Self-Assembly of Magnetic Bacillus-Shaped Bilayer Vesicles in

Sep 16, 2016 - Bacillus-shaped bilayer vesicles of nanoscale size are very rare structures of stable surfactant self-assembly, because they are both t...
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Self-Assembly of Magnetic Bacillus-Shaped Bilayer Vesicles in Catanionic Surfactant Solutions Lu Xu,† Wenrong Zhao,† Jingcheng Hao,*,† Yurong Zhao,‡ Dong Wang,‡ Hai Xu,‡ and Jian R. Lu‡,§ †

Key Laboratory of Colloid and Interface Chemistry and Key Laboratory of Special Aggregated Materials, Shandong University, Jinan 250100, China ‡ Centre for Bioengineering and Biotechnology, China University of Petroleum (East China), Qingdao 266555, China § Biological Physics, School of Physics and Astronomy, University of Manchester, Manchester M13 9PL, United Kingdom S Supporting Information *

ABSTRACT: Bacillus-shaped bilayer vesicles of nanoscale size are very rare structures of stable surfactant self-assembly, because they are both thermodynamically and electrostatically unfavorable in solution. It is evidently demonstrated that appropriately aqueous mixtures of single-tailed cationic and anionic (catanionic) surfactants can produce rigidly bacillusshaped bilayer vesicles with both flat parts and bent edges. The crucial requirement for forming bacillus-shaped bilayer vesicles is the use of cationic surfactants with relatively hydrophobic [FeCl3Br]− as counterions. [FeCl3Br]− can strongly associate with cationic surfactants to partition into the hydrophobic bilayer of bacillus-shaped bilayer vesicles, significantly increasing the edge energy of cationic surfactants to make them distribute in the low curvature part of bilayers. This causes the formation of bacillus-shaped bilayer vesicles, but not completely bent spherical vesicles, in the case of cationic surfactant excess. The specificity of hydrophobic counterions, [FeCl3Br]−, could also make the catanionic mixtures do not precipitate at the stoichiometric point. This new self-assembly on catanionic systems is culminated in the discovery of beautifully structured colloidal objects which are of practical use for molecular templating and controlled drug or DNA release. bionics.13−15 In salt-free catanionic solutions, Hoffmann once observed strongly repulsive charged cylinders of limited length.7 Zemb and his team discovered two specific aggregates, flat nanodiscs16,17 and regular hollow icosahedra.17,18 They closely analyzed both new structures and elucidated how various interactions conspire to produce them.19 In salt-free catanionic systems, the charges of excess surfactants cannot be screened, and excess ionic surfactants form the edges of flat disc-like micelles or the vertex of icosahedras, but not finite bacillusshaped bilayer vesicles.19 In catanionic solutions with excess salts, at the stoichiometric mole of cationic and anionic surfactants, a large amount of precipitates are produced. These beautiful microstructures formed by surfactants spontaneously; Agudo-Canalejo and Lipowsky report a strategy for changing the curvature of vesicles via adjusting the charge localization of adsorbed nanoparticles.20−22 In this study, we presented a conventionally catanionic system of five components, containing two counterions [FeCl3Br]− and Na+ from cationic and anionic surfactants, with overall electroneutrality. At a constant temperature (T)

1. INTRODUCTION In typical mixtures of surfactants, the aqueous solution of single-tailed cationic and anionic (catanionic) surfactants, above the mixed critical micelle concentration (cmc), ion pairing of the two counterions of catanionic surfactants strongly reduces the area per headgroup to induce the formation of molecular bilayers at concentrations far below the cmcs of both surfactants. A large number of stable aggregates, including spherical micelles and vesicles, worms, folded bilayers, and hexagons, have been reported.1−5 Two kinds of catanionic systems were identified in the late 19th century, and beautiful aggregates were discovered, one of which is the catanionic surfactant system containing the excess salts in aqueous solution formed from the combination of the two compounds and the other is the “true”, salt-free catanionic surfactant system having the acidic (H+) and basic (OH−) as the counterions for the cationic and anionic surfactants, respectively, which form water by the combination of the counterions.6,7 Surfactant bilayer microstructures in excess salt-containing aqueous solution such as planar lamella, closed uni- or multilamellar vesicles, and sponge structures have been widely investigated, and pioneering studies of vesicle formation were reported.2,7−12 These microstructures show importance in molecular templating,8 preparing hard materials,8 and molecular delivery and © XXXX American Chemical Society

