Liquid Crystal Cubic Phases Constructed from Sophorolipids Micelles

Biosurfactants are considered to be desirable alternatives to synthetic surfactants. Sophorolipids produced by nonpathogenic yeast strains are one of the main types of glycolipid biosurfactants and have various applications. In this work, the aqueous phase behavior of the glycolipid-based biosurfactant sophorolipids (SL) was investigated using polarized microscopy, small-angle X-ray diffraction (SAXS), nuclear magnetic deuterium spectroscopy (²H NMR), transmission electron microscopy (TEM), freeze-etched transmission electron microscopy (FF-TEM), and dynamic light scattering (DLS). The binary phase diagram of the SL/H₂O system was constructed, and a liquid crystalline cubic phase constructed by sophorolipids micelles was observed. A micellar phase at low concentrations (<50 wt %) was found. As the concentration increases, after a transition phase, it is a strictly micellar cubic phase at concentrations up to 70 wt %. The micellar cubic phase is an isotropic, highly viscous liquid crystal composed of three-dimensionally ordered arrangements of spherical micelles, which are arranged in simple cubic (Cub_I/<i>Pm</i>3<i>m</i>) or body-centered cubic (Cub_I/<i>Im</i>3<i>m</i>). The rheological properties at different concentrations and temperatures were studied. The micellar cubic phase is highly viscoelastic, and the viscosity tends to decrease uniformly with increasing temperatures (15-90 °C) and then returns to its original state after cooling, indicating that the micellar cubic phase possesses satisfactory reversibility at high temperature. The results are expected to be instructive for the application of the sophorolipids.