J. Phys. Chem. 1996, 100, 11113-11119
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Morphology of Microphase Separation in Arachidic Acid/Cadmium Arachidate Langmuir-Blodgett Multilayers M. L. Kurnaz and D. K. Schwartz* Department of Chemistry, Tulane UniVersity, New Orleans, Louisiana 70118 ReceiVed: March 4, 1996; In Final Form: April 16, 1996X
It is well-known that the fraction of cadmium salt incorporated into arachidic acid Langmuir-Blodgett (LB) films deposited from a dilute CdCl2 subphase increases from 0 to 1 over the pH range of about 4.8 to 6.2. We report a systematic change in the surface morphology of such LB multilayers over this pH range using atomic force microscopy (AFM). At pH e 5.0 (low pH) the surface displays increasing coverage of stripes (ridges), 0.6 ( 0.2 nm above the surrounding area, aligned in the dipping direction. At pH ) 5.8 (high pH), the surface is pockmarked with irregular but compact indentations about 1.2 ( 0.3 nm deep (in addition to numerous monolayer and bilayer deep holes). At intermediate pH values, the surface is covered by alternating bands (perpendicular to the dipping direction) of the low- and high-pH textures. These bands represent an example of “substrate-mediated condensation”. When films are soaked in benzene, dissolving away the free acid, ridges 2-2.5 nm high remain in the low-pH films; however, structures 5-6 nm high remain in the high-pH structure. Also, the low-pH structure is easily damaged by AFM scanning, while the high-pH structure is more robust and molecular resolution images are obtained. This implies that, at low pH, correlations are only within the top monolayer; however, at high pH, structures are correlated with those in the neighboring layer.
Introduction Langmuir-Blodgett (LB) films are multilayers of amphiphilic molecules transferred layer-by-layer from the surface of an aqueous solution by dipping a solid substrate.1 Various applications of these assemblies have been proposed; some exploiting their layered nature, such as nonlinear optical devices and soft X-ray monochromators; and others that involve the ability to immobilize molecules in well-defined orientations, such as biosensors.2 In recent years, increased availability of structural information has made LB films interesting model systems for studying low-dimensional statistical physics and molecular organization.3-6 The most-studied and prototypical LB system is that of longchain saturated carboxylic acids (stearic, arachidic, behenic) and the divalent salts (e.g., Cd2+, Ca2+, Pb2+, Ba2+, etc.) of these acids. It is known empirically that the incorporation of these divalent cations dramatically increases the film stability and ease of deposition.7,8 The cations are added to the film simply by dissolving them in the subphase in submillimolar concentration. Since the amphiphile is acidic, however, at low pH it will not dissociate and the cations do not affect the monolayer on the water surface, nor do they become incorporated into the deposited LB film. At high pH, on the other hand, the acidic amphiphiles dissociate and are converted completely to the salt (soap) by the association of the dissolved cations. In the intermediate pH range the relative fractions of free acid and soap are sensitive functions of pH.9-13 Although the intrinsic pKa of the arachidic acid monolayer has been estimated at about 5.4,14 the particular range of pH over which the conversion takes place is specific to the individual cation. However, it has been thoroughly studied for several common cations. Varying the pH within this intermediate range, therefore, can be interpreted as a variation of the relative fractions of two components of a * To whom correspondence should be addressed. X Abstract published in AdVance ACS Abstracts, June 1, 1996.
S0022-3654(96)00665-X CCC: $12.00
Figure 1. Isotherms obtained of arachidic acid deposited on a 5 × 10-4 M CdCl2 solution at pH ) 4.8, 5.0, 5.2, 5.4, 5.8, and 6.2, respectively, top to bottom. Note that the diagonal region representing the L2 phase becomes less prominent with increasing pH until it finally disappears. The “lift-off” point, i.e. the area at which the surface pressure begins to deviate from zero, is an indicator of the relative fractions of arachidic acid and the cadmium soap.
