Partial Chain Deuteration as an IRRAS Probe of Conformational Order

Feb 7, 1996 - Daniel K. Beaman , Ellen J. Robertson , and Geraldine L. Richmond. The Journal of Physical Chemistry C 2011 115 (25), 12508-12516...
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Langmuir 1996, 12, 758-762

Partial Chain Deuteration as an IRRAS Probe of Conformational Order of Different Regions in Hexadecanoic Acid Monolayers at the Air/Water Interface Arne Gericke and Richard Mendelsohn* Department of Chemistry, Rutgers University, 73 Warren Street, Newark, New Jersey 07102 Received July 21, 1995. In Final Form: October 2, 1995X External infrared reflection-absorption spectroscopy (IRRAS) of hexadecanoic acid-d17 (half deuterated at the tail end) monolayers at the air/water interface has been used to investigate conformational order in both halves of the molecule. The symmetric and asymmetric CH2 stretching vibrations in the liquidcondensed phase were found to be 0.8 and 1.3-1.8 cm-1 lower in frequency respectively than the same vibrations in hexadecanoic acid-d3 (terminal methyl deuterated), suggesting substantially higher conformational order near the polar headgroup. Theoretical simulations of the bands reveal that the effect cannot arise from different IRRAS intensities for these vibrations in the two molecules. IRRAS measurements at a series of temperatures between 14 and 27 °C reveal a different average response to increased temperature in the two halves of the chain: conformational disorder is introduced at the tail end at a lower temperature. This work demonstrates that current state-of-the-art IRRAS measurements provide sufficient signal-to-noise ratios to monitor conformational order in different regions of amphiphile chains, thus providing a sound experimental base for comparison with theoretical models of chain structure.

Introduction In recent years, substantial progress has been made in the structural characterization of monomolecular films at the air/water (A/W) interface. Fluorescence1,2 and Brewster angle3,4 microscopy reveal details of domain structure in Langmuir films. A wide variety of different forms and sizes has been observed depending on factors such as the type of film-forming molecules, temperature, and state of compression. X-ray determinations5,6 of molecular tilt angles and subcell structures define a rich polymorphism. Infrared reflection-absorption spectroscopy (IRRAS)7 provides in situ information concerning molecular tilt angle, headgroup structure, and conformational order of the polymethylene chains (trans/gauche population ratio). The latter is estimated from the position of the asymmetric and symmetric methylene stretching vibrations (νasym(CH2) and νsym(CH2)) at ≈2920 and 2850 cm-1, respectively. Introduction of conformational disorder leads to increases in the frequencies of both these bands, although quantitative correlations between the extent of conformational disorder and the exact wavenumber have remained elusive. It is known that the increased wavenumbers of the CH2 stretching vibrations due to gauche rotamer formation are caused by coupling between the carbon atoms and the methylene hydrogen in the C-C-C plane (in contrast, for the all-trans conformation all methylene hydrogens are out of the C-C-C plane). * Author to whom correspondence should be addressed. E-mail address: [email protected]. X Abstract published in Advance ACS Abstracts, January 1, 1996. (1) McConnell, H. M. Annu. Rev. Phys. Chem. 1991, 42, 171. (2) Stine, K. J. Microsc. Res. Technol. 1994, 27, 439. (3) Ho¨nig, D.; Mo¨bius, D. J. Phys. Chem. 1991, 95, 4590. (4) Overbeck, G. A.; Ho¨nig, D.; Mo¨bius, D. Thin Solid Films 1994, 242, 213. (5) Mo¨hwald, H. Annu. Rev. Phys. Chem. 1990, 41, 441. (6) Knobler, C. M.; Desai, R. C. Annu. Rev. Phys. Chem. 1992, 43, 207. (7) Mendelsohn, R.; Brauner, J. W.; Gericke, A. Annu. Rev. Phys. Chem. 1995, 46, 305. (8) MacPhail, R. A.; Strauss, H. L.; Snyder, R. G.; Elliger, C. A. J. Phys. Chem. 1984, 97, 334. (9) McKean, D. C.; Biedermann, S.; Bu¨rger, H. Spectrochim. Acta, Part A 1974, 30, 845. (10) Snyder, R. G.; Aljibury, A. L.; Strauss, H. L.; Casal, H. L.; Gough, K. M.; Murphy, W. F. J. Chem. Phys. 1984, 81, 5352.

