γ-Radiolysis of Room-Temperature Ionic Liquids: An EPR Spin-Trapping Study

Abstract

The radiolytic stability of a series of room-temperature ionic liquids (ILs) composed of bis(trifluoromethylsulfonyl)imide anion (Tf 2 N − ) and triethylammonium, 1-butyl-1-methylpyrrolidinium, trihexyl(tetradecyl)phosphonium, 1-hexyl-3-methylpyridinium, and 1hexyl-3-methylimidazolium (hmim) cations was studied using spin-trap electron paramagnetic resonance (EPR) spectroscopy with a spin-trap α-(4-pyridyl N-oxide)-N-tert-butylnitrone (POBN). The trapped radical yields were measured as a function of POBN concentration and as a function of radiation dose by double integration of the broad unresolved lines. Well-resolved motionally narrowed EPR spectra for the trapped radicals were obtained by dilution of the ILs with CH 2 Cl 2 after irradiation. The trapped radicals were identified as mainly carboncentered alkyl and • CF 3 , and their ratio varies greatly across the series of ILs. Expected nitrogen-centered radicals derived from Tf 2 N − were not observed. The hmim liquid proved most interesting because a large part of the trapped radical yield (entirely carbon-centered) grew in over several hours after irradiation. We also discovered a complicated narrow-line stable radical signal in this neat IL with no spin trap added, which grows in over several hours after irradiation and decays over several weeks.

Predicting Degradation Mechanisms in Lithium Bistriflimide “Water‐In‐Salt” Electrolytes For Aqueous Batteries

“…On the other hand, the following reactions involving the anions subjected to ionizing radiation can be proposed based on our findings and literature data focusing on the radiation chemistry of ionic liquids possessing the same anion [45–48] . These processes are all the more important as molality increases.…”

“…When the anion (the Li + cation being inert towards irradiation) constitutes the major part of the sample, its direct interaction with ionizing radiation leads to excitation and ionization, producing an electron and a $${{\rm T}{\rm F}{{\rm S}{\rm I}}^{\bullet }}$$ neutral radical according to Eq. : [46,47] …”

“…On the other hand, the following reactions involving the anions subjected to ionizing radiation can be proposed based on our findings and literature data focusing on the radiation chemistry of ionic liquids possessing the same anion. [45][46][47][48] These processes are all the more important as molality increases. When the anion (the Li + cation being inert towards irradiation) constitutes the major part of the sample, its direct interaction with ionizing radiation leads to excitation and ionization, producing an electron and a TFSI � neutral radical according to Eq.…”

Improvement on the Radiation Stability of Multi-scale Supramolecular Assembly in Ionic Liquid-Based Extraction

“…It was found that the imide anions are relatively stable due to N–N σ 2 σ* 1 bond formation involving the parent anion, − although a small amount of free radical CF 3 · and TfN· – is produced after radiation. In contrast, the cation 1,3-dialkyl imidazolium is not as so stable as the imide anions, and its radiation stability has been widely studied. − The γ radiolysis of imidazolium ILs most likely leads to the formation of 2-imidazolyl radicals, which either are protonated or react with the parent cation forming C(2)–C(2) σσ*-bound dimer radicals. The elimination of substituted alkyl groups at the N­(1,3) sites was also observed for radiation effects. − In addition, Adhikari et al , demonstrated that the introduction of a hydroxyl group into the alkyl side chain of the imidazolium moiety can result in a significant change in the physicochemical properties of ILs and promote various degradation channels.…”

“…In contrast, the cation 1,3-dialkyl imidazolium is not as so stable as the imide anions, and its radiation stability has been widely studied. − The γ radiolysis of imidazolium ILs most likely leads to the formation of 2-imidazolyl radicals, which either are protonated or react with the parent cation forming C(2)–C(2) σσ*-bound dimer radicals. The elimination of substituted alkyl groups at the N­(1,3) sites was also observed for radiation effects. − In addition, Adhikari et al , demonstrated that the introduction of a hydroxyl group into the alkyl side chain of the imidazolium moiety can result in a significant change in the physicochemical properties of ILs and promote various degradation channels. The radiation stability of 1-(2-hydroxyethyl)-3-methylimidazolium was suggested to be lower than that of 1-ethyl-3-methylimidazolium. , However, the specific reactions and products of hydroxyl-functionalized imidazolium induced by radiation are still unclear.…”