Phospholipids as Emulsion Stabilizers

The equilibrium phase behavior and the macroemulsion type and stability of oil-water-lecithin mixtures was studied. For dioleyl phosphatidylcholine (DOPC) and n-alkanes as the oil component, the phase equilibrium is characterized by an extended inverse micellar region in equilibrium with water (Winsor II). On the other hand, n-C8F18-DOPC-water and soybean oil-DOPC-water mixtures show a three phase equilibrium of almost pure water, almost pure oil, and lamellar phase Lalpha (Winsor III). Equilibrium phases of DOPC systems can be emulsified in each other. Inverse W/O macroemulsions are favored for all the n-alkanes (C6-C14) studied; O/W emulsions are rather unstable. On the other hand, n-perfluorooctane and soybean oil produced very stable O/W emulsions. The phase behavior and emulsion stabilizing properties of an egg yolk phospholipid mixture is similar to those of DOPC. The phase equilibrium of the saturated analogue of DOPC: distearoyl phosphatidylcholine (DSPC) in mixtures with alkanes and water at room temperature is different and characterized by a Winsor III equilibrium of a Lbeta lamellar phase in a gel state, oil and water. Accordingly, the O/W emulsions are strongly favored to inverse systems. The pattern of the phase equilibrium and macroemulsion stability becomes similar to that of DOPC at elevated temperatures. The macroemulsion stability pattern versus the phospholipid packing type is discussed in relation to the recently proposed theory of emulsion stability to coalescence (Kabalnov, A. and Wennerstrom, H., Langmuir 12, 276 (1996)).