Ionic Liquids and Ionizing Radiation: Reactivity of Highly Energetic

PERSPECTIVE pubs.acs.org/JPCL

Ionic Liquids and Ionizing Radiation: Reactivity of Highly Energetic Species James F. Wishart* Chemistry Department, Brookhaven National Laboratory, Upton, New York 11973-5000, United States

ABSTRACT Due to their unique properties, ionic liquids present many opportunities for basic research on the interactions of radiation with materials under conditions not previously available. At the same time, there are practical applied reasons for characterizing, understanding, and being able to predict how ionicliquid-based devices and industrial-scale systems will perform under conditions of extreme reactivity, including radiation. This perspective discusses current issues in ionic liquid physical chemistry, provides a brief introduction to radiation chemistry, draws attention to some key findings in ionic liquid radiation chemistry, and identifies some current hot topics and new opportunities.

ions.5 Functionalized side chains (aryl, fluorous, alcohol, ether, etc.) can potentially form other types of domains. Each of these domains can influence and direct reactions taking place within them, and the interfaces between the domains can have their own intrinsic characteristics that influence reactivity. These regions can be particularly important for reactions that create or annihilate charge, due to the specific solvation of neutral and charged species. Adding to the intrinsic complexity of ionic liquids are the properties of their interfaces with vacuum, gases, liquids, and solids. As with other fluids, experimental and theoretical studies have shown ordering of ionic liquids at interfaces that departs significantly from the bulk. The intricacy in the case of ionic liquids is that differences in composition, such as anion substitution, cation aromaticity, or alkyl chain length variation, greatly change the structure of the ionic liquid interface. Charged surfaces such as electrodes have extremely profound effects on local ionic liquid structure, as one would expect.6

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n recent years, ionic liquids have become so widely known1-3 and studied that it is no longer necessary to start every paper explaining what they are. They are of compelling interest because they have unusual and widely adjustable combinations of properties that permit a vast array of fundamental investigations on the properties and behavior of condensed matter, and additionally they are robust and diverse enough to be used in myriad industrial and technical applications that can positively impact the way we obtain, use, and expend energy and material resources. The utility of ionic liquids,4 owing to the exquisite tunability of their properties, suggests that they will often be used in conjunction with reactive, energetic chemical species. These applications include exposure to ionizing radiation (as in the separation and chemical processing of spent nuclear fuel), electrodeposition of reactive metals (including metal-air and lithium metal batteries), and photoinduced processes (photoelectrochemistry, charge transfer, photocatalysis), to name a few. The benefits of successful applications in these areas include the expansion of sustainable nuclear power production through the closure of the nuclear fuel cycle (by efficient recycling in order to reduce the need to mine fresh uranium and to decrease the amount of waste generated and its aggregate radioactive lifetime), improved efficiency and energy density of batteries, and the chemical conversion of solar energy into fuels or electricity. It is therefore crucial to characterize and understand the reactivity of ionic liquids under the extreme conditions anticipated, and to describe and ultimately predict the effects of ionic liquids on the chemistry of reactive, energetic species. These are not simple tasks, for many reasons. Ionic liquids are binary materials that can be composed from an extremely large assortment of anions and cations, each having their own intrinsic reactivity profile that can in turn be influenced by the choice of counterion. Furthermore, depending on their composition, ionic liquids often contain extensive nonpolar domains formed through the aggregation of aliphatic side chains, interspersed among or (at higher volume fraction) interwoven with the polar domain formed by the charged cores of the constituent

r 2010 American Chemical Society

Ionic liquids are robust and diverse enough to be used in myriad industrial and technical applications that can positively impact the way we obtain, use, and expend energy and material resources.

