We report the transformation of ionic liquid films from isotropic bulk to a fluid-ordered state over micron length-scales. Data from infrared and non-linear spectroscopy measurements show clear transitions that, for varying ionic liquids, occur over time frames of ten minutes to two hours. These maturation times depend linearly on the chosen ionic liquids' bulk viscosities. Interestingly, the ionic liquids do not form solids upon ordering, but do exhibit strong preferential alignments of molecules that persist throughout the fluid films' thicknesses. Our measurements characterize this ordering process; and show that it is largely insensitive to substrate surface chemistry or small amounts of absorbed water. Additional experiments show the transition is observed across several of the most common ionic liquid cations; and that the process is completely reversible. The driving force for this organization is attributed to electrostatic and steric forces combined with a slow shearing of the viscous ionic liquid. These interactions work together to slowly bring the molecules within the film to a preferred, global orientation. The physical length and time scale of this transformation is unexpected and intriguing, and invites additional studies to develop understanding and control of ionic liquid materials' behavior, particularly near surfaces, to benefit their uses in lubrication, capacitive energy storage, and heterogeneous catalysis.
This research output has an Altmetric Attention Score of 64. This is our high-level measure of the quality and quantity of online attention that it has received. This Attention Score, as well as the ranking and number of research outputs shown below, was calculated when the research output was last mentioned on 19 May 2016.
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