pubs.acs.org/Langmuir © 2009 American Chemical Society
Transition from Vesicle Phase to Lamellar Phase in Salt-Free Catanionic Surfactant Solution Zaiwu Yuan,†,‡,§ Shuli Dong,† Weimin Liu,‡ and Jingcheng Hao*,†,‡ †
State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, People’s Republic of China, ‡Key Laboratory of Colloid and Interface Chemistry, Shandong University, Ministry of Education, Jinan 250100, People’s Republic of China, and §Graduate School of the Chinese Academy of Sciences, Beijing 100080, People’s Republic of China Received February 24, 2009. Revised Manuscript Received April 29, 2009 A salt-free cationic and anionic (catanionic) surfactant system was formed by mixing a double-tailed di-(2-ethylhexyl) phosphoric acid (DEHPA, commercial name P204), which is an excellent extractant of rare earth metal ions, with a single-tailed cationic trimethyltetradecylammonium hydroxide (TTAOH) in water. With the mole ratio (r) of DEHPA to TTAOH varying from 0.9 to 1, the phase transition occurred from a densely stacked vesicle phase (LRv) to a lamellar phase (LRl). Macroscopic properties, such as polarization and rheology, were measured and changed greatly during the course of the phase transition. When r was 0.96 or 0.98, the steady state shear curves exhibited two yield stress values, indicating the coexistence of the LRv phase and the LRl phase. The LRl phase formed in the salt-free and zero-charged system (r = 1.0) is defective and undulating, which was confirmed by cryogenic transmission electron microscopy (cryoTEM). The deuterium nuclear magnetic resonance spectra (2H NMR) showed that a single peak (singlet) split into two symmetric peaks (doublet) gradually, indicating the phase transition from the LRv phase to the LRl phase. Correspondingly, phosphorus nuclear magnetic resonance spectra (31P NMR) presented changes in both the chemical shift and the peak width, indicating that these two types of bilayer structures are of different anisotropy degrees and different viscosities. When the LRl phase is subjected to a certain shear force, it can be reversed to a LRv phase again, which was proved by rheological, 2H NMR, and 31P NMR measurements. Furthermore, a theoretical consideration about the formation of the defective and undulating LRl phase was taken into account from a viewpoint of energy.
Introduction Bilayer structures (or LR phases) formed by surfactants in aqueous solution can mimic the real biological membranes in a simplified manner. Therefore, the LR phase has been drawing much attention.1-4 The LR phase is characterized by alternating stacked sheets of surfactant molecular bilayers consisting of water domains. In the LR phase, generally, the bilayer membranes can either adopt a zero-curvature to form a planar lamellar phase (denoted by the LRl phase) or be curled up into a closed bilayer structure, that is, vesicle phase (denoted by the LRv phase). Experiments have proved that the LRv phase may form spontaneously, that is, at thermodynamic stability.5-7 However, the LRv phase also may be strongly dependent on the sample history, that is, at thermodynamic metastability.8-12 For example, the LRv phase can be formed by a LRl phase through the introduction of *To whom correspondence should be addressed. Tel/Fax: +86-53188366074. E-mail:
[email protected]. (1) Gennis, R. B. Biomembranes: Molecular Structure and Function; SpringerVerlag: New York, 1989. (2) Rosoff, M. Vesicles. Surfactant Science Series; Marcel Dekker Inc.: New York, 1996; Vol. 62. (3) Sackmann, E.; Lipowsky, R. Handbook of Biological Physics; Elsevier: Amsterdam, 1995; Vol. 1. (4) Wilson, M. A.; Pohorille, A. J. Am. Chem. Soc. 1996, 118, 6580. (5) Jung, H. T.; Lee, S. Y.; Kaler, E. W.; Coldren, B.; Zasadzinski, J. A. Proc. Natl. Acad. Sci. 2002, 99, 15318. (6) Kaler, E. W.; Herrington, K. L.; Murthy, A. K.; Zasadzinski, J. A. J. Phys. Chem. 1992, 96, 6698. (7) Safran, S. A.; Pincus, P.; Andelman, D. Science 1990, 248, 354. (8) Bergmeier, M.; Hoffmann, H.; Thunig, C. J. Phys. Chem. B 1997, 101, 5767. (9) Hao, J.; Hoffmann, H.; Horbaschek, K. J. Phys. Chem. B 2000, 104, 10144. (10) Hao, J.; Yuan, Z.; Liu, W.; Hoffmann, H. J. Phys. Chem. B 2004, 108, 5105. (11) Lasic, D. D. J. Colloid Interface Sci. 1990, 140, 302. (12) Lasic, D. D. Nature 1991, 351, 613.
