Organized Collapse Structures in Mixtures of Chiral Ethyl 2-Azido-4

collapse structures which were visualized with scanning force microscopy. ... Irreversible Collapse of Poly(vinyl stearate) Monolayers at the Airâ...
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Langmuir 2006, 22, 1428-1435

Organized Collapse Structures in Mixtures of Chiral Ethyl 2-Azido-4-fluoro-3-hydroxystearates Silke Steffens,† Jens Oldendorf,‡ Gu¨nter Haufe,*,‡ and Hans-Joachim Galla*,† Institut fu¨r Biochemie der UniVersita¨t Mu¨nster, Wilhelm-Klemm Strasse 2, and Organisch-Chemisches Institut der UniVersita¨t Mu¨nster, Corrensstrasse 40, 48149 Mu¨nster, Germany ReceiVed August 8, 2005. In Final Form: December 5, 2005 Monolayers of enantiomeric compounds as well as diastereomeric mixtures and racemic/diastereomeric mixtures of ethyl 2-azido-4-fluoro-3-hydroxystearates have been investigated using surface pressure-area isotherms and Brewster angle microscopy. All monolayers collapse out of the liquid-expanded phase, forming 3D collapse structures which were visualized with scanning force microscopy. The enantiomeric compound and the diastereomeric mixtures form unique fiber-like network structures with heights between 20 and 40 nm. Interestingly, the shape of the enantiomeric fiber structures is straight, whereas the diastereomeric mixtures exhibit curved fibers of different sizes. The racemic mixture however forms circular 10 nm high and 20-50 µm broad structures. The shape of unconventional collapse structures could be changed by using distinct ratios of diastereomeric or racemic/diastereomeric mixed compounds.

Introduction The controlled assembly of molecular structures on the nanometer scale is a focus of current interest. Due to their size, such structures are not available by conventional lithographic techniques.1,2 On the other hand, they are too large to be designed by chemical synthesis. Self-assembled systems are an exciting possibility to overcome this problem and build up complex defined assemblies of functional nanostructures. Biomolecules may be used as building blocks, clearly mimicking the construction principle of natural systems such as the so-called s-layers covering the surface of bacterias and archaea.3-5 Thereby, a main feature for the formation of twodimensional protein structures is the chirality of the molecules. Besides proteins, which are the biological prototype of structurally defined, noncovalent aggregated, macromolecular assemblies, also membrane lipids are of major interest to tune surface properties.6,7 At the air/water interface it is possible to create 2D but also 3D structures designed mainly by hydrogen bonding. In chiral compounds this leads to chiral discrimination8-10 and/ or the formation of chiral domain structures.11,12 2D and 3D structures mainly observed in artificial surface layer systems have a direct biological correlation to the alveolar surfactant supporting the breathing cycle.13,14 However, 3D collapse * To whom correspondence should be addressed. (H.-J.G.) Phone: +49251-83 33201. Fax: +49-251-83 33206. E-mail: [email protected]. (G.H.) Phone: +49-251-83 33281. Fax: +49-251-83 39772. E-mail: [email protected]. † Institut fu ¨ r Biochemie. ‡ Organisch-Chemisches Institut. (1) Xia, Y. N.; Whitesides, G. M. Angew. Chem., Int. Ed.1998, 37, 551. (2) Ito, T.; Okazaki, S. Nature 2000, 406, 1027. (3) Sleytr, U. B.; Messner, P.; Pum, D.; Sa´ra, M. Angew. Chem., Int. Ed. 1999, 38, 1035. (4) Howorka, S.; Sara, M.; Wang, Y. J.; Kuen, B.; Sleytr, U. B.; Lubitz, W.; Bayley, H. J. Biol. Chem. 2000, 275, 37876. (5) Engelhardt, H.; Peters, J. J. Struct. Biol. 1998, 124, 276. (6) Giocondi, M. C.; Vie´, V.; Lesniewska, E.; Milhiet, P. E.; Zinke-Allmang, M.; Le Grimellec, C. Langmuir 2001, 17, 1653. (7) Lee, D. H.; Kim, D.; Oh, T.; Cho, M. Langmuir 2004, 20, 8124. (8) Andelman, D. J. Am. Chem. Soc. 1989, 111, 6536. (9) Hu¨hnerfuss, H.; Neumann, V.; Stine, K. J. Langmuir 1996, 12, 2561. (10) Nandi, N.; Vollhardt, D. Chem. ReV. 2003, 103, 4033. (11) Vollhardt, D.; Emrich, G.; Gutberlet, T.: Fuhrhop, J.-H. Langmuir 1996, 12, 5659. (12) Nandi, N.; Vollhardt.; D. J. Phys. Chem. B 2003, 107, 3464. (13) Perez-Gil, J. Pediatr. Pathol. Mol. Med. 2001, 20, 445.

