(709e) The Molecular Structure of Ionic Liquids at the IL/Solid Interface:Uncovering the Effect of Water

Due to the high thermal stability and excellent physiochemical characteristics, ionic liquids (ILs) are promising for various applications such as coating and energy storage, where the IL/solid interface is involved. It is critical to uncover how water impacts the wettability of ILs at the IL/solid interface since water absorption is inevitable under ambient conditions. Wettability of [Bmim][FAP], [Bmim][BF₄] and [Bmim][NTf₂] on mica was investigated by time-dependent contact angle measurement and ATR-FTIR. Different relative humidity levels were controlled by applying saturated salt solutions in an environmental chamber. [Bmim][FAP] was found to wet the surface completely at humidity >48% because of the charging of the mica surface and the self-assembly of the ions in ILs, while the contact angles of [Bmim][BF₄] and [Bmim][NTf₂] only dropped slightly at humidity >80%. Ultrathin ionic liquid films were also applied on the mica and silica surfaces by dip-coating, and the molecular-level morphology was characterized by tapping-mode AFM and ellipsometry. When the nanofilms got thicker, a transition from an island structure to a sponge-like structure to nanodroplets was observed for [Bmim][FAP], while a transition from a smooth surface to nanodroplets was observed for [Bmim][BF₄] and [Bmim][NTf₂]. The relation between the chemistry, morphology and the wettability has been established.