DYNAMIC LIGHT-SCATTERING OF CTAB-NASAL THREAD-LIKE MICELLES IN THE SEMIDILUTE REGIME .2. EFFECT OF SURFACTANT CONCENTRATION

Dynamic light scattering measurements are made on networks formed by elongated threadlike micelles of cetyltrimethylammonium bromide (CTAB) in aqueous sodium salicylate (NaSal) solutions at 25-degrees-C. The surfactant concentration C(D) of the samples is varied from 0.006 to 0.3 M and the ratio of the salt concentration C(s) to C(D) is fixed at unity. The time correlation function A(q)(t) of light intensity scattered from the solutions exhibits transition from the unimodal to the bimodal distribution of the decay rate GAMMA at around C(D) = 0.05 M. The dependence of the first cumulant GAMMA(e) on the scattering vector q for the samples with C(D) less-than-or-equal-to 0.03 M is described by the dynamic scaling law. The cooperative diffusion coefficient D(c) is obtained from extrapolation of GAMMA(e)/q2 for the samples with C(D) less-than-or-equal-to 0.03 M and of GAMMA(f)/q2 for C(D) greater-than-or-equal-to 0.05 M where GAMMA(f) is the first cumulant from the fast mode. The D(c) is found in proportion to C(D)0.45, being in agreement with the theoretical prediction for a rigid rod in the semidilute regime by the scaling law. The decay rate GAMMA(s) characteristic of the slow mode is independent of q, and GAMMA(s)-1 roughly agrees with the mechanical relaxation time tau estimated from a fit of the dynamic viscoelastic data of the same samples by a Maxwell type of model with the single relaxation time tau.