Ion-Pair Specific Response of Water to Electric Fields

The response of an aqueous solution to electric fields is strongly impacted by the presence of electrolytes. The electrolyte effect is conventionally described by the ionic strength and by means of continuum models. These descriptions are less adequate for concentrated solutions, where ion solvation shells and ion-ion correlations matter. Here, we investigate these effects via molecular dynamics simulations of two salt solutions, CsCl and ${\mathrm{MgCl}}_2$ , of varying concentration, in the presence of an oscillating electric field. We find that the response of water to the field is ion-pair-specific. These effects are rationalized by dissecting theoretical spectra into individual contributions from anion, cation, and shared solvation shells and compared to the results of THz absorption measurements. We find evidence that the solvation shell water of ${\mathrm{Cl}}^{-}$ itself is chameleonic, changing its response to an electric field depending on the environment created by the counterions. This effect could be especially relevant for the balance of ion-surface interactions at biological and electrochemical interfaces.