Prediction of solubility parameters of lignin and ionic liquids using multi-resolution simulation approaches

Abstract

The solubility parameter (SP) of a molecular species is a vital feature that indicates polarity and quantifies the ‘like-seeks-like’ principle, which is used in chemistry to screen solvents for dissolution. Recent studies have demonstrated that ionic liquids (ILs) and deep eutectic solvents (DESs) efficiently solubilize lignocellulosic biomass and promote enzymatic saccharification into sugars used for the production of biofuels and value-added chemicals. Understanding the solubility of plant biopolymers, particularly lignin, in ILs and DESs is critical for selecting candidate ILs and DESs for biomass pretreatment; however, experimentally measuring SPs is challenging. Thus, the present study investigates lignin dissolution mechanisms in IL/DES and prediction of the solubility parameters (Hildebrand and Hansen) of lignin, ILs, and DESs using multi-resolution simulation approaches. Solubility parameters of the studied compounds were predicted using molecular dynamics (MD) simulations, and the SP of lignin was determined to be 23–27 MPa^1/2, which was close to the polymeric lignin solubility parameter (24.3–25.5 MPa^1/2). The SPs of ILs namely [Ch][Lys], [Ch][Oct], and [Emim][Lys] were predicted to be ∼26 MPa^1/2, which is close to lignin's SPs and resulted in increased biomass delignification. The MD simulated SPs were validated by both the COSMO-RS model and experimental investigations, with the results showing a close agreement between the predicted and experimentally obtained SPs. In addition, the enthalpy of vaporization (ΔH_vap) of ILs/DESs was predicted based on the potential energy of the system, and the ΔH_vap of ILs/DESs was around 40–65 kcal mol⁻¹, which is 5–8 times higher than that of traditional organic solvents.