Radiation Chemistry of Ionic Liquids: Reactivity of Primary Species

Chapter 31

Radiation Chemistry of Ionic Liquids: Reactivity of Primary Species

Downloaded by COLUMBIA UNIV on July 27, 2012 | http://pubs.acs.org Publication Date: August 26, 2003 | doi: 10.1021/bk-2003-0856.ch031

James F. Wishart Chemistry Department, Brookhaven National Laboratory, Upton, NY 11973

An understanding of the radiation chemistry of ionic liquids is important for development of their applications in radioactive material processing and for the application of pulse radiolysis techniques to the general study of chemical reactivity in ionic liquids. The distribution of primary radiolytic species and their reactivities determine the yields of ultimate products and the radiation stability of a particular ionic liquid. This chapter introduces some principles of radiation chemistry and the techniques used to perform radiolysis experiments. Kinetic studies of primary products and their reactions on short time scales are described and future challenges in ionic liquid radiation chemistry are outlined.

© 2003 American Chemical Society In Ionic Liquids as Green Solvents; Rogers, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2003.


382 Radiation chemistry has proven to be an invaluable tool for the understanding of chemical reaction mechanisms (/, 2, 3). Pulse radiolysis is a particularly useful technique for the measurement of fast redox reactions and reactions of radicals and other energetic transient species. Many of the techniques of radiation chemistry have parallels in photochemistry, but the methods differ significantly in the way the chemical reactions are induced. These differences are complementary and allow many chemical systems to be approached from several directions to provide greater insight into reaction mechanisms. Often, the two techniques can be combined to circumvent their limitations when used in isolation. Despite their versatility and usefulness, many people concerned with understanding chemical reactivity are unaware of radiation chemistry and pulse radiolysis techniques, largely because the necessary equipment is located at a limited number of facilities in the U. S. and abroad. In the hope of making the advantages of radiation chemistry more widely known, this chapter is offered as a general introduction to the field with specific reference to some recent results on radiolysis of ionic liquids and their implications for specific ionic liquid applications. The rationale for investigating the radiation chemistry of ionic liquids is several fold. First, their solvent properties, non-volatility and combustion resistance make them a very attractive medium for chemical transformations of radionuclides, particularly in the nuclear fuel and waste cycles, as substitutes for volatile organic or aqueous systems. For example, British Nuclear Fuels, Ltd. is exploring the use of electron transfer reactions in ILs to recycle spent nuclear fuel (4). Several families of ionic liquids contain good thermal neutron poisons such as boron and chlorine. Calculations from a Los Alamos group indicate that the minimum critical concentrations (above which a solution in a large container would go critical) for plutonium in representative tetrachloroaluminate and tetrafluoroborate BLs are 20 to 100 times greater, respectively, than in water (5). Use of ILs could dramatically decrease the risk of criticality accidents such as the one that occurred in Japan in 1999. Successful application of ionic liquids to these problems can only be accomplished if they are sufficiently stable under exposure to high radiation doses. An investigation into the radiochemical stability of certain imidazolium ionic liquids has been conducted using product studies and microsecond-timescale pulse radiolysis (6). This approach can be substantially augmented by pico- and nanosecond observations of the primary radiolytic species and the dependence of their yields and reactivities on the composition of the ionic liquid. Second, applications of ionic liquids as chemical reaction media are expanding at a phenomenal pace (7, S, 9, 10). In order to properly exploit the potential of these new solvents, traditional methods of chemical kinetics studies must be applied, and in some cases adapted, to the study of chemical reactivity in ionic liquids. Pulse radiolysis is a particularly useful technique for the measurement of fast redox reactions and reactions of radicals and other energetic

In Ionic Liquids as Green Solvents; Rogers, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2003.