structures have been observed when the area of the monolayer is reduced beyond the equilibrium spreading pressure.15 Under such a condition the monolayer is driven into a metastable state which relaxes into 3D structures initiated by a nucleation step. Such 3D structures will disturb the fabrication of well-ordered multilayer structures. On the other hand, the coexistence between 2D and 3D structures may be essential for the function of biological systems. Thus, the controlled formation of collapse structures is important for both the understanding of the formation of complex biological assemblies and the fabrication of defectfree surface layers. Extended studies on enantiomeric and racemic 2,3-dihydroxystearates16-19 led to the interesting observation that these compounds are able to build up a hydrogen-bonded network which is modulated by the position and configuration of the vicinal hydroxyl groups forming the hydrogen bridges. The homochirality observed for 2,3-dihydroxystearates is attributed to the formation of antipodal pairs. The general objective of our study is to elucidate the effect of a single fluorine atom in the 4-position of ethyl 2,3dihydroxystearates on the phase behavior by comparison with that of the unfluorinated compounds.20 Our results indicate a strengthening of intermolecular interaction due to a more distinctive 2D hydrogen-bonded network formed in the ethyl 4-fluoro-2,3-dihydroxystearates. This led us to the question of whether a sterically demanding linear group in the 2-position would disturb the intermolecular hydrogen bridges and therefore enhance the strength of the intramolecular hydrogen interactions. Our choice was an azido group due to the compliance to the demanded conditions. In this paper ethyl 2-azido-4-fluoro-3hydroxystearates (EAFHSs)21 were investigated as a new class (14) von Nahmen, A.; Post, A.; Galla, H.-J.; Sieber, M. Eur. Biophys. J. 1997, 26, 359. (15) Gopal, A.; Lee, K. Y. C. J. Phys. Chem. B 2001, 105, 10348. (16) Fix, M.; Sieber, M.; Overs, M.; Scha¨fer, H.-J.; Galla, H.-J. Phys. Chem. Chem. Phys. 2000, 2, 4515. (17) Overs, M.; Fix, M.; Jacobi, S.; Chi, L. F.; Sieber, M.; Scha¨fer, H.-J.; Fuchs, H.; Galla, H.-J. Langmuir 2000, 16, 1141. (18) Chi, L. F.; Jacobi, S.; Anczykowski, B.; Overs, M.; Scha¨fer, H.-J. AdV. Mater. 2000, 12. (19) Jacobi, S.; Chi, L. F.; Plate, M.; Overs, M.; Scha¨fer, H.-J.; Fuchs, H. Thin Solid Films 1998, 327, 180. (20) Steffens, S.; Meyer, M.; Schroth, S.; Chen, X., Chi, L. F.; Haufe, G.; Galla, H.-J. Manuscript in preparation. (21) Oldendorf, J. Thesis, University of Mu¨nster, Germany, 2002.

10.1021/la0521589 CCC: $33.50 © 2006 American Chemical Society Published on Web 01/11/2006

Organized Collapse Structures in EAFHS Mixtures

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Scheme 1. Configurations of Investigated Ethyl 2-Azido-4-fluoro-3-hydroxystearates in Different Mixtures of Racemic or Highly Enantioenriched Diastereomers

