(447a) Water Structure And Dynamics In Thin Interfacial Layers At The SiO2 And Graphite Surfaces

The structure of the first few molecular layers of aqueous solutions at solid-liquid interfaces determines a number of phenomena, typically identified by the general classification of ?hydrophobic' and ?hydrophilic' surface properties. Such phenomena are important in a variety of fields including geology (transport of heavy metal ions through rocks), biology (ion channels), and engineering (lab on a chip). As a consequence, aqueous solutions at interfaces have generated, and continue to generate, significant research interest [Verdaguer, Sacha, Bluhm, and Salmeron, Chem. Rev. 2006, 1478]. We employed all-atom equilibrium molecular dynamics simulations in the canonical ensemble to investigate the structure and dynamics of water, as well as that of aqueous solutions of NaCl, within thin interfacial layers (maximum 10 molecular layers) near a few selected free-standing surfaces. As a model hydrophobic surface we considered graphite. As technologically more important surfaces we considered those carved from perfect SiO2 crystals. Different crystallographic surfaces [(1 1 1), (0 0 1), and (1 0 0)] of the SiO2 cristobalite crystal were considered to assess the effect of surface roughness on the properties of interest. Moreover various amounts of H from non-bridging O atoms on the SiO2 surfaces were removed, randomly, to control the surface charge density. The water-water interactions were modeled using the SPC/E model and the solid layers (graphite and SiO2) were treated as rigid. Density profiles, radial distribution functions, and diffusion coefficients were calculated to study the effect of surface properties on the characteristics of interfacial water and on the formation of the electric double layer. In brief our results show that both the surface roughness and chemical heterogeneity affect the density profile and the orientation of the interfacial water molecules, as well as their rate of exchange with bulk water and reorientation at the surface. We will discuss how our simulation results compare to experimental quasi-elastic neutron scattering data obtained at various hydration levels, if available.