Modeling the Gouy-Chapman Diffuse Capacitance with Attractive Ion-Surface Interaction

The interfacial capacitance of a metal electrode in contact with a dilute electrolyte is generally expected to follow the behavior predicted by the Gouy-Chapman-Stern model. Recent experiments [Angew. Chem. Int. Ed. 2020, 59, 711], however, have shown that a deviation from the Gouy-Chapman behavior is observed even in dilute electrolytes on platinum and gold single-crystal electrodes. Such deviations are usually attributed to an interaction between the surface and the electrolyte ions. However, a quantitative model showing that the strong deviations from the Gouy-Champan behavior observed for Pt can be ascribed to such an effect is still lacking, particularly as other experimental observables do not indicate a strong ion adsorption. Here, we propose a double-layer model that is capable of reproducing the main experimental findings in a simple and (in parts) analytical way. The analytical model thereby includes the attractive ion-surface interaction via an additional capacitive element connected in parallel to the Gouy-Chapman capacitance. By comparing the model predictions to experiment, we subsequently infer characteristics of the ion-surface interaction. In particular, we find that the model predicts the attractive interaction to be weak (weaker than a typical chemical bond formed when contact adsorbing) and that the interaction has to be very similar for all ions. Furthermore, for a good agreement with experiment, ion-size effects are suggested to play a role in determining the potential of minimum capacitance.