Phase Equilibria of Fluids in Narrow Pores

Abstract

Despite its practical importance the nature of the phase equilibria of fluids confined in narrow pores remains poorly understood. Much experimental effort is focussed on gas adsorption measurements in materials such as vycor glass or carbon powders. Such measurements can, in principle, yield important information concerning the character of the confined fluid. However, the theory of adsorption in mesoporous solids is still in its infancy. While many of the difficulties in real materials are associated with the complex connectivity of the pores and the fact that these are not of a uniform size, even the idealized case of a single, infinitely long pore has received relatively little attention from theoreticians. Recently we have embarked upon a detailed study of phase equilibria, adsorption and related phenomena of simple fluids confined in slit-like and cylindrical capillaries which act as models of a single pore. Our approach is based on a mean-field, density functional theory for inhomogeneous fluids that treats the walls of the capillary as exerting an external potential on the fluid molecules. This type of approach has already proved successful for a variety of other interfacial problems, providing much physical insight into wetting transitions, contact angles and adsorption isotherms for both solid-fluid and fluid-fluid interfaces.