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Various Bilayer Organizations in a Single-Tail Nonionic Surfactant: Unilamellar Vesicles, Multilamellar Vesicles, and Flat-Stacked Lamellae O. Regev*,† and F. Guillemet‡ Department of Chemical Engineering, Ben-Gurion University of the Negev, P.O. Box 653, 84105 Beer-Sheva, Israel, and Centre d’Application de Levallois, Elf-Atochem, 95, rue Danton, 92300 Levallois Perret, France Received July 24, 1998. In Final Form: March 16, 1999 Bilayer structural evolution of a single-tail nonionic surfactant cocodiethanolamide-water system is studied using cryo-transmission electron microscopy (cryo-TEM), small-angle X-ray scattering, light scattering, and rheological measurements. It is found that, with increasing surfactant concentration, unilamellar vesicles turn to multilamellar vesicles while at higher concentrations a fully expressed lamellar phase is clearly observed in the TEM micrographs. The results will be discussed in terms of surfactant geometry and membrane flexibility.
Introduction Surfactant molecules are known to self-assemble in a variety of structures.1 In some cases, the basic building unit is locally a bilayer of surfactant molecules. Examples of such structures are the lamellar, LR, the sponge, L3, or the vesicle, L4, phases. The stability of these various organizations can be understood in the framework of the membrane elasticity theory. According to Helfrich,2 the free energy, dEel, necessary to bend a bilayer area element, dA, of principal curvatures c1 and c2 can be written as
dEel )
[21κ(c + c ) + κjc c ] dA 2
1
2
1 2
(1)
where κ and κj are, respectively, the mean and the Gaussian curvature rigidity moduli. κ, which is of the order of kT for fluid membranes, controls the rigidity of the bilayer, while κj plays an important role in the structure of the membrane. The relations between the moduli define three distinct regimes: when κj < -2κ (assuming that κ is positive), the membrane prefers to curve into disconnected aggregates such as vesicles. When -2κ < κj < 0, the membrane is planar and flat-stacked lamellae are stabilized. Finally, when κj > 0, highly connected surfaces with handles are favored and the sponge phase forms. Therefore, a transition from the vesicle phase, L4, to the smectic lamellar phase, LR, to the sponge phase, L3, is theoretically predicted as κj is increased. This sequence of phases has been observed experimentally in pseudo ternary systems as a function of an increased alcoholto-surfactant ratio.3,4 In these systems the addition of alcohol molecules could be used for a fine-tuning of the aggregate curvature; however, it is not necessarily the only driving force for phase transition. * To whom correspondence is addressed. † Ben-Gurion University of the Negev. ‡ Centre d’Application de Levallois. (1) Gelbart, W. H.; Ben-Shaul, A.; Roux, D. Micelles, membranes, microemulsions and monolayers; Springer-Verlag: New York, 1994. (2) Helfrich, W. Z. Naturforsch 1973, 28c, 693. (3) Herve, P.; Roux, D.; Bellocq, A.-M.; Nallet, F. J. Phys. II 1993, 3, 1255. (4) Hoffmann, H.; Thunig, C.; Schmiedel, P.; Munkert, U. Langmuir 1994, 10, 3972.
