Phase Separation in Two-Dimensional αs-Casein/β-Casein/Water Ternary Film at the Air−Water Interface

Abstract

Two-dimensional phase separation in αs-casein/β-casein/water ternary film adsorbed at the air−water interface has been studied using an epi-fluorescence microscopy technique. The equilibrium composition of saturated mixed monolayer films of αs-casein/β-casein at the air−water interface at various bulk concentration ratios did not follow a Langmuir-type competitive adsorption model. This was due to a change in the binding affinity of the proteins to the interface as a result of incompatibility of mixing of the proteins in the mixed monolayer. This manifested itself in two-dimensional phase separation of αs-casein and β-casein in the mixed protein film. αs-Casein always tended to be the dispersed phase and β-casein the continuous phase in the film. Also, phase separation occurred only when the film at the air−water interface was aged for several days, suggesting that the process was kinetically limited by lateral diffusion of the protein components in the viscous film.