Dynamic Rearrangement of Stearic Acid Molecules Adsorbed on a

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Dynamic Rearrangement of Stearic Acid Molecules Adsorbed on a Gold Surface Induced by Ambient Water Molecules Studied by Infrared Spectroscopy Takafumi Shimoaka,† Yuki Itoh,‡ and Takeshi Hasegawa*,† †

Laboratory of Solution and Interface Chemistry, Division of Environmental Chemistry, Institute for Chemical Research, Kyoto University, Gokasho, Uji, Kyoto 611-0011, Japan ‡ Department of Chemistry, Graduate School of Science and Engineering, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo 152-8551, Japan ABSTRACT: Molecular adsorbates of stearic acid on a gold surface prepared as an imperfect Langmuir−Blodgett (LB) film is found to exhibit dynamic molecular rearrangement when the humid atmosphere about the sample is changed. The molecular adsorbates stored in a thoroughly dried sample room of FT-IR is found to have a unique adsorption structure; the hydrocarbon chains have a nearly parallel orientation to the substrate surface while the molecules are highly packed to have the orthorhombic subcell packing, which is confirmed by infrared reflection−absorption (RA) spectrometry. When the sample is pulled into an ambient air, the adsorption structure exhibits a drastic change in about only 15 min, which is pursued by polarization-modulation infrared reflection−absorption spectrometry (PM-IRRAS). The spectra clearly indicate that the molecular stance has largely been changed to have a standing-up orientation, whereas the molecular conformation is largely degraded. When the sample is got back to the dried sample room, the molecular conformation largely improves while the standing orientation is kept. These irreversible changes are induced by ambient water molecules adsorbed on the lying stearic acid molecules, which was monitored by analyzing absorption bands of the hydronium ion.

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INTRODUCTION For highly sensitive measurements of infrared spectra of an ultrathin film, the reflection−absorption (RA) spectrometry1−3 is an essential and established technique. The RA technique requires a metallic surface, on which the analyte thin film is deposited. When infrared ray is irradiated at a grazing angle of incidence, the incident ray and the reflected one are interfered with each other to generate an intensive electric field at the surface, whose direction is normal to the surface. As a result, the RA technique has a great benefit of the surface-selection rule as well as the high sensitivity thanks to the metallic property of the surface. In principle, any metal can be employed for the infrared RA measurements, but in many cases, gold is chosen for the material because of the chemically inert character. A gold surface has, however, an intrinsic issue for deposition of a thin film that the surface practically exhibits a relatively hydrophobic character. The surface property of gold was a controversy many years ago, but at about 1980, many papers reached a common conclusion that a clean metal surface is “hydrophilic” in both cases of amorphous and crystal.4−6 In theory, the wettability depends on the surface free energy, and all the metal surfaces are categorized to have a “high” surface energy,6 which means that all the metal surfaces are hydrophilic. As argued already, however, even the inert material of gold is readily contaminated by adsorption of small molecules, and keeping a clean surface is difficult: refreezing of melted gold in vacuum,4,5 electrochemical treatment,6 and strong chemical cleaning7 are representative techniques to obtain the clean © 2012 American Chemical Society

surface. When a metal surface receives a submonolayer level contamination after the cleaning, the hydrophilicity is drastically lost.4−6 On a practical basis, therefore, a goldevaporated glass substrate is sometimes not suitable for an LB (vertical dipping) deposition of a monolayer of a certain compound. To fabricate a structurally ordered and homogeneous LB film, a metal salt of stearic acid or arachidic acid is a good choice. For example, cadmium stearate (CdSt) is known to form a structurally stable LB film with a high reproducibility for both single-monolayer and multilayered LB films.8−13 Another notable benefit of using this compound is that the contact of the monolayer with gold is quite good even when the gold surface is cleaned by a moderate method such as solvent sonication.8,9 Since the CdSt monolayer also deposits on an infrared transparent material such as calcium fluoride and germanium, this compound is conveniently used as a good standard to check both infrared transmission and RA techniques.12−14 In the present study, a poor deposition of a free stearic acid monolayer on gold is focused on. When a gold surface was cleaned by a sonicator using several kinds of solvents as stated in Experimental Section, the surface was found adequately clean in terms of infrared spectroscopy. When a spread monolayer Received: June 14, 2012 Revised: July 21, 2012 Published: July 24, 2012 17142

