On the Nature of the Heterogeneity of Real Solid Surfaces

This chapter discusses the nature of the heterogeneous surface mechanism of real solid compounds. The heterogeneous nature of real solid surfaces is a consequence either of their mode of formation or of their history. If a solid is formed by the sintering of a powder, the structure will be determined by the temperature of heat treatment. Alternatively, heat treatment may lead to the partial or total recrystallization of the solid as in the case of the graphitization of carbon blacks. The nature of the heterogeneity is determined by the temperature at which the surface is formed. Since the temperature at which adsorption measurements are made is different, the surface under adsorption conditions is a nonequilibrium one and it may relax slowly towards a more equilibrated structure. The effects of surface topography arise from the interactions between adsorbed molecules so in the case of ideal adsorbed phases, the topography has no effect on the thermodynamic properties of the system. The behavior of the isosteric enthalpy of adsorption at low temperatures can be a source of information concerning the nature of the heterogeneity of surfaces.