J. Phys. Chem. B 2004, 108, 19163-19168
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Aggregate Transition from Nanodisks to Equilibrium among Vesicles and Disks Jingcheng Hao,*,†,‡,§ Zaiwu Yuan,† Weimin Liu,‡ and Heinz Hoffmann§ Key Laboratory of Colloid and Interface Chemistry (Shandong UniVersity), Ministry of Education, Jinan 250100, P. R. China, State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physical, Chinese Academy of Sciences, Lanzhou 730000, P. R. China, and Physikalische Chemie I, Bayreuth UniVersita¨t, Bayreuth 95440, Germany ReceiVed: April 15, 2004; In Final Form: July 22, 2004
A dilute, salt-free cationic/anionic/nonionic surfactant system was prepared. The cationic surfactant is derived from a zwitterionic tetradecyldimethylamine oxide (C14DMAO) charged by a strongly acidic anionic surfactant Texapon N70-H [CH3(CH2)11(CH2CH2O)2.5SO3H]. The aggregate transition from nanodisks to the coexistence of disks and unilamellar vesicles to the equilibrium among disks, unilamellar vesicles, and multilamellar vesicles with polydisperse size distributions was observed by freeze-fracture transmission electron microscopy (FF-TEM) in the dilute, salt-free cationic/anionic/nonionic surfactant mixtures. At equilibrium, dynamic laser light scattering (DLLS) measurements show three peaks, which are attributed to the nanodisks, unilamellar vesicles, and multilamellar vesicles, respectively. Control and optimization of self-assembled aggregates under such experimental conditions should allow the realization and applications of novel nanostructures.
Introduction Above the critical micelle concentration (cmc), ionic surfactants in aqueous solutions self-assemble into micelles. The experimental conditions under which micelles grow and take a shape distinctly different from spheroids is dependent on several parameters: temperature, pressure, ionic strength, nature of the counterion, size of the surfactant headgroup, and length and number of alkyl chains per surfactant. Generally, an increase of surfactant concentration or ionic strength results in extensive micelle growth. A large number of stable shapes such as spherical micelles, bilayer cylinders with hemispherical end caps, and lamellar structures have been reported in the literature.1 Cationic and anionic (catanionic) surfactant mixtures in aqueous solutions form entangled micelles at low concentrations and readily form lamellar phases because of the reduction in area per headgroup resulting from ion pairing. Mixtures of oppositely charged surfactants exhibit novel solution and interface properties, and the aggregation in aqueous mixtures of cationic and anionic surfactants occurs at considerably lower concentrations than the cmc of each individual surfactant. The coadjustment of electrostatic effects and surfactant molecular geometry allows a rich diversity of phase behavior. The phase behavior and microstructures of many catanionic mixtures have been explored in the 15 years since spontaneous vesicles were prepared in 1989 by Kaler et al.2 Ali Khan and co-workers have made significant contributions to the work of catanionic surfactant systems.3 However, the novel disks of finite size are a very rare form of stable surfactant self-assembly in aqueous solution, and equilibrium among the variety of aggregates such as bilayer cylinders, spheres, and open, flat disks is only possible for systems with a spontaneous curvature, Ro, and a positive Gaussian curvature modulus, kh.4,5 Few aggregates such as disks have been reported, and little information is in the literature concerning aggregate transitions in surfactant mixtures.
