Anion dependence of camel-shape capacitance at the interface between mercury and ionic liquids studied using pendant drop method

The electrocapillarity and zero-frequency differential capacitance, Cd, have been studied using pendant drop method, at the Hg interface of an ionic liquid (IL), 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)amide, [C2mim+][TFSA−], and have been compared with those of [C2mim+]BF4−, an IL with the common cation and a different anion, to focus on the anion dependence of zero-frequency Cd. The Hg interface of [C2mim+][TFSA−], the IL of the larger anion in the present study, exhibits greater zero-frequency Cd than that of [C2mim+]BF4−, the IL of the smaller anion. This behavior contradicts a simple expectation in which larger ion leads to smaller Cd. This apparent contradiction is explained by proximity of the charged moiety of TFSA− to the electrode surface compared with that of BF4−. The potential dependence of zero-frequency Cd for the two ILs both exhibits one-hump camel shape around the potential of zero charge (Epzc), which has been predicted to be specific behavior of the electrical double layer of ILs by theory and simulation. The humps are located at potentials more negative than Epzc. From a mean-field lattice-gas theory for the EDL in ILs, this negative shift can be interpreted that the charged moiety for C2mim+ is more easily condensed in the EDL than those for BF4− and TFSA−. •Zero-frequency differential capacitance is evaluated at Hg interface of ionic liquids (ILs).•One-hump camel-shape capacitance as a function of potential is obtained for the two ILs studied.•The camel shape is compared and discussed with a model of the electrical double layer (EDL) in ILs.•The condensation degree of the IL ions in the EDL is evaluated by comparing data with theory.