Cd2+ System on the Aqueous Cadmium Acetate

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J. Phys. Chem. B 2001, 105, 7723-7729

7723

CH3(CH2)nCOOH/Cd2+ System on the Aqueous Cadmium Acetate Solution Investigated in Situ by Polarization Modulation Infrared Spectroscopy Yanzhi Ren, Md. Mufazzal Hossain, Ken-ichi Iimura, and Teiji Kato* Satellite Venture Business Laboratory, Utsunomiya UniVersity,Yoto 7-1-2, Utsunomiya, 321-8585, Japan ReceiVed: February 22, 2001; In Final Form: May 22, 2001

Polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS) was applied to investigate the classical CH3(CH2)nCOOH/Cd2+ system (abbreviated as CdSt, CdA, and CdB for n ) 16, 18, and 20, respectively) in situ on the aqueous cadmium acetate subphase of pH ) 6.7 in a Langmuir trough. The alkanoic acids were deprotonated and spontaneously formed crystalline domains after spreading to the subphase. The spectra were recorded at a resolution of 4 cm-1 and the crystal lattice information contained in the CH2 scissoring mode was unambiguously concluded. It is found that the crystal lattice has a systematic dependency on the monolayer temperature and chain length. The CdSt monolayer possesses a hexagonal packing at 293283 K and pseudohexagonal packing at 274 K. The CdA monolayer possesses a hexagonal unit cell at 293 K and an orthorhombic one at 283-274 K. The CdB monolayer always has an orthorhombic unit cell at 293-274 K. Both temperature lowering and chain lengthening can increase the interchain interaction and tend to render an orthorhombic unit cell. After being transferred from the aqueous subphase onto solid substrates at 283 K, the orthorhombic monolayer of either CdA or CdB relaxes into a hexagonal structure. Finally, barrier compression has no effect on the crystal lattice of the uniform monolayer but does affect that of the collapsed films.

Introduction The crystal structure of lipid monolayers floating at the air/ water interface is of broad interest to the scientific community.1-3 The technique to make such monolayers involves dissolving an amphiphile into an evaporative solvent and then spreading it onto the water surface. Among all the lipids, long chain alkanoic acids are regarded as standard materials by the Langmuir-Blodgett (LB) community.4 The aqueous solution containing Cd2+ ions is regarded as a standard subphase, on which alkanoic acids have an ideal film forming property. The Cd2+ ion has a condensing effect on alkanoic acid monolayers. It is a familiar experience that LB films of alkanoic acids on the aqueous Cd2+ subphase are easier to make than on the pure water subphase. The crystal lattice of arachidic acid (CH3(CH2)18COOH)/Cd2+ (CdA) has been investigated by synchrotron X-ray total reflection spectroscopy5 and polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS).6,7 We recently published two letters describing briefly the behenic acid (CH3(CH2)20COOH)/Cd2+ (CdB) system8 and the CdA system.9 One goal of this paper is to present a complete account of alkanoic acid/Cd2+ systems from the room temperature of 293 K down to the lowest achievable temperature of 274 K, from the initial molecular area of 0.40 nm2 to 0.10 nm2, and from the CdB system to the stearic acid (CH3(CH2)16COOH)/Cd2+ (CdSt) system. The PM-IRRAS technique will be applied to investigate the cadmium alkanoate monolayers at a resolution of 4 cm-1. Information about the crystalline packing of hydrocarbon chains is contained in the methylene scissoring (δ(CH2)) mode.10 * Author to whom correspondence should be addressed. Fax: +81-28689-6179. E-mail: [email protected].

To determine the subcell packing unambiguously, it is imperial to record the spectra at a high resolution of 4 cm-1. The orthorhombic packing of hydrocarbon chains gives a doublet of δ(CH2) bands at approximately 1473 and 1462 cm-1,10 which cannot be resolved at a resolution of 8 cm-1. Previous ordinary IRRAS studies of the CdSt system were carried out at a resolution of 8 cm-1 and, consequently, the subcell packing information cannot be drawn therefrom.11-13 In fact, the ordinary IRRAS approach,14-16 without using a polarization modulator, usually records the δ(CH2) region at a resolution of 8 cm-1. It faces some difficulty in recording this region at a 4 cm-1 resolution, where the signal-to-noise ratio is not satisfactory due to air moisture absorption. The PM-IRRAS approach, advocated by Blaudez et al.,17 can easily measure the spectra at a 4 cm-1 resolution and simultaneously determine the monolayer orientation. Absorption due to the air moisture and the water subphase has an isotropic nature and can be eliminated effectively by the PM technique. We recently demonstrated that under carefully controlled experimental conditions, the PM-IRRAS spectra in the δ(CH2) region can be obtained at a 4 cm-1 resolution with an excellent signalto-noise ratio.8 Another goal of this paper is to provide a model study for the following paper about the surface micelles of CF3(CF2)m(CH2)nCOOH on the aqueous cadmium acetate subphase. The classical and simple CH3(CH2)nCOOH monolayer on the aqueous cadmium acetate subphase can serve as a good model system for the study of the surface micelles of partially fluorinated amphiphiles. Experimental Section The stearic, arachidic, and behenic acids were gifts from Nippon Fat and Oil Co. with a guaranteed purity of 99.9%. The

