Probing Cation-Induced Ion Gating and Surface pKa in the Stern Layer Using SERS Spectroscopy on 4-MBA-Functionalized...

Understanding proton activity at electrified interfaces is central to electrochemical energy conversion, catalysis, and interfacial chemistry; however, direct measurements of local pH, surface acid-base equilibria, and the structure of the electrical double layer (EDL) remain challenging. Here, we report an in situ spectroscopic determination of the surface pKa at electrode-electrolyte interfaces using surface-enhanced Raman scattering (SERS) spectroscopy of the surface-bound 4-mercaptobenzoic acid (4-MBA). By monitoring the potential-dependent evolution of protonated (COOH), deprotonated (COO-), and metallic-bound (COO-) vibrational modes relative to a pH-insensitive aromatic-ring reference, we quantitatively extract surface protonation fractions and the corresponding interfacial acid-base equilibrium, while inferring cation-dependent EDL restructuring under electrochemical control. Furthermore, we show that interfacial protonation behavior depends strongly on electrolyte composition. In cation-free systems, the 4-MBA monolayer remains predominantly protonated across the investigated potential window, leading to an increase in surface pKa with increasingly negative applied potential. In contrast, introducing K+ ions at the same pH produces a markedly different response, characterized by partial protonation and an opposite dependence of surface pKa on potential. These observations provide direct experimental evidence that alkali cations reorganize the EDL, suppress proton access at negative potentials, and shift the surface acid-base equilibrium at the electrode surface. The observed cation-dependent shift in surface pKa reflects an ion-gating mechanism in which nanoscale ionic organization within the EDL modulates interfacial electric fields and ion availability, thereby directly controlling surface protonation and interfacial chemistry. This behavior is analogous to electrostatic gating in electric-double-layer transistors, where ionic structuring at the interface enables substantial carrier modulation. Overall, this work establishes a molecular-scale framework for probing surface acid-base equilibrium and EDL structure using 4-MBA as a surface reporter, providing insights into electrolyte-dependent regulation in electrochemical systems.