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external factors, such as shear force. In fact, besides the classical LRl phase and the closed LRv phase, another LR form, that is, the defective LRl phase, has also been widely studied recently.13-27 In the defective LRl phase, the continuous and integral membranes are perforated by water-filled pores or channels as a result of all of the water domains becoming interconnected. The formation of water-filled defects in the planar bilayers was regarded as related to the formation of so-called intermediate phases between classical lamellar phase and rodlike micellar structures.16-19,26 The formation of defects was thought of as a simple way to lower free energy through introducing curvature into the membranes without destroying the overall planar (13) Baciu, M.; Holmes, M. C.; Leaver, M. S. J. Phys. Chem. B 2007, 111, 909. (14) Baciu, M.; Olsson, U.; Leaver, M. S.; Holmes, M. C J. Phys. Chem. B 2006, 110, 8184. (15) Bagdassarian, C. K.; Roux, D.; Ben-Shaul, A.; Gelbart, W. M. J. Phys. Chem. 1991, 94, 3030. (16) Buhler, E.; Mendes, E.; Boltenhagen, P.; Munch, J. P.; Zana, R.; Candau, S. J. Langmuir 1997, 13, 3096. (17) Gustafsson, J.; Ora¨dd, G.; Almgren, M. Langmuir 1997, 13, 6956. (18) Gustafsson, J.; Ora¨dd, G.; Lindblom, G.; Olsson, U.; Almgren, M. Langmuir 1997, 13, 852. (19) Gustafsson, J.; Ora¨dd, G.; Nyden, M.; Hansson, P.; Almgren, M. Langmuir 1998, 14, 4987. (20) Holmes, M. C.; Leaver, M. S.; Smith, A. M. Langmuir 1995, 11, 356. (21) Hubbard, P. L.; McGrath, K. M.; Callaghan, P. T. Langmuir 2005, 21, 4340. (22) Kadi, M.; Hansson, P.; Almgren, M. J. Phys. Chem. B 2004, 108, 7344. (23) Krishnaswamy, R.; Ghosh, S. K.; Lakshmanan, S.; Raghunathan, V. A.; Sood, A. K. J. Colloid Interface Sci. 1981, 81, 150. (24) Matsuzaki, K.; Sugishita, K.; Ishibe, N.; Ueha, M.; Nakata, S.; Miyajima, K.; Epand, R. M. Biochemistry 1998, 37, 11856. (25) Nieh, M. P.; Raghunathan, V. A.; Wang, H.; Katsaras, J. Langmuir 2003, 19, 6936. (26) Ora¨dd, G.; Gustafsson, J.; Almgren, M. Langmuir 2001, 17, 3227. (27) Zipfel, J.; Berghausen, J.; Lindner, P.; Richtering, W. J. Phys. Chem. B 1999, 103, 2841.
Published on Web 05/18/2009
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geometry.15 Some of surfactant molecules reside around the pores to enjoy a more curved environment in comparison with those in the planar bilayers, which is consistent with the idea of molecular packing constraints.13,14 The study of phase diagrams plays an important role in a good understanding of the defective LRl phase. In the systems containing either nonionic surfactants or ionic ones, a continuous transition from the micellar to the LRl phase usually occurs through an intermediate perforated LRl phase as the composition ratio, concentration, or temperature changed. So, it was assumed as a matter of course that the curved defects, pores or channels, are introduced in bilayer membranes to achieve higher curvature interfaces. Some intuitive techniques, such as cryogenic transmission electron microscopy (cryo-TEM), were employed to reveal the microstructure in surfactant aqueous solutions, such as the vesicle phase, the classical LRl phase, and even the LRl phase with defects.17-19 Different surfactant self-assemblies can be distinguished by examining the anisotropy degree through 2H NMR (deuterium nuclear magnetic resonance) measurement. Baciu et al. have figured out that the quadrupolar splitting from heavy water is directly proportional to that from the R-deuterated surfactant in 2H NMR spectrum. Therefore, the more easily available heavy water can safely be used to monitor the variation of microstructures.14 The chemical shift of 31P NMR is also orientation-dependent, and the line shape is governed by chemical shift anisotropy (CSA) and viscosity.18,28 In our previous investigation on a salt-free catanionic surfactant system of double-tailed di-(2-ethylhexyl) phosphoric acid (DEHPA) with a single-tailed cationic surfactant trimethyltetradecylammonium hydroxide (TTAOH) in water, densely stacked and remarkably deformed multilamellar vesicles were mainly observed when TTAOH was slightly in excess (r < 1, where the molar ratio of the anionic surfactant to the cationic surfactant r = cDEHPA/cTTAOH).29 As our previous report, the vesicle phase was a viscoelastic solution. However, the solution samples can lose their viscoelastic property in a very narrow tongue in the phase diagram at near equimolar amounts of TTAOH and DEHPA (r = 1). In the present work, the experiments showed that the viscoelastic vesicle phase can be transitioned into the LRl phase with a trace addition of the hydrophobic DEHPA. Rheological measurements, polarizing optical microscopy (POM) observations, and the measurements of 2H NMR and 31P NMR spectra indicated the phase transition. Cryo-TEM images showed that the bilayer membranes in the LRl phase were undulating and perforated by random pores. If a certain shear force was introduced into the LRl phase, however, the LRl phase could be transformed back into a vesicle phase. Different from the theory of molecular packing constraints, the saddle-splay modulus, and the curvature modulus, the viewpoint of energy was considered to explain the curved defects in the bilayer membranes.
