Langmuir 1997, 13, 1907-1917
1907
10,10′-Bis(2-ethylhexyl)-9,9′-bianthryl (BOA) Molecule: The First Free Aromatic Probe for the Core of Micelles H. Laguitton-Pasquier,*,† R. Pansu,† J.-P. Chauvet,‡ P. Pernot,§ A. Collet,‡ and J. Faure† Laboratoire de Photophysique et Photochimie Supramole´ culaire et Macromole´ culaire, ENS CACHAN, U.R.A. 1906 CNRS, 61 avenue du Pre´ sident Wilson, 94235 Cachan, France, Laboratoire de Ste´ re´ ochimie et Interactions mole´ culaires, CNRS-ENS Lyon, UMR 117, 46 alle´ e d’Italie, 69364 Lyon Cedex 07, France, and Laboratoire de Physicochimie des Rayonements, U.R.A. 75 CNRS, Bat 350, Campus d’Orsay, 91405 Orsay Cedex, France Received July 18, 1996. In Final Form: December 20, 1996X The spectroscopic properties of the recently synthesized molecule 10,10′-bis(2-ethylhexyl)-9,9′-bianthryl (BOA) and of its parent molecule 9,9′-bianthryl (BA) have been studied in micelles. A time-resolved red shift in the transient fluorescence spectra has been interpreted as a diffusion process of the excited probe from the apolar core to the water interface of the micelle where the excited probe is trapped. The dynamics of the diffusion process are governed by the chemical potential profile of the excited probe through the Smoluschowski equation. The diffusion of excited molecules of BA occurs on a potential surface presenting an attractive well about the water interface, whereas an energy barrier prevents the diffusion of BOA toward the interface. The height of this barrier has been estimated at 7kT in CTACl micelles. The diffusive nature of the red shift is shown by its dependence on viscosity. In CTACl micelles the viscosities measured from the rotational and the translational movement of the probe have the same activation energy. Finally we have confirmed that the solubilization site of BA in its ground state is localized in different points of the micelle, either in the micellar core or at the level of the water interface, near the polar heads. However BOA is found to largely remain located in the micellar core.
Introduction Over the past three decades, fluorescence probe techniques have been widely used to investigate various micellar parameters such as viscosity,1-4 polarity,1-5 the amount of water incorporated,2,6-8 and quenching efficiencies.2,3,5,9-12 In order to provide a sound basis on which to form an unambiguous interpretation of the experimental fluorescence emission data of a specific probe, it is important to characterize the fluorescence behavior of the probe and to identify the solubilization site with some degree of confidence. The knowledge of the solubilization site of molecules within the micelle is of particular interest in the identification of the catalytic activities of micellar systems. * To whom correspondence should be addressed. Current address: Chemistry Department, Laboratory for Molecular Dynamics and Spectroscopy, K.U. Leuven, Celestijnenlaan 200F, 3001 Heverlee, Belgium. † Laboratoire de Photophysique et Photochimie Supramole ´ culaire et Macromole´culaire. ‡ Laboratoire de Ste ´ re´ochimie et Interactions mole´culaires. § Laboratoire de Physicochimie des Rayonements. X Abstract published in Advance ACS Abstracts, March 1, 1997. (1) Blatt, E.; Ghiggino, K. P.; Sawyer, W. H J. Phys. Chem. 1982, 86, 4461. (2) Fendler, J. H.; Fendler, E. J. Catalysis in micellar and macromolecular systems; Academic Press: New York, 1975. (3) Singer, L. In Solutions behavior of surfactants; Mittel, K. L., Fendler, E. J., Eds.; Plenum Press: New York, 1982; Vol. 2. (4) Grieser, F.; Drummond, C. J. J. Chem. Phys. 1988, 92, 5580. (5) Almgren, M.; Grieser, F.; Thomas, J. K. J. Am. Chem. Soc. 1979, 101, 279. (6) Gnash, K. N.; Mitra, P.; Balasubramanian, D. J. Phys. Chem. 1982, 86, 4291. (7) Melo, E. C. C.; Costa, S. M. B.; Mac¸ anita, A. L.; Santos, H. J. Colloid Interface Sci. 1990, 141, 439. (8) Muller, N.; Birkhahn, R. H. J. Phys. Chem. 1967, 71, 957. (9) Van der Auweraer, M.; Roelants, E.; Verbeek, A.; De Schryver, F. C. In Surfactants in solution; Mittal, K. L., Eds.; Plenum Publishing Corporation: New York, 1989; Vol. 7, p 140. (10) Dederen, J. C.; Van der Auweraer, M.; De Schryver, F. C. J. Phys. Chem. 1981, 85, 1198. (11) Gehlen, M. H.; De Schryver, F. C. Chem. Rev. 1993, 93, 199. (12) Roelants, E.; Gelade´, E.; Smid, J.; De Schryver, F. C. J. Colloid Interface Sci. 1985, 107, 337.
