Room-temperature decomposition of the ethaline deep eutectic solvent

TL;DR

This study reveals that the green solvent ethaline decomposes at room temperature into toxic compounds, challenging its environmental benefits and highlighting the need for careful solvent design.

Contribution

We experimentally and computationally analyze the decomposition mechanism of ethaline, providing insights into its instability and guiding future green solvent development.

Findings

Ethaline decomposes into toxic chloromethane and dimethylaminoethanol at room temperature.

Hydrogen bond fluctuations facilitate chloride's role in initiating decomposition.

Choline chloride is not recommended for green solvent applications due to its instability.

Abstract

Environmentally-benign, non-toxic electrolytes with combinatorial design spaces are excellent candidates for green solvents, green leaching agents, and carbon capture sources. Here, we examine one particular green solvent, ethaline, a 2:1 molar ratio of ethylene glycol and choline chloride. Despite its touted green credentials, we find partial decomposition of ethaline into toxic chloromethane and dimethylaminoethanol at room temperature, limiting its sustainable advantage. We experimentally characterize these decomposition products and computationally develop a general, quantum chemically-accurate workflow to understand decomposition. We find that fluctuations of the hydrogen bonds bind chloride near reaction sites, initiating the reaction between choline cations and chloride anions. In summary, in the design of green solvents, we do not recommend the use of choline chloride due to its susceptibility to undergo decomposition in strongly hydrogen-bound mixtures.