Langmuir 1999, 15, 7993-7997
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Rotational Motion and Lateral Migration of Surfactants in Threadlike Micelles Shin-ichiro Imai and Toshiyuki Shikata* Department of Macromolecular Science, Osaka University, Toyonaka, Osaka 560-0043, Japan Received January 6, 1999. In Final Form: July 12, 1999 Molecular motion in a spherical and a threadlike micelle formed by cetyltrimethylammonium bromide (CTAB) and sodium p-toluenesulfonate (NapTS) in an aqueous solution was examined by fluorescence anisotropy using fluorescent probe molecules. The shape of micelles in the solution changes from spherical to long threadlike with the molar ratio of NapTS to CTAB. In a spherical micelle obtained at the ratio less than unity, a detergent ionic pair consisting of CTA+ and Br- rotates around its long tail and migrates laterally along the micellar surface. On the other hand, in a threadlike micelle the Br- of the detergent ionic pair is replaced by pTS-. The rotational relaxation time of the detergent ionic pair in the threadlike micelle is twice as long as that in the spherical micelle. Moreover, the lateral diffusion constant for the detergent ionic pair in the threadlike micelle is about a half of that in the spherical micelle. These results suggest that molecular motion in the threadlike micelle is slower than in a spherical micelle.
Introduction Certain cationic surfactants, for example cetyltrimethylammonium bromide, form enormously long threadlike micelles in aqueous solutions containing additives such as sodium salicylate.1-3 Long threadlike micelles produce concentrated entanglement networks and exhibit pronounced viscoelastic behavior similar to that of polymer molecules in semidilute to concentrated conditions. A threadlike micelle behaves as a flexible long thread with many entangling points, each possessing a finite lifetime.3,4 However, viscoelastic behavior of the entangling threadlike micellar system, especially of the slowest relaxation mechanism that is related to the entanglement release, is obviously different from the relaxation mechanisms of concentrated polymer systems.4 In concentrated polymer systems, the slowest relaxation mode shows a box type relaxation spectrum, which implies a broad distribution of relaxation modes.5 On the contrary, the slowest relaxation mode of a threadlike micellar system shows a very sharp relaxation spectrum like that of a Maxwell element, which implies that the system possesses a single relaxation time in the slowest mode.3 This difference in the slowest relaxation mechanism reflects an essential difference in structural features of these threadlike materials. In the case of polymer chain molecules, chain units are tightly bound to each other by covalent chemical bonding. On the other hand, threadlike micelles do not form such strong intermolecular interactions with surfactant molecules. Instead, the interaction is a relatively weaker hydrophobic interaction. Therefore, because the micelle-forming hydrophobic interaction is much weaker than a covalent chemical bond, it is possible that the micelle is cut into two pieces and the two threadlike micelles cross each other at entanglement points. We have proposed the phantom crossing model4 to understand the unique viscoelasticity of the threadlike micellar system. In the model we assumed that every entanglement point has a finite lifetime equal to the (1) Glavsholts, S. J. Colloid Interface Sci. 1976, 57, 575. (2) Hoffmann, H.; Rehage, H. Mol. Phys. 1989, 5, 1225. (3) Shikata, T.; Hirata, H.; Kotaka, T. Langmuir 1987, 3, 1081. (4) Shikata, T.; Hirata, H.; Kotaka, T. Langmuir 1988, 4, 354. (5) Ferry, J. D. Viscoelastic Properties of Polymers, 3rd ed.; Wiley: New York, 1980.
