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Transition from Nanotubes to Micelles with Increasing Concentration in Dilute Aqueous Solution of Potassium N-Acyl Phenylalaninate Akio Ohta,*,† Radostin Danev,‡ Kuniaki Nagayama,‡ Toshimitsu Mita,† Tsuyoshi Asakawa,† and Shigeyoshi Miyagishi† DiVision of Material Sciences, Graduate School of Natural Science and Technology, Kanazawa UniVersity, Kanazawa, Ishikawa 920-1192, Japan, and Okazaki Institute for IntegratiVe Bioscience, National Institutes of Natural Sciences, Okazaki 444-8787, Japan ReceiVed April 25, 2006. In Final Form: July 12, 2006 An aggregation behavior of potassium N-acyl phenylalaninate in dilute aqueous solution was investigated. It was found that this surfactant formed large aggregates at lower concentrations, which were then transformed to micelles at higher concentrations. Fluorescence intensity measurements using a probe were used to examine the effects of alkali concentration, acyl chain length, and solvent isotope on the aggregation behavior. The influence of the alkali concentration suggested that formation of an acid-soap dimer brought about the construction of the large particles at very dilute concentrations. Increases in both the acyl chain length and replacement of H2O with D2O resulted in stronger hydrophobic interactions; consequently, the large aggregate formation was enhanced. This aggregation behavior has not been observed when racemic modification of N-acyl phenylalaninate has taken place. By using cryo-transmission electron microscopy (TEM) with a Zernike differential contrast phase plate, it was found that the large aggregates were tubes with bilayer structures, which were then transformed into spherical micelles via threadlike micelles with increasing concentration due to a drastic increase in the concentration of ionic species in the aggregate.
Introduction Self-assembling amphiphiles form various aggregates such as vesicles and micelles in aqueous solution. To explain the structural formation of these aggregates, it has been proposed by Israelachivili et al. that a geometric concept called the packing parameter is a significant factor.1,2 According to the packing parameter theory, a single-tailed surfactant tends to form spherical micelles, while a double-tailed surfactant generally leads to a bilayer structure in aqueous solution. Over the past decade, much attention has been given to the spontaneous formation of thermodynamically stable vesicles, which result from oppositely charged singletailed surfactant mixtures in aqueous solution.3-9 In such binary surfactant systems, a vesicle is formed first, and then a vesiclemicelle transition occurs at a higher concentration when the total concentration of surfactants is increased at a constant mole fraction. This apparently strange behavior, in which a more highly ordered structure such as a vesicle is transformed into a less highly ordered structure such as a micelle with increasing * To whom correspondence should be addressed. Mailing address: Akio Ohta, Division of Material Sciences, Graduate School of Natural Science and Technology, Kanazawa University, Kakuma, Kanazawa, Ishikawa 9201192, Japan. E-mail:
[email protected]. † Kanazawa University. ‡ National Institutes of Natural Sciences. (1) Israelachvili, J. N.; Mitchell, D. J.; Ninham, B. W. J. Chem. Soc., Faraday Trans. 2 1976, 72, 1525-1529. (2) Israelachvili, J. N. Intermolecular and Surface Forces; Academic Press: London, 1985. (3) Kaler, E.; W.; Herrington, K. L.; Iampietro, D. J.; Coldren, B. A.; Jung, H.; Zasadzinski, J. A. In Mixed Surfactant Systems, 2nd ed.; Abe, M., Scamehorn, J. F., Eds.; Marcel Dekker: New York, 2005; p 289. (4) Svenson, S. Curr. Opin. Colloid Interface Sci. 2004, 9, 201-212. (5) Shang, Y.; Xu, Y.; Liu, H.; Hu, Y. J. Dispersion Sci. Technol. 2006, 27, 105-108. (6) Yin, H.; Huang, J.; Lin, Y.; Zhang, Y.; Qiu S.; Ye, J. J. Phys. Chem. B 2005, 109, 4104-4110. (7) Nan, Y.; Liu, H.; Hu, Y. Colloids Surf. A 2005, 269, 101-111. (8) Tsuchiya, K.; Nakanishi, H.; Sakai, H.; Abe, M. Langmuir 2004, 20, 21172122. (9) Karukstis, K. K.; Zieleniuk, C. A.; Fox, M. J. Langmuir 2003, 19, 1005410060, and references therein.
