The subtle but substantial distinction between ammonium- and phosphonium-based deep eutectic solvents

In the search for greener solvents to replace the commercially organic volatile solvents that caused negative impact on both human beings and the environment, researchers investigated a number of potential alternatives, e.g. ionic liquids (ILs), supercritical CO₂, deep eutectic solvents (DESs), etc. DESs are mixtures of two or more compounds that due to interaction forces have melting points less than that of any of their constituents. With the wise selection of their constituents, DESs can have attractive physiochemical properties, e.g. low volatility, high thermal stability, moderate electrical conductivity, extremely low toxicity, and biodegradability. In addition, the DESs' properties could be tuned by selecting constituents with suitable chemical structures and by changing the molar ratio of the constituents. In this review, we investigated the main differences between two important classes of DESs, namely, ammonium and phosphonium based DESs. We focused our interest on the effect of both DES structure on physiochemical properties, such as density, viscosity, surface tension, electrical conductivity, freezing temperature and thermal stability, as well as their respective performance in selected applications. Mainly, it was found that in most cases the ammonium based DESs have more favorable properties compared to their phosphonium counterparts, e.g. lower melting point and lower viscosity. However, there was no clear trend for the performance of these DESs in the applications reported in the literature. Finally, we discussed the main challenges and limitations that must be taken into consideration before using DESs industrially.