Aggregation of dialkyl-substituted diphosphonic acids and its effect on metal ion extraction

Solvent extraction reagents containing the diphosphonic acid group exhibit an extraordinary affinity for tri-, tetra-, and hexavalent actinides. Their use has been considered for actinide separation and preconcentration proce dures. Solvent extraction data obtained with P,P'-di(2-ethylhexyl) methane- , ethane-, and butanediphosphonic acids exhibit features that are difficult to explain without knowledge of the aggregation state of the extractants. Information about the aggregation of the dialkyl-substituted diphosphonic a cids in aromatic diluents has been obtained using the complementary techniq ues of vapor pressure osmometry, small-angle neutron scattering, infrared s pectroscopy, and molecular mechanics. The results from these techniques pro vide an understanding of the aggregation behavior of these extractants that is fully compatible with the solvent extraction data. The most important r esults and their relevance to solvent extraction are reviewed in this paper .