Micellar interactions in nonionic/ionic mixed surfactant systems

To study the influence of the chemical nature of headgroups and the type of counterion on the process of micellization in mixed surfactant systems, th e cmc's of several binary mixtures of surfactants with the same length of h ydrocarbon tail but with different headgroups have been determined as a fun ction of the monomer composition using surface tension measurements. Based on these results, the interaction parameter between the surfactant species in mixed micelles has been determined using the pseudophase separation mode l. Experiments were carried out with (a) the nonionic/anionic C12E6/SDS ((h exa(ethyleneglycol) mono-n-dodecyl ether)/(sodium dodecyl sulfate)), (b) am photeric/anionic DDAO/SDS ((dodecyldimethylamine oxide)/(sodium dodecyl sul fate)), and (c) amphoteric/nonionic C12E6/DDAO mixed surfactant systems. In the case of the mixed surfactant systems containing DDAO, experiments were carried out at pH 2 and pH 8 where the surfactant was in the cationic and nonionic form, respectively. It was shown that the mixtures of the nonionic surfactants with different kinds of headgroups exhibit almost ideal behavi or, whereas for the nonionic/ionic surfactant mixtures, significant deviati ons from ideal behavior (attractive interactions) have been found, suggesti ng binding between the head groups. Molecular orbital calculations confirme d the existence of the strong specific interaction between (1) SDS and noni onic and cationic forms of DDAO and between (2) C12E6 and the cationic form of DDAO. In the case for the C12E6/SDS system, an alternative mechanism fo r the stabilization of mixed micelles was suggested, which involved the low ering in the free energy of the hydration layer. (C) 2000 Academic Press.