Surface active species and their reaction schemes in extraction solution systems identified from practical behaviors of interfacial tension

The interfacial tensions in two-phase solution systems for extraction of metallic ions from aqueous phase to organic phase by di(2-ethyl-hexyl) phosphoric acid were measured by the Wilhelmy plate method and partly by laser-light scattering method to confirm the accuracy. For copper and iron extraction solution systems, the surface-active species and their interfacial concentrations were identified by comparing the actual behavior of interfacial tension, the pH and extractant concentration dependencies, with the Gibbs–Langmuir equation for adsorption. It was shown that the extractant molecules should be adsorbed at the interface in the form of monomer and its dehydrogenated anion and that in copper extraction solution system non-surface-active 1:2 intermediate complexes should be formed, and in iron extraction solution system strongly surface-active 1:2 intermediate complexes should be formed at the interface. Based on the insights related to the intermediate complexes, reaction schemes at the interface are also discussed.