to enhance the compatibility between MimMa and SBR. As a consequence, the mechanical performance of SBR/silica composites was improved effectively. With increasing MimMa loading, the abrasion resistance, tensile strength, tear strength, and the modulus at 300% relative elongation were improved. The composites can be used as damping materials owing to increased mechanical loss under dynamic load. The strong interactions between MimMa and silica were partially linked, due to hydrogen bonding between the imidazolium cation of the MimMa and the Si-O-Si groups of silica, and partially to hydrogen bonding between the anion of the MimMa and the Si-OH groups on the silica's surface, which was well-described in [37]. SEM and TEM confirmed the improved dispersion of silica in the IL-functionalized SBR, and that agglomerates with reduced size were uniformly distributed into the SBR matrix. This was attributed to interfacial interactions that were induced by the incorporation of functional MimMa into the rubber chains. The presence of MimMa affected the vulcanization behavior of SBR compounds. With an increasing loading of MimMa, the value of minimum rheometric torque during vulcanization was consequently lowered, and the maximum torque required a maximum of 1 phr MimMa and then decreased significantly for higher content of this ionic liquid. It was expected that MimMa would cover the silica surface and reduce the adsorption of dicumyl peroxide (DCP) onto the silica, but on the other hand, it would consume a portion

stability of vulcanizates and their aging resistance under the influence of UV radiation.

BIIR nanocomposites was observed compared with that of the unfilled BIIR.

. Moreover, an improvement of the thermal conductivity of GO-BmiPF<sup>6</sup>

Silicone rubber is a special rubber with improved electrical insulation, high elasticity and flexibility, and low modulus. Xu, Wang, et al. fabricated a novel, flexible piezoresistive material by mechanical grinding and two-roll mixing of silicone-rubber (SR), conductive CB and the ionic liquid 1-hexadecyl-3-methylimidazolium bromide (HdmiBr) that was applied to improve the dispersibility of the filler in SR matrix [24]. CB and HdmiBr were effectively co-grinded in a mortar and mixed with silicone rubber. A uniform dispersion of CB particles was observed in both composites (CB/SR and CB-HdmiBr/SR), but the size of the CB particles in the SR that contained ionic liquid was much smaller. Some of the HdmiBr was distributed on the surface of the CB, and attractive interactions between the imidazolium cation of the HdmiBr and the π-electrons on the CB surface were reported to improve the compatibility between the SR matrix and the CB and, consequently, the extent of dispersion in the elastomer. Additionally, these composites were characterized by a lower percolation threshold compared with the composite without HdmiBr. About 5 vol.% of CB-HdmiBr filler was sufficient to obtain higher piezoresistivity, shorter time of relaxation, and better cyclic repeatability of the composites, due to the plasticizing effect of the HdmiBr which resulted in the forced motion of the CB particles in the SR elastomeric network.

ILs were also reported to interact with a specific type of filler—graphene. A perfect form of graphene consists ideally of a flat, single layer material. However, in reality, some ripples are formed due to thermal fluctuations, which along with associated waviness, may affect its ability to reinforce composite materials [25]. Generally, elastomer/graphene and graphite composites can be fabricated by solution mixing, melt blending, and in situ polymerization methods, but it is difficult to incorporate chains of rubber macromolecules directly into the interlayers of graphite. There are many surfactants that can facilitate the dispersion of graphene in polymers [26]. The increased recognition gained by ILs is due to their unique capacity of imparting surface charges to the graphene sheet and thereby increasing colloidal stability [27].

Xiong et al. demonstrated that adding modified ionic liquid graphene oxide (GO-ILs) to bromobutyl rubber (BIIR) improved the thermal stability and thermal conductivity of the BIIR [28]. The thermal stability of BIIR has always been a concern because of the double bonds, which could be generated by eliminating HBr from its backbone [29]. Suitably prepared GO was modified with 1-butyl-3-methylimidazolium hexafluorophosphate (BmiPF<sup>6</sup> ) and then incorporated into a BIIR matrix. BmiPF<sup>6</sup> was successfully intercalated into the interlayer of GO, resulting in an increased degree of GO exfoliation, which consequently improved the dispersion in the BIIR matrix due to the strong interfacial interactions between GO-BmiPF<sup>6</sup> and BIIR rubber chains. Addition of GO intercalated with BmiPF<sup>6</sup> increased the thermal stability of composites as well as the Tg of BIIR, due to the attractive interactions of the elastomer chains with the large surface area of the GO-BmiPF<sup>6</sup> . Moreover, an improvement of the thermal conductivity of GO-BmiPF<sup>6</sup> / BIIR nanocomposites was observed compared with that of the unfilled BIIR.

reinforcement of the elastomers in the presence of CB particles that were modified with ILs could not explain the differences in the frequency dependencies of the storage and the loss modulus. Therefore, it was concluded that particles of CB can form gels in the presence of ILs, similar to CNTs. AMICl appeared to react either with vulcanization systems or with the double bonds of the rubber matrix, especially for EPDM, which decreased the crosslink density and thus affected the mechanical properties of the composites. For S-SBR/BR blends with AMIClmodified CB, the elongation at break was increased slightly, but there was no influence on the rupture strength. For EPDM vulcanizates, AMICl-CB caused a huge increase in the elongation at break and a significant increase in the rapture strength compared to samples without ILs. Silicone rubber is a special rubber with improved electrical insulation, high elasticity and flexibility, and low modulus. Xu, Wang, et al. fabricated a novel, flexible piezoresistive material by mechanical grinding and two-roll mixing of silicone-rubber (SR), conductive CB and the ionic liquid 1-hexadecyl-3-methylimidazolium bromide (HdmiBr) that was applied to improve the dispersibility of the filler in SR matrix [24]. CB and HdmiBr were effectively co-grinded in a mortar and mixed with silicone rubber. A uniform dispersion of CB particles was observed in both composites (CB/SR and CB-HdmiBr/SR), but the size of the CB particles in the SR that contained ionic liquid was much smaller. Some of the HdmiBr was distributed on the surface of the CB, and attractive interactions between the imidazolium cation of the HdmiBr and the π-electrons on the CB surface were reported to improve the compatibility between the SR matrix and the CB and, consequently, the extent of dispersion in the elastomer. Additionally, these composites were characterized by a lower percolation threshold compared with the composite without HdmiBr. About 5 vol.% of CB-HdmiBr filler was sufficient to obtain higher piezoresistivity, shorter time of relaxation, and better cyclic repeatability of the composites, due to the plasticizing effect of the HdmiBr which resulted in the forced motion of the CB

ILs were also reported to interact with a specific type of filler—graphene. A perfect form of graphene consists ideally of a flat, single layer material. However, in reality, some ripples are formed due to thermal fluctuations, which along with associated waviness, may affect its ability to reinforce composite materials [25]. Generally, elastomer/graphene and graphite composites can be fabricated by solution mixing, melt blending, and in situ polymerization methods, but it is difficult to incorporate chains of rubber macromolecules directly into the interlayers of graphite. There are many surfactants that can facilitate the dispersion of graphene in polymers [26]. The increased recognition gained by ILs is due to their unique capacity of imparting sur-

Xiong et al. demonstrated that adding modified ionic liquid graphene oxide (GO-ILs) to bromobutyl rubber (BIIR) improved the thermal stability and thermal conductivity of the BIIR [28]. The thermal stability of BIIR has always been a concern because of the double bonds, which could be generated by eliminating HBr from its backbone [29]. Suitably prepared GO was mod-

an increased degree of GO exfoliation, which consequently improved the dispersion in the BIIR

was successfully intercalated into the interlayer of GO, resulting in

) and then incorporated

and BIIR rubber chains.

face charges to the graphene sheet and thereby increasing colloidal stability [27].

ified with 1-butyl-3-methylimidazolium hexafluorophosphate (BmiPF<sup>6</sup>

matrix due to the strong interfacial interactions between GO-BmiPF<sup>6</sup>

particles in the SR elastomeric network.

126 Recent Advances in Ionic Liquids

into a BIIR matrix. BmiPF<sup>6</sup>

Silica is widely used as a filler for elastomers because of its excellent mechanical properties, high thermal and chemical stability, a suitable pore structure, the presence of silanol groups on the surface and its high specific surface area [30]. ILs can be used to modify the surface of silica [31], mainly due to the π-π interactions between the cations and anions of the ILs with active sites on the silica's surface [32]. Generally, nanoparticles of silica after modification with ILs are monodispersed and smooth spheres. The structure of the applied ILs significantly influences the surface and properties of modified silica nanoparticles, thereby improving the hydrophobicity to encourage the stability of dispersions and suspensions of this filler in proper dispersants [33].

ILs can be used to increase the degree of dispersion of silica fillers and vulcanization activators (zinc oxide) nanoparticles, as was reported by Maciejewska et al. [34]. The addition of ILs (salts of benzylimidazolium, alkylpyridinium, alkylpyrrolidinium, or alkylpiperidinium) produced a homogeneous dispersion of silica and zinc oxide nanoparticles in the SBR rubber. This resulted in reduced time and temperature of vulcanization and increased the crosslink density, thermal stability of vulcanizates and their aging resistance under the influence of UV radiation.

ILs with specific structures can be employed as interfacial modifiers for various silica-filled polymer composites [35]. Lei and Tang reported using 1-methylimidazolium methacrylate (MimMa) as a modifier for SBR to improve the dispersion of silica and increase the interfacial interactions between SBR and the filler [36]. MimMa was polymerized radically and grafted onto rubber chains during vulcanization, forming the graft product poly(SBR-g-MimMa), to enhance the compatibility between MimMa and SBR. As a consequence, the mechanical performance of SBR/silica composites was improved effectively. With increasing MimMa loading, the abrasion resistance, tensile strength, tear strength, and the modulus at 300% relative elongation were improved. The composites can be used as damping materials owing to increased mechanical loss under dynamic load. The strong interactions between MimMa and silica were partially linked, due to hydrogen bonding between the imidazolium cation of the MimMa and the Si-O-Si groups of silica, and partially to hydrogen bonding between the anion of the MimMa and the Si-OH groups on the silica's surface, which was well-described in [37]. SEM and TEM confirmed the improved dispersion of silica in the IL-functionalized SBR, and that agglomerates with reduced size were uniformly distributed into the SBR matrix. This was attributed to interfacial interactions that were induced by the incorporation of functional MimMa into the rubber chains. The presence of MimMa affected the vulcanization behavior of SBR compounds. With an increasing loading of MimMa, the value of minimum rheometric torque during vulcanization was consequently lowered, and the maximum torque required a maximum of 1 phr MimMa and then decreased significantly for higher content of this ionic liquid. It was expected that MimMa would cover the silica surface and reduce the adsorption of dicumyl peroxide (DCP) onto the silica, but on the other hand, it would consume a portion of the DCP during the vulcanization process.

The improved dispersion of silica nanoparticles in the SBR elastomer was confirmed by Maciejewska and Zaborski [38] for ILs with benzalkonium or didecyldimethylammonium cations and saccharinate, acesulfame, or lactate anions, which contributed to a considerable increase in the mechanical properties of the SBR composites.

agents on its surface, thus increasing the efficiency of vulcanization. When an IL was absorbed on the HNT, the interfacial bonding and the dispersion were improved, and consequently, the

The same authors investigated two other functional ILs, 1-methylimidazolium mercaptopropionate (MimMP) and bis(1-methylimidazolium) mercaptosuccinate (BMimMS), as modifiers for tailoring the interfacial structure of SBR/HNTs composites [48]. Incorporation of functional groups, e.g., double bonds and thiol groups, into IL molecules improved the performance of rubber composites. MimMP was reported to interact with Al-O, Si-OH and Al-OH on the HNT's surface by hydrogen bonding. Hydrogen bonding between Si-O, Al-OH, or Si-OH and the imidazolium cation of BMimMS was also confirmed. With loading of functional ILs, the scorch time of SBR was reduced due to restricted adsorption of curatives on the HNT's surface. Additionally, the presence of a thiol group in the ILs could activate the vulcanization significantly by effectively lowering the activation energy of this process. Incorporation of ILs increased the maximum torque values during vulcanization, which was attributed to the filler networking into the SBR matrix and filler-rubber interfacial interactions by hydrogen bonding, as well as to increasing the vulcanizates crosslink density with ILs loading. It was also confirmed that incorporation of MimMP or BMimMS into SBR/HNTs composites effectively restrained the agglomeration of HNTs in the elastomer matrix, especially in the case of BMimMS which promoted the fine and homogeneous dispersion of filler. The addition of functional ILs improved considerably the tensile and tear strengths of the vulcanizates. Possible explanations that were proposed for this phenomenon included: the increased crosslink density of SBR, the improved dispersion of HNTs in the rubber matrix, and the strengthened interfacial interactions between HNTs and SBR. The efficiency of the described functional ILs in modifying SBR/

HNTs vulcanizates was also enhanced compared with other modifiers [49].

O3

**3. Conducting rubber composites containing ILs**

novel electrolytes as an alternative to commonly used lithium salts [52].

imidazolium tetrafluoroborate (HMIMBF<sup>4</sup>

of magnetic fillers, such as Fe<sup>2</sup>

(OMIMPF<sup>6</sup>

Furthermore, it was reported that the addition of hydrophobic ILs, such as 1-hexyl-3-methyl-

), and 1-methyl-3-methylimidazolium hexafluorophosphate (HMIMPF<sup>6</sup>

meric blends based on an ethylene-propylene copolymer (EPM), prevented the agglomeration

duced with other ingredients to the pre-plasticized rubber. The elastomeric composites that were produced exhibited good magnetic and mechanical properties. Moreover, uniformly distributed particles of magnetite protected the composites against elevated temperatures and UV rays [50].

Solid polymer electrolytes (SPEs) have been used in various electrochemical devices such as sensors, actuators, supercapacitors, and rechargeable batteries [51]. To improve their mechanical properties, SPEs based on elastomers, such as natural rubber (NR), NBR or SBR containing lithium salts, were developed. This is an effective process for obtaining polymer electrolytes with high tensile strength and elasticity. However, due to their high ionic conductivity, good electrochemical stability, and non-volatility, ILs could be applied successfully as

), 1-methyl-3-octylimidazolium hexafluorophosphate

Applications of Ionic Liquids in Elastomeric Composites: A Review

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129

, and improved the degree of their dispersion. ILs were intro-

), to elasto-

tensile strength of the SBR vulcanizates increased.

Over the last two decades, the published literature regarding polymer composites with layered silicates, especially montmorillonite (MMT) and halloysite and nanoclays, has grown [39, 40]. MMT is a hydrophilic material that can be used as a filler in polymeric nanocomposites to enhance mechanical strength, modulus, heat resistance, anti-flammability, anti-gas permeability, and service life at elevated temperatures [41]. However, the key factor that is required to achieve these properties of polymer composites is ensuring a homogeneous dispersion of MMT in the polymer matrix accompanied with the exfoliation of MMT layers. The modification of clays with ILs can be considered as a green method because it does not require a solvent [42] or a melt intercalation process [43] to be successfully completed.

Fontana et al. [44] reported the use of MMT mechano-chemically co-intercalated with hexadecyltrimethylammonium bromide (HDTMA) and the ionic liquid 1-methyl-3-octylimidazolium bis(trifluoromethanesulfonyl)imide (OMImTf<sup>2</sup> N) as a filler for acrylonitrile-butadiene elastomers (NBR). Applying MMT intercalated with ILs improved the thermal stability of the NBR composites, which was explained by the extra interactions between the cations present in the clay layers and the elastomer chains. Based on SEM analysis, it was shown that the presence of ILs was necessary to improve the dispersion of the MMT into rubber. There were numerous agglomerates in the vulcanizate with small amounts of OMImTf2 N, but after increasing its loading, the dispersion of MMT became more homogeneous. Therefore, it was confirmed that the organophilization of MMT enhanced the interfacial interactions between the MMT and NBR elastomer, due to the higher compatibility of intercalated MMT with the elastomeric matrix. MMT modified with ILs added to NBR was classified as a good reinforcing filler, which improved the efficiency of the vulcanization process, and consequently, the mechanical and physical properties of the elastomer.

Halloysite nanotubes (HNTs) are naturally occurring types of mineral clays with a nanotubular structure and were studied as promising fillers for polymer composites. Overcoming the poor interfacial interactions between polymers and HNTs remains a great challenge for processing polymer/HNTs nanocomposites with desired performance [45]. Guo et al. reported that BmiPF<sup>6</sup> was strongly adsorbed onto HNT surfaces and consequently improved the strength of interfacial bonding and the dispersion of the HNTs in the elastomeric matrix [46]. It was reported that clays may adsorb ILs by an ion exchange mechanism [47]. The authors claimed that ILs may be adsorbed onto HNT surfaces by hydrogen bonding between hydrogen atoms of the imidazolium ring of the ionic liquid and the surface siloxane groups of the HNTs. The IL-coated HNTs (m-HNTs) were used as a reinforcement for SBR. The rheometric torque during vulcanization of SBR composites increased with loading of HNTs, probably due to the increase in the crosslink density of the vulcanizates and the improved interaction between SBR and m-HNTs. Moreover, using m-HNTs, the scorch time and the optimal vulcanization time of SBR compounds shortened in comparison with neat SBR. This resulted from the adsorption of IL on the surface of the HNTs, which restricted the adsorption of curing agents on its surface, thus increasing the efficiency of vulcanization. When an IL was absorbed on the HNT, the interfacial bonding and the dispersion were improved, and consequently, the tensile strength of the SBR vulcanizates increased.

The improved dispersion of silica nanoparticles in the SBR elastomer was confirmed by Maciejewska and Zaborski [38] for ILs with benzalkonium or didecyldimethylammonium cations and saccharinate, acesulfame, or lactate anions, which contributed to a considerable

Over the last two decades, the published literature regarding polymer composites with layered silicates, especially montmorillonite (MMT) and halloysite and nanoclays, has grown [39, 40]. MMT is a hydrophilic material that can be used as a filler in polymeric nanocomposites to enhance mechanical strength, modulus, heat resistance, anti-flammability, anti-gas permeability, and service life at elevated temperatures [41]. However, the key factor that is required to achieve these properties of polymer composites is ensuring a homogeneous dispersion of MMT in the polymer matrix accompanied with the exfoliation of MMT layers. The modification of clays with ILs can be considered as a green method because it does not require

Fontana et al. [44] reported the use of MMT mechano-chemically co-intercalated with hexadecyltrimethylammonium bromide (HDTMA) and the ionic liquid 1-methyl-3-octylimidazo-

elastomers (NBR). Applying MMT intercalated with ILs improved the thermal stability of the NBR composites, which was explained by the extra interactions between the cations present in the clay layers and the elastomer chains. Based on SEM analysis, it was shown that the presence of ILs was necessary to improve the dispersion of the MMT into rubber. There

increasing its loading, the dispersion of MMT became more homogeneous. Therefore, it was confirmed that the organophilization of MMT enhanced the interfacial interactions between the MMT and NBR elastomer, due to the higher compatibility of intercalated MMT with the elastomeric matrix. MMT modified with ILs added to NBR was classified as a good reinforcing filler, which improved the efficiency of the vulcanization process, and consequently, the

Halloysite nanotubes (HNTs) are naturally occurring types of mineral clays with a nanotubular structure and were studied as promising fillers for polymer composites. Overcoming the poor interfacial interactions between polymers and HNTs remains a great challenge for processing polymer/HNTs nanocomposites with desired performance [45]. Guo et al. reported

strength of interfacial bonding and the dispersion of the HNTs in the elastomeric matrix [46]. It was reported that clays may adsorb ILs by an ion exchange mechanism [47]. The authors claimed that ILs may be adsorbed onto HNT surfaces by hydrogen bonding between hydrogen atoms of the imidazolium ring of the ionic liquid and the surface siloxane groups of the HNTs. The IL-coated HNTs (m-HNTs) were used as a reinforcement for SBR. The rheometric torque during vulcanization of SBR composites increased with loading of HNTs, probably due to the increase in the crosslink density of the vulcanizates and the improved interaction between SBR and m-HNTs. Moreover, using m-HNTs, the scorch time and the optimal vulcanization time of SBR compounds shortened in comparison with neat SBR. This resulted from the adsorption of IL on the surface of the HNTs, which restricted the adsorption of curing

was strongly adsorbed onto HNT surfaces and consequently improved the

N) as a filler for acrylonitrile-butadiene

N, but after

a solvent [42] or a melt intercalation process [43] to be successfully completed.

were numerous agglomerates in the vulcanizate with small amounts of OMImTf2

increase in the mechanical properties of the SBR composites.

128 Recent Advances in Ionic Liquids

lium bis(trifluoromethanesulfonyl)imide (OMImTf<sup>2</sup>

mechanical and physical properties of the elastomer.

that BmiPF<sup>6</sup>

The same authors investigated two other functional ILs, 1-methylimidazolium mercaptopropionate (MimMP) and bis(1-methylimidazolium) mercaptosuccinate (BMimMS), as modifiers for tailoring the interfacial structure of SBR/HNTs composites [48]. Incorporation of functional groups, e.g., double bonds and thiol groups, into IL molecules improved the performance of rubber composites. MimMP was reported to interact with Al-O, Si-OH and Al-OH on the HNT's surface by hydrogen bonding. Hydrogen bonding between Si-O, Al-OH, or Si-OH and the imidazolium cation of BMimMS was also confirmed. With loading of functional ILs, the scorch time of SBR was reduced due to restricted adsorption of curatives on the HNT's surface. Additionally, the presence of a thiol group in the ILs could activate the vulcanization significantly by effectively lowering the activation energy of this process. Incorporation of ILs increased the maximum torque values during vulcanization, which was attributed to the filler networking into the SBR matrix and filler-rubber interfacial interactions by hydrogen bonding, as well as to increasing the vulcanizates crosslink density with ILs loading. It was also confirmed that incorporation of MimMP or BMimMS into SBR/HNTs composites effectively restrained the agglomeration of HNTs in the elastomer matrix, especially in the case of BMimMS which promoted the fine and homogeneous dispersion of filler. The addition of functional ILs improved considerably the tensile and tear strengths of the vulcanizates. Possible explanations that were proposed for this phenomenon included: the increased crosslink density of SBR, the improved dispersion of HNTs in the rubber matrix, and the strengthened interfacial interactions between HNTs and SBR. The efficiency of the described functional ILs in modifying SBR/ HNTs vulcanizates was also enhanced compared with other modifiers [49].

Furthermore, it was reported that the addition of hydrophobic ILs, such as 1-hexyl-3-methylimidazolium tetrafluoroborate (HMIMBF<sup>4</sup> ), 1-methyl-3-octylimidazolium hexafluorophosphate (OMIMPF<sup>6</sup> ), and 1-methyl-3-methylimidazolium hexafluorophosphate (HMIMPF<sup>6</sup> ), to elastomeric blends based on an ethylene-propylene copolymer (EPM), prevented the agglomeration of magnetic fillers, such as Fe<sup>2</sup> O3 , and improved the degree of their dispersion. ILs were introduced with other ingredients to the pre-plasticized rubber. The elastomeric composites that were produced exhibited good magnetic and mechanical properties. Moreover, uniformly distributed particles of magnetite protected the composites against elevated temperatures and UV rays [50].

#### **3. Conducting rubber composites containing ILs**

Solid polymer electrolytes (SPEs) have been used in various electrochemical devices such as sensors, actuators, supercapacitors, and rechargeable batteries [51]. To improve their mechanical properties, SPEs based on elastomers, such as natural rubber (NR), NBR or SBR containing lithium salts, were developed. This is an effective process for obtaining polymer electrolytes with high tensile strength and elasticity. However, due to their high ionic conductivity, good electrochemical stability, and non-volatility, ILs could be applied successfully as novel electrolytes as an alternative to commonly used lithium salts [52].

Marwanta et al. [35] used NBR as a matrix and ILs as an ion source for SPEs. N-ethylimidazolium salts of tetrafluoroborate (EImBF<sup>4</sup> ), bis(trifluoromethanesulfonyl)imide (EImTFSI) and benzenesulfonate (EImBS) were included as electrolytes. The EImTFSI was the only one miscible with NBR, which was confirmed by the single T<sup>g</sup> present in the DSC curve. This was attributed to ion-dipole interactions between the nitrile groups of NBR and EImTFSI ions. The best miscibility of ILs with TFSI anions was confirmed by Likozar [53] for hydrogenated nitrile elastomers. In the case of other ILs, completely phase-separated composites were obtained. The Tg of NBR/EImTFSI composites decreased with increasing ionic liquid content, thereby demonstrating the plasticizing effect of ILs on the elastomeric matrix. The conductivity of NBR/EImTFSI composites increased with EImTFSI content, which resulted from both the increased carrier ion number and the low Tg of the composites. Moreover, the composites containing from 10 to 50 phr of EImTFSI exhibited good elasticity and high ionic conductivity (up to 1.2 × 10−5 S/cm at 30°C). The addition of lithium salts with the TFSI anion (LiTFSI) produced NBR conductive composites with even better mechanical strength than the composites without lithium salts. However, in this case, the composites revealed microscopic phase separation that was induced by the addition of a lithium salt. Li+ ions in the LiTFSI should function as couplers between the NBR and the EImTFSI, since they can interact with both nitrile groups in the elastomer and the TFSI anion from the LiTFSI. As a result of microphase separation in the NBR/EImTFSI/LiTFSI composites, their ionic conductivity was approximately 100 times greater than that of composites without lithium salt, and the conductivity increased with the content of this salt up to 20 mol%.

