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Mesoscopic Tapes Formed by Amphiphilic Two-Dimensional Colloids on Water Surface Masahito Sano,* Ayumi Kamino, and Seiji Shinkai* Chemotransfiguration ProjectsJST, 2432 Aikawa, Kurume, Fukuoka 839-0861, Japan Received September 24, 1998. In Final Form: October 14, 1998 A two-dimensional (2D) analogue of 3D fibrous supramolecular assemblies was formed spontaneously on the water surface by a short chain carboxyazobenzene derivative. It is a mesoscopic tape 1 nm thick, 80 nm wide, and can be as long as 0.1 mm. The tapes are obtained as Langmuir-Blodgett (LB) films on mica and examined by atomic force microscopy (AFM). AFM images exhibit complex internal structures, indicating that the tapes are not single crystals. They are not produced by LB processes, such as compression and lifting, and are not induced by the mica surface. The mesoscopic tape is self-contained and exists independent of surface pressure whose equilibrium value is nearly zero. UV-vis reflection spectroscopy on the water surface shows a peak at 305 nm. Increasing the water temperature to above 30 °C or leaving the film on the water surface for 14 h caused the tapes to disintegrate into small clusters and the UV-vis peak to shift to 365 nm. The concentration at which the solvent evaporates controls the tape formation. At low concentrations, the tapes coexist with the domains of an ordinary phase. Then, there is a critical aggregation concentration at 8 nm2/molecule that the tape fraction increases abruptly. These results suggest that the tape is closely related to associations of 2D clusters.
In aqueous solution, synthetic amphiphilic molecules assemble spontaneously to form self-contained aggregates with distinct shapes, such as tubular micelles and spherical vesicles. They may further develop into supramolecular objects such as helical fibrils extending over micrometers. As for two-dimensional (2D) systems, structured patterns have been observed in amphiphilic monolayers on the water surface1-8 and in block copolymers9 and thiols10 on solid surfaces. In many cases, they are stable only in periodic continuum under nonvanishing pressure. Otherwise, they exist as 2D crystals or adsorbed crystalline films. In this paper, we report a formation of 80 nm wide and 100 µm long monolayer tapes that are 2D analogues of 3D fibrils. In particular, morphology, spontaneous formation, and an existence of critical aggregation concentration11 are described. The film properties of the carboxyazobenzene compound of a form
have been studied previously by the surface pressurearea isotherms and UV-vis reflection spectroscopy.12,13 (1) Knobler, C. M. Science 1990, 249, 870-874. (2) McConnell, H. M. Annu. Rev. Phys. Chem. 1991, 42, 171-195. (3) Overbeck, G. A.; Ho¨nig, D.; Mo¨bius, D. Thin Solid Films 1994, 242, 213-219. (4) Brezesinski, G.; Dietrich, A.; Dobner, B.; Mo¨hwald, H. Prog. Colloid Polym. Sci. 1995, 98, 255-262. (5) Evert, L. L.; Leckband, D.; Israelachvili, J. N. Langmuir 1994, 10, 303-315. (6) Chi, L. F.; Anders, M.; Fuchs, H.; Johnston, R. R.; Ringsdorf, H. Science 1993, 259, 213-216. (7) Seul, M.; Andelman, D. Science 1995, 267, 476-483. (8) (a) Bercegol, H.; Gallet, F.; Langevin, D.; Meunier J. J. Phys. (Paris) 1989, 50, 2277-2289. (b) Flament, C.; Graf, K.; Gallet, F.; Riegler, H. Thin Solid Films 1994, 243, 411-414 and references listed therein. (c) Sikes, H. D.; Schwartz, D. K. Langmuir 1997, 13, 4704-4709. (9) Kumaki, J.; Hashimoto, T. J. Am. Chem. Soc. 1998, 120, 423424. (10) Poirier, G. E.; Pylant, E. D. Science 1996, 272, 1145-1148. (11) Israelachvili, J. N. Intermolecular and Surface Forces, 2nd ed.; Academic: New York, 1992.
Figure 1. AFM image of mesoscopic tapes. The azobenzene solution was spread at 2.4 nm2/molecules, then transferred after compression to 0.6 nm2/molecules. Scale bar is 5 µm.
The short chain compounds (C4-C6) produced no surface pressure at all areas/molecule and condensed into 3D crystallites upon spreading. The long chain homologues (C10 and longer) gave the stable isotherms and crystalline films. We now know that they form pancake-shaped domains in the condensed phase. Compound C7 was intermediate in a sense that it showed no evidence of 3D aggregation, yet it had no stable isotherms. The surface pressure strongly depended on how we measured it. To investigate the structures of C7 on the water surface, (12) Sano, M.; Kunitake, T. Paper presented at the 44th meeting of Colloid and Surface Chemistry, the Chemical Society of Japan, Saitama, 1991; pp 56-57. (13) Sano, M.; Sasaki, D. Y.; Isayama, M.; Kunitake, T. Langmuir 1992, 8, 1893-1902.
