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Microdynamic Behavior in Threadlike Micelles Toshiyuki Shikata* and Yotaro Morishima Department of Macromolecular Science, Osaka University, Toyonaka, Osaka 560, Japan Received April 11, 1996. In Final Form: August 6, 1996X Microscopic dynamics of a salicylate anion, Sal-, in long rodlike or threadlike micelles formed by cetyltrimethylammonium bromide, CTAB, in aqueous solution was examined by use of fluorescence techniques. Long and stable threadlike micelles are formed at the equimolar ratio of CTAB and sodium salicylate, NaSal. Enhancement of fluorescence intensity of Sal- was observed with increasing the molar ratio of CTAB/NaSal up to unity, while a further increase in the ratio beyond unity did not affect the fluorescence intensity. We also found that Sal- anions had two (long and short) distinctive fluorescence lifetimes when the molar ratio was smaller than unity, and had long one only when the ratio was larger than unity. The long and short lifetimes are due to Sal- residing at a rather nonpolar micellar interior site and in the bulk aqueous phase, respectively. The enhancement of fluorescence intensity is related to the increase in the fraction of the long lifetime component. Furthermore, fluorescence anisotropy of Sal- was also measured and the rotational relaxation time, τφmic, at the micellar interior site was estimated as a function of the molar ratio. Since τφmic showed a long value (∼1.6 ns) at a molar ratio below unity, frequency of molecular motion of Sal- in the threadlike micelle is highly reduced. However, the τφmic is still in a nanosecond range, and the molecular motion in the micelle must be rather fast and dynamic even in the fully established threadlike micelle.
Introduction Some detergent molecules could make very long and stable rodlike or threadlike micelles with or without additives in aqueous solution.1-5 Cationic surfactant cetyltrimethylammonium bromide, CTAB, is one of the typical examples of forming threadlike micelles with additives. CTAB could make threadlike micelles with sodium salicylate, NaSal, in aqueous solution even at rather low concentrations.4,5 As one would imagine from the long threadlike shape of the micelles, the system (CTAB:NaSal/W) shows profound viscoelastic behavior due to entangling among the formed threadlike micelles. According to the previous studies on the CTAB:NaSal/W systems, plateau modulus observed in a frequency range of 101-102 s-1 is proportional to a square of the concentration of CTAB just like concentrated polymer systems.4 Entanglement among the threadlike micelles would be an essential reason for the elasticity as well as the polymer systems. However, the system shows a single mode longest relaxation process with a characteristic time which would be related to entanglement release and was affected by the concentration of free salicylate, Sal-, anions not bound to the micelles.5 The single relaxation mode is unusual for concentrated polymer systems6,7 in which diffusion of the polymer chains along themselves must be the essential dynamics for the entanglement release. The mechanism for the longest single mode relaxation of the threadlike micellar systems is still in hot controversy,5,8,9 while one could imagine the threadlike micelle as a long thread with a finite radius which would obey the Gaussian dynamics and statistics as well as flexible X
Abstract published in Advance ACS Abstracts, October 1, 1996.
(1) Gravsholt, S. J. Colloid Interface Sci. 1976, 57, 575. (2) Candau, S. J.; Hirsch, E.; Zana, R. J. Colloid Interface Sci. 1985, 105, 521. (3) Rehage, H.; Hoffmann, H. Mol. Phys. 1991, 74, 933. (4) Shikata, T.; Hirata, H.; Kotaka, T. Langmuir 1987, 3, 1081. (5) Shikata, T.; Hirata, H.; Kotaka, T. Langmuir 1988, 4, 354. (6) Ferry, J. D. Viscoelastic Properties of Polymers, 3rd ed.; John Wiley & Sons: New York, 1980. (7) Doi, M.; Edwards, S. F. The Theory of Polymer Dynamics; Oxford University Press: London, 1986. (8) Lequeux, F. Europhys. Lett. 1992, 19, 675. (9) Cates, M. E. Structure and Flow in Surfactant Solutions; ACS Symposium Series 578; Herb, C. A., Prud’homme, R. K., Eds.; Amercian Chemical Society: Washington, DC, 1994; Chapter 2, p 31.
