Electrolyte Cation Effects on Interfacial Acidity and Electric Fields

Electrolyte cation identity can have large effects on electrocatalysis by influencing the interfacial electric field, structure, or local pH. However, the specific role(s) of cations is not clear. Herein, we probed electrolyte cation effects on interfacial acidity and electric fields as a function of applied potential ( ${\varphi }_{app}$ ) for mixed self-assembled monolayers (SAMs) of 4-mercaptobenzoic acid and 4-mercaptobenzonitrile on gold using Li⁺, Na⁺, K⁺, ND₄ ⁺, or tetrabutylammonium (TBA⁺) phosphate electrolytes by surface-enhanced infrared absorption spectroscopy. The small cations showed only subtle differences in their effects on acidity and electric fields. In contrast, electrolytes containing the large TBA⁺ gave much higher electric fields that were less sensitive to ${\varphi }_{app}$ , and much more basic SAMs with a more ${\varphi }_{app}$ -responsive $p{K}_{1/2}$ . Varied cation identity and ${\varphi }_{app}$ together can shift interfacial acid/base equilibria by >3 orders of magnitude. The impeded approach of TBA⁺ to the surface results in less electric screening within the SAM and steeper potential decays into the electrolyte solution compared to other cations. Our data also suggest specific cation/surface COO^- interactions and cation identity effects on SAM heterogeneity. These insights on electrolyte cation effects on interfacial structure, proton transfer, and electric field are of fundamental interest for a range of electrochemical applications.