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

A variety of imidazolium, quaternary ammonium, and phosphonium cation based ionic liquids were irradiated with γ-rays, 2-15 MeV protons and 5-20 MeV helium ions in order to examine their relative radiation stability and potential hazards for application in advanced nuclear fuel cycles. Molecular hydrogen production can be taken as an overall indicator of radiation stability, and was found to be considerably lower for the γ-irradiated aromatic imidazolium based compounds when compared to the other aliphatic based media. Increasing the length of the aliphatic side chain increases the H2 yields for all the compounds examined. Little difference is found in the production of H2 between the quaternary ammonium and phosphonium based ionic liquids with similar length side chains. Yields of H2 increase substantially from γ-rays to 5 MeV He ions for the imidazolium based ionic liquids, but little variation with radiation type is observed for the quaternary ammonium and phosphonium based ionic liquids. The imidazolium based ionic liquids show a darkening with increasing dose and the UV-Visible spectra show an increase in absorption from 240 to 400 nm that is probably due to induced changes in the cation. FTIR spectra show little variation with radiolysis, which is consistent with the low H2 yields. The formation of a new peak at 1658 cm−1 is attributable to the formation of acyclic disubstituted alkene bonds in the irradiated imidazolium based compounds.