Solvation Dynamics in Aqueous Anionic and Cationic Micelle

Solvation dynamics of the fluorescence probe, coumarin 102, in anionic surfactant, sodium alkyl sulfate (CnH2n+1SO4Na; n = 8, 10, 12, and 14), and cat...
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Langmuir 2005, 21, 3757-3764

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Solvation Dynamics in Aqueous Anionic and Cationic Micelle Solutions: Sodium Alkyl Sulfate and Alkyltrimethylammonium Bromide Yushi Tamoto,†,‡ Hiroshi Segawa,†,§ and Hideaki Shirota*,§,| Department of Applied Chemistry, Graduate School of Engineering, and Department of General Systems Sciences, Graduate School of Arts & Sciences, University of Tokyo, 3-8-1 Komaba, Meguro-ku, Tokyo 153-8902, Japan Received December 12, 2004. In Final Form: February 9, 2005 Solvation dynamics of the fluorescence probe, coumarin 102, in anionic surfactant, sodium alkyl sulfate (CnH2n+1SO4Na; n ) 8, 10, 12, and 14), and cationic surfactant, alkyltrimethylammonium bromide (CnH2n+1N(CH3)3Br; n ) 10, 12, 14, and 16), micelle solutions have been investigated by a picosecond streak camera system. The solvation dynamics in the time range of 10-10-10-8 s is characterized by a biexponential function. The faster solvation time constants are about 110-160 ps for both anionic and cationic micelle solutions, and the slower solvation time constants for sodium alkyl sulfate and alkyltrimethylammonium bromide micelle solutions are about 1.2-2.6 ns and 450-740 ps, respectively. Both the faster and the slower solvation times become slower with longer alkyl chain surfactant micelles. The alkyl-chain-length dependence of the solvation dynamics in both sodium alkyl sulfate and alkyltrimethylammonium bromide micelles can be attributed to the variation of the micellar surface density of the polar headgroup by the change of the alkyl chain length. The slower solvation time constants of sodium alkyl sulfate micelle solutions are about 3.5 times slower than those of alkyltrimethylammonium bromide micelle solutions for the same alkyl-chain-length surfactants. The interaction energies of the geometry optimized mimic clusters (H2O-C2H5SO4- and H2O-C2H5N(CH3)3+) have been estimated by the density functional theory calculations to understand the interaction strengths between water and alkyl sulfate and alkyltrimethylammonium headgroups. The difference of the slower solvation time constants between sodium alkyl sulfate and alkyltrimethylammonium bromide micelle solutions arises likely from their different specific interactions.

1. Introduction Self-organized molecular assemblies are often found in biological systems. Molecular assembly systems are formed by the system stabilization due to hydrophobic interactions of long hydrophobic alkyl or alkenyl chains of surfactants and the hydrogen-bonding interactions between surfactant polar headgroups and water molecules.1-4 In self-organized molecular assembly systems, water molecules are often confined within or at the surface of self-organized molecular assemblies.5 Properties of bound water are very much different from those of bulk water. The specific properties of hydration lead to biological, physical, and chemical functions in self-organized molecular assembly systems. Therefore, dynamical aspects of hydration water have been paid a great attention.5,6 * To whom correspondence should be addressed. E-mail: [email protected]. † Department of Applied Chemistry, University of Tokyo. ‡ Present address: Sato Pharmaceutical Co., Ltd., 6-8-5 HigashiOhi, Shinagawa-ku, Tokyo 140-0011, Japan. § Department of General Systems Sciences, University of Tokyo. | Present address: Department of Chemistry & Chemical Biology, Rutgers University, 610 Taylor Road, Piscataway, NJ 08854. (1) Tanford, C. The Hydrophobic Effect; John Wiley & Sons: New York, 1973. (2) Israelachvili, J. N. Intermolecular and Surface Forces, 2nd ed.; Academic Press: London, 1992. (3) Physics of Amphiphiles: Micelles, Vesicles and Microemulsions; Degiorgio, V., Corti, M., Eds.; North-Holland Physics Publishing: Amsterdam, 1985. (4) Biomembrane Electrochemistry; Blank, M., Vodyanoy, I., Eds.; American Chemical Society: Washington, DC, 1994. (5) Nandi, N.; Bhattacharyya, K.; Bagchi, B. Chem. Rev. 2000, 100, 2013. (6) Vajda, S.; Jimenez, R.; Rosenthal, S. J.; Fidler, V.; Fleming, G. R.; Castner, E. W., Jr. J. Chem. Soc., Faraday Trans. 1995, 91, 867.

One of the simplest and typical self-organized molecular assembly systems is a micelle.1-3,7,8 Because a micelle can be regarded as a simple model of biological lipid membranes, the dynamical features of aqueous micelle solutions have been extensively investigated. Interestingly, these results show extremely slow dynamics of hydration water in comparison with bulk water for specific interactions and confined geometry structures.5,9-12 Time-dependent fluorescence Stokes shift measurement is a powerful spectroscopic technique to characterize dynamical features of solvent,13-15 as well as complex condensed phases.9-12,16 This spectroscopic method monitors the solvent reorganization process by using an instantaneous electronic perturbation of a solvatochromic fluorescence probe. An advantage of this method is the broad time range detection: 10-13-10-8 s (temporal response depends on the spectroscopic technique and the light source). Time-dependent fluorescence Stokes shift measurement has been frequently used to investigate dynamical aspects of micelles,17-28 as well as other surfactant assembly systems, such as reverse micelles,29-55 (7) Micellization, Solubilization, and Microemulsions; Mittal, K. L., Ed.; Plenum Press: New York, 1976. (8) Lindman, B.; Wennerstrom, H.; Eicke, H.-F. Micelles; SpringerVerlag: Berlin, 1980. (9) Bhattacharyya, K.; Bagchi, B. J. Phys. Chem. A 2000, 104, 10603. (10) Levinger, N. E. Curr. Opin. Colloid Interface Sci. 2000, 5, 118. (11) Pal, S. K.; Peon, J.; Bagchi, B.; Zewail, A. H. J. Phys. Chem. B 2002, 106, 12376. (12) Bhattacharyya, K. Acc. Chem. Res. 2003, 36, 95. (13) Barbara, P. F.; Jarzeba, W. Adv. Photochem. 1990, 15, 1. (14) Maroncelli, M. J. Mol. Liq. 1993, 57, 1. (15) Horng, M. L.; Gardecki, J. A.; Frankland, S. J. V.; Maroncelli, M. J. Phys. Chem. 1995, 99, 17311. (16) Richert, R. J. Chem. Phys. 2000, 113, 8404. (17) Sarkar, N.; Datta, A.; Das, S.; Bhattacharyya, K. J. Phys. Chem. 1996, 100, 15483.