Recently, some fascinating results in catanionic surfactant mixtures have been obtained by the groups of Kaler and Zasadzinski4,5 and Zemb.6,7 Zemb et al. observed the self-assembly of flat nanodisks6 and regular hollow icosahedra7 in salt-free catanionic surfactant solutions by FF-TEM and small-angle neutron scattering (SANS). The high osmotic pressure induced by unscreened electrostatic repulsions due to the use of H+ and OHas counterions levels off the rigid disk colloidal solution. Regular hollow icosahedra are formed owing to defect-free repetition of strong interactions, as one of the fascinating consequences of the quasiequivalence principle.7 Kaler and Zasadzinski et al.4,5 observed the equilibrium among novel bilayer cylinders with hemispherical end caps and open, flat disks and spherical unilamellar vesicles in mixtures of cetyltrimethylammonium bromide (CTAB) and sodium perfluorooctanoate in aqueous solution. They showed the coexistence of cylindrical and spherical vesicles with nearly monodisperse radii by cryo-TEM. In this paper, we show the aggregate transition in surfactant solutions with different ratios (rC14DMAO/Texapon N70-H) of C14DMAO and strongly acidic dodecylethoxysulfonic acid [CH3(CH2)11(CH2CH2O)2.5SO3H, Texapon N70-H]. Salt-free mixed solutions were formed from the combination of these nonionic and anionic surfactants. The cationic surfactant is formed from the zwitterionic C14DMAO being charged by the strongly acidic Texapon N70-H, thus forming a complicated salt-free cationic/ anionic/nonionic surfactant system in the present of excess C14DMAO. The protonation reaction is shown below
* To whom correspondence should be addressed. Phone: +86-5318366074(o). Fax: +86-531-8564750(o). E-mail:
[email protected]. † Shandong University. ‡ Lanzhou Institute. § Bayreuth Universita ¨ t.
10.1021/jp048350l CCC: $27.50 © 2004 American Chemical Society Published on Web 11/17/2004
19164 J. Phys. Chem. B, Vol. 108, No. 50, 2004 Using FF-TEM, we have measured the equilibrium transition from nanodisks to coexisting disks and unilamellar vesicles to equilibrium among disks, unilamellar vesicles, and multilamellar vesicles. Dynamic laser light scattering (DLLS) measurements were carried out on the equilibrium samples. CONTIN analysis8 of the DLLS measurements show the polydisperse size distribution of the different aggregates in the cationic/anionic/nonionic surfactant mixtures. Experimental Section Chemicals. Tetradecyldimethylamine oxide (C14DMAO) was a gift from the Clariant AG Gendorf (Frankfurt Am Main, Germany) and was delivered as a 25% solution. It was crystallized twice from acetone and characterized by melting point (∼130.2-130.5 °C) and cmc (1.4 × 10-4 mol‚L-1, see Figure 1). Sodium dodecylethoxysulfate [CH3(CH2)11(CH2CH2O)2.5SO3Na, Texapon N70] with a cmc value of 2.25 × 10-3 mol‚L-1 (Figure 1) was a gift from Henkel (Du¨sseldorf, Germany) and used without further purification. Sodium dodecyl sulfonate [C12H25SO3Na, SDS] was obtained from the Xinhua Chemical Factory, Shanghai, China. It was crystallized twice from the mixtures of acetone and ethanol (1:1), and cmc was determined to be 8.25 × 10-3 mol‚L-1. Preparation of Texapon N70-H. Texapon N70-H stock solution was prepared from Texapon N70 solution (120 mM) by passing through a strong acidic cation exchange column (ion exchanger I, Merck) at room temperature. Na+ could not be detected by K[Sb(OH)6] solution Na+ + Sb(OH)6- f NaSb(OH)6V in the Texapon N70-H stock solutions. The only detectable impurity in the stock solutions was at ,0.1%; therefore, the ion exchange with hydrogen was much greater than 99.9%. The cmc of Texapon N70-H was determined by surface tension measurements to be 3.41 × 10-3 mol‚L-1 at 25 °C (Figure 1). Surface Tension Measurements. Surface tension was measured by a Processor Tensiometer K12 (Swiss) and the de Nou¨y ring method using a Lauda TE 1C ring tensiometer. The surface tensions at various concentrations were measured after stirring and equilibration at a constant temperature of 25 ( 0.1 °C. The surface tension value reported at each concentration is the average value from three measurements, and the results were within (0.01 mN‚m-1. Conductivity Measurements. The conductivity measurements of the samples were performed