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J. Phys. Chem. C 2007, 111, 18326-18333

Measurements of Differential Capacitance at Mercury/Room-Temperature Ionic Liquids Interfaces 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: July 24, 2007; In Final Form: September 14, 2007

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.

Introduction Room-temperature ionic liquids (RTILs) have emerged as attractive solvents as their physicochemical properties can be tuned easily by changing the component ions.1-3 Unlike aqueous or usual organic solvents, they offer us the opportunity to prepare solvents of our desired characteristics ranging from highly hydrophilic to completely hydrophobic or from highly viscous to less viscous just by changing the length of the alkyl substituent while keeping the basic structure of the ions the same. Because of the environmentally benign characteristics (e.g., nonvolatility, nontoxicity, etc.), they have the potential to displace some conventional organic solvents with volatility and toxicity in various practical processes. In fact, RTILs are being used extensively in gas separation, liquid-liquid extraction, biphasic catalysis process, fuel cells, and a wide range of chemical and electrochemical investigations.2-7 However, some fundamental aspects remain to be solved including the structure of electrical double layer at the electrode/RTILs interface. The structure of electrical double layer is mainly elucidated with the aid of some surface-related spectroscopic techniques, measurement of interfacial capacitances, and surface tensions. Baldelli and co-workers8-11 and Nanbu et al.12 have made use of spectroscopic techniques for determining the influence of the molecular structure and charge on the orientation of the RTILs ions at the interface. Nanjundiah et al.13 first measured the differential capacitances at Hg in 1-ethyl-3-methylimidazolium based RTILs where they demonstrated the effects of anion on the overall capacitance-potential curve as well as the potential of zero charge (PZC). In our recent report,14 we have presented * To whom correspondence should be addressed. Tel: +81-45-9245404; fax: +81-45-9245489; e-mail: [email protected].

essentially different capacitance-potential curves measured at three different electrode (Hg, Au, GC) substrates in 1-propyl3-methylimidazolium tetrafluoroborate (PMIBF4). However, knowledge about the structure of electrical double layer at the electrode/RTIL interface is still limited. Thus, experimental data as well as theoretical model of the electrical double layer are needed to explore its structure. Previous models of the electrical double layer (e.g., GouyChapman theory) on the basis of dilute-solution approximation are no longer applicable in RTIL systems because of the high concentration of the ions, the strong Coulombic interaction between them, and the direct contact of the charged ions with the electrode surface.2,3,15-17 Thus, the theory of double layer in ionic liquids should be built differently. Very recently, Kornyshev18 developed a new theory on double layer at ionic liquid/metal electrode interfaces which results in a simple analytical expression for the double-layer capacitance providing the first advance beyond the Gouy-Chapman formula. He has predicted that the capacitance-potential curve should be bellor camel-shaped for the ionic liquids with similar anion and cation sizes depending on the so-called lattice-saturation parameter. 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 camel-shaped. In addition, it has been also predicted18 that for ionic liquids with asymmetric ion size the maximum of the capacitance will not coincide with the PZC. In this study, our attention is focused chiefly on how the interfacial capacitance (i.e., structure) varies with potential under various interactions of the ions of RTILs with the electrode, and a series of RTILs are used for this purpose. This is the first report on the unexpected resemblance of the capacitance-

10.1021/jp075808l CCC: $37.00 © 2007 American Chemical Society Published on Web 11/17/2007