of amphiphilic molecules at the air/water interface using film balance measurements. To gain more detailed information on 2D phase transformations and monolayer morphology, Brewster angle microscopy (BAM) and scanning force microscopy (SFM) were applied. These methods revealed the formation of new 3D collapse structures which can be modulated by using different enantiomeric, diastereomeric, or racemic/diastereomeric mixtures. These EAFHSs prove to be interesting substrates to tune surface properties. Materials and Methods Materials. All different EAFHSs were synthesized as described elsewhere.21 The enantiomeric, diastereomeric, or racemic/diastereomeric EAFHSs investigated in this paper are presented in Scheme 1. The relative stereochemistry as well as the enantiomeric purity of the examined amphiphiles was determined by 19F NMR experiments. Ultrapure water (pH 5.5) was obtained from a Millipore system (Millipore, Bedford, MA) comprising reverse osmosis, ion exchange, and 0.22 µm ultrafiltration cartridges. Lipid stock solutions were prepared with chloroform (HPLC grade CHCl3 was purchased from Sigma-Aldrich, Steinheim, Germany). Isotherm Measurements. Surface pressure vs area isotherms (or simply “π-A isotherms”) were obtained with film balances with an accuracy of 0.1 mN/m and a Wilhelmy plate system. The total surface area was 175 cm2 (Riegler & Kirstein, Wiesbaden, Germany) or 690 cm2 (NIMA, Coventry, U.K.). Monolayers were prepared by spreading the appropriate chloroform solutions onto ultrapure water using a 25 µL syringe (ILS, Stu¨tzerbach, Germany). After 10 min of solvent evaporation the monolayer was compressed with a velocity of 5.8 cm2/min (Riegler & Kirstein) or 10.0 cm2/min (NIMA). Brewster Angle Microscopy. The monolayers were visualized by a Brewster angle microscope (BAM 2Plus, Nanofilm Technology GmbH, Go¨ttingen, Germany), mounted on the NIMA trough. The light source of the BAM instrument was a Nd:YAG laser. The laser beam is reflected at the air/water interface at the Brewster angle. The reflected beam is recorded with a CCD camera, and the data are digitally saved. The 10-fold objective of the microscope provides a diffraction-limited lateral resolution of approximately 2 µm. The size of the BAM images in this paper is 430 × 537 µm2. Langmuir-Blodgett Transfer. Langmuir-Blodgett (LB) films were prepared by spreading a compound mixture dissolved in chloroform onto a pure water subphase of a Wilhelmy film balance (Riegler & Kirstein, Mainz, Germany) with an operational area of 39 cm2. Prior to spreading, a cleaved mica sheet (Electron Microscopy

Figure 1. Surface pressure-area isotherms of different mixtures of EHAFSs on ultrapure water at 20 °C. Inset: Expansioncompression cycles of the RDIA EHAFS. Science, Munich, Germany) was dipped into the subphase. After an equilibration period of 10 min, the film was compressed with a velocity of 1.8 cm2/min to a defined surface pressure (in general ∼30 mN/m) and transferred onto the mica sheet (0.64 mm/min) under almost constant surface pressure (∆π ) max ( 0.15 mN/m). This was guaranteed by regulating the barrier speed manually. Scanning Force Microscopy. Surface images of LB films were obtained at ambient conditions using a Nanoscope IIIa Dimension 3000 microscope from Digital Instruments (Santa Barbara, CA) operating in tapping mode. Silicon tips (BS-Tap 300, Nanoscience Instruments Inc., Phoenix, AZ) with a resonance frequency of 250300 kHz were used. The mean height of the surface was analyzed by WSxM software (Nanotec Electronica, Barcelona, Spain).

Results and Discussion In Scheme 1 an overview of the synthesized EAFHSs is presented. Our synthetic route21 using a Sharpless dihydroxylation22 as a key step for the preparation of precursors (Scheme 1A) leads to two diastereomeric pairs of enantiomers. The azido group in the 2-position and the hydroxyl function in the 3-position are always in an anti configuration (Scheme 1, 1-4). Different mixtures of diastereomers 1-4 with R or S configuration of the fluorine in the 4-position were investigated in this study. The RSR/RSS mixture 5, named RDIA, possesses a diastereomeric ratio of 62:38. This diastereomeric mixture was separated by HPLC, yielding one enantiopure EAFHS named ENAN. The (22) Gao, Y.; Sharpless, K. B. J. Am. Chem. Soc. 1988, 110, 7538.