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and Langmuir layers.56-59 Very recently, sophisticated femtosecond spectroscopic techniques, such as optical Kerr effect spectroscopy60-62 and IR three-pulse photon echo spectroscopy,63 have also been demonstrated to investigate the ultrafast dynamics in microemulsions. Using time-resolved fluorescence Stokes shift measurements with the temporal response of about 50 ps, Bhattacharrya and co-workers extensively studied the hydration water dynamics in aqueous micelle solutions.17,18 They found that extremely slow solvation components appear in aqueous micelle solutions. The observed slowest solvation component of micelle solutions is two or three (18) Datta, A.; Mandal, D.; Pal, S. K.; Das, S.; Bhattacharyya, K. J. Mol. Liq. 1998, 77, 121. (19) Mandal, D.; Sen, S.; Bhattacharyya, K.; Tahara, T. Chem. Phys. Lett. 2002, 359, 77. (20) Sen, P.; Mukherjee, S.; Halder, A.; Bhattacharyya, K. Chem. Phys. Lett. 2004, 385, 357. (21) Hara, K.; Kuwabara, H.; Kajimoto, O. J. Phys. Chem. A 2001, 105, 7174. (22) Hara, K.; Baden, N.; Kajimoto, O. J. Phys.: Condens. Matter 2004, 16, S1207. (23) Chakrabarty, D.; Hazra, P.; Chakraborty, A.; Sarkar, N. Chem. Phys. Lett. 2004, 392, 340. (24) Shirota, H.; Tamoto, Y.; Segawa, H. J. Phys. Chem. A 2004, 108, 3244. (25) Balasubramanian, S.; Bagchi, B. J. Phys. Chem. B 2001, 105, 12529. (26) Balasubramanian, S.; Bagchi, B. J. Phys. Chem. B 2002, 106, 3668. (27) Pal, S.; Balasubramanian, S.; Bagchi, B. J. Chem. Phys. 2002, 117, 2852. (28) Pal, S.; Balasubramanian, S.; Bagchi, B. J. Chem. Phys. 2004, 120, 1912. (29) Zhang, J.; Bright, F. V. J. Phys. Chem. 1991, 95, 7900. (30) Zhang, J.; Bright, F. V. J. Phys. Chem. 1992, 96, 5633. (31) Zhang, J.; Bright, F. V. J. Phys. Chem. 1992, 96, 9068. (32) Lundgren, J. S.; Heitz, M. P.; Bright, F. V. Anal. Chem. 1995, 67, 3775. (33) Sarkar, N.; Das, K.; Datta, A.; Das, S.; Bhattacharyya, K. J. Phys. Chem. 1996, 100, 10523. (34) Das, S.; Datta, A.; Bhattacharyya, K. J. Phys. Chem. A 1997, 101, 3299. (35) Mandal, D.; Datta, A.; Pal, S. K.; Bhattacharyya, K. J. Phys. Chem. B 1998, 102, 9070. (36) Pal, S. K.; Mandal, D.; Sukul, D.; Bhattacharyya, K. Chem. Phys. Lett. 1999, 312, 178. (37) Riter, R. E.; Undiks, E. P.; Kimmel, J. R.; Levinger, N. E. J. Phys. Chem. B 1998, 102, 7931. (38) Riter, R. E.; Willard, D. M.; Levinger, N. E. J. Phys. Chem. B 1998, 102, 2705. (39) Riter, R. E.; Undiks, E. P.; Levinger, N. E. J. Am. Chem. Soc. 1998, 120, 6062. (40) Willard, D. M.; Riter, R. E.; Levinger, N. E. J. Am. Chem. Soc. 1998, 120, 4151. (41) Willard, D. M.; Levinger, N. E. J. Phys. Chem. B 2000, 104, 11075. (42) Pant, D.; Riter, R. E.; Levinger, N. E. J. Chem. Phys. 1998, 109, 9995. (43) Pant, D.; Levinger, N. E. Langmuir 2000, 16, 10123. (44) Corbeil, E. M.; Levinger, N. E. Langmuir 2003, 19, 7264. (45) Shirota, H.; Horie, K. J. Phys. Chem. B 1999, 103, 1437. (46) Shirota, H.; Segawa, H. Langmuir 2004, 20, 329. (47) Raju, B. B.; Costa, S. M. B. Phys. Chem. Chem. Phys. 1999, 1, 5029. (48) Hazra, P.; Sarkar, N. Chem. Phys. Lett. 2001, 342, 303. (49) Hazra, P.; Chakrabarty, D.; Sarkar, N. Chem. Phys. Lett. 2002, 358, 523. (50) Hazra, P.; Sarkar, N. Phys. Chem. Chem. Phys. 2002, 4, 1040. (51) Hazra, P.; Chakrabarty, D.; Chakraborty, A.; Sarkar, N. Chem. Phys. Lett. 2003, 382, 71. (52) Faeder, J.; Ladanyi, B. M. J. Phys. Chem. B 2000, 104, 1033. (53) Faeder, J.; Ladanyi, B. M. J. Phys. Chem. B 2001, 105, 11148. (54) Faeder, J.; Albert, M. V.; Ladanyi, B. M. Langmuir 2003, 19, 2514. (55) Satoh, T.; Okuno, H.; Tominaga, K.; Bhattacharyya, K. Chem. Lett. 2004, 33, 1090. (56) Benderskii, A. V.; Eisenthal, K. B. J. Phys. Chem. B 2000, 104, 11723. (57) Benderskii, A. V.; Eisenthal, K. B. J. Phys. Chem. B 2001, 105, 6698. (58) Benderskii, A. V.; Eisenthal, K. B. J. Phys. Chem. A 2002, 106, 7482. (59) Vieceli, J.; Benjamin, I. J. Phys. Chem. B 2003, 107, 4801.

