Langmuir 1997, 13, 2935-2942
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Micelle Aggregating Condition of Fluorocarbon-Hydrocarbon Hybrid Surfactants in Aqueous Solution Atsushi Ito,† Keiji Kamogawa,‡,§ Hideki Sakai,†,‡ Katsumi Hamano, Yukishige Kondo,‡,⊥ Norio Yoshino,‡,⊥ and Masahiko Abe*,†,‡ Faculty of Science and Technology, Science University of Tokyo, 2641 Yamazaki, Noda, Chiba 278, Japan, Institute of Colloid and Interface Science, Science University of Tokyo, 1-3 Kagurazaka, Shinjuku, Tokyo 162, Japan, Faculty of Engineering, Science University of Tokyo, 1-3, Kagurazaka, Shinjuku, Tokyo 162, Japan, and Elementary & Secondary Education Bureau, The Ministry of Education, Sports and Culture, Kasumigaseki 3-2-2, Chiyoda, Tokyo 100, Japan Received March 13, 1996. In Final Form: March 24, 1997X The micelle aggregating condition of sodium 1-oxo-1-[(4-fluoroalkyl)phenyl]-2-alkanesulfonate (FCmHCn; m ) 4, 6, n ) 2, 4, 6), hybrid surfactants containing a fluorocarbon chain and a hydrocarbon chain in the same molecule, has been investigated by means of the pyrene fluorescence probe method and Raman spectroscopy. Micropolarity of the hydrocarbon moiety for the FCm-HCn micelles is examined with the I1/I3 ratio in the fluorescence spectra of pyrene added as an indicator. The ratios for the FC6-HCn and FC4-HC6 solutions decrease significantly even above their critical micelle concentration (cmc), suggesting that these micelles gradually change their aggregation state above the cmc with concentration increment. The wavenumber of CH3 asymmetric stretching vibration for the FC6-HCn series in Raman spectra shifts to lower wavenumber at this concentration range, while that of FC4-HCn is almost constant. These results suggest that FC6-HCn and FC4-HC6 form a wet micelle first but another dehydrated micelle coexists above the second cmc.
Introduction Hybrid surfactants, which have a fluorocarbon chain and a hydrocarbon chain in the same molecule, have been studied extensively.1-4 For instance, a series of hybrid anionic surfactants, CmF2m+1CH(OSO3Na)CnH2n+1 (m ) 6-9, n ) 1-9), was synthesized, and its solution property was investigated by Guo et al.1,2 However, the surfactants slowly hydrolyzed in air due to adsorption of moisture. Thereby, we have synthesized novel hybrid surfactants which never hydrolyze in air by introducing a phenyl group between the hydrocarbon and fluorocarbon moieties and analyzed the Krafft point, surface activity, and critical micelle concentration (cmc) of these aqueous solutions.3 Many studies on mixed systems of fluorocarbon and hydrocarbon surfactants have reported that extremely heterogeneous micelles are formed in aqueous solutions.5-16 Such heterogeneity may be also reproduced with chemi* To whom all correspondence should be addressed: telephone, 81-471-24-8650; fax, 81-471-24-8650; e-mail, abemasa@ koura01.ci.noda.sut.ac.jp. † Faculty of Science and Technology, Science University of Tokyo. ‡ Institute of Colloid and Interface Science, Science University of Tokyo. § Elementary and Secondary Education Bureau, The Ministry of Education, Sports, and Culture. ⊥ Faculty of Engineering, Science University of Tokyo. X Abstract published in Advance ACS Abstracts, May 1, 1997. (1) Guo, W.; Li, Z.; Fung, B. M.; O’Rear, E. A.; Harwell, J. H. J. Phys. Chem. 1992, 96, 6738. (2) Guo, W.; Fung, B. M.; O’Rear, E. A. J. Phys. Chem. 1992, 96, 10068. (3) Yoshino, N.; Hamano, K.; Omiya, T.; Kondo, Y.; Ito, A.; Abe, M. Langmuir 1995, 11, 466. (4) Inoue, H.; Arai, S.; Kakuta, Y.; Taki, M.; Masuda, H.; Moronuki, N.; Yamada, M. Mem. Fac. Technol., Tokyo Metrop. Univ. 1992, No. 42, 4511. (5) Mukerjee, P.; Yang, A. Y. S. J. Phys. Chem. 1976, 80, 1388. (6) Shinoda, K.; Nomura, T. J. Phys. Chem. 1980, 84, 365. (7) Funasaki, N.; Hada, S. J. Phys. Chem. 1980, 84, 736. (8) Carlfors, J.; Stilbs, P. J. Phys. Chem. 1984, 88, 4410. (9) Asakawa, T.; Miyagishi, S.; Nishida, M. J. Colloid Interface Sci. 1985, 104, 279.