concentrations of the constituents, can be explained as follows. The vesicle particles are derived from dimers of the two surfactants resulting from the electrostatic interaction between the oppositely charged headgroups, and these dimers behave like double-tailed surfactants. Since the composition of vesicles formed in a cationic and anionic binary surfactants system is very different from that of the prepared composition, one type of surfactant molecule is present in excess in the bulk solution when the concentration is increased, and then the excess surfactants finally form micelles. Such behavior has also been reported in cationic single- and double-tailed surfactant binary systems.10,11 We have also reported that fluorescence intensity (using probes) and size distribution measurements show that such aggregation behavior takes place in an alkali aqueous solution of potassium N-tetradecanoyl L-phenylalaninate (K C14-L-Phe, see Figure 1a) despite this being a single-surfactant system.12 Since the fluorescence spectra of some dye molecules change when the molecules are solubilized in aggregates of surfactants, the ratio of the fluorescence intensity of a dye in an aqueous solution containing no surfactant (I0) to that in a surfactant solution (I) has often been used to determine the critical aggregation concentration (cac). Figure 1b shows I/I0 for auramine as a probe versus the concentration of alkali aqueous K C14-L-Phe solution at 298.2 K. For common surfactants, the value of I/I0 is unity at concentrations under the critical micelle concentration (cmc) and then increases monotonically after the cmc. However, in this case, after a characteristic peak, I/I0 increased abnormally with increasing concentration. As shown in Figure 1c, the size distribution measurements show that relatively large particles appeared at 0.2 mM, which is the peak concentration in Figure 1b, while normal micelle particles about 2 nm in size were present (10) Viseu, M. I.; Edwards, K.; Campos, C. S.; Costa, S. M. B. Langmuir 2000, 16, 2105-2114. (11) Kodama, T, Ohta, A.; Toda, K.; Katada, T.; Asakawa, T.; Miyagishi, S. Colloids Surf. A 2006, 277, 20-26. (12) Ohta, A.; Tani-I, K.; Hoshiba, A.; Asakawa, T.; Miyagishi, S. Chem. Lett. 2005, 34, 560-561.
10.1021/la0611110 CCC: $33.50 © 2006 American Chemical Society Published on Web 08/24/2006
From Nanotubes to Micelles of K C14-L-Phe
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Figure 1. (a) Chemical structure of K C14-Phe. (b) Fluorescence intensity ratios of auramine vs concentration for K C14-L-Phe solution in 1 mM KOH. (c) Size distribution of aggregates in alkali aqueous solutions of K C14-L-Phe at 0.2 mM (solid line) and 1.0 mM (dotted line).
in the solution at higher concentrations. This was discussed in our previous study as follows: first, large aggregate-like vesicles composed of acid-soap dimers were formed, and second, micelles were formed by the excess ionic-type surfactant molecules. Although it is well-known that acid-soap dimers of fatty acids self-organize into vesicles at intermediate pH,13,14 there are few studies on the formation of large aggregates (like vesicle particles) in alkali solutions. However, the true character of the large aggregates has not been identified yet, and the mechanism of this aggregation behavior has still not been determined. As this remarkable growth of aggregates has not been observed in corresponding alkali solutions of other N-acyl amino acid surfactants, i.e., those with alanine, valine, leucine, glutamic acid, and so on, or with fatty acid type surfactants, it is presumed that formation of the aggregate is unusually stabilized by interactions between phenyl groups. It is remarkable that there are some reports on the formation of such large aggregates by surfactants containing phenyl groups in their structures.15-18 In this study, the effects of the length of the hydrophobic chain of potassium N-acyl phenylalaninate and the concentration of KOH on this aggregation behavior were investigated systematically. In addition, a solvent isotope effect and a racemate effect on this behavior were also examined, by fluorescence measurements. By using cryo-TEM with a Zernike differential contrast phase plate, we observed the large aggregates directly. The details of these instruments and the principles of the phase contrast TEM have been described elsewhere.19,20 It is possible to obtain (13) Gebicki, J. M.; Hicks, M. Nature (London) 1973, 243, 232-235. (14) Namani, T.; Walde, P. Langmuir 2005, 21, 6210-6219, and references therein. (15) Gonzalez, Y. I.; Nakanishi, H.; Stjerndahl, M.; Kaler, E. W. J. Phys. Chem. B 2005, 109, 11675-11682. (16) Roy, S.; Khatua, D.; Dey, J. J. Colloid Interface Sci. 2005, 292, 255-264. (17) Mohanty, A.; Dey, J. Langmuir 2004, 20, 8452-8459. (18) Wang, C.; Gao, Q.; Huang, J. Langmuir 2003, 19, 3757-3761. (19) Danev, R.; Nagayama, K. Ultramicroscopy 2001, 88, 243-252.
