- Email: [email protected]
Electrochemical behavior of clay modified electrodes in the presence of cationic surfactant
343
J. Ekctroanal. Chem., 267 (1989) 343-349 Elsevier Sequoia S.A., Lausanne - Printed in The Netherlands
Preliminary note
Electrochemical behavior of clay modified electrodes presence of cationic surfactant
in the
Brahim Brahimi, Pierre Labbe and Gilbert Reverdy * Laboratoire de Photochimie, Unit4 de recherche: Chimie et Ingdnierie de I’Environnement, Savoie, B.P. 1104, 7301 I Chambkry Cedex (France)
UniversitJ de
(Received 14 June 1989)
INTRODUCTION
The electrochemistry of electrodes modified by coverage with different inorganic materials such as clay [l-8], zeolite [9-121, aluminium oxide [13,14] or hydrotalcite [15] has recently received increased attention. In addition to their great chemical and mechanical stability, the use of inorganic materials for coating electrode surfaces may provide new patterns of reactivity to incorporated reactants. Because of their unique structure, smectic clay colloids have been used to prepare clay modified electrodes (CMEs) [l-8]. These electrodes present a large surface area, a high cation exchange capacity (ccc), swelling and intercalating properties. The primary use of CMEs is as cation-exchange minerals. Cationic metal complexes [l-3] as well as microparticulate catalysts such as Pt [3] and RuO, [2] have been immobilized in these clay films and employed as electrocatalysts. However, the use of these electrodes is limited since anionic or neutral species cannot be absorbed in these clay films [3,6,8]. The use of cationic micelles and CMEs to organize an electrocatalytic reaction has been reported recently by Rusling et al. [16]. They suggest that catalyst and substrate react on the micelle-coated surface of the CME. We present here our own preliminary results concerning the electrochemical behavior of a laponite clay modified electrode when the cationic surfactant cetyltrirnethylammonium (CTA+) is present in the electrolytic solution at the critical micellar concentration (cmc). Cationic surfactants adsorb on clay colloids [17-191 and can form a double layer which reverses the charge of the particles from negative to positive: the first layer results from the binding of the quaternary ammonium ion to the cation-exchange sites of the clay, while the second layer is
* To whom correspondence should be addressed. 0022-0728/89/$03.50
0 1989 Elsevier Sequoia S.A.
344
adsorbed by van der Waals interactions with the hydrophobic first layer surrounding the particles. The clay colloid becomes hydrophobic and can solubilize neutral organic compounds [18,19]. CTA+ adsorbs readily in the same way and forms a positive double layer on the surface of the particles comprising the film of a CME (this electrode will be called CTA+-CME). The build-up of a surfactant-CME can be described well by using the electroactive (ferrocenylmethyl)dodecyldimethylammonium (FDDA+). A single layer of incorporated FDDA+ is electroinactive, whereas a double layer is electroactive (FDDA+-CME). Unlike the initial CME, the formation of a cationic surfactant bilayer confers anion-exchange properties to a CTA+-CME: an anionic compound such as ferricyanide (Fe(CN)i-) is concentrated into the CTA+-CME; in contrast, cations such as ferricinium or methylviologen (MV2’) are not adsorbed by the CTA+-CME. Moreover, because of the existence of a CTA+ double layer, a hydrophobic compound such as ferrocene (Fc) is concentrated into the CTA+-CME. EXPERIMENTAL
