Langmuir monolayers of lignins obtained with different isolation methods

The characteristics of Langmuir monolayers of lignins isolated via distinct methods, viz. acetone-oxygen-organosolv (termed Lignin-1), Kraft (Lignin-2), acetic acid-oganosolv (Lignin-3) and acetone-organosolv (Lignin-4), are investigated using surface pressure and surface potential measurements. The overall behavior of these lignins is essentially the same, but some differences were observed which are attributed to the relative amount of carbonyl groups in the lignin skeleton. Lignin-1 and 3 displayed pressure-area isotherms considerably more expanded because they possess the largest amounts of carbonyls that are forced out of the water upon monolayer compression in the condensed phase. These carbonyls contribute negatively to the surface potential. For Lignin-1, where such groups appear in COOH moieties, the surface potential is much smaller than for the other lignins. Hysteresis and stability experiments showed that the lignin monolayers are metastable possibly owing to molecular reorganization at the air-water interface. (C) 1999 Elsevier Science S.A. All rights reserved.