Examination of ion size effects on the potential drop across the diffuse layer using Monte Carlo simulations

Monte Carlo simulations of the diffuse double layer have been carried out for two unrestricted electrolytes in which the component ions have ionic radii equal to the Shannon and Prewitt radii for Na+, Cs+, and Cl−. In this way the effect of ionic size on the diffuse layer potential drop ϕd has been examined at negative charge densities for the simple 1:1 electrolytes NaCl and CsCl. It is shown that the departure of this potential drop from the Gouy–Chapman estimate increases with decrease in the radius of the cation. It is also shown that the Fawcett–Smagala model for ion size effects in the diffuse double layer gives an excellent description of the Monte Carlo data. The significance of the present results is discussed with respect to the effect of alkali cation nature on double layer properties at negative charge density, and on the reduction of the proton at mercury in the same potential region.