A comparison of water adsorption on ordered and disordered silica substrates

The adsorption properties of water adsorbed on various silica substrates are investigated by way of Grand Canonical Monte Carlo simulations (GCMC). The SPC and PN-TrAZ potential are used to describe water-water and water-silica interactions. The numerical sample of mesoporous silica glass (pore size: 3.6 nm) was obtained by off-lattice reconstruction, known to reproduce in a realistic way the geometrical complexity of high specific surface Vycor. The chemistry of the surface is made realistic by hydroxylation. The simulated adsorption isotherm and isosteric differential enthalpy of adsorption compare well to experimental data for Vycor, showing the ability of the PN-TrAZ potential to describe the hydrophilic properties of silica surfaces. This study was extended to several crystallographic faces of cristobalite. Their adsorption properties differ widely from each other. It is shown that the hydrophilic properties are not simply related to surface hydroxyl density but are also related to the local structure of the silica surface. In spite of these large variations, it is possible to reproduce the adsorption isotherm of the mesoporous disordered sample by applying a natural averaging procedure over the different crystallographic faces of cristobalite.