Matchawet et al. fabricated elastomeric composites with increased electrical conductivity using epoxidized natural rubber (ENR) and 1-ethyl-3-methylimidazolium chloride (EmiCl) as the ionic liquid [57]. The electrical conductivity of ENR composites increased from 10−8 S/m (for neat ENR) to 10−4 S/m for the composites with 7 phr of EmiCl, due to the mobility of cations and anions in the EmiCl, which served as additional charge carriers. The effect of EmiCl on both the curing behavior of ENR and its crosslink density was also reported. The scorch time and optimal vulcanization time significantly decreased for EmiCl loadings below 5 phr and then increased with the content of EmiCl due to the microphase separation of the EmiCl that was dispersed in the ENR matrix. The addition of EmiCl increased the crosslink density of ENR, probably as a result of π-π and dipole interactions in the ENR with the imidazolium moiety of the EmiCl. The disadvantage of using EmiCl was the significant reduction of tensile

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131

Hydrophobic ILs, such BmiTFSI, were used to produce composites of carboxylated nitrile rubber (XNBR) that contained hydrotalcite (HT), which exhibited an increase in the ionic conductivity from 10−10 S/cm (neat XNBR) to 10−7 S/cm [58]. Moreover, BmiTFSI did not affect the tensile strength or elongation at break of XNBR significantly, although a plasticizing effect of BmiTFSI was confirmed. The improvement of the ionic conductivity of XNBR/HT composites resulted not only from the ionic conductivity of BmiTFSI and its concentration but was also influenced by the plasticizing effect of the ionic liquid, which increased the mobility of the rubber chain segments of the XNBR that contained BmiTFSI. Additionally, BmiTFSI

In recent years, the use of MWCNTs has increased as a method for producing conductive elastomeric composites [21, 59, 60]. Unfortunately, because of their tendency to agglomerate due to van der Waals interactions between individual nanotubes, a fine, uniform dispersion of MWCNTs and their network formation in the elastomer is a significant challenge from a technological point of view. Different techniques have been applied to disperse MWCNTs in polymer matrices, including functionalization with the use of harmful solvents. In addition to the harmful effect of these volatile organic solvents, they also have several disadvantages, such as destruction of the π-electron network of MWCNTs, resulting in a reduction of their inherent electrical conductivity [61]. To preserve the π-electrons on the surface of MWCNTs and to maintain the inherent electrical conductivity, ILs can be used to non-covalently modify the surface of nanotubes [21, 59]. Subramaniam et al. reported the development of new elastomeric conductors that employ CR rubber and MWCNTs that were modified with ILs [59]. BmiTFSI was used for surface modification of the MWCNTs and the resultant mix was directly admixed into CR without the use of an organic solvent. The Raman spectrum confirmed the interactions between π-electrons of the nanotubes and cations of the BmiTFSI, or the perturbation of π-π stacking of multi-walls of the nanotubes. Therefore, it was concluded that the modified MWCNTs were activated physically, without a chemical impairment of the nanotubes. The use of modified MWCNTs produced CR composites whose electrical conductivity increased with the amount of filler. Moreover, the higher the ratio of MWCNTs to BmiTFSI, the higher was the conductivity of the obtained composites. The increase in the conductivity of the CR composites was attributed to a synergistic effect of electrons and ions as well as to an improved dispersion and formation of a percolating network by the BmiTFSI-modified

strength and elongation at break of the ENR.

improved the dispersion of HT in the elastomeric matrix.

Ion conductive elastomeric composites were also prepared by mixing NBR with lithium salts and ILs with an alkylimidazolium cation and a TFSI anion, where both cation and anion were tethered in the form of a zwitterion [54]. Addition of 9.2 wt% zwitterion enhanced the ionic conductivity approximately 8-fold compared with composites without zwitterions. The zwitterion was thought to provide additional high ion conductive paths that could improve the ionic conductivity. Moreover, zwitterions can effectively reduce the interaction between nitrile groups of the NBR and Li<sup>+</sup> ions of the lithium salt as a result of a strong interaction between zwitterions and lithium ions. The reduction of NBR/Li<sup>+</sup> interactions could increase the number of free Li+ ions that can enhance the ionic conductivity of the NBR composites. No less important was microphase separation of the prepared composites, which was responsible for creating an ion conduction path. Flexible and mechanically stable NBR-based SPEs were also fabricated using BmiTFSI [55]. The highest ionic conductivity was achieved with composites that contained NBR with 40 mol% acrylonitrile. ILs were reported to act as electrolytes and plasticizers that enhanced the mobility of the elastomer chains and also increased the ionic conductivity.

Another approach that could produce a soft, flexible SPE with good dimensional stability made use of interpenetrating polymer networks (IPNs) that were formed by two crosslinked polymers, such as NBR and poly(ethylene oxide) (PEO) and the ionic liquid N-ethylmethylimidazolium bis(trifluoromethanesulfonyl)imide (EmiTFSI) as the electrolyte [56]. EmiTFSI was chosen because of its high affinity for polymers with polar groups, which rendered it compatible with NBR and PEO [58]. Swelling of NBR/PEO IPNs with EmiTFSI produced materials with an ionic conductivity greater than 10−4 S/cm at room temperature. Such materials could be used successfully in actuators and other electrochemical devices.

Matchawet et al. fabricated elastomeric composites with increased electrical conductivity using epoxidized natural rubber (ENR) and 1-ethyl-3-methylimidazolium chloride (EmiCl) as the ionic liquid [57]. The electrical conductivity of ENR composites increased from 10−8 S/m (for neat ENR) to 10−4 S/m for the composites with 7 phr of EmiCl, due to the mobility of cations and anions in the EmiCl, which served as additional charge carriers. The effect of EmiCl on both the curing behavior of ENR and its crosslink density was also reported. The scorch time and optimal vulcanization time significantly decreased for EmiCl loadings below 5 phr and then increased with the content of EmiCl due to the microphase separation of the EmiCl that was dispersed in the ENR matrix. The addition of EmiCl increased the crosslink density of ENR, probably as a result of π-π and dipole interactions in the ENR with the imidazolium moiety of the EmiCl. The disadvantage of using EmiCl was the significant reduction of tensile strength and elongation at break of the ENR.

Marwanta et al. [35] used NBR as a matrix and ILs as an ion source for SPEs. N-ethylimidazolium

zenesulfonate (EImBS) were included as electrolytes. The EImTFSI was the only one miscible

uted to ion-dipole interactions between the nitrile groups of NBR and EImTFSI ions. The best miscibility of ILs with TFSI anions was confirmed by Likozar [53] for hydrogenated nitrile elastomers. In the case of other ILs, completely phase-separated composites were obtained.

containing from 10 to 50 phr of EImTFSI exhibited good elasticity and high ionic conductivity (up to 1.2 × 10−5 S/cm at 30°C). The addition of lithium salts with the TFSI anion (LiTFSI) produced NBR conductive composites with even better mechanical strength than the composites without lithium salts. However, in this case, the composites revealed microscopic phase sepa-

tion as couplers between the NBR and the EImTFSI, since they can interact with both nitrile groups in the elastomer and the TFSI anion from the LiTFSI. As a result of microphase separation in the NBR/EImTFSI/LiTFSI composites, their ionic conductivity was approximately 100 times greater than that of composites without lithium salt, and the conductivity increased

Ion conductive elastomeric composites were also prepared by mixing NBR with lithium salts and ILs with an alkylimidazolium cation and a TFSI anion, where both cation and anion were tethered in the form of a zwitterion [54]. Addition of 9.2 wt% zwitterion enhanced the ionic conductivity approximately 8-fold compared with composites without zwitterions. The zwitterion was thought to provide additional high ion conductive paths that could improve the ionic conductivity. Moreover, zwitterions can effectively reduce the interaction between nitrile

ions that can enhance the ionic conductivity of the NBR composites. No less impor-

tant was microphase separation of the prepared composites, which was responsible for creating an ion conduction path. Flexible and mechanically stable NBR-based SPEs were also fabricated using BmiTFSI [55]. The highest ionic conductivity was achieved with composites that contained NBR with 40 mol% acrylonitrile. ILs were reported to act as electrolytes and plasticizers that enhanced the mobility of the elastomer chains and also increased the ionic conductivity.

Another approach that could produce a soft, flexible SPE with good dimensional stability made use of interpenetrating polymer networks (IPNs) that were formed by two crosslinked polymers, such as NBR and poly(ethylene oxide) (PEO) and the ionic liquid N-ethylmethylimidazolium bis(trifluoromethanesulfonyl)imide (EmiTFSI) as the electrolyte [56]. EmiTFSI was chosen because of its high affinity for polymers with polar groups, which rendered it compatible with NBR and PEO [58]. Swelling of NBR/PEO IPNs with EmiTFSI produced materials with an ionic conductivity greater than 10−4 S/cm at room temperature. Such materials could be used

ions of the lithium salt as a result of a strong interaction between

 of NBR/EImTFSI composites decreased with increasing ionic liquid content, thereby demonstrating the plasticizing effect of ILs on the elastomeric matrix. The conductivity of NBR/EImTFSI composites increased with EImTFSI content, which resulted from both the

), bis(trifluoromethanesulfonyl)imide (EImTFSI) and ben-

present in the DSC curve. This was attrib-

of the composites. Moreover, the composites

ions in the LiTFSI should func-

interactions could increase the number

salts of tetrafluoroborate (EImBF<sup>4</sup>

130 Recent Advances in Ionic Liquids

The Tg

with NBR, which was confirmed by the single T<sup>g</sup>

increased carrier ion number and the low Tg

with the content of this salt up to 20 mol%.

zwitterions and lithium ions. The reduction of NBR/Li<sup>+</sup>

successfully in actuators and other electrochemical devices.

groups of the NBR and Li<sup>+</sup>

of free Li+

ration that was induced by the addition of a lithium salt. Li+

Hydrophobic ILs, such BmiTFSI, were used to produce composites of carboxylated nitrile rubber (XNBR) that contained hydrotalcite (HT), which exhibited an increase in the ionic conductivity from 10−10 S/cm (neat XNBR) to 10−7 S/cm [58]. Moreover, BmiTFSI did not affect the tensile strength or elongation at break of XNBR significantly, although a plasticizing effect of BmiTFSI was confirmed. The improvement of the ionic conductivity of XNBR/HT composites resulted not only from the ionic conductivity of BmiTFSI and its concentration but was also influenced by the plasticizing effect of the ionic liquid, which increased the mobility of the rubber chain segments of the XNBR that contained BmiTFSI. Additionally, BmiTFSI improved the dispersion of HT in the elastomeric matrix.

In recent years, the use of MWCNTs has increased as a method for producing conductive elastomeric composites [21, 59, 60]. Unfortunately, because of their tendency to agglomerate due to van der Waals interactions between individual nanotubes, a fine, uniform dispersion of MWCNTs and their network formation in the elastomer is a significant challenge from a technological point of view. Different techniques have been applied to disperse MWCNTs in polymer matrices, including functionalization with the use of harmful solvents. In addition to the harmful effect of these volatile organic solvents, they also have several disadvantages, such as destruction of the π-electron network of MWCNTs, resulting in a reduction of their inherent electrical conductivity [61]. To preserve the π-electrons on the surface of MWCNTs and to maintain the inherent electrical conductivity, ILs can be used to non-covalently modify the surface of nanotubes [21, 59]. Subramaniam et al. reported the development of new elastomeric conductors that employ CR rubber and MWCNTs that were modified with ILs [59]. BmiTFSI was used for surface modification of the MWCNTs and the resultant mix was directly admixed into CR without the use of an organic solvent. The Raman spectrum confirmed the interactions between π-electrons of the nanotubes and cations of the BmiTFSI, or the perturbation of π-π stacking of multi-walls of the nanotubes. Therefore, it was concluded that the modified MWCNTs were activated physically, without a chemical impairment of the nanotubes. The use of modified MWCNTs produced CR composites whose electrical conductivity increased with the amount of filler. Moreover, the higher the ratio of MWCNTs to BmiTFSI, the higher was the conductivity of the obtained composites. The increase in the conductivity of the CR composites was attributed to a synergistic effect of electrons and ions as well as to an improved dispersion and formation of a percolating network by the BmiTFSI-modified MWCNTs. The dispersion of MWCNTs in the CR matrix was enhanced with an increasing proportion of BmiTFSI due to a reduction in the intertubular attraction between nanotubes in the presence of the BmiTFSI. The network formation of the exfoliated nanotubes in the elastomer matrix is destroyed with an increase in the strain amplitude in DMA measurements, as confirmed by the Payne effect.

**4. Use of ILs for crosslinking elastomers**

rial was obtained with a much lower Tg

65–67].

Catalytic activity of ILs in interfacial reactions and their dispersing action could be used successfully as elastomer crosslinkers [64] or other components of crosslinking systems [37,

Behera et al. [64] fabricated a novel ionic liquid-crosslinked flexible polyurethane elastomer using a one-pot polymerization method. Tris(2-hydroxyethyl)methylammonium methylsulfate (THMAMS) was used as a crosslinker. FT-IR analysis confirmed the presence of THMAMS in the polyurethane backbone, which significantly suppressed the hydrogen bonding interactions of the obtained polyurethane elastomer. As a result, a highly flexible and tough elastomeric mate-

(TPU) or the elastomer that was crosslinked using a non-ionic crosslinker. Moreover, THMAMScrosslinked polyurethane elastomers exhibited significantly higher tensile strengths and elongation at break, and lower hardness, compared with conventional TPUs or elastomers prepared without THMAMS as the crosslinker. The higher tensile strength of THMAMS-crosslinked elas-

ILs with a suitably designed structure could be successfully applied as accelerators in the sulfur vulcanization of unsaturated elastomers [65–68]. These ILs consist of the 2-mercaptobenzothiazolate anion derived from the traditionally used 2-mercaptobenzothiazole (MBT) vulcanization accelerator and various organic cations, mainly benzalkonium, alkylammonium or phosphonium and alkylimidazolium. All of these are thermally stable at commonly used vulcanization temperatures (150–180°C). Benzalkonium, tetradecyltrihexylphosphonium, or alkylimidazolium 2-mercaptobenzothiazolates were used in the vulcanization of silica-filled NBR as an alternative to the traditionally used MBT accelerator [66]. This resulted in a two- or even three-fold (for benzalkonium salt) reduction in the NBR vulcanization time at 160°C while maintaining the torque increase during vulcanization and a crosslink density slightly higher or comparable to that of the rubber compound crosslinked in the presence of MBT. Therefore, it was concluded that these ILs acted as vulcanization accelerators. Moreover, vulcanizates containing most of these ILs exhibited tensile strengths higher than or similar to that of NBR vulcanized with MBT. Additionally, vulcanizates with ILs exhibited an increased resistance to thermo-oxidative aging. The activity of azolanic ILs and their influence on vulcanization kinetics resulted from not only the catalytic effect of ILs but also the improvement in the degree of dispersion of curatives in the NBR composites. It is also very important that by using salts of MBT as vulcanization accelerators, approximately 30–70% less MBT was introduced into the rubber compound than by using pure MBT. The activity of benzalkonium and alkylammonium 2-mercaptobenzothiazolates as vulcanization accelerators was also studied for SBR composites that were filled with pyrogenic silica [67]. The increase in vulcanization efficiency in the presence of ILs can be attributed not only to the aforementioned factors, such as catalytic activity and the improved degree of curative dispersion in the elastomer, but also to the adsorption of ILs on the silica surface, which reduces the ability of the silica to adsorb curatives and water. Therefore, it is possible that ILs could play roles as both accelerators and shielding agents, increasing the efficiency of vulcanization. An important aspect of IL applications is the improvement in SBR resistance to thermo-oxidative aging

tomers was reported to be due to the inter-ionic interaction between hard segments.

than the linear thermoplastic polyurethane elastomer

Applications of Ionic Liquids in Elastomeric Composites: A Review

http://dx.doi.org/10.5772/intechopen.76978

133

Sabu etal. developed SBR composites that can attenuate electromagnetic radiation. To achieve this, MWCNT surfaces were modified with the 1-benzyl-3-methylimidazolium chloride (BenMICl) and subsequently incorporated in the SBR matrix [60]. The cation-π interaction between the imidazolium cation of the BenMICl and the π electronic surface of the carbon nanotubes that resulted in the reduction of the size of the MWCNT agglomerates after modification, and an improvement of their dispersion in the elastomer matrix, has been reported. An increase in the dielectric constant of SBRs that contain MWCNTs was observed due to the large difference between dielectric permittivity of the elastomer matrix and the nanotubes, which caused the accumulation of charge carriers at the elastomer/carbon nanotubes interphase. Electrical percolation was determined between 3 and 5 wt% MWCNTs. At this concentration of MWCNTs, a sudden and sharp increase in the conductivity was observed, indicating the formation of an electrically conductive three-dimensional continuous network of carbon nanotubes. Moreover, SBR composites that contain MWCNTs that are functionalized with BenMICl exhibited the shielding effect to electromagnetic radiation, which increased with the content of the MWCNTs. After the percolation threshold, the formation of continuous networks of MWCNTs enhanced the mobility of charge carriers; as a result, the attenuation of electromagnetic radiation by absorption was observed. This was reported to be a synergistic effect of BenMICl that is in electrical contact with carbon nanotubes [61]. The shielding effectiveness increased with an increase of the ratio between MWCNTs and the BenMICl, because the ionic liquid provides additional charge carriers that increase the polarizability of the SBR composites. Therefore, the combination of ILs with MWCNTs produced a new soft rubber material that is suitable for the attenuation of electromagnetic radiation. The same authors reported developing highly conducting and mechanically durable SBR composites that contained MWCNTs that were modified with the 1-ethyl-2,3-dimethylimidazolium bis(trifluoromethylsulfonyl)imide [62]. The significant increase in the dielectric constant of SBR composites was attributed to the uniform dispersion of the MWCNTs in the elastomeric matrix, resulting in the formation of micro capacitor networks of carbon nanotubes in the presence of ILs. The cation-π interactions between ILs and MWCNTs were also confirmed.

MWCNTs pretreated with BmiTFSI were also used to produce stretchable conductive polymer nanocomposites (CPCs) of thermoplastic elastomers (polyester-based thermoplastic polyurethanes TPU) [63] that could be used as strain sensors. It was reported that entangled single-walled CNTs or MWCNTs bundles could be exfoliated into much finer bundles by grinding them with BmiTFSI, after which a gel-like material was formed. Physical interactions or the formation of chemical bonds between BmiTFSI-modified MWCNTs and TPU chains enhanced the uniform dispersion of the MWCNTs in the TPU matrix; as a result, the composites were able to withstand high strain as opposed to the TPU without BmiTFSI. The resulting CPCs exhibited a high level of strain (100%) without a noticeable degradation of the conductivity after many stretching/relaxing cycles. Their resistivity was reported to be recoverable, and the strain sensing properties were stable.

#### **4. Use of ILs for crosslinking elastomers**

MWCNTs. The dispersion of MWCNTs in the CR matrix was enhanced with an increasing proportion of BmiTFSI due to a reduction in the intertubular attraction between nanotubes in the presence of the BmiTFSI. The network formation of the exfoliated nanotubes in the elastomer matrix is destroyed with an increase in the strain amplitude in DMA measurements, as

Sabu etal. developed SBR composites that can attenuate electromagnetic radiation. To achieve this, MWCNT surfaces were modified with the 1-benzyl-3-methylimidazolium chloride (BenMICl) and subsequently incorporated in the SBR matrix [60]. The cation-π interaction between the imidazolium cation of the BenMICl and the π electronic surface of the carbon nanotubes that resulted in the reduction of the size of the MWCNT agglomerates after modification, and an improvement of their dispersion in the elastomer matrix, has been reported. An increase in the dielectric constant of SBRs that contain MWCNTs was observed due to the large difference between dielectric permittivity of the elastomer matrix and the nanotubes, which caused the accumulation of charge carriers at the elastomer/carbon nanotubes interphase. Electrical percolation was determined between 3 and 5 wt% MWCNTs. At this concentration of MWCNTs, a sudden and sharp increase in the conductivity was observed, indicating the formation of an electrically conductive three-dimensional continuous network of carbon nanotubes. Moreover, SBR composites that contain MWCNTs that are functionalized with BenMICl exhibited the shielding effect to electromagnetic radiation, which increased with the content of the MWCNTs. After the percolation threshold, the formation of continuous networks of MWCNTs enhanced the mobility of charge carriers; as a result, the attenuation of electromagnetic radiation by absorption was observed. This was reported to be a synergistic effect of BenMICl that is in electrical contact with carbon nanotubes [61]. The shielding effectiveness increased with an increase of the ratio between MWCNTs and the BenMICl, because the ionic liquid provides additional charge carriers that increase the polarizability of the SBR composites. Therefore, the combination of ILs with MWCNTs produced a new soft rubber material that is suitable for the attenuation of electromagnetic radiation. The same authors reported developing highly conducting and mechanically durable SBR composites that contained MWCNTs that were modified with the 1-ethyl-2,3-dimethylimidazolium bis(trifluoromethylsulfonyl)imide [62]. The significant increase in the dielectric constant of SBR composites was attributed to the uniform dispersion of the MWCNTs in the elastomeric matrix, resulting in the formation of micro capacitor networks of carbon nanotubes in the presence of ILs. The cation-π interactions between ILs and MWCNTs were also confirmed.

MWCNTs pretreated with BmiTFSI were also used to produce stretchable conductive polymer nanocomposites (CPCs) of thermoplastic elastomers (polyester-based thermoplastic polyurethanes TPU) [63] that could be used as strain sensors. It was reported that entangled single-walled CNTs or MWCNTs bundles could be exfoliated into much finer bundles by grinding them with BmiTFSI, after which a gel-like material was formed. Physical interactions or the formation of chemical bonds between BmiTFSI-modified MWCNTs and TPU chains enhanced the uniform dispersion of the MWCNTs in the TPU matrix; as a result, the composites were able to withstand high strain as opposed to the TPU without BmiTFSI. The resulting CPCs exhibited a high level of strain (100%) without a noticeable degradation of the conductivity after many stretching/relaxing cycles. Their resistivity was

reported to be recoverable, and the strain sensing properties were stable.

confirmed by the Payne effect.

132 Recent Advances in Ionic Liquids

Catalytic activity of ILs in interfacial reactions and their dispersing action could be used successfully as elastomer crosslinkers [64] or other components of crosslinking systems [37, 65–67].

Behera et al. [64] fabricated a novel ionic liquid-crosslinked flexible polyurethane elastomer using a one-pot polymerization method. Tris(2-hydroxyethyl)methylammonium methylsulfate (THMAMS) was used as a crosslinker. FT-IR analysis confirmed the presence of THMAMS in the polyurethane backbone, which significantly suppressed the hydrogen bonding interactions of the obtained polyurethane elastomer. As a result, a highly flexible and tough elastomeric material was obtained with a much lower Tg than the linear thermoplastic polyurethane elastomer (TPU) or the elastomer that was crosslinked using a non-ionic crosslinker. Moreover, THMAMScrosslinked polyurethane elastomers exhibited significantly higher tensile strengths and elongation at break, and lower hardness, compared with conventional TPUs or elastomers prepared without THMAMS as the crosslinker. The higher tensile strength of THMAMS-crosslinked elastomers was reported to be due to the inter-ionic interaction between hard segments.

ILs with a suitably designed structure could be successfully applied as accelerators in the sulfur vulcanization of unsaturated elastomers [65–68]. These ILs consist of the 2-mercaptobenzothiazolate anion derived from the traditionally used 2-mercaptobenzothiazole (MBT) vulcanization accelerator and various organic cations, mainly benzalkonium, alkylammonium or phosphonium and alkylimidazolium. All of these are thermally stable at commonly used vulcanization temperatures (150–180°C). Benzalkonium, tetradecyltrihexylphosphonium, or alkylimidazolium 2-mercaptobenzothiazolates were used in the vulcanization of silica-filled NBR as an alternative to the traditionally used MBT accelerator [66]. This resulted in a two- or even three-fold (for benzalkonium salt) reduction in the NBR vulcanization time at 160°C while maintaining the torque increase during vulcanization and a crosslink density slightly higher or comparable to that of the rubber compound crosslinked in the presence of MBT. Therefore, it was concluded that these ILs acted as vulcanization accelerators. Moreover, vulcanizates containing most of these ILs exhibited tensile strengths higher than or similar to that of NBR vulcanized with MBT. Additionally, vulcanizates with ILs exhibited an increased resistance to thermo-oxidative aging. The activity of azolanic ILs and their influence on vulcanization kinetics resulted from not only the catalytic effect of ILs but also the improvement in the degree of dispersion of curatives in the NBR composites. It is also very important that by using salts of MBT as vulcanization accelerators, approximately 30–70% less MBT was introduced into the rubber compound than by using pure MBT. The activity of benzalkonium and alkylammonium 2-mercaptobenzothiazolates as vulcanization accelerators was also studied for SBR composites that were filled with pyrogenic silica [67]. The increase in vulcanization efficiency in the presence of ILs can be attributed not only to the aforementioned factors, such as catalytic activity and the improved degree of curative dispersion in the elastomer, but also to the adsorption of ILs on the silica surface, which reduces the ability of the silica to adsorb curatives and water. Therefore, it is possible that ILs could play roles as both accelerators and shielding agents, increasing the efficiency of vulcanization. An important aspect of IL applications is the improvement in SBR resistance to thermo-oxidative aging and long-term UV radiation that resulted from a reduction in the increase in crosslink density due to aging factors. The greatest activity, from this point of view, was exhibited by alkylammonium salts of MBT. ILs can also be applied as components of new systems that activate the sulfur vulcanization of unsaturated elastomers (SBR, NBR, and EPDM). These systems are based on the use of nanosized ZnO in combination with ILs with different cations and anions [38, 69–71]. Obtaining a homogeneous dispersion of zinc oxide nanoparticles in an elastomer is a technological challenge. Selecting the appropriate dispersing agents remains an unresolved issue. ILs could be useful in this role. Moreover, due to catalytic activity in interfacial reactions, ILs could additionally increase the crosslinking efficiency and reduce the time and temperature of vulcanization. For example, commercially available 1-ethyl-3-methylimidazolium or 1-butyl-3-methylimidazolium bromides, chlorides, tetrafluoroborates, and hexafluorophosphates with nanosized ZnO were used to activate the vulcanization of SBR filled with pyrogenic silica [69, 71]. Nanosized ZnO, despite its 60% lower content in the SBR compound than that of micro-ZnO, reduced the vulcanization time by 20 min and increased the crosslink density of the vulcanizates. ILs, especially chlorides and hexafluorophosphates, produced a further reduction in the vulcanization time to 12 min, and additionally increased the number of crosslinks in the elastomer network. Moreover, ILs decreased the onset temperature of the SBR vulcanization process by 20–30°C compared to that of rubber compounds without ionic liquids. The tensile strength of SBR vulcanizates that contain ZnO nanoparticles and alkylimidazolium salts was similar to or higher than the TS value of conventionally crosslinked vulcanizates. Furthermore, an improvement in the resistance of SBR to thermo-oxidative aging and UV radiation was achieved by reducing the intensity of the crosslinking reactions during the aging process. The activity of ILs and their influence on the useful properties of vulcanizates depends on the type of elastomer. In the case of SBR, the most significant effect was obtained with chlorides, whereas for EPDM elastomer, the most active ILs were chlorides and tetrafluoroborates. This could be a result of the superior miscibility of these ILs with SBR and EPDM elastomers, which was also observed during the preparation of rubber compounds using two roll mills.

time with respect to that of the rubber compounds that contained microsized ZnO, and a two-fold reduction compared with compounds that contained nanosized ZnO. The greatest activity in the crosslinking process was produced with didecyldimethylammonium lactate (DDAL), which allowed for the shortest time, lowest temperature, and highest energy effect of vulcanization, thereby demonstrating the high intensity of the vulcanization process. All of the GRAS ILs that were studied improved the tensile strength of vulcanizates. They did not affect the dynamic properties of SBR at temperatures above 25°C or the resistance of SBR to thermo-oxidative aging or long-term UV radiation. A similar relationship was observed with

Applications of Ionic Liquids in Elastomeric Composites: A Review

http://dx.doi.org/10.5772/intechopen.76978

The accelerating effect of ILs on the curing process was also reported by Marzec et al. for XNBR/HT composites that contain 1-butyl-3-methylimidazolium tetrachloroaluminate

higher crosslink densities than the reference without an ionic liquid, and the crosslink density

The effect of the type and concentration of hydrophilic imidazolium ILs, such as 1-ethyl-3-methylimidazolium thiocyanate (EmiSCN) and 1-methyl-3-octylimidazolium chloride (OmiCl), on the curing kinetics of NBR filled with silica was also studied [75]. The torque values during vulcanization of rubber compounds were reported to be inversely proportional to the content of ILs due to their plasticizing action. The accelerating effect of ILs was demonstrated because their inclusion caused a reduction in the scorch time and significantly shortened the curing time of rubber compounds. However, NBR composites with high loadings of both ILs (20 and 30 phr) exhibited reversion during vulcanization. The tensile strength decreased with increasing amounts of both ILs, whereas the elongation at break was the highest for NBRs containing

Apart from the main applications of ILs in elastomer composites described above, there are other interesting reports that concern additional applications of ILs in elastomer science or

Tang et al. [76] utilized the phosphonium ionic liquid octadecyltriphenylphosphonium iodide (ODtppI) in a silica-filled SBR matrix as a novel catalyst for the silanization reaction between silica and bis(3-triethoxysilylpropyl)-tetrasulfide (TESPT), a commonly used silane in the tire industry. The silanization of silica plays a vital role in enhancing the compatibility between silica and a rubber matrix, and hence the improvement of the composites' properties. ODtppI reacted with silanol groups on the silica surface to yield more nucleophilic silanolate anions, which promoted a condensation reaction with the ethoxy groups of TESPT, and as a result, improved the extent of silanization. Consequently, the dispersion of silica in the SBR matrix and the interfacial interaction between silica and rubber chains were improved. Moreover, treatment of the silica surface with ODtppI rendered it more hydrophobic; the silica surface thus

.

