Partitioning Mechanism of Lignin in Biphasic Solvent Systems: Combined Experiments with Theoretical Calculations

Biphasic solvent pretreatment selectively partitions lignocellulosic compositions between immiscible phases, facilitating fractionation and valorization. However, the molecular determinants of lignin partitioning remain unclear. Herein, lignin partitioning in n-pentanol/H2O and methyl isobutyl ketone (MIBK)/H2O biphasic systems was investigated across pH 1–13 using combined experiments and molecular dynamics (MD) simulations. Higher proton activity (lower pH) promoted lignin transfer into the organic phase, whereas alkaline conditions deprotonated lignin and drove its accumulation in the aqueous phase. In the n-pentanol/H2O system, lignin fragments enriched in syringyl (S) units preferentially partitioned into the n-pentanol phase, followed by guaiacyl (G) and p-hydroxyphenyl (H) units. MD simulations showed that the total interaction energies between lignin units and the n-pentanol/H2O system were stronger than those in the MIBK/H2O system, especially for S and G units, while the difference for H units was marginal. van der Waals forces dominated the lignin-n-pentanol interactions, with strengths in the order S > G > H. n-Pentanol also exhibited stronger affinity for ferulate (FA) than for p-coumarate (PCA). This study elucidated lignin partitioning mechanisms in biphasic solvents, providing a theoretical basis for optimizing biphasic pretreatment and tuning lignin structure.