Phase Transition of a Binary Room-Temperature Ionic Liquid

Jun 16, 2009 - The initial decrease with x of the interfacial tension (γ) at the .... First, the singularity at ln x ) -1.6 (x ) 0.2) is highlighted ...
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J. Phys. Chem. B 2009, 113, 9321–9325

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Phase Transition of a Binary Room-Temperature Ionic Liquid Composed of Bis(pentafluoroethanesulfonyl)amide Salts of Tetraheptylammonium and N-Tetradecylisoquinolinium and Its Surface Properties at the Ionic Liquid|Water Interface Ryoichi Ishimatsu, Yuki Kitazumi, Naoya Nishi, and Takashi Kakiuchi* Department of Energy and Hydrocarbon Chemistry, Graduate School of Engineering, Kyoto UniVersity, Kyoto 615-8510, Japan ReceiVed: March 25, 2009; ReVised Manuscript ReceiVed: May 2, 2009

A binary room-temperature ionic liquid (RTIL) composed of bis(pentafluoroethanesulfonyl)amide (C2C2N-) salts of tetraheptylammonium (THpA+) and N-tetradecylisoquinolinium (C14Iq+) undergoes a phase transition upon increasing the mole fraction of C14Iq+ (x) in the bulk RTIL. The initial decrease with x of the interfacial tension (γ) at the interface between water (W) and the binary RTIL reaches a break point at x = 0.2 irrespective of the values of the phase-boundary potential. The surface tension at RTIL|air interface and the conductivity of the binary RTIL support that the break point at x ) 0.2 at the RTIL|W interface is attributable to the change of the bulk property. However, unlike the micelle formation of a surfactant solution, a further increase in x gives rise to a further change in γ. Whereas the phase transition at x ) 0.2 does not depend on the applied potential (E) across the RTIL|W interface, the mode of the change in γ at x > 0.2 strongly depends on E and the apparent deficit of C14Iq+ at the interface is more pronounced when E is closer to the point of zero charge. TABLE 1: Densities of Water-Saturated RTIL (d/g cm-3) at 25°C

Introduction A sufficiently hydrophobic room-temperature ionic liquid (RTIL) that is immiscible with water (W) can form a polarized interface upon contact with W.1,2 The structure of the electrical double layers at the RTIL|W interface depends on the phaseboundary potential (∆φ).3 Previously, we reported the electrocapillarity at the polarized RTIL|W interface by externally controlling ∆φ to investigate the electrical double-layer structure.4,5 When the RTIL consists of a symmetrical quaternary ammonium ions, such as tetrahexylammonium (THxA+), the specific adsorption of Cl- at the interface from the W phase is negligible.4 In contrast, when the RTIL consists of surface-active cations, such as N-tetradeylisoquinolinium (C14Iq+), the interfacial ion-pairing with Cl- takes place at the RTIL|W interface.5 The nature of the RTIL-constituent ions can thus significantly alter the structure of the electrical double layer and the surface properties, accordingly. Recently, the physicochemical properties of binary mixtures of RTILs have been studied to expand the tuning capability of RTILs.6-11 Applications of such binary RTILs include electrochemical devices12-15 and gas chromatography.16 Presumably, the surface properties of such binary RTILs should depend on the type of the RTIL-constituting ions and the mixing ratio of two cations or anions. In the present paper, we describe the electrocapillarity at the electrochemically polarized interface between W and a binary RTIL composed of tetraheptylammonium (THpA+) and C14Iq+ as cations and bis(pentafluoroethanesulfonyl)amide (C2C2N-) as the common anion, and present evidence for the phase transition of RTIL upon increasing the mole fraction of C14Iq+. The phase transition in the bulk RTIL affects the structure of the RTIL|W interface depending on the magnitude of ∆φ. * To whom correspondence should be addressed. Telephone: +81-75383-2489. Fax: +81-75-383-2490. E-mail: [email protected].

