Orogenic Displacement of Protein from the Air/Water Interface by Competitive Adsorption

Abstract

The displacement of proteins from an air/water interface by surfactant has been visualized by atomic force microscopy (AFM) through the imaging of Langmuir-Blodgett films formed on mica. Three different proteins were studied: beta-casein, a largely random coil protein, and two globular proteins, beta-lactoglobulin and alpha-lactalbumin. The proteins were displaced from both spread and coadsorbed films using the nonionic surfactant Tween 20. The combined use of AFM with studies of surface tension and surface rheology have revealed the mechanism of protein desorption from the air/water interface. The surfactant is found to adsorb at defects in the protein network and these nucleated sites then grow, compressing the protein network. At sufficiently high surface pressures the network fails, releasing proteins that then desorb from the interface. We have called this mechanism orogenic displacement. Stress propagation through beta-casein films is homogeneous resulting in the growth of circular surfactant domains. beta-Lactoglobulin and alpha-lactalbumin form stronger networks and stress propagation is restricted resulting in the growth of irregular (fractal) surfactant domains. The AFM images also provide direct evidence for the formation of elastic (gel-like) protein networks at the air/water interface. Copyright 1999 Academic Press.