The dynamics of liquid 1-ethyl-3-methylimidazolium acetate measured with implanted-ion ${}^8$Li $\beta$-NMR

Abstract

We demonstrate the application of implanted-ion $\beta$-detected NMR as a probe of ionic liquid molecular dynamics through the measurement of ${}^8$Li spin-lattice relaxation (SLR) and resonance in 1-ethyl-3-methylimidazolium acetate. The motional narrowing of the resonance, and the local maxima in the SLR rate, $1/T_1$, imply a sensitivity to sub-nanosecond Li${}^{+}$ solvation dynamics. From an analysis of $1/T_1$, we extract an activation energy $E_A=74.8\pm 1.5$ meV and Vogel-Fulcher-Tammann constant $T_{\mathrm{VFT}}=165.8\pm 0.9$ K, in agreement with the dynamic viscosity of the bulk solvent. Near the melting point, the lineshape is broad and intense, and the form of the relaxation is non-exponential, reflective of our sensitivity to heterogeneous dynamics near the glass transition. The depth resolution of this technique may later provide a unique means of studying nanoscale phenomena in ionic liquids.