Electric Double Layer of Au(100)/Imidazolium-Based Ionic Liquids Interface: Effect of Cation Size

We have carried out differential capacitance measurements and in-situ scanning tunneling microscope (STM) characterizations to investigate the effect of the length of alkyl side chains on an electric double layer of Au(100)/imidazolium-based ionic liquids interface. In ionic liquids consisting of BMI+ cation (1-butyl-3-methylimidazolium), differential capacitance curves present an obvious bell-shaped feature. In ionic liquids with PMI+ (1-methyl-3-propylimidazolium) or OMI+ (1-methyl-3-octylimidazolium) cations, the rising of capacitance from about -0.5 V disturbs the bell-shaped feature. In-situ STM characterizations reveal the generality of surface etching and micelle-like adsorption of imidazolium cations on Au(100) at potential around the peaks of the bell-shaped feature, demonstrating that the potential of zero charge (PZC) should locate at the potential close to the peaks. Because of the longer side chain length and stronger interaction with Au(100) substrate, an extra capacitance peak appears at the potential as negative as -1.65 V in OMIPF6 and a corresponding order-disorder transformation of OMI+ cation adlayer is revealed by STM, indicating a correlation between differential capacitance curve and STM.