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In Situ Investigation of Dynamical Nanophase Separation Mohamed M. S. Abdel-Mottaleb,† Steven De Feyter,*,† Michel Sieffert,‡ Markus Klapper,‡ Klaus Mu¨llen,‡ and Frans C. De Schryver*,† Department of Chemistry, Katholieke Universiteit Leuven (KULeuven), Celestijnenlaan 200-F, 3001 Leuven, Belgium, and Max-Planck Institute for Polymer Research, Ackermannweg 10, D-55128 Mainz, Germany Received March 28, 2003. In Final Form: July 11, 2003 The nanophase separation behavior of two semi-fluorinated isophthalic acid derivatives, F8H10-ISA and F12H11-ISA, at the liquid-solid interface is studied by in situ scanning tunneling microscopy. Two polymorphs of F8H10-ISA were observed at the interface. One polymorph, which is not stable in time, showed nanosegregation of the perfluorinated segments of the alkyl chains, while the second polymorph showed a completely interdigitating lamellar structure with solvent co-deposition. This co-deposition of solvent molecules is believed to induce extra stability in the monolayer which in turn aids the molecules in overcoming the repulsive interactions induced by the perfluorinated segment of the molecule. F12H11ISA did not exhibit nanosegregation at the liquid-solid interface, despite the longer perfluorinated segment (increase by 4 carbon atoms) with respect to the non-fluorinated segment (increase by 1 carbon atom). Instead, clustering of the molecules into units consisting of eight molecules was observed. This is explained in terms of a compromise between opposing interactions arising from the chemical nature of the molecule. Whereas the perfluorinated-non-fluorinated segments tend to induce nanosegregation in the monolayer, the hydrogen-bonding network formed by the isophthalic acid groups in combination with solvent codeposition tends to induce an interdigitated lamellar structure.
Introduction In the field of supramolecular chemistry, a variety of noncovalent synthesis strategies or approaches have been established to organize matter at the molecular level, not only in solution but also on surfaces by self-assembly.1 Self-assembled monolayers at surfaces can be obtained by either chemisorption2 or physisorption.3 Noncovalent interactions have a profound effect on the molecular organization in the obtained monolayers. For example, hydrogen bonding and metal coordination, due to the strength and directionality of the interactions, have been extensively used to control such organization.4,5 On the other hand, fluorophobic/fluorophilic interactions have not been exploited to the same extent to control the structure of monolayer thin films on surfaces.6 * To whom correspondence should be addressed. E-mail:
[email protected]. † Katholieke Universiteit Leuven. ‡ Max-Planck Institute for Polymer Research. (1) Comprehensive supramolecular chemistry; Atwood, J. L., Davies, J. E. D., Macnicol, D. D., Vogtle, F.; Pergamon: New York, 1996. (2) (a) Ulman, A. Chem. Rev. 1996, 96, 1533 and references therein. (b) Lewis, P. A.; Donhauser, Z. J.; Mantooth, B. A.; Smith, R. K.; Bumm, L. A.; Kelly, K. F.; Weiss, P. S. Nanotechnology 2001, 12, 231 and references therein. (3) (a) Rabe, J. P.; Buchholz, S. Science 1991, 253, 424. (b) Frommer, J. Angew. Chem., Int. Ed. Engl. 1992, 31, 1298. (c) Cyr, D. M.; Venkataraman, B.; Flynn, G. W. Chem. Mater. 1996, 8, 1600. (d) Giancarlo, L. C.; Flynn, G. W. Annu. Rev. Phys. Chem. 1998, 49, 297. (e) De Feyter, S.; Gesquie`re, A.; Abdel-Mottaleb, M. M. S.; Grim, P. C. M.; Meiners, C.; Sieffert, M.; Valiyaveettil, S.; Mu¨llen, K.; De Schryver, F. C. Acc. Chem. Res. 2000, 33, 520. (4) De Feyter, S.; Larsson, M.; Schuurmans, S.; Verkuijl, B.; Zoriniants, G.; Gesquie`re, A.; Abdel-Mottaleb, M. M.; van Esch, J.; Feringa, B. L.; van Stam, J.; De Schryver, F. Chem.sEur. J. 2003, 9, 1198 and references therein. (5) (a) Semenov, A.; Spatz, J. P.; Mo¨ller, M.; Lehn, J.-M.; Sell, B.; Schubert, D.; Weidl, C. H.; Schubert, U. S. Angew. Chem., Int. Ed. 1999, 38, 2547. (b) Ziener, U.; Lehn, J. M.; Mourran, A.; Mo¨ller, M. Chem.sEur. J. 2002, 8, 951. (c) Abdel-Mottaleb, M. M. S.; Schuurmans, N.; De Feyter, S.; Van Esch, J.; Feringa, B. L.; De Schryver, F. C. Chem. Commun. 2002, 1894. (6) Hipps, K. W.; Scudiero, L.; Barlow, D. E.; Cooke, M. P. J. Am. Chem. Soc. 2002, 124, 2126.
