The adsorption of proteins to gas-liquid interfaces

A kinetic model of protein adsorption to gas-liquid interfaces is described. The model is based on a formalism similar to that used to describe adsorption at gas-solid interfaces, but with proteins we allow for both tight adsorption of a first layer and loose packing of a second layer of proteins. The model is fit to the adsorption isotherm data of Graham and Phillips for the three proteins: β-casein, bovine serum albumin, and lysozyme. We also fit the model to adsorption rate data for the two proteins lysozyme and β-casein. The experimental adsorption rates are considerably faster than those for molecular diffusion alone. The rate of mass transfer of protein to the interface was accordingly modeled by an effective mass-transfer coefficient. The ratio of the mass-transfer coefficients for two different proteins can be predicted by the free-convection correlation of Globe and Dropkin, suggesting that small temperature gradients may have existed in the vessel during the measurements. Finally, we have predicted desorption rates for exchange experiments in which fresh solution replaces bulk protein solution after equilibrium adsorption has been reached. Depending on the residence time of the bulk fluid in the reservoir, our results indicate desorption can be very difficult to measure, particularly at low surface concentrations and even if equilibrium is assumed between the interface and the solution just below the surface. Thus, there appears to be no need to postulate irreversible adsorption of protein to account for the available experimental data on protein adsorption and desorption.