The Langmuir isotherm for localized unimolecular adsorption is generalized (using statistical methods) in order to discuss adsorption on a heterogeneous surface, with and without interactions between adsorbed molecules. The configurational entropy, and the nature of the phase changes on a random heterogeneous surface are discussed. In particular, on this type of surface, the theory predicts, at least for one special case, that condensation will occur in two steps (instead of the customary one step) at low enough temperatures.
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