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Structures of Collapsed Polysiloxane Monolayers Investigated by Scanning Force Microscopy Jiyu Fang, Michael Dennin, and Charles M. Knobler* Department of Chemistry and Biochemistry, UniVersity of California, Los Angeles, California 90059-1569
Yu. K. Godovsky KarpoV Institute of Physical Chemistry, UI. VorontzoVo Pole 10, Moscow 103064, Russia
N. N. Makarova Institute of Organoelement Compounds RAS, VaViloVa Strasse 28, Moscow, Russia
Hiroshi Yokoyama Molecular Physical Section, Electrotechnical Laboratory, 1-1-4 Umezono, Tsukuba, Ibaraki 305, Japan ReceiVed: October 24, 1996; In Final Form: February 17, 1997X
Surface pressure-area isotherms of monolayers of cyclolinear polysiloxanes on water exhibit several plateaus that have been associated with the formation of multilayers. This stepwise collapse has been studied in one of these polymers by scanning force microscopy under topographic, frictional, and electric force modes. Topographic images show that the growth of the second layer begins with the nucleation of three-dimensional islands, which then appear to spread in the form of ribbons 0.5 µm wide and 7.5 Å high. The number of ribbons increases as the extent of the second layer increases, but the width of the ribbons remains constant. Subsequent layers do not exhibit this morphology.
Introduction Monolayers at the air-water interface (Langmuir monolayers) provide ideal systems for the study of two-dimensional phase transitions.1 These monolayers exhibit a rich phase behavior with transitions between gaseous, liquid, liquid-crystalline, and crystalline phases. When the monolayers are compressed above the equilibrium spreading pressure, however, they can undergo transitions to three-dimensional phases. This process, which is called collapse, has been studied for decades,2-16 but is not well understood. Two general scenarios are observed: a direct transition from the monolayer to a bulk phase or a progressive transition through a series of multilayers. In relatively simple amphiphiles, such as saturated fatty acids and esters, there is evidence for both the formation of multilayers and nucleation of the three-dimensional phase. In a study of surface pressure-area isotherms, Lundquist2 observed evidence of multilayer formation in acetates. Ries3 examined collapsed monolayers of fatty acids by transferring them onto solid substrates and using electron microscopy to show that the films contained monolayer and trilayer regions. The formation of multilayers has also been confirmed by neutron reflectivity.4 In contrast, the direct formation of three-dimensional nuclei has been observed on the water surface by Brewster angle microscopy (BAM)5 and by atomic force microscopy (AFM)6 on a transferred film. There is convincing evidence that smectic liquid crystals, which have a layered structure in the bulk, always form stable multilayers before complete collapse to a three-dimensional phase. For example, Rapp and Gruler,7 on the basis of surface pressure-area isotherm measurements, reported layer growth by a series of collapses of a Langmuir monolayer of the smectic X
Abstract published in AdVance ACS Abstracts, April 1, 1997.
S1089-5647(96)03319-6 CCC: $14.00
phases of the ferroelectric liquid crystal p-hexyloxybenzylidenep′-amino-2-chloro-R-propyl cinnamate (HOBACPC). Xue et al.8 used ellipsometry and optical second harmonic generation to study collapsed monolayers of the smectic phase of 4′-noctyl-4-cyanobiphenyl (8CB) and observed a first-order phase transition from a stable monolayer to a stable trilayer. Collapse of 8CB monolayers has been directly visualized by BAM, confirming the formation of the uniform trilayer.9,10 Ibn-Elhaj et al.11 found layering in the smectic C phase in collapsed monolayers of triblock organosiloxane smectogens. Layer growth was also observed in a mixed monolayer of a crown ether liquid crystal and a fatty acid, where liquid crystal molecules were squeezed out of the mixed monolayer to form a bilayer.12 Polymer thin films are interesting systems because of their potential applications.17 Bilayer formation at the air-water interface has been observed for polymer monolayer systems such as polypeptides13 and polysiloxanes.14-16 There is considerable evidence14 that the collapse of linear polysiloxanes proceeds through the coiling of the chains into helices. On the other hand, Granick et al.15 found nearly identical patterns of collapse in isotherms of linear and cyclic polysiloxanes, which suggested a similar mechanism of collapse despite the fact that helix formation in systems consisting of small rings is unlikely. More recently, Godovsky and co-workers18,19 found up to seven plateaus in pressure-area isotherms of cyclolinear polysiloxanes. The plateaus occur at ratios of the area that are consistent with stepwise layer growth from monolayer to bilayer, bilayer to trilayer, etc., continuing up to seven layers. This is clear evidence of successiVe multilayer transitions in a material that does not have a bulk smectic liquid crystal phase. BAM images of collapsing linear siloxanes16 showed that there were domains of different discrete thicknesses and also suggested collapse involving multilayer formation. © 1997 American Chemical Society
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Figure 1. Surface pressure-area isotherms of the polysiloxane monolayer during a compression (full line)-expansion (dashed line) cycle. The different points for film transfer are marked by arrows. The structure of the monomer is shown in the insert.
