Monte Carlo simulation study on the structure and reaction at metal-electrolyte interface. II. Mechanism of nonlinear electrode reactions

Using the two-dimensional free energy surfaces which were obtained previous ly by the Monte Carlo simulations for metal-electrolyte interfaces by allow ing the movement of reactant [N. Goto et al.: J. Phys, Sec. Jpn. 66 (1997) 1825], we have analyzed the mechanism of nonlinearity in the electrode reac tion in detail. The nonlinearity was defined as a square of ratio between w idths of the energy gap laws for the neutralization and ionization reaction s. We developed a method to derive the nonlinearity due to only fluctuation s of motion of solvent molecules and electrolyte ions for each distance of the reactant from the metal surface. We found that the nonlinearity was the largest (4.8) at the distance 6 Angstrom of the reactant from the metal su rface, and was the smallest (2.2) at the distance 2 Angstrom where the reac tant is in contact with the metal surface and at the distance larger than 9 Angstrom where Gouy-Chapman diffuse layer ends. We also found that the mov ement of the ions and solvents directly adsorbed to the metal-surface is st rongly restricted, showing a feature of dielectric saturation. Combining th ese facts, we conclude that the strongly adsorbed layer of ions and solvent molecules (Helmholtz double layer) causes a large nonlinear phenomenon of the electrode reaction due to breakdown of the central limit theorem especi ally for the reactant contacting the adsorbed layer.