Specific Salt Effect on the Interaction between Rare Earth Ions and Trioctylphosphine Oxide Molecules at the Organic-Aqueous Two-Phase Interface: Experiments and Molecular Dynamics Simulations

Aqueous Interfaces in Chemical Separations

Chemical separations play a vital role in refinery and reprocessing of critical materials, such as platinum group metals, rare earths, and actinides. The choice of separation system─whether it is liquid-liquid extraction (LLE), sorbents, or membranes─depends on specific needs and applications. In almost all separation processes, the desired metal ions adsorb or transfer across an aqueous interface, such as the solid/liquid interface in sorbents or oil/water interfaces in LLE. Despite these separation technologies being extensively used for decades, our understanding of the molecular-scale mechanisms governing ion adsorption and transport at interfaces remains limited. This knowledge gap presents a significant challenge in meeting the increasing demands for these critical materials due to their growing use in advanced technologies. Fortunately, recent advancements in surface-specific experimental and computational techniques offer promising avenues to bridge this gap and facilitate the development of next-generation separation systems. Interestingly, unanswered questions regarding interfacial phenomena in chemical separations hold great relevance to various fields, including energy storage, geochemistry, and atmospheric chemistry. Therefore, the model interfacial systems developed for studying chemical separations, such as amphiphilic molecules assembled at a solid/water, air/water, or oil/water interface, may have far-reaching implications, extending beyond separations and opening doors to addressing a wide range of scientific inquiries. This perspective discusses recent interfacial studies elucidating amphiphile-ion interactions in chemical separations of metal ions. These studies provide direct, molecular-scale information about solute and solvent behavior at aqueous interfaces, including multivalent and complex ions in highly concentrated solutions, which play key roles in LLE of critical materials.

Recent Advances in the Study of Trivalent Lanthanides and Actinides by Phosphinic and Thiophosphinic Ligands in Condensed Phases

The separation of trivalent actinides and lanthanides is a key step in the sustainable development of nuclear energy, and it is currently mainly realized via liquid-liquid extraction techniques. The underlying mechanism is complicated and remains ambiguous, which hinders the further development of extraction. Herein, to better understand the mechanism of the extraction, the contributing factors for the extraction are discussed (specifically, the sulfur-donating ligand, Cyanex301) by combing molecular dynamics simulations and experiments. This work is expected to contribute to improve our systematic understanding on a molecular scale of the extraction of lanthanides and actinides, and to assist in the extensive studies on the design and optimization of novel ligands with improved performance.

Effect of Sulfate, Citrate, and Tartrate Anions on the Liquid-Liquid Equilibrium Behavior of Water + Surfactant

Cloud point extraction is a versatile method aimed at separating compounds from complex mixtures and arouses great technological interest, particularly among the biochemical industries. However, one must have deep knowledge of the liquid–liquid equilibrium behavior of systems to properly use the method. Thus, we used thermodynamic parameters to evaluate the effect of citrate, sulfate, and tartrate anions on the phase separation of water + Triton X-114® mixtures at 283.2 K, 293.2 K, and 303.2 K. In these systems, increasing the temperature and the anion molar fraction expanded the biphasic region in the following order: C6H5O73-> SO42- >  C4H4O62−. Unlike other studies based on the Hofmeister series, the Gibbs free energy of micellization correlated the anion effect on the biphasic region with the spontaneity of the micelle formation. The water molecules structured around these anions were evaluated according to the shell volume of the immobilized water by electrostriction, volume of water around the hydration shell, Gibbs free energy of hydration, and Gibbs free energy of electrostriction (ΔGel12). The citrate anion presented a higher ΔGel12 of −1781.49 kJ mol−1, due to the larger number of electrons around it. In addition, the partition coefficient of the surfactant in the two liquid phases revealed a linear dependence upon the anion mole fractions by following the previous anion sequence and temperature in the phase separation.

Origins of Clustering of Metalate-Extractant Complexes in Liquid-Liquid Extraction

Effective and energy-efficient separation of precious and rare metals is very important for a variety of advanced technologies. Liquid-liquid extraction (LLE) is a relatively less energy intensive separation technique, widely used in separation of lanthanides, actinides, and platinum group metals (PGMs). In LLE, the distribution of an ion between an aqueous phase and an organic phase is determined by enthalpic (coordination interactions) and entropic (fluid reorganization) contributions. The molecular scale details of these contributions are not well understood. Preferential extraction of an ion from the aqueous phase is usually correlated with the resulting fluid organization in the organic phase, as the longer-range organization increases with metal loading. However, it is difficult to determine the extent to which organic phase fluid organization causes, or is caused by, metal loading. In this study, we demonstrate that two systems with the same metal loading may impart very different organic phase organizations and investigate the underlying molecular scale mechanism. Small-angle X-ray scattering shows that the structure of a quaternary ammonium extractant solution in toluene is affected differently by the extraction of two metalates (octahedral PtCl₆^2- and square-planar PdCl₄^2-), although both are completely transferred into the organic phase. The aggregates formed by the metalate-extractant complexes (approximated as reverse micelles) exhibit a more long-range order (clustering) with PtCl₆^2- compared to that with PdCl₄^2-. Vibrational sum frequency generation spectroscopy and complementary atomistic molecular dynamics simulations on model Langmuir monolayers indicate that the two metalates affect the interfacial hydration structures differently. Furthermore, the interfacial hydration is correlated with water extraction into the organic phase. These results support a strong relationship between the organic phase organizational structure and the different local hydration present within the aggregates of metalate-extractant complexes, which is independent of metalate concentration.

