Surface tension of solids. Structure-mechanical approach

Abstract

Problems of applying the classic and generalized Lippmann equations to adsorption studies on solid electrodes are shortly reviewed. Long-time thermodynamic discussion results in no clear-cut decision. A new nonthermodynamic approach is proposed considering the principal possibility and role of elastic and plastic deformations of the electrode surface during adsorption. The extremely thin electrode surface layers affected electrically and mechanically by adsorbate are supposed to be free of dislocations because of volume restriction. The nearest structure-mechanical analogs of such layers are the whisker crystals whose side surface could have one-and two-dimensional defects, but no active dislocations. Like whiskers, surface metal layers should possess a high ultimate strength close to the theoretical one and a purely elastic deformation. Special attention has been paid to the concepts of “reversible wholly plastic deformation (as in cleavage)” used for derivation of the generalized Lippmann equations for solids. This concept was shown to be an idealized one having no relation to the real plastic deformation of metals. It is a case of mixed notions since “cleavage” is not a kind of deformation. It is a kind of irreversible fracture mostly without the steps of plasticity. Affected only by adsorbate, the thin surface electrode layer should be considered as absolutely elastic body, whose plastic deformation is impossible, i.e. the Lippmann equation and other equations containing terms of plastic deformation cannot be used in thermodynamics of adsorption on the solid metals.