Nonideal mixing of dodecyltrimethyl-ammonium halides and nonionic surfactant in adsorbed films and micelles

The ion-dipole interaction between dodecyltrimethylammonium cations and nonionic surfactant molecules in adsorbed films and micelles was investigated by concentrating on the difference in the degree of counterion binding by employing dodecyltrimethylammonium chloride (DTAC)-octyl methyl sulfoxide (OMS) and dodecyltrimethylammonium bromide (DTAB)-OMS mixtures. The phase diagrams of adsorption and micelle formation were constructed and then the non-ideal mixing of different species of surfactants was demonstrated in terms of the excess Gibbs free energies of adsorption and micelle formation, and the surface excess areas. Furthermore the dependence of them on the counterion was clearly shown. All these results were found to support our previous view that the direct interaction between surfactant cation and the dipole of the hydrophilic part of a nonionic surfactant is essential in cationic-nonionic surfactant mixtures, i.e., the DTAC system with a lower counterion biding has more negative excess thermodynamic quantities than the DTAB system with a higher one.