A predictive model of reverse micelles solubilizing water for solvent extraction

Insights into water extraction and aggregation mechanisms of malonamide-alkane mixtures

Structure at the nanoscale in the organic phase of liquid-liquid extraction systems is often tied to separation performance. However, the weak interactions that drive extractant assembly lead to poorly defined structures that are challenging to identify. Here, we investigate the mechanism of water extraction for a malonamide extractant commonly applied to f-element separations. We measure extractant concentration fluctuations in the organic phase with small angle X-ray scattering (SAXS) before and after contact with water at fine increments of extractant concentration, finding no qualitative changes upon water uptake that might suggest significant nanoscopic reorganization of the solution. The critical composition for maximum fluctuation intensity is consistent with small water-extractant adducts. The extractant concentration dependence of water extraction is consistent with a power law close to unity in the low concentration regime, suggesting the formation of 1 : 1 water-extractant adducts as the primary extraction mechanism at low concentration. At higher extractant concentrations, the power law slope increases slightly, which we find is consistent with activity effects modeled using Flory-Huggins theory without introduction of additional extractant-water species. Molecular dynamics simulations are consistent with these findings. The decrease in interfacial tension with increasing extractant concentration shows a narrow plateau region, but it is not correlated with any change in fluctuation or water extraction trends, further suggesting no supramolecular organization such as reverse micellization. This study suggests that water extraction in this system is particularly simple: it relies on a single mechanism at all extractant concentrations, and only slightly enhances the concentration fluctuations characteristic of the dry binary extractant/diluent mixture.

Insights into the spontaneous multi-scale supramolecular assembly in an ionic liquid-based extraction system

Herein, we report a four-step mechanism for the spontaneous multi-scale supramolecular assembly (MSSA) process in a two-phase system concerning an ionic liquid (IL). The complex ions, elementary building blocks (EBBs), [EBB]_n clusters and macroscopic assembly (MA) sphere are formed step by step. The porous large-sized [EBB]_n clusters in the glassy state can hardly stay in the IL phase and they transfer to the IL-water interface due to both electroneutrality and amphiphilicity. Then, the clusters undergo random collision in the interface driven by the Marangoni effect and capillary force thereafter. Finally, a single MA sphere can be formed owing to supramolecular interactions. To our knowledge, this is the first example realizing spontaneous whole-process supramolecular assembly covering microscopic, mesoscopic and macroscopic scales in extraction systems. The concept of multi-scale selectivity (MSS) is therefore suggested and its mechanism is revealed. The selective separation and solidification of metal ions can be realized in a MSSA-based extraction system depending on MSS. In addition, insights into the physicochemical characteristics of ILs from microscopic, mesoscopic to macroscopic scales are provided, and especially, the solvation effect of ILs on the large-sized clusters leading to the phase-splitting is examined. It is quite important that the polarization of uranyl in its complex, the growing of uranyl clusters in an IL as well as the glassy material of uranyl are investigated systematically on the basis of both experiment and theoretical calculations in this work.

Molecular Forces in Liquid-Liquid Extraction

The phase transfer of ions is driven by gradients of chemical potentials rather than concentrations alone (i.e., by both the molecular forces and entropy). Extraction is a combination of high-energy interactions that correspond to short-range forces in the first solvation shell such as ion pairing or complexation forces, with supramolecular and nanoscale organization. While the latter are similar to the long-range solvent-averaged interactions in the colloidal world, in solvent extraction they are associated with lower characteristic lengths of the nanometric domain. Modeling of such complex systems is especially complicated because the two domains are coupled, whereas the resulting free energy of extraction is around k_BT to guarantee the reversibility of the practical process. Nevertheless, quantification is possible by considering a partitioning of space among the polar cores, interfacial film, and solvent. The resulting free energy of transfer can be rationalized by utilizing a combination of terms which represent strong complexation energies, counterbalanced by various entropic effects and the confinement of polar solutes in nanodomains dispersed in the diluent, together with interfacial extractant terms. We describe here this ienaics approach in the context of solvent extraction systems; it can also be applied to further complex ionic systems, such as membranes and biological interfaces.

