Tailoring kraft lignin for high-performance nanoparticles: from structure to function

In the context of biorefineries, kraft lignin (KL) has emerged as an underexplored yet promising resource. Its phenolic structures are crucial, offering a wide range of applications, from advanced materials to nanotechnology. This study investigates the sequential fractionation of KL in acetic acid (HOAc), a green solvent, to produce lignin nanoparticles (LNP). KL fractions were obtained at HOAc concentrations ranging from 10 % to 60 %. The most promising fractions, namely KL_30%, KL_40%, KL_50%, and the final residue (FR), were selected based on high extraction yield and distinct molar masses (M_w) for LNP preparation using the antisolvent precipitation method. The study explores the influence of M_w, chemical composition, and functionality of LNPs on their properties. The results demonstrated an inverse correlation between M_w and the hydrodynamic diameter of LNPs, where lower M_w fractions (KL_30% and KL_40%) resulted in larger particle sizes, while KL_50%, despite its higher M_w, produced the smallest LNPs. This finding reinforces the idea that M_w alone does not dictate particle size, and the chemical functionality of lignin - particularly the balance between phenolic and aliphatic groups - plays a crucial role in determining aggregation behavior, surface charge, and colloidal stability. A deeper understanding of these structure-function relationships opens new opportunities for optimizing lignin fractionation and tailoring LNPs for specific applications in nanotechnology and advanced materials.