Aqueous solutions of [EMIM] 1,1,2,2-tetrafluoroethanesulfonate and [EMIM] trifluoromethanesulfonate: A thermodynamic study

While ionic liquids (ILs) based on a trifluoromethanesulfonate ([OTF]−) anion and their mixtures with other compounds have been relatively well explored, the information about structurally similar 1,1,2,2-tetrafluoroethanesulfonate-based ([TFES]−) ILs is either very scarce or, as in the case of their mixtures with water, totally lacking. We herein thus present a broad thermodynamic characterization of aqueous solutions of [EMIM][TFES] and [EMIM][OTF], with the study of aqueous solutions of the latter serving as a benchmark. First, the peculiar solid-liquid phase transition behavior of the neat [EMIM][TFES] was examined and its density was determined over a temperature range encompassing both the subcooled and the stable liquid. For the aqueous solutions of the two ILs, we performed extensive measurements at several temperatures in the range from (288.15 to 318.15) K of water activity (aw) across the entire composition range, mixing enthalpy and density specifically in the highly dilute IL region, and the solution freezing temperatures at water mole fractions greater than 0.6. The ample aw data were fitted (i) by a two-step procedure and (ii) simultaneously with the excess enthalpy data using an extended NRTL-type model. Although both treatments provide adequate representations of the data and prove to yield reasonable estimates of derivative thermal properties, the latter approach to the global description of energetics of the systems is preferred for its handiness in practical calculations. We duly proved its reliability in extrapolation and/or prediction against experimental data on various properties, among others, SLE. The analysis of volumetric data by the Redlich-Meyer equation allowed us to obtain accurate values of infinite dilution partial molar volumes of the ILs and revealed the nature of their hydration as rather hydrophobic. The solution behavior of the systems studied was found to be essentially non-ideal: their small S-shaped excess Gibbs energy (GE) actually results from the compensation of their large positive excess enthalpy (HE) and entropy (SE). Thus, despite being completely miscible with water, the two ILs are not typically hydrophilic, but intermediate on an imaginary (hydrophobicity/hydrophilicity) scale. The structural variation in the anion, though influencing HE and SE considerably, is only subtly reflected in GE and activity coefficients. Due to their somewhat lower values and a weaker decrease of apparent molar volume with increasing molality observed for [EMIM][TFES], [TFES]− is identified to be slightly more hydrophilic than [OTF]−.