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orders of magnitude slower than that of bulk water. They also investigated the ultrafast solvation dynamics in micelle solutions by femtoseocnd laser spectroscopy.19 The solvation dynamics in neutral, cationic, and anionic micelles are different. Specifically, the neutral Triton X-100 (hydrophilic part is poly(ethylene oxide) with the polymerization degree of about 10) micelle solution shows a quite large nanosecond solvation component in comparison with anionic (sodium dodecyl sulfate, SDS) and cationic (cethyltrimethylammonium bromide) micelle solutions. A large amplitude of the nanosecond solvation process was also observed in the aqueous unimolecular polymer micelle, which consists mostly of poly(ethylene oxide), solution.64 The solvation dynamics in liquid poly(ethylene glycol)s with the polymerization degrees of 3 to ∼14 also shows a nanosecond component.65,66 Chakrabarty et al. investigated the solvation dynamics of Brij 35 (dodecyl hydrophobic chain and hydrophilic poly(ethylene oxide) with the polymerization degree of about 23) and Brij 58 (hexadecyl hydrophobic group and hydrohydrophilic poly(ethylene oxide) with the polymerization degree of about 20) micelle solutions using coumarins 102 and 490. The solvation dynamics in the Brij 35 micelle is slightly faster than that in Brij58.23 Kumbhakar et al. compared the solvation dynamics in Triton X-100 and Triton X-165 (hydrophilic poly(ethylene oxide) with the polymerization degree of about 16) micelle solutions.67 Interestingly, the Triton X-165 micelle solution shows faster solvation dynamics than that of the Triton X-100 micelle solution. This chain effect in the neutral surfactant micelles is opposite to the trend of liquid poly(ethylene glycol)s.65,66 Temperature dependence of the solvation dynamics in Triton X-100 micelles was also investigated by Bhattacharrya and co-workers.20 Hara et al. showed interesting results about the pressure effect on the solvation dynamics in the aqueous Triton X-100 and SDS micelle solutions.21,22 The solvation dynamics in the Triton X-100 micelle solution becomes faster with higher pressure. On the other hand, the pressure dependence of solvation dynamics in SDS micelle solutions shows the opposite trend. They suggested that the pressure effect leads to the decrease in hydrogenbonding interaction strength of water molecules and Triton X-100 micelles. In contrast, the hydrogen-bonding interaction strength in SDS micelles becomes stronger with higher pressure. We reported the surfactant concentration dependence and solvent isotope effect on the solvation dynamics of aqueous SDS micelle solutions.24 The solvation time constants with the values of about 130 ps and 2.5 ns are almost independent of the surfactant concentration though the amplitude of the slow solvation components arising from micelles is increasing with the higher surfactant concentration. Furthermore, these solvation components become slightly slower by deuterium isotopic substitutions of water, as well as in simple hydrogen-bonding molecular liquids. (60) Hunt, N. T.; Jaye, A. A.; Meech, S. R. Chem. Phys. Lett. 2003, 371, 304. (61) Hunt, N. T.; Jaye, A. A.; Meech, S. R. J. Phys. Chem. B 2003, 107, 3405. (62) Hunt, N. T.; Jaye, A. A.; Hellman, A.; Meech, S. R. J. Phys. Chem. B 2004, 108, 100. (63) Maekawa, H.; Ohta, K.; Tominaga, K. Phys. Chem. Chem. Phys. 2004, 6, 4074. (64) Frauchiger, L.; Shirota, H.; Uhrich, E. K.; Castner, E. W., Jr. J. Phys. Chem. B 2002, 106, 7463. (65) Shirota, H.; Segawa, H. J. Phys. Chem. A 2003, 107, 3719. (66) Shirota, H.; Segawa, H. Chem. Phys. 2004, 306, 43. (67) Kumbhakar, M.; Nath, S.; Mukherjee, T.; Pal, H. J. Chem. Phys. 2004, 121, 6026.

Aqueous Micelle Solution Solvation Dynamics

Langmuir, Vol. 21, No. 9, 2005 3759 Table 1. Steady-State Absorption νjabs and Fluorescence νjfl Maxima and Stokes Shifts ∆νj of C102 in Aqueous Micelle Solutions, Cyclohexane, Glycerol, and Methanol (a) Micelle Solutions

Figure 1. Chemical structure of coumarin 102 (C102).

Although some studies in terms of the solvation dynamics in aqueous micelle solutions have been reported as mentioned above, many detailed molecular aspects of the solvation dynamics in micelle solutions are still unknown. In this article, we report the effect of the alkylchain-length dependence of both the cationic and anionic surfactants on the solvation dynamics in aqueous micelle solutions. The surfactant controls some physical parameters, such as micelle size and aggregation number. It is important to study the solvation dynamics of micelles with the different physical properties, because it is expected that the physical properties affect the dynamical features of water molecules in micelles. This information will be helpful in understanding the details of solvation dynamics in aqueous micelle systems. We also compare the solvation dynamics in anionic micelles and cationic micelles. To find the specific interactions between the surfactant polar headgroups and water molecules, the interaction energies for the optimized clusters of water and simple models of the headgroups of surfactants (C2H5SO4- and C2H5N(CH3)3+) are estimated by density functional theory calculations. The experimental results of the solvation dynamics in anionic and cationic micelle solutions are compared with density functional theory calculations. 2. Experimental Section Laser-grade coumarin 102 (C102, Exciton, Figure 1) was used without further purification. Sodium alkyl sulfates (sodium octyl sulfate, C8SO4Na, >99%, Merck; sodium decyl sulfate, C10SO4Na, >99%, Merck; SDS, C12SO4Na, >99%, Nacalai Tesque; sodium tetradecyl sulfate, C14SO4Na, 95%, ACROS) and alkyltrimethylammonium bromides (decyltrimethylammonium bromide, C10TAB, >99%, Tokyo Kasei; dodecyltrimethylammonium bromide, C12TAB, >99%, Aldrich; tetradecyltrimethylammonium bromide, C14TAB, >98%, Tokyo Kasei; hexadecyltrimethylammonium bromide, C16TAB, >98%, Tokyo Kasei) were used as received. Water with the conductivity of 18.2 MΩ‚cm was obtained from a Milli-Q system. The concentration of C102 in micelle solutions was kept at about 0.02 mM. The concentrations of surfactants were kept at about twice their critical micelle concentrations (Table 1). The sample solutions were mixed by sonication and filtered through a 0.45-µm pore poly(tetrafluoroethylene) filter. The steady-state absorption and fluorescence spectra of C102 in aqueous micelle solutions were measured with a JASCO V-570 UV/vis/near-IR spectrometer and a JASCO FP777 spectrofluorometer, respectively. Details of the picosecond streak camera system used in this study were reported elsewhere.46,65 Briefly, the fundamental oscillator light of Spectra Physics Hurricane at 800 nm with an average power of about 770 mW was used as the light source. A combined type doubler and pulse picker (Spectra Physics, model 3980) was used to reduce the repetition frequency (from 82 to 4 MHz) and to produce the second harmonic light at 400 nm for the excitation of a sample. The sample was excited by the second harmonic light after passing though a Glan-Laser polarizer to set vertical polarization. The fluorescence of the sample was passed though a 2-mm slit attached with a 1-cm cell, a GlanLaser polarizer set at the magic angle to the pump beam polarization, and a polychromator (Jobin Yvon CP-200). The passed fluorescence was detected by a streak camera (Hamamatsu Photonics, C4334). The full width at half-maximum of the instrument’s responses were 25-35 ps for 2-ns full-scale detection, 35-45 ps for 5-ns full-scale detection, 100-120 ps for 10-ns full-scale detection, and 200-300 ps for 20-ns full-scale detection. A total of 5000 scans were collected for one data set. Time-resolved