Received: April 26, 2016 Revised: September 5, 2016

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scattering angular range from 20° to 140° and of intensity−intensity digital photon correlation over a similar angular range (DLS and depolarized DLS). About 2−3 mL of sample solution was transferred into a special dust-free light scattering cell 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. DLS 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) as follows:26,27

and pressure (P), single- and multiple-phase regions can be observed from the equilibrium phase diagram. The highly stable colloidal self-assembly objects can be obtained due to high osmotic pressure from the unscreened electrostatic repulsions. The bacillus-shaped bilayer vesicles with inherent magnetism can be obtained in catanionic solution by mixing 50 mmol·L−1 cationic cetyltrimethylammonium trichloromonobromoferrate and 40 mmol·L−1 anionic Texapon N70.

2. EXPERIMENTAL SECTION 2.1. Chemicals and Materials. Cetyltrimethylammonium trichloromonobromoferrate (C16H33−N+(CH3)3[FeCl3Br]−, (CTA)Fe, 98%) was synthesized by mixing equal molar amounts of (CTA)Br and FeCl3 in methanol and stirring overnight at room temperature.23,24 The solvent was then evaporated and the product dried at reduced pressure at 80 °C for 12 h yielding brown/yellow solids. Cationic (CTA)Fe is a paramagnetic compound; its cmc and dissociation constant (β) are 0.42 mmol·L−1 and 0.66, respectively.25 Cetyltrimethylammonium bromide (98%) and iron trichloride (FeCl3, 99%) were purchased from Sinopharm Chemical Reagent Co. Ltd., Beijing, China and used without further purification. Texapon N70 (C12H25(C2H4O)2.5SO3Na+, 98%) was a gift of Henkel (Dusseldorf, Germany) and used without further purification. Its cmc is 3.41 mmol·L−1.26 Thrice-distilled water was used to prepare all sample solutions except when D2O was used for 2H NMR and SANS instead. All measurements were performed at 25 ± 0.1 °C. 2.2. Preparation of Bacillus-Shaped Bilayer Vesicles in Catanionic Surfactant Solutions. Catanionic solutions were prepared by mixing calculated amounts of (CTA)Fe and Texapon N70 micellar stock solutions in tubes with a fixed final total catanionic concentration of 90 mmol·L−1. Each sample was equilibrated for at least 1 month. Proper sonication and centrifugation were used to help solutions disperse homogeneously. The vesicles can also be produced by directly mixing the (CTA)Fe solids and Texapon N70 liquids in water. 2.3. Cryogenic Transmission Electron Microscope (cryoTEM) Observations. Carbon film grids with a hole size between 1 and 12 mm were used for specimen preparation. A 4 μL drop of the sample solution was put on the untreated coated TEM grid (copper grid, 3.02 mm, 200 meshes). Most of the liquid was removed with blotting paper leaving a thin film stretched over the holes. The specimens were instantly shock frozen by plunging them into liquid ethane in a temperature-controlled freezing unit (Zeiss, Oberkochen, Germany). After freezing the specimens, the remaining ethane was removed using blotting paper. The specimens were inserted into a cryo-transfer holder (Gatan 626) and transferred to a JEOL JSM-1400 TEM under liquid nitrogen to avoid the formation of ice. Examinations were carried out at a constant temperature of 90 K. The TEM observations were operated at an accelerating voltage of 120 kV, and a Gatan multiscan CCD was used to record every image. The average size and bilayer thickness of catanionic vesicles were measured using the software Nano Measurer. 2.4. Dynamic Laser Light Scattering Measurements..26,27 The average hydrodynamic diameters (2Rh) of catanionic elongated (CTA)Fe/Texapon N70 bilayer vesicles were measured by a BI-200SM instrument (Brookhaven) at a constant scattering angle of 90° at 298 K. To prepare dust-free solutions for dynamic light scattering (DLS) measurements, the sample solutions were filtered directly into dustfree light scattering cells through a Millipore sterile membrane filter depending on the concentrations and the sizes of the aggregates. The light scattering cells had been rinsed inside and outside with distilled (dust-free) acetone to ensure a dust-free condition before use. A standard laboratory-built laser light scattering spectrometer equipped with a Coherent Radiation 200 mW diode pumped solid-state (DPSS) 532 laser, operating at 532 nm, and a Brookhaven Instruments (BI200SM) correlator was used for the DLS measurements. The spectrometer is capable of making measurements of both the angular dependence of the absolute integrated scattered intensity over a

