Ion pair formation of 1-alkyl-3-methylimidazolium salts in water

Ion pair formation in water of a series of 1-alkyl-3-methylimidazolium cations (1-butyl-, C(4)mim(+); 1-hexyl-, C(6)mim(+); 1-octyl-, C(8)mim(+)) with BF4-, PF6-, bis(trifluoromethylsulfonyl)imide (Tf2N-), and picrate (Pic(-)) anions has been investigated by capillary electrophoresis at 25 degrees C. The formation constants at infinite dilution (K-IP degrees) of the 1:1 ion pairs have been determined. For all the anions, the K-IP degrees value increases with increasing alkyl chain length of the cation (i.e., C(4)mim(+) < C(6)mim(+) < C(8)mim(+)). Except for Pic(-), the anion dependence of the K-IP degrees value is always BF4- < PF6- < Tf2N-. The Pic(-) anion has a larger K-IP degrees value with C(8)mim(+) than the other anions, whereas the K-IP degrees value of Pic(-) is smaller than or nearly equal to that of Tf2N- when the cation is C(4)mim(+) or C(6)mim(+); the K-IP degrees value for Pic(-) varies most sensitively with the alkyl chain length of the cation. The K-IP degrees values of the picrate salts are compared with those of tetraalkylammonium picrates previously reported.