Semi-fluorinated n-alkanes, F(CF2)n(CH2)mH, have been studied both in the melt and in the solid state7 as well as in solution.8 Phase separation is observed between perfluorinated n-alkanes and non-fluorinated analogues in solution, due to the strong incompatibility between the components of the system. This type of phase separation is not possible if the incompatible moieties are part of the same molecule.8 The three-dimensional (3D) crystalline structures of perfluoroalkylalkanes in their different solid and liquid-crystalline phases and in the solution phase have been extensively investigated.7-9 These studies demonstrate that the miscibility of fluorocarbon and hydrocarbon molecules is generally poor. Chain molecules consisting of hydrocarbon and fluorocarbon segments of at least six to eight carbons in each segment tend to organize in ordered bilayer structures consisting of microdomains of fluorinated and hydrogenated segments. Few studies have been carried out on the twodimensional (2D) self-assembly and phase behavior of fluorinated n-alkanes. Infrared absorption spectroscopy and atomic force microscopy (AFM) experiments on selfassembled monolayers of fluorinated alkanethiolates on gold confirmed the larger rigidity of partially fluorinated alkyl chains with respect to non-fluorinated alkyl chains.10 This property was used in an AFM experiment to differentiate between two molecular classes in Langmuir(7) (a) Twieg, R.; Rabolt, J. F. J. Polym. Sci., Polym. Lett. Ed. 1983, 21, 901. (b) Rabolt, J. F.; Russell, T.; Twieg, R. Macromolecules 1984, 17, 2786. (c) Minoni, G.; Zerbi, G. J. Polym. Sci., Polym. Lett. Ed. 1984, 22, 533. (8) Ho¨pken, J.; Pugh, C.; Richtering, W.; Mo¨ller, M. Makromol. Chem. 1988, 189, 911. (9) (a) Twieg, R. J.; Rabolt, J. F.; Russell, T. P. Polym. Prepr. (Am. Chem. Soc., Div. Polym. Chem.) 1985, 26, 234. (b) Twieg, R. J.; Rabolt, J. F. Macromolecules 1988, 21, 1806. (c) Viney, C.; Russell, T. P.; Depero, L. E.; Twieg, R. J. Mol. Cryst. Liq. Cryst. 1989, 168, 63. (d) Turberg, M. P.; Brady, J. E. J. Am. Chem. Soc. 1988, 110, 7797. (e) Ishikawa, Y.; Kuwahara, H.; Kunitake, T. J. Am. Chem. Soc. 1989, 111, 8530. (f) Dorset, D. L. Macromolecules 1990, 23, 894. (g) Russell, T. P.; Rabolt, J. F.; Twieg, R. J.; Siemens, R. L. Macromolecules 1986, 19, 1135. (10) (a) Chidsey, C. E. D.; Lioacono, D. N. Langmuir 1990, 6, 682. (b) Alves, C. A.; Porter, M. D. Langmuir 1993, 9, 3507.
