Viscoelastic solution of long polyoxyethylene chain phytosterol/monoglyceride/water systems

We have studied the rheology and structure of a mixed long polyoxyethylene chain phytosterol (PhyEO30) and monoglyceride (monolaurin (ML) or monopalmitin (MP)) surfactants in an aqueous system. Both ML and MP are insoluble in 10 wt.% PhyEO30 solution at normal room temperature, but their solubility is found to increase with the rise of temperature and an isotropic solution is formed at higher temperature. A maximum viscosity as a function of temperature appears as ML or MP is solubilized in the micellar solution of PhyEO30. With increasing temperature, viscosity increases rapidly for higher monoglyceride content and forms viscoelastic solution. The oscillatory rheological behavior of the viscoelastic solution can be described by Maxwell model at low-frequency region, which is the typical pattern of entangled wormlike micelles. Upon successive increase in temperature, the viscosity decreases and ultimately a phase separation occurs. Small-angle X-ray scattering measurements were performed to provide a supportive structural evidence for the rheological data.