- Email: [email protected]
Contrasting the solvation properties of protic ionic liquids with different nanoscale structure
Journal of Molecular Liquids 290 (2019) 111361
Contents lists available at ScienceDirect
Journal of Molecular Liquids journal homepage: www.elsevier.com/locate/molliq
Contrasting the solvation properties of protic ionic liquids with different nanoscale structure Igor A. Sedov ⁎, Timur M. Salikov, Boris N. Solomonov Chemical Institute, Kazan Federal University, Kremlevskaya Str., 18, Kazan 420008, Russian Federation
a r t i c l e
i n f o
Article history: Received 25 April 2019 Received in revised form 6 July 2019 Accepted 12 July 2019 Available online 13 July 2019 Keywords: Protic ionic liquids Limiting activity coefficient Solvation Nanostructure
a b s t r a c t The limiting activity coefficients of several hydrocarbons and aliphatic alcohols in protic ionic liquids: 2hydroxyethylammonium nitrate (EOAN), propylammonium nitrate (PAN), butylammonium nitrate (BAN), and butylammonium thiocyanate (BASCN), were determined at 298 K using gas chromatographic headspace analysis. The activity coefficients of hydrocarbons in EOAN turned out to be much higher than in BASCN or alkylammonium nitrates. The methylene group increments for the Gibbs free energies of solvation of various homological series in EOAN are also much higher than in BASCN, BAN, PAN as well as in aprotic ionic liquids or molecular solvents except water. Water-like solvation properties of EOAN are linked with the presence of a threedimensional network of hydrogen bonds, high concentration of ion pairs, and the absence of nanoscale domain structure that exists in liquid alkylammonium salts and favors solvation of hydrocarbons in apolar domains. The sensitivity of solvation properties of PILs to their nanostructure suggests that the presence of apolar and polar domains in a liquid can be probed with the studies of solvation thermodynamics. © 2019 Elsevier B.V. All rights reserved.
1. Introduction Ionic liquids (ILs) are organic salts with a melting point below 100 °C. They are considered as promising green solvents for industrial processes. Many ILs have a very low vapor pressure and high thermal stability [1,2]. Due to the numerous possible combinations of cation and anion, it is possible to design ILs with desired physic-chemical properties [3]. The areas of possible applications of ILs include their use as solvents for extraction and extractive distillation processes as well as supramolecular self-assembly media and catalytic media [4–6]. Good separation ability of ionic liquids for the mixtures of alkanes and alkenes, alkanes and arenes [7], alcohols and alkanes [8], alcohols and alkenes [9], and the prospects for their use in industrial processes such as fuel desulfurization or extraction of metals ions from aqueous solutions were noted. For successful applications of ILs in separation processes, the data on the equilibrium interfacial distribution of the separated substances are necessary. Thermodynamic functions of the solvation of organic substances, which are used to calculate the selectivity and capacity factors in separation processes are of particular interest. In the last two decades, a huge attention was paid to the experimental measurement of limiting activity coefficients in ionic liquids [10–12]. Among many ILs, there is a group of protic ionic liquids (PILs), which have an acidic hydrogen atom in the cation. As a result, they can form hydrogen bonds between the cation and the anion. Like in the case of ⁎ Corresponding author. E-mail address: [email protected] (I.A. Sedov).
https://doi.org/10.1016/j.molliq.2019.111361 0167-7322/© 2019 Elsevier B.V. All rights reserved.
molecular solvents, the presence of a network of hydrogen bonds leads to a significant change in the structure and properties of ionic liquids. At the same time, the solvation properties of PILs in contrast to the aprotic ionic liquids (AILs) have almost not been studied. The classic gas-chromatographic technique for the measurement of the activity coefficients at infinite dilution, which implies the capillary columns with ILs as a stationary phase, turned out to be unsuitable for PILs. This is due to the relatively low boiling temperatures of PILs and their high vapor pressure [13]. Since the presence of a network of hydrogen bonds in molecular solvents always leads to a decrease in solubility and an increase in the activity coefficients of low-polar compounds [14], one could suggest that hydrocarbons should have significantly higher activity coefficients in PILs than in AILs. However, in our recent work [15], the opposite tendency has been observed for typical PILs – alkylammonium nitrates. The values of the limiting activity coefficients of hydrocarbons were measured in butylammonium nitrate (BAN) and propylammonium nitrate (PAN) by gas chromatographic headspace analysis and were found to be significantly lower than expected for the AILs with the similar molar volume values [16]. The effect of increased solubility of hydrocarbons in BAN and PAN was explained by the peculiarities of the nanostructure of these liquids, namely the presence of polar and apolar domains [17,18]. The solvation occurs predominantly in apolar domains, which causes a decrease in the activity coefficients and an increase in solubility of hydrocarbons. At the same time, there are nanohomogeneous liquids among PILs that do not have a domain structure. An example is 2-
2
I.A. Sedov et al. / Journal of Molecular Liquids 290 (2019) 111361
hydroxyethylammonium nitrate [19]. Its cation has an additional OHgroup in comparison with that of ethylammonium nitrate. Nothing is known about the solvation properties of such ionic liquids. In the present paper, we study the activity coefficients of hydrocarbons and alcohols in 2-hydroxyethylammonium nitrate and butylammonium thiocyanate. According to the neutron diffraction studies [20], the latter has a domain nanostructure similarly to BAN. The results are compared with the corresponding values in PAN and BAN in order to analyze the influence of the structure of cation and anion of ionic liquids and the nanostructure of their bulk phase on the solvation properties. 2. Experimental 2.1. Chemicals Fig. 1. FTIR-ATR spectra of EOAN (above) and BASCN (below).
