(531e) Molecular Structure of Solid-Confined Ionic Liquids

Since the ionic liquid (IL)/solid interfaces always play a fundamental role in coating, lubrication, CO₂ capture, energy storage, and catalysis, it is critical to understand the molecular structures of ILs on solid surfaces. Here we report direct experimental evidence showing that the growth of the nanometer-thick [Bmim][FAP] film on the mica surface is unique. AFM results and the corresponding line profiles reveal that the thickness of the IL nanofilm on the mica surfaces increases in a quantized manner as the film grows. Initially, the nanofilm thickness remains constant at ~1 monolayer (ML) of [Bmim][FAP] as the nanofilm grows, and the IL film only covers more solid surface areas. After exceeding a critical point, the thickness increases discontinuously by the double layering of the IL ion pairs. Then the nanofilm thickness remains constant at ~ 2 ML until the next jump of the film thickness to ~4 ML. ARXPS is conducted to understand the underlying mechanism of the double-layering molecular structure, and to uncover the molecular orientation of the alternating layers of cations and anions in the IL nanofilms. These results potentially provide more insight into the molecular arrangement of ILs confined on the solid surfaces.