Langmuir 2002, 18, 1999-2003
1999
Solubilization of Pyrene in CnE7 Micelles Chikako Honda,* Miwa Itagaki, Rie Takeda, and Kazutoyo Endo Showa Pharmaceutical University, Higashi-Tamagawagakuen 3-3165, Machida, Tokyo 194-8543, Japan Received August 10, 2001. In Final Form: November 8, 2001 The locations and numbers of solubilized pyrene molecules in heptaethyleneoxide monoalkyl ether CnE7 (n ) 10, 12, 14, and 16) surfactant micelles were studied as functions of the alkyl chain length, micelle concentration, aggregation number, and pyrene concentration by means of steady-state fluorescence spectroscopy. It was found that the monomeric fluorescence intensity ratio (Im3/Im1) of the third to the first vibrational bands of pyrene in the micelles depends on the volume of the hydrophobic core, that is, the alkyl chain length. The ratio of pyrene excimer fluorescence intensity to the intensity of the first vibrational band (Ie/Im1) was found to increase with the CnE7 micelle concentration and to reach a maximum ratio at a particular micellar concentration. The micellar concentration at the maximum value of Ie/Im1 depended on the alkyl chain length and shifted to a lower micellar concentration when the alkyl chain length increased. The number of solubilized pyrene molecules in the unit core volume at the maximum value of Ie/Im1 was found to be linearly proportional to the aggregation number of the micelles.
Introduction 1-13
has been carried A considerable amount of research out on the physicochemical properties of aggregates by means of UV,14-17 Raman,18 NMR,19-22 and ESR23,24 spectroscopies. For studies using UV spectroscopy, the fluorescence probe method is an effective techenique for estimating the properties and locations of the solubilizates. Pyrene is a suitably effective fluorescence probe because of its photophysical properties. The ratio between the third (386 nm) and the first (375 nm) monomer emission intensities (Im3/Im1) of the pyrene spectrum depends on the environmental polarity of the solubilized pyrene * To whom correspondence should be addressed. Tel.: +81-42721-1566. Fax: +81-42-721-1588. E-mail:
[email protected]. (1) Tokiwa, F. J. Phys. Chem. 1968, 72, 1214. (2) Weber, G. Annu. Rev. Biophys. Bioeng. 1972, 1, 553. (3) Dorrance, R. C.; Hunter, T. F. J. Chem. Soc., Faraday Trans. 1 1972, 68, 1312. (4) Hantala, R.; Schore, N.; Turro, N. J. J. Am. Chem. Soc. 1973, 95, 5508. (5) Infelta, P.; Gratzel, M.; Thomas, J. K. J. Phys. Chem. 1974, 78, 190. (6) Fendler, J. H.; Fendler, E. J. Catalysis in Micellar and Macromolecular Systems; Academic Press: New York, 1975. (7) Chem, R., Edelhoch, H., Eds. Biochemical Fluorescence; Marcel Dekker: New York, 1975. (8) Maestrri, M.; Infelta, P.; Gra¨tzel, M. J. Chem. Phys. 1978, 69, 1522. (9) Atik, S.; Thomas, J. K. J. Am. Chem. Soc. 1981, 103, 3543. (10) Malliaris, A.; Lang, J.; Zana R. J. Phys. Chem. 1986, 90, 655. (11) Turro, N. J.; Kuo, P.-L. Langmuir 1986, 2, 438. (12) Zana, R., Ed. Surfactant Solutions; Surfactant Science Series Vol. 22; Marcel Dekker: New York, 1987. (13) Grieser, F.; Drummond, C. J. J. Phys. Chem. 1988, 92, 5580. (14) Eriksson, J. C.; Gilberg, G. Acta Chem. Scand. 1966, 20, 2019. (15) Takenaka, T.; Harada, K.; Nakagawa, T. Bull. Inst. Chem. Res. Kyoto Univ. 1975, 53, 173. (16) Cardinal, J. R.; Mukarjee, P. J. Phys. Chem. 1978, 82, 1614. (17) Mukarjee, P.; Cardinal J. R. J. Phys. Chem. 1978, 82, 1620. (18) Zachariasse, K. A.; Van Phuc, N.; Kozankiowicz, B. J. Phys. Chem. 1981, 85, 2676. (19) Ohnishi, S.; Cyr, T. J. R.; Fukushima, H. Bull. Chem. Soc. Jpn. 1970, 43, 673 (20) Atherton, N. M.; Strach, S. J. Chem. Soc., Faraday Trans. 2 1972, 68, 374. (21) Lianos, P.; Viriot, M. L.; Zana, R. J. Phys. Chem. 1984, 88. 1098. (22) Mackay, R. A. Nonionic Surfactants; Schick M. J., Ed.; Marcel Dekker: New York, 1987; p 308. (23) Glushko, V.; Thaler, M. S. R.; Karp, C. D. Arch. Biochem. Biophys. 1981, 210, 33. (24) Dong, D. C.; Winnik, M. A. Can. J. Chem. 1984, 62, 2560.
