Condensation d'ion sur une surface chargee analyse par dynamique moleculaire de la couche de Stern pour une interface eau-silice

National audienceNous etudions la couche de Stern a l'interface eau-silice chargee en calculant l'interaction ion-surface a partir de simulations de dynamique moleculaire biaisee de type umbrella-sampling [1] permettant ainsi de determiner les profils d'interaction des ions avec les surfaces d'ou decoulent les constantes d'association. Nous avons fait varier l'hydrophilicite d'une surface de silice en faisant varier le nombre de silanols en surface (Fig.1). Chaque surface comporte une charge unique portee par un oxygene deprotone qOc = -1 e

Condensation at a charged surface: a molecular dynamics investigation of the Stern layer for water-silica interfaces

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Scrutinizing Electro-Osmosis and Surface Conductivity with Molecular Dynamics

International audienceWe present a simulation and modeling study of electro-osmotic flow of an aqueous cesium chloride solution confined in a charged amorphous silica slot. Contrasting traditional models of the electric double layer, molecular dynamics simulations indicate that there is no stagnant layer, no Stern layer conduction, and no outer Helmholtz layer. The description of the interface requires two considerations. First, a distinction has to be made between free and surface-bonded ions. The latter do not form a physical layer but rather a set of ion–surface contact pairs. Second, the mobility of the free ions is reduced relative to their bulk value. This hydrodynamic effect needs to be included. These two concepts, coupled to simple macroscopic equations, are sufficient to describe surface conductivity and electro-osmotic flow in the frame of classical mean-field treatment. We show that surface conduction is negative at high concentration, and the Bikerman formula is only valid at low concentration

How Ion Condensation Occurs at a Charged Surface: A Molecular Dynamics Investigation of the Stern Layer for Water–Silica Interfaces

International audienceWe investigate the Stern layer of charged silica–water interfaces by calculating the ion–surface interaction from molecular dynamics simulations. The McMillan–Mayer 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