Investigating Surface pK_a and pH Using Surface-Enhanced Raman Scattering Spectroscopy with 4-Mercaptobenzoic Acid in...

Our research presents spectroscopic measurements of the surface pK_a at electrode/electrolyte interfaces using surface-enhanced Raman scattering spectroscopy of 4-mercaptobenzoic acid (4-MBA). As the electrochemical potential is varied from negative to positive, the Raman intensity of the -COOH functional group (at 1700 cm^-1) decreases while the -COO^- Raman intensity (at 1410 cm^-1) increases. The protonation-deprotonation of this surface-bound molecule reflects an electrochemically induced shift to more acidic conditions at negative potentials and more basic conditions at positive potentials. By fitting the data to a normalized sigmoid function, we obtain the percentage of surface protonation/deprotonation, which can be related to the surface pK_a of the system. The percentage of surface protonation, which gives a proxy of the two-dimensional surface pK_a, follows the Fermi-Dirac distribution as a function of the applied potential. The electrolyte-electrode pH-neutral conditions at the interface are extracted by the linear fitted intercepts of -log(COO^-/COOH) as a function of the applied potential based on the Nernst equation, which are 0.25, 0.07, 0.08, and -0.46 V for DI water and 0.5 M sodium bicarbonate solutions with and without CO₂ purging, respectively. The shift of surface neutral conditions toward more positive voltages in the electrolytes with CO₂ purging indicates that the bulk solutions dissolved in the CO₂-dissolved form become more acidic. The 25% reduction of protonation at negative applied potentials in CO₂-purged DI water suggests that the direct reduction of hydronium ions and/or carbonic acid increases the surface pK_a in the microenvironment. Adding alkali cations (Na⁺) attracts more proton donors toward the working electrode, resulting in the protonation capacity near the electrode surface, approximately -1.9 V^-1, being double that of DI water, which is around -1 V^-1. Hydrogen evolution reaction pathways are not detected in neutral or basic conditions due to the low concentration of hydronium ions (<10^-6 M). The independence of the carbonic acid concentration with applied negative potentials, as measured by the surface pK_a in the Helmholtz plane, indicates that changes in the local pH/surface pK_a under neutral or basic bulk conditions are governed by the acid-base equilibrium of water, carbonic acid, bicarbonate, and carbonate ions.