Electroactive Deposits of Anthraquinone-Attached Micelle- and

Jan 22, 1997 - Soma De, Vinod K. Aswal, Prem S. Goyal, and Santanu Bhattacharya. The Journal of Physical Chemistry B 1998 102 (32), 6152-6160...
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Langmuir 1997, 13, 153-160

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Electroactive Deposits of Anthraquinone-Attached Micelleand Vesicle-Forming Surfactant Assemblies on Glassy Carbon Surfaces Marappan Subramanian, Sisir K. Mandal,† and Santanu Bhattacharya* Department of Organic Chemistry, Indian Institute of Science, Bangalore 560012, India Received February 13, 1996. In Final Form: September 9, 1996X Three cationic surfactants with covalently attached anthraquinone moieties at the headgroup region, 1-3, were synthesized. The single chain surfactant, N,N-dimethyl-N-octadecyl-N-((9,10-dihydro-9,10dioxoanthracen-2-yl)methyl)ammonium bromide, 1, upon comicellization with cetyltrimethylammonium bromide (CTAB) in water gave micellar aggregates with anthraquinone residues attached at the exposed aqueous interfaces. The two double chain amphiphiles, N,N-dioctadecyl-N-methyl-N-((9,10-dihydro-9,10-dioxoanthracen-2-yl)methyl)ammonium bromide, 2, and N,N-dimethyl-N-(1,2-bis(palmitoyloxy)propanyl)-N-((9,10-dihydro-9,10-dioxoanthracen-2-yl)methyl)ammonium bromide, 3, however, on suspension with either dioctadecyldimethylammonium bromide (DODAB) or alone in aqueous media gave vesicular assemblies with anthraquinone residues attached both at the inner and at the outer aqueous vesicular surfaces. Holomicellar 1 or comicellar 1/CTAB deposited on to the glassy carbon electrode surface during cyclic voltammometric studies. Above their critical temperatures for the gel to liquid-crystalline-like phase transitions, the vesicular systems also formed electroactive layers on glassy carbon electrodes. In this paper, we present in detail various electrochemical aspects concerning these anthraquinone aggregates and the role of introduction of electrochemically inert cosurfactants in the formation of such electroactive deposits.

1. Introduction Current efforts to achieve irreversible adsorption of different self-assembled monolayers utilize amphiphiles containing a -SH or -SiCl3 group at the end of polymethylene chains which attach themselves respectively on gold or glassy surfaces.1,2 Amphiphilic molecules lacking such specific functional groups also self-organize at the electrode/solution interfaces.3,4 Thus exposure of a clean electrode surface to an aqueous dispersion of redox-active amphiphiles results in the formation of monolayers of redox-active surfactants.5 Over the last several years a large number of studies were devoted to the examination of both electroactive and electrochemically inert micelles and other aggregates on various electrode interfaces.6-11 Mousty and co-workers examined7 how various charges on the surfactants influence the reduction of organic substances. It was shown that besides efficient solubilization of organic molecules within such aggregates, * To whom correspondence may be addressed: e.mail, [email protected]; fax, +91-80-3341683. † Presently at the Alchemie Research Center, P.O. Box No. 155, Thane Belapur Rd., Thane 400 601, Maharashtra, India. X Abstract published in Advance ACS Abstracts, November 15, 1996. (1) (a) Hickman, J. J.; Ofer, D.; Zou, C.; Wrighton, M. S.; Laibnis, P. E.; Whitesides, G. M. J. Am. Chem. Soc. 1991, 113, 1128. (b) Dubois, L. H.; Nuzzo, R. G. Annu. Rev. Phys. Chem. 1991, 43, 437. (2) Doblhofer, K.; Figura, J.; Fuhrhop, J.-H. Langmuir 1992, 8, 1811. (3) Gileadi, E., Ed. Electrosorption; Plenum Press: New York, 1967. (4) Murray, R. W. In Electroanalytical Chemistry; Bard, A. J., Ed.; Marcel Dekker: New York, 1984; Vol. 13, p 191. (5) Facci, J. S. Langmuir 1987, 3, 525. (6) (a) Nordyke, L. L.; Buttry, D. A. Langmuir 1991, 7, 380. (b) Schneider, T. W.; Buttry, D. A. J. Am. Chem. Soc. 1993, 115, 12391. (7) Mousty, C.; Mousset, G. J. J. Colloid Interface Sci. 1989, 128, 427. Mousty, C.; Cheminat, B.; Mousset, G. J. J. Org. Chem. 1989, 54, 5377. (8) Rojas, M. T.; Han, M.; Kaifer, A. E. Langmuir 1992, 8, 1627. Zhang, L.; Lu, T.; Gokel, G. W.; Kaifer, A. E. Langmuir 1993, 9, 1404. Rojas, M. T.; Kaifer, A. E. J. Am. Chem. Soc. 1995, 117, 5883. (9) Garcia, E.; Texter, J. J. Colloid Interface Sci. 1994, 162, 262. Garcia, E.; Texter, J. Langmuir 1993, 9, 2782. (10) Rusling, J. F.; Nassar, A. E. F. J. Am. Chem. Soc. 1993, 115, 11891. (11) Rusling, J. F. In Electroanalytical Chemistry; Bard, A. J., Ed.; Marcel Dekker: New York, 1994; Vol. 18, p 1.