S0743-7463(96)00348-4 CCC: $12.00
polymer chains on a time scale slower than 102 s-1. In this paper, we investigate microscopic dynamics in the threadlike micelles of much faster time scale than 102 s-1 and of much smaller spatial scale comparable to the dimension of the detergent molecule and Sal- anion in the threadlike micelles. Nuclear magnetic resonance, NMR, spectroscopy is wellknown as one of the typical techniques to investigate microscopic behavior of molecules consisting of the threadlike micelles.5,10,11 Actually, sites of additive molecules bound in the micelles could be assigned in detail from their chemical shift data.5,10 If one pays attention to relaxation of NMR signals of the molecules, microscopically dynamic information of the molecules in the micelles indeed could be obtained.11 Fluorescence spectroscopy is also a powerful method for investigation when one has good fluorescence probes in the systems.12,13 Fortunately, Sal- is strongly fluorescent in aqueous solution12 with an adequate fluorescence lifetime to carry out experiments with ordinary equipment. Because fluorescence intensity and lifetime should be good indicators for information about microenvironments or chemical circumstances around fluorescence probes, a change in the intensity and the lifetime should be exact proof of alteration of the site occupied by the probe molecules. Although fluorescence anisotropy and the rotational relaxation time are good measures of frequency of molecular motion of probe molecules, in the case of quite tiny and quickly rotating molecules such as Sal-, fluorescence anisotropy results are usually too small to discuss with high accuracy. However, motion of Sal- stuck or bound in the micellar interior would be more highly restricted (10) Bunton, C. A.; Minch, M.; Hidalgo, J.; Sepulveda, L. J. Am. Chem. Soc. 1973, 95, 3262. (11) Molecular motions of surfactant molecules in micelles are discussed in, for example, Monduzzi, M.; Olsson, U.; So¨derman, O. Langmuir 1993, 9, 2914. Anisotropic rotational motions of aromatic rings are reported in the following: Anet, F. A. N. J. Am. Chem. Soc. 1986, 108, 7102. Menger, F. M.; Jerkunica, J. M. J. Am. Chem. Soc. 1978, 100, 688. Stark, R. E.; Storrs, R. W.; Kasakevich, M. L. J. Phys. Chem. 1985, 89, 272. (12) Verman, B.; Valanlinkar, B. S.; Manohar, C. J. Sci. Technol. 1987, 3, 19. (13) For example, Kinoshita, K.; Kawamoto, S.; Ikegami, A. Biophys. J. 1977, 20, 289.
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than in the free aqueous state; therefore, significant fluorescence anisotropy of Sal- in the micelle could be anticipated. Experimental Section Materials. CTAB was purchased from Wako Pure Chemical Industries, Ltd. (Osaka, Japan), and was purified by recrystallization from a solution of a mixture of ethanol and acetone. Extra pure grade NaSal was also purchased from the same company and used without any further purification. Water for solvent was deionized with a Milli Q SP system (Millipore) and had a specific resistance higher than 14 MΩ cm-1. NaSal concentration, CS, was kept at 2.0 mM, because its fluorescence intensity was perfectly proportional to CS in the range up to 5 mM. On the other hand, CTAB concentration, CD, was varied from 0 to 10.3 mM. Glycerine was used as a solvent to measure the instantaneous fluorescence anisotropy, r0, of Sal-, which means the fluorescence anisotropy at time equal to zero, because Sal- was slightly soluble in glycerine, which had a high viscosity and could be a glassy liquid below -10 °C. The concentration of NaSal in glycerine to get r0 was lower than 1 mM. One must be very careful about fluorescence quenching by oxygen molecules and heavy atoms existing in systems. Actually, our system including CTAB possesses Br- anions which could work as strong quenchers in some cases. However, we confirmed that Br- no longer affected fluorescence intensity in aqueous solutions of NaSal with NaBr varying NaBr concentrations up to 30 mM, and we concluded that Br- was not a quencher for Sal-. The reason for that would be the same charge for both anions and strong electrostatic repulsion between them. We did not pay special attention to the effect of oxygen molecules, because the lifetime of Sal- was rather short ( 380 nm light was placed before a photomultiplier. The same quartz cell was employed for the lifetime measurements, and temperature was kept at 25 °C. Absorption of light at the maximum wavelength of 295 nm, which is close to the excitation wavelength of 320 nm, was monitored with a V-520-SR spectrometer (JASCO) at 25 °C. For sample solutions including NaSal whose absorbances are so high, a thin quartz cell with a light pass of 2 mm was used.