Measurements of Differential Capacitance

J. Phys. Chem. C, Vol. 111, No. 49, 2007 18327

potential curve at Hg/EMIBF4 interface to that of inner Helmholtz layer at Hg in aqueous solution in the absence of specific adsorption of electrolytes. Moreover, this is the first systematic investigation on the structure of electrical double layer at Hg/RTILs interface. Along with the competitive interactions (i.e., electrostatic and hydrophobic) of the ionic moieties with the Hg surface, the physical state of Hg (as Hg drop and film electrodes) has been also taken into account in explaining the capacitance curves. Attempt has been made to explain and corroborate different aspects of capacitancepotential curves with the aim of understanding an interfacial structure at the Hg/RTILs interface. A comparison with the theoretical prediction on electrical double layer in RTILs18 has also been included. Experimental Section Reagents. Dimethyl sulfoxide (DMSO) and tetraalkylammonium salts [(tetramethylammonium perchlorate (TMAP), tetraethylammonium perchlorate (TEAP), tetrabutylammonium perchlorate (TBAP), and tetraethylammonium tetrafluoroborate (TEABF4)] were purchased from Kanto Chemical Co. Inc. Prior to use, DMSO was dried by activated molecular sieve (4A 1/16, Wako Pure Chemicals Industries). All the RTILs of EMIBF4 (1-ethyl-3-methylimidazolium tetrafluoroborate), BMIBF4 (1butyl-3-methylimidazolium tetrafluoroborate), HMIBF4 (1hexyl-3-methylimidazolium tetrafluoroborate), and N,N-DEMMEABF4 [N,N-diethyl-N-methyl-N-(2-methoxyethyl)ammonium tetrafluoroborate] with a purity of more than 99% and less than 30 ppm (i.e., ca. 2.1 mM) water were obtained from either Stella Chemifa Co. (Japan) or from Kanto Chemical Co., Inc (Japan). Highly pure N2 (99.99%) gas was supplied by Nippon Sanso Co., Inc. (Japan). All of the chemicals were of reagent grade and were used without further purification. Electrochemical Measurements. Electrochemical experiments were performed in a three-electrode cell containing a hanging mercury drop [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) as reference electrode. The preparation of this reference electrode and the stability of its potential are described elsewhere.14 Each experiment was carried out at a freshly formed Hg drop unless otherwise noted. A continuous water flowing thermostatic bath was used for measuring the capacitances at elevated temperatures. The Hgfilm electrode (Hg/Au), used as working electrode, was prepared by dipping Au electrode in pure liquid Hg for 2 min in N2atmosphere. The coated electrode was then shaken strongly to form a mirrorlike film of Hg on Au.19 Solarton SI 1260 and SI 1287 were used as impedance/gain phase analyzer and electrochemical interface, respectively, for the measurement of capacitances. The time constants of the cell (RsCd; Rs, solution resistance; Cd, double-layer capacitance) containing EMIBF4, BMIBF4, and N,N-DEMMEABF4 were calculated to be 0.061, 0.22, and 0.29 ms, respectively, using the values of Rs and Cd obtained from the Nyquist plots (not shown here) measured at -1 V. Impedance measurements were done at a constant frequency (200 Hz) by scanning the electrode potential (i.e., dc potential) from negative to positive direction at a scan rate of 5 mV/s, and the ac potential with 5 mV peakto-peak amplitude was superimposed on 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.20,21 The drop-time measurements were done with homemade dropping Hg electrode in an electrochemical cell with the same

Figure 1. Capacitance versus potential curve measured at N2-saturated Hg/EMIBF4 interface.

arrangement of reference and counter electrodes as described above. In these experiments, Hg drops were allowed to fall through the capillary from a height of 56.5 cm to N2-saturated BMIBF4 and N,N-DEMMEABF4 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 three 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. All the experiments were carried out at room temperature (25 ( 2 °C) unless otherwise mentioned. Electrolytes were deoxygenated by purging pure N2 gas for about 30 min, and the gas was kept flowing over the liquids during the electrochemical measurements. In accordance with our pervious report14 and the in situ spectrochemical data of Aliaga and Baldelli10,11 and Nanbu et al.,12 a minimum of the capacitance-potential curve is ascribed to PZC throughout this study. This assignment of PZC is in nice agreement with the maximum of the corresponding electrocapillary curve measured at dropping Hg electrode. Results and Discussion Differential Capacitance at Hg/EMIBF4 Interface. Figure 1 illustrates the differential capacitance curve measured at N2saturated Hg/EMIBF4 interface as a function of potential. The shallow minimum that arises in the capacitance-potential curve is designated as the PZC (-0.21 V) in agreement with the electrocapillary experiment done by our group22 where the electrocapillary maximum was found to be -0.23 V versus Ag/ AgCl. The minimum between zones III and IV is a characteristic feature of Hg in the absence of specific adsorption of electrolytes and by no means represents the PZC. The capacitance-potential curve measured at the Hg/EMIBF4 interface is analogous to that of the inner Helmholtz layer at Hg/aqueous interface in the absence of specific adsorption of electrolyte.15,23 Possible causes for the resemblance of the curves will be discussed later. The appearance of a hump in zone II of Figure 1 could be the consequence of adsorption of ions as previously proposed in some other cases.16 Since it appears on the cathodic side of the PZC, the specific adsorption of anions should be negligible. However, preferential nonspecific adsorption of neutral alkyl group or positively charged imidazolium moiety could be a cause and will be taken into consideration one by one. First, capacitance versus potential curves were measured at different

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Figure 2. Temperature-dependent capacitance versus potential curves measured at N2-saturated Hg/EMIBF4 interfaces. Capacitances were recorded at (9) 22, (b) 40, (2) 60, (1) 80 °C.

