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Langmuir 2005, 21, 516-519
“Micro-Pottery”sMarangoni Effect Driven Assembly of Amphiphilic Fibers Janhavi S. Raut,* Pradeep Bhattad, Aditi C. Kulkarni, and Vijay M. Naik Unilever Research India & Hindustan Lever Research Centre, 64 Main Road, Whitefield, Bangalore-560066, India Received May 17, 2004. In Final Form: August 6, 2004 We report spontaneous supra-assembly of fibrous surfactant crystallites at the air-solution interface resulting in spectacular arrays of two-dimensional spiral and three-dimensional “micro-pottery”-like superstructures. Surface pressure differential driven bending of the embryonic fiber nuclei and Marangoni convection driven fiber migration/alignment appear to be the causal factors behind this phenomenon. The assemblies form at specific crystal-growth velocities dictated by the relative time scales for fiber bending/ alignment and their rigidification/immobilization as they grow. Although our studies are restricted to a specific class of amphiphiles, namely, alkaline metal salts of linear fatty acids, the phenomenon should be generic to amphiphilic molecules that crystallize into flexible fibers.
Nature is replete with examples of spontaneously formed, spectacular shapes of crystalline supramolecular assemblies, arising solely out of either the intrinsic packing characteristics of molecules or their combination with patterns of external energy and mass transport. Amphiphilic molecules in particular organize themselves into a range of beautiful liquid crystalline or crystalline forms.1-6 Crystallization of one class of amphiphiles, namely, alkaline metal salts of linear fatty acids, have been the subject of a large number of scientific studies, for nearly a century, because of their ready availability and commercial importance.7-9 It is well documented that they crystallize from aqueous solutions in the form of flat platelets, ribbons, or twisted fibers.9-11 Despite being a subject of such exhaustive study, we report for the first time observations of harmonious intricate designs of pottery-like tertiary structures formed spontaneously by crystalline fibers of one typical member of the salt family, namely, sodium myristate (NaMy). In addition to its aesthetic charm, we believe that once the underpinning factors and mechanisms are fully understood, the phenomenon can be harnessed for potential applications in fabrication of encapsulates, microporous membranes, templates for microelectronics, and so forth. When an isotropic aqueous solution of NaMy is cooled from a temperature well above its Kraft boundary (59 °C),9 ribbonlike flat fibers are seen to crystallize out of solution.10,11 The fiber structure that has been well * To whom the correspondence should be addressed. E-mail:
[email protected]. (1) Yan, D.; Zhou, Y.; Hou, J. Science 2004, 303 (5654), 65-67. (2) Dubois, M.; Deme, B.; Krzywicki, T. G.; Dedieu, J. C.; Vautrin, C.; Desert, S.; Perez, E.; Zemb, T. Nature 2001, 411, 672-675. (3) Zhang, S. Nat. Biotechnol. 2003, 21, 1171-1178. (4) Zastavker, Y. V.; Asherie, N.; Lomakin, A.; Pande, J.; Donovan, J. M.; Schnur, J. M.; Benedek, G. B. Proc. Natl. Acad. Sci. U.S.A. 1999, 96, 7883-7887. (5) Fuhrhop, J.-H.; Helfrich, W. Chem. Rev. 1993, 93, 1565-1582. (6) Kaler, E. W.; Murthy, A. K.; Rodriguez, B. E.; Zasadzinski, J. A. N. Science 1989, 245, 1371-1374. (7) Madelmont, C.; Perron, R. Colloid Polym. Sci. 1976, 254, 581595. (8) Laughlin, R. G. The Aqueous Phase Behavior of Surfactants; Academic Press: San Diego, CA, 1994. (9) Darke, W. F.; McBain, J. W.; Salmon, C. S. Proc. R. Soc. London, Ser. A 1920, 395-409. (10) Trager, O.; Sowade, S.; Bottcher, C.; Fuhrhop, J. H. J. Am. Chem. Soc. 1997, 119, 9120-9124. (11) Liang, J.; Ma, Y.; Zheng, Y.; Davis, T. H.; Chang, H. T.; Binder, D.; Abbas, S.; Hsu, F. L. Langmuir 2001, 17, 6447-6454.
Figure 1. Parts a-c show SEM surface micrographs of a 10% NaMy solution deposited on a glass slide and cooled at (a) 4, (b) 2, and (c) 1 °C/min. Scale bar ) 50 µm. (d) Cross-sectional view of the surface pattern, scale bar ) 20 µm.
characterized in the past is lamellar in nature with a d spacing of 36.2 Å as obtained from X-ray diffraction studies.11 Time-resolved microscopy experiments have shown that the fibers first grow in length and then in thickness.9 When the concentration of NaMy is in excess of 5% (w/w), the crystallized fibers are seen to form a solidified random network (“gel”) having a pore space filled with the equilibrated solution phase.11 The tertiary fiber assemblies were observed while carrying out detailed studies on the NaMy microstructures, crystallized under different conditions. Figure 1a-c shows surface micrographs of 10% NaMy solutions deposited on a glass slide cooled to ambient temperature at varying rates. At a cooling rate of 4 °C/min a random network of fibers was seen (Figure 1a). However, when the cooling rate was lowered to 2 °C/min, we started to see some unusual phenomena (Figure 1b). The fibers now tended to align themselves into bundles, as well as showed bending and curling. Some fibers formed circular ring structures. When the cooling rate was further reduced to 1 °C/min, a neat array of close-packed spiral patterns of fibers was observed in the micrographs (Figure 1c). Thus, changing the cooling
10.1021/la0487848 CCC: $30.25 © 2005 American Chemical Society Published on Web 09/17/2004
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Figure 2. (a) Emergence of three-dimensional micro-pottery structures at the free surface and (b) enhanced density of the structures on application of a vacuum. The surface folds are because of drying; scale bar ) 200 µm. (c) Zoomed view of a three-dimensional micro-pottery structure; scale bar ) 10 µm. (d) Spiral patterns and “micro-pottery” structure on the inner surface of an air bubble in NaMy; scale bar ) 50 µm.
rate resulted in transition from a random network to a well-organized spiral self-assembly of fibers at the free surface. A cross section of the surface showed that each of the surface spirals extended about 10-15-µm deep into the bulk (Figure 1d), forming a bobbin-like structure with fibers tightly wound around it. The bulk region underneath showed the presence of a random fiber assembly. Thus, the fiber assembly appeared to be a pure surface phenomenon. Image analysis of the surface spiral structure showed that the spirals had an average hole of 1.2-µm diameter (with a standard deviation of 0.5). The spiral assemblies could be obtained for concentrations ranging from 5-30%. Higher concentrations close to the liquid crystalline region were not studied. At lower concentrations of NaMy (
