J. Phys. Chem. B 2008, 112, 15313–15319
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Phase-Separated Structures of Mixed Langmuir-Blodgett Films of Fatty Acid and Hybrid Carboxylic Acid Hideto Kimura,† Satoshi Watanabe,† Hirobumi Shibata,† Reiko Azumi,‡ Hideki Sakai,§ Masahiko Abe,§ and Mutsuyoshi Matsumoto*,† Departments of Materials Science and Technology and of Pure and Applied Chemistry, Tokyo UniVersity of Science, Yamazaki 2641, Noda 278-8510, Japan, and Photonics Research Institute, National Institute of AdVanced Industrial Science and Technology (AIST), 1-1 Higashi, Tsukuba 305-8565, Japan ReceiVed: August 9, 2008; ReVised Manuscript ReceiVed: October 2, 2008
Phase separation often occurs in mixed Langmuir-Blodgett (LB) films. Usually circular domains at the micrometer length scale form in the LB films. The size and shape of the domains are governed by a compromise between two competing interactions of line tension and dipole-dipole interaction. An attempt was made to control the line tension by varying systematically the hydrophobic moieties of the film-forming molecules. Phase-separated structures of two-component mixed LB films of fatty acid [CkH2k+1COOH (HkA)] and hybrid carboxylic acid [CmF2m+1CnH2nCOOH (FmHnA)] were investigated. IR spectra of the mixed LB films of H17A and F8H10A revealed that the alkyl chains were in an all-trans conformation and that the molecular orientation remained unchanged when the two components were mixed. Nanowires formed in the mixed LB films of HkA and F8H10A. The width of the nanowires increased with an increase in k. Domain size and shape in the mixed LB films of H17A and FmHnA depended strongly on the values of m and n. Circular domains at the micrometer length scale formed in the region m + n < 16. In contrast, domains at the nanometer length scale formed in the region m + n g 16 except for F6H10A. These results were explained by using a lattice model that considers the effect of the hydrophobic moieties of fatty acid and hybrid carboxylic acid on the line tension. Introduction Self-assembly enables us to control nanostructures and microstructures of various materials such as ultrathin organic films,1-18 polymer films,18-21 and inorganic nanoparticle assemblies.22,23 Complex superstructures consisting of organic molecules have been fabricated using the availability of molecular design to control the molecular properties and intermolecular interactions of organic materials. Lateral patterning of organic materials has been performed through top-down20,24-26 and bottom-up1-19,21 methods. Self-assembling processes using bottom-up methods have been attracting considerable attention because these processes usually require less energy, less environmental burden, and fewer natural resources than the processes using top-down methods. Microphase separation of block copolymers has been used to form patterns for the applications to data storage.18-21 The Langmuir-Blodgett (LB) technique uses self-assembling processes for the fabrication of ultrathin organic films with welldefined structures.27-29 LB films are fabricated by transferring Langmuir films of amphiphiles at the air-water interface onto solid substrates. Phase separation often occurs in mixed LB films, resulting in the formation of patterned surfaces with wettability and/or chemical reactivity varying at the micrometer or nanometer length scale.1-7,12-18 This feature allows for the confinement of materials reflecting the patterns of the phase* To whom correspondence should be addressed. E-mail: m-matsu@ rs.noda.tus.ac.jp. † Department of Materials Science and Technology, Tokyo University of Science. ‡ AIST. § Department of Pure and Applied Chemistry, Tokyo University of Science.
