First water-to-ligand substitution dictates Co²⁺/Ni²⁺ extraction selectivity

Solvent extraction is a crucial technology for the large-scale recovery of critical metals. One key challenge is to develop ligands with high ionic selectivity, especially for ions with similar properties. Manipulating ionic solvation emerges as a viable approach to enhance the selectivity. However, such manipulation requires a thorough understanding of the thermodynamics underlying ligand-induced variation in ionic solvation. We investigated the thermodynamics governing the binding of two organophosphorus ligands (Cyanex 272 and D2EHPA) with Co²⁺ and Ni²⁺ at the aqueous-organic liquid-liquid interface using a combination of experiments and computations. UV-vis and IR spectroscopy confirmed that extraction induces solvation structure changes of Ni²⁺ and Co²⁺. The free-energy landscapes obtained from well-tempered metadynamics illustrated two plausible routes for water-ligand substitution during extraction. In the first route, the ligand occupies the vacancy created by the departing water molecule. In the second route, the ligand associates with the ion to form an oversaturated solvation state that drives water expulsion. The ligand-binding process is a competition between the two routes. The free-energy landscapes further revealed that substitution of the first water molecule in the ionic solvation shell plays a determining role in ligand-induced ionic selectivity. Our results suggest a target for the rational design of Co²⁺/Ni²⁺ separation technology: engineering the free energy associated with substitution of the first water molecule.