ADSORPTION OF BIOPOLYMERS

Of the different biopolymers, most studies of adsorption have been mad e with proteins. As a result of their marked polar/non-polar compositi on, proteins adsorb strongly at most of the interfaces commonly studie d. This property is not shared by polysaccharides and polynucleotides to the same extent. The adsorption of these biopolymers has therefore been comparatively little studied, and requires very specific types of interface. The use of proteins and fluid interfaces provides certain advantages for arriving at a general understanding of the mechanism of the adsorption process. Firstly, at fluid interfaces, it is possible to measure parameters such as area per molecule precisely and to verif y that the interface is clean and free from surface-active impurities. Secondly, proteins are useful because, when they are pure, they are o f constant size, unlike most polymer preparations. In addition, their relatively low diffusion coefficients facilitate the measurements requ ired to elucidate the role of diffusion in adsorption. These measureme nts are very sensitive to the presence of impurities, both in the prot ein sample and in the two phases that form the interface. Great care i s required to ensure purity as well as reproducible diffusion conditio ns. Other aspects of the adsorption process can be studied using the f ilm balance to calculate the adsorption kinetics from measurements of the rate of increase of area at constant interfacial pressure. In this way, information may be gained about the nature of the transition com plex in adsorption. The same techniques can be used to gain correspond ing information about the desorption process. At moderate interfacial pressures, the activation energy barrier to adsorption is relatively l ow compared with that for desorption.