The effect of ions on water alignment and its nonlinear optical response at metal electrodes

Ions are more than spectators in electrochemical reactions. By modulating how water responds to the surface field, they reshape the solvation environment in close proximity to the electrode-where chemistry takes place. Here, we combine recent advances in molecular dynamics and theoretical non-linear optical spectroscopy to obtain mechanistic insight into how such reorganizations occur. We observe a rich diversity of behaviors that are strongly ion-pair specific and voltage dependent. This diversity originates from the coupling of three processes, whose relative balance depends sensitively on the positions where different ions adsorb at the electrode and on their ion-water interaction strength. Our findings provide intuitive design rules for how ions can be exploited to engineer the interfacial solvation environment in electrocatalysis.