Data-Centric Development of Lignin Structure–Solubility Relationships in Deep Eutectic Solvents Using Molecular Simulations

Lignin is a natural source of aromatic chemicals with
significant
potential as an abundant, renewable feedstock for value-added products.
Deep eutectic solvents (DES)solvents composed of a hydrogen
bond donor (HBD) and acceptor (HBA) in varying ratioshave
emerged as a highly tunable class of solvents for lignin solubilization.
However, the variety of possible DES compositions and limited molecular-scale
understanding of lignin solubility makes solvent selection a challenge
without laborious trial-and-error experimentation. To address these
challenges, we use classical molecular dynamics (MD) simulations to
study the interactions of lignin model compounds with various DES–water
systems. Quantitative parameters (descriptors) were calculated by
postprocessing the MD results and used to train a regression model
that predicts experimentally determined solubilities of lignin model
compounds. This approach revealed that the most important descriptors
of solubility are the system temperature, solute hydrophilicity, and
metrics quantifying hydrogen bonding. Maximizing the interactions
between solute–HBD (hydrophobic group), water–HBD (hydrophilic
group), and water–HBA molecules led to the highest model compound
solubility. Our results support a hydrotropic mechanism in which extensive
DES–water hydrogen bonding and favorable HBD interactions with
the solute promote high solubility. We applied the regression model
derived using model compounds to predict the solubility of representative
lignin oligomers. The model predicted lignin oligomers’ solubilities
in good agreement with experiments, indicating that the simulations
of model compounds can be extended to predict the solubility of larger
lignin compounds across a range of solvent compositions and temperatures.
These findings provide new molecular-scale insight into lignin solubilization
mechanisms and a new method for computationally screening potential
solvent systems for lignin valorization.