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Unique liquid crystal behavior in water of anionic fluorocarbon–hydrocarbon hybrid surfactants containing oxyethylene units
Journal of Colloid and Interface Science 357 (2011) 400–406
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Unique liquid crystal behavior in water of anionic fluorocarbon–hydrocarbon hybrid surfactants containing oxyethylene units Masanobu Sagisaka ⇑, Yoshie Fujita, Yusuke Shimizu, Chie Osanai, Atsushi Yoshizawa Department of Frontier Materials Chemistry, Graduate School of Science and Technology, Hirosaki University, 3 Bunkyo-cho, Hirosaki, Aomori 036-8561, Japan
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Article history: Received 19 October 2010 Accepted 8 February 2011 Available online 15 February 2011 Keywords: Lamella phase Dehydration Hybrid surfactant Microsegregation Ion channel
a b s t r a c t This study reports the unique aqueous lyotropic liquid crystal behavior of an anionic hybrid surfactant, 8F-B2ES, which has 2-[2-(butyloxy)ethyloxy]ethyl and 1H,1H,2H,2H-perfluorodecyl tails. An 8F-B2ESanalog hybrid surfactant with no oxyethylene units (8F-DeS) and a symmetric fluorinated double-tail surfactant with two 2-(1H,1H,2H,2H-perfluorohexyloxy)ethyl tails (4FEOS) were used as control surfactants in examining the effects of the oxyethylene units and of the hybrid structure on the liquid crystal behavior. Polarized microscopic observations showed the formation of a lamellar liquid crystal phase for each surfactant/water mixture at surfactant concentrations higher than 10 wt.%. In the case of the 30 wt.% 8FB2ES/water mixture, two types of spherical aggregates were observed at temperatures higher than 40 °C: one was a typical lamella liquid crystal with a maltese cross-texture, and the other was optically isotropic. Interestingly, when the 8F-B2ES lamellar phase was cooled to below 40 °C, the lamellar aggregates were distorted and the isotropic droplets became anisotropic. As this unique liquid crystal behavior was not observed for aqueous mixtures of the control surfactants, the oxyethylene units in the hybridized hydrocarbon tail play an important role in the behavior. This study also examined the effect of the oxyethylene units on microenvironmental polarity in the hybrid surfactant bilayer via fluorescence spectral measurements of pyrene solubilized in each lamellar phase. The polarity of the 8F-B2ES bilayer at 70 °C was found to be that of a hydrocarbon surfactant lamellar phase, and increased gradually with decreasing temperature. The polarity became the same as that of hydrophilic spherical micelles below 40 °C, despite the presence of the lamellar aggregates. Since the polarity in the 8F-DeS bilayer was independent of temperature, and as low as that of a typical hydrocarbon surfactant bilayer, hydration of the 8F-B2ES oxyethylene units would increase the polarity, and then loosen the 8F-B2ES packing within the bilayer. This probably led to distortion of the lamellar aggregates. Ó 2011 Elsevier Inc. All rights reserved.
1. Introduction Biological supramolecular structures spontaneously self-assemble in aqueous media via noncovalent interactions such as electrostatic interactions, hydrogen bonding, dipole–dipole interactions, and hydrophobic association. Ion channels, which are integral membrane proteins or, more typically, an assembly of several proteins, regulate the flow of ions across all cell membranes. Such ‘‘multisubunit’’ assemblies usually involve a circular arrangement of homologous proteins closely packed around a water-filled pore through the plane of the membrane [1]. In some ion channels, passage through the pore is governed by a ‘‘gate,’’ which may be opened or closed by chemical or electrical signals, temperature, or mechanical force, depending on the type of channel. These biological self-assemblies have attracted considerable attention for the construction of supramolecular aggregates [2]. ⇑ Corresponding author. Fax: +81 172 39 3569. E-mail address: [email protected] (M. Sagisaka). 0021-9797/$ - see front matter Ó 2011 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2011.02.024
Since 1977, many studies have reported that a variety of singletail and double-tail surfactants self-assemble to form stable monolayers and bilayers [3,4]. In the case of double-tail surfactants, the aggregate morphology usually comprised vesicles and lamellar bilayers [5]. Similar results were obtained for bilayer-forming single-tail surfactants [6–8]. In our earlier studies [9–11], we synthesized fluorinated double-tail surfactants with oxyethylene spacers and a sulfosuccinate group (sodium 1,2-bis(x-perfluoroalkyldioxyethylenecarbonyl)-1-ethanesulfonate, nFEOS, fluorocarbon chain length n = 4, 6, 8), and examined their lyotropic liquid crystal behavior in water. Despite the strong hydrophobic double fluoroalkyl tails, their Krafft temperatures were <0 °C, and some of the single nFEOS solutions spontaneously formed spherical vesicles at room temperature, even without the use of a specific preparation technique. This is an interesting observation since vesicle formation in common surfactants is usually driven by external forces such as ultrasonic irradiation, addition of a cosurfactant (or cosolvent), or heating [9–11]. We therefore concluded that the oxyethylene spacer between the fluorocarbon chain and the
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sulfosuccinate group allows the rigid fluorocarbon chain to orientate reasonably in the curved bilayer of the vesicle and enhances the stability of the vesicular structure. These results imply that the aggregate morphology of double-tail surfactants can be controlled by appropriately modifying the surfactant structures. To prepare ‘‘biological molecular assembly-like’’ functional materials, some interesting studies were conducted by using synthesized surfactants bearing three or more different parts (for example, fluorophilic, oleophilic, and hydrophilic groups) [12,13]. These surfactants were successfully formed microsegregated domains (or layers) composed of each group in one assembly. Many studies on vesicles have aimed at controlling the release rate of encapsulated materials (drug or active ingredient) for applying them to drug delivery systems (DDS) [14–19]. One method to control material release is the use of a lamellar gel (Lb) to liquid crystalline (La) phase transition [19–23]. At the transition temperature, rigidity and surfactant-packing density of the bilayers drastically lowered, and the encapsulated and solubilized compounds tend to be released from the vesicle interior [19]. Then, material-release rates can be controlled by change in temperature at around the phase transition temperature. In contrast with the material-release mechanism by the Lb–La phase transition, if on cooling the rigidity and surfactant-packing density are able to be lowered drastically at below a certain temperature, it could be a new carrier capable of material releasing at only low temperatures (for example, drug delivery systems to avoid frostbite and capsules for an antifreezing agent). To obtain effective and easy control for material-release rate by temperature, many papers have been focused on thermo-sensitive polymers (e.g., block copolymers with poly(N-isopropylacrylamide) or poly[2-(2-ethyloxy)ethoxyethyl vinyl ether]) [14–18]; those polymers were added in a typical vesicle solution, or used as an amphiphile to form vesicles. Some of these studies demonstrated a controllable drug-release rate by change in temperature beyond a lower critical solution temperature (LCST) of those block copolymers [14–18]. In this study, with the aim of yielding new functions, a quasiion channel and a low-temperature-promoting material release, we designed a hybrid surfactant, 8F-B2ES, shown in Fig. 1, which was composed of two isomers: sodium 1-(1H,1H,2H,2H-perfluorodecyloxycarbonyl)-2-{2-[2-(butyloxy)ethyloxy]ethyloxycarbonyl}ethanesulfonate and sodium 1-{2-[2-(butyloxy)ethyloxy]ethyloxycarbonyl}-2-(1H,1H,2H,2H-perfluorodecyloxycarbonyl)ethanesulfonate. 8F-B2ES has different types of tails in one molecule. In general, fluorocarbon–hydrocarbon miscibility is very low, and this immiscibility could introduce microsegregation of fluorocarbon and hydrocarbon tails in the hybrid surfactant bilayer [24], as shown in Fig. 2. The microsegregated 2-(butyloxy)ethyloxy]ethyl
O
O NaO3S
O O
4FEOS 8F-B2ES
X1 X2
O and
NaO3S
O
X1 X2
O O X1 = X2 = ?CH2CH2OCH2CH2(CF2)4F X1 = ?(CH2)2(CF2)7CF3 X2 = ?(CH2CH2O)2(CH2)3CH3
8F-DeS
X1 = ?(CH2)2(CF2)7CF3 X2 = ?(CH2)9CH3
Fig. 1. Molecular structures of 8F-B2ES, 8F-DeS, and 4FEOS. 8F-B2ES and 8F-DeS consisted of two isomers with different C–S bond locations.
