The physico-chemical and thermodynamic properties of the choline chloride-based deep eutectic solvents

This paper reports the physical (density, dynamic viscosity, electrical conductivity and refractive index) and thermodynamic (thermal expansion coefficient, molecular volume, lattice energy and heat capacity) properties of choline chloride (ChCl):propylene glycol, ChCl:1,3-dimethylurea and ChCl:thiourea deep eutectic solvents (DESs) (1:2 molar ratio) at atmospheric pressure as a function of temperature over the range of 293.15-363.15 K. Their properties were also compared with those of some already characterized ChCl-based DESs, namely ChCl:ethylene glycol, ChCl:glycerol and ChCl:urea (1:2 molar ratio). Density, viscosity and refractive index of all DESs decrease with increasing temperature while electrical conductivity increases. Viscosity and conductivity of the tested DESs were fitted by both Arrhenius and Vogel-Tamman-Fulcher equations. The molar enthalpy, entropy and Gibbs energy of activation, determined using the Eyring theory, demonstrates the interactional factor as predominant over the structural factor for all DES systems. The fractional Walden rule, used to correlate molar conductivity and viscosity, showed an excellent linear behavior. It was shown that ChCl:propylene glycol DES had properties similar to ChCl:ethylene glycol and ChCl:glycerol DESs. However, the properties (density, viscosity and electrical conductivity) of ChCl:1,3-dimethylurea and ChCl:thiourea DESs were inferior to those of the ChCl:urea DES.