Photochemical Control of Molecular Assembly Formation in a

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Photochemical Control of Molecular Assembly Formation in a Catanionic Surfactant System Atsutoshi Matsumura,† Koji Tsuchiya,‡,§ Kanjiro Torigoe,† Kenichi Sakai,†,§ Hideki Sakai,*,†,§ and Masahiko Abe†,§ † Faculty of Science and Technology, Tokyo University of Science, 2641 Yamazaki, Noda, Chiba 278-8510, Japan, ‡Faculty of Science, and §Institute of Colloid and Interface Science, Tokyo University of Science, 1-3 Kagurazaka, Shinjuku, Tokyo 162-8601, Japan

Received September 12, 2010 Photochemical control of vesicle disintegration and reformation in aqueous solution was examined using a mixture of 4-butylazobenzene-40 -(oxyethyl)trimethylammonium bromide (AZTMA) as the photoresponsive cationic surfactant and sodium dodecylbenzenesulfonate (SDBS) as the anionic surfactant. Spontaneous vesicle formation was found in a wide-ranging composition of the trans-AZTMA/SDBS system. AZTMA molecules constituting vesicles underwent reversible trans-cis photoisomerization when irradiated with ultraviolet and visible light. Transmission electron microscopy observations using the freeze-fracture technique (FF-TEM) showed that UV light irradiation caused the vesicles to disintegrate into coarse aggregates and visible light irradiation stimulated the reformation of vesicles (normal control). A detailed investigation of the phase state and the effects of UV and visible light irradiation on the AZTMA/ SDBS system with the use of electroconductivity, dynamic/static light scattering, and surface tension measurements and FF-TEM observations revealed that in the AZTMA-rich composition (AZTMA/SDBS 9:1) a micellar solution before light irradiation became a vesicular solution after UV light irradiation and visible light irradiation allowed the return to a micellar solution (reverse control). Thus, we could photochemically control the disintegration (normal control) and reformation (reverse control) of vesicles in the same system.

Introduction Small corpuscles with a closed bimolecular membranous structure composed of amphiphilic molecules (i.e., vesicles) such as in surfactants and phospholipids are expected to be useful as biomembrane models,1 carriers in drug-delivery systems (DDS),2 and in new fields of organic chemical reaction.3,4 Recent studies report the preparation of nanometer-sized inorganic particles in the inner aqueous phase of vesicles5-7 and the high-order filtration of poisonous organic substances using bimolecular vesicle membranes.8,9 If the formation and disintegration of molecular assemblies such as vesicles and micelles could be reversibly controlled by external stimuli, then the method would be applicable to the sustained release of perfume and medicine trapped in the interior of molecular assemblies and to controlled drug release in DDS. Attempts to control the formation of molecular assemblies and interfacial properties using external stimuli include the use of a *Corresponding author. E-mail: [email protected].

(1) Singer, S. J.; Nicolson, G. L. Science 1972, 175, 720. (2) Ostro, M. J.; Cullis, P. R. Am. J. Hosp. Pharm. 1989, 46, 1576. (3) Shimomura, M.; Kunitake, T. J. Am. Chem. Soc. 1982, 104, 1982. (4) Moss, R. A.; Bizzigott, G. O. J. Am. Chem. Soc. 1981, 103, 6512. (5) Mann, S.; Hannington, J. P.; Williams, R. J. P. Nature 1986, 324, 565. (6) Yaacob, I. I.; Nunes, A. C.; Shah, D. O. J. Colloid Interface Sci. 1994, 168, 289. (7) Hentze, H.-P.; Raghavan, S. R.; McKelvey, C. A.; Kaler, E. W. Langmuir 2003, 19, 1069. (8) Abe, M.; Kondo, Y. Mater. Technol. 1992, 10, 275. (9) Kondo, Y.; Abe, M.; Ogino, K.; Uchiyama, H.; Scamehorn, J. F.; Tucker, E. E.; Christian, S. D. Langmuir 1993, 9, 899. (10) Shinkai, S.; Shigematsu, K.; Kusano, Y.; Manabe, O. J. Chem. Soc., Perkin Trans. 1 1981, 3279. (11) Shinkai, S.; Ogawa, T.; Kusano, Y.; Manabe, O.; Kikukawa, K.; Goto, T.; Matsuda, T. J. Am. Chem. Soc. 1982, 104, 1940. (12) Tsukube, H. J. Chem. Soc., Perkin Trans. 1 1983, 29. (13) Izatt, R. M.; Ramb, J. D.; Hawkins, R. T.; Brown, P. R.; Izatt, S. R.; Christensen, J. J. J. Am. Chem. Soc. 1983, 105, 1782.

