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Surface Micelles of CF3(CF2)7(CH2)10COOH on Aqueous La3+ Subphase Investigated by Atomic Force Microscopy and Infrared Spectroscopy Yanzhi Ren, Ken-ichi Iimura, Akiko Ogawa, and Teiji Kato* Satellite Venture Business Laboratory, Utsunomiya UniVersity, Yoto 7-1-2, Utsunomiya, 321-8585, Japan ReceiVed: December 14, 2000; In Final Form: March 1, 2001
CF3(CF2)7(CH2)10COOH spontaneously assembles into monodispersed nanometer-sized micelles after spreading onto the aqueous lanthanum acetate subphase in a Langmuir trough at 283 K, according to the atomic force microscope (AFM) images. Polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS) and ordinary polarized infrared spectroscopy are applied to study the surface micelles in situ on the aqueous subphase and ex situ on the solid substrate, respectively. The long axes of the -(CH2)7- helix and -(CH2)10chain are slightly inclined, with tilt angles of 25 ( 3°, on the solid substrate and basically so inclined on the aqueous subphase. The micelle formation comes from a subtle interplay of the steric hindrance imposed by the bulky -(CH2)7- helix and the van der Waals interaction between the underlying -(CH2)10- chains. The micelle has a CH2 antisymmetric stretching [νas(CH2)] frequency of 2918.0 ( 0.1 cm-1 on the solid substrate and 2916.4 ( 0.2 cm-1 on the aqueous subphase, corresponding to a predominantly trans zigzag planar chain conformation. This trans zigzag chain has a hexagonal packing in the micelle. Elevating the subphase temperature from 283 to 303 K and keeping the temperature at 303 K for some time (30-120 min) causes the micelles to fuse into each other, according to the AFM images. In the fused monolayer, the hydrocarbon segment has a higher νas(CH2) frequency of ∼2920 cm-1, implying that some gauche kinks appear during the thermal treatment. It is found that micelle fusion occurs preferentially at zero surface pressure in loosely packed monolayers rather than at 20 mN/m in tightly packed monolayers. Finally, the combined in situ and ex situ infrared characterization of the surface micelles eliminates transfer artifacts concerning the headgroup orientation.
Introduction Interest in Langmuir-Blodgett (LB) films of partially fluorinated long-chain fatty acids stems from Naselli, Swalen, and Rabolt,1 who transferred monomolecular layers of CF3(CF2)7(CH2)10COOH (11-perfluoroctylundecanoic acid, abbreviated C19F17) from the aqueous Cd2+ subphase onto solid substrates. The LB technique involves, first, the spreading of the material in chloroform solution onto water surface; then, the compression of the monomolecular layer by movable barriers; and finally, the deposition of the compressed monolayer onto a solid substrate.2 The transferred LB films were confirmed to be cadmium salts of C19F17 rather than the acid itself. Kato et al.3 made the first discovery that this material actually forms monodispersed nanometer-sized micelles at the interface of air and aqueous Cd2+ solution. The micelles were transferred onto solid substrates in a single monolayer and observed under an atomic force microscope (AFM). Four other similar materials, CF3(CF2)m(CH2)nCOOH with (m, n) ) (7, 16), (7, 22), (5, 22), and (3, 22), also form monodispersed micelles on the aqueous Cd2+ subphase. The AFM observation reveals that the micelle size and shape change systematically with (m, n). This phenomenon has been explained in terms of the intermolecular interactions between the hydrocarbon and fluorocarbon parts of the molecules.4,5 In addition, we have demonstrated that single monolayers of these partially fluorinated fatty acids have excellent lubrication properties in a wide temperature range * To whom correspondence should be addressed. E-mail: teiji@ cc.utsunomiya-u.ac.jp.
