Grand canonical Monte Carlo simulation study of water structure on hydrophilic mesoporous and plane silica substrates

The radial distribution functions obtained by grand canonical Monte Carlo simulations of water adsorbed on disordered mesoporous and ordered plane silica surfaces at 300 K are investigated. The plane surfaces are crystallographic faces of cristobalite. The disordered mesoporous Vycor-like glass was previously obtained by off-lattice reconstruction known to reproduce in a realistic way the geometrical complexity of the real Vycor of high specific surface, in terms of surface area, chord distribution, and correlation peak in small angle neutron scattering spectra. The SPC model is used for water, and water-substrate interactions are described by the PN-TrAZ model without any adjustment of parameter. It is shown that the water confined in Vycor is slightly more structured than bulk water, mainly due to the interaction of the first layer with the substrate. The vapor pressure is reduced so as to obtain a mono-layer coverage for each surface. On ordered crystalline faces, the water mono-layer has long range correlations, that are absent in the case of a disordered silica (Vycor-like sample): The main change being the displacement of the second peak towards larger distances (4.5 Angstrom in bulk, 5 Angstrom in mono-layer), characteristic of bi-dimensional structure of water. (C) 2003 American Institute of Physics.