Langmuir 1996,11, 3243-3250
3243
Barrier Properties of a Thin Film of 4-Pentadecylpyridine Coated at Au(111) Electrode Surface D. Bizzotto, A. McAlees, J. Lipkowski,* and R. McCrindle Guelph Waterloo Centre for Graduate Work in Chemistry, Department of Chemistry and Biochemistry, University of Guelph, Guelph, Ontario N1G 2W1, Canada Received February 7, 1995. I n Final Form: May 11, 1995@ The reduction of Fe(CN)63- at the Au(ll1) electrode coated with a thin film of 4-pentadecylpyridine ((215-4-Py)was investigated. The C15-4-Pymolecules form a multilayer at theAu(ll1)surface which may be repeatedly adsorbed and desorbed by varying the electrode potential. At E > -0.2 V (SCE) the film has a very compact structure. Its capacit is lower than 1pF cm-2 and it exhibits strong blocking behavior wifh respect to the reduction of Fe(CN)$ . At potentials -0.6 V (SCE) E -0.3 V (SCE) the film is less rigid. Its capacity increases to about 2 pF cm-2 and it is more porous. The rate of Fe(CN)& reduction at the film covered electrode is characteristic of nonlinear diffusion to active pores separated by inactive areas. The average pore size changes between 10 pm and about 30 pm and the pore separation varies between 200 and 500 pm for -0.6 V (SCE) E -0.3 V (SCE). In this range the pore density changes from 100 to 6 x lo3 poreslcm2.
Introduction The objective of this work was to describe the barrier properties of a thin film of 4-pentadecylpyridine (C15-4Py)deposited a t the Au(ll1) electrode surface by the socalled horizontal touching technique. We have recently demonstrated t h a t a film of a n insoluble surfactant can be transferred to a n electrified gold/solution (metal/ solution, M/S) i n t e r f a ~ e l -using ~ this technique. The insoluble surfactants can be deposited at M/S from the gaslsolution (GIs)interface with the transfer ratio ofunity at potentials close to the potential of zero charge (pzc). When a negative voltage scan is applied to the electrode initially covered by insoluble surfactant, the film may be desorbed from the electrode surface at sufficiently negative polarization and later readsorbed when the direction of the voltage scan is reversed. The adsorbed monolayer decreases the capacity of the interface significantly and hence the adsorption and desorption of surfactant may be conveniently investigated by recording either cyclic voltammograms (CV) and/or differential capacity curves. Several properties of the organic monolayers deposited a t the electrode surface by the horizontal touching technique make them an attractive alternative to the films produced by the self-assembly t e ~ h n i q u e .Namely, ~ the physical state of the monolayer and its film pressure may be controlled and the initial conditions of the film in an experiment may be easily restored due to its repeatable adsorptioddesorption behavior. Thin organic films may find application in fabrication of sensors, biosensors, and chemically modified electrodes and may be used as protective layers impervious to deleterious species in the environment (e.g., corrosion inhibitor^).^ These applications make use of the barrier properties of organic films which stem from their ability to prevent ionic or electronic transfer between the metal and the electrolyte solution. Investigations of barrier Abstract published in Advance A C S Abstracts, July 1, 1995. (1)Noel, J.;Bizzotto, D.; Lipkowski, J . J. Electroanal. Chem. 1993, 344,343-354. (2)Bizzotto, D.;Noel, J.;Lipkowski, J. Thin Solid Films 1993,248, 69-77. (3)Bizzotto, D.; Noel, J.J.; Lipkowski, J.J. Electroanal. Chem. 1994, 369,259-265. (4)Swalen, J.; Allara, D.; Andrade, D.; Chandross, E. A.; Garoff, S.; Isrealachvili, J.; McCarthy, T. J.; Murray, R.; Pease, R. F.; Rabolt, J. F.; Wynne, K. J.; Yu, H. Langmuir 1987,3,932. @
