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Thermodynamic properties

The thermodynamic properties associated with phase boundaries may be defined in terms either of excess quantities relative to a suitably chosen reference system, or of an interfacial layer of thickness $ {\tau}$ and volume $ V^s={\tau}A_s$, in a manner analogous to that used for the definition of excess amounts and total amounts of substance adsorbed.

When expressed in forms which are invariant to the choice of dividing surface, on the one hand, or to the thickness of the interfacial layer on the other, the two methods lead to identical results. In the development of statistical mechanical theories of adsorption, however, many authors have preferred to express the problem in terms of an interfacial layer. For completeness, therefore, the appropriate definitions are given in relation to both formulations.