Letter pubs.acs.org/JPCL
Controlling the Intermediate Structure of an Ionic Liquid for f‑Block Element Separations Carter W. Abney,*,† Changwoo Do,‡ Huimin Luo,† Joshua Wright,§,∥ Lilin He,‡ and Sheng Dai†,¶ †
Oak Ridge National Laboratory, One Bethel Valley Road, P.O. Box 2008, Oak Ridge, Tennessee 37831, United States Biology and Soft Matter Division, Neutron Sciences Directorate, Oak Ridge National Laboratory, P.O. Box 2008, Oak Ridge, Tennessee 37831, United States § Advanced Photon Source, Argonne National Laboratory, 9700 Cass Avenue, Lemont, Illinois 60439, United States ∥ Illinois Institute of Technology, 3300 South Federal Street, Chicago, Illinois 60616, United States ¶ Department of Chemistry, University of Tennessee, Knoxville, Tennessee 37966, United States ‡
S Supporting Information *
ABSTRACT: Recent research has revealed molecular structure beyond the inner coordination sphere is essential in defining the performance of separation processes; nevertheless, such structure remains largely unexplored. Here we apply small-angle neutron scattering (SANS) and X-ray absorption fine structure (XAFS) spectroscopy to investigate the structure of an ionic liquid system studied for f-block element separations. SANS data reveal dramatic changes in the ionic liquid microstructure (∼150 Å) which we demonstrate can be controlled by judicious selection of counterion. Mesoscale structural features (>500 Å) are also observed as a function of metal concentration. XAFS analysis supports formation of extended aggregate structures, similar to those observed in traditional solvent extraction processes, and suggests additional parallels may be drawn from further study. Achieving precise tunability over the intermediate features is an important development in controlling mesoscale structure and realizing advanced new forms of soft matter.
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the metal-bound nitrate or a noninnocent ionic liquid anion.14 The common theme in these examples is that the structure of the extractant system outside of the primary coordination sphere directly participates in successful separation processes. Therefore, the ability to manipulate systems at these intermediate spatial regimes is essential for the design of advanced separations processes and development of new softmatter materials. In this work we report control over the micro- to nanoscale structure of a task-specific ionic liquid (IL) system designed for the extraction of trivalent lanthanides from spent nuclear fuel processing effluent. The extraction system is composed of two IL precursors (Figure 1a, inset). The first IL is the metal extractant and is composed of an alkyl-substituted phosphonium cation ([Pnnn(14)]+, where the subscript denotes the alkyl chain length and n = 4, 6) and the conjugate base of the extensively investigated di(2-ethylhexyl)phosphoric acid ([DEHP]−). This IL extractant is dissolved in a second IL solvent composed of 1-alkyl-3-methylimidizolium bis(trifluoromethylsulfonyl)imide ([Cnmim][NTf2], n = 4, 6, 8, 10). Previous work has revealed the system displays impressive performance for the extraction of trivalent lanthanides
raditional paradigms in separation processes focus on the binding of individual ions either by well-designed receptors possessing complementary local structure, or by solvation of the target ion. However, behavior emerging at spatial regimes starting beyond the first coordination sphere and extending to assemblies in the low micrometers have recently been identified as possessing an indispensable role in successful separation processes.1,2 For example, fundamental insights into third phase formation, a well-documented impediment to liquid−liquid separations, arise from the uncontrolled aggregation of micelles formed by amphiphilic extractants.3 These mesoscaled assemblies have since been demonstrated to drive metal ion extraction,4 with intermolecular dipole interactions in adjacent aggregates effecting the observed structure.5,6 Similarly, investigations of uranyl binding modes by polymer-based adsorbents diverge from those proposed from computational and crystallographic studies, suggesting the mesoscale morphology of the polymer may directly affect interactions at the molecular level.7,8 Studies focused on in situ formation of selective binding pockets, such as supramolecular sandwich complexes,9,10 organic capsules,11,12 and intertwined foldamers13 reveal performance relies on intraligand contacts outside of the primary coordination sphere. Our recent report of a dicarboxamide-functionalized phenanthroline task-specific ionic liquid also suggests interaction of the cationic carboxamide functionalities with either © 2017 American Chemical Society
Received: March 29, 2017 Accepted: April 19, 2017 Published: April 19, 2017 2049
DOI: 10.1021/acs.jpclett.7b00755 J. Phys. Chem. Lett. 2017, 8, 2049−2054
Letter
The Journal of Physical Chemistry Letters
Figure 1. (a) SANS data for [P666(14)][DEHP] in [C4mim][NTf2] after contact with TALSPEAK simulant containing 0.0025 M Lu(NO3)3. Dashed lines denote the slope of the scattering data. The orange line is the fit of the intermediate q-range. Shown in the inset are the molecular structures of the ionic liquids used in this study. (b) Simultaneous fit of [P666(14)][DEHP] in [C4mim][NTf2] contacted with TALSPEAK simulant, with data for D2O displayed in orange and H2O displayed in green. The lines are the simultaneous fits afforded by the solid cylinder model. The inset presents change in Δρ2 as a function of f water.
