Vapor-Liquid and Vapor-Solid Phase Equilibria for United-Atom Benzene Models near Their Triple Points:The Importance of Quadrupolar Interactions

摘要

Gibbs ensemble Monte Carlo simulations were used to calculate the vapor-liquid and vapor-solid coexistence curves for benzene using two simple united-atom models.An extension of the Gibbs ensemble method that makes use of an elongated box containing a slab of the condensed phase with a vapor phase along one axis was employed for the simulations of the vapor-solid equilibria and the vapor-liquid equilibria at very low reduced temperatures.Configurational-bias and aggregation-volume-bias Monte Carlo techniques were applied to improve the sampling of particle transfers between the two simulation boxes and between the vapor and condensed-phase regions of the elongated box.An isotropic united-atom representation with six Lennard-Jones sites at the positions of the carbon atoms was used for both force fields,but one model contained three additional out-of-plane partial charge sites to explicitly represent benzene's quadrupolar interactions.Both models were fitted to reproduce the critical temperature and density of benzene and yield a fair representation of the vapor-liquid coexistence curve.In contrast,differences between the models are very large for the vapor-solid coexistence curve.In particular,the lack of explicit quadrupolar interactions for the 6-site model greatly reduces the energetic differences between liquid and solid phases,and this model yields a triple point temperature that is about a factor of 2 too low.In contrast,the 9-site model predicts a triple point of benzene at T=253+-6 K and p=2.3+-0.8 kPa in satisfactory agreement with the experimental data (T=278.7 K and p=4.785 kPa).