10.1021/jp010689+ CCC: $20.00 © 2001 American Chemical Society Published on Web 07/20/2001

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Ren et al.

CHART 1

spreading solvent was chloroform. The subphase was aqueous cadmium acetate solution of concentration 5 × 10-4 M with a self-buffered pH of 6.7. The Wilhelmy plate was a piece of filter paper. The compression ratio was 7% per minute. The trough had a dimension of 19 cm × 9 cm × 0.7 cm. Its bottom was made of copper, to which large amounts of Peltier elements were attached. The subphase temperature can be conveniently varied from room temperature down to below the ice point. The temperature of the Langmuir film was measured by placing a platinum wire resistance sensor of length 7 mm at the air/ subphase interface, i.e., 3.5 mm in the air and 3.5 mm in the subphase. The temperature stability was controlled to be within (0.1 K. Chart 1 shows the side view and top view of the experimental setup. The discontinuous π-A isotherm was always recorded before the PM-IRRAS scan and the surface pressure was always monitored during the PM-IRRAS scan. The PM-IRRAS spectra were recorded at 4 cm-1 resolution and 200 scans using a Nicolet Magna 860 FTIR spectrometer, equipped with a PM part. The incidence angle of the IR beam onto the subphase was 80° with respect to the surface normal. The principles of PM-IRRAS measurements had been described by many authors.18 The beam was first s-polarized and then passed through a ZnSe photoelastic modulator (Hinds, PEM-90), which modulated at a frequency of ω ) 50 kHz. The maximum phase retardation at 7100 nm (1404 cm-1) was set to π. The light intensity “I” arriving at the mercury cadmium telluride (MCT) detector was demodulated to give Ip and Is, representing the pand s-polarized parts, respectively. The PM-IRRAS spectrum was recorded as S ) (Ip - Is)/(Ip + Is). The spectra are reported as the PM-IRRAS intensity S(d)/S(o) - 1, where d and o stand for the monolayer covered and uncovered subphase, respectively. No spectra are smoothed. The important δ(CH2) and ν(CH2) frequencies are determined by the “center of mass” method, using the software Grams/386. In the present and the following paper the conventional symbol ν will be used to stand for a stretching mode. A one-layer LB film of cadmium stearate was deposited onto a gold-coated glass substrate (abbreviated as Au substrate) at 20 mN/m. Its grazing incidence reflection (GIR) spectrum was recorded at a 4 cm-1 resolution and 1000 scans, with p-polarized light incident on the substrate surface at 85°. The spectrum is reported as the reflection absorbance -log[R/R(o)], where R is the reflectivity of the monolayer-covered Au substrate and R(o) is that of the bare Au substrate. A one-layer LB film of cadmium arachidate and behenate was deposited onto CaF2 substrates at

Figure 1. GIR spectrum of one-layer CdSt on an Au substrate deposited at 20 mN/m (a) and normal transmission spectrum of bulk CdSt on a CaF2 substrate (b). The absorbance in spectrum (b) has an arbitrary unit.