mechanical relaxation time, and at the end of this period, the two threadlike micelles cross each other at the entanglement point. Other groups have proposed different models2 based on the idea that scission and re-formation of a threadlike micelle occur throughout the micelle. When one looks at the dynamics of a threadlike micelle between entanglement points at a time scale much shorter than the slowest relaxation time, no difference is observed between the viscoelastic behavior of a concentrated polymer system and of an entangling threadlike micellar system.6 Both the polymer chain and the threadlike micelle behave as a flexible threadlike material that can bend rather freely between entanglement points. In a threadlike micelle, surfactants and additives exhibit relatively fast molecular motion. These molecules alter their positions and rotate quickly in the micelle. In the present study, we focus on the microdynamics of threadlike micelles. The microdynamics of a threadlike micelle can be studied using fluorescent probes as substitutes for micelle-forming surfactant and additive molecules. We used fluorescence anisotropy to investigate rotational relaxation times of the fluorescent probes as an indicator of the molecular motions of the micelle-forming substances. In previous investigations of physicochemical features of threadlike micellar systems, we used cetyltrimethylammonium bromide (CTAB) and sodium salicylate (NaSal)3 as micelle-forming substances. However, the fluorescence of NaSal is too intense for examining the behavior of molecular probes incorporated into a threadlike micelle.7 Thus, we employed sodium p-toluenesulfonate (NapTS), which is weakly fluorescent, in conjunction with CTAB to form the threadlike micelle. Threadlike micelles formed in an aqueous solution of CTAB and NapTS (CTAB:NapTS/W) are known to exhibit rheological behavior similar to that observed in the aqueous CTAB and NaSal system.8 Thus, the microdynamics of the threadlike micelle formed in the CTAB: NapTS/W system are essentially identical to the CTAB and NaSal system. (6) Shikata, T.; Niwa, H.; Morishima, Y. Nihon Reoroji Gakkaishi (Trans. Soc. Rheol, Jpn.) 1997, 25, 19. (7) Shikata, T.; Morishima, Y. Langmuir 1996, 12, 5307; 1997, 13, 5229. (8) Shikata, T. Unpublished data.
10.1021/la9900066 CCC: $18.00 © 1999 American Chemical Society Published on Web 09/03/1999
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Experimental Section Materials. A cationic surfactant, CTAB, was purchased from Wako Chemical Co. Ltd. (Osaka, Japan) and was purified by recrystallization in a mixture of methanol and acetone. NapTS was also purchased from the same company and was used without further purification. Highly deionized water with a specific resistance g14 MΩ cm-1 was obtained using the MilliQ SP system. A fluorescent molecule, sodium 2-hydroxyl-3-naphthoate (NaHN), was purchased from Wako Chemical Co., Ltd., and 1-[4(trimethylamino)phenyl]-6-phenylhexa-1,3,5-triene bromide (TMADPH) was purchased from Molecular Probes, Inc. (Eugene, OR). Another fluorescent probe molecule, cetylacridinium orange bromide (CAOB), was synthesized according to the method proposed by Miethke and Zanker9 and was recrystallized in methanol. The concentration of CTAB (CD) was kept constant at 3, 10, and 50 mM, while the concentration of NapTS (CS) was altered from 0 to 300 mM. The concentration of NaHN was about 1 mM and was much lower than the CD value. On the other hand, the concentrations of TMA-DPH and CAOB were lower than 1 mM. Methods. Steady-state fluorescence anisotropy (r) of the fluorescent molecular probes incorporated into the micelle was completed at 25 °C using a conventional fluorescence photometer (F-4500, Hitachi, Tokyo) equipped with a polarizer in front of the excitation window and an analyzer in front of the emission window. Excitation wavelengths for the probes, NaHN, CAOB, and TMA-DPH, were 365, 498, and 360 nm, respectively. Furthermore, emission wavelengths for these probes were 433, 512, and 415 nm, respectively. The value of r was estimated using eq 1 and fluorescence intensities, Ivv and Ivh. The subscripts
r)
Ivv - gIvh Ivv + 2gIvh
(1)
(v, vertical, and h, horizontal) of the intensities indicate the direction of the polarizer and of analyzer, respectively. The factor g means the correction factor for the polarizing character of the fluorescence photometer used in the present study and can be estimated by g ) Ivh/Ihh. Fluorescence lifetimes (τlife) of the probe molecules in the micelle were measured by a conventional time correlation single photon counting apparatus (NAES-500, Horiba, Kyoto, Japan) equipped with a high-pressure hydrogen flash lamp. The half-width of the flash pulse was ca. 2 ns. A deconvolution program was used to calculate the τlife values. Rotational relaxation times (τφ) of the fluorescent molecular probes were evaluated using eq 2, based on the values of r, τlife and the instantaneous fluorescence anisotropy (r0), which was determined in an extraordinarily viscous medium such as glycerin at low temperature.
