Simulating the Competitive Ion Pairing of Hydrated Electrons with Chaotropic Cations

Experiments show that the absorption spectrum of the hydrated electron (e_hyd^-) blue-shifts in electrolyte solutions compared with what is seen in pure water. This shift has been assigned to the e_hyd^-'s competitive ion-pairing interactions with the salt cation relative to the salt anion based on the ions' positions on the Hofmeister series. Remarkably, little work has been done investigating the e_hyd^-'s behavior when the salts have chaotropic cations, which should greatly change the ion-pairing interactions given that the e_hyd^- is a champion chaotrope. In this work, we remedy this by using mixed quantum/classical simulations to analyze the behavior of two different models of the e_hyd^- in aqueous RbF and RbI electrolyte solutions as a function of salt concentration. We find that the magnitude of the salt-induced spectral blue-shift is determined by a combination of the number of chaotropic Rb⁺ cations near the e_hyd^- and the number of salt anions near those cations so that the spectrum of the e_hyd^- directly reflects its local environment. We also find that the use of a soft-cavity e_hyd^- model predicts stronger competitive interactions with Rb⁺ relative to I^- than a more traditional hard cavity model, leading to different predicted spectral shifts that should provide a way to distinguish between the two models experimentally. Our simulations predict that at the same concentration, salts with chaotropic cations should produce larger spectral blue-shifts than salts with kosmotropic cations. We also found that at high salt concentrations with chaotropic cations, the predicted blue-shift is greater when the salt anion is kosmotropic instead of chaotropic. Our goal is for this work to inspire experimentalists to make such measurements, which will help provide a spectroscopic means to distinguish between simulations models that predict different hydration structures for the e_hyd^-.