Dynamics of Imidazolium Ionic Liquids from a Combined Dielectric Relaxation and Optical Kerr Effect Study: Evidence for Mesoscopic Aggregation

Abstract

The authors have measured the intermol. dynamics of the 1,3-dialkylimidazolium-based room-temp. ionic liqs. (RTILs) [emim][BF4], [emim][DCA], and [bmim][DCA] at 25 °C from <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. Of greatest interest though, is an intense low frequency (sub-α) relaxation that the authors show is in accordance with recent simulations that have reported mesoscopic structure arising from aggregates or clusters-structure that explains the anomalous and inconveniently high viscosities of these liqs.