383 transient species. For water, and to a lesser extent other conventional solvents, elegant methods have been developed to convert primary radiolytic species into specific intermediates for the study of many types of reactions. A similar knowledge base must be assembled for ionic liquids if the versatile methods of pulse radiolysis are to be applied to general questions of chemical reactivity in this exciting new area. Third, ionic liquids provide a new environment to test the details of theoretical descriptions of charge transfer and other reactions. This can lead to deeper understanding of reactivity in conventional solvents. An example is the reactivity of pre-solvated electrons, which our observations show to feature much more prominently in the radiation chemistry of ionic liquids than in conventional solvents. Pre-solvated electron reactivity is particularly important in concentrated solutions that are exposed to radiation, such as found in nuclear fuel production and waste management. Radiolytic investigations of ionic liquids have two general goals: understanding the chemistry of ionic liquids under ionizing radiation and learning how pulse radiolysis techniques can be used to study general problems of chemical reactivity in ionic liquids. Some questions which need to be answered are: what primary reactive species are formed when ionic liquids are irradiated, how do their yields depend on the liquid's composition, what reactions do the primary species undergo, and how can their reactivity be exploited to achieve maximum radiation stability, or conversely, provide a high yield of specific products that can be used to study chemical reactions in ionic liquids.

Primary Physical and Chemical Effects of Radiation

Forms of Ionizing Radiation and Modes of Energy Deposition The term "radiation" can properly be used to refer to emissions in any part of the electromagnetic spectrum, however in common usage it has come to represent photons (X-rays and gamma rays) or energetic particles (electrons, protons, neutrons, alpha particles ( He ), and higher nucléons (e.g., C , 0 )) capable of ejecting electrons from molecules. The vernacular usage will be adopted in other parts of this chapter, although the term "ionizing radiation** is more precise. Ionizing radiation is ubiquitous in the universe and represents a significant hazard for long-term space flight. Exposure to high levels of radiation can lead to immediate and long-term illness, yet living organisms cope quite well with chronic exposure to normal background levels from cosmic sources and 4

I6

2+

8+


I2

6+


384 terrestrial radioactivity. Despite the hazards of ionizing radiation there are many beneficial applications in the diagnosis and therapy of diseases, sterilization of spices, foodstuffs and medical devices, modification of materials, radiography, and (not least) the study of chemical kinetics. Sources of ionizing radiation for chemical experimentation come in several types (/, / / ) . Most time-resolved, radiation-induced kinetic studies are done with particle accelerators capable of producing short pulses (picoseconds to nanoseconds) of high-energy (1-30 MeV) electrons. Experimental detection systems are similar to those used with laser flash photolysis installations. Product studies are often done through exposure of samples to radioactive sources, such as ^Co which produces gamma rays with an average energy of 1.2 MeV. Radiolysis facilities for heavier ions such as protons and nucléons are far less common, but important for understanding how the physics of radiolysis affects product yields as explained below. The actions of the various types of ionizing radiation follow similar lines with the exception of neutrons. (12, 13) Having no charge, neutrons do not directly ionize molecules but instead act through the charged particles that result from collisions of the neutron with atomic nuclei. X-rays produce energetic electrons through the photoelectric effect and through elastic Compton scattering, processes that occur through interaction of the photon with electrons of the medium. Above energies of 3 MeV the photons may interact with atomic nuclei to produce electron-positron pairs or to excite photonuclear reactions which produce other charged particles. Energetic charged particles ionize molecules by knocking bound electrons loose through coulombic interactions. The result of all these interactions is to convert the primary incident radiation to a cascade of energetic (> 5 keV) secondary electrons, which then lose their kinetic energy through ionization and excitation of the medium. Each primary photon or particle thus produces a "track" of energy deposition events in the irradiated material. The spatial distribution of these events is determined by the velocity and charge of the particle, or the interaction cross section for a photon of a given energy. Interactions transferring large amounts of energy produce electrons capable of generating their own secondary "branch" tracks. Slower and higher-charged particles interact more frequently per unit distance, resulting in high local densities of ionized species. On the picosecond timescale energy deposition is very inhomogeneous, with most of the medium and almost all the solutes completely unperturbed. This is the origin of an important distinction between radiolysis and photolysis. In radiolysis, the energy is deposited in the bulk material, which defines the primary chemistry of reactive intermediates. In photolysis the energy is absorbed by solute chromophores whose photophysics and photochemistry define the subsequent reactivity. Thus, it is clear that the descriptive radiation chemistry of ionic liquids is highly dependent on the composition of the solvent in ways that


385 photochemistry is not. Different classes of ionic liquids will produce different reactive intermediates, which is advantageous considering the wide range of potential applications. Furthermore, with the exception of very high solute concentrations, the primary radiation chemistry is independent of the nature of the solute and initial product yields will remain consistent when different solutes are investigated.


