SANS study of aggregation of the complexes formed by selected metal cations and P,P′-di(2-ethylhexyl) ethane- and butane-diphosphonic acids

The aggregation of several metal complexes formed during solvent extraction with P,P'-di(2-ethylhexyl) ethanediphosphonic acid, H2DEH[EDP], and by P,P'-di(2-ethylhexyl) butanediphosphonic acid, H2DEH[BuDP], in deuterated toluene, has been investigated by small angle neutron scattering (SANS). With H2DEH[EDP], the extraction of Ca(II), La(III) and U(VI) does not disrupt the cyclic hexameric structure of the ligand in solution. Fe(III) and Th(IV) complexes of H2DEH[EDP], on the other hand, exhibit a very modest tendency to aggregate but only at very high metal loading in the organic phase. With H2DEH[BuDP], the extraction of Ca(II), La(III), U(VI) and Th(IV) is not accompanied by significant aggregation of the metal complexes. The Fe(III)-H2DEH[BuDP] complexes, however, form long cylindrical aggregates similar to those previously observed with P,P'-di(2-ethylhexyl) methanediphosphonic acid, H2DEH[MDP]. The aggregation behavior of the various metal-extractant species is discussed in light of the information obtained from earlier solvent extraction, vapor pressure osmometry, and infrared spectroscopy studies.