Laponite, a synthetic hectorite (monovalent cation exchange capacity, ccc = 0.74 mmol g-‘) obtained from Laporte Industries Ltd., cetyltrimethylammonium bromide and ferrocene from Merck, methylviologen dichloride hydrate and potassium ferricyanide from Aldrich, and sodium sulfate from Fluka were used as supplied. Water was purified as described previously [20]. (Ferrocenylmethyl)dodecyldimethylammonium bromide was synthesized [21]. The electrochemical instrumentation consisted of a PAR model 362 potentiostat and a Sefram model TRP X-Y chart recorder. Cyclic voltammetric experiments were conducted in an undivided thermostated three-electrode cell comprising a glassy carbon electrode (0.17 cm*), a Pt counter electrode and a saturated calomel reference electrode (SCE). The supporting electrolyte was 0.05 M Na,SO,. Electrochemical experiments were carried out at 28” C for CTA+ and 26” C for FDDA+ under a nitrogen atmosphere. Under these conditions, the cmc of CTA+ and FDDA+ is 10e4 M and 6 X lop4 M, respectively. Clay modified electrodes were made by dropping 30 ~1 of a 0.5 g 1-l laponite colloid onto the glassy carbon disk of the working electrode and drying in air. The CME was then allowed to swell in 0.05 M Na,SO, solution for 3 min. Incorporation of CTAf or FDDA+ into the CME was accomplished by soaking the CME in an electrolytic solution containing low4 M CTA+ or 5 X lop4 M FDDA+ for 40 min; the CTA+-CME was then activated by cycling between - 1 and + 0.6 V for 15 min before being transferred into an electrolytic solution containing low4 M CTA+ and the desired amount of electroactive species. The peak current, which is diffusion controlled in Figs. lc and 5c below, was used to calculate the apparent diffusion coefficient for FDDA+ and Fc incorporated in the clay. The number of moles of electroactive species was determined by integrating the area under the oxidation peak at 5 mV s-l and the concentration was calculated assuming a laponite true density of 2.53 (data from Laporte Industries Ltd.).
345 RESULTS AND DISCUSSION
The cyclic voltammogram of 5 X 10m4 M FDDA+ at a glassy carbon electrode (Fig. la) shows a reversible one-electron oxidation step with a half-wave potential (EtJ of +400 mV vs. SCE at a scan rate, u, of 50 mV s-l. E1,2 was determined as described in ref. 21 by the relationship E,,2 = E,, + 29 mV which takes into account the fact that FDDA+ is weakly adsorbed while FDDA2+ is not adsorbed. Figure lb presents the cyclic voltammogram (first scan, 50 mV s-l) of 5 X lop4 M FDDA+ at a CME after 40 min of soaking under open-circuit conditions. The increase of the anodic peak current (I,,) with soaking time and its stabilization after 40 min (Fig. 2) indicate that FDDA+ is progressively incorporated into the CME. The high Ipa value, 136 PA, at the CME compared to the low Ipa value, 13 PA, at the bare glassy carbon electrode demonstrates that FDDA+ is concentrated on the clay surface. The adsorption isotherm at 26 ’ C of FDDA+ on non-colloidal laponite particles in suspension in a 0.05 M Na,SO, aqueous solution shows that the maximum uptake of FDDA+, corresponding to an amount of 3.2 ccc, occurs when the FDDA+ bulk concentration is 5 X 10e4 M. Assuming a similar adsorption process, we conclude that the maximum uptake of FDDA+ in the film electrode is 3.2 ccc (such an electrode will be called FDDA+-CME). We observed that a CME containing a monolayer of FDDA+‘(in a quantity smaller than or equal to the ccc) does not present a cyclic voltammetric response. Only FDDA+ adsorbed in excess of the ccc in a double layer is electroactive. These observations are in agreement with the assumptions of King et al. [8] who demonstrate that cations bound
h
4 5
100..
4(2.
0..
-40..
*
0
0.3
0.6
E IV
vs.SCE
Fig. 1. Cyclic voltammograms of 5 X 10e4 M FDDA+ in 0.05 M Na,S04 at 26OC. (a) At the uncoated electrode; (b) at the FDDA+-CME. Scan rate: 50 mV s-‘. (c) Plot of Zrpaand (d) plot of Zpa/Zw ratio vs. u’/* at the FDDA+-CME. Since FDDA’*+ IS . ejected from the electrode, each point of curves (c) and (d) was obtained after 40 mitt of soaking under open-circuit conditions in order to re-equilibrate the electrode.
