Ionic Liquid Structure Dependent Electrical Double Layer at the

Jan 25, 2008 - Structures of the electrical double layer at Hg|room-temperature ionic liquid (RTIL) interfaces were studied by measuring the different...
0 downloads 0 Views 95KB Size
Download PDF
J. Phys. Chem. C 2008, 112, 2601-2606

2601

Ionic Liquid Structure Dependent Electrical Double Layer at the Mercury Interface Muhammad Tanzirul Alam, Md. Mominul Islam, Takeyoshi Okajima, and Takeo Ohsaka* Department of Electronic Chemistry, Interdisciplinary Graduate School of Science and Engineering, Tokyo Institute of Technology, Mail Box G1-5, 4259 Nagatsuta, Midori-ku, Yokohama 226-8502, Japan ReceiVed: October 8, 2007; In Final Form: NoVember 19, 2007

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.

Introduction Room-temperature ionic liquids (RTILs) are being increasingly studied, as demonstrated by their widespread use in practical applications such as electrochemical capacitors and energy storage devices as well as in many fundamental investigations, due to their unique physicochemical properties (e.g., nonvolatility, acceptable ionic conductivity, high thermal stability, and wide electrochemical potential window), environmentally benign characteristics, and ease of their preparation with desired properties.1-6 However, little attention has been paid in delving into the structure of the electrical double layer (EDL) at the electrode interface7-15 and in understanding its effect on heterogeneous electron-transfer kinetics.16-18 A comprehensive understanding of the general aspects of the structure of the EDLs and their correlation with the structures of the RTILs requires related systematic investigations in a wide variety of ionic liquids, for example, by changing their cationic and anionic assemblies. In our previous works,7,8 we studied the interfacial capacitances of a series of RTILs with a variety of alkyl groups. Both the charged and the alkyl moieties of the imidazolium-based cations interact concurrently with the Hg electrode surface. Interaction of the positively charged imidazolium ring with the Hg surface diminishes gradually with increasing the length of the alkyl group due to the preferential hydrophobic interaction of the longer alkyl group with the Hg surface, and thus, capacitance-potential curves at the Hg electrode were found to change remarkably with the variation of the alkyl-chain length. Baldelli et al.9-12 and Nanbu et al.13 have made use of spectroscopic techniques for determining the influence of the molecular structure and charge on the orientation of the RTIL ions at the interface. Nanjundiah et al.14 first measured the differential capacitances at Hg in 1-ethyl-3* Corresponding author. Tel.: +81-45-9245404; fax: +81-45-9245489; e-mail: [email protected].

methylimidazolium-based RTILs, where they demonstrated the effects of anions on the overall capacitance-potential curve as well as the potential of zero charge (PZC). Very recently, Kornyshev15 developed a new theory on double layers at ionic liquid/metal electrode interfaces. For the ionic liquid with symmetric ion sizes, the PZC can be obtained from the maximum of the capacitance if the curve is bell-shaped or from the local minimum between the humps if the curve is camelshaped based on the so-called lattice saturation parameter. In addition, it also has been predicted15 that for ionic liquids with asymmetric ion sizes, the maximum of the capacitance will not coincide with the PZC. Thus far, there has been no report on the measurements of differential capacitance-potential curves and the corresponding electrocapillary curves in RTILs containing larger alkyl chains. The side-chain length of the cations has been known to significantly influence the physical and chemical properties of the ionic liquids, especially their liquid crystal formation.4,5,19 Ionic liquids are reported to maintain a solid-like crystalline structure even in their liquid state with different degrees of stability of the conformers and varying orientations of the neutral alkyl groups and charged moieties of the ions.19-24 Increasing the length of the alkyl groups has been found to largely influence the crystal formation of RTILs and consequently to cause spatial heterogeneity in ionic liquids.19 Thus, it is anticipated that unlike aqueous or conventional organic solvents containing electrolytes with larger alkyl groups, the variation of the alkyl group in RTILs would have a profound effect on the double layer structure at the Hg|RTIL interface, and a comprehensive study is needed in this regard. In this study, an attempt was made to gain an understanding of the interfacial structure at the Hg|RTIL interface depending on the interaction of the alkyl residue of the cation with the Hg surface and the structure of the RTILs themselves. Differential capacitance and electrocapillary curves were measured under

