The electrical double layer. III. Modified Gouy−Chapman theory with unequal ion sizes

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Abstract
This article examines the consequence of assigning to the cations and anions different distances of closest approach to the charged surface, within the simple Poisson–Boltzmann theory of the diffuse part of the electrical double layer. This introduces asymmetry into electrocapillary and differential capacitance curves. It also leads to a diffuse double layer with zero charge on the surface, and thus to a concentration-dependent potential of zero charge (pzc). These phenomena are usually discussed in terms of solvent-structure next to the surface and of adsorption isotherms for anions on the surface, but it seems that a contribution to such effects must come from the diffuse layer itself.