Characterization of the State of Order of Octadecylsilane Chains on

Oct 29, 1999 - Raman spectroscopy was used to investigate the state of conformational and packing order for octadecyltrichlorosilane (OTS) attached to...
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Langmuir 2000, 16, 568-576

Characterization of the State of Order of Octadecylsilane Chains on Fumed Silica Rongwei Wang,† Jin Guo,† George Baran,‡ and Stephanie L. Wunder*,† Departments of Chemistry and Mechanical Engineering, Temple University, Philadelphia, Pennsylvania 19122 Received June 23, 1999. In Final Form: September 3, 1999 Raman spectroscopy was used to investigate the state of conformational and packing order for octadecyltrichlorosilane (OTS) attached to high surface area fumed silica as a function of the hydration state of the silica. The fumed silica had a surface area of 50 m2/g and a primary particle size of ∼40 nm. Room temperature and temperature-dependent spectra were obtained for OTS attached to the fumed silica and compared with those of polyoctadecyl siloxane (POS). Both the skeletal optical region, between 1000 and 1150 cm-1, and the high-frequency CH2 stretching region, between 2800 and 3000 cm-1, were monitored. The intensity ratios I(1060)/I(1080), which is indicative of the ratio of trans/gauche bonds, and I(2850)/ I(2885), which is indicative of lateral packing order, were plotted as a function of temperature. For OTS attached to completely dehydrated and dehydroxylated surfaces, only disordered structures of alkyl chains, similar to those of molten alkane chains, were observed. With increasing hydration of the silica surface, there was a marked increase in trans bonds and lateral packing. However, for approximately monolayer coverage of OTS on the silica surface, the packing order of the chains at room temperature never reached the level obtained for POS. POS formed an hexagonal closely packed structure with a differential scanning calorimetry (DSC) melting point at 63 °C, which corresponded to the inflection points of the plots of I(2850)/I(2885) or I(1060)/I(1080) versus temperature. A very much broader and shallower endotherm was observed for OTS attached to “superhydrated” fumed silica, and the ordered structures disappeared by 60 °C. In addition, at room temperature, the packing order of the chains on this “superhydrated” surface was similar to that observed for small unilamellar vesicles of lipid bilayers. The decreased packing order of OTS on “superhydrated” fumed silica compared with POS may be due to the constraints imposed by the underlying silica substrate or to the curvature of the silica particles.

Introduction The hydrophilic and hydrophobic surfaces of fumed silica have been of interest in scientific and technical applications for several decades.1-8 The silanization of silica surfaces with alkyl trichlorosilanes (R-SiCl3) or alkyl trialkoxysilanes (R-Si(OR′)3), which was designed for chromatographic applications 50 years ago,9,10 is still useful and has been extensively studied. Generally, the pretreated silica is added to a solution of the silanizing agent in a suitable organic solvent, such as toluene, hexane, pentane, or cyclohexane. This reaction mixture is stirred at elevated or room temperature, with a post cure period at elevated temperature to effect cross-linking of the silanes. To accelerate the reaction at room temperature in liquid solvents and promote the attachment * To whom correspondence should be addressed. † Department of Chemistry. ‡ Department of Mechanical Engineering. (1) Gun′ko, V. M.; Turov, V. V.; Zarko, V. I.; Dudnik, V. V.; Tischenko, V. A.; Kazakova, O. A.; Voronin, E. F.; Siltchenko, S. S.; Barvinchenko, V. N.; Chuiko, A. A. J. Colloid Interface Sci. 1997, 192, 166. (2) Tripp, C. P.; Veregin, R. P. N.; McDougall, M. N. V.; Osmond, D. Langmuir 1995, 11, 1858. (3) Linseisen, F. M.; Hetzer, M.; Brumm, T.; Bayerl, T. M. Biophys. J. 1997, 72, 1659. (4) Jones, K. J. Chromatogr. 1987, 392, 1. (5) Hennion, M. C.; Picard, C.; Caude, M. J. Chromatogr. 1978, 166, 21. (6) Vansant, E. F.; Voort, V. D. P.; Vrancken, K. C. Studies in Surface and Catalysis; Elsevier: Amsterdam, 1995; Vol. 93. (7) Kiselv, A. V.; Lygin, V. I. Infrared Spectra of Surface Compounds; Wiley: New York, 1975. (8) Iler, R. K. The Chemistry of Silica; Wiley: New York, 1979. (9) Bigelow, W. C.; Pickett, D. L.; Zisman, W. A. J. Colloid Sci. 1946, 1, 513. (10) Legrand, A. P. The Surface Properties of Silicas; John Wiley & Sons: New York, 1998.