(20-140 °C) and can be used to coat hard disks.4,5 This application constitutes a practical motivation for our work on the partially fluorinated materials. In this paper, we study C19F17 micelles formed on an aqueous lanthanum acetate subphase both in situ and ex situ. AFM and ordinary infrared spectroscopy are applied to investigate the surface micelles deposited onto solid substrates ex situ. Polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS)6,7 is applied to detect the micelle structure on the subphase surface in situ. We have demonstrated the potential of PM-IRRAS technique in detecting the structural features of CF3(CF2)3(CH2)22COOH micelles on the aqueous cadmium subphase.8 Experimental Details The material C19F17 was purchased from Wako Chemical Co. with a guaranteed purity of 99%. The subphase was a lanthanum acetate solution with a concentration of 5 × 10-4 M and a self-buffered pH of 6.7. The compression ratio was 10% per minute. The substrates were CaF2, cover glass, and goldcoated glass slides. In the following, we simply call the goldcoated glass substrate the Au substrate. The cover glass used for AFM observations was sufficiently smooth, having a surface roughness of 0.3 nm. The AFM images were obtained with a Nanoscope III instrument (Digital Instruments Co.) in the tapping mode using etched silicon cantilevers. The deposition speed was 1 mm/min. Infrared spectra of the deposited monolayer were recorded at a resolution of 4 cm-1 and as averages over 500 scans each
10.1021/jp004502a CCC: $20.00 © 2001 American Chemical Society Published on Web 04/24/2001
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SCHEME 1
Figure 1. π-A isotherm of C19F17 on the aqueous lanthanum acetate subphase at a concentration of 5 × 10-4 M and a self-buffered pH of 6.7 at 283 K.
using Nicolet Magna 860 FTIR spectrometer, equipped with a mercury-cadmium-telluride detector and a Whatman 75-52 JA 100 purge gas generator. The normal transmission spectra were measured for two single monolayers on both sides of the CaF2 substrate. The reported transmission absorbance has been divided by 2. The grazing-incidence reflection (GIR) spectra were measured using the Series 501 reflection accessory (Spectra-Tech, Inc.), with p-polarized light incident on the Au substrate surface at an angle of 85°. The GIR spectra were reported as the reflection absorbance of -log(R/R0), where R is the reflectivity of the monolayer-coated Au substrate and R0 is the reflectivity of the bare Au substrate. The PM-IRRAS spectra were recorded at a resolution of 8 cm-1 and as averages of 800 scans each. The beam was first s-polarized and then passed through a ZnSe photoelastic modulator (Hinds, PEM-90). The maximum phase retardation, φ0, at 7100 nm (1404 cm-1) was set to π. The purpose of this setting was to record the two regions of 3000-2800 cm-1 and 1600-1100 cm-1 simultaneously. The PM-IRRAS spectrum was recorded as S ) (Rp - Rs)J2(φ0)/[(Rp + Rs) - (Rp - Rs)J0(φ0)] and reported as -S(d)/S(o), where Rp and Rs represent the p- and s-polarized reflectances, respectively; J2 and J0 represent the second- and zero-order Bessel functions, respectively; and d and o represent the monolayer-covered and uncovered subphases, respectively. The important CH2 antisymmetric stretching frequency was determined by the “centerof-mass” method, using the software Grams/386. The Langmuir trough dedicated to the PM-IRRAS measurements had an excellent temperature-regulating ability.9 The trough bottom was made of copper, to which eight units of integrated Peltier elements (thermomodule) were attached. The subphase temperature was controlled to an accuracy of (0.1 K. The temperature reported in this paper was measured by placing a sheathed platinum wire resistance temperature sensor at the air/subphase interface. The surface pressure was monitored during the PM-IRRAS scan in order to check the monolayer state. X-ray photoelectron spectroscopy (XPS) measurements were performed with an ESCA 5600 spectrometer (Physical Electronics) using a monochromatized Al KR source. The takeoff angle was 45°. The binding energies were referenced to the C 1s spectrum at 284.6 eV. Results We performed two experiments: constant- and cyclictemperature experiments, as depicted in Scheme 1. In the constant-temperature experiments, the La3+ subphase temperature was kept at 283 K from the start of spreading to the end of deposition. 1. Constant-Temperature Experiments. Figure 1 shows the surface pressure-area (π-A) isotherm of C19F17 on the
Figure 2. AFM images of the monolayer deposited at points a and b along the π-A isotherm of Figure 1 in the constant-temperature experiment at 283 K.
aqueous La3+ subphase at 283 K. Figure 2a and b shows the AFM images of the monolayer deposited at points a (0.60 nm2, 0 mN/m) and b (0.33 nm2, 20 mN/m), respectively, along this π-A isotherm. The AFM images show two-dimensional roundshaped nanometer-sized micelles. Figure 2b differs from Figure
Surface Micelles of CF3(CF2)7(CH2)10COOH
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Figure 3. Infrared transmission spectrum of bulk C19F17 cast from chloroform solution onto a CaF2 substrate.
Figure 4. Infrared spectra of the lanthanum salts of C19F17 deposited from the aqueous La3+ subphase at 283 K in the constant-temperature experiments. (a) Transmission spectrum of the isotropic bulk salts smeared from the subphase onto a CaF2 substrate after monolayer collapse. (b) Transmission spectrum of the micelles deposited at 20 mN/m onto a CaF2 substrate. The absorbance is to be read directly from the ordinate. (c) GIR spectrum of the micelles deposited at 20 mN/m onto an Au substrate. The reflection absorbance is 13.8 times the ordinate.