S0743-7463(96)00708-1 CCC: $14.00
The position of solubilized additives within the micelle depends on their relative hydrophobic or hydrophilic natures. The hydrophobic core of the micelle is generally considered to be the site of solubilization for very nonpolar molecules such as n-alkanes.13-15 Compounds of relatively high polarity such as alcohols or carboxylic acids are located in the interfacial region of the micelle with the polar group at the micellar interface and the nonpolar hydrocarbon groups incorporated in the micellar core.14 However, the site of solubilization of aromatic molecules which are only slightly polar such as pyrene, benzene and its alkyl derivatives has been a matter of some controversy.2 From 1H-NMR experiments Ericksson and Gilbert16 in their pioneer works have shown that benzene is located close to the water interface of the micelle when it is dissolved in small amounts of benzene in cethyltrimethylammonium bromide (CTABr) micelles. As the molar solubilization ratio of benzene increases, the interfacial region of micelles appears saturated and the incorporation of aromatic molecules in the micellar core occurs. From pulse radiolysis measurements Fendler and Patterson17 concluded that benzene is located in the interfacial region in CTABr micelles whereas in SDS micelles the micellar core is the main localization site. Rehfeld18 led to quite different conclusions from UV studies of benzene in CTABr and SDS micelles. He concluded that in both micellar systems the benzene molecule is mainly solubilized in the micellar core. These conclusions were contradicted by the investigations of (13) Aamodt, M.; Landgren, M.; Jonsson, B. J. Phys. Chem. 1992, 96, 945. Landgren, M.; Aamodt, M.; Jonsson, B. J. Phys. Chem. 1992, 96, 950. (14) Chevalier, Y.; Zemb, T. Rep. Prog. Phys. 1990, 55, 279. (15) Nagarajan, N.; Charko, M. A.; Ruckenstein, E. J. Phys. Chem. 1984, 88, 2916. (16) Eriksson, J. C.; Gilbert, G. Acta Chem. Scand. 1966, 20, 2019. (17) Fendler, J. H.; Patterson, L. K. J. Phys. Chem. 1970, 74, 4608. Fendler, J. H.; Patterson, L. K. J. Phys. Chem. 1971, 75, 3907. (18) Rehfeld, S. J. J. Phys. Chem. 1970, 74, 117. Rehfeld, S. J. J. Phys. Chem. 1971, 75, 3905.
© 1997 American Chemical Society
1908 Langmuir, Vol. 13, No. 7, 1997
Mukerjee and Cardinal19 on UV spectra of benzene in micelles. They showed that, at low concentration, benzene is located in a quite polar part of both CTABr and SDS micelles. In order to reconcile some of the apparently conflicting observations and to provide a unified picture which would apply to both low and high concentrations of additive in micelles, they proposed a two-state model for the solubilized aromatic species. This involves a distribution of the additive between a nonpolar “dissolved state” associated with the hydrocarbon core and a polar “adsorbed state” associated with the micellar interface. The distribution between the two states is affected by the nature of the polar head groups of the surfactant as well as by the amount of the solubilized species in the micelle. More recent studies using fluorescence or absorption techniques5,7,10,20-22 as well as 1H-NMR7,6,20 have been undertaken in order to determine the site of solubilization of aromatic derivatives such as toluene, pyrene, anthracene, perylene, and xylene in neutral or ionic micelles.2,15,20 Agreement has been found on the location of the additive in the outer rather than in the inner hydrophobic core of the micelle. However, to obtain the incorporation of probes in the micellar core some of them have been grafted onto the alkyl chain of surfactants.1,3,4,7,23-27 This approach has led to successful results for membranes,26,28-31 but studies involving micelles have been the subject of much controversy.32 In the present work we have used a solvatochromic aromatic probe substituted by bulky octyl groups, 10,10′-bis(2-ethylhexyl)-9,9′-bianthryl (BOA). From the spec-
Laguitton-Pasquier et al.
