FTIR Studies of C30 Self-Assembled Monolayers on Silica, Titania

File failed to load: https://cdn.mathjax.org/mathjax/contrib/a11y/accessibility-menu.js. ADVERTISEMENT · Log In Register · Cart · ACS · ACS Publicatio...
0 downloads 0 Views 141KB Size
Download PDF
1746

Langmuir 2004, 20, 1746-1752

FTIR Studies of C30 Self-Assembled Monolayers on Silica, Titania, and Zirconia Gokulakrishnan Srinivasan,† Matthias Pursch,‡ Lane C. Sander,§ and Klaus Mu¨ller*,† Institut fu¨ r Physikalische Chemie, Universita¨ t Stuttgart, Pfaffenwaldring 55, D-70569 Stuttgart, Germany, Dow Deutschland Incorporated, Industriestrasse 1, 77836 Rheinmu¨ nster, Germany, and Analytical Chemistry Division, Chemical Science and Technology Laboratory, National Institute of Standards and Technology (NIST), Gaithersburg, Maryland 20899 Received August 12, 2003. In Final Form: December 19, 2003 The conformational order of alkyl chains in four different C30 self-assembled monolayers (SAMs) has been examined using Fourier transform infrared spectroscopy. The C30 SAMs used in the present study were prepared by reacting C30H61SiCl3 with the humidified surfaces of zirconia, titania, and two different silica gels. The conformational order of the alkyl chains that are attached to the solid substrates is derived from the analysis of well-known conformation-sensitive IR regions, that is, CH2 symmetric and antisymmetric stretching modes as well as CH2 wagging bands. The CH2 symmetric and antisymmetric stretching band positions provide a qualitative measure of the conformational order. The CH2 wagging bands are used to determine the relative amounts, that is, integral values over the whole chains of kink/gauche-transgauche, double-gauche, and end-gauche conformers in the alkyl chain regions. The results derived from the present IR data analysis demonstrate that the substrate plays an important role for the conformational order of alkyl chains that are attached to inorganic oxide surfaces. C30 SAMs on titania are found to possess a higher degree of conformational order as compared to C30 SAMs on zirconia and silica.

Introduction Self-assembled monolayers (SAMs) can be prepared through a variety of approaches on planar and microparticulate sufaces, by reaction and/or immobilization of alkyl siloxanes, fatty acids, and alkanethiolates. SAMs are widely utilized in applications on metal and oxide surfaces,1,2 biosensors,3 electronics,4 optoelectronics,5 Langmuir-Blodgett films,6,7 and chemically modified silicas for various types of chromatography.8-10 The alkyl chain structure and interfacial properties influence function and utility of these materials. Modified interphases are commonly used as stationary phase materials for liquid chromatography and solid-phase extraction, and the conformational order of alkyl chain moieties governs the efficiency and selectivity of separations.11-13 Highly * To whom correspondence should be addressed. Phone: (+49) 711 685 4470. Fax: (+49) 711 685 4467. E-mail: k.mueller@ ipc.uni-stuttgart.de. † Universita ¨ t Stuttgart. ‡ Dow Deutschland Inc. § National Institute of Standards and Technology. (1) Templeton, A. C.; Wuelfing, W. P.; Murray, R. W. Acc. Chem. Res. 2000, 33, 27-36. (2) Ulman, A. An Introduction to Ultrathin Organic Films from Langmuir-Blodgett to Self-Assembly; Academic Press: San Diego, 1991. (3) Haupt, K.; Mosbach, K. Chem. Rev. 2000, 100, 2495-2504. (4) Kagan, C. R.; Mitzi, D. B.; Dimitrakopoulos, C. D. Science 1999, 286, 945-947. (5) Kataoka, T.; Konda, T.; Ito, R.; Sasaki, S.; Uchida, K.; Miura, N. Phys. B 1994, 201, 423-426. (6) Mendelsohn, R.; Moore, D. J. Chem. Phys. Lipids 1998, 96, 141157. (7) Roberts, G. Langmuir-Blodgett Films; Plenum Press: New York, 1990. (8) Packings and Stationary Phases in Chromotographic Techniques; Unger, K. K., Ed.; Chromatographic Science Series 47; Marcel Dekker: New York, 1990. (9) Grushka, E.; Kikta, E. J., Jr. Anal. Chem. 1977, 49, 1004A. (10) Bonded Stationary Phases in Chromatography; Grushka, E., Ed.; Ann Arbor Science Publishers: Ann Arbor, MI, 1974. (11) Dorsey, J. G.; Cooper, W. T. Anal. Chem. 1994, 66, A857-A867.

