Effect of the reservoir size on gas adsorption in inhomogeneous porous media

Abstract

We study the influence of the relative size of the reservoir on the adsorption isotherms of afluid in disordered or inhomogeneous mesoporous solids. We consider both an atomisticmodel of a fluid in a simple, yet structured pore, whose adsorption isotherms are computedby molecular simulation, and a coarse-grained model for adsorption in a disorderedmesoporous material, studied by a density functional approach in a local mean-fieldapproximation. In both cases, the fluid inside the porous solid exchanges matterwith a reservoir of gas that is at the same temperature and chemical potentialand whose relative size can be varied, and the control parameter is the totalnumber of molecules present in the porous sample and in the reservoir. Varyingthe relative sizes of the reservoir and the sample within experimental range maychange the shape of the hysteretic isotherms, leading to a ‘re-entrant’ behaviorcompared to the grand-canonical isotherm when the latter displays a jump in density.We relate these phenomena to the organization of the metastable states thatare accessible for the adsorbed fluid at a given chemical potential or density.