In addition to these three bilayer-containing phases, it has been found that the lamellar phase can be divided into two distinguishable regions of different supramolecular structures.4-9 Observation under optical polarizing microscopy shows two types of textures: oily streaks at a high alcohol-to-surfactant ratio (for the systems mentioned above), characteristic of focal conic domains of the first type (FCD I) with negative Gaussian curvature (c1c2 < 0) and Maltese crosses at a low alcohol-to-surfactant ratio, characteristic of focal conic domains of the second type (FCD II) with positive Gaussian curvature (c1c2 > 0). This latter bilayer organization consists of multilamellar vesicles and is sometimes referred to as the onion phase. It is of particular interest since it exhibits elastic rheological properties.4,10 A similar onion phase can also be produced by shearing a classical lamellar phase.11,12 We note that the onion phase is a synonym for a multilamellar vesicle phase. This phase is only made of these types of objects and hence defect-free. In this paper, we describe an experimental investigation of the L4 to LR phase transition. The motivation is to investigate whether unilamellar vesicles turn, with increasing concentration, directly into flat-stacked lamellae or an intermediate onion phase exists. In the present work, we study the phase behavior of a nonionic, bilayer-forming surfactant as a function of concentration. The simplicity of this binary water-surfactant system allows us to detect the pure effect of surfactant concentration on the morphology and on the rheological behavior. In particular, we show the evolution of various structures of the bilayer, from unilamellar to multilamellar vesicles and finally to a flat-stacked lamellar phase. (5) Benton, W. J.; Miller, C. A. J. Phys. Chem. 1983, 87, 4981. (6) Gomati, R.; Appell, J.; Bassereau, P.; Marignan, J.; Porte, G. J. Phys. Chem. 1987, 91, 6203. (7) Boltenhagen, P.; Lavrentovitch, O. D.; Kleman, M. Phys. Rev. A 1992, 46, 1743. (8) Auguste, F.; Douliez, J.-P.; Bellocq, A.-M.; Dufourc, E. J.; GulikKrzywicki, Th. Langmuir 1997, 13, 666. (9) McGrath, K. M. Langmuir 1997, 13, 1987. (10) Oberdisse, J.; Couve, C.; Appell, J.; Berret, J. F.; Ligoure, C.; Porte, G. Langmuir 1996, 12, 1212. (11) Diat, O.; Roux, D.; Nallet, F. J. Phys. II 1993, 3, 1427. (12) Panizza, P.; Roux, D.; Vuillaume, V.; Lu, C.-Y. D.; Cates, M. E. Langmuir 1996, 12, 248.
10.1021/la980935h CCC: $18.00 © 1999 American Chemical Society Published on Web 05/22/1999
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Figure 1. Geometry of the shear cell. The solution passes from left to right and is sheared in the center of the cell where the cross-sectional area is reduced (1-mm wide). The narrowing channel is observed through crossed polarizers (cf. Figure 5).
Experimental Section Materials. The nonionic surfactant used in this study is N,Nbis(2-hydroxyethyl)cocamide, CnH2n+1CON(C2H4OH)2, and is referred to as CDEA (cocodiethanolamide). It is purchased from McIntyre under the commercial name Mackamide LMD. The hydrophobic moiety is a mixture of 80% n ) 12 and 20% n ) 14 chains. The surfactant is used as supplied without purification. The density is 0.981 g/mL at 25 °C and the Kraft temperature is 22 °C. The water used is of ultrapure quality filtered through Millipore cartridges. Sample Preparation, Optical Microscopy, and Phase Diagram. In this paper, the surfactant concentration is given in wt %. To construct the phase diagram, samples are prepared at 25 °C by mixing an appropriate amount of surfactant and water in small screw-capped vials. The solutions are gently stirred in order to avoid foam formation and then stored at 25 °C for equilibration for at least 2 days. To check if any phase separation process is taking place, the samples were centrifuged at 3000 rpm for 4 h. The samples are also prepared in various ways (heating and cooling, dilution). Samples which remain homogeneous after the above treatments are considered monophasic. The samples are observed by polarizing light microscopy using a Leitz microscope. Characteristic defects of the lamellar phase,7 i.e., Maltese cross (FCD II) and oily streaks (FCD I), are visualized by optical polarizing microscopy. Some samples exhibit strong flow birefringence and were studied with a flow cell (cf. Figure 1) under crossed polarizers. Surface Tension. Surface tension, γ, is measured by a Lauda tensiometer (open-frame geometry) after equilibration of the solution for typically 2 h. The surface tension data below the critical micelle concentration, CMC, are fitted by a linear curve for the determination of the slope. The maximum surface excess concentration, Γ, and the polar-head surface area at the CMC, a0, are calculated from the Gibbs equation:13
Γ ) (-1/RT) (dγ/d ln C)
(2)
a0 ) 1/NnΓ
(3)
where C is the concentration of the surfactant, N the Avogadro number, R the gas constant, and T the temperature. Light Scattering. Dynamic light scattering experiments (DLS) are carried out using a PCS 100M Malvern spectrometer. The light source is a helium-neon laser (60 mW at 632.8 nm) and the scattered light intensity is detected at 90°. The samples are prepared by mixing the pure surfactant and water, both previously filtered at 0.2 µm. The autocorrelation functions of the scattering curves are analyzed using the CONTIN program developed by Provencher14 to give the distribution of the apparent diffusion coefficient, Dapp. The apparent hydrodynamic radius, RH, is then calculated using the Stokes-Einstein relationship:
RH ) kT/6πηDapp
(4)
where k is the Boltzmann constant, T the absolute temperature, and η the viscosity of the solvent. (13) Rosen, M. J. Surfactants and interfacial phenomena, 2nd ed.; John Wiley and Sons, Inc.: New York, 1989. (14) Provencher, S. W. Makromol. Chem. 1985, 82, 632.
