© Copyright 1997 by the American Chemical Society
VOLUME 101, NUMBER 10, MARCH 6, 1997
LETTERS Novel Gel Phase: A Cubic Phase of Densely Packed Monodisperse, Unilamellar Vesicles M. Gradzielski, M. Bergmeier, M. Mu1 ller, and H. Hoffmann* Lehrstuhl fu¨ r Physikalische Chemie I, UniVersita¨ t Bayreuth, D-95440 Bayreuth, Germany ReceiVed: NoVember 25, 1996; In Final Form: January 14, 1997X
A novel type of gel phase has been investigated in the ternary surfactant system sodium oleate/octanol/water. It occurs at relatively high octanol content (molar ratio of ∼2:1 for octanol/oleate) and for a total concentration of amphiphile in the range of 5-15 wt %. This phase is transparent, isotropic, and highly viscous and possesses elastic properties (“ringing gel”). Microstructural investigations by means of freeze-fracture electron microscopy and small-angle neutron scattering show that it is mainly composed of small unilamellar vesicles. The observed size is also in good agreement with the measured rheological properties. These vesicles are remarkably monodisperse and form a highly ordered phase (presumably of cubic type); i.e., this is the first time that such a liquid crystalline array of vesicles has been observed.
Investigations of vesicle phases have attracted a lot of interest in recent years. Vesicles represent simple model systems for biological membranes.1,2 They can occur in different states, like unilamellar vesicles or multilamellar vesicles, and their degree of polydispersity can vary widely. Originally they were formed mainly by methods like ultrasonification or the microfluidization technique,3 but now many surfactant systems are known in which they form spontaneously.4-7 In general, a tendency for the transformation of a conventional lamellar phase into the vesicle state is to be expected for the case of charging the amphiphilic membrane.8 One purpose of our study was to investigate whether such vesicle phases also occur with classical, ionic carboxylate surfactants. Typically, formation of vesicle phases can be induced by addition of a medium chain alcohol to the surfactant system. In our study we chose sodium oleate as surfactant and added 1-octanol as cosurfactant. Similar studies on mixtures of the zwitterionic surfactant tetradecyldimethylamine oxide (TDMAO) and the cationic tetradecyltrimethylammonium bromide (TTABr) have been done before, and in these systems it was observed that the addition X
Abstract published in AdVance ACS Abstracts, March 1, 1997.
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of the cosurfactant induces first a growth of rodlike micellar aggregates, which is followed by the formation of vesicles at higher cosurfactant concentration.9-11 A principally similar behavior is found for the case of adding octanol to sodium oleate. However at fairly high octanol concentrations formation of a highly viscous (gel-like) and isotropic phase is observed. This transparent gel phase also possesses elastic properties and shows a mechanical resonance phenomenon (“ringing effect”), as it has typically been found for cubic phases in surfactant systems.12,13 This highly elastic gel phase occurs over a fairly large range of surfactant and cosurfactant concentration, and its location in the phase diagram is given in Figure 1 (here we have distinguished between a highly viscous region and the “gel” region, where the samples possess a yield stressshowever, there is continuous transition between both regions). In particular, it should be noted that the solid-like gel can already be formed for volume fractions as low as ∼8% of dispersed amphiphilic material. The gel phase is surrounded by a two-phase region which indicates that here really a first-order phase transition takes place. One finds that typically a molar ratio of ∼2 for octanol/oleate is required for the formation of the gel phase; i.e., the amphiphilic film that is © 1997 American Chemical Society
1720 J. Phys. Chem. B, Vol. 101, No. 10, 1997
Figure 1. Phase diagram of the system sodium oleate/1-octanol/water at 25 °C. The isotropic gel phase is given as the shaded region.
Figure 2. Magnitude of the complex viscosity |η*| (9), storage modulus G′ (b), and loss modulus G′′ (2) as a function of the angular frequency ω for a sample of composition 200 mM sodium oleate + 7.5 wt % (560 mM) octanol.