346
Fig. 2. Plot of current response vs. soaking time under open-circuit conditions, for a CME in 5 x low4 M FDDA+ +0.05 M Na,SO, at 26” C. Scan rate: 50 mV s-l.
electrostatically to the exchange sites of the clay are rigorously inactive. The electroactivity of clay films arises from cations which are bound at the clay surface in excess of the ccc by an ion pairing mechanism [S] or by van der Waals interactions in an electroactive surfactant double layer. Ipa at a FDDA+-CME is linearly related to the square root of the potential scan rate (20-200 mV s-l), indicating that the anodic current is controlled by a diffusional process within the coating at these scan rates (Fig. lc). The estimated apparent diffusion coefficient of FDDA+ in the film is 9.1 X 10-l’ cm’ s-l. The clay coating stabilizes the lower oxidation state of the redox couple FDDA+/FDDA2+ since E1,2 = 480 mV at the CME as compared to E1,2 - 400 mV at the bare electrode. The oxidized form FDDA2+ is ejected from the CME, as demonstrated by a peak ratio IPa/lpc = 2 (50 mV s-l), i.e. higher than unity, and by the fact that this ratio increases for lower potential scan rates (Fig. Id). This observation can be interpreted by the fact that FDDA2+ is not an amphiphilic compound [21] and presents a greater hydrophilic character than FDDA+ [21]. Consequently, FDDA2+ acts as a cation and is repelled by the positive charges of the cationic bilayer surrounding the clay particles. Soaking a CME in a 5 x 1O-4 M FDDA2+ electrolytic solution does not lead to substantial incorporation of FDDA’ 2+ since a current of only 6.5 PA (50 mV s-l) is observed. This confirms that FDDA2+ does not form bilayered aggregates on the clay surface. The CTA+ adsorption isotherm at 28 o C on a laponite suspension, determined in the presence of 0.05 M Na,SO,, shows that the maximum uptake of CTA+ by laponite, which occurs at a cmc of low4 It4, is 3.2 ccc. Adsorption in excess of the ccc results in the formation of a positive double layer [18,19]. As observed previously with FDDA+, soaking a CME in a 10P4 M CTA+ electrolytic solution also leads to the formation of a CTA+ double layer on the surface of the particles comprising the clay film. The electrochemical behavior of the CTA+-CME is typical of the existence of a CTA+ double layer which renders the clay particles’ surface positive. The CTA+-CME presents a very strong affinity for anionic Fe(CN)zsince under conditions where Fe(CN)iis undetectable at the bare
347
4
/
ZL
3..
b
& a
0..
+
-3..
I -0.2
*
w 0.4
0
E / V VS.SCE
Fig. 3. Cyclic voltammograms of 4~10~~ M Fe(CN)z+10e4 M CTA+ +0.05 M Na,SO, at 28“C. Scan rate: 50 mV s-‘. (a) At the uncoated electrode; (b) at the CTA+-CME. (c) Plot of Ipc vs. u at the CTA+-CME.
electrode (Fig. 3a), an electrochemical response (1, = 2 PA) is observed (Fig. 3b). Wave shapes characteristic of thin film behavior and linear dependence of the Fe(CN)icathodic peak current, I,,, on the sweep rate, u (Fig. 3c), are observed at small u. Direct transfer of the Fe(CN)iexchanged electrode to solutions containing 10e4 CTA+ + 0.05 M Na,SO, causes the peak current to be attenuated only slightly. With continued soaking, the electrode response remains constant, indicating that the electroactive anion is held strongly by the CTA+-CME. In contrast, the
16.
a
0.
16 i
I
w
-1
-0.5
0
E I V vs.SCE
Fig. 4. Cyclic voltammograms of 3 X 10F4 MV 2+10-4 M CTA+ +0.05 M Na,SO, 50 mV s-‘. (a) At the uncoated electrode. (b) at the CTA+-CME.
at 28°C.
