stabilized by phospholipids have been known for several
The equilibrium phase behavior and the macroemulsion type decades, the stabilization mechanism remains controversial and stability of oil-water-lecithin mixtures was studied. For di- (3)(4)(5)(6)(7)(8). oleyl phosphatidylcholine (DOPC) and n-alkanes as the oil com-It is known that the micellar phase always tends to be ponent, the phase equilibrium is characterized by an extended the continuous phase of a macroemulsion (9). Recently a inverse micellar region in equilibrium with water (Winsor II). On mechanistic interpretation for this correlation was proposed, the other hand, n-C 8 F 18 -DOPC-water and soybean oil-DOPCaccording to which the surfactant packing type (spontaneous water mixtures show a three phase equilibrium of almost pure curvature) affects both the phase behavior of microemulsions water, almost pure oil, and lamellar phase L a (Winsor III). Equilibrium phases of DOPC systems can be emulsified in each other. and the coalescence energy barrier of macroemulsions (10). Inverse W/O macroemulsions are favored for all the n-alkanes At positive spontaneous curvatures H 0 , the coalescence bar-(C 6 -C 14 ) studied; O/W emulsions are rather unstable. On the rier for O/W emulsions is large and that of W/O emulsions other hand, n-perfluorooctane and soybean oil produced very stais small. At negative H 0 , the coalescence barrier of W/O ble O/W emulsions. The phase behavior and emulsion stabilizing emulsions is large and that of O/W emulsions is small. The properties of an egg yolk phospholipid mixture is similar to those behavior in the vicinity of the balanced point, where H 0 Ç of DOPC. The phase equilibrium of the saturated analogue of 0, is dependent on the bending modulus of the monolayer DOPC: distearoyl phosphatidylcholine (DSPC) in mixtures with k. For flexible monolayers, k Ç kT, both O/W and W/O alkanes and water at room temperature is different and characteremulsions are unstable. For rigid monolayers, k Ç 10-100 ized by a Winsor III equilibrium of a L b lamellar phase in a gel kT, both O/W and W/O emulsions may be stable (Fig. state, oil and water. Accordingly, the O/W emulsions are strongly favored to inverse systems. The pattern of the phase equilibrium 1). The model is in good agreement with experiments for and macroemulsion stability becomes similar to that of DOPC at poly(oxyethylene) surfactants, which form flexible monoelevated temperatures. The macroemulsion stability pattern versus layers at the oil-water interface (11). the phospholipid packing type is discussed in relation to the re-It would be challenging to examine if the model works cently proposed theory of emulsion stability to coalescence (Kabalequally well for macroemulsions stabilized by rigid phosnov, A. and Wennerstro ¨m, H., Langmuir 12, 276 (1996)). ᭧ 1996 pholipid monolayers. Although the concept of phospholipid Academic Press, Inc.
shape was introduced in lipid science more than 20 years ago