The organic and the third phase in the system hno₃/tbp/n-dodecane: no reverse micelles

The composition and speciation of the organic and third phases in the system HNO₃/TBP/n-dodecane have been examined by a combination of gravimetry, 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, whilst 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 centres, 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 stabilising the polar TBP/HNO₃ aggregates in the organic matrix, and not as a strongly bound hydrate species in aggregates with defined stoichiometry.