LINEAR VISCOELASTICITY OF THE DIRECT HEXAGONAL LIQUID-CRYSTALLINE PHASE FOR A HEPTANE NONIONIC SURFACTANT/WATER SYSTEM/

A linear viscoelastic characterization by small amplitude oscillatory shear and creep compliance tests was carried out for heptane/polyoxyet hylene glycol nonylphenyl ether (EO(a) = 10)/water systems exhibiting a direct hexagonal liquid-crystalline structure. The influence of comp osition and temperature on the dynamic functions was considered. By ad ding heptane to the binary system, an increase in the critical Ninham- Israelachvili ratio defining the appearance of the hexagonal mesophase is likely to occur. Heptane solubilization at 25 degrees C gave rise to the appearance of a ''plateau'' region in the frequency dependence of the dynamic functions, which was wider as heptane concentration inc reased and as temperature decreased. The Generalized Maxwell model fit s the frequency dependence of the dynamic functions fairly well. The B SW-CW does not describe, in most cases, the relaxation spectrum of the hexagonal phase, except for binary systems at 20-25 degrees C. The st eady-state limiting viscosity of the hexagonal phase shows an Arrheniu s-like dependence on temperature. (C) 1997 Academic Press.