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Study of Long-Range Tilt Orientation in Fatty Acid Monolayers by Dynamic Scanning Force Microscopy L. F. Chi,* M. Gleiche, and H. Fuchs Physikalisches Institut, Westfa¨ lische Wilhelms-Universita¨ t, Wilhelm-Klemm-Strasse 10, D-48149 Mu¨ nster, Germany Received May 28, 1997. In Final Form: October 16, 1997 Segments in the liquid condensed (LC) domains of stearic acid, induced by long-range tilt orientation, were visualized on solid substrates by applying scanning force microscopy (SFM) using the force modulation mode (FMM). To understand the image contrast, a model based on the asymmetric apex of the SFM tip and the tilted phase of the molecules on the solid substrate after film transfer is presented. As a result, the apparent local elasticity is found to vary for molecules having the same tilt angle but different azimuthal angles. The observed textures are in good agreement with the theoretically predicted ones.
Introduction In the past decade, liquid condensed (LC) domains formed during the transition from a liquid phase to a condensed phase in Langmuir monolayers were intensively investigated with respect to the long range tilt orientational order as related to their rich and defined textures.1-8 Originally, the observations were done by polarized fluorescence microscopy (PFM).1-3 Recently, Brewster angle microscopy (BAM) allowed the direct observation of tilted phases of various amphiphiles, including for example a fatty acid,4 a fatty acid methyl ester,5 a fatty acid ethyl ester,6 and a 1-monoglyceride.7,8 The textures of the ester monolayers often show “star” defects,5,6 while the fatty acids show a “boojum” texture.4 The observed textures can be predicted from the Landau free energy for tilted hexatic phases of monolayers, supplemented by a boundary energy.9 These studies reveal that Langmuir monolayers often self-organize into structures in which the molecular azimuthal tilt is ordered over mesoscopic distances.10 The textures are often similar to those observed in thin liquid crystal films.11 These kinds of studies were concentrated on Langmuir monolayers at the air/water interface. Only a few investigations concerning these phenomena were done on LangmuirBlodgett films on solid substrates.12 LB films supported by solid substrates have been the subject of numerous scanning force microscopy (SFM) * To whom correspondence should be addressed. Telephone: 49251-8333651. Fax: +49-251-8333602. E-mail:
[email protected]. (1) Moy, V. T.; Keller, D. J.; Gaub, H. E.; McConnell, H. M. J. Phys. Chem. 1986, 90, 3198-3202. (2) Qiu, X.; Ruiz-Garcia, J.; Stine, K. J.; Knobler, C. M. Phys. Rev. Lett. 1991, 67, 703-706. (3) Fischer, B.; Tsao, M. W.; Ruiz-Garcia, J.; Fischer, T. M.; Schwartz, D. K.; Knobler, C. M. J. Phys. Chem. 1994, 98, 7430-7435. (4) Henon, S.; Meunier, J. J. Chem. Phys. 1993, 98, 9148-9154. (5) Overbeck, G. A.; Ho¨nig, D.; Mo¨bius, D. Thin Solid Films 1994, 242, 213-219. (6) Weidemann, G.; Vollhardt, D. Langmuir 1996, 12, 5114-5119. (7) Vollhardt, D.; Gehlert, U.; Siegel, S. Colloids Surf., A 1993, 6, 187-195. (8) Brezesinski, G.; Scalas, E.; Struth, B.; Mo¨hwald, H.; Bringezu, F.; Gehlert, U.; Weidemann, G.; Vollhardt, D. J. Phys. Chem. 1995, 99, 8759-8762. (9) Fischer, Th. M.; Bruinsma, R. F.; Knobler, C. M. Phys. Rev. E 1994, 50, 413-428. (10) Ruiz-Garcia, J.; Qiu, X.; Tsao, M. W.; Marshall, G.; Knobler, C. M.; Overbeck, G. A.; Mo¨bius, D. J. Phys. Chem. 1993, 97, 6955-6957. (11) Dierker, S. B.; Pindak, R. Phys. Rev. Lett. 1986, 56, 1819-1822. (12) Ho¨nig, D.; Mo¨bius, D. Chem. Phys. Lett. 1992, 195, 50-52.
