Wetting Behavior of Ionic Liquid on Mesoporous Titanium Dioxide Surface by Atomic Force Microscopy

Abstract

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][PF6] 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 CO2 capture, gas separation, and the lubrication of micro/nanoelectromechanical systems (M/NEMs).

Ionic liquids under nanoscale confinement

“…The interactions taking place at the interface between a liquid and a solid matrix result in physical and chemical behavior of the confined liquid, which can be very different from the ones characterizing the liquid in the bulk. Different phase transition behavior, wetting, layering near surface walls, shift in glass transition, melting and freezing points have been reported for ILs in confined geometries [18,20,31,32,62,63].…”

“…An understanding of the properties of the ILs confined in a nanoscaleconstrained geometry is of both fundamental and practical interest. In particular, the spatial restriction and low dimensionality promote a strong interaction of the ILs with the walls of the confining matrix, resulting in physicochemical behavior of the confined ILs, in terms of phase transition behavior [29][30][31], wetting [32], layering near surface walls [33,34], as well as mobility [35][36][37], much different from the corresponding bulk properties.…”

Combined Nano- and Macrotribology Studies of Titania Lubrication Using the Oil-Ionic Liquid Mixtures

“…Titanium and its alloys have wide applications in industry due to their high strength-to-weight ratio and high melting point. , An oxidized titania layer is typically present on the titanium surface which prevents the bulk titanium from further corroding and oxidizing . However, the titania surface on top of the bulk titanium is prone to high friction and wear, and currently there are no suitable lubricants for titanium surfaces, , which restricts their widespread use in moving parts. , Conventional mineral oil based lubricants adhere weakly to titania, so they are easily squeezed out during sliding, leading to wear .…”

“…The lubrication of pure ILs on titania surfaces have been rarely investigated, ,− and to date, the lubrication of titania using IL–oil mixtures has not been studied. It is not clear how IL ions interact with titania surfaces and lubricate the surface at either nano- or macroscales.…”

Nanofriction of Graphene/Ionic Liquid-Infused Block Copolymer Homoporous Membranes

“…SLIPS makes it possible to stabilize ILs to minimize the “rheological thinning” weakness, thus forming a slippery interface better suitable for higher load applications. Previously, we employed mesoporous TiO 2 films to provide porous surface matrix to further immobilize ILs, but no significant frictional difference was observed from the nonimmobilized ones. The hydration layers (water molecules surrounded) bound to polymer surfaces turn out to be able to support large pressures without being squeezed out and have remarkable properties in the context of lubrication. − Hence, it is highly desirable to find one kind of substrates with homogeneous and uniform pore structures to stabilize and hold the lubricating ILs film.…”