Reaction of Methylsilanols with Hydrated Silica ... - ACS Publications

Christian D. Lorenz, Michael Chandross, Gary S. Grest, Mark J. Stevens, and Edmund .... Michael D. K. Ingall, Scott J. Joray, Daniel J. Duffy, David P...
0 downloads 0 Views 964KB Size
Download PDF
Langmuir 1995,11, 149-155

149

Reaction of Methylsilanols with Hydrated Silica Surfaces: The Hydrolysis of Trichloro-, Dichloro-, and Monochloromethylsilanesand the Effects of Curing C. P.Tripp* and M. L. Hair Xerox Research Centre of Canada, 2660 Speakman Drive, Mississauga, Ontario L5K 2L1, Canada Received January 18, 1994. I n Final Form: October 14, 1994@ It is well-established that the tendency of alkylsilanols to condenseincreases with the number of hydroxyl groups of the silicon atom. We have used infrared spectroscopy to provide direct evidence that shows that at the solidgas interface methylchlorosilanes are fully hydrolyzed to methylsilanols by surface water on a hydrated silica and the percentage ofthe adsorbed methylsilanolsthat condense with the surface hydroxyl groups proceeds in the order (CH3)3SiOH> (CH3)2Si(OH)2> CH3Si(OH)3. This is opposite to the expected trend and the discrepancy is explained in terms of a difference between the kinetics of self-condensation and reaction with the surface hydroxyl groups. Although the tendency of self-condensationincreases with the number of hydroxyl groups attached to the silicon atom the tendency for reaction the surface hydroxyl groups follows the reverse order. The transformations in the adsorbed species upon curing have also been investigated. At room temperature an adsorbed methyltrisilanol does not condensewith the surface hydroxyl groups and is partially cross-linked via intermolecular Si-0-Si bonds. Curing completes the crosslinking process and also results in a few attachments to the surface through a Si-0-Si bond. Both the dimethyldisilanol and the trimethylsilanol condense fully at room temperature to form surface Si-0-Si bonds. No change in these adsorbed species occurs with curing.

Introduction Chlorosilanes of the form R4-,SK, where R is an organo group and X is chlorine, are widely used materials in the surface modification of s i l i c a ~ . l -The ~ properties imparted by the silane are defined by the selection of the organo group and by the nature of the attachment to the surface. The main obstacle that limits the widespread use of chlorosilanes is that there exists a wide variability in the quality of the films. Most of this variability can be traced to the acute sensitivity of the reaction to surface preparation and, particularly, to the level of surface This critical dependence on water can be avoided by performing a gas phase reaction at elevated temperature. At high temperatures ('300 "C) a direct reaction of the chlorosilane with the surface hydroxyl groups (SisOH) occurs to form a strong Sis-0-Si surface bond." The reaction proceeds monofunctionally,

Si,OH

or difunctionally,

+ R,SiCl,-, Si,(OH), + R,SiCl,-,

2Si,OH

- Si,OSiR,Cl,-,

+ HC1

- (SisO),SiR,C1,-,

+ 2HC1 + 2HC1

+ R,SiCl,-,

-

SisO,SiR,C1,-,

(1)

(2)

Thus, the more common approach is to perform the silanization in a nonaqueous solvent near or at room temperature. In these solution reactions a variance in film quality exists because the surface water is a key component in the reaction scheme. In the absence of surface water, it has been shown that there is no reaction between the alkyltrichlorosilane and the silica surface at these lower temperatures in s o l ~ t i o n .(An ~ , ~exception to this general rule occurs when R contains a fluoroalkyl group on the y carbon atom.12) The most common scheme for the reaction a t the lower temperature in solution is as follow^:'^ In the first step, the adsorbed chlorosilane is hydrolyzed to the alkylsilanol by water which is either preadsorbed on the surface or dispersed in the solvent nH,O

* Abstract published in Advance A C S Abstracts, December 15, 1994. (1)Noll, W. Chemistry and Technology of Silicones; Academic Press: New York, 1968. (2) Plueddemann, E.P. Silane CouplingAgents;Plenum: New York, 1982. (3) Iler, R. K. The Chemistry of Silica; John Wiley and Sons: New York, 1979. (4) Unger, K. K. Porous Silica; Elsevier: Amsterdam, 1971. (5) Tripp, C. P.; Hair, M. L. Langmuir 1992, 8, 1961. (6) Tripp, C. P.; Hair, M. L. Langmuir 1992, 8, 1120. (7) Brzoska, J. B.; Shahidzadeh, N.; Rondelez, F. Nature 1992,360, 719. (8)Angst, D. L.; Simmons, G. W. Langmuir 1991, 7, 2236

0743-7463/95/2411-0149$09.00/0

-

R,-,Si(OH),

+ nHCl

(4)

In the second step, the adsorbed alkylsilanol then condenses with the surface SisOH groups to form a surface Sis- 0- Si bond. Si(OH),

+ R,-,Si(OH),

(3)

where Sis refers to a surface silicon atom. However, these harsh reaction temperatures limit the number of potential silanizing agents to those with sufficient vapor pressure.