x

d

x

d

x

d

x

d

0 0.005 0.01 0.02

1.153 1.153 1.155 1.156

0.03 0.04 0.05 0.1

1.157 1.159 1.160 1.165

0.2 0.3 0.5 0.7

1.175 1.189 1.219 1.248

0.8 0.9 0.95 1

1.263 1.278 1.290 1.297

Experimental Section Materials. N-Tetradecylisoquinolinium chloride (C14IqCl) was synthesized from isoquinoline (Aldrich) and 1-chlorotetradecane (Aldrich), as described elsewhere.17 [C14Iq+][C2C2N-] and tetraheptylammonium bis(pentafluoroethanesulfonyl)amide ([THpA+][C2C2N-]) were prepared by mixing approximately equimolar amounts of C14IqCl or THpABr (Tokyo Chemical Industry Co., Ltd., Extra Pure) and HC2C2N(70% aqueous solution, Central Glass Co., Ltd.) in methanol. After the solution was stirred for 1 h, methanol was evaporated using an evaporator. The residual waxy solid (a mixture of [C14Iq+][C2C2N-] and HCl, or [THpA+][C2C2N-] and HBr) was washed with water repeatedly until Cl- or Br- was not detected when a few drops of an aqueous AgNO3 solution were added in the supernatant solution. The RTILs were then decolorized with a column packed with active charcoal and silica gel according to the method proposed by Earle et al.18 RTILs saturated with water were used throughout this study. The density of RTILs (d) measured with a pycnometer at 25C° is listed in Table 1. The solubility of water in [C14Iq+][C2C2N-] and [THpA+][C2C2N-] (SW/R), and that of [C14Iq+][C2C2N-] in W (SR/W) measured with the Karl Fischer method and UV-vis adsorption spectroscopy, respectively, were 0.40 and 0.26 wt %, and 70 µ mol dm-3. Binary RTILs were prepared by mixing [THpA+][C2C2N-] and [C14Iq+][C2C2N-] with a magnetic stirrer for one day. Interfacial Tension Measurements. The interfacial tension (γ) at the RTIL|W interface was measured with a pendant-drop

10.1021/jp9027035 CCC: $40.75  2009 American Chemical Society Published on Web 06/16/2009

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method. A video image of an RTIL drop in W was processed to extract the shape of the drop. The details of the apparatus and data processing have been described elsewhere.19 The electrochemical cell employed for voltammetry and electrocapillarity measurements in the present study is represented as

where x stands for the mole fraction of C14Iq+. The potential of the right-hand-side terminal of the cell with respect to the left, E, was controlled using a four-electrode potentiostat with positive feedback for the ohmic drop compensation. The glass cell for (I) with electrode setting has been described elsewhere.19 In recording electrocapillary curves, E was changed every 10 mV from the positive end of the polarized potential window (ppw) to the negative. Since at least 30 s was required to obtain a stable value of γ after setting a value of E, the shape of an RTIL drop was acquired every 2 s successively between 40 and 60 s after the potential step. The video images were processed to calculate γ as the average of the γ values from ten successive images. Electrocapillary curves were measured quadruplicately at a given x for sixteen values of x () 0, 0.005, 0.01, 0.02, 0.03, 0.04, 0.05, 0.1, 0.2, 0.3, 0.5, 0.7, 0.8, 0.9, 0.95, and 1). γ at the interface between water-saturated RTIL and the air was measured three times for a given value of x. For ion-transfer voltammetry, a flat RTIL|W interface having the area of 0.03 cm-2 was employed. All measurements were performed at 25 ( 0.5 °C. Conductivity Measurements. The conductivity of the watersaturated binary RTIL was measured with a conductivity meter (GM-117, Kyoto Electronics Manufacturing Co., Ltd.) at 25.0 ( 0.2 °C. The binary RTIL in a glass cell for the conductivity measurement was kept in a thermostat bath for 24 h prior to the measurement. Results and Discussion Polarized Potential Window. First, we examined the degree of the interactions of Cl- and C14Iq+ in the RTIL phase using voltammetry of the facilitated transfer of Cl- in W to the RTIL phase through the ion-pairing between Cl- and C14Iq+. The degree of this interaction in the bulk RTIL phase can be estimated from the width of the ppw. Figure 1 shows cyclic voltammograms at the interface between [THpA+ + C14Iq+][C2C2N-] and the aqueous solution of 0.05 mol dm-3 LiCl at six values of x (0, 0.05, 0.1, 0.3, 0.5, and 1). The scan rate of the applied voltage was 100 mV s-1. The positive ends of the ppw are independent of x, and are limited by the transfer of C2C2N- from RTIL to W.20,17 In contrast, the negative ends of the ppw shifted to the positive potentials with increasing x. The widths of the ppw were 290 (x ) 0), 250 (x ) 0.05), 240 (x ) 0.1) 230 (x ) 0.3), 210 (x ) 0.5), and 170 mV (x ) 1). Here we defined the positive (negative) end of a ppw as the E values when the current reached 3 (-3) µA in the scanning of E to the positive (negative) direction. An increase in the concentration of LiCl in W resulted in a decrease in the width of the ppw at the negative end (data not shown), which fact confirms that the negative end of the ppw is determined by the transfer of Clfrom W to RTIL when x > 0.5 From the plot of the ppw as a function of ln x in the inset of Figure 1 we can see that the width of the ppw linearly decreases