Received Date: August 5, 2010 Accepted Date: October 18, 2010 Published on Web Date: October 26, 2010

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DOI: 10.1021/jz101096b |J. Phys. Chem. Lett. 2010, 1, 3225–3231


On top of these issues lies the fact that in real-world applications ionic liquids are used in the presence of water and other solvents, gases, solutes of various kinds, and even other ionic liquids. Not only will these materials undergo reactions in ionic liquids, but it is also well demonstrated that they can affect the physical properties of the ionic liquids in which they are dissolved and influence reactivity in that way as well.7 The paragraphs above outline a daunting set of factors to consider when trying to understand the reactivity of energetic species in neat ionic liquids and in mixtures containing ionic liquids. Nevertheless, much progress can be made by starting with fundamental investigations and building in complexity over time. Because of the wide variety of ionic liquids, it is not possible to paint a single picture to fit all cases. However, it is possible to elucidate underlying principles that pertain to the majority of ionic liquids and to survey the descriptive reactivity of, and reaction chemistry in, ionic liquids containing various classes of anions and cations. Systematic reactivity patterns can also arise when the descriptive chemistry is compared across ionic liquid classes. Three commonly used experimental methods to generate highly energetic chemical species and study their reaction kinetics are radiolysis, photolysis, and electrochemistry. These three methods have complementary strengths and weaknesses. A detailed discussion and examples of the complementarity of radiation chemistry and photochemistry approaches to kinetic studies can be found in Volume 254 of the Advances in Chemistry Series.8 In short, ionizing radiation deposits energy in the bulk material to generate ionizations and excitations, while photolysis deposits energy in specific light-absorbing species, typically (but not always) a solute. Photolysis thus generates excitations, and in some cases ionizations, with a spatial distribution determined by the distribution of the absorbing species. On the other hand, radiolytic energy deposition is spatially inhomogeneous and clustered around the track of each incident high-energy particle or photon and the tracks of the secondary electrons it ionizes from the medium. Locally high concentrations of reactive species are created along the tracks, resulting in higher probabilities for cross-reactions and recombinations of the initially generated species and product distributions that vary with the type of radiation because the energy deposition densities differ between radiation types and energies. Due to the spatial heterogeneity of radiolytic energy distribution, photolysis experiments cannot completely mimic the response of matter to ionizing radiation, and dedicated equipment for pulse radiolysis kinetics studies is required. Further details about radiation chemistry and the methods of pulse radiolysis kinetic studies can be found in recently published books.9-11 Radiolysis and electrochemistry techniques are also complementary. Radiolytic ionization events often create strongly oxidizing or reducing species that can subsequently undergo redox reactions with solutes of interest on nanosecond or microsecond time scales. Radiolysis is thus useful for following the mechanisms of redox-induced chemistry that can in many cases be too fast to follow with electrochemical instrumentation and for following these reactions in the regime of low local reactant concentration, which is not possible at electrode surfaces. The reactivities of energetic transient species generated by photochemical processes and at electrochemical interfaces


Figure 1. Scheme showing the major reactions following radiolytic ionization of an ionic liquid. S and S0 designate different types of electron scavengers, where S reacts efficiently with presolvated electrons but not with partially or fully solvated ones (reaction arrows crossed out). The numbered arrows correspond to the following processes: (1) electron solvation; (2 and 20 ) presolvated electron scavenging by S and S0 , respectively; (3 and 30 ) solvated electron scavenging by S and S0 ; (4) electron-hole recombination to produce a ground or excited state of the ionic liquid anion or cation (IL or IL*); (5) hole scavenging by hole scavenger S00 ; (6) hole fragmentation. X and Y designate any type of charged or uncharged fragments.

have recently been reviewed.12-14 This perspective will cover primarily the progress that has been made in understanding the general principles of ionic liquid radiation chemistry and how it differs from the radiation chemistry of regular solvents. The particular reactivities of various classes of anions and cations will also be examined. To save space, some aspects of ionic liquid radiation chemistry are included here by reference to previous reviews.15-19 It is useful to organize the major processes that occur in irradiated ionic liquids according to the schemes shown in the Table of Contents graphic at the top of the article and in Figure 1 (while acknowledging that the schemes are not comprehensive and other processes occur as well). Referring first to the graphic above, deposition of energy from the incident radiation to the medium (in this case an ionic liquid) generates a range of excited states (IL*) and ionizations that eject electrons from the ions of the ionic liquid, leaving behind electron vacancies (“holes”). The ejected, excess electrons “thermalize” by losing their excess kinetic energy to the medium (often inducing more excitations and ionizations) and come to rest over a distribution of distances from their parent holes. These electrons are called “pre-solvated” or “dry”electrons because they are not yet equilibrated with their surroundings. As the solvation process proceeds (blue curved arrows) the electrons, which were initially weakly trapped and highly mobile, become more localized through stronger interactions with the solvent, and consequently less mobile. The solvation process in ionic liquids occurs over phases, the fastest corresponding to inertial movement of the solvent on the femtosecond time scale, and slower components that are due to translational and orientational diffusion of the anions and cations.20 In normal molecular solvents at room temperature the solvation process is complete within a few picoseconds, but it lasts for hundreds of picoseconds to many nanoseconds in ionic liquids due to their higher viscosities. The average solvation times measured using emission Stokes shifts of solvatochromic dyes correlate well with the bulk viscosities of ionic liquids. A review of ionic liquid solvation dynamics can be found in a recent perspective by Samanta.21 To a first approximation, the solvation dynamics of excess electrons and solvatochromic dyes in ionic liquids are similar because they are dominated by the response of the solvent to a change in