10.1021/la034535s CCC: $25.00 © 2003 American Chemical Society Published on Web 08/20/2003
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Blodgett (LB) films deposited from mixtures of semifluorinated and non-fluorinated fatty acids.11 These results indicate the high degree of incompatibility between fluorocarbons and hydrocarbons. Scanning tunneling microscopy (STM) is the method of choice for highresolution investigation of the structural properties of twodimensional adlayers on conductive surfaces such as graphite. Note that in this kind of setting, the molecules have their molecular backbones parallel to the substrate. Theoretical calculations according to a resonant tunneling model12 indicate that the tunneling current over fluorinated methylene groups is lower than that over hydrogenated methylene groups in monolayers physisorbed on graphite, as confirmed experimentally.13 Monolayers of completely fluorinated alkanols were never observed, most likely because a crystalline monolayer of this type of molecules is not formed at room temperature on the surface of graphite. However, monolayers of partially fluorinated alkanols could be observed with STM. The perfluorinated part of the alkyl chain appears with dark contrast (indicating a lower tunneling current) at both negative and positive sample bias.14 We have previously reported on the two-dimensional organization of a semi-fluorinated isophthalic acid derivative (5-((ω-perfluorohexyl)undecyloxy)isophthalic acid; F6H11-ISA) at the liquid-solid interface.15 The perfluorinated segments could be clearly distinguished by their characteristic dark contrast. There was no evidence of nanophase segregation in the monolayers. The molecules were fully interdigitating, where each perfluorinated segment had two non-fluorinated segments as nearest neighbors. Moreover, mixed monolayers of the semi-fluorinated molecules and a non-fluorinated analogue showed complete miscibility between the two components. The semi-fluorinated molecules were adsorbed in the monolayer of the non-fluorinated molecules in a completely random fashion. It was suggested that the length of the perfluorinated segment (six fluorinated methylene groups) was not long enough to induce segregation in the monolayer. Furthermore, taking advantage of the characteristic contrast of the perfluorinated segments in STM images, under constant-height (variablecurrent) conditions, an in situ investigation of the adsorption-desorption dynamics of individual molecules at the liquid-solid interface was made possible. The semifluorinated segment can be regarded as a marker. The specific contrast of the perfluorinated moieties allows the semi-fluorinated molecules in a mixed monolayer with non-fluorinated analogues to be distinguished. This specific contrast can be used to probe adsorptiondesorption dynamics at the liquid-solid interface. The molecule residence time in the mixed monolayer was found to vary from several seconds to a few minutes, although the exchange process itself is fast and takes less than 4.3 ms. Molecular mechanics calculations indicated a substantially smaller adsorption energy of a semi-fluorinated molecule compared to a non-fluorinated molecule.16 (11) Overney, R. M.; Meyer, E.; Frommer, J.; Brodbeck, D.; Lu¨thi, R.; Howald, L.; Gu¨ntherodt, H.-J.; Fujihira, M.; Takano, H.; Gotoh, Y. Nature 1992, 359, 133. (12) (a) Lambin, G.; Calderone, A.; Lazzaroni, R.; Bre´das, J. L.; Clarke, T. C.; Rabe, J. P. Mol. Cryst. Liq. Cryst. 1993, 235, 75. (b) Lazzaroni, R.; Calderone, A.; Bre´das, J. L.; Rabe, J. P. J. Chem. Phys. 1997, 107, 99. (13) Stabel, A.; Dasaradhi, L.; O’Hagan, D.; Rabe, J. P. Langmuir 1995, 11, 1427. (14) Claypool, C. L.; Faglioni, F.; Goddard, W. A., III; Gray, H. B.; Lewis, N. S.; Marcus, R. A. J. Phys. Chem. B 1997, 101, 5778. (15) Gesquie`re, A.; Abdel-Mottaleb, M. M. S.; Sieffert, M.; Mu¨llen, K.; De Schryver, F. C. Langmuir 1999, 15, 6821.
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Figure 1. Chemical structures of F8H10-ISA and F12H11ISA.
