Alexander Stoppa

Dynamics of Imidazolium Ionic Liquids from a Combined Dielectric Relaxation and Optical Kerr Effect Study: Evidence for Mesoscopic Aggregation

We have measured the intermolecular dynamics of the 1,3-dialkylimidazolium-based room-temperature ionic liquids (RTILs) [emim][BF(4)], [emim][DCA], and [bmim][DCA] at 25 degrees C from below 1 GHz to 10 THz by ultrafast optical Kerr effect (OKE) spectroscopy and dielectric relaxation spectroscopy (DRS) augmented by time-domain terahertz and far-infrared FTIR spectroscopy. This concerted approach allows a more detailed analysis to be made of the relatively featureless terahertz region, where the higher frequency diffusional modes are strongly overlapped with librations and intermolecular vibrations. Of greatest interest though, is an intense low frequency (sub-alpha) relaxation that we show is in accordance with recent simulations that have reported mesoscopic structure arising from aggregates or clusters--structure that explains the anomalous and inconveniently high viscosities of these liquids.

Temperature Dependence of the Dielectric Properties and Dynamics of Ionic Liquids

Dielectric spectra were measured for eight, mostly imidazolium-based, room temperature ionic liquids (RTILs) over a wide range of frequencies (0.2 < or = nu/GHz < or = 89) and temperatures (5 < or = theta/degrees C < or = 65). Detailed analysis of the spectra shows that the dominant low frequency process centred at ca. 0.06 to 10 GHz (depending on the salt and the temperature) is better described using a symmetrically broadened Cole-Cole model rather than the asymmetric Cole-Davidson models used previously. Evaluation of the temperature dependence of the static permittivities, effective dipole moments, volumes of rotation, activation energies, and relaxation times derived from the dielectric data indicates that the low frequency process cannot be solely due to rotational diffusion of the dipolar imidazolium cations, as has been thought, but must also include other contributions, probably from cooperative motions. Analysis of the Debye process observed at higher frequencies for these RTILs is not undertaken because it overlaps with even faster processes that lie outside the range of the present instrumentation.

Interactions and Dynamics in Ionic Liquids

Precise dielectric spectra have been determined at 25 degrees C over the exceptionally broad frequency range of 0.1 <or= nu/GHz <or= 3000 for the imidazolium-based room-temperature ionic liquids (RTILs) [bmim][BF4], [bmim][PF6], [bmim][DCA], and [hmim][BF4]. The spectra are dominated by a low-frequency process at approximately 1 GHz with a broad relaxation time distribution of the Cole-Davidson or Cole-Cole type, which is thought to correspond to the rotational diffusion of the dipolar cations. In addition, these RTILs possess two Debye relaxations at approximately 5 GHz and approximately 0.6 THz and a damped harmonic oscillation at approximately 2.5 THz. The two higher-frequency modes are almost certainly due to cation librations, but the origin of the approximately 5 GHz mode remains obscure.

Electrical conductivity and translational diffusion in the 1-butyl-3-methylimidazolium tetrafluoroborate ionic liquid

Broadband dielectric and terahertz spectroscopy (10(-2)-10(+12) Hz) are combined with pulsed field gradient nuclear magnetic resonance (PFG-NMR) to explore charge transport and translational diffusion in the 1-butyl-3-methylimidazolium tetrafluoroborate ionic liquid. The dielectric spectra are interpreted as superposition of high-frequency relaxation processes associated with dipolar librations and a conductivity contribution. The latter originates from hopping of charge carriers on a random spatially varying potential landscape and quantitatively fits the observed frequency and temperature dependence of the spectra. A further analysis delivers the hopping rate and enables one to deduce--using the Einstein-Smoluchowski equation--the translational diffusion coefficient of the charge carriers in quantitative agreement with PFG-NMR measurements. By that, the mobility is determined and separated from the charge carrier density; for the former, a Vogel-Fulcher-Tammann and for the latter, an Arrhenius temperature dependence is obtained. There is no indication of a mode arising from the reorientation of stable ion pairs.

