CHARACTERIZING STRUCTURES FROM THE ATOMIC SCALE TO THE NANOSCALE
One example of how the state-of-the-science approaches described in this report can be used to gain detailed insight into structures from the atomic scale to the nanoscale is the exploitation of scattering to access information on interatomic correlations. Atom–atom correlations are obtained by a Fourier transformation of scattering data to obtain pair distribution functions, wherein peak positions represent correlation distances and peak intensities are related to coordination numbers (Egami and Billinge, 2003). Obtaining adequate peak resolution to understand chemical speciation requires the use of a spallation neutron source or a synchrotron.
With this method, one can characterize adsorbed gases in nanoporous membrane materials (Gallis et al., 2017) and study interpenetrating networks of relevance to carbon dioxide sequestration (Mulfort et al., 2010). High-energy x-ray scattering (energy > 60 keV) patterns have been used to determine thermodynamic stability constants for uranyl chloride complexation (see Figure C-1). The advantage of this approach over more standard techniques is that the results provide a direct link between the targeted ion’s coordination structure and thermodynamic free energies (Soderholm et al., 2011). Further development and use of this approach can provide modelers with the structural information necessary to understand the thermodynamic energy changes that drive mass transfer.
Extended x-ray absorption fine structure (EXAFS) spectroscopy has become a tool of choice to elucidate the local structure and bonding of target species. An exemplary study to identify an ionic liquid-based solvent to extract cobalt (Co) from samarium (Sm) used EXAFS to determine that the extraction occurs mainly via an ion pair rather than an ion-exchange mechanism (Sobekova Foltova et al., 2019). The competing roles of chloride, thiocyanate, and nitrate anions for complexation with Sm and Co were determined by comparing the coordination environments of these metals after extraction into the ionic liquid. The results provide direction for further development of an environmental-friendly approach to the recycling and recovery of high-purity metals from wasted permanent magnets. A similar study, coupling small-angle neutron scattering (SANS) with EXAFS, demonstrated the important role of extended aggregates and solvent structuring in the performance of a task-specific ionic liquid designed for the targeted extraction of trivalent lanthanides from spent nuclear-fuel waste (Abney et al., 2017).
CHARACTERIZING STRUCTURE, DYNAMICS, AND FIELDS AT INTERFACES
Experimental techniques that probe the speciation of surfaces and their adsorbates have been largely restricted to high-vacuum environments and cryogenic temperatures. Questions have been continually raised about the applicability of the experimental results to operando systems. Complicating the difficulties for liquid–solid and liquid–liquid processes is that changes in the targeted moiety’s speciation occur across a buried phase boundary, a problem not limited to separation science but relevant to a wide variety of chemical and physical systems from biological membranes to catalysis to geochemical transport. The result of the widespread need for experimental probes of interfaces under operando environments has been the recent development of a wide variety of techniques that yield information on the structure of interfaces on the atomic scale.
The opportunities for the separations community are just now being conceptualized, with recent papers reporting interesting and underappreciated techniques for studying the structure of interfaces that might eventually provide a mechanism for controlling a separation outcome. For example, studies have demonstrated an unexpected two-step process for PtCl62- adsorption at an air–water interface (Uysal et al., 2017). A recent study hints at the power of a multimodal approach to surface studies; it combines x-ray surface techniques with sum frequency generation to reveal the surface-water structure in a system related to solvent extraction (Rock et al., 2018). The combination of x-ray and neutron
reflectivity has been shown to be a powerful probe of a separation interface. Using that approach, researchers identified the mixing of water and extractant molecules within an interphase region in an oil and water solvent extraction system (Scoppola et al., 2015).
INTERFACES WITH SOLIDS
Accessing atomic-level detail of binding at sites at a solid interface under operando conditions is often complicated by multiscale roughness or porosity, the presence of multiple reactive site types, and reactive behavior convoluted with transport processes, such as advection and diffusion. Similar challenges are encountered in other disciplines, such as heterogeneous catalysis, electrochemistry, and geochemistry. Those disciplines have helped to advance characterization techniques that overcome such obstacles in situ.