both sides. The transmission absorbance reported in this paper has been divided by two. Results and Discussion 1. The Stearic Acid/Cd2+ System. The CdSt system has been addressed by several authors at room temperature and a resolution of 8 cm-1, who studied the coordination mode11,12 and chain orientation.13 We shall focus on other aspects of this system, namely, the crystal lattice, the temperature effect, and the collapse behavior. Below we shall first present an ex situ study and then an in situ study of the CdSt system. Figure 1a shows the GIR spectrum of one-layer CdSt deposited at 20 mN/m onto an Au substrate. Umemura et al.19 have reported the GIR spectra of CdSt LB multilayers on silver substrates and the bands in Figure 1a are assigned according to their paper. Of first note is that the CH2 stretching intensities at 2919 and 2850 cm-1 are suppressed, as small as the CH3 stretching bands at 2963 and 2877 cm-1. Empirically, such a feature implies that the hydrocarbon chain axis is oriented almost along the surface normal. According to the surface selection rule of GIR measurements, the reflection absorbance is proportional to cos2θ, where θ is the angle between the transition dipole moment (TDM) and the surface normal. The νas(CH2) and νs(CH2) TDMs are estimated to have tilt angles near 90°. They are perpendicular to each other and both perpendicular to the hydrocarbon chain axis. The bands at 2932 and 2877 cm-1 are both assigned to the νs(CH3) mode, whose splitting has been caused by Fermi resonance with the first overtone of the CH3 bending mode. The nine bands from 1333 to 1187 cm-1 are known as “methylene progression bands” and their number is consistent with the total number (2 × 9 ) 18) of carbon atoms in stearic acid.20 Figure 1b shows the transmission spectrum of bulk CdSt, manually smeared from the Cd2+ subphase onto a CaF2 substrate. Two bands at 1473 and 1462 cm-1 due to the CH2 scissoring mode appear, implying an orthorhombic subcell packing. The orthorhombic structure of CdSt in powders and multilayers is known to be rendered by the headgroup interaction and one-layer CdSt on a solid substrate, which does not possess

CH3(CH2)nCOOH/Cd2+ System on Aqueous Cadmium Acetate

J. Phys. Chem. B, Vol. 105, No. 32, 2001 7725

Figure 2. Surface pressure-area isotherms of stearic acid on the aqueous cadmium acetate subphase at 293 K measured during continuous (dashed line) and discontinuous (solid line) compression.

such an interaction, is hexagonal.21 It is seen that no band appears in the region of 1800-700 cm-1 in Figure 1b, corresponding to the CdO stretching modes of either free or hydrogen-bonded carboxylic acids. Therefore, the stearic acids have been totally deprotonated on the aqueous cadmium acetate solution of pH ) 6.7. It has been reported that the pH value around 6 can ensure a total deprotonation of stearic acids on the Cd2+ subphase, where the carboxylate group interacts predominantly covalently with the Cd ion and forms a chelating bidentate coordination complex.11,12 The νs(COO-) band at 1432 cm-1 is rather strong, while the νas(COO-) band at 1543 cm-1 is rather weak in the one-layer GIR spectrum. This feature is reversed in the isotropic bulk spectrum of Figure 1b, as can be seen from the intensity ratio of νs(COO-)/νas(COO-). The νs(COO-) and νas(COO-) TDMs are then estimated to have tilt angles near 0° and 90°, respectively. The νs(COO-) TDM is along the C2 axis of the COO- group. Thus, the COO- group is symmetrically anchored on the Au surface with its C2 axis along the surface normal. Figure 2 compares the π-A isotherms of the CdSt system measured during continuous compression and discontinuous compression which is interrupted by the PM-IRRAS scan. During the PM-IRRAS scan the moving barrier has to be stationary, causing the surface pressure to drop and the π-A isotherm to be discontinuous. It is seen that the discontinuous and continuous isotherms reflect different collapse behavior. The former shows a collapse point at “e”, where the surface pressure drops to 4 mN/m after the barrier stop. The latter shows no drop of surface pressure with compression from 0.20 to 0.17 nm2. We regard it as a faithful action to correlate the discontinuous π-A isotherm with the PM-IRRAS spectra. Figure 3 shows the PM-IRRAS spectra corresponding to the break points of Figure 2. The surface pressures, displayed in Figure 3, refer to the relaxed values where the spectra were measured. The νas(CH2) frequency is 2916.3 ( 0.1 cm-1 in Figure 3a-c, determined by the “center of mass” method. Such a νas(CH2) frequency implies an all-trans zigzag planar conformation of the hydrocarbon chain.22 It is noted that even at the initial molecular area of 0.40 nm2, the hydrocarbon chains can take an ordered conformation. This observation is consistent with the knowledge that long chain fatty acids spontaneously form crystalline domains after spreading to the aqueous Cd2+ subphase. In Figure 3d the νas(CH2) frequency shifts appreciably to 2917.5 cm-1. It is well-documented that the νas(CH2) frequency (not the νs(CH2) frequency) can be empirically correlated with the trans/gauche ratio of the hydrocarbon chain.23 Thus, barrier compression from the point “c” (0.23 nm2, 13 mN/ m) to “d” (0.21 nm2, 43 mN/m) along the π-A isotherm