Figure 2. Surface pressure relaxation with time for the collapsed monolayers at plateau regions E (solid line) and C (dashed line).
The cyclolinear polysiloxanes have bulk columnar liquid crystal phases,20 and it has been proposed that the multilayers form by a sliding mechanism.19 In order to understand the mechanism of collapse better and to examine the validity of this model, we have studied the collapse of monolayers of a cyclolinear polysiloxane by several methods. The pressurearea isotherms have been reexamined, images of floating monolayers undergoing collapse have been obtained by BAM, and three types of scanning force microscopy have been employed to obtain images of films that have been transferred to solid supports. These complementary methods have provided us with new insights into the collapse process. Experimental Section Sautter et al.19 studied a series of cyclolinear polysiloxanes in which rings containing six -Si-O units were separated by linear spacers of varying lengths. We have studied only the polymer shown in the insert in Figure 1, in which the spacer is an oxygen atom. The sample investigated had a molecular weight of 46 000, which corresponds to a degree of polymerization of about 100. It was spread from a 0.7 mg/mL chloroform (Fisher spectranalyzed) solution onto a pure water subphase (Millipore Milli-Q system, 18 MΩ, pH 5.7) in a NIMA Type 611 trough held at a constant temperature of about 24 °C. Silicon oxide substrates were cleaned by immersion in a mixture of H2O2/H2SO4 (1:1) held at 100 °C for 30 min and
Figure 3. Topographic images of a monolayer transferred onto mica from point 1. The images differ in magnification as indicated by the bars. The regular organization of the chains seen in (c) may have been induced by the scanning process.
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Figure 4. Topographic images of collapsed monolayers transferred onto mica from point 3 (a), 4 (b), 5 (c), and 6 (d).
then rinsed with Millipore water. Mica substrates were cleaved with adhesive tape. The collapsed monolayers were transferred onto these substrates by vertical dipping at a constant surface pressure. Unless otherwise noted, the dipping speed was 1 mm min-1. Topographic and frictional force images of collapsed monolayers were obtained with a scanning force microscope (Park Scientific Instruments) using 5 and 100 µm scanners. Microfabrication triangular Si3N4 cantilevers with a normal spring constant of 0.05 N m-1 (Park Scientific Instruments) were used for the measurements. All topographic and frictional force images were obtained in the constant-force mode. The applied loading forces were in the range 1-5 nN. Images of the lateral variation in the surface potential were obtained by scanning Maxwell stress microscopy (SMM).21 The SMM system is a modified NanoScope III AFM (Digital Instruments). A Si3N4 cantilever tip (Digital Instruments) with a spring constant of 1 N m-1 was coated with platinum to make it electrically conductive. An ac voltage was applied between the tip and silicon substrate, and the oscillation of the cantilever was detected by a double lock-in amplifier system, instead of the
repulsive force as in AFM. A 50 µm scanner was employed, and the scan rate was 0.2 Hz. Brewster angle microscopy (BAM) measurements were performed with an instrument that has been previously described in detail.22 The light polarized in the plane of incidence (ppolarized) was reflected off the surface at the Brewster angle for pure water and detected by a CCD camera. The presence of a monolayer changes the Brewster angle for the system, and variations in either the index of refraction or thickness across the polymer monolayer are detected as intensity variations in the image. Results and Discussion Isotherms. A surface pressure-area isotherm of the polysiloxane at 24 °C is shown in Figure 1. The compression isotherm agrees quantitatively with that reported by Sautter, et al.19 In region A, the surface pressure exhibits a continuous increase with decreasing area per monomer, corresponding to a monolayer coverage. Extrapolation to zero pressure in region A gives an area per monomer of 130 Å2 on the water surface.