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Fig. 2. Schematic models of the 8F-B2ES bilayer in water at (a) higher, and (b) lower temperatures.
tails of 8F-B2ES would be hydrophilic at lower temperatures and hydrophobic at higher ones because dehydration of oxyethylene units is promoted with increasing temperature. Thus 2-(butyloxy)ethyloxy]ethyl domains could behave as temperature-responsive nano-aqueous pores, in other words, quasi-ion channels, which open with decreasing temperature. If the 8F-2BES vesicles formed with temperature-responsive nano-aqueous pores, the material release of encapsulated materials in the vesicle interior would be promoted at lower temperatures. In addition, as the microdomain size of oxyethylene units should be smaller than nanometer order and 8F-B2ES vesicles could stably remain at low temperatures, the rate of material release may be controlled easily by temperature, compared with the vesicles with thermo-sensitive polymers reported earlier [14–18]. Most ionic fluorocarbon–hydrocarbon hybrid surfactants reported previously employed a stimuli-insensitive group (e.g., n-alkyl) as one of the hybrid-structured tails [25–41], although only two ionic hybrid surfactants had a stimuli-sensitive group; one had a redox-responsive ferrocene group at the terminal end of nalkyl tail [42]. Then, there is no information on the interfacial properties and liquid crystal behavior for an ionic hybrid surfactant having temperature-sensitive oxyethylene units in one hybridstructured alkyl tail. If new functions never reported appear in the 8F-B2ES lamellar aggregates, this surfactant molecular design will be useful for producing those functions, and then it broaden the industrial applications of lamellar aggregates. In this study, we synthesized 8F-B2ES and examined its aqueous liquid crystal behavior by optical microscopy and fluorescence spectral measurements with pyrene as the fluorescence probe. To elucidate the effects of the oxyethylene units in the hybridized hydrocarbon tail on liquid crystal formation, two control surfac-
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tants were also examined; one is a hybrid 8F-B2ES-analog surfactant without oxyethylene units (8F-DeS), and the other is a nonhybrid 8F-B2ES-analog surfactant with oxyethylene units (4FEOS). 2. Experimental 2.1. Materials In this study, we used two hybrid surfactants, 8F-B2ES (a mixture of two isomers: sodium 1-(1H,1H,2H,2H-perfluorodecyloxycarbonyl)-2-{2-[2-(butyloxy)ethyloxy]ethyloxycarbonyl}ethanesulfonate and sodium 1-{2-[2-(butyloxy)ethyloxy]ethyloxycarbonyl}-2-(1H,1H,2H,2H-perfluorodecyloxycarbonyl)ethanesulfonate) and 8F-DeS (a mixture of two isomers: sodium 1-(1H,1H,2H,2Hperfluorodecyloxycarbonyl)-2-(decyloxycarbonyl)ethanesulfonate and sodium 1-(decyloxycarbonyl)-2-(1H,1H,2H,2H-perfluorodecyloxycarbonyl)ethanesulfonate). Both hybrid surfactants were synthesized in our laboratory and purified as described in Section 2.2. The structures of the final products were elucidated by infrared (IR) spectroscopy (FTS-30, Bio-Rad Laboratories, Berkeley, CA, USA) and proton nuclear magnetic resonance (1H NMR) spectroscopy (JNM-GX270, JEOL, Tokyo, Japan). The purity of the final compounds was confirmed by elemental analysis (EA 1110; CE Instruments Ltd, Wigan, UK) and normal-phase high-performance liquid chromatography (HPLC) (SIL 150A-5 column; Intersil) with a UV detector (k = 254 nm). Chloroform was used as the eluent in the HPLC analysis. Ultrapure water with a resistivity of 18.2 MX cm was obtained from a Millipore Milli-Q Plus system (Millipore, Billerica, MA, USA) and used for the experiments. A symmetric double fluorinated-tail surfactant with oxyethylene units, 4FEOS, was provided by Dr. Yukishige Kondo at Tokyo University of Science [10,11]. Cetyltrimethylammonium bromide (CTAB) and sodium octylsulfate (SOS) were used as control surfactants for fluorescence spectral measurements with pyrene. 2.2. Syntheses 8F-B2ES and 8F-DeS were synthesized and purified using the following procedures. The synthetic scheme is shown in Fig. 3. 2.2.1. Synthesis of (Z)-4-oxo-4-(1H,1H,2H,2H-perfluorodecyloxy)but2-enoic acid A mixture of 1H,1H,2H,2H-heptadecafluoro-1-decanol (10.0 g, 21.6 mmol) and maleic anhydride (10.1 g, 103.0 mmol) was stirred at 90 °C for 6 h. After the reaction, brine was added to the residue and the white precipitate in the aqueous mixture was obtained by filtration. The precipitate was then washed sufficiently with dichloromethane and an aqueous HCl solution at pH2 to remove the unreacted fluorinated alcohol and to exchange the Na+ and H+
counterions. Drying the precipitate under vacuum gave a white powder, (Z)-4-oxo-4-(1H,1H,2H,2H-perfluorodecyloxy)but-2-enoic acid (yield 9.85 g, 81.3%): melting point 103.8–104.7 °C; 1H NMR (400 MHz, CF3COOD, TMS) dH/ppm: 2.60–2.75 (m, 2H, CF2CH2 ), 4.74 (t, 2H, OCH2 , J = 6.3 Hz), 6.57 (d, 1H, HCCH , J = 6.1 Hz), 6.66 (d, 1H, HCCH , J = 6.2 Hz); IR (KBr) mmax/cm 1: 2888, 1734, 1701, 1202. 2.2.2. Synthesis of maleic acid diester 2-(2-Butoxyethoxy)ethyl (1H,1H,2H,2H-perfluorodecyl) maleate: (Z)-4-oxo-4-(1H,1H,2H,2H-perfluorodecyloxy)but-2-enoic acid (3.00 g, 6.0 mmol), 2-(2-butoxyethoxy)ethanol (0.866 g, 5.34 mmol), and p-toluenesulfonic acid monohydrate (0.203 g, 1.07 mmol) in 15 mL toluene were refluxed with stirring at 110 °C for 48 h. During the reaction, the water liberated was removed azeotropically from the reaction system to shift the equilibrium of the esterification reaction. After the reaction was complete, ethyl acetate (60.0 mL) was added to the residue, and the mixture was washed sufficiently with brine to remove p-toluenesulfonic acid and unreacted maleic acid monoester; it was then dried over Na2SO4. After removal of the drying agent, toluene and ethyl acetate in the mixture were removed by evaporation. The residue was purified by column chromatography on silica gel with a dichloromethane–ethanol (30:1) mixture, giving a yellow liquid, 2-(2-butoxyethoxy)ethyl (1H,1H,2H,2H-perfluorodecyl) maleate (yield 0.64 g, 17.0%): 1H NMR (400 MHz, CDCl3, TMS) dH/ppm: 0.91 (t, 3H, CH3 , J = 7.5 Hz), 1.36 (sextet, 2H, aliphatic-H, J = 7.48 Hz), 1.57 (quintet, 2H, aliphatic-H, J = 7.1 Hz), 2.50–2.59 (m, 2H, CF2CH2 ), 3.45 (t, 2H, –CH2O–, J = 6.8 Hz), 3.57–3.65 (m, 4H, –CH2O–), 3.74–3.75 (m, 2H, –CH2O–), 4.33–4.35 (m, 2H, – CH2OCO–) 4.50 (t, 2H, CH2OCO , J = 6.6 Hz), 6.25–6.32 (m, 2H, –HCCH–). Decyl (1H,1H,2H,2H-perfluorodecyl) maleate: (Z)-4-oxo4-(1H,1H,2H,2H-perfluorodecyloxy)but-2-enoic acid (4.00 g, 6.0 mmol), 2-(2-butoxyethoxy)ethanol (0.950 g, 6.00 mmol), and ptoluenesulfonic acid monohydrate (0.230 g, 1.20 mmol) in toluene (15 mL) were refluxed with stirring at 120 °C for 48 h in a Dean– Stark apparatus. The purification procedure was the same as that for 2-(2-butoxyethoxy)ethyl (1H,1H,2H,2H-perfluorodecyl) maleate, giving a yellow liquid, decyl (1H,1H,2H,2H-perfluorodecyl) maleate (yield 2.56 g, 60.5%): 1H NMR (400 MHz, CDCl3, TMS) dH/ ppm: 0.88 (t, 3H, CH3 , J = 6.90 Hz), 1.21–1.38 (m, 14H, aliphaticH), 1.66 (quintet, 2H, aliphatic-H, J = 6.9 Hz), 2.46–2.61 (m, 2H, CF2CH2 ), 4.15–4.22 (m, 2H, –CH2O–), 4.50 (t, 2H, CH2O , J = 6.6 Hz), 6.22–6.30 (m, 2H, –HCCH–). 2.2.3. Synthesis of hybrid surfactants 8F-B2ES and 8F-DeS A mixture of sodium 1-(1H,1H,2H,2H-perfluorodecyloxycarbonyl)-2-{2-[2-(butyloxy)ethyloxy]ethyloxycarbonyl}ethanesulfonate
Fig. 3. Scheme for hybrid surfactant synthesis.