S0743-7463(96)00242-9 CCC: $14.00
cally bound systems. Thus, it is important to study the aggregation state of the hybrid surfactant micelle for a model of fluorocarbon-hydrocarbon mixed micelle. The aggregation state of a fluorocarbon-hydrocarbon hybrid surfactant micelle has been studied by Guo et al.1 and Inoue et al.4 by means of 1H- and 19F-NMR. Guo et al.1 reported that both the fluorocarbon and the hydrocarbon chains were incorporated in the interior of the micelle when the hydrocarbon chain bears three carbons or more. Inoue et al.4 studied the surfactants which have a short fluorocarbon chain and a long hydrocarbon chain, CnF2n+1CONHCH2CH2N+(CH3)2C16H33Br- (n ) 1-3) and have shown that fluoroalkyl groups extended their chains straight forward to the micellar surface and that the terminal CF3 groups directly face the bulk phase water. Of course, these result from the synthesized control, where the alkyl groups were long enough to singly form the hydrophobic core and expel the short fluorocarbon chain to locate at the surface. In our previous paper,3 we reported that hybrid surfactants such as sodium 1-oxo1-[(4-fluoroalkyl)phenyl]-2-alkanesulfonate (FCm-HCn; m ) 4, 6, n ) 2, 4, 6) changed their aggregation state by increasing the concentration of the surfactants. In this paper, we have examined the internal micelle environment in aqueous solutions of the hybrid surfactants bearing different hydrocarbon and fluorocarbon chains by the fluorescence probe method and Raman spectroscopic method. (10) Muto, Y.; Esumi, K.; Meguro, K.; Zana, R. J. Colloid Interface Sci. 1987, 120, 162. (11) Kalyanasundaram, K. Langmuir 1988, 4, 942. (12) Burkitt, S. J.; Ingram, B. T.; Ottewill, R. H. Prog. Colloid Polym. Sci. 1988, 76, 247. (13) Matsuki, H.; Ikeda, N.; Aratono, M.; Kaneshina, S.; Motomura, K. J. Colloid Interface Sci. 1992, 150, 331. (14) Guo, W.; Guzman, E. K.; Heavin, S. D.; Li, Z.; Fung, B. M.; Christian, S. D. Langmuir 1992, 8, 2368. (15) Abe, M.; Yamaguchi, T.; Shibata, Y.; Uchiyama, H.; Ogino, K.; Yoshino, N.; Christian, S. D. Colloids Surf. 1992, 67, 29. (16) Kamogawa, K.; Tajima, K. J. Phys. Chem. 1993, 97, 9506.
