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Langmuir 1998, 14, 1786-1798
Monodisperse Two-Dimensional Nanometer Size Clusters of Partially Fluorinated Long-Chain Acids Teiji Kato,* Makoto Kameyama, Masayoshi Ehara, and Ken-ichi Iimura Department of Applied Chemistry, Faculty of Engineering, Utsunomiya University, Ishii 2753, Utsunomiya, 321-8585 Japan Received August 25, 1997. In Final Form: December 29, 1997 Two-dimensional molecular clusters of a few tens nanometer size were found in spread monolayers of a series of partially fluorinated long-chain acids. Atomic force microscopy images have revealed that the size of the clusters is sharply monodisperse. The size changes systematically with changing structure of the hydrophobic chain of the amphiphiles. The smallest cluster has a circular shape of 17 nm diameter. One cluster is composed of about 700 film molecules. These clusters gather to form macroscopic domains of millimeter size without compression. A formation mechanism of these molecular clusters is discussed. Clusters are formed during the spreading process due to the instability of the film materials at the spreading process. It was made clear that cluster formation during the spreading is rather general for amphiphiles under the conditions where condensed monolayers are formed.
Introduction Insoluble monolayers at the water surface are the most basic existence of amphiphilic molecules. Many investigators have studied properties and structures of them since the early works of Harkins and Langmuir.1 Recently, insoluble monolayers are studied extensively as the precursors of Langmuir-Blodgett (LB) films. Many kinds of advanced instruments have been applied to investigate the structure of insoluble monolayers in situ or ex situ. These include glancing-angle X-ray diffraction using synchrotron orbital radiation, Brewster angle microscope, in situ application of Fourier-transform infrared (FT-IR) and Raman spectroscopy, second harmonic generation, and ex situ application of transmission electron microscopy (TEM), scanning electron microscopy (SEM), scanning tunneling microscopy (STM), and atomic force microscopy (AFM). The last one is especially attractive to investigate surface structures of monolayers up to molecular resolution after being transferred onto smooth solid substrates because many of the insoluble monolayers are not conductive. Recently, we have found that there are two-dimensional molecular clusters of nanometer size in spread monolayers of partially fluorinated long-chain acids using AFM.2 The size of the smallest one is about 17 nm in diameter and the size distribution is sharply monodisperse. Existence of surface micelles in insoluble monolayers has repeatedly discussed since the first idea was proposed by Langmuir in the 1930s.3-5 However, no one has established the existence of surface micelles in insoluble monolayers since then because of experimental difficulties. Mann and his collaborators have reported formation of * To whom correspondence should be addressed: Phone +8128-689-6170; fax, +81-28-689-2883; e-mail,
[email protected]. (1) Gains, G. L., Jr., Insoluble Monolayers at Liquid-Gas Interfaces; John Wiley & Sons: New York, 1966. (2) Kato, T.; Kameyama, M.; Kawano, M. Thin Solid Films 1996, 273, 232-235. (3) Birdi, K. S. Lipid and Biopolymer Monolayers at Liquid Interfaces; Plenum: New York and London, 1989; section 4.7.1. (4) Yue, B. Y.; Jackson, C. M.; Taylor, J. A. G.; Mingins, J.; Perthica, B. A. J. Chem. Soc., Faraday Trans. 1 1976, 72, 2685-2693; 1982, 78, 323-339. (5) Albrecht, O.; Gruler, H.; Sackmann, E. J. Phy. (Paris) 1978, 39, 301-313.
surfactant hemimicelles adsorbed on the hydrophobic solid surfaces in aqueous solutions observed by AFM.6,7 Ducker and his collaborators have also reported hemicylindrical sodium dodecyl sulfate (SDS) micelles adsorbed onto a graphite surface from aqueous solutions, observed with AFM.8,9 These micelles on the solid surfaces are adsorbed in equilibrium with aqueous solutions. Eisenberg and his group have reported surface micelle formation at the air/water interface from nonionic diblock copolymers. They investigate aggregation numbers of surface micelles by analyzing TEM observed images.10-14 Chi and her collaborators have found that stearic acid monolayers exhibit a phase transition from an expanded to a condensed state on the subphase containing polycations. They observed that many micrograins of 0.030.05 µm and macrograins of a few micrometers are coexistent at the transition region.15,16 All of these surface micelles are discussed as equilibrium structures. However, there is another possibility that these surface micelle structures are formed by instability of the amphiphilic molecules when they are spread on the water surface from organic solutions. This paper presents structural studies of two-dimensional molecular clusters of nanometer size in monolayers of a series of partially fluorinated long-chain acids and related amphiphiles observed with AFM. We discuss the (6) Aksay, I. A.; Trau, M.; Manne, S.; Honma, I.; Yao, N.; Zhou, L.; Fenter, P.; Eisenberg, P. M.; Gruner, S. M. Science 1996, 273, 892-898. (7) (a) Manne, S.; Cleveland, J. P.; Gaub, H. E.; Stucky, G. D.; Hansma, P. K. Langmuir 1994, 10, 4409-4413. (b) Jaschke, M.; Butt, H.-J.; Gaub, H. E.; Manne, S. Langmuir 1997, 13, 1381-1384. (8) Wanless, E. J.; Ducker, W. A. J. Phys. Chem. 1996, 100, 32073214. (9) Ducker, W. A.; Wanless, E. J. Langmuir 1996, 12, 5915-5920. (10) (a) Zhu, J.; Lennox, R. B.; Eisenberg, A. Langmuir 1991, 7, 15791584. (b) Li, S.; Hanley, S.; Valshney, S. K.; Eisenberg, A.; Lennox, R. B. Langmuir 1993, 9, 2243-2246. (11) Zhu, J.; Eisenberg, A.; Lennox, R. B. J. Am. Chem. Soc. 1991, 113, 5583-5588. (12) Zhu, J.; Eisenberg, A.; Lennox, R. B. Macromolecules 1992, 25, 6547-6555, 6556-6562. (13) Zhu, J.; Lennox, R. B.; Eisenberg, A. J. Phys. Chem. 1992, 96, 4727-4730. (14) Zhu, J.; Hanley, S.; Eisenberg, S.; Lennox, R. B. Makromol. Chem., Makromol. Symp. 1992, 53, 211-220. (15) Chi, L. F.; Anders, M.; Fuchs, H.; Johnston, R. R.; Ringsdorf, H. Science 1993, 259, 213-216. (16) Chi, L. F.; Anders, M.; Fuchs, H.; Johnston, R. R.; Ringsdorf, H. Thin Solid Films 1994, 242, 151-156.
