Dynamics of protein and mixed protein/surfactant adsorption layers at the water/fluid interface

The adsorption behaviour of proteins and systems mixed with surfactants of different nature is described. In the absence of surfactants the proteins m ainly adsorb in a diffusion controlled manner. Due to lack of quantitative models the experimental results are discussed partly qualitatively. There a re different types of interaction between proteins and surfactant molecules . These interactions lead to protein/surfactant complexes the surface activ ity and conformation of which are different from those of the pure protein. Complexes formed with ionic surfactants via electrostatic interaction have usually a higher surface activity, which becomes evident from the more tha n additive surface pressure increase. The presence of only small amounts of ionic surfactants can significantly modify the structure of adsorbed prote ins. With increasing amounts of ionic surfactants, however, an opposite eff ect is reached as due to hydrophobic interaction and the complexes become l ess surface active and can be displaced from the interface due to competiti ve adsorption. In the presence of non-ionic surfactants the adsorption laye r is mainly formed by competitive adsorption between the compounds and the only interaction is of hydrophobic nature. Such complexes are typically les s surface active than the pure protein. From a certain surfactant concentra tion of the interface is covered almost exclusively by the non-ionic surfac tant. Mixed layers of proteins and lipids formed by penetration at the wate r/air or by competitive adsorption at the water/chloroform interface are fo rmed such that at a certain pressure the components start to separate. Usin g Brewster angle microscopy in penetration experiments of proteins into lip id monolayers this interfacial separation call be visualised. A brief compa rison of the protein adsorption at the water/air and water/n-tetradecane sh ows that the adsorbed amount at the water/oil interface is much stronger an d the change in interfacial tension much larger than at the water/air inter face. Also some experimental data on the dilational elasticity of proteins at both interfaces measured by a transient relaxation technique are discuss ed on the basis of the derived thermodynamic model. As a fast developing fi eld of application the use of surface tensiometry and rheometry of mixed pr otein/surfactant mixed layers is demonstrated as a new tool in the diagnost ics of various diseases and for monitoring the progress of therapies. (C) 2 000 Elsevier Science B.V. All rights reserved.