on a DDS-307 conductivity meter at 25.0 ( 0.1 °C. Freeze-Fracture TEM. The microstructures of the samples with the birefringence between the polarizers were examined by FF-TEM. A small amount of sample (∼4 µL) was placed on a 0.1-mm-thick copper disk and 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 with liquid nitrogen. For fracturing and replication, a freeze-fracture apparatus (Balzer BAF 400, Germany) was used at a temperature of -140 °C. Pt/C was deposited at a 45° angle. The replicas were examined with a Zeiss CEM 902 transmission electron microscope operated at 80 kV. Dynamic Laser Light Scattering Measurements. To prepare dust-free solutions for DLLS measurements, the sample solutions were filtered directly into dust-free light scattering cells through Millipore sterile membrane filters depending on the concentrations and the sizes of the aggregates. The light scattering cells were rinsed inside and outside with distilled (dust-free) acetone to ensure a dust-free condition before use. A standard Brookhaven commercial laser light scattering
Hao et al. spectrometer equipped with a Coherent Radiation 200 mW diode pumped solid-state (DPPS) 532 laser, operating at 532 nm, and a Brookhaven Instruments Corporation (BI-9000AT) correlator was used for the DLLS measurements. The spectrometer is capable of making measurements of both the angular dependence of absolute integrated scattered intensity over a scattering angular range of 20° to 140° and of intensity-intensity digital photon correlation over a similar angular range (DLLS and dynamic depolarization light scattering). About 2-3 mL of the sample solutions were transferred into scattering cells for light scattering measurements. The scattering cells were held in a brass thermostat block filled with refractive-index-matching silicone oil. The temperature was controlled to within (0.05 °C. DLLS measures the intensity-intensity time correlation function G(2)(Γ) in the self-beating mode where Γ is the characteristic line width. The G(2)(Γ) can be related to the electric field time correlation function g(1)(τ)
G(2)(Γ) ) A[1 + b| g(1)(τ)|2]
(1)
where A and b are, respectively, the background (baseline) and a coherence factor (a parameter depending on the detection coherence). The electric field time correlation function, |g(1)(τ)|, was analyzed by the constrained regularized CONTIN method8,9 to yield the characteristic line width distribution G(Γ) by inversion of
|g(1)(τ)| )
∫0∞ G(τ)e-Γτ dΓ
(2)
The first and second moments of G(Γ) are 〈Γ〉 ) ∫ ∞0 ΓG(Γ) dΓ and µ2 ) ∫ ∞0 (Γ - 〈Γ〉)2G(Γ) dΓ, respectively. The value of µ2/〈Γ〉2 is a measure of the particle polydispersity. If the relaxation is diffusive, Γ can be related to the average apparent diffusion coefficient (D)
D ) Γ/q2
(3)
where q is the magnitude of the scattering wave vector, which could be given by q ) (4πn0/λ0) sin(θ/2), n0 is the refractive index of the liquid, λ0 is the wavelength of the laser in a vacuum (here, λ0 ) 532 nm), and θ is the scattering angle. The apparent hydrodynamic radius, Rh, can be obtained via the StokesEinstein equation
Rh )
kBT 6πηD
(4)
where kB is the Boltzmann constant and η is the solvent viscosity at temperature T. On the basis of eqs 3 and 4, a characteristic line width distribution G(Γ) corresponds to a distribution of an apparent hydrodynamic radii from which, for example, the average apparent hydrodynamic radius Rh can be determined. The DLLS measurements were performed at finite concentrations, and interparticle interactions have been neglected. All samples were allowed to equilibrate at 25 °C for several weeks (at least 4 weeks). Results and Discussion Surface Activities of Individual Surfactants. Surface tensions as a function of surfactant concentrations at 25 °C are shown in Figure 1, where four surfactants, C14DMAO, Texapon N70-H and its precursor Texapon N70, and C12H25SO3Na were studied. C12H25SO3Na was studied to compare with Texapon N70-H and Texapon N70. Figure 1 indicates that the surface
Aggregate Transition from Nanodisks
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Figure 2. Surface tensions of C14DMAO/Teaxpon N70-H and C14DMAO/Texapon N70 mixtures. T ) 25.00 ( 0.01 °C; c ) 10 mM. Figure 1. Surface tension as a function of surfactant concentration.
