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/polyoxyethylene glycol nonylphenyl ether (EO(a) = 10)/water systems exhibiting a direct hexagonal liquid-crystalline structure. The influence of composition and temperature on the dynamic functions was considered. By adding 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 increased and as temperature decreased. The Generalized Maxwell model fits the frequency dependence of the dynamic functions fairly well. The BSW-CW does not describe, in most cases, the relaxation spectrum of the hexagonal phase, except for binary systems at 20-25 degrees C. The steady-state limiting viscosity of the hexagonal phase shows an Arrhenius-like dependence on temperature. (C) 1997 Academic Press.