Partition Behavior of Chlorophenols and Nitrophenols between Hydrophobic Ionic Liquids and Water

The partition of various chlorinated and nitrated phenols between hydrophobic ionic liquids and aqueous solutions of different pH values was investigated at 298.2 K. Ionic liquids used are 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)amide ([BMIm][NTf2]), 1-butyl-1-methylpyrrolidinium bis(trifluoromethanesulfonyl)amide ([BMPyr][NTf2]), 1-butyl-3-methylimidazolium hexafluorophosphate ([BMIm][PF6]), 1-methyl-3-octylimidazolium bis(trifluoromethanesulfonyl)amide ([OMIm][NTf2]), and 1-methyl-3-octylimidazolium tetrafluoroborate ([OMIm][BF4]). Partition coefficients (PIL/W) of the electrically neutral form (ArOH) and the dissociated anionic form (ArO–) were determined. For a given phenol, the PIL/W(ArOH) value in the ionic liquid/water system is generally independent of the kind of the ionic liquid and smaller than that in the 1-octanol/water system. Exceptionally, the PIL/W(ArOH) value in the [OMIm][BF4]/water system is larger than those in the other ionic liquid/water systems and comparable to or greater than that in the 1-octanol/water system. The interaction energies of the ionic liquids with the phenolic hydroxyl group were evaluated. It is shown from the dependence of the interaction energy on the acid-dissociation constant of the phenol that the ionic liquid behaves as both an electron-pair donor and an acceptor like water rather than 1-octanol. The PIL/W(ArO–) value depends largely on the kind of ionic liquid, showing that an ionic liquid formed by a more hydrophobic cation and a more hydrophilic anion has a higher extractability for the phenolate anions. The substituent effects on PIL/W(ArO–) are also different from those on PIL/W(ArOH).