of the silane to the silica via a SissO-Si bond (where Sis is a surface silica atom), base catalysis using amines, such as pyridine, triethylamine, or ammonia, is employed.11-14 Octadecyltrichlorosilane (OTS) is the most extensively used silane surface-active reagent for modifying silica surfaces. The mechanism of the silanization reaction has been commonly accepted as follows.15 The aliphatic chains are strongly attracted to the silica surface via the trichlorosilane group, which acts like the polar head of an amphiphilic molecule. Because the surface contains silanol groups, it is covered with a water film that is one or several layers thick. The trichlorosilane groups are hydrolyzed when they get near to the surface. The hydrolyzed chains further react (via elimination of water) either with the surface silanols to anchor the chains on the silica surface or with their close neighbors to form a network. Depending on the pretreatment of the silica surface, it can contain mobile water, hydrogen-bonded water, and/or geminal, vicinal, and isolated silanol groups. The silanization reaction can proceed with any of these species, giving rise to a surface layer covalently bonded to the surface at one extreme, and to a film attached only to the surface water and thus detachable from the silica at the other extreme. Under certain conditions, well-ordered monolayers, described as self-assembled monolayers (SAMs), can be formed.16 (11) Blitz, J. P.; Murthy, R. S. S.; Leyden, D. E. J. Colloid Interface Sci. 1988, 126, 387. (12) Kinkel, J. N.; Unger, K. K. J. Chromatogr. 1984, 316, 193. (13) Tripp, C. P.; Hair, M. L. J. Phys. Chem. 1993, 97, 5693. (14) Melander, W. R.; Horvath, C. High-Performance Liquid Chromatography Advances and Perspectives; Academic: New York, 1980; Vol. 2. (15) Sagiv, J. J. Am. Chem. Soc. 1980, 102, 92.

10.1021/la9908081 CCC: $19.00 © 2000 American Chemical Society Published on Web 10/29/1999

Order of Octadecylsilane Chains on Fumed Silica

The importance of the water content on the substrate surface in determining the reactivity, surface coverage, and chain packing of trichlorosilanes has been emphasized by Silberzan et al.17 and Angst and Simmons.18 Angst and Simmons were able to obtain a tightly packed monolayer on a fully hydrated oxidized silicon wafer (Si/SiO2), whereas reaction with a dry silicon wafer gave a disordered monolayer with lower surface coverage at room temperature. Silberzan et al.17 concluded that OTS reacted with the water layer on the surface of Si/SiO2 and formed very few bonds with the surface at 18 °C. On the other hand, Le Grange and co-workers19 concluded that a fully hydrated surface was not essential for complete coverage by OTS, and suggested that islands of well-ordered chains formed around water clusters on the surface. Tripp et al.20 and McGovern et al.21 also believed that the surface water played an important role in the adsorption process. Tripp22 showed that OTS remained in solution and was not adsorbed by the silica surface until the surface water was present in more than monolayer quantities (i.e., on “superhydrated” fumed silica). Allara et al.16,23 suggested that a mobile water layer on the surface was necessary to decouple the OTS monolayer from the underlying substrate allowing improved lateral packing of the hydrocarbon tails. However, on flat surfaces, it was not possible to directly measure the amount of adsorbed water or to determine the hydration state of the substrate. The structures of the OTS layers that are formed, namely their thickness, uniformity, and state of order of the alkyl chains, have been investigated on flat silica surfaces using methods such as ellipsometry, X-ray reflectivity, and vibrational spectroscopy. In the latter case, Fourier transform infrared spectroscopy (FTIR) has been used most frequently. The frequencies of the methylene symmetric (d+) and antisymmetric (d-) stretching vibrations are sensitive to chain conformational order. The value of d+ is in the range 2846-2850 cm-1 for the all-trans chain24,25 and occurs at around 2856 cm-1 for liquidlike disordered chains.26 The value of d- is between 2918 and 2919 cm-1 for the all-trans chain and at 2928 cm-1 for the disordered chain. For SAMs of C-8 and C-18 alkyl silanes on fused silica glass plates,27 infrared-visiblesum-frequency generation spectroscopy (IV-SFG) showed that there were more cis-trans chain conformational defects in the C-8 monolayer compared with the C-18 monolayer. Dichroic ratio measurements of OTS deposited onto a silicon attenuated total reflection (ATR) prism showed that28 at low coverage more liquidlike conformations of the alkane chain were seen, whereas at high coverage there were more trans conformations. IR measurements of OTS monolayers on well-hydrated oxidized (16) Parikh, A. N.; Allara, D. L.; Azouz, I. B.; Rondelez, F. J. Phys. Chem. 1994, 98, 7577. (17) Silberzan, P.; Leger, L.; Ausserre, D.; Benattar, J. J. Langmuir 1991, 7, 1647. (18) Angst, D. L.; Simmons, G. W. Langmuir 1991, 7, 2236. (19) LeGrange, J. D.; Markham, J. L.; Kurkjian, C. R. Langmuir 1993, 9, 1749. (20) Tripp, C. P.; Hair, M. L. Langmuir 1995, 11, 1215. (21) McGovern, M. E.; Kallury, K. M. R.; Thompson, M. Langmuir 1994, 10, 3607. (22) Tripp, C. P.; Hair, M. L. Langmuir 1992, 8, 1961. (23) Brzoska, J. B.; Azouz, I. B.; Rondelez, F. Langmuir 1994, 10, 4367. (24) Snyder, R. G.; Schatschneider, J. H. Spectrochim. Acta 1963, 19, 85. (25) MacPhail, R. A.; Strauss, H. L.; Snyder, R. G.; Elliger, C. A. J. Phys. Chem. 1984, 88, 334. (26) Snyder, R. G.; Strauss, H. L.; Elliger, C. A. J. Phys. Chem. 1982, 86, 5145. (27) Huang, J. Y.; Song, K. J.; Lagoutchev, L.; Yang, P. K.; Chuang, T. J. Langmuir 1997, 13, 58. (28) Banga, R.; Yarwood, J.; Morgan, A. M. Langmuir 1995, 11, 618.

