Langmuir 1998, 14, 3003-3010
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Self-Assembled Monolayers of Branched Thiols and Disulfides on Gold: Surface Coverage, Order and Chain Orientation Victor Chechik,† Holger Scho¨nherr,‡ G. Julius Vancso,‡ and Charles J. M. Stirling*,† Centre for Molecular Materials and Department of Chemistry, The University of Sheffield, Sheffield S3 7HF, U.K., and Faculty of Chemical Technology, Polymer Materials Science and Technology, University of Twente, P.O. Box 217, 7500 AE Enschede, The Netherlands Received October 6, 1997. In Final Form: March 5, 1998 Self-assembly of several branched thiols possessing two long alkane chains and corresponding disulfides on gold surfaces are described. The self-assembled monolayers (SAMs) obtained were investigated by contact angle measurements, Fourier transform infrared spectroscopy (FT-IR), surface plasmon resonance (SPR), and atomic force microscopy (AFM). Monolayers formed by the disulfides were shown to be significantly thinner (SPR) and much more disordered (FT-IR, contact angles) than SAMs of the thiol counterparts. The presence of polar functional groups and complementary H-bond donors/acceptors in the alkane chains of branched disulfides was shown to assist the formation of better packed monolayers. Compared to SAMs of octadecanethiol, the branched thiols investigated in this study gave SAMs with a significantly reduced tilt angle, as seen in the FT-IR spectra. AFM revealed the lattice of one of the thiols on Au(111) with molecular (lattice) resolution showing a reduced area per molecule (as compared to octadecanethiol) which is consistent with a reduced tilt angle.
Introduction The self-assembly of thiols on noble metals has been studied intensively during the last 15 years.1 It has been established by a multitude of experimental methods that these organosulfur compounds form stable, well-ordered self-assembled monolayers (SAMs) on various metals. In a number of cases, well-packed quasi-crystalline monolayers were obtained.1 In SAMs of thiols on Ag(111) the alkane chains are oriented perpendicular to the surface,2 whereas in SAMs on Au(111) the alkane chains are tilted ca. 30° with respect to the surface normal direction.3 The reason for this tilt angle is considered to be a consequence of the arrangement of the sulfur atoms on the gold surface. The distance between adjacent sulfur atoms on Au(111) (ca. 5 Å) is larger than the distance between closely packed alkane chains (4.2-4.4 Å).1c,4 Therefore, the alkane chains tilt in order to achieve a close-packed structure. In SAMs of thiols on silver, however, the sulfur-sulfur distance is only ca. 4.6 Å. Thus the tilt angle of the molecules is close to zero.4 The validity of this interpretation has been proven by investigations on the tilt angle and the 2-dimensional crystal lattices of SAMs of fluorinated thiols and disulfides on Au(111).5-8 The increased diameter of the helical fluorocarbon segment (ca. 5.5 Å)9 (as compared to a † ‡
The University of Sheffield. University of Twente.
(1) (a) Ulman, A. An Introduction to Ultrathin Organic Films: From Langmuir-Blodgett to Self-Assembly; Academic Press: New York, 1991. (b) Bishop, A. R.; Nuzzo, R. G. Curr. Opin. Colloid Interface Sci. 1996, 1, 127-136. (c) Dubois, L. H.; Nuzzo, R. G. Annu. Rev. Phys. Chem. 1992, 43, 437-463. (2) (a) Samant, M. G.; Brown, C. A.; Gordon, J. G. Surf. Sci. 1996, 365, 729-734. (b) Ehler, T. T.; Malmberg, N.; Noe, L. J. J. Phys. Chem. B 1997, 101, 1268-1272. (3) (a) Sinniah, K.; Cheng, J.; Terrettaz, S.; Reuttrobey, J. E.; Miller, C. J. J. Phys. Chem. 1995, 99, 14500-14505. (b) Hoffmann, H.; Mayer, U.; Brunner, H.; Krischanitz, A. Vibr. Spectrosc. 1995, 8, 151-157. (4) Ulman, A. Chem. Rev. 1996, 96, 1533-1554.
