Electrochemical investigation of cetylpyridinium cation micelle adsorption at the Hg | aqueous solution interface

Journal of Electroanalytical Chemistry, 367 (1994) 239-246

239

Electrochemical investigation of cetylpyridinium cation micelle adsorption at the Hg 1aqueous solution interface P. Nikitas, A. Pappa-Louisi

and S. Antoniou

Laboratory of Physical Chemistry, Department of Chemistry, Aristotle University of Thessaloniki, 54006 Thessaloniki (Greece)

(Received 1 June 1993; in revised form 19 July 1993)

Abstract Differential capacitance measurements at a mercury electrode in contact with 0.1 M Na,SO, aqueous solutions containing cetylpyridinium cations (CP+) are carried out to examine the formation of micelles on the electrode surface and their phase transitions in the various polarization regions. Analysing the experimental data by means of theoretical treatments presented in J. Phys. Chem., 96 (1992) 8453 and J. Electroanal. Chem., 348 (1993) 59, it is shown that, below the critical micelle concentration (CMC), a monolayer film of surface micelles is formed at positive potentials, in the region from about +O.lO to 0 V with respect to the saturated calomel electrode. At concentrations above the CMC and up to 1 X 1O-3 M, the micellar film is formed in the region between + 0.20 and + 0.05 V, and extends to three dimensions, forming a bilayer on the mercury surface. Finally, at concentrations above 1 x 10e3 M, the formation of a polylayer micellar film is observed. This film, either monolayer or polylayer, covers the electrode surface up to -0.95 V, where it is destroyed as a result of the onset of the electrochemical reduction of CP+. Thus, the micellar film of CP+ cations is transfgrmed into a film of uncharged monomer species of the reduction product. This film is not particularly stable and, in the region between - 0.95 and - 1.4 V, it collapses to a compact layer. At potentials more negative than about - 1.4 V, the compact layer is destroyed and the reduction product is desorbed from the electrode. The substance, in the form of CP+, is also desorbed at potentials more positive than +0.2 V, except at concentrations above 1 X lo-* M, where a compact, possibly polylayer is formed in this polarization region.

1. Introduction Cetylpyridinium chloride (CPC) is well known as a strongly adsorbable cationic surfactant and it has been used repeatedly as a suppressor of the polarographic maxima which appear during the reduction process of several compounds [l-3]. Recently, CPC has also been used in studies of electrosorption from pseudobinary surfactant mixtures [4,5]. However, it is surprising that these communications did not take into account the fact that CPC undergoes an electrochemical reduction, although the reduction of N-alkylpyridinium cations has already been mentioned in the literature [6-g]. As far as we know, no systematic study of the electrochemical behaviour of CPC on a mercury electrode has been reported yet, although there are spectroelectrochemical investigations of cetylpyridinium cations (CP’> adsorption on glassy carbon [lo] and silver electrodes [ill. It should be noted that the electrosorption of surfactants forming a micellar medium, such as CPC, is extremely important in a variety of fields. In particular, micellar surfactants are used in 0022-0728/94/$7.00 SSDZ 0022-0728(93)03044-P

electrocatalysis [12-161 and electroanalysis [171 not only for their ability to solubilize non-polar organic compounds in aqueous media within the micelles [18-201 but also for their effect on the electrochemical behaviour of organic molecules, since micellar surfactants as adsorbed materials constitute modified electrodes [21-241. However, research advances in these fields are closely related to a better understanding of the interaction between the surfactant and the electrode surface, as well as the structure of the adsorbed species. For these reasons, we decided to investigate the adsorption of CPC at a mercury electrode, by means of differential capacitance measurements, as an extension of our interest in the study of the interfacial properties of micellar surfactants [25-301. 2. Experimental Differential capacitance measurements and cyclic voltamrnograms were obtained using a hanging mercury drop electrode (HMDE; Metrohm with a surface 0 1994 - Elsevier Sequoia. All rights reserved

