Thermal, chemical, electrochemical, radiolytic and biological stability of ionic liquids and deep eutectic solvents

Ionic liquids (ILs) and deep eutectic solvents (DESs) are regarded as two kinds of novel solvents with high tunability and they exist in liquid-state for a wide range of temperature....

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

The radiolytic stability of a series of room-temperature ionic liquids (ILs) composed of bis(trifluoromethylsulfonyl)imide anion (Tf₂N^-) and triethylammonium, 1-butyl-1-methylpyrrolidinium, trihexyl(tetradecyl)phosphonium, 1-hexyl-3-methylpyridinium, and 1-hexyl-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 CH₂Cl₂ after irradiation. The trapped radicals were identified as mainly carbon-centered alkyl and ^•CF₃, and their ratio varies greatly across the series of ILs. Expected nitrogen-centered radicals derived from Tf₂N^- 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.

In Situ Tracking of Organic Reactions at the Vapor/Liquid Interfaces of Ionic Liquids

The molecular structures of ionic liquids at interfaces play a crucial role in determining their chemical activities in applications. In situ X-ray photoelectron spectroscopy (XPS) was used to track the evolution of X-ray irradiation-induced chemical reactions in a series of ionic liquids ([C_n mim][AuCl₄ ]; n=4, 6, 8, 10) on the Si (111) single-crystal surface. Analyses of microstructure and chemical bonding based on the XPS results indicated that reactions occurred at the vapor/liquid interfaces of the ionic liquids. The time-resolved XPS spectra revealed that with increasing irradiation time, the intensity of the peak corresponding to trivalent Au anion decreased for the four ionic liquids as Au was continually reduced to a lower chemical state and finally converted to gold nanoparticles. The rate and conversion of the reaction were associated with the length of the alkyl chain of the ionic liquids cation. Molecular dynamics simulations further revealed that the alkyl chain of the cation in the ionic liquids was oriented towards the vacuum environment at the vapor/liquid interface. Our results provide a real-time atomic-scale experimental evidence of organic reactions at the vapor/liquid interfaces of ionic liquids. The findings are important for understanding the roles of ionic liquids in catalysis, separation, electrochemistry, functional materials, and so on.

Evidence for Indirect Action of Ionizing Radiation in 18-Crown-6 Complexes with Halogenous Salts of Strontium: Simulation of Radiation-Induced Transformations in Ionic Liquid/Crown Ether Compositions

Ionic liquid/crown ether compositions are an attractive alternative to traditional extractants in the processes for spent nuclear fuel and liquid radioactive wastes reprocessing. These compositions are exposed to ionizing radiation, and their radiation stability, especially in the presence of metal salts, is a crucial issue. In the present study, the macrocyclic 18C6·Sr(BF₄)₂ and 18C6·Sr(PF₆)₂ complexes simulating the components of metal loaded ionic liquid/crown ether extractants were synthesized and their structures were characterized by FTIR spectroscopy and single-crystal X-ray diffraction analysis. Inclusion of Sr²⁺ cation into the 18C6 cavity resulted in more symmetric D_3d conformations of the macrocycle. The structural transformations of the crown ether were accompanied by an elongation of polyether C-O bonds that could increase the possibility of radiolytic cleavage of the macrocycle. However, EPR study of the synthesized compounds subjected to X-ray irradiation revealed predominant formation of macrocyclic -CH₂-ĊH-O- radicals. This result demonstrated an evidence for indirect action of ionizing radiation on individual components of the complexes and was reasonably described by a positive "hole" transfer from primary macrocyclic radical cation to fluorous anion at the primary stages of radiolysis and a subsequent interaction of fluorine atom with 18C6 macrocycle in secondary radical reactions. The observed effects may be partially responsible for enhanced sensitivity of the ionic liquid/crown ether extractants to ionizing radiation due to chemical blocking of the crown ether with radiolytic HF, radiation-chemical degradation of the 18C6, and precipitation of a low-soluble SrF₂.

Radiolysis of alkyl substituted tridentate 2,6-bis(1,2,4-triazine-3-yl)pyridines: an experimental study with DFT validation

The radiolysis products of alkyl substituted tridentate 2,6-bis(1,2,4-triazine-3-yl)pyridines (BTPs) were studied by an experimental study with DFT validation.

Thermal, electrochemical and radiolytic stabilities of ionic liquids

Ionic liquids show instability when exposed to high temperature, to high voltage as electrolytes, or under irradiation.

