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 BF 4 -, PF 6 -, bis(trifluoromethylsulfonyl)imide (Tf 2 N - ), and picrate (Pic - ) anions has been investigated by capillary electrophoresis at 25 °C. The formation constants at infinite dilution ( K IP °) of the 1:1 ion pairs have been determined. For all the anions, the K IP ° 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 ° value is always BF 4 - < PF 6 - < Tf 2 N - . The Pic - anion has a larger K IP ° value with C 8 mim + than the other anions, whereas the K IP ° value of Pic − is smaller than or nearly equal to that of Tf 2 N − when the cation is C 4 mim + or C 6 mim +; the K IP ° value for Pic - varies most sensitively with the alkyl chain length of the cation. The K IP ° values of the picrate salts are compared with those of tetraalkylammonium picrates previously reported.