ordered, high-density stationary phases have been shown to possess enhanced shape selectivity for planar polycyclic aromatic hydrocarbons (PAHs), while less ordered stationary phase materials are characterized by reduced geometric selectivity.14,15 The description of the alkyl chain conformational order on a molecular level is thus crucial to an understanding of such materials in their respective chromatographic applications. Efforts have been directed toward the study of the formation, structure, and dynamics of C18 SAMs16 and alkyl-modified silica gels.17,18 In contrast, very few studies of long-chain (C30) SAMs have been reported so far.19-21 Silica-based C30 phases designed for liquid chromatography have received increased attention after the successful application to the separation of carotenoids, vitamin A isomers, fullerenes, tocopherols, and tocotrienols.22-24 Titania and zirconia provide an alternative to silica as chromatographic packing materials and exhibit high mechanical stability, separation ef(12) Sander, L. C.; Wise, S. A. Anal. Chem. 1995, 67, 3284-3292. (13) Sander, L. C.; Wise, S. A. Anal. Chem. 1984, 56, 504-510. (14) Ellwanger, A.; Sander, L. C.; Bell, C. M.; Handel, H.; Albert, K. Solid State Nucl. Magn. Reson. 1997, 9, 191-201. (15) Sander, L. C.; Pursch, M.; Wise, S. A. Anal. Chem. 1999, 71, 4821-4830. (16) Badia, A.; Demers, L.; Cuccia, L.; Lennox, R. B. J. Am. Chem. Soc. 1997, 119, 2682-2692. (17) Zeigler, R. C.; Maciel, G. E. J. Phys. Chem. 1991, 95, 73457353. (18) Zeigler, R. C.; Maciel, G. E. J. Am. Chem. Soc. 1991, 113 (17), 6349-6358. (19) Boehm, C.; Leveiller, F.; Moehwald, H.; Kjaer, K.; Als-Nielsen, J.; Weissnuch, I.; Leiserowitz, L. Langmuir 1994, 10, 830-836. (20) Bierbaum, K.; Grunze, M.; Baksi, A. A.; Chi, L. F.; Schrepp, W.; Fuchs, H. Langmuir 1995, 11, 2143-2150. (21) Bierbaum, K.; Linzler, M.; Woell, C. H.; Grunze, M.; Haehner, G.; Heid, S.; Effenberger, F. Langmuir 1995, 11, 512-518. (22) Sander, L. C.; Sharpless, K. E.; Craft, N.; Wise, S. A. Anal. Chem. 1994, 66, 1667-1674. (23) Emenhiser, C.; Sander, L. C.; Schwartz, S. J. J. Chromatogr. 1995, 707, 205-216. (24) Emenhiser, C.; Englert, G. E.; Sander, L. C.; Ludwig, B.; Schwartz, S. J. J. Chromatogr., A 1996, 719, 333-343.

10.1021/la0354739 CCC: $27.50 © 2004 American Chemical Society Published on Web 02/04/2004

