Partition of Substituted Benzenes between Hydrophobic Ionic Liquids and Water: Evaluation of Interactions between Substituents and Ionic Liquids

The partition coefficients (P) of benzene and its various derivatives between hydrophobic ionic liquids and water were determined at 298.2 K. The ionic liquids used are 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)amide ([BuMeIm][NTf2]), 1-butyl-1-methylpyrrolidinium bis(trifluoromethanesulfonyl)amide ([BuMePyrr][NTf2]), and 1-butyl-3-methylimidazolium hexafluorophosphate ([BuMeIm][PF6]). For a given solute, the P value in the [BuMePyrr][NTf2]/water system is nearly equal to that in the [BuMeIm][NTf2]/water system. The P values of the alkylated benzenes and the chlorinated ones are slightly smaller in the [BuMeIm][PF6]/water system than in the [BuMePyrr][NTf2]/water system, whereas for the derivatives having polar substituents the P values in both the systems are comparable to each other. As compared to 1-octanol, all of the ionic liquids exhibit a higher extraction ability for aniline, benzonitrile, acetophenone, nitrobenzene, and anisole and a lower one for benzoic acid, phenol, and benzyl alcohol. The P values were analyzed by using the regular solution theory, and the contributions of solubility parameters, molar volumes, and solute-solvent specific interactions were evaluated. Each ionic liquid exhibits both the donor and the acceptor properties in the interaction with the polar substituents; the strength of the interaction decreases in the following substituent sequence: -COOH >= -OH > -OH (aliphatic) > -NH2 > -CN > -COCH3 >= -NO2 > -OCH3 > -Cl.