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 $\left[\mathrm{NaSal}\right]/\left[\mathrm{CTAC}\right]=\mathrm{0.6.}\mathrm{}$ While all the solutions behave as a Maxwell fluid, the measured zero-shear 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 ${\dot{\gamma }}_p{\tau }_R$ 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 $-\mathrm{0.9.}$ 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.