Hexane (Sigma-Aldrich, 99%) (hereinafter the mass fraction purity is specified), heptane (Acros Organics, 99%), octane (Sigma-Aldrich, 99%), nonane (Acros Organics, 99%), benzene (Komponent-Reaktiv, 99.8%), toluene (Sigma-Aldrich, 99.8%), ethylbenzene (Fluka, 99%), propylbenzene (Acros Organics, 98%), butylbenzene (Acros Organics, 99%), methanol (Vekton, 99.5%), ethanol (Ecros, 99.5%), 1-propanol (Ecos-1, 99%), 1-butanol (Ecos-1, 99.8%), 1-pentanol (Sigma-Aldrich, 99%), 1-hexanol (Sigma-Aldrich, 98%), 2-hydroxyethylamine (Ecos-1, 99%), butylamine (Aldrich, 99%), nitric acid (Komponent-Reaktiv, 65% aqueous solution) and ammonium thiocyanate (Lenreaktiv, 99%) were purchased from a commercial supplier. They were used without additional purification except alcohols which were dried with calcium hydride and then distilled. The absence of significant amounts of impurities and water in hydrocarbons and alcohols was confirmed by gas chromatography and Karl-Fischer titration. 2.2. Synthesis of ionic liquids Synthesis and physical properties of propylammonium nitrate (PAN) and butylammonium nitrate (BAN) were described in our previous work [15]. A similar procedure was used to synthesize 2hydroxyethylammonium nitrate (EOAN) from 2-hydroxyethylamine and nitric acid. Butylammonium thiocyanate (BASCN) was prepared as follows. A small excess of n-butylamine was added dropwise while stirring to an aqueous solution of ammonium thiocyanate in a flask equipped with a Liebig condenser. The temperature was maintained at 75 °C. Stirring was continued for 24 h, then a small portion of butylamine (4 ml) was added. Cessation of ammonia release indicates the end of the reaction. The synthesized ILs were purified from water and the excess of amine by rotary evaporation in vacuo during 20 h at 70 °C. BASCN at 298.15 K is a yellow liquid with ρ = 1.024 g·cm−3 (literature values 1.011 [21] and 0.949 [22] g·cm−3), nD = 1.5225 (literature values 1.5170 [21] and 1.5264 [22]), and mp 8°С (measured using DSC at 10 K/min). EOAN at 298.15 K is a supercooled yellowish liquid with ρ = 1.363 g·cm−3 (literature values 1.38 [23] and 1.265 [24] g·cm−3), nD = 1.4852, and mp 50°С (literature value 51°С [24]). The IR spectra of BASCN and EOAN shown in Fig. 1 indicate that only traces (b0.1%) of water are present. 2.3. Measurement of limiting activity coefficients Limiting activity coefficients of hydrocarbons and alcohols were determined by means of gas chromatographic analysis of the headspace over dilute solutions. The headspace technique is well suitable for even much more volatile solvents than PILs [25–27]. In a typical experiment, 2–10 μl of a solute was put into a vial containing 5 ml of solvent. The vials were crimp sealed, shaken and thermostated at 298.15 K for several hours. An autosampler takes small samples of the equilibrium vapor over a solution and transfers them through a heated quartz glass line into the gas chromatograph. The areas of the chromatographic
peaks of a solute S are proportional to its equilibrium vapor pressure p. In a separate experiment, the same procedure is repeated with 5 ml of pure solute in a vial. The peak area in obtained chromatogram S0 is proportional to the saturated vapor pressure of a solute Psat. The limiting acp S ¼ , where tivity coefficients can be calculated by equation γ ∞ ¼ psat x S0 x x is the molar fraction of a solute in the equilibrium liquid phase. In case of poorly soluble solutes, a significant fraction of initially dissolved solute may evaporate. The value of x after equilibration can be calculated from the initial molar fraction of a solute x0: x ¼ x0 −
Spsat V free S0 RTv
ð1Þ
where Vfree = 17 ml is the volume of a headspace in a vial, ν is the number of moles of solvent in a vial. The standard molar Gibbs free energies of solvation in a molar fraction-based standard state are related to γ∞ through equation: Δsolv G ° ¼ RT ln
∞ γ psat p°
ð2Þ
where the standard state pressure p° = 1 bar. The obtained values of ΔsolvG° were averaged over 6 repetitions for each system at different concentrations of a solute. No significant dependence of the activity coefficients at different from concentration was observed, which allows to conclude that infinite dilution is reached. The obtained results are given in Table 1. 3. Discussion The solubility of hydrocarbons in EOAN turned out to be much lower in comparison with alkylammonium nitrates. Thus, we could not measure the activity coefficients of alkanes with sufficient accuracy using the headspace analysis technique. The activity coefficients of alkylbenzenes in EOAN are much higher than in PAN and BAN. In contrast, the values of activity coefficients of hydrocarbons in BASCN are very close to those in BAN. This suggests the existence of a very strong solvophobic effect in EOAN in comparison with BAN, PAN, or BASCN. Earlier [16] we have shown that there is a linear correlation between the Gibbs free energies of solvation of hydrocarbons in different AILs and the reciprocal of their molar volume, which is a measure of molar concentration of ion pairs in a liquid. The data points corresponding to solutions of hydrocarbons in BAN and PAN fall below this line (see Fig. 2a–c), which was explained [15] by their preferential solvation in the apolar domains that exist in these ionic liquids. Fig. 2b and c shows that the data points for solutions of aromatic hydrocarbons in EOAN lie above the correlation for AILs
I.A. Sedov et al. / Journal of Molecular Liquids 290 (2019) 111361
3
Table 1 Limiting activity coefficients and the Gibbs free energies of solvation in protic ionic liquids at T = 298.15 Ka. Solvent
Solute
γ∞
PAN PAN PAN PAN PAN PAN PAN PAN PAN PAN PAN PAN PAN PAN PAN BAN BAN BAN BAN BAN BAN BAN BAN BAN BAN BAN BAN BAN BAN BAN EOAN EOAN EOAN EOAN EOAN EOAN EOAN EOAN BASCN BASCN BASCN BASCN BASCN BASCN BASCN BASCN BASCN BASCN BASCN BASCN BASCN BASCN
Hexane Heptane Octane Nonane Benzene Toluene Ethylbenzene Propylbenzene Butylbenzene Methanol Ethanol 1-Propanol 1-Butanol 1-Pentanol 1-Hexanol Hexane Heptane Octane Nonane Benzene Toluene Ethylbenzene Propylbenzene Butylbenzene Methanol Ethanol 1-Propanol 1-Butanol 1-Pentanol 1-Hexanol Benzene Toluene Ethylbenzene Methanol Ethanol 1-Propanol 1-Butanol 1-Pentanol Hexane Heptane Octane Nonane Benzene Toluene Ethylbenzene Propylbenzene Butylbenzene Methanol Ethanol 1-Propanol 1-Butanol 1-Pentanol
117b 247b 445b 910b 8.37b 13.7b 22b 43.2 63.7b 0.53 0.88 1.25 1.65 2.21 3.31 38.8b 62b 86.9b 154b 4.62b 7.62b 9.94b 16 19.4b 0.52 0.82 0.99 1.12 1.46 1.65 142 416 1052 0.98 3.44 8.55 22.2 58.5 44 60.5 80 114 3.71 5.85 9.2 12 16.7 0.38 0.61 0.74 0.82 0.9
u(γ∞)
ΔsolvG°/(kJ·mol−1)
8.0 11 17 29 0.32 0.4 0.6 1.0 2.2 0.02 0.03 0.03 0.03 0.05 0.05 1.5 2.3 3.1 10 0.12 0.22 0.24 0.27 0.3 0.02 0.02 0.03 0.03 0.03 0.04 6.5 25 50 0.03 0.05 0.2 0.8 1.8 2.0 2.5 4.0 5.0 0.1 0.1 0.4 0.5 0.6 0.01 0.03 0.03 0.05 0.05
7.83 6.73 5.26 4.18 0.14 −1.63 −3.14 −3.98 −5.97 −5.97 −6.60 −8.31 −10.45 −12.48 −13.62 5.09 3.31 1.22 −0.22 −1.33 −3.08 −5.11 −6.44 −8.91 −6.02 −6.78 −8.88 −11.41 −13.51 −15.35 7.15 6.83 6.44 −4.45 −3.23 −3.54 −4.01 −4.37 5.40 3.25 1.01 −0.97 −1.88 −3.73 −5.30 −7.16 −9.28 −6.80 −7.51 −9.60 −12.18 −14.71
a Standard uncertainty for temperature u(T) = 0.2 K. The experimental pressure inside vials p = 2.38 bar, u(p) = 0.01 bar. b Results from our previous work [15].