parameters of this inhomogeneity.29 A coarse approximation from inspection of the q-range reveals aggregates greater than 500 Å in dimension. In contrast, SANS data of the IL system prior to contact with TALSPEAK simulant or for the constituent ILs do not display significant scattering features at any spatial range (Figure S1). Unexpectedly, the data for the analogous IL system contacted with D2O-substituted TALSPEAK simulant displayed a decrease in scattering intensity, most apparent in the intermediate q range below 0.01 Å−1 (Figure 1b). This is indicative of reduced scattering contrast with D2O, which can only be ascribed to a change in the scattering length density (ρ) of the intermediate structure. Due to the significantly larger ρ for D2O than H2O (Table S2), an increase in contrast upon inclusion of D2O is typically expected, and the observed decrease in contrast thus requires colocalization of the extracted water with an IL component. Of the IL constituents, only [C4mim]+ displays significant solubility in water, having been identified in previous work as the exchanged species affording charge balance following metal ion extraction.30,31 Preservation of local charge balance and mitigation of Coulombic repulsion demands pairing with the [DEHP]−, favored because of the polar head as well as a priori knowledge of the extreme hydrophobicity of [NTF2]−.32 The ρ of the structure (ρstructure) can thus be expressed as the average contribution of [C4mim][DEHP] (ρ(IL) ) and the included water. The scattering contrast was plotted as a function of water volume fraction ( f) assuming equal extraction for both H2O and D2O, revealing the observed contrast reduction can be observed only for f < 45% (Figure 1b, inset). Furthermore, ρstructure(D2O) can be expressed as a function of ρstructure(H2O), as shown in eq 1 and derived in the Supporting Information:
(Ln(III)) from simulant representative of TALSPEAK (Trivalent Actinide Lanthanide Separation with Phosphorusreagent from Aqueous Komplexes) conditions,15 achieving 7 orders of magnitude improvement over HDEHP (protonated [DEHP]−) deployed under traditional conditions.16,17 However, the aforementioned work relies solely on macroscopic extraction data to infer mechanistic and structural behavior of the extracted Ln(III) species. Computational18−21 and experimental22,23 approaches have both demonstrated some degree of mesoscopic ordering occurs even in neat ionic liquids and is influenced through modification of alkyl chains24 or through the inclusion of solutes and IL mixing.22,25 Nevertheless, the structural effects observed in an IL following application in a chemical separation remain largely unexplored,26 and to the best of our knowledge this is the first demonstration of direct control over the intermediate structure of an IL extraction system. To identify and characterize the mesoscopic structural features occurring following extraction of Ln(III) during processing of spent nuclear fuel, small-angle neutron scattering (SANS) data were collected using EQ-SANS at the Spallation Neutron Source of Oak Ridge National Laboratory on [P666(14)][DEHP] dissolved at 0.4 M in [C4mim][NTf2]. After contact with TALSPEAK simulant containing Lu(NO3)3 (defined in detail in the Supporting Information), a representative Ln(III), the SANS data displayed a scattering profile revealing a new structural formation over the q-range of 0.01−0.5 Å−1, while a significant increase of scattering intensity was also observed at q-ranges
