Micro-Phase Separation in Binary Mixed Langmuir Monolayers of n

Langmuir 2002, 18, 10183-10190

10183

Micro-Phase Separation in Binary Mixed Langmuir Monolayers of n-Alkyl Fatty Acids and a Perfluoropolyether Derivative Ken-ichi Iimura, Tatsuya Shiraku, and Teiji Kato* Department of Applied Chemistry, Faculty of Engineering, Utsunomiya University, 7-1-2 Yoto, Utsunomiya 321-8585, Japan Received July 15, 2002 Micro-phase separation in binary mixed Langmuir monolayers of cadmium salts of n-alkyl fatty acids (CH3(CH2)n-2COOH; Cn (n ) 18, 20, 22, 24)) and a perfluoropolyether surfactant (F(CF(CF3)CH2O)3CF(CF3)COOH, PFPE) is studied by film balance measurement and atomic force microscopy (AFM). At different temperatures, mixtures of Cn/PFPE in chloroform were spread onto the aqueous Cd2+ subphase and deposited on silicon wafers. AFM images showed that Cn and PFPE separate into microscopic domains of condensed phase and a surrounding matrix of expanded phase, respectively, in their mixed monolayers. The morphological feature of phase-separated structures was characterized by characteristic length (λ) expressing the periodicity of two-phase distribution and fractal dimension (D) of Cn domains reflecting the complexity of domain shape, which were determined through AFM image analyses. It was found that the monolayer morphologies systematically vary with alkyl chain length of Cn and temperature of the water surface; circular-shaped condensed phase microdomains with D of about 1.1 are formed at λ of 7-9 µm when the mixed monolayers are prepared using a shorter alkyl chain fatty acid and/or at a higher temperature, whereas branched narrow domains with D close to 2 are formed at λ of approximately 4 µm when a longer chain fatty acid is used at a lower temperature.

Introduction Lateral structuring in Langmuir monolayers at the airwater interface has been a topic of long-standing interest.1-6 Researchers are motivated in part by scientific profits of knowledge on organized monomolecular structures and their thermodynamic properties in relation to the biological phenomena and functions of cell membranes. Another incentive arises from the potential applications of organic ultrathin films in the fields of molecular electronics and engineering. Since most of the expected functions are based on uniform orientation and arrangement of functionalized molecules in monolayers, it is essential for efficient generation of functions to fabricate extensively uniform and ordered monomolecular films. Recent development of microscopic observation techniques, such as fluorescence microscopy7,8 and Brewster angle microscopy9,10 for monolayers on water surfaces and electron microscopy11,12 and scanning probe microscopy13,14 * To whom correspondence should be addressed. Phone: +81-28-689-6170. Fax: +81-28-689-6179. E-mail: teiji@ cc.utsunomiya-u.ac.jp. (1) Gains, L. G. Insoluble Monolayers at Liquid-Gas Interfaces; Interscience Publishers: New York, 1969. (2) Langmuir-Blodgett Films; Roberts, G. G., Ed.; Plenum Press: New York, 1990. (3) Organic Thin Films and Surfaces: Directions for the Nineties; Ulman, A., Ed.; Academic Press: San Diego, 1995. (4) Seul, M.; Andelman, D. Science 1995, 267, 476-483. (5) Petty, C. M. Langmuir-Blodgett Films, An Introduction; Cambridge University Press: Cambridge, 1996. (6) Birdi, K. S. Self-Assembly Monolayer Structures of Lipids and Macromolecules at Interfaces; Kluwer Academic/Plenum Publishers: New York, 1999. (7) Lo¨sche, M.; Sackmann, E.; Mo¨hwald, H. Ber. Bunsen-Ges. Phys. Chem. 1983, 87, 848-852. (8) McConnell, H. M.; Tamm, K.; Weis, R. M. Proc. Natl. Acad. Sci. U.S.A. 1984, 81, 3249-3253. (9) Ho¨nig, D.; Mo¨bius, D. J. Phys. Chem. 1991, 95, 4590-4592 (10) He´non, S.; Meunier, J. Rev. Sci. Instrum. 1991, 62, 936-939. (11) Fischer, A.; Sackmann, E. J. Phys. 1984, 45, 517-527. (12) Fischer, A.; Sackmann, E. Nature 1985, 313, 299-301.

