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Solution Properties of Double-Tailed Cationic Surfactants Having Ferrocenyl Groups in Their Hydrophobic Moieties Yasushi Kakizawa,† Hideki Sakai,† Katsuhiro Nishiyama,† and Masahiko Abe*,†,‡ Faculty of Science and Technology, Science University of Tokyo, 2641 Yamazaki, Noda, Chiba 278, Japan, and Institute of Colloid and Interfacial Science, Science University of Tokyo, 1-3 Kagurazaka, Shinjuku, Tokyo 162, Japan
Hideki Shoji,§ Yukishige Kondo,§ and Norio Yoshino‡,§ Faculty of Engineering, Science University of Tokyo, 1-3 Kagurazaka, Shinjuku, Tokyo 162, Japan Received July 20, 1995. In Final Form: November 16, 1995X Solution properties of novel double-tailed cationic surfactants with two ferrocenyl groups introduced to the end of each alkyl chain ({Fc(CH2)n}2N+(CH3)2Br- (n-BFDMA; n ) 5, 11)) have been investigated by light scattering, relative conductivity, and polarized optical microscope measurements. The aggregation form of n-BFDMA is fairly dependent on its concentration. Scattered light intensity from a diluted (10-3 mol/L) solutions of 5- and 11-BFDMA by polarized optical microscopy shows the formation of liquid crystals dispersed in water and liquid crystals with high viscosity, respectively.
Introduction Reversible control of formation and deformation of surfactant micelles has been the subject of significant attention with a view to application as a drug delivery system (DDS) and removal of organic impurities dissolved in water.1-5 For instance, the reversible control of the critical micelle concentration (cmc) with photoirradiation has been reported recently utilizing photoinduced transcis isomerization of the surfactant molecules modified with an azobenzene moiety.1 Conformational changes of surfactant molecules induced by reversible redox reactions may also affect the micelle formation in the surfactant solution. Surfactants modified with the ferrocene moiety have been studied intensively for this purpose.2-6 The solution properties of these surfactants, e.g., surface tension, cmc, and relative conductivity, change drastically with the oxidation of the ferrocenyl group. By utilizing the change in the solution properties by the redox reactions, the ferrocene-modified surfactants have been applied to the micelle electrolysis * To whom all correspondence should be addressed: Tel, 81-471-24-8650; FAX, 81-471-24-8650; e-mail, abemasa@ koura01.ci.noda.sut.ac.jp. † Faculty of Science and Technology, Science University of Tokyo. ‡ Institute of Colloid and Interfacial Science, Science University of Tokyo. § Faculty of Engineering, Science University of Tokyo. X Abstract published in Advance ACS Abstracts, February 1, 1996. (1) Hayashita, T.; Kurosawa, T.; Miyata, T.; Tanaka, K.; Igawa, M. Colloid Polym. Sci. 1994, 272, 1611. (2) Saji, T.; Hoshino, K.; Aoyagui, S. J. Am. Chem. Soc. 1985, 107, 6865. (3) Saji, T.; Hoshino, K.; Aoyagui, S. J. Chem. Soc., Chem. Commun. 1985, 865. (4) Saji, T.; Hoshino, K.; Ishii, Y.; Goto, M. J. Am. Chem. Soc. 1991, 113, 450. (5) Saji, T. Yukagaku 1990, 39, 717. (6) Tajima, K.; Huxur, T.; Imai, Y.; Motoyama, I., Nakamura, A.; Koshinuma, M. Colloids Surf. A 1995, 94, 243.
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method,2 which is a promising method for preparing organic thin films. Various kinds of ferrocene-modified surfactants with a single alkyl chain have been synthesized, and their solution properties have been studied in detail.2-6 However, no study on the solution properties of surfactants bearing two alkyl chains and two ferrocenyl groups in one molecule has yet been reported. In the present study, the solution properties of novel cationic surfactants introducing ferrocenyl groups to the ends of the hydrophobic moieties of a double-tailed alkyl ammonium salt7 have been investigated by the measurement of surface tension, light scattering, relative conductivity, and so on. Experimental Section 1. Syntheses and Materials. Synthetic processes of bis(n-ferrocenylalkyl)dimethylammonium bromide ({Fc(CH2)n}2N+(CH3)2Br- (n-BFDMA; n ) 5, 11)) are the same as reported previously.7 The synthetic products were characterized by use of FT-IR spectra, pulsed Fourier transform 500-MHz 1H NMR, and mass spectra. Didodecylammonium bromide was purchased from Tokyo Kasei, Co., and water for injection was supplied by Otsuka Pharmaceutical Co. 2. Measurements. Surface Tension Measurements. The surface tension of n-BFDMA aqueous solutions was determined with a surface tensiometer (Kyowa Interface Science Co., Tokyo, Japan, Model CBVP-A3) using a platinum plate at 30 °C. Light-Scattering Measurements. The diameter and the lightscattering intensity of the surfactant’s molecular aggregates in aqueous solution were determined with a submicrometer particle analyzer (Malvern Instrument, Worcestershire, U.K., Model 4700) with an argon ion laser operating at 488 nm (Coherent Co., Palo Alto, CA, Model Innova 90). Measurements were performed at a scattering angle of 90° at 30 °C, and a glass cell and an optical quartz cell were used for the dynamic light(7) Yoshino, N.; Shouji, H.; Kondo, Y.; Kakizawa, Y.; Sakai, H.; Abe, M. Yukagaku, submitted for publication.
