Ultraslow Phase Transitions in an Anion–Anion Hydrogen-Bonded Ionic Liquid

Abstract

A Raman spectroscopy study of 1-ethyl-3-methylimidazolium hydrogen sulfate, [CCim][HSO], as a function of temperature, has been performed to reveal the role played by anion-anion hydrogen bond on the phase transitions of this ionic liquid. Anion-anion hydrogen bonding implies high viscosity, good glass-forming ability, and also moderate fragility of [CCim][HSO] in comparison with other ionic liquids. Heating [CCim][HSO] from the glassy phase results in cold crystallization at ∼245 K. A solid-solid transition (crystal I → crystal II) is barely discernible in calorimetric measurements at typical heating rates, but it is clearly revealed by Raman spectroscopy and X-ray diffraction. Raman spectroscopy indicates that crystal I has extended ([HSO]) chains of hydrogen-bonded anions but crystal II has not. Raman spectra recorded at isothermal condition show the ultraslow dynamics of cold crystallization, solid-solid transition, and continuous melting of [CCim][HSO]. A brief comparison is also provided between [CCim][HSO] and [CCim][HSO], as Raman spectroscopy shows that the latter does not form the crystalline phase with extended anion-anion chains.

Supramolecular Self‐Assembly of Nanoconfined Ionic Liquids for Fast Anisotropic Ion Transport

“…Note that the phase transition temperatures suggested by the DSC with its high sweep rate cannot directly be compared with the phase transition temperatures observed using POM or X‐ray studies, which require small temperature sweep rates to reach the equilibrium. That DSC showed several phase transitions was, in fact, not surprising, as seen for other imidazolium ILs with an HSO 4 − ‐counter‐anion …”

“…This has previously led to hydrogen bonded HSO 4 − ‐chains in CsHSO 4 at low temperatures . Furthermore, in other imidazolium ILs, this counter‐ion has a strong role to control the assembly due to formation of hydrogen bonds between the counter‐ions . Not surprisingly, also in the present DiC 9 ‐IL, the calculated structure suggests formation of hydrogen bonded chains of HSO 4 − in the b ‐direction with SS distances of 0.4 nm and corresponding H‐bond distance of 0.14 nm (Figure S9, Supporting Information).…”

Low-Temperature Phase Transitions of the Ionic Liquid 1-Ethyl-3-methylimidazolium Dicyanamide

“…[6][7][8][9] Calorimetric measurements have shown that ionic liquids may exhibit complicated patterns of phase transitions including crystallization, glass transition, solid-solid transitions, and complex pre-melting behavior. 10,11,12,13,14,15 At least three characteristic patterns of thermal behavior have been found by differential scanning calorimetry (DSC) of ionic liquids: i. crystallization; ii. supercooling and glass transition; iii.…”

“…[C2C1im][HSO4], the glass forming ability follows as a consequence of the high viscosity. 14,24 Then the interesting issue is the reason of high viscosity, in the case of [C2C1im][HSO4] being the occurrence of anion-anion hydrogen bonded structures. 14,24 When the ethyl chain is replaced by a benzyl group (1-benzyl-3methylimidazolium, [BzC1im] + ) low viscosity liquids can be obtained, e.g.…”

“…Understanding the physicochemical properties of ionic liquids in terms of the molecular structure of the constituent ions is needed for the growing scope of technological applications of these interesting systems. − Studies concerning phase transitions of ionic liquids, that are obviously important for practical purposes, have unveiled how ionic interactions and molecular conformation determine the local arrangement of ions in the liquid phase and crystalline structure. − Calorimetric measurements have shown that ionic liquids may exhibit complicated patterns of phase transitions including crystallization, glass transition, solid–solid transitions, and complex pre-melting behavior. − At least three characteristic patterns of thermal behavior have been found by differential scanning calorimetry (DSC) of ionic liquids: (i) crystallization; (ii) supercooling and glass transition; and (iii) devitrification by reheating the glass (the so-called cold-crystallization). It should be noted that this is not a strict distinction because a given ionic liquid undergoes crystallization or glass transition depending on slight change in the cooling rate or different thermal history experienced by the sample .…”