properties of a film may also be used to characterize its quality. Specifically, they may provide information about the absence or presence of defects and about the defect size and density. The mechanisms of electron and ion transfer through organic films were described in a recent review.5 The first descriptions of nonlinear mass transport toward a n electrode partially covered by a n organic film were given for rotating disk electrode,6 for chronopotent i ~ m e t r y for , ~ c h r o n o a m p e r ~ m e t r y for , ~ ~chronopotenti~ ometry and chronoamperometry,lOJ1 for cyclic voltammetry,12J3and for impedance spedroscopy.14J5 The theory for cyclic voltammetry12J3 and ac impedance14 was then employed by Sabatini and Rubinstein,16J7 Rubinstein et al.,18-21Finklea et al.,22-24and Millel.25-27to describe redox reactions at gold electrodes covered by self-assembled (5) Lipkowski, J. Ion and Electron Transfer across Monolayers of Organic Surfactants. In Modern Aspects of Electrochemistry; Conway, B. E., Bockris, J . O'M, White, R. E., Eds.; Plenum Press: New York, 1992:Vol. 23. DD 1-99. (6jScheller,F.;Muller, S.;Landsberg, R.; Spitzer, H.J. J.Electroanal. Chem. 1968,19,187-198. (7)Lindemann, J.;Landsberg, R. J. Electroanal. Chem. 1971,29, 261-268. (8)Lindemann, J.; Landsberg, R. J. ElectroanaL Chem. 1971,30, 79-85. (9)Guidelli, R.; Piccardi, G. Anal. Chem. 1971,43,1639. (10)Lindemann, J.;Landsberg, R. J. Electroanal. Chem. Interfacial Electrochem. 1971,31,107-111. (11)Gueshi, T.; Tokuda, K.; Matsuda, H.J. Electroanal. Chem. 1978, 89,247-260. (12)Gueshi, T.; Tokuda, K.; Matsuda, H. J. Electroanal. Chem. 1979, 101,29-38. (13)Amatore, C.; Saveant, J.; Tessier, D. J. Electroanal. Chem. 1983, 147,39-51. (14)Tokuda, K.;Gueshi, T.; Matsuda, H. J. Electroanal. Chem. 1979, 102,41-48. (15)Jehring, H.; Retter, U.; Horn, E. J. Electroanal. Chem. 1983, 149. 153-166. (16)Sabatani, E.; Rubinstein, I.; Maoz, R.; Sagiv, J. J. Electroanal. Chem. 1987,219,365-371. (17)Sabatani, E.; Rubinstein, I. J. Phrs. Chem. 1987,91,66636669. (18)Rubinstein, I.;Steinberg, S.;Tor,Y.;Shanzer, A.;Sagiv, J.Nature 1988,332,426-429. (19)Rubinstein,I.;Steinberg,S.;Tor,Y.;Shanzer,A.;Sagiv, J.Nature 1989,337,216-217, (20)Steinberg, S.; Tor, Y.; Sabatani, E.; Rubinstein, I. J . Am. Chem. SOC.1991,113,5176-5182. (21)Steinberg, S.;Rubinstein, I. Langmuir 1992,8,1183-1187. (22)Finklea, H. 0.;Avery, S.;Lynch, M.; Furtsch, T. Langmuir 1987, 3,409. (23)Finklea, H. 0.;Snider, D. A.; Fedyk, J. Langmuir 1990,6,371. (24)Finklea, H.0.;Snider, D. A.; Fedyk, J.;Sabatani, E.; Gafni, Y.; Rubinstein, I. Langmuir 1993,9,3660-3667. ~
0743-746319512411-3243$09.0010 0 1995 American Chemical Society
3244 Langmuir, Vol. 11, No. 8, 1995 alkanethiol monolayers. Blocking of Mo(CN)S~-'~redox reaction by a monolayer of octadecanoic acid on a HPOG electrode was characterized by Uchida.28 Monolayers with molecular sized channels were designed by Bilewicz and MajdaZ9and employed as microarray electrodes to study very fast electron transfer reactions. The influence of the physical state of the self-assembled monolayers on the rate of Fe(CN)e3- redox reaction was recently described by Lenno~.~O In this paper we will describe the influence of C15-4-Py molecules deposited at the Au(ll1) surface on the rate of Fe(CN)63- reduction. We will show that, depending on the electrode potential, the film either blocks completely or slows down significantly the rate of the redox reaction. We will demonstrate t h a t the faradaic current a t the filmcovered electrode, if measurable, is controlled by nonlinear mass transport of Fe(CN)63- ions to active pores in the film. We will utilize the model developed by Gueshi et al.l1JZJ4and Amatore et al.13 which treats the electrode as a series of active pores surrounded by a n inactive (blocked) surface (microelectrode array) to describe the rate of the faradaic reaction. The average pore size and density of pores will be determined from chronoamperometric transients using the theory developed by Gueshi et al.
