Measurements of differential capacitance at mercury/room-temperature ionic liquids interfaces

Differential capacitances were measured at Hg/room-temperature ionic liquids (RTILs) interfaces as a function of potential with the aim of getting an insight of their interfacial structures. Capacitance-potential curve measured at Hg in 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIBF4) resembles well the inner layer capacity at the Hg/aqueous solution interface containing nonspecifically adsorbing electrolyte. In both cases, the hump decreases with an increase in temperature which is discussed in the light of the previous theory. Both the alkyl group and the charged moiety of the cation of 1-alkyl-3-methylimidazolium based RTILs are found to interact concurrently with the Hg surface with the possible change of their orientation in response to the applied potential, and the appearance of a shallow minimum in the capacitance-potential curve related to potential of zero charge (PZC) depends on the extent of their interaction. PZC shifts to the negative direction of potential with increasing the chain length of alkyl residue of the cationic moiety because of the constraint in the orientational change needed for the interaction of positively charged imidazolium ring with Hg surface. Electrocapillary curves were also measured to determine the PZC. Throughout this study, a minimum of the capacitance-potential curve is designated as the PZC in agreement with the maximum of the corresponding electrocapillary curve. Different aspects of the capacitance-potential curves are interpreted satisfactorily on the basis of the hitherto proposed concept of electrical double layer structure.