Scan rate:
348
5
w -0.2
0
cl4
E I V vs.SCE
Fig. 5. Cyclic voltammograms of 3.5 X lo-’ M Fc+ low4 M CTA+ +O.OS M Na,SO, at 28” C. Scan rate: 50 mV s-r. (a) At the uncoated electrode, (b) at the CTA+-CME. (c) Plot of Zpa vs. u’/~ at the CTA+-CME. Since ferricinium cation is ejected from the electrode, each point of curve (c) was obtained after 40 min of soaking under open-circuit conditions in order to re-equilibrate the electrode.
dication MV* i is not adsorbed significantly by the CTA+-CME, which presents a smaller cyclic voltammetric response than the bare electrode (Fig. 4). MV*+, which would be expected to be repelled by the positive double layer of adsorbed CTA+, penetrates the clay film through channels between the clay particles. When the CTA+-CME is removed from the MV*+ solution and placed in pure electrolyte solution (lOA it4 CTA+ + 0.05 M Na,CO,) and a voltammogram is taken immediately, the MV*+ waves disappear, indicating rapid loss of MV*+ from the film. Neutral Fc is concentrated by the CTA+-CME, which presents a larger response (I,, = 6.4 PA) (Fig. 5b) than the bare electrode (I,, = 1.8 PA) (Fig. 5a). The CTA+ double layer provides the CTA+--CME with hydrophobic adsorption sites; it is then able to concentrate neutral compounds such as ferrocene. The ratio IPa/lpc = 6.4, being greater than unity, shows that cationic ferricinium is ejected from the CTA+-CME. The linear plot of the anodic peak current, I,,, of Fc as a function of the square root of the potential scan rate (20-200 mV s-l) indicates that the anodic peak current is controlled by a diffusional process when ferrocene is incorporated into the coating (Fig. 5~). The apparent diffusion coefficient of ferrocene in the film is 2.9 x lo-” cm* s-l. More detailed results will appear soon in a full paper. REFERENCES 1 P.K. Ghosh and A.J. Bard, J. Am. Chem. Sot., 17 (1983) 5691. 2 P.K. Ghosh, A.W.-H. Mau and A.J. Bard, J. Electroanal. Chem., 169 (1984) 315. 3 D. Ege, P.K. Ghosh, J.R. White, J.-F. Eguey and A.J. Bard, J. Am. Chem. Sot., 107 (1985) 5644.
349 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21
J.R. White and A.J. Bard, J. Electroanal Chem., 197 (1986) 233. M.T. Carter and A.J. Bard, J. Ekxtroanal. Chem., 229 (1987) 191. H.-Y. Liu and F.C. Anson, J. EIectroanaI. Chem., 184 (1985) 411. P.V. Kamat, J. Electroanal Chem., 163 (1984) 389. R.D. King, D.C. Nocera and T.J. Pinnavaia, J. Electroanal. Chem., 236 (1987) 43. C.G. Murray, R.J. Nowak and D.R. Rolison, J. Electroanal. Chem., 164 (1984) 205. H.A. Gemborys and B.R. Shaw, J. Electroanal Chem., 208 (1986) 95. B. De Vismes, F. Bedioui, J. Devynck and C. Bied-Charreton, J. Electroanal Chem., 187 (1985) 197. Z. Li, C. Lai and T.E. MaIlouk, Inorg. Chem., 28 (1989) 178. C.J. Miller, CA. Widrig, D.H. Charych and M. Majda, J. Phys. Chem., 92 (1988) 1928. CA. Goss, C.J. Miller and M. Majda, J. Phys. Chem., 92 (1988) 1937. K. Itaya, H.-C. Chang and I. Uchida, Inorg. Chem., 26 (1987) 624. J.F. Rusling, C.-N. Shi and S.L. Suib, J. EIectroanaI. Chem., 245 (1988) 331. H. Van Olphen, An Introduction to Clay Colloid Chemistry, Wiley Interscience, New York, 1977. R.A. Dellaguardia and J.K. Thomas, J. Phys. Chem., 87 (1983) 3550. T. Nakamura and J.K. Thomas, Langmuir, 3 (1987) 234. P. Labbt and G. Reverdy, Langmuir, 4 (1988) 419. T. Saji, K. Hoshino and S. Ayogui, J. Am. Chem. Sot., 107 (1985) 6865.
