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Raman Spectroscopy of Langmuir Monolayers at the Air-Water Interface James R. Chamberlain and Jeanne E. Pemberton* Department of Chemistry, University of Arizona, Tucson, Arizona 85721 Received September 30, 1996. In Final Form: April 4, 1997X A new approach for the acquisition of Raman spectra of Langmuir layers at the air-water interface is presented. Colloidal Ag is grown electrochemically under layers of the anionic surfactants stearic acid and dipalmitoylphosphatidic acid sodium salt (DPPA) on an aqueous AgNO3 subphase. The Ag colloid layer supports surface-enhanced Raman scattering (SERS) of the Langmuir layers with minimal interference from the subphase. Spectra of good signal-to-noise ratio have been acquired. These spectra are interpreted in terms of the headgroup chemistry and the degree of alkane chain order as a function of surface pressure.
Introduction Langmuir-Blodgett techniques have been used for decades to prepare ordered assemblies of organic molecules on the surface of water. When transferred to solid substrates, these ordered assemblies have unique properties well-suited for uses in electronic, nonlinear optical, and chemical sensing devices.1 The chemistry of such ordered assemblies while they remain at air-water or related liquid-liquid interfaces remains of critical importance to the study of surfactant behavior and of model biological membranes. A number of techniques can be used to study LangmuirBlodgett films (i.e., compressed assemblies transferred to solid substrates) including FTIR, fluorescence, and electrochemistry. FTIR spectroscopy has been used to investigate the degree of ordering of alkane chains in transferred films.2,3 Cyclic voltammetry has been used to study the redox behavior and aggregation of electrochemically-active species.4,5 Fluorescence spectroscopy has been used to ascertain the orientation of fluorophoredoped alkanoic acid Langmuir-Blodgett films.6 The goals of such studies have been to elucidate molecular orientation and interaction between neighboring molecules in these films. Very few techniques, however, can probe alkane chain order or disorder while the Langmuir layer remains at the air-water interface. Until recently, both X-ray spectroscopy and fluorescence spectroscopy were the primary tools used in elucidating Langmuir layer behavior.7-10 X-ray diffraction patterns of Langmuir layers have shown that the crystal lattice length goes through a definite transition as the surface pressure is increased through the liquid-expanded to liquid-condensed * Author to whom correspondence should be addressed. X Abstract published in Advance ACS Abstracts, May 15, 1997. (1) Langmuir-Blodgett Films; Roberts, G., Ed.; Plenum Press: London, 1990. (2) Ohnishi, T.; Ishitani, A.; Yamamoto, N.; Tsubomura, H. J. Phys. Chem. 1978, 82, 1989. (3) Kimura, F.; Umemura, J.; Takenaka, T. Langmuir 1986, 2, 96. (4) Kobayashi, N.; Lam, H.; Nevin, W. A.; Janda, P.; Leznoff, C. C.; Koyama, T.; Monden, A.; Shirai, H. J. Am. Chem. Soc. 1994, 116, 879. (5) Jehoulet, C.; Obeng, Y. S.; Kim, Y. T.; Zhou, F.; Bard, A. J. J. Am. Chem. Soc. 1992, 114, 4237. (6) Edmiston, P. L.; Lee, J. E.; Wood, L. L.; Saavedra, S. S. J. Phys. Chem. 1996, 100, 775. (7) Kjaer, K.; Als-Nielson, J.; Helm, C. A.; Laxhuber, L. A.; Mo¨hwald, H. Phys. Rev. Lett. 1987, 58, 2224. (8) Kjaer, K.; Als-Nielson, J.; Helm, C. A.; Tippman-Krayer, P.; Mo¨hwald, H. Thin Solid Films 1988, 159, 17. (9) Wolf, S. G.; Landau, E. M.; Lahav, M.; Leiserowitz, L.; Deutsch, M.; Kjaer, K.; Als-Nielsen, J. Thin Solid Films 1988, 159, 29. (10) Vaidyanathan, S.; Patterson, L. K.; Mo¨bius, D.; Gru¨niger, H. J. Phys. Chem. 1985, 89, 491.
