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Diffusion studies using nuclear magnetic resonance (NMR) spectroscopy wereconducted on two model surfactant solutions of cetyltrimethylammoniumbromide/sodium salicylate (CTAB/NaSal) and cetylpyridinium chloride/sodiumsalicylate (CPCl/NaSal). By increasing the salt-to-surfactant concentrationratio, these systems display two peaks in the zero-shear viscosity andrelaxation time, which are indicative of transitions from linear to branchedmicellar networks. The goal of this work is to assess the sensitivity of NMRdiffusometry to different types of micellar microstructures and identify themechanism(s) of surfactant self-diffusion in micellar solutions. At lowsalt-to-surfactant concentration ratios, for which wormlike micelles arelinear, the surfactant self-diffusion is best described by a mean squareddisplacement, Z2, that varies as Z2 ~ Tdiff 0.5, where Tdiff is the diffusiontime. As the salt concentration increases to establish branched micelles, Z2 ~Tdiff, indicating a Brownian-like self-diffusion of surfactant molecules inbranched micelles. This result indicates that NMR diffusometry is capable ofdifferentiating various types of micellar microstructure. In addition, theself-diffusion coefficient of the surfactant molecules in linear and branchedmicelles are determined, for the first time, by comparing to the existingrestricted diffusion models, and shown to be much slower than the diffusion ofproton molecules in the bulk. Moreover, in linear and moderately branchedwormlike micelles, the dominant mechanism of surfactant self-diffusion isthrough the curvilinear diffusion of the surfactant molecules along the contourlength of the micelles, whereas in the branched micelles, before the secondviscosity maxima, the surfactant self-diffusion could arise from a combinationof micellar breakage, exchange between micelles and/or the bulk.

Nuclear Magnetic Resonance Diffusometry of Linear and Branched Wormlike Micelles