Solvation of uranyl-CMPO complexes in dry vs. humid forms of the [BMI][PF6] ionic liquid. A molecular dynamics study.

The solvation of the UO(2)(NO(3))(CMPO) and [UO(2)(NO(3))(2)(CMPO)(2)] complexes (CMPO = octyl(phenyl)-N,N-diisobutylmethylcarbamoyl phosphine oxide) is investigated by molecular dynamics in the "dry" and "humid" forms of a room temperature ionic liquid (IL) based on the 1-butyl-3-methylimidazolium (BMI(+)) cation and the hexafluorophosphate (PF(6)(-)) anion. The simulations reveal the importance of the solvent anions in "dry" conditions and of water molecules in the "humid" solvent. For the UO(2)(NO(3))(CMPO) complex, the monodentate vs. bidentate coordination modes of CMPO are compared, and the first solvation shell of uranyl is completed by 1-3 PF(6)(-) anions in the dry IL and by 2-3 water molecules in the humid IL, leading to a total coordination number close to 5. The energy analysis shows that interactions with the IL stabilize the UO(2)(NO(3))(bi)(CMPO)(mono) form (with bidentate nitrate and monodentate CMPO) in the dry IL and the UO(2)(NO(3))(mono)(CMPO)(mono) form (with monodentate nitrate and CMPO) in the humid IL. The extracted compound characterized by EXAFS is thus proposed to be the UO(2)(NO(3))(mono)(CMPO)(mono)(H(2)O)(3) species. Furthermore we compare the [UO(2)(NO(3))(2)(CMPO)(2)] complex in its associated and dissociated forms (UO(2)(NO(3))(mono)(CMPO)(mono) + CMPO + NO(3)(-)) and discuss the results in the context of uranyl extraction by CMPO to ionic liquids.