J. Phys. Chem. B 1999, 103, 10737-10740
10737
Photochemical Switching of Vesicle Formation Using an Azobenzene-Modified Surfactant Hideki Sakai,*,†,‡ Atsutoshi Matsumura,† Shoko Yokoyama,§ Tetsuo Saji,| and Masahiko Abe†,‡ Faculty of Science and Technology, Science UniVersity of Tokyo, 2641, Yamazaki, Noda, Chiba 278-8510, Japan, Institute of Colloid and Interface Science, Science UniVersity of Tokyo, 1-3, Kagurazaka, Shinjuku-ku, Tokyo 162-0825, Japan, Kyoritsu College of Pharmacy 1-5-30, Shibakoen, Minato-ku, Tokyo 162-8601, Japan, and Department of Applied Chemistry, Tokyo Institute of Technology, Ohokayama, Meguro-ku, Tokyo 152-8552, Japan ReceiVed: August 3, 1999; In Final Form: October 12, 1999
Formation and disruption of vesicles could be photochemically controlled in aqueous mixtures of a “photoswitchable” azobenzene-modified cationic surfactant (4-butylazobenzene-4′-(oxyethyl)trimethylammonium bromide; AZTMA) and an anionic surfactant (sodium dodecylbenzenesulfonate; SDBS). Vesicles were formed spontaneously in a wide composition range in aqueous trans-AZTMA/SDBS mixtures. AZTMA molecules constituting vesicles underwent reversible trans-cis photoisomerization upon alternate UV and visible-light irradiation. Transmission electron microscopic observations via freeze replica technique demonstrated the disruption of the vesicles into larger aggregates (precipitate) with UV-light irradiation (cis formation) and the following visible-light irradiation (trans formation) resulted in vesicle reformation. Furthermore, the release rate of aqueous compounds encapsulated in vesicles was shown to be photochemically controllable in the present AZTMA/SDBS mixed system.
Introduction Vesicles are lipid bilayer shells consisting of surfactants and phospholipids that have widely been used as model membranes,1 capsules for drug delivery systems,2 and microreactors for synthesis of functional polymer particles and capsules.3-6 More recently, vesicles have also been applied to the withdrawal of harmful organic compounds from wastewater making use of solubilization of hydrophobic compounds into the bilayer membrane.7,8 Reversible control of formation and disruption of vesicles by external stimuli, e.g., thermal, electrical, and optical ones, has been a subject of significant attention with the view to apply it to the controlled release of drugs and perfumes, and to the removal of organic impurities dissolved in water. Among various ways of control, optical control is promising since it requires no addition of a third component to the system. Meanwhile, vesicles consisting of double-tailed surfactants are normally in a metastable state and an ultrasonic treatment is essential for their preparation. Even if these vesicles can be disrupted with an external stimulation, their reformation by removing the stimulation would be impossible. Recently, however, Kaler et al.9-12 have reported a general method for producing equilibrium phases of vesicles. The vesicle forms spontaneously upon mixing surfactants with oppositely charged headgroups (catanionic surfactants).17,18 It would be possible, therefore, to control vesicle formation and disruption reversibly if we can prepare this kind equilibrium (spontaneous) vesicles using “switchable” surfactants. Actually, we have recently reported a reversible control of vesicle formation with an aqueous mixture of a “redox switchable” ferrocene-modified cationic surfactant and a simple anionic surfactant.13 * Author to whom correspondence should be addressed. Phone and Fax: 81-471-24-8650. E-mail:
[email protected]. † Faculty of Science and Technology. Science University of Tokyo. ‡ Institute of Colloid and Interface Science. Science University of Tokyo. § Kyoritsu College of Pharmacy. | Department of Applied Chemistry. Tokyo Institute of Technology.
SCHEME 1. Molecular Structure of AZTMA
In the present study, spontaneously forming vesicles are prepared by mixing a cationic surfactant modified with “photo switchable” azobenzene moiety and a simple anionic surfactant. The effect of trans-cis photoisomerization of the azobenzenemodified surfactant on the aggregation state of vesicles is also studied by means of microscopic observations and spectroscopic measurements. Experimental Section 4-Butylazobenzene-4′-(oxyethyl)trimethylammonium bromide (AZTMA, Scheme 1) was used as the cationic surfactant. AZTMA was synthesized and purified as described previously.14,15 Monomeric and micelle-forming AZTMA molecules have been reported to undergo revsersible trans-cis photoisomerization. Anionic sodium dodecylbenzenesulfonate (SDBS) was used as received from Tokyo Kasei. Sample preparations were done by first making stock solutions of AZTMA and SDBS at desired concentrations in deionized water. These stock solutions were equilibrated at room temperature and then samples were prepared by vortex-mixing stock solutions of AZTMA and SDBS at desired ratios for 3 s. All samples were equilibrated in a thermostated bath at 25 °C. Except for gentle stirring, the sample solutions were not subjected to any type of mechanical agitation. Results and Discussion The ternary phase diagram of dilute (less than 3 wt %) aqueous mixtures of AZTMA and SDBS at 25 °C is shown in Figure
10.1021/jp9927505 CCC: $18.00 © 1999 American Chemical Society Published on Web 11/20/1999
10738 J. Phys. Chem. B, Vol. 103, No. 49, 1999
Figure 1. Ternary phase diagram for AZTMA/SDBS/H2O at 25 °C: micelle region (M), precipitate (P), two-phase region (vesicles and lamellar phase,(V + L)), three-phase region (vesicles, lamellar phase, and precipitate, (V + L + P)).