adsorption of surfactants at the electrode interfaces significantly influences such electrochemical processes. Kaifer and co-workers studied the effects of lipid composition on the electrochemistry of incorporated viologen amphiphiles on the electrodes that were modified with cast lipid films.8 Texter reported how Faradaic electron transfer could be modulated by controlling the amounts of cosurfactant present in highly resistive water-AOTtoluene reverse microemulsion.9 Rusling examined the kinetics of electron transfer involving electroactive films for their performance.10 These early pioneering studies led to the design of the electrode coatings for specific applications and has recently emerged into a major area of research.11 Other interesting applications are in the protection from surface corrosion,12 in the preparation of materials with modified electroactive properties,13 and also in the generation of systems that produce vectorial transmembrane charge separation for phototransduction or photostorage purposes or in the molecular nanoelectronics.14 However, often the rationale behind each of these applications differs widely.15 Consequently, the elucidation of structure-property relationships requires careful examination and subsequent preparation of different supramolecular, electroactive systems and their detailed characterization. Leidner and co-workers reported16 the synthesis of some quinone functionalized phospholipid systems. However, (12) Volmer-Vebing, M.; Stratmann, M. Appl. Surf. Sci. 1992, 55, 19. (13) Wrighton, M. S. Science 1986, 231, 32. Medina, J. C.; Gay, I.; Chen, Z.; Echegoyen, L.; Gokel, G. W. J. Am. Chem. Soc. 1991, 113, 365. Rusling, J. F.; Couture, E. C. Langmuir 1990, 6, 425. Lee, L. Y. C.; Hurst, J. K.; Politi, M.; Kurihara, K.; Fendler, J. H. J. Am. Chem. Soc. 1993, 115, 370. Patterson, B. C.; Hurst, J. K. J. Chem. Soc., Chem. Commun. 1990, 1137. van Esch, J. H.; Hoffman, M. A. M.; Nolte, R. J. M. J. Org. Chem. 1995, 60, 1599. Laibnis, P. E.; Hickman, J. J.; Wrighton, M. S.; Whitesides, G. M. Science 1989, 245, 845. Bilewicz, R.; Sawaguchi, T.; Chamberlain, R. V., II; Majda, M. Langmuir 1995, 11, 2256. (14) (a) Center, F. L., Siatkowski, R. E., Wohlgen, H., Eds. Molecular Electronic Devices; North Holland: Amsterdam, 1988. (b) Hurst, J. K. In Kinetics and Catalysis in Microheterogeneous Systems; Gratzel, M., Kalyanasundaram, K., Eds.; Marcel Dekkar: New York, 1991; p 183. (15) Aviram, A. Molecular Electronics-Science and Technology; Engineering Foundation Publications: New York, 1989.

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the corresponding micellar analogues were not prepared, and therefore, comparative studies of the quinone bound micellar and vesicular systems under related experimental conditions were not available. Due to our continuing involvement with the supramolecular assemblies,17-20 we have now prepared a new set of cationic, chemically stable, electroactive amphiphilic molecular assemblies bearing covalently attached anthraquinone units at the polar headgroups. In our design, we emphasize the differences in reactivities at the micellar and vesicular interfaces.21 Micelles are highly dynamic structures and possess a polar interface of high curvature which is exposed to bulk water. On the contrary, vesicular assemblies contain both inner (protected from exogenous ions) and outer (exposed) aqueous compartments providing scopes for surfacespecific reactivities. Thus access to both micelle-forming (single-chain) and vesicle-forming (double-chain) anthraquinone bound systems with a positive charge headgroup would allow comparison of their specific redox and electrochemical properties under related conditions. In this paper, we examine separately the electrochemical properties of the films produced on glassy carbon electrode surfaces from the micellar dispersion of the cationic anthraquinone bound single-chain surfactant, 1. We also describe the findings of the corresponding studies involving vesicular dispersions of two different double-chain cationic surfactants. One contained dimethyldialkylammonium architecture with anthraquinone at the level of the headgroup of the surfactant, 2, and the other had a cationic NMe2+ headgroup attached with an anthraquinone unit of the lipid which flanked diester chains from a pseudoglyceryl backbone, 3. Note that since the two vesicle forming systems 2 and 3 differ in terms of their molecular architectures, this study offers opportunity to closely probe the effect of specific molecular features on their electroactive film formation capacities. We also examined the electroactive film formation and surface coverages on glassy carbon electrodes in the presence of electrochemically “inert” cosurfactant cetyltrimethylammonium bromide or in the presence of “inert” double-chain cosurfactant dioctadecyldimethylammonium bromide, where anthraquinone-attached amphiphiles were doped in a matrix of host cosurfactant assemblies.

(16) (a) Leidner, C. R.; Liu, M. D. J. Am. Chem. Soc. 1989, 111, 6859. (b) Leidner, C. R.; Simpson, H. O.; Liu, M. D.; Horvath, K. M.; Howell, B. E.; Dolina, S. J. Tetrahedron Lett. 1990, 31, 189. (c) Liu, M. D.; Leidner, C. R. J. Chem. Soc., Chem. Commun. 1990, 383. (d) Liu, M. D.; Patterson, D. H.; Jones, C. R.; Leidner, C. R. J. Phys. Chem. 1991, 95, 1858. (17) (a) Bhattacharya, S.; De, S. J. Chem. Soc., Chem. Commun. 1995, 651. (b) Bhattacharya, S.; De, S. J. Chem. Soc., Chem. Commun. 1996, 1283. (c) De, S.; Aswal, V.; Goyal, P. S.; Bhattacharya, S. J. Phys. Chem. 1996, 100, 11664. (18) (a) Ragunathan, K.; Bhattacharya, S. Chem. Phys. Lipids 1995, 77, 13. (b) Bhattacharya, S.; Subramanian, M.; Hiremath, U. S. Chem. Phys. Lipids 1995, 78, 177. (19) Bhattacharya, S.; Haldar, S. Langmuir 1995, 11, 4748. (20) Bhattacharya, S.; Snehalatha, K. Langmuir 1995, 11, 4653. (21) Fendler, J. H. In Membrane Mimetic Chemistry; Wiley: New York, 1982.