concentration νjabs, 103 cm-1 νjfl, 103 cm-1 ∆νj, (mM) (λabs, nm) (λfl, nm) 103 cm-1

surfactant C8H17SO4Na C10H21SO4Na C12H25SO4Na C14H29SO4Na C10H21N(CH3)3Br C12H25N(CH3)3Br C14H29N(CH3)3Br C16H33N(CH3)3Br

260 64 16 4.2 136 32 6.8 2.0

25.16 (397.4) 25.19 (397.0) 25.20 (396.9) 25.21 (396.7) 25.35 (394.5) 25.36 (394.3) 25.38 (394.0) 25.39 (393.8)

20.91 (478.2) 20.92 (478.1) 20.95 (477.3) 20.96 (477.2) 20.92 (478.0) 20.94 (477.5) 20.95 (477.3) 20.95 (477.3)

4.25 4.27 4.25 4.25 4.43 4.42 4.43 4.44

(b) Pure Solvents solvent

s

glycerol methanol cyclohexane

46.5 33.0 2.02

νjabs, 103 cm-1 (λabs, nm)

νjfl, 103 cm-1 (λfl, nm)

∆νj, 103 cm-1

25.03 (399.5) 25.68 (389.4) 27.66 (361.5) 26.57 (376.4)

20.91 (478.2) 21.26 (470.4) 25.32 (394.9) 24.50 (408.2)

4.12 4.42 2.20

fluorescence spectra were obtained by slicing the data along the frequency axis at each time. Two data sets were averaged for analysis. All the measurements were made at ambient temperature (297 ( 2 K). The optimized geometry calculations of mimics of the polar headgroups of sodium alkyl sulfates and alkyltrimethylammonium bromides (C2H5SO4- and C2H5N(CH3)3+) were carried out using the Gaussian 03 program package (revision B.03).68 The calculations performed were based on the Becke’s threeparameter gradient-corrected exchange69 and Lee-Yang-Parr gradient-corrected correlation functions (B3LYP)70 with the basis set of 6-311++G(d,p). A self-consistent reaction field theory (the integral equation formalism polarized continuum method: IEFPCM) provides a simple solvent model.71-74 The value of the dielectric constant was chosen as 36.64 (acetonitrile), because the static fluorescence Stokes shifts of C102 in micelle solutions are nearly between those in methanol (s ) 33.0) and those in glycerol (s ) 46.5; vide infra). C2H5SO4- and C2H5N(CH3)3+ were chosen as simple models of the polar headgroups of sodium alkyl sulfate and alkyltrimethylammonium bromide in this calculation. H2O, C2H5SO4-, and C2H5N(CH3)3+ monomers and their clusters were calculated to find the interactions of water and surfactant polar headgroups.

3. Results 3.1. Steady-State Absorption and Fluorescence Spectra. Figure 2 shows the steady-state absorption (68) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03, revision B.03; Gaussian, Inc.: Pittsburgh, PA, 2003. (69) Becke, A. D. J. Chem. Phys. 1993, 98, 5648. (70) Lee, C.; Yang, W.; Parr, R. G. Phys. Rev. B 1988, 37, 785. (71) Mennucci, B.; Tomasi, J. J. Chem. Phys. 1997, 106, 5151. (72) Mennucci, B.; Cances, E.; Tomasi, J. J. Phys. Chem. B 1997, 101, 10506. (73) Cances, M. T.; Mennucci, B.; Tomasi, J. J. Chem. Phys. 1997, 107, 3032. (74) Tomasi, J.; Mennucci, B.; Cances, E. J. Mol. Struct. (THEOCHEM) 1999, 464, 211.

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Figure 3. Electrolyte concentration dependence of the steadystate absorption spectrum of coumarin 102 in aqueous (a) Na2SO4 and (b) N(CH3)4Br solutions. Solid line, dotted line, broken line, and dotted and broken line indicate the electrolyte concentrations of 0.00 (pure water), 0.01, 0.1, and 1.0 M, respectively. The saturated concentration of coumarin 102 increases with lower Na2SO4 concentration and higher N(CH3)4Br concentration.

