ON THE MECHANISM OF UNEXPECTED POTENTIOMETRIC RESPONSE TO NEUTRAL PHENOLS BY LIQUID MEMBRANES BASED ON QUATERNARY AMMONIUM-SALTS - SYSTEMATIC EXPERIMENTAL AND THEORETICAL APPROACHES

Changes in membrane potentials (potentiometric responses) induced by u ndissociated, neutral phenols (ArOH), though unexpected in terms of th e conventional response mechanism for charged species, were systematic ally investigated using poly(vinyl chloride) (PVC) matrix liquid membr anes containing quaternary ammonium and phosphonium salts (Q(+)X(-)) a s the sensory elements. The observed anionic responses were characteri stic in that they accompanied large response slopes (-65 to -200 mV-de cade(-1)) and selectivities reflecting the acidity and lipophilicity o f phenolic compounds. In two phase systems, the extracted and complexe d ArOH (Q(+)X(-) + ArOH --> Q(+)X(-). ArOH) further underwent proton d issociation (Q(+)X(-). ArOH-->Q(+)ArO(-) + HX) with concomitant ejecti on 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 memb rane interface, was proposed. A theoretical treatment based on the abo ve model reproduced the potentiometric response behaviors for undissoc iated phenols. This model was further supported by optical second harm onic generation (SHG), which enabled direct observation of the process es occurring at the interface of a liquid membrane and an aqueous solu tion.