Speciation and organic phase structure in nitric acid extraction with trioctylamine

We report on the extraction of nitric acid from its aqueous solutions into organic solutions of trioctylamine (TOA) in toluene, investigated with spectroscopic, X-ray scattering, and computational tools to understand the molecular speciation in the organic phase and its relation to the nanoscale structure of the organic phase. Trends in acid and water extraction clearly show two and three regimes, respectively, but the speciation of nitric acid, water, and the amine in these regimes is not apparent. ¹H-NMR of the organic phase shows that there are at least two distinct acidic protons in the organic phase while ATR-FTIR results show that the organic phase with excess acid extraction is a mixture of trioctylammonium-nitrate ion-pairs (TOAH.NO₃) , and undissociated nitric acid molecules. IR spectra computed from DFT calculations performed with clusters of undissociated nitric acid molecule hydrogen-bonded with the anion of the trioctylammonium nitrate ion-pair are consistent with the ATR-FTIR results. We also show that the chain-like configurations of TOAH.NO₃.HNO₃.H₂O are favored over TOAH.NO₃$.H₂O.HNO₃, i.e., direct interaction between the two extracted nitric acid molecules is more favored compared to a water-mediated interaction. SAXS results of the organic phases were modeled as sums of Ornstein-Zernike scattering and a pre-peak feature at higher Q region, corresponding to features from density fluctuations and extractant packing, respectively. The extraction of undissociated nitric acid by the ion-pairs leads to an increased X-ray scattering contrast in the organic phase without any significant change in the Ornstein-Zernike correlation length. These results show that the organic phase nanostructure is more sensitive to the concentration of TOAH.NO₃ and is relatively unaffected by excess acid extraction. These findings will enable a molecular understanding of the mechanisms behind metal extractions from acidic media with basic extractants.