+ R4-,SiC1,

-

+

SisOSiR,-,Si(OH),-, H,O ( 5 )

In both steps, the amount of surface water is critical; insufficient water leads to an incomplete monolayer whereas a thick water layer gives rise to a polymerized silane layer which then sits on top of the surface water layer. In the latter, the silane layer can easily be floated Thus, it is important that the chlorosilane is securely anchored to the surface through surface Sis-0-Si linkage if robust films are to be obtained. (9) Silberzan, P.; LBger, L.; Ausserre, D.; Benattar, J. J. Langmuir 1991. - _ _ -7. 1647. 7

. I

(10) Kessel, C.R.;Granick, S. Langmuir 1991, 7,532. (11) Tripp, C.P.; Hair, M.L.Langmuir 1991, 7,923. (12) Tripp, C.P.; Veregin, R. P. N.; Hair, M.L.Langmuir 1993, 9, 251 F( ---I.

(13) Sagiv, J. J. Am. Chem. SOC.1980, 102, 92.

0 1995 American Chemical Society

Tripp and Hair

150 Langmuir, Vol. 11,No. 1, 1995

It is well-known that the tendency of alkylsilanols to condensation increases with the number of hydroxyl groups attached to the silicon atom.' Therefore, the tendency to react with the surface hydroxyl group outlined in eq 5 might be expected to follow the order RSi(OH), > R,Si(OH), > R,SiOH Alkylsilanes containing trichlorosilyl groups are expected to easily form a surface bond and have been most widely used. Also, a trisilanol that is bound to the surface through an Sis-0-Si bond can still participate in intermolecular condensation between adjacent molecules to form a lateral polymerized network. Forming a cross-linked polymerized silane layer enhances the stability of the film. Although the condensation of the first hydroxyl group of a trisilanol readily occurs, the condensation ofthe second and third hydroxyl groups attached to the same silicon atom become increasingly more diffi~ult.'~J~ Often a third step is introduced to enhance the cross-linking process to produce a more densely packed and stable film. This third step of the process is a curing or baking of the coated surface by heating the dried sample in air a t temperatures between 100 and 200 "C. The curing completes the lateral condensation reaction between adjacent silane molecules to form a (2)-dimensional cross-linked silane layer. It might be expected that the curing also assists in chemically bonding more of the film to the surface SisOH groups. R

R

I

0-Si-OH

I

HO-Si-0

I

d

0

Li8

Sia

A

R

R + HzO

OJi-O-&-O

ol

b

hi,

Lie

(6)

In the present study, we use IR spectroscopy to provide direct evidence of the species formed upon adsorption of trichloro-, dichloro-, and monochloromethylsilanes on a hydrated silica at the solidgas interface and focus on the transformations occurring in these adsorbed species during the curing process. In recent articles from our l a b ~ r a t o r y , ~ ~ ~we J 'have J ~ ~ 'described ~ the first two steps in the reaction of alkyltrichlorosilanes with high surface area fumed silica at both the solifliquid and solidgas interface. We concentrated on alkylsilanes containing trichlorosilyl groups because of their wide usage. Using a thin film IR spectroscopic technique, we were able to obtain direct evidence of surface reaction a t the solidgas interface by followingthe changes which occurred in bands due to SiOH, Si-0-Si, Si-C, and Si-C1 modes. The ability to obtain spectra in the region between 1300 and 200 cm-l was critical. Our findings showed that alkyltrichlorosilanes are hydrolyzed to alkyltrisilanols with the surface water. However, few if any of the adsorbed alkyltrisilanol on the surface underwent a reaction with the surface SisOH group to form a surface Sis-0-Si bond. The alkyltrisilanol polymerized in solution and dropped out onto the surface as a thick silane layer. Ellipsometric data of Trau et a1.16on silica plates and IR studies by Le Grange et al." on oxidized silicon wafers agree with our IR findings on the high surface area silica particles. A glimpse into the chemical processes which occur during the curing process has been provided by Siz9NMR. (14)Osterholtz, 127.

F.D.; Pohl, E. R. J . Adhesion Sci. Technol. 1992,6,

(15)Tripp, C. P.; Hair, M. L. J . Phys. Chem. 1993,97, 5693. (16) Trau, M.;Murray, B. S.; Grant, IC;Grieser, F.J. Colloid Interface Sci. 1992,148, 182. (17) Le Grange, J. D.; Markham, J. L.; Kurkjian, C. R. Langmuzr 1993,9,1749.

Caravajel et al.le have shown that the curing step results in the formation of an increased number of Si-0- Sibonds. However, the NMR data could not distinguish whether these bonds were due to surface Sis-0-Si or to intermolecular Si-0-Si formation. The key enabler provided by the thin film IR technique used in this work is that it can be used to differentiate surface Sis-0-Si from the Si-0-Si which is due to cross-linking. Thus, an examination of the adsorption of trichloro-, dichloro-, and monochloromethylsilanes and the transformations occurring during the curing step is a natural extension of our previous thin film work.