Using Microemulsions: Formulation Based on Knowledge of Their Mesostructure

Microemulsions, as thermodynamically stable mixtures of oil, water, and surfactant, are known and have been studied for more than 70 years. However, even today there are still quite a number of unclear aspects, and more recent research work has modified and extended our picture. This review gives a short overview of how the understanding of microemulsions has developed, the current view on their properties and structural features, and in particular, how they are related to applications. We also discuss more recent developments regarding nonclassical microemulsions such as surfactant-free (ultraflexible) microemulsions or ones containing uncommon solvents or amphiphiles (like antagonistic salts). These new findings challenge to some extent our previous understanding of microemulsions, which therefore has to be extended to look at the different types of microemulsions in a unified way. In particular, the flexibility of the amphiphilic film is the key property to classify different microemulsion types and their properties in this review. Such a classification of microemulsions requires a thorough determination of their structural properties, and therefore, the experimental methods to determine microemulsion structure and dynamics are reviewed briefly, with a particular emphasis on recent developments in the field of direct imaging by means of electron microscopy. Based on this classification of microemulsions, we then discuss their applications, where the application demands have to be met by the properties of the microemulsion, which in turn are controlled by the flexibility of their amphiphilic interface. Another frequently important aspect for applications is the control of the rheological properties. Normally, microemulsions are low viscous and therefore enhancing viscosity has to be achieved by either having high concentrations (often not wished for) or additives, which do not significantly interfere with the microemulsion. Accordingly, this review gives a comprehensive account of the properties of microemulsions, including most recent developments and bringing them together from a united viewpoint, with an emphasis on how this affects the way of formulating microemulsions for a given application with desired properties.

Thermodynamic Modeling of Salting Effects in Solvent Extraction of Cobalt(II) from Chloride Media by the Basic Extractant Methyltrioctylammonium Chloride

The design and optimization of solvent extraction processes for metal separations are challenging tasks due to the large number of adjustable parameters. A quantitative predictive solvent extraction model could help to determine the optimal parameters for solvent extraction flow sheets, but such predictive models are not available yet. The main difficulties for such models are the large deviations from ideal thermodynamic behavior in both the aqueous and organic phases due to high solute concentrations. We constructed a molecular thermodynamic model for the extraction of CoCl₂ from different chloride salts by 0.2 mol L^-1 trioctylmethylammonium chloride in toluene using the OLI mixed-solvent electrolyte (OLI-MSE) framework. This was accomplished by analyzing the water and hydrochloric acid content of the organic phase, measuring the water activity of the system, and using metal complex speciation and solvent extraction data. The full extractant concentration range cannot be modeled by the OLI-MSE framework as this framework lacks a description for reversed micelle formation. Nevertheless, salting effects and the behavior of hydrochloric acid can be accurately described with the presented extraction model, without determining specific Co(II)-salt cation interaction parameters. The resulting model shows that the salting effects originate from indirect salt cation-solvent interactions that influence the availability of water in the aqueous and organic phases.

Thermodynamics of Malonamide Aggregation Deduced from Molecular Dynamics Simulations

The aggregation of malonamide extractants diluted in an aliphatic solvent phase has been studied in the presence of water by molecular dynamics simulation. Using association criteria based on distances between molecules and graphs theory, the aggregate distribution has been computed and the corresponding Gibbs energy of aggregates and mass action law constants have been determined. Finally, a model allowing us to the compute critical micelle concentration and osmotic data for a variable concentration of extractants, with or without a correction of the organic phase activity, was developed. It appears however that the accurate depiction of the aggregation allows modeling the thermodynamics of the solution even without an explicit calculation of the activity: both models give results in good agreement with the experiments.

Colloidal Model for the Prediction of the Extraction of Rare Earths Assisted by the Acidic Extractant

We propose the statistical thermodynamic model for the prediction of the liquid-liquid extraction efficiency in the case of rare-earth metal cations using the common bis(2-ethyl-hexyl)phosphoric acid (HDEHP) extractant. In this soft matter-based approach, the solutes are modeled as colloids. The leading terms in free-energy representation account for: the complexation, the formation of a highly curved extractant film, lateral interactions between the different extractant head groups in the film, configurational entropy of ions and water molecules, the dimerization, and the acidity of the HDEHP extractant. We provided a full framework for the multicomponent study of extraction systems. By taking into account these different contributions, we are able to establish the relation between the extraction and general complexation at any pH in the system. This further allowed us to rationalize the well-defined optimum in the extraction engineering design. Calculations show that there are multiple extraction regimes even in the case of lanthanide/acid system only. Each of these regimes is controlled by the formation of different species in the solvent phase, ranging from multiple metal cation-filled aggregates (at the low acid concentrations in the aqueous phase), to the pure acid-filled aggregates (at the high acid concentrations in the aqueous phase). These results are contrary to a long-standing opinion that liquid-liquid extraction can be modeled with only a few species. Therefore, a traditional multiple equilibria approach is abandoned in favor of polydisperse spherical aggregate formations, which are in dynamic equilibrium.

