Application of the Baxter model for hard spheres with surface adhesion to SANS data for the U(VI)-HNO₃, TBP-n-dodecane system

Small-angle neutron scattering (SANS) data for the tri-n-butyl phosphate (TBP)-n-dodecane, HNO₃-UO₂(NO₃)₂ solvent extraction system have been interpreted using the Baxter model for hard spheres with surface adhesion. The increase in the scattering intensity in the low Q range observed when increasing amounts of HNO₃ or UO₂(NO₃)₂ are transferred into the organic phase has been interpreted as arising from interactions between solute particles. The SANS data have been reproduced using a 12-16 Å diameter of the hard sphere, d_hs, and a 5.6k_BT-7.1k_BT stickiness parameter, τ^-1. When in contact with an aqueous phase, TBP in n-dodecane forms small reverse micelles containing three TBP molecules. Upon extraction of water, HNO₃, and UO₂(NO₃)₂, the swollen micelles interact through attractive forces between their polar cores with a potential energy of about 2k_BT and an effective Hamaker constant of about 4k_BT. The intermicellar attraction, under suitable conditions, leads to third-phase formation. Upon phase splitting, most of the solutes in the original organic phase (water, TBP, HNO₃, and UO₂(NO₃)₂) separate in a continuous phase containing interspersed layers of n-dodecane.