Langmuir 1998, 14, 7091-7094
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Articles Diffusion Coefficients of Micelles Composed of Fluorocarbon Surfactants with Cyclic Voltammetry Tsuyoshi Asakawa,* Hirokazu Sunagawa, and Shigeyoshi Miyagishi Department of Chemistry and Chemical Engineering, Faculty of Engineering, Kanazawa University, Kanazawa 920-8667, Japan Received November 17, 1997. In Final Form: September 10, 1998 Cyclic voltammetry of (ferrocenylmethyl)trimethylammonium bromide (FcTAB) was applied on anionic micelle systems in order to reveal the solution properties of micelles composed of fluorocarbon surfactants. Diffusion coefficients of FcTAB decreased with micelle formation because the cationic FcTAB diffuses together with micelles. The diffusion coefficient abruptly decreased with the addition of sufficient salt, suggesting an increase in micelle size and a transition of micelle shape. The diffusion coefficient for mixed micelles composed of fluorocarbon and hydrocarbon surfactants decreased considerably compared to those for their single surfactant systems. The micelle size of fluorocarbon surfactants, especially for lithium 1,1,2,2-tetrahydroheptadecafluorodecyl sulfate, significantly depended on the salt concentrations and the compositions of the mixtures. Cyclic voltammetry is a simple and useful method to evaluate the micelle diffusion coefficients of fluorocarbon surfactants which can hardly be estimated by dynamic-light-scattering measurements.
Introduction The size and shape of micelles are usually determined by a light-scattering method.1 However, it is difficult to determine the micelle size of the fluorocarbon surfactant in aqueous solution by a conventional light-scattering measurement, because the difference in refractive index between the fluorocarbon surfactant and water is extremely small. Thus Hoffmann et al. applied a small angle neutron-scattering (SANS) method to this system.2,3 Burkitt et al. also investigated mixed micelles of fluorocarbon and hydrocarbon surfactants by the SANS method.4,5 The model of segregation between fluorocarbon and hydrocarbon surfactants was proposed within a mixed micelle of cylindrical shape. Pulsed-gradient spin-echo Fourier transform NMR is a useful method to determine the self-diffusion coefficients of surfactant molecules.6 Since there is a fast exchange between monomers and micelles on the NMR time scale, the observed diffusion coefficient is identical to the average diffusion coefficient. The actual monitoring of micelle selfdiffusion was performed on signals from added trace amounts of tetramethylsilane which can be assumed to be completely solubilized in the micelle.6 The polarographic method was also used to estimate the diffusion coefficient of micelles containing electroactive probe * Corresponding author. E-mail:
[email protected]. (1) Imae, T.; Ikeda, S. J. Phys. Chem. 1986, 90, 5216 and references therein. (2) Hoffman, H.; Ulbricht, W.; Tagesson, B. Z. Phys. Chem. 1978, 113, 17. (3) Hoffmann, H.; Kalus, J.; Thurn, H. Colloid Polym. Sci. 1983, 261, 1043. (4) Burkitt, S. J.; Ottewill, R. H.; Hayter, J. B.; Ingram, B. T. Colloid Polym. Sci. 1987, 265, 619. (5) Burkitt, S. J.; Ottewill, R. H.; Hayter, J. B.; Ingram, B. T. Colloid Polym. Sci. 1987, 265, 628. (6) Carlfors, J.; Stilbs P. J. Phys. Chem. 1984, 88, 4410.
