Microstructure and stability of a lamellar liquid crystalline and gel phase formed by a polyglycerol ester mixture in dilute aqueous solution

The self-assembly behavior of a commercial mixture of polyglycerol fatty acid esters (PGE) and water is investigated as a function of temperature and surfactant content. The phase diagram of this pseudo-binary mixture was characterized using a combination of cross-polarized light and freeze-fracture electron microscopy (cryo-SEM), X-ray diffraction (XRD), small-angle neutron scattering (SANS), and differential scanning calorimetry (DSC). Our experiments show that the morphology of the supramolecular aggregates is lamellar and present in the form of a continuous or dispersed phase (multilamellar vesicles) depending on the water content of the system. Under the effect of temperature, the short- and long-range order of the bimolecular layers successively changes from a biphasic surfactant dispersion to a lamellar liquid-crystalline (Lalpha) and a stable lamellar gel phase (Lbeta) upon cooling; this transition is found to be irreversible. Formation of the lamellar aggregates can be related to the average molecular structure and shape factor of PGE. The stability of the resulting gel phase (Lbeta) appears to be due to the presence of small amounts of unreacted ionic co-surfactant, namely, fatty acid soaps, in this per se nonionic commercial mixture.