10.1021/la981319o CCC: $18.00 © 1999 American Chemical Society Published on Web 12/04/1998
14 Langmuir, Vol. 15, No. 1, 1999
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Figure 2. AFM images of mesoscopic tapes: (a) a magnified image showing the bumpy perimeter and a groove running along the long axis, bar 100 nm; (b) a highly magnified view of the tapes, made with the standard conditions, bar 100 nm; (c) broken and skeletonized tapes. The LB film was made by the horizontal lifting mode. The stress during LB processes, high temperature, and aging often resulted in disintegration of tapes to double strands of small clusters. Bar indicates 1 µm.
Langmuir-Blodgett (LB) films were fabricated14 and observed by atomic force microscopy (AFM). Unlike usual LB methods, there was no reliable surface pressure to refer to. Thus, a single upstroke transfer to mica was made at the fixed area (the equilibrium surface pressure was almost always zero) without moving the barrier.15 AFM observations reveal that C7 forms mesoscopic tapes with uniform thickness of 1 nm and average width of 80 nm (Figure 1).16,17 The length ranges from several micrometers to at least 100 µm. Due to lack of molecular resolutions for larger area scans in AFM, it is not known how long the longest tape can be. Higher magnifications show that the perimeter of a tape is not straight and has many bumps (Figure 2a). There is a cracklike groove running along the long axis on every tape. The grooves are not always at the middle of tapes. The AFM images taken at still higher magnifications exhibit many small clusters of 20-30 nm tracing a tape shape (Figure 2b).16 Although there is a possibility of AFM tips disturbing the tape during scanning, the images suggest that the small 2D clusters play a role in the tape assembly. The tapes on water have an UV-vis reflection peak at 305 nm, in a H-like aggregate state.18,19 This corresponds to a model of each molecule standing up nearly straight from the water surface with high orientational order. Symmetry of molecular packing in the lateral direction, which is important for anisotropic aggregation of small clusters, is not known presently. We have varied the LB conditions and the spreading solvents to confirm that the tape structure is not made by the LB processes or specific interactions of C7 with the solvent.20 We also verify that the tapes are first formed (14) In the standard conditions, a chloroform solution of C7 was spread on the pure water surface at 1.2 nm2/molecule. After aging for 15 min at 20 °C, a barrier was moved to compress the film to 0.45 nm2/molecule. The film was transferred with a vertical lifting mode at 5 mm/min. (15) Since the substrate surface area was much smaller than the total film area on the trough, this did not produce noticeable inhomogeneity. (16) It is well-known that AFM images of an object whose size is comparable to the tip apex are significantly affected by the tip contour. In particular, lateral sizes of objects tend to be broadened. Thus, the tape width of 80 nm and the cluster diameter of 20 nm should be taken as the upper limits of true values. (17) Similar tape structures are observed in polysiloxane monolayers (Fang, J.; Dennin, M.; Knobler, C. M.; Godovsky, Y. K.; Makarova, N. N.; Yokoyama, H. J. Phys. Chem. B 1997, 101, 3147-3154.) when it is collapsed. (18) Hochstrasser, R. M.; Kasha, M. Photochem. Photobiol. 1964, 3, 317-331. (19) Kawai, T.; Umemura, J.; Takenaka, T. Langmuir 1989, 5, 13781383.
