Critical fluctuations in liquid–liquid extraction organic phases controlled by extractant and diluent molecular structure

Bonnett, Brittany; Sheyfer, Dina; Wimalasiri, Pubudu N.; Nayak, Srikanth; Lal, Jyotsana; Zhang, Qingteng; Seifert, Söenke; Stephenson, G. B.; Servis, Michael J.

doi:10.1039/d3cp01029e

Cited by 9 publications

(10 citation statements)

References 113 publications

(178 reference statements)

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“…At 0.and 1 M of TOAH • NO 3 , the O-Z scattering is weak and we could not obtain useful fits to the L-G model. This behavior of ξ passing through a maximum with the solute concentration corresponds well with the scattering due to critical fluctuations,50,56,57 with the critical TOAH • NO 3 concentration being between 0.25 and 0.4 M. We note that none of the samples showed phase splitting even at temperatures as low as -19 • C in line with the suppression of phase separation with aromatic solvents. This implies that the theoretical critical temperature is out of the experimentally accessible temperatures and thus we could not ascertain whether the phase behavior of the samples follow the SAXS trends.In Figure5(b), the pre-peak is strengthened with increasing TOAH • NO 3 concentration without a significant shift in the peak position.…”

supporting

confidence: 75%

“…I 0 appears to have a maximum between 0.25M and 0.4M (∼ 0.1 to 0.17 volume fraction of TOAH • NO 3 ), which is lower than the corresponding extractant concentration for neutral extractants such as TBP and DMDPMA in the absence of acid. 50,56,57 The origins of this difference is not clear, but it could be characteristic of TOA, the presence of acid, or some combination thereof.…”

Section: Atr-ftir and Dft Study Of Excess Hno 3 Extracted With Toah • Nomentioning

confidence: 99%

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Speciation and organic phase structure in nitric acid extraction with trioctylamine

Nayak,

Servis,

Combs

et al. 2023

Preprint

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Understanding chemical speciation and intermolecular interactions in multicomponent liquids is essential to understanding their phase behavior, which underpins chemical separation processes including solvent extraction. Here 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 relationship with the nanoscale structure of the organic phase. Trends in acid and water extraction clearly show two and three regimes, respectively, but the speciation of HNO3, water, and 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 HNO3 molecules. Comparison with DFT-computed IR spectra 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 HNO3 molecules is more favored compared to a water-mediated interaction. SAXS of the organic phases were modeled as sums of Ornstein-Zernike (O-Z) scattering and a prepeak feature in the higher Q region, corresponding to features from density fluctuations and extractant packing, respectively. The extraction of undissociated HNO3 by the ion pairs leads to an increased X-ray scattering contrast in the organic phase without any significant change in the O-Z 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.

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supporting

confidence: 75%

Section: Atr-ftir and Dft Study Of Excess Hno 3 Extracted With Toah • Nomentioning

confidence: 99%

Speciation and organic phase structure in nitric acid extraction with trioctylamine

Nayak,

Servis,

Combs

et al. 2023

Preprint

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“…Here both the TOAH·NO 3 concentration and excess acid concentration significantly affect the nanoscale structure of the liquid. I 0 appears to have a maximum between 0.25 and 0.4 M (∼0.1–0.17 volume fraction of TOAH·NO 3 ), which is lower than the corresponding extractant concentration for neutral extractants such as TBP and DMDPMA in the absence of acid. − The origin of this difference is not clear, but it could be characteristic of TOA, the presence of an acid, or some combination thereof.…”

Section: Resultsmentioning

confidence: 96%

“…The presence of charged and polar protic species can also cause extensive noncovalent interactions which can affect the nanostructure of the organic phase and metal speciation. Hydrogen bonding networks have been found to be particularly important in nanoscale structuring, − but in the presence of charged species other interactions can also be significant. − Aggregation of amine and ammonium-based extractants has been related to metal extraction and separation trends. ,, This aggregation phenomenon has been principally studied using X-ray and neutron scattering techniques and the data fit to colloidal models, wherein the polar and nonpolar species in the organic phase self-assemble into reverse-micellar structures. , Recently, application of such colloidal models has been challenged, proposing instead that scattering profiles are due to density or concentration fluctuations in the organic phase in the vicinity of its critical point. − Presumably, the two models imply different energetic landscapes with regard to the nanoscale structuring of the organic phase, with differing impacts on metal extraction. Hence, understanding the speciation of the extracted acid, its interactions with other molecules in the organic phase, and the resulting nanoscale structure is essential to understanding the mechanism of metal extraction from acidic solutions with amines.…”