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and sodium 1-{2-[2-(butyloxy)ethyloxy]ethyloxycarbonyl}-2-(1H, 1H,2H,2H-perfluorodecyloxycarbonyl)ethanesulfonate, 8F-B2ES: 2-(2-Butoxyethoxy)ethyl 1H,1H,2H,2H-perfluorodecyl maleate (0.55 g, 0.8 mmol) was dissolved in 1,4-dioxane (14 mL); the mixture was heated to 50 °C. A solution of sodium hydrogensulfite (0.30 g, 3.3 mmol) in water (5 mL) was added to the mixture. The reaction mixture was stirred under reflux for 60 h. The reaction solvent was removed by evaporation under vacuum at room temperature, and the residue was then washed with 1,4-dioxane to remove the unreacted diester. The product was extracted in a Soxhlet apparatus with acetone to remove excess NaHSO3, and washed with toluene. A white powder, 8F-B2ES, was obtained after vacuum drying (yield 0.176 g, 32.0%); 1H NMR (400 MHz, CF3COOD, TMS) dH/ppm: 0.86 (t, 3H, CH3 , J = 7.3 Hz), 1.30 (sextet, 2H, aliphaticH, J = 7.5 Hz), 1.60 (quintet, 2H, aliphatic-H, J = 7.2 Hz), 2.41–2.57 (m, 2H, CF2CH2 ), 3.25–3.41 (m, 2H, –CH2COO–), 3.70 (t, 2H, CH2O , J = 7.0 Hz), 3.84–3.96 (m, 6H, –CH2O–), 4.35–4.41 (m, 2H, –CH2OCO–), 4.44–4.52 (m, 2H, CH2OCO ), 4.54–4.65 (m, 1H, –HC(SO3Na)–). ; IR (KBr) mmax/cm 1: 2965, 2876, 1740, 1242, 1215; elemental analysis for C22H24F17 NaO9S: found C, 32.1; H, 2.4; S, 4.0; calcd. C, 32.6; H, 3.0; S, 4.0. A mixture of sodium 1-(1H,1H,2H,2H-perfluorodecyloxycarbonyl)-2-(decyloxycarbonyl)ethanesulfonate and sodium 1-(decyloxycarbonyl)-2-(1H,1H,2H,2H-perfluorodecyloxycarbonyl)ethanesulfonate, 8F-DeS: Decyl 1H,1H,2H,2H-perfluorodecyl maleate (2.00 g, 2.8 mmol) was dissolved in 1,4-dioxane (50 mL); the mixture was heated to 50 °C. A solution of sodium hydrogensulfite (1.23 g, 11.8 mmol) in water (15 mL) was added to the mixture. The reaction mixture was stirred under reflux for 60 h. The purification procedure for 8F-DeS was the same as that for 8F-B2ES. A white powder, 8F-DeS, was obtained (yield 1.47 g, 65.1%); 1H NMR (400 MHz, CF3COOD, TMS) dH/ppm: 0.89–0.94 (m, 3H, CH3 ), 1.29–1.49 (m, 14H, aliphatic-H), 1.74–1.85 (m, 2H, aliphatic-H), 2.55–2.73 (m, 2H, CF2CH2 ), 3.41–3.56 (m, 2H, – CH2COO–), 4.28–4.43 (m, 2H,–CH2OCO–), 4.61–4.78 (m, 3H, CH2OCO and –HC(SO3Na)–).; IR (KBr) mmax/cm 1: 2963, 2931, 2859, 1735, 1243, 1219; elemental analysis for C24H28F17NaO7S: found C, 35.6; H, 3.5; S, 3.9; calcd. C, 35.7; H, 3.5; S, 4.0. 2.3. Liquid crystal properties The initial phase assignments for surfactant/water mixtures were determined by thermal optical microscopy using a polarizing microscope (BX-51, Olympus) equipped with a thermal stage (TS62, INSTEC), and a temperature controller (STC200, INSTEC). 4FEOS and 8F-B2ES solutions for microscopic observation were prepared at room temperature, with no external force except stirring. As 8F-DeS had a low affinity with water, preparation of sample solutions was carried out at 70 °C. In order to avoid evaporation of water from the mixture, a polymer gel was used to seal a cover glass onto the glass slide on which the mixture was placed. The heating and cooling rates were both 2 °C min 1. As phase transition of the liquid crystals was found for the aqueous 8F-B2ES mixtures, the turbidity of these mixtures was measured with a double-beam spectrometer (U-2810, Hitachi High-Technologies, Co., Tokyo, Japan) as a function of temperature. Absorbance at 700 nm was employed for turbidity. To examine changes in microenvironmental polarity in the hydrophobic region of the liquid crystals, fluorescence spectral measurements were conducted at temperatures of 15–70 °C with a fluorescence spectrometer (RF-5300, SHIMADZU, Kyoto, Japan, resolution: 0.2 nm at room temperature) and pyrene as a fluorescence probe. The microenvironmental polarities around the pyrene molecule were found from the ratio of the first (373 nm) to the third (384) vibronic peak intensities (I1/I3) in the fluorescence spectrum for pyrene [31,43–45].
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For fluorescence spectral measurement, a hybrid surfactant/ pyrene solution was prepared using the following procedure. A pyrene/ethanol solution (0.40 mL, 0.10 mM) was placed in a test tube, and the ethanol was then completely removed by evaporation under vacuum. After ethanol removal, a 10 wt.% surfactant/ water mixture (4.0 mL) was added to the test tube, and the mixture was slowly stirred for 1 h. After allowing the sample solution to stand for 1 day at room temperature, the fluorescence spectra of pyrene were measured between 335 and 600 nm at an excitation wavelength of 335 nm. Measurements were also made on 10 wt.% surfactant/water mixtures without pyrene to obtain the actual pyrene fluorescence spectra.
3. Results and discussion The liquid crystal behavior of the aqueous surfactant solutions at concentrations of 10–30 wt.% were examined as a function of temperature by optical microscopy. In the case of the fresh 4FEOS solutions prepared at room temperature, optically anisotropic capsular aggregates with multilayer structures were found without irradiation or any other eternal forces except stirring, as shown in Fig. 4, representing the formation of multilamellar vesicles (MLVs). Some of the MLVs were quite large (ca. 100 lm) and stable for at least 3 days. Although on heating MLVs become smaller and the population decreased by increasing the surfactant solubility, some MLVs remained at 70 °C. However, no specific morphology change was observed as MLV on heating and cooling processes for the 4FEOS solutions. The aqueous solutions of 8F-DeS and 8F-B2ES at 10–30 wt.% exhibited spherical aggregates with maltese cross-textures in the polarized micrographs (Fig. 5), which are often found in typical MLV systems [46,47]. For the 8F-DeS lamellar aggregates, the textures became unclear at temperatures higher than 70 °C. Loss of textural clarity at higher temperatures is usual for a typical liquid crystal system, and results from a lowering of the order parameter caused by increases in thermal molecular motion [48]. In contrast to the 8F-DeS lamellas, the 8F-B2ES lamella textures were clear at higher temperatures, even at 70 °C (Fig. 5d). At temperature higher than 40 °C, optically isotropic aggregates were also observed (Fig. 5e and i), and identified as a sponge or bicontinuous cubic liquid crystal [49], which has a bilayer structure as well as a lamellar phase. The coexistence of two types of liquid crystal (lamellar and isotropic liquid crystal phases) in one solution is quite rare, and probably results from the presence of two 8FB2ES isomers. Interestingly, at temperatures lower than 40 °C, the maltese cross-textures of the lamellas were distorted (Fig. 5f and iv) and the isotropic aggregates became anisotropic (Fig. 5f and ii). The textures in (iv) and (ii) in Fig. 5 reverted to those of (iii) and (i), respectively, when the temperature was increased. This study could not determine which type of liquid crystal the anisotropic one was (Fig. 5ii). Although texture changes from (i) to (ii) and from (iii) to (iv) in Fig. 5 were not observed for the lamellar phases of 8F-DeS and 4FEOS, the unique phase transition could come from oxyethylene units in the hybridized hydrocarbon tail. The turbidity of a surfactant solution depends on the refractive index of the molecular assembly, for example, changes in morphology, molecular packing, and ordering [50]. Through turbidity measurements of the aqueous 8F-B2ES solutions, the temperatures at which phase transition occurred were studied at surfactant concentrations of 15 wt.% and 30 wt.%. Fig. 6 shows the changes in turbidity as a function of temperature. The arrows in the graph show the kinks which were observed in the linear relationships between turbidity and temperature. These temperatures were identified as phase transition temperatures: 36 °C for 15 wt.% and 39 °C for 30 wt.% 8F-B2ES solutions. Although a similar decrease in turbidity
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Fig. 4. Optical micrographs of aqueous 10 wt.% 4FEOS solution at 30 °C. Micrographs (a and b) were taken under normal and crossed Nicol prism conditions, respectively.
(a)
(b)
(c)
(d)
(e)
(f)
(i) (iv)
(iii) (ii)
(ii)
(i)
(iii)
(iv)
Fig. 5. Polarized micrographs of aqueous 30 wt.% hybrid surfactant solutions at (a, d) 70 °C, (b, e) 40 °C, and (c, f) 25 °C. Textures in (e and f) were magnified and shown at the bottom in (i–iv). The textures (a–c) are for 8F-DeS and the others are for 8F-B2ES.
2.4
Turbidity / a.u.
2.0 15 wt% 30 wt%
1.6
1.2
0.8
10
20
30
40
50
60
70
Temperature / ºC Fig. 6. Changes in turbidity for 15 or 30 wt.% aqueous 8F-B2ES mixtures as a function of temperature.
with increasing temperature was reported for a typical liposome system when a lamellar gel (Lb) to liquid crystalline (La) phase transition occurs [19–23], the turbidity decrease for the 8F-B2ES lamellar phase would come from changes in oxyethylene unit hydration, not from an Lb-to-La phase transition, because of the highly hydrated 8F-B2ES lamellar bilayer at temperatures below 40 °C, as noted later. To confirm whether or not the hydration mechanism shown in Fig. 2 occurs, I1/I3 values of pyrene solubilized in the surfactant lamellar phase were measured. Fig. 7 shows the relationship between temperature and I1/I3 for pyrene in lamellar aggregate solutions. High turbidity at 30 wt.% solutions affected the fluorescence spectra of pyrene, and then 10 wt.% with a low turbidity was employed instead of 30 wt.%. For pyrene in pure water, I1/I3 was almost the same (2.02–2.06) at temperatures of 25–65 °C. These values suggest a high polar environment surrounding the pyrene molecules when compared with I1/I3 values in typical organic solvents [44]: 0.85 for n-octanol, 0.90 for n-hexanol, and 0.99 for n-
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2.2
Conformation change
(I)
2.0
(II)
I1 / I3
1.8 1.6
X2
10 wt% 8F-B2ES solution 10 wt% 8F-DeS solution pure water
1.4
2 hydrophobic and 1 hydrophilic groups
1.2 1.0
20
30
40
50
X1
60
70
1 hydrophobic and 2 hydrophilic groups
Fig. 8. Steric structures of the 8F-B2ES isomers (X2OCOCH2CH(SO3Na)COOX1, where X1 = C8F17CH2CH2– and X2 = C4H9OCH2CH2OCH2CH2–).