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photoisomerization reaction,10,11 pH change,12,13 temperature change,14 and a redox reaction.15-18 In utilizing the oxidationreduction reaction, Saji et al. reported the micelle electrolysis method in micellar systems,19-21 and Gokel et al. reported that vesicles obtained by oxidizing and then ultrasonically irradiating newly synthesized amphiphilic molecules with a ferrocenyl group were destroyed by reduction.22 In the latter case, however, vesicle formation required an external physical force such as ultrasonic irradiation, thus the reformation of vesicles disintegrated by reduction was impossible. Meanwhile, Kaler et al. reported that vesicles formed spontaneously as a thermodynamically stable system when an anionic surfactant solution and a cationic surfactant solution were mixed in a certain mixing ratio,23-26 which has attracted attention as a simple method for vesicle preparation. If such spontaneously formed vesicles were used, then the vesicles destroyed by an external stimulus would reform through the removal of the external (14) Shinkai, S.; Nakamura, S.; Tachiki, S.; Minabe, O. J. Am. Chem. Soc. 1985, 107, 1782. (15) Gallardo, S. B.; Metcalfe, L. K.; Abbott, L. N. Langmuir 1996, 12, 4116. (16) Gallardo, S. B.; Abbott, L. N. Langmuir 1997, 13, 203. (17) Aydogan, N.; Gallardo, S. B.; Abbott, L. N. Langmuir 1999, 15, 722. (18) Luk, Y.; Abbott, L. N. Science 2003, 301, 5633. (19) Saji, T.; Hoshino, K.; Aoyagi, S. J. Am. Chem. Soc. 1985, 107, 6865. (20) Saji, T.; Hoshino, K.; Ishii, Y.; Goto, M. J. Am. Chem. Soc. 1991, 113, 450. (21) Saji, T.; Ebata, K.; Sugawara, K.; Liu, S.; Kobayashi, K. J. Am. Chem. Soc. 1994, 116, 6053. (22) Medina, J. C.; Chen, I. G. Z.; Gokel, G. W. J. Am. Chem. Soc. 1991, 113, 1991. (23) Kaler, E. W.; Murthy, A. K.; Rodriguez, B.; Zasadzinski, A. N. Science 1989, 245, 1371. (24) Kaler, E. W.; Herington, K. L.; Murthy, A. K.; Zasadzinski, J. A. N. J. Phys. Chem. 1992, 96, 6698. (25) Herrington, K. L.; Kaler, E. W.; Miller, D. D.; Zasadzinski, J. A. N.; Chiruvolu, S. J. Phys. Chem. 1993, 97, 13792. (26) Yatcilla, M. T.; Herrington, K. L.; Brasher, L. L.; Kaler, E. W.; Chiruvolu, S.; Zasadzinski, J. A. J. Phys. Chem. 1996, 100, 5874.

