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Electroreflectance study of potential dependent phase changes of dodecyl sulfate adlayer on a Au(1 1 1) Electrode
Electrochimica Acta 162 (2015) 4–10
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Electroreflectance study of potential dependent phase changes of dodecyl sulfate adlayer on a Au(111) Electrode Takamasa Sagara *, Kenji Izumi Division of Chemistry and Materials Science, Graduate School of Engineering, Nagasaki University, Bunkyo 1-14, Nagasaki 852-8521, Japan
A R T I C L E I N F O
A B S T R A C T
Article history: Received 15 August 2014 Received in revised form 21 October 2014 Accepted 22 October 2014 Available online 27 October 2014
Electroreflectance (ER) methods (potential-modulated UV-visible reflectance spectroscopy) were used to track the potential dependent phase changes of an n-dodecyl sulfate (DS) adlayer on a Au(111) electrode surface. In the aqueous solution containing Na-DS (SDS) at a concentration lower than cmc, two step non-faradaic phase changes, which have been well known so far, were clearly observed as ER signals, although the surfactant is colorless. On the ER spectral structure basis, the origin of the signal was the electroreflectance from Au, which relies on the change of surface free electron density on the Au surface. The second harmonic frequency ER signal helped us gain perspective on the extent of non-linearity and kinetics of the reflectance change in response to the potential modulation. Results of the ER measurements enabled us to find that adsorptive hemi-micelle formation-desorption at -0.18 V (Ag/AgCl/ sat-KC1) is faster process than the phase change process between interdigitated bilayer and the hemimicellar phase at +0.50 V. The phase change from the interdigitated bilayer to the hemi-micellar phase exhibited a nucleation-growth type nature in response to a potential step, and this phase change appeared slower than the reverse change. ã 2014 Elsevier Ltd. All rights reserved.
Keywords: Sodium dodecyl sulfate Au(1 1 1) Adsorption Electroreflectance Phase change
1. Introduction Potential-driven phase changes of surfactant adlayers on electrode surfaces are among typical dynamic processes of adsorbed organic molecules at electrified interfaces [1–3]. Deep understanding and fine control of potential-dependent phase changes, especially those which proceed in both ways, may provide us with the opportunities to learn about interplay of intermolecular interactions at the interface and with essential tools for wetsystem fabrication of organic molecular devices. To mention just a typical water-soluble anionic surfactant, sodium dodecyl sulfate (SDS), its dynamic multi-step phase changes at Hg and welldefined solid electrode|solution interfaces have been extensively studied [1,2,4–12]. We also focus on the behavior of dodecyl sulfate ion (DS) in the present work. It is known that DS takes, in a specific range of potential on a Au(111) electrode surface, a hemimicellar structure, viz. an array of hemi-cylindrical stripes, which can be used as a template to electrodeposit a metal wire [12]. Advanced measurement techniques such as neutron reflection [2,9], in situ Atomic Force Microscopy (AFM) [8], Electrochemical Scanning Tunneling Microscopy (EC-STM) [10], and sophisticated
* Corresponding author. Tel.: +81 95 8192676; fax: +81 95 8192676. E-mail address: [email protected] (T. Sagara). http://dx.doi.org/10.1016/j.electacta.2014.10.109 0013-4686/ ã 2014 Elsevier Ltd. All rights reserved.
IR reflection spectroscopy [11] have been applied to track the response of DS to the electrode potential. Classical ac potential modulation methods, especially the interfacial differential capacity measurement, are still indispensable as quick, in-depth, and simple-to-use analytical tools. In this paper, we describe the first attempt, to the best of our knowledge, to track potential-driven dynamics of a surfactant using ac potential-modulated UV-visible reflectance spectroscopic methods, namely electroreflectance (ER) methods [13], in the absence of any chromophore. ER method detects the chemically reversible changes of interfacial optical properties through light reflection in response to the ac potential modulation. For a faradaic process, a molecular color change upon the redox reaction usually gives rise to the ER signal [13]. Non-faradaic processes at the electrode/solution interfaces also give rise to the ac reflectance change, if the processes are chemically reversible to the ac potential change. If one could achieve sensitive enough measurements, the ac reflectance change was observed as an ER signal. Such detectable non-faradaic processes by ER methods include the change of surface electron density of an electrode substrate, adsorptiondesorption, Stark effect, the change of adlayer or film thickness, and orientation change of molecules or dipoles, as well as the change of the complex refractive indexes of the electrode substrate, surface adlayer, or solution in the close proximity to the electrode surface [13–18]. Using ER methods, we can gain an
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made by a lock-in amplifier (EG&G 5210). The fundamental frequency component of the ac intensity of the reflected light Iac is represented by a form of DIac exp[j(vt ’)] where DIac is the zeroto-peak amplitude of Iac and ’ is the phase of Iac with respect to Eac. The ER signal (DR/R)ER is defined as
insight into the details of phase changes that cannot be probed only by current-potential relationship measurements or slow timescale in situ methods. In this paper, we describe the results of ER measurements at a Au(111) electrode/SDS aqueous solution interface. It is known that two main processes take place there [5,7–12]: (1) adsorptive hemimicelle formation of DS and its desorptive disruption and (2) interdigitated bilayer formation and its phase change back to hemi-micellar phase. The former process (1) is observed at approximately 0.5 V more negative potential than the potential of zero charge (pzc) of a bare Au(111) electrode (pzc = 0.27 V vs. Ag/ AgCl/sat-KCl [19]), while the latter process (2) at slightly more positive potential than the pzc. For consistency with previous reports, processes (1) and (2) are hereafter denoted as a_ and b_, respectively. The question is whether the ER method is a powerful tool to track the phase changes of a_ and b_, even though the surfactant, DS, is colorless.
where Idc is the time averaged dc intensity of the reflected light. An ER spectrum (ERS) is a plot of both real (in-phase to Eac) and imaginary (90 out-of-phase) parts of (DR/R)ER as a function of the wavelength of the incident light, l. To record the ERS, l was scanned while monitoring (DR/R)ER. In an ER voltammogram (ERV), both real and imaginary parts of (DR/R)ER obtained during a linear scan of Edc with a potential sweep rate of v were plotted. The second harmonic frequency ER signal (SH-ER signal) was measured at the same phase adjustment setting as the fundamental ac response by the lock-in detection.