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 method28,29 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 2 2 and μ2 = ∫ ∞ 0 (Γ − ⟨Γ⟩) G(Γ) dΓ, respectively. The value of μ2⟨Γ⟩ is a measure of the particle polydispersity. If the relaxation is diffusive, Γ can be related to the average apparent diffusion coefficient (D):28,29

D = Γ/q2

(3)

The apparent hydrodynamic radius, Rh, can be obtained via the Stokes−Einstein equation:26−30

R h = 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 DLS measurements were performed at finite concentrations, and interparticle interactions have been neglected. 2.5. 2H Nuclear Magnetic Resonance Measurements. 2H nuclear magnetic resonance (2H NMR) spectra were recorded on a Bruker Avance 400 spectrometer equipped with pulse field gradient module (Zg2h.2) operating at a deuterium frequency of 61.4 MHz in a 9.4 T magnetic field and was controlled by Bruker Topspin 2.0. The samples were prepared in D2O and placed in 5 mm NMR tubes left at 298 K for at least 1 month for equilibration. Spectra were gained by Fourier transformation using a single-pulse sequence, without fieldfrequency lock. A count of 128 transients with a recycle rate of 1 s were averaged before Fourier transformation. 2.6. Small-Angle Neutron Scattering Measurements. Smallangle neutron scattering (SANS) measurements were conducted at LOQ, ISIS Neutron Facility, Rutherford Appleton Laboratory (Oxford, U.K.). The samples were dissolved in D2O and equilibrated for at least 1 month. Each sample was transferred into 2.0 mm pathlength disc-shaped silica cells for characterization. Neutron incident wavelengths were controlled from 2.2 to 10 Å at 25 Hz. A 64 cm2 detector with 5 mm resolution was placed at a distance of 4.05 m from the samples, resulting in a q range of 0.008−0.25 Å−1. Data were collected for the wavelength dependence of the incident spectrum, the measured sample transmission, and relative detector efficiencies, prior to subtraction of the solvent background (D2O). Absolute scaling was gained by comparing to the scattering from a partially deuterated polystyrene standard. The data were fitted using the SansView 2.1.1 program offered by RAL. 2.7. Model-Fitting Analysis of SANS Measurements. For the flat lamella model, the scattering intensity I(q) can be expressed as16,17,23 I(q) = 2π(Δρ)2 φδ /q2 ∝ q−2 B

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Figure 1. (a) Cryo-TEM images of bacillus-shaped bilayer vesicles of 50 mmol·L−1 (CTA)Fe/40 mmol·L−1 Texapon N70 mixtures in water. Bacillusshaped bilayer vesicles with both elongated parts and bent edges (b) and thick bilayers (c). Cryo-TEM images of the catanionic mixtures of 45 mmol·L−1 (CTA)Fe/45 mmol·L−1 Texapon N70 (d) and 40 mmol·L−1 (CTA)Fe/50 mmol·L−1 Texapon N70 (e). (f) FF-TEM image of the sample solution of 20 mmol·L−1 (CTA)Fe and 70 mmol·L−1 Texapon N70. where δ is the bilayer thickness, φ is the volume fraction, and Δρ is the difference of scattering length densities of the lamellar layers and the surrounding solvent. 2.8. Polarizer Microscope Observations. Birefringent lamellar textures were characterized by a polarizer microscope (PLM), Axioskop 40/40 FL (Zeiss). 2.9. Rheological Measurements. The measurements were carried out with a Haake Rheostress RS75 rheometer using a concentric cylinder system Z41-Ti (Haake, Vreden, Germany) at 25.0 ± 0.1 °C. The viscoelastic properties were determined by oscillatory measurements from 0.01 to 10 Hz. 2.10. ζ Potential Measurements. The ζ potential of catanionic vesicles was measured with a Zeta PALS potential analyzer instrument (Brookhaven, Holtsville, NY, USA) with parallel-plate platinum black electrodes spaced 5 mm apart and a 10 mm path-length rectangular organic glass cell. All samples were measured using a sinusoidal voltage of 80 V with a frequency of 3 Hz. Both the average values and standard errors are calculated by the instrument automatically with 10 times repeated measurements.