TABLE 1: cmc and γcmc Values for Four Surfactants in Water at 25.00 ( 0.01 °C surfactant
cmc [mol‚L-1]
γcmc [mN‚m-1]
C14DMAO C11(CH2CH2O)2.5SO3H C11(CH2CH2O)2.5SO3Na C12SO3Naa
1.4 × 10-4 3.41 × 10-3 2.25 × 10-3 8.25 × 10-3
30.82 43.21 40.25 34.35
a
Measurements were carried out at 40.00 ( 0.01 °C.
tension-concentration plot has no minimum, as is usually the case in the presence of surface-active impurities. Extrapolations from the linear regions above and below the break points in the curves yield the cmcs and the surface tensions at surface saturation, for which the values are listed in Table 1. Even though Na+ was replaced by H+, the two surfactants, Texapon N70-H and its precursor Texapon N70, have similar surface activities. Properties of Dilute C14DMAO/Texapon N70-H Mixtures. In our previous work,10 properties such as the phase behavior, rheology, and conductivities, but not the surface tension measurements of concentrated C14DMAO/Texapon N70-H mixtures (total concentration more than 100 mM), were measured. In the present study, the properties of dilute C14DMAO/Texapon N70-H mixtures (total concentration less than 30 mM) were considered. First, we measured the mixed surface tensions of C14DMAO/Texapon N70-H mixtures at ctotal ) 10mM with different ratios of C14DMAO/Texapon N70-H. To compare the mixed surface properties, the surface tensions of two systems C14DMAO/Texapon N70-H and C14DMAO/ Texapon N70 are shown in Figure 2, where the surface tensions are plotted against the mole fraction of xTexaponN70-H and xTexaponN70. Despite the low concentrations, the samples of C14DMAO/Texapon N70-H with 0.175 e xTexaponN70-H e 0.815 were slightly turbid, slightly viscous, and birefringent but did not phase separate. This situation did not affect the surface tension measurements. The surface tensions in the mixture of C14DMAO and Texapon N70-H show lower values than those of the two individual surfactants, C14DMAO with γcmc ) 30.82 mN‚m-1 and Texapon N70-H with γcmc ) 43.21 mN‚m-1. At the minimum, the surface tension is about 29.00 mN‚m-1, which shows that there is a synergism in the C14DMAO/Texapon N70-H mixtures. However, for the C14DMAO/Texapon N70 mixtures, the surface tensions show considerably higher surface tensions than C14DMAO alone and increase almost linearly with xTexaponN70. In mixtures of C14DMAO/Texapon N70, we did not find phase separation, turbidity, or birefringence under these conditions.
Figure 3. FF-TEM image of 30 mM total surfactant aqueous mixtures at rC14DMAO/Texapon N70-H ) 1. Polydisperse multilamellar vesicles were observed, and nonspherical elongated vesicles appeared.
