A Grand Canonical Monte Carlo study of argon adsorption/condensation in mesoporous silica glasses

We have studied adsorption of argon in a mesoporous silica controlled porous glass (CPG) over a wide range of temperatures by means of Grand Canonical Monte Carlo (GCMC) simulation. A numerical sample of the CPG adsorbent has been obtained by using an off-lattice reconstruction method recently introduced to reproduce topological and morphological properties of correlated disordered porous materials. The off-lattice functional of Vycor is applied to a simulation box containing silicon and oxygen atoms of cubic cristoballite with a homothetic reduction factor of 2.5 so as to obtain 36 Å CPG samples. The Ar adsorption/desorption isotherms are calculated at 77, 87, 100, 120 and 130 K. They exhibit a hysteresis loop characteristic of the capillary condensation phenomenon. As often encountered in gas adsorption experiments with this type of porous solid, the adsorption branch is characterized by a finite slope, in contrast to that theoretically obtained in simple geometries such as slits and cylinders. As the temperature increases, the hysteresis loop is displaced toward the higher (reduced) pressure range and shrinks in agreement with experiment; the temperature, Tcc, for which the isotherm is fully reversible is 132.7 K. This behavior is similar to that found for slit-shaped or cylindrical pores for which the capillary condensation/evaporation transition is found to be first order. We have shown that the hysteretic coexistence curve obtained from desorption data is characteristic of disordered systems with energetic heterogeneity. In the system of reduced units (ρcc and Tcc, where ρcc is the confined fluid density at Tcc), we have also found that the hysteretic coexistence curves close to Tcc for polar and non-polar fluids in silica ordered and disordered mesoporous sytems of the same mean pore size, fall on a single master curve.