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Langmuir 1997, 13, 666-672
Evidence for Multilamellar Vesicles in the Lamellar Phase of an Electrostatic Lyotropic Ternary System. A Solid State 2H-NMR and Freeze Fracture Electron Microscopy Study Fre´de´ric Auguste, Jean-Paul Douliez, Anne-Marie Bellocq,* and Erick J. Dufourc* Centre de Recherche Paul PascalsCNRS, av. A. Schweitzer, 33600 Pessac, France
Tadek Gulik-Krzywicki Centre de Ge´ ne´ tique Mole´ culairesCNRS, 91190 Gif-sur-Yvette, France Received July 11, 1996. In Final Form: November 12, 1996X Deuterium solid state NMR and freeze fracture electron microscopy experiments have been carried out in the lamellar LR phase of the water-sodium dodecyl sulfate-octanol system. Within the lamellar phase two types of bilayer organizations have been found. At high surfactant and alcohol concentrations, the lamellar phase is made of a stack of flat parallel bilayers while in the dilute part, it consists of multilayered vesicles of large radius (ca. 10 000 Å). These latter structures, commonly called spherulites, which appear spontaneously at low octanol contents and/or for high water dilution, can be described as textural defects of the lamellar phase. The location of this onion-like structure region is discussed within the framework of the membrane elasticity theory. As a side result, the presence of 33% glycerol in some of the freeze fracture experiments is shown to barely affect the bending modulus of the film but rather disorder the molecular packing of the bilayers.
Introduction Amphiphilic molecules in solution form a wide range of self-assembling structures which have motivated many theoretical and experimental developments.1,2 Upon specific conditions of composition and temperature, these molecules aggregate into bilayers sometimes designated as membranes. These two-dimensional aggregates can organize in space either as a long range ordered structure, the lamellar phase LR,3,4 or as isotropic disordered phases like the vesicle L45-9 or the sponge L310,11 phase. In the vesicle phase, the bilayers form unilamellar or multilamellar spherical objects, while they are randomly connected in the sponge phase. Theoretical studies9,12-14 * Author to whom correspondence should be addressed. Fax: 56 84 56 00. X Abstract published in Advance ACS Abstracts, January 15, 1997. (1) See: Physics of amphiphilic layers; Meunier, J., Langevin, D., Boccara, N., Eds.; Springer-Verlag: New York, 1987. (2) See: Micelles, membranes, microemulsions and membranes; Gelbart, W. M., Ben-Shaul, A., Roux, D., Eds.; Springer-Verlag: New York, 1994. (3) Bellocq, A. M. In Microemulsions: fundamentals and applied aspects; Kumar, P., Mittal, K. L., Eds.: Marcel Dekker: New York, in press. (4) Ekwall, P. In Advances in liquid crystals; Brown, G. M., Ed.; Academic Press: New York, 1975. (5) (a) Brady, J. E.; Evans, D. F.; Kachar, B.; Ninham, B. W. J. Am. Chem. Soc. 1984, 106, 4279. (b) Dubois, M.; Zemb, T. Langmuir 1991, 7, 1352. (6) Hauser, H.; Gains, N.; Lasic, D. In Physics of amphiphiles: micelles, vesicles and microemulsions; Degiorgio, V., Corti, M., Eds.; North Holland: Amsterdam, 1985. (7) Kaler, E. W.; Murthy, K. A.; Rodriguez, B. E.; Zasadzinski, J. A. N. Science 1989, 245, 1371. (8) (a) Munkert, U.; Hoffmann, H.; Thunig, C.; Meyer, H. W.; Richter, W. Prog. Colloid Polym. Sci. 1993, 93, 137. (b) Hoffmann, H.; Munkert, U.; Thunig, C.; Valiente, M. J. Colloid Interface Sci. 1994, 163, 217. (9) Herve´, P.; Roux, D.; Bellocq, A. M., Nallet, F.; Gulik-Krzywicki, T. J. Phys. II 1993, 3, 1255. (10) Porte, G.; Marignan, J.; Bassereau, P.; May, R. J. Phys. (Paris) 1988, 49, 511. (11) Gazeau, D.; Bellocq, A. M.; Roux, D.; Zemb, T. Europhys. Lett. 1989, 5, 447. (12) Huse, D. A.; Leibler, S. J. Phys. (Paris) 1988, 49, 605. (13) Cates, M. E.; Roux, D.; Andelman, D.; Milner, S. T.; Safran, S. A. Europhys. Lett. 1988, 5, 733.
S0743-7463(96)00683-X CCC: $14.00
Figure 1. Partial phase diagram of the water-SDS-octanol system at T ) 25 °C: L1, micellar phase; L4, vesicle phase; HR, hexagonal phase; LR, lamellar phase; region I, stack of parallel flat bilayers; region II, multilayered vesicles. The dashed line separates region I from region II and the dotted-dashed line represents a typical dilution line in our experiments (see text).
based on the framework of the membrane elasticity theory15 have shown that the ultimate structure of the aggregates existing in solution depends strongly on two important intrinsic parameters of the bilayers, the bending elastic constants κ and κj which control respectively the mean curvature and the Gaussian curvature. The mean bending modulus κ characterizes the rigidity of the bilayer, it is generally of the order of kBT in alcoholsurfactant-water systems.16 The Gauss-Bonnet theorem17 states that the integral of the Gaussian curvature (14) Simons, B. D.; Cates, M. E. J. Phys. II 1992, 2, 1439. (15) Helfrich, W. Z. Naturforsch. 1973, 28c, 693. (16) For a review see: Roux, D.; Safinya, C.; Nallet, F. In ref 2, p 303. (17) See for instance: David, F. In Statistical mechanics of membranes and surfaces; Nelson, D., Piran, T., Weinberg, S., Eds.; World Scientific: Singapore, 1989.
© 1997 American Chemical Society
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Figure 2. Variation of the quadrupolar splitting vs the smectic repeat distance for the water-SDS (R-deuterated)-octanol system along the four dilution lines defined as (a) A/S ) 1.8, (b) A/S ) 2.4, (c) A/S) 2.6, and (d) A/S ) 2.9. Arrows show the position of the discontinuity in the variation (see text). In (b) is also shown the variation obtained for the water + glycerol (33%)-SDS-octanol system.
for a given surface is a topological invariant which depends only on nc and nh, the number of disconnected components of the surface and the number passages or handles, respectively. As a consequence, the Gaussian curvature is only a function of the topology and the Gaussian bending constant κj plays an important role in phase transitions which involve topological transformations. When κj is sufficiently negative, the surface forms many disconnected aggregates such as vesicles; in contrast when κj is positive, highly connected surfaces with many handles are favored and the sponge phase is stabilized. These considerations indicate that while increasing κj we expect the following sequence of phases: vesicle phase (L4)-lamellar smectic (LR)-sponge phase (L3). Experimentally the L4-LR-L3 sequence of phases has been found in the sodium dodecyl sulfate (SDS)-octanolbrine system.9 At low SDS content (
