engleski

Study of the ion dynamics in ionic liquids by fluidity spectroscopy

Ionic liquids (ILs) are purely ionic, salt-like materials, which are by definition liquids below 100 oC. Their unique properties, such as non-volatility, non-flammability, chemical and thermal stability and high ionic conductivity, make them promising materials for electrochemical devices and as solvents in “green” chemistry. Generally, ILs show a non-Arrhenius temperature dependence of the viscosity, η, and the DC conductivity, σDC, characteristic for fragile glass-forming liquids. Recent investigations on the ionic liquid BMIm-BF4 have revealed that curves of σDC and fluidity f (inverse viscosity, 1/η), have the same shape and when suitably shifted, can be exactly superimposed. To describe the non-Arrhenius f and σDC, the most popular approach is the empirical Vogel-Tammann-Fulcher (VTF) relation. However, when using equations derived for the modeling of conductivity spectra with the MIGRATION concept, we obtain an activation energy E* for an elementary displacive step of an ion, which is the same for both fluidity and conductivity. This strongly suggests that the electrical and mechanical properties can be traced back to the same elementary steps and, consequently, these properties should show similar frequency dependence. Based on this premise, we have measured the fluidity spectra, f(ν) of two ionic liquids, 1-butyl-3-methylimidazolium tetrafluoroborate (BMIm-BF4), and 1-hexyl-3-methylimidazolium tetrafluoroborate (HMIm-BF4), and compared them with their conductivity spectra σ(ν) in order to obtain a deeper insight into the motions of ions that determine their electrical and mechanical properties.

Ion dynamics; Fluidity spectroscopy; Conductivity spectroscopy

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