binary mixture. In this study we are concerned with the phase behavior and structure of this mixture in the deposited multilayer. Experimental Details Arachidic (eicosanoic) acid was spread from chloroform solution to an area of about 40 Å2/mol on a 5 × 10-4 M CdCl2 solution (Millipore Milli-Q UV+ water was used) contained in a Teflon NIMA LB trough held at 22 ( 0.5 °C. The pH was adjusted between 4.8 and 5.6 by addition of HCl and to 6.2 and above by addition of NaHCO3. Three molecular layers were transferred to mica substrates by successive vertical dipping at 1.6 or 5 mm/min, while the monolayer was held at constant surface pressure (π ) 25 or 30 mN/m). Representative π-A isotherms as a function of pH are shown in Figure 1. Mica substrates were freshly cleaved immediately before use. Transfer ratios were typically 100 ( 5% at pH e 5.8; however, at © 1996 American Chemical Society
11114 J. Phys. Chem., Vol. 100, No. 26, 1996
Kurnaz and Schwartz
higher pH, the third layer had a transfer ratio of about 80% at 1.6 mm/min and 100% at 5 mm/min. Films were skeletonized by immersion in gently-stirred benzene for 10-30 min. Imaging was performed using a Nanoscope III atomic force microscope (AFM) under ambient conditions using a 15 µm × 15 µm scanner and a silicon nitride tip on an integral cantilever with spring constant 0.12 N/m in contact mode. Images were obtained from at least five macroscopically-separated areas on each sample. Representative images are presented below. We have been careful to consider the effect of tip geometry in the measurement of the depths of observed pits. In particular, the apparent depth of a pit whose bottom appears sharp in a crosssectional height profile may be inaccurate because the tip was unable to reach the bottom. That is, the width to depth aspect ratio of the pit must be greater than that of the AFM tip in order to get an accurate measurement. The only reliable way we have found to be certain of this is to ignore pits with sharp bottom profiles as “tip limited” and to rely on wider pits for quantitative information about depth. Results Figure 1 shows representative surface pressure-area isotherms as a function of pH. The isotherm at pH ) 4.8 is indistinguishable from an isotherm of arachidic acid on pure water (with no Cd2+ present and with no pH adjustment). At areas greater than about 21 Å2/mol, the monolayer is in a coexistence state between a two-dimensional gas and the socalled L2 phase, a tilted condensed mesophase (for a review of fatty acid Langmuir monolayer phase behavior, see ref 15). At about 21 Å2/mol the monolayer is completely in the L2 phase, which is fairly compressible, and the pressure rises gradually until about 18 Å2. At a pressure of about 24 mN/m a kink in the isotherm signals a transition to another phase, in this case the “superliquid” or LS phase (an untilted mesophase). The LB films are deposited from this phase. As the pH is increased, the L2 phase is observed over a smaller range of π and area per molecule until at pH ) 6.2 the L2 phase is altogether absent. Increasing the pH above 6.2 does not result in any additional change in the isotherm. This series of isotherms is consistent with fatty acid salt isotherms in the literature.16,17 We will define a convenient order parameter associated with the fraction of soap as 0 0 0 χ ) (ApH)4.8 - A0)/(ApH)4.8 - ApH)6.2 )
where A0 is the area per molecule at which the extrapolated L2 phase section of the isotherm intersects the x-axis.18 The surface morphology of three layer LB films prepared at pH ) 4.8 and 5.0 is shown in Figure 2a,b respectively. In both cases narrow stripes or ridges appear. They are more prominent and closer together in the pH ) 5.0 film. In all the films we have observed, the stripes are aligned with the dipping direction within our ability to measure it ((3°). Figure 2c shows a typical cross-sectional map of the surface of the pH ) 5.0 film, demonstrating that the high areas are about 0.4-0.8 nm above the low areas. The films prepared at these low-pH values were quite delicate and were successfully imaged only at forces below about 1 nN; higher forces damaged the film and dug holes. Attempts to acquire molecular resolution images were unsuccessful for the same reason; the very small scans required damaged the film. In the following discussion we will call the morphology of these films the “low pH” structure or texture. Figure 3a shows a film prepared at pH ) 5.0 that has been “skeletonized” (soaked in benzene) to remove the free acid
Figure 2. AFM images of three-layer LB films of arachidic acid/ cadmium arachidate, 1 µm × 1 µm, deposited at (a) pH ) 4.8 and (b) pH ) 5.0. Narrow ridges about 0.6 nm high are visible in both cases. The ridges are separated by about 100 nm, on average, at pH ) 4.8 and by about 50 nm at pH ) 5.0. The ridges are aligned with the dipping direction which is indicated by an arrow on (b). (c) Height profile associated with the line drawn on (b) displaying the heights of typical ridges.