S0743-7463(97)00951-7 CCC: $15.00 © 1998 American Chemical Society Published on Web 03/13/1998
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Table 1. Materials materials (chemical formula) 1 heptadecafluorononadecanoic acid (CF3(CF2)7(CH2)10COOH) 2 heptadecafluoropentacosanoic acid (CF3(CF2)7(CH2)16COOH) 3 heptadecafluorotriacontanoic acid (CF3(CF2)7(CH2)22COOH) 4 nonafluoroheptacosanoic acid (CF3(CF2)3(CH2)22COOH) 5 tridecafluorononacosanoic acid (CF3(CF2)5(CH2)22COOH) 6 perfluoroundecanoic acid (CF3(CF2)9COOH)
abbreviation C19F17 C25F17 C31F17 C27F9 C29F13 C11F21
plausible formation mechanisms of these clusters during the spreading process of monolayers. Materials and Methods Film materials used in this study are summarized in Table 1. Partially fluorinated long-chain acids were purchased from Wako Chemical Co. They were used without further purification. They are abbreviated as C19F17, C25F17, and so on, based on the total carbon numbers and total fluorine numbers. Materials 1, 2, and 3 compose the material series 1 where the total fluorine number is the same while the total carbon number increases systematically. Materials 4, 5, and 3 compose series 2 where the total methylene group number is the same while the total fluorine number increases systematically. Perfluoroundecanoic acid was purchased from Daikin Chemical Co. and was used as a diluent in monolayers of partially fluorinated long-chain acids. Stearic acid and dipalmitoylphosphatidic acid (DPPA) were purchased from Nippon Fat and Oil Co. Guaranteed purity was 99.9%. Spectrograde chloroform (Dojin Chemicals) was used as a spreading solvent for the partially fluorinated long-chain acids. A mixed solvent of spectrograde chloroform and methanol (9:1 in volume) was used for preparing the spreading solution of DPPA. Spectrograde hexane was used as the spreading solvent for the stearic acid. Concentration of the spreading solutions was around 0.4 × 10-6 mol/mL. Ultrapure water from the Elgastadt system was used for the subphase. pH of the subphase was adjusted by adding an aqueous solution of guaranteed grade potassium hydroxide. To form metal salts of long-chain acids, cadmium acetate and lanthanum acetate of guaranteed grade were used to prepare the subphase containing metal ions. Details of the instrument for the measurements of π-A isotherms and for preparing the samples of AFM observation are published elsewhere,17 but are explained here briefly. A very shallow Langmuir trough with inner size of 40 cm length, 16 cm width, and 2 mm depth, constructed with PTFE rims and thin platinum base-sheet on a copper base plate of 6 mm thickness was developed. Eight units of integrated Peltier elements (thermomodule) were attached to the back of the copper base plate. A microcomputer controls the direction and amount of dc current to keep constant or to change the temperature of the water surface in the trough as functions of time. Two barriers which confine monolayers are controlled symmetrically by another microcomputer to keep surface pressure constant or to compress monolayers under the all compression modes including constant strain rates. The physical meaning and the importance of the compression of monolayers under the constant strain rates for measurement of π-A isotherms are reported elsewhere.18 A Pyrex-made glass plate for the Wilhelmy-type surface balance and a platinum-wire resistance temperature sensor of very small heat capacity sealed in a piece of nickel-plated copper plate were set at the center of the trough. A sensitive stainless steel spring and an eddy current displacement sensor constitute the surface (17) Kato, T.; Tatehana, A.; Suzuki, N.; Iimura, K.; Araki, T.; Iriyama, K. Jpn. J. Appl. Phys. 1995, 34, L911-914. (18) (a) Kato, T. Langmuir 1990, 6, 870-872. (b) Kato, T.; Hirobe, Y.; Kato, M. Langmuir 1991, 7, 2208-2212.
Figure 1. pH-dependency of π-A isotherms of C19F17 monolayers on the subphase containing Cd2+ ions at the concentration of 5.0 × 10-4 mol/L at 15 °C. balance. Surface pressure data and temperature data are taken into the computer through an analog-to-digital converter. Compression of monolayers started about 60 min after spreading to ensure complete evaporation of the spreading solvents. Single-layer LB films were transferred onto the cover glass of an optical microscope with “the horizontal scooping up method”, which was devised to transfer monolayers without accompanying flow of them during the transfer process.19 In the horizontal scooping up method, a solid substrate of completely hydrophilic surface is supported almost horizontally with a thin platinum wire support just beneath the water surface. Then, it is raised at a slow speed (1 mm/min) through a monolayer, keeping horizontal set of the substrate, after the conditions of the monolayer are set. Surface roughness of the cover glass used was less than 0.3 nm, which is smooth enough for the substrate of AFM observation. Almost all of the AFM images were obtained with a Nanoscope III (Digital Instrument Co.) by the tapping mode using etched silicon cantilevers. However, the contact mode was also used to obtain images of molecular resolution using cantilevers of silicon nitride. Brewster angle microscope (BAM) used to observe macroscopic structure of monolayers on the water surface was constructed in our laboratory. It is composed of a 10 mW He-Ne laser, a GranThompson polarizer, an analyzer, a zooming microscope of long working distance (Seiwa Optical Industry Co.), and a CCD camera of very high sensitivity (Flobel Optics Co., HCC-600M, 0.01 lux at the highest sensitivity). Surface compressibility of monolayers (κs) was calculated with numerical differentiation of the isotherm data (data number was about a few thousands) using a linear regression analysis.