Experimental Section A detailed description of the infrared cell, spectrometer, and thin iilm technique is described in detail elsewhere.6J1The fumed silica was Aerosil 380 obtained from Degussa A. G. and had a measured surface area of 375 m2g-l. It was dispersed as a thin film (about 0.2 mg/cmZ) on a CsI window. A dehydrated silica was produced by evacuation at 400 "C for 30 min followed by coolingto room temperature. This treatment gives rise to a silica containing only isolated/geminal hydroxyl groups at a density of about 1.4 OWnm2.19 The absorbance spectra shown in the figures were calculated by using a spectrum of the silica recorded prior to addition of the silane as the reference spectrum. In the curing experiments with a hydrated silica, the thin film of silica was first evacuated at 200 "C and cooled to room temperature. A spectrum was then recorded and this was used as a reference for the absorbance spectrum recorded after curing at 200 "C. The hydrated silica was then produced by exposingthe silica to air for 10min followed by a short evacuation of about 1min. This procedure removed approximately 90% of the original amount of adsorbed water. A second reference spectrum was recorded and this was used in the calculation ofthe absorbance spectrum for the silane addition to a hydrated silica at room temperature. By using spectra of the silica as a reference, the negative bands produced in the absorbance spectra are due to bonds that have been removed from the silica and the positive bands are due to bonds formed on the surface. All spectra were recorded at room temperature. Trichloromethylsilane (TCMS), dichlorodimethylsilane (DCDMS),and trimethylchlorosilane (TMCS)were obtained from Hiils America Inc. All reagents were transferred to glass bulbs and degassed several times using freeze-thaw cycles. IR spectra of the reagents were recorded periodicallyto monitor purity. An excess amount of silane vapor (about 4 mmol g-l) was added to the hydrated silica for 5 min at room temperature and this was followed by evacuation for 5 min. During curing, the silica was heated to 200 "C under evacuation for 15 min before being cooled to room temperature. AM1 semiempirical calculations were performed on MM+ optimized geometries using Hyperchem software version 2.0 for SGI.

Results and Discussion (1)Trichloromethylsilane (TCMS).The difference spectrum obtained upon adding TCMS to a hydrated silica at room temperature is shown in Figure l a . The reaction was instantaneous and complete. There was no change in this spectrum following prolonged evacuation for 2 h. Hydrolysis of the TCMS with the surface water has occurred and is evidenced by the disappearance of the HzO deformation mode at 1620 cm-' and formation of HC1 (not shown). We have previously shown that no reaction occurs when TCMS is added to a totally dehydrated silica a t room t e m p e r a t ~ r e .Although ~ the spectrum shown in Figure l a was recorded after evacuation of the excess TCMS, it is important to note that the band at 1620 cm-l disappeared prior to the evacuation step. Thus, the surface water was removed by reaction with (18)Caravajal, G. S.; Leyden, D. E.; Quinting, G. R.; Maciel, G. E. Anal. Chem. 1988,60, 1776. (19) Morrow, B.A.;McFarlan, A. J. Langmuir 1991,7,1695.

Hydrolysis of Chloromethylsilanes

Langmuir, Vol. 11, No. 1, 1995 151 Table 1. Intensity Ratios of Infrared Bands

Reactionn

106011265

1060f3747

126513747

0.10 1.0 0.18 0.18 0.71 0.43 0.63 0.82 0.28 1.27

0.01 0.056 0.035 0.035 0.057 0.033 0.032 0.048 0.01 0.052

0.14 0.11 0.056 0.23 0.23 0.08 0.077 0.050 0.058 0.048 0.041

TCMS (RT)

TCMS (Curing) TCMS (400 "C) DCDMS (RT) DCMS (Curing) DCDMS (400 "C) TMCS (RT) TMCS (Curing) TMCS (400 "C) Et3SiOH (RT) Et3SiOH (Curing)

RT = reaction at room temperature, Curing = aRer curing at 200 "C, 400 "C = gas phase reaction at 400 "C.

4000

3000

2000

1000

cm-1

Figure 1. Addition of TCMS to (a) a hydrated silica at room temperaturefollowed by (b)evacuationat 200 "C. Curve labeled c is after reaction of TCMS at 400 "C with a dehydrated silica.