for monolayers on solid substrates, enables us to directly observe microscopic structures in films. These techniques provide not only a wealth of information useful for elucidation of biological matters and fabrication of homogeneous layered structures but also new insights into the monolayer nature for advanced applications of organized thin films. One of the most noticeable topics derived from newly discovered experimental facts is the formation of various kinds of two-dimensional patterns through selforganization processes such as growth of condensed phase domains in an expanded phase at the phase-transition region during compression in single-component monolayers and micro-phase separation in mixed monolayers.15-21 These patterns are composed of regions differentiated in molecular density, orientation, and chemical species, and thus each of the regions has intrinsic physical and/or chemical properties even on the same monolayer surfaces. Accordingly, the patterns have the possibility to generate functions due to following two points: the spatial regularity of structures and the different properties of regions. This, in turn, brings a novel idea of fabricating functionalized surfaces by taking advantage of patterns with contrasting surface properties. (13) Binnig, G.; Rohrer, H.; Gerber, Ch.; Weibel, E.; Phys. Rev. Lett. 1982, 49, 57-61. (14) Binnig, G.; Quate, F. C.; Gerber, Ch. Phys. Rev. Lett. 1986, 56, 930-933. (15) Weis, M. R.; McConnell, M. H. J. Phys. Chem. 1985, 89, 44534459. (16) Fischer, A.; Sackmann, E. J. Colloid Interface Sci. 1986, 112, 1-14. (17) Heckel, W. M.; Mo¨hwald, H. Ber. Bunsen-Ges. Phys. Chem. 1986, 90, 1159-1167. (18) Lipp, M. M.; Lee, C. Y. K.; Zasadzinski, A. J.; Waring, J. A. Science 1996, 273, 1196-1199. (19) Frommer, J.; Lu¨thi, R.; Meyer, E.; Anselmetti, D.; Dreier, M.; Overney, R.; Gu¨ntherodt, H.-J.; Fujihira, M. Nature 1993, 364, 198198. (20) Duschl, C.; Liley, M.; Corradin, G.; Vogel, H. Biophys. J. 1994, 67, 1229-1237. (21) Fang, J.; Knobler, M. C. Langmuir 1996, 12, 1368-1374.

10.1021/la020643n CCC: $22.00 © 2002 American Chemical Society Published on Web 11/20/2002

10184

Langmuir, Vol. 18, No. 26, 2002

Construction of architectures of supramolecular assemblies or complexes with a desired spatial disposition in ultrathin films has been a big challenge in surface science. Several approaches have been attempted by utilizing lateral structuring in Langmuir and LangmuirBlodgett (LB) monolayers. For instance, Frommer et al. used a phase-separated surface of LB monolayers containing circular domains of a long hydrocarbon-chain amphiphile surrounded by a sea region of a fluorocarbon compound for adsorption of tobacco mosaic virus.19 They found that the virus preferentially adsorbs at points of the highest energy, or at sidewalls of the hydrocarbon domains where the methylene groups are exposed. Highly structured surfaces fabricated by the LB technique can be combined with the chemisorption technique using sulfur-containing or organosilane compounds in order to give wide applications of surface patterns with improved robustness and different functionalities. Duschl et al. reported a combined fabrication route using a mixed Langmuir monolayer of palmitic acid and a thiolipid.20 Upon compression, the monolayer demixes into condensed phase domains predominately containing the palmitic acid and a surrounding expanded phase of the thiolipid. The average size and surface density of domains depend on experimental parameters such as mixing ratio, surface pressure, subphase temperature, and ionic strength. In the monolayer deposited on a gold substrate, the palmitic acid physisorbs but the thiolipid covalently binds to the substrate surface. After removal of palmitic acid domains by a rinse with an organic solvent, newly exposed gold surfaces can be used for chemisorption of pendant-tail thiols and subsequent self-assembly of proteins, metalcomplexes, and so on. Another construction route using phase-separated structures in LB monolayers was reported by Fang and Knobler.21 They showed formation of condensed phase microdomains of n-alkyl organosilane in LB monolayers transferred on mica from a liquidexpanded state of the spread monolayer. In this case, the domain formation is ascribed to dewetting of the water film after monolayer deposition, and the monolayer structure is altered by changing deposition conditions such as transfer speed and surface pressure. The monolayers were then exposed to chemisorption of a fluorinated silane compound on the gaps between the hydrocarbon domains. Inversely, LB monolayers of the fluorinated silane were used as precursors for self-assembly of the alkylsilane. A protein, bovine serum albumin, adsorbed preferentially to methyl-terminated regions of the phase-separated surfaces. As known from the examples mentioned above, control of shape, size, and distribution of molecular assemblies in self-organized surface patterns will lead to the controlled construction of two-dimensional supramolecular architectures. Although there are several monolayer systems which enable one to create patterned surfaces, we take up binary mixed Langmuir monolayers of n-alkyl fatty acids (Cn, n ) 18, 20, 22, 24) and a perfluoropolyether derivative (PFPE) in the present work. Since Cn and PFPE form a fully condensed phase and a largely expanded phase at the air-water interface, respectively, their mixed monolayer is a promising system to produce the so-called twodimensional island-sea structure composed of condensed phase microdomains of Cn and a surrounding region of the expanded phase of PFPE. This type of micro-phase separation has been described in an early publication by Gains1 and actually observed since a pioneer work by Fischer and Sackmann using electron microscopy.16 However, fundamental principles on formation mechanisms and on desired control of structures have not been

Iimura et al.