Section: Introductionmentioning

confidence: 99%

Speciation and Organic Phase Structure in Nitric Acid Extraction with Trioctylamine

Nayak,

Servis,

Combs

et al. 2024

J. Phys. Chem. B

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Understanding chemical speciation and intermolecular interactions in multicomponent liquids is essential to understanding their phase and chemical equilibria, which underpin chemical separation processes, including solvent extraction. Here 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 molecular speciation in the organic phase and its relationship with the nanoscale structure of the organic phase. Trends in acid and water extraction clearly show two and three regimes, respectively, indicating different stoichiometric relationships, but speciation of HNO 3 , water, and amine in these regimes is not apparent. 1 H 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•NO 3 ), and undissociated HNO 3 molecules. Comparison with DFT-computed IR spectra show that the chain-like configurations of TOAH•NO 3 •HNO 3 •H 2 O are favored over TOAH•NO 3 •H 2 O•HNO 3 , i.e., direct interaction between the nitrate and HNO 3 molecules is more favored compared to a water-mediated interaction. SAXS of the organic phases were modeled as sums of Ornstein−Zernike (O-Z) scattering and a prepeak feature in the higher Q region that corresponds to extractant packing. The extraction of undissociated HNO 3 by the ion pairs leads to an increased X-ray scattering contrast in the organic phase without any significant change in the correlation length. These results show that the organic phase nanostructure is more sensitive to the concentration of TOAH•NO 3 and is relatively unaffected by excess acid extraction. These findings will enable a molecular understanding of the mechanisms behind metal extraction from acidic media with basic extractants.

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“…Effects of Alkyl Tail Length on Malonamides. The functionalization of longer alkyl chains provides better solubility in the organic phase, 42,43 improving the metal loading capacity. However, the potential effect of alkyl length on conformational flexibility is not understood.…”

Section: Effects Of Alkyl Substitution On Malonamide α-Carbonmentioning

confidence: 99%

Using Metadynamics to Reveal Extractant Conformational Free Energy Landscapes

Wang,

Servis

2023

J. Phys. Chem. B

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Understanding the impact of extractant functionalization on metal-binding energetics in liquid−liquid extraction is essential to guide the development of better separation processes. Traditionally, computational extractant design uses electronic structure calculations on metal−ligand clusters to determine the metal-binding energy of the lowest energy state. Although highly accurate, this approach does not account for all of the relevant physics encountered under experimental conditions. Such methodologies often neglect entropic contributions such as temperature effects and ligand flexibility, in addition to approximating solvent−extractant interactions with implicit solvent models. In this study, we use classical molecular dynamics simulations with an advanced sampling method, metadynamics, to map out extractant molecule conformational free energies in the condensed phase. We generate the complete conformational landscape in solution for a family of bidentate malonamidebased extractants with different functionalizations of the headgroup and the side chains. In particular, we show how such alkyl functionalization reshapes the free energy landscape, affecting the free energy penalty of organizing the extractant into the cis-like metal-binding conformation from the trans-like conformation of the free extractant in solution. Specifically, functionalizing alkyl tails to the center of the headgroup has a greater influence on increasing molecular rigidity and disfavoring the binding conformation than functionalizing side chains. These findings are consistent with trends in metal-binding energetics based on experimentally reported distribution ratios. We also consider a different bidentate extractant molecule, carbamoylmethylphosphine oxide, and show how the choice of solvent can further reshape the conformational energetic landscape. This study demonstrates the feasibility of using molecular dynamics simulations with advanced sampling techniques to investigate extractant conformational energetics in solution, which, more broadly, will enable extractant design that accounts for entropic effects and explicit solvation.

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Critical fluctuations in liquid–liquid extraction organic phases controlled by extractant and diluent molecular structure

Abstract: Extractant aggregation in liquid–liquid extraction organic phases impacts extraction energetics and is related to the deleterious efficiency-limiting liquid–liquid phase transition known as third phase formation.

Cited by 9 publications

References 113 publications

Speciation and organic phase structure in nitric acid extraction with trioctylamine

Speciation and organic phase structure in nitric acid extraction with trioctylamine

Speciation and Organic Phase Structure in Nitric Acid Extraction with Trioctylamine

Using Metadynamics to Reveal Extractant Conformational Free Energy Landscapes

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