Microstructural probing of phosphonium-based ionic liquids on gold electrode using colloid probe AFM

Atomic force microscopy (AFM) with gold colloid probe modeled as the electrode surface is employed to directly capture the contact resonance frequency of two phosphonium-based ionic liquids (ILs) containing a common anion [BScB]? and differently lengthened cations ([P6,6,6,14]+ and [P4,4,4,8]+). The comparative interfacial studies are performed by creating IL films on the surface of gold, followed by measuring the wettability, thickness of the films, adhesion forces, surface morphology and AFM-probed contact resonance frequency. In addition, cyclic voltammetry and impedance spectroscopy of the neat ILs are measured on the surface of gold electrode. The IL with longer cation alkyl chains exhibits a well-defined thin film on the electrode surface and enhanced capacitance than the shorter-chained IL. AFM contact resonance frequency and force curves reveal that the longer IL prefers to form stiffer ion layers at the gold electrode surface, suggesting “…anion–anion–cation–cation…” bilayer structure, in contrast, the shorter-chained IL forms softer cation–anion alternating structure, i.e., “…anion–cation–anion–cation…”.