Formation of Homogeneous Lignin Nanoparticles from Industrial Kraft Lignin via Fractionation Combined with Antisolvent Precipitation

Processing lignin into nanoparticles (LNPs) offers a promising utilization strategy; however, its structural and molecular weight heterogeneity poses challenges in the formation of uniform LNPs. In this study, industrial kraft lignin was fractionated in stepwise molecular weight (Mw) from low to high and from which LNPs were fabricated via antisolvent precipitation. The results showed that lignin with high Mw benefits the formation of uniform and smaller-sized LNPs. Particularly, the lignin fraction with Mw of 2016 g·mol^-1 fails to form LNPs. The main mechanism is that the higher content of hydrophilic groups (mainly phenolic hydroxyl groups) on the lower molecular weight lignin hinders the formation of LNPs. This hypothesis is supported by the successful formation of homogeneous LNPs after low molecular weight lignin acetylation. Fractionation effectively reduces lignin heterogeneity and promotes the formation of LNPs, which would favor the chemical reactivity and properties, enhancing the utilization of industrial lignin.