Phase Behavior of Salt-Free Catanionic Surfactant Aqueous Solutions

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J. Phys. Chem. B 2007, 111, 7719-7724

7719

ARTICLES Phase Behavior of Salt-Free Catanionic Surfactant Aqueous Solutions with Fullerene C60 Solubilized Hongguang Li and Jingcheng Hao* Key Laboratory of Colloid and Interface Chemistry, Shandong UniVersity, Ministry of Education, Jinan 250100, P. R. China ReceiVed: February 16, 2007; In Final Form: May 9, 2007

A salt-free catanionic surfactant system, tetradecyltrimethylammonium laurate (TTAL), was constructed by mixing tetradecyltrimethylammonium hydroxide (TTAOH) and lauric acid (LA). The H+ and OH- counterions form water (TTAOH + LA f TTAL + H2O), leaving the solution salt-free. The phase behaviors at fixing the total surfactant concentration (cTTAL) to be 33.0 and 55.0 mmol L-1, respectively, were studied through varying the molar ratio of r ) nLA/nTTAOH from 0.70 to 1.20. With an increasing value of r, one observed an L1-region, an LR/L1 two-phase region with a birefringent LR-phase at the top, and finally a single LR-phase. The ability to solubilize a fullerene mixture of C60 and C70 of different phases in different regions was tested. The colloidal stability and phase behavior of different phases with embedded fullerenes were investigated as a function of r, cTTAL, and weight ratio of fullerene to surfactant (WF/WTTAL). The 33.0 or 55.0 mmol L-1 zero-charged vesicle-phase at r ) 1.00 could solubilize a considerable amount of fullerenes without macroscopic phase separation and obvious vesicular structure breakage. However, these colloidal solutions became unstable at lower concentrations of surfactants, and a precipitate would be observed at the bottom. The micellar (L1phase) solubilization at the TTAOH-rich side was less pronounced compared to the vesicular solubilization of the zero-charged vesicle-phase, and the solubilizing ability decreased at higher r values. In the LR/L1 twophase region, a brown or dark-brown LR-phase was usually found at the top of a colorless or yellowish L1-phase, indicating that most of the fullerenes were embedded in the upper LR-phase. The influence of fullerene incorporation on the property of the zero-charged TTAL vesicle-phase was also investigated, and evidence has been found that the system tended to be more fluid after fullerenes were incorporated into the hydrophobic microdomains of aggregates.

Introduction Cationic and anionic (catanionic) surfactant mixtures can form vesicles spontaneously in dilute aqueous solution as reported by Kaler and co-workers in 19891 and research in this area has been a hotspot since then. However, as the concentrations of cationic and anionic surfactants increase, the excess salt formed by their counterions often induces precipitate formation, especially when the stoichiometry between the cationic and anionic components is exactly 1.2 This makes the phase behavior of catanionic surfactant mixtures rather simple and often brings big obstacles for practical applications. In recent years, increasing attention has been paid to salt-free catanionic surfactant mixtures by using OH- and H+ as counterions.3-5 Compared to conventional catanionic surfactant mixtures, salt-free systems could be prepared at much higher concentrations without precipitate formation and usually have rich phase behavior. Typical examples are mixtures of alkyltrimethylammonium hydroxide and alkyl carboxylic acids, where uni- and multilamellar vesicles were clearly observed by a freeze-fracture transmission electron microscope (FF-TEM) when the mixing * Corresponding author. Phone/Fax: E-mail: [email protected].