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stimulus. Actually, we investigated the electrochemical control of vesicle formation using an anionic/cationic mixed surfactant system and succeeded in reversibly controlling the formation and disintegration of vesicles by means of an oxidation-reduction reaction.27,28 Nevertheless, the electrochemical control method has several disadvantages, including the need to add a third substance to the system, such as supporting electrolytes, and a certain duration of control. If it were possible to control vesicle formation by using light as the external stimulus, then we could expect high-speed control without the addition of a third substance. There have been several reports on the control of material properties using light as the external stimulus. For instance, azobenzene-modified surfactants were newly synthesized and UV light irradiation was found to cause the decomposition of surfactants through the loss of their surface activity.29 Veronese et al. reported vesicle formation when the phospholipids they synthesized were decomposed by UV light irradiation.30 Okahata et al. prepared an artificial cell membrane model using a nylon capsular membrane (diameter 2 mm, membrane thickness 1-5 μm) and coated the membrane pores with lipid bimolecular films. They then controlled the transport of substances between the inside and outside of the membrane by making use of the liquid crystal-crystal phase transition in the bimolecular films caused by external stimuli, including changes in temperature and pH in the outside solution and the addition of calcium ions.31,32 Although many studies have been done on the control of membrane properties and shapes using the thermoisomerization or photoisomerization of azobenzene,33-39 no studies have addressed the reversible photochemical control of the formation and disintegration of vesicles composed of azobenzenemodified surfactants. In a previously reported study,40 we synthesized an azobenzenemodified cationic surfactant (AZTMA) that exhibits changes in the geometrical structure of the molecules through reversible trans-cis photoisomerization induced by UV/vis irradiation and attempted spontaneous vesicle formation by mixing the cationic surfactant with an anionic surfactant (SDBS). We succeeded in controlling the disintegration and reformation of vesicles using UV/vis irradiation. This article describes the detailed examination of the trans-cis photoisomerization of AZTMA that affects the phase state of the AZTMA/SDBS aqueous mixed solution. The examination led to the finding that a micellar solution of the mixture without light irradiation changes to a vesicular solution through UV light irradiation and then returns to a micellar solution through visible irradiation. Thus, the proper choice of the mixing ratio for (27) Sakai, H.; Imamura, H.; Kakizawa, Y.; Yukishige, K.; Yoshino, N.; Harwell, J. H. Denki Kagaku 1997, 65, 669. (28) Sakai, H.; Imamura, H.; Kondo, Y; Yoshino, N; Abe, M. Colloids Surf., A 2004, 232, 221. (29) Dunkin, I. R.; Gettinger, A.; Sherrington, D. C.; Whittaker, P. J. Chem. Soc., Perkin Trans. 2 1996, 1837. (30) Veronese, A.; Berclaz, N.; Luisi, P. L. J. Phys. Chem. B 1998, 102, 7078. (31) Okahata, Y.; Lim, H. J.; Hachiya, S. Makromol. Chem., Rapid Commun. 1983, 4, 303. (32) Okahata, Y.; Lim, H. J.; Hachiya, S. J. Chem. Soc., Perkin Trans. 2 1984, 989. (33) Buwalda, R. T.; Stuart, M. C. A.; Engberts, J. B. F. N. Langmuir 2002, 18, 6507. (34) Liu, S.; Gonzalez, Y. I.; Kaler, E. W. Langmuir 2003, 19, 10732. (35) Kuiper, J. M.; Engberts, J. B. F. N. Langmuir 2004, 20, 1152. (36) Hamada, T.; Sato, T. Y.; Yoshikawa, K. Langmuir 2005, 21, 7626. (37) Hubbard, F. P.; Santonicola, G.; Kaler, E. W.; Abotto, N. L. Langmuir 2005, 21, 6131. (38) Yu, W. Y; Yang, Y. M.; Chang, C. H. Langmuir 2005, 21, 6185. (39) Zhai, L.; Zaho, M.; Sun, D.; Hao, J.; Zhang, L. J. Phys. Chem. B 2005, 109, 5627. (40) Sakai, H.; Matsumura, A.; Yokoyama, S.; Saji, T.; Abe, M. J. Phys. Chem. B 1999, 103, 10737.

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AZTMA and SDBS allows vesicle disintegration (normal control) and vesicle formation (reverse control) in the same system.

Materials and Methods 4-Butylazobenzene-40 -(oxyethyl)trimethylammonium bromide (AZTMA, Scheme 1) was synthesized and purified as described previously.41-43 Sodium dodecylbenzenesulfonate (SDBS) was purchased from Tokyo Chemical Industry Co., Ltd. Scheme 1. Molecular Structure of AZTMA

Sample preparation was carried out as follows. First, the respective appropriate amounts of AZTMA and SDBS were dissolved in deionized water to give stock solutions of each surfactant at the desired concentration. Then, various volumes of the stock solutions were vortex mixed for 3 s to yield stock solutions of surfactant mixtures at the desired mixing ratio, which were used as samples. All samples were equilibrated in a thermostatted bath at 25 °C and were not subjected to any type of mechanical agitation, except gentle stirring. A 200 W mercury-xenon lamp (San-Ei Supercure 203S) was employed as the light source to induce the photoisomerization of AZTMA. An ultraviolet light filter (Kenko U-340, transmitting wavelength 260-390 nm) and a visible light filter (Kenko L-42, transmitting wavelength >410 nm) were used with the light source for ultraviolet and visible light irradiation, respectively. The light intensity was fixed at 10 mW/cm2. Monitoring of the trans-cis isomerization of AZTMA in aqueous solution was conducted through UV/vis absorption spectrum measurements (700-200 nm) using a UV/vis spectrophotometer (Hitachi U-3310) equipped with a quartz cell (optical length 1.0 mm). Transmission electron microscopy observation was performed using the freeze-fracture (FF) technique. Samples were frozen in liquid nitrogen and fractured at -120 °C using a freeze-fracture device (Hitachi FR7000A). The fractured surfaces were immediately replicated by evaporating platinum at an angle of 45°, followed by a carbon film at normal incidence, to increase the mechanical stability of the replica. The replicas thus prepared were examined using a transmission electron microscope (JEOL TEM-1200EX). Vesicles can trap water-soluble compounds (drugs, perfumes, etc.) in the inner aqueous phase. If the release of trapped compounds to the outside medium could be controlled by light irradiation, then vesicles would then be provided with a new function. In this connection, the effect of light irradiation on the trapping efficiency of AZTMA/SDBS vesicles was investigated using the glucose dialysis technique. Briefly, AZTMA/SDBS vesicles were prepared in an aqueous 0.28 mol/L glucose solution. Nontrapped glucose was removed by dialysis using a cellulose tube (Viscose Sales Corp.). The vesicles were then disintegrated by the addition of ethanol, and the amount of glucose inside the tube was determined by the mutarotase-GOD method.45,46 To estimate the trapping efficiency of the solutions of cis and reformed trans vesicles, dialysis was performed while irradiating with UV and visible light, respectively. (41) Saji, T.; Hoshino, K.; Ishii, Y.; Goto, M. J. Am. Chem. Soc. 1991, 113, 450. (42) Saji, T.; Ebata, K.; Sugawara, K.; Liu, S.; Kobayashi, K. J. Am. Chem. Soc. 1994, 116, 6053. (43) Hayashita, T.; Kurosawa, R.; Miyata, T.; Tanaka, K.; Igawa, M. Colloid Polym. Sci. 1994, 272, 1611. (44) Yang, L.; Takisawa, N.; Hayashita, T.; Shirahama, K. J. Phys. Chem. 1995, 99, 8799. (45) Miwa, I.; Okuda, J.; Maeda, K.; Okuda, G. Clin. Chim. Acta 1972, 37, 538. (46) Okuda, J.; Miwa, I. Protein, Nucleic Acid, Enzyme 1972, 17, 216.