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(Langmuir (L) film) of stearic acid on pure water was transferred on the sonicated gold substrate by the LB technique, regardless, the meniscus of water at the contact with the gold surface was almost linear during the LB deposition, which strongly indicated the hydrophobic character of the gold surface. As expected, the transfer ratio was quite poor at 0.14, which is largely different from that for deposition of a metal salt LB film on gold. Thus far, the surface property has never been discussed spectroscopically in relation to the LB deposition, and the molecular structure of the molecular adsorbates on gold as a result of the poor deposition stays unclearly understood. During the present study, infrared spectroscopy has revealed that the molecular adsorbates on gold exhibit molecular dynamic rearrangement induced by the humidity around the sample. The irreversible and dramatic changes of the adsorbed structure on gold in a drying process were studied by the infrared RA technique, whereas the film in an ambient wet condition was studied by using the polarization-modulation infrared reflection absorption spectrometry (PM-IRRAS). The spectral changes suggest that the ambient water adsorbed on the carboxylic groups of the lying molecular aggregates plays a key role to induce the dynamic molecular rearrangement.

(Madison, WI) Magna550 FT-IR spectrometer equipped with a Harrick (Pleasantville, NY) VR1-NIC variable angle reflection accessory mounted with a Harrick PWG-U1R wire-grid polarizer for passing the p-polarization only. The modulation frequency of FT-IR was 60 kHz, and the IR ray was detected by a liquid-N2-cooled MCT detector. The wavenumber resolution was 4 cm−1. The number of accumulations of the interferogram was 2000. PM-IRRAS Measurements. The main bench of the PMIRRAS measurements was a Thermo Fischer Scientific Nicolet 6700 FT-IR spectrometer. The FT-modulated IR ray was led out of the spectrometer, and it was passed though the photoelastic modulator (PEM) with the resonance frequency of 50 kHz generating modulated polarization rays operated by a HINDS Instruments (Hillsboro, OR) PEM-90 PEM controller. The modulated IR ray was directly introduced onto the LB film deposited on gold, and the reflected ray was led to another MCT detector through a CaF2 lens. No mirror was used in the optics to prevent disturbance of polarizations. The angle of incidence was 76°, and the half-wave-retardation frequency was set to 2900 cm−1 for the measurements of the C−H region.15−17 The modulation frequency of the interferometer was 20 kHz, and the band resolution was 8 cm−1. The accumulation number of the interferogram collection was at maximum 1000 to obtain a better signal-to-noise (SN) ratio, which took about 2000 s. After collecting the demodulated signals, IDC and IAC, the PM-IRRAS spectra were obtained by calculating the ratio of [S(d) − S(0)]/S(0). Here, S(d) is the so-called “ratio spectrum” defined as IAC/IDC, and S(0) is the ratio spectrum of bare substrate with no film. For the details of the notations, the reader may refer to earlier references.18−20 Since the substrate is metallic, the surface selection rule of the measurements is the same as that of the RA measurements.18 According to Buffeteau et al., [S(d) − S(0)]/S(0) can be converted to the pseudo-absorbance that has a common ordinate scale with the RA spectra, which works quite good for direct comparison of PM-IRRAS and RA spectra.21,22 To an experimental limit of our optical equipment, however, the conversion was not performed. Instead, the relative band intensity is discussed through the present study. Infrared MAIRS Measurements. The single-monolayer LB film of stearic free acid ideally deposited on a germanium substrate was measured by the infrared MAIRS technique.13,23,24 The spectra are useful to understand the RA and PM-IRRAS spectra. MAIRS spectra are a set of IP (in-plane; parallel vibrations to the substrate surface) and OP (out-ofplane; perpendicular to the surface) spectra that correspond to the conventional transmission and RA spectra of a thin film, respectively. With a conventional technique, the comparison of the fully covered LB film on germanium to the molecular adsorbates on gold is quite difficult because the surface selection rules are totally different from each other. Infrared MAIRS enables us to get over the limit, and discussion of the RA spectra of the molecular adsorbates becomes much easier. The measurements were performed by using a Thermo-Fisher Scientific (Yokohama, Japan) automatic MAIRS equipment (TN10-1500). For the detail of the MAIRS technique, the reader may refer to the literature.13,23,24

