Langmuir 1995,11,1215-1219
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Direct Observation of the Surface Bonds between Self-AssembledMonolayers of Octadecyltrichlorosilane and Silica Surfaces: A Low-Frequency IR Study at the Solid/Liquid Interface C.P.Tripp* and M.L.Hair Xerox Research Centre of Canada, 2660 Speakman Drive, Mississauga, Ontario, Canada L5K 2Ll Received October 14, 1994. In Final Form: January 26, 1995@ We describe an infrared technique for studying reactions on silica surfaces in CC4 that provides access to the spectral region below 1300 cm-l. The region below 1300 cm-' is important in the study of the adsorption of self-assembled monolayers of alkylchlorosilanesbecause it contains the characteristic bands due to Si-0-Si, Si-0, Si-C and Si-C1 modes. It is these bands that provide direct evidence of the type of adsorbed species and on the nature of the bonding with the surface of the silica. The potential of this technique is demonstrated using the reaction of octadecyltrichlorosilane (OTS) with water on a fumed silica. It is confirmed that OTS does not adsorb on silica in CC4 and hydrolyses when surface water is present. Evidence is also presented which shows that the type of adsorbed species varies with the amount of surface water initially present on the surface but that, in all cases, these adsorbed species form few, if any, Si-0-Si bonds with the surface.
Introduction Over the past three decades, IR spectroscopy has played a pivotal role in gaininginsight to the reaction of molecules on silica Much of the success of IR spectroscopy can be traced to its ability to distinguish various types ofhydroxyl groups on the surface. While important for most reactions, the spectral changes occurring in the surface hydroxyl region provide little or no information of the nature of the bonding of alkylchlorosilanes to the silica surface. Key to understandingthe bonding between the adsorbed chlorosilane and the surface is the spectral region below 1300 cm-l because this region contains the characteristic bands due to Si-0-Si, Si-0, Si-C, and Si-C1 modes. It is these bands that provide direct evidence of the species formed on the surface. Until recently, access to this spectral region has not been possible because silica is opaque below 1300 cm-' due to the presence of strong Si-0-Si bulk modes. To overcome this limitation, a thin film technique6.6was developed that expanded the transparency of the silica (for example, see Figure la) from 1300 cm-' to the FTIR cutoff of 200 cm-l. In a number of recent studies, we have used the thin film method to provide direct evidence of the bonds formed in the reaction of alkylchl~rosilanes~~~ and their hydrolyzed counterparts, alkylsilanols,4 with the surface of silica particles at the solidlgas interface. Gaining access to the spectral region below 1300 cm-' did impose several experimental constraints. The main limitation was that the thin film experiments were only Abstract published in Advance ACS Abstracts, April 1, 1995. (1)Iler, R.K. The Chemistry of Silica; John Wiley and Sons: New York, 1979. (2) Hair, M. L. Infrared Spectroscopy in Surface Chemistry; Marcel Dekker: New York, 1967. (3) Kiselev, A. V.; Lygin, V. I. Infrared Spectra ofsurfme Compounds; John Wiley and Sons: New York, 1975. (4)Morrow, B. A. In Spectroscopic Analysis of Heterogeneous Catalysts, Part A: Methods of Surface Analysis; Fierro, J. L. G., Ed.; Elsevier: Amsterdam, 1990. (5) Morrow, B. A.; Tripp, C. P.; McFarlane,R. A. J.Chem. SOC.,Chem. Commun. 1984, 1282. (6)Tripp, C. P.; Hair, M. L. Langmuir 1991, 7 , 923. (7) Tripp, C. P.; Hair, M. L. Langmuir 1992, 8, 1961. (8)Tripp, C. P.; Veregin, R. P. N.; Hair, M. L. Langmuir 1993, 9, 3518. (9) Tripp, C. P.;Hair, M. L. Langmuir 1996, 1 1 , 149. @
possible a t the solidlgas interface and therefore limited
to those adsorbates with suf'ficient vapor pressure. In practice, most silanization reactions are performed at or near room temperature in nonaqueous solutions. Such reaction conditions in solution are difficult to mimic with solidgas studies especially in the duplication of the level of surface hydration. Experiments at the solidlgas interface require partial evacuation of the silica prior to exposure to the chlorosilane vapor and this partial evacuation removes about 90% of the amount of adsorbed water that is normally present on the silica in air. Thus, experiments at the solidlgas interface are performed at a much lower water content than those typically present during silanization treatment in solution. It is also evident that the importance of solvent effects is not addressed in solidlgas studies. In this paper, we describe an infrared cell and methodology that extends the thin film technique to the solid / liquid interface. Specifically, we report our finding on the adsorption of octadecyltrichlorosilane (CHa(CH&7SiCl3, OTS)with a fumed silica. OTS is the most widely used chlorosilane in the preparation of self-assembled monolayers (SAM's). This work is an extension of an earlier IR studyloof the OTS reaction on silica particles suspended in CC4. This previous study was limited to the spectral region above 1300 cm-' and therefore provided no clear evidence of the type of speciesformed or the nature of the attachment to the surface. We now provide this spectroscopic evidence needed to identify the species formed during the reaction of OTS with dry and hydrated silica using CCL as the liquid phase.
