Multiscale modeling of solvent extraction and the choice of reference state: Mesoscopic modeling as a bridge between nanoscale and chemical engineering

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.

Hierarchical Aggregation in a Complex Fluid─The Role of Isomeric Interconversion

There is an ever-increasing body of evidence that metallic complexes involving amphiliphic ligands do not form normal solutions in organic solvents. Instead, they form complex fluids with intricate structures. For example, the metallic complexes may aggregate into clusters, and these clusters themselves may aggregate into superclusters. To gain a deeper insight into the mechanisms at play, we have used an improved force field to conduct extensive molecular dynamics simulations of a system composed of zirconium nitrate, water, nitric acid, tri-n-butyl phosphate, and n-octane. The important new finding is that a dynamic equilibrium between the cis and trans isomers of the metal complex is likely to play a key role in the aggregation behavior. The isolated cis and trans isomers have similar energies, but simulation indicates that the clusters consist predominantly of cis isomers. With increasing metal concentration, we hypothesize that more clustering occurs and the chemical equilibrium shifts toward the cis isomer. It is possible that such isomeric effects play a role in the liquid-liquid extraction of other species and the inclusion of such effects in flow sheet modeling may lead to a better description of the process.

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 Dynamics Study of the Aggregation Behavior of N,N,N',N'-Tetraoctyl Diglycolamide

Liquid–liquid extraction is a commonly used technique to separate metals and is a process that has particular relevance to the nuclear industry. There has been a drive to use environmentally friendly ligands composed only of carbon, hydrogen, nitrogen, and oxygen. One example is the i-SANEX process that has been developed to separate minor actinides from spent nuclear fuel. The underlying science of such processes, is, however, both complex and intriguing. Recent research indicates that the liquid phases involved are frequently structured fluids with a hierarchical organization of aggregates. Effective flow-sheet modeling of such processes is likely to benefit from the knowledge of the fundamental properties of these phases. As a stepping stone toward this, we have performed molecular dynamics simulations on a metal free i-SANEX system composed of the ligand N,N,N′,N′-tetraoctyl diglycolamide (TODGA), diluent hydrogenated tetrapropylene (TPH), and polar species water and nitric acid. We have also studied the effects of adding n-octanol and swapping TPH for n-dodecane. It would seem sensible to understand this simpler system before introducing metal complexes. Such an understanding would ideally arise from studying the system’s properties over a wide range of compositions. The large number of components, however, precludes a comprehensive scan of compositions, so we have chosen to study a fixed concentration of TODGA while varying the concentrations of water and nitric acid over a substantial range. Reverse aggregates are observed, with polar species in the interior in contact with the polar portions of the TODGA molecules and the organic diluent on the exterior in contact with the TODGA alkyl chains. These aggregates are irregular in shape and grow in size as the amount of water and nitric acid increases. At a sufficiently high polar content, a single extended cluster forms corresponding to the third phase formation. No well-defined bonding motifs were observed between the polar species and TODGA. The cluster size distribution fits an isodesmic model, where the Gibbs energy change of adding a TODGA molecule to a cluster ranges between 4.5 and 7.0 kJ mol^–1, depending on the system composition. The addition of n-octanol was found to reduce the degree of aggregation, with n-octanol acting as a co-surfactant. Exchanging the diluent TPH for n-dodecane also decreased the aggregation. We present evidence that this is due to the greater penetration of n-dodecane into the reverse aggregates. It is known, however, that the propensity for the third phase formation is greater with n-dodecane as the diluent than is the case with TPH, but we argue that these two results are not contradictory. This research casts light on the driving forces for aggregation, informs process engineers as to what species are present, and indicates that flow-sheet liquid–liquid extraction modeling might benefit by incorporating an isodesmic aggregation approach.

Advancing Chemical Separations: Unraveling the Structure and Dynamics of Phase Splitting in Liquid-Liquid Extraction

Liquid-liquid extraction (LLE), the go-to process for a variety of chemical separations, is limited by spontaneous organic phase splitting upon sufficient solute loading, called third phase formation. In this study we explore the applicability of critical phenomena theory to gain insight into this deleterious phase behavior with the goal of improving separations efficiency and minimizing waste. A series of samples representative of rare earth purification were constructed to include each of one light and one heavy lanthanide (cerium and lutetium) paired with one of two common malonamide extractants (DMDOHEMA and DMDBTDMA). The resulting postextraction organic phases are chemically complex and often form rich hierarchical structures whose statics and dynamics near the critical point were probed herein with small-angle X-ray scattering and high-speed X-ray photon correlation spectroscopy. Despite their different extraction behaviors, all samples show remarkably similar critical behavior with exponents well described by classical critical point theory consistent with the 3D Ising model, where the critical behavior is characterized by fluctuations with a single diverging length scale. This unexpected result indicates a significant reduction in relevant chemical parameters at the critical point, indicating that the underlying behavior of phase transitions in LLE rely on far fewer variables than are generally assumed. The obtained scalar order parameter is attributed to the extractant fraction of the extractant/diluent mixture, revealing that other solution components and their respective concentrations simply shift the critical temperature but do not affect the nature of the critical fluctuations. These findings point to an opportunity to drastically simplify studies of liquid-liquid phase separation and phase diagram development in general while providing insights into LLE process improvement.

