Connection between dielectric constant and total number of hydrogen-bond groups per cation–anion pair in ionic liquids

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This paper reports a quantitative connection between the dielectric constant and calculated total number of hydrogen-bond groups per cation–anion pair for 44 ionic liquids, including 15 ionic liquids with high dielectric constants exceeding 20. The total number of hydrogen-bond groups per cation–anion pair was equal to the addition of the hydrogen-bond group number per cation and per anion. The methylene group could reduce the group number capable of forming hydrogen bonds, and one methylene group caused a decrease of 1 hydrogen-bond number. Regardless of whether a liquid was ionic or non-ionic liquid, approximately linear relationship were observed between dielectric constant and total hydrogen-bond number per molecule. For ionic liquids, an increase of 1 in the total number of hydrogen-bond groups per cation–anion pair resulted in an increase of approximately 8.5 in the dielectric constant. For non-ionic liquids, the value was approximately 21.9. The average molar density of nine hydrogen-bonded ionic liquids with high dielectric constants was 9.3 × 103 mol/m3, while the average molar density of eight non-ionic liquids with hydrogen-bonds was 21.1 × 103 mol/m3. The 21.1 molar density coefficient of the non-ionic liquids was almost equal to the 21.9 coefficient in the dielectric constant equation, and the 9.3 molar density coefficient of the ionic liquids was very close to the 8.5 coefficient in its dielectric constant equation. This result indicated that the effect mechanisms of the intermolecular hydrogen-bond structure on dielectric constant were similar for the two types of liquids.