The Effect of Intermicellar Interactions on Interpretations of Micellar Diffusivities by Dynamic Light Scattering

Using the method of dynamic light scattering, we have determined the diffusion coefficients of micelles in aqueous solution as a function of surfactant and salt concentrations for three related systems: CTAB (C16H33N(CH3)3Br) + NaBr, MyTAB(C14H29N(CH3)3Br) + NaBr and CTACl(C16H33N(CH3)3Cl) + NaCl. For each system there is a well-defined salt concentration below which the micellar diffusivity increases linearly with surfactant concentration. In this low-salt region, in which the net intermicellar interaction is repulsive, we have obtained good fits to the diffusivity data using linear interaction/DLVO theory. From this procedure we have determined the “minimum-sphere” micellar radius as a function of temperature as well as estimates of the micellar fractional ionization α and the Hamaker constant A for the above systems as well as for SDS(C12H25SO4Na) + NaCl. Hence, by this approach one is able to relate variations in these physical parameters to subtle changes in surfactant composition (e. g. alkyl chain length) and counterion properties. Interestingly, significant micellar growth apparently commences at the salt concentration at which the net intermicellar interaction shifts from repulsive to attractive.