A Telescoping View of Solute Architectures in a Complex Fluid System

Short- and long-range correlations between solutes in solvents can influence the macroscopic chemistry and physical properties of solutions in ways that are not fully understood. The class of liquids known as complex (structured) fluids-containing multiscale aggregates resulting from weak self-assembly-are especially important in energy-relevant systems employed for a variety of chemical- and biological-based purification, separation, and catalytic processes. In these, solute (mass) transfer across liquid-liquid (water, oil) phase boundaries is the core function. Oftentimes the operational success of phase transfer chemistry is dependent upon the bulk fluid structures for which a common functional motif and an archetype aggregate is the micelle. In particular, there is an emerging consensus that mass transfer and bulk organic phase behaviors-notably the critical phenomenon of phase splitting-are impacted by the effects of micellar-like aggregates in water-in-oil microemulsions. In this study, we elucidate the microscopic structures and mesoscopic architectures of metal-, water-, and acid-loaded organic phases using a combination of X-ray and neutron experimentation as well as density functional theory and molecular dynamics simulations. The key conclusion is that the transfer of metal ions between an aqueous phase and an organic one involves the formation of small mononuclear clusters typical of metal-ligand coordination chemistry, at one extreme, in the organic phase, and their aggregation to multinuclear primary clusters that self-assemble to form even larger superclusters typical of supramolecular chemistry, at the other. Our metrical results add an orthogonal perspective to the energetics-based view of phase splitting in chemical separations known as the micellar model-founded upon the interpretation of small-angle neutron scattering data-with respect to a more general phase-space (gas-liquid) model of soft matter self-assembly and particle growth. The structure hierarchy observed in the aggregation of our quinary (zirconium nitrate-nitric acid-water-tri-n-butyl phosphate-n-octane) system is relevant to understanding solution phase transitions, in general, and the function of engineered fluids with metalloamphiphiles, in particular, for mass transfer applications, such as demixing in separation and synthesis in catalysis science.

Multicomponent Model for the Prediction of Nuclear Waste/Rare-Earth Extraction Processes

We develop a minimal model for the prediction of solvent extraction. We consider a rare earth extraction system for which the solvent phase is similar to water-poor microemulsions. All physical molecular quantities used in the calculation can be measured separately. The model takes into account competition complexation, mixing entropy of complexed species, differences of salt concentrations between the two phases, and the surfactant nature of extractant molecules. We consider the practical case where rare earths are extracted from iron nitrates in the presence of acids with a common neutral complexing extractant. The solvent wetting of the reverse aggregates is taken into account via the spontaneous packing. All the water-in-oil reverse aggregates are supposed to be spherical on average. The minimal model captures several features observed in practice: reverse aggregates with different water and extractant content coexist dynamically with monomeric extractant molecules at and above a critical aggregate concentration (CAC). The CAC decreases upon the addition of electrolytes in the aqueous phase. The free energy of transfer of an ion to the organic phase is lower than the driving complexation. The commonly observed log-log relation used to determine the apparent stoichiometry of complexation is valid as a guideline but should be used with care. The results point to the fact that stoichiometry, as well as the probabilities of a particular aggregate, is dependent on the composition of the entire system, namely the extractant and the target solutes' concentrations. Moreover, the experimentally observed dependence of the extraction efficiency on branching of the extractant chains in a given solvent can be quantified. The evolution of the distribution coefficient of particular rare earth, acid, or other different metallic cations can be studied as a function of initial extractant concentration through the whole region that is typically used by chemical engineers. For every chemical species involved in the calculation, the model is able to predict the exact equilibrium concentration in both the aqueous and the solvent phases at a given thermodynamic temperature.

Charge transport by inverse micelles in non-polar media

Charged inverse micelles play an important role in the electrical charging and the electrodynamics of nonpolar colloidal dispersions relevant for applications such as electronic ink displays and liquid toner printing. This review examines the properties and the behavior of charged inverse micelles in microscale devices in the absence of colloidal particles. It is discussed how charge in nonpolar liquids is stabilized in inverse micelles and how conductivity depends on the inverse micelle size, water content and ionic impurities. Frequently used nonpolar surfactant systems are investigated with emphasis on aerosol-OT (AOT) and poly-isobutylene succinimide (PIBS) in dodecane. Charge generation in the bulk by disproportionation is studied from measurements of conductivity as a function of surfactant concentration and from generation currents in quasi steady-state. When a potential difference is applied, the steady-state situation can show electric field screening or complete charge separation. Different regimes of charge transport are identified when a voltage step is applied. It is shown how the transient and steady-state currents depend on the rate of bulk generation, on insulating layers and on the sticking or non-sticking behavior of charged inverse micelles at interfaces. For the cases of AOT and PIBS in dodecane, the magnitude of the generation rate and the type of interaction at the interface are very different.

Stability of reverse micelles in rare-earth separation: a chemical model based on a molecular approach

A new molecular approach based on molecular dynamics simulations is proposed to investigate the stability of reverse micelles containing strategic metals in organic solution.