Simon Schrödle

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.

Broadband dielectric response of the ionic liquid N-methyl-N-ethylpyrrolidinium dicyanamide

Dielectric relaxation measurements as a function of temperature, and of concentration in a non-coordinating solvent, the first reported for an ionic liquid, indicate a crossover in the relaxation mechanism due to varying levels of ion aggregation and the interplay of formation kinetics and relaxation dynamics of associates.

Effect of the Chain Length on the Inter- and Intramolecular Dynamics of Liquid Oligo(ethylene glycol)s.

Precise complex permittivity spectra over the frequency range 0.15 GHz </= nu </= 89 GHz are reported for monodisperse (EGn, 1 </= n </= 6) and polydisperse (PEG174, PEG300, PEG400) oligo(ethylene glycol)s at 25 degrees C. Up to about 20 GHz, the relaxation behavior of all samples can be reasonably described with empirical functions that reflect a broad and asymmetric relaxation time distribution, like the Havriliak-Negami function. However, these functions deviate systematically at higher frequencies and do not allow one to rationalize the concentration dependence of the spectra on dilution in dichloromethane or when going from the diol to the corresponding dimethyl ether. It is shown that a coherent description can be achieved by using a superposition of Debye-type relaxation processes. This approach allows the separation of end-group effects connected with the relaxation of the hydrogen bond network from intramolecular dipole relaxation processes caused by the reorientation of chain segments.

Specific Ion Effects of Salt Solutions at the CaF2/Water Interface

Calcium fluoride is a slightly soluble compound commonly extracted from ores via flotation at elevated pH, where surfactant molecules bind with hydroxylated surface sites. The addition of F-(aq) suppresses surfactant adsorption by replacing these sites. In this paper, we look at the effects of aqueous Cl-, Br-, F-, and SO4(2-) on the water structure at the CaF2/H2O interface at a pH where surface hydroxylation has not yet occurred. Using static and time-resolved vibrational sum-frequency spectroscopy (VSFS), we find that aqueous Cl- and Br- have only electrostatic screening effects on the interface and do not perturb the interfacial water or surface structure. Sulfate, which we find to be strongly attracted to the interface, affects the interfacial water more than Cl- or Br-. This is in contrast to F- ions that directly interact with the surface and alter the water structure and bonding at the CaF2 surface in addition to screening the surface charge.

High Frequency Dielectric Response of the Ionic Liquid N-Methyl-N-ethylpyrrolidinium Dicyanamide

A study of the room-temperature ionic liquid N-methyl-N-ethylpyrrolidinium dicyanamide by dielectric relaxation spectroscopy over the frequency range 0.2 GHz ≤ ν ≤ 89 GHz has revealed that, in addition to the already known lower frequency processes, there is a broad featureless dielectric loss at higher frequencies. The latter is probably due to the translational (oscillatory) motions of the dipolar ions of the IL relative to each other, with additional contributions from their fast rotation.

Scandium Sulfate Complexation in Aqueous Solution by Dielectric Relaxation Spectroscopy

Ion association in aqueous solutions of scandium sulfate has been investigated at 25 degrees C and at concentrations from 0.01 to 0.8 M by broadband dielectric spectroscopy over the frequency range 0.2 <or= nu/GHz <or= 89. Detailed analysis of the spectra reveals the presence of both inner- and outer-sphere 1:1 [ScSO 4] (+)(aq) complexes, similar to solutions of other high-valent metal sulfates. Outer-outer-sphere 1:1 complexes are probably also formed, but their contribution is swamped by the presence of higher-order inner-sphere complexes. The latter predominate in the more concentrated solutions, causing major changes to the low-frequency end of the spectrum. The data, while not definitive, are consistent with fac-[Sc(SO 4) 3(OH 2) 3] (3-) as the major species present. The speciation is strikingly different from that recently reported for aluminum sulfate solutions and indicates that the often-postulated similarity between the aqueous chemistry of Al(III) and Sc(III) has to be treated with caution.