molecules.25-27 The Im3/Im1 ratio decreases as the solvent polarity increases. The critical micelle concentration (cmc) can be determined using the Im3/Im1 ratio, because the environment experienced by pyrene changes upon aggregation, causing the Im3/Im1 ratio to change.3 Formation of the pyrene dimer, which emits fluorescence in the vicinity of 470 nm, depends on the microviscosity conditions of the pyrene molecules and the occupancy numbers of the solubilized pyrene in the micelle.28-34 Nagarajan and Ganash35 studied the theory of solubilization and predicted all of the equilibrium characteristics such as the core radius, the shell thickness, the radius of the solubilized pool, the volume, etc., of micelles through the minimization of the free energy per molecule in the micelles. More extensive and systematic studies, particularly experimental approaches, are needed to develop a better understanding of the solubilization of pyrene in nonionic surfactant micelles. In the present study, fluorescence spectra of pyrene in aqueous solutions of heptaethyleneoxide monoalkyl ether (CnE7, n ) 10, 12, 14, and 16) were measured as a function of surfactant concentration. The number of solubilized pyrene molecules in the micelles, the core volume of the micelles, and the critical micelle concentration were evaluated for aqueous solutions of CnE7 (n ) 10, 12, 14, and 16) surfactants. It was clarified from the Im3/Im1 ratio that the location of the solubilized pyrene molecules in the CnE7 micelles is a function of both the core alkyl chain length and the surfactant concentrations. It was also clarified from the Ie/Im1 ratio that the location of the solubilized pyrene molecules in the CnE7 micelles is a (25) Nakajima, A. Bull. Chem. Soc. Jpn. 1971, 44, 3272. (26) Kalyanasundaram, K.; Gratzel, M.; Thomas, J. K. J. Am. Chem. Soc. 1974, 96, 7869. (27) Kalyanasundaram, K.; Thomas, J. K. J. Am. Chem. Soc. 1977, 99, 2039. (28) Pownall, H. J.; Smith, L. C. J. Am. Chem. Soc. 1973, 95, 3136. (29) Selinger, B. K.; Watkins, A. R. J. Photochem. 1981, 16, 321. (30) Miller, D. J. Ber. Bunsen-Ges. Phys. Chem. 1981, 85, 337. (31) Selinger, B. K.; Watkins, A. R. J. Photochem. 1982, 20, 319. (32) Almeida, L. M.; Vaz, W. L. C.; Zachariasse, K. A.; Madeira, V. M. C. Biochemistry 1982, 21, 5972. (33) Lianos, P.; Viriot, M.-L.; Zana, R. J. Phys. Chem. 1984, 88, 1098. (34) Zana, R. J. Phys. Chem. B 1999, 103, 9117. (35) Nagarajam, R.; Ganesh, K. J. Chem. Phys. 1993, 98, 7440.
