Monte Carlo calculations of phase diagrams for a fluid confined in a disordered porous material

A Monte Carlo simulation study of the phase diagram of an off-lattice molecular model of a fluid in a disordered porous material is presented. The molecular model consists of a Lennard-Jones 12-6 fluid confined in a rigid matrix of spheres with size parameters representative of methane in a silica xerogel. The matrix spheres are arranged in a configuration from an equilibrium hard-sphere system, although in some cases a fee arrangement was considered in order to study the effect of translational order in the matrix. Various strengths of attraction between the fluid molecules and matrix particles have been considered, including the case of complete repulsion. The fluid-phase diagram shows effects of confinement, wetting, and matrix disorder. The results of this study provide evidence for two fluid-phase transitions. One transition is analogous to the bulk vapor-liquid transition, while the second is related to the wetting properties of the fluid in the more confined regions of the matrix. A key feature of our results is the inhomogeneity and disorder of the equilibrium phases in the system.