A comparative study of thermophysical properties between choline chloride-based deep eutectic solvents and imidazolium-based ionic liquids

Abstract

In respond to the queries of deep eutectic solvent (DES) as a better analogue over ionic liquid (IL), this research aims in contributing a comparative study on the thermophysical properties of choline chloride-based DESs and imidazolium-based ILs to provide a general understanding on the similarities and differences of their properties. The thermal conductivity, viscosity and ionic conductivity of choline chloride-based DESs with glycerol (Gly) and ethylene glycol (EG) as hydrogen bond donors (HBDs) are characterized and compared with those of 1-ethyl-3-methylimidazolium chloride (EmimCl) and 1-hexyl-3-methylimidazolium chloride (HmimCl). It is observed that the thermal conductivity of DESs increases with salt: HBD molar ratio while that of ILs increases with alkyl chain length. Meanwhile, the viscosity of DESs is depending on the nature of HBD. DESs with glycerol as HBD show an incremental trend in viscosity with increasing molar ratio, while DESs with ethylene glycol as HBD depict an opposite trend. For ILs, longer alkyl chain length results in greater viscosity. The ionic conductivity of both DESs and ILs increase as their viscosity decrease. The effect of temperature is insignificant on the thermal conductivity of DESs and ILs. The viscosity of both DESs and ILs is reduced while their ionic conductivity is enhanced with increase of temperatures. When aqueous form of DESs and ILs is considered, water has greatly enhanced the thermal conductivity and ionic conductivity of DESs and ILs. However, the viscosity of DESs and ILs is drastically reduced with the presence of water content. In general, the thermophysical properties of both DESs and ILs are comparable, including in their aqueous forms. A prefeasibility study is conducted to further evaluate the potential and practicality of DESs and ILs in comparison with the conventional lubricant and battery electrolyte.