Adsorption of water on TiO2 and SnO2 surfaces:: Molecular dynamics study

The structure and thermodynamics of water adsorbed at the ( 110) surface of rutile (alpha-TiO2) and cassiterite (alpha-SnO2) were studied by means of molecular dynamics simulations with atomic interactions represented by a classical forcefield based on the SPC/ E model of water. To investigate the effect of surface water dissociation on the adsorption of additional layers of water, two extreme cases of completely hydroxylated and nonhydroxylated surfaces were considered. Axial density distributions and adsorption Helmholtz free energies of water for different types of surfaces were compared and related to thermal gravimetric analysis data from literature. We found that the dissociation of water in the first layer considerably changes the affinity of additional water to the surface, weakening hydrogen bonding between the first and second layer and strengthening cohesion between the second and third layer. Comparison with the experimental measurements of adsorption indicates that water dissociates on cassiterite while it stays associated on rutile. The degree of dissociation in the first layer is not strongly affected by the adsorption of additional water.