Lignin Fractionation for Reduced Heterogeneity in Self-Assembly Nanosizing: Toward Targeted Preparation of Uniform Lignin Nanoparticles with Small Size

Lignin heterogeneity, including complex chemical structure and wide molecular-weight distribution, results in the inhomogeneous and modest performance of lignin, which substantially restricts its value-added applications. For an evaluation of the effects of lignin heterogeneity on the self-assembly nanosizing behaviors, three lignin fractions subdivided from enzymatic hydrolysis lignin (EHL) were used as the raw material for lignin micro-/nanoparticles (LMNPs) preparation, and afterward, the properties of these obtained LMNPs were compared. The three lignin fractions (denoted as F1, F2, and F3) presented reduced heterogeneity compared to the parent EHL, and a gradual increasing of molecular weight accompanied by decreasing hydrophilic group content were found from F1 to F3. The LMNPs prepared from the three fractions exhibited totally different morphologies: F1 mostly formed incomplete spherical particles with large size (450–650 nm), F3 produced only small compact nanoparticles (about 50 nm) with a quite uniform size distribution, and then two distinct particles with a large hollow structure (500–700 nm) and a small compact structure (100–250 nm) were fabricated using F2 as the raw materials. Among the three fractions, F3 showed the highest yield, and the obtained F3 nanoparticles exhibited excellent water dispersion stability because of their small/uniform particles size and high negative zeta potential. The formation mechanism of different nanosizing behaviors among the three lignin fractions was proposed on the basis of the nanoemulsion formed by amphiphilic low-molecular-weight lignin and the hydrophobic aggregation of high-molecular-weight lignin. Overall, this work demonstrates that lignin heterogeneity has significant effects on the self-assembly nanosizing behaviors of lignin, and the nanoparticle properties can be substantially improved using fractionated lignin with high molecular weight.