Critical Insight into Mechanochemical and Thermal Degradation of Imidazolium-Based Ionic Liquids with Alkyl and Monomethoxypoly(ethylene glycol) Side Chains

Abstract

Processes of mechanochemical degradation of imidazolium bis((trifluoromethyl)sulfonyl)imide ionic liquids with alkyl and monomethoxypoly(ethylene glycol) (mPEG) side groups were studied using the novel technique of mechanically stimulated gas emission spectroscopy. Mechanical stimulus caused degradation of both the anionic and cationic moieties. For the latter, the degradation was concentrated on the alkyl and ether chains rather than on the imidazole. For the anionic moiety, various degradation steps associated with the emission of CH x F y volatile products followed by SO2 and SO3 emission were identified. Simulation of frictional heat dissipation revealed that tribochemical reactivity was induced by the mechanical energy supply rather than the temperature increase. Thermal degradation of the same ionic liquids studied using thermogravimetric analysis and Fourier transform infrared spectroscopy was concentrated mainly on the cationic moiety. Thermal stability significantly depended on the side chains. The decomposition of mPEG-functionalized ionic liquids was a two-step process, where the lower temperature step corresponded to mPEG decomposition.

Time-Resolved Characterization of Dynamic Tribochemical Processes for Dicationic Imidazolium Ionic Liquid

“…connecting chain [28][29][30][31] that was associated with better tribological properties, higher stability, lower toxicity and corrosiveness of the dicationic ILs [14,[31][32][33][34][35][36][37][38][39][40][41]. In the literature, there is a consensus of opinions that the good friction-reduction and wearresistant properties of ILs are owed to their ability to form stable adsorbed layers and protective tribofilms [4,27,[42][43][44]. For ILs with fluorinated anions, Jimenez et al [26] noted that the formation of a metal fluoride-rich boundary tribolayer is necessary to achieve good tribological performance.…”

“…A t-test (H 0 : dms i,j = 0 and H 1 : dms i,j = 0) was used to check the statistical significance of the values of DMS [27]. A statistics…”

“…Generally, the increase of the length of the alkyl side chains attached to a cation head group leads to the improvement of wear resistance, but this effect is not straightforward [2,22,26]. Substitution of alkyl chains by poly(ethylene glycol) (PEG) improved the tribological performance of the IL, although at the cost of decreasing its thermal stability [8,27]. Another approach relies on bridging two cationic head groups by PEG chain.…”

“…In the literature, there is a consensus of opinions that the good friction-reduction and wear-resistant properties of ILs are owed to their ability to form stable adsorbed layers and protective tribo-films [4,27,[42][43][44]. For ILs with fluorinated anions, Jimenez et al [26] noted that the formation of a metal fluoride-rich boundary tribolayer is necessary to achieve good tribological performance.…”

“…The products of tribochemical reactions were identified using statistical and correlation analyses. The results were compared with the tribochemical behavior of an homologous monocationic ionic liquid [27,44].…”