10.1021/la011274i CCC: $22.00 © 2002 American Chemical Society Published on Web 02/12/2002
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Figure 1. Emission spectra of pyrene in C14E7 micelles. Pyrene concentration ) 1 × 10-4 M, excitation wavelength: 335 nm.
function of the aggregation number and the surfactant concentration. Experimental Section Nonionic surfactant samples of heptaethyleneoxide monoalkyl ether (C10E7, C12E7, C14E7, and C16E7) were purchased from Nikko Chemicals (Tokyo, Japan). These samples were used without further purification. Pyrene was purchased from Wako Pure Chemical Industries (Ohsaka, Japan) and was purified once by recrystallization from ethanol. Aqueous surfactant solutions with pyrene (1 × 10-4 M) were kept in a supersonic bath for approximately 2 h and then stirred continually for 3 days in a thermostatic bath at 30 °C. The steady-state fluorescence emission spectra of pyrene, solubilized in aqueous nonionic surfactant solution, were measured by means of a fluorescence photometer (FP-777 Jasco) with a xenon lamp. All steady-state fluorescence spectra were obtained at an excitation wavelength of 335.0 nm and were scanned in the range of 350-550 nm. The Im3/Im1 ratio, with the intensity of the third vibronic band (Im3) divided by that of the first (0-0) vibronic band (Im1) (Figure 1) located near 386 and 375 nm, respectively, in the emission spectrum of monomeric pyrene, provides an estimate of the polarity in the pyrene environment. The fluorescence intensity (Ie) in the region of 450-550 nm, ascribed to excimer emissions, was measured at a wavelength of 470 nm.
Results and Discussion Figure 1 shows the spectra of pyrene solubilized in aqueous solutions of C14E7 micelles at various C14E7 concentrations. The first and third monomer vibronic peaks (Im1 and Im3, respectively) increase at low C14E7 concentrations, and, after reaching a maximum intensity, decrease with increasing C14E7 concentration. The excimer fluorescence intensity at about 470 nm increases with the C14E7 concentration. Figure 2 shows the Im3/Im1 ratio of pyrene solubilized in the nonionic surfactant micelles against the CnE7 concentration. The Im3/Im1 ratio increases rapidly at low surfactant concentrations, indicating the cmc, and then reaches a plateau. Pyrene monomer fluorescence spectra are highly influenced by the solvent polarity.26,36,37 The fluorescence intensity ratio of the third (386 nm) to the first (0-0, 375 nm) vibronic peaks, Im3/Im1, depends on the environment of the pyrene molecules, specifically, the solute-solvent interactions and/or the effective dielectric constant of the solvent. The Im3/Im1 ratio decreases when the polarity of the solvent increases. The Im3/Im1 ratio is influenced not only by the solvent polarity but also by the aggregation number, core cavity, core thickness of the micelles, (36) Turro, N. J.; Kuo, P.-L. J. Phys. Chem. 1986, 90, 4205. (37) Matzinger, S.; Hussy, D. M.; Fayer, M. D. J. Phys. Chem. B 1998, 102, 7216.
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Figure 2. Im3/Im1 dependence on the CnE7 concentration (n ) 10, 12, 14, and 16). The inset shows a plot of Im3/Im1 vs the logarithmic CnE7 concentration.