Figure 1. Film molecules used. (a) n-Alkyl fatty acids (Cn): C18 ) stearic acid (octadecanoic acid), C20 ) arachidic acid (eicosanoic acid), C22 ) behenic acid (docosanoic acid), and C24 ) lignoceric acid (tetracosanoic acid). (b) Perfluoro-2,5,8trimethyl-3,6,9-trioxadodecanoic acid (PFPE).

fully clarified yet. In this work, we provide a clue to elucidation of such unknown principles. Monolayer behavior at the air/water interface was studied by surface pressure (π)-molecular area (A) isotherm measurements, and surface structures were observed using an atomic force microscope. It is found that micro-phase-separated structures change systematically depending on the n-alkyl chain length of the fatty acids and the temperature of the water surface. Considering the relation between experimental parameters and resultant surface structures, the underlying principles controlling the structures are discussed and structure development mechanisms are proposed. Experimental Section Figure 1 shows molecular structures of film materials used in this work. Highly purified n-alkyl fatty acids (Cn) (>99.9%) were obtained from the Research Institute of Biological Materials, Japan. A carboxyl-terminated perfluoropolyether, perfluoro-2,5,8trimethyl-3,6,9-trioxadodecanoic acid (PFPE) (>97%), was purchased from PCR. Spectro-grade chloroform (Dojin Chemicals) was used as a spreading solvent for spreading solutions. All materials were used without further purification. Mixtures of Cn and PFPE were made up into spreading solutions under defined molar ratios to give a total concentration of 2.5 mM. The water used for all experiments was ultrapure water obtained from an Elgastat UHQ-PS system. Monolayers were spread onto a temperature-controlled aqueous subphase surface using a gastight type microsyringe and allowed to stand for 30 min before starting compression at a constant strain rate of 10% min-1. Average molecular areas at spreading were 0.7, 1.5, 1.75, 2.0, 2.25, and 2.5 nm2 molecule-1 at mixing ratios of Cn/PFPE ) 10/0, 8/2, 6/4, 4/6, 2/8, and 0/10, respectively, unless otherwise specified. The subphase was a 0.5 mM aqueous solution of cadmium acetate dihydrate (>98%, Wako Chemicals) adjusted to pH 7.0 with potassium hydrogen carbonate (special grade, Kanto Chemicals). Monolayer deposition was performed by the horizontal scooping-up method in which a hydrophilic solid substrate was supported almost horizontally by thin platinum wires just beneath the water surface and raised at a slow speed of 1 mm min-1 through the monolayer, maintaining the horizontal orientation of the substrate. The solid substrate used was a highly polished silicon wafer having a natural oxide layer (MEMC). Prior to deposition, the substrate was cleaned by the RCA cleaning method and then stored in ultrapure water until use. A microcomputer-controlled Langmuir trough was constructed in our laboratory.22 Two barriers confining monolayers were driven symmetrically under a constant strain rate (i.e., constant time of observation) mode of compression according to a control program. The temperature of the subphase surface was controlled using a large number of integrated Peltier element modules (22) Kato, T.; Tatehana, A.; Suzuki, N.; Iimura, K.; Iriyama, K. Jpn. J. Appl. Phys. 1995, 34, L911-L914.

Micro-Phase Separation in Langmuir Monolayers

Figure 2. π-A isotherms of C20/PFPE mixed monolayers on the 0.5 mM Cd2+ subphase (left) and average molecular area as a function of the molar fraction of PFPE at given surface pressures (right): (a,b) at 5 °C; (c,d) at 20 °C; (e,f) at 30 °C. An inset of (c) shows a π-A isotherm of the C20/PFPE (8/2) mixed monolayer at 20 °C measured by starting compression at the spreading molecular area of 0.6 nm2 molecule-1 for the maximum trough area. attached to the back of a base plate of the trough and was detected precisely by platinum wire resistance temperature sensors of a very small heat capacity. Atomic force microscopy (AFM) observation was carried out under ambient conditions with a Nanoscope III (Digital Instruments) in a tapping mode, using a 125 × 125 µm2 scanning head and a silicon tip on a rectangular-shaped cantilever (129 µm long, spring constant of 37-61 N/m). In all observations, the force applied to the sample surface with the tip was reduced to the smallest possible value at which the surfaces could be imaged steadily and reproducibly. A spatial resolution of 256 × 256 pixels was adopted for usual imaging. However, 30 × 30 µm2 images were obtained under the resolution of 512 × 512 pixels and binarized in order to extract information about surface structures using a digital image analysis software, Win ROOF (Mitani Corp.).

