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

Abstract

The structure of ionic liquids (ILs) at the electrochemical IL|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|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_dvs. 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.