Wetting behavior of ionic liquid on mesoporous titanium dioxide surface by atomic force microscopy

Ionic liquids based on 1-butyl-3-methylimidazolium hexafluoro-phosphate (ILs [Bmim][PF₆]) has been employed to wet the mesoporous and dense titanium dioxide (TiO₂) films. It has been found from atomic force microscopy (AFM) analysis that ILs [Bmim][PF₆] can form a wetting phase on mesoporous TiO₂ films, but nonwetting and sphere shaped droplets on dense films. AFM topography, phase images, and adhesion measurements suggest a remarkable dependence of wetting ILs [Bmim][PF₆] films on the TiO₂ porous geometry. On mesoporous TiO₂ films, the adhesive force of ILs [Bmim][PF₆] reaches at 40 nN, but only 4 nN on dense TiO₂ films. The weak interacting ILs [Bmim][PF₆] on dense TiO₂ films forms rounded liquid spheres (contact angle as 40), which helps to reduce friction locally but not on the whole surface. The stronger adhesive force on mesoporous TiO₂ films makes ILs [Bmim][PF₆] adhere to the surface tightly (contact angle as 5), and this feature remains after five months. The stable spreading ILs [Bmim][PF ₆] films provide low friction coefficient (0.0025), large wetting areas, and short CO₂ diffusion distance on the whole mesoporous TiO₂ surface, avoiding the significant decelerating effect through equilibrium limitations to enable CO₂ capture rate up to 1.6 and 10 times faster than that on dense TiO₂ and pure ILs, respectively. And importantly, ILs wetted on mesoporous TiO₂ shorten the time reaching to the maximum adsorption rate (2.8 min), faster than that on mesoporous TiO₂ (6.1 min), and dense TiO₂ (11.2 min). This work provides an important guidance for the improvement of the efficiency of CO ₂ capture, gas separation, and the lubrication of micro/nanoelectromechanical systems (M/NEMs).