In this paper, we extend our investigations to monocomponent semi-fluorinated systems that differ in the relative length of the perfluorinated segment with respect to the non-fluorinated one. We investigate the interplay between the hydrogen-bond formation and the fluorophilic/ fluorophobic interactions in the two-dimensional phase behavior. We report on the observation of a kinetically controlled nanophase segregation and a solvent-mediated thermodynamically controlled miscibility of the molecules at the liquid-solid interface. Experimental Section Prior to imaging, compounds under investigation were dissolved in 1-octanol (Aldrich, 99%) and a drop of this solution was applied on a freshly cleaved surface of highly oriented pyrolytic graphite (HOPG). The presented STM images were acquired in the variable-current mode (constant height) under ambient conditions. In the STM images, white corresponds to the highest and black to the lowest measured tunneling current. STM experiments were performed using a Discoverer scanning tunneling microscope (Topometrix Inc., Santa Barbara, CA) with a typical frame acquisition time of 7 s, along with an external pulse/function generator (model HP 8111 A), with negative sample bias. Tips were etched electrochemically from Pt/Ir wire (80%/20%; diameter, 0.2 mm) in a 2 N KOH/6 N NaCN solution in water. The experiments were repeated in several sessions using different tips to check for reproducibility and to avoid artifacts. Note that during the experiments, the STM tip is immersed in the supernatant solution. Different settings for the tunneling current and the bias voltage were used, ranging from 0.7 to 1 nA and -0.1 to -1.1 V, respectively. After registration of an STM image of the monolayer structure, the underlying graphite lattice was recorded at the same position by decreasing the bias voltage, serving as an in situ calibration. All STM images contain raw data and are not subjected to any manipulation or image processing.
Results and Discussion Submolecular investigation of the two-dimensional organization of 5-(11,11,12,12,13,13,14,14,15,15,16,16, 17,17,18,18,18-heptadecafluoro-n-octadecyloxy)isophthalic acid (F8H10-ISA) and 5-(12,12,13,13,14,14,15,15,16, 16,17,17,18,18,19,19,20,20,21,21,22,22,23,23,23-pentacosafluoro-n-tricosyloxy)isophthalic acid (F12H11-ISA) has been carried out by means of STM at the liquid-solid interface. The molecular structure of the compounds under investigation is depicted in Figure 1. Figure 2a shows an STM image of a physisorbed monolayer consisting of F8H10-ISA molecules adsorbed from a solution in 1-octanol onto the basal plane of graphite. The molecules are arranged in a bilayer type structure with the lamellar width (∆L1) equal to 43.3 ( 3.8 Å. The observed bright spots (white circles) correspond (16) Gesquie`re, A.; Abdel-Mottaleb, M. M. S.; De Feyter, S.; Sieffert, M.; Mu¨llen, K.; De Schryver, F. C.; Calderone, A.; Lazzaroni, R.; Bre´das, J. L. Chem.sEur. J. 2000, 6, 3739.
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Abdel-Mottaleb et al. Chart 1. Overview of the Different Types of Patterns Observed and Their Relation to Phase Separation on the Nanoscalea
a The molecules are depicted as follows: isophthalic acid groups (open circles); non-fluorinated segments (lines); perfluorinated segments (bars). (a) Nanophase separation: the isophthalic acid groups, the non-fluorinated segments, and the perfluorinated segments form separate zones. (b) No nanophase separation: interdigitation of the semi-fluorinated alkyl chains. (c) No nanophase separation: interdigitation of the semifluorinated alkyl chains in addition to the formation of clusters, indicating the stress enforced by the incompatibility of the nonfluorinated and perfluorinated segments.
Figure 2. (a) STM image of the F8H10-ISA monolayer deposited from 1-octanol solution onto the basal plane of graphite. The image is submolecularly resolved. Alternating dark and bright rows can be clearly distinguished. The dark row corresponds to the fluorinated part of the molecule, while the bright row corresponds to the non-fluorinated part plus the isophthalic acid group (indicated by white circles). ∆L1 indicates a lamella. The image size is 10.6 × 10.6 nm2. Iset ) 1.0 nA, and Vbias ) -0.72 V. (b) Molecular model for the area indicated in panel a.
