Langmuir 2006, 22, 6053-6055
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The Unusual Micelle Micropolarity of Partially Fluorinated Gemini Surfactants Sensed by Pyrene Fluorescence Tsuyoshi Asakawa,* Tomohisa Okada, Tsutomu Hayasaka, Keiko Kuwamoto, Akio Ohta, and Shigeyoshi Miyagishi DiVision of Material Sciences, Graduate School of Natural Science & Technology, Kanazawa UniVersity, Kanazawa 920-1192, Japan ReceiVed March 24, 2006. In Final Form: May 8, 2006 New gemini surfactants having two fluorocarbon chains were prepared by refluxing partially fluorinated alkyl bromide with N,N,N′,N′-tetramethyl-1,6-diaminohexane in acetonitrile. The partially fluorinated gemini surfactants containing a six-methylene spacer chain are easily soluble in water. The critical micelle concentrations (cmc’s) were determined by various fluorescent probe methods. The hydrophobicity of a CF2 group was estimated to be 1.5 times that of a CH2 group according to the cmc values. The micelle micropolarity of a fluorocarbon gemini sensed by pyrene fluorescence was unusually high, suggesting an apparent iceberg-like environment in the location of pyrene. The significantly small micelle aggregation numbers of fluorinated gemini surfactants were ascertained by the pyrene fluorescence quenching method. The micelle ionization degree estimated by fluorescence quenching of 6-methoxyN-(3-sulfopropyl)quinolinium (SPQ) gave tendencies similar to those of the corresponding hydrocarbon geminis.
Introduction Fluorinated surfactants have unique properties that are different from those of hydrocarbon surfactants.1 The fluorocarbon chain is more rigid than the hydrocarbon chain because of the size of fluorine atoms. The hydrophobicity of a CF2 group has been roughly estimated to be 1.5 times that of a CH2 group.2 Consequently, fluorinated surfactants have much lower critical micelle concentrations (cmc’s) and show characteristic micellar aggregation behavior. Recently, unusual aggregation properties of fluorinated gemini surfactants have been reported by Oda et al.3 The geminis containing two heptadecafluorododecyl chains have extremely low cmc’s and have a tendency to form elongated and stacked bilayer structures. An unusually slow exchange of surfactant monomers between the micelle and bulk phases was observed by NMR measurements. They shed light on the immiscibility of hydrocarbon and fluorocarbon cationic geminis and the miscibility of a hybrid gemini with both hydrocarbon and fluorocarbon geminis. The effect of spacer chain of gemini surfactants on solution properties has been investigated by Zana.4 The Krafft temperature was found to be low at s ) 5-10 methylene spacer chains of 12-s-12 geminis. The aggregation properties are particularly interesting; for instance, 12-6-12 forms spherical micelles, while 12-2-12 and 12-16-12 are prone to form threadlike micelles and vesicles, respectively.5 We found that fluorocarbon cationic geminis containing a six-methylene spacer chain have lower Krafft temperatures in comparison with short-spacer geminis. The intermediate length of the spacer chain has a tendency to prevent the formation of bilayer structures. Our attention can be focused on the aggregation behavior of water-soluble fluorocarbon geminis. Solution properties such as cmc have been investigated by using a fluorescent probe.6,7 Pyrene fluorescence is convenient * To whom correspondence should be addressed. E-mail: asakawa@ t.kanazawa-u.ac.jp. (1) Kissa, E. Fluorinated Surfactants; Marcel Dekker, Inc.: New York, 1994. (2) Shinoda, K.; Nomura, T. J. Phys. Chem. 1980, 84, 365. (3) Oda, R.; Huc, I.; Danino, D.; Talmon, Y. Langmuir 2000, 16, 975. (4) Zana, R. J. Colloid Interface Sci. 2002, 252, 259. (5) Danino, D.; Talmon, Y. Zana, R. Langmuir 1995, 11, 1456.