Experimental Section Materials. TheAu(ll1)single crystal workingelectrode (WE) was a gold bead with a radius of 2 mm attached to a stem made of a gold wire (diameter 1 mm). The bead was cut in half to expose the (111)surface (surface area 0.1130 cm2). The electrode was prepared by flame annealing before experiment as described e l ~ e w h e r e .The ~ ~ annealing involved heating the crystal in a flame of a Bunsen burner at a temperature corresponding to the appearance of a dark red glow for a period of about 1min. The crystal was then cooled slowly in argon to the ambient temperature. The counter electrode (CE) was a gold coil cleaned by flame annealing before use. The reference electrode (RE) was a saturated calomel electrode (SCE)isolated from the electrolyte solution by a salt bridge. All water used was purified in a Millipore system (> 18MS2.cm). The supporting electrolyte was 0.05 M KC104, purified as described previ~usly.~~ The pH of the electrolyte (8.1) was controlled by the addition of 1 mM KHco3. The supporting electrolyte was deaerated with A r which had passed through an activated charcoal purifier (Supelco). The surfactant solution was prepared by dissolving (C15-4-Py)in chloroform (Spectrograde Fisher) to a concentration of approximately 1mg/mL. All measurements were conducted at a temperature of 20 f 1C". Preparation of 4-Pentadecylpyridine. The method used was analogous to that described in ref 32 for the synthesis of related alkylpyridine derivatives. A suspension of lithium diisopropylamide (0.16 mol) was prepared by the addition of n-butyllithium in hexane (100mL, 0.16mol) to diisopropylanime (16.2 g, 0.16 mol) in dry diethyl ether (100 mL) at 0 "C under Nz. The mixture was then transferred (cannula)into a stirred solution of 4-methylpyridine (14.9 g, 0.16 mol) in dry diethyl ether (100 mL) maintained at -20 "C under nitrogen. The resulting yellow mixture was stirred for 30 min; then 1-bromotetradecane (44.4 g, 0.16 mol) was added. The yellow color was discharged. The mixture was stirred for a further 1 h at -20 "C; then the temperature was allowed to rise and stirring was continued (25)Miller, C. J.;Cuendet, P.; Gratzel, M. J . Phys. Chem. 1991,95, 877-888. (26)Miller, C. J.;Gratzel, M. J . Phys. Chem. 1991,95,5225-5233. (27)Becka, A.M.; Miller, C. J. J. Phys. Chem. 1992,96,2657-2668. (28)Uchida, I.;Ishiho,A.; Matsue, T.; Itaya, K. J.EZectrounuZ. Chem. 1989,266,455. (29)Bilewicz, R.; Majda, M. J . Am. Chem. SOC.1991, 113, 54645466.
(30)Badia, A.;Back, R.; Lennox, R. B. Angew. Chem., Int. Ed. Engl., in press. (31)Richer, J.; Lipkowski, J. J. EZectrochem. Soc. 1986,133, 121. (32)Sudholter, E. J.R.; Engberts, J. B. F. N.; de Jeu, W. H. J.Phys. Chem. 1982,86,1908-1913.
