Structure and Energetics of Thin Film Water

We present a molecular dynamics (MD) simulation study of the structure and energetics of thin films of water adsorbed on solid substrates at 240 K. By considering crystalline silica as a model hydrophilic surface, we systematically investigate the effect of film thickness on the hydrogen bonding, density, molecular orientation, and energy of adsorbed water films over a broad surface coverage range (delta). At the lowest coverage investigated (delta = 1 monolayer, >90% of water molecules form three hydrogen bonds (H-bonds) with surface silanol groups and none with other water molecules; when delta = 1 ML, the most probable molecular orientation is characterized by both the molecular dipole and the OH vectors being parallel to the surface. As 6 increases, water-water and water-surface interactions compete, leading to the appearance of an orientational structure near the solid-liquid interface characterized by the dipole moment pointing toward the silica surface. We find that the water-surface H-bond connectivity and energetics of the molecular layer nearest to the solid liquid interface do not change as delta increases. Interfacial water molecules, therefore, are able to reorient and form water-water H-bonds without compromising water-surface interactions. The surface-induced modifications to the orientational structure of the adsorbed film propagate up to similar to 1.4 nm from the solid-liquid interface when delta = 15.1 ML (a film that is similar to 2.3 run thick). For the thinner adsorbed films (delta <= 4.3 ML, thickness <= 0.8 nm) orientational correlations imposed by the solid liquid and liquid-vapor interfaces are observed throughout.