(456e) Liquid-Vapor Coexistence Properties of Complex Molecules Using Grand-Canonical and Isothermal-Isobaric Temperature Expanded Ensemble Simulations

The chemical industry is rapidly evolving with the discovery of new tunable molecules and novel applications of conventional compounds. The knowledge of liquid-vapor coexistence of such compounds is necessary for the design of numerous technological applications. In this presentation we describe a Monte Carlo simulation based approach to evaluate the liquid-vapor co-existence properties of complex molecules over a broad range of temperatures. We address the general difficulties encountered in performing Monte Carlo simulations of large, flexible and associating molecules. To improve sampling of configuration space we use a combination of reservoir GCMC and expanded ensemble addition and deletion moves. Hybrid Monte Carlo moves are used to sample the intramolecular degrees of freedom of large molecules and distance-biased moves are used to sample strongly associating species like ionic liquids. We first discuss the direct determination of coexistence properties via grand canonical and isothermal-isobaric simulations. We then discuss the use of temperature expanded ensemble simulations for tracing coexistence curves over a wide range of temperatures. This strategy reduces computational expenses and avoids the need to overcome large free energy barriers between liquid and vapor phases at low temperatures. Results are presented for the liquid-vapor phase coexistence properties of neutral molecules like octane, squalene and pyrene as well as polar molecules like propanol and water and ionic liquids like 1,3-dimethylimidazolium tetrafluoroborate ([C₁mim][BF₄]).