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charge distribution. (To higher order, the quantum nature of the electron must be considered.) Figure 1 shows the most important processes that occur to the primary species produced in the ionization event. As mentioned above, when a presolvated electron becomes solvated (process 1 in Figure 1), its mobility decreases as does its thermodynamic potential. As a result, its reactivity profile also changes (processes 2, 20 , 3, and 30 in Figure 1). Energetic presolvated electrons may be able to reduce some scavengers (designated “S”) that cannot be reduced by more solvated forms, by overcoming large activation barriers or by directly producing scavenger anion excited states (S-*). Intermediate, partly solvated electrons can show their own reactivity profiles (black arrows in Figure 1). In water, for example, the differences in reactivity profiles between presolvated and solvated electrons are welldocumented22 even though solvation occurs in less than a picosecond. In ionic liquids, solvation processes can be up to 1000 times slower than in water; therefore the importance of presolvated electron reactivity in determining the behavior of ionic liquids under ionizing radiation is undeniable.

Figure 2. Pulse radiolysis transient absorption signals at 900 nm of electron capture by various concentrations of benzophenone in the ionic liquid C4pyrr NTf2, measured by the optical fiber singleshot pulse radiolysis technique.30 Each trace is the average of 25 shots. (Unpublished results. Noise levels have been significantly reduced since this data was taken.).

assumption holds in ionic liquids.23 In the ionic liquid methyltributylammonium bis(trifluoromethylsulfonyl)amide (N1444 NTf2), solvated electrons react with the arenes 20 times slower than H atoms (1.3-1.7108 M-1 s-1 vs 2.9-3.8109 M-1 s-1), whereas in conventional solvents such as water, electron reactions are about 10 times faster than those for H atoms with the same substrates.25 At present, additional examples of direct electron and H-atom rate comparisons in ionic liquids are lacking; however, similar slow but diffusion-limited rate constants (1.55.9 108 M-1 s-1) for electron capture by bis(oxalato)borate anion,26 by neutral pyrene,26,27 and by imidazolium cations28 were obtained in less viscous quaternary ammonium and pyrrolidinium bis(trifluoromethylsulfonyl)amide ionic liquids (indicating no particular dependence on charge type as might be expected for these high-salt conditions). The operative assumption is that solvated electron diffusion in ionic liquids is slowed by Coulombic interactions with the constituent ions of the liquid, in much the same way that ion self-diffusion is correlated with the motions of the surrounding disordered ionic lattice. The relative sluggishness of solvated electron reactions in ionic liquids emphasizes the role played by presolvated electron scavenging in determining overall electron capture product yields and their impact on the radiation stability of ionic liquid-based processing systems. Moreover, from a fundamental physical chemistry standpoint, the dramatically extended time scale of ionic liquid solvation dynamics at room temperature provides a golden opportunity to conveniently study the mechanisms of presolvated electron capture by time-resolved detection methods to an extent not previously possible. In water and other conventional solvents at room temperature, solvation is too fast to observe presolvated electron capture kinetics within the time resolution of pulse radiolysis instrumentation (0.2 ps in the best case but generally 5-15 ps). Remarkably, at Argonne National Laboratory a cryogenic flow system (-60 °C) was used to slow the solvation response in 1-propanol and ethanol to the time scale of hundreds of picoseconds for the study of solvated and presolvated electron scavenging by picosecond stroboscopic optical detection, so