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Experimental data concerning conformational order in different regions of the polymethylene chains of Langmuir films are limited. Computer simulations comparing perfluorinated and protiated dodecanoic acid in condensed phases led to the following predictions:11 The central section of the molecule exhibits the highest degree of order. For a headgroup smaller than the hydrocarbon chain (perfluorinated saturated carboxylic acids or saturated 1-alkanols), the polymethylene chain adjacent to the headgroup is less ordered than the tail end. The opposite is found if the headgroup is larger than the tail. Karaborni and Toxvaerd12 studied hexadecanoic acid monolayers by molecular dynamics simulations at 300 K. At low areas/ molecule (0.185-0.190 nm2/molecule), the overall conformational order was high, with very little disorder adjacent to the headgroup and stronger disorder at the tail end. At 0.200 nm2/molecule, gauche defects start to diffuse from both ends toward the middle of the molecule. Schmid and Schick13 extended these studies by comparing liquid-condensed (LC) and -expanded (LE) phases. In contrast to the LC-phase, the LE-phase had the highest degree of conformational disorder in the middle of the molecule, with substantially more order at the tail end. In a series of papers, Thomas and co-workers14 used neutron reflection spectroscopy to investigate deuteriumlabeled cationic and anionic surfactant monolayers adsorbed at the air/water interface. They acquired information concerning the thickness of different regions of the layer and the separation of different fragments within the layer. This paper reports an IRRAS investigation of hexadecanoic acid-d17 (deuterated at the tail end) monolayers at the air/water interface to monitor conformational order in different regions of the molecule. The CH2 stretching vibrations probe the chain conformation near the acid moiety, while the CD2 stretching vibrations monitor the tail sections of molecule. The results are referenced to the respective band positions of hexadecanoic acid16,16,16-d3 monolayers. The possibility that the observed differences found in the band positions arise from the (11) Collazo, N.; Shin, S.; Rice, S. A. J. Chem. Phys. 1992, 96, 4735. (12) Karaborni, S.; Toxvaerd, S. J. Chem. Phys. 1992, 97, 5876. (13) Schmid, F.; Schick, M. J. Chem. Phys. 1995, 102, 2080. (14) Lyttle, D. J.; Lu, J. R.; Su, T. J.; Thomas, R. K.; Penfold, J. Langmuir 1995, 11, 1001.

