  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, Z2, that varies as Z2 ~ Tdiff 0.5, where Tdiff is the diffusion
time. As the salt concentration increases to establish branched micelles, Z2 ~
Tdiff, 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 microstructure. In addition, the
self-diffusion coefficient of the surfactant molecules in linear and branched
micelles are determined, for the first time, by comparing to the existing
restricted diffusion models, and 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.
