Probing Voltage- and Electrolyte-Dependent Monolayer Dynamics with 2D-IR Spectroscopy

Little is known about structural dynamics at electrode surfaces under applied potential. Here, we report 2D-IR spectra of a 4-mercaptobenzonitrile monolayer on a gold electrode, revealing chemical exchange between two subensembles on sub- to tens-of-picoseconds time scales. Dynamics are measured in 100 mM MgCl₂, LiCl, and KCl at -200 and +300 mV vs Ag/AgCl. At +300 mV, all electrolytes exhibited ≥48 ps exchange, while at -200 mV, the slower exchange component ranged from ≥33 to ≥43 ps. The dynamics correlate with ion concentration, according to radial distribution functions calculated from molecular dynamics simulations, suggesting that local ion densities, regardless of valency or sign, slow dynamics. Results show solvation, electric double layer formation, and monolayer reorientation. This work reveals how electrolyte composition modulates molecular reorientation and hydrogen bonding at functionalized electrodes.