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2. Experimental Section 2.1. General Methods. UV-visible absorption spectra were recorded using a UV-vis spectrophotometer (Shimadzu Model 2100) equipped with a thermoelectric temperature controller, TCC-260. Vesicles obtained from the amphiphilic-AQ 2 and 3 were prepared by the sonication method using an immersion probe sonicator at a power setting of 25 W, using a Model XL2020 Heat Systems Ultrasonic Processor. Column chromatrography was performed using silica gel of 60-120 mesh (Merck). 2.2. Electrochemical Measurements. All the electrochemical experiments were performed as described in detail in the following. 2.2.1. Instrument. Electrochemical measurements were made at specified temperatures using a BAS 100A electrochemical workstation interfaced with a DMP-40 digital plotter. 2.2.2. Cell. A single-compartment glass cell was used for all the investigations. 2.2.3. Electrode Assembly. A conventional three-electrode cell was employed in all measurements which consisted of a platinum wire auxiliary electrode, a glassy carbon working electrode, and a KCl saturated calomel reference electrode (SCE). All voltammograms were recorded in pure water (18 Ω) under nitrogen atmosphere using KCl (0.1 M) as the supporting electrolyte. Imported glassy carbon (GC) of 9 mm2 surface area was used in all the cyclic voltammetric experiments. 2.2.4. Techniques. A modified GC electrode was prepared by dipping the precleaned GC electrode in solutions of respective electroactive surfactants in 0.1 M KCl for 10 min and rinsing the electrode with deionized water (Millipore) repetitively. Before each experiment, the GC was polished with finely coated Al2O3 paper (BAS) followed by sonication in a mixed solvent system of CHCl3-MeOH (50:50 v/v) and finally treated with 0.1 M HCl solution at a potential of -1.5 V and rinsed with deionized water (Millipore). Solution resistance was compensated prior to each experiment, and the maximum uncertainties in the potential measurements were within (5 mV. Temperature was maintained above the thermotropic phase-transition temperature of vesicular 3 (48 ( 2 °C). For vesicular 2 and micellar 1 the temperature was also maintained at 48 ( 2 °C in order to provide comparable conditions for electrochemical measurements. A constant nitrogen flow was maintained over the solution with stirring to remove any adventitiously dissolved traces of oxygen. Areas under the cathodic peak, i.e., the charges (Q), were obtained by converting the voltage axis (E) into the time axis. The surface coverage (Π) was calculated using the equation, |Q|/ nFA. Cyclic voltammetric data were uncorrected for junction potentials. The capacitance values were obtained upon dividing the sum of cathodic anodic currents by twice of the scan rates from current-voltage (i-E) plots at -50 mV upon scanning of the region from +100 to -200 mV. Geometric surface area of the electrode was used, and the data were not corrected for roughness factors. 2.2.5. Sample Preparation for Electrochemical Measurements. Micellar dispersions of the amphiphilic anthraquinones 1 and vesicular dispersions of the amphiphilic anthraquinones 2 and 3 for electroactive deposition were prepared in the following way. The comicellar solution of amphiphilic-AQ, 1, was prepared by dissolving the requisite amount of 1 (either 10, 20, or 30 mol %) with respect to the nonelectroactive cosurfactant cetyltrimethylammonium bromide (CTAB) in water. First, 1 was dissolved in ethanol (10 µL), and then it was injected into a clear, freshly prepared micellar solution of CTAB (1 × 10-3 M) prepared in aqueous 0.1 M KCl (10 mL) solution. The dispersion was briefly sonicated using an immersion probe sonifier at 25 W power setting using a Heat Systems ultrasonic processor (Model XL-2020) for 2 min at room temperature. The clear comicellar solution thus obtained containing specified mole percents of 1 were used immediately for electrochemical measurements. In the case of amphiphilic-AQ 2 and 3, the optically clear vesicular solutions containing either 10, 20, or 30 mol % of anthraquinone-attached surfactant with respect to cosurfactant DODAB (1 × 10-3 M) were prepared by dissolving the corresponding amount of the amphiphiles in ethanol (20 µL). The ethanolic solutions were injected into separate, clear vesicular solution samples prepared from DODAB alone in aqueous 0.1 M KCl. The resulting mixtures were individually sonicated for 6 min at a temperature above their phase transition temperatures (∼60 °C). The covesicular

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solutions thus formed were optically stable for long time and were employed for further studies. Similarly, clear holomicellar dispersions of 1 and holovesicular solutions of 2 and 3 were prepared in the absence of any cosurfactants. Micellar solution of 1 (0.1 mmol) was prepared by dissolving the appropriate amount of the anthraquinone in ethanol (10 µL), and then the ethanolic solution was injected into a 10 mL aqueous 0.1 M KCl solution prepared in deionized water (Millipore). The dispersion was sonicated using the an immersion probe sonicator for 2 min to obtain optically stable, clear micelles. In the case of amphiphilic-AQ 2 and 3, the liposomal solutions of concentration (0.1 mmol) were prepared by dissolving the requisite amount of the amphiphile in ethanol (10 µL) followed by injection into separate 10 mL 0.1 M aqueous KCl solutions. The resulting dispersions were sonicated 10 min above ∼60 °C to obtain clear holovesicular solutions. The micellar and vesicular solutions thus prepared were used immediately for electrochemical studies. 2.3. Materials. All the starting materials were obtained from the best known commercial suppliers and used without further purification. Dioctadecyldimethylammonium bromide (DODAB) was prepared from stearyl bromide (Aldrich) following a reported procedure.22 All the solvents used herein are freshly distilled prior their use. Although the synthesis of the three anthraquinone-bearing amphiphiles has been described,27 the detailed synthetic methods are given below for convenience. 2.4. Methods Related to the Synthesis of Anthraquinone Surfactants. Melting points were measured using open capillaries and are uncorrected. IR spectra were recorded using a Perkin-Elmer 781 spectrometer and 1H-NMR spectra were recorded using either Bruker WH 270 (270 MHz) or Jeol FX-90Q (90 MHz) spectrometers. The chemical shifts for 1H-NMR spectra are reported in parts per million (ppm) relative to tetramethylsilane (TMS) as internal standard, and J values are reported in Hertz (Hz). Elemental analyses were performed using a Carlo Erba 1106 analyzer. 2-(Bromomethyl)anthraquinone. To a mixture of 2-methylanthraquinone (4.4 g, 0.02 mol) and N-bromosuccinimide (3.56 g, 0.02 mol) in freshly distilled, dry CCl4 (50 mL) was added a catalytic amount of benzoyl peroxide (∼50 mg), and the mixture was refluxed for 10 h. The colorless succinimide (produced during bromination) floating on the surface of the solution was filtered, and the solvent was removed under vacuum. The crude yellow solid thus obtained was purified by column chromatography over silica gel using CHCl3-MeOH (95:5) as an eluent: mp 198-200 °C (lit.23 mp 199-201 °C); yield, 5.4 g (90%). N,N-Dimethyl-N-octadecyl-N-((9,10-dihydro-9,10-dioxoanthracen-2-yl)methyl)ammonium bromide, 1. To a solution of N-octadecyl-N,N-dimethylamine (297 mg, 1 mmol) in dry EtOH (20 mL), was added 2-(bromomethyl)anthraquinone (300 mg, 1 mmol), and the resulting mixture was refluxed for ∼20 h. Then the solvent was evaporated and the crude solid thus obtained was purified by passing through a silica gel column using CHCl3MeOH (95:5). Upon solvent removal from the fractions, a pale yellow crystalline solid of 1 was obtained in 92% yield. Mp 199201 °C. IR (neat) (ν): 1660 cm (quinone carbonyl), 1580, 1460, 1280 cm-1. 1H-NMR (CDCl3, 270 MHz) δ: 0.87 (t, 3H, J ) 6.8 Hz), 1.23 (br, m, 26H), 1.36 (crude t, 2H), 1.78-1.88 (m, 4H), 3.42 (s, 6H), 3.62 (t, 2H), 5.43 (s, 2H), 7.77-7.80 (m, 2H, Ar-H), 8.198.43 (m, 5H, Ar-H). Anal. Calcd for C35H52O2NBr: C, 70.21; H, 8.75; N, 2.33. Found: C, 70.18; H, 8.79; N, 2.32. N,N-Dioctadecyl-N-methyl-N-((9,10-dihydro-9,10-dioxoanthracen-2-yl)methyl)ammonium Bromide, 2. A solution of a mixture of N,N-dioctadecyl-N-methylamine (270 mg, 0.5 mmol) and 2-bromomethylanthraquinone (150 mg, 0.5 mmol) in absolute EtOH (20 mL) was refluxed for ∼16 h. The solid obtained upon evaporation of the solvent after reflux was purified by silica gel column chromatography using CHCl3-MeOH (95:5). This gave 2 as a pale yellow crystalline solid: mp 138-140 °C; yield 375 mg (95%). IR (neat) (ν): 1670 (quinone carbonyl), 1580, 1460, 1320, 1280 cm-1. 1H-NMR (CDCl3, 270 MHz) δ: 0.87 (t, 6H, J ) 6.7 Hz), 1.25-1.37 (br, m, 56H), 1.80 (br, m, 8H), 3.28 (s, 3H), 3.44 (t, 4H, J ) 8.0 Hz), 5.30 (s, 2H), 7.80-7.81 (m, 2H, Ar-H),

