Nanoscale view of assisted ion transport across the liquid–liquid interface

Abstract

During solvent extraction, amphiphilic extractants assist the transport of metal ions across the liquid-liquid interface between an aqueous ionic solution and an organic solvent. Investigations of the role of the interface in ion transport challenge our ability to probe fast molecular processes at liquid-liquid interfaces on nanometer-length scales. Recent development of a thermal switch for solvent extraction has addressed this challenge, which has led to the characterization by X-ray surface scattering of interfacial intermediate states in the extraction process. Here, we review and extend these earlier results. We find that trivalent rare earth ions, Y(III) and Er(III), combine with bis(hexadecyl) phosphoric acid (DHDP) extractants to form inverted bilayer structures at the interface; these appear to be condensed phases of small ion-extractant complexes. The stability of this unconventional interfacial structure is verified by molecular dynamics simulations. The ion-extractant complexes at the interface are an intermediate state in the extraction process, characterizing the moment at which ions have been transported across the aqueous-organic interface, but have not yet been dispersed in the organic phase. In contrast, divalent Sr(II) forms an ion-extractant complex with DHDP that leaves it exposed to the water phase; this result implies that a second process that transports Sr(II) across the interface has yet to be observed. Calculations demonstrate that the budding of reverse micelles formed from interfacial Sr(II) ion-extractant complexes could transport Sr(II) across the interface. Our results suggest a connection between the observed interfacial structures and the extraction mechanism, which ultimately affects the extraction selectivity and kinetics.

Water follows polar and nonpolar protein surface domains

“…water | proteins | surface | solubility | AMOEBA force field A ccurate knowledge of the properties of water on a surface is essential for understanding processes such as chemical reactions (1,2), the transport of ions (3,4) and water (5,6), enzymatic activity (7), charge transfer (8), and various cellular functions (9). In particular, hydration on a protein surface plays a crucial role in solubilizing proteins (10)(11)(12), hydrolyzing enzymatic substrates (13,14), and ligand recognition (10,15,16), as well as protein assembly (17,18).…”

Interfaces and mixing: Nonequilibrium transport across the scales

“…This issue reports significant successes that have been achieved recently in understanding interfaces, mixing, and nonequilibrium dynamics based on theoretical analysis, large-scale numerical simulations, laboratory experiments, and technology development (20)(21)(22)(23)(24)(25)(26)(27)(28)(29)(30)(31)(32)(33)(34). These include theoretical approaches for handling complex multiscale, nonlocal, and statistically unsteady dynamics and boundary value problems; developments of efficient Eulerian and Lagrangian methods of large-scale numerical modeling; advancements in laboratory experiments in low-and high-energy density regimes; and possibilities for dramatic improvements in precision, accuracy, dynamic range, reproducibility, and data acquisition rate with the use of modern technologies (20)(21)(22)(23)(24)(25)(26)(27)(28)(29)(30)(31)(32)(33)(34).…”

“…These works represent frontier research in interaction of molecules and chemical reactions, interfacial dynamics and nonequilibrium processes, fluid turbulence and turbulent mixing, supernovas and nuclear synthesis, formation of fluid phases at molecular scales, first-principlesbased reaction kinetics, ion transport of nanoscales, electric and magnetic fields structures, dynamics of high-energy density plasmas, subdiffusive and superdiffusive transport, formation of vortices in geophysical flows, diffusiophoresis of charged particles, and electron transport in macromolecules. They motivate the discussions of rigorous mathematical problems, theoretical approaches, and state-of-the-art numerical simulations along with advanced experimental methods and technological applications (20)(21)(22)(23)(24)(25)(26)(27)(28)(29)(30)(31)(32).…”

“…The contributions in this issue include 6 perspective papers and 7 research papers (20)(21)(22)(23)(24)(25)(26)(27)(28)(29)(30)(31)(32). They are ordered from perspective to research papers, from macroscopic to microscopic scales, and from fundamentals to applications.…”

“…The research paper by Liang et al (26) studies the transport of metal ions across the liquid-liquid interface between an aqueous ionic solution and an organic solvent. To probe fast molecular processes on nanometer-length scales in the presence of interfaces, the authors apply state-of-the-art techniques to characterize these processes using X-ray surface scattering of intermediate states and observe directly the essentially nondiffusive transport of ions across the interface.…”