NEUTRON REFLECTIVITY STUDY OF COMPETITIVE ADSORPTION OF BETA-LACTOGLOBULIN AND NONIONIC SURFACTANT AT THE AIR-WATER-INTERFACE

We have measured the neutron reflectivity of beta-lactoglobulin films adsorbed at the air-water interface in the presence of varying concent rations of the nonionic surfactant hexaoxyethylene dodecylether (C12E6 ). Applying a model-independent Guinier analysis to the measurements o f reflectivity as a function of the momentum transfer vector normal to the surface, we have obtained estimates of surface coverage and adsor bed layer thickness. Exploiting the differing neutron scattering power s of hydrogen and deuterium, we have used surfactant with a fully deut erated hydrocarbon chain to obtain details of surfactant adsorption in the presence of protein, mapping both protein displacement and surfac tant adsorption. The results show extensive adsorption of surfactant w ith little protein displacement and only slight increases in layer thi ckness. Further adsorption of surfactant leads to displacement and an increase in layer thickness before complete protein displacement. All of the surface coverage observations are time dependent, with protein levels increasing over a period of hours of measurement at the expense of surfactant being displaced from the initially formed film. (C) 199 8 Published by Elsevier Science Ltd. All rights reserved.