Temperature dependence of viscosity for aqueous NaCl solutions of nonionic rod-like micelles in dilute and semidilute regimes

Abstract

The viscosity for aqueous micellar solutions of heptaoxyethylene alkyl ethers (C n E 7, n = 12, 14, 16) has been measured at different micelle and NaCl concentrations and at several temperatures under different shear rates. Rod-like micelles entangle one another above the threshold micelle concentration, ( c − c 0) η∗, which is higher than the threshold value of overlap, ( c − c 0)∗, and decreases with a rise in temperature. The intrinsic viscosity at zero shear rate and at ( c − c 0) η∗ increases with the temperature, indicating the micellar growth. The relative viscosity at zero shear rate for aqueous NaCl solutions of entangled rod-like micelles in semidilute regime can be scaled against the micelle concentration, on the basis of the reptation model. The characteristic exponent, v η , increases as the temperature rises. This indicates the increase in rigidity or expansion of rod-like micelles. The complicated shear rate dependence on the relative viscosity is observed for micellar solutions above ( c − c 0) η∗, depending on the v η values of the rod-like micelles. The relation between the temperature dependence of viscosity and the liquid-liquid phase separation is discussed.