temperatures for confirming the type of interaction responsible for the generation of the hump in Figure 1. Figure 2 represents the temperature dependency of the differential capacitance curves measured at N2-saturated Hg/ EMIBF4 interfaces as a function of potential. Capacitances were measured at four different temperatures, 22, 40, 60, and 80 °C. The hump at -0.34 V diminishes gradually with increasing temperature whereas the curves in the potential region more negative than ca. -0.4 V remain essentially the same. The hump disappears almost completely at 80 °C. Interaction of the specifically adsorbed ions with the electrode surface is always thought to be quite strong in nature, and thus this much remarkable change of the pattern of the capacitance-potential curve, that is, almost complete ceasing of the hump, is not expected. Very similar temperature-dependent capacitancepotential curves were also reported by Grahame15 and others16 for the inner part of the electrical double layer measured at Hg in contact with aqueous solution of sodium fluoride where the inner Helmholtz layer is composed of only adsorbed water molecules and the intrusion of the charged ions in this layer is considered to be insignificant. The hump that appears usually in dilute solution of electrolyte disappears gradually with increasing the concentration of ions because of the gradual compactness of the diffuse layer resulting in higher value of its capacitance around the PZC.15-17,23 However, in this temperature-dependent experiment at Hg/EMIBF4 interface, such capacitance elevation around the PZC is not observed (Figure 2). Instead, the hump starts to diminish gradually with increasing temperature indicating its relation with the nonspecific adsorption of EMI+ at the Hg electrode surface. The lessened capacitance at the hump could be the consequence of partial desorption of imidazolium ring with concomitantly increased interaction of the neutral alkyl group with the electrode surface. Differential Capacitance at Hg/BMIBF4 Interface. For justifying the above reasoning, capacitance versus potential curve was measured at N2-saturated Hg/BMIBF4 interface and is shown in Figure 3a. This curve is completely analogous to one measured at the Hg/EMIBF4 interface at 80 °C (Figure 2). In both cases, there is no hump and also no distinct point of PZC. Therefore, it can be assumed that the enhanced interaction of the neutral alkyl group with the electrode surface at least partially prevents the imidazolium moiety from being in contact with the electrode surface, which results in no hump unlike the case at the Hg/EMIBF4 interface at 25 °C (Figure 1). As per

Figure 3. (a) Capacitance versus potential and (b) electrocapillary curves measured at N2-saturated Hg/BMIBF4 interface.

the recent report of Umebayashi et al.,24 EMI+ ions exist in two conformers (in equilibrium), namely, planar and nonplanar with respect to the C2-N1-C6-C7 dihedral angle (Scheme 1), and the nonplanar conformer is favored over the planar one with slightly lower energy (2-4 kJ mol-1). The dihedral angle changes with temperature, with the kind of counteranion, and with the molecular arrangement in crystal. In the case of EMIBF4, the preferred electrostatic interaction of the charged imidazolium moiety over the hydrophobic interaction of the ethyl group with the Hg surface is responsible for the generation of the hump. At high temperature, the equilibrium between these conformers seems to increase the portion of the planar conformer compared to that at room temperature which in turn enhances the chance of interaction of ethyl group with the Hg surface. Therefore, the comparatively reduced electrostatic interaction of the imidazolium moiety with the electrode surface results in decreasing the value of capacitance at the hump (Figure 2). In the case of BMIBF4, the nonplanar conformer is even more stable because of the higher energy barrier between the conformers compared to EMIBF4 which may give more room for the preferential interaction of the alkyl group with the Hg surface. On the other hand, the large alkyl groups are reported to form hydrophobic region directing away from the imidazolium ring.25-27 So, the scope of the imidazolium ring to interact with the electrode surface decreases in BMIBF4. Thus, the comparatively lesser extent of electrostatic interaction of the



SCHEME 1: Structures of the Cations Constituting the RTILs Used in This Study

Figure 4. Capacitance versus potential curve measured at N2-saturated Hg/HMIBF4 interface.

