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

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<p>The composition and speciation of the organic and third phases in the system HNO<sub>3</sub>/TBP (tri-<i>n-</i>butyl phosphate)/<i>n</i>-dodecane have been examined by a combination of gravimetric, Karl Fischer analysis, chemical analysis, FTIR, and <sup>31</sup>P 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 HNO<sub>3</sub> 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⋅HNO<sub>3</sub> and the hemi-solvate TBP⋅2HNO<sub>3</sub>, respectively. Furthermore, the <sup>31</sup>P NMR spectra of organic and third phase show no significant chemical differences at the phosphorus centers, suggesting that the second HNO<sub>3</sub> molecule in the third phase is bound to HNO<sub>3</sub> rather than TBP. The third-phase FTIR spectra reveal stronger vibrational absorption bands at 1028, 1310, 1653, and 3200–3500 cm<sup>−1</sup>, reflecting higher concentrations of H<sub>2</sub>O, HNO<sub>3</sub>, and TBP. The molecular dynamics simulation data predict structures in accord with the spectroscopically identified speciation, indicating inequivalent HNO<sub>3</sub> 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. H<sub>2</sub>O appears to be present as a disordered hydrogen-bonded solvate stabilizing the polar TBP/HNO<sub>3</sub> aggregates in the organic matrix, and not as a strongly bound hydrate species in aggregates with defined stoichiometry.</p>
Date made available10 Jul 2017
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