Interactions and Dynamics in Ionic Liquids

Abstract

Precise dielec. spectra were detd. at 25° over the exceptionally broad frequency range of 0.1 ≤ ν/GHz ≤ 3000 for the imidazolium-based room-temp. ionic liqs. (RTILs) [bmim][BF4], [bmim][PF6], [bmim][DCA], and [hmim][BF4]. The spectra are dominated by a low-frequency process at ∼1 GHz with a broad relaxation time distribution of the Cole-Davidson or Cole-Cole type, which is thought to correspond to the rotational diffusion of the dipolar cations. These RTILs possess two Debye relaxations at ∼5 GHz and ∼0.6 THz and a damped harmonic oscillation at ∼2.5 THz. The two higher-frequency modes are almost certainly due to cation librations, but the origin of the ∼5 GHz mode remains obscure.