the TCMS and not during the evacuation of the excess TCMS. The broad bands at 3400 cm-' (SiO-H) and 898 cm-' (Si-OH) are due to the alkylsilano15 and their appearance, coupled with a very weak Si-C1 band from the parent compound at 574 cm-l, shows that the attached silane is almost completely hydrolyzed. The positive band at 3400 cm-l cannot be due to an OH stretching mode of adsorbed water because the HzO deformation mode at 1620 cm-l is in the negative direction. The negative band at 3747 is due to the SiO-H stretching mode ofthe isolated geminal surface silanols and its disappearance shows that there is an interaction of the surface SisOH groups and the alkylsilanol. A negative Sis-OH stretching modell should accompany the negative band at 3747 cm-l and this band a t 973 cm-l is clearly evident in the spectrum shown in Figure IC.This negative band at 973 cm-l is not as apparent in Figure l a because it is superimposed on two positive peaks located at 1007 and 898 cm-l. This interaction with the surface hydroxyl groups can arise from the physisorption of an alkylsilane species, from hydrogen bonding with the alkylsilanol or from chemisorption via the formation of a surface Sis-0-Si linkage. The key spectral region used to decipher between these possibilities is the region between 1200 and 1000 cm-l because this region contains the Si-0-Si modes. Figure l a shows that there is one band located at 1007 cm-' in this spectral region. Assignment of bands in the Si-0Si spectral region is difficult because a Si-0-Si bond can arise from several species. A Si-0-Si bond can be due to a reaction with the surface SisOH groups, from crosslinking between adjacent alkylsilanols, or from polymerization products depositing on the surface. The assignment is further complicated in the thin film studies because the bands in this region are superimposed on the intense bulk mode of silica. Subtle changes in the bulk Si-0-Si modes near the adsorbed species could produce spectral changes in the 1200-1000 cm-l region. Assignment of the band at 1007 cm-l to a polymerized silane layer can be eliminated. Known polymeric compounds of methyltrichlorosilanes containing (Si-0-Si), in cyclic compounds where x = 3 or 4 have the Si-0-Si band split in two components at about 1090 and 1020 cm-l and linear compounds show two bands at 1080 and 1020 cm-1.20,21On this basis, we rule out deposition of a siloxane polymer on the surface. (20) Bellamy, L. J. The Infra-red Spectra of Complex Molecules, 3rd ed.; Chapman and Hall Ltd.: Thetford, LJK, 1975. (21) Smith, A. L.Spectrochim. Acta 1960,16, 87.

Assignment of the 1007 cm-l band to a surface Sis0-Si species can also be discarded by comparison with the spectrum shown in Figure IC. That spectrum was produced by the reaction of TCMS at 400 "C with a completely dehydrated silica, as reported previously.ll The bands in Figure ICare assigned as follows: negative 3747 cm-l (SisO-H stretching), 2970,2920 cm-l (CH3 stretching), 1428, 1275 cm-' (CH3 bending), 1060 cm-' (Sis0-Si stretching),negative 973 cm-' (Sis-OH stretching), 827, 803 cm-l, (CH3 rocking), 574 and 473 cm-I (Si-C1 stretching). In the reaction at 400 "C, all adsorbed TCMS molecules are bound to the surface by a Sis-0-Si bond which is evidenced by the broad band located at 1060cm-l. It is evident from this comparison to the reaction at 400 "C that the band a t 1007 cm-l is not due to a surface Sis-0-Si bond. Thus, removal of surface SisOH groups (i.e., the negative bands at 3747 and 973 cm-l in Figure l a ) cannot be due to the formation of a surface Sis-0-Si linkage. It is possible that a few of the silanols react to produce a Sis-0-Si surface bond. The relative number of adsorbed TCMS molecules that are either bound to the surface or cross-linked to each other by Si-0-Si bonds can be estimated by a comparison with the spectrum shown in Figure IC. Since the reaction at 400 "C proceeds according to eqs 1-3, the ratio of the integrated intensities ofthe 106011275cm-l bands in Figure l a to those in Figure ICcan be used to estimate the relative number of TCMS molecules attached to the surface by Sis-0-Si bonds in the room temperature reaction with a hydrated silica. Similarly, the 127513747cm-' ratio provides information on the relative number of TCMS molecules per SisOH site, and the 106013747ratio gives the relative number of perturbed SisOH groups converted to Sis-0-Si bonds. By analogy, these ratios are applicable to the adsorption of DCDMS and TMCS, and the results are listed in Table 1. (The Si-CH3 bending mode of TCMS, DCDMS, and TMCS are a t 1275,1267, and 1259 cm-l, respectively. To reduce confusion we will henceforth refer to these bands collectively as 1265 cm-l.) Acomparison ofthe 106013747ratio in Figure la,c shows that the number of surface Sis-0-Si bonds is below our detection limit (i.e., a maximum of 5% of the perturbed surface SisOH groups could be involved in a condensation reaction with the methyltrisilanol). Therefore, it is clear that the adsorption of TCMS occurs mainly by hydrogen bonding interactions of the methylsilanol with the surface SisOH groups. However, this alone cannot account for the total number of adsorbed TCMS molecules, as a comparison of the 126513747 ratios shows that there are about 2.5 times more TCMS per SisOH group on the hydrated sample than on the dry sample. Some caution must be exercised when using the intensity of the 3747 cm-l band in these comparisons. The value is probably underestimated because the chlo-

