Activity coefficients at infinite dilution measurements for organic solutes in the ionic liquid 1-hexyl-3-methyl-imidazolium bis(trifluoromethylsulfonyl)-imide using g.l.c. at T = (298.15, 313.15, and 333.15) K

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Abstract

The activity coefficients at infinite dilution, γ13 (where 1 refers to the solute and 3 to the solvent), for both polar and non-polar solutes (alkanes, alk-1-enes, alk-1-ynes, cycloalkanes, benzene, carbon tetrachloride, and methanol) in the ionic liquid 1-hexyl-3-methyl-imidazolium bis(trifluoromethylsulfonyl)-imide [HMIM][Tf2N] at three temperatures T = (298.15, 313.15, and 333.15) K have been determined by gas–liquid chromatography. The interaction at the gas–liquid interface between the solutes and the solvent was examined by varying solvent liquid loading on the column. Corrected retention values, taking carrier gas and solute imperfections into account, were determined and used to calculate the activity coefficients at infinite dilution. The results have been used to predict the solvent potential for the hexane/benzene separation from calculated selectivity values. The results were compared to γ13 for similar systems found in the literature in an attempt to understand the effect of the nature of the cation and anion has on solute–solvent interactions.

The partial molar excess enthalpies at infinite dilution values ΔH1E were calculated from the experimental γ13 values obtained over the temperature range.

Introduction

Activity coefficients at infinite dilution γ13 (where 1 refers to the solute and 3 to the solvent), provide a useful tool for solvent selection in extractive distillation or solvent extraction processes. It is sufficient to know the separation factor at infinite dilution, of the components to be separated, in order to determine the applicability of a compound (in this work, a new ionic liquid) as a selective solvent. This work is a continuation of our investigation on ionic liquids to determine activity coefficients at infinite dilution. Our group has previously measured the γ13 for organic volatile solutes in the ionic solvents: 1-methyl-3-octyl-imidazolium chloride [1], 1-hexyl-3-methyl-imidazolium tetrafluoroborate [2], 1-hexyl-3-methyl-imidazolium hexafluorophosphate [3], and 1-ethyl-3-methyl-imidazolium bis(trifluoromethylsulfonyl) imide [4], or 1-butyl-3-methyl-imidazolium 2-(2-methoxyethoxy) ethyl sulfate, [BMIM][MDEGSO4], [5], or 1-butyl-3-methyl-imidazolium octylsulfate, [BMIM][OcOSO3] [6].

Recently, ionic liquids (ILs) similar to those studied in this work were under intense investigation. The QSPR method for the analysis of γ13 values obtained in different laboratories was used for the correlation and prediction [7]. This method has shown that QSPR is a powerful tool for extending experimental activity coefficient data, especially for new solutes.

In this work, the activity coefficients at infinite dilution, γ13 have been determined for alkanes, alk-1-enes, alk-1-ynes, cycloalkanes, aromatic hydrocarbons, carbon tetrachloride, and methanol in the ionic liquid 1-hexyl-3-methyl-imidazolium bis(trifluoromethylsulfonyl) imide [HMIM][Tf2N] using gas–liquid chromatography at the temperatures T = (298.15, 313.15, and 333.15) K.

The selectivity Sij is the ratio of activity coefficients at infinite dilution and is given by the equation Sij=γi3/γj3, where i and j refer to the liquids to be separated and in this work refer to hexane and benzene, respectively. The selectivity value is used to determine the potential of the ionic solvent for extractive distillation in the separation of aromatic compounds from aliphatic compounds [8].

Gas–liquid chromatography is a well-established and accurate method that is used to obtain γ13 [9], [10]. The partial molar excess enthalpies at infinite dilution values ΔH1E were also calculated from the γ13 values obtained over the temperature range.

Section snippets

Materials

The ionic liquid [HMIM][Tf2N] had a purity of mass fraction >0.98 and was supplied by Merck KGaA. The ionic liquid was further purified by subjecting the liquid to a very low pressure of about 5 · 10−3 Pa for approximately 30 h. This procedure removed any volatile chemicals and water from the ionic liquid. A Karl-Fischer titration showed that the water concentration in the ionic liquid was less than 0.001 mass percent. The solutes were obtained from Saarchem, Acros Organics, Aldrich, Janssen

Theory

The equation developed by Everett [14] and Cruickshank et al. [15]lnγ13=lnn3RTVNP1-(B11-V1)P1RT+(2B12-V1)PoJ23RT,was used in this work to calculate the γ13 of solutes in the ionic liquid. The VN denotes the net retention volume of the solute, Po the outlet pressure, PoJ23 the mean column pressure, n3 the number of moles of solvent on the column packing, the column temperature T, P1 the saturated vapour pressure of the solute at temperature T, B11 the second virial coefficient of pure

Results and discussion

Table 1 lists the average γ13 values for the varying amounts of solvent on the solid packing at T = (298.15, 313.15, and 333.15) K. The γ13 value for the n-alkanes, alk-1-enes, alk-1-ynes, cycloalkanes and alcohols increases with an increase in carbon number, which is typical for many ionic liquids [2], [3], [4], [5], [6], [21]. For benzene, the value of γ13 is small (0.674 at T =  298.15 K) and is the lowest for all solvents tested in this work. By lengthening the alkyl chain at the imidazole

Acknowledgements

The authors thank the FRD (South Africa) and KBN (Poland) for financial support for this work according to Polish-South African agreement of co-operation. Authors A. Marciniak, M. Marciniak and U. Domańska have been supported by the Polish Committe for Scientific Research (Grant No. 3T09B00427).

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