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, -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.Glossary of symbols W/A Water-air interface - W/O Water-oil interface - E/A Electrolyte-air interface - L-C Liquid-condensed - A_c Area per molecule obtained by conventional extrapolation of the -A isotherm at close-packing - A_e Experimentally determined area per molecule - A_t Theoretically predicted area per molecule - A_v Area per molecule obtained by vertical extrapolation of the -A isotherm at close-packing - A₀ Head group area term - f_i Activity coefficient of water in surface region - i Constant - x_i Mol fraction of componenti - Z Compressibility factor=A/kT - Interfacial tension - Surface pressure - _i Partial molar area of component i