8.20-8.32 (m, 4H, Ar-H), 8.43-8.46 (d, 1H, Ar-H, J ) 8.1 Hz). Anal. Calcd for C52H86NO2Br: C, 74.60; H, 10.35; N, 1.67. Found: C, 74.47; H, 10.57; N, 1.88. N,N-Dimethyl-N-(1,2-bis(palmitoyloxy)propanyl)-N-((9,10-dihydro-9,10-dioxoanthracen-2-yl)methyl)ammonium Bromide, 3. a. 3-(N,N-Dimethylamino)-1,2-propanediol. A solution of 3-chloro-1,2-propanediol (1.5 g, 1.13 mL, 14 mmol) in dry MeOH (35 mL) in a screw-top-sealed tube was cooled to -5 °C. Dry Me2NH gas was gently through the stirred methanolic solution until the volume of the resulting solution was raised by 6 mL. The resulting mixture was then kept at room temperature for 20 h. After that, the solvent was removed under vacuum in a rotary evaporator. The amine hydrochloride thus formed, was treated with a stoichiometric amount of sodium ethoxide at 0 °C. The precipitated sodium chloride was filtered off. Removal of the ethanol gave a light yellowish thick oil which was characterized as 3-(dimethylamino)1,2-propanediol, 1.57 g (95%) yield. The material produced by the above described procedure gave spectral and analytical data consistent with that reported for the same in literature.24 b. (()-1,2-Bis(palmitoyloxy)-3-(dimethylamino)propane, 4. To a stirred solution of 3-(dimethylamino)-1,2-propanediol (240 mg, 2 mmol), a solution of palmitic acid (1.07 g, 4.2 mmol) and DCC (865 mg, 4.2 mmol) in freshly distilled dry CHCl3 (30 mL) was added to DMAP (100 mg) at room temperature and kept for 24 h. The dicyclohexylurea precipitated after this period was filtered, and the solvent from the filtrate was concentrated under vacuum. The solid thus obtained was purified by column chromatography over silica gel using CHCl3-MeOH (95:5). Evaporation of the solvent gave 4 as a colorless solid, mp 48-50 °C, 950 (mg (80%) yield. 1H-NMR (CDCl3, 90 MHz) δ: 0.88 (t, 6H, 9 Hz), 1.22 (br, m, 48H), 1.42-1.65 (m, 4H), 2.28 (s, 10H), 2.40-2.56 (d of d, 2H, J ) 7.2 Hz, 2.4 Hz), 4.1-4.3 (two dd, 2H, J ) 7.2 Hz, 3.5 Hz), 5.1-5.26 (m, 1H). c. (1,2-Bis(palmitoyloxy)-3-propyl)dimethyl((9,10-dihydro-9,10-dioxoanthracen-2-yl)methyl)ammonium Bromide, 3. To a solution of dl-1,2-bis(palmitoyloxy)-3-(dimethylamino)propane, 4 (300 mg, 0.5 mmol), in absolute ethanol (20 mL), 2-(bromomethyl)anthraquinone (150 mg, 0.5 mmol) was added and refluxed for 15 h. Solid obtained upon evaporation of the solvent was purified by passing through a silica gel column using CHCl3-MeOH (94:6). The quaternary anthraquinone salt 3 was obtained as a pale yellow crystalline, hygroscopic solid, mp 115117 °C, 390 mg (87%) yield. IR (neat) ν: 1730 (ester carbonyl), 1660 (quinone carbonyl), 1580, 1450, 1280 cm-1. 1H-NMR (CDCl3, 270 MHz) δ: 0.87 (t, 6H, 6.7 Hz), 1.22 (br, m, 44H), 1.60-1.70 (m, 8H), 2.28-2.37 (m, 4H), 3.38-3.48 (two s, 6H), 3.85-3.92 (d of d, 1H), 4.1-4.2 (d of d, 1H), 4.54 (m, 2H), 5.32-5.43 (dd, 1H), 5.75 (s, 2H), 7.81 (d, 2H), 8.22 (m, 2H), 8.35 (s, 2H), 8.42 (s, 1H). Anal. Calcd for C52H82O6NBr‚1.5H2O: C, 67.58; H, 9.27; N, 1.51. Found: C, 67.79; H, 9.12; N, 1.77. 2.5. Phase Transition Temperatures (Tm) of Vesicular Dispersions. The thermotropic phase transition temperatures (Tm), for the transition from the “frozen” gel-like states to the “fluid” liquid crystalline states, were determined by following the changes in the temperature-dependent keto-enol tautomerism of vesicle-doped benzoylacetoanilide (BAA).18b,25 To a solution of BAA in EtOH (2 × 10-3 M, 100 µL), was dissolved amphiphiles 2 or 3 of concentration (5 × 10-5 M), and the mixed solution was injected into 10 mL of deionized water. The suspended solution was sonicated using an immersion probe sonifier (at 25 W power setting using Heat Systems, Ultrasonic Processor, Model XL-2020) for above ∼60 °C for 3 min. The liposomes thus obtained were transferred to a sample cuvette and same concentration of the “empty” liposomal solution without BAA was kept in the reference cell. The temperature of the system was brought to 25 °C and left for 10 min to attain thermal equilibrium. After that, the UV absorbance spectra of the sample were recorded in the range from 200 to 500 nm at different temperatures. The intensities of the absorbances due to keto and enol forms of BAA at 250 nm and at 315 nm were measured. The temperature of the system was then gradually increased at intervals of 2 °C from 25 to 65 °C, and the spectra were recorded.

