Aggregation of P,P′-di(2-ethylhexyl) methanediphosphonic acid and its Fe(III) complexes

The aggregation of P,P'-di(2-ethylhexyl) methanediphosphonic acid, H2DEH[MDP], dissolved in deuterated toluene has been investigated by small angle neutron scattering (SANS). The title compound was shown to exist in solution as a dimer under all conditions. Dimer formation is independent of the acidity of the aqueous solution with which the organic solution is pre-equilibrated. Solutions of H2DEH[MDP] containing increasing amounts of Fe(III) extracted from aqueous solutions in the acidity range from 0.1 to 5 (M) under bar were also investigated. The SANS results confirm the tendency of H2DEH[MDP]-Fe(III) species in aromatic diluents to form rod-like aggregates characterized by a constant cross section and a length which increases with increasing amounts of metal in the organic phase. Particle growth is independent of the acidity of the aqueous solutions from which the metal is extracted. These results are important for understanding the chemistry underlying the solvent extraction behavior of H2DEH[MDP] and its practical application in separation procedures.