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EXPERIMENTAL SECTION Chemicals. Octadecanoic acid (stearic acid) with a purity higher than 99% was provided by Sigma-Aldrich (St. Louis, MO), and it was used without further purification. For preparation of a spread monolayer on water (Langmuir (L) film), a chloroform solution of stearic acid at a concentration of ca. 1.0 mg mL−1 was used. The solvent, chloroform, was a spectra grade reagent purchased from Wako Pure Chemical Industries Ltd. (Osaka, Japan). The surface pressure−surface area (π−A) isotherm measurements and the LB deposition were performed on a KSV NIMA (Espoo, Finland) Langmuir−Blodgett deposition trough with a size of 364 × 75 × 4 mm3 by spreading the chloroform solution on pure water at pH of ca. 5.8. The subphase water was obtained by a Millipore (Molsheim, France) Elix UV-3 purewater generator and a Yamato (Tokyo, Japan) Autopure WT100U water purifier, which is a compatible model with Milli-Q. The water exhibited an electric resistivity higher than 18.2 MΩ cm, and the surface tension was 72.8 mN m−1 at 25 °C, which guaranteed that the water was free from contaminants. LB Film Preparation. The LB film studied was prepared by transferring a Langmuir monolayer of stearic acid on pure water onto the gold substrate surface using the LB (vertical dipping) method at 30 mN m−1. The gold-evaporated glass substrate was purchased from Geomatec (Yokohama, Japan), which has the gold layer with a thickness of 300 nm on a stabilization layer of chromium of 50 nm deposited on the glass substrate. The layered structure was fabricated on a single side of a glass slide, and the other side was a bare glass. This gold-evaporated glass was cleaned in a conventional manner: successive sonication in pure water, ethanol, acetone, and dichloroethane for about 1 min each. All the solvents were guaranteed reagents. The surface cleanness was checked by measuring the RA spectrum with a background of air, and no peak was recognized in the spectrum. In other words, the surface was clean enough in terms of infrared spectroscopy. RA Measurements. Infrared reflection−absorption (RA) measurements were performed on a Thermo Fischer Scientific

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RESULTS AND DISCUSSION Preparation of Molecular Adsorbates on a Gold Surface. Figure 1 presents the π−A isotherm of stearic acid 17143

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decrease appears in an almost linear manner, but the slope for the Ge surface is largely different from that for the gold one. The slope directly reflects the transfer ratio that is defined by a ratio of the total area decrease to the coverage area of the substrate surface. The transfer ratio of the LB transfer onto Ge was 1.02. When the transfer ratio is close to unity, the LB transfer is recognized to be done in an ideal manner with an ignorable film collapse. Therefore, the experimental result using the Ge substrate indicates that (1) the contact between the L film and the Ge surface is good and (2) the film was stable during the time period. On the other hand, the transfer ratio on the gold surface is calculated to be 0.65, which is largely degraded. Note that the gold substrate has a single-sided gold-evaporated surface and the other side of bare glass. Since the transfer ratio on a glass slide was found at 1.02 through our experiments, the ratio of 0.65 can be divided into the contribution of the glass surface of 0.51 and the other one of the gold surface of 0.14, roughly speaking. Therefore, on the gold surface, no “film” is expected, and instead “molecular adsorbates” are expectedly available. Molecular Adsorption Structure in a Dried Condition. To reveal the structure of the molecular adsorbates on the gold surface, the infrared RA technique was employed. The spectrum is presented in Figure 3. Although the quantity of the molecular adsorbates should be very low (largely less than monolayer), the spectrum is clearly obtained with a good signal-to-noise ratio in a wide wavenumber range thanks to the highly sensitive characteristic of the RA technique. When the surface selection rule of the RA method is taken into account, however, the spectrum looks unusual as discussed below. For better understanding of the RA spectrum, the monolayer on Ge presented in Figure 2a was subjected to infrared MAIRS analysis. The spectra are presented in Figure 4. Infrared MAIRS provides IP and OP spectra simultaneously obtained from an identical sample, whose surface-selection rules are the same as the conventional transmission and RA spectrometries, respectively. Since the LB film on Ge is expected to have an ordered structure judging from the good transfer ratio (1.02), the molecules should have a nearly perpendicular stance to the substrate surface, which should yield surface parallel orientation of the antisymmetric and symmetric CH2 stretching vibration (νa(CH2) and νs(CH2), respectively) modes. In fact, both the νa(CH2) and νs(CH2) bands strongly appear in the IP spectrum at 2917 and 2849 cm−1, respectively, whereas they are largely suppressed in the OP one. Here, we have to note that the RA

