On the definition of interfacial excesses in a system consisting of an insoluble solid adsorbent and a binary liquid mixture

The paper deals with the problem whether the interfacial excess volumeV ^σ has to be set zero in the case of the systems in question as stated by D. H. Everett, or eventually different from zero as proposed by G. Schay. First it is shown that the current operational definitions of the interfacial excess amounts as relative or reduced excesses are quite independent of the choice of the reference volume (or amount) of equilibrium bulk liquid. Everett operates with the Gibbs dividing surface, letting it coincide with the surface of the solid, the corresponding operational definition of the interfacial excess amount being then that ofn ^σ(v). It is shown that this definition, while compatible with those of the relative or reduced excesses, is not cogent. Relative and reduced excesses are usually interpreted as the values resulting when the adsorption of either one selected component or the total adsorption is set arbitrarily zero. With this interpretation, the assumptionV ^σ =0 for the total system could be maintained only by admitting an adsorption excess of the solid different from zero which appears, however, unnatural.