Protein adsorption at a static and expanding air–water interface: a neutron reflection study

Using the technique of contrast variation in neutron reflectometry, we determined the volume fraction profile of the milk protein β-casein, adsorbed at the air–water interface in great detail. For bulk concentrations ranging from 10 to 1000 mg l−1, the total thickness of the adsorbed layer is approximately 15 nm. It consists of a very dense region at the interface and a middle region with a volume fraction of about 45%. Thicknesses of both regions increase with increasing bulk concentration. With an overflowing cylinder, designed to be used in combination with neutron reflectometry, we create an expanding surface with adjustable relative expansion rate. With this technique the adsorption dynamics has been studied, and here we present the first experimental results.