CHARACTERIZATION OF A SUBMICRON-SIZED OIL-IN-WATER EMULSION

The present study was initiated to analyse the structure of a submicron-sized soybean oil-in-water (o/w) emulsion and to find a useful selection of independent methods for the characterization of submicron-sized emulsions. The emulsion under investigation was stabilized by fractionated egg phospholipids and the majority of particles had diameters in the nanometer size range. Systems with a comparable chemical composition and size distribution are used in the pharmaceutical field for intravenous administration. The influence of the high energy input during production on the structure of such phospholipid-stabilized oil-in-water emulsions is of special interest since toxic side-effects have been described which seem to be related to an excess of phospholipids and structural parameters. During production and storage (for more than 1 year) lysophospholipids and free fatty acids were formed by the degradation of the fractionated egg phospholipids and thereby influenced the actual composition of the emulsifier. Although about 12 mol% of the phospholipids were lysophospholipids and at least comparable amounts of partly ionized free fatty acids existed in the emulsion, micellar or mixed micellar structures were not detected. Nuclear magnetic resonance data as well as zeta potential measurements indicated that the majority of lysophospholipids and ionized fatty acids were incorporated in the emulsifier layers and influenced the actual surface charge density of the emulsion droplets. In contrast to earlier findings on cruder o/w emulsions, the predominant particle structure of the emulsion droplets was an oil core covered by a phospholipid monolayer. The excess of emulsifier did not form predominantly liquid crystalline lamellar structures at the oil-water interface as described for cruder systems but small unilamellar vesicles (SUVs). Large numbers of SUVs coexisted in the system with real emulsion droplets. Large unilamellar vesicles (LUVs) were not found, probably owing to the high energy input during homogenization. Additionally, few double-emulsion particles and large multiple vesicles could be detected. The results indicate that the submicron-sized o/w emulsion under investigation is a complex system containing various types of particle. It seems that in submicron-sized o/w emulsions the excess of phospholipids does not preferably form liquid crystalline lamellar structures at the oil-water interfaces but SUVs, owing to the high energy input during their production. The absence of micelles in combination with the introduction of charges by the continuous formation of ionized free fatty acids and their incorporation into the emulsifier layers may explain the stability of the system. It turned out that P-31 NMR spectroscopy was a powerful tool for (quantitative) structural analysis, especially in combination with small-angle X-ray scattering measurements, ultracentrifugation experiments and transmission electron microscopy of freeze-fractured specimen.