Hydrotropy and Cosolvency in Lignin Solubilization with Deep Eutectic Solvents

The mechanisms responsible for the good solubility of lignin in aqueous solutions of deep eutectic solvents are here investigated using both monomer model compounds and technical lignins (kraft and organosolv). The results show the ability of deep eutectic solvents to act either as hydrotropes or cosolvents, enhancing the solubility of poorly soluble solutes (technical lignins or their monomers). Hydrotropy was shown to lead to a remarkable enhancement of organosolv lignin solubility of 474.7 ± 2.7 times using an aqueous solution of propionic acid:urea (2:1), while for the systems where the solubilization mechanism was cosolvency, the best solubility enhancement was 194.2 ± 4.1 times using ethylene glycol:tetrabutylphosphonium chloride (2:1). The solubility of kraft lignin was also enhanced using an aqueous solution of propionic acid:urea (2:1) (228.3 ± 8.2 times) and ethylene glycol:tetraethylammonium chloride (2:1) (163.0 ± 16.1 times) by the same mechanisms.