Effect of organic salts on micellar growth and structure studied by rheology

We report a rheological study on the effect of adding organic salts [sodium tosylate (NaTos) and benzoic acid potassium salt (BaPs)] on the micellar g rowth and structure of aqueous solutions of cethyltrimethylammonium chlorid e (CTAC) at a constant molar concentration ratio [salt]/[CTAC]. The rheolog ical data show two well-defined domains of growth characterized by scaling laws for the surfactant concentration. The addition of NaTos leads to an un usual maximum in the viscosity-surfactant concentration curve. Before the m aximum (domain 1), the analysis of the data (eta(0), tau(R) and G(0)) sugge sts the presence of branched micelles (connections). After the maximum (dom ain 2), however, the exponents of the scaling laws do not reflect either th e relaxation of this branched structure or that of an entangled transient n etwork structure. A faster mechanism of relaxation, not yet elucidated gove rns their dynamics. The exponents of the power laws in the presence of the BaPs are found, however, to be in accordance with the theory of equilibrium polymers.