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silicon and gold substrates showed peaks at 2850 and 2917 cm-1,29 which are indicative of chains in a predominantly all-trans conformation. These values were obtained under conditions where the water layer was believed to decouple the monolayer film from the underlying substrate, allowing for better in-plane lateral reorganization. The d- frequencies were 1 cm-1 lower than observed previously,18,30-33 indicating improved conformational ordering for the films. In the case of spherical surfaces, methods such as ellipsometry and X-ray reflectivity are not applicable, and only vibrational techniques such as FTIR spectroscopy have been used to investigate the state of order of the alkane chains. However, FTIR measurements alone are useful only for monitoring trans/gauche sequences in the alkyl chains, and have not been used to monitor lateral interaction packing. Raman spectroscopy has proved to be a useful tool in studying long alkyl chains. The Raman spectra of alkyl chains have characteristic features that can be used to identify both their conformational and packing order in the liquid and solid states, in biomembrane systems, and in lipids, surfactants, and polymers.34,35 The intensity distribution in the CH2 stretching region, particularly the peak height ratio I(2850)/I(2885), has been found to be sensitive to the physical state of those systems, with its value being affected by chain conformation, mobility, and packing. In addition, an alkyl chain has characteristic features in the region from 1000 to 1150 cm-1 in the Raman spectrum, which can be correlated with the population of trans and gauche conformers along the chain. Fumed silica has no spectral features in either the conformationally sensitive backbone region or in the high wavenumber CH2 stretching region. Raman spectroscopy thus provides a convenient method to detect conformational and packing changes of long alkyl chains on the surface of fumed silica. Raman spectroscopy has been used to characterize OTS layers on thin silica films and Al2O3 surfaces,36,37 and polymerized OTS (POS) in the absence of a silica surface.38 In this paper, Raman spectroscopy was used to investigate the effects of hydration state of the fumed silica on the chain packing and conformation of OTS attached to the silica surface. Previous Raman investigations of OTS attached to planar substrates and POS showed similar packing of the alkyl chains. However, the former data were also consistent with a three-dimensional POS network formed in solution and then adsorbed onto the planar surface. When alkyl silanes are attached to the surfaces of nanometer size fumed silica, the packing order of the chains may be expected to be different compared with that on flat glass substrates. In fact, the size of the primary particles for the fumed silica frequently used in spectroscopic investigations [Aerosil OX50 (40 nm) or Aerosil 380 (7 nm) from Degussa], is in the range of small unilamellar vesicles (SUVs) of lipids, where it is known (29) Allara, D. L.; Parikh, A. N.; Rondelez, F. Langmuir 1995, 11, 2357. (30) Netzer, L.; Sagiv, J. J. Am. Chem. Soc. 1983, 105, 674. (31) Tillman, N.; Ulman, A.; Schildkraut, J. S.; Penner, T. L. J. Am. Chem. Soc. 1988, 110, 6136. (32) Carson, G. A.; Granick, S. J. Mater. Res. 1990, 5, 1745. (33) Finklea, H. O.; Robinson, L. R.; Blackburn, A.; Richter, B. Langmuir 1986, 2, 239. (34) Wallach, D. F. H.; Verma, S. P.; Fookson, J. Biochim. Biophys. Acta 1979, 559, 153. (35) Lippert, J. L.; Peticolas, W. L. Proc. Natl. Acad. Sci. U.S.A. 1971, 68, 1572. (36) Thompson, W. R.; Pemberton, J. E. Langmuir 1995, 11, 1720. (37) Thompson, W. R.; Pemberton, J. E. Anal. Chem. 1994, 66, 3362. (38) Parikh, A. N.; Schivley, M. A.; Koo, E.; Seshadri, K.; Ayrentz, D.; Mueller, K.; Allara, D. L. J. Am. Chem. Soc. 1997, 119, 3135.