Figure 2. Steady-state absorption and fluorescence spectra of coumarin 102 in (a) aqueous sodium alkyl sulfate micelle solutions (from top, C14SO4Na, C12SO4Na, C10SO4Na, and C8SO4Na), (b) aqueous alkyltrimethylammonium bromide micelle solutions (from top, C16TAB, C14TAB, C12TAB, and C10TAB), and (c) pure solvents (cyclohexane, dotted lines; methanol, solid lines; glycerol, broken lines).

and fluorescence spectra of C102 in (a) sodium alkyl sulfate micelle solutions (from top, C14SO4Na, C12SO4Na, C10SO4Na, and C8SO4Na) and (b) alkyltrimethylammonium bromide micelle solutions (from top, C16TAB, C14TAB, C12TAB, and C10TAB). The steady-state absorption spectra of C102 in cyclohexane (dotted lines), methanol (solid lines), and glycerol (broken lines) are also shown in Figure 2c for references. As shown in Figure 2 a,b, the steady-state absorption and fluorescence spectra of C102 in micelles do not vary much with the different alkylchain-length surfactants. The steady-state absorption νjabs and fluorescence νjfl maxima and fluorescence Stokes shift (∆νj ) νjabs - νjfl) of C102 in aqueous micelle solutions are summarized in Table 1. Table 1 also lists the νjabs, νjfl, and ∆νj of C102 in methanol, glycerol, and cyclohexane. The values of ∆νj for C102 in aqueous micelle solutions are between methanol and glycerol. Na2SO4 and N(CH3)4Br are simple model compounds of the headgroups of sodium alkyl sulfates and alkyltrimethylammonium bromides. The saturated concentrations of C102 in the aqueous electrolyte solutions are estimated. The steady-state absorption spectra of the saturated C102 in aqueous Na2SO4 and N(CH3)4Br solutions with the electrolyte concentrations of 0 (pure water), 0.01, 0.1, and 1 M are shown in Figure 3. It is clear from the figure that the solubility of C102 in the aqueous Na2SO4

Figure 4. Time-resolved fluorescence spectra of coumarin 102 in the aqueous C10TAB micelle solution at t ) 50 (black), 150 (blue), 500 (green), 1000 (orange), and 3000 (red) ps.

solution is decreasing with higher Na2SO4 concentration but that in aqueous N(CH3)4Br solution it increases with higher N(CH3)4Br concentration. 3.2. Time-Dependent Fluorescence Stokes Shift. Figure 4 shows the typical examples of the time-resolved fluorescence spectra of C102 in the C10TAB micelle solution at t ) 50, 150, 500, 1000, and 3000 ps. The fluorescence peak of C102 in the aqueous C10TAB micelle solution shifts to a longer wavelength with subnanosecond time evolution. Aqueous micelle solutions of the other surfactants studied here also show the subnanosecond time scale fluorescence Stokes shifts. On the other hand, we did not observe any fluorescence peak shift of C102 in pure water by the present streak camera system24 because the solvation dynamics in pure water is much faster than the temporal response of this spectroscopy setup.75 The slow fluorescence spectral shift in the subnanosecond time scale is (75) Jimenez, R.; Fleming, G. R.; Kumar, P. V.; Maroncelli, M. Nature 1994, 369, 471.

Aqueous Micelle Solution Solvation Dynamics

Langmuir, Vol. 21, No. 9, 2005 3761 Table 2. Fit Parameters and Observed Solvation Components for Solvation Dynamics of C102 in Aqueous Micelle Solutions

Figure 5. Comparison of the decays of the solvation correlation function S(t) for coumarin 102 in aqueous C12SO4Na micelle (open circles) and C12TAB micelle (filled circles) solutions.

surfactant

as1a

τs1b (ns)

as2c

τs2d (ns)

〈τs〉 (ns)

obs. comp.

C8H17SO4Na C10H21SO4Na C12H25SO4Na C14H29SO4Na C10H21N(CH3)3Br C12H25N(CH3)3Br C14H29N(CH3)3Br C16H33N(CH3)3Br

0.79 0.77 0.77 0.77 0.74 0.73 0.75 0.76

0.11 0.12 0.14 0.15 0.13 0.15 0.15 0.16

0.21 0.23 0.23 0.23 0.26 0.27 0.25 0.24

1.25 1.52 2.14 2.57 0.45 0.54 0.65 0.74

0.35 0.44 0.60 0.71 0.21 0.26 0.28 0.30

0.28 0.26 0.26 0.24 0.41 0.39 0.38 0.37

a Fit error is (5%. b Fit error is (10%. c Fit error is (10%. error is (15%.

d

Fit

due to the presence of micelles and not due to the bulk water region. The maximum of the time-resolved fluorescence spectrum at each time is estimated from the fit by a log-normal line shape function,76

[

If (νj) ) If0 exp - ln(2)

(

ln[1 + 2b(νj - νjp)/∆] b

)] 2

(1)

where If0, νjp, b, and ∆ are the peak height, peak frequency, asymmetric parameter, and width parameter, respectively. When 2b(νj - νjp)/∆ is less than -1, If(νj) is taken as 0. The fit curves are also shown in Figure 4. From the estimated fluorescence maximum at each time, the solvation dynamics is characterized using the following equation:

S(t) )

νj(t) - νj(∞) νj(0) - νj(∞)

(2)

where νj(0), νj(∞), and νj(t) are the maximum frequencies of the fluorescence spectra of the probe molecule at time 0, ∞, and t, respectively. The time 0 is defined as the time when a probe is excited by a laser pulse, and the time ∞ is defined as the time when the system achieves the equilibrium state. Although the νj(∞) is often used as the maximum frequency of the steady-state fluorescence spectra, the value of νj(∞) in here is tentatively estimated by extrapolation of the fit function to νj(t) at infinite time. The reason is that the slower solvation component (τs2) is competitive to the fluorescence lifetime of C102 in micelle solutions (about 5 ns). Figure 5 shows the comparison of the decays of S(t) for C102 in aqueous C12SO4Na micelle (open circles) and C12TAB micelle (filled circles) solutions. It is clear from this comparison that the C12SO4Na micelle solution shows slower solvation dynamics than the C12TAB micelle solution. Decyl and tetradecyl surfactant micelle solutions also show the same feature. The biexponential fit parameters for the solvation dynamics in micelle solutions are listed in Table 2. Table 2 also lists the average solvation time constants, which are defined as the time integral of the biexponential fit function (〈τs〉 ) as1τs1 + as2τs2). The faster solvation time constants τs1 are nearly same for both the cationic and the anionic surfactant micelle solutions, but the slower solvation time constants τs2 are quite different. τs2 in alkyltrimethylammonium bromide micelle solutions is 460-750 ps, but that in sodium alkyl sulfate micelle solutions is 1.2-2.2 ns. Figure 6 shows the decays of S(t) for C102 in (a) sodium alkyl sulfate micelle solutions (n ) 8, purple; 10, green; 12, blue; and 14, red) and (b) alkyltrimethylammonium (76) Siano, D. B.; Metzler, D. E. J. Chem. Phys. 1969, 51, 1856.

