Electrocapillarity and zero-frequency differential capacitance at the interface between mercury and ionic liquids measured using the pendant drop method

摘要

The structure of ionic liquids (ILs) at the electrochemical IL vertical bar Hg interface has been studied using the pendant drop method. From the electrocapillarity (potential dependence of interfacial tension) differential capacitance (C-d) at zero frequency (in other words, static differential capacitance or differential capacitance in equilibrium) has been evaluated. The potential dependence of zero-frequency C-d at the IL vertical bar Hg interface exhibits one or two local maxima near the potential of zero charge (E-pzc), depending on the cation of the ILs. For 1-ethyl-3-methylimidazolium tetrafluoroborate, an IL with the cation having a short alkyl chain, the C-d vs. potential curve has one local maximum whereas another IL, 1-octyl-3-methylimidazolium tetrafluoroborate, with the cation having a long alkyl chain, shows two maxima. These behaviors of zero-frequency C-d agree with prediction by recent theoretical and simulation studies for the electrical double layer in ILs. At negative and positive potentials far from E-pzc, the zero-frequency C-d increases for both the ILs studied. The increase in zero-frequency C-d is attributable to the densification of ionic layers in the electrical double layer.