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Figure 2. BAM images (scale bar 100 µm) of different EHAFSs in the plateau region: (A) ENAN, 21.0 mN/m, 0.44 nm2 (inset, 16.5 mN/m, 33 nm2); (B) RDIA, 24.2 mN/m, 0.40 nm2 (inset, 24.1 mN/m, 0.48 nm2); (C) SDIA, 27.0 mN/m, 0.20 nm2; (D) RAC, 30.0 mN/m, 0.25 nm2.

absolute configuration could not be determined, and therefore, this compound is used for comparison with the diastereomeric mixtures. SDIA 6 is a 61:39 mixture of SRS/SRR diastereomers. All compounds within the diastereomeric mixtures were practically enantiopure (>98% ee). Another synthetic route (Scheme 1B) via a different dihydroxylation procedure (KMnO4) of a precursor yielded the racemic mixture RAC, which is composed of two enantiomeric pairs (RSR/SRS and RSS/SRR) in a 62:38 diastereomeric ratio, 7. Film Balance Measurements. Figure 1 shows π-A isotherms of RAC, RDIA, SDIA, and ENAN at 20 °C on a MilliQ subphase and, as an inset, three compression-expansion cycles of RDIA. All compounds show surface activity and behave similarly on compression. The lift-off area is 0.75 nm2 for RDIA, 0.8 nm2 for ENAN, and 0.9 nm2 for RAC and SDIA. The pressure increases on reduction of the area. At a characteristic point the π-A isotherm course changes into a plateau region with constant surface pressure and an almost infinite compressibility. Generally, a plateau indicates a phase transition. In the case of the examined EAFHSs a collapse from the liquid-expanded phase is assumed although the pressure is considerably high. This has been observed with other compounds before.23 All compounds could be compressed beneath a molecular area of 0.2 nm2 without an increase of surface pressure. The highest plateau pressure was observed for RAC with a value of 30 mN/m. Both mixtures of diastereomers form a plateau at 27 mN/m. ENAN however develops a plateau pressure of 20 mN/m. Interestingly, for RDIA and ENAN a spike in the π-A isotherm was observed at 0.4 and 0.6 nm2, respectively. Such a spike is generally attributed to slow rearrangement of headgroups following nucleation.24 By increasing the barrier speed to 10 cm2/min, the spike of the RDIA isotherm was suppressed. Similar (23) Sackmann, H.; Do¨rfler, H.-D. Z. Phys. Chem. 1972, 251, 303. (24) McFate, C.; Ward, D.; Olmsted, J., III. Langmuir 1993, 9, 1036.

spikes or overshoots in the π-A variation on compression have been previously observed and described in several other isotherm studies,25-27 for example, for amino acid amphiphiles.28 The inset in Figure 1 is evidence for the reversibility of the RDIA compound as an example for all other compounds. The three expansion-compression cycles clearly show that almost no loss of material occurs. The other compression-expansion cycles of ENAN, SDIA, and RAC show the same reversible behavior. The monolayer of RAC is slightly more expanded than that of RDIA and SDIA. Furthermore, both diastereomeric mixtures are more expanded than the pure enantiomer. Such behavior would be expected for monolayers exhibiting homochirality.29 The isotherms show differences in their lift-off areas as well as in the shape and height of the surface pressure plateau. For the examined compounds, however, it is not possible to decide which kind of preference is exhibited, because we investigated diastereomeric and racemic/diastereomeric mixtures in comparison with the enantiomer. Since the isotherms clearly show different behavior, we consider chiral discrimination. Brewster Angle Microscopy. To characterize the lateral organization of EAFHS films within the plateau region, BAM investigations were performed. At large areas per molecule all investigated films do not show domain formation. Only when the plateau pressure is reached is the formation of defined structures observed. From their appearance they can be classified as collapse structures. Examples of the different collapse structures formed upon compression of the EAFHSs are shown in Figure 2. (25) Hui, S. W.; Yu, H.; Xu, Z. C.; Bittman, R. Langmuir 1992, 8, 2724. (26) Makino, M.; Kamiya, M.; Ishii, T.; Yoshikawa, K. Langmuir 1994, 10, 1287. (27) Mertesdorf, C.; Ringsdorf, H. Liq. Cryst. 1989, 5, 1757. (28) Parazak, D. P.; Uang, J. Y.-J.; Turner, B.; Stine, K. J. Langmuir 1994, 10, 3787. (29) Gehlert, U.; Weidemann, G.; Vollhardt, D.; Brezesinski, G.; Wagner, R.; Mo¨hwald, H. Langmuir 1998, 14, 2112.

