A computational study of interfacial phenomena in dissociative water confined by nanostructured silica

Molecular Dynamics (MD) Simulation techniques were used to build a dissociative model for water that accurately represents the structure, vibration spectrum and thermal expansion curves over a wide range of temperatures and pressure. The structural changes and interaction of this water model when confined by nano layers of silica were observed. Hydronium formation was observed and the structure and diffusion properties between confined water, bulk water, and water far from the silica interface were compared.;The water model was based on a pair potential and atomic water which allows for dissociation of water and its interaction with silica to form silanols. An interaction parameter representing the O-H distance (&zgr;r-OH) was adjusted based on temperature and pressure as a strong correlation was observed between changes in the OH distance and the structure, density and energies of simulated water. The properties of water were close to the experimentally observed physical properties of water.;An atomic model for vitreous silica was also built based on the same potential and using the same parameters for oxygen-oxygen interactions in silica as that of water. The cross species interactions (Si-H and Si-O) were determined to accurately predict the structure of vitreous silica and low energy structures of interacting silicic acid -- water clusters.;Based on the above potentials, a 3nm water film was placed between layers of vitreous silica and MD simulations of the above system were carried out for seven temperatures. The structure of water far away from the interface was closer to that of bulk water and the structure of the penetrated water had features of bulk water at higher temperature and pressure. The self diffusion coefficient of the penetrated water molecules was observed to an order of magnitude lower than that of bulk water. The confined water was also observed to respond differently to changes in temperature as compared to bulk water thus changing the averaged properties of water and exhibiting variation in phase behavior.