C30 Self-Assembled Monolayers

ficiency, and chemical inertness at elevated temperatures and over a pH range of 1-14.25 The present work addresses the conformational properties of C30 self-assembled monolayers attached to zirconia, titania, and two different silica gels. Fourier transform infrared (FTIR) spectroscopy is a powerful and versatile analytical method, which has been utilized to study conformational order in a wide range of alkyl materials. For example, FTIR absorption frequencies, band intensities, and band shapes have been shown to reflect the molecular conformation, configuration, and chain packing in polyethylene chains.26 In a pioneering work, Snyder calculated and tabulated the IR frequencies for an extensive number of vibrations in alkane systems, including some that are specific to localized bent structures containing gauche bonds.27 The normal mode calculations detail the assignment of various C-H stretching, scissoring, rocking, and wagging modes. The intensities and positions of these bands provide quantitative information about various defect structures within the chains that contribute to the overall disorder of the system. These conformational defects have also been observed in other more complex hydrocarbon assemblies, such as model membranes and biomembranes28 as well as micellar systems.29 Sander et al.30 reported the first application of FTIR spectroscopy to the study of the conformational structure of C1-C22 n-alkyl modified silica surfaces though semiquantitative assessment of C-H stretching and wagging mode bands. Since this early work, relatively few efforts have utilized FTIR spectroscopy to characterize conformational details of chromatographic stationary phases. Jinno et al.31,32 reported differences in alkyl chain order and rigidity for monomeric and polymeric C18 stationary phases, as assessed with diffuse reflectance FTIR spectroscopy, and Lochmu¨ller and co-workers33 demonstrated the use of Fourier transform infrared photoacoustic spectroscopy to characterize octadecylmodified surfaces. Recently, Singh et al.34 described a comprehensive FTIR investigation of alkyl chain conformational order in alkyl-modified silica gels. The influence of alkyl chain length, bonded phase density, and temperature were examined. In the present work, a variable temperature FTIR study was carried out to assess the conformational order of four different C30 SAMs. A qualitative statement about the conformational order is possible based on the analysis of symmetric and antisymmetric CH2 stretching bands.35-37 The frequency shift of the band maxima in the symmetric/ (25) Nawrocki, J.; Rigney, M. P.; McCormick, A.; Carr, P. W. J. Chromotogr., A 1993, 657, 229-282. (26) Mendelsohn, R.; Snyder, R. G. Biological Membranes; Merz, K. M., Jr., Roux, B., Eds.; Birkha¨user: Boston, 1996. (27) Snyder, R. G. J. Chem. Phys. 1967, 47, 1316-1360. (28) Mendelsohn, R.; Davies, M. A.; Brauner, J. W.; Schuster, H. F.; Dluhy, R. A. Biochemistry 1989, 28, 8934-8939. (29) Farida, H.; James, B. C. J. Am. Chem. Soc. 1989, 93, 20532058. (30) Sander, L. C.; Callis, J. B.; Field, L. R. Anal. Chem. 1983, 55, 1068-1075. (31) Jinno, K.; Ibuki, T.; Tanaka, N.; Okamoto, M.; Fetzer, J. C.; Biggs, W. R.; Griffiths, P. R.; Olinger, J. M. J. Chromatogr. 1989, 461, 209-227. (32) Jinno, K.; Wu, J.; Ichikawa, M.; Takata, I. Chromotographia 1993, 37 (11-12), 627-634. (33) Lochmu¨ller, C. H.; Marshall, S. F.; Wilder, D. R. Anal. Chem. 1980, 52, 19-23. (34) Singh, S.; Wegmann, J.; Albert, K.; Mu¨ller, K. J. Phys. Chem. B 2002, 106, 878-888. (35) Parikh, A. N.; Leidberg, B.; Atre, S. V.; Ho, M.; Allara, D. L. J. Phys. Chem. 1995, 99, 9996-10008. (36) Kojio, K.; Ge, S.; Takahara, A.; Kajiyama, T. Langmuir 1998, 14, 971-974. (37) Britt, D. W.; Hlady, V. Langmuir 1999, 15, 1770-1776.

Langmuir, Vol. 20, No. 5, 2004 1747 Table 1. Sample Properties of the Present C30 SAMs surface mean specific % C of coverage pore surface size (Å) area (m2/g) C30 SAM (µmol/m2)

support silica (LiChrospher) silica (ProntoSil) titania (Sachtopore) zirconia (Zirchrom phase)

300 260 286 290

77 109 17 31

8.77 16.77 3.58 6.51

3.57 5.22 6.10 6.44

antisymmetric regions provides information about the changes in the conformational order as a function of sample temperature and solid support. Quantitative information about the presence and amounts of various gauche conformers (kink/gauche-trans-gauche (gtg), doublegauche, and end-gauche conformers) in the C30 SAMs has been accessed by the analysis of the wagging band intensities between 1330 and 1400 cm-1. The results from the present FTIR study on four different C30 SAMS are discussed and compared with the published data on other n-alkyl-modified silica gels, pure hydrocarbons, and biological membranes. Experimental Section56 Sample Preparation and Variable Temperature Measurements. The self-assembled monolayer materials in this work are as previously reported38 except that the zirconia SAM was synthesized expressly for the current study and has a slightly altered surface coverage (6.44 vs 5.06 µmol/m2). The essential parameters and properties of the C30 self-assembled monolayers are summarized in Table 1. The samples were measured by means of the KBr pellet technique. The pellets of the SAMs and KBr (1/10 to 1/15 w/w) of 1 mm thickness were prepared under a vacuum using a hydraulic press. The pellets of the respective samples were placed in a brass cell equipped with an external thermocouple in close vicinity to the sample. The same thermocouple was also used for monitoring the actual sample temperature. The brass cell was thermostated in a variable temperature transmission cell (L.O.T., Oriel GmbH, Langenberg, Germany) equipped with KBr windows. The temperature was regulated with an automatic temperature control unit with an accuracy of (0.5 °C. IR Measurements. IR spectra were recorded on a Nicolet Nexus 470 FTIR spectrometer with a nitrogen-purged optical bench (Nicolet, Madison, WI) equipped with a DTGS detector. Typically, 256 interferograms covering a spectral range from 4000 to 400 cm-1 at a resolution of 2 cm-1 were collected within a temperature range from 193 to 353 K. The recorded interferograms were apodized with a triangular function and Fourier transformed with two levels of zero filling. Correction for background absorption was done by recording the background spectrum of the empty cell (measured with twice the number of interferograms as that used for the sample). The background spectrum was automatically subtracted from the spectra of the SAMs. Data from three independent samples were acquired at all temperatures for all the samples studied, and the FTIR spectra were measured twice for each sample (n ) 6). IR Data Analysis. The processing and analysis of the spectra for CH2 stretching band analysis was done with the OMNIC E.S.P.5.1 software (Nicolet). The frequencies of the CH2 stretching vibrations were performed from the interpolated zero crossing in the first derivative spectra. Processing and analysis of the spectra in the CH2 wagging regions were performed using the GRAMS 32 software (Galactic, Salem, NH). A quadratic baseline correction was applied in the spectral region from 1330 to 1400 cm-1. The experimental spectra were fitted using four vibration bands. Their initial positions were 1378 cm-1 (symmetric methyl deformation mode), 1368 cm-1 (kink (gtg′) and gtg sequences), 1354 cm-1 (double-gauche sequences), and 1341 cm-1 (end-gauche sequences). During the (38) Pursch, M.; Vanderhart, D. L.; Sander, L. C.; Gu, X.; Nguyen, T.; Wise, S. A.; Gajewski, D. A. J. Am. Chem. Soc. 2000, 122, 69977011.