2. Experimental
3. Results
Milli-Q water with 18 MV cm resistivity and less than 2 ppb content of total oxidizable carbon was used to prepare all the aqueous solutions. The glassware was boiled in a mixed acid (conc. H2SO4: conc. HNO3 = 1:1 vol/vol) and rinsed by copious amount of Milli-Q water until acid leaching became undetectable. Potassium perchlorate (KClO4) was recrystallized fromwater and dried in vacuo. SDS purchased from Kishida Chemical Co. Ltd. was recrystallized from ethanol and analyzed by 1H NMR before use. Wetted Ar gas (99.9995%) was used to deaerate the electrolyte solution. A Au(111) of a single crystal disk (Techno Chemics Inc. facet precision < 1) with a surface area of 0.502 cm2 was used as a working electrode. Before each experiment, it was flame-annealed and quenched with Milli-Q water. The quality of the Au(111) electrode surface was checked by recording the cyclic voltammogram (CV) and differential capacitance–potential (C–E) curve in a surfactant-free 50 mM KClO4 aqueous solution. The reference electrode was a Ag/AgCl/sat-KCl in a separate compartment which was connected to the main compartment of the electrochemical cell through a salt bridge. All potentials reported in this paper are referenced to this electrode. The counter electrode was a flame-annealed gold coil. The base electrolyte solution used in the experiments with SDS was 50 mM KClO4. The concentration of SDS was 0.50 mM. All of the electrochemical control was carried out at room temperature, 23 2C , being higher than the Krafft point of SDS. For the potential-step chronoamperometry measurements, the transient currents were recorded with a time resolution of 0.2 ms. Measurements of ER signals were carried out by normal incidence of monochromatic light at a hanging meniscus (H-M) configuration. Keeping the H-M configuration set by horizontal touching a Au(1 1 1) electrode to an Ar gas|SDS solution interface, the probe and reflected light passed through the flat quartz cell bottom. The details of the ER instrumentation and spectroelectrochemical cell were given in our previous publication [13]. Briefly, the light source was an air-cooled 300 W halogen lamp (Xenophot-HLX64663). The light was monochromated by a grating, filtered, and focused to the electrode surface. The detector was a photo-multiplier (R928, Hamamatsu Photonics). The potential modulation is described as pffiffiffiffiffi (1) E ¼ Edc þ Eac ¼ Eac þ 2 DEac expðjvtÞ
Fig. 1a and b shows, respectively, a CV and C-E curve together with model depictions of phase change clarified by Burgess and colleagues [8,9] in Fig. 1c. The C-E curve (Fig. 1b) well reproduced previously reported results [8,9], showing a_ and b_ responses. At more negative potential than -0.4 V, no adsorption of DS was observed as indicated by the same capacity values as those for a bare Au(111) electrode. The potential region between the two peaks in Fig. 1a corresponds to the flat and almost constant capacity region in Fig. 1b. In this range from -0.1 V to 0.4 V, adsorbed DS molecules form hemi-micelle structures on the Au surface. In literature, this range of potential has been denoted as state I. The change of the hemi-micellar phase to an interdigitated bilayer, b_ process, took place at 0.44 V (Fig. 1a and b). The C value as low as 3 mF cm2 at 0.65 V is indicative of a compact film formation with low dielectric constant with almost no defect, as modeled as the interdigitated bilayer. In literature, this range of potential has been denoted as state II. Fig. 1c shows an ERV at 500 nm. At this wavelength, regardless of the presence of SDS, a Au(111) electrode exhibited a peak of a broad ER band with negative-going real part (Fig. 2). This band is well-known as an “electroreflectance response of Au” due to the change of the surface electron density upon the ac potential modulation [13,14,16,17,20,21]. Two ER voltammetric peaks observed around -0.22 V (a_) and 0.50 V (b_) in Fig. 1c are in good agreement with the voltammograms in Fig. 1a and b in regard to the peak potentials. At a capacity peak, the change of the electrode surface charge density, sm, is the steepest, thus gives also the voltammetric peak of the ER signal at the identical potential. This description is in line with a simplified expression of the differential of reflectance (R) with respect to E [16]:
where E is the electrode potential, Edc is the dc potential, Eac is the ac potential, DEac is the zero-to-peak root-mean-square (rms) amplitude, v = 2pf is the angular frequency (f is the p frequency of ffiffiffiffiffiffiffi the potential modulation), t is the time, and j = 1. Phasesensitive detection of the ac intensity of the reflected light was
(DR/R)ER = DIac exp(j’)/Idc
dR ds m dR dR ¼ ¼C dE ds m dE ds m
(2)
(3)
However, the practical value of R is a complex function of both E and sm through a set of optical constants at the interface [14,20]. Therefore, above description is actually an approximate representation. We observed an ER signal of almost constant real part of -2 to -4 104 level, which is even lying under the a_ ERV peak around -0.22 V, in the potential range between -0.50 V and 0.40 V (Fig. 1c). The imaginary part of this potential-insensitive response was much weaker than the real part. Rather small imaginary-part/real-part ER signal ratio of this response indicates that the process originating this ER signal is so fast that it has minor delay to the potential modulation
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Fig. 2. ER spectra of a Au(111) electrode in 0.05 M KClO4 solution with DEac = 70 mVrms and f = 14 Hz. Subscripts for the indications in the figure: r, real part; i, imaginary part. Upper figure: (a) Edc = 0.20 V with 0.05 mM SDS, and (b) -0.18 V with 0.05 mM SDS. Lower figure: (c) Edc = +0.70 V without SDS, and (d) in Edc = +0.70 V with 0.05 mM SDS.
Fig. 1. Voltammograms for a Au(111) electrode (A = 0.502 cm2) in a H-M contact with 0.50 mM SDS + 0.05 M KClO4 solution with the initial potential of 0.70 V. (a) CV at v = 0.02 V s1, (b) C-E curve (DEac = 5 mVrms,f = 14 Hz, v = 0.005 V s1), (c) ERV (l = 500 nm, DEac = 70 mVrms, f = 14 Hz, v = 0.002 V s1). Thin line in (a), broken line in (b), and ERV in Fig. 1d are for a bare Au(111) electrode in the absence of SDS. In ERVs, closed circles are for real part signals, while open ones are for imaginary part signals. On Web red marks are for negative scan and blue ones are for positive scan.
at 14 Hz [13]. This signal exhibited an ER spectrum of Fig. 2a, which is apparently due to Au electroreflectance response associated with the double-layer charging-discharging. The spectrum showed a similar spectral structure as Fig. 2b (a_ process for DS) and Fig. 2c (+0.70 V without DS). The ER signal synchronizes with ac change of the interfacial potential drop. Even in comparison to the bare Au(111) response shown in Fig. 1d, which has a peak around pzc and OH adsorption/desorption feature at E > 0.5 V, the potential dependence of the underlying ER signals in Fig.1c in the range between -0.5 V and 0.4 V is rather flat and featureless. At the more positive potentials than the peaks around 0.5 V, namely in the region II, the ER signals became a level lower than 4 105, in harmony with a very low interfacial capacity. This can be also seen in Fig. 2d (+0.70 V with DS). Note that Fig. 1c in comparison to Fig. 1d, and Fig. 2b as well, revealed that, even at 14 Hz, both a_ and b_ processes have a large imaginary part of the ER signal relative to the real part. This fact clearly indicates slower kinetics of the phase change than that of the double-layer charging outside of the potential regions around a_ and b_ peaks. In particular, the b_ peak of ERV in Fig. 1c exhibited much greater imaginary part than its real counterpart,
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demonstrating that the process b_ is so slow that it cannot completely follow the potential modulation of 14 Hz. It means that we can discuss the kinetics of these processes in the measurable frequency range. Fig. 3 shows the frequency dependencies of solution resistancecompensated ER signals. The method of the resistance compensation was described in detail elsewhere [13,22,23], but for clarity, the procedure is briefly given below. The equivalent circuit of the cell involves an uncompensated solution resistance, Rs, in series with an interfacial impedance component. Using the measurable total impedance Zt and Rs, one can obtain the ac voltage across the interfacial impedance component, eac, as Rs (4) eac ¼ Eac 1 Zt Then, the corrected ER signal, (DR/R)Corr, is given using Eq. (2) as
DR=R
Corr
Eac ¼ DR=R ER eac
(5)
In the absence of SDS (Fig. 3, open circles), the real part signal level became constant with lowering f accompanied by diminishing of the imaginary part signal. In the presence of SDS, the ER signal
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onset at nearly the same frequency as in the absence. However in sharp contrast, the real part signal was still increasing down to 1 Hz accompanied by increasing imaginary part. These results indicate the involvement of very slow components, which cannot catch up to 1 Hz potential modulation, in the adlayer phase changes of both a_ and b_. Although the voltammetric peaks due to adlayer phase change appeared sharp (Fig. 1a and b), we should have used 70 mVrms ac amplitude, being comparable or greater than the phase transition voltammetric peak widths, to achieve reasonable signal-to-noise ratio for the ER signal measurements. Therefore, we need to evaluate the extent of non-linearity of the ER signals. For this purpose, we recorded the second harmonic ER (SH-ER) response to the ac potential modulation (Fig. 4). The SH-ER signal is sensitive to mechanism and kinetics as in the case of SH ac voltammetric or polarographic measurements [24–26]. Of importance in the use of SH-ER signal is that the double-layer charging, if it is not modified by non-linear molecular adsorption-desorption or assembling structural change in response to potential, is rather linear to the modulation and thus does not show the SH-ER component [24]. Because above adsorption-desorption and structural change are, in principle, asymmetrical processes, they can give SH-ER signals semiselectively even with a rather large ac amplitude. In this work, we could measure only at low frequencies because of the sensitivity limitation, allowing only qualitative discussion. What we learned from the SH-ER voltammogram in Fig. 4 are: 1. For process a_, bipolar responses with opposite sign for real and
imaginary parts were observed. The bipolar wave is in line with peak-shaped dc voltammetric response as the rudiments of SH ac polarography [24,25]. The imaginary/real signal magnitude ratio was approximately 2.5, being greater than that of the fundamental frequency response. It reveals that the process producing non-linear component is a slower process than that producing linear component. 2. In the potential region I, a broad response with a peak around 0.2 V was observed, although the fundamental frequency response (Fig. 1c) was almost constant. This signal may originate from a curvature of sm-E curve (presumably because of nonideal linearity of double-layer charging response). 3.