2.11. Small-Angle X-ray Scattering Measurements. Smallangle X-ray scattering (SAXS) measurements were carried out using a SAXSess MC2 high flux SAXS instrument (Anton Paar, Graz, Austria; Cu Kα, λ = 0.154 nm), equipped with a Kratky block-collimation system and using an image plate (IP) as the detector. The X-ray generator was operated at 40 kV and 50 mA. A standard temperature control unit (Anton-Paar TCS 120) connected to the SAXSess instrument was used to regulate the temperature and maintain it at the desired level. Samples were transferred into 1 mm standard quartz capillaries. An exposure time of 2 h was long enough to provide a good signal-to-noise ratio. The scattering curve of solvent water in the same capillaries was recorded as background. The data were normalized to the same incident primary beam intensity and corrected for background scattering from the capillaries and water. 2.12. Freeze−Fracture TEM Observations. An ∼4 μL amount of sample solution was dropped on a 0.1 mm thick copper disc covered with a second copper disc. This sandwich was plunged into liquid propane, which had been cooled by liquid nitrogen, and the copper sandwich was frozen with the sample. Fracturing and replication were conducted at a temperature of −140 °C. Pt/C was C

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Figure 2. (a) DLS data showing the size distribution of the vesicles. (b) 2H NMR spectrum of the vesicle solution. The peak width is 131.98 Hz as noted. T = 298 K.

Figure 3. (a) A SANS profile of 50 mmol·L−1 (CTA)Fe/40 mmol·L−1 Texapon N70 catanionic complex solution. The solid line refers to the fitting curves with the model and parameters shown in the text. (b) PLM image of the bilayer vesicle sample, indicating the stripe texture of the birefringent phase at T = 298 K. The scale bar is equal to 50 μm. deposited at an angle of 45°. The replicas were examined with a JEOL JSM-1400 TEM at 120 kV. 2.13. Surface Tension Measurements. Surface tension measurements were performed on a Krüss K100 (Hamburg, Germany) surface tensiometer using the plate method. The temperature was controlled with a Haake K10 super constant temperature trough, and the error was within ±0.1 °C. All the surface tension was measured after stirring and equilibration and was repeated at least twice until the error was negligible. 2.14. SQUID Magnetometry. Dried samples of (CTA)Fe/ Texapon N70 complexes were placed in sealed polypropylene tubes and mounted inside a plastic straw for measuring in a magnetometer with a superconducting quantum interference device (MPMSXL, Quantum Design, San Diego, CA, USA) and a reciprocating sample option (RSO). The data were collected at 300 K.

hydrodynamic diameter (2Rh) of bacillus-shaped bilayer vesicles is determined as 407.4 ± 20.3 nm. In a 2H NMR spectrum (Figure 2b), a single broad peak with a Δv = 131.98 Hz was found, suggesting that the catanionic aggregates form a strongly anisotropic dispersion phase.31 The 2 H NMR spectra are governed by the electric quadrupole interaction between the quadrupole moment of the spin I = 1 nucleus and the electric-field gradient at the site of the nucleus. This interaction is averaged by the rapid anisotropic motion of the water molecules, resulting in a reduced quadrupole splitting,31 as described in

3. RESULTS AND DISCUSSION Catanionic mixture solution of 50 mmol·L−1 cationic (CTA)Fe and 40 mmol·L−1 anionic Texapon N70 was the model sample that was studied. Cryogenic transmission microscopy (cryoTEM) images (Figures 1a−c and Supporting Information Figure S1) show that the catanionic complexes in solution mainly exist as numerous dispersive, regular, bacillus-shaped bilayer vesicles with both elongated parts and bent edges. These “bacillus-shaped” bilayer vesicles have the length of long axis of elongated parts of 351.3 ± 31.7 nm and length of short axis of 112.3 ± 12.5 nm. The thickness of the double bilayers and the flat hollow is 49.2 ± 4.3 nm, evidently predicating the existence of bacillus-shaped bilayer vesicles of (CTA)Fe/Texapon N70 mixtures.16 Dynamic light scattering data (Figure 2a) reveal the size distribution of the bacillus-shaped bilayer vesicles, there is no existence of multiscale aggregates in the solution, and the

in which δ (δ = e qQ/h; e is the elementary charge, eQ is the electric quadrupole moment, eq is the electric filed gradient, and h is Planck’s constant) is the motionally averaged quadrupole coupling constant and β is the angle between the director and the external magnetic field. Both the orientational distribution of the domains and the size of the domains affect the line shape.31 The orientation of the domains parallel to the external magnetic field generates a doublet with a maximum splitting of (3/2) δ, whereas the orientation perpendicular to the magnetic field gives rise to a doublet with half the maximum value. In a strong anisotropic dispersion such as a vesicle system, the orientation is nonuniform and shows a continuous change, which may lead to a single broad peak in a 2H NMR spectrum.31 The size of aggregates also affects the line shape. If the domain size is large enough that diffusion of water between domains with different orientations takes place during the measurement, the value of δ