separated LB films and for the applications of these materials to data storage, switching, and sensing. Phase-separated structures of mixed Langmuir and LB films are governed by two competing interactions of line tension and dipole-dipole interaction.1,2,15,30,31 Line tension favors the formation of large and/or circular domains. In two-dimensional systems of Langmuir films in which amphiphiles are oriented, dipole-dipole interactions effective in the determination of the shape and size of the domains are those between the dipoles situated at both ends of the molecules.32,33 Dipole-dipole interaction between the same chemical species in Langmuir films is repulsive and favors the formation of elongated and/or small domains. Circular domains at the micrometer length scale form in the usual phase-separated LB films.3,5,6,12,14,16 Only few exceptions have been reported. Iimura et al. have investigated the phaseseparated structures of the mixed LB films of a series of fatty acids and an amphiphilic perfluoropolyether.16 The phaseseparated structures depend both on the alkyl chain length of the fatty acid and on the fabrication temperature. Branched narrow domains form in the mixed LB films of docosanoic acid and the perfluoropolyether when fabricated at low temperatures, whereas circular domains at the micrometer length scale form when fabricated at high temperatures. These phenomena have been explained in terms of low mobility of molecules at low temperatures. Domain structures of Langmuir films of L-Rmyristoylphosphatidic acid (DMPA) with a small quantity of cholesterol have been reported to change reversibly with temperature.1,2 Circular domains change into wire domains via banana-shaped domains on decreasing the temperature. The domain width decreases with the elongation of the domains. These results have been understood in terms of a larger
10.1021/jp807118v CCC: $40.75 2008 American Chemical Society Published on Web 11/11/2008
15314 J. Phys. Chem. B, Vol. 112, No. 48, 2008 contribution of dipole-dipole interaction originating from a higher degree of the orientation of DMPA at lower temperatures. In a preliminary study, we have reported that the phaseseparated structures of the mixed LB films of fatty acid and hybrid carboxylic acid having both perfluoroalkyl and alkyl moieties depend strongly on the chemical species of the components and the fabrication conditions.15 We obtain nanowires in some of the mixed LB films. By adding an amphiphilic silane coupling agent as a third film-forming component, we fabricate templates having patterns at the nanometer length scale. This methodology is important because circular domains at the micrometer length scale form in the usual mixed LB films containing silane coupling agent.5,6,12,14,15 Here, we have demonstrated that it is possible to control the line tension by chemical modification of the film-forming molecules. This allows for the controlling of the phase-separated structures of the mixed LB films. We investigate the phaseseparated structures of the mixed LB films of fatty acid and hybrid carboxylic acid. The hydrophobic moieties of fatty acid and hybrid carboxylic acid are systematically varied, and the phase-separated structures of the mixed LB films are studied. A lattice model has been used to consider the effect of the hydrophobic moieties of fatty acid and hybrid carboxylic acid on the line tension in the mixed LB films. Experimental Section Materials. The amphiphiles used in this study and the abbreviations are shown below. The numbers following H and F are the lengths of the hydrocarbon and the perfluorocarbon, respectively. “A” stands for a carboxylic group. Fatty acid CkH2k+1COOH and hybrid carboxylic acid CmF2m+1CnH2nCOOH are abbreviated as HkA and FmHnA, respectively. Octadecanoic acid [C17H35COOH (H17A)], eicosanoic acid [C19H39COOH (H19A)], and docosanoic acid [C21H43COOH (H21A)] were purchased from Fluka, Acros Organics, and Fluka,respectively.12,12,13,13,14,14,15,15,16,16,17,17,18,18,19, 19,19-Heptadecafluorononadecanoic acid [C8F17C10H20COOH (F8H10A)] was purchased from Wako Pure Chem. Ind., Ltd. All of the other FmHnA molecules were synthesized according to the literature.34 Fabrication of LB Films. The monolayer measurements were performed using a Lauda Filmwaage (FW1) at given subphase temperatures. The spreading solvent was hexane for fatty acid and hexane/THF (99/1, v/v) for hybrid carboxylic acid unless otherwise stated. A spreading solution at a total concentration of 1.0 × 10-3 M was spread on pure water (electrical resistivity >18.2 MΩ cm). The molecules were compressed at a speed of 0.4 × 10-2 nm2 molecule-1 min-1 after 30 min of evaporation time. Single-layer LB films were fabricated by transferring Langmuir films at 10 mN m-1 using the vertical dipping method at a withdrawal speed of 5 mm min-1 onto Si wafers with oxidized surfaces. The Si wafers were kept in aqueous NH4OH and H2O2 at 98 °C for 10 min and rinsed with water before use. Characterization. IR spectra of single-layer LB films were measured using a Perkin-Elmer Spectrum 2000 FTIR. The spectrometer was purged with nitrogen gas to minimize the amount of water vapor present in the sample chamber. The spectra were recorded at a 4 cm-1 resolution by coadding 256 scans in the 3200-800 cm-1 region. Atomic force microscopic (AFM) observations were carried out with an SPA 300 microscope (SII, Japan). Noncontact mode (dynamic force mode) topographic images were acquired at a scan rate of 1 Hz using silicon nitride
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Figure 1. Surface pressure-area isotherms of H17A (thick solid line) and F8H10A (thin solid line) and their mixtures at molar ratios of 3/1 (dashed line), 1/1 (dotted line), and 1/3 (dashed-dotted line) at 20 °C.