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Figure 6. BAM images of a multilayer at the air-water interface before (a) and after (b) the compression was stopped. The size of the images is 300 × 500 µm2.
Figure 5. Topographic images of the breakup of multilayers held at a constant area. The multilayers were transferred onto mica after 2 (a) and 10 min (b) after compression.
We expect that in a close-packed monolayer each monomer ring will lie flat with its oxygen groups anchored in the water surface and its methyl side groups extending upward. Molecular modeling with the program23 Macromodel 5.0 gives a ring diameter of 12.7 Å, which would correspond to a monolayer area of 127 Å2, in good agreement with the isotherms. The picture is also confirmed by height and contact angle measurements of transferred films (see below). Further compression leads to several plateaus at surface pressures of 9.4, 10.3, 11.7 and 13.6 mN m-1, respectively. The area per monomer at the end of these plateaus is about 0.50, 0.32, 0.27, and 0.19 times that of the monolayer coverage, indicating that a complete monolayer transforms into a bilayer, a trilayer, and a tetralayer during the lateral compression process. The plateau regions (B-E) can be seen as n-layer and (n + 1)-layer coexistence regions (where n ) 1, 2, 3, and 4). The plateau pressures are independent of temperature over the range 8.7-30 °C and compression speed in the range 2-70 Å2 min-1. This is in contrast to the behavior found by Gourier24 in
monolayers of 10,12-pentacosadynoic acid for which the plateau pressure depends linearly on the compression speed. Similar behavior has been observed in HOBACPC,7 but the collapse pressure of 8CB is independent of barrier speed.24 Studies of hysteresis are somewhat complicated by the solubility of the polysiloxane, but the loss of the polymer can be diminished by carrying out isotherms on a subphase that had previously been in contact with a monolayer. As shown in Figure 2a, if a compression is stopped in first plateau, the pressure decreases rapidly by 0.4 mN m-1. (On a fresh subphase this would be followed by a slow decrease in pressure that we attribute to the solubility.) In an expansion, the plateau lies 0.6 mN m-1 below the compression curve, and when the expansion is stopped the pressure rises rapidly by 0.2 mN m-1. Subsequent cyclic compressions and expansions trace out the same curves. The hysteresis is much greater in compressions beyond the first plateau. If the expansion is begun in any of the other plateaus, the surface pressure rapidly drops to 8.9 mN m-1. If the expansion is stopped, the pressure rises by 0.2 mN m-1. The pressure during the expansion remains constant at 8.9 mN m-1 until the area per monomer reaches 100 Å2, at which point the pressure begins to fall to zero (Figure 1). Subsequent compression-expansion cycles follow the same curves. This general behavior is similar to that seen in acetates by Lundquist,2 but it is in contrast to that observed in smectic liquid crystals where the successive plateaus appear to represent equilibrium pressures.7 The fall of the pressure to 8.9 mN m-1 during compressions beyond the monolayer and the upward relaxation to the same pressure during expansion suggests that the monolayer-bilayer coexistence regime can be in equilibrium with the bulk. This
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Figure 7. Surface potential image of a collapsed monolayer transferred onto a silicon wafer from point 6.
conjecture is verified by spreading excess polymer on the surface or by allowing a fully collapsed monolayer to equilibrate, from which we find Πq ) 8.9 ( 0.1 mN m-1. Scanning Microscope Studies. Figure 3a shows a topographic image of a polysiloxane monolayer that was transferred onto mica from region A (point 1). (This image and all other topographic images in this paper have been flattened but have not otherwise been treated; none of the images obtained by the other SFM techniques have undergone any treatment.) The monolayer is quite uniform except for a few pinhole defects. Height profiles measured across these holes reveal that they are 7.5 ( 0.2 Å deep, compatible with the thickness (7 Å) of a flat monomer ring determined by molecular modeling. At higher magnification, oriented bundles 50 nm wide become evident (Figure 3b). It is clear that these bundles correspond to aggregates of polymer chains, because they are much larger than a single chain. An image at molecular resolution (Figure 3c) shows regularly aligned individual polymer chains with an interchain spacing of about 11 Å. In some monomer rings, holes a few angstrom in diameter are also observed. It is likely that this high degree of order is the result of the tip scanning and was not present in the unimaged sample. In the first plateau, we observe a pattern composed of ribbons separated by uniform regions of lower height. Examples of this pattern are shown in Figure 4, where parts a and b are transfers from the middle of the plateau (points 3 and 4 in Figure 