Bizzotto et al. overhight at ambient temperature. Water (200 mL) was then added, the mixture was transferred to a separating funnel, and the aqueous and organic layers were separated. The aqueous layer was extracted with diethyl ether (50 mL then 2 x 25 mL), the combined organic layers were dried (Na2S04),and the solvents were evaporated to leave a viscous liquid. This was dissolved in diethyl ether (200 mL) and the resulting solution was treated with concentrated HCl(30 mL, ca. 0.36 mol ofHC1). Upon stirring and cooling in an ice-salt bath, a voluminous precipitate of hydrochloride salts was obtained. It was collected by filtration, washed thoroughly with diethyl ether, and crystallized from acetone to give 4-pentadecylpyridine hydrochloride as shining white leaflets. This material was recrystallized from acetone (ca. 125mug) to give 16g of the hydrochloride with mp 128-130 "C. The hydrochloride was stirred with excess aqueous sodium hydroxide (5gin 100mL), the resulting mixture was transferred to a separating funnel along with pentane (150 mL), and the aqueous and the organic layers were separated. The organic layer was dried (Na~S04) and distilled, first at ordinarypressure to remove pentane and then at reduced pressure (ca 0.05 mmHg) to give 4-pentadecylpyridine (11g) as a colorless, slightly cloudy, viscous liquid. The surfactant was recrystallized from acetone and cooled in an ice bath a number of times until the sample was >99.9% pure as determined by GC-MS. The product was analyzed by 200MHz NMR and GC-MS. The molecular ion peak was present at MIE = 289. The surfactant displayed MS features consistent with the fragmentation pattern of a long alkyl chain (AMIE = 14 - CH2 fragments) The largest peak (MIE = 106)represented the 4-ethylpyridine fragment, confirming the single alkyl substitution on the aromatic ring. Equipment. A HEKA potentiostat and a PAR Model 129A lock-in amplifier were used to perform electrochemical measurements. The data acquisition was performed under computer controlutilizing a RC Electronics acquisition board and in-house software. The compression isotherm for (2-15-4-9. was measured using a home made Langmuir trough. The barrier movement and the film pressure at the gas-solution interface of the trough were controlled and measured using KSV equipment and software. The light scattering experiments were performed using an optical setup made of components supplied by Sciencetech (London, ON, Canada). It included a C-T type monochromator with a 1200 linedmm grating blazed at 400 nm, a polarizer (cutoff 220 nm), quartz optics, a PMT detector and a 150-WXe arc lamp. Methodology. Two cells were utilized in these experiments. The first contained only the supporting electrolyte and buffer and was used to deposit the film at the Au electrode surface and to describe its properties by recording CVs and the differential capacity curves. The second cell contained the same supporting electrolyte with the addition of 1mM &Fe(CN)G and was used to study the barrier properties of the film. The electrode was flame annealed, introduced into the first cell and brought in contact with the electrolyte using the horizontal touching technique. Initially, CPs and differential capacity curves were recorded in the absence of surfactant, to ensure that the electrolyte was clean and that shape of CV matched the fingerprint ofthe Au(ll1)electrode. The electrode was then removed from the cell. A 6-pL aliquot of surfactant (enough to cover the free surface twice over) was placed onto the surface of the electrolyte. M e r sufficient waiting time for the chloroform to evaporate (10 mid, the film attained a film pressure of 33.5mN m-l measured with a Wilhemy plate. The film pressure remained constant over a period of many hours needed to perform the experiments. The properties of this film were characterized in an independent experiment performed using a Langmuir trough. Figure 1shows a compression isotherm for C15-4-F'yspread at the surface of the investigated electrolyte in the Langmuir trough, recorded at a compression rate 10 &/min. The isotherm shows that the film has a liquid expanded character up to the collapse pressure which is equal to the 33.5 mN m-l. In this case the collapse pressure is equal to the equilibrium spreading pressure measured for the film spread in the electrochemical cell. The film spread in the electrochemical cell had, therefore, a certain concentration of lenses or droplets that contained the surfactant in excess of the amount corresponding to one monolayer. Since the overall
Thin Film Barrier Properties 35
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Mean Molecular Area / A2 Figure 1. Compression isotherm for C15-4-Pyon 0.05 M Kc104 with 1mM m c o 3 , compressed at a compression rate 10 (A2/ min)/molecule.
