Microscopic Structures of Tri-n-butyl Phosphate/n-Octane Mixtures by X-ray and Neutron Scattering in a Wide q Range

The pervasive impact of critical fluctuations in liquid-liquid extraction organic phases

Liquid-liquid extraction is an essential chemical separation technique where polar solutes are extracted from an aqueous phase into a nonpolar organic solvent by amphiphilic extractant molecules. A fundamental limitation to the efficiency of this important technology is third phase formation, wherein the organic phase splits upon sufficient loading of polar solutes. The nanoscale drivers of phase splitting are challenging to understand in the complex hierarchically structured organic phases. In this study, we demonstrate that the organic phase structure and phase behavior are fundamentally connected in a way than can be understood with critical phenomena theory. For a series of binary mixtures of trialkyl phosphate extractants with linear alkane diluents, we combine small angle x-ray scattering and molecular dynamics simulations to demonstrate how the organic phase mesostructure over a wide range of compositions is dominated by critical concentration fluctuations associated with the critical point of the third phase formation phase transition. These findings reconcile many longstanding inconsistencies in the literature where small angle scattering features, also consistent with such critical fluctuations, were interpreted as reverse micellar-like particles. Overall, this study shows how the organic phase mesostructure and phase behavior are intrinsically linked, deepening our understanding of both and providing a new framework for using molecular structure and thermodynamic variables to control mesostructure and phase behavior in liquid-liquid extraction.

Polarization analysis for small-angle neutron scattering with a 3He spin filter at a pulsed neutron source

Neutron polarization analysis (NPA) for small-angle neutron scattering (SANS) experiments using a pulsed neutron source was successfully achieved by applying a 3He spin filter as a spin analyzer for the neutrons scattered from the sample. The cell of the 3He spin filter gives a weak small-angle scattering intensity (background) and covers a sufficient solid angle for performing SANS experiments. The relaxation time of the 3He polarization is sufficient for continuous use for approximately 2 days, thus reaching the typical duration required for a complete set of SANS experiments. Although accurate evaluation of the incoherent neutron scattering, which is predominantly attributable to the extremely large incoherent scattering cross section of hydrogen atoms in samples, is difficult using calculations based on the sample elemental composition, the developed NPA approach with consideration of the influence of multiple neutron scattering enabled reliable decomposition of the SANS intensity distribution into the coherent and incoherent scattering components. To date, NPA has not been well established as a standard technique for SANS experiments at pulsed neutron sources such as the Japan Proton Accelerator Research Complex (J-PARC) and the US Spallation Neutron Source. It is anticipated that this work will contribute significantly to the accurate determination of the coherent neutron scattering component for scatterers in various types of organic sample systems in SANS experiments at J-PARC, particularly for systems involving competition between the coherent and incoherent scattering intensity.

Competitive extraction of Li, Na, K, Mg and Ca ions from acidified aqueous solutions into chloroform layer containing diluted alkyl phosphates

Alkyl phosphates were extensively used in liquid-liquid extraction of lanthanides and actinides, but to a lesser extent for alkali and alkaline earth metals. The high amount of alkyl phosphate, which is usually used in the organic layer (>40 wt%), is not favoured due to its corrosive effect and toxicity. In the present work, diluted chloroform solutions (20.0 mM) of tri-n-butyl phosphate (TBP), tris(2-ethylhexyl) phosphates (TRIS) and bis(2-ethylhexyl) phosphate (BIS) were investigated for their extraction of Li, Na, K, Mg and Ca ions. The extraction experiments were conducted on 7.0 M HNO₃ aqueous solutions containing 60.0 mM of metal ions in binary (Li⁺ and Mg²⁺), ternary (Li⁺, Na⁺ and K⁺) and quinary (Li⁺, Na⁺, K⁺, Mg²⁺ and Ca²⁺) mixtures. The Li⁺ selectivity over Mg²⁺ was very high in the binary system. Remarkably, increasing HNO₃ concentration in the aqueous layer had opposing effect on the extraction of Li⁺ (positive) and Mg²⁺ (negative). However, the selectivity for Li⁺ became less dramatic in the case of ternary and quinary system, though the selectivity varied with initial metal concentrations. The amounts of water and NO₃^- transferred into the organic layer demonstrated their synergistic effect on extracting metal ions. In the ternary and quinary systems, the total concentrations of metal ions in the organic layer (ranged from 49 to 85 mM) were higher than the concentration of ligand in the organic layer (20.0 mM), suggesting that metal ions may be extracted into water/ligand/NO₃^- aggregates in the organic layer. TBP, TRIS and BIS do not have significant difference in their extraction behaviour. The FTIR results indicated formation of P⁺-O^-M⁺/M²⁺ in the solid TBP/metal complex.

