How Phase Modifiers Disrupt Third-phase Formation in Solvent Extraction Solutions

The structure of complex organic solutions based on N,N,N',N'-tetraoctyldiglycolamide (TODGA) used in actinide separation processes were characterized after water and neodymium extraction. The organic phases were characterized using FTIR spectroscopy, ESI-MS spectrometry, Small and Wide Angle X-ray Scattering (SWAXS) and molecular dynamic simulations. The role of the "phase modifier" like 1-octanol (used in the innovative SANEX process) or the "co-extractant" like DMDOHEMA (used, for example, in the EURO-GANEX process) added to the organic phase to avoid third-phase formation was investigated at molecular and supramolecular scales. Without metallic cations in 0.2 mol/L TODGA solution in n-heptane, TODGA molecules are organized in monomers and small aggregates containing mainly two or three TODGA molecules. After Nd(NO3)3 extraction, complexes or aggregates are formed in which each neodymium cation is surrounded in average by 2 to 4 TODGA, 3 nitrate ions and 1 to 2 water molecules. Polymetallic species (aggregates) may also be formed. Adding 5%Vol 1-octanol or 0.5 mol/L DMDOHEMA to 0.2 mol/L TODGA solution in n-heptane leads to the formation of mixed aggregates containing TODGA and octanol molecules or TODGA and DMDOHEMA molecules. These molecules both interact directly with the extracted metallic cation in the organic phase, modifying the structure of the neodymium species, and improving their solubility in this organic phase. At least one TODGA molecule remains in the cation first coordination sphere in both cases and preserves its significant role in the complexation/selectivity of the cations. But it is clear that both 1-octanol and DMDOHEMA have a role in the speciation of the cations in the organic phase: 1-octanol replaces water molecules in the first coordination sphere of Nd, acting as a co-surfactant; DMDOHEMA drives the organization and DMDOHEMA molecules partially replace TODGA in the first coordination sphere of Nd.