Exploring Lignin Conformation in Organic and Deep Eutectic Solvents Using Small-Angle Neutron Scattering

Lignin, a structurally intricate and heterogeneous phenolic biopolymer, holds considerable promise as a sustainable alternative to petrochemical-derived materials across diverse applications in the energy and materials sectors. However, precise lignin molecular weight and structure determination remains challenging due to its intrinsic tendency to aggregate in solution and the absence of chemically analogous polymer standards for chromatographic techniques. By employing small-angle neutron scattering, this study aims at precise measurement of lignin's polymeric conformation, aggregation behavior, and radius of gyration in organic gel permeation chromatography/NMR solvent, tetrahydrofuran (THF), and in an emerging class of solvent systems known as deep eutectic solvents (DES). These "designer" solvents, formed from tailored hydrogen bond donors and acceptors, are gaining importance for lignin extraction from biomass and analytical characterization. However, their influence on lignin conformation in solutions remains unexplored. Our study reveals that both organosolv and Indulin AT kraft lignin in THF exhibit loosely associated polymeric conformations. Upon D₂O addition, Indulin AT undergoes moderate swelling, suggestive of partial dissolution, while organosolv lignin undergoes substantial elongation with directional ordering, resulting in flexible rod-like structures. Lignin oil from a reductive catalytic fractionation process (RCF), in contrast, remains well-dispersed in THF and shows minimal structural change with solvent polarity modulation via D₂O addition. Indulin AT and organosolv lignin solvated in the choline chloride/oxalic acid/ethylene glycol DES adopt dense, cylindrical morphologies. These structures show moderate temperature sensitivity and notable resistance to D₂O-induced structural perturbation, highlighting strong lignin-DES interactions. Additionally, lignin extracted from cocoa bean shells using a diol-based DES and subsequently dissolved in the same solvent demonstrates a fractal-like morphology, which evolves with D₂O content and temperature, revealing a complex solvation landscape. These results offer molecular-level insight into lignin's solvent-dependent structural transitions, enabling more accurate molecular weight estimation and supporting optimization of lignin processing for high-performance biobased formulations and advanced materials.