NEUTRON REFLECTIVITY STUDY OF THE COMPETITIVE ADSORPTION OF BETA-CASEIN AND WATER-SOLUBLE SURFACTANT AT THE PLANAR AIR-WATER-INTERFACE

Application of the technique of specular neutron reflection to the study of adsorbed layer structure is illustrated for the case of beta-casein at the air-water interface in the presence and absence of nonionic water-soluble surfactant C12E6. Guinier analysis of reflectivity data for a 5 x 10(-3) wt% solution of pure beta-casein in air-contrast-matched water (8 vol% D2O) at pH 7.0 gives a time-independent adsorbed amount of 2.05 +/- 0.10 mg/m2 and an adsorbed layer thickness of 1.65 +/- 0.07 nm; these values are found to increase quite substantially as the pH is reduced towards the protein's isoelectric point. In the presence of surfactant the loss of reflectivity is a direct measure of protein displacement from the interface because the hydrogenated surfactant is almost contrast matched to the aqueous phase. At a C12E6 concentration in the range 2-2.5 x 10(-4) wt% (surfactant: protein molar ratio, R almost-equal-to 2.2), there is roughly half the protein lost from the interface but little change in adsorbed layer thickness as inferred from the slopes of the Guinier plots. Protein is effectively completely removed from the surface for R greater than or similar to 10. These results are in semi-quantitative agreement with complementary competitive adsorption data for beta-casein in soya oil-in-water emulsions.