Experimental Results Figure 1 shows the pH dependency of π-A isotherms of C19F17 monolayers on the subphase containing cadmium acetate at the concentration of 5.0 × 10-4 mol/L. All isotherms were measured under the constant strain rate of 5.0%/min at 15 °C. The molecular areas of the condensed region shift to smaller values, and the collapse pressures increase with increasing pH of the subphase. This is due to the formation of the cadmium salt of the long-chain acid with increasing pH. π-A isotherms at pH 6, 7, and (19) Kato, T.; Matsumoto, N.; Kawano, M.; Suzuki, N.; Araki, T.; Iriyama, K. Thin Solid Films 1994, 242, 223-228.
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Figure 2. π-A isotherms and κs-A isotherms of C19F17, C25F17, and C31F17 monolayers at pH 7. Other experimental conditions are the same as those in Figure 1. Arrows A to I on the isotherms mark the points where monolayers were transferred onto cover glass for AFM observation.
8 coincide well with each other. This means that almost all amphiphilic molecules form cadmium salt in the monolayer at higher pH above 6. All of other partially fluorinated long-chain acids in Table 1 also show almost the same dependency of isotherms of monolayers on pH by forming cadmium salts. Figure 2 summarizes π-A isotherms and κs-A isotherms of (A) C19F17, (B) C25F17, and (C) C31F17 monolayers at pH 7 under the same conditions as those in Figure 1, respectively. Here, κs is the surface compressibility, calculated from numerical differentiation of the isotherms. The π-A isotherm of C19F17 exhibits two kinks around 10 and 30 mN/m, and this reflects to the three-minima shape of the κs-A isotherm. The minimum κs value of the C19F17 monolayer around the molecular area of 0.33 nm2, 4.5 × 10-3 m/mN, is larger than those of monolayers of long-chain hydrocarbonic acids such as stearic acid or arachidic acid under the same conditions. The minimum surface compressibility of arachidic acid monolayers on the subphase containing cadmium ions at pH 7 is less than 1 × 10-3 m/mN. This is the reflection of difference in molecular packing between the partially fluorinated long alkyl group and the long hydrocarbonic alkyl group in monolayers at higher surface pressures. The π-A isotherm of C25F17 shows one kink around 30 mN/m and the κs-A isotherm has two minima corresponding to the two condensed regions before and after the kink. The minimum compressibility around the molecular area of 0.34 nm2 is 3.9 × 10-3 m/mN, which is also larger than that of arachidic acid monolayers. In the case of C31F17, the π-A isotherm shows no clear kink and the κs-A isotherm exhibits only one minimum around the molecular area of 0.35 nm2. The minimum surface compressibility, 4.3 × 10-3 m/mN is much larger than that of arachidic acid. One-layer LB films of C19F17, C25F17, and C31F17 were transferred from monolayers on the water surface onto solid substrates with the scooping up method at the points indicated by arrows A to I on the isotherms of parts A-C in Figure 2, respectively, at the points just before the pressure rising, at 20 and at 35 mN/m. AFM images were obtained with the tapping mode using etched silicon cantilevers to avoid damage of the surface by reducing force between the top of cantilever and the surface to the order of few nanometer. Parts A-C of Figure 3 exhibit bird’s eye view images of AFM showing surface structure of the one-layer LB films of C19F17 transferred at points A, B, and C, respectively. Sizes of these images are 500 × 500 nm2 in lateral and
20 nm full scale in vertical. The most striking features of these images are two-dimensional molecular clusters of less than 20 nm diameter and the uniformity of the size of them. At point A before detection of the surface pressure rising, there is a gap among the clusters and the surface of the solid substrate is still observed. At point B, the clusters contact each other with compression and there is no gap observed among them. At point C at 35 mN/m, density of the clusters increases and the shape of the clusters is deformed from circular to hexagonal by the higher lateral pressure. The array of clusters themselves is also hexagonal and is shown in detail later. Parts D-F and G-I of Figure 3 are bird’s eye view images of the surface structure of C25F17 and C31F17 monolayers, respectively. In these cases also, we can see molecular clusters of uniform sizes, but the size and the shape are different from those of C19F17; average size of C25F17 is about 33 nm and that of C31F17 is around 70 nm, and the shape of clusters changes from the circular (C19F17) to the irregular one (C31F19) with increasing chain length. Figure 4A demonstrates an extended AFM image of the contact mode of the same sample of C19F17 monolayer in Figure 3C. Sizes of this image are 50 × 50 nm2 in lateral and 3 nm full scale in vertical. Figure 4B is a cross section profile along a line in Figure 4A. The cross section profile reveals that the diameter of the cluster is about 17 nm in average and the height difference between top of the clusters and the bottom at the valley among the clusters is about 1 nm. From these figures, we can estimate the cross section of the clusters as a very flat half oval shape. From the average diameter of the clusters, we can also estimate that one cluster is composed of about 700 film molecules. Parts A and B of Figure 5 show π-A isotherms and κs-A isotherms of C27F9 and C29F13 monolayers on the water surface at the same conditions of Figure 2, respectively. Both isotherms exhibit one loose kink around 20 mN/m and the κs-A isotherms are those of two-minima type as that of C25F17 monolayers. The minimum surface compressibility of the C27F9 monolayer around the molecular area of 0.34 nm2 is 3.7 × 10-3 m/mN and that of C29F13 monolayer around the molecular area of 0.33 nm2 is 4.4 × 10-3 m/mN, and both are also much larger than that of arachidic acid monolayers. One-layer LB samples were transferred at the points indicated by the arrows A to F on the isotherms in Figure 5. Parts A-C and D-F of Figure 6 are bird’s eye view images of surface structures of C27F9 and C29F13 monolayers transferred at three corresponding points,
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Figure 3. Bird’s-eye view images of AFM of C19F17 (A, B, and C), C25F17(D, E, and F), and C31F17 (G, H, and I) one-layer LB films on the coverglass, transferred at corresponding points marked in Figure 2. Image sizes are 500 × 500 nm2 in lateral and 20 nm full scale in height.
