Electrostatic Adsorption in the Gouy Layer

Abstract

In the preceding chapter we have derived the system of equations (81), (82), which describes the electrostatic potential distribution in the EDL of dilute electrolyte solutions. Here we shall continue to analyse this system and show that it includes two dimensionless parameters: the dimensionless ion charge $\tilde{e}$ and the dimensionless electrode charge ${{\tilde{Q}}_{e}}$. As the electrolyte concentration decreases, $\tilde{e}$ tends to zero, while ${{\tilde{Q}}_{e}}$ increases. Therefore, in the case of dilute electrolytes a solution of system (81), (82) can be sought as a power series in $\tilde{e}$. In the zero-order approximation in $\tilde{e}$ this system is reduced to the Poisson-Boltzmann equation underlying the Gouy-Chapman theory for the continuous electrical double layer, and in the next approximation (linear in $\tilde{e}$) to the system of equations, which is a generalization of the Wagner-Onsager-Samaras theory of the discrete double layer to the case of non-zero electrode charge. Solving the latter system, we can find a correction to the Gouy-Chapman formula for the Gouy layer capacity. Comparison of the theory thus improved with experiment shows that ions with specific negative adsorption (NaF on X mercury) approach the electrode surface at a distance of 1–2 Å (which practically coincides with their radius) and that no dielectric interlayer with a lower dielectric permittivity exists at the metal-electrolyte interface.