molecules.7 The electrochemical behavior of the micellesolubilized probe was investigated in order to evaluate the micelle size.8 This method requires that the introduction of a small amount of probe molecule per micelle will not alter the size and shape of the micelle. The probe should be completely solubilized in micelles. The rate constant for electron transfer between probe and electrode should be large enough to give the probe’s translational diffusion-determining step toward the electrode surface. The aim of this study is to evaluate the diffusion coefficient of fluorocarbon micelles using a cyclic voltammetric method with a suitable probe under these conditions. Ferrocene is well-known to give a reversible one-electron reaction with cyclic voltammetry. The diffusion coefficient of ferrocene can be estimated by using the Randles-Sevcik equation with the cyclic voltammetry peak current.9 Rusling et al. estimated the diffusion coefficient of the micelle and free ferrocene in water by the two-state model.10 Amphiphilic ferrocenes were also investigated in order to reveal the mechanism of electron transfer between ferrocene and the electrode surface.11-14 The electron transfer was sufficiently fast for the ferrocene probe in the presence of micelles. The solubilization power of the fluorocarbon micelle toward organic compounds is known to be small. But if (7) Zana, R.; Mackey, R. A. Langmuir 1986, 2, 109. (8) Mandal, A. B. Langmuir 1993, 9, 1932. (9) Chokshi, K.; Qutubuddin, S.; Hussam, A. J. Colloid Interface Sci. 1989, 129, 315. (10) Rusling, J. F.; Shi, C.-N.; Kumosinski, T. F. Anal. Chem. 1988, 60, 1260. (11) Abbott, A. P.; Gounili, G.; Bobbitt, J. M.; Rusling, J. F.; Kumosinski, T. F. J. Phys. Chem. 1992, 96, 11091. (12) Gounili, G.; Bobbitt, J. M.; Rusling, J. F. Langmuir 1995, 11, 2800. (13) Mackay, R. A.; Mayers, S. A.; Bodalbhai, L.; Brajiter-Toth, A. Anal. Chem. 1990, 62, 1084. (14) Electrochemistry in Colloids and Dispersions; Mackay, R. A., Texter, J., Eds.; VCH Press: Cambridge, 1992.
10.1021/la9712552 CCC: $15.00 © 1998 American Chemical Society Published on Web 11/13/1998
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Figure 1. Cyclic voltammograms of a 1 mM FcTAB aqueous solution in the absence and presence of LiHFDeS at the scan rate 10 mV/s: (s) 0.1 M LiCl; (- - -) 0.1 mM LiHFDeS in 0.1 M LiCl; (- - -) 10 mM LiHFDeS in 0.1 M LiCl. The arrows show directions of the scans.
Figure 2. Plot of ipa versus v1/2 for a 1 mM FcTAB aqueous solution in the absence and presence of LiHFDeS at the scan rates 1, 2, 5, 10, and 20 mV/s: (O) 0.1 M LiCl; (b) 0.1 M LiHFDeS in 0.1 M LiCl.
the electrochemical probe is completely solubilized by electrostatic interaction into the micelles, the diffusion coefficient of the probe can be used to estimate the micelle size with the Stokes-Einstein equation. In this paper, a cationic ferrocene derivative, (ferrocenylmethyl)trimethyammonium bromide (FcTAB), was solubilized into an anionic surfactant micelle by electrostatic interaction. The cyclic voltammetry technique with this probe was examined in aqueous micelle solutions of surfactants having various hydrophilic groups and hydrophobic carbon chain lengths. The salt-induced increase in micelle size was investigated, especially for fluorocarbon surfactant systems. Moreover, the mixing effect of two surfactants on micelle size was also examined in order to reveal the effect on micellization of the specific characteristics of the rigid fluorocarbon chain.
interaction. Figure 1 shows cyclic voltammograms for 1 mM FcTAB in aqueous 0.1 M LiCl solutions. It exhibits a reversible one-electron step, judging from the separation between the cathodic and anodic peak potentials (∆Ep ) Epa - Epc ) 70 mV) and the ratio of cathodic to anodic peak currents (ipa/ipc ) 1.0). The cyclic voltammograms for FcTAB containing 0.1 mM LiHFDeS were almost unaffected by the presence of anionic surfactant monomer in comparison to that of FcTAB in the absence of surfactant. The ipa values remained constant within the experimental error when the surfactant concentrations were below the cmc of LiHFDeS. On the other hand, the ipa value for 10 mM LiHFDeS considerably decreased due to the micelle solubilization of FcTAB, as shown in Figure 1. The ipa for one-electron reversible systems is given by Randles-Seveik equation,9
Experimental Section Materials. Fluorocarbon and hydrocarbon surfactants were prepared by the same procedures as reported previously.15 Abbreviations for the surfactants are as follows: lithium perfluorononanoate (LiPFN), C8F17COOLi; lithium perfluorooctanesulfonate (LiFOS), C8F17SO3Li; lithium 1,1,2,2-tetrahydroheptadecafluorodecyl sulfate (LiHFDeS), C8F17CH2CH2SO4Li; lithium dodecyl sulfate (LiDS), C12H25SO4Li; lithium tridecyl sulfate (LiTrS), C13H27SO4Li; lithium tetradecyl sulfate (LiTS), C14H29SO4Li. (Ferrocenylmethyl)trimethylammonium bromide (FcTAB, Tokyo Kasei Kogyo Co., Ltd.) was used as received. The other reagents were of guaranteed grade. Measurements. Cyclic voltammetry (CV) was performed by using an electrochemical analyzer, HAB-151 (Hokuto Denko Co. Ltd.). The working electrode was a highly polished glassy carbon disk electrode (GCE, A ) 0.071 cm2). A platinum wire was used as a counter electrode, and the reference electrode was Ag/AgCl wire with a salt bridge containing a 3.3 M KCl aqueous solution. The cell was thermostated to 25.0 ( 0.1 °C. The electrode (GCE) surface was polished before each voltammetric scan. Reversible cyclic voltammetry at the scan rates 1-20 mV/s from 0.01 to 0.6 V was used to estimate the apparent diffusion coefficient for FcTAB. FcTAB is easily solubilized into an anionic micelle solution containing a salt (LiCl) concentration of 0.1-1.5 M.