imidazolium moiety with the electrode surface is considered to be responsible for the absence of the hump in Figure 3a. For determining the PZC, electrocapillary measurement was carried out in N2-saturated BMIBF4, and the result is shown in Figure 3b. The electrocapillary curve is parabolic like those in aqueous media with a maximum at -0.32 V which is theoretically designated as the PZC.28 Increase of the alkyl chain length from ethyl in EMIBF4 to butyl in BMIBF4 shifted the PZC from -0.23 V to -0.32 V. This somewhat positive value of PZC in EMIBF4 is also an indication of the comparatively greater extent of interaction of the positively charged imidazolium ring with the electrode surface and is responsible for the generation of the hump in Figure 1. The figures presented above confirm the relation of the hump with the nonspecific adsorption of the positively charged imidazolium moiety at the Hg surface. However, as the hump appears very close to the PZC, the contribution of neutral alkyl group to its generation cannot be discarded completely. So, to further get an insight of the interfacial structure and to confirm the fate of the interaction of alkyl group with the Hg surface, the capacitance-potential curve was measured in RTILs containing a larger alkyl group. Differential Capacitance at Hg/HMIBF4 Interface. Figure 4 stands for the differential capacitance curve measured at N2saturated Hg/HMIBF4 interface as a function of potential. The PZC is estimated as -0.35 V from the shallow minimum of this capacitance curve. The significant features of this curve are the sharp capacitance rise on the anodic side of the PZC, the shallow minimum at the PZC followed by capacitance rise in the form of a hump, and the almost constant capacitance on the cathodic side of the hump in analogy with the capacitancepotential curves at Hg electrode in aqueous media containing surfactants.29,30 The position of the hump in HMIBF4 differs largely from that in EMIBF4, that is, -0.53 and -0.34 V in HMIBF4 and EMIBF4, respectively. This difference of 0.19 V is a clear indication of the different origins of the generation of the humps in HMIBF4 and EMIBF4. The hump comes out as a consequence of the increase in (i.e., dielectric constant) because of the orientational change of the ions resulting from the possible partial desorption of alkyl chain from the Hg surface. A decrease in with increasing the proportion of neutral

alkyl group at the interface is recognized generally and is responsible for the overall lower capacitance value of the capacitance-potential curve at the Hg/HMIBF4 interface compared to that at the Hg/EMIBF4 (Figure 1). The extent of adsorption of neutral alkyl group is reported to be high at the PZC, and the appearance of the hump close to the PZC corroborates it nicely. Hayter and Hunter29,30 had reported that the adsorption of large alkyl group affects the capacitancepotential curve both by lowering the value of capacitance and by shifting the PZC. Almost constant capacitance on the cathodic side of the hump is a manifestation of the structural constraint of the bulky cation and preferential adsorption of the neutral alkyl group on the Hg surface. The above factors impede the orientational change of the positively charged imidazolium ring required to come close to the electrode surface for capacitance rise on the cathodic side of the hump. Ionic liquids seem to be unique in that they have definite local structures despite their homogeneous appearance.25-27,31 The negative shift of the PZC in HMIBF4 compared to that in EMIBF4 is the result of greater extent of interaction of the alkyl group with the Hg electrode surface or, in other words, lesser extent of interaction of the positively charged imidazolium ring with the Hg. Usually, the greater the interaction of the positively charged ion with the electrode, the more the PZC shifts to the positive direction of potential. For confirming the relation of the hump with the alkyl chain of the imidazolium moiety of RTILs, differential capacitances were measured at Hg electrode in N2-saturated DMSO containing 0.01 M tetraalkylammonium perchlorate salts of various chain length, and the results are presented in Figure 5a where the symbols 9, b, and 2 stand for TMAP, TEAP, and TBAP, respectively. The systematic increase of the capacitance at the hump with increasing the alkyl chain length strongly suggests its relevancy to the hump. The PZC was found to be the same in all the cases. This is to be expected because the inner part of the double layer is almost fully composed of DMSO molecules and the effect of the chain length of neutral alkyl group on the PZC is considered to be negligible. Capacitance curves were also measured by changing the anion while keeping the cation the same for confirming that the hump is solely a consequence of hydrophobic interaction of the neutral alkyl group with the Hg electrode surface. Figure 5b represents the capacitance versus potential curves measured at Hg electrode in N2-saturated DMSO containing 0.01 M (9) TEAP and (b) TEABF4. The curves on the cathodic side of the PZC are essentially the same


Figure 5. Capacitance versus potential curves measured at Hg in contact with N2-saturated DMSO containing 0.01 M of tetraalkylammonium salt. (a) TMAP (9), TEAP (b), TBAP (2); (b) TEAP (9), TEABF4 (b).