From ionic liquid to electrolyte solution: dynamics of 1-N-butyl-3-N-methylimidazolium tetrafluoroborate/dichloromethane mixtures.

Dielectric spectra have been measured at 25 degrees C for mixtures of the room temperature ionic liquid 1- N-butyl-3- N-methylimidazolium tetrafluoroborate (IL) with dichloromethane (DCM) over the entire composition range at frequencies 0.2 less than or approximately nu/GHz < or = 89. The spectra could be satisfactorily fitted by assuming only two relaxation modes: a Cole-Cole process at lower frequencies and a Debye process at higher frequencies. However, detailed analysis indicated that both spectral features contain additional modes, which could not be resolved due to overlaps. The spectra indicate that the IL appears to retain its chemical character to extraordinarily high levels of dilution ( x IL greater than or approximately 0.5) in DCM. At even higher dilutions ( x IL less than or approximately 0.3), the IL behaves as a conventional but strongly associated electrolyte.

On the collective network of ionic liquid/water mixtures. II. Decomposition and interpretation of dielectric spectra

This study deals with the dielectric spectra of mixtures of the ionic liquid 1-butyl-3-methyl-imidazolium (BMIM(+)) tetrafluoroborate with water at three selected mole fractions 0.767<or=x(H(2)O)<or=0.967. The focus lies on the comparison of experimental and computational data. On the one hand, a computational analysis permits a complete decomposition of spectra, both with respect to dynamical behavior (translation and rotation) as well as to composition of the mixture (cation, anion, and water). Thereby, not only the peak assignment in experimental spectra is enabled but one can also learn more about solvation properties. Of particular importance is the interplay of the dielectric constant and the conductivity representing a measure of collective rotational and translational motion. On the other hand, the comparison with experimental spectra is essential for the validation of the force fields used in simulation. The satisfying agreement between corresponding peaks in the dielectric spectra confirms not only computed dielectric relaxation times but also other collective dynamical properties such as the viscosity. Nevertheless, the detailed fine structure of the conductivity regime reveals specific ion-pair effects not covered by the simulation. A possible confinement of dynamical heterogeneity as a consequence of a system size effect is also indicated.

Dipole correlations in the ionic liquid 1-N-ethyl-3-N-methylimidazolium ethylsulfate and its binary mixtures with dichloromethane

Dielectric spectra over the frequency range of 0.2 less, similar nu/GHz < or = 89 have been measured for the room-temperature ionic liquid 1-N-ethyl-3-N-methylimidazolium ethylsulfate ([emim][EtSO(4)], IL) and its mixtures with dichloromethane (DCM) at temperatures of 5 < or = vartheta/ degrees C < or = 65 and 25 degrees C respectively. The spectra of the neat IL at all temperatures and those of the mixtures could be satisfactorily fitted by assuming three relaxation modes, a Cole-Cole process at lower frequencies and two Debye processes at higher frequencies. Consistent with previous studies, detailed analysis of the first (lowest-frequency) process, centered at 0.2-2 GHz depending on temperature and composition, indicated that it is mainly due to the reorientation of the dipolar [emim](+) cations. At high dilutions in the mixtures (x(IL) less, similar 0.2), contact ion pairs also contribute to this mode. The second mode at approximately 8 GHz, which is absent from the dielectric spectra of previously studied imidazolium salts and their mixtures with DCM, is assigned to reorientation of the dipolar [EtSO(4)](-) anions. The highest-frequency mode (located at approximately 80 GHz) in the mixtures is a composite of low-energy intermolecular vibrations originating from the IL and the rotational diffusion of DCM molecules. Detailed analysis of the spectra reveals marked orientational correlations of the IL components, with the cation dipoles showing a strong preference for parallel and the anions showing preference for antiparallel arrangements. These effects are the probable cause of the unusually high dielectric constant of [emim][EtSO(4)]. The structure of the IL appears to be maintained up to quite high dilutions (x(IL) > or = 0.2) in DCM.

