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

TL;DR

This paper explores how lignin's structure changes in different solvents using neutron scattering, revealing insights for sustainable material development.

Contribution

The study provides molecular-level insights into lignin conformation in organic and deep eutectic solvents using small-angle neutron scattering.

Findings

Lignin in THF shows loosely associated conformations, with Indulin AT swelling and organosolv lignin elongating.

DES-solvated lignin adopts dense cylindrical structures with resistance to D2O perturbation.

Cocoa bean lignin in diol-based DES exhibits fractal-like morphology sensitive to D2O and temperature.

Abstract

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.

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