Figure 2. UV/vis spectra of aqueous AZTMA/SDBS mixtures (total surfactant concentration 0.05 wt %). (a) AZTMA:SDBS ) 6:4 (wt %), (b) AZTMA:SDBS ) 4:6 wt %.
1. Mixed micellar phases (M) existed on the binary surfactantwater axes in Figure 1. Precipitates (P) were formed along the equimolar line (AZTMA:SDBS ) 54.7:45.3 (wt %)). Spontaneously formed vesicles (V) were observed in a relatively wide range of mixing compositions on both cation-rich and anionrich sides (V+L and V+L+P regions). Aqueous mixtures with compositions in V + L regions seemed a single vesicular phase just after sample preparation. However, as the samples aged
Letters for weeks, small amounts of lamellar clouds were formed and became visible. Therefore, these samples were assigned to a vesicle/lamellar (V + L) phase. Freeze replica TEM observation and glucose dialysis experiments17 also confirmed vesicle formation. The effect of photoinduced trans-cis isomerization of AZTMA on the phase behavior of the catanionic system was studied. Figures 2 represents UV/vis absorption spectra of diluted (total surfactant concentration; 0.05 wt %) aqueous mixtures of AZTMA and SDBS. The spectroscopic characteristics of AZTMA in aqueous mixtures with SDBS were almost the same as those of AZTMA alone in its aqueous solutions as reported.18,19 An absorption band characteristic of trans-azobenzene was observed at 344 nm for as-prepared mixed solutions. The absorption of this band decreased after UV-light irradiation (260-390 nm) with a Hg-Xe lamp (San-ei Supercure-203S), and a weak absorption band due to n-π* transition of cis isomer appeared at about 440 nm. Visible-light irradiation (>410 nm) following the UV-light irradiation made cis-AZTMA revert to the trans form, but not completely,19 indicating that the photostationary state of the cis isomer remains to some extent in this system. Repeated UV and visible-light irradiation, however, resulted in a reversible trans-cis isomerization that corresponds to the change between the spectra (II) and (III) in Figure 2. These results show that the AZTMA molecules forming the bilayer structure (vesicles) can undergo reversible trans-cis photoisomerization. It should be noted that the turbidity of the cation-rich solution (Figure 2a, AZTMA:SDBS ) 6:4) increased after UV-light irradiation, while such increase in turbidity was not observed for the anion-rich solution (Figure 2b, AZTMA:SDBS ) 4:6). The aggregation state of aqueous cis-AZTMA/SDBS mixtures on the cation (AZTMA)-rich side are thus considered to be significantly different from that of trans-AZTMA/SDBS mixtures. The effect of light irradiation on the aggregation state of cation-rich aqueous mixtures of AZTMA and SDBS (total concentration; 0.05 wt %, AZTMA:SDBS ) 6:4) was then directly observed using a transmission electron microscope via freeze replica technique. The samples were frozen in liquid nitrogen and fractured at -120 °C using a freeze fracture device (Hitachi, FR-7000A). The fractured surfaces were immediately replicated by evaporating platinum at an angle of 45°, followed by carbon film at normal incidence, to increase the mechanical stability of the replica. The replicas thus prepared were examined on an electron microscope (JEOL TEM1200EX). In a micrograph of the as-prepared solution (trans form, Figure 3a), spherical vesicles with an average size of 50-100 nm were observed. After 2 h of UV-light irradiation (Figure 3b), spherical vesicles disappeared and large elongated molecular aggregates were observed, though the nature of these large molecular aggregates has not yet been identified except that this solution is highly turbid. Furthermore, subsequent visible light irradiation resulted in reformation of vesicles with an average size of ca. 50 nm (Figure 3c). These results clearly demonstrate that vesicle formation and disruption can be reversibly controlled with photoirradiation in the present catanionic AZTMA/SDBS system. Vesicles can encapsulate aqueous compounds (drugs, perfumes, etc.) in their inner aqueous phase. Hence, if the release of these encapsulated compounds from the inside of vesicles to the outside can be controlled by photoirradiation, this should give a novel functionality to vesicles. The effect of photoirradiation on the trapping efficiency of AZTMA/SDBS vesicles was investigated with the glucose dialysis technique.17 Briefly, AZTMA/SDBS vesicles were prepared in 0.28 mol/L aqueous
Letters
J. Phys. Chem. B, Vol. 103, No. 49, 1999 10739
Figure 3. Freeze replica TEM micrographs for aqueous AZTMA/SDBS mixed solutions. (AZTMA:SDBS ) 6:4, total surfactant concentration; 0.05 wt %). (a) As-prepared, (b) after 2 h UV-irradiation, (c) after 2 h visible-light irradiation.