Experimental Section CCl4 was obtained from Caledon Laboratories Ltd. and octadecyltrichlorosilane (OTS) was from Huls Petrarch. The silica was Aerosil380, a nonporous fumed silica obtained from Degussa A.G. It had a measured BET (Nz)surface area of 375 m2/g. Fumed silica is used because the surface chemistryis well understood2-4 and because it exists as small particles (7 nm) with high surface area. The high surface area of the silica allows for easy detection of surface species and the small particle size (10)Tripp, C. P.; Hair, M. L. Langmuir 1992, 8, 1120.
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1216 Langmuir, Vol. 11, No. 4, 1995
silica from stock without pretreatment. The amount of surface water on the hydrated silica is roughly 2.5-4 H20/nm2.10A silica sample referred as "superhydrated" contained about 1.8 times the amount of surface water as the hydrated silica. To produce this higher surface water level,the silica from stock was dispersed in water and then dried by evaporation at room temperature. The spectrum of the superhydrated silica is shown in Figure 2c. "he amount of adsorbed water on the silica is estimated from the integrated intensity of the water deformation mode at 1620
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Figure 1. Thin film of hydrated silica recorded using the thin film liquid cell (a) in air and (b) in CC4.
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Figure 2. Hydroxyl spectral region of a (a) hydrated, (b) dehydrated, and (c) "superhydrated" silica. limits scattering ofthe IR beam by the sample. The low scattering has enabled earlier experiments to be performed in the transmission mode using a pressed disk ofthe fumed si1ica.l' However, the thinnest pressed disk of silica has spectral regions below 1300 cm-I that are opaque.6 To provide partial transparency over the entire region below 1300 cm-l requires lower amounts of silica than the minimum needed to press a disk. In essence, it is this requirement that forms the basis of the thin film technique. We briefly outline the spectral features of silica before tackling the details ofthe thin film technique because the silica bands are important in defining the technique. A typical spectrum of a thin film of fumed silica showing partial transparency over the entire region between 4000 and 200 cm-l is plotted in Figure la. Assignment of the various infrared bands of fumed silica is well e ~ t a b l i s h e d . ~The - ~ silica is covered by a layer of adsorbed water which gives the broad band centered about 3450 cm-l and a deformation mode at 1620 cm-l. The band at 1620 cm-l is superimposed on a region between 2000 and 1300 cm-l which contains various Si-0 combination and overtone modes. The sharp band at 3747 cm-I is due to single and geminal silanol groups. Below 1300 cm-l, there are three strong bands at 1100, 822, a n d 450 cm-l which are due to Si-0-Si bulk modes. Attention has mainly been on the hydroxyl spectral region and spectra are shown in Figure 2. Evacuation of the silica at room temperature removes the adsorbed water from the surface exposing the underlying hydrogen bonded silanols represented by a broad band at 3550 cm-' (See Figure 2b). In previous experiments, we examined the reaction of OTS with silica containing various amounts of surface water. A dehydrated silica sample was prepared by evacuation ( torr) for 1h at room temperatures and a hydrated silica refers to the (11)The scattering limitation is removed for transmission studies in CC14 because of the similar refractive index of silica ( n =~ 1.45) and CC14 ( n =~ 1.46).Therefore, porous silicas, such as silica gels used in chromatography can be studied by transmission in CCll rather than the traditional method of diffuse reflectance IR (DRIFT).
