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 e longated threadlike micelles of cetyltrimethylammonium bromide (CTAB) in aqueous sodium salicylate (NaSal) solutions at 25-degrees-C. The su rfactant 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 un ity. The time correlation function A(q)(t) of light intensity scattere d from the solutions exhibits transition from the unimodal to the bimo dal 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 f or 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 regi me by the scaling law. The decay rate GAMMA(s) characteristic of the s low mode is independent of q, and GAMMA(s)-1 roughly agrees with the m echanical relaxation time tau estimated from a fit of the dynamic visc oelastic data of the same samples by a Maxwell type of model with the single relaxation time tau.