hydrogen-bond-acceptor basicities, and coordinative covalency.36 In addition, nonpolar derivatives solubilized in micelles are distributed in two solubilization loci. Matzinger et al. showed that probe molecules are distributed in the Stern layer and the hydrocarbon core in the both long and short lifetimes of probe molecules in ionic trimethylammonium chloride and bromide micelles.37 Turro et al. concluded that pyrene exists in the palisade layer of nonionic micelles.36 In contrast, Mukerjee suggested that many solubilized benzoic acid derivatives and phenolic preservatives in nonionic surfactants micelles are distributed in two loci, namely, in the hydrocarbon core and in the polyoxyethylene mantle.38 In the present work (Figure 2), the Im3/Im1 ratio increases as the alkyl chain length of the surfactant increases in the plateau region. The average value of the Im3/Im1 ratio at the plateau region is shown in Table 1. The shift of the Im3/Im1 ratio to higher plateau values as the alkyl chain length increases indicates that more of the solubilized pyrene molecules reside in a hydrophobic environment as the core volume increases. The dependence of the Im3/Im1 ratio at the plateau on the alkyl chain length also implies that the pyrene molecules reside not only in the hydrophilic palisade layer but also in the hydrophobic inner core of the micelles. If the pyrene molecules were distributed only in the palisade layer, the Im3/Im1 ratio would not change when the alkyl chain length increased, as the ethylene oxide length is the same for every surfactant used. Table 1 shows the core radius, the micelle radius with the configuration of the corona in both plain zigzag and meander structures,39 and the hydrodynamic radius measured by dynamic light scattering for each surfactant micelle.40-42 According to Imae,40 in the case that the alkyl chain length is less than 14 carbons in no-salt aqueous solutions, the size of the micelles remains constant at 25 °C even if the micelle concentration increases. It has been reported that C16E7 micelles grow with temperature and concentration and that Z45 observed by light scattering measurements exhibits values somewhat greater than unity, i.e., 1.1-1.2 at lower concentrations (below 1.3 × 10-2 M) at 25 °C.43 The deviation is not large, if the (38) Mukerjee, P. J. J. Pharm. Sci. 1971, 60, 1528. (39) Degiorgio, V., Corti, M., Eds. Physics of Amphiphiles: Micelles, Vesicles and Microemalsions; North-Holland Physics Publishing: Amsterdam, 1985; p 303. (40) Imae, T.; Ikeda, S. J. Phys. Chem. 1986, 90, 5216. (41) Imae, T. Colloid Polym. Sci. 1989, 267, 707. (42) Imae, T. J. Colloid Interface Sci. 1989, 127, 256. (43) Elworthy, P. H.; Macfarlane, C. B. J. Chem. Soc. 1963, 907.
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Table 1. Radius of Micelles with the Corona Configuration
sample
core radius (nm)
C10E7 C12E7 C14E7 C16E7
1.33 1.60 1.87 2.13
a
micelle radius (nm) with plain with meander zigzag corona corona 3.83 4.10 4.37 4.63
hydrodynamic radius (nm)a
core volume/ total volume (%)
average value of Im3/Im1 ratio in plateau regionb
2.72 3.06 3.58 4.13
11.8 14.3 14.2 13.8
0.8302 0.8396 0.8516 0.8665
2.59 2.86 3.13 3.39
Obtained from refs 40-42. b These values were the averages for concentrations higher than 4 × 10-3 M. Table 2. cmc’s of CnE7 Surfactants cmc × 105 (M)
a
Figure 3. Linear relationship between Im3/Im1 and the core volume of CnE7 (n ) 10, 12, 14, and 16).