a realistic image without suffering from the influences of chemical fixation, dehydration, staining, and vapor deposition by this method. Therefore, it is a very powerful method for acquiring micro- and nanostructural information, not only on biological samples but also on chemical materials, especially soft matter.21-24 Experimental Section Materials. N-Hexadecanoyl, N-tetradecanoyl, and N-dodecanoyl
L-phenylalanines (Cn-L-Phe: n ) 16, 14, and 12) were synthesized by the reaction of L-phenylalanine with each acyl chloride as described
previously25 and were recrystallized from their mixtures in acetonemethanol at least three times. The purities of these compounds were checked by HPLC. They were then dissolved in aqueous potassium hydroxide solution at several concentrations. First, the most concentrated solution of K Cn-Phe was prepared, and second, other more dilute solutions were made by mixing the concentrated solution with an appropriate volume of KOH solution. Auramine (N,N,N′,N′tetramethyl-4,4′-diamino-diphenylketoimine hydrochloride) was purchased from Kanto Kagaku and used as a fluorescence probe to determine the cac. The concentration of the probe molecule was 1 × 10-5 M. Fluorescence Measurements. Fluorescence measurements were carried out on an Hitachi fluorescence spectrophotometer F-2000 to determine the cac values of surfactants at 298.2 K. It is known that the ratio of the fluorescence intensity in an aqueous solution containing no surfactant (I0) to that of a surfactant solution (I) can (20) Danev, R.; Okawara, H.; Usuda, N.; Kametani, K.; Nagayama, K. J. Biol. Phys. 2002, 28, 627-635. (21) Kaneko, Y.; Danev, R.; Nagayama, K.; Nakamoto, H. J. Bacteriol. 2006, 188, 805-808. (22) Kaneko, Y.; Danev, R.; Nitta, K.; Nagayama, K. J. Electron Microsc. 2005, 54, 79-84. (23) Tosaka, M.; Tsuji, M.; Ogawa, T.; Kitano, H.; Nakano, K.; Kojima, S.; Danev, R.; Nagayama, K. Polymer 2006, 47, 951-955. (24) Tosaka, M.; Danev, R.; Nagayama, K. Macromolecules 2005, 38, 78847886. (25) Ohta, A.; Hata, Y.; Mizuno, Y.; Asakawa, T.; Miyagishi, S. J. Phys. Chem. B 2004, 108, 12204-12209.
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Figure 2. Fluorescence intensity ratios of auramine vs concentration for K Cn-Phe solution in 1 mM KOH. Full circles and solid line: optically active systems; open circles and dotted line: racemate systems. be used as an indicator of the microviscosity of aggregates in aqueous solution because the fluorescence yield of auramine increases as a result of restriction of its rotational motion when auramine molecules are solubilized.26,27 The excitation and emission wavelengths of auramine were 422 and 490 nm, respectively, and band-passes were 10 nm. Electron Microscopy. The experiments were carried out on a JEOL JEM-3100FFC transmission electron microscope operated at an acceleration voltage of 300 kV and equipped with a Zernike phase plate. The surfactant solutions were dropped on a copper grid coated with holey carbon film. After removal of excess liquid, the sample was frozen rapidly in liquid ethane using a LEICA rapidfreezing device (LEICA EM CPC). The grid was transferred to the specimen chamber of the apparatus using a cryo-transfer system. Atomic Force Microscopy. A separated material in the surfactant solution was investigated using an SPA-400 (Seiko Instrument Co.) in tapping mode using silicon nitride tips with a nominal spring constant of 3 N m-1. The samples were dropped on a cover glass and excess solution was removed carefully with the tip of a filter paper. Dried samples, which had been left overnight in a desiccator, were used for the experiment.