Figure 1. π−A isotherm of stearic acid spread on pure water at 25 °C. The LB depositions on germanium and gold surfaces were performed at 30 mN m−1, which is higher than the kink.

(free acid) L film prepared on a pure water surface at 25 °C with a pH of ca. 5.8. The isotherm has a kink at 26 mN m−1 located at the connection point between the liquid expansion (LE) and liquid condensed/solid (LS) parts.25 Since a high molecular packing is achieved in the LS part, the LB transfer was conducted at a surface pressure of 30 mN m−1, which is higher than the kink. Figure 2 presents the decreased area measured during the LB depositions of the stearic acid L film on (a) Ge and (b) gold

Figure 2. Change in decrease of the surface area of the L film during an LB deposition. The dotted and solid lines are results on the (a) germanium and (b) gold surfaces, respectively.

surfaces at 30 mN m−1. The area decrease is mostly due to the LB transfer, and the rest small portion is due to the film collapse under the applied surface pressure. In Figure 2, during the period of about 350 s needed for the deposition, the area

Figure 3. Infrared RA spectra of the poorly transferred LB film of stearic acid on gold. 17144

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structure involving the ring dimer, which is consistent with the molecules being highly aggregated to form crystallites. This lying-crystallite model readily explains the appearance of the band at 2954 cm−1. This band is assigned to the out-ofskeleton asymmetric CH3 stretching vibration (νa(CH3)os) mode, and it is nearly invisible in an RA spectrum when the molecule has a perpendicular stance.27 As a result, all the bands tell us that the molecules adsorbed on the gold surface after the LB disposition lie parallel to the gold surface. Since the molecules in the L film on water have a perpendicular stance to the water surface,28 the molecules on gold are found to change the orientation drastically. Judging from the high crystallinity, the orientation change should have occurred during the drying process after the LB deposition. In other words, the lying crystallite was generated via the self-arrangement as a result of the negligible interaction with the hydrophobic substrate surface, but with the strong molecular interaction property between the hydrocarbon chains. Structural Change in Ambient Air. If the molecular selfarrangement mechanism related to the dried hydrophobic surface is true, our interest goes to the change when the dried sample is put into an ambient air. To measure infrared spectra in an ambient condition, in the present study, PM-IRRAS was employed. Figure 5 presents two PM-IRRAS spectra measured

Figure 4. Infrared MAIRS spectrum of the ideally transferred singlemonolayer LB film of stearic acid on the germanium substrate. The surface selection rules of the IP and OP spectra are the same as those of transmission and RA spectrometries, respectively.

