Partitioning of a nearly insoluble lipid monolayer into its aqueous subphase

Pertinent analysis of surface activity data recorded with palmitoyloleoylphosphatidylcholine (POPC) at the air/water interface reveals that in contrast to common belief a generally non-negligible infiltration into the aqueous bulk volume may occur. A plot of the amount of spread lipid vs. the interfacial area taken at equilibrium and constant surface pressure results in a well-defined straight line. Its slope and ordinate intercept turn out to be equal to the actual surface concentration, and the amount of lipid having entered the subphase, respectively. The latter quantity depends on the individual measuring conditions. Accordingly, the apparent area per molecule in the interface may appreciably vary among different experimental set-ups. We have evaluated the film pressure as a function of the true surface concentration as well as the partitioning isotherm. These data are quantitatively discussed in terms of thermodynamic relations. The findings suggest that a structural transition of the interfacial lipid is induced beyond about 30 mN m(-1).