Linear and nonlinear rheology of an aqueous concentrated system of cethyltrimethylammonium chloride and sodium salicylate

This work reports a rheological study on concentrated solutions of cethyltrimethylammonium chloride (CTAC) and sodium salicylate (NaSal) at a fixed molar concentration ratio [NaSal]/[CTAC]= 0.6. While all the solutions behave as a Maxwell fluid, the measured zero-sheat viscosity reveals a maximum with the surfactant concentration. The increase of the viscosity shows a power law dependence with an exponent 1.1 followed by a negative scaling law with an exponent -2.1 after the maximum. The nonlinear rheological properties identified by the reduced stress sigma(p)/G(0) and the Weissenberg number gamma(p) tau(R) show two different behaviors: these quantities are nearly constant for low concentrations, while they show a scaling law dependence with the surfactant concentration for the higher ones, with respectively the exponents -1.4 and -0.9. These results which do not reflect the dynamic behavior of entangled wormlike micelles are interpreted as a result of the formation of a multiconnected network structure.