Figure 2. Transmission IR spectra of CH stretching vibrations (3000-2800 cm-1) of single-layer LB films of H17A (thick solid line) and F8H10A (thin solid line) and those of single-layer mixed LB films at molar ratios of 3/1 (dashed line), 1/1 (dotted line), and 1/3 (dashed-dotted line) fabricated at 20 °C.
cantilevers with a spring constant of 15 N m-1 and a resonance frequency of 127 kHz. Results Phase-Separated Structures of Mixed Langmuir and LB Films of H17A and F8H10A. Figure 1 shows the surface pressure-area isotherms of H17A and F8H10A and their mixtures at 20 °C. The isotherm of F8H10A rises at large molecular area compared with that of H17A because F8H10A has a bulky fluorocarbon chain. The isotherms of the mixtures are located between the isotherm of H17A and that of F8H10A and are shifted gradually to the large molecular area region with increasing molar fraction of F8H10A. At constant surface pressures in the region 5-30 mN m-1, the additivity rule holds for mean molecular area with respect to the molar fraction of F8H10A. This result indicates the occurrence of ideal mixing or phase separation in the mixed Langmuir films. Transmission IR spectra of CH stretching vibrations of singlelayer LB films of H17A and F8H10A and those of single-layer mixed LB films are shown in Figure 2. Distinct peaks due to νa(CH3), νa(CH2), and νs(CH2) vibrations are present. The peaks of the νa(CH2) and νs(CH2) vibrations are located at 2917 and 2850 cm-1, respectively. The peak positions of the νa(CH2) and νs(CH2) vibrations are sensitive to the conformation of alkyl chains35 and indicate that the alkyl chains in the LB films take an all-trans conformation. The intensities of these peaks increase with increasing molar fraction of H17A, reflecting the difference in molecular structures of H17A and F8H10A. The changes in the intensities are treated quantitatively considering the additivity rule for the surface pressure-area isotherms. We consider a two-component
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Figure 3. 1/{I(t) - I0} plotted as a function of the molar fraction of H17A, t. Open circles and open squares are for νa(CH2) and νs(CH2) vibrations, respectively.
mixed system of A and B with A ) H17A and B ) F8H10A. Let t be the molar fraction of A. The intensity of an absorption band I(t) is given as follows:
I(t) ) {tAAIA + (1 - t)ABIB}/{tAA + (1 - t)AB} (1) where AA and AB are the molecular area of A and B in the singlecomponent Langmuir films, respectively, and IA and IB are the intensities of the absorption band in the IR spectra of the singlecomponent LB films of A and B, respectively. Rearrangement of eq 1 gives eq 2.
Figure 4. AFM images of the mixed LB films of H17A and F8H10A at molar ratios of (A) 3/1, (B) 1/1, and (C) 1/3 transferred at 10 mNm-1 at 20 °C. (D) Cross-sectional view along the horizontal line in the center of part A. The scanned area is 5 µm × 5 µm each.