1, respectively) and part c is from the end (point 5 in Figure 1). Cross-sectional profiles of the ribbons reveal that their height is constant, at ∼7.5 Å above that of the uniform regions, and measurements of height profiles across pinhole defects in the uniform regions reveal that they are ∼7.5 Å deep. Thus, the uniform regions consist of a monolayer and the ribbons are the second layer. As the area/unit is decreased, the width of the ribbons remains constant at 0.5 µm, but their density increases until a complete bilayer has been formed at the end of the plateau. As expected for coexistence between a monolayer and a bilayer, the density of the ribbons obeys a lever rule, with the coverage of ribbons increasing in proportion to the distance across the plateau. As in the first layer shown in Figure 3a,
the surface topography of the complete bilayer that is transferred at point 6 is flat and homogeneous (Figure 4d). All of the transfers shown in Figure 4 were made after allowing the system to relax to the equilibrium pressure of 8.9 mN m-1 but transfers made at the dynamic pressure of 9.4 mN m-1 revealed similar structures. The same ribbon structures were also observed in transfers from the middle of the first plateau at 10 min and 1 h after reaching equilibrium. Because the relaxation of the pressure is not accompanied by any obvious change in the structure of the system, the ribbon pattern appears to be the equilibrium state in the first plateau. The situation for the higher plateaus is quite different. As discussed above, the surface pressure of the films in the higher plateaus also relaxes quickly to Πeq, implying a coexistence of the monolayer with the multilayer phases. Figure 5a shows that this is indeed the case and that the decrease in pressure is accompanied by a tearing of the multilayer into islands separated by the monolayer. As the system evolves, one is left with a monolayer network (Figure 5b). Because no such tearing was observed for the transfers from the first plateau, it is reasonable to assume that the structures presented in Figure 5 were present on the water surface. There is a large body of evidence, however, that the transfer process can affect the structures of the films,25-28 and for this reason we studied the multilayer formation on the air-water interface with the BAM. The images in Figure 6 were taken after the system had been compressed to the middle of the plateau region E. The different intensity levels represent different numbers of layers, with the brightest regions corresponding to the greatest number of layers. The image in Figure 6a was taken just before the compression was stopped, so that no tearing had occurred; that in Figure 6b was taken after the system had been slightly expanded to enhance the tearing and corresponds to the region shown in Figure 6a. It is evident from these images that the tearing of the monolayer occurs on the water surface. In contrast to the pressure-area measurements made under static conditions, the isotherms obtained under continuous compression exhibit plateaus whose relative areas are consistent
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Figure 8. Frictional force images of collapsed monolayers transferred onto mica from points 4 (a) and 6 (b).
with layering transitions, even when the compression is carried out over a period of 1 h. (This should be compared with the time of a few minutes required for the pressure to fall and the multilayer to break apart once the compression is stopped.) A possible explanation for the observed discrepancies is that the breakup of the multilayers under constant area is due to a relatively rapid nucleation of small amounts of the threedimensional phase. As material is lost from the multilayers to the three-dimensional phase, the film “opens up” and forms the monolayer network. Under compression, there is still nucleation of the three-dimensional phase, but the constant compression keeps the multilayer structure together. Furthermore, the amount of material lost to the three-dimensional phase is small enough that the ratio of the areas per monomer of the plateaus closely approximates the n to n + 1 transition. This scenario is further supported by transfers made at constant pressure in the second and third plateaus, which reveal uniform multilayers. Significantly, in these higher plateaus, there is no evidence of the ribbon pattern that occurs during the bilayer transition. It is clear from the isotherms and the morphology that the
Figure 9. Topographic images of a collapsed monolayer transferred onto mica from point 2. (a) immediately below the dipping line; (b) 150 µm below dipping line; (c) 300 µm below dipping line.
second layer is markedly different from the higher layers. The orientation of the rings in the second layer can be deduced from
Structures of Collapsed Polysiloxane Monolayers
Figure 10. Topographic images of a collapsed monolayer transferred onto mica from point 6. (a) immediately below the dipping line; (b) 150 µm below the dipping line.