© 1997 American Chemical Society
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Chart 1. Molecular Structure of Fluorocarbon-Hydrocarbon Hybrid Surfactants
Experimental Section Materials. Hybrid surfactants with different fluorocarbon and hydrocarbon chains, sodium 1-oxo-1-[(4-fluoroalkyl)phenyl]2-alkanesulfonate (FCm-HCn; m ) 4, 6, n ) 2, 4, 6), graphically shown in Chart 1, were synthesized as mentioned in our previous paper.3 Sodium dodecyl sulfate (SDS, C12H25OSO3Na) was purchased from Tokyo Kasei Kogyo Co., Ltd., Tokyo, Japan. It was more than 99.5% pure. This material was extracted with ether and recrystallized from ethanol. Sodium perfluorooctanoate (SPFO, C7F15COONa) was prepared by neutralizing perfluorooctanoic acid with sodium hydrogen carbonate solution. Pyrene used as a fluorescence probe was purchased from Wako Pure Chemical Industries Co., Ltd., Tokyo, Japan. It was recrystallized three times from ethanol, dissolved in cyclohexane and passed through silica gel, and dissolved in the surfactant solutions up to 2.0 × 10-5 mol/L concentration. Water used in this experiment was distilled water for injection, Japanese Pharmacopoeia, obtained from Otsuka Pharmacy Co., Ltd., Tokyo, Japan. Measurements. Fluorescence spectra of pyrene monomers in the surfactant solution at 335 nm excitation were recorded with a RF-5000 spectrofluorophotometer (Shimadzu Co., Tokyo, Japan) and with a quartz cell. Raman spectra were measured by spectrometer with a multireflection cell and with an optical-fiber light-collecting device.17 A 514.5 nm Ar+ ion laser (Inova70A, Coherent Co., 1000 mW), a single monochromator (CT-50C, JASCO), and an intensified photodiode-array detector (1420HQ, EG&G) were used. Both the sample and reference spectra (Ne lamp) were measured quasi-simultaneously for calibration of wavelength and white emission for correction of sensitivities among different channels of the diode array detector, and the data were sent to a 16-bit computer (PC-9801VM21, NEC) for further analysis. A holographic edge filter (Physical Optics Corp.) was placed to reject the Rayleigh scattering down to ca. 200 cm-1. Central positions of the concerned Raman bands were analyzed with the curve-fitting procedure at the least-squares condition. In this study, the sum of a Gaussian function and an asymmetric baseline function were fitted to the observed curve. This baseline function can extract all the envelopes of the neighboring bands or background without specifying the number and types of the envelopes. This minimizes artificial uncertainty and gets universal accuracy for a wide range of concentrations.
Figure 1. Surface tension against concentration of FCm-HCn.
Surface Tension vs Concentration. Figure 1 shows the relationship between surface tension and concentration of FCm-HCn in aqueous solutions. Surface tension for each FCm-HCn decreased with an increase in surfactant concentrations and the curve inflected to give each cmc, which was equal to the value obtained from a static light scattering measurement. It should be noted that each surface tension decreases continuously to a smaller extent, even above each cmc. Such a stepwise decrease of surface tension has been reported for mixed micelle solutions of fluorocarbon and hydrocarbon surfactants, and for some single surfactants such as SDS and AOT.18 In the former case the secondary decrease is interpreted to indicate the coexistence of two different types of micelles, one being a fluorocarbon surfactant rich micelle and the
other a hydrocarbon surfactant rich micelle. In the case of single surfactants such as FCm-HCn, however, two species of homogeneous micelles are hardly imagined because of the fixed number ratio of the fluorocarbon chain to the hydrocarbon chain. Nevertheless, this is probably because the micellar structure should be changed with a concentration increment even above cmc. Effect of the Surfactant Concentration on the Micropolarity. To evaluate the change in the micelle aggregating condition of the hybrid surfactant according to its concentration increment, the internal polarity of the micelle has been monitored with a pyrene fluorescence measurement. Fluorescence spectrum of pyrene has five predominant vibronic peaks, numbered 1-5 from shorter to longer wavelength. The intensity ratio of the first peak to the third one, I1/I3, indicates the micropolarity around pyrene molecules at the solubilizing site.19,20 Figure 2 shows the concentration dependence of the I1/I3 ratios of the FCm-HCn solutions. In the figure, the I1/I3 ratios in FCm-HCn solutions below the cmc’s are close to the value in water (1.57). As the concentration increased, the ratio decreased to reach 1.05-1.20 just or above the cmc concentration. The zipping point is much higher than the cmc obtained from Figure 1. The concentration dependence of the micropolarity was compared among the typical surfactants. Figure 3 shows the concentration dependence of the I1/I3 ratio of solutions of SDS, a typical hydrocarbon surfactant, of SPFO, a typical fluorocarbon surfactant, and of a mixture system of SDS and SPFO at a 1:1 molar ratio. In all solutions, the I1/I3 ratios below cmc were about 1.56, which is equal to the value in water. In SDS and SDS-SPFO solutions, the I1/I3 ratios decreased rapidly to about 1.02 when the concentration just reaches the cmc (SDS, 8 × 10-3 mol/L; SDS-SPFO, 9 × 10-3 mol/L). This depression of the I1/I3 values accompanied by the micelle formation may be caused by the transfer of pyrene molecules from bulk water to the inside of the micelles. In SPFO solutions, the ratio decreased gradually above the cmc (2 × 10-2 mol/L). It is considered that pyrene is hard to solubilize to the SPFO micelle because the hydrophobic chain of SPFO is also
(17) Kamogawa, K.; Takamura, M.; Matsuura, H.; Kitagawa, T. Spectrochim. Acta 1994, 50, 1513. (18) Purcell, I. P.; Thomas, R. K.; Penfold, J.; Howe, A. M. Colloids Surf. 1995, 94, 125.
