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

TL;DR

This study investigates how the size of the reservoir affects gas adsorption isotherms in inhomogeneous porous media, revealing that reservoir size variations can alter hysteresis behavior and metastable state organization.

Contribution

It introduces a combined atomistic and coarse-grained modeling approach to analyze the impact of reservoir size on adsorption in disordered porous materials, highlighting reentrant isotherm behavior.

Findings

Reservoir size influences the shape of adsorption isotherms.

Varying reservoir size can induce reentrant hysteresis.

Metastable state organization explains observed phenomena.

Abstract

We study the influence of the relative size of the reservoir on the adsorption isotherms of a fluid in disordered or inhomogeneous mesoporous solids. We consider both an atomistic model of a fluid in a simple, yet structured pore, whose adsorption isotherms are computed by molecular simulation, and a coarse-grained model for adsorption in a disordered mesoporous material, studied by a density functional approach in a local mean-field approximation. In both cases, the fluid inside the porous solid exchanges matter with a reservoir of gas that is at the same temperature and chemical potential and whose relative size can be varied, and the control parameter is the total number of molecules present in the porous sample and in the reservoir. Varying the relative sizes of the reservoir and the sample may change the shape of the hysteretic isotherms, leading to a "reentrant" behavior compared to the grand-canonical isotherm when the latter displays a jump in density. We relate these phenomena to the organization of the metastable states that are accessible for the adsorbed fluid at a given chemical potential or density.