2a in terms of the density of the micelles. Thus, the barrier compression from point a to point b has no effect on the morphology of the micelles, although the surface pressure has increased dramatically. It just brings the already formed micelles together. The micelles are deemed to have formed spontaneously after spreading and before compression. After spreading, 20 min was left for the deprotonation of C19F17 on the La3+ subphase. Figure 3 shows the IR spectrum of the material C19F17 cast from its chloroform solution onto a CaF2 substrate. The band at 1701 cm-1 is due to the antisymmetric CdO stretching mode of carboxylic acids in hydrogen-bonded dimers.10 Figure 4a shows the IR spectrum of the collapsed films manually smeared from the La3+ subphase onto a CaF2 substrate. It is seen that the ν(CdO) band at 1701 cm-1 disappears, indicating complete deprotonation of C19F17 on the La3+ subphase. Figure 4b and c shows the normal transmission and p-polarized GIR spectra of the micelles deposited in a single monolayer, respectively. Table 1 lists the assignment of the bands in Figure 4c. As illustrated in Figure 5, the νas(CH2) and νs(CH2) bands at 2918 and 2850 cm-1, respectively, are due to the trans zigzag methylene sequences, whereas the νas(CH2) and νs(CH2) bands at 2945 and 2873 cm-1, respectively, come from the one or
TABLE 1: Assignment of the IR Bands in Figure 4c for the Lanthanum Salt of C19F17 modea
frequency (cm-1)
νas(CH2) νas(CH2) νs(CH2) νs(CH2) νas(COO) νs(COO) ν(CF3) νas(CF2) νas(CF2) νs(CF2)
2945 2918 2873 2850 1565 1460 1372, 1333 1250 1214 1153
polarizationb ⊥ the C2 axis of CH2 ⊥ the trans zigzag chain axis | the C2 axis of CH2 ⊥ the trans zigzag chain axis ⊥ the C2 axis of COO | the C2 axis of COO ⊥ the long axis of -(CF2)7⊥ the long axis of -(CF2)7⊥ the long axis of -(CF2)7-
νs ) symmetric stretch; νas ) antisymmetric stretch. b “⊥” means “perpendicular to”, and “|” means “parallel to”. a
two connecting CH2 groups adjacent to the fluorocarbon part (see below). It is well-documented that the νas(CH2) frequency is conformation-sensitive and that it can be empirically correlated with the trans/gauche ratio of the hydrocarbon chains.11 The low frequency of 2918 cm-1 is indicative of a preferential trans zigzag planar conformation. Using the center-of-mass method, this frequency is further determined to be 2918 ( 0.1
4308 J. Phys. Chem. B, Vol. 105, No. 19, 2001
Figure 5. Schematic illustration of C19F17 orientation in the twodimensional micelle deposited in the constant-temperature experiment.
cm-1. The CH2 scissoring band [δ(CH2)] appears at 1469 cm-1 in Figure 4b, with a full width at half-maximum (fwhm) of 6.5 cm-1, implying a hexagonal packing of the trans zigzag CH2 sequences.12 The three bands at 1250, 1214, and 1153 cm-1 are due to ν(CF2) modes, according to Chidsey13 and Rabolt’s14 study about the self-assembled monolayer of CF3(CF2)7(CH2)nSH on gold, which contains the same fluorocarbon part as our C19F17. The GIR spectrum in Figure 4c was divided by an optical enhancement factor Eop ) 13.8 for ease of comparison. The physical meaning of Eop is the ratio of the reflection absorbance of a hypothetical isotropic film in the GIR spectrum to the transmission absorbance of the respective film. For the monolayer of CF3(CF2)7(CH2)2SH-Au, Chidsey et al.13 determined an enhancement factor of 11.5 at 3000 cm-1 when going from the transmission spectrum of a free-standing film to the GIR spectrum of the respective gold-supported isotropic film. It is known from Umemura et al.15 that the transmission absorbance of a CaF2-supported film will decrease by a factor of 0.83 in comparison with that of the corresponding free-standing film. Combining Chidsey et al.’s factor of 11.5 and Umemura et al.’s factor of 0.83, we obtain an enhancement factor of Eop ) 11.5/ 0.83 ) 13.8 on going from CaF2 to gold substrate. From a direct comparison of parts c and b of Figure 4, it is seen that the methylene stretching intensities are severely reduced when measured in the GIR mode. Empirically, such a feature implies that the hydrocarbon chain is roughly vertically aligned on the substrate surface, since the electric field is perpendicular to the substrate surface in the p-polarized GIR mode. Assuming a uniaxial orientation of C19F17 in the LB films, the ratio of the transmission absorbance AT to the reflection absorbance AR can be expressed as15