In ionic liquids, solvation processes can be up to 1000 times slower than in water; therefore the importance of presolvated electron reactivity in determining the behavior of ionic liquids under ionizing radiation is undeniable. The significance of presolvated electron reactivity in ionic liquids was illustrated by a pair of early pulse radiolysis studies on the reactions of the solvated electron23 and hydrogen atom24 with aromatic scavengers, including pyrene, phenanthrene, and benzophenone. Response-limited prompt (timezero) absorption features assignable to arene anions grew with increasing scavenger concentration; at the same time, the prompt absorbance of the solvated electron decreased, indicating that some electrons were being scavenged before they could become solvated.23 When hydrogen atom reactions were investigated, the rate constants obtained through direct transient absorption kinetics measurements did not agree with the rate ratios estimated from competition kinetics product assays. The root cause of the discrepancy was the fact that presolvated electron capture by pyrene or phenanthroline, followed by protonation of the arene radical anion, provided a significant alternate pathway for product formation in addition to the reaction of the H atom with the arene. Another striking result that emerged from these studies was the comparison of the solvated electron and hydrogen atom reaction rates with the same pyrene and phenanthrene scavengers. In conventional solvents, both reactions are considered to be diffusion controlled, and there is justification to believe that


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groups through its spectral resemblance to the product of the aqueous electron with the C4mimþ cation;33,34 however, other imidazolium-derived radicals give similar spectra.33 Low-temperature EPR studies of irradiated imidazolium IL glasses indicate that the reduced cation becomes distorted from planarity into a σ-radical located at the pyramidal C(2)-carbon between the nitrogens.36 This radical is predicted to be reactive, and it has been suggested that in certain cases it can form a dimer linked to the C(2)-carbon of a second imidazolium cation.36,37 Numerous reactions can ensue, leading to higher oligomers. Such mechanisms can account for dimers observed in electrospray mass spectroscopy38,39 and oligomers observed in electrolytic solvent breakdown. However, the kinetics of these proposed reactions have yet to be experimentally observed, although the IR and Raman spectra of key intermediates have been predicted.36 In the near future it may be possible to test these hypotheses because nanosecond timeresolved infrared spectroscopy detection capability will be added to BNL's LEAF pulse radiolysis facility,40 and pulse radiolysis transient EPR detection capability is planned to return to the Notre Dame Radiation Laboratory. Both techniques promise to add the necessary structural specificity to resolve some of these mechanistic issues. Despite its significance, the reactivity of electrons is only one part of the chemistry induced in ionic liquids by ionizing radiation. Equally important is the chemistry of the radical holes left behind during the ionization process. Referring back to Figure 1, the holes may recombine with electrons in various states of solvation to reconstitute the starting material in the ground or excited states (process 4). While production of the ground state leads to no net radiation damage, the reactions of ionic liquid excited states, whether produced directly by radiolytic energy transfer or indirectly through electron-hole recombination, can lead to some products that are not accessible through the ionization pathway (vide infra). Holes may also be reduced by electron-donating scavengers (process 5) or undergo some type of bond scission to produce fragments (process 6). A few radical hole species produced in ionic liquids have characteristic electronic absorption spectra that permit their identification and the measurement of their reaction kinetics using standard pulse radiolysis transient absorption techniques. Notable among this group are bromide,41,42 iodide,43,44 and pseudohalides such as thiocyanate,42,45 azide,45 and dicyanamide (N(CN)2-).36 In these cases, the neutral radical X• formed by “ionization” of the anion rapidly reacts with a second anion X- to produce a dimer anion radical X2•- that typically has a strong absorption feature (eq 1). Br2•- can also be made in nonbromide ionic liquids through dissociative electron detachment of 1,2-dibromoethane.41,42 Dihalide radical anions may subsequently disproportionate to produce less reactive species (eq 2).44-46 X• þ X - f X2 • ð1Þ

