Interpretation of Third Phase Formation in the Th(IV)–HNO₃, TBP–n‐Octane System with Baxter's “Sticky Spheres” Model

Small‐angle neutron scattering (SANS) data for the tri‐n‐butylphosphate (TBP)–n‐octane, HNO3–Th(NO3)4 solvent extraction system, obtained under a variety of experimental conditions, have been interpreted using two different models. The particle growth model led to unrealistic results. The Baxter model for hard‐spheres with surface adhesion, on the other hand, was more successful. According to this model, the increase in scattering intensity in the low Q range observed when increasing amounts of Th(NO3)4 are extracted into the organic phase, has been interpreted as arising from interactions between small reverse micelles containing three TBP molecules. Upon extraction of Th(NO3)4, the micelles interact through attractive forces between their polar cores with a potential energy of up to about 2 kBT. The intermicellar attraction, under suitable conditions, leads to third phase formation. Upon phase splitting, most of the solutes of the original organic phase separate in a continuous phase containing interspersed layers of n‐octane.