Adsorption at the air/water interface in dodecylammonium chloride/sodium dodecyl sulfate mixtures

The composition and properties of the adsorption films of dodecylammonium chloride/sodium dodecyl sulfate at the air/water interface depend on interactions between the film molecules and equilibria in the bulk phase (monomer-micelle and/or monomerprecipitate equilibria).The negative value of surface molecular interaction parameter ^mon calculated using the regular solution theory indicates strong attractive interactions between adsorbed molecules. Electrostatic interactions between oppositely charged ionic head groups enhance the adsorption of surfactants and decrease the minimum molar area of surfactant molecules at the air/water interface. The addition of an oppositely charged surfactant enhances packing at the air/water interface and transition from a liquid expanded to a liquid condensed state. Surface potential measurements reveal positive values for the mixtures investigated, implying the cationic surfactant ions are closer to the surface than the anionic ones.