while the difference was found only at the PZC and on its anodic side as expected from the structure of diffuse layer and the interaction of the ions with the electrode surface. Thus, it can be said that the interaction of alkyl group of HMIBF4 with the Hg electrode surface is responsible for the generation of the hump in Figure 4. Differential Capacitance at Hg/N,N-DEMMEABF4 Interface. Figure 6a demonstrates the capacitance versus potential curve measured at Hg electrode in N2-saturated N,N-DEMMEABF4. This curve is almost analogous to that measured at the Hg/HMIBF4 interface (Figure 4) except for the presence of a relatively broader and intense hump though a more intense hump was expected for HMIBF4 because of the presence of longer alkyl chain (hexyl) and its (anticipated) relatively strong interaction with the electrode surface. Capacitance-potential curves measured in DMSO containing tetraalkylammonium salts (Figure 5) also give an intense hump almost at the same potential as in HMIBF4. The three-dimensional tetrahedral structure of the cation of N,N-DEMMEABF4 impedes its positive charge center from being in direct contact with the electrode surface which is responsible for the generation of the intense hump. Unlike tetraalkylammonium salts, the cationic moiety of HMIBF4 contains only two alkyl groups, one is methyl and the other is hexyl, allowing some room for the positively charged imidazolium ring to interact with the electrode surface with the possible change of its orientation. A relatively small hump in

Alam et al.

Figure 6. (a) Capacitance versus potential and (b) electrocapillary curves measured at N2-saturated Hg/N,N-DEMMEABF4 interface.

HMIBF4 (Figure 4) is an indication of the concomitant hydrophobic and electrostatic interactions of the alkyl and imidazolium moieties of the HMI+ cation, respectively, with the Hg electrode surface. The electrostatic interaction is a little stronger in HMIBF4 than in N,N-DEMMEABF4 (Figure 6a) and DMSO containing tetraalkylammonium salts (Figure 5) which is responsible for the appearance of a small hump in HMIBF4. Figure 6b illustrates the electrocapillary curve measured at dropping Hg electrode in N2-saturated N,N-DEMMEABF4 as a function of potential. This curve shows a maximum at ca. -0.33 V which theoretically corresponds to the PZC and which is in accordance with the minimum (-0.34 V) of the corresponding capacitance-potential curve (Figure 6a). Aliaga and Baldelli10,11 have also assigned the minimum of the capacitance-potential curve as the PZC at Pt/RTILs interface using sum frequency generation in situ spectroelectrochemical technique. Thus, by comparing the data obtained in this study with our previous results14,22 and the spectroelectrochemical data,8-12 the minimum of the capacitance-potential curve is ascribed to the PZC throughout this study though it is unlikely that the theory of diffuse double layer would be straightforwardly applicable to RTILs. The PZCs along with the corresponding values of capacitance are summarized in Table 1. PZC shifts to the negative direction of potential in the order of EMIBF4, BMIBF4, and HMIBF4 because of the reduced interaction of the positively charged imidazolium moieties with the Hg electrode surface. The capacitance-potential curve is neither bell-shaped nor camel-shaped, being different from the theoretical prediction



TABLE 1: Comparison of the PZC and the Hump at Hg Electrode in the Studied RTILs RTILs

hump (V vs Ag/AgCl)

cause of the hump

PZC (V vs Ag/AgCl)

capacitance at PZC (µF cm-2)

EMIBF4

-0.34

nonspecific adsorption of imidazolium ring

-0.23

19.7

PMIBF4d BMIBF4 HMIBF4 N,N-DEMMEABF4

no hump no hump -0.53 -0.58

a a adsorption of alkyl group adsorption of alkyl group

-0.31 -0.32 -0.35 -0.33

17.9 17.7 8.9 10.5

remark capacitance-potential curve similar to that of the inner Helmholtz layer at Hg/aqueous interface b b c c

a None of the interactions (e.g., electrostatic and hydrophobic interactions of the imidazolium ring and alkyl group of the cation, respectively, with the Hg surface) is strong enough to give rise to a hump. b Capacitance-potential curves do not have any minimum around the potential of electrocapillary maximum because of the concurrent electrostatic and hydrophobic interactions of both the imidazolium ring and the alkyl group of the cation with the Hg surface. c Preferential interactions of alkyl groups with the Hg surface result in a lower value of capacitance in the capacitancepotential curves with the appearance of the hump and shifting of PZC to the negative direction of potential. d From ref 14.