Figure 3. PM-IRRAS spectra of CdSt on the aqueous cadmium acetate subphase at 293 K. Spectra (a) to (g) correspond to the points “a” to “g” along the discontinuous π-A isotherm of Figure 2 at 0.40, 0.30, 0.23, 0.21, 0.20, 0.14, and 0.10 nm2, respectively.

produces some gauche kinks in the hydrocarbon chain. It is noted that the surface pressure drops from 43 to 22 mN/m after the moving barrier stops at “d”. Probably, the surface pressure of 43 mN/m is too high and the CdSt monolayer has partially collapsed. The δ(CH2) band is found at 1468.9 ( 0.1 cm-1 in Figure 3b-d with an fwhm of 6 ( 1 cm-1. Such a δ(CH2) band is known as implying a hexagonal packing of the hydrocarbon chains. With the event of monolayer collapse, the fwhm of the δ(CH2) band increases considerably to 10 cm-1 in Figure 3e and 14 cm-1 in Figure 3f. The broadening of the δ(CH2) band should imply a loss of the interchain packing order. At the same time, the νas(CH2) frequency increases from 2917.5 cm-1 in Figure 3e to 2917.8 cm-1 in Figure 3f, indicating a loss of the chain conformational order. In Figure 3g the δ(CH2) band finally splits into a doublet at 1473 and 1462 cm-1, indicative of an orthorhombic unit cell. This orthorhombic packing is usually explained as due to the headgroup interaction. The νas(COO-) band at 1538 cm-1 in Figure 3d gradually shifts to 1543 cm-1 in Figure 3g, reflecting a change in the environment of the carboxylate headgroup. Before collapse the COO- group is surrounded by an aqueous ionic medium and after collapse it leaves the subphase. The νs(COO-) mode gives a negative band at 1433 cm-1 relative to the baseline. According to the PM-IRRAS selection rule for water surface,6,7 the PM-IRRAS intensity of a vibrational mode will reach its positive maximum and negative maximum when its TDM has a tilt angle of 90° and 0°, respectively. The absolute value of the negative maximum is approximately half of the positive maximum. Owing to the small extinction coefficient of the νs(COO-) mode, as can be seen from the isotropic bulk spectrum of Figure 1b), its TDM and hence the C2 axis of the COO- group, is empirically estimated to have a tilt angle of 0° in Figure 3g at 0.10 nm2. This feature suggests that the collapsed films at 0.10 nm2 are not randomly packed but oriented. According to Nakamoto,24 the frequency separation ∆ between the νas(COO-) and νs(COO-) bands can be used as a diagnostic tool to determine the interaction type between the carboxylate head and the metal ion. There are four types of

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Figure 4. PM-IRRAS spectra of CdSt on the aqueous cadmium acetate subphase at 274 K. Spectra (a) to (g) are recorded at 0.40, 0.30, 0.23, 0.21, 0.20, 0.14, and 0.10 nm2, respectively.

interactions: monodentate, bridging bidentate, chelating bidentate, and ionic interactions. 4 is the smallest for the chelating bidentate interaction (80-110 cm-1, see Figure 6 and Table 2 of ref 12). In the present case 4 )1543 - 1433 ) 110 cm-1, corresponding to the chelating bidentate coordination. The π-A isotherm and the PM-IRRAS spectra of the CdSt system at 283 K (Supplementary Figures 1 and 2) are similar to those at 293 K. The δ(CH2) band has a frequency of 1468.9 ( 0.1 cm-1 with an fwhm of 6 ( 1 cm-1 before monolayer collapse. It is broadened after collapse and finally splits into two bands at 1473 and 1462 cm-1, accompanied by the νas(COO-) frequency shift to 1543 cm-1 and the appearance of a negative νs(COO-) band. Figure 4 shows the PM-IRRAS spectra of the CdSt system at 274 K. The corresponding π-A isotherm, which resembles Figure 2 in most aspects, is displayed in Supplementary Figure 3. Of note is that the δ(CH2) band is rather broad, with an fwhm of about 11 cm-1 in Figures 4b-d, indicating that no genuine hexagonal unit cell exists. Judging from the δ(CH2) frequency and the fwhm, we propose that the subcell packing in the monolayer at 274 K is pseudohexagonal. The broadening of the δ(CH2) band at 274 K is unexpected, since temperature lowering is usually expected to sharpen any vibrational band. The following study about the temperature effect on the CdA system will provide an explanation for this observation. 2. The Arachidic Acid/Cd2+ System. Figure 5 shows the PM-IRRAS spectra of the CdA system at 293 K. The corresponding π-A isotherm is displayed in Supplementary Figure 4. They are similar to those of CdSt at 293 K in the following aspects. The δ(CH2) band has a frequency of 1468.9 ( 0.1 cm-1 with an fwhm of 6 ( 1 cm-1 before monolayer collapse. It is broadened after collapse and finally splits into two bands at 1473 and 1462 cm-1. The νas(CH2) frequency stays at 2916.3 ( 0.1 cm-1 from Figure 5b to Figure 5d and increases to 2917.3 cm-1 in Figure 5e, implying a collapse-induced chain conformational disorder. The δ(CH2) band reflects a hexagonal packing of CdA on the aqueous subphase at 293 K. Previous authors have also claimed a hexagonal unit cell for one-layer CdA deposited on solid substrates, on the basis of the electron diffraction data25

