J. Phys. Chem. B 1999, 103, 435-444
435
Carboxylate-Counterion Interactions and Changes in These Interactions during Photopolymerization of a Long-Chain Diacetylene Monocarboxylic Acid at Air-Water Interfaces: External Infrared Reflection Absorption Spectroscopic Study C. Ohe, H. Ando, N. Sato, Y. Urai, M. Yamamoto, and K. Itoh* Department of Chemistry, School of Science and Engineering,Waseda UniVersity, Shinjuku-ku, Tokyo 169-8555, Japan ReceiVed: September 10, 1998; In Final Form: NoVember 16, 1998
External infrared reflection absorption spectroscopy was used to study the carboxylate-counterion interactions and changes in these interactions during the photopolymerization of a long-chain diacetylene monocarboxylic acid, 10,12-pentacosadiynoic acid (DA), at air-water interfaces in the presence of divalent metal ions, Ba2+ (pH 7.7), Cd2+ (pH 6.8), and Pb2+ (pH 6.0). Upon reducing the molecular area (0.80 f 0.18 nm2/molecule), the DA monolayer on each subphase exhibited discrete frequency changes of an IR band due to a carboxylate antisymmetric stretching vibration (νas(COO-)), indicating discrete changes in the coordination and/or association states of the carboxylate groups. Ab initio molecular orbital calculations were applied to confirm the empirical relationships between the frequency differences of the asymmetric and symmetric stretching bands of the carboxylate group and the modes of coordination of the group to metal ions. On the basis of the relationships, it was proposed that, when the molecular area is reduced, the carboxylate group in the DA monolayer on the Ba2+ subphase changes its coordination mode from a bridging state to a bidentate one, while the carboxylate group in the DA monolayer on the Cd2+ subphase keeps a bidentate coordination state. It was also suggested that the DA monolayer on the Pb2+ subphase exhibits a coordination change from a bridging to bidentate state upon compression. IR spectral changes in the νas(COO-) and νs(COO-) regions observed during UV-irradiation-induced polymerization of the DA monolayers were similar to the IR spectral changes observed by compressing the monolayers without irradiation. The results indicated that the polymerization induces more densely packed states of the carboxylate groups in the monolayers.
Introduction The Langmuir-Blodgett (LB) films of diacetylene derivatives of monocarboxylic acids such as 10,12-pentacosadiynoic acid (CH3(CH2)11CtC-CtC(CH2)8COOH, abbreviated to DA; see Figure 1) is known to undergo polymerization under UV irradiation to form the LB assemblies of polydiacetylenes (PDA).1-3 The mechanism of the polymerization and the structures of the LB assemblies have been extensively studied because of their unique optical nonlinear effects.3 It is also known that the monolayers of the derivatives undergo the UVinduced polymerization under certain conditions. Mino et al.4 applied absorption spectroscopy to monitor the polymerization of a DA monolayer on a water subphase containing the Ca2+ ion, indicating that, when the molecular area was reduced from 0.29 to 0.22 nm2/molecule by increasing the pH value of the subphase from 5.0 to 6.8 at 20 mN/m, a PDA monolayer formed on the subphase shifted its absorption maximum from 650 to 540 nm. The blue shift was interpreted in terms of a conversion of the PDA backbone from an ordered state to a less-ordered one. Yamada and Shimoyama5 could form a large single crystal of PDA at an air-water interface (pH 6.7-6.8, [CdCl2] ) 2.5 × 10-4 mol/L) by UV irradiation of an ordered domain of a DA monolayer, which was prepared by employing a rather concentrated developing solution in benzene (10-2 mol/L); according to the authors, the use of the concentrated solution is a key factor to obtain the ordered domain because, when a more diluted developing solution was used, fast diffusion of the solvent caused the formation of an amorphous monolayer of
Figure 1. Structures and abbreviations of the samples used in this study.
DA, preventing the formation of the ordered monolayer. Although these studies suggested that the coordination states, orientations, and packing of DA monolayers at the air-water interfaces determine the reactivity of the monolayers and the properties of PDA, the structural information has remained almost unknown because of the lack of adequate techniques. Since Dluhy and co-workers6,7 successfully used external infrared reflection spectroscopy to elucidate a series of amphiphile monolayers at the interfaces, it has been established as one of the most powerful methods to study the structures of monolayers at air-water interfaces.8 Gericke and Hu¨hnerfuss9-11 applied the spectroscopy to clarify the structures and coordination modes of octadecanoic acid monolayers on water subphases containing a series of divalent cations (e.g., Ca2+, Ba2+, Cd2+, and Pb2+) at various pH values. They measured IR bands due to antisymmetric and symmetric stretching vibrations (νas(COO-) and νs(COO-)) of the carboxylate group and concluded, for example, that Ca2+ interacts mainly ionically with the carboxy-
10.1021/jp983669p CCC: $18.00 © 1999 American Chemical Society Published on Web 01/05/1999
436 J. Phys. Chem. B, Vol. 103, No. 3, 1999 late group, while Cd2+ and Pb2+ are largely coordinated covalently with the carboxylate group.10 In the present paper, we measured the IR spectra of DA monolayers on water subphases containing divalent metal ions, Ba2+, Cd2+, and Pb2+, and compared the IR spectra with those of the monolayers of a series of saturated long-chain fatty acids, pentadecanoic acid (CH3(CH2)13COOH, abbreviated to PA), octadecanoic acid (CH3(CH2)16COOH, abbreviated to OA), and eicosanoic acid (CH3(CH2)18COOH, abbreviated to EA) (see Figure 1). The spectral features of the monolayers are strongly dependent on measurement conditions (pH, temperature, kinds and concentrations of the divalent metal ions, etc.).10,11 We therefore measured the IR spectra of the long-chain fatty acid monolayers under exactly the same conditions employed for the measurements of the DA monolayers in order to clarify differences in the carboxylate-counterion interactions between the DA and fatty acid monolayers. Ab initio molecular orbital calculations were performed to get explicit information about the relationship between the antisymmetric and symmetric