Linear Viscoelasticity of Wormlike Micellar Solutions Found in the Vicinity of a Vesicle−Micelle Transition

Linear viscoelastic behavior of micellar solutions of cetyltrimethylammonium hydroxynaphthalene carboxylate is studied at temperatures above the vesicle−micelle transition under salt-free conditions and also in the presence of salt (sodium bromide and potassium acetate). The salt-free solutions, beyond a crossover volume fraction φ*, obey the micellar growth model of Mackintosh et al. The low values of φ* indicate that the effective charge on the micelles is very low. The decrease of the terminal relaxation time upon increasing surfactant concentration for volume fractions larger than ∼6φ* is attributed to the formation of intermicellar connections, favored by the diminution of the electrostatic contributions. A similar effect is found by the addition of a salt that does not bind with the surfactant. The phase diagrams shown in this study suggest that the branched micelles constitute the intermediate structures between linear micelles and bilayers.