1/{I(t) - I0} ) t/{I0(k2 - k1)} + k1 /{I0(k2 - k1)} (2) where I0 ) (AAIA - ABIB)/(AA - AB), k1 ) AB/(AA - AB), and k2 ) ABIB/(AAIA - ABIB). 1/{I(t) - I0} is plotted as a function of t in Figure 3. These plots give straight lines for both the ν(CH2) and νs(CH2) vibrations, indicating that the orientation of both H17A and F8H10A remains unchanged when they are mixed in the LB films. This suggests that the two components are ideally mixed or phase-separated. Figure 4 shows the AFM images of the mixed LB films of H17A and F8H10A transferred at 10 mNm-1 and at 20 °C. Phase-separated structures are evident in all of the images. Wiretype domains with widths at the nanometer length scale are present, which is in contrast to the formation of circular domains at the micrometer length scale in the usual mixed LB films.3,5,6,12,14,16 Nanowire formation should be due to the incorporation of hybrid carboxylic acid. The area fraction of nanowires increases with increasing molar fraction of H17A, suggesting that the nanowires consist of H17A. This is consistent with previous reports that the domains are formed by long chain fatty acids in mixed LB films because long chain fatty acids have large line tension.5,6,12-16 Figure 4D shows the crosssectional view along the horizontal line positioned in the center of Figure 4A. The height difference between the domains and the surrounding region is ca. 1 nm. Phase-Separated Structures of Mixed LB Films of Fatty Acid and F8H10A. We have investigated the phase-separated structures of the mixed LB films of long chain fatty acid and hybrid carboxylic acid. Figure 5 shows the AFM images of the mixed LB films of H17A and F8H10A at a molar ratio of 1/1 fabricated at 10 and 30 °C. The AFM image of a mixed LB film fabricated at 20 °C is shown in Figure 4B. Domains at the nanometer length scale are present in the three images. The domain length is large for the LB film fabricated at high
Figure 5. AFM images of the mixed LB films of H17A and F8H10A at a molar ratio of 1/1 fabricated at 10 mN m-1 (A) at 10 °C and (B) at 30 °C.
temperatures. This should be due to the high mobility of molecules at high temperatures, suggesting that the structures of the mixed Langmuir films are reflected in those of the mixed LB films and that the domain structures remain unchanged during and after the transfer. The effect of the fabrication temperature on the domain structures was investigated in more detail. We fabricated a mixed Langmuir film of H17A and F8H10A at a molar ratio of 1/1 at 10 mN m-1 at 10 °C, raised the subphase temperature up to 30 °C while keeping the surface pressure constant (isobarically), and fabricated a mixed LB film. The AFM image of the resultant LB film was similar to that of a mixed LB film fabricated isothermally at 30 °C, with the formation of elongated nanowires. Then, we fabricated a mixed Langmuir film of H17A and F8H10A at 10 mN m-1 at 30 °C, lowered the subphase temperature down to 10 °C isobarically, and fabricated a mixed LB film. The AFM image of the resultant LB film was similar to that of a mixed LB film fabricated isothermally at 30 °C and not to that of a mixed LB film fabricated isothermally at 10 °C. These results suggest that the stable domain structures in the mixed Langmuir films of H17A and F8H10A in the range of 10-30 °C are nanowires. Similar results have been reported for the mixed LB films of fatty acid and a perfluoropolyether.16
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Kimura et al. acid is long. In total, circular domains at the micrometer length scale form in the region m + n < 16, while domains at the nanometer length scale form in the region m + n g 16 except for F6H10A. The strong correlation between the chemical structure of FmHnA and the characteristic length of the domains indicates that the characteristic length is governed by the intermolecular interaction. Discussion
Figure 6. AFM images of the mixed LB films of HkA and F8H10A at a molar ratio of 1/1 fabricated at 30 °C: (A) H19A and F8H10A; (B) H21A and F8H10A. The scanned area is 5 µm × 5 µm each.
