The impact of nonelectrostatic physisorption of ions on free energies and forces between redox electrodes: ion-specific repulsive peaks

The DLVO-style theory of surface energies and surface forces is extended to describe the charge transfer energy due to redox processes at an electrode, including nonelectrostatic ion interactions. Formulae for the corresponding disjoining pressure are given. Illustrative calculations are made for a coupled 0.01M Fe||Cu concentration cell. Nonelectrostatic interactions are found to reduce electrode potentials but increase electrode charges. Forces between graphite electrodes are calculated in two geometries: between two cathodes in the Fe cell and between two anodes in the Cu cell. Nonelectrostatic redox ion interactions, in particular strong nonelectrostatic physisorption of the Cu+ ion combined with a peak in the electrode charge, introduce a repulsive peak at a Debye-length separation (2 nm), that transforms the cathode–cathode interaction from attractive to repulsive and magnifies an existing repulsive peak in the anode–anode interaction. The strength of the peak varies with a Hofmeister series in the “inert” counterion.