respectively. These images were obtained with the tapping mode. Both materials show also two-dimensional molecular clusters of regular shape and size. The average sizes of clusters are about 70 nm in diameter and the shape is almost circular in both cases. The most surprising feature of these clusters is that almost all clusters have holes of a few tens nanometer diameter at the center of them especially for the C27F9 monolayers. With compression, the shape of clusters deforms from circular to hexagonal or polygonal by the strong lateral external pressure, but the holes do not disappear even at the highest surface pressure of 35 mN/m. Discussion Figure 7 shows surface structures of C19F17 monolayers at 35 mN/m on the subphases containing K+ ions, Cd2+ ions, and La3+ ions of the concentration of 5.0 × 10-4 mol/L at pH 7. For the case of potassium ions, the cluster structure is not clear, but the outline of the clusters becomes sharp with increasing valency of the metal ions without changing the size. Note that the image size of (C) (1000 × 1000 nm2) is different from those of (A) and (B). Figure 8A is a power spectrum of the signal along a line in Figure 7C. There is a sharp peak corresponding to the uniform diameter of the clusters of 17 nm. Figure 8B is a two-dimensional Fourier transform spectrum of the AFM image of Figure 7C. The sharp ring corresponding to the “Debye-Scherrerring” suggests that the cluster arrangement is that in “polycrystalline”. Figure 8C is a result of autocorrelation analysis of the AFM image of Figure 7C. Autocorrelation analysis shows how long or how wide the positional correlation of clusters from the cluster set at
the center of the image reaches. The result suggests that the positional correlation reaches over only several clusters. In contrast to this result, clusters form a twodimensional “monocrystalline” state when certain amount of C11F21 is mixed as described later. Why has such a uniform distribution of size of clusters been realized? Figure 9 show schematically the elemental processes of formation of monolayers during spreading under the conditions where the film materials form condensed monolayers. The processes of formation of the two-dimensional molecular clusters at the water surface are estimated as follows. When a drop of a spreading solution in a volatile solvent such as hexane or chloroform of 0.005 mL is set on the clean water surface, it expands instantaneously to a circle of about 10 cm diameter (Figure 9A). As the thickness of the solution is less than a micrometer, the solvent of the thin layer of solution evaporates rapidly in a very short time of less than a second. Just after the evaporation of the solvent, film molecules are left in a uniform distribution at the water surface (Figure 9B). They are very unstable thermodynamically under the conditions where film materials form condensed monolayers because the equilibrium spreading pressures (ESPs) of them are very low under these conditions. Two-dimensional densities of the film molecules are much higher than those of monolayers at ESPs and the degree of supersaturation is very high. Then, two-dimensional nucleation of the condensed phase starts to occur at once (Figure 9C). Film molecules surrounding the nuclei move around laterally by the thermal motion and collide against the nuclei. If we assume that the nuclei capture the film molecules
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Figure 4. (A) An expanded AFM image of the C19F17 monolayer transferred at point C in Figure 2. Image sizes are 50 × 50 nm2 in lateral and 3 nm full scale in vertical. (B) A cross section profile along the line in Figure 4A.
whenever the film molecules collide with them, the nuclei grows in two-dimensions simultaneously and rapidly. When almost all of the film molecules surrounding the nuclei are exhausted by the capturing, growing of clusters stops (Figure 9D). Clusters gather to form macroscopic irregular-shaped domains by van der Waals attraction among the clusters which we can observe with a Brewster angle microscope (BAM) (Figure 9E). BAM observation of the monolayers of these materials, however, reveals that contrast of images is very low. We can observe macroscopic domain structures on the CRT, but it is difficult to print out because of the low contrast. It is well-known in polymer physics that introduction of fluorine atoms by substitution into molecules will lead to lowering of the refractive indices of the materials in general due to the large atomic volume of fluorine. The refractive indices of the partially fluorinated long-chain acids are not measured yet, but are clearly smaller than those of long-chain hydrocarbonic acids such as stearic or arachidic acid and become near to that of water. This is the reason that the contrast of the BAM images of these materials is much lower than that of hydrocarbonic film materials. However, we can already see macroscopic condensed domains of irregular shape in all cases before detecting the surface pressure. When the nuclei start to grow by capture of film molecules colliding with them, two-dimensional density of film molecules surrounding the nuclei goes down rapidly. Then, nucleation is suppressed by rapid lowering of degree of supersaturation. This is the reason that a very uniform
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size of two-dimensional molecular clusters is formed because successive nucleation and growth will lead to the polydispersed distribution of the cluster size. Figure 10A shows a further expanded AFM image of the same sample in Figure 4A of 20 × 20 nm2. In this much expanded image, resolution of AFM attains to the molecular size, but we could not observe any regularity of molecular arrangement such as that found in twodimensional molecular crystals. Figure 10B is the Fourier transform spectrum of the cross section signal along a line in Figure 10A. The spectrum exhibits a peak corresponding to a 0.65 nm repeating distance. This is strong evidence that the repeating distance of 0.65 nm is not a spurious signal caused by the electric noise of the AFM imaging process. Bunn and Howells have reported a molecular structure of poly(tetrafluoroethylene) based on the X-ray diffraction measurements of a fiber sample of the material.20 They proposed a 71 helical structure of the fluorocarbon chain because a fluorine atom is much larger than hydrogen atoms. Main-chain carbon-carbon bonds cannot take the complete trans zigzag conformation due to the steric hindrance of the fluorine atoms, but the fluorinated parts are almost fully extended. The shape of the fluorocarbon chain looks almost a circular cylinder in contrast to the flat cross section of the trans zigzag hydrocarbon chains. Bunn and Howells calculated the diameter of the fluorocarbon chain as 0.65 nm. The repeating distances obtained by the Fourier transform spectra in Figure 10 coincided well with the molecular diameter of the fluorocarbon chain obtained by them. However, the image in Figure 10A seems to show no regularity of the molecular arrangement in the clusters as described above. Let us examine some numerical consideration of the cluster formation processes during the spreading of the C19F17 monolayer as an example. Initial conditions are as follows: A drop of 0.005 mL of a spreading solution of 0.4 × 10-6 mol/mL expands to a circle of 10 cm diameter instantaneously and molecules are uniformly distributed just after the evaporation of the solvent. Under these initial conditions, the average intermolecular distance is 2.9 nm and molecules occupy 6.5 nm2 in average. As about 700 molecules constitute one cluster, there was a growing nucleus for every 4500 nm2 in average. If it is assumed that nuclei of clusters are uniformly distributed at the water surface, there is one nucleus for every circular area of 38 nm radius. All of the molecules surrounding the growing nuclei are captured by traveling 38 nm at most with self-diffusion mechanism. If we assume that molecules are captured whenever the molecules collide with the growing nuclei, we can estimate the time to complete the formation of clusters using the reported twodimensional diffusion coefficient in monolayers. In the explanation of cluster formation processes described above, it is necessary to suggest what mechanism will give an origin that growing nuclei are separated by a constant distance, a constant separation of about 80 nm among the nuclei for the C19F17 case as described in the previous paragraph. One plausible explanation of this is as follows. When solvent of the spreading solution evaporates from the exceedingly thin solution layer very rapidly within a second or less as shown schematically in Figure 9, a very thin layer of subphase water surface is cooled suddenly by the heat of evaporation of the solvent. As this process proceeds almost adiabatically, an inverted layer is formed where temperature of the top thin layer is much lower (20) Bunn, C. W.; Howells, E. R. Nature 1954, 174, 549-563.