Figure 4. Dependence of trapping efficiency of aqueous AZTMA/ SDBS mixtures on photoirradiation (total surfactant concentration 1.00 wt %).
glucose solution. The unencapsulated glucose was separated by dialysis using a cellulose tube (Viscase Sales Corp.). The vesicles inside the tube were then disrupted by the addition of ethanol. Finally, the amount of glucose inside the tube was quantized using the mutarotase•GOD method.20,21 For determining the trapping efficiencies of the cis form and re-produced trans form solutions, dialysis was done while irradiating with UV and visible light, respectively. Figure 4 represents the trapping efficiency of AZTMA/SDBS mixed solutions (total surfactant concentration; 1 wt %) as a function of their composition. The trapping efficiency of asprepared (trans form) solutions was high (2-4% against total glucose amount) at the compositions where vesicles are formed (AZTMA compositions are 0.30-0.50 and 0.60-0.75), whereas it was almost zero at the equimolar composition (the AZTMA ratio is 0.547). After cis-form formation induced by UV-light irradiation for 12 h, the trapping efficiency decreased drastically. For instance, it decreased from 3.1% to 0.3% by UV-light irradiation for the AZTMA/SDBS solution (AZTMA:SDBS ) 6:4). Furthermore, the following visible-light irradiation for 12 h (in advance of dialysis) caused re-increase in the trapping
efficiency to 3.2%. These results confirm the disruption and reformation of vesicles induced by the UV- and visible-light irradiation, and also suggest that the release of aqueous compounds encapsulated in the vesicles can be controlled by the photochemical reaction of AZTMA. The allowed packing of surfactant molecules is governed by the “surfactant number22” V/aolc, where V is the volume of the hydrophobic portion of the surfactant, lc is the length of the hydrophobic group, and ao is the headgroup area of the surfactant molecule. In this scheme, when the surfactant number is less than 1/3, spherical micelles are the preferred form of aggregation, as the surfactant headgroup area is large in comparison to the volume of the surfactant tail. Cylindrical micelles form when the number is between 1/3 and 1/2 and highly curved bilayer vesicles and then flat bilayers are formed when it is greater than 1/2. In terms of the surfactant mixtures of interest here, the dynamic ion pairing of ionic single-tailed surfactants apparently brings about a pseudo double-tailed zwitterionic surfactant, which results in a smaller headgroup and a larger hydrophobic region than those in the individual surfactants. This ion pairing was confirmed by Kaler et al.12 with surface tension and conductivity measurements. The dynamic pairing of trans-AZTMA and SDBS should roughly double the surfactant number, leading to a transition from spherical micelles in the pure component systems to vesicles in the mixed surfactant systems. On the other hand, the formation of cis form by UV-light irradiation causes an increase in the critical micelle concentration (cmc).18,19 Thus, the cmc of trans-AZTMA corresponds to that of alkyltrimethylammonium bromide with carbon chain length of 16, while the cmc of cis-AZTMA corresponds to that of alkyltrimethylammonium bromide with carbon chain length of 14. Furthermore, the volume of the hydrophobic portion (V) increases and the length of the hydrophobic group (lc) decreases through cis-form formation. This would produce an increase in the surfactant number of AZTMA, thereby causing transformation from vesicles to a planar lamellar structure. Kaler et al.12 have also reported that the stability of catanionic vesicles is higher when the difference in the alkyl tail length between cationic and anionic surfactants is larger. The alkyl chain length of cationic trans-AZTMA is significantly longer than that of anionic SDBS, while the alkyl chain length of cis-AZTMA is
10740 J. Phys. Chem. B, Vol. 103, No. 49, 1999 close to that of SDBS. This may cause the dispersion instability of the cis-AZTMA-SDBS ion pair. Acknowledgment. This work was partially supported by Grant-in-Aid for Scientific Research on Priority Area of Electrochemistry of Ordered Interfaces from the Ministry of Education, Science, Sports and Culture, Japan. References and Notes (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) Abe, M.; Kondo, Y. Mater. Technol. 1992, 10, 275. (8) Kondo, Y.; Abe, M.; Ogino, K.; Uchiyama, H.; Scamehorn, J. F.; Tucker, E. E.; Christian, S. D. Langmuir 1993, 9, 899. (9) Kaler, E. W.; Murthy, A. K.; Rodriguez, B.; Zasadzinski, A. N. Science 1989, 245, 1371.
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