© 1996 American Chemical Society

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optical properties of the substrate is evaluated. Secondly, conformational order adjacent to the headgroup and at the tail end is compared at several temperatures between 14 and 27 °C. Finally, the conformational order of hexadecanoic acid-d17 monolayers in the presence of Pb2+ and Zn2+ subphase cations is studied. Experimental Section The spreading solvent was n-hexane (ACS certified, Fisher, Springfield, NJ). For a discussion of the influence of different spreading solvents on the properties of Langmuir films the reader is referred to Gericke et al.15 Hexadecanoic-9,9,10,10,11,11,12,12,13,13,14,14,15,15,16,16,16-d17 acid (g98% isotopic purity) was obtained from MSD isotopes (St. Louis, MO), while hexadecanoic acid-16,16,16-d3 (g98% isotopic purity) was purchased from Cambridge Isotope Laboratories (Cambridge, MA). All substances were used as received. HPLC grade H2O was used (Fisher Scientific). IRRAS was performed on a Biorad FTS 40A (Cambridge, MA) spectrometer equipped with a MCT detector using a home-built attachment placed on a Newport I-2000 table (Fountain Valley, CA). The IR beam is focused onto the water surface by a concave mirror with a focal length of 120 mm. The angle of incidence was set to 35 ( 1°; unpolarized radiation was used. Spectra were acquired with a resolution of 4 cm-1 by co-addition of 1024 interferograms. One level of zero filling was employed. The thermostatable ((0.1 °C) Langmuir trough was homemade. Surface tension was monitored by use of a NIMA electrobalance (NIMA, Coventry, UK). The accuracy of the surface pressure measurement was 0.1 mN/m, while the area/ molecule was only determined to (0.005 nm2/molecule, due to limited accuracy in the determination of the barrier position. Therefore, we refer mainly to surface pressures rather than to areas/molecule. Surface active compounds were spread at an area of ≈0.45 nm2/molecule (for measurements at 27 °C the film was spread at ≈0.55 nm2/molecule). Prior to the spreading, the area was reduced and surface active contaminants were suctioned off. An initial clean surface was indicated by zero surface pressures at low trough areas. Humidity in the sample compartment was held constant by an adjustable stream of dry air, resulting in a good water vapor match between sample and reference spectra. During the scanning procedure, film compression was interrupted (for a discussion of different compression techniques see ref 16). The initial stepwidth for each compression step was 0.004 nm2/molecule. This was reduced to 0.002 nm2/ molecule in regions of high surface pressure. After each compression step, the film was allowed to relax for 5 min prior to data collection, while the latter takes approximately another 5 min. The total time for running one complete isotherm was ≈10 h. IR peak positions were determined from a center-ofgravity algorithm17 and/or second derivative spectra using software provided by the National Research Council of Canada. It should be noted that due to its low intensity, the measurement of the asymmetric CD2 stretching band (νasym(CD2)) position for hexadecanoic acid-d17 is likely to be disturbed by the nearby CD3 stretching band, which, due to the half-deuteration, is quite strong relative to the CD2 bands. Therefore, νasym(CD2) was determined by first generating the (inverted) second derivative spectrum from the absorption spectrum then utilizing the center-of-gravity algorithm. νsym(CD2) is not as well suited for conformational investigations as this vibration may be affected by Fermi resonance with CD2 scissoring mode overtones; the latter fundamentals may in turn be coupled with C-C stretching vibrations.

Figure 1. IRRAS of hexadecanoic acid-d17 (A) and hexadecanoic acid-d31 (B) between 3000 and 1000 cm-1.

1. Comparison of Hexadecanoic Acid-d17 and Hexadecanoic Acid-d3 in Condensed Phases. In Figure 1, the IRRAS of a hexadecanoic acid-d17 monolayer

is displayed for 20 °C, pH 2, and a surface pressure of 5.6 mN/m. νasym(CH2) and νsym(CH2) are located at ≈2915 and 2849 cm-1, respectively, while νasym(CD2) and νsym(CD2) appear at ≈2192 and 2089 cm-1. A broad feature found at ≈1723 cm-1 arises from the overlapped bands for the unprotonated, singly protonated, and doubly protonated carbonyl groups (for a detailed discussion of the fatty acid monolayer headgroup modes see ref 18). The CH2 scissoring band is observable at ≈1466 cm-1. We refrain from detailed discussions of this feature as its peak position will be strongly affected by residual water vapor bands. However, we do suggest that the absence of a splitting of this feature reveals that the subcell structure is not orthorhombic. The CD2 scissoring mode is barely discerned above the background noise at 1089 cm-1. In Figure 1B the spectrum for perdeuterated hexadecanoic acid is shown for comparison. νasym(CD2) and νsym(CD2) are found at ≈2192 and 2089 cm-1, while the CD2 scissoring band is located at 1089 cm-1. The lack of splitting for this mode is consistent with the absence of an orthorhombic subcell structure. Deuterated compounds exhibit a Π/A isotherm slightly shifted to larger areas/molecule compared with their protiated analogues (Brumm et al.19 ). At 20 °C, the liftoff of the isotherm for half-deuterated hexadecanoic acid is found at ≈0.248 nm2/molecule, while the LC f S transition was observed at ≈0.194 nm2/molecule (not shown). Although these data appear slightly shifted compared with protiated hexadecanoic acid,20 we reiterate that the accuracy of our area determination is limited to (0.005 nm2/molecule. In Figure 2a,b, νasym(CH2) and νsym(CH2) are displayed for hexadecanoic acid-d17 monolayers in the LC and solid (S) phases at 20 °C. The wavenumbers found for hexadecanoic acid-d17 are considerably lower than those for hexadecanoic acid on an H2O subphase18. To determine whether the lower wavenumbers are caused by the absence of the (protiated) methyl stretching bands (note that the tail end is deuterated), which overlap the spectral region where the CH2 stretching modes occur, the results are compared to hexadecanoic acid-16,16,16d3 monolayers. For both fatty acids, the wavenumbers of

