Elucidating the extraction behaviour of tri-n-alkyl phosphates and tris(2-methylbutyl) phosphate with nitrates of uranyl and tetravalent metal ions from infrared spectroscopy and dynamic light scattering studies: A distinct correlation from conformational changes and variation in size of aggregates

Abstract

Extraction of tetravalent Zr(IV), Ce(IV), Th(IV) and Pu(IV) and hexavalent U(VI) nitrates, from their aqueous solutions by tri-n-butyl phosphate (TBP), tri-n-amyl phosphate (TAP) and tris-2-methyl-butyl phosphate (T2MBP) diluted to 30% with n-dodecane has been studied using infrared (IR) spectroscopy and dynamic light scattering (DLS) measurements. A comprehensive interpretation of observations, from the molecular perspectives offered by IR spectroscopy and DLS techniques, allows understanding the behaviour exhibited by different extractant-metal nitrate systems. Formation of metal nitrate-ligand complex involving phosphoryl oxygen as a neutral electron donor stands affirmed by the significant red-shifting of vibrational wavenumber associated with the characteristic P=O stretching mode. Conformational interconversion (repopulation) with the enrichment of ‘trans’ cluster of conformations within the organic phase as a consequence of loading of metal nitrates has been proposed as a probable cause for, the counter-intuitive, blue-shifting of the vibrational wavenumber associated with phosphoryl stretching. This conformational interconversion could be instrumental in the onset of formation of a third phase. Conclusive evidence, for preferential stabilization of ‘trans’ cluster of conformations in a polar environment, has been obtained from the combined use of Density Functional Theory (DFT) and IR spectroscopy. Coupling the scatter profile of aggregates with the molecular information reveals, a compromise of the polar volume of an aggregate and its non-polar surface area, which in turn determine the sizes to which aggregates could grow for a given extractant-metal nitrate system.