Temperature / ºC Fig. 7. Temperature dependencies of I1/I3 for pyrene solubilized in 10 wt.% hybrid surfactant/water mixtures or pure water.
propanol at 25 °C. In the case of the 8F-DeS lamellar phase, I1/I3 was 1.28 and almost independent of temperature. The I1/I3 value was similar to that for an aqueous hydrocarbon surfactant lamellar phase. For example, this study found the I1/I3 values to be 1.85–1.9 for SOS micelles (spherical [51]), 1.39–1.42 for mixed CTAB/SOS micelles (rod-like [51], CTAB:SOS molar ratio = 4:1), and 1.26– 1.28 for mixed CTAB/SOS lamellar systems (vesicular [51], CTAB:SOS molar ratio = 1:4) at a total surfactant concentration of 1 wt.% at 25–75 °C. In addition, DPPC liposome solutions were reported to have I1/I3 of 1.10–1.05 at temperatures from 28 °C (Lb phase) to 38 °C (Pb phase) [52]. Despite 8F-DeS having a strong hydrophobic fluorocarbon tail, the I1/I3 values were similar to those of a hydrocarbon surfactant lamellar phase, compared with those of fluorocarbon systems. For example, I1/I3 values were reported to be 1.00 in perfluorooctane [53], and 1.05 in the mixed micelle of the equimolar hexadecytrimethylammonium chloride(CTAC)/ 1,1,2,2-tetrahydroheptadecafluorodecylpyridinium chloride (HFDePC) mixture, which was significantly lower than 1.30 in CTAC micelle [54]. This suggests that the pyrene molecules were surrounded by microsegregated hydrocarbon chains (i.e., a hydrocarbon microdomain) of 8F-DeS molecules, avoiding contact with oleophobic fluorocarbon chains. In addition, the constant I1/I3 values for 8F-DeS imply that the partition of pyrene molecules between the water and the 8F-DeS lamellar bilayer did not change at any temperature. In contrast, for the 8F-B2ES lamellar phase, I1/I3 at 70 °C was found to be 1.45, which was slightly higher than that of 8F-DeS, and close to that of the rod-like micelles in the mixed CTAB/SOS system. However, the I1/I3 value increased with decreasing temperature, and became almost constant (1.9) at temperatures below 40 °C. The I1/I3 value of 1.9 is the same as that of a highly hydrated SOS micelle, even though 8F-B2ES lamellar aggregates remained. Similar to 8F-B2ES lamellar aggregates, an increase in I1/I3 with decreasing temperature was observed for nonionic micelles of Pluronic PEO-PPO-PEO (PEO, poly(ethylene oxide); PPO, poly(propylene oxide)) block copolymers (P65, P85, and P105) in water; I1/I3 values of these micelles increased from 1.4 to 2.0 when temperature decreased from 50 to 10 °C [45]. This indicates that, with decreasing temperature, hydration of the oxyethylene units led to the interior of the 8F-B2ES bilayer becoming more hydrophilic, suggesting generation of nano-aqueous pores, as shown in Fig. 2b. In addition, we noted that the temperature at which I1/I3 starts to become constant on cooling was coincident with that causing distortion of the fine 8F-B2ES lamellar aggregates. This suggests that the hydrated oxyethylene microdomains at temperatures below 40 °C loosened the molecular packing and then caused distortion of the lamellar aggregates. With regard to distortion of the 8F-B2ES lamellar aggregates, not only the nano-aqueous
pore but also any other effects were expected to occur at temperatures below 40 °C. As noted in Section 2.1, 8F-B2ES was composed of two isomers: (i) X1OCOCH2CH(SO3Na)COOX2, and (ii) X2OCOCH2CH(SO3Na)COOX1, where X1 = C8F17CH2CH2– and X2 = C4H9OCH2CH2OCH2CH2–. For isomer (ii), as shown in Fig. 8, the X2 tail could orientate from the fluorocarbon tail side (I) to the sulfonate one (II) along the H2C–CH(SO3Na) bond axis. When hydration of the oxyethylene units was promoted at lower temperatures, the conformation ratio of (II):(I) was expected to increase, which would decrease the apparent packing parameter of 8FB2ES. This could be one of the reasons for distortion of the 8FB2ES lamellar aggregates.
4. Conclusion With the aim of forming quasi-ion channels within a bilayer, this study designed and synthesized the hybrid surfactant 8FB2ES, which has a fluorocarbon tail and a hydrocarbon tail with two oxyethylene units. 8F-B2ES was expected to yield oxyethylene microdomains in its bilayers because of the strong oleo- and hydrophobicities of the fluorocarbon tail in the hybrid structure; the microdomain would then act as a quasi-ion channel. Lowering the temperature increases hydration of the oxyethylene units, and the well-hydrated microdomains could allow passage of small ions or hydrophilic compounds. A symmetric double fluorinated-tail surfactant with one oxyethylene unit in each tail (4FEOS) and a 8F-B2ES-analog surfactant with no oxyethylene units (8F-DeS) were used as control surfactants. For two control surfactant/water mixtures, textures showing lamellar aggregates were clear at low temperatures, but became unclear with increasing temperature. In contrast with those of the control surfactant systems, the 8F-B2ES lamellar aggregates were distorted at temperatures below 40 °C, although the textures were fine at higher temperatures. The fluorescence spectra of pyrene solubilized in 8F-B2ES and 8F-DeS lamellar phases were used to investigate the polarity in the bilayer interiors to confirm whether or not the oxyethylene units acted as the well-hydrated microdomain. The interiors of the 8F-DeS bilayers exhibited a low polarity, similar to that of a typical hydrocarbon surfactant lamellar phase, and no temperature dependence for the polarity. On the other hand, although the polarity of the 8F-B2ES bilayer at 70 °C was close to that for a hydrocarbon surfactant lamellar phase, it gradually increased with decreasing temperature, and at temperatures below 40 °C became similar to that for a typical hydrophilic micelle. The presence of highly hydrophilic microenvironments in the 8F-B2ES bilayer suggested formation of nano-aqueous pores with oxyethylene units. At temperatures below 40 °C, the polarity was independent of temperature. On cooling, for 8F-B2ES lamellar aggregates, the temperature at which the polarity became constant was almost consistent
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with that for distortion of lamellar aggregates. As the 4FEOS did not display distorted lamellar aggregates at any temperature, hydrated oxyethylene microdomains within the 8F-B2ES bilayers could be a trigger for distortion of the lamellar aggregates. For a simple binary mixture of an ionic surfactant and water [3– 54], the increased microenvironmental polarity with lowering temperature is unusual if lamellar aggregates remain at any temperature. In addition, the drastic distortion of lamellar aggregates at a certain temperature was quite rare on the cooling process. From these standpoints, the new surfactant design giving 8F2BES, namely introduction of oxyethylene units into hybrid-structured alkyl tail, could lead to these interesting phenomena, suggesting formation of temperature-responsive nano-aqueous pores and softening of the 8F-2BES bilayers on cooling. In the case of the fluorocarbon–hydrocarbon hybrid surfactant with a temperature-sensitive ferrocene group [42], oxidation as well as an increase in temperature was found to induce interesting changes in interfacial properties (critical micelle concentration and surface tension of aqueous solution) and aggregation morphology (transitions of coil-like to vesicle-like aggregates or vesicle-like to no aggregates), but these were not connected with results in this study. Our further study will synthesize novel hybrid surfactants, and examine changes in their interfacial properties, nanostructure, and material-release property of liquid crystal as a function of temperature. To yield liquid crystals with an unusual morphology/nanostructure, or liquid crystals that are both lyotropic and thermotropic (amphitropic liquid crystal), thermotropic mesogens would be an interesting group to consider as an alternative to fluorocarbons in the hybrid structure, as they also have low miscibility with hydrocarbon and oxyethylene units [48]. These will advance our molecular design for hybrid surfactant to achieve formation of a quasi-ion channel and its specific application as a new carrier (e.g., DDS to avoid frostbite and capsules for an antifreezing agent). In addition, if the conformation-change rate of the hybrid surfactant (as shown in Fig. 8) can be controlled by an advanced surfactant design, it might give a dynamic and time-periodic change of lyotropic liquid crystal morphology (or a dissipative structure) though it has been often reported in thermotropic systems [48]. Acknowledgments This work was partly supported by a Grant for Priority Research Designated by a Grant for Hirosaki University Institutional Research, a Grant for Priority Research designated by Dean of Graduate School of Science Technology, and Oil & Fat Industry Kaikan Foundation. We thank Dr. Y. Kondo for providing a fluorinated double-tail surfactant 4FEOS. References [1] G.J. Siegel, B.W. Agranoff, R.W. Albers, S.K. Fisher, M.D. Uhler, Basic Neurochemistry: Molecular, Cellular, and Medical Aspects, Lippincott-Raven, Philadelphia, 1999. [2] J.-P. Sauvage, in: M.W. Hosseini (Ed.), Comprehensive Supramolecular Chemistry, vol. 9, Pergamon, UK, 1996. [3] T. Kunitake, Y. Okahata, J. Am. Chem. Soc. 99 (1977) 3860. [4] T. Kunitake, Y. Okahata, K. Tamaki, F. Kumamaru, M. Takayanagi, Chem. Lett. (1977) 387. [5] J.H. Fendler, Acc. Chem. Res. 13 (1980) 7. [6] T. Kunitake, Y. Okahata, J. Am. Chem. Soc. 102 (1980) 549.