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that the curvature of the bilayer affects the packing order of the alkyl chains.39 In the present investigation, completely disordered liquidlike alkane chains have been observed on dehydrated and dehydroxylated fumed silica surfaces. In the presence of mobile water, ordered structures are formed that never achieve the degree of order observed for POS. Temperature-dependent Raman spectra were obtained for both POS and OTS on fumed silica to further differentiate between these two structures. Experimental Section Materials. Fumed silica (Aerosil OX50) was obtained from Degussa AG and had a surface area of 50 ( 15 m2/g and an average primary particle size of 40 nm. Anhydrous pentane (30 h; a portion was subsequently equilibrated in air for several days. To begin the reaction, 10 g of OTS were added to ∼100 mL of anhydrous pentane, containing 2 g of fumed silica, in a glovebox that was purged >10 times with argon and kept under a positive pressure of argon. The reaction mixture was stirred for 3.5 h at 20 ( 2 °C. Before the end of the reaction, 3 mL of TEA was added and the mixture was stirred for another half an hour. The reaction mixture was transferred to centrifuge tubes in the glovebox. The stoppered tubes were centrifuged outside the glovebox and moved back to the glovebox to decant the supernate. Fumed silica was washed with fresh anhydrous pentane to remove unreacted OTS. This procedure was repeated more than three times. Fumed silica was further rinsed with 300 mL of pentane and 200 mL of methanol in a Bu¨chner funnel. The methanol was used to remove the amine salt that formed. The fumed silica was then dried at 60∼90 °C under reduced pressure for >2 h and at room temperature overnight. POS was directly made from polymerization of OTS. OTS is a water-sensitive compound that can hydrolyze and further polymerize in air to form a three-dimensional network. OTS was dropped with a pipet onto a glass slide or into a small vial and it spontaneously polymerized in air to form a white solid polymer. Raman Spectroscopy. Raman spectra were measured using a computer-controlled double monochromator (Spex 1403) with a thermoelectrically cooled photomultiplier tube. The samples were excited with ∼30 mwatts of the 514.5 nm line of an argonion laser. All of the spectra were recorded in backscattering geometry with an 80x microscope objective and 5 cm-1 resolution, and signals from >20 scans were averaged. FTIR Spectroscopy. FTIR spectra were recorded using a Mattson Research Series FTIR spectrometer. Spectra were obtained in a sample compartment purged with dry air and were referenced to a background dry air spectrum. Spectra were obtained using 10 scans and a resolution of 1 cm-1. Thermogravimetric Analysis (TGA) and Differential Scanning Calorimetry (DSC) Measurements. TGA was performed on a TA Instruments Hi-Res TGA 2950 Thermogravimetric Analyzer using a ramp rate of 10 °C/min. Samples were held at 100 °C for 0.5 h and then heated to 800 °C. The melting point of POS was measured with TA Instruments DSC 2920 differential scanning calorimeter at a heating rate of 10 °C/min. (39) Gaber, B.; Peticolas, W. L. Biochim. Biophys. Acta 1977, 465, 260.

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Results and Interpretation Characterization of Fumed Silica Surface. Because IR spectra of fumed silica with different thermal histories and the resulting different amounts and kinds of hydroxyl groups attached to the surface have been reported in the literature, the similar FTIR spectra of Aerosil OX50 are not shown here. It is well known that40 the sharp band at 3747 cm-1 is due to isolated noninteracting silanol groups, and the small broad band centered near 3650∼3660 cm-1 has been attributed to bridged or hydrogen-bonded silanols or silanols that are perturbed due to interparticle contact. The perturbed silanols are largely inaccessible to many reactants. The broad peak between 3400 and 3500 cm-1 is due to physically adsorbed water. The band due to hydrogen-bonded pairs or chains of silanols is underneath the water band and is centered at ∼3520 cm-1. The changes in the FTIR spectra indicate that at temperatures 150 °C, dehydroxylation via condensation of the H-bonded groups starts and continues up to 450 °C; and at >450 °C the isolated silanol groups start to condense. For silica with physically adsorbed water, the amount of water is usually ∼2.5-4.0 H2O/nm2 and the density of surface hydroxyl groups is ∼3.1 OH/nm2 41 for “as is” fumed silica and 4.9 OH/nm2 for fully hydrated silica.42 In the literature, this is often referred to as “as received” silica, which is considered to have roughly a monolayer of adsorbed water on the surface. Fumed silica, with ∼1.8 times the amount of water adsorbed to an “as received” silica surface, is referred to as “superhydrated” silica. After the loss of physically adsorbed water, “dehydrated” silica has only 3.1 OH/nm2 exposed. With 450 °C treatment, there are no water molecules or vicinal hydroxyl group present on the surface. Only geminal and isolated hydroxyl groups remain on the surface, and the hydroxyl group density is about 1.4 OH/nm2. This is denoted as “dehydroxylated” silica. TGA Results. The weight loss measurement of OTS from dehydroxylated fumed silica was 4.6%. For two different hydrated “as is” samples, values of 5.5% and 5.9% were obtained. For a “superhydrated” fumed silica sample, the weight loss was 7.4%. To obtain an estimate of the extent of coverage of OTS on the surface, 100% coverage was calculated assuming a maximum possible value of 5 molecules/nm2, and using 50 m2/g as the surface area of Aerosil OX50 fumed silica. Therefore, the percent coverage for dehydroxylated silica was 48%, that for hydrated “as is” silica was 58-62%, and that for “superhydrated” fumed silica was 78%. Characterization of OTS and POS with Raman Spectroscopy. The Raman spectra of POS and liquid OTS in the skeletal stretching (1000∼1150 cm-1) region are shown in Figure 1. In the case of POS at 4. The Raman spectra of liquid OTS (Figure 3d) is typical of that observed in alkane melts. The characteristically sharp peak at 2848 cm-1, the symmetric CH2 stretching mode, becomes the strongest feature in the spectrum, and the broad 2930 cm-1 band appears with slightly increased intensity [I(2848)/I(2930) ) 1.7]. The most dramatic effect, however, is the broadening and shift to higher frequency (2888 cm-1) of the antisymmetric stretching mode. In long alkane chains, this change results mainly from the coupling of the antisymmetric stretching mode to the increased torsional oscillations of the chain.46 The intensity ratio I(2848)/I(2900) is equal to 1.25, which is the same value as that observed for polyethylene (PE) and alkane melts. (46) Zerbi, G.; Roncone, P.; Longhi, G.; Wunder, S. L. J. Chem. Phys. 1988, 89, 166.