1748

Langmuir, Vol. 20, No. 5, 2004

Figure 1. Experimental CH2 stretching bands for C30 SAMs on four different substrates at 353 K. curve fit analysis, the band intensities and widths were varied independently. The integrated intensities of the CH2 wagging bands were further normalized with respect to the methyl deformation band. The amount of specific gauche sequences was calculated according to the procedure given in the literature,39 which is based on reference measurements on alkanes and a theoretical approach using the rotational isomeric state40 (RIS) model. Calculations reflect the fact that the surface-linked alkyl chains contain only one methyl group per chain in contrast to liquid n-alkanes (which contain two methyl groups per chain). The total number of gauche conformers per chain was obtained by taking into account that two gauche bonds are necessary to comprise one kink or double-gauche sequence and one gauche bond for the formation of an end-gauche sequence. The estimated coefficient of variation for the various gauche conformers is 1015%. A signal at 1384 cm-1 was observed with all samples; however, this transition has not been attributed to alkyl wagging or vibrational modes and is not considered in this analysis.

Results The conformational order of the four C30 alkyl modified substrates was determined by the analysis of the CH2 symmetric and antisymmetric stretching modes as well as the CH2 wagging bands in a temperature range from 193 to 353 K. Changes in the FTIR spectra reflect subtle differences in the morphology of the alkyl chains resulting from modification of the various substrates. CH2 Symmetric and Antisymmetric Stretching Band Analysis. The position of the CH2 symmetric and antisymmetric stretching band maxima (at 2853-2846 and 2926-2912 cm-1, respectively) provides qualitative information about the conformational order of the alkyl chains. For completely disordered “spaghetti-like” structures, the frequency of the CH2 antisymmetric stretching band is close to that of a liquid alkane (νa ) 2924 cm-1). For well-ordered systems, the frequency is shifted to lower wavenumbers and is close to that of crystalline alkanes (νa ) 2915-2918 cm-1). Figure 1 illustrates the FTIR spectrum for the C30 SAMs in the antisymmetric stretching band region between 2940 and 2900 cm-1. The CH2 antisymmetric stretching band maximum varies by 10 cm-1 between the C30 SAM on titania and the C30 SAMs on silica, which reflects significant differences in conformational order for these materials. The absorption band maxima of all samples shifted toward higher wavenumbers with increasing (39) Senak, L.; Davies, M. A.; Mendelsohn, R. J. Phys. Chem. 1991, 95, 2565-2571. (40) Flory, P. J. Statistical Mechanics of Chain Molecules; Wiley: New York, 1969.

Srinivasan et al.

Figure 2. Temperature dependence of the CH2 symmetric and antisymmetric stretching modes of the C30 SAM on silica (ProntoSil).

Figure 3. Symmetric CH2 stretching band positions in various C30 SAM systems.

temperature. Band maxima positions and variations with temperature were different for the various substrates. In addition, the CH2 stretching bandwidth increases with increasing sample temperature, as reported earlier for n-alkanes and biomembranes41 (Figure 2). Absorption frequencies of the CH2 symmetric and antisymmetric stretching band regions are summarized in Figures 3 and 4. Inspection of these figures reveals that the frequencies of the symmetric and antisymmetric stretching bands of the C30 SAMs on titania are lower than for the other C30 SAMs. Furthermore, the CH2 symmetric and antisymmetric stretching band positions for the C30 SAMs on the two silica substrates are quite similar. Based on the assessment of the CH2 stretching band positions, the conformational order of the C30 SAMs can be ranked titania > zirconia > silica. CH2 Wagging Band Analysis. The conformationdependent wagging modes of interest appear near 1368, 1353, and 1341 cm -1 and arise from kink/gtg, doublegauche, and end-gauche sequences, respectively, as shown in Figure 5. The most intense band in the spectral range from 1300 to 1400 cm-1 is due to the methyl group umbrella deformation mode at 1378 cm-1, which is insensitive to the conformational order and is used as an internal reference (see Experimental Section). In Figure 6, a representative experimental FTIR spectrum for C30 SAMs (41) Cameron, D. G.; Casal, H. L.; Mantsch, H. H.; Boulanger, Y.; Smith, I. C. P. Biophys. J. 1981, 35, 1-16.

