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Self-Assembled Monolayers of Cu(II) Metallosurfactants on GC and HOPG Giovanna Ghirlanda Department of Organic Chemistry, University of Padova, Via Marzolo 1, 35131 Padova, Italy
Paolo Scrimin* Department of Chemical Sciences, University of Trieste, via Giorgieri 1, 34127 Trieste, Italy
Angel E. Kaifer* and Luis A. Echegoyen* Department of Chemistry, University of Miami, Coral Gables, Florida 33124 Received January 17, 1996X The aggregation properties of a series of three amphiphilic copper(II) complexes on a glassy carbon (GC) electrode were examined by means of cyclic voltammetry (CV) and Osteryoung square wave voltammetry (OSWV). The complexes examined consisted of a copper-binding polar head, modeled after the metalbinding site of the antibiotic Bleomycin, connected to a C16 aliphatic tail. In the pH range explored, 6.0-7.3, all the complexes were found to self-assemble on the rough GC surface. Comparison with similar behavior observed on highly oriented pyrolytic graphite (HOPG) for one of the complexes (L1‚Cu(II)) allowed a quantitative treatment of the data. The data fit to the Langmuir isotherm allowed determination of the electrode surface coverage, Γsat, which was consistent with the formation of a monolayer. Preliminary characterization of the structure formed on HOPG by STM confirms the presence of a monolayer.
Introduction Surfactants have been the focus of widespread interest over decades due to their ability to self-assemble in supramolecular structures such as micelles, liposomes, and Langmuir-Blogett (LB) films.1 The aggregates formed create sharp polarity gradients at the interface and define clear hydrophobic regions in an aqueous solution.2 Those properties are of fundamental importance for the creation of new materials;3 the same properties have been exploited by several groups for the acceleration of organic reactions.1,4 Besides, the aggregates, particularly the more ordered ones, such as liposomes and LB films, are considered to be good models of biological membranes.1,5 Metalloaggregates6-8 are made of surfactants that combine a metal-coordinating polar head to a hydrophobic tail. The polar head of the surfactant is functionalized with metal-coordinating groups. These aggregates are particularly interesting, since the metal ions are bound X
Abstract published in Advance ACS Abstracts, June 15, 1996.
(1) Fendler, J. H. Membrane Mimetic Chemistry; Wiley: New York, 1982. (2) Bunton, C. A.; Nome, F.; Quina, F. H.; Romsted, L. S. Acc. Chem. Res. 1991, 224, 357. (3) Uhlman, A. An Introduction to Ultrathin Organic Films; Academic Press: San Diego, CA, 1991. (4) (a) Fendler, J. H.; Fendler, E. J. Catalysis in Micellar and Macromolecular Systems; Academic Press: New York, 1975. (b) Bunton, C. A.; Savelli, G. Adv. Phys. Org. Chem. 1986, 22, 213. (5) Jain, M. K. Introduction to Biological Membranes; WileyInterscience: New York, 1982. (6) (a) Gellman, S. H.; Petter, R.; Breslow, R. J. Am. Chem. Soc. 1986, 108, 238. (b) Di Furia, F.; Fornasier, R.; Tonellato, U. J. Mol. Catal. 1983, 19, 81. (c) Gutsche, C. D.; Mei, G. C. J. Am. Chem. Soc. 1985, 107, 7964. (d) Tagaki, W.; Ogino, K. Top. Curr. Chem. 1985, 128, 144. (e) Scrimin, P.; Tonellato, U. In Surfactants in Solution; Mittal, K. L., Shah, D. O., Eds.; Plenum Press: New York, 1991; Vol. 11, p 349. (f) Scrimin, P.; Tecilla, P.; Tonellato, U.; Vendrame, T. J. Org. Chem. 1989, 54, 5988. (g) Kunitake, T.; Ishikawa, Y.; Shimoumura, W. J. Am. Chem. Soc. 1986, 108, 327. (7) (a) Tagaki, W.; Ogino, K.; Tanaka, O.; Machiya, K.; Kashihara, N.; Yoshida, T. Bull. Chem. Soc. Jpn. 1991, 64, 74. (b) Ogino, K.; Kashihara, N.; Veda, T.; Isaka, T.; Yoshida, T.; Tagaki, W. Bull. Chem. Soc. Jpn. 1992, 65, 373. (c) Scrimin, P.; Tecilla, P.; Tonellato, U. J. Am. Chem. Soc. 1992, 114, 5086.