S0743-7463(96)00947-X CCC: $14.00
phase transition. Fluorescence experiments have shown the formation of aggregates by the splitting of fluorescence peaks10 as well as phase boundaries within a film by fluorescence microscopy.7 In the past several years, sum frequency generation (SFG)11,12 and Fourier transform infrared external reflectance-absorbance spectroscopy (FT-IRAS)13-15 have been used to acquire vibrational spectra from Langmuir layers. SFG is limited to collection of data in specific spectral regions due to current laser limitations. FT-IRAS suffers from similar limitations due to subphase interferences in various spectral regions. Raman spectroscopy would be a desirable tool to add to the existing vibrational spectroscopy arsenal applied to the study of these systems, since it can add complementary information about chain order and tilt angle to the FTIRAS and SFG data in hand. In addition, Raman spectroscopy may also provide information about headgroup interactions both with neighboring surfactant molecules and with subphase molecules. Two recent studies reported Raman spectroscopy of Langmuir layers of deuterated surfactants in the ν(CD) region. In the study of Kawai and co-workers, spectra acquired on deuterated films on a custom-built quartz Langmuir trough by internal reflection Raman spectroscopy were reported.16 Surprisingly, essentially no differences in alkyl chain order were detected with changing surface pressure, in contrast to the results of the vibrational spectroscopic investigations cited above. No further studies by these researchers using this approach have appeared in the recent literature. In a more recent study, Castaings and co-workers reported the use of a CCD-based Raman spectroscopy system for the acquisition of normal Raman spectra from Langmuir layers of deuterated cadmium arachidate in the ν(CD) region.17 The signalto-noise ratios in the ν(CD) region were quite adequate in both studies; however, no spectral data were presented for regions of the spectrum other than the ν(CD) region which might provide important insight into surfactant headgroup chemistry. In the past few years, many advances have been made in the nucleation and crystallization of materials under (11) Shen, Y. R. Nature 1989, 9, 519. (12) Guyot-Sionnest, P.; Hunt, J. H.; Shen, Y. R. Phys. Rev. Lett. 1987, 14, 1597. (13) Dluhy, R. A. J. Phys. Chem. 1986, 90, 1373. (14) Gericke, A.; Simon-Kutscher, J.; Hu¨hnerfuss, H. Langmuir 1993, 9, 2119. (15) Gericke, A.; Simon-Kutscher, J.; Hu¨hnerfuss, H. Langmuir 1993, 9, 3115. (16) Kawai, T.; Umemura, J.; Takenaka, T. Chem. Phys. Lett. 1989, 162, 243. (17) Castaings, N.; Blaudez, D.; Desbat, B.; Turlet, J. M. Thin Solid Films 1996, 284-285, 631.
© 1997 American Chemical Society
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Langmuir, Vol. 13, No. 12, 1997 3075
Figure 1. Electrochemical setup on Langmuir-Blodgett trough.
