Small-angle neutron scattering studies of the aggregation of Pr(NO₃)₃-CMPO and PrCl₃-CMPO complexes in organic solvents

Small-angle neutron scattering studies have been carried out on praseodymium-CMPO [octyl(phenyl)(N,N-diisobutylcarbamoylmethyl)phosphine oxide] complexes in organic solvents to understand the solution structures of the complexes formed in the TRUEX (transuranic extraction) process. In this investigation, the effects of the concentration of the extractant, the nature of the solvent, the nature of the anions and the bifunctional character of CMPO on the aggregation properties of the extracted species were considered. The results reveal that the PrX₃-CMPO (X = NO₃, Cl) complexes in C₆D₆ form large aggregates which have an ellipsoidal shape, although CMPO in C₆D₆ is a monomer. The size of the aggregates depends upon the concentration of the CMPO. The anion accompanying the praseodymium into the organic phase and coextracted water or acid have little influence on the morphology of the aggregates. The nature of the diluent and the concentration of CMPO, however, strongly influence the size of the aggregates formed. Unlike CMPO, the monofunctional phosphoryl extractant DAAP (diamyl amylphosphonate) does not form large aggregates, suggesting that the aggregation may be associated with the bifunctional nature of the CMPO.