EFFECT OF THE MOLAR RATIO OF SURFACTANTS IN THE SOLUTION ON THE COMPOSITION AND MUTUAL INTERACTIONS IN ANIONIC CATIONIC ADSORPTION FILMS

The composition and mutual interactions of the components in anionic-cationic films adsorbed from solutions of composition varying over a wide range have been calculated by using a regular solution model of the mixed monolayer on the basis of surface tension measurements. Mixtures of surfactants of similar surface activity (sodium decylsulfonate and decylpyridinium chloride) as well as those differing markedly in their surface activity (dodecylpyridinium chloride and sodium octylsulfonate), containing a large excess of inorganic electrolyte (0.1 M NaCl), have been investigated. Both types of mixtures show negative deviations from ideal behavior. The attractive interaction between adsorbed components is strongest in the case of adsorption from solutions containing equimolar amounts of both surfactants. This interaction is stronger in the case of mixtures of surfactants containing hydrocarbon chains of the same length. Over a wide composition range these mixtures form mixed films of almost equimolar composition. However, in the case of mixtures of surfactants differing in their surface activity, mixed adsorption films are formed which consist predominantly of the component with the longer hydrocarbon chain even if its content in the solution is very small. The composition of mixed monolayers and the mutual interactions between their components are dependent on the surface tension in a significant way.