To optimize the wetting performance of ionic liquids (ILs) on solid surfaces, which is important in catalysis, lubrication, and energy storage, it is critical to control the molecular arrangement of ILs at the IL/solid interface. Here we report direct experimental evidence showing that tuning humidity is a novel and effective approach to manipulate the molecular arrangement and thus controlling the macroscopic wettability of ILs on the mica surface. Attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR), contact angle testing, and atomic force microscopy (AFM) results showed that, under various relative humidity (RH), the molecular arrangement of nanometer-thick ILs on mica could be droplet, “pancake”, or extended layering. Meanwhile, the macroscopic wettability increases with RH. With the increase of RH, more water adsorbs on the mica surface, which dissolves and mobilizes K<sup>+</sup> on the mica. As a result, the cations of ILs occupy the empty spot left by the K<sup>+</sup> and initiate the layering of ILs. The water-enabled ion exchange and IL layering processes result in not only the decrease of the IL contact angle on mica but also the time-dependent contact angle. Additionally, it has been revealed that the growth of the nanometer-thick IL film on mica is unique. Initially, the IL film only covers more solid surface area while the thickness of the covered area remains constant. Then the thickness increases by double layering of the ion pairs. The findings here potentially provide new dimensions tailoring the performance of ILs at the IL/solid interface.

From molecular arrangement to macroscopic wetting of solid-confined ionic liquids

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.