A NEUTRON REFLECTIVITY STUDY OF THE ADSORPTION OF BETA-CASEIN AT FLUID INTERFACES

Direct information on the thickness and structure of adsorbed protein layers at oil/water and air/water interfaces is necessary for a full understanding of the role of proteins in the stabilization of emulsions and foams. This paper shows that the technique of specular neutron reflectivity can reliably give such information. By use of the CRISP instrument at the Rutherford-Appleton Laboratory (Oxford, U.K.), measurements of neutron reflectivity have been made for monolayers of the disordered milk protein beta-casein at the n-hexane-water and air-water interfaces. For a bulk protein concentration of 5 x 10(-3) wt %, the surface concentration at the air/water interface is estimated as 3.8 mg m-2 and the first moment of the segment density distribution is calculated to be 3.1 nm with the origin at the water surface. Based on modeling techniques to obtain best-fit segment density profiles, we find that the distribution of protein normal to both oil/water and air/water interfaces is well described by a dense inner layer of ca. 2 nm thickness directly at the interface and a more tenous secondary layer of thickness 5-7 nm extending into the aqueous phase. This structural description of the adsorbed beta-casein layer is broadly consistent with recent measurements of film properties derived from adsorption isotherms, dynamic light scattering, and proteolytic digestion of the adsorbed protein.