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

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.