Capillary condensation and hysteresis in disordered porous materials

In this article we consider capillary condensation and hysteresis in molecular models of fluids confined in a disordered porous materials. Capillary phase diagrams for these systems show that the phase behavior is substantially modified by confinement in the porous material. Isotherms from grand canonical Monte Carlo simulations of these systems exhibit hysteresis loops which resemble those seen in experiments on adsorption in silica xerogels. The hysteresis in the Monte Carlo simulations is associated with metastability of low density states on the adsorption branch and high density states on the desorption branch of the isotherm. It is suggested that further investigation of the stability of these states in Monte Carlo simulations and their relationship with those seen in experiments is worthwhile. In this regard an algorithm for simulation of adsorption via diffusive mass transfer based on the grand canonical molecular dynamics with control volume method is described. An illustrative application to the hysteresis in the bulk vapor-liquid equilibrium for the Lennard-Jones 12-6 fluid is presented.