10.1021/jp7098043 CCC: $40.75 © 2008 American Chemical Society Published on Web 01/25/2008

2602 J. Phys. Chem. C, Vol. 112, No. 7, 2008

Alam et al.

certain conditions by taking these factors into consideration. This is the first report on the dependence of the structure of EDL on the crystal structure of the RTILs containing larger alkyl groups. Until now, there has been no satisfactory theoretical model of the EDL at the electrode|RTIL interface,15 which enables us to account for different aspects of its structure under the application of potential in different RTILs. Thus, it is deemed that the present experimental data will help to obtain insight into the interfacial structure at Hg in RTILs and to develop the theoretical model of the EDL. Experimental Procedures Reagents. Dimethyl sulfoxide (DMSO), octanol, and tetraethylammonium tetrafluoroborate (TEABF4) were purchased from Kanto Chemical Co. Inc. DMSO was dried by an activated molecular sieve (4A 1/16, Wako Pure Chemical Industries). All the RTILs of EMIBF4 (1-ethyl-3-methylimidazolium tetrafluoroborate), HMIBF4 (1-hexyl-3-methylimidazolium tetrafluoroborate), and OMIBF4 (1-octyl-3-methylimidazolium tetrafluoroborate) were obtained from either Stella Chemifa Co. or Wako Pure Chemical Industries, Ltd. N2 (99.99%) gas was of ultrahigh purity and was supplied from Nippon Sanso Co., Inc. All of the chemicals were of reagent grade and used without further purification. Electrochemical Measurements. Electrochemical experiments were performed in a three-electrode cell containing a hanging mercury drop (Hg) [HMDE; model CGME 900, Bioanalytical Systems, Inc. (BAS); area 0.018 cm2] as a working electrode, a spiral platinum wire as a counter electrode, and a homemade silver|silver chloride (Ag|AgCl) system as the reference electrode. The preparation of this reference electrode and the stability of its potential have been described elsewhere.7 Each experiment was carried out at a freshly formed Hg drop unless otherwise noted. Solarton SI 1260 and SI 1287 were used as the impedance|gain phase analyzer and electrochemical interface, respectively, for the measurement of capacitances. Impedance measurements were performed at a constant frequency (500 Hz) by scanning the electrode potential (i.e., dc potential) from a negative to positive direction at a scan rate of 5 mV s-1, and the ac potential with a 5 mV peak-to-peak amplitude was superimposed on the dc potential. From the impedance measurement data, the value of the capacitance (C) was derived using the equation -Z| ) 1/(2πfC), where Z| is the imaginary component of impedance.25,26 The drop time measurement was performed with a homemade natural dropping Hg electrode in an electrochemical cell with the same arrangement of reference and counter electrodes as described previously. In this experiment, the Hg drops were allowed to fall through the capillary from a height of 56.5 cm to a N2-saturated solution under varied applied potentials. The lifetime of a drop (i.e., drop time) was estimated as an average by counting the total time required for 10 drops to fall at a certain applied potential. The measurement at each applied potential was repeated 3 times to confirm the reproducibility of the obtained drop time. In a similar way, the drop times were measured in the potential range of -1.2 to +0.55 V at a potential interval of 0.1 or 0.05 V. Electrolytes were deoxygenated by purging pure N2 gas for about 30 min, and the gas was kept flowing over the liquids during electrochemical measurements. All the experiments were carried out at room temperature (25 ( 2 °C) unless otherwise noted. The time constants of the cell (RsCd: Rs, solution resistance and Cd, double layer capacitance) containing EMIBF4, HMIBF4, and OMIBF4 were calculated to be 0.061, 0.63, and 1.12 ms,

