Electrochemical Reaction in an Aqueous Solution of a Ferrocene

ferrocenyl surfactant, was studied by cyclic voltammetry using a glassy carbon electrode in its aqueous mixtures with sodium dodecylbenzenesulfonate (...
0 downloads 0 Views 299KB Size
Download PDF
Langmuir 2003, 19, 9343-9350

9343

Electrochemical Reaction in an Aqueous Solution of a Ferrocene-Modified Cationic Surfactant Mixed with an Anionic Surfactant Koji Tsuchiya,† Hideki Sakai,*,†,‡ Tetsuo Saji,§ and Masahiko Abe†,‡ Faculty of Science and Technology, Tokyo University of Science, 2641 Yamazaki, Noda, Chiba 278-8510, Japan, Institute of Colloid and Interface Science, Tokyo University of Science, 1-3 Kagurazaka, Shinjuku, Tokyo 162-8601, Japan, and Department of Chemistry and Materials Science, Tokyo Institute of Technology, Ohokayama, Meguro-ku, Tokyo 152-8552, Japan Received April 7, 2003. In Final Form: July 23, 2003 The electrode reaction of (11-ferrocenylundecyl)trimethylammonium bromide (FTMA), a cationic ferrocenyl surfactant, was studied by cyclic voltammetry using a glassy carbon electrode in its aqueous mixtures with sodium dodecylbenzenesulfonate (SDBS), an anionic surfactant. In FTMA-rich mixtures, mixed micelles were formed at low mixing ratios ([SDBS]/[FTMA]), while vesicles (and lamellar liquid crystals) were formed at high mixing ratios except for in the vicinity of the equimolar ratio. Cyclic voltammograms showed that the oxidation peak due to FTMA/SDBS complexes mainly forming vesicles (and lamellar liquid crystals) has an anodic potential which is more positive compared with that due to uncomplexed (free) FTMA molecules forming mixed micelles. At mixing ratios of [SDBS]/[FTMA] e 0.30, the oxidation current was caused by the diffusion-controlled process, while at the ratios of 0.40 e [SDBS]/ [FTMA] e 0.80, the oxidation process was dominated mainly by adsorption species on the glassy carbon electrode because vesicles (and lamellar liquid crystals) deposited onto the hydrophobic carbon electrode due to a high hydrophobicity of vesicle bilayers. In addition, the anodic peak current was strongly affected by the phase behavior of the mixtures. Electrochemical measurements in aqueous solution with a ferrocenyl surfactant would allow us to determine the various aggregation states such as micelles and vesicles.

Introduction Ferrocenyl surfactants have attracted attention because their formation of aggregates such as micelles1-11 and vesicles12-18 and interfacial properties19-22 can be controlled by their redox reactions. This is because the * To whom all correspondence should be addressed. Phone +814-7124-1501 (ext. 3621). Fax +81-4-7122-1442. E-mail: hisakai@ rs.noda.tus.ac.jp. † Faculty of Science and Technology, Tokyo University of Science. ‡ Institute of Colloid and Interface Science, Tokyo University of Science. § Tokyo Institute of Technology. (1) Saji, T.; Hoshino, K.; Ishii, Y.; Goto, M. J. Am. Chem. Soc. 1991, 113, 450. (2) Saji, T.; Hoshino, K.; Aoyagui, S. J. Am. Chem. Soc. 1985, 107, 6865. (3) Hoshino, K.; Saji, T. J. Am. Chem. Soc. 1987, 109, 5881. (4) Saji, T.; Hoshino, K.; Aoyagui, S. J. Chem. Soc., Chem. Commun. 1985, 865. (5) Hoshino, K.; Saji, T. Chem. Lett. 1987, 1439. (6) Saji, T. Chem. Lett. 1988, 693. (7) Takeoka, Y.; Aoki, T.; Sanui, K.; Ogata, N.; Yokoyama, M.; Okano, T.; Sakurai, Y.; Watanabe, M. J. Controlled Release 1995, 33, 79. (8) Takeoka, Y.; Aoki, T.; Sanui, K.; Ogata, N.; Watanabe, M. Langmuir 1996, 12, 487. (9) Kakizawa, Y.; Sakai, H.; Abe, M.; Kondo, Y.; Yoshino, N. Mater. Technol. 2001, 19, 259. (10) Kakizawa, Y.; Sakai, H.; Yamaguchi, A.; Kondo, Y.; Yoshino, N.; Abe, M. Langmuir 2001, 17, 8044. (11) Takei, T.; Sakai, H.; Kondo, Y.; Yoshino, N.; Abe, M. Colloids Surf., A 2001, 183-185, 757. (12) Medina, J. C.; Gay, I.; Chen, Z.; Echegoyen, L.; Gokel, G. W. J. Am. Chem. Soc. 1991, 113, 365. (13) Mun˜oz, S.; Gokel, G. W. J. Am. Chem. Soc. 1993, 115, 4899. (14) Wang, K.; Mun˜oz, S.; Zhang, L.; Castro, R.; Kaifer, A. E.; Gokel, G. W. J. Am. Chem. Soc. 1996, 118, 6707. (15) Wang, K.; Gokel, G. W. J. Phys. Org. Chem. 1997, 10, 323. (16) Kakizawa, Y.; Sakai, H.; Nishiyama, K.; Abe, M.; Shoji, H.; Kondo, Y.; Yoshino, N. Langmuir 1996, 12, 921. (17) Kakizawa, Y.; Sakai, H.; Yamaguchi, A.; Kondo, Y.; Yoshino, N.; Abe, M. Langmuir 2001, 17, 8044.