Turbidity. Turbidity is measured with a UV-visible Hach 2100A turbidimeter. X-ray Measurements. Small-angle X-ray scattering (SAXS) curves are measured by a rotating anode X-ray generator using a Cu KR radiation (λ ) 1.54 Å) selected by means of Nichols filters. The sample-to-detector distance is 0.43 or 1.33 m. The detectable wave vector q ranges between 0.02 and 0.33 Å-1, where q ) (4π/λ) sin(θ/2) and θ is the scattering angle. The samples are flame-sealed in 1-mm diameter quartz capillaries. The scattering data, accumulated over 8 h, are corrected for background scattering by subtracting the intensity of water in a capillary. Cryo-TEM Technique. Cryo-transmission electron microscopy (cryo-TEM) is an important tool for morphological studies in the liquid phase. Because of recent improvements in specimen preparation techniques,15,16 low-temperature TEM (known as “cryo-TEM”) has become an important tool for obtaining structural information of molecular objects in solutions. The specimen is prepared by blotting a 5-µL drop of the sample on a carboncoated holey polymer film supported on a 300-mesh grid (Ted Pella Inc.) in a controlled environment vitrification system (CEVS), where the temperature is controlled by a bulb and the relative humidity is kept above 95% by a wet sponge to prevent sample evaporation. The specimen is blotted using filter paper and allowed to relax a few seconds in the CEVS. Later, the specimen is vitrified very quickly in liquid ethane. The CEVS operation retains the original composition of the sample so that the original microstructures remain unaltered in the vitrified specimen. The specimen is then transferred to a JEOL 1200EXII electron microscope equipped with a Gatan cold stage and examined under acceleration voltage of 100 kV in conventional TEM mode with a nominal underfocus of a few microns. The working temperature was below -170 °C and the images were recorded on a Kodak SO-163 film. A minimum dose system (MDS) of the electron beam is used. Viewing specimens by a normal electron beam (intense radiation) causes considerable damage mainly to the organic parts of the film. However, with the use of the MDS operation, a microstructure of highly labile type materials can be recorded with minimum destruction of its structure. Rheology. The rheological measurements are performed with a Carrimed CSL 100 controlled stress rheometer using a cone and plate geometry (2° angle, 40-mm diameter). The temperature is kept constant at 25 °C with a Peltier device. Flow experiments are carried out by applying an increasing stress to the sample during a period of typically 30 min. For a yield stress determination, creep experiments are performed in order to measure very small strain. The viscoelastic properties, i.e., the shear storage modulus G′ and the shear loss modulus G′′, are determined by oscillatory measurements from 0.01 to 10 Hz. The strain amplitude is kept constant at 5% to remain in the linear range of viscoelasticity.
Results Surface Tension. As shown in Figure 2, the surface tension monotonically decreases with surfactant concentration up to 0.0025%. Beyond this point, a break in the slope is observed and the surface tension levels off to a value of 26.5 mN‚m-1. This change in behavior corresponds to the aggregation of the surfactant and the critical aggregation concentration, CAC, is thus 0.0025%. This value is rather common for nonionic surfactants. For instance, the CAC of tetraethyleneglycol dodecyl ether is 0.0017%.13 From the Gibbs equation (2), the polar headgroup area, a0, has been calculated (eq 3) to be 0.42 ( 2 nm2. According to Israelachvili et al.,17 the surfactant aggregate geometry in water results from the balance of two antagonist effects: on one hand, the hydrophobic interactions between the alkyl chains tend to decrease (15) Bellare, J. R.; Davis, H. T.; Scriven, L. E.; Talmon, Y. J. Electron Microsc. Technol. 1988, 10, 87. (16) Adrian, M.; Heggeler-Bordier, B.; Wahli, W.; Stasiak, A. Z.; Stasiak, A.; Dubochet, J. EMBO 1990, 9, 4551. (17) Israelachvili, J. N.; Mitchell, D. J.; Ninham, B. W. J. Chem. Soc., Faraday Trans. 2 1976, 72, 1525.