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Figure 1. Ternary phase diagram for AZTMA/SDBS/H2O at 25 °C: vesicle region (V), micelle region (M), precipitate (P), twophase region (vesicles and lamellar phase) (V þ L), and three-phase region (vesicles, lamellar phase, and precipitate) (V þ L þ P).

Results and Discussion Control of Disintegration/Reformation of Vesicles by UV/Vis Light Irradiation. Figure 1 shows the ternary phase diagram for dilute (less than 3 wt %, about 75 mM) aqueous mixtures of AZTMA/SDBS at 25 °C obtained on the basis of visual and optical microscopic observations of the system. Sample solutions in the region denoted by the symbol M were transparent and optically isotropic as indicated by optical microscopy observations, and no large molecular assemblies were seen, confirming that the solutions are composed of mixed micelles.24-26 This suggests that when either of the surfactants is present in large excess, the solutions contain mixed micelles. The formation of precipitates was seen in the region denoted by the symbol P where the molar ratio of AZTMA to SDBS was around unity. The cationic quaternary ammonium group of AZTMA and the anionic sulfonic acid group of SDBS quite likely neutralized each other to form an ion pair, leading to the formation of a surfaceinactive, water-insoluble salt. The region denoted by the symbol V was assigned as a vesicle region because doughnut-shaped molecular assemblies characteristic of vesicles were observed with a differential interference microscope.24-26 As indicated in the Figure, vesicles were spontaneously formed over a relatively wide composition range on both the cation-rich and anion-rich sides of region V (V þ L and V þ L þ P). Vesicles and lamellar liquid crystals were found to coexist in the region denoted by V þ L because small lamellar clouds appeared after several weeks in standing solution that initially contained only vesicles. The UV/vis absorption spectra for dilute aqueous mixtures of AZTMA/SDBS at a total surfactant concentration of 0.05 wt % (about 1.25 mM) were studied. A peak ascribed to the π f π* transition of the azo group at 344 nm and a feeble peak assigned to the n f π* transition at around 440 nm were observed, respectively, for the solution containing the trans surfactant before light irradiation. These peaks are consistent with those previously reported for derivatives of azobenzene.43,44,47 After UV light irradiation, the peaks ascribed to the trans surfactant at 344 nm (47) Shimomura, M.; Ando, R.; Kunitake, T. Ber. Bunsen-Ges. Phys. Chem. 1983, 87, 1134.

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Figure 2. Turbidity change for AZTMA/SDBS mixed aqueous solutions caused by UV/vis irradiation. Total surfactant concentration = 0.05 wt %. AZTMA/SDBS = (O) 6:4, (0) 7:3, (2) 5:5, and (b) 2:8.

