The Stabilization of the Monoglyceride Alpha-gel Phase and its Potential Application in Cosmetic Creams
This research is aimed at increasing the stability of the metastable α-gel phase formed by long-chain saturated monoglycerides (MGs) in water and emulsion systems. Increasing the stability of the α-gel phase will enhance the water swelling capacity and stability of MG-structured systems therefore it will improve the shelf life of emulsion systems structured with saturated MGs. The application of MG-structured emulsions as a petroleum-free cosmetic cream was also considered. The polymorphic behaviour of a saturated MG, glycerol monostearate (GMS), was studied first in relatively simple GMS-water systems and then in more complex oil-in-water emulsions. The thermal and structural properties of GMS-structured water and emulsion systems were examined with differential scanning calorimetry (DSC), and with X-ray diffraction (XRD), light microscopy, and scanning electron microscopy (SEM). Water mobility in MG-structured emulsions was examined using nuclear magnetic resonance (NMR) analysis. After understanding the polymorphic behaviour of MG-structured systems, the rheological and sensory properties of the MG-structured emulsion were compared with a commercially available cosmetic cream. Results showed that the stability of the α-gel phase is affected by similar factors in MG-water and MG-structured emulsions. The structure and concentration of co-emulsifiers used with GMS affect the stability of the α-gel phase. A negatively charged co-emulsifier that is naturally stable in the α phase, such as sodium stearoyl lactylate (SSL), could improve the stability of GMS-structured systems. Using slow cooling rates without shear and storage at refrigeration temperatures were also able to enhance the stability of the α-gel phase. In MG-structured emulsions, the addition of xanthan gum reduced water syneresis upon aging thus improved emulsion stability. During the study of the polymorphic behaviour of MG-water systems, the thermal and structural properties of the sub-α-gel phase were clearly identified for the first time. Sub-α Coagel Index (CIsub-α), a factor to quantitatively characterize the degree of polymorphic transformation of MG-water systems was proposed. CIsub-α was later successfully used to quantify coagel formation in MG-structured emulsion systems. Because SSL showed promising functionality in improving the stability of the α-gel phase, a phase diagram of GMS and SSL neat systems was also composed. The petroleum-free MG-structured emulsion showed comparable cosmetic properties to a commercially available petroleum-based skin cream. Similar rheological behaviour under small and large amplitude oscillatory shear experiments was collected for the two skin creams. The two skin creams also showed similar moisture barrier function to prevent water evaporation from human skin. Consumer acceptance tests also suggested similar preference between the two skin creams.