Results Monolayer Behavior at the Air/Water Interface. As an example of π-A isotherms measured for Cn/PFPE mixed monolayers, the isotherms of C20/PFPE mixtures at 5, 20, and 30 °C are shown in Figure 2, parts a, c, and e, respectively. At every temperature, C20 singlecomponent monolayers undergo transition directly from the gaseous/solid coexistent state to the solid state during compression. Brewster angle microscopic observation of the C20 monolayers indicated that bright macroscopic domains of condensed phase were formed in the dark

Langmuir, Vol. 18, No. 26, 2002 10185

gaseous phase just after spreading (images are not shown). The macroscopic domains were, then, forced to fuse by compression, leading to disappearance of the gaseous phase and a simultaneous steep increase of surface pressure at around the cross-sectional area of the n-alkyl chain. Both the fully condensed characteristics in the isotherms and the spontaneous formation of huge condensed phase domains are indicative of a strong attractive interaction among C20 molecules on the water surface. A slight expansion is seen at a low-pressure region in the isotherm at 5 °C. This expansion reflects the strengthened intermolecular interaction between C20 molecules due to lowering temperature, resulting in an increase of the rigidity of the condensed phase domains to prevent the fusion at low surface pressure during compression. C20 exists as its cadmium salt dimer under the present experimental conditions, referring to the previous works on cadmium salt formation of the fatty acids in their Langmuir monolayers.23 The liquid-expanded film region, which is observed in an isotherm for an arachidic acid monolayer at an acidic condition such as pH 3, disappears in the isotherms of Figure 2. This fact also indicates the cadmium salt formation of the film molecules on the subphase surface at the present experimental conditions. In contrast to C20, PFPE takes a considerably expanded state at the air/water interface over the temperature range examined. This can be explained by a strong affinity to water in comparison with a weak attractive intermolecular interaction of PFPE, certainly due to the perfluoropolyether groups in the hydrophobic part of the molecule. A broad break at around 40 mN m-1 corresponds to a gradual collapse of the monolayer. Cadmium salt formation of PFPE was confirmed by X-ray photoelectron spectroscopy. The measurement was made for a film transferred onto a silicon wafer at 0.36 nm2 molecule-1 (after monolayer collapse) at 5 °C, using a Physical Electronics ESCA 5600 with a monochromatic Al KR beam (14 kV, 150 W) at a takeoff angle of 45°. The atomic concentration ratio obtained is F/Cd ) 97.2:2.8 ((0.13), in good agreement with the value expected for Cd(PFPE)2.24 The isotherms of mixed monolayers change systematically with the mixing ratio of two film components. With increasing molar fraction of C20, the isotherm shifts to the lower molecular area side. However, these isotherms do not show the whole behavior of the mixed monolayers because the lower molecular area regions of the isotherms could not be measured due to the instrumental limitations of our Langmuir trough. An inset of Figure 2c presents a π-A isotherm of the C20/PFPE (8/2) monolayer prepared by spreading the mixtures to give a compression-starting molecular area of 0.6 nm2 molecule-1 for the maximum trough area. In the isotherm, an initial surface pressure, increased to about 1.3 mN m-1 by the overspreading of film molecules, was set to zero. One can see a two-step collapse of the mixed monolayer; that is, two collapses appear at surface pressures corresponding to those of single-component monolayers. This can be interpreted as an independent contribution of individual film components to the monolayer property; an expanded region of the isotherm is governed by the PFPE component, and after the collapse of PFPE monolayer, the feature of the C20 component emerges at a lower molecular area.1 (23) Kobayashi, K.; Takaoka, T.; Ochiai, S. Thin Solid Films 1988, 159, 267-273. (24) The atomic density ratio of elements is calculated by taking the ratio of photoelectron intensities divided by sensitivity factors. The sensitivity factors used are 55.509 and 192.323 for F1s and Cd3d5/2 photoelectrons, respectively. For further details of the sensitivity factors, refer to: Handbook of X-ray Photoelectron Spectroscopy; Chastain, J., Ed.; Perkin-Elmer Corp.: Eden Prairie, MN, 1992.

10186


Iimura et al.

Figure 3. 10 × 10 µm2 AFM images of C20/PFPE mixed monolayers at 20 °C: C20/PFPE ) (a) 2/8, (b) 4/6, (c) 6/4, (d) 8/2. The monolayers were transferred on Si wafers at 0.7 nm2 molecule-1.

Average molecular areas experimentally determined at different surface pressures are plotted against the molar fraction of PFPE in the mixture in Figure 2, parts b, d, and e at 5, 20, and 30 °C, respectively. The average molecular areas become a linear function of composition when two components are ideally miscible or completely immiscible in mixed monolayers.1 Straight lines drawn in the figures express the compositional dependence of mean molecular area, A12, following the additivity rule,