Since there are many common features in the thin film technique at both gas and liquid interfaces, it is appropriate to briefly review those aspects of the experiment at the solidgas interface that are pertinent for this study. The details of the infrared cell and the thin film technique in experiments performed at the solidgas interface are found elsewhereS6A thin film of silica is prepared by spreading a small amount on an infrared transmitting disk. In our case, CsI is used as the disk material because it provides the necessary transparency to the FTIR cutoff at 200 cm-I. A small amount of silica (about 1mg) is placed in the center of a 1in. diameter CsI disk. With a second 1in. glass disk, the silica is spread out on the CsI disk by applying minimal pressure while moving the glass plate radially from the center to the edges of the CsI disk. The silica adheres readily to the infrared disk using this procedure but the thickness of the thin film is not uniform across the surface of the infrared disk. The desired thickness of silica is judged from the strong bulk Si0-Si mode at 1100 cm-' and is set so that this band is about 1absorbance unit. This would correspond to a uniform amount of silica of about 0.25 mg/cm2. In the solidgas studies the thin film of silica on the CsI disk is then mounted in an vacuum infrared cell. Sample pretreatment usually consists of evacuation at elevated temperatures. After pretreatment, a reference spectrum of the thin silica film is recorded. The silica is then exposed to an excess quantity of chlorosilane vapor. After a specified interval, the excess silane is evacuated and an absorbance spectrum is then recorded. Because the reference spectrum includes the thin film of silica, the absorbance spectrum is then a measure of the changes occurring on the surface due to the presence of the adsorbed species. Negative bands refer to bonds removed and positive bands to those formed. This procedure is used so that it is easy to extract the much weaker bands of the adsorbed species from the three strong bulk Si-0-Si modes at 1100, 822, and 450 cm-'. For the same reason, it is imperative that the experiment be performed in situ in the infrared beam. Because the thin film is nonuniform,any small change in sample position would produce an apparent change in thickness. This change in the strong Si-0-Si bulk modes would then mask out the weaker bands arising from the adsorption of the chlorosilane. The same constraints apply when the thin film technique is used at a liquid interface. Evacuation capabilities are desired to control surface quality especially with respect to the amount of adsorbed water, and because of the nonuniformity in thickness, the experiments must be performed in situ in the IR beam. While satisfying these constraints in the presence of a solvent are difficult, there are additional obstacles that are unique to the presence of a solvent. Most prominent among these is that nonaqueous solvents are strong infrared absorbers and thus require narrow path lengths. This latter issue is addressed in the infrared cell shown in Figure 3. The cell is essentially a hybrid of two other cells we have previously described. The top section is from a "mixing cell"' and the bottom from a variable path length "in situ liquid cell".'O A cross section of the lower portion is shown in Figure 4. Two CsI windows are held in place with an epoxy resin. One window is on a threaded sleeve that allows the path length between the two windows to be varied from about 25pm to 1cm. The threaded sleave is removed from the cell and a thin film of silica is deposited on the window. Again, the desired thickness of the film is determined in the same manner as described above in solidgas studies. The sleeve is reattached to the lower portion and the entire cell canthen be evacuated (if required). Evacuation (12)Hambleton, F. H.; Hockey, J. A.; Taylor, J. A. G. Trans.Faraday SOC.1968,62, 795.
Silica Surfwe Bonds
Langmuir, Vol. 11, No. 4, 1995 1217 ,ADSORBATE SOLVENT
4 I
Figure 3. The thin film liquid cell.