152 Langmuir, Vol. 11, No. 1, 1995 rosilane is hydrolyzed with the surface water layer. The removal of this water layer can change the number of isolated hydroxyl groups because the water layer is hydrogen bonded to the surface. For example, a dehydrated silica contains about 1.1 isolatedgeminal groups per nm2, and this value is only about 0.65per nm2 on the silica which is "as received" from stock.22 If we consider the possibility that an adsorbed alkylsilanolreplaces some of the SisOH groups which are hydrogen bonded with the water with hydrogen bonded alkylsilanols, then there would be no change in intensity at 3747 cm-l and no band in the difference spectrum. In our experiments the hydrated silica contains about 0.25 HzO moleculeslnm2 (i.e.,about 10% of the water level of an "as received" silica). For the level of water removed in Figure l a (as measured from the negative H2O deformation mode at 1620 cm-9 the corresponding change at 3747 cm-l, on a separate hydrated silica sample that has been evacuated, caused a change at 3747 cm-' that was about 10% of the band intensity shown in Figure la. Thus, the effect of water removal from silica during the reaction would at best underestimate the 126513747and 106013747ratios by 10%. In the gas phase reaction of TCMS with silica at 400 "C, the chlorosilane can react monofunctionally as depicted in eq 1 or difunctionally as shown in eqs 2 and 3. Thus, part of the difference in the 126513747ratios could be due to a change in the proportion of TCMS molecules reacting monofunctionally and difunctionally. However, even if we assume that all TCMS reacts monofunctionally with the hydrated silica, this would still not account for the almost 2.5times excess. This excess TCMS is resilient to removal by prolonged evacuation and must bind to the surface by a mechanism other than direct interaction with the surface SisOH groups. It is possible that the excess TCMS which is adsorbed per surface &-OH group and the resilience of the product to removal is due to self-condensation with formation of an Si-0-Si bond between adjacent alkylsilanols. We will show later in this paper that curing the sample (a process known to increase cross-linking) results in an increase in intensity of the band at 1007 cm-l and a decrease in intensity at 3400 and 898 cm-l. (The two latter bands are due to the SiO-H and Si-OH vibrations of the alkylsilanol.) No other changes occur in the spectrum during the curing step. Furthermore, we also show that a band near 1007 cm-' forms when dichlorodimethylsilane (DCDMS) is adsorbed but does not appear with trimethylchlorosilane (TCMS). With TCMS, a cross-linked Si-0-Si bond between adsorbed alkylsilanols is not possible and thus this evidence is strong support for assigning the 1007 cm-' band to a Si-0-Si bond between adjacent adsorbed alkylsilanols. It is noted that spectroscopic arguments suggest that a band at 1007 cm-l is low in frequency for Si-0-Si modes. Bands at this frequency are only found in compounds containing highly strained Si-0-Si linkages such as those of cyclic trimers of dimethylsiloxane.20~23 However, it is not unreasonable that at the level of adsorbed TCMS (about 2.5per surface SiOH)there exists an intermolecular Si-0-Si bond between adsorbed alkylsilanols, and this would be in a highly constrained geometry. We have reported15an Si-0-Si band a t 1035 cm-l which is formed from cross-linking when a multisilane layer extends out from the surface. In the multisilane layer, the Si-0-Si network extending from the surface would be less constrained than that formed between adjacent alkylsilanols adsorbed on the surface and would occur at the higher frequency. (22)Tripp, C. P.;Hair, M.L.Langmuir 1993,9, 3523. (23)Smith, A. L.;Anderson, D. R. Appl. Spectrosc. 19&4, 38, 822.

Tripp and Hair

We previously suggested5that the band at 1007 cm-l could be the high-frequency component of a broad band found near 900 cm-l which is due to the Si-0 stretching mode of the alkylsilanol. A separate band at 1007 cm-' was the result of the superposition of a negative band located at 978cm-l (Sis-OH stretch). However, evidence from curing shows that this assignment cannot be correct. We cannot discount the possibility that the band at 1007 cm-l is due to changes in the underlying Si-0-Si network modes and this is especially true because of the close proximity of the negative feature at 978 cm-'. However, the appearance of a band at 1007 cm-' with the di- and trisilanols (but not the monosilanol), its nonformation in the 400 "C gas phase reaction, and its increase in intensity with curing, lead us to conclude that this band is indicative of the formation of a highly strained Si-0-Si network between adjacent adsorbed alkylsilanols on the surface. The picture that emerges is that the TCMS is fully hydrolyzed at room temperature and about 40% of the molecules (from the 126513747ratios) are anchored to the surface by hydrogen-bondinginteractions with the surface SisOH groups. The remaining hydrolyzed TCMS are not directly attached to the surface and form a partially polymerized layer through Si-0-Si bonds between molecules. This is similar to the Si29NMR conclusions reported by Sindorf and M a ~ i e l Although .~~ their experimental protocol was different from ours, Sindorf and Maciel found that the TCMS adsorbed on a hydrated silica was in excess of the number of surface SisOH groups, and this difference was attributed to polymerization of silane on the surface. To study the effect of curing, the sample giving the spectrum shown in Figure la was then evacuated at 50 "C intervals up to 200 "C. Changes in the spectrum begin to appear to 100 "C and are complete by 200 "C. The spectrum obtained aRer evacuation at 200 "C is shown in Figure lb. The band at 1265cm-' is of the same intensity, showing that the curing process did not change the number of methyl groups and therefore the amount of silane attached to the surface. Upon curing, there is a reduction (about 95%) in the intensity of the silanol bands at 3400 and 898cm-' and this is accompanied by a doubling in the intensity of the band at 1007 cm-'. A weak band at 3500 cm-l appears and this could be due to residual hydrogenbonded surface silanols. Evidently, the curing completes the condensation process through the formation of Si0-Si bonds between adsorbed molecules. There is also evidence of a very weak, broad band at 1060 cm-l, showing that some of the surface SisOH groups condense to form a surface Sis-0-Si linkage, and this is accompanied by about a 25% increase (measured from the negative band at 3747cm-l) in the number ofhydroxyl groups interacting with the silane. Although an exact comparison of the 3747 cm-l band in parts a and b of Figure 1 is not quite correct because the reference spectra are different in the two cases, we note that there would be a maximum of a 10% underestimation in the number of surface silanols involved in hydrogen bonding with the adsorbed alkylsilanol. This is well below the measured 25% change. Angst and Simmons8have shown that the amount of water adsorbed on an oxidized silicon wafer treated with octadecyltrichlorosilane (OTS)was higher on an uncured sample than on a cured sample. Although the outer surfaces of both the cured and uncured OTS layers are equally hydrophobic, the authors suggested that the amount of water adsorbed at the interface depends on the number of unreacted hydroxyl groups present. Curing would decrease the number of hydroxyl groups available (24)Sindorf, D.W.; Maciel, G.E.J . h . Chem. SOC.1983,105,3767.