240

P. Nikitas et al. / CP + micelle adsorption on Hg electrode

area of 0.026 cm2>. A detailed account of the electrochemical system used for capacitance measurements has appeared elsewhere [31]. The frequency at which the capacitances were measured was 120 Hz. Preliminary experiments using signals of 60 and 80 Hz did not show any appreciable effect of the frequency on the shape of the capacitance curves. The electrochemical instruments were interfaced to an AT-compatible computer (AS1 286) via the interface card of the lock-in amplifier and a 1Cbit AD-DA card. The data acquisition software enabled complete control of the lock-in amplifier functions. The polarographic measurements were carried out using a dropping mercury electrode (DME) with a flow rate of 1.2 mg s-l and drop time of 3 s. All the potentials are related to a saturated calomel electrode (SCE) and the electrochemical measurements were obtained at 25 f O.l”C, keeping an inert atmosphere in the cell. As a supporting electrolyte, a 0.1 M Na,SO, solution was chosen. The Na,SO, and CPC were commercial products of analytical grade and, for this reason, they were used without further purification. The adsorption properties of CPC were studied in a wide concentration range from the premicellar to postmicellar region. The critical micelle concentration (CMC) value of CPC in 0.1 M Na,SO, aqueous solutions was determined by surface measurements with the ring method, using a Du Notiy Model Cambridge Tensiometer at 25 & 0.2”C, and it was found to be equal to 3.9 x 10e5 M. To investigate the adsorption of CPC at positive potentials, we had to transform this chloride salt into the corresponding sulphate salt. Details of the procedure for this transformation of CPC are described in ref. 30. 3. Results and discussion Pyridinium ions are generally polarographically reducible and their cathodic reduction is of increasing interest, because these anions are model substances for biologically important compounds. In several publications, the following cathodic coupling of N-pyridinium salts has been reported [6-91:

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Fig. 1. Differential capacitance vs. applied potential curves () recorded at an HMDE with a potential scan rate of 2 mV s-l, in contact with aqueous 0.1 M Na,SO, solutions of CPC at the following concentrations (in mol dmm3): (1) 0; (2) 1 x 10m5; (3) 2X 10m5; (4) 5 x lo-‘; (5) 5 x 10W4;(6) 5 x 10e3. Curves (-.-.and - - -) show cyclic voltammograms of 1 x 10W3M CPC recorded at 50 mV s-‘.

Thus, the electrochemical redox pattern of these salts involves an initial cathodic one-electron step, followed by a rapid dimerization of the pyridinyl radical. Whereas the cathodic reaction of the lower homologues of N-alkylpyridinium cations seems to be fairly clear, the electrochemistry of pyridinium cations containing long-chain alkyl groups, such as CP+, remains unclarified on many points. This might occur because the reaction pathway of the cathodic reaction of CP+ in aqueous solutions is strongly affected by adsorption, which leads to complicated cyclic voltammograms. In fact, the cyclic voltammogram obtained for a 1 X 10d3 M CPC solution using an HMDE (Fig. 1) shows a total of two reduction peaks and no oxidation peak in the potential region scanned, which indicates the non-reversibility of the electrode process. Moreover, during the reverse scan, instead of an oxidation peak, the current gives a negative peak at the same potential where the second reduction peak appears, i.e. at a potential around - 1.25 V. It should be noted that the shape of that reduction peak is rather unusual, with a steep negative side. This anomalous voltametric response will be discussed in more detail below. Here, we only note that it is caused by the surface activity of a product being formed in the course of the electrode reduction, as can be deduced from the shape of the capacitance C vs. potential E curves, obtained for various concentrations of CPC in that polarization region (Fig. 1). Thus, in the range of potentials from -0.95 to - 1.4 V, even at dilute concentrations of CPC, the capacitance forms a pit on the C vs. E curves. With increasing CPC concentration, the depth of the capacitance pit remains invariant but its width grows. The value of the capacitance within the pit is extremely low-in fact, lower

P. Nikitas et al. / CP

0.05

0.0

0.5

’ micelle adsorption on Hg electrode

1.0

t/min Fig. 2. Capacitance transients at an HMDE in contact with an aqueous 0.1 M Na$O, + 2 X lo-’ M CPC solution at the following electrode potentials (in V vs. SCE): (1) -0.2; (2) -0.3; (3) -0.5; (4) -0.7; (5) -0.8.

