Uranyl and strontium salt solvation in room-temperature ionic liquids. A molecular dynamics investigation

Abstract

Using molecular dynamics simulations, we compare the solvation of uranyl and strontium nitrates and uranyl chlorides in two room-temperature ionic liquids (ILs): [BMI][PF6] based on 1-butyl-3-methylimidazolium +,PF6- and [EMI]-[TCA] based on 1-ethyl-3-methylimidazolium+, AlCl4-. Both dissociated M2+,2NO3- and associated M(NO 3)2 states of the salts are considered for the two cations, as well as the UO2Cl2 and UO2Cl 42- uranyl complexes. In a [BMI]-[PF6] solution, the "naked" UO22+ and Sr2+ 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 Sr2+. In an [EMI][TCA] solution, UO22+ and Sr2+ coordinate 5.0 and 7.4 Cl atoms of AlCl 4-, 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 UO2Cl 2 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.