INTERACTIONS OF A NONIONIC ABA COPOLYMER SURFACTANT WITH PHOSPHOLIPID MONOLAYERS - POSSIBLE RELEVANCE TO EMULSION STABILIZATION

The pi-A isotherms of phospholipid monolayers in the presence of ABA polyoxyethylene, polyoxypropylene block copolymer (poloxamer) in the aqueous subphase exhibit a marked increase in surface pressure, indicating that poloxamer molecules are localized at the air-water interface and are intercalated between phospholipid molecules. While the surface pressure at collapse remained practically constant at poloxamer concentrations lower than the CMC, it increased at poloxamer concentrations above the CMC. The molecular area values were independent of poloxamer concentration. These results suggest that ejection of poloxamer molecules from phospholipid monolayers takes place at high compressions. In independent studies on the stability of emulsions using the same phospholipid-poloxamer combination, it was noted that an optimal concentration of these emulsifiers was necessary to stabilize the emulsion. The emulsion stability data which corroborate the results of surface pressure measurements confirm the existence of an association between poloxamer and phospholipid molecules. At poloxamer concentrations slightly higher than the CMC, poloxamer aggregates appear to form a hydrophilic environment close to the dispersed oily droplets, thus favoring emulsion stabilization. A model of molecular arrangements at the mixed monolayer-water interface is proposed.