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Figure 3. AFM images of the different EFAHSs (1, RAC; 2, ENAN; 3, SDIA) in 2D (1a, 2a, 3a) and 3D (1b, 2b, 3b) views. Profiles (1c, 2c, 3c) are marked in every 2D image as a blue line. The zoom-in image of the blue marked square in panel 3a is shown in Figure 4.

ENAN shows snowflake structures within the spike range (Figure 2A inset). At an area below the spike a reorganization process was observed which resulted in the elongation of the collapse structure at their tips, forming a distinctive network (Figure 2A). RDIA shows similar behavior within the spike range in comparison with ENAN. At an area slightly larger than the spike range snowflake-like structures were found. In the spike, everything vanished completely and single spider-like structures were observed (Figure 2B inset). The formed collapse structures have a center and form 4-6 left-handed spider-like arms. The amount of structures increases with decreasing area (Figure 2B). The collapse structures appearing in the SDIA mixture resemble the RDIA structures, although no spike has been observed. The formation process of the collapse structures of SDIA was therefore elucidated with BAM. At the beginning of the plateau very small spider-like structures are formed, which increase only in size by further compression, but not in number (Figure 2C). Interestingly, RAC monolayers exhibited round domains instead of elongated network structures. The structures of RAC shown by BAM are bright and circular and increase in size during compression. The circular collapse structures show different intensities of brightness. This is often interpreted as the difference in the azimuth within domains30 or as multilayer formation31 (30) de Mul, M. N. G.; Mann, J. A., Jr. Langmuir 1998, 14, 2455. (31) Angelova, A.; Vollhardt, D.; Ionov, R. J. Phys. Chem. 1996, 100, 10710.

indicating different heights. For the collapse structures however an interpretation as height differences is more likely. The BAM image (Figure 2D) clearly shows that the boundaries of the circular structures are even brighter, indicating the formation of more dense material. As is discernible from BAM images, the EAFHSs display clearly different morphologies and collapse structures depending on the stereoisomeric composition. These differences can be attributed to distinctive interactions, especially hydrogen bonding, within the headgroup region. From infrared reflection absorption spectroscopy (IRRAS) measurements we know that the network of hydrogen bonds in enantiomeric monolayers is more structured and thus leads to different collapse structure formation. This is concluded from the shift of the IR peak of the azide band from 2110 cm-1 of ENAN in comparison with 2113.5 cm-1 of the RAC mixture. This will be discussed in detail in a forthcoming paper. Interestingly, the formation of the observed structures is highly reversible under expansion as can be deduced from compressionexpansion isotherms of all compounds. Scanning Force Microscopy. To gain more structural information on the formed collapse structures, monolayer films were transferred via the Langmuir-Blodgett method on mica sheets. The SFM results show for all EAFHSs the formation of 3D structures with heights between 10 and 150 nm and lateral extensions on the micrometer scale. In the overall appearance,

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Figure 4. Zoom-in AFM image of SDIA (1) in Figure 3a marked with the blue rectangle in 2D (1a) and 3D (1b) views. (2) Analysis of the fiber structure marked in panel 1a with a white rectangle in 2D (2a) and 3D (2b) views. Fiber profile 2c is indicated in panel 2a with a blue line. (3) Analysis of the fiber heights marked with a green box in 2D (3a) and 3D (3d) views near the center (c) and at the tip (d). Profiles 3c and 3d are indicated as blue lines in panel 3a.

all obtained SFM images correspond to the BAM observations, clearly showing that the transfer process did not disturb the structures formed on the aqueous surface of the film balance. The RAC mixture shows perfect circular patterns with an overall height of 10 nm and little buds with heights of over 100 nm (Figure 3, panels 1a-c). The diameter of the circular structures is between 20 and 50 nm and can be correlated with the intermediate diameter of the collapse structures observed in BAM. Since the regions of enhanced brightness in BAM images seem to correspond to the buds observed in SFM images, we conclude that, in this case, differences in brightness detected by BAM can actually be related to height differences within the circular structures. For ENAN, fiber networks are observed with an intermediate fiber height of 35 nm (Figure 3, panels 2a-c). All fiber arms are straight and have kinks within the fiber mainly at an angle of about 120°. The pattern resembles the structures observed with BAM. SFM images of solid supported SDIA layers reveal a pattern that differs characteristically from the 3D structures formed by ENAN (Figure 3, panels 3a-c). In contrast to those of ENAN the fiber arms are curved and spider-like as was observed by

