Structure and dynamics of uranyl(VI) and plutonyl(VI) cations in ionic liquid/water mixtures via molecular dynamics simulations.

A fundamental understanding of the behavior of actinides in ionic liquids is required to develop advanced separation technologies. Spectroscopic measurements indicate a change in the coordination of uranyl in the hydrophobic ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMIM][Tf2N]) as water is added to the system. Molecular dynamics simulations of dilute uranyl (UO2(2+)) and plutonyl (PuO2(2+)) ) solutions in [EMIM][Tf2N]/water mixtures have been performed in order to examine the molecular-level coordination and dynamics of the actinyl cation (AnO2(2+)) ); An = U, Pu) as the amount of water in the system changes. The simulations show that the actinyl cation has a strong preference for a first solvation shell with five oxygen atoms, although a higher coordination number is possible in mixtures with little or no water. Water is a much stronger ligand for the actinyl cation than Tf2N, with even very small amounts of water displacing Tf2N from the first solvation shell. When enough water is present, the inner coordination sphere of each actinyl cation contains five water molecules without any Tf2N. Water also populates the second solvation shell, although it does not completely displace the Tf2N. At high water concentrations, a significant fraction of the water is found in the bulk ionic liquid, where it primarily coordinates with the Tf2N anion. Potential of mean force simulations show that the progressive addition of up to five water molecules to uranyl is very favorable, with ΔG ranging from −52.3 kJ/mol for the addition of the first water molecule to −37.6 kJ/mol for the addition of the fifth. Uranyl and plutonyl dimers formed via bridging Tf2N ligands are found in [EMIM][Tf2N] and in mixtures with very small amounts of water. Potential of mean force calculations confirm that the dimeric complexes are stable, with relative free energies of up to −9 kJ/mol in pure [EMIM][Tf2N]. We find that the self-diffusion coefficients for all the components in the mixture increase as the water content increases, with the largest increase for water and the smallest increase for the ionic liquid cation and anion. The velocity autocorrelation functions also indicate changes in structure and dynamics as the water content changes.