(22) Kunitake, T.; Okahata, Y.; Tanaki, K.; Kumanaru, F.; Takayanagai, M. Chem. Lett. 1977, 387. (23) Blankespoor, R. L.; Lan, A. N. K.; Miller, L. L. J. Org. Chem. 1984, 49, 4441.

(24) Scrimin, P.; Tecila, P.; Tonellato, U. J. Am. Chem. Soc. 1992, 114, 5086. (25) Ueno, M.; Katoh, S.; Kobayashi, S.; Tomoyama, E.; Obata, R.; Nakao, H.; Ohsawa, S.; Koyama, N.; Morita, Y. Langmuir 1991, 7, 918.

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The ratio of enol to keto absorbances, Aenol/Aketo, with temperature gave a pronounced break and was taken as Tm values for each individual liposomal system. The Tm values measured for covesicular 2/DODAB and holovesicular 3 were found to be 41 ((1) and 48 ((1) °C, respectively. The value of Tm for covesicular 2/DODAB virtually did not change ((2 °C) with the variation in the molar ratio of 2 over DODAB from 10 to 30 mol %. Transmission Electron Microscopy. Electron microscopy (TEM, Jeol 200-CX instrument) with 0.5% uranyl acetate stained vesicles revealed that vesicle diameters ranged from ∼1000 to 1200 Å in the case of covesicular 2/DODAB. Holovesicular 3 diameters were larger, ranging from ∼1450 to 1600 Å. Closer examinations of these micrographs further revealed the presence of multilamellar structures in either of the vesicular organizations. Molecular Modeling. Computations of optimized structures were carried out using Insight II, version 2.3.5 version (Biosym Technology), software using Discover Forcefield and steepest descent algorithm.26

3. Results and Discussion Redox Processes Induced by Chemical Reagents. The anthraquinone-bearing micellar or vesicular systems can be reduced and reoxidized rapidly and practically reversibly by addition of appropriate reducing or oxidizing agents. We employed reducing agents such as, sodium dithionite (SDT) or sodium borohydride in order to effect the conversion of anthraquinone to the corresponding hydroquinones. An absorption peak was seen at ∼328 nm due to the presence of anthraquinone units of 1 in micellar solution prepared by doping 1 (5 × 10-5 M) in CTAB (5 × 10-4 M) in phosphate buffer at pH 7.8. The reduction of the anthraquinones to the anthrahydroquinones of the amphiphile 1 was studied by addition of SDT (10-fold molar excess with respect to 1) in phosphate buffer pH 7.8. The anthrahydroquinone peak appeared at ∼389 nm with a time-dependent decrease in the absorption intensity of the anthraquinone peak at 326 nm (figure not shown). The reduction process was found to be quantitative and the reduction followed an apparent first-order kinetic path with a half-time of ∼52 s at 25 °C. Attempts to produce stable vesicles at the same pH with a phosphate buffer did not succeed. Due to this reason, vesicles of 2 or 3 were generated in Tris buffer at pH 7.8 or 8 using 10-fold molar excess of host surfactant DODAB. These vesicles were sufficiently stable and allowed examination of their redox processes by optical spectroscopy. Addition of a 10-fold molar excess of SDT (with respect to 2) to the covesicular 2 (5 × 10-5 M)/DODAB (5 × 10-4 M) in Tris buffer (pH 7.8, 0.01 M KCl) at 25 °C (below Tm) afforded an incomplete reduction of anthraquinones (332 nm). Similarly with 3.5 × 10-5 M holovesicular 3 in Tris buffer (pH 8, 0.01 M KCl) at 25 °C (below Tm) the anthraquinone peak due to vesicular 3 at 328 nm decreased as the corresponding anthrahydroquinones were produced at 390 nm. Thus, below the phase transition temperatures, the reductions in vesicles were not complete. Note that upon melting of the vesicles above their respective phase transition temperatures gave practically quantitative reduction (∼98%). The precise determination of the kinetic details, however, under these circumstances (above Tm) were not possible due to concomitant onset of turbidity and phase separation.27 Analysis of the time-courses of the reduction of vesicular anthraquinones revealed that reactions on vesicular systems did not follow the monoexponential process. This (26) For computations of optimized structures see, BIOSYM program manual, Insight II 2.3.5 version. These programs are available from Biosym Technologies, 9685 Scranton Rd., San Diego, CA 92121-3752. (27) The details of the chemical reduction of different anthraquinonebound aggregates and the corresponding kinetic studies have been published. See: Bhattacharya, S.; Subramanian, M. J. Chem. Soc., Perkin Trans. 2 1996, 2027.

Figure 1. Energy minimized structural drawings (INSIGHT) of the amphiphilic anthraquinones 1, 2, and 3.

became particularly apparent when the points of these kinetic traces were subjected to fitting either with a mono- or biexponential analysis. With both instances, the single exponential fits were quite poor, suggesting that these reactions followed biphasic processes. On the basis of the above findings, we believe that in either case, the first, rapid process accounts for the reduction of the anthraquinones exposed at the outer surfaces of the vesicular aggregates to the dithionite in bulk aqueous solution and according to the above interpolation, these rapid kinetic phases amount to ∼36% and ∼38% for 2/DODAB and 3, respectively, of the total (maximum theoretical absorbance). The slower processes observed with vesicles should then be due to the reductions of the residual ∼64% or ∼62% anthraquinones “protected” within the endovesicular loci of 2/DODAB and 3, respectively. These explanations are quite likely as the vesicles were found to be multilamellar on the basis of electron micrography (TEM). Such one-sided reactivities of anthraquinone vesicles are nicely complementary to the earlier reports on “monolayer” membranes composed of bolaamphiphilic quinones developed by Fuhrhop and co-workers.28 Optimized Surfactant Conformations. Energyminimized conformational features of the electroactive amphiphilic anthraquinones 1-3 were calculated by extension or modification of the structures considering the optimum conformations of the dialkyldimethylammonium ion lipids as suggested by the published crystal structure data of dioctadecyldimethylammonium bromide monohydrate (DODAB)29 and dipalmitoylphosphatidylcholine (DPPC).30 Figure 1 shows the conformation of the anthraquinone amphiphiles 1, 2, and 3. The optimized architectures for 2 and 3 were derived from the crystal structure data of DODAB and DPPC respectively by (28) Fuhrhop, J.-H.; Hungerbuhler, H.; Siggel, U. Langmuir 1990, 6, 1295. (29) Okuyama, K.; Soboi, Y.; Iijima, N.; Hirabayashi, K.; Kunitake, T.; Kajiyama, T. Bull. Chem. Soc. Jpn. 1988, 61, 1485. (30) Hauser, H.; Pascher, I.; Pearson, R. H.; Sundell, S. Biochim. Biophys. Acta 1981, 650, 21.