(15) Gericke, A.; Simon-Kutscher, J.; Hu¨hnerfuss, H. Langmuir 1993, 9, 2219. (16) Gericke, A.; Simon-Kutscher, J.; Hu¨hnerfuss, H. Langmuir 1993, 9, 3115. (17) Cameron, D. C.; Kauppinen, J. K.; Dougls, J. M.; Mantsch, H. Appl. Spectrosc. 1982, 36, 245.

(18) Gericke, A.; Hu¨hnerfuss, H. J. Phys. Chem. 1993, 97, 12899. (19) Brumm, T.; Naumann, C.; Sackmann, E.; Rennie, A. R.; Thomas, R. K.; Kanellas, D.; Penfold, J.; Bayerl, T. M. Eur. Biophys. J. 1994, 23, 289. (20) Gaines, G. L., Jr. Insoluble monolayers at liquid-gas interfaces; Interscience: New York, 1966.

Results and Discussion

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Figure 2. νasym(CH2) and νsym(CH2) wavenumbers vs surface pressure for hexadecanoic acid-d17 (O) and -d3 (b).

Figure 3. Simulated νasym(CH2) band centered in transmission at 2917 cm-1 for different intensities. Simulation parameters for the monolayer are nmax ) 1.5, kmax was varied between 0.05 and 1.5, the molecules are assumed to be untilted, and the surface layer thickness was set to 2.19 nm. The substrate refractive indices n2 and k2 are taken from ref 26 and interpolated for the desired stepwidth.

the methylene stretching vibrations decrease throughout the LC-phase and are approximately constant in the S-phase. It is noted that fatty acid monolayers show partial collapse in the S-phase, thus limiting the utility of data collected from that region of the isotherm. The methylene stretching vibrations of hexadecanoic acid-d3 in the LC-phase are found to be 1.3-1.8 cm-1 (νasym(CH2)) and ≈0.8 cm-1 (νsym(CH2)) higher than their respective values for half-deuterated hexadecanoic acid. IRRAS band positions may shift compared with the transmission spectrum of the same compound, depending on the refractive index of the substrate in that spectral region, the absorption coefficient k, the angle of incidence, and the state of polarization of the incident beam.7 The substrate refractive index influences the shape and intensity of a band in a manner that depends on the initial (unperturbed) band intensity. The intensities of the CH2 stretching vibrations for half-deuterated hexadecanoic acid monolayers are considerably lower than their counterparts for hexadecanoic acid-d3 films. To decide whether intensity differences per se may lead to the observed shifts in these frequencies (as a result of a differential disturbance by the optical properties of the substrate), νasym(CH2) was simulated for different intensities by varying the film absorption coefficient kmax (Figure 3). For the simulation, formulas 3 and 4 of the paper by Gericke et al.21 are used. A detailed discussion of IRRAS optical models at the A/W interface is given in ref 7. For the simulation, it was assumed that the molecules are untilted (whether or not the molecule is tilted will not affect the

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Figure 4. Comparison of the νasym(CH2) (A) and νasym(CD2) (B) wavenumbers of hexadecanoic acid-d17 monolayers at different surface pressures for pH ≈ 6 (O) and pH 2 (2).