Hydrolysis of Chloromethylsilanes

Langmuir, Vol. 11, No. 1, 1995 153

.." t 4000

-

-.-

3000

.

-

. - ~ - .

2000

1

.

1000

i cm-1

Figure 2. Addition of DCDMS to (a) a hydrated silica at room temperaturefollowed by (b)evacuationat 200 "C. Curve labeled cis after reaction of DCDMS at 400 "C with a dehydrated silica.

through condensation and this would result in a smaller adsorption of water on the surface. Our results show a 25% reduction in the number of hydroxyl groups with curing and this finding is supportive of the Angst and Simmons interpretation. Assuming the same stoichiometry as in the 400 "C reaction, the 106013747 ratio shows that the amount of Sis-0-Si formed accounts for only about 20% of the reduction in the band at 3747 cm-l due to surface SisOH groups. Thus, there are still a large number of surface SisOH groups that are perturbed by the adsorbed silane layer. This perturbation probably arises from the physisorption of the cross-linked silane layer and by hydrogenbonding interactions with the remaining few alkylsilanol groups. Nevertheless, the above results clearly show that curing completes the cross-linking process, reduces the number of available surface SisOHgroups and causes some condensation between the methylsilanol and the surface SisOH groups. However, even after curing, only a few of the adsorbed TCMS molecules are anchored via Sis-0Si bonds, and a comparison of the 106011265 ratios in Figure lb,c suggests that less than 10%of the adsorbed molecules are linked through a surface Sis-0-Si bond. (2) Dichlorodimethylsilane (DCDMS). When DCDMS and TCMS condense we would anticipate differences both in the rate of condensation and in the mode of lateral polymerization. Cross-linking of TCMS occurs to form a dendritic pattern whereas polymerization of DCDMS proceeds in a linear chain. The spectrum obtained with the addition ofDCDMS to a hydrated silica is shown in Figure 2a. The absence of a Si-C1 band from the parent compound at 485 cm-I shows that the DCDMS is completely hydrolyzed. However, unlike the TCMS described previously, very little of the hydrolyzed DCDMS remains on the surface. The weak band at 898 cm-l shows that there are very few dimethylsilanol molecules, and the appearance ofbands at 1060 and 1000 cm-I is evidence that a large proportion of the dimethylsilanol molecules have condensed with the surface SisOH groups and crosslinked to form both surface and intermolecular Si-0-Si bonds at room temperature. By comparing the 106013747 ratio with that obtained in the high-temperature gas phase reaction (see Figure 2c) it can be estimated that about 75% of the surface SisOH groups on the hydrated silica are due to Sis-0-Si formation. Furthermore, the ratio of the 126513747 intensities shows that about 2.8 times as many DCDMS molecules are adsorbed per perturbed hydroxyl group on the hydrated silica than is found in the 400 "C gas phase reaction. Since the lateral polymeri-

4000

3000

2000

10'00

cm-1

Figure 3. Addition of TMCS to (a) a hydrated silica at room temperature followed by (b)evacuqtion at 200 "C. Curve labeled c is after reaction of TMCS at 400 "C with a dehydrated silica.