significantly diminished whereas that assigned to the cis surfactant at 440 nm increased in height. The peak ascribed to the azo group is reported to shift toward the shorter-wavelength side at concentrations higher than the critical micelle concentration (cmc) on the basis of UV/vis absorption spectrum measurements on solutions of AZTMA alone. That is, the absorption peak for AZTMA molecules is located at a shorter wavelength when forming micelles than when they are in the monomeric form.47 The same situation is also seen in which the absorption peak at 348 nm due to monomeric AZTMA exhibits a shift to the shorterwavelength side (344 nm) when mixed with SDBS, suggesting the formation of molecular assemblies (vesicles). Hence, UV/vis irradiation causes AZTMA molecules forming bimolecular films (vesicles) to undergo trans-cis photoisomerization. In addition, the photoisomerization ratio of the trans form was estimated to be about 80% from the UV/vis absorption spectra, independent of AZTMA/SDBS composition. Note here that UV light irradiation increased the turbidity of the AZTMA-rich solution (AZTMA/SDBS = 7:3, 6:4) on the longer-wavelength side, whereas no such change in turbidity was observed for SDBS-rich solution. This finding suggests that UV light irradiation produces an appreciable change in the phase state of the aqueous mixture on the AZTMA-rich side. The effect of light irradiation on the turbidity of the aqueous mixture was investigated while fixing the total surfactant concentration at 0.05 wt % (about 1.25 mM) and changing the mixing ratio of the surfactants. Figure 2 shows the results, which indicate a noticeable change in turbidity caused by UV light irradiation for solutions with mixing ratios of AZTMA/SDBS = 7:3 and 6:4 on the AZTMA-rich side, whereas no such change in turbidity was observed for the solution with an SDBS-rich mixing ratio of 2:8. This confirms the finding described above that UV light irradiation causes an appreciable change in the phase state of solutions on the AZTMA-rich side. Differential interference microscopy was used to examine the phase state of a solution with a composition of AZTMA/SDBS = 6:4 at a total surfactant concentration of 0.05 wt % before and after UV light irradiation. The solution was transparent before irradiation (Figure 3a), and no large molecular assemblies were detected by optical microscopy, whereas 10-20 μm aggregates were found after UV light irradiation. This shows a significant change in the phase state caused by UV light irradiation. Langmuir 2011, 27(5), 1610–1617

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Figure 5. Turbidity change for AZTMA/SDBS mixed aqueous solutions by UV/vis irradiation. AZTMA/SDBS = 6:4. Total surfactant concentration = (O) 1.00, (0) 0.75, (Δ) 0.50, (9) 0.25, and (b) 0.05 wt %. Table 1. Dependence of the Trapping Efficiency of AZTMA/SDBS Mixed Aqueous Solutions on Photoirradiation (Total Surfactant Concentration = 1.00 wt %)

Figure 3. Differential interference microscopy observations of the AZTMA/SDBS aqueous mixture. Total surfactant concentration=0.05 wt % and AZTMA/SDBS=6:4. (a) As prepared and (b) UV irradiated for 2 h.

Figure 4. Freeze-replica TEM micrographs for AZTMA/SDBS mixed aqueous solutions. Total surfactant concentration = 0.05 wt % and AZTMA/SDBS = 6:4. (a) As prepared and (b) UV irradiated for 2 h.

TEM observation using the FF technique was performed to examine the results in Figure 3 in more detail. Figure 4a shows an FF-TEM micrograph of the sample before UV light irradiation, indicating the presence of vesicles of about 50 nm in size. This verifies the formation of vesicles in the solution in which no large micrometer-sized aggregates were observed by optical microscopy. Hence, UV light irradiation likely caused nanometer-sized vesicles to disintegrate into micrometer-sized large aggregates. Figure 4b shows an FF-TEM micrograph of the sample after 2 h of UV light irradiation, showing the formation of elongated aggregates that are similar in size to those shown in Figure 3b. When the UV-irradiated sample was irradiated with visible light for 2 h, the reformation of vesicles was observed. These results clearly demonstrate that the disintegration and reformation of vesicles can be reversibly controlled by UV/vis irradiation. Langmuir 2011, 27(5), 1610–1617

mixed ratio of AZTMA

as prepared (%)

UV irradiated (%)

vis irradiated (%)

