The eutectic point in choline chloride and ethylene glycol mixtures

The choline chloride (ChCl) and ethylene glycol (EG) mixture has become established as a paradigmatic deep eutectic solvent (DES). Here, we present measurements of the phase behaviour of this mixture over a wide composition range, and provide an extended phase diagram. The eutectic point was found to lie at −28 ± 1 °C and 0.01 < x_ChCl < 0.02, sharply contrasting with the previously expected eutectic composition. Our observations confirm that the eutectic temperature is not ‘deep’ compared to the ideal solution theory prediction. We also observe a cold-crystallisation at T = −65 °C which may have been formerly misinterpreted as the eutectic point.