Desorption transition at charged interfaces: Theoretical approach and experimental evidence

A field theory is used to describe an ionic solution in contact with a charged and adsorbing wall. The Hamiltonian, a functional of the ionic density fields, contains the entropy, the electrostatic energy, a nonlocal Van der Waals type contribution preventing sharp density variations, and an adsorption potential. The mean-field equations are solved numerically. However, they can be recasted so as to put in evidence a one parameter Lie group structure, which is a generalization of the charge-translation symmetry present in the Gouy-Chapman theory. There is a region in the charge-adsorption parameter space where this symmetry is broken, which corresponds to a desorption transition for the ionic species. The properties of this transition are investigated. Finally, this desorption phenomenon provides a simple explanation for a general feature in the properties of metal-electrolyte interfaces: the branching pattern observed in the experimental capacitance curves for a series of electrolytes. The part of the capacitance curves which is independent of the nature of the ions is related to the absence of interaction of the ionic species with the wall once the desorption takes place.