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Phase Transition of Octadecylurea at the Air-Water Interface Studied by Infrared External Reflection Spectroscopy Yoshie Urai, Chikaomi Ohe, and Koichi Itoh* Department of Chemistry, School of Science and Engineering, Waseda University, Tokyo 169-8555, Japan
Masaaki Yoshida, Ken-ichi Iimura, and Teiji Kato Department of Applied Chemistry, Faculty of Engineering, Utsunomiya University, Utsunomiya 321-8585, Japan Received June 14, 1999. In Final Form: January 26, 2000 External infrared reflection spectroscopy was applied to study a phase transition of the monolayer of octadecylurea (OU) at the air-water interface. Upon compression at a constant temperature (6 °C in this study) the monolayer undergoes the transition from a condensed phase (β phase) with a larger limiting area (0.250 nm2/molecule) to another condensed phase (R phase) with a smaller limiting area (0.195 nm2/ molecule). The formation of the β phase and the phase transition accompany discrete IR spectral changes in the νas(CH2), νs(CH2), amide I, amide II, and δ(NH2) vibration regions. The spectral changes indicated that (i) upon compression of the monolayer to form the β phase the alkyl side chain is converted from an irregular state containing gauche conformations to an ordered one consisting of the all-trans conformation and at the same time the peptide bond in the head group is transformed from a fully hydrated state to a hydrogen-bonded one and (ii) upon further compression to the R phase and/or a collapsed state the alkyl side chains are changed again to the irregular state and a part of the peptide bonds is converted to the hydrated state. These results are contrasted with the case of the monolayer of N-methyl-N-octadecylurea (MOU) at the air-water interface, which gives the amide I and δ(NH2) bands characteristic of a fully hydrated state irrespective of the molecular area. Thus, the IR spectral changes provided direct proofs for the explanation that the endothermic first-order β f R phase transition of the OU monolayer takes place through a partial collapse of the two-dimensional ordered array consisting of the hydrogen-bonded network between the peptide bonds.
Introduction A number of studies have been performed on the phase transition of octadecylurea (OU, Figure 1) at air-water interfaces by measuring surface pressure-molecular area (π-A) isotherms, molecular area-temperature (A-T) isobars, and evaporation resistances of the monolayer.1-6 The π-A isotherms indicated that upon compression the OU monolayer undergoes a phase transition from a condensed phase (β phase) with a limiting area of 0.250 nm2/molecule to another condensed phase (R phase) with an area of 0.195 nm2/molecule. The A-T isobars under constant surface pressures5,6 demonstrated that upon increasing the temperature the OU monolayer undergoes a first-order phase transition from the β phase to the R phase with an appreciable area contraction. As the surface pressure increases from 5 to 20 mN/m, the transition temperature decreases from 30 to 23 °C. The analysis of the data proved that the molar enthalpy change of the transition from the β phase to the R phase is 15.2 kJ mol-1 at 10 mN/m,6 indicating that the transition is an endothermic process. In contrast to the contraction-type transition of the OU monolayer, the monolayer of Nmethyl-N-octadecylurea (MOU, Figure 1) at the air-water (1) Adam, N. K. Proc. R. Soc. London, Ser. A 1922, 101, 452. (2) Alexander, A. E. Proc. R. Soc. London, Ser. A 1941, 179, 470. (3) Glazer, J.; Alexander, A. E. Trans. Faraday Soc. 1951, 47, 401. (4) Hunter, D. S.; Barnes, G. T.; Godfrey, J. S.; Grieser, F. J. Colloid Interface Sci. 1990, 138, 307. (5) Kato, T.; Akiyama, H.; Yoshida, M. Chem. Lett. 1992, 565. (6) Shimizu, M.; Yoshida, M.; Iimura, K.; Suzuki, N.; Kato, T. Colloids Surf., A: Physicochem. Eng. Aspects 1995, 102, 69.
