Applications of ionic liquids in biphasic separation: Aqueous biphasic systems and liquid–liquid equilibria

Accepted Manuscript Title: Applications of ionic liquids in biphasic separation: aqueous biphasic systems and liquid-liquid equilibria Authors: Shashi Kant Shukla, Shubha Pandey, Siddharth Pandey PII: DOI: Reference:

S0021-9673(17)31503-0 https://doi.org/10.1016/j.chroma.2017.10.019 CHROMA 358924

To appear in:

Journal of Chromatography A

Received date: Revised date: Accepted date:

31-7-2017 5-10-2017 6-10-2017

Please cite this article as: Shashi Kant Shukla, Shubha Pandey, Siddharth Pandey, Applications of ionic liquids in biphasic separation: aqueous biphasic systems and liquid-liquid equilibria, Journal of Chromatography A https://doi.org/10.1016/j.chroma.2017.10.019 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Review article

Applications of ionic liquids in biphasic separation: aqueous biphasic systems and liquid-liquid equilibria Shashi Kant Shukla1, Shubha Pandey2 and Siddharth Pandey1*

1

Department of Chemistry, Indian Institute of Technology Delhi, Hauz Khas, New Delhi 110016

2

Department of Science and Technology, Ministry of Science and Technology, Technology

Bhawan, New Mehrauli Road, New Delhi 110016, India

*Corresponding author: email: [email protected] (SP) __Research

Highlights

 Applications of ionic liquids (ILs) in biphasic separation is reviewed.  IL-based aqueous biphasic systems (ABSs) and liquid-liquid equilibria are discussed.  Phase diagrams of IL-based ABSs are presented.  Factors influencing biphasic separation in IL-based ABSs are outlined.  Applications of IL-based ABSs in extraction of biomolecules/metal ions are highlighted. ____________________________________________________________________________ ABSTRACT

1

Ionic liquids (ILs) have been receiving much attention in many fields of analytical chemistry because of their various interesting properties which distinguish them from volatile organic compounds. They offer both directional and non-directional forces towards a solute molecule and therefore act as excellent solvents for a wide range of polar and non-polar compounds. Because of the presence of various possible interactions, ILs easily undergo biphasic separation with water and other less polar/non-polar organic solvents. Their ability to create biphasic splitting makes them a promising candidate for liquid-liquid separation processes, such as aqueous biphasic systems and liquid-liquid equilibria. Various aspects of ILs in these separation methods are discussed in view of the origin of physical forces responsible for the biphasic interactions, the effect of structural components, temperature, pressure, pH and additives. The specific advantages of using ILs in aqueous biphasic systems and liquid-liquid equilibria in binary and ternary systems are discussed with a view to defining their future role in separation processes by giving major emphasis on developing non-toxic ILs with physical and solution properties tailored to the needs of specific sample preparation techniques.

Keywords: Ionic liquids; Aqueous biphasic systems; Liquid-liquid equilibria; Separation; Solvents in separation.

1. Introduction From the last few decades, ionic liquids (ILs) research has experienced a massive upsurge of interest because of their application in ‘green’ chemistry, for example, as replacement of organic compounds and volatile organic solvents (VOSs) in chemical industry for the easy chemical transformation with the minimum chemical waste and environment pollution.1 ILs are a molten analog of inorganic salts in which asymmetrical ions are held by directional forces. The initial interests in ILs emerged because of their several advantageous properties, such as negligible vapor pressure, low melting point, high thermal stability, wide electrochemical window, inflammability, recyclability, and so on.2-6 Since the inception of ILs in 1914, they are extensively employed in various fields, viz., organic, inorganic, physical, and biological 2

chemistry; however, very recently, they have been explored as a promising solvent in the extraction and separation of organic compounds, biomolecules, cell organelles and transition and inner-transition metal ions.7-10 The potential of ILs in extraction and biphasic separation processes is facilitated by their amphiphilic nature, i.e., polar and non-polar moieties and tailorability. The polarizability of IL anion governs the miscibility with water and other molecular solvents. In general, ILs containing less polarizable anion exhibit lower solubility with water thereby enabling the demixing/biphasic separation of the two layers which is a quintessential requirement for extraction and separation. The alkyl chain length on cation also helps in tuning the miscibility of ILs in water and polar solvents. The nature of anion and size of alkyl chain on cationic core determine physical properties, such as polarity, viscosity, density and surface tension. The techniques which employ ILs for extraction and separation of organic compounds, biomolecules and metal ions are chromatography, capillary electrophoresis (CE), aqueousbiphasic systems (ABSs) and liquid-liquid equilibria (LLE).11 In chromatography, ILs exhibit an unusual selectivity with a ‘dual nature’, where they separate both polar and nonpolar compounds because of their amphiphilic nature. In reverse-phase chromatography, ILs have been used as the mobile phase with low-viscos molecular solvents. In capillary electrophoresis, ILs have been used as electrolyte additives, running buffer modifiers and supported coatings on the capillary walls. In aqueous-biphasic systems (ABSs), hydrophobic ILs are used as a separate phase either with water or with an aqueous solution of the polymer. Later, hydrophilic ILs along with kosmotropic salt in water were employed as a biphasic medium in separation. In liquid-liquid equilibria, IL co-exists with another phase at a given composition and temperature.

3

In the present review, we have embarked mainly on the two IL-based separation methods, viz., aqueous-biphasic systems and liquid-liquid equilibria. The structure of ILs which are employed in these methods are depicted in Table 1. Many review articles, over the years, have been published regarding the significance of ILs in analytical applications.

Table 1 Structure of cations and anions of representative ILs employed in ABSs and LLE Cations

Anions

4

5

2. Physical forces involved in biphasic separation The basis of separation in a two-phase system is the selective distribution of substances between two phases.12 Biphasic separation in aqueous-two-phase systems involves two major operations: equilibration and phase separation. Equilibration is rapid and involves mixing of the components that constitute the phase system with the material subjected to partitioning. When the two components attain equilibrium of phase composition in the given conditions separation occurs. The phase separation under gravity is not rapid and is influenced by the density and viscosity of the mixing phases. Haynes et al. formulated Flory-Huggins theory and derived various analytical expressions to identify and quantify the fundamental driving forces governing phase behavior and solute partitioning for the aqueous-two phase systems.13 The useful parameters that were used for the prediction of solute partition and phase separation are combinatorial entropy change (∆Sc), enthalpy of mixing (∆Hmix) and partition coefficient of solute (Ks). Combinatorial entropy change is the entropy change during mixing of two phases. For biphasic separation, ∆Sc decreases whereas for the homogenous mixing ∆Sc approaches zero.

6

The ∆Hmix is the enthalpy required to form the biphasic systems from two phases and depends on the “self-energy” of both the phases. The self-energy is the energy of interaction of the two average lattice site of a phase. If the interaction between two lattice sites is of attractive nature the corresponding self-energy will be negative or less positive and vice-versa. The positive selfenergy values of the phases of aqueous-two-phase system favor the phase separation and solute partitioning. Solute exclusively participates in the phase with the highest “self-energy”. Partition coefficient (Ks) of a solute between phases is the ratio of the volume fraction of solute in top phase (𝜙𝑠𝑡 ) to that in the bottom phase (𝜙𝑠𝑡 ) and is influenced by the degree of polymerization of phase-forming components and temperature. In general, in absence of the enthalpic effects, solute partitioning will be more to that phase which has the highest number of particles per unit volume, i.e., the charge density. Nagaraja and Iyyaswami studied the effect of density, viscosity and surface tension on the phase separation in aqueous-biphasic solutions in terms of the change in the tie-line length of the phase diagram.14 The density difference between the phases was observed to increase with the concentration of phase-forming components and decrease with their molecular mass. As the tie line length increases the density difference between the phases forming aqueous-two-phase system increases. Consequently, composition difference between phases takes and demixing occurs. The demixing of phases depends on the difference in viscosity of the phases. In the viscous phase, friction between the aggregated particles and phase remains higher than the less viscous phase. This affects the rate of transfer of the droplets in more viscous phase and retards phase separation. The higher concentration of the component which constitutes more viscous phase offers more restrictions for the phase separation. Salamanca et al. observed that the composition of the less viscous lower phase also controls the demixing of phases.15 The drops

7

from the highly viscous upper phase coalesce easily in the continuous lower phase and thus the demixing is instantaneously maintained. Interfacial tension between phases affects the phase separation. At lower surface tension, stable and small droplet forms easily and initiates phase separation than that at higher surface tension. The interfacial tension increases with the concentration difference of the phase-forming constituents and molecular weight of the component constituting the upper phase. Apart from all these physical properties, the volume ratio of the phase forming components also determines the phase separation. For example, for polyethylene 2000-salt system optimum separation was noticed for the volume ratio 1.5. Biphasic separation in ILs solutions with molecular solvents and water occurs above a critical temperature referred to as the cloud-point temperature/critical solution temperature (CST). Above cloud point, a homogenous solution of IL separates into the two equilibrium phases. Based on the interplay of the intermolecular interaction, two different kinds of CSTs are observed in the liquid-liquid equilibria determination of the IL solutions, namely; lower critical solution temperature (LCST) and upper critical solution temperature (UCST). Unlike aqueous systems and polymer solutions, which exhibit LCST and UCST phase behavior, respectively, both LCST and UCST behaviors were seen in the polymer solution of ILs. The LCST phase separation is driven by the unfavorable entropy of mixing and is caused by two reasons, viz., associating interactions and compressibility. The LCST behavior is observed in IL systems which are exothermic in nature. In such systems strong associating interactions between components such as strong polar interactions and hydrogen bonding prevent random mixing. At lower temperature, the magnitude of intermolecular forces between the two liquids becomes stronger than any interactions between the like molecules, hence the components become