zation of DCDMS occurs in a linear chain, the molecules bound to the surface through a Sis-0-Si bond must anchor the silane and terminate the polymer chain. On average, about two DCDMS molecules are attached through intermolecular Si-0-Si bonds for each DCDMS which is attached to the surface. This oversupply of DCDMS molecules per SisOH is similar to that found for TCMS and is probably reflective of the equivalent level of adsorbed water initially present in both samples. When the above sample is cured by evacuation at 200 "C, there is only a small change in the adsorbed amount. However, the adsorbed DCDMS species do not undergo any chemical transformation during the curing. This is not surprising since the spectrum in Figure 2a shows that a nearly complete condensation of the dimethylsilanol had already occurred at room temperature. Further comment on the differences between TCMS and DCDMS with a hydrated silica is warranted and this is differed to the section entitled Summary and General Trends. (3)Trimethylchlorosilane (TMCS). TMCS differs both from TCMS and DCDMS in that lateral polymerization is not possible and does not occur. This is clearly evident by the absence of a band at 1007cm-l in the spectra of both the uncured and cured sample shown in Figure 3a,b. From Figure 3a, it is also evident that TMCS reacts with the surface SisOH groups of the hydrated silica at room temperature. Again, the negative band at 1620cm-l and the absence of bands due to Si-C1 modes from the parent compound in Figure 3a show that the TMCS is completely hydrolyzed. The band at 898 cm-I due to the trimethylsilanol is not evident and this, coupled with the appearance of a band at 1060 cm-', shows that most ofthe trimethylsilanol undergoes a reaction with surface SisOH groups. Comparison of the 106013747 ratio obtained here with those obtained from the 400 "C gas phase reaction indicates that about 70%ofthe trimethylsilanols react with the surface SisOH groups to form a Sis-0-Si bond. To account for the remaining 30% it is noted that there is a band at 3715 cm-l which is attributed to weakly perturbed hydroxyl groups arising from the physisorption of an adsorbate on the surface SisOH groups. This band disappears when the sample is cured and this is accompanied by a 15%drop in the amount of adsorbed TCMS. After curing, the 126713747 ratio was close to the ratio obtained in the 400 "C gas phase reaction. This indicates that the same reaction occurs at room temperature with a hydrated sample. The curing of this sample simply removes the weakly adsorbed species from the surface.

Tripp and Hair

154 Langmuir, Vol. 11, No. 1, 1995 Summary and General Trends

The above results show that the three methylchlorosilanes, TCMS, DCDMS, and TMCS, are fully hydrolyzed to methylsilanols upon contact with the surface water on silica. The results also show (from the 1060/3747ratio) that the tendency for reaction of the adsorbed methylsilanols hydrogen bonded with the surface SiSOH groups follows the order (CH,),SiOH = (CH,),Si(OH), > CH,Si(OH), (7) This is opposite to the trend which is expected from selfcondensationreactions in solution. It is generally accepted that the tendency of organosilanols to self-condensation increases with the number of hydroxyl groups attached to the silicon atom and decreases with the number and size of the organic gr0up.l In solution, the mechanism for self-condensation is by nucleophilic attack of an 0 atom on a Si atom of a silane through a pentacoordinate transition state and this attack is favored at a Si atom with a higher number of attached hydroxyl group^.^^,^^ The self-condensationsof alkylsilanols are also extremely sensitive to steric hindrance. For instance, silanediols containing tert-butyl groups are extremely resistance to condensation as compared to di-n-alky1silanediols.l It is our contention that the mechanism for solution condensation is completely different from the mechanism with surface SiOH groups and that a steric effect predominants. In solution, a pentacoordinate intermediate is formed in the reaction by attack of the 0 at 180"or 90" with respect to the leaving These geometries are not possible with the attack of 0 of an alkylsilanol on the Si surface atom, and it is energetically improbable that the surface silicon is able to alter its conformation and achieve a pentacoordinate structure. This argument finds support from previous studies comparing base catalysis in solution versus surface rea~ti0n.l~ It has been shown that triethylamine preadsorbed on surface SisOH groups could promote the room temperature reaction of chlorosilanes with the surface. In this case, the triethylamine does not form a pentacoordinate intermediate with the incoming silane but rather forms a strong hydrogen bond with the surface SisOH groups. It is this hydrogen bonding that renders the 0 of the surface SisOH group a better nucleophile for attack on a Si atom of the chloroalkylsilane. For the reaction between hydroxyl groups of the surface and the alkylsilanol, an acid-base mechanism can be envisaged. In principle, this can occur two ways: the oxygen atom eliminated can come either from the surface SisOsH

+ HOSiR, - Sis-0-SiR, + H,O,

(8)

or from the alkylsilanol SisOsH

+ HOSiR, - Sis-Os-SiR3 + H 2 0

(9)

The relative acidity of the silanol groups will decide the reaction. The question of surface acidity has been addressed previously and it has been shown that the surface pKa values can be estimated by measuring frequency shifts for a series of adsorbed molecules. It is found that the hydroxyl stretching frequencies for silica2' (25)Corriq R. J. P.; Guerin, C. J. Organomet. Chem., 1980,198, 231. (26) Sommer, L. H. Stereochemistry, mechanism and silicon: An introduction to the dynamic stereochemistry and reaction mechanisms of silicon centers; McGraw-Hill Inc.: NY, 1965. (27) Hair,M. L.; Hertl, W. J.Phys. Chem. 1969,74, 91.

Table 2. Calculated Si-0 Bond Lengths (A) of Methylsilanols nonprotonated protonated (CHdsSiOH 1.78 1.96 1.95 1.91

(3690cm-l) and trimethylsilanoP (3688cm-l) in CCL shift 395 and 238 cm-', respectively, by hydrogen bonding to diethyl ether. This shows that the surface SisOH groups are more acidic than the SiOH of the methylsilanols and pKa values of 7.1 for silica and 11 for trimethylsilanol have been obtained.29 Thus, the surface reaction will proceed according to eq 9; the water produced will contain a proton from the surface SisOH group and an OH from the alkylsilanol. In essence, the condensation reaction involves breaking the 0-H bond on the surface and the Si-0 bond of the approaching silanol. If it is assumed that the reaction does proceed by attack of the oxygen atom of the surface SisOH group on the silicon atom of the alkylsilanol, then a single transition state structure as shown below is plausible. AM1 calcula-

