Deep eutectic solvents on a tightrope: balancing the entropy and enthalpy of mixing

The large melting point depressions characterising deep eutectic solvents (DESs) are related to negative deviations from ideal mixing behaviour characterised by the excess Gibbs energy. Favourable excess Gibbs energies result from a balance between the excess entropy and enthalpy of mixing, which was experimentally determined for three choline chloride (ChCl) based mixtures using calorimetry. While the excess Gibbs energy of H₂O + ChCl is enthalpy dominated, those of ethylene glycol (EG) + ChCl and 1,3-propanediol + ChCl are entropy dominated. Molecular dynamics simulations using polarisable force-fields show intermolecular hydrogen bonds between DES constituents for H₂O + ChCl and EG + ChCl. Hence, inter-species hydrogen bonding does not guarantee enthalpy-dominated melting point depressions. We suggest future research to focus on tuning the entropy-enthalpy balance via the chemical nature of the DES constituents.