Organic and Third Phase in HNO₃/TBP/n-Dodecane System: No Reverse Micelles

ABSTRACT

The composition and speciation of the organic and third phases in the system HNO₃/TBP (tri-n-butyl phosphate)/n-dodecane have been examined by a combination of gravimetric, Karl Fischer analysis, chemical analysis, FTIR, and ³¹P NMR spectroscopy, with particular emphasis on the transition from the two-phase to the three-phase region. Phase densities indicate that third-phase formation takes place for initial aqueous HNO₃ concentrations above 15 M, while the results from the stoichiometric analysis imply that the organic and third phases are characterized by two distinct species, namely the mono-solvate TBP⋅HNO₃ and the hemi-solvate TBP⋅2HNO₃, respectively. Furthermore, the ³¹P NMR spectra of organic and third phase show no significant chemical differences at the phosphorus centers, suggesting that the second HNO₃ molecule in the third phase is bound to HNO₃ rather than TBP. The third-phase FTIR spectra reveal stronger vibrational absorption bands at 1028, 1310, 1653, and 3200–3500 cm⁻¹, reflecting higher concentrations of H₂O, HNO₃, and TBP. The molecular dynamics simulation data predict structures in accord with the spectroscopically identified speciation, indicating inequivalent HNO₃ molecules in the third phase. The predicted structures of the organic and third phases are more akin to microemulsion networks rather than the distinct, reverse micelles assumed in previous studies. H₂O appears to be present as a disordered hydrogen-bonded solvate stabilizing the polar TBP/HNO₃ aggregates in the organic matrix, and not as a strongly bound hydrate species in aggregates with defined stoichiometry.

KEYWORDS:

• Third phase formation
• PUREX
• solvent extraction
• nitric acid
• tributyl phosphate
• molecular dynamics simulations

Acknowledgments

All data supporting this study are provided either in the Results and Discussion section of this article or in the supplementary information accompanying it. The authors also thank to Dr. Stephanie Cornet for assistance with the NMR measurements. Sin-Yuen Chang thanks the University of Manchester as well as Mr. and Mrs. Clews for the Robert Clews Presidential PhD scholarship.

Supplemental data

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Funding

We thank EPSRC for providing financial support under grant EP/I002855/1.

Additional information

Funding

We thank EPSRC for providing financial support under grant EP/I002855/1.