SANS Study of Third Phase Formation in the HCl‐TBP‐n‐Octane System

Third phase formation in the extraction of HCl by tri‐n‐butyl phosphate (TBP) in n‐octane has been investigated. The extraction of the acid is accompanied by the simultaneous extraction of large amounts of water and brings about organic phase splitting when the equilibrium HCl concentration in the aqueous phase is higher than 7.6 M. For a description of mechanism and energetics of these phenomena, a combination of coordination chemistry and concepts and techniques that are typical of colloid chemistry is needed. SANS data for organic phases loaded with progressive amounts of HCl and water have been interpreted using the Baxter model for hard spheres with surface adhesion. According to this model, the increase in scattering intensity in the low Q range observed when increasing amounts of solutes are extracted into the organic phase, arises from van der Waals interactions between reverse micelles containing up to seven TBP molecules. At the onset of third phase formation, the potential energy of attraction between micelles is about twice the average thermal energy. The results obtained for HCl extraction are compared with those reported previously for the extraction of metal nitrates under the same conditions.