Interactions of a non-ionic surfactant with mixed phospholipid—oleic acid monolayers. Studies under dynamic conditions

Abstract

Surface pressure studies of pure and mixed monolayers of phosphatidylcholine (PC), phosphatidylethanolamine (PE), and oleic acid (OA) spread at the air—water interface were performed in the presence and in the absence of ABA polyoxyethylene-polyoxypropylene block copolymer (non-ionic surfactant, POL) in the aqueous subphase. The results reveal that PC—PE mixtures (PL) form homogeneous monolayers.It was found that while the mean surface areas of the mixed PL—OA monolayers followed the additivity rule, the surface pressures (π) corresponded to the phospholipid π values. It was concluded that the non-ideal behavior of OA—PL mixed films resulted from the molecular interactions occurring between the hydrophobic parts of the film-forming components, and that the double bond of oleic acid is involved in these interactions. The mixing energies and interaction parameters at intermediate and collapse pressures were calculated according to the Goodrich and Joos approaches. Negative values of these parameters indicated that OA—PL mixed films were thermodynamically more stable than the films of each component. The addition of POL into the aqueous subphase yielded films capable of supporting higher compressions, a fact attributed to the partial penetration of POL molecules into the lipid monolayers. From these data, models of PL—OA—POL arrangements in the interfacial region are proposed for the expanded and condensed states of monolayers.