of the micelle. Thus, this is a promising probe for the detection of quenchers in the hydrophobic core of the micelle. The spectroscopic properties of BOA in micelles are compared to those of its parent molecule 9,9′-bianthryl (BA), which is known as a TICT molecule and has been intensively studied in homogeneous solvents.33-44 The fluorescence from BA or BOA is emitted from two distinct excited states, a locally excited (LE) state and a charge transfer (CT) state. The CT state originates from the LE state through an equilibrium on the picosecond time scale35-37 depending on the polarity of the microenvironment. An increase of this polarity enhances the formation of the CT excited form and introduces a red shift to the fluorescence emission spectrum of BA or BOA. The diffusion process of the excited state of BA in CTACl micelles has been previously reported.45 However, the recent estimation of the dipole moment for the excited states of BA and BOA in apolar solvents from spectroscopic measurements43,44 implies a revision of the analysis of the time-resolved spectroscopy of BA in micelles. In this work we propose a new model for the diffusion process with an extension to the excited forms of BOA. For this second probe the diffusion model implies an energy barrier which prevents the probe diffusion toward the micelle interface. This appears to be due to the hydrophobic nature of the octyl groups of BOA. Experimental Section
troscopic and kinetic data obtained in micelles we have shown that this probe, in the ground state as well as in the excited state, is trapped inside the hydrophobic core (19) Cardinal, J. R.; Mukerjee, P. J. Phys. Chem. 1978, 82, 1614. Mukerjee, P.; Cardinal, J. R. J. Phys. Chem. 1978, 82, 1621. Mukerjee, P. In Solution Chemistry of Surfactants; Mittal, K. L., Eds.; Plenum Press: New York, 1979; Vol. 1. (20) Lianos, P.; Viriot, M.; Zana, R.; Gratzel, M.; Thomas, J. K. J. Phys. Chem. 1984, 88, 1098. (21) Luo, H.; Boens, N.; Van der Auweraer, M.; De Schryver, F. C.; Malliaris, A. J. Phys. Chem. 1989, 93, 3244. (22) Heindl, A.; Strnad, J.; Kohler, H.-H. J. Phys. Chem. 1993, 97, 742. (23) Minch, M. J.; Sadiq Shah, S. J. Org. Chem. 1979, 44, 3252. (24) Atik, S. S.; Singer, L. A. J. Am. Chem. Soc. 1979, 101, 5696. (25) Atik, S. S.; Singer, L. A. J. Am. Chem. Soc. 1979, 101, 6759. (26) Blatt, E.; Sawyer, W. H. Biochim. Biophys. Acta 1985, 822, 42. Blatt, E.; Ghiggino, K. P.; Sawyer, W. H. J. Chem. Phys. 1988, 92, 2301. (27) Zachariasse, K. A.; Kozankiewicz, B.; Kuhnle, W. In Photochemistry and Photobiology; Zewail, A., Ed.; Hardwood: London, 1983; Vol. 2, p 941. (28) Bangham, A. D.; Standish, M. M.; Watkins, J. C. J. Mol. Biol. 1965, 13, 238. (29) Hubbel, W. L.; McConnell, H. M. J. Am. Chem. Soc. 1971, 93, 314. (30) Podo, F.; Blaise, J. K. Proc. Natl. Acad. Sci. U.S.A. 1977, 74, 1032. (31) Handa, T.; Matsuzaki, K.; Nakagaki, M. J. J. Colloid Interface Sci. 1987, 116, 50. (32) Baglioni, P.; Bongiovanni, R.; Rivara-Minten, E.; Kevan, L. J. Phys. Chem. 1989, 93, 5574. Baglioni, P.; Rivara-Minten, E.; Kevan, L. J. Phys. Chem. 1990, 94, 4296.
