General Relationships of the Adsorption Behavior of Surfactants at the Water/Air Interface

The structure of a surfactant is in direct correlation with the properties of an adsorption layer formed at a liquid interface. By use of thermodynamic adsorption models, such as the Frumkin, reorientation and aggregation isotherms, characteristic parameters of adsorption layers at the surface of aqueous solutions are determined for the following 12 homologous series of surfactants: alcohols, diols, fatty acids, soaps, alkyldimethylphosphine oxides, oxyethylated alcohols, alkyl sulfates, alkylammonium salts, and others. The results obtained for all analyzed homologous series, essentially in terms of the Frumkin isotherm, support the general idea of the increment theory, which works for ionic as well as nonionic surfactants. The standard free energy per methylene group for all types of surfactants varies in a narrow range between −2.6 and −3.3 kJ/mol. The free energy increment for a non-oxyethylated polar group of −4.25 kJ/mol is independent of the type and nature of the surfactant.