Thermodynamics, Structure, and Dynamics in Room Temperature Ionic Liquids:  The Case of 1-Butyl-3-methyl Imidazolium Hexafluorophosphate ([bmim][PF₆])

Abstract

A detailed investigation of the phase diagram of 1-butyl-3-methyl imidazolium hexafluorophosphate ([bmim][PF(6)]) is presented on the basis of a wide set of experimental data accessing thermodynamic, structural, and dynamical properties of this important room temperature ionic liquid (RTIL). The combination of quasi adiabatic, continuous calorimetry, wide angle neutron and X-ray diffraction, and quasi elastic neutron scattering allows the exploration of many novel features of this material. Thermodynamic and microscopic structural information is derived on both glassy and crystalline states and compared with results that recently appeared in the literature allowing direct information to be obtained on the existence of two crystalline phases that were not previously characterized and confirming the view that RTILs show a substantial degree of order (even in their amorphous states), which resembles the crystalline order. We highlight a strong connection between structure and dynamics, showing the existence of three temperature ranges in the glassy state across which both the spatial correlation and the dynamics change. The complex crystalline polymorphism in [bmim][PF(6)] also is investigated; we compare our findings with the corresponding findings for similar RTILs. These results provide a strong experimental basis for the exploration of the features of the phase diagram of RTILs and for the further study of longer alkyl chain salts.

Structural organization and phase behaviour of 1-butyl-3-methylimidazolium hexafluorophosphate: an high pressure Raman spectroscopy study

“…Two of these phases were previously investigated. 34,35 These experimental results are in contrast with recent results from high pressure DTA and Raman experiments, where only one crystalline phase was discernible up to 2 GPa. 19,20 The existence of different crystalline polymorphs at comparable thermodynamic conditions points to the occurrence of several local conformational minima in the potential energy surface.…”

Raman Spectroscopy and Ab-Initio Model Calculations on Ionic Liquids

“…Wide-angle X-ray scattering results on the [C 4 mim][I] room temperature ionic liquids show prominent peaks in the residual radial distribution curve [118], indicating certain periodical arrangements of the iodide anions. The existence of different rotamers and local structures has also been found by interpretation of results from optical heterodynedetected Raman-induced Kerr-effect spectroscopy (OHD-RIKES) [139], from neutron scattering and diffraction experiments versus temperature [111][112][113]140], from Coherent anti-Stokes Raman Scattering (CARS) [141] and by theoretical molecular dynamics calculations [43,47,[142][143][144]. The local structures may also lead to other unique properties of ionic liquids; for example, if magnetic anions are aligned in RTILs, novel magnetic liquids will be created [145,146].…”

“…The structural analysis helps the understanding of solvation of nonpolar, polar, and associating solutes in these media [140,148]. The existence of an extended hydrogen-bonded network structure was suggested by Abdul-Sada et al [65] for 1-alkyl-3-methylimidazolium halides based on results from fast-atom bombardment mass spectroscopy.…”

Novel 2-alkyl-1-ethylpyridinium ionic liquids: synthesis, dissociation energies and volatility

“…It was also observed that the degree of charge ordering becomes less pronounced for large anions and asymmetrical cations. [10][11][12][13] Recently, a combination of infrared (IR) spectroscopic techniques was used to characterize the degree of charge organization present in the imidazolium based ionic liquid family. 14 It was found that longer alkyl side chains attached onto the imidazolium ions increase the short-range repulsive interactions among the ions and disrupt the charge organization, in agreement with previous studies.…”