Experimental Section Chemicals. Tetradecyltrimethylammoniumbromide (TTABr) was purchased from Sigma-Aldrich Chemical Company. DEHPA was purchased from Shanghai Chemical Company (China). Both were directly used without further purification. An ion exchanger III was obtained from Merck Company. D2O (99.9%, deuterated) was obtained from Cambridges Isotope Laboratories, Inc. and used for 2H and 31P NMR measurements. Water was triply distilled. (28) Dong, S.; Xu, G.; Hoffmann, H. J. Phys. Chem. B 2008, 112, 9371. (29) Yuan, Z.; Yin, Z.; Sun, S.; Hao, J. J. Phys. Chem. B 2008, 112, 1414.
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Preparation of Cationic TTAOH Stock Solution. TTAOH stock solution was prepared from commercial bromide form (TTABr) aqueous solution (140 mmol L-1) with use of a strong base exchanger (ion exchanger III, Merck) at 40.0 ( 0.1 °C. Bromide ions could not be detected by AgNO3 in the TTAOH stock solution with excess HNO3 (Ag+ + Br- f AgBr, yellow precipitate), so the ion exchange with hydroxide was >99%. The total concentration of the stock TTAOH solution was determined by acid-base titration with 0.10 mol L-1 HCl to be 122.1 mmol L-1. The critical micelle concentration (cmc) of TTAOH was determined to be 1.8 mmol L-1. Surface Tension Measurements. The surface tensions were determined at a Processor Tensiometer-K12 (Swiss) with the Wilhelmy plate at method at 25.0 ( 0.1 °C. Phase Diagram. On the basis of observations collected on more than 150 samples with 10 mmol L-1 < cTTAOH < 100 mmol L-1 after equilibration for 10 weeks, the phase diagram of TTAOH/DEHPA/H2O at 25.0 ( 0.1 °C was established. Phase boundaries were delineated based on visual observations, which remained unchanged over an extended period of time, and were also carefully determined by conductivity measurements. POM Observations. Preliminary studies involved visual inspection either with or without crossed polarizers to verify the homogeneity and birefringence of the sample solutions. POM measurements of birefringent sample solutions were performed using a Carl Zeiss Axioskop 40 light microscope (Germany). Conductivity Measurements. The conductivity measurements were performed on a conductivity meter of DDSJ-308A (China) at 25.0 ( 0.5 °C. Two-phase samples were stirred during the conductivity measurements. Rheological Measurements. The rheological properties of the system were measured using a dynamic shear rheometer with a concentric cylinder measurement cell (Rheostress RS75 HAAKE). The slit between the inner cylinder and the outer cylinder was 3 mm. Samples were placed in the temperaturecontrolled measurement vessel and allowed to equilibrate to the required temperature (25 °C) for 5 min prior to the measurements. An oscillatory frequency test was used to measure the magnitude of the elastic modulus (G0 ), viscous modulus (G00 ), and the complex viscosity (η*) of the system. Cryo-TEM Images. Carbon film grids with a hole size between 1 and 12 mm were used for specimen preparation. A 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 vitrified by plunging them into liquid ethane in a temperature-controlled freezing unit (Zeiss, Oberkochen, Germany). After the specimens were vitrified, the remaining ethane was removed using blotting paper. The specimens were inserted into a cryogenic-transfer holder (Zeiss, Oberkochen, Germany) and transferred to a Zeiss CEM 902, equipped with a cryo-stage. Examinations were carried out at a constant temperature of 90 K. The TEM was operated at an accelerating voltage of 80 kV. Zeroloss filtered images (ΔE = 0 eV) were taken under low dose conditions. 2 H and 31P NMR Measurements. All of the sample solutions were dissolved in D2O. 2H NMR and 31P NMR spectra were recorded on a Bruker Avance 400 spectrometer equipped with pulse field gradient module (Z-axis) using a 5 mm NMR sample tube. All of the experiments were operated at 25.0 ( 0.1 °C. For the sample solutions with a certain shear force, 2H NMR and 31 P NMR experiments were carried out as soon as the shear was completed with the rheometer to avoid as possible that these sample solutions revert to the lamellar state of thermodynamic stability. DOI: 10.1021/la900662w
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Figure 1. Phase diagram of TTAOH/DEHPA/H2O system at T = 25.0 ( 0.5 °C. At cTTAOH = 90 mmol L-1 (the dot line) and with the gradual addition of DEHPA, the solutions undergo an L1 phase, an LRv-L1 two phase, a birefringent LR phase (including LRv phase and LRl phase), and then a two phase sequentially. The coexistence region, including both the LRv and the LRl phases without phase separation, is near the dashed line.