molecules from the film.8 As in the unskeletonized film at the same pH, parallel ridges, approximately 50 nm apart, are aligned in the dipping direction. However, as demonstrated by the height profile (Figure 3b), the ridges are 2-2.5 nm above the lower adjacent areas. This height difference is consistent with the length of a single molecule (fully extended length is 2.8 nm) and implies that monolayer patches of arachidic acid have been removed. The surface of films prepared at pH ) 5.8, on the other hand, appears qualitatively different (see Figure 4a). The film is marked by pits of several different depths. There are a significant number of holes 2.5 ( 0.3 nm deep as well as holes 5.0 ( 0.5 nm deep. These monolayer and bilayer deep defects have been previously observed19-25 and are present as well in
Arachidic Acid/Cadmium Arachidate LB Multilayers
J. Phys. Chem., Vol. 100, No. 26, 1996 11115
Figure 3. (a) AFM image of a three-layer LB film of arachidic acid/ cadmium arachidate, 1 µm × 1 µm, deposited at pH ) 5.0 and later skeletonizedssoaked in benzene for 20 min. Ridges about 2-2.5 nm high are observed separated by about 50 nm. The ridges are aligned with the dipping direction, which is indicated by an arrow. (b) Height profile associated with the line drawn on (a) displaying the heights of typical ridges.
the film prepared at pH ) 6.2 (see Figure 4b). However, there are numerous pits in the surface of the pH ) 5.8 film that are 0.8-1.5 nm deep as well; these do not appear in the pH ) 6.2 film. The cross-sectional profile shown in Figure 4c displays several of these shallow pits as well as a monolayer deep defect. These shallow pits are irregular in shape but compact (not elongated), and we take them as distinctive of this “high-pH” texture. Molecular resolution images can be obtained routinely on the flat raised regions of these films, and although we have not made a systematic attempt to measure the surface lattice quantitatively, the molecular arrangement is consistent with our previous images of the cadmium arachidate surface lattice.26,27 It is worth remarking that the density of monolayer and bilayer defects in the pH ) 6.2 film is surprisingly high. By increasing the dipping speed from 1.6 to 5 mm/min, we were able to significantly decrease the appearance of the large diameter monolayer holes. However, the very small holes are still observed. Because of their small diameter we are unable to determine the actual depth of these holes; in fact they are visible with only about one-third of the tips we tried. If the tip was dull or if it became contaminated, the small holes became difficult to see or disappeared entirely. Films prepared at pH ) 6.5 and 7.2 were identical in appearance to those at pH ) 6.2. We suspect that these small defects are characteristic of the film and may be related to the frequently-observed decrease in the transfer ratio on the upstroke (so-called XY LB transfer).28-30 Future studies will explore the relationship between such defects and the transfer ratio of the layer in more detail. A film prepared at pH ) 5.8 and skeletonized in benzene is shown in Figure 5a. The morphology appears as labyrinths of high regions separated by lower regions. The height profile in Figure 5b shows that the raised areas are about 5-6 nm higher
Figure 4. AFM images of three-layer LB films of arachidic acid/ cadmium arachidate, 1 µm × 1 µm, deposited at (a) pH ) 5.8 and (b) pH ) 6.2. In both parts numerous holes are visible with depths of about 2.5 or 5.5 nm corresponding to monolayer and bilayer defects. In addition, pits 1.2 ( 0.3 nm deep are observed in (a). (c) Height profile associated with the line drawn on (a) displaying the depths of several pits and one monolayer hole.