present in these systems is to a large degree made up of the cosurfactant octanol. The isotropic gel phase exhibits a yield stress (with typical values in the range of 50-200 Pa); e.g., air bubbles are trapped in such samples. Its rheological properties may quantitatively be characterized by a rheogram as given in Figure 2. In this plot one observes a constant value for the storage modulus G′ (that describes the elastic properties of the system) for the frequency range investigated. Such a behavior is similarly observed for all samples from this phase region. The shear moduli are relatively large, i.e., typically in the range of several 1000 Pa, and thereby much higher (by a factor of ∼100-1000) than that normally found for vesicle phases of similar total concentration (compare for instance ref 14). However, the rheological behavior compares reasonably well with that of cubic phases although the shear moduli for those might be higher by about a factor of 100.15-17 Of course, the question of the microstructure of this gel phase arises; i.e., how does a system with a volume fraction of 5-15% of dispersed material form such a stiff gel? In order to address this question, electron micrographs via the freezefracture technique18 were obtained from this system. A typical picture for this phase is given in Figure 3 where one sees that the system is mainly composed of spherical aggregates with a diameter of 300-350 Å, while some larger aggregates of about 2000 Å are also present. From this electron micrograph it can be estimated that approximately 5% of the amphiphilic material is contained in the larger, multilamellar vesicles (MLV), which consist of 2-4 shells. The smaller,
Letters unilamellar aggregates (ULV) are densely packed, and in some parts of the picture a hexagonal packing of spheres around one central sphere can be seen, as must be the case for a cubic lattice. From the size of these aggregates (and also from a close inspection of the electron micrographs), it is fairly clear that these have to be vesicles and their size distribution is relatively monodisperse if compared to other vesicular systems (e.g., see ref 14). That the cubic packing is less evident in our electron micrograph in comparison to electron micrographs of cubic phases made up from micellar aggregates has to be attributed to the fact that in the latter case compact spherical aggregates are present whereas in our case the vesicles are only a hollow sphere of amphiphilic material. For this reason they will just be broken in the plane of fracture during the preparation process, whereas in contrast a compact hydrophobic particle will always be seen fully since fracturing will take place not through the particle but along its surface. Therefore, of course, the cubic arrangement is less visible since one cuts along an arbitrary plane through the particles. A complementary structural picture can be obtained from a scattering experiment, and in Figure 4 a small-angle neutronscattering (SANS) curve is given for a sample of composition 200 mM sodium oleate + 6.5 wt % (490 mM) 1-octanol. Here a sharp peak (note that figure 4 is on a logarithmic scale) followed by a less pronounced peak is observed, a scattering pattern that demonstrates that a highly ordered structure has to be present. Of course, this is not conclusive evidence for the presence of a long range cubic structure, and one could not completely exclude the possibility of a glassy structure. However, in comparison to our earlier studies on cubic phases, the scattering pattern is very similar,13,16,17 and further studies to confirm this point are in progress. From the position of the primary peak at a q-value of 0.0255 1/Å, a mean spacing of 250 Å between the vesicular aggregates can be calculated. Since evidently this system has to be densely packed (otherwise its highly elastic properties could not be accounted for), the diameter of the vesicles has to be close to the spacing as deduced from the scattering maximum, i.e., 250 Å, a value that corresponds well to the value obtained from the electron micrographs. The high q-region should be indicative of the local structure of the aggregates present. This part of the scattering curve can be fitted well with the simple model of a lamellar structure, a model that should be valid for such vesicular aggregates since at a level of 10-100 Å, which is the range probed in this q-range, the vesicle looks like a flat object. The scattering law for a flat lamella reads19
I(q) )
(
)
sin(qd/2) 2π Φ∆F2 d 2 qd/2 q
2
(1)
where q is the scattering vector (q ) (4π/λ) sin(Θ/2)), Φ the volume fraction, ∆F the contrast difference between solvent and amphiphilic film, and d the thickness of the lamella. From the fit of the lamellar model, one obtains a thickness of 21.1 Å. This value is much lower than that one might expect for a bilayer of a C18-surfactant, but here one should keep in mind that the bilayer is mainly formed by the much shorter octanol, which explains the relative thinness of the vesicle shell. It might be noted here that similar values for the thickness of the lamellae of lamellar phases (LR) with high content on alcohol as cosurfactant have been observed before.20-22 All these experimental results demonstrate that the investigated gel phase is composed of vesicular aggregates. This can
Letters
J. Phys. Chem. B, Vol. 101, No. 10, 1997 1721
Figure 3. Freeze-fracture electron micrograph of a sample of 200 mM sodium oleate + 7.5 wt % (560 mM) octanol (in an aqueous solution containing 20 wt % glycerol to avoid crystallization). The bar represents 100 nm, and some places where the hexagonal packing is visible are marked by hexagons.
Figure 4. Small-angle neutron scattering (SANS) curve for a 200 mM sodium oleate sample that contained 6.5 wt % (490 mM) octanol (all in D2O) at 25 °C (fitted curve for a lamellar model given as a solid line).
