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

Abstract

The critical temperature of ionic liquids is predicted by scaling-law, Guggenheim, and Eo ¨tvo ¨s approaches, 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 (E inter ) 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 E inter . The ionic liquids with the BF 4 anion, which consistently have the highest critical point temperature, also have the largest absolute value of E inter . 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.

Surface tensions of ionic liquids: Non-regular trend along the number of cyano groups

“…Weiss et al [60] favoured Guggenheim's expression over the Eötvös, as it could yield accurate estimates of T c and it is capable of describing the behavior in the near-critical region. Also Ghatee et al [61] determined similar T c values by these two methods, although the Eötvös equation also depends on the accuracy of the density data. In general, and although not completely accurate due to the inability of determining the critical temperatures of ILs, these values can be used as a first approximation to characterize the volatility of ILs.…”

On the prediction of critical temperatures of ionic liquids: Model development and evaluation

“…The critical temperature of a substance is the temperature at and above which vapor of the substance cannot be liquefied, no matter how much pressure is applied. The behavior of a fluid near its critical point is a specific property necessary to develop the thermodynamic models for fluids [38]. Due to presence of high electrostatic interactions and Coulombic forces, as well as short-range van der Waals interactions in ionic liquids, the critical properties of ionic liquids, cannot be measured as they are decomposed before their normal boiling temperature (T b ) are reached [39,40].…”

“…These equations usually can estimate the T c of organic compounds pretty well; so it is assumed that they are applicable for ionic liquids too. This approach has been followed by the majority of researchers [38,40,[43][44][45][46][47] to calculate and report the critical temperature of ionic liquids.…”

A new fragment contribution‐corresponding states method for physicochemical properties prediction of ionic liquids

“…Moreover, they first extended the two equations to estimate the critical temperature of ILs. Several researchers subsequently quoted the Eotvos and Guggenheim equations and calculated the T c of ILs. These pioneering works provide ideas and methods for predicting the critical temperatures of ILs, whereas the derived values needed further investigations to justify their validity.…”