) [58]. This ionic liquid considerably shortened the scorch time and optimal vulcani-

exhibited

135

alkylimidazolium, pyrrolidinium, piperidinium, and pyridinium salts [34, 70].

zation time of rubber compounds. Moreover, XNBR/HT composites with BmiAlCl<sup>4</sup>

increased with the concentration of the BmiAlCl<sup>4</sup>

30 phr of ILs, which was attributed to their plasticizing effect.

**5. Additional applications of ILs in elastomeric composites**

(BmiAlCl<sup>4</sup>

technology.

The next area of interest of new systems for activating sulfur vulcanization is related to the application of ILs to the GRAS group (generally regarded as safe). These ILs are generally recognized as environmentally friendly and safe for human health. ILs of interest for this purpose include anions of nonnutritive sweeteners such as saccharinate and acesulfame, or pharmaceutically active anions such as lactate [72–74]. These are currently used as additives in food or pharmaceutical products and are approved by most national health agencies. Moreover, these ILs are thermally stable at vulcanization temperatures (160°C). It was observed that, similar to the ILs discussed above, these salts exhibited catalytic activity during the vulcanization process and improved the degree of dispersion of zinc oxide nanoparticles and filler (silica) in SBR [38]. It can be postulated that the IL/ZnO interactions consist of hydrogen bonding between Zn-OH or Zn-O located on the outside of the zinc oxide crystals and the anion or cation, respectively, of the ionic liquid. Similar interactions were described for Si-OH (Al-OH) or Si-O (Al-O) groups present on HNTs [48]. Hydrogen bonding in IL/ZnO results in reduced interactions between zinc oxide particles, preventing them from agglomerating. The GRAS ILs used in this research produced a 4-fold reduction in the SBR vulcanization time with respect to that of the rubber compounds that contained microsized ZnO, and a two-fold reduction compared with compounds that contained nanosized ZnO. The greatest activity in the crosslinking process was produced with didecyldimethylammonium lactate (DDAL), which allowed for the shortest time, lowest temperature, and highest energy effect of vulcanization, thereby demonstrating the high intensity of the vulcanization process. All of the GRAS ILs that were studied improved the tensile strength of vulcanizates. They did not affect the dynamic properties of SBR at temperatures above 25°C or the resistance of SBR to thermo-oxidative aging or long-term UV radiation. A similar relationship was observed with alkylimidazolium, pyrrolidinium, piperidinium, and pyridinium salts [34, 70].

and long-term UV radiation that resulted from a reduction in the increase in crosslink density due to aging factors. The greatest activity, from this point of view, was exhibited by alkylammonium salts of MBT. ILs can also be applied as components of new systems that activate the sulfur vulcanization of unsaturated elastomers (SBR, NBR, and EPDM). These systems are based on the use of nanosized ZnO in combination with ILs with different cations and anions [38, 69–71]. Obtaining a homogeneous dispersion of zinc oxide nanoparticles in an elastomer is a technological challenge. Selecting the appropriate dispersing agents remains an unresolved issue. ILs could be useful in this role. Moreover, due to catalytic activity in interfacial reactions, ILs could additionally increase the crosslinking efficiency and reduce the time and temperature of vulcanization. For example, commercially available 1-ethyl-3-methylimidazolium or 1-butyl-3-methylimidazolium bromides, chlorides, tetrafluoroborates, and hexafluorophosphates with nanosized ZnO were used to activate the vulcanization of SBR filled with pyrogenic silica [69, 71]. Nanosized ZnO, despite its 60% lower content in the SBR compound than that of micro-ZnO, reduced the vulcanization time by 20 min and increased the crosslink density of the vulcanizates. ILs, especially chlorides and hexafluorophosphates, produced a further reduction in the vulcanization time to 12 min, and additionally increased the number of crosslinks in the elastomer network. Moreover, ILs decreased the onset temperature of the SBR vulcanization process by 20–30°C compared to that of rubber compounds without ionic liquids. The tensile strength of SBR vulcanizates that contain ZnO nanoparticles and alkylimidazolium salts was similar to or higher than the TS value of conventionally crosslinked vulcanizates. Furthermore, an improvement in the resistance of SBR to thermo-oxidative aging and UV radiation was achieved by reducing the intensity of the crosslinking reactions during the aging process. The activity of ILs and their influence on the useful properties of vulcanizates depends on the type of elastomer. In the case of SBR, the most significant effect was obtained with chlorides, whereas for EPDM elastomer, the most active ILs were chlorides and tetrafluoroborates. This could be a result of the superior miscibility of these ILs with SBR and EPDM elastomers, which was also observed during the preparation of rubber compounds

The next area of interest of new systems for activating sulfur vulcanization is related to the application of ILs to the GRAS group (generally regarded as safe). These ILs are generally recognized as environmentally friendly and safe for human health. ILs of interest for this purpose include anions of nonnutritive sweeteners such as saccharinate and acesulfame, or pharmaceutically active anions such as lactate [72–74]. These are currently used as additives in food or pharmaceutical products and are approved by most national health agencies. Moreover, these ILs are thermally stable at vulcanization temperatures (160°C). It was observed that, similar to the ILs discussed above, these salts exhibited catalytic activity during the vulcanization process and improved the degree of dispersion of zinc oxide nanoparticles and filler (silica) in SBR [38]. It can be postulated that the IL/ZnO interactions consist of hydrogen bonding between Zn-OH or Zn-O located on the outside of the zinc oxide crystals and the anion or cation, respectively, of the ionic liquid. Similar interactions were described for Si-OH (Al-OH) or Si-O (Al-O) groups present on HNTs [48]. Hydrogen bonding in IL/ZnO results in reduced interactions between zinc oxide particles, preventing them from agglomerating. The GRAS ILs used in this research produced a 4-fold reduction in the SBR vulcanization

using two roll mills.

134 Recent Advances in Ionic Liquids

The accelerating effect of ILs on the curing process was also reported by Marzec et al. for XNBR/HT composites that contain 1-butyl-3-methylimidazolium tetrachloroaluminate (BmiAlCl<sup>4</sup> ) [58]. This ionic liquid considerably shortened the scorch time and optimal vulcanization time of rubber compounds. Moreover, XNBR/HT composites with BmiAlCl<sup>4</sup> exhibited higher crosslink densities than the reference without an ionic liquid, and the crosslink density increased with the concentration of the BmiAlCl<sup>4</sup> .

The effect of the type and concentration of hydrophilic imidazolium ILs, such as 1-ethyl-3-methylimidazolium thiocyanate (EmiSCN) and 1-methyl-3-octylimidazolium chloride (OmiCl), on the curing kinetics of NBR filled with silica was also studied [75]. The torque values during vulcanization of rubber compounds were reported to be inversely proportional to the content of ILs due to their plasticizing action. The accelerating effect of ILs was demonstrated because their inclusion caused a reduction in the scorch time and significantly shortened the curing time of rubber compounds. However, NBR composites with high loadings of both ILs (20 and 30 phr) exhibited reversion during vulcanization. The tensile strength decreased with increasing amounts of both ILs, whereas the elongation at break was the highest for NBRs containing 30 phr of ILs, which was attributed to their plasticizing effect.

#### **5. Additional applications of ILs in elastomeric composites**

Apart from the main applications of ILs in elastomer composites described above, there are other interesting reports that concern additional applications of ILs in elastomer science or technology.

Tang et al. [76] utilized the phosphonium ionic liquid octadecyltriphenylphosphonium iodide (ODtppI) in a silica-filled SBR matrix as a novel catalyst for the silanization reaction between silica and bis(3-triethoxysilylpropyl)-tetrasulfide (TESPT), a commonly used silane in the tire industry. The silanization of silica plays a vital role in enhancing the compatibility between silica and a rubber matrix, and hence the improvement of the composites' properties. ODtppI reacted with silanol groups on the silica surface to yield more nucleophilic silanolate anions, which promoted a condensation reaction with the ethoxy groups of TESPT, and as a result, improved the extent of silanization. Consequently, the dispersion of silica in the SBR matrix and the interfacial interaction between silica and rubber chains were improved. Moreover, treatment of the silica surface with ODtppI rendered it more hydrophobic; the silica surface thus became more compatible with a nonpolar hydrocarbon rubber matrix. The SBR composites that contained silica functionalized with TESPT in the presence of an ODtppI exhibited shorter optimal vulcanization times and higher crosslink densities compared with SBR filled with pure silica or with TESPT-modified silica without ODtppI. It was confirmed that silanization prevented the adsorption of curing agents onto the silica surface. Additionally, the phosphonium cation of the ODtppI could act as a secondary accelerator, thereby increasing the curing rate and enhancing the crosslink density of the SBR. Moreover, TESPT acts as a sulfur donor that increases the amount of covalent crosslinks in the elastomer network. Finally, the resulting SBR composites exhibited greater tensile strength, abrasion resistance, or decreased energy loss during rolling of the rubber wheel.

**Acknowledgements**

**Conflict of interest**

**Author details**

**References**

C6GC01677D

ie000689g

S.A. for supporting this work.

The authors declare no conflict of interest.

Anna Sowinska and Magdalena Maciejewska\*

2008. ISBN 978-3-527-31239-9

\*Address all correspondence to: magdalena.maciejewska@p.lodz.pl

Springer-Verlag; 1999. pp. 223-236. DOI: 10.1007/s100980050036

Polymer Chemistry. 2015;**6**:4059-4066. DOI: 10.1039/C5PY00423C

Institute of Polymer and Dye Technology, Lodz University of Technology, Lodz, Poland

[1] Wasserscheid P, Welton T, editors. Ionic Liquids in Synthesis. New York: Wiley-VCH;

[2] Holbrey JD, Seddon KR. Ionic liquids. In: Clean Products Processes. 1st ed. Belfast:

[3] Kubisa P. Ionic liquids in the synthesis and modification of polymers. Journal of Polymer Science Part A: Polymer Chemistry. 2005;**43**:4675-4683. DOI: 10.1002/pola.20971

[4] Singha NK, Pramanik NB, Behera PK, Chakrabarty A, Mays JW. Tailor-made thermoreversible functional polymer via RAFT polymerization in an ionic liquid: A remarkably fast polymerization process. Green Chemistry. 2016;**18**:6115-6122. DOI: 10.1039/

[5] Pernak J, Czepukowicz A, Pozniak R. New ionic liquids and their antielectrostatic properties. Industrial & Engineering Chemistry Research. 2001;**40**:2379-2383. DOI: 10.1021/

[6] Yuan C, Guo J, Sia Z, Yan F. Polymerization in ionic liquid-based microemulsions.

[7] Vijayaraghavan R, MacFarlane DR. Living cationic polymerization of styrene in an ionic

liquid. Chemical Communications. 2004;**6**:700-701. DOI: 10.1039/B315100J

The authors wish to acknowledge the French companies TOTAL S.A. and HUTCHINSON

Applications of Ionic Liquids in Elastomeric Composites: A Review

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137

Mouawia et al. [77] developed a process for controlling the metathetic depolymerization of NR in trihexyl-(tetradecyl)phosphonium chloride or N,N′-dioctylimidazolium bromide. This process can produce telechelic polyisoprene oligomers from waste tires. The depolymerization of NR was performed using olefin metathesis reactions in the IL phase under soft conditions, e.g., low temperature, the quantity of ILs, and short reaction times, which promoted the efficient production of telechelic oligomers with low Ru contamination. The catalytic IL phase could be recycled five times, and in each cycle, an efficient and controlled depolymerization of NR to polyisoprene oligomers occurred.

Chen et al. [78] developed highly stretchable, nonvolatile, transparent, and stable ionogels by radical polymerization of acrylic acid in the 1-ethyl-3-methylimidazolium ethylsulfate. This ionogel consisted of a three-dimensional polymer network, which provided an elastic solid form, and the IL enabled electrical conduction. The conductivity of the ionogel was adequate to fabricate electromechanical transducers when used with a dielectric elastomer. Additionally, this ionogel exhibited a low elastic modulus, a large rupturing stretch as well as good mechanical reversibility and negligible degradation after cyclic stretches of large amplitude. These studies confirmed the possibility of using ILs as nonvolatile compliant ionic conductors for dielectric elastomeric transducers.

#### **6. Conclusions**

Owing to their unique properties, such as thermal and chemical stability, low vapor pressure, non-flammability and high ionic conductivity, ILs have attracted much attention for applications not only in thermoplastics or resins composites, but also in elastomer technologies. An analysis of the recent literature reports indicates that ILs are widely used in elastomeric composites as dispersing agents of fillers, conductive additives, crosslinkers or components of the crosslinking system (vulcanization accelerators or activators), catalysts for the silanization reaction, solvents for the depolymerization of natural rubber, or for the production of highly stretchable ionogels. Because the structure of ILs can be designed for specific applications, it can be expected that the use of ILs in elastomeric composites will continue to increase.

#### **Acknowledgements**

became more compatible with a nonpolar hydrocarbon rubber matrix. The SBR composites that contained silica functionalized with TESPT in the presence of an ODtppI exhibited shorter optimal vulcanization times and higher crosslink densities compared with SBR filled with pure silica or with TESPT-modified silica without ODtppI. It was confirmed that silanization prevented the adsorption of curing agents onto the silica surface. Additionally, the phosphonium cation of the ODtppI could act as a secondary accelerator, thereby increasing the curing rate and enhancing the crosslink density of the SBR. Moreover, TESPT acts as a sulfur donor that increases the amount of covalent crosslinks in the elastomer network. Finally, the resulting SBR composites exhibited greater tensile strength, abrasion resistance, or decreased energy loss dur-

Mouawia et al. [77] developed a process for controlling the metathetic depolymerization of NR in trihexyl-(tetradecyl)phosphonium chloride or N,N′-dioctylimidazolium bromide. This process can produce telechelic polyisoprene oligomers from waste tires. The depolymerization of NR was performed using olefin metathesis reactions in the IL phase under soft conditions, e.g., low temperature, the quantity of ILs, and short reaction times, which promoted the efficient production of telechelic oligomers with low Ru contamination. The catalytic IL phase could be recycled five times, and in each cycle, an efficient and controlled depolymerization

Chen et al. [78] developed highly stretchable, nonvolatile, transparent, and stable ionogels by radical polymerization of acrylic acid in the 1-ethyl-3-methylimidazolium ethylsulfate. This ionogel consisted of a three-dimensional polymer network, which provided an elastic solid form, and the IL enabled electrical conduction. The conductivity of the ionogel was adequate to fabricate electromechanical transducers when used with a dielectric elastomer. Additionally, this ionogel exhibited a low elastic modulus, a large rupturing stretch as well as good mechanical reversibility and negligible degradation after cyclic stretches of large amplitude. These studies confirmed the possibility of using ILs as nonvolatile compliant ionic con-

Owing to their unique properties, such as thermal and chemical stability, low vapor pressure, non-flammability and high ionic conductivity, ILs have attracted much attention for applications not only in thermoplastics or resins composites, but also in elastomer technologies. An analysis of the recent literature reports indicates that ILs are widely used in elastomeric composites as dispersing agents of fillers, conductive additives, crosslinkers or components of the crosslinking system (vulcanization accelerators or activators), catalysts for the silanization reaction, solvents for the depolymerization of natural rubber, or for the production of highly stretchable ionogels. Because the structure of ILs can be designed for specific applications, it can be expected that the use of ILs in elastomeric composites will

ing rolling of the rubber wheel.

136 Recent Advances in Ionic Liquids

of NR to polyisoprene oligomers occurred.

ductors for dielectric elastomeric transducers.

**6. Conclusions**

continue to increase.

The authors wish to acknowledge the French companies TOTAL S.A. and HUTCHINSON S.A. for supporting this work.

#### **Conflict of interest**

The authors declare no conflict of interest.

#### **Author details**

Anna Sowinska and Magdalena Maciejewska\*

\*Address all correspondence to: magdalena.maciejewska@p.lodz.pl

Institute of Polymer and Dye Technology, Lodz University of Technology, Lodz, Poland

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**Chapter 8**

**Provisional chapter**

**Metal Extraction with Ionic Liquids-Based Aqueous**

**Metal Extraction with Ionic Liquids-Based Aqueous** 

Although ionic liquids (ILs) have excellent properties, their use as extractants in solvent extraction has not completely overcome the problems encountered when organic solvents are used. In conventional solvent extraction, a hydrophobic IL should be used to establish an IL/water biphasic system to replace the conventional organic solvent with ILs. However, the number of water-immiscible ILs is currently limited, and most contain fluorinated anions which are expensive and environmentally nonbenign. Furthermore, the use of an organic solvent as a diluent agent cannot be avoided because of the very high viscosity of ILs. An IL-based aqueous two-phase system (ATPS) can overcome these drawbacks. This chapter summarizes the use of an IL-based ATPS for the separation of

**Keywords:** aqueous two-phase system, metal extraction, solvent extraction, ionic liquid

Solvent extraction is the most commonly used method to separate metal ions. This technique is performed by mixing the aqueous phase containing metal salt with an organic phase containing an extraction agent (extractant) [1]. The simplicity with which the parameters controlling extraction, such as pH of the aqueous solution, extractants and diluent, can be changed is a major advantage of solvent extraction [2]. However, it is not environmentally friendly as it requires a large volume of organic solvents which are often toxic and/ or flammable. Using ILs, with their excellent properties such as near-zero vapor pressure, good chemical and thermal stability and the tunability of their physicochemical properties by altering the substitutive groups, may overcome the problems of organic solvents [3].

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

© 2018 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.

DOI: 10.5772/intechopen.77286

**Two-Phase System**

**Abstract**

**1. Introduction**

**Two-Phase System**

Pius Dore Ola and Michiaki Matsumoto

Pius Dore Ola and Michiaki Matsumoto

http://dx.doi.org/10.5772/intechopen.77286

Additional information is available at the end of the chapter

Additional information is available at the end of the chapter

metals used in various areas of human life.

#### **Metal Extraction with Ionic Liquids-Based Aqueous Two-Phase System Metal Extraction with Ionic Liquids-Based Aqueous Two-Phase System**

DOI: 10.5772/intechopen.77286

Pius Dore Ola and Michiaki Matsumoto Pius Dore Ola and Michiaki Matsumoto

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

http://dx.doi.org/10.5772/intechopen.77286

#### **Abstract**

Although ionic liquids (ILs) have excellent properties, their use as extractants in solvent extraction has not completely overcome the problems encountered when organic solvents are used. In conventional solvent extraction, a hydrophobic IL should be used to establish an IL/water biphasic system to replace the conventional organic solvent with ILs. However, the number of water-immiscible ILs is currently limited, and most contain fluorinated anions which are expensive and environmentally nonbenign. Furthermore, the use of an organic solvent as a diluent agent cannot be avoided because of the very high viscosity of ILs. An IL-based aqueous two-phase system (ATPS) can overcome these drawbacks. This chapter summarizes the use of an IL-based ATPS for the separation of metals used in various areas of human life.

**Keywords:** aqueous two-phase system, metal extraction, solvent extraction, ionic liquid

#### **1. Introduction**

Solvent extraction is the most commonly used method to separate metal ions. This technique is performed by mixing the aqueous phase containing metal salt with an organic phase containing an extraction agent (extractant) [1]. The simplicity with which the parameters controlling extraction, such as pH of the aqueous solution, extractants and diluent, can be changed is a major advantage of solvent extraction [2]. However, it is not environmentally friendly as it requires a large volume of organic solvents which are often toxic and/ or flammable. Using ILs, with their excellent properties such as near-zero vapor pressure, good chemical and thermal stability and the tunability of their physicochemical properties by altering the substitutive groups, may overcome the problems of organic solvents [3].

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 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.

In conventional solvent extraction, a hydrophobic IL should be used to create an IL/water biphasic system to replace the conventional organic solvent with ILs. However the number of water-immiscible ILs is currently limited, and most contain fluorinated anions which are expensive and environmentally nonbenign, such as PF<sup>6</sup> − , which can decompose to a dangerous HF gas in the presence of water [4]. Furthermore, the use of organic solvent as a diluent cannot be avoided because of the very high viscosity of ILs [5]. On the other hand, the use of ILs as an extractant in conventional solvent extraction has not completely resolved the drawbacks encountered when organic solvents are used as diluents. An IL-based aqueous two-phase system (ATPS) can overcome these disadvantages. This chapter summarizes the use of an IL-based ATPS for the separation of metals. Metals are widely used in many aspects of human life, and their existence in the environment at high concentrations is a cause for concern. For a better understanding of this topic, we will begin with a brief discussion of ILs and ATPSs.

**3. Ionic liquid and aqueous two-phase system**

Ionic liquids (ILs), according to the widely accepted definition, are molten salts under 100°C.The most investigated ILs are comprised of organic cation such as imidazolium, pyridinium, pyrrolidinium, phosphonium and ammonium, and their counterions can be either inorganic (e.g. tetrafluoroborate, hexafluorophosphate, chloride) or organic (e.g. trifluoromethylsulfonate, bis[(trifluoromethyl) sulfonyl]imide) anions [11]. ILs possess many unique physicochemical properties such as low vapor pressure, high thermal stability, high viscosity, good solvation ability, wide electrochemical windows, wide liquid range and tunable polarity [3, 12]. Tuning of these properties by combining the cations and anions makes ILs unique compounds for applications in different areas [13]. With these properties, using ILs instead of organic solvents

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as extractants in solvent extraction can reduce the negative impact on the environment.

The application of ILs in metal ion extraction is very promising. The main problem is their solubility in the aqueous phase, which can decrease the extraction efficiency. To overcome this issue, a hydrophobic IL can be used. However, hydrophobic ILs are partly soluble in the acidic aqueous solutions from which metal ions are extracted, which results in a costly undesirable loss [14]. In addition, hydrophobic ILs are still limited, and most contain fluorinated anions which can be expensive and environmentally nonbenign [5, 15]. Due to the very high viscosity of ILs, the diffusion of solute from the aqueous to the organic phase probably is slow, thus requiring either a longer stirring time [16] or an organic solvent which can be flammable and toxic as diluent agent. An ATPS can resolve the issues associated with the hydrophobic properties and high viscosity limitations. By using the ATPS, a more hydrophilic IL can be employed as the extractant because this technique needs a water-soluble IL. The ILs used in the ATPS are listed in **Table 1**.

The ATPS was accidentally discovered by Martinus Willem Beijerinck (1896) whilst mixing an aqueous solution of starch and gelatin. However, its real application was developed by Per-Åke

TBAB Tetrabutylammonium bromide (b) (c)

(a) (d)

(a) (d)

**Acronym Chemical name Molecule structure**

] Choline bis(trifluoromethylsulfonyl)

] Betainium bis(trifluoromethylsulfonyl)

imide

imide

[Chol][NTf<sup>2</sup>

[Hbet][NTf2

#### **2. Extraction of metal ions**

Metal ion extraction is a hot topic and is important economically and environmentally. Metals are obtained from ores or scraps through metallurgical processes and then manufactured into final products used by humans, either directly or indirectly. The processing of raw materials into final products produces waste containing metal ions. These products are utilized and later discarded as waste. Metal ion extraction is an effort to recover the metal ions both from ores and from waste due to their limited availability in nature. In addition, metal ion extraction from metal-containing waste can reduce the level of environmental pollution. Therefore, an effective, efficient, economic and environmentally friendly method for the recovery of metal ions from ores and waste is an absolute necessity.

