Vapor–Liquid Coexistence and Critical Behavior of Ionic Liquids via Molecular Simulations

Abstract

Vapor–liquid coexistence curves and critical points are of great practical and fundamental importance. Our understanding of these phenomena is well-developed for most fluids but is severely lacking for ionic liquids, a class of salts that are liquid near ambient temperatures. Thermal stability limitations virtually eliminate direct experimental determination of these properties. In this Letter, we report the first vapor–liquid phase diagrams and critical points for ionic liquids obtained in silico with an atomistic force field. We show that within a homologous series of imidazolium-based ionic liquids, the critical temperature, critical density, critical pressure, boiling point, and enthalpy of vaporization all decrease with increasing length of the cation alkyl chain, while the saturation pressure increases with chain length. These trends are opposite to what is observed for alkanes and other nonionic polar compounds such as alcohols. In the vapor phase, we find that ions are distributed across clusters of different sizes with neutral ion pairs being the predominant aggregation state.