Ren et al.

Figure 5. PM-IRRAS spectra of CdA on the aqueous cadmium acetate subphase at 293 K. Spectra (a) to (g) are recorded at 0.40, 0.30, 0.20, 0.195, 0.18, 0.15, and 0.10 nm2, respectively.

Figure 6. PM-IRRAS spectra of CdA on the aqueous cadmium acetate subphase at 283 K. Spectra (a) to (g) are recorded at 0.40, 0.30, 0.22, 0.20, 0.18, 0.15, and 0.10 nm2, respectively. Bottom shows the normal transmission spectrum of one-layer CdA deposited from the subphase at 283 K onto a CaF2 substrate. Its absorbance is 1/50 of the ordinate.

and scanning tunneling microscope images.26 Schwartz et al.27 transferred bilayers of CdA at 295.0 ( 0.5 K and 30 mN/m onto solid substrates. Using atomic force microscopes, they observed that two layers of CdA packed in a head-to-head way possess an orthorhombic structure, while two layers packed in a tail-to-tail fashion do not. They concluded that the interlayer interaction between the carboxylate groups is responsible for the orthorhombic packing in CdA multilayers. Blaudez et al.28 have also reached this conclusion for deuterated CdA multilayers using infrared spectroscopy. Figure 6 shows the PM-IRRAS spectra of the CdA system at 283 K. The corresponding π-A isotherm is displayed in Supplementary Figure 5. The δ(CH2) mode splits into two bands

CH3(CH2)nCOOH/Cd2+ System on Aqueous Cadmium Acetate

J. Phys. Chem. B, Vol. 105, No. 32, 2001 7727

SCHEME 1

at 1472 and 1464 cm-1 in Figure 6b-d, indicative of an orthorhombic unit cell at 283 K. The two short lines in Figure 6c serve as peak indicators. One orthorhombic unit cell contains two hydrocarbon chains whose C-C-C trans zigzag planes are perpendicular to each other, as depicted in Scheme 1. The methylene scissoring vibration of one chain can occur either in-phase or out-of-phase with respect to that of another chain, resulting in the δ(CH2) splitting. The orthorhombic unit cell of the CdA monolayer is of great interest to the scientific community.27 By lowering the monolayer temperature from 293 to 283 K, we have observed a hexagonal-to-orthorhombic transition of the CdA system on the aqueous Cd2+ subphase. For one-layer deuterated CdA deposited on a solid substrate, Blaudez et al.28 have observed the hexagonal-to-orthorhombic transition by lowering the substrate temperature to 77 K, evidenced by the δ(CD2) splitting. This transition has been explained as due to the increased interchain constraint with temperature lowering. Going from the hexagonal packing of CdSt to the orthorhombic packing of CdA at the same temperature of 283 K, we see that chain lengthening can also result in a hexagonal-to-orthorhombic transition. The van der Waals interaction between trans zigzag planar hydrocarbon chains is known to increase with chain lengthening. As a summary, both temperature lowering and chain lengthening can increase the interchain interaction and hence render an orthorhombic unit cell to the cadmium alkanoate monolayer. It is reminded that temperature lowering of the CdSt system from 283 to 274 K causes a transition from hexagonal to pseudohexagonal packing. We hypothesize that if the monolayer temperature could be further lowered to below 274 Ks for example, 264 K,sthe CdSt system might undergo a transition from pseudohexagonal to orthorhombic packing. We admit that the monolayer temperature of 274 K, i.e., the subphase temperature of 271 K, is the lowest achievable temperature in our laboratory. After the monolayer collapse, the δ(CH2) mode gives a broad band at 1466.5 cm-1 with an fwhm of about 12 cm-1 in Figure 6e, possibly corresponding to a disordered interchain packing. The bottom of Figure 6 shows the transmission spectrum of a single monolayer of CdA transferred onto a CaF2 substrate at 283 K and 20 mN/m. The δ(CH2) band is found at 1469.0 cm-1 with an fwhm of 5.6 cm-1, corresponding to a hexagonal subcell packing. Therefore, the orthorhombic unit cell on the aqueous subphase at 283 K has relaxed into a hexagonal one after being transferred onto the solid substrate. Figure 7 shows the PM-IRRAS spectra of the CdA system at 274 K. The corresponding π-A isotherm is displayed in Supplementary Figure 6. The δ(CH2) mode always has a twoband nature. The νas(COO-) frequency stays at 1534 cm-1 throughout Figure 7, indicating that the carboxylate group remains under the subphase at the molecular area of 0.10 nm2. Comparing the νas(COO-) frequency in Figures 7g, 6g, and 5g at 0.10 nm2, we reach the impression that the lower the temperature, the later the COO- group leaves the subphase. Lowering the temperature and hence reducing the molecular