+86-531-88366074 (office).

molar ratio of the cationic and anionic surfactant is 1:1.5b When the cationic or anionic component is in excess, aggregate transition from micelles to vesicles and other unique selfassembled structures such as flat nanodiscs and regular hollow icosahedra will be observed.4 Fullerenes are highly hydrophobic, which is a big obstacle for their potential applications in biological and material science. Attempts have been made in the past decades to enhance the solubility of fullerenes, especially for C60 in water, through a colloid method including cyclodextrine inclusion 6 as well as micellar or vesicular solubilization.7-13 Recently, we have found that the salt-free zero-charged TTAL vesicle-phase (r ) 1) could greatly enhance the solubility of fullerene C60 in water.14 As a typical result, 0.588 mg mL-1 C60 has been successfully solubilized into a 50 mmol L-1 (22.75 mg mL-1) salt-free zerocharged TTAL vesicle-phase, which is higher than the biggest value reported in the literature (∼0.4 mg mL-1). Since saltfree catanionic surfactant mixtures such as TTAL show a rich phase behavior in aqueous solution and different kind of aggregates may form with the variation of r and total surfactant concentration, further investigations are very necessary to obtain a better understanding of the properties of fullerene/surfactant hybrids and to broaden the scope of possible applications of

10.1021/jp071332u CCC: $37.00 © 2007 American Chemical Society Published on Web 06/19/2007

7720 J. Phys. Chem. B, Vol. 111, No. 27, 2007 fullerene/surfactant hybrids in biological and material science. Also of great concern is the property change of salt-free TTAL aqueous solution with the increasing amount of fullerenes incorporated into their hydrophobic domains. In this article, the colloid stability and phase behavior of fullerene/TTAL/H2O hybrids were investigated as a function of the mixing molar ratio of LA to TTAOH, the concentration of TTAL, and the weight ratio of fullerene to TTAL. The rheological properties of the equimolar mixture of LA and TTAOH with an increasing amount of fullerene embedded were also carried out. Experimental Section Chemicals and Materials. Tetradecyltrimethylammonium bromide (TTABr, analytical grade) was purchased from Merck and used without further purification. Lauric acid (LA, chemical grade, >98%) and toluene were purchased from Shanghai Shiyi Chemicals Reagent Co. Ltd. and used without further purification. The mixture of fullerenes was a gift from FutureCarbon GmbH (Bayreuth, Germany). It contains ∼20% C70 and ∼1% higher fullerenes besides C60 and was purified from toluene before use. Preparation of Salt-Free Catanionic Surfactant Mixtures. TTAOH stock solution was prepared from TTABr aqueous solution by anion exchange (Ion exchanger III, Merck) and followed the procedures described previously.14 The concentration was determined to be 135.3 mmol L-1 by acid-base titration by using standard 111.3 mmol L-1 HCl aqueous solution. A portion of this stock solution was then divided into several equal parts, to which different amounts of solid LA and water were added. Because LA was water-insoluble and could not dissolve totally when it was in excess, heating to ∼60 °C and frequent shaking was performed on the samples with r > 1.00 during sample preparation to get homogeneous solutions. By this means, TTAL aqueous solutions with different concentrations and r values between 0.70 and 1.20 were prepared. The samples were then kept at 25.0 ( 0.1 °C for about four weeks for phase equilibrium, and the determination of phase sequences was carried out by visual inspection and crossed polarizers. After that, each part of these samples was dried below 40 °C to get solid TTAL for further use. During sample preparation, a N2 stream was used where necessary to prevent TTAOH from reacting with CO2 involved in the air. Preparation of Fullerene/TTAL/H2O Hybrids. Salt-free catanionic TTAL aqueous solutions with fullerenes embedded were prepared according to the procedures described previously14 with slight modifications. In brief, to yellow-brown solutions of a fullerene mixture in toluene (note the difference with the color of pure C60 in toluene, which is purple) were added different amounts of solid TTAL with different r values. In most cases, solid TTAL could not dissolve totally and remained at the bottom, especially when r < 1.00. So the mixed samples were then heated to ∼40 °C to get homogeneous solutions before toluene was evaporated with the help of a flower (the maximum temperature during the evaporation process is ∼60 °C). By this means, brown solid fullerene/TTAL hybrids with different r values and different weight ratios of fullerene to TTAL were obtained, which were then weighed accurately and mixed with different amounts of water to get desired concentrations. After being stirred with a glass rod or sonicated at low frequency to accelerate the dissolving process, the samples were kept at 25.0 ( 0.1 °C for more than four weeks before phase behavior and other properties were investigated. Compared to the procedures described previously,14 the main modification of the current procedure is that we performed our

