Validation of Structural Grounds for Anomalous Molecular Mobility in Ionic Liquid Glasses

Abstract

Ionic liquid (IL) glasses have recently drawn much interest as unusual media with unique physicochemical properties. In particular, anomalous suppression of molecular mobility in imidazolium IL glasses vs. increasing temperature was evidenced by pulse Electron Paramagnetic Resonance (EPR) spectroscopy. Although such behavior has been proven to originate from dynamics of alkyl chains of IL cations, the role of electron spin relaxation induced by surrounding protons still remains unclear. In this work we synthesized two deuterated imidazolium-based ILs to reduce electron–nuclear couplings between radical probe and alkyl chains of IL, and investigated molecular mobility in these glasses. The obtained trends were found closely similar for deuterated and protonated analogs, thus excluding the relaxation-induced artifacts and reliably demonstrating structural grounds of the observed anomalies in heterogeneous IL glasses.

Glass-like behavior of intercalated organic solvents in graphite oxide detected by spin-probe EPR

“…4C) may be related to the glass-like cross-over in confined acetonitrile. Indeed, for bulk molecular glasses such as orthoterphenyl, 43 ethanol, 44 dibutyl phthalate, 45 or ionic liquids, 44,45 a correlation between the glass transition temperature T g and the enhancement of stochastic librations was observed. For confined solvents, the so-called dynamical cross-over is observed for various systems, including water in between GO planes.…”

Effects of Zwitterions on Structural Anomalies in Ionic Liquid Glasses Studied by EPR

“…First of all, we have reliably shown that the dynamics of alkyl chains, included in the IL structure, play a crucial role in the anomaly, which is fundamentally important [41]. Next, the influence of the spin-probe structure was excluded [43], and the anomaly was observed in glasses of other alkyl-chain-containing molecules (ionic liquids, phthalates, alkyl alcohols, and alkyl benzenes), thus making this phenomenon rather ubiquitous [44]. Furthermore, we studied the influence of water content in IL and found self-assembled domains around solutes in binary IL/water mixtures [47].…”

“…A recent series of IL studies using pulse electron paramagnetic resonance (EPR) spectroscopy revealed peculiar molecular dynamics in the glassy state near the glass transition temperature (T g ) [39][40][41][42][43][44][45][46]. Such "structural anomaly" was first found in imidazolium-based ILs, and was later observed for other ILs, phthalates, alkyl alcohols, and alkyl benzenes [44].…”

Active Pharmaceutical Ingredient-Ionic Liquids (API-ILs): Nanostructure of the Glassy State Studied by Electron Paramagnetic Resonance Spectroscopy

“…In addition to characterizing the local rigidity/stiffness/fragility of ILs, the developed methodology also reveals so-called structural anomalies in ILs [29,30,[32][33][34]. In such anomalous temperatures regions near the glass transition, the molecular mobility decreases upon a temperature increase, which opposes general expectation for most media and materials [29,35].…”