The effect of the spreading solvent on the domain structures was investigated. We fabricated a mixed LB film of H17A and F8H10A at 20 °C under the experimental conditions similar to those for the LB film whose AFM image is shown in Figure 4B. The only difference was that toluene (boiling point: 110 °C) was used as a spreading solvent instead of hexane (boiling point: 69 °C). The AFM image of the LB film using toluene as a spreading solvent was similar to that shown in Figure 5B and not to that shown in Figure 4B. This result shows that a spreading solvent with a high boiling point allows for the movement of the molecules at the air-water interface for a long period of time, facilitating the evolution of domain structures. The effect of the length of the alkyl chain of fatty acid on the domain structures is clearly shown in Figures 5B and 6. We consider the phase-separated structures of the mixed LB films fabricated at 30 °C because the evolution of the domain structures is not sufficient for LB films fabricated at low temperatures. With increasing length of the hydrocarbon chain of HkA, the width of the domains increases. The widths of the nanowires are ca. 40, 70, and 100 nm for k ) 17, 19, and 21, respectively. The domain edges are curved for k ) 17 and 19, while domains with linearly shaped boundaries are formed for k ) 21. These results show that the absolute value of the intermolecular interaction between H21A molecules is larger than that between H17A molecules and that between H19A molecules. Phase-Separated Structures of Mixed LB Films of H17A and FmHnA. The effect of the hydrophobic moieties of FmHnA on the domain structures was investigated. Figure 7 shows the AFM images of the mixed LB films of H17A and FmHnA at a molar ratio of 1/1 transferred at 30 °C with variations in the values of m and n. The shape and size of the domains depend strongly on the hydrophobic moieties of FmHnA. For example, circular domains at the micrometer length scale are formed in the mixed LB films of H17A and F6H8A, while nanowires are formed in the mixed LB films of H17A and F8H12A. We consider the effect of the length of the perfluoroalkyl chain and that of the hydrocarbon of FmHnA on the domain structures in more detail. We define the characteristic length of domains in the phase-separated structures as the diameter for circular domains and as the width for wires. On going from the left to the right with the same value of n, e.g., n ) 8 or 10, in Figure 7, the characteristic length of the domain tends to be smaller. In other words, domains at the nanometer length scale form when the length of the perfluoroalkyl chain of FmHnA is long. On going from the top to the bottom with the same value of m, e.g., m ) 6 or 8, in Figure 7, the characteristic length of the domain has a tendency to be smaller. In other word, domains at the nanometer length scale form when the length of the alkyl chain of hybrid carboxylic
We have demonstrated that the phase-separated structures of the two-component mixed LB films of HkA and FmHnA are governed by several factors. Characteristic lengths of the phaseseparated structures increase with an increase in the length of hydrocarbon of HkA when HkA is mixed with FmHnA having the same values of m and n. Characteristic lengths decrease with increasing values of m and/or n when FmHnA is mixed with HkA having the same value of k. Phase-separated structures depend also on the fabrication conditions such as the subphase temperature and molar ratio. We discuss the effect of the chemical structures of HkA and FmHnA on the phase-separated structures of the mixed LB films. Phase-separated structures in two-dimensional Langmuir films are determined by two competing interactions of line tension and dipole-dipole interaction.30,31 Line tension is a twodimensional version of interfacial tension and is free energy necessary to create a unit length of domain wall. Large line tension favors the formation of large circular domains. A covalent bond between different atomic species in a molecule can be considered to form a dipole. Dipole-dipole interactions effective in two-dimensional Langmuir films with well-defined molecular orientation are those between dipoles located at terminal positions in the molecules, such as those of methyl and carboxyl groups in HkA and of trifluoromethyl and carboxyl groups in FmHnA. Due to the molecular orientation in Langmuir films, dipole-dipole interaction is repulsive, favoring the formation of elongated and/or small domains. Line tension is dominant in the systems where circular domains at the micrometer length scale are formed. In contrast, dipole-dipole interaction plays a major role in the formation of nanowires in the mixed LB films. The width of nanowires increases with an increase in the value of k in the mixed LB film of HkA and F8H10A. This phenomenon is explained by considering that an increase in k increases the stabilizing energy due to dispersive interaction between HkA molecules. Further consideration is required to understand the phase-separated structures of the mixed LB films of H17A and FmHnA. First, we discuss the effect of the molecular structures on the line tension. We consider a two-dimensional lattice model consisting of two equally sized molecules A and B. The numbers of molecules A and B are NA and NB, respectively. The total number of molecules is N. The Gibbs energy of mixing ∆Gmix is expressed as follows
∆Gmix ) ∆Hmix - T∆Smix
(3)
where ∆Hmix and ∆Smix are the enthalpy and entropy of mixing, respectively. The molar fractions of A and B are defined as xA ) NA/N and xB ) NB/N, respectively. The entropy of mixing is given in eq 4.
∆Smix ) -NkB(xA ln xA + xB ln xB) where kB is the Boltzmann constant.
(4)
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Figure 7. AFM images of the mixed LB films of H17A and FmHnA at a molar ratio of 1/1 transferred at 10 mN m-1 at 30 °C.