Ion Association of Imidazolium Ionic Liquids in Acetonitrile

Molar conductivities, Λ, of dilute solutions of the ionic liquids (ILs) 1-ethyl-3-methylimidazolium tetrafluoroborate ([emim][BF4]), 1-butyl-3-methylimidazolium tetrafluoroborate ([bmim][BF4]), 1-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF6]), 1-hexyl-3-methylimidazolium tetrafluoroborate ([hmim][BF4]), and 1-hexyl-3-methylimidazolium bis-(trifluoromethanesulfonyl)amide ([hmim][NTf2]) in acetonitrile (AN) were determined as a function of temperature in the range 273.15-313.15 K. The data were analyzed with Barthel’s lcCM model to obtain limiting molar conductivities, Λ(∞)(T), and association constants, K(A)°(T) of these electrolytes. The temperature dependence of these parameters, as well as the extracted limiting cation conductivities, λ(i)(∞), were discussed. Additionally, dielectric spectra for [hmim][NTf2] + AN were analyzed in terms of ion association and ion solvation and compared with the inference from conductivity. It appears that in dilute solutions the imidazolium ring of the cations is solvated by ∼6 AN molecules that are slowed by a factor of ∼8-10 compared to the bulk-solvent dynamics. Ion association of imidazolium ILs to contact ion pairs is only moderate, similar to common 1:1 electrolytes in this solvent.

Terahertz Dynamics of Ionic Liquids from a Combined Dielectric Relaxation, Terahertz, and Optical Kerr Effect Study: Evidence for Mesoscopic Aggregation

To exploit the great potential of room-temperature ionic liquids (RTILs) as solvents that offer both low environmental impact and product selectivity, an understanding of the liquid structure, the microscopic dynamics, and the way in which the pertinent macroscopic properties, such as viscosity, thermal conductivity, ionic diffusion, and solvation dynamics depend on these properties, is essential. We have measured the intermolecular dynamics of the 1,3-dialkylimidazoliumbased RTILs [emim][BF4], [emim][DCA], and [bmim][DCA], at 25 °C from below 1 GHz to 10 THz by ultrafast optical Kerr effect (OKE) spectroscopy and dielectric relaxation spectroscopy (DRS) augmented by time-domain terahertz and far-infrared FTIR spectroscopy. This concerted approach allows a more detailed analysis to be made of the relatively featureless terahertz region, where the higher frequency diffusional modes are strongly overlapped with librations and intermolecular vibrations. In the terahertz region, the signal-to-noise ratio of the OKE spectra is particularly high and the data show that there is a greater number of librational and intermolecular vibrational modes than previously detected. Of greatest interest though, is an intense low frequency (sub-alpha) relaxation that we show is in strong accordance with recent simulations that observe mesoscopic structure arising from aggregates or clusters; structure that explains the anomalous and inconveniently-high viscosities of these liquids.

Toward the Rationalization of Facilitated Hydrotropy: Investigation with the Ternary Dimethyl Isosorbide / Benzyl Alcohol / Water System

Solubility enhancement has been achieved by facilitated hydrotropy for the dimethyl isosorbide (DMI) / benzyl alcohol / water system. Facilitated hydrotropy has been studied via three different approaches: the solubilization in water of a hydrophobic dye, the evolution of the surface tension and dynamic light scattering, all as a function of the benzyl alcohol concentration. The facilitated hydrotropy has been rationalized from the solubilization properties of the system according to the ratio between the insoluble hydrotrope (here benzyl alcohol, a preservative used in parenteral injections) and the bio-sourced co-solvant (here the dimethyl isosorbide ether, DMI, a solvent used in pharmaceutical formulation). The presence of self-associated nanostructures has been detected by dynamic light scattering (DLS). It appears that the cosolvent, DMI, has an antagonistic action: DMI increases the facilitated hydrotrope (benzyl alcohol) solubility in the aqueous solution (favoring solute solubilization) but simultaneously decreases the hydrotropic efficiency of benzyl alcohol.