5.4.1. Voltammetric studies for sulfate anion
I shows the CV for Au(1 1 1) electrode with HD prepared by Procedure A in 0.09 M H2SO4 aqueous solution in Fig. 5-8. The basic behavior of SO42− anions at a Au(1 1 1) electrode surface is same as that of Br− anions except for that these peaks appear at more positive potentials. The potential at 0.0 V vs. Hg/HgSO4 (sat-K2SO4) as a reference electrode is equal to the 0.46 V vs. Ag/AgCl (sat-KCl). The sharp peak at E between
91
−0.20 V and −0.05 V may be identified as corresponding to lifting of the (1×23) reconstruction of the Au(1 1 1) surface [13]. The peak at −0.05 and +0.30 V corresponds to the formation of a disordered adlayer of Br−. The CV display also a small peak between +0.35 and +0.42 V which corresponding to the phase transition between the disordered and ordered (√3 ×√7) adlayer of SO42− [14]. It is known that the asymmetry between the sections of CVs recorded using a positive going and a negative going potential sweep is caused by a slow kinetics of lifting and restoring the reconstructed (1×23) structure of the Au surface [15]. When the HD was prepared by touching method or a HD 1 L droplet was deposited on Au(1 1 1) electrode surface, the peaks lost sharpness but difference between the bare electrode and the electrode with HD was not be observed. These behaviors of SO42−
anions were not affected by whether the HD existed or not. This result corresponds to that the CV in Fe(CN)63−/4− solution has redox peaks.
Fig. 5-9 shows a typical contact angle ()–potential (E) curve obtained by potential scan method for a HD droplet deposited on a Au(1 1 1) electrode surface in a H-M configuration in 0.09 M H2SO4 aqueous solution. The initial potential of scan was −0.3 V and was constant in negative and positive going potential scan until 0.0 V. After turning the potential and the potential reached to 0.0 V, increased, the droplet started retracting. increased between
Figure 5-8. CV for a bare Au(1 1 1) electrode in 0.09 M H2SO4 solution obtained at v = 20 mVs−1.
92 0.0 V and 0.2 V and reached to 69.5° as a maximum value. After turning the direction of potential scan at +0.6 V, the began to decrease at 0.0 V and was back to the initial value.
The HD 1 L droplet on Au(1 1 0) electrode surface did not change its surface in H2SO4
aqueous solution because sulfate anions did not absorb at the electrode surface. The specific adsorption of sulfate anions can be a driving force to change the of HD droplet on Au(1 1 1) electrode surface.
Fig. 5-10 shows a plot of cos versus M for a HD droplet on Au(1 1 1) electrode surface in K2SO4 2 mM + KClO4 44 mM aqueous solution. The initial potential (Ei) was −0.6 V for the step to positive direction to obtain the data points for spreading processes (opened square), whereas Ei = 0.4 V for those to negative direction to obtain the points for retracting processes (closed square). In both spreading and retracting processes, the s were M dependent between 20 C cm−2 and 50 C cm−2. On the other hand, in the region from 0 C cm−2 to 20 C cm−2, the s showed a plateau.
Figure 5-9. Contact angle () as the function of the electrode potential (E) of HD 1 L droplet on Au(1 1 1) electrode surface in 0.09 M H2SO4 solution.
I also found a plateau region of in the absence of K2SO4 (Fig. 5-3) and its range was
m > -12 C sm-2. The plateau level of the contact angle was 58° in K2SO4 solution, being nearly equal to that in KBr solution (67°) and much greater than the value in KClO4 solution,
93 38°. The plateau region in K2SO4 solution is within the potential range of sulfate anion adsorption as same as in KBr solution. The difference of the plateau level of those shows that the becomes high in the presence of the adlayer of sulfate anions.
The data points obtained in the retracting processes in the negative M region have inflection point around −7 C cm-2 in Figure 5-9. In the spreading process, corresponding inflection was not observed. The curve negative to the inflection point overlapped the curve of spreading processes in K2SO4 solution and the curve obtained in KClO4 solution without K2SO4. Before the inflection point, the was determined by the adlayer of sulfate anions and water molecules ordering over the adlayer. After the inflection point, the was determined by water molecules ordering induced by the interfacial electric field. Overall, these characteristic behaviors represent that the inflection point pinpoints the desorption potential of sulfate anions.