BAM. The fiber height ranges from 6 to 50 nm (Figure 3, panel 3b), whereas the average height of most fibers is 25-35 nm (Figure 3, panel 3c). By zooming in on the SDIA collapse structure (Figure 4, panels 1a and 1b), it becomes obvious that each fiber is distinctively substructured. An example of the nanoscale morphology is given in Figure 4, panels 2a-c. The fiber is divided into three different height sections, with the steps occurring all at one side (Figure 4, panel 2b). The first fiber step was determined to be 6 nm. The second and third steps displayed heights of 20 and 35 nm, respectively (Figure 4, panel 2c). Because the length of one stretched molecule in the zigzag conformation is about 3 nm, it is possible that the 6 nm step consists of a bilayer. The next step would consist of six layers if not much hydration water between the layers is assumed, seven layers if all molecules are stretched and the film is slightly squeezed by the SFM tip, or eight, nine, or more layers if some interdigitation of the alkyl chains occurss a feature which has been observed for sulfite octadecanoate esters.32 Within the highest step (35 nm) the layer number is uncertain as well for the same reasons. Nevertheless, the height differences of the layer formation might be attributed to a stepwise folding of the monolayer which is presented in a model later on.

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Figure 5. Schematic model of the fiber formation at the air/water interface.

The zoom-in picture shows the height and step height difference of a fiber which grew from a nucleation center of the nanoscale structures (Figure 4, panels 3a-d). Also in this fiber there are two distinct steps (Figure 4, panel 3b). At the tip of the fiber, the ground height is less than 2.5 nm and the first step is 5 nm (Figure 4, panel 3d). The top has a height of 10 nm. The steps occur on both sides of the fiber. In profile 3c the development of the heights in comparison with that in profile 3d is shown. The height of the steps increases if the center of the structure is approached. The formation of collapse structures is often discussed with different mechanisms such as the one proposed by Ries33 for 2-hydroxytetracosanoic acid. This collapse mechanism begins with buckles in the film (weakening) and the formation of a head-head column under compression, which is folded over and subsequently topples to form a trilayer (a bilayer on top of the monolayer), breaking into disconnected multilayers. In contrast to the disconnected multilayers, the standard view of reversible monolayer collapse is slightly different. Reversible collapses often show that the surface pressure builds up steeply until a spike or “wall” is reached. The first steps of the mechanism follow the predictions of Ries, except that the layers after the toppling slide over one another by pressure relief.24,34 The main difference in this mechanism is that the trilayer (3D phase) stays in contact with the monolayer (2D phase). These mechanisms do not fit with our structural results within the fibers. We therefore present a modified mechanism which includes the knowledge of the literature and explains the structure (32) Chen, X.; Wiehle, S.; Weygand, M.; Brezesinski, G.; Klenz, U.; Chi; L. F.; Galla, H.-J.; Fuchs, H.; Haufe, G. J. Phys. Chem. B 2005, 109, 19866. (33) Ries, H. E. Nature 1979, 281, 287. (34) Ybert, C.; Lu, W. X.; Mo¨ller, G.; Knobler, C. M. J. Phys. Chem B 2002, 106, 2004.

of the amazing pattern of the EAFHSs. Hydration water and interdigitation of alkyl chains are not considered in the model but remain a question. As in other described mechanisms first a weakening (buckling) (Figure 5A) of the monolayer due to a change in curvature occurs. In the first picture a monolayer defect is shown which acts as the nucleation site for the formation of 3D structures. Then the formation of a head-to-head double layer which faces the air might occur due to strong interaction within the headgroups of the molecules (Figure 5B). The double layer might be formed during compression and bend to one side (Figure 5C). The height differences of a thus formed collapse structure would correlate with those found in the SFM images of the fiber tips (2.5-3 nm). During compression, both layers could slide over the monolayer until the friction between adjacent layers is so high that another head-to-head double layer is formed. This new bud would appear at the border of an already formed layer complex acting as a second nucleation center. An additional tail-to-tail double layer might be formed by bending of this layer to one side (Figure 5D). According to the step heights found with SFM measurements, this folding is probably repeated several times (Figure 5E). Decreasing the pressure, by expanding the barriers, leads to a relaxation of the film and a sliding down of all layers. The observed reversibility of the structure formation is another indicator for the proposed multilayer formation. Additionally, this mechanism explains the formation of steps on opposite sides of the fiber. A folded head-to-head layer can bend to either side and create steps on each side of a fiber. The proposed model is able to explain the formation of the fiber structures of ENAN, RDIA, and SDIA. To explain the observation of the straight (ENAN) and curved (SDIA, RDIA) fiber structure, a 3D model of the headgroup