(19) Kalyanasundaram, K.; Thomas, J. K. J. Am. Chem. Soc. 1977, 99, 2039. (20) Turro, N. J.; Kuo, P. L.; Somasundaran, P.; Wong, K. J. Phys. Chem. 1986, 90, 288.
Results and Discussion
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a
b
c
d
e
f
Figure 2. Concentration dependence of I1/I3 ratio for FCm-HCn aqueous solutions of (a) FC4-HC2 (25 °C), (b) FC4-HC4 (25 °C), (c) FC4-HC6 (25 °C), (d) FC6-HC2 (25 °C), (e) FC6-HC4 (25 °C), and (f) FC6-HC6 (50 °C).
lipophobic. Therefore, at this stage, the high I1/I3 value in Figure 2 may suggest a low solubility of pyrene to the FCm-HCn micelles. However, we also noticed that the curves in Figure 2 were convex to upper, while those in Figure 3 were convex to below. In addition, we have reported previously that pyrene molecules are solubilized in the hydrocarbon chain interior, i.e., the hydrocarbon chain and benzyl groups21
of the FCm-HCn micelles, over the entire concentration region. Therefore, curves in Figure 2 reflect two different micellar environments, more than the solubilizing isotherm between bulk water and a single kind of micelle. That is, the micelle of the hybrid surfactant gradually changes its aggregation condition, e.g., orientation of the (21) Ito, A.; Kamogawa, K.; Sakai, H.; Abe, M.; Hamano, K.; Kondo, Y.; Yoshino, N. J. Jpn. Oil Chem. Soc. To be submitted.
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Figure 3. Concentration dependence of I1/I3 ratio of SDS, SPFO, and SDS-SPFO mixture systems at 25 °C.
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Figure 5. Temperature dependence of I1/I3 ratio for FC6-HC4 solutions.
since molecular motion becomes more vigorous. Moreover, the concentration where the I1/I3 ratio begins to decrease was about 1 × 10-3 mol/L, independent of the temperature. Second cmc. If the micellar structures of FCm-HCn change according to their concentration increments, they may have a second cmc. Suppose that they form a micelle at the first cmc (c1) and form another type of micelle at a second cmc (c2), the I1/I3 value observed consists of the I1/I3 ratio in bulk (I1/I3)0, that in the first micelle (I1/I3)1, and that in the second micelle (I1/I3)2. Below the cmc, the I1/I3 value observed is expressed in eq 1. Here C is the total surfactant concentration. Moreover, it is expressed in eq 2 in the concentration range of c1 to c2, and eq 3 is expressed above c2.
when C < c1
()
I1 I1 ) I3 I3 Figure 4. Relationship between I1/I3 ratio and C/cmc for FCmHCn solutions at 25 °C (FC6-HC6, 50 °C).