Figure 5-10. a plot of cos versus M for a HD droplet on Au(1 1 1) electrode surface in K2SO4 2 mM + KClO4 44 mM aqueous solution.
94 References
[1] O. M. Magnussen, Ordered anion adlayers on metal electrode surfaces, Chem. Rev. 2002, 102, 679.
[2] J. Lipkowski, Z. Shi, A. Chen, B. Pettinger, C. Bilger, Ionic adsorption at the Au(1 1 1) electrode, Electrochim. Acta 1998, 43, 2875.
[3] Z. Shi, J. Lipkowski, S. Mirwald, B. Pettinger, Electrochemical and second harmonic generation study of bromide adsorption at the Au(1 1 1) electrode surface, J. Chem. Soc., Faraday Trans. 1996, 92, 3737.
[4] A. A. Kornyshev, A. R. Kucernak, M. Marinescu, C. W. Monroe, A. E. S. Sleightholme, M. Urbakh, Ultra-low-voltage electrowetting, J. Phys. Chem. C 2010, 114, 14885.
[5] C. Korzeniewski, V. Climent, J. M. Feliu, Electrochemistry at platinum single crystal electrodes, in: A. J. Bard, C. Zoski (Eds.), Electroanalytical chemistry: a series of advances, Vol. 24, Ch. 2 CRC Press, 2011, p. 75.
[6] J. Lipkowski, L. Stolberg, Molecular adsorption at gold and silver electrodes, Chap. 4, in:
J. Lipkowski, P.N. Ross (Eds.), Adsorption of Molecules at Metal Electrodes, VCH, N.Y.
1992, pp. 171–238.
[7] S. Wu, J. Lipkowski, O. M. Magnussen, B. M. Ocko, Th. Wandlowski, The driving force for (p × √3)↔(1 × 1) phase transition of Au(111) in the presence of organic adsorption: a combined chronocoulometric and surface X-ray scattering study, J. Electoanal. Chem.
1998, 446, 67.
[8] B. Pettinger, J. Lipkowski, S. Mirwald, In situ SHG studies of adsorption induced surface reconstruction of Au(1 1 1) electrodes, Electrochim. Acta 1995, 40, 133.
[9] O. M. Magnussen, B. M. Ocko, J. X. Wang, R. R. Adzic, In-situ X-ray diffraction and STM studies of bromide adsorptin on Au(1 1 1) electrodes, J. Phys. Chem. 1996, 100, 5500.
[10] D. M. Kolb, reconstruction phenomena at metal-electrolyte interfaces, Prog. Surf, Sci.
1996, 51, 109.
[11] D. Bizzotto, J. L. Shepherd, Epi-fluorescence microscopy studies of potential controlled changes in adsorbed thin organic films at electrode surfaces in: R. C. Alkire, D. K. Kolb, J. Lipkowski, P. N. Ross (Eds.), Advances in Electrochemical Science and Engineering:
Diffraction and Spectroscopic Methods in Electrochemistry, Vol. 9, Ch. 3, Weinheim, Wiley-VCH Verlag, 2006, p. 97.
95 [12] D. Bizzotto, J. Lipkowski, Electrochemical and spectroscopic studies of the mechanism of monolayer and multilayer adsorption of an insoluble surfactant at the Au(111)|electrolyte interface, J. Electroanal. Chem. 1996, 409, 33.
[13] K. Ataka, M. Osawa, In Situ Infrared Study of Water-Sulfate Coadsorptioin on Gold(1 1 1) in Sulfuric Acid Solutions, Langmuir 1998, 14, 951-959.
[14] S. Mirwald, B. Pettinger, J. Lipkowski, Surf. Sci. 1995, 335, 264-272.
[15] Z. Shi, J. Lipkowski, S. Mirwald, B. Pettinger, Electrochemical and second harmonic generation study of SO42- adsorption at the Au(1 1 1) electrode, J. Electroanal. Chem.