Results and Discussion As mentioned in the previous section, the I/I0 vs concentration curve of K C14-L-Phe has a characteristic peak appearing on the increasing section of the curve. Hence, it is possible to use the appearance of this peak as an index of the existence of the large aggregate, which differs from common micelles, in the solution. Therefore, the I/I0 values were plotted against the concentration of surfactant in Figure 2 for the aqueous systems of K C12-LPhe, K C14-L-Phe, and K C16-L-Phe in 1 mM KOH solutions. It is seen that the cac decreases with increasing acyl chain length. For the K C16-L-Phe system, furthermore, the I/I0 vs concentration curve has a larger characteristic peak than that for the K C14L-Phe system. On the other hand, there are no peaks in the K C12-L-Phe system on the increasing section of the curve. This result suggests that the large aggregate formation is enhanced by an increase in the hydrophobic interactions between acyl chains, and the K C12-L-Phe molecules cannot form the large aggregates, only normal micelles, in a 1 mM KOH solution. Second, we examined the effect of the KOH concentration on the aggregation behavior. Since we have already found that the characteristic peak on the I/I0 vs concentration curve vanishes in the K C14-L-Phe system in 10 mM KOH solution, we considered KOH concentrations lower than 1 mM for the K C12-L-Phe system and KOH concentrations higher than 1 mM (26) Conrad, R. H.; Heinz, J. R.; Brand, L. Biochemistry 1970, 9, 1540-1543. (27) Miyagishi, S.; Kurimoto, H.; Ishikawa, Y.; Asakawa, T. Bull. Chem. Soc. Jpn. 1994, 67, 2398-2402.
Ohta et al.
Figure 3. Fluorescence intensity ratios of auramine vs concentration for K C12-L-Phe and K C16-L-Phe solutions. Crosses: K C16-L-Phe in 10 mM KOH system; open squares: K C12-L-Phe in 50 µM KOH system; full circles and solid line: respective solutions in 1 mM KOH.
Figure 4. Fluorescence intensity ratios of auramine vs concentration for K C12-L-Phe solutions in 1 mM KOH. Open circles: in D2O; full circles: in H2O.
for the K C16-L-Phe system. It can be seen from Figure 3 that the I/I0 vs concentration curve for the K C16-L-Phe system has no peak in 10 mM KOH solution, and the characteristic peak appears even for the K C12-L-Phe system in 50 µM KOH solution. This strongly supports the belief that the large aggregates are formed from acid-soap dimers.12 That is because the acid-soap dimer formation becomes easier at lower alkali concentrations, and the characteristic peak is pronounced at lower concentrations of KOH. Third, the chiral effect on the aggregation behavior was investigated by using a racemate of K Cn-Phe in 1 mM KOH. These results have been inserted into Figure 2. It can easily be seen that there are no peaks in the racemate systems. This suggests that formation of the large aggregates requires chiral constituent amphiphiles. Of course, it has been confirmed that the large aggregates are formed in K C16-D-Phe and K C14-D-Phe systems under the same conditions. Finally, the solvent isotope effect on the aggregation behavior was examined for the K C12-L-Phe system in 1 mM KOH solution. The length of a hydrogen bond in D2O is essentially identical to that in H2O, while the energy of a hydrogen bond in D2O is about 5% larger than that in H2O.28 It is expected, therefore, that the hydrophobic interaction is enhanced by changing the solvent from H2O to D2O and that the large aggregate formation will also be accelerated. As shown in Figure 4, the characteristic peak, which was not present for the 1 mM KOH solution in H2O, was obtained when D2O was used as the solvent. Unexpectedly, a floating fibrous material was observed visually in the solution of K C12-L-Phe at around the concentration at the top of the peak of the I/I0 vs concentration curve 1-2 weeks after the sample preparation. Figure 5a shows photographs of (28) Nenethy, G.; Scheraga, H. A. J. Chem. Phys. 1964, 41, 680-688.
From Nanotubes to Micelles of K C14-L-Phe
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Figure 5. (a) View of some K C12-L-Phe solutions with 1 mM KOH in D2O after 4 weeks. From left to right: 0.3, 1.2, 2.0, and 5.0 mM solutions. (b) AFM topographical image of the fibrous material in K C12-L-Phe solution at 1.2 mM. The lateral dimension of the shown area is 10 µm.
