Liquid-vapor equilibrium and critical parameters of the ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate from molecular dynamics simulations

The liquid-vapor equilibrium properties of the room temperature ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate, often abbreviated as [bmim][PF6] or [C4mim][PF6], have been determined using a classical all-atom force field in molecular dynamics simulations. Extending our earlier work, the behavior at elevated temperatures (875-1075 K) was studied to obtain the following refined estimates of the critical parameters: Tc = 1105 ± 25 K, ρc = 227 ± 19 kg m− 3, and Pc = 3.1 ± 1.4 bar. In the above-mentioned temperature range, the vapor pressure can be calculated directly with reasonable accuracy, giving us indirect access also to the enthalpy of vaporization. From these new data, several corresponding-states properties, which are believed to be generic for ionic liquids and which do not depend too sensitively on any specific potential model, can be obtained in addition to Guggenheim's reduced surface tension which we reported previously. The critical compressibility factor is found to be Zc ≈ 0.04 ± 0.02, Guggenheim's ratio ΔvapHapp/(RTc) ≈ 9.9 ± 0.5, the relative normal boiling point Tb/Tc = 0.90 ± 0.05, and Trouton's constant ΔvapS(Tb) ≈ 71 ± 11 J mol− 1 K− 1. The implications of these findings are discussed.