Electrical Double Layer in Mixtures of Room-Temperature Ionic Liquids

The interfacial structure at the Hg electrode in mixtures of room-temperature ionic liquids (1-ethyl-3-methylimidazolium tetrafluoroborate (EMIBF4) and 1-octyl-3-methylimidazolium tetrafluoroborate (OMIBF4)) has been studied for the first time by the measurement of surface tension and differential capacitance. Surface tension, charge density on the electrode surface, and capacitance decrease with increasing the addition of OMIBF4 in EMIBF4, which has been found to be the consequence of the preferential adsorption of the octyl group on the Hg surface and increasing extent of heterogeneity of the mixtures. The continuous widening of the electrocapillary maxima (ECM) with increasing addition of OMIBF4 in EMIBF4 also supports the above reasoning. In accordance with the ECM, the potential corresponding to the minimum of the capacitance-potential curve is assigned as the potential of zero charge (PZC). The probable cause for the appearance of the PZC at the minimum of the capacitance-potential curve has been discussed.