A12 ) (1 - x2)A1 + x2A2

(1)

where x2 is the molar fraction of PFPE in the mixtures, and A1 and A2 are the molecular areas of C20 and PFPE in single-component monolayers at given surface pressures, respectively. It is evident that the experimental data points lie almost completely on the straight lines. These results together with the two-step character of the isotherm lead to the conclusion that C20 and PFPE are almost completely immiscible in the mixed monolayers in the whole compression range. π-A isotherms were measured for all other combinations of Cn/PFPE at mixing molar ratios of 10/0, 8/2, 6/4, 4/6, 2/8, and 0/10 at 5, 20, and 30 °C. Although the slight expansion of the isotherm, like that seen for the C20 monolayer at 5 °C shown in Figure 2a, was observed for single-component monolayers of C22 at 5 and 20 °C and of C24 at all temperatures examined, the behavior of mixed monolayers of the combinations was essentially the same as that of C20/PFPE monolayers. Irrespective of temperature, the Cns showed only a solid film region in the isotherm as the stable two-dimensional phase, and the isotherms of Cn/PFPE mixtures revealed regular shifts with change in the mixing ratio, resulting in a linear correlation between the average molecular area at constant surface pressure and the composition of the mixed monolayer. Surface Morphology of Mixed Monolayers Observed by AFM. In the present monolayer system, the experimental parameters that would affect the monolayer morphology are mixing ratio, temperature of the water surface, n-alkyl chain length of Cn, and molecular area (surface pressure). Hereafter, we describe some experimental results obtained under one variable with the other parameters fixed, to obtain an entire insight into phase separation processes in our mixed monolayers. Mixing ratio dependency of the surface morphology is examined for C20/PFPE mixed monolayers transferred at 0.7 nm2 molecule-1 at 20 °C. AFM images in Figure 3 demonstrate that the two components are in a microphase-separated state. Apparently, surface coverage with islandlike domains increases with an increase in the molar fraction of C20, suggesting that brighter domains and a surrounding continuous region can be attributed to the condensed phase of C20 and the expanded phase of PFPE,

respectively. The cross sections of the images indicated that the C20 domains are 1.5 ( 0.1 nm higher than the surrounding PFPE monolayer. According to Sugi et al., the bilayer long spacing, d nm, in LB films of cadmium salts of n-alkyl fatty acids increases with the number of carbon atoms, n, as d ) 0.53 + 0.25n.25 Assuming the thickness of the C20 domain to be half of the bilayer long spacing, the thickness of the surrounding PFPE phase is calculated to be about 1.3 nm. Dependency of the morphology on water surface temperature and chain length of Cn was examined for Cn/ PFPE (2/8) mixed monolayers transferred at 0.7 nm2 molecule-1. AFM images are summarized in Figure 4. It turns out that the phase-separated structures strongly depend on the two experimental parameters. Although the C18/PFPE (2/8) mixture forms circular-shaped condensed phase domains irrespective of water surface temperature, irregular-shaped domains are formed in the C20/PFPE (2/8) monolayer at 5 °C. This shape deformation of condensed phase domains is already seen at 20 °C in the C22/PFPE (2/8) monolayer, and temperature lowering leads to the formation of elongated narrow domains of C22. For the C24/PFPE mixed monolayer, circular domains are no longer observed even at 30 °C, and only irregular-shaped or branched elongated domains are observed here. Evidently, the linearly developed narrow domains are formed for longer chain Cns and/or at lower water surface temperatures while the round islands preferentially appear for shorter chain Cns and/or at higher temperatures. The irregular-shaped domains are seen in the intermediate conditions. It is also found that the spatial frequency of the domains varies with the change in the domain shape. If a straight line is drawn from one end to the other on the image, it crosses more times the condensed phase domains with branched shape than those with circular shape. Structural Regimes of Cn/PFPE (2/8) Mixed Monolayers. The above-mentioned findings lead to an idea that the monolayer morphologies can be characterized by some quantities which reflect the distribution of phases and the shape of condensed phase domains. In this work, we adopt characteristic length, λ, and fractal dimension, D, determined from binarized 30 × 30 µm2 AFM images of Cn/PFPE (2/8) mixed monolayers, for characterization of surface structures. The λ is defined as the sum of the average length of white segments (Cn phase domains) and that of black segments (PFPE phase regions) on vertical and horizontal scanning lines (512 × 512 lines) of the binarized images. The D is determined by applying the following definition:26,27 (25) Matsuda, A.; Sugi, M.; Fukui, T.; Iizima, S.; Miyahara, M.; Otsubo, Y. J. Appl. Phys. 1977, 48, 771-774. (26) Lovejoy, S. Science 1982, 216, 185-187. (27) Wang, Z.-Y.; Konno, M.; Saito, S. J. Chem. Eng. Jpn. 1991, 24, 256-258.


Langmuir, Vol. 18, No. 26, 2002 10187

Figure 4. 10 × 10 µm2 AFM images of Cn/PFPE (2/8) (n ) 18, 20, 22, 24) mixed monolayers at 5, 10, 20, and 30 °C. The monolayers were transferred on Si wafers at 0.7 nm2 molecule-1.