Fig. 3. Frequency dependencies of real and imaginary parts of ER signals of a_ and b_ processes using solution resistance-compensated ER signals, (DR/R)Corr (see text) for a Au(111) electrode. Measurement parameters of l = 500 nm, DEac = 70 mVrms, f = 14 Hz were fixed. Edc was -0.18 V for a_ process (a) and 0.50 V for b_ process (b). Close circles: in the presence of 0.50 mM SDS in 0.05 M KClO4 solution, open circles: in the absence of SDS.
Fig. 4. Second harmonic ER (SH-ER) voltammogram obtained under fundamental frequency potential modulation of 14 Hz (l = 500 nm, DEac = 70 mVrms, v = 0.002 V s1 negative scan) for a Au(111) electrode in a H-M contact with 0.50 mM SDS + 0.05 M KClO4 solution. Solid line: SH-ER real part, dotted line: SH-ER imaginary part.
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The bipolar response for process b_ exhibited same sign for real and imaginary parts, indicating that this process involves a very slow non-linear response [26]. We also conducted the measurements of single potential step current transients to gain insight into the kinetics of phase transitions. We applied potential steps to jump over a_ potential, for example between–0.28 V and –0.08 V. Then, we obtained a single exponential decay (not shown here) for both negative and positive step directions. The time periods needed to 95% decay from the initial current maxima were approximately 20 ms for both step directions. The decay curves were symmetrical for both step directions. Fig. 5 shows current transients between I and II regions by using potential steps to jump over b_ potential. We found not only much slower decay of current compared with the decay at a bare electrode but also the appearance of a current hump on the negative step transient. This waveform with a hump is typical for the sharp phase change such as a first order phase transition representing the occurrence of a nuclear-growth process [27]. In the present case, the hump occurred only in the negative potential step. The initial event upon the change of potential to less-positive is the formation of nuclei from which the partial dissolution process of DS propagates two-dimensionally or the re-nucleation growth process of the hemi-micellar phase takes place. The partial dissolution definitely takes place, because the surface concentration of DS is greater in state II (interdigitated bilayer) than in state
I (hemi-micellar phase) [2]. Because the formation of state I through a_ process does not show any nucleation-growth nature, it is likely that the dissolution kinetics is the origin of the nucleargrowth character of current decay. This is a typical dissolution process of a two-dimensional condensed adlayer as a solid-like film [28]. Similar hole nucleation and growth processes have been observed at crystalline Au electrode surfaces for uracil [29], cytosine [30], and uradine [31,32] monolayers for order-todisorder phase changes. In the present case, we did not see any annealing or aging effect at least in the time order of minutes. The transient curves did not depend on the time period spent in state II before the potential step. The positive step response from 0.40 V to 0.60 V took approximately 35 ms for 95% decay. Note that the negative step current transient for the process b_ showed a long tailing down to 120 ms. At 120 ms after giving the positive potential step, the current already reached zero. Taken together, the process of a_ is faster than b_, and the negative step response of the process b_ exhibits much longer decay time than the positive step. The above-mentioned experimental results revealed that, in the process b_, the formation of the interdigitated bilayer is more rapid process than its phase change back to hemi-micellar phase. This is rather surprising, because usually, a desorption process and a phase change to less dense state are faster than the counterpart processes. In the adsorption process, a mass transfer such as diffusion may participate in a rate determining step, because adsorption induces depletion of the solution species to be adsorbed in close proximity of the electrode surface. In addition, in the adsorption process, adsorption precursor species should search about right vacant sites. On the other hand, desorption or dissolution step does not have these obstacles, giving faster rate [28]. Therefore, the inverse relation of the kinetics that we found herein, I!II phase change is faster than II!I, is unusual. This may be due to low monomeric solubility of DS slowing down the hole growth rate, or highly stable structure of the DS interdigitated bilayer as discussed by Burgess and colleagues [9] so that nucleation rate appears sluggish due to a high activation barrier for dissolution. 4. Discussion
Fig. 5. Current transients obtained by potential step chronoamperometry a Au (111) electrode (A = 0.502 cm2) in a H-M contact with 0.50 mM SDS + 0.05 M KClO4 solution: (a) Negative step from +0.60 V to +0.40 V, (b) reverse step. For both, broken lines were measured in the absence of SDS.
Because of the presence of non-negligible non-linearity in the 70 mVrms-modulated ER response and asymmetricity of the phase changes of a_ and b_ between cathodic and anodic processes, attempts of rigorous kinetic analysis using present frequency dependent ER data should be abandoned. Nevertheless, it is still meaningful to compare the obtained non-faradaic ER signals with a simplified semi-quantitative model to gain a deeper insight. We start from an assumption that non-faradaic ac changes have two different components originating the ER signal: one is due to the change of sm and other is due to adsorption amount change of DS. The former component is the electroreflectance of Au surface, which is present even in the absence of SDS. The latter component of ER signal is due to the difference of optical dielectric constant between the assembly of alkyl chains of DS molecules income and the water phase displaced. In addition, we should also consider the contributions to the change of sm from adsorption and assembling of DS through water exclusion by the formation of alkyl phase, anion approach to the electrode surface, and dipole reorientation. If the origin of the ER component is solely the optical dielectric change, the ER signal comes from the change of distribution of the refractive index of the adlayer but not Au surface itself. Then, the ER spectrum is not necessarily the same as the Au electroreflectance waveform. On the other hand, if the origin involves the abovementioned additional contribution to the change of sm, ER
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spectrum should have the same waveform as of Au electroreflectance. Based on the results of ERS and ERV measurements in Figs. 1 and 2, as well as the fact that ER spectrum at 100 Hz (not shown here) showed the same spectral features as the spectra in Fig. 2, it is likely that the additional contribution to sm is the main origin at the ER signal peaks upon phase transition. Irrespective of which the true origin is, the light reflectance, R, should be a function of both E and the amount of adsorbed DS, G , and we can write: dR @R @R @R @s m @R dG dt ¼ þ @G ¼ þ dE @E G @G E @E @s m G @E G @G E dt dE (6) Note that sm and G in Eq. (6) are not of the equilibrium with respect to the electrode potential but are either linearly or nonlinearly ac modulated around Edc. Both two terms in Eq. (6) may have phase delays to Eac. The first term synchronizes to the doublelayer charging and gives the underlying signal in Fig. 1c. Therefore, the second term in Eq. (6) represents the phase change kinetics around the ERV peaks. We assume also that, at the ERV peak potential, the linear part of G is described as 1 2
G ¼ Gsat þ uac expðjvtÞ
(7)
where G sat is the saturated amount of DS in the adlayer as either hemi-micellar phase or interdigitated bilayer, and uac is the amplitude of the change of G multiplied by a phase factor, exp(-j’). We use the first-order Langmuir-type adsorption-desorption rate equation for simplicity:
dG ¼ kads ½DS surf Gsat G kdes G dt
(8)
Assuming activation-type rate constants, we may write 0
(9)
0
(10)
kads ¼ kads exp½Ea =kT with Ea ¼ ra ðE E0 Þ
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At lower frequencies, the magnitude of both real and imaginary part ER signals gradually increased. In Fig. 6, we can view the lower frequency region so that there are overlapping semi-circles. Presumably, the ER signals in these frequency regions are due to the adsorption/desorption or phase change. This region for b_ appeared at lower frequencies and gave greater imaginary/real ratio compared with that for a_. This fact revealed that b_ is a slower process than a_. Current transients in Fig. 5 also support this, because total decay time for b_ (> 130 ms at 0.4 V) is much longer than that for a_ (approximately 90 ms at 0.08 V in the positive step). We found that clear ER signals can be obtained for the potentialdriven phase changes of colorless surfactants at electrified interfaces. However, ER data analysis in the frequency domain to extract kinetic information in this work remained insufficient even to a semi-quantitative level. We tentatively used for the fundamental frequency response both Eq. (8) and the linear response approximation. The former unrealistically ignored intermolecular interactions, and the latter was in doubt in the uses of large DEac values. Nevertheless, we could find the approximate time domain of very slow components in the phase change processes. To see whether ER methods help gain deeper insight or not, we need measurements of the dependencies of incident light polarization, incident angle, and DS concentration, as well as development of SH-ER measurements. We are in preparation to these examinations. 5. Conclusions ER techniques were used to track the potential dependent phase changes of aDS adlayer on a Au(111) electrode surface. We found that the ER voltammograms at fundamental frequencies well correspond to C-E curves. The ER signals obtained were totally of non-faradaic responses. Although the signals included several
and kdes ¼ kdes exp½Ed =kT with Ed ¼ rd ðE E0 Þ
where kads and kdes are adsorption and desorption rate constants, respectively, DS concentration is taken at the surfaces with a 0 0 constant value approximation to be [DS]surf, kads and kdes are the standard rate constants, ra and rd are the coefficients representing the relationships between the electrode potential difference from E0 and the activation energies, Ea and Ed, respectively for adsorption and desorption processes, and E0 is the potential at which adsorption amount is equal to 12Gsat . We further assume that the potential modulation amplitude is small enough and enables us to use the linear response approximation for Eqs. (9) and (10). The ER response as a function of frequency of the present case should be the same as a previously reported case of adsorptiondesorption process at a liquid-liquid interface measured by potential-modulated fluorescence spectroscopy as derived by Nagatani and colleagues [33]. In reference to their formulation, it follows that the trajectory of (DR/R)ER on a complex plane of the present case should be a semicircle. @s m @R The first term of Eq. (6), @s @E G , should also show m G another semi-circle as nearly the same frequency range as the bare electrode does. This has already been confirmed by the experiments at a bare Au(1 1 1) electrode using the data in Fig. 3. Taking a look once again at Fig. 3 from higher to lower frequencies, the onset of the ER signal is similar to the bare electrode. This as well as the complex plane plot in Fig. 6 indicates that the ER signals higher than ca. 80 Hz for a_ and 20 Hz for b_ are mainly due to the first term of Eq. (6).
