Four-layer complexation model for ion adsorption at energetically heterogeneous metal oxide/electrolyte interfaces

Adsorption equations are developed for the four-layer model assuming that the anions and cations of the inert electrolyte (1:1) are adsorbed in two distinct layers, located at different distances from the surface; furthermore, the energetic heterogeneity of the oxide surface is taken into account. The theoretical development has been based on the idea of the triple-layer model along with the 2 pK charging mechanism. Because of some compensating effects, the titration curves are almost insensitive to surface energetic heterogeneity. It is also shown that electrokinetic data and radiometrically measured individual adsorption isotherms of ions are very sensitive and can even make discrimination among various models possible. The developed expressions for the four-layer model taking into account the energetic heterogeneity of the surface predict the difference between PZC and IEP values, similar to the homogeneous four-layer model. This new theoretical approach leads to adsorption equations which are easy to use like those developed for the popular triple layer model, but in both cases a simple computer program for performing calculations is needed.