Li and Hao experiment in air without using a N2 atmosphere and the temperature during the solvent evaporation process is a bit higher. UV-vis measurements carried out on a yellow-brown toluene solution of fullerene mixture before and after treatment by the current procedure (i.e., heated to ∼40 °C, evaporating toluene by a flower at ∼60 °C in the air and then redissolved in toluene) showed that the two absorption curves were identical and no obvious changes were found, indicating that the current procedure did not have any detectable influences on the properties of fullerenes and fullerene/TTAL hybrids. A KQ-250DB-type ultrasonicator with a maximum frequency of 40 kHz and a centrifuge with a maximum rotation speed of 4000 rpm were used where necessary during sample preparation. Freeze-Fracture Transmission Electron Microscopy (FFTEM) Observations. A small amount of solution to be characterized was placed on a 0.1 mm thick copper disk covered with a second copper disk. The copper sandwich with the sample was frozen by plunging this sandwich into liquid propane which had been cooled by liquid nitrogen. Fracturing and replication were carried out at a temperature of -140 °C. Pt/C was deposited at an angle of 45°. The replicas were examined in a CEM 902 electron microscope (Zeiss, Germany) operated at 80 kV. Negative-Staining TEM Observations. The structures of vesicular aqueous solutions containing C60 were determined by negative-staining TEM observations. Uranyl acetate was used as negative stained-dye. Approximately 4 µL of aqueous vesicular solution containing C60 was dropped on a TEM grid (copper grid, 3.02 mm, 200 mesh, coated with Formvar film), and stained with ∼4 µL of 2% uranyl acetate aqueous solution. After drying the solution in air, TEM images were taken on a JEOL JEM 100-CXII microscope (Japan) at an accelerating voltage of 80 kV. UV-Vis Measurements. UV-vis measurements were carried out on a computer-controlled spectrometer (UV-vis 4100, HITACHI HIGH-TECHNOLOGIES Co., Japan) at room temperature in a 1 cm path length quartz cell. Conductivity Measurements. Conductivity measurements were carried out on a DDSJ-308A-type instrument at room temperature. Rheological Measurements. Rheological measurements were carried out on a HAAKE RS75 rheometer with a wheeling canister or a cone plate measuring system, depending on the viscosity of the samples. TGA and Polarizer Measurements. Thermogravimetric analysis (TGA) experiments were performed on an SDT Q600 V8.0 Build 95-type instrument. The temperature was increased from room temperature to 500 or 700 °C with a speed of 10 °C/min. Polarized microscopy observations were carried out on an AXIOSKOP 40/40 FL (ZEISS, Germany) microscope. All the experiments were carried out at room temperature unless otherwise stated. Results and Discussion Phase Behavior of Salt-Free Catanionic TTAL Aqueous Solution. For comparison, it is necessary to study the phase behavior of salt-free catanionic TTAL aqueous solutions without fullerene embedded first prior to investigating the phase behavior of fullerene/TTAL/H2O hybrids. In our current experiments, samples were prepared by fixing the TTAL concentration to be constant and by varying r from 0.70 to 1.20. Typically, two series samples with TTAL concentration of 55.0 and 33.0 mmol L-1 were prepared. When r < 1.00, homogeneous solutions could be obtained only by gentle shaking during preparation,

Behavior of Salt-Free Catanionic Surfactant Systems

Figure 1. Phase sequence at the total concentration of cTTAL ) 55.0 mmol L-1 with varied ratios of r between 0.70 and 1.20. The conductivity data (9) and phase volumes (O) were also included. The conductivity data at r ) 0.90 and 0.95 were the averaged values of the upper LR-phase and the bottom L1-phase.