some solutions of K C12-L-Phe with 1 mM KOH in D2O after 4 weeks. It was found that the fibrous material grew over time only at the concentration at the top of the peak of the I/I0 vs concentration curve, while the other samples, both those that were more dilute and those that were more concentrated, remained transparent. The fibrous materials were separated from solution and dried carefully on a cover glass in a desiccator, and then the prepared sample was observed by tapping-mode AFM with a 3 Nm-1 cantilever. Figure 5b shows an AFM topographical image of the dried sample of K C12-L-Phe aggregate. This confirmed that the fibrous material is a bundle of tubes with diameters of about 100 nm and lengths of over 10 µm. We then examined the microstructures of some samples of K C12-L-Phe with 1 mM KOH in D2O 3 days after preparation, by phase contrast TEM. These samples appeared to still be clear. Parts a-c of Figure 6 show TEM images of the 1.2 mM K C12-L-Phe solution. This concentration is that at the top of the peak of the I/I0 vs concentration curve in Figure 4. As shown in Figure 6a, many tubule structures with diameters 50-80 nm and wall thicknesses of about 10 nm were observed. In addition to these tubule structures, we also obtained helically coiled ribbons from the same sample, as shown by an arrow in Figure 6b. It was suggested, therefore, that the tubule structure was caused by growth of the helical ribbon.29,30 Considering that chiral selfassembly brings about the helical structures, the result presented in Figure 2, where the racemate of N-acyl phenylalaninate salt does not form the large aggregates, stands to reason. That is to say, since the respective enantiomers assemble in a random fashion (29) Simizu, T.; Masuda, M.; Minamikawa, H. Chem. ReV. 2005, 105, 14011443. (30) Svenson, S.; Messersmith, P. B. Langmuir 1999, 15, 4464-4471.
Figure 6. Phase contrast TEM images of 1.2 mM K C12-L-Phe solution with 1 mM KOH in D2O. The scale bars are 100 nm, and concentric circles in all TEM images were artifacts. (a) Image of nanotubes with diameters of about 80 nm; (b) image of helical ribbon and ropelike aggregates constituted of the threadlike micelles; (c) image of branched nanotube.
in the racemate system, the regular helical sheet cannot be formed and, consequently, sufficient free energy of association cannot be gained by the intermolecular interaction.31-34 An image of a branched tube was also obtained and is shown in Figure 6c. A branched tube might be grown when the tip of one tube collides with another tube body. It was noted, furthermore, that the surface of the tubes in Figure 6c was less smooth and the contrast of the (31) Fuhrhop, J.-H.; Schnider, P.; Rosenberg, J.; Boekema, E. J. Am. Chem. Soc. 1987, 109, 3387-3390. (32) Fuhrhop, J.-H.; Boettcher, C. J. Am. Chem. Soc. 1990, 112, 1768-1776. (33) Fuhrhop, J.-H.; Demoulin, C.; Rosenberg, J.; Boettcher, C. J. Am. Chem. Soc. 1990, 112, 2827-2829. (34) Sommerdijk, N. A. J. M.; Buynsters, P. J. J. A.; Akdemir, H.; Geurts, D. G.; Pistorius, A. M. A.; Feiters, M. C.; Nolte, R. J. M.; Zwanenburg, B. Chem.Eur. J. 1998, 4, 127-136.
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Figure 7. Phase contrast TEM images of K C12-L-Phe solutions with 1 mM KOH in D2O. Image of nanotube in 0.3 mM solution. The scale bar is 100 nm.
tube wall was less clear than for those in Figure 6a. A more remarkable example, which was very different from the tubes in terms of the molecular packing, was obtained in Figure 6b. Here, three ropelike aggregates made of many threadlike micelles with diameters of about 10 nm were present. It seems, moreover, that the three aggregates have been unraveled from one large ropelike aggregate. Because there are few pieces of threadlike micelle around the ropelike aggregate, it is hard to consider that the threadlike micelles gather spontaneously to form the ropelike aggregate. It might be concluded, therefore, that the ropelike aggregate was transformed from the tubule structure due to a decline in the molecular packing on the membrane. Figure 7 shows micrographs of 0.3 mM solution of K C12-L-Phe containing 1 mM KOH in D2O. This corresponds to an onset concentration of the peak of the I/I0 vs concentration curve in Figure 4. Although it was hard to find an image of aggregate at 0.3 mM owing to its low concentration, the image of one tube, shown in Figure 7, was obtained. On the other hand the images of threadlike micelles were not verified as well as those of tubes or ribbons at 2.0 mM. These facts support that the tube is formed first and is then transformed into micelles via threadlike micelles, with increasing concentration. Recently, Yan et al. have shown that threadlike micelles are an intermediate structure in a sodium dodecyl sulfate-induced vesicle-micelle transition for a mixed aqueous ABA-type silicone surfactant system.35 This is quite similar to our observations. Figure 8 was drawn to summarize the predicted aggregation behavior of K Cn-L-Phe in KOH solution against the concentration of surfactant, according to the results obtained in this study. Since these surfactants are weak electrolytes, the aqueous solutions can be regarded as a mixed solution of an ionic (carboxylate) and a nonionic (carboxylic acid) species. It should be noted that the amount of ionic species is overwhelmingly in excess of the amount of nonionic species. Actually, taking into account the pH value of about 11 for the 1 mM aqueous solution of potassium hydroxide, while the pKa of K C12-Phe is ca. 7, the mole fraction of ionic species was about 0.9999 in the 1 mM KOH. Despite such a severe imbalance, an acid-soap dimer, in which the ionic (35) Yan, Y.; Hoffmann, H.; Drechsler, M.; Talmon, Y.; Makarsky, E. J. Phys. Chem. B 2006, 110, 5621-5626.