Vesicle Formation in the Equimolar C14DMAO/Texapon N70-H Mixtures. A salt-free catanionic surfactant system can be constructed when C14DMAO and Texapon N70-H are mixed at rC14DMAO/Texapon N70-H ) 1. Conductivity (κ) measurements for the equimolar mixtures of C14DMAO and Texapon N70-H, where the total surfactant concentrations are lower than 30 mM, show that the solutions have very low conductivity of about 12.6 µS‚cm-1. This is a very small value compared with that of Texapon N70-H (κ ≈ 0.62 mS‚cm-1 at c ) 30 mM) but similar to that of 30 mM C14DMAO solution. The conductivity value corresponds to an ionic concentration on the order of 10 µM. This ionic strength leads to a Debye length (Lde) of about 100 nm.11 The sample with a total concentration of 30 mM at rC14DMAO/Texapon N70-H ) 1 is slightly bluish, somewhat turbid and viscous, and birefringent but does not phase separate. The FF-TEM image of the solution after two months of equilibrium is shown in Figure 3. Polydisperse multilamellar vesicles were observed. The vesicle phase could be prepared by changing the total concentration between ctotal ) 10 mM and 25 mM with the rC14DMAO/Texapon N70-H around 1 (0.96 to 1.18). When rC14DMAO/Texapon N70-H < ∼0.96, phase separation and a micellar phase are observed, and when rC14DMAO/Texapon N70-H > 1.18, the aggregate transition was observed as in the following FF-TEM images (Figures 4-6). Usually, the cationic-anionic surfactant systems precipitate when the stoichometry between the cationic and anionic surfactants is exactly 1.12 In the present samples, we prepared the homogeneous solutions at rC14DMAO/Texapon N70-H ) 1, which are stable regardless of the sample history and the mixing routes to the desired concentration, such as mixing the two L1 phases of C14DMAO and Texapon N70-H or mixing the pure solid
19166 J. Phys. Chem. B, Vol. 108, No. 50, 2004
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Figure 4. FF-TEM images of 20 mM total surfactant aqueous mixtures at different C14DMAO/Texapon N70-H ratios. Disks are present in each image but there were also a small number of spherical vesicles in Figure 4c. (a) r)90:10; (b) r)85:15; (c) r)80:20.
Figure 5. FF-TEM images of 30 mM total surfactant aqueous mixtures at different C14DMAO/Texapon N70-H ratios. Disklike aggregates and spherical unilamellar vesicles are present in each image. A significant population of spherical unilamellar vesicles is present in samples with higher charge. (a) r ) 75:25; (b) r ) 73:27; (c) r ) 71:29.
Figure 6. FF-TEM images of 40 mM total surfactant aqueous mixtures at different C14DMAO/Texapon N70-H ratios. Disks, unilamellar vesicles, and multilamellar vesicles coexist in each image. Miltilamellar vesicles are present because of higher charge. (a) r ) 65:35; (b) r ) 60:40; (c) r ) 55:45.
C14DMAO with the L1 phase of Texapon N70-H. We made five samples at different total concentrations, and the polydisperse multilamellar vesicles could be observed repeatedly by FF-TEM. The features of the vesicles are apparent: (i) Only polydisperse multilamellar vesicles were observed; (ii) the multilamellar vesicles have diameters ranging from several nanometers to more than 1.0 µm; (iii) a few nonspherical vesicles, or elongated vesicles, could be seen. FF-TEM Observation of the Transition from Nanodisks to Equilibrium among Vesicles and Disks in Dilute Cationic/
anionic/nonionic C14DMAO/Texapon N70-H Mixtures. Direct observations of the aggregate transition were carried out on dilute solutions with rC14DMAO/Texapon N70-H * 1. At the nonequimolar ratios of C14DMAO/Texapon N70-H, cationic/ anionic/nonionic surfactant mixtures were prepared, because C14DMAO is partly protonated by Texapon N70-H to C14DMAOH+. Figure 4a-c shows the structures observed for the equilibrium mixtures with 20 mM total surfactant and rC14DMAO/Texapon N70-H ) 90:10 (Figure 4a), 85:15 (Figure 4b), and 80:20 (Figure 4c). The three samples are transparent,