Figure 1. Cyclic voltammograms at the interface between an aqueous 0.05 mol dm-3 LiCl solution and various values of cationic mole ratio of C14Iq+ (x) at 0 (1), 0.05 (2), 0.1 (3), 0.3 (4), 0.5 (5), and 1 (6) with a scan rate of 100 mV s-1. The interfacial area was 0.03 cm-2. The arrows show a scan direction. The inset shows the width of the polarized potential window (ppw) as a function of ln x. The dotted line in the insert is intended to guide the eye.

Figure 2. Electrocapillary curves at the interface between an aqueous 0.05 mol dm-3 LiCl solution and various x, 0 (O), 0.01 (b), 0.03 (∆), 0.05 (2), 0.1 (0), 0.3 (9), 0.5 (]), 0.7 ([), 0.8 (∇), 0.9 (1), and 1 (.). The vertical bar exemplifies the standard deviation.

with ln x from x ) 0 to the break point at 0.3 (ln x ) -1.2), and then the slope becomes more than 5 times steeper. Since the degree of narrowing the ppw with the ion pairing of Cland C14Iq is proportional to ln KIP, where KIP is the formation 5 constant of ion pair between Cl- and C14Iq+ in the RTIL, the strength of the ion pair becomes distinctively stronger beyond x ) 0.3 up to pure [C14Iq+][C2C2N-]. The manner of narrowing the ppw thus suggests the phase transition of the [THpA+ + C14Iq+][C2C2N-] with the change in x that creates the different environments for ion pairing. Electrocapillary Curves. The electrocapillary curves at the [THpA+ + C14Iq+][C2C2N-]|W interface for eleven different values of x () 0, 0.01, 0.03, 0.05, 0.1, 0.3, 0.5, 0.7, 0.8, 0.9, and 1) are shown in Figure 2. Each point represents the average value of quadruplex measurements. A typical standard deviation is shown as a vertical bar on the leftmost point for x ) 0.03. The curves at x ) 0, 0.01, 0.02, and 0.03 exhibit electrocapillary maxima corresponding to the points of zero charge (pzc). Around the pzc, the γ values at x ) 0 and 1 are both 10-20% higher than those of our previous work for the same RTILs.5 This rise is probably because of the purification of RTILs we made in the present work, as described above. Figure 3a shows γ as a function of x at six different values of E () -150, -200, -250, -300, -350, and -400 mV). γ initially decreased rapidly with x up to x = 0.2, and then, the rate of a decrease in γ became distinctively gradual beyond that

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-

1 ∂γ RT ∂ln x

( )

T,p,E

) ΓC14Iq+ -

x ≡ ΓC* 14Iq+ Γ 1 - x THpA+

(2)

Figure 3. Plots of γ against x (a) or ln x (b) at E ) -150 (O), -200 (b), -250 (]), -300 ([), -350 (0), and -400 mV (9). The dotted lines are intended to guide the eye. The vertical bar exemplifies the standard deviation.

