Structuring of the solvent at metal/solution interfaces and components of the electrode potential

Abstract

The electrode potential can be split into four components: the electron work function, the electronic term, σ X M, the dipolar term, g S(dip), and the molecular term, g S M(ion). While the first is a property of the solid phase, the other three are properties of the interface. The three components are discussed in the light of “traditional” approaches and of new developments. In particular, it is shown that σ X M is probably overestimated in calculations, but it cannot be neglected anyway. There is evidence that σ X M and g S(dip) may run parallel being related in a simple way. This means that all discussions neglecting σ X M should retain their qualitative validity. The dipolar term g S(dip) is a measure of the metal-solvent interaction. Thermodesorption studies in the gas phase suggest weak surface bonds. It is shown that this is not in contrast with electrochemical concepts and results provided the nature of the surface bond is taken into account. In particular, it is shown that large electrical effects do not necessarily imply large energetic modifications. The cases of sp- and d-metals are discussed in some details. Finally, the possible effect of the surface atomic configuration on g S M(ion) as recently suggested in the literature is examined by extending this concept to a series of metals. It is shown that there is not enough data either to prove or disprove this idea, but this effect may also be a manifestation of surface roughness at an atomic level which cannot be detected experimentally, and may be somewhat related to the hardness of the metals, as measured for instance by the melting point.