The initial stages of radiation damage in ionic liquids and ionic liquid-based extraction systems

Radical intermediates generated in radiolysis and photoionization of ionic liquids (ILs) composed of ammonium, phosphonium, pyrrolidinium, and imidazolium cations and bis(triflyl)amide, dicyanamide, and bis(oxalato)borate anions have been studied using magnetic resonance spectroscopy. Large yields of terminal and penultimate C-centered radicals are observed in the aliphatic chains of the phosphonium, ammonium, and pyrrolidinium cations, but not for imidazolium cation. This pattern is indicative of efficient deprotonation of a hole trapped on the parent cation (the radical dication) that competes with rapid electron transfer from a nearby anion. This charge transfer leads to the formation of stable N- or O-centered radicals; the dissociation of parent anions is a minor pathway. Addition of 10-40 wt % of trialkyl phosphate (a common extraction agent), has relatively little effect on the fragmentation of the ILs. The yield of the alkyl radical fragment generated by dissociative electron attachment to the trialkyl phosphate is < 4% of the yield of the radical fragments derived from the IL solvent. The import of these observations for radiation stability of the prospective nuclear cycle extraction systems based upon the ILs is discussed.