AT/AR ) sin 2 θ/(2Eop cos 2 θ) ) tan 2 θ/2Eop where θ is the tilt angle of a certain transition dipole moment (TDM) with respect to the substrate normal. The θas and θs angles of the νas(CH2) and νs(CH2) TDMs are calculated to be 74° and 72°, respectively. The νas(CH2) mode in Figure 4b contains contributions from both the trans zigzag segment and the CH2 groups adjacent to the -(CF2)7- part (see Figure 5). Because of its two-band nature, its integrated intensity cannot be determined exactly. The tilt angle R of the trans zigzag chain axis is calculated to be 25 ( 3° from cos 2 θas + cos 2 θs + cos 2 R ) 1. Chidsey et al.13 determined an enhancement factor of 17.1 at 1200 cm-1 for a gold-supported isotropic film and a freestanding film. Combining Chidsey et al.’s factor of 17.1 and Umemura et al.’s factor of 0.83, we obtain an enhancement factor of Eop ) 17.1/0.83 ) 20.6 at 1200 cm-1. The θas and θs angles of the νas(CF2) and νs(CF2) TDMs at 1214 and 1153 cm-1 are calculated to be 74° and 72°, respectively. The fluorocarbon long axis, being perpendicular to the νas(CF2) and νs(CF2) TDMs, thus has a tilt angle of 25 ( 3°.
Ren et al. According to Bunn and Howells,16 the -(CF2)7- helix looks like a circular cylinder of cross-sectional area ∼0.32 nm2 when viewed from the long axis. This value is larger than the 0.2 nm2 area occupied by a vertically oriented hydrocarbon chain. In the C19F17 monolayer, the -(CF2)7- helix and the -(CH2)10- chain have similar tilt angles of 25 ( 3°. The mismatch of areas occupied by the fluorocarbon and hydrocarbon parts in the micelles logically entails that adjacent CF3(CF2)7- groups overlap with each other. The bulky CF3(CF2)7- will act to prevent the -(CH2)10- segments from packing closely, hence reducing the intermolecular van der Waals interaction. Similar to our conclusion for the cadmium micelle of C19F17, the lanthanum micelle should have formed as a result of the competition between the steric constraint dictated by the fluoroalkyl helix and the van der Waals interaction between adjacent -(CH2)10- chains. By contrast, stearic acid, without the fluoroalkyl spacer, forms macroscopic domains on the aqueous La3+ subphase. 2. Cyclic-Temperature Experiments. The LB operations in this experiment are illustrated in Scheme 1. The La3+ subphase temperature was 283 K during the 5 min of spreading and 20 min of waiting. After this waiting time, the micelles were considered to have formed on the subphase. Then, the subphase temperature was raised to 303 K and kept there for some time (30-120 min). Finally, the subphase temperature was lowered back to 283 K, and the compression and deposition were carried out at 283 K. Figure 6a and b shows the AFM images of the monolayer treated at (2 nm2/molecule, 0 mN/m) and 303 K for 30 and 120 min, respectively. It is seen that the micelles observed in Figure 2 have fused into each other completely after a thermal treatment of 120 min. The following experiment suggests that micelle fusion occurs preferentially at zero rather than high surface pressure. First, the micelles were compressed to 20 mN/m at 283 K. Second, the subphase temperature was elevated to 303 K and kept at 303 K for 60 min. Third, the temperature was lowered back to 283 K, and the monolayer was transferred to a cover glass substrate. AFM images show almost no symptom of fusion. Thus, the external pressure in tightly packed monolayers severely suppresses the fusion of micelles. Figure 7b and c shows the transmission and GIR spectra, respectively, of the fused monolayer. The