E / mV vs SCE Figure 2. Cyclic voltammograms for a clean (-) and covered by C15-4-Py (- - -) Au(ll1) electrode in 0.05 M KC104 1 mM KHC03.
amount of surfactant spread at surface of the electrochemical cell amounted to about two monolayers only, the concentration of lenses or droplets was extremely small and the experiments performed on such films were essentially performed at a monolayer of n = 33.5 mN m-l. The electrodewas flame annealed, cooledin the Ar atmosphere, and then positioned in a horizontal touching configuration under potential control (E = 0 mV (SCE)). The adsorption ofthe C154-Py was characterized by CV and differential capacity measurements. The electrode was emersed at E = 0 mV (SCE)and a small droplet of solution adhering to its surface was removed. The electrodewas again brought in contact with the film-covered surface of the electrolyte solution under potential control, employing the horizontal touching technique. The properties of the filmdeposited at the electrodesurfacewere then characterized by recording CV and differential capacity curves. The films prepared by the double touching procedure have an approximate thickness of 100A which is characteristic ofmultilayer adsorption of C15-4-Py . The adsorption properties of the film are described in a separate publication.33 The electrode covered by the film of C15-4-Py was emersed under potential control (f300 mV) and the small droplet of electrolyte adhering to its surface was again removed. The electrode was then introduced into the second cell containing 1 mM &Fe(CN)a at E = $300 mV (SCE), a potential where no significant reduction of Fe(CN)e3- occurs. The kinetics of Fe(CN)e3- reduction at the film-covered electrode was then investigated by means of CV and potential step experiments. The potential step experiments consisted of initially polarizing the electrode at f300 mV for 30 s (time long enough for the Fe(CN)c4-oxidation current to reach zero) then stepping to a more negative potential and recording the current over the initial 1000ms. The negative step lasted only 2 s and then the potential returned t o +300 mV. The stability of the system was investigated by comparing the CV before and after the potential step experiments.
a) Y
Results and Discussion Characterization of the Film. The cyclic voltammetry and differential capacity curves were recorded to describe adsorption of C15-4-Py onto Au(ll1). Figure 2 shows CVs recorded at an electrode that was film free (solid line)and covered by the film produced by the double touchingtechnique(dashedline).TheCVs showninFigure 2 correspond to the double layer region of the gold electrode and display capacitive currents only. The significant decrease of the capacitive current at potentials close to the pzc (+300mV (SCE)) indicates that surfactantadsorbs strongly in this region. The surfactant desorbs from the (33) Bizzotto, D.; Lipkowski, J. In preparation.
+
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15 \
electrode surface at very negative potentials (-900 mV (SCE)). The presence of two peaks in either the positive going or negative going half cycle indicates that adsorption ofC15-4-9. has a two-state character. The most negative peak (taller peak) corresponds to adsorptioddesorption of the surfactant molecules. The smaller (more positive) peak, which is split in the positive going half cycle, corresponds to the phase transition between two states of the film. The asymmetry of the adsorptioddesorption peaks and the peaks corresponding to the phase transition in the film indicate that both the adsorptioddesorption reactions and the phase changes are slow. Figure 3a shows the corresponding differential capacity curves. At the negative end of potentials, the curves measured at the electrode which was film free and at the electrode which was initially covered by the film merge together indicating that the surfactant is desorbed from the electrode surface at these negative polarizations. The capacity pits seen at more positive potentials indicate adsorption of C15-4-fi molecules. The curve recorded in
Bizzotto et al.
3246 Langmuir, Vol. 11, No. 8, 1995 the positive going half cycle shows features characteristic of two state adsorption of the surfactant molecules seen also in the CVs in Figure 2 . In state I (seen a t negative potentials) the capacity drops to a value of about 2 p F cm+ . In state I1 (observed at positive potentials) the capacity drops to a very low value which is smaller than 1p F cm-2, Ellipsometric studies of the film, described in a separate p~blication,3~ indicate that the film produced by the double touching technique is therefore made of two or more monolayers of C15-4-Py molecules. Only one pit is seen on the differential capacity curves recorded in the negative going half cycle. In contrast, the negative going section of CV (Figure 2) displays two peaks and two regions of different value of the capacity. The absence of the pseudocapacity peak on the differential capacity curve, corresponding to the transition from state I1 to state I of the film, indicates that the transition may be slow in the time scale corresponding to the frequency of the ac perturbation. The repetitive electrode potential cycles can be applied for hours to the electrode initially covered by the film of C15-4-Py to produce identical CVs and differential capacity curves. This feature indicates that the film may be desorbed and adsorbed repeatedly and its properties may be easily restored in that way. In addition the electrode covered by the compact film formed a t E = 300 mV (SCE) may be removed a t the controlled potential from the first