Figure 6. Decays of the solvation correlation function S(t) for coumarin 102 in (a) sodium alkyl sulfate micelle solutions (C8SO4Na, purple; C10SO4Na, green; C12SO4Na, blue; and C14SO4Na, red) and (b) alkyltrimethylammonium bromide micelle solutions (C10TAB, green; C12TAB, dark blue; C14TAB, red; and C16TAB, light blue).

bromide micelle solutions (n ) 10, green; 12, blue; 14, red; and 16, light blue). Biexponential fit curves are also shown in Figure 6. The solvation dynamics for both sodium alkyl sulfate and alkyltrimethylammonium bromide micelle solutions become slower with longer alkyl chain of the surfactants. Because the temporal instrument’s response of the spectroscopy system used in this study is 30 ps, the observed solvation components are estimated. The component of the observed solvation process in the whole solvation process is estimated from the time-resolved fluorescence spectra at t ) 0 detected by streak camera and steady-state absorption and fluorescence spectra of C102 in aqueous micelle solutions and nonpolar solvent (cyclohexane).77 The estimated values are listed in Table 2. The observed solvation component for sodium alkyl sulfate micelle solutions is about 25%, and that for (77) Fee, R. S.; Maroncelli, M. Chem. Phys. 1994, 183, 235.

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Tamoto et al. Table 3. Calculated Energies, Interaction Energies, and Hydrogen-Bonding Geometries of H2O, C2H5OSO3-, C2H5N(CH3)3+, and Their Clusters at the B3LYP/ 6-311++G(d,p) Optimization Geometry

monomer H2O C2H5OSO3C2H5N(CH3)3+

Figure 7. H2O, C2H5SO4-, and C2H5N(CH3)3+ monomers and C2H5SO4-/H2O and C2H5N(CH3)3+/H2O clusters at the B3LYP/ 6-311++G(d,p) optimized geometry. Acetonitrile is chosen as the dielectric medium for the optimized geometry calculations (IEF-PCM, s ) 36.64). White, gray, red, blue, and yellow denote hydrogen, carbon, oxygen, nitrogen, and sulfur, respectively.

alkyltrimethylammonium bromide micelle solutions is about 40%. 3.3. Density Functional Theory Calculations. The optimized geometries of the monomers (H2O and mimics of the headgroups of alkyl sulfate C2H5SO4- and alkyltrimethylammonium C2H5N(CH3)3+) and their clusters (H2O-C2H5SO4- and H2O-C2H5N(CH3)3+) estimated by the B3LYP/6-311G++(d,p) level of calculation are illustrated in Figure 7. The calculated optimization energies (Em for the monomers and Ec for the clusters), interaction energies (Ei), hydrogen-bond distances, and hydrogenbond angles of the clusters are listed in Table 3. Ei is defined as the difference between the energy of cluster Ec and the sum of the energies of monomers Em,n:

Ei ) Ec -

∑n Em,n

(3)

Table 3 summarizes the energies for both gas-phase conditions and acetonitrile medium conditions (s ) 36.64). Although the microenvironment of micelles is not simple and the counterions are absent in the calculations, it is worth discussing qualitative features in the microenvironment of micelles. As shown in Table 3, the interaction of the C2H5SO4-/H2O cluster in the dielectric medium is stronger than that of the C2H5N(CH3)3+/H2O cluster. 4. Discussion 4.1. Static Solvation. The steady-state absorption and fluorescence spectra give the solvent reorganization energy λs,78

λs ) (∆νj - ∆νjref)/2

(4)

where ∆νj and ∆νjref are the steady-state fluorescence Stokes shifts of the fluorescence probe in the target medium and in a nonpolar solvent (about 2200 cm-1 in cyclohexane). ∆νj of C102 in aqueous sodium alkyl sulfate micelle solutions and in aqueous alkyltrimethylammonium bromide micelle solutions are about 4250 and 4430 cm-1, respectively (Table 1). The value of ∆νj of C102 in the micelle solutions does not much depend on the alkyl chain length of the surfactant for both the sodium alkyl sulfate and alkyltrimethylammonium bromide within experimental error. The values of ∆νj of C102 in methanol and glycerol are 4420 and 4120 cm-1, respectively. The reorganization energies of C102 in sodium alkyl sulfate micelle solutions are between those in methanol and (78) van der Zwan, G.; Hynes, J. T. J. Phys. Chem. 1985, 89, 4181.

cluster H2O-C2H5OSO3H2O-C2H5N(CH3)3+

environment

Em (kJ/mol)

dipole moment (D)

gas acetonitrile gas acetonitrile gas acetonitrile

-200 741.9 -200 777.4 -2 043 885.6 -2 044 143.6 -665 696.0 -665 892.8

2.16 2.55 5.39 8.28 1.21 1.49

environment

Ec (kJ/mol)

gas -2 244 689.6 acetonitrile -2 244 935.2 gas -866 501.3 acetonitrile -866 679.9