be seen in the electron micrograph (Figure 3), which also already shows that these vesicles are unilamellar and unusually monodisperse. This high degree of monodispersity is then further evidenced by the very pronounced scattering pattern, i.e., in particular the sharp scattering peak. This has to be the result of a highly ordered structure that can only be formed by relatively monodisperse particles. The diameter of the aggregates as determined by the different methods is in good agreement, and the vesicles have a diameter of 300 Å. Finally it should be noted that from this size in combination with the thickness of the vesicle shell the enclosed volume fraction of the vesicles can be calculated. For the systems investigated by us simple geometry gives a volume fraction of 68-75%, i.e., exactly the value that one expects for densely packed spherical objects. It might be noted here that the formation of small vesicles of narrow size distribution has been predicted theoretically for highly asymmetric surfactant mixtures, i.e., mixtures of surfactants where the chain length of the two components is largely
different, and where these vesicles are energetically stabilized.23 These calculations have been done for oppositely charged surfactant mixtures; however, one might expect that this would similarly apply to a mixture of a charged and an uncharged surfactant since this effect is largely due to the differences in the length of the alkyl chain. However, this is exactly the situation in our case, where we have a C18-surfactant (anionic) in combination with a C8-cosurfactant (nonionic). Finally the proposed structure of a cubic phase of vesicles is also able to explain the rheological properties discussed above. Normally shear moduli G0 in the range of several 105 Pa are observed for cubic phases.15-17 Of course, these are much higher than in our present system, yet this is easily explained by the fact that the observed G0 is proportional to the number density of the structural units. However, the size (diameter) of the structural units for the other cubic phases is 50-80 Å.16,17 Since our vesicles are about a factor of 4-6 larger, this explains nicely why the corresponding values for G0 are about a factor of 100 smaller than those found for these other cubic phases of surfactant systems. If one takes together all the experimental evidence, it is clear that the unique properties of the investigated gel phase lead to the conclusion that it consists of a highly ordered (cubic), dense array of monodisperse, unilamellar vesicles. To our knowledge this is the first time that such a cubic phase of highly ordered vesicles has been observed. This finding should open up a variety of interesting studies on such vesicle systems, since so far only relatively disperse systems of vesicles have been studied. Of course, for the analysis of experimental data in terms of theoretical models, such a very well-defined system, as present in our system, should be much more amenable for a comparison with predicted theoretical behavior. In addition, formation of monodisperse and well-characterized vesicles might be useful for a variety of applications. Furthermore, the possibility to form a highly viscous phase of these aggregates (without having to employ large quantities of dispersed material) will be useful for all applications where such gel phases are desired and in which
1722 J. Phys. Chem. B, Vol. 101, No. 10, 1997 either hydrophilic or hydrophobic agents are dispersed with the purpose of slow release (e.g., in pharmaceuticals). Acknowledgment. The SANS measurements were performed on the instrument D22 of the institute Laue-Langevin, Grenoble, France, and we are grateful to B. Farago for help with the experiments. References and Notes (1) Handbook of Biological Physics; Lipowsky, R., Sackmann, E., Eds.; Elsevier: Amsterdam, 1995; Vol. 1. (2) Vesicles; Rosoff, M., Ed.; Surfactant Science Series, 62; Marcel Dekker Inc.: New York, 1996. (3) Vuillemard, J. C. J. Microencapsulation 1991, 8, 547. (4) Carnie, S.; Israelachvili, J. N.; Pailthorpe, B. A. Biochim. Biophys. Acta 1979, 554, 340. (5) Talmon, Y.; Evans, D. F.; Ninham, B. W. Science 1983, 221, 1047. (6) Kaler, E. W.; Murthy, A. K.; Rodriguez, B. E.; Zasadzinski, J. A. N. Science 1989, 245, 1371. (7) Cantu, L.; Corti, M.; Musolino, M.; Salina, P. Prog. Colloid Polym. Sci. 1991, 84, 21. (8) Winterhalter, M.; Helfrich, W. J. Phys. Chem. 1992, 96, 327. (9) Hoffmann, H.; Thunig, C.; Valiente, M. Colloids Surf. 1992, 67, 223.
Letters (10) Valiente, M.; Thunig, C.; Munkert, U.; Lenz, U.; Hoffmann, H. J. Colloid Interface Sci. 1993, 160, 39. (11) Hoffmann, H.; Munkert, U.; Thunig, C.; Valiente, M. J. Colloid Interface Sci. 1994, 163, 217. (12) Nu¨rnberg, E.; Pohler, W. Prog. Colloid Polym. Sci. 1984, 69, 48. (13) Gradzielski, M.; Hoffmann, H.; Oetter, G. Colloid Polym. Sci. 1990, 268, 167. (14) Hoffmann, H.; Thunig, C.; Schmiedel, P.; Munkert, U. Langmuir 1994, 10, 3972. (15) Bohlin, L.; Ljusberg-Wahren, H.; Miezis, Y. J. Colloid Interface Sci. 1989, 103, 294. (16) Gradzielski, M.; Hoffmann, H. in The Structure, Dynamics and Equilibrium Properties of Colloidal Systems; Bloor, D. M., Wyn-Jones, E., Eds.; Kluwer Academic Publishers: Dordrecht, 1990; p 427. (17) Gradzielski, M.; Hoffmann, H.; Panitz, J.-C.; Wokaun, A. J. Colloid Interface Sci. 1995, 169, 103. (18) Robards, A. W.; Sleytr, U. B. In Practical Methods in Electron Microscopy; Glauert, A. M., Ed.; Elsevier: Amsterdam, 1985; Vol. 10. (19) Porod, G. In Small Angle X-ray Scattering; Glatter, O., Kratky, O., Eds.; Academic Press: London, 1982. (20) Thunig, C.; Hoffmann, H.; Platz, G. Prog. Colloid Polym. Sci. 1989, 79, 297. (21) Bassereau, P.; Marignan, J.; Porte, G. J. Phys. 1987, 48, 673. (22) Maldonado, A.; Urbach, W.; Ober, R.; Langevin, D. Phys. ReV. E 1996, 54, 1774. (23) Yuet, P. K.; Blankschtein, D. Langmuir 1996, 12, 3819.
