Vapour-liquid equilibria in two-dimensional Lennard-Jones fluids: unperturbed and substrate-mediated films

Abstract

The Gibbs ensemble Monte Carlo simulation technique has been applied to study vapour-liquid equilibrium for two-dimensional (2D) fluids of Lennard-Jones (LJ) molecules. Both unperturbed 2D films and films on a model graphite substrate are investigated; in the latter case the fluid parameters are chosen to model methane. Large systems are used, up to 8000 molecules in the case of the unperturbed 2D films. Finite-size effects are accounted for in the region near the critical point, and the critical temperatures and densities are estimated for both systems. For the unperturbed 2D LJ films we find the reduced critical temperature and density to be T*_c = 0·498 ± 0·002, and ρ*_c = 0·360 ± 0·005. The corresponding values for the monolayer film on graphite are T*_c = 0·500 ± 0·002, and ρ*_c = 0·368 ± 0·005. The influence of the graphite substrate on the coexistence curve is found to be small, as expected. Comparisons of the simulated coexistence curve with experimental data for methane on graphite show good agreement in the critical region, and for the coexisting liquid phase densities, but the simulated gas phase densities are smaller than experimental values.