ADSORPTION OF FLUIDS IN DISORDERED POROUS-MEDIA FROM INTEGRAL-EQUATION THEORY

We consider the application of the integral equation theory developed by Madden and Glandt to a recently developed model of methane adsorbed in a silica xerogel. At higher temperatures, above the bulk fluid cri tical temperature, the theory yields very good predictions of the adso rbate-matrix and adsorbate-adsorbate distribution functions. At lower temperatures, where the attractive intermolecular forces play a more d ominant role in determining the microstructure, the agreement between simulation and theory deteriorates somewhat. It also becomes increasin gly difficult to obtain solutions of the integral equations at low tem peratures. We have found that for this system and others where the mat rix particles are much larger than the adsorbate particles the adsorba te-matrix and adsorbate-adsorbate correlation functions differ only sl ightly from those of an equilibrium mixture of matrix and adsorbate pa rticles. We offer an explanation for this result on the basis of cance llation of diagrams in the cluster expansion of the total correlation function. We discuss the determination of adsorption isotherms from in tegral equation theory using expressions developed in the context of d ensity functional theory.