1995, 396, 115-124.
* The contents of this chapter have been published: T. Morooka, T. Sagara, Journal of Physical Chemistry C 2018, 122, 25964-25973.
96
Chapter 6
Effect of Oil Droplet Coexistence upon Potential-Dependent Phase Change of Surfactant Adlayer on Gold Electrode: Interplay of Hexadecane Droplets and Dodecyl Sulfate Adlayer
ABSTRACT. When a small immiscible oil droplet is placed on an electrode surface in a surfactant aqueous solution, a drastic decrease in the interfacial differential capacitance of the electrode is observed. Interplay of an n-hexadecane (HD) droplet with a dodecyl sulfate anion (DS−) adlayer on an Au electrode was described to clarify this behavior at a molecular level.
When coexisting HD droplets and the DS− adlayer, 1μL droplet of HD showed a stronger tendency to spread on the Au(111) electrode surface than when HD alone is present, because DS− adsorption took place at both HD/water and electrode/water interfaces. When the electrode surface with many HD microdroplets (ϕ <50μm) was immersed in Na-DS (SDS) solution, we found that the formation of a mixed adlayer consisting of HD and DS− significantly lowered the interfacial differential capacitance down to 5 μF cm−2. ATR-SEIRAS revealed that the alkyl phase of this mixed adlayer was more liquid-like than the DS− adlayer alone and more solid-like than the HD droplet alone. At positive potentials, the interdigitated bilayer of DS− was formed regardless of whether HD droplets were present on the electrode surface. At very positive potentials, a HD droplet can spread over the DS− adlayer.
97 6.1. Introduction
The focus in this chapter is placed on the effect of HD droplet coexistence upon the potential dependent phase change of dodecyl sulfate anion (DS－) adlayer on a gold electrode. Especially, interplay between HD droplet and DS－ adlayer should be highlighted. I use a Au electrode with HD droplets on it in an aqueous solution of SDS.
DS－ absorbs at a HD/water interface and decreases its interfacial tension. Upon adsorption of DS－ on the electrode surface, the electrode/water interfacial tension is decreased by formation of a self-assembled adlayer of DS－, which forms into hemi-micelles, or interdigitated bilayer. These effects on both oil/water and solid/water interfacial tensions may play an important role for emergence of new potential-dependent changes of the adlayer structure of DS－ and of the shape of HD droplet. In addition, HD molecules also form an ordered adsorption monolayer at a Au(1 1 1) electrode surface around the potential of zero charge (pzc) [1, 2] as described in Chapter 2. Synergetic effect may emerge because of coexistence of these well-investigated two molecules, which form individual ordered adlayers on the single crystal electrode surface. Therefore, the study for coexistence of HD and DS－ molecules on a metal surface will reveal the molecular-level conditions for interaction between oil and surfactant, whichever they tend to mix or to be phase-separated. In this chapter, the surface enhanced infrared reflection absorption spectrum (SEIRAS) measurement was used as a main approach to reveal the coexistence state of HD and DS－ from molecular point of view.
SEIRAS measurements detect the vibrational mode of the adsorbed species, the dipole moment of which changes perpendicular to the metal surface when vibrating. Because of this surface selection rule, SEIRAS has been used to reveal the orientation of molecules just above the metal surface [3-30]. In SEIRAS measurements, electromagnetic wave amplitude of the infrared beam is enhanced by a factor of ~102~3. In situ combined use of SEIRAS measurements with cyclic voltammetry and pulse voltammetry have been carried out with no problem, because the enhancement can be attained with internal reflection setup [21, 22]. Intriguingly, a Au thin film electrode fabricated by electroless deposition has also single crystal facets on the particles at the same level as the sputtered Au thin film electrode [23, 29, 30]. Based on these reports, I used the chemically deposited Au thin film electrode as a working electrode in this study.