GIR spectra of Figures 7c and 4c show similar intensities for the ν(CH2) and ν(CF2) bands, indicating that the fluorocarbon and hydrocarbon orientations do not change perceptibly during micelle fusion. The νas(CH2) frequency is found to be 2920.0 ( 0.2 cm-1, appreciably larger than the 2918.0 ( 0.1 cm-1 value for the micelle. A higher νas(CH2) frequency is always regarded as indicative of a worse conformational order along the hydrocarbon chain, i.e., a lower trans/gauche ratio. Therefore, more gauche kinks have appeared during the thermal treatment. There has been a vast amount of literature about temperaturevariable infrared studies of organized monolayers containing trans zigzag hydrocarbon chains on solid substrates.2,4,5 The common phenomenon is that, below a certain temperature, the conformation and orientation of the hydrocarbon chain are stable and the νas(CH2) frequency undergoes no shift. Although this temperature is substrate-dependent and material-specific, it is usually higher than the room temperature of 303 K. For a cadmium stearate monolayer5 deposited on a Au substrate, the νas(CH2) band undergoes no change below 313 K. The present treating temperature of 303 K on the La3+ subphase does produce a discernible νas(CH2) shift, accompanied by a reorientation of the hydrocarbon chain. It seems that the monolayer
Surface Micelles of CF3(CF2)7(CH2)10COOH
J. Phys. Chem. B, Vol. 105, No. 19, 2001 4309 On the other hand, the νas(CH2) and νs(CH2) intensities undergo no discernible changes from part a to part b of Figure 8, indicating that the tilt angle of the hydrocarbon long axis undergoes no discernible changes after experiencing 303 K for 2 h. According to the PM-IRRAS selection rule,6 the strong appearance of the νas(CH2) and νs(CH2) bands in both parts a and b of Figure 8 implies that their TDMs preferentially align parallel to the subphase surface. In other words, the hydrocarbon segment is roughly vertically aligned on the subphase for both the surface micelles and the fused domains. This conclusion agrees well with that obtained for the C19F17 monolayer deposited on solid substrates. The C-F stretching intensities at 1241, 1204, and 1149 cm-1 undergo no discernible changes from part a to part b of Figure 8, indicating that the -(CF2)7- orientation on the subphase undergoes no perceptible change after experiencing the thermal treatment. The CF2 stretching bands at 1204 and 1149 cm-1 have strong and positive intensities in Figure 8a and b, indicating that their TDMs are preferentially horizontally oriented on the subphase. In other words, the -(CF2)7- helix is roughly vertically aligned on the subphase for both the surface micelles and the fused domains. This conclusion agrees well with that obtained for the C19F17 monolayer deposited on solid substrates. Discussions
Figure 6. AFM images of the monolayer deposited at (0.60 nm2/ molecule, 0 mN/m) in the cyclic-temperature experiment: (a) treated at 303 K for 30 min, (b) treated at 303 K for 120 min.
on the aqueous subphase is less stable than that deposited on solid substrates. We have examined the thermal stability of the lanthanum micelles on solid substrates by both AFM and infrared spectroscopy. Thermal treatment at 303 K on solid substrates for any elongated time has no effect either on the micelle morphology or on the νas(CH2) frequency. 3. In Situ PM-IRRAS Study. Figure 8a and b shows the in situ PM-IRRAS spectra of the surface micelle in the constanttemperature experiment and the fused monolayer in the cyclictemperature experiment at 20 mN/m on the La3+ subphase, respectively. The νas(CH2) frequency is found to be 2916.4 ( 0.2 cm-1 for the surface micelle and 2920.2 ( 0.2 cm-1 for the fused domain. The remarkable increase in the νas(CH2) frequency suggests that some gauche methylene kinks have been included during the thermal treatment at 303 K. This conclusion is consistent with that obtained for the monolayers deposited on solid substrates.
