Functional groups in geminal imidazolium ionic compounds and their influence on thermo-physical properties

To enhance the thermal stability of ionic liquids (ILs) and increase the latent heat, the effect of functional groups for geminal dicationic ionic compounds were investigated and compared to that of monocationic ionic liquids (MILs) analogues. A series of new functionalized ionic compounds were synthesized by introducing vinyl group, cyanomethyl group, hydroxyl group, and carboxyl group to the imidazolium cation. The structures of these ionic compounds were confirmed by ¹H NMR, ¹³C NMR and FT-IR. The thermal stability was determined by TGA; the melting temperature, heat of fusion and heat capacity were investigated by DSC for the synthetic ionic compounds. The thermal analysis results indicate that the functionalized ionic compounds show excellent thermal stability. The decomposition temperature and melting temperature can be up to 593.17 K and 484.99 K, respectively, and the latent heat can reach 64.40 KJ·mol⁻¹. It's increased on average by 6.30%, 34.0% and 93.93%, respectively, as compared to alkyl chain ionic liquid (C₂mim)Br. In addition, the heat capacity and thermal storage density of the functionalized ionic compounds are higher than those of the MILs. These superior properties are attributed to the additional hydrogen bonds and electrostatic energy in the functionalized ionic compounds.