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Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
Unique liquid crystal behavior in water of anionic fluorocarbon–hydrocarbon hybrid surfactants containing oxyethylene units Masanobu Sagisaka ⇑, Yoshie Fujita, Yusuke Shimizu, Chie Osanai, Atsushi Yoshizawa Department of Frontier Materials Chemistry, Graduate School of Science and Technology, Hirosaki University, 3 Bunkyo-cho, Hirosaki, Aomori 036-8561, Japan
a r t i c l e
i n f o
Article history: Received 19 October 2010 Accepted 8 February 2011 Available online 15 February 2011 Keywords: Lamella phase Dehydration Hybrid surfactant Microsegregation Ion channel
a b s t r a c t This study reports the unique aqueous lyotropic liquid crystal behavior of an anionic hybrid surfactant, 8F-B2ES, which has 2-[2-(butyloxy)ethyloxy]ethyl and 1H,1H,2H,2H-perfluorodecyl tails. An 8F-B2ESanalog hybrid surfactant with no oxyethylene units (8F-DeS) and a symmetric fluorinated double-tail surfactant with two 2-(1H,1H,2H,2H-perfluorohexyloxy)ethyl tails (4FEOS) were used as control surfactants in examining the effects of the oxyethylene units and of the hybrid structure on the liquid crystal behavior. Polarized microscopic observations showed the formation of a lamellar liquid crystal phase for each surfactant/water mixture at surfactant concentrations higher than 10 wt.%. In the case of the 30 wt.% 8FB2ES/water mixture, two types of spherical aggregates were observed at temperatures higher than 40 °C: one was a typical lamella liquid crystal with a maltese cross-texture, and the other was optically isotropic. Interestingly, when the 8F-B2ES lamellar phase was cooled to below 40 °C, the lamellar aggregates were distorted and the isotropic droplets became anisotropic. As this unique liquid crystal behavior was not observed for aqueous mixtures of the control surfactants, the oxyethylene units in the hybridized hydrocarbon tail play an important role in the behavior. This study also examined the effect of the oxyethylene units on microenvironmental polarity in the hybrid surfactant bilayer via fluorescence spectral measurements of pyrene solubilized in each lamellar phase. The polarity of the 8F-B2ES bilayer at 70 °C was found to be that of a hydrocarbon surfactant lamellar phase, and increased gradually with decreasing temperature. The polarity became the same as that of hydrophilic spherical micelles below 40 °C, despite the presence of the lamellar aggregates. Since the polarity in the 8F-DeS bilayer was independent of temperature, and as low as that of a typical hydrocarbon surfactant bilayer, hydration of the 8F-B2ES oxyethylene units would increase the polarity, and then loosen the 8F-B2ES packing within the bilayer. This probably led to distortion of the lamellar aggregates. Ó 2011 Elsevier Inc. All rights reserved.
1. Introduction Biological supramolecular structures spontaneously self-assemble in aqueous media via noncovalent interactions such as electrostatic interactions, hydrogen bonding, dipole–dipole interactions, and hydrophobic association. Ion channels, which are integral membrane proteins or, more typically, an assembly of several proteins, regulate the flow of ions across all cell membranes. Such ‘‘multisubunit’’ assemblies usually involve a circular arrangement of homologous proteins closely packed around a water-filled pore through the plane of the membrane [1]. In some ion channels, passage through the pore is governed by a ‘‘gate,’’ which may be opened or closed by chemical or electrical signals, temperature, or mechanical force, depending on the type of channel. These biological self-assemblies have attracted considerable attention for the construction of supramolecular aggregates [2]. ⇑ Corresponding author. Fax: +81 172 39 3569. E-mail address: [email protected] (M. Sagisaka). 0021-9797/$ - see front matter Ó 2011 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2011.02.024
Since 1977, many studies have reported that a variety of singletail and double-tail surfactants self-assemble to form stable monolayers and bilayers [3,4]. In the case of double-tail surfactants, the aggregate morphology usually comprised vesicles and lamellar bilayers [5]. Similar results were obtained for bilayer-forming single-tail surfactants [6–8]. In our earlier studies [9–11], we synthesized fluorinated double-tail surfactants with oxyethylene spacers and a sulfosuccinate group (sodium 1,2-bis(x-perfluoroalkyldioxyethylenecarbonyl)-1-ethanesulfonate, nFEOS, fluorocarbon chain length n = 4, 6, 8), and examined their lyotropic liquid crystal behavior in water. Despite the strong hydrophobic double fluoroalkyl tails, their Krafft temperatures were <0 °C, and some of the single nFEOS solutions spontaneously formed spherical vesicles at room temperature, even without the use of a specific preparation technique. This is an interesting observation since vesicle formation in common surfactants is usually driven by external forces such as ultrasonic irradiation, addition of a cosurfactant (or cosolvent), or heating [9–11]. We therefore concluded that the oxyethylene spacer between the fluorocarbon chain and the
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sulfosuccinate group allows the rigid fluorocarbon chain to orientate reasonably in the curved bilayer of the vesicle and enhances the stability of the vesicular structure. These results imply that the aggregate morphology of double-tail surfactants can be controlled by appropriately modifying the surfactant structures. To prepare ‘‘biological molecular assembly-like’’ functional materials, some interesting studies were conducted by using synthesized surfactants bearing three or more different parts (for example, fluorophilic, oleophilic, and hydrophilic groups) [12,13]. These surfactants were successfully formed microsegregated domains (or layers) composed of each group in one assembly. Many studies on vesicles have aimed at controlling the release rate of encapsulated materials (drug or active ingredient) for applying them to drug delivery systems (DDS) [14–19]. One method to control material release is the use of a lamellar gel (Lb) to liquid crystalline (La) phase transition [19–23]. At the transition temperature, rigidity and surfactant-packing density of the bilayers drastically lowered, and the encapsulated and solubilized compounds tend to be released from the vesicle interior [19]. Then, material-release rates can be controlled by change in temperature at around the phase transition temperature. In contrast with the material-release mechanism by the Lb–La phase transition, if on cooling the rigidity and surfactant-packing density are able to be lowered drastically at below a certain temperature, it could be a new carrier capable of material releasing at only low temperatures (for example, drug delivery systems to avoid frostbite and capsules for an antifreezing agent). To obtain effective and easy control for material-release rate by temperature, many papers have been focused on thermo-sensitive polymers (e.g., block copolymers with poly(N-isopropylacrylamide) or poly[2-(2-ethyloxy)ethoxyethyl vinyl ether]) [14–18]; those polymers were added in a typical vesicle solution, or used as an amphiphile to form vesicles. Some of these studies demonstrated a controllable drug-release rate by change in temperature beyond a lower critical solution temperature (LCST) of those block copolymers [14–18]. In this study, with the aim of yielding new functions, a quasiion channel and a low-temperature-promoting material release, we designed a hybrid surfactant, 8F-B2ES, shown in Fig. 1, which was composed of two isomers: sodium 1-(1H,1H,2H,2H-perfluorodecyloxycarbonyl)-2-{2-[2-(butyloxy)ethyloxy]ethyloxycarbonyl}ethanesulfonate and sodium 1-{2-[2-(butyloxy)ethyloxy]ethyloxycarbonyl}-2-(1H,1H,2H,2H-perfluorodecyloxycarbonyl)ethanesulfonate. 8F-B2ES has different types of tails in one molecule. In general, fluorocarbon–hydrocarbon miscibility is very low, and this immiscibility could introduce microsegregation of fluorocarbon and hydrocarbon tails in the hybrid surfactant bilayer [24], as shown in Fig. 2. The microsegregated 2-(butyloxy)ethyloxy]ethyl
O
O NaO3S
O O
4FEOS 8F-B2ES
X1 X2
O and
NaO3S
O
X1 X2
O O X1 = X2 = ?CH2CH2OCH2CH2(CF2)4F X1 = ?(CH2)2(CF2)7CF3 X2 = ?(CH2CH2O)2(CH2)3CH3
8F-DeS
X1 = ?(CH2)2(CF2)7CF3 X2 = ?(CH2)9CH3
Fig. 1. Molecular structures of 8F-B2ES, 8F-DeS, and 4FEOS. 8F-B2ES and 8F-DeS consisted of two isomers with different C–S bond locations.
