Phase Diagrams of Nonideal Fluid Mixtures from Monte Carlo Simulation

A methodology is presented for the direct determination of phase equilibria in mixtures from Monte Carlo simulation. The Widom test-particle expression is used for the evaluation of the chemical potential. The novel features of the proposed methodology include use of fluctuation theory that allows one to “observe” the approach of the system to a stability limit and introduction of an interchange algorithm to facilitate the calculation of properties for highly nonideal mixtures. The key attributes of simulations for the determination of the chemical potential and the phase diagrams are illustrated with simple Lennard-Jones fluids. It is shown that the interaction energies between unlike molecules are particularly important in determining the tendency of a system to phase-separate or to form an azeotrope. Molecular size differences between components were found to be less important. Finally, by choosing potential parameters that reproduce the critical constants of carbon dioxide and acetone, we obtained a simulated phase diagram that captures all the essential elements of the phase equilibrium behavior of the real mixture.