Characterization of Monoolein Liquid Crystals Using Oscillatory Rheology and Strain Rate Frequency Superposition

Glycerol monooleate (GMO) is a commonly used surfactant in the food and pharmaceutical industries. The admixture of GMO and water can form liquid crystals (LCs) with a controllable microstructure upon changes in temperature or GMO:water ratio. Our research hypotheses are that (a) the LC phase directionality dominates the yielding and large deformation behaviour of LCs and that (b) strain-rate frequency superposition (SRFS) may be used to directly measure both the long and short relaxation time of GMO-water LC structures. The microstructure and rheology of GMO-based LCs were characterized as a function of temperature and composition. The structure was assessed using X-ray scattering and polarized light microscopy. Small and large deformation rheology was determined using frequency and amplitude sweeps as well as large amplitude oscillatory shear and SRFS. A binary GMO-water phase diagram was used to identify the presence of lamellar and two cubic phases at temperatures below 65 °C, irrespective of the water:GMO ratio, with a hexagonal phase dominating at higher temperatures. Rheology revealed unique yielding and relaxation times for each of these phases. Via amplitude sweeps, the cubic phases demonstrated a distinct G′′ overshoot that was absent in the lamellar and hexagonal phases. Parameters extracted from the Lissajous plots, namely the stiffening and thickening ratios, provided further insights into their structural deformation. Finally, SRFS showed that these LCs measurably differed in structural relaxation times. We confirmed our hypothesis that each LC phase has unique rheological behaviour upon large deformation and, by linking rheology with X-ray scattering data, showed that their symmetry defined their rheology.