Ei d angle (kJ/mol) (Å) (deg) -62.1 -14.2 -63.4 -9.7

2.09 1.84 N/A N/A

146.1 174.5 N/A N/A

glycerol, and those in alkyltrimethylammonium bromide micelle solutions are similar to that in methanol. ∆νj of C102 in pure water is 5000 cm-1 which is larger than that in the micelle solutions.24 On the other hand, ∆νj of C102 in aqueous micelle solutions is much greater than that of a nonpolar solvent (cyclohexane). This result indicates that C102 does not exist in both the free bulk water region and the hydrophobic micelle core. Furthermore, the steady-state fluorescence spectra of C102 in both the sodium alkyl sulfate and alkyltrimethylammonium bromide micelle solutions do not much depend on the excitation light wavelength for the S1 band of C102 in aqueous micelle solutions. We, therefore, conclude that the microenvironment of C102 in aqueous micelle solutions is rather homogeneous on a fluorescence time scale, and C102 should be located in the Stern layer, which consists of counterions, polar headgroups, and hydration water molecules, around micelles on the fluorescence probing time scale. Furthermore, C102 solubility in aqueous electrolyte solutions suggests that C102 in aqueous sodium alkyl sulfate micelle solutions should more strongly interact with the micelle hydrophobic core than in aqueous alkyltrimethylammonium bromide micelle solutions because of the solubility difference between sodium sulfate and ammonium bromide electrolyte microenvironments. Because the hydrophobic core is less polar, ∆νj of C102 in sodium alkyl sulfate micelle solutions is smaller than in alkyltrimethylammonium bromide micelle solutions. 4.2. Solvation Dynamics. 4.2.1. Comparison between Anionic and Cationic Micelle Solutions. As shown in Figure 5, the solvation dynamics in alkyltrimethylammonium bromide micelle solutions is faster than that in sodium alkyl sulfate micelle solutions. The difference of the slower solvation time constants τs2 of alkyltrimethylammonium bromide micelle solutions and sodium alkyl sulfate micelle solutions is significant (Table 2): τs2 in sodium alkyl sulfate micelle solutions is nanoseconds and τs2 in alkyltrimethylammonium bromide micelle solutions is on the subnanosecond time scale. Sarkar et al. also reported that the longest solvation time in the C16TAB micelle solution is faster than that in C12SO4Na micelle solutions.17 As mentioned above, hydrogen bonds affect the solvation dynamics in aqueous micelle solutions. Hara et al. found that the nanosecond solvation dynamics of the neutral micelle (Triton X-100) is faster with higher pressure while that of the SDS micelles is slower.21,22 We also reported the retardation of the solvation dynamics in SDS micelle solutions by deuterium substitutions of water.24 Therefore, it is expected that hydrogen bonds strongly affect the observed solvation dynamics in micelle solutions.

Aqueous Micelle Solution Solvation Dynamics

The anionic sulfate group can directly make hydrogen bonds with water molecules. In contrast, the cationic trimethylammonium group is impossible to bind directly with water molecules via hydrogen bonds. Therefore, the interaction strength between the sulfate group and water is most likely stronger than that between the trimethylammonium group and water. From the density functional theory calculation results for the clusters of water and surfactant polar headgroup mimics (Table 3), the calculated interaction energy between the sulfate group and water is about 4.5 kJ/mol greater than that between the trimethylammonium group and water. This result indicates that water around the sulfate group is less mobile than that around the trimethylammonium group due to the strong interaction for the anionic sulfate group in comparison with the cationic trimethylammonium group. Nandi and Bagchi theoretically studied the dielectric relaxation of hydration water around a mimic of biomacromolecules on the basis of a model of dynamic equilibrium between free water and bound water.79 The dielectric relaxation processes of biological water depend strongly on the excess hydrogen-bonding interaction energy of water with a biomolecule (the subnanosecond to nanosecond time scale relaxation is 7.5 times slower by the change of the excess free energy from -11.7 kJ/mol to -16.7 kJ/mol). Although the solvation dynamics is the collective solvent reorganization process, it is reasonable to assume that the difference of the solvation dynamics between the sodium alkyl sulfate micelle and the alkyltrimethylammonium bromide micelle solutions should be due to the interaction energy difference of water and surfactant polar headgroups. Namely, the stronger interaction between the sulfate headgroup and water than that between the trimethylammonium headgroup and water could lead to less collective mobility in the interfacial region of the sodium alkyl sulfate micelles than that of the alkyltrimethylammonium bromide micelles. 4.2.2. Alkyl-Chain-Length Dependence. As shown in Figure 6 and Table 2, both the faster and slower solvation time constants in aqueous micelle solutions depend on the alkyl chain length of the surfactant: the longer alkyl chain yields a longer solvation time. Both anionic sodium alkyl sulfate and cationic alkyltrimethylammonium bromide surfactants show this feature. Namely, the mobility of water molecules around micelles becomes less with the longer-alkyl-chain surfactant’s micelle solution. The different surfactant concentrations might provide the different solvation time constants in micelle solutions. However, the solvation time constants in SDS micelle solutions with the concentrations above the critical micelle concentration do not show significant surfactant concentration dependence.24 Therefore, the alkyl chain length dependence of the solvation times is not likely from the different surfactant concentrations. Recently, Pal and co-workers compared the solvation dynamics of Triton X-100 and Triton X-165 micelle solutions using coumarin 153.67 The micelle size of Triton X-100 is larger than that of Triton X-165. The solvation dynamics of the aqueous Triton X-100 micelle solution is slower than that of the Triton X-165 micelle solution. They suggested that the difference of the solvation dynamics between Triton X-100 and Triton X-165 micelle solutions is attributed to the different looseness of hydration (Palisade) layers of Triton X-100 and Triton X-165 micelles. The relatively loose structure of the Palisade layer of the Triton X-165 micelle to that of Triton X-100 micelle arises (79) Nandi, N.; Bagchi, B. J. Phys. Chem. B 1997, 101, 10954.

Langmuir, Vol. 21, No. 9, 2005 3763 Table 4. Summary of Critical Micelle Concentrations (cmc), Aggregation Numbers (Nag), Hydrophobic Core Volumes (V), Radii (r), Surface Area of Micelle (Am), and Surface Area Per Polar Headgroup (Ag) for Sodium Alkyl Sulfate and Alkyltrimethylammonium Bromide Surfactants surfactant

cmca (mM)

Nagb

C8H17SO4Na 130 21 (15-27) C10H21SO4Na 33 40 (30-50) C12H25SO4Na 8.1 64 (50-77) C14H29SO4Na 2.0 86 (80-91) C10H21N(CH3)3Br 66 38 (36-40) C12H25N(CH3)3Br 15 55 (44-65) C14H29N(CH3)3Br 3.5 79 (60-97) C16H33N(CH3)3Br 0.9 110 (75-144) a

V (Å3)

r (Å)

Am (Å2)

Ag (Å2)