Ion-specific clustering of metal-amphiphile complexes in rare earth separations

The nanoscale structure of a complex fluid can play a major role in the selective adsorption of ions at the nanometric interfaces, which is crucial in industrial and technological applications. Here we study the effect of anions and lanthanide ions on the nanoscale structure of a complex fluid formed by metal-amphiphile complexes, using small angle X-ray scattering. The nano- and mesoscale structures we observed can be directly connected to the preferential transfer of light (La and Nd) or heavy (Er and Lu) lanthanides into the complex fluid from an aqueous solution. While toluene-based complex fluids containing trioctylmethylammonium-nitrate (TOMA-nitrate) always show the same mesoscale hierarchical structure regardless of lanthanide loading and prefer light lanthanides, those containing TOMA-thiocyanate show an evolution of the mesoscale structure as a function of the lanthanide loading and prefer heavy lanthanides. The hierarchical structure indicates the presence of attractive interactions between ion-amphiphile aggregates, causing them to form clusters. A clustering model that accounts for the hard sphere repulsions and short-range attractions between the aggregates has been adapted to model the X-ray scattering results. The new model successfully describes the nanoscale structure and helps in understanding the mechanisms responsible for amphiphile assisted ion transport between immiscible liquids. Accordingly, our results imply different mechanisms of lanthanide transport depending on the anion present in the complex fluid and correspond with anion-dependent trends in rare earth separations.

Cation Effect of Chloride Salting Agents on Transition Metal Ion Hydration and Solvent Extraction by the Basic Extractant Methyltrioctylammonium Chloride

The addition of a nonextractable salt has an important influence on the solvent extraction of metal ions, but the underlying principles are not completely understood yet. However, relating solute hydration mechanisms to solvent extraction equilibria is key to understanding the mechanism of solvent extraction of metal ions as a whole. We have studied the speciation of Co(II), Zn(II), and Cu(II) in aqueous solutions containing different chloride salts to understand their extraction to the basic extractant methyltrioctylammonium chloride (TOMAC). This includes the first speciation profile of Zn(II) in chloride media with the three Zn(II) species [Zn(H₂O)₆]²⁺, [ZnCl₃H₂O]⁻, and [ZnCl₄]^2–. The observed differences in extraction efficiency for a given transition metal ion can be explained by transition metal ion hydration due to ion–solvent interactions, rather than by ion–solute interactions or by differences in speciation. Chloride salting agents bearing a cation with a larger hydration Gibbs free energy reduce the free water content more, resulting in a lower hydration for the transition metal ion. This destabilizes the transition metal chloro complex in the aqueous phase and increases the extraction efficiency. Salting agents with di- and trivalent cations reduce the transition metal chloro complex hydration less than expected, resulting in a lower extraction efficiency. The cations of these salting agents have a very large hydration Gibbs free energy, but the overall hydration of these salts is reduced due to significant salt ion pair formation. The general order of salting-out strength for the extraction of metal ions from chloride salt solutions is Cs⁺ < Rb⁺ < NH₄⁺ ≈ K⁺ < Al³⁺ ≈ Mg²⁺ ≈ Ca²⁺ ≈ Na⁺ < Li⁺. These findings can help in predicting the optimal conditions for metal separation by solvent extraction and also contribute to a broader understanding of the effects of dissolved salts on solutes.

Addition of a nonextractable salt influences the stability and solvent extraction efficiency of metal complexes. Cations of different chloride salts reduce the solution free water content as a function of their increasing hydration energy and decreasing tendency for ion pair formation with chloride anions. These ion−solvent interactions reduce the hydration of metal complexes, increasing their distribution ratios. These effects influence aqueous transition metal complexes more than direct ion−solute interactions and changes in complex speciation.

