Calculation of ionic surface excess concentrations in the diffuse double layer at low field strengths for a restricted primitive model electrolyte

Analytical expressions have been derived for the ionic surface excesses of a restricted primitive-model z–z electrolyte near a charged wall, by combination of a thermodynamic result with a recently derived equation for the surface potential as a function of salt concentration and surface charge density within the context of the Fawcett–Henderson method. This approximate result accounts for the finite size of ions and applies in the limit of low field strengths at the surface, corresponding to absolute surface charge densities not exceeding ca. 10 μC cm−2 and concentrations of at least 0.1 M. The theoretical expression has been tested via comparison with predictions based on Gouy–Chapman theory and data obtained with Monte Carlo simulations, and has been found to show a superior performance at high concentrations and large ion sizes in the case of 1–1 electrolytes and, to a lesser extent, 2–2 electrolytes.