hydrophilic-lipophilic balance of the surfactant changes drastically when a hydrophobic ferrocenyl group in its molecular form (reduced form) is oxidized to form a hydrophilic ferricinium cation. Many researchers have then studied this peculiar property. Saji et al.1-6 reported a novel method to prepare organic thin films on electrodes by electrolysis of ferrocenyl surfactant micellar solutions, in which hydrophobic dyes and pigments are solubilized. Abbott and co-workers19-24 described a particular conformation of ferrocenyl surfactants and its control on an air-liquid interface. We9-11 investigated the electrochemical reversible control of micellar solubilization of an oily substance using a ferrocenyl surfactant. The formation-disruption control of vesicles12-18 is considerably interesting from the viewpoint of applying ferrocenyl surfactants as a drug carrier and to the removal of organic impurities dissolved in water. Gokel and co-workers12-15 studied the control of vesicle formation using newly synthesized ferrocenyl surfactants. Vesicles formed by a single surfactant, however, usually require the application of mechanical agitation like ultrasonic treatment for their preparation, and they are thermodynamically metastable in water.25 Reversible control of vesicle formation and interfacial properties is difficult in such nonequilibrated systems. (18) Sakai, H.; Imamura, H.; Kakizawa, Y.; Abe, M.; Kondo, Y.; Yoshino, N.; Harwell, J. H. Denki Kagaku 1997, 65, 669. (19) Aydogan, N.; Abbott, N. L. Langmuir 2001, 17, 5703. (20) Aydogan, N.; Abbott, N. L. Langmuir 2002, 18, 7826. (21) Aydogan, N.; Gallardo, B. S.; Abbott, N. L. Langmuir 1999, 15, 722. (22) Gallardo, B. S.; Abbott, N. L. Langmuir 1997, 13, 203. (23) Gallardo, B. S.; Hwa, M. J.; Abbott, N. L. Langmuir 1995, 11, 4209. (24) Gallardo, B. S.; Metcalfe, K. L.; Abbott, N. L. Langmuir 1996, 12, 4116.

10.1021/la0301442 CCC: $25.00 © 2003 American Chemical Society Published on Web 10/01/2003

9344

Langmuir, Vol. 19, No. 22, 2003 Scheme 1. Redox Reaction of FTMA

Recently, Kaler et al.26-33 have reported that thermodynamically stable vesicles are spontaneously formed by mixing only cationic and anionic surfactants in water. Electrochemical control of reversible formation and disruption of vesicles would be possible if we have spontaneously formed vesicles in aqueous surfactant mixtures with a ferrocenyl surfactant as one component. Actually, we have succeeded in obtaining vesicles spontaneously formed in aqueous mixed solutions of a cationic ferrocenyl surfactant [11-(ferrocenylundecyl)trimethylammonium bromide, FTMA; Scheme 1] and an anionic surfactant (sodium dodecylbenzenesulfonate, SDBS) and in controlling reversibly the formation-disruption of vesicles by the redox reaction.18 To put this interesting phenomenon into practical application, the elucidation of the process of its redox reaction in the mixed surfactant system is essential. Composition-dependent formation of various molecular assemblies such as vesicles, micelles, and rodlike micelles is another interesting property of aqueous mixtures of cationic and anionic surfactants when compared with the other surfactant mixtures such as anionic/anionic and anionic/nonionic systems.26-39 The electrochemical behavior of the ferrocenyl surfactant would change depending on its aggregation state, for example, micelle or vesicle, because each aggregation state gives a different environment (polarity, mobility, etc.) to the surfactant molecule. In addition, if the relation between the electrochemical and the phase behaviors is revealed, the various ag(25) Deasy, P. B. Microencapsulation and related drug processes; Marcel Dekker, Inc.: New York, 1984. (26) Kaler, E. W.; Murthy, A. K.; Rodriguez, B. E.; Zasadzinski, J. A. N. Science 1989, 245, 1371. (27) Herrington, K. L.; Kaler, E. W.; Miller, D. D.; Zasadzinski, J. A. N. J. Phys. Chem. 1993, 97, 13792. (28) Yatcilla, M. T.; Herrington, K. L.; Brasher, L. L.; Kaler, E. W.; Zasadzinski, J. A. N. J. Phys. Chem. 1996, 100, 5874. (29) Kaler, E. W.; Herrington, K. L.; Murthy, A. K.; Zasadzinski, J. A. N. J. Phys. Chem. 1992, 96, 6698. (30) Brasher, L. L.; Herrington, K. L.; Kaler, E. W. Langmuir 1995, 11, 4267. (31) Chiruvolu, S.; Israelachvile, J. N.; Naranjo, E.; Xu, Z.; Zasadzinski, J. A.; Kaler, E. W.; Herrington, K. L. Langmuir 1995, 11, 4256. (32) Iampietro, D. J.; Brasher, L. L.; Kaler, E. W.; Stradner, A.; Glatter, O. J. Phys. Chem. B 1998, 102, 3105. (33) Brasher, L. L.; Kaler, E. W. Langmuir 1996, 12, 6270. (34) Xia, Y.; Goldmints, I.; Johnson, P. W.; Hatton, T. A.; Bose, A. Langmuir 2002, 18, 3822. (35) Villeneuve, M.; Kaneshina, S.; Imae, T.; Aratono, M. Langmuir 1999, 12, 2029. (36) Kondo, Y.; Uchiyama, H.; Yoshino, N.; Nishiyama, K.; Abe, M. Langmuir 1995, 11, 2380. (37) Marques, E.; Khan, A.; Graca Miguel, M.; Lindman, B. J. Phys. Chem. 1993, 97, 4729. (38) Bergtro¨m, M.; Perdersen, J. S. Langmuir 1998, 14, 3754. (39) Tsuchiya, K.; Sakai, H.; Kwon, K.; Takei, T.; Abe, M. J. Oleo Sci. 2002, 51, 133.