8

miscible in all proportions. Another factor which controls the LCST phase behavior is compressibility. At higher pressure, as the IL system approaches CST the components separate into two phases due to the different magnitude of intermolecular interactions between phases. Schalley et al. studied the phase behavior of imidazolium-based ILs with acetone and observed that the LCST is controlled by concentration, the choice of cation, anion, and solvent and supramolecular host-guest complex formation.16 Lodge et al. have investigated the LCST phase behavior of poly(n-butylmethacrylate) in mixtures of imidazolium-based ILs containing different alkyl chain.17 They observed an increase in the cloud point temperature with the proportion of longer alky chain imidazolium IL in the mixture and suggested that longer alkyl chain leads to stronger intra-IL and/ or polymer-IL interactions. These interactions increase the entropic penalty and enthalpic driving force for mixing. Watanabe et al. investigated the LCST phase behavior of different polyethers in various classes of ILs.18 For ILs containing the same anions, polyethers were miscible in imidazolium- and pyridinium-based ILs, whereas they were insoluble in the ammonium- and phosphonium-based ILs under similar conditions. For a given IL, LCST was noted to increase with the polarity of polyethers and decreases by replacing the hydrogen atom on imidazolium cation by an alkyl group. The UCST phase behavior is exhibited by the systems in which the magnitude of like interactions is higher than the magnitude of unlike interactions. At higher temperature, unlike interactions overcome like interactions and two components become miscible. The overall randomness of the system increases and the system becomes miscible. Crosthwaite et al. showed that the UCST can be tuned by the length of the alkyl chain both in IL and alcohol.19-21 Solubility of alcohol in IL decreases as the size of alkyl chain on IL cation decreases; however increases for

9

the shorter alcohols. Domanska and Marciniak have shown that among lower alcohols primary alcohol exhibits lower solubility in imidazolium ILs than the secondary alcohol.22 3. Ionic liquid-based aqueous biphasic systems Aqueous biphasic systems (ABSs) are formed when two immiscible phases, soluble in water, are brought into the contact with each-other at a certain temperature. They are used as clean alternative for the traditional organic-water solvent extraction. ABSs are constituted either by mixing two polymers, one polymer and one kaotropic salt or two salts (in which one is chaotropic). Since the formation of first ABS in 1958 by Albertsson, several combinations of polymer and salts have been used to construct efficient ABSs by several workers.23,24 The efficacy of ABSs depends on the extent of phase separation which in turn is controlled by the polarity difference of the constituent phases. The polarity difference between polymer-based ABSs was tuned by the addition of kosmotropic salts. In 2003, the pioneering work of Rogers and coworkers showed that an ABS can be created between hydrophilic IL and an aqueous solution of a kaotropic salt.25 Since then, considerable efforts have been made towards the use of IL as a suitable substitute in replacing the salt-rich and polymer-rich phases.26-45 Because of the large availability of cation and anion, apposite ILs with a set of desired properties can be synthesized to make them more applicable in separation processes. Thus IL-based ABSs clearly have an edge over the other traditional polymer and salt-based ABSs.

3.1. Chemical composition of ABSs Three major IL-based ABSs have been used extensively by different workers for the metathesis reaction, separation, and recovery of materials. These are 1) IL-inorganic salt-ABSs, 2) IL-carbohydrate-ABSs, and 3) IL-polymer-based ABSs. The properties of these three classes

10

of IL-based ABSs depend on the charge density of salts, structure of carbohydrate, and molar mass and functionality of polymer. ABS systems composed of inorganic salts and ILs are the most sought after IL-based ABSs. In this system, IL, which is a weak electrolyte, is salted out from aqueous solution by the strong inorganic salts. Though hydrophobic ILs separate from water without the addition of inorganic salts, they are not used for the ABS formation. Hydrophilic ILs possess higher solubility in water than inorganic salts and do not precipitate upon addition of salts. During the biphasic separation, hydrophilic ILs carry large numbers of water molecules and thus facilitate partitioning of the solute in one phase. ABSs composed of carbohydrates and ILs constitute the most environmentally-benign class of ABSs. The biphasic separation in IL-carbohydrate-based ABSs depends on the solubility of carbohydrate and do not require inorganic salt. In carbohydrate-IL-based ABSs, the hydrophilic IL salt-out the less water soluble carbohydrates and remain present as the lower phase.29 The polymer-IL-based ABSs constitute the third major class of ABSs. In this class of ABSs, both IL and polymer can salt-out from water depending on the structure and functionality of the other component.46 Hydrophobicity of ILs can be tailored by manipulating the structure of anion and alkyl chain on the cation. Compared to the polymer-salt and polymer-polymer-based ABSs, where the immiscibility of the phases remain poor due to the nearly similar polarity of polymer and salt, IL-polymer-based ABSs have an advantage as they show greater immiscibility because of the large variation in IL polarity.

3.2. Phase diagram of binary and ternary ABSs

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ABSs are ternary systems composed of water and two salts. In order to know the mixing/demixing behavior between two phases, phase diagram is required. In literature, two different types of phase diagrams, namely orthogonal and triangular are used. In the orthogonal phase diagram, both axes indicate concentration (wt %) of phase forming substance (Fig. 1).

Fig 1. Orthogonal ternary phase diagram for a hypothetical system composed of polymer + inorganic salt + water (weight fraction units).

Water is omitted as a component of the orthogonal phase diagram and its composition is determined from the difference of 100 - composition of components (wt%) on both x-and y-axes. The triangular phase diagram represents all the components and is shown by the equilateral triangle or Gibbs-Roozeboom triangle (Fig. 2). In the triangular phase diagram, the apex of the equilateral triangle represents pure component, while the side denotes the equilibrium between two components. In both representations of the phase diagram, the binodal curve represents the boundary between the monophasic and biphasic regions. At phase composition below the binodal curve the phases do not separate and it thus represents one phase while the phase

12

composition above this curve only indicates single phase. The mixture composition along the binodal curve is given by the tie line. Along a binodal curve, mixture composition remains constant. The binodal curve in the ternary phase diagram in ABSs is determined by the cloudpoint method.47 The cloud-point is achieved by adding the aqueous solution of the salting out agent to the aqueous solution of the other phase forming components until the appearance of the cloud-point. The homogenous mixture can also be achieved by gradually adding water to the mixture of components.

Fig 2. Triangular phase diagram for a hypothetical system composed of IL + inorganic salt + water (weight fraction units).

Rogers et al. used the three-parameter equation (Eq. 1), which was initially proposed by Merchuk et al., to fit the binodal data and determine the tie line by a weight balance equation.25,48 The compositions of the top and bottom phases and the composition of the system are determined by an empirical lever-arm rule. 13

Y= 𝐴 exp[(𝐵 • 𝑋 0.5 ) − (C • 𝑋 3 )]

(1)

where Y and X denote the percentage of weight fraction of IL and other phase forming component. A, B and C are the fitting parameters obtained by least square regression. Tie-lines (TLs) are determined by using four unknown values from four equations (Eqs. 2-5) given below. 𝑌𝑇 = 𝐴 exp[(𝐵 • 𝑋𝑇0.5 ) − (C • 𝑋𝑇3 )] 𝑌𝐵 = 𝐴 exp[(𝐵 • 𝑋𝐵0.5 ) − (C • 𝑋𝐵3 )] 𝑌𝑇 =

𝑌𝑀

𝑋𝑇 =

𝑋𝑀

𝛼

𝛼

-

1− 𝛼 𝛼 1− 𝛼 𝛼

(2) (3)

• 𝑌𝐵

(4)

•𝑋𝐵

(5)

where YT and YB denote the percentage weight fraction of IL and another component in the top and bottom phases, respectively. M and α designate the initial mixture composition and the ratio of the weight of the top phase to the total weight of the mixture. The effects of pH and temperature on the binodal curve were accounted by using modified Merchuk equations.49,50 𝑎 = 𝑎0 + 𝑏 = 𝑏0 + 𝑐 = 𝑐0 +

𝑎1 𝑝𝐻 𝑏1 𝑝𝐻 𝑐1 𝑝𝐻

(6) (7) (8)

𝑎 = 𝑎0 + 𝑎1 (𝑇 − 𝑇0 )

(9)

𝑏 = 𝑏 0 + 𝑏1 (𝑇 − 𝑇0 )

(10)

𝑐 = 𝑐 0 + 𝑐1 (𝑇 − 𝑇0 )

(11)

Equations 6, 7 and 8 are used to account the effect of pH on the binodal curve. In these equations, a, a0, b, b0, c and c0 are pH-independent adjustable parameters. Similarly, equations 9, 10 and 11 are applied to analyze the effect of temperature on a binodal curve. T0 denotes 14

absolute/reference temperature (273.15 K) while a, a0, b, b0, c and c0 are temperatureindependent adjustable parameters. In both cases, these adjustable parameters were obtained by comparing the experimental binodal data with the corresponding standard deviation.

3.3. Factors influencing biphasic separation in IL-based ABSs The formation and stability of IL-based ABSs vary with the structure of ILs, temperature, pH, and nature and functionality of other phase forming component. A detailed account of all these effects is given below.

