An investigation of the effects of the structure of gel materials on their adsorptive properties using a simple lattice–gas model

Abstract

We have used a simple lattice–gas model solved in the mean-field approximation to study the effects of material structure on adsorption and desorption isotherms in simple models of silica aerogels and xerogels. We have varied independently the gel particle radius, the density, the surface wettability and the degree of long-range structural correlation in the gel, and in each case obtained adsorption and desorption data at a series of temperatures. These data are discussed in the context of standard adsorption theories and classifications. Kelvin-like behaviour is observed over the entire range of models studied, for both randomly generated gel structures and diffusion-limited aggregates. The shape of the hysteresis loops in these systems appears to vary smoothly with gel structure and is principally determined by the gel density and gel particle size. We have also measured the structure factor at each pressure along each isotherm, for which we discuss data in developing a microscopic understanding of adsorption in these systems. The relative merits of small-angle scattering data and geometrically defined pore size distributions for understanding adsorptive behaviour are discussed. For the material models considered here, a geometric measure of pore size distribution appears to be the best predictor of adsorptive behaviour.