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
Introduction
Activity coefficients at infinite dilution (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 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 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, 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 is the ratio of activity coefficients at infinite dilution and is given by the equation , 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 [9], [10]. The partial molar excess enthalpies at infinite dilution values were also calculated from the 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]was used in this work to calculate the of solutes in the ionic liquid. The VN denotes the net retention volume of the solute, Po the outlet pressure, the mean column pressure, n3 the number of moles of solvent on the column packing, the column temperature T, 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 values for the varying amounts of solvent on the solid packing at T = (298.15, 313.15, and 333.15) K. The 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 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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