A theoretical basis is developed for the classification of adsorption isotherms for solutes in dilute solution

Three of the four main classes (S, L, H) are accounted for by differences in relative magnitude of the activation energies of desorption of solutes and solvent. The S-shaped isotherm is also accounted for by an additional concentration-dependence of this parameter, implying cooperative adsorption. The subgroups of each class are also explained.