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Figure 1. Surface tension (γ) of n-BFDMA solutions as a function of surfactant concentration. scattering method and the static light-scattering method, respectively. Specific Conductivity Measurement. The specific conductivity of the n-BFDMA solution was measured with a conductivity meter (TOA Electronics Ltd., Tokyo, Japan, Model CM-60S) at 30 °C. Observation of Liquid Crystal Formation. The phase behavior of n-BFDMA/water mixtures was observed with a polarizing plate and a differential interference optical microscope (Olympus Optical Co., Tokyo, Japan, Model IMT-2). The liquid crystalline phase was observed using the optical microscope under an optically crossed nicol condition. Observation of Vesicle Formation. Vesicles were observed with a transmission electron microscope (TEM) (JEOL, Tokyo, Japan, Model JEM-100SX). TEM observation was performed with the negatively-staining method.8,9 As soon as the surfactant solution and an aqueous solution of 2% phosphotungstic acid (PTA, pH ) 7) were mixed volumetrically at a ratio of 2:1, the resultant solution was added dropwise to a 150-mesh copper grid coated with collodion sprayed with a carbon film. Excess droplets were instantly removed using filter paper, and then the grid was dried in a vacuum desiccator for 6 h as a TEM sample. Glucose Trapping Experiment. The glucose trapping experiments were also employed to confirm the existence of vesicles in the solution. BFDMA/water mixtures, prepared by using 0.28 mol/L glucose aqueous solutions instead of solvent water, were put into a regenerated cellulose tube for dialysis (13 000 Dalton molecular weight cutoff, Viskase Sales Co., Chicago, IL). The unencapsulated glucose was transferred into 4 L of an isotonic Na2SO4 aqueous solution (0.12 mol/L) over a 10-h period (using 1 L of Na2SO4 solution at 2.5-h intervals) at 0 °C. After dialysis, the glucose encapsulated in the vesicles was detected by using a color-product agent for glucose, glucose CII-test Wako (Wako Pure Chemical Industry Co., Tokyo, Japan). The color-producing process is as follows.10 The glucose is oxidized by glucose oxidase in the CII-test solution, and then hydrogen peroxide is formed. The hydrogen peroxide works quantitatively as a catalyst to the reaction of 4-aminoantipyrine and phenol in the test solution and consequently produces an absorbency at 505 nm).
Results and Discussion Properties of Dilute n-BFDMA Solutions. The Krafft point of 5-BFDMA was below 0 °C. On the other hand, the Krafft point of 11-BFDMA was 21 °C, suggesting that the solubility of n-BFDMA decreases with increasing chain length of the hydrophobic moiety. Figure 1 represents the surface tension of the n-BFDMA solutions as a function of the concentration at 30 °C. The surface tension decreases by increasing the n-BFDMA concentration, and critical micelle concentrations (cmc) are estimated to be 3 × 10-5 mol/L (n ) 5) and 6 × 10-6 mol/L (n ) 11), respectively. These values are smaller (8) Lau, A. L. Y.; Chan, S. I. Biochemistry 1974, 13, 4942. (9) Schullery, S. E.; Schmidt, C. F.; Felgner, P.; Tillack, T. W.; Thompson, T. E. Biochemistry 1980, 19, 3919. (10) Miwa, I.; Okuda, J.; Maeda, K.; Okuda, G. Clin. Chim. Acta 1972, 37, 538.
Figure 2. Change in the intensity of light scattered by n-BFDMA solutions as a function of the surfactant concentration.
