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 (UO22+) and plutonyl (PuO22+) solutions
in [EMIM]­[Tf2N]/water mixtures have been performed in order
to examine the molecular-level coordination and dynamics of the actinyl
cation (AnO22+); 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.