Humidity-accelerated spreading of ionic liquids on a mica surface

The role of relative humidity (RH) on the kinetic spreading and equilibrium contact angles of droplets of two [Rmim][NTf₂] ionic liquids (ILs) on a mica surface was investigated using contact angle measurement and atomic force microscopy (AFM) imaging. Firstly, with contact angle measurement, we found that increasing RH accelerates the kinetic spreading and reduces the equilibrium contact angles of the droplets of two studied ILs on mica. Then by analyzing the mica surface with AFM imaging, we observed that the increasing RH leads to enhanced water vapor adsorption on mica and accordingly enhanced coverage of the IL precursor film on mica. It is essentially the enhanced coverage of the IL precursor film that induces the acceleration of IL droplet spreading and the reduction of their equilibrium contact angles. The analysis of experimental results also suggests that the surface diffusion coefficients of IL precursor films as well as the equilibrium contact angles of IL droplets have a threshold at a RH of ∼70%, which indicates the formation of a complete water monolayer on mica. Such a water layer facilitates the formation of a uniform IL precursor film on the mica surface, which would modify the surface energy of mica and thus the contact angle of IL droplets on mica. The findings from the present investigation are instrumental in the fundamental understanding of the wetting behavior of ionic liquids in humid environments and also offer practical insights into applications of ionic liquids in lubrication and electrochemical energy systems.