Orogenic displacement of protein from the oil/water interface

Orogenic displacement has been shown to be a mechanism by which protein can be removed from an interface by small surfactant molecules. This paper describes the progressive displacement of two different proteins from an oil/water interface by a nonionic surfactant. The process has been visualized by atomic force microscopy (AFM). Measurement of surface tension and AFM imaging of Langmuir-Blodgett (LB) films formed on mica are used to demonstrate the mechanism of protein desorption from the interface. This paper extends previous work which demonstrated a new orogenic mechanism of protein displacement from an air/water interface. The two proteins used in the present study were beta-casein, a largely random coil protein, and beta-lactoglobulin, a globular protein. The proteins were displaced from both spread and coadsorbed films using the water-soluble nonionic surfactant Tween 20. The AFM images also provide direct evidence for the formation of a heterogeneous protein layer at the interface. The heterogeneity of the protein film is important in allowing the initial adsorption of the surfactant onto the interface. These nucleated surfactant sites then expand, compressing the protein network, which initially increases in density without increasing in thickness. Once a certain critical density is reached, further compression of the protein layer results in the thickness increasing in order that protein film volume is maintained constant as the surfactant domains expand. At sufficiently high surface pressures, the network fails, releasing proteins which then desorb from the interface.