(1) Materials and Methods. BA was synthesized and purified as described elsewhere.45 BOA was synthesized by R. Lapouyade. The surfactants hexadecyltrimethylammonium chloride (CTACl) (Tokio Kasai) and Triton X100 (TX100) (Merck) were used as received. The surfactant hexadecyltrimethylammonium tosylate (CTAOTOS) was recrystallized twice from an ether/ethanol mixture46 while the surfactants hexadecyltrimethylammonium bromide (CTABr), sodium dodecyl sulfate (SDS), and N-hexadecylpyridinium chloride (CPC) were recrystallized twice from ethanol. The purity of the samples (probes solubilized in micelles) was checked by NMR, HPLC, and fluorescence spectroscopy. The micelle concentration ([Mi]) of 2 × 10-4 mol‚dm-3 was the same for the different aqueous solutions of surfactants used in this study. The surfactant concentrations (Cs) were deduced from the corresponding values of the critical micelle (33) Schneider, F.; Lippert, E. Ber. Bunsen-Ges. Phys. Chem. 1968, 72, 1155. (34) Schneider, F.; Lippert, E. Ber. Bunsen-Ges. Phys. Chem. 1970, 74, 624. (35) Visser, R. J.; Weisenborn, P. C. M.; van Kan, P. J. M.; Huizer, B. H.; Varma, C. A. G.; Warman, J. M.; De Haas, M. P. J. Chem. Soc., Faraday Trans. 2 1985, 81, 689. (36) Migita, N.; Okada, T.; Nataga, N.; Sakata, Y.; Misumi, S.; Nakashima, N.; Yoshihara, K. Bull. Chem. Soc. Jpn. 1981, 54, 3304. (37) Anthon, D. W.; Clark, J. H. J. Phys. Chem. 1987, 91, 3530. (38) Okada, T.; Mataga, N.; Baumann, W.; Siemiarczuk, A. J. Phys. Chem. 1987, 91, 4490. (39) Nakashima, N.; Murakawa, M.; Mataga, N. Bull. Chem. Soc. Jpn. 1976, 49, 854. (40) Kang, T. J.; Kahlow, M. A.; Swallen, S.; Nagarajan, V.; Jarzeba, W.; Barbara, P. F. J. Phys. Chem. 1988, 92, 6800. (41) Kang, T. J.; Jarzeba, W.; Barbara, P. F.; Fonseca, T. Chem. Phys. 1990, 149, 81. (42) Nagarajan, V.; Brearley, A. M.; Kang, T. J.; Barbara, P. F. J. Chem. Phys. 1987, 86, 3183. (43) Laguitton-Pasquier, H.; Pansu, R.; Chauvet, J.-P.; Collet, A.; Faure, J.; Lapouyade, R. Chem. Phys. 1996, 212, 437. (44) Laguitton-Pasquier, H. Ph.D. Thesis, Ecole Normale Supe´rieure de Cachan, Cachan, 1995. (45) Pansu, R. B.; Yoshihara, K. J. Phys. Chem. 1991, 95, 10123. (46) Sepulvada, L.; Cabrera, W.; Gamboa, C.; Meyer, M. J. Colloid Interface Sci. 1987, 117, 460.
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Table 1. Critical Micellar Concentration (cmc) and Aggregation Number (na) As Reported in the Literature of Surfactants Used in this Present Work
concentration2,47-49 (cmc) and aggregation number2,47,48 (na) reported in the literature (Table 1):
[Mi] )
Cs - cmc na
Probe (BA and BOA) concentrations were kept small (5 × 10-6 mol‚dm-3) for steady state and time-resolved fluorescence measurements in order to avoid probe-probe interactions. According to the Poisson distribution only 2.44% of micelles contain one probe and a negligible proportion of micelles (0.03%) will contain two probe molecules or more. At the micelle concentration employed, the micelles formed in the aqueous solution of surfactants were spherical with a core radius close to 20 Å70 and a total radius up to 25 Å.70 As the Van der Waals radii of BA and BOA have been estimated to be 6 Å33,34,43,44 and 7.5 Å,43,44 respectively, their volume represents respectively either 1.4% or 2.7% of the total micellar volume, assuming a spherical form for BA and BOA molecules. BA and BOA in water form non fluorescent microcrystals.44,45 By steady state anisotropy measurements44,45 we have verified (47) Dennis, E. A.; Ribeiro, A. A.; Roberts, M. F.; Robson, R. J. In Solution Chemistry of Surfactants; Mittal, K. L., Eds.; Plenum Press: New York, 1979; Vol. 1, p 175. (48) Kwan, C. L.; Atik, S.; Singer, L. A. J. Am. Chem. Soc. 1978, 100, 4783. (49) Fendler, J. H. In Membrane Mimetic Chemistry; John Wiley: New York, 1982.