We employ a lattice model in which only the change of enthalpy between adjacent molecules is considered. This approximation ignores the change in interaction between molecules that are not in direct contact. The homointeraction energy and the heterointeraction energy represent the interaction between the same chemical species and that between different chemical species, respectively. Let the homointeraction energies of molecules A and B be εAA and εBB, respectively, and the heterointeraction energy between molecules A and B be εAB. When molecules A and B are in contact with each other, the total energy of the system changes by ∆ε ) εAB - (εAA + εBB)/ 2. Let the number of molecules surrounding one molecule be z. By assuming homogeneous mixing of molecules A and B, the number of molecules B surrounding a molecule A is zNB/ N. This shows that the total number of A-B pairs is NAzNB/N or NzxAxB. The enthalpy of mixing is expressed in eq 5.
∆Hmix ) NzxAxB∆ε
(5)
The Gibbs energy of mixing is represented as follows
∆Gmix ) N{zxAxB∆ε - kBT(xA ln xA + xB ln xB)} (6) We consider the change in ∆Gmix, ∆(∆Gmix), caused by varying the structures of hydrophobic moieties of HkA and FmHnA. We take into account only the change in dispersive
interaction. We define ∆k(∆Gmix) as the change in ∆Gmix produced by an increase in k by 1. Let the molecules A and B be HkA and FmHnA, respectively. ∆k(∆Gmix) is given in eq 7.
∆k(∆Gmix) ) NzxAxB∆k(∆ε) ) NzxAxB{∆kεAB - (1/2)∆kεAA} (7) where ∆k(∆ε), ∆kεAA, and ∆kεAB are the change in ∆ε, that in the homointeraction of HkA, and that in the heterointeraction, respectively, produced by an increase in k by 1. AFM images show that the surface of HkA domains is higher than that of the region of FmHnA. This indicates that the heterointeraction can be approximated to remain unchanged when the value of k is increased. Under this approximation, eq 7 is reduced into eq 8.
∆k(∆Gmix) = -(1/2)NzxAxB∆kεAA > 0
(8)
An increase in k decreases the homointeraction and increases the Gibbs energy of mixing. This means that the two molecules are less miscible with each other when the length of the hydrocarbon of HkA is larger. In other words, an increase in the length of the hydrocarbon of HkA increases the line tension, thereby decreasing the total peripheral length. This is consistent with the results of AFM.
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∆n(∆Gmix) ) NzxAxB{∆nεAB - (1/2)∆nεBB} = (1/2)NzxAxB∆nεBB < 0 (10) An increase in n decreases the Gibbs energy of mixing, showing that the two molecules are more miscible with each other when the length of the hydrocarbon of FmHnA is larger. In other words, an increase in the length of the hydrocarbon of FmHnA decreases the line tension, thereby favoring the formation of elongated and/or small domains. This is consistent with the results of AFM. We next consider ∆m(∆Gmix), the change in ∆Gmix caused by an increase in m by 1.
∆m(∆Gmix) ) NzxAxB{∆mεAB - (1/2)∆mεBB}
(11)
where ∆mεAB and ∆mεBB are the change in the heterointeraction and that in the homointeraction of FmHnA, respectively, produced by an increase in m by 1. Figure 8B shows a schematic illustration of changes in the homo- and heterointeractions on elongation of the perfluoroalkyl chain length of FmHnA. An increase in m decreases both the homo- and heterointeractions due to the dispersive interaction between the perfluoromethylene units of FmHnA in the former and between the methylene units of HkA and the perfluoromethylene unit of FmHnA in the latter. The dispersive interaction between a methylene unit and a perfluoromethylene unit is negative and is smaller than that between two perfluoromethylene units.36 This shows ∆mεAB < ∆mεBB. Equation 11 is approximated as follows:
∆m(∆Gmix) ) NzxAxB{∆mεAB - (1/2)∆mεBB} < (1/2)NzxAxB∆mεBB < 0 (12)
Figure 8. Schematic illustration of changes in the homo- and heterointeractions in the cases in which the hydrocarbon length (A) and the perfluoroalkyl chain length (B) of FmHnA are elongated.
We consider ∆n(∆Gmix), the change in ∆Gmix caused by an increase in n by 1.
