Superficial work and surface stress at solid electrodes: a thermodynamic assessment

Abstract

A Gibbs adsorption equation under surface strain is derived, that holds whether or not chemical equilibrium is established. The thermodynamic derivation does not make use of the ad hoc introduction of the ‘generalized surface parameter’, nor of the fictitious separation of surface deformation into plastic and elastic contributions. Moreover, no use is made of Euler’s theorem in connection with the functional dependence of the surface excess of internal energy upon the surface area A and the mole numbers of all components adsorbed at the interphase. Conversely, in the case of solid electrodes, account is taken of the lack of proportionality between A and the number of moles of at least one interfacial component, i.e. the surface atoms of the metal substrate. At equilibrium, this approach leads to a ( γ − Y )d ε term in the Gibbs adsorption equation, where γ is the superficial work, Y is the surface stress and ε is the surface deformation. This term is shown to be negligible under all practical conditions. Hence, the Lippmann equation and the electrocapillary equation, strictly valid for liquid electrodes, can also be applied to solid electrodes, for measuring changes of γ and surface excesses of chemisorbed species. On the other hand, experimental measurements of changes of Y induced by chemisorption have no bearing on the electrocapillary equation; rather, they are related to partial charge transfer accompanying chemisorption.