Ionic liquid long-term stability assessment and its contribution to toxicity and biodegradation study of untreated and altered ionic liquids

Abstract

In contrast to well understood degradation mechanisms in conventional lubricants generally promoted by thermo-oxidative stress, the degradation of ionic liquids is widely unknown although they are considered as promising novel types of lubricants. Hence, the ionic liquid long-term stability has been evaluated by small scale artificial alteration experiments under thermo-oxidative conditions. The ionic liquid selection was based on non-functionalized and functionalized ammonium type cations with three different counter anions. The identification of ionic liquid degradation products accomplished by high end mass spectrometric methods, namely time-of-flight/reflectron time-of-flight mass spectrometry and linear quadrupole trap-orbitrap-mass spectrometry, revealed that ionic liquids which were composed of functionalized cation moieties were prone to degradation. Furthermore, the amounts of the most abundant degradation product formed under various artificial alteration conditions have been quantified by ultra high pressure liquid chromatography coupled to electrospray ionization linear quadrupole ion trap orbitrap mass spectrometry, suggesting that the ionic liquid degradation preferably takes place at higher temperatures after a longer period of time. The proposed degradation mechanism requires the presence of nucleophilic species such as methanesulphonate anion. The (eco)toxicological impact of the selected ionic liquids have been evaluated by comprehensive toxicity studies and biodegradation experiments. As the evaluation of selected ionic liquids revealed contrary assessments from stability and (eco)toxicological studies, the need for mutual and complementary consideration of the ionic liquids for a successful implementation of ionic liquid lubricants has been disclosed.