uncertainty involved in the measurements is taken into consideration. Therefore, it might be considered reasonable that C16E7 forms approximately spherical micelles analogous to those of CnE7 (n < 16). In the present work, the measurement was performed at room temperature (25 °C). Consequently, the micelle growth was small for increased surfactant concentrations (10-3-10-2M), and the core volume could be evaluated in the same way as in the case of CnE7 (n < 16) even if the micelle concentration increased. The ratio of the core volume to the total volume, which is calculated by the hydrodynamic radius, does not seem to be obviously dependent on the alkyl chain length, suggesting that the corona configuration changes from meander to plain zigzag with increasing alkyl chain length. Specifically, the hydrodynamic radius of C10E7 micelles is close to the radius of the meander corona. In contrast, the hydrodynamic radius of C16E7 micelles is closer to the radius of plain zigzag than to that of meander. Therefore, as the alkyl chain lengthens, the corona configuration of CnE7 becomes extended zigzag, and the pyrene molecules easily penetrate into the core part of the micelles. The relationship between the Im3/Im1 ratio and the core volume of the micelles was examined, as it was assumed that the pyrene molecules were distributed into the threedimensional micelles. As seen in Figure 3, the Im3/Im1 ratio increases linearly with the micelle core volume; therefore, the core volume of the nonionic surfactant micelles is the one of the important factors in the solubilization of the pyrene molecules. Turro and Kuo reported the Im3/Im1 ratios of pyrene in Triton (C9Ph En, n ) 7.5-20) micelles with different ethylene oxide chain lengths and concluded from the pyrene fluorescence spectra that the pyrene molecules reside in the inner layer of Triton micelles at 25 °C. 44 The Im3/Im1 ratios of pyrene in Triton (C9Ph En, n ) 7.5-20) range from 0.71 to 0.68 and are smaller than those (0.83-0.86) found in CnE7 micelles in the present work (Table 1). The Im3/Im1 ratio extrapolated to zero core (44) Turro, J. N.; Kuo P.-L. Langmuir 1985, 1, 170.
sample
this work
literature valuea
C10E7 C12E7 C14E7 C16E7
70 6.5 1.0 0.30
98 6.9 0.90 0.20
Obtained from refs 43 and 45.
volume in Figure 3 was determined to be 0.82, indicating that the pyrene molecules distribute mainly in the core (or between the core and the headgroup). Dong and Winnik reported that the Im3/Im1 ratios of pyrene molecules in triethylene oxide and in ethylene glycol are 0.63 and 0.61, respectively. 24 Because 0.82 is considerably above the 0.63 value for triethylene oxide (C0E3), pyrene must be in a more nonpolar medium than the ethylene oxide headgroup region. The observed Im3/Im1 ratios (0.83-0.86 in Figure 3) show that the distribution of the probe molecules is greater in the core region than in the corona layer of heptaethyleneoxide. The Im3/Im1 ratios of C10E7 and C12E7 micelles increase rapidly with increasing logarithmic surfactant concentrations, as can be seen in the inset of Figure 2. The surfactant concentration at which the Im3/Im1 ratio increases rapidly corresponds to the cmc.6 The cmc’s of C10E7 and C12E7 surfactants were obtained from the intersection of two lines: the baseline and the rapidly rising Im3/Im1 line (see Table 2). The values represented by the intersections of these two lines are in good agreement with literature values43,45 that were measured by light scattering technique. In contrast, the Im3/Im1 ratios of C14E7 and C16E7 micelles increase gradually with the surfactant concentration. If the cmc’s of these surfactants are determined as the surfactant concentrations at which the Im3/Im1 values leave the baseline, their cmc’s correspond to the literature values obtained by light scattering measurements (see Table 2). The excimer fluorescence intensity at about 470 nm increases with increasing C14E7 concentration in Figure 1. Similar behaviors were also observed