Abstract
Since the fundamental work of Gibbs, many models have been proposed to describe the interfacial properties of adsorbed surfactant layers. The historical models of von Szyszkowski, Langmuir, and Frumkin are presented here together with their advantages and shortcomings. Only during the last few decades have additional adsorption models been developed by Fainerman and co-workers. These models allow describing particular details of surfactants adsorbed at liquid interfaces. They are derived from the fundamental thermodynamic principle of Braun–Le Châtelier and allow us to assume that surfactant molecules may adsorb in different adsorption states or form small two-dimensional aggregates. The most recently discussed multistate model assumes more than two coexisting adsorption states, which has been shown to be particularly suitable for surfactant molecules capable of undergoing conformational changes upon adsorption. The classical models of Langmuir and Frumkin, as well as the newer multistate models, are all of practical importance for the respective types of surfactants.