Analytical Sciences

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

Shoichi KATSUTA, Naoko YAMAGUCHI, Ryuji OGAWA, Yoshihiro KUDO, and Yasuyuki TAKEDA

Graduate School of Science, Chiba University, 1-33 Yayoi-cho, Inage, Chiba 263-8522, Japan

The distribution behavior of the salts of a series of 1-alkyl-3-methylimidazolium cations (RMeIm⁺; R = butyl, hexyl, and octyl) with tetrafluoroborate (BF₄^-), hexafluorophosphate (PF₆^-), bis(trifluoromethanesulfonyl)amide (NTf₂^-), and 2,4,6-trinitrophenolate (Pic^-) anions has been investigated in a dichloromethane-water system at 25°C. The distribution constants (K_D) of the ion pairs and the transfer activity coefficients (^oγ^w) of the single ions were determined. For the ion pairs with a given anion, the log K_D value increases linearly with the number of methylene groups (N_CH2) in the cation, which can be explained by using the regular solution theory. A similar relationship was observed between log ^oγ^w and N_CH2 for the free RMeIm⁺ ions, and the result was discussed by decomposing the transfer activity coefficient into the Born-type electrostatic contribution and the non-electrostatic one. For the free anions and their ion pairs with a given cation, the ^oγ^w and K_D values increase with increasing molar volume of the anion: i.e., BF₄^- < PF₆^- < Pic^- < NTf₂^-. The features of the RMeIm⁺ salts in the liquid-liquid distribution and the ion-pair formation in water are also discussed by comparing the present results with those of tetraalkylammonium salts previously reported.

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