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Figure 4. Raman spectra in skeletal stretching region of OTS attached to the surfaces of Aerosil OX50: (a) “dehydrated” or “dehydroxylated” fumed silica and same as the spectrum obtained for liquid OTS; (b) fumed silica with a “monolayer” of water, “as is” fumed silica; and (c) “superhydrated” fumed silica.

The Raman bands in the CH2 stretching region and their ratios are often used as measures of chain packing of alkyl chains. There are frequency shifts between compounds and between liquids and crystals. When comparing compounds and following changes in band intensity ratios, the sharp symmetric stretching frequencies, d+, will be referred to as the 2850 and the 2930 cm-1 bands. The antisymmetric stretching frequency, d-, will be referred to as the 2885 cm-1 band. Characterization of OTS Attached to the Silica Surface. Figure 4 shows the Raman spectra in the skeletal stretching region from 1000 to 1150 cm-1 for OTS attached to the 40-nm fumed silica for three pretreatment conditions. In Figure 4a the trichloroalkyl chains were attached to a fully dehydrated and dehydroxylated silica surface, in which there are only isolated and geminal OH, and the silica surface is predominantly hydrophobic. The Raman spectrum in this region is identical to those of a molten alkane chain and liquid OTS, and indicates that the chains adopt a disordered, melt-like structure. Figure 4b shows the Raman spectrum for OTS attached to hydrated “as is” fumed silica expected to have a “monolayer” of hydrogenbonded water on the surface. In this case, the OH group density increases to ∼3.1 OH/nm2 and there is a slight increase in the trans (1060, 1130 cm-1) bands compared with the gauche envelope (1080 cm-1). When excess surface water is present on the “superhydrated” fumed silica, the trans bands of OTS become more pronounced, as shown in Figure 4c. Figure 5 shows the Raman spectra in the CH2 stretching region for OTS on the surfaces of Aerosil OX50 pretreated under the same conditions as for Figure 4. The liquid spectrum of OTS is also presented (spectrum a, Figure 5)

Order of Octadecylsilane Chains on Fumed Silica

Figure 5. Raman spectra in CH2 stretching region, 28003000 cm-1, of (a) liquid OTS, and OTS attached to (b) “dehydroxylated” fumed silica heated to 450 °C for 1 h, (c) “dehydrated” fumed silica heated to 150 °C for 1 h, (d) fumed silica with a “monolayer” of water, “as is” fumed silica, and (e) “superhydrated” fumed silica.

for comparison. The Raman spectra of OTS attached to a dehydroxylated hydrophobic silica surface (spectrum b, Figure 5) or attached to a dehydrated silica surface (spectrum c, Figure 5) are similar to the spectrum of liquid OTS. These spectra are characteristic of those for completely disordered alkyl chains with no lateral packing order. The ratio of I(2850)/I(2885) is between 1.2 and 1.25, and I(2850)/I(2930) is between 1.6 and 1.7. When water is present on the fumed silica, as in spectra d and e in Figure 5, the sharp antisymmetric stretching vibration appears at 2885 cm-1. With increasing water content, the ratio I(2850)/I(2885) decreases, indicating both the increases in trans sequences and increased lateral ordering among the chains. For spectrum d in Figure 5, I(2850)/ I(2885) ) 1.1 and I(2850)/I(2930) ) 1.6. For spectrum e in Figure 5, OTS on “superhydrated” silica surface, I(2850)/ I2885) ) 1.0 and I(2850)/I(2930) ) 1.3. In both cases, these values indicate less order at room temperature than for POS or for crystalline alkane chains. Temperature Dependence of Vibrational Modes. The temperature dependence of the skeletal vibrations for POS was shown in Figure 1. The plots of the temperature dependence of these modes for OTS attached to “superhydrated” and hydrated “as is” fumed silicas are shown in Figures 6 and 7, respectively. The spectra of OTS attached to dehydrated and dehydroxylated fumed silica are the same as those for alkane melts. They are completely disordered, as expected for the Boltzmann distribution of trans/gauche populations at room temperature. At low temperature for POS and for OTS attached to “superhydrated” and hydrated “as is” fumed silica, there are two sharp trans bands, at 1060/1124 cm-1, and also a band at 1090 cm-1, which is indicative of gauche

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Figure 6. Raman spectra of OTS attached to “superhydrated” fumed silica as a function of temperature in the skeletal stretching (1000∼1150 cm-1) region.

Figure 7. Raman spectra of OTS attached to “as is” fumed silica as a function of temperature in the skeletal stretching (1000∼1150 cm-1) region.

sequences adjacent to long trans sequences. For both OTS samples, there is also a band centered at ∼1080 cm-1, which is indicative of gauche sequences adjacent to short trans sequences. With increasing temperature for all the

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Figure 8. I(1060)/I(1080) versus temperature for POS (circle), OTS attached to “superhydrated” fumed silica (square), and OTS attached to “as is” fumed silica with a “monolayer” of water (triangle and rectangle, two different samples).