(
)
3τlife 1 1 ) 1+ r r0 τφ
(2)
Results and Discussion τO of Probe Molecules. If one assumes a radius of 2.5 nm10 for a CTAB spherical micelle in an aqueous system, the overall rotational relaxation time for the spherical micelle is calculated to be ca. 50 ns using the StokesEinstein relationship. Moreover, the time constant for the overall rotation and/or bending motion of a threadlike micelle is even longer than this value. The values of τφ for all the fluorescent molecular probes in the present study were less than 5 ns as calculated using eq 2. A probe molecule incorporated into a spherical micelle has a rotational relaxation time (τφm) related to its microdynamics, and a micelle has an overall rotational relaxation time (τφr). Thus, τφ can be expressed as 1/τφ ) (9) Miethke, E.; Zanker V. Phys. Chem. (Frankfurt am Main) 1958, 18, 375. (10) Goyal, P. S.; Menon, S. V. G.; Dasannacharaya, B. A.; Rajagopalan, V. Chem. Phys. Lett. 1993, 211, 559.
Figure 1. Dependence of fluorescence lifetime (τlife) for HNon the concentration ratio (CS/CD) between CTAB and NapTS in the CTAB:NapTS/W system at 25 °C.
1/τφm + 1/τφr. From this, the τφ value obtained in the present study for both the spherical and threadlike micellar systems essentially corresponds to τφm, because the τφr value (ca. 50 ns for the spherical micelle) is much greater than τφm. Fluorescent Molecular Probes for pTS-. A previous NMR investigation of a CTAB and NapTS system in D2O revealed that in a threadlike micelle Br- of CTAB was perfectly replaced by dissociated pTS- from NapTS.8 The molar ratio of CTA+ to pST- in the threadlike micelle was estimated to be unity. Thus, the ratio of CS/CD is a good indicator of the shape of micelles. A region with a CS/CD value between 0 and unity is occupied by spherical and short rodlike micelles. On the other hand, a region with CS/CD > 1 is occupied by long threadlike micelles. A fluorescent probe molecule, such as NaHN, is incorporated into a micelle as a dissociated anion, HN-. This is confirmed by the fluorescence lifetime (τlife) of the probe. The value of τlife for NaHN in pure water was 1.5 ns and in the micelle 5-6.5 ns. Fluorescence decay curves for all the solutions examined were fitted well with single τlife values dependent on the ratio of CS/CD. The τlife value for NaHN in the spherical micelle was determined to be 5 ns and in the threadlike micelle 6.5 ns (Figure 1). In many cases, the τlife values for fluorescence probes reflect hydrophobicity around the probes themselves, and higher τlife values imply greater hydrophobicity.11 Therefore, the interior of a threadlike micelle is more hydrophobic than that of a spherical micelle. Because NaHN is similar in size to NapTS and, like NapTS, NaHN has an aromatic group, NaHN may substitute for NapTS in the micelle. HN- has two hydrophilic groups (OH and COO-). Thus, HN- can be incorporated into the micellar interior such that these two hydrophilic groups are orientated toward the bulk aqueous phase, as shown in Figure 2a. The direction of the transition moment of NaHN is as shown in the same figure. The value of r0 for NaHN was determined to be 0.34 in glycerin at -10 °C7 similar to the limiting value of 0.4 at which the transition moment and the axis of fluorescence emission are parallel. Therefore, the rotational motion around the axis may be the most effective means of relaxing the fluorescence anisotropy (Figure 2a). The relationship between τφ for NaHN incorporated into the micelle and the ratio of CS/CD is plotted in Figure 3. The stepwise increase in τφ around CS/CD ) 1 implies a change in the rate of molecular motion of the HN- anion (11) Grieser, F.; Drummond, C. J. J. Phys. Chem. 1988, 92, 5580.
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Figure 4. Dependence of τlife for TMA+-DPH on the CS/CD ratio in the CTAB:NapTS/W system at 25 °C.
Figure 2. Schematic representation of the location of fluorescent molecular probes in the micelle and the direction of the transition moments: (a) 2-hydroxyl-3-naphthoate anion (HN-); (b) 1-[4-(trimethylamino)phenyl]-6-phenylhexa-1,3,5-triene cation (TMA+-DPH); (c) cetylacridinium orange cation (CAO+).
Figure 5. Dependence of τlife ratio for CAO+ on the CS/CD ratio in the CTAB:NapTS/W system at 25 °C.
