ION-SELECTIVE MONOLAYER MEMBRANES BASED UPON SELF-ASSEMBLING TETRADENTATE LIGAND MONOLAYERS ON GOLD ELECTRODES .2. EFFECT OF APPLIED POTENTIAL ON ION BINDING

The role of the externally applied potential in the functioning of self-assembled organized monolayers on electrodes is investigated for the first time. It is shown that the selective ion binding to monolayer membranes comprising thiobis(ethyl acetoacetate) (TBEA) is crucially dependent upon the applied potential. Thus, while at a certain applied potential (0.15 V vs SCE) no ion binding can be detected electrochemically in acid or neutral solutions, the amount of bound ions increases markedly at either positive or negative excursions from this potential. These findings are supported by independent optical and wettability measurements. The potential of minimal ionic binding is experimentally identified with the potential of zero charge (the PZC), and a mechanism is suggested which accounts for the influence of the applied potential on the ionic binding to TBEA monolayers. The mechanism involves field-driven enolization and complexation at positive rational potentials and the formation of weaker, positively charged complexes at negative rational potentials.