1. Band Assignments. In our previous paper about the cadmium micelle of C19F17,4 we assigned the strongest band at 1204 cm-1 between the two ν(CF2) bands at 1241 and 1149 cm-1 in Figures 3 and 4b to the C-C-C deformation mode of the fluorocarbon helix and assumed that it was polarized parallel to the helix long axis. This assignment follows Naselli, Swalen, and Rabolt.1 In view of our recent study on the LB films of CF3(CF2)16COOH, this assignment needs revision. The bands at ∼1210 and ∼1150 cm-1 due to the -(CF2)16- helix are about three times stronger than those due to the -(CF2)7- helix of C19F17 in the transmission spectra, whereas they are much weaker than the C19F17 bands in the GIR spectra (not shown). The long-chain CF3(CF2)16COOH is deemed to be almost vertically aligned on the aqueous Cd2+ subphase. Therefore, these two bands must be polarized perpendicular to the fluorocarbon helix axis. In the C-F stretching region 13001100 cm-1 of the transmission spectrum, only two bands at ∼1210 and ∼1150 cm-1 are observed for CF3(CF2)16COOH. We then immediately assign the bands at ∼1210 and ∼1150 cm-1 to the νas(CF2) and νs(CF2) modes, respectively. Chidsey et al.13 also claimed that the band around 1210 cm-1 is polarized perpendicular to the -(CF2)7- helix axis. Rabolt seems to adopt their assignment in a later paper.14 The spectrum of pure C19F17, cast from its chloroform solution, gives two νas(CH2) bands at ∼2945 and 2918 cm-1 in Figure 3. Judging from its low frequency, the band at 2918 cm-1 is readily ascribed to trans zigzag planar methylene sequences. The shoulder band at ∼2945 cm-1 has also been noted by Rabolt, who assigns it to the CH2 groups near the fluorocarbon part of the molecule. It is reasonable to assume that the stretching vibrations due to the several CH2 groups near the CF3(CF2)7- part have a frequency different from that of the other CH2 groups. A broad band ranging from 3200 to 2600 cm-1 is superimposed on the three C-H stretching bands at 2945, 2918, and 2849 cm-1. The broad absorption from 3200 to 2600 cm-1 is known to be due to the O-H stretching modes of carboxylic acids in hydrogen-bonded dimers.10 The CH2 scissoring band appears at 1469 cm-1 with an fwhm of 6.5 cm-1,
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Figure 7. Infrared spectra of lanthanum salts of C19F17 deposited from the aqueous La3+ subphase in the cyclic-temperature experiment. (a) Transmission spectrum of the isotropic bulk salts smeared from the subphase onto a CaF2 substrate after monolayer collapse. (b) Transmission spectrum of the fused monolayer deposited at 20 mN/m onto a CaF2 substrate. The absorbance is to be read directly from the ordinate. (c) GIR spectrum of the micelles deposited at 20 mN/m onto an Au substrate. The reflection absorbance is 13.8 times the ordinate.
Figure 8. PM-IRRAS spectra of C19F17 monolayers measured in situ on the aqueous lanthanum acetate subphase at 20 mN/m and 283 K: (a) surface micelles in the constant-temperature experiment, (b) fused domains in the cyclic-temperature experiment. The spectra have been baseline corrected and smoothed by the Savitsky-Golay method with polynomial 2 and five points in the region of 3000-1280 cm-1.
indicating a hexagonal packing of the trans zigzag CH2 segment in the cast films. 2. Subcell Packing. As concluded above, the trans zigzag CH2 segment has a hexagonal subcell packing in the surface micelle deposited on solid substrates. It is an open problem that the molecular packing in Langmuir monolayers might undergo some changes during transfer, even though the deposition is carried out at a constant surface pressure. An in situ PM-IRRAS examination is deemed necessary. The PM-IRRAS spectrum of Figure 8a gives the CH2 scissoring band at 1469 cm-1 with an fwhm of 6.5 cm-1, also corresponding to a hexagonal packing of the trans zigzag CH2 sequences. The present PM-IRRAS spectrum was recorded at a resolution of 8 cm-1, rather than 4 cm-1, in order to increase the signalto-noise ratio. The resolution of 8 cm-1 allows for the
determination of the hexagonal or triclinic packing of the hydrocarbon chains, leaving the issue of orthorhombic packing unsettled.17 Hydrocarbon chains have mainly three kinds of crystalline packing: triclinic, hexagonal, and orthorhombic. The orthorhombic packing of hydrocarbon chains gives a doublet of δ(CH2) bands at 1474 and 1462 cm-1,12 which can not be resolved at a resolution of 8 cm-1. We have reported that a single monolayer containing a -(CH2)n- chain with n e 16 cannot exhibit orthorhombic packing on an aqueous subphase at 283 K,18 because of the small interchain constraint. The present material C19F17, bearing a short -(CH2)10- chain, obviously cannot have an orthorhombic packing. In addition, previous authors have all proposed a hexagonal packing for the hydrocarbon chain of C19F17 in its cadmium salt monolayers.1,4