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Figure 7. Structural models for the molecular interaction of the fiber formation: (A) ENAN symbolized as a triangle or a quadrangle compared with a modified AFM image; (B) SDIA symbolized by a triangle and a quadrangle compared with a modified AFM image.

Figure 6. Configuration of the EAFHS headgroup shown by models in side and top views (without hydrogen atoms): (A) SRR symbolized by a quadrangle; (B) SRS symbolized by a triangle.

configuration of the SDIA is given as an example in Figure 6. It also shows the hydrogen-bonding between the carbonyl and hydroxyl groups in position 2 calculated with force field methods. The diastereomers only differ in the fluorine configuration. The top view allows insight into the headgroup as it appears at the air/water interface. It can be seen from the view without hydrogen atoms that the azido group blocks the fluorine function in the case of the SRS enantiomer, due to its size and rigidity. In the case of the SRR diastereomer no blocking occurs. The diastereomers have an overall required space represented by a triangle and a quadrangle in Figure 6. Taking into account specific interactions between identical headgroups in ENAN and different headgroups in the diaster-

eomeric mixtures, a simplified model explaining the observed fiber structures can be proposed. In the case of ENAN, on a molecular scale, the specific interactions lead to a linear alignment of the molecules. Assuming that 3D structure formation appears along this molecular alignment acting as a 2D defect line in the monolayer, this model predicts the fiber structures as detected by SFM (Figure 7A, enhanced contrast of Figure 3A). Specific interactions between the triangle-shaped molecules, on one hand, and only quadrangle-shaped molecules, on the other hand, would always lead to such linear fibers. The appearance of kinks with an angle of 120° can be explained by defects occurring during packing within the alignment. The existence of curved fibers, as was observed for SDIA, can also be explained by specific interactions, this time between two differently shaped molecules, a triangle-shaped one and a quadrangle-shaped one. The 2D arrangement of these specimens naturally leads to curved line defects, which can similarly be transferred from the molecular scale to the micrometer scale observable with SFM (Figure 7B). Finally, we can presume that the distortion of a distinct interaction due to the fluorine within the headgroup is the key for the different fiber structures of ENAN and the diastereomeric mixtures.

Conclusion EAFHSs proved to be exciting surface-active compounds with an unconventional 3D collapse behavior. They all display a characteristic thermodynamic behavior of expanded films and collapse from the liquid-expanded phase under formation of 3D

Organized Collapse Structures in EAFHS Mixtures

collapse structures. These structures show a distinct behavior. This could be shown by examining the air/water interface through BAM and, after Langmuir-Blodgett transfer, also with SFM. The azido group of the compounds seems to be the main factor for the reversible and stable 3D pattern formation. Other substituents in position 2 with similarity to the azido function will be investigated in due course. Moreover, we could modify the shape and structure of fascinating 3D patterns by using different diastereomeric or racemic/diastereomeric mixtures of ethyl 2-azido-4-fluoro-3hydroxystearates compared with the observed structures exhibited by the enantiopure compound. The investigations of mixtures of diastereomers lead to surprising features in the case of EFAHSs. The stereoisomeric composition determines the shape of the 3D

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structures and has a slight effect on the behavior of the monolayer, which has also been observed before. Acknowledgment. We acknowledge H. Fuchs and L. F. Chi for continuous discussions and for their help with the BAM experiments. This work was supported by the Deutsche Forschungsgemeinschaft (Grant SFB 424). Special thanks are due to Udo Ho¨weler, Andreas Kerth, and Michaela Meyer for their fruitful contributions and their contagious enthusiasm. Supporting Information Available: Description of the synthesis of the compounds studied and the characterization of the ethyl 2-azido4-fluoro-3-hydroxystearates. This material is available free of charge via the Internet at http://pubs.acs.org. LA0521589