that the relative contribution of each process could be assessed for various types of scavengers.29 Figure 2 illustrates the differential reactivity of presolvated and solvated electrons toward benzophenone in the ionic liquid N-butyl-N-methylpyrrolidinium bis(trifluoromethylsulfonyl)amide (C4pyrr NTf2). The kinetics were recorded with an early version of a remarkable new detection system that uses a fiber optic bundle to generate 100 (presently 142) laser probe pulses with different delays in a single shot.30 The absorbance rise at 900 nm in the neat ionic liquid is due to the blue shift of the electron's spectrum as it becomes solvated (the peak is at 1100 nm). As the benzophenone concentration is increased, this rising feature is quickly attenuated, indicating that the presolvated electrons responsible for the rise are preferentially scavenged. The kinetics observed in Figure 2 resemble those obtained for acetone in flowing cold propanol, including a decrease in the prompt absorbance indicative of scavenging during the earliest stages of solvation;29 however, they were taken at room temperature using static cuvettes that require as little as 125 μL per sample. This new “optical fiber single shot” (OFSS) technique,30 which has already been improved with a higherperformance fiber bundle, opens the door to detailed studies of electron solvation and scavenging processes on the picosecond time scale currently underway in our laboratory. This information will be very useful for learning how to predict and control the distribution of radiolysis products in real-world processes using ionic liquids.

Radiation will generate chemistry in any system used for separations; the key thing is whether the radiation damage can be controlled to avoid degradation of the performance of the separation system. It is important to point out that the above discussion applies primarily to ionic liquids with saturated quaternary aliphatic cations (ammonium, pyrrolidinium, phosphonium, etc.) that react very slowly or not at all with solvated electrons. In ionic liquids with aromatic cations such as imidazolium or pyridinium, the electrons are captured by the cations very rapidly, even before they can become solvated. The electron adducts of pyridinium cations are reasonably stable and have been used for studies of redox processes and diffusion in ionic liquids.31,32 On the other hand, despite the fact that it has been observed since the earliest radiation chemistry experiments on roomtemperature ionic liquids,33-35 the nature of the product of the reaction between the electron and the imidazolium cation, its mechanism of formation, and its early time evolution remain objects of active study, and the picture has not yet fully converged. Clearly, the existence of a cation-bound electron adduct in imidazolium ionic liquids was demonstrated by two


X2 • - þ X2 • - f X - þ X3 - ðX ¼ I, BrÞ

ð2Þ

X2 • - þ Y f 2X - þ Y•þ

ð3Þ

However, the dimer anion radicals of halides and pseudohalides are strong oxidants that will generally be reduced by

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electron donors,41 providing a convenient method to oxidatively initiate redox reactions for studies of electron transfer in ionic liquids. Another hole species with a distinctive optical absorption feature is the ring-centered radical of the imidazolium dication (Cnmim•2þ, where Cn = alkyl) formed during radiolysis of imidazolium ionic liquids. Spectroscopic assignment of the imidazolium radical dication in irradiated C4mim PF6 was made by Neta and co-workers.33 As is often the case with aromatic radicals, the spectra of the one-electron oxidized species (C4mim•2þ) and the one-electron reduced species (C4mim•)33,34 strongly resemble each other. Despite the above examples, the majority of radicals formed in the radiolysis of ionic liquids have poor or inconveniently located electronic absorption spectra, and conventional transient absorption spectroscopy is ineffective for their study. Grodkowski and Neta succeeded in measuring the reaction kinetics of trifluoromethyl radical •CF3 in ionic liquids by following the absorption growth of its adducts with pyrene and phenanthrene.47 Most of what we know about the reactions of radiation-induced radicals in ionic liquids has been inferred from the detailed electrospray ionization mass spectroscopy (ESI-MS) investigations of ionic liquid radiolysis products by Moisy's group38,39,48 and electron paramagnetic resonance (EPR) studies of irradiated and photolyzed ionic liquid glasses.15,36,37,49 The methods are complementary because ESI-MS gives composition and limited structural information, while EPR, combined with electronic structure calculations, is more structurally specific. Mechanistic information is also obtainable. The ESI-MS product analysis indicates that •CF3 is produced in the radiolysis of NTf2- ionic liquids,38,39,48 consistent with the kinetic work of Grodkowski and Neta.47 Comparison of EPR spectra under radiolysis and photolysis conditions with selected scavengers showed that NTf2- destruction and • CF3 production occur through dissociation of excited anion states formed from recombination of electrons with the NTf2• radical and not from unimolecular decomposition of the neutral NTf2• radical.49 Sulfur dioxide fragments coproduced in this mechanism have been shown to interfere with the extraction of Sr2þ ions from aqueous solution.50 This last observation brings us back to the question of whether ionic liquids will be useful in separation processes to enable the recycling of spent nuclear fuel. It has become clear in recent years that increasing nuclear power production will be in the near-term an important component of policies to accommodate increasing worldwide energy demand while reducing the use of carbon-based fuels, whatever the eventual potential of renewable energy sources.51 Although it is possible to continue for the next 30-50 years to mine new uranium for fuel and to store the spent fuel for eventual disposal, beyond that time frame such practices are not sustainable, and recycling of spent fuel will be necessary.51 The benefits of recycling (or “closure” of the nuclear fuel cycle) include a significant reduction of the volume, activity and lifetime of the radioactive waste (through the destruction of long-lived nuclides in specially designed reactors) and the extension of nuclear fuel resources by several centuries. Today's chemistries and separation technologies for nuclear fuel reprocessing were developed within the context of the Cold