for evaluating the cmc as well as micelle micropolarity.8 Fluorescence quenching by hexadecylpyridinium is utilized to determine the aggregation number.9 4-Chloro-7-nitrobenzofurazan (NBD-Cl) enables amino compounds to be observed using a fluorescent probe by labeling.10 Benzofurazans give broad fluorescence spectra in the region of visible light. The fluorescence behavior could be used to evaluate the cmc as well as the micelle micropolarity in the solubilization site. The fluorescence spectra of 6-methoxy-N-(3-sulfopropyl)quinolinium (SPQ) exhibit a broad band maximum at 443 nm, and are quenched by halide ions.11,12 The fluorescence intensity was unaffected by the solvent polarity. We recently reported that SPQ fluorescence quenching was useful to simultaneously determine both the cmc and the degree of micellar counterion dissociation (R).13,14 The purpose of this paper is to elucidate the aggregation behavior of cationic, partially fluorinated gemini surfactants by employing various fluorescent probes. As compared with hydrocarbon geminis, we can clarify the obvious differences in micelle microenvironment between the hydrocarbon and fluorocarbon geminis in aqueous solutions. Experimental Procedures Materials. SPQ (Molecular Probes, Inc.) and pyrene (Aldrich) were used as received. Tris(hydroxymethyl)methylamino-7-nitrobenzofurazan (Tris-NBD) was obtained by the incubation of NBDCl (Dojinodo) with Tris(hydroxymethyl)aminomethane (Wako Pure Chemical Industries, Ltd.) in ethanol at 60°C. Replacement reactions of 3-(perfluorohexyl)propanol (Kanto Kagaku) with thionyl bromide (6) Grieser, F.; Drummond, C. J. J. Phys. Chem. 1988, 92, 5580. (7) Gehlen, M. H.; Schryver, F. C. D. Chem. ReV. 1993, 93, 199. (8) Kalyanasundaram, K. Langmuir 1988, 4, 942. (9) Turro, N. J.; Gratzel, M.; Braun, A. M. Angew. Chem., Int. Ed. Engl. 1980, 19, 675. (10) Onoda, M.; Uchiyama, S.; Santa, T.; Imai, K. Anal. Chem. 2002, 74, 4089. (11) Verkman, A. S.; Seller, M. C.; Chao, A. C.; Leung, T.; Ketcham, R. Anal. Biochem. 1989, 178, 355. (12) Biwersi, J.; Tulk, B.; Verkman, A. S. Anal. Biochem. 1994, 219, 139. (13) Kuwamoto, K.; Asakawa, T.; Ohta, A.; Miyagishi, S. Langmuir 2005, 21, 7691. (14) Asakawa, T.; Kubode, H.; Ozawa, T.; Ohta, A.; Miyagishi, S. J. Oleo Sci. 2005, 54, 545.
10.1021/la060787s CCC: $33.50 © 2006 American Chemical Society Published on Web 06/10/2006
6054 Langmuir, Vol. 22, No. 14, 2006 in diethyl ether produced 1-bromo-2,2,3,3,4,4-hexahydroperfluorononane, which was then vacuum distilled. By refluxing the twice moles of bromoalkane with N,N,N′,N′-tetramethyl-1,6-diaminohexane in acetonitrile, C6F13(CH2)3N+(CH3)2(CH2)6N+(CH3)2(CH2)3C6F13 (2F6C3-6) was obtained, which was then purified by recrystallization from acetone-ethanol mixtures. C4F9(CH2)6N+(CH3)2(CH2)6N+(CH3)2(CH2)6C4F9 (2F4C6-6) was also prepared by the same procedure described above. C12H25N+(CH3)2(CH2)6N+(CH3)2C12H25 (2C12-6) was obtained by the use of bromododecane in a similar procedure.13 Measurements. The aqueous solutions of surfactants were made up in 1.0 × 10-6 M SPQ, 3.0 × 10-7 M pyrene, and 1.0 × 10-5 M Tris-NBD. The fluorescence intensities of SPQ were measured at 443 nm by the excitation at 346 nm using a Hitachi F-3010 spectrophotometer.13 The fluorescence intensity without quencher (I0) was used as a standard. The fluorescence spectra of pyrene were measured scanning from 350 to 450 nm by the excitation at 335 nm. The broad fluorescence spectra of Tris-NBD were observed by the excitation at 468 nm. The fluorescence intensities at 533 nm increased with increasing surfactant concentrations without spectral shift. The conductivity measurements of surfactant aqueous solutions were carried out using a conductivity meter, model CM-20S (TOA Electronics Ltd.).
Asakawa et al.