0.90 0.60 0.55 0.40 0.20

0.13 3.08 0.27 2.05 0.06

0.09 0.28 0.33 1.06 0.16

0.06 3.21 0.37 2.04 0.24

The dependence of turbidity on the total surfactant concentration was studied while fixing the mixing ratio of AZTMA to SDBS at 6:4 (Figure 5). The turbidity changed markedly because of UV light irradiation at concentrations below 0.25 wt %, above which the change diminished as the concentration increased and finally disappeared at 1.00 wt %. In addition, UV light irradiation caused no change in the particle size distribution measured via dynamic light scattering at this total surfactant concentration. TEM observation using the FF technique at this concentration verified the formation of vesicles of around 50 nm in size. When a 12 h UV-irradiated solution was observed, spherical vesicles were seen, indicating no vesicle disintegration. The results mentioned so far demonstrate that UV light irradiation produces no vesicle disintegration at a total surfactant concentration of 1.00 wt % and a composition of AZTMA/SDBS = 6:4. However, certain changes in the properties of the bimolecular membranes may have been induced even if no vesicle disintegration was caused by the formation of cis surfactant molecules. In relation to this, an investigation was conducted on the effect of light irradiation on the trapping efficiency of aqueous AZTMA/SDBS mixtures at a total surfactant concentration of 1.00 wt % (Figure 1, line A). The value of the trapping efficiency for glucose was 3.1% at a mixing ratio of 6:4, evidencing the presence of vesicles in the solution (Table 1, Figure 6). UV light irradiation of the solution reduced the trapping efficiency from 3.1 to 0.27%, which is proof that irradiation caused the release of entrapped solute through the formation of cis-AZTMA molecules. Successive visible light irradiation followed by dialysis caused the solution to regain a trapping efficiency value of 3.2%, almost identical to the original value, indicating the reformation of vesicles with solute-entrapping ability. This decrease in trapping efficiency would be due to the increased permeability of vesicle membranes, as a result of the DOI: 10.1021/la104731w

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Figure 8. Particle size distribution of AZTMA/SDBS mixed aqueous solutions measured by the dynamic light scattering method. Total surfactant concentration = (a) 1.00 and (b) 6.25 mM and AZTMA/SDBS = 9:1. Figure 6. Dependence of the trapping efficiency on the photoirradiation of AZTMA/SDBS mixed aqueous solutions. Total surfactant concentration = 1.00 wt % and AZTMA/SDBS = (b) 6:4, (9) 4:6, and (O) 5.5:4.5.

Figure 9. Freeze-replica TEM micrograph for an AZTMA/SDBS mixed aqueous solution. Total surfactant concentration = 1.00 mM and AZTMA/SDBS = 9:1.

Figure 7. Change in scattered light intensity by AZTMA/SDBS mixed aqueous solutions as a function of the total surfactant concentration. (a) AZTMA/SDBS = (O) 7:3, (0) 6:4, and (Δ) 4:6. (b) AZTMA/SDBS = (b) 9:1.

UV-irradiation-induced formation of folded cis-AZTMA molecules because no remarkable change occurred in the shape of the vesicles after UV light irradiation. The recovery of the trapping efficiency after visible light irradiation would have been caused by the regained trans-AZTMA molecules, thereby making the membranes rigid to permit the retention of glucose by the vesicles. Because vesicles were not destroyed even after cistype formation and this photoinduced change in the trapping 1614 DOI: 10.1021/la104731w

efficiency is reversible in the concentrated solution, cis and trans forms of the azobenzene-containing surfactant do not segregate between/within the aggregates. The experimental findings mentioned above demonstrate that UV/vis irradiation of AZTMA molecules constituting vesicles enables them to control the release and uptake of entrapped solute reversibly. Control of the Micelle-Vesicle Transition by UV/Vis Light Irradiation. So far, we have mentioned surfactant systems in which UV light irradiation causes prepared vesicles to disintegrate. If, conversely, we had a surfactant system where UV light irradiation allowed vesicle formation, then we would be able to apply such a system to recover hazardous or poisonous substances from aqueous solutions. The possibility of this reverse control using the AZTMA/ SDBS system was investigated. Figure 7 shows the dependence of scattered light intensity on the total surfactant concentration for aqueous mixed AZTMA/ SDBS solutions. The light intensity started to increase at around 0.3 mM, above which it continued to increase as the concentration increased for mixtures of AZTMA/SDBS = 7.3, 6:4, and 4:6 (a). This 0.3 mM concentration coincided with the concentration where the static surface tension-concentration curve showed a kink. In contrast, the light intensity began to increase at about 0.01 mM and continued to increase further with increasing concentration up to 3 mM, where it showed an abrupt drop and thereafter continued to increase for the AZTMA/SDBS = 9:1 mixture (b). Hence, the discontinuous change in light intensity suggests a change in the phase state of the mixture at this surfactant concentration. Langmuir 2011, 27(5), 1610–1617

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Figure 10. Particle size distribution of AZTMA/SDBS mixed aqueous solutions measured by the dynamic light scattering method. Total surfactant concentration: (a-c) 1.00 mM, (d-f) 6.25 mM. AZTMA/SDBS = 9:1. (a, d) Nonirradiated, (b, e) UV irradiated for 2 h, and (c, f) visible light irradiated for 2 h.