Figure 1. Structures of octadecylurea (OU) and N-methylN-octadecylurea (MOU).
interface undergoes a phase transition from a condensed state to an expanded one upon increasing the temperature. These results have been interpreted as follows. The β phase of the OU monolayer takes on an ordered hydrogen-bonded network between the urea head groups. The temperature increase causes a disruption of the network in the OU monolayer, resulting in the contraction. Thus, the β to R phase transition is a two-dimensional counterpart of the melting process of ice. The absence of the contractiontype transition in the MOU monolayer suggests that the hydrogen bonding in the β phase is formed between the NH and CO groups of the peptide bond. Although these explanations conform to the experimental facts, there has been no experimental proof for the existence of the ordered
10.1021/la990759s CCC: $19.00 © 2000 American Chemical Society Published on Web 03/09/2000
Phase Transition of Octadecylurea
hydrogen-bonded network in the β phase and for the disruption of the network associated with the β to R phase transition. Since Dluhy and co-workers7,8 successfully used external IR reflection spectroscopy to elucidate the structures of a series of amhiphile monolayers at air-water interfaces, it has been established as one of the most powerful methods for studying the structures of monolayers at the air-water interfaces.9 For example, Gericke and Hu¨hnerfuss10-12 applied IR external reflection spectroscopy to study the coordination modes of octadecanoic acid monolayers on water subphases containing a series of divalent cations (e.g., Ca2+, Ba2+, Cd2+, and Pb2+). Sakai and Umemura13 studied the molecular orientation changes of stearic acid and cadmium stearate on the water surface at various surface areas by analyzing polarized IR external reflection spectra. Ohe et al.14 used spectroscopy to clarify the carboxylate-counterion interactions and the changes in these interactions during the photopolymerization of a long-chain diacetylene monocarboxylic acid at air-water interfaces in the presence of divalent metal ions, Ba2+, Cd2+, and Pb2+. In this paper we tried to delineate the nature of the β to R phase transition of the OU monolayer at molecular levels by using external infrared (IR) reflection spectroscopy. IR spectral changes were observed for the OU monolayer during the phase transition induced by compression and compared with the IR spectral changes of the MOU monolayer measured at various molecular areas. As mentioned below, the results gave some new insight into the structural aspects of the phase transition of the OU monolayer at the air-water interface. Experimental Section Materials. OU and MOU were prepared and purified by the method already reported.5,6 Methylurea (MU) and ethylurea (EU) were purchased from Tokyo Chemical Industry Co., Ltd., and propylurea (PU) was purchased from Kanto Chemicals Co., Ltd. They were used without further purification. Water used as the subphase was purified by a Millipore water purification system (Milli-Q, 4-bowl). Chloroform used as a solvent for the IR spectral measurements of MU, EU, and PU was of spectral grade and was obtained from Wako Chemicals Co., Ltd. Measurement of External IR Reflection Spectra. The measurements were performed by the method already reported.14 Briefly, a Bio-Rad FTS-45A Fourier transform infrared spectrometer with an MCT detector was used in conjunction with a modified external reflection attachment (a JEOL IR-RSC110), which contains a homemade Teflon-coated small trough (2 cm × 12 cm × 0.5 cm). The air-water interface was kept at 6 ( 0.5 °C by circulating thermostated water in order to reduce thermal agitation of a monolayer at the interface and get IR spectra of the monolayer with reasonable S/N ratios. The monolayer was compressed to each sampling point at a constant velocity (about 0.05 nm2 molecule-1 min-1), and the surface pressure was monitored with a Wilhelmy balance (Nippon Laser & Electronics Lab., model NL-004-PS). All the measurements were performed with unpolarized IR beams at an incident angle of 30°. An aperture of 6 mm in diameter was used to reduce the width of (7) Dluhy, R. A.; Cornell, D. G. J. Phys. Chem. 1985, 89, 3195. (8) Dluhy, R. A.; Cornell, D. G. In Fourier Transform Infrared Spectroscopy in Colloids and Interface Science; Scheuing, D. G., Ed.; ACS Symposium Series 447; American Chemical Society: Washington, DC, 1991; p 192. (9) Mendelsohn, R.; Brauner, J. W.; Gericke, A. Annu. Rev. Phys. Chem. 1995, 46, 305. (10) Gericke, A.; Hu¨hnerfuss, H. J. Phys. Chem. 1993, 97, 12899. (11) Gericke, A.; Hu¨hnerfuss, H. Thin Solid Films 1994, 245, 74. (12) Simon-Kutscher, J.; Gericke, A.; Hu¨hnerfuss, H. Langmuir 1996, 12, 1027. (13) Sakai, H.; Umemura, J. Bull. Chem. Soc. Jpn. 1997, 70, 1027. (14) Ohe, C.; Ando, H.; Sato, N.; Urai, Y.; Yamamoto, M.; Itoh, K. J. Phys. Chem. B 1999, 103, 435.
