Ion Pair Formation of Alkylimidazolium Ionic Liquids in Dichloromethane

Abstract

The 1:1 ion pair formation constants (K IP 0 ) of 1-alkyl-3-methylimidazolium ([RMeIm] + ; R ) butyl, hexyl, and octyl) and 1-butyl-2,3-dimethylimidazolium ([BuMe 2 Im] + ) ions with tetrafluoroborate ([BF 4 ] -), hexafluorophosphate ([PF 6 ] -), bis(trifluoromethanesulfonyl)amide ([NTf 2 ] -), and 2,4,6-trinitrophenolate (picrate, [Pic] -) ions have been determined conductometrically in dichloromethane at 25 °C. The K IP 0 determinations have also been made for symmetric tetraalkylammonium ions ([R 4 N] + ; R ) methyl, ethyl, propyl, and butyl) for comparison. For a given anion, the K IP 0 value of the [RMeIm] + salt is almost independent of the length of the alkyl chain (R), whereas that of the [R 4 N] + salt decreases with increasing alkyl chain length. Such a difference in the alkyl chain length dependence of the ion pair formation ability can be explained on the basis of the structures of the ion pairs calculated by density functional theory. The K IP 0 values of [BuMeIm] + , [BuMe 2 Im] + , and [Et 4 N] + , which are similar in the van der Waals volume, are in the order of [BuMeIm] + . [BuMe 2 Im] + ≈ [Et 4 N] + , showing that the C2-H atom on the imidazolium ring makes an important contribution to the strong ion pair formation ability of [RMeIm] + . For a given cation, the K IP 0 value is generally smaller for the larger anion, i.e., [BF 4 ]g [PF 6 ]g [NTf 2 ] -> [Pic]for [Et 4 N] + and [BuMe 2 Im] + , and [BF 4 ] -> [PF 6 ]g [Pic]g [NTf 2 ]for [RMeIm] + .

Atom Substitution Effects in Ionic Liquids: A Microscopic View by Femtosecond Raman-Induced Kerr Effect Spectroscopy

“…As seen in Table 1, we can confirm that the molar volume in the ILs becomes larger as the cation and anion become heavier. Although the self-diffusion, interionic interaction, and ion-pair formation do not always correlate to the ionic volume (Tsuzuki et al, 2005;Katsuta et al, 2008), it is suggested that discussing the interionic interaction with the effect of the ion volume is appropriate when comparing the present ions because the structures of the ions are relatively less influenced by a heavy atom substitution compared to the case in which a whole ion is exchanged. Polarizability is another factor responsible for the intermolecular (or interionic) interaction.…”

Ionic association and conductance of ionic liquids in dichloromethane at temperatures from 278.15 to 303.15 K

“…It results from the fact that the relative permittivity of dichloromethane is low, its ionization properties are relatively weak, and electrolytes dissolved in DCM are strongly associated [11][12][13][14][15]. Therefore, it is necessary to use the extremely low concentrations of the electrolyte.…”

Distribution of 1-Alkyl-3-methylimidazolium Ions and Their Ion Pairs between Dichloromethane and Water

“…8 Water was deionized with a Milli-Q Labo system (Millipore) after distillation. Dichloromethane (Kanto Chemical, GR grade) was washed with deionized water for three times.…”

“…The cation dependence of the ion-pair formation constant in dichloromethane reflects that of the direct cation-anion interaction in the gas phase. 8,9 It can therefore be concluded that the direct cation-anion interaction is also a major factor determining the larger aqueous ion-pair formation constants for RMeIm + compared to those for R4N + .…”

“…4 The results show that the ionpair formation constants in water are considerably different in magnitude and tendency from those in dichloromethane. 8 The aqueous ion-pair formation is expected to be strongly affected by the hydration of ions and ion pairs. The distribution constants of solutes between a low or non-polar organic solvent and water provide useful information on the solute-water interaction.…”