Figure 1. Surface tension vs potential curves measured at the droppingHg electrode in N2-saturated (9) EMIBF4, (b) HMIBF4, and (2) OMIBF4.

respectively, using the values of Rs and Cd obtained from the Nyquist plots (not shown here) measured at -1.0 V. The viscosities of EMIBF4, HMIBF4, and OMIBF4 are 31.8, 223.8, and 421.9 cp, respectively. (Data were supplied by Kanto Chemical Co. Inc.). Thus, to confirm the equilibration of the interfacial restructuring, electrocapillary and capacitancepotential curves were measured in OMIBF4 by varying the mercury column height (27 and 56.5 cm) and frequency (200 and 500 Hz), respectively. The results indicated that the electrocapillary curves obtained at mercury column heights of 27 and 56.5 cm are almost the same and also that the capacitance-potential curves measured at 200 and 500 Hz are almost the same. The measurement of surface tension (γ) from the drop time experiment at a dropping mercury electrode is obtained from the following relation:27

tmax )

2πrc γ mg

(1)

The weight of the drop at the end of its lifetime is gmtmax, where m is the mass flow rate of the mercury issuing from the capillary, g is the gravitational force, and tmax is the lifetime of the drop. This force is counterbalanced by the surface tension γ acting around the circumference of the capillary with radius rc. Surface charge density (σM) was estimated by differentiating the electrocapillary curve as per the following equation:28

σM ) -

(∂E∂γ)

T,P,µ

(2)

In this equation, the subscript µ implies that the chemical potentials (activities) of all the components are held constant during each measurement. The slope of the electrocapillary curve at any point is numerically equal and opposite in sign to the value of σM. Results and Discussion Figure 1 illustrates the electrocapillary curves measured at a dropping Hg electrode in N2-saturated (9) EMIBF4, (b) HMIBF4, and (2) OMIBF4. As E is changed from negative to positive values, the surface tension (γ) first increases, then passes through an electrocapillary maximum (ECM), and finally decreases. The ECM is the most important feature of a single

Electrical Double Layer at Mercury Interface electrocapillary curve; it corresponds to the unique situation in the double layer for which the electronic charge density on the metal electrode (in this case Hg) is zero (σM ) 0). An electrocapillary curve is therefore a kind of map that indicates as to how the double layer changes with electrode potential. The value of the surface tension at ECM in HMIBF4 and OMIBF4 (346 dyn cm-1) is quite low as compared to that in EMIBF4 (368 dyn cm-1), although the concentration of the ions in EMIBF4 (6.46 M) is higher than that in HMIBF4 and OMIBF4 (ca. 4.64 and 4.11 M, respectively). The general feature of electrocapillary curves in conventional molecular solvents containing electrolytes is the overall lowering of the value of surface tension with increasing the electrolyte concentration.27-29 However, the reverse was observed in Figure 1. Thus, it is evident that the preferential hydrophobic interaction of the longchain alkyl groups of HMI+ and OMI+ with the Hg surface is responsible for the overall lower value of surface tension at the Hg|HMIBF4 and OMIBF4 interfaces. Alternatively, the value of the dielectric constant () was reported to decrease with increasing the chain length of the alkyl group,30 and thus, the long-chain alkyl groups are considered to play a significant role in reducing the surface tension in the previously mentioned systems as capacitance is directly proportional to . The curve in EMIBF4 is parabolic, similar to that in aqueous and conventional organic media containing electrolytes. Analogous curves were also obtained by Nanjundiah et al.14 in a series of 1-alkyl-3-methylimidazolium-based RTILs with different anions. Thus, the PZC at Hg in EMIBF4 was determined as -0.23 V. On the other hand, the electrocapillary curves in HMIBF4 and OMIBF4 are broad without clear maxima, and thus, it is actually impossible to estimate the PZC values from these curves. The broadness of the electrocapillary curve around the ECM is considered to be a unique property reflecting the structure of the EDL at Hg|RTILs containing a large alkyl group as the substituent of the cationic moiety. In conventional molecular solvent systems, such broadness of the ECM was found to be related to the strong adsorption of the alkyl group on the Hg surface.27-29 Thus, to confirm the origin of the broadness, electrocapillary curves were measured in DMSO containing octanol. Figure 2 represents the (a) electrocapillary and (b) corresponding surface charge density versus potential curves measured at the dropping-Hg electrode in N2-saturated (9) OMIBF4 and (b) DMSO containing 2 M octanol and 0.1 M TEABF4. In both cases, the maxima are quite broad as compared to those usually observed in aqueous media containing inorganic salts27,28 and also in other RTILs (1-propyl-3-methylimidazolium tetrafluoroborate, 1-butyl-3-methylimidazolium tetrafluoroborate, and N,N-diethyl-N-methyl-N-(2-methoxyethyl)ammonium tetrafluoroborate) containing relatively shorter alkyl chains.7,8 In addition, we can see that the curves on the cathodic side of the electrocapillary maxima (Figure 2a) are almost the same, demonstrating the resemblance of the interfacial structures. This resemblance is a manifestation of the preferential adsorption of the alkyl group (i.e., octyl group) on the hydrophobic Hg surface in both cases. Long-chain alkyl groups are reported to form a hydrophobic film on the Hg surface excluding the solvent from the inner part of the double layer, which is responsible for the broadness of the ECM in molecular solvents containing electrolytes with larger alkyl groups.27-29,31,32 Ionic liquids containing long-chain alkyl groups are reported to form a hydrophobic region directed away from the positively charged moiety (e.g., imidazolium ring).19-24 Thus, the preferential interaction of the octyl group of OMIBF4 with the Hg surface

