Infinite dilution activity coefficients of volatile organic compounds in two ionic liquids composed of the tris(pentafluoroethyl)trifluorophosphate ([FAP]) anion and a functionalized cation

Abstract

Interactions of volatile organic compounds with two ionic liquids (ILs) containing tris(pentafluoroethyl)trifluorophosphate ([FAP]) anion and a functionalized cation, 1-(2-hydroxyethyl)-3-methylimidazolium ([HO-EMIM]) and 1-(2-methoxyethyl)-1-methylpyrrolidinium ([MO-EMPYR]), were explored through systematic GLC retention measurements. Infinite dilution activity coefficients γ1∞ and gas–liquid partition coefficients KL of 30 selected solutes in [HO-EMIM][FAP] and [MO-EMPYR][FAP] were determined at five temperatures in the range from (318.15 to 353.15)K. Partial molar excess enthalpies and entropies at infinite dilution were derived from the temperature dependence of the γ1∞ values. The Linear Free Energy Relationship (LFER) solvation model was used to correlate the KL values. The LFER correlation parameters and excess thermodynamic functions were analyzed to identify molecular interactions operating between the ILs and the individual solutes. By comparing the behaviors of the studied ILs and of their closely similar unfunctionalized analogs, net effects imparted by cation functionalization were also disclosed. The cohesivity of the two ILs was shown to differ dramatically: while [MO-EMPYR][FAP] ranks among ILs to the least cohesive, [HO-EMIM][FAP] belongs to the most cohesive ones. Both [HO-EMIM][FAP] and [MO-EMPYR][FAP] are capable of interacting with solutes specifically through dipolarity/polarizibility and hydrogen bonding, but apparently lack the ability to interact with solute lone electron pairs. The proton donating capability of [HO-EMIM][FAP], undoubtedly brought by the hydroxyl functionality, is enormous and imparts to this IL extraordinary potential for use in solvent-aided separations. As we have demonstrated on some model aliphatic/aromatic separation pairs, [HO-EMIM][FAP] as a solvent for the separation of aliphatic hydrocarbons from aromatics gives superior performance, its performance index (product of selectivity and capacity) surpassing distinctly that of conventional solvents and of most ILs studied so far. [HO-EMIM][FAP] and [MO-EMPYR][FAP] could also serve as efficient entrainers in separations of other non-petrochemical azeotropic systems of industrial importance by extractive distillation as exemplified for some mixtures of alcohols with aprotic oxygenates.