Chart 1
hydrocarbon segment, 4.2-4.4 Å) resulted in a reduced tilt angle (as seen by FT-IR,5,6 NEXAFS,5 and grazing incidence X-ray diffraction7) and an increase in the nearest neighbor distances (as seen by AFM).6-8,10 In this paper we present the results of an alternative approach to manipulate the tilt angle of the alkane segment in SAMs of thiols on Au(111). In branched thiols containing two hydrocarbon segments (see Chart 1), the alkane chains occupy roughly twice as large an area as for corresponding n-alkanethiols. Furthermore, the sulfur (5) Lenk, T. J.; Hallmark, V. M.; Hoffmann, C. L.; Rabolt, J. F.; Castner, D. G.; Erdelen, C.; Ringsdorf, H. Langmuir 1994, 10, 46104617. (6) Alves, C. A.; Porter, M. D. Langmuir 1993, 9, 3507-3512. (7) Liu, G.-Y.; Fenter, P.; Chidsey, C. E. D.; Ogletree, D. F.; Eisenberger, P.; Salmeron, M. J. Chem. Phys. 1994, 101, 4301-4306. (8) Jaschke, M.; Scho¨nherr, H.; Wolf, H.; Butt, H.-J.; Bamberg, E.; Besocke, M. K.; Ringsdorf, H. J. Phys. Chem. 1996, 100, 2290-2301. (9) The diameter of a poly(tetrafluoroethylene) polymer chain can be concluded from the crystal structure of poly(tetrafluoroethylene): Polymer Handbook, 3rd ed.; Brandrup, J., Immergut, E. H., Eds.; Wiley: New York, 1989; p. V/37. (10) Scho¨nherr, H.; Vancso, G. J. Langmuir 1997, 13, 3769-3774.
S0743-7463(97)01090-1 CCC: $15.00 © 1998 American Chemical Society Published on Web 05/05/1998
3004 Langmuir, Vol. 14, No. 11, 1998
Figure 1. (a) Structure of an alkanethiol-gold monolayer with sulfur atoms occupying the 3-fold hollow sites on the gold surface.4 (b) Branched thiols are expected to form a monolayer with chains perpendicular to the gold surface, even though some binding sites on the gold surface remain unoccupied. Sulfur atoms are dark gray, alkane chains are light gray, and gold atoms are white.
atoms are not expected to arrange in a close-packed manner as in the well-known hexagonal x3 × x3 R 30° adlayer of n-alkanethiols, commensurate with the underlying Au(111) surface.1a Therefore, a monolayer structure with a significantly reduced tilt angle with respect to the surface normal was expected (see Figure 1). Experimental Section Materials. HPLC grade dichloromethane and ethanol were purchased from Aldrich and used as such. Deionized water was used in all experiments. Preparation of Monolayers. Gold substrates were prepared by thermal evaporation of 5 nm of Cr followed by 100 nm of Au onto silicon wafers that were precleaned by heating in piranha solution (7:3 mixture of concentrated sulfuric acid and 30% hydrogen peroxide) at 90 °C for 1 h. Caution! Piranha solution reacts violently with almost any organic materials and should be handled with the utmost care!11 Prior to deposition of monolayers, the gold substrates were treated with concentrated HNO3 for 10 min, washed exhaustively with water and ethanol and dried. Monolayers were prepared by dipping the precleaned substrates into 1 mM solutions of compounds 1-7 in dichloromethane for 16 h at room temperature. The substrates were then rinsed with dichloromethane and ethanol and dried. Instrumentation. FT-IR spectra of monolayers were determined with a Perkin-Elmer 1725 X instrument fitted with an MCT detector and a grazing angle accessory (Spectra-Tech Inc.). Freshly cleaned (concentrated HNO3, 10 min) bare gold slides were used as the background. The sample compartment of the spectrometer was purged with nitrogen. The positions of the weak peaks in the spectra were determined as follows. Several spectra (6-10) of the same monolayer were recorded and the spectra were smoothed by a factor of 3 using the Savitsky-Golay algorithm. Automatically picked peak positions in all spectra were within the range of 2 cm-1, and the values quoted in the text are the averages of all measurements. Surface plasmon resonance (SPR) data were acquired using an instrument built in these laboratories giving a resonance angle resolution of 0.006°. The values of thickness were calculated from the shift of the resonance angle using the standard Fresnel equation12 assuming the dielectric constant of the film to be 2.1. In one case, the value of the dielectric constant was calculated to be 2.12 using the SPR signal in gas and solution by the method described by Georgiadis et al.13 Contact angles were recorded with a CCD camera attached to a Power Macintosh computer. Water drops (1 µL) were generated with a micrometer syringe (Agla). Electronic images of sessile, advancing, and receding drops were stored in the computer and analyzed using ClarisDraw software. At least three drops were analyzed for each slide. The atomic force microscopy (AFM) images were recorded with a NanoScope II and a NanoScope III AFM (Digital Instruments (11) Bain, C. D.; Troughton, E. B.; Tao, Y.-T.; Evall, J.; Whitesides, G. M.; Nuzzo, R. G. J. Am. Chem. Soc. 1989, 111, 321-335. (12) Hansen, W. N. J. Opt. Soc. Am. 1968, 58, 380-390. (13) Peterlinz, K. A.; Georgiadis, R. Langmuir 1996, 12, 4731-4740.
