Kinetics of lysozyme adsorption at the air-buffer interface

The adsorption of lysozyme at the air-buffer interface has been monitored previously by a radiotracer method. A major experimental feature of these studies is that the amount of protein adsorbed is a significant fraction of the protein in the bulk at the beginning of the adsorption process. In the present study, several kinetic models of adsorption are derived which take into account the bulk protein concentration and a possible attraction of the protein in solution by the protein molecules forming the interfacial adsorption layer. Moreover, the data are analyzed assuming either an exponential regime (the rate of adsorption is controlled by the distance from equilibrium) or a diffusive regime (the rate of adsorption is controlled by the diffusion of protein molecules in the solution). The full model (depletion of protein from the bulk and attraction by the previously adsorbed molecules) leads to an expression for the rate of adsorption of the third degree. Only the third degree gives a good fit to the experimental data where two kinetic phases are obvious. However, the accuracy of the data does not allow us to determine whether the adsorption regimes are diffusive or exponential. It may be concluded that the adsorption of lysozyme at the air-buffer interface is very probably a 'cooperative' process.