Due to an intrinsically amphipathic nature, many proteins adsorb at fluid-fluid interfaces such as air-water or oil-water interfaces. In addition, some proteins form laterally bonded cohesive films at interfaces, and these interfacial protein films can be important in the stabilisation of emulsions and foams. The properties of interfacial protein films have typically been studied using methods of interfacial dilatational and shear rheology. We have used unique instrumentation, the Cambridge Interfacial Tensiometer (CIT), to directly determine the strength of protein films via a full stress-strain curve to high strain (up to 1000%), without the need for assumptions about the viscoelastic properties of the material.

Interfacial films formed by a range of proteins at the air-water interface under differing conditions of pH and ionic strength were investigated. Washout experiments with one protein, hen egg white lysozyme, illustrated the role played by ongoing adsorption from the bulk phase in the development of interfacial film strength over time. These results also suggest the presence of a reversibly adsorbed protein sub-layer that takes part in rearrangement or ‘healing' of an interfacial protein film when subjected to high strain. The CIT, by testing the response of protein films to large strains over long time periods, complements existing techniques and will lead to further understanding of the nature of self-assembled protein films at interfaces.