Chechik et al. (DI), Santa Barbara, CA) in the contact mode. The measurements were carried out in a liquid cell (DI) filled with ethanol (p.a., Merck). The silicon nitride cantilevers (DI) used in this study had a nominal spring constant of 0.06 N/m. Existence of large atomically flat Au(111) terraces is crucial for successful AFM imaging with molecular resolution; therefore, SAMs of 2 were assembled on annealed Au(111).14 The lattice constants and symmetry were evaluated according to a procedure reported earlier.14,15 Since the images of SAMs of 2 were noisier than images of SAMs of octadecanethiol, all images presented here were filtered by calculating the autocovariance pattern.16 Molecular modeling was performed with Macromodel software using an MM2 force field. Synthesis. The cysteine/cystine-based tripeptides 1, 2, 6, and 7 were synthesized using standard procedures of peptide chemistry (Scheme 1).17 The secondary thiols/disulfides 3 and 4 were prepared from di-n-tridecyl ketone using the method described by Wilson et al.18 Disulfide 1. The trifluoroacetate of cystine p-nitrophenyl ester 8 (160 mg, 4.5 mmol) was dissolved in 10 mL of dry THF. To this solution, heptadecanoyl chloride (143 mg, 4.9 mmol) was added in THF followed by triethylamine (114 mg, 11.25 mmol). The mixture was warmed to ca. 50 °C and stirred at this temperature for 2 h. It is important to maintain a high temperature to prevent precipitation of intermediates. Hexadecylamine (120 mg, 4.9 mmol) in THF was added to the reaction mixture, and it was left aside for 16 h. The next day the mixture was evaporated, mixed with 50 mL of warm DCM, filtered, and evaporated. The solid residue was washed with ethanol and dried. Crude product was purified by flash chromatography from DCM/ether 15/1 (Rf 0.4). Yield: 70 mg (26%). Mp: 117-119 °C. 1H NMR (CDCl ): δ 8.21 (br t, 1H, NHCOCH), 6.41 (br d, 1H, 3 NHCH), 5.32 (m, 1H, CH), 3.22 (m, 2H, CH2N), 2.90 (m, 2H, CH2S), 2.25 (t, 2H, CH2CO), 1.15-1.75 (m, 28H, CH3CH2CH2 + CH2CH2CH2), 0.87 (t, 6H, CH3). IR (cast film), cm-1: ν(NH) 3287, amide I 1636, amide II 1538. Anal. Calcd for C72H142N4O4S2: C, 72.55; H, 12.01; N, 4.70. Found: C, 72.35; H, 12.13; N, 4.60. Thiol 2. Zn powder (50 mg) was added to the stirred solution of disulfide 1 in 10 mL of THF containing 5 mL of acetic acid. The mixture was refluxed for 2 h, diluted with water, extracted with DCM, dried, and evaporated. Crude product was purified by flash chromatography from DCM/ether 15/1 (Rf 0.25) and recrystallized from ethanol. Yield: 40 mg (67%). Mp: 112 °C. 1H NMR (CDCl ): δ 6.44 (br d, 1H, NHCH), 6.30 (br t, 1H, 3 NHCH2), 4.52 (m, 1H, CH), 3.24 (q, 2H, CH2N), 3.04 and 2.66 (m, 2H, CH2S), 2.25 (t, 2H, CH2CO), 1.15-1.75 (m, 29H, SH + CH3CH2CH2 + CH2CH2CH2), 0.87 (t, 6H, CH3). IR (cast film), cm-1: ν(NH) 3287, amide I 1636, amide II 1538. Anal. Calcd for C36H72N2O2S: C, 72.42; H, 12.16; N, 4.69. Found: C, 72.44; H, 12.23; N, 4.52. Thiol 4. (a) S-Ketal Intermediate 10. Di-n-tridecyl ketone 9 (1 g, 2.53 mmol) was refluxed overnight with ethanedithiol (0.24 g, 2.55 mmol) and 10 mg of p-toluenesulfonic acid in 50 mL of benzene with azeotropic distillation of water. The next day the reaction mixture was poured into water, and the organic layer was separated, washed, dried, and evaporated to give the oil 10, which was used without further purification. 