Aggregate Transition from Nanodisks isotropic nonbirefringent solutions. In the three FF-TEM images, only nanodisks exist at all C14DMAO/Texapon N70-H ratios, and the size of the disks changed with the change in composition. At a C14DMAO/Texapon N70-H ratio of 90:10 (Figure 4a), the disks are quite polydisperse with a thickness of about 26 ( 4.5 nm, and diameters of the small disks are ∼30 nm, while the large ones are around 400 nm. Some flexural disks are also clearly observed. At rC14DMAO/Texapon N70-H ) 85:15 (Figure 4b), the disks are also quite polydisperse with thicknesses from ∼15 to ∼70 nm, and the diameters of the small disks are ∼20 nm and the large ones around 670 nm. These disks are quite rigid, and no flexural disks were observed. At rC14DMAO/Texapon N70-H ) 80:20 (Figure 4c), the disks are much narrower than those of the above solutions. In addition to the disks in Figure 4c, there were also a small number of spherical vesicles. Figure 5a-c shows that disks and unilamellar vesicles coexist in solutions with ctotal ) 30 mM and C14DMAO/Texapon N70-H ratios of 75:25 (Figure 5a), 73:27 (Figure 5b), and 71:29 (Figure 5c). These three samples are different from the three samples in Figure 4. The solutions are slightly bluish in appearance. Disklike aggregates and unilamellar vesicles are observed in the FF-TEM images at the three C14DMAO/Texapon N70-H ratios. The size distributions of the disklike aggregates and unilamellar vesicles change with the changing composition and C14DMAO/Texapon N70-H ratios. At rC14DMAO/Texapon N70-H ) 75:25 (Figure 5a), some flexural disks are present, and there are a few unilamellar vesicles. The spherical unilamellar vesicles were monodisperse with a mean radius of 35 ( 3.9 nm, while the disklike aggregates are quite polydisperse with thicknesses of 20 ( 3.2 nm. The diameters of small disks ranged from ∼30 nm for the smallest ones to around 380 nm for the largest. At rC14DMAO/Texapon N70-H ) 73:27 (Figure 5b), the mean radius of the spherical unilamellar vesicles (∼10 to >20 nm) is smaller compared with those in Figure 5a. The disks are also polydisperse with thicknesses of 15 ( 2.9 nm, and the diameters ranged from ∼20 to >150 nm. At rC14DMAO/Texapon N70-H ) 71:29 (Figure 5c), the size distributions of the spherical unilamellar vesicles and the disklike aggregates are essentially unchanged compared with the sample with rC14DMAO/Texapon N70-H ) 73:27. A significant population of spherical unilamellar vesicles is present in Figure 5b,c. Figure 6a-c presents the FF-TEM images for the equilibrium aqueous mixtures with ctotal ) 40 mM at rC14DMAO/Texapon N70-H ) 65:35 (Figure 6a), 60:40 (Figure 6b), and 55:45 (Figure 6c). The three samples are slightly birefringent, somewhat viscous, and bluish solutions. The FF-TEM images clearly demonstrate that the disklike aggregates, unilamellar vesicles, and multilamellar vesicles coexist. The size distributions of the spherical unilamellar vesicles and the disks are essentially unchanged compared with the images in Figure 5. However, the multilamellar vesicles have rather polydisperse size distributions, with diameters ranging from about 500 nm to 1.5 µm, and interlamellar spacing between two adjacent bilayers is approximately 50 nm. On the basis of the observations from Figure 4a-c to Figure 6a-c, it is clear that the compositions determined by the ratios of C14DMAO/Texapon N70-H play an important role in the aggregate transition. The nonionic surfactant molecule, C14DMAO, should modulate the electrostatic interaction between cationic C14DMAOH+ and anionic CH3(CH2)11(CH2CH2O)2.5SO3in the aggregate formation. To our knowledge, this is the first time the aggregate transition has been observed in aqueous saltfree cationic/anionic/nonionic surfactant mixtures.
J. Phys. Chem. B, Vol. 108, No. 50, 2004 19167
Figure 7. Typical CONTIN analysis of DLLS measurements on three samples at different C14DMAO/Texapon N70-H ratios and concentrations.
DLLS Measurements. To verify the FF-TEM observations and size distribution of the different aggregates, a commercial Brookhaven Instrument light scattering spectrometer was used for DLLS (with a BI-9000AT digital Autocorrelator) measurements. The CONTIN method9 was used to analyze the DLLS data to calculate the hydrodynamic radius (Rh) of the nanostructures from the characteristic line width Γ. The DLLS experimental results for the aqueous salt-free cationic/anionic/nonionic surfactant mixtures were reproduced after the samples were equilibrated at 25.0 ( 0.1 °C. The Rh data were the average values of 15 CONTIN analyses, and the sense of the error should be with an average error below ∼10%. DLLS measurements provide information on the nanostructure size distribution in solution from a plot of Γ‚G(Γ) versus Rh. DLLS measurements show the different modes to the variety of aggregates in solution.
