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

A detailed investigation of the binary mixtures of the ionic liquids (ILs) 1-N-R-3-N-methylimidazolium tetrafluoroborate (R = ethyl, n-butyl, n-hexyl) with the important molecular solvent acetonitrile (AN) over the entire composition range has been made at 25 degrees C using broadband dielectric spectroscopy. All spectra showed two modes: a Cole-Cole (CC) mode centered at similar to 2 GHz and a Debye mode centered at similar to 50 GHz. However, detailed analysis indicated both relaxations were composites. The Debye mode arises from the rotational diffusion of free AN molecules 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 molecules solvating them. At very low IL concentrations 1:1 contact ion pairs are dominant. Overall, these IL + AN mixtures can be divided into two broad regions: at IL mole fraction (x(IL)) less than or similar to 0.2 the IL behaves as a rather weakly associated conventional electrolyte while at x(IL) greater than or similar to 0.2 it takes on its IL characteristics, lubricated by the AN.