Simulating the Competitive Ion Pairing of Hydrated Electrons with Chaotropic Cations

Experiments show that the absorption spectrum of the hydrated electron () blue-shifts in electrolyte solutions compared with what is seen in pure water. This shift has been assigned to the '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 's behavior when the salts have chaotropic cations, which should greatly change the ion-pairing interactions given that the 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 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 and the number of salt anions near those cations so that the spectrum of the directly reflects its local environment. We also find that the use of a soft-cavity 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 .