The size of the molecular assemblies formed below and above the surfactant concentration at which the scattered light intensity abruptly decreases in Figure 7b was investigated using dynamic light scattering measurements to examine in more detail the phase state of the mixture at AZTMA/SDBS = 9:1. Figure 8 shows the particle size distribution for the mixture at a total surfactant concentration of 1.00 mM (below the concentration at which the scattered light intensity decreases) (a) and at 6.25 mM (above this concentration) (b). The formation of molecular assemblies that are 50-200 nm in size, seemingly vesicles, was suggested at 1.00 mM, and the presence of small molecular assemblies of about 6 nm in size, probably micelles, was implied at 6.25 mM. TEM observation using the FF technique on a sample prepared from the mixture at 1.00 mM verified the formation of vesicles of around 50 nm in size (Figure 9). This observation, together with the dynamic light scattering measurements, indicated the transition from vesicles to micelles with increasing surfactant concentration. (The FF-TEM observation of micelles of less than 10 nm is impossible using our system.) Figure 10 shows the effects of UV/vis light irradiation on the size distribution of molecular assemblies of the mixture (AZTMA/ SDBS = 9:1) at 1.00 and 6.25 mM. No change was detected in the size distribution at 1.00 mM even after 2 h of UV light irradiation when the photoisomerization of AZTMA molecules seemed to reach a stationary state. By contrast, the size distribution after 2 h of UV light irradiation changed greatly at 6.25 mM, showing the formation of large assemblies of 50-100 nm in size. TEM observation using the FF technique on the sample after UV light irradiation confirmed the formation of vesicles (Figure 11). Further irradiation of visible light for 2 h caused the large assemblies to decrease in size and return to small ones that were 5 to 6 nm in size (the state before light irradiation, Figure 10d). These experimental findings indicate that a micellar sample solution before UV light irradiation turns into a vesicular solution after UV light irradiation and returns to a micellar solution after visible light irradiation, a photoswitching Langmuir 2011, 27(5), 1610–1617

Figure 11. Freeze-replica TEM micrographs for a AZTMA/SDBS mixed aqueous solution after UV light irradiation. Total surfactant concentration = 6.25 mM and AZTMA/SDBS = 9:1.

process in the opposite direction to that reported in earlier papers.40 Changes in scattered light intensity caused by UV light irradiation were further investigated at varying total surfactant concentrations for the AZTMA/SDBS = 9:1 mixture (Figure 12). The intensity was found to increase after UV light irradiation in the concentration range of 4-10 mM. Figure 13 shows the concentration dependence of the electroconductivity of the mixed solution before and after UV light irradiation. The electroconductivity curve had a kink point at 3 mM before UV light irradiation, and this concentration coincided with that showing a sharp increase in scattered light intensity in Figure 7b. This suggests vesicle formation in the surfactant concentration range between that of molecular assembly formation and the electroconductivity break and a change of vesicles to micelles above the kink point. The kink point shifted to 7 mM on the higher-concentration side DOI: 10.1021/la104731w

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Figure 13. Specific conductivity of AZTMA/SDBS mixed aqueous solutions as a function of the total surfactant concentration. AZTMA/SDBS = 9:1. (b) Nonirradiated and (9) UV irradiated.

Figure 12. Change in the scattered light intensity by AZTMA/ SDBS mixed aqueous solutions as a function of the total surfactant concentration. AZTMA/SDBS = 9:1. (b) Nonirradiated and (O) UV irradiated.

after UV light irradiation. These findings indicate that a micellar solution before light irradiation changes to a vesicular solution after UV light irradiation and returns to a micellar solution after visible light irradiation in the surfactant concentration range between that corresponding to the kink points on the electroconductivity curve before and after UV light irradiation. Figure 14 shows the ternary phase diagrams before and after UV light irradiation, respectively, for the AZTMA/SDBS/H2O system in a very dilute surfactant concentration range below 15 mM (about 0.6 wt %, 25 °C), prepared on the basis of the data obtained by TEM observations using the FF technique, electroconductivity measurements, static/dynamic light scattering measurements, and static surface tension measurements. The phase diagram before UV light irradiation revealed the following facts. Vesicles were formed even in these dilute solutions in either AZTMA-rich or SDBS-rich mixing ratios although micelle formation was observed in solutions with similar mixing ratios at around 25 mM (= 1 wt %) and the vesicular solution changed to a solution containing both vesicles and micelles (V þ M, AZTM/SDBS = 8:2, 7:3, and 6:4) or a micellar solution (M, AZTM/SDBS = 9:1) above the surfactant concentration corresponding to the kink point on the electroconductivity curve. These results are consistent with those obtained by Villeneuve et al. for a sodium decyl sulfate (SDeS)/decyltrimethylammonium bromide (DeTAB) system.48 UV light irradiation (48) Villeneuve, M.; Kaneshina, S.; Imae, T.; Aratono, M. Langmuir 1999, 15, 2029.

