Near-surface dynamics of the ionic liquid EMIM-Ac above and below the glass transition

Abstract

In our prior work, we showed that β-detected nuclear magnetic resonance (β-NMR) was a good probe of bulk room temperature ionic liquid (RTIL) dynamics and dynamic heterogeneity. We now investigate how the surface modifies these properties, presenting the first depth-resolved β-NMR measurements in 1-ethyl-3-methylimidazolium acetate as a liquid, supercooled liquid, and glass. This interfacial region is important for understanding how constrained dimensionality affects dynamics. We show that both the surface and the glass transition have a large impact on molecular dynamics, which in many aspects differs greatly from our expectations based on polymer glasses. For example, in the glassy phase the surface dynamics appear to be faster than in the bulk (i.e., liquid-like), yet just as heterogeneous (i.e., glass-like).