Bulk water, potential energy surfaces

Here, and p are the electric potentials in the metal, in the bulk solution and at the inner Helmholtz plane, x = / rw max is the surface concentration of the adsorbed water molecules in the absence of chemisorption and f(Fwmax - v/J) is their activity in the presence of chemisorption. In Eq. (24) the electrostatic contributions of the adsorbed molecules H2O and S to their electrochemical potentials also include the electrostatic potential energy of their dipoles in the interfacial electric field, -dftdx and are average values of the normal components of these dipoles, regarded as positive when their positive end is directed towards the solution. By substituting Eq. (24) into Eq. (23), rearranging terms, and differentiating jus with respect to [Pg.315]

Do, a, and y are parameters that characterize the adsorption energy, the energetic corrugation, and the anisotropy, respectively, zj and zj describe the equilibrium distance of the molecule from the surface, Pq and Pu the curvature of the potential, x, y and 2 are the atom coordinates and Lx and Ly are the box dimensions, m and n denote the number of elementary cells along the x and y directions of the periodic box thus, Lx/m and Ly/n are the lattice constants of the surface elementary cell. For the simulation of bulk water in contact with a crystal, this assumption does not lead to serious defects, considering the very approximate nature of the currently used... 

It has been recognized that the structure of water near the interface determines the adsorption behavior of ions on the metal surface in specific ways [24, 30, 31]. Therefore, realistic models of the metal phase are needed in order to describe the inhomogeneity and orientational anisotropy in the aqueous phase adequately. Contrary to the situation for bulk liquid where reliable interaction potentials, from empirical parametrizations or from ab initio calculations, are available, the quantum chemical description of interactions between molecular adsorbates and metal substrates poses substantial problems due to the complexity of the system. Systematic studies contribute to the understanding of the key factors that determine the structure and dynamics at the electrochemical interface. In the present work the influence of water adsorption energy (for many transition metal surfaces a known experimental quantity [32]), surface corru-... 

Near the surface, AG is dominated by the Coulomb energy profile and, therefore, it is approximately equal to the difference of the electrostatic potentials at the proton positions before and after the transfer. This difference depends strongly on the distance of the proton from the surface. Values of AG were found in the range of 0.5 eV. This value decreases, however, to the activation energy of proton transport in bulk water when the proton-surface separation exceeds 3 A (the thickness of one monolayer of water). Moreover, the electrostatic activation energy is a function of the separation between surface charges, which lies in the range of 7 to 15 A. 

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