Abstract Third phase formation in the extraction of Pu(IV) nitrate by 30% tri‐
n‐butyl phosphate (TBP) dissolved in
n‐dodecane or in the highly branched diluent hydrogenated polypropylene tetramer (HPT), which may also be known as 4,4 dipropyl heptane or tétrapropylène hydrogéné, was investigated through small‐angle neutron scattering (SANS) measurements. The SANS data were interpreted using the Baxter model for hard‐spheres with surface adhesion. According to this model, the increase in scattering intensity observed when increasing amounts of Pu(NO
3)
4 are extracted into the organic phase, is due to interactions between small reverse micelles containing three to five TBP molecules. In
n‐dodecane, the micelles interact through attractive forces between their polar cores with a potential energy of up to ?2.6 k
BT. This strong intermicellar attraction leads to organic phase splitting with the separation of most of the solutes of the original organic phase into a distinct phase containing interspersed layers of
n‐dodecane. When HPT is the diluent, the intermicellar attraction energy calculated from the SANS data is much lower, and no third phase formation is observed under comparable chemical conditions. However, when a significant amount of the initial aqueous plutonium is in the form of plutonyl ions, PuO
2 2+, the critical energy potential is reached even in HPT. A potential explanation of the effect of Pu(VI) involves the formation of a plutonyl trinitrato complex.
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