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a

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b

Figure 2. (a) Cyclic voltammogram of the modified glassy carbon electrode produced upon dipping the electrode for 10 min into an aqueous solution of 1 × 10-4 M single chain AQ surfactant, 1, containing 0.1 M KCl. The voltammogram was recorded at a scan rate of 50 mV s-1 with respect to the SCE. (b) Cyclic voltammetric responses of the modified GC electrode obtained upon dipping into a 3/DODAB (3 × 10-4 M/7 × 10-4 M) covesicular solution containing 0.1 M KCl at scan rates (1) 50 mV s-1 and (2) 100 mV s-1 with respect to the SCE.

appropriately modifying the headgroup with electroactive anthraquinone moiety and other connector region as were necessary using the methods described.19 Formation of Electroactive Deposits. We then examined the electrochemical behavior of the newly synthesized anthraquinone-attached surfactant aggregates in aqueous media. Consequently we carried out cyclic voltammetric experiments with the above mentioned surfactant aggregates in 0.1 M KCl solution using a glassy carbon electrode. Spontaneous adsorption of the surfactant-bound anthraquinones (covalently attached with the surfactant at the level of the polar headgroup) 1, 2, and 3 from the respective aggregate dispersions occurred onto the glassy carbon electrode (GC). This took place either from the aqueous micellar 1 at room temperature and also at higher temperatures. For vesicular 2 and 3 the temperature was maintained at 48 ( 2 °C (above their phase transition temperatures) during cyclic voltammetric experiments to give reproducible results. In order to provide comparable conditions for electrochemical measurements, further experiments with micellar solution of 1 or 1/CTAB containing 0.1 M KCl were also carried out at 48 ( 2 °C. This phenomenon led to the formation of deposits of electroactive layers onto the GC surfaces in contradistinction to the well-known Langmuir-Blodgett deposition.31 “Immobilized” electroactive species such as deposited amphiphilic anthraquinones at the electrode surfaces are expected to show a voltammogram32 quite different from that of the “normal” anthraquinones when they are present in the bulk medium in the form of a homogeneous solution. Thus, with the adsorbed anthraquinone amphiphiles on the GC surfaces, instead of the anticipated, two widely separated reduction waves corresponding to the quinone/semiquinone and semiquinone/hydroquinone couples, only one broad or two very closely spaced peaks were observed as shown in Figure 2a. Dipping the GC electrode in comicellar or covesicular 1, 2, or 3 (30 mol %) in aqueous 0.1 M KCl solutions for 10 min followed by rinsing the electrode in pure water also produced voltammograms with minimal peak separations and equal cathodic and anodic coverages (area under the curve) expected for surface-bound electroactive species.26 A representative voltammogram due to the deposit formed from 3/DODAB is shown in Figure 2b. A plot of iPa/iPc vs v1/2 (figure not shown) also indicated33 that the reactants were getting adsorbed primarily onto the (31) (a) Bain, C. D.; Troughton, E. B.; Tao, Y.; Evall, J.; Whitesides, G. M.; Nuzzo, R. G. J. Am. Chem. Soc. 1989, 111, 321. (b) Kuhn, H. Mol. Cryst. Liq. Cryst. 1985, 125, 233. (32) Carter, M. T.; Rowe, G. K.; Richardson, J. N.; Tender, L. M.; Terrill, R. H.; Murray, R. W. J. Am. Chem. Soc. 1995, 117, 2896. (33) Bard, A. J.; Faulkner, L. R. In Electrochemical Methods; John Wiley & Sons: New York, 1990.

Figure 3. A plot of the cathodic peak current, iPc (µA) vs v (scan rate, V s-1) of the modified glassy carbon electrode produced upon dipping for 10 min into a covesicular solution of 2/DODAB (1 × 10-4 M/9 × 10-4 M) in the presence of 0.1 M KCl.

GC surface. The manifestation of the adsorption of the electroactive species is further corroborated from the fact that the adsorbed quinone produced a voltammogram on which a linear relationship is obtained when ipc (cathodic peak current) is plotted against v (scan rate) (Figure 3). It appears that the formation of electroactive deposits on the GC surfaces take place even when the electroactive surfactants 1-3 were doped in matrix dispersions of excess electrochemically inert cosurfactants such as CTAB or DODAB (see below). In order to understand the relative surface coverages of the adsorbed anthraquinones under identical conditions, the charges (Q) corresponding to the area under the cathodic peaks were separately measured for each of the amphiphiles 1, 2, and 3 based on two-electron transfer (n ) 2). All the anthraquinone-bearing surfactants contained a Me2+-CH2-AQ unit at the headgroup irrespective of whether they produced micellar or vesicular assemblies. Consequently we assumed that the amphiphiles had similar surface area for their headgroup (70 Å2/molecule) as obtained from the calculation of their energy minimized structures (INSIGHT). Adsorption of quinone surfactants in a perpendicular mode onto the surface of the GC in a “head-down” fashion, also gave estimates of the surface coverages. In fact is has been shown34,35 that the glassy (34) (a) Witkowski, A.; Brajter-Toth, A. Anal. Chem., 1992, 64, 635. (b) Witkowski, A.; Freund, M. S.; Brajter-Toth, A. Anal. Chem. 1991, 63, 622. (35) Rusling, J. F.; Hu, N.; Zhang, H.; Howe, D.; Miaw, C.-L.; Couture, E. In Electrochemistry in Colloids and Dispersions; Mackay, R. A., Texter, J., Eds.; VCH Publishers: New York, 1992; p 303.

