Fluids adsorbed in narrow pores: phase equilibria and structure

Abstract

When gases or liquids are adsorbed in narrow pores or capillaries their properties are significantly different from those in a bulk phase. This article reviews recent developments in the statistical theory and computer simulation of simple fluids confined in model pores, emphasizing the microscopic structure and phase equilibria. The structure reflects the packing of atoms or molecules in confining geometries while the phase behavior reflects the presence of surface and bulk contributions to the fluid's free energy. Confinement shifts first-order transitions, such as condensation or freezing, away from their location in bulk; it also alters the location and nature of the bulk critical point reducing the effective dimensionality. Sometimes surface phase transitions such as layering and prewetting compete with shifted bulk transitions giving rise to rich phase diagrams. The extent to which the theorists' results for fluids in single idealized pores might be relevant for solvation force studies probing liquids between crossed mica cylinders and for gas adsorption studies in real mesoporous solids such as VYCOR is mentioned briefly.