hydrocarbon chain and/or packing density of the surfactant molecules in the micelles with increasing concentration. Effect of Hydrophobic Chain Length in the Surfactant on the Micropolarity. The effect of hydrophobic chain length in the surfactant on the micropolarity has been investigated. I1/I3 values for FCm-HCn solutions in Figure 2 are replotted as a function of relative concentrations to their cmc values and shown in Figure 4. The concentrations where the I1/I3 ratio for the FC4-HCn and FC6-HCn series began to decrease were around each cmc, similarly to SDS and SPFO and their mixture. In both series, moreover, the zipping concentration increased with an increase in hydrocarbon chain length. Effect of Temperature on Micropolarity. Figure 5 shows the temperature dependence of the I1/I3 ratio for an HC6-HC4 solution. In the lower concentration, the I3/I3 ratio decreased with increasing temperature, which is consistent with the fact that the I1/I3 ratio of pyrene dissolved in water decreases with the concentration increase. At the higher concentration (2.0 × 10-2 mol/L), however, the I1/I3 ratio increased with increasing temperature. This is because it is easier for water molecules to invade the micelle interior with increasing temperature
(1)
0
when c1 < C < c2
() ()
I1 I1 c1 + (C - c1) I1 I3 0 I3 1 ) I3 C I1 I1 1 I1 ) c1 + I3 0 I3 1 C I3
(( ) ( ) ) ( )
(2) 1
when c2 < C
() ()
()
I1 I1 I1 c + (c - c1) + (C - c2) I3 0 1 I3 1 2 I3 2 I1 ) I3 C I1 I1 I1 I1 I1 1 ) c1 + c2 + I3 0 I3 1 I3 1 I3 2 C I3
{ (( ) ( ) ) (( ) ( ) )} ( )
2
(3)
From these equations, when the I1/I3 ratio and the reciprocal of surfactant concentration are taken as the axes of ordinates and abscissas, respectively, changes in the micelle condition are obtained by examining the slope of the straight line.
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a
b
c
d
e
f
Figure 6. Relationship between I1/I3 ratio and reciprocal of concentration of (a) FC4-HC2 (25 °C), (b) FC4-HC4 (25 °C), (c) FC4-HC6 (25 °C), (d) FC6-HC2 (25 °C), (e) FC6-HC4 (25 °C), and (f) FC6-HC6 (50 °C).
Figure 6 shows the relationship between the I1/I3 ratio and reciprocal of concentration of FCm-HCn. In the case of FC4-HC2 (Figure 6a), and FC4-HC4 (Figure 6b), the concentration (c1) corresponding to the intersection of two lines is similar to the cmc obtained by the surface tension measurement. The second inflection is not clear enough or close to c1. For the case of FC4-HC6 (Figure 6c) and FC6-HCn (Figure 6d-f), however, two intersections are obtained, one is c1 and the other is c2. This implies that they have the second cmc. Highly critical behavior of these
surfactants is ascribable to the length of the hydrocarbon or fluorocarbon chains. The values of cmc obtained from their surface tension measurements and c1, c2, (I1/I3)0, (I1/ I3)1, and (I1/I3)2 obtained from the pyrene fluorescence spectra are summarized in Table 1. The (I1/I3)0 values of FCm-HCn are similar to the I1/I3 ratio of pyrene dissolved in water. (I1/I3)1 of FC4-HC6 and FC6-HCn series were 1.46-1.55, which means the micropolarity of the hydrocarbon chain is high in the first micelle. That is, water molecules penetrated deeply into
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Table 1. cmc, c1, c2, (I1/I3)0, (I1/I3)1, and (I1/I3)2 of FCm-HCn Aqueous Solutions at 25 °C cmc/ mol‚L-1 FC4-HC2 FC4-HC4 FC4-HC6 FC6-HC2 FC6-HC4 FC6-HC6a a
8.2 × 10-3 3.5 × 10-3 1.2 × 10-3 8.3 × 10-4 2.3 × 10-4 5.5 × 10-5
c1/ c2/ mol‚L-1 mol‚L-1 (I1/I3)0 (I1/I3)1 (I1/I3)2 4 × 10-3 5 × 10-3 1 × 10-3 8 × 10-4 4 × 10-3 5 × 10-4 5 × 10-3 1 × 10-4 2 × 10-3
1.52 1.51 1.55 1.55 1.56 1.49
1.13 0.99 1.22 1.49 1.51 1.46
1.06 1.15 0.89
At 50 °C.