Fig. 6. Complex plane plots of ER signal frequency dependence data in Fig. 3. (a) Edc = -0.18 V for a_ process, (b) Edc = 0.50 V for b_ process.
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origins, the spectral feature was uniquely of the Au electroreflectance type, which relies on the change of surface free electron density on the Au surface. We may conclude that the Au surface electron density change with potential plays a role of a mirror to show up the interfacial processes such as molecular adsorption and assembling. Therefore, it was not necessary to have a chromophore in the interfacial molecular assemblies on the electrode surface to obtain ER signal. ER observation, including the second harmonic ER signal measurements, enabled us to find that adsorptive hemi-micelle formation/desorption at -0.18 V is a faster process than phase change process between interdigitated bilayer and hemi-micellar phase at +0.50 V. The phase change from the interdigitated bilayer to the hemi-micellar phase exhibited a nucleation-hole-growth type nature. This process is slower than the reverse process. Acknowledgements This work was financially supported by Grant-in-aid for Scientific Research from MEXT, Japan (No. 24550158). We thank Mr. Tetsuro Moroka, School of Engineering, Nagasaki University, for his helpful discussion. References [1] D. Bizzotto, V. Zamlynny, I. Burgess, C.A. Jeffrey, H.-Q. Li, J. Rubinstein, R.A. Merril, J. Lipkowski, Z. Galus, A. Nelson, B. Pettinger, Amphiphilic and Ionic Surfactants at Electrified Interfaces, in: A. Wieckowski (Ed.), Interfacial Electrochemistry, Marcel Dekker, New York, 1999, pp. 405–426. [2] M. Chen, I. Burgess, J. Lipkowski, Potential controlled surface aggregation of surfactants at electrode surfaces – A molecular view, Surf. Sci. 603 (2009) 1878–1891. [3] T. Sagara, Dynamic Behaviors of Molecular Assemblies and Nano-Substances at Electrified Interfaces, in: K. Ariga, H. S. Nalwa (Eds.), Bottom-up Nanofabrication: Supramolecules, Self-Assemblies, and Organized Films, Vol. 3 (Self-Assemblies-I), American Scientific Publishers, Valencia, 2009, Chap. 13, pp. 347–373. [4] K. Eda, Structure of Adsorbed Layers at the Interface of Mercury-Surfactant Solution. III. Structure of Adsorbed Layers of Sodium Decyl, Dodecyl, and Tetradecyl Sulfate, Nippon Kagaku Zasshi 80 (1959) 349–352. [5] K. Eda, K. Takahashi, Structure of Adsorbed Layers at the Interface MercurySurfactant Solution. X. Study of the Competitive Adsorption of Two Surfactants by the Method of Differential Double Layer Capacity, Nippon Kagaku Zasshi 85 (1964) 828–832. [6] P. Skoluda, The voltammetric study of the Au(100) electrode in the presence of alkyl sulfates, J. Electroanal. Chem. 406 (1996) 235–238. [7] T. Wandlowski, M. Hromadova, R. de Levie, On the Kinetics of Adsorption of Dodecyl Sulfate at the Mercury-Water Interface, Langmuir 13 (1997) 2766–2772 and references therein. [8] I. Burgess, C.A. Jeffrey, X. Cai, G. Szymanski, Z. Galus, J. Lipkowski, Direct Visualization of the Potential-Controlled Transformation of Hemimicellar Aggregates of Dodecyl Sulfate into a Condensed Monolayer at the Au(111) Electrode Surface, Langmuir 15 (1999) 2607–2616. [9] I. Burgess, V. Zamlynny, G. Szymanski, J. Lipkowski, J. Majewski, G. Smith, S. Satija, R. Ivkov, Electrochemical and Neutron Reflectivity Characterization of Dodecyl Sulfate Adsorption and Aggregation at the Gold-Water Interface, Langmuir 17 (2001) 3355–3367. [10] M. Petri, D.M. Kolb, Nanostructuring of a sodium dodecyl sulfate-covered Au (111) electrode, Phys. Chem. Chem. Phys. 4 (2002) 1211–1216. [11] J.J. Leitch, J. Collins, A.K. Friedrich, U. Stimming, J.R. Dutcher, J. Lipkowski, Infrared Studies of the Potential Controlled Adsorption of Sodium Dodecyl Sulfate at the Au(111) Electrode Surface, Langmuir 28 (2012) 2455–2464.
[12] T. Ye, K. Seo, E. Borguet, Electrodeposition of Metal Wires onto a Molecular Scale Template: An In Situ Investigation, Langmuir 25 (2009) 5491–5495. [13] T. Sagara, UV-visible reflectance spectroscopy of thin organic films at electrodesurfaces, in: R.C. Alkire, D.M. Kolb, J. Lipkowski, P.N. Ross (Eds.), Advances in Electrochemical Science and Engineering, vol. 9, Wiley-VCH, Weinheim, 2006, pp. 47–95. [14] J.D.E. McIntyre, Electrochemical Modulation Spectroscopy, Surf. Sci. 37 (1973) 658–682. [15] R.M. Lazorenko-Manevich, E.B. Brick, Y.M. Kolotyrkin, Influence of adsorption on the optical properties of the electrode-solution interface, Electrochim. Acta 22 (1977) 151–160. [16] C. Hinnen, C. Nguyen Van Huong, A. Rousseau, J.P. Dalbera, A study of the gold/ aqueous electrolyte interphase by modulated reflection spectroscopy, J. Electroanal. Chem. Interfacial Electrochem. 95 (1979) 131–146. [17] C. Nguyen Van Huong, C. Hinnen, J.P. Dalbera, R. Parsons, Adsorption of ethyl ether on polycrystal and the (110) single crystal face of gold by admittance measurements and modulated reflection spectroscopy, J. Electroanal. Chem. Interfacial Electrochem. 125 (1981) 177–192. [18] P.H. Schmidt, W.J. Plieth, Adsorption of p-nitroaniline on platinum studied by modulated reflectance spectroscopy, J. Electroanal. Chem. Interfacial Electrochem. 201 (1986) 163–174. [19] A.S. Dakkouri, D.M. Kolb, Reconstruction of Gold Surfaces, in: A. Wieckowski (Ed.), Interfacial Electrochemisty, Marcel Dekker, New York, 1999, pp. 151–173. [20] J.D.E. McIntyre, Specular Reflection Spectroscopy of the Electrode-Solution Interphase, in: R. Muller (Ed.), Advances in Electrochemistry and Electrochemical Engineering, vol. 9, Wiley, New York, 1973, pp. 61–166 Chap. 2. [21] D.M. Kolb, UV-Visible Reflectance Spectroscopy, in: R.J. Gale (Ed.), Spectroelectrochemistry, Theory and Practice, Plenum Press, New York, 1988, pp. 87–183. [22] A. Toyota, N. Nakashima, T. Sagara, UV–visible transmission–absorption spectral study of Au nanoparticles on a modified ITO electrode at constant potentials and under potential modulation, J. Electroanal. Chem. 565 (2004) 335–342. [23] T. Sagara, N. Kato, N. Nakahsima, Electroreflectance Study of Gold Nanoparticles Immobilized on an Aminoalkanethiol Monolayer Coated on a Polycrystalline Gold Electrode Surface, J. Phys. Chem. B 106 (2002) 1205–1212. [24] D.E. Smith, H.H. Bauer, Recent Developments in Alternating Current Polarography, Crit. Rev. Anal. Chem. 2 (1971) 247–343. [25] A.M. Bond, Modern Polarographic Methods in Analytical Chemistry, Marcel Dekker, New York, 1980. [26] C.P. Andrieux, P. Hapiot, J. Pinson, J.M. Saveant, Determination of formal potentials of chemically unstable redox couples by second-harmonic alternating current voltammetry and cyclic voltammetry. Application to the oxidation of thiophenoxide ions, J. Am. Chem. Soc. 115 (1993) 7783–7788. [27] Th. Wandlowski, Phase Transtions in Two-dimensional Adlayers at Electrode Surfaces: Thermodynamics, Kinetics, and Structural Aspects, in: E. Gileadi, M. Urbakh (Eds.), Thermodynamics and Electrified Interfaces, in: A. J. Bard, M. Stratmann (Eds.), Encyclopedia of Electrochemistry, Wiley-VCH, Weinheim, 2003, pp. 383–467. [28] S. Buess-Herman, M. Baré, M. Poelman, M. Van krieken, Ordered Organic Adlayers at Electrode Surfaces, in: A, Wieckowski (Eds.), Interfacial Electrochemisty, Marcel Dekker, New York, 1999, pp. 427–447. [29] M.H. Hölzle, Th. Wandlowski, D.M. Kolb, Structural transitions in uracil adlayers on gold single crystal electrodes, Surf. Sci. 335 (1995) 281–290. [30] D. Th. Wandlowski, S.M. Lampner, Lindsay, Structure and stability of cytosine adlayers on Au(111): an in-situ STM study, J. Electroanal. Chem. 404 (1996) 215–226. [31] M. Van krieken, C. Buess-Herman, On the dissolution kinetics of twodimensional uridine layers adsorbed on gold single-crystal electrodes, Electrochim. Acta 43 (1998) 2831–2841. [32] S. Baré, C. Buess-Herman, On the formation and dissolution kinetics of twodimensional uracil ordered phases at the gold single crystal electrode-aqueous solution interface, Colloid Surface A 134 (1998) 181–191. [33] H. Nagatani, D.J. Fermín, H.H. Girault, A Kinetic Model for Adsorption and Transfer of Ionic Species at Polarized Liquid|Liquid Interfaces as Studied by Potential Modulated Fluorescence Spectroscopy, J. Phys. Chem. B 105 (2001) 9463–9473.
