A Comprehensive Presentation of the Thermodynamics of Adsorption Excess Quantities

Abstract

Abstract A common general framework of thermodynamic relations between interfacial excess quantities in multicomponent systems is derived, based essentially on the fundamental Gibbsian treatment of adsorption phenomena. Both adjoining bulk phases are regarded as equivalent constituents of the system concerned—not only in the case of fluid interfaces, which are approached in this way in any current exact treatment, but also in the case of S/G and S/L interfaces, respectively. The algebraic mode of definition of the surface excesses of the components and of the coupled thermodynamic quantities is adopted, but without reference to imaginary Gibbs dividing surfaces positioned within the actual system, in order to avoid the necessity of admitting the concept of an adsorption excess of the solid, when considering the most common mode of evaluation of adsorption from liquid solutions by immersion experiments. Applications of the general relations to L/G, S/G and S/L interfaces respectively are demonstrated. In connection with the latter two, no use is made of the controversial concept of surface tension of the solid. Instead an extra interfacial free energy term is introduced, as a mean specific quantity for adsorbents with a fixed colloidal texture, but whose surfaces are inhomogeneous, both geometrically as well as energetically. The aim of the proposed representation is not the derivation of fundamentally new thermodynamic relations, but to obtain a unified picture of adsorption at different kinds of interfaces, and to rely as far as possible only on operational definitions.