3.3.1. Effect of IL structure Bridges et al. have studied the effect of different cationic cores on the formation of ABS.26 They evaluated the phase behavior of imidazolium-, pyridinium-, quaternary ammoniumand phosphonium-based ILs with aqueous K3PO4, K2HPO4, K2CO3, KOH and (NH4)2SO4. ABS formation was observed to increase with the increasing hydrophobicity of cationic core. For a given salt, say K3PO4, ordering of different cationic core is as follows: [P4444][Cl] > [N4444][Cl] > [C4Py][Cl] > [C4C1im][Cl]. Compared to quaternary ammonium- and phosphonium-based ILs, the positive charge on imidazolium- and pyridinium-based ILs is more delocalized over the ring. This allows imidazolium and pyridinium cores to interact with water and thus disfavor the demixing. Louros et al. have also found phosphonium-based ILs to be more promising than any other class of ILs in ABS formation.51 Ventura et al. have assessed the role of nitrogen containing cyclic cationic ILs (imidazolium-, pyridinium-, pyrrolidinium- and piperidiniumbased) on the ABS formation.37 In order to keep the pH unchanged, they used a combination of

15

two salts (K2HPO4-KH2PO4) as a buffer. The charge density of these cationic cores, molar volume was noted to affect the ABS formation. Six-membered ILs (pyrrolidinium and piperidinium) were salted out preferably and hence easily formed ABS than those of fivemembered ILs. Neves et al. studied the effect of alkyl chain length on the ABS formation with imidazolium halide ILs and K3PO4.32 They noted that ILs having longer alkyl chain on imidazolium cation easily undergo phase separation than the shorter alkyl chain ILs because of the lower hydration entropy of the longer alkyl chain ILs. The lower hydration entropy of the longer alkyl chain ILs results in higher hydrophobicity and thus aggregate easily during demixing.52 Because of this reason, longer chain ILs require less salt for salting-out and easily phase separate from the aqueous salt-rich phase to the IL phase. Several other studies have suggested that the locus of the homogeneous region extends with the increase in alkyl chain length.53-56 Compared to the studies on the influence of cation and alkyl chain length, there are few reports on the influence of the IL anion on the ABS formation. Pie et al. noted that the ABS takes place easily with the bromide-based imidazolium ILs than the chloride-based ILs.55 Ventura et al. studied the effect of chloride, bromide, acetate, hydrogensulfate, methanesulfonate, methylsulfate, ethylsulfate, trifluomethanesulfonate, trifluoroacetate, and dicyanamide anions having common 1-butyl-3-methylimidazolium cation in the presence of aqueous K3PO4. The results indicated that the ability of these ILs on the ABS formation closely follows the decrease in the hydrogen bond accepting strength or the increase in the hydrogen bond acidity of the IL anion.36 During ABS formation, there occurs competition between salt ions and IL ions for the formation of hydration complexes. Anions with lower hydrogen bond

16

basicity values (β) present lower abilities to form coordinate bonds with water, and therefore, are more easily salted-out by conventional salts.36 In other attempts, Coutinho and coworkers have studied the influence of different IL anions containing common cations on the ABS formation with different inorganic salts. The aptitude of different IL anions in promoting the ABS formation is universal and observed unaltered of the type of salt used. In all cases, anion with lower hydrogen bond formation ability required least amount of salt exhibited higher ability to phase separate.28,38

3.3.2. Effect of temperature Several workers have reported the influence of temperature on the phase diagram of imidazolium-based ABSs combined with salts such as NaCl, NaH2PO4, Na2CO3, Na2SO3, Na2SO4, Na3PO4, Na3C6H5O7, KOH, K2HPO4, K2CO3, K3PO4, K3C6H5O7 and (NH4)2SO4.54,55,57,58 Shekaari and coworkers have studied the influence of temperature on the phase behavior of 1-butyl-3-methylimidazolum bromide ([bmim]Br) + aqueous tri-sodium citrate.33 They observed a decrease in the existence of two-phase region with the increase in temperature. The onset of phase demixing at elevated temperature was observed to shift towards higher temperature and salt concentration. In addition, free energy of phase separation for ILs was calculated from the temperature-dependent cloud-point data. Compared to the polymer-saltbased ABSs which showed higher value of free energy for phase separation, very low free energy changes were noted for the IL-salt-based ABSs. However, this observation of Shekaari and coworkers was contradicted by Han et al. and Wang et al.54,57,59 They observed easier phase demixing at a higher temperature because of the transfer of water molecules from IL-rich phase towards aqueous salt phase. Zafarani-Moattar and Hamzehzadeh in their study reported that the

17

temperature has no influence on the tie-line slopes for the IL-based ABSs.42,49 Similar to ILwater-inorganic salt-based ABSs, smaller immiscibility regions were noted at a higher temperature for IL-water-amino acid- and IL-water-carbohydrate-based ABSs.27,41,60-63 Wells and co-workers noted that [bmim][BF4] may break the equilibrium concerning the cyclic structures and chain structures of sucrose through complexation of [bmim]+ and sucrose.64 Wu et al. concluded that in the case of IL-water-carbohydrate-based ABS, [bmim]+ cause destabilization of the cyclic structural form into chain form.41 This IL-induced decyclization of sugar dispels the compatibility between the OH groups on ring carbohydrate molecules and hydrogen bond structure of water and thus help in ABS formation. This IL-induced decyclization of carbohydrates decrease with increasing temperature. On the basis of the temperature-dependent study Wu et al. inferred that the interaction between [bmim][BF4] and sugar is exothermic in nature and the strength of this interaction decreases with temperature. At higher temperature, more [bmim][BF4] would be required for the phase separation. This observation was consistent with work of Rebelo and coworkers.65 However, this observation contradicts with the findings of Freire et al. who observed that the formation of a hydrated complex is the driving force for salting-out carbohydrate from the system.29 The effect of temperature on the phase diagram of IL-water-polymer-based ABS was first studied by Rebelo and coworkers who observed the change in LCST locus of aqueous polyethylene glycol (PEG) as a function of IL concentration.66 Friere et al. reported lower immiscibility of PEG 2000 and [C2C1im][Cl] and [C2C1im][C1COO] with the rise in temperature.43 The extent of demixing with the rise in temperature influence on the ABS formation was noted to be higher for [C2C1im][C1COO] than [C2C1im][Cl] due to the higher saltout ability of acetate for PEG 2000. Contrary to the PEG 2000 with acetate- and chloride-based

18

ILs, an opposite trend was observed for the polypropylene 400 (PPG 400).43,67 This contradiction in the phase behavior of PEG 2000 and PPG 400 infers that ABS formation is a complex phenomenon and it largely depends on the structural components. The difference is phase forming behavior with PEG 2000 and PPG 400 is because of the large difference in polarity and hydrogen bond donating ability. The temperature-dependent observations for the IL-waterpolymer ABSs are similar to that of polymer-polymer, polymer-salt, and IL-salt systems. This suggests that, in general, almost similar mechanism prevails in all these cases, however, more studies are required to completely understand the effect of temperature on the phase separation of IL-based ABSs.

3.3.3. Effect of pH The use of ABS with a distinct pH can have importance in extracting specific biomolecules, such as proteins, which exhibit lower dissociation constant in acidic condition. However, the effect of pH on the phase demixing is limited to the IL-water-salt-based ABS. The alkalinity of the medium promotes the phase demixing in the IL-water-salt-based ABSs, while acidity and neutrality conditions are poorly understood on the biphasic separation. ZafaraniMoattar and Hamzehzadeh have studied the effect of aqueous pH on the phase separation in [bmim][Br] + potassium citrate system by the addition of kosmotropic salt.50 In general, a decrease in the pH leads to an increase in the area of the monophasic region. Similar observations were noted by the pH-dependent phase separation of [bmim][Cl] with aqueous alkaline solution (K2HPO4, K3PO4, K2CO3, KOH, Na2HPO4 and NaOH) and acidic and neutral salt (KH2PO4, K2SO4, (NH4)2SO4, KCl or NaCl) solution by He et al. and Li et al, respectively.31,68 Contrary to these observations, Claudio et al. noted that the phase demixing in

19

ABS formation can be accelerated by incorporating the suitable structural feature in IL structure.28 The authors concluded that though kosmotropic salts follow the Hofmeister series, lower pH reduces their efficacy towards liquid–liquid demixing.28At lower pH, the number of ILs that are able to suffer phase splitting is largely reduced.

3.3.4. Effect of salt, polymer and carbohydrate The extent of the biphasic separation in ABSs depends largely on the selection of the second phase forming component, viz., salt, polymer and carbohydrate. Proper selection of these phase forming components facilitates the necessary driving force for biphasic separation. The pioneering work of Rogers and coworkers demonstrated that it is possible to create biphasic separation using a water-soluble IL and a water-structuring inorganic salt (the salting-out agent).25 This work upsurge stupendous increase in hydrophilic IL-water-salt-based ABSs owing to the availability of various combinations of cations and anions with kaotropic salts. A difference in the partition coefficients of the hydrophobic IL and kaotropic salt eases the transfer of a solute from the aqueous phase to the IL phase. A large difference in the partition coefficients of IL cause precipitation of the IL phase from the aqueous salt phase and inhibit the solute transfer. The partition coefficient difference between hydrophobic IL and salt is maximum and therefore these ILs are not very useful in the ABS formation. In the IL-water-salt systems, both the phase forming components are ionic speciation of these electrical entities cannot be avoided. Therefore, in order to maintain the electroneutrality of the IL-water-salt systems concentration of “free ions” obtained due to the speciation of the ionic species in aqueous solution is used in place of the total concentration of ions.11 Shahriari et al. have shown that for a given IL ABS formation eases with the salting-out ability of the inorganic salt.34 The ability of the salt-ions was

20

noted to follow the Hofmeister series. Moreover, the ordering of the ions in the Hofmeister series closely follows the order of the Gibbs free energy of hydration of the ions. However, it was demonstrated by some workers that some deviations can occur to the well-established series and that mainly depend on the interactions occurring among the salt ions and on the ions speciation in aqueous media.69-72 While aiming at evaluating the impact of polyvalent salt cations on the formation of IL-based ABSs Kurnia and coworkers demonstrated that the extent of biphasic separation in IL based ABSs is not only dominated by the ability of the strong and “free” saltingout ions to interact with water creating thus hydration complexes but also a result of the interactions occurring between the different ions and, particularly, on their speciation in aqueous solutions.73 They also noted that the inclusion of the “free” ion concentration on biphasic separation also helps in establishing the Hofmeister ordering of the cations and anions. Most of the authors claimed that the ability of salt ions in promoting the formation of IL-based ABS depends on their Gibbs free energy of hydration, a systematic investigation conducted by Shahriari et al., however, revealed that the ABS formation ability largely depends instead on the molar entropy of hydration of the salt ions.26,31,34,56,58,68,74,75 Contrary to other biphasic systems, IL-water-polymer-based ABS has an advantage as both components can be salted-out from the solution depending upon the proper selection of IL and polymer. Thus for IL-water-polymer-based ABSs, both “upper polymer rich” and “upper IL rich” phases are possible. However, the ABSs consist of IL and polymer are only studied with two classes of polymers namely PEG and PPG. For a given IL, it is noted that PPG easily undergoes phase separation in comparison to the PEG of identical molecular weight.76-78 PPG contains a longer hydrocarbon tail and methyl group also hinder the hydrogen bond formation with water molecule and ether oxygen of the adjacent moiety, thereby making itself more