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Figure 5. π-A isotherms and κs-A isotherms of C27F9 and C29F13 monolayers at pH 7. Other experimental conditions are the same as those in Figure 2. One layer of monolayers was transferred by the scooping up method at the points indicated by arrows A to F on the isotherms.
Figure 6. Bird’s-eye view images of C27F9(A, B, and C) and C29F13(D, E, and F) one-layer LB films on the cover glass, transferred at corresponding points marked in Figure 5. Image sizes are 500 × 500 nm2 in lateral and 20 nm full scale in height.
than that of the next beneath layer. Convection starts to occur and transient micro-Benard cells are formed within the very thin inverted layer. This regular pattern of Benard cells causes regular singular points at the water surface where temperature is somehow lower than that of the surrounding surface. These points may in turn become origins of nucleation of clusters. Furthermore, the Benard cells cause local flow of the water surface to concentrate amphiphiles to the regular low-temperature points. This also facilitates nucleation at these singular points. As it is well-known that the lateral size of Benard cells is almost equal to the thickness
of the inverted layer, it is estimated reasonably that thickness of the inverted layer of the above discussion may be the order of 40 nm. Hyuk Yu et al. have studied lateral diffusion of a probe lipid in L-R-dilauroylphosphatidylcholine monolayers by the fluorescence recovery after the photobleaching technique and have reported the diffusion coefficient at the liquid-expanded state is 1.2 × 10-10 m2/s.21 The time necessary to capture almost all of film molecules using this diffusion coefficient is estimated to be about 1 ms at most. (21) Tamada, K.; Kim, S.; Yu, H. Langmuir 1993, 9, 1545-1550.
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Figure 7. Top view images of AFM of C19F17 monolayers at 35 mN/m on the subphase containing (A) K+, (B) Cd2+, and (C) La3+ of the concentration of 5.0 × 10-4 mol/L at pH 7. Image sizes are 500 × 500 nm2 for (A) and (B) and 1000 × 1000 nm2 for (C).
However, as it is assumed that all the molecules that collide with the growing nucleus are captured, the density of film molecules just outside of the nucleus becomes very low and this accelerates the access of molecules to the nucleus as shown schematically in Figure 11. This effect makes shorter the time necessary for almost all molecules to be captured than that for the case of the usual selfdiffusion mechanism. The set situation may be oversimplified because some assumptions such as film molecules are always captured when colliding with nuclei or nucleation starts just after the completion of solvent evaporation are not realistic. The last stage of the solvent evaporation and the nucleation process should overlap each other. We can understand, however, that the formation process of the clusters during spreading is dramatically fast and the size of clusters is determined not thermodynamically but by a dynamic balance. Thus, the size of the molecular clusters is determined primarily by the two-dimensional density of growing nuclei, which is relevant to the degree of supersaturation of film molecules just after the evaporation of the solvent, and the exhaustion of film molecules surrounding the nuclei stops the growing of clusters by capturing. Even for oleic acid monolayers on the water surface at 25 °C, the surface pressure of the gas phase equilibrated with the liquid-expanded phase is less than 40 µN/m and the average molecular area at this point is larger than 30
nm2/molecule.22 The surface pressures of the gas phases of the partially fluorinated long-chain acids equilibrated with solids at 10 °C are much smaller than that of oleic acid because these materials form solid films at this temperature and the average molecular areas in gas phase should be much larger than that of oleic acid. Furthermore, formation of divalent metal salts of the long-chain acids lowers the equilibrium surface pressures of the gas phases much more. Thus, molecular densities of the gas phases surrounding the clusters should be almost zero and the gap surface among the clusters should be that of almost bare solid substrates. Figure 12 summarizes top view images of AFM of the five partially fluorinated amphiphiles sampled at points A, D, and G in Figure 2 and points A and D in Figure 5, and the cross section profiles along lines in the corresponding images. In these images, we can see some gaps among the clusters except in the case of C19F17, and we can read heights of clusters from the basal planes of the glass substrates as the reason described above. Table 2 lists the range of heights of clusters and the lengths of molecules when all of the carbon-carbon links take the trans zigzag conformation. From the figures in Table 2, we can see that the heights of clusters are always shorter than those of the fully extended molecules. Cross section molecular areas of the fluorinated parts are estimated to be 0.33 nm2 from π-A isotherms or (22) Rakshit, A. K., J. Colloid Interface Sci. 1981, 80, 467-472
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Figure 8. (A) A power spectrum of the signal along a line in Figure 7C showing a strong peak corresponding to the cluster diameter (18 nm). (B) A two-dimensional Fourier transform spectrum of the AFM image of Figure 7C, showing a ring which suggests “polycrystal” structure of the cluster array. (C) An image of autocorrelation analysis of the AFM image of Figure 7C. This image suggests that positional correlation of clusters do not extend over several at most.
