Dip-Coating of a Nanometer-Thick Fluorinated Imidazolium Ionic Liquid: Layer Structure, Formation Mechanism and...

In this work, the film formation mechanisms and wetting behavior have been investigated for a nanometer-thick highly fluorinated imidazolium-based ionic liquid (HFILOH) deposited on silica substrate via dip-coating. Through systematic variation of solution concentration and dwell time, it is found that the nanometer-thick HFILOH forms a dual-layer structure consisting of a self-limiting bonded layer and a concentration-dependent mobile layer. Kinetic modeling reveals that the formation of the bonded layer follows reversible pseudo-first-order adsorption kinetics, with the equilibrium bonded thickness remaining constant at ∼0.8 nm across all concentrations, while characteristic time constants decrease from ∼2080 s to ∼ 314 s with increasing concentration. The mobile layer thickness exhibits excellent linear correlation with concentration (R² = 0.99), consistent with Landau-Levich hydrodynamic theory for dilute solutions. These results demonstrate that HFILOH film formation in dip-coating is governed by the dual mechanisms of site-limited adsorption and viscous flow entrainment. Contact angle measurements reveal hydrophilic-oleophobic wetting behavior, with hexadecane contact angles remaining consistently high (∼70-75°) while water contact angles increase modestly from ∼ 30° to ∼ 40° with dwell time, indicating time-dependent molecular reorganization and preferential orientation of fluorinated chains at the HFILOH-air interface. This interpretation of a fluorine-rich outer surface is in good agreement with angle-resolved X-ray photoelectron spectroscopy and dynamic contact angle measurements previously reported for nanometer-thick HFILOH coatings on silica, which directly show fluorinated segments enriched at the film-air interface and stable hydrophilic-oleophobic wetting. Together, these findings provide mechanistic insight into the structure-property relationships controlling ionic liquid nanofilm functionality and establish HFILOH as a promising candidate for applications like antifogging, self-cleaning, and oil-water separation.