Figure 5. Double-logarithmic plots of area and perimeter of condensed phase domains for determination of fractal dimension, D: (a) C18/PFPE (2/8) monolayer at 20 °C; (b) C24/PFPE (2/8) monolayer at 20 °C.

S1/2 ∝ P1/D

(2)

where S and P are the area and perimeter of condensed phase domains, respectively. Examples of double-logarithmic plots of S versus P are shown in Figure 5 for (a) the C18/PFPE (2/8) monolayer at 20 °C and for (b) the C24/PFPE (2/8) monolayer at 20 °C. It is found that a linear relation between log S and log P is well established for both monolayers in a wide range of the plots. As shown below, the fractal dimension takes a value close to unity for the round domains with smooth boundaries, whereas it increases with an increase of the perimeter for elongated domains of the same areas, or complication of domain shape.

The λ and D are plotted as a function of water surface temperature for each mixing combination in Figure 6, parts A and B, respectively. Each of the data in the figures is the average of values obtained from at least 10 macroscopically separate areas on at least two samples. The plots imply that the two indices reflect well the structural variation of the mixed monolayers depending on the chain length and temperature. We can divide the structural evolution into three regimes according to changes of λ and D values. The three regimes appear in the plots for C22/PFPE (2/8) monolayers and are designated in the figures for clarity. The regime boundary temperatures, T1 and T2, are experimentally determined by taking the intersection of two approximated lines in the λ versus T plot and in the D versus T plot, respectively, as drawn in the figures. The boundary temperatures obtained for C22/PFPE (2/8) monolayers are T1 ) 13.4 °C and T2 ) 21.3 °C. The temperature range below T1 is referred to as regime I, where the λ takes values as small as about 4 µm and the D keeps the largest value close to 2, reflecting the characteristic surface structure composed of branched narrow domains of condensed phase distributed with a high spatial frequency. A temperature region above T2 is defined as regime III. The monolayer structure in this regime is characterized by λ > 7 µm and D ∼ 1, that is, a low population of round domains with smooth boundaries. Regime II corresponds to a transition region between regimes III and I. Shortening the chain length of Cn leads to a shift of the regimes to the low-temperature side. For C20/PFPE (2/8) mixed monolayers, regime I moves to a temperature range below the range studied,

10188


Iimura et al.

Figure 7. 10 × 10 µm2 AFM images of C22/PFPE (2/8) mixed monolayers deposited (a) after heating from 5 to 30 °C and (b) after cooling from 30 to 5 °C at rates of (1.0 °C min-1 at 0.7 nm2 molecule-1.

Figure 6. (A) Characteristic length, λ, and (B) fractal dimension, D, for Cn/PFPE (2/8) mixed monolayers as a function of water surface temperature: Cn ) (a) C18, (b) C20, (c) C22, and (d) C24. Each of the data is the average of values obtained from at least 10 AFM images (30 × 30 µm2) at macroscopically separate areas on at least two samples. The monolayer deposition area was 0.7 nm2 molecule-1. In the plots for the C22/PFPE (2/8) mixed monolayer, structural regimes are designated for clarity.

and only regimes II and III emerge. In this case, the T2 is determined to be 9.0 °C undoubtedly from the D versus T plot, but the T1 is expected to be 0.6 °C on the supposition of a straight line λ ) 4 in the λ versus T plot. The differences of each boundary temperature between C20/PFPE and C22/PFPE monolayers are ∆T1 ) 12.8 °C and ∆T2 ) 12.3 °C, which are almost consistent with each other. C18/ PFPE (2/8) mixed monolayers show only regime III, indicating that the condensed phase domains have only a circular shape. On the other hand, an increase of the alkyl chain length of Cn shifts the boundary temperatures to the higher side. The T1 and T2 for C24/PFPE (2/8) monolayers are 25.0 and 33.7 °C, respectively. Again, we observe a regular shift of the regimes by about 12 °C with a change in two methylene units in the alkyl chain, ensuring the existence of a regular progression in the structure evolution as a function of alkyl chain length and temperature. Discussion The equilibrium size and shape of two-dimensional domains are determined by competition between the line tension at the two-phase boundary and long-range electrostatic repulsive forces within the domains.28,29 If the line tension is high enough, the domains would take a (28) McConnel, H. M.; Moy, V. T. J. Phys. Chem. 1988, 92, 45204525. (29) McConnel, H. M.; De Koker, R. J. J. Phys. Chem. 1992, 96, 71017103.