S0743-7463(96)00052-2 CCC: $12.00
to and surrounded by a hydrophobic region, similar to the situation found in metalloproteins.9 Several groups have been studying the properties of metalloaggregates as catalysts for the cleavage of activated carboxylic7 and phosphoric acid esters.8 A natural extension of this work was to explore the possibility of exploiting the properties of these metalloaggregates as catalysts for redox reactions as well. The underlying idea was that the hydrophobic environment would have some influence on the redox potential of the metal ion10 and that it would be possible to control the latter by controlling the degree of organization of the aggregates. We thus began an investigation on the electrochemistry of a series of amphiphilic copper(II) complexes, aiming to compare their electrochemical properties with those of nonsurfactant analogues. However, direct electrochemical studies of the aggregates are not easy, since surfactants can be adsorbed on the surface of electrodes of different materials (Au, Pt, glassy carbon), thus influencing the electrochemical response.11 This property has been widely exploited in order to control the molecular architecture at the electrode-solution interface. Most of the work found in the literature focuses on the self-assembly of alkyl organosulfur or alkylsilane compounds on gold substrates.12 This process is usually irreversible and has been (8) (a) Menger, F. M.; Gan, L. H.; Johnson, E.; Durst, D. H. J. Am. Chem. Soc. 1987, 109, 2800. (b) Scrimin, P.; Tecilla, P.; Tonellato, U. J. Org. Chem. 1991, 56, 161. (c) Weijnen, J. G.; Komdijs, A.; Engbersen, F. J. J. Chem. Soc., Perkin Trans. 2 1991, 1121. (d) Weijnenand, J. G.; Engbersen, F. G. Recl. Trav. Chim. Pays-Bas 1993, 112, 351. (9) Lippard, S. J.; Berg, J. M. Principles of Bioinorganic Chemistry; University Science Books: Mill Valley, 1994. (10) Golub, G.; Cohen, H.; Paoletti, P.; Bencini, A.; Messori, L.; Bertini, I.; Meyerstein, D. J. Am. Chem. Soc. 1995, 117, 8353. (11) (a) Rusling, J. F. Acc. Chem. Res. 1991, 24, 75. (b) Besia, G. J.; Prud’homm, R. K.; Bensinger, J. B. Langmuir 1988, 4, 140. (c) Dong, S.; Zhu, J.; Chang, G. Langmuir 1991, 7, 389. (12) (a) Dubois, L. H.; Nuzzo, R. G. Annu. Rev. Phys. Chem. 1992, 43, 437. (b) Chidsey, C. E. D.; Loiacono, D. N. Langmuir 1990, 6, 682. (c) Hockett, L. A.; Creager, S. E. Langmuir 1995, 11, 2318. (d) Thoden van Velzen, E. U.; Engbersen, J. F. J.; deLange, P. J.; Mahy, J. W. G.; Reinhoudt, D. N. J. Am. Chem. Soc. 1995, 117, 6853. (e) Hockett, L. A.; Craeger, S. E. Langmuir 1995, 11, 2318.
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used to permanently modify the electrode surface with groups that act as chemical receptors13 or sensors.14 On the other hand, the electrode can be modified also by simply exposing it to a dilute solution of surfactant.15 If the surfactant is functionalized with an electroactive tag, this can be used to monitor the formation of the aggregates on the surface of the electrode, driven by hydrophobic interactions rather than by the chemical interactions of, say, the thiol moieties with the gold surface. The main goal of this paper was to investigate the aggregation of the copper(II) functionalized surfactants on the surface of different electrodes. Experimental Section Electrochemical Measurements. A BAS-100W electrochemical analyzer interfaced with a Hewlett-Packard ColorPro plotter was used to record the voltammetric experiments. Ag/ AgCl/sat KCl was used as the reference electrode. A platinum wire served as counter electrode. A glassy carbon (GC) working electrode (3 mm in diameter) was used unless otherwise specified, after being polished with 1/4 mm diamond polishing compound (Metadi II) from Buehler. Cyclic voltammograms were recorded at the scan rate specified for each experiment. Osteryoung square wave voltammograms were obtained using a sweep width of 25 