Using Monte Carlo Simulations to Compute Liquid-Vapor Saturation Properties of Realistic Models of Room Temperature Ionic Liquids

Understanding the liquid-vapor saturation properties of room temperature ionic liquids (RTILs) is important for several industrial applications of these fluids. In this presentation, we discuss the use of Monte Carlo simulation to evaluate liquid-vapor saturation properties of the homologous series 1-alkyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([C_nmim][NTf₂]) at temperatures ranging from room temperature to those very near the predicted critical points. We also relate the trends observed in these properties to the microstructure of the saturated liquid and vapor phase. To compute these properties, we perform direct grand canonical Monte Carlo simulations at two temperatures near the critical point. These are followed by isothermal-isobaric temperature expanded simulations to trace the saturation curves to low temperatures. To address the challenges generally encountered in simulating complex molecules like those of RTILs, we use advanced simulation tools like Hybrid Monte Carlo, reservoir Monte Carlo and distance-biased moves. We present results for the above series with n ranging from 2 to 12. The calculated properties include saturated liquid and vapor densities, vapor pressures, enthalpies and entropies of vaporization. We also show how the liquid phase microstructure, especially the nano-domains observed for some of these fluids evolve over the temperature range investigated.