19168 J. Phys. Chem. B, Vol. 108, No. 50, 2004 Typical CONTIN analysis of DLLS measurements for three samples at different scattering angles are shown in Figure 7a-c. Just one mode could be identified in Figure 7a with ctotal ) 20 mM at rC14DMAO/Texapon N70-H ) 85:15. The average Rh values of about 115 and 158 nm were obtained at scattering angles of 30° and 60°, respectively. The average Rh values have angular dependence, suggesting the presence of asymmetric spherical aggregates in solution (i.e., disks), which is consistent with the observation of disklike aggregates by FF-TEM in Figure 4b. The DLLS CONTIN analysis shows two peaks in Figure 7b, Rh ) ∼46 and ∼170 nm, respectively, which suggest two modes, because the disks and unilamellar vesicles are coexisting in solution, and is consistent with the FF-TEM observation in Figure 5b with ctotal ) 30 mM at rC14DMAO/Texapon N70-H ) 73: 27. For 40 mM total surfactant aqueous mixtures with rC14DMAO/Texapon N70-H ) 60:40, DLLS results at different scattering angles indicate the coexistence of disks, unilamellar vesicles, and multilamellar vesicles in solution. The first peak shows an average hydrodynamic radius Rh of about 40 nm without angle dependence, which is the unilamellar mode (spherically symmetric aggregates). The second, much narrower peak suggests the disklike aggregate mode with Rh ) 181.2 nm at 45°, 160.7 nm at 60°, and 159.2 nm at 90°, respectively. The angle dependence of the second peak suggests spherically asymmetric aggregates in the solutions (i.e., disks). The third peak has Rh ) 922.4 nm at 45°, 825.5 nm at 60°, and 812.3 nm at 90°, which is the multilamellar vesicle mode. The multilamellar vesicle polydispersity index, µ2/〈Γ〉2, is about 0.34. Conclusions Stable surfactant self-assemblies of different geometries can be created as control parameters such as temperature or osmotic conditions are varied.5 Kaler and Zasadzinski et al.3,4 determined that the variety of aggregates in surfactant mixtures, such as unilamellar and multilamellar vesicles,13,14 vesicles and micelles,15,16 disks, lamellar phase, or vesicles,5 depending on the surfactant or counterion chemistry or concentrations, can coexist at equilibrium only if the variation of curvature energies among the various aggregates is on the order of kBT. Recently, Maeda’s group reported the formation of pH-induced charged vesicles of oleyldimethylamine oxide (oleylDMAO).17 They investigated the vesicle-micelle transition and the vesicle formation of C14DMAO because of the binding of aromatic counterions.18 However, surfactant self-assemblies still present a number of theoretical and experimental mysteries. One example is that flat disks of finite size are a very rare form of stable surfactant selfassembly, because the disk geometry implies the coexistence of highly curved regions on the edges and also regions with zero curvature.5 A very recent review by Hyde et al.19 summarized the recent developments in theoretical and experimental studies of amphiphilic lyotropic intermediate mesophase formation. In our case, we have shown that a zwitterionic surfactant containing a