to the location of the isophthalic acid groups. The occurrence of a higher tunneling current above an aromatic moiety, as predicted by theoretical calculations,17 is a general finding, which has been observed for a large variety of organic adsorbates on graphite. The distance from one isophthalic acid group to the next one along the lamella axis (the intermolecular distance) is 9.6 ( 0.4 Å. The nonfluorinated part of the alkyl chains appears darker than the isophthalic acid groups. The perfluorinated part of the alkyl chains can clearly be distinguished as a black band, due to the decreased tunneling current detected over the fluorinated methylene groups. This is in agreement with theoretical calculations.12 Individual perfluorinated alkyl chains cannot be distinguished. No evidence of solvent co-deposition between the isophthalic acid groups was observed in these monolayers. The difference in the apparent contrast (brightness) between (17) Lazzaroni, R.; Calderone, A.; Lambin, G.; Rabe, J. P.; Bre´das, J. Synth. Met. 1991, 525, 41.
the non-fluorinated alkyl chains at the left and right side of a row of isophthalic acid groups is a scanning artifact.18 The non-fluorinated alkyl chains form an angle of 45 ( 4° with respect to the lamella axis, which does not allow for interdigitation of the chains, leading to phase separation on the nanoscale (rows of isophthalic acid groupsnon-fluorinated segments-fluorinated segments) (see Chart 1a). A model for the observed packing pattern is presented in Figure 2b. Nanophase separation in this kind of system is not trivial, in the sense that all 5-alkoxyisophthalic acid derivatives investigated so far show full interdigitation of the alkylated part.19 In time, during the imaging session, the monolayer structure changes into an interdigitated lamellar one. Figure 3a shows an STM image of such a monolayer. The specific contrast of F8H10-ISA can be clearly observed. The molecules now appear to be interdigitating. Thus there is a dynamic change in the monolayer packing pattern, (18) Although the measurements occur in the constant-height mode, the feedback loop is not fully disabled and the z-piezo responds upon changes in the detected tunneling current. If an increased tunneling current is recorded (e.g., on top of the isophthalic acid moieties), the z-piezo will retract, and on average in the course of a line scan, the distance between tip and monolayer will be larger after the tip has passed over a “bright” feature. Therefore, in the sequence non-fluorinated alkyl chains-isophthalic acid groups-isophthalic acid groups-nonfluorinated alkyl chains, with the tip scanning from left to right, the non-fluorinated alkyl chains at the left will appear brighter than those at the right. (19) Solvent co-deposition was previously observed within our research group: Vanoppen, P.; Grim, P. C. M.; Ru¨cker, M.; De Feyter, S.; Moessner, G.; Valiyaveettil, S.; Mu¨llen, K.; De Schryver, F. C. J. Phys. Chem. 1996, 100, 19636.
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Figure 3. (a) STM image of a monolayer of F8H10-ISA with no nanophase separation observed. ∆L1 indicates a F8H10ISA lamella, while ∆L2 indicates a solvent lamella. The image size is 10.1 × 10.1 nm2. Iset ) 1.0 nA, and Vbias ) -0.67 V. (b) Molecular model of the STM image shown in panel a.