Contents lists available at ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Electroreflectance study of potential dependent phase changes of dodecyl sulfate adlayer on a Au(111) Electrode Takamasa Sagara *, Kenji Izumi Division of Chemistry and Materials Science, Graduate School of Engineering, Nagasaki University, Bunkyo 1-14, Nagasaki 852-8521, Japan
A R T I C L E I N F O
A B S T R A C T
Article history: Received 15 August 2014 Received in revised form 21 October 2014 Accepted 22 October 2014 Available online 27 October 2014
Electroreflectance (ER) methods (potential-modulated UV-visible reflectance spectroscopy) were used to track the potential dependent phase changes of an n-dodecyl sulfate (DS) adlayer on a Au(111) electrode surface. In the aqueous solution containing Na-DS (SDS) at a concentration lower than cmc, two step non-faradaic phase changes, which have been well known so far, were clearly observed as ER signals, although the surfactant is colorless. On the ER spectral structure basis, the origin of the signal was the electroreflectance from Au, which relies on the change of surface free electron density on the Au surface. The second harmonic frequency ER signal helped us gain perspective on the extent of non-linearity and kinetics of the reflectance change in response to the potential modulation. Results of the ER measurements enabled us to find that adsorptive hemi-micelle formation-desorption at -0.18 V (Ag/AgCl/ sat-KC1) is faster process than the phase change process between interdigitated bilayer and the hemimicellar phase at +0.50 V. The phase change from the interdigitated bilayer to the hemi-micellar phase exhibited a nucleation-growth type nature in response to a potential step, and this phase change appeared slower than the reverse change. ã 2014 Elsevier Ltd. All rights reserved.
Keywords: Sodium dodecyl sulfate Au(1 1 1) Adsorption Electroreflectance Phase change
1. Introduction Potential-driven phase changes of surfactant adlayers on electrode surfaces are among typical dynamic processes of adsorbed organic molecules at electrified interfaces [1–3]. Deep understanding and fine control of potential-dependent phase changes, especially those which proceed in both ways, may provide us with the opportunities to learn about interplay of intermolecular interactions at the interface and with essential tools for wetsystem fabrication of organic molecular devices. To mention just a typical water-soluble anionic surfactant, sodium dodecyl sulfate (SDS), its dynamic multi-step phase changes at Hg and welldefined solid electrode|solution interfaces have been extensively studied [1,2,4–12]. We also focus on the behavior of dodecyl sulfate ion (DS) in the present work. It is known that DS takes, in a specific range of potential on a Au(111) electrode surface, a hemimicellar structure, viz. an array of hemi-cylindrical stripes, which can be used as a template to electrodeposit a metal wire [12]. Advanced measurement techniques such as neutron reflection [2,9], in situ Atomic Force Microscopy (AFM) [8], Electrochemical Scanning Tunneling Microscopy (EC-STM) [10], and sophisticated
* Corresponding author. Tel.: +81 95 8192676; fax: +81 95 8192676. E-mail address: [email protected] (T. Sagara). http://dx.doi.org/10.1016/j.electacta.2014.10.109 0013-4686/ ã 2014 Elsevier Ltd. All rights reserved.
IR reflection spectroscopy [11] have been applied to track the response of DS to the electrode potential. Classical ac potential modulation methods, especially the interfacial differential capacity measurement, are still indispensable as quick, in-depth, and simple-to-use analytical tools. In this paper, we describe the first attempt, to the best of our knowledge, to track potential-driven dynamics of a surfactant using ac potential-modulated UV-visible reflectance spectroscopic methods, namely electroreflectance (ER) methods [13], in the absence of any chromophore. ER method detects the chemically reversible changes of interfacial optical properties through light reflection in response to the ac potential modulation. For a faradaic process, a molecular color change upon the redox reaction usually gives rise to the ER signal [13]. Non-faradaic processes at the electrode/solution interfaces also give rise to the ac reflectance change, if the processes are chemically reversible to the ac potential change. If one could achieve sensitive enough measurements, the ac reflectance change was observed as an ER signal. Such detectable non-faradaic processes by ER methods include the change of surface electron density of an electrode substrate, adsorptiondesorption, Stark effect, the change of adlayer or film thickness, and orientation change of molecules or dipoles, as well as the change of the complex refractive indexes of the electrode substrate, surface adlayer, or solution in the close proximity to the electrode surface [13–18]. Using ER methods, we can gain an
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made by a lock-in amplifier (EG&G 5210). The fundamental frequency component of the ac intensity of the reflected light Iac is represented by a form of DIac exp[j(vt ’)] where DIac is the zeroto-peak amplitude of Iac and ’ is the phase of Iac with respect to Eac. The ER signal (DR/R)ER is defined as
insight into the details of phase changes that cannot be probed only by current-potential relationship measurements or slow timescale in situ methods. In this paper, we describe the results of ER measurements at a Au(111) electrode/SDS aqueous solution interface. It is known that two main processes take place there [5,7–12]: (1) adsorptive hemimicelle formation of DS and its desorptive disruption and (2) interdigitated bilayer formation and its phase change back to hemi-micellar phase. The former process (1) is observed at approximately 0.5 V more negative potential than the potential of zero charge (pzc) of a bare Au(111) electrode (pzc = 0.27 V vs. Ag/ AgCl/sat-KCl [19]), while the latter process (2) at slightly more positive potential than the pzc. For consistency with previous reports, processes (1) and (2) are hereafter denoted as a_ and b_, respectively. The question is whether the ER method is a powerful tool to track the phase changes of a_ and b_, even though the surfactant, DS, is colorless.
where Idc is the time averaged dc intensity of the reflected light. An ER spectrum (ERS) is a plot of both real (in-phase to Eac) and imaginary (90 out-of-phase) parts of (DR/R)ER as a function of the wavelength of the incident light, l. To record the ERS, l was scanned while monitoring (DR/R)ER. In an ER voltammogram (ERV), both real and imaginary parts of (DR/R)ER obtained during a linear scan of Edc with a potential sweep rate of v were plotted. The second harmonic frequency ER signal (SH-ER signal) was measured at the same phase adjustment setting as the fundamental ac response by the lock-in detection.
