Deep eutectic solvents (DESs) are best defined as mixtures of ionic and non-ionic solid compounds with melting points much lower than the pure compounds. They are comprised of a hydrogen bond donor (HBD) - which is generally the organic salt - and a hydrogen bond acceptor (HBA). They are similar to ionic liquids in turns of their physiochemical properties but are cheaper and easier to make. One of their main tuning parameters is the hydrogen-bonded network since it affects their resulting physiochemical properties. In an effort to characterize the hydrogen-bonded network of various classes of DESs we used multi-(¹H, ²H, ¹³C and ¹⁷O) Nuclear Magnetic Resonance (NMR) relaxometry and diffusometry techniques as functions of temperature, frequency and pressure. We studied the ion-ion, ion-solvent and solvent-solvent interactions by focusing on the incorporated dilute probe solute molecules which includes isotopically enriched (¹³C and ¹⁷O) and deuterated (²H) benzene, toluene and carbon dioxide. These measurements were compliment by density, viscosity and ionic conductivity, thereby providing more insight into the effect of intermolecular interactions on the resulting dynamics.

Results show that in spite of their high viscosities, the ionic conductivity values (see Fig. 1a) for the DESs were comparable to those obtained for selected ionic liquids comprised of the common 1-alkyl-3-methylimidazolium (C_nMIM⁺) cation and bis(trifluoromethylsulfonyl)amide, bis(pentafluoroethylysulfonyl)amide or bis(fluorosulfonyl)amide anion, which are considered as some of the most conductive. Additionally, their Walden Product (see Fig. 1b) shows their classification as ‘subionic’, suggesting there is either significant cation-anion pairing, or their respective motions are correlated. These results and more will be discussed in detail during our presentation.

Figure 1. (a) Variable temperature ionic conductivity data and (b) Walden Products for selected DESs.