unlike the points for solutions in BASCN and alkylammonium nitrates. The values of the Gibbs free energy of solvation in EOAN are higher than those expected in AILs with similar molar volumes, which is likely to be linked with the additional enhancement of the solvophobic effect by the three-dimensional network of hydrogen bonds existing in EOAN. At the same time, there are no apolar domains in EOAN [19], which favor the solvation of hydrocarbons in the case of alkylammonium nitrates. As for BASCN, the existence of domain nanostructure in this solvent and the molar volume value close to that of BAN leads to very similar solvation properties of BASCN and BAN despite the different composition, geometric and electronic structure of thiocyanate and nitrate anions. Fig. 3 shows the plots of the Gibbs free energies of solvation of homologous series of alkanes, alkylbenzenes, and linear saturated alcohols in the studied PILs against the number of carbon atoms in the alkyl
Fig. 2. The standard molar Gibbs free energy of solvation of a) n-octane, b) benzene, c) toluene against the concentration of the ion pairs in different ionic liquids at 298.15 K. Empty circles are the data in AILs. Lines correspond to the correlations for solvation in AILs.
chain. Since these quantities are linearly correlated, it is possible to define so-called increments of the methylene group ΔΔG°(CH2), which are the average differences between the Gibbs free energies of solvation of the neighboring members of these series. The values of ΔΔG°(CH2) calculated as the slopes of correlation lines from Fig. 3 are given in Table 2. It is clear that the increments for alkylbenzenes and alcohols series in EOAN are significantly higher than in other PILs. The increments of methylene groups for the Gibbs free energies of solvation of homologous series of organic solutes in the majority of molecular solvents lie in the range from −2 to −3.5 kJ·mol−1 [28,29]. In aprotic ionic liquids, these values usually fall into the range from −1.8 to −2.5 kJ·mol−1 [16]. In formamide and ethylene glycol, which exhibit strong solvophobic effects, these increments are from −1.5 to −2 kJ·mol−1 [30,31]. For water, the values of ΔΔG°(CH2) are
4
I.A. Sedov et al. / Journal of Molecular Liquids 290 (2019) 111361
Fig. 3. Standard molar Gibbs free energies of solvation for n-alkanes, n-alkylbenzenes, and n-alcohols in PAN, BAN, EOAN, and BASCN against the number of carbons in alkyl chain.
significantly higher, averaging at +0.75 kJ·mol−1 [28,32,33]. Thus, the observed values of ΔΔG°(CH2) about −0.3 kJ·mol−1 for EOAN are higher than in any other common solvents except water. The solvation properties of EOAN appear to be the most “water-like” among other solvents, while BASCN, BAN, and PAN are similar to the aprotic ionic liquids and organic solvents in terms of methylene group increments. The values of ΔΔG°(CH2) for solvation of alkylbenzenes in another nanostructured PIL, ethylammonium nitrate (EAN) [19], are −2.6 kJ·mol−1 in the case of alkylbenzenes and −2.1 kJ·mol−1 in the case of alcohols [34], which is close to other alkylammonium nitrates. Interestingly, earlier EAN was called the “water-like” solvent. This was primarily due to very low critical micelle concentration (CMC) values for a number of surfactants, despite much lower aggregation numbers than in water [35,36], while in aprotic ionic liquids micelles either do not form at all or form at several orders of magnitude higher surfactant concentrations than in water [37]. However, in EOAN the CMC values can be much lower than in EAN. In the work [38], it was shown that the CMCs of cationic surfactants n-alkylmethylimidazolium chlorides and n-alkyldimethylethanolammonium chlorides in EOAN were 30–70 times lower than in EAN and even 5–7 times lower than in water. Such low CMC values in EOAN are observed not for all surfactants. Some of them have significantly higher CMCs than in water [39] or even form micelles in EAN but not in EOAN [40]. It should be understood that not only the solvophobic effects but also other types of interactions between surfactants and solvents promote micelle formation. The activity coefficients of hydrocarbons in EOAN are still much lower than in water. For example, benzene in water at 298 K has 17
Table 2 Methylene group increments ΔΔG°(CH2)/(kJ ⋅ mol−1) for the Gibbs free energies of solvation of different homological series in protic ionic liquids at T = 298.15 K. Ionic liquid PAN BAN BASCN EOAN
Alkanes
Alkylarenes
Alcohols
−1.24 −1.80 −2.14 –
−1.46 −1.85 −1.82 −0.36
−1.82 −2.18 −2.42 −0.30
times higher activity coefficient (about 2400), toluene has one about 22 times higher in water than in EOAN, and ethylbenzene – 31 times higher [41]. However, the solvophobic effects in EOAN are very strong as compared to other ionic liquids. This will result in higher selectivity factor of separation of less polar compounds from more polar ones, which is equal to the ratio of the activity coefficients of the separated compounds. In a previous paper [15], we have shown that PAN and BAN have inferior selectivity of separation of alkanes from aromatic hydrocarbons than many AILs. EOAN is a more promising solvent for this and other separation problems. 4. Conclusion 2-Hydroxyethylammonium nitrate shows significantly different solvation properties in comparison with propyl- and butylammonium nitrates as well as with butylammonium thiocyanate. We observed very low solubilities and very large activity coefficients of hydrocarbons in EOAN allowing to call it the most “water-like” solvent from all considered ionic liquids. Strong solvophobic effects in this solvent are linked with the high concentration of ion pairs, existence of a spatial network of hydrogen bonds, and the absence of domain nanostructure. The last factor explains a difference between the solvation thermodynamics in EOAN and alkylammonium salts. The latter have apolar domains formed by alkyl groups which can preferentially solvate hydrocarbons. An important consequence is that one can judge about the existence or absence of the domain structure in protic ionic liquids from the values of activity coefficients obtained using relatively simple thermodynamic experiments. Acknowledgement We thank Timur Magsumov for his help with headspace experiments, Dr. Alexander Klimovitsky and Dr. Marat Ziganshin for their help in measuring IR-spectra and DSC curves. This work was performed according to the Russian Government Program of Competitive Growth of Kazan Federal University. Igor Sedov acknowledges the Russian Federation President Grant MK-6547.2018.3.