2. Experimental
3. Results
Milli-Q water with 18 MV cm resistivity and less than 2 ppb content of total oxidizable carbon was used to prepare all the aqueous solutions. The glassware was boiled in a mixed acid (conc. H2SO4: conc. HNO3 = 1:1 vol/vol) and rinsed by copious amount of Milli-Q water until acid leaching became undetectable. Potassium perchlorate (KClO4) was recrystallized fromwater and dried in vacuo. SDS purchased from Kishida Chemical Co. Ltd. was recrystallized from ethanol and analyzed by 1H NMR before use. Wetted Ar gas (99.9995%) was used to deaerate the electrolyte solution. A Au(111) of a single crystal disk (Techno Chemics Inc. facet precision < 1) with a surface area of 0.502 cm2 was used as a working electrode. Before each experiment, it was flame-annealed and quenched with Milli-Q water. The quality of the Au(111) electrode surface was checked by recording the cyclic voltammogram (CV) and differential capacitance–potential (C–E) curve in a surfactant-free 50 mM KClO4 aqueous solution. The reference electrode was a Ag/AgCl/sat-KCl in a separate compartment which was connected to the main compartment of the electrochemical cell through a salt bridge. All potentials reported in this paper are referenced to this electrode. The counter electrode was a flame-annealed gold coil. The base electrolyte solution used in the experiments with SDS was 50 mM KClO4. The concentration of SDS was 0.50 mM. All of the electrochemical control was carried out at room temperature, 23 2C , being higher than the Krafft point of SDS. For the potential-step chronoamperometry measurements, the transient currents were recorded with a time resolution of 0.2 ms. Measurements of ER signals were carried out by normal incidence of monochromatic light at a hanging meniscus (H-M) configuration. Keeping the H-M configuration set by horizontal touching a Au(1 1 1) electrode to an Ar gas|SDS solution interface, the probe and reflected light passed through the flat quartz cell bottom. The details of the ER instrumentation and spectroelectrochemical cell were given in our previous publication [13]. Briefly, the light source was an air-cooled 300 W halogen lamp (Xenophot-HLX64663). The light was monochromated by a grating, filtered, and focused to the electrode surface. The detector was a photo-multiplier (R928, Hamamatsu Photonics). The potential modulation is described as pffiffiffiffiffi (1) E ¼ Edc þ Eac ¼ Eac þ 2 DEac expðjvtÞ
Fig. 1a and b shows, respectively, a CV and C-E curve together with model depictions of phase change clarified by Burgess and colleagues [8,9] in Fig. 1c. The C-E curve (Fig. 1b) well reproduced previously reported results [8,9], showing a_ and b_ responses. At more negative potential than -0.4 V, no adsorption of DS was observed as indicated by the same capacity values as those for a bare Au(111) electrode. The potential region between the two peaks in Fig. 1a corresponds to the flat and almost constant capacity region in Fig. 1b. In this range from -0.1 V to 0.4 V, adsorbed DS molecules form hemi-micelle structures on the Au surface. In literature, this range of potential has been denoted as state I. The change of the hemi-micellar phase to an interdigitated bilayer, b_ process, took place at 0.44 V (Fig. 1a and b). The C value as low as 3 mF cm2 at 0.65 V is indicative of a compact film formation with low dielectric constant with almost no defect, as modeled as the interdigitated bilayer. In literature, this range of potential has been denoted as state II. Fig. 1c shows an ERV at 500 nm. At this wavelength, regardless of the presence of SDS, a Au(111) electrode exhibited a peak of a broad ER band with negative-going real part (Fig. 2). This band is well-known as an “electroreflectance response of Au” due to the change of the surface electron density upon the ac potential modulation [13,14,16,17,20,21]. Two ER voltammetric peaks observed around -0.22 V (a_) and 0.50 V (b_) in Fig. 1c are in good agreement with the voltammograms in Fig. 1a and b in regard to the peak potentials. At a capacity peak, the change of the electrode surface charge density, sm, is the steepest, thus gives also the voltammetric peak of the ER signal at the identical potential. This description is in line with a simplified expression of the differential of reflectance (R) with respect to E [16]:
where E is the electrode potential, Edc is the dc potential, Eac is the ac potential, DEac is the zero-to-peak root-mean-square (rms) amplitude, v = 2pf is the angular frequency (f is the p frequency of ffiffiffiffiffiffiffi the potential modulation), t is the time, and j = 1. Phasesensitive detection of the ac intensity of the reflected light was
(DR/R)ER = DIac exp(j’)/Idc
dR ds m dR dR ¼ ¼C dE ds m dE ds m
(2)
(3)
However, the practical value of R is a complex function of both E and sm through a set of optical constants at the interface [14,20]. Therefore, above description is actually an approximate representation. We observed an ER signal of almost constant real part of -2 to -4 104 level, which is even lying under the a_ ERV peak around -0.22 V, in the potential range between -0.50 V and 0.40 V (Fig. 1c). The imaginary part of this potential-insensitive response was much weaker than the real part. Rather small imaginary-part/real-part ER signal ratio of this response indicates that the process originating this ER signal is so fast that it has minor delay to the potential modulation
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Fig. 2. ER spectra of a Au(111) electrode in 0.05 M KClO4 solution with DEac = 70 mVrms and f = 14 Hz. Subscripts for the indications in the figure: r, real part; i, imaginary part. Upper figure: (a) Edc = 0.20 V with 0.05 mM SDS, and (b) -0.18 V with 0.05 mM SDS. Lower figure: (c) Edc = +0.70 V without SDS, and (d) in Edc = +0.70 V with 0.05 mM SDS.
Fig. 1. Voltammograms for a Au(111) electrode (A = 0.502 cm2) in a H-M contact with 0.50 mM SDS + 0.05 M KClO4 solution with the initial potential of 0.70 V. (a) CV at v = 0.02 V s1, (b) C-E curve (DEac = 5 mVrms,f = 14 Hz, v = 0.005 V s1), (c) ERV (l = 500 nm, DEac = 70 mVrms, f = 14 Hz, v = 0.002 V s1). Thin line in (a), broken line in (b), and ERV in Fig. 1d are for a bare Au(111) electrode in the absence of SDS. In ERVs, closed circles are for real part signals, while open ones are for imaginary part signals. On Web red marks are for negative scan and blue ones are for positive scan.