J. Phys. Chem. C, Vol. 112, No. 7, 2008 2603

Figure 2. (a) Surface tension vs potential and (b) corresponding surface charge density vs potential curves measured at the dropping-Hg electrode in N2-saturated (9) OMIBF4 and (b) DMSO containing 2 M octanol and 0.1 M TEABF4.

is responsible for the generation of the electrocapillary curve (on the cathodic side) similar to that in DMSO containing octanol. The significant feature of the curve in DMSO is the sharp declination of the surface tension on the anodic side, although the reverse was expected. The concentration of the tetrafluoroborate anion in OMIBF4 is ca. 4.11 M, whereas 0.1 M TEABF4 was added deliberately in DMSO. Therefore, a higher extent of adsorption of BF4- anions was expected (due to its high concentration) to result in a comparatively sharper diminution of the surface tension on the anodic side of the electrocapillary curve in OMIBF4. However, the reverse was observed actually. Surface charge density versus potential curves in Figure 2b also support the above anomaly. σM (the negative value) on the cathodic side of the ECM increases gradually as the potential is increased to the negative direction. This fact could be related to the weak interaction of the positively charged moiety with the electrode surface in response to the applied potential. An interesting aspect of the curve in OMIBF4 is the almost zero charge density in a wide range of potentials around the maximum of the electrocapillary curve (i.e., -0.4 to -0.1 V), whereas in DMSO, the range is quite narrow. For example, the values of σM at 0 V are 1.9 and 7.6 µC cm-2 in OMIBF4 and DMSO containing octanol and TEABF4, respectively. The broad ECM and comparatively lower value of the surface charge

