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The composition and mutual interaction of the components in anionic-cationic adsorption films have been determined by using a regular solution model for the mixed monolayers. Equimolar mixtures of surfactants that differ markedly in their surface activity (decylpyridinium chloride and sodium butylsulfonate) have been investigated. A marked asymmetry of the composition of mixed adsorption films, as well as mutual attraction of the adsorbed components (negative values of surface interaction parameter beta) have been found in these systems. The effect of the ionic strength and kind of the inorganic electrolyte on the surface interactions is similar, as in the case of mixtures of decylpyridinium chloride and sodium alkylsulfonates with hydrocarbon chains longer than the butyl ones that were previously investigated. Copyright 1998 Academic Press.

The composition and mutual interaction of the components in mixed adsorption films have been determined on the basis of the results of surface tension measurements obtained for solutions of single anionic and cationic surfactants with hydrocarbon chains of the same length (sodium decylsulfonate and decylpyridinium chloride) and their mixtures. An equation analogous to one derived on the basis of a regular solution model, however without the controversial assumption that the excess entropy of mixing is zero, has been used in calculations. Investigations have been made for mixtures of different compositions and for different concentrations of added inorganic electrolyte (NaCl).

The effect of sodium bromide concentration on the simultaneous adsorption of dodecylpyridinium bromide and sodium octylsulfonate on the free surface of their aqueous solutions has been investigated. The composition of the mixed films adsorbed from equimolar solutions has been evaluated on the basis of the Gibbs adsorption equation and by using the regular solution model, it has been stated that this composition depends on the surface tension value and the concentration of added inorganic electrolyte. The same factors influence the value of the surface interaction parameter β; negative values indicate a strong attraction between the adsorbed components. The concentration ratio of both surfactants in the solution significantly influences the composition of anionic-cationic adsorption films and the mutual interactions of their components. A distinct difference in the effects of NaBr and NaCl on the properties of the mixed monolayers has been also found.

The composition and mutual interaction of the components in anionic-cationic adsorption films have been determined by using to the mixed monolayers a regular solution model. The effect of the inorganic electrolyte concentration and kind on the properties of monolayers adsorbed from solutions containing decylpyridinium chloride and sodium decylsulfonate in concentration ratios varying over a wide range has been investigated. The calculations have been performed both neglecting and taking into consideration the effect of surface interactions on the partial molar areas of surfactants in the mixed monolayer. It has been found that although calculation results are very different, all conclusions concerning the effect of the solution composition are almost the same in both cases.

On the basis of surface tension measurements carried out at fixed ionic strength (0.1 M ) at different temperatures, the changes of standard thermodynamic functions ( ΔG o , ΔS o , and ΔH o ) have been calculated for the adsorption from solutions containing equimolar quantities of decylpyridinium chloride and sodium alkylsulfonates with different lengths of the hydrocarbon chain. Applying the Gibbs adsorption equation, the surface excess values have also been calculated for both types of surface-active ions which are adsorbed from solutions containing equimolar quantities of anionic and cationic detergents. It has been found that in the case of mixtures of detergents of different hydrocarbon chain lengths, the adsorption films are enriched in the more surfaceactive component, which is particularly marked at low surface coverage.

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.