spectrum in Figure 3 has a largely different shape from that of the MAIRS-OP spectrum, although the surface selection rule is common. On the contrary, the RA spectrum is similar to the MAIRS-IP spectrum, which is quite unusual when an LB film is fabricated at a high surface pressure in the LS region. This result thus strongly implies that the molecules preferentially lie on the substrate. The fingerprint region (below 1800 cm−1) of the RA spectrum supports the discussion of the higher wavenumber region. The CH 2 scissoring (δ(CH 2 )) vibration band apparently appears as a doublet peak at 1472 and 1464 cm−1. Since the direction of the transition moment of this mode is perpendicular to the molecular axis, the apparent appearance of the doublet peaks in the RA spectrum strongly denies the perpendicular molecular stance to the gold surface. The doublet peaks, at the same time, indicate that the molecules are in the orthorhombic subcell packing,26 which indicates that the molecules aggregate with a high degree of crystallinity. This discussion agrees with the band positions of the νa(CH2) and νs(CH2) modes found at 2916 and 2849 cm−1, respectively. These positions are well-known conformation markers, indicating that the hydrocarbon chains are packed in the all-trans zigzag manner.27 In this fashion, the RA spectrum with an aid of the discussion using the MAIRS spectra gives us a model that the molecular adsorbates on gold are of crystallites, in which molecules lie parallel to the substrate surface. Here, we have to note that this parallel orientation of “molecules” does not mean a parallel orientation of the “molecular plane” of stearic acid. If the molecular plane lies parallel to gold surface, the νs(CH2) band should be invisibly weak because of the surface selection rule. The fact that the relative intensity ratio of the νs(CH2) band to νa(CH2) one in Figure 3 is similar to the MAIRS-IP spectrum suggests that the molecules lie on the surface with random orientation about the molecular axis. If this discussion is true, the CO stretching vibration (ν(CO)) band at about 1700 cm−1 should appear in the RA spectrum apparently. As expected, we find an apparent peak at 1703 cm−1 accompanying shoulder peaks at 1739 and 1685 cm−1. The randomly lying model of crystallite is thus supported. The high-wavenumber shoulder at 1739 cm−1 suggests that a portion of the molecules are hydrogen-bonding free, while the major components are in a hydrogen-bonded

Figure 5. Time-dependent PM-IRRAS spectra measured in an ambient air. The sample is the same as used for the measurement for Figure 3. The time after removing the sample from the dried sample room of FT-IR into an ambient condition is (a) 15 and (b) 80 min.

on (a) 15 and (b) 80 min after putting the sample in the ambient air. Since the optimal wavenumber of PM-IRRAS was set to 2900 cm−1, only the C−H stretching vibration region was measured due to a reason for spectral quality. The spectrum at 15 min is found largely different from that in Figure 3: the bands of the all-trans zigzag conformation at 2916 and 2849 cm−1 largely decrease, and the gauche-conformation bands at 2925 and 2854 cm−1 are recognized to have comparable heights to the all-trans ones. This trend proceeds further, and the gauche bands are found dominant peaks at 80 min. In addition, the νa(CH3)os band becomes invisible at 80 min. These apparent changes should be attributed to the ambient gas, since other conditions are common to the RA measurements in the dried atmosphere. The most possible candidate of the gas is water. To understand the molecular rearrangement mechanism in the ambient air, a schematic illustration is presented in Figure 6. The lying crystallites have bare carboxylic groups to the air as indicated by the yellow balls. When the air has humid, therefore, the water molecules easily access the carboxylic 17145

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Figure 6. Schematic illustrations of the adsorbed stearic acid molecules on gold as a function of the environmental condition.

Figure 7. Infrared RA spectra of the molecular adsorbates of stearic acid on gold measured during the redrying process. For the details, please see text.

groups because of the hydrophilic property. Once the water molecules are attached to the carboxylic groups, a hydrated headgroup comprises −COO− (carboxylate ion) and H3O+ (hydronium ion) groups, which can be recognized to be a salt. Since an ionic salt of stearic acid is known to stand on the gold surface, the hydrated stearic acid is expected to be apart from the crystallite and move onto the gold surface. The conformational degradation with time discussed above is thus understandable by considering this moving molecules model. This speculation is readily confirmed by checking the change in the relative band intensity of the νa(CH2) mode to the νa(CH3) one. In Figure 3, the relative intensity is found at 5.1, whereas the ratio becomes down to 2.3 in Figure 5a. The intensity decrease of the νa(CH2) band indicates that the lying molecules are standing up with time as expected in Figure 6. Do the standing-up molecules with the poor conformation go back to the lying crystallite or stay standing when the sample is got back to the dried atmosphere? To investigate the reversibility in orientation and conformation, the sample was put back in the sample room of FT-IR, and spectral changes were pursued by using the RA technique. After Getting Back to the Dried Condition. The sample that yields the spectrum in Figure 5b is now back in the dried sample room of FT-IR, and the time-dependent spectral variation has been measured in more than a week. The

