Penetration of poly(oxyethylene)—poly (oxypropylene) block copolymer surfactant into soya phospholipid monolayers

Abstract

The surface tension of aqueous solutions of ABA poly (oxyethylene)-poly (oxypropylene) block copolymer nonionic surfactant, has been measured as a function of time. The kinetics and final equilibrium values were surfactant concentration dependent. From these values, using the simple form of the Gibbs equation, the area per molecule has been calculated. The small molecular area (40.7 Å 2) suggests that surfactant molecules form close-packed coil monolayers with hydrophilic poly (oxyethylene) groups immersed in the aqueous phase. The penetration of the surfactant into monolayers of soya phosphatidylcholine spread at the air-water interface was followed by the variation of the surface pressure (△π) at constant area. As revealed by the results at low surfactant concentrations, the adsorption of surfactant is enhanced by the presence of the phospholipid film. Analysis of equilibrium penetration values, obtained by the use of different thermodynamic approaches, shows that the equations of Pethica and Alexander-Barnes are better adapted for calculation of surfactant penetration into the monolayers only in their condensed state (102–122 Å 2/molecule). Conversely, the equation of Motomura et al. enabled the calculation of the adsorbed surfactant quantities, for the expanded state of phospholipid monolayers (140–230 Å 2/molecule).