Langmuir, Vol. 16, No. 8, 2000 3921 the area of the monolayer irradiated by the IR beam in order to reduce curvature (or meniscus effect) on the spectra. The ordinates of all the spectra were expressed by -log(R/R0), where R0 and R are the reflectivities of the pure and film-covered water surface, respectively. The measurements of R and R0 were performed by coaddition of 1024 scans with a resolution of 8 cm-1. A 1.0 × 10-4 mol/L solution of a sample in a mixed solvent of benzene and ethanol (9:1) was added dropwise to the trough water by using a microsyringe, and the solvent was allowed to evaporate for at least 1 h before the IR spectral measurements were started. Measurement of IR Spectra of the Aqueous and Chloroform Solutions of MU, EU, and PU. The IR spectra of the aqueous solutions of MU, EU, and PU were measured by using a Bio-Rad FTS-45A Fourier transform infrared spectrometer equipped with a DTGS detector and demountable fluorite cells. The thickness of the cell was adjusted to 6-12 µm with polyvinylidene chloride spacers. To eliminate the contribution of absorptions due to water, the IR spectrum of water was subtracted, after multiplying by an appropriate factor, from the spectra of the sample solutions. Measurement of Surface Pressure-Area Isotherms. Surface pressure-area isotherms were measured with a microcomputer-controlled Langmuir trough constructed in our laboratory. Isotherms were measured at a constant strain rate of 10% min-1. This means that the compression speed of two barriers confining monolayers is changed exponentially with time. The temperature of the water surface in the trough was kept at 6 °C by a large number of integrated Peltier element modules attached to the back of the base plate of the trough. Details of the instrument were described elsewhere.15
Results Surface Pressure-Area (π-A) Isotherms of OU and MOU Measured at 6 °C. Figure 2, A and B, exhibits the π-A isotherms of OU and MOU monolayers, respectively, on pure water measured at 6 °C. The OU monolayer exhibits the phase transition from the high-area condensed phase, β, with a limiting area of 0.250 nm2/molecule to the low-area condensed phase, R, with an area of 0.195 nm2/ molecule. The result is almost identical to that already reported.4,16 We performed also Brewster angle microscopy observation on the same OU monolayer.17 The result indicated that, in the region of the molecular area (σ), 0.94 g σ g 0.42 nm2/molecule, where the surface pressure (π) is zero, the monolayer contains a lot of island structures (or two-dimensional associated states). There may be a gaseous phase between the islands. Since the gaseous phase has been known to generally have a molecular area > 20 nm2/molecule, the surface concentration of the islands is very small and compression in the region 0.94 g σ g 0.42 nm2/molecule results only in the increase in surface concentration of the island structures. At σ e 0.38 nm2/ molecule the islands begin to coalesce with each other, which causes a slight increase in π (see Figure 2A). In the region 0.27 g σ g 0.26 nm2/molecule there occurs an abrupt increase in π, suggesting that the β phase formation takes place. Around σ ) 0.25 nm2/molecule the increase is terminated, indicating the completion of the coalescence (or the completion of the β phase formation). In contrast to the case of the OU monolayer, the isotherm of the MOU monolayer exhibits a transition from a gaseous to a condensed phase with a limiting area of 0.30 nm2/molecule, as can be seen from Figure 2B. (15) Kato, T.; Tatehana, A.; Suzuki, N.; Iimura, K.; Araki, T.; Iriyama, K. Jpn. J. Appl. Phys. 1995, 34 (Part 2), L911. (16) The limiting area of the R phase is difficult to determine only from the π-A isotherm in Figure 2A. So we borrowed the value of 0.195 nm2/molecule, which had been determined by Hunter et al.4 on the basis of the detailed analyses of the π-A isotherms of the OU monolayer at various temperatures. (17) To be published.
