Development of low melting ionic liquid compositions using mixtures of imidazolium and pyrazolium ionic liquids

Abstract

Solid-liquid phase diagrams of binary mixtures of eight ionic liquids were determined. Results showed a high decrease in melting point compared with the lower melting component, when mixtures of imidazolium and pyrazolium tetrafluoroborate ionic liquids were used. Equimolar mixture of 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIBF4) and 1-ethyl-2-methylpyrazolium tetrafluoroborate (EMPBF4) has a melting point of - 16°C which is 29°C below the melting point of EMIBF 4. Its conductivity of 10.8 mS/cm at 25°C, is higher than those of the low melting ionic liquids 1-methyl-3-propylimidazolium tetrafluoroborate (MPIBF4, 5.9 mS/cm, mp -17°C) and 1-butyl-3-methylimidazolium tetrafluoroborate (BMIBF4, 3.5 mS/cm) produced by introducing larger alkyl groups onto the imidazolium ring. Results suggest that the difference in the distribution of alkyl groups on the imidazolium and pyrazolium rings plays a significant role in the observed melting point depression. Anion also affects the melting point depression; for the ionic liquids investigated, the tetrafluoroborate ion is superior to the bis(trifluoromethylsulfonyl)imide ion. Melting points of ionic liquids containing small symmetric cations such as 1,3-dimethylimidazolium and 1,2-dimethylpyrazolium, can also be decreased by the use of mixtures. Therefore, lower melting binary ionic liquids with better properties can be produced by the use of mixtures of imidazolium and pyrazolium ionic liquids, when compared with the low melting ionic liquids (single salts) produced by introducing large alkyl groups onto the cation.