Δv =

⎛3⎞ 2 ⎜ ⎟δ(3 cos β − 1) ⎝4⎠

(6)

2

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Figure 4. (a) SAXS spectra of 50 mmol·L−1 (CTA)Fe/40 mmol·L−1 Texapon N70 solutions at different temperatures. (b) PLM image of the sample containing 20 mmol·L−1 (CTA)Fe and 70 mmol·L−1 Texapon N70, showing the Maltese crosses of the birefringent phase at T = 298 K. The bar is equal to 50 μm.

catanionic vesicles in solution are spherical or nearly spherical; some reports revealed that the cylindrical or tubular vesicles which contain a stretched part are instable.36,37 As a result, we needed to determine whether elongated bilayer vesicles are the most stable shape of the catanionic mixtures at this concentration. It is necessary to note that each sample was equilibrated for at least 1 month before performing measurements; no sign of sedimentation or phase separation was discovered during the equilibrium process. ζ potential measurements were applied to estimate the thermodynamic stability of colloid particle dispersions.38,39 In principle, the high values, >30 or 698 Å, because q−2 decay extends to q < 0.009 Å−1.16,32 Further data analysis from Figure 3a revealed that a flat lamella model could provide the best fit to the measured SANS profile of (CTA)Fe/ Texapon N70 vesicles, determining the bilayer thickness δ = 2.9 ± 0.1 nm. The δ of a bilayer composed of the extended, upright surfactant molecules of catanionic (CTA)Fe/Texapon N70 mixtures can be calculated, δ± = 3.5 nm.33 The difference between δ and δ± suggests that surfactants in the bilayers of bacillus-shaped bilayer vesicles arrange in an intertwist or tilt formation. To better support our assumption, a core−shell hollow sphere model as well as a core−shell hollow cylinder model were utilized to fit with our SANS data (see details in the Experimental Section in the SI). Both of the models present obvious deviation from our spectra (Figure S2), proving that the (CTA)Fe/Texapon N70 aggregates are not spherical or cylindrical vesicles. An extreme situation was also considered, in which the (CTA)Fe/Texapon N70 mixtures are cylindrical vesicles, but with flat part > 698 Å. In this condition, the SANS data should fit well with the flat lamella and hollow cylinder complex model. Although the fitting data (Figure S2c) are quite similar to that of the catanionic complexes, the given δ is −3.68 ± 1.7 nm and the given scale factor is −1.02 ± 17.42, it is completely impossible. On the basis of the above results, it is reasonable to conclude that (CTA)Fe/Texapon N70 catanionic mixtures aggregate as bacillus-shaped bilayer vesicles containing both flat parts and bent edges, i.e., flat vesicles, rather than bended hollow cylinders or spheres. The dispersions of catanionic bacillus-shaped bilayer vesicles from (CTA)Fe/ Texapon N70 mixtures possess uniform, orange-yellow solutions due to the [FeCl3Br]− in macroscopic present birefringent (Figure S3, Lα phase) and are stable for several months without any changes at room temperature. A typical PLM image in Figure 3b shows the typical striped texture of the birefringent phase, which is an indicator of a lamellar phase.34 Based on the thermodynamics of surfactant self-assembly in solution, the stability of these elongated bilayer vesicles is very improbable because the flat parts and highly curved edges should coexist in a microstructure.16,17,35 Most shapes of E

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Figure 5. (a) SAXS spectra of catanionic mixtures with different XTexapon N70. (b) Variations of the 2Rh of catanionic complexes vs the mole fraction of anionic surfactants, X. T = 298 K. (c) ζ potential changes of the catanionic mixtures upon the addition of Texapon N70 at 298 K.

