Dipole Correlations in the Ionic Liquid 1-N-Ethyl-3-N-methylimidazolium Ethylsulfate and Its Binary Mixtures with Dichloromethane

Abstract

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.

Do H-bonds explain strong ion aggregation in ethylammonium nitrate + acetonitrile mixtures?

“…All mixtures exhibit a maximum in the static permittivity at low IL content, which is an indication of ion-pair formation. 11,13,42 However, whilst only a small increase was observed for the static permittivity of imidazolium IL + AN mixtures, followed by a monotonic decrease to the value of the pure IL, the present mixtures show a huge rise from e s = 35.84 for pure AN to 54.71 at x EAN = 0.12 ( Fig. 3a).…”

“…Compared to mixtures of 1-alkyl-3-methylimidazolium ILs with AN 13 and dichloromethane (DCM), 11,42 where the spectra can be fitted with a CC + D model, some similarities but also marked differences are apparent. From the evolution of the resolved amplitudes and relaxation times it is obvious that for all mixtures studied so far, including the present EAN + AN system, the lower-frequency mode (1) is associated with the IL.…”

“…7a). Therefore, similar to other IL + aprotic solvent mixtures, 11,13,42 the simultaneous presence of free cations and ion pairs can be reasonably assumed, although the individual contributions of both dipolar species to the dielectric spectra of the mixtures cannot be resolved. However, their existence is clearly manifested in the concentration dependence of relaxation time, t 1 (Fig.…”

Dynamics of RTILs: A comparative dielectric and OKE study

“…Already published material can be found in Refs. [12,33,34,35,36,37,38,39]. From a review of these results the general characteristics discussed below can be observed.…”

“…Generally, the amplitude of this mode is considerably smaller than expected from the dipole moment of the anion, whereas the value of S DRS α is increased compared to the corresponding RTIL with non-polar anion. This reveals strong orientational correlations of cation and anion dipole moments in such RTILs, with a significant fraction of the anions slaved in their motions to cation dynamics [36]. We are still working on the detailed evaluation of our data for non-imidazolium RTILs but it appears that, depending on their structure, both cations and anions contribute to the α mode but with different relative weights for DR and OKE.…”