Molecular insights into pyrolytic lignin solubility and Bio-Oil phase stability in representative organic solvents for sustainable Bio-Energy Applications.

Phase separation in fast-pyrolysis bio-oil (FPBO) arises partly from the limited solubility of pyrolytic lignin in the organic-solvent (OS) fraction and in water. Using molecular dynamics (MD), we examine lignin solubility in five representative OS, acetic acid (AA), phenol (PH), methanol (ME), hydroxyacetone (HA), furfural (FU), and their binary mixtures. In non-aqueous systems, lignin solubility follows ME > AA ≈ PH > HA > FU, reflecting ME's rapid diffusion and strong hydrogen bonding (H-bonding) capability. Under aqueous competition, the order becomes PH > AA ≈ ME > HA > FU; PH maintains π-π interaction with lignin, while AA preserves favorable polarity matching. Binary mixtures enriched in PH, AA or ME possess good lignin solubility, whereas FU/HA blends underperform, especially with water. Overall, lignin solubility is heavily influenced by polarity, H-bonding, and π-π interaction. These molecular insights provide mechanistic guidelines for designing bio-oil formulations that minimize phase separation.