Tsuchiya et al.

gregates observed in the mixtures would easily be identified by electrochemical measurements using a ferrocenyl surfactant as an electrochemical probe. In a previous paper,39 we have reported that the electrochemical behavior of FTMA changes depending on the phase behavior of FTMA/SDBS aqueous mixtures. Cyclic voltammograms showed that the oxidation potential of FTMA in mixed micellar solutions was nearly equal to that in a pure FTMA aqueous solution, while that of the surfactant in vesicles was observed at a more anodic potential. The difference in the electrochemical behavior of FTMA in these cases, however, has not been elucidated. In the present paper, we report the process of the electrochemical reaction of the ferrocenyl surfactant (FTMA) in the various molecular aggregates formed in FTMA/SDBS aqueous mixtures revealed by electrochemical and interfacial techniques. Experimental Section Materials. FTMA and SDBS were purchased from Dojin Co. and Tokyo Kasei Kogyo Co., Ltd., respectively, and were used without purification. Sodium bromide (Wako Pure Chemical Industries, Ltd.) was used as the supporting electrolyte because it has the same counterions as FTMA and SDBS. Distilled water for injection (Otsuka Pharmaceutical Co., Ltd.) was used as the water. Preparation of FTMA/SDBS Mixed Solutions. Both FTMA and SDBS aqueous solutions were prepared in an N2 atmosphere using 0.02 M NaBr aqueous solutions pretreated by bubbling N2 for 30 min. First, a 7 mM FTMA aqueous solution and SDBS solutions with various concentrations were prepared, and then they were gently mixed to give sample solutions at desired molar ratios. All the sample solutions were not sonicated but were stirred with a magnetic stirrer, and they were equilibrated in a thermostated bath at 25 °C. Cyclic Voltammetry. Cyclic voltammetry was conducted with a three-electrode cell at 25 °C using a glassy carbon electrode as the working electrode, a platinum wire as the counter electrode, and a saturated calomel electrode (SCE) as the reference electrode. Before each measurement, samples were bubbled with N2 for 15 min. A potentiostat (model HA-301, Hokuto Denko Co.) and a function generator (model HB-111, Hokuto Denko Co.) were used to control the potential. The potential was scanned from 0 to 0.6 V versus SCE. Dynamic Light Scattering (DLS) Measurement. Sample solutions were prepared by mixing FTMA and SDBS stock solutions after being made dust-free by passing them through a polycarbonate filter with a pore size of 0.2 µm. The particle size and its distribution were measured with a DLS measuring apparatus (NICOMP 380 ZLS, Particle Sizing Systems) at a scattering angle of 90° using a wavelength of 535 nm. Freeze-Fracture Transmission Electron Microscopy (FF-TEM).29,31,35 Transmission electron microscopy (TEM) observations were carried out by the freeze-fracture (FF) method. Sample solutions were quickly frozen in liquid propane with a cryo preparation system (LEICA EM CPC, LEICA microsystems), and the frozen samples were fractured with a glass knife at -120 °C using a freeze-replica preparing apparatus (FR 7000A, Hitachi Science Systems, Ltd.). A replica film was prepared by successively evaporating platinum-carbon at 45° and carbon at 90° on the fractured-face of the sample. After being washing several times with acetone and distilled water, the film was transferred onto a 300-mesh copper grid. The replica thus prepared was observed with a transmission electron microscope (JEM-1200EX, JEOL). Trapping Efficiency Measurement. Vesicles can encapsulate aqueous compounds in their inner water phase. The trapping efficiency of FTMA/SDBS vesicles was determined by the glucose dialysis technique.40 FTMA/SDBS aqueous mixtures were prepared using a 0.28 M D-(+)-glucose/0.02 M NaBr aqueous solution instead of a 0.02 M NaBr aqueous solution as the solvent. The samples prepared were placed into a cellulose tube (Viskase (40) Brode, W. R.; Gould, J. H.; Wyman, G. M. J. Am. Chem. Soc. 1952, 74, 4641.

Ferrocene-Modified Cationic Surfactant

Langmuir, Vol. 19, No. 22, 2003 9345

Figure 2. Particle size distribution for aqueous mixtures of FTMA and SDBS as a function of the molar ratio (7 mM FTMA, 0.02 M NaBr).

Figure 1. Particle size distribution for aqueous mixtures of FTMA and SDBS measured with quasi-elastic light scattering (7 mM FTMA, 0.02 M NaBr, [SDBS]/[FTMA] ) (a) 0.05, (b) 0.10, (c) 0.15, (d) 0.30, (e) 0.40, and (f) 0.60). Co., Inc.), through which solute molecules with molecular weights of 13 000 or more cannot pass. The unencapsulated glucose was removed by dialysis against 0.16 M NaBr aqueous solutions for 12 h. Vesicles inside the tube were disrupted by the addition of ethanol, and then the amount of glucose left in the tube was determined by the mutarotase-GOD method41 using Glucose CIITest Wako (Wako Pure Chemical Industries, Ltd.).

Results and Discussion Phase Behavior in Aqueous Solutions of FTMA and SDBS. We investigated the phase behavior of FTMA/ SDBS mixed solutions with the FTMA concentration fixed at 7 mM. Figures 1 and 2 show respectively the size distribution and the mean size of the molecular assemblies as a function of the molar ratio ([SDBS]/[FTMA]) measured by the DLS method. FTMA/SDBS mixtures at compositions of 0 e [SDBS]/[FTMA] e 0.02 gave yellowish isotropic solutions. The critical micelle concentration (cmc) of FTMA is reported to be 0.07 mM in aqueous solution in the presence of 0.2 M Li2SO4.1 In the present study, the concentration of FTMA (7 mM) was sufficiently higher than the cmc, although NaBr was used as the electrolyte instead of Li2SO4 and micelles were formed in pure FTMA aqueous solutions. In this region, the assembly size could not be measured by DLS because the intensity of scattered light was too low. In addition, no vesicle-like structure was observed in FF-TEM observations. These results show that mixed micelles (M) are formed at molar ratios of 0 e [SDBS]/[FTMA] e 0.02. At compositions of 0.05 e [SDBS]/[FTMA] e 0.30, a size distribution peak around 4-8 nm (A) was observed and the mean size increased with increasing mixing ratio. Aydogan and Abbott19 have reported that a hydrodynamic (41) Miwa, I.; Okuda, J.; Maeda, K.; Okuda, G. Clin. Chim. Acta 1972, 37, 538.