I.A. Sedov et al. / Journal of Molecular Liquids 290 (2019) 111361
Boris Solomonov acknowledges funding from Ministry of Science and Higher Education of Russia (4.5618.2017/6.7). References [1] N. Meine, F. Benedito, R. Rinaldi, Green Chem. 12 (2010) 1711. [2] P. Wasserscheid, T. Welton, Ionic Liquids in Synthesis, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim, Germany, 2007. [3] N.V. Plechkova, K.R. Seddon, in: P. Tundo, A. Perosa, F. Zecchini (Eds.), Methods Reag. Green Chem, John Wiley & Sons, Inc., Hoboken, NJ, USA 2007, pp. 103–130. [4] P. Madhavan, R. Sougrat, A.R. Behzad, K.-V. Peinemann, S.P. Nunes, J. Membr. Sci. 492 (2015) 568–577. [5] T. Welton, Coord. Chem. Rev. 248 (2004) 2459–2477. [6] S.P.M. Ventura, F.A. e Silva, M.V. Quental, D. Mondal, M.G. Freire, J.A.P. Coutinho, Chem. Rev. 117 (2017) 6984–7052. [7] A. Marciniak, Fluid Phase Equilib. 294 (2010) 213–233. [8] T.M. Letcher, N. Deenadayalu, B. Soko, D. Ramjugernath, P.K. Naicker, J. Chem. Eng. Data 48 (2003) 904–907. [9] T.M. Letcher, P. Reddy, Fluid Phase Equilib. 219 (2004) 107–112. [10] M. Krummen, P. Wasserscheid, J. Gmehling, J. Chem. Eng. Data 47 (2002) 1411–1417. [11] G. Foco, M.D. Bermejo, A.J. Kotlewska, F. van Rantwijk, C.J. Peters, S.B. Bottini, J. Chem. Eng. Data 56 (2011) 517–520. [12] R. Kato, J. Gmehling, Fluid Phase Equilib. 226 (2004) 37–44. [13] T.L. Greaves, C.J. Drummond, Chem. Rev. 115 (2015) 11379–11448. [14] I.A. Sedov, B.N. Solomonov, J. Struct. Chem. 54 (2013) S262–S270. [15] I.A. Sedov, T.I. Magsumov, T.M. Salikov, B.N. Solomonov, Phys. Chem. Chem. Phys. 19 (2017) 25352–25359. [16] I.A. Sedov, B.N. Solomonov, Fluid Phase Equilib. 425 (2016) 9–14. [17] Y. Umebayashi, W.-L. Chung, T. Mitsugi, S. Fukuda, M. Takeuchi, K. Fujii, T. Takamuku, R. Kanzaki, S. Ishiguro, J. Comput. Chem. Jpn. 7 (2008) 125–134. [18] R. Atkin, G.G. Warr, J. Phys. Chem. B 112 (2008) 4164–4166. [19] R. Hayes, S. Imberti, G.G. Warr, R. Atkin, Phys. Chem. Chem. Phys. 13 (2011) 3237–3247.
5
[20] R. Hayes, S. Imberti, G.G. Warr, R. Atkin, J. Phys. Chem. C 118 (2014) 13998–14008. [21] C.F. Poole, B.R. Kersten, S.S.J. Ho, M.E. Coddens, K.G. Furton, J. Chromatogr. A 352 (1986) 407–425. [22] P.H. Shetty, P.J. Youngberg, B.R. Kersten, C.F. Poole, J. Chromatogr. A 411 (1987) 61–79. [23] D. Wakeham, P. Niga, C. Ridings, G. Andersson, A. Nelson, G.G. Warr, S. Baldelli, M.W. Rutland, R. Atkin, Phys. Chem. Chem. Phys. 14 (2012) 5106. [24] T.L. Greaves, A. Weerawardena, C. Fong, I. Krodkiewska, C.J. Drummond, J. Phys. Chem. B 110 (2006) 22479–22487. [25] J.H. Park, A. Hussam, P. Couasnon, D. Fritz, P.W. Carr, Anal. Chem. 59 (1987) 1970–1976. [26] I.A. Sedov, T.I. Magsumov, B.N. Solomonov, J. Mol. Liq. 224 (2016) 1205–1209. [27] I.A. Sedov, T.I. Magsumov, J. Phys. Chem. B 119 (2015) 8773–8780. [28] A.K. Sengupta, Y. Marcus, Ion Exchange and Solvent Extraction: A Series of Advances, Dekker, New York, 2001. [29] P.W. Carr, J. Li, A.J. Dallas, D.I. Eikens, L.C. Tan, J. Chromatogr. A 656 (1993) 113–133. [30] I.A. Sedov, M.A. Stolov, B.N. Solomonov, Fluid Phase Equilib. 354 (2013) 95–101. [31] I.A. Sedov, M.A. Stolov, B.N. Solomonov, J. Chem. Thermodyn. 64 (2013) 120–125. [32] Y.A. Mirgorod, Russ. J. Phys. Chem. A 75 (2001) 352–355. [33] S. Cabani, P. Gianni, V. Mollica, L. Lepori, J. Solut. Chem. 10 (1981) 563–595. [34] S.P. Verevkin, D.H. Zaitsau, B. Tong, U. Welz-Biermann, Phys. Chem. Chem. Phys. 13 (2011), 12708. [35] D.F. Evans, A. Yamauchi, G.J. Wei, V.A. Bloomfield, J. Phys. Chem. 87 (1983) 3537–3541. [36] D.F. Evans, A. Yamauchi, R. Roman, E.Z. Casassa, J. Colloid Interface Sci. 88 (1982) 89–96. [37] T.L. Greaves, C.J. Drummond, Chem. Soc. Rev. 37 (2008) 1709. [38] Y. Pei, L. Hao, J. Ru, Y. Zhao, H. Wang, G. Bai, J. Wang, J. Mol. Liq. 254 (2018) 130–136. [39] E.C. Wijaya, F. Separovic, C.J. Drummond, T.L. Greaves, Phys. Chem. Chem. Phys. 18 (2016) 24377–24386. [40] H.J. Jiang, P.A. FitzGerald, A. Dolan, R. Atkin, G.G. Warr, J. Phys. Chem. B 118 (2014) 9983–9990. [41] U.N. Wiesmann, S. DiDonato, N.N. Herschkowitz, Biochem. Biophys. Res. Commun. 66 (1975) 1338–1343.
