Combined THz and Microwave Dielectric Spectroscopy of Intermolecular Interactions in Homologous Protic Ionic Liquids

We report on a detailed and systematic study of the intermolecular dynamics of a homologous series of protic alkylammonium-formate ionic liquids in a broad spectral window, combining THz time-domain and microwave dielectric methods. The joint experiments yield the complex dielectric function over more than four orders of magnitude in frequency. The dielectric absorption data smoothly join with data from Fourier transform far-infrared (FT-FIR) spectroscopy. All dielectric spectra were fitted to a multimodal relaxation model. The observed processes are assigned to low-frequency re-orientational relaxation of the dipolar ions and to intermolecular vibrations between the cations and anions. The dielectric spectra show a dependence on the length and OH-functionalization of the alkyl residues of the cation.