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Figure 3. Textures observed by POM. The images in panels a and b are the polarizing patterns for the two samples 1 and 4 in Figure 2. Panel c is the polarizing pattern of sample 4 subjected to a steady : shear process with shear rate γ = 50 s-1 for 0.5 h.
Figure 2. Photographs of two sample solutions within LR phase domains without (left row) and with (right row) polarizers at 25 °C. Letters a and a0 represent sample 1 at r = 0.9, and letters b and b0 represent 4 at r = 1 in the phase diagram of Figure 1.
Results and Discussion In our previous work,29 we have delineated the detailed phase diagram of the ternary salt-free catanionic surfactant system, the TTAOH/DEHPA/H2O system, at ctotal e 200 mmol L-1. It is necessary to redisplay the phase diagram, as shown in Figure 1. Different amounts of an excellent extractant, double-tailed DEHPA, were gradually added into a series of single-tailed TTAOH aqueous solutions with constant concentrations. The samples with cTTAOH = 90 mmol L-1 were chosen to observe the phase sequence. A transparent and low-viscosity single L1 phase, that is, a spherical micelle phase, formed at cDEHPA < 52 mmol L-1. After a diphase equilibrium at 52 mmol L-1 < cDEHPA < 70 mmol L-1, the birefringent LRv phase containing densely stacked and remarkably deformed multilamellar vesicles was confirmed by cryo-TEM. However, when TTAOH is kept at 90 mmol L-1, within the birefringent LR phase domain, one can find that both the polarization and the rheological properties change obviously with the increase of the concentration of DEHPA. When the mole ratio r reaches to near 1, the properties become greatly different from that with r = 0.9. One can see that both samples 1 and 4 denoted on the dotted line in Figure 1, at r = 0.9 and r = 1, respectively, are birefringent, as viewed in Figure 2 between crossed polarizers. Nevertheless, sample 1 at r = 0.9 is weakly birefringent, which was a “Schlieren” texture. Sample 4 at r = 1 is strongly birefringent, which was a “domainlike” texture,30-33 where all of the colors of the visible spectrum can be seen. The polarizing (30) Bergmeier, M.; Gradzielski, M.; Hoffmann, H.; Mortensen, K. J. Phys. Chem. B 1999, 103, 1605. (31) Haas, S.; Hoffmann, H.; Thunig, C.; Hoinkis, E. Colloid Polym. Sci. 1999, 277, 856. (32) Hoffmann, H.; Thunig, C.; Schmiedel, P.; Munkert, U. Langmuir 1994, 10, 3972. (33) Horbaschek, K.; Hoffmann, H.; Hao, J. J. Phys. Chem. B 2000, 104, 2781.
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Figure 4. Rheological curves of the vesicle phase of sample 1 at r = 0.9. The oscillation frequency sweep mode (a) and the steady state shear mode (b).
patterns are observed with POM for both sample 1 at r = 0.9 and sample 4 at r = 1, as shown in Figure 3. One can clearly see the crosslike spherulites, indicating the existence of bilayer structures. Generally, the microstructures should be reflected by the macroscopic properties. For example, the surfactant self-assemblies in aqueous solutions can lead to the change of the rheology of the solutions. We employed steady state stress and viscosity curves to prove that the LR phase solutions at r = 0.9 are of yield values, which are necessary for vesicles to overcome the bending energy of deformation before flow. This indicates that the LR phase is composed of vesicles, that is, the LRv phase. We employed the CS (control stress) mode in the oscillatory frequency sweep experiment to investigate the LRv phase, as shown in Figure 4. We selected the shear stress to be 0.03 Pa, which has been proved to be in the linear viscoelastic region. From the rheograms, it can be seen that the storage modulus G0 and the loss modulus G00 remain more or less constant at about 20 and 2 Pa, respectively, and G0 is about 1 order of magnitude higher than G00 . The complex Langmuir 2009, 25(16), 8974–8981
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Figure 5. Rheological curves of the LRl phase of sample 4 at r = 1.