A number of processes have been explored for metal ion recovery, such as precipitation [6], reverse osmosis [7], adsorption [8], ion exchange [9] and solvent extraction using organic solvents [10]. Solvent extraction (liquid-liquid extraction) is the most commonly used method for the separation of metal ions [2]. In this technique, the aqueous solution of metal salt is mixed with an organic solvent containing an extraction agent (extractant). The metal ions form a hydrophobic complex with the extractant and migrate to the organic phase. The migration of complexes from the aqueous to the organic phase is driven by the difference of the complexes' affinity towards the aqueous phase and organic phase, as well as the relative solubility of the complexes in both phases. Solvent extraction can be implemented in a continuous mode and is suitable for the processing of high metal feed concentrations. However, the main disadvantage of this method is that it uses a large amount of organic solvents, such as kerosene, toluene, dichloromethane or diethyl ether, which are often toxic and/or flammable and therefore environmentally unfriendly [2]. Not only are their volatility and flammability an issue, but these organic solvents also have a negative impact on human health if their vapors are emitted into the air. ILs can be used to solve the problem faced when organic solvents are used as extractants in conventional liquid-liquid extraction.

#### **3. Ionic liquid and aqueous two-phase system**

In conventional solvent extraction, a hydrophobic IL should be used to create an IL/water biphasic system to replace the conventional organic solvent with ILs. However the number of water-immiscible ILs is currently limited, and most contain fluorinated anions which are

ous HF gas in the presence of water [4]. Furthermore, the use of organic solvent as a diluent cannot be avoided because of the very high viscosity of ILs [5]. On the other hand, the use of ILs as an extractant in conventional solvent extraction has not completely resolved the drawbacks encountered when organic solvents are used as diluents. An IL-based aqueous two-phase system (ATPS) can overcome these disadvantages. This chapter summarizes the use of an IL-based ATPS for the separation of metals. Metals are widely used in many aspects of human life, and their existence in the environment at high concentrations is a cause for concern. For a better understanding of this topic, we will begin with a brief discus-

Metal ion extraction is a hot topic and is important economically and environmentally. Metals are obtained from ores or scraps through metallurgical processes and then manufactured into final products used by humans, either directly or indirectly. The processing of raw materials into final products produces waste containing metal ions. These products are utilized and later discarded as waste. Metal ion extraction is an effort to recover the metal ions both from ores and from waste due to their limited availability in nature. In addition, metal ion extraction from metal-containing waste can reduce the level of environmental pollution. Therefore, an effective, efficient, economic and environmentally friendly method for the recovery of

A number of processes have been explored for metal ion recovery, such as precipitation [6], reverse osmosis [7], adsorption [8], ion exchange [9] and solvent extraction using organic solvents [10]. Solvent extraction (liquid-liquid extraction) is the most commonly used method for the separation of metal ions [2]. In this technique, the aqueous solution of metal salt is mixed with an organic solvent containing an extraction agent (extractant). The metal ions form a hydrophobic complex with the extractant and migrate to the organic phase. The migration of complexes from the aqueous to the organic phase is driven by the difference of the complexes' affinity towards the aqueous phase and organic phase, as well as the relative solubility of the complexes in both phases. Solvent extraction can be implemented in a continuous mode and is suitable for the processing of high metal feed concentrations. However, the main disadvantage of this method is that it uses a large amount of organic solvents, such as kerosene, toluene, dichloromethane or diethyl ether, which are often toxic and/or flammable and therefore environmentally unfriendly [2]. Not only are their volatility and flammability an issue, but these organic solvents also have a negative impact on human health if their vapors are emitted into the air. ILs can be used to solve the problem faced when organic solvents are used as

−

, which can decompose to a danger-

expensive and environmentally nonbenign, such as PF<sup>6</sup>

metal ions from ores and waste is an absolute necessity.

extractants in conventional liquid-liquid extraction.

sion of ILs and ATPSs.

146 Recent Advances in Ionic Liquids

**2. Extraction of metal ions**

Ionic liquids (ILs), according to the widely accepted definition, are molten salts under 100°C.The most investigated ILs are comprised of organic cation such as imidazolium, pyridinium, pyrrolidinium, phosphonium and ammonium, and their counterions can be either inorganic (e.g. tetrafluoroborate, hexafluorophosphate, chloride) or organic (e.g. trifluoromethylsulfonate, bis[(trifluoromethyl) sulfonyl]imide) anions [11]. ILs possess many unique physicochemical properties such as low vapor pressure, high thermal stability, high viscosity, good solvation ability, wide electrochemical windows, wide liquid range and tunable polarity [3, 12]. Tuning of these properties by combining the cations and anions makes ILs unique compounds for applications in different areas [13]. With these properties, using ILs instead of organic solvents as extractants in solvent extraction can reduce the negative impact on the environment.

The application of ILs in metal ion extraction is very promising. The main problem is their solubility in the aqueous phase, which can decrease the extraction efficiency. To overcome this issue, a hydrophobic IL can be used. However, hydrophobic ILs are partly soluble in the acidic aqueous solutions from which metal ions are extracted, which results in a costly undesirable loss [14]. In addition, hydrophobic ILs are still limited, and most contain fluorinated anions which can be expensive and environmentally nonbenign [5, 15]. Due to the very high viscosity of ILs, the diffusion of solute from the aqueous to the organic phase probably is slow, thus requiring either a longer stirring time [16] or an organic solvent which can be flammable and toxic as diluent agent. An ATPS can resolve the issues associated with the hydrophobic properties and high viscosity limitations. By using the ATPS, a more hydrophilic IL can be employed as the extractant because this technique needs a water-soluble IL. The ILs used in the ATPS are listed in **Table 1**.

The ATPS was accidentally discovered by Martinus Willem Beijerinck (1896) whilst mixing an aqueous solution of starch and gelatin. However, its real application was developed by Per-Åke



system is derived by the steric exclusion of large aggregates generated by the interaction between the polymer and water. With a polymer-salt ATPS, the salt absorbs a large amount

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Conventional polymer-based ATPSs have been largely exploited since the 1980s and mainly consist of mixtures of two incompatible polymers or a polymer and a salt that induces salting out. However, the use of polymer-based ATPS for separation is restricted by the similarity in the polarities of the two phases. By using ILs instead of polymer, it is easier to adjust the

In the past few years, research on biphasic systems with ILs has received crucial attention for the development of novel and more efficient separation processes [21]. The phase separation of systems composed of ILs and water can be induced by temperature. These systems are characterized by either an upper critical solution temperature (UCST) or a lower critical solution temperature (LCST). At a temperature higher than the UCST, the organic phase (ILs) is completely miscible with water, and a homogeneous solution is obtained. Application of this method for metal ion separation allows the reaction between the metal complex and ILs to occur at the entire volume of the solution and does not depend on the metal complex' diffusion from the aqueous to the organic phase, which is slower due to the high viscosity of ILs. Therefore, mixture stirring and organic solvents are unnecessary. In other words, the reaction rate is only determined by the kinetics of the chemical reactions [22]. Upon cooling, the phases separate, and the metal complex is extracted to the organic phase. In a system with an LCST, a homogeneous mixture is formed at temperatures below the LCST, and it returns to a two-phase system when its temperature is higher than the LCST. These temperature-dependent phase transitions, known as homogeneous liquid-liquid extraction (HLLE), coalescence extraction or phase-transition extraction, have proven to be highly advantageous in the selective separation of metals [23]. Because HLLE is composed mainly of water and no organic solvent is needed, which is a feature of the classical ATPS, we believe that HLLE can be categorized as a more recent ATPS. In HLLE, a salting-out agent is not needed because phase separation is induced by temperature (temperature-dependent separation). In the next section, we will discuss several articles reporting the extraction of metals using an ATPS com-

As with ATPSs in general, an ATPS composed of ILs and a salting-out agent has a unique phase diagram under a particular set of conditions, such as pH and temperature (**Figure 1**). This phase diagram, known as a binodal curve, is like a fingerprint identifying the potential working area of the ATPS. In this example (**Figure 1**), the total composition of the mixture, the composition of each phase and the critical point are defined as M, D, B and C, respectively. The total mixture compositions above the binodal curve fall into the biphasic regime, whereas mixture compositions below the solubility curve are homogeneous [15]. The salting-out agent

of water, and the similar steric exclusion occurs [19].

posed of ILs with and without a salting-out agent.

**4. IL-based ATPS for metal separation**

**4.1. ATPS with salting-out agent**

polarity phases by changing the substitutive groups of ILs [15, 20].

**Table 1.** Ionic liquids employed in ATPS for separation of metal ions with (a) or without (b) an extra extractant other than IL, with (c) or without (d) a salting-out agent.

Albertsson [17]. Under specific thermodynamic conditions, an ATPS is spontaneously formed by mixing the aqueous solutions of two chemically different hydrophilic polymers or by combining the aqueous solutions of a polymer and an electrolyte, which in turn separate two immiscible aqueous phases in equilibrium-a polymer-enriched top phase and a polymer or an electrolyte-enriched bottom phase [18]. The phase separation of the polymer-polymer system is derived by the steric exclusion of large aggregates generated by the interaction between the polymer and water. With a polymer-salt ATPS, the salt absorbs a large amount of water, and the similar steric exclusion occurs [19].

Conventional polymer-based ATPSs have been largely exploited since the 1980s and mainly consist of mixtures of two incompatible polymers or a polymer and a salt that induces salting out. However, the use of polymer-based ATPS for separation is restricted by the similarity in the polarities of the two phases. By using ILs instead of polymer, it is easier to adjust the polarity phases by changing the substitutive groups of ILs [15, 20].

In the past few years, research on biphasic systems with ILs has received crucial attention for the development of novel and more efficient separation processes [21]. The phase separation of systems composed of ILs and water can be induced by temperature. These systems are characterized by either an upper critical solution temperature (UCST) or a lower critical solution temperature (LCST). At a temperature higher than the UCST, the organic phase (ILs) is completely miscible with water, and a homogeneous solution is obtained. Application of this method for metal ion separation allows the reaction between the metal complex and ILs to occur at the entire volume of the solution and does not depend on the metal complex' diffusion from the aqueous to the organic phase, which is slower due to the high viscosity of ILs. Therefore, mixture stirring and organic solvents are unnecessary. In other words, the reaction rate is only determined by the kinetics of the chemical reactions [22]. Upon cooling, the phases separate, and the metal complex is extracted to the organic phase. In a system with an LCST, a homogeneous mixture is formed at temperatures below the LCST, and it returns to a two-phase system when its temperature is higher than the LCST. These temperature-dependent phase transitions, known as homogeneous liquid-liquid extraction (HLLE), coalescence extraction or phase-transition extraction, have proven to be highly advantageous in the selective separation of metals [23]. Because HLLE is composed mainly of water and no organic solvent is needed, which is a feature of the classical ATPS, we believe that HLLE can be categorized as a more recent ATPS. In HLLE, a salting-out agent is not needed because phase separation is induced by temperature (temperature-dependent separation). In the next section, we will discuss several articles reporting the extraction of metals using an ATPS composed of ILs with and without a salting-out agent.

#### **4. IL-based ATPS for metal separation**

#### **4.1. ATPS with salting-out agent**

**Acronym Chemical name Molecule structure**

(b) (d)

(b) (c)

(b) (d)

(b) (d)

(b) (d)

[P44414][Cl] Tributyltetradecyl phosphonium chloride

> ] 1-Hexyl-3-methyl imidazolium dodecyl sulfonate

> > (Hydrazinocarbonylmethyl) trimethylammonium

bis(trifluoromethylsulfonyl)imide

methoxyethylphosphonium bis(2-

Tri-n-butyl[2-(2-methoxyethoxy) ethyl]phosphonium bis(2-ethylhexyl)

Tri-n-butyl-{2-[2-(2-methoxyethoxy) ethoxy]ethyl}phosphonium bis(2-

ethylhexyl)phosphate

ethylhexyl) phosphate

phosphate

IL, with (c) or without (d) a salting-out agent.

(Hydrazinocarbonylmethyl)pyridinium bis(trifluoromethylsulfonyl)imide

im]Cl 1-Butyl-3-methylimidazolium chloride (b) (c)

**Table 1.** Ionic liquids employed in ATPS for separation of metal ions with (a) or without (b) an extra extractant other than

Albertsson [17]. Under specific thermodynamic conditions, an ATPS is spontaneously formed by mixing the aqueous solutions of two chemically different hydrophilic polymers or by combining the aqueous solutions of a polymer and an electrolyte, which in turn separate two immiscible aqueous phases in equilibrium-a polymer-enriched top phase and a polymer or an electrolyte-enriched bottom phase [18]. The phase separation of the polymer-polymer

] 1,3-Dihexylimidazolium nitrate (b) (d)

[C<sup>6</sup> C1

im][C12SO3

148 Recent Advances in Ionic Liquids

[Nxyzhcm][NTf2

Girard's T cation

[Phcm] [NTf2

[C<sup>4</sup> C1

[C<sup>6</sup> C6 im][NO3

Girard's P cation

]

]

[P444E][DEHP] Tri-n-butyl-2-

As with ATPSs in general, an ATPS composed of ILs and a salting-out agent has a unique phase diagram under a particular set of conditions, such as pH and temperature (**Figure 1**). This phase diagram, known as a binodal curve, is like a fingerprint identifying the potential working area of the ATPS. In this example (**Figure 1**), the total composition of the mixture, the composition of each phase and the critical point are defined as M, D, B and C, respectively. The total mixture compositions above the binodal curve fall into the biphasic regime, whereas mixture compositions below the solubility curve are homogeneous [15]. The salting-out agent

content in the mixture up to 11 wt%. On the other hand, Co (II) extraction improved, with *D* values up to 100 at 11 wt% NaCl. This is because Co (II) easily forms extractable anionic chlo-

In the case of polymer salting out, Zheng et al. reported the use of polyethylene glycol (PEG)

the extraction of gold (III) from an aqueous solution. Gold (III) was quantitatively extracted to the IL phase in the range of pH 1.13–1.90 with extraction percentages of 97.56% (PEG 6000), 76.60% (PEG 4000) and 71.83% (PEG 2000) in 5 min, and thereafter the extraction was not related to vibration time. It was also confirmed that the extractability of gold (III) was derived

An IL-based ATPS with a salting-out agent was also successfully applied to the separation of

natAg target with a α-particle from bulk Ag. The salt-rich phase was predominantly alkaline as a

to the IL cation is greater than Cd(OH)<sup>4</sup>

of bulk Ag was higher. Under optimum conditions, the extraction percentage of both the bulk Ag and NCA <sup>109</sup>Cd in the IL phase reached 87% for bulk Ag and 4% for NCA <sup>109</sup>Cd, respectively, and an overall separation of 91% NCA 109Cd free from bulk Ag was finally achieved by re-

Thus far in our search, an ATPS with a salting-out agent for metal ion separation is still limited. However, the application of ILs in collaboration with a salting-out agent reported in the above studies confirms that an ATPS consisting of ILs and a salting-out agent is a highly

Chemical separation methods aim to simplify and miniaturize sample preparation procedures to consume fewer solvents and reagents and drastically reduce laboratory waste [32]. Unconventional liquid-liquid extractions such as HLLE have been developed with this intent [33]. Probably because HLLE does not require a salting-out agent, this method is more often used in metal ion separation than an ATPS with a salting-out agent. IL is mixed with an aqueous solution of metal ions and heated to a temperature higher than the UCST (for mixtures with a UCST) or cooled to lower than the LCST (for mixtures with an LCST) to obtain a homogeneous solution. The temperature of the solution is then returned to its original temperature allowing for phase separation. This technique sometimes requires an extra extractant in addition to the ILs in order to increase the extraction percentage. In this case, the IL only acts as a separator agent. Choline hexafluoroacetylacetonate and betaine, whose structures are shown in **Figure 2**, have been widely applied as extractants for metal ion separation using the HLLE technique.

fluoroacetylacetonate [Chol][hfac] as specific chelating agents mixed with water that displays thermomorphic behaviour with a UCST of 72°C, has been employed for neodymium (III)

to produce KOH and H3

separated no-carrier-added (NCA) <sup>109</sup>Cd which was produced by irradiation of the

PO4

C1

http://dx.doi.org/10.5772/intechopen.77286

Metal Extraction with Ionic Liquids-Based Aqueous Two-Phase System

. Therefore, both Ag+

N], in combination with choline hexa-

2− which then reacted with the cation of IL. However,

im][C12SO3

]) IL for

151

C1 im]Cl

and Cd2+ were

2−; thus, the extractability

ride complexes, whilst Ni (II) does not, even at high chloride concentrations [29].

by the reaction between the cation of IL with the gold chloro complex anion [30].

extraction twice with 0.1 mL IL to free from the bulk Ag of the salt-rich phase [31].

radioactive elements. An ATPS composed of 1-butyl-3-methylimidazolium chloride, [C<sup>4</sup>

incorporated with 1-hexyl-3-methyl imidazolium dodecyl sulfonate ([C<sup>6</sup>

HPO4

and Cd(OH)<sup>4</sup>

−

efficient and selective means of separating metal ions.

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

**4.2. ATPS without salting-out agent (HLLE)**

−

and K2

HPO4

result of the hydrolysis of K2

the affinity of Ag(OH)<sup>2</sup>

in anionic form, i.e., Ag(OH)2

**Figure 1.** Phase diagram for a hypothetical system composed of polymer + inorganic salt + water (weight fraction units).

in ATPS is employed to separate the more hydrophobic agents from the more hydrophilic one. The general salting-out agents used in combination with ILs include inorganic salt [24], carbohydrates [25] and polymer [26]. However, to the best of our knowledge, carbohydrates have not been used as salting-out agents for metal ion extraction in combination with ILs.

Ammonium sulphate was used as a salting-out agent incorporated with tetrabutylammonium bromide (TBAB) for Cr (VI) separation. The results showed that the extraction of Cr (VI) was influenced by pH due to the existence of HCrO<sup>4</sup> − as the predominant species at pH 5.5. Therefore, Cr (VI) was satisfactorily extracted into the upper phase at pH 5, where its extraction percentage was found to be 93–98% with the following extraction mechanism:

On the other hand, the extraction percentage of Cr (III) was below 1.0% in all cases, indicating that this system was very selective when used for separating Cr (VI) from Cr (III). The (NH<sup>4</sup> ) 2 SO4 increase resulted in an increase in the extraction of Cr (VI) attributed to the progressive increase in the hydrophobicity of upper phase [27]. This system was also used to separate cadmium from cobalt, copper, iron (III) and zinc. The upper phase permitted the complete extraction of Cd2+ over the widest possible ranges of pH (1–10) with recovery in the range of 91–99%. Almost complete extraction of Cd2+ was obtained even though the concentration of Cd2+ in the mixture was far lower (20 μg) than the concentration of Co2+, Cu2+, Fe3+ and Zn2+ (2 mg for each) which coexisted with Cd2+. The proposed method also successfully detected trace Cd in zinc nitrate [28]. In collaboration with tributyl(tetradecyl)phosphonium chloride [P44414][Cl], a certain concentration of sodium chloride also underwent temperature-induced phase separation (TIPS); thus it can be considered a thermomorphic mixture applicable to HLLE (IL-ABS-HLLE).

A system composed of 40% [P44414][Cl] and lower than 11% NaCl showed the LCST and therefore has been used to separate Co (II) and Ni (II). Ni (II) was only poorly extracted to the IL-rich phase (distribution ratio, *D* = 0.1), and extraction did not improve with increased NaCl content in the mixture up to 11 wt%. On the other hand, Co (II) extraction improved, with *D* values up to 100 at 11 wt% NaCl. This is because Co (II) easily forms extractable anionic chloride complexes, whilst Ni (II) does not, even at high chloride concentrations [29].

In the case of polymer salting out, Zheng et al. reported the use of polyethylene glycol (PEG) incorporated with 1-hexyl-3-methyl imidazolium dodecyl sulfonate ([C<sup>6</sup> C1 im][C12SO3 ]) IL for the extraction of gold (III) from an aqueous solution. Gold (III) was quantitatively extracted to the IL phase in the range of pH 1.13–1.90 with extraction percentages of 97.56% (PEG 6000), 76.60% (PEG 4000) and 71.83% (PEG 2000) in 5 min, and thereafter the extraction was not related to vibration time. It was also confirmed that the extractability of gold (III) was derived by the reaction between the cation of IL with the gold chloro complex anion [30].

An IL-based ATPS with a salting-out agent was also successfully applied to the separation of radioactive elements. An ATPS composed of 1-butyl-3-methylimidazolium chloride, [C<sup>4</sup> C1 im]Cl and K2 HPO4 separated no-carrier-added (NCA) <sup>109</sup>Cd which was produced by irradiation of the natAg target with a α-particle from bulk Ag. The salt-rich phase was predominantly alkaline as a result of the hydrolysis of K2 HPO4 to produce KOH and H3 PO4 . Therefore, both Ag+ and Cd2+ were in anionic form, i.e., Ag(OH)2 − and Cd(OH)<sup>4</sup> 2− which then reacted with the cation of IL. However, the affinity of Ag(OH)<sup>2</sup> − to the IL cation is greater than Cd(OH)<sup>4</sup> 2−; thus, the extractability of bulk Ag was higher. Under optimum conditions, the extraction percentage of both the bulk Ag and NCA <sup>109</sup>Cd in the IL phase reached 87% for bulk Ag and 4% for NCA <sup>109</sup>Cd, respectively, and an overall separation of 91% NCA 109Cd free from bulk Ag was finally achieved by reextraction twice with 0.1 mL IL to free from the bulk Ag of the salt-rich phase [31].

Thus far in our search, an ATPS with a salting-out agent for metal ion separation is still limited. However, the application of ILs in collaboration with a salting-out agent reported in the above studies confirms that an ATPS consisting of ILs and a salting-out agent is a highly efficient and selective means of separating metal ions.

#### **4.2. ATPS without salting-out agent (HLLE)**

**Figure 1.** Phase diagram for a hypothetical system composed of polymer + inorganic salt + water (weight fraction units).

in ATPS is employed to separate the more hydrophobic agents from the more hydrophilic one. The general salting-out agents used in combination with ILs include inorganic salt [24], carbohydrates [25] and polymer [26]. However, to the best of our knowledge, carbohydrates have not been used as salting-out agents for metal ion extraction in combination with ILs.

Ammonium sulphate was used as a salting-out agent incorporated with tetrabutylammonium bromide (TBAB) for Cr (VI) separation. The results showed that the extraction of Cr (VI)

Therefore, Cr (VI) was satisfactorily extracted into the upper phase at pH 5, where its extrac-

On the other hand, the extraction percentage of Cr (III) was below 1.0% in all cases, indicating that this system was very selective when used for separating Cr (VI) from Cr (III). The (NH<sup>4</sup>

increase resulted in an increase in the extraction of Cr (VI) attributed to the progressive increase in the hydrophobicity of upper phase [27]. This system was also used to separate cadmium from cobalt, copper, iron (III) and zinc. The upper phase permitted the complete extraction of Cd2+ over the widest possible ranges of pH (1–10) with recovery in the range of 91–99%. Almost complete extraction of Cd2+ was obtained even though the concentration of Cd2+ in the mixture was far lower (20 μg) than the concentration of Co2+, Cu2+, Fe3+ and Zn2+ (2 mg for each) which coexisted with Cd2+. The proposed method also successfully detected trace Cd in zinc nitrate [28]. In collaboration with tributyl(tetradecyl)phosphonium chloride [P44414][Cl], a certain concentration of sodium chloride also underwent temperature-induced phase separation (TIPS); thus it can be considered a thermomorphic mixture applicable to HLLE (IL-ABS-HLLE).

A system composed of 40% [P44414][Cl] and lower than 11% NaCl showed the LCST and therefore has been used to separate Co (II) and Ni (II). Ni (II) was only poorly extracted to the IL-rich phase (distribution ratio, *D* = 0.1), and extraction did not improve with increased NaCl

tion percentage was found to be 93–98% with the following extraction mechanism:

−

as the predominant species at pH 5.5.

) 2 SO4

was influenced by pH due to the existence of HCrO<sup>4</sup>

150 Recent Advances in Ionic Liquids

Chemical separation methods aim to simplify and miniaturize sample preparation procedures to consume fewer solvents and reagents and drastically reduce laboratory waste [32]. Unconventional liquid-liquid extractions such as HLLE have been developed with this intent [33]. Probably because HLLE does not require a salting-out agent, this method is more often used in metal ion separation than an ATPS with a salting-out agent. IL is mixed with an aqueous solution of metal ions and heated to a temperature higher than the UCST (for mixtures with a UCST) or cooled to lower than the LCST (for mixtures with an LCST) to obtain a homogeneous solution. The temperature of the solution is then returned to its original temperature allowing for phase separation. This technique sometimes requires an extra extractant in addition to the ILs in order to increase the extraction percentage. In this case, the IL only acts as a separator agent. Choline hexafluoroacetylacetonate and betaine, whose structures are shown in **Figure 2**, have been widely applied as extractants for metal ion separation using the HLLE technique.

Choline bis(trifluoromethylsulfonyl)imide [Chol][Tf<sup>2</sup> N], in combination with choline hexafluoroacetylacetonate [Chol][hfac] as specific chelating agents mixed with water that displays thermomorphic behaviour with a UCST of 72°C, has been employed for neodymium (III)

ions and Y (III). Furthermore, the ATR-FTIR spectra of the IL phase of the extraction mixture, loaded with varying amounts of scandium, showed that coordination of the IL to the scandium ion occurs via the carboxylic acid function of the cation. Sc(III) was extracted in a ligandto-metal ratio of 3:1, as suggested from the calculations based on the maximum loading [35].

