Formation and characterization of spread lignin layers at the air/water interface

The structure and surface properties of spread layers of lignins at the air/water interface have been studied by neutron reflectivity, spectroscopic ellipsometry, and static and dynamic tensiometry. The layers hold 70% water, and the surface concentration of lignin measured by neutron reflectivity is less than the amount deposited. When the spread amount increases from 1 to 16 mg m-2, 65−12.5% of the deposited lignin is recovered at the interface. The absolute value of the Brewster ellipticity decreases slowly after spreading and tends toward quasi-equilibrium after 30 h, a fact which could be explained by a slow desorption from the interfacial layer to the bulk. After a compression of the interfacial layers, the surface pressure and the Brewster ellipticity exhibit a strong relaxation due, at least in part, to a desorption. However, the diffusion of lignin molecules from the layer to the substrate is limited because at equilibrium the neutron reflectivity and ellipsometry data show that a significant lay...