from the nanosegregated patterns (Figure 2a) to the interdigitating one (Figure 3a). The interdigitating pattern (Chart 1b) was observed to be stable over extended periods; monolayers with the nanosegregated pattern were never observed again in the same session. In Figure 3a, two different lamellar widths can clearly be distinguished. The wide lamellae (∆L1 ) 34.2 ( 1.2 Å) consist of F8H10ISA molecules, while the narrow lamellae (∆L2 ) 16.3 ( 0.5 Å) are built up by solvent molecules. Interestingly, in all cases where the interdigitation of the alkyl chains was observed, the solvent molecules are coadsorbed. This suggests that as a result of solvent coadsorption, the hydrogen bonding involving the isophthalic acid groups overcomes the repulsive interactions between perfluorinated and non-fluorinated alkyl segments, leading to an interdigitated pattern. Figure 3b shows a model for the packing pattern observed in Figure 3a. Co-deposition of 1-alcohol molecules was observed for a substantial number of 5-alkoxy-isophthalic acid derivatives.19 In a number of cases, we observed that the codeposition process leads to thermodynamically more stable 2D patterns. Often the first images obtained in a session do not show co-deposition of solvent molecules while later
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on, more co-deposited 1-alcohol solvent lamellae are formed. In a number of cases, it was even possible to follow this co-deposition process in real time.16 Based on the STM results obtained for F8H10-ISA molecules, one would expect that increasing the length of the perfluorinated segment results in favoring nanosegregation within the monolayer. Motivated by this idea, F12H11-ISA was investigated at the liquid-solid interface. It is clear that the expected packing pattern was not achieved as shown in a typical large-scale STM image of a F12H11-ISA monolayer physisorbed at the liquid-solid interface (Figure 4a). The molecules appear to be fully interdigitating. Interestingly, the interdigitated pattern differs from the one of F8H10-ISA. The grouping of the molecules into units (Chart 1c) consisting of eight molecules is evident. One such unit is indicated by ∆M. Each unit is shifted by 4.2 ( 0.4 Å with respect to the preceding unit. The wider lamellae (∆L1 is 38.9 ( 1.7 Å) consist of F12H11-ISA molecules, while the narrow lamellae are built up by solvent molecules (∆L2 ) 15.8 ( 0.8 Å).16 The perfluorinated segments appear with a dark contrast. In this particular image, the isophthalic acid groups are not as well resolved as in the higher resolution image in Figure 4b where the bright spots correspond to the location of the isophthalic acid groups. The intermolecular distance, the distance between equivalent isophthalic acid groups, within a unit was found to be 10.2 ( 0.7 Å, which is slightly bigger than that typically found for isophthalic acid derivatives.19 This might be due to the longer perfluorinated segment, which will have more influence on the packing than in the case of F8H10-ISA. A molecular model is provided in Figure 4c. As indicated earlier, each unit consists of eight fully interdigitating molecules. Two white arrows (Figure 4b,c) indicate a unit. At the interface between two adjacent units, the molecules are oriented in such a way to allow favorable perfluorinated-perfluorinated and non-fluorinated-non-fluorinated interactions. The hydrogen-bonding network is partially preserved across the units. In addition, this arrangement provides a good explanation of the intermolecular distance variation at the interface between the units, as indicated by the white arrows in Figure 4c. During the different sessions, solvent-free domains were never observed. Although we cannot rule out that they are formed, this suggests that they are not stable and easily converted to an adlayer structure with solvent codeposition and interdigitation of the alkyl chains. Experiments in 1-phenyloctane, which is not known to co-deposit, did not lead to the formation of stable monolayers. The STM image shown in Figure 4d is of lower resolution with respect to the alkyl chains than that in Figure 4a,b, whereas the isophthalic acid groups and solvent lamellae are well resolved. This is due to the mobility of the alkyl chains. In this image, the coadsorbed solvent lamellae are easily identified (yellow arrows). At the edges of these solvent lamellae, bright spots appear. From the analysis of the STM data, the distance between two successive solvent lamellae (∆L1) corresponds to the width of a lamella consisting of F12H11-ISA molecules, within the experimental error. From this analysis and a comparison of images a and d in Figure 4, the bright spots are identified as the isophthalic acid groups of the F12H11-ISA molecules (white circles). The white arrows indicate the location of the alkyl chains. Apparently, increasing the length of the perfluorinated chain is the reason for the grouping of F12H11-ISA molecules into units. The incompatibility induced by the presence of the F12 segments should be strong enough to cause nanosegregation, but due to the long non-fluorinated
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Figure 4. (a) Large-scale STM image of the monolayer of F12H11-ISA molecules physisorbed at the 1-octanol-graphite interface. The grouping of the molecules into units of eight molecules is evident. One unit is indicated by ∆M. ∆L1 indicates an F12H11-ISA lamella, while ∆L2 indicates a solvent lamella. The image size is 20.8 × 20.8 nm2. Iset ) 0.9 nA, and Vset ) -0.7 V. (b) Small-scale STM image of a monolayer formed by physisorption of F12H11-ISA molecules at the 1-octanol-graphite interface. White arrows point at the interface between two units. White circles indicate the location of some isophthalic acid groups. ∆L1 indicates a F12H11-ISA lamella, while ∆L2 and the yellow arrows indicate solvent lamellae. The image size is 10.4 × 10.4 nm2. Iset ) 0.8 nA, and Vset ) 1.0 V. (c) Molecular model of the F12H11-ISA lamella flanked by two solvent lamellae, indicated by the two yellow arrows in panel b. (d) STM image of a monolayer of F12H11-ISA. Co-deposited solvent lamellae (∆L2) are indicated by yellow arrows. White circles indicate the location of isophthalic acid groups. The black arrows indicate different units. White arrows point to the area where the F12H11 part of the molecules is located.