for the other surfactants. In contrast, the fluorescence intensity of monomer pyrene is reduced at high C14E7 concentrations. Figure 4 shows the dependence of the Ie/Im1 ratio on the surfactant concentration. The Ie/Im1 ratio first reaches a maximum with increasing surfactant concentration and then decreases rapidly. The Ie/Im1 ratio starts to increase as the Im3/Im1 ratio starts to increase with the surfactant concentration and then reaches a maximum value before the Im3/Im1 ratio reaches a plateau. The number of solubilized pyrene molecules increases with the number of micelles in solution, and the number of pyrene excimers is proportional to the number of solubilized pyrene molecules. However, the number of solubilized pyrene molecules is limited because the pyrene concentration is (45) Becher, P., Schick, M. J., Eds. Nonionic Surfactants; Marcel Dekker: New York, 1967; p 478
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concentration of pyrene molecules, kinetic analysis of this reaction leads to
Qe Ie Kfe kaC ) k) Qm Im Kfm kd + 1/τe
(2)
with
ka )
Figure 4. Dependence of Ie/Im1 of pyrene in CnE7 micelles on the CnE7 concentration. Table 3. Micelle Concentration, Occupancy Number of Pyrene (〈np〉), and Aggregation Number (Nagg) at the Concentration Corresponding to the Maximum in Ie/Im1
a
sample
micelle conc × 105 (M)
〈np〉
Nagga at 25 °C
C10E7 C12E7 C14E7 C16E7
3.31 1.61 0.926 0.134
3.0 6.2 10.8 74.6
61 89 107 594
held constant (1 × 10-4 M). As the number of micelles continues to increase, the pyrene molecules disperse in the individual micelles, the probe molecules achieve a Poisson distribution over the surfactant aggregates,46 the number of pyrene molecules per micelle decreases, and also the Ie/Im1 ratio decreases rapidly. The surfactant concentration at the maximum Ie/Im1 ratio is higher for surfactants with relatively shorter alkyl chain lengths. The surfactants with shorter alkyl chain lengths have small aggregation numbers (see Table 3); consequently, at higher surfactant concentrations, the number of micelles increases significantly. The maximum Ie/Im1 ratio is larger for surfactants with longer alkyl chain lengths. The concentrations at the maximum Ie/Im1 ratio are lower than the reported threshold concentration of overlap or entangled micelles.47-50 Therefore, micelles are independently present in the solution at the surfactant concentration corresponding to the maximum Ie/Im1 ratio. According to Birk et al.,51,52 intermolecular excimer formation can be described by the equation ka
Py + Py {\ } (PyPy) k d
/
(1)
where Py/, Py, and (PyPy)/ denote a pyrene molecule in the excited singlet state, a pyrene molecule in the ground state, and an excimer, respectively. The parameters ka and kd represent the rate constants for excimer formation and dissociation, respectively. With C being the effective (46) Infelta, P. P.; Grazel, M. J. Chem. Phys. 1979, 70, 179. (47) Kato, T.; Anzai, S.; Seimiya, T. J. Phys. Chem. 1986, 90, 3159. (48) Imae, T.; Ikeda, S. Colloid Polym. Sci. 1987, 256, 1090. (49) Imae, T. J. Phys. Chem. 1988, 92, 5721. (50) Honda, C.; Kiuchi, Y.; Nose, T. Nippon Kagaku Kaishi 1992, 11, 1301. (51) Brisk, J. B.; Dyson, D. J.; Munro, I. H. Proc. R. Soc. A 1963, 275, 575. (52) Brisk, J. B.; Dyson, D. J.; Munro, I. H. Proc. R. Soc. A 1964, 280, 289.
(3)
where Qe, Qm, Ie, Im, k, Kfe, Kfm, and τe are the quantum yield of excimer fluorescence, the quantum yield of monomer fluorescence, the intensity of excimer emission, the intensity of monomer emission, a proportionality constant, the radiative rate constant for the excimer, the radiative rate constant for the exited monomer, and the excimer lifetime, respectively. R, T, and η j are the ideal gas constant, the absolute temperature, and the microviscosity, respectively. If the dissociation rate constant for the excimer becomes negligible, then kd , 1/τe can be established, and eq 2 can be simplified to
Ie Kfe Qe ) k) k Cτ Qm Im Kfm a e
Obtained from refs 40-43.