C30 Self-Assembled Monolayers

Langmuir, Vol. 20, No. 5, 2004 1749

Figure 4. Antisymmetric CH2 stretching band positions in various C30 SAM systems. Figure 7. Experimental FTIR spectra (CH2 wagging band region) for C30 SAMs on ProntoSil at three different temperatures.

Figure 5. Alkyl chain conformations. The nonplanar conformations shown give rise to localized mode vibrations, which are observed in FTIR spectra (wagging band region).

Figure 8. Number of end-gauche conformers per chain in the C30 SAMs.

Figure 6. Experimental FTIR spectrum (CH2 wagging band region) for the C30 SAM on ProntoSil at 353 K. The underlying curves result from a curve fitting analysis.

Figure 9. Number of kink/gtg conformers per chain in the C30 SAMs.

on ProntoSil is shown along with the theoretical curves from the curve fitting analysis. Figure 7 portrays typical FTIR spectra (only the CH2 wagging band region) of C30 SAMs on ProntoSil at different temperatures, from which the temperature dependence of the wagging band intensities is clearly evident. The results obtained from the curve fitting analysis of the CH2 wagging bands are shown in Figures 8-10. The number of end-gauche conformers per chain remains

almost unaffected by the sample temperature for all C30 SAMs examined. In contrast, the number of kink/gtg and double-gauche conformers per chain is strongly temperature dependent for all C30 SAMs. The most pronounced changes with temperature are observed for the kink/gtg conformers, particularly above room temperature. For the C30 SAMs on ProntoSil, the number of kink/gtg conformers per chain is between 0.68 and 0.87 at temperatures between 193 and 353 K, whereas for C30 SAMs on titania,

1750

Langmuir, Vol. 20, No. 5, 2004

Figure 10. Number of double-gauche conformers per chain in the C30 SAMs.

Figure 11. Variation of the total number of gauche conformers per chain as a function of temperature in C30 SAMs.

the values lie between 0.53 and 0.67 over the same temperature interval. For LiChrospher, the number of kink/gtg conformers per chain varies between 0.65 and 0.85. In general, at low temperatures the fraction of kink/ gtg conformers per chain is lower than the corresponding values for double-gauche conformers. The total number of gauche conformers per chain for the four different C30 SAMs is depicted in Figure 11. As expected, the total number of gauche conformers per chain increases with temperature for all C30 SAMs. The most alkyl chain disorder is observed for C30 SAMs on Prontosil, with between 3.1 and 3.7 gauche conformers per chain. The fewest defects per chain are found for C30 SAMs on titania, with values between 2.8 and 3.1. Inspection of Figure 11 further reveals that the effect of temperature is less pronounced for C30 SAM on titania than on silica (ProntoSil and LiChrospher), which is similar to the trends observed for CH2 stretching frequencies. Discussion In the present investigation, the conformational order of immobilized C30 alkyl chains was studied by FTIR spectroscopy as a function of temperature and for four different solid supports. The analysis of the experimental spectra was performed in the same manner as shown earlier for other systems bearing alkyl chains, such as pure hydrocarbons,27 phospholipids,28 and n-alkyl-modified silica gels.34 FTIR spectroscopy as well as NMR spectroscopy are very suitable tools to probe the conformational order in

Srinivasan et al.