J. Ekctroanal. Chem., 267 (1989) 343-349 Elsevier Sequoia S.A., Lausanne - Printed in The Netherlands
Preliminary note
Electrochemical behavior of clay modified electrodes presence of cationic surfactant
in the
Brahim Brahimi, Pierre Labbe and Gilbert Reverdy * Laboratoire de Photochimie, Unit4 de recherche: Chimie et Ingdnierie de I’Environnement, Savoie, B.P. 1104, 7301 I Chambkry Cedex (France)
UniversitJ de
(Received 14 June 1989)
INTRODUCTION
The electrochemistry of electrodes modified by coverage with different inorganic materials such as clay [l-8], zeolite [9-121, aluminium oxide [13,14] or hydrotalcite [15] has recently received increased attention. In addition to their great chemical and mechanical stability, the use of inorganic materials for coating electrode surfaces may provide new patterns of reactivity to incorporated reactants. Because of their unique structure, smectic clay colloids have been used to prepare clay modified electrodes (CMEs) [l-8]. These electrodes present a large surface area, a high cation exchange capacity (ccc), swelling and intercalating properties. The primary use of CMEs is as cation-exchange minerals. Cationic metal complexes [l-3] as well as microparticulate catalysts such as Pt [3] and RuO, [2] have been immobilized in these clay films and employed as electrocatalysts. However, the use of these electrodes is limited since anionic or neutral species cannot be absorbed in these clay films [3,6,8]. The use of cationic micelles and CMEs to organize an electrocatalytic reaction has been reported recently by Rusling et al. [16]. They suggest that catalyst and substrate react on the micelle-coated surface of the CME. We present here our own preliminary results concerning the electrochemical behavior of a laponite clay modified electrode when the cationic surfactant cetyltrirnethylammonium (CTA+) is present in the electrolytic solution at the critical micellar concentration (cmc). Cationic surfactants adsorb on clay colloids [17-191 and can form a double layer which reverses the charge of the particles from negative to positive: the first layer results from the binding of the quaternary ammonium ion to the cation-exchange sites of the clay, while the second layer is
* To whom correspondence should be addressed. 0022-0728/89/$03.50
0 1989 Elsevier Sequoia S.A.
344
adsorbed by van der Waals interactions with the hydrophobic first layer surrounding the particles. The clay colloid becomes hydrophobic and can solubilize neutral organic compounds [18,19]. CTA+ adsorbs readily in the same way and forms a positive double layer on the surface of the particles comprising the film of a CME (this electrode will be called CTA+-CME). The build-up of a surfactant-CME can be described well by using the electroactive (ferrocenylmethyl)dodecyldimethylammonium (FDDA+). A single layer of incorporated FDDA+ is electroinactive, whereas a double layer is electroactive (FDDA+-CME). Unlike the initial CME, the formation of a cationic surfactant bilayer confers anion-exchange properties to a CTA+-CME: an anionic compound such as ferricyanide (Fe(CN)i-) is concentrated into the CTA+-CME; in contrast, cations such as ferricinium or methylviologen (MV2’) are not adsorbed by the CTA+-CME. Moreover, because of the existence of a CTA+ double layer, a hydrophobic compound such as ferrocene (Fc) is concentrated into the CTA+-CME. EXPERIMENTAL