2604 J. Phys. Chem. C, Vol. 112, No. 7, 2008 density on the anodic side seem to reflect the structural constraint of OMIBF4, which may impede BF4- anions from being in contact with the electrode surface. Crystal structures of several imidazolium-based RTILs explored by different groups showed that the side-chain length of the cations largely influences their liquid crystal formation.19-24 In the case of RTILs with sufficient side-chain lengths, neutral alkyl groups of cations aggregate to form spatially heterogeneous domains of the alkyl chain, while in the case of RTILs with short side chains, the charged moieties of cations and anions distribute as uniformly as possible due to the strong electrostatic interactions.19-24 At a certain temperature, when the alkyl chain is short, the distribution of cations is mainly decided by its charged moiety, and the alkyl groups do not have enough collective attractive energy to aggregate. On the contrary, when the alkyl chain is long adequately, the hydrophobic interaction between the chains becomes more significant, and therefore, the spatial heterogeneity in the ionic liquid medium becomes prominent.19-24 Thus, in HMIBF4 and OMIBF4, the length of the alkyl groups (i.e., hexyl and octyl) is long enough to introduce such spatial heterogeneity in their liquid structure forming hydrophobic regions of the alkyl groups directed away from the charged moieties of the cations and anions. Because of the strong hydrophobicity of the Hg surface, OMIBF4 and HMIBF4 interact with the Hg electrode through their alkyl groups, and the charged moieties (i.e., both the imidazolium ring and the BF4- anion) remain away from the electrode surface. The structural constraint of these RTILs may obstruct the anions from being in contact with the electrode surface with the change of potential up to certain extent (ca. -0.4 to -0.1 V), and this is supposed to be responsible for the broadness of the electrocapillary maxima and almost zero charge density over a wide potential range in Figures 1 and 2. Thus, if the above reasoning is true, a prominent change of the structure of the electrical double layer is expected to arise with the breakdown of the liquid crystal structure of RTILs. Figure 3 shows the (a) electrocapillary and (b) corresponding surface charge density versus potential curves measured at the dropping-Hg electrode in different percentage ratios (v/v) of OMIBF4 and DMSO: (9) 100% OMIBF4, (b) 80% OMIBF4, (2) 50% OMIBF4, and (1) 10% OMIBF4. The values of surface tension (γ) and charge density (σM) on the cathodic side of both panels are essentially the same irrespective of their percentage ratio, while a sharp change in γ and σM was observed on the anodic side of the curves with the dilution of OMIBF4 by DMSO. Ions (i.e., OMI+ and BF4-) leave their crystal structure with dilution and become soluble. The more DMSO that is added, the more the ions become dissolved (until all OMIBF4 is dissolved), increasing the concentration of the individual ions in DMSO. Because of the strong hydrophobic interactions, alkyl groups are capable of forming a hydrophobic film around the Hg drop even at a low concentration. Thus, dilution of OMIBF4 does not have a noticeable effect on the cathodic side of the electrocapillary and surface charge density versus potential curves (Figure 3). On the other hand, adsorption of BF4- anions on the Hg surface is quite low but depends on the concentration. For example, the value of σM at -0.15 V in 100% OMIBF4 is zero, whereas in 50% diluted OMIBF4, it is 4.2 µC cm-2. Thus, it can be assumed that BF4- anions leave the crystalline structure of OMIBF4 with dilution, resulting in an increase of charge density on the anodic side (Figure 3b). Moreover, from the negative shift of the ECM with increasing the concentration of BF4- anions (via dilution of OMIBF4) as shown in Figure 3a,

Alam et al.

Figure 3. (a) Surface tension vs potential and (b) corresponding surface charge density vs potential curves measured at the dropping-Hg electrode in different percentage ratios (v/v) of OMIBF4 and DMSO under a N2 atmosphere. (9) 100% OMIBF4, (b) 80% OMIBF4, (2) 50% OMIBF4, and (1) 10% OMIBF4.