Radiation Effect of Carboxyl-Functionalized Task-Specific Ionic Liquids on UO22+ Removal: Experimental Study with DFT Validation

Experimental study with DFT validation on the effect of radiation on 1-carboxymethyl-3-methylimidazolium bis(trifluoromethylsulfonyl) imide ([HOOCCH₂MIM][NTf₂]) as a solvent and extractant in rapid homogeneous extraction of UO₂²⁺ was performed for the first time. The radiolytic products of the anions and cations of [HOOCCH₂MIM][NTf₂] were identified by ¹⁹F NMR and high-resolution ESI-MS, respectively, and they were attributed to a decrease in UO₂²⁺ partitioning. Experimental study with DFT validation for complexing reactions between [HOOCCH₂MIM][NTf₂] and radiolytic products proved that F^- competition was one of the main reasons for the decrease in UO₂²⁺ partitioning. However, UO₂²⁺ partitioning in irradiated [HOOCCH₂MIM][NTf₂] can largely be recovered after thorough water washing because of the removal of radiolytic products of [NTf₂]^-.

Photocatalytic degradation of imidazolium ionic liquids using dye sensitized TiO2/SiO2 composites

The photodegradation efficiency of imidazolium ILs reach 95% with DCQ-TiO₂/SiO₂ as the photocatalyst under simulated solar light.

Optical properties of irradiated imidazolium based room temperature ionic liquids: new microscopic insights into the radiation induced mutations

Considering the future perspectives of room temperature ionic liquids (RTILs) in areas involving high radiation fields (such as the nuclear fuel cycle and space applications), it is essential to probe and have a microscopic understanding of the radiation induced perturbations in the molecular structures and the intrinsic bonding interactions existing in the ILs. Herein, a focused investigation concerning the photophysical behavior of post-irradiated FAP (fluoroalkyl phosphate) imidazolium ILs revealed considerable rearrangements and bonding realignments of the ionic moieties in the ILs on irradiation, however, their physicochemical properties do not change significantly even at high absorbed doses. Most interestingly, the well-established excitation wavelength dependent fluorescence (FL) behavior of the ILs was considerably perturbed on irradiation and this is attributed to the radiation induced decoupling of pre-existing different associated structures of ions, and the subsequent formation of oligomers and other species containing multiple bond order groups. This was further substantiated by vibrational studies, where peaks appearing in the range 1600-1800 cm(-1) indicated the formation of double bonded products. Furthermore, for the hydroxyl functionalized (in the alkyl side chain of the imidazolium cation) IL, a blue shift in the O-H stretching frequency was observed for the -OH group H-bonded to the FAP anion (νOH···[FAP](-)), while a red shift was observed for the H-bonded -OH groups in the cationic clusters. The FL lifetime values were found to increase with irradiation, which clearly indicates the enhancement in the rigidity level in the vicinity of the ions, thereby hindering the non-radiative decay processes. Such studies could contribute to the fundamental understanding of the radiation driven perturbations in the structure-property relationships, which eventually affect the radiolytic degradation pathways and the product distribution in RTILs.

Radiolysis of crown ether–ionic liquid systems: identification of radiolytic products and their effect on the removal of Sr2+ from nitric acid

The quantitative analysis and identification of the radiolytic products of dicyclohexano-18-crown-6 (DCH18C6), 4′,4′′(5′′)-di-tert-butyldicyclohexano-18-crown-6 (DtBuCH18C6) and benzo-18-crown-6 (B18C6) in 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl) imide ([C₄mim][NTf₂]) ionic liquid after irradiation were performed for the first time.

The chemical stability of phosphonium-based ionic liquids under gamma irradiation

Photographs of irradiated [P₁₄₆₆₆][dca] and the corresponding UV-vis and Raman spectra, and conductivities as a function of irradiation time. Black, blue and red lines are for 0, 96 and 192 h of irradiation, respectively.

The role of structural and fluidic aspects of room temperature ionic liquids in influencing the morphology of CdSe nano/microstructures grown in situ

