Competitive Displacement of β-Lactoglobulin from the Air/Water Interface by Sodium Dodecyl Sulfate

In this work, the displacement of a globular protein from the air/water interface by sodium dodecyl sulfate (SDS) was followed using a combination of the Langmuir−Blodgett method and atomic force microscopy. The results are compared to displacement of the same protein by the nonionic surfactant Tween 20. The displacement by SDS is characterized by the same orogenic mechanism but the charge interactions between the protein and surfactant served to increase the surface pressure at which displacement occurred. Two types of interfacial film transfer were used in order to check that features seen were not caused by the method of transfer. The displacement was also followed using two surface rheological techniques: shear and dilation. The results from the rheological studies are correlated with the data seen with the AFM and show that the mixed film was much weaker than either of the component films. Both surface shear and dilation rheology showed similar trends. Also demonstrated is the appearance of larger circular holes in the protein film during the latter stages of displacement.