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Langmuir 1996, 12, 1204-1207
Experimental Verification of Demixing Micelles Composed of Fluorocarbon and Hydrocarbon Surfactants via the Fluorescence-Quenching Method Tsuyoshi Asakawa,* Hirofumi Hisamatsu, and Shigeyoshi Miyagishi Department of Chemistry and Chemical Engineering, Faculty of Engineering, Kanazawa University, Kanazawa 920, Japan Received September 11, 1995. In Final Form: November 22, 1995X A new cationic surfactant, 1,1,2,2-tetrahydroheptadecafluorodecylpyridinium chloride (HFDePC), quenches the fluorescence emission from pyrene in pure micelles but hardly quenches it in mixed micelles composed of fluorocarbon and hydrocarbon surfactants. Pyrene is located in hydrocarbon-rich micelles, and its fluorescence is quenched by cetylpyridinium chloride. However, HFDePC is mainly solubilized in fluorocarbon-rich micelles, and a collision probability between pyrene and HFDePC would be small within the lifetime of the excited pyrene. That is, the depression of quenching by HFDePC could be ascribed to the coexistence of two kinds of mixed micelles. The effects of addition of salt and temperature toward the depression of quenching were also examined.
Introduction The demixing of micelles was proposed for the nonideal mixtures of fluorocarbon and hydrocarbon surfactants by the analysis of the maximal mixture cmc curves.1-6 The plausible model of separated hydrocarbon-rich and fluorocarbon-rich micelles under certain conditions was verified by NMR self-diffusion data and gel filtration.7,8 The microscopic aspect of the micellar miscibility was also investigated by fluorescence, ESR, and NMR methods.9-12 The micellar demixing should be verified directly from measurements of microscopic properties. Analogous with the miscibility of fluorocarbon and hydrocarbon liquids, the demixing micelles may disappear with increasing temperature. The counterions may also play a significant role in the miscibility of ionic fluorocarbon and hydrocarbon surfactants mixtures. Minimizing the electrostatic repulsion between hydrophilic groups should induce the increase in micellar size with ordered packing of hydrophobic chains in a micelle. The phobic interaction between fluorocarbon and hydrocarbon chains would be expected to be emphasized in such a case. Under such a condition, a segregation in a micelle was proposed to minimize the interfacial area between fluorocarbon and hydrocarbon chains in a large aggregate. We shall give much attention to the microenvironment of demixing micelles through an appropriate method. The fluorescence probe method provide useful information on the micellar environment which is not available by other methods. Nonpolar arenes such as pyrene tend to partition almost completely into micelles with exit rates smaller than their fluorescence decay rates. The fluorescence behavior of pyrene can be used to reveal the X Abstract published in Advance ACS Abstracts, February 15, 1996.
(1) Mukerjee, P.; Yang, A. Y. S. J. Phys. Chem. 1976, 80, 1388. (2) Shinoda, K.; Nomura, T. J. Phys. Chem. 1980, 84, 365. (3) Funasaki, N.; Hada, S. J. Phys. Chem. 1980, 84, 736. (4) Kamrath, R. F.; Franses, E. I. Ind. Eng. Chem. Fundam. 1983, 22, 230. (5) Asakawa, T.; Johten, K.; Miyagishi, S.; Nishida, M. Langmuir 1985, 1, 347. (6) Hoffman, H.; Possnecker, G. Langmuir 1994, 10, 381. (7) Carfors, J.; Stilbs, P. J. Phys. Chem. 1984, 88, 4410. (8) Asakawa, T.; Miyagishi, S.; Nishida, M. Langmuir 1987, 3, 821. (9) Kalyanasundaram, K. Langmuir 1988, 4, 942. (10) Kamogawa, K.; Tajima, K. J. Phys. Chem. 1993, 97, 9506. (11) Burkitt, S. J.; Ottewill, R. H.; Hayter, J. B.; Ingram, B. T. Colloid Polym. Sci. 1987, 265, 628. (12) Clapperton, R. M.; Ottewill, R. H. Langmuir 1994, 10, 51.
microscopic aspects of intramicellar events. Thus the fluorescence-quenching method can be used to evaluate the micellar aggregation number. The pair of pyrene and cetylpyridinium chloride has been used for various micellar systems.13-15 In this paper, we have prepared a new quencher, 1,1,2,2-tetrahydroheptadecafluorodecylpyridinium chloride (HFDePC), which quenches pyrene emission due to the presence of the pyridinium group. The fluorescencequenching behavior of pyrene by HFDePC was measured in fluorocarbon and hydrocarbon surfactants mixtures. The effects of the addition of salt and temperature toward quenching behavior were also investigated. Experimental Section Materials. Lithium perfluorononanoate (C8F17COOLi, LiPFN), lithium perfluoro-1-octanesulfonate (C8F17SO3Li, LiFOS), sodium dodecyl sulfate (C12H25SO4Na, SDS), lithium dodecyl sulfate (C12H25SO4Li, LiDS), and lithium tetradecyl sulfate (C14H29SO4Li, LiTS) were prepared by the same procedures reported previously.16 Cetylpyridinium chloride (CPC) was obtained from Tokyo Kasei Co. and recrystallized from acetone. 1,1,2,2-Tetrahydroheptadecafluorodecyl iodide (C8F17CH2CH2I) was obtained from PCR Inc. and was refluxed with pyridine to prepare HFDePC in the same procedure reported previously.17 Pyrene (Wako Pure Chemical Ind., Ltd.) was used as received. The used reagents were of guaranteed grade. Measurements. The solubilization of pyrene in surfactant micellar solutions was performed by sonicating for 10 min and shaking overnight. Then, the quencher (CPC or HFDePC) was accurately added to a part of the micellar solution solubilizing pyrene. Steady-state fluorescence spectra of 1.0 × 10-5 M pyrene were recorded by using a Hitachi F-3010 spectrometer. The typical pyrene monomer spectra were observed by excitation at 335 nm (excitation slit width 5 nm, emission slit width 1.5 nm). The fluorescence intensities at 372 nm in the absence of quencher (I0) and in the presence of it (I) were measured in mixed micellar solutions.
Results and Discussion Molecular probes were introduced into micellar systems for the purpose of elucidating the local environment of (13) Hashimoto, S.; Thomas, J. K. J. Colloid Interface Sci. 1984, 102, 152. (14) Sapre, A.; Rao, K. S.; Rao, K. N. J. Phys. Chem. 1980, 84, 2281. (15) Malliaris, A.; Lang, J.; Zana, R. J. Chem. Soc., Faraday Trans 1 1986, 82, 109. (16) Asakawa, T.; Fukita, T.; Miyagishi, S. Langmuir 1991, 7, 2112. (17) Asakawa, T.; Hisamatsu, H.; Miyagishi, S. Langmuir 1995, 11, 478.
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Fluorescence Quenching of Micelles
Figure 1. Fluorescence quenching of pyrene solubilized in micelles as a function of quencher concentration at fixed 10 mM SDS in 0.1 M NaCl: CPC (O), HFDePC (b) at 20 °C; CPC (4), HFDePC (2) at 30 °C; CPC (0), HFDePC (9) at 40 °C.
the host micelles. This approach is very popular due to the high sensitivity and the easy handling but requires several cautions to evaluate the information on the microenvironment. The pair of probes, pyrene and cetylpyridinium chloride, have been well investigated by many investigators.13-15 Pyrene is localized almost completely into micelles due to extremely low solubility toward water.18 CPC also tends to partition almost completely into micelles due to the attractive interactions toward anionic micelles. In general, the exit rates of arenes (
