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Langmuir 1996, 12, 4042-4043
Superstructures from Didodecyldimethylammonium Bromide and Poly(acrylic acid) Marcel D. Everaars, Armanda C. Nieuwkerk, Solenne Denis, Antonius T. M. Marcelis, and Ernst J. R. Sudho¨lter* Department of Organic Chemistry, Wageningen Agricultural University, Dreijenplein 8, 6703 HB Wageningen, The Netherlands Received January 16, 1996. In Final Form: May 1, 1996
Biological tissues consist of lipid membrane cells which are inter- and intracellularly stabilized by biopolymers like proteins and polysaccharides.1 The study of the interaction between water-soluble polymers and bilayer forming surfactants is therefore highly relevant to a better understanding of the organization of these complex structures.2,3 However, studies on the interactions between ionic surfactants and oppositely charged polyelectrolytes4-9 in aqueous solution are often hampered by the formation of precipitates.10-12 Recently, the solid state structure and material properties of these complexes have become a focus of interest. The lamellar bilayer structure is usually preserved in the bulk and the polymer acts as an external stabilizer.13-18 We have observed that upon heating the insoluble complex of didodecyldimethylammonium bromide (DDAB) and poly(acrylic acid) (PAA) in water, multivesicular superstructures are formed, which morphologically strongly resemble biological tissues. A thin film of DDA-PAA complex was prepared by casting an ethanolic solution of DDAB and PAA (1:10 monomeric units) onto a glass microscope slide.19 After addition of water this initially clear film becomes turbid and remains tightly adsorbed onto the microscope slide. Subsequent heating of the water-covered film for 1 min at 80 °C results in the spontaneous formation of immense numbers of vesicles. The vesicles are immobilized in the surrounding matrix of DDA-PAA hydrogel (Figure 1a). In some locations the vesicles are so closely packed that (1) Darnell, J.; Lodish, H.; Baltimore, D. Molecular Cell Biology; Scientific American Books: New York, 1986. (2) Goddard, E. D. Colloids Surf. 1986, 19, 255-300. (3) Breuer, M. M.; Robb, I. D. Chem. Ind. 1972, 530-535. (4) Kiefer, J. J.; Somasundaran, P.; Ananthapadmanabhan, K. P. Langmuir 1993, 9, 1187-1192. (5) Hayakawa, K.; Kwak, J. C. T. J. Phys. Chem. 1983, 87, 506-509. (6) Hayakawa, K.; Santerre, J. P.; Kwak, J. C. T. Macromolecules 1983, 16, 1642-1645. (7) Santerre, J. P.; Hayakawa, K.; Kwak, J. C. T. Colloids Surf. 1985, 13, 35-45. (8) Goddard, E. D. Colloids Surf. 1986, 19, 301-329. (9) Thalberg, K.; Lindman, B. Langmuir 1991, 7, 277-283. (10) Ohbu, K.; Hiraishi, O.; Kashiwa, I. J. Am. Oil Chem. Soc. 1982, 59, 108-112. (11) Goddard, E. D.; Hannan, R. B. J. Am. Oil Chem. Soc. 1977, 54, 561-566. (12) Seki, M.; Morishima, Y.; Kamachi, M. Macromolecules 1992, 25, 6540-6546. (13) Antonetti, M.; Kaul, A.; Thu¨nemann, A. Langmuir 1995, 11, 2633-2638. (14) Antonetti, M.; Burger, C.; Effing, J. Adv. Mater. 1995, 7, 751753. (15) Okahata, Y.; Enna, G. J. J. Phys. Chem. 1988, 92, 4546-4551. (16) Okahata, Y.; Enna, G. J.; Taguchi, K.; Seki, T. J. Am. Chem. Soc. 1985, 107, 5300-5301. (17) Tal’roze, R.; Kuptsov, S. A.; Sycheva, T. I.; Bezborodov, V. S.; Plate, N. A. Macromolecules 1995, 28, 8689-8691. (18) Tirell, D. A.; Turek, A. B.; Wilkinson, D. A.; McIntosh, T. J. Macromolecules 1985, 18, 1512-1513. (19) The samples were prepared as follows: a solution of DDAB (0.02 M) and PAA (0.2 M in monomeric units; Mw ) 90 000) in ethanol was cast onto a glass microscope slide to give a thin film. After drying, this film was covered with pure water and subsequently heated at 80 °C for 1 min on a Mettler FP82HT hot stage. No corrections of the pH were made (pH ) 3). After cooling to room temperature the turbid films which were still covered with water were inspected by optical microscopy using an Olympus BH-2 microscope.
S0743-7463(96)00047-9 CCC: $12.00
Figure 1. (a) Optical micrograph of vesicles embedded in a blob of DDA-PAA matrix adsorbed on a microscope slide. (b) Optical micrograph of densely packed vesicles in the hydrated DDA-PAA film. (c) Optical micrograph showing single vesicles surrounded by a layer of DDA-PAA matrix. Some of the vesicles are indicated by the arrows.
