Physicochemical characterization of a model intravenous oil-in-water emulsion

Photon correlation spectroscopy, nuclear magnetic resonance spectroscopy, electron microscopy, and small-angle X-ray scattering were used for the structural characterization of a model oil-in-water emulsion containing particles in the submicrometer size range. Additionally, small-angle X-ray diffraction, wide-angle X-ray diffraction, and differential scanning calorimetry were applied to raw materials and to binary mixtures. The majority of emulsion droplets have the characteristic of an ideal emulsion droplet, that is, a liquid lipid core covered by an emulsifier monolayer. However, the system contains a certain excess of emulsifier. Particles with bi- and/or oligolayer structures can be deduced. Double-emulsion droplets were detected. Large unilamellar vesicles were not found; however, the existence of small unilamellar liposomes (also referred to as small unilamellar vesicles or SUVs) seems likely. The proportion of all small nonmonolayer structures was quantified by nuclear magnetic resonance spectroscopy. No mixed micellar structures were detectable. Lysophospholipids were not detected in the aqueous phase, indicating their predominant incorporation into the emulsifier layers. Water-soluble phospholipid degradation products were found in the water phase. The existence of at least several monolayers of phospholipids does not seem to be a prerequisite for a stable soybean oil-in-water emulsion, in general.