Ions Under an External Field: Insights from Non-Equilibrium Molecular Simulations*

Enabling future separation technologies that reduce energy consumption and chemical waste requires accessing "beyond equilibrium diffusion" to achieve higher selectivity and efficiency. This is crucial for separating rare earth elements (REEs) from unconventional feedstocks (e.g., mine tailings) where targeted ion concentrations are <10^-2 ppm. Existing separation processes relying on equilibrium conditions are energy and chemically intensive at low concentrations for large-scale implementation. Therefore, accessing non-equilibrium processes is imperative to accelerate the separation and concentration of REEs, making them adaptable to existing industries.

Lanthanides are critical for high-tech and clean energy applications, making their efficient separation and processing a priority. Using non-equilibrium molecular dynamics (NEMD) simulations, we investigated lanthanide (Ln³⁺) ion behavior under applied electric fields and flow conditions. Our findings reveal that electric fields significantly influence ion solvation structure, with changes in the first coordination shell being more pronounced for Ln³⁺ than Na⁺. Coupling the electric field to the flow results in asymmetrical velocity profiles and changes in viscosity and surface wettability, largely dependent on the nature of the salt and its solvation properties. Our results highlight the importance of ion-specific interactions in determining transport properties under non-equilibrium conditions.