Results Fluorescence Intensity of the CTAB:NaSal/W System. Fluorescence intensity, I, of the CTAB:NaSal/W system at 420 nm was very sensitive to the molar ratio of CD/CS as shown in Figure 1a. One must be very careful about the change of the magnitude of optical density, OD, of absorption of the excitation light at 320 nm caused by some reasons when exact values of the fluorescence intensity are considered. Since the exact fluorescence intensity should be proportional to the absorption of excitation light, reduced fluorescence intensity, I/OD, must be shown, as a function of CD/CS. Figure 1b shows the relationship between I/OD and CD/CS, where OD at a maximum absorption wavelength of 295 nm is accepted, and one could recognize that the fluorescence intensity is
Figure 1. (A) Fluorescence intensity of Sal- anions excited at 320 nm and monitored at 420 nm as a function of the molar ratio of CD/CS for the CTAB:NaSal/W systems with CS ) 2 mM at 25 °C. (B) The dependence of reduced fluorescence intensity, I/OD, on the molar ratio.
Figure 2. Typical profiles of fluorescence intensity decay for the CTAB:NaSal/W system with CS ) 2 mM at 25 °C.
exactly proportional to CD/CS in the range of 0 < CD/CS < 1, and it becomes constant when CD/CS > 1. Fluorescence Lifetime. Typical profiles of fluorescence decay (excited at 320 nm) monitored at λ > 380 nm are shown in Figure 2. The decay rate of the fluorescence intensity of Sal- was strongly dependent on CD/CS; for example, a slope at CD/CS ) 0 is much steeper than that at CD/CS > 1. This means that the fluorescence lifetime, τ, of Sal- in a system at CD/CS ) 0 in which there is no micelle and all Sal- anions are free in the bulk aqueous phase is shorter than those at CD/CS > 1. It is interesting to note that a slope of a decay curve belonging to CD/CS ) 0.35 alters from a steeper value similar to that at CD/CS ) 0 to a value parallel to that at CD/CS > 1. The fluorescence decay data were best-fitted with a double-exponential function by use of the nonlinear leastsquares method combined with a deconvolution technique. Decay curves at CD/CS ) 0 and ratios CD/CS > 1 are welldescribed with only one τ with high accuracy. However, decay curves at CD/CS < 1 are well-resolved into two decay components with high accuracy. A short fluorescence lifetime, τshort, was about 4 ns and was essentially independent of CD/CS, while the fraction, of the short component, Fshort, varied as a function of CD/CS. The other
Microdynamic Behavior in Threadlike Micelles
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Figure 5. Dependence of fluorescence anisotropy, r, on the molar ratio of CD/CS for the CTAB:NaSal/W system with CS ) 2 mM at 25 °C.
Figure 3. Relationship between evaluated two fluorescence lifetimes, τshort and τlong, from time dependent decay proflies of intensity with a deconvolution technique and the molar ratio of CD/CS.
) 1 and is consequently followed by decreasing with the ratio. When Sal- is placed in the bulk aqueous phase, it feels only the very low viscosity of water and could rotate freely. The very low r value at CD/CS ) 0 reflects free rotation of Sal-. On the other hand, the first increase and the consequent decrease in r value with CD/CS imply that Sal- feels strong restriction in rotational motion and its rotational diffusion constant is highly depressed around CD/CS ) 1. Discussion
Figure 4. Relationship between fraction of long fluorescence lifetime component, Flong, and the molar ratio of CD/CS.