alkyl chains. However, they are distinguished by their experimental time-scales. That is, IR measurements are essentially a snapshot of different bond conformations, and thus of chain disorder, in a given area of irradiation, while NMR spectroscopy provides an insight into the motional processes which accompany the chain disordering. A major advantage of FTIR spectroscopy is the high sensitivity and the relatively easy access to the desired information about the conformational order. However, often FTIR spectroscopy is not applicable for the study of the alkyl conformational order, since the vibrational bands are obscured by other strong FTIR bands of the materials under investigation. NMR spectroscopy is superior, since, as mentioned earlier, in principle information about both the chain dynamics and the chain ordering is accessible. On a qualitative basis, the chain ordering is obtained via the analysis of 13C NMR spectra, where “trans” and “gauche” NMR signals can be clearly distinguished.38 However, quantitative data about these parameters are only available after performing a detailed data analysis of the NMR line shapes or relaxation data.42 In summary, the application of FTIR and NMR techniques can be used to get complementary information on systems bearing n-alkyl chains. The influences of both the sample type (i.e., substrate composition and/or C30 bonding density) and the sample temperature are clearly evident from the present CH2 stretching data and wagging data. In the case of the CH2 stretching bands, a pronounced shift of the absorption maxima toward higher wavenumbers is visible upon increasing sample temperature. Although the CH2 stretching data can only be discussed on a qualitative basis, the frequency shifts in CH2 stretching bands and the changes in line broadening that are observed with increasing temperature are consistent with decreases in the conformational order. This stretching band data can be compared directly with data from a recent FTIR study on various n-alkyl chemically modified silica gels by Singh et al.34 In that study, which included investigations on deuterated and nondeuterated alkyl chains, it was shown that the conformational disorder in the n-alkyl moieties is very sensitive to the temperature, the proximity of particular CH2 segments to the silica surface, the length of the n-alkyl chain, and the bonding density of the alkyl chains. For instance, it was demonstrated that the conformational disorder in shorter n-alkyl chains (C8C12) is much greater than in their longer counterparts (C22-C30). A further comparison of the present CH2 stretching band data with those from the C30 alkyl modified silica gels,34 prepared via solution polymerization,43 reveals almost identical data for all silica supports despite the differences in surface coverage and synthetic pathway. The resemblance of these data for the various silica gels also includes the pronounced temperature dependence at elevated temperatures. Another comparison of the present CH2 stretching data can be made with the published data of hexadecanethiolate monolayers on a gold surface, in which the authors concluded that the number of gauche defects in the hexadecanethiolate chains is relatively low.44 In a related study, Porter et al.45 utilized reflection-absorption infrared (RAIR) spectroscopy to study a wide range of (42) Neumann-Singh, S.; Villanueva-Garibay, J.; Mu¨ller, K. J. Phys. Chem. B, in press. (43) Pursch, M.; Sander, L. C.; Albert, K. Anal. Chem. 1996, 68, 41074113. (44) Nuzzo, R. G.; Fusco, F. A.; Allara, D. L. J. Am. Chem. Soc. 1987, 109, 2358-2368. (45) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. Soc. 1987, 109, 3559-3568.

C30 Self-Assembled Monolayers

alkanethiolate monolayers (chain lengths, C2-C23) on a 2D gold surface. They compared CH2 symmetric and antisymmetric stretching for the alkanethiolate monolayers with those from crystalline hexadecanethiol (2918 cm-1, 2851 cm-1) and liquid heptanethiol (2924 cm-1, 2855 cm-1). The authors concluded that monolayers with chains longer than hexanethiol are highly ordered, whereas the shorter chain lengths resemble the liquid state with a greater fraction of gauche defects. Our present CH2 stretching data indicate an even higher conformational alkyl chain order for the C30 SAMs compared with these former studies on alkanethiolate monolayers. These differences can probably be attributed to the longer chain lengths of the SAM systems used in the present work. Fadeev et al. studied a series of C18H37SiH3 SAMs prepared on various transition metal oxides by FTIR spectroscopy.46 They observed antisymmetric stretching bands between 2916 and 2919 cm-1 for monolayers on TiO2, ZrO2, and HfO2, which is consistent with ordering of the organosilicon hydride chains. By comparison, CH2 stretching values for C30 SAMs in the current work indicate an even higher degree of conformational order. A distinct increase of the IR bandwidth with sample temperature is observed for all C30 SAMs, irrespective of the actual solid support. Such a behavior is in qualitative agreement with data reported by Griffiths et al. for alkali salts of ascorbyl palmitate.47 Below 313 K, the CH2 stretching bands are narrower, which is indicative of low acyl chain mobility. In addition, the absolute peak wavenumbers are characteristic of almost fully extended all-trans chains. The increase of the CH2 stretching bandwidth with temperature is attributed to an increase in alkyl chain mobility along with a decrease in conformational order. Quite similar bandwidth changes were also reported from a FTIR study on phospholipid membranes.41 Quantitative information about alkyl ordering can be derived from analysis of the CH2 wagging bands within the interval 1330-1400 cm-1. Bands attributed to specific gauche defect sequences were obtained through deconvolution of the spectra over this interval using a fourband model. Details of the approach have been described in a previous study.34 In general, trends observed from the (quantitative) wagging band analysis are in agreement with the (qualitative) data from the CH2 stretching bands. The number of end-gauche conformers per chain was found to be nearly independent of temperature (and solid support), while a distinct temperature dependence is apparent for double-gauche and kink/gtg conformers. These observations for the C30 SAMs are consistent with the RIS model40 developed for liquid alkanes, which predicts that the number of end-gauche conformers per chain is independent of the alkyl chain length, almost independent of temperature, and much lower than for the double-gauche and kink/gtg conformers. These predictions are consistent with a previous IR investigation on (longchain) n-alkanes,48 for which some evidence exists of nonlinear spectral changes in the vicinity of the melting point of the corresponding bulk alkane. This change reflects an increase in the number of nonplanar (gauche) conformers as the melting point is approached. The wagging mode band at 1368 cm-1 deserves additional discussion. Senak et al.39 and Snyder27 noted that (46) Fadeev, A. Y.; Helmy, R.; Marcinko, S. Langmuir 2002, 18, 75217529. (47) Griffiths, P. R.; de Haseth, J. A. Fourier Transform Infrared Spectroscopy; Wiley-Interscience: New York, 1986. (48) Maroncelli, M.; Qi, S. P.; Strauss, H. L.; Snyder, R. G. J. Am. Chem. Soc. 1982, 104, 6237-6247.