the first micelle or the hydrocarbon chain involved in the micelles exists near the bulk phase. In contrast, low values of (I1/I3)1 or (I1/I3)2 are almost the same as the I1/I3 value of pyrene in SDS micellar solution, which means the hydrocarbon chain locates at a hydrophobic site in the first or the second micelles. Correspondingly, two intersections reflect formation of a wet, preliminary micelle prior to the oridinary one, more than poor adsorption of pyrene on a single kind of micelle, which is seen for SPFO. Raman Spectra. To investigate the change in the microenvironment of the micellar interior, Raman spectroscopy has been measured in the high concentration range above the cmc’s, because Raman intensity of FCnHCn below the cmc’s is too weak to measure. Figure 7 shows Raman subtract spectra in the range of 2850-3000 cm-1 of FC6-HC4 solutions in various concentrations at 25 °C. CH3 symmetric stretching vibration at 2882-2884 cm-1 and CH3 asymmetric stretching vibration at 2947-2952 cm-1 are observed. These Raman peaks are assigned according to the refs 22-29. As can be seen in Figure 7, the relative intensity of CH3 symmetric stretching vibration to its asymmetric stretching vibration increases with increasing the concentration only for the FC6-HCn series, which means the degree of disorder of the hydrocarbon chain increases with increasing surfactant concentration.30,31 Figure 8 shows the concentration dependence of the wavenumber of CH3 asymmetric stretching vibration. Central positions of the band are read after removal of the neighboring components under the best-fit condition. In the case of FC4-HCn the wavenumber is almost constant with an increase in concentration of the surfactants. However, for the case of FC6-HCn, the wavenumber shifts to a lower one as the concentration increases and becomes invariant around the second cmc obtained previously. The similar trends are also found for the CH3 symmetric stretching bands. This behavior clearly confirms the presence of the second cmc, because this result is free from a probe (like pyrene) related to an adsorption isotherm. Kamogawa et al. have reported that dehydration of C-H results in reduction of wavenumber.32 Namely, dehydration of alkyl chain involved in the (22) Rosenholm, J. B.; Stenius, P.; Danielsson, I. J. Colloid Interface Sci. 1976, 57, 551. (23) Kalyanasundaram, K.; Thomas, J. K. J. Phys. Chem. 1976, 80, 1462. (24) Okabayashi, H.; Kitagawa, T. J. Phys. Chem. 1978, 82, 1830. (25) Brooker, M. H.; Jobe, D. J.; Reinsborough, V. C. J. Chem. Soc., Faraday Trans. 1 1984, 80, 73. (26) Picquart, M. J. Phys. Chem. 1986, 90, 243. (27) Takenaka, T.; Harada, K.; Nakanaga, T. Bull. Inst. Chem. Res., Kyoto Univ. 1975, 53, 173. (28) Matsuura, H.; Fukuhara, K.; Takashima, K.; Sakakibara, M. J. Phys. Chem. 1991, 95, 10800. (29) Ohno, K.; Naganobu, T.; Matsuura, H.; Tanaka, H. J. Phys. Chem. 1995, 99, 8477. (30) Amorim da costa, A. M.; Geraldes, C. F. G. C.; Teixeira-dias, J. J. C. J. Colloid Interface Sci. 1982, 86, 254. (31) Maitra, A.; Jain, T. K. Colloids Surf. 1987, 28, 19. (32) Kamogawa, K.; Tajima, K.; Hayakawa, K.; Kitagawa, T. J. Phys. Chem. 1984, 88, 2494.