long lifetime, τlong, was about 8 ns and also was independent of CD/CS. This situation is shown in Figure 3 as a function of CD/CS. It is noteworthy that τ for the system at a ratio of 0 is identical to the short one. On the other hand, the lifetime at CD/CS > 1 is identical to the long one. In Figure 4 the fraction of a component of the long fluorescence lifetime, Flong ()1 - Fshort), is plotted as a function of CD/CS. It is quite clear that the proportionality in Figure 4 is completely similar to that in Figure 1b. Thus, the increase in fluorescence intensity with the ratio results from the increase of τ or, in the other words, from the increase of the fraction of the long lifetime component. Fluorescence Anisotropy. Fluorescence anisotropy, r, is defined by eq 1 below. Ivv denotes fluorescence
r)
Ivv - gIvh Ivv + 2gIvh
(1)
intensity when the system is excited by vertically polarized light and emission is monitored through a vertical polarizer, and Ivh denotes the intensity excited by vertical light and monitored through a horizontal polarizer. The g factor in eq 1 is defined by the relationship of Ihv/Ihh and means a polarization character of a spectrometer used. The g factor is a function of monitored emission wavelength, and in our case g is 1.06 at 410 nm. The ratio of CD/CS dependence of r of Sal- is plotted in Figure 5. At CD/CS ) 0, the system shows a very low r value, lower than 0.001, and r increases with CD/CS and reaches the maximum value close to 0.02 around CD/CS
Two-State Model. To interpret behavior shown in Figures 1-4, we propose a two-site model which is consistent with a model used in the previous considerations for the same CTAB:NaSal/W system based on chemical shifts of NMR signals5 of Sal- in the micelles. In the twosite (or -state) model, we assume that Sal- has two states in the micellar system. One is simply a state in the bulk aqueous phase in which Sal- can rotate freely feeling very low viscosity of water and has the short fluorescence lifetime of 4 ns. The other state is in the interior of the micelle, in which rotational motion of Sal- is restricted by surrounding surfactant molecules and Sal- has the longer fluorescence lifetime of 8 ns due to nonpolar microenvironments created by lipophilic surfactant tails. Because Sal- bears two hydrophilic groups of OH and COO-, it should be placed not in a deep micellar interior but near the surface of the micelles beside ammonium groups.5,10 According to this model, the fraction of the long fluorescence component, Flong, shown in Figure 4 is identical to the fraction of the Sal- component occupying the state of the micellar interior, Fmic. The change of Flong in the range of 0 < CD/CS < 1 implies that Sal- anions exist at the two states. Since in the time scale of fluorescence lifetime (∼10 ns or ∼108 Hz in this case) two states could be precisely separated as shown in Figure 4, resident time for the Sal- anions at both the states is much longer than their fluorescence lifetime. However, in the previous discussion based on NMR data5 for the same system in the regime of 0 < CD/CS < 1, resonance signals of protons in Sal- could not be separated into distinctive chemical shifts corresponding to both the micellar, δmic, and free, δfree, states, but only one resonance signal was observed. Then, the fraction of the micellar state component, Fmic, could be evaluated with the equation Fmic ) (δ - δmic)/(δfree - δmic).5 From the time scale of NMR, Sal- could change their states so quickly (much faster than 103 Hz or 1 ms). Thus, the rate of exchange of Sal- anions between the micellar interior and the free state could not be discussed, and it should be on an intermediate time scale between the fluorescence and NMR. According to the previous NMR studies,5 the threadlike micelle is constructed with a stoichiometric pair of CTA+
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and Sal-. Therefore, in the regime of CD/CS < 1 there should be the perfect threadlike micelles with essentially infinite length and excess Sal- free in the bulk aqueous phase. However, in the regime of CD/CS > 1 the amount of Sal- is not enough to make the perfect threadlike micelles, and formed micelles would not be so long but rather imperfect and short or spherical. Mobility of Salin the micelles would be affected by completion of the threadlike micelle or micellar shape. Therefore, in the regime of CD/CS > 1, mobility of Sal- in the micelles could alter with the ratio. Thus, the fact that the τlong corresponding to the micellar interior state is not essentially influenced by CD/CS implies that the fluorescence lifetime of Sal- is determined only by microenvironments and not by their mobility in the micelles. Imae et al. reported electrophoretic behavior of the threadlike micelles consisting of tetradecyltrimethylammonium salicylate, C14TASal, with NaSal in aqueous solution, and they concluded that the threadlike micelles had another site which can be occupied by Sal- anions at CS > CD and surface charge of the micelles was able to become negative.14 Cassidy et al. measured the surface charge of the threadlike micelles by use of a molecular probe, 4-heptadecyl-7-hydroxycoumarin, and they did not support that the threadlike micelles could be a stoichiometric 1:1 complex between a C14TA+ cation and a Salanion.15 Because the condition of Imae’s system14 is belonging to a fully entangling threadlike micelle formation and the formed micelles is not able to flow due to the entanglement, the electrophoretic behavior in the system would not reflect real surface charge on the micelles. Cassidy’s method to detect the surface charge of the micelles needs pH change in the micellar systems. However, there is the possibility that pH affects the structure of the micelles. From these, the two-site model applied in this study is not denied by their data. Rotational Relaxation Time. The fluorescence anisotropy, r, measured under the steady excitation condition could be related to the rotational relaxation time, τφ, of the probe fluorescent molecule with the well-know Perrin equation (eq 2),16 if free rotation of the probe could be supposed.
