Understanding Lignin Dissolution with Urea and the Formation of a Lignin Nano-Aggregate: A Multiscale Approach

Abstract

This study employs a combined computational and experimental approach to elucidate the mechanisms governing the interaction between lignin and urea, impacting lignin dissolution and subsequent aggregation behavior. Molecular dynamics (MD) simulations reveal how the urea concentration and temperature influence lignin conformation and interactions. Higher urea concentrations and temperatures promote lignin dispersion by disrupting intramolecular interactions and enhancing solvation. Density functional theory (DFT) calculations quantitatively assess the interaction energy between lignin and urea, supporting the findings from MD simulations. Anti-solvent precipitation demonstrates that increasing the urea concentration hinders the self-assembly of lignin nanoclusters. The findings provide valuable insights for optimizing lignin biorefinery processes by tailoring the urea concentration and temperature for efficient extraction and dispersion. Understanding the influence of urea on lignin behavior opens up avenues for designing novel lignin-based materials with tailored properties. This study highlights the potential for the synergetic application of MD simulations and DFT calculations to unravel complex material interactions at the atomic level.

Keywords: lignin conformation; lignin molecular dynamics; lignin nanoparticles; self-assembly of lignin.

Figure 1

(a) Schematic 2D structure of linear corncob lignin model. Snapshots of 4 lignin molecules (each color represents a lignin molecule, Licorice model) dissolving in (b) water or (c) 10, (d) 20, (e) 30, (f) 40, or (g) 50 wt% urea solution at 353 K. (h) Solvent’s accessible surface area, (i) lifetime, and (j) number of hydrogen bonds.

Figure 2

Inter-/intramolecular interaction of lignin dissolved in different solvent systems, and interaction energy between lignin and different solvent molecules.

Figure 3

Density contour images of benzene ring–ring distance (between the mass center of all interacting (within 6 Angstrom) pairs of rings) versus benzene ring–ring stacking angle within lignin in different urea concentration systems at 353 K after 100 ns.

Figure 4

Snapshots after 100 ns of molecular dynamics simulation of four lignin molecule clusters dissolving in distinct temperatures and concentrations of urea aqueous solution (four colors represent four molecules of lignin model, Licorice model, with implicit urea aqueous solvent).

Figure 5

Inter-/intramolecular interaction of lignin dissolved in different solvent systems, and the interaction energy between lignin and different solvent molecules.

Figure 6

Density contour images of benzene ring–ring distance (between the mass centers of all interacting (within 6 Angstrom) pairs of rings) versus benzene ring–ring stacking angle within lignin in water, for the 30 and 50 wt% urea concentration systems at 25, 40, 60, 80, and 100 °C after 100 ns.

Figure 7

(a) Hydrodynamic diameter of lignin nano-aggregate dispersed in water from the lignin–urea aqueous solution. (b–f) AFM images of CLNPs.

Figure 8

Theoretical configurations of the lignin dimer model with (a,d) 1, (b,e) 2, and (c,f) 3 urea molecules representing distinct concentration solvent systems. (a–c) Iso-surface (value = 0.5) and (d–f) scatter graph of reduced density gradient (RDG) of GGE configurations in the implicit water solvent (GGE: Licorice model, purple; Urea: Licorice model, green; black box region: hydrogen bonding).

Figure 9

(a–c) Iso-surface (value = 0.05) and (d–f) scatter graph of independent gradient model (IGM) based non-covalent interaction between GGE and simulated urea aqueous solution (including 2 urea and 10 water molecules). Interactions between (a) GGE and solution, (b) GGE and urea molecules, and (c) GGE and only water molecules (Licorice model, C—cyan, H—white, O—red, N—blue).