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The role of anions in charge transport and localized dipolar relaxations in tributyloctylphosphonium ionic liquids is investigated by broadband dielectric spectroscopy and rheology. The dielectric spectra are quantitatively described by a combination of the random barrier model which accounts for ion transport and empirical Havriliak-Negami functions to characterize dipolar relaxations. Two secondary relaxations are observed at temperatures below the calorimetric glass transition temperature, where the primary structural relaxation is essentially frozen at the relevant experimental time scales. The faster process has an anion independent activation energy of 30 kJ/mol and is attributed to libration motion of the phosphonium cation. The slower relaxation is similar to a process previously assigned to a Johari-Goldstein relaxation in imidazolium-based ionic liquids; however, the activation energy is significantly higher in the phosphonium systems. For the charge transport dominated regime, it is observed that variation of the anion results in differences in the dc ionic conductivity and characteristic charge transport rates by ∼2.5 decades. Upon scaling by the calorimetric glass transition temperature, both transport quantities are observed to coincide. From these results, a picture of glass transition assisted hopping emerges as the underlying microscopic mechanism of ion conduction, in agreement with recent results obtained for other classes of aprotic ionic liquids.

Fourier Transform Infrared Spectroscopy and Broadband Dielectric Spectroscopy are combined to trace kinetics of mutarotation in L-fucose. After quenching molten samples down to temperatures between T = 313 K and 328 K, the concentrations of two anomeric species change according to a simple exponential time dependence, as seen by an increase in absorbance of specific IR-vibrations. In contrast, the dielectric spectra reveal a slowing down of the structural (α-) relaxation process according to a stretched exponential time dependence (stretching exponent of 1.5 ± 0.2). The rates of change in the IR absorption for α- and β-fucopyranose are (at T = 313 K) nearly one decade faster than that of the intermolecular interactions as measured by the shift of the α-relaxation. This reflects the fact that the α-relaxation monitors the equilibration at a mesoscopic length scale, resulting from fluctuations in the anomeric composition.