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We have developed a new method, called the reaction Gibbs ensemble Monte Carlo (RGEMC) method, for the computer simulation of the phase equilibria for multicomponent mixtures, given an intermolecular potential model for the constituent molecular species. The approach treats the phase equilibrium conditions as a special type of chemical reaction and incorporates knowledge of the pure-substance vapor pressure data into the simulations. Unlike macroscopic thermodynamic-based approaches like the Wilson and the universal quasichemical functional group activity coefficients (UNIFAC) approximations, no experimental information concerning the mixtures is required. In addition to the PTxy phase equilibrium data, the volumetric properties of the mixture are calculated. We developed intermolecular potential models based on the optimized potentials for liquid simulations (OPLS) of Jorgensen and used the …