Anions Enhance Rare Earth Adsorption at Negatively Charged Surfaces

Anions are expected to be repelled from negatively charged surfaces. At aqueous interfaces, however, ion-specific effects can dominate over direct electrostatic interactions. Using multiple in situ surface sensitive experimental techniques, we show that surface affinities of SCN^- anions are so strong that they can adsorb at a negatively charged floating monolayer at the air-aqueous interface. This extreme example of ion-specific effects may be very important for understanding complex processes at aqueous interfaces, such as chemical separations of rare earth metals. Adsorbed SCN^- ions at the floating monolayer increase the overall negative charge density, leading to enhanced trivalent rare earth adsorption. Surface sensitive X-ray fluorescence measurements show that the surface coverage of Lu³⁺ ions can be triple the apparent surface charge of the floating monolayer in the presence of SCN^-. Comparison to NO₃^- samples shows that the effects are strongly dependent on the character of the anion, providing further evidence of ion-specific effects dominating over electrostatics.

Binding of lanthanide salts to zwitterionic phospholipid micelles

As the use of lanthanide salts in biophysical systems increases and the separation of lanthanides from nuclear and other wastes with extraction processes has become an important technological challenge, a deeper understanding of the behavior of lanthanides at lipid interfaces is urgently needed. In this work the interaction of lanthanide salts with zwitterionic phospholipids is probed using aqueous micelles of the surfactant dodecyl phosphocholine (DPC), which are useful membrane-mimetic model systems, widely used for the solubilization of membrane proteins in aqueous solutions. Because more than one species exists in lanthanide salt solutions, even at the pH value of 4 used in this experiment, the major goal of this investigation is to examine which species are actually binding to the micelles. Using static and time-dependent europium fluorescence, strong indications are obtained that both the Eu³⁺ cation and its 1:1 chloride, nitrate, or sulfate complexes bind to the micelles, whereas the europium species do not appear to interact strongly with DPC molecules below the cmc. From isothermal titration calorimetry (ITC) measurements it is found that the lanthanide interaction with DPC micelles increases to the right of the lanthanide series and is - surprisingly - endothermic, underlying the important role of hydration effects in the interaction. The anion of the lanthanide salt strongly influences the thermodynamics: perchlorate and sulfate salts give extraordinary results, switching the interaction to exothermic. A multi-level phenomenological electrostatic model of the europium fluorescence lifetimes strongly suggests that in the case of nitrate salts both Ln³⁺ and LnNO₃²⁺ ions bind to the micelles. Overall a detailed molecular picture of the complexity of lanthanide-lipid interactions at interfaces is emerging from these experiments and the associated modelling effort.

In situ study of the competitive adsorption of ions at an organic-aqueous two-phase interface: the essential role of the Hofmeister effect

Understanding of the microcosmic essence of the competitive adsorption of different ions at liquid/liquid interfaces is of crucial importance for the elucidation of the unique chemical reactivities or selectivities of ions in numerous heterogeneous chemical processes. However, the knowledge of the microscopic mechanism behind the competitive adsorption of ions at the liquid/liquid interface is lacking. Herein, the competitive adsorption of various inorganic salt anions at organic-aqueous two-phase interfaces has been investigated as compared to that of the CrO42- ions by total internal reflection UV-visible (TIR-UV) spectroscopy since CrO42- ions are detectable by UV-visible spectroscopy and have a relatively poor interface propensity as compared to other chaotropic ions. Experimental results indicate that the interface propensities of different salt anions to the organic/aqueous phase interface follow the Hofmeister series. Molecular dynamics simulations further provided molecular-level evidence for role of the Hofmeister series of ions in the competitive adsorption of salt anions at organic-aqueous two-phase interfaces; the present study provided the first-hand experimental evidence demonstrating the occurrence of the Hofmeister series effect at the organic/aqueous two-phase interfaces, influencing the competitive adsorption of different salt ions; moreover, it is expected to offer a basis for the development of new strategies for the regulation of the chemical reactivity and selectivity of ions at organic/aqueous phase interfaces by introduction of other ions for competitive adsorption.

The nature of salt effect in enhancing the extraction of rare earths by non-functional ionic liquids: Synergism of salt anion complexation and Hofmeister bias

Separation and recycling of rare-earths using ionic liquids as extractant are becoming a promising approach to replace traditional volatile organic solvents in recent years. Generally, the addition of some special salts could improve the extraction efficiency of rare-earths by numerous non-functional ionic liquids. However, knowledge behind the nature of the salt effect is limited. Here, we found that the enhancement in the extraction of rare-earth ions, Pr³⁺ ions, using non-functional ionic liquid, [A336][NO₃] (Tricaprylmethylammonium nitrate) was driven by the synergism of Hofmeister bias and complexation behaviors of salt anions with Pr³⁺ ions. Molecular dynamic simulations offered a new insight into the interaction mechanism of the ionic liquid with Pr³⁺ ions at liquid/liquid interface. It was revealed that salt anions could perform as a bridge to connect Pr³⁺ ions and the ionic liquid, so that promoted the extraction of Pr³⁺ ions. Therefore, the strong complexation ability of salt anions with Pr³⁺ ions and poor hydration of salt anions faciliated the transport of Pr³⁺ ions across liquid/liquid interface.