than the lower adjacent areas. This is consistent with the thickness of a molecular bilayer, implying that bilayer patches of arachidic acid have been removed. The evolution of the surface morphology with pH is summarized in Figure 6a-e. At values of pH between 5.0 and 5.8, the film is characterized by alternating bands, perpendicular to the dipping direction, of two distinctive textures. The relative fraction of the surface covered by the two textures varies systematically with pH. The dominant texture at pH ) 5.2 is lower on average, with striped or labyrinthine high areas. These “low” bands appear to have sharper edges on the side that emerged first from the water on the upstroke. Sometimes they have an especially dendritic character, as in Figure 4c. This texture is strongly reminiscent of the film morphology at pH ) 5.0. In fact, in the pH ) 5.2 films (Figure 6b) the striped high
11116 J. Phys. Chem., Vol. 100, No. 26, 1996
Kurnaz and Schwartz more and more pits. These “high” bands cover a greater fraction of the surface with increasing pH and are similar in appearance to the surface texture at pH ) 5.8. Therefore, to a first approximation, we may regard the pH range between 5.2 and 5.6 as a sort of coexistence region, with alternating bands of low-pH and high-pH texture obeying a sort of “lever rule” regarding the relative fractional coverage. Isolated “low” bands are observed occasionally in films prepared at pH ) 5.8, but they cover a very small fraction of the surface. This implies that pH ) 5.8 is near, but not past, the high pH end of the coexistence region of the two textures. The mechanical stability of the film is distinctly different in the different bands. In particular, the “low” bands are much more delicate. Parts a-c of Figure 7 show a series of images of the same film area in which the imaging force was gradually increased. Damage to the film seems to nucleate at monolayer or bilayer holes in the low band and grow throughout the band. This type of damage was typically seen during imaging; holes were torn in the low bands if the imaging force was not kept very low. In addition, molecular resolution was obtained on the high bands, but not on the low bands. The consistency between the film stability of and molecular packing in the low and high bands and the uniform films at low and high pH, respectively, reinforces our hypothesis of the banding representing a sort of coexistence.
Figure 5. (a) AFM image of a three-layer LB film of arachidic acid/ cadmium arachidate, 1 µm × 1 µm, deposited at pH ) 5.8 and later skeletonizedssoaked in benzene for 20 min. Trenches 5-6 nm deep are observed. (b) Height profile associated with the line drawn on (a) displaying the depths of typical trenches. Note that the depths of the last two trenches are not particularly reliable since the profile bottoms are sharp, indicating that they may be tip-limited.
areas in these bands are aligned in the dipping direction. The bands alternating with these are higher on average, and although they are fairly flat at pH ) 5.2, as the pH is increased, contain
Discussion The systematic trend in the behavior of the isotherms shown in Figure 1 with pH has traditionally been interpreted as due to an increasing fraction of cadmium soap in the monolayer. In fact, if we plot χ (as defined above) versus pH (Figure 8) , there is excellent agreement with the mole fraction of cadmium soap incorporated in an LB film directly measured by surface analytical techniques such as XPS, FTIR, and neutron activation
Figure 6. AFM images of three-layer LB films of arachidic acid/cadmium arachidate, 5 µm × 5 µm, deposited at (a) pH ) 5.0, (b) pH ) 5.2, (c) pH ) 5.4, (d) pH ) 5.6, and (e) pH ) 5.8. The dipping direction is indicated by an arrow on (a), (c), and (d). Parts (b)-(d) display alternating bands of two distinctive textures, one similar to the surface shown in part (a) and the other similar to the surface shown in part (e). The fraction of the surface covered by the two textures changes systematically with pH.