For example, researchers have made substantial progress in probing the multiscale structure of solid surfaces down to the atomic scale directly in a fluid medium of interest. Dynamics can also be probed. For example, three-dimensional crystal truncation rod x-ray scattering techniques with supporting ab initio molecular dynamics simulations based on density functional theory have disentangled relationships between time-averaged interfacial atomic structure and dynamic processes, such as ligand exchange in complex aqueous electrolytes at a given pH (McBriarty et al., 2017, 2018). In situ scanning probe methods, such as atomic force microscopy (AFM), can image molecular dynamics at video-rate speeds. For example, AFM with a high scan rate is opening a window into such nanoscale processes as nucleation and growth, molecular translocation, and self-assembly (Kodera et al., 2010; Uchihashi and Scheuring, 2018). Another example is the use of nuclear magnetic resonance (NMR) and molecular in silico simulations using coarse-grained ligand docking to determine which region of a protein binds to multimodal ligand chromatographic beads (Chung et al., 2010).
Under operando conditions, the average interfacial structure might depend on the interplay of relatively static atoms at the solid surface with relatively dynamic molecules in an overlying fluid. Time-averaged structure captured by x-ray scattering can now be scrutinized and interpreted in terms of fast molecular dynamic processes, such as water-ligand exchange on metal oxide surfaces using ab initio molecular dynamic simulations (see Figure C-2).
In situ x-ray scattering techniques have been used to probe interface structure under equilibrium conditions. Recently, those techniques were extended into the more challenging case of far-from-equilibrium conditions, revealing how interfacial atomic structure responds to changes in electrostatic potential and adsorbed ion distributions (McBriarty et al., 2018). Conceptually, such measurements can be extended to steady-state flow conditions to examine more complex operando conditions.
Opportunities regarding the liquid–liquid interface (LLI), and in particular probing and understanding the interface structure and dynamics, have arisen. The opportunities include understanding the extent of heterogeneity within the interface and mechanism of transinterface mass transport (Zhou et al., 2017). In principle, understanding of the LLI could be used to design interface structure and properties. The LLI plays three roles in liquid–liquid extraction systems: mass transport, dispersion behavior, and soft-matter interactions. Thus, understanding interfacial structure could be used for kinetic control of selectivity and control of drop formation and coalescence. Understanding of the LLI could affect the effectiveness and utility of liquid–liquid extraction, supported liquid membranes, emulsion liquid membranes, polymer-inclusion membranes, membrane electrodes, membrane-assisted solvent extraction, two-phase aqueous extraction, and extraction chromatography.
X-ray techniques similar to those mentioned above for solid–fluid interfaces, such as x-ray reflectivity and x-ray fluorescence near-total reflection, that are available at synchrotron light sources have allowed a much more detailed inspection of the arrangement of extractant molecules at LLIs (Bu et al., 2014). Optical techniques have also proved valuable as probes to reveal the nature of species and supramolecular ordering at the LLI. In one example, an apparatus called a centrifugal liquid membrane has facilitated analysis by reducing the bulk-phase thicknesses to less than 100 μm (Watarai, 2014). When highly absorbing species, such as porphyrins, are used as extractant molecules, subtle interfacial speciation effects can be observed. In that way, ultraviolet-visible absorbance and fluorescence spectroscopies have provided detailed kinetic and mechanistic information not accessible with x-ray reflectance techniques.
Determining interfacial structure has been difficult. Fluorescence methods have detected single molecules at the LLI and measured lateral diffusion under different conditions. Other informative techniques include Raman spectroscopy, second harmonic generation, vibrational sum frequency generation, and optical chirality methods (Kocsis et al., 2018). When combined with molecular simulations, those techniques can reveal significant detail regarding interfacial structure. For example, recent work has shown that after adsorption, PtCl62- complexes partially retain their first and second hydration spheres, and it is possible to identify three types of water molecules around them on the basis of their orientational structures and hydrogen bonding strengths. Those results have important implications for relating interfacial water structure and hydration enthalpy to the general understanding of specific ion effects (Rock et al., 2018).