Langmuir, Vol. 20, No. 5, 2004 1751

this wagging mode arises from gtg′ and gtg sequences within alkyl chains. The former sequence (gtg′) is often referred to as a “kink” conformation, whereas the gtg sequence represents a more bent conformation (see Figure 5). Because the relative contributions of the two conformers cannot be evaluated from the 1368 cm-1 wagging mode, this band cannot be used rigorously to quantify kink conformers alone. Senak et al. reasoned that kink conformers probably dominate over gtg conformers due to chain packing considerations in alkyl liquids.39 For tethered alkyl chains (e.g., SAMs), spacial requirements would further restrict formation of gtg conformers. Kink/ gtg conformers in the present C30 SAMs exhibit the strongest temperature dependence of the gauche defects studied. As a result, the temperature dependence of the combined gauche conformers per chain is dominated by the formation of the kink/gtg conformers. It is interesting to examine differences in the influence of temperature on conformational defects for the four samples (Figures 8-10). Because the SAMs were prepared in the same manner for the four substrates, it might be expected that the materials would exhibit similar alkyl organization at the surfaces. Clearly, however, differences exist in the numbers of gauche defects per chain and temperature dependence of the alkyl conformations. The two silica-based SAMs exhibit similar properties in terms of kink/gtg defects, even though the surface coverages differ significantly. The zirconia and titania SAMs represent more ordered systems. The temperature dependence of the double-gauche and kink/gtg conformers is somewhat different for the titania SAM than for the other three materials, which suggests that a different organization of the alkyl chains exists on the substrate surface. In this connection, the question might arise of whether the observed differences in chain conformational order are a consequence of the surface coverage. On the basis of the corresponding data in Table 1, such a conclusion cannot be drawn for the present systems. Another important contribution to the packing density of the alkyl chains and thus the alkyl conformational order is the uniformity of the surface coverage, that is, whether the attached alkyl chains are clustered or whether they are distributed uniformly across the solid surface. The same holds for the constitution of the solid surface (roughness, specific surface area, porosity, etc.), which may also affect the alkyl chain properties. Unfortunately, there are not enough experimental data available in order to discuss the influence of the latter two contributions in an appropriate way. The end-gauche conformational data in Figure 8 are consistent with the results reported by Singh et al. for n-alkyl-modified silica gels.34 They observed that the number of end-gauche conformers per chain was 0.35, irrespective of chain length and temperature. In addition, the influence of temperature on the number of doublegauche conformers per chain was small for the C30modified silica gels, which was attributed to the higher chain packing and lower conformational disorder in these long alkyl chains. For shorter alkyl phases, the number of double-gauche conformers and their temperature dependence were found to be much more pronounced. In the current work, elevated temperature was found to influence gauche conformational defects to a greater extent than subambient temperature (see Figures 9 and 10). The data for the various amounts of gauche conformers, as evaluated from the CH2 wagging band data, can be further compared with previous investigations on phospholipids, where such FTIR techniques have been employed extensively. Thus, the liquid crystalline phases of phospholipid bilayers from 1,2-dipalmitoyl-phosphatidyl-

