In situ conversion of electrostatic repulsion into attraction and its regulation induced by external potential on the surface of self-assembled monolayers†

Investigating the influence of external potential on electrostatic interactions at electrode surfaces helps us understand their crucial role in electrochemical processes. To achieve this goal, we constructed a self-assembled monolayer of 3-mercaptopropionic acid (3-MPA) on the gold electrode surface. Meanwhile, ferrocene formic acid (FcCOOH) was selected as the redox medium to prepare a structurally clear electrostatic interaction model at the molecular level. Using scanning electrochemical microscopy (SECM), the in situ conversion and regulation of electrostatic interactions were studied in detail. We found that external potential caused in situ conversion of electrostatic repulsion into electrostatic attraction between molecules. This conversion was attributed to the electrostatic attraction between the oxidation products (Fc⁺COO⁻) and the deprotonation products (3-MPA⁻) at the membrane surface. We also found that the changes in potential can effectively regulate the electrostatic attraction between molecules. This is due to the fact that the increase in potential reduces the negative charge density on the membrane surface, leading to protonation of 3-MPA⁻ to form 3-MPA and release some Fc⁺COO⁻. Moreover, theoretical calculation results supported our explanation from an energy point of view, and the key sites of electrostatic interactions were visualized. It was also confirmed that anions have a synergistic effect on the regulation process. This work reveals the conversion and regulation of electrostatic interactions on electrode surface at the molecular level, thereby advancing the fundamental understanding of electrochemical interface processes.