Laponite, a synthetic hectorite (monovalent cation exchange capacity, ccc = 0.74 mmol g-‘) obtained from Laporte Industries Ltd., cetyltrimethylammonium bromide and ferrocene from Merck, methylviologen dichloride hydrate and potassium ferricyanide from Aldrich, and sodium sulfate from Fluka were used as supplied. Water was purified as described previously [20]. (Ferrocenylmethyl)dodecyldimethylammonium bromide was synthesized [21]. The electrochemical instrumentation consisted of a PAR model 362 potentiostat and a Sefram model TRP X-Y chart recorder. Cyclic voltammetric experiments were conducted in an undivided thermostated three-electrode cell comprising a glassy carbon electrode (0.17 cm*), a Pt counter electrode and a saturated calomel reference electrode (SCE). The supporting electrolyte was 0.05 M Na,SO,. Electrochemical experiments were carried out at 28” C for CTA+ and 26” C for FDDA+ under a nitrogen atmosphere. Under these conditions, the cmc of CTA+ and FDDA+ is 10e4 M and 6 X lop4 M, respectively. Clay modified electrodes were made by dropping 30 ~1 of a 0.5 g 1-l laponite colloid onto the glassy carbon disk of the working electrode and drying in air. The CME was then allowed to swell in 0.05 M Na,SO, solution for 3 min. Incorporation of CTAf or FDDA+ into the CME was accomplished by soaking the CME in an electrolytic solution containing low4 M CTA+ or 5 X lop4 M FDDA+ for 40 min; the CTA+-CME was then activated by cycling between - 1 and + 0.6 V for 15 min before being transferred into an electrolytic solution containing low4 M CTA+ and the desired amount of electroactive species. The peak current, which is diffusion controlled in Figs. lc and 5c below, was used to calculate the apparent diffusion coefficient for FDDA+ and Fc incorporated in the clay. The number of moles of electroactive species was determined by integrating the area under the oxidation peak at 5 mV s-l and the concentration was calculated assuming a laponite true density of 2.53 (data from Laporte Industries Ltd.).
345 RESULTS AND DISCUSSION
The cyclic voltammogram of 5 X 10m4 M FDDA+ at a glassy carbon electrode (Fig. la) shows a reversible one-electron oxidation step with a half-wave potential (EtJ of +400 mV vs. SCE at a scan rate, u, of 50 mV s-l. E1,2 was determined as described in ref. 21 by the relationship E,,2 = E,, + 29 mV which takes into account the fact that FDDA+ is weakly adsorbed while FDDA2+ is not adsorbed. Figure lb presents the cyclic voltammogram (first scan, 50 mV s-l) of 5 X lop4 M FDDA+ at a CME after 40 min of soaking under open-circuit conditions. The increase of the anodic peak current (I,,) with soaking time and its stabilization after 40 min (Fig. 2) indicate that FDDA+ is progressively incorporated into the CME. The high Ipa value, 136 PA, at the CME compared to the low Ipa value, 13 PA, at the bare glassy carbon electrode demonstrates that FDDA+ is concentrated on the clay surface. The adsorption isotherm at 26 ’ C of FDDA+ on non-colloidal laponite particles in suspension in a 0.05 M Na,SO, aqueous solution shows that the maximum uptake of FDDA+, corresponding to an amount of 3.2 ccc, occurs when the FDDA+ bulk concentration is 5 X 10e4 M. Assuming a similar adsorption process, we conclude that the maximum uptake of FDDA+ in the film electrode is 3.2 ccc (such an electrode will be called FDDA+-CME). We observed that a CME containing a monolayer of FDDA+‘(in a quantity smaller than or equal to the ccc) does not present a cyclic voltammetric response. Only FDDA+ adsorbed in excess of the ccc in a double layer is electroactive. These observations are in agreement with the assumptions of King et al. [8] who demonstrate that cations bound
h
4 5
100..
4(2.
0..
-40..
*
0
0.3
0.6
E IV
vs.SCE
Fig. 1. Cyclic voltammograms of 5 X 10e4 M FDDA+ in 0.05 M Na,S04 at 26OC. (a) At the uncoated electrode; (b) at the FDDA+-CME. Scan rate: 50 mV s-‘. (c) Plot of Zrpaand (d) plot of Zpa/Zw ratio vs. u’/* at the FDDA+-CME. Since FDDA’*+ IS . ejected from the electrode, each point of curves (c) and (d) was obtained after 40 mitt of soaking under open-circuit conditions in order to re-equilibrate the electrode.