∆n(∆Gmix) ) NzxAxB∆n(∆ε) ) NzxAxB{∆nεAB - (1/2)∆nεBB} (9)
where ∆n(∆ε), ∆nεAB, and ∆nεBB are the change in ∆ε, that in the heterointeraction, and that in the homointeraction of FmHnA, respectively, produced by an increase in n by 1. Figure 8A shows a schematic illustration of changes in the homo- and heterointeractions on elongation of the hydrocarbon length of FmHnA. An increase in n decreases both the homo- and the heterointeractions due to the dispersive interaction between the methylene units of FmHnA in the former and those of HkA and FmHnA in the latter. This shows ∆nεAB = ∆nεBB. Equation 9 is reduced to eq 10.
An increase in m decreases the Gibbs energy of mixing, showing that the two molecules are more miscible with each other when the length of the perfluoroalkyl chain of FmHnA is larger. In other words, an increase in the length of the perfluoroalkyl chain of FmHnA decreases the line tension, thereby favoring the formation of elongated and/or small domains. This is consistent with the results of AFM. Finally, we consider the effect of the structures of the hydrophobic moieties of the molecules on the dipole-dipole interaction. The dipole-dipole interaction within the domains should be dominant in the nanostructure formation. This shows that we should consider mainly the dipole-dipole interaction between HkA molecules. This means that the dipole-dipole interaction can be approximated to be constant in the mixed LB films of H17A and FmHnA. We consider only the mixed LB films of HkA and F8H10A. Each HkA molecule has effective dipoles at both ends. These dipoles form two layers of dipoles in the domains, one consisting of dipoles of carboxyl groups and the other consisting of dipoles of methyl groups. An increase in the hydrocarbon length increases the distance between the two layers of dipoles by ca. 1 nm at most in the present study. The interaction of the dipoles within the same layer does not depend on the value of k. Each of the two layers of dipoles produces a constant electric field normal to the layer if the boundary effect is neglected. This shows that the interaction between the dipoles positioned in the different layers does not depend on the value of k. These results demonstrate that a lattice model considering only the changes in the line tension with variations in the hydrophobic moieties of fatty acid and hybrid carboxylic acid
Phase-Separated Mixed LB Films can explain qualitatively the phase-separated structures of the mixed LB films. Conclusions Domain structures of phase-separated LB films are governed by a compromise between two competing interactions of line tension and dipole-dipole interaction. Systematic variations in the structures of the hydrophobic moieties of HkA and FmHnA allow for systematic variations of the homo- and heterointeractions, leading to a systematic variation in the line tension. In contrast, dipole-dipole interaction remains unchanged with variations in the hydrophobic moieties. These results demonstrate that the domain structures of the phase-separated LB films can be controlled by varying the hydrophobic moieties of the component molecules. When the line tension is large, circular domains at the micrometer length scale form. Small line tension leads to the formation of domains at the nanometer length scale. In particular, nanowires can be formed systematically. Templates can be fabricated by using three-component mixed LB films containing a silane coupling agent, followed by heat treatment and rinse with organic solvent. The templates have patterns that are controlled by the chemical structures of the component molecules and allow for the confinement of functional material due to the differences in wettability and chemical reactivity at the nanometer length scale.15 The present methodology facilitates the control over the patterns of the templates using self-assembly and will be important in the construction of future materials for the applications to data storage, switching, and sensing. Acknowledgment. This work was partly supported by the Japan Society for the Promotion of Science (Grant No.18350077). References and Notes (1) Mo¨hwald, H. Annu. ReV. Phys. Chem. 1990, 41, 441. (2) McConnell, H. M. Annu. ReV. Phys. Chem. 1991, 42, 171. (3) Overney, R. M.; Meyer, E.; Frommer, J.; Brodbeck, D.; Luthi, R.; Howald, L.; Gunterodt, H.-J.; Fujihira, M.; Takano, H.; Gotoh, Y. Nature 1992, 359, 133. (4) Duschl, C.; Liley, M.; Vogel, H. Angew. Chem., Int. Ed. Engl. 1994, 33, 1274. (5) Ge, S.; Takahara, A.; Kajiyama, T. Langmuir 1995, 11, 1341. (6) Takahara, A.; Kojio, K.; Ge, S.; Kajiyama, T. J. Vac. Sci. Technol., A 1996, 14, 1747.
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