(591h) Time-Dependent Self-Diffusion of Surfactant Ions Due to Breakup and Reformation of Micelles in Micellar Systems

Pulsed field gradient (PFG) NMR technique was used to study diffusion of surfactant ions in the aqueous solutions of anionic surfactant SDS and the mixture of SDS with the cationic surfactant C12TAB. PFG NMR measurements at high magnetic field gradient strength provided separate sets of data on diffusion of SDS and C12TAB ions for a broad range of diffusion times. For small diffusion times at least two surfactant components with different effective diffusivities were observed for each type of surfactant ions. The fast-diffusing component is tentatively assigned to the surfactant ions which experience breakup or reformation of micelles during the diffusion time of the PFG NMR measurements. The slow-diffusing component is assigned to the surfactant ions that do not experience such events during the diffusion time. This assignment was found to be in agreement with the observed patterns of change of the fractions and diffusivities of these components with increasing diffusion time. The fractions of the slow-diffusing component of SDS in the pure SDS sample were observed to be smaller than those in the SDS/C12TAB sample. This observation can be explained by stabilization of micelles due to an addition of the cationic surfactant C12TAB to the aqueous solution of the anionic surfactant SDS.

Acknowledgements

NMR data were obtained at the Advanced Magnetic Resonance Imaging and Spectroscopy (AMRIS) facility in the McKnight Brain Institute of the University of Florida. The surfactant samples were prepared in the group of Professor Dinesh Shah, University of Florida by Dr. Monica A. James-Smith. The authors of this paper are very grateful to Professor Shah for multiple discussions and useful suggestions regarding this work.