Abstract
The elementary treatment of interactions and first order phase changes in localized and mobile monolayers is reviewed. Calculations on multimolecular adsorption and capillary condensation, which take into account horizontal interactions in an approximate manner, are presented. Several other topics are discussed briefly, including the quasi‐chemical method and reversible hysteresis. It is suggested, on the basis of the capillary condensation calculations, that reversible hysteresis involves the use of metastable portions of the adsorption isotherm. The theory predicts, in agreement with experiment, that the adsorption curve should be below the desorption curve.

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