Surface Micelles of CF3(CF2)7(CH2)10COOH SCHEME 2
3. Headgroup Reorientation and Coordination. The extinction coefficient of the νas(COO) band at around 1530 cm-1 is about two times that of the νs(COO) band at around 1410 cm-1, as can be estimated from the isotropic bulk spectrum of Figure 4a. In the GIR spectrum of Figure 4c, the intensity ratio of νas(COO)/νs(COO) is approximately 1/2, implying that the νas(COO) TDM is preferentially parallel to the substrate surface and that the νs(COO) TDM is preferentially perpendicular to it. Here, an in situ PM-IRRAS examination is deemed necessary, as the LB transfer process might cause some headgroup reorientations. In the PM-IRRAS spectrum of Figure 8a, the νas(COO) band appears strongly at 1540 cm-1, whereas the νs(COO) mode gives weak and positive features at 1430 and 1410 cm-1. The PMIRRAS selection rule is such that a vibrational mode reaches its positive and negative maximum intensities when its TDM has tilt angles of 90 and 0°, respectively.6 At around 39°, the band will actually disappear from the spectrum. Judging from their intensities, the νas(COO) TDM should have a tilt angle near 90°, whereas the νs(COO) TDM is expected to have a tilt angle slightly larger than 39°. In other words, the C2 axis of the COO group on the subphase surface is largely inclined. This orientational feature is different from that on the solid substrate, where the C2 axis of the COO group is preferentially vertically aligned. Therefore, reorientation of the carboxylate headgroup occurs during the LB transfer process. In addition, the νas(COO) band in Figure 8b of the fused domains is obviously weakened in comparison with that in Figure 8a, suggesting that reorientation of the carboxylate headgroup takes place during micelle fusion. According to Nakamoto,19 the frequency separation ∆ ) νas(COO) - νs(COO) can be used as a tool to determine the coordination type in metal carboxylates. The ionic, monodentate, chelating bidentate, and bridging bidentate coordination complexes of anhydrous acetates have ∆ values of 164, 200-300, 40-80, and 140-170 cm-1, respectively. In the present case, the frequency separation ∆ ) 1530-1410 ) 120 cm-1 is close to that of the bridging bidentate coordination, as illustrated in Scheme 2. X-ray photoelectron spectroscopy measurements of the monolayer on Au substrates show that the atomic ratio of fluorine to lanthanum is 13.7 ( 0.5. Here, the inaccuracy (0.5 has been calculated from three measurements. Because the material C19F17 contains 17 fluorine atoms, the molar ratio of the CF3(CF2)-7 helix to La is (13.7 ( 0.5)/17 ≈ 0.80 ( 0.03. The La atom is located at the monolayer bottom, and its photoelectrons are expected to be attenuated by the overlayering material. Its XPS intensity should be weakened in comparison with that of the outer fluorine atom.20 Often, this problem can be overcome by setting the takeoff angle to 0° (the present takeoff angle is 45°). Consequently, the atomic ratio of F to La derived from XPS measurements should be slightly larger than the actual value of 13.7 ( 0.5. The actual ratio R of the CF3(CF2)7- helix to La should be no larger than 0.80 ( 0.03, i.e., R e 0.80 ( 0.03. This formula immediately eliminates an ionic interaction between the COO- and La3+ ions, whose charge balance rule
J. Phys. Chem. B, Vol. 105, No. 19, 2001 4311 would require R ) 3. The possibilities of monodentate and chelating bidentate coordination types, which would entail R ) 1, are also eliminated. It is seen from Scheme 2 that only the bridging bidentate coordination can give R < 1. R ) (n + 2)/ (n + 3), where n is the repeat number. With R ) (n + 2)/(n + 3) ) 0.80, n is determined to be in a safe range of 0-2. Conclusion Two experiments, constant- and cyclic-temperature experiments, have been performed on the lanthanum monolayer of C19F17 with the aid of AFM, XPS, and ex situ and in situ infrared measurements. From the constant-temperature experiment done at 283 K, the following conclusions are reached. C19F17 is totally deprotonated and forms