Langmuir layers.18 These studies are potentially significant for semiconductors and in studies of precious metal crystallization and flocculation. Particularly noteworthy in this regard is the recent work of Fendler and coworkers.18,19 These researchers have shown that Ag colloids of reproducible size (ca. 50-70 nm diameter as determined by transmission electron microscopy) can be grown electrochemically underneath layers of anionic surfactants. The colloidal nature of these Ag films is exploited in the study reported here to support-surface enhanced Raman scattering (SERS) from the Langmuir layer on top of the colloid layer. This approach is used to generate Raman spectra from surfactant layers of stearic acid (SA) and dipalmitoylphosphatidic acid (DPPA) on an aqueous AgNO3 subphase. Experimental Section Materials. AgNO3 (Alfa, 99.9+%), stearic acid (Sigma, 99%), and dipalmitoyl phosphatidic acid sodium salt (Avanti Polar Lipids) were used as received. Dilutions of surfactants and cleaning of the Langmuir trough were performed using HPLCgrade chloroform (Aldrich, 99.9%). Water for all experiments was purified to a resistance of 18.2 MΩ/cm using a Milli-RO 10 plus prefiltration system; organics were removed with a Milli-Q UV system (Millipore). Instrumentation. Preliminary isotherms of stearic acid were produced on a Nima model 2022 Langmuir-Blodgett trough (CTC Nima Technologies). All other Langmuir layers were prepared using an RK 3 Langmuir-Blodgett trough system (Riegler & Kirstein GmbH). The electrochemical setup is shown in Figure 1. Both the working and counter electrodes were 0.5 mm diameter Ag wire (Johnson-Matthey, Puratronic, 99.999%). The reference electrode was a Ag/AgCl electrode prepared by standard methods. The potential was controlled during Ag colloid formation using an IBM model EC/225 potentiostat. The Raman spectroscopic setup is shown in Figure 2. Raman spectra were obtained using 150 mW of 514.5 nm radiation from a Coherent Radiation Innova 90-5 Ar+ laser. The beam was incident on the surface at an angle of ca. 60° with respect to the surface normal. The beam was focused onto the surface using a 40 cm focal length biconvex lens to an elliptical spot ca. 400 µm long by 200 µm wide. Raman scattering was focused onto the collection end of a 0.22 NA, 100 µm fiber core diameter, sevenfiber, round-to-slit fiber optic bundle (Nimbus Optics) using a 12 cm focal length planoconvex lens. A 514.5 nm holographic edge filter (Physical Optics Corporation) was used to remove the Rayleigh-scattered radiation and the specularly reflected excitation beam before it entered the fiber optic bundle. Removal of these light sources is essential to prevent the production of Raman scattering from the fiber optic glass. The image from the slit end of the fiber optic bundle was focused onto the front entrance slit of a Spex 1877 Triplemate Spectrometer using an f/1.2 Minolta camera lens. The front slit of the filter stage was 7.0 mm wide, (18) Fendler, J. H.; Meldrum, F. C. Adv. Mater. 1995, 7, 607. (19) Kotov, N. A.; Zaniquelli, M. E. D.; Meldrum, F. C.; Fendler, J. H. Langmuir 1993, 9, 3710.
Figure 2. Optical sampling arrangement for Raman spectroscopy of Langmuir layers. and the entrance slit to the spectrograph stage was 150 µm wide for all experiments. Gratings of 600 gr/mm and 1200 gr/mm were used in the filter and spectrograph stages, respectively. The final image was focused onto a RTE-1100-PB thinned, backilluminated CCD camera (Princeton Instruments) of 1100 × 330 pixels cooled to -110 °C. Procedure. After filling the trough with water, AgNO3 was dissolved in a small portion of water and added to the trough to achieve a subphase concentration of ca. 20 mM AgNO3. The subphase was allowed to equilibrate for several minutes. The reference and counter electrodes were then placed in the solution prior to spreading of the surfactant solution. At this point, the subphase was thoroughly cleaned of any surface contamination. Films were prepared by depositing 250 µL of a 1.0 mg/mL chloroform solution of surfactant across the surface of the subphase. The chloroform was allowed to evaporate for 2-3 min before compression of the monolayer at 130 cm2/min until the desired surface pressure was obtained. After monolayer formation at the desired surface pressure, the working electrode was slowly lowered toward the subphase surface using the trough dipping mechanism until the electrode just touched the subphase surface. A potential of -0.8 V versus Ag/AgCl was then applied to the working electrode. After the potential was applied, a Ag colloid layer formed at ca. 0.5 cm2/s. The colloid layer formation followed the path of the counter electrode situated under the subphase surface. Once the colloid island reached a size that easily allowed laser beam access to the surface, the electrochemical cell was set to open circuit potential and Raman spectra were acquired.
