Abstract
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 (KD) of the ion pairs and the transfer activity coefficients (oγw) of the single ions were determined. For the ion pairs with a given anion, the log KD value increases linearly with the number of methylene groups (NCH2) in the cation, which can be explained by using the regular solution theory. A similar relationship was observed between log oγw and NCH2 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 oγw and KD 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.