Results and Discussion Ferrocene is partitioned between micelles and bulk water because of its poor solubility in water.16 On the other hand, cationic (ferrocenylmethyl)trimethylammonium bromide (FcTAB) is soluble in water, but it can be solubilized into anionic micelles due to electrostatic (15) Asakawa, T.; Hashikawa, M.; Amada, K.; Miyagishi, S. Langmuir 1995, 11, 2376. (16) Ohsawa, Y.; Aoyagui, S. J. Electroanal. Chem. 1982, 136, 353.
ipa ) 0.4463FACp(F/RT)1/2v1/2D1/2
(1)
where A is the area of the electrode, F is Faraday’s constant, R is the gas constant, T is the absolute temperature, Cp is the concentration of the electroactive probe, D is the diffusion coefficient, and v is the scan rate. Equation 1 was used to estimate the diffusion coefficient from the slopes of ipa vs v1/2 plots at fixed 1 mM FcTAB. It has already been reported that the electron transfer is sufficiently fast for the ferrocene probe in the presence of surfactant micelles.11 With micelle systems involving an electroactive probe completely solubilized in the micelles, the diffusion coefficient would correspond to the micelle diffusion coefficient DM, since the probe diffuses along with the micelle. The ipa versus v1/2 plots gave straight lines both in the absence and in the presence of LiHFDeS micelles, as shown in Figure 2. From its slope, a value of 5.9 × 10-6cm2,s-1 was obtained for the diffusion coefficient of FcTAB in water. The hydrodynamic radius of the FcTAB equivalent sphere was calculated by the Einstein-Stokes equation
r ) kBT/6πη0D
(2)
where kB is the Boltzmann constant and η0 is the viscosity of the solvent. The hydrodynamic radius of FcTAB was 4.1 Å, which corresponded to 3.8 Å for ferrocene.11 When FcTAB is partitioned between micelles and bulk water, the obtained value of 6.3 × 10-7cm2 s-1 could be considered as the apparent diffusion coefficient of the LiHFDeS micelle.
Diffusion Coefficients by Cyclic Voltammetry
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Figure 3. Influence of FcTAB concentration on diffusion coefficient: (O) 0.1 M LiCl; (b) 0.1 M LiPFN in 0.1 M LiCl; (4) 0.1 M LiDS in 0.1 M LiCl.
Figure 4. Influence of surfactant concentration on the apparent micelle diffusion coefficient of LiDS in 0.1 M LiCl. The concentration ratio of FcTAB per LiDS was fixed at 0.01.