segment (H11) plus the hydrogen-bonding network, nanosegregation did not occur and interdigitated patterns (with solvent co-deposition) were observed. However, the incompatibility induced by the F12 chains is expressed at two levels. First, the molecules group into units, until the lattice cannot tolerate the incompatibility induced by the F12 groups. It should be stressed that at the interface between two units, the intermolecular interactions are optimized, resulting in close contacts between the perfluorinated-perfluorinated and the nonfluorinated-non-fluorinated parts of the adjacent molecules, Figure 4b. Second, within the units, the contact between the non-fluorinated and fluorinated parts leads to less favorable enthalpic interactions, resulting in an increased mobility of the alkyl chains (Figure 4d). Thus, the increased mobility is a result of and a clear demonstration of incompatibility between the perfluorinated segments and the non-fluorinated ones. The grouping of molecules into units has been previously observed for alkylcyanobiphenyls (mCB, m ) 8, 10, 12).20 Incommensurate packing was used to explain the formation of units, and the packing pattern of the mCB molecules at
the liquid-solid interface was explained as a compromise between the molecule-molecule interactions and the molecule-substrate interactions.20 Incommensurate packing is also proposed here to explain the clustering of F12H11-ISA molecules into units of eight at the liquidsolid interface. The strong incompatibility of perfluorinated and non-fluorinated segments and the weak perfluorinated segments-substrate interactions tend to induce nanosegregation in the monolayer. At the same time, the interaction of non-fluorinated segments with the graphite surface in addition to the hydrogen-bonding network formed between the isophthalic acid groups and solvent molecules tends to induce interdigitation. Hence, the observed packing pattern of F12H11-ISA molecules at the liquid-solid interface is a subtle compromise between these opposing interactions within the monolayer. (20) Foster, J.; Frommer, J. Nature 1988, 333, 542. Frommer, J. Angew. Chem., Int. Ed. Engl. 1992, 31, 1298. Hara, M. RIKEN Rev. 2001, 37, 48.
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Conclusion In this paper, we report on the two-dimensional organization of two semi-fluorinated isophthalic acid derivatives physisorbed at the liquid-solid interface. For F8H10-ISA monolayers, two packing patterns were observed. In the first packing pattern, nanophase separation of the perfluorinated segments of the molecules was observed. The monolayer showed a bilayer structure. In time, this packing changed into a lamellar structure, where the molecules are fully interdigitating. Hence, complete miscibility of the perfluorinated and non-fluorinated segments was observed. Increasing the length of the perfluorinated segment by four carbon atoms and the nonfluorinated segment by one carbon atom (F12H11-ISA) did not result in favoring nanophase separation. Instead, a staggered structure was observed. In this packing pattern, the molecules are grouped into units of eight molecules each. This pattern is interpreted as a com-
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promise between opposing interactions arising from the chemical structure of the molecules, where the perfluorinated-non-fluorinated interactions tend to induce nanophase separation while the non-fluorinated segments and the hydrogen-bonding network of the isophthalic acid groups tend to form an interdigitating lamellar structure. Acknowledgment. The authors thank the DWTC through IUAP-V-03, the FWO (Flemish Ministry of Education), the STWW through the IWT project “Molecular Nanotechnology”, the German Ministry of Education and Research, the Volkswagenstiftung, and ESF SMARTON for financial support. S. De Feyter is a Postdoctoral Researcher of the Fund of Scientific Researchs Flanders. The collaboration was made possible thanks to the TMR project SISITOMAS and a Max-Planck Research Award. Dr. A. Gesquie`re is thanked for fruitful discussions. LA034535S