Another mixture having thermomorphic properties with the UCST is demonstrated by the IL

mixture of IL and aqueous solution of metal ions was heated at 60°C for 20 min. The order of distribution coefficients was Cu (II) (28586)> > Ni (II) (767) > Co (II) (36) > Cr (5.4), which reflects the well-known Irving-Williams series [36]. Nitrate IL (1,3-dihexylimidazolium nitrate, [C<sup>6</sup>

a water-saturated sample, but the viscosity was still lower than 50 mPa·s at room temperature, which is far lower than water-saturated trihexyltetradecylphosphonium nitrate (265 mPa·s). Firstrow transition metals were far less efficiently extracted than the rare earths. Among the tested transition metals, no clear trend could be observed; all percentage extractions were around 30%. For the rare earths, as the charge density increased (La3+ < Nd3+ < Sm3+ < Sc3+), the %E decreased.

there was also a significant co-extraction of Co (II), of about 30%. The extraction was not affected by the shaking time or the pH of the aqueous solution. La (III)/Ni (II) pairs were investigated to mimic nickel metal hydride batteries. A synthetic solution of 9.6 g L−1 La (III) and 35.4 g L−1 Ni (II) was prepared. La (III) was fully extracted, and only 15% of Ni(II) was

HLLE with a UCST has been also applied to U (VI) extraction, which has a strong radioactive

an extraction percentage of 60%. The *D* value in both HLLE and conventional liquid-liquid

decrease in *D* value was derived by decreasing the deprotonated carboxyl group due to the

Previous applications of an IL-based HLLE have been a mixture of ILs with water with a UCST. Some ILs that form a homogeneous phase below the critical solution temperature were also applied in metal ion separation. Gras et al. [39] reported the formation of a two-phase system by mixing concentrated hydrochloric acid with tributyl(tetradecyl) phosphonium

]. This mechanism was also confirmed by the fact that the *D* values were almost

N]) [38].

confirmed by the fact that the *D* value clearly decreases with an increase in [H+

seems to follow a linear relationship with a slope of 1.2; it is independent of [NO3

This system was also applied to Sm (III)/Co (II) separation to mimic a SmCo<sup>5</sup>

2+ and [Cr(OH<sup>2</sup>

] was selected as the extractant for extraction of four common metal ions, viz.,

) 6 ]

]) was used in metal ion separation for the first time. Most extraction experiments were

]), in which hcm is hydrazinocarbonylmethyl.

Metal Extraction with Ionic Liquids-Based Aqueous Two-Phase System

3+. To create a homogeneous solution, the

http://dx.doi.org/10.5772/intechopen.77286

, wherein the solubility of IL was much

] and were almost consistent with

takes part in the extraction of U (VI) has been

solution had a slightly higher viscosity than

, the more Sm (III) was extracted, facilitated by the

, nearly 100% of the Sm (III) was extracted. However,

solution. In this system, betainium bis(trifluoromethylsulfonyl)

N] does not only act as a solvent but also as an extractant with

C6 im] 153

magnet. The

]; the plot

− ]. The

analogues of Girard's reagents ([Nxyzhcm][NTf2

2+, [Cu(OH<sup>2</sup>

performed with an aqueous phase containing 6 M NaNO3

extraction decreased with an increase in the initial [HNO3

each other. That the carboxyl group of [Hbet]+

zero using an unfunctionalized IL ([TMPA][Tf2

lower (< 0.5 wt%). IL presaturated with a 6 M NaNO3

) 6 ]

) 6 ]

higher the molar concentration of NaNO3

inner salting-out effect. At 6 M NaNO<sup>3</sup>

co-extracted into the IL phase [37].

nature from the aqueous HNO3

imide ionic liquid [Hbet][Tf2

increase in [H+

[N222hcm][NTf2

2+, [Co(OH<sup>2</sup>

[Ni(OH2 ) 6 ]

[NO3

**Figure 2.** Molecule structures of the extractants applied in HLLE for metal ion extraction.

extraction. The results demonstrate that the heating temperature of mixture greatly influences the extraction percentage. Under heating temperature lower than 45°C, almost no Nd (III) was extracted. The extraction of Nd (III) was less than 50% at 60°C due to decreasing viscosity, and by heating the mixture to 80°C (higher than the UCST), almost 100% of Nd (III) was extracted after shaking for 5 s when 60 mmol kg−1 [Chol][hfac] was contained in the organic phase. Nd (III) was extracted at pH 5.5 and sharply decreased to pH 3–5 because of the protonation of [hfac]<sup>−</sup> anion. Extraction stoichiometry was confirmed by slope analysis, revealing that Nd (III) was extracted by [Chol][hfac] in a 1:4 ratio [34].

Unlike the variation of the temperature below the UCST, the application of different temperatures around and above the UCST had no influence on the extraction equilibrium and the percentage extraction of Nd (III) by a HLLE system composed of betainium bis(trifluoromethylsulfonyl) imide [Hbet][NTf2 ] as the organic phase and betaine as the extractant with a UCST of 55°C. As with the heating temperature, settling temperature tested at 6–50°C had no effect on the extractability. The complex formed during extraction can be described as [Nd2 (bet)3 (H2 O)y ] 3+.

Electrical neutrality can be achieved by bis(trifluoromethylsulfonyl)imide (or chloride or nitrate) counterions [5]. This similar system was also applied to metal ion separation including La3+, Pr3+, Nd3+, Dy3+, Ho3+, Er3+, Y3+, Sc3+, Ga3+, In3+, Mn2+, Ni2+, Cu2+, Zn2+ and Ag+ , Sc3+, Ga3+ and In3+ were almost completely extracted to the organic phase, and the distribution ratios of Mn2+, Ni2+, Zn2+ and Ag+ are very low (almost no extraction). Cu2+ was reasonably well extracted to the IL phase, indicating the possibility of separating Cu2+ from Ni2+, Zn2+ and Mn2+. The distribution ratios for lanthanide ions were about 10 with no significant differences between the different lanthanide ions [16].

Another study of this system specifically focused on scandium (III) extraction from red mud leachates. Individual study on Sc (III) extraction indicates that the distribution of the Sc (III)− [Hbet][NTf2 ] complex is solely controlled by the difference in solubility in the aqueous phase and the IL phase. Red mud consists of the major elements iron, aluminum, titanium, calcium, sodium and silicon and some minor constituents including scandium and other rare-earth elements. Therefore, the tested metal ions include the rare-earth elements Y (III), La (III), Ce (III), Nd (III), and Dy (III) and the major elements Al (III), Fe (III), Ca (II), Ti (IV) and Na (I) and were compared to the extraction of Sc (III). A high affinity for Sc (III) (extraction percentage, %E > 90%) as compared to the other rare-earth metal ions (%E between 4% and 12% at an initial pH of 3, depending on the rare-earth element) was shown by [Hbet][NTf2 ], as well as a very low affinity for almost all the major elements present in red mud, except for Fe(III), which is very similar to the %E of Sc (III). The higher %E for Fe (III) and Sc (III) was derived by the smaller ionic radius, and thus there are higher charge densities than the trivalent lanthanide ions and Y (III). Furthermore, the ATR-FTIR spectra of the IL phase of the extraction mixture, loaded with varying amounts of scandium, showed that coordination of the IL to the scandium ion occurs via the carboxylic acid function of the cation. Sc(III) was extracted in a ligandto-metal ratio of 3:1, as suggested from the calculations based on the maximum loading [35].

Another mixture having thermomorphic properties with the UCST is demonstrated by the IL analogues of Girard's reagents ([Nxyzhcm][NTf2 ]), in which hcm is hydrazinocarbonylmethyl. [N222hcm][NTf2 ] was selected as the extractant for extraction of four common metal ions, viz., [Ni(OH2 ) 6 ] 2+, [Co(OH<sup>2</sup> ) 6 ] 2+, [Cu(OH<sup>2</sup> ) 6 ] 2+ and [Cr(OH<sup>2</sup> ) 6 ] 3+. To create a homogeneous solution, the mixture of IL and aqueous solution of metal ions was heated at 60°C for 20 min. The order of distribution coefficients was Cu (II) (28586)> > Ni (II) (767) > Co (II) (36) > Cr (5.4), which reflects the well-known Irving-Williams series [36]. Nitrate IL (1,3-dihexylimidazolium nitrate, [C<sup>6</sup> C6 im] [NO3 ]) was used in metal ion separation for the first time. Most extraction experiments were performed with an aqueous phase containing 6 M NaNO3 , wherein the solubility of IL was much lower (< 0.5 wt%). IL presaturated with a 6 M NaNO3 solution had a slightly higher viscosity than a water-saturated sample, but the viscosity was still lower than 50 mPa·s at room temperature, which is far lower than water-saturated trihexyltetradecylphosphonium nitrate (265 mPa·s). Firstrow transition metals were far less efficiently extracted than the rare earths. Among the tested transition metals, no clear trend could be observed; all percentage extractions were around 30%. For the rare earths, as the charge density increased (La3+ < Nd3+ < Sm3+ < Sc3+), the %E decreased.

extraction. The results demonstrate that the heating temperature of mixture greatly influences the extraction percentage. Under heating temperature lower than 45°C, almost no Nd (III) was extracted. The extraction of Nd (III) was less than 50% at 60°C due to decreasing viscosity, and by heating the mixture to 80°C (higher than the UCST), almost 100% of Nd (III) was extracted after shaking for 5 s when 60 mmol kg−1 [Chol][hfac] was contained in the organic phase. Nd (III) was extracted at pH 5.5 and sharply decreased to pH 3–5 because of the protonation of

**Figure 2.** Molecule structures of the extractants applied in HLLE for metal ion extraction.

anion. Extraction stoichiometry was confirmed by slope analysis, revealing that Nd

] as the organic phase and betaine as the extractant with a UCST of 55°C. As

are very low (almost no extraction). Cu2+ was reasonably well

(bet)3 (H2 O)y ] 3+.

, Sc3+, Ga3+

], as well as a

Unlike the variation of the temperature below the UCST, the application of different temperatures around and above the UCST had no influence on the extraction equilibrium and the percentage extraction of Nd (III) by a HLLE system composed of betainium bis(trifluoromethylsulfonyl)

with the heating temperature, settling temperature tested at 6–50°C had no effect on the

Electrical neutrality can be achieved by bis(trifluoromethylsulfonyl)imide (or chloride or nitrate) counterions [5]. This similar system was also applied to metal ion separation includ-

and In3+ were almost completely extracted to the organic phase, and the distribution ratios

extracted to the IL phase, indicating the possibility of separating Cu2+ from Ni2+, Zn2+ and Mn2+. The distribution ratios for lanthanide ions were about 10 with no significant differences

Another study of this system specifically focused on scandium (III) extraction from red mud leachates. Individual study on Sc (III) extraction indicates that the distribution of the Sc (III)−

and the IL phase. Red mud consists of the major elements iron, aluminum, titanium, calcium, sodium and silicon and some minor constituents including scandium and other rare-earth elements. Therefore, the tested metal ions include the rare-earth elements Y (III), La (III), Ce (III), Nd (III), and Dy (III) and the major elements Al (III), Fe (III), Ca (II), Ti (IV) and Na (I) and were compared to the extraction of Sc (III). A high affinity for Sc (III) (extraction percentage, %E > 90%) as compared to the other rare-earth metal ions (%E between 4% and 12% at an

very low affinity for almost all the major elements present in red mud, except for Fe(III), which is very similar to the %E of Sc (III). The higher %E for Fe (III) and Sc (III) was derived by the smaller ionic radius, and thus there are higher charge densities than the trivalent lanthanide

initial pH of 3, depending on the rare-earth element) was shown by [Hbet][NTf2

] complex is solely controlled by the difference in solubility in the aqueous phase

extractability. The complex formed during extraction can be described as [Nd2

ing La3+, Pr3+, Nd3+, Dy3+, Ho3+, Er3+, Y3+, Sc3+, Ga3+, In3+, Mn2+, Ni2+, Cu2+, Zn2+ and Ag+

[hfac]<sup>−</sup>

imide [Hbet][NTf2

152 Recent Advances in Ionic Liquids

[Hbet][NTf2

of Mn2+, Ni2+, Zn2+ and Ag+

between the different lanthanide ions [16].

(III) was extracted by [Chol][hfac] in a 1:4 ratio [34].

This system was also applied to Sm (III)/Co (II) separation to mimic a SmCo<sup>5</sup> magnet. The higher the molar concentration of NaNO3 , the more Sm (III) was extracted, facilitated by the inner salting-out effect. At 6 M NaNO<sup>3</sup> , nearly 100% of the Sm (III) was extracted. However, there was also a significant co-extraction of Co (II), of about 30%. The extraction was not affected by the shaking time or the pH of the aqueous solution. La (III)/Ni (II) pairs were investigated to mimic nickel metal hydride batteries. A synthetic solution of 9.6 g L−1 La (III) and 35.4 g L−1 Ni (II) was prepared. La (III) was fully extracted, and only 15% of Ni(II) was co-extracted into the IL phase [37].

HLLE with a UCST has been also applied to U (VI) extraction, which has a strong radioactive nature from the aqueous HNO3 solution. In this system, betainium bis(trifluoromethylsulfonyl) imide ionic liquid [Hbet][Tf2 N] does not only act as a solvent but also as an extractant with an extraction percentage of 60%. The *D* value in both HLLE and conventional liquid-liquid extraction decreased with an increase in the initial [HNO3 ] and were almost consistent with each other. That the carboxyl group of [Hbet]+ takes part in the extraction of U (VI) has been confirmed by the fact that the *D* value clearly decreases with an increase in [H+ ]; the plot seems to follow a linear relationship with a slope of 1.2; it is independent of [NO3 − ]. The decrease in *D* value was derived by decreasing the deprotonated carboxyl group due to the increase in [H+ ]. This mechanism was also confirmed by the fact that the *D* values were almost zero using an unfunctionalized IL ([TMPA][Tf2 N]) [38].

Previous applications of an IL-based HLLE have been a mixture of ILs with water with a UCST. Some ILs that form a homogeneous phase below the critical solution temperature were also applied in metal ion separation. Gras et al. [39] reported the formation of a two-phase system by mixing concentrated hydrochloric acid with tributyl(tetradecyl) phosphonium chloride ([P44414][Cl]), which is water miscible below the critical solution temperature. The use of concentrated hydrochloric acid allows for the simultaneous leaching and extraction of metal ions. This system was used to separate Fe (III), Pt (IV), Ni (II) and Co (II). After stirring in a rotator for 2 hours until complete dissolution, the mixture was then placed in a heating bath for 6 hours until phase separation. Fe (III) and Pt (IV) were close to quantitatively extracted towards the IL-rich phase for all experimental mixtures for temperatures of all system. Co (II) was also efficiently extracted from 75 to 97% in mixtures composed of 10.2% IL, 19% HCl and 70.8% water and 18.6% IL and 15.7% HCl and 65.7% water, respectively, at 50°C, whilst the majority of Ni (II) remained in the HCl-rich phase. The extraction mechanism was derived by the anion exchange between a metalchlorocomplex with Cl− of IL. The selectivity of this system was evaluated towards Co (II) and Mn (II), which is relevant for the recycling of NiMH batteries. Mixtures containing 17.8% IL, 25.9% HCl, 56.3% H<sup>2</sup> O and 18.1% IL, 20.8% HCl and 61.1% H<sup>2</sup> O at 50°C yielded a nearly pure Co(II) in the IL-rich phase, with a separation factor of 400 and 376, respectively. These promising results demonstrate that an acidic aqueous biphasic system can be used simply and efficiently for the critical separation of Co (II) and Ni (II) directly from the HCl leachates of NiMH batteries [39].

**Conflict of interest**

No conflict of interest.

Kyotanabe, Kyoto, Japan

**References**

Pius Dore Ola and Michiaki Matsumoto\*

DOI: 10.1039/c3gc40198g

2012. pp. 305-316

ijms141121353

cej.2012.02.073

DOI: 10.1039/b304400a

\*Address all correspondence to: mmatsumo@mail.doshisha.ac.jp

Department of Chemical Engineering and Materials Science, Doshisha University,

[1] Kislik V. Solvent Extraction, Classical and Novel Approaches. Amsterdam: Elsevier; 2011 [2] Hoogerstraete TV, Wellens S, Verachtert K, Binnemans K. Removal of transition metals from rare earths by solvent extraction with an undiluted phosphonium ionic liquid: Separations relevant to rare-earth magnet recycling. Green Chemistry. 2013;**15**:919-927.

Metal Extraction with Ionic Liquids-Based Aqueous Two-Phase System

http://dx.doi.org/10.5772/intechopen.77286

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[3] Matsumoto M. Ionic liquid-based supported liquid membranes. In: Mohanty K, Purkait MK, editors. Membrane Technology and Applications. Boca Raton, USA: CRC Press;

[4] Swatloski RP, Holbrey JD, Rogers RD. Ionic liquids are not always green: Hydrolysis of 1-butyl-3-methylimidazolium hexafluorophosphate. Green Chemistry. 2003;**5**:361-363.

[5] Hoogerstraete TV, Onghena B, Binnemans K. Homogeneous liquid–liquid extraction of rare earths with the betaine—Betainium bis(trifluoromethylsulfonyl)imide ionic liquid system. International Journal of Molecular Sciences. 2013;**14**:21353-21377. DOI: 10.3390/

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2011;**370**:1-22. DOI: 10.1016/j.memsci.2010.12.036

mental Research and Development. 2014;**4**:41-48

**Author details**

Another system, composed of IL-water with a dependent temperature homogeneous below the critical solution temperature, was obtained by mixing ether-functionalized ILs with bis(2-ethylhexyl)phosphate (DEHP), which is well known as a metal extractant anion with water. The cations investigated were tri-n-butyl-2-methoxyethylphosphonium ([P444E1 ]), trin-butyl[2-(2-methoxyethoxy) ethyl]phosphonium ([P444E2 ]) and tri-n-butyl-{2-[2-(2-methoxyethoxy)ethoxy]ethyl} phosphonium ([P444E3 ]). All synthesized chloride ILs ([P444E]Cl) were fully miscible with water, whilst all synthesized DEHP ILs [P444E][DEHP] displayed LCSTphase behaviour. To determine the distribution ratios, four different aqueous solutions of approximately 5000 ppm of the metal (CoCl<sup>2</sup> , CuCl<sup>2</sup> , NiCl<sup>2</sup> and ZnCl<sup>2</sup> ) at a pH of around 3.5 were mixed with the IL [P444E3 ][DEHP] in the homogeneous phase. The distribution ratio of Co (II), Ni (II), Cu (II) and Zn (II) were 4.4, 19, 34 and 25, respectively, which meets the Irving-Williams series. Indium and some rare earths formed the precipitation with the IL probably due to the strong complexation with the anion of IL [40].

#### **5. Conclusion**

This chapter reviewed several articles discussing the use of an IL-based ATPS for metal ion extraction. In general, an ATPS composed of ILs and a salting-out agent is excellent for metal ion separation because of its efficiency, selectivity and environmental friendliness. Due to the temperature dependence of a mixture comprised of ILs with water, it has been manipulated for metal ion extraction known as homogeneous liquid-liquid extraction (HLLE). With this technique, a salting-out agent is not necessary. In some cases, ILs in HLLE act as both an extractant and separator agent simultaneously. In other cases, the IL acts only as a separator agent. Therefore, an extractant should be added to the system in order to increase the extraction percentage of metal ions. HLLE also showed high efficiency and selectivity in the metal ion extraction. Metal ions can be extracted by both an ATPS and HLLE, including transition metals, rare-earth elements and radioactive substances.

#### **Conflict of interest**

chloride ([P44414][Cl]), which is water miscible below the critical solution temperature. The use of concentrated hydrochloric acid allows for the simultaneous leaching and extraction of metal ions. This system was used to separate Fe (III), Pt (IV), Ni (II) and Co (II). After stirring in a rotator for 2 hours until complete dissolution, the mixture was then placed in a heating bath for 6 hours until phase separation. Fe (III) and Pt (IV) were close to quantitatively extracted towards the IL-rich phase for all experimental mixtures for temperatures of all system. Co (II) was also efficiently extracted from 75 to 97% in mixtures composed of 10.2% IL, 19% HCl and 70.8% water and 18.6% IL and 15.7% HCl and 65.7% water, respectively, at 50°C, whilst the majority of Ni (II) remained in the HCl-rich phase. The extraction mechanism was

of this system was evaluated towards Co (II) and Mn (II), which is relevant for the recycling

factor of 400 and 376, respectively. These promising results demonstrate that an acidic aqueous biphasic system can be used simply and efficiently for the critical separation of Co (II) and

Another system, composed of IL-water with a dependent temperature homogeneous below the critical solution temperature, was obtained by mixing ether-functionalized ILs with bis(2-ethylhexyl)phosphate (DEHP), which is well known as a metal extractant anion with water. The cations investigated were tri-n-butyl-2-methoxyethylphosphonium ([P444E1

fully miscible with water, whilst all synthesized DEHP ILs [P444E][DEHP] displayed LCSTphase behaviour. To determine the distribution ratios, four different aqueous solutions of

Co (II), Ni (II), Cu (II) and Zn (II) were 4.4, 19, 34 and 25, respectively, which meets the Irving-Williams series. Indium and some rare earths formed the precipitation with the IL probably

This chapter reviewed several articles discussing the use of an IL-based ATPS for metal ion extraction. In general, an ATPS composed of ILs and a salting-out agent is excellent for metal ion separation because of its efficiency, selectivity and environmental friendliness. Due to the temperature dependence of a mixture comprised of ILs with water, it has been manipulated for metal ion extraction known as homogeneous liquid-liquid extraction (HLLE). With this technique, a salting-out agent is not necessary. In some cases, ILs in HLLE act as both an extractant and separator agent simultaneously. In other cases, the IL acts only as a separator agent. Therefore, an extractant should be added to the system in order to increase the extraction percentage of metal ions. HLLE also showed high efficiency and selectivity in the metal ion extraction. Metal ions can be extracted by both an ATPS and HLLE, including transition

, CuCl<sup>2</sup>

, NiCl<sup>2</sup>

O at 50°C yielded a nearly pure Co(II) in the IL-rich phase, with a separation

of IL. The selectivity

O and 18.1% IL, 20.8%

) at a pH of around 3.5

]) and tri-n-butyl-{2-[2-(2-methoxye-

]). All synthesized chloride ILs ([P444E]Cl) were

and ZnCl<sup>2</sup>

][DEHP] in the homogeneous phase. The distribution ratio of

]), tri-

derived by the anion exchange between a metalchlorocomplex with Cl−

of NiMH batteries. Mixtures containing 17.8% IL, 25.9% HCl, 56.3% H<sup>2</sup>

Ni (II) directly from the HCl leachates of NiMH batteries [39].

n-butyl[2-(2-methoxyethoxy) ethyl]phosphonium ([P444E2

due to the strong complexation with the anion of IL [40].

metals, rare-earth elements and radioactive substances.

thoxy)ethoxy]ethyl} phosphonium ([P444E3

approximately 5000 ppm of the metal (CoCl<sup>2</sup>

were mixed with the IL [P444E3

**5. Conclusion**

HCl and 61.1% H<sup>2</sup>

154 Recent Advances in Ionic Liquids

No conflict of interest.

#### **Author details**

Pius Dore Ola and Michiaki Matsumoto\*

\*Address all correspondence to: mmatsumo@mail.doshisha.ac.jp

Department of Chemical Engineering and Materials Science, Doshisha University, Kyotanabe, Kyoto, Japan

### **References**


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[C<sup>1</sup> C4 im][Tf2


[34] Onghena B, Jacobs J, Meervelt LV, Binnemans K. Homogeneous liquid-liquid extraction of neodymium(III) by choline hexafluoroacetylacetonate in the ionic liquid choline bis(trifluoromethylsulfonyl)imide. Dalton Transactions. 2014;**43**:11566-11578. DOI: 10.1039/c4dt01340a

**Chapter 9**

**Provisional chapter**

), methane

) molecules are trapped

,

**Kinetic Assessment of Tetramethyl Ammonium**

**Kinetic Assessment of Tetramethyl Ammonium** 

**and Binary Mix Gas Hydrates**

**and Binary Mix Gas Hydrates**

Muhammad Saad Khan, Bavoh B. Cornelius, Bhajan Lal and Mohamad Azmi Bustam

Muhammad Saad Khan, Bavoh B. Cornelius, Bhajan Lal and Mohamad Azmi Bustam

Additional information is available at the end of the chapter

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.77262

, and mixed 50% CO2

**Abstract**

(CH4

CH4

and CH4

**1. Introduction**

**Keywords:** CH4

**Hydroxide (Ionic Liquid) for Carbon Dioxide, Methane**

**Hydroxide (Ionic Liquid) for Carbon Dioxide, Methane** 

This present work highlights the impact of ammonium-based ionic liquid tetramethylam-

time, the initial apparent rate of formation and the total gas consumed are the kinetic parameters used to evaluate the performance of TMAOH as KHI. The results are further compared with commercial KHI (PVP), at higher subcooling condition of 1°C and 1 wt% of all the studied gaseous systems. Furthermore, the KHI performance of TMAOH is also evaluated via the relative inhibition performance (RIP) compared with other ILs for CO2

 hydrates. Results revealed that TMAOH delays the induction time for all the considered systems. The presence of TMAOH also reduced the total gas consumed and

hydrate, ionic liquids, mix gas hydrate, KHI, RIP

), and their binary mixed gas (50–50 mole%) hydrates. The TMAOH (IL) is applied in varying concentrations (0.5, 1, and 2 wt%) at different experimental temperatures, i.e., 1 and 4°C. The kinetic experiments are conducted in a high-pressure reactor equipped with two-bladed impeller, to provide sufficient agitation. The experimental pressures of CO<sup>2</sup>

were 3.50, 8.0, and 6.50 MPa, respectively. Induction

and CO2

monium hydroxide (TMAOH) on the formation kinetics of carbon dioxide (CO2

+ 50% CH4

the initial rate of hydrate formation in most of the studied systems.

hydrate, CO2

Gas hydrates are solid crystals formed when gas (C1

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

© 2018 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,

, C2 , C3 , C4

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

inside the hydrogen-bonded water cages under thermodynamically favourable conditions

DOI: 10.5772/intechopen.77262


#### **Kinetic Assessment of Tetramethyl Ammonium Hydroxide (Ionic Liquid) for Carbon Dioxide, Methane and Binary Mix Gas Hydrates Kinetic Assessment of Tetramethyl Ammonium Hydroxide (Ionic Liquid) for Carbon Dioxide, Methane and Binary Mix Gas Hydrates**

DOI: 10.5772/intechopen.77262

Muhammad Saad Khan, Bavoh B. Cornelius, Bhajan Lal and Mohamad Azmi Bustam Muhammad Saad Khan, Bavoh B. Cornelius, Bhajan Lal and Mohamad Azmi Bustam

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

http://dx.doi.org/10.5772/intechopen.77262

#### **Abstract**

[34] Onghena B, Jacobs J, Meervelt LV, Binnemans K. Homogeneous liquid-liquid extraction of neodymium(III) by choline hexafluoroacetylacetonate in the ionic liquid choline bis(trifluoromethylsulfonyl)imide. Dalton Transactions. 2014;**43**:11566-11578. DOI:

[35] Onghena B, Binnemans K. Recovery of scandium(III) from aqueous solutions by solvent extraction with the functionalized ionic liquid betainium bis(trifluoromethylsulfonyl) imide. Industrial and Engineering Chemistry Research. 2015;**54**:1887-1898. DOI: 10.1021/

[36] Blesic M, Gunaratne HQN, Jacquemin J, Nockemann P, Olejarz S, Seddon KR, Strauss CR. Tunable thermomorphism and applications of ionic liquid analogues of Girard's

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[38] Mori T, Takao K, Sasaki K, Suzuki T, Arai T, Ikeda Y. Homogeneous liquid–liquid extrac-

[39] Gras M, Papaiconomou N, Schaeffer N, Chainet E, Tedjar F, Coutinho JAP, Billard I. Ionic-liquid-based acidic aqueous biphasic systems for simultaneous leaching and extraction of metallic ions. Angewandte Chemie, International Edition. 2018;**57**:1563-

[40] Depuydt D, Liu L, Glorieux C, Dehaena W, Binnemans K. Homogeneous liquid-liquid extraction of metal ions with non-fluorinated bis(2-ethylhexyl)phosphate ionic liquids having a lower critical solution temperature in combination with water. Chemical

imide ionic liquid and recovery of extracted U(VI). Separation and Purification

aqueous solution to betainium bis(trifluoromethylsulfonyl)

reagents. Green Chemistry. 2014;**16**:4115-4121. DOI: 10.1039/c4gc01159g

Technology. 2015;**155**:133-138. DOI: 10.1016/j.seppur.2015.01.045

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10.1039/c4dt01340a

10.1039/c7cc01685a

tion of U(VI) from HNO<sup>3</sup>

1566. DOI: 10.1002/anie.201711068

ie504765v

158 Recent Advances in Ionic Liquids

This present work highlights the impact of ammonium-based ionic liquid tetramethylammonium hydroxide (TMAOH) on the formation kinetics of carbon dioxide (CO2 ), methane (CH4 ), and their binary mixed gas (50–50 mole%) hydrates. The TMAOH (IL) is applied in varying concentrations (0.5, 1, and 2 wt%) at different experimental temperatures, i.e., 1 and 4°C. The kinetic experiments are conducted in a high-pressure reactor equipped with two-bladed impeller, to provide sufficient agitation. The experimental pressures of CO<sup>2</sup> , CH4 , and mixed 50% CO2 + 50% CH4 were 3.50, 8.0, and 6.50 MPa, respectively. Induction time, the initial apparent rate of formation and the total gas consumed are the kinetic parameters used to evaluate the performance of TMAOH as KHI. The results are further compared with commercial KHI (PVP), at higher subcooling condition of 1°C and 1 wt% of all the studied gaseous systems. Furthermore, the KHI performance of TMAOH is also evaluated via the relative inhibition performance (RIP) compared with other ILs for CO2 and CH4 hydrates. Results revealed that TMAOH delays the induction time for all the considered systems. The presence of TMAOH also reduced the total gas consumed and the initial rate of hydrate formation in most of the studied systems.