Arachidic Acid/Cadmium Arachidate LB Multilayers
J. Phys. Chem., Vol. 100, No. 26, 1996 11117 using the expression
∆Gexcess ) ∫0
πmax
Figure 7. Series of AFM images of the same region, 4 µm × 4 µm, of a three-layer LB film of arachidic acid/cadmium arachidate prepared at pH ) 5.6 showing damage caused by the AFM tip as the normal force is gradually increased. (a) Typical low-imaging forces of about 1 nN. (b) Imaging force increased to about 10 nN and damage seen to nucleate at preexisting monolayer and bilayer defects in the low bands. (c) Imaging force increased to about 22 nN and the damage increased and extended throughout the low bands (However, the high bands are still undamaged).
Figure 8. Mole fraction of cadmium arachidate as extracted from the isotherms in Figure 1.
analysis.11,12 With this assumption about relative mole fractions of free acid and soap, the series of isotherms in Figure 1 can be analyzed to obtain the excess Gibbs free energy of mixing31-34
(A12 - N1A1 - N2A2) dπ
where A1, A2, and A12 are the areas per molecule at pressure π of pure arachidic acid, pure cadmium arachidate, and a mixture of the two with mole fraction N1 of arachidic acid and N2 of cadmium arachidate. For all values of pH, the excess free energy of mixing obtained by this calculation has a magnitude less than 0.05 kJ/mol or about 0.02 kBT. Therefore, we have no reason to suspect, on the basis of thermodynamic measurements, that the two species will phase separate on the water surface. The question of acid/soap mixing versus phase separation on the water surface remains open. No grazing incidence X-ray diffraction measurement of a CdA monolayer on an aqueous subphase has been performed as a function of pH. Such an experiment has, however, been performed on calcium and copper(II) fatty acid salts by Lin and co-workers.35 In both cases, the lattice parameters of the two-dimensional crystal changed continuously with pH over the range in which the acid is converted to soap. Coexistence of multiple structures was not observed, which suggests that the monolayer is in a single phase at the air/water interface. We cannot, however, exclude the possibility that a two-dimensional liquid or disordered phase coexists with the crystalline phase in these systems, since such a phase would not have a diffraction signature. Small angle X-ray scattering (SAXS) measurements of arachidic acid and cadmium arachidate (CdA) LB multilayers show clearly that the two pure substances have different structures. LB multilayers of the free acid (like crystals of the acid) exhibit crystalline polymorphism as a function of preparation conditions and temperature.36,37 However, in all cases the long-chain n-alkanoic acids have a layered crystal structure in which the alkyl chain is tilted with respect to the layer normal. The tilt angle can have several values between about 20° and 35°, depending on which structure is observed. Thus, a bilayer of arachidic acid can be expected to have a thickness between 4.4 and 5.1 nm. The most often reported and thermodynamically stable structure is the so-called C form,36-39 which has a bilayer thickness of 4.4 nm. LB multilayers of the cadmium salts of these acids, on the other hand, are known to have their alkyl chains parallel to the layer normal (untilted) within a few degrees, giving a bilayer thickness of 5.55 nm.13,40,41 In addition, there has been at least one report of coexisting layer spacings in an LB multilayer of cadmium behenate prepared at pH ) 5.7,13 which would tend to support a conclusion of phase separation. However, other SAXS measurements of CdA prepared at pH ) 5.7 showed only a single set of diffraction peaks corresponding to a single layer spacing.42-44 We believe that our current AFM images clearly demonstrate phase separation at various length scales at values of pH between 4.8 and 5.8. The textures we have called low pH and high pH both indicate phase separation at small length scales, although probably of different structural phases. A comparison of films prepared at pH ) 5.0 and pH ) 5.8 yields several distinctive differences. First the morphology is different, elongated high domains at low pH and compact low domains at high pH. Also, the height difference between neighboring high and low domains is different, about 0.6 nm at low pH and 1.2 nm at high pH. The mechanical stability is much greater in the high-pH films, and molecular resolution images of a surface lattice is observed only at high pH. Finally, when the free acid molecules are removed, trenches about 2.3 nm deep (monolayer) are observed at low pH, while trenches about 5.5 nm deep (bilayer) are observed at high pH.
