Effect of interactions between the adsorbed species on the properties of single and mixed-surfactant monolayers at the air/water interface

Experimental data on surface tension available from the literature and generated in the present study are analyzed to estimate the applicability of adsorption models, based on the Frumkin equation, to nonionic and ionic surfactants and their mixtures. Optimization programs based on the least-squares method in media of Delphi V and Pascal VII are used. The effect of interactions between the adsorbed species on surface tension is considered in all cases. The results are compared to those obtained with the simpler Szyszkowski equation, employed in numerous studies of nonionic surfactants, when interactions are neglected. Cases where the Frumkin model can be successfully employed with ionic surfactants and mixtures are presented and the conditions of its applicability are analyzed. Related characteristic quantities (maximum adsorption, standard free energy of surfactant adsorption, energy of interaction between adsorbed species, standard free energy of counterion adsorption, degree of coverage by surfactant/counterion associates) are established as a function of 1. The number of methylene groups in the surfactant molecule; 2. The position of the polar head on the hydrophobic tail; 3. The type of (alkali) counterion. The properties of an adsorption layer from a mixture of nonionic and ionic surface-active species are compared to those of the single surfactants. (C) 2002 Elsevier Science (USA)