of Kornyshev.18 This may result from the specific adsorption of the alkyl group on the Hg surface which has not been taken into account in his theoretical approach. It has also been predicted that in ionic liquids with asymmetric ion size the maximum (hump) will be shifted away from the PZC.18 Capacitance-potential curve measured at Hg in EMIBF4, HMIBF4, and N,N-DEMMEABF4 (Figures 1, 4, and 6) also has a hump (maximum) close to the PZC, which is in harmony with the theoretical predication, and it shifts to the negative direction of potential with increasing the size of the cation (Table 1). It should be emphasized, however, that the hump in HMIBF4 and N,N-DEMMEABF4 is related to the specific adsorption of alkyl group on Hg electrode surface. Cause of Hump and Valley. As mentioned above, two types of capacitive humps were observed, Figures 1 (in EMIBF4) and 4 (in HMIBF4), reflecting the structure-dependent preferential interactions of the positively charged imidazolium moiety and the hydrophobic alkyl group with the Hg surface, respectively. The absence of the hump in BMIBF4 (Figure 3a) similar to that observed in EMIBF4 is considered as a consequence of lesser extent of interaction of the imidazolium moiety with the Hg surface because of the larger size and increased hydrophobicity of the butyl group in BMIBF4 compared to ethyl group in EMIBF4. The hump in the capacitance-potential curve in DMSO containing tetraalkylammonium salt (Figure 5) was found to increase with a small increase in the alkyl chain length, while in RTILs (EMIBF4 and BMIBF4) containing alkyl groups of similar length, such a hump (because of the adsorption of alkyl groups) was not observed. So, to investigate the cause of the disappearance of such a hump, capacitances were measured at the Hg-film (Hg/Au) electrode in EMIBF4 and HMIBF4. Figure 7 represents the differential capacitance curves measured at the Hg-film electrode in N2-saturated (a) HMIBF4 and (b) EMIBF4 as a function of potential. The shape of the capacitance-potential curve at the Hg-film electrode in HMIBF4 is almost similar to that at the Hg drop electrode (Figure 4) with a little shift of the PZC to the negative direction of potential. The PZC at the drop electrode is found to be -0.35 V whereas at the Hg-film electrode it is -0.4 V. This small shift could be a result of Hg-film formation on Au electrode and, thus, it can be said that the Hg-film on the Au electrode possesses most of the characteristics of bulk liquid mercury except its fluidity. Except this small shift in PZC, other aspects such as the constant capacitance on the cathodic side of the hump, the height of the hump, and the capacitance rise on the anodic side of the PZC were found to be the same. The capacitance-potential curve at the Hg-film electrode in EMIBF4 differs significantly from that at the Hg drop electrode (Figure 1). The PZC (-0.4 V) was found to be shifted by 0.19 V to the negative direction of potential compared to that at the

Figure 7. Capacitance versus potential curves measured at Hg-film (Hg/Au) electrode in contact with N2-saturated (a) HMIBF4 and (b) EMIBF4.

Hg drop electrode (-0.21 V) with the appearance of the hump at -0.51 V. Interestingly, the positions of both the hump and the PZC are almost the same as those in the capacitancepotential curves at the Hg/HMIBF4 (Figure 4) and Hg/DMSO (Figure 5) interfaces which can be considered as a consequence of preferential adsorption of ethyl group of EMI+ on the Hg surface in its solidlike film state. This also justified that the hump found at Hg drop electrode in EMIBF4 (Figure 1) is not related to the adsorption of ethyl group (as mentioned above). Moreover, the negative shift of PZC and the appearance of the hump, like in HMIBF4, are also indications of the lesser extent


Alam et al.

C)

Figure 8. Capacitance versus potential curves measured at Hg in contact with N2-saturated DMSO containing 0.01 (9), 0.025(b), 0.05 (2), 0.1 (1), and ([) 0.5 M NH4BF4.

of interaction of the positively charged imidazolium moiety with the Hg-film surface. Alternatively, the interactions of the positively charged moiety and the alkyl group of the imidazolium-based RTILs with the Hg surface are related to the physical state of Hg (i.e., its liquid drop or solidlike film). The preferential interaction of the positively charged imidazolium ring of EMIBF4 with Hg in its drop state is responsible for the disappearance of the hump related to the adsorption of ethyl group. Further work is in progress in this regard. To further substantiate the cause of the disappearance of the hump in BMIBF4 related to the adsorption of alkyl group on Hg drop, capacitance curves were measured in DMSO containing NH4BF4 of various concentrations, and the results are shown in Figure 8. The hump was found to become ambiguous with increasing the concentration because of the elevation of capacitance around the valley of the PZC. A similar type of effect is also expected to be observed in RTILs. The concentrations of HMIBF4 and BMIBF4 themselves are 4.64 and 5.35 M, respectively. Thus, on the basis of the theory of electrical double layer,28 in these much concentrated electrolytic systems there should not be any distinguishable valley related to the PZC in the capacitance-potential curves. However, HMIBF4 behaves like a low-concentration solution probably owing to the fact that the positively charged imidazolium moieties reside far from the electrode surface because of its structural constraint to orientational change and preferential hydrophobic interaction with the Hg electrode surface. On the contrary, the relatively shorter alkyl chains in BMIBF4 enable the charged moiety to be in stronger contact with the electrode surface compared to HMIBF4. The relatively reduced hydrophobic interaction of BMI+ cation with the Hg surface in BMIBF4 may also increase the chance of electrostatic interaction. Thus, it is thought that the higher extent of interaction of the charged moiety with the Hg surface in BMIBF4 compared to that in HMIBF4 is responsible for the disappearance of hump and valley corresponding to the PZC in the capacitance-potential curve obtained in the former ionic liquid. The capacitance-potential curve at the Hg/EMIBF4 interface (Figure 1) is analogous to that of the inner Helmholtz layer at the Hg/aqueous interface15-17,23 in the absence of specific adsorption of electrolytes. According to Kornyshev,18 the innerlayer capacitance is given as