that our samples contain no microcrystals. Thus, the fluorescence is only emitted by the probe solubilized inside the micelle. The processes observed in this work last no more than 100 ns while the residence time of a surfactant in a micelle is on the order of 0.2-10 µs. Consequently it can be assumed that the micelle organization is frozen and the dynamic properties of micelles can be neglected. (2) Sample Preparation. Probe and surfactant were dissolved in CHCl3 (spectroscopic grade solvent). Sample solutions were dried under argon then under vacuum. Water was added, and the sample was kept at 25 °C until the fluorescence measurements had been obtained. (3) Apparatus. The stationary fluorescence spectra have been recorded with a SPEX spectrofluorimeter. Fluorescence quantum yields and lifetimes of aerated solutions of BA and BOA were measured at room temperature. For quantum yield determination, a degassed solution of 9,10-diphenylanthracene in cyclohexane (ΦF ) 0.91 ( 0.02)50 was used as a standard. Timeresolved fluorescence studies have been performed using the timecorrelated single-photon counting method,51 as previously described.43 For population decay measurements, the laser polarization was rotated by 34° with respect to the vertical. The fluorescence was collected through a depolarizer that was mixing equally the parallel and the perpendicular components of the (50) (a) Hamai, S.; Hirayama, F. J. Phys. Chem. 1983, 87, 83. (b) Meech, S. R.; Phillipps, D. J. Photochem. 1983, 23, 193. (51) O’Connor, D. V.; Phillips, D. In Time-correlated Single Photon Counting; Academic Press: New York, 1984.
1910 Langmuir, Vol. 13, No. 7, 1997
Figure 1. Fluorescence emission spectra of BA in various micelles at room temperature and upon excitation at 370 nm. Spectra are normalized to a constant area fixed to unity.
Laguitton-Pasquier et al.
Figure 3. Fluorescence emission decays of BA in CTACl micelles at the emission wavelengths in the range 380-550 nm at room temperature and upon excitation at 370 nm. Table 2. Fluorescence Quantum Yield of BA and BOA in Various Micelles at Room Temperature CPC SDS TX100 CTABr
Figure 2. Fluorescence emission spectra of BOA in various micelles at room temperature and upon excitation at 380 nm. Spectra are normalized to a constant area fixed to unity. fluorescence emission. For anisotropy relaxation measurements, the fluorescence decays were recorded for the vertical and the horizontal polarization of the laser. Anisotropy measurements were calculated according to
a(t) ) (IV - IH)/(IV + 0.5IH) ) (I| - I⊥)/(I| + 2I⊥) This setup is brighter than the traditional polarization equipment. A full qualification of it has been done and will be published in the near future.
Experimental Results (1) Steady-State Fluorescence Measurements. The fluorescence emission spectra from BA and BOA incorporated in various anionic, cationic, or neutral micelles are reported in Figures 1 and 2, respectively. As expected BA is very sensitive to the nature of the micelle. Contrarily the fluorescence spectrum of BOA in these various micelles is only weakly dependent on the micelle nature. Indeed, the fluorescence spectra of BOA in various micelles and in cyclohexane43,44 are quite similar. This result suggests that during its fluorescence lifetime BOA remains mainly localized in the hydrophobic core of the micelles. These observations are confirmed by the fluorescence quantum yield values of the probes in the various micelles
BA
BOA
0.04 0.41 0.46 0.26
0.5 0.43 0.61 0.51
studied. The fluorescence quantum yield of BOA is quite independent of the nature of the micelle and close to that observed in nonpolar solvents.43,44 The fluorescence quantum yield for BA in micelles decreases with the increase of the fluorescence Stokes shift as it occurs in homogeneous solvents. Only CPC micelles do not follow this trend. The quantum yield for BA in CPC micelles is low with respect to those obtained in other micelles (Table 2). As the pyridinium chloride group is known as an efficient quencher of fluorescence,11 consequently, only the fluorescence emission of probes solubilized in the hydrophobic core of CPC micelles would be observed. This explains the similarity of the fluorescence spectrum obtained from BA in CPC and in CCl4,43 a nonpolar solvent. The quantum yield of BOA in CPC is of the same order as those obtained for other micelles, which confirms that emitting molecules of BOA are not interacting with the Stern layer of the micelles as suggested above. The polarity of the solubilization site of each molecule of probe in micelles cannot be evaluated only from their stationary fluorescence emission spectra. Indeed various processes such as probe diffusion,45 solvent reorganization, or chemical reaction, etc. do occur during the probe lifetime. Consequently the stationary fluorescence emission spectra reflect