stretching frequencies of the carboxylate groups and its coordination modes to the divalent metal ions. The relationship was used to know how the coordination modes in the DA monolayers change with the reduction of the molecular areas. UV-irradiation-induced IR spectral changes were also measured for the DA monolayers at the air-water interfaces to elucidate changes in the coordination modes and local environments of the carboxylate groups associated with the conversion from the DA to PDA monolayers. Experimental Section Materials. DA, obtained from Wako Chemicals Co. Ltd., and PA, OA (stearic acid), and EA (arachidic acid) obtained from Aldrich Chemicals Co. Ltd. were used without further purification. Chloroform purchased from Kanto Chemicals Co. Ltd. was of spectral grade and used without purification. Other chemicals obtained form commercial sources are of reagent grade and were used as received. Measurement of External Infrared Reflection Spectra. External infrared reflection spectral measurements were performed by using a Bio-Rad FTS-45A Fourier transform infrared spectrometer equipped with an MCT detector and a modified external reflection attachment of a JEOL IR-RSC110, which contains a homemade Teflon-coated small trough (2 cm × 15 cm × 0.5 cm). The attachment and the trough were placed in a compartment into which dry air was flowed at a constant rate to keep the humidity over the air-water interface at a constant level; this allowed us an appropriate match for water vapor bands compensation to get the IR spectra of monolayers with reasonable S/N ratios. The air-water interface was kept at a constant temperature within (0.5 °C by circulating thermostated water around the trough. The monolayers were compressed to each sampling point at a constant velocity (about 0.05 nm2 molecule-1 min-1), and the surface pressure was monitored by a Wilhelmy balance (Nippon Laser & Electronics Lab, model NL-004-PS). After each compression step, the monolayer was allowed to relax for at least 5 min before starting the IR measurement. All the measurements were performed by using unpolarized infrared beams at an incidence angle of 30°. An aperture of 4 mm in diameter was used to reduce the width of the area of the airwater interface irradiated by the IR beam in order to avoid curvature (meniscus) effects on the spectra. The ordinates of all spectra were expressed by -log(R/R0), where Ro and R are the reflectivities of the pure and film-covered water surface, respectively. The spectra were taken by coadding of 1024 scans
Ohe et al. with a resolution of 8 cm-1. To reduce thermal agitation of a monolayer at the air-water interface, the trough temperature was kept at 6 °C. A 1.0 × 10-3 mol/L chloroform solution of a sample was added to the trough water surface by using a microsyringe, and the solvent was allowed to evaporate for at least 30 min before the IR spectral measurements were started. The divalent metal salts were added to the trough waters, and the concentrations and pH values of their solutions were as follows: BaCl2 (2 × 10-4 mol/L, pH 7.7), CdCl2 (2 × 10-4 mol/L, pH 6.8), PbCl2 (1 × 10-3 mol/L, pH 6.0). The pH values were adjusted by adding NaHCO3 and NaOH. The degree of dissociation of the carboxyl groups of the monolayers depends on the pH values of the subphases as well as the kind of divalent metal ions.12 The pH value for each divalent metal ion was determined so that almost all the carboxyl groups of the monolayers were in the dissociated state. Measurements of Raman Spectra of Monolayers at Water-Air Interfaces. Raman spectra of a PDA monolayer at an air-water interface was measured by using a trough similar to that used for the IR measurements and a Kaiser Optical System Holospec f/1.8 spectrometer equipped with a liquidnitrogen-cooled CCD detector (Princeton Instruments Co. Ltd., model CCD-1100-PB). A cw Nd:YAG laser (λ ) 532 nm, Spectron Laser Systems, model DTL-116A) was used as an excitation source. The s-polarized excitation laser light was introduced through a window at the bottom of the trough at an incident angle of 50° (in a total reflection mode) with respect to the surface normal of the interface. Raman scattering light was collected by using a collection lens system with f ) 50 mm in the direction normal to the air-water interface and introduced into the spectrometer. The power of the excitation light was reduced to less than 1 mW at the window of the trough. Measurement of Surface Pressure-Area Isotherms. The surface pressure/area isotherms were recorded by using a Langmuir trough (Kyowa Kaimen Co. Ltd., model HBM-AP2) equipped with a Wilhelmy balance under the corresponding conditions used for the IR spectral measurements. The compression velocity was about 0.005 nm2 molecule-1 min-1. Irradiation of UV Light. A high-pressure mercury lamp (Oriel Co. Ltd., model 6283) was used for irradiation of a monolayer of DA at the air-water interface. A quartz lens (f ) 10 cm) was used to adjust the light intensity to 11 mW/cm2 at the interface. Computational Procedures Ab initio molecular orbital calculations were performed by using the Gaussian 94 program13 on an Alpha 500D/256Ua workstation (Aspen Systems). The self-consistent reaction field (SCRF) method was employed to take into consideration the effects of surrounding dielectric media.14,15 All the calculations were carried out at the Hartree-Fock (HF) level. The 6-31+G** basis sets were used for hydrogen, carbon, and oxygen atoms, and a Huzinaga’s basis set,16 Ba[5s3p2d](43333/4333/43), was used for the Ba2+ ion. The cavity radii employed in the SCRF calculations were determined by using the “volume” key word in the program. The dielectric constant of the surrounding medium has been assumed to be 85.4 (the value at 6 °C). All the calculated frequencies were multiplied by 0.8929 to get a better fit to the observed frequencies. Results and Discussion Surface Pressure/Area (π-A) Isotherms of DA and Pentadecanoic Acid (PA). Figure 2A exhibits the π-A isotherms of DA monolayers measured at 6 °C in the presence of the Ba2+
Carboxylate-Counterion Interactions
Figure 2. π-A isotherms observed for DA (A) and PA (B) at 279 K measured under the following conditions: pH 12 without divalent metal ions; 2 × 10-4 mol/L Ba2+, pH 7.7; 2 × 10-4 mol/L Cd2+, pH 6.8; 1 × 10-3 mol/L Pb2+, pH 6.0.