A critical component for understanding phase transfer during a liquid–liquid separation is the target ion’s speciation, including any change in ligation when it is bound by the extractant molecule. A recent study answered some questions. It reported the extraction of erbium (Er) from an acidic aqueous phase by using dihexadecyl phosphate (DHDP) dissolved in dodecane as an extractant. The system was used as a model system. Coupling several related x-ray surface techniques with EXAFS spectroscopy and using temperature to arrest the separation process revealed a counterintuitive interfacial arrangement of extractants in an intermediate state: an inverted bilayer of extractants with headgroups complexing and shielding the Er3+ ions from the organic phase. That
CONFINEMENT BETWEEN INTERFACES
As in other fields, confinement between interfaces can be important in separation processes. For example, many of the proposals to use nanoporous materials to separate ethanol from water in biofuel purification rely on the fact that ethanol responds to confinement entirely differently from water (Wang et al., 2014).
The theory behind the interfacial forces under confinement are well established. However, techniques that can isolate and measure their magnitudes for a given pair of interacting oriented material surfaces are few and are restricted to flat or nearly flat interacting surfaces. Scanning probe microscopies and spectroscopies constitute one proven path forward (Zhang et al., 2017, 2018). For example, dynamic force spectroscopy (DFS) has recently demonstrated the self-organization of confined solvent molecules during the approach of solid surfaces of a defined crystallographic structure and twist angle. The self-organization was related to the net interaction force (see Figure C-4) (Zhang et al., 2017).
CHARACTERIZING STRUCTURES FROM THE NANOSCALE TO THE MICROSCALE
Understanding reverse micelles and other aggregates requires extending structural studies from the atomic to the nanoscale and mesoscale. Besides observing cloud points optically (Quina et al., 1999), small-angle x-ray scattering (SAXS) and SANS have been used to characterize these and other reverse-micelle systems more fully (Jensen et al., 2007; Hollamby et al., 2010; Ellis et al., 2013). SAXS and SANS can also be used to characterize aggregate structures, including membrane and polymer pores and channels, as well as the role of reverse micelles in separation efficacy. Researchers have combined x-ray absorption spectroscopy (to understand the role of metal coordination) with SANS data to observe the role of the counterion on ionic liquid-based solvent microstructure separation (Abney et al., 2017). The results show important mechanistic parallels between ionic liquids and classic solvents and suggest opportunities for process improvements. Application of dynamic light scattering (DLS) combined with SAX and SANS has proved a powerful option for analyzing spatial and temporal fluctuations in polymer gels (Shibayama, 1998) but remains a topic of opportunity for sample characterization. Questions concerning uptake and fouling are particularly amenable to a combination of SAXS and SANS data: the complementary scattering could be used effectively to probe the roles of membrane and analyte structuring.
NEW OPPORTUNITIES FOR CHARACTERIZING STRUCTURAL DYNAMICS
Most structural characterization related to separation systems is time-averaged, so quantitative information on system dynamics and its potential effect on chemical separations is scarce. For liquid–liquid systems, dynamic properties are of low enough energy that they could affect selectivity or capacity. For example, control or limiting of the splitting of the organic phase into two distinct phases (third-phase formation) in liquid–liquid extraction could depend on structural dynamics.
NMR has provided information on dynamic processes and, although the technique is limited to slower time scales and the structural information is indirect, its potential effect is apparent (Witherspoon et al., 2018). DLS could be extended into a temporal regime that touches on complex solution structuring. Emerging opportunities are presented by the recent developments in x-ray photon correlation spectroscopy (XPCS) (Shpyrko, 2014), a technique that uses the coherent x-ray beams only now becoming available at synchrotron sources. Newly developing capabilities are enabling access to information on the nanometer and microsecond scales that are relevant to separations (see Figure C-5).
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