Influence of breakup and reformation of micelles on surfactant diffusion in pure and mixed micellar systems

Pulsed field gradient (PFG) NMR technique was used to study diffusion of surfactant ions in the following two micellar systems: (i) aqueous solution of an anionic surfactant sodium dodecyl sulfate (SDS), and (ii) aqueous solution of a mixture of SDS and a small amount of the cationic surfactant N-dodecyltrimethylammonium bromide (C12TAB). PFG NMR measurements provided separate sets of data on diffusion of SDS and C12TAB surfactant ions for a broad range of diffusion times. For each type of surfactants at least two components with different effective diffusivities were observed at sufficiently small diffusion times. The faster component was assigned to the surfactants that experience breakup or reformation of micelles during the diffusion time of the PFG NMR measurement, while the slower component was assigned to the surfactants that did not participate in such events during the diffusion time. The observed changes of the fractions and diffusivities of these components with increasing diffusion time were found to be in a qualitative agreement with such assignment. Fundamental understanding of surfactant diffusion in micellar system is important due to an increasing use of such systems for synthesis of porous materials where micelles are used as templates as well as for many other applications.