98 6.2. Results and Discussion
6.2.1. Voltammogram in SDS solution with or without HD on Au(1 1 1)
Fig. 6-1 shows typical CVs and C-E curves for a Au(1 1 1) electrode with HD microdroplets on the electrode surface. The HD-covered electrode was prepared by Procedure A. The aqueous solution contains 0.50 mM SDS + 50 mM KClO4. The SDS concentration is lower than its critical micelle concentration (ca. 0.54 mM [31]) in 50 mM KClO4 solution. In the absence of HD, when sweeping the potential from −0.7 V to positive, the first current peak (_ peak) appeared at −0.21 V in CV. A corresponding capacitance _ peak was observed in the C-E curve. The _ peak represents adsorption-desorption of DS− on the bare Au(1 1 1) electrode/solution interface. In the C-E curve, the _ peak was followed by an almost constant capacitance region of a level of ca. 18 F cm−2 in −0.20 V ~ +0.54 V (potential region-I). In the potential region-I, the DS− adlayer forms hemi-micelles on the electrode/water interface [32-43]. Sweeping the electrode potential to more positive, observed was the _ peak (+0.55 V) which is assigned to the adlayer phase transition of the hemi-micellar DS− adlayer to an interdigitated bilayer [32-43]. At the potential region-II, in which the interdigitated bilayer covered the electrode surface, the value of C was ca. 15 F cm−2.
When a Au(1 1 1) electrode with HD microdroplets was prepared using Procedure A and brought into contact to the SDS solution, the _ and _ peaks still appeared in CV at nearly the same potentials as in the absence of HD but lost their sharpness and became broader. The CV demonstrates that the DS− adlayers undergo phase transitions similarly to the behavior in the absence of HD. In the absence of SDS, the electrode prepared by Procedure A to have many HD microdroplets on it did not exhibit considerable blocking of the redox reaction of solution species such as hexacyanoferrate; the total area occupied by the HD small droplets was less than 11% as judged from the decrease of the slope of the plot of the CV peak current vs. square root of the potential sweep rate, leaving continuous bare Au parts on the surface. Therefore, DS− is capable of adsorbing there.
In C-E curves (Fig. 6-1), the values of C in the potential region-I (< 5 F cm−2) were drastically lower than those in the absence of HD. These low values of C in the potential region-I remained after repeating the potential scans. If the microdroplets of pure HD remained in this region, occupying the same sub-total area as that when SDS had been absent, and if they were independent of the DS− adlayer at the electrode surface, then the
99 total value of C should have been a sum of the capacitance of the area under HD microdroplets and the capacitance of the area covered with a DS− adlayer. In this situation, the observed total C should have been still greater than ca. 15 F cm−2, because the sub-total area occupied by HD is as low as 11%. This does not explain the experimental results. Therefore, I should invoke following two models:
(i) The area occupied by HD and that occupied by SDS are independently share the total electrode surface so that no mixing of HD and SDS takes place. The area occupied by HD, however, is much greater than that when SDS is absent. Even if the capacitance of the
Figure 6-1. Voltammograms for a HD microdroplet-attached Au(1 1 1) electrode in 5 mM SDS + 50 mM KClO4 aqueous solution: (a) CV at a potential sweep rate of 20 mV s−1, (b) C-E curve at a sweep rate of 5 mV s−1. The gray lines in (a) and (b) were obtained for a bare Au(1 1 1) electrode without HD.
100 area occupied by HD is assumed to be near zero, over 80% of the total electrode area should be occupied by HD. The presence of SDS affected HD droplet to let HD droplet greatly spread.
(ii) A new structure of interactive HD + DS− adlayer is formed at the entire electrode/water interface. The new adlayer exhibits a capacitance as low as ca. 4 F cm−2. The new layer should be thicker or of smaller dielectric constant than the DS− adsorption in the region-I.
6.2.2. Contact angle change of HD in SDS solution
To understand how SDS affects the spreading HD droplet, I measured of of HD 1 L droplet on Au(1 1 1) electrode surface in 0.5 mM SDS + 50 mM KClO4 aqueous solution as a function of the electrode potential. If Procedure A is used for HD on a Au(1 1 1) single crystal electrode surface, many microdroplets of HD are formed as described in Chapter 5.