issue under discussion). The simulation is presented for s-polarized radiation, because unpolarized radiation at an angle of incidence of 35° behaves comparably to s-polarized radiation (i.e., the component of the electric field normal to the water surface is quite small). It is found that the peak position shifts to lower wavenumbers upon increasing the band intensity (Figure 3). Since for the half-deuterated hexadecanoic acid, the methylene stretching bands are shifted to lower wavenumbers, under conditions of decreasing band intensities compared with hexadecanoic acid-d3, the observed shift cannot be caused by the optical properties of the substrate. For compounds labeled with a single CD2 group, the frequency of the CD2 stretching vibration was found to depend on the location of the CD2 group within the chain,22 i.e., the headgroup influences the stretching frequency of nearby CD2 groups. In the current case, the headgroup may slightly influence the frequency position of the first couple of methylene units. Overall, the influence is expected to be minor, because coupling between adjacent CH2 units will reduce the magnitude of any isolated frequency alteration. Since the CH2 stretching modes monitor conformational order in the half of the molecule adjacent to the headgroup, it is concluded that chain order from the middle of the molecule toward the headgroup is higher than the overall order of the molecule. Rice and co-workers23 studied fully equilibrated 1-alkanol monolayers at the air/water interface for a series of temperatures down to 5 °C. The lowest frequencies found for νasym(CH2) and νsym(CH2) of solid phase henicosanol monolayers at 5 °C were ≈2915.9 and 2848.3 cm-1, respectively. The frequencies found in the current work for the region of the chain adjacent to the headgroup are slightly lower. Thus, even in comparison to highly condensed monolayers, an additional process that lowers the CH2 stretching frequencies adjacent to the headgroup must be assumed. 2. Conformational Order in Condensed Phases at Different Temperatures and pHs. In Figure 4, νsym(CH2) and νasym(CH2) for hexadecanoic acid-d17 are compared for pH 6 and pH 2. Lowering the pH increases the headgroup size due to protonation, which, according to the models described in the Introduction, should result in (21) Gericke, A.; Michailov, A. V.; Hu¨hnerfuss, H. Vib. Spectrosc. 1993, 4, 335. (22) Cameron, D. G.; Casal, H. L.; Mantsch, H. H.; Boulanger, Y.; Smith, I. C. P. Biophys. J. 1981, 34, 1. (23) Buontempo, J. T.: Rice, S. A. J. Chem. Phys. 1993, 98, 5835. Buontempo, J. T., Rice, S. A. J. Chem. Phys. 1993, 99, 7030. Buontempo, J. T. Rice, S. A. Karaborni, S. Siepmann, J. I. Langmuir 1993, 9, 1604.

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Figure 7. IRRAS of half-deuterated hexadecanoic acid-d17 monolayers in the presence of 1 mM Zn2+.

Figure 5. Comparison of the νasym(CH2) (A) and νasym(CD2) (B) wavenumbers of hexadecanoic acid-d17 monolayers at different surface pressures for the temperatures 17 (2) and 20 °C (O).

Figure 6. Comparison of the νasym(CH2) (A) and νasym(CD2) (B) wavenumbers of hexadecanoic acid-d17 monolayers at different surface pressures for the temperatures 20 (O), 23 °C (9), and 27 °C (4).

increased conformational order adjacent to the headgroup and reduced order at the tail end. Enhanced hydrogen bond formation between the headgroups should yield the same effect. However, the data in Figure 4 reveal no significant pH dependence of the chain conformational order, i.e., the wavenumbers for both the CH2 and CD2 stretching vibrations (Figure 4) are the same within experimental error. The slightly greater “scatter” of the data for pH 2 may arise from a greater tilt angle away from the normal and therefore less intense IRRAS bands. In Figure 5 the wavenumbers for the asymmetric CH2 and CD2 stretching bands of hexadecanoic acid-d17 monolayers at 17° and 20° are displayed vs surface pressure. The data for 14 °C are essentially unchanged from the data for 17 °C (former not shown). νasym(CH2) values are essentially unchanged for 17 and 20 °C (Figure 5A) and decrease throughout the LC-phase, reaching a minimum value of 2915.1 ( 0.1 cm-1. In contrast, the wavenumbers for the asymmetric CD2 stretching vibrations differ by approximately 1 cm-1 (Figure 5B) at the two temperatures. This result is indicative of a different average response to increased temperature in the two halves of the chain, i.e., the disorder is more easily introduced at the tail end upon raising the temperature. In Figure 6 the respective values are shown for 20, 23, and 27 °C. νasym(CH2) increases in this temperature interval. For the asymmetric CD2 stretching vibration the situation is less clear, because the broadening and diminishing intensity of the