mV, a frequency of 15 Hz, and a step size of 4 mV. The electrochemical cell used for the experiments consisted generally of a 10 mL beaker sealed with a Teflon cap with holes to fit the electrodes. All experiments were performed under an argon atmosphere. For some experiments high oriented pyrolytic graphite (HOPG) was used as the working electrode. A sheet of graphite was held in a vertical position and clamped with an “O” ring to a connector glass-blown to a beaker, to yield a T-shaped cell. The distance between the HOPG and the body of the cell, where the reference and counter electrode were placed, was approximately 3 cm. Stirring was achieved by a magnetic bar placed in the connector. A drawing of the cell is illustrated in Chart S1 in the Supporting Information. Materials. Ligands L1-L3 were prepared following a variation of the procedure used to prepare symmetrically and asymmetrically 2,5 functionalized pyridines16 and fully characterized.17 The supporting electrolyte (NaNO3, purity >99%) was purchased from Aldrich, recrystallized three times from water, and dried in vacuo prior to use. 2-(N-Morpholino)ethanesulfonic acid (MES) and N-2-hydroxyethylpiperazine-N′2-ethanesulfonic acid (HEPES) buffers were purchased from Sigma and used without further purification. Cu(NO3)2 was purchased from Aldrich; a stock solution 5.11 × 10-2 M was prepared and titrated with ethylenediamine tetraacetic acid (EDTA).18 1,1′-Ferrocenyldicarboxylic acid was purchased from Aldrich and used without further purification; trimethyl(ferrocenylmethyl)ammonium chloride was prepared according to a published method.19 HOPG was obtained from Union Carbide. (13) (a) Rojas, M. T.; Kaifer, A. E. J. Am. Chem. Soc. 1995, 117, 5883. (b) Rojas, M. T.; Ksˇniger, R.; Stoddart, J. F.; Kaifer, A. E. J. Am. Chem. Soc. 1995, 117, 336. (14) (a) Hickman, J. J.; Ofer, D.; Laibinis, P. E.; Whitesides, G. M.; Wrighton, M. S. Science 1991, 252, 688. (b) Wang, J.; Wu, H.; Angnes, L. Anal. Chem. 1993, 65, 1893. (15) (a) Gomez, M.; Li, J.; Kaifer, A. E. Langmuir 1991, 7, 1797. (b) Facci, J. S. Langmuir 1987, 3, 525. (c) Donohue, J. J.; Buttry, D. A. Langmuir 1989, 5, 671. (d) Lee, C. W.; Bard, A. J. J. Electroanal. Chem. 1988, 239, 441. (e) Diaz, A.; Kaifer, A. E. J. Electroanal. Chem. 1988, 239, 333. (16) (a) Cristini, M.; Scrimin, P.; Tonellato, U. Tetrahedron Lett. 1989, 30, 2987. (b) Scrimin, P.; Tecilla, P.; Tonellato, U. Org. Prep. Proced. Int. 1991, 23, 204. (17) Ligands L1-L3 were characterized by 1H and 13C NMR and elemental analysis; the formation of copper complexes was established by UV-vis spectroscopy and ESR. The aggregates formed were characterized by determination of the cmc by surface tension measures. Micelle sizes were determined by dynamic light scattering using as light source an argon laser. Full details of the synthesis and characterization will be published elsewhere. (18) Fornasier, R.; Scrimin, P.; Tecilla, P.; Tonellato, U. J. Am. Chem. Soc. 1989, 111, 224. (19) Lombardo, A.; Bieber, T. I. J. Chem. Educ. 1983, 60, 1080.
Ghirlanda et al. Procedures. In a typical experiment a blank solution containing the supporting electrolyte (0.1 M) and the buffer (0.05 M) was prepared and degassed thoroughly by bubbling humidified argon. After a background voltammogram had been obtained, the complex under study was added. For the surface coverage experiments two stocks of Lx‚Cu(II) complex were prepared, namely 8.5 × 10-5 and 8.5 × 10-4 M. Additions were made according to the concentration desired.
Results and Discussion Interfacial Self-Assembly of Cu(II) Complexes of Ligands L1-L3 on GC Electrodes. The electrochemistry of amphiphilic Cu(II) complexes of ligands L1-L3 was explored by means of cyclic voltammetry (CV) and Osteryoung square wave voltammetry (OSWV) on Au, Pt, or GC as working electrodes. The best results in terms of reversibility and reproducibility were obtained with GC.