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Figure 6. Oscillation frequency sweep curves for sample 2 at r =
The oscillatory frequency sweep mode (a) and the steady state shear mode (b).
0.96 (a) and sample 3 at r = 0.98 (b). From the curves, two yield stress values can be seen.
viscosities η* decrease over the whole frequency range investigated. The rheological properties should characterize a typical vesicle phase.32,34,35 However, when the concentration of DEHPA is the same as that of TTAOH, that is, r = 1, some substantial changes in the rheological behavior occur. Figure 5a shows the CR mode by the oscillation experiment; the storage modulus G0 and the loss modulus G00 do not keep constant any longer but increase with oscillatory frequency v. Moreover, G00 keeps higher than G0 , which means that the flow ability dominates the rheological behavior under the small shear stress (0.03 Pa), while the following steady state viscosity curve in Figure 5b behaves more interestingly. At first, the apparent viscosity η decreases with increasing shear stress, showing shear-thinning behavior, until it reaches a minimum. Then, η turns to increase sharply, and there obviously exists a peak value. In the subsequent shearing stage, η decreases again with an increase of shear rate, in which the rheological properties are completely different from the case in Figure 4b. As proved by the following cryo-TEM and 2H NMR, sample 4 at r = 1 is a typical lamellar phase, that is, a LRl phase. Unlike the vesicle phase with r = 0.9, which needs to overcome the deforming energy (yield stress) before flowing, the LRl phase can flow under an extremely low shear stress, because the sliding motion of lamellas against each other becomes much easier than that of vesicles. Therefore, the rheological property characterizes nice flow ability under a small shear stress. The flow behavior of the LRl phase is shear-thinning upon shear. With further increasing of shear rate, however, the LRl phase would be transformed into vesicles by the input of the outside shear force, which has been studied in detail in our previous work and other studies.9,30,32,34 Therefore, the apparent viscosity η will increase with shear stress increase until the lamellar phase is basically transformed into vesicles, and then, the η will reach a maximum value. The vesicle phase behaves shear thinning property again with further shear.
The variation in textures also verified the transition from the LRl phase to the LRv one under the effect of shear force. The patterns observed with POM in Figure 3 still showed crosslike spherulites after a steady shear process with shear rate of γ γ· = 50 s-1 for 0.5 h (Figure 3). However, the “domainlike” texture turned into a “Schlieren” (not shown here) between polarizers, which indicated the transition from a LRl phase to a LRv phase. More interestingly, two yield stress values can be seen if the concentration is intermediate and lies in the coexistence regions of the LRv and LRl phases. The first yield stress τ01 mainly originates from the original vesicles before shear, while the second yield stress τ02 contributes to the original vesicles with other structures induced by shear force. Figure 6 shows the rheological properties of two samples at mole ratios of r = 0.96 (sample 2) and r = 0.98 (sample 3), respectively, with steady state shear force. One can see that the apparent viscosity η decreases upon the increase of shear rate γ at first, reaches a maximum value, and finally decreases gradually again. During the shearing process, the LRl phase transits into the LRv phase gradually, which leads η to increase and reaches a maximum value. During the transition course, accordingly, the shear stress increases fast and then increases slowly after reaching a knee point. Different from the pure lamellar phase, two yield stress values appear due to the coexistence of vesicle and lamellar phases. The first yield stress τ01 of sample 2 is apparently higher than that of sample 3 due to containing more vesicles relatively before shear. At the same time, one can notice that the second yield stress τ02 of sample 2 is also higher than that of sample 3 due to loading more positive charges on the surface of vesicles. Because great changes of macroscopic behavior occurred both in rheology and in polarization, the change of the microstructure could be expected. Cryo-TEM was employed to determine the microstructure of the system 90 mmol L-1 TTAOH/90 mmol L-1 DEHPA. One can clearly see the lamellar structure in the system from the cryo-TEM images (Figure 7), where optical density contrast can be seen due to the hierarchical lamellas. Therefore, for the systems with excess TTAOH, a trace addition of DEHPA
(34) Escalante, J. I.; Gradzielski, M.; Hoffmann, H.; Mortensen, K. Langmuir 2000, 16, 8653. (35) Li, X.; Dong, S.; Jia, X.; Song, A.; Hao, J. Chem.;Eur. J. 2007, 13, 9495.