Improved Recovery and Selectivity of Lanthanide-Ion-Binding Cyclic Peptide Hosts by Changing the Position of Acidic Amino Acids

The development of an effective host molecule to separate lanthanide (Ln) ions and a method for predicting its guest recognition/self-assembly behavior based on primary chemical structures are highly sought after in both academia and industry. Herein, we report the improvement of one-pot Ln ion recovery and a performance prediction method for four new cyclic peptide hosts that differ in the position of acidic amino acids. These cyclic peptide hosts could recognize Ln3+ directly through a 1:1 complexation–precipitation process and exhibited high Lu3+ selectivity in spite of similar ion size and electronegativity when the positions of the acidic amino acids were changed. This unpredictable selectivity was explained by considering the dipole moment, lowest unoccupied molecular orbital, and cohesion energy. In addition, a semi-empirical function using these parameters was proposed for screening the sequence and estimating the isolated yields without long-time molecular dynamics calculations. The insights obtained from this study can be employed for the development of high-performance peptides for the selective recovery of Ln and other metal ions, as well as for the construction of diverse supramolecular recognition systems.

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.

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.

Mesostructuring in Liquid-Liquid Extraction Organic Phases Originating from Critical Points

Organic phase structure plays an important role in solute extraction energetics and phase behavior of liquid-liquid extraction (LLE) systems. For a binary extractant (amphiphile)/solvent mixture of relevance to LLE, we find that the organic phase mesostructuring is consistent with extractant concentration fluctuations as the compositional isotherm traverses the Widom line above its liquid-liquid critical point. This reveals a different mechanism for the well-documented heterogeneities in LLE organic phases that are typically attributed to micellization.

Impact of Water Extraction on Malonamide Aggregation: A Molecular Dynamics and Graph Theoretic Approach

Solution structure in liquid-liquid extraction affects the efficacy of separation; however, even for simplified organic phases, structural characterization and attribution of aggregation to intermolecular interactions are fundamental challenges. We investigate water uptake into organic phases for two malonamides commonly applied to actinide and lanthanide separations. Extracted water induces reorganization of the amphiphilic extractant molecules, although we find this rearrangement is not strongly manifested in small-angle X-ray scattering making it challenging to probe without methods such as atomistic simulation. Using a graph theoretic approach to define hydrogen bonded water/malonamide aggregates from molecular dynamics simulations, we find evidence of a characteristic aggregate size by water number that results from geometric accommodation of the surrounding malonamide molecules. This implies a degree of size selectivity inherent to these water-in-oil aggregates. Conversely, we find no evidence of a characteristic size of the aggregates with respect to their malonamide number. By defining a separate graphical representation of self-association of the amphiphilic malonamides, we quantify how water affects the local and nonlocal topology of the malonamide network, providing a basis for characterization of the structure and impact of polar solutes in increasingly complex organic phases.

From Water Solutions to Ionic Liquids with Solid State Nanopores as a Perspective to Study Transport and Translocation Phenomena

Solid state nanopores are single-molecular devices governed by nanoscale physics with a broad potential for technological applications. However, the control of translocation speed in these systems is still limited. Ionic liquids are molten salts which are commonly used as alternate solvents enabling the regulation of the chemical and physical interactions on solid-liquid interfaces. While their combination can be challenging to the understanding of nanoscopic processes, there has been limited attempts on bringing these two together. While summarizing the state of the art and open questions in these fields, several major advances are presented with a perspective on the next steps in the investigations of ionic-liquid filled nanopores, both from a theoretical and experimental standpoint. By analogy to aqueous solutions, it is argued that ionic liquids and nanopores can be combined to provide new nanofluidic functionalities, as well as to help resolve some of the pertinent problems in understanding transport phenomena in confined ionic liquids and providing better control of the speed of translocating analytes.

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.

Phase diagrams and microstructures of aqueous short alkyl chain polyethylene glycol ether carboxylate and carboxylic acid triblock surfactant solutions

HYPOTHESIS

The surfactant C₈EO₈CH₂COOH (Akypo LF2) and its salts have a small hydrophobic and a significantly longer hydrophilic part. As a consequence, there must be a significant steric constraint, once these surfactant molecules form micelles. In addition, the partially charged headgroups should bring some additional fine-tuning via electrostatic interactions to this "essentially non-ionic" surfactant.

EXPERIMENTS

Phase diagrams of binary mixtures of water and C₈EO₈CH₂COOH are established over large concentration and temperature ranges, also at different pHs and in the presence of sodium and calcium ions. Surface tensions and osmotic pressures are measured to understand the systems. To evaluate the microstructures, also Dynamic Light Scattering and Small-Angle X-ray Scattering are performed.

FINDINGS

Apart from the formation of coacervates at very low surfactant concentrations, spherical micelles persist over the whole concentration and temperature range and do not change in size and shape. At very high surfactant concentrations, above 60% by weight, where the headgroups are no longer fully hydrated, the standard core-shell structure of micelles vanishes and highly stabilized aggregates of 8-26 octyl chains are suspended in interdigitated polyoxyethylene layers and form an "osmotic brush". When the acid is partially transformed to a sodium salt, the repulsion between the micelles increases, whereas bridging between micelles prevails, when the counterions are calcium cations. Remarkably, the negative charges of the headgroups are randomly distributed in the hydrophilic ethylene oxide shell. Altogether, a phase diagram without lyotropic liquid crystalline phases and an extreme shift of the cloud-point in temperature and composition is found, similar to the phase diagram of C₈EO₈OH already known in literature. The phase properties can be explained by the curvature and packing constraints together with the Lindemann rule applied to short hydrocarbon chains.