band with increasing temperature tend to produce scatter in the data as a result of an increasingly less accurate peak picking. However, it appears that the values are approximately constant. Therefore, the following sequence of events occurs over the temperature range investigated: At low temperatures (14-20 °C) the chain begins to disorder at the tail end. As the temperature increases, the order at the tail end remains constant, while the region adjacent to the headgroup becomes more disordered, i.e., different parts of the chain disorder differentially as the temperature increases. These results are in accord with some computer simulations.11-13 3. Conformational Order of Half-Deuterated Hexadecanoic Acid Monolayers in the Presence of Zn2+ and Pb2+ Subphase Cations. In Figure 7, the IRRAS of half-deuterated hexadecanoic acid monolayers on a subphase of 1 mM Zn2+ is shown. The carboxylic acid bands (≈1705-1739 cm-1) are replaced by the strong and sharp asymmetric (1541.6 cm-1) and symmetric (1396.5 cm-1) carboxylate bands. The position of the carboxylate band and the type of complex formed with the Zn2+ cation are discussed in detail by Simon-Kutscher et al.24 In that paper, unusually low wavenumbers for the methylene stretching vibrations are described, e.g., νasym(CH2) was located at 2914.3 cm-1. R. G. Snyder, in several discussions with the authors, has pointed out that different subcell structures in ordered phases may cause reduced frequencies for CH2 stretching bands. The location of the methylene deformation mode, 1471 cm-1, suggests a triclinic subcell. In the context of this work, it was interesting to determine whether the CH2 stretching wavenumbers represent the lowest wavenumbers for a triclinic structure. The CH2 stretching vibrations for halfdeuterated hexadecanoic acid in the presence of Zn2+ were found at 2911.8 ( 0.2 and 2848.9 ( 0.1 cm-1, while νasym(CD2) was located at 2188.2 ( 0.2 cm-1. It is interesting to note that the wavenumber reduction of νasym(CH2) is more pronounced than the reduction of νsym(CH2). Different subcell structures may account for part of the observed differences in the asymmetric CH2 stretching frequencies. Gericke and Hu¨hnerfuss25 investigated octadecanoic acid monolayers on Pb2+-containing subphases and found a strong ordering effect induced by the cation, presumably due to the bridging character of the bidentate chelate formed between the cation and the carboxylate headgroup, which results in reduced headgroup mobility. In the current case, the positions found for νasym(CH2) and νsym(CH2) for hexadecanoic acid-d17 (Pb2+ subphase) are 2914.2 ( 0.2 and 2848.1 ( 0.2 cm-1, respectively, while νasym(CD2) is found at 2191.0 ( 0.2 cm-1. All bands indicate (24) Simon-Kutscher, J.; Gericke, A.; Hu¨hnerfuss, H. Langmuir, in press. (25) Gericke, A.; Hu¨hnerfuss, H. Thin Solid Films 1994, 245, 74. (26) Bertie, J. E.; Ahmed, M. K.; Eysel, H. H. J. Phys. Chem. 1989, 93, 2210.

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Figure 8. Wavenumbers of the symmetric methylene stretching vibration vs area/molecule of hexadecanoic acid-d17 monolayers at 14 (b) and 27 °C (4).

Figure 9. Wavenumbers of the asymmetric methylene stretching vibration vs area/molecule of a hexadecanoic acid-d17 monolayer at 14 °C.