**Keywords:** CH4 hydrate, CO2 hydrate, ionic liquids, mix gas hydrate, KHI, RIP

#### **1. Introduction**

Gas hydrates are solid crystals formed when gas (C1 , C2 , C3 , C4 and CO2 ) molecules are trapped inside the hydrogen-bonded water cages under thermodynamically favourable conditions

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 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.

(low-temperature and high-pressure conditions) [1, 2]. Typically, three types of gas hydrate structures are known, sI, sII and sH hydrates, depending on the type and size of the encaged gas molecules. For example, pure CH4 and CO2 mostly form sI hydrate, while C2 H6 forms sII hydrate.

and complex process [19]. Commercially employed KHIs are water-soluble polymers such as polyvinylpyrrolidone (PVP) and polyvinyl caprolactam (PVCAP). Karaaslan and Parlaktuna [20] reported that PVP and PEO work as kinetic hydrate inhibitors but have carcinogenic materials which are capable of causing severe health and safety impact on human health. However, the impact of PEO is relatively less compared to PVP [20]. To address these environmental defies of current inhibitors, the quest for greener inhibitors is actively ongoing. Several chemicals such as ILs (mostly imidazolium-based ILs) have been tested for gas hydrate mitigation. ILs could effi-

Kinetic Assessment of Tetramethyl Ammonium Hydroxide (Ionic Liquid) for Carbon Dioxide…

http://dx.doi.org/10.5772/intechopen.77262

161

Xiao and Adidharma [21] initiated the research on ILs as gas hydrate inhibitors and found a dual functional effect through imidazolium-based ILs. Since ILs are salts in a molten state, their ability to exhibit electrostatic interaction and form hydrogen bonds with water molecules enhances their hydrate crystalline surface adsorption ability by retarding hydrate nucleation process [21]. Kim and Kang [22] used a high-pressure cell to evaluate pyrrolidinium- and morpholinium-based ionic liquids such as N-hydroxyethyl-N-methyl pyrrolidinium chloride ([HEMP][Cl]), N-hydroxyethyl-N-methyl pyrrolidinium tetrafluoroborate ([HEMP]

]), N-butyl-N-methyl pyrrolidinium bromide ([BMP][Br]), N-(2-hydroxyethyl)-N-methyl morpholinium bromide ([HEMM][Br]) and N-(2-hydroxyethyl)-N-methyl morpholinium

] acts as better KHI than [BMIM][MS]. However, poor THI behaviour was observed from

] in comparison with [BMIM][MS]. Norland and Kelland [24] and Lee et al. [25]

and PVCAP at 0.1, 1 and 10 wt%. Furthermore, Nazari and Ahmadi [23] studied the effects

elucidate that the type of anion/cation has a significant impact on IL effectiveness. They further suggested that ILs synergistically enhance the kinetic inhibition impact of PVCAP [25]. However, quaternary ammonium salts have also reported as potential KHIs, which open the

An earlier study suggested that quaternary ammonium salts (QAs) can reduce the nucleation time and hydrate growth rate of tetrahydrofuran (THF) hydrates [26]. The inhibition influence of trimethylpentane, trimethyl hexane and trimethyl octane (QAs) is attributed to

be adsorbed on the hydrate crystalline surface [27]. Most QAs were found to exhibit kinetic inhibition and anti-agglomeration tendency. However, prior study from Storr et al. [27] recommended that these chemicals should be tested in mixed gas or natural gas hydrate system to evaluate their suitability for field applications. The best kinetic inhibition performance for the studied QAs on THF hydrates were achieved by tetrapentylammonium bromide (TPAB)

On the other hand, a limited number of studies are found on the effect of ammonium-based ionic liquids (AILs) as gas hydrate inhibitors [4, 12, 31–33]. Few studies have reported that AILs could efficiently induce thermodynamic hydrate inhibition [3, 4, 14, 34]; however, kinetic evaluations of AILs are rarely available in open literature [15]. Tariq et al. [15] previously

more water hydrogen bond cleavage, thus delaying hydrate formation nucleation time. As the

]) that can delay methane hydrate nucleation time than PVP

) groups in a polar moiety form, which shows high tendency to

anion in ILs has a higher inhibition effect as it facilitates

hydrate formation and suggested that [BMIM]

ciently work as dual-functional hydrate inhibitors (THIs alongside KHIs).

] and [BMIM][MS] on the CH4

followed by tetrabutylammonium bromide (TBAB) [28–30].

door for the second-generation IL-based KHIs.

[BF4

[BF4

of [BMIM][BF4

[BMIM][BF4

tetrafluoroborate ([HEMM][BF<sup>4</sup>

the presence of alkyl (CH3

reported that the presence of OH<sup>−</sup>

Gas hydrate formation in the pipelines is considered as one of the most perennial flow assurance problems, which consumes about 70% of flow assurance resources. The accumulation of gas hydrates plugs oil and gas pipelines, disturbing hydrocarbon flow, and causes several safety issues [3–5]. The removal of hydrate plugs from transmission pipeline amount to about 1 million/day shutdown [1]. The hydrate plugging risk increases when producing and transporting high carbon dioxide (CO2 ) content natural gas, as CO2 readily forms hydrates than methane (CH4 ) at the same pressure [6, 7]. High CO2 content natural gas reservoirs are frequently encountered in various areas around the world, such as Central European Pannonian Basin, Colombian Putumayo Basin, Gulf of Thailand, South China Sea, Ibleo platform, Taranaki Basin, Sicily, North Sea South Viking Graben and New Zealand [8]. Malaysia is among the leading natural gas exporters in the world [8]. The J5 and K5 gas fields located in offshore Eastern Malaysia produces about 70–87 mol% CO<sup>2</sup> content natural gas [9]. This significant amount of CO<sup>2</sup> together with the harsh offshore conditions poses various complications for the exploration and transportation of these enormous reserves. One of the core concerns from CO2 -enriched gas systems are their susceptibility to forming gas hydrates in the natural gas production lines [6, 7, 10]. Therefore, in-depth understanding of the mix gas hydrate systems will primarily provide an avenue for safe flow assurance operations when transporting such natural gas systems. Also, this will also provide some fundamental knowledge to design CO2 separation system for the natural gas system together with the storage opportunity for CO2 captured from richer CO2 content natural gas [4, 11, 12].

Four methods can be used to combat gas hydrate formation in flow assurance; this includes removal of water, pressure reduction, thermal heating and chemical injection [4]. In most of the cases, chemical injection is the utmost economical preventive method among the others [8, 13]. These gas hydrate preventive chemicals are known as hydrate inhibitors and are extensively used in oil and gas transmission pipelines. There are three types of gas hydrate inhibitors; thermodynamic hydrate inhibitors (THIs), which mainly shift the hydrate equilibrium curves towards lower-temperature and higher-pressure regions. THIs are mostly required in large concentrations (10–50 wt% of water cuts). Commonly used THIs are methanol and glycols. Although these chemicals are still used in practical field applications till date, they face many drawbacks such as their high operational cost (transportation, storage, injection and pumping quantities and regeneration units) [14, 15].

The drawbacks of above THIs motivated researchers to develop a new kind of hydrate of inhibitors known as low-dosage hydrate inhibitors (LDHIs). This class of inhibitors are typically applied in very less concentration (<2 wt%). LDHIs consist of kinetic hydrate inhibitors (KHIs) and antiagglomerates (AA). Kinetic hydrate inhibitors (KHIs) are primarily engrossed in delaying the hydrate nucleation time and formation growth rate. It remains quite problematic to evaluate the kinetics of hydrate formation [16–18], exclusively in the presence of KHIs, as it is a very dynamic and complex process [19]. Commercially employed KHIs are water-soluble polymers such as polyvinylpyrrolidone (PVP) and polyvinyl caprolactam (PVCAP). Karaaslan and Parlaktuna [20] reported that PVP and PEO work as kinetic hydrate inhibitors but have carcinogenic materials which are capable of causing severe health and safety impact on human health. However, the impact of PEO is relatively less compared to PVP [20]. To address these environmental defies of current inhibitors, the quest for greener inhibitors is actively ongoing. Several chemicals such as ILs (mostly imidazolium-based ILs) have been tested for gas hydrate mitigation. ILs could efficiently work as dual-functional hydrate inhibitors (THIs alongside KHIs).

(low-temperature and high-pressure conditions) [1, 2]. Typically, three types of gas hydrate structures are known, sI, sII and sH hydrates, depending on the type and size of the encaged

Gas hydrate formation in the pipelines is considered as one of the most perennial flow assurance problems, which consumes about 70% of flow assurance resources. The accumulation of gas hydrates plugs oil and gas pipelines, disturbing hydrocarbon flow, and causes several safety issues [3–5]. The removal of hydrate plugs from transmission pipeline amount to about 1 million/day shutdown [1]. The hydrate plugging risk increases when

ral gas reservoirs are frequently encountered in various areas around the world, such as Central European Pannonian Basin, Colombian Putumayo Basin, Gulf of Thailand, South China Sea, Ibleo platform, Taranaki Basin, Sicily, North Sea South Viking Graben and New Zealand [8]. Malaysia is among the leading natural gas exporters in the world [8]. The J5 and K5 gas fields located in offshore Eastern Malaysia produces about 70–87 mol% CO<sup>2</sup>

tions poses various complications for the exploration and transportation of these enormous

to forming gas hydrates in the natural gas production lines [6, 7, 10]. Therefore, in-depth understanding of the mix gas hydrate systems will primarily provide an avenue for safe flow assurance operations when transporting such natural gas systems. Also, this will also

Four methods can be used to combat gas hydrate formation in flow assurance; this includes removal of water, pressure reduction, thermal heating and chemical injection [4]. In most of the cases, chemical injection is the utmost economical preventive method among the others [8, 13]. These gas hydrate preventive chemicals are known as hydrate inhibitors and are extensively used in oil and gas transmission pipelines. There are three types of gas hydrate inhibitors; thermodynamic hydrate inhibitors (THIs), which mainly shift the hydrate equilibrium curves towards lower-temperature and higher-pressure regions. THIs are mostly required in large concentrations (10–50 wt% of water cuts). Commonly used THIs are methanol and glycols. Although these chemicals are still used in practical field applications till date, they face many drawbacks such as their high operational cost (transportation, storage, injection and

The drawbacks of above THIs motivated researchers to develop a new kind of hydrate of inhibitors known as low-dosage hydrate inhibitors (LDHIs). This class of inhibitors are typically applied in very less concentration (<2 wt%). LDHIs consist of kinetic hydrate inhibitors (KHIs) and antiagglomerates (AA). Kinetic hydrate inhibitors (KHIs) are primarily engrossed in delaying the hydrate nucleation time and formation growth rate. It remains quite problematic to evaluate the kinetics of hydrate formation [16–18], exclusively in the presence of KHIs, as it is a very dynamic

mostly form sI hydrate, while C2

) at the same pressure [6, 7]. High CO2

) content natural gas, as CO2

together with the harsh offshore condi-

separation system for the natural gas


captured from richer CO2

H6

forms sII

readily

con-

content

content natu-

and CO2

gas molecules. For example, pure CH4

forms hydrates than methane (CH4

producing and transporting high carbon dioxide (CO2

tent natural gas [9]. This significant amount of CO<sup>2</sup>

provide some fundamental knowledge to design CO2

pumping quantities and regeneration units) [14, 15].

system together with the storage opportunity for CO2

reserves. One of the core concerns from CO2

natural gas [4, 11, 12].

hydrate.

160 Recent Advances in Ionic Liquids

Xiao and Adidharma [21] initiated the research on ILs as gas hydrate inhibitors and found a dual functional effect through imidazolium-based ILs. Since ILs are salts in a molten state, their ability to exhibit electrostatic interaction and form hydrogen bonds with water molecules enhances their hydrate crystalline surface adsorption ability by retarding hydrate nucleation process [21]. Kim and Kang [22] used a high-pressure cell to evaluate pyrrolidinium- and morpholinium-based ionic liquids such as N-hydroxyethyl-N-methyl pyrrolidinium chloride ([HEMP][Cl]), N-hydroxyethyl-N-methyl pyrrolidinium tetrafluoroborate ([HEMP] [BF4 ]), N-butyl-N-methyl pyrrolidinium bromide ([BMP][Br]), N-(2-hydroxyethyl)-N-methyl morpholinium bromide ([HEMM][Br]) and N-(2-hydroxyethyl)-N-methyl morpholinium tetrafluoroborate ([HEMM][BF<sup>4</sup> ]) that can delay methane hydrate nucleation time than PVP and PVCAP at 0.1, 1 and 10 wt%. Furthermore, Nazari and Ahmadi [23] studied the effects of [BMIM][BF4 ] and [BMIM][MS] on the CH4 hydrate formation and suggested that [BMIM] [BF4 ] acts as better KHI than [BMIM][MS]. However, poor THI behaviour was observed from [BMIM][BF4 ] in comparison with [BMIM][MS]. Norland and Kelland [24] and Lee et al. [25] elucidate that the type of anion/cation has a significant impact on IL effectiveness. They further suggested that ILs synergistically enhance the kinetic inhibition impact of PVCAP [25]. However, quaternary ammonium salts have also reported as potential KHIs, which open the door for the second-generation IL-based KHIs.

An earlier study suggested that quaternary ammonium salts (QAs) can reduce the nucleation time and hydrate growth rate of tetrahydrofuran (THF) hydrates [26]. The inhibition influence of trimethylpentane, trimethyl hexane and trimethyl octane (QAs) is attributed to the presence of alkyl (CH3 ) groups in a polar moiety form, which shows high tendency to be adsorbed on the hydrate crystalline surface [27]. Most QAs were found to exhibit kinetic inhibition and anti-agglomeration tendency. However, prior study from Storr et al. [27] recommended that these chemicals should be tested in mixed gas or natural gas hydrate system to evaluate their suitability for field applications. The best kinetic inhibition performance for the studied QAs on THF hydrates were achieved by tetrapentylammonium bromide (TPAB) followed by tetrabutylammonium bromide (TBAB) [28–30].

On the other hand, a limited number of studies are found on the effect of ammonium-based ionic liquids (AILs) as gas hydrate inhibitors [4, 12, 31–33]. Few studies have reported that AILs could efficiently induce thermodynamic hydrate inhibition [3, 4, 14, 34]; however, kinetic evaluations of AILs are rarely available in open literature [15]. Tariq et al. [15] previously reported that the presence of OH<sup>−</sup> anion in ILs has a higher inhibition effect as it facilitates more water hydrogen bond cleavage, thus delaying hydrate formation nucleation time. As the search for useful IL hydrate inhibitors is still ongoing, in our recent works [3, 4], tetramethylammonium hydroxide (TMAOH) was reported as an effective THI for both CH<sup>4</sup> and CO2 gases with average suppression temperature (∆Ŧ) of 1.53 and 2.23*°*C, respectively. However, its ability to delay hydrate formation kinetics has not been investigated in the open literature.

Therefore, herein, the kinetic influence of TMAOH on CO<sup>2</sup> , CH4 and binary mixed gas hydrate (50–50 CO2 + CH4 ) formation at different concentrations (0.5, 1 and 2 wt%) is comprehensively evaluated and reported for the first time. Induction time, moles consumption and initial apparent rate of hydrate formation are the kinetic parameters that are used to assess the performance of TMAOH at different subcooling experimental temperatures of 1 and 4°C. The obtained results for all gaseous systems are further compared with PVP (commercial KHI) at 1 wt% and 1°C (since the best and commercially applicable concentration of PVP is 1 wt%, and 1°C gives the highest subcooling conditions in the study.) conditions. Also, the performance evaluation of TMAOH via RIP with other ILs is reported. Also, an attempt is made to describe the TMAOH kinetic inhibition mechanism in the presence of the studied hydrate systems.

#### **2. Methodology**

#### **2.1. Materials**

**Table 1** shows the materials used in this study. Only deionized water is used for all the sample preparations during the experiments. The sample concentration measurements are performed using a gravimetrical analytical balance.

#### **2.2. Details of experimental equipment**

A high-pressure stainless steel cell [3, 12, 32] with an internal volume of 650 mL, alongside a maximum working pressure of 30 MPa, is used in this study. The cell is immersed in a water bath to regulate the temperature in the cell at desired conditions during experimentations. Two thermocouples are used to measure the temperature inside the cell at an accuracy of ±0.01°C. To achieve appropriate agitation in the liquid phase, two-bladed pitch impeller stirrer is positioned in the equipment with a constant speed of 400 rpm (optimum speed) in all experimentations. The pressure inside the cell is noticed via a pressure transducer with an accuracy of ±0.001 MPa.

system temperature to the experimentally desired subcooling temperature of 1 or 4*°*C). The

O Deionized

**No Chemical name Symbol Purity Chemical structure**

**1** Methane CH4 99.99 mole %

**2** Carbon dioxide CO2 99.95 mole %

**3** Mixed gas CO2 + CH4 50.009–49.991

**5** Polyvinylpyrrolidone PVP 99 wt%

stant at 1 and 4*°*C for all studied systems and concentrations. All the experiments are repeated for three times, and averages are reported. The kinetic inhibition influence of TMAOH is assessed within the range of 0.5–2 wt%, similar to the typical industrial application of KHIs.

In all experiments, the hydrate formation is observed through a sudden pressure drop in the pressure-time plot recorded by the data recording system. The experiments are considered complete after observing a constant pressure in the cell at each experimental temperature for about 3–5 hours. A typical pressure and temperature verse time profile during hydrate formation is illustrated in **Figure 1**. In **Figure 1**, the general hydrate formation process has three major stages: nucleation, crystal/hydrate growth and complete hydrate formation due to mass transfer limitation. The objective of effective KHIs is to increase the hydrate nucleation time (induction time) and at the same time reduce the hydrate crystal growth rate and the total amount of gas consumed into hydrates [35]. Hence, the kinetic inhibition influence

hydrate systems, respectively. The experimental temperatures are kept con-

mole %

Kinetic Assessment of Tetramethyl Ammonium Hydroxide (Ionic Liquid) for Carbon Dioxide…

solution

TMAOH 25 wt% aqueous

, CH4

—

http://dx.doi.org/10.5772/intechopen.77262

163

and binary mixed gas

experimental pressures are set at 3.50, 8.00 and 6.50 MPa for CO2

of TMAOH is evaluated based on these three parameters.

50–50 CH4 + CO2

**2.4. Kinetic measurement parameter**

**6** Water H2

**Table 1.** Materials used for kinetic study of gas hydrates.

**4** Tetramethylammonium hydroxide

#### **2.3. Kinetic measurement procedure**

Before each experimental run, the cell is meticulously cleaned with distilled water, and then a 100 mL liquid phase sample (with or without TMAOH) is loaded into the cell. The cell is then introduced into the water bath, and the system is put under vacuum for about an hour to ensure there are no traces of the air in the cell. The desired gas is then pressurized into the cell to the desired experimental operating pressures; after that, the system is allowed to stabilize to the desired initial experimental pressure and temperature conditions by leaving it for about an hour. The mechanical stirrer is turned on at 400 rpm, and data logging system is started simultaneously with the commencement of the experiment (i.e. by reducing the


**Table 1.** Materials used for kinetic study of gas hydrates.

search for useful IL hydrate inhibitors is still ongoing, in our recent works [3, 4], tetrameth-

gases with average suppression temperature (∆Ŧ) of 1.53 and 2.23*°*C, respectively. However, its ability to delay hydrate formation kinetics has not been investigated in the open literature.

sively evaluated and reported for the first time. Induction time, moles consumption and initial apparent rate of hydrate formation are the kinetic parameters that are used to assess the performance of TMAOH at different subcooling experimental temperatures of 1 and 4°C. The obtained results for all gaseous systems are further compared with PVP (commercial KHI) at 1 wt% and 1°C (since the best and commercially applicable concentration of PVP is 1 wt%, and 1°C gives the highest subcooling conditions in the study.) conditions. Also, the performance evaluation of TMAOH via RIP with other ILs is reported. Also, an attempt is made to describe the TMAOH kinetic inhibition mechanism in the presence of the studied hydrate systems.

**Table 1** shows the materials used in this study. Only deionized water is used for all the sample preparations during the experiments. The sample concentration measurements are per-

A high-pressure stainless steel cell [3, 12, 32] with an internal volume of 650 mL, alongside a maximum working pressure of 30 MPa, is used in this study. The cell is immersed in a water bath to regulate the temperature in the cell at desired conditions during experimentations. Two thermocouples are used to measure the temperature inside the cell at an accuracy of ±0.01°C. To achieve appropriate agitation in the liquid phase, two-bladed pitch impeller stirrer is positioned in the equipment with a constant speed of 400 rpm (optimum speed) in all experimentations. The pressure inside the cell is noticed via a pressure transducer with an

Before each experimental run, the cell is meticulously cleaned with distilled water, and then a 100 mL liquid phase sample (with or without TMAOH) is loaded into the cell. The cell is then introduced into the water bath, and the system is put under vacuum for about an hour to ensure there are no traces of the air in the cell. The desired gas is then pressurized into the cell to the desired experimental operating pressures; after that, the system is allowed to stabilize to the desired initial experimental pressure and temperature conditions by leaving it for about an hour. The mechanical stirrer is turned on at 400 rpm, and data logging system is started simultaneously with the commencement of the experiment (i.e. by reducing the

, CH4

) formation at different concentrations (0.5, 1 and 2 wt%) is comprehen-

and CO2

and binary mixed gas hydrate

ylammonium hydroxide (TMAOH) was reported as an effective THI for both CH<sup>4</sup>

Therefore, herein, the kinetic influence of TMAOH on CO<sup>2</sup>

formed using a gravimetrical analytical balance.

**2.2. Details of experimental equipment**

(50–50 CO2 + CH4

162 Recent Advances in Ionic Liquids

**2. Methodology**

accuracy of ±0.001 MPa.

**2.3. Kinetic measurement procedure**

**2.1. Materials**

system temperature to the experimentally desired subcooling temperature of 1 or 4*°*C). The experimental pressures are set at 3.50, 8.00 and 6.50 MPa for CO2 , CH4 and binary mixed gas 50–50 CH4 + CO2 hydrate systems, respectively. The experimental temperatures are kept constant at 1 and 4*°*C for all studied systems and concentrations. All the experiments are repeated for three times, and averages are reported. The kinetic inhibition influence of TMAOH is assessed within the range of 0.5–2 wt%, similar to the typical industrial application of KHIs.

#### **2.4. Kinetic measurement parameter**

In all experiments, the hydrate formation is observed through a sudden pressure drop in the pressure-time plot recorded by the data recording system. The experiments are considered complete after observing a constant pressure in the cell at each experimental temperature for about 3–5 hours. A typical pressure and temperature verse time profile during hydrate formation is illustrated in **Figure 1**. In **Figure 1**, the general hydrate formation process has three major stages: nucleation, crystal/hydrate growth and complete hydrate formation due to mass transfer limitation. The objective of effective KHIs is to increase the hydrate nucleation time (induction time) and at the same time reduce the hydrate crystal growth rate and the total amount of gas consumed into hydrates [35]. Hence, the kinetic inhibition influence of TMAOH is evaluated based on these three parameters.

