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

Abstract

To exploit the great potential of room-temp. ionic liqs. (RTILs) as solvents that offer both low environmental impact and product selectivity, an understanding of the liq. structure, the microscopic dynamics, and the way in which the pertinent macroscopic properties, such as viscosity, thermal cond., ionic diffusion, and solvation dynamics depend on these properties, is essential. We have measured the intermol. dynamics of the 1,3-dialkylimidazoliumbased RTILs [emim][BF4], [emim][DCA], and [bmim][DCA], at 25 °C from below 1 GHz to 10 THz by ultrafast optical Kerr effect (OKE) spectroscopy and dielec. relaxation spectroscopy (DRS) augmented by time-domain terahertz and far-IR FTIR spectroscopy. This concerted approach allows a more detailed anal. to be made of the relatively featureless terahertz region, where the higher frequency diffusional modes are strongly overlapped with librations and intermol. 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 no. of librational and intermol. 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 liqs.