Effect of temperature on the rheology of wormlike micelles in a mixed surfactant system

The formation and theological behavior of a viscoelastic wormlike micellar solution in an aqueous solution of a mixed surfactant system of alkyl ethoxylate sulfate (AES), C12H25(OCH2CH2)(3)OSO3-Na+, and polyoxyethylene dodecyl ether, C12EO3, and the unusual effect of temperature on the rheological behavior have been studied. Upon successive addition Of C12EO3 to the dilute micellar solution of AES, viscosity increases swiftly and reaches its peak where a viscoelastic solution with nearly Maxwellian behavior is formed. With the further addition Of C12EO3, viscosity decreases sharply, which is attributed to the formation of micellar joints. With increasing temperature, the extent of micellar growth increases and the viscosity maximum is achieved at a lower mixing fraction of C12EO3, but the maximum viscosity attained by the system decreases. The evolution of relaxation time and network density of the viscoelastic network also suggests that with increasing temperature, enhanced micellar growth takes place, but an additional, faster relaxation mechanism becomes increasingly favorable at high concentrations Of C12EO3. These results can be explained in terms of the increase in free energy of hemispherical end-caps (end-cap energy) of the micelles with increasing temperature. (C) 2007 Elsevier Inc. All rights reserved.