In the absence of SDS, the potential-dependent shape change of the microdroplet was in line with that of a 1 L droplet as found in using fluorescence microscopy. Herein, I use a 1 L droplet to semi-quantitatively see the effect of SDS through the change of .
Fig. 6-2 shows a typical -E curve of HD 1 L droplet on a Au(1 1 1) electrode surface in 0.5 mM SDS + 50 mM KClO4 aqueous solution. The initial potential of the potential scan was 0.0 V. When sweeping the potential to negative, began to increase from 22° at
−0.3 V to 45° at −0.6 V, the negative turn-round potential. The maximum of was 47° at
−0.3 V after the turning of the potential scan. Then, decreased slowly from −0.2 to +0.2 V, and exhibited a sudden decrease at +0.5 V. The potentials showing decrements were the same as the _ and _ peak potentials. In addition, all values of with SDS in the potential range between −0.6 V and 0.7 V were lower than those without SDS, revealing stronger tendency to spread on a Au(1 1 1) electrode surface than when HD alone there. It indicates that DS− adsorption on an oil/water and an electrode/water interfaces decreased the value of .
To confirm whether the HD droplet can spread over DS− interdigitated bilayer, I conducted the following experiment. The Au (1 1 1) electrode was dipped in SDS aqueous solution, and its potential was held at +0.7 V. Onto the electrode surface, a HD droplet (1.0 L) was placed in SDS solution using a syringe. The droplet exhibited values
101 around 20°, being as the same as at E = +0.6 and 0.7 V in Fig. 6-2. This result indicates that a HD droplet can spread on the DS− interdigitated bilayer at the _ peak potential, as far as the HD droplet did not wash out the DS− adsorption layer. These results show that the adsorbed DS− lowers the interfacial tensions of both HD/water and the electrode/water interfaces. HD droplet can spread to a greater area with DS− than without DS− because of the adsorption of DS− on HD/water and the electrode/water interfaces.
Figure 6-2. –E curves with typical corrected photo images obtained by potential sweep at a potential sweep rate of 10 mV s−1 for a HD 1.0 L droplet on a Au(1 1 1) electrode surface in 5.0 mM SDS + 50 mM KClO4 aqueous solution (black circles) and in 50 mM KClO4
aqueous solution (gray circles). The droplet was prepared using Procedure C.
In Fig. 6-2, was 22-40° in the potential region-I with SDS, whereas was 36-45° at the same potential region without SDS. This decrease of corresponds to 55% increase of the HD-covered area by coexistence of DS− that lowered the interfacial tensions of both HD/water and the electrode/water interfaces. Herein, I assume that the microdroplets
102 change their shape, and thus , with potential as the same manner as 1 L droplet and that the model (i) is applied. The HD-covered area by Procedure A in the absence of SDS was experimentally obtained to be 30% using fluorescence microscopy. Based on these assumptions and experimental results, I estimated that the HD-covered area increased from 30% without SDS to 47% with SDS. With this 17% coverage change in mind, the model (i) did not explain the significant capacitance decrease to the level as low as 5 F cm−2 in the potential region-I.
Although the model (ii) is considered more appropriate than the model (i), I cannot describe the state of the adlayer at a molecular level only from the characteristic CVs, C-E curves and -E curves. Therefore, SEIRAS studies were conducted to gain more in-depth insight into the molecular-level adlayer structure.
6.2.3. Coexistence of HD and DS− on the Au electrode: SEIRAS study 6.2.3.1 Cyclic Voltammetry of Au thin film electrode
SEIRAS study should enable us to gain the surface-selective molecular view of HD and DS− coexisting on the Au electrode surface. First, I need to confirm whether the potential dependent behavior of DS− on the Au thin film electrode mirrors that at a Au(1 1 1) single crystal electrode. In Fig. 6-3, line-a is the CV of a Au thin film electrode without HD in 0.5 mM SDS + 50 mM KClO4 aqueous solution recorded at a potential sweep rate of 20 mV s−1. In the comparison to the CV of Au(1 1 1) electrode (line-b in gray), the Au thin film electrode showed both characteristic _ and _ peaks corresponding to the adsorption-desorption of DS- and the phase transition of the adlayer, although their potentials were negatively shifted by 200 mV.
