(537h) Free Energy Simulations of Uranyl Extraction

Extractant design is fundamental to developing solvent extraction processes for actinide separations and often focuses on optimizing the direct extractant-metal interactions. However, a more detailed understanding of energetic drivers of separations beyond primary metal coordination is often lacking, including the role of solvent in the extractant phase. In this work, we use molecular dynamics simulations with umbrella sampling and find that the organic solvent can reshape the energetics of the extractant's intramolecular conformational landscape. We calculate free energy profiles of different conformations of a representative bidentate extractant, n-octyl(phenyl)-N,N-diisobutyl carbamoyl methyl phosphinoxide (CMPO), in four different solvents: dodecane, tributyl phosphate (TBP), and dry and wet ionic liquid (IL) 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMIM][Tf₂N]). By promoting reorganization of the extractant molecule into its binding conformation, our findings suggest how particular solvents can ameliorate this energetically unfavorable step of the metal separation process. In particular, the charge alternating nanodomains formed in ILs substantially reduce the free energy penalty associated with extractant reorganization. Using alchemical free energy calculations, we find that this stabilization persists even when we explicitly include the extracted cation. We also explore the use of alchemical free energy calculations to predict the total free energy of the uranyl extraction process. By explicitly modeling the cation exchange mechanism, where the IL cation is exchanged into the aqueous phase, we can predict how choice of IL cation affects the overall free energy of extraction. These findings provide insight into the energic drivers of metal ion separations and potentially suggest a new approach to designing effective separations using a molecular-level understanding of solvent effects.