Application of the Baxter Model for Hard Spheres with Surface Adhesion to SANS Data for the U(VI)−HNO₃, TBP−n-Dodecane System

Abstract

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 B T−7.1k B T 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 B T and an effective Hamaker constant of about 4k B T. 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.

Small‐Angle Neutron Scattering Study of Plutonium Third Phase Formation in 30% TBP/HNO3/Alkane Diluent Systems

“…Literature data was not available for PuO 2 (NO 3 ) 2 , subsequently the molar volume calculated for UO 2 (NO 3 ) 2 was used. [17] …”

“…This result was unexpected, based on the similarity of behavior between Pu(VI) and U(VI), since it has been previously shown that U(VI) does not exhibit a strong tendency toward third phase formation when extracted by TBP in n-dodecane. [11,17] The effectiveness of Pu(VI) in promoting third phase formation in TBP systems has been potentially linked to the trinitrato complex of Pu(VI), PuO 2 (NO 3 ) 3 2 . [2] It has been reported that this complex has a significantly higher stability than the corresponding U(VI) complex, and, therefore, starts to form at relatively low HNO 3 concentrations (.4 M).…”

Influence of the extracted solute on the aggregation of malonamide extractant in organic phases: Consequences for phase stability

“…He indicated that the occurrence of phase splitting was due to the formation of reverse micelles, or microemulsion, in the organic diluent by the aggregation of TBP complexes. The concept of a colloidal approach enabled a better understanding of the organic solution of extractants [7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22]. The first direct characterization of reverse micelles with interaction potential came from the modeling of small angle scattering intensities on absolute scale by Madic et al [17][18][19].…”

Characterization of the Species Formed during the Extraction of Thorium Employing Tri-n-Butyl Phosphate and N,N-Dihexyl Octanamide as Extractants by Laser Desorption/Ionization Time-of-Flight Mass Spectrometry

“…These techniques have helped in identifying the physical/chemical parameters (for example, aggregation, nature and concentration of acids, diluents) responsible for third-phase formation and also identifying species formed in the heavier organic phases. [3][4][5][6][7]27,[32][33][34][35] SANS measurements on TBP solutions loaded with only HNO 3 or with increasing amounts of U(VI) or Th(IV) revealed the presence of ellipsoidal aggregates (reverse micelle type) before phase splitting. The presence of smaller aggregates in the light organic phase upon third-phase formation was observed, while the heavy organic phase contained pockets of diluent.…”