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Figure 2. Surface tension as a function of CDEA concentration.
the surface area available to the polar head; on the other hand, the hydrophilic interactions between the heads themselves (hydration forces for nonionic surfactants and electrostatic forces for ionic surfactants) tend to increase it. As a result, an “optimal surface area” per polar headgroup emerges, which corresponds to a minimum in the free energy. The “packing parameter” is defined by
P ) V/a0lc
Figure 3. Size distribution in a 0.005% CDEA solution obtained by dynamic light scattering. The samples are prepared either by gentle mixing or by ultracentrifugation at 12 000 rpm for 90 min or sonication for 30 min.
(5)
where a0 is the polar headgroup area at the CAC, V the hydrocarbon volume, and lc the critical chain length.17 The packing parameter determines the geometry of surfactant aggregates. The surfactant is expected to form spherical micelles for P < 1/3, rodlike micelles for 1/3 < P < 1/2, vesicles for 1/2 < P < 1, and planar bilayers for P ) 1. The volume (in nm3) of the hydrophobic chain of the amphiphile can be calculated from the equation V ) 0.0274 + 0.0269n where n is the number of carbon atoms of the alkyl chain.18 As a rough estimate, the critical chain length, lc, can be compared to the fully extended length of the alkyl chain, l, which is given in nanometers by l ) 0.154 + 0.1265n, assuming that the molecule is in an all-trans configuration.18 The calculations of the values of the hydrophobic chain and the critical chain length for CDEA give V ) 0.361 nm3 and lc e l ) 1.72 nm. Substituting the measured a0 at the CAC in eq 5 yields P ) 0.5. This value is a slight underestimation as we used l instead of lc. However, according to the model, the CDEA molecules should form vesicular aggregates at the CAC. Light Scattering. To check the average size of aggregates present in solution slightly above the CAC, DLS experiments are carried out on a 0.005% CDEA solution. The analysis of the autocorrelation function indicates the existence of two relaxation modes. They are converted into two size distributions, assuming spherical particles. To know whether the above bimodal size distribution of the aggregates corresponds to an equilibrium state, we apply two different premeasurement protocols to the samples: (1) ultracentrifugation at 12 000 rpm for 90 min at 25 °C and (2) sonication for 30 min at 25 °C. After this treatment, the samples, still showing macroscopical homogeneity, are placed in the light scattering cell. Figure 3 shows the variation of the size distribution with the preparation mode of the sample. Two populations of sizes coexist in the solution: the smaller is fairly monodispersed (18) Tanford, C. The Hydrophobic Effect; Wiley: New York, 1980.
Figure 4. Turbidity as a function of CDEA concentration. The lower inset presents the results obtained by visual inspection of the samples between crossed polarizers: they can be isotropic (I), flow birefringent (FB), or birefringent at rest (B). The birefringence under shear has been determined by means of the shear cell (cf. Figures 1 and 5). The upper inset shows the results obtained with optical microscopy under polarized light: the sample can be isotropic (I) or it can exhibit characteristic textures such as Maltese crosses, oily streaks, and marble-like textures (cf. Figure 6).
and centered around 75 nm while the larger is rather polydispersed and centered around 600 nm. The size distribution is based on a scattering intensity analysis so that more weight is given to the larger scattering particles. We note that the size distribution of the smaller particles is almost unaffected by the sample preparation method while the larger particles become more homogeneous and smaller in diameter with sonication (330 nm) and centrifugation (450 nm). Turbidity. The turbidity of the CDEA solution is measured as a function of the surfactant concentration (cf. Figure 4). The turbidity curve passes through a sharp maximum at a surfactant concentration of 1% and rises smoothly again beyond 7.5%. At very low surfactant concentrations, the turbidity vanishes. To verify that the maximum in turbidity is not attributed to a macroscopic
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Figure 5. Picture of the narrowing channel of the shear cell when a 5% CDEA solution is at rest (a) and under shear (b). The walls of the cell are made of Plexiglas and the left one appears slightly birefringent while the right one is completely isotropic.
