Regulating Lignin Condensation and Extraction in Acidic Deep Eutectic Solvents through Controlled Hydration

Acidic deep eutectic solvents (DESs) facilitate lignocellulosic fractionation, yet their high viscosity and acidity often limit mass transfer and promote lignin condensation and byproduct formation, particularly for compositionally complex feedstocks. In this work, water was introduced into a p -toluenesulfonic acid/choline chloride (PTSA/ChCl) DES system to regulate lignin extraction and product properties during the fractionation of protein-rich brewer’s spent grains (BSG). At a water content of 20%, efficient lignin separation was achieved under mild conditions (80 °C, 45 min), with a lignin removal rate of 81.2% and a cellulose retention rate of 87.4%. Structural analyses show that the extracted lignin exhibits favorable structural integrity, with reduced condensation and limited solvent incorporation. Molecular dynamics simulations indicate that water restructures hydrogen-bonding interactions and suppresses dehydration-driven lignin condensation without compromising solubility. These results identify water as a key regulator in acidic DES systems, enabling efficient and selective lignin fractionation from compositionally complex biomass.