studies in recent years.13-17 High-resolution images on barium arachidate multilayers show that SFM offers a unique method to resolve the different molecular lattices on the same sample.13 The SFM investigations of LB films transferred in the coexistence region of the liquidcondensed phase and the liquid-expanded (LE) phase, with one14,15 or more different chemical components,16,17 gave detailed information about the morphology, the mechanical properties, and the phase separation. The rich textures in LC domains observed by PFM and BAM at the air/water interface are not expected to be observed by SFM in topographical imaging, since the molecules in different segments only differ in their azimuthal angles and thus exhibit the same height. But, the inner structures of a sixfold star-shaped domain were observed with friction force imaging.18 In the present work, we report the observation of the segments in LC domains of stearic acid, prepared on an aqueous subphase containing a polymeric counterion, applying SFM operating in the force modulation mode (FMM). Previously, the domain formation of this system was investigated by fluorescence microscopy at the air/ water interface.19 Applying BAM to this system, the anisotropy of the domains can also be observed20 without adding a fluorescent dye. The domains investigated here are one order of magnitude smaller (ca. 10 µm) than those normally investigated by BAM. Thus, the spatial resolution of BAM is not high enough to resolve the structure and to allow a detailed analysis. We will demonstrate here that the FMM offers an alternative and complementary method to investigate the long-range tilt orientation of molecules on solid substrates. To understand the image contrast, an asymmetric SFM tip has to be taken (13) Schwartz, D. K.; Viswanathan, R.; Zasadzinski, J. A. N. Langmuir 1993, 9, 1384-1391. (14) Chi, L. F.; Anders, M.; Fuchs, H.; Johnston, R. R.; Ringsdorf, H. Science 1993, 259, 213-216. (15) Chi, L. F.; Fuchs, H.; Johnston, R. R.; Ringsdorf, H. J. Vac. Sci. Technol. B 1994, 12 (3), 1967-1972. (16) Overney, R. M.; Mayer, E.; Frommer, J.; Brodbeck, D.; Lu¨thi, R.; Howald, L.; Gu¨ntherodt, H.-J.; Fujihira, M.; Takano, H.; Gotoh, Y. Nature 1992, 359, 133-135. (17) Chi, L. F.; Jacobi, S.; Fuchs, H. Thin Solid Films 1996, 284/285, 403-407. (18) Santesson, L.; Wong, T. M. H.; Taborelli, M.; Descouts, P.; Liley, M.; Duschl, C.; Vogel, H. J. Phys. Chem. 1995, 99, 1038-1045. (19) Chi, L. F.; Johnston, R. R.; Ringsdorf, H. Langmuir 1991, 7, 2323-2329. (20) Gleiche, M. Diplom Thesis, University of Mu¨nster, 1997.
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into account. In this case, the interaction between the tip and the azimuthally tilted molecules is expected to be anisotropic. The variation in symmetry induced by polymer counterions compared with that by using the similar molecule on the pure water subphase is discussed. Experimental Section Materials. Stearic acid (C18, purity 99.9%) was purchased from Sigma. The substance was dissolved in chloroform (HPLC, Baker) with a concentration of 5 × 10-3 M. Branched poly(ethyleneimine) with an average molecular weight of Mn ) 1800 (PEI1800, purity 99%) purchased from Polyscience was dissolved in Millipore water (water resistance 18.2 MΩ), providing an aqueous solution with a concentration of 1 and 2 × 10-3 M. The substances were used without further purification. LB Films on Solid Substrates. The LB films were prepared with commercial LB troughs (Lauda FW2 and Nima 6100) and transferred in the coexisting regime of the LE and LC phases. A transfer speed of 10 mm min-1 was used for all the films investigated. Freshly cleaved mica and an Si wafer cleaned with the standard RCA method21 were used as solid substrates for the film transfer. SPM Investigations. The SPM inspection of the LB films was done with a commercial instrument (Digital Instrument, Nanoscope III, Dimension 3000). Triangular cantilevers with Si3N4 tips purchased from Nanosensor and Topometrix were used for elasticity measurements and the simultaneously recorded topographic measurements. For the elasticity measurements, the instrument was operated in force modulation mode (FMM).22 FMM measures local elasticity by oscillating a probe such that the tip indents slightly into a sample. Harder sites on the sample surface cause increased cantilever response and higher amplitude and are rendered as darker areas of the image, whereas softer sites cause a reduced cantilever response and lower amplitude and are rendered as lighter areas in the image.