Contents lists available at ScienceDirect
Journal of Molecular Liquids journal homepage: www.elsevier.com/locate/molliq
Contrasting the solvation properties of protic ionic liquids with different nanoscale structure Igor A. Sedov ⁎, Timur M. Salikov, Boris N. Solomonov Chemical Institute, Kazan Federal University, Kremlevskaya Str., 18, Kazan 420008, Russian Federation
a r t i c l e
i n f o
Article history: Received 25 April 2019 Received in revised form 6 July 2019 Accepted 12 July 2019 Available online 13 July 2019 Keywords: Protic ionic liquids Limiting activity coefficient Solvation Nanostructure
a b s t r a c t The limiting activity coefficients of several hydrocarbons and aliphatic alcohols in protic ionic liquids: 2hydroxyethylammonium nitrate (EOAN), propylammonium nitrate (PAN), butylammonium nitrate (BAN), and butylammonium thiocyanate (BASCN), were determined at 298 K using gas chromatographic headspace analysis. The activity coefficients of hydrocarbons in EOAN turned out to be much higher than in BASCN or alkylammonium nitrates. The methylene group increments for the Gibbs free energies of solvation of various homological series in EOAN are also much higher than in BASCN, BAN, PAN as well as in aprotic ionic liquids or molecular solvents except water. Water-like solvation properties of EOAN are linked with the presence of a threedimensional network of hydrogen bonds, high concentration of ion pairs, and the absence of nanoscale domain structure that exists in liquid alkylammonium salts and favors solvation of hydrocarbons in apolar domains. The sensitivity of solvation properties of PILs to their nanostructure suggests that the presence of apolar and polar domains in a liquid can be probed with the studies of solvation thermodynamics. © 2019 Elsevier B.V. All rights reserved.
1. Introduction Ionic liquids (ILs) are organic salts with a melting point below 100 °C. They are considered as promising green solvents for industrial processes. Many ILs have a very low vapor pressure and high thermal stability [1,2]. Due to the numerous possible combinations of cation and anion, it is possible to design ILs with desired physic-chemical properties [3]. The areas of possible applications of ILs include their use as solvents for extraction and extractive distillation processes as well as supramolecular self-assembly media and catalytic media [4–6]. Good separation ability of ionic liquids for the mixtures of alkanes and alkenes, alkanes and arenes [7], alcohols and alkanes [8], alcohols and alkenes [9], and the prospects for their use in industrial processes such as fuel desulfurization or extraction of metals ions from aqueous solutions were noted. For successful applications of ILs in separation processes, the data on the equilibrium interfacial distribution of the separated substances are necessary. Thermodynamic functions of the solvation of organic substances, which are used to calculate the selectivity and capacity factors in separation processes are of particular interest. In the last two decades, a huge attention was paid to the experimental measurement of limiting activity coefficients in ionic liquids [10–12]. Among many ILs, there is a group of protic ionic liquids (PILs), which have an acidic hydrogen atom in the cation. As a result, they can form hydrogen bonds between the cation and the anion. Like in the case of ⁎ Corresponding author. E-mail address: [email protected] (I.A. Sedov).
https://doi.org/10.1016/j.molliq.2019.111361 0167-7322/© 2019 Elsevier B.V. All rights reserved.
molecular solvents, the presence of a network of hydrogen bonds leads to a significant change in the structure and properties of ionic liquids. At the same time, the solvation properties of PILs in contrast to the aprotic ionic liquids (AILs) have almost not been studied. The classic gas-chromatographic technique for the measurement of the activity coefficients at infinite dilution, which implies the capillary columns with ILs as a stationary phase, turned out to be unsuitable for PILs. This is due to the relatively low boiling temperatures of PILs and their high vapor pressure [13]. Since the presence of a network of hydrogen bonds in molecular solvents always leads to a decrease in solubility and an increase in the activity coefficients of low-polar compounds [14], one could suggest that hydrocarbons should have significantly higher activity coefficients in PILs than in AILs. However, in our recent work [15], the opposite tendency has been observed for typical PILs – alkylammonium nitrates. The values of the limiting activity coefficients of hydrocarbons were measured in butylammonium nitrate (BAN) and propylammonium nitrate (PAN) by gas chromatographic headspace analysis and were found to be significantly lower than expected for the AILs with the similar molar volume values [16]. The effect of increased solubility of hydrocarbons in BAN and PAN was explained by the peculiarities of the nanostructure of these liquids, namely the presence of polar and apolar domains [17,18]. The solvation occurs predominantly in apolar domains, which causes a decrease in the activity coefficients and an increase in solubility of hydrocarbons. At the same time, there are nanohomogeneous liquids among PILs that do not have a domain structure. An example is 2-
2
I.A. Sedov et al. / Journal of Molecular Liquids 290 (2019) 111361
hydroxyethylammonium nitrate [19]. Its cation has an additional OHgroup in comparison with that of ethylammonium nitrate. Nothing is known about the solvation properties of such ionic liquids. In the present paper, we study the activity coefficients of hydrocarbons and alcohols in 2-hydroxyethylammonium nitrate and butylammonium thiocyanate. According to the neutron diffraction studies [20], the latter has a domain nanostructure similarly to BAN. The results are compared with the corresponding values in PAN and BAN in order to analyze the influence of the structure of cation and anion of ionic liquids and the nanostructure of their bulk phase on the solvation properties. 2. Experimental 2.1. Chemicals Fig. 1. FTIR-ATR spectra of EOAN (above) and BASCN (below).
