A Systematic Study on the Complexation of Quaternary Ammonium Salts and Neutral Phenols

抄録

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 (K_s) 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 pK_a and higher log P_oct, 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 K_s 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.