We investigate the Stern layer of charged silicawater interfaces by calculating the ionsurface interaction from molecular dynamics simulations. The McMillanMayer potentials of mean force between a charged oxygen site and a lithium or cesium cation have been calculated. Contact ion pairs (CIPs) are important for the adsorption and desorption of ions, especially for lithium. An activation energy appears, which can result in a large estimated relaxation time. In the case of lithium, time scales needed to bind or unbind ions to and from the surface are found to be very long (up to the order of seconds for some surfaces), which implies that molecular dynamics cannot always be fully equilibrated. This work provides a new image of the Stern layer: it is not a continuous layer but a set of Bjerrum pairs. As a matter of fact, quantitative (macroscopic) treatments of such systems with localized surface charges require a three-dimensional model, contrary to the more commonly used one- or two-dimensional theoretical treatments.

VL - 120 UR - http://pubs.acs.org/doi/10.1021/acs.jpcc.5b08836 IS - 2 JO - J. Phys. Chem. C ER - TY - JOUR T1 - Water self-diffusion at the surface of silica glasses: effect of hydrophilic to hydrophobic transition JF - Molecular Physics Y1 - 2013 A1 - Bertrand Siboulet A1 - Molina, J. A1 - Benoit A. Coasne A1 - Turq, P. A1 - Jean-Francois Dufreche AB -We study water dynamics at a silica aqueous interface. Both hydrophilic (hydroxylated) surfaces and hydrophobic surfaces (dehydroxylated upon irradiation) have been generated from atomistic simulations. A new method for the calculation of the normal self-diffusion coefficients based on the calculation of mean first passage times is proposed. It uses the Smoluchowski theory of Brownian motion and it takes proper account of the layering of the molecules. In the case of parallel self-diffusion coefficients, a decrease is found compared to the bulk values. It can be described in terms of hydrodynamic boundary conditions induced by the surface confinement. This hydrodynamic explanation is not enough to interpret the case of normal self-diffusion coefficients for which an important diminution is found. Normal self-diffusion coefficients appear to depend strongly on the hydrophilicity of the surface. They tend towards their bulk value only at long distances from the surfaces. The first layer of water molecules is found to be partially adsorbed.

VL - 111 IS - 22-23 JO - Molecular Physics ER -