Hexane (Sigma-Aldrich, 99%) (hereinafter the mass fraction purity is specified), heptane (Acros Organics, 99%), octane (Sigma-Aldrich, 99%), nonane (Acros Organics, 99%), benzene (Komponent-Reaktiv, 99.8%), toluene (Sigma-Aldrich, 99.8%), ethylbenzene (Fluka, 99%), propylbenzene (Acros Organics, 98%), butylbenzene (Acros Organics, 99%), methanol (Vekton, 99.5%), ethanol (Ecros, 99.5%), 1-propanol (Ecos-1, 99%), 1-butanol (Ecos-1, 99.8%), 1-pentanol (Sigma-Aldrich, 99%), 1-hexanol (Sigma-Aldrich, 98%), 2-hydroxyethylamine (Ecos-1, 99%), butylamine (Aldrich, 99%), nitric acid (Komponent-Reaktiv, 65% aqueous solution) and ammonium thiocyanate (Lenreaktiv, 99%) were purchased from a commercial supplier. They were used without additional purification except alcohols which were dried with calcium hydride and then distilled. The absence of significant amounts of impurities and water in hydrocarbons and alcohols was confirmed by gas chromatography and Karl-Fischer titration. 2.2. Synthesis of ionic liquids Synthesis and physical properties of propylammonium nitrate (PAN) and butylammonium nitrate (BAN) were described in our previous work [15]. A similar procedure was used to synthesize 2hydroxyethylammonium nitrate (EOAN) from 2-hydroxyethylamine and nitric acid. Butylammonium thiocyanate (BASCN) was prepared as follows. A small excess of n-butylamine was added dropwise while stirring to an aqueous solution of ammonium thiocyanate in a flask equipped with a Liebig condenser. The temperature was maintained at 75 °C. Stirring was continued for 24 h, then a small portion of butylamine (4 ml) was added. Cessation of ammonia release indicates the end of the reaction. The synthesized ILs were purified from water and the excess of amine by rotary evaporation in vacuo during 20 h at 70 °C. BASCN at 298.15 K is a yellow liquid with ρ = 1.024 g·cm−3 (literature values 1.011 [21] and 0.949 [22] g·cm−3), nD = 1.5225 (literature values 1.5170 [21] and 1.5264 [22]), and mp 8°С (measured using DSC at 10 K/min). EOAN at 298.15 K is a supercooled yellowish liquid with ρ = 1.363 g·cm−3 (literature values 1.38 [23] and 1.265 [24] g·cm−3), nD = 1.4852, and mp 50°С (literature value 51°С [24]). The IR spectra of BASCN and EOAN shown in Fig. 1 indicate that only traces (b0.1%) of water are present. 2.3. Measurement of limiting activity coefficients Limiting activity coefficients of hydrocarbons and alcohols were determined by means of gas chromatographic analysis of the headspace over dilute solutions. The headspace technique is well suitable for even much more volatile solvents than PILs [25–27]. In a typical experiment, 2–10 μl of a solute was put into a vial containing 5 ml of solvent. The vials were crimp sealed, shaken and thermostated at 298.15 K for several hours. An autosampler takes small samples of the equilibrium vapor over a solution and transfers them through a heated quartz glass line into the gas chromatograph. The areas of the chromatographic
peaks of a solute S are proportional to its equilibrium vapor pressure p. In a separate experiment, the same procedure is repeated with 5 ml of pure solute in a vial. The peak area in obtained chromatogram S0 is proportional to the saturated vapor pressure of a solute Psat. The limiting acp S ¼ , where tivity coefficients can be calculated by equation γ ∞ ¼ psat x S0 x x is the molar fraction of a solute in the equilibrium liquid phase. In case of poorly soluble solutes, a significant fraction of initially dissolved solute may evaporate. The value of x after equilibration can be calculated from the initial molar fraction of a solute x0: x ¼ x0 −
Spsat V free S0 RTv
ð1Þ
where Vfree = 17 ml is the volume of a headspace in a vial, ν is the number of moles of solvent in a vial. The standard molar Gibbs free energies of solvation in a molar fraction-based standard state are related to γ∞ through equation: Δsolv G ° ¼ RT ln
∞ γ psat p°
ð2Þ
where the standard state pressure p° = 1 bar. The obtained values of ΔsolvG° were averaged over 6 repetitions for each system at different concentrations of a solute. No significant dependence of the activity coefficients at different from concentration was observed, which allows to conclude that infinite dilution is reached. The obtained results are given in Table 1. 3. Discussion The solubility of hydrocarbons in EOAN turned out to be much lower in comparison with alkylammonium nitrates. Thus, we could not measure the activity coefficients of alkanes with sufficient accuracy using the headspace analysis technique. The activity coefficients of alkylbenzenes in EOAN are much higher than in PAN and BAN. In contrast, the values of activity coefficients of hydrocarbons in BASCN are very close to those in BAN. This suggests the existence of a very strong solvophobic effect in EOAN in comparison with BAN, PAN, or BASCN. Earlier [16] we have shown that there is a linear correlation between the Gibbs free energies of solvation of hydrocarbons in different AILs and the reciprocal of their molar volume, which is a measure of molar concentration of ion pairs in a liquid. The data points corresponding to solutions of hydrocarbons in BAN and PAN fall below this line (see Fig. 2a–c), which was explained [15] by their preferential solvation in the apolar domains that exist in these ionic liquids. Fig. 2b and c shows that the data points for solutions of aromatic hydrocarbons in EOAN lie above the correlation for AILs
I.A. Sedov et al. / Journal of Molecular Liquids 290 (2019) 111361
3
Table 1 Limiting activity coefficients and the Gibbs free energies of solvation in protic ionic liquids at T = 298.15 Ka. Solvent
Solute
γ∞
PAN PAN PAN PAN PAN PAN PAN PAN PAN PAN PAN PAN PAN PAN PAN BAN BAN BAN BAN BAN BAN BAN BAN BAN BAN BAN BAN BAN BAN BAN EOAN EOAN EOAN EOAN EOAN EOAN EOAN EOAN BASCN BASCN BASCN BASCN BASCN BASCN BASCN BASCN BASCN BASCN BASCN BASCN BASCN BASCN
Hexane Heptane Octane Nonane Benzene Toluene Ethylbenzene Propylbenzene Butylbenzene Methanol Ethanol 1-Propanol 1-Butanol 1-Pentanol 1-Hexanol Hexane Heptane Octane Nonane Benzene Toluene Ethylbenzene Propylbenzene Butylbenzene Methanol Ethanol 1-Propanol 1-Butanol 1-Pentanol 1-Hexanol Benzene Toluene Ethylbenzene Methanol Ethanol 1-Propanol 1-Butanol 1-Pentanol Hexane Heptane Octane Nonane Benzene Toluene Ethylbenzene Propylbenzene Butylbenzene Methanol Ethanol 1-Propanol 1-Butanol 1-Pentanol
117b 247b 445b 910b 8.37b 13.7b 22b 43.2 63.7b 0.53 0.88 1.25 1.65 2.21 3.31 38.8b 62b 86.9b 154b 4.62b 7.62b 9.94b 16 19.4b 0.52 0.82 0.99 1.12 1.46 1.65 142 416 1052 0.98 3.44 8.55 22.2 58.5 44 60.5 80 114 3.71 5.85 9.2 12 16.7 0.38 0.61 0.74 0.82 0.9
u(γ∞)
ΔsolvG°/(kJ·mol−1)
8.0 11 17 29 0.32 0.4 0.6 1.0 2.2 0.02 0.03 0.03 0.03 0.05 0.05 1.5 2.3 3.1 10 0.12 0.22 0.24 0.27 0.3 0.02 0.02 0.03 0.03 0.03 0.04 6.5 25 50 0.03 0.05 0.2 0.8 1.8 2.0 2.5 4.0 5.0 0.1 0.1 0.4 0.5 0.6 0.01 0.03 0.03 0.05 0.05
7.83 6.73 5.26 4.18 0.14 −1.63 −3.14 −3.98 −5.97 −5.97 −6.60 −8.31 −10.45 −12.48 −13.62 5.09 3.31 1.22 −0.22 −1.33 −3.08 −5.11 −6.44 −8.91 −6.02 −6.78 −8.88 −11.41 −13.51 −15.35 7.15 6.83 6.44 −4.45 −3.23 −3.54 −4.01 −4.37 5.40 3.25 1.01 −0.97 −1.88 −3.73 −5.30 −7.16 −9.28 −6.80 −7.51 −9.60 −12.18 −14.71
a Standard uncertainty for temperature u(T) = 0.2 K. The experimental pressure inside vials p = 2.38 bar, u(p) = 0.01 bar. b Results from our previous work [15].
