A Telescoping View of Solute Architectures in a Complex Fluid System

Abstract

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.

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

“…Small angle X-ray and neutron scattering (SAXS and SANS, respectively) have been the principal techniques in this effort and they show the link between aggregation structures on extraction efficiency and third phase formation (an undesired fluid-fluid phase transition and splitting due to high metal loading) in different extraction systems. [26][27][28][29][30][31][32][33][34] A significant portion of these studies used Baxter's sticky hard sphere model to describe the interactions between reverse micellar (RM) aggregates. In Baxter model, spherical aggregates attract each other when they are very close and show hard sphere repulsion when the distance between their centers is less than or equal to their hard sphere diameter (See results and discussion for details).…”

“…32,36 Motokawa et al, have developed a model for the clustering interactions between the metal-extractant complexes based on molecular level details 34. Their model does not include the effects of metal-free aggregates on clustering.…”

Proton Chelating Ligands Drive Improved Chemical Separations for Rhodium

“…The best-fit curves for the SANS profiles for L2 were obtained using the hard-sphere model, with optimized solute structures provided by DFT calculations 14 (see Supporting Information), with the radii varying slightly from 9.7 ± 1.3 Å for (L2D•Cl)2 to 10.9 ± 1.…”

A microfluidic study of synergic liquid–liquid extraction of rare earth elements

“…13,15 Ion transfer between two phases and its driving force have been the subject of many kinetic, thermodynamic, theoretical, and modeling studies which require years of research with yet many remaining unknowns. 11,[16][17][18][19][20][21][22] In addition to this complexity, even when promising hydrometallurgy processes are described, their use by the recycling industry is hindered by the cost associated with the use of very low pH and the variability in the elemental composition from one waste lot to the other. The industry is therefore not only in need of more efficient approaches that can work at higher pH and show higher performances but also of methods enabling a faster speed of process development or adjustment.…”