(2 × 10-4 mol/L, pH 7.7), Cd2+ (2 × 10-4 mol/L, pH 6.8), and Pb2+ (1 × 10-3 mol/L, pH 6.0) ions together with the isotherm of a DA monolayer measured at 6 °C and pH 12. The isotherms of a PA monolayer measured under the same conditions employed for the measurement of the isotherms of DA are shown in Figure 2B for comparison purposes. Upon compression, the isotherm of the DA monolayer at pH 12 initially shows a slow rise of the surface pressure, suggesting the existence of a liquid expanded (LE) phase.17 At about 0.35 nm2/molecule, the rise of the pressure with compression is a little accelerated, which is ascribable to a transition to a liquid condensed (LC) phase. The π-A isotherm of the DA monolayer recorded in the presence of Cd2+ ion is similar to that reported by Tieke et al.1 In contrast to the case at pH 12.0, the isotherm gives a steep rise with a limiting area of 0.26 nm2/molecule, indicating the existence of a solid (S) phase. In addition, a shoulder of the isotherm at about 0.24 nm2/molecule suggests that there occurs either a partial collapse of the S phase or a transition from the S phase to another solid phase. The isotherms measured in the presence of both the Ba2+ and Pb2+ ions give similar features
J. Phys. Chem. B, Vol. 103, No. 3, 1999 437 to those observed for a low density form of a DA monolayer on a subphase containing the Ca2+ ion.18 The isotherms at first show a slow rise ascribable to an LE phase and/or an LE/LC transition and then a rather sharp rise, indicating a transition from the LC phase to an S phase. Shoulders observed for the isotherms at a molecular area of about 0.25 nm2/molecule (19 mN/m) on the Ba2+ subphase and at about 0.26 nm2/molecule (27 mN/m) on the Pb2+ subphase suggest that either a partial collapse of the solid phase or a transition from the S state to another solid phase takes place at these molecular areas. The π-A isotherms of a PA monolayer are virtually identical with those already reported.19 The isotherm observed for PA in an ionic state at pH 12 shows a gradual rise in the region of 0.40.3 nm2/molecule, indicating an LE/LC phase transition, and a sharp rise in the region below 0.25 nm2/molecule, ascribable to an LC/S phase transition. The π-A isotherms observed for the PA monolayer in the presence of the Pb2+, Cd2+, and Ba2+ ions exhibit features characterized by the S phases with limiting areas of 0.19, 0.22, and 0.23 nm2/molecule, respectively. IR Spectra of DA Monolayers at Air-Water Interfaces at pH 12 and in the Presence of Ba2+, Cd2+, and Pb2+ Ions. Figure 3 illustrates the IR spectra measured as a function of molecular area for the monolayers of DA at pH 12 (A) and in the presence of the Ba2+ (B) (pH 7.7), Cd2+ (C) (pH 6.8), Pb2+ (D) (pH 6.0). The spectra give IR bands in the 1565-1491, 1474-1467, and 1425-1407 cm-1 regions, which are ascribable to COO- antisymmetric stretching (νas(COO-)), CH2 scissoring (δ(CH2)), and COO- symmetric stretching (νs(COO-) vibrations, respectively.10,20 As the molecular area of the DA monolayer on each subphase is reduced, the δ(CH2) and νs(COO-) bands do not shift appreciably, while the νas(COO-) band shows a discrete frequency lowering. The IR spectrum measured at pH 12 gives rise to a broad band around 1565 cm-1, due to the νas(COO-) mode at molecular areas above ca. 0.30 nm2/molecule, and a sharp band at 1559 cm-1 upon compression. Comparison of the spectral change with the π-A isotherm indicates that the broad band is ascribable to the carboxylate group of the monolayer forming the LE phase and the sharp band to the group of the monolayer forming the LC phase. In the LE phase, the carboxylate groups
Figure 3. Molecular area dependence of the IR spectra in the range of 1800-1350 cm-1 measured for the monolayer of DA at 279 K: (A) pH 12 without divalent metal ions; (B) pH 7.7, Ba2+ (2 × 10-4 mol/L); (C) pH 6.8, Cd2+ (2 × 10-4 mol/L); (D) pH 6.0, Pb2+ (1 × 10-3 mol/L). The vertical line indicates a scale of the ordinate of each spectrum given by -log(R/R0) (see text). The number at the right-hand side of each spectrum indicates a molecular area.
438 J. Phys. Chem. B, Vol. 103, No. 3, 1999 exist in an inhomogeneous or irregular state, causing the broad feature of the νas(COO-) band. Upon compression to the LC phase, the carboxylate groups form an associated state containing a more or less ordered arrangement of the carboxylate groups in the monolayer, giving the sharp νas(COO-) band. The DA monolayer on the subphase containing the Ba2+ ion (Figure 3B) at molecular areas above ca. 0.30 nm2/molecule gives rise to a broad 1541 cm-1 band due to νas(COO-), which shifts to 1532 cm-1 upon further compression. When the molecular area is reduced below 0.25 nm2/molecule, there appears a sharp νas(COO-) band at 1513 cm-1 in addition to the 1532 cm-1 band. The broad 1541 cm-1 band corresponds to a phase in which the monolayer does not show any detectable surface pressure (or a gas phase) and to the LE and LC phases, while the 1532 cm-1 band is due to the S phase. The sharp feature at 1513 cm-1 may be ascribed either to the collapsed phase or the second solid phase. These frequencies are appreciably lower than those observed for the monolayers of DA in the free ionic state (1565 and 1559 cm-1), indicating that there exists a bonding interaction between the Ba2+ ion and the carboxylate group. The nature of the bonding will be discussed later based on the results of ab initio MO calculation. The IR spectra of the DA monolayer on the subphase containing the Cd2+ ion (Figure 3C) gives rise to a broad νas(COO-) band centered at 1530 cm-1 at molecular areas above 0.40 nm2/molecule. When the monolayer is compressed further, the νas(COO-) band becomes sharp and shifts to 1527 cm-1. The frequency lowering of these bands compared to the corresponding bands measured for the monolayer in the free ionic state at pH 12 (1565 and 1559 cm-1; see Figure 3A) indicates that the carboxylate groups are coordinated to the Cd2+ ion, as will be discussed later. As already explained, the π-A isotherm of the DA monolayer on the Cd2+ subphase (Figure 2A) indicates a LC/S transition near 0.26 nm2/molecule and either a partial collapse of the solid state or a transition to another solid phase near 0.24 nm2/molecule. The fact that the νas(COO-) band is observed at an almost constant frequency (1527 cm-1) below 0.30 nm2/molecule suggests that the coordination mode and/or a local environment of the carboxylate group of the DA monolayer remains unchanged during the transitions. Figure 3D indicates that the IR spectra of the monolayer of DA on the subphase containing the Pb2+ ion exhibit successive frequency lowering of the νas(COO-) band upon compression. The correspondence between the frequency changes and the molecular areas can be summarized as follows: 1528 cm-1 (for molecular area (A) g ∼0.40 nm2/molecule) f 1508 cm-1 (0.40 nm2/molecule > A g ∼0.30 nm2/molecule) f 1502 cm-1 (0.30 nm2/molecule > A g ∼0.25 nm2/molecule) f 1491 cm-1 (A < 0.25 nm2/molecule). Comparing these results with the π-A isotherm observed in the presence of the Pb2+ ion (Figure 2A), we can recognize that the 1528 cm-1 band corresponds to a gas phase, in which the monolayer does not show any detectable surface pressure, the 1508 cm-1 band corresponds to the LE and LC phases, and the 1502 cm-1 band corresponds to the S phase. The 1491 cm-1 band may be ascribable either to the collapsed phase or to the second S phase. Table 1 summarizes the discrete frequency changes observed under various conditions for the νas(COO-) band induced by the compression of the DA monolayer together with the corresponding changes for the νs(COO-) bands. To get further insight into the correlation between the νas(COO-) frequencies and the molecular areas for the DA monolayers, we measured the molecular area dependence of the IR spectra of the monolayers of a series long-chain fatty acids at pH 12 as well as in the presence of the Ba2+, Cd2+, and Pb2+ ions.
