Dynamic light scattering of CTAB:NaSal threadlike micelles in the semidilute regime. II. 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°C. The surfactant concentrationC _D of the samples is varied from 0.006 to 0.3 M and the ratio of the salt concentrationC _s toC _D is fixed at unity. The time correlation functionA _q (t) of light intensity scattered from the solutions exhibits transition from the unimodal to the bimodal distribution of the decay rate at aroundC _D=0.05 M. The dependence of the first cumulant _e on the scattering vectorq for the samples withC _D0.03 M is described by the dynamic scaling law. The cooperative diffusion coefficientD _c is obtained from extrapolation of _e/q² for the samples withC _D0.03 M and of _f/q² forC _D0.05 M where _f is the first cumulant from the fast mode. TheD _c is found in proportion toC _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 _s characteristic of the slow mode is independent ofq, and _s ^–1 roughly agrees with the mechanical relaxation time estimated from a fit of the dynamic viscoelastic data of the same samples by a Maxwell type of model with the single relaxation time .