Lignin Dissolution and Direct Ultrasmall-Lignin-Nanoparticle Formation in Acidic and Alkaline Deep Eutectic Solvents: A Molecular-Level Insight

Although deep eutectic solvents (DESs) have demonstrated significant potential in lignin processing, their influence on molecular stacking and conformational evolution during lignin dissolution and nanoparticle formation remains insufficiently understood. Here, we develop a green, straightforward, and single-step approach to produce self-assembled lignin nanoparticles (LNPs). The LNPs obtained using the acidic DES method exhibited a great size reduction, with an average size approximately one-ninth of that produced by conventional solvent-exchange methods. To gain mechanistic insights into the reconstruction, dissolution, and self-assembly of lignin in DESs, we integrate structural characterization with molecular dynamics simulations. Specifically, we simulate the dynamic behavior and configurational states of high-molar-mass lignin models (4,182 g mol⁻¹) in aqueous solvent systems. The results reveal the critical role of molecular structure, intra/intermolecular π–π interactions, stacked conformations, solvent-specific effects in determining the size and compactness of LNPs. Notably, the DES stabilizes lateral-shifted configurations, promoting the formation of small and compact LNPs. In contrast, the tetrahydrofuran/H₂O solvent system favors offset-stacked configurations and hydrophobic interactions, leading to larger, spherical LNPs. Overall, our findings offer new insights into the underlying mechanisms of LNP formation using DESs, demonstrating the possibility of regulating and controlling lignin assemblies through solvent parameters.