Influence of surfactant concentration and counterion to surfactant ratio on rheology of wormlike micelles

The influence of concentration on rheological properties, including shear v iscosity, sheer instability, transient stress start-up and relaxation, appa rent extensional viscosity, viscoelastic behavior, and microstructure by cr yo-TEM, were studied with surfactant Ethoquad O/12, commercialized oleyl me thyl bishydroxyethyl chloride, with counterion sodium salicylate. Counterio n to surfactant molar ratios, xi, were 1.0 and 2.5. Concentrations for the xi = 1 series are 5 mM/5 mM, 10 mM/10 mM, 50 mM/50 mM, 100 mM/100 mM, and 2 00 mM/200 mM (surfactant/counterion); those for the I = 2.5 series are 5 mM /12.5 mM, 10 mM/25 mM, 50 mM/125 mM, 100 mM/250 mM, and 200 mM/500 mM, The experimental results showed complicated rheological behavior with concentra tion changes. Shear viscosity decreases with increases in concentration for the xi = 1 series. At xi = 2.5 apparent viscosity increases with concentra tion above 10 mM. Viscoelasticity of the solutions also decreases with incr eases in surfactant concentration. At high concentration, a high shear rate is needed to induce viscoelasticity. A high extensional rate induces super micellar structures, Gelation was observed during shear for the 100 mM/250 mM and 200 mM/500 mM solution in the cone-and-plate geometry. Cryo-TEM resu lts revealed that all of the solutions examined had wormlike network micell e microstructures. (C) 2001 Academic Press.