/
8RT 2000η j
(4)
Therefore, the major factors influencing the Ie/Im1 ratio are the microviscosity, the occupancy number of pyrene molecules in a micelle, and τe. In addition, τe is not sensitive to variations in the microviscosity of the solvating medium.53 If the alkyl chain in the core region has the same fluidity as it does in the bulk, then the macroscopic viscosity should increase dramatically with the alkyl chain length. Therefore, ka in eq 3 rapidly decreases with increasing alkyl chain length. According to Oster and Nishijima, the fluorescence intensity is linearly proportional to the viscosity of the medium.54 As the viscosity of the medium increase, ka decreases. Although the Ie/Im1 ratio of C16E7 is larger than that of the other surfactants, as seen in Figure 4, it is unlikely that the microviscosity of the C16E7 core region is lower than that of other surfactants. Therefore, the excimer fluorescence intensity primarily depends on the solubilized pyrene concentration in the CnE7 micelles. The average occupancy number, 〈np〉, of pyrene in a micelle is given by55
〈np〉 )
[Py]‚Nagg (Cs - cmc)
(5)
where [Py], Cs, and Nagg are the bulk pyrene concentration, the surfactant concentration, and the aggregation number, respectively. Table 3 shows the number of pyrene molecules solubilized per micelle, assuming that all of the pyrene molecules at the surfactant concentration corresponding to the maximum in Ie/Im1 are solubilized in the micelles. The aggregation numbers for CnE7 micelles for calculating the pyrene occupancy number were obtained from refs 40-43. C16E7 micelles have a larger aggregation number, and its number of solubilized pyrene molecules per micelle is the largest among the surfactant micelles studied. A (53) Oster, G.; Nishijima, Y. J. Am. Chem. Soc. 1956, 78, 1581. (54) Zachariasse, K. A.; Kozankiowicz, B.; Kuhnle, W. Photochemistry and Photobiolozy; Zewail, A. H., Ed.; Harwood: London, 1983; Vol. 2, p 941. (55) Kawaguchi, S.; Yekta, A.; Duhamel, J.; Winnik, M. A.; Ito, K. J. Phys. Chem. 1994, 98, 7891.
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Figure 5. Average occupancy number 〈np〉 of pyrene molecules in CnE7 (n ) 10, 12, 14, and 16) micelles at the concentration where Ie/Im1 reaches a maximum.
Figure 6. Ie/Im1 dependence on C14E7 concentration at various pyrene concentrations.
linear relationship between the aggregation number and the average occupancy number of pyrene molecules was found for CnE7 (n ) 10, 12, 14, and 16), as shown in Figure 5. Although the quantity [Py]/(Cs - cmc) in eq 5 is not, of course, constant for all of the surfactants, the linear relationship obtained in Figure 5 indicates that the average occupancy number of pyrene is proportional to the aggregation number of the surfactants used in the present study. As the bulk pyrene concentration increases, the surfactant concentration at the maximum Ie/Im1 ratio shifts to a higher value (Figure 6). The number of solubilized pyrene molecules per micelle, determined from the maximum concentration, is constant, assuming that all of the pyrene molecules have been solubilized. The saturated number of solubilized pyrene molecules per micelle obviously depends on the micelle size and/or the aggregation number of the nonionic surfactant micelles.
16) micelles are as follows. The Im3/Im1 ratio of pyrene monomer fluorescence increases with the CnE7 concentration and reaches a plateau. The Im3/Im1 ratio in the CnE7 micelles in the plateau region depends on the volume of the micellar hydrophobic core. The pyrene molecules in the CnE7 micelles distribute mainly in the core region, as indicated by the Im3/Im1 ratio. The cmc’s of C10E7 and C12E7 surfactants, the micelles of which have relatively small aggregation numbers, could be obtained from the intersection of two lines: the baseline and the rapidly rising Im3/Im1 line. In contrast, the cmc’s of C14E7 and C16E7 surfactants were determined as the surfactant concentrations at which the Im3/Im1 values left the baseline. The dependence of the intensity ratio of pyrene excimer to monomer (Ie/Im1) on the CnE7 concentration shows a maximum for a given pyrene concentration. The CnE7 concentration at the maximum in Ie/Im1 is related to the aggregation number for each CnE7 micelle, and the occupancy number of pyrene molecules in a micelle has linear relationship to the aggregation number.
Conclusions The main conclusions to be drawn from the fluorescence spectra of pyrene molecules in CnE7 (n ) 10, 12, 14, and
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