Figure 3. Relationship between specific conductivity and 11BFDMA concentration.
than that for the ferrocene-modified surfactant with a single hydrophobic tail (8 × 10-5 mol/L for n ) 11). The lowest values of the surface tension in the presence of 5-BFDMA and 11-BFDMA are 42.5 mN/m; i.e., they do not depend on the tail length of hydrophobic moieties. The intensity of the light scattered by the aqueous n-BFDMA solution is measured as a function of the concentration, which is shown in Figure 2. In this figure, the intensity represents the value of the scattered light intensity from the surfactant solutions relative to that from pure benzene. The scattered light intensity from 5-BFDMA aqueous solution shows an abrupt increase at a concentration greater than 3 × 10-5 mol/L, and this concentration agrees with the lowest concentration that gave the minimum value of surface tension (Figure 1). In this concentration range the 5-BFDMA solution is isotropic, suggesting that 5-BFDMA molecules form micelles at a concentration greater than 3 × 10-5 mol/L. The intensity of the scattered light from 11-BFDMA solution increases at concentrations greater than 1 × 10-7 mol/L (C1 in Figure 2), which is followed by a temporary decrease at a concentration around 4 × 10-6 mol/L (C2); the intensity increases again at concentrations above 6 × 10-6 mol/L (C3). The intensity of the scattered light may reflect the number and the size of the molecular aggregates. Thus the results obtained here suggest that the aggregates forming at concentrations between C1 and C2 change their form at the C2 concentration. In order to investigate the change in aggregation forms in more detail, measurements of the conductivity of the 11-BFDMA solution have been carried out. Figure 3 shows the relationship between the concentration of 11-BFDMA and the solution conductivity. Panels a and b of Figure
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Figure 5. Phase diagram for the system of n-BFDMA/water at 30 °C.
Figure 4. TEM micrographs of vesicles formed from 1.25 × 10-2 mol/L 11-BFDMA solution: (a) without sonication (magnification, 10000×); (b) after 5 min of sonication (magnification, 50000×).
3 show the solution conductivity at concentrations (a) below C2 (around 4 × 10-6 mol/L) and (b) below C3 (around 10 × 10-6 mol/L), respectively. As shown in Figure 3a, the solution conductivity increases linearly with increasing the concentration of 11-BFDMA up to 4 × 10-7 mol/L, and above this concentration the slope of the line changes. The concentration at which the change in the slope was observed agrees with the C1 obtained by the lightscattering measurements. The conductivity decreases once around the C2 concentration and then increases again at concentrations above C3. The average sizes of the 11-BFDMA aggregates evaluated by the light-scattering measurements were 80 nm below the C2 concentration and 300 nm at the C3 concentration, respectively. Optical microscopic observation using polarized light shows that aqueous solutions of 11-BFDMA above the C3 concentration have optical anisotropy. Furthermore, small particles with a ca. 1 µm size are observed under optical microscopic observation. TEM Observation of Vesicle Formation. Figure 4a represents a TEM picture of the 11-BFDMA aggregates at the C3 concentration. As shown in Figure 4, the bilayer structure and the existence of the inner aqueous phase are observed, confirming that 11-BFDMA is capable of forming a spontaneous vesicle at this concentration. Furthermore, vesicles with smaller size are obtained by sonication of the solution. Glucose retention experiments confirm the existence of the inner aqueous phase and indicate the measured retention efficiency to be ca. 3%. The change in aggregation forms of 11-BFDMA with increasing concentration can be summarized as follows. At concentrations above C1 (1 × 10-7 mol/L), small molecular aggregates with an average particle size of ca. 80 nm are formed. The aggregate gradually changes its form at concentrations between C2 and C3 and gradually forms the spontaneous vesicle above the C3 (6 × 10-6 mol/ L) concentration. Observation of the Phase Behavior of Concentrated n-BFDMA Solutions. Figure 5 shows the phase diagram of concentrated 5- and 11-BFDMA aqueous
Figure 6. Lamellar phase texture observed by polarized optical microscope at room temperature (6.23 × 10-2 mol/L 11-BFDMA solution, 650×).
