The influence of charged lipids on the flocculation and coalescence of oil-in-water emulsions. II: Electrophoretic properties and monolayer film studies.

The influence of calcium ions on the electrophoretic properties of phospholipid stabilized emulsions containing various quantities of the sodium salts of oleic acid (SO), phosphatidic acid (SPA), phosphatidylinositol (SPI), and phosphatidylserine (SPS) was examined. The critical flocculation concentration of calcium corresponded to a critical zeta potential in all but one of the systems. Systems of approximately equal zeta potential in 0 mM Ca++ had different zeta potentials in dilute solutions of Ca++. A comparison of emulsions of similar polydispersity suggests that these differences may be largely related to differences in particle size and surface area of the emulsions. The influence of Ca++ on the monolayer properties of mixed films containing phosphatidylcholine (PC) with either SPA or SO was also examined at the air-water interface. Films containing PC with SPA were more expanded on a subphase containing calcium compared to a subphase with no calcium. In addition, the compression of films containing PC with SO demonstrated two collapse pressures while SPA was relatively more miscible in the film. This suggests that phase separation of interfacial lipids occurs more easily in systems containing PC and SO. These results may help to explain differences in the flocculation and coalescence of emulsions stabilized by lipid films of different composition.