A SCANNING ANGLE REFLECTOMETRY INVESTIGATION OF BLOCK-COPOLYMER ADSORPTION TO INSOLUBLE LIPID MONOLAYERS AT THE AIR-WATER-INTERFACE

We modified the technique of scanning angle reflectometry to measure t he surface excess concentration as water-soluble polystyrene-poly(ethy lene oxide) (PS-PEO) diblock copolymers adsorb from solution to dipalm itoylphosphatidylcholine (DPPC) monolayers spread at the air-water int erface. Polymers adsorb by penetrating the lipid monolayer. Surface pr essure data and fluorescence microscopy indicate that PS-PEO adsorptio n drives the liquid expanded-to-liquid condensed phase transition in t he DPPC monolayer by decreasing the available area per lipid. PS-PEO a dsorption is therefore functionally equivalent to mechanical compressi on of the monolayer. Accordingly, the extent of PS-PEO adsorption acco mmodated by DPPC monolayers in different regimes of the surface pressu re-area isotherm correlates with the compressibility of the penetrated monolayers. Polymer adsorption to Liquid expanded monolayers increase s the interface compressibility to equal that of the phase transition regime. Polymer segments that adsorb to monolayers in the phase transi tion and the liquid expanded regimes force lipids from the expanded to the condensed state, thereby creating accessible interfacial area for the polymer. Polymer surface concentrations in those monolayer regime s are indistinguishable from those attained at the air-water interface in the absence of a spread monolayer. Liquid condensed monolayers can not accommodate polymer segments in this way, and as a result, the ext ent of polymer adsorption is diminished.