21

susceptible to salt-out by the IL from the water. This affinity/miscibility pattern also corroborates the lower solubility of PPG in IL than that of PEG.79 For a given IL, it was also noted that the position of binodal curve shifts towards the origin as the molecular weight of the polymer increased. The shift of the binodal curve towards the origin indicates that lower concentration of polymer is required for the creation of ABS. In general, it is observed that the higher molecular weight polymers exhibit lower affinity for water and are preferentially saltedout by the IL. Rodriguez et al. demonstrated that the higher molecular weight polymers do not solubilize in IL and hence phase-out easily.46 Carbohydrate-water-IL-based ABSs are environmentally-benign and offer promising technology for purification and separation. The extended network of hydrogen bonding hydroxyl group in carbohydrates inhibits them to form ABS with ILs, and hence, IL-water-carbohydratebased ABSs are less explored as compared to the other salt and polymer-based ABSs.27,29,40,61 Wu et al. investigated the influence of glucose, fructose, sucrose and xylose on the ABS formation with [bmim][BF4] and noted that the demixing ability directly depends on the number of the equatorial hydroxyl group in the carbohydrates. Among various carbohydrates, the calculated number of equatorial hydroxyl groups was maximum in sucrose (6.2) and minimum in fructose (2.56). Carbohydrates containing higher number of equatorial hydroxyl group undergo ABS formation easily than those having a lower number of equatorial hydrogens. Friere et al. noted that equatorial hydroxyl group easily gets hydrolyzed than axial hydroxyl groups.29 The ease of ABS formation by carbohydrates follows the order: glucose > galactose > mannose. Carbohydrates with a higher number of hydroxyl group exhibit greater affinity for hydrogen bonding with water and thereby easily salt-out the chaotropic IL. Ferrera et al. showed that not only the number of equatorial hydroxyl group but the size of cation core and the length of the

22

alkyl group either on cation or anion also favor the ABS formation.80 Apart from the number of equatorial hydroxyl groups, ring conformation also affects the ABS formation. Maltose easily undergoes ABS formation in contrast to sucrose as the former has pyranose conformation while later has furanose conformation.29 In the case of polyols, the ABS formation tendency increases with the number of the hydroxyl group. The higher ability of polyols in ABS formation is also partially due to the conversion of the cyclic structure into the linear one. This involves the conversion of aldoses and ketoses into the hydroxyl, thereby further promoting the kosmotropic ability.29

4. Applications of IL-based ABSs in extraction of biomolecules and metal ions Almost six decades ago Albertsson, for the first time, employed ABSs in the separation of biological molecules which was later proved to be of immense utility in chemical as well as in biochemical and cell biological basic and applied research.23 ABS provides a technically simple, easily scalable, energy-efficient, mild and nontoxic separation technique for product recovery in biotechnology.81 The conventional methods that have been used for the separation of biomolecules (amino acid and protein) are ionic exchange process, reverse micelle method and liquid membrane extraction process; however, organic solvents which employed in these methods poses problems such as, high volatility, flammability and toxicity.47,82,83 Therefore, recovery of biomolecules with IL-based aqueous-two-phase systems may be a suitable alternate to more traditional methods. The initial interest in IL-based ABSs is because of their insignificant vapor pressure, low flammability, wide liquidus range, high thermal and chemical stability and last but not the least recyclability. The judicious selection of cations and anions allows fine tuning of the physicochemical properties of IL for achieving enhanced yield of

23

product recovery.32,36,84 The efficacy of IL-based ABSs depends on the properties of solute and phase forming components and their polarities. However, solute partitioning in one phase of the IL-based ABSs can be manipulated by changing the phase forming components, their concentrations and adding co-solvent, amphiphilic solvent, or antisolvent.11 Application of various IL-based ABSs in separation is summarized in Table 2.

Table 2 Application of IL-based ABSs in separation S. N. IL-based ABSs

Measurement of ABS

1

Cloud point titration Cloud point titration

2

[C4C1im]Cl + aq. K3PO4 [C4C1im][BF4] + aq. Sucrose/maltose

3

[C4C1im][CF3SO3]

4

[C4C1im]Cl + aq. K2HPO4 [C4C1im][BF4] + aq. NaH2PO4

5

6 7

8

9

10

Imidazolium-based ILs + PEG/Na2SO4 [C2C1im]Cl + aq. PEG 1500/2000/3400 Phosphonium-based ILs + aq. K3PO4

[C4C1im][BF4] + aq. glycine/L-serine/Lproline [C4C1im][BF4] + aq. fructose

Cloud point titration Molecular fluorescence spectrophotom etry Cloud point titration Cloud point titration Cloud point titration

Application in separation

Predictive equation

Ref. 25

Othmer-Tobias and Bancroft equations

27

L-tryptophan, caffeine and βcarotene Testosterone and epitestosterone Acetylspiramycin

29

L-tryptophan

44

Cellulose and lignin

46

L-tryptophan, βcarotene, rhodamine 6 G and caffeine

51

31 39

Cloud point titration

60

Cloud point titration

Othmer-Tobias and Bancroft equations 24

63

11

[C2C1im][Br + aq. PPG 400

Cloud point titration

L-tryptophan and L-tyrosine

12

[C4C1im][BF4] + aq. salt [AC1im]Cl + aq. K3PO4, K2HPO4 and K2CO3 [AC1im]Cl / [C4C1im]Ac / [C4C1im]Cl + aq. PPG Cholinium-based ILs + PPG 400/1000

Cloud point titration Cloud point titration

Codeine and papaverine Recovery of hydrophilic ILs

Cloud point titration

Recovery/enrichme nt of aq. hydrophilic ILs

Perfluoroalkylsulpho nate-based ILs + aq. Mono-, di- and polysaccharides Hydrophobic ILs + aq. potassium citrate [C4444N]Br + aq. (NH4)2SO4 [C4C1im]Br + PEG 600 + tri-potassium citrate Hexaalkyl- and tetralkylguanidinium -based ILs + aq. K3PO4 1,1,3,3tetramethylguanidini um-based ILs + aq. K2HPO4 IL Ammoeng 100, 101 and 110 + aq. K2HPO4/KH2PO4 [C4C1im]-based hydrophobic ILs + crown ether Betaine[Hbet][N(Tf)2] [Hbet][N(Tf)2] + water

Cloud point titration

Food dyes

80

Cloud point titration

L-tryptophan

84

Cr(VI) from Cr(III)

86

13

14

15

16

17 18 19

20

21

22

23

24 25

Cloud point titration

Non-random two-liquid (NRTL) model

67

68 Bancroft and Othmer-Tobias equation Bancroft and Othmer-Tobias equation

75

Bancroft and Othmer-Tobias equation

77

Cloud point titration

L-tyrosine

Cloud point titration

Lysozyme, trypsin, ovalbumin and bovine serum albumin (BSA) Bovine serum albumin (BSA)

90

Alcohol dehydrogenases (ADHs) Sr(II)

102

Nd(III)

107

Pd(II), Rh(III) and Ru (III)

110

Cloud point titration

Cloud point titration

Cloud point titration Cloud point titration

25

Non-random two-liquid (NRTL) model

76

87

91

103

26

Imidazoliumpyridinium- and pyrrolidinium-based ILs + water

Tetrachloroaurate and tetrabromoaurate

119

Initially the extraction of essential amino acid L-tryptophan was studied in imidazoliumwater-K3PO4- and phosphonoium-water-K3PO4-based ABSs and results were analyzed in terms of the change of IL structure and nature of salts.32,36,51 Later, the strongest salting-out agent, K3PO4 was replaced by the “greener alternative” potassium citrate (K3C6H5O7) which is biodegradable, non-toxic and can be discharged into the water.85 The partitioning of biomolecules in IL-water-salt-based ABSs was dependent on hydrophobic interactions, electrostatic forces, solubility and affinity for both phases and nature of biomolecules. Coutinho and coworkers noted that imidazolium-based ILs containing double bond, hydroxyl group and presence of aromatic side chain facilitated higher partition coefficient of biomolecules in the IL phase.32 Lower partition coefficient was observed for those ILs which were either unsubstituted or monosubstituted. Based on these observations they suggested that the partition coefficient of L-tryptophan in IL phase is largely governed by the π-π stacking interactions, hydrogen bonding and hydrophobic interactions.32 In regard to the influence of IL anion, they noted that ILs containing halide anion (Cl-, Br-) or more hydrophobic anion (N(CN)2-) possess higher ability to accommodate L-tryptophan. The ordering of IL anions in the partitioning of L-tryptophan was in accordance with the Hofmeister series.32 Li et al. observed the effect of alkyl side chain on the partitioning behavior of L-tryptophan for a series of alkylimidazolium acetate IL-based ABSs.85 The observed results indicated no definite order in alkyl chain as expected based on their increasing hydrophobicity upon the increment of methylene group (-CH2) successively. The highest partitioning of L-tryptophan was obtained in [C6C1im][C1CO2] while the least

26

partitioning was in [C8C1im][C1CO2].85 However, the self-aggregation of IL interferes with the solute partitioning and therefore the reliable partitioning was noted in the ILs containing hexyl chain on imidazolium cation.86 Louros and coworkers demonstrated nearly double partitioning of L-tryptophan for the ABS composed of [P1444][C1SO4] and K3PO4.51 ABSs containing phosphonium-IL possess lower density than water and hence can be easily separated during work up, thermally more stable and have no acidic proton than that of imidazolium-based ILs. Passos et al. investigated the role of ABSs composed of ammonium, phosphonium and imidazolium ILs and potassium citrate in the extraction of L-tryptophan.84 Ammonium and phosphonium-based ILs exhibited greater aptitude in the partitioning of L-tryptophan than imidazolium-based ILs. The results indicated that besides the π-π interactions, hydrophobic interaction and amino acid-IL interaction, the partition coefficient of L-tryptophan and composition of phases determines the extraction of amino acid.84 The partitioning of L-tryptophan among ABSs composed of imidazolium-based ILs and potassium citrate salt was lower than that of imidazolium IL + K3PO4-based ABSs.36,84 Freire et al. measured the partitioning behavior of model amino acid L-tryptophan in 1butyl-3-methylimidazolium trifluoromethanesulfonate ([C4C1im][CF3SO3]).29 When compared with the PEG-polysaccharide-based ABSs, IL-carbohydrate-based ABSs were found superior for the separation of L-tryptophan. This suggested that [C4C1im][CF3SO3]-polymer ABS possess higher extraction ability than PEG-polymer ABS. However, the maximum extraction ability of [C4C1im][CF3SO3] was obtained with monosaccharides and disaccharides.29 Surprisingly, the extraction ability was found independent of the type of carbohydrate involved due to the absence of interaction between carbohydrate and model amino acid L-tryptophan. However, he