Results and Discussion Stearic Acid. Stearic acid was chosen as the initial surfactant for study due to its simple structure. Upon compression, however, the isotherms of SA films on AgNO3 subphases exhibited unconventional behavior. Surface pressure-area isotherms for SA produced on both trough geometries are shown in Figure 3. In the isotherm produced on the circular trough, a shift of ca. 10 Å2/ molecule to smaller values relative to a pure water subphase is observed for the isotherm on the AgNO3 subphase. Furthermore, a dip in surface pressure appears just prior to the transition from the gas phase to what appears to be the liquid-expanded phase. The shift in Å2/molecule is not as pronounced in the isotherm produced on the rectangular trough, nor is the dip in surface pressure present. However, isotherms on both troughs produced over AgNO3 subphases show a considerably smaller (ca. 50%) area/molecule for stearic acid than isotherms produced over pure water. The influence of subphase cations on surface areapressure isotherms has been studied extensively for surfactants such as SA and related systems. Cationic species such as Cd2+ in the subphase typically result in
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Figure 3. Stearic acid surface pressure-area isotherms on (a) a water subphase and (b and c) an aqueous AgNO3 subphase. Isotherms on AgNO3 are shown for both the rectangular (RK) trough (b) and the circular (Nima) trough (c).
a small increase in area/molecule of ca. +5% for such surfactants.1 The difference in isotherm behavior between subphase cations such as Cd2+, which are not complexed by the carboxylate group, and Ag+, which are readily complexed by the carboxylate group, is proposed to be due to silver stearate precipitation on the AgNO3 subphase. Both troughs produce visible precipitates when SA is compressed on aqueous AgNO3 subphases. The resulting film is proposed to consist of larger regions of SA Langmuir layers surrounded by smaller regions of silver stearate precipitate. The film is not free-flowing upon suction cleaning of the subphase surface. The difference in isotherms between the two troughs is proposed to be due to compression geometry. The observed dip in surface pressure on the circular trough is probably due to precipitation of silver stearate onto the Wilhelmy balance. We speculate that this effect is more pronounced on the circular trough, once a crystalline, nonflowing layer is formed on the subphase surface, by acceleration of precipitation due to the placement of the Wilhelmy balance in the section of the trough with the greatest angular velocity. Faster precipitate formation in the area around the balance would result in more unreliable readings earlier in the isotherm. Despite the precipitation of silver stearate on the subphase surface, Ag colloid layers can be grown under these SA Langmuir layers. However, for surface pressures above ca. 25 mN/m, surface-confined Ag colloid layers can only be produced ca. 50% of the time. The remainder of the time, dendritic Ag metal deposits grow out from the working electrode. For surface pressures lower than ca. 25 mN/m, the production of surface-confined colloid layers becomes increasingly difficult. The observation of a threshold in surface pressure for successful growth of surface-confined Ag colloid layers is attributed to achievement of a minimum proximity between SA molecules that will support the correct nucleation rate for colloid formation (as opposed to dendrite formation) for the electrochemical conditions employed for Ag reduction. Raman spectra of solid SA and a Langmuir layer of SA are shown for the ν(CH) region in Figure 4. Peak frequencies and assignments are given in Table 1. The band in the spectrum of solid SA at 2846 cm-1 corresponds to a νs(CH2) mode. The bands at 2881 and 2897 cm-1 correspond to νa(CH2) modes.20 The band at 2936 cm-1 (20) Snyder, R. G.; Strauss, H. L.; Elliger, C. A. J. Phys. Chem. 1982, 86, 5145.
