Characterization of Lignin Extracted from Willow by Deep Eutectic Solvent Treatments

Purity, morphology, and structural characterization of synthesized deep eutectic solvent (DES)-lignins (D-6h, D-9h, D-12h, D-18h, D-24h) extracted from willow (Salix matsudana cv. Zhuliu) after treatment with a 1: 10 molar ratio of choline chloride and lactic acid at 120 degrees C for 6, 9, 12, 18, and 24 h were carried out. The purity of DES-lignin was similar to 95.4%. The proportion of hydrogen (H) in DES-lignin samples increased from 4.22% to 6.90% with lignin extraction time. The DES-lignin samples had low number/weight average molecular weights (1348.1/1806.7 to 920.2/1042.5 g/mol, from D6h to D24h) and low particle sizes (702-400 nm). Atomic force microscopy (AFM) analysis demonstrated that DES-lignin nanoparticles had smooth surfaces and diameters of 200-420 nm. Syringyl (S) units were dominant, and total phenolic hydroxyl content and total hydroxyl content reached their highest values of 2.05 and 3.42 mmol.g(-1) in D-12h and D-6h, respectively. beta-Aryl ether (beta-O-4) linkages were eliminated during DES treatment.