Evidence of secondary relaxations in the dielectric spectra of ionic liquids

TL;DR

This study uses broadband dielectric spectroscopy to reveal secondary relaxation processes in ionic liquids, showing similarities to molecular glasses and identifying an intrinsic Johari-Goldstein relaxation.

Contribution

It provides the first detailed evidence of secondary relaxations in ionic liquids and characterizes their activation energies and dependence on anion type.

Findings

Secondary relaxations appear below T_g in ionic liquids.

The slower process is identified as a Johari-Goldstein relaxation.

Activation energy of the secondary process is approximately 24k_B T_g.

Abstract

We investigated the dynamics of a series of room temperature ionic liquids based on the same 1-butyl-3-methyl imidazolium cation and different anions by means of broadband dielectric spectroscopy covering 15 decades in frequency (10^(-6)-10^9 Hz), and in the temperature range from 400 K down to 35 K. An ionic conductivity is observed above the glass transition temperature T_{g} with a relaxation in the electric modulus representation. Below T_{g}, two relaxation processes appear, with the same features as the secondary relaxations typically observed in molecular glasses. The activation energy of the secondary processes and their dependence on the anion are different. The slower process shows the characteristics of an intrinsic Johari-Goldstein relaxation, in particular an activation energy E_{beta}=24k_{B}T_{g} is found, as observed in molecular glasses.