β-casein adsorption kinetics on air-water and oil-water interfaces studied by ellipsometry

In this work beta-casein adsorption kinetics at air/water and oil/water surfaces is studied by ellipsometry. Double layer formation is observed; the process is not influenced by the type of the hydrophobic phase. A dense protein layer initially forms, and then the adsorption continues in a second more diffuse layer, which extends in the aqueous phase. The surface coverage (i.e. the adsorption, Gamma) in the second layer approaches that in the first dense layer at long times, but the thickness and the refractive index of the second layer testify for lower volume density of protein there. The well known ellipsometric problem of correlating parameters in the case of measurement in a very thin layer (on a substrate which does not absorb light) has been overcome by using different upper phases (oil and air). This gives the opportunity to determine separately the refractive index and the thickness of the layer. The same procedure is applicable whenever a very thin layer structure with two unknown parameters is under investigation. (C) 2000 Elsevier Science B.V. All rights reserved.