collected spectra are presented in Figure 7. For comparison, the initial spectrum shown in Figure 3 is shown again at the top of the figure (denoted as “dried”). Note that the first-day spectrum is measured immediately after exposing the sample to the ambient air during the PM-IRRAS measurements for the time period of 80 min. The first-day RA spectrum is therefore similar to the spectrum at Figure 5b. In the spectra from the first day to the eighth day, an apparent change is observed: the gauche conformer bands of the νa(CH2) and νs(CH2) bands at 2928 and 2856 cm−1, respectively, decrease while the all-trans zigzag conformers exhibit an increase at the same time. This spectral variation indicates that the molecular order in the aggregates is being improved. In addition, on the eighth day, all the CH2 stretching vibration bands weakly appear, which implies that the molecular stance is close to perpendicular when considering the surfaceselection rule. The redried sample has shown therefore that the standing-up stance is kept, and improvement of molecular conformation is found in about 1 week. Regardless, both alltrans and gauche bands are comparably recognized, which raises a question about the molecular picture: (1) both the trans and gauche conformers are available in a molecule or (2) phase-separated molecular aggregates comprised of the ordered and disordered molecules. 17146

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1383 cm−1 is apparently stronger than that of the νs(CH3) band (Figure 7), the new band cannot be assigned to the δs(CH3) mode. We have noted an assignment mentioned in literature by Bellamy32 that a carbonate ion (HCO3−) has the symmetric COO− stretching vibration band at 1385 cm−1. In the discussion above, most of the ambient water molecules are attached to the carboxylic groups of the lying crystallites, but directly attached water molecules on the gold surface are also expected. Such adsorbed water molecules would be another help to have the standing-up orientation. Since the gold surface has a relatively hydrophobic property, regardless, many water molecules are not expected to adsorb on the surface directly. In fact, no water-related bands, such as the O−H stretching and deformation vibration bands (∼3300 and 1650 cm−1 , respectively), do not appear at all. Instead, the complex of water and carbon dioxide could be generated and adsorbed on gold. To experimentally confirm that the band is not from stearic acid, but it is truly from the carbonate ion species, another experiment was performed: an incompletely dried gold surface was measured (S(d)) in an ambient air by using PM-IRRAS with the use of a thoroughly dried gold surface as the background (S(0)). The incompletely dried surface was prepared by dipping a gold-evaporated glass in pure water, and the glass was immediately placed on the sample stage of PM-IRRAS facing the gold surface to the infrared incidence. The measured spectrum is presented in Figure 8. Only a peak

In Figure 7, no band progression is recognized. Therefore, the molecular picture of a mixture of the trans and gauche conformers is chosen. In addition, the significant development of the ω(CH2) band strongly suggests that the molecules are getting to have a perpendicular stance to the substrate with time, which also agrees with the previous discussion. The most impressive change in the fingerprint region is, however, the disappearance of the ν(CO) band. The band intensity change can be discussed by using the surface selection rule, but the complete disappearance requires a perfectly parallel orientation of the CO group, which is unnatural. Instead, once the carboxylic groups are hydrated, as discussed in Figure 6, the ν(CO) band reasonably disappears, since the carboxylic group is changed to be the carboxylate anion group. The combined water molecules on the carboxylate groups become hydronium ion (H3O+), which may accompany other water molecules. Bokij et al.30 performed precise analysis of such a hydrated hydronium ion based on a factor-group analysis coupled with infrared spectroscopic experiments. According to them, the hydronium ion yields a band at about 1430 cm−1 mostly due to the symmetric OH3 deformation (δs(OH3)) mode with a large bandwidth reflecting the complicated configuration of the accompanying water molecules. Therefore, the disappearance of the ν(CO) band and the appearance of the broad band at about 1450 cm−1 strongly suggest the generation of the hydronium ions. This discussion is supported by another report:31 one of the degenerated modes (the asymmetric OH3 deformation; δa(OH3)) has a band near 1720 cm−1. The gradual appearance near 1700 cm−1 along with the increase of the broad band at about 1433 cm−1 is thus reasonably attributed to the hydronium ions. Here, the fact that the δa(OH3) band is considerably weaker than the δs(OH3) one is an additional experimental proof of the standing-up orientation of the hydrated stearic acid. The δa(OH3) band corresponds to a degenerated mode that belongs to the irreducible representation of E under C3v, which means that the transition moment of this mode has components along the x- and y-axes equivalently when the z is defined to direct along the C3 axis. Therefore, the δa(OH3) band becomes weak only when the headgroup stands nearly perpendicular to the gold surface. The symmetric COO− stretching (νs(COO−)) vibration band falls in the same region, which also emphasizes this trend, since the transition moment of the νs(COO−) mode is considered to have a similar direction to that of the δs(OH3) mode. As a result, the simultaneous increase of the δa(OH3) and δs(OH3) bands keeping a similar ratio suggests an increase of the standing-up molecules with time. This discussion is supported by the development of a band at 3238 cm−1 in Figure 7. According to Bokij et al.,30 the symmetric OH3 stretching vibration (νs(OH3)) band appears in a region of 3000−3500 cm−1, which are concluded by a theoretical calculation and an infrared spectrum. Although the band region is too wide to assign the band at about 3238 cm−1 to the νs(OH3) band, the growth of the band is synchronous to the bands of δa(OH3) and δs(OH3). Therefore, the band can reasonably be accepted as the νs(OH3) band. The presence of the hydronium ion has thus been confirmed experimentally. Another notable band is a newly appeared peak at 1383 cm−1. This band may be attributed to the CH3 symmetric deformation (δs(CH3)) vibration mode32 in stearic acid. When we refer to a KBr transmission spectrum of stearic acid,7 the band intensity of the δs(CH3) mode is found comparable to that of the νs(CH3) mode. Since the intensity of the band at