than 0.01 F rnm2-indicating a condensation of the reduction product in the double layer. The negative boundary of the pit region can be observed at potentials of - 1.3 V or more negative, beyond which the C vs. E curves for solutions of CPC almost coincide with the background electrolyte curve. It is worth noting that, at the desorption potential, the capacitance changes abruptly without showing any appreciable peaks, owing to the two-dimensional condensation of the adsorbate within the pit region [25,32]. It also can be seen from Fig. 1 that there is a steady increase in the recorded double-layer capacitance with increasing CPC concentration in the potential region between -0.2 V and the onset of the faradaic reduction (about -0.95 V>. The origin of this adsorption effect, which has been observed in our previous studies on micellar surfactants [33,34], has been attributed to surface aggregation. A further indication supporting the fact that CPC aggregates on the electrode to form micelles is the appearance of a characteristic well in the capacitance transients obtained for a premicellar concentration of CPC at various potentials in the range from -0.2 to - 0.8 V (Fig. 2). In our earlier papers [29,30,33,34], this finding was interpreted by assuming an interfacial micellization process which occurs via initial adsorption of monomers. At this stage, it may be pointed out that the appearance of a well in the capacitance transients and/or the increase in capacitance with increasing surfactant concentration observed with the C vs. E curves only indicate accumulation in the interface. However, previous experience with micelle-forming surfactants suggests that the above indications could be characterized as reliable empirical criteria for surface micellization.

241

We have recently shown [25,26] that a rigorous criterion for micellization within the adsorption layer is the appearance of a deformed and/or a split capacitance peak in the C vs. E curves. In addition, the shape of this peak, which commonly depends upon the potential scan rate and direction, characterizes the extension of the aggregation phenomena along the interface. Under normal circumstances, once micellization takes place, a micellar film will cover the electrode surface and will remain on it until another deformed and/or split demicellization capacitance peak appears on the recorded C vs. E curves at some negative potentials. In other words, a micellar film is present at the electrode I solution interface at polarizations between two capacitance peaks, with features already discussed above and in refs. 25 and 26. However, in our system, such deformed capacitance peaks cannot be detected at negative potentials, as a result of the onset of the reduction process involving CPC. Thus, in searching for a deformed peak which would indicate micellization across the interface, we have to inspect the adsorption of CP+ at positive potentials. For this reason, we extend our study to differential capacitance measurements of cetylpyridinium sulphate, i.e. (CP),SO,, in the presence of 0.1 M Na,SO,. In the following sections, we will present extensively the features of the observed double-layer properties of CP+ in the polarization region before and beyond the onset of the electrochemical reduction of the surfactant under study. 3.1. Adsorption of cetylpyridinium cations in the nonfaradaic region An overall picture of the adsorption behaviour of CP+ in the potential range 0.3 2 E 2 -0.3 V is given in Fig. 3, which depicts C vs. E curves for various concentrations of CP+ below and above the CMC. It can be seen that, up to a concentration of 2 x 10m5 M, a single wide capacitance peak is recorded at about 0 V, which is shifted to more positive potentials with increasing concentration of surfactant. At a concentration immediately above the CMC, i.e. 5 X 10e5 M, a split peak is developed in the potential region between about 0.2 and 0.05 V (Fig. 3(a)). This split peak remains almost unchanged-with the exception that the height of its left-hand side increases as the concentration rises-for a range of concentrations above the CMC, and changes only at high surfactant concentrations. Thus, at CP+ concentrations above 5 x lop4 M, this peak becomes enormous and extremely deformed (Fig. 3(b)). With fu r th er increases in the CP+ concentration, the left-hand side of this deformed peak decreases abruptly, giving rise to a very sharply defined capacitance pit over the potential range 0.3-0.2 V with

P. Nikitas et al. / CP ’ mice& adsorption on Hg electrode

0.6

I

I

I

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0.0

I

0.3

016

0.9

-E/V Fig. 4. Differential capacitance vs. applied potential cutves recorded at an HMDE in contact with aqueous solutions of 0.1 M Na,SO, +5 x 10e6 M CP+, using potential scan rates of 1 mV s-l and (inset) 10 mV s-l.