point. γ reaches a minimum at x = 0.9 and then γ increased when x > 0.9 at all values of E (E e -300 mV) where γ was measurable over the entire range of x. The discontinuous change in γ with x is more clearly seen in Figure 3b, where γ is plotted as a function of ln x. First, the singularity at ln x ) -1.6 (x ) 0.2) is highlighted in Figure 3b. Interestingly, the location of this singularity is independent of E. The appearance of such a break point in γ vs x or γ vs ln x is common at air|W interfaces where the micelle formation of surfactant takes place in the adjacent bulk phase.23-27 The invariance of the location of the break point in Figure 3b with the change of E strongly suggests that this singularity is associated with the change in bulk properties of the RTIL phase and a micelle-like structure is formed in the RTIL. However, the anomaly in Figure 3b is distinctively different from the cases at the air|W interface; beyond x ) 0.2, γ further decreased at higher x values, and more extraordinarily, γ increased with ln x when E ) -150 (O) and -200 (b) mV between ln x ) -1.6 (x ) 0.2) and -1.2 (x ) 0.3). The change in γ in Figure 3b is better understood in terms of surface thermodynamics. The electrocapillary equation for cell (I) at the constant temperature and pressure can be written as28

-dγ ) qW dE + ΓC14Iq+RT d ln x + ΓTHpA+RT d ln(1 - x) (1) where qW, Γi, R, and T are the excess surface charge density in W, the relative surface excess of i (where i ) THpA+ or C14Iq+), the gas constant, and the absolute temperature, respectively. From eq 1, we obtain

The slope of the curves in Figure 3b is thus related to the adsorption of C14Iq+ relativized to that of THpA+. Figure 4 shows the plots of Γ*C14Iq+ vs x at E ) -150 (O), -200 (b),-250 (]), -300 ([), -350 (0), and -400 mV (9). Up to x ) 0.2, ΓC* 14Iq+ almost linearly increased with x, showing strong surface activity of C14Iq+. At the singularity at x ) 0.2, the differentiation is not possible, but we show the points at x ) 0.2 in the round brackets, which were obtained from fitting experimental point in x < 0.2 or x > 0.2, just to show the magnitude of ΓC* 14Iq+ in the vicinity of the singularity. Upon increasing x, ΓC* 14Iq+ reaches ∼2.5 × 10-10 mol cm-2 just before x ) 0.2, which corresponds to ca. two-thirds of the maximum adsorption of C14Iq+, as judged from a maximum adsorption of C14Iq+, 4.0 × 10-10 mol cm-2, calculated from a molecular modeling. Since γ increases with x beyond x ) 0.2 when E > -250 mV and then decreases again with x at all E values before reaching x ) 0.9, ΓC* 14Iq+ discontinuously decreases when x > 0.2. In the rising part of γ vs ln x curves (-1.6 < ln x < -1.2 at E ) -150 and -200 mV), ΓC14Iq+ < x/(1 - x)ΓTHpA+ and a further increase in x up to x ) 0.9 again brings about the situation, ΓC14Iq+ > x/(1 - x)ΓTHpA+. When x ) 0.5, ΓC* 14Iq+ ) ΓC14Iq+ - ΓTHpA+ and from Figures 3b and 4 it is seen that this value is positive at all E values e.g., 1.2 × 10-10 mol dm-3 at E ) -150 and -200 mV. This clearly indicates that C14Iq+ prevails at the interface and it continues to be so up to x ) 0.9. When x ) 0.9, Figure 3a shows that dγ/dx ) (1/x)(dγ/d ln x) ) 0, that is, ΓC* 14Iq+ ) ΓC14Iq+ - 9ΓTHpA+ ) 0. Since THpA+ is hardly surface active, ΓTHpA+ should be negative. This result then suggests that ΓC14Iq+ becomes negative when x g 0.9. At this point, we have no clear-cut explanation for the anomalies at x ) 0.2 and 0.2 < x, in particular, 0.2 < x < 0.3. The increase in γ with increasing concentration of surfactant in the bulk has been observed at the air|W interface and is interpreted in terms of the formation of the vesicle-like or bilayer-type structures in the vicinity of the interface.25-27 The decrease in the adsorbed surfactant after the formation of micelles at air|W interface does not cause the increase in γ at the air|W interface, because the micelles formed leave the interface to the bulk and the interface is replenished by the surfactant transported from the bulk solution phase while keeping γ constant. Unlike conventional electrolyte solutions, the RTIL phase does not contain any solvent, except water in the present case at the RTIL-W two-phase system. It would not be surprising if the phase transition in the RTIL phase does not allow the presence of “free” C14Iq+ in the bulk RTIL phase when x > 0.2. If the phase transition has a certain mechanism that attracts the surface active C14Iq+ into the bulk RTIL phase, the adsorbed amount of C14Iq+ may decrease and even become negative when x > 0.2. The adsorption of C14Iq+ at the RTIL|W interface is presumably dependent on ∆φ, whereas the phase transition in the bulk RTIL phase should not. The change in ∂γ/∂ln x beyond x ) 0.2 with ∆φ (E) in Figure 3b conforms to one of characteristic features of the adsorption of ionic surfactant at the interface,28 that is, the shift of ∆φ to the negative directions enhances the adsorption of C14Iq+ at the interface. Interfacial Tension at the Binary RTIL|Air Interface. To further confirm our postulate that the break point at x ) 0.2 in Figure 3, panels a and b, is due to the phase transition in the bulk RTIL phase, we conducted the γ measurement at the RTIL|air interface and also the conductivity measurement of