circular shape in order to reduce their perimeter since there is a large energetic cost on increasing the boundary length. A relative decrease of the line tension with respect to the electrostatic forces allows an increase of the domain boundary/area ratio, leading to an elongation of the domains or formation of stripe-patterned domain structures. Such characteristic morphologies have been observed in mixed monolayers containing a line tension lowering material, such as cholesterol15,17 and a amphiphilic protein, SP-B,18 using fluorescence microscopy. The observations have clarified that the width of elongated domains is influenced by experimental parameters such as molecular area, the concentration of line tension lowering material in the mixtures, pH and ionic strength of the subphase, and temperature. A noteworthy finding is the effect of temperature observed for a L-R-dimyristoylphosphatidic acid (DMPA) monolayer containing 2 mol % cholesterol and 1 mol % fluorescent dye amphiphile.17 In this monolayer system, a lowering of water temperature at a fixed molecular area causes a reversible deformation of domain texture; initially circular condensed phase domains of highly charged DMPA change to stripes via a banana shape with decreasing temperature, accompanying a continuous change of the domain width from about 22 to 2 µm. This elongation of domains is ascribed to a relative increase in electrostatic repulsive force due to enhanced alignment of polar DMPA molecules at low temperatures. We have also observed an elongation of Cn domains at lower temperatures in our mixed monolayers, and this phenomenon itself is very similar to a change induced by increased electrostatic repulsion. However, no similarity in formation mechanism exists as evidenced by the observations for C22/PFPE (2/8) monolayers shown in Figure 7. The monolayer in Figure 7a was spread at 5 °C and heated to 30 °C on the water surface at a heating rate of 1 °C min-1 after compression up to 0.70 nm2 molecule-1, and then it was deposited on a silicon wafer. On the contrary, the monolayer of Figure 7b was spread at 30 °C and deposited at 5 °C. AFM images reveal that heating changes originally elongated domains to round ones, but originally circular domains are never elongated or branched by cooling. This observation demonstrates that decrease of water surface temperature does not produce enough electrostatic repulsive forces for elongation of the condensed phase domains, and hence the domain shapes are predominantly governed by the line tension in our mixed monolayers. The relatively small contribution of electrostatic forces should be caused by charge neutralization of the carboxyl headgroups of the film molecules through cadmium salt formation. The line tension arises from a difference in the intermolecular interactions between two adjacent phases. Since Cn and PFPE form a fully condensed and a largely expanded phase, respectively, irrespective of water surface


Figure 8. Schematic diagram showing the evolution of phaseseparated structure in the binary mixed Langmuir monolayer of Cn/PFPE. In the left side cartoons of monolayer surfaces, white and black regions indicate Cn and PFPE phases, respectively. Linearly developed Cn phase domains formed just after monolayer spreading should transform into circular ones in order to minimize the line energy. However, the structural transformation can be stopped by reduced mobility of Cn molecules in their domains; after complete solvent evaporation, the surface structures are frozen at a certain stage of the deformation process (regime I to regime III). The mobility of Cn molecules increases with decreasing chain length of Cn and increasing temperature of the water surface. The transformation process is irreversible; once formed, circular domains are no longer deformed into the elongated ones.

temperature, there should be an essentially high line tension at the two-phase boundaries, resulting in formation of circular domains of Cn. Furthermore, at constant temperature, the line tension is expected to increase with the chain length of Cn, so that round domains should be formed preferentially in mixed monolayers containing a longer chain Cn as one film component. However, it is apparent that the surface morphology observed in the present work does not always follow this theoretical expectation. It seems reasonable to assume that Cn molecules are sufficiently mobile only when they are fully solvated by the spreading solvent, and the mobility of Cn molecules on the water surface is drastically reduced by complete evaporation of the solvent and simultaneous incorporation of the film molecules into condensed phase domains. If Cn molecules still have a sufficient mobility in the condensed phase domains, the domains should take a circular shape to minimize the perimeters. However, under the conditions where molecular motions are extremely suppressed, the domain structure would be quenched immediately after complete solvent evaporation. The linearly developed condensed phase domains observed in regime I can be regarded as structures formed by such a quenching process and are apparently nonequilibrium ones. Indeed, as shown in Figure 7, the elongated domains can be transformed into the circular shape when a certain degree of mobility is introduced by heating, whereas the circular domains are not deformed into the elongated shape by cooling. The most plausible mechanism of structural growth in our mixed monolayers is schematically shown in Figure 8. At an early stage of the phase separation, a characteristic structure consisting of linearly developed branched Cn phase domains surrounded by an expanded phase of PFPE is formed. If Cn molecules are sufficiently mobile at this stage, the elongated domains would become circular or be