0- Ring
Figure!4. Cross section of the thin film liquid cell optical compartment. was at room temperature only. Thus, the silica was fully hydroxylated at all times and only the level of surface water was controlled. CC4 is used as a solvent because it is optically transparent in the infiared region. The CC14 contained in an evacuated bulb is transferred by gravity to the cell (under vacuum without exposure to air, if required) through the solvent outlet located on the upper portion of the cell. The amount of Cc1.1 added to the cell is sufficientto fully immerse the silica sample. To obtain the necessary transparency the path length between the two windows is then adjusted and fixed to about 1.2mm by narrowing the distance between the two windows until the absorbance of
the CCL overtone mode at 2300 cm-l is about 0.015 absorbance unit. At this path length, there ispartial transparency (seeFigure lb) over the entire region between 4000 and 200 cm-l except for two narrow sectionsbetween 1600and 1500cm-l (C-Cl overtone) and between 850 and 700 cm-l (C-Cl stretching). Note that physisorption of CC14 shifts the band at 3747 cm-l to 3690cm-l. After the path length is set, a reference spectrum is recorded through the silica/CCl4 combination. From this point onward, it is important that the experiment be performed without disturbing the position of the cell. Thus, the adsorbatdsolvent was added to the ccl4 in the cell and simply allowed to diffuse into the window region. The first sign of the adsorbate in the spectrum (i.e., in the region between the two infrared windows) usually appeared within 2to 3min. With a vacuum maintained, the adsorbate can be introduced from an evacuated solvent/ adsorbate bulb attached to the upper portion of the cell. Alternatively, the cell can be backfilled with dry air (prior to recordingthe referencespectrum) and the solvent/adsorbatecould be injected directly into the solvent using a syringe. Although contact of the OTS with the silica occurred within 2 to 3 min from injection, spectra were recorded after an incubation period of 15-30 min. This contact time is well above the 2 to 3 min quoted for the formation of self-assembled monolayersof OTS on glass surfaces.13 In general, the incubation time will vary for different alkylchlorosilanes because the rate of hydrolysis and condensation is highly dependent on the size and nature of the alkyl group and on the number of chlorine. atoms attached to the silicon atom.14 Since the aim of this study was to obtain direct spectroscopic evidence of the species formed in the presence of surface water, it was important to have control experiments where the level of water was eliminated or minimized. The removal of water for the control experiments required considerable effort whereas those experiments conducted in the presence of surface water were relatively straightforward. In experiments with surface water, freshly distilled CC14 (in air) was added to the infrared cell containing the thin film of silica. The OTS was vacuum distilled and was added directly to the ccl4 using a syringe. For the control experiment, the followingprocedure was used to minimize exposure to water. The ccl4 was dried over CaC12 for a minimum period of 24 h and then refluxed and distilled over P205in dry argon. The ccl4 was transferred directly under dry argon to a dried glass bulb, evacuated using several freezethaw cycles and then attached to the solvent outlet on the infrared cell. The cell containing the thin silica film was evacuated Torr) for 1h a t room temperature. CC14 was then added to the cell under vacuum and this was immediately followed by backfilling with dry air supplied from a Balston 75-60air dryer. For the entire duration of the experiment there was a constant flow of dry air into the cell entering from the solvent inlet and exiting from one of the adsorbate inlets. The need for all these precautions in handling the ccl4 was derived from a set of control experiments using the in situ mixing cell.' The top portion of this cell is the same as that shown in Figure 3 and the bottom portion consists of a closed cylindrical glass tubing 32mm in diameter containing two infrared windows held in place with an epoxy resin. The long path length (32mm) between the windows allows for easy detection of minute quantities of water in the CCl4. Figure 5a shows the spectrum of CC14 from stock recorded in the in situ mixing cell. The bands at 3707 and 3614 cm-l are due to molecular water1°J6 and these bands do not diminish with distillation in air. Figure 5b is the spectrum of cc14 recorded after taking the precautions to eliminate water as outlined above. The level of water measured from the band at 3707 cm-l is a maximum of 3% of the stock concentration. The water in the stock CC4 is assayed by Caledon (Karl Fischer titration) at no greater than 0.01%. In general, bands due to water in CC14 were present or appeared during the time period required to perform a thin film experiment if the above steps were not followed. Without these precautions, it was not possible to obtain thin film spectra of silica without some evidence of hydrolysis and adsorption of the OTS.
