Uranyl and Strontium Salt Solvation in Room-Temperature Ionic Liquids. A Molecular Dynamics Investigation

Using molecular dynamics simulations, we compare the solvation of uranyl and strontium nitrates and uranyl chlorides in two roomtemperature ionic liquids (ILs): [BMI][PF6] based on 1-butyl-3-methylimidazolium^+,PF6^- and [EMI][TCA] based on 1-ethyl-3methylimidazolium^+,AlCl4^-. Both dissociated M^2+,2NO3^- and associated M(NO3)2 states of the salts are considered for the two cations, as well as the UO2Cl2 and UO2Cl42^- uranyl complexes. In a [BMI][PF6] solution, the "naked" UO2^2+ and Sr^2+ ions are surrounded by 5.8 and 10.1 F atoms, respectively. The first-shell PF6- anions rotate markedly during the dynamics and are coordinated, on the average, monodentate to UO22^+ and bidentate to Sr^2+. In an [EMI][TCA] solution, UO22^+ and Sr^2+ coordinate 5.0 and 7.4 Cl atoms of AlCl4^-, respectively, which display more restricted motions. Four Cl atoms sit on a least motion pathway of transfer to uranyl, to form the UO2Cl42^- complex. The free NO3- anions and the UO2Cl42^- complex are surrounded by imidazolium+ cations (4 and 6-9, respectively). The first shell of the M(NO3)2 and UO2Cl2 neutral complexes is mostly completed by the anionic components of the IL, with different contributions depending on the solvent, the M^2+ cation, and its counterions. Insights into energy components of solvation are given for the different systems.