Figure 7. Raman subtract spectra in range of 2850-3000 cm-1 for FC6-HC4 aqueous solutions at 25 °C: (a) 2.0 × 10-2 mol/L; (b) 1.0 × 10-2 mol/L; (c) 4.0 × 10-3 mol/L; (d) 2.0 × 10-3 mol/L; (e) 1.0 × 10-3 mol/L; (f) 4.0 × 10-4 mol/L. The intensity scale is arbitrary. Table 2. Wavenumber of C-C Skelton Vibration of Benzene and Benzene in Water at 25 °C wavenumber/cm-1 benzene benzene in water
995.33 ( 0.02 995.22 ( 0.03
surfactants may take place in a high concentration of FC6HCn solutions. Next, the C-C stretching vibration of a phenyl ring observed at 980-1000 cm-1 and SO3 symmetric stretching vibration observed at about 1050 cm-1 are analyzed. Figure 9 shows the relationship between the wavenumber of the C-C stretching vibration of a phenyl ring and the concentration of FCm-HCn. The wavenumber decreases with an increase in concentration of the surfactants. Table 2 summarizes the wavenumber of the C-C stretching vibration of pure benzene and benzene dissolved in water. The wavenumber is lower for the benzene in water than for that of the pure one. Correspondingly, the phenyl ring of FCm-HCn is supposed to be more hydrated at the higher concentration. Figure 10 shows the concentration dependence of the wavenumber of SO3 symmetric stretching vibration. The wavenumber of SO3 symmetric stretching vibration increases with an increase in concentration of the surfactants, which means the interaction between the SO3 and Na ion is enhanced with increasing surfactant concentrations.33 Micelle Aggregating Condition of FCm-HCn. From the above results, in the case of FC6-HCn and FC4-HC6, for which the second cmc was observed, water molecules may invade the interior of the first micelle above the cmc (micelle 1) to give the loose structure, resulting in the high I1/I3 values, whereas in higher concentrations above the second cmc (micelle 2), water molecules are removed (33) Kawai, T.; Umemura, J.; Takenaka, T. Colloid Polym. Sci. 1984, 262, 61.
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Figure 8. Concentration dependence of Raman wavenumber of CH3 asymmetric stretching band for (a) FC4-HCn and (b) FC6-HCn at 25 °C.
Figure 9. Concentration dependence of Raman wavenumber of C-C stretching band of phenyl ring for (a) FC4-HCn and (b) FC6-HCn at 25 °C.
Figure 10. Concentration dependence of Raman wavenumber of SO3 symmetric stretching band for (a) FC4-HCn and (b) FC6-HCn at 25 °C.
from the micelle interior, giving the low I1/I3 values of pyrene fluorescence. On the other hand, the first type of micelle is rather unstable, and only the latter type of micelle appears dominantly for other FC4-HCn solutions. The instability is understood as a result of shorter chain length of FC or HC. Some points must be clarified in this model as follows. (1) How can a single kind of FC6-HCn or FC4-HC6
surfactant construct two types of micelles with different aggregation number and environment? (2) Why do the phenyl ring and S-O stretching vibrations indicate the ring hydration and enhanced S-O interaction in the dehydrated micelle? Micelle-micelle clustering may account for (1). However, formation of a new hydrophobic core is not expected in such micellar clustering. Furthermore, the fixed form of a single kind of surfactant
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should result in a specific aggregation number. Namely, formation of two kinds of micelle needs some kind of difference for the composing surfactant molecules. From (1) and (2), some driving mechanism should be involved in the formation of two types of micelles from a single kind of surfactant. One key may be the additive effect between the FC and HC chains in Figure 3. The c2 or the zipping (falling down) concentration was in the order of FC4-HC2 < FC4-HC4 < FC4-HC6 and FC6-HC2 < FC6-HC4 < FC6-HC6. This reveals the colaborating effect between FC and HC frames in the micelle 1 stabilization. This suggests the adjacent form, which gives rise to a loosely packed structure. At high concentration however, micelle 2 is much more dehydrated than micelle
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1. A single kind of surfactant can hardly construct two types of micelles. The other conformation, such as the distant form with longer hydrophobic frame, may be mixed with the adjacent form in micelle 2, expelling the penetrated water. Assisted by such rotational isomerism, dehydration of the second micelle seems to proceed. Acknowledgment. This study was supported in part by a Grant-in-Aid for Scientific Research (No. 6640754) from the Ministry of Education, Science and Culture, Japan. LA9602426