1752

Langmuir, Vol. 20, No. 5, 2004

choline (DPPC), 1,2-dimyristoyl-phosphatidylcholine (DMPC), and 1,2-dipalmitoyl-phosphatidylethanolamine (DPPE) 49,50 are characterized by a lower fraction of doublegauche conformers, while end-gauche and kink/gtg conformers are very similar to the values determined for the present C30 SAMs. The total number of gauche conformers, and thus the conformational disorder, is generally lower in DPPC, DMPC, and DPPE bilayers than in C30 SAMs, although the alkyl chain lengths in these systems are quite different (i.e., C14 or C16 in the phospholipids vs C30 in the SAMs). The titania- and silica-based C30 SAMs have been examined in a previous study by liquid chromatography, atomic force microscopy, ellipsometry, and solid-state 13C NMR spectroscopy.38 Solid-state 13C NMR spectroscopy was used to derive information about the conformational order of the attached alkyl chains by measuring the relative intensities of carbon signals attributed to trans and gauche conformations. The chemical shift for the trans peak of the C30 SAMs (33.4 ppm) is nearly identical to that of the interior methylene carbons of long-chain alkanes in the crystalline state, while the gauche peak at 30.6 ppm agrees with the chemical shift value in the isotropic alkane melt. Similar observations have also been reported for variable temperature 13C NMR investigations of stationary C30-modified silica gels,51 in which the interconversion of the two conformational states was monitored. At elevated temperatures, a larger fraction of gauche conformers was detected, as with the melting process of solid polyethylene and solid n-alkanes.52,53 It should be emphasized that the results from the present FTIR study are consistent with the 13C NMR data by Pursch et al.38 on the same systems, where also higher conformational order for the titania and zirconia SAMs was reported as compared to the silica SAMs. In addition, the 13C NMR line width of the trans peak for the titania SAM was reported to be approximately 3 times higher than for the corresponding interior methylene resonance in crystalline C19H40 and C32H66, which was attributed to a general higher disorder (i.e., greater chemical shift dispersion) as well as a higher mobility, causing line broadening, in the SAMs as compared with the crystalline alkane phases. In principle, the existence of all-trans chains should be visible in FTIR spectroscopy via the formation of wagging band progressions26 in the region between 1350 and 1100 cm-1. This phenomenon has been reported for pure (49) Senak, L.; Davies, M. A.; Mendelsohn, R. J. Phys. Chem. 1991, 95, 2565-2571. (50) Wolfangel, P.; Meyer, H. H.; Bornscheuer, U. T.; Mu¨ller, K. Biochim. Biophys. Acta 1999, 1420, 121-138. (51) Sander, L. C.; Raitza, M.; Pursch, M.; Strohschein, S.; Albert, K. GIT Lab. J. 1998, 2, 237-241. (52) Earl, W. L.; Vanderhart, D. L. Macromolecules 1979, 12, 762767. (53) Vanderhart, D. L. J. Magn. Reson. 1981, 44, 117-125.

Srinivasan et al.

hydrocarbons48 in the crystalline state and phospholipid membranes54,55 in the gel state. In the present study on the C30 SAMs, such wagging band progressions were not detected. It is likely that sufficient numbers of gauche conformers exist in these materials to prevent the formation of progression bands.55 Conclusion Variable temperature FTIR studies were performed on four different C30 SAMs. From the analysis of both the CH2 stretching and CH2 wagging bands, it was found that alkyl chain flexibility and conformational disorder increase with increasing sample temperature. The highest degree of conformational order exists for SAMs prepared on titania, followed by zirconia and by the silica supports. From an analysis of the wagging bands, the number of kink, double-gauche, and end-gauche conformers and the total number of gauche conformers per chain were calculated. It could be shown that the observed temperature dependence of the conformational order is dominated by the changes in the number of kink/gtg conformers. The molecular origin of the somewhat higher conformational order for the titania support so far is unknown. It certainly cannot be related to the surface coverage, as both the zirconia and the titania supports exhibit a very similar surface loading. It might be speculated whether the titania sample exhibits a higher surface inhomogeneity, expressed by areas of local high bonding density (“islands”). This then would give rise to a higher chain packing density in these areas along with the observed higher conformational order. However, so far there is no direct proof for this assumption. Furthermore, the question might arise of why a similar surface inhomogeneity does not exist for the other solid supports as well. It is therefore quite obvious that much more work is necessary in order to provide a satisfactory explanation of the derived alkyl chain order in the systems examined here. Further work along this line, comprising studies on related systems as well as the influence of pressure and solvents, is in progress. Acknowledgment. Financial support for this project from DFG (Deutsche Forschungsgemeinschaft) is gratefully acknowledged. G. Srinivasan is thankful to the Graduiertenkolleg “Chemie in Interphasen” for a doctoral fellowship. LA0354739 (54) Senak, L.; Moore, D.; Mendelsohn, R. J. Phys. Chem. 1992, 96, 2749-2754. (55) Chia, N.; Mendelsohn, R. J. Phys. Chem. 1992, 96, 10543-10547. (56) Certain commercial equipment, instruments, or materials are identified in this report to specify adequately the experimental procedure. Such identification does not imply recommendation or endorsement by the National Institute of Standards and Technology, nor does it imply that the materials or equipment identified are necessarily the best available for the purpose.