COADSORPTION OF BETA-CASEIN AND BOVINE SERUM-ALBUMIN AT THE AIR-WATER-INTERFACE FROM A BINARY MIXTURE

Competitive adsorption of bovine serum albumin (BSA) and beta-casein from binary solutions to the air-water interface have been studied. Adsorption of these proteins followed a kinetically controlled, instead of a Langmuirian-type thermodynamically controlled, competitive adsorption mechanism. Molecular area calculations indicated that the excluded area between protein molecules in the binary protein film at the air-water interface was greater than that in saturated monolayers of single protein systems, indicating thermodynamic incompatibility of mixing of these proteins. Adsorption of 3 molecules of BSA excluded adsorption of 16 molecules of beta-casein from the interface and vice versa. Sequential adsorption experiments showed that beta-casein can displace beta-casein from the interface, whereas it cannot displace adsorbed BSA. In contrast, BSA can neither displace beta-casein nor exchange with adsorbed BSA. The results showed that the protein component that arrives first to the interface adsorbs first, and the late-arriving protein component cannot displace the adsorbed component regardless of its surface active properties.