Activity coefficients at infinite dilution measurements for organic solutes and water in the ionic liquid 1-ethyl-3-methylimidazolium tetracyanoborate
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, [1]. In the literature, there are numerous studies on for different organic solutes, such as alcohols, in imidazolium-based ILs. We are aware of only two reports dealing with 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 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, , 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 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 for solutes in [EMIM][TCB]
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, is the saturated vapor pressure of the solute at temperature T, B11 is the second virial coefficient of pure solute, is the
Results and discussion
Table 1 lists the average values for different solutes in [EMIM][TCB] over the temperature range from (298.15 to 358.15) K. The values of for solute homologues increase with an increase in alkyl chain length. The highest 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 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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