Experimental and theoretical study on infinite dilution activity coefficients of various solutes in ionic liquid 1-propyl-2,3-dimethylimidazolium bis(trifluoromethylsulfonyl)imide

Abstract

The infinite dilution activity coefficients (γi∞) and gas–liquid partition coefficients (KL) of solutes in the ionic liquids (ILs) are important parameters of thermodynamic properties. They can evaluate the separation selectivity (Sij∞) of ILs and screen out suitable extractants for difficult separation systems. The retention times of solutes in IL 1-propyl-2,3-dimethylimidazolium bis(trifluoromethylsulfonyl)imide [PMMIM][NTf2] were measured by gas chromatography (GC) to calculate the γi∞ and the ΚL values of organic solutes (eg. n-alkanes, alkyl benzenes, acetonitrile, tetrahydrofuran, ethyl acetate, etc.) in [PMMIM][NTf2] at 303.15 K-353.15 K. The density of [PMMIM][NTf2], as a function of temperature, was measured at p = 101.3 kPa. The values of the partial molar excess enthalpies at infinite dilution (H-iE,∞) were derived from the T dependence of the lnγi∞. The entropies (TrefS-iE,∞) and Gibbs free energies (G-iE,∞) of organic solutes in [PMMIM][NTf2] at a reference temperature Τref = 298.15 K can be calculated from the γi∞ values. The Hildebrand’s solubility parameters of the IL [PMMIM][NTf2] were obtained from the regular solution theory (RST) combined with Flory “combinatorial” equation. At 323.15 K, the selectivity (Sij∞) of n-hexane (i)/benzene (j) and cyclohexane (i)/benzene (j) and the capacity (kj∞) at infinite dilution of benzene have been determined. The results were analyzed by comparing with literature data for alkylimidazolium-based ILs with [NTf2]−.