Thermodynamics Research Center / ThermoML | Journal of Chemical Thermodynamics

ABSTRACT

Thermodynamic behavior of aqueous solutions of two ionic liquids (ILs), [EMIM][SCN] and [EMIM][DCA], was thoroughly characterized. Vapor liquid equilibria (water activity), mixing enthalpy, density, and viscosity were systematically measured in the entire composition range and/or specifically in the highly dilute IL region, as a function of temperature in the range from (288.15 to 318.15) K. Pertinent information available in the literature was gathered and critically examined. Data on system energetics, measured in this work and selected from the literature, were simultaneously correlated with an NRTL-type model. Despite complicated courses of excess energetic functions, their compact, thermodynamically consistent and adequate representation was achieved and the model description was proved to provide reasonable extrapolations. Excess volumes (positive) and viscosity deviations (negative) as functions of composition and temperature were correlated with Pade approximants with T-dependent parameters yielding adequate representation of data even for the M-shaped curves encountered for the excess volume of (water + [EMIM][SCN]). The aqueous solutions of [EMIM][SCN] and [EMIM][DCA] were found to exhibit a closely similar energetic behavior. Both systems display large negative excess Gibbs energy and S-shaped excess enthalpy and excess heat capacity which are positive in the IL dilute region and change to negative and close-to-zero, respectively, as the IL content increases. The inferred excess entropy is markedly positive, reflecting vast disorganization of nano-scale structures upon mixing the ILs with water. A detailed inspection of the energetic functions of the two systems revealed consistently more positive values for [EMIM][SCN] indicating that [DCA] is slightly more hydrophilic anion than [SCN] . Comparison to [EMIM][tricyanomethanide] studied previously showed that [DCA] and [SCN] are considerably more hydrophilic than [TCM] .