(13)Flinn, D.H.;Guzonas,D. A.; Yoon, R. H. Colloids Surf 1994, 87,163. (14)Noll, W. Chemistry and Techmbgy of Silicones; Academic Press: New York, 1968. (15)Bascom, W. D. J. Phys. Chem. 1972,76,3188.
1218 Langmuir, Vol. 11,No.4,1995 .4
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Figure 6. Spectrum of CCl4 from (a)stock and (b) dried using the protocol described in the text.
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Figure 7. Expanded low frequency region of (a)Figure 6b, (b) Figure 6c, (c) Figure 6d, and (d) polymerized OTS.
the curve shown in Figure 6a. Furthermore, the previous study showed that when using a hydrated silica there was a decrease in the deformation mode of HzO at 1620 cm-1 and this was accompanied by a small decrease in the band a t 3690 cm-1 (surface SiOH) and the formation of a band at 3400 cm-I due to SiOH of an alkylsilanol. The adsorbed amount was small (0.058 OTS/nm2),much lower than the hydroxyl concentration of 3.1 OWnm2 or the amount ofwater (2.5to 4 H20/nm2on the hydrated sample) and well below the value of about 5 OTS/nm2for tightly packed monolayers.16 At the higher surface water levels (factor of 1.8)found on a superhydrated silica the amount of adsorbed OTS increased 10-foldto about 0.5 OTS/nm2. The adsorption of OTS was clearly sensitive to the level of surface water and it is not unreasonable to expect that this would produce different types of adsorbed species. This possibilitywas tested in three experiments on three different silica samples. For all experiments, the CC4 was distilled in air prior to use. This distillation in air did not change the residual water level from the level in the stock CC14. In the first experiment, the silica was evacuated for 1h at room temperature. In this case, the only source of water is from the solvent. The presence of the small quantity of water in the solvent has a noticeable effect on the adsorption of OTS. The spectrum is shown in Figure 6b and the low frequency region is plotted in Figure 7a. Hydrolysis of the OTS is evidenced by a reduced 590/2920 ratio with respect to the ratio measured for OTS in CC4. The exact value of this ratio is not significant because there is a continuous migration of OTS into the beam area and because the spectrum does not distinguish between species adsorbed on the silica and those species in solution. Both can vary from sample to sample. Nevertheless a reduced value in the 590/2920 ratio is evidence of hydrolysis. The broad band at 3400 cm-l (SiO-H stretch) and 898 cm-I (Si-OH stretch) show that the hydrolyzed OTS contains silanol groups and therefore has not fully polymerized. It is likely that the result is a mixture ofpartially hydrolyzed (i.e., C I ~ H ~ ~ S ~ C ~ ~ ( O H ) ~ and partially polymerized species. For simplicity,we will refer to all possible species collectivelyas octadecylsilanol. The negative bands at 3690 and 973 cm-l (both surface SiOH modes) show that the hydrolyzed OTSinteracts with the surface hydroxyl groups and thus there is some adsorption of the octadecylsilanol on the surface. The presence of a negative band at 973 cm-l in Figure 7a is deceptive because it is superimposed on positive bands in this r e g i ~ n .The ~ ? ~spectral features in the region between 1200 and 800 cm-l in Figure 7a are very similar to those obtained with the addition of trichloromethylsilane(TCMS)
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Figure 6. Addition of OTS to (a)dry silica in contact with dry CC14. Addition of OTS to air distilled cc14 in contact with (b) dry silica, (c) hydrated silica, and (d) superhydrated silica. The OTS was vacuum distilled prior to use and stored in a dry bulb under dry argon. Access to the bulb was through a rubber septum. The OTS was extracted (50yL)using an air-tightsyringe and injected into the cell through the adsorbate inlet under a positive pressure of dry air. Periodic checks on the OTS showed no infrared bands attributed to Si-0-Si or Si-OH. Infrared spectra were recorded on a Bomem Michelson 102 FTIR using a CsI beamsplitter and DTGS detector. All spectra were recorded at 4 cm-l resolution using 100 scans. Each scan required about 6 s to record.
