(363e) Solvent-Controlled Ligand Conformational Energetics in Liquid-Liquid Extraction: A Combined Simulation and Experimental Study on Molecular Solvents and Ionic Liquids

Solvent extraction is the most widely used technique in recovering radioactive metals and reprocessing aqueous phase of nuclear fuels. Designing better extractants in solvent extractions is a research frontier in metal ion separations that typically focuses on the direct extractant-metal interactions. However, a more detailed understanding of energetic drivers of separations beyond primary metal coordination is often lacking, especially including the role of solvent in the extractant phase. In this work, we propose a new mechanism for enhancing metal extraction energetics with nanostructured solvents. Using molecular dynamics simulations to estimate free energies, we find that the organic solvent can reshape the energetics of the extractant's intramolecular conformational landscape. Our investigations have covered some multidentate extractants, including malonamides and CMPO, in different types of solvents, from traditional molecular solvents such as dodecane and TBP, to state-of-the-art ionic liquids (IL). By promoting reorganization of the extractant molecule into its binding conformation, our findings reveal how particular solvents can ameliorate this unfavorable step of the metal extraction process. In particular, the charge alternating nanodomains formed in ILs substantially reduce the free energy penalty associated with extractant reorganization. These findings provide insights into the potential energy drivers underpinning the notable separation improvements reported with ILs and suggest a new approach to designing effective separations using a molecular-level understanding of the solvent effects.