Probing the interfacial structure and ion mobility in structurally-related ionic liquids via dynamic wetting measurements

Hypothesis: Understanding the structure of ionic liquids (ILs) near solid surfaces is essential for advancing their implementation in engineering applications, including as electrolytes in energy storage devices and lubricants. The structure of IL/solid interfaces has extensively been evaluated through scanning probe microscopy and surface force apparatus measurements; however, these techniques only probe the static interfacial properties of ILs when nanoconfined between two solid surfaces. The underpinning hypothesis of this work is that dynamic wetting is an effective method for gaining insights into the structure of IL/solid interfaces and drawing links between the IL architecture and their interfacial behavior without nanoconfinement between two surfaces. Experiment: Sessile drop dynamic wetting measurements were conducted to assess the static and dynamic interfacial structure of a homologous series of ILs, which contain cations with varying alkyl chain length, on oxidized silicon. The dynamic wetting data were modeled using the molecular kinetic theory. Findings: The dynamic wetting results revealed that the jump length of the IL ions at the three-phase contact line decreases with increasing cation alkyl chain length. This finding indicates a progressive change in interfacial ion organization, with cations increasingly oriented perpendicularly to the substrate, as the cation alkyl chain length increases, allowing for a higher surface packing density. Furthermore, the viscosity of ILs was found to significantly increase (58-105x) near the solid surface, particularly for ILs with cations containing short alkyl chains. The results of this work advance our understanding of the structural and dynamic properties of ILs near solid surfaces.