Figure 8. PM-IRRAS spectrum of the artificially reproduced molecular adsorbates of ambient gas on a gold surface.

appears at 1385 cm−1, which reproduces the band in the RA spectra assigned to the νs(COO−) band. In this PM-IRRAS spectrum, no other band appears except this band. This experiment thus proves that the band is not attributed to stearic acid, but a water-related compound, which is most possibly the carbonate ion.

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CONCLUSION For obtaining a good LB film on gold, the substrate surface should thoroughly be cleaned to have a hydrophilic property. Therefore, the imperfect LB deposition on a moderately cleaned surface has reasonably been ignored thus far, and no study has been performed on the molecular adsorbates on it. In the present study, the molecular adsorbates are revealed to exhibit dynamic molecular rearrangement induced by humid in an ambient air, and the mechanism of the rearrangement is proposed. 17147

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(25) MacRitchie, F. Chemistry at Interfaces; Academic Press: San Diego, 1990. (26) Snyder, R. G. J. Mol. Spectrosc. 1961, 7, 116−144. (27) Parikh, A. N.; Allara, D. L. J. Chem. Phys. 1992, 95, 927−945. (28) Muro, M.; Itoh, Y.; Hasegawa, T. J. Phys. Chem. B 2010, 114, 11496−11501. (29) Snyder, R. G. J. Chem. Phys. 1967, 47, 1316−1360. (30) Bokij, G. B.; Arkhipenko, D. K. Phys. Chem. Miner. 1977, 1, 233−242. (31) Nakamoto, K. Infrared and Raman Spectra of Inorganic and Coodination Compounds Part A: Theory and Applications in Inorganic Chemistry, 5th ed.; Wiley: Chichester, 1997; pp 173−178. (32) Bellamy, L. J. The Infrared Spectra of Complex Molecules, 3rd ed.; Chapman and Hall: Norfolk, 1975; Vol. 1.

The most significant driving force of the rearrangement is the adsorption of water molecules onto the carboxylic groups of the lying crystallites on the substrate. The salt of the carboxylate anion and the hydronium cation accompanying water molecules becomes an interacting site to move onto the gold surface, which changes both molecular orientation and conformation. Another minor factor is given by the molecular complex of water and carbon dioxide directly adsorbed on gold.

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]; Fax +81 774 38 3074. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by Grant-in-Aid for Scientific Research (B) (No. 23350031) from Japan Society for the Promotion of Science, and Priority Areas (23106710) from the Ministry of Education, Science, Sports, Culture, and Technology, Japan.

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