Preliminary investigation of 1 mM solutions of the complexes at pH 6.0 (MES, 0.05 M) and I ) 0.2 M (NaNO3) revealed a single redox couple, consistent with the reduction of the Cu(II) complexes to Cu(I). The shape of the cyclic voltammetric waves clearly suggests that the electroactive species are adsorbed on the electrode surface. Ideally, a surface-confined electroactive species should show symmetric current peaks, equal peak potentials (i.e. the difference between the cathodic and anodic current peak potentials, ∆Ep ) 0), and identical wave shapes for the cathodic and anodic current-potential curves.20 Moreover, the observed peak current ip should vary linearly with the potential sweep rate, as derived from eq 1:
ip )
(nF)2AΓν 4RT
(1)
where ip is the observed peak current, n is the number of electrons involved in the electrochemical process, F is the Faraday constant, A is the surface area of the electrode, Γ is the electroactive surface coverage, ν stands for the scan rate used in the experiment, R is the gas constant, and T is the absolute temperature. By contrast, the ip for a diffusion-controlled process varies linearly with the square root of the scan rate. Plots of the cathodic peak currents vs the scan rates were linear for all three hydrophobic complexes in the range explored (approximately 20-5000 mV/s); these plots are shown in Figure 1. The above results, the wave shapes observed, the small ∆Ep values measured, and the scan rate dependence of the currents allow us to conclude that the amphiphilic copper complexes are adsorbed on the surface of the GC electrode at the concentrations examined. (20) Bard, A. J.; Faulkner, L. R. Electrochemical Methods. Fundamentals and Applications; Wiley: New York, 1980.
SAMs of Cu(II) Metallosurfactants on GC and HOPG
Figure 1. Plots of ip vs scan rate of the complexes L1‚Cu(II) (2), L2‚Cu(II) (b), and L3‚Cu(II) (9); values for ip were obtained from the cathodic peak current recorded for 0.5 mM solutions of ligand at I ) 0.2 (NaNO3) at pH 6.3 (MES 0.05 M) for L1‚ Cu(II) and L3‚Cu(II) and at pH 7.2 (HEPES 0.05 M) for L2‚ Cu(II). Table 1. Formal Potential Values Recorded at pH 6.0 and 7.3 for the Different Cu(II) Complexesa ligand (pH)
E(osw), mV
Ep,c, mV
Ep,a, mV
E1/2, mV
∆E, mV
L1 (6) L1 (7.3) L2 (6) L2 (7.3) L3 (6) L3 (7.3)
-344 -452 -330 -376 -390 -456
-362 -465 -348 -387 -421 -452
-334 -448 -294 -349 -367 -442
-348 -456 -321 -368 -394 -447
28 17 54 38 54 10
a Conditions: 1 mM Cu(II) complexes, I ) 0.2 (NaNO ), T ) 25 3 °F; pH was buffered with MES (pH 6) or HEPES (pH 7.3) 0.05 M.
Further investigations were carried out at pH 7.3 (HEPES) to examine the strong pH dependence of the stability of the Cu(II) complexes. Again, the voltammograms obtained indicate the presence of a surface-confined species. Complexes L2‚Cu(II) and L3‚Cu(II) seem to be paticularly sensitive to the pH of the solution. In fact, the peaks observed at pH ) 6 are quite broad while their shape gets sharper at pH ) 7.3. Consequently, the peakto-peak separation approaches the ideal limit of 0 for a surface-confined species at higher pH values. However, for all the ligands and at all the pH values explored, ∆Ep is always larger than zero; see Table 1. Deviations from ideality are quite common,21 and when ∆Ep is observed to increase with the scan rate, as in our case, the effect can be ascribed to a slow electrochemical process, since the Cu(II) to Cu(I) reduction involves a geometric rearrangement and a change in coordination.22 Further characterization of the aggregates obtained on the surface of the electrode was carried out by studying the adsorption isotherms. In a typical experiment, to a buffered solution of supporting electrolyte increasing quantities of a stock solution of the ligand-Cu(II) complex under investigation were added. The range of concentrations was between 0 and 1.5 mM, and for each concentration the cyclic voltammogram and Osteryoung square wave voltammogram were recorded at different scan rates. (21) Murray, R. W. In Molecular Design of Electrode Surface; Murray, R. W., Ed.; Wiley: New York, 1992. (22) Hathaway, B. J. In Comprehensive Co-ordination Chemistry; Wilkinson, G., Gillard, R. D., McCleverty, A., Eds.; Pergamon Press: Oxford, 1987; Vol. 5, pp 596-634.
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Figure 2. Cyclic voltammograms of L1‚Cu(II) on GC at increasing concentrations of complex: (a) 2 × 10-6, 5 × 10-6, 2 × 10-5, 1.7 × 10-4 M; (b) 5 × 10-4, 1 × 10-3, 1.6 × 10-3 M. Conditions: I ) 0.2 (NaNO3), pH 6.3 (MES 0.05 M), scan rate 100 mV/s.