**Figure 1.** Pressure and temperature vs. time relationship (50–50 mixed gas hydrate).

#### *2.4.1. Induction time measurement*

The induction time is the most significant indicator to assess initiation of gas hydrate crystallization and growth. It is the time elapse for the occurrence of visible hydrate crystals of the critically stable-sized hydrate nucleus. Longer induction time than the fluid retention time would result in hydrate-free transportation for hydrocarbons in pipelines [36]. However, induction time is a probabilistic phenomenon which depends upon the heterogeneous nucleation parameters [2]. The nucleation rate could influence by the numerous factors, such as the existence of particles and impurities in the sample, the roughness of the cell wall and the presence of driving force. Thus, diverse experimental approaches would apparently provide different outcomes. For comparison of measurements of the induction time of hydrate inhibitors, the same apparatus is highly recommended/appropriate, and experimental method should is employed [15]. The induction time is measured in this study from the pressure–time data plotted in **Figure 1** as described in the literature [37, 38] as

$$t\_i = t\_s - t\_h \tag{1}$$

*2.4.2. Initial apparent rate of hydrate formation*

where *k* is the initial apparent rate of any studied gas, *n0*

hydrate formation can be given by

*ns*

consumed gas.

*2.4.3. Moles of gas consumed*

Δ *ngas* = \_\_

**3. Results and discussion**

growth stages.

The initial apparent rate of hydrate formation determines the rate of hydrate crystallization after the hydrate nucleation [39]. The initial apparent rate is measured with regard to the rate at which the initial moles of gas is consumed into hydrate formation, thereby defining fast hydrate crystal growth that takes place [40]. In these experiments, the initial rate of hydrate formations is accounted for initial 10 min of hydrate formations. The initial apparent rate of

Kinetic Assessment of Tetramethyl Ammonium Hydroxide (Ionic Liquid) for Carbon Dioxide…

*dn*/*dt* = *k*(*n*<sup>0</sup> − *ns*) (2)

 is the initial moles of consumed gas. The Peng-Robinson equation is employed for calculation of compressibility factor *z*, and the real gas equation is also used to calculate the mole of

Moles of gas consumed during hydrate formation determine the dissolved gas in hydrate phase which could form hydrate plug in the subsea condition. The total amount of consumed

where *V, R, P* and *T* denote the system gas phase volume, universal gas constant, pressure and temperature, respectively. *∆ngas* denotes the moles of gas consumed, *z* is the compressibility factor of the gas determined from the Peng-Robinson equation of state and the subscripts '*<sup>0</sup>*

denote the number of n of moles of gas at the time, zero and time of complete hydrate formation. In the mixed gas hydrate system, the final gas composition in the gas phase is different from the original mix gas composition due to the guest cage occupancy ratio of the mix gas composition. A gas chromatograph (PerkinElmer) is used to accurately calculate the final composition of the mixed gas in the gas phase after complete hydrate formation. The gas chromatograph results are employed to calculate the moles of mixed gas in the gas phase after hydrate completion (denoted as '*f*' in Eq. (3)). Also, the gas chromatograph values indicated that gas composition is more in the hydrate phase in the presence and absence of TMAOH.

The kinetic inhibition effect of TMAOH on the hydrate induction time, formation growth rate and total gas consumption on the hydrate systems is assessed in this work. This is to efficiently evaluate the kinetic inhibition of TMAOH on the complete hydrate formation and

gas for complete hydrate formation is calculated by applying the real gas law [38, 41]:

*V <sup>R</sup>*[( \_\_\_*P zT*)<sup>0</sup> − ( \_\_\_*P*

is the mole of gas at initial stage 0 and

http://dx.doi.org/10.5772/intechopen.77262

165

*zT*)*f*] (3)

' and '*<sup>f</sup>* '

where ts is the time taken for the system pressure to decrease the experimental pressure and t <sup>h</sup> and ti is the actual induction time for hydrate formation (**Figure 1**). Usually, the induction time is recognized by the point at which there is a drastic drop in the reactor pressure accompanied by a corresponding sudden spike in the reactor temperature, thus indicating the onset of hydrate formation (see **Figure 1** at point th).

#### *2.4.2. Initial apparent rate of hydrate formation*

The initial apparent rate of hydrate formation determines the rate of hydrate crystallization after the hydrate nucleation [39]. The initial apparent rate is measured with regard to the rate at which the initial moles of gas is consumed into hydrate formation, thereby defining fast hydrate crystal growth that takes place [40]. In these experiments, the initial rate of hydrate formations is accounted for initial 10 min of hydrate formations. The initial apparent rate of hydrate formation can be given by

$$dn/dt = k \langle n\_0 - n\_s \rangle \tag{2}$$

where *k* is the initial apparent rate of any studied gas, *n0* is the mole of gas at initial stage 0 and *ns* is the initial moles of consumed gas. The Peng-Robinson equation is employed for calculation of compressibility factor *z*, and the real gas equation is also used to calculate the mole of consumed gas.

#### *2.4.3. Moles of gas consumed*

*2.4.1. Induction time measurement*

164 Recent Advances in Ionic Liquids

plotted in **Figure 1** as described in the literature [37, 38] as

**Figure 1.** Pressure and temperature vs. time relationship (50–50 mixed gas hydrate).

*t*

of hydrate formation (see **Figure 1** at point th).

where ts

t <sup>h</sup> and ti

The induction time is the most significant indicator to assess initiation of gas hydrate crystallization and growth. It is the time elapse for the occurrence of visible hydrate crystals of the critically stable-sized hydrate nucleus. Longer induction time than the fluid retention time would result in hydrate-free transportation for hydrocarbons in pipelines [36]. However, induction time is a probabilistic phenomenon which depends upon the heterogeneous nucleation parameters [2]. The nucleation rate could influence by the numerous factors, such as the existence of particles and impurities in the sample, the roughness of the cell wall and the presence of driving force. Thus, diverse experimental approaches would apparently provide different outcomes. For comparison of measurements of the induction time of hydrate inhibitors, the same apparatus is highly recommended/appropriate, and experimental method should is employed [15]. The induction time is measured in this study from the pressure–time data

> *<sup>i</sup>* = *t <sup>s</sup>* − *t*

time is recognized by the point at which there is a drastic drop in the reactor pressure accompanied by a corresponding sudden spike in the reactor temperature, thus indicating the onset

is the time taken for the system pressure to decrease the experimental pressure and

is the actual induction time for hydrate formation (**Figure 1**). Usually, the induction

*<sup>h</sup>* (1)

Moles of gas consumed during hydrate formation determine the dissolved gas in hydrate phase which could form hydrate plug in the subsea condition. The total amount of consumed gas for complete hydrate formation is calculated by applying the real gas law [38, 41]:

$$
\Delta n\_{gas} = \frac{V}{R} \left[ \left( \frac{P}{zT} \right)\_0 - \left( \frac{P}{zT} \right)\_f \right] \tag{3}
$$

where *V, R, P* and *T* denote the system gas phase volume, universal gas constant, pressure and temperature, respectively. *∆ngas* denotes the moles of gas consumed, *z* is the compressibility factor of the gas determined from the Peng-Robinson equation of state and the subscripts '*<sup>0</sup>* ' and '*<sup>f</sup>* ' denote the number of n of moles of gas at the time, zero and time of complete hydrate formation.

In the mixed gas hydrate system, the final gas composition in the gas phase is different from the original mix gas composition due to the guest cage occupancy ratio of the mix gas composition. A gas chromatograph (PerkinElmer) is used to accurately calculate the final composition of the mixed gas in the gas phase after complete hydrate formation. The gas chromatograph results are employed to calculate the moles of mixed gas in the gas phase after hydrate completion (denoted as '*f*' in Eq. (3)). Also, the gas chromatograph values indicated that gas composition is more in the hydrate phase in the presence and absence of TMAOH.

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

The kinetic inhibition effect of TMAOH on the hydrate induction time, formation growth rate and total gas consumption on the hydrate systems is assessed in this work. This is to efficiently evaluate the kinetic inhibition of TMAOH on the complete hydrate formation and growth stages.

#### **3.1. Effect of TMAOH on the kinetics of CO<sup>2</sup> hydrates**

The experimental pressure for CO2 hydrates has been fixed at 3.50 MPa with operating temperatures of 1 and 4*°*C (subcooling of 8.32 and 5.32*°*C). These conditions are selected to provide sufficient driving force for hydrate formation as typically encountered in offshore operations of oil and gas production. The effect of various concentrations (0.5, 1 and 2 wt %) of TMAOH on the induction time of CO2 hydrate at different experimental temperatures (1 and 4°C) is presented in **Figure 2(a)** and **(b)**, respectively. Results reveal that TMAOH could delay the growth of CO2 hydrates at almost all studied systems by hindering its nucleation process. In the absence of TMAOH, the induction time of water is observed as 14.33 min at 1*°*C (see **Figure 2(a)**), while 4*°*C condition showed even lesser induction time of 12.35 min for pure water as evident in **Figure 2(b)**. The presence of 0.5 wt% TMAOH shows a slight improvement in hydrate nucleation time (induction time) with the base sample (pure water) for both 1 and 4*°*C, which are 23.2 and 21.45%, respectively. Upon increasing the concentration to 1 wt%, the induction time of pure water is improved up to 32.5 and 78.1% at 1 and 4*°*C, respectively, indicating an inhibition ability of TMAOH at the studied subcooling conditions. However, the inhibition impact at 4*°*C is relatively higher compared to 1*°*C, primarily due to the significant driving force existed at 1*°*C (8.32*°*C). Further increase in TMAOH concentration to 2 wt% resulted in a negligible delay of CO2 hydrate induction time compared to the 1 wt% sample. Thus, the optimum TMAOH concentration to delay CO2 hydrate nucleation is at 1 wt% as shown in **Figure 2**. For comparison purposes, the induction time data is also equated with PVP at 1*°*C condition in **Figure 2(a)**. The comparison results suggest that PVP still possess better hydrate holding efficacy than studied aqueous TMAOH solution perhaps due to the enhanced polymer linkage on the liquid-gas surface compared to TMAOH.

The RIP values determine the kinetic inhibitory efficacy of the KHIs, as the RIP>0 corresponds to the better inhibitory performance of KHI. The RIP value of 1 wt% TMAOH concentration at

system. The RIP results are further compared with PVP and some reported ILs, namely,

RIP results further enlighten the kinetic inhibition performance of TMAOH. In fact, TMAOH

lium-based IL. However, [EMIM][Cl] provides much better inhibition performance due to the

TMAOH concentrations at 1 and 4*°*C, respectively. It is found that the rate of hydrate formation is inhibited in the presence of TMAOH compared with the pure water sample at all studied con-

temperature than 1*°*C, especially at 0.5 wt% system. This is attributed to the higher driving force existed at 1*°*C [15, 21]. At both 1 and 4*°*C, the samples with 0.5 wt% TMAOH concentra-

0.00065 min−1, respectively. Unlike the induction time, whose inhibition impact is enhanced with

with increasing TMAOH concentrations from 0.5 to 1 and 2 wt% as shown in **Figure 4**. It is suggested that the hydrate kinetic inhibition impact of TMAOH is concentration and subcooling temperature dependent. The obtained formation rate data of PVP (see **Figure 4(a)**) suggests that 1 wt% TMAOH performed considerably superior to PVP at 1*°*C conditions perhaps due to

into hydrate phase. The maximum inhibition effect on mole consumption of TMAOH for CO<sup>2</sup>

superior steric hindrances ensued in very least formation rates for all the studied CO2

depicted in **Figure 5**. All concentrations of TMAOH solution reduce the CO2

anion in its structure which leads towards better induction time [15]. However,

Kinetic Assessment of Tetramethyl Ammonium Hydroxide (Ionic Liquid) for Carbon Dioxide…

] from Bavoh et al. [38] and Chun et al. [43], respectively. The

hydrate formation rate is reduced more at 4*°*C experimental

consumption during hydrate formation in TMAOH solution is

hydrate

167

] (0.27) earlier studied imidazo-

http://dx.doi.org/10.5772/intechopen.77262

hydrate formation in the presence of various

hydrate formation rates up to 0.0013 and

formation rate is condensed

hydrates at 1°C, the solid line (0.00 RIP)

systems.

uptakes (moles)

a higher subcooling experimental temperature (1°C) is presented in **Figure 3** for CO2

possess enhanced RIP value (0.32) compared to [EMIM][BF4

PVP performs better than all studied ILs in **Figure 3**.

tion shows the maximum inhibition impact on CO2

increasing TMAOH concentration, the inhibition impact on CO2

**Figure 3.** Influence of 1 wt% TMAOH on relative inhibition power (RIP) of CO<sup>2</sup>

represents pure water, and results are compared with commercial inhibitor (PVP) and ILs.

**Figure 4** presents the initial rate constant of CO2

[EMIM][Cl] and [EMIM][BF4

centrations (see **Figure 4**). The CO2

The total moles of CO2

presence of Cl<sup>−</sup>

Since hydrate formation kinetic is a complex phenomenon, it is hard to compare with the previous studies due to the different experimental conditions and apparatus designs. However, [42] found the efficient method to compare different types of the system via relative inhibition power (RIP): *RIP* <sup>=</sup> Induction time of KHI‐Induction time of water \_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_ Induction time of water (4)

**Figure 2.** Induction time of CO2 hydrates in the presence of water (straight line) and TMAOH at different experimental temperatures (a) 1 and (b) 4°C.

The RIP values determine the kinetic inhibitory efficacy of the KHIs, as the RIP>0 corresponds to the better inhibitory performance of KHI. The RIP value of 1 wt% TMAOH concentration at a higher subcooling experimental temperature (1°C) is presented in **Figure 3** for CO2 hydrate system. The RIP results are further compared with PVP and some reported ILs, namely, [EMIM][Cl] and [EMIM][BF4 ] from Bavoh et al. [38] and Chun et al. [43], respectively. The RIP results further enlighten the kinetic inhibition performance of TMAOH. In fact, TMAOH possess enhanced RIP value (0.32) compared to [EMIM][BF4 ] (0.27) earlier studied imidazolium-based IL. However, [EMIM][Cl] provides much better inhibition performance due to the presence of Cl<sup>−</sup> anion in its structure which leads towards better induction time [15]. However, PVP performs better than all studied ILs in **Figure 3**.

**3.1. Effect of TMAOH on the kinetics of CO<sup>2</sup>**

The experimental pressure for CO2

delay the growth of CO2

166 Recent Advances in Ionic Liquids

power (RIP):

**Figure 2.** Induction time of CO2

temperatures (a) 1 and (b) 4°C.

of TMAOH on the induction time of CO2

tion to 2 wt% resulted in a negligible delay of CO2

1 wt% sample. Thus, the optimum TMAOH concentration to delay CO2

 **hydrates**

temperatures of 1 and 4*°*C (subcooling of 8.32 and 5.32*°*C). These conditions are selected to provide sufficient driving force for hydrate formation as typically encountered in offshore operations of oil and gas production. The effect of various concentrations (0.5, 1 and 2 wt %)

and 4°C) is presented in **Figure 2(a)** and **(b)**, respectively. Results reveal that TMAOH could

process. In the absence of TMAOH, the induction time of water is observed as 14.33 min at 1*°*C (see **Figure 2(a)**), while 4*°*C condition showed even lesser induction time of 12.35 min for pure water as evident in **Figure 2(b)**. The presence of 0.5 wt% TMAOH shows a slight improvement in hydrate nucleation time (induction time) with the base sample (pure water) for both 1 and 4*°*C, which are 23.2 and 21.45%, respectively. Upon increasing the concentration to 1 wt%, the induction time of pure water is improved up to 32.5 and 78.1% at 1 and 4*°*C, respectively, indicating an inhibition ability of TMAOH at the studied subcooling conditions. However, the inhibition impact at 4*°*C is relatively higher compared to 1*°*C, primarily due to the significant driving force existed at 1*°*C (8.32*°*C). Further increase in TMAOH concentra-

is at 1 wt% as shown in **Figure 2**. For comparison purposes, the induction time data is also equated with PVP at 1*°*C condition in **Figure 2(a)**. The comparison results suggest that PVP still possess better hydrate holding efficacy than studied aqueous TMAOH solution perhaps

Since hydrate formation kinetic is a complex phenomenon, it is hard to compare with the previous studies due to the different experimental conditions and apparatus designs. However, [42] found the efficient method to compare different types of the system via relative inhibition

*RIP* <sup>=</sup> Induction time of KHI‐Induction time of water \_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_ Induction time of water (4)

hydrates in the presence of water (straight line) and TMAOH at different experimental

due to the enhanced polymer linkage on the liquid-gas surface compared to TMAOH.

hydrates has been fixed at 3.50 MPa with operating

hydrates at almost all studied systems by hindering its nucleation

hydrate at different experimental temperatures (1

hydrate induction time compared to the

hydrate nucleation

**Figure 4** presents the initial rate constant of CO2 hydrate formation in the presence of various TMAOH concentrations at 1 and 4*°*C, respectively. It is found that the rate of hydrate formation is inhibited in the presence of TMAOH compared with the pure water sample at all studied concentrations (see **Figure 4**). The CO2 hydrate formation rate is reduced more at 4*°*C experimental temperature than 1*°*C, especially at 0.5 wt% system. This is attributed to the higher driving force existed at 1*°*C [15, 21]. At both 1 and 4*°*C, the samples with 0.5 wt% TMAOH concentration shows the maximum inhibition impact on CO2 hydrate formation rates up to 0.0013 and 0.00065 min−1, respectively. Unlike the induction time, whose inhibition impact is enhanced with increasing TMAOH concentration, the inhibition impact on CO2 formation rate is condensed with increasing TMAOH concentrations from 0.5 to 1 and 2 wt% as shown in **Figure 4**. It is suggested that the hydrate kinetic inhibition impact of TMAOH is concentration and subcooling temperature dependent. The obtained formation rate data of PVP (see **Figure 4(a)**) suggests that 1 wt% TMAOH performed considerably superior to PVP at 1*°*C conditions perhaps due to superior steric hindrances ensued in very least formation rates for all the studied CO2 systems.

The total moles of CO2 consumption during hydrate formation in TMAOH solution is depicted in **Figure 5**. All concentrations of TMAOH solution reduce the CO2 uptakes (moles) into hydrate phase. The maximum inhibition effect on mole consumption of TMAOH for CO<sup>2</sup>

**Figure 3.** Influence of 1 wt% TMAOH on relative inhibition power (RIP) of CO<sup>2</sup> hydrates at 1°C, the solid line (0.00 RIP) represents pure water, and results are compared with commercial inhibitor (PVP) and ILs.

as overseen in CO2

time of CH4

however slightly enhanced CH4

by Nashed et al. [44] (see **Figure 7**).

of TMAOH increases, the rate of CH4

(see **Figure 4**), at 1*°*C the formation rates of CH4

The initial rates of CH4

was observed in CO2

section (see **Figure 4(a)**).

**Figure 6.** Induction time of CH4

(a) 1 and (b) 4°C.

pared with PVP data in **Figure 6(a)**. Similar to CO2

more than TMAOH.

hydrate systems, the maximum TMAOH inhibition impact is observed at

Kinetic Assessment of Tetramethyl Ammonium Hydroxide (Ionic Liquid) for Carbon Dioxide…

hydrate nucleation observed at 1*°*C. Increasing the TMAOH

hydrate formation with and without TMAOH mass concentrations

hydrate in the presence of water (straight line) and TMAOH at different temperatures

hydrate formation decreases. The similar behaviour

hydrates are found in non-linear trend (see

hydrate, PVP is able to enhance the induction

enhanced nearly 41.4 and 81.1% for 1 and 4*°*C, respectively, compared to the pure water sample. However, the presence of 0.5 wt% TMAOH ensued little induction time value at 4*°*C and

**Figure 6**). Furthermore, the induction data of 1 wt% TMAOH at 1*°*C condition is further com-

Additionally, obtained induction time results of 1 wt% TMAOH concentration of CH4 hydrates at 1*°*C are also compared with recently published imidazolium-based IL data of Nashed et al. [44] in the form of RIP and perceived in **Figure 7**. Moreover, RIP data of studied AILs could easily be compatible with the previous study [44] as shown in **Figure 7**. TMAOH possess better RIP values compared to most earlier studied imidazolium-based ILs reported

(0.5, 1 and 2 wt%) at 1 and 4*°*C are illustrated in **Figures 8(a)** and **(b)**, respectively. At 4*°*C condition, the formation rates of TMAOH found for concentration are driven; as the quantity

**Figure 8(a)**) which demonstrates that the inhibition impact of TMAOH is not only concentration dependent but also significantly dependent on the type of hydrate formerly present. The presence of low driving force (subcooling = 7*°*C) at 4*°*C enhanced the inhibition of the rate of hydrate formation than at 1*°*C, which holds the higher driving force (subcooling = 10*°*C)

Furthermore, the formation rate of 1 wt% TMAOH is also compared with PVP data in **Figure 8(a)**. The formation rate data revealed that TMAOH is able to reduce the formation

resulting in catastrophic crystal growth as discussed in the preceding study [45].

rate moderately than PVP as also evident in the above results of CO2

hydrate systems as well (**Figure 4(a)**). However, unlike CO2

concentration above 1 wt% reduced the induction time inhibition impact on CH4

hydrate induction time is

http://dx.doi.org/10.5772/intechopen.77262

hydrates (see

169

hydrates

hydrates in an earlier

1 wt% at 1 and 4*°*C. In the presence of 1 wt% TMAOH, the CH4

**Figure 4.** The initial apparent rate of CO2 hydrate formation in the presence of water (straight line) and TMAOH at different temperatures (a) 1 and (b) 4°C.

hydrate is observed at 1 wt% for both experimental conditions. The CO2 uptake values are found to be 0.132 and 0.124 moles for 1 and 4*°*C, respectively. The complete moles consumed at 4*°*C at all concentration are significantly higher than 1*°*C, again affirming the effect of the active driving force at 1*°*C. This considerable driving force attributed further hydrate formation environs cause the formation of more CO2 hydrates. Additionally, the CO2 uptake result of 1 wt% TMAOH is further compared with PVP in **Figure 5(a)**. Referring to the comparison results of CO2 consumed, it is detected that TMAOH can reduce the mole consumption of CO2 more than PVP which signifies its usefulness as a potential KHI for CO<sup>2</sup> hydrate formation.

#### **3.2. Effect of TMAOH on the kinetics of CH<sup>4</sup> hydrates**

CH4 hydrate formation is a bit different from CO<sup>2</sup> hydrate formation and required higher pressure (8.0 MPa) compared to CO2 hydrate (3.5 MPa). **Figure 6** presents the influence of TMAOH on the induction time of CH4 hydrate formation at various concentrations (0.5, 1 and 2 wt%) and different experimental temperatures (1 and 4*°*C).

In contrary to CO2 , higher concentrations of TMAOH do not reflect the linear induction time with mass concentration perhaps due to probabilistic nature of hydrate formation. Similarly,

**Figure 5.** Moles of CO2 consumption in the presence of water (straight line) and TMAOH at different temperatures (a) 1 and (b) 4°C.

as overseen in CO2 hydrate systems, the maximum TMAOH inhibition impact is observed at 1 wt% at 1 and 4*°*C. In the presence of 1 wt% TMAOH, the CH4 hydrate induction time is enhanced nearly 41.4 and 81.1% for 1 and 4*°*C, respectively, compared to the pure water sample. However, the presence of 0.5 wt% TMAOH ensued little induction time value at 4*°*C and however slightly enhanced CH4 hydrate nucleation observed at 1*°*C. Increasing the TMAOH concentration above 1 wt% reduced the induction time inhibition impact on CH4 hydrates (see **Figure 6**). Furthermore, the induction data of 1 wt% TMAOH at 1*°*C condition is further compared with PVP data in **Figure 6(a)**. Similar to CO2 hydrate, PVP is able to enhance the induction time of CH4 more than TMAOH.

Additionally, obtained induction time results of 1 wt% TMAOH concentration of CH4 hydrates at 1*°*C are also compared with recently published imidazolium-based IL data of Nashed et al. [44] in the form of RIP and perceived in **Figure 7**. Moreover, RIP data of studied AILs could easily be compatible with the previous study [44] as shown in **Figure 7**. TMAOH possess better RIP values compared to most earlier studied imidazolium-based ILs reported by Nashed et al. [44] (see **Figure 7**).

The initial rates of CH4 hydrate formation with and without TMAOH mass concentrations (0.5, 1 and 2 wt%) at 1 and 4*°*C are illustrated in **Figures 8(a)** and **(b)**, respectively. At 4*°*C condition, the formation rates of TMAOH found for concentration are driven; as the quantity of TMAOH increases, the rate of CH4 hydrate formation decreases. The similar behaviour was observed in CO2 hydrate systems as well (**Figure 4(a)**). However, unlike CO2 hydrates (see **Figure 4**), at 1*°*C the formation rates of CH4 hydrates are found in non-linear trend (see **Figure 8(a)**) which demonstrates that the inhibition impact of TMAOH is not only concentration dependent but also significantly dependent on the type of hydrate formerly present. The presence of low driving force (subcooling = 7*°*C) at 4*°*C enhanced the inhibition of the rate of hydrate formation than at 1*°*C, which holds the higher driving force (subcooling = 10*°*C) resulting in catastrophic crystal growth as discussed in the preceding study [45].

Furthermore, the formation rate of 1 wt% TMAOH is also compared with PVP data in **Figure 8(a)**. The formation rate data revealed that TMAOH is able to reduce the formation rate moderately than PVP as also evident in the above results of CO2 hydrates in an earlier section (see **Figure 4(a)**).

**Figure 6.** Induction time of CH4 hydrate in the presence of water (straight line) and TMAOH at different temperatures (a) 1 and (b) 4°C.

**Figure 5.** Moles of CO2

1 and (b) 4°C.

results of CO2

In contrary to CO2

CH4

consumption in the presence of water (straight line) and TMAOH at different temperatures (a)

hydrate is observed at 1 wt% for both experimental conditions. The CO2

more than PVP which signifies its usefulness as a potential KHI for CO<sup>2</sup>

tion environs cause the formation of more CO2

**Figure 4.** The initial apparent rate of CO2

different temperatures (a) 1 and (b) 4°C.