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Figure 2. Surface pressure-molecular area isotherms for octadecylurea (OU) (A) and N-methyl-N-octadecylurea (MOU) (B) measured at 6 °C.
IR Spectra in the 3000-2800 cm-1 Region of OU and MOU Monolayers at the Air-Water Interface. Figure 3 exhibits the IR spectral changes in the 30002800 cm-1 region induced by compression of the OU and MOU monolayers at 6 °C. Negative peaks at 2920 and 2851 cm-1 are due to the CH2 antisymmetric (νas(CH2)) and symmetric stretching (νs(CH2)) vibrations. Negative peaks are usually observed for long alkyl chains of monolayers at the air-water interfaces by using an unpolarized IR beam at an incident angle of 30°.7-13 As the monolayers are compressed, the amplitudes of the νas(CH2) and νs(CH2) bands in Figure 3 increase; this is due to the increase in the surface density of the OU and MOU molecules. The 2920 cm-1 band (νas(CH2)) of the OU monolayer observed at molecular areas (σ) > 0.42 nm2/ molecule shifts to a lower frequency side upon compression, giving a 2916 cm-1 band at 0.38 g σ g 0.25 nm2/molecule; on further compression to 0.21 nm2/molecule the band shifts back to 2920 cm-1. On the other hand, the νas(CH2) band of the MOU monolayer in Figure 3B does not show any frequency shift upon compression. As already pointed out,11,13 the νas(CH2) frequency of the monolayers is conformation sensitive,18,19 giving the band below 2918 cm-1 for the all-trans conformation and the band above 2920 cm-1 for irregular structures containing gauche conformations. Then, the above-mentioned results indicate that the alkyl side chain of the OU monolayer takes on a more or less irregular structure containing gauche conformations at σ g 0.42 nm2/molecule and the highly ordered all-trans conformation at 0.38 g σ g 0.25 nm2/ (18) Snyder, R. G.; Hsu, S. L.; Krimm, S. Spectrochim. Acta, Part A 1978, 34, 395. (19) MacPhail, R. A.; Strauss, H. L.; Snyder, R. G.; Elliger, C. A. J. Phys. Chem. 1984, 88, 334.
Figure 3. IR spectral changes in the region 3000-2800 cm-1 observed during the compression of an OU monolayer (A) and an MOU monolayer (B) at the air-water interface (6 °C). Molecular area is indicated at the right-hand side of each spectrum. The vertical line shows an absorption scale.
molecule. Upon further compression to 0.21 nm2/molecule the side chain of the OU monolayer is converted again to an irregular structure. On the other hand, the alkyl side chain of the MOU monolayer exists in an irregular structure irrespective of the molecular area. Figure 4, A and B, plots the relative intensities of the νas(CH2) and νs(CH2) bands in Figure 3, A and B, respectively, against the molecular area. As Figure 4A shows, the intensities of both the νas(CH2) and νs(CH2) bands increase monotonically as the area decreases in the region σ g 0.42 nm2/molecule, indicating that the alkyl side chain of the OU monolayer takes a random (or uniaxial) orientation. In the smaller area region (0.38 g σ g 0.25 nm2/molecule), however, the intensity of the νas(CH2) band continues the monotonic increase, while the intensity plots of the νs(CH2) band deviate appreciably from the monotonic increase. If the alkyl side chain still
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Figure 4. Relative intensity changes of the νas(CH2) and νs(CH2) bands plotted against the molecular area of the OU (A) and MOU (B) monolayer at the air-water interface (6 °C). The intensities are normalized by those measured at the smallest area: (b) intensity of the νas(CH2) band; (2) intensity of the νs(CH2) band.
