Kinetics of adsorption of globular proteins at an air-water interface.

Adsorption of globular proteins at an air-water interface from an infinite stagnant medium was modeled as one-dimensional diffusion in a potential field. The interaction potential experienced by an adsorbing molecule consisted of contributions from electrostatic interactions, work done against the surface pressure to clear area at the interface in order to anchor the adsorbed segments, and the change in the free energy due to exposure of penetrated surface hydrophobic functional groups to air. The assumption of irreversible adsorption is employed in the present analysis. The energy barrier to adsorption, present at sufficiently large surface pressures, was found to be higher for smaller surface hydrophobicities, larger surface pressures, larger size molecules, and oblate orientation of an ellipsoidal molecule. Consequently, more adsorption occurred at larger surface hydrophobicities, smaller size molecules, and for prolate orientation of ellipsoidal molecules. The subphase concentration has been shown to be zero at short times, increasing with time at larger times, and eventually becoming close to the bulk concentration as a result of increasing energy barrier to adsorption. The predicted evolution of surface concentration with time for adsorption of lysozyme at an air-water interface agreed well with the experimental data of Graham and Phillips (1979a).