On the Radiation Stability of Crown Ethers in Ionic Liquids

Abstract

Crown ethers (CEs) are macrocyclic ionophores used for the separation of strontium-90 from acidic nuclear waste streams. Room temperature ionic liquids (ILs) are presently being considered as replacements for traditional molecular solvents employed in such separations. It is desirable that the extraction efficacy obtained with such solvents should not deteriorate in the strong radiation fields generated by decaying radionuclides. This deterioration will depend on the extent of radiation damage to both the IL solvent and the CE solute. While radiation damage to ILs has been extensively studied, the issue of the radiation stability of crown ethers, particularly in an IL matrix, has not been adequately addressed. With this in mind, we have employed electron paramagnetic resonance (EPR) spectroscopy to study the formation of CE-related radicals in the radiolysis of selected CEs in ILs incorporating aromatic (imidazolium and pyridinium) cations. The crown ethers have been found to yield primarily hydrogen loss radicals, H atoms, and the formyl radical. In the low-dose regime, the relative yield of these radicals increases linearly with the mole fraction of the solute, suggesting negligible transfer of the excitation energy from the solvent to the solute; that is, the solvent has a "radioprotective" effect. The damage to the CE in the loading region of practical interest is relatively low. Under such conditions, the main chemical pathway leading to decreased extraction performance is protonation of the macrocycle. At high radiation doses, sufficient to increase the acidity of the IL solvent significantly, such proton complexes compete with the solvent cations as electron traps. In this regime, the CEs will rapidly degrade as the result of H abstraction from the CE ring by the released H atoms. Thus, the radiation dose to which a CE/IL system is exposed must be maintained at a level sufficiently low to avoid this regime.

Flat crown ethers with planar tetracoordinate carbon atoms

“…[ 50–54 ] Because of this versatility, more than 10 000 crown ether molecules have been synthetically characterized, [ 55–59 ] and their potential applications in phase transfer catalysis, [ 60 ] chemical separations, [ 61 ] analytical methods, [ 62 ] and in nuclear waste management have already been well documented. [ 63–65 ] Motivated by crown ethers and also by molecules with a ptC atom, new “flat crown ether molecules” have been designed here, and their chelation behavior has been explored computationally. Embedding crown ethers in graphene [ 66 ] or carbon nanostructures [ 67 ] has been emerging to control their low structural rigidity.…”

Gamma and heavy ion radiolysis of ionic liquids: A comparative study

“…[21][22][23][24] Several research groups have investigated the radiation stability of different hydrophobic ILs using various methodologies. [18,[25][26][27][28][29][30][31][32][33][34][35][36][37][38][39][40][41][42] An assessment of the radiation effects of ,  and  on imidazolium cation based hydrophilic ILs show less than 1 % degradation of the media even when exposed to a dose of 400 kGy, which suggests very high radiation stability for these ILs. [25] Some studies reported changes in the physical properties such as density, viscosity, conductivity, surface tension and refraction index after -radiolysis.…”

“…[43][44][45][46][47] Shkrob et al have investigated radical generated species by EPR spectroscopy, showing different degradation pathways for aromatic and aliphatic ILs. [18,[35][36][37][38][39][40][41][42] Degradation pathways have been proposed on the basis of final product identification using spectrometric techniques such as NMR, electrospray ionization mass spectrometry (ESI-MS) and liquid chromatography coupled with ESI-MS. [26,27] The anion effect on the stability of imidazolium based ILs has been reported using spectrometric techniques. [29] All of these studies suggest that many radiolysis products are formed from ILs, but in small quantities, and that the mechanisms are difficult to quantify.…”

Organomagnesium Crown Ethers and Their Binding Affinities with Li+, Na+, K+, Be2+, Mg2+, and Ca2+ Ions - a Theoretical Study

“…More than 10,000 crown ether molecules have been reported to date [1][2][3][4][5] since the original discovery of crown ethers accidentally happened in 1967 by Pedersen. 6,7 However, the concept of crown ethers remains fascinating due to the potential applications of these molecules in various fields including (but not limited to) phase transfer catalysis, 8 ion-sensing, 9 nuclear waste management, [10][11][12] and analytical methods. 13 The ability of crown ethers to recognize and trap different metal ions depending upon the size of the macrocyclic ring, type of the donor atom (N, O, and S and thus hard-soft interactions), and polarity of the medium strengthened their utility in various aspects.…”