Figure 3. Typical freeze-replica TEM micrograph of an aqueous mixture of FTMA and SDBS (1:0.08 FTMA/SDBS, 7 mM FTMA, 0.02 M NaBr).

diameter of a micelle is 6 ( 2 nm in pure FTMA aqueous solutions (>0.1 mM). Because our results are in good agreement with theirs, we inferred that mixed micelles were formed at these molar ratios. A particle size distribution peak (B) was observed around 15-80 nm at compositions of 0.05 e [SDBS]/[FTMA] e 0.60, as shown in Figure 1. A typical FF-TEM micrograph of a FTMA/ SDBS aqueous solution ([SDBS]/[FTMA] ) 0.08, Figure 3) also shows the formation of vesicle-like aggregates with diameters of 30-50 nm. The diameter of the aggregates observed by TEM was consistent with the mean size in distribution peak B found in DLS measurements. These results confirm the vesicle (maybe small unilamellar vesicle) formation in the aqueous mixtures with compositions 0.05 e [SDBS]/[FTMA] e 0.6. At compositions of 0.15 e [SDBS]/[FTMA] e 0.60, a peak (C) in the particle size distribution is observed around 200-400 nm in DLS measurement (Figure 1). FF-TEM micrographs ([SDBS]/[FTMA] ) 0.60, Figure 4a,b) show the existence of vesicles with diameters 400-600 nm. The steplike structure (indicated by the arrows in the figures) indicates that these large vesicles have a multilamellar

9346

Langmuir, Vol. 19, No. 22, 2003

Tsuchiya et al.

Figure 5. Phase behavior of aqueous solutions of FTMA and SDBS (with 0.02 M NaBr). The FTMA concentration is fixed at 7 mM (M, micelle; V, vesicle; L, lamellar phase; P, precipitate).

Figure 4. Typical freeze-replica TEM micrograph of an aqueous mixture of FTMA and SDBS (1:0.60 FTMA/SDBS, 7 mM FTMA, 0.02 M NaBr).

membrane. The diameter of the vesicles observed in the TEM micrographs agrees well with the mean diameter in the distribution (C) obtained in DLS measurements. In addition, vesicles of sizes (70-100 nm) consistent with the distribution (B) were also observed by TEM as shown in Figure 4c. At compositions of [SDBS]/[FTMA] above 0.15, the turbidity of the aqueous mixtures drastically increased and a small amount of turbid cloud wisp deposited in a few days after preparation of the mixtures. The birefringence could be detected for the aggregate by polarizing microscopy. These observations show the existence of a lamellar phase. Kaler and co-workers28-30 reported the formation of similar turbid clouds in aqueous mixtures of cationic and anionic surfactants (CTAB/SOS, etc.), and the aggregates were identified as fragmented lamellar liquid crystals. These results indicate the formation of multilamellar vesicles and lamellar liquid crystals in the region 0.15 e [SDBS]/[FTMA] e 0.60. At compositions of 0.90 e [SDBS]/[FTMA] e 1, yellowish crystalline precipitates were formed in the aqueous mixtures.

On the basis of the results previously described, the phase behavior of FTMA/SDBS aqueous mixtures with the FTMA concentration fixed at 7 mM can be summarized as shown in Figure 5: (i) 0 e [SDBS]/[FTMA] e 0.02, micelle (M) region; (ii) 0.05 e [SDBS]/[FTMA] e 0.10, micelle and (small unilamellar) vesicle (M + V) region; (iii) 0.15 e [SDBS]/[FTMA] e 0.30, micelle, vesicle, and lamellar liquid crystal (M + V + L) region; (iv) 0.40 e [SDBS]/[FTMA] e 0.80, vesicle and lamellar liquid crystal (V + L) region; and (v) 0.90 e [SDBS]/[FTMA] e 1, precipitate (P) region. Kaler et al.26-33 studied in detail the spontaneous formation of vesicles in aqueous mixtures of cationic and anionic surfactants, especially in dilute solutions. When a cationic surfactant and an anionic surfactant are mixed with each other in aqueous solutions, the electrostatic interaction between their hydrophilic groups causes the mixtures to form a pseudo double-tailed complex. The complex has a geometric structure favorable to vesicle formation.24,42,43 For the cationic and anionic surfactant mixtures, the nonideal mixing of pseudo double-tailed complexes and excess uncomplexed surfactants in the inner and outer monolayers in vesicles allows the formation of spontaneous curvatures. This is one of the factors for the spontaneous formation of vesicles. In the case of FTMA/SDBS mixtures, when SDBS was added to pure FTMA micellar solutions, the pseudo doubletailed FTMA/SDBS complex, which has a high hydrophobicity compared with that of each of the surfactants, was formed in aqueous solutions. At considerably high proportions of FTMA (0 < [SDBS]/[FTMA] e 0.02), excess uncomplexed FTMA molecules formed mixed micelles, in which FTMA/SDBS complexes are incorporated. With an increasing concentration of SDBS in the mixture, the size of the mixed micelles increases as a result of an increase in the number of FTMA/SDBS complexes in the mixed micelle (Figure 2). Vesicles were formed at proportions of SDBS ([SDBS]/[FTMA]) above 0.05. In this case, uncomplexed FTMA molecules are oriented in the outer monolayer of vesicles composed mainly of FTMA/SDBS complexes and, hence, the bilayer has a spontaneous curvature. In addition, this prevents the vesicles from flocculation by the electrostatic repulsion between the cationic surfactant molecules. With a further increase in the SDBS proportion, the number of FTMA/SDBS complexes in the vesicle increased, which results in formation of multilamellar vesicles and lamellar liquid crystals in aqueous solutions. In the vicinity of the equimolar mixing ratio (0.9 < [SDBS]/[FTMA] e 1), crystalline precipitates eventually appeared in the aqueous solutions because of the neutralization of electric charges between FTMA and SDBS. Cyclic Voltammetry (CV). Figure 6 shows cyclic voltammograms [scan speed (ν) ) 10 mV/s] for aqueous mixtures of FTMA and SDBS ([FTMA] is fixed at 7mM). An anodic current peak (I) was observed at 0.2 V versus SCE (Figure 6A) in the case of a pure FTMA aqueous solution. The electrochemical behavior of a pure FTMA aqueous solution was reported by Saji and co-workers.1,2

