Dynamics of the Electrolyte-to-Metal Transition in Aqueous Solutions of Alkali Metals

Electrolyte-to-metal transitions in liquids represent a critical process in the evolution of excess electrons from localized species into a delocalized conduction band. Alkali metal--ammonia solutions are the textbook example, displaying a gradual progression from blue electrolytes to metallic gold liquids. In contrast, preparing the analogous aqueous solutions was long deemed impossible as alkali metals ignite or even explode on contact with water. Moreover, individual electrons solvated in water are more strongly bound than in liquid ammonia, which suggests that electrolyte-to-metal transitions may be inhibited, or only occur at very high alkali metal concentrations. Here we combine optical spectroscopy with ab initio molecular dynamics to show that aqueous alkali metal solutions can not only be prepared by adsorbing water vapor on alkali metal alloys of varying compositions but that they also undergo a dynamic, fluctuating electrolyte-to-metal transition. Simulations reveal femtosecond-scale flipping between electrolyte and metallic states, while experiments confirm the evolution of the metallic character via plasmonic signatures of distinct colors. Water can thus not only form a metallic solution but also exhibits a dynamic, rapidly fluctuating non-metal-to-metal transition.