Neutron reflectivity study of competitive adsorption of β-lactoglobulin and nonionic surfactant at the air-water interface

We have measured the neutron reflectivity of β-lactoglobulin films adsorbed at the air-water interface in the presence of varying concentrations of the nonionic surfactant hexaoxyethylene dodecylether (C₁₂E₆). Applying a model-independent Guinier analysis to the measurements of reflectivity as a function of the momentum transfer vector normal to the surface, we have obtained estimates of surface coverage and adsorbed layer thickness. Exploiting the differing neutron scattering powers of hydrogen and deuterium, we have used surfactant with a fully deuterated hydrocarbon chain to obtain details of surfactant adsorption in the presence of protein, mapping both protein displacement and surfactant adsorption. The results show extensive adsorption of surfactant with little protein displacement and only slight increases in layer thickness. 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.