27

partitioning of L-tryptophan was noted to vary with the extent of π-π interaction and hydrogen bonding.29 Contrary to the IL-salt- and IL-carbohydrate-based ABSs, IL-polymer-based ABSs show lower aptitude in the partitioning of amino acid. However, when used as adjuvants in the classical polymer-salt-based ABSs, ILs were noted to accelerate the partitioning of the Ltryptophan and L-tyrosine because of the modification of the characteristics of the polymer-rich phase. Pereira et al. have studied the effect of the 5 % (w/w) of IL to the PEG-600 + Na2SO4 ABS and discussed the results in terms of the change of cation, anion, alkyl chain length and presence of the different functional group on the alkyl side chain.44 In this study hydrophilic ILs, such as, imidazolium chloride ([im][Cl]), 1-methylimidazolium chloride ([C1im][Cl]), 1-ethyl-3methylimidazolium chloride ([C2C1im][Cl]), 1-hydroxyethyl-3-methylimidazolium chloride ([OHC2C1im][Cl]) and 1-allyl-3-methylimidazolium chloride ([aC1im][Cl]) were noted to decrease the partitioning of L-tyrosine in the PEG-600 phase when added as adjuvants.44 The lower aptitude of these hydrophilic ILs in promoting the extraction of amino acid towards the PEG-rich phase was attributed to their salting-out characteristics which made the upper phase PEG-deficient. Contrary to this, ILs containing longer alkyl chain and the substituent at the C-2 carbon promoted the transfer of amino acid in the PEG-rich upper phase. In short, insertion of IL in PEG phase promotes the efficacy of PEG-polymer-based ABSs.44 The influence of anion on the partitioning of L-tryptophan was in accordance with their position in the Hofmeister series. Hamzehzadeh and Abbasi investigated the influence of the 1-butyl-3-methylimidazolium bromide ([C4C1im][Br]) on the partitioning of L-tyrosine in the ABS formed by PEG-600 and K3C6H5O7.87 The efficiency of the ABS was higher when [C4C1im][Br] was used as adjuvants rather than as a phase. In another attempt Hamzehzadeh and Vasiresh measured the effect of

28

[C4C1im][Br] on the partitioning of L-tryptophan in the ABS composed of PEG-400 and K3C6H5O7 in terms of the change in the tie-line length.88 Higher partitioning of L-tyrosine in PEG- K3C6H5O7-based ABS was fortified by the increase in the tie-line length. Unlike the extraction of amino acid, protein purification protocols require the development of more specific, robust and cost-effective methods.89 IL-based ABSs find importance in the extraction of protein molecules as they offer mild operating conditions which is necessary to maintain their nativity and functionality. IL-based ABSs has several advantageous characteristics such as nonvolatile nature, polarity, viscosity, solvent miscibility and most importantly tailorability than the classical polymer-polymer- and polymer-salt-based ABSs. In all studies, ILs formed by imidazolium and guanidinium cation along with chloride, bromide, tetrafluoroborate, butylsulfate and ethylsulfate anions are employed with kosmotropic salts to create ABSs for protein extraction.90-96 Protein extraction with IL-salt-based ABSs largely depends on the structure, functionality and the concentration of IL.97 Large protein molecule such as RuBisCo, formed aggregate at IL concentration 30% v/v while the small protein Bovine serum albumin (BSA) started aggregating when IL concentration reached 50% v/v.97 Small angle neutron scattering (SANS) revealed that at 50% v/v [C4C1im][Cl] induce aggregate formation in human serum albumin (HAS) and cytochrome c with retained structure and functionality.98 Fujita et al. showed that choline dihydrogen phosphate improve thermal and long-term stability of cytrochome c owing to the higher kosmotropicity of ILs.99 Several attempts were made to correlate the stability of proteins in terms of the kosmotropicity and chaotropicity of anion and cation, respectively. However, Debeljuh et al. showed an inverse Hofmeister trend of IL ions during the amyloid fibrilization of Aβ16-22.100 In fact, protein stability in ILs is an

29

overall consequence of hydrogen bonding, electrostatic interaction, dispersive interactions and hydrophobicity. Cao et al. studied the activity of enzyme horseradish peroxidase in imidazolium-based ABSs.101 The enzyme was exclusively partitioned in the aqueous-IL rich phase; however, the amount of enzyme in the aqueous-IL phase was observed to vary with the size of alkyl chain on cation and concentration of IL. Higher enzyme concentration was found with the ABS formed by the IL containing shorter alkyl chain length at higher concentration.101 Dreyer et al. investigated the extraction behavior of enzyme alcohol dehydrogenase in ammonium IL (Ammoeng100, 101 and 110)-based ABSs.102 Ammoeng ILs contain an oligo-ethylene side chain in the cation. This facilitates higher partitioning of the enzyme in aqueous-IL phase at a lower temperature. The ABSs employed for the extraction of metal ions is different than the traditional ABSs and are constructed by adding mineral acid (HNO3), salt or an extracting agent. After the pioneering work of Dai et al., which showed that ILs exhibit four-time higher extraction efficiency for Sr(II) than the organic solvents, several groups accounted the benefits of involving these coulombic media over the volatile organic compounds (VOCs).103-106 In all ABSs, engaged in the extraction of metals, monophasic to biphasic conversion is brought about either by altering the temperature of the system or by varying concentration of one constituent. The subjected IL for metal extraction should exhibit LCST/UCST behavior with water. Conversely to the IL-saltbased ABSs, where salt controls the salting-in/salting-out ability of the system, the role of the extracting agent, mineral acid, and the metal salt is to bring down the UCST to benefit extraction. A tremendous increase in the performance of metal extraction upon the decrease in UCST is noted in several studies.107-109 ABS composed of betanium cation and bistriflimide anion ([Hbet][NTf2]) in aqueous state is used for the extraction of U(VI), Pd(II), Rh(III), Ru(III),

30

Sc(III) and Fe(III).110-112 In all these studies, extraction of metal ions was brought about by heating the system above UCST (60 oC) and then cooled back to room temperature. Binnemans and coworkers added the extracting agent betaine (bet) in the aqueous [Hbet][NTf2] and extracted the Cu(II), Y(III), Dy(III), Er(III), Ho(III), La(III), Pr(III), Nd(III), Ga(III), In(III) and Sc(III) by contacting the aqueous and IL phases followed by vigorous shaking at room temperature.113 Binnemans and coworkers obtained nearly 100% extraction for Sc(III) by when bringing the biphasic separation below UCST under vigorous shaking.113 In absence of betaine negligible extraction of Lanthanide ions was seen by the aqueous [Hbet][NTf2]. The addition of the betaine (13% w/w) improved the extraction of Cu(II) and Sc(III) to 30-fold and 600-fold, respectively. In absence of betaine, nitric acid (HNO3) was also noted to increase the extraction of Pd(II), Rh(III) and Ru (III) to 100 to 95%, 70 to 40% and 40 to 20%, respectively, depending on the concentration of acid.110 Nockemann and coworkers employed cholinium bistriflimide ([Chol][NTf2]) which in aqueous state has UCST 72 oC.114 Akama and coworkers have used tetraalkylammonium halide along with aqueous sodium salts for biphasic extraction of Cd(II), Pd(II), Co(II), Cu(II), Fe(III) and Zn(II).115 Choi and coworkers used ABS constituted by aqueous tetrabutylammonium bromide ([TBA][Br]) and ammonium sulfate ((NH4)2SO4) for the separation of Ru in a “microfluidic manner”.116 Bridges and coworkers obtained pertechnate anion ([TcO4-]) by using [C4C1im][Cl]-salt (K3PO4, K2CO3 and K2HPO4)-based ABSs.117,118 Various imidazolium, pyridinium and pyrrolidinium halide-based ILs in presence of salt and mineral acid are utilized in extraction of Au(III), Pt(IV) and Ir(IV).119-121 In all ABSs, extraction efficiency was observed to improve with the increasing concentration of mineral acid. Binnemans and coworkers used tetraalkylphosphonium-based IL (40% w/w) with alkali salt solution for the extraction of Co, Ni, Cu, Zn and Sc.112,122 In phosphonium IL-based ABSs, metal

31

separation were performed by cooling the system below LCST and then heated up for creating biphase.

5. Liquid-liquid equilibria (LLE) involving ILs The design of safe and environmentally-benign separation processes has an increasingly important role in the development of clean manufacturing processes. In this regard, various approaches have been identified using water, supercritical fluids and ILs.123,124 Liquid-liquid equilibria (LLE) is a common phenomenon exhibited in binary solutions. At LLE, system becomes univariant according to the Gibbs phase rule and thus depends on the change in one variable, i.e., temperature, pressure or composition of the system. If any of these variables is fixed, all other variables can be determined at equilibrium. Knowledge of phase equilibria (LLE) of various IL-systems is important in evaluating and effectively employing IL-systems in potential applications like extraction and separation.1,2,125-130

5.1. Chemical composition and phase diagram of binary and ternary systems LLE is a simple procedure in which different types of solute, dissolved in one phase, is removed preferentially by an immiscible liquid in which solute has a high affinity in comparison to others. The solution which is subjected to the extraction is called feed and the liquid with which the feed is contacted is called solvent. The mixing of two liquids results in biphasic solution in which solvent rich phase is known as extract and the residual liquid remains after the extraction is known as raffinate.