Ohe et al. TABLE 1: Frequencies (cm-1) of the νas(COO-) and νs(COO-) Bands and Frequency Differences (∆ν ) νas(COO-) - νs(COO-)) Observed for the Monolayers of CH3(CH2)11CtC-CtC(CH2)8COOH (DA), PA (CH3(CH2)nCOOH, n ) 13), OA (n ) 16), and EA (n ) 18) in an Ionic State and in the Presence of Ba2+, Cd2+, and Pd2+ Ionsa DA ionic 1565 f 1559 (1405) ∆ν 160 f 154 Ba2+ 1541 f 1532 f 1513 (1405 f 1407) ∆ν 136 f 125 f 106 Cd2+ 1530 f 1527 (1410) ∆ν 120 f 117 Pb2+ 1528 f 1508 f 1502 f 1491 (1425) ∆ν 103 f 83 f 77 f 66
PA
OA
EA
1556 (1405) 1560 (1405) 151 155 1539 (1405) 1540 f 1523 (1402 f 1405) 134 138 f 118 1539 (1410) 1539 (1410) 129 129 1515 (1409) 1512 (1402)
135 1539 (1420) 119 1512 (1420)
106
92
100
1558 (1405) 153 1544 (1409)
a
The numbers in parentheses indicate the frequencies of the νs(COO-) bands.
IR Spectra of the Monolayers of Pentadecanoic Acid (PA), Octadecanoic Acid (OA), and Eicosanoic Acid (EA) at the Air-Water Interface at pH 12 and in the Presence of Ba2+, Cd2+, and Pb2+ Ions. Figures 4 and 5 show the molecular area dependence of the IR spectra of the monolayers of PA and OA (see Figure 1), respectively, on the water subphases at pH 12 and on those containing the Ba2+ (pH 7.7), Cd2+ (pH 6.8), and Pb2+ (pH 6.0) ions. Table 1 compares the νas(COO-) and νs(COO-) frequencies observed for DA monolayers with those for the PA, OA, and EA monlayers. (The spectra of the EA monolayer are not shown in this paper because the spectral changes are similar to those observed for the PA and OA monolayers.) The IR spectra of the monolayer of OA on water subphases containing Ba2+ (1 × 10-3 mol/L, pH 5.5), Cd2+(1 × 10-3 mol/L, pH 6.0), and Pb2+(1 × 10-3 mol/L, pH 6.0) have already been reported by Gericke and Hu¨hnnerfuss10 and Simon-Kutscher et al.11 The spectra measured in the presence of the Ba2+ ion at the molecular areas of 0.22 and 0.197 nm2/ molecule gave the νas(COO-) bands at 1560, 1542, and 1512 cm-1;11 these features are appreciably different from those in Figure 5B, which exhibits the bands at 1540 and 1523 cm-1 (a shoulder band). The former spectra were measured at pH 5.5 and the latter at pH 7.7. According to Kabayashi et al.,12 the ratio of the barium salts of the carboxyl groups to the total amount of the groups in the LB films of EA depends on the pH values of the water subphase, giving about 20% at pH 5.5 and about 70% at pH 7.7. Then, the discrepancy of the IR spectra may be due to the difference in the degree of the dissociation of the OA monolayers. To elucidate the reason for the discrepancy, however, we need to study more detailed pH dependence of the IR spectra of the monolayer. The spectra measured in the presence of the Cd2+ ion10 are virtually identical with those in Figure 5C, giving a prominent νas(COO-) band at 1537 cm-1. In the case of subphases containing the Pb2+ ion, the IR spectra measured by Gericke and Hu¨hnnerfuss10 gave the νas(COO-) band, which is composed of a main component at 1512 cm-1 and shoulder ones at 1542 and 1523 cm-1, while the spectra in Figure 5D gives a 1512 cm-1 band irrespective of the molecular area. The latter spectra were measured at 279 K, while the former spectra were recorded at 294 K. Presumably, at the lower temperature, the monolayer is stabilized to form only one structure associated with the main component at 1512 cm-1. Figures 4 and 5 and Table 1 indicate that the molecular area dependence of the IR spectra of the monolayers on each subphase exhibits general trends irrespective of the lengths of
Carboxylate-Counterion Interactions
J. Phys. Chem. B, Vol. 103, No. 3, 1999 439
Figure 4. Molecular area dependence of the IR spectra in the range of 1800-1350 cm-1 measured for the monolayer of PA at 279 K: (A) pH 12 without divalent metal ions; (B) pH 7.7, Ba2+ (2 × 10-4 mol/L); (C) pH 6.8, Cd2+(2 × 10-4 mol/L); (D) pH 6.0, Pb2+ (1 × 10-3 mol/L). The vertical lines and the number at the right-hand side of each spectrum are the same as those in Figure 3.
Figure 5. Molecular area dependence of the IR spectra in the range of 1800-1350 cm-1 measured for the monolayer of OA at 279 K: (A) pH 12 without divalent metal ions; (B) pH 7.7, Ba2+ (2 × 10-4 mol/L); (C) pH 6.8, Cd2+ (2 × 10-4 mol/L); (D) pH 6.0, Pb2+ (1 × 10-3 mol/L). The vertical lines and the number at the right-hand side of each spectrum are the same as those in Figure 3.
the alkyl chains of the fatty acids. The trends can be summarized as follows. (i) In the cases of subphases at pH 12, all the monolayers give a broad νas(COO-) band at 1556-1565 cm-1 irrespective of the molecular areas. (ii) The frequencies of the νas(COO-) bands of the monolayers on the subphases containing the Ba2+, Cd2+, and Pb2+ ions do not change upon compression, except for the case of OA in the presence of Ba2+, where a 1523 cm-1 band appears at molecular areas below 0.20 nm2/ molecule in addition to a main band at 1540 cm-1 (Figure 5B). (iii) The frequencies of the monolayers on the subphases containing the divalent metal ions are appreciably smaller than those observed for the monolayers in the absence of the metal ions, indicating the existence of bonding interactions between the metal ions and the carboxylate groups. (iv) The frequencies and widths of the νas(COO-) bands of the monolayers depend on the kinds of the divalent metal ions in the subphases. The monolayers on the Ba2+ subphase give a broad νas(COO-) band at 1539-1544 cm-1, while the monolayers on subphases containing the Cd2+ and Pb2+ ions give the νas(COO-) bands
at 1539-1542 and 1512-1515 cm-1, respectively. The widths of the νas(COO-) bands observed for the Ba2+ subphase remain almost constant upon compression, while the widths observed for the Cd2+ and Pb2+ subphases become sharper as the monolayers are compressed. As illustrated by the π-A isotherm of PA in Figure 2B, the monolayer in a free ionic state exhibits an LC/S transition as the molecular area is decreased. Then, the independence of the spectral features on the molecular area (trend (i)) can be interpreted by considering that the carboxylate groups already form an associated state in the LC phase and, as the monolayer is compressed, the associated states coalesce with each other to form the S phase. Trend (ii) indicates that the νas(COO-) bands of the fatty acid monolayers in the presence of the divalent metal ions do not show any appreciable frequency shifts associated with the (LE + LC)/S transition. The result is in a marked contrast with the case of the DA monolayers on the Ba2+ and Pb2+ subphases, where the discrete changes in the widths and frequencies of the
440 J. Phys. Chem. B, Vol. 103, No. 3, 1999
Ohe et al. TABLE 2: Comparison of the Calculated and Observed Frequencies in the 1600-1000 cm-1 Region for Propionate Ion
Figure 6. Types of interaction between a carboxylate ion and a metal cation: (A) an ionic state; (A′) a hydrated ionic state; (B) a bridging state; (C) a bidentate state; (D) a unidentate state.