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3

4 2

1

-0.3 (b)

-0.1

0.1

0.3

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Fig. 3. Differential capacitance vs. applied potential curves recorded at an HMDE with a potential scan rate of 2 mV s-t, in contact with aqueous 0.1 M Na,SO, solutions of CP+ at the following concentrations (in mol drn-?z (a) (1) 0; (2) 5 x 10e6; (3) 2 x 10e5; (4) 5 x 10W5; (5) 1x1O-4; (6) 5~10-~; (b) (1) 0; (2) 1~10-~; (3) 5~10-~; (4) 1x10-‘; (5) 2x10-‘; (6) 5x10-‘.

a constant saturation capacity, C < 0.02 F m-*. It is probable that, in the region where the pit occurs, a very compact film is formed. The above observations and, in particular, the fact that at all CP+ concentrations the C vs. E curves do show deformed and/or split peaks, are further evidence supporting our contention that micellization occurs in the interface. However, to inspect better the micellization of CP+ within the adsorption layer, a more detailed examination of these peaks is necessary. Thus, the single capacitance peak recorded at a premicellar concentration depends strongly upon the scan rate and direction, as is depicted in Fig. 4, which gives a detailed picture of the formation of this peak in the case of 5 x 10m6 M CP+, when different scan rates are used. According to our theory of surface micellization [25,26], this type of behaviour of a single capacitance peak is associated with the build-up of aggregates in the interface from monomers, resulting in at

least a monolayer micellar film. This film should extend from the region of the micellization capacitance peak towards negative potentials. With regard to the formation of the characteristic split peak developed over a concentration range from the CMC up to 5 x 10e4 M CP+, it seems to be accompanied by some hysteresis phenomena, which are virtually independent of the scan rate (Fig. 5). It is presumed that, at these concentrations and in the region of the split capacitance peak, a micellar film is formed which, according to the predictions of the theory [25], may extend up to at least two successive layers along the interface. However, at high concentrations of CP+, i.e. above 1 x lo-* M, the split of the deformed peak as well as its strong dependence upon the scan rate and scan direction (Fig. 6) indicates the formation of a multi-

-0.3

0.0

0.3

0.6

0.9

-E/V Fig. 5. Differential capacitance vs. applied potential curves recorded at an HMDE in contact with aqueous solutions of 0.1 M Na,S04 +5 x 10e4 M CP+, using potential scan rates of 1 mV s-l and (inset) 10 mV s-l.

P. Nikitas et al. / CP ’ micelle adsorption on Hg electrode

243

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layer of micelles at potentials more negative than that peak. For the pit that occurs at a potential positive of the deformed peak, according to the theory presented in ref. 25, the adsorption layer in that pit region may result from a multilayer stacking of monomers on the electrode surface. If this is the case, then this deformed peak should separate two different types of adsorbed polylayers: one existing at potentials positive of the peak, and consisting of monomers, while the other is formed by micelles at potentials negative of that micellization peak. The time dependence of the capacitance at positive potentials indicates the formation of a polylayer, even in the case of 1 X lo-’ M CP+ (curves 1 and 2 in Fig. 7). The shape of these capacitance transients reveals that, at potentials more positive than 0.22 V, the adsorption equilibrium between the electrode and surfac-

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Fig. 8. Differential capacitance vs. applied potential curves recorded at an HMDE with a potential scan rate of 2 mV s-l, in contact with aqueous 0.1 M Na2S0, solutions of CPf at the following concentrations (in mol dmT3): 1 X lo-‘; 1 X 10-4; 1 X 10e3; 1 X lo-’ (from bottom to top).

tant is established in a time of 0.4-l min. This is why the above solution did not exhibit any capacitance pit on the C vs. E curves depicted in Fig. 3. Figure 8 clearly shows that the increase in CP+ concentration causes a significant increase in the capacitance values obtained in the polarization region between 0 and -0.8 V. This finding is similar to that observed in previous studies of micellar surfactants and has been tentatively interpreted in terms of a stacking association of the surfactant in the interface [33,34]. It should be noted that the minimum value of the differential capacitance at this region is around 0.1 F m-‘, which is in complete agreement with previous results for other cationic surfactants [29,30,33,34]. 3.2. Adsorption of cetylpyridinium cations in the faradaic region As the potential

2

0.4 -E/V

Fig. 6. Differential capacitance vs. applied potential curves recorded at an HMDE in contact with aqueous solutions of 0.1 M Na,SO, + 2 X 10e2 M CP+, using potential scan rates of 1 mV s-* and (inset) 10 mV s-t.

