Various eutectic systems including eutectic metals, eutectic salts, and deep eutectic solvents have been proposed and applied in engineering, energy and environmental fields.

The development of effective solvents for lignin is of high importance. Herein, two types of novel deep eutectic solvents (DESs) were designed employing lactic acid and N-methylthiourea as hydrogen bond donors and four different quaternary ammonium salts as hydrogen bond acceptors. The designed DESs could efficiently dissolve different types of lignin, including dealkaline lignin (DAL), enzymatic hydrolysis lignin (EHL), sodium lignosulfonate (SL), and organosolv lignin (OL). The investigation of the mechanism revealed that the functional groups in the DESs significantly affected their ability to dissolve lignin, and a positive correlation was found between the solubility of lignin and the subtraction value of α and β (Kamlet–Taft empirical parameters) for the dissolution of EHL, DAL and OL. Additionally, the unique dissolution behaviour of SL was due to its salt-like structure.

Deep eutectic solvents are found to be highly hygroscopic when exposed to the atmosphere.

We report the synthesis of nanostructured Fe₃S₄ from deep eutectic solvents via a one-step solvothermal method. The as-obtained Fe₃S₄ catalyst was capable of electrochemically reducing N₂ to NH₃ under ambient conditions, and exhibits a high NH₃ yield (75.4 μg h⁻¹ mg⁻¹ _cat.) and faradaic efficiency (6.45%) at −0.4 V vs. a reversible hydrogen electrode.

DESs play a Janus role (chemical or physical) in the preparation of materials. The physical aspect includes solvating, exfoliating, dispersing and confining, while the chemical part includes reacting, composing, polymerizing and modifying.