Alcohol-Decorated Lignins for Nanoparticle Formation through Reactive Fractionation in Ternary Deep Eutectic Solvent Systems

This study explores the use of ternary deep eutectic solvent (DES) systems composed of choline chloride (ChCl), oxalic acid (OA), and ethylene glycol (EG) for the efficient fractionation of lignocellulosic biomass and isolation of lignins with diverse structural features and EG incorporation levels. Reactive fractionation of birchwood under optimized conditions resulted in a high lignin yield (66%), with up to 75% retention of β-O-4 aryl ether linkages. Systematic variation of temperature (80–200 °C), reaction time, and DES composition showed that EG incorporation is both temperature- and time-dependent, with optimal structural preservation observed at 140–160 °C and short reaction times. Furthermore, 2D HSQC NMR and GPC analyses revealed that increasing temperatures promote the cleavage of aryl ether linkages, while a higher EG content in the DES mitigates structural degradation. The resulting EG-decorated lignins were successfully applied to the controlled synthesis of lignin nanoparticles (LNPs) via hydrotropic and pH-induced flash precipitation methods. The latter allowed one to obtain LNPs with size tunability, colloidal stability, and favorable surface charge, highlighting their potential for material applications. Overall, this study provides critical insights into the structure-processing relationships of DES-isolated lignins and establishes a promising approach to their valorization into functional nanomaterials.