Mixed Micellization and the Dissociated Margules Model for Cationic/Anionic Surfactant Systems

The first and the second critical micelle concentration (CMC1 and CMC2) for three alkyltrimethylammonium bromide (C(n)TAB)/sodium dodecylsulfonate (AS)/H2O mixed systems, and CMC1 for trimethylene-1,3-bis(dodecyldimethylammonium bromide) (12-3-12)/AS/H2O mixed system have been measured. The largest negative beta(m) value means the strongest synergism between 12-3-12 and AS. The CMC1 and CMC2 for the C(n)TAB/AS/H2O mixed systems decrease with the increase of n. The equimolar mixed systems give the smallest CMC1 values, whereas the CMC2 values decrease with the increase of the composition of the surfactant with higher surface activity in the C(n)TAB/AS/H2O mixed systems. For the C(16)TAB/AS mixed systems far from equimolar, specific counterion effect on lowering CMC1 enhances according to the Hofmeister series. There is slightly or no salt effect on the CMC1 of the other wide composition range of C(16)TAB/AS/H2O mixed system. The pseudophase separation model coupled with the dissociated Margules model has been proposed and gives satisfactory description of the mixed CMC1, the calculated micellar compositions are in well accordance with composition information from the zeta potential measurements. The addition of salt into the C(16)TAB/AS/H2O mixed system, leads to a certain degree of decrease in CMC2. In addition to counterion effect, the co-ion effect on CMC2 of the mixed system was explained using Collins' concept of matching water affinities.