Next, we examined the effect of FcTAB concentration on the diffusion coefficient, as shown in Figure 3. The diffusion coefficients of FcTAB were almost constant within the experimental error in 0.1 M LiCl aqueous solution. With micelle systems the diffusion coefficients were also constant against FcTAB concentration. This suggests that FcTAB is completely solubilized into the micelles. The diffusion coefficients were unaffected by the average number of FcTAB molecules per micelle, that is, about 0.5-1 in LiDS micelles. Such a small amount of probe will not alter the micelle size. The equilibrium of the probe between micelle and water has been expressed by the two-state model in eq 3,10
Dapp ) fWDW + fMDM
(3)
where fW and fM are the fractions of the probe in water and in the micelle and DW and DM are the diffusion coefficients of the probe in water and in the micelle, respectively. The equilibrium may be formulated as
M + P f MP
K ) [MP]/[M][P]
(4)
Substituting the equilibrium expressions for fW and fM into eq 3,
Dapp )
DW + K[M]DM 1 + K[M]
(5)
Rearranging eq 5, the reciprocal relation is given by
1 1 1 ) + DW - Dapp DW - DM (DW - DM)K[M] )
1 n + DW - DM (DW - DM)K(C - cmc)
(6)
where C is the surfactant concentration, cmc is the critical micelle concentration and n is the micelle aggregation number. The plot of 1/(DW - Dapp) versus 1/(C - cmc) gave a straight line, as shown in Figure 4. From this intercept a value of 1.12 × 10-6 cm2 s-1 was obtained for DM of LiDS. The DM was almost identical to the value of Dapp in 0.1 M LiDS. The Dapp in 0.1 M surfactant could be regarded as DM due to the almost complete solubilization of FcTAB in micelles. (17) Asakawa, T.; Imae, T.; Ikeda, S.; Miyagishi, S.; Nishida, M. Langmuir 1991, 7, 262. (18) Asakawa, T.; Ikehara, J.; Ikeda, S.; Miyagishi, S. J. Am. Oil Chem. Soc. 1996, 73, 21.
Figure 5. Apparent micelle diffusion coefficient as a function of added LiCl concentrations in 0.1 M surfactant containing 1 mM FcTAB: (b) LiHFDeS; (2) LiFOS; (9) LiPFN; (O) LiDS.
We have reported the salt-induced micelle growth especially for anionic fluorocarbon surfactants.17,18 The viscosity measurements suggested that the transition of micelle shape may occur at 0.5 and 1.0 M LiCl in LiPFN and LiFOS, respectively.18 The salt-induced micelle growth of fluorocarbon surfactants is significant in comparison to that of the corresponding hydrocarbon surfactants. The behavior can be reexamined by the cyclic voltammetry method designed in this study. Figure 5 shows the variation of Dapp of surfactant micelles as a function of LiCl. The Dapp for LiDS gradually increased up to 0.5 M LiCl. The Dapp is small at lower concentration of LiCl due to an electroviscous effect.19 The Dapp for LiDS micelles significantly decreased above 1.2 M LiCl, while the Dapp for LiFOS micelles gradually decreased with increasing LiCl concentration and became almost constant with the sufficient addition of LiCl. The decrease in Dapp for LiPFN micelles was larger than that for LiFOS micelles, corresponding to the viscosity measurements. The Dapp for LiHFDeS micelles abruptly decreased only with low LiCl concentration. The micelle growth of LiHFDeS was significantly enhanced in comparison to that of LiDS containing the same hydrophilic group and a longer chain length. It has been reported that the aggregation numbers of fluorocarbon micelles are rather small in the absence of salt due to the bulkiness of the short fluorocarbon chain.20,21 Addition of salt was very effective in increasing the micelle aggregation number due to the suppression of (19) Birdi, K. S. Prog. Colloid Polym. Sci. 1985, 70, 23. (20) Turro, J.; Lee, P. C. C. J. Phys. Chem. 1982, 86, 3367. (21) Muto, Y.; Esumi, K.; Meguro, K.; Zana, R. J. Colloid Interface Sci. 1987, 120, 162.
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Table 1. Effect of Salt on Micelle Growth of Single-Surfactant Systems surfactanta C8F17COOLi (LiPFN) C8F17SO3Li (LiFOS)
C8F17CH2CH2SO4Li (LiHFDeS) C12H25SO4Li (LiDS) C13H27SO4Li (LiTrS) C14H29SO4Li (LiTS)
LiCl/M
106D/cm2 s-1
rb/nm
0.1 0.5 1.0 0.1 0.5 1.0 1.5 0.1 0.2 0.1 1.0 1.5 0.1 0.1
1.01 0.41 0.22 0.98 0.89 0.29 0.22 0.63 0.13 1.13 1.02 0.26 0.98 0.83
2.3 5.6 9.9 2.5 2.6 7.3 9.0 3.9 18.3 2.1 2.1 12.6 2.5 2.9
a 0.1 M surfactant aqueous solutions containing 1 mM FcTAB at 25 °C. b r ) kBT/6πη0D, the apparent hydrodynamic micelle radius.