νas(COO-) bands are observed as the molecular areas are reduced. As already indicated,10,19 there may be a strong tendency to form an ordered array of the alkyl groups within the monolayers of the saturated normal fatty acids, which leads to the formation of a more or less rigid two-dimensional associated state, even when the monolayers are in either a gas phase or an LE phase. Upon compression, the associated states are assembled with each other to form the S phase. Trend (ii) tells that the strong tendency to form the associated states determines the local environments (or intermolecular interactions) of the carboxylate groups and the type of bonding between the groups and the divalent metal ions. As can be seen from parts A and B of Figure 2, the π-A isotherms of the DA monolayers on the Ba2+ and Pb2+ subphases exhibit more expanded film characteristics compared to those of the PA monolayers. The existence of the diacetylene moiety prevents the formation of the associated states, making the carboxylate groups in the DA monolayers, especially on the Ba2+ and Pb2+ subphases, more labile to environmental changes; this may cause the relaxation of the groups to discrete coordination and/or association states as the molecular areas are compressed. The discrete frequency changes observed for the DA monolayers and the dependence of the frequencies on the kind of the divalent metal ions for the fatty acid monolayers (Trend (iv)) should be explained in terms of the types of association and bonding to the metal ions of the carboxylate group. This will be discussed in the following sections. Interaction Modes of the Carboxylate Groups with the Metal Ions of the Monolayers on the Air-Water Interfaces. The types of interactions of metal ions with the carboxylate groups can be classified into ionic (Figure 6, A or A′), bridging (Figure 6B), bidentate (Figure 6C), and unidentate (Figure 6D) states.21 Tackett22 recorded the νas(COO-) and νs(COO-) frequencies of acetate ions in aqueous solutions containing alkali metal, alkali earth metal, and transition metal ions and proposed an empirical relationship between the frequency difference, ∆ν ) νas(COO-) - νs(COO-), and the types of bonding, which can be summarized as follows: ionic (164 cm-1), bridging (140-170 cm-1), bidentate (40-80 cm-1), unidentate (200300 cm-1). Gericke and Hu¨hnerfuss10 analyzed the IR spectra of the OA on the subphases containing Ca2+, Cd2+, and Pb2+ ions by using a similar correlation modified to long-chain saturated fatty acids, i.e., ionic (158 cm-1), bridging (150-200 cm-1), bidentate (80-110 cm-1), and unidentate (130-160 cm-1). On the basis of the modified correlation, they concluded that the Ca2+ ion forms mainly the ionic structure, while the Cd2+ and Pb2+ ions take on the bidentate structure. Before applying this kind of correlation to analyze the results of the present paper, we performed ab initio MO calculations in order to examine the reliability of the correlations. Ab Initio Molecular Orbital Calculation of the Correlation between the νas(COO-) and νs(COO-) Frequencies and the Coordination Modes of the Ba2+ Ion to the Carboxylate Group. The calculations were performed on a series of normal
obsd frequency (cm-1)a
calcd frequency (cm-1)b
1552 1468
1572 1446 1437 1418 1396 1371 1281 1238 1071
1426 1415 1375 1302 1243 1079
assignmentsc νas(COO-) [94] δd(CH3) [83] δd(CH3) [95] δ (CH2) [94] νs(COO-) [59], ν (C-C) [25] δs(CH3) [94] ω(CH2) [63], νs(COO-) [22] τ(CH2) [64], F(CH3) [16] F(CH3) [30], F(CH2) [28], τ(CH2) [16]
a Data taken from ref 25. b Calculated frequencies for the model in Figure 6A′ (see text). c νas(COO-) and νs(COO-) denote COOantisymmetric and symmetric stretching modes, respectively. δd(CH3), δs(CH3), and F(CH3) denote degenerate deformation, symmetric deformation, and in-plane rocking modes of the methyl group, respectively. δ (CH2), τ(CH2), and ω(CH2) show scissoring, twisting, and wagging modes of the methylene group, respectively. ν(C-C) is a stretching mode of the C-C bond connecting the methyl and methylene groups, and F(CH3) and F(CH2) are rocking modes of the CH3 and CH2 groups, respectively. The number in brackets after each vibrational mode indicates the potential energy distribution (%).
fatty acids, CH3(CH2)nCOO- (n ) 1-4), in their ionic states as well as in their coordination states, with the Ba2+ ion taking the structures shown in Figure 6. (The unidentate coordination state was excluded from the calculation because the mode is improbable for the coordination structures of the barium carboxylates.10,22) The effectiveness of the SCRF method at the Hartlee-Fock level, which was employed by the present calculation, has been well proved by Nara et al.,23 who applied the method to calculate the normal frequencies of the acetate ion interacting with a series of metal ions (Na+, Mg2+, and Ca2+) in various modes. In the present paper, the normal frequency calculations were at first performed to reproduce the vibrational frequencies observed for the aqueous solutions of the normal fatty acids in a free ionic state. The calculated frequencies for the structures optimized by assuming the structure in Figure 6A′ gave a much better fit to the observed frequencies than those for the structures optimized by assuming the structure in Figure 6A.24 Thus, hydrogen bonding of a water molecule to the carboxylate group must be taken into consideration in addition to the reaction field effect to reproduce the observed frequencies for the fatty acids in the ionic state. Table 2 compares the observed frequencies in the 1800-1000 cm-1 region for sodium propionate (n ) 1) in an aqueous solution25 with the calculated ones for the ionic state of the acid. The calculated frequencies coincide with the observed frequencies including those of νas(COO-) and νs(COO-) bands within (22 cm-1. The result indicated also that the symmetric stretching mode of the COO- group is coupled with vibrational modes of the alkyl side chains including a C-C stretching mode to give an IR band assigned to the νs(COO-) band. The effect of the coupling can be clarified by listing the calculated frequencies of the νas(COO-) and νs(COO-) bands for the series of fatty acids in the hydrogen-bonded ionic state (Figure 6A′), as summarized in Table 3. It is clear from Table 3 that the calculated frequency difference, ∆ν ) νas(COO-) - νs(COO-), for the ionic state gives a value in the 172-164 cm-1 region for n g 2. The optimized structures of the series of fatty acids in the bridging as well as bidentate coordination states with the Ba2+ ion were also determined, and the normal frequency calculation was performed on each structure. The results of the
Carboxylate-Counterion Interactions
J. Phys. Chem. B, Vol. 103, No. 3, 1999 441
TABLE 3: Calculated and Observed Frequencies (cm-1) of the νas(COO-) and νs(COO-) Bands for a Series of Fatty Acids (CH3(CH2)nCOO-, n ) 1-5) in Ionic and Ba2+ Coordination States n ) 1a n ) 2b n ) 3b n ) 4b ionic state
(A′)c
bridging state (Ba2+) (B)c bidentate state (Ba2+) (C)c
νas(COO-)
calcd obsd νs(COO-) calcd obsd ∆νd calcd obsd νas(COO-) calcd νs(COO-) calcd ∆νd calcd νas(COO-) calcd νs(COO-) calcd ∆νd calcd
1572 1552 1396 1415 186 138 1555 1433 122 1578 1431 144
1570 1551 1398 1408 172 143 1550 1431 119 1558 1449 109
1563 1551 1399 1408 164 143 1555 1440 115 1548 1454 94
1564 1551 1398 1408 166 143 1555 1439 126 1557 1459 98
a Data taken from ref 25. b Data taken from ref 26. c Refer to Figure 6. d ∆ν ) νas(COO-) - νs(COO-).
