Thermodynamics of aqueous solutions of ionic liquids composed of [BMPYR] or [BMIM] cations and tetraflate or dicyanamide anions

This paper explores the thermodynamic behavior of aqueous solutions of four ionic liquids (ILs), namely 1-butyl-1-methylpyrrolidinium dicyanamide [BMPYR][DCA], 1-butyl-3-methylimidazolium dicyanamide [BMIM][DCA], 1-butyl-1-methylpyrrolidinium 1,1,2,2-tetrafluoroethanesulfonate [BMPYR][TFES], and 1-butyl-3-methylimidazolium 1,1,2,2-tetrafluoroethanesulfonate [BMIM][TFES]. The ionic liquids with [TFES]- anion (tetraflates) have been scarcely researched so far and therefore were characterized here with 1H NMR spectroscopy and DSC calorimetry. In all four systems studied, water activity was systematically measured, the determinations being carried out in the whole composition range and at seven temperatures in the range from 288.15 K to 318.15 K. In addition, the mixing enthalpies in the IL dilute region for (water + DCA IL) systems and the solid-liquid equilibria for (water + TFES IL) systems were experimentally determined. The dependences of activity coefficients on temperature and composition were correlated (i) separately with appropriate flexible equations and (ii) simultaneously with an extended NRTL model. Both correlation approaches provide adequate description of mixture energetics and, as documented by comparisons with other relevant data, allow extrapolations of activity coefficients to both higher and lower temperatures as well as good predictions of excess enthalpy. Furthermore, the volumetric and viscosity behavior of highly dilute solutions of the ILs was explored. The concentration dependences of the apparent volume and viscosity were fitted well by the respective classical Redlich-Meyer and Jones-Dole equations. The values of partial molar volumes at infinite dilution were found to perfectly obey the ion additivity and correlate well with the intrinsic volume of IL molecule. Thermodynamic behavior of studied systems was compared to each other and discussed with respect to molecular interactions and nanostructure as well as to how involved ions affect it.