Ferrocene-Modified Cationic Surfactant

Langmuir, Vol. 19, No. 22, 2003 9347

Figure 7. Anodic peak potential of peak I divided by the square root of the scan rate [ip/(ν1/2)] plotted against the square root of the scan rate (ν1/2).

Figure 6. Cyclic voltammograms for aqueous mixtures of FTMA and SDBS (scan rate ) 10 mV/s, 7 mM FTMA, 0.02 M NaBr).

At concentrations of FTMA far above the cmc (0.07 mM), the anodic peak observed for pure FTMA aqueous solutions arises from the oxidation of FTMA molecules forming micelles. When FTMA monomers in the vicinity of the working electrode are oxidized, FTMA micelles diffuse from the bulk solution to the electrode owing to a concentration gradient of reduced-FTMA formed between the electrode and the bulk solution. The concentration of reduced-FTMA in the neighborhood of the electrode becomes lower than the cmc at the potential more anodic than the equilibrium potential, and FTMA molecules leave micelles as monomers as a result of a shift in the micellemonomer equilibrium. Dissociated FTMA monomers react electrochemically on the electrode to cause the reaction to proceed further.1,2 As shown in Figure 6A, cyclic voltammograms for FTMA/SDBS mixed micellar solutions in the M and M + V regions in Figure 5 gave an anodic peak I with a peak potential nearly equal to that observed in the pure FTMA aqueous solution. The current of peak I decreased with increasing molar ratio [SDBS]/[FTMA]. Surprisingly, another anodic peak (II) was observed at about 0.3 V versus SCE at the compositions (0.05 e [SDBS]/[FTMA] e 0.70) where vesicles were formed. The current of peak II increased as the molar ratio ([SDBS]/[FTMA]) increased. To analyze the electrochemical behavior of FTMA that gives these anodic peaks, the anodic peak current divided by the square root of the scan rate [ip/(ν1/2)] was plotted against ν1/2. These plots allow us to judge whether the electrochemical reaction is dominated by the electrochemical reactant (FTMA) diffused to the electrode or by that adsorbed on the electrode surface.8,44 Figure 7 shows (42) Safran, S. A.; Pincus, P.; Andelman, D. Science 1990, 248, 354.

the ip/(ν1/2) versus ν1/2 plots for peak I. In pure FTMA solutions with a concentration of 7 mM (far above the cmc), ip/(ν1/2) showed a remarkable decrease with increasing ν1/2. A similar result was reported by Watanabe and co-workers8 in aqueous solutions of a nonionic ferrocenyl surfactant [R-(ferrocenylundecyl)-ω-hydroxyoligo(ethylene oxide), FPEG]. They described that this decrease in ip/(ν1/2) is caused by the dissociation of FPEG molecules from micelles preceding the electron-transfer reaction (CE reaction).8 The present result on the ip/(ν1/2) versus ν1/2 plot for pure FTMA aqueous solutions is presumably brought about by a similar process. At compositions of 0.02 e [SDBS]/[FTMA] e 0.30, the ip/(ν1/2) values were almost constant independent of ν1/2. This suggests that the oxidation reaction of FTMA giving peak I is dominated mainly by a diffusion-limited process. On the other hand, ip/(ν1/2) for peak I increased with increasing ν1/2 in the region of 0.40 e [SDBS]/[FTMA] e 0.70. Thus, a contribution of the adsorption wave of FTMA molecules attached to the glassy carbon electrode is suggested in this composition range. Figure 8 shows the ip/(ν1/2) versus ν1/2 plots for anodic peak II observed in Figure 6. At mixing ratios of 0.05 e [SDBS]/[FTMA] e 0.30, the plots exhibited a diffusioncontrolled behavior and ip/(ν1/2) decreased with increasing ν1/2 at [SDBS]/[FTMA] ) 0.10. On the other hand, ip/(ν1/2) for peak II increased linearly with increasing ν1/2 in the region 0.40 e [SDBS]/[FTMA] e 0.70, which suggests a contribution of the adsorption waves to peak II. As shown in Figures 7 and 8, the oxidation process of FTMA (giving both peaks I and II) was found to change at the compositions where the phase shifts from the M + V to the M + V + L region. The process of the oxidation reaction of FTMA (peaks I and II) is discussed separately for the two regions 0.05 e [SDBS]/[FTMA] e 0.30 and 0.40 e [SDBS]/[FTMA] e 0.70. In the region 0.05 e [SDBS]/[FTMA] e 0.30, the oxidation reaction corresponding to peaks I and II is dominated by a diffusion process. Because the potential for peak I in the mixtures is nearly equal to that in the pure FTMA aqueous solution, it is suggested that peak I is due to the oxidation of uncomplexed FTMA molecules (43) Safran, S. A.; Mackintosh, F. C.; Pincus, P. A.; Andelman, D. A. Prog. Colloid Polym. Sci. 1991, 84, 3. (44) Southampton Electrochemical Group. Instrumental Methods in Electrochemistry; Ellis Horwood: Chichester, 1985; Chapter 6.