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Figure 7. Cryo-TEM images: (a) An image of a multilamellar LRv phase sample of 90 mmol L-1 TTAOH/81 mmol L-1 DEHPA, (b) a
magnified image of panel a, (c) an image of defective and undulating LRl phase sample of 90 mmol L-1 TTAOH/90 mmol L-1 DEHPA, and (d) a magnified image of panel c. From panels c and d, the swelling phenomenon is also distinct.
induced the densely stacked multilamellar vesicle phase to transit into the lamellar phase. Another characteristic of the lamellar structure could also be obviously observed; that is, there existed a lot of random pore defects with radii between 10 and 40 nm in the bilayer membranes. Meanwhile, the swelling phenomenon was also distinctly observed from the cryo-TEM images. Noting that the bilayer membranes exhibit lacelike edges and intraoptical density-contrast fluctuation, we conclude that the membranes are greatly undulated. Because of the stoichiometric mixing of the catanionic system, there are no excess surface charges to stabilize the colloid solution. So, the Helfrich undulation repulsion force must be a dominant stabilizing factor. The anisotropy of surfactant self-assemblies in aqueous solutions can be determined by the 2H NMR. For macroscopically anisotropic systems formed by surfactants in aqueous solutions, such as a long-range ordered LRl phase, water molecules in the solutions can be thought of as divided into two species.17 Some are bound to the anisotropic surfactant aggregates, and the others are free in an isotropic environment. Of course, fast exchange may occur between the water molecules in the two sites. When D2O molecules are used to prepare the anisotropic surfactant solutions instead of H2O ones in the 2H NMR experiment, D2O molecules in the anisotropic site result in a nonzero average of the 8978 DOI: 10.1021/la900662w
quadrupolar interaction. Therefore, doublet quadrupole splitting can be seen in the birefringent LRl phase. However, the case for the vesicle phase is different from the LRl phase. The vesicle phase is composed of closed bilayers of surfactants, and the D2O molecules at the interface of the closed bilayers exhibit local anisotropy rather than macroscopic anisotropy. So, the electric quadrupole couplings between the deuterium nuclei (spin I = 1) and the electric field gradients are interneutralized to be a zero average, leading to a single peak on 2H NMR spectra. It can be seen from the spectrum profile in Figure 8, which is a singlet of the densely stacked vesicle phase of sample 1 at r = 0.9. When the vesicle phase transits into the LRl phase, that is, sample 4 with r = 1, the single peak splits into two symmetric peaks completely. We even observed the intermediate state of the peak splitting at the coexistence regions of vesicles and lamellas at r = 0.96 and 0.98, respectively. When sample 4 is subject to a certain shear stress, however, the doublet reverts to a singlet, indicating that the LRl phase transits into a vesicle phase again. The physicochemical environment of a surfactant molecule is expected to be quite different for different surfactant self-assembled structures. The line profile of 31P NMR spectra was also determined for the four samples at different mole ratios denoted on the dashed line in Figure 1. From Figure 9, a broad resonance peak is observed in the 31P NMR spectrum for sample 1 in the Langmuir 2009, 25(16), 8974–8981
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Figure 8. Quadrupolar nuclear magnetic resonance spectra recorded from the vesicle phase (a) to the coexistence of the vesicle and LRl phase (b and c), and then to the LRl phase (d). Parts a, b, c, and d correspond to samples 1, 2, 3, and 4, respectively. Part e corresponds to the sample 4 after shear imposed.
vesicle region, reflecting the time-averaged environment of the anionic DEHPA and indicating that the exchange of DEHPA molecules between vesicles is slow on the NMR time scale due to the higher viscosity. As compared with the signal from the vesicle phase, a narrow peak for sample 4 was obtained, indicating a fast exchange of the anionic DEHPA molecules between the lamellar self-assemblies due to the lower viscosity. Furthermore, the peak of sample 4 shifts toward the low frequency side. This is because the chemical shift of 31P NMR is orientation-dependent, and the LRl phase is of higher CSA than the vesicle phase. The CSA of samples 2 and 3 that fall into the coexistence region containing both vesicle and lamellar phase is between the two instances mentioned above from Figure 9. When sample 4 of the lamellar phase underwent a shearing process, the peak shifted to a higher frequency and became broad again due to the transition from the LRl phase of good fluidity to the viscoelastic vesicle phase. The continuous phase sequence of the DEHPA/TTAOH/H2O system at cTTAOH = 90 mmol L-1 can be seen from the phase diagram: from a micellar phase to a strongly deformed multilamellar vesicle phase and then to a defective lamellar phase with the gradual addition of DEHPA into TTAOH solutions. Generally, the phase sequence of other surfactant systems, both nonionic and ionic amphiphilic systems,17-19,22,26,37 is from a spherical micellar phase via a rodlike micellar phase, then to a transitional defective lamellar phase, and finally to a classical (36) Medronho, B.; Shafaei, S.; Szopko, R.; Miguel, M. G.; Olsson, U.; Schmidt, C. Langmuir 2008, 24, 6480. (37) Porte, G.; Gomati, R.; El Haitamy, O.; Appell, J.; Marignan, J. J. Phys. Chem. 1986, 90, 5746.