11118 J. Phys. Chem., Vol. 100, No. 26, 1996 All of these differences are consistent with the basic premise that the top monolayer is uncorrelated with the underlying layer at low pH, while at high pH the top two layers are correlated. The height difference between domains is 1.2 nm at high pH, approximately equal to the expected difference in bilayer thickness between the C form of arachidic acid and the untilted CdA structure. At low pH the height difference is only about half this value. Furthermore, when the free acid is removed by skeletonization, trenches one monolayer deep are observed at low pH, implying that regions of arachidic acid in the top layer were sitting on top of regions of the cadmium salt not removed by skeletonization. However, at high pH, entire bilayers are removed, implying that the phase separation is correlated through at least a bilayer. Also, mechanical stability and the ability to obtain molecular resolution AFM images in CdA has been previously shown to be connected with the headgroup/ headgroup interface in combination with the cation.26 As for the difference in morphology at low and high pH, we speculate that it may be due to the difference in long-range dipolar interactions. Striped modulated phases are well-known in various quasi-two dimensional dipolar systems in which the dipole moment is perpendicular to the plane (for a review, see ref 45). The long-range dipolar repulsion competes with an effective line tension and results in a modulated structure with a well-defined length scale. Examples include ferrofluids, magnetic garnet films, and phase-separated Langmuir monolayers on water. However, one generally expects that a bilayer in an LB film will have a symmetrical structure, which implies that there can be no component of a dipole moment normal to the layer. If, however, a layer is inhomogeneous and uncorrelated to the neighboring layer, there can be local variation in the dipole moment leading to a modulated stripe phase. At a molecular level we can imagine that, at low pH, each Cd2+ ion is associated with two acid molecules that are in the same layer, while at high pH each Cd2+ ion bridges across neighboring layers. The alignment of the stripes at low pH with the dipping direction may be due to one of several factors. They may preexist on the water surface and become aligned due to monolayer shear created during deposition. This effect is commonly used to form aligned polymeric LB films which can then be used as a control surface for liquid crystal alignment.46-48 Another possibility is that the stripes form upon deposition and grown preferentially in the dipping direction. This would be analogous to the many observations that the tilt direction of molecules is correlated with the dipping direction (although a broad distribution is generally reported).49-51 The bands observed at pH ) 5.2, 5.4, and 5.6 (and occasionally at pH ) 5.8) are similar to those observed by Riegler and co-workers in two-component LB films.10,13 They have labeled the process “substrate-mediated condensation” and explain it as the preferential adsorption of one of the two components (call it the A component) on the solid substrate. This leads to a gradual buildup of the other component (B) near the three-phase line until the excess is so large that a meniscus instability leads to the deposition of a B-rich band. This process is cyclical, leading to alternating bands of A-rich and B-rich material. They also make an analogy with thermotropic solidification of a binary alloy where the deposited layer is analogous to the solid alloy, the floating monolayer is analogous to the liquid alloy, and the three-phase line represents the liquid/ solid alloy interface.47 This explanation for the banding is applicable only if the monolayer on the water surface consists of a single phase. If the monolayer on the water surface is already phase-separated, the origin of the banding is not clear.