calculations for the νas(COO-) and νs(COO-) bands are listed also in Table 3. The calculated ∆ν values for the bridging state are about 120 cm-1, irrespective of the chain lengths of the alkyl groups (n), while those for the bidentate state decrease as the chain length increases, giving 98 cm-1 for n ) 4. Assuming that the calculated values for n ) 4 can be applied to the ionic states of the longer fatty acids, we can predict that the ∆ν values for the ionic, bridging, and bidentate states are of the following order: ∆ν (166 cm-1, ionic) > ∆ν (126 cm-1, bridging) > ∆ν (98 cm-1, bidentate). Thus, the calculated ∆ν values for the ionic and bidentate states correspond well to those proposed by Tackett22 (164 and 40-80 cm-1) and Gericke and Hu¨hnerfuss10 (158 and 80-110 cm-1). On the other hand, the calculation predicts the appreciably smaller ∆ν value for the bridging state compared to those already proposed by Tackett21 (140-170 cm-1) and Gericke and Hu¨hnerfuss10 (150-200 cm-1). The alkaline earth metal ions such as Ca2+ and Ba2+ have the electronic configuration of a rare gas atom, while the Cd2+ and Pb2+ ions have the configurations involving d and f orbitals. On the basis of these facts, it has been considered that in the presence of the alkaline earth ions the carboxylate group is mainly in an ionic state and that the Cd2+ and Pb2+ ions tend to form a bridging or bidentate coordination bond with substantial covalent character. The results of the calculations, however, indicated that (i) the alkaline earth metal ions may form stable bridging and bidentate coordination bonds, (ii) the ∆ν values predicted for these coordination states are substantially smaller than those for the ionic states, and (iii) the ∆ν values can be used to identify a coordination state of a carboxylate-Ba2+ bond from the observed ∆ν values. As can be seen from Table 1, the ∆ν values observed for the monolayers at pH 12 are in the range of 151-160 cm-1; the values correspond well to the calculated ones (166 cm-1), confirming that the carboxylate groups exist in an ionic state (probably in a hydrated state). In the presence of the Ba2+ ion, the monolayers of DA and OA at larger molecular areas and those of PA and EA in the whole range of the area give the ∆ν values of 135-136 cm-1. These values are similar to the calculated frequency difference for the bridging coordination state (126 cm-1), indicating that the carboxylate groups of the monolayers are in this coordination state. The lowering in the ∆ν value of 136 f 125 cm-1 associated with compression observed for the DA monolayer and that of 136 f 125 cm-1 observed for the OA monolayer (Table 1) are too small to be explained in terms of a change in the coordination state (e.g., a
change from the bridging to bidentate state). Presumably, the lowerings are due to a change in the local environment of the carboxylate group associated with the LC/S transition. The further lowering of 125 f 106 cm-1 observed in the DA monolayer on the Ba2+ subphase may be caused by a coordination change from the bridging to bidentate state because the difference of 106 cm-1 coincides well with the calculated frequency difference for the bidentate state (98 cm-1). The discontinuity or the shoulder observed for the π-A isotherm near 0.25 nm2/molecule corresponds to this coordination change. As already explained, Simon-Kutscher et al.11 measured the IR spectra of the OA monolayer on the Ba2+ subphase, which gave the νas(COO-) bands at 1560, 1540, and 1512 cm-1. According to the authors, the 1560 and 1540 cm-1 bands are indicative of ionic interactions and the 1512 cm-1 band is indicative of a preferential covalent interaction. These conclusions are partly in line with those of the present study. Table 1 indicates that the ∆ν values observed for the monolayers on the Cd2+ subphase are in the range of 129-117 cm-1, irrespective of the kinds of monolayers and the molecular areas. It is clear from parts A and B of Figure 2 that, although the limiting area of the DA monolayer on the Cd2+ subphase (about 0.25 nm2/molecule) is slightly larger than that of the PA monolayer on the Cd2+ subphase (0.22 nm2/molecule), the π-A isotherms of these monolayers are similar to each other, giving a steep rise associated with the formation of the S phase. The strong condensation effect of the Cd2+ ion10,19 on the DA monolayer may cause the similarity of the isotherms and the formation of the coordination state of the monolayer almost identical with those of the PA, OA, and EA monolayers. According to Gericke and Hu¨hnerfuss,10 the OA monolayer on the Cd2+ subphase exists in the bidentate state. We therefore concluded that the DA, PA, and EA monolayers on the Cd2+ subphase also take on the bidentate state. The ∆ν values observed for the monolayers on the Pb2+ subphase exhibit discrete changes (103 f 83 f 77 f 66 cm-1) as in the case of the DA monolayer on the Ba2+ subphase, although the values for the former case are appreciably smaller than those for the latter. In addition, the π-A isotherm of the DA monolayer on the Pb2+ subphase exhibits similar features to that of the monolayer on the Ba2+ subphase. On the basis of these similarities, we tentatively concluded that the monolayer on the Pb2+ subphase at first takes on a bridging coordination state, giving the ∆ν value of 108 cm-1, and that upon compression it is converted to a bidentate state with ∆ν ) 66 cm-1. The ∆ν changes of 103 f 83 f 77 cm-1 may be ascribed to changes in the environment of the carboxylate group in the bridging coordination state. The ∆ν values observed for the PA, OA, and EA monolayers on the Pb2+ subphase are similar to that for the DA monolayer in the bridging coordination state, suggesting that the carboxylate group in the former monolayers also takes on the bridging coordination state. The reason the νas(COO-) frequencies (and then the ∆ν values) observed for the monolayers on the Cd2+ and Pb2+ subphases are appreciably smaller than those for the monolayers on the Ba2+ subphase may be ascribed to the fact that the transition metal ions containing the d and f electrons tend to form stronger coordination bonds with the carboxylate group than the Ba2+ ion. To confirm the relationship between the ∆ν values and the coordination modes of the Cd2+ and Pb2+ ions, we need to perform the ab intio molecular orbital calculation of the νas(COO-) and νs(COO-) frequencies for cayboxylatetransition metal complexes in various coordination states; the calculation is under way in our laboratory.
