Rheological behaviour of a lamellar liquid crystalline surfactant-water system

The rheological behaviour of a lamellar liquid crystalline surfactant-water system was investigated using a Haake RS-100 viscometer. The repeat distance of lamellae as a function of surfactant concentration and temperature was measured by small-angle X-ray scattering. The rheological characteristics were measured at different temperatures with systems containing surfactant in different concentrations. The frequency-dependent storage and loss modulus were found to be characteristic to the lamellar phase in the linear viscoelastic region. The results are analysed on the basis of Jones-McLeish slip-plane theory. Trends of the fitted constants are discussed based on the general knowledge on the interactions in dispersions stabilized by non-ionic surfactants, and the structure of lamellar liquid crystalline samples. Time-dependent compliance was also measured. The instantaneous elasticity measured in the creep tests was compared with that predicted from oscillatory tests. Burger's model was used to describe the time-dependent compliance. Viscosity measurements in the non-linear region were also done. A modified Carreau equation is used to describe the viscosity versus shear rate curves. The changes in sample under shear is described briefly. (C) 1998 Elsevier Science B.V. All rights reserved.