the weighted average of the polarities of the various probe solubilization sites reached. Time-resolved fluorescence studies should be able to provide further information. (2) Time-Resolved Fluorescence Measurements. The time-resolved fluorescence decays of BA and BOA in the various micelles studied are multiexponential and depend on the emission wavelength (Figure 3). However, about 20 ns after excitation for BA and 40 ns after excitation for BOA, at room temperature in CTACl micelles, the fluorescence decays become monoexponential and independent of the emission wavelength, which indicates that an equilibrium is reached. Moreover, the shape of the nonexponential part of the fluorescence decays and the time necessary to reach the final equilibrium depend on the sample temperature, which suggests that
Free Aromatic Probe for the Core of Micelles
Figure 4. Transient fluorescence emission spectra of BA in CTACl micelles at room temperature (λexc ) 370 nm). These spectra are corrected for the detection sensitivity and the fluctuations of the excitation laser and normalized to a constant area.
Langmuir, Vol. 13, No. 7, 1997 1911
associated with a strong decrease in the fluorescence intensity at the red edge of the transient spectrum with a time lapse of 2.5 ns. We have observed that the efficiency of this quenching depends on the nature of the counterion. This fast process is assumed to be the quenching by the polar head (quaternary ammonium and/or counterion) of the part of BA molecules solubilized in the Stern layer just before excitation. More insight into this isoemissive point and this quenching process will be published elsewhere. In the present work, we will consider only the second isoemissive point at 445 nm, which results from a red shift of the transient spectra with a time lapse of 17.5 ns at room temperature in CTACl micelles. For BOA only one isoemissive point is observed regardless of the micelle nature, and the process associated with this isoemissive point corresponds to a red shift of the transient spectra and lasts about 40 ns in CTACl micelles at room temperature. (3) Fluorescence Depolarization. The direction of the transition dipole moment is the same in the two excited states, LE and CT, for both probes BA52,53 and BOA. Thus, the polarisation of the fluorescence is independent of the nature of the excited state and the fluorescence depolarization is controlled by the probe rotation. Consequently from the fluorescence depolarization measurements, we are able to determine the intramicellar viscosity (η) according to the Debye-Stokes-Einstein4,54-58 equation (eq 1), and the translational diffusion coefficient Dt can also be deduced from the usual Stokes-Einstein57 equation (eq 2):
τr )
ηV 1 ) kT 6Dr
(1)
kT 6πηa
(2)
Dt )
Figure 5. Transient fluorescence emission spectra of BOA in CTACl micelles at room temperature (λexc ) 380 nm). These spectra are corrected for the detection sensitivity and the fluctuations of the excitation laser and normalized to a constant area.
the short time process (5kT), then the relaxation of the population is monoexponential. However, for smaller barrier heights or for a diffusion process toward a potential well, a multiexponential decay is expected. Thus, the multiexponentiality of the inner population decay of BA in CTACl micelles suggests that the diffusion of BA in its E1 form occurs on a potential surface with a well at the interface (Scheme 1) in agreement with preceding results.45 This result should also be extended to the other cationic or neutral micelles studied where the inner population decay of BA is multiexponential.43,44 The potential well on the chemical potential profile of BA in its E1 form can be interpreted by the attractive interaction between BA in its E1 form and dipoles and charges localized in the Stern layer, favored by the polar nature of BA in its E1 form. In SDS micelles this interaction is repulsive, as suggested by the existence of an energy barrier.45 Several possible reasons for the monoexponential shape of the inner population of BOA can be considered: (i) the intramicellar viscosity is too large and prevents the diffusion of BOA toward the Stern layer; (ii) a repulsive interaction such as the hydrophobic interaction between the polar heads and the hydrophobic chains of BOA can occur which reduces the affinity of excited BOA for the water interface. When the contribution of the intramicellar viscosity is removed from the inner population decays of BA and BOA in each micellar system, we observe that, at the same viscosity of the medium, the inner population decays of BA are more rapid than those of BOA (67) Kramers, H. A. Physica 1940, 7, 284. (68) Mel’nikov, V. I.; Meshkov, S. V. J. Chem. Phys. 1986, 85, 1018. (69) Zhou, H. X. Chem. Phys. Lett. 1989, 164, 285. (70) Larsen, J. W.; Magid, L. J. J. Am. Chem. Soc. 1974, 96, 5775.