Langmuir, Vol. 18, No. 26, 2002 10189

broken up into circular pieces due to the line tension, a two-dimensional version of the capillary instability. However, when the mobility is not sufficient, the domain structure would be frozen at a certain stage during the transformation process because of complete solvent evaporation. The mobility of Cn molecules is largely dependent upon the alkyl chain length and the temperature of the water surface and increases in the order of regime I to regime III. Although it seems reliable that the phase-separated structures develop from the elongated domains, the genetic mechanism for the phase separation still remains open to question. There are two possible mechanisms for the phase separation in general, the nucleation and growth (NG) and the spinodal decomposition (SD) mechanism.30 By whichever mechanism the phase separation takes place, there exists a possibility for elongated domains to emerge with completely immiscible components. If the phase separation occurs through the NG mechanism, the only way for elongated condensed phase domains to be formed is that the phase separation starts with the first exclusion of the PFPE component from the spreading solution during the spreading process. When PFPE molecules are excluded, they should start to expand while pushing the remaining solution, like a two-dimensional bubble growing, resulting in elongated branching development of Cn domains after solvent evaporation. On the other hand, it is also true that the characteristic structure in regime I is very similar in appearance to the structure expected to be formed by the SD mechanism. As is well-known in the field of two-component polymer systems or two-component metal alloys, the SD mechanism produces very characteristic periodic morphology, the so-called bicontinuous structure or percolating network structure, by temperature jump31,32 or evaporation of cosolvent.33,34 Nakai et al. investigated the phase separation and subsequent structure deformation process of binary polymer mixtures occurring in films as thin as about 10 µm.31,32 When the mixture was exposed to a temperature jump, it separated into two phases according to the SD. They found that the phase separation progresses through the following stages in time: (i) a self-similar growth of the percolating network of two phases, (ii) disruption of the network and shrinkage of the disrupted fragments of one phase into droplets, and (iii) diffusion and coalescence of the droplets. This structure evolution also quite resembles that seen in this work. If the SD works in our mixed spread monolayer systems, it also should take place in a very thin layer of spreading solution during rapid evaporation of the solvent. The Cn component and the PFPE component separate into a two-dimensional bicontinuous structure, and then PFPE expands and Cn shrinks to form the structure in regime I. Conclusions We found systematic variation of micro-phase-separated structures in binary mixed Langmuir monolayers composed of Cn and PFPE, depending on chain length and water surface temperature. Monolayer morphologies are evaluated by using characteristic length, λ, and fractal (30) Strobl, R. G. The Physics of Polymers; Springer: Berlin, 1996. (31) Nakai, A.; Shiwaku, T.; Wang, W.; Hasegawa, H.; Hashimoto, T. Macromolecules 1996, 29, 5990-6001. (32) Nakai, A.; Wang, W.; Ogasawara, S.; Hasegawa, H.; Hashimoto, T. Macromolecules 1998, 31, 5391-5398. (33) Inoue, T.; Ougizawa, T.; Yasuda, O.; Miyasaka, K. Macromolecules 1985, 18, 57-63. (34) Miyake, Y.; Sekiguchi, Y.; Kohjiya, S. J. Chem. Eng. Jpn. 1993, 26, 543-550.

10190


dimension, D, determined from AFM image analyses. According to the change in the two indices, the monolayer structures can be divided into three regimes. Although we cannot determine yet what mechanism works during the spreading of mixed monolayers of two completely immiscible components, it becomes clear that Cn domain structures develop from branched narrow lines with short repeat distances (regime I) to round shapes (regime III) through the transition stage (regime II). The degree of structural development is determined by the mobility of Cn molecules on the water surface, related to the chain length of Cn and the water surface temperature. The elongated structure of condensed phase domains formed in regime I is regarded as a nonequilibrium one frozen at an initial stage of structural development processes due to the reduced mobility of Cn molecules after complete evaporation of the spreading solvent. On the contrary, the increased mobility in regime III allows formation of equilibrium circular domains. We have previously reported a micro-phase separation in binary mixed Langmuir monolayers of combinations of a fluorinated silazane derivative, which forms an expanded monolayer, and longchain trichlorosilanes, which form condensed monolayers.35 Though the trichlorosilanes are specific compounds since they can two-dimensionally polymerize with each (35) Iimura, K.; Kato, T. Mol. Cryst. Liq. Cryst. 1998, 322, 117-122.

Iimura et al.

other via hydrolysis of the trichlorosilyl headgroups on the water surface, the structure evolution in micro-phase separation is basically almost the same as that seen in this paper. This indicates that the phase-separation process mentioned above should be common for binary mixed monolayer systems of fully condensed and expanded components, regardless of chemical species. From the viewpoint of monolayer applications, the advantage of two-dimensional phase separation in mixed Langmuir monolayers is its potential to produce surfaces patterned with a spatial distribution of film materials and thus of chemical and/or physical properties. Our work implies how the phase-separated patterns, made of condensed phase islands and an expanded phase sea region, can be controlled by manipulating experimental parameters. Acknowledgment. The authors thank Professor N. Suzuki (Utsunomiya University, Japan) and Professor H. Mo¨hwald (Max-Planck Institute of Colloids and Interfaces, Germany) for helpful discussions. A part of this research was supported by the Satellite Venture Business Laboratory at Utsunomiya University and by a Grant-in-Aid for Scientific Research (12555240) in 2000-2002 from the Ministry of Education, Culture, Sports, Science, and Technology. LA020643N

Micro-Phase Separation in Binary Mixed Langmuir Monolayers of n

Recommend Documents