Figure 7. PM-IRRAS spectra of CdA on the aqueous cadmium acetate subphase at 274 K. Spectra (a) to (g) are recorded at 0.30, 0.25, 0.22, 0.21, 0.18, 0.17, and 0.10 nm2, respectively.

mobility can cause the monolayer to adhere more strongly to the subphase. Comparing the νas(COO-) frequencies of CdA and CdSt in Figures 7g and 4g at 274 K and 0.10 nm2, we reach the conclusion that the longer the hydrocarbon chain, the later the COO- group leaves the subphase. Increasing the chain length and hence reducing the molecular mobility can also cause the monolayer to adhere more strongly to the subphase. Comparing the π-A isotherms of CdA in Supplementary Figures 6, 5, and 4, it is noted that the surface pressure hardly drops at 274 K but drops to a near zero value at 283 and 293 K after the moving barrier stops at the just-collapsed point (0.18 nm2). The same phenomenon is also noted for the CdSt system by comparing the surface pressure drop at 274, 283, and 293 K after the moving barrier stops at the just-collapsed point. As a summary, for both CdA and CdSt at 274 K, the surface pressure can maintain a value above 30 mN/m after the monolayer collapse event. We tentatively explain that the collapse event at 274 K only causes the part of monolayer in front of the moving barrier to collapse, leaving the part of monolayer around the Wilhelmy plate still uniformly and tightly packed (see Chart 1). Again, the physical implication is that temperature lowering causes the monolayer to adhere strongly to the subphase. This point becomes evident if one compares the CH2 stretching intensities in Figures 5e and 5g, in Figures 6e and 6g, and in Figures 7e and 7g. At 293 K in Figure 5 and 283 K in Figure 6, the integrated intensities of νas(CH2) and νs(CH2) bands are seen to be nearly doubled with compression from 0.18 to 0.10 nm2/molecule. However, at 274 K in Figure 7, the νas(CH2) and νs(CH2) intensities hardly increase although the chain density has been nearly doubled with compression from 0.18 to 0.10 nm2/molecule. The CdA films at 0.10 nm2 are then estimated to be two-molecules thick at 293-283 K but still onemolecule thick at 274 K. Again, we explain that at 0.10 nm2 and 274 K, the part of monolayer under the infrared irradiation is still a monolayer. The same conclusion holds for the CdSt system if one examines the evolution of CH2 stretching intensities from 0.18 to 0.10 nm2/molecule at 293, 283, and 274 K. The νas(CH2) and νs(CH2) intensities are nearly doubled with compression

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TABLE 1: PM-IRRAS Frequencies of the νas(CH2), νs(CH2), and δ(CH2) Bands and the δ(CH2) fwhm of the Uniform Monolayer on the Aqueous Cadmium Acetate Subphase. (Otherwise stated, the inaccuracy is (0.1 cm-1.) CdSt cm-1 νas(CH2) νs(CH2) δ(CH2) fwhm

CdA

293 K 283 K 2916.3 2850.1 1468.9 6(1

2916.3 2850.1 1468.9 6(1

274 K 2916.3 2850.1 1469 ( 1 11 ( 1

CdB

293 K 283 K 274 K 293 K 283 K 2916.3 2850.1 1468.9 6(1

2916.3 2850.1 split split

2916.3 2850.1 split split

2916.3 2850.1 split split

2916.3 2848.8 split split

Figure 8. PM-IRRAS spectra of CdB on the aqueous cadmium acetate subphase at 283 K. Spectra (a) to (g) are recorded at 0.40, 0.25, 0.22, 0.18, 0.15, 0.10, and 0.08 nm2, respectively. Bottom shows the normal transmission spectrum of one-layer CdB deposited from the subphase at 283 K onto a CaF2 substrate. Its absorbance is 1/50 of the ordinate.