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Figure 4. Amount of ΓC* 14Iq+ as a function of x at five different values of E, -150 (O), -200 (b), -250 (]), -300 ([), and -350 (0) mV. Symbols in parentheses at x ) 0.2 were calculated from the slope at 0.1 e x e 0.2 and 0.2 e x e 0.3. The dotted lines are intended to guide the eye.

Figure 5. Plots of γ against x (a) or ln x (b) at the RTIL|air interface. The vertical bar exemplifies the standard deviation. The dotted lines are intended to guide the eye.

the binary RTIL. Figure 5, panels a and b, shows γ at the interface between the binary RTIL and air against x and ln x, respectively. The γ increased with increasing x and decreased when x > 0.9. The break point at x = 0.2 is clearly discerned, which fact well agrees with the results at the RTIL|W interface (Figure 3, panels a and b). Li et al. reported that the surface tension of a mixture of 1-butyl-3-methylimidazolium tetrafluoroborate ([C4mim+][BF4-]) and 1-alkyl-3-methylimidazolium bromide ([Cnmim+]Br, where n ) 10, 12, 14, and 16) shows a break point with increasing the concentration of [Cnmim+]Br and suggested the formation of micelles.29 It is interesting to see that in their data γ continued to decrease beyond the “critical micelle concentration” (cmc), as in the case we encountered (Figures 3a and 5a). Conductivity of the Binary RTIL as a Function of x. Figure 6 shows the conductivity of the water-saturated binary RTIL as a function of x. The conductivity decreased until x ) 0.3

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Figure 6. Conductivity of the binary RTIL saturated with water at 25 °C. The dotted line is intended to guide the eye.