9348

Langmuir, Vol. 19, No. 22, 2003

Tsuchiya et al.

I. On the basis of this assumption, DCV was calculated using

C (mol/dm3) ) 7 × 10-3(1 - [SDBS]/[FTMA])

Figure 8. Anodic peak potential of peak II divided by the square root of the scan rate [ip/(ν1/2)] plotted against the square root of the scan rate (ν1/2).

Figure 9. Comparison of diffusion coefficients calculated from DLS data with those calculated from the anodic peak current of peak I from the cyclic voltammograms.

forming molecular assemblies, especially mixed micelles. The diffusion coefficient (DCV) calculated from the anodic peak current for peak I in the cyclic voltammograms (Figure 6) is compared with that of the molecular assemblies estimated from particle size distributions determined by DLS (Figure 2) in Figure 9. The diffusion coefficient DDLS (cm2/s) is calculated by the StokesEinstein expression, DDLS ) kT/6πηR, where k is the Boltzmann constant, T is the absolute temperature, η is the viscosity of the solvent, and R is the mean radius of the spherical aggregates estimated by DLS. On the basis of the assumption that anodic peak I is a reversible diffusion wave, the diffusion coefficient DCV (cm2/s) is calculated using the following equation:

Ip ) 0.4463 × 10-3n3/2F3/2A(RT)-1/2DCV1/2Cν1/2

(1)

where A (cm2) is the surface area of the electrode, n is the electron number involved in the electrochemical reaction, C (mol/dm3) is the concentration of the reactant, and ν (V/s) is the scan rate. In the present study, the concentration of FTMA is fixed at 7 mM in all the mixtures studied. We assumed simply that all the SDBS molecules, a poor component in the aqueous mixtures, form a pseudo doubletailed complex with the FTMA molecules and excess uncomplexed FTMA molecules are responsible for peak

As shown in Figure 9, when the scan rate was 200 mV/ s, the values of the diffusion coefficient (DCV) calculated from the current of anodic peak I were in good agreement with those (DDLS) of mixed micelles determined by DLS (peak A in Figure 1), while the DCV values decreased and approached the DDLS values of the vesicles (peak B in Figure 1) with a decreasing scan rate at lower scan rates. As previously described, the electrochemical reaction proceeds owing to the dissociation of FTMA molecules from the micelles in the case of pure FTMA micelles. Similarly, the vesicle formation in aqueous solutions of cationic and anionic surfactants is reported by many authors27,29-30,45-46 to be in equilibrium with their monomers; that is, vesicles are formed spontaneously. However, the rate of surfactant dissociation from the vesicles as monomers is much slower than that from (mixed) micelles. At low scan rates, the diffusion coefficient calculated from the current of peak I was smaller than that for mixed micelles but larger than that for vesicles because not only uncomplexed (free) FTMA molecules in mixed micelles but also those in vesicles dissociated into FTMA monomers as a result of the FTMA concentration gradient in the vicinity of the electrode surface. On the other hand, at a high scan rate (ν ) 200 mV/s), only uncomplexed FTMA molecules in mixed micelles but not those in vesicles dissociated into monomers and underwent the electrochemical reaction. The value of DCV is, thus, consistent with that of DDLS at the high scan rate (200 mV/s). Because peak II was observed in FTMA/SDBS aqueous solutions where vesicles were formed, the peak is suggested to be due to FTMA/SDBS complexes mainly constituting bilayers (vesicles and lamellar liquid crystals). The electrochemical reaction for peak II occurs in a way similar to that in the pure FTMA micellar system. As FTMA/SDBS complexes in the vicinity of the electrode are consumed by the oxidation reaction, the vesiclemonomer dissociation equilibrium shifts from the vesicle side to the monomer side. Hence, vesicles in the bulk solution diffuse to the electrode and then dissociate into monomers to be oxidized on the electrode surface. Figures 7 and 8 show that both oxidation currents (peak I, peak II) are produced by adsorbed FTMA (or the FTMA/ SDBS complex) at compositions of 0.40 e [SDBS]/[FTMA] e 0.70. As shown in Figure 2, mixed micelles are not formed in the mixtures at these compositions (V + L). As described in the previous section, the spontaneous vesicle formation results from formation of the pseudo doubletailed FTMA/SDBS complex, which has a high hydrophobicity compared with uncomplexed FTMA molecules because of the neutralization of the electric charges on the ionic headgroups. The number of FTMA/SDBS complexes in a vesicle is larger than that of FTMA (free) molecules at compositions of 0.40 e [SDBS]/[FTMA] e 0.70, and the surface of the vesicle (bilayer) has a high hydrophobicity. Because the surface of the glassy carbon electrode is hydrophobic, a surfactant having a high hydrophobicity is easier to adsorb on the electrode. Vesicles and lamellar liquid crystals in bulk solution are, therefore, deposited (adsorbed) onto the glassy carbon electrode, and uncomplexed FTMA and FTMA/SDBS complexes in the (45) Steller, K. L.; Amante, J. C.; Scamehorn, J. F.; Harwell, J. H. J. Colloid Interface Sci. 1988, 123, 186. (46) Laughlin, R. G.; Colloids Surf., A 1997, 128, 27-38.