while in the cases of r > 1.00, the water-insoluble LA was very difficult to dissolve totally at room temperature. After being heated to higher temperatures with frequent shaking, homogeneous solutions could also be obtained; once formed, these homogeneous solutions were found to be very stable at room temperature and LA could not separate out again. For the samples with r >1.00, excessive dilution by water often leads to macroscopic phase separation with a white excess LA layer at the top. The higher the r value is, the easier is the phase separation. A similar problem has also been pointed out by Zemb et al. in another salt-free catanionic surfactant system.4b This is very important for understanding the phase behavior of fullerene/ TTAL/H2O hybrids. The phase behaviors of the two series of samples with cTTAL ) 33.0 and 55.0 mmol L-1, respectively, were very similar to each other, and Figure 1 gives the phase sequence of the samples with cTTAL ) 55.0 mmol L-1 and 0.70 e r e 1.20. With increasing r value, one observes an L1-phase (the L1-phase is a micellar phase) whose viscosity increased with increasing r value, an LR/L1 two-phase region with a transparent birefringent LR-phase (LR-phase is a vesicle-phase or a stacked bilayer phase) at the top, and finally a bluish or slightly turbid LR-phase exhibiting weakly flowing birefringence. In the two-phase region, the volume ratio of the LR-phase to the L1-phase increased gradually with increasing r value. The conductivity decreased continuously from 2.26 to 0.06 mS cm-1 in the TTAOH-rich side and then remained almost constant in the LArich side, indicating a rather low dissociation degree of excess LA. This phase sequence was similar to that of the previously studied system by fixing the concentration of TTAOH to be 100 mmol L-1 and adding solid LA at room temperature.5b The LR-phase at r ) 1.00 consists of vesicles which can be determined by FF-TEM observations. A typical FF-TEM image of 55.0 mmol L-1 zero-charged TTAL birefringent solution is shown in Figure 2. Uni- and multilamellar vesicles can be clearly observed, which has been discussed about 50.0 and 100.0 mmol L-1 TTAL birefringent solutions.5b,14 The unilamellar vesicles have a rather polydisperse distribution with diameters ranging from about 30 to more than 300 nm and an average diameter of around 100 nm. The multilamellar vesicles have an average diameter of about 500 nm. Phase Behavior of the Zero-Charged TTAL LR-Phase with Fullerenes Embedded at r ) 1.00. The LR-phase at r ) 1.00, that is, the salt-free zero-charged TTAL vesicle-phase was selected to test the solubilizing ability of fullerenes. In our previous report, we found that 50 or 100 mmol L-1 salt-free

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Figure 2. A typical FF-TEM image of the 55.0 mmol L-1 zero-charged TTAL LR-phase at r ) 1.00.

zero-charged TTAL vesicle-phases could solubilize a considerable amount of C60 without macroscopic phase separation and obvious vesicular structure breakage.14 In the current study, we have found the vesicle-phases with cTTAL ) 55.0 and 33.0 mmol L-1, respectively, could also solubilize considerable amounts of fullerene mixtures without macroscopic phase separation, and photos of three typical samples with and without polarizers are given in Figure 3. The concentration of TTAL was fixed to be 55.0 mmol L-1 while the weight ratio of fullerenes to surfactants, WF:WTTAL, was varied to be 1:1000 (a), 1:200 (b), and 1:50 (c), respectively. With the increasing WF:WTTAL, one can see that the color of the sample changed from yellowish to dark brown. All three samples are single phases without a precipitate at the bottom and show weakly flowing birefringence between crossed polarizers. TEM observations revealed the existence of many globular vesicular structures with diameters usually on the order of several hundred nanometers (Figure 3, right image). This is consistent with the observations in Figure 2, indicating that the vesicles had not been destroyed during fullerene solubilization. The ill-defined objects (red arrows) were probably caused by fusion, deformation, or disruption of vesicles during the drying process of sample preparation for TEM observations. The samples with cTTAL ) 55.0 and 33.0 mmol L-1 were rather stable around room temperature. However, these colloidal solutions became unstable at lower concentrations of surfactants and macroscopic phase separation would be observed. Figure 4, samples a-e, shows the phase-behavior variation induced by a decrease of the TTAL concentration. On the contrary, at higher concentrations such as at cTTAL ) 110.0 mmol L-1, a stable gel-like phase was obtained which could not flow induced by its own gravity (Figure 4, sample e). The weight ratios of fullerene to TTAL for the five samples were fixed to be 1:100. With decreasing concentration of TTAL, the upper aqueous solution changed from brown to yellowish or colorless with more and more precipitates deposited at the bottom. When the concentration of TTAL was below a critical value, the phase transition occurs and the solution could lose the ability to solubilize fullerenes. TGA analysis on a sample with cTTAL ) 17.6 mmol L-1 and WF:WTTAL ) 1:50, as shown in Figure 5, demonstrated the upper yellowish aqueous phase mainly composed of surfactants. However, for the dark-brown precipitates at the bottom, WF: WTTAL was found to be about 1:2, which was much higher than that of the original value which is 1:50. The TGA curve of the original fullerene/TTAL solid mixture is also given in Figure 5 for comparison. The loss of the weight below 100 °C was due to the evaporation of water entrapped in the solid mixtures. Phase Behavior of the Isotropic L1-Phases with Fullerenes Embedded at r e 0.85. Vesicle solubilization of fullerenes