takes the random orientation, the intensities of the νas(CH2) and νs(CH2) bands should increase in a similar manner. So, the result suggests that on reducing the molecular area there occurs some changes in orientation and/or ordering of the alkyl chain. In contrast to the case of the OU monolayer, the intensity changes observed for the νas(CH2) and νs(CH2) bands of the MOU monolayer (Figure 4B) increase almost monotonically with the molecular area in the whole range (0.90 g σ g 0.20 nm2/ molecule). The frequency and intensity changes of the νas(CH2) and νs(CH2) bands observed for the OU monolayer (Figures 3A and 4A) correspond to the π-A isotherm and BAM observations explained in the previous section. In the region σ g 0.42 nm2/molecule, where the monolayer consists mainly of the island structures, the intensities of the νas(CH2) and νs(CH2) bands increase monotonically, giving the 2920 and 2851 cm-1 bands characteristic of an irregular alkyl side chain containing gauche conformations. In the region 0.38 g σ g 0.25 nm2/molecule, where the coalescence of the islands and the β phase formation take place, the νas(CH2) band shifts to 2916 cm-1, characteristic of the alkyl side chain in the all-trans conformation, and the intensity change of the νs(CH2) band shows the deviation from the monotonic increase, which can be interpreted as due to an orientation change and/or a change in the ordering of the alkyl side chains associated with the coalescence of the islands.
Figure 5. IR spectral changes in the region 1800-1400 cm-1 observed during the compression of an OU monolayer (A) and an MOU monolayer (B) at the air-water interface (6 °C). Molecular area is indicated at the right-hand side of each spectrum. The vertical line shows an absorption scale.
Upon compression both the νas(CH2) and νas(CH2) bands of the MOU monolayer increase in intensity monotonically and remain at 2920 and 2851 cm-1, respectively. As already explained, the π-A isotherm (Figure 2B) indicates that the monolayer undergoes the transition from a gaseous to a condensed phase. These results indicate that the orientation and the structure of the alkyl side chain in the MOU monolayer remain in an irregular state during the transition. IR Spectra in the 1800-1400 cm-1 Region of OU and MOU Monolayers at the Air-Water Interface. Figure 5, A and B, exhibits the IR spectral changes in the 1800-1400 cm-1 region induced by compression of the OU and MOU monolayers, respectively, at 6 °C. The S/N ratios of the spectra, especially in the region above 1660 cm-1, are not high; this is mainly due to the fact that the vibration-rotation spectrum of water vapor in this region
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was so strongly observed that its contribution could not be compensated by calculating -log(R/R0) (see Experimental Section). In the region below 1660 cm-1, however, the spectra observed in the temperature range 6-10 °C gave almost reproducible features except for bands near 1615 and 1551 cm-1,20 guaranteeing that we can discuss the structures of the monolayers on the basis of the spectra in the 1660-1400 cm-1 region. Positive broad peaks around 1680 cm-1 in Figure 5, which are typical of IR spectra of thin films at the air-water interface are due to the strong change in the refractive index of water.10-12,14 As can be seen from Figure 5A, the IR spectra of the OU monolayer exhibit discrete changes with compression. In the region σ g 0.42 nm2/molecule, the monolayer gives broad features around 1643, 1582, and 1570 cm-1. In the region 0.38 g σ g 0.25 nm2/molecule, the spectral changes occur as follows: the 1643 cm-1 band splits into a broad band centered around 1651 cm-1 and a sharp band at 1632 cm-1; the 1570 cm-1 band becomes indiscernible, because the band shifts to the higher frequency side and combines with a broad feature originating from the 1582 cm-1 band; the 1582 cm-1 band shifts gradually to the higher frequency side, giving a broad 1597 cm-1 band. Upon further compression to 0.21 nm2/molecule, bands corresponding to those at 1643 and 