Electrochemically Driven Reorientation of Three Ionic States of p-Aminobenzoic Acid on Ag(111)

Abstract

Broad band sum frequency generation spectroscopy is used to probe the orientation of three pH accessible ionic states of p-aminobenzoic acid (PABA) on an Ag(111) electrode surface as a function of electrochemical potential. Experimentally measured phase differences between resonant and nonresonant SFG signals confirm features in electrochemical capacitance and cyclic voltammograms correspond to ordering and reorientation of the PABA molecule on the electrode surface as the surface potential is varied. While the zwitterionic form rotates 180° after the potential is moved through the potential of zero charge (PZC), such is not the case for the cationic or anionic forms. In the former, the molecule rotates only 90°. In the later, the molecule does not rotate at all (because the PZC is not reached). Additionally, optical methods are utilized to show that the PZC of the surface changes dramatically following adsorbate adsorption. The surface potential at which PABA reorients is quite different from the PZC determined in neat electrolyte solutions. The measured reorientation potentials for zwitterionic PABA (ca. -300 mV) and anionic PABA (-800 mV) reflect a substantial shift of the PZC of the electrode with the adsorbed dipolar organic species.