Search

International audience; In this article Part II, we consider the application of ienaic modeling for solvent extraction of rare earths in the case of the mixed DMDOHEMA/HDEHP extractant system. The system exhibits a synergistic extraction effect depending on the acid concentration and the extractant mole fraction, as demonstrated in the experimental article Part I. In this work, we directly compare experimental findings with theoretical predictions of the droplet model. The model considers the effective free energy of transfer as a combination of competing molecular forces, but contrary to previous micelle models that focus only on the dominant stoichiometry, it allows calculations of free energies of every possible spherical aggregate. The resulting image of the extraction process is that different behaviors can be obtained depending on the acidity and mole fraction of extractants, which are associated with different aggregation regimes in a complex free energy landscape of the system. Nevertheless, self-assembly is tuned by the extraction of all solutes, and not only the target metal cations.

International audience; In solvent extraction, the self-assembly of amphiphilic molecules into an organized structure is the phenomenon responsible for the transfer of the metal ion from the aqueous phase to the organic solvent. Despite their significance for chemical engineering and separation science, the forces driving the solute transfer are not fully understood. Instead of assuming the simple complexation reaction with predefined stoichiometry, we model synergistic extraction systems by a colloidal approach that explicitly takes into account the self-assembly resulting from the amphiphilic nature of the extractants. Contrary to the current paradigm of simple stoichiometry behind liquid–liquid extraction, there is a severe polydispersity of aggregates completely different in compositions, but similar in the free energy. This variety of structures on the nanoscale is responsible for the synergistic transfer of ions to the organic phase. Synergy can be understood as a reciprocal effect of chelation: it enhances extraction because it increases the configurational entropy of an extracted ion. The global overview of the complex nature of a synergistic mixture shows different regimes in self-assembly, and thus in the extraction efficiency, which can be tuned with respect to the green chemistry aspect.

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.

International audience; 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 kBT 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.

This article highlights current paradigms and challenges in modeling of lanthanides and actinides solvent extraction by lipophilic extractants. Within the multiscale approach, complex phenomena that occur in solvent systems can be rationalized at different length scales. Strengths and drawbacks of quantum and classical simulations, as well as mesoscopic modeling, are presented. In the multiscale modeling, the definition of standard states is of paramount importance because it dictates the amount of collective effects included within calculations. Mesoscopic modeling of the transfer and the aggregation free energies can be used to successfully predict properties of extraction systems at phenomenological scale and to assist chemical engineering of separation industry.

International audience; 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.

International audience; Charging of material surfaces in aqueous electrolyte solutions is one of the most important processes in the interactions between biomaterials and surrounding tissue. Other than a biomaterial, titania nanotubes (TiO2 NTs) represent a versatile material for numerous applications such as heavy metal adsorption or photocatalysis. In this article, the surface charge properties of titania NTs in NaNO3 solution were investigated through electrophoretic mobility and polyelectrolyte colloid titration measuring techniques. In addition, we used high-resolution transmission electron microscopy imaging to determine the morphology of TiO2 NTs. A theoretical model based on the classical density functional theory coupled with the charge regulation method in terms of mass action law was developed to understand the experimental data and to provide insights into charge properties at different physical conditions, namely, pH and NaNO3 concentration. Two intrinsic protonation constants and surface site density have been obtained. The electrostatic properties of the system in terms of electrostatic potentials and ion distributions were calculated and discussed for various pH values. The model can quantitatively describe the titration curve as a function of pH for higher bulk salt concentrations and the difference in the equilibrium amount of charges between the inner and outer surfaces of TiO2 NTs. Calculated counterion (NO3–) distributions show a pronounced decrease of NO3– ions for high bulk pH (both inside and outside TiO2 NT) because of the strong electric field. With the decrease of bulk pH or the increase of the salt concentration, NO3– is able to accumulate near the TiO2 NTs surfaces.