442 J. Phys. Chem. B, Vol. 103, No. 3, 1999
Ohe et al.
Figure 7. IR spectral changes during UV irradiation of the DA monolayer with the molecular area of 0.40 nm2/molecule on water subphases under the following conditions: (A) pH 7.7, Ba2+ (2 × 10-4 mol/L); (B) pH 6.8, Cd2+ (2 × 10-4 mol/L); (C) pH 6.0, Pb2+ (1 × 10-3 mol/L). The vertical lines are the same as those in Figure 3. The irradiation time is indicated at the right-hand side of each spectrum (s, second; m, minute).
Figure 8. IR spectral changes during UV irradiation of the DA monolayer with the molecular area of 0.25 nm2/molecule on water subphases under the following conditions: (A) pH 7.7, Ba2+ (2 × 10-4 mol/L); (B) pH 6.8, Cd2+(2 × 10-4 mol/L); (C) pH 6.0, Pb2+ (1 × 10-3 mol/L). The vertical lines are the same as those in Figure 3. The irradiation time is indicated at the right-hand side of each spectrum (s, second; m, minute).
IR Spectral Changes Induced by Photopolymerization of DA Monolayers at Air-Water Interfaces in the Presence of the Ba2+, Cd2+, and Pb2+ Ions. Parts A-C of Figure 7 illustrate the spectral changes observed during UV irradiation of the DA monolayers at the molecular area of 0.40 nm2/ molecule in the presence of the Ba2+, Cd2+, and Pb2+ ions, respectively. The irradiation results in the broadening of the νas(COO-), δ(CH2), and νs(COO-) bands, and in the case of the Pb2+ subphase, all the bands disappear after a prolonged irradiation. These spectral changes may be ascribed to a photodegradation of the monolayers. On the other hand, the monolayers at a more compressed state (0.25 nm2/molecule) exhibit explicit spectral changes due to the UV irradiation, as can be seen from parts A-C of Figure 8. After a few seconds of irradiation, the frequency shift from 1533 to 1525 cm-1 and the reduction of the bandwidth are observed for the νas(COO-) band of the monolayer on the Ba2+ subphase (Figure 8A). The irradiation does not cause any frequency shift but causes appreciable narrowing of the νas(COO-) band for the DA monolayer on the Cd2+ subphase (Figure 8B). In the case of
the DA monolayer on the Pb2+ subphase, the irradiation results in the frequency shift of the νas(COO-) band from 1510 to 1491 cm-1 (Figure 8C). Similar spectral changes are also observed by compressing the DA monolayers without irradiation; that is, as parts B-D of Figure 3 show, the compression from 0.48 to 0.21 nm2/molecule causes the frequency lowering of 1541 f 1532 cm-1 and 1508 f 1491 cm-1 for the νas(COO-) band of the DA monolayers on the Ba2+ and Pb2+ subphases, respectively, while the compression results in an appreciable narrowing of the νas(COO-) band for the DA monolayer on the Cd2+ subphase. To know what kind of photochemical process takes place during the UV irradiation of the DA monolayers, the resonance Raman (RR) spectral measurements were performed. Parts A-D of Figure 9 show the surface area dependence of the RR spectra measured for the DA monolayer on the Ba2+ subphase after UV irradiation for 10 min, and parts E-G of Figure 9 show the corresponding dependence observed for the DA monolayer on the Cd2+ subphase. The DA monolayer on the Ba2+ subphase at 0.32 nm2/molecule does not show any RR bands (Figure 9A).
Carboxylate-Counterion Interactions
Figure 9. Molecular area dependence of the resonance Raman spectra measured after UV irradiation for 10 min of the DA monolayers on water subphases (300 K) containing the Ba2+ ion (A-D) and the Cd2+ ion (E-G); The concentrations of the ions are 2 × 10-4 mol/L: (A) 0.32 nm2/molecule; (B) 0.24 nm2/molecule; (C) 0.20 nm2/molecule; (D) 0.15 nm2/molecule; (E) 0.28 nm2/molecule; (F) 0.21 nm2/molecule; (G) 0.12 nm2/molecule.