measurements of the contact angle. At 24 °C, the contact angle for water on a complete bilayer is 81 ( 2°. This hydrophobic behavior suggests that most of the methyl groups in the second layer are exposed, and the oxygen atoms must then be in contact with the methyl-terminated surface of the first layer; i.e., a Z-type bilayer is formed. This orientation of the second layer is confirmed by SMM measurements, which show that, in general, the surface potential of bilayer is twice that of the monolayer. As seen in Figure 7, however, there are some 2-3 µm diameter dark spots in the SMM image that do not appear as features in topographic images. These regions of the bilayer therefore have the same height as their surroundings but exhibit a lower dipole moment. They therefore must correspond to a Y-type organization in which the methyl groups on adjacent layers are in contact. The ribbons possess a high degree of anisotropy. Figure 8a shows a frictional force image of the pattern. Ribbons that are parallel to the scanning direction exhibit high frictional force (bright), while those oriented perpendicular to the scanning direction exhibit low frictional force (dark). This anisotropy shows that the chains are oriented within the ribbons. We expect
J. Phys. Chem. B, Vol. 101, No. 16, 1997 3153 the frictional force to depend on the orientation of the chains with respect to the scan direction. We therefore infer from the images that the polymer chains are oriented perpendicular to the ribbon direction. Frictional anisotropy is also observed on the surface of the complete bilayer (Figure 8b), reflecting local variations in the orientation of the chains in the plane of the second layer. Kinetics of Collapse. We attempted to study the nucleation and growth of the ribbons on the water surface with the BAM, but we were unable to observe any structure, apparently because the resolution of the microscope, which is about 2 µm, is too low. We recognized, however, that the Langmuir-Blodgett transfer provides a mechanism to time-resolve the growth process. If the transfer is begun immediately after a compression, the evolution of the film can be seen in the way that the pattern changes with the distance along the substrate in the dipping direction. By scanning points on the substrate that are a known distance apart, we can obtain images with a known separation in time. The separation in time between two points can be changed by varying the dipping speed. Figure 9 is a series of images taken from a single substrate with a spacing of 150 µm along the dipping direction, which, given the dipping speed of 2 mm min-1, corresponds to a time interval of about 0.5 s. The transfer was made at the collapse point (point 2). Figure 9a reveals that the initial state of the layer growth is the formation of islands 1-1.5 µm in diameter and 21-28 Å in height, which corresponds to 3 or 4 times the monolayer thickness. The spreading of ribbons from the islands becomes evident with time (Figure 9b) until only the ribbons are observed (Figure 9c). By relating distance on the surface to the dipping speed, we find that the ribbons grow at about 100 nm s-1. Topographic images of the spreading of the ribbon from the islands show that its height decreases smoothly to that of a monolayer over a distance of about 1 µm with no evidence of steps. Ribbonlike textures have also been observed29 in monolayers of a polystyrene-poly(dimethylsiloxane) block copolymer that was spread onto water and then transferred to solid supports. Electron micrographs and TEM images show that these monolayer ribbons are about 0.1 µm wide and 25 µm long. It is believed that they grow by the aggregation of two-dimensional micelles, an unlikely mechanism in the system we have studied. The nucleation of the three-dimensional islands may arise from the structural defects in the first layer. The high strain force that is produced at such defects during the compression might peel some molecules off the monolayer to form multilayer islands. However, we only observed nucleation sites for transfers which were made at point 2, and it is not clear why they were not observed across the entire plateau. The anisotropic spreading from the nucleated