Oil-in-Water Emulsions Stabilized by Acylglutamic Acid–Alkylamine Complexes as Noncovalent-Type Double-Chain Amphiphiles

We have studied the preparation and stabilization mechanism of oil-in-water-type emulsions in the presence of amphiphilic 1:1 stoichiometric complexes of acylglutamic acids (C n Glu) with tertiary alkylamines (C n DMA). Relatively stable emulsions were obtained when C16Glu–C16DMA (or C18Glu–C18DMA), hexadecane, and water were homogenized at 80 °C and then stored at room temperature. The gel–liquid crystal phase transition temperatures ( T c ) of C16Glu–C16DMA and C18Glu–C18DMA dispersed in water were determined to be ca. 39 and 53 °C, respectively. This indicates that the complexes form an adsorbed layer at the oil/water interface during the homogenization process above the T c and then change into a gel during storage at room temperature. The gel phase formed at the oil/water interface prevents the oil droplets from coalescing. In contrast, shorter chain analogues (C10Glu–C10DMA and C12Glu–C12DMA) did not yield stable emulsions because their adsorption layers were not able to prevent coalescence of the oil droplets (i.e., the T c of these analogues was below the room temperature). We have also demonstrated that the dispersion stability of these emulsion systems can be controlled by changing the aqueous pH.