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.

Structures of the electrical double layer at Hg|room-temperature ionic liquid (RTIL) interfaces were studied by measuring the differential capacitance and electrocapillary curves as a function of potential. Maxima of the electrocapillary curves measured at the Hg|1-hexyl-3-methylimidazolium tetrafluoroborate (HMIBF4) and 1-octyl-3-methylimidazolium tetrafluoroborate (OMIBF4) interfaces demonstrate an unusual broadness on the anodic side of the potential of zero charge (PZC), which is significantly different from those obtained at Hg in RTILs containing shorter alkyl chains or in conventional molecular solvents containing electrolytes. This broadness of the electrocapillary curve was found to depend on the crystal structure and spatial heterogeneity of the RTILs containing larger alkyl groups, which impede the charged moieties from being in contact with the electrode surface within a certain potential range. Cleaving of the liquid crystal structure by the dilution of OMIBF4 with dimethyl sulfoxide, which is reflected on the electrocapillary and surface charge density versus potential curves, supports the above reasoning. This is the first report on the dependence of the interfacial structure at the Hg electrode on the structure of the RTIL itself. A schematic model of the structure of the electrical double layer is also given.

A systematic and comprehensive study on cyclic voltammetric anodic current oscillation (CVACO) at a hanging mercury drop electrode (HMDE) was carried out for the redox reactions of molecular oxygen (O2), nitrobenzene (NB), 1,4-dinitrobenzene (DNB), benzoquinone (BQ), 2,3,5,6-tetramethylbenzoquinone (TMBQ), benzophenone (BP), azobenzene (AB), 2,1,3-benzothiadiazole (BTD), 7,7,8,8-tetracyanoquinodimethane (TCNQ), methyl viologen dichloride (MV2+), and tris(2,2‘-bipyridine)ruthenium(II) dichloride [Ru(bpy)3 2+] in dimethyl sulfoxide (DMSO) solutions containing 0.1 M tetraethylammonium perchlorate (TEAP). From the electrocapillary curve (ECC) obtained using a dropping mercury electrode as well as the capacitance versus potential curves measured using electrochemical impedance technique, the value of the potential of zero charge (PZC) was estimated to be −0.27 V versus Ag|AgCl|NaCl (sat.) in a DMSO solution containing 0.1 M TEAP. CVACO was found to occur only for the redox couples (i.e., BP0/BP•-, O2 0/O2 •-, AB0/AB•-, Ru(bpy)3 2+/Ru(bpy)3 +, BTD0/BTD•-, NB0/NB•-, DNB0/DNB•-, DNB•-/DNB2-, TMBQ0/TMBQ•-, MV2+/MV•+, and BQ•-/BQ2-) having the formal potentials (E 0‘ values) more negative than the PZC. CVACO was largely dependent on the concentrations of redox species and TEAP; for example, in the case of BTD the intensity of CVACO increased with increasing concentration, and CVACO ceased at high concentrations of TEAP (≥0.5 M). Furthermore, CVACO was not observed for the BQ0/BQ•- redox couple having E 0‘ (= −0.31 V) near the PZC, and a pronounced cathodic maximum was observed for the TCNQ•-/TCNQ2- redox couple with E 0‘ (= −0.16 V) more positive than the PZC. These observations and the factors governing the CVACO are discussed on the basis of the theory presented for the polarographic maxima of the first kind. The observed CVACO and the cathodic maximum obtained for the TCNQ•-/TCNQ2- redox couple could be explained in terms of the so-called streaming effect.