Preferential Formation of Layered Structure of Ionic Liquid at Ionic Liquid Aqueous Solution / Graphite Electrode Interfaces Observed by Frequency-Modulation Atomic Force Microscopy

摘要

Local structure of ionic liquid (IL), 1-butyl-3-methylimidazolium tetrafluoroborate (BMIM-BF4), at IL + water mixtures / highly-oriented pyrolytic graphite (HOPG) interfaces has been investigated using frequency-modulation atomic force microscopy (FM-AFM). The imaging study of surface topography and energy dissipation of IL + water / HOPG interfaces suggested that even the hydrophilic IL molecules favored the HOPG substrate forming stable layers of ILs whose layer thickness corresponded to the averaged ion-pair size. A threshold molar ratio (χIL) for the first IL layer formation was between 0.01 and 0.05 and the HOPG surface was completely covered by IL layer at as low as χIL = 0.1. Force curve measurements in various concentrations of IL + water mixtures also suggested that the liquid side nature such as the apparent viscosity near the interfaces changed around the critical concentration. Dissolution of the IL layer was found to be a slow process, suggesting that the interface structure was history-dependent. Furthermore, electrochemical potential of the HOPG substrate affected the IL layer formation. Particularly application of positive potential resulted in growth of IL layer at the interface. [DOI: 10.1380/ejssnt.2014.89]