Hierarchical phenomena in multicomponent liquids: simulation methods, analysis, chemistry

Complex, multicomponent, solutions have often been studied solely through the lens of specific applications of interest. Yet advances to both simulation methodologies (enhanced sampling, etc.) and analysis techniques (network analysis algorithms and others), are creating a trove of data that reveal transcending characteristics across vast compositional phase space. This perspective discusses technical considerations of the reliable and accurate simulations of complex solutions, followed by the advances to analysis algorithms that elucidate coupling of different length and timescale behavior (hierarchical phenomena). The different manifestations of hierarchical phenomena are presented across an array of solution environments, emphasizing fundamental and ongoing science questions. With a more advanced molecular understanding in hand, a quintessential application (solvent extraction) is discussed, where significant opportunities exist to re-imagine the technical scope of an established technology.

Molecular Insights into Water Clusters Formed in Tributylphosphate-Di-(2-ethylhexyl)phosphoric Acid Extractant Systems from Experiments and Molecular Dynamics Simulations

Di-(2-ethylhexyl)phosphoric acid (D2EHPA) and tributylphosphate (TBP) are two of the most studied and researched organophosphorous extractants. D2EHPA is an acidic extractant, offering both hydrogen bond donor and acceptor sites while TBP, a neutral extractant, only offers a single acceptor site per molecule. In spite of this, it is observed that 1 M D2EHPA in dodecane is a poorer extractant for water than 1 M TBP in dodecane. The objective of present work is to look into the molecular interactions that cause such behavior. Experiments were carried out with varying molar ratios of TBP and D2EHPA in the organic dodecane phase. Total extractant concentration was kept constant at 1 M with dodecane as diluent. Water extraction was quantified by measuring the moisture content of the organic phase after equilibration. ¹H and ³¹P NMR spectra of the organic phase samples were recorded to study the change in the chemical environment upon extraction. Small angle X-ray scattering data of water saturated extractant phases were analyzed for the possibility of a reverse micellar aggregate formation. Molecular dynamics simulations could calculate free energies in quantitative agreement with experiments. Experimental and simulation studies showed that aggregation in the organic phase was promoted by the presence of water. This combined approach, of experiments and simulation, has shown that water is indispensable for the formation of ordered aggregates of extractants in nonpolar organic solvents. It is seen that, in the organic phase, around 80% of water's hydrogen bonds are with extractant molecules rather than with itself. The analysis clearly indicates that, rather than forming an aqueous core surrounded by extractant, water acts as a bridge between extractant molecules.

Determination of the Structures of Uranyl-Tri-n-butyl-Phosphate Aggregates by Coupling Experimental Results with Molecular Dynamic Simulations

