Molecular Structure and Dynamics in Thin Water Films at Metal Oxide Surfaces: Magnesium, Aluminum, and Silicon Oxide Surfaces

All-atom equilibrium molecular dynamics simulations were employed to investigate the structural and dynamical properties of interfacial water on the magnesium oxide surface. The solid support was modeled utilizing two different formalisms, both based on the CLAYFF force field. In one case, the atoms in the MgO substrate are allowed to vibrate, whereas in the other they are maintained rigid. The properties of water within the thin film are assessed in terms of density profiles in the direction perpendicular to the substrate as well as along planes parallel to the substrate, in-plane radial distribution functions, density of hydrogen bonds, residence times in contact with the substrate, and orientation distribution of interfacial water molecules. The contact angle for a small droplet on various substrates (MgO, SiO2, Al2O3) was also calculated and compared with experimental observations. On MgO, the substrate in which the atoms are maintained fixed is the one that most closely reproduces experimental contact angles. This contrasts with results on other substrates, for example, silicon dioxide, on which the vibrations of the solid atoms were found to be useful for better predicting experimental observations. These differences suggest that proper force-field validation is necessary before investigating the structure of interfacial water on solid substrates. In the case of MgO, our analysis suggests that the vibrations of the solid atoms yield atomic-scale roughness. This, in turn, causes water molecules to spread on the substrate. A brief comparison of water properties on MgO, alumina, and silica is provided.