Figure 6. Schematic model showing the microstructure of a elongated bilayer vesicle which is drawn by the long axis. The bilayer vesicles were produced by cationic surfactant (CTA)Fe (orange) and anionic surfactant Texapon N70 (purple). The flat parts of elongated bilayer vesicles have an average length of long axis of 351.3 ± 31.7 nm and length of short axis of 112.3 ± 12.5 nm. The thickness of the double bilayers and the flat hollow is 49.2 ± 4.3 nm; the bilayer thickness, δ = 2.9 ± 0.1 nm. A section of right edge of an elongated bilayer vesicle has been removed for enhanced visibility.

but the length of long axis of elongated parts decreases to 313.3 ± 18.8 nm. It means that increasing the XTexapon N70 may cause a reduction of the equilibrium size of the elongated part of the vesicles. PLM observations on the sample solution of XTexapon N70 = 0.78 presents typical Maltese crosses (Figure 4b). The texture is quite different from that of a vesicle solution in Figure 3b, meaning that the aggregation behavior of catanionic mixtures in the two sample solutions of Lα- and Lα′-phase regions is probably different. Freeze-fracture TEM (FF-TEM) observations (Figure 1f) reveal that these catanionic mixtures of Lα′ phase form spherical vesicles at this molar ratio; the size of these vesicles is 68.0 ± 11.5 nm. The SAXS data (Figure 5a) of the catanionic mixtures at this ratio do not follow the q−2 behavior, indicating no appearance of elongated bilayer vesicles in the dispersion. The SAXS spectra of (CTA)Fe/Texapon N70 complexes at various molar ratios stepwise vary from that of elongated bilayer vesicles to that of spherical vesicles upon increasing the XTexapon N70 (Figure 5a), and almost remain constant as the XTexapon N70 reaches 0.67. This behavior predicates that there exists a transition from elongated bilayer vesicles to spherical ones as the molar fraction of anionic surfactant increases. DLS measurements show that the 2Rh of catanionic aggregates decreases stepwise vs the XTexapon N70

(Figure 5b), also confirming a transition from large-sized elongated bilayer vesicles to small-sized spheres. The ζ potential data (Figure 5c) of catanionic mixtures exhibit significant decrease as the transition of phase behavior from Lα to Lα′ occurs. An interesting phenomenon was found that the mixtures present weakly negatively charged at the stoichiometric point, suggesting that the catanionic aggregates are not electroneutral when cationic and anionic surfactants are in equal amount. Cryo-TEM image, as shown in Figure 1e, also verifies that catanionic aggregates at XTexapon N70 = 0.56 are in a sphereelongated coexisting state. On the basis of the above experimental results, one can conclude that adding excess anionic surfactants reduces the content as well as the equilibrium size of elongated bilayer vesicles and increases the whole curvature of catanionic aggregates. It is reasonable to infer that the highly curved edges of elongated bilayer vesicles are mainly composed of anionic surfactants. Within all of the approximations involved, a proposed schematic diagram satisfying all of the experimental observations and parameters for the elongated bilayer vesicles of (CTA)Fe/Texapon N70 mixtures was shown in Figure 6. The results of oscillatory shear experiments reveal that at a relatively high concentration the elongated bilayer vesicles F

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weakly negatively charged for absorbing [FeCl3Br]− as demonstrated by ζ potential data in Figure 5c, which enhances the charge repulsion among bilayers. The ionic strength in solution then becomes lower than that of conventional saltcontaining systems, which weakens the electrostatic shielding effect of counterions. The interplay between the two effects causes the absence of precipitation at the stoichiometric point of catanionic surfactants. Because of electrostatic interaction, the binding degree of [FeCl3Br]− with cationic surfactants should be much higher than that with anionic surfactants. The combination with largesized f-block anions can not only significantly increase the volume of the hydrophobic moiety of cationic (CTA)Fe, making cationic surfactants possess much higher edge energy and become more difficult to arrange in the highly curved parts of vesicles, but also screen the charge repulsion between (CTA) Fe molecules even in the absence of anionic surfactants. This behavior makes cationic (CTA)Fe approach each other to produce the flat parts of elongated bilayer vesicles at low edge energy cost. Due to the charge of partial (CTA)Fe neutralized by the hydrophobic counterions, the charge of some anionic surfactants cannot be neutralized. This part of anionic surfactants, which possesses low edge energy, repulses each other to produce the highly curved edges of elongated bilayer vesicles. To better demonstrate the importance of [FeCl3Br]− in inducing the formation of bacillus-shaped catanionic vesicles. A control experiment using mixtures of 50 mmol L−1 (CTA)Br and 40 mmol L−1 Texapon N70 was performed. As shown in Figure 7, the catanionic mixtures at this concentration and ratio