cell and transferred to another cell without affecting the integrity of the film. The integrity of the film is preserved even if the surfactant is not spread a t the gas-solution interface of the second cell. The CV and differential capacities of the electrode recorded in the second cell, after the transfer, are identical to those recorded in the first cell irrespective of whether the surfactant is present at the gas solution interface or not. Apparently, the film transferred from the gassolution interface onto the electrode surface is trapped under the electrode and is no longer in equilibrium with the film a t the gas-solution interface. To understand the mechanism by which the surfactant is repeatedly adsorbed and desorbedfrom the electrode surface, we have recorded U V spectra of adsorbed surfactant molecules and measured light scattering by the desorbed surfactant molecules. Details ofthese experiments will be described in a separate p ~ b l i c a t i o n .These ~ ~ results indicate that, at the negative potentials, desorbed C15-4-97 molecules remain in the subsurface region of the electrode, probably in the form of micelles. This is illustrated by Figure 3b which shows changes of the scattered light intensity as a function of the electrode potential under experimental conditions identical to those used to record the differential capacity curves presented in Figure 3a. At potentials where surfactant spreads a t the electrode surface and where a characteristic pit is observed on the differential capacity curves (Figure 3a), the amount of light scattered is small and equal to a constant background observed also in experiments performed with the film-free electrodes. At potentials where the differential capacity rises steeply and the surfactant desorbs from the electrode surface, the amount of scattered radiation increases quickly above the background level. The scattered radiation attains a limitingvalue a t the most negative potentials where the film is totally desorbed from the electrode surface. When the direction of the voltage sweep is reversed, the amount of scattered radiation drops back to the background level at potentials at which the surfactant is readsorbed at the electrode surface and where the capacity pits reappear on the differential capacity curve. Identical potential-induced changes of the amount of scattered radiation were observed when the electrode potential was cycled continuously between the positive
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E / mV VI SCE Figure 4. Cyclic voltammograms of the reduction of 1 mM &Fe(CN)e for a clean (- - -) and covered by C15-4-Py (-1 Au(ll1) electrode in 0.05 M KC104 + 1mM KHC03. Inset, a section of CV in the vicinity of the redox potential of the Fe(CN)e3-/Fe(CN)e4-Couple. and negative limits indicated in Figure 3b. The spectroelectrochemical experiments (which will be described in fuil in ref 33) indicate that the repeatable desorption and adsorption of the surfactant molecules involve micelle formation a t the negative potentials and spreading of the surfactant molecules from the micelle onto the electrode surface at less negative potentials. Barrier Properties of the F".The barrier properties of the film formed by C15-4-97 were investigated using reduction of Fe(CN)63-as a test reaction. The CVs for the reduction of Fe(CN)e3- at a film-covered (solid line) and a n uncovered electrode (dashed line) are shown in Figure 4. In the absence of the film, the reduction of Fe(CN)63was fast and the shape of CV was typical for a reversible mass transport controlled reaction. The film of C15-4-Py apparently blocks faradaic currents at potentials a t which the capacitive pits are seen in Figure 3a. At the filmcovered electrode, the reduction of Fe(CN)63-is effectively delayed up to the potentials where the surfactant is desorbed from the electrode surface. Hence, a sharp Fe(CN)e3-reduction peak, whose height is comparable to the height of the peak recorded at the film-free electrode, could be seen at potentials where the differential capacity jumps to the film-free value. When the direction of the voltage sweep is reversed, the reduction current is again attenuated a t potentials a t which state I1 of the film is formed. This behavior was stable when repetitive voltage sweeps were applied to the electrode for hours. It is useful to emphasize that Figure 4 describes quite an interesting system in which the surfactant is repeatedly adsorbed desorbed to produce film which blocks the reduction of Fe(CN)e3-as efficiently as the well-known self-assembled monolayers. The blocking of the faradaic reaction by the film of C154-97 is not complete, however. Inset to Figure 4 shows a section of the CV recorded in the vicinity of the redox potential of the Fe(CN)63-/Fe(CN)64-couple at a n enlarged sensitivity. For comparison, the CV recorded a t the filmfree electrode is fitted into the curve recorded a t the filmcovered electrode. The currents measured in the presence of the filmare about 50 times smaller than in the unblocked case. However, the positions of the voltammetric peaks are essentially identical at the film-free and the filmcovered electrodes. Moreover, the current recorded a t the film-covered electrode in the negative going half cycle displays a quasiplateau. These properties qualitatively resemble the behavior expected for a reaction controlled
Thin Film Barrier Properties
Figure 5. Model of a partially blocked electrode represented by a set of N circular, active centers of radius rl, surrounded by concentric rings covered by surfactanthaving an inner radius of rl and an outer radius of r2.