Ferrocene-Modified Cationic Surfactant

Langmuir, Vol. 19, No. 22, 2003 9349 Table 1. Amount of FTMA Adsorbed onto the Glassy Carbon Electrode

Figure 10. Trapping efficiencies measured by the glucose dialysis method for FTMA/SDBS aqueous mixtures (7 mM FTMA, 0.02 M NaBr).

adsorbed membrane would contribute to the oxidation current for peaks I and II, respectively. The cathodic currents of peaks I and II were smaller than the corresponding anodic currents, which indicates that the adsorbed amount of oxidized FTMA molecules is less than that of the reduced ones because the oxidized form is more hydrophilic than the reduced form. The anodic potentials of peaks I and II were different from the corresponding cathodic potentials of the peaks. The interaction between the adsorbed molecules and the lower rate of the electrontransfer reaction might have caused the difference in the potentials. The contribution of the adsorption wave for the mixtures with compositions of 0.05 e [SDBS]/[FTMA] e 0.30 was considerably low compared with that in the composition range 0.40 e [SDBS]/[FTMA] e 0.70. This is because there are more uncomplexed (free) FTMA molecules entrapped in a vesicle in the range 0.05 e [SDBS]/ [FTMA] e 0.30. Therefore, in this composition range, vesicles and lamellar liquid crystals are difficult to adsorb onto the glassy carbon electrode owing to their high hydrophilicity. The results mentioned so far in this section will be summarized as follows. At compositions of 0.05 e [SDBS]/ [FTMA] e 0.30, the oxidation peaks I and II arise mainly from the diffusion wave of uncomplexed FTMA molecules forming molecular assemblies (especially mixed micelles) and that of FTMA/SDBS complexes forming vesicles (and lamellar liquid crystals), respectively. On the other hand, in the region 0.40 e [SDBS]/[FTMA] e 0.70, vesicles and lamellar liquid crystals adsorb on the glassy carbon electrode as a result of their relatively high hydrophobicity. The uncomplexed FTMA molecules and FTMA/SDBS complexes in the adsorption membrane contribute to the oxidation current peaks I and II, respectively. Thus, the results indicate that the electrochemical behavior of FTMA in FTMA/SDBS aqueous mixtures is affected by the aggregation state, such as mixed micelles and vesicles. If the results obtained in the present study are further developed, the electrochemical behavior of FTMA would help us to understand better the phase behavior of the mixed surfactant systems. Trapping Efficiency of Molecular Assemblies. The electrochemical behavior of FTMA-constituting vesicles mentioned in the previous section suggests that the physical properties of FTMA/SDBS vesicles with compositions of 0.05 e [SDBS]/[FTMA] e 0.30 are different from those at compositions of 0.40 e [SDBS]/[FTMA] e 0.70. To investigate the difference in the membrane properties, the trapping efficiency of the assemblies is examined using the glucose dialysis technique (Figure 10). The trapping efficiency means the amount of glucose left in the

FTMA/SDBS

Γ (10-10 mol/cm2)

1:0.40 1:0.50 1:0.60 1:0.70

6.71 11.2 14.9 22.1

assemblies after dialysis divided by the total amount of glucose used. The trapping efficiency was almost 0 at compositions of 0.05 e [SDBS]/[FTMA] e 0.25 even though vesicles were formed in this composition range as shown in Figures 2 and 3, while the vesicles formed in the region 0.30 e [SDBS]/[FTMA] e 0.80 had a high trapping efficiency. This is because the solute trapped in the inner water phase of the vesicles in the region 0.05 e [SDBS]/ [FTMA] e 0.25 leaks out into the bulk solution during dialysis. Vesicle bilayers in the region 0.05 e [SDBS]/ [FTMA] e 0.25 are sufficiently loose because uncomplexed FTMA molecules are incorporated in the outer vesicle monolayers mainly composed of FTMA/SDBS complexes, while the bilayers at 0.30 e [SDBS]/[FTMA] e 0.80 are rather rigid because the vesicles are constituted mostly by FTMA/SDBS complexes. The results of trapping efficiency measurements support the previous considerations about the electrochemical process. The inconsistency in the results of trapping efficiency and cyclic voltammogram measurements at the composition of [SDBS]/[FTMA] ) 0.3 could be due to the fact that the composition is close to the boundary between the M + V and the M + V + L regions. Adsorption Amount of FTMA on the Glassy Carbon Electrode. At compositions of 0.40 e [SDBS]/[FTMA] e 0.70, vesicles (and lamellar liquid crystals) attached to the glassy carbon to form adsorbed films as shown in Figures 7 and 8. Table 1 shows the adsorption amount of the FTMA molecules on the electrode calculated from anodic current peak II (ν ) 20 mV/s). It is reported8 that a nonionic ferrocenyl surfactant (FPEG) adsorbs on a glassy carbon electrode and forms a monolayer, and its saturated adsorption amount is 2.65 × 10-10 mol/cm2. The saturated adsorption amount of a cationic ferrocenyl surfactant [(ferrocenylmethyl)dimethyloctadecylammonium hexafluorophosphate] on a platinum electrode is also reported8 to be 3.7 × 10-10 mol/cm2. The adsorbed amounts calculated in the present work are considerably large compared with the value obtained in the previous results.8 Diociaiuti et al. have described that liposomes formed by a certain phospholipid in bulk solutions adsorb and form a multilayer on the surface of carbon.47 As the trapping efficiency measurements showed, vesicles in the composition range 0.40 e [SDBS]/[FTMA] e 0.70 have a rigid bilayer. In addition, the bilayer is formed mainly by FTMA/ SDBS complexes with a higher hydrohobicity. These events would promote aggregation (and deformation) of the vesicles (and lamellar liquid crystals) on the electrode. Consequently, vesicles formed in bulk solutions are likely to adsorb and form a multilayer on the glassy carbon electrode in the composition range 0.40 e [SDBS]/[FTMA] e 0.70. Anodic Peak Current versus Molar Ratio. Figure 11 shows the dependence of the anodic peak current (peak I) on the molar ratio ([SDBS]/[FTMA]). The anodic peak current also strongly depends on the phase behavior of the system. In the M + V region (0.05 e [SDBS]/[FTMA] (47) Diociaiuti, M.; Molinari, A.; Ruspantini, I.; Gaudiano, M. C.; Ippoliti, R.; Lendaro, E.; Bordi, F.; Chistolini, P.; Arancia, G. Biochim. Biophys. Acta 2002, 1559, 21.