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Li and Hao

Figure 3. (Left) Photos of three typical 55.0 mmol L-1 zero-charged vesicle-phases with increasing ratio of WF to WTTAL: 1:1000 (a), 1:200 (b), and 1:50 (c). Top is without and bottom is with polarizers, respectively. (Right) A typical TEM micrograph for sample c.

Figure 4. Fullerene C60 precipitates appear due to the decrease of TTAL concentration at WF:WTTAL ) 1:100. cTTAL: 33.0 mmol L-1 (a), 17.6 mmol L-1 (b), 4.4 mmol L-1 (c), 1.1 mmol L-1 (d), and 110.0 mmol L-1 (e).

Figure 5. TGA analysis of the fullerene/TTAL solid mixture with WF: WTTAL ) 1:50 (a), the solid mixture obtained by drying the upper yellowish aqueous solution (b), and the bottom dark-brown precipitates (c), respectively.

opens the door for possible biological use of fullerenes because vesicles are expected to be good drug carriers and represent simple model systems of membranes. Micellar solubilization of fullerenes was also very interesting both fundamentally and practically, and some results have been reported by different groups.7-12 Using the same sample preparation method, we have successfully solubilized fullerenes in the L1-phases at r e 0.85, and four typical samples are shown in Figure 6. One can see that the amount of precipitates at the bottom of each sample increased with increasing r values, indicating a decrease of their ability to solubilize fullerenes. This was probably caused by the different kinds of dominating aggregates at different r values, which was confirmed by steady-state shear rheological measurements. Table 1 shows the apparent viscosities of the L1-phases at different shear rates and different surfactant concentrations for TTAL aqueous solutions without fullerenes embedded, from which one can see that the apparent viscosity of the L1-phase for the two series samples with cTTAL ) 55.0 and 33.0 mmol L-1 both increased with increasing r value. The L1-phases at

Figure 6. Typical L1-phases embedded fullerenes C60. cTTAL ) 33.0 mmol L-1 and WF:WTTAL ) 1:100 for samples at different r: 0.70 (a), 0.75 (b), 0.80 (c), and 0.85 (d).

Figure 7. UV-vis absorption of ∼0.02 mg mL-1 fullerene mixture in toluene (a) and the upper clear solutions of two samples with r ) 0.70 (b) and 0.85 (c), respectively. cTTAL ) 17.6 mmol L-1 and WF: WTTAL ) 1:300.