calculated from the molecular diameter, 0.65 nm estimated from the data in Figure 10B. This may be due to the fact that the cross section molecular area of the fluorinated
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Figure 9. Schematic illustration of the two-dimensional molecular cluster formation during the spreading of monolayers at the water surface. (A) A drop of spreading solution of 0.005 mL expands to a circle of about 10 cm diameter. (B) Film molecules are uniformly distributed just after evaporation of spreading solvent. (C) Nucleation of the condensed phase starts to occur at once. (D) Nuclei grow by capturing film molecules surrounding them. (E) Clusters gather to form macroscopic domains. (See text for detail.)
part is much larger than that of the hydrocarbon part and occupation areas of the amphiphiles in clusters are determined by the fluorinated parts. Then, many of the C-C bonds in the hydrocarbon parts take gauche conformation rather than trans conformation to incorporate the molecular space in the clusters. C19F17, C25F17, and C31F17 compose the material series 1 where length of the fluorinated part is constant while that of the hydrocarbon part increases systematically. Cluster size increases from about 17 nm of C19F17
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Figure 10. (A) A highly expanded top view AFM image of the C19F17 monolayer at point C in Figure 2. Image size is 20 × 20 nm2. (B) A Fourier transform spectrum of the signal along a line in Figure 10A.
to about 70 nm of C31F17 and shape is also changed from circular to irregular in this series with increasing the length of hydrocarbon part. The height of clusters also increases steadily from around 1.8 nm of C19F17 to around 2.7 nm of C31F17. The height of C19F17 clusters is abnormally small. This is because the gap among the clusters is narrow and the top of etched silicon cantilever (nominal radius of the tip top is 10 nm) cannot reach to the basal plane in this case. C27F9, C29F13, and C31F17 compose the material series 2 in this study where length of the hydrocarbon part is the same while that of the fluorinated part increases systematically. The size of clusters in this series is almost the same to be around 70 nm, but the shape changes from circular to irregular with increasing the length. The height of clusters in this series also increases steadily from around
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2.5 nm of C27F9 to around 2.7 nm of C31F17. As a whole, molecular lengths in the clusters are shorter than those expected from the extended molecular models. The cross section profile of C19F17 clusters is a very flat half oval shape as shown in Figure 4B. However, the shape of the top of clusters of other amphiphiles look rather flat as the cross section profiles in Figure 13 show. Allen and his collaborators have reported that cross section profiles of AFM images are hardly distorted by the convolution of the tip top, especially when the size of the object is comparable to the tip top diameter.23,24 Butt and Gerharz have also reported the same distortion effect on AFM images when they observed two-dimensional arrangement of nanospheres of the size comparable to the tip top size.25 Thus, the very flat half oval shape of the cross section profile of C19F17 clusters may be the result of distortion by the convolution of the cantilever because the size of the clusters, about 17 nm, is almost comparable to the nominal diameter of the top of the etched silicon cantilever, 20 nm. This problem should be checked by other experimental technique such as SEM or STM observation of the osmium-replicated samples. C27F9 and C29F13 monolayers show the very characteristic cluster shape of a two-dimensional doughnut. Almost all clusters of C27F9 have holes of a few tens nanometer diameter at the center of them. As the nominal diameter of the top of the cantilever is 20 nm, hole size in the images may become smaller than the real one because of the convolution of the tip top in all cases. Nevertheless, holes of nanometer size in the clusters of C27F9 and C29F13 become small with compression but do not disappear even at the highest surface pressure of 35 mN/m as shown in Figure 6. We cannot suggest yet what is the formation mechanism of the holes in the clusters. Figure 13 shows an expanded image of the C27F9 shown in Figure 6A, and the height difference is emphasized here. From this image, we can imagine how the doughnut-shaped molecular clusters are formed. Thus, it looks as if the doughnut-shaped clusters are composed of several smaller size clusters. And the small size clusters may fuse into a doughnut shape under the special conditions. It was made clear that doughnutshaped clusters are formed under the very narrow window of experimental conditions such as concentration of the spreading solution, kind and concentration of the metal ion, spreading temperature, etc. The AFM samples in Figures 3 and 6 are transferred 60 min after the spreading. We are now preparing some samples with time after the spreading going back to 2 min to pursue the formation mechanism of the two-dimensional doughnuts in nanometer size. The size of the molecular clusters is determined mainly by the two-dimensional density of growing nuclei as described previously, and the number or density of the nuclei depends on the degree of supersaturation at the spreading conditions. This in turn depends on the molecular structures. Shape of the clusters is mainly controlled by the line tension between the condensed phase and the surrounding phase. As the line tension becomes large, the shape of clusters becomes compact and circular. In material series 1, the size of clusters becomes large with increasing length of the hydrocarbon part and the shape of them changes from circular to irregular. For the (23) Allen, M. J.; Hud, N. V.; Balooch, M.; Tench, R. J.; Siekhaus, W. J.; Balhorn, R. Ultramicroscopy 1992, 42-44, 1095-1100. (24) (a) Markiewicz, P.; Goh, M. C. Langmuir 1994, 10, 5-7. (b) Wilson, D. L.; Kump, K. S.; Eppell, S. J.; Marchant, R. E. Langmuir 1995, 11, 265-272. (25) Butt, H. J.; Gerharz, B. Langmuir 1995, 11, 4735-4741.
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Figure 11. Schematic illustration of the growing process of the clusters by capturing of film molecules surrounding the nuclei with time: (A) uniform distribution of molecules just after the evaporation of solvent; (B) nucleation starts; (C) growing of a nucleus by capturing; (D) stop growing by exhaustion.