and turned to increase. This turning point is close to the singularity in Figure 3. The conductivity measurements of an aqueous solution containing surfactants show inflection points due to the formation of aggregations of a surface active in W.27,30-32 Aratono et al. reported a local minimum of the conductivity, and explained that small aggregations contained in a large vesicle does not contribute the conductivity.27 Several conductivity curves of the binary RTIL show a maximum33 or a monotonic increase.12,34 The result in this study in Figure 6 is distinctively different and suggests the uniqueness of the phase transitions of the present binary system. The v-shaped curve in Figure 6 is different from those seen in the determination of the cmc of surfactant solutions.30,27 The initial decrease suggests that the conductivity due to the [THpA+][C2C2N-] is hampered by the addition of C14Iq+ into the [THpA+][C2C2N-] phase that forms a continuous phase and [C14Iq+][C2C2N-] is likely to be segregated. The increase in the conductivity beyond x ) 0.3 suggests that [C14Iq+][C2C2N-] becomes the continuous phase. This type of the transition may be likened to inversion of the continuous phase in oil-water emulsion systems.35 Comparison between the Surface and the Bulk Properties of the Binary RTIL. It is interesting to see that the interfacial tension measurements at both RTIL|W and RTIL|air interfaces suggest the phase transition of the RTIL phase at x ) 0.2, whereas the facilitated transfer of Cl- and the conductivity suggest the phase transition at x ) 0.3. It seems as if the surface properties of the binary RTIL forecast the drastic change in the bulk properties upon increasing x. Actually, in the opposite change of x from 1 to 0 the change in the bulk properties precedes the change in surface properties and the interfacial tension is not a harbinger of the bulk transition. A gap between the locations of the break points has been known in the determinations of cmc using various techniques. For example, the cmc value of sodium dodecyl sulfate in the aqueous phase from conductivity is ca. 1.5 times greater than those estimated from surface and interfacial tension measurements.36 As C14Iq+ is much more surface active than THpA+ in the RTIL, the similarity in the gap of the cmc values and the points of the phase transition in the RTIL may imply the presence of a certain common denominator that gives rise to the difference in surface and bulk properties in the phase transition of electrolyte solutions. Conclusions The binary RTIL composed of the cations, C14Iq+ and THpA+, with the common anion, C2C2N-, exhibits the phase transition in the bulk RTIL phase, probably forming, aside from C2C2N-, the C14Iq+-rich domains surrounded by THpA+ at

Phase Transition of a Binary RTIL smaller x values. The surface properties of the RTIL|W interface is affected by the phase transition and the adsorption of C14Iq+ depends not only on x but also E, resulting in the change in the surface charge and the structure of the electrical double layers. Acknowledgment. This research was supported by Grantin-Aid for Priority Area (No. 20031017) from the Ministry of Educations, Sports, Science, and Technology, Japan. Support by the Global COE Program, “International Center for Integrated Research and Advanced Education in Materials Science” (No. B-09) from the Ministry of Education, Culture, Sports, Science and Technology of Japan is highly appreciated. Supporting Information Available: Surface charge density, Gibbs energy for the adsorption of C14Iq+, and relative surface excess of C14Iq+. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Kakiuchi, T.; Tsujioka, N. Electrochem. Commun. 2003, 5, 253. (2) Katano, H.; Tatsumi, H. Anal. Sci. 2003, 19, 651. (3) Kakiuchi, T.; Shigematsu, F.; Kasahara, T.; Nishi, N.; Yamamoto, M. Phys. Chem. Chem. Phys. 2004, 6, 4445. (4) Ishimatsu, R.; Shigematsu, F.; Hakuto, T.; Nishi, N.; Kakiuchi, T. Langmuir 2007, 23, 925. (5) Ishimatsu, R.; Nishi, N.; Kakiuchi, T. Langmuir 2007, 23, 7608. (6) Fletcher, K. A.; Baker, S. N.; Baker, G. A.; Pandey, S. New J. Chem. 2003, 27, 1706. (7) Lopes, J. N. C.; Cordeiro, T. C.; Esperamc¸a, J. M. S. S.; Guedes, H. J. R.; Huq, S.; Rebelo, L. P. N.; Seddon, K. R. J. Phys. Chem. B 2005, 109, 3519. (8) Xiao, D.; Rajian, J. R.; Li, S.; Bartsch, R. A.; Quitevis, E. L. J. Phys. Chem. B 2006, 110, 16174. (9) Navia, P.; Troncoso, J.; Romani, L. J. Chem. Eng. Data 2007, 52, 1369. (10) Annat, G.; MacFarlane, D. R.; Forsyth, M. J. Phys. Chem. B 2007, 111, 9018. (11) Xiao, D.; Rajian, J. R.; Hines, L. G.; Li, S.; Bartsch, R. A.; Quitevis, E. L. J. Phys. Chem. B 2008, 112, 13316. (12) Egashira, M.; Nakagawa, M.; Watanabe, I.; Okada, S.; Yamaki, J. J. Power Sources 2005, 146, 685.