Figure 1. Stern-Volmer plots for SPQ fluorescence quenching in surfactant aqueous solutions: (O) 2C12-6, (b) 2F6C3-6, (2) 2F4C6-6. Table 1. Micellar Solution Properties of Gemini Surfactants at 25 °C surfactant
SPQ
κ
2C12-6 2F6C3-6 2F4C6-6
1.1 1.0 2.0
1.0 1.0 2.0
Results and Discussion We prepared partially fluorinated gemini surfactants containing at least three methylene groups between the perfluorinated alkyl chain and the ammonium headgroup because -CF2CH2CH2N+ is prone to β elimination, as pointed out by Oda et al.3 The fluorocarbon cationic geminis containing long spacer chains gave low Krafft temperatures in comparison with short spacer geminis. Aqueous solutions of 2F6C3-3 were slightly opaque at room temperature and became clear above 40 °C. In contrast, aqueous solutions of 2F6C3-6 were easier to handle than those of 2F6C3-3. We intend to verify the differences in solution properties between fluorocarbon and hydrocarbon surfactants, which have the same headgroup and six-methylene spacer chains. SPQ has been used for the determination of the concentration of halide ions.11,12 SPQ fluorescence quenching behavior will make it possible to determine both the cmc and the micelle ionization degree. We have demonstrated that SPQ fluorescence is quenched by halide ions dissociated from the surfactant without trapping of the probe into cationic micelles.13,14 The SPQ fluorescence gave a single broad emission peak centered at 443 nm by the excitation at 346 nm. The fluorescence is quenched by free halide ions with a linear Stern-Volmer relation, I0/I ) 1 + KSV[Br], where I0 and I are the fluorescence intensities in the absence and in the presence of the quencher, respectively, and KSV is the Stern-Volmer constant. The gemini surfactants are bisquaternary ammonium containing six-methylene spacer chains, which have a bromide counterion quencher. The fluorocarbon geminis are characterized by their water-solubility in comparison with geminis containing a threemethylene spacer chain. Figure 1 shows the SPQ fluorescence quenching in aqueous solutions of gemini surfactants. The distinct inflection point gave the cmc in a manner similar manner to that of conductivity measurements for aqueous surfactant solutions. The fluorescence of SPQ was unaffected by not only the surfactant monomers but also the bound bromide electroneutralized with micelles, as reported previously.14 The decreases in slopes for the Stern-Volmer plot can be ascribed to the bromide counterion binding toward cationic micelles. The degree of micelle ionization, R, can be taken as the ratio of the slopes above and below the cmc in a manner similar manner to that of conductivity data.15 (15) Wang, X.; Wang, J.; Yan, H.; Li, P.; Thomas, R. K. Langmuir 2004, 20, 53.
Ra
cmc /mM b
Py
Tris-NBD
SPQ
κ
I1/I3c
Naggd
0.9 1.0 2.0
1.0 1.0 2.0
0.37 0.36 0.35
0.35 0.34 0.34
1.54 2.18 1.91
22.2 6.7 5.6
a R is the degree of micelle ionization. b κ is the conductivity method. I1/I3 is the pyrene intensity ratio at 3 mM surfactant. d Nagg is the micelle aggregation number at 3mM surfactant.
c
Figure 2. Conductivity for aqueous solutions of gemini surfactants: (O) 2C12-6, (b) 2F6C3-6, (2) 2F4C6-6.
The values of the cmc and the degree of micelle ionization are shown in Table 1. The cmc of 2F6C3-6 was almost identical with that of 2C12-6. This is because the hydrophobicity of the CF2 group would be 1.5 times that of the CH2 group, as pointed out by Shinoda et al.2 The R values were almost identicalas well. The cmc and R values were also determined from conductivity measurements, as shown in Figure 2. The micelle ionization degree was evaluated by assuming that the micelles do not contribute significantly to the conductivity. Thus, the slopes of the conductivity slopes above and below the cmc against the surfactant concentration have often been used to evaluate the micelle ionization degree.15 The cmc and R values were in fairly good agreement with the data of the SPQ fluorescence probe method. The intensity ratio of the first and third vibronic peaks for pyrene fluorescence has been used for the evaluation of micelle micropolarity.8 The I1/I3 ratio in water was 1.88, whereas the values decreased along with the solubilization of pyrene molecules into the micelles. Figure 3 shows the surfactant concentration dependence of the I1/I3 values for pyrene fluorescence. The usual decreases were observed around the cmc of 2C12-6, while significant increases in the I1/I3 values were first observed for the micelle formation of 2F6C3-6. On the other hand, there were
Partially Fluorinated Gemini Surfactants
Figure 3. Fluorescence intensity ratio of pyrene in surfactant aqueous solutions: (O) 2C12-6, (b) 2F6C3-6, (2) 2F4C6-6.
