Surfactant-protein interactions at air-water and oil-water interfaces observed by atomic force microscopy.

Both proteins and surfactants can stabilise foams or emulsions. However, the mechanisms of stabilisation are very different and, when surfactant is added to a protein stabilised foam or emulsion, the effect is antagonistic, and the foam or emulsion breaks down A convenient model system for studying proteins or surfactants at interfaces is the Langmuir trough If the interfacial layers are pulled onto mica using Langmuir-Blodgett techniques then it becomes possible to study the molecular structure of the films using the new technique of Atomic Force Microscopy (AFM). By combining AFM measurements with conventional interfacial biophysical methods it has been possible to investigate the destabilisation of dairy protein (beta -casein & beta -lactoglobulin) films, formed at air-water and oil-water interfaces, by the non-ionic surfactant Tween 20. The mechanisms of displacement are novel involving nucleation and growth of surfactant domains within a protein network. At a given surfactant concentration the area occupied by the surfactant domains grows until further growth is balanced by the mechanical energy stored in the protein lattice. The reduced area of the surface occupied by the protein is compensated for by an increased protein film thickness. Rupture of the protein network permits displacement of the protein from the surface. The details of the breakdown process are sensitive to the type and structure of the protein, the method of film formation and the type of interface. Using the AFM it is also possible to visualise the dynamics of the surfactant displacement of protein from surfaces in real time.