401
Fig. 2. Schematic models of the 8F-B2ES bilayer in water at (a) higher, and (b) lower temperatures.
tails of 8F-B2ES would be hydrophilic at lower temperatures and hydrophobic at higher ones because dehydration of oxyethylene units is promoted with increasing temperature. Thus 2-(butyloxy)ethyloxy]ethyl domains could behave as temperature-responsive nano-aqueous pores, in other words, quasi-ion channels, which open with decreasing temperature. If the 8F-2BES vesicles formed with temperature-responsive nano-aqueous pores, the material release of encapsulated materials in the vesicle interior would be promoted at lower temperatures. In addition, as the microdomain size of oxyethylene units should be smaller than nanometer order and 8F-B2ES vesicles could stably remain at low temperatures, the rate of material release may be controlled easily by temperature, compared with the vesicles with thermo-sensitive polymers reported earlier [14–18]. Most ionic fluorocarbon–hydrocarbon hybrid surfactants reported previously employed a stimuli-insensitive group (e.g., n-alkyl) as one of the hybrid-structured tails [25–41], although only two ionic hybrid surfactants had a stimuli-sensitive group; one had a redox-responsive ferrocene group at the terminal end of nalkyl tail [42]. Then, there is no information on the interfacial properties and liquid crystal behavior for an ionic hybrid surfactant having temperature-sensitive oxyethylene units in one hybridstructured alkyl tail. If new functions never reported appear in the 8F-B2ES lamellar aggregates, this surfactant molecular design will be useful for producing those functions, and then it broaden the industrial applications of lamellar aggregates. In this study, we synthesized 8F-B2ES and examined its aqueous liquid crystal behavior by optical microscopy and fluorescence spectral measurements with pyrene as the fluorescence probe. To elucidate the effects of the oxyethylene units in the hybridized hydrocarbon tail on liquid crystal formation, two control surfac-
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tants were also examined; one is a hybrid 8F-B2ES-analog surfactant without oxyethylene units (8F-DeS), and the other is a nonhybrid 8F-B2ES-analog surfactant with oxyethylene units (4FEOS). 2. Experimental 2.1. Materials In this study, we used two hybrid surfactants, 8F-B2ES (a mixture of two isomers: sodium 1-(1H,1H,2H,2H-perfluorodecyloxycarbonyl)-2-{2-[2-(butyloxy)ethyloxy]ethyloxycarbonyl}ethanesulfonate and sodium 1-{2-[2-(butyloxy)ethyloxy]ethyloxycarbonyl}-2-(1H,1H,2H,2H-perfluorodecyloxycarbonyl)ethanesulfonate) and 8F-DeS (a mixture of two isomers: sodium 1-(1H,1H,2H,2Hperfluorodecyloxycarbonyl)-2-(decyloxycarbonyl)ethanesulfonate and sodium 1-(decyloxycarbonyl)-2-(1H,1H,2H,2H-perfluorodecyloxycarbonyl)ethanesulfonate). Both hybrid surfactants were synthesized in our laboratory and purified as described in Section 2.2. The structures of the final products were elucidated by infrared (IR) spectroscopy (FTS-30, Bio-Rad Laboratories, Berkeley, CA, USA) and proton nuclear magnetic resonance (1H NMR) spectroscopy (JNM-GX270, JEOL, Tokyo, Japan). The purity of the final compounds was confirmed by elemental analysis (EA 1110; CE Instruments Ltd, Wigan, UK) and normal-phase high-performance liquid chromatography (HPLC) (SIL 150A-5 column; Intersil) with a UV detector (k = 254 nm). Chloroform was used as the eluent in the HPLC analysis. Ultrapure water with a resistivity of 18.2 MX cm was obtained from a Millipore Milli-Q Plus system (Millipore, Billerica, MA, USA) and used for the experiments. A symmetric double fluorinated-tail surfactant with oxyethylene units, 4FEOS, was provided by Dr. Yukishige Kondo at Tokyo University of Science [10,11]. Cetyltrimethylammonium bromide (CTAB) and sodium octylsulfate (SOS) were used as control surfactants for fluorescence spectral measurements with pyrene. 2.2. Syntheses 8F-B2ES and 8F-DeS were synthesized and purified using the following procedures. The synthetic scheme is shown in Fig. 3. 2.2.1. Synthesis of (Z)-4-oxo-4-(1H,1H,2H,2H-perfluorodecyloxy)but2-enoic acid A mixture of 1H,1H,2H,2H-heptadecafluoro-1-decanol (10.0 g, 21.6 mmol) and maleic anhydride (10.1 g, 103.0 mmol) was stirred at 90 °C for 6 h. After the reaction, brine was added to the residue and the white precipitate in the aqueous mixture was obtained by filtration. The precipitate was then washed sufficiently with dichloromethane and an aqueous HCl solution at pH2 to remove the unreacted fluorinated alcohol and to exchange the Na+ and H+
counterions. Drying the precipitate under vacuum gave a white powder, (Z)-4-oxo-4-(1H,1H,2H,2H-perfluorodecyloxy)but-2-enoic acid (yield 9.85 g, 81.3%): melting point 103.8–104.7 °C; 1H NMR (400 MHz, CF3COOD, TMS) dH/ppm: 2.60–2.75 (m, 2H, CF2CH2 ), 4.74 (t, 2H, OCH2 , J = 6.3 Hz), 6.57 (d, 1H, HCCH , J = 6.1 Hz), 6.66 (d, 1H, HCCH , J = 6.2 Hz); IR (KBr) mmax/cm 1: 2888, 1734, 1701, 1202. 2.2.2. Synthesis of maleic acid diester 2-(2-Butoxyethoxy)ethyl (1H,1H,2H,2H-perfluorodecyl) maleate: (Z)-4-oxo-4-(1H,1H,2H,2H-perfluorodecyloxy)but-2-enoic acid (3.00 g, 6.0 mmol), 2-(2-butoxyethoxy)ethanol (0.866 g, 5.34 mmol), and p-toluenesulfonic acid monohydrate (0.203 g, 1.07 mmol) in 15 mL toluene were refluxed with stirring at 110 °C for 48 h. During the reaction, the water liberated was removed azeotropically from the reaction system to shift the equilibrium of the esterification reaction. After the reaction was complete, ethyl acetate (60.0 mL) was added to the residue, and the mixture was washed sufficiently with brine to remove p-toluenesulfonic acid and unreacted maleic acid monoester; it was then dried over Na2SO4. After removal of the drying agent, toluene and ethyl acetate in the mixture were removed by evaporation. The residue was purified by column chromatography on silica gel with a dichloromethane–ethanol (30:1) mixture, giving a yellow liquid, 2-(2-butoxyethoxy)ethyl (1H,1H,2H,2H-perfluorodecyl) maleate (yield 0.64 g, 17.0%): 1H NMR (400 MHz, CDCl3, TMS) dH/ppm: 0.91 (t, 3H, CH3 , J = 7.5 Hz), 1.36 (sextet, 2H, aliphatic-H, J = 7.48 Hz), 1.57 (quintet, 2H, aliphatic-H, J = 7.1 Hz), 2.50–2.59 (m, 2H, CF2CH2 ), 3.45 (t, 2H, –CH2O–, J = 6.8 Hz), 3.57–3.65 (m, 4H, –CH2O–), 3.74–3.75 (m, 2H, –CH2O–), 4.33–4.35 (m, 2H, – CH2OCO–) 4.50 (t, 2H, CH2OCO , J = 6.6 Hz), 6.25–6.32 (m, 2H, –HCCH–). Decyl (1H,1H,2H,2H-perfluorodecyl) maleate: (Z)-4-oxo4-(1H,1H,2H,2H-perfluorodecyloxy)but-2-enoic acid (4.00 g, 6.0 mmol), 2-(2-butoxyethoxy)ethanol (0.950 g, 6.00 mmol), and ptoluenesulfonic acid monohydrate (0.230 g, 1.20 mmol) in toluene (15 mL) were refluxed with stirring at 120 °C for 48 h in a Dean– Stark apparatus. The purification procedure was the same as that for 2-(2-butoxyethoxy)ethyl (1H,1H,2H,2H-perfluorodecyl) maleate, giving a yellow liquid, decyl (1H,1H,2H,2H-perfluorodecyl) maleate (yield 2.56 g, 60.5%): 1H NMR (400 MHz, CDCl3, TMS) dH/ ppm: 0.88 (t, 3H, CH3 , J = 6.90 Hz), 1.21–1.38 (m, 14H, aliphaticH), 1.66 (quintet, 2H, aliphatic-H, J = 6.9 Hz), 2.46–2.61 (m, 2H, CF2CH2 ), 4.15–4.22 (m, 2H, –CH2O–), 4.50 (t, 2H, CH2O , J = 6.6 Hz), 6.22–6.30 (m, 2H, –HCCH–). 2.2.3. Synthesis of hybrid surfactants 8F-B2ES and 8F-DeS A mixture of sodium 1-(1H,1H,2H,2H-perfluorodecyloxycarbonyl)-2-{2-[2-(butyloxy)ethyloxy]ethyloxycarbonyl}ethanesulfonate
Fig. 3. Scheme for hybrid surfactant synthesis.
