Thermodynamic characteristics of anionic—cationic adsorption films based on a butler-type equation

It was stated that the simplified form of an integral adsorption isotherm based on the Butler's equation and derived previously (D. Goralczyk and B. Waligora J. Colloid Interface Sci. 82, 1 (1981)), describes satisfactorily the surface behavior of the solutions containing equimolar amounts of anionic and cationic surfactants with various surface ativity only within the range of great surface pressures. On the other hand, in the case of a small degree of surface coverage, deviation appear due to the presence of excess of ions with longer alkyl chain in the mixed adsorption film. The values of ΔG of adsorption for anionic—cationic surfactants are within the limits of experimental error equal to the sum of the values characteristic for single ionic substances being components of the mixture. The presence of adsorption film influences on the water structure in the surface layer which causes nonzero values of ΔGH2O.