© 1996 American Chemical Society
Notes
they form tissue-like superstructures (Figure 1b). The diameter of the vesicles varies from smaller than 1 µm to 30 µm. Less developed superstructures can also be prepared by heating the water-covered precipitate obtained by mixing aqueous solutions of DDAB and PAA or by heating a cast DDAB film covered with an aqueous PAA solution. In the latter case sometimes spherical matrix particles including one or more vesicles detach from the surface and migrate freely through the solution (Figure 1c). The vesicles are then clearly observed in the interior of the matrix particle. Sometimes the enveloping matrix layer is very thin and the particles resemble normal bilayer vesicles.20-22 Proof for the presence of water-containing vesicles was obtained from the following observations: (1) heating of the film in the absence of water does not result in the formation of superstructures; (2) upon evaporation of water the vesicle structures collapse; (3) osmotic shrinkage and subsequent collapse of the larger vesicle structures are observed after addition of a hypertonic salt solution.23,24 The smaller vesicle structures only deteriorate under these conditions. Good quality tissue-like samples are obtained when PAA and DDAB are mixed in a ratio of approximately ten acrylate units to one DDAB molecule. Upon increasing the ratio of DDAB to PAA, the formation of myelin structures and free bilayer vesicles becomes predominant upon heating of the submersed cast film, as is also observed for pure DDAB.23 Under optimal conditions the binding sites of the polymer are only partially occupied by DDA cations. The DDA cations are thus able to migrate along the polymer backbone, facilitating the reorganization of the material into multivesicular superstructures. The formation of tissue-like superstructures is observed between pH 1 and 10 although the tissues become increasingly unstable at both extreme pH values. Increasing the ionic strength of the supernatant solution also decreases the tissue stability, indicating that ionic interactions play an important role. When other polyelectrolytes are used like polysulfonates,25 alginic acid, or hydrophobically modified poly(maleic acid),26 no tissue(20) Brady, J. E.; Evans, D. F.; Kachar, B.; Ninham, B. W. J. Am. Chem. Soc. 1984, 106, 4279-4280. (21) Regen, S. L.; Shin, J. S.; Yamaguchi, K. J. Am. Chem. Soc. 1984, 106, 2446-2447. (22) Fukuda, H.; Diem, T.; Stefely, J.; Kezdy, F. J.; Regen, S. L. J. Am. Chem. Soc. 1986, 108, 2321-2327. (23) Carmona-Ribeiro, A. M.; Chaimovich, H. Biophys. J. 1986, 50, 621-628. (24) Menger, F. M.; Balachander, N. J. Am. Chem. Soc. 1992, 114, 5862-5863. (25) Poly(2-acrylamido-2-methyl-1-propanesulfonic acid) was used.
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like structures are observed. Presumably the interaction between the surfactant and the polyelectrolyte must not be too strong in order to allow a reorganization of the material. Using dioctadecyldimethylammonium bromide instead of didodecyldimethylammonium bromide also does not result in the formation of tissue-like structures. Probably, the low monomer solubility of this compound with respect to DDAB hampers an easy reorganization of the complex. The formation of tissue-like structures therefore seems to be determined by a delicate balance of interactions. We suggest that upon being heated, the DDA cations reorganize into extended bilayer structures. The tissuelike superstructure is then stabilized by the presence of the negatively charged PAA which binds and interconnects the positively charged DDA bilayer vesicles. This concept is based on the fact that besides tissues, freely migrating vesicle structures are also observed which look very similar to normal bilayer vesicles. Furthermore, the tissues show birefringence under crossed polarizers indicating the presence of ordered structures. The presence of bilayer structures in the tissues has however not been proven and no phase transition was observed by DSC. The stability of the superstructure is further enhanced by a strong adhesion of the matrix to the glass substrate. Our observations strongly suggest that vesicles in the DDA-PAA hydrogel are formed via a fundamentally different mechanism than that observed for pure crystalline vesicle forming surfactants. In the latter case the hydrated surface of the crystalline surfactant forms myelin structures, from which the vesicles detach.27-29 In the DDA-PAA hydrogel the vesicles are formed in the hydrated bulk of the amorphous material. The DDA-PAA complex has thus been shown to form ordered multivesicular assemblies which resemble the architecture of biological tissues. In biological tissues polypeptides and polysaccharides are known to keep the phospholipid walled cells together.1 It is suggested that in our system DDA vesicles are joined by a PAA containing matrix. We have thus made a step forward from the study of synthetic bilayer vesicles, which are model systems for single cells, to the more complex multivesicular assemblies which could be regarded as model systems for biological tissues. LA960047R (26) Poly(maleic acid-co-alkyl vinyl ether) (II-6) was used. Nieuwkerk, A. C.; Marcelis, A. T. M.; Sudho¨lter, E. J. R. Macromolecules 1995, 28, 4986-4990. (27) Lasic, D. D. Biochem. J. 1988, 256, 1-11. (28) Lasic, D. D. J. Colloid Interface Sci. 1988, 124, 428-435. (29) Lasic, D. D. J. Colloid Interface Sci. 1990, 140, 302-304.