electrostatic repulsion between the ionic headgroups. The aggregation numbers of LiDS micelles increased with increasing LiCl concentration, and the micelle growth of LiDS could be attended by the transition from sphere to prolate ellipsoidal micelles according to small angle neutron scattering (SANS) results.22 The apparent hydrodynamic radii of micelles are summarized in Table 1.23 The measured intrinsic viscosity [η] of LiDS in 0.11.0 M LiCl was 4.2 cm3 g-1, while that of LiDS in 1.6 M LiCl was 5.0 cm3 g-1. That is, the salt-induced micelle growth became significant above 1.5 M LiCl for LiDS. The SANS and viscosity measurements suggested similar behavior concerning salt-induced micelle growth in accordance with cyclic voltammetry measurements. The [η] values of LiPFN and LiFOS in 0.1 M LiCl were 2.7 and 2.5 cm3 g-1, respectively. The [η] of LiPFN in 1.0 M LiCl and LiFOS in 1.6 M LiCl were 16.3 and 12.0 cm3 g-1, respectively. The addition of salt is very effective for micelle growth of fluorocarbon surfactants in comparison to that of hydrocarbon surfactants. Fluorocarbon surfactants are characterized by lower cmc values than those for the corresponding hydrocarbons due to a higher hydrophobicity of the fluorocarbon chain. Both a greater van der Waals volume and a more extended average configuration of the fluorocarbon chain will have an influence on the aggregate geometry. Burkitt et al. proposed a cylindrical micelle in which the headgroups are hexagonally close-packed and the fluorocarbon chains form helical rows.4 Berr and Jones reported that sodium perfluorooctanoate, with the small aggregation number 23, forms spherical micelles whereas ammonium perfluorooctanoate, with a larger aggregation number, forms ellipsoidal micelles due to the reduced electrostatic repulsion between the headgroups.24 Fluorocarbon and hydrocarbon surfactants should be miscible at least partially in micelles. Some commercial hydrocarbon surfactants have been mixed with fluorocarbon surfactants to improve their performance. The (22) Bendedouch, D.; Chen, S.-H., J. Phys. Chem. 1984, 88, 648. (23) Physical Chemistry of Macromolecules; Tanford, C., Ed.; Wiley: New York, 1961. (24) Berr, S. S.; Jones, R. R. M. J. Phys. Chem. 1989, 93, 2555.
Figure 6. Apparent micelle diffusion coefficient as a function of the mole fraction of LiHFDeS (LiFOS, LiPFN, LiTrS) at a fixed 0.1 M total surfactant concentration in the presence of 1 mM FcTAB and 0.1 M LiCl. (b) LiHFDeS-LiDS; (2) LiFOSLiDS; (9) LiPFN-LiDS; (O) LiTrS-LiDS.
synergy will be governed by the interactions between the rigid fluorocarbon and more flexible hydrocarbon chains in micelles. The mixed micelle of ammonium perfluorooctanoate and ammonium decanoate was found to be cylindrical with micelle growth compared to that for single surfactant systems.5 The Dapp values for fluorocarbonhydrocarbon surfactant mixtures were significantly decreased in comparison to those for single surfactant systems, as shown in Figure 6. The Dapp values for LiHFDeS-LiDS mixtures were about one-third those of a pure LiHFDeS micelle; those for LiTrS-LiDS (hydrocarbon-hydrocarbon) mixtures were almost intermediate between those for pure LiTrS and LiDS micelles. The incorporation of a rigid fluorocarbon chain would enhance the ordered packing of hydrophobic chains in micelles, which induced the micelle growth along with the micelle shape transition. Such a micelle growth may lead to segregation between fluorocarbon and hydrocarbon surfactants with longer hydrophobic chains due to their immiscibility. Conclusions The knowledge of fluorocarbon micelles is sparse in contrast to the extensive information about hydrocarbon micelles. The micelle solutions of fluorocarbon surfactants have been characterized by less sensitive or more complicated techniques because the fluorocarbon micelles are more difficult to investigate. The simple cyclic voltammetry method was useful to evaluate the micelle diffusion coefficient using (ferrocenylmethyl)trimethylammonium bromide (FcTAB). Diffusion coefficients of FcTAB were suppressed along with micelle growth because the cationic FcTAB diffuses along with large anionic micelles. The diffusion coefficients for fluorocarbon micelles significantly decreased with the addition of salt in comparison to those for hydrocarbon micelles. The diffusion coefficients for mixed micelles composed of fluorocarbon and hydrocarbon surfactants were decreased considerably compared with those for hydrocarbon and hydrocarbon surfactant mixtures. The micelle sizes of fluorocarbon surfactants were found to be large in the presence of salt and mixtures with hydrocarbon surfactants. LA9712552
