Nuclear Magnetic Resonance Diffusometry of Linear and Branched Wormlike Micelles

Diffusion studies using nuclear magnetic resonance (NMR) spectroscopy were conducted on two model surfactant solutions of cetyltrimethylammonium bromide/sodium salicylate (CTAB/NaSal) and cetylpyridinium chloride/sodium salicylate (CPCl/NaSal). By increasing the salt-to-surfactant concentration ratio, these systems display two peaks in the zero-shear viscosity and relaxation time, which are indicative of transitions from linear to branched micellar networks. The goal of this work is to assess the sensitivity of NMR diffusometry to different types of micellar microstructures and identify the mechanism(s) of surfactant self-diffusion in micellar solutions. At low salt-to-surfactant concentration ratios, for which wormlike micelles are linear, the surfactant self-diffusion is best described by a mean squared displacement, Z(2), that varies as Z(2) proportional to T-diff(0.5), where T-diff is the diffusion time. As the salt concentration increases to establish branched micelles, Z(2) proportional to T-diff, indicating a Brownian-like self-diffusion of surfactant molecules in branched micelles. This result indicates that NMR diffusometry is capable of differentiating various types of micellar microstructures. In addition, the self-diffusion coefficient of the surfactant molecules in linear and branched micelles are determined, for the first time, by comparing the existing restricted diffusion models and are shown to be much slower than the diffusion of proton molecules in the bulk. Moreover, in 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 branched micelles, before the second viscosity maxima, the surfactant self-diffusion could arise from a combination of micellar breakage, exchange between micelles and/or the bulk.

Automated summary

NMR diffusometry is capable of differentiating various types of micellar microstructures and identifying the mechanisms of surfactant self-diffusion in micellar solutions. The self-diffusion coefficient of surfactant molecules in linear and branched micelles are slower than proton diffusion in the bulk, indicating different diffusion mechanisms. Surfactant self-diffusion can occur through curvilinear diffusion in linear and moderately branched micelles, and through micellar breakage, exchange between micelles and/or the bulk in branched micelles.