NMR Relaxation and Self-Diffusion Study at High and Low Magnetic Fields of Ionic Association in Protic Ionic Liquids

Abstract

NMR relaxation and diffusion characterization of several protic ionic liquids at high and low magnetic fields are reported. The dynamics of cations and anions were similar at both frequencies, with similar trends and magnitudes for a fixed component paired with oppositely charged species. An Arrhenius relationship was displayed between the molecular motion and the glass transition temperature. The diffusion of ions showed a strong degree of ion correlation between cation and anion, and Arrhenius plots of relaxation and diffusion indicated that the ions diffused as a pair. At high field diffusion was dominated by mobile species that followed Stokes-Einstein behavior. Conversely, diffusion observed at low field emphasized relatively immobile species that displayed fractional Stokes-Einstein behavior. No evidence was found to indicate the influence of magnetic field on structural and dynamic properties of the studied ILs; however, variation between diffusion coefficients at different magnetic fields indicated dynamic heterogeneities (or temporal aggregates) within the ionic liquid.