Dynamic light scattering of CTAB:NaSal threadlike micelles in the semidilute regime. II. effect of surfactant concentration

Abstract

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 concentrationCD of the samples is varied from 0.006 to 0.3 M and the ratio of the salt concentrationCs toCD is fixed at unity. The time correlation functionAq (t) of light intensity scattered from the solutions exhibits transition from the unimodal to the bimodal distribution of the decay rate Γ at aroundCD=0.05 M. The dependence of the first cumulant Γe on the scattering vectorq for the samples withCD≦0.03 M is described by the dynamic scaling law. The cooperative diffusion coefficientDc is obtained from extrapolation of Γe/q2 for the samples withCD≦0.03 M and of Γf/q2 forCD≧0.05 M where Γf is the first cumulant from the fast mode. TheDc is found in proportion toCD0.45, being in agreement with the theoretical prediction for a rigid rod in the semidilute regime by the scaling law. The decay rate Γs characteristic of the slow mode is independent ofq, and Γs−1 roughly agrees with the mechanical relaxation time τ estimated from a fit of the dynamic viscoelastic data of the same samples by a Maxwell type of model with the single relaxation time τ.