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0.0

-- -_

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---___a

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Fig. 7. Capacitance transients at an HMDE in contact with an aqueous 0.1 M Na,SO, + 1 X 10e2 M CP+ solution at the following electrode potentials (in V vs. SCE): (1) 0.26; (2) 0.24; (3) 0.22; (4) 0.20 (5) 0.16; (6) 0.10; (7) 0.0.

is shifted towards more negative values, i.e. negative of -0.95 V, the micellar film on the electrode surface is destroyed by the onset of the electrochemical reduction of CP +. Figure 9 shows cyclic voltammograms of 1 X lop3 M CP++ 0.1 M Na,SO, at an HMDE and the corresponding polarographic curve. Similar plots are obtained for all the concentrations examined above 5 x 10m5 M CP+. It is seen that CP+ is electrochemically reduced at about -0.95 V WE). However, its reduction is strongly affected by adsorption effects. Thus, the shape of the polarographic curves is deformed by the existence of two maxima. The first maximum is located in the region from - 1.05 to - 1.3 V and it is characterized by a very abrupt negative side. This maximum has been observed already and discussed by Mairanovskii 1351,who attributed it to the formation of an insoluble

244

P. Nikitas et al. / CP ’ micelle adsorption on Hg electrode

reduction product which covers the mercury drop nonuniformly. The second maximum appears at potentials negative of - 1.6 V and is probably related to the further reduction of this substance, as suggested in the literature [7,8]. The cyclic voltammograms exhibit two peaks. The first peak corresponds to the reduction of CP+ at -0.95 V. The second peak appears only when the potential scan rate is higher than 10 mV s- ‘. If we compare the polarographic curves with the cyclic voltammograms, we readily conclude that the second peak is closely related to the first polarographic maximum. In particular, it is totally deformed with an abrupt side located at the same potential as the corresponding side of the polarographic maximum. The shapes of the voltammetric curves recorded upon scan reversal depend on the potential at which the reverse scan starts. Thus, if the reverse scan starts at a potential beyond - 1.3 V, then no oxidation peak is detected (Fig. 9(a)>. In contrast, an oxidation peak is observed when the reverse scan starts immediately after the first reduction peak (Fig. s(b)>. In all cases, the electrochemical reduction of CP+ is an irreversible process. The value E1,2= -0.95V detected for CP+ reduction is considerably different from the value E,,,=

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I a7

I 0.9

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Fig. 9. Cyclic voltammograms and (inset) polarogram at an HMDE for the reduction of 1 x 10W3 M CP+ from 0.1 M Na,SO, aqueous solutions. Potential scan rates (in mV s-t): (1) 10; (2) 20; (3) 50; (4) 100.

01

0

I

I

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10 t/s

Fig. 10. Current (I) vs. time

(t) transients at an HMDE and (inset) a

DME for the reduction of 1~10~~ M CP+ from 0.1 M Na*SO, aqueous solutions at the following electrode potentials (in V vs. scE): (HMDE) (1) - 1.10; (2) -1.20; (3) -1.22; (4) - 1.25 (5) - 1.28; (6) - 1.32; (DME) (1) -0.95; (2) - 1.00; (3) - 1.05; (4) - 1.25; (5) - 1.28.

- 1.3 V (SCE) recorded for the electrochemical reduction of the lower homologues of this series [6,7,36]. This difference may reflect the strong adsorption of the reduction product. However, the first peak of the cyclic voltammograms does not vary linearly with the potential scan rate 1371. Moreover, it does not increase linearly with the square root of the scan rate, indicating a rather complex effect of the adsorption on the reduction of this substance. The complex effect of adsorption also may be responsible for current and capacity oscillations. In particular, at potential scan rates around 20 mV s-l, oscillations are observed during the formation of the second voltammetric peak (Fig. 9(a)). Current oscillations are also observed when current vs. time transients are recorded either at an HMDE or at a single drop of the DME (Fig. 10). The frequency of the oscillations depends upon the CP+ concentration and the applied potential. At low concentrations and at potentials which correspond to the left-hand side of the polarographic maximum, the frequency is small (Fig. 11). The faradaic current oscillations result in oscillations of the capacitive current and, therefore, the recorded capacitance oscillations following the current oscillations. As the concentration increases and/or at potentials close to the abrupt side of the polarographic maximum, the frequency increases considerably (Fig. 11(a), curve 4). These oscillations are not depicted in the capacitance transients (see curve 4 of Fig. 11(b)), owing to instru-

