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 °C for 6, 9, 12, 18, and 24 h were carried out. The purity of DES-lignin was ~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 D(6h) to D(24h)) 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. β-Aryl ether (β-O-4) linkages were eliminated during DES treatment.