Thermophysical properties of aqueous solutions of the 1-ethyl-3-methylimidazolium tricyanomethanide ionic liquid

Abstract

Thermophysical behavior of the binary system [water+1-ethyl-3-methylimidazolium tricyanomethanide ionic liquid (IL)] was thoroughly characterized through systematic measurements of (vapor+liquid) equilibria (water activity aw), mixing enthalpy, density, viscosity, and refractive index. The measurements were performed in the entire composition range and/or specifically in the highly dilute IL region, at T=298.15K or as a function of temperature in the range from (288.15 to 318.15)K. Effective experimental methods minimizing IL sample consumption, using flow arrangements, instrument couplings and high degree of automation were preferably employed. In particular, the aw determination based on the chilled-mirror dew point technique and implemented by an AquaLab 4TE instrument was identified as a generally superior approach to study VLE of (water+IL) systems. Excess thermodynamic properties (Gibbs free energy, enthalpy, heat capacity, and volume) and property deviations from the linear mixing rule (viscosity, refractive index) were evaluated, Padé approximants being used to correlate adequately their complex composition dependences. The extensive aw data were processed by a two-step procedure fitting first the temperature dependence at each isopleth and subsequently the composition dependence at each isotherm. Good estimates could be thus obtained for derivative thermal properties (enthalpy, heat capacity). Alternatively, the water activity and excess enthalpy data were correlated simultaneously by a NRTL-type model, providing their compact, thermodynamically consistent and adequate representation. Despite small absolute values of excess Gibbs free energy (GE), the system is revealed to be highly nonideal, the small GE resulting from close compensation of its large enthalpy and entropy contributions. Large endothermic effects and an enhanced increase of entropy upon mixing found for this system indicate relative weakness of interactions between unlike molecules and a massive structure breakage in the solution. Positive values of excess volume and negative values of viscosity and refractive index deviations found in the major part of the composition range corroborate this general energetic and structural pattern, although the situation appears to be more complicated in the highly dilute IL region, where these properties congruently exhibit a sign inversion.