material series 2, the size of the clusters is almost the same but the shape changes from a compact to irregular one. These facts may suggest that the degree of supersaturation is almost the same when hydrocarbon parts of the amphiphiles are the same. Some investigators have hypothesized an existence of two-dimensional micelles in insoluble monolayers and have discussed them,3-5 but no one could establish the existence of them experimentally because of lack of the observing technique. Recently, Israelachivili has proposed an idea to explain the nonhorizontal character of the π-A isotherms of insoluble monolayers in first-order phase transition regions by assuming the existence of the surface micelles.26 Manne and his collaborators have reported direct observation of surfactant hemimicelles adsorbed onto hydrophobic substrates from aqueous solutions by atomic force microscopy.6,7 Ducker and co-workers have also reported hemicylindrical aggregate structures of SDS adsorbed onto a graphite surface from aqueous solutions and double hemimicelle structures from mixed aqueous solutions of dodecyldimethylammoniopropanesulfonate and dodecyltrimethylammonium bromide adsorbed onto mica surface, revealed by atomic force microscopy.8,9 All of these hemimicelle structures on the solid substrates, however, are formed by adsorption from aqueous solutions as equilibrium processes. These hemimicelle structures resemble the two-dimensional molecular clusters reported in this paper as a whole, but the formation mechanism of the former is quite different from the latter by the reason described previously. Chi and her collaborators have reported that stearic acid monolayers on the subphase containing poly(ethyleneimine) as a polycation exhibit a phase transition from liquid-expanded to liquid-condensed state while stearic acid on the subphase without polycation shows only a phase transition from liquid-condensed to solid phase as is well-known. They observed structures of the monolayers with AFM by transferring monolayers onto mica surface by the LB technique. It is very interesting to point out here that at the liquid-expanded state, there are only molecular clusters of the several tens nanometer size in lateral, but after the transition, molecular clusters and macroscopic domains of a few micrometers are coexistent with, and it looks as if molecular clusters fuse into macroscopic domains with compression. Their situation is rather complex because surface structures of their system change not only with surface pressure or time on (26) Israelachvili, J. Langmuir 1994, 10, 3774-3781.
the water surface but also with the time after the transfer onto mica surface because poly(ethyleneimine) adsorbed on a mica surface absorbs water from the ambient atmosphere and stearic acid molecules can move even after the transfer on the mica surface.15,16 There is another explanation of the formation mechanism of two-dimensional regular structures in spread monolayers in nanometer size, i.e., two-dimensional spinodal decomposition during the rapid evaporation of spreading solvent. Spinodal decomposition is well-known in the field of two-component polymer systems or twocomponent metal alloys to form some characteristic regular structures such as the bicontinuous phases by changing temperature rapidly.27-30 A spinodal decomposition mechanism is also applied to the one-component systems when temperature is suddenly lowered from above the critical temperature to the spinodal decomposition region where phases are unstable. At the evaporation process of the spreading solution, the number of solvent molecules decreases rapidly with time after spreading. Interaction among the film molecules increases with decreasing the number of solvent molecules. This situation corresponds to the sudden lowering of temperature of the monolayer system and the system becomes metastable (nucleation and growth will occur) or unstable (spinodal decomposition will occur), depending on the many factors such as the kind of film materials, the solubility parameter of the solvent (degree of affinity between the solvent and the amphiphile), spreading temperature, initial density of the film molecules (concentration of the spreading solution), kind and concentration of metal ions in the subphase water, etc. The cluster shape in Figure 3G or 12E closely resembles to the computer simulation pattern of the two-dimensional spinodal decomposition.31 This may suggest that some clusters in this paper are the result of a two-dimensional spinodal decomposition mechanism rather than the nucleation and growth mechanism. We can classify the two-dimensional molecular cluster formation in the spread monolayers into the nucleation and growth mechanism (27) (a) Cahn, J. W. J. Chem. Phys. 1965, 42, 93-102. (b) Cahn, J. W. Trans. Metall. Soc. AIME 1968, 242, 166-175. (28) Nishi, T.; Wang, T. T.; Kwei, T. K. Macromolecules 1975, 8, 227-234. (29) Sariban, A.; Binder, K. Polym. Commun. 1989, 30, 205-211. (30) Kawakatsu, T.; Kawasaki, K.; Furusaka, M.; Okabayashi, H.; Kanaya, T. J. Chem. Phys. 1993, 99, 8200-8217. (31) Koch, S. W.; Desai, R. C.; Abraham, F. F. Phys. Rev. A 1983, 27, 2152-2167.
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Figure 12. Top-view images and cross section profiles along the corresponding lines in the images of (A) C19F17, (B) C25F17, (C) C27F9, (D) C29F13, and (E) C31F17. All samples were transferred at the points just before the pressure rising.
and the two-dimensional spinodal decomposition mechanism by changing the experimental conditions systematically. Lennox and his collaborators have reported formation of surface micelles in monolayers of polyelectrolytes of styrene and vinylpyridine block copolymers.10-14 They discussed structures of the surface micelles based on the observation of LB films with a transmission electron
microscope. In their discussion, they estimate aggregation numbers in the micelles based on the premise that the surface micelles are formed in equilibrium. There is another possibility here that these surface micelles are also formed by the two-dimensional spinodal decomposition during the spreading processes because block copolymers are the most representative materials for the spinodal decomposition in three-dimensions by the rapid
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Table 2. Heights of Clusters and Fully Extended Molecular Lengths materials
fully extended molecular length (nm)
range of cluster height (nm)
C19F17 C25F17 C27F9 C29F13 C31F17
2.26 3.02 3.27 3.52 3.77
1.7-2.0 2.2-2.4 2.4-2.6 2.6-2.8 2.6-2.8
Figure 13. Expanded top view AFM image of Figure 6A. Image sizes are 250 × 250 nm2 in lateral and 10 nm full scale in vertical.
evaporation of the common solvent of the two components of blocks or by sudden change of temperature of the system. We describe here some attempt on the disposition control of the clusters by mixing with other amphiphiles for C19F17 as an example. When ethyl stearate is used as a diluent, AFM observation revealed that clusters of almost the same size to those in Figure 3A are formed. However, ethyl stearate does microphase separation from C19F17 and it cannot affect the disposition control by mixing. C11F21 forms expanded monolayers even at 15 °C, and judging from isotherms of mixtures, C11F21 mixes with C19F17 well in the whole range of mixing ratios. Size of C19F17 clusters does not change with mixing ratio but intercluster distance increases monotonically with increasing mixing ratio of C11F21 up to 80 mol %. However, autocorrelation analysis of the AFM images of mixed monolayers revealed that disposition correlation among the clusters is completely lost above the mixing ratio of 40 mol %.32 It is worthwhile to note here that a certain amount of C11F21 mixed with C19F17 greatly improves regularity of the arrangement of C19F17 clusters than that of the pure C19F17 case. For example, when 10 mol % C11F21 is mixed with C19F17, we can observe a hexagonally arrayed “monocrystalline” region of clusters of C19F17 whose size is larger than 500 × 500 nm2. These results should be compared with those of pure C19F17 in Figure 7. Thus, a certain amount of C11F21 may weaken the van der Waals attraction among the clusters and facilitate the clusters to be arranged very regularly over (32) Kato, T.; Ehara, M.; Iimura, K. Mol. Cryst. Liq. Cryst. 1997, 295, 167-170.
