Dicationic imidazolium-based dicarboxylate ionic liquids: Thermophysical properties and solubility

Dicationic ionic liquids have been attracting attention due to their high thermal storage density, which is attributed to an increase in the number of cations. However, there is no information about the effect that the dianionic structure has on the thermophysical properties of dicationic ionic liquids. Thus in this study, imidazolium-based dicationic dicarboxylate ionic liquids (Cn (MIM)2[Cm(CO2)2], n = 4, 6, 8, 10 and m = 0, 1, 2, 3, 4, 5) were synthesized and their thermal properties were determined. The results showed that the dicarboxylate's spacer length has no effect on the melting point; however, a trend in which the heat of fusion of ionic liquids increases with the increase in the spacer chain length of the dication and dianions was observed. Moreover, dicationic dicarboxylate ionic liquids have a greater heat capacity than monocationic ionic liquids, but a smaller heat capacity than analogous dicationic ionic liquids with bromine. Our results also showed that the anion has a strong effect on the heat capacity of dicationic ionic liquids. In terms of thermal storage, [C6(MIM)2][C2] and [C6(MIM)2][C5] were the most promising ionic liquids, because they have thermal storage higher than monocationic ILs and mineral oil and compared with analogues ILs with bromine, they are potentially less toxic. All of the ionic liquids were soluble in protic polar solvents and DMSO, and were insoluble in aprotic and nonpolar–polar solvents. The study and determination of these properties is extremely important for future applications.