Electron-hole Pairs and Geminate Recombination Figure 1 is a depiction of the early processes following a single ionization event induced by a high energy electron or other charged particle. Kinetic energy transfers of up to a few tens of electron volts produce energetic electrons which travel tens of nanometers before they have lost all of their excess kinetic energy to the medium. The "thermalized" electrons are spatially delocalized and interact weakly with the medium at first, but the medium rapidly relaxes to accommodate and solvate the excess charge. During this process the electron may pass through one or more phases of partial relaxation, referred to as pre-solvated or "dry" electron states. These phases are typically associated with the characteristic electronic, vibrational, librational and translational responses of the medium. In water or alcohols the entire solvation process may take only a few picoseconds, (14, 15) whereas in viscous or glassy materials solvation may not be complete for milliseconds (16).

Figure 1. Thermalization and solvation processes following ionization.



386 The net result of ionization is to produce a pair of reactive intermediates consisting of the solvated electron and the ionized species from which it came, often referred to as the "hole". (In the radiation chemistry of molecular materials it is common to refer to the hole as a cation, but in the context of ionic liquids that term becomes confusing and will be avoided here.) The population of electron-hole pairs are distributed over a range of thermalization distances depending on the frequency of energy transfer events producing ionization and the efficiency of energy loss to the medium from the energetic electron. The efficiency will depend on the strength of coulombic interactions between the electron and the dipoles or ions within the medium, as well as the density of electronic states available to act as energy transfer acceptors. The fact that ionic liquids consist of a disordered lattice of charged species implies that thermalization via coluombic interactions may be extremely efficient and result in a relatively short distribution of electron-hole pair distances. Electron-hole pair distances are a critical factor in determining the survival probability of the pairs and consequently the ultimate yields of radiolysis products (17, 18). Short separation distances increase the probability that motion of the hole and electron will bring them close enough to recombine, resulting in no net chemical change. Recombination can be driven by the Coulomb attraction between the electron and hole if the hole is a cation, or as may obtain in the case of ionic liquids, a neutral radical occupying an anionic lattice site. The coulombic attraction is screened by the dielectric properties of the solvent. Beyond the Onsager radius, defined as the distance at which the screened coulombic potential is equal to the thermal energy, recombination occurs solely through diffusive motion and the survival probability of the "free radicals" (or "free ions" in molecular systems) is high. In molecular solvents with low dielectric constants such as cyclohexane (ε = 2.02), the Onsager radius (28 nm) is large compared to the mean thermalization distance (6-7 nm) (19, 20) and the total yield of free ions is low due to loss from coulombic recombination, on the order of 0.15 ion pairs per 100 eV of absorbed radiation. In water, where the dielectric constant is 78 and the Onsager radius is only 0.7 nm, the equivalent free ion yield is 2.7 per 100 eV. The nature of ionic liquids would seem to make them particularly inappropriate subjects for description by dielectric continuum models. However in the absence of a more detailed theoretical treatment, empirical correlations of their solvatochromic properties with those of molecular solvents allow estimation of their effective polarities and serve as a basis for predicting their effects on free radical yields. Several groups have used solvatochromic dyes such as Reichardt's betaine dye (21) or polarity-sensitive fluorophores such as pyrene to estimate that the effective polarities of several imidazolium-containing ionic liquids lie in the range between acetonitrile and ethanol (22, 23, 24). The yield of free ions from electron radiolysis in ethanol is 1.8 pairs per 100 eV.



387

The treatment above has been restricted to the idealized case of isolated electron-hole pairs. As described in the previous section, energy deposition within the radiolysis track is locally dense, and a significant probability exists of neighboring ion pairs occurring within a distance comparable to the thermalization length. The density of ionizations is particularly high at "track ends" when a particle is moving slowly and the frequency of interactions is high, and when the incident particle is a alpha particle or nucléon. The cross reactions that can occur when ion pairs overlap reduce the yield of free radicals but result in a wider variety of products by virtue of radical-radical reactions. Since the frequency of overlapping pairs directly influences the yields of radiolysis products, researchers who are concerned with exposures to multiple types of radiation, as may occur in a fuel or waste processing facility, must determine the yields from each of the different radiation types which apply (6).