When HD was deposited by Procedure A on the Au film electrode (Fig. 6-3, line-c), the CV exhibited the current peak of _, although it was less sharp than those in line-a.
The _ peak was ill-defined in line-c. The loss of the sharpness by HD coexistence was also observed in the CV of Au(1 1 1) (Fig. 6-1). The double layer charging current of line-c over +0.1 V was significantly smaller than that without HD (line-a). Overall, in the coexistence of HD and DS−, the potential-dependent processes occurred on the Au film electrode and on the Au(1 1 1) single crystal electrode were the same. If a new adlayer is formed as described by the model (ii) (vide supra) in the potential region-I, the adlayer
103 should be a denser, thicker, or lower-dielectric-constant layer than the layer of DS− alone, to explain the low double layer charging current.
Figure 6-3. Cyclic voltammograms at a potential sweep rate of 20 mV s−1 in 5 mM SDS + 50 mM KClO4 aqueous solution for (a) a bare Au thin film electrode (blue line), (b) a Au(1 1 1) electrode (gray line), (c) a Au thin film electrode with a HD microdroplets using Procedure A (red line).
104 6.2.3.2 SEIRAS of O-H stretching mode of interfacial water
Fig. 6-4 shows potential dependent SEIRAS spectra for three systems as entitled in 6-4-a through 6-4-c. Fig. 6-5 shows the potential-dependence of peak wavenumbers of the OH-stretching (3500−3200 cm−1), HOH-bending (1650 cm−1), CH-stretching (as(CH2) around 2916 cm−1, s(CH2) around 2850 cm−1), and SO4 asymmetric stretching (as(SO4)A
around 1230 cm−1) modes in Fig. 6-4-a and b. The potential-dependence of signal intensities obtained at a bare Au thin film electrode in SDS aqueous solution are shown in Fig. 6-6-a and those with HD microdroplets prepared by using Procedure A are shown in Fig. 6-6-b; the presentation was made in a difference absorbance unit for all the five modes shown in Fig. 6-5. Note that the values for CH-stretching modes were multiplied by 10 in Fig. 6-6. I set ER = −0.7 V at which neither adsorbed HD nor DS− exists on the Au electrode surface. At −0.7 V, the bottom area of a HD droplet in contact with Au surface was minimum (Fig. 3), and no H2 evolution took place. Using the definition of A in the experimental section as the signals with the well-suited ER, I can clearly detect the changes induced by adsorption, desorption, and adlayer phase transitions. At a glance, it is obvious that the spectra for SDS + HD are not a weighted sum of the spectra for HD and spectra for SDS adlayer. Although this enables us to perceive model (i) in a negative light, extensive analysis of SEIRAS is indispensable to figure out the adlayer structures.
SEIRAS for a HD-free Au thin film electrode in SDS solution is shown in Fig. 6-4-a.
The OH stretching mode of water molecules has IR absorption bands in 3800-3000 cm−1 region, and the HOH bending mode does in 1650-1610 cm−1 region. The band observed at 3468 cm−1 in Fig. 6-4-a is assigned to the O-H stretching mode of interfacial water in contact with the thin Au film electrode surface. In SEIRAS, the negative-going signal indicates that the amount of the adsorbed species of interest is smaller at the sample potential (Es) than that at the reference potential (ER = −0.7 V in Fig. 6-4), and vice versa the positive-going signal does. The band of water at Es > −0.4 V in Fig. 6-4-a were negative-going, indicative of tilting of the dipole moment to flat-lying with the electrode surface, or a decrease in the amount of interfacial water molecules from the electrode surface because of the displacement of them by the surface hemi-micelles of DS−. The decrease of band intensity as the potential increasing at Es > −0.4 V (Fig. 6-6-a) also indicates that the number or strength of hydrogen bonds among water molecules increased [44]. In Fig. 6-6-a, the negative-going band intensity became greater because of the
105 formation of hemi-micelles at −0.2 V < Es < 0.0 V. Subsequently, the intensity at 0.1 V <
Es < 0.2 V became smaller than that at −0.2 V < Es < 0.0 V. When the phase transition of DS− adlayer occurred at 0.1 V< Es < 0.2 V in Fig. 6-6-a, the superficial concentration of DS− drastically increased together with the increase of an IR absorption by water. The amplitude of the negative-going signal became stronger as Es was set more positive potentials than 0.3 V. This behavior reflects further displacement of interfacial water molecules by the development and complete formation of the interdigitated bilayer at Es
more positive than the _ peak. This is because of less need of interfacial water for the formation of the bilayer than the formation of the water-rich hemi-micelle layer between _
and _ peaks. The spectra and their potential dependence in Fig. 6-4-a are almost in line the SEIRAS reported by Grossutti et al. obtained at a sputtered Au thin film electrode in SDS aqueous solution [14]. A minor difference from the report by Grossutti et al. is that the 3675 cm−1 positive-going band assigned to a monomeric water molecule was not found in our experiments.