TABLE 2: PM-IRRAS Frequencies of the νas(CH2), νs(CH2), and νas(COO-) Bands of the Collapsed Orthorhombic Films on the Aqueous Cadmium Acetate Subphase at 0.10 nm2 (The inaccuracy is (0.1 cm-1 for the νas(CH2) and νs(CH2) bands and (1 cm-1 for the νas(COO-) band) CdSt cm-1

CdA

CdB

293 K 283 K 274 K 293 K 283 K 274 K 293 K 283 K

νas(CH2) 2918.1 2918.1 2917.6 2918.1 2918.1 2916.3 2917.6 2916.3 νs(CH2) 2850.1 2850.1 2848.8 2850.1 2850.1 2850.1 2850.1 2848.8 νas(COO-) 1543 1543 1543 1543 1543 1534 1543 1534

from 0.18 to 0.10 nm2/molecule at 293 and 283 K, but hardly increase at 274 K. 3. The Behenic Acid/Cd2+ System. We have reported the PM-IRRAS spectra of the CdB system at 293 K.8 The δ(CH2) splitting and hence the orthorhombic packing of CdB, already occurs at the room temperature of 293 K. Table 1 summarizes the δ(CH2) bands obtained from our experiments. There is a systematic dependency of the δ(CH2) splitting on the chain length and monolayer temperature. The CdSt monolayer can never possess an orthorhombic unit cell from 293 to 274 K. On the other hand, the CdB monolayer always adopts an orthorhombic packing from 293 to 274 K. Figure 8 shows the PM-IRRAS spectra of the CdB system at 283 K. The corresponding π-A isotherm is displayed in Supplementary Figure 7. The νas(COO-) frequency in Figure 8f remains to be at 1534 cm-1. Table 2 lists the νas(COO-) frequency of the CdSt, CdA, and CdB collapsed films at 0.10

nm2, which has a systematic dependency on the chain length and monolayer temperature. For CdSt at 0.10 nm2, the COOgroup always leaves the subphase from 293 to 274 K. For CdA and CdB at 0.10 nm2, the COO- group can stay under the subphase within a temperature range that extends with chain lengthening. Either lowering the temperature or increasing the chain length can reduce the molecular mobility and hence gives rise to stronger adhering of the monolayer to the subphase. Examining the CdB system at 0.10 nm2 and 283 K with the naked eye, we see that the Langmuir film within 1.0 cm from the moving barrier are white and thick, while that under the infrared beam or around the Wilhelmy plate is still invisible. Judging from the νas(CH2) frequency of 2916.3 cm-1 in Figure 8f, which is characteristic of the uniform monolayer, the Langmuir film under irradiation by the infrared beam probably seems not yet collapsed. Again, we would like to consider that barrier compression from 0.20 to 0.10 nm2 just causes the part of monolayer in the immediate front of the moving barrier to collapse. The infrared beam has a diameter of ∼1 cm at the air/subphase interface and is at least 3 cm away from the moving barrier (Chart 1). From Figure 8f to 8g, the νas(COO-) band finally shifts to 1543 cm-1, indicating that the COO- group, under irradiation of the infrared beam, leaves the subphase with compression from 0.10 to 0.08 nm2. Simultaneously, the νas(CH2) frequency shifts from 2916.3 to 2917.6 cm-1. The bottom of Figure 8 shows the normal transmission spectrum of one-layer CdB transferred onto a CaF2 substrate at 283 K and 20 mN/m. The δ(CH2) band is found at 1469.2 cm-1 with an fwhm of 8.5 cm-1, corresponding to a pseudohexagonal packing. Tables 1 lists the νas(CH2) and νs(CH2) frequencies of the cadmium alkanoate monolayer. The cadmium alkanoate monolayer has a common νas(CH2) frequency of 2916.3 ( 0.1 cm-1 within the temperature range examined. It seems that the cadmium ion is responsible for rendering the CdSt, CdA, and CdB monolayers such a common νas(CH2) frequency. The νas(CH2) and νs(CH2) frequencies of the uniform monolayer are not so sensitive to temperature variation and chain lengthening as the δ(CH2) band. There seems to be no correlation between the unit cell type and the ν(CH2) frequencies. Table 2 lists the νas(CH2) frequency of the collapsed orthorhombic films at 0.10 nm2, which are sensitive to both temperature variation and chain lengthening. The νas(CH2) frequency of CdSt decreases from 2918.1 to 2917.6 cm-1 with temperature lowering from 293 to 274 K. At the same temperature of 293 K, the νas(CH2) frequency decreases from 2918.1 to 2917.6 cm-1 with chain lengthening from CdSt to CdB. These effects are reasonable, since the decrease of the νas(CH2) frequency should imply an increase in the chain conformational order. Conclusion The infrared spectra of three famous cadmium alkanoate systems have been recorded in situ at a resolution of 4 cm-1 on the aqueous cadmium acetate subphase in a molecular area range of 0.40-0.10 nm2 and a temperature range of 293-274 K. The crystal lattice information contained in the CH2 scissoring mode was unambiguously concluded. A systematic dependency of the crystal lattice on the monolayer temperature and chain length has been discovered. The CdSt monolayer possesses a hexagonal packing at 293-283 K and a pseudohexagonal packing at 274 K. The CdA monolayer is hexagonal at 293 K and orthorhombic at 283-274 K. The CdB monolayer always has an orthorhombic unit cell at 293-274 K. The hexagonal unit cell of the CdSt and CdA monolayer has a common δ(CH2) frequency of 1468.9

