Progress in the Understanding of the Structure of the Metal Electrode/Solution Interface. Evolution of the Concept of Hydrophilicity*

The metal-water interaction strength (»hydrophilicity«) plays a major role in deterrnining both the structure of the interfacial region and the energetics of adsorption phenomena. A »hydrophilicity« scale can be established on the basis of a phenomenological approach for a series of sp-metals. It is shown that recent results for sd-metals (Cu, Ag, Au) cannot be organized in the same picture. This has lead to a reconsideration of the factors governing the behaviour of an electrode interface. The role of the surface electron gas, thus far underestimated, should be revaluated. This is what »physical models« emphasize as opposite to »molecular models« for which the response of the interface is simply that of a monolayer of solvent molecules. It is stressed that these conceptual developments do not clash with conclusions of the phenomenological approach based on the correlation of a few experimental parameters with some physicochemical properties of metals.