phase separation, the samples are ultracentrifuged at 12 000 rpm for 3 h. We have not observed a macroscopic phase separation after that treatment. Visual Inspection and Optical Microscopy. Samples in the concentration range between 0.5 and 40% are visually inspected between crossed polarizers and then under the optical microscope in thin flat capillary tubes. The observations are reported in the insets of Figure 4. Five regions can be distinguished as a function of the surfactant concentration. In zone 1 (C < 0.5%), the samples are isotropic and slightly opalescent. In zone 2 (0.5% < C < 3%), they are highly turbid, isotropic at rest, but birefringent at flow. A few Maltese crosses in an isotropic phase are observed with polarizing optical microscopy. In zone 3 (3% < C < 9%), the samples are slightly opalescent, weakly birefringent at rest (bluish), and strongly flow birefringent (cf. Figure 5). The appearance of birefringence is instantaneous under shear and its loss is immediate after cessation of the flow. The Maltese crosses are also present as can be seen at 3% (Figure 6a). When gently stirred, the sample can trap some air bubbles which remain within the sample at rest, indicating elastic behavior. In zone 4 (9% < C < 20%), the samples are slightly opalescent, birefringent with iridescent colors at rest, strongly flow birefringent, and gel-like with air bubbles trapped in the vial. A marbled texture is observed under polarizing optical microscopy (cf. Figure 6b). Finally, samples in zone 5 (20% < C < 40%) present the same characteristic as those of zone 4 except that it is possible now to observe oily streaks under polarizing optical microscopy characteristic of FCD I (cf. Figure 6c). SAXS Measurements. SAXS scattering curves I(q) vs q are shown in Figure 7 for various concentrations of CDEA. Above surfactant concentration of 10% it is possible to extract two narrow Bragg peaks characteristic of a smectic lamellar structure. At higher scattering vectors, q, a broad peak corresponding to d ) 4.6 nm is observed for every concentration. According to Nallet et al.,19 such a broad peak can be attributed to thermal fluctuations of the bilayers. Indeed, at high q, the structure factor tends toward 1 so that the form factor can be observed as a diffused peak. As the concentration is increased, the position of this peak remains unchanged but its intensity increases as can be seen on Figure 7. Below 10% no Bragg peak is detected but the diffused peak at a wide angle is still present. At higher surfactant concentrations Bragg peaks are observed at scattering vectors annotated qB. (19) Nallet, F.; Laversanne, R.; Roux, D. J. Phys. II 1993, 3, 487.
The interlamellar distance, d ) 2π/qB, is plotted as a function of the reciprocal surfactant volume fraction, φ, in Figure 8 and its evolution is consistent with a onedimensional swelling of a conventional lamellar phase: d ) δ/φ, where δ is the bilayer thickness. This equation leads to δ ) 3.1 nm which is in agreement with the value of the fully extended length previously estimated (δ ) 2 l ) 3.4 nm). Rheological Measurements. The rheological properties of two samples at 5 and 15% are illustrated in Figures 9 and 10. The shear storage and loss moduli are plotted as a function of the frequency of the applied stress on Figure 10. The storage modulus is much larger than the loss modulus over the whole frequency range and is slightly dependent on the frequency. The system behaves like a soft gel.4,10,12 As shown in Figure 9 with the flow experiments, the system has actually a real yield stress, which tremendously increases with the concentration (0.02 Pa at 5% and 20 Pa at 15%). The yield stress is determined by creep experiments where the strain is measured as a function of time for a given stress. Below the yield stress, the deformation depends on the applied stress but not on time. Above the yield stress, the deformation increases with time. Using this method, the existence of a yield stress is unambiguous. Figure 9 shows that, above the yield stress, the system behaves like a shear-thinning liquid. Cryo-TEM. The binary system is imaged at 1, 2, 5, and 15% as shown in Figures 11 and 12. At 1% CDEA, one finds unilamellar spherical vesicles around two diameters: 100 ( 30 and 500 ( 200 nm (cf. Figure 11a). The smaller vesicles are smoother than the larger ones (Figures 11a,b). This observation can be attributed to the possibility of fluctuations in large and thus less constrained vesicles. With increasing surfactant concentrations, multilamellar vesicles containing up to four bilayers begin to appear with a diameter ranging between 100 and 800 nm and an interlamellar spacing of 30-40 nm (cf. Figure 11b). As the concentration is further increased, the dominant objects in the solution are multilamellar vesicles, also termed onions (cf. Figure 11c). Their diameter lies between 150 and 400 nm, the interlamellar distance is about 25 nm, and the lamellarity (the number of bilayers in a vesicle) increases. The vesicles have a high flexibility: they are arranged along the contour of the hole of the polymer support and undulations of the membrane are clearly observed (see arrow in Figure 11c). When the CDEA concentration reaches 15%, a fully expressed lamellar
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Figure 6. Characteristic textures observed with optical microscopy under polarized light: (a) Maltese crosses in an isotropic phase for a 5% CDEA solution, (b) marbled texture for 15% CDEA solution, and (c) oily streaks for a 30% CDEA solution. Bar ) 100 µm.