on water then laid on mica.21 By lifting mica during solvent evaporation and imaging along the lifting direction, we saw a sudden emergence of tape structures just when all solvent molecules were evaporated. Thus, the tapes are formed spontaneously on water upon solvent evaporation. The mesoscopic tapes are formed independent of surface pressure. Once formed, the tapes remain distinct and resist fusing on pure water. The LB transfers deviating extremely from the standard conditions result in broken and skeletonized tapes (Figure 2c). Spreading at a slightly higher temperature of 30 °C or aging for 14 h at 20 °C causes most of the tapes to disintegrate into the dispersed small 2D clusters.22,23 These 2D clusters are different from those shown earlier. The UV-vis peak is shifted to 350 nm, in a less oriented state. Since all short chain homologues give a peak at 410 nm as floating 3D crystallites,12 the disintegrated state is considered to be still in 2D. Thus, the tape state is only kinetically stable on water, similar to some 3D colloidal particles.11 In contrast, they are stable on mica over a period of days. The clear evidence demonstrating that the tapes belong to a state different from the ordinary phase separated domains of monolayers is given by the sample prepared at a low concentration (Figure 3). The tapes (marked A in Figure 3) coexist with the pancake-shaped domains (B) that have been seen in an ordinary amphiphilic phase of the longer homologues . Coalescence and collapse to 3D aggregation (C) occur only on the ordinary domains and not on the tapes. The area/molecule at which the solvent evaporates controls the fraction of the tape component. A (20) Changing the lifting mode (vertical or horizontal), the lifting speed (5-50 mm/min), the transferred area (0.1-4.8 nm2/molecule), and with or without compression (additions of the solution only) made no difference in the tape formation. Also, there was no preferential orientation of the tapes with respect to the lifting direction. Chloroform and benzene as a spreading solvent have yielded the identical result. Absence of chlorine peak in XPS indicated that there was no residual solvent molecules in a film. (21) The tapes were found to orient indifferent to mica defects and lie over the mica steps without any discontinuity. Transferring onto a hydrophilic glass caused the tapes to stick together randomly, suggesting that the tapes could not adhere onto the surface as water drained. (22) This temperature is much lower than the lowest phase transition temperature of C7 at 154 °C in bulk. (Sano, M.; Kunitake, T. Langmuir 1992, 8, 320-323.) (23) Small 2D clusters have been observed in partially fluorinated fatty acid monolayers (Kato, T.; Kameyama, M.; Ehara, M.; Iimura, K. Langmuir 1998, 14, 1786-1798.) and in a block copolymer monolayer (Li, S.; Hanley, S.; Khan, L.; Varshney, S. K.; Eisenberg, A.; Lennox, R. B. Langmuir 1993, 9, 2243-2246.). The 2D micelles are mentioned in these papers, based only on morphological appearances without giving physical characteristics.
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Langmuir, Vol. 15, No. 1, 1999 15
Figure 5. A hierarchy of self-organization in the 2D colloid model. Also indicated are the peak wavelengths in the UV-vis reflection spectra. The perimeter of each object is drawn purposely to remind of an importance of the interface between the interior of a cluster and a free water surface, analogous to the surface of a colloidal particle in 3D solution.
Figure 3. AFM image (10 × 10 µm) showing coexistence of tapes (A), pancake-shaped ordinary domains (B), and 3D collapsed regions (C). The solution was spread at 11.1 nm2/ molecule. The tapes appear wider than one made with the standard conditions, partly because of the tip-sample interactions during AFM. Other AFM artifacts like contrast inversion have been observed reproducibly with the samples made around the transition concentration.
Figure 4. A plot of the normalized fraction of the tape area over the total film area (A/(A + B) in Figure 3) against a log of the concentration at spreading, at 20 °C, pH ) 5.8. The uncertainty comes mainly from the difficulties of achieving uniform spreading and evaporation.
plot of the fraction of tapes against the concentration (reciprocal of the area/molecule) reveals a sharp increase of the tape fraction at approximately 8 nm2/molecule (Figure 4), which is over 30 times larger than the limiting area of this type of compound.13,24 This represents the critical aggregation concentration of the tapes or its constituent clusters.
This also indicates that the tape formation is a thermodynamically driven spontaneous aggregation and not kinetically controlled crystal growth on water.8 If a tape is viewed as a single crystal, line tension may play an important role. In this case, a theory predicts a formation of a sharp cusp at each end of tapes and a correlation between width and length.25 Lack of these properties as well as structures with bumpy perimeters and grooves indicate that the tape formations are not crystal growth. The present results suggest a hierarchical model of two levels self-organization (Figure 5). First, molecules gather to form small 2D clusters. Within an cluster, molecules are standing nearly straight from the water surface in a H-like aggregate state. Then, the small clusters associate to form tapes. These clusters are not stable, however. Reorientation of molecules occur within the clusters to transfer away from the H-like state, resulting in disintegration of the tapes. Subsequent experiments have shown that the subphase pH affects the shape of the tapes, whereas an addition of the inorganic salts induces critical coagulation of the tapes. Within the 2D colloid model, these effects imply that the collective electrostatic interactions govern the forces between the tapes. The detailed accounts of these effects will be reported elsewhere. Lastly, it should be noted that the compound C7 belongs to a class of common materials that have been ignored previously for a reason of “poor film-making” capability. LA981319O (24) Durbin, M. K.; Malik, A.; Richter, A. G.; Yu, C. J.; Eisenhower, R.; Dutta, P. Langmuir 1998, 14, 899-903. (25) Muller, P.; Gallet, F. J. Phys. Chem. 1991, 95, 3257-3262.
