Ellipsometric study of the displacement of milk proteins from the oil–water interface by the non-ionic surfactant C10E8

Ellipsometry was employed to study the adsorption to the hexadecane–water interface of the simple non-ionic hydrocarbon surfactant C10E8 and the two milk proteins β-casein and β-lactoglobulin, as well as the competitive adsorption of each protein with the surfactant. The interfacial excess of the pure surfactant was determined by tensiometry. Modelling of the ellipsometric response of the pure surfactant monolayer shows that the polyethylene oxide headgroups are hydrated with approximately 40% of the headgroup layer occupied by water. Adsorbed layers of C10E8 at the hexadecane–water, triolein–water and air–water interfaces are structurally similar. Both proteins form dense layers at the oil–water interface with a volume fraction of water in the protein film of <60%. Competitive adsorption between the surfactant and protein was investigated in two ways: co-adsorption from solution or injection of surfactant solution into the subphase of a preformed protein film. The long-time ellipsometric response was independent of the preparation procedure. The protein and surfactant films at oil–water interfaces generate ellipticities of opposite sign, which enabled direct determination of the concentration at which the surfactant completely displaces protein from the interface.