solutions. 5-BFDMA solutions are optically isotropic at concentrations below 1.5 wt % (0-2.37 × 10-2 mol/L), while above this concentration the solution shows anisotropic properties. The fluidity of the concentrated solution, however, does not show a drastic decrease, suggesting that in this concentration range liquid crystals are dispersed in the solution (Lc + W phase). On the contrary, 11-BFDMA solutions, which can form spontaneous vesicles in the diluted concentration range, lose their fluidity at a concentration of 1.6 wt % (2.00 × 10-2 mol/L) and change into liquid crystals with high viscosity. Polarized microscopic observation of a 5 wt % (6.23 × 10-2 mol/L) 11-BFDMA solution gives a mosaic texture pattern as shown in Figure 6, suggesting the formation of lamellar phase liquid crystals. The ferrocenyl moiety itself is known to act as a mesogen of liquid crystal.11 Thus, liquid crystal formation of the 11-BFDMA solution at a relatively low concentration may be due to the increase of the hydrophobicity by introduction of the ferrocenyl moiety and also to the enhanced molecular interaction between ferrocenyl groups. Discussion Aggregation States of 11-BFDMA. The relative conductivity of 11-BFDMA solutions decreased at concentrations between 4 × 10-6 and 1 × 10-5 mol/L as shown in Figure 3. In this concentration region, formation of spontaneous vesicles was observed. In order to investigate the relationship between vesicle formation and the decrease of solution conductivity in more detail, the change in solution conductivity was measured when the vesicles of DDAB (didodecylammonium bromide), a typical surfactant molecule with two hydrophobic tails, are formed by ultrasonic treatment. Figure 7 shows the change in the specific conductivity of a 10 mM DDAB solution as a function of sonication time. The change in the vesicle (11) Singh, P.; Rausch, M. D.; Lentz, R. W. Liq. Cryst. 1991, 9, 19.
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Figure 7. Time course change in specific conductivity and particle size induced by the “sonication”-induced vesicle formation of DDAB molecules.
size is also shown in the same figure. By sonication treatment within less than 3 s, vesicles ca. 100 nm in size were formed and the vesicle size gradually decreased with increasing sonication time. On the other hand, the relative solution conductivity showed an abrupt decrease accompanied by vesicle formation and it slightly increased again after long-time sonication. From the results it was shown that vesicle formation with the surfactant molecules decreased the solution conductivity. Thus the decrease in the specific conductivity of 11-BFDMA solutions at concentrations from 4 × 10-6 to 1 × 10-5 mol/L is caused by spontaneous vesicle formation. The reason for the conductivity decrease by vesicle formation may be due to the fact that the counterions in the inner aqueous phase are separated by the alkyl chains of the bilayer membrane, resulting in a decrease of the electric charge. As the vesicle size becomes smaller with increasing sonication time, the solution conductivity increases again as shown by Figure 7. This may be caused by enhanced dispersion stability of the small-size vesicles. Molecular Geometry Required for Spontaneous Vesicle Formation. It is generally accepted that the structure of molecular aggregates strongly depends on the geometrical properties of the surfactant molecules. For example, DDAB, which has the same molecular structure as BFDMA except that the ferrocenyl group is
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replaced by the methyl group, is a cylinder type molecule. This surfactant molecule forms a lamellar type liquid crystal and cannot form vesicles spontaneously. 11BFDMA has two ferrocenyl groups, which have a larger volume than the methyl group; thus the molecular geometry is expected to be a corn type. Aqueous solutions of 11-BFDMA, however, form lamellar type liquid crystals, suggesting that the ferrocenyl groups of the molecule interact with each other and the molecular geometry of 11-BFDMA becomes similar to a cylinder type rather than a corn type. Furthermore, spontaneous vesicle formation of 11-BFDMA above the C2 concentration may be caused by the following: the area occupied by the hydrophilic moiety decreases with increasing surfactant concentration, i.e., increasing ion strength. This results in a change in the molecular geometry to a rod-like one, which is an optimum geometry to form vesicles. We are currently studying the reversible formation and deformation of 11-BFDMA vesicles by utilizing the redox reactions of ferrocene moieties, which will be reported elsewhere. Conclusion Novel cationic surfactants modified with ferrocenyl groups introduced to the end of hydrophobic moieties (nBFDMA, n ) 5, 11) were synthesized. The phase behavior of the n-BFDMA aqueous solution was investigated, and the following results were obtained. 1. 5-BFDMA changes its aggregation form from monomer, through micelle, to liquid crystal dispersed in the isotropic solution with increasing concentration. On the other hand, the aggregation form of 11-BFDMA changes from monomer, through small molecular aggregate, a dispersed liquid crystal, to a lamellar phase liquid crystal with high viscosity. 2. Vesicles are formed spontaneously in the dispersed liquid crystal phase of the 11-BFDMA solution. 3. Spontaneous vesicle formation may be caused by interaction between the ferrocenyl groups, which makes the geometry of the 11-BFDMA molecule rod-like. LA950600P