it can be said that the adsorption of the BF4- anion on the Hg surface is stronger than that of the positively charged imidazolium ring. Thus, the electrocapillary curves follow the EsinMarkov effect27 quite nicely. Esin-Markov effect is an indicator of the specific adsorption of charged species, which implies that the preferential cationic and anionic adsorption causes the positive and negative shift of the ECM, respectively, with the concentration. Figure 4 represents the capacitance-potential curve measured at the Hg electrode in N2-saturated OMIBF4. This capacitancepotential curve is analogous to that measured at the Hg electrode in HMIBF4.8 The significant features of the curve are the shallow minimum at -0.35 V followed by a capacitance rise in the form of a hump and almost constant capacitance on the cathodic side of the hump. Causes of the appearance of a hump and shallow minimum in RTILs containing larger alkyl groups are described in our previous paper.8 The potential (-0.35 V) at the shallow minimum of Figure 4 can be considered to approximately correspond to the maximum of the electrocapillary curve (Figure 1), although the ECM is not determined precisely because of its broadness. Thus, in this case, the PZC is ca. -0.35 V. Determination of PZC from the capacitance-potential curves in other RTILs was also found to be in nice agreement with that based on the electrocapillary curves.7,8 This experimentally observed harmony is important as we do not have satisfactory theory15 that instructs us as to which point of the capacitance-

Electrical Double Layer at Mercury Interface

Figure 4. Capacitance vs potential curve measured at the N2-saturated Hg|OMIBF4 interface.

potential curve should be the PZC in a given RTIL system with asymmetric ion size and strong specific adsorption of ions on the electrode surface. The almost constant capacitance on the cathodic side of the hump also supports the previous reasoning, that is, due to the preferential interaction of the octyl group with the Hg surface and the structural constraint of OMIBF4, the imidazolium ring cannot approach the electrode surface in response to the potential change, resulting in the almost potential independent capacitance. On the other hand, the capacitance changes largely with potential on the anodic side of the curve. An increase of the capacitance on the anodic side of PZC could be related to the decrease of the distance (from the electrode) of the closest approach for BF4- anions with their possible orientational change with potential. However, in the potential range of ca. -0.45 to -0.1 V, it seems that the orientational change of BF4ions is not large enough for them to leave the liquid crystal structure of OMIBF4 and to be in contact with the electrode surface as the surface charge density was found to be almost zero (Figure 2b). The capacitance-potential curve is neither bell-shaped nor camel-shaped, being different from the theoretical prediction of Kornyshev.15 This may result from the specific adsorption of the alkyl group on the Hg surface, which was not taken into account in his theoretical approach. There is a strong asymmetry in ion sizes of the RTILs used in this study, and thus, as per the prediction of Kornyshev, the maximum (hump) will be shifted away from the PZC, that is, the value of the capacitance at the PZC will no longer be the maximum. However, it is worthwhile to mention here that the hump (maximum) in Figure 4 is related to the specific adsorption of the alkyl group8 on the Hg surface and cannot be correlated with the PZC. Figure 5 shows the capacitance-potential curves measured at the Hg electrode in different percentage ratios (v/v) of OMIBF4 and DMSO. In accordance with the electrocapillary and surface charge density versus potential curves (Figure 3), the capacitance (C) was found to increase drastically on the anodic side of the hump with dilution of OMIBF4, whereas on the cathodic side, it remained essentially the same. For instance, the value of C at -0.1 V in OMIBF4 is 19.7 µF cm-2, whereas the values are 35.3 and 40.7 µF cm-2 in 80 and 50% OMIBF4, respectively. The same explanations, given previously for substantiating the change of the surface tension and charge density with dilution, are also applicable to this case. The hump that appears on the anodic side (ca. -0.05 V) of the capacitance-

J. Phys. Chem. C, Vol. 112, No. 7, 2008 2605

Figure 5. Capacitance vs potential curves measured at the Hg electrode in different percentage ratios (v/v) of OMIBF4 and DMSO. (9) 100% OMIBF4, (b) 80% OMIBF4, (2) 50% OMIBF4, and (1) 10% OMIBF4.