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carbon surface is relatively hydrophilic and therefore cationic amphiphiles should absorb head-down. The surface coverages were found to increase in the order 1 < 2 < 3. The relative stabilities of the deposits should be primarily governed by (i) the interactions between the electrode surface and the polar head groups of the surfactants and (iii) the mutual interactions among the monomeric surfactants (hydrophobic effects). The electrostatic interactions at the GC surfaces might be assumed to be nearly comparable for 1, 2, or 3 since they have identical features at the level of head groups. However, the hydrophobic interactions and the packing efficiencies among surfactant monomers should differ from surfactant to surfactant. On the basis of these considerations, we reasoned that the packing organization in micelles (1) is “looser”21 than the same with the surfactant assemblies of 2 or 3 containing double hydrocarbon chains. This could result in the reduced surface coverages with 1/CTAB as compared with that of 2 or 3. The tighter packing in vesicular organization is well-known.21 But the observed differences in the surface coverages between covesicular 2/DODAB and covesicular 3/DODAB are more intriguing. The enhanced surface coverages observed with covesicular 3/DODAB relative to 2/DODAB could be due to several reasons. The presence of diester chain/headgroup connector region in 3 promotes additional intermolecular interaction via water-promoted hydrogen-bonding networks as was concluded based on extensive studies involving cationic mixed-chain and diester amphiphiles.19,36 Furthermore, the MeN+(CH2AQ)-CH2CHOC(O)C15H31 chains in 3 are ∼2 Å longer than the AQCH2N+Me(CH2)17CH3 in 2 based on the calculations of their energyminimized structures. This small but additional chain elongation in 3 offers scope for interdigitation. The lipid architectures with naturally occurring glycerol backbones have been shown to be more efficiently packed in membranes than their dialkyldimethylammonium ion counterparts.36 Role of the Presence of Electrochemically Inert Cosurfactants on Surface Coverage. It should be noted, however, that these experiments were done in the presence of nonelectroactive, supporting surfactants such as CTAB in the case of 1 or DODAB in the case of 2 or 3. These electrochemically inert host surfactants should also participate in the formation of similar deposits on the electrode surfaces although they are not electroactive. In order to clearly understand the effect of the presence of cosurfactants, the GC electrode was first dipped in 1 × 10-3 M either pure micellar CTAB or pure vesicular DODAB solutions separately. After 10 min of dipping in such a solution, the respective electrode samples were thoroughly rinsed with pure water and then the GC was again dipped in either 30 mol % 1/CTAB, 2/DODAB, or 3/DODAB coaggregate solutions for another additional 10 min. Then the resulting GC’s were again rinsed with pure water and the voltammograms were recorded under the identical conditions as described earlier. The measured coverage was found to be ∼50-60% less than the one observed earlier without the pretreatment of electrodes in pure micellar CTAB or pure vesicular DODAB solution. These findings provide experimental evidence that CTAB or DODAB also adsorb onto the GC surface as one would anticipate. At the same time, it appears that the adsorption of 1, 2, or 3 was preferred over their surfactant counterparts that do not contain electroactive anthraquinone moieties. In order to further examine whether predeposited single chain CTAB or double chain DODAB on the GC electrode (36) Moss, R. A.; Ganguli, S.; Okumura, Y.; Fujita, T. J. Am. Chem. Soc. 1990, 112, 6391.

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interfere with the adsorption of electroactive quinone amphiphiles on to the GC surfaces, the voltammograms were recorded after dipping the CTAB or DODAB deposited electrode in pure 0.1 M micellar 1 or pure vesicular 2 or 3 solutions alone in 0.1 M KCl, respectively. Higher measured coverages were observed in the absence of any predeposition of cosurfactants such CTAB or DODAB. It is noteworthy to mention that the surface coverages still, however, follow the above mentioned order (1 < 2 < 3) indicating that the interference due to the presence of cosurfactants was almost comparable in all these cases. Role of the Amphiphile Charge on Surface Coverage. In another experiment, it was found that the formation of the electrode coverage is also dependent on the surface charge of the headgroups. For cationic quinone-bound surfactant 1 (30 mol %) when doped in a matrix of an anionic surfactant (SDS) micelle, a drastic reduction of the surface coverage was seen. The cationic surfactant doped in an anionic organized media is expected to form hydrophobic ion pairs.37 Under such conditions the dissociation of the electroactive species from the ionpaired complexes could be difficult. This could reduce the number of quinones that could be accessible for adsorption on to the GC surface resulting in the lower measured coverage (only ∼20% compared to the coverages observed from 30 mol % 1 in CTAB). Heterogeneous Electron Transfer. Quinone-functionalized vesicles or micelles can be reduced or oxidized rapidly and reversibly by the application of external agents as described earlier. However, the electrochemical method can be conveniently used as a means to study the heterogeneous electron transfer process in simple model membranes. The GC adsorbed functionalized quinones of variable structure and controllable aggregation properties offer potential to examine their behavior as interfacially layered assemblies. In turn this offers opportunity to probe the presently described systems as models for studies of various aspects such as heterogeneous charge transfer, ion transfer, double layer phenomena, etc. Keeping this in mind the different micellar and vesicular samples in solution as well as the surface-modified layered quinones were then subjected to further electrochemical studies. Since upon dipping a freshly cleaned glassy carbon electrode into either micellar or vesicular solution of anthraquinone-bearing amphiphiles produced spontaneously amphiphilic anthraquinone-coated surface-modified GC, upon electrolysis the resulting voltammograms must represent either a mediated or electrocatalyzed event.16c A representative voltammogram is shown in Figure 4 describing the effect of anthraquinone deposit on glassy carbon electrode in the presence of a vesicular solution containing anthraquinone-functionalized surfactant. The waves in Figure 4 appear as “broad”, and the broadness is attributed to the appearance of anthraquinone/anthrasemiquinone and anthrasemiquinone/ anthrahydroquinone couples. The existence of similar type of differences has been repeated earlier.38,39 The formal potentials for 1, 2, and 3 were found to be 0.63, 0.66, and 0.67 V, respectively. To understand the factors affecting the heterogeneous electron transfer rates, cyclic voltammetric investigations were then carried out using the correspondingly surface-modified electrodes in the absence of any micellar or vesicular media (only in presence of 0.1 M KCl solution). The data are (37) (a) Kaler, E. W.; Murthy, A. K.; Rodriguez, B. E.; Zasadzinski, J. A. N. Science 1989, 245, 1371. (b) Fukuda, H.; Kawata, K.; Okuda, H.; Regen, S. L. J. Am. Chem. Soc. 1990, 112, 1635. (38) Gorton, L.; Johannson, G. J. Electroanal. Chem. 1980, 113, 151. (39) Durfor, C. N.; Yenser, B. A.; Bowers, M. L. J. Electroanal. Chem. 1988, 224, 287.