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Figure 7. X-ray scattering pattern of three CDEA solutions at 5, 10, and 15%. Sharp Bragg peaks are observed except for the lowest concentration. A broad peak is located at a wide angle.
Figure 9. Rheograms of two CDEA solutions at 5 and 15%. The shear stress is plotted as a function of the shear rate. The yield stress increases from 0.020 Pa at 5% to 20 Pa at 15%. It has been determined with creep experiments until the strain increasing with time was observed.
Figure 8. Evolution of the interlamellar distance, d, extracted from the location of qB, the Bragg peaks, as a function of the volume fraction, φ, for the CDEA solution at 10, 15, 20, and 25%. The one-dimensional swelling (d ) δ/φ) gives a bilayer thickness, δ, of 3.1 nm.
Figure 10. Viscoelastic properties of CDEA at 5 and 15%. The shear storage, G′, and loss, G′′, moduli are plotted as a function of the frequency.
phase is observed (cf. Figures 12a,b). However, onions of about 200 nm in diameter are observed in the micrographs. The interlamellar spacing, 10 nm, is only on the same order of magnitude of that obtained by SAXS measurements (19 nm). However, one should bear in mind that the cryo-TEM technique supplies mainly qualitative direct local structural information and therefore has to be combined with an indirect quantitative technique (e.g., SAXS). This lamellar structure is somewhat different from the traditional schematic view of lamellar ordering, i.e., long, flat, and regularly ordered bilayers. The curvature of the bilayers in this system is rather high and the lamellar phase is highly defected. It has not been possible to image the solution above 15% with cryo-TEM because of the high content of organic compounds resulting in radiation damage. Discussion The phase behavior of the CDEA solutions has been investigated by means of various techniques. We will now
recombine the information to propose a possible evolution of structure as a function of concentration. Vesicle Phase: L4. The surfactant molecules below 0.0025% (CAC) are in the monomeric state. Above this concentration, they aggregate into unilamellar vesicles and form the L4 phase which extends until 0.5%. The aggregation is consistent with the prediction according to the packing parameter, which suggests an association in closed bilayers. The vesicle diameter is about 100 nm but larger vesicles are also observed with both DLS and cryoTEM. One should note, however, that these bigger objects are fewer and that their size strongly depends on sample preparation. Similar organization states have been reported in dilute systems of bilayer-forming surfactants such as ionic double-chain surfactants,20,21 single-chain fluorocarbon surfactants,22 a mixture of oppositely charged surfactants23-25 or a pseudo ternary system containing a surfactant and alcohol.3 The mixed chain length of the surfactant in the present study could result in partition of the C12 and C14 chains between the inner and the outer layers of the bilayers in which the curvature is different.
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Figure 12. Cryo-TEM micrographs of CDEA solutions at 15%: (a) polyhedral packing of onion structures; (b) onion-like structures embedded in the lamellae sheets. Bar ) 100 nm.
Figure 11. Cryo-TEM micrographs of CDEA solutions at various concentrations: (a) unilamellar vesicles at 1%; (b) transition between unilamellar and multilamellar vesicles at 2%; (c) multilamellar vesicles at 5%. The arrow indicates a membrane undulation. Bar ) 100 nm.
