Structure and Dynamics of Uranyl(VI) and Plutonyl(VI) Cations in Ionic Liquid/Water Mixtures via Molecular Dynamics Simulations

Abstract

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.

Identification of Uranyl Minerals Using Oxygen K‐Edge X‐Ray Absorption Spectroscopy

“…Soft X-ray spectromicroscopy can also be used to confirm the presence of uranium specifically. Although the oxygen Kedge spectra of uranyl minerals are clearly distinct from the alkali metal complexes used for comparison (Figures 1, 3 and 4), it may be more difficult to distinguish uranyl spectra from that of other heavy metal oxides, such as plutonium, which forms its own plutonyl (PuO 2 2+ ) complexes (Maerzke et al 2013). In practice, the specific presence of uranium can be confirmed by also scanning over the uranium N 5edge at 736.2 eV, an energy that would be accessible by most soft X-ray and STXM beamlines.…”

Atomistic Insight into Tetraalkylphosphonium Bis(oxalato)borate Ionic Liquid/Water Mixtures. 2. Volumetric and Dynamic Properties

“…All these simulation results indicate a gradual change of the local ionic coordination environment from IL-rich to waterconcentrated mixtures as more water molecules are introduced into [P 6,6,6,14 ][BOB] IL/water mixtures, akin to a distinct dynamic quantities of ionic species in binary mixtures consisting of imidazolium-based ILs and small molecule solvents (water, methanol, ethanol, etc.). 11,20,37,38,51,61 Translational Diffusion. For a liquid or mixture under thermodynamic equilibrium conditions, the thermal agitation causes translational motion of liquid molecules, which is called self-diffusivity.…”

The properties of residual water molecules in ionic liquids: a comparison between direct and inverse Kirkwood–Buff approaches

“…An extensive discussion of the resulting effects can be found in recent reviews 16,28 . The outcomes of most studies revealed that low water concentrations induce the formation of residual water states in the IL 9,18 . Further studies highlighted that water molecules accumulate around the hydrophobic parts of the ions with a strong affinity towards the anions 29,30 .…”