Micelle formation in mixtures of nonionic and cationic detergents

The effect of a homologous series of polyoxyethylene n‐dodecanols on the critical micelle concentration (cmc) of mixtures with n‐dodecyl trimethyl ammonium bromide (DTAB) and of polyoxyethylene n‐hexadecanols on the cmc of mixtures with n‐hexadecyl trimethyl ammonium bromide (CTAB) has been studied in terms of the composition of the mixtures. The cmc of the nonionic component of the mixed micelles is approximately one hundredth of that of the cationic component. Only a gradual increase in the cmc values of the mixed micelles above the values of the nonionic components was observed in the composition range of 0舑75 mole % or 0舑90 mole %, respectively, of cationic detergent. This is followed by an abrupt transition to the high cmc values of the cationic component. This abrupt transition is considerably reduced by addition of NaBr. It is postulated that in the absence of added electrolyte the degree of ionic repulsion of the cationic component in mixed micelles is markedly decreased as the proportion of nonionic component reaches a threshold range of 25 or 10 mole %, respectively. Swamping of charges reduces ionic repulsion in NaBr solutions and, consequently, the abrupt transition in the cmc vs. composition curves is reduced.