by a nonlinear mass transport to active centers present in the film.13 Such active centers may be seen as defects ("pores")randomly distributed in the film which effectively form a n array of microelectrodes surrounded by larger inactive regions. Landsberg et a1.6-8J0and Guidelli and Piccardig were the first to describe a nonlinear mass transport to a partially covered electrode. They gave approximate solutions to the problem which may be applied to describe reactions a t a blocked electrode in the case of moderate coverages. More recently Gueshi et al.11J2J4 andhatore and Saveant13 provided a more exact description of diffusion to a partially blocked electrode. This solution may be applied to the case of high coverage (8 > 0.9) and hence we may employ it to estimate the size and density of active regions (pores) in the film of C15-4-Py. The data analysis is particularly straightforward for potentiostatic conditions. Therefore, we performed the potential step experiments (described in the Experimental Section) to quantitatively investigate the reduction of Fe(CN)63- a t the C15-4-Py covered electrode. Two series of measurements were made. In the first series, the electrode was covered by the film transferred from the gas-solution interface of the cell not containing the Fe(CN)& using the double touching procedure. Next, the film-covered electrode was transferred to the cell that contained Fe(CN)c3- a t a controlled potential f0.3 V (SCE). The blocking properties of this film were then investigated by stepping the potential from an initial value +0.3 V (SCE) to progressively more negative potentials. The surfactant molecules were not desorbed and readsorbed prior to the potential step experiments. Hence these properties are characteristic of the film which is initially formed a t the gas-solution interface and then transferred onto the gold electrode surface (as-formed film). The second series of the potential step experiments were performed just after completion of the first one. This time a full voltage cycle between E = +0.3 V and E = -0.9 V W E ) was applied and the surfactant molecules were desorbed from and later readsorbed onto the electrode prior to the second series of potential step experiments (re-formed film). The blocking properties determined in this series describe the film which is formed by spreading of micelles onto the electrode surface. Gueshi et al." have shown t h a t for 8 > 0.9 the chronoamperometric transient recorded a t the partially blocked electrode is given by
Langmuir, Vol. 11, No. 8, 1995 3247 where j is the measured current, j d = FcD1n/(zt)lBis the limiting diffusion current recorded a t the film-free electrode (Cottrell equation), u = 8 ( l - 61, and fll - 6 ) is a function which depends on the particular model for the distribution of the active and inactive centers at the electrode surface. The simplest model describes the surface of a partially blocked electrode as a set ofN circular active centers of radius rl, surrounded by concentric rings covered by surfactant having a n inner radius rl and an outer radius r2 as shown in Figure 5. The fraction of the uncovered (active) electrode area is then given by:
1- e = (r1/r2)'
(2)
According to Amatore et al.,13 for 8 > 0.9, the functionfll - 8) may then be taken as equal t o f l l - 8) =D62/0.36r12(1 - 8). (The functionfll - 8 ) given by Gueshi et al.ll is somewhat different than t h a t reported above. However, Amatore et al.13pointed out t h a t Gueshi et al. made a n assumption of uncoupled diffusion parallel and perpendicular to the electrode surface which is inappropriate for the case of 8 > 0.9). Equation 1is quite complex and contains two unknowns r1 and 8. However, if the time window used to record the chronoamperometric curves incorporates values oft t h a t are sufficiently short for the thickness of the diffuse layer, 6 = (nDt)*, to be smaller than the radius of the active center r1 (6 < rl), than the initial section of the chronoamperometric transient is controlled by a linear diffision to the active centers and eq 1 may be reduced to
j =j&l- 8 )
for (6 < rl)
(3)
If in addition, the time window is sufficiently long so that at the end of the transient the thickness of the diffise layer becomes much larger than rl but smaller than r2 (r1