Results and Discussion Segment Contrast. Applying SPM to inspect freshly prepared LC domains of C18 PEI1800, spaced mica, segmentation inside of the domains is clearly observed in FMM images, as shown in Figure 1. We call this contrast “segment contrast” for simplification. By using different SPM tips, two basic variations in contrast were obtained: in hexagonal domains, opposite segments exhibit the same contrast in one case (Figure 1a), whereas “left” and “right” segments show an opposite contrast for the other case (Figure 1b). The former one is observed more frequently than the latter one. No corresponding segment contrast was observed in the simultaneously recorded topographical imaging, as shown in Figure 2. The observed contrast is independent of scan direction, as shown in Figure 2a (0° and Figure 2b (90°). When the images were taken in trace and retrace, no contrast inversion was observed. The theoretically predicted star defects with chiral shapes of the defect lines (Figure 3a), as well as the point defects (Figure 3b and 3c), are all observable in FMM images of this system. The segment contrast can only be obtained on a fresh sample. About 2 h after the film transfer, the contrast in FMM images disappears, as shown in Figure 4. Using the same SPM tip to inspect other fresh samples, the same kind of segment contrast will be seen again. This indicates that the loss of the segment contrast is due to a real change of the sample by aging, probably induced by the evaporation of water in the polymer layers rather than by a change of the probing tip. In this system, the segment contrast is not observed in lateral force (friction) (21) Brzezinski, V.; Peterson, I. R. J. Phys. Chem. 1995, 99, 1254512552. (22) Radmacher, M.; Tillmann, R. W.; Gaub, H. E. Biophys. J. 1993, 64, 735-742.
Figure 1. Segment contrast in FMM images. Contrast type I: opposite segments have the same contrast in the hexagonal domains (a). Contrast type II: segments on the right side and the left side of the domains have opposite contrast (b). The segment contrast of the quasi-pentagonal domains and hexagonal domains marked with arrows drawn with solid lines can be explained with a model presented here (see text).
imaging. Similar results were obtained when silicon was used as substrate instead of mica. Textures. From BAM and PFM studies it is known that the textures of long-chain esters and acids are different (see refs 4-6). The “ester texture” often shows sixfold star defects with sharp segment boundaries, while the “acid texture” often shows “boojum” defects without sharp segment boundaries. In our system, we observe the typical defects of esters, although the amphiphile used here is a acid. The change from acid to ester type of domains must be induced from the polymeric ions in the subphase. We suppose that the head-head interaction of stearic acid is changed by the adsorption of PEI at the interface layers, which lets this complex system appear more esterlike than acidlike. Here we summarize phenomenogically the textures of LC domains in this system:
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Figure 2. Images of topography (left) and elasticity (right) taken in different scan directions: (a) 0° and (b) 90°. The segment contrast is independent of the scan direction, and it is not observable in the topographical images.
Chiral Textures. Chiral textures are occurring in most cases in hexagonal domains of the C18/PEI system instead of the “basic motif” with straight defect lines observed in corresponding ester system, such as methyl octadecanoate.2 Point Defects. Point defects are observed in quasipentagonal (Figure 3c) and defected quasi-pentagonal domains (Figure 3b). Domains with three defect lines emerging from a point at the center of a domain boundary and ending at the LC-LE boundary are less common (Figure 3c). The most common ones are those with two wedges and four or five defect lines starting at one point of the domain boundary with wedges. The numbers of domains with star and point defects are almost equivalent and are not influenced by the kinetics of the film formation, for example by compressing the film with different speeds. Model of the Contrast Mechanism. The strong segment contrast observed in FMM is not expected, since (i) only one substance is involved here and (ii) it is known that the molecules in different segments only differ in their relative azimuthal angle rather than in their tilt
anglesthis is the basis for the theoretical description of such hexatic phases. Supposing a tip apex in a circular shape with a radius of 5-40 nm, the interaction between such a symmetric SPM tip and azimuthal-oriented molecules must be isotropic. As a consequence no segment contrast should be observed. Thus, the segment contrast in the FMM images indicating different local elastic properties shown above can only be understood if (i) the mean tilt angles of molecules in different segments are not the same; (ii) the packing density of molecules varies in different segments; or (iii) the apex of the SFM tip is not symmetric. The assumption of spatial variation in the mean tilt angle at the air/water interface is not reasonable, at least in the case of lowenergy textures.9 The variation in packing density on the defect lines is possible but should not occur over all the segments. Therefore, we have to consider the anisotropic interaction between the SPM tip and the sample. By applying force modulation, the SFM tip will indent the sample by a depth of δ under a force F (the relation between δ and
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Figure 3. Different defect lines in LC domains: (a) chiral deformation in hexagonal domains; (b-c) point defects in quasipentagonal domains.
