Solvation of uranyl-CMPO complexes in dry vs.humid forms of the [BMI][PF_6] ionic liquid.A molecular dynamics study

摘要

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_2O)_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.