the long distance. C11F21 molecules seem to act as a molecular “lubricant” for clusters. From the context of discussion on the formation mechanism of molecular clusters, it is natural to consider that cluster formation is a rather general phenomenon in spread monolayers especially when amphiphiles are spread under the conditions where they form condensed monolayers. In fact, we have observed molecular clusters in monolayers of the most representative film forming materials such as stearic acid and DPPA. Figure 14 shows three top images of AFM of stearic acid monolayers on the pH 3 subphase. Image sizes of Figure 14A are 4 × 4 µm2 in lateral and 20 nm in vertical. There are many irregular-shaped molecular clusters of the size ranging from a few hundreds nanometers to several hundreds nanometers. Image (B) shows the gathering process of clusters to fuse into macroscopic domains. At the top left of the image, we can see the primary clusters of the size in a few tens nanometer diameter. They combine with each other to fuse into the secondary irregular-shaped clusters of the size of a few hundreds nanometers. The size distribution of clusters is polydisperse in this case. Images (B) and (C) display a formation mechanism of macroscopic domains from clusters by fusion. We can see a characteristic finger structures at the circumference of the macroscopic domains and holes inside the domains. Thus, clusters of nanometer size are formed at the spreading process of stearic acid and they fuse into macroscopic domains with time without compression, which we can observe with BAM. Dipalmitoylphosphatidic acid also exhibits cluster formation of the size ranging from a few hundreds nanometer to several hundreds nanometers. Thus, cluster formation is a rather general phenomenon in spreading of monolayers under the conditions where the materials form condensed monolayers (ESPs are low). However, whether once formed clusters may fuse into macroscopic domains or not depends on the structure of the amphiphiles and on the spreading conditions such as temperature, solvent of spreading solutions, kind and concentration of metal ions in the subphase, etc. For most of amphiphiles, clusters fuse into macroscopic domains and these domains fuse into uniform monolayers with compression or with time. The clusters of the partially fluorinated long-chain acids, however, do not fuse even under the high surface pressure near collapse of monolayers. All samples for the AFM observation in this study were transferred at 60 min after spreading. To pursue the formation mechanism of clusters, time course sampling of monolayers from the time just after the spreading is now in progress. To make clear the formation dynamics of clusters, size control, disposition control, and shape control of the two-dimensional clusters in nanometer size are the next important subjects of our study in the future. Conclusions Two-dimensional molecular clusters of nanometer size were found in spread monolayers of five kinds of partially fluorinated long-chain acids by AFM. AFM observation reveals that sizes of the clusters are sharply monodisperse in all cases. The smallest size cluster of C19F17 has a circular shape of 17 nm diameter and is composed of about 700 molecules. The size and shape of clusters change with changing structure and length of molecules. These clusters are formed during the spreading processes due to the instability of film molecules just after the rapid evaporation of spreading solvent (nucleation and growth mechanism). Transient micro-Benard cells formed during
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Figure 14. Top view images of stearic acid monolayers on the pH 3 subphase before compression. Image sizes are 4 × 4 µm2 for (A), and 10 × 10 µm2 for (B) and (C). Top left of image (B) shows primary clusters of the size of a few tens nanometers. Clusters in image (A) are the secondary one which are formed by fusion of the primary clusters. Images (B) and (C) display a formation mechanism of macroscopic domains by fusion of clusters. Image (C) exhibits a very characteristic finger structure around the domain. (See text for detail).
the spreading processes may determine the size and size distribution of the clusters. Another possible mechanism of cluster formation is the spinodal decomposition from the unstable state of film molecules into spinodal decomposition regions by rapid evaporation of the solvent. Both of the transient micro-Benard cells and the spinodal decomposition are typical dissipation phenomena. Thus, it is worthwhile to point out here that two-dimensional cluster formation in spread monolayers may be the appearance or results of dissipation phenomena, dissipation structures. C27F9 and C29F13 forms two-dimensional doughnutshaped clusters of nanometer size. Nanoholes inside the clusters do not disappear even at the highest surface pressure of 35 mN/m. A formation mechanism of these very characteristic clusters is not suggested yet. There is a possibility that smaller size clusters may fuse into this shape under particular conditions. A two-dimensional “monocrystalline” state of C19F17 clusters over a distance longer than 500 nm was attained by mixing of a certain amount of perfluoroundecanoic acid. Perfluoroundecanoic acid molecules may act as molecular lubricant to facilitate mutual movement of clusters at the water surface by weakening the van der Waals attraction among them.
The cluster formation during spreading is a rather general phenomenon when materials are spread under the conditions where the materials form condensed monolayers. This is verified by AFM observation of clusters of very representative materials of insoluble monolayer formation such as stearic acid or dipalmitoylphosphatidic acid. Fusion of clusters with time or with temperature and/or surface pressure increase depends on the film materials. Clusters of most of amphiphiles fuse into macroscopic domains with time or surface pressure increase. However, the clusters of partially fluorinated long-chain acids are very stable and they do not fuse to each other even at high surface pressures near collapse of monolayers. Acknowledgment. This study was financially supported by the Grant-in-Aid from Ministry of Education, Sports and Culture of the Japanese Government which was given to one of the authors (T. Kato) in 1996 and 1997 (No. 08404047), and this was much appreciated. This work was also financially supported by the fund from Hitachi Institute of Hitachi Co. given from 1992 to 1997. This was also much appreciated. The authors wish to express their thanks to Dr. T. Kawakatsu for his encouraging discussion about two-dimensional spinodal decomposition. LA970951Z
