Abstract
Complexation of quaternary ammonium salts (Q+X-) and undissociated, neutral phenols (ArOH) in homogeneous organic solutions was systematically investigated to afford a reliable quantitative basis for a new model of anionic potentiometric response to neutral phenols, displayed by organic liquid membranes based on Q+X-. IR, 1H- and 13C-NMR, and UV studies indicated complexation of Q+X- and ArOH in 1 : 1 stoichiometry in nonpolar organic solvents such as benzene and chloroform. The stability constant (Ks) of the complex was affected by the acidity and lipophilicity of the ArOH component as well as the size of X- of the Q+X- component; stronger complexes were formed by ArOH with a lower pKa and higher log Poct, as well as by Q+X- with a smaller ionic radius for X-. These results suggest that the major driving force for complexation is hydrogen bonding interactions between X- and the phenolic OH. The Ks values were found to be parallel with the magnitudes of anionic potentiometric responses by membranes based on Q+X-, indicating that the complexation of ArOH by Q+X- plays an important role in the anionic potentiometric responses. The significance of proton dissociation from the Q+X-·ArOH complex was also discussed.
