On the Mechanism of Unexpected Potentiometric Response to Neutral Phenols by Liquid Membranes Based on Quaternary Ammonium SaltsSystematic Experimental and Theoretical Approaches

Changes in membrane potentials (potentiometric responses) induced by undissociated, neutral phenols (ArOH), though unexpected in terms of the conventional response mechanism for charged species, were systematically investigated using poly(vinyl chloride) (PVC) matrix liquid membranes containing quaternary ammonium and phosphonium salts (Q+X-) as the sensory elements. The observed anionic responses were characteristic in that they accompanied large response slopes (−65 to −200 mV decade-1) and selectivities reflecting the acidity and lipophilicity of phenolic compounds. In two phase systems, the extracted and complexed ArOH (Q+X- + ArOH → Q+X-·ArOH) further underwent proton dissociation (Q+X-·ArOH → Q+ArO- + HX) with concomitant ejection of HX to the aqueous phase. Based on these experimental results, a model for potentiometric responses to neutral phenols, which explains the anionic responses on the basis of a decrease in the amount of the cationic and anionic species that are charge-separated across the membrane interface, was proposed. A theoretical treatment based on the above model reproduced the potentiometric response behaviors for undissociated phenols. This model was further supported by optical second harmonic generation (SHG), which enabled direct observation of the processes occurring at the interface of a liquid membrane and an aqueous solution.