Figure 6. Schematic illustration of the Hg|OMIBF4 interface where the lines (||||||||) and (------) represent the electrostatic interaction and hydrogen bonding, respectively.

potential curve in diluted OMIBF4 is related to DMSO as a similar type of hump was also observed by Payne33 and our group8 almost at the same potential in DMSO. Moreover, such a hump was not observed in 100% OMIBF4 (Figure 4). Figure 6 is a schematic presentation of the Hg|OMIBF4 interface at a potential around the ECM. This illustration is based on the structures of different imidazolium-based ionic liquids explored by other groups.19-24 Long-chain alkyl groups are reported to aggregate due to the collective short-range interaction within them forming hydrophobic regions that are shown to interact directly with the Hg surface in this diagram. In the bulk of OMIBF4, these long-chain alkyl groups are not aligned in the same direction as shown in Figure 6. Thus, it seems that the hydrophobic interaction between the Hg surface and the alkyl group is strong enough to bring an orientational change

2606 J. Phys. Chem. C, Vol. 112, No. 7, 2008

Alam et al.

necessary for alignment of the alkyl groups pointing toward the Hg surface. Anions reside away from the electrode surface (in the vicinity of the imidazolium ring) due to the electrostatic interaction and the ease of forming a hydrogen bond with the protons of the imidazolium ring.19-24 It is presumed that the above-mentioned interaction impedes the anions from being in contact with the electrode surface with a small change of potential to the positive direction from the PZC, being responsible for the broadness of the electrocapillary maxima in Figure 1. However, at sufficiently positive potentials (more positive than 0.05 V) and with dilution by DMSO, BF4- anions were found to interact with the electrode surface with a concomitant change of the surface charge density (Figures 2b and 3b). The simplest equation for double layer capacitance is27

0 Cd ) d

(3)

where  is the dielectric constant of the medium that has an approximate value30 of 8 for OMIBF4, 0 (8.85 × 10-12 F m-1) is the permittivity of the free space, and as per the model of the interfacial structure presented in Figure 6, the distance (d) is assumed to be equal to the van der Waals diameter of the cation, that is, ca. 0.75 nm.4 Thus, the calculated capacitance at the Hg|OMIBF4 interface at the PZC is equal to 9.4 µF cm-2, which is close to the experimental value of 9 µF cm-2. Here, it should be mentioned that the dielectric constant of OMIBF4 in contact with a charged electrode would differ from (and possibly be lower than) that of the bulk of OMIBF4. Conclusion Differential capacitance and electrocapillary curves were measured at a Hg electrode in HMIBF4 and OMIBF4 RTILs as a function of potential. The interfacial structure was found to differ significantly from that at Hg in RTILs containing shorterchain alkyl groups. Electrocapillary curves in HMIBF4 and OMIBF4 demonstrate an unusual broadness of the maxima on the anodic side of the PZC and are a unique characteristic of the double layer structure at the Hg|RTILs interface. Because of the strong hydrophobicity of the Hg surface, the alkyl groups of HMIBF4 and OMIBF4 interact with it predominantly, while the charged moieties of the cation and the anion (BF4-) reside away from the electrode surface. Structural constraints in orientational change and spatial heterogeneity in their liquid structure impede the charged moieties from being in contact with the Hg surface within a certain potential range and are responsible for the observed broadness of the ECM. The surface charge density versus potential curves also support the above reasoning, that is, the charge density is almost zero in a wide range of potentials around the ECM. However, the electrocapillary curves become sharper with the dilution of OMIBF4 by DMSO. The addition of DMSO into OMIBF4 results in a breakdown of the liquid crystal structure and allows the anions to be in contact with the electrode surface. The shift of the ECM