F is given in ref 23). In the case of LB films exhibiting tilted phases, the indentation δ may originate from two terms: one is the change of the tilt angle, and the other is the deformation of the alkyl chains. When the tilt angle of molecules located directly under the SPM tip is changed from θ to θ′, the nearest neighbor molecule which is not under the SFM tip also has to change its tilt angle to keep the lattice packing of the head groups unaffected. This will induce a kind of “Domino effect”. It is known that the carboxyl head group of stearic acid is larger than the alkyl chain. Thus the change in the tilt angle should not be infinite. To estimate the encountered region, we make the following assumptions: (i) the indentation δ is supposed to be 2.5 Å, which is only contributed by the change in tilt angle for simplification; (ii) the original tilt angle is 15° relative to the surface normal, and the changed tilt angle is 30°; (iii) the head group is 10% larger in (23) Johnson, K. L. Contact Mechanics; Cambridge University Press: Cambridge, 1994.
diameter than the alkyl chain; (iv) the lattice constant is 5 Å; and (v) the tip radius is 50 nm. Then the encountered region is calculated to be 6 nm wide, and the region directly under the SFM tip is calculated to be 10 nm in a onedimensional projection, as shown in Figure 5 schematically. Now, if we take an asymmetric apex of the SFM tip, for example an ellipsoidal instead of spherical apex, we will get different effective interacting regions depending on the azimuthal orientation relative to the apex of the tip, as shown schematically in Figure 6a. The shadowed regions are resulting from the “Domino effect”. The arrows in these regions indicate the directors of the tilted molecules. Thus, indenting the same depth into the film with an asymmetric tip, the resistance forces due to the tilt of molecules on both edges of the tip will not be the same: molecules acting on the long edge behave as “harder” while those acting on the short edge behave as “softer”, since different numbers of molecules are involved. As a consequence, when the force is kept constant, the
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Figure 4. Images of topography (left) and elasticity (right) on C18/PEI/mica 2 h after the film transfer. Segment contrast is no longer visible.
Figure 5. Schematic presentation of changing tilt angles when the tilted molecules get into contact with the SFM tip.
the segment contrast shown in Figure 1b, we have to assume another asymmetric tip as shown in Figure 6b. In this case, the molecules acting on the triangular side of the tip behave as “harder” and those acting on the opposite side as “softer”. The segment contrast in some domains shown in Figure 1a, denoted with dotted lines, could not be explained with this model. This may originate from the textures in higher energy levels. The identical mean tilt angles in the segments for the low-energy textures might not hold for those with higher energy. Further experimental and theoretical work has to be done to clarify this point. SFM tips, consisting of Si or Si3N4, are produced by etching the single crystals of Si or Si3N4. It is not unusual to produce structures with slightly asymmetric geometry by etching single crystals. The extent of the asymmetry will depend on the etching parameters. We do see weak or strong segment contrast when we use different SFM tips, especially from different companies. Due to the higher lateral resolution, the method presented here complements well the conventional optical methods to investigate tilted hexatic phases of monolayers on solid substrates. For this purpose, asymmetric apices of SFM tips with defined shape will be needed. A quantitative study will only be possible if the geometry of the tip is well characterized. Conclusions
Figure 6. Schematic illustration of the interaction areas between the asymmetric apex of SFM tips and azimuthally tilted molecules. (a) ellipsoidal tip; (b) quasi-triangular tip. The arrows in the shadow of the tip apex denote the directors of the tilt of the molecules.
indentation in different segments will not be the same. By using this model, supposing that the long axis of the ellipsoid is parallel to the cantilever and the domains are in splay type, we can explain the contrast observed in almost all the quasi-pentagonal domains and the contrast observed in some of the hexagonal domains shown in Figure 1a, which are denoted with arrows. To understand
Segments in LC domains of stearic acid bound by a polymer were observed with SFM operating in force modulation mode. The contrast can partially arise from the variation in effective elasticity induced by the anisotropic interaction between an asymmetric apex of the SFM tip and azimuthally rotated tilted molecules. A spatial variation of the mean tilt angle cannot be excluded in order to understand the segment contrast, which could not be explained by the model presented. The loss of segment contrast with time indicates the transition from a tilted phase to a vertical phase after the film was transferred from the air/water interface to the air/solid substrate interface, on a time scale of about 2 h. LA9705569
