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 (δ). At the lowest coverage investigated (δ = 1 monolayer, ML), >90% of water molecules form three hydrogen bonds (H-bonds) with surface silanol groups and none with other water molecules; when δ = 1 ML, the most probable molecular orientation is characterized by both the molecular dipole and the OH vectors being parallel to the surface. As δ 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 δ 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 ∼1.4 nm from the solid-liquid interface when δ = 15.1 ML (a film that is ∼2.3 nm thick). For the thinner adsorbed films (δ ≥ 4.3 ML, thickness ≥0.8 nm) orientational correlations imposed by the solid-liquid and liquid-vapor interfaces are observed throughout.