Density, Viscosity, Electrical Conductivity, and Specific Heat Capacity of Dicationic IL-Based Deep Eutectic Solvents

In this study, six pure deep eutectic solvents (DESs) were prepared using dicationic ionic liquids ([C2(MIM)2Br2] or [C4(MIM)2Br2]) as hydrogen bond acceptors and ethylene glycol (EG) as the hydrogen bond donor, with HBA/HBD molar ratios of 1:4, 1:5, and 1:6. The melting points of the synthesized ILs, DESs, and EG were determined. The density, viscosity, electrical conductivity, and specific heat capacity of these DESs were measured over the temperature range of 288.15∼323.15 K, and the thermal expansion coefficients were derived. Results show that the density decreases with temperature, while the viscosity, electrical conductivity, and specific heat capacity increase. The density and specific heat capacity correlate well with the empirical models, and the Vogel–Fulcher–Tammann equation accurately describes the viscosity and electrical conductivity behavior. With an increase of the HBD/HBA molar ratio, the density and viscosity decrease, while the electrical conductivity and specific heat capacity increase. Due to the interplay between chain length and molecular flexibility, the density and viscosity follow the trend of [C2(MIM)2Br2]/EG < [C4(MIM)2Br2]/EG, and the electrical conductivity and specific heat capacity follow the trend of [C4(MIM)2Br2]/EG < [C2(MIM)2Br2]/EG. These properties were compared with those of [C3(MIM)2Br2]/EG. This study supports the potential application of dicationic IL-based DESs in extraction.