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Eroded Myelin Figures M. A. Arunagirinathan, C. Manohar, and Jayesh R. Bellare* Department of Chemical Engineering, Indian Institute of Technology Bombay, Powai, Mumbai 400 076, India Received November 3, 2003. In Final Form: March 17, 2004 Myelin figures with unusual surface morphology were observed on contacting Tween85 with water. Myelins, which are normally smooth rodlike forms in other surfactants, are in this system found to be with an irregular, rough surface with vesiclelike structures adhered to the myelin tubes. Besides these, smooth myelin figures were also observed. We term the myelin figures with a rough surface eroded myelin figures. The same myelin could show a coexistence of smooth and rough areas with a sharp boundary between the smooth surface at one end whereas the other end shows a rough texture. The transformation of smooth myelins into eroded forms were observed often whereas the reverse is quite rare. In the later stage, the tip of the eroded myelin figures transforms into tentacles and acts as a source for new myelins and the growth of vesiclelike structures which were expelled into the surrounding medium. The eroded myelin figures are stable for a longer period in comparison to simple, smooth rodlike forms. By studying the myelin growth at different temperatures, it was found that eroded myelin figures were stable in the temperature range of 22-42 °C and at g42 °C only smooth myelin figures were observed.
1. Introduction 1-8
formed by The extensive study on myelin figures sparingly soluble surfactants or lipids on contact with water has allowed them to acquire a remarkable place as the nonequilibrium surfactant microstructures. The normal myelin figures are concentrically arranged bilayers with alternating layers of water around the central axis of the medium. The myelins are termed intermediate structures formed during the transformation of lipid into hydrated bilayers. The transition of lipid in the presence of excess water f myelin figures f vesicles f bilayers with excess water9 has been indicated by Lasic. The myelins so far observed in different surfactant systems such as lecithin,1,7 pentaethyleneglycol lauryl ether,10 amine oxide/alcohol,11 and triethyleneglycol lauryl ether6 are elongated tubes with a smooth surface and coiled and twisted structures which transform into the mosaiclike structure at later stages. Here, we report the first observation of elongated tubes of myelins with a rough surface and ragged appearance, which we term eroded myelins. These eroded myelins are stable for a longer period in comparison to the ordinary smooth myelin figures. The stability of the microstructure as evident from optical microscopic observations seems to make it a promising microstructure for potential applications in * Corresponding author. E-mail:
[email protected]. (1) Sakurai, I.; Kawamura, Y. Biochim. Biophys. Acta 1984, 777, 347-351. (2) Mishima, K.; Yoshiyama, K. Biochim. Biophys. Acta 1987, 904, 149-153. (3) Mori, F.; Lim, J. C.; Raney, O. G.; Elsik, C. M.; Miller, C. A. Colloids Surf. 1989, 40, 323-345. (4) Lim, J. C.; Miller, C. A. Langmuir 1991, 7, 2021-2027. (5) Buchanan, M.; Arrault, J.; Cates, M. E. Langmuir 1998, 14, 73717377. (6) Buchanan, M.; Egelhaaf, S. U.; Cates, M. E. Langmuir 2000, 16, 3718-3726. (7) Sakurai, I.; Shibata, I.; Minobe, M.; Kawamura, Y. Mol. Cryst. Liq. Cryst. 2001, 363, 157-165. (8) Dave, H.; Surve, M.; Manohar, C.; Bellare, J. J. Colloid Interface Sci. 2003, 264, 76-81. (9) Lasic, D. D. Liposomes: from physics to applications; Elsevier: Amsterdam, 1993. (10) Benton, W. J.; Raney, K. H.; Miller, C. A. J. Colloid Interface Sci. 1986, 110, 363-388. (11) Rang, M. J.; Lim, J. C.; Miller, C. A.; Hoffmann, H. F. J. Colloid Interface Sci. 1995, 175, 440-445.
