The molar surface Gibbs energy and polarity of ether-functionalized ionic liquids

Ether groups are well-known for their unique contribution to low viscosity and high conductivity, hence ether-functionalized ionic liquids (EFILs) have been widely studied and successfully employed in various applications. Three EFILs 1-(2-methoxyethyl)-3-methylimidazolium chloride, 1-(2-ethoxyethyl)-3-methy limidazolium chloride and 1-(2-ethoxyethyl)-3-methylimidazolium bromide were prepared and characterized. Their surface tension, density and refractive index in the range of T = (288.15-328.15) K were measured and the molar surface Gibbs energy, g(s), were calculated. Combined g(s) with Lorentz-Lorenz equation, an expression for predicting surface tensions of ionic liquids was obtained, and the predicted values were in good agreement with the experimental ones. According to the g(s), the vaporisation enthalpies were predicted and the association enthalpies of ionic liquids were estimated. A new method for estimating the difference of heat capacities between the vapour phase and the liquid phase of ionic liquids was put forward, and the estimated values were in the same order of magnitude as the Verevkin method. Moreover, the concept of the polarity coefficient P for ionic liquids was proposed and the polarity of many ionic liquids was estimated. (C) 2019 Elsevier Ltd.