On the other hand, the monolayers on the Ba2+ subphase at 0.24 and 0.15 nm2/molecule give RR bands at 1516 and 1490 cm-1, respectively (parts B and D of Figure 9). These RR bands are mainly due to a CdC stretching vibration (ν(CdC)) of the conjugated backbone of PDA,27,28 indicating that the photopolymerization takes place for the monolayers on the Ba2+ subphase at the area e0.24 nm2/molecule. Presumably, at 0.32 nm2/molecule, the diacetylene moieties take on an orientation and/or packing state unfavorable for the polymerization. According to Shand et al.,29 the frequency of the ν(CdC) peak can be related to the conjugation length of the PDA backbone; i.e., the lower the frequency, the longer the conjugation length. Then, PDA formed from the monolayer at 0.15 nm2/molecule contains more ordered backbones (i.e., with longer conjugation lengths) than the polymer formed from the monolayer at 0.24 nm2/molecule. The DA molecules in the former monolayer may exist in a polycrystalline state, resulting in the formation of the ordered PDA backbones. On the other hand, PDA formed from the DA monolayers on the Cd2+ subphase give the RRS band at 1516 cm-1, irrespective of the surface areas in the 0.280.12 nm2/molecule region (parts E-G of Figure 9), which indicates that the orientation and/or packing states of the DA molecules change little upon compression. This result conforms to the IR spectral observation. The RR spectral measurements proved the photopolymerization of the DA monolayers of parts A-C of Figure 8. (The polymerization of the DA monolayer on the Pb2+ subphase was also prove by the RR spectral measurements.) Thus, we can conclude from the spectral changes in Figure 8 that the photopolymerization of the DA monolayers induce the ordered and/or closely packed states of the carboxyl groups. Conclusions The frequencies of the νas(COO-) and νs(COO-) bands observed for the DA monolayers on the water subphases containing the Ba2+, Cd2+, and Pb2+ ions exhibited discrete
J. Phys. Chem. B, Vol. 103, No. 3, 1999 443 frequency shifts upon reducing the molecular areas from 0.80 to 0.18 nm2/molecule, reflecting changes in the coordination as well as association states of the carboxylate groups. The comparison of the spectral changes of the DA monolayers with the spectral changes of the monolayers of the saturated fatty acids on the Ba2+, Cd2+, and Pb2+ subphases indicated that the diacetylene moiety increases the flexibility of the DA monolayers, causing discrete changes in the coordination and association states, which correspond to the phase changes of the monolayers revealed by the π-A isotherms. Ab initio molecular orbital calculations predicted that the correlation between the frequency difference, ∆ν ) νas(COO-) - νs(COO-), and the coordination modes of the Ba2+ ion with the carboxylate groups of long-chain fatty acids is as follows: ∆ν (166 cm-1, ionic) > ∆ν (126 cm-1, bridging) > ∆ν (98 cm-1, bidentate). On the basis of the correlation, we concluded that the DA monolayer on the Ba2+ subphase at first takes on a bridging coordination state, which is converted to a bidentate state upon compression. On the other hand, the coordination state in the DA monolayer on the Cd2+ subphase remains unchanged as the molecular area is reduced; probably, the carboxylate group takes on a bidentate state. It was also suggested that the carboxylate group in the DA monolayer on the Pb2+ subphase undergoes a conversion from a bridging to bidentate coordination state upon compression, as in the case of the monolayer on the Ba2+ subphase. To substantiate these conclusions, however, we need to perform more detailed ab initio molecular orbital calculations. RR spectroscopy was successively used to prove UVirradiation-induced polymerization of DA monlayers on the Ba2+, Cd2+, and Pb2+ subphases at 0.25 nm2/molecule. The IR spectral changes in the νas(COO-) and νs(COO-) regions observed during the photopolymerization indicated that the polymerization result in the formation of an ordered or closely packed environment of the carboxylate groups in the PDA monolayers. References and Notes (1) Tieke, B.; Lieser, G.; Wegner, G. J. Polym. Sci., Polym. Chem. Ed. 1979, 17, 1631. (2) Tieke, G.; Lieser, G. J. Colloid Interface Sci. 1982, 88, 471. (3) Polydiacetylene; Cantow, H.-J., Ed.; Springer-Verlag: Berlin, 1984. (4) Mino, N.; Tamura, H.; Ogawa, K. Langmuir 1992, 8, 594. (5) Yamada, S.; Shimoyama, Y. Jpn. J. Appl. Phys. 1996, 35, 4480. (6) Dluhy, R. A.; Cornell, D. G. J. Phys. Chem. 1985, 89, 3159. (7) Dluhy, R. A.; Cornell, D. G. Fourier Transform Infrared Spectroscopy in Colloid and Interface Science; Scheuing, D. G., Ed.; ACS Symp. Ser. 447; American Chemical Society: Washington, DC, 1991; p 192. (8) Mendelsohn, R.; Brauner, J. W.; Gericke A. Annu. ReV. Phys. Chem. 1995, 46, 305. (9) Gericke, A.; Hu¨hnerfuss, H. J. Phys. Chem. 1993, 97, 12899. (10) Gericke, A.; Hu¨hnerfuss, H. Thin Solid Films 1994, 245, 74. (11) Simon-Kutscher, J.; Gericke, A.; Hu¨hnerfuss, H. Langmuir 1996, 12, 1027. (12) Kobayashi, K.; Takaoka, K.; Ochiai, S. Thin Solid Films 1988, 159, 267. (13) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Gill, P. M. W.; Johnson, B. G.; Robb, M. A.; Cheeseman, J. R.; Keith, T.; Petersson, G. A.; Montgomery, J. A.; Raghavachari, K.; Al-Laham, M. A.; Zakrzewski, V. G.; Ortiz, J. V.; Foresman, J. B.; Cioslowski, J.; Stefanov, B. B.; Nanayakkara, A.; Challacombe, M.; Peng, C. Y.; Ayala, P. Y.; Chen, W.; Wong, M. W.; Andres, J. L.; Replogle, E. S.; Gomperts, R.; Martin, R. L.; Fox, D. J.; Binkley, J. S.; Defrees, D. J.; Baker, J.; Stewart, J. P.; HeadGordon, M.; Gonzalez, C.; Pople, J. A. Gaussian 94, revision C.2; Gaussian, Inc.: Pittsburgh, PA, 1995. (14) Wong, M. W.; Frisch, M. J.; Wiberg, K. B. J. Am. Chem. Soc. 1991, 113, 4776. (15) Wong, M. W.; Wiberg, K. B.; Frisch, M. J. J. Chem. Phys. 1991, 95, 8991. (16) Huzinaga, S. Gaussian Basis Sets for Molecular Calculations; Elsevier: New York, 1980. (17) Mo¨hwald, H. Annu. ReV. Phys. Chem. 1990, 41, 441.
444 J. Phys. Chem. B, Vol. 103, No. 3, 1999 (18) Ogawa, K. Polym. Int. 1992, 28, 25. (19) Yazdanian, M.; Yu, H.; Zografi, G. Langmuir 1990, 6, 1093. (20) The maxima in the baselines around 1680 cm-1 in Figure 3 (also in Figures 4, 5, 7, and 8, vide infra) are due to a strong change in the refractive index of water.10 One of the reviewers pointed out that the intensities or the amplitudes of the maxima increase as the monolayers are compressed, indicating that the maxima should be related to bound water near the carboxylate groups. Since the origin of the intensity increase is unknown, we do not discuss the maxima further in the present paper. (21) Nakamoto, K. Infrared and Raman Spectra of Inorganic and Coordination Compounds, 4th ed.; Wiley: New York, 1986; p 232. (22) Tackett, J. E. Appl. Spectrosc. 1989, 43, 483.
Ohe et al. (23) Nara, M.; Torii, H.; Tasumi, M. J. Phys. Chem. 1996, 100, 19812. (24) The detailed results of calculations including the optimized structures will be reported in a separate paper by C. Ohe and K. Itoh. (25) Spinner, E.; Yang, P.; Wong, P. T. T.; Mantsch, H. H. Aust. J. Chem. 1986, 39, 475. (26) Gotoh, Y.; Takenaka, T. Nippon Kagakukai Zasshi 1963, 84, 392. (27) Batchelder, D. N.; Bloor, D. In AdVances in Infrared and Raman Spectroscopy; Clark, R. J. H., Hester, R. E., Eds.; Wiley-Heyden: Chichester, U.K., 1984; Vol. 11, p 133. (28) Shirai, E.; Urai, Y.; Itoh, K. J. Phys. Chem. B 1998, 102, 3765. (29) Shand, M. L.; Chance, R. R.; LePostollec, M.; Schott, M. Phys. ReV. B 1982, 25, 4431.