Results and Discussion The spectrum of OTS added to a dehydrated silica (using the thin film liquid cell) is shown in Figure 6a and it is identical to the spectrum of OTS in CC4. The intensity ratio of 590 cm-l (Si-Cl stretchofOTS)to 2920 cm-' (CH2 stretch) is the same as OTS in CC4, proof that the OTS has not hydrolyzed. This is confirmed by the absence of any SiOH bands at 3400 or 898cm-l due to an alkylsilanol. The nonreaction of the OTS with silica is shown by the absence of any change to the band due to surface silanol groups (i.e., no negative band at 3690 cm-l) and by the lack of bands in the Si-0-Si region between 1200 and 1000 cm-l. This nonreactivity of OTS with silica is consistent with previous work using silica particles suspended in CCI4.Io In this earlier work, the evidence for adsorption was derived from the measured adsorbed amount and from spectral changes occurring above 1300 cm-', specifically changes to the band at 3690 cm-l. We showed that in the absence of water there is no reaction of OTS with the hydroxyl groups of silica. That result is consistent with
(16)Maoz, R.;Sagiv, J. J. Colloid Interface Sci. 1984, 100, 465.
Langmuir, Vol. 11, No. 4, 1995 1219
Silica Surface Bonds
to a hydrated silica at the solidgas interface.' In that case, the absence of a band at 1060 cm-l showed that there was no Si-0-Si bond formed with the surface and the band at 1007 cm-' was indicative of a strained Si0-Si bond from self-condensation between adsorbed TCMSg Similar arguments are used in assigning the bands in Figure 7a recorded in the presence of CCL. Although the absence of a surface Si-0-Si bond is derived from comparison to solidgas spectra, we now have evidence of a strong band at 1060 cm-l when OTS is chemically bound via a surface Si-0-Si bond through a base promoted process with dry silica in CCL.l7 Therefore, the octadecylsilanol is not covalently bound to the surface but adsorbs by a hydrogen bonding interaction with the surface silanol groups. All of the above spectral changes occur because of the presence of molecular water in the CC4. It is possible that these changes arise from the hydrolysis of OTS in solution and then adsorption onto the silica. Very recently, McGovern et. al.ls showed that the amount of OTS deposited on a hydrated glass in contact with dry solvents increased with the ability of the solvent to extract water from the surface. They concluded that the water in the solvent hydrolyzes the OTS which then deposits on the silica. The presence of a hydrated OTS species in solution is in agreement with our data. Evidence for this comes from the rate of growth of bands due to octadecylsilanol (3400 and 898 cm-l) verses the rate of decrease in the surface Si-OH groups. It is found that the increase in intensity of the bands at 3400 and 898 cm-l occurs faster than the decrease in intensity of the band at 3690 cm-' (i.e., more octadecylsilanol is produced with increasing contact time than can be accounted for by adsorption on the surface hydroxyl groups). Nevertheless, we believe that the surface water also plays an important role in the adsorption process. In all cases, the molecular water in the solvent is in equilibria with the adsorbed water on the silica. In the experiments reported by McGovern et al., the dry solvents extract some of the water from the surface. In our experiments, the reverse occurs, a portion of the water in the CCL is adsorbed by the dry silica. We found that the bands a t 3707 and 3614 cm-l due to molecular water diminish in intensity upon contact with dry silica.1° The importance of surface water is shown in a separate control experiment in which the reaction steps giving rise to Figures 6b and 7a were carried out in the thin filmliquid cell in the absence of silica. In this case, there was no evidence of hydrolysis of OTS over several hours. Although the hydrolysis of OTS may occur in solution, in the end it is the equilibria between surface water and molecular water that dictates the nature of the adsorbed OTS species. Although the water adsorbed from the solvent on the dry silica is necessary for the adsorption of OTS, the amount of water needed to adsorb on the silica to cause these spectral changes could be quite small. Similar features in the 1200 to 800 cm-' region were obtained in the gas phase reaction of TCMS on a silica containing only about 10%of the original water level.' In the latter, hydrolysis of the TCMS occurred instantaneously with all adsorbed water molecules. This explains the extreme precautions needed to preclude all water to avoid hydrolysis and adsorption of chlorosilanes on siliceous materials. (17) Manuscript in preparation. (18) McGovern, M. E.;Kallury, K. M. R.; Thompson, M. Langmuir 1994, 10, 3607.