Figure 4. Variations in the fluorescence intensity ratio of pyrene as a function of temperature: (O) H2O, (b) 3 mM 2F6C3-6.
only slight changes around the cmc of 2F4C6-6. The high I1/I3 values for the 2F6C3-6 micelle system were confirmed by the temperature dependences of the I1/I3 values, as shown in Figure 4. The I1/I3 values linearly increased with decreasing temperature. The I1/I3 value in H2O closely approached 2.0 by extrapolation at 0 °C, whereas it attained this value at about 47 °C for the 2F6C3-6 micelle system. This suggests the apparent iceberg-like environment for 2F6C3-6 micelles in the location of pyrene. We could speculate that there is stronger hydrogen bonding around pyrene molecules, as judged also by the high I1/I3 value of 1.98 at 25 °C in D2O, with the stronger hydrogen bonding between D2O molecules.16 The observed high I1/I3 value suggests that the location of pyrene would be closer to the micelle surface in contact with water molecules. The location of pyrene will be restricted around lipophilic methylene chains of 2F6C3-6 by the lipophobicity of the fluorocarbon micellar core. The abnormal environment around the location of pyrene may be realized by the small fluorocarbon micelles of 2F6C3-6 in contact with water molecules. The micelle aggregation numbers of gemini surfactants were ascertained by the pyrene fluorescence quenching method using a hexadecylpyridinium chloride quencher. The micelle aggregation numbers for fluorinated gemini surfactants were significantly small (Table 1). This behavior can be recognized by simple
Langmuir, Vol. 22, No. 14, 2006 6055
Figure 5. Fluorescence intensity for Tris-NBD in surfactant aqueous solutions: (O) 2C12-6, (b) 2F6C3-6, (2) 2F4C6-6.
geometric considerations.17 If we assume a spherical micelle with a radius of one alkyl chain length, a small number of gemini surfactant molecules is sufficient because of the bulky fluorocarbon chains. In such a small micelle, appreciable water contact would exist in the location of pyrene around the micelle surface. The hydrophobic hydration around the fluorocarbon chains in small micelles would affect the pyrene intensity ratio I1/I3. The iceberg-like environment in the location of pyrene would result in an unusually high micropolarity for 2F6C3-6 micelles. NBD-Cl reacts with amino groups to yield a highly fluorescent derivative.10 The NBD-labeled derivatives have been used to report the molecular environments from the fluorescence behavior; for example, the fluorescence intensity increases with decreasing solvent polarity.18,19 Tris(hydroxymethyl)aminomethane was labeled with NBD-Cl to use the fluorescent probe in micelle systems. The appearance of an absorption band around 468 nm was observed along with NBD-labeling. Tris-NBD gave broad fluorescence spectra at around 533 nm by the excitation at 468 nm. The fluorescence intensities increased with increasing surfactant concentrations without spectral shift. Figure 5 shows the variation in fluorescence intensity as a function of surfactant concentration. The fluorescence intensity I0 in water was used as a standard. The fluorescence intensity increased upon micelle formation, corresponding to the solubilization of Tris-NBD in micelles. The fluorescence intensity increased with increasing surfactant concentration above the cmc, then it became almost constant at higher concentrations. The I/I0 values of 2F6C3-6 were lower than those of 2C12-6. Since the fluorocarbon chains have a low solubility of organics in comparison with hydrocarbon ones, the location of the water-soluble Tris-NBD will be at a micelle surface. LA060787S (16) Ozawa, T.; Asakawa, T.; Garamus, V. M.; Ohta, A.; Miyagishi, S. J. Oleo Sci. 2005, 54, 585. (17) Israelachvilli, J. N.; Mitchell, D. J.; Ninham, B. W. J. Chem. Soc., Faraday Trans. 2 1976, 72, 1525. (18) Tsukanova, V.; Grainager, D. W.; Salesse, C. Langmuir 2002, 18, 5539. (19) Uchiyama, S.; Matsumura, Y.; Silva, A. P.; Imai, K. Anal. Chem. 2003, 75, 5926.
