Competitive adsorption of lysozyme and C12E5 at the air/liquid interface

Abstract

We have studied the adsorption of lysozyme and pentaethylene glycol monododecyl ether (C12E5) at the air/water interface using neutron reflection and surface tension measurements. The effect of C12E5 concentration was examined at three fixed lysozyme concentrations of 0.01, 1 and 4 g dm−3. The surface tension showed little variation with the addition of C12E5 over the low surfactant concentration region, but with the increase of C12E5 concentration, the surface tension gradually became identical to that corresponding to pure C12E5. These results suggest a progressive replacement of lysozyme by C12E5 and that the observed surface event is dominated by competitive adsorption. The parallel neutron measurements showed that, at low surfactant concentration, the surface was predominantly occupied by lysozyme. At intermediate C12E5 concentrations, the surface layer consisted of both lysozyme and C12E5, with the C12E5 eventually completely replacing the adsorbed lysozyme as the surfactant concentration was further increased. While the neutron results confirm the inference from surface tension measurement, structural analysis clearly showed the partial breakdown of the globular structure of lysozyme induced by the nonionic surfactant. Furthermore, neutron data showed that the adsorbed C12E5 molecules are present at the top surface layer only, suggesting no preferential association or binding between the surfactant and any immersed protein fragments at the interface.