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lamellar phase. For the current catanionic surfactant system, the single-tailed cationic surfactant, TTAOH, can self-assemble into globular micelles with a spontaneous radius of curvature equal to its molecular length. With the addition of DEHPA, the strong acid-base reaction between the two components caused DEHPA to slowly dissolve into the TTAOH micelle solutions. Simultaneously, the increase of spontaneous radius of curvature induced the formation of molecular bilayers at low concentrations due to the strong reduction in area per headgroup resulting from catanionic ion pairing. So, there was not a rodlike micellar phase in the present system when the composition ratios varied as reported elsewhere. This verified the strong interaction between TTAOH and DEHPA molecules as a result that the spontaneous radius of curvature would increase largely, and molecular bilayers were formed without the transition of a rodlike micellar phase. The viscoelastic and multilamellar vesicle phase was retained until arriving at a near stoichiometric ratio, where there were nearly no excess charges, and the defective lamellar phase was formed. Correspondingly, the macroscopic properties such as the birefringence and the rheological properties of the solutions changed abruptly. As the concentration of DEHPA increased, the OH- counterions of TTAOH were neutralized by the H+ ions dissociated by DEHPA. On the other hand, a part of charged ions was enclosed in the water cores of the vesicles after the LRv phase was formed. So, the conductivity decreased quickly before arriving at a stoichiometric ratio between DEHPA and TTAOH, and both the ionic strength and the structure charge density decreased as well. When arriving at a stoichiometric ratio, there were nearly no excess ions in the system, the conductivity was still kept small DOI: 10.1021/la900662w
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Figure 9. Line profile of 31P NMR spectra for samples 1-4; parts a, b, c, and d correspond to the samples 1, 2, 3, and 4, respectively. Part e corresponds to sample 4 after shear imposed.
(data not shown here), and the defective and undulating lamellar phase was formed in the salt-free and zero-charged catanionic surfactant solutions. The classical expression about the bending elastic energy per unit area associates the energy of surfactant membranes with their optimal morphologies:38 1 1 1 2 2 1 fc ¼ K þ þK 2 R1 R2 R0 R1 R2
ð1Þ
where fc is the curvature free energy per unit area of the membrane, R1 and R2 are the principle radii of the curvature of the structures, R0 is the spontaneous radius of the curvature, κ is the curvature modulus, and κh is the saddle-splay modulus. At a stoichiometric ratio of cat- and anionic surfactants (r = 1), the systems should belong to the case of low salt concentration and low charge density, the so-called salt-free and zero-charged phase. Previous theoretical studies have indicated that the electrostatic contribution to the bending modulus of mean curvature κ of surfactant membranes, that is, δκ, can in general be smaller than the intrinsic value of the bending modulus in this regime.39-42 Both the thermal fluctuation that dominates over the bending elasticity of the membranes and the Helfrich forces that dominate over the other interactions are responsible for the flexible and (38) Jung, H. T.; Coldren, B.; Zasadzinski, J. A.; Iampietro, D. J.; Kaler, E. W. Proc. Natl. Acad. Sci. 2001, 98, 1353. (39) Harden, J. L.; Marques, C.; Joanny, J. F.; Andelman, D. Langmuir 1992, 8, 1170. (40) Higgs, P. G.; Joanny, J. F. J. Phys. (Paris) 1990, 51, 2307. (41) Pincus, P.; Joanny, J. F.; Andelman, D. Europhys. Lett. 1990, 11, 763. (42) Winterhalter, M.; Helfrich, W. J. Phys. Chem. 1992, 96, 327.