unlike the points for solutions in BASCN and alkylammonium nitrates. The values of the Gibbs free energy of solvation in EOAN are higher than those expected in AILs with similar molar volumes, which is likely to be linked with the additional enhancement of the solvophobic effect by the three-dimensional network of hydrogen bonds existing in EOAN. At the same time, there are no apolar domains in EOAN [19], which favor the solvation of hydrocarbons in the case of alkylammonium nitrates. As for BASCN, the existence of domain nanostructure in this solvent and the molar volume value close to that of BAN leads to very similar solvation properties of BASCN and BAN despite the different composition, geometric and electronic structure of thiocyanate and nitrate anions. Fig. 3 shows the plots of the Gibbs free energies of solvation of homologous series of alkanes, alkylbenzenes, and linear saturated alcohols in the studied PILs against the number of carbon atoms in the alkyl
Fig. 2. The standard molar Gibbs free energy of solvation of a) n-octane, b) benzene, c) toluene against the concentration of the ion pairs in different ionic liquids at 298.15 K. Empty circles are the data in AILs. Lines correspond to the correlations for solvation in AILs.
chain. Since these quantities are linearly correlated, it is possible to define so-called increments of the methylene group ΔΔG°(CH2), which are the average differences between the Gibbs free energies of solvation of the neighboring members of these series. The values of ΔΔG°(CH2) calculated as the slopes of correlation lines from Fig. 3 are given in Table 2. It is clear that the increments for alkylbenzenes and alcohols series in EOAN are significantly higher than in other PILs. The increments of methylene groups for the Gibbs free energies of solvation of homologous series of organic solutes in the majority of molecular solvents lie in the range from −2 to −3.5 kJ·mol−1 [28,29]. In aprotic ionic liquids, these values usually fall into the range from −1.8 to −2.5 kJ·mol−1 [16]. In formamide and ethylene glycol, which exhibit strong solvophobic effects, these increments are from −1.5 to −2 kJ·mol−1 [30,31]. For water, the values of ΔΔG°(CH2) are
4
I.A. Sedov et al. / Journal of Molecular Liquids 290 (2019) 111361
Fig. 3. Standard molar Gibbs free energies of solvation for n-alkanes, n-alkylbenzenes, and n-alcohols in PAN, BAN, EOAN, and BASCN against the number of carbons in alkyl chain.
significantly higher, averaging at +0.75 kJ·mol−1 [28,32,33]. Thus, the observed values of ΔΔG°(CH2) about −0.3 kJ·mol−1 for EOAN are higher than in any other common solvents except water. The solvation properties of EOAN appear to be the most “water-like” among other solvents, while BASCN, BAN, and PAN are similar to the aprotic ionic liquids and organic solvents in terms of methylene group increments. The values of ΔΔG°(CH2) for solvation of alkylbenzenes in another nanostructured PIL, ethylammonium nitrate (EAN) [19], are −2.6 kJ·mol−1 in the case of alkylbenzenes and −2.1 kJ·mol−1 in the case of alcohols [34], which is close to other alkylammonium nitrates. Interestingly, earlier EAN was called the “water-like” solvent. This was primarily due to very low critical micelle concentration (CMC) values for a number of surfactants, despite much lower aggregation numbers than in water [35,36], while in aprotic ionic liquids micelles either do not form at all or form at several orders of magnitude higher surfactant concentrations than in water [37]. However, in EOAN the CMC values can be much lower than in EAN. In the work [38], it was shown that the CMCs of cationic surfactants n-alkylmethylimidazolium chlorides and n-alkyldimethylethanolammonium chlorides in EOAN were 30–70 times lower than in EAN and even 5–7 times lower than in water. Such low CMC values in EOAN are observed not for all surfactants. Some of them have significantly higher CMCs than in water [39] or even form micelles in EAN but not in EOAN [40]. It should be understood that not only the solvophobic effects but also other types of interactions between surfactants and solvents promote micelle formation. The activity coefficients of hydrocarbons in EOAN are still much lower than in water. For example, benzene in water at 298 K has 17
Table 2 Methylene group increments ΔΔG°(CH2)/(kJ ⋅ mol−1) for the Gibbs free energies of solvation of different homological series in protic ionic liquids at T = 298.15 K. Ionic liquid PAN BAN BASCN EOAN
Alkanes
Alkylarenes
Alcohols
−1.24 −1.80 −2.14 –
−1.46 −1.85 −1.82 −0.36
−1.82 −2.18 −2.42 −0.30
times higher activity coefficient (about 2400), toluene has one about 22 times higher in water than in EOAN, and ethylbenzene – 31 times higher [41]. However, the solvophobic effects in EOAN are very strong as compared to other ionic liquids. This will result in higher selectivity factor of separation of less polar compounds from more polar ones, which is equal to the ratio of the activity coefficients of the separated compounds. In a previous paper [15], we have shown that PAN and BAN have inferior selectivity of separation of alkanes from aromatic hydrocarbons than many AILs. EOAN is a more promising solvent for this and other separation problems. 4. Conclusion 2-Hydroxyethylammonium nitrate shows significantly different solvation properties in comparison with propyl- and butylammonium nitrates as well as with butylammonium thiocyanate. We observed very low solubilities and very large activity coefficients of hydrocarbons in EOAN allowing to call it the most “water-like” solvent from all considered ionic liquids. Strong solvophobic effects in this solvent are linked with the high concentration of ion pairs, existence of a spatial network of hydrogen bonds, and the absence of domain nanostructure. The last factor explains a difference between the solvation thermodynamics in EOAN and alkylammonium salts. The latter have apolar domains formed by alkyl groups which can preferentially solvate hydrocarbons. An important consequence is that one can judge about the existence or absence of the domain structure in protic ionic liquids from the values of activity coefficients obtained using relatively simple thermodynamic experiments. Acknowledgement We thank Timur Magsumov for his help with headspace experiments, Dr. Alexander Klimovitsky and Dr. Marat Ziganshin for their help in measuring IR-spectra and DSC curves. This work was performed according to the Russian Government Program of Competitive Growth of Kazan Federal University. Igor Sedov acknowledges the Russian Federation President Grant MK-6547.2018.3.
