Abstract:We have investigated structure and relaxation phenomena for ionic liquids 1-octyl-3-methylimidazolium hexafluorophosphate (C8mimPF6) and bis(trifluoromethylsulfonyl)imide (C8mimTFSI) by means of neutron diffraction and neutron spin echo (NSE) techniques. The diffraction patterns show two distinct peaks appeared at scattering vectors Q of 0.3 and 1.0 Å(-1). The former originates from the nanoscale structure characteristic to ionic liquids and the latter due to the interionic correlations. Interestingly, the int… Show more
“…14 , we interpret our data in terms of caged dynamics of the probe sphere: we see the MSD plateau as due to the dominantly elastic response of the medium followed at longer time by a cage relaxation mechanism. This similarity in the behavior of the probe in BMIM-TFSI and in a micellar system is interesting as a micelle-like structure has been proposed 15 for the structure of an other imidazolium IL.Figure 5( a ) Dynamic Light Scattering ( λ
DLS = 647 nm, Q = 4.0 × 10 −3 nm −1 ): MSD of a nanometric probe (latex, 220 nm radius) embedded in water (blue) and bulk BMIM-TFSI (red). While in water the particle MSD is proportional to time (pure Fickian diffusion process), in the IL, around 10 4
μs , the probe experiences a transient cage-like localization.…”
Ionic Liquids (ILs) are a specific class of molecular electrolytes characterized by the total absence of co-solvent. Due to their remarkable chemical and electrochemical stability, they are prime candidates for the development of safe and sustainable energy storage systems. The competition between electrostatic and van der Waals interactions leads to a property original for pure liquids: they self-organize in fluctuating nanometric aggregates. So far, this transient structuration has escaped to direct clear-cut experimental assessment. Here, we focus on a imidazolium based IL and use particle-probe rheology to (i) catch this phenomenon and (ii) highlight an unexpected consequence: the self-diffusion coefficient of the cation shows a one order of magnitude difference depending whether it is inferred at the nanometric or at the microscopic scale. As this quantity partly drives the ionic conductivity, such a peculiar property represents a strong limiting factor to the performances of ILs-based batteries.
“…14 , we interpret our data in terms of caged dynamics of the probe sphere: we see the MSD plateau as due to the dominantly elastic response of the medium followed at longer time by a cage relaxation mechanism. This similarity in the behavior of the probe in BMIM-TFSI and in a micellar system is interesting as a micelle-like structure has been proposed 15 for the structure of an other imidazolium IL.Figure 5( a ) Dynamic Light Scattering ( λ
DLS = 647 nm, Q = 4.0 × 10 −3 nm −1 ): MSD of a nanometric probe (latex, 220 nm radius) embedded in water (blue) and bulk BMIM-TFSI (red). While in water the particle MSD is proportional to time (pure Fickian diffusion process), in the IL, around 10 4
μs , the probe experiences a transient cage-like localization.…”
Ionic Liquids (ILs) are a specific class of molecular electrolytes characterized by the total absence of co-solvent. Due to their remarkable chemical and electrochemical stability, they are prime candidates for the development of safe and sustainable energy storage systems. The competition between electrostatic and van der Waals interactions leads to a property original for pure liquids: they self-organize in fluctuating nanometric aggregates. So far, this transient structuration has escaped to direct clear-cut experimental assessment. Here, we focus on a imidazolium based IL and use particle-probe rheology to (i) catch this phenomenon and (ii) highlight an unexpected consequence: the self-diffusion coefficient of the cation shows a one order of magnitude difference depending whether it is inferred at the nanometric or at the microscopic scale. As this quantity partly drives the ionic conductivity, such a peculiar property represents a strong limiting factor to the performances of ILs-based batteries.
“…Neutron spin echo (NSE) measurements of relaxation dynamics at the nearest neighbor length scales have shown the presence of a nanosecond time scale for a few neat ILs at room temperature containing [Omim] + with anions bis(trifluoromethylsulfonyl)imide, hexafluorophosphate, and chloride (abbreviated respectively as [TFSI] − , [PF 6 ] − , and Cl − ) and has been attributed to the center-of-mass motion of the ions. 55 Interestingly, this slow time scale has also been found in time-resolved fluorescence measurements 12 of the IL, [Omim] [TFSI]. All these data motivate us to explore the interconnection between the DH time-and length scales on one hand, as well as the slow time scale in the nanosecond and beyond observed in different relaxation measurements on the other.…”
Composition dependence of four-point dynamic susceptibilities, overlap functions, and other dynamic heterogeneity (DH) parameters have been investigated by using all-atom molecular dynamics simulations for aqueous solutions of the ionic liquid (IL), 1-octyl-3-methyl imidazolium tetrafluoroborate ([Omim][BF4]) covering the pure-to-pure range. Upon addition of water in the IL, the DH time scales become faster and the four-point dynamic susceptibility time scale softens. Evidences for jump motions for both water and ions have been found from the simulated single particle displacements that show strong deviation from Gaussian distribution. Estimated dynamic correlation length for water reflects effects of IL, whereas those for ions remain largely insensitive to the mixture composition. Simulated structural aspects and DH time scales provide microscopic explanations to the existing experimental observations from time-resolved fluorescence and Kerr spectroscopic measurements.
Low energy excitations of 1-alkyl-3-methylimidazolium ionic liquids (ILs) have been investigated by means of neutron spectroscopy. In the spectra of inelastic scattering, a broad excitation peak referred to as a "boson peak" appeared at 1-3 meV in all of the ILs measured. The intensity of the boson peak was enhanced at the Q positions corresponding to the diffraction peaks, reflecting the in-phase vibrational nature of the boson peak. Furthermore the boson peak energy (E BP ) was insensitive to the length of the alkyl-chain but changed depending on the radius of the anion. From the correlation among E BP , the anion radius, and the glass transition temperature T g , we conclude that both E BP and T g in ILs are predominantly governed by the inter-ionic Coulomb interaction which is less influenced by the alkyl-chain length. We also found that the E BP is proportional to the inverse square root of the molecular weight as observed in molecular glasses. Please cite this article as: M. Kofu, et al., Inelastic neutron scattering study on boson peaks of imidazolium-based ionic liquids, J. Mol. Liq. (2015), http://dx.
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