War and are not sustainable for the greater demands of the future from the economic and environmental standpoints. Presently there are many opportunities for new chemistries to replace existing technologies in this area and the time horizon is right (20-50 years to implementation) for fundamental and applied studies in support of these goals. Ionic liquid-based separation processes are one potential alternative for spent fuel recycling, due to the superb tunability of their solvent properties, including solute solubility, extraction mechanisms and phase separation with other solvents, and their intrinsic conductivity that facilitates electrochemical separations.15 Under fuel recycling conditions, the ionic liquids will be exposed to the radioactivity of the fuel and its activation and fission products, although the radiation flux will depend on the specific activity of the elements being separated in a particular process. The radiation flux could be high for short-lived fission products and low for long-lived actinides. From the examples given above it is clear that radiation can induce a wide variety of chemistry in ionic liquids depending on their composition; however, that is by no means a disqualifier. Radiation will generate chemistry in any system used for separations; the key thing is whether the radiation damage can be controlled to avoid degradation of the performance of the separation system. Here, the outlook is encouraging due to the extremely wide compositional variety of ionic liquids, which allows selection for particular radiolytic properties. Indeed, earlier work has shown that ionic liquids could protect an extractant (tributylphosphate) from radiolytic damage, whereas the standard kerosene solvent used in the PUREX process (to separate U and Pu) directs damage into the extractant.49 Ionic liquids are consequently a very promising avenue to follow in pursuit of sustainable spent fuel recycling technologies.15 To conclude, there are many fascinating aspects to the chemistry of highly reactive species in ionic liquids. Due to space limitations, only a few selected topics are offered here to emphasize the fundamental issues controlling reactivity in ionic liquids, and apologies are offered for those topics and various details that could not be included. Radiation chemistry offers some excellent approaches to investigating energetic reactions in ionic liquids, and new technical developments are enabling previously impossible investigations. Ultrafast photochemistry can be a valuable partner in these studies, particularly in the femtosecond regime. The insights thus gained will help ionic liquids fulfill their promise for the development of technologies to efficiently generate and use energy.

Radiation chemistry offers some excellent approaches to investigating energetic reactions in ionic liquids, and new technical developments are enabling previously impossible investigations.

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AUTHOR INFORMATION

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Corresponding Author:

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*E-mail: [email protected].

Biographies James F. Wishart is a Staff Scientist in the Chemistry Department of Brookhaven National Laboratory and Supervisor of the BNL LaserElectron Accelerator Facility. His interests include ionic liquids, radiation chemistry, electron and energy transfer, development of new accelerators and new pulse radiolysis detection techniques, and green and sustainable chemistry. More details about the author can be found at http://www.chemistry.bnl.gov/SciandTech/PRC/wishart/ wishart.html.

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ACKNOWLEDGMENT The author thanks Edward W. Castner, Jr., Andrew R. Cook, Ilya A. Shkrob, and Tomasz Szreder for helpful comments. This work was supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences, and Biosciences under Contract # DEAC02-98CH10886.

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