346
Fig. 2. Plot of current response vs. soaking time under open-circuit conditions, for a CME in 5 x low4 M FDDA+ +0.05 M Na,SO, at 26” C. Scan rate: 50 mV s-l.
electrostatically to the exchange sites of the clay are rigorously inactive. The electroactivity of clay films arises from cations which are bound at the clay surface in excess of the ccc by an ion pairing mechanism [S] or by van der Waals interactions in an electroactive surfactant double layer. Ipa at a FDDA+-CME is linearly related to the square root of the potential scan rate (20-200 mV s-l), indicating that the anodic current is controlled by a diffusional process within the coating at these scan rates (Fig. lc). The estimated apparent diffusion coefficient of FDDA+ in the film is 9.1 X 10-l’ cm’ s-l. The clay coating stabilizes the lower oxidation state of the redox couple FDDA+/FDDA2+ since E1,2 = 480 mV at the CME as compared to E1,2 - 400 mV at the bare electrode. The oxidized form FDDA2+ is ejected from the CME, as demonstrated by a peak ratio IPa/lpc = 2 (50 mV s-l), i.e. higher than unity, and by the fact that this ratio increases for lower potential scan rates (Fig. Id). This observation can be interpreted by the fact that FDDA2+ is not an amphiphilic compound [21] and presents a greater hydrophilic character than FDDA+ [21]. Consequently, FDDA2+ acts as a cation and is repelled by the positive charges of the cationic bilayer surrounding the clay particles. Soaking a CME in a 5 x 1O-4 M FDDA2+ electrolytic solution does not lead to substantial incorporation of FDDA’ 2+ since a current of only 6.5 PA (50 mV s-l) is observed. This confirms that FDDA2+ does not form bilayered aggregates on the clay surface. The CTA+ adsorption isotherm at 28 o C on a laponite suspension, determined in the presence of 0.05 M Na,SO,, shows that the maximum uptake of CTA+ by laponite, which occurs at a cmc of low4 It4, is 3.2 ccc. Adsorption in excess of the ccc results in the formation of a positive double layer [18,19]. As observed previously with FDDA+, soaking a CME in a 10P4 M CTA+ electrolytic solution also leads to the formation of a CTA+ double layer on the surface of the particles comprising the clay film. The electrochemical behavior of the CTA+-CME is typical of the existence of a CTA+ double layer which renders the clay particles’ surface positive. The CTA+-CME presents a very strong affinity for anionic Fe(CN)zsince under conditions where Fe(CN)iis undetectable at the bare
347
4
/
ZL
3..
b
& a
0..
+
-3..
I -0.2
*
w 0.4
0
E / V VS.SCE
Fig. 3. Cyclic voltammograms of 4~10~~ M Fe(CN)z+10e4 M CTA+ +0.05 M Na,SO, at 28“C. Scan rate: 50 mV s-‘. (a) At the uncoated electrode; (b) at the CTA+-CME. (c) Plot of Ipc vs. u at the CTA+-CME.
electrode (Fig. 3a), an electrochemical response (1, = 2 PA) is observed (Fig. 3b). Wave shapes characteristic of thin film behavior and linear dependence of the Fe(CN)icathodic peak current, I,,, on the sweep rate, u (Fig. 3c), are observed at small u. Direct transfer of the Fe(CN)iexchanged electrode to solutions containing 10e4 CTA+ + 0.05 M Na,SO, causes the peak current to be attenuated only slightly. With continued soaking, the electrode response remains constant, indicating that the electroactive anion is held strongly by the CTA+-CME. In contrast, the
16.
a
0.
16 i
I
w
-1
-0.5
0
E I V vs.SCE
Fig. 4. Cyclic voltammograms of 3 X 10F4 MV 2+10-4 M CTA+ +0.05 M Na,SO, 50 mV s-‘. (a) At the uncoated electrode. (b) at the CTA+-CME.
at 28°C.
