previously11,30,111,129,171,183 (i.e. rotational diffusion is not the underlying relaxation mech-anism).
To summarize, the combination of DR and OKE spectroscopy allows fitting of the spectra more accurately and reliably than previously possible. At THz frequencies has been shown that there is a greater number of librational/intermolecular modes than detected previously, mainly due to the high signal-to-noise ratio of the OKE experiment. However, the degree of overlap of the individual modes still prevents a more detailed analysis.§ The presence of an intense low-frequency mode in the OKE spectra can be explained by a mesoscale structure.
This means that there is a certain degree of heterogeneity in the liquids, resulting in a broadening of the α relaxation. The underlying mechanism of the α relaxation is due to large angle jumps, as suggested by computer simulations.179
3.4 Conclusions
More detailed insights into the dynamical as well as the structural and static properties of RTILs can be obtained by combination of optical Kerr effect and dielectric spectra over a very broad frequency range. This concerted analysis allowed the resolution of nine modes at 0.2.ν/GHz.10000, including intramolecular modes. The most interesting feature is the large-amplitude low-frequency mode in the OKE spectra due to the meso-scale structure.
It was not possible to resolve this mode just on the basis of DR data due to its low DR amplitude.
Based on only DR data, five modes were sufficient to model the experimental spectra.
Nevertheless, the parameters obtained agree well with the parameters obtained from the concerted analysis. Thus, it is possible to infer the dynamical and structural properties
— connected with these processes — on the basis of just DR data. However, especially at THz frequencies, the low signal-to-noise ratio prevents a more detailed analysis.
Covering only frequencies up to 89 GHz allows a reliable characterization of the microwave modes. The parameters determining these low frequency modes agree well with the broad-band studies and allow an analysis of the rotational modes, even though the high frequency modes are not properly resolved. The effect of temperature on these modes enabled the derivation of activation parameters and gave additional insight into the relaxation mecha-nisms. Due to current limitations of the THz apparatus the temperature dependence can only be studied at frequencies ofν ≤89GHz. Ongoing improvements of the THz setup will allow more insight into the structural and dynamical properties as function of temperature to be gained in them.
§One may doubt the presence of well-defined individual modes. A continuum of intermolecular vibra-tions and libravibra-tions can also be a plausible origin of the observed THz spectra.
Binary mixtures
4.1 Introduction
While most of the earlier papers about RTILs were naturally focused on synthesis and the use of RTILs as reaction media,219,220 there has been growing interest in their physical, dynamical and structural properties.31,221Because the information that can be gained from a physicochemical study of a single RTIL is necessarily limited, most current studies either report data on sensibly-related series of salts222–224 or on the effects of other variables, such as temperature.225,226 Another way of gaining insights into the nature of RTILs is to study their mixtures with co-solvents whose properties are well known. Such investigations are particularly pertinent to potential technical applications of RTILs, as reaction media for industrial-scale syntheses or in batteries, where they would not normally be present in neat form.
Figure 4.1: The scheme of Dupont227 for the dilution of an ionic liquid by a co-solvent; the arrows represent increasing co-solvent content.
Of the vast range of RTILs currently available, those containing substituted imidazolium cations have been the most intensively studied,31 but relatively few of these investiga-tions have focused on their mixtures with co-solvents. According to Dupont,227 neat
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4.1. INTRODUCTION 87
imidazolium-based RTILs form an extended hydrogen-bond network that strongly resem-bles the corresponding solid structures.228 Upon addition of a polar co-solvent, Dupont has proposed that this network breaks up, initially forming “supramolecular” aggregates of the constituent ions of the RTIL (fig. 4.1). With increasing dilution such aggregates are succeeded in turn by triple ions (TIs), contact ion pairs (CIPs) and, ultimately, solvent-separated ion pairs (SSIPs) and free ions.227This scheme has intuitive appeal since, except perhaps for the first step, it is essentially the mirror image of what is known to occur in conventional electrolyte solutions with increasing concentration.229,230 However, while some of the features of Dupont’s scheme have been verified for some RTIL/co-solvent mix-tures29,231,232 others remain speculative.
The range of techniques available to study the structure and dynamics of RTILs and their mixtures is limited. Thermodynamic (solubility, excess volume, etc.) and transport (conductivity, viscosity, etc.) measurements are useful but provide only indirect insights into the nature of solutions at the molecular level. The powerful spectroscopic techniques (NMR, Raman, etc.) on the other hand, generally provide information only about short range (bonding) interactions, and have a specific weakness with respect to the detection of SSIPs.233
Dielectric relaxation spectroscopy (DRS) is particularly suited for the investigation of the long and medium range ordering234 that is implied, at least in RTIL-rich solutions, by Dupont’s scheme.227 DRS also has unique abilities to detect and quantify the formation of all ion pair types in solution.229,230A modest number of papers have reported the dielectric properties of neat RTILs,28–30,145–147including some containing imidazolium cations.28,30,146 However, studies investigating RTIL/co-solvent mixtures are scarce and have been limited with respect to the range of frequencies and/or compositions investigated.29