samples, the 1060/1124 cm-1 trans bands decrease and the 1080 cm-1 band grows in intensity; eventually the 1090 cm-1 band becomes incorporated into the broad gauche envelope. At corresponding temperatures, the intensity of the trans bands compared with the gauche bands is in the order POS > “superhydrated” fumed silica > “hydrated” fumed silica > dehydrated fumed silica ) dehydroxylated fumed silica, until the chains are completely disordered at elevated temperatures. Figure 8 is a plot of the trans/gauche ratio versus temperature for POS, and for OTS attached to “superhydrated” and hydrated fumed silica particles. The trans/ gauche ratio in the molten state (0.6) corresponds roughly to that found in liquid OTS, although the signal-to-noise (S/N) ratios are poor. When OTS is attached to the silica surface, the amount of water affects the trans/gauche ratio of the conformers. At every temperature, there are more trans conformers in OTS attached to the silica surface that had more water initially on its surface. At high temperatures, the trans/gauche ratio converges for all of the samples. Unlike the sigmoidal dependence of the intensity ratio versus temperature observed for POS, the change in this ratio decreases smoothly with temperature for OTS attached to fumed silica. The temperature dependence of the CH2 stretching modes for POS and OTS attached to “superhydrated” and hydrated fumed silica was also determined. The ratio of the two strongest Raman bands in the CH2 stretching region, the 2885 and the 2850 cm-1 vibrations, is often used as a measure of chain packing, especially in investigations of the lipid component in biomembranes.35 With increasing temperature for each sample, the intensity of the 2850 cm-1 symmetric stretching band increases with respect to the 2885 cm-1 antisymmetric stretching band, and the intensity of the 2930 cm-1 band increases. These changes indicate increased disordering and decreased lateral order of the chains with increasing temperature. In addition, at comparable temperatures, the intensity ratio I(2850)/I(2885) is smallest for POS and increases with decreasing water content of the fumed silica. Figure 9 is a plot of I(2850)/I(2885) as a function of temperature for POS, and for OTS attached to “superhydrated” and hydrated “as is” fumed silica. For POS, the inflection point of the curve corresponds to the calorimetrically determined melting temperature as discussed previously. From the temperature-dependent Raman spectra of POS, not shown here, it can also be seen that

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Figure 9. I(2850)/I(2885) versus temperature for POS (circle), and OTS attached to “superhydrated” fumed silica (triangle) and “as is” fumed silica with a “monolayer” of water (square and cross).

the shift in frequency of the 2885 cm-1 band occurs at this temperature. For the two OTS samples, there is no inflection point, but instead a smooth variation with the temperature. In addition, the asymptotic value of I(2850)/ I(2885) appears to be different for the three samples. As a point of reference, I(2850)/I(2885) is 1.2∼1.25 at room temperature for alkane melts and liquid OTS. For the rotator phase of alkanes and for alkane chains in lipid bilayers, this ratio is ∼1.1 and decreases to Tm) dispersions of multilamellar vesicles and SUVs of dipalmitoyl phosphatidylcholine (DPPC).35 The relative intensity I(2850)/I(2885) of these two bands for POS at high temperature is ∼1.1. For OTS attached to “as is” fumed silica, the intensity ratio approaches that of the completely disordered alkane melt. As discussed previously, the chains for dehydrated and dehydroxylated silica surfaces are completely disordered, even at room temperature, and the spectra are identical to those of liquid alkanes. Attachment of OTS to Surfaces and Role of TEA. The nature of the attachment of OTS to the silica surface was investigated by a comparison with the compounds shown in Table 1. All of the silanizations were carried out at room temperature in pentane solution. To effect removal of the silane, the silanized fumed silica were rinsed many times with the solvents pentane and methanol, and evacuated at 80 °C for 4 h and/or at 200 °C for 4 h. In addition, the state of order of the alkyl chains for OTS and dimethyloctadecylchlorosilane (DMOCS) is indicated in the table. The results presented in Table 1 indicate that for the low molecular weight monochlorosilane, trimethylchlorosilane (TMCS), no chemical bonding to the silica surface takes place at room temperature without the addition of TEA. This result is in agreement with those of Tripp and Hair.22 In all other cases [i.e., for trichloromethylsilane (TCMS), DMOCS, and OTS], the silane cannot be removed from the silica surface, whether or not TEA is added to the reaction mixture, and the chain conformation of the longer alkanes is that of a disordered melt-like chain. At higher temperatures, it is possible that bonding of the silane to the surface occurs. Tripp and Hair22 also observed that TCMS in solution reacts quickly with water (even rogue water in the system) to form the silanol, which condenses to a polymeric material. Because covalent attachment to silica does not occur without TEA at room

Order of Octadecylsilane Chains on Fumed Silica

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Table 1. Role of Triethylamine (TEA) silica surface pretreatment silanizing reagent

hydrated “as is”

trimethylchlorosilane (TMCS) (CH3)3sSisCl without TEA with TEA trichloromethylsilane (TCMS) (CH3)sSisCl3 without TEA with TEA dimethyloctadecylchlorosilane (DMOCS) (CH3)s(CH2)17sSis(CH3)2sCl without TEA with TEA octadecyltrichlorosilane (OTS) (CH3)s(CH2)17sSisCl3 without TEA with TEA

hydrophobic (450 °C)

at 80 °C in vacuum removed cannot be removed

at 80 °C in vacuum removed cannot be removed

at 80 °C in vacuum cannot be removed or with solvent cannot be removed or with solvent

at 80 °C in a vacuum cannot be removed or with solvent cannot be removed or with solvent