32

Phase demixing in binary systems constituted by ILs and hydrocarbons exhibit both LCST and UCST behavior. The equilibrium constant for the reversible distribution in binary systems is determined by the Nernst distribution law, 𝑋𝐴 ↔ 𝑋𝐵 𝐾𝐷 =

(12)

𝑋𝐵 𝑋𝐴

(13)

where 𝑋𝐴 and 𝑋𝐵 are concentrations of solute in phase A and B. The extracting conditions should afford maximum value of 𝐾𝐷 indicating complete transfer of solute from phase A to B. Few hydrocarbons, such as chloroform, benzene and thiophene showed both LCST and UCST phase behavior at different temperatures. Domanska and coworkers have found LCST phase behavior for aromatic hydrocarbons and thiophene along with ILs containing thiocyanate and tricyanomate anion anions and alkylpyrrolidinium and alkylmorpholonium cations.131-133 1Butyl-3-methylimidazolium tricyanomate ([C4C1im][TCM]) with thiophene undergoes UCST to LCST phase behavior higher and lower temperatures. The LCST and UCST phase behavior depends on the magnitude of excess enthalpy (𝐻 𝐸 ). UCST phase behavior is shown by the binary systems for which 𝐻 𝐸 > 0 while for the LCST phase behavior 𝐻 𝐸 < 0. The nature of intermolecular interactions decides the fate of 𝐻 𝐸 . Strong intermolecular interactions favors dissolution of IL and hydrocarbon as the temperature increase and results UCST phase behavior. Unfavorable intermolecular interactions between IL and hydrocarbon lowers the solubility of phases. The magnitude of unfavorable interactions diminishes as the temperature decreases resulting LCST phase behavior. A binary phase diagram in LLE is a representation of cloud point temperature vs composition. Wu et al. investigated the phase equilibria for the binary mixture of 1-alkyl-3-methylimidazolium hexafluorophosphate (alkyl = butyl, pentyl, hexyl and

33

octyl) with 1-butanol.134 The mixture showed an UCST at mole fraction of butan-1-ol (𝜒𝑏𝑢𝑡𝑎𝑛−1−𝑜𝑙 ) = 0.9. They found a decrease in the UCST upon increasing the alkyl chain length on imidazolium cation. LLE in ternary systems containing an IL, aromatic hydrocarbon and aliphatic hydrocarbon, is studied by several workers and a ternary phase diagram is represented in orthogonal form. The apex of equilateral triangle denotes initial amounts (100%) of components. The concentration of components varies along the sides of the triangle. Along the sides of equilateral triangle concentration of the components at two ends vary while the concentration of the third component remains constant. The concentration of hydrocarbons in the IL-phase is obtained by the Gas chromatography and NMR techniques. LLE for the ternary systems containing 1-ethyl-3-methylimidazolium triiodate ([C2C1im][I3]), toluene and heptane at 318.2 K is shown in Fig 3.135 The efficiency of extraction in the ternary system is determined in terms of the partition coefficient (β) and selectivity (S). Wilson et al. defined partition coefficient as the ratio of mole fraction of one component (say toluene) in the extract (x(1)𝐸 ) to the mole fraction of the same component in the raffinate (x(1)𝑅 ).136 𝛽=

𝑆=

x(1)𝐸 x(1)𝑅

x(1)𝐸 /x(1)𝑅 x(2)𝐸 /x(2)𝑅

(14)

(15)

where, x(1) and x(2) represents the mole fractions of component 1 and 2, respectively. A high value of β indicates that less amount of extracting solvent (say IL) is required for the extraction of solute (say toluene). High β value results in high selectivity.

34

Fig. 3. Liquid−liquid equilibrium for the system toluene + heptane + 1-butyl-3methylimiazolium triiodide ([BuMeIm][I3]) at 308.2 K (35 °C). [Key:  ·, experimental data, experimental tie line, − − −, calculated tie line from the NRTL equation]. (“Reprinted with permission from135, copyright (2000) American Chemical Society”)

5.2. Effect of IL structure on LLE To design an IL for a specific separation problem the knowledge of the influence of cation and anion structure on solubility is required. ILs are considered as suitable extractant for sulfur compounds from fuels, therefore, a knowledge of aliphatic and aromatic compounds solubility in IL is studied by several groups.137-140 Marciniak and Karczemna studied the LLE for a binary mixture of trifluoromethanesulfonate-based ILs with aliphatic (n-hexane, n-heptane and cyclohexane) and aromatic (benzene and toluene) hydrocarbons using “cloud-point” method.141 The LLE phase behavior for the binary systems of aliphatic hydrocarbons were similar to the 35

typical UCST diagrams while the phase diagrams of aromatic hydrocarbons were identical to the LCST. Aromatic hydrocarbons showed better solubility in ILs because of the π-π interactions than those of aliphatic hydrocarbons where such favorable interactions are lacking. However when compared the efficiency of IL cations in the extraction of hydrocarbons, alkylpyridinium IL was observed most effective while alkylimidazolium IL was noted least effective. The influence of same alkyl chain on different cationic core has no major influence over the extraction of hydrocarbons. All aliphatic hydrocarbons were more soluble in alkylpyrrolidinium IL because of its more aliphatic character while all aromatic hydrocarbons were highly soluble in alkylpyridinium IL than alkylimidazolium IL owing to the higher aromatic nature of former than the latter.141 Calver et al. investigated the LLE for aliphatic and aromatic hydrocarbons with 1butyl-3-methylimidazolium ([C4C1im]+) cations and two different anions bis(trifluoromethanesulfonyl)imide ([NTf2]-) and methylsulfate ([MeSO4]+).142 The miscibility of both hydrocarbons were higher in [C4C1im][NTf2] than that of [C4C1im][MeSO4]. Greater solubility of hydrocarbons in [C4C1im][NTf2] is supported by higher selectivity (S) and partition coefficient values and is attributed to the ([NTf2]-) anion.142 Effect of IL structure on the extraction of hydrocarbons in ternary systems was studied by several groups. Marciniak and Królikowski studied LLE for three ternary systems containing ILs 4-(2-methoxyethyl)-4-methylmorpholinium bis(trifluoromethylsulfonyl)imide, 1-(2-methoxy ethyl)-1-methylpiperidinium bis(trifluoromethylsulfonyl)imide, 1-(2-methoxyethyl)-1-methyl pyrrolidinium bis(trifluoromethylsulfonyl)imide) + thiophene + n-heptane.143 The solubility of thiophene in all ILs was higher than that of n-heptane. 4-(2-methoxyethyl)-4methylmorpholinium bis(trifluoromethylsulfonyl)imide was the most efficient in the extraction of thiophene from n-hexane indicating the influence of the IL cation.143 In another study,

36

Marciniak and Królikowski measured the LLE for ternary mixtures containing 4-(2methoxyethyl)-4-methylmorpholinium bis(trifluoromethylsulfonyl)imide, 1-(2-methoxyethyl)-1methylpiperidinium bis(trifluoromethyl sulfonyl)imide, 1-(2-methoxyethyl)-1methylpyrrolidinium bis(trifluoromethylsulfonyl)imide) + methanol + heptane.144 High selectivity and partition coefficient values were obtained in all ILs indicating susceptibility of these ILs in the separation of methanol + heptane mixture. Out of all the cationic structures, 4-(2methoxyethyl)-4-methylmorpholinium cation has shown higher aptitude in the separation of methanol + heptane mixture.144 Królikowski studied the LLE equilibria for a ternary mixture containing 1-butyl-1-methyl-morpholinium tricyanomethanide/1-butyl-3-methylimidazolium tricyanomethanide + p-xylene + octane or decane.145 The most efficient system for the p-xylene + octane/decane extraction was one with IL 1-butyl-1-methyl-morpholinium tricyanomethanide.

5.3. Effect of pressure on LLE Gutkowski et al. studied high pressure phase behavior of binary system composed of 1octyl-3-methylimidazolium tetrafluoroborate ([C8C1im][BF4]) + carbon dioxide (CO2) in the pressure range 0.1-100 MPa.146 The mutual solubility of CO2 in the IL was found to increase up to the mole fraction of CO2 = 0.6. Beyond the CO2 mole fraction 0.6, an increase in the equilibrium pressure was observed suggesting lower solubility of CO2 in [C8C1im][BF4]. Najdanovic-Visak et al. measured the LLE up to 400 bar using “cloud-point” method for ternary systems containing [C4C1im][PF6] + ethanol + water and [C4C1im][NTf2] + 2-methylpropanol + water.147 High pressure favored mutual solubility of alcohol with both [C4C1im][PF6] and [C4C1im][NTf2] which is also revealed by low cloud-point temperature. However, at elevated pressure miscibility of water with ILs was noted to decrease. Zhang et al. investigated the effect

37

of pressure on the phase behavior of CO2/acetone/1-butyl-3-methyl-imidazolium hexafluorophosphate ([C4C1im][PF6]) system.148 At pressures from 4.9 to 8.1 MPa, a three phase region appeared in the system indicating lower miscibility. The distribution coefficient values at different pressure suggested lower solubility of CO2 in IL at elevated pressure. An increased solubility of acetone in the IL was observed with the rise in pressure.148 Fu et al. determined the concentration of naphthalene in CO2 rich phase using IL 1-butyl-3-methylimidazolium hexafluorophosphate ([C4C1im][PF6]) at pressure range 8-20 MPa.149 The concentration of naphthalene in CO2 was increased more with increasing pressure from 8-12 MPa in presence of IL. A small increase in the miscibility of naphthalene with CO2 was seen in the pressure range 12-20 MPa.