Figure 2 displays the results obtained for complex L1‚ Cu(II). The current associated with the waves increases with increasing concentration of added complex in the range (0-1) × 10-4 M. There are a few scattered points around the critical micelle concentration (cmc) of L1‚Cu(II),23 and then the current levels off and is roughly constant up to 8 × 10-4 M. In this concentration range the appearance of the voltammograms is typical of a surface-confined species. However, as the concentration of the complex is increased up to and over 1 mM, the current increases again, while the shapes of the voltammograms resemble those of a diffusion-controlled process. Note the increased ∆Ep and the increased tailing Faradaic current observed at potentials more negative than the peak potential. For clarity, the voltammograms recorded in the concentration range below the cmc are plotted separately than those above the cmc. However, it should be pointed out that the adsorption process is not directly related to the cmc. It depends primarily on the nature of the surface, although the solution equilibria perturb the process. This point will be further discussed below. Complex L3‚Cu(II) displays the same behavior, with the intensity of the current of the surface-confined species increasing up to a limiting value and leveling off and then increasing again and assuming a more “diffusional” appearance. Complex L2‚Cu(II) does not display such a clear behavior at pH 6.0 (MES); however, further investigations at pH 7.3 (HEPES) yielded clear indications of a surface-confined species, and it followed the same pattern observed for L1 and L3. Surface Coverage Studies. Since we are clearly dealing with surface-confined species, we investigated the type of structure formed on the electrode surface. Useful information can be obtained by studying the dependence of the surface coverage values, Γi, on the concentration of the monomer in solution. We can obtain these values from eq 1, where the peak current recorded is proportional to the extent of adsorption of the electroactive species on the surface of the electrode. Alternatively, the apparent surface coverage can be derived from the total charge transferred by direct integration of the catodic wave24 (23) The cmc of L1‚Cu(II) determined at I ) 0.2 (NaNO3), pH 6.3 (MES 0.05 M) is 7 × 10-5 M. (24) Gomez, M. E.; Kaifer, A. E. J. Chem. Educ. 1992, 69, 503.
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Figure 4. Dependence of the surface coverage (Γi) on the concentration of L1‚Cu(II) on HOPG. Conditions: I ) 0.2 (NaNO3), pH 6.3 (MES 0.05 M), scan rate 500 mV/s.
Figure 3. Cyclic voltammograms of L1‚Cu(II) on HOPG at increasing concentrations: (a) 2 × 10-6, 5 × 10-6, 2 × 10-5, 5 × 10-5 M; (b) 8 × 10-5, 1.5 × 10-4, 5 × 10-4, 8 × 10-4 M. Conditions: I ) 0.2 (NaNO3), pH 6.3 (MES 0.05 M), scan rate 100 mV/s.
according to eq 2:
Q ) nAΓF
(2)
This latter approach is, in principle, more accurate because ip may be affected by the broadening of the waves as a consequence of limiting diffusion rates. However, in our case at high concentration direct integration becomes difficult because the residual current is not negligible. To get reliable values, we used, whenever possible, both approaches and compared the results obtained. In order to be able to extrapolate the exact surface coverage values, Γi, from the experimental voltammograms, the true area of the electrode must be known. It has been reported that for glassy carbon the roughness factor can range from 1.7 to 4.1;21 thus the use of the geometric area will lead to great uncertainty in the Γi values calculated. To circumvent this problem we used a different electrode: HOPG. HOPG provides a flat and smooth surface of graphite, thus seeming a good substitute for GC. We designed a special electrochemical cell in order to employ a sheet of HOPG approximately 1 cm × 2 cm long as the working electrode. The HOPG electrode was placed vertically between two “O” rings, exposing only a well defined area to the solution. The isotherm experiment was repeated using this cell, and results similar to those observed with the GC electrode were obtained. At low concentrations of L1‚Cu(II) the cathodic peak current increases as the concentration is increased, until a limiting value is reached. Further addition of the complex results in no increase in ip, up to a value when the ip increases again. In the low range of concentrations the voltammograms have the shape typical of a surface-confined species, while at higher concentrations (above the plateau) the appearence of the CV approaches that of a diffusional system (see Figure 3). The ∆Ep’s are significantly higher than those observed on GC at all the concentrations explored, being in the range 40-50 mV but still below the theoretical value for pure diffusional systems. The increased ∆Ep’s can be due to resistance effects resulting from the cell geometry: the control and reference electrodes are quite far from the working electrode (approximately 5 cm); moreover, the area of the HOPG electrode is nearly 1 cm2.