at 200 °C in vacuum

at 200 °C in vacuum

cannot be removed or with solvent; disordered cannot be removed or with solvent; disordered

cannot be removed or with solvent; disordered cannot be removed or with solvent; disordered

cannot be removed by heat or with solvent; disordered cannot be removed by heat or with solvent; very slightly ordered

cannot be removed by heat or with solvent; disordered

temperature, it is possible that many weak hydrogen bonds between the polysilanol and the silica surface make it improbable that all of the points of attachment detach at one time. The inability to remove the monochlorosilane DMOCS although no covalent bonds are formed indicates that the alkane chains are nonspecifically adsorbed onto the hydrophobic silica surface. In the case of OTS, Tripp and Hair22 have shown that there is no adsorption at all on an “as is” fumed silica from CCl4 solution in the absence of TEA. It is possible in the present case that the use of the more hydrophobic solvent pentane favors an assembly of the OTS with the Si-(OH)3 portion oriented toward the polar silica surface. Nonspecific adsorption of the OTS polymer may make it difficult to remove, as observed for TCMS, or some polycondensation may occur. In a separate experiment, hydrated “as is” fumed silica was reacted with OTS in the absence of TEA using pentane that was not dry. The alkyl chains on the silica surface were disordered. There were no characteristic “trans” markers for the POS chain, which implies that in the presence of excess water under acidic conditions a polymer that subsequently adsorbs to the silica surface does not form in solution. Discussion The results of the Raman investigation of the attachment of OTS to fumed silica particles provide insight into the state of order of the alkane chains. In particular, the data indicate that the conformation and packing of the chains are affected by the surface hydroxyl groups and the hydration state of the silica surface to which it is attached. On a molecular level, the details of the alkyl silane layer formed are not unambiguously determined by the spectroscopic results because of the uncertainty in the type of silanol groups on the surface, the hydration state of the silica, and the type of network formed by the silane. In the case of dehydroxylated or dehydrated fumed silica surfaces, the Raman data indicate that completely disordered OTS monolayers are formed. The inability to remove the OTS with solvent or by heating also suggests that the chains are nonspecifically adsorbed onto the hydrophobic regions of the silica surface. This suggestion is supported by TGA experiments that show greater coverage of OTS than would be expected if a single OTS molecule were attached to each of the 1.4 OH sites/nm2 of the dehydroxylated silica surface. Weight losses of 2.9%

cannot be removed by heat or with solvent; disordered

are expected for coverage of 1.4 molecule/nm2, but weight losses of 4.6% were observed. Based on the very low water adsorption characteristics of DMOCS on dry Si/SiO2 (even less than observed for bare dry Si/SiO2), Angst and Simmons18 also suggested that the DMOCS blocked sites (i.e., surface silanols) for water adsorption. In the presence of an excess of a monolayer of water on the silica surface, that is, for hydrated “as is” fumed silica and especially in the case where the fumed silica is “superhydrated”, the Raman spectroscopic data indicate that there is a concomitant increase in the population of trans conformers in the alkane chain. The CH2 stretching region has an appearance similar to that observed in the packing of the alkyl chains in lipid bilayers prepared as SUVs.39 This result is in contrast with the Raman spectra of POS that appear similar to those observed for lipid bilayers prepared as dispersions, which exhibit negligible curvature. The latter systems have been described as having a hexagonal close packing of the alkyl chains, similar to that observed for the rotator phase of n-alkanes. Allara et al.38 and Pemberton et al.37observed the same Raman spectrum for POS, and the former presented IR, X-ray diffraction (XRD), and small-angle X-ray diffraction (SAXRD) data to confirm that the chains were organized in bilayer stacks of highly trans C18H37 chains in a hexagonal array. Pemberton et al.38,39 have observed the same Raman spectra for OTS attached to planar thin silica films immobilized on silver substrates and on planar Al2O3 surfaces. However, ellipsometry data for the former were also consistent with a collapsed three-dimensional polymeric layer rather than an SAM. The more disordered structure observed in the case of OTS on fumed silica compared with POS may indicate that the curvature of the silica affects the packing of the alkyl chains, as has been observed for SUVs. However, it must be noted that the one reported Raman spectra obtained for OTS on a planar substrate may in fact be that of POS. Therefore, it is possible that the Raman spectra of OTS on a planar silica substrate may look identical to the one obtained for OTS attached to “superhydrated” fumed silica. In this case, the differences in packing of the alkyl chains for OTS attached to fumed silica compared with POS may be due to the constraints imposed by the underlying silica substrate in the former case, as discussed by several researchers.47-49 In addition, (47) Stevens, M. J. Langmuir 1999, 15, 2773.