6. Use of predictive models for correlation and interpretation of IL LLE data The huge number of potential ILs make their selection impossible in the extraction/separation simply based on the experimental results. However, the adequate selection of IL can be made possible by developing predictive models and correlations that can be employed in the comprehension of phase equilibria. A number of predictive models have been used to evaluate interactions between components of the solution. Two classical local composition models, namely Non-Random Two Component (NRTL) and UNIQUAC, have been used in several studies to correlate by the experimental data.140,150-156 NRTL model is popularly used for LLE data correlation of IL-based binary and ternary systems.157 Binary interactions between IL and molecular solvents using NRTL is calculated by using Eq. 16. 𝜏𝑖𝑗 = 𝑎𝑖𝑗 + 𝑏𝑖𝑗 /𝑇,

𝐺𝑖𝑗 = exp(−𝑎𝑖𝑗 𝜏𝑖𝑗 ),

(16)

38

𝑎𝑖𝑗 = 𝑎𝑗𝑖

where, where 𝜏𝑖𝑗 and 𝐺𝑖𝑗 are NRTL parameters calculated by using the non-randomness parameter 𝑎𝑖𝑗 and the parameters 𝑎𝑖𝑗 and 𝑏𝑖𝑗 (K). The model parameters of the NRTL equation (𝑎𝑖𝑗 , 𝑎𝑖𝑗 , 𝑎𝑗𝑖 , 𝑏𝑖𝑗 (K) and 𝑏𝑗𝑖 (K)) can be determined by data regression using ASPEN Plus 12.1. In the state of equilibrium, activity coefficient of the 𝑖 component in both raffinate and extract phase can be used to establish the referring equation. 𝑥𝑖𝑅 =

𝛾𝑖𝐸

𝛾𝑖𝑅

𝑥𝑖𝐸

(17) where, 𝑥𝑖𝑅 and 𝑥𝑖𝐸 represent the composition of 𝑖 in the raffinate phase and extract phase, respectively. 𝛾𝑖𝐸 and 𝛾𝑖𝑅 denote activity coefficient of component 𝑖 in the extract phase and in the raffinate phase, respectively. Meindersma and coworkers studied LLE for binary and ternary systems for composed of sulfolane and four ILs (4-methyl-N-butylpyridinium tetrafluoroborate ([C1C4py]BF4]) or 1-ethyl3-methylimidazoliumethylsulfate ([C2C1im][C2H5SO4]) or 1,3-dimethylimidazolium methyl sulfate ([C1C1im][CH3SO4]) or 1-butyl-3-methylimidazolium methylsulfate ([C4C1im][CH3SO4]) + toluene + n-heptane at 313.2 and 348.2 K temperatures and 0.1 MPa pressure.158 LLE data for both binary and ternary systems were correlated with the NRTL model. Compared to sulfolane, which is one of the most common solvents for hydrocarbon separation, higher selectivity for toluene/n-heptane mixtures was obtained in all four ILs. Of all these ILs, ([C1C4py][BF4]) showed the highest distribution coefficient and high toluene/n-heptane selectivity. Domańska and Walczak regressed LLE data using the NRTL method for desulfurization of heptane from thiophene/benzothiophene using alkylpiperidinium- and alkylphosphonium-based ILs.159 The model exhibited excellent fit to the phase composition and experimental tie-lines with an average 39

RMSD value between 0.03 – 0.07. Shiflet and Yokozeki used NRTL model for binary solutions of hydrofluoroethers (HFE) and [C2C1im][NTf2].160 With the increasing concentration of ILs, miscibility of HFE increases. This is also supported by lower excess molar volume at a higher concentration of [C2C1im][NTf2]. In another effort, Shiflet and Niehaus fitted LLE data for the binary mixtures of [C2C1im][NTf2] with substituted benzene (toluene, ethylbenzene, propylbenzene, xylene isomers, trimethylbenzene isomers, aniline, nitrobenzene, phenol, and benzaldehyde) using NRTL model.161 The miscibility of substituted benzene with [C2C1im][NTf2] was noted to increase with their polarity. This was further supported by the negative value of the excess molar volume (-1 to -5 cm3/mol) of the mixture using NRTL model. A negative excess molar volume was also found by Shiflet et al. when halogenated benzene (chlorobenzene, bromobenzene, and iodobenzene) were extracted with [C2C1im][NTf2].162 LLE experimental data of 1,3-propanediol with three ILs 1-butyl-3-methylimidazolium hexafluorophosphate [C4C1im][PF6], 1-butyl-3-methylimidazolium tetrafluoroborate [C4C1im][BF4], and 1-ethyl-3-methylimidazolium tetrafluoroborate [C2C1im][BF4] were regressed using NRTL method.163 All three binary mixtures exhibited UCST phase behavior and their excess molar volume in the ionic-liquid-rich side solutions increased in the order [C4C1im][BF4] < [C2C1im][BF4] < [C4C1im][PF6]. UNIQUAC model has been used to correlate the liquid-liquid phase equilibria for ternary systems. This model is expressed by two contributions to determine the activity coefficient γi of each component 𝑖 in each phase. ln 𝛾𝑖 = ln 𝛾𝑖𝐶 + ln 𝛾𝑖𝑅 (18)

40

where, ln 𝛾𝑖𝐶 represents the contributions of the molecular volume and surface area attributes of the compounds to the activity coefficient while ln 𝛾𝑖𝑅 shows interactions between the different species i in the mixture. Value of ln 𝛾𝑖𝐶 and ln 𝛾𝑖𝑅 can be determined by using Eq. 19 and 20, respectively. ln 𝛾𝑖𝐶 = ln

𝜙𝑖 𝑥𝑖

+

𝑧 2

𝑞𝑖 ln

𝜃𝑖 𝜙𝑖

+ 𝑙𝑖 −

𝜙𝑖 𝑥𝑖

Σ 𝑥𝑖 𝑙𝑖

(19) where z is the coordination number, which is usually taken to be 10, and for any component 𝑖, 𝑥𝑖 represents its mole fraction in the mixture, while 𝜙𝑖 , 𝜃𝑖 , and 𝑖𝑖 represents volume fraction, area fraction and bulk, factor, respectively. ln 𝛾𝑖𝑅 = 𝑞𝑖 {1 − ln(𝛴𝜃𝑗 𝜏𝑗𝑖 ) − Σ

𝜃𝑗 𝜏𝑖𝑗 Σ𝑘 𝜃𝑘 𝜏𝑘𝑗

}

(20) where, 𝜏𝑖𝑗 and 𝜏𝑗𝑖 are the adjustable parameters. Santiago and Aznar fitted LLE data for IL-based ternary systems at different temperatures using UNIFAC method.164 This method satisfactorily accounted the influence of structural parameters and functional groups in ILs. Klamt and coworkers proposed an alternative approach, COnductor-like Screening Model for Real Solvents (COSMO-RS), to predict the phase equilibria of the IL systems.165-167 In COSMO-RS, molecules are treated as perfect/ideal conductor and assumed as surrounded by a virtual conductor environment. The interactions are completely made on the conductor interface, taking into account the electrostatic screening and the back-polarization of the solute molecule. This provides a discrete surface around the solute molecule which is characterized by its geometry and screening charge density (𝜎). This iteratively corresponds to a energy minima at 41

the conductor. For two electrochemically interacting segments (𝜎, 𝜎′) or (𝜎𝑎𝑐𝑐𝑒𝑝𝑡𝑜𝑟 , 𝜎𝑑𝑜𝑛𝑜𝑟 ), the electrostatic misfit energy, Emisfit, and the hydrogen bounding energy, EHB, are the most relevant strengths. The van der Waals energy (𝐸vdW ) is included depending on the type of atoms involved in molecules. 𝐸𝑚𝑖𝑠𝑓𝑖𝑡 (𝜎, 𝜎′) = 𝑎𝑒𝑓𝑓

𝛼′ 2

(𝜎 + 𝜎′)2

(21) 𝐸𝐻𝐵 = 𝑎𝑒𝑓𝑓 𝑐HB (0; min(0; 𝜎donor + 𝜎HB )) ∗ max(0; 𝜎acceptor − 𝜎HB ) (22) ′ 𝐸vdW = 𝑎eff (𝜏vdW + 𝜏vdW )

(23) ′ where, 𝑎𝑒𝑓𝑓 , 𝛼 ′ , 𝑐HB , 𝜎HB ; and 𝜏vdW and 𝜏vdW are five adjustable parameters and denote

effective contact area between two surface segments, interaction parameter, hydrogen bond strength, threshold for hydrogen bonding; and element specific van der Waals interaction parameters, respectively. Green and coworkers studied the phase equilibria for different binary systems comprising with a lipidic IL, [oleyl-C1im][NTf2] and hydrocarbons (n-hexane, n-octane, n-decane, cyclohexane, methylcyclohexane, 1-octene).168 The experimental data were fitted using the COSMO-RS model to obtain the liquid-liquid phase equilibria and for the dependence of structural features on IL on the miscibility with hydrocarbons. Ferreira et al. fitted the LLE data for the binary mixture of IL + hydrocarbon using COSMO-RS.169 The results showed a good qualitative and semi-quantitative description of the structural effects of different hydrocarbons and ILs in the mutual solubility of the studied systems. The influence of IL anion on the mutual 42

solubility with molecular solvents could not be satisfactorily accounted by the COSMO-RS. Wang et al. used UNIQUAC model to interpret the phase equilibria data of the IL + water + acetic acid system.170 The most efficient IL in the extraction of acetic acid from water was one comprising of the hydrophobic cation and highly coordinating anion. While comparing the efficacy of IL in extraction with molecular solvents such as, methyl tert-butyl ether (MTBE) and methyl isobutyl ketone (MIBK), they noted higher efficiency of molecular solvents over narrow composition range than that of IL ([CnC1im][NTf2], n = 2,4,6,8 and 10). However, for an acetic acid mole fraction of 0.1 in water-rich phase at 293.15 K [P666,14]Cl, [C4mpyrr][NTf2] and [Cnmim][NTf2] (with n > 4) give higher selectivity MTBE and MIBK and therefore demonstrate the potential of ILs for the removal of acetic acid from wastewater. Freire and coworkers predicted the phase equilibria for alkylimidazolium-based ILs with alcohols using COSMO-RS model.171 This model lacks accuracy in prediction with the increasing length of alky chain both on IL and in alcohol. However, for the average-sized ILs and alcohols, COSMO-RS model fits well and thus can be used in the selection of optimum IL for the extraction of alcohol. Ferriera and coworkers screened liquid-liquid phase equilibria for ternary mixtures with ILs + hydrocarbons with COSMO-RS.172 This overview allowed to create a global picture of the influence of ILs and hydrocarbons on their mutual solubilities. In general, ILs containing shorter alkyl chain facilitate extraction of hydrocarbon. Among several possibility of constituent ions, aromatic nitrogen-based cations and anions with low hydrogen-bond basicity, such as [EtSO4]−,[MeSO4]−, [SCN]−, and [DCA]−, are preferred. Lower experimental temperature facilitate aromatic-aliphatic separation because of the reduced energy consumption. In another overview these authors used COSMO-RS for predicting the influence of structural features of ILs and hydrocarbons and experimental conditions on the miscibility for the binary mixtures of

43

ILs + hydrocarbons.173 COSMO-RS prediction resulted higher miscibility of aromatic hydrocarbons with ILs than that of the cyclic and acyclic aliphatic hydrocarbons. The mutual solubility of ILs with both aliphatic and aromatic hydrocarbons reduced with increasing number of carbon chain. For a given hydrocarbon, ILs with all types of cationic cores exhibited fairly good miscibility. The increasing number of carbon on the cationic core further increased solubility. However the role of IL anion on mutual solubility was not very evident by the COSMO-RS.