Chemical Effects of Radiation Radical pair creation and recombination are very important processes which occur during the first few nanoseconds after radiolysis, but there are several other significant processes to consider as shown in Figure 2. Energetic particles can directly create excited singlet and triplet electronic states of molecules or ions in the bulk medium through energy transfers too weak for ionization (18). The excited states may then undergo subsequent physical and

Excitation

^

Λ Ι

^

Ionization +e

XY + e"

XY* Fluorescence

XY + hv

Thermalization

Radiationless decay

XY

Solvation

Dissociation

χ· + γ·

Scavenging

Quenching

XY + S*

e

"th

m

>

e

"dry

m

>

e

"solv

+e

Recombination

XY +e ->XY, XY*

Dissociation

Χ· + Υ

+

Figure 2. EarlyReactions in Radiolysis



388 chemical processes, such as dissociation or energy transfer to a solute, which may lead to net chemical changes. Excited-state yields are significantly larger than free ion yields in hydrocarbons, particularly aromatic ones. In some cases a large proportion of the excited state yield results from electron-hole recombination to produce excited states (18, 20). Along the ionization pathway, the production of net radiolysis products proceeds through scavenging of primary radicals by solutes, dissociation or fragmentation of holes and excited states, and cross reactions of radicals. In imidazolium and pyridinium ionic liquids the electrons produced by ionization are very rapidly captured by the solvent cations, as observed by the characteristic spectra of imidazoyl and pyridinyl radicals (25, 26, 27). Radiation-induced fragmentation of the solvent anion in methyltributylammonium bis(trifluoromethylsulfonyl)imide [MtBA]*[NTf F, produces *CF radicals (27, 28). The mechanism may proceed through dissociation of the oxidized-anion "hole" species [NTfJ* or dissociation of the excited state [NTf ]*~ formed from geminate recombination of the hole with an electron. 2

3

2

+

Radiolysis of [MtBA] [NTf F 2

Methyltributylammonium bis(trifluoromethylsulfonyl)imide is an ionic liquid with many desirable properties for the study of fast radiation-induced reactions. It is liquid at room temperature, easy to prepare in relatively pure, colorless form and has a wide electrochemical window: +1.5 to -3 V vs. Ag/Ag (29). The very negative cathodic limit permits the solvated electron to exist as a discrete species in solution until it is scavenged by solutes or impurities. The liquid is hydrophobic but hygroscopic and has a viscosity of -700 cP at room temperature (27). The visible and NIR spectra of the initial transients generated by electron pulse radiolysis of [MtBA] [NTf ]~ are shown in Figure 3 (30). The data were obtained at the Brookhaven National Laboratory Laser-Electron Accelerator Facility (LEAF), using the 9 MeV, picosecond electron gun accelerator (/, / / , 31). Transients were recorded by digitizer-based transient absorption spectroscopy using silicon and InGaAs photodiodes and fit to single- or doubleexponential kinetics as appropriate. The radiolytic dose on the sample was normalized using known values for water in order to express the measured absorbance of the transients as the product of the yield per 100 eV absorbed (G) and the extinction coefficient (ε). Only two transient species are observed within this wavelength range. Both species appear immediately after the electron pulse. A very broad absorption

+

+

2



band peaking at 1400 nm is observed to decay by a first order process with a lifetime (l/& bs) of 170-300 ns, depending on the batch of ionic liquid. This transient decays faster when electron scavengers are added to the sample and lives slightly longer with addition of the cation scavenger triethylamine. The breadth and shape of the absorption band strongly resembles known spectra of solvated electrons in molecular solvents. The assignment of this species as the solvated electron is therefore difficult to dispute. This is the first observation of the solvated electron in an ionic liquid. The maximum of the solvated electron absorption spectrum in [MtBA] [NTf F (1400 nm) occurs at much longer wavelengths than in water (715 nm) and alcohols (580-820 nm), but not as low energy as in ammonia (1850 nm) and alkylamines (1900-1950 nm) (32, 33). The peak in the ionic liquid is practically the same as the peak of the solvated electron in acetonitrile (1450 nm). Another transient absorption band appears on top of the solvated electron spectrum at wavelengths below 700 nm. This species has a lifetime of 50 ns and does not react with electron scavengers. It is most likely a hole species such as NTf \ however further work is required for identification. One microsecond after irradiation of neat [MtBA] [NTf ]" there is no significant absorbance anywhere between 400 and 1700 nm. 0

