MONTE-CARLO SIMULATION OF THE BINARY-LIQUID MIXTURE WATER-METHANOL

Monte Carlo statistical mechanics simulations of water-methanol mixtur es were performed in the isothermal-isobaric ensemble (NPT) at T = 298 K and p = 1.0 atm; canonical ensemble (NVT) simulations were performe d at this same temperature and experimental densities. Configurational averages were obtained by using Metropolis importance sampling and tr uncated octahedron box boundary conditions. To calculate the intermole cular energy, the TIP4P and a tree-site model with united atom represe ntation for the methyl group were used for water and methanol molecule s respectively. The potential energy surface for water-methanol intera ction was calculated using combining rules and the original potential parameters for the pure liquids. Volume contraction and exothermic mix ing were obtained in the present calculation, in fair agreement with e xperimental results. The radial distribution functions obtained show f eatures indicating a large population of hydrogen bonded complexes. Th e analysis of site-site coordination numbers shows an enhancement of t he average number of hydrogen bonded complexes at a methanol compositi on near 25%. This result agrees with the experimental observation that the water-methanol system has its largest deviation from the ideal mi xture behavior at this same composition.