materials science such as in the synthesis of surfactanttemplated porous and nonporous composite materials which will be useful in catalytic and gas occlusion processes. 2. Experiment and Methods The nonionic surfactant polyethoxy sorbitan trioleate (Tween85) was obtained from Merck Co., U.S.A., without further purification, and the distilled Millipore water was used for the penetration scan studies. The optical microscopic observations were carried out at 22-50 °C. The samples (surfactant, water) were equilibrated in the room that was maintained at 22 and 25 °C for a minimum of 24 h, and a drop (2 µL) of Tween85 was placed on the glass slide and covered with a coverslip. The gap of 160 µm was maintained between the coverslip and the slide by using cellophane tape as spacers. About two drops (10 µL) of water were kept at the edge of the coverslip and allowed to contact the surfactant by capillary action. For temperatures greater than 28 °C, a Linkam 350 hot stage with temperature control having a precision of (0.1 °C was used. The outgrowths that take place at the surfactant/water interface were continuously monitored and recorded as digital images by using a Sony-XC77CE, WAT202D, or Olympus DP-11 digital camera (resolution 2.38 megapixels) fitted with the optical microscope.
3. Results and Discussion The surfactant Tween85 at room temperature that was isotropic transformed into a liquid crystalline phase on contact with water, from which the eroded myelin figures (Figure 1) emerged into the contacting medium. Three kinds of myelin figures with different surface morphologies were observed, namely, (1) myelins with a rough surface (Figure 1), (2) simple rodlike myelins with a smooth surface (Figure 2), and (3) myelins with part of the surface being smooth and remaining part being rough (Figure 3a,b). The formation of a lamellar liquid crystalline phase on contact with water by Tween85 is known, but its subsequent evolution into myelin figures with a rough surface morphology which we call an eroded myelin is reported here for the first time. Myelins with a smooth surface have been reported earlier in many systems, whereas the observation of myelin with a rough surface has not been reported. The transition of a smooth myelin tube into an eroded surface was observed in most cases with the tip of the
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Figure 1. Eroded myelin figures observed within 7 min at 26 °C on contacting Tween85 with water. The scale bar represents 100 µm.
Figure 2. Coexistence of eroded and smooth myelin (indicated by the arrow) obtained at 27 °C. The scale bar represents 100 µm.
initially smooth myelin starting to transform into an eroded form and continuing toward the root of the myelin (Figure 3a) and vice versa; namely, the eroded form into the smooth myelin as shown in Figure 3b was also observed but not often. Indeed, the growth of a longer, smooth myelin tube as a branch from the eroded form was observed (Figure 3c). The observation of myelin figures at the interface was carried out at different temperatures (27 °C, 30 °C, 3542 °C with a 1° degree interval, 45 °C, and 50 °C), revealing the stability of the eroded myelin figures (Figure 4). From the study, it is clear that as the temperature increases above 35 °C the population of the eroded myelin figure decreases whereas that of the smooth myelin figures increases. The population of eroded myelin figures increases slowly in the temperature range 22-35 °C and decreases drastically from 36 to 42 °C. In the temperature range of 36-42 °C, it can be seen that the number of eroded myelins decreases to very few, and at 42 °C no eroded myelins were observed. At and above 42 °C, smooth myelin figures were only observed. Eroded myelin figures bear resemblance to rods that are corroded and have their outer layers peeled off from the surface. In addition to the pitlike depressions, an almost crumbled state of appearance and irregular size of the eroded myelin rod were evident from the optical microscopic observations. Erosion in the form of patches, as well as being uniform, was found on the surface of the eroded myelin figures. As the time lapsed, the transformation of the tip of the eroded myelin rod into a cluster of vesiclelike structures was observed (Figure 5a). The vesicles that had formed at the tip of the myelin rods later expelled into the contacting medium. This implies that vesicles are one among the intermediate structures during
Figure 3. Transformation in the morphology of the myelins in the later stages of myelin growth. (a) Smooth myelin (indicated by the arrow) transforms into eroded myelin from the tip of the myelin tube and continues toward the root of the myelin. (b) Eroded myelin transforms into smooth myelin (indicated by the arrow). (c) Growth of smooth myelin (indicated by the arrow) as a branch from the eroded myelin. The scale bar represents 100 µm.