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and sodium 1-{2-[2-(butyloxy)ethyloxy]ethyloxycarbonyl}-2-(1H, 1H,2H,2H-perfluorodecyloxycarbonyl)ethanesulfonate, 8F-B2ES: 2-(2-Butoxyethoxy)ethyl 1H,1H,2H,2H-perfluorodecyl maleate (0.55 g, 0.8 mmol) was dissolved in 1,4-dioxane (14 mL); the mixture was heated to 50 °C. A solution of sodium hydrogensulfite (0.30 g, 3.3 mmol) in water (5 mL) was added to the mixture. The reaction mixture was stirred under reflux for 60 h. The reaction solvent was removed by evaporation under vacuum at room temperature, and the residue was then washed with 1,4-dioxane to remove the unreacted diester. The product was extracted in a Soxhlet apparatus with acetone to remove excess NaHSO3, and washed with toluene. A white powder, 8F-B2ES, was obtained after vacuum drying (yield 0.176 g, 32.0%); 1H NMR (400 MHz, CF3COOD, TMS) dH/ppm: 0.86 (t, 3H, CH3 , J = 7.3 Hz), 1.30 (sextet, 2H, aliphaticH, J = 7.5 Hz), 1.60 (quintet, 2H, aliphatic-H, J = 7.2 Hz), 2.41–2.57 (m, 2H, CF2CH2 ), 3.25–3.41 (m, 2H, –CH2COO–), 3.70 (t, 2H, CH2O , J = 7.0 Hz), 3.84–3.96 (m, 6H, –CH2O–), 4.35–4.41 (m, 2H, –CH2OCO–), 4.44–4.52 (m, 2H, CH2OCO ), 4.54–4.65 (m, 1H, –HC(SO3Na)–). ; IR (KBr) mmax/cm 1: 2965, 2876, 1740, 1242, 1215; elemental analysis for C22H24F17 NaO9S: found C, 32.1; H, 2.4; S, 4.0; calcd. C, 32.6; H, 3.0; S, 4.0. A mixture of sodium 1-(1H,1H,2H,2H-perfluorodecyloxycarbonyl)-2-(decyloxycarbonyl)ethanesulfonate and sodium 1-(decyloxycarbonyl)-2-(1H,1H,2H,2H-perfluorodecyloxycarbonyl)ethanesulfonate, 8F-DeS: Decyl 1H,1H,2H,2H-perfluorodecyl maleate (2.00 g, 2.8 mmol) was dissolved in 1,4-dioxane (50 mL); the mixture was heated to 50 °C. A solution of sodium hydrogensulfite (1.23 g, 11.8 mmol) in water (15 mL) was added to the mixture. The reaction mixture was stirred under reflux for 60 h. The purification procedure for 8F-DeS was the same as that for 8F-B2ES. A white powder, 8F-DeS, was obtained (yield 1.47 g, 65.1%); 1H NMR (400 MHz, CF3COOD, TMS) dH/ppm: 0.89–0.94 (m, 3H, CH3 ), 1.29–1.49 (m, 14H, aliphatic-H), 1.74–1.85 (m, 2H, aliphatic-H), 2.55–2.73 (m, 2H, CF2CH2 ), 3.41–3.56 (m, 2H, – CH2COO–), 4.28–4.43 (m, 2H,–CH2OCO–), 4.61–4.78 (m, 3H, CH2OCO and –HC(SO3Na)–).; IR (KBr) mmax/cm 1: 2963, 2931, 2859, 1735, 1243, 1219; elemental analysis for C24H28F17NaO7S: found C, 35.6; H, 3.5; S, 3.9; calcd. C, 35.7; H, 3.5; S, 4.0. 2.3. Liquid crystal properties The initial phase assignments for surfactant/water mixtures were determined by thermal optical microscopy using a polarizing microscope (BX-51, Olympus) equipped with a thermal stage (TS62, INSTEC), and a temperature controller (STC200, INSTEC). 4FEOS and 8F-B2ES solutions for microscopic observation were prepared at room temperature, with no external force except stirring. As 8F-DeS had a low affinity with water, preparation of sample solutions was carried out at 70 °C. In order to avoid evaporation of water from the mixture, a polymer gel was used to seal a cover glass onto the glass slide on which the mixture was placed. The heating and cooling rates were both 2 °C min 1. As phase transition of the liquid crystals was found for the aqueous 8F-B2ES mixtures, the turbidity of these mixtures was measured with a double-beam spectrometer (U-2810, Hitachi High-Technologies, Co., Tokyo, Japan) as a function of temperature. Absorbance at 700 nm was employed for turbidity. To examine changes in microenvironmental polarity in the hydrophobic region of the liquid crystals, fluorescence spectral measurements were conducted at temperatures of 15–70 °C with a fluorescence spectrometer (RF-5300, SHIMADZU, Kyoto, Japan, resolution: 0.2 nm at room temperature) and pyrene as a fluorescence probe. The microenvironmental polarities around the pyrene molecule were found from the ratio of the first (373 nm) to the third (384) vibronic peak intensities (I1/I3) in the fluorescence spectrum for pyrene [31,43–45].
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For fluorescence spectral measurement, a hybrid surfactant/ pyrene solution was prepared using the following procedure. A pyrene/ethanol solution (0.40 mL, 0.10 mM) was placed in a test tube, and the ethanol was then completely removed by evaporation under vacuum. After ethanol removal, a 10 wt.% surfactant/ water mixture (4.0 mL) was added to the test tube, and the mixture was slowly stirred for 1 h. After allowing the sample solution to stand for 1 day at room temperature, the fluorescence spectra of pyrene were measured between 335 and 600 nm at an excitation wavelength of 335 nm. Measurements were also made on 10 wt.% surfactant/water mixtures without pyrene to obtain the actual pyrene fluorescence spectra.
3. Results and discussion The liquid crystal behavior of the aqueous surfactant solutions at concentrations of 10–30 wt.% were examined as a function of temperature by optical microscopy. In the case of the fresh 4FEOS solutions prepared at room temperature, optically anisotropic capsular aggregates with multilayer structures were found without irradiation or any other eternal forces except stirring, as shown in Fig. 4, representing the formation of multilamellar vesicles (MLVs). Some of the MLVs were quite large (ca. 100 lm) and stable for at least 3 days. Although on heating MLVs become smaller and the population decreased by increasing the surfactant solubility, some MLVs remained at 70 °C. However, no specific morphology change was observed as MLV on heating and cooling processes for the 4FEOS solutions. The aqueous solutions of 8F-DeS and 8F-B2ES at 10–30 wt.% exhibited spherical aggregates with maltese cross-textures in the polarized micrographs (Fig. 5), which are often found in typical MLV systems [46,47]. For the 8F-DeS lamellar aggregates, the textures became unclear at temperatures higher than 70 °C. Loss of textural clarity at higher temperatures is usual for a typical liquid crystal system, and results from a lowering of the order parameter caused by increases in thermal molecular motion [48]. In contrast to the 8F-DeS lamellas, the 8F-B2ES lamella textures were clear at higher temperatures, even at 70 °C (Fig. 5d). At temperature higher than 40 °C, optically isotropic aggregates were also observed (Fig. 5e and i), and identified as a sponge or bicontinuous cubic liquid crystal [49], which has a bilayer structure as well as a lamellar phase. The coexistence of two types of liquid crystal (lamellar and isotropic liquid crystal phases) in one solution is quite rare, and probably results from the presence of two 8FB2ES isomers. Interestingly, at temperatures lower than 40 °C, the maltese cross-textures of the lamellas were distorted (Fig. 5f and iv) and the isotropic aggregates became anisotropic (Fig. 5f and ii). The textures in (iv) and (ii) in Fig. 5 reverted to those of (iii) and (i), respectively, when the temperature was increased. This study could not determine which type of liquid crystal the anisotropic one was (Fig. 5ii). Although texture changes from (i) to (ii) and from (iii) to (iv) in Fig. 5 were not observed for the lamellar phases of 8F-DeS and 4FEOS, the unique phase transition could come from oxyethylene units in the hybridized hydrocarbon tail. The turbidity of a surfactant solution depends on the refractive index of the molecular assembly, for example, changes in morphology, molecular packing, and ordering [50]. Through turbidity measurements of the aqueous 8F-B2ES solutions, the temperatures at which phase transition occurred were studied at surfactant concentrations of 15 wt.% and 30 wt.%. Fig. 6 shows the changes in turbidity as a function of temperature. The arrows in the graph show the kinks which were observed in the linear relationships between turbidity and temperature. These temperatures were identified as phase transition temperatures: 36 °C for 15 wt.% and 39 °C for 30 wt.% 8F-B2ES solutions. Although a similar decrease in turbidity
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Fig. 4. Optical micrographs of aqueous 10 wt.% 4FEOS solution at 30 °C. Micrographs (a and b) were taken under normal and crossed Nicol prism conditions, respectively.
(a)
(b)
(c)
(d)
(e)
(f)
(i) (iv)
(iii) (ii)
(ii)
(i)
(iii)
(iv)
Fig. 5. Polarized micrographs of aqueous 30 wt.% hybrid surfactant solutions at (a, d) 70 °C, (b, e) 40 °C, and (c, f) 25 °C. Textures in (e and f) were magnified and shown at the bottom in (i–iv). The textures (a–c) are for 8F-DeS and the others are for 8F-B2ES.
2.4
Turbidity / a.u.
2.0 15 wt% 30 wt%
1.6
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20
30
40
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70
Temperature / ºC Fig. 6. Changes in turbidity for 15 or 30 wt.% aqueous 8F-B2ES mixtures as a function of temperature.
with increasing temperature was reported for a typical liposome system when a lamellar gel (Lb) to liquid crystalline (La) phase transition occurs [19–23], the turbidity decrease for the 8F-B2ES lamellar phase would come from changes in oxyethylene unit hydration, not from an Lb-to-La phase transition, because of the highly hydrated 8F-B2ES lamellar bilayer at temperatures below 40 °C, as noted later. To confirm whether or not the hydration mechanism shown in Fig. 2 occurs, I1/I3 values of pyrene solubilized in the surfactant lamellar phase were measured. Fig. 7 shows the relationship between temperature and I1/I3 for pyrene in lamellar aggregate solutions. High turbidity at 30 wt.% solutions affected the fluorescence spectra of pyrene, and then 10 wt.% with a low turbidity was employed instead of 30 wt.%. For pyrene in pure water, I1/I3 was almost the same (2.02–2.06) at temperatures of 25–65 °C. These values suggest a high polar environment surrounding the pyrene molecules when compared with I1/I3 values in typical organic solvents [44]: 0.85 for n-octanol, 0.90 for n-hexanol, and 0.99 for n-
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2.2
Conformation change
(I)
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50
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1 hydrophobic and 2 hydrophilic groups
Fig. 8. Steric structures of the 8F-B2ES isomers (X2OCOCH2CH(SO3Na)COOX1, where X1 = C8F17CH2CH2– and X2 = C4H9OCH2CH2OCH2CH2–).