1 4π ξ(d/2) - zσ

where is the effective dielectric constant, d is the diameter of an ion, the factor ξ takes into account the distance of the closest approach of an ion to the ionic skeleton of the metal, and zσ is the position of the center of mass of the excess charge on the electrode relative to the edge of the metal ionic skeleton. For a specified system, the inner-layer capacitance will change on the basis of the effect of the electrode charge (potential) on the center of mass of the excess charge (zσ) and the distance of the closest approach ξ(d/2). It has been predicted18,32,33 that at negative potential the abovementioned two parameters compete with each other, while at positive potential they work together, and thus the increase in the capacitance at positive charges is faster and higher than at negative charges, that is, the whole capacitance-potential curve becomes asymmetric with respect to PZC. Thus, the asymmetry of the observed capacitancepotential curve can be understood on the basis of this idea as previously pointed out by Kornyshev and Vorotyntsev32 for the well-known asymmetry of the capacitance curves obtained in aqueous media. Although the values of the abovementioned parameters are unknown for ionic liquids, it can be assumed that the capacitance-potential curve at Hg in EMIBF4 is mainly controlled by zσ and ξ(d/2) (inner layer) upon charging, and the effect of the diffuse layer is negligible. For the inner-layer capacitance-potential curve at Hg in aqueous media, the reorientation of water molecules is the cause of the generation of hump.32-36 Thus, the orientation of the ions at the Hg/EMIBF4 interface under applied potential may be concerned with the generation of the hump, and some recent studies8-12 based on the spectro-electrochemical techniques have revealed this possibility. Baldelli and co-workers8-11 have shown that the imidazolium rings tip at different angles in response to the applied potential with the orientational change of the ions. The chemisorption of water molecule at the Hg surface through the oxygen atom has been reported to restrict the change of its orientation with potential,36 whereas in EMIBF4 the weak electrostatic interaction of imidazolium ring with the Hg surface has proved to be the cause of the appearance of the hump in Figure 1. The temperature dependency of the hump and the absence of strong specific adsorption of EMI+ at the Hg surface in EMIBF4 also support the above reasoning (Figure 2). Additionally, the adsorption affinity of BF4- anion toward Hg has been reported to be very low.16,17 Thus, a combination of all the abovementioned effects causes EMIBF4 to behave like the nonspecifically or weakly adsorbing electrolyte at Hg interface, allowing the ions to orient in response to the applied potential like water molecules at the inner Helmholtz layer of the Hg/aqueous solution interface. The similarity of the abovementioned capacitance-potential curves thus suggests that the capacitance-potential curve in EMIBF4 is mainly controlled by the inner-layer capacity and that the effect of the diffuse layer is negligible. However, the characteristic capacitancepotential curve under consideration was observed only in EMIBF4 and not in other RTILs used in this study. This could be the result of alteration of the surface electronic property as well as orientation of the imidazolium ring with potential because of the preferential hydrophobic interaction of the alkyl group with the Hg electrode surface. Further work needs to be done in this regard. Conclusions Differential capacitances of the electrical double layer at Hg electrode in 1-alkyl-3-methylimidazolium based RTILs were

Measurements of Differential Capacitance measured with the aim of getting an insight of its structure and of determining the values of PZCs. The capacitance-potential curve at the Hg/EMIBF4 interface was found to be similar to that of the inner layer at Hg/aqueous system containing nonspecifically adsorbing electrolyte. In both cases, the hump disappeared with increasing the temperature. Electrocapillary measurements were also performed for the determination of PZC in BMIBF4 and N,N-DEMMEABF4. The minimum of the capacitance-potential curve is ascribed to the PZC in accordance with the maximum of the corresponding electrocapillary curve. The values of the PZC were found to shift to the negative direction of potential with increasing the chain length of the alkyl residue of the cationic moiety. In addition, appearance or disappearance of the minimum corresponding to the PZC was found to depend on the chain length of the alkyl residue of the cations, reflecting the competitive interactions of the charged moiety and the hydrophobic alkyl group of the cations with the Hg electrode surface with their potentialdependent orientational change. Our experimental results do not follow the theoretical prediction completely which could be a consequence of the specific adsorption of alkyl group on the Hg electrode surface. Acknowledgment. The present work was financially supported by Grant-in-Aids for Scientific Research on Priority Areas (No. 417) and Scientific Research (A) (No. 19206079) to T.O. 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) Anderson, J. L.; Armstrong, D. W.; Wei, G. T. Anal. Chem. 2006, 78, 2893.