RTILs as media to synthesize a variety of nanomaterials are gaining momentum owing to their unique physicochemical properties. However, the fundamental questions regarding the role of the inherent structure of the IL in directing the morphology and the growth mechanism of the nanoparticles are still unexplored. Therefore, an attempt was made in this respect wherein CdSe nanoparticles were synthesized in a neat room temperature ionic liquid (RTIL), 1-ethyl-3-methyl imidazolium ethylsulfate ([EMIM][EtSO4]), under ambient conditions. The IL was found to play three roles, as a solvent, as a stabilizing agent and as a shape directing template. The primary nanoparticles were of the sizes in the range of 2-5 nm, as determined by HR-TEM. These primary nanoparticles grow into nanoflake-like units which further self-assemble and transform into a mixture of anisotropic nanostructures (predominantly 2D sheets and flower-like 3D patterns) as revealed by the SEM studies. The co-existence as well as the stability of these nanomorphologies point towards the intrinsic microheterogeneity prevailing in the IL. Furthermore, the vibrational spectroscopic studies comprising of FT-IR and Raman spectroscopy clearly indicate a sort of accord involving the π-π stacked aromatic geometry and the hydrogen bonding network (between the cation and the anion) of the IL with the CdSe nanoparticles. Therefore, a suitable mechanism has been provided for the resulting anisotropic nanostructures on the basis of the structural and the fluidic aspects of the IL in conjunction with the surface properties of the transient morphologies involved in the process. To further supplement this, control experiments were facilitated by diluting the IL with different amounts of water and the morphology of the CdSe nanostructures was examined at respective mole fractions of water as well as at different time intervals.

Radiation induced physicochemical changes in FAP (fluoro alkyl phosphate) based imidazolium ionic liquids and their mechanistic pathways: influence of hydroxyl group functionalization of the cation

Future applications of ionic liquids (ILs) in a variety of areas, especially those having high radiation fields such as the nuclear fuel cycle and in space technology, are under serious consideration nowadays. For such applications to be possible, however, radiation stability of the ILs is an important issue that needs to be addressed. We envisaged that the ultra-hydrophobic, bulky and hydrolytically stable FAP (tris(perfluoroalkyl)trifluorophosphate) anion might shield the radiolytically vulnerable imidazolium cations from degradation and our result shows that these anions indeed enhance their radiolytic stability. However, introduction of a hydroxyl group into the alkyl side chain of the imidazolium moiety resulted in significant changes in the physical properties of the IL with respect to onset temperatures, conductivity and the electrochemical window. Furthermore, a nonlinear trend in absorbance with an increase in radiation dose accompanied by NMR (nuclear magnetic resonance) and mass spectrometry studies clearly demonstrated that the presence of the hydroxyl group promotes various degradation channels. Interestingly, a perturbation of the hydrogen bond between the hydroxyl group (present in the side chain of the cation) and the fluorine atom of the anion (OHF) was evident in the case of irradiated hydroxyl functionalized FAP ILs. Besides, the hydrogen gas yields of the ILs were determined and found to be comparable to those of a radiolytically stable aromatic compound, benzene. Finally, through transient spectroscopic studies we could delineate the mechanism of the radiation induced changes in the physicochemical properties of the non-hydroxyl and hydroxyl containing FAP ILs. We have clearly demonstrated that a simple functionalization of the molecular structure of the FAP based imidazolium ILs might cause marked differences in the reactivity, reaction center and the nature of the radiolytic products, which eventually lead to significant changes in their physicochemical properties.

α-Radiolysis of ionic liquid irradiated with helium ion beam and the influence of radiolytic products on Dy3+extraction

Helium ion beam produced by a heavy ion linear accelerator was used to simulate α-rays for studying the radiation effect on 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ionic liquid.

Towards understanding the color change of 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide during gamma irradiation: an experimental and theoretical study

The radiation-induced color change of [BMIm][NTf₂] originates from the formation of double bonds in cations and various associated species containing [BMIm-H₂][NTf₂].

Influence of radiation effect on extractability of an isobutyl-BTP/ionic liquid system: quantitative analysis and identification of radiolytic products

Approaches were established for assessing the influence of radiation effect on the extractability of the 2,6-di(5,6-diisobutyl-1,2,4-triazin-3-yl)pyridine (isobutyl-BTP)/1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl) imide ([C₂mim][NTf₂]) extraction system.

Radiation stability of cations in ionic liquids. 4. Task-specific antioxidant cations for nuclear separations and photolithography

Three families of "task-specific" antioxidant organic cations that include designated sites to facilitate deprotonation following electronic excitation and ionization have been introduced. The deprotonation from the excited state is reversible, leading to minimal damage of the cation, whereas the deprotonation from the oxidized cation yields persistent aroxyl and trityl radicals. This protection improves radiation stability of ionic compounds in 2.5 MeV electron beam radiolysis. Apart from the use of such cations as structural components of room temperature ionic liquid (IL) diluents for nuclear separations, their ionic compounds involving bases of superacids are well suited for use as chemically amplified acid generator resists in electron beam lithography and extreme ultraviolet photolithography.