+

2

2

+

2


390 The reactivity of the solvated electron with pyrene, a typical aromatic electron acceptor, is illustrated in Figure 4. As the concentration of pyrene is increased the decay of the solvated electron observed at 1030 nm gets faster, as does the build-up of pyrene anion absorbance at 490 nm. The second-order rate constant for solvated electron capture obtained from the concentration dependence is 1.7 χ 10 M" s" ., about a hundred times slower than observed in alcohols where the reaction is diffusion-limited. Reactions of the solvated electron with other solutes listed in Table 1 show the same general trend, indicating the effect of the substantial viscosity of [MtBA] [NTf ]". 8

1

1

+


2

Dry electron reactivity It is also evident from inspection of Figure 4 that the initial solvated electron yield, as indicated by the peak absorbance of the transient at 1030 nm, decreases

0.0

0.2

0.4 0.6 Time, μ&

0.8

1.0

0.2

0.4 0.6 Time, |is

Figure 4. Transient absorption traces showing the reactions of dry and solvated electrons with pyrene in [MtBA ]*[NTf f. The decay of the solvated electron is observed at 1030 nm and the formation ofpyrene anion at 490 nm. Pyrene concentrations: 0, 8, 21 and 33 mM from top to bottom on the left and from bottom to top on the right. 2

Table 1. Rate constants for solvated electron capture and C values for dry electron capture by several scavengers in [MtBA] [NTf ]~. 3 7

+

2

Scavenger Benzophenone Pyrene Phenanthrene Indole H 0 (as 70% HC10 ) +

3

4

Ç, , M 0.062 0.063 0.084 0.22 0.20 7

1.6 χ 10" 1.7 χ 10 1.3 χ 10 4.3 χ 10 6.2 χ 10 8

8

7

7


391 as the pyrene concentration is increased. This is due to scavenging of the presolvated, "dry" electron states depicted in Figure 1, to form pyrene anions at very short times (34, 35, 36, 37). The amount of pyrene anion absorption at 490 nm that appears almost instantaneously increases with increasing pyrene concentration. If the pyrene concentration is high enough, some fraction of the electrons produced by solvent ionization will be captured by the pyrene molecules before the electrons can become fully solvated. The fractional yield of solvated electrons remaining after this process can be expressed as a function of the scavenger concentration by the relation GJGq = exp(-C/C ), where G is the yield of solvated electrons at a given scavenger concentration C, G is the yield in the absence of scavenger, and C is the concentration where only 1/e (37%) of the electrons survive to be solvated. (Some authors prefer to use the reciprocal Qyj = 1/C as a measure of the quenching efficiency.) The values of C37 reported in Table 1 for benzophenone, pyrene and phenanthrene are much lower than those reported (34, 35, 36, 37) for efficient dry electron scavengers in water, ethanol and 1-propanol. There is one report of a comparable value, for benzophenone in decanol, -50 mM (38). The higher scavenging efficiency in the ionic liquid is most likely due to less effective competition from the slower electron solvation process, consistent with the higher viscosity of the ionic liquid. Indication of the slower electron solvation process can be seen in the top trace in Figure 4 for the absorbance of the solvated electron at 1030 nm in the absence of pyrene, where the rounded peak reveals a time-dependent Stokes shift with a time constant of 4 ns at room temperature. Dry electron capture by pyrene provides a convenient method to estimate the initial yield of the solvated electron in [MtBA] [NTf ]~. A plot of the 1030 nm absorbance of the solvated electron extrapolated to time zero versus the extrapolated 490 nm absorbance of the pyrene anion from dry electron capture fits well to a line of slope -0.34. Combined with the reported extinction coefficient of pyrene anion, 5.0 χ 10 M" cm" (39), the observed Ge value at 1030 nm in Figure 1 results in an estimated solvated electron yield of 0.7 electrons per 100 eV absorbed in [MtBA] [NTf ]~. This a very interesting result which places the yield well below most polar solvents such as alcohols but well above most hydrocarbons (17, 18). The solvated electron yield is comparable to that observed in the materials used to process plutonium, indicating that nuclear fuel cycle applications of ionic liquids may be viable. Dry electron capture has enormous implications for the chemistry of ionic liquids in radiation fields. Quantities of solutes which may seem to be innocent in the context of their solvated electron reactivity may have substantial impact on the radiation chemistry of the ionic liquid because of efficient dry electron capture. This phenomenon must be studied in detail. Because of the broad range of solutes which can be dissolved in ionic liquids, it will be possible for the first 37