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Figure 9. Inner population decays of BA and BOA recorded as a function of the reduced time DE1t at 30 °C in CTACl, CTABr, CTAOTOS, and TX100 micelles (DE1 is determined as reported in Figure 8).
independent of the micelle nature (Figure 9). This means that for a similar surrounding viscosity the diffusion of excited BA toward the Stern layer is much more efficient than the corresponding diffusion of BOA in its excited state. Consequently, hypothesis i cannot be involved in the control of the BOA diffusion process, and the existence of an energy barrier which prevents the diffusion of BOA in its E1 form toward a more polar surrounding (zone 2), namely the Stern layer (Scheme 1), is confirmed. The barrier height can be estimated from the time decays (τint) of the inner population of BOA in CTACl micelles recorded at different temperatures:
Ib1,λ(t) ) I0e-t/τint ) I0e-(kKram+1/τ(E1))t
(12)
where τ(E1) is the fluorescence lifetime of BOA in its E1 form and kKram designates the rate constant of the barrier crossing process. The kKram expression is given by the Kramers equation:58
kKram )
{[ ( ) ]
η 2ωq 1 + η 2ωq
2 1/2
}
-1
ω qq -E0q/RT e 2π q
(13)
where η represent the viscosity of the medium and Eq0 the activation energy. ω and ωq characterize the curvature of the potential profile respectively at the energy minimum of the well and at the energy maximum of the barrier; q and qq designate the partition function of respectively the initial and transition states.
In the regime η . ωq, relation 13 becomes62
kKram )
ωωq qq -E0q/RT e πη q
(14)
Consequently, according to eqs 12 and 14, we obtain
kint ) kKram +
1 1 ωωq qq -E0q/RT + ) e πη q τ(E1) τ(E1)
(15)
with kint ) 1/τint. Considering the temperature dependence of the viscosity (relation 4) , relation 15 can be written as
(
) (
)
Eq0 + Ear 1 ωωq qq ) ln ln kint πη0 q RT τ(E1)
(16)
In this relation τ(E1), Eq0, and ln(ωωq/πη0)(qq/q) are unknown parameters. Thus, we have adjusted these parameters in order to get the best straight line. We obtain τ(E1) ) 8.1 ns, Eq0 ) 18 kJ‚mol-1, and ln(ωωq/πη0)(qq/q) ) 30.36 with a linear regression coefficient value of 0.99 and using Ear ) 15.9 kJ‚mol-1. The energy barrier value of 7kT (18 kJ‚mol-1) obtained in this study is quite similar to the rotational activation energy determined from the analysis of the rotational relaxation time as a function of the temperature. This value is also of the same order as the energy barrier determined for BA in SDS micelles.45 It should be noticed that the height of the potential barrier shown for BOA in micelles depends on the micelle nature,
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Langmuir, Vol. 13, No. 7, 1997 1917
Table 4. Time Decay of the Inner Population of BOA in Various Micelles Adjusted by a Monoexponential Function τint (ns)
SDS
TX100
CTABr
CTAOTOS
CTACl
7.5
8.6
8.0
6.5
6.9
as is suggested by the diversity of the time decays of the inner population of BOA in the various micelles studied (Table 4). This energy barrier seems to be a consequence of the hydrophobic nature of the lateral chains of BOA which can act in two convergent ways: the hydrophobic chains of BOA can induce an affinity for the micellar core, which is more important for BOA than for BA, as well as an hydrophobic interaction with the polar heads. The analysis of the final transient spectra of BA and BOA leads to a similar conclusion. Nevertheless, the small affinity of BOA for the water interface could also result from the large volume of BOA which compels the charged polar head groups to come closer to each other when BOA is localized about the water interface. This leads to an increase of the electrostatic repulsion between the head groups and to an unfavorable increase of the interfacial energy,13 implying a destabilization of the micellar structure. (2.2) Concentration Profile of the Probe in the Ground State in Micelles. From the initial transient spectra of a probe in micelles we can deduce its molecular density of distribution in its ground state. Whatever the nature of the micelle, the initial transient spectra of BOA are similar to the stationary fluorescence emission spectrum of BOA in cyclohexane.43,44 For BA the initial transient spectra and the stationary fluorescence spectrum in 1,4-dioxane are quite similar. The polarity of the micellar core is expected to be close to that of cyclohexane.3,18,64 Consequently, such an initial transient spectrum for BA can be