Monte Carlo studies of the freezing and condensation transitions of confined fluids

Abstract

The results are presented of a Monte Carlo simulation study of the freezing transition of a Lennard-Jones fluid confined in a single slit pore. Using an ensemble in which the pressure Pz normal to the walls is fixed, phase equilibria are determined by calculating the Gibbs free energy, for various wall—fluid potentials. For purely repulsive walls the freezing line is shifted down in temperature relative to the bulk, while for weakly attractive walls the shift is much smaller in magnitude and is towards higher temperatures at high pressures and lower temperatures at low pressures: the freezing line crosses that of the bulk at a reduced temperature of 0.65. The liquid—gas transition also is studied, observing capillary evaporation for purely repulsive walls and capillary condensation for attractive walls. When the wall—fluid attraction is strong, the techniques for determining the free energy of the confined liquid phase cannot be applied due to the occurrence of surface phase transitions leading to solid-like adsorbed layers. The results for the structure of the confined liquid and solid are similar to those found in a recent grand canonical Monte Carlo study. Results are also presented for the freezing line of a confined model fluid in which the fluid—fluid pair potential is purely repulsive. Brief comments are made concerning the possible relevance of simulation studies for idealized pores for experimental investigations of the freezing of fluids in mesoporous solids.