Investigations on the stability and ecotoxicity of selected ionic liquid cations and anions

Abstract

The field of ionic liquids (ILs) is rapidly growing and several applications have already been discussed or applied. For a long time, such substances were announced to be green . This was solely based on their low vapor pressure and the accompanying enhanced operational safety. This general term was refined when first evaluations on the ecotoxicity and biodegradability of ILs were published, indicating a certain environmental hazard for representatives. Within this thesis investigation on hydrolytic stability, biodegradation potential and ecotoxicological properties were extended to anions and cations which have not yet been studied. This includes three cyano based and three (per)fluorinated anions, five ammonium and several pyrrolidinium, morpholinium, piperidinium, imidazolium and pyridinium cations, as well as a first assessment for bivalent cations. An analytical method based on ion chromatography was developed for all of these compounds in order to monitor the degree of degradation. The studies presented here contribute to the hazard assessment of ILs and support their design with reduced hazard. The investigated anions showed considerable drawbacks since none of them was degraded via hydrolysis or microorganisms and the perfluorinated ones were especially toxic to aquatic organisms. The dicationic ILs seem to be an effective alternative. Their outstanding physico-chemical properties, specifically, the high thermal stability combined with a lower toxicity relative to monocationic analogues are already a step towards safer chemicals while fulfilling the application related profile. However, their biodegradability still needs to be improved. Here, ammonium and pyrrolidinium based ILs are preferable because they are readily biodegradable. This reveals the feasibility to design ILs with reduced hazard potential. However, this work represents an initial hazard assessment and further studies, e.g. long-term toxicity tests of ILs in order to determine chronic effects, are required for concluding evaluations.