at 14 Hz [13]. This signal exhibited an ER spectrum of Fig. 2a, which is apparently due to Au electroreflectance response associated with the double-layer charging-discharging. The spectrum showed a similar spectral structure as Fig. 2b (a_ process for DS) and Fig. 2c (+0.70 V without DS). The ER signal synchronizes with ac change of the interfacial potential drop. Even in comparison to the bare Au(111) response shown in Fig. 1d, which has a peak around pzc and OH adsorption/desorption feature at E > 0.5 V, the potential dependence of the underlying ER signals in Fig.1c in the range between -0.5 V and 0.4 V is rather flat and featureless. At the more positive potentials than the peaks around 0.5 V, namely in the region II, the ER signals became a level lower than 4 105, in harmony with a very low interfacial capacity. This can be also seen in Fig. 2d (+0.70 V with DS). Note that Fig. 1c in comparison to Fig. 1d, and Fig. 2b as well, revealed that, even at 14 Hz, both a_ and b_ processes have a large imaginary part of the ER signal relative to the real part. This fact clearly indicates slower kinetics of the phase change than that of the double-layer charging outside of the potential regions around a_ and b_ peaks. In particular, the b_ peak of ERV in Fig. 1c exhibited much greater imaginary part than its real counterpart,
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demonstrating that the process b_ is so slow that it cannot completely follow the potential modulation of 14 Hz. It means that we can discuss the kinetics of these processes in the measurable frequency range. Fig. 3 shows the frequency dependencies of solution resistancecompensated ER signals. The method of the resistance compensation was described in detail elsewhere [13,22,23], but for clarity, the procedure is briefly given below. The equivalent circuit of the cell involves an uncompensated solution resistance, Rs, in series with an interfacial impedance component. Using the measurable total impedance Zt and Rs, one can obtain the ac voltage across the interfacial impedance component, eac, as Rs (4) eac ¼ Eac 1 Zt Then, the corrected ER signal, (DR/R)Corr, is given using Eq. (2) as
DR=R
Corr
Eac ¼ DR=R ER eac
(5)
In the absence of SDS (Fig. 3, open circles), the real part signal level became constant with lowering f accompanied by diminishing of the imaginary part signal. In the presence of SDS, the ER signal
7
onset at nearly the same frequency as in the absence. However in sharp contrast, the real part signal was still increasing down to 1 Hz accompanied by increasing imaginary part. These results indicate the involvement of very slow components, which cannot catch up to 1 Hz potential modulation, in the adlayer phase changes of both a_ and b_. Although the voltammetric peaks due to adlayer phase change appeared sharp (Fig. 1a and b), we should have used 70 mVrms ac amplitude, being comparable or greater than the phase transition voltammetric peak widths, to achieve reasonable signal-to-noise ratio for the ER signal measurements. Therefore, we need to evaluate the extent of non-linearity of the ER signals. For this purpose, we recorded the second harmonic ER (SH-ER) response to the ac potential modulation (Fig. 4). The SH-ER signal is sensitive to mechanism and kinetics as in the case of SH ac voltammetric or polarographic measurements [24–26]. Of importance in the use of SH-ER signal is that the double-layer charging, if it is not modified by non-linear molecular adsorption-desorption or assembling structural change in response to potential, is rather linear to the modulation and thus does not show the SH-ER component [24]. Because above adsorption-desorption and structural change are, in principle, asymmetrical processes, they can give SH-ER signals semiselectively even with a rather large ac amplitude. In this work, we could measure only at low frequencies because of the sensitivity limitation, allowing only qualitative discussion. What we learned from the SH-ER voltammogram in Fig. 4 are: 1. For process a_, bipolar responses with opposite sign for real and
imaginary parts were observed. The bipolar wave is in line with peak-shaped dc voltammetric response as the rudiments of SH ac polarography [24,25]. The imaginary/real signal magnitude ratio was approximately 2.5, being greater than that of the fundamental frequency response. It reveals that the process producing non-linear component is a slower process than that producing linear component. 2. In the potential region I, a broad response with a peak around 0.2 V was observed, although the fundamental frequency response (Fig. 1c) was almost constant. This signal may originate from a curvature of sm-E curve (presumably because of nonideal linearity of double-layer charging response). 3.
Fig. 3. Frequency dependencies of real and imaginary parts of ER signals of a_ and b_ processes using solution resistance-compensated ER signals, (DR/R)Corr (see text) for a Au(111) electrode. Measurement parameters of l = 500 nm, DEac = 70 mVrms, f = 14 Hz were fixed. Edc was -0.18 V for a_ process (a) and 0.50 V for b_ process (b). Close circles: in the presence of 0.50 mM SDS in 0.05 M KClO4 solution, open circles: in the absence of SDS.
Fig. 4. Second harmonic ER (SH-ER) voltammogram obtained under fundamental frequency potential modulation of 14 Hz (l = 500 nm, DEac = 70 mVrms, v = 0.002 V s1 negative scan) for a Au(111) electrode in a H-M contact with 0.50 mM SDS + 0.05 M KClO4 solution. Solid line: SH-ER real part, dotted line: SH-ER imaginary part.
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The bipolar response for process b_ exhibited same sign for real and imaginary parts, indicating that this process involves a very slow non-linear response [26]. We also conducted the measurements of single potential step current transients to gain insight into the kinetics of phase transitions. We applied potential steps to jump over a_ potential, for example between–0.28 V and –0.08 V. Then, we obtained a single exponential decay (not shown here) for both negative and positive step directions. The time periods needed to 95% decay from the initial current maxima were approximately 20 ms for both step directions. The decay curves were symmetrical for both step directions. Fig. 5 shows current transients between I and II regions by using potential steps to jump over b_ potential. We found not only much slower decay of current compared with the decay at a bare electrode but also the appearance of a current hump on the negative step transient. This waveform with a hump is typical for the sharp phase change such as a first order phase transition representing the occurrence of a nuclear-growth process [27]. In the present case, the hump occurred only in the negative potential step. The initial event upon the change of potential to less-positive is the formation of nuclei from which the partial dissolution process of DS propagates two-dimensionally or the re-nucleation growth process of the hemi-micellar phase takes place. The partial dissolution definitely takes place, because the surface concentration of DS is greater in state II (interdigitated bilayer) than in state
I (hemi-micellar phase) [2]. Because the formation of state I through a_ process does not show any nucleation-growth nature, it is likely that the dissolution kinetics is the origin of the nucleargrowth character of current decay. This is a typical dissolution process of a two-dimensional condensed adlayer as a solid-like film [28]. Similar hole nucleation and growth processes have been observed at crystalline Au electrode surfaces for uracil [29], cytosine [30], and uradine [31,32] monolayers for order-todisorder phase changes. In the present case, we did not see any annealing or aging effect at least in the time order of minutes. The transient curves did not depend on the time period spent in state II before the potential step. The positive step response from 0.40 V to 0.60 V took approximately 35 ms for 95% decay. Note that the negative step current transient for the process b_ showed a long tailing down to 120 ms. At 120 ms after giving the positive potential step, the current already reached zero. Taken together, the process of a_ is faster than b_, and the negative step response of the process b_ exhibits much longer decay time than the positive step. The above-mentioned experimental results revealed that, in the process b_, the formation of the interdigitated bilayer is more rapid process than its phase change back to hemi-micellar phase. This is rather surprising, because usually, a desorption process and a phase change to less dense state are faster than the counterpart processes. In the adsorption process, a mass transfer such as diffusion may participate in a rate determining step, because adsorption induces depletion of the solution species to be adsorbed in close proximity of the electrode surface. In addition, in the adsorption process, adsorption precursor species should search about right vacant sites. On the other hand, desorption or dissolution step does not have these obstacles, giving faster rate [28]. Therefore, the inverse relation of the kinetics that we found herein, I!II phase change is faster than II!I, is unusual. This may be due to low monomeric solubility of DS slowing down the hole growth rate, or highly stable structure of the DS interdigitated bilayer as discussed by Burgess and colleagues [9] so that nucleation rate appears sluggish due to a high activation barrier for dissolution. 4. Discussion
Fig. 5. Current transients obtained by potential step chronoamperometry a Au (111) electrode (A = 0.502 cm2) in a H-M contact with 0.50 mM SDS + 0.05 M KClO4 solution: (a) Negative step from +0.60 V to +0.40 V, (b) reverse step. For both, broken lines were measured in the absence of SDS.