interpreted as follows: (i) The localization site of BA in its ground state is a single site whose polarity is close to the 1,4-dioxane one. Thus, this site is located between the hydrophobic core of the micelle and the polar heads. (ii) BA is distributed between two zones of different polarity: the apolar core of the micelle and a more polar zone near the polar heads. This means that in fact the initial transient spectrum of BA is a superposition of two types of emission spectrum. This last case is in agreement with most of our results and also with the model of two solubilization sites proposed by Mukerjee and Cardinal.19 Indeed, the strong quenching of the red fluorescence of BA observed just after probe excitation in cationic micelles44 suggests that some of the BA molecules in the ground state are solubilized near the polar heads (Scheme 2). Such a quenching is not observed in the case of BOA, which confirms that BOA in its ground state is solubilized only in the apolar core of the micelle (Scheme 2), as suggested by the initial transient spectra of BOA. Micellar solubilization of BA follows a pattern rather similar to that of other aromatic molecules like pyrene, benzene, anthracene, and naphthalene.2,15,20 It is now well established that at low concentration aromatic molecules are mainly located at or very near to the water micelle interfacial region. Consequently BOA is the first “free” aromatic probe which does not conform to this general behavior and acts as a probe for the center of micelles. With BOA a new indirect way for studying the localization of other solutes in microheterogeneous media can be explored. This can involve, for example, a quenching of the fluorescence emission of BOA by a solute of unknown localization.
Scheme 2. Schematic Representation of the Molecular Density Distribution of BA and BOA in their Ground States in Micellesa
a Although BA is dispersed in micellar domains either in the micellar core or in the Stern layer, BOA remains mainly located in the micellar core.
Conclusion The time-resolved red shift of the transient fluorescence spectra of BA and BOA in micelles is due to the diffusion of the probe in its E1 form from the apolar hydrophobic core toward a more polar site of the micelle, namely the Stern layer for BA. This diffusion process is confirmed by the dependence of the decay rates on viscosity. Beyond the interfacial region between the hydrophobic core and the Stern layer an important stabilization of the charge transfer state of the probe occurs which induces the E1 f E2 transformation. This diffusion process results mainly from the polar nature and the polarizability of BA and BOA in their E1 forms. From the inner population decays we have been able to determine the qualitative chemical potential profile of BA and BOA in micelles. The diffusion of BA in neutral and cationic micelles occurs on a potential surface with an energy well at the level of the interface. However, regardless of the nature of the micelle, in the case of BOA there is an energy barrier which prevents the diffusion of BOA toward the water interface. This energy barrier seems to originate from the hydrophobic nature of the hydrocarbon chains of BOA, which reduces its affinity for the hydrophilic part of the micelle. The barrier height has been estimated at 7kT in CTACl micelles. The qualitative molecular density profiles of BA and BOA in their ground states in micelles have been deduced. We show that BA is solubilized either in the micellar core or in the Stern layer of the micelle. This behavior is similar to the one expected for an aromatic probe, but it is the first time that we are able to show the distribution of a probe between the micellar core and the water interface at low probe concentration. On the other hand, BOA in its ground state is solubilized in the hydrophobic core of the micelle. Consequently BOA is the first aromatic molecule which is able to probe the micellar core. The representation of the inner population decay as a function of the reduced time DE1t provides us a convenient method to remove the contribution of the intramicellar viscosity from the dynamics of the diffusion process. The intramicellar viscosity estimated from the analysis of the fluorescence depolarization decays leads to a good estimation of the translational diffusion coefficient. Acknowledgment. The Ministe`re de la Recherche et de l’Enseignement Supe´rieur is greatly acknowledged for its financial support. LA960708K