Fig. 6-4-b shows the SEIRAS for a Au thin film electrode with HD in SDS solution.
The negative-going band with a potential dependent peak of OH stretching (3495−3335 cm−1) became significantly stronger with changing Es to more positive from
−0.5 V to 0.0 V (see also Fig. 6-6-b). At Es > +0.1 V, the intensity remained constant.
Therefore, the potential region with small amount of superficial water in direct contact to Au surface (E > +0.1 V) was observed in the IR spectra, and this region is in accord with the potential region of low charging current (line c, Fig. 6-3). This accordance was seen when HD and DS− coexist on the Au (1 1 1) electrode surface (C-E curve for HD + SDS in Fig. 6-1) at E > +0.1 V.
For the HD-free Au thin film electrode in contact with SDS aqueous solution (Fig.
6-5-a), the peak at 3468 cm−1 (s(OH)) grew in negative-going while remained at the same wavenumber between −0.6 V and 0.5 V. On the other hand, when the electrode with HD was in contact with SDS aqueous solution, the s(OH) peak shifted to lower energy as the potential was made more positive from −0.6 V to 0.0 V (Fig. 6-5-e). This fact indicates that the hydrogen bonding of interfacial water is stronger in the presence of HD than in the absence. It is known that, if water molecules form an icelike structure, a positive band appears at 3100-3000 cm−1 [5]. The peak shift of s(OH) in the presence of HD (Fig.
6-5-e) is mainly originated from the band at 3100-3000 cm−1, whereas it is not the case in
106 Figure 6-4. Series of infrared spectra, obtained by a Au thin film electrode surface as a function of applied potential: (a) a bare Au thin film electrode in 5 mM SDS + 50 mM KClO4 aqueous solution (the mark “I” represents the potential region-I), (b) a Au thin film electrode with a HD microdroplets using Procedure A in 5 mM SDS + 50 mM KClO4 aqueous solution, (c) a Au thin film electrode with a HD microdroplets using Procedure A in 50 mM KClO4 aqueous solution without SDS (the mark “OM” represents the range of ordered monolayer forming). For all the data, reference potential was ER = −0.70 V.
107 the absence of HD. In other words, the icelike structure was extensively developed on the electrode surface in the presence of HD.
Based on these results, it is likely that a new adlayer consisting of both HD molecules and DS− formed on the thin film electrode surface so that interfacial water was excluded by the new adlayer. This supports the model (ii) in the previous section.
Figure 6-5. Plots of the peak position of the OH-stretching, HOH-bending, CH-stretching and SO4 stretching mode as a function of potential. The data have been obtained from the experiments shown in Figure 5. The gray part represents the potential region-I.
108 Figure 6-6. Plots of the peak intensity of the OH-stretching, HOH-bending, CH-stretching (multiplied by 10) and SO4 stretching modes as a function of potential, obtained at a bare Au thin film electrode in 5 mM SDS + 50 mM KClO4 aqueous solution (a) and at a Au thin film electrode with a HD microdroplets using Procedure A in 5 mM SDS + 50 mM KClO4
aqueous solution (b). The data have been obtained from the experiments shown in Figure 5.