solutions do not present viscoelastic Bingham fluid properties as other vesicle systems of catanionic mixtures,40 in which the storage modulus G′ is higher than the loss modulus G″. In the current system, at low frequency, G′ is much lower than G″ and sharply increases vs the shear force (Figure S6). A shear thickening can be discovered at high frequency, suggesting a low viscosity of vesicular dispersions in macroscopic. This result indicates that the vesicular particles do not associate with each other to form closely condensed, polydispersive vesicles which present viscoelasticity,40 but maintain a good monodispersibility at high concentration; for example, ctotal = 90 mmol·L−1. As reported, cationic (CTA)Fe with a longer hydrophobic chain has higher edge energy than that of anionic Texapon N70 having a shorter alkyl chain, which is less suitable for arranging in the region of high curvature.19,41,42 As confirmed by Eastoe et al.32 and our previous work,43 the [FeCl3Br]− is of strong hydrophobicity and can even partition into the core of a cationic micelle through the hydrophobic interaction with the alkyl chains. In our system, the strong hydrophobicity of [FeCl3Br]− could be verified by surface tension (ST) measurements (Figure S5a) by comparing the critical aggregation concentrations (cacs) of (CTA)Fe/Texapon N70 and (CTA)Br/Texapon N70 catanionic mixtures with a fixed molar ratio of 5:4 (cationic:anionic). The aggregation of cationic and anionic surfactants is a consequence of the electrostatic interaction between the polar groups and the hydrophobic interaction between the alkyl chains.41,42 Obtaining the same polar groups and hydrophobic chains, the cac of catanionic complexes with (CTA)Fe (0.15 μmol·L−1) is almost 1 order of magnitude lower than that of mixtures with (CTA) Br (1.1 μmol·L−1). It seems that the association between [FeCl3Br]− and hydrophobic chains of surfactants enhances the whole hydrophobicity of catanionic mixtures to favor the aggregation of the two oppositely charged surfactants. Simultaneously, the large amount of [FeCl3Br]− counterions having bigger volume can compress and even partition into the bilayers of the elongated bilayer vesicles via the hydrophobic interaction. The formation of some vesicles having much thicker bilayers (δ = 4.6 ± 0.1 nm) that was found in Figure 1a,c may be a result of a large amount of hydrophobic [FeCl3Br]− entering in the bilayers. The entry of the more hydrophobic [FeCl3Br]− counterions into the hydrophobic bilayers of (CTA)Fe/Texapon N70 vesicles could cause: (i) partially cationic surfactants to exist as (CTA)Fe molecules instead of CTA+ ions, and the combination with [FeCl3Br]− may make anionic surfactants become more negatively charged and more repulsive; (ii) at the stoichiometric point, vesicles may become weakly negatively charged for absorbing some [FeCl3Br]− ions; and (iii) the ionic strength in solution would be weaker than that of a conventional salt-containing catanionic surfactant solution. According to DLVO theory, electrostatic repulsion among bilayers constitutes the main driving force in stabilizing vesicular dispersion.40 In a salt-free system, the repulsion between vesicles is much stronger because there are hardly any counterions in solution. In fact, salt-free catanionic surfactant mixtures do not form precipitates as the molar ratio of both components approaches 1:1.16−18,40 In a salt-containing system, because of the large amount of counterions, i.e., high ionic strength, it screens the charge repulsion among bilayers; vesicles could tend to aggregate and even coagulate. In the current system, because of the hydrophobic counterions partitioning into the bilayers of vesicles, vesicles become

Figure 7. Cryo-TEM image of cylinder micelles resulting from the selfassembly of 50 mmol·L−1 (CTA)Br and 40 mmol·L−1 Texapon N70.

cannot form bacillus-shaped vesicles; only short cylinderical micelles were obtained. According to packing constant theory,44 p = v0/al0, where v0 and l0 refer to the volume and the length of the hydrophobic chain and a refers to the molecular average area at the interface; the aggregation behavior of surfactants in solution depends a lot on their packing constants.44 It usually requires higher p for surfactants to form lamellar structures including vesicles than that for surfactants to form micelles.44 After replacing Br− with the large coordination ion [FeCl3Br]−, both the l0 and a of the catanionic mixtures remain unchanged. But because of the strong hydrophobicity of [FeCl3Br]−, the binding of large-sized [FeCl3Br]− ions on the alkyl chains may significantly increase the v0 of the surfactant (CTA)Fe, thereby highly increasing the p of (CTA)Fe/Texapon N70 mixtures and inducing the complexes to form vesicles instead of micelles. In most cases G