Kurnaz and Schwartz Conclusion Phase separation at small (50-100 nm) length scales is observed by AFM in mixed LB films of arachidic acid and cadmium arachidate as a function of pH in the range 4.8-5.8 as the acid is converted to the cadmium soap. At the low-pH side of this region the surface is covered by stripes, 0.6 ( 0.2 nm high, aligned in the dipping direction. When the free acid molecules are removed from these films by soaking in benzene, ridges 2.3 ( 0.3 nm (one monolayer high) remain. On the highpH side of the region isolated compact pits are observed about 1.2 ( 0.3 nm below the surrounding areas. After removal of free acid molecules the height differences observed between neighboring domains are 5.5 ( 0.5 nm (bilayer thickness). Molecular resolution is obtained only at high pH, and the highpH films are much stronger mechanically. From these observations we conclude that the observed domains are correlated only within the top monolayer at low pH, while they extend at least one bilayer at high pH. The striped texture at low pH is explained by analogy to striped phases in other dipolar twodimensional systems. A dipole moment normal to the layer is only possible in an asymmetric bilayer consistent with our conclusions for the low-pH structure. At intermediate values of pH, alternating bands of low-pH and high-pH textures are observed with the bands running perpendicular to the dipping direction. These bands are an example of substrate-mediated condensation, which has previously been observed in twocomponent LB monolayers. Acknowledgment. The authors gratefully acknowledge support from the Camille and Henry Dreyfus New Faculty Award Program; the donors of Petroleum Research Fund, administered by the American Chemical Society; and the Center for Photoinduced Processes funded by the National Science Foundation and the Louisiana Board of Regents. References and Notes (1) Swalen, J. D.; et al. Langmuir 1987, 3, 932. (2) Roberts, G. G. AdV. Phys. 1985, 34, 475. (3) Zasadzinski, J. A.; Viswanathan, R.; Madsen, L.; Garnaes, J.; et al. Science 1994, 263, 1726. (4) Schwartz, D. K.; Viswanathan, R.; Zasadzinski, J. A. J. Chem. Phys. 1994, 101, 7161. (5) Viswanathan, R.; Zasadzinski, J. A. N.; Schwartz, D. K. Nature 1994, 368, 440. (6) Viswanathan, R.; Madsen, L. L.; Zasadzinski, J. A. N.; Schwartz, D. K. Science 1995, 269, 51. (7) Blodgett, K. B. J. Am. Chem. Soc. 1935, 57, 1007. (8) Blodgett, K. B.; Langmuir, I. Phys. ReV. 1937, 51, 964. (9) Spratte, K.; Riegler, H. Makromol. Chem., Macromol. Symp. 1991, 46, 113. (10) Spratte, K.; Chi, L. F.; Riegler, H. Europhys. Lett. 1994, 25, 211. (11) Spink, J. A. J. Colloid Interface Sci. 1967, 23, 9. (12) Riegler, J. E.; LeGrange, J. D. Phys. ReV. Lett. 1988, 61, 2492. (13) Riegler, H.; Spratte, K. Thin Solid Films 1992, 210/211, 9. (14) Betts, J. J.; Pethica, B. A. Trans. Faraday Soc. 1956, 52, 1581. (15) Knobler, C. M.; Desai, R. C. Annu. ReV. Phys. Chem. 1992, 43, 207. (16) Yazdanian, M.; Yu, H.; Zografi, G. Langmuir 1990, 6, 1093. (17) Spink, J. A.; Sanders, J. V. Trans. Faraday Soc. 1955, 51, 1154. (18) Tippmann-Krayer, P.; Kenn, R. M.; Mo¨hwald, H. Thin Solid Films 1992, 210/211, 577. (19) Hansma, H. G.; Gould, S. A. C.; Hansma, P. K.; Gaub, H. E.; et al. Langmuir 1991, 7, 1051. (20) Viswanathan, R.; Schwartz, D. K.; Garnaes, J.; Zasadzinski, J. A. N. Langmuir 1992, 8, 1603. (21) Linde´n, M.; Rosenholm, J. B. Langmuir 1995, 11, 4499. (22) Fuchs, H.; Chi, L. F.; Eng, L. M.; Graf, K. Thin Solid Films 1992, 210/211, 655. (23) Chi, L. F.; Eng, M.; Graf, K.; Fuchs, H. Langmuir 1992, 8, 2255. (24) Garnaes, J.; Schwartz, D. K.; Viswanathan, R.; Zasadzinski, J. A. N. Synth. Met. 1993, 55-57, 3795. (25) Schaper, A.; Wolthaus, L.; Mo¨bius, D.; Jovin, T. M. Langmuir 1993, 9, 2178.
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