Equilibrium Between Large Three- and Two-Dimensional Phases: Adsorption Phenomena

Abstract

In Sect. 2.8 we have already discussed the way of calculating the thermodynamic parameters and state functions of surface phases, which are the adsorption layers. The Gibbs definition is, however, too general to be used in many cases of practical interest. For example, it supposes that the excess of matter in the surface phase comes from both phases touching at the interface. This is not the case, e.g., of adsorption without chemical reaction of a gas on a foreign solid substrate. Furthermore, according to the Gibbs definition, the adsorption layer is not limited in thickness, although its concentration must tend to that of the bulk phase with increasing distance from the surface. In fact, as we shall see in the case of the gas-solid surface at not too high a temperature, adsorption is limited to a monomolecular layer until its completion. Subsequently this condensed layer acts as a substrate, i.e. a new monomolecular layer can be formed on top of it, and so on. It should be pointed out, however, that in other systems, e.g. aqueous solutions, where the concentration of the adsorbate in the bulk phase could be much higher than that in a gas, adsorption layers can be thicker, their diffuse parts extending far beyond the molecular dimension.