Amphifunctionally electrified interfaces: Coupling of electronic and ionicsurface-charging processes

Solid/liquid interfaces can acquire a charge by two principally different p rocesses, i.e., by charging the interface via an external electric source o r by specific adsorption/desorption of ionic species from/to the liquid. He re, we analyze the properties of electric double layers (DL) that are gover ned by the simultaneous functioning and coupling of these two charging mech anisms. For such interfaces, which we will denote as amphifunctional, the q uantitative dependencies of the DL parameters on the electric variables and bulk composition are computed. The interface between a partially oxidized metal and an electrolyte solution is an example of an amphifunctional inter face. Particular situations occur at the point of zero charge (pzc) and at the isoelectric point, which refer to specific values of the electrode pote ntial and the pH of the solution. Limiting cases of the amphifunctional mod el correspond to the familiar DL behavior of either fully polarized metal/e lectrolyte interfaces or fully relaxed oxide/electrolyte interfaces. Our an alysis can successfully be applied in the interpretation of recent atomic f orce microscopy force measurements on the gold/solution interface. It also allows a new approach in the understanding of the dependence on pH of the p otential of zero charge of metals and the dependence on the potential of pz c (pH value) of oxide surfaces.