Adsorbed protein layers at fluid interfaces: interactions, structure and surface rheology

Properties of adsorbed layers of proteins at air-water and oil-water interfaces are reviewed. Emphasis is placed on recent developments in understanding the adsorption and surface rheology of individual globular proteins, especially the milk protein beta-lactoglobulin, in the absence and presence of other proteins or small-molecule surfactants. Problems of modelling adsorbed layer structure and surface equation of state against a background of irreversibility and molecular complexity are addressed. Similarities and differences are described between impenetrable solid and mobile fluid surfaces, between air-water and oil-water interfaces, between shear and dilatational surface deformations, and between globular proteins and disordered proteins (caseins). A new simulation model is discussed in which the adsorbed globular protein monolayer is modelled mesoscopically as a flexible bonded network of spherical particles. The model can describe various characteristic properties of viscoelastic gel-like protein layers, including time-dependent changes in properties arising as a result of large-scale dilatational deformations. Competitive displacement of beta-lactoglobulin from the air-water interface by non-ionic surfactant has been demonstrated directly by neutron reflectivity. While some good correlations have now been established between properties of mixed protein + surfactant layers and the stability of foam and emulsion systems, the detailed relationship between surface rheology and colloid stability is still poorly understood. (C) 1999 Elsevier Science B.V. All rights reserved.