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 layer covers the interface. Thus, the desorption is limited as shown also by the large values of the dilational modulus at quasi-equilibrium, suggesting that large interactions occur between adsorbed molecules at the interface. The refractive index and extinction coefficient spectra of lignin layers were calculated by a point by point numerical resolution of the ellipsometric data assuming isotropic and homogeneous layers and using the thickness obtained by neutron reflectivity. The absorption coefficient spectrum calculated from the extinction coefficient of the interfacial layer compares well with the bulk absorption spectrum of lignin solutions in a dioxane/water mixture. Thus, spectroscopic ellipsometry measurements are convenient for the characterization of the adsorbed lignins and show that they are not chemically modified at the air/water interface.