1H NMR (CDCl3): δ 3.50 (s, 4H, SCH2), 1.89 (m, 2H, CH2CS), 1.23-1.50 (m, 22H, CH2CH2CH2 + CH3CH2CH2), 0.86 (s, 3H, CH3). (b) n-Butyllithium (15 mL of 1.6 M solution in hexane) was added under argon to a solution of the crude product 10 from the previous synthesis (1.22 g, 2.53 mmol) in dry ether, and the (14) Scho¨nherr, H.; Vancso, G. J.; Huisman, B.-H.; van Veggel, F. C. J. M.; Reinhoudt, D. N. Langmuir 1997, 13, 1567-1570. (15) Briefly, cross-sectional plots of autocovariance filtered images were evaluated quantitatively. The calculated averages were recalibrated as described in ref 8. (16) The autocovariance is calculated as the inverse 2-D Fourier transform of the product of Fourier transform and corresponding complex conjugate Fourier transform of the image. In contrast to Fourier “manual” filtering, this filtering eliminates the influence of the operator. (17) Meienhofer, J. In The Peptides: Analysis, Synthesis, Biology; Gross, E., Meienhofer, J., Eds; Academic Press: New York, 1979; Vol. 1; pp 263-314. (18) Wilson, S. R.; Georgiadis, G. M.; Khatri, H. N.; Bartmess, J. E. J. Am. Chem. Soc. 1980, 102, 3577-3583.
SAMs of Thiols and Disulfides on Au
Langmuir, Vol. 14, No. 11, 1998 3005 Scheme 1
mixture was stirred under argon overnight. The next day it was cautiously quenched with water and acidified, and the organic layer was separated, washed, dried, and evaporated. Crude product 4 was purified by flash chromatography from petroleum ether (Rf 0.57). Overall yield: 85%. Mp: 40 °C. 1H NMR (CDCl3): δ 2.77 (m, 1H, CHSH), 1.20-1.70 (m, 24H, CH2), 1.35 (d, 1H, SH), 0.87 (s, 3H, CH3). Anal. Calcd for C27H56S: C, 78.56; H, 13.67. Found: C, 78.61; H, 13.83. Disulfide 3. Thiol 4 (100 mg) was dissolved in 5 mL of ether. To this mixture, a solution of iodine in ethanol was added until persistent yellow color was observed for 30 min. The excess of iodine was destroyed with an aqueous solution of sodium thiosulfate, the mixture was poured into water, and the organic layer separated, washed with water, dried, and evaporated. Crude product was purified by flash chromatography from petroleum ether (Rf 0.8). Yield: 90%. Mp: 37.5 °C. 1H NMR (CDCl3): δ 2.59 (m, 1H, CHS), 1.15-1.65 (m, 24H, CH2), 0.87 (s, 3H, CH3). Anal. Calcd for C54H110S2: C, 78.75; H, 13.46. Found: C, 78.66; H, 13.43. Disulfide 6. (a) N-Boc-cystinylhexanolamine 12. Bis-(N-Boc)cystine 11 (2.2 g, 5 mmol) was dissolved in dry THF (50 mL), and the solution was cooled to -15 °C. To this mixture, isobutyl chloroformate (1.36 g, 10 mmol) was added followed by triethylamine (1.01 g, 10 mmol). The mixture was stirred at -15 °C for 15 min, 6-aminohexanol (1.17 g, 10 mmol) was added, and the solution was stirred for further 2 h. The mixture was filtered, dried, and evaporated. The crystals were washed with water, dried, and crystallized from ethyl acetate. Yield of amide 12 was 2 g (60%). Mp: 135.5-136 °C. 1H NMR (CDCl3), δ, ppm: 7.70 (br t, 1H, NHCH2), 5.55 (br d, 1H, NHCH), 4.74 (m, 1H, CH), 3.62
