Structure and Dynamics of 1-N-Alkyl-3-N-Methylimidazolium Tetrafluoroborate + Acetonitrile Mixtures

Abstract

A detailed investigation of the binary mixts. of the ionic liqs. (ILs) 1-N-R-3-N-methylimidazolium tetrafluoroborate (R = Et, Bu, n-hexyl) with the important mol. solvent acetonitrile (AN) over the entire compn. range has been made at 25 °C using broadband dielec. spectroscopy. All spectra showed two modes: a Cole-Cole (CC) mode centered at ∼2 GHz and a Debye mode centered at ∼50 GHz. However, detailed anal. indicated both relaxations were composites. The Debye mode arises from the rotational diffusion of free AN mols. with contributions from ultrafast vibrations and librations of the ILs. The CC mode corresponds to the jump rotation of the imidazolium cations and the hindered rotational diffusion of "slow" AN mols. solvating them. At very low IL concns. 1:1 contact ion pairs are dominant. Overall, these IL + AN mixts. can be divided into two broad regions: at IL mole fraction (xIL) .ltorsim. 0.2 the IL behaves as a rather weakly assocd. conventional electrolyte while at xIL gtrapprox 0.2 it takes on its IL characteristics, "lubricated" by the AN.