Investigating Surface pKa and pH Using Surface-Enhanced Raman Scattering Spectroscopy with 4-Mercaptobenzoic Acid in Deionized Water and Sodium Bicarbonate Electrolytes

Our research presents spectroscopic measurements of the surface pKa 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 pKa of the system. The percentage of surface protonation, which gives a proxy of the two-dimensional surface pKa, 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 pKa 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 pKa in the Helmholtz plane, indicates that changes in the local pH/surface pKa under neutral or basic bulk conditions are governed by the acid–base equilibrium of water, carbonic acid, bicarbonate, and carbonate ions.