TABLE 1: Apparent Viscosities Obtained from Steady-State Shear Flow at a Shear Rate (γ) of 0.3 and 1 s-1, Respectively, for the Samples with Different r Values cTTAL ) 55.0 mmol L-1

cTTAL ) 33.0 mmol L-1

r

γ ) 0.3 s-1

γ ) 1 s-1

γ ) 0.3 s-1

γ ) 1 s-1

0.70 0.75 0.80 0.85

0.0023 0.0030 0.0137 5.121

0.0023 0.0030 0.0137 2.315

0.0020 0.0023 0.0033 1.116

0.0020 0.0023 0.0033 0.724

low r value exhibited a Newton behavior, while at higher r value such as 0.85, an obvious shear-thinning behavior was observed, indicating the appearance of a longer relaxation mode. It was undoubtedly a sign of phase transition, probably from globular micelles to worm-like micelles or long, rod-like micelles. Current study led to the conclusion, for the first time, to our knowledge, that globular micellar phases seemed to be better candidates to solubilize fullerenes than worm-like or long, rodlike micelles. The ability of solubilizing fullerenes for the L1-phases at r e 0.85 seemed to be less pronounced than that of the zerocharged vesicle-phase. We concluded here that this was partially

Behavior of Salt-Free Catanionic Surfactant Systems

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Figure 8. Typical photos of TTAL aqueous solutions with fullerenes embedded within the two-phase region. Without polarizers: (a) cTTAL ) 17.6 mmol L-1, WF:WTTAL ) 1:100, r ) 0.90; (b) cTTAL ) 33.0 mmol L-1, WF:WTTAL ) 1:100, r ) 0.92; (c) cTTAL ) 55.0 mmol L-1, WF:WTTAL ) 1:100, r ) 0.90; (d and e) photos taken from samples b and c between crossed polarizers.

due to the more hydrophilic character of the surfactant mixtures because of a smaller content of water-insoluble LA. UV-vis measurements on the upper clear solutions with cTTAL ) 17.6 mmol L-1, WF:WTTAL ) 1:300, and r values of 0.70 and 0.85, respectively, are shown in Figure 7. One can note that all the characteristic absorptions of fullerenes had been lost, which was consistent with the previous observations for C60/TTAL hybrids.14 The molar absorption coefficient of the sample with r ) 0.70 was larger than that of the sample with r ) 0.85, indicating a higher content of fullerene/TTAL in the upper solution. The probable reason for the absorption change after fullerene solubilization is that the high symmetry of C60 was broken, that is, C60 molecules were solubilized to a vesiclephase which resulted in the interaction of C60 molecules and the HOMO (the highest occupied molecular orbitals)-LUMO (the lowest unoccupied molecular orbitals) transition of the C60 molecules becomes allowed and the spectrum of C60 should be changed. Some effects of the alkylammonium cations on the spectrum of C60 adding into a vesicle-phase were found.14 Studies in this direction were currently under progress and would be reported separately soon. Phase Behavior of the LR/L1 Two-Phase Region with Fullerenes Embedded. We observed that the volume fraction of the upper clear, birefringent LR-phase in the LR/L1 two-phase region of salt-free TTAL aqueous solutions would decrease at higher temperatures. When the temperature was above ∼50 °C, a transition from a two-phase region to a single homogeneous phase would be observed. After fullerenes were embedded into the two-phase region, a more-or-less change of phase boundaries and volume fraction of the LR-phase was also observed, but this influence induced by fullerene solubilization was much less pronounced than that of temperature change, even for the samples with high WF:WTTAL. Figure 8 gives typical photos of aqueous solutions of fullerenes embedded within the two-phase region with and without polarizers, respectively. There is more color in the upper LR-phase than in the bottom L1-phase for each sample, indicating that the content of fullerenes embedded in the upper LR-phase was much higher than that embedded in the bottom L1-phase. This was consistent with the conclusion that the ability of solubilizing fullerenes for the L1-phases at r e 0.85 was less pronounced than that of the zero-charged vesicle-phase at r ) 1.00 (LR-phase). This different distribution makes fullerenes act as an interesting marker for easily distinguishing the LR-phase and L1-phase. The upper phase showed birefringence between crossed polarizers and showed bright strips under a polarized microscope, indicating the existence of a lamellar phase (Figure 9). In some cases, precipitates were also observed at the bottom of the tubes (Figure 8, samples b and c).