1570 cm-1 appear again and the 1597 cm-1 band gives a much sharper feature. At the same time the 1651 and 1632 cm-1 bands seem to decrease in intensity. As Figure 5B shows, the spectra of the MOU monolayer give rise to broad bands near 1643 and 1593 cm-1, the intensities of which increase monotonically with compression. To assign the bands in Figure 5, the spectra of methyl-, ethyl-, and propylurea (MU, EU, and PU) in aqueous solutions were measured. Figure 6, A and B, illustrates the concentration dependence of the spectra of MU and PU, respectively. Both spectra give bands near 1647 and 1607 cm-1, which can be assigned to amide I of the peptide group and the NH2 deformation mode (δ(NH2)).21 The 1572 cm-1 band observed for PU in Figure 6B can be assigned to amide II of the peptide group.21 The corresponding amide II mode of MU shifts to a higher frequency side, giving a shoulder band on the lower frequency side of the 1609 cm-1 band in Figure 6A. The shift can be explained as due to a difference in the vibrational coupling of a NH inplane bending mode of the peptide bond and a deformation mode of alkyl chains between PU and MU. Figure 6 shows that the intensity ratios and frequencies of the amide I, amide II, and δ(NH2) bands do not change in a wide concentration range, (1.0-6.25) × 10-2 mol/L, indicating that all the bands can be ascribable to fully hydrated head groups. The frequencies observed for the hydrated head groups of MU, EU, and PU are listed in Table 1 together with the amide I, amide II, and δ(NH2) frequencies of the samples in chloroform (0.01 mol/L). In the chloroform solutions the MU, EU, and PU molecules exist in a free (or non-hydrogen-bonded) state. The frequency shifts associated with the conversion from the free to the fully hydrated state of PU are -33 cm-1 (amide I), +42 cm-1 (amide II), and +12 cm-1 (δ(NH2)). (20) The IR spectral measurements of the OU and MOU monolayers were performed several times by changing temperatures in the 6-10 °C range. The IR bands assigned to amide I, amide II, and δ(NH2) modes (vide infra) appear reproducibly, while IR bands due to water vapor change their relative intensities each time. So, we could separate the former bands from the latter superfluous bands. The 1615 cm-1 band, which is observed for the OU monolayer at 0.42 nm2/molecule, and the 1551 cm-1 band, which appears in the spectra of the OU and MOU monolayers (Figure 5), are also due to the water vapor, because the intensities of these bands were not reproducible in the spectra measured at 6-10 °C. (21) Mido, Y.; Murata, H. Nippon Kagaku Zasshi 1969, 90, 254.
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Figure 6. Concentration dependence of the IR spectra of aqueous solutions of methylurea (A) and propylurea (B) at room temperature. The concentrations of the samples for the spectra from the top to bottom are as follows: 1.0, 0.5, 0.25, 0.125, and 6.25 × 10-2 mol/L. Table 1. Frequencies (cm-1) of the Amide I, Amide II, and δ(NH2) Bands of Methyl- (MU), Ethyl- (EU), and Propylurea (PU) MU
EI
PU
in chloroform (0.01 mol/L) in water (1 - 6.25 × 10-2 mol/L) in chloroform (0.01 mol/L) in water (1 - 6.25 × 10-2 mol/L) in chloroform (0.01 mol/L) in water (1 - 6.25 × 10-2 mol/L)
amide I
amide II
δ(NH2)
1682
1537
1599
1647
1609
1680
1528
1599
1647
1572
1607
1680
1530
1593
1647
1572
1605
Comparing the results summarized in Table 1 with the spectra in Figure 5A, we can assign the 1643 cm-1 band
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Table 2. Frequencies (cm-1) and Behaviors of IR Bands Observed for Each State of the Octadecylurea (OU) Monolayer at the Air-Water Interface (6 °C) island structuresa molecular area (σ) in nm2/molecule νas(CH2) νs(CH2) amide I amide II δ(NH2) a
β phase formationa
R phase and/or collapsed phasea
σ g 0.42
0.38 g σ g 0.25
σ ) 0.21
2920 2851 1643 1570 1582 (broad)
shifts to 2916 2851 splits to 1651 (broad) and 1632 (sharp) bands shows intensity decrease and broadening shifts to 1597 (broad)
shifts to 2920 2851 reappears at 1643 reappears at 1570 1597 (sharp)
See text.
