Terahertz dynamics of ionic liquids from a combined dielectric relaxation, terahertz, and optical Kerr effect study: evidence for mesoscopic aggregation

Turton, David A. and Hunger, Johannes and Stoppa, Alexander and Hefter, Glenn and Thoman, Andreas and Walther, Markus and Buchner, Richard and Wynne, Klaas; Sadwick, LP and OSullivan, CMM, eds. (2010) Terahertz dynamics of ionic liquids from a combined dielectric relaxation, terahertz, and optical Kerr effect study: evidence for mesoscopic aggregation. In: Conference on Terahertz Technology and Applications III, 2010-01-27 - 2010-01-28.

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Abstract

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-dialkylimidazolium-based RTILs [emim][BF4], [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. 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.

ORCID iDs

Turton, David A., Hunger, Johannes, Stoppa, Alexander, Hefter, Glenn, Thoman, Andreas, Walther, Markus, Buchner, Richard and Wynne, Klaas ORCID logoORCID: https://orcid.org/0000-0002-5305-5940; Sadwick, LP and OSullivan, CMM