Figure 9. Typical texture of the upper birefringent phase of sample c in Figure 8. The existence of bright strips was assigned to be the character of a lamellar structure.

Figure 10. Typical samples in the LA-rich side with cTTAL ) 33.0 mmol L-1 and WF:WTTAL ) 1:100 at different r: 1.00 (a, zero-charged vesicle-phase), 1.05 (b), 1.10 (c), 1.15 (d), and 1.20 (e).

Phase Behavior of the Single LR-Phase with Fullerenes Embedded at r g 1.05. The single LR-phase at r g 1.05 was found to be unsuitable to solubilize fullerenes. Figure 10 shows typical samples for cTTAL ) 33.0 mmol L-1 and WF:WTTAL ) 1:100 with r g 1.05. For comparison, a zero-charged vesiclephase with cTTAL 33.0 mmol L-1 and WF:WTTAL ) 1:100 is also given in Figure 10. Unlike the brown solution of the zerocharged vesicle-phase sample a, macroscopic phase separation was observed for all the single LR-phase with fullerenes embedded at r g 1.05. When the r value was low, such as for sample b, a yellowish solution could still be seen at the top with floccules at the bottom. No obvious phase boundary was observed. With increasing r value, brown precipitates were found to deposit at the bottom of the tubes or float in the bulk solution and the upper solution became ivory-white. This indicates that when LA was in excess, the system would solubilize the excess LA first and hence lose the ability to solubilize fullerenes. If the r value was further increased or the samples were diluted by water, a white top layer would be observed which was assigned to be the excess LA dissociated from the bulk solutions. Influence of Fullerene Incorporation on the Macroscopic Properties of the Zero-Charged TTAL Vesicle-Phase. The zero-charged TTAL vesicle-phase has a better ability to solubilize fullerenes compared to other TTAL mixtures in cases of r deviating from 1.00. Although no macroscopic phase separation and obvious vesicular structure breakage were observed for fullerene/TTAL/H2O at suitable concentrations of TTAL, we speculate that subtle influences induced by fullerene solubilization might also exist. Steady-state shear rheological measurements on four samples with cTTAL ) 110.0 mmol L-1 and increasing WF:WTTAL values showed that the shear stress needed to get the same shear rate for each sample decreased with increasing WF:WTTAL values (Figure 11). This indicates

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Li and Hao Acknowledgment. This work was supported by the NSFC (20625307, 20473049, and 20533050) and by the Specialized Research Fund for the Doctoral Program of Higher Education (20050422009). References and Notes

Figure 11. Plot of shear stress to shear rate obtained from steadystate shear rheological measurements for 110.0 mmol L-1 zero-charged TTAL vesicle-phase with increasing WF to WTTAL ratio: 1:200 (0), 1:150 (O), 1:100 (4), and 1:50 (3).

that a zero-charged TTAL vesicle-phase would become more fluid after fullerenes were embedded. Small-angle X-ray scattering (SAXS) measurement on a sample with cTTAL ) 100 mmol L-1 and WF:WTTAL ) 1:40 showed that the scattering peaks became very weak and difficult to detect (data not shown). This was in contrast with the previous result of the 100 mmol L-1 pure zero-charged TTAL vesiclephase where four peaks with a relative position ratio of 1:2:3:4 were found.14 It was supposed that the solubilization of fullerenes had induced the more fluid character of the systems as revealed by rheological measurements. Conclusions The colloidal stability and phase behavior of fullerene/TTAL/ H2O hybrids were found to be dominated by the mixing molar ratio of LA to TTAOH, the concentration of TTAL, and the weight ratio of fullerene to TTAL. The zero-charged vesiclephase was the best candidate to solubilize fullerenes. Vesicular structures could still be seen clearly for the zero-charged vesiclephase after fullerene solubilization, as revealed by TEM, but rheological measurements showed that the samples became more fluid after fullerenes were incorporated into the hydrophobic microdomains of the aggregates.

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