6.2.3.3 C-H stretching mode of Alkyl Chains of DS− and HD
In the spectral range of 3000 cm−1 to 2600 cm−1 in Fig. 6-4-a, the bands from alkyl chains of DS− appeared. Usually, the methylene bands are of the asymmetric stretching (as(CH2) at ∼ 2916 cm−1) and of symmetric (s(CH2) at 2850 cm−1) stretching [14]. The dipole moments of these CH stretching modes are oriented perpendicular to the axis defined by a trans segment of the alkyl chains. In Fig. 6-6-a, intensity of these positive-going bands increased as the potential was set to positive direction. This indicated that the superficial concentration of DS− increased as more positive potential was applied, and that the alkyl chains of DS− in hemi-micelle adlayer largely involved flat-lying alkyl chains on the electrode surface in potential region-I. In Fig. 6-5-c, the peak wavenumber of the symmetric (s(CH2)) methylene stretching got closer to 2850 cm−1 with changing Es from −0.6 V to 0.5 V, indicating that the self-assembled film of DS− on the electrode without HD became more tightly packed and better-ordered with the all-trans alkyl chains in the interdigitated bilayer, in reference to the assignment of IR spectra by Leitch [13]. The observed spectra were in good agreement with those reported by Grossutti et al. [14].
109 In the same wavenumber region in Fig. 6-4-b, the increase of the positive-going signals of C-H stretching modes was less steep than that of Fig. 6-4-a (see also Fig. 6-6). This is because, even at −0.7 V, the HD molecules are in contact with the Au thin film electrode surface, as confirmed by fluorescence microscopic imaging using the fluorescence from dye containing in the HD liquid as described in Chapter 4. In Fig. 6-5-g, the peak wavenumber of the symmetric (s(CH2)) methylene stretching appeared at 2854 cm−1 between −0.2 V and 0.2 V. These results suggest that the alkyl chains of both DS− and HD molecules in the adsorption layers are more disordered than the DS— in the interdigitated bilayer. The wavenumber of s(CH2) in between −0.2 V and 0.2 V in Fig. 6-4-b revealed the mixture of gauche and trans conformers.
In Fig. 6-4-c for HD alone, SEIRAS curves have the peak of the asymmetric stretching of CH3 (a(CH3) at 2950 cm−1). This peak did not appear in HD + SDS system (Figure 6-4-b). The peak center 2867 cm−1 of the asymmetric stretching of methylene in Figure 6-4-c (as(CH2)) was at a higher wavenumber than that of 2843 cm−1 in HD+SDS system in Fig. 6-4-b, indicating that the HD molecules in close proximity to the Au surface at the bottom of HD microdroplets without DS− are more liquid-like than the adlayer consisting of HD + DS−. Therefore, alkyl chains in the mixed adlayer of HD + DS− is more ordered than those in just alkane liquid alone.
In the O-H vibration region in Fig. 6-4-c, the negative-going signal in the range from 3450 to 3370 cm−1 increased from Es = −0.4 V to Es = 0.0 V, and remained constant at Es >
0.1 V. At Es > −0.1 V, the peak shifted to higher energy and the shoulder appeared at 3100 cm−1. As mentioned above, if water molecules form an icelike structure when water molecules are in contact with more hydrophobic medium, a positive-going band appears at 3100-3000 cm−1. At Es > −0.1 V, the HD droplets spread to cover a wider electrode surface, and water molecules formed an icelike structure over ultra-thin layer including adsorbed HD molecules. These results indicated that the HD droplet spread wider at -0.4 V < Es < 0.0 V than at more negative potentials, and the droplet shape did not change at Es > 0.1 V as supported by the -E plot in Fig. 6-2. The interfacial water was displaced by the wider spreading of HD droplet at -0.4 V < Es < 0.5 V. At Es > 0.1 V, the fractional bare Au electrode surface area did not decrease because the HD droplet did not spread farther.
In Fig. 6-4-c, a positive-going peak of the CH2 bending mode at 1223 cm−1 appeared.
This peak significantly increased at Es > 0.0 V, indicating that the ordered HD adsorption