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.