I.A. Sedov et al. / Journal of Molecular Liquids 290 (2019) 111361
Boris Solomonov acknowledges funding from Ministry of Science and Higher Education of Russia (4.5618.2017/6.7). References [1] N. Meine, F. Benedito, R. Rinaldi, Green Chem. 12 (2010) 1711. [2] P. Wasserscheid, T. Welton, Ionic Liquids in Synthesis, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim, Germany, 2007. [3] N.V. Plechkova, K.R. Seddon, in: P. Tundo, A. Perosa, F. Zecchini (Eds.), Methods Reag. Green Chem, John Wiley & Sons, Inc., Hoboken, NJ, USA 2007, pp. 103–130. [4] P. Madhavan, R. Sougrat, A.R. Behzad, K.-V. Peinemann, S.P. Nunes, J. Membr. Sci. 492 (2015) 568–577. [5] T. Welton, Coord. Chem. Rev. 248 (2004) 2459–2477. [6] S.P.M. Ventura, F.A. e Silva, M.V. Quental, D. Mondal, M.G. Freire, J.A.P. Coutinho, Chem. Rev. 117 (2017) 6984–7052. [7] A. Marciniak, Fluid Phase Equilib. 294 (2010) 213–233. [8] T.M. Letcher, N. Deenadayalu, B. Soko, D. Ramjugernath, P.K. Naicker, J. Chem. Eng. Data 48 (2003) 904–907. [9] T.M. Letcher, P. Reddy, Fluid Phase Equilib. 219 (2004) 107–112. [10] M. Krummen, P. Wasserscheid, J. Gmehling, J. Chem. Eng. Data 47 (2002) 1411–1417. [11] G. Foco, M.D. Bermejo, A.J. Kotlewska, F. van Rantwijk, C.J. Peters, S.B. Bottini, J. Chem. Eng. Data 56 (2011) 517–520. [12] R. Kato, J. Gmehling, Fluid Phase Equilib. 226 (2004) 37–44. [13] T.L. Greaves, C.J. Drummond, Chem. Rev. 115 (2015) 11379–11448. [14] I.A. Sedov, B.N. Solomonov, J. Struct. Chem. 54 (2013) S262–S270. [15] I.A. Sedov, T.I. Magsumov, T.M. Salikov, B.N. Solomonov, Phys. Chem. Chem. Phys. 19 (2017) 25352–25359. [16] I.A. Sedov, B.N. Solomonov, Fluid Phase Equilib. 425 (2016) 9–14. [17] Y. Umebayashi, W.-L. Chung, T. Mitsugi, S. Fukuda, M. Takeuchi, K. Fujii, T. Takamuku, R. Kanzaki, S. Ishiguro, J. Comput. Chem. Jpn. 7 (2008) 125–134. [18] R. Atkin, G.G. Warr, J. Phys. Chem. B 112 (2008) 4164–4166. [19] R. Hayes, S. Imberti, G.G. Warr, R. Atkin, Phys. Chem. Chem. Phys. 13 (2011) 3237–3247.
5
[20] R. Hayes, S. Imberti, G.G. Warr, R. Atkin, J. Phys. Chem. C 118 (2014) 13998–14008. [21] C.F. Poole, B.R. Kersten, S.S.J. Ho, M.E. Coddens, K.G. Furton, J. Chromatogr. A 352 (1986) 407–425. [22] P.H. Shetty, P.J. Youngberg, B.R. Kersten, C.F. Poole, J. Chromatogr. A 411 (1987) 61–79. [23] D. Wakeham, P. Niga, C. Ridings, G. Andersson, A. Nelson, G.G. Warr, S. Baldelli, M.W. Rutland, R. Atkin, Phys. Chem. Chem. Phys. 14 (2012) 5106. [24] T.L. Greaves, A. Weerawardena, C. Fong, I. Krodkiewska, C.J. Drummond, J. Phys. Chem. B 110 (2006) 22479–22487. [25] J.H. Park, A. Hussam, P. Couasnon, D. Fritz, P.W. Carr, Anal. Chem. 59 (1987) 1970–1976. [26] I.A. Sedov, T.I. Magsumov, B.N. Solomonov, J. Mol. Liq. 224 (2016) 1205–1209. [27] I.A. Sedov, T.I. Magsumov, J. Phys. Chem. B 119 (2015) 8773–8780. [28] A.K. Sengupta, Y. Marcus, Ion Exchange and Solvent Extraction: A Series of Advances, Dekker, New York, 2001. [29] P.W. Carr, J. Li, A.J. Dallas, D.I. Eikens, L.C. Tan, J. Chromatogr. A 656 (1993) 113–133. [30] I.A. Sedov, M.A. Stolov, B.N. Solomonov, Fluid Phase Equilib. 354 (2013) 95–101. [31] I.A. Sedov, M.A. Stolov, B.N. Solomonov, J. Chem. Thermodyn. 64 (2013) 120–125. [32] Y.A. Mirgorod, Russ. J. Phys. Chem. A 75 (2001) 352–355. [33] S. Cabani, P. Gianni, V. Mollica, L. Lepori, J. Solut. Chem. 10 (1981) 563–595. [34] S.P. Verevkin, D.H. Zaitsau, B. Tong, U. Welz-Biermann, Phys. Chem. Chem. Phys. 13 (2011), 12708. [35] D.F. Evans, A. Yamauchi, G.J. Wei, V.A. Bloomfield, J. Phys. Chem. 87 (1983) 3537–3541. [36] D.F. Evans, A. Yamauchi, R. Roman, E.Z. Casassa, J. Colloid Interface Sci. 88 (1982) 89–96. [37] T.L. Greaves, C.J. Drummond, Chem. Soc. Rev. 37 (2008) 1709. [38] Y. Pei, L. Hao, J. Ru, Y. Zhao, H. Wang, G. Bai, J. Wang, J. Mol. Liq. 254 (2018) 130–136. [39] E.C. Wijaya, F. Separovic, C.J. Drummond, T.L. Greaves, Phys. Chem. Chem. Phys. 18 (2016) 24377–24386. [40] H.J. Jiang, P.A. FitzGerald, A. Dolan, R. Atkin, G.G. Warr, J. Phys. Chem. B 118 (2014) 9983–9990. [41] U.N. Wiesmann, S. DiDonato, N.N. Herschkowitz, Biochem. Biophys. Res. Commun. 66 (1975) 1338–1343.