Adsorption of Molecules on a Charged Polarizable Surface in an Electrolyte

We present a method for the calculation of the binding and rotational energies of neutral (H2S) and charged (HS-) molecules impinging upon a charged and polarizable (Cu <100>) surface in the presence of an electrolyte. A molecular surface is constructed surrounding the H2S and HS- molecules forming boundary elements. A coupled Schrödinger-Poisson-Boltzmann iterative procedure treats the electronic structure of the molecules at the 6-31G**/MP2 level of theory and includes solvation effects through the single and double layers of charge induced by the electronic distribution. The molecule, together with its charged layers, forms a molecular single and double layer, an object which then interacts with a polarizable Gouy-Chapman plane within the electrolyte. The induced charge at the molecular surface resulting from this external electric field is obtained by solving a second set of boundary element equations. The induced polarization of the solid surface created by the impinging molecular ion is treated by a modified method of images. Repulsive interactions between the atoms of the molecule and those of the surface are obtained using a rigid-ion Hartree-Fock method. Binding energies of the molecule to the surface are determined as a function of the real surface charge imposed and also the ionic strength of the solution. It is found that surface charges can completely (180 degrees) reorient these molecules and that the counterions in the solution can completely screen binding effects of even large surface charges. Solid surface polarization can be significant in low dielectric constant solvents and is also reduced by counterions. Copyright 1998 Academic Press.