Figure 4. Raman spectra in the ν(CH) region of (a) solid SA and (b) a Langmuir layer of SA at 35 mN/m (integration times 300 s). Table 1. Raman Spectral Bands and Assignments for Solid SA and Langmuir Layers of SA assignmenta solid SA SA Langmuir layerb νs(CH2) νs(CH2) νs(CH3) νa(CH2) νa(CH2) νs(CH2)FR νs(CH3)FR νa(CH3)
2846 2854 2872 2881 2897 2925 2936 2965
SA SAMc on Ag
2849
2851(R), 2848(IR)
2873 2883 2900 2928
2877(IR) 2882(R), 2914(IR)
2965
2965(IR)
2924(R)
a
νs ) symmetric stretch, νa ) asymmetric stretch. b At a surface pressure of 35 mN/m. c SAM ) self-assembled monolayer.
corresponds to a Fermi resonance νs(CH3) mode, and the band at 2925 cm-1, to the νs(CH2) mode for the methylene group adjacent to the carboxylic acid.20,21 The band at 2965 cm-1 corresponds to the νa(CH3) mode for the terminal methyl groups. The peaks in the spectrum of the Langmuir layer are shifted to slightly higher frequencies by several inverse centimeters from the values for the spectrum of the solid, suggesting that the layer possesses a level of order approaching, although not identical to, that of the crystalline solid. In other words, this layer exhibits liquid-crystalline-like characteristics. As is discussed in greater detail below, in bulk alkane chain systems, the ratio of the intensity of νa(CH2) to νs(CH2) modes is often used as a measure of the degree of order in alkane chains.20,21 The relatively large intensity of the νa(CH2) mode in the spectra for the SA Langmuir layer also suggests that the layer is in a liquid-crystalline state. The degree of order in the alkane chains of the SA Langmuir layers at this air-water interface is similar to, although slightly greater than, that observed in selfassembled layers of alkanoic acids on solid surfaces such as Ag and Al2O3.22-24 Comparison of the peak frequencies for the ν(CH2) modes for solid SA, SA Langmuir layers, and self-assembled SA layers on Ag in Table 1 suggests that the greatest order in the alkane chains (lowest frequencies) occurs for solid SA, and the least order (21) Wong, P. T. T. Annu. Rev. Biophys. Bioeng. 1984, 13, 1. (22) Thompson, W. R.; Pemberton, J. E. Langmuir 1995, 11, 1720. (23) Tao, Y. T. J. Am. Chem. Soc. 1993, 115, 4350. (24) Schlotter, N. E.; Porter, M. D.; Bright, T. B.; Allara, D. L. Chem. Phys. Lett. 1986, 132, 93.
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Figure 5. DPPA surface pressure-area isotherms on (a) a water subphase and (b) an aqueous AgNO3 subphase.
(highest frequencies) occurs in the self-assembled layers with the order of the alkane chains in the Langmuir layers intermediate between the two. It should be noted, however, that the absolute differences are small, suggesting that both the Langmuir and self-assembled layers are correctly considered to be in a liquid-crystalline state.20 Dipalmitoylphosphatidic Acid. Surface pressurearea isotherms for DPPA on pure water and on an aqueous 20 mM AgNO3 subphase are shown in Figure 5. The isotherm for DPPA on pure water shows that the layer is compressed to ca. 55 Å2/molecule before transition to the liquid-expanded state. The layer then enters the liquidcondensed state at ca. 44 Å2/molecule and finally collapses at 42 Å2/molecule. The isotherm for DPPA on 20 mM AgNO3 shows that the layer remains in the gaseous state until ca. 52 Å2/ molecule. The layer is then compressed in the liquidexpanded state to 40 Å2/molecule. Finally, the layer is compressed in the liquid-condensed state to ca. 38 Å2/ molecule, at which point the layer collapses. The isotherm obtained for DPPA on the AgNO3 subphase deviates slightly from the isotherm on pure water by a shift of ca. 3 Å2/molecule to smaller surface areas. This shift suggests that tighter packing is occurring for the DPPA molecules in the presence of Ag+ at a given surface pressure. However, the absolute difference between the two isotherms is small (