c

0


3 7

37

+

2

4

1

1

+

2


392 time to compare hydrophilic inorganic scavengers and hydrophobic aromatic scavengers in the same medium, which will improve understanding of the process in molecular liquids as well. In addition to its potential impact on radiolytic yields, it also provides a method to rapidly produce reactive intermediates for studies of fast electron transfer and ground- and excited-state reactivity of radicals by avoiding the limitations of diffusion-controlled secondorder formation reactions of the intermediates.


Effects of Functionalization Figure 5 shows how the absorption spectrum and solvation processes of the electron are affected by introduction of an hydroxyl functional group (40). The fiilly-solvated electron spectrum closely resembles the spectra of the solvated electron in alcohols, which tend to peak around 700 nm. The blue shift due to relaxation of the intermediate solvation states that takes about 50 ns to complete in this relatively viscous liquid occurs in about 18 ps in ethanol (35) and 300 ps in octanol (41). Even at 2 ns a significant amount of electron solvation has occurred since the spectrum is shifted to higher energy than observed for the equilibrated solvated electron in [MtBA] [NTf ]". The ability to use functionalization to differentiate between various pre-solvated electron states will be a very important tool in the study of dry electron capture. Functionalization also confers a degree of control over the stabilization of the solvated electron, its thermodynamics and reactivity patterns. +

2

Conclusion The preliminary results reported here demonstrate that there is an enormous potential for development of radiation chemistry in the study of reactivity in ionic liquids and their application to critical technologies of the future. We have measured dry and solvated electron capture by several solutes in one particular ionic liquid. The observation of very efficient dry electron scavenging was a fascinating discovery which we plan to use to test various hypotheses of dry electron scavenging mechanisms, by exploring the competition between electron solvation and electron capture processes. These studies will be very useful also in understanding dry electron capture in molecular solvents where the phenomenon is much harder to observe. The structure of an ionic liquid solvent differs from those of molecular solvents in ways that may significantly influence the solvation process. Neutron diffraction (42) and proton NMR (43) have verified the intuitive expectation that ionic liquids have a high degree of order typical of a crystalline lattice. The self-imposed regularity of the ionic lattice may reduce the population and depth of pre-formed shallow traps for localization of the thermalized electron relative to molecular solvents. Using the capabilities



393

400

600

800

1000

1200

1400

1600

Wavelength, nm Figure 5. Relaxation of the solvated electron in an ionic liquid with hydroxyl functionalities.


394 of the LEAF facility we will attempt to observe weakly-localized electrons and follow their evolution. Solvation of the negatively charged electron involves displacement and reorganization of the ionic lattice, which will be influenced by the translational component of the solvent dynamics. In that case, the activation energy for the solvation process should be the same as activation energy for viscous flow. If the solvent dynamics are slow, scavengers can compete effectively for the dry electrons, and it may be possible to use our observations to characterize the reactive dry electron states.


Acknowledgments The work was performed at Brookhaven National Laboratory under contract DE-AC02-98CH10886 with the U.S. Department of Energy and supported by its Division of Chemical Sciences, Office of Basic Energy Sciences. The author would like to thank Pedatsur Neta, Robert Engel, Sharon Lall, Ravinder Raju and Sherly Bellevue for their collaboration on radiolysis of ionic liquids. The author would also like to thank Richard Holroyd, Sergei Lymar and John Miller for helpful discussions.

References 1.

2. 3.

4. 5. 6.

7. 8. 9.

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Radiation Chemistry of Ionic Liquids: Reactivity of Primary Species

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