Interfacial properties of mixtures of lecithin with a block copolymer surfactant at the water/air and water/oil interfaces

Surface pressure-area isotherms have been determined for both a pure lecithin (L, α-dipalmitoyl phosphatidyl choline) and an impure lecithin (soya bean lecithin) at the water/air and water/oil interfaces. Equations of state have been applied and an equation of Gaines was found to be particularly successful in describing the isotherms. Mixed monolayers with an ABA nonionic block copolymer surfactant (A is poly(12-hydroxystearic) acid and B is poly(ethylene oxide)) were also investigated. The additivity rule was obeyed only at high surface pressures; inefficient packing was observed at low surface pressures. The polymer may promote a horizontal headgroup orientation in the lecithin, which gives rise to this effect. The presence of electrolyte up to very high concentrations in the aqueous phase (8.75 mol dm−3 NH4NO3) was shown to expand the lecithin monolayer.