An Insight into Third-Phase Formation during the Extraction of Thorium Nitrate: Evidence for Aggregate Formation from Small-Angle Neutron Scattering and Validation by Computational Studies

Abstract

Small-angle neutron scattering (SANS) studies were carried out to compare the aggregation behavior of 1.1 M solutions of tributyl phosphate (TBP) and N,N-dihexyl octanamide (DHOA) dissolved in different deuterated diluents, viz., n-dodecane, chloroform, and benzene, during the extraction of Th(IV) from nitric acid medium. The scattering data was treated using the Baxter sticky spheres model. The third phase formed in the case of DHOA displayed higher aggregation tendency compared to that of TBP. These studies have demonstrated that the nature of the diluents plays an important role in the aggregation behavior of the extracted species (reverse micelles). No third phase was observed in the case of chlorinated and aromatic diluents like chloroform and benzene during the extraction of Th(IV) from nitric acid medium. Theoretical calculations were also performed to gain insights into the binding of thorium nitrate with TBP and DHOA models. These calculations suggest that two to three molecules of both DHOA and TBP strongly coordinate to Th(NO3)4. It is noted that the highly charged Th(IV) cations are screened by nitrates and extractants which enables the interaction of second unit of such complex through noncovalent interactions.