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Langmuir, Vol. 16, No. 2, 2000

caution must be applied when coming to more detailed conclusions about the actual morphology of the alkyl silane layer on the fumed silica. It would not be possible using the current data to distinguish between a single uniform packing morphology and one in which there were two phases, each contributing to the vibrational spectra, especially because the phases could each have very small domains. This problem also exists in the interpretation of biomembrane spectra. It is always possible that a three-dimensional polymer chain forms in solution, either by itself or attached by one or more bonds to the surface silanes. However, no DSC melting curve similar to that observed for POS was found, and the amount of the alkyl silane adsorbed to the surface was less than needed for monolayer coverage, even for the “superhydrated” fumed silica. Preliminary results indicate that the DSC trace of the OTS on the “superhydrated” fumed silica shows a very shallow broad endotherm. It is clear from the analysis of the Raman spectra for “superhydrated” fumed silica that there is a significant trans bond population and that the packing of the alkane chains occurs at best in a loose, slightly disordered hexagonal structure. The preparation method used in this work allows the OTS molecules to self-assemble on the hydrated silica surface. Subsequent addition of TEA promotes self-condensation between adjacent chains because it has been shown that, at least in the gas phase, addition of TEA after silane hydrolysis does not promote covalent attachment to the silica surface. Although some of the three possible covalent attachments for each OTS molecule are expected to be to the silica surface, most are to other OTS molecules or to water, the latter resulting in terminal OH groups. Recently, Stevens47 pointed out that condensation between adjacent OTS molecules is not possible for monolayers at full coverage due to steric effects. In particular, the Si-O-Si bond formed in the polymerization reaction is shorter than required to accommodate nonrepulsive van der Waal radii of carbon atoms attached to adjacent silica atoms. Allara et al.50 have shown that the Si-OH peak grows significantly with increasing silane coverage and that the Si-O-Si cross-linking band decreases with coverage in excess of 50%. When there are many OH groups, the distance between the alkyl chains is not as rigidly fixed as if the attachment was to the silica surface or to a perfectly cross-linked siloxane structure. In fact, the Si-O bond is known to be particularly flexible in the sense that there is a low energy barrier to rotation about the Si-O bond. Many Si-OH groups could result in short, one-dimensional polymer structures winding on the silica surface, combined with some covalent attachment to the surface, eliminating steric overlap and permitting the alkyl chains to maximize their packing order. It is interesting to observe that the high-temperature spectra of POS, and to a lesser extent that of OTS attached to “superhydrated” silica, never indicate melting to a completely disordered state. It is known that the antisymmetric stretching mode couples to both rotational and torsional motions of the chain, and that with increased torsional oscillations or rotational motions, the bandwidth increases. It is thus reasonable to conclude that compared with the neat liquid, these motions are more hindered in the case of POS, where the alkane chains are restricted (48) Rye, R. R.; Nelson, G. C.; Dugger, M. T. Langmuir 1997, 13, 2965. (49) Ulman, A. Adv. Mater. 1990, 2, 573. (50) Parikh, A. N.; Liedberg, B.; Atre, S. V.; Ho, M.; Allara, D. J. Phys. Chem. 1995, 99, 9996.

Wang et al.

by the three-dimensional network formed by the condensation of the silanols, or the case of OTS attached to “superhydrated” fumed silica, where the chains may be restricted by the attachment of the chains to the surface. For both of these cases, there is thus not as great of a bandwidth broadening and a concomitant peak maxima decrease of the antisymmetric stretching band at 2880 cm-1, so that I(2850)/I(2885) is less than that for the disordered liquid. Gaber and Peticolas39 attributed very similar differences in the CH2 stretching region between DPPC bilayers that were dispersions (large size, low curvature) or vesicles (small size, high curvature) to retention of some type of chain order in the former case. Conclusions Raman spectroscopy was used to investigate the conformation and lateral chain-chain interactions of alkyl chains on the surface of fumed silica. In particular, the alkyl chains of OTS attached to 40-nm fumed silica particles (Aerosil OX50 from Degussa) were monitored in two different wavenumber regions (i.e., the skeletal and CH2 stretching regions). Different thermal histories of fumed silica resulted in pretreatment surfaces that varied from being hydrophobic, with only isolated OH groups, to hydrophilic, with greater than equilibrium quantities of surface water. Attachment of OTS to fumed silica with variable OH and H2O concentrations affected the packing and conformation order of the alkyl chains. Disordered chains were observed attached to fumed silica with hydrophobic surfaces and fumed silica with less than a monolayer of water adsorbed on the surface. For a hydrophobic silica surface, the alkyl chains may be physisorbed in a disordered state to the Si-O-Si surface groups. With increasing water content, the number of trans conformations in the chains increased, and increased lateral interactions between the chains were observed. The skeletal stretching region was used to monitor trans/ gauche content, and the CH2 stretching region was used to monitor the packing order of the chains. The Raman spectrum of POS was monitored in the same spectral regions. POS had a higher percent of trans conformations and increased lateral packing compared with OTS attached to fumed silica. The Raman spectrum in the CH2 stretching region was characteristic of a hexagonal closepacked crystal structure, and the DSC results exhibited a well-defined melting endotherm at 63 °C. The characteristic features of POS could be used to detect whether POS was attached to fumed silica, and in the present investigation POS was shown not to adsorb to the fumed silica particles. In the case of high water content on fumed silica, well-assembled monolayers were formed that appeared to have the slightly disordered hexagonal packing structure of alkane chains observed in SUV liposomes. DSC melting curves showed a very shallow broad endotherm, and the trans structures disappeared by 60 °C. When used for chromatographic purposes, OTS could therefore change conformational order and, thus, binding characteristics as a function of temperature. Acknowledgment. The authors gratefully acknowledge the support of NIH grant AR45472, and Temple University for the purchase of the Argon-ion laser. LA9908081