Phase stability and nucleation kinetics of salts in confinement

Abstract

Of fundamental importance to energy storage, electrochemistry and separation processes, ionic liquids in confinement exhibit unexpected thermodynamic behaviors. Such complex behaviors highlight the need for multi-scale descriptions of the thermodynamics and dynamics of nanoconfined ionic liquids. Here, we probe the impacts of confinement on the freezing point and nucleation kinetics of a simple molten salt which shares important features with ionic liquids. Using molecular modeling techniques, the melting temperature of salt in confinement is found to be larger than its bulk counterpart. Relying on conventional thermodynamic models (Clapeyron and Gibbs-Thomson equations), we capture the crystal/liquid coexistence in bulk and confinement with simple parameters derived from molecular simulations. Then, by considering different nucleation mechanisms – bulk, surface, and confined nucleation, the metastability barrier upon salt formation is shown to decrease upon confinement, therefore providing theoretical insight into the larger nucleation rate constants observed in experiments on nanoconfined ionic liquids. We also discuss the effects of wettability on capillary freezing of ionic liquids at various confining surfaces through varying the contact angle. These findings provide a practical means to predict the melting point, wettability, and freezing kinetics of ionic liquids at various surface conditions from simple thermodynamic data.