Surface properties of mixed phospholipid-stearylamine monolayers and their interaction with a non-ionic surfactant (poloxamer)

Surface pressure and surface potential studies of phosphatidylcholine-stearylamine (PL-ST) monolayers were performed in the presence and absence of poloxamer in the aqueous phase. For pure PL, the surface pressure area (π-A) isotherm was found to be independent of whether the monolayer was obtained by continuous compression or by aliquot adding. In sharp contrast to PL, ST molecules orient vertically when compressed under dynamic conditions and have a tilted configuration when compressed at constant area. Mixed PL-ST monolayers exhibit a non-ideal behavior which results in a significant increase in surface potential above that corresponding to the pure compounds. This deviation from ideality is mainly due to strong interactions between the positively charged ST head group and a phospholipid polar group occurring in the electric double layer. As revealed by surface pressure measurements, poloxamer molecules seem to be unable to penetrate into the mixed PL-ST monolayers, even when the molar ratio of ST to PL is 0.74. The surface potential data do not corroborate the surface pressure data. An important increment of the surface potential of these monolayers observed on the injection of poloxamer into the aqueous subphase can thus be attributed to the interaction occurring at the level of the hydrophilic groups of the spread lipid.