monodispersed micelles on the aqueous lanthanum acetate subphase at 283 K, within 20 min after spreading. In the micelles, the hydrocarbon segment and the fluorocarbon rod are inclined at similar tilt angles. The area mismatch between the two parts of the molecule demands that adjacent CF3(CF2)7- groups should overlap with each other. The hydrocarbon segment takes a predominantly trans zigzag conformation and has a hexagonal subcell packing. The cyclic-temperature experiments were designed delicately so that the effect of the thermal treatment can be unambiguously concluded. Whereas the thermal treatment was done at 303 K and zero surface pressure, the subsequent compression and deposition were carried out at 283 K. Elevation of the subphase temperature from 283 to 303 K prior to compression causes the micelles to fuse into large domains. The micelle fusion involves a reorientation of the headgroup and a disordering of the hydrocarbon chain. Finally, possible artifacts associated with the LB transfer process were excluded from the conclusions with the aid of in situ PM-IRRAS measurements. Acknowledgment. The authors appreciate the financial support from the Satellite Venture Business Laboratory of Utsunomiya University. References and Notes (1) Naselli, C.; Swalen, J. D.; Rabolt, J. F. J. Chem. Phys. 1989, 90, 3855. (2) Ulman, A. An Introduction to Ultrathin Organic Films from Langmuir-Blodgett to Self-Assembly; Academic Press: New York, 1991 and references therein. (3) Kato, T.; Kameyama, M.; Ehara, M.; Iimura, K.-I. Langmuir 1998, 14, 1786. (4) Ren, Y.; Iimura, K.-I.; Kato, T. J. Chem. Phys. 2000, 113, 1162. (5) Ren, Y.; Asanuma, M.; Iimura, K.-I.; Kato, T. J. Chem. Phys. 2001, 114, 923. (6) Blaudez, D.; Turlet, J.-M.; Dufourcq, J.; Bard, D.; Buffeteau, T.; Desbat, B. J. Chem. Soc., Faraday Trans. 1996, 92, 525. (7) Ren, Y. Z.; Hossain, Md. M.; Iimura, K.-I.; Kato, T. Chem. Phys. Lett. 2000, 325, 503. (8) Ren, Y. Z.; Iimura, K.-I.; Kato, T. Chem. Phys. Lett. 2000, 332, 339. (9) Kato, T.; Tatehana, A.; Suzuki, N.; Iimura, K.; Araki, T.; Iriyama, K. Jpn. J. Appl. Phys. 1995, 34, L911. (10) Lin-Vien, D.; Colthup, N. B.; Fateley, W. G.; Grasselli, J. G. Infrared and Raman Characteristic Frequencies of Organic Molecules; Academic Press Inc.: San Diego, CA, 1991; pp 137-140 (about the O-H stretching and CdO stretching modes of hydrogen-bonded carboxylic acid dimers). (11) Mendelsohn, R.; Brauner, J. W.; Gericke, A. Annu. ReV. Phys. Chem. 1995, 46, 305. (12) Weers, J. G.; Scheuing, D. R. In Fourier Transform Infrared Spectroscopy in Colloid and Interface Science; Scheuing, D. R., Ed.; American Chemical Society: Washington, D.C., 1991; p 91 [about the hexagonal (a singlet δ(CH2) band at 1469-1468 cm-1), triclinic (a strong, narrow, and sharp singlet δ(CH2) band at 1474-1470 cm-1), and orthorhombic (two δ(CH2) bands at ∼1474 and ∼1462 cm-1) packing of trans zigzag hydrocarbon chains. Disordered hydrocarbon chains give a broad δ(CH2) band at 1465 cm-1].
4312 J. Phys. Chem. B, Vol. 105, No. 19, 2001 (13) Chidsey, C. E. D.; Loiacono, D. N. Langmuir 1990, 6, 682. The following values were used in evaluating the optical enhancement factor at 3000 cm-1: film refractive index n˜ ) 1.45 + 0.0i and 1.45 + 0.1i, corresponding to a transparent reference film and an absorbing film, respectively; film thickness d ) 1.0 nm; substrate refractive index n˜ ) 2.0 + 21.0i. The following values were used in evaluating the optical enhancement factor at 1200 cm-1: film refractive index n˜ ) 1.33 + 0.0i and 1.33 + 0.1i, corresponding to a transparent reference film and an absorbing film, respectively; film thickness d ) 1.0 nm; substrate refractive index n˜ ) 9.0 + 48.0i. The enhancement factor was averaged over grazing incidence angles from 82.4° to 87.6°, i.e., assuming a 5° divergence around the incidence angle of 85°.
Ren et al. (14) Tsao, M.-W.; Hoffmann, C. L.; Rabolt, J. F.; Johnson, H. E.; Castner, D. G.; Erdelen, C.; Ringsdorf, H. Langmuir 1997, 13, 4317. (15) Umemura, J.; Kamata, T.; Kawai, T.; Takenaka, T. J. Phys. Chem. 1990, 94, 62. (16) Bunn, C. W.; Howells, E. R. Nature 1954, 174, 549. (17) Simon-Kutscher, J.; Gericke, A.; Hu¨hnerfuss, H. Langmuir 1996, 12, 1027. (18) Ren, Y. Z.; Iimura, K.-I.; Kato, T. J. Chem. Phys. 2001, 114, 1949. (19) Nakamoto, K. Infrared and Raman Spectra of Inorganic and Coordination Compounds; Wiley: New York, 1986. (20) Briggs, D.; Seah, M. P. Practical Surface Analysis; Wiley: New York, 1983.