4500 10 800 20 700 32 400 10 200 17 800 29 800 47 400

10.2 13.7 17.0 19.8 13.5 16.2 19.2 22.5

1310 2360 3630 4930 2290 3300 4630 6360

62.4 59.0 56.7 57.3 60.3 60.0 58.6 57.8

Reference 2. b References 80-89.

from the small micelle size and aggregation number of Triton X-165 compared with those of Triton X-100. The micelle properties, such as radius, aggregation number, and surface area per polar headgroup, depend on the alkyl chain length of surfactant constituting the micelle. Table 4 summarizes the micelle hydrophobic core diameter r, aggregation number Nag, surface area Am () 4πr2), and surface area per polar headgroup for the surfactants Ag () Am/Nag) used in this study. The hydrophobic core radius r in angstroms is estimated by the following equation:1,2

r ) (3Nag(27.4 + 26.9n)/4π)1/3

(5)

where Nag is the aggregation number and n is the number of carbons of the surfactant’s alkyl chain. Typical reported values of Nag for sodium alkyl sulfate and alkyltrimethylammnoium bromide surfactants are listed in Table 4.80-89 The average values of Nag are used for the estimation of r, Am, and Ag. Although Ag varies with the different Nag values in the reported value range, the entire trend of the alkyl-chain-length dependence of Ag shows smaller Ag with longer alkyl chain of surfactant for both sodium alkyl sulfate and alkyltrimethylammonium bromide surfactants. The solvation dynamics in aqueous micelle solutions becomes slower with longer alkyl chain surfactant. Because the solvation dynamics in micelle solutions probes the microenvironment of micelle surface, it is expected that the solvation dynamics depends on Ag. From the comparison between the solvation dynamics and Ag, it is clear that they are reasonably correlated: Ag decreases and then 〈τs〉 increases. Namely, the present result of the alkyl-chain-length dependence of the solvation dynamics implies that the mobility of water molecules around micelles becomes less with increasing number density of polar headgroups of the micelle surface. (80) Tartar, H. V. J. Phys. Chem. 1955, 59, 1195. (81) Hayter, J. B.; Penfold, J. Colloid Polym. Sci. 1983, 261, 1022. (82) Herrington, K. L.; Kaler, E. W.; Miller, D. D.; Zasadzinski, J. A.; Chiruvolu, S. J. Phys. Chem. 1993, 97, 13792. (83) Rafati, A. A.; Gharibi, H.; Iloukhani, H.; Safdari, L. Phys. Chem. Liq. 2003, 41, 227. (84) Yoshida, N.; Matsuoka, K.; Moroi, Y. J. Colloid Interface Sci. 1997, 187, 388. (85) Berr, S. S. J. Phys. Chem. 1987, 91, 4760. (86) Malliaris, A.; Moigne, J. L.; Sturn, J.; Zana, R. J. Phys. Chem. 1985, 89, 2709. (87) Dorrance, R. C.; Hunter, T. F. J. Chem. Soc., Faraday Trans. 1 1974, 70, 1572. (88) Ogino, K.; Kakihara, T.; Abe, M. Colloid Polym. Sci. 1987, 265, 604. (89) Aniansson, E. A. G.; Wall, S. N.; Almgren, M.; Hoffman, H.; Kielmann, I.; Ulbricht, W.; Zana, R.; Lang, J.; Tondre, C. J. Phys. Chem. 1976, 80, 905.

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Another possible factor is the alkyl chain length of the surfactant. Maroncelli and co-workers studied the solvation dynamics of n-alcohols.15 The solvation dynamics of alcohol depends strongly on alkyl chain length: longer alkyl chain alcohols yield slower solvation dynamics. Liquid poly(ethylene glycol)s also show a strong polymer chain length dependence on the nanosecond solvation dynamics.65,66 Although the micelle solution systems are not simple alcohol and polymer liquids, a similar chain length effect of the solvation dynamics may also occur in surfactant systems. It is possible that the less local mobility of longer alkyl chain surfactant causes the slower dynamics in the interfacial region of the micelle solution. 5. Conclusions We have reported the solvation dynamics in aqueous micelle solutions of sodium alkyl sulfate and alkyltrimethylammonium bromide surfactants with several different alkyl chain lengths (n ) 8-14 for CnH2n+1SO4Na and n ) 10-16 for CnH2n+1N(CH3)3Br) measured with a picosecond streak camera system. The observed solvation dynamics in the aqueous micelle solutions shows a biexponential feature. In comparison between anionic sodium alkyl sulfate and cationic alkyltrimethylammonium bromide surfactants with the same alkyl chain length, the solvation dynamics in the cationic micelle solution is faster than that in the anionic micelle solution. The slower solvation time in alkyltrimethylammonium bromide micelle solutions is about 3.5 times faster than that in sodium alkyl sulfate micelle solutions. To see the specific interaction strengths between water and polar headgroups of the anionic and cationic surfactants, the interaction energies of model clusters of water and

Tamoto et al.

surfactant mimics (C2H5SO4- and C2H5N(CH3)3+) have been estimated by density functional theory calculations at the level of B3LYP/6-311++G(d,p). The interaction energy between water and the anionic headgroup mimic of sodium alkyl sulfate is about 4.5 kJ/mol larger than that between water and the cationic headgroup mimic of alkyltrimethylammonium bromide. The slower solvation dynamics in the sodium alkyl sulfate micelle solution than that in the alkyltrimethylammonium bromide micelle solution should be due to the stronger hydration of the anionic sulfate group than that of the cationic trimethylammonium group. We have also found that both the faster and the slower solvation time constants become slightly longer with increasing alkyl chain length of both sodium alkyl sulfate and alkyltrimethylammonium bromide surfactants. From the comparison between the present experimental result and the estimated surface density of the surfactant headgroups, the alkyl-chain-length dependence of solvation dynamics in aqueous micelle solutions seems to be correlated with the trend of the number density of the polar headgroup of the micellar surface by the change of the surfactant alkyl chain length. Acknowledgment. We thank Dr. Christian D. Grant (Rutgers University) for critical reading of this manuscript. This work is partially supported by the Shiseido Fund for Science and Technology and the Mitsubishi Chemical Corporation Fund (H. Shirota). We also acknowledge the Ministry of Education, Culture, Sports, Science and Technology of Japan (Grant-in-Aid for Scientific Research on Priority Areas: 417). LA046953I