Onion Phase: L′r. It consists of densely packed multilamellar vesicles which are stable between 3 and 9%. The multilamellar vesicles give rise to weak birefringence and Maltese crosses (FCD II) under polarizing light microscopy. Nevertheless, no Bragg peak is detected with SAXS while cryo-TEM micrographs indicate a 25nm interlamellar distance. This ordering has only two to (20) Regev, O.; Khan, A. Prog. Colloid Polym. Sci. 1994, 97, 298. (21) Regev, O.; Kang, C.; Khan, A. J. Phys. Chem. 1994, 98, 6619. (22) Wu¨rtz, J.; Hoffman, H. J. Interface Colloid Sci. 1994, 175, 304. (23) Regev, O.; Khan, A. J. Colloid Interface Sci. 1996, 182, 95. (24) Khan, A.; Marques, E. In Catanionic Surfactants; Robb, I. D., Ed.; Blackie Academic and Professional, an imprint of Chapman & Hall: London, 1997; pp 37-74. (25) Herrington, K. L.; Kaler, E. W.; Miller, D. M.; Zasadzinski, J. A.; Chiruvolu, S. J. Phys. Chem. 1993, 97, 13792.
four repetitions, which is not enough to create a contrast necessary for an X-ray Bragg peak.26 These multilamellar vesicles are flexible and easily oriented either along the grid walls (cf. Figure 11c) or in the shear cell (cf. Figure 5). This phase behaves like a soft solid easily broken under shear. The existence of a yield stress may be due to the close packing of the vesicles at this volume fraction and to the compressibility of the vesicle because of their onion structure. A similar elastic phase of densely packed multilamellar vesicles have been observed in systems made of a lamellar phase doped by an ionic surfactant.4,10,27 In particular, these systems exhibit strong flow birefringence, possess a weak and bluish birefringence at rest, and are characterized by the presence of Maltese crosses.4 We conclude from this convergence in behavior that the L′R phase observed in this work is of the same nature. Classical Lamellar Phase: Lr. This phase made of flat-stacked lamellae emerges above 20%. Oily streaks characteristic of defects in a smectic ordering are observed (26) Cullity, B. D. Elements of X-ray Diffraction, 2nd ed.; AddisonWesley: Reading, MA, 1978. (27) Oberdisse, J.; Regev, O.; Porte, G. J. Phys. Chem. B 1998, 102, 1102.
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in this phase. First-, second-, and third-order Bragg peaks are detected and the interlamellar distance is consistent with a one-dimensional swelling (cf. Figure 8). This smectic ordering extends over large distances as it can be seen from the narrowness of the Bragg peaks. The LR phase is birefringent with iridescent colors and behaves like a gel with a yield stress. Unfortunately, it has not been possible to image this phase with cryo-TEM because of its high concentration of organic components. Transition Region: L4 + L′r. Between 0.5% and 3%, the turbidity of the solution exhibits a pronounced maximum. This concentration domain can be considered a transition region between unilamellar (L4) and multilamellar (L′R) vesicles. Therefore, the increased turbidity is attributed to the wider size distribution of objects as also evidenced by cryo-TEM. Note, however, that no macroscopic phase separation takes place even with ultracentrifugation. Two-Phases Region: L′r + Lr. In this concentration domain lying between 9 and 20%, multilamellar vesicles (L′R) are embedded in flat-stacked lamellae (LR) as clearly evidenced by cryo-TEM (cf. Figures 12b). The texture observed with polarizing light microscopy is marble-like and has also been reported in similar biphasic lamellar systems.9 It should be noted that the pattern of the texture strongly depends on sample treatment (way of mixing, heating). However, no macroscopic phase separation occurs which can be ascribed to similar densities of both phases: indeed, the interlamellar distance appears to be
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the same in onions or flat-stacked lamellae (cf. Figure 12a,b and the single Bragg peak on Figure 7). In this region, samples are elastic and possess a yield stress. Conclusions The single-tail nonionic surfactant CDEA self-assembles into bilayer aggregates with a morphology that strongly depends on the concentration. Indeed, three distinct phases have been identified. The L4 phase forms in the dilute regime and is made of unilamellar vesicles. The L′R phase appears above the overlap concentration of the vesicles and is made of multilamellar vesicles also termed onions. Finally, at much higher concentrations, a classical lamellar phase, LR, made of flat-stacked lamellae is observed. Both L′R and LR possess elastic properties with a yield stress and a frequency-independent elastic modulus. As the concentration is increased, this lamellar phase exhibits a one-dimensional swelling. The transition between these two phases is first-order. In the biphasic region, onions are embedded in flat-stacked lamellae. However, no macroscopic phase separation has been achieved, probably because of the similar interlamellar distances of the onions and flat-stacked lamellae. This unique structural evolution in a simple binary system from L4 to L′R and then to LR stems from the high flexibility of the nonionic surfactant and from an increase in the Gaussian curvature. LA980935H
