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 stu dy of adsorbed layer structure is illustrated for the case of beta-cas ein 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-mat ched water (8 vol% D2O) at pH 7.0 gives a time-independent adsorbed am ount 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 t he pH is reduced towards the protein's isoelectric point. In the prese nce of surfactant the loss of reflectivity is a direct measure of prot ein displacement from the interface because the hydrogenated surfactan t is almost contrast matched to the aqueous phase. At a C12E6 concentr ation 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 inferre d from the slopes of the Guinier plots. Protein is effectively complet ely removed from the surface for R greater than or similar to 10. Thes e results are in semi-quantitative agreement with complementary compet itive adsorption data for beta-casein in soya oil-in-water emulsions.