8980 DOI: 10.1021/la900662w
swollen LRl phase. Comparing the case of r < 1 with that of r = 1, the Debye screening length, which is associated with a minimal conductivity, becomes relatively large. The unscreened very small surface charge density softened the bilayer membranes.43 So, the vesicle phase transitioned into the undulating LRl phase with a decrease in membrane rigidity. We speculated that the strong undulation could correspond to the lacelike edges and the optical density-contrast fluctuation presented on the cryo-TEM pictures due to the variation of membrane thickness along electronic transmission direction, which might be attributed to the heavily reduced electrostatic repulsion. The defective pores in the LRl phase have ever been discussed a lot by others. One viewpoint classified onionlike vesicles and defective lamellas in terms of extrinsic and intrinsic defect types, respectively,17-19,26 and the existence of curvature defects in the LRl phase were regarded as one mean to lower the local free energy of surfactant packing. In consideration of the topology, the defective pores were thought of as semitoroids embedded into the bilayer membranes. The semitoroids are cylindrical sections with one surfactant length as radius and have a high spontaneous curvature. Every semitoroid pore is the inner half of an intact toroid (Figure 10). Besides the high and positive spontaneous curvature, as a matter of fact, the semitoroid pores are of negative Gaussian curvatures. The values of Caussian curvature (K = 1/R1R2) of the semitoroid pores can be calculated with the following equation: K ¼
cos u ðb > aÞ aðb þ a cos uÞ
ð2Þ
(43) Oberdisse, J.; Couve, C.; Appell, J.; Berret, J. F.; Ligoure, C.; Porte, G. Langmuir 1996, 12, 1212.
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a shift in the balance of κ domination to κh domination due to the increase of the anionic surfactant.26,43,47
Figure 10. Models of defective and undulating lamellae, a pore, and an intact toroid that are individed into an inner half and an outer half. Some available parameters are given in the toroid model.
where, the parameters a, b, and u can be referred to as the toroid model in Figure 10. Because the u value of the inner half of toroids is between 90° and 270°, the values of Gaussian curvature of these defective pores are negative. One can compare the K of negative values for pores with that of positive values for disk edges,10,44-46 which is the outer half of toroids and u is between -90° and 90° from the model in Figure 10. So, both the spontaneous curvature and the Gaussian curvature shifted with the increase of the concentration of DEHPA. A similar observation of punctured planes was obtained in the dilute catanionic solutions of myristic acid (C13H27COOH) and cetyltrimethylammonium hydroxide [C16H33N(CH3)3OH],10,44-46 and the authors held that a part of water-insoluble anionic surfactant molecules can separate and reside around the pores in membranes. Therefore, the variation of the morphology from onion vesicles to defective lamellas suggests (44) Dubois, M.; Deme, B.; Gulik-Krzywicki, T.; Dedieu, J. C.; Vautrin, C.; Desert, S.; Perez, E.; Zemb, Th. Nature 2001, 411, 672. (45) Dubois, M.; Lizunov, V.; Meister, A.; Gulik-Krzywicki, T.; Verbavatz, J. M.; Perez, E.; Zimmerberg, J.; Zemb, Th. Proc. Natl. Acad. Sci. 2004, 101, 15082. (46) Zemb, Th.; Dubois, M.; Deme, B.; Gulik-Krzywicki, T. Science 1999, 283, 816.
Langmuir 2009, 25(16), 8974–8981
Conclusions We studied the phase transition between the vesicle phase and the lamellar phase based on the previous study on the densely stacked and deformed vesicle phase in a salt-free system. Strongly basic single-tailed TTAOH was neutralized by double-tailed DEHPA. Two types of LR phase formed successively in the system: an onion lamellar phase with excess positive charges and a defective and undulating lamellar phase with zero-charge. Phase transition occurred between the two LR phases when the mole ratios varied or when a certain shear force was imposed. When the two surfactants were mixed at a stoichiometric ratio, the surface charge density decreased to a minimum. Without surface charge on the bilayer membranes, the electrostatic contribution to the bending modulus of mean curvature of surfactant membranes is small and the thermal fluctuation dominates over the intrinsic bending elasticity of the membranes. Thus, the membranes show great undulation as compared with the onion vesicles charged positive electricity. Both the spontaneous curvature and the Gaussian curvature of the membranes can change when the defective and undulating lamellar phase is formed. The system at stoichiometric ratio is analogous to nonionic surfactant ones, but the difference is that the microsegregation can occur between the cat- and the anionic surfactant molecules, which may perhaps lead a part of anionic surfactant molecules to separate and reside around the pores in the membranes. Acknowledgment. We are thankful for the financial support by the NSFC (Grant No. 20625307) and National Basic Research Program of China (973 Program, 2009CB930103). We are grateful to Prof. Dr. Yeshayahu (Ishi) Talmon of RBNI Nanotechnology, Technion, Israeli, for the useful discussions about the cryoTEM images. (47) Herve, P.; Roux, D.; Bellocq, A. M.; Nallet, F.; Gulik-Krzywicki, T. J. Phys. II France 1993, 3, 1255.
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