Infinite dilution activity coefficients in the smectic and isotropic phases of tetrafluoroborate-based ionic liquids

Abstract

Inverse gas chromatography was used to obtain the activity coefficients at infinite dilution (γ13∞) of several organic solutes and water in the thermotropic ionic liquid crystal phases of both [C12mim][BF4] and [C14mim][BF4] and their isotropic phases. In the smectic to isotropic transition, a change in the linear representation of the natural logarithm of γ13∞ with the reciprocal temperature was observed for [C14mim][BF4], and was more evident for alkanes, alkylbenzenes, esters, and alcohols, that can be related to structural modifications of the stationary phase. Results are interpreted in terms of the enthalpic and entropic contributions of solute-IL interactions. Selectivities and capacities of important separation problems, such as octane/benzene and cyclohexane/benzene, were calculated and compared with literature values for separating agents such as N-methyl-2-pyrrolidinone, sulfolane, and ionic liquids presenting the [BF4]− anion combined with several imidazolium-based cations. It is shown that to achieve maximum separation efficiency, imidazolium cations with short alkyl chain lengths such as [C2mim][BF4] should be used, whereas high capacities require larger alkyl chain lengths.