For comparison, the spectra obtained &r addition of OTS to both hydrated and superhydrated silica as well as that of polymerized OTS (produced upon exposureof OTS to air or OTS in wet CC4)are plotted in Figures 6 and 7. The spectral changes occurring above 3000 cm-l are complex and difficult to decipher and again the key to the spectra lies in the region between 1200 and 800 cm-l. For example,in Figure 6c, the presence of an octadecylsilanol is evident by the band at 898 cm-' even though there is only a weak band located at 3400 cm-l. The negative band at 1620 cm-' clearly shows the removal of surface water. This must also reduce the overall intensity at 3400 cm-l and so the spectrum shows only a weak feature in this region. In a similar vein it is tempting to speculate that fewer SiOH groups are perturbed by the adsorbed OTS on the hydrated silica in Figure 6c than on the dry silica shown in Figure 6b. However, the number of single/ geminal SiOH groups will also vary with the degree of hydration because these groups are perturbed by the adsorbed water layer. This is clearly seen from the intensity of the band at 3747 cm-' in the spectra shown in Figure 2. On a dehydrated silica19the isolated silanols (single and geminal) have a density of about 1.1OWnm2, which is lowered to about 0.6-0.7 OWnm2on a hydrated silica,20and from Figure 2c they are nearly all covered on the superhydrated silica. Since the adsorption of OTS results in removal of water, the net effect of the band due to isolatedgeminal groups is difficult to assess. Nevertheless, an examination of the 1200 to 800 cm-' region provides clear evidence showing that the adsorbed species on the hydrated and superhydrated silica are similar in structure and differ from the species formed on the dry silica or of polymerized OTS. The adsorption of OTS on both hydrated and superhydrated samples is dominated by a band located at 1020 cm-'. A band at 1020 cm-l is consistent with the formation of a crosslinked Si-0-Si network but one that is dissimilar from an OTS polymer. Known polymeric compounds of hydrolyzed chlorosilanes containing(Si-0-Si), split the Si0-Si band in two components at about 1090 and 1020 cm-1.21,22The Si-0-Si band in the spectrum of the polymerized OTS shown in Figure 7d has two components located at 1120 and 1020cm-l. Furthermore,the absence of a strong band at 1060 cm-l shows that few Si,-0-Si surface bonds are formed.
Conclusion We have developeda thin film liquid cell that has proven useful in qualitatively assessing the nature of adsorbed species on the surface of silica in C C 4 . Specifically, we have shown that the type of adsorbed OTS species on silica is sensitive to the level of water present on silica or in the solvent. In all cases, few if any surface Si,-0-Si bonds form between the hydrolyzed OTS and the silica surface. While providing a qualitative picture of the interactions, the in situ liquid cell does not provide information of adsorbed amounts. In this respect, it complementsan in situ mixing cell7and the use of these two cells in tandem will be explored in future work on the reaction of alkylsilanes with siliceous materials. LA940800L (19) Morrow, B. A.; McFarlan, A. J. Langmyir 1991, 7 , 1695. (20) Tripp, C. P.; Hair, M. L. Langmuir 1993, 9, 3523. (21) Bellamy, L. J. The Infra-red Spectra ofcomplex Molecules, 3rd ed.; Chapman and Hall, Ltd.: Thetford, U.K., 1975. (22) Smith, A. L. Spectrochim. Acta 1960, 16, 87.
