H 2 S and HS - Adsorption on a Charged Cu Surface in an Electrolyte: Effect of Ionic Strength
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reorient a neutral molecule in an electrolyte can be studied. Because ionic strength effects are intrinsic to the model, the binding of charged species in the presence of salt of various concentrations is obtained. METHOD OF CALCULATION The total energy of interaction between a molecule and a charged surface is here comprised of a) a classical electrostatic interaction between the single and double layer surface charges on the molecule (the MSDL) and a charged Gouy-Chapman plane; b) an interaction due to overlap of the electronic distributions of the atoms in the molecule and those in the solid which comes from solving the SchrOdinger equation and c) the energy of interaction of the surface charge induced by the Gouy-Chapman field [8-10]. This latter induced surface charge is obtained from the solution to a set of boundary element equations. It is this polarization which creates the reaction electric field of a cavity in a dielectric medium in an applied field; for a spherical cavity, the problem can be solved in closed form [11]. This aspect of our calculations has been validated by comparison with
this known solution. The method of solution of the coupled Schrodinger-Poisson-Boltzmann equation has been described previously [7]. The method couples a Green's function-based solution of the PoissonBoltzmann equation for the induced surface charge layers on a molecule iteratively to the solution of the Schr6dinger equation for the energy and also for the electric field at the boundary elements. It has been successful in obtaining the solvation energy of a C0 32- molecule as a function of ionic strength of a solution including electronic relaxation effects. The iterative method has also been applied to a study of the effect of charged lattice defects on molecular adsorption in an electrolyte including covalent (chemical reaction) effects [12]. Here the method is applied to the calculation of single and double layers of charge induced on discretized molecular surfaces of H2S and HS- molecules. The charge layers are determined as a function of the Debye length (ionic strength) of the electrolyte. The Schr6dinger equation is solved using 6-3 lG** wavefunctions at the MP2 level of theory. The electric fields so determined are used as source terms in a Poisson-Boltzmann solver for the surrounding single and double layer charges. These charges are then employed as background charges in the next iteration solution of the Schr6dinger equation. The procedure converges (MP2 energy changes by
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