to the negative direction of potential and the increase of the capacitance on the anodic side of the capacitance-potential curves with the dilution of OMIBF4 also support the previous reasoning. Acknowledgment. The present work was financially supported by Grant-in-Aids for Scientific Research on Priority Areas (417) and Scientific Research (A) (19206079) to T.O. and from the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan. M.T.A. gratefully acknowledges the Government of Japan for a Monbu-kagakusho Scholarship. References and Notes (1) Wasserscheid, P.; Keim, W. Angew. Chem., Int. Ed. 2000, 39, 3772. (2) Welton, T. Chem. ReV. 1999, 99, 2071. (3) Binnemans, K. Chem. ReV. 2005, 105, 4148. (4) Tokuda, H.; Hayamizu, K.; Ishii, K.; Susan, M. A. B. H.; Watanabe, M. J. Phys. Chem. B 2005, 109, 6103. (5) Tokuda, H.; Ishii, K.; Susan, M. A. B. H.; Tsuzuki, S.; Hayamizu, K.; Watanabe, M. J. Phys. Chem. B 2006, 110, 2833. (6) Oyama, T.; Okajima, T.; Ohsaka, T. J. Electrochem. Soc. 2007, 154, 322. (7) Alam, M. T.; Islam, M. M.; Okajima, T.; Ohsaka, T. Electrochem. Commun. 2007, 9, 2370. (8) Alam, M. T.; Islam, M. M.; Okajima, T.; Ohsaka, T. J. Phys. Chem. C 2007, 111, 18326. (9) Rivera-Rubero, S.; Baldelli, S. J. Phys. Chem. B 2004, 108, 15133. (10) Romero, C.; Baldelli, S. J. Phys. Chem. B 2006, 110, 6213. (11) Aliaga, C.; Baldelli, S. J. Phys. Chem. B 2006, 110, 18481. (12) Baldelli, S. J. Phys. Chem. B 2005, 109, 13049. (13) Nanbu, N.; Sasaki, Y.; Kitamura, F. Electrochem. Commun. 2003, 5, 383. (14) Nanjundiah, C.; McDevitt, S. F.; Koch, V. R. J. Electrochem. Soc. 1997, 144, 3392. (15) Kornyshev, A. A. J. Phys. Chem. B 2007, 111, 5545. (16) Lagrost, C.; Preda, L.; Volanschi, E.; Hapiot, P. J. Electroanal. Chem. 2005, 585, 1. (17) Fietkau, N.; Clegg, A. D.; Evans, R. G.; Villagran, C.; Hardacre, C.; Compton, R. G. ChemPhysChem. 2006, 7, 1041. (18) Lynden-Bell, R. M. Electrochem. Commun. 2007, 9, 1857. (19) Wang, Y.; Voth, G. A. J. Am. Chem. Soc. 2005, 127, 12192. (20) Saha, S.; Hayashi, S.; Kobayashi, A.; Hamaguchi, H. Chem. Lett. 2003, 32, 740. (21) Matsumoto, K.; Tsuda, T.; Hagiwara, R.; Ito, Y.; Tamada, O. Solid State Sci. 2002, 4, 23. (22) Hayashi, S.; Ozawa, R.; Hamaguchi, H. Chem. Lett. 2003, 32, 498. (23) Matsumoto, K.; Hagiwara, R.; Mazej, Z.; Benkic, P.; Zemva, B. Solid State Sci. 2006, 8, 1250. (24) Hunt, P. A.; Kirchner, B.; Welton, T. Chem.sEur. J. 2006, 12, 6762. (25) Islam, M. M.; Okajima, T.; Ohsaka, T. J. Phys. Chem. B 2004, 108, 19425. (26) Saha, M. S.; Ohsaka, T. Electrochim. Acta 2006, 50, 4746. (27) Bard, A. J.; Faulkner, L. R. Electrochemical Methods, Fundamentals, and Applications, 2nd ed.; Wiley Inc.: New York, 2001. (28) Bard, A. J. Electroanalytical Chemistry, A Series of AdVances; Marcel Dekker Inc.: New York, 1966; Vol. 1. (29) Devanathan, M. A. V.; Tilak, B. V. K. S. R. A. Chem. ReV. 1965, 65, 635. (30) Wakai, C.; Oleinikova, A.; Ott, M.; Weingartner, H. J. Phys. Chem. B 2005, 109, 17028. (31) Hayter, J. B.; Hunter, R. J. J. Electroanal. Chem. 1972, 37, 71. (32) Hayter, J. B.; Hunter, R. J. J. Electroanal. Chem. 1972, 37, 81. (33) Payne, R. J. Am. Chem. Soc. 1967, 89, 489.