Mixed micellization and the dissociated Margules model for cationic/anionic surfactant systems.

The first and the second critical micelle concentration (CMC(1) and CMC(2)) for three alkyltrimethylammonium bromide (C(n)TAB)/sodium dodecylsulfonate (AS)/H(2)O mixed systems, and CMC(1) for trimethylene-1,3-bis(dodecyldimethylammonium bromide) (12-3-12)/AS/H(2)O mixed system have been measured. The largest negative β(m) value means the strongest synergism between 12-3-12 and AS. The CMC(1) and CMC(2) for the C(n)TAB/AS/H(2)O mixed systems decrease with the increase of n. The equimolar mixed systems give the smallest CMC(1) values, whereas the CMC(2) values decrease with the increase of the composition of the surfactant with higher surface activity in the C(n)TAB/AS/H(2)O mixed systems. For the C(16)TAB/AS mixed systems far from equimolar, specific counterion effect on lowering CMC(1) enhances according to the Hofmeister series. There is slightly or no salt effect on the CMC(1) of the other wide composition range of C(16)TAB/AS/H(2)O mixed system. The pseudophase separation model coupled with the dissociated Margules model has been proposed and gives satisfactory description of the mixed CMC(1), the calculated micellar compositions are in well accordance with composition information from the ζ potential measurements. The addition of salt into the C(16)TAB/AS/H(2)O mixed system, leads to a certain degree of decrease in CMC(2). In addition to counterion effect, the co-ion effect on CMC(2) of the mixed system was explained using Collins' concept of matching water affinities.