Influence of extractant aggregation on the extraction of trivalent f-element cations by a tetraalkyldiglycolamide

The influence of nitric acid extraction on the aggregation state of 0.10M N,N,N',N'-tetra-n-octyl-3-oxapentane-1,5-diamide (TODGA) in n-octane or n-heptane was studied by small-angle neutron scattering (SANS) and vapor pressure osmometry (VPO). When the equilibrium concentration of nitric acid in the aqueous phase is less than 0.7 M, TODGA exists as a mixture of monomers and dimers. As the aqueous phase acidity is increased, the extractant molecules form higher aggregates containing up to an average of seven molecules of TODGA. The formation of the larger TODGA aggregates takes place over the same range of aqueous acidities where the extraction of trivalent f-element cations displays a hyperstoichiometric sixth power nitric acid dependence. This suggests that acid-driven aggregation of TODGA is responsible for the unusual acid and extractant dependencies observed for the extraction of trivalent metal nitrates with this ligand.