Lignin molecular arrangements in Langmuir and Langmuir–Blodgett films: the influence of extraction processes

The influence of the extraction process has been investigated in the formation of Langmuir and Langmuir–Blodgett films of lignins. In order to focus on the extraction process, lignins from a single source, sugar cane bagasse, were extracted via four methods, viz. acetone–oxygen (AO), soda (SO), ethanol–water (EW) and acetone/water/sulfuric acid (SAC). Langmuir films were spread onto ultra pure water by dissolving the lignin in a mixture of N,N-dimethylacetamide (DMA) and chloroform. The major effect arises from the presence of strong polar groups, which depends on the extraction process. Lignins with larger contents of carbonyl, such as AO and EW, yielded more expanded surface pressure–area isotherms and lower surface potentials (or dipole moments). Carbonyl groups therefore contribute negatively to the surface potential, i.e. the oxygen in the carbonyl linkage somehow points to the air when the film is in the compressed state. When the subphase conditions were altered by changing temperature, pH and ionic strength, the monolayers from EW lignin were the most affected, presumably because of the different degrees of difficulty in removing the COOH groups from the water/film interface. For example, EW displays one of the lowest dipole moments at pH = 6 but one of the highest at pH = 1, as the non-dissociated COOH may no longer be removed from the interface. Confirming previous studies, all monolayers are amenable to transfer as LB films only with special procedures, particularly a low dipping speed. The molecular arrangement in the LB films also depended on the contents of polar groups, which is reflected in the film roughness. AFM images of five-layer LB films deposited on mica indicated a much lower roughness for the lignins with higher contents of COOH groups, with the root mean square roughness being 0.24, 0.30, 1.25 and 3.80 nm for a scanned area of 5 μm × 5 μm for EW, AO, SO and SAC lignins, respectively.