Figure 3. Dependence of rotational relaxation time (τφ) on the CS/CD ratio for HN- in the CTAB:NapTS/W system at 25 °C.
in the micelle caused by a change in the micellar shape. Because the τφ value for HN- incorporated into the micelle is indicative of the rotational relaxation time around the axis, the data in Figure 3 indicate that the rotational rate of HN- in the threadlike micelle is approximately half of that in the spherical micelle. This fact reflects differences in the inner viscosities of the two micelles, since the rate of the rotational Brownian motion is inversely proportional to the viscosity of the medium surrounding a probe molecule. Thus, we conclude that the inner viscosity of a threadlike micelle is twice as high as that of a spherical micelle. Fluorescent Molecular Probes for CTA+. TMA-DPH and CAOB are similar in structure to CTAB. Fluorescence decay curves for both TMA-DPH and CAOB in all the examined solutions were fitted by single τlife values
dependent on the ratio of CS/CD. TMA-DPH has a fluorescence lifetime, τlife, less than 0.3 ns in water, whereas in the micellar interior τlife increases to 1-1.5 ns. On the other hand, although CAOB is insoluble in water, it can be incorporated into the micelle formed by CTAB and has a τlife value of 1-1.5 ns. The dependencies of the TMA-DPH and CAOB τlife values on the CS/CD ratio are shown in Figures 4 and 5, respectively. Again, the threadlike micellar interior is more hydrophobic than the spherical micellar interior. Both TMA-DPH and CAOB work well as substitute fluorescent probe molecules in the micelle. Their incorporation into the micelle might be as dissociated forms, TMA+-DPH and CAO+. Because the direction of the transition moment of TMADPH is aligned in the long molecular axis connecting the two phenyl rings as shown schematically in Figure 2b and the value of r0 is 0.39,12 the effective molecular motion in the micelle for fluorescence anisotropy relaxation is the translational motion along the curvilinear micellar surface and/or the wobbling motion. On the other hand, the direction of the CAO+ transition moment is parallel to an axis connecting the nitrogen and 9-carbon in an acridinium ring as shown in Figure 2c. Thus, the translational motion along the micellar surface and/or the wobbling motion should also effectively relax the fluorescence anisotropy for CAO+ in the micellar interior. The τφ values for TMA+-DPH and CAO+ are plotted as functions of the CS/CD ratio in Figures 6 and 7, respectively. (12) Lakowicz, J. R.; Prendergast, F. G.; Hogen, D. Biochemistry 1979, 18, 520.
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Figure 6. Dependence of τφ for TMA+-DPH on the CS/CD ratio in the CTAB:NapTS/W system at 25 °C.
Figure 8. (a) Schematic representation of migration modes for a fluorescent molecular probe: (I) a straight migration mode along the long threadlike micelle; (II) a circular migration mode along the contour of the slice of the threadlike micelle. Relaxation of fluorescence anisotropy is observed only in (II). (b) Migration mode of a detergent ionic pair consisting of CTA+ and pTS- in the threadlike micelle.
CAO+
Figure 7. Dependence of τφ for CTAB:NapTS/W system at 25 °C.
on the CS/CD ratio in the
The CS/CD ratio dependencies of these data in the figures roughly resemble those in Figure 3. This indicates that rates of molecular motion for both TMA+-DPH and CAO+ change in accordance with the micellar shape. Therefore, we conclude that, in the threadlike micelle, the translational motion along the micellar surface and/or the wobbling motion of the CTA+ cation is twice as slow as that in the spherical micelle. The fact that data in Figures 6 and 7, obtained by different probe molecules, are in agreement with each other strongly suggests the stepwise change in τφ is essential for the transformation of a micelle from a sphere to a thread. Because the rate of the translational motion is directly related to the lateral diffusion constant (Dlat), the stepwise increase in τφ in Figures 6 and 7 implies a stepwise decrease in Dlat for the CTA+ cation on the surface of the spherical micelle to that on the threadlike micellar surface. Lateral Diffusion Constant of CTA+ in the Micelle. The direction of translational motion of the CTA+ cation on the surface of a spherical micelle is always random. On the other hand, the translational motion on the rodlike surface of the threadlike micelle can be divided into two modes:7 a straight migration mode along the long threadlike micellar axis and a circular migration mode along the contour of the slice of the threadlike micelle, as shown schematically in Figure 8a. When substitute fluorescence molecules such as TMA-DPH and CAOB exhibit a transition moment parallel to the long molecular axis, a circular migration mode effectively relaxes the fluorescence anisotropy. However, a straight migration mode does not