J. Phys. Chem. C, Vol. 111, No. 49, 2007 18333 (5) Galinski, M.; Lewandowski, A.; Stepniak, I. Electrochim. Acta 2006, 51, 5567. (6) Binnemans, K. Chem. ReV. 2007, 107, 2592. (7) Parvulescu, V. I.; Hardacre, C. Chem. ReV. 2007, 107, 2615. (8) Rivera-Rubero, S.; Baldelli, S. J. Phys. Chem. B 2004, 108, 15133. (9) Romero, C.; Baldelli, S. J. Phys. Chem. B 2006, 110, 6213. (10) Aliaga, C.; Baldelli, S. J. Phys. Chem. B 2006, 110, 18481. (11) Baldelli, S. J. Phys. Chem. B 2005, 109, 13049. (12) Nanbu, N.; Sasaki, Y.; Kitamura, F. Electrochem. Commun. 2003, 5, 383. (13) Nanjundiah, C.; McDevitt, S. F.; Koch, V. R. J. Electrochem. Soc. 1997, 144, 3392. (14) Alam, M. T.; Islam, M. M.; Okajima, T.; Ohsaka, T. Electrochem. Commun. 2007, 9, 2370. (15) Grahame, D. C. J. Am. Chem. Soc. 1957, 79, 2093. (16) Devanathan, M. A. V.; Tilak, B. V. K. S. R. A. Chem. ReV. 1965, 65, 635. (17) Grahame, D. C. Chem. ReV. 1947, 41, 441. (18) Kornyshev, A. A. J. Phys. Chem. B 2007, 111, 5545. (19) Islam, M. M.; Saha, M. S.; Okajima, T.; Ohsaka, T. J. Electroanal. Chem. 2005, 577, 145. (20) Islam, M. M.; Okajima, T.; Ohsaka, T. J. Phys. Chem. B 2004, 108, 19425. (21) Saha, M. S.; Ohsaka, T. Electrochim. Acta 2006, 50, 4746. (22) Islam, M. M.; Alam, M. T.; Okajima, T.; Ohsaka, T. J. Phys. Chem. B 2007, 111, 12849. (23) Grahame, D. C. J. Am. Chem. Soc. 1954, 76, 4819. (24) Umebayashi, Y.; Fujimori, T.; Sukijaki, T.; Asada, M.; Fujii, K.; Kanzaki, R.; Ishiguro, S. J. Phys. Chem. A 2005, 109, 8976. (25) Saha, S.; Hayashi, S.; Kobayashi, A.; Hamaguchi, H. Chem. Lett. 2003, 32, 740. (26) Matsumoto, K.; Hagiwara, R.; Mazej, Z.; Benkic, P.; Zemva, B. Solid State Sci. 2006, 8, 1250. (27) Hunt, P. A.; Kirchner, B.; Welton, T. Chem. Eur. J. 2006, 12, 6762. (28) Bard, A. J.; Faulkner, L. R. Electrochemical Methods, Fundamental and Application, 2nd ed.; Wiley Inc.: New York, 2001. (29) Hayter, J. B.; Hunter, R. J. J. Electroanal. Chem. 1972, 37, 71. (30) Hayter, J. B.; Hunter, R. J. J. Electroanal. Chem. 1972, 37, 81. (31) Hayashi, S.; Ozawa, R.; Hamaguchi, H. Chem. Lett. 2003, 32, 498. (32) Kornyshev, A. A.; Vorotyntsev, M. A. J. Electroanal. Chem. 1984, 167, 1. (33) Kornyshev, A. A. Electrochim. Acta 1989, 12, 1829. (34) Damaskin, B. B.; Frumkin, A. N. Electrochim. Acta 1974, 19, 173. (35) Parsons, R. J. Electroanal. Chem. 1975, 59, 229. (36) Conway, B. E.; Bockris, J. O’M. Modern Aspects of Electrochemistry; Plenum press: New York, 1979; Vol. 13.

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