Microscopic Study of Ionic Liquid-H2O Systems: Alkyl-Group Dependence of 1-Alkyl-3-Methylimidazolium Cation

We have investigated the microscopic aspects for ionic liquid-H2O systems by Raman and IR spectroscopies. Ionic liquids studied here are a series of 1-alkyl-3-methylimidazolium cations (alkyl groups: ethyl, butyl, hexyl, octyl, and decyl groups) with the anions of tetrafluoroborate and bis(trifluoromethanesulfonyl)amide. Polarities of the ionic liquid-H2O systems have also been measured by means of a solvatochromic dye, betaine 33. Vibrational bands of the anions shift to the higher frequency with the higher water content. We have also found that the magnitude of the frequency shifts of the anions' vibrational modes by adding water becomes smaller with the longer alkyl group of cation. From the comparison of the vibrational spectroscopic result with the result of the solvatochromic experiment, it has become clear that the frequency shift of the vibrational modes of anions almost correlates with the polarity. On the other hand, the feature of the vibrational band of water stretching mode is not really changed among the cations with the different alkyl groups. This evidence implies that the water aggregations localize at ionic regions and the water state does not really depend on the alkyl group of cation.