Activity coefficients at infinite dilution measurements for organic solutes and water in the ionic liquid 1-ethyl-3-methylimidazolium tetracyanoborate

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Abstract

The activity coefficients at infinite dilution, γ13, for 36 solutes, including alkanes, cycloalkanes, alkenes, alkynes, aromatic hydrocarbons, alcohols, thiophene, tetrahydrofuran, ethers, acetone, and water, in the ionic liquid 1-ethyl-3-methylimidazolium tetracyanoborate, [EMIM][TCB], were determined by gas–liquid chromatography at temperatures from 298.15 K to 358.15 K. These values are compared to those previously published for selected solutes in the same ionic liquid. The values of the partial molar excess Gibbs free energy ΔG1E,, enthalpy ΔH1E,, and entropy ΔS1E, at infinite dilution were calculated from the experimental γ13 values obtained over the temperature range. Three gas–liquid partition coefficients, KL were calculated for all solutes and the Abraham solvation parameter model is discussed. The values of the selectivity for different separation problems were calculated from γ13 and compared to literature values for N-methyl-2-pyrrolidinone (NMP), sulfolane, 1-decyl-3-methylimidazolium tetracyanoborate, [DMIM][TCB], and additional ionic liquids.

Research highlights

► Measurements of activity coefficients at infinite dilution using GLC. ► 36 organic solvents and water in the ionic liquid 1-ethyl-3-methylimidazolium tetracyanoborate, [EMIM][TCB]. ► Possible entrainer for different separation processes. ► The partial molar excess thermodynamic functions at infinite dilution were calculated.

Introduction

Ionic liquids (ILs), due to their unique properties and those that can be tailored, have become the subjects of intensive study in recent years. In particular, they are expected to play a growing role as replacements for conventional volatile (and often flammable and toxic) organic solvents in the chemical industry. Among those currently under investigation, imidazolium-based ILs feature prominently. Simply by changing the anion or the alkyl chain on the alkylimidazolium cation, a wide range of solvent properties can be attained. In their various manifestations, ILs are capable of dissolving a diverse range of inorganic, organic, or biomaterials to a useful extent. The solute–solvent interactions in solution are controlled by the nature and interplay of the cation and anion pair comprising the IL. These complex and dynamic interactions are the consequence of various energetic and geometric factors leading to uniquely organized, hydrogen-bonded, and self-segregated solvent nanostructures. Discerning trends relating the chemical structure of an IL with its thermophysical and physico-chemical properties are key to the efficient application of ILs and for the identification of promising synthetic targets. For extraction processes, essential solvent properties include selectivity and capacity, which can be directly calculated for different separation problems from the activity coefficients at infinite dilution, γ13 [1]. In the literature, there are numerous studies on γ13 for different organic solutes, such as alcohols, in imidazolium-based ILs. We are aware of only two reports dealing with γ13 for tetracyanoborate (TCB) anion-based ILs: namely, a recent publication from our group on 1-decyl-3-methylimidazolium tetracyanoborate, [DMIM][TCB] [2], and a report on 1-ethyl-3-methylimidazolium tetracyanoborate, [EMIM][TCB], that emerged during the preparation of the current manuscript [3].

Our research group has provided systematic measurements of γ13 for organic solutes and water in various ILs based on the imidazolium cation paired to different anions [4], [5], [6], [7], [8], [9]. In three of these papers, we noted that ILs based on the thiocyanate anion present the best extraction properties in many separation problems [5], [7], [9]. However, careful analysis of our results indicates that the capacity of thiocyanate-based ILs has room for improvement [10]. Recently, Mahurin et al. [11] showed that supported IL membranes based on [EMIM][TCB] yielded the highest known permeance while exhibiting excellent CO2/N2 separation selectivity over 50. Clearly, these results suggest that cyano-containing ILs deserve further scrutiny and consideration in chemical separations. Thus, a number of groups have initiated studies of the activity coefficients for ILs in which the cyano group is hosted by the anion (e.g., 1-ethyl-3-methylimidazolium dicyanamide, [EMIM][N(CN)2] [12]), the cation (e.g., 1-cyanopropyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, [CN-C3MIM][NTf2], and 1-cyanopropyl-dimethylimidazolium bis(trifluoromethylsulfonyl)imide, [CN-C3MMIM][NTf2] [13]), or both cation and anion (e.g., 1-cyanopropyl-3-methylimidazolium dicyanamide, [CN-C3MIM][N(CN)2], and 1-cyanopropyl-dimethyl imidazolium dicyanamide, [CN-C3MMIM][N(CN)2] [14]). These various ILs reveal high values of selectivity in the separation of aromatic and aliphatic hydrocarbons. Unfortunately, the capacities for these ILs remain low. In previous work, we elected to investigate 1-decyl-3-methylimidazolium tetracyanoborate, [DMIM][TCB] [2], because we expected the long alkane chain affixed to the cation to increase the capacity.

We report here on activity coefficients at infinite dilution, γ13, for 36 solutes, including diverse alkanes, cycloalkanes, alkenes, alkynes, aromatic hydrocarbons, alcohols, thiophene, tetrahydrofuran (THF), ethers, acetone, ketones, and water, in the IL 1-ethyl-3-methylimidazolium tetracyanoborate, [EMIM][TCB]. Values of γ13 were determined by gas–liquid chromatography at 10 K intervals from T = 298.15 K to T = 358.15 K. This work also provides an opportunity to make comparisons with previously-published [EMIM][TCB] results found in the literature [3].

Section snippets

Materials

The ionic liquid [EMIM][TCB] had a mass fraction purity of >0.99 and was supplied by Merck, KGaA. The sample was dried for several days at T = 353 K under reduced pressure to remove volatile impurities and trace water, resulting in a water content of <0.0002 mass fraction, as determined by Karl Fisher titration. The different solutes, purchased from Aldrich or Fluka, had purities better than 0.99 mass fraction and were used without further purification due to the fact that the GLC technique

Theoretical basis

The equations developed by Everett [16] and Cruickshank et al. [17] were used in this work to calculate γ13 for solutes in [EMIM][TCB]lnγ13=lnn3RTVNP1-P1B11-V1RT+PoJ23(2B12-V1)RT.

In this expression, n3 is the number of moles of solvent on the column packing, R is the gas constant, T is the column temperature, VN denotes the net retention volume of the solute, P1 is the saturated vapor pressure of the solute at temperature T, B11 is the second virial coefficient of pure solute, V1 is the

Results and discussion

Table 1 lists the average γ13 values for different solutes in [EMIM][TCB] over the temperature range from (298.15 to 358.15) K. The values of γ13 for solute homologues increase with an increase in alkyl chain length. The highest γ13 values are observed for the n-alkanes, cycloalkanes, and alkenes. This behavior is typical of ILs, including the subset of those based on the imidazolium cation. High values of γ13 signify very weak interactions between solute and solvent. Cyclic alkanes show

Conclusions

Activity coefficients at infinite dilution for 43 solutes in the IL [EMIM][TCB] were measured by gas–liquid chromatography at the temperatures from (298.15 to 358.15) K and compared to recently-published data from another laboratory [3], as well as to other ILs studied by our group and others. It was found that [EMIM][TCB] shows higher selectivity and capacity at infinite dilution than the generally used organic solvents such as NMP or sulfolane, as well as many other ILs containing the same

Acknowledgment

This work has been supported by the Warsaw University of Technology.

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