9350

Langmuir, Vol. 19, No. 22, 2003

Tsuchiya et al.

Figure 11. Anodic peak current of peak I as a function of the molar ratio of FTMA to SDBS.

e 0.10), the current of anodic peak I decreased with increasing molar ratio [SDBS]/[FTMA]. Peak I is mainly a diffusion-controlled wave, as mentioned previously. In this case, the peak current is proportional to the concentration of electrochemical reactants and the square root of the scan rate according to eq 1. If the decrease in the current of anodic peak I only depends on the concentration of uncomplexed FTMA molecules, the peak current should linearly decrease and become 0 at the equimolar composition. However, the anodic peak current exhibited a remarkably nonlinear decrease, as shown in Figure 11. This suggests that the decrease is caused by a decrease in the diffusion coefficient of the electrochemical reactant (FTMA). The total numbers of mixed micelles and vesicles would respectively decrease and increase with increasing molar ratio ([SDBS]/[FTMA]). Hence, the mean size of the aggregates (including both mixed micelles and vesicles) increases and the average diffusion coefficient decreases with the molar ratio. In addition, the diameters of the mixed micelles and vesicles increase with increasing molar ratio, respectively (Figure 2). Consequently, the decrease in peak current I against the molar ratio in the M + V region would be caused by (1) a decrease in the concentration of uncomplexed FTMA due to FTMA/SDBS complex formation, (2) a decrease in the diffusion coefficient of either mixed micelles or vesicles, and (3) a decrease in the average diffusion coefficient of aggregates owing to the respective decrease and increase in the number of mixed micelles and vesicles. In the M + V + L region, the negative slope of the plots in Figure 11 for anodic current I was less steep than that in the M + V region. This is because a mixed micellevesicle-lamellar liquid crystal equilibrium is established as a result of lamellar liquid crystal formation. Because changes in the total number of mixed micelles and vesicles are suppressed by this equilibrium, an increase in the average particle size is also depressed. An increase in the molar ratio ([SDBS]/[FTMA]) from 0.3 to 0.4 caused an increase in the peak current for peak I at ν g 50 mV/s. This increase in the current was more significant when the scan rate was higher. As described previously, the electrochemical process in the V + L region (0.40 e [SDBS]/ [FTMA] e 0.70) is controlled mainly by the adsorption wave, unlike the situation of [SDBS]/[FTMA] e 0.40. The peak currents of the diffusion wave and adsorption wave are proportional to ν1/2 and ν, respectively. Hence, the difference in the peak current between the diffusion and the adsorption waves becomes larger with increasing scan rate. These results, shown by Figure 11, also reveal that the process of the electrochemical reaction for peak I in the V + L region (0.40 e [SDBS]/[FTMA] e 0.70) are

Figure 12. Anodic peak current of peak II as a function of the molar ratio of FTMA to SDBS.

different from those in the other regions (0 e [SDBS]/ [FTMA] e 0.30). Figure 12 shows the molar ratio ([SDBS]/[FTMA]) dependence of anodic peak current II. In the M + V and M + V + L regions, the current of anodic peak II was much lower than that of anodic peak I. This is because the total number of FTMA/SDBS complexes is fewer than that of the uncomplexed FTMA molecules and the diffusion coefficient of the vesicles is considerably lower than that of the mixed micelles. The anodic peak current for peak II showed a remarkable increase with increasing molar ratio of [SDBS]/[FTMA] in the V + L region (0.40 e [SDBS]/ [FTMA] e 0.70). Bilayers in bulk solutions adsorb onto the glassy carbon electrode as multilayers at these compositions because the number of FTMA/SDBS complexes in vesicles is larger than that in the M + V and M + V + L regions and the bilayers (vesicles and lamellar liquid crystals) have a higher hydrophobicity. This is the reason for the steep increase in the current of anodic peak II in the V + L region. The results mentioned in the present work indicate that the electrochemical behavior of FTMA sensitively reflects the change in the phase behavior of the system. Also, a possibility is suggested that examinations of the electrochemical behavior of other mixed surfactants systems with FTMA as a component would allow us to determine the aggregation states of the component in the systems with various compositions and concentrations with ease. Conclusions The present study shows that the electrochemical behavior of FTMA is strongly affected by the phase behavior of FTMA/SDBS aqueous mixtures. In cyclic voltammetry, the oxidation peak potential due to FTMA/ SDBS complexes mainly forming vesicles (and lamellar liquid crystals) is more anodic than that due to uncomplexed (free) FTMA molecules forming molecular assemblies (especially mixed micelles). At [SDBS]/[FTMA] e 0.30, the oxidation peaks are mainly diffusion-controlled waves, while the electrochemical process of the peaks at compositions of 0.40 e [SDBS]/[FTMA] e 0.80 are mainly dominated by adsorption species, which constitute the adsorbed layers formed by vesicles (and lamellar liquid crystals) in bulk solutions on the electrode. In addition, the peak current for the peaks plotted against the composition strongly reflects the change in the phase behavior of the mixtures. LA0301442