Ohta et al.
Figure 8. Schematic diagrams of the morphological transition from acid-soap dimer to spherical micelle. All images are taken from our phase contrast cryo-TEM experimental results.
species interacts with the nonionic one by hydrogen bonding, is assembled as a unit at the start. It is known that vesicle formation occurs in an aqueous sodium decyl sulfate-decyltrimethylammonium bromide mixture even at a mole fraction of cationic surfactant of 0.9999.36 Then, a helical bilayer sheet is formed because the acid-soap dimer behaves like a chiral double-tailed surfactant. In that case, formation of the helical bilayer sheet of K Cn-L-Phe is unusually stabilized by interactions between phenyl groups in addition to those between acyl chains and between amide groups. The helical sheet grows with increasing surfactant concentrations and is then transformed into nanotubes, while the composition of these bilayer structures is enriched with the ionic surfactant. Hence, the molecular packing on the membrane gradually becomes weak, owing to the electrostatic repulsion between the ionic species. However, the membrane density of the bilayer does not become uniformly thin, but it becomes thin locally, as shown in Figure 6c. Therefore, it can be presumed that the tube is deformed to a bundle of threadlike micelles via a tube with a perforated layer structure because of an increase in the population of the ionic species. It seems that this morphological transition resembles the lamellar to gyroid transition via modulation of the fluctuating layer structure in a nonionic surfactant-water system, in which the transition occurs with decreasing temperature at about 60 wt % of surfactant.37,38 Furthermore, Kadi et al. has also proposed a model that glycerol monooleate vesicles are transformed into threadlike micelles by addition of a cationic surfactant via a formation of pores in a bilayer.39 Finally, the threadlike micelles were transformed into spherical micelles constituted almost entirely of the ionic species. It should be noted that this drastic morphological transition occurs over a narrow concentration range. Incidentally, it was supposed that a lot of the tubule structures shown in Figure 5a were developed in the solution since the pH value decreased due to (36) Villeneuve, M.; Kanashina, S.; Imae, T.; Aratono, M. Langmuir 1999, 15, 2029-2036. (37) Imai, M.; Saeki, A.; Teramoto, T.; Kawaguchi, A.; Nakaya, K.; Kato, T.; Ito, K. J. Chem. Phys. 2001, 115, 10525-10531. (38) Imai, M.; Kawaguchi, A.; Saeki, A.; Nakaya, K.; Kato, T.; Ito, K.; Amemiya, Y. Phys. ReV. E 2000, 62, 6865-6874. (39) Kadi, M.; Hansson, P.; Almgren, M. J. Phys. Chem. B 2004, 108, 73447351.
From Nanotubes to Micelles of K C14-L-Phe
absorption of atmospheric carbon dioxide. In fact, the pH values of all the solutions in this picture were about 9.2, although they had been about 11 immediately after preparation.
Conclusions It was found that potassium N-acyl phenylalaninate first formed large aggregates at lower concentrations, and these were then transformed into micelles at higher concentrations. It was suggested that formation of an acid-soap dimer brought about the construction of such large particles at very dilute concentrations. Both an increase in the acyl chain length and replacement of H2O with D2O increased the hydrophobic interactions, and consequently large aggregate formation was enhanced. Furthermore, this aggregation behavior does not occur when racemically
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modified N-acyl phenylalaninate is used. By using phase contrast cryo-TEM, it was found that the large aggregates were tubes with a bilayer structure, and these were then transformed into spherical micelles via threadlike micelles with increasing concentration because of a drastic increase in the proportion of ionic species in the aggregate. Acknowledgment. The authors are grateful to Prof. Takahashi and Mr. Shigeyama at Kanazawa University for providing the AFM apparatus. A part of this work was supported by the “Nanotechnology Support Project” of the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan. LA0611110
