Background: The transition toward biorefineries requires efficient, non-toxic solvents for lignocellulosic biomass fractionation. Deep eutectic solvents (DES), particularly those based on choline chloride (ChCl), have emerged as promising green alternatives to volatile organic compounds and ionic liquids due to their low toxicity, biodegradability, and tunable physicochemical properties. However, the fundamental relationship between DES hydrogen bonding networks, thermophysical behavior, and biomass dissolution efficiency remains poorly quantified.

Objective: This study aims to systematically characterize the density, viscosity, refractive index, and electrical conductivity of three ChCl-based DES systems across a temperature range of 293–353 K, and to correlate these properties with molecular interactions via FTIR spectroscopy and COSMO-RS modeling to predict lignin extraction performance.

Method: Three DES were prepared by mixing ChCl with urea (U), glycerol (Gly), and lactic acid (LA) at optimal molar ratios. Thermophysical properties were measured using digital densitometry, rotational viscometry, Abbe refractometry, and conductometry. FTIR spectra were analyzed to identify H-bond donor-acceptor interactions. Computational screening was performed using COSMO-RS to estimate activity coefficients and excess enthalpies.

Key Results: All DES exhibited significant melting point depression relative to individual components. Viscosity followed the order ChCl-U > ChCl-Gly > ChCl-LA, ranging from 482 mPa·s to 38 mPa·s at 303 K. Density decreased linearly with temperature (R² > 0.998). FTIR revealed strong H-bonding between Cl⁻ anions and HBD hydroxyl/carboxyl protons, with ChCl-LA showing the strongest interaction network. COSMO-RS predicted superior lignin affinity for ChCl-LA, correlating with experimental delignification yields of 78% vs. 42% for ChCl-U.