Effect of molecular orientation angle of imidazolium ring on frictional properties of imidazolium-based ionic liquid

Direct observation of the double-layering quantized growth of mica-confined ionic liquids

Since the interface between ionic liquids (ILs) and solids always plays a critical role in important applications such as coating, lubrication, energy storage and catalysis, it is essential to unravel the molecular structure and dynamics of ILs confined to solid surfaces. Here we report direct observation of a unique double-layering quantized growth of three IL (i.e. [Emim][FAP], [Bmim][FAP] and [Hmim][FAP]) nanofilms on mica. AFM results show that the IL nanofilms initially grow only by covering more surface areas at the constant film thickness of 2 monolayers (ML) until a quantized increase in the film thickness by another 2 ML occurs. Based on the AFM results, we propose a double-layering model describing the molecular structure of IL cations and anions on the mica surface. The interesting double-layering structure can be explained as the result of several competing interactions at the IL-mica interface. Meanwhile, the time-dependent AFM results indicate that the topography of IL nanofilms could change with time and mobility of the nanofilm is lower for ILs with longer alkyl chains, which can be attributed to the stronger solvophobic interaction. The findings here have important implications on the molecular structure and dynamics of ILs confined to solid surfaces.

Interfacial structuring of non-halogenated imidazolium ionic liquids at charged surfaces: effect of alkyl chain length

Control of the interfacial structures of ionic liquids (ILs) at charged interfaces is important to many of their applications, including in energy storage solutions, sensors and advanced lubrication technologies utilising electric fields.

Analysis of the Interfacial Molecular Behavior of a Lubrication Film of n-Dodecane Containing Stearic Acid under Lubricating Conditions by Sum Frequency Generation Spectroscopy

The molecular behavior of n-dodecane with added stearic acid at a friction interface was studied using sum frequency generation (SFG) spectroscopy and a tribometer. In the case of n-dodecane with stearic acid, under dynamic conditions, a strong peak from the symmetric stretching vibrational mode of methylene, which was not observed under static conditions, appears. However, this strong methylene peak was not observed in the case of only n-dodecane. The SFG spectrum in the C-H stretching mode region of n-dodecane-d₂₆ with stearic acid in the dynamic condition was analogous to that in the static condition. These results indicate that the interfacial structure of stearic acid does not change under sliding condition. The n-dodecane on a stearic acid adsorption film is highly aligned. Moreover, from the sliding direction dependence of the SFG measurements, the molecular orientation of n-dodecane was deduced: n-dodecane on stearic acid adsorption films orient parallel to the sliding direction. These results have shown that the stearic acid adsorption film behaves as solid-like, which has also been mentioned in previous studies. Further, our results revealed a new function of stearic acid: the stearic acid adsorption film induces the formation of a well-defined n-dodecane interfacial structure and forces the n-dodecane molecules to orient along the sliding direction at the friction interface.