Critical Point Temperature of Ionic Liquids from Surface Tension at Liquid-Vapor Equilibrium and the Correlation with Interaction Energy

Abstract

The critical temperature of ionic liquids is predicted by scaling-law, Guggenheim, and Eotvos approaches,<br><br>using surface tension data measured in the temperature range of 293-393 K. The available surface tension data for imidazolium-, phosphonium-, and ammonium-based ionic liquids, with different anions content, show that the predicted critical temperature is a function of cation type and its alkyl chain length as well as the anion type. According to this dependence on the nature of the ionic liquid, the anion-cation interaction energy (Einter) was calculated by quantum mechanical density functional theory and the correlation with the predicted critical temperature was studied. The predicted critical temperature has a direct correlation to the absolute value of Einter. The ionic liquids with the BF4<br><br>- anion, which consistently have the highest critical point temperature, also have the largest absolute value of Einter. As the alkyl chain length increases, the critical temperature decreases. When the surface tension is measured under a liquid-vapor equilibrium, the prediction has the meaningful feature of producing the critical-point temperature.