(
)
1 1 3τ ) 1+ r r0 τφ
(2)
where τ and r0 are the fluorescence lifetime and the instantaneous fluorescence anisotropy of the probe, respectively. Generally, r0 could be determined with time-resolved fluorescence spectroscopy. However, the equipment we could operate for the fluorescence lifetime measurement had a time resolution of sub-nanosecond, which was not short enough to get r0 for Sal- even in the micellar interior site. The other method to determine r0 is that r of the probe dissolved in extremely high viscous media could be regared as r0 with high accuracy. Fortunately, NaSal is slightly soluble in glycerine, which is a sufficiently viscous glass around -10 °C, to get r0. Thus, we could estimate r0 of Sal- to be 0.32 because r of Sal- in glycerine from -10 to -15 °C was constant. Equation 2 could be used directly to evaluate τφ, if the system has only one τ as in the regime of CD/CS > 1. However, in the case where the system has a mixed state of two sites, eq 2 should be modified somehow. Sal- anions existing in the micellar interior and in the free bulk state (14) Imae, T.; Kohsaka, T. J. Phys. Chem. 1992, 96, 10030. (15) Cassidy, M. A.; Warr, G. G. J. Phys. Chem. 1996, 100, 3237. (16) Perrin, F. J. Phys. Radium 1926, 7, 39.
Figure 6. Dependence of the rotational relaxation time of Salcorresponding to the micellar interior site, τφmic, evaluated with eq 3 in the text on the molar ratio of CD/CS.
Figure 7. A schematic diagram representing the direction of transition moment and the axis for rational motion of Sal- anion belonging to the micellar interior state.
should have different rmic and rfree, and the average value of the both states is observed as r ) Fmicrmic + (1 Fmic)rfree. Because of very quick rotation, rfree in the free state is much smaller than rmic and is a negligibly small value ( 1, the rotation of Sal- is accelerated with increasing the ratio CD/CS. This means that the rotation of Sal- is faster in a micelle which has imperect threadlike structure but has a shape with rather low aspect ratio, especially in the spherical shape micelle. Axis of Rotation of Sal- in the Micellar Interior Site. The direction of transition moment in Sal- could be roughly estimated as shown in Figure 7 from consideration of electronic spectra.17 Thus, rotation of a virtual line connecting the 3 and 6 carbons in Sal- could be active to fluorescence anisotropy. As discussed previously, Salhas two hydrophilic groups which must stick out radially from the surface of the micelle into the bulk aqueous phase. (17) Burawoy, A.; Cais, M.; Chamberlain, J. T.; Liversedge, F.; Thompson, A. R. J. Chem. Soc. 1955, 3721, 3727.
Microdynamic Behavior in Threadlike Micelles
The axis of rotation of Sal- anions essentially contributing to the fluorescence anisotropy should be parallel to the direction connecting the 2 and 4 carbons as shown in Figure 7. Of course, Sal- anions can migrate easily along the surface of the micelle; therefore, their rotational motions occur simultaneously, not at the same place in the threadlike micelle. The axis of rotation of Sal- anions in the threadlike micelles was discussed by Anet on the basis of NMR data.11 He concluded that rotation or tumbling of Sal- anions around the axis connecting the 1 and 4 carbons was much faster than the perpendicular axis. The rotational relaxation time estimated from Anet’s NMR data is comparable to the τφmic obtained here. The orientation of Salanions in the threadlike micelles proposed by Anet is that the axis connecting the 1 and 4 carbons is normal to the micellar surface and the axis is a little tilting from that proposed here. However, the difference between them
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would not be so important for essential discussion of the microdynamics in the micelles at this stage. Conclusions The Sal anion has two different states to exist in the CTAB:NaSal/W threadlike micellar system. One is free state in the bulk aqueous phase which leads to a short fluorescence lifetime and a short rotational relaxation time. The other is the micellar interior state in which Sal- has a long fluorescence lifetime and a long rotational relaxation time. The long fluorescence lifetime is not affected by the completion of the threadlike micellar structure, but the relaxation time is strongly related to that. Rotation of Sal- anions in the micellar interior state becomes faster with imperfection in structure of the threadlike micelle. -
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