Transport of an adsorbing vapour in a model silica system

Abstract

A simple, though realistic, model of the structure of microporous silica is used in simulation studies of the properties of a Lennard-Jones (12–6) vapour adsorbed in a confined space over a wide range of pore filling conditions. The silica medium is composed of randomly distributed interconnected microspheres of SiO₂ reminiscent of a gel-like material formed from a colloidal suspension. Each microsphere in the assembly is generated from a molecular model of bulk vitreous SiO₂ which in turn is based on a modified Born–Mayer–Huggins pair potential and the Stillinger–Weber three-body potential for the silicon–oxygen system.

Vapour sorption within the voids of this microporous medium is investigated via molecular dynamics and Monte Carlo simulations. Of particular interest in these studies is the dynamical behaviour of moderately dense adsorbed phases in the transition region between “monolayer” coverage and complete pore filling. In this intermediate region, adsorbed fluid transport properties are known to undergo a radical change in character, ranging from free-molecule activated diffusion at low pore filling to both diffusive and bulk collective motion at high densities. Detailed microscopic information on the dynamics of such systems is currently unavailable, and in this paper the mechanisms associated with this transition region are examined with the aid of dynamical self-and cross-correlation functions of the adsorbed vapour. The time-dependent properties of the adsorbed phase are also investigated using an inert, nonadsorbing molecular probe. The results from the latter studies provide an interesting insight into the percolation behaviour of porous media in the presence of immobile and mobile adsorbed films.