CH3(CH2)nCOOH/Cd2+ System on Aqueous Cadmium Acetate ( 0.1 cm-1. The orthorhombic unit cell of the CdA and CdB monolayer has the characteristic δ(CH2) splitting at 1472 and 1464 cm-1. After deposition of the CdA and CdB monolayer onto solid substrates, this orthorhombic packing is completely lost, irrespective of the deposition temperature. Temperature lowering can increase the interchain constraint and chain lengthening can increase the interchain van der Waals interaction. The enhanced interchain constraint or interaction tends to render the cadmium alkanoate monolayer an orthorhombic packing. On the other hand, temperature lowering and chain lengthening certainly reduce the molecular mobility and hence cause the molecule to adhere more strongly to the subphase. The molecular area at which the COO- group leaves the aqueous subphase with barrier compression also has a systematic dependency on temperature and chain length. The shift of the νas(COO-) frequency to 1543 cm-1 has been used as a symptom to identify the leaving of the COO- group from the subphase. Finally, post-collapse compression of the CdSt, CdA, and CdB systems to 0.10 nm2/molecule at 293-274 K always results in an orthorhombic packing due to the emergence of headgroup interactions. Acknowledgment. The authors appreciate much the financial support from the Venture Business Laboratory of Utsunomiya University. Supporting Information Available: Full description of the supplementary figures. These figures are available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Dutta, P.; Peng, J. B.; Lin, B.; Ketterson, J. B.; Prakash, M.; Georgopoulos, P.; Ehrlich, S. Phys. ReV. Lett. 1987, 58, 2228. (2) Bedzyk, M. J.; Bommarito, G. M.; Caffrey, M.; Penner, T. L. Science 1990, 248, 52. (3) Leveiller, F.; Jacquemain, D.; Lahav, M.; Leiserowitz, L.; Deutsch, M.; Kjaer, K.; Als-Nielsen, J. Science 1991, 252, 1532. (4) Ulman, A. An Introduction to Ultrathin Organic Films from Langmuir-Blodgett to Self-Assembly; Academic Press: New York, 1991, and references therein. (5) (a) Kjaer, K.; Als-Nielsen, J.; Helm, C. A.; Tippmann-Krayer, P.; Mo¨hwald, H. J. Phys. Chem. 1989, 93, 3200. (b) Tippmann-Krayer, P.; Kenn, R. M.; Mo¨hwald, H. Thin Solid Films 1992, 210, 577. (c) Leveiller, F.; Bo¨hm, C.; Jacquemain, D.; Mo¨hwald, H.; Leiserowitz, L.; Kjaer, K.; Als-Nielsen, J. Langmuir 1994, 10, 819. (6) Blaudez, D.; Turlet, J.-M.; Dufourcq, J.; Bard, D.; Buffeteau, T.; Desbat, B. J. Chem. Soc., Faraday Trans. 1996, 92, 525. (7) Buffeteau, T.; Blaudez, D.; Pe´re´, E.; Desbat, B. J. Phys. Chem. B 1999, 103, 5020. (8) Ren, Y. Z.; Hossain, M.; Iimura, K.; Kato, T. Chem. Phys. Lett. 2000, 325, 503.

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