Molecular dynamics of supercooled ionic liquids studied by light scattering and dielectric spectroscopy
Introduction
Low melting point molten salts, also known as room temperature ionic liquids, that consist of usually bulky organic cations and smaller anions of different sorts, show a series of unusual properties, like low or even negligible vapour pressure, high chemical stability, high conductivity and a large electrochemical window, which make them attractive candidates for various applications [2]. But also from a fundamental perspective RTILs are interesting, e.g., due to the variety of competing interactions found in some anion/cation combinations, which for example may lead to complex nanoscopic structure formation in RTILs [3]. Such nanostructures in a concentrated ionic system in turn may influence the effectively available ion concentration and thus the conductivity beyond its simple viscosity dependence [4]. One example would be substantial ion-pair formation that, if sufficiently long lived, produces some amount of electrically neutral species, which temporarily is unavailable for charge transport [5]. Structure formation not only occurs in neat ionic liquids [3], but becomes even more important in mixtures of RTILs with water [6], [7], or in the presence of surfaces and interfaces, where, e.g., pronounced layering is observed [8].
In order to investigate such structure/dynamics relationships it is important to be able to access properties like ion transport and molecular mobility independently. In many cases therefore several experimental methods are combined, like, e.g., field gradient NMR and broadband dielectric spectroscopy [9]. However, in the latter case there is no agreement in the literature on how the dielectric spectra of RTILs should be interpreted regarding their molecular origin: In general charge transport is assumed to be the predominant contribution [10], [11], [12], frequently described in the framework of the Dyre model of charge carrier hopping [13], sometimes with an additional process, the origin of which is not clear [14], [15], and is either assigned to electrode polarization [16], [17], or to a Maxwell-Wagner-Sillars polarization due to chain aggregations [18]. Also, in some cases electrode polarization is thought to be the main process observed in dielectric spectra [19] or rotational dynamics, mostly of the cation, with an additional dc conductivity contribution is considered [20], [21], [22], [23].
In order to shed more light on this problem, with the final aim of being able to clearly disentangle rotational dynamics from conductivity contributions and polarization effects in dielectric spectra, in the present paper we combine depolarized dynamic light scattering (DDLS) with broadband dielectric spectroscopy in frequency and time domain. Such a comparison is particularly suitable for our purpose because DDLS is sensitive to the reorientation of the molecular optical anisotropy without being influenced by the overall charge transport. The experimental details, that need to be correctly taken into account to allow for a quantitative comparison of both methods are outlined in Section 2. In Section 3 we exemplify the procedure for two prototypical ionic liquids, one protic and one aprotic, and demonstrate, how such a comparison even allows for conclusions regarding the motional mechanism of molecular reorientation in the sample.
Section snippets
Experimental
N,N-Diethyl-N-methyl-N-(2-methoxyethyl) ammonium bis(trifluoromethylsulfonyl) imide (DEME-NTF2) with a purity of 99.7% and 1-(2-Hydroxyethyl)-3-methylimidazolium tetrafluoroborate (HEMIM-BF4) with a purity of 98% was purchased from Iolitec (Fig. 1).
Prior to use the samples were kept in a vacuum oven for two days at 70 °C to remove any water. After that they were quickly filled into a glass cuvette for light scattering experiments or in a stainless steel sample cell for dielectric measurements,
Results and discussion
In order to compare the data from PCS (recorded in the time domain) and BDS (recorded in the frequency domain) directly, we performed a Fourier transform of the PCS data as may be found in standard textbooks [31]:where is the complex susceptibility, . The integrals are approximated by the Filon rule [32]. Since and are not accessible by PCS, these values had to be taken from BDS data. In general, can be read off from the spectrum in the
Conclusion
We performed depolarized light scattering and dielectric spectroscopy measurements of supercooled room temperature ionic liquids down to the glass transition. We followed the slowing down of the dynamics from a few nanosecond to . By direct comparison of the raw data of these two techniques, we unambiguously identified the part of the dielectric spectrum that is due to molecular reorientations. We showed that, for the two systems under study, the decoupling of the relaxation times obtained
Acknowledgements
We thank Catalin Gainaru, Dortmund, for stimulating discussions. We are also grateful to Ernst Rössler, Bayreuth, for making the dielectric time domain setup available to us. Financial support of the Deutsche Forschungsgemeinschaft (DFG) under Grant No. BL923/1 and in the framework of FOR 1583 under Grant No. BL1192/1 is gratefully acknowledged.
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