168 Recent Advances in Ionic Liquids

**3.2. Effect of TMAOH on the kinetics of CH<sup>4</sup>**

pressure (8.0 MPa) compared to CO2

TMAOH on the induction time of CH4

hydrate formation is a bit different from CO<sup>2</sup>

2 wt%) and different experimental temperatures (1 and 4*°*C).

found to be 0.132 and 0.124 moles for 1 and 4*°*C, respectively. The complete moles consumed at 4*°*C at all concentration are significantly higher than 1*°*C, again affirming the effect of the active driving force at 1*°*C. This considerable driving force attributed further hydrate forma-

of 1 wt% TMAOH is further compared with PVP in **Figure 5(a)**. Referring to the comparison

with mass concentration perhaps due to probabilistic nature of hydrate formation. Similarly,

consumed, it is detected that TMAOH can reduce the mole consumption of CO2

, higher concentrations of TMAOH do not reflect the linear induction time

 **hydrates**

hydrates. Additionally, the CO2

hydrate formation in the presence of water (straight line) and TMAOH at

hydrate (3.5 MPa). **Figure 6** presents the influence of

hydrate formation at various concentrations (0.5, 1 and

uptake values are

uptake result

hydrate formation.

hydrate formation and required higher

**Figure 7.** Influence of 1 wt% TMAOH on relative inhibition power (RIP) of CH<sup>4</sup> hydrates at 1°C, the solid line (0.00 RIP) represents pure water, and results are compared with commercial inhibitor (PVP) and ILs.

**3.3. Effect of TMAOH on the kinetics of binary mixed gas hydrates (50–50 mole%** 

thus showing a significant inhibition impact for the mixed gas hydrates.

quantities of aqueous TMAOH solutions is discussed in this section. The hydrate induction time values for the mix gas hydrate systems are perceived in **Figure 10**. Results revealed that as TMAOH concentration increases the mixed gas hydrate induction time is further delayed,

consumption in the presence of water (straight line) and TMAOH at different temperatures (a)

Kinetic Assessment of Tetramethyl Ammonium Hydroxide (Ionic Liquid) for Carbon Dioxide…

At the lowest concentration (0.5 wt%), TMAOH delayed the base sample induction time by about 1.2 times at both 1 and 4*°*C, while at 2 wt% about 1.5 and 2.3 times at 1 and 4*°*C, respectively. Additionally, the KHI impact (induction time) of 1 wt% TMAOH aqueous solution of the mixed gas is also compared with PVP data in **Figure 10(a)**. The comparisons of data are interestingly piercing that TMAOH can delay hydrate nucleation time further than commercial KHI inhibitor PVP. The potential of the improved delay time observed is perhaps due to the larger subcooling (11*°*C) conditions of studied mixed gas system compared to pure water

**Figure 10.** Induction time of binary mixed gas hydrate in the presence of water (straight line) and TMAOH at different

hydrates in the presence of various

http://dx.doi.org/10.5772/intechopen.77262

171

The kinetic formation of mixed gas 50–50 CO2 + CH4

**CO<sup>2</sup> + CH<sup>4</sup>**

1 and (b) 4°C.

**)**

temperatures (a) 1 and (b) 4°C.

**Figure 9.** Moles of CH4

**Figure 9** presents the CH4 mole consumed into hydrate with and without TMAOH. The presence of TMAOH poorly inhibited the CH4 consumption into hydrate formation at both studied experimental temperatures. Especially at 1*°*C, all tested TMAOH concentrations significantly enhanced CH4 hydrate formation, thus showing hydrate promotional impact. However, the moles of CH4 consumed into the hydrate at 4*°*C are similar to the water sample; this may be due to the low driving force existing at that temperature (4*°*C). Therefore, the presence of TMAOH in CH4 hydrate can inhibit the hydrate induction time and formation rate but will cause hydrate plug upon formation with time due to its CH4 mole consumption promotion effect. Previous studies have shown that some conventional KHIs such as PVP have similar behaviour [46]. They form large hydrate plugs upon hydrate formation (i.e. after hydrate nucleation).

**Figure 8.** Rate of CH4 hydrate formation in the presence of water (straight line) and TMAOH at different temperatures (a) 1 and (b) 4°C.

Kinetic Assessment of Tetramethyl Ammonium Hydroxide (Ionic Liquid) for Carbon Dioxide… http://dx.doi.org/10.5772/intechopen.77262 171

**Figure 9.** Moles of CH4 consumption in the presence of water (straight line) and TMAOH at different temperatures (a) 1 and (b) 4°C.

#### **3.3. Effect of TMAOH on the kinetics of binary mixed gas hydrates (50–50 mole% CO<sup>2</sup> + CH<sup>4</sup> )**

The kinetic formation of mixed gas 50–50 CO2 + CH4 hydrates in the presence of various quantities of aqueous TMAOH solutions is discussed in this section. The hydrate induction time values for the mix gas hydrate systems are perceived in **Figure 10**. Results revealed that as TMAOH concentration increases the mixed gas hydrate induction time is further delayed, thus showing a significant inhibition impact for the mixed gas hydrates.

At the lowest concentration (0.5 wt%), TMAOH delayed the base sample induction time by about 1.2 times at both 1 and 4*°*C, while at 2 wt% about 1.5 and 2.3 times at 1 and 4*°*C, respectively. Additionally, the KHI impact (induction time) of 1 wt% TMAOH aqueous solution of the mixed gas is also compared with PVP data in **Figure 10(a)**. The comparisons of data are interestingly piercing that TMAOH can delay hydrate nucleation time further than commercial KHI inhibitor PVP. The potential of the improved delay time observed is perhaps due to the larger subcooling (11*°*C) conditions of studied mixed gas system compared to pure water

**Figure 10.** Induction time of binary mixed gas hydrate in the presence of water (straight line) and TMAOH at different temperatures (a) 1 and (b) 4°C.

**Figure 8.** Rate of CH4

**Figure 9** presents the CH4

170 Recent Advances in Ionic Liquids

enhanced CH4

moles of CH4

in CH4

ence of TMAOH poorly inhibited the CH4

**Figure 7.** Influence of 1 wt% TMAOH on relative inhibition power (RIP) of CH<sup>4</sup>

represents pure water, and results are compared with commercial inhibitor (PVP) and ILs.

plug upon formation with time due to its CH4

(a) 1 and (b) 4°C.

hydrate formation in the presence of water (straight line) and TMAOH at different temperatures

mole consumed into hydrate with and without TMAOH. The pres-

hydrate formation, thus showing hydrate promotional impact. However, the

consumed into the hydrate at 4*°*C are similar to the water sample; this may be due

experimental temperatures. Especially at 1*°*C, all tested TMAOH concentrations significantly

to the low driving force existing at that temperature (4*°*C). Therefore, the presence of TMAOH

studies have shown that some conventional KHIs such as PVP have similar behaviour [46].

They form large hydrate plugs upon hydrate formation (i.e. after hydrate nucleation).

hydrate can inhibit the hydrate induction time and formation rate but will cause hydrate

consumption into hydrate formation at both studied

hydrates at 1°C, the solid line (0.00 RIP)

mole consumption promotion effect. Previous

systems (CO2 = 7.32*°*C; CH<sup>4</sup> = 10*°*C). This observation also highlighted the limitation of PVP (commercial inhibitor) efficacy over harsh deep-water condition which loses its strength at higher subcooling conditions [38, 47].

**Figure 11** refers the initial formation rate of 50–50 CO2 + CH4 mixed gas hydrates. Results revealed that initial rate of hydrate formation for mixed gas hydrates reduced with increasing TMAOH mass concentrations for both 1 and 4*°*C. The presence of 1 wt% TMAOH shows the optimal hydrate rate diminution. The hydrate formation rate is found to be reduced about five times at 1 wt% TMAOH for all subcooling compared with the mixed gas hydrate (pure water sample). Furthermore, the result of 1 wt% TMAOH is further compared with PVP in **Figure 11(a)** at a similar concentration. Like the pure gases hydrates, TMAOH can further reduce the initial formation rate compared to the PVP-based system.

In **Figure 12**, it is observed that the presence of TMAOH significantly reduces the amount of mixed gas consumption in both experimental (subcooling) conditions, unlike the case of pure CH4 hydrates. The inhibition of mixed gas consumed into hydrate is practically observed at all concentrations. However, the much significant inhibition impact is obtained at 2 wt% aqueous TMAOH solution. At 2 wt%, the amount of mixed gas consumed into hydrate is reduced about 1.7 times compared to the pure water system. This notable inhibition together with the THI impact [3] shows the potentials of applying TMAOH as a novel dual functional inhibitor for practical field operation, especially in high CO<sup>2</sup> content reservoir productions of fluid transportation and procession.

Confirming that CO<sup>2</sup>

hydrate formation.

temperatures (a) 1 and (b) 4°C.

**3.4. Kinetic mechanism of TMAOH**

CO2

hydrates are more formed in mixed gas hydrates compared to CH4

The kinetic inhibition influence of TMAOH observed for all studied gas systems in this work presents TMAOH as a potential KHI. TMAOH is speculated to demonstrate a kinetic inhibitory mechanisms effect via (1) coulombic forces of interactions and (2) hydrogen bonding affinity, which probably results in its kinetic hydrate inhibition influence. Firstly, TMAOH is an ionic liquid which can form hydrogen bonding with water molecules and thus disturb the hydrogen-bonded structure of water molecules owing to the presence of its anion and cation

**Sample Concentration (wt%) Experimental temperature (***°***C) CH<sup>4</sup> CO<sup>2</sup>** Pure water 0.00 4 53.26 46.74 Pure water 0.00 1 54.1 45.9 TMAOH 0.50 4 53.1 46.9

TMAOH 0.50 1 53 47

PVP 1.00 1 55.3 44.7

**Table 2.** Gas chromatography (GC) data for the average composition of binary mixed gas hydrates after complete

1.00 4 55.38 44.62 2.00 4 54.1 45.9

1.00 1 55.8 44.17 2.00 1 54 46

**Figure 12.** Moles of binary mixed gas consumption in the presence of water (straight line) and TMAOH at different

gas.

Kinetic Assessment of Tetramethyl Ammonium Hydroxide (Ionic Liquid) for Carbon Dioxide…

which is due to experimental pressure, provides more driving force to CO2

is more prone to hydrate formation compared to CH4

ions [4, 12, 15, 48]. TMAOH ionizes in aqueous solution as TMA+

hydrate,

173

hydrates as

anions which

than CH4

http://dx.doi.org/10.5772/intechopen.77262

cation and OH<sup>−</sup>

A gas chromatography (GC) is used to explore the mixed gas composition after hydrates are entirely formed in all the binary mixed gas experiments. The data are used to calculate the amount of gas remaining in the gas phase after hydrate formation. However, the GC readings are also useful to understand the percentage of each gas entrapped in the hydrate phase in the presence and absence of TMAOH. The inhibition selectivity of TMAOH for the binary mixed gas composition may be suggested. The measured GC readings are tabularized in **Table 2**. The data revealed that CO2 compositions in the binary mixed gas are diminished after complete hydrate formation compared to original compositions (50–50% CO2 + CH4 ) for all experiments.

**Figure 11.** Initial apparent rate of binary mixed gas hydrate formation in the presence of water (straight line) and TMAOH at different temperatures (a) 1 and (b) 4°C.

Kinetic Assessment of Tetramethyl Ammonium Hydroxide (Ionic Liquid) for Carbon Dioxide… http://dx.doi.org/10.5772/intechopen.77262 173

**Figure 12.** Moles of binary mixed gas consumption in the presence of water (straight line) and TMAOH at different temperatures (a) 1 and (b) 4°C.

Confirming that CO<sup>2</sup> hydrates are more formed in mixed gas hydrates compared to CH4 hydrate, which is due to experimental pressure, provides more driving force to CO2 than CH4 hydrates as CO2 is more prone to hydrate formation compared to CH4 gas.

#### **3.4. Kinetic mechanism of TMAOH**

**Figure 11.** Initial apparent rate of binary mixed gas hydrate formation in the presence of water (straight line) and

systems (CO2 = 7.32*°*C; CH<sup>4</sup> = 10*°*C). This observation also highlighted the limitation of PVP (commercial inhibitor) efficacy over harsh deep-water condition which loses its strength at

revealed that initial rate of hydrate formation for mixed gas hydrates reduced with increasing TMAOH mass concentrations for both 1 and 4*°*C. The presence of 1 wt% TMAOH shows the optimal hydrate rate diminution. The hydrate formation rate is found to be reduced about five times at 1 wt% TMAOH for all subcooling compared with the mixed gas hydrate (pure water sample). Furthermore, the result of 1 wt% TMAOH is further compared with PVP in **Figure 11(a)** at a similar concentration. Like the pure gases hydrates, TMAOH can further reduce the initial

In **Figure 12**, it is observed that the presence of TMAOH significantly reduces the amount of mixed gas consumption in both experimental (subcooling) conditions, unlike the case of pure

A gas chromatography (GC) is used to explore the mixed gas composition after hydrates are entirely formed in all the binary mixed gas experiments. The data are used to calculate the amount of gas remaining in the gas phase after hydrate formation. However, the GC readings are also useful to understand the percentage of each gas entrapped in the hydrate phase in the presence and absence of TMAOH. The inhibition selectivity of TMAOH for the binary mixed gas composition may be suggested. The measured GC readings are tabularized in **Table 2**. The

compositions in the binary mixed gas are diminished after complete

 hydrates. The inhibition of mixed gas consumed into hydrate is practically observed at all concentrations. However, the much significant inhibition impact is obtained at 2 wt% aqueous TMAOH solution. At 2 wt%, the amount of mixed gas consumed into hydrate is reduced about 1.7 times compared to the pure water system. This notable inhibition together with the THI impact [3] shows the potentials of applying TMAOH as a novel dual functional

mixed gas hydrates. Results

content reservoir productions of

) for all experiments.

TMAOH at different temperatures (a) 1 and (b) 4°C.

higher subcooling conditions [38, 47].

172 Recent Advances in Ionic Liquids

fluid transportation and procession.

data revealed that CO2

CH4

**Figure 11** refers the initial formation rate of 50–50 CO2 + CH4

formation rate compared to the PVP-based system.

inhibitor for practical field operation, especially in high CO<sup>2</sup>

hydrate formation compared to original compositions (50–50% CO2 + CH4

The kinetic inhibition influence of TMAOH observed for all studied gas systems in this work presents TMAOH as a potential KHI. TMAOH is speculated to demonstrate a kinetic inhibitory mechanisms effect via (1) coulombic forces of interactions and (2) hydrogen bonding affinity, which probably results in its kinetic hydrate inhibition influence. Firstly, TMAOH is an ionic liquid which can form hydrogen bonding with water molecules and thus disturb the hydrogen-bonded structure of water molecules owing to the presence of its anion and cation ions [4, 12, 15, 48]. TMAOH ionizes in aqueous solution as TMA+ cation and OH<sup>−</sup> anions which


**Table 2.** Gas chromatography (GC) data for the average composition of binary mixed gas hydrates after complete hydrate formation.

interacts with the dipoles of the aqueous molecules with strong coulombic forces in additions to its hydrogen bonding ability [49]. This combined effect easily overcomes the hydrogen bond structure or the van der Waals forces which causes clustering of water molecules in hydrate formation nucleation and growth [3, 4, 12]. As the water structures are distorted, it promotes the hindrance/delay in hydrate nuclei clustering and gas consumption into hydrate. Additionally, TMAOH possesses four methyl cations [CH3+] which is the least hydrophobic alkyl radical among other alkyl groups, which makes tetramethylammonium [TMA+ ] cation relatively more hydrophilic [3, 50–52]. Similarly, hydroxyl [OH<sup>−</sup> ] anion is well known for its strong hydrogen bonding affinity for water molecules [3, 4, 32]. Therefore, TMAOH could adsorb on the gas-water interface, and retarded hydrate nucleation and crystal growth (through hydrogen bonding by the amide group and anion) and sterically block the hydrate formation [15, 53–56].

gases; however, for binary mixed gas, the induction time of TMAOH is found to be higher than PVP due to enhanced subcooling conditions. The initial formation rate and mole consumption are found to be enhanced with TMAOH compared to the commercial counterpart. The TMAOH inhibitory mechanisms are driven by their coulombic forces of interactions and hydrogen bonding affinity for water molecules in hydrate formation environment. Therefore, the findings of this study highlighted the kinetic impact of TMAOH, which should be beneficial for gas hydrate-based technological applications such as storage and gas transportation,

Kinetic Assessment of Tetramethyl Ammonium Hydroxide (Ionic Liquid) for Carbon Dioxide…

http://dx.doi.org/10.5772/intechopen.77262

175

PETRONAS Research Sdn Bhd (PRSB) financially supports this work under the Grant No.

Muhammad Saad Khan1,2, Bavoh B. Cornelius1,2, Bhajan Lal1,2\* and Mohamad Azmi Bustam1,2

[1] Koh CA. Towards a fundamental understanding of natural gas hydrates. Chemical Society

1 Chemical Engineering Department, Universiti Teknologi of PETRONAS, Bandar Seri

Research Centre (CO2RES), Bandar Seri Iskandar, Perak, Malaysia

flow assurance and energy productions.

**Acknowledgements**

053C1–024.

**Nomenclature**

**Author details**

Iskandar, Perak, Malaysia

Reviews. 2002;**31**(3):157-167

2 CO2

**References**

ILs ionic liquids

LDHI low-dosage hydrate inhibitor

RIP relative inhibition performance

TMAOH tetramethylammonium hydroxide

\*Address all correspondence to: bhajan.lal@utp.edu.my

KHI kinetic hydrate inhibitors PVP polyvinyl pyrrolidinium

It is already proven from previous studies [1, 13, 35, 57, 58] that hydrate formation depends on the presence of a guest molecule. Therefore, the inhibition impact of TMAOH also differs according to the type of gas systems. For instance, CO2 hydrates showed lesser induction time in comparison with CH4 hydrates and 50–50 CO2 -CH4 mix gas hydrates in this study. The overall better KHI performance of TMAOH is found with 50–50 CO<sup>2</sup> -CH4 mix gas hydrates for both experimental temperature conditions (1 and 4*°*C) for induction time, hydrate formation rate alongside moles of gas consumed. Additionally, although the kinetic inhibition is concentration dependent, as aqueous TMAOH concentration increases, the KHI performance enhances for most of the studied systems. However, for optimum performance evaluation, one% (1 wt%) TMAOH seems to be more efficient as not much differently observed (above results) between 1 and 2 wt% concentrations. It is also evident for all studied system that at higher experimental temperature condition 4*°*C TMAOH performs better in comparison to lower-temperature condition (1*°*C). It can be concluded that TMAOH successfully worked as a KHI inhibitor for all the studied system; earlier [3], TMAOH has already been proven as an efficient THI as well for CH4 and CO2 gas hydrates. Therefore, TMAOH possibly will work as dual-functional hydrate inhibitor and therefore could efficiently be contemplated for flow assurance strategies.

#### **4. Conclusions**

In the present work, ammonium-based ionic liquid, TMAOH, is tested as a potential kinetic inhibitor for pure CO2 , CH4 and binary mixed (50–50) CO2 + CH4 hydrate systems at different mass concentrations (0.5, 1.0 and 2.0 wt%). The effect of subcooling was also investigated at two different experimental temperatures of 1 and 4*°*C at moderate pressures for all the studied TMAOH systems. Experimental results revealed that TMAOH kinetically inhibits hydrate formation by delaying hydrate nucleation through the enhancement of hydrate induction time and TMAOH also lessens the initial apparent rate of hydrate formation and accordingly decreased the mole consumptions for almost all studied systems. Influence of subcooling was also observed as the subcooling increased the hydrate formation due to the presence of excessive driving force. The effect of TMAOH concentration and the type of guest molecules under study are found to affect the TMAOH hydrate inhibition impact significantly. Furthermore, the experimental results of TMAOH were further compared with PVP at 1 wt% concentration at 1*°*C conditions. The induction time of PVP seems to be higher than TMAOH for both pure gases; however, for binary mixed gas, the induction time of TMAOH is found to be higher than PVP due to enhanced subcooling conditions. The initial formation rate and mole consumption are found to be enhanced with TMAOH compared to the commercial counterpart. The TMAOH inhibitory mechanisms are driven by their coulombic forces of interactions and hydrogen bonding affinity for water molecules in hydrate formation environment. Therefore, the findings of this study highlighted the kinetic impact of TMAOH, which should be beneficial for gas hydrate-based technological applications such as storage and gas transportation, flow assurance and energy productions.

#### **Acknowledgements**

interacts with the dipoles of the aqueous molecules with strong coulombic forces in additions to its hydrogen bonding ability [49]. This combined effect easily overcomes the hydrogen bond structure or the van der Waals forces which causes clustering of water molecules in hydrate formation nucleation and growth [3, 4, 12]. As the water structures are distorted, it promotes the hindrance/delay in hydrate nuclei clustering and gas consumption into hydrate. Additionally, TMAOH possesses four methyl cations [CH3+] which is the least hydrophobic alkyl radical

gen bonding affinity for water molecules [3, 4, 32]. Therefore, TMAOH could adsorb on the gas-water interface, and retarded hydrate nucleation and crystal growth (through hydrogen bonding by the amide group and anion) and sterically block the hydrate formation [15, 53–56]. It is already proven from previous studies [1, 13, 35, 57, 58] that hydrate formation depends on the presence of a guest molecule. Therefore, the inhibition impact of TMAOH also differs


gas hydrates. Therefore, TMAOH possibly will work as dual-functional hydrate

experimental temperature conditions (1 and 4*°*C) for induction time, hydrate formation rate alongside moles of gas consumed. Additionally, although the kinetic inhibition is concentration dependent, as aqueous TMAOH concentration increases, the KHI performance enhances for most of the studied systems. However, for optimum performance evaluation, one% (1 wt%) TMAOH seems to be more efficient as not much differently observed (above results) between 1 and 2 wt% concentrations. It is also evident for all studied system that at higher experimental temperature condition 4*°*C TMAOH performs better in comparison to lower-temperature condition (1*°*C). It can be concluded that TMAOH successfully worked as a KHI inhibitor for all the studied system; earlier [3], TMAOH has already been proven as an efficient THI as well for

inhibitor and therefore could efficiently be contemplated for flow assurance strategies.

In the present work, ammonium-based ionic liquid, TMAOH, is tested as a potential kinetic

mass concentrations (0.5, 1.0 and 2.0 wt%). The effect of subcooling was also investigated at two different experimental temperatures of 1 and 4*°*C at moderate pressures for all the studied TMAOH systems. Experimental results revealed that TMAOH kinetically inhibits hydrate formation by delaying hydrate nucleation through the enhancement of hydrate induction time and TMAOH also lessens the initial apparent rate of hydrate formation and accordingly decreased the mole consumptions for almost all studied systems. Influence of subcooling was also observed as the subcooling increased the hydrate formation due to the presence of excessive driving force. The effect of TMAOH concentration and the type of guest molecules under study are found to affect the TMAOH hydrate inhibition impact significantly. Furthermore, the experimental results of TMAOH were further compared with PVP at 1 wt% concentration at 1*°*C conditions. The induction time of PVP seems to be higher than TMAOH for both pure

and binary mixed (50–50) CO2 + CH4

] cation relatively more

mix gas hydrates for both

hydrate systems at different

] anion is well known for its strong hydro-

hydrates showed lesser induction time

mix gas hydrates in this study. The over-


among other alkyl groups, which makes tetramethylammonium [TMA+

hydrates and 50–50 CO2

all better KHI performance of TMAOH is found with 50–50 CO<sup>2</sup>

hydrophilic [3, 50–52]. Similarly, hydroxyl [OH<sup>−</sup>

according to the type of gas systems. For instance, CO2

in comparison with CH4

174 Recent Advances in Ionic Liquids

CH4

and CO2

**4. Conclusions**

inhibitor for pure CO2

, CH4

PETRONAS Research Sdn Bhd (PRSB) financially supports this work under the Grant No. 053C1–024.

#### **Nomenclature**


#### **Author details**

Muhammad Saad Khan1,2, Bavoh B. Cornelius1,2, Bhajan Lal1,2\* and Mohamad Azmi Bustam1,2

\*Address all correspondence to: bhajan.lal@utp.edu.my

1 Chemical Engineering Department, Universiti Teknologi of PETRONAS, Bandar Seri Iskandar, Perak, Malaysia

2 CO2 Research Centre (CO2RES), Bandar Seri Iskandar, Perak, Malaysia

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## *Edited by Mohammed Muzibur Rahman*

*Recent Advances in Ionic Liquids* contains research on the preparation, characterization, and potential applications of stable ionic liquids (ILs). ILs are a class of low- and stable-melting point, ionic compounds that have a variety of properties allowing many of them to be sustainable green solvents. It is promising novel research from top to bottom and has received a lot of interest over the last few decades. It covers the advanced topics of physical, catalytic, chemical, polymeric, and potential applications of ILs. This book features interesting reports on cutting-edge science and technology related to the preparation, characterization, polymerization, and potential applications of ILs. This potentially unique work offers various approaches on the R&D implementation of ILs or related ionic catalysts and their conjugates. With this in mind, the authors present R&D on the preparation, properties, potential applications, and utility of ILs. The chapters describe important applications in a wide variety of contexts, including polymerization, devices, electrochemistry, and biotechnology. Both the theoretical and practical, it can be passed from the stable as well as molten salt to the ionic liquid and vice versa. Experimental and theoretical techniques for examining these studies are elaborated based on the methods for preparation, utilization, applications, and analysis. This book epitomizes the transfer of these techniques and methods between the differing temperature regimes, and is a major contribution to the future of both fields. The book presents an overview of current ILs fundamentals: preparation, polymerization, substantial applications, and enhancement of research worldwide. The techniques of ILs preparation, total characterization, and possible applications related with ILs as well as modified or conjugated material research are investigated. *It is hoped that it will be an important book for research organizations, governmental research centers, and academic libraries engaged in recent R&D of ILs.*

Published in London, UK © 2018 IntechOpen © Paul-Daniel Florea / iStock

Recent Advances in Ionic Liquids

Recent Advances in

Ionic Liquids

*Edited by Mohammed Muzibur Rahman*