(439d) Surfactant Self-Diffusion in Linear and Branched Wormlike Micellar Fluids As Studied By Nuclear Magnetic Resonance Diffusometry

We report nuclear magnetic resonance (NMR) diffusometry experiments on two model wormlike micellar solutions of cetyltrimethylammonium bromide/sodium salicylate (CTAB/NaSal) and cetylpyridinium chloride/sodium salicylate (CPyCl/NaSal). By increasing the salt to surfactant concentration ratio, these systems display two peaks in the zero-shear viscosity and the relaxation time, which are indicative of microstructural transitions from linear to branched to highly branched micellar networks. The goal of this work is to assess the sensitivity of the NMR diffusometry to different types of micellar microstructures, and to identify the mechanism(s) of surfactant self-diffusion in micellar solutions. At low salt to surfactant concentration ratios, for which worm-like micelles are linear, the self-diffusion of surfactants is best described by a mean squared displacement, Z²(T_diff), that varies as Z²(T_diff) ∝ T_diff ^0.5, where T_diff is the diffusion time. As the salt concentration increases to establish branched micelles, Z²(T_diff) ∝ T_diff, indicating a Brownian-like self-diffusion of surfactant molecules in branched micelles. This result clearly indicates that the NMR diffusometry is sensitive to the type of micellar microstructure. In addition, the self-diffusion coefficient of the surfactant molecules in the linear and branched micelles is estimated and shown to be much slower than the diffusion of proton molecules in the bulk, indicating a highly restricted surfactant self-diffusion. Moreover, we find that in the linear and moderately branched wormlike micelles, the dominant mechanism of surfactant self-diffusion is through the curvilinear diffusion of the surfactant molecules along the contour length of the micelles, whereas in the highly dense network of micelles, before the second maxima, the surfactant self-diffusion could arise from a combination of micellar breakage, exchange between micelles and/or the bulk.