We used a microscope to measure directly the geometric area of the electrode, i.e. of the portion of the HOPG surface exposed to the solution, and we assumed it to be a good approximation of the true area of the electrode. The area was 0.994 cm2. The Γi values were then calculated for each concentration using the ip current only because with the HOPG electrode the integration of the surface under the wave was totally hampered by a very high residual current even at very low concentrations due to the geometrical cell design. The Γi values obtained were then plotted as a function of the L1‚Cu(II) complex concentration. The plot clearly shows that the system reaches a limiting coverage value of 2.2 × 10-10 mol/cm2 (Figure 4). This value is in the range expected for the presence of a self-assembled monolayer on the surface of the HOPG electrode. This was further verified by using the area of the polar head of the L1‚Cu(II) complex, as derived from Corey-Pauling-Koltun (CPK) molecular models, assumed to be a rectangle of 6.5 × 10.5 Å. The corresponding area is thus 68.2 Å2. The theoretical Γmax can be calculated from the area: the reciprocal of the area gives the number of electroactive molecules adsorbed on the surface of the electrode per square centimeter. By dividing by Avogadro’s number, the coverage is expressed in mol/cm2, i.e. Γmax (eq 3)
Γmax )
1 ) 2.4 × 10-10 mol/cm2 ANA
(3)
where A is the area of the polar head of the L1‚Cu(II) complex, NA is Avogadro’s number, and Γmax is the theoretical saturation coverage. The agreement with the figure obtained above supports the formation of a monolayer. Further evidence of the formation of a monolayer coverage on the HOPG electrode was obtained by direct observation of the HOPG surface by STM, reported as Supporting Information (see Figure S1). It was possible to see the deposition of regularly spaced molecules on the surface. The diagonal of the repeating cells was approximately 10 Å, in perfect agreement with the CPK models of L1‚Cu(II). However, this does not give any indication of the orientation of the monomers on the surface of the HOPG (heads down or heads up). Having assessed the similarity of the behavior of L1‚ Cu(II) on both GC and HOPG electrodes enabled us to compare the surface coverage data obtained for L1‚Cu(II) on GC and HOPG and to evaluate the true area of the GC electrode. It was assumed that the Γmax value would in principle be the same on the two electrodes, being determined by the packing properties of the surfactant and, on a first approximation, being independent of the
SAMs of Cu(II) Metallosurfactants on GC and HOPG
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Figure 6. Scan rate dependence of the surface coverage Γmax of a 8 × 10-5 M solution of L1‚Cu(II) on GC. Conditions: I ) 0.2 (NaNO3), pH 6.3 (MES 0.05 M).
Figure 5. Dependence of the surface coverage (Γi) on the concentration of the different ligands on GC as determined with the ip method (a) and the wave integration method (b): (b) L1‚Cu(II), I ) 0.2 (NaNO3), pH 6.3 (MES 0.05 M), 500 mV/s; (2) L2‚Cu(II), I ) 0.2 (NaNO3), pH 7.3 (HEPES 0.05 M), 100 mV/s; (O) L3‚Cu(II), I ) 0.2 (NaNO3), pH 6.3 (MES 0.05 M), 500 mV/s.
nature of the surface. Consequently, the difference observed experimentally should exclusively reflect the roughness of the GC electrode. The isotherm data plot for L1‚Cu(II) on GC gave an experimental Γmax of 4.3 × 10-10 mol/cm2 with the ip approach and of 4.6 × 10-10 mol/ cm2 by surface area integration when using the geometrical area of 0.0702 cm2. The ratio of the two Γmax values should then give the roughness coefficient of the GC electrode, found to be 1.91 or 2.09, respectively. These values are in agreement with the data found in the literature.21 Multiplying the geometrical area by the roughness coefficient gave the true area of the electrode, 0.135 or 0.148 cm2. These values were used in all further calculations on the surface coverage. Figure 5 displays the plot of Γi vs [Lx‚Cu(II)], calculated by using the true area of GC (i.e. the geometrical area corrected by the roughness factor) as obtained using both procedures. For all the complexes investigated, Γi increases steeply up to a concentration close to the cmc of the complexes and then it decreases slightly and eventually levels off (see Figure 5). We ascribe the anomalies observed around the cmc to the rearrangement of the aggregates being formed at the surface of the electrode as the molecules in the bulk solution start to form micelles. At low concentration (