Temperature / ºC Fig. 7. Temperature dependencies of I1/I3 for pyrene solubilized in 10 wt.% hybrid surfactant/water mixtures or pure water.
propanol at 25 °C. In the case of the 8F-DeS lamellar phase, I1/I3 was 1.28 and almost independent of temperature. The I1/I3 value was similar to that for an aqueous hydrocarbon surfactant lamellar phase. For example, this study found the I1/I3 values to be 1.85–1.9 for SOS micelles (spherical [51]), 1.39–1.42 for mixed CTAB/SOS micelles (rod-like [51], CTAB:SOS molar ratio = 4:1), and 1.26– 1.28 for mixed CTAB/SOS lamellar systems (vesicular [51], CTAB:SOS molar ratio = 1:4) at a total surfactant concentration of 1 wt.% at 25–75 °C. In addition, DPPC liposome solutions were reported to have I1/I3 of 1.10–1.05 at temperatures from 28 °C (Lb phase) to 38 °C (Pb phase) [52]. Despite 8F-DeS having a strong hydrophobic fluorocarbon tail, the I1/I3 values were similar to those of a hydrocarbon surfactant lamellar phase, compared with those of fluorocarbon systems. For example, I1/I3 values were reported to be 1.00 in perfluorooctane [53], and 1.05 in the mixed micelle of the equimolar hexadecytrimethylammonium chloride(CTAC)/ 1,1,2,2-tetrahydroheptadecafluorodecylpyridinium chloride (HFDePC) mixture, which was significantly lower than 1.30 in CTAC micelle [54]. This suggests that the pyrene molecules were surrounded by microsegregated hydrocarbon chains (i.e., a hydrocarbon microdomain) of 8F-DeS molecules, avoiding contact with oleophobic fluorocarbon chains. In addition, the constant I1/I3 values for 8F-DeS imply that the partition of pyrene molecules between the water and the 8F-DeS lamellar bilayer did not change at any temperature. In contrast, for the 8F-B2ES lamellar phase, I1/I3 at 70 °C was found to be 1.45, which was slightly higher than that of 8F-DeS, and close to that of the rod-like micelles in the mixed CTAB/SOS system. However, the I1/I3 value increased with decreasing temperature, and became almost constant (1.9) at temperatures below 40 °C. The I1/I3 value of 1.9 is the same as that of a highly hydrated SOS micelle, even though 8F-B2ES lamellar aggregates remained. Similar to 8F-B2ES lamellar aggregates, an increase in I1/I3 with decreasing temperature was observed for nonionic micelles of Pluronic PEO-PPO-PEO (PEO, poly(ethylene oxide); PPO, poly(propylene oxide)) block copolymers (P65, P85, and P105) in water; I1/I3 values of these micelles increased from 1.4 to 2.0 when temperature decreased from 50 to 10 °C [45]. This indicates that, with decreasing temperature, hydration of the oxyethylene units led to the interior of the 8F-B2ES bilayer becoming more hydrophilic, suggesting generation of nano-aqueous pores, as shown in Fig. 2b. In addition, we noted that the temperature at which I1/I3 starts to become constant on cooling was coincident with that causing distortion of the fine 8F-B2ES lamellar aggregates. This suggests that the hydrated oxyethylene microdomains at temperatures below 40 °C loosened the molecular packing and then caused distortion of the lamellar aggregates. With regard to distortion of the 8F-B2ES lamellar aggregates, not only the nano-aqueous
pore but also any other effects were expected to occur at temperatures below 40 °C. As noted in Section 2.1, 8F-B2ES was composed of two isomers: (i) X1OCOCH2CH(SO3Na)COOX2, and (ii) X2OCOCH2CH(SO3Na)COOX1, where X1 = C8F17CH2CH2– and X2 = C4H9OCH2CH2OCH2CH2–. For isomer (ii), as shown in Fig. 8, the X2 tail could orientate from the fluorocarbon tail side (I) to the sulfonate one (II) along the H2C–CH(SO3Na) bond axis. When hydration of the oxyethylene units was promoted at lower temperatures, the conformation ratio of (II):(I) was expected to increase, which would decrease the apparent packing parameter of 8FB2ES. This could be one of the reasons for distortion of the 8FB2ES lamellar aggregates.
4. Conclusion With the aim of forming quasi-ion channels within a bilayer, this study designed and synthesized the hybrid surfactant 8FB2ES, which has a fluorocarbon tail and a hydrocarbon tail with two oxyethylene units. 8F-B2ES was expected to yield oxyethylene microdomains in its bilayers because of the strong oleo- and hydrophobicities of the fluorocarbon tail in the hybrid structure; the microdomain would then act as a quasi-ion channel. Lowering the temperature increases hydration of the oxyethylene units, and the well-hydrated microdomains could allow passage of small ions or hydrophilic compounds. A symmetric double fluorinated-tail surfactant with one oxyethylene unit in each tail (4FEOS) and a 8F-B2ES-analog surfactant with no oxyethylene units (8F-DeS) were used as control surfactants. For two control surfactant/water mixtures, textures showing lamellar aggregates were clear at low temperatures, but became unclear with increasing temperature. In contrast with those of the control surfactant systems, the 8F-B2ES lamellar aggregates were distorted at temperatures below 40 °C, although the textures were fine at higher temperatures. The fluorescence spectra of pyrene solubilized in 8F-B2ES and 8F-DeS lamellar phases were used to investigate the polarity in the bilayer interiors to confirm whether or not the oxyethylene units acted as the well-hydrated microdomain. The interiors of the 8F-DeS bilayers exhibited a low polarity, similar to that of a typical hydrocarbon surfactant lamellar phase, and no temperature dependence for the polarity. On the other hand, although the polarity of the 8F-B2ES bilayer at 70 °C was close to that for a hydrocarbon surfactant lamellar phase, it gradually increased with decreasing temperature, and at temperatures below 40 °C became similar to that for a typical hydrophilic micelle. The presence of highly hydrophilic microenvironments in the 8F-B2ES bilayer suggested formation of nano-aqueous pores with oxyethylene units. At temperatures below 40 °C, the polarity was independent of temperature. On cooling, for 8F-B2ES lamellar aggregates, the temperature at which the polarity became constant was almost consistent
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with that for distortion of lamellar aggregates. As the 4FEOS did not display distorted lamellar aggregates at any temperature, hydrated oxyethylene microdomains within the 8F-B2ES bilayers could be a trigger for distortion of the lamellar aggregates. For a simple binary mixture of an ionic surfactant and water [3– 54], the increased microenvironmental polarity with lowering temperature is unusual if lamellar aggregates remain at any temperature. In addition, the drastic distortion of lamellar aggregates at a certain temperature was quite rare on the cooling process. From these standpoints, the new surfactant design giving 8F2BES, namely introduction of oxyethylene units into hybrid-structured alkyl tail, could lead to these interesting phenomena, suggesting formation of temperature-responsive nano-aqueous pores and softening of the 8F-2BES bilayers on cooling. In the case of the fluorocarbon–hydrocarbon hybrid surfactant with a temperature-sensitive ferrocene group [42], oxidation as well as an increase in temperature was found to induce interesting changes in interfacial properties (critical micelle concentration and surface tension of aqueous solution) and aggregation morphology (transitions of coil-like to vesicle-like aggregates or vesicle-like to no aggregates), but these were not connected with results in this study. Our further study will synthesize novel hybrid surfactants, and examine changes in their interfacial properties, nanostructure, and material-release property of liquid crystal as a function of temperature. To yield liquid crystals with an unusual morphology/nanostructure, or liquid crystals that are both lyotropic and thermotropic (amphitropic liquid crystal), thermotropic mesogens would be an interesting group to consider as an alternative to fluorocarbons in the hybrid structure, as they also have low miscibility with hydrocarbon and oxyethylene units [48]. These will advance our molecular design for hybrid surfactant to achieve formation of a quasi-ion channel and its specific application as a new carrier (e.g., DDS to avoid frostbite and capsules for an antifreezing agent). In addition, if the conformation-change rate of the hybrid surfactant (as shown in Fig. 8) can be controlled by an advanced surfactant design, it might give a dynamic and time-periodic change of lyotropic liquid crystal morphology (or a dissipative structure) though it has been often reported in thermotropic systems [48]. Acknowledgments This work was partly supported by a Grant for Priority Research Designated by a Grant for Hirosaki University Institutional Research, a Grant for Priority Research designated by Dean of Graduate School of Science Technology, and Oil & Fat Industry Kaikan Foundation. We thank Dr. Y. Kondo for providing a fluorinated double-tail surfactant 4FEOS. References [1] G.J. Siegel, B.W. Agranoff, R.W. Albers, S.K. Fisher, M.D. Uhler, Basic Neurochemistry: Molecular, Cellular, and Medical Aspects, Lippincott-Raven, Philadelphia, 1999. [2] J.-P. Sauvage, in: M.W. Hosseini (Ed.), Comprehensive Supramolecular Chemistry, vol. 9, Pergamon, UK, 1996. [3] T. Kunitake, Y. Okahata, J. Am. Chem. Soc. 99 (1977) 3860. [4] T. Kunitake, Y. Okahata, K. Tamaki, F. Kumamaru, M. Takayanagi, Chem. Lett. (1977) 387. [5] J.H. Fendler, Acc. Chem. Res. 13 (1980) 7. [6] T. Kunitake, Y. Okahata, J. Am. Chem. Soc. 102 (1980) 549.
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