Micelle formation in mixtures of nonionic and anionic detergents

This paper describes the effect of a homologous series of polyoxyethylene n-dodecanols on the critical micelle concentration (CMC) of sodium n-dodecyl alcohol sulfate as a function of composition of the mixtures and temperature. The CMC of the nonionic component of the mixed micelles is about one-hundredth of that of the anionic. 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 0–90 mole % anionic detergent. This is followed by an abrupt transition to the high CMC values of the anionic component. The gradual increase of the CMC values in the range below 90 mole % anionic detergent of n-dodecanol+ 4 EO exceeds that of the higher homologs containing 7, 23 and 30 ethylene oxide units. It is postulated that the degree of ionic repulsion of the ionic component in mixed micelles is markedly decreased as the proportion of nonionic component reaches a threshold range of 10 mole %. This effect is more pronounced with large ethylene oxide coils operating at the periphery of the micelle core than with short ethylene oxide coils. Thermodynamic data have been included.