the transformation of myelin figures into a final mosaiclike structure, which is a fully hydrated planar bilayer film. The formation of tentaclelike structures at the tip of the myelin rods was also observed at the later stages of transformation of the eroded myelin figures. The closer observation of the surface of the eroded myelins and the transformation of smooth myelin figures into the eroded form at later stages revealed the presence of polygon structured aggregates on the surface of the myelin (Figure 5). When some of the smooth myelin figures transformed into eroded myelin figures, it was found that polygon structures have formed at different places of the smooth myelin during transition of the smooth myelin into an eroded myelin rod. This type of transformation was clearly seen at 41 °C, where the initially formed smooth myelin figures started transforming into eroded myelin figures with the formation of polygon structures at the surface of the smooth myelin rod. We feel that the polygon aggregates that had formed on the smooth myelin rods may exist by peeling off enough of an amount of the
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Figure 4. Myelin figures observed at various temperatures. Eroded myelin figures at (a) 30, (b) 35, and (c) 40 °C. (d) Smooth myelin figures observed at 41 °C. The scale bar represents 100 µm.
Figure 6. Myelin figures with vesiclelike structures adhered to the surface of the tube. The arrow indicates the vesicles clinging to the tip of the myelin. The scale bar represents 100 µm.
Figure 5. (a) Transformation of eroded myelins (1). Eroded myelin fully transformed into vesiclelike sructures (indicated by the arrow; 2). Partly transformed eroded myelin in which the polygon-shaped aggregates (indicated by the arrow) are seen on the entire surface of the eroded tube along with the vesiclelike structures that have formed. (b) Smooth myelin partly transforming into the eroded form by precipitating the aggregated structures (indcated by the arrow) on the surface of the myelin tube. The scale bar represents 50 µm.
bilayers from the surface of the myelin in the form of aggregates, which might have led to pitlike depressions on the smooth surface of the myelin. However, the presence of decomposition products of the polyethoxy sorbitan
trioleate as an impurity cannot be ruled out, which may induce the formation of the aggregated structures that might have crystallized on the surface of the myelin tubes at a temperature less than 42 °C. The rough myelin figures with vesiclelike structures adhered to the surface of the myelin rod were also observed (Figure 6). In some cases, the outgrowth of the smooth myelin figure from the rough myelin figure was observed; it seems that the tip of the rough myelin tube was split open, and through this the smooth myelin figure had emerged. The opening of the rough myelin tip indicates that there would have been a formation of a thicker film with enough elasticity to constitute the surfactant aggregates in their hydrated form and engulp the myelin tube, imparting a rough appearance. As the growth of the myelin continues, it overcomes the threshold elastic strength of the film and bursts out into the medium as smooth myelin tubes; however, the split opening of the tip of the rough myelin tube is very rarely observed.
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4. Conclusions The myelin figures with a rough surface morphology were observed for the first time. The tip and the surface of the eroded myelin figures transform into a cluster of vesicles at the later stages of myelin growth. On closer observation of the eroded myelin figures, the patches of eroded surface as well as polygon structures adhered to the surface are visible. The polygon structures which emanate from the surface of the smooth myelin rod might have created pitlike depressions on the surface of the myelin rod and impart an eroded appearance to the myelin figures. From the temperature scan study, we found that
“eroded” myelin figures are stable up to 42 °C, beyond which smooth myelin alone were observed. Detailed investigations of the surface characteristics of the eroded myelin figures by cryo-electron microscopy and the effect of various additives on the surface morpholgy of the myelin figures are in progress. Acknowledgment. We would like to thank Dr. Dhanuka and Dr. Peter Garrett of Unilever for their interest in this work and Dr. Anil Kumar for allowing us to use the heating stage Linkam 350. LA036069C