(observed at σ g 0.42 nm2/molecule and σ ) 0.21 nm2/ molecule) and the 1651 and 1632 cm-1 bands (0.38 g σ g 0.25 nm2/molecule) to amide I, the 1570 cm-1 band (σ g 0.42 nm2/molecule and σ ) 0.21 nm2/molecule) to amide II, and the 1582 (σ g 0.42 nm2/molecule) and 1597 cm-1 (σ e 0.38 nm2/molecule) bands to δ(NH2). The bands at 1643 and 1593 cm-1 in Figure 5B are due to amide I and δ(NH2), respectively. The assignments of the IR bands and the spectral changes associated with the phase transition of the OU monolayer are summarized in Table 2. Discussion Changes in Orientation and Association States of the Alkyl Side Chains during the Phase Transition of the OU Monolayer. At σ g 0.42 nm2/molecule, where the OU monolayer takes on the island structures, the alkyl side chains are in an irregular state containing gauche conformations. At 0.38 g σ g 0.25 nm2/molecule, where the islands coalesce with each other to form the β phase, the alkyl chains are converted to an ordered state consisting of the all-trans conformation. Presumably, the coalescence increases external pressure to the islands, resulting in the conversion to the ordered state. The conversion of the alkyl groups from the irregular to the ordered state conforms to the relative intensity changes of the νas(CH2) and νs(CH2) bands illustrated in Figure 4A. The alkyl groups at σ g 0.42 nm2/molecule should be in a randomly (or uniaxially) oriented state. In this case the relative intensities should increase in a similar and monotonic way upon compression, which is actually observed in Figure 4A. In the region 0.38 g σ g 0.25 nm2/ molecule, however, the intensities of the νas(CH2) and νs(CH2) bands increase in appreciably different manners. Although the detailed analysis of the polarized IR spectra22 is necessary to determine the orientation of the alkyl groups, the result can be explained by considering that at least some of the alkyl groups forming the islands associate with each other to form the β phase consisting of mosaic structures, each of which has a biaxial symmetry (or an anisotropy). In this case the νas(CH2) and νs(CH2) bands may have different directions of average transition moments, resulting in the different dependence of the intensities on the molecular area. Upon transition to the R phase and/or a collapsed phase (σ ) 0.21 nm2/molecule), the νas(CH2) band appears again at 2920 cm-1, indicating that the transition accompanies disruption of the ordered state.23 Changes in the Hydrogen-Bonded State of the Head Group in the OU Monolayer during the Phase Transition. As summarized in Table 2, the OU monolayer forming the island structure (σ g 0.42 nm2/molecule) gives the amide I, amide II, and δ(NH2) bands at 1643, 1570, (22) The measurements and the analyses of the s- and p-polarized IR spectra of the OU monolayer at the air-water interface are under way in our laboratory.
