Aggregation of the aqueous dodecyl-trimethylammonium bromide-didodecyldimethylammonium bromide system at low concentration

The dodecyltrimethylammonium bromide (DTAB)-didodecyldimethylammonium bromide (DDAB)-water system was studied to elucidate whether the mixed surfactant aggregates are mainly micelles or vesicles, and whether the surfactant-surfactant interaction is ideal or not. The regular solution theory of mixed micelles and the Israelachvili-Mitchell-Ninham (IMN) packing parameter of the aggregated mixture were used. The interaction between dissimilar surfactant species was found to be non-ideal, with a positive value of the regular solution theory interaction parameter. The steric hindrance interaction parameter of Huang and Somasundaran theory indicated that the introduction of DTAB into DDAB layers was favoured at very low DTAB content, but became highly unfavourable when the proportion of DTAB increased. The aggregates were systematically much richer in DDAB than the solution of interaggregates. The IMN values systematically were compatible with flat surfaces, i.e., lamellar liquid crystals and vesicles. The results may be explained by a different composition of the inner and outer monolayers in the bilayer structure of the vesicles