(t, 2H, CH2O), 3.25 (m, 2H, CH2N), 2.94 (m, 2H, CH2S), 1.7 (br s, 1H, OH), 1.55 (m, 4H, CH2CH2N + CH2CH2O), 1.45 (s, 9H, CH3), 1.35 (m, 2H, CH2CH2CH2CH2CH2). IR (cast film), cm-1: ν(NH + OH) 3335 (br), ν(CsMe) 1393, 1366, ν(CdO) 1687, ν(C-O) 1171, amide I 1658, amide II 1522. Anal. Calcd for C28H54N4O8S2: C, 52.64; H, 8.52; N, 8.77. Found: C, 52.33; H, 8.63; N, 8.95. (b) (N-Boc-cystinyl)(tosyloxy)hexylamine 13. N-Boc-cystinylhexanolamine 12 (1 g, 1.56 mmol) was dissolved in dry pyridine (5 mL), and the solution was cooled to 0 °C. Tosyl chloride (0.717 g, 3.76 mmol) was added, and the reaction mixture was stirred at 0 °C for 7 h. The solution was diluted with DCM, washed with water, dried, and evaporated. Crude product 13 was purified by flash chromatography (EtOAc:DCM ) 1:6). Rf 0.22. Yield: 720 mg (49%). Mp: 98-99 °C. 1H NMR (CDCl3), δ, ppm: 7.77 (d, 2H, CHArCSO2), 7.68 (br t, 1H, NHCH2), 7.35 (d, 2H, CHArCMe), 5.52 (br d, 1H, NHCH), 4.74 (m, 1H, CH), 3.97 (t, 2H, CH2O), 3.20 (m, 2H, CH2N), 2.90 (m, 2H, CH2S), 2.45 (s, 3H, CH3Ph), 1.62 (m, 4H, CH2CH2N + CH2CH2O), 1.45 (s, 9H, C(CH3)3), 1.30 (m, 2H, CH2CH2CH2CH2CH2). IR (cast film), cm-1: ν(NH) 3333, ν(CsMe) 1391 (w), ν(CsMe) + νa(SO2) 1362, ν(CdO) 1685, ν(CsO) + νs(SO2) 1175, νs(SO2) 1189, amide I 1652, amide II 1520. Anal. Calcd for C42H66N4O12S4: C, 53.25; H, 7.02; N, 5.91. Found: C, 53.38; H, 7.13; N, 5.69. (c) (N-Boc-cystinyl)(tosyloxy)hexylamine 13 (115 mg, 0.24 mmol) was treated with trifluoroacetic acid (1 mL) for 1 min. The mixture was evaporated and triturated with ether, and the solvent was evaporated. The solid was dissolved in THF (10 mL), and the solution was cooled to 0 °C. To this mixture, dodecanoyl
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Chechik et al.
Table 1. Contact Angles of Monolayers 1-7 contact angle, deg monolayer
sessile
advancing
receding
hysteresis, deg
1 2 3 4 5 6 7
94 104 74 101 106 79 89
100 110 86 106 112 90 92
79 94 54 90 96 71 78
21 16 32 16 16 19 14
chloride (57 mg, 0.26 mmol) was added followed by triethylamine (76 mg, 0.75 mmol). The mixture was stirred for 1 h, filtered, and evaporated. Crude product 6 was purified by flash chromatography (ether/DCM ) 1/4). Rf 0.57. Yield: 120 mg (90%). Mp: 121.5-122.5 °C. 1H NMR (CDCl3), δ, ppm: 8.18 (br t, 1H, NHCH2), 7.76 (d, 2H, CHArCSO2), 7.32 (d, 2H, CHArCMe), 6.40 (br d, 1H, NHCH), 5.30 (m, 1H, CH), 3.98 (t, 2H, CH2O), 3.20 (m, 2H, CH2N), 2.90 (m, 2H, CH2S), 2.45 (s, 3H, CH3Ph), 2.22 (t, 2H, CH2CO), 1.70-1.20 (m, 26H, CH2CH2CH2 + CH3CH2CH2), 0.86 (t, 3H, CH3). IR (cast film), cm-1: ν(NH) 3293, νa(SO2) 1356, νs(SO2) 1175, 1188, amide I 1636, amide II 1538. Anal. Calcd for C56H94N4O10S4: C, 60.51; H, 8.52; N, 5.04. Found: C, 60.76; H, 8.71; N, 4.97. Thiol 7. Zn powder (50 mg) was added to the stirred solution of disulfide 6 (60 mg) in 10 mL of THF containing 5 mL of acetic acid. The mixture was refluxed for 2 h, diluted with water, extracted with DCM, dried, and evaporated. Crude product was purified by flash chromatography from DCM/ether 4/1 (Rf 0.47). Yield: 35 mg (60%). Mp: 92-92.5 °C. 1H NMR (CDCl3), δ, ppm: 7.77 (d, 2H, CHArCSO2), 7.34 (d, 2H, CHArCMe), 6.42 (m, 2H, NH), 4.53 (m, 1H, CH), 4.01 (t, 2H, CH2O), 3.22 (q, 2H, CH2N), 3.02 + 2.70 (m, 2H, CH2S), 2.45 (s, 3H, CH3Ph), 2.23 (t, 2H, CH2CO), 1.70-1.20 (m, 27H, HS + CH2CH2CH2 + CH3CH2CH2), 0.86 (t, 3H, CH3). IR (cast film), cm-1: ν(NH) 3289, νa(SO2) 1357, νs(SO2) 1175, 1189, amide I 1634, amide II 1541. Anal. Calcd for C28H48N2O5S2: C, 60.40; H, 8.69; N, 5.03. Found: C, 60.66; H, 8.83; N, 4.91.