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Figure 8. Effect of a NdFeB magnet (1 T) through dodecane on a 50 mmol·L−1 (CTA)Fe/40 mmol L−1 Texapon N70 vesicular solution. The magent was slowly moved up and down by hand, the entire sequence left to right took about 30 s.

of the solvent medium. The morphology, the size, and surface potential of this new type of colloid particles may be considered for molecular templating. These elongated bilayer vesicles with flat parts could better fit with negatively charged rigid stretched biomacromolecules, such as DNA and protein, in geometry than conventional spherical vesicles and are more suitable to act as vectors for these biomolecules. We envision that these novel magnetic elongated bilayer vesicles could find wide applications in the fields of nanotechnology, material science, and biochemistry.

of catanionic mixtures, spherical vesicles are obtained because most of the counterions are hydrophilic;1,2,5,7,26,27,38,39 they can hardly partition into the hydrophobic bilayer and cause cationic surfactants less suitable for arranging in the high curvature part of vesicles to favor the formation of bacillus-shaped vesicles. Based on the above facts, one can conclude that the [FeCl3Br]− plays an important role in the formation of bacillus-shaped vesicles. SQUID magnetometry (Figure S5b) shows that the prepared bilayer nanovesicles are ferromagnetic complexes, with a typical “hysteresis loop” being traced,43 and the effective magnetic moment was determined to be 0.15 emu·g−1. The strong magnetism can be extended to magnetize and control the liquid surface. As shown in Figure 8, inserting a NdFeB magnet (1 T) into a two-phase solution which contains dodecane (upper phase) and a 50 mmol·L−1 (CTA)Fe/40 mmol·L−1 Texapon N70 elongated bilayer vesicle aqueous solution (bottom phase) was able to overcome both gravity and the oil−water interfacial tension (∼50 mN·m−1) and pull some of the magnetic fluids through the upper organic phase to be attracted by the applied magnet. This phase inversion is energetically unfavorable because it increases the oil/water interfacial area and inverts the density of the two liquids. The presence of this uncommon phenomenon may be caused by the formation of “magnetoactive” ordered elongated bilayer vesicles at the interface, which lowers the magnetic energy of the system.40 After removing the magnetic field, the original two-phase solution can be recovered. This ability to control the physicochemical properties and the orientation of colloid dispersions noninvasively and reversibly by simply switching the applied magnetic field “on” and “off” makes this kind of novel colloid particles applicable for manipulating the targeted delivery of cargo molecules, offering clean recovery of expensive products, as well as facile recyclability of the solvent medium.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.6b01564. Cryo-TEM images of elongated bilayer vesicles, SANS profiles, photographs of the sample solutions, phase diagram, surface tensions, SQUID magnetometry, and effect of a NdFeB magnet on magnetic migration of elongated bilayer vesicles (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +86-531-88366074. Fax: +86531-88564750. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

This work was funded by the National Natural Science Foundation of China (Grant Nos. 21420102006 and 21273134).

4. CONCLUSIONS In summary, we demonstrate the formation of elongated bacillus-shaped catanionic bilayer vesicles via simply mixing cationic surfactant (CTA)Fe and anionic surfactant Texapon N70 in aqueous solution, in which (CTA)Fe is in excess or equal amount. The hydrophobic [FeCl3Br]− ion from (CTA) Fe can partition into the hydrophobic bilayers via the hydrophobic interaction with the alkyl chains, thereby significantly increasing the edge energy of cationic (CTA)Fe to make them distribute in the elongated part of bilayers. It induces the formation of bacillus-shaped catanionic vesicles with both flat parts and bent edges. Besides, the partition of [FeCl3Br]− into bilayers also causes the catanionic complexes not to precipitate at the stoichiometric point. The bacillusshaped catanionic vesicles have strong magnetism, making them show potential applications in regulation of the targeted cargo delivery, clean recovery of expensive products, and recyclability

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DOI: 10.1021/acs.langmuir.6b01564 Langmuir XXXX, XXX, XXX−XXX