The complex structure of a plutonium uranium refining by extraction (PUREX) process organic phase was characterized by combining results from experiments and molecular dynamics simulations. For the first time, the molecular interactions between tri-n-butyl phosphate (TBP) and the extracted solutes, as well as TBP aggregation after the extraction of water and/or uranyl nitrate, were described and analyzed concomitantly. Coupling molecular dynamics simulations with small- and wide-angle X-ray scattering (SWAXS) experiments can lead to simulated organic solutions that are representative of the experimental ones, even for high extractant and solute concentrations. Furthermore, this coupling is well adapted for the interpretation of SWAXS experiments without preliminary hypothesis on the size or shape of aggregates. The results link together previous literature studies obtained for each level of depiction separately (complexation or aggregation). Without uranium, or at low metal concentration, almost no aggregation was observed. At high uranium concentration, organic phases contain small [UO₂ (NO₃ )₂ (TBP)₂ ]_n polymetallic aggregates (with n=2 to 4), in which the 1:2 U/TBP stoichiometry is preserved.

Phosphoric acid recovery from concentrated aqueous feeds by a mixture of di-isopropyl ether (DiPE) and tri-n-butylphosphate (TBP): extraction data and modelling

This paper reports experimental data and modelling about the extraction of H₃PO₄ from concentrated aqueous feeds (3 to 14 M) by the mixture of DiPE and TBP at 25 °C: distribution ratios of H₃PO₄ and water, and associated change of phase volume ratio.

Structural insights into the multinuclear speciation of tetravalent cerium in the tri-n-butyl phosphate–n-dodecane solvent extraction system

Macroscopic phase behaviors in the liquid–liquid extraction are explained by microscopic, reverse micellar fluid structures containing tetranuclear Ce(iv) clusters revealed by use of X-ray spectroscopy and scattering of the light and dense organic phases.

Characterization of the species formed during the extraction of thorium employing tri-n-butyl phosphate and N,N-dihexyl octanamide as extractants by laser desorption/ionization time-of-flight mass spectrometry

Laser desorption/ionization-time-of-flight mass spectrometry (LDI-ToF-MS) has been applied to identify and characterize the organic phase species formed during the extraction of thorium nitrate by 1.1 M tri-n-butyl phosphate (TBP) and N,N-dihexyl octanamide (DHOA) solutions in n-dodecane. The aqueous phase thorium concentrations (at 4M HNO3) have been suitably chosen to get Loaded organic phases with/without third-phase. The extracted species have been characterized for the first time using LDI-ToF-MS. The results show feasibility of the use of this technique for understanding the extraction mechanisms and third-phase formation behavior of different extractants. The different chemical species observed using this technique are consistent with those observed by small-angle neutron scattering (SANS).

Coordination structures and supramolecular architectures in a cerium(III)-malonamide solvent extraction system

The process chemistry and solution structures investigated in the title system bridge the three ostensibly disparate fields of separation sciences, soft matter research, and coordination chemistry. We have explored this subject with synchrotron radiation research and advanced analyses leading to original insights into aggregation phenomena in solvent extraction. Herein we present findings showing the coagulation of reverse micelles into wormlike aggregates in organic phases (N,N'-dimethyl-N,N'-dibutyltetradecylmalonamide-abbreviated as DMDBTDMA-in n-dodecane) obtained by liquid-liquid extraction following contact with acidic and neutral aqueous media containing trivalent cerium. The growth of solute architectures was shown to prelude phase transition (i.e., the formation of a "third phase"). The presence of acid was shown to promote the growth of these micellar chains and, therefore, promoted third-phase formation. Acid was also shown to hydrate and swell the reverse micelle units, preorganizing them to allow for incorporation of cerium, leading to different coordination structures and enhanced metal extraction. The approach of linking both the coordination environment and supramolecular structures to the process properties of a solvent extraction system in a single study provides perspectives that are not available from independent, uncorrelated experimentation. Moreover, the analysis of small-angle X-ray scattering data from a solvent extraction system using the generalized indirect Fourier transform method to gain real-space information led to insights not otherwise available, showing that micellar assemblies are larger and more ordered than previously thought. This multipronged and multidisciplinary investigation opens new avenues in the evolving understanding of solute architectures in organic phases of practical relevance to solvent extraction and, simultaneously, of fundamental relevance to structured fluids and, in particular, phase transition phenomena.