Because of the presence of non-negligible non-linearity in the 70 mVrms-modulated ER response and asymmetricity of the phase changes of a_ and b_ between cathodic and anodic processes, attempts of rigorous kinetic analysis using present frequency dependent ER data should be abandoned. Nevertheless, it is still meaningful to compare the obtained non-faradaic ER signals with a simplified semi-quantitative model to gain a deeper insight. We start from an assumption that non-faradaic ac changes have two different components originating the ER signal: one is due to the change of sm and other is due to adsorption amount change of DS. The former component is the electroreflectance of Au surface, which is present even in the absence of SDS. The latter component of ER signal is due to the difference of optical dielectric constant between the assembly of alkyl chains of DS molecules income and the water phase displaced. In addition, we should also consider the contributions to the change of sm from adsorption and assembling of DS through water exclusion by the formation of alkyl phase, anion approach to the electrode surface, and dipole reorientation. If the origin of the ER component is solely the optical dielectric change, the ER signal comes from the change of distribution of the refractive index of the adlayer but not Au surface itself. Then, the ER spectrum is not necessarily the same as the Au electroreflectance waveform. On the other hand, if the origin involves the abovementioned additional contribution to the change of sm, ER
T. Sagara, K. Izumi / Electrochimica Acta 162 (2015) 4–10
spectrum should have the same waveform as of Au electroreflectance. Based on the results of ERS and ERV measurements in Figs. 1 and 2, as well as the fact that ER spectrum at 100 Hz (not shown here) showed the same spectral features as the spectra in Fig. 2, it is likely that the additional contribution to sm is the main origin at the ER signal peaks upon phase transition. Irrespective of which the true origin is, the light reflectance, R, should be a function of both E and the amount of adsorbed DS, G , and we can write: dR @R @R @R @s m @R dG dt ¼ þ @G ¼ þ dE @E G @G E @E @s m G @E G @G E dt dE (6) Note that sm and G in Eq. (6) are not of the equilibrium with respect to the electrode potential but are either linearly or nonlinearly ac modulated around Edc. Both two terms in Eq. (6) may have phase delays to Eac. The first term synchronizes to the doublelayer charging and gives the underlying signal in Fig. 1c. Therefore, the second term in Eq. (6) represents the phase change kinetics around the ERV peaks. We assume also that, at the ERV peak potential, the linear part of G is described as 1 2
G ¼ Gsat þ uac expðjvtÞ
(7)
where G sat is the saturated amount of DS in the adlayer as either hemi-micellar phase or interdigitated bilayer, and uac is the amplitude of the change of G multiplied by a phase factor, exp(-j’). We use the first-order Langmuir-type adsorption-desorption rate equation for simplicity:
dG ¼ kads ½DS surf Gsat G kdes G dt
(8)
Assuming activation-type rate constants, we may write 0
(9)
0
(10)
kads ¼ kads exp½Ea =kT with Ea ¼ ra ðE E0 Þ
9
At lower frequencies, the magnitude of both real and imaginary part ER signals gradually increased. In Fig. 6, we can view the lower frequency region so that there are overlapping semi-circles. Presumably, the ER signals in these frequency regions are due to the adsorption/desorption or phase change. This region for b_ appeared at lower frequencies and gave greater imaginary/real ratio compared with that for a_. This fact revealed that b_ is a slower process than a_. Current transients in Fig. 5 also support this, because total decay time for b_ (> 130 ms at 0.4 V) is much longer than that for a_ (approximately 90 ms at 0.08 V in the positive step). We found that clear ER signals can be obtained for the potentialdriven phase changes of colorless surfactants at electrified interfaces. However, ER data analysis in the frequency domain to extract kinetic information in this work remained insufficient even to a semi-quantitative level. We tentatively used for the fundamental frequency response both Eq. (8) and the linear response approximation. The former unrealistically ignored intermolecular interactions, and the latter was in doubt in the uses of large DEac values. Nevertheless, we could find the approximate time domain of very slow components in the phase change processes. To see whether ER methods help gain deeper insight or not, we need measurements of the dependencies of incident light polarization, incident angle, and DS concentration, as well as development of SH-ER measurements. We are in preparation to these examinations. 5. Conclusions ER techniques were used to track the potential dependent phase changes of aDS adlayer on a Au(111) electrode surface. We found that the ER voltammograms at fundamental frequencies well correspond to C-E curves. The ER signals obtained were totally of non-faradaic responses. Although the signals included several
and kdes ¼ kdes exp½Ed =kT with Ed ¼ rd ðE E0 Þ
where kads and kdes are adsorption and desorption rate constants, respectively, DS concentration is taken at the surfaces with a 0 0 constant value approximation to be [DS]surf, kads and kdes are the standard rate constants, ra and rd are the coefficients representing the relationships between the electrode potential difference from E0 and the activation energies, Ea and Ed, respectively for adsorption and desorption processes, and E0 is the potential at which adsorption amount is equal to 12Gsat . We further assume that the potential modulation amplitude is small enough and enables us to use the linear response approximation for Eqs. (9) and (10). The ER response as a function of frequency of the present case should be the same as a previously reported case of adsorptiondesorption process at a liquid-liquid interface measured by potential-modulated fluorescence spectroscopy as derived by Nagatani and colleagues [33]. In reference to their formulation, it follows that the trajectory of (DR/R)ER on a complex plane of the present case should be a semicircle. @s m @R The first term of Eq. (6), @s @E G , should also show m G another semi-circle as nearly the same frequency range as the bare electrode does. This has already been confirmed by the experiments at a bare Au(1 1 1) electrode using the data in Fig. 3. Taking a look once again at Fig. 3 from higher to lower frequencies, the onset of the ER signal is similar to the bare electrode. This as well as the complex plane plot in Fig. 6 indicates that the ER signals higher than ca. 80 Hz for a_ and 20 Hz for b_ are mainly due to the first term of Eq. (6).
Fig. 6. Complex plane plots of ER signal frequency dependence data in Fig. 3. (a) Edc = -0.18 V for a_ process, (b) Edc = 0.50 V for b_ process.
10
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origins, the spectral feature was uniquely of the Au electroreflectance type, which relies on the change of surface free electron density on the Au surface. We may conclude that the Au surface electron density change with potential plays a role of a mirror to show up the interfacial processes such as molecular adsorption and assembling. Therefore, it was not necessary to have a chromophore in the interfacial molecular assemblies on the electrode surface to obtain ER signal. ER observation, including the second harmonic ER signal measurements, enabled us to find that adsorptive hemi-micelle formation/desorption at -0.18 V is a faster process than phase change process between interdigitated bilayer and hemi-micellar phase at +0.50 V. The phase change from the interdigitated bilayer to the hemi-micellar phase exhibited a nucleation-hole-growth type nature. This process is slower than the reverse process. Acknowledgements This work was financially supported by Grant-in-aid for Scientific Research from MEXT, Japan (No. 24550158). We thank Mr. Tetsuro Moroka, School of Engineering, Nagasaki University, for his helpful discussion. References [1] D. Bizzotto, V. Zamlynny, I. Burgess, C.A. Jeffrey, H.-Q. Li, J. Rubinstein, R.A. Merril, J. Lipkowski, Z. Galus, A. Nelson, B. Pettinger, Amphiphilic and Ionic Surfactants at Electrified Interfaces, in: A. Wieckowski (Ed.), Interfacial Electrochemistry, Marcel Dekker, New York, 1999, pp. 405–426. [2] M. Chen, I. Burgess, J. Lipkowski, Potential controlled surface aggregation of surfactants at electrode surfaces – A molecular view, Surf. Sci. 603 (2009) 1878–1891. [3] T. Sagara, Dynamic Behaviors of Molecular Assemblies and Nano-Substances at Electrified Interfaces, in: K. Ariga, H. S. Nalwa (Eds.), Bottom-up Nanofabrication: Supramolecules, Self-Assemblies, and Organized Films, Vol. 3 (Self-Assemblies-I), American Scientific Publishers, Valencia, 2009, Chap. 13, pp. 347–373. [4] K. Eda, Structure of Adsorbed Layers at the Interface of Mercury-Surfactant Solution. III. Structure of Adsorbed Layers of Sodium Decyl, Dodecyl, and Tetradecyl Sulfate, Nippon Kagaku Zasshi 80 (1959) 349–352. [5] K. Eda, K. Takahashi, Structure of Adsorbed Layers at the Interface MercurySurfactant Solution. X. Study of the Competitive Adsorption of Two Surfactants by the Method of Differential Double Layer Capacity, Nippon Kagaku Zasshi 85 (1964) 828–832. [6] P. Skoluda, The voltammetric study of the Au(100) electrode in the presence of alkyl sulfates, J. Electroanal. Chem. 406 (1996) 235–238. [7] T. Wandlowski, M. Hromadova, R. de Levie, On the Kinetics of Adsorption of Dodecyl Sulfate at the Mercury-Water Interface, Langmuir 13 (1997) 2766–2772 and references therein. [8] I. Burgess, C.A. Jeffrey, X. Cai, G. Szymanski, Z. Galus, J. Lipkowski, Direct Visualization of the Potential-Controlled Transformation of Hemimicellar Aggregates of Dodecyl Sulfate into a Condensed Monolayer at the Au(111) Electrode Surface, Langmuir 15 (1999) 2607–2616. [9] I. Burgess, V. Zamlynny, G. Szymanski, J. Lipkowski, J. Majewski, G. Smith, S. Satija, R. Ivkov, Electrochemical and Neutron Reflectivity Characterization of Dodecyl Sulfate Adsorption and Aggregation at the Gold-Water Interface, Langmuir 17 (2001) 3355–3367. [10] M. Petri, D.M. Kolb, Nanostructuring of a sodium dodecyl sulfate-covered Au (111) electrode, Phys. Chem. Chem. Phys. 4 (2002) 1211–1216. [11] J.J. Leitch, J. Collins, A.K. Friedrich, U. Stimming, J.R. Dutcher, J. Lipkowski, Infrared Studies of the Potential Controlled Adsorption of Sodium Dodecyl Sulfate at the Au(111) Electrode Surface, Langmuir 28 (2012) 2455–2464.
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