Results and Discussion Self-assembled monolayers prepared from the thiols and disulfides shown in Chart 1 were analyzed by contact angle measurements, surface plasmon resonance, and grazing incidence FT-IR spectroscopy. SAMs of octadecanethiol 5 were used as a reference. The contact angle measurements (Table 1) revealed that there is a pronounced difference between SAMs prepared from branched alkanethiols 2 and 4 and SAMs prepared from the corresponding disulfides 1 and 3. The low contact angles for disulfides 1 and 3 suggest that these compounds form only incomplete SAMs. The large hysteresis is indicative of a disordered layer.19 The hysteresis observed for the parent thiols 2 and 4 is equal to the hysteresis for reference thiol 5. Thus, the thiols form much more ordered SAMs than the corresponding disulfides. The thicknesses of the SAMs were measured by surface plasmon resonance (SPR) spectroscopy.20 In Table 2 the experimentally determined thickness is compared with the theoretical thickness estimated by using CoreyPauling-Koltun (CPK) models and assuming a perpendicular orientation on the surface. The thickness of the SAMs prepared from thiols, as measured by SPR, is consistently greater than of those prepared from the corresponding disulfides. This can be explained by a lower coverage of the Au surface in SAMs prepared from the disulfides. From a comparison with the estimated maximum thickness it becomes evident that disulfides 1 and 3 form incomplete layers. (19) Bain, C. D.; Whitesides, G. M. J. Am. Chem. Soc. 1989, 111, 7164-7175. (20) Knoll, W. MRS Bull. 1991, 16, 29-39.
Table 2. Thickness of Monolayers 1-6 monolayer
SPR thickness, Åa
max theor thickness, Åb
1 2 3 4 5 6 7
10 20c d 17 23 11 15
26 26 19 19 25 20 20
a The dielectric constant of the monolayer is assumed to be 2.1 unless stated otherwise. b Estimated using CPK models. c The dielectric constant of this monolayer was determined to be 2.12 (see Experimental Section). d Disulfide 3 gives incomplete monolayers with variable thickness.
Figure 2. Grazing angle FT-IR spectra of the monolayers of octadecanethiol 5 (a), disulfides 1 (b) and 3 (c), thiols 2 (d) and 4 (e), and disulfide 1 exposed to solution of octadecanethiol for 16 h (f).
On the assumption that the SPR response is linearly proportional to the amount of material adsorbed,21 one can estimate a coverage of ca. 50% for SAMs of 1. Taking into account the very low contact angles and the large hysteresis of SAMs of 3 (vide supra), this estimate of 50% may even be too high. To further characterize monolayers of compounds 1-4, their FT-IR spectra were analyzed (Figure 2). In this analysis the following features of the C-H stretching frequencies were taken into account: (i) The position of the νa(CH2) peak is sensitive to packing of alkane chains (interchain interactions). The location of this peak at 2918 or >2922 cm-1 is typical of well and loosely packed alkane chains, respectively.22 (ii) The intensity of the νa(CH3) peak is almost independent of tilt and is therefore to a first approximation (21) DeBono, R. F.; Loucks, G. D.; Manna, D. D.; Krull, U. J. Can. J. Chem. 1996, 74, 677-688. (22) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. Soc. 1987, 109, 3559-3568.
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Langmuir, Vol. 14, No. 11, 1998 3007
Table 3. Relative Intensity of the νa(CH3) Peak and νa(CH3)/νa(CH2) Intensity Ratio
monolayer
rel intens of the νa(CH3) peaka
νa(CH3)/νa(CH2) intens ratio (corrected for the no. of CH2 groups)b
1 2 3 4 5
0.4 1.1