sites is unusual. It may reflect a highly anisotropic line tension between the bilayer and the monolayer, consistent with the observed orientation of the polymer units within a ribbon. Another possibility is that the order present in the underlying monolayer results in directions of preferred growth. We have also applied the time-resolved technique to the study of the formation of the third layer. The transfer was again carried out at constant pressure immediately after the film was compressed to the start of the second plateau. When a dipping speed of 5 mm min-1 was employed and the tip force did not exceed 1 nN, we were able to obtain the images shown in Figure 10. Jagged islands of the third layer are evident immediately below the dipping line (Figure 10a). The size of the islands increases with distance below the dipping line (Figure 10b), evidence of their growth. Only a uniform layer is observed beyond 600 µm. From the change of size with distance we
3154 J. Phys. Chem. B, Vol. 101, No. 16, 1997 estimate that the growth rate is 10-20 nm s-1. It appears that, unlike the second layer, the third layer is formed by the nucleation and growth of islands, a mechanism that has been observed in BAM studies of liquid crystalline films.10 Acknowledgment. This work was supported by the National Science Foundation and the Civilian Research and Development Foundation. We thank Jian Liu for carrying out the molecular modeling calculations. References and Notes (1) For a review, see: Knobler, C. M.; Desai, R. C. Annu. ReV. Phys. Chem. 1992, 43, 207. (2) Lundquist, M. Chem. Scr. 1971, 1, 5. (3) Ries, H. E., Jr. Nature 1979, 281, 287. Ries, H. E.; Walker, D. C. J. Colloid Sci. 1961, 16, 361. (4) Richardson, R. M.; Roser, S. J. Langmuir 1991, 7, 1458. (5) Siegel, S.; Ho¨nig, D.; Vollhardt, D.; Mo¨bius, D. J. Phys. Chem. 1992, 96, 8157. Vollhardt, D.; Gutberlet, T. Colloids Surf. 1995, 102, 257. (6) Kato, T.; Matsumoto, N.; Kawano, M.; Suzuki, N.; Araki, T.; Irigama, K. Thin Solid Films 1994, 242, 223. (7) Rapp, B.; Gruler, H. Phys. ReV. A 1990, 42, 2215. (8) Xue, J. Z.; Jun, C. S.; Kim, M. W. Phys. ReV. Lett. 1992, 69, 474. (9) Friedenberg, M. C.; Fuller, G. G.; Frank, C. W.; Robertson, C. R. Langmuir 1994, 10, 1251. (10) deMul, M. N. G.; Mann, J. A., Jr. Langmuir 1994, 10, 2311. (11) Ibn-Elhaj, M.; Riegler, H.; Mo¨hwald, H. J. Phys. I 1996, 6, 969. (12) Fang, J. Y.; Uphaus, R. A. Langmuir 1994, 10, 1005. (13) Takeda, T.; Matsumota, M.; Takenaka, T.; Fujiyoshi, Y.; Uyeda, N. J. Colloid Interface Sci. 1983, 91, 267. Malcolm, B. R. J. Colloid
Fang et al. Interface Sci. 1985, 104, 267. (14) Noll, W.; Steinbach, H.; C. Sucker, C. J. Polym. Sci. 1971, C34, 123. (15) Granick, S.; Clarson, S. J.; Formoy, T. R.; Semlyen, J. A. Polymer 1985, 26, 925. (16) Mann, E. K.; He´non, S.; Langevin, D.; Meunier, J. J. Phys. II 1992, 2, 1683. (17) For a review, see: Swalen, J. D.; Allara, D. L.; Andrade, J. D.; Chandross, E. A.; Garoff, S.; Israelachvili, J.; McCarthy, T. J.; Murray, R.; Pease, R. F.; Rabolt, J. F.; Wynne, K. J.; Yu, H. Langmuir 1987, 31, 923. (18) Sautter, E.; Belousov, S. I.; Pechold, W.; Makarova, N. N.; Godovsky, Yu. K. Russ. Polym. Sci. 1996, A38, 39. (19) Belousov, S. I.; Sautter, E.; Godovsky, Yu. K.; Makarova, N. N.; Pechhold, W. Russ. Polym. Sci. 1996, A38, 1532. (20) Godovsky, Y. K.; Makarova, N. N. Philos. Trans. R. Soc. London A 1994, 348, 45. (21) Yokoyama, H.; Inoue, T. Thin Solid Films 1994, 242, 33. Yokoyama, H.; Saito, K.; Inoue, T. Mol. Electron. Bioelectron. 1992, 3, 71. (22) Fischer, B.; Tsao, M. W.; Ruiz-Garcia, J.; Fischer, T. M.; Schwartz, D. K.; Knobler, C. M. J. Phys. Chem. 1994, 98, 7430. (23) Muhamadi, F.; Richards, N. G. J.; Guida, W. C.; Liskamp, R.; Lipton, M.; et al. J. Comput. Chem. 1990, 11, 440. (24) Gourier, C. PhD Thesis, University of Paris VI, 1996. (25) Riegler, J. E.; LeGrange, J. D. Phys. ReV. Lett. 1988, 61, 2492. (26) Spratte, K.; Chi, L. F.; Riegler, H. Europhys. Lett. 1994, 25, 211. (27) Fang, J. Y.; Knobler, C. M. J. Phys. Chem. 1995, 99, 10425. (28) Sikes, H. D.; Woodward, J. T.; Schwartz, D. K. J. Phys. Chem. 1996, 100, 9093. (29) Li, S.; Hanley, S.; Khan, I.; Varshney, S. K.; Eisenberg, A.; Lennox, R. B. Langmuir 1993, 9, 2243.