higher conformational order compared to the cationfree case. The much lower CD2 frequencies in the presence of the Zn2+-compared to the Pb2+-containing subphases may indicate a higher order and thus less mobility, at the tail end. We note that different subcell structures for Zn2+- and Pb2+-containing subphases result in different effects on the band positions of the CH2 stretching vibrations, i.e., although the asymmetric CH2 stretching band is found at lower wavenumbers for Zn2+ subphases in comparison to Pb2+ containing subphases, the symmetric stretching band is located at higher wavenumbers. Further work is evidently required to determine the effects of different subcell structures on the CH2 frequencies in ordered structures. 4. Conformational Order in Two-Phase Regions. The experimental investigation of two-phase regions and less ordered phases is of great interest for testing the predictions of computer modeling. Due to increasing disorder, the methylene stretching bands become less intense and broadened compared to those condensed phases. Therefore, it is difficult to obtain precise frequency measurements for the CD2 stretching bands of hexadecanoic acid-d17 for disordered phases. In Figure 8, the wavenumbers of νsym(CH2) are shown for different areas/ molecule at 14 and 27 °C, respectively. At low temperatures, the wavenumbers decrease slightly (≈0.5 cm-1) upon compressing the film, while at 27 °C the decrease is substantial. In the two-phase region above ≈0.25 nm2/ molecule both data sets exhibit strong scatter. The values for 27 °C are clearly higher than those for 14 °C, indicating increasing disorder with temperature for regions adjacent to the headgroup, a result consistent with that for the condensed phases. No clear transition can be inferred. However, the curve for 27 °C flattens slightly as the film is compressed toward the LC-region. The data points for 17 and 20 °C are superimposible within the limits of error with the data for 14 °C, while the respective values for 23 °C appear to be slightly higher (not shown). The results obtained for νasym(CH2) are less intuitive and are shown for the 14 °C data in Figure 9 (similar behavior was found for all investigated temperatures). For large areas/

Gericke and Mendelsohn

molecule , the values are scattered although the wavenumbers seem to reach a maximum around the lift-off pressure in the Π/A isotherm. For unpolarized radiation at the angle of incidence used, vibrations exhibiting strong absorption parallel to the water surface will show high intensity. Thus, a possible interpretation of the above observation would be that for large areas/molecule only the more ordered and less tilted film molecules are sampled. Upon decreasing the available area/molecule, the less ordered molecules become less tilted and slightly more ordered, both of which result in an increased intensity of that component. As a result, the less ordered component exerts a greater influence on the overall band shape and position. This interpretation is in line with the work of Gericke and Hu¨hnerfuss18 who found that the half-width of the νasym(CH2) band for octadecanoic acid monolayers increases prior to lift-off of the Π/A isotherm. However, the question persists as to why the symmetric and asymmetric CH2 stretching bands show different behavior. In the first case a gradual decrease of the wavenumbers is found, while in the latter case a maximum is observed. In general, the position of the asymmetric CH2 band is more strongly influenced by conformational order than the symmetric one. As a result, the CH2 bands of two or more conformational states will have a greater wavenumber separation for the asymmetric vibration. Therefore, it should be possible to separate component bands from very ordered (e.g., LC-domains) and quite disordered conformational species (e.g., gaseous phase). It appears that the asymmetric CH2 stretching band has a shoulder at large areas/molecule. However, the low signal-to-noise ratio persuades us to refrain from an unequivocal assignment of this feature. For the symmetric CH2stretching band, where the two components are not separated, a broadened band in an intermediate wavenumber position results. This behavior was not observed for fully protiated carboxylic acid monolayers, which may be a result of the presence of at least four conformational species, i.e., LC- and gaseous phases have different conformational order and within these phases the two halves of the molecules may also have different order. Conclusions IRRAS measurements of hexadecanoic acid-d17 (half deuterated at the tail end) monolayers at the air/water interface provide an efficient means for investigation of conformational order in both halves of the molecule. The symmetric and asymmetric CH2 stretching vibrations in the liquid-condensed phase were found to be 0.8 and 1.31.8 cm-1 lower in frequency, respectively, than the same vibrations in hexadecanoic acid-d3 (terminal methyl deuterated), suggesting substantially higher conformational order near the polar headgroup. The observation of more conformational order near the headgroup is in good agreement with the molecular dynamics simulations of Karaborni and Toxvaerd.12 IRRAS measurements for films at a series of temperatures between 14 and 27 °C clearly reveal a different average response to increased temperature in the two halves of the chain; conformational disorder is introduced at the tail end at a lower temperature. This work demonstrates that current IRRAS measurements provide sufficient signal-to-noise ratios to monitor conformational order in different regions of amphiphile chains, thus providing a sound experimental basis for comparison with theoretical models of chain structure. Acknowledgment. This work was supported by grant GM 29864 from the Public Health Service to R.M. A.G. was supported by a grant from the German Research Foundation (DFG). LA950608Z