relax the fluorescence anisotropy, because the direction of the transition moment does not alter with time. If the probe molecules follow a straight migration mode, more time is required to relax the fluorescence anisotropy than if they follow a circular mode. Thus, the value of τφ for TMA+-DPH and CAO+ is essentially governed by the circular migration mode. If one neglects a numerical front factor for simplicity, Dlat of the fluorescence probes in a threadlike micelle can be roughly estimated by eq 3 using the radius (a) of the
Dlat ≈
a2 τφ
(3)
threadlike micelle.7 Assuming a ) 2.5 nm, the Dlat values for CTA+ in the spherical and threadlike micelles can be estimated as 2 × 10-5 and 1 × 10-5 cm2 s-1, respectively, from Figures 6 and 7. These values are larger, by a factor of 5-50, than the reported Dlat values for surfactants in micellar systems estimated by other techniques.13,14 This discrepancy presumably arises because the numerical front factor is neglected and the quick wobbling motion of the probe molecules contributes to the fluorescence anisotropy relaxation. If a time-resolved fluorescence anisotropy relaxation experiment is completed using a narrow excitation pulse, the contribution of the quick wobbling motion to the fluorescence anisotropy relaxation can be separated from the total fluorescence anisotropy decay curve and the (13) So¨derman, O.; Henriksson, U.; Olsson, U. J. Phys. Chem. 1987, 91, 116. (14) Alsins, J.; Almgren, M. J. Phys. Chem. 1990, 94, 3062.
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contribution of the translational migration mode of the probe molecules alone can be identified.15 Ionic Pairs. In a threadlike micelle, the ratio of CTA+ to pTS- is unity. Thus, CTA+ and pTS- do not migrate independently of each other in a threadlike micelle but form an ionic pair and migrate together in the micelle. The presence of a detergent ionic pair between CTA+ and Br- in the spherical micellar system16 and between CTA+ and Sal- (or pTS-) in the threadlike micellar system17 has been recently confirmed using a dielectric relaxation experiment. The dielectric relaxation time due to rotation of these detergent ionic pairs in the micelle was estimated to be ca. 1 ns. Since the dielectric relaxation time for the detergent ionic pairs is essentially identical to τφ for HNin physical meaning, reasonable agreement between these two relaxation times evaluated in different ways strongly supports that CTA+ forms an ionic pair with HN- in the threadlike micelle. When the average life span of the detergent ionic pair is exceeded, a pTS- anion in a detergent ionic pair will be exchanged for another pTS- from a different ionic pair or for a free pTS- anion from the bulk aqueous phase. The TMA+-DPH (or CAO-) substitute for CTA+ should have a τφ value similar to that of the CTA+ and pTS- ionic pair. (15) Kinoshita, S.; Tanaka, I.; Kushida, T.; Kinoshita, Y.; Kimura, S. J. Phys. Soc. Jpn. 1982, 51, 598. (16) Shikata, T.; Imai, S. Langmuir 1998, 14, 6804. (17) Imai, S.; Shikata, T. Langmuir, submitted for publication. (18) Israelachivili, J. N. Intermolecular & Surface Forces, 2nd ed.; Academic Press: New York, 1991.
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Therefore, the τφ values for HN- reflect the rotational relaxation times around the long alkyl tail of the CTA+ and pTS- ionic pair, whereas τφ’s for TMA+-DPH and CAOreflect the lateral diffusion constant of the ionic pair (Figure 8b). Stepwise Change in τO. The size of the head portion of a surfactant molecule is a decisive parameter for the shape of micelles. Israelachvili described a criterion for distinguishing between a spherical and threadlike micelle based on a change in the packing parameter of the detergent molecules.18 However, the ratio of the head size of the detergent ionic pair to the length of the detergent alkyl tail is also an important parameter for determining the shape of the micelle. Dielectric relaxation data for CTAB and NaBr aqueous solutions suggest a decrease in the size of the head portion of detergent ionic pairs at the transition from a spherical to a threadlike micelle.17 Since this decrease in the size of head portion of the detergent ionic pair implies an increase in the density of the head portion on the micellar surface, the rate of molecular motion in the interior of a threadlike micelle is less than that of a spherical micelle. Therefore, in the CTAB:NapTS/W system this depression in the rate of rotational molecular motion, which results from increased density of ionic pairs, may counteract and overwhelm the increased rate of rotation, which results from a decrease in the size of the head portion of the detergent ionic pair. LA9900066