and 1582 cm-1, respectively. The frequencies of the amide I and II bands correspond well to those observed for the hydrated head group of PU in aqueous solutions (1647 and 1572 cm-1, see Table 1), indicating that the peptide group of the OU monolayer exists in the hydrated state. On the other hand, the frequency of the δ(NH2) band is appreciably lower than that of the corresponding band observed for the hydrated head group of PU (1605 cm-1, Table 1). The former frequency is even lower than that observed for a free state of PU in the chloroform solution (1593 cm-1, Table 1). Although the reason for the abnormally low frequency of the δ(NH2) band remains unknown, we tentatively conclude the following: the hydration to the peptide group results in formation of a kind of a “ cavity ” around the NH2 group at the air-water interface, which may cause the abnormality in the δ(NH2) frequency. In the molecular area range 0.38 g σ g 0.25 nm2/ molecule, where the β phase formation takes place, the amide I band splits into the 1651 (broad) and 1632 (sharp) cm-1 bands, the amide II band becomes indiscernible because of the higher frequency shift, and the δ(NH2) band shifts to 1597 cm-1. These results indicate that both the peptide and NH2 groups undergo appreciable changes in their association states and/or their environments. The frequency lowering of the amide I band to the sharp component at 1635 cm-1 and the frequency increase of the amide II band suggest the formation of a hydrogenbonded state between the peptide bonds. Thus, the spectral changes observed at 0.38 g σ g 0.25 nm2/molecule prove the conversion of the peptide bonds from the hydrated state to the hydrogen-bonded one. The appearance of the broad feature near 1651 cm-1 suggests that some of the peptide groups are converted to an irregularly associated or weakly hydrogen-bonded state concomitantly with the formation of the hydrogen bond network. The shift of the δ(NH2) band to the higher frequency side may be explained by considering that the neighboring NH2 groups in the β phase are associated with each other via water molecules through hydrogen bonding. At σ ) 0.21 nm2/molecule, where the OU monolayer undergoes conversion to the R phase and/or a collapsed phase, amide I gives a sharp band at 1643 cm-1 in addition to the 1651 and 1532 cm-1 components, amide II appears again at 1570 cm-1, and the δ(NH2) band remains at 1597 cm-1, giving a much sharper feature compared to that observed for the β phase. The 1643 and 1570 cm-1 bands are ascribable to the hydrated peptide bond. Thus, upon compression of the OU monolayer a part of the hydrogen(23) The π-A isotherm in Figure 2A indicates that the collapse takes place at a molecular area < 0.21 nm2/molecule. The isotherm measurement performed with the small trough used by the IR spectral measurements, however, is not accurate enough to make an accurate comparison with the isotherm in Figure 2A. In addition, the R phase is thermodynamically unstable at 6 °C and upon completion of the R phase formation the phase is readily converted to a collapsed phase. So, we tentatively consider that at least a part of the OU monolayer exists in a collapsed state at 0.21 nm2/molecule.
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bonded network of the head group is destroyed, resulting in the formation of the hydration state. The intensity increase and sharpening of the 1597 cm-1 band suggests that upon conversion to the R phase and/or the collapsed state the hydrogen bonding between the NH2 groups via water molecules is stabilized. In contrast to the case of the OU monolayer, the IR spectra of the MOU monolayer do not show any discrete change, giving broad bands centered at 1643 and 1593 cm-1, which are due to amide I and δ(NH2), respectively. The result indicates that during the transition from a gaseous to a condensed phase the head group of the monolayer exists in an identical state, that is, a fully hydrated state. Conclusion The important facts elucidated by the spectra are as follows: (i) at relatively larger molecular areas, where the OU and MOU monolayers exist in islands and/or a gaseous state, the alkyl groups take on an irregular structure containing gauche conformations with a more or less random or uniaxial orientation with respect to the surface normal and the peptide bonds exist in a fully hydrated state; (ii) when the MOU monolayer is compressed to the condensed phase, the monolayer keeps the
Urai et al.
same structure and orientation of the alkyl group and the same hydration states as those observed for the islands and/or the gaseous state, which is contrasted with the case of the OU monolayer; (iii) when the OU monolayer forming the islands is compressed to the β phase with the larger limiting area (0.250 nm2/molecule), the alkyl groups are converted to a more or less ordered association state consisting of the all-trans conformation and the peptide bonds in the head groups are converted to the hydrogenbonded state; (iv) upon further compression of the OU monolayer to the R phase with the smaller limiting area (0.21 nm2/molecule) and/or a collapsed state, the alkyl side chains are converted again to an irregular state containing gauche conformations and a part of the peptide bonds in the head group are converted to the hydrated state. Thus, the external IR reflection spectroscopy of the OU monolayer at the air-water interface provided direct proofs for the formation of the two-dimensional ordered array consisting of the hydrogen-bonded peptide bonds in the β phase and for the collapse of the network associated with the β to R phase transition. LA990759S