Hao et al. group and a strongly acidic surfactant containing a -(CH2CH2O)2.5SO3H group can produce rigid nanodisks whose diameters are continuously adjustable from a length of ∼10 nm to hundreds of nanometers and with radii around 50 nm, as well as unilamellar and multilamellar vesicles. The crucial requirement for obtaining bilayer and disk self-assemblies is satisfied by mixing these two surfactants to form ion pairs in the absence of salt, and thus, the conductivity is lowered to about 12.6 µS‚cm-1. In fact, we observed that the nanodisks transfer to a thermodynamic equilibrium among unilamellar vesicles, multilamellar vesicles, and disks in dilute solutions. These nanostructural self-assemblies should allow the realization and application of novel nanostructures and be of practical value for template applications and controlled drug or DNA release. Acknowledgment. J. Hao gratefully acknowledges support of this work by the NFSC, by the program of Hundreds of Talents of the Chinese Academy of Sciences, the Alexander von Humboldt-Foundation when Dr. Hao was an AvH Fellow guided by Professor Dr. H. Hoffmann at Bayreuth University (Germany), the young-middle age Scientists’ Awards of (Shandong, 03BS083), and by Shandong University. References and Notes (1) (a) Schwarz, U. S.; Swamy, K.; Gompper, G. Europhys. Lett. 1996, 36 (2), 117-122. (b) Kawasaki, H.; Imahayashi, R.; Tanaka, S.; Almgren, M.; Karlson, G.; Maeda, H. J. Phys. Chem. B 2003, 107, 8661-8668. (c) Holmqvist, P.; Alexandridis, P.; Lindman, B. J. Phys. Chem. B 1998, 102, 1149-1158. (d) Watansbe, K.; Nakama, Y.; Yanaki, T.; Hoffmann, H. Langmuir 2001, 17, 7219-7224. (2) Kaler, E. W.; Murthy, A. K.; Rodriguez, B. E.; Zasadzinski, J. A. Science 1989, 245, 1371-1374. (3) Khan, A.; Marque`s, E. In Specialist surfactants; Robb, I. D., Ed.; Kluwer: Dordrecht, 1997; pp 37-80. (4) Jung, H. T.; Coldren, B.; Zasadzinski, J. A.; Lampietro, D. J.; Kaler, E. W. PNAS 2001, 98 (4), 1353-1357. (5) Jung, H. T.; Lee, S. Y.; Kaler, E. W.; Coldren, B.; Zasadzinski, J. A. PANS 2002, 99 (24), 15318-15322. (6) Zemb, Th.; Dubois, M.; Deme´, B.; Gulik-Krzywick, Th. Science 1999, 283, 816-819. (7) Dubois, M.; Deme´, B.; Gulik-Krzywick, Th.; Dedieu, J. C.; Vautrin, C.; De´sert, S.; Perez, E.; Zemb, Th. Nature 2001, 411, 672-675. (8) Provencher, S. W. Biophys. 1976, 16, 29-47. (9) Provencher, S. W. J. Chem. Phys. 1976, 64, 2772. (10) Hao, J.; Hoffmann, H.; Horbaschek. K. J. Phys. Chem. B 2000, 104 (44), 10144-10153. (11) (a) Antropov, L. I. Theoretical electrochemistry (English Translation); Mir Publishers: Moscow, 1977. (b) Hiemenz, P. C.; Rajagopalan, R. Principles of colloid and surface chemistry; Marcel Dekker: New York, 1997. (12) Horbaschek, K.; Hoffmann, H.; Hao, J. J. Phys. Chem. B 2000, 104, 2781-2784. (13) Herve, P.; Roux, D.; Bellocq, A. M.; Nallet, F.; Gulik-Krzywicki, T. J. Phys. II 1993, 3, 1255-1270. (14) Miller, D. D.; Bellare, J. R.; Evans, D. F.; Talmon, Y.; Ninham, B. W. J. Phys. Chem. 1987, 91, 674-685. (15) Miller, D. D.; Evans, D. F.; Warr, G. G.; Bellare, J. R.; Ninham, B. W. J. Colloid Interface Sci. 1987, 116 (2), 598-601. (16) Dubois, M.; Zemb, Th. Curr. Opin. Colloid Interface Sci. 2000, 5, 27-37. (17) Kawasaki, H.; Souda, M.; Tanaka, S.; Nemoto, N.; Karlsson, G.; Almgren, M.; Maeda, H. J. Phys. Chem. B 2002, 106, 1524-1527. (18) Kawasaki, H.; Imahayashi, R.; Tanaka, S.; Almgren, M.; Karlsson, G.; Maeda, H. J. Phys. Chem. B 2003, 107, 8661-8668. (19) Hyde, S. T.; Schro¨der, G. E. Curr. Opin. Colloid Interface Sci. 2003, 8, 5-14.