J. Phys. Chem. B, Vol. 113, No. 27, 2009 9325 (13) Wang, P.; Wenger, B.; Humphry-Baker, R.; Moser, J.-E.; Teuscher, J.; Kantlehner, W.; Mezger, J.; Stoyanov, E. V.; Zakeeruddin, S. M.; Grätzel, M. J. Am. Chem. Soc. 2005, 127, 6850. (14) Zistler, M.; Wachter, P.; Wasserscheid, P.; Gerhard, D.; Hinsch, A.; Sastrawan, R.; Gores, H. J. Electrochim. Acta 2006, 52, 161. (15) Fredin, K.; Gorlov, M.; Pettersson, H.; Hagfeldt, A.; Kloo, L.; Boschloo, G. J. Phys. Chem. C 2007, 111, 13261. (16) Baltazar, Q. Q.; Leininger, S. K.; Anderson, J. L. J. Chromatogr. A 2008, 1182, 119. (17) Nishi, N.; Imakura, S.; Kakiuchi, T. Anal. Chem. 2006, 78, 2726. (18) Earle, M. J.; Gordon, C. M.; Plechkova, N. V.; Seddon, K. R.; Welton, T. Anal. Chem. 2007, 79, 758. (19) Kitazumi, Y.; Kakiuchi, T. Langmuir, Article ASAP, DOI: 10.1021/ la9005696. (20) Kakiuchi, T.; Tsujioka, N.; Sueishi, K.; Nishi, N.; Yamamoto, M. Electrochemistry 2004, 72, 833. (21) Ishimatsu, R.; Nishi, N.; Takashi, T. Chem. Lett. 2007, 36, 1166. (22) Kakiuchi, T. Anal. Chem. 2007, 79, 6442. (23) Shinoda, K.; Yamaguchi, T.; Hori, R. Bull. Chem. Soc. Jpn. 1961, 34, 237. (24) Menger, F. M.; Littau, C. A. J. Am. Chem. Soc. 1993, 115, 10083. (25) Villeneuve, M.; Kaneshina, S.; Imae, T.; Aratono, M. Langmuir 1999, 15, 2029. (26) Bordes, R.; Vedrenne, M.; Coppel, Y.; Franceschi, S.; Perez, E.; Rico-Lattes, I. ChemPhysChem 2007, 8, 2013. (27) Aratono, M.; Onimaru, N.; Yoshikai, Y.; Shigehisa, M.; Koga, I.; Wongwailikhit, K.; Ohta, A.; Takiue, T.; Lhoussaine, B.; Strey, R.; Takata, Y.; Villeneuve, M.; Matsubara, H. J. Phys. Chem. B 2007, 111, 107. (28) Kakiuchi, T.; Kobayashi, M.; Senda, M. Bull. Chem. Soc. Jpn. 1987, 60, 3109. (29) Li, N.; Zhang, S.; Zheng, L.; Dong, B.; Li, X. W.; Yu, L. Phys. Chem. Chem. Phys. 2008, 10, 4375. (30) Wright, K. A.; Abbott, A. D.; Sivertz, V.; Tartar, H. V. J. Am. Chem. Soc. 1939, 61, 549. (31) Goddard, E. D.; Benson, G. C. Can. J. Chem. 1957, 35, 986. (32) Kawamura, H.; Manabe, M.; Nomura, M.; Inoue, T.; Murata, Y.; Sasaki, Y. Nippon Kagaku Kaishi 1996, 861. (33) Every, H.; Bishop, A. G.; Forsyth, M.; MacFarlane, D. R. Electrochim. Acta 2000, 45, 1279. (34) Jarosik, A.; Krajewski, S. R.; Lewandowski, A.; Radzimski, P. J. Mol. Liq. 2006, 123, 43. (35) Davies, J. T.; Rideal, E. K. Interfacial Phenomena; Academic Press: London, 1961. (36) Adamson, A. W. Physical Chemistry of Surfaces, 3rd ed.; John Wiley and Sons: New York, 1976; pp 477.

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