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produced the following changes in the phase diagram. Vesicles turned into precipitates in the region denoted by P on the AZTMArich side (Figure 14b), indicating vesicle disintegration. Visible light irradiation of the solution allowed the precipitates to return to vesicles. Moreover, UV light irradiation widened the vesicle region, thereby causing micelles to change into vesicles at AZTMA-rich mixing ratios such as AZTMA/SDBS = 9:1. Thus, the proper choice of mixing ratio and total surfactant concentration could make it possible to cause vesicle disintegration by UV light irradiation and vesicle formation by UV light irradiation, completely opposite to changes in the AZTMA/ SDBS/H2O system. The shape of molecular assemblies in the surfactants can be predicted by the use of the critical packing parameter (CPP).49 CPP is given by v/a0lc, in which v is the volume of the hydrophobic group, a0 is the surface area of the hydrophilic group, and lc is the length of the hydrophobic group. When the value of CPP is smaller than 1/3, spherical micelles are formed. If the value lies between 1/3 and 1/2, then cylindrical micelles or vesicles with large curvature are formed. Planar bimolecular membranes are formed when CPP takes values larger than 1/2. In the present system, cationic AZTMA and anionic SDBS electrostatically bind to each other when they are mixed and give a pseudo-double-chain surfactant, thereby increasing the CPP value, but they form only micelles in a single-component solution. This would be the reason that these surfactants form vesicles when they are mixed with each other at mixing ratios of around 1:1. We have reported that UV light irradiation raised the cmc of AZTMA.50 The hydrophobic group of trans-AZTMA corresponds to C16 of a straight-chain quaternary ammonium salt, and that of cis-AZTMA corresponds to C1444 of the salt. The formation of cis-AZTMA increases the volume of the hydrophobic group and decreases its length. The stability of vesicles formed in anionic/cationic mixed surfactant systems is reported by Kaler et al. to be higher if the symmetry in the hydrophobic chain length is lower.44,51,52 When the azobenzene part of AZTMA changes (49) Israelachvili, J. N.; Mitchell, D. J.; Ninham, B. W. J. Chem. Soc., Faraday Trans. 1976, 72, 1525. (50) Orihara, H.; Matsumura, A.; Saito, Y.; Ogawa, N.; Saji, T.; Yamaguchi, A.; Sakai, H.; Abe, M. Langmuir 2001, 17, 6072. (51) Yuet, P. K.; Blankschtein, D. Langmuir 1996, 12, 3819. (52) Regev, O.; Khan, A. J. Colloid Interface Sci. 1996, 182, 95.

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Matsumura et al.

Article

Figure 14. Water-rich corner ternary phase diagram for AZTMA/SDBS/H2O at 25 °C: vesicle region (V), micelle region (M), precipitate (P), and two-phase region (vesicles and lamellar phase) (V þ M). (a) Nonirradiated and (b) UV irradiated.

from trans to cis through photoisomerization induced by UV light irradiation, the pseudo-double-chain surfactant becomes bulkier, making the value of CPP larger. This would lead to higher symmetry in the hydrophobic chain length, thereby lowering the stability of the vesicles and causing vesicle disintegration. Mixed micelles are formed when either of the surfactants is present in excess. UV light irradiation causes AZTMA molecules to undergo photoisomerization from trans to cis and induces the transition from micelles to vesicles by increasing the CPP value even in the mixed micelle region before light irradiation.

Conclusions The ternary phase diagram of the photoswitcable azobenzenemodified quarternary-ammonium-type cationic surfactant (AZTMA), anionic sodium dodecylbenzenesulfate (SDBS), and water was prepared, and the effect of trans-cis photoisomerization on the phase behaviors was studied. In the phase diagram of

Langmuir 2011, 27(5), 1610–1617

trans-AZTMA/SDBS/water (before UV light irradiation), vesicles formed at relatively wide compositions near the equimolar line and mixed micelles were observed in the AZTMA-rich compositions. When the composition was AZTMA/SDBS = 6:4 (total concentration = 0.05 wt %), the vesicles were disintegrated to form precipitates by UV light irradiation and vesicles formed again by visible light irradiation. In more concentrated solutions (1 wt %), vesicles kept their structure even after UV light irradiation, but the trapping efficiency against a water-soluble model drug was drastically decreased after UV light irradiation. However, mixed micelles formed in AZTMA-rich dilute solutions (AZTMA/ SDBS = 9: 1) were shown to transform to vesicles by UV light irradiation (cis-type formation). These photoinduced phase transitions to and from vesicles were able to be explained by the increase in the CPP of the pseudo-double-tailed surfactant formed by AZTMA and SDBS as a result of the bulky structure of cis-AZTMA.

DOI: 10.1021/la104731w

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