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Figure 4. A representative cyclic voltammetric response of covesicular 2/DODAB (3 × 10-4 M/7 × 10-4 M) solution containing 0.1 M KCl at scan rates of (1) 100, (2) 200, (3) 300, (4) 400, and (5) 500 mV s-1, respectively, with respect to the SCE.

Figure 6. Epc - E0′ vs -ln v plots due to the modified GC electrodes produced upon dipping the same for 10 min into a solution of 1 × 10-4 M 1 (b), 1 × 10-4 M 2 (0), and 1 × 10-4 M 3 (O). All the aggregate solutions contained 0.1 M KCl as supporting electrolyte. Inset: Epc - E0′ vs -ln v plots due to a modified GC electrode resulted from dipping the same for 10 min into either a comicellar 1/CTAB (3 × 10-4 M/7 × 10-4 M) solution (b) or into a covesicular 2/DODAB (3 × 10-4 M/7 × 10-4 M) solution (0) and or a covesicular 3/DODAB solution (3 × 10-4 M/7 × 10-4 M) solution (O). Table 2. Electrochemical Data of the Redox Couples for 1, 2, and 3

Figure 5. A representative cyclic voltammogram of a GC electrode in holovesicular solution of 2 (1 × 10-4 M) in the presence of 0.1 M KCl at scan rates of (1) 100, (2) 200, and (3) 300 mV s-1.

amphiphile

scan rate (mV/s)

∆Ep (mV)c

1a

50 100 200 300 100 200 300 400 50 100 200 300 400 50 100 200 300 400 50 100 200 300 100 200 300 400

34 40 48 56 76 90 102 116 92 124 156 192 236 32 44 54 64 70 50 70 86 98 142 228 272 302

2a

3a

Table 1. Electrochemical Data of the GC Modified Electrodesa amphiphile 1 2 3 1c 2c 3c

capacitance (µF, cm-2)

Γ0 (mol cm-2)

no. of layersb

16 48 40 92 47 24

1.28 × 3.41 × 10-11 4.60 × 10-10 2.72 × 10-10 4.10 × 10-10 7.33 × 10-10

0.05 0.14 0.94 1.15 1.72 3.1

10-11

a Modified GC electrodes were prepared by dipping the GC electrode onto a 30 mol % micellar 1 (30:70, 1/CTAB (mol:mol)) or vesicular dispersion of 30 mol % 2 or 3/70 mol % DODAB for a period of 10 min. See text for details. b Estimated number of layers deposited on the GC surfaces. c Modified GC electrodes were prepared following the above procedure using 100% AQ surfactants.

presented in Table 1, and a representative diagram is shown in Figure 5. It is clear from the Table 1 that the surface coverage for the modified electrodes prepared in the absence of cosurfactants follow the order 1 > 2 > 3 as observed earlier, whereas the surface coverages follow the reverse order (1 < 2 < 3). Electron transfer in a densely packed layer could be sluggish due to the kinetic barrier imposed in the layer. Involvement of similar kinetic barrier has also been proposed for related quinone systems previously.40 The peak widths at half maxima (PWHM) were found to be 100, 113, and 118 mV for 1, 2, and 3, respectively. On the basis of two-electron transfer considering Nernstian behavior, the theoretical PWHM would be 45.3 mV. The loss of the Nernstian behavior is attributable to the (40) Li, T. T. T.; Weaker, M. J. J. Am. Chem. Soc. 1989, 106, 6107.

1b

2b

3b

a Data in the presence of cosurfactants. b Data for 100% amphiphilic AQ. c ∆Ep ) Epc - Epa.

surface-bound close-packed deposits which are also susceptible toward intermolecular interactions, and the loss of activity could be due to high concentration as a consequence of tight packing of the electroactive species. Similar broadening also has been well described in the literature.32 The peak to peak potential (∆Ep) in the case of 1 was found to be ∼40 mV for 50 mV s-1 scan rate and the ∆Ep increases with the increase in scan rates. This again is indicative of the fact that there may be a kinetic barrier for electron transfer. The heterogeneous electron transfer rate k0 can be calculated from the plot of -(Epc - E0′) vs -ln v adapted for this behavior. A typical plot is given in Figure 6. Least-squares regression analysis yielded straight lines. The pertinent parameters are presented in Table 2.

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The capacitance measurements offer an additional measure of layer thickness as well as permeability of the layer to small ionic species. It is found that the capacitances for individual systems decrease in the order 1 > 2 > 3 for either of the cases where coaggregates of anthraquinone and nonanthraquinone surfactant were used or aggregates containing anthraquinone surfactants alone were employed. This indicates that the reduced permeability of the layers are not caused by the interference effects due to the presence of external ions or small molecules. These could be due to the layer thickness or the strength of supramolecular organization, which is anticipated to follow the order 1 < 2 < 3. Our results demonstrate the spontaneous molecular deposition at the liquid-solid interface, thereby forming surface adsorbed organized structures with oriented quinone functionalities. These coverages and thicknesses onto the GC could be systematically varied and controlled upon suitable choice of the type of the surfactants. Summary. The chemical reagent mediated redox processes could be furnished quantitatively on the micellar anthraquinone systems. The vesicular anthraquinone systems, however, protect below phase transition tem-

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peratures and anthraquinone units that reside at the inner vesicular loci. Amphiphilic anthraquinones deposit onto the GC surfaces to form electroactive layers. The electroactive coverages onto the GC surface increase with the increase in the chain length and the number of chains per surfactant molecules. We examined herein whether the presence of cosurfactants that are derived of any anthraquinone moieties could influence the preferential adsorption of the electroactive surfactant on the GC surfaces. Surface coverages depend on the time of dipping the electrode into electroactive aggregate dispersion and the concentration of the electroactive species. The surface coverages are increased with concentration and dipping time. Acknowledgment. We are grateful to the Department of Science and Technology for financial support of this work. We thank Supercomputer Education Research Center at the Indian Institute of Science, Bangalore, for the molecular modeling facilities. LA9601317