Water adsorption on hydrophilic mesoporous and plane silica substrates: A grand canonical Monte Carlo simulation study

Abstract

The adsorption properties of water on silica surfaces at 300 K are evaluated by way of grand canonical Monte Carlo simulations, a well suited technique to investigate thermodynamical properties of interfacial fluids. The PN–TrAZ potential is used to describe atomic interactions between water and substrate, and the SPC model for water–water interactions. To measure confinement effects, we compare adsorption on different plane surfaces to a realistic disordered mesoporous material previously obtained by off lattice reconstructions known to reproduce in a realistic way the geometrical complexity of real Vycor of high specific surface, in terms of surface area, chord distribution, and correlation peak in small angle neutron scattering spectra. The adsorption isotherm, and isosteric heat of adsorption are calculated and compared to experimental data found in the literature for Vycor. They show good agreement, the PN–TrAZ potential for the adsorbate–surface interaction being able to give a satisfactory description of hydrophilic properties of plane and curved silica surfaces. For pressure less than 0.65 times the vapor pressure of SPC water at 300 K, the model shows identical adsorption properties for plane and mesoporous surfaces, validating the t-plot concept for such materials, i.e., the fact that the amount of fluid adsorbed is proportional to the surface area of the substrate irrespective of the pore size, as long as the surface chemistry (the OH density in this case) is kept constant. However, for very low relative pressure (10−3) an analysis of the isosteric heat of adsorption shows the presence of highly energetic sites on the mesoporous Vycor sample probably related to curvature, defects or roughness absent from perfectly plane surfaces. For relative pressure higher than 0.65 a sudden rise in Vycor adsorption is the signature of capillary condensation in the mesoporosity of the material, in agreement with experimental measurements in Vycor. Such a rise is absent on plane surfaces as expected for this confinement-induced capillary condensation.