7. Applications of IL LLE in extraction and separation of organic mixtures Naphtha, a mixture of refined, partly refined or unrefined petroleum products and liquid products of natural gas, is used as automotive fuel, engine fuel, and jet-B. For the economic separation of the components, a number of aliphatic products (C4 – C10) in the aromatic stream should be as low as possible. However, the separation of aliphatic hydrocarbons from the aromatic stream cannot be performed by simple or advanced distillation processes as both aliphatic and aromatic hydrocarbon possess similar boiling points and form azeotropic mixtures over all composition. Other options for the extraction of aliphatic hydrocarbon from the aromatic stream are extraction, adsorption, membrane permeation and extractive distillation. Out of all these processes, extraction of hydrocarbons using ILs is an economically-feasible process. Application of IL-based LLE in separation is enlisted in Table 3.

Table: 3 Application of IL-based LLE in separation S. N. IL-based LLE

LLE measurement

44

Application in separation

Predictive equation

Ref.

1

Cloud point titration

Aromatic hydrocarbon from aliphatic hydrocarbon

Non-random two-liquid (NRTL) model Non-random two-liquid (NRTL) model

Cloud point titration

Aromatic hydrocarbon from aliphatic hydrocarbon and lower alcohol from higher alcohol

perturbed-chain 132 statistical associating fluid theory (PCSAFT)

4

[C4C1py][TCM] and Cloud point [C4C1Mor][TCM] + titration alcohols/ hydrocarbons

Aromatic hydrocarbon from aliphatic hydrocarbon and lower alcohol from higher alcohol

Non-random two-liquid (NRTL) model

133

5

toluene + heptane + 1ethyl-3methylimidazolium triiodide and e toluene + heptane + 1-butyl-3methylimidazolium triiodide

Aromatic hydrocarbon from aliphatic hydrocarbon

Non-random two-liquid (NRTL) model

135

6

[C4C1im]Cl/AlCl3 and [C2C1im]Cl/AlCl3 [C4C1im][OcSO4] [C2C1im][EtSO4] [C4C1im][TCM] and [C4C1Mor][TCM] + pxylene + octane, or decane [C2C1im][EtSO4] + benzene + alkanes

Cloud point titration

10

[C2C1im][N(Tf)2] + substituted benzenes

Cloud point titration

11

[N4441][N(Tf)2] + octane/decane + benzene

Cloud point titration

2

3

7 8

9

Alkylpyridinium-ILs + 1-propanol/1butanol/1-hexanol [C2C1im][EtSO4]+ nalkanes/ aromatic hydrocarbon/ ketones/ ethers/DMSO [C4C1py][N(CN)2] + nheptane, benzene, toluene, ethylbenzene, thiophene, 1-butanol, 1-hexanol, and 1octanol

Cloud point titration

Cloud point titration

139

Hydrodesulfurization

140 NRTL and UNIQUAC model

NRTL and UNIQUAC model Separation of toluene, NRTL model ethylbenzene, xylene, aniline, phenols from their mixtures Benzene from NRTL model octane/decane mixture 45

128

Desulfurization

Extraction of pxylene from octane/decane mixture Separation of benzene from alkane

Cloud point titration

126

145

152

161

183

Rogers and coworkers were first to report that aqueous IL 1-butyl-3-methylimidazolium hexafluorophosphate ([C4C1im][PF6]) can be used for the extraction of benzene derivatives.174 In their investigation they noted that the distribution of solutes between IL and water phase depends on a charge of solute. Najdanovic-Visak et al. studied LLE for the ternary systems composed of 1-hydroxyethyl-3-methylimidazolium-based ILs, dichloromethane and 2-propanol. ILs containing [PF6]- and [BF4]- anions showed partial miscibility for 2-propanol while a phase splitting was observed for all IL + dichloromethane based on which temperature-composition phase diagram for all systems was constructed.175 A moderate pressure effect on the cloud-point temperature was detected for the ternary system [C2OHC1im][PF6] + dichloromethane + 2propanol. Hansmeier et al. studied the LLE data for the ternary systems of aliphatic and aromatic hydrocarbons with 3-methyl-N-butylpyridinium dicyanamide ([C4C1py][N(CN)2]) at 303.15 and 328.15 K and atmospheric pressure.176 The order of extraction of 3-methyl-N-butylpyridinium dicyanamide for different aromatics followed the order: benzene > p-xylene > cumene. Gonzάlez et al. studied the separation heptane, octane and nonane from toluene by employing IL 1-ethyl-3methylpyridinium ethylsulfate ([C2C1py][EtSO4]) at ambient conditions.177 The LLE data were analyzed in terms of the selectivity, % removal of aromatic, and solute distribution ratio. Seoane et al. studied the LLE data for various ILs, such as, 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl) imide ([C2C1im][NTf2]), 1-propyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([C3C1im][NTf2]), 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([C4C1im][NTf2]) and 1-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([C6C1im][NTf2]) along with ethanol and heptane at ambient conditions.178 The potential of these ILs in the extraction of ethanol from heptane was analyzed

46

in terms of selectivity and solute distribution ratio. García et al. used binary mixture of two ILs namely, N-butylpyridinium tetrafluoroborate ([C4py][BF4]) and 1-butyl-3-methylpyridinium bis(trifluoromethylsulfonyl) imide ([C4C1py][NTf2]) in the extraction of toluene from n-heptane at 313.15 K and atmospheric pressure.179 The aptitude of ILs mixture in terms of selectivity and extraction capability was found to be intermediate than those of the constituent ILs. Domanska et al. extracted a mixture of aliphatic and aromatic hydrocarbons at room temperature using ammonium IL ethyl(2-hydroxyethyl)dimethylammonium bis{(trifluomethyl)sulfonyl}imide (C2NTf2).180 The extraction of aromatic hydrocarbons from a mixture of hexane and heptane was supported by the selectivity and distribution ratio values. González et al. performed extraction of xylene isomers from hexane with the use of extracting medium 1-ethyl-3-methylpyridinium ethylsulfate ([C2C1py][EtSO4]) at ambient conditions.181 The LLE data showed that the efficiency of [C2C1py][EtSO4] in extracting xylene isomers from hexane follows the order: oxylene > p-xylene > m-xylene. Higher selectivity and distribution coefficient values of xylene isomers suggest that [C2C1py][EtSO4] is a potential medium for their separation from hexane mixture. Removal of ethanol from its mixture with hexane and heptane was performed by measuring the phase equilibria with 1-ethyl-3-methylpyridinium bis(trifluoromethylsulfonyl)imide ([C2C1py][NTf2]) and 1-methyl-3-propyl pyridinium bis(trifluoromethylsulfonyl)imide ([C3C1py][NTf2]) at 298.15 K temperature and atmospheric pressure.182 The solute distribution ratio and selectivity values for the applied ILs were higher even at a low mole fraction of ethanol. The increasing length of the alkyl chain on pyridinium cation has a little effect on the solute distribution ratio and selectivity. Requejo and coworkers used separated benzene from its mixture with octane and decane by employing tributylmethylammonium bis(trifluoromethylsulfonyl)imide ([N4441][NTf2]) at T = 298.15 K and

47

atmospheric pressure.183 The LLE data for the ternary systems (octane/decane + benzene + [N4441][NTf2]) and quaternary system (octane + decane + benzene + [N4441][NTf2]) were determined and solute distribution ratio and selectivity were calculated for these combinations. The highest value of the solute distribution ratio in the ternary systems suggests that [N4441][NTf2] can be used as a solvent for the removal benzene from the mixture of hexane or heptane. However, the selectivity value for both systems was comparable. Ahmed and coworkers performed the removal of thiophenic compounds from hexadecane, which is a representative for diesel fuel, using pyrrolidinium- and phosphonium-based ILs.184 LLE data for all four ternary systems were measured and used for the calculation of solute distribution ratio and selectivity. For benzothiophene, the highest solute distribution coefficient (2.754) was observed with 1hexyl-3-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide ([C6C1pyrr][NTf2]), while a solute distribution coefficient of 3.772 was noted with tetrabutylphosphonium methanesulfonate ([P4444][MeSO3]). Domínguez et al. used 1-butyl-3-methylimidazolium methylsulfate ([C4C1im][MeSO4]), 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([C4C1im][NTf2]) and 1-methyl-3-propylimidazolium bis(trifluoromethylsulfonyl)imide ([C1C3im][NTf2]) for the separation of benzene from heptane.185 Selectivity and solute distribution ratio were derived from the experimental LLE data. Higher solute distribution ratio was obtained with [C4C1im][NTf2] and [C1C3im][NTf2], whereas higher selectivity was noted with [C4C1im][MeSO4]. This suggests that [NTf2]-based ILs are the most efficient in the separation of benzene from heptane.

8. Future outlook

48

IL-mediated separation methods have been a subject of extensive research since their inception. The insignificant vapor pressure of ILs authorizes them as a “green” media. This propels ILs as potential candidate in replacing the volatile organic compounds (VOCs) from various physical, chemical and biological processes. However, the environmental benefits of ILs need to be carefully considered. Not all ILs are safe and nontoxic, especially during their synthesis. Another issue with the use of ILs in various separation processes is sustainability. In order to attain sustainability of ABS and LLE methods, the efficient IL recycling should be investigated in order to make these processes cost-effective.

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