Scan rate:
348
5
w -0.2
0
cl4
E I V vs.SCE
Fig. 5. Cyclic voltammograms of 3.5 X lo-’ M Fc+ low4 M CTA+ +O.OS M Na,SO, at 28” C. Scan rate: 50 mV s-r. (a) At the uncoated electrode, (b) at the CTA+-CME. (c) Plot of Zpa vs. u’/~ at the CTA+-CME. Since ferricinium cation is ejected from the electrode, each point of curve (c) was obtained after 40 min of soaking under open-circuit conditions in order to re-equilibrate the electrode.
dication MV* i is not adsorbed significantly by the CTA+-CME, which presents a smaller cyclic voltammetric response than the bare electrode (Fig. 4). MV*+, which would be expected to be repelled by the positive double layer of adsorbed CTA+, penetrates the clay film through channels between the clay particles. When the CTA+-CME is removed from the MV*+ solution and placed in pure electrolyte solution (lOA it4 CTA+ + 0.05 M Na,CO,) and a voltammogram is taken immediately, the MV*+ waves disappear, indicating rapid loss of MV*+ from the film. Neutral Fc is concentrated by the CTA+-CME, which presents a larger response (I,, = 6.4 PA) (Fig. 5b) than the bare electrode (I,, = 1.8 PA) (Fig. 5a). The CTA+ double layer provides the CTA+--CME with hydrophobic adsorption sites; it is then able to concentrate neutral compounds such as ferrocene. The ratio IPa/lpc = 6.4, being greater than unity, shows that cationic ferricinium is ejected from the CTA+-CME. The linear plot of the anodic peak current, I,,, of Fc as a function of the square root of the potential scan rate (20-200 mV s-l) indicates that the anodic peak current is controlled by a diffusional process when ferrocene is incorporated into the coating (Fig. 5~). The apparent diffusion coefficient of ferrocene in the film is 2.9 x lo-” cm* s-l. More detailed results will appear soon in a full paper. REFERENCES 1 P.K. Ghosh and A.J. Bard, J. Am. Chem. Sot., 17 (1983) 5691. 2 P.K. Ghosh, A.W.-H. Mau and A.J. Bard, J. Electroanal. Chem., 169 (1984) 315. 3 D. Ege, P.K. Ghosh, J.R. White, J.-F. Eguey and A.J. Bard, J. Am. Chem. Sot., 107 (1985) 5644.
349 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21
J.R. White and A.J. Bard, J. Electroanal Chem., 197 (1986) 233. M.T. Carter and A.J. Bard, J. Ekxtroanal. Chem., 229 (1987) 191. H.-Y. Liu and F.C. Anson, J. EIectroanaI. Chem., 184 (1985) 411. P.V. Kamat, J. Electroanal Chem., 163 (1984) 389. R.D. King, D.C. Nocera and T.J. Pinnavaia, J. Electroanal. Chem., 236 (1987) 43. C.G. Murray, R.J. Nowak and D.R. Rolison, J. Electroanal. Chem., 164 (1984) 205. H.A. Gemborys and B.R. Shaw, J. Electroanal Chem., 208 (1986) 95. B. De Vismes, F. Bedioui, J. Devynck and C. Bied-Charreton, J. Electroanal Chem., 187 (1985) 197. Z. Li, C. Lai and T.E. MaIlouk, Inorg. Chem., 28 (1989) 178. C.J. Miller, CA. Widrig, D.H. Charych and M. Majda, J. Phys. Chem., 92 (1988) 1928. CA. Goss, C.J. Miller and M. Majda, J. Phys. Chem., 92 (1988) 1937. K. Itaya, H.-C. Chang and I. Uchida, Inorg. Chem., 26 (1987) 624. J.F. Rusling, C.-N. Shi and S.L. Suib, J. EIectroanaI. Chem., 245 (1988) 331. H. Van Olphen, An Introduction to Clay Colloid Chemistry, Wiley Interscience, New York, 1977. R.A. Dellaguardia and J.K. Thomas, J. Phys. Chem., 87 (1983) 3550. T. Nakamura and J.K. Thomas, Langmuir, 3 (1987) 234. P. Labbt and G. Reverdy, Langmuir, 4 (1988) 419. T. Saji, K. Hoshino and S. Ayogui, J. Am. Chem. Sot., 107 (1985) 6865.