One of the unique properties of ionic liquids (ILs) is that in the normal liquid region, ILs exhibit behavior, such as non-Arrhenius viscosities and non-exponential relaxation, that is typical of glass-formers in the viscous region near the glass transition temperature Tg. These effects are nominally attributed to dynamic heterogeneity. Given the theoretical and experimental work to date on understanding dynamic heterogeneity in glass-formers near Tg, studying rotational and translational motion in ILs near Tg might help further our understanding of the behavior of ILs in the normal liquid region. To this end we have studied the rotational and translational motion of optical probes in 1-alkyl-3-methylimidazolium cation ([CnC1im]+)-based ILs near Tg using fluorescence recovery after photobleaching (FRAP) techniques. In this talk, we will discuss results from studies of probe size, anion, alkyl chain length, and cation symmetry effects on rotational motion near Tg. Fits of the Kohlrausch-Williams-Watts function to the observed rotational correlation functions obtained from these FRAP measurements provides a way of quantifying relationships between the structure of probe-IL systems and the relaxation-time dispersion and the temperature dependence of the rotational correlation time. Another one of the characteristic phenomena that occurs in glass-formers near Tg associated with dynamic heterogeneity is the breakdown of the Stoke-Einstein (SE) equation below ~1.2Tg, where the probe translational diffusion coefficient is enhanced over the value predicted by the SE equation at high temperatures. It will be shown how this breakdown depends on the length of the Cn chain.

Dynamic heterogeneity in ionic liquids near the glass transition: Fluorescence recovery after photobleaching measurements of probe rotational and translational diffusion in ionic liquids

248th National Meeting (2014)

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