Abstract
In the framework of spent nuclear fuel reprocessing, N,N‑dialkylamides are molecules of interest for the selective recovery of uranium and plutonium. Two monoamides, DEHBA (N,N‑di‑2‑ethylhexyl‑butyramide) and MOEHA (N,N‑methyl, octyl‑2‑ethylhexanamide), were investigated to evaluate the effect of their structure on the molecular and supramolecular speciation in organic phase. The structure of the organic phases after water and uranyl nitrate extraction was characterized by combining experimental SWAXS with molecular dynamic simulations. After water extraction, N,N‑dialkylamides solutions are mainly composed of monomers and dimers. In the case of MOEHA solutions, dimers and bigger aggregates containing 3 to 5 ligands molecules surrounding nano-cluster of water molecules are observed. The proportion of aggregates increases with the N,N‑dialkylamide concentration. After extraction of high uranyl nitrate concentration, 0.5 mol·L−1 N,N‑dialkylamide solutions in n-heptane are composed of large and dense aggregates of uranyl cations linked together by bridging nitrates and solvated by the monoamide. For 1.5 mol·L−1 N,N‑dialkylamide solutions, the organization is quite different with the presence of a multitude of smaller aggregates. This study validates the approach coupling MD simulation and SWAXS to study the complexity of extractant organic phases.