Diffusion coefficients of ionic surfactants with different molecular structures in aqueous solutions

Aqueous micellar solutions of dodecyl- and hexadecyltrimethylammonium bromides, hexade- cylpyridinium bromide and chloride, domiphen bromide ((dodecyldimethyl- N -2-phenoxyethyl)ammonium bromide), hexadecyltriphenylphosphonium bromide, and sodium dodecyl sulfate have been investigated using dynamic light scattering. A local increase in the diffusion coefficient has been discovered for all ionic surfactants at concentrations higher than the first critical micelle concentration (CMC 1 ). It has been shown that, at the same alkyl chain length and counterion nature, the increase in the diffusion coefficient with surfactant content becomes more pronounced with a reduction in CMC 1 . The concentration dependences of the diffusion coefficients have been explained in terms of a previously proposed theory formulated for ideal micellar systems. The existence of the local minimum in the diffusion coefficient near CMC 1 has been shown to be a general regularity for binary surface-active electrolytes.