P. Nikitas et al. / CP ’ micelle adsorption on Hg electrode

mental limitations-the lock-in amplifier we used for the capacitance measurements cannot respond to changes in C with high frequency. It is worth noting that, in all cases, the oscillations last until the formation of the capacitance pit (Fig. 11(b)). At concentrations above 1 X 10e4 M, the capacitance pit is formed when the potential varies with a scan rate up to 10 mV s-l (Fig. 12). However, we have already shown that, at such low scan rates, the second voltammetric peak is not observed. Thus, we can conclude that the formation of the capacitance pit destroys not only the oscillations but also the second voltammetric peak; therefore, these two phenomena may have a common origin. It should be noted that, when the potential scan rate is higher than 10 mV s-l, instead of the pit, a characteristic peak is recorded in the plot of C vs. E. The location and shape of this non-equilibrium capacitance peak closely resembles the second peak which appeared in the cyclic voltammograms. At this point, we should mention the work of Miiller et al. [4]. These authors studied the adsorption of CPC

245

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-

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k

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0.2 1

0.0

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1.2 1.6 -E/V Fig. 12. Differential capacitance vs. applied potential curves recorded at an HMDE in contact with aqueous solutions of 0.1 M Na,SO, + 1 X 1O-3 M CP+, using potential scan rates of (a) 50 and (b) 10 mV s-1.

(b)

d G 0.1

---_____________ O0

0.5

1

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2

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0.2

0.1

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(b)

0.5

1

1.5

2

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Fig. 11. (a) Current and (b) capacitance transients at an HMDE in contact with 0.1 M Na,SO, + 1 X 10m3 M CP+ aqueous solutions at the following electrode potentials (in V vs. SCE): (1) - 1.16; (2) - 1.20; (3) - 1.22; (4) - 1.26.

on a DME, where the time’to achieve equilibrium was t = 2.5 s from 0.2 M NaOH aqueous solutions, and recorded two capacitance peaks at negative potentials. They interpreted these peaks in terms of adsorption and desorption maxima. However, the present results, despite the different supporting electrolyte, cannot support this interpretation, since these peaks should be related to the reduction of CPC. The strong adsorption effects discussed above did not allow the determination of the number of electrons required for the reduction of CPf and, consequently, did not allow the verification of reaction (1) proposed for the lower homologues of N-alkyl pyridinium cations. However, we can conclude that the reduction product must be an uncharged species, similar to the dimer in reaction (1). This conclusion arises from the sharp decrease in the differential capacitance within the pit region, which indicates the adsorption of a neutral organic compound with a relatively high molar mass. Moreover, the abrupt vertical steps in capacitance at the boundaries of the pit and the hysteresis recorded

246

P. Nikitas et al. / CP + micelle adsorption on Hg electrode

show that the reduction product does not follow a simple adsorption process but, after an initial stage of normal adsorption, the adsorbed layer undergoes a phase transition which leads to the formation of a very compact layer on the electrode surface. Therefore, at potentials negative of -0.95 V, both the composition and the state of the adsorbed layer change abruptly. The micellar film of CP+ cations on the electrode surface is transformed into a film of uncharged monomer species, possibly the dimers of reaction (1). This film is not particular stable and collapses to a compact layer when the potential varies with a scan rate of 10 mV s-l or less. At potentials more negative than about - 1.4 V, the compact layer is destroyed and the reduction product is desorbed from the electrode. For this reason, the capacitance curves coincide with that of the supporting electrolyte. It should be noted that reverse scans in capacitance reveal a depression of the differential capacitance at potentials more positive than - 1.2 V, suggesting that the reduction product remains on the electrode surface throughout the polarization region positive of - 1.2 V (Fig. 12).

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