INTERFACIAL PROPERTIES OF MIXTURES OF LECITHIN WITH A BLOCK-COPOLYMERSURFACTANT AT THE WATER AIR AND WATER/OIL INTERFACES/

Surface pressure-area isotherms have been determined for both a pure l ecithin (L, alpha-dipalmitoyl phosphatidyl choline) and an impure leci thin (soya bean lecithin) at the water/air and water/oil interfaces. E quations of state have been applied and an equation of Gaines was foun d to be particularly successful in describing the isotherms. Mixed mon olayers with an ABA nonionic block copolymer surfactant (A is poly(12- hydroxystearic) acid and B is poly(ethylene oxide)) were also investig ated. The additivity rule was obeyed only at high surface pressures; i nefficient 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 show n to expand the lecithin monolayer.