(10)

/"

Si,

tions of the bond length are given in Table 2 and show that the Si-0 bond length decreases in the order

(CH,),SiOH

>

(CH,)2Si(OH), > CH3Si(OH),

(11)

Thus, a condensation reaction with the surface silanols would likely proceed according to the above trend since the behavior of the hydroxyl groups on the alkylsilanol in terms of a good leaving group also follows this trend. Of the three chlorosilanes,the methyltrisilanol of TCMS is the only adsorbed species that had not fully condensed and reacted with the surface at room temperature. This is a consequence of the competition between selfcondensation and surface reaction and it is this difference in kinetics (self-condensation vs reaction with surface) that controls the nature ofthe adsorbed species. Although the tendency of self-condensation increases with the number of hydroxyl groups attached to the silicon atom, the tendency for condensation with the surface hydroxyl groups follows the reverse order. In the first step of the condensation of the methyltrisilanol, self-condensationis favored over surface reaction. One hydroxyl group of the trisilanol reacts with the hydroxyl group of a second trisilanol. Reaction of the second and third hydroxyl groups of the trisilanol becomes increasingly more diff i ~ u l t , ' ~and J ~therefore, a curing step is needed to complete the self-condensation process and give some surface reaction. Figure 1shows that the curing step eliminates the remaining methylsilanols, and this effectively doubles the number of intermolecular Si-0-Si bonds (i.e., intermolecular condensation has gone from one to two bonds per silicon atom). In this discussion, we have neglected the possible role of HC1 as a catalyst in the reaction schemes. HC1 may have little or no effect on the room temperature reaction because the water needed to ionize the HC1 is consumed (28) West, R.; Baney, R. H. J.Am. Chem. SOC.1969,81,6145. (29) Hair, M. L. Infrared Spectroscopy in Surface Chemistry;Marcel Dekker: New York, 1967.

Hydrolysis of Chloromethylsilanes

Langmuir, Vol. 11, No. I, 1995 155

Figure 4. Addition of triethylsilanol t o (a) a hydrated silica at room temperature followed by (b) evacuation at 200 "C. in the hydrolysis step. Under anhydrous conditions, HC1 does not catalyze the reaction.14 Furthermore, in our experiments,the acid has no role in the curing step because this is done under evacuation. Acids catalyze the condensation reaction by attachment of the proton t o the silanol oxygen, rendering the silicon atom a better electrophile for nucleophilic attack by a second silanol.1,14 0

H I 0

~i-OH-H ~i-O-H

+

HOSi(- - 3 i - O - S i $

+

0

H + H,O

(12)

In effect, the proton weakens the Si-0 bond. Calculations (see Table 2) of the protonated methylsilanols show longer Si-0 bond lengths than for the unprotonated methylsilanols. However, the trend in bond lengths between the three methylsilanols has not changed with protonation. Thus, acid, if present, would catalyze the condensation reaction but would not change the relative order in which it proceeds. Nevertheless to test the role of the acid in the hydrolysis and room temperature condensation a final experiment was performed in which triethylsilanol was added to a dehydrated silica surface. By using triethylsilanol, the hydrolysis step is bypassed, surface water is not needed, and so HC1 is not produced. The results of performing this experiment are shown in Figure 4. The addition of triethylsilanol at room temperature leads to the appearance of a band at 1060 cm-', showing that a condensation with the surface SisOH has taken place. In addition, there

are a large number of hydrogen-bonded triethylsilanol molecules attached to the surface, and these are removed by curing at 200 "C. However the number of triethylsilanols that were chemically anchored via a surface Sis0-Si bond remained constant: the band at 1060 cm-l did not change with curing. Thus a similar picture emerges for the reaction ofTMCS and triethylsilanol on silica which leads to the conclusion that the catalytic effect of the HC1 produced in the hydrolysis step in our experimental protocol is negligible. Conclusion In this paper, we have shown that the methylsilanols that are formed when the corresponding chloromethylsilanes are hydrolyzed by surface water have differing reactions with the hydroxyl groups on a silica surface. The reaction product from the TCMS is a partially crosslinked species which reacts little, if at all, with the surface groups a t room temperature. Curing at 200 "C completes the cross-linking process and results in a few direct attachments to the surface. At room temperature, both the dimethyldisilanol and the trimethylsilanol condense fullywith the surface. For DCDMS, the amount adsorbed is nearly 3 times that expected from gas phase studies and is presumably related to the amount of water on the original surface. In both cases, there is extensive reaction (about 75%) with the surface hydroxyl groups. The percentage of the methylsilanols reacting with the surface SisOH is in the order (CH,),SiOH > (CH,),Si(OH), > CH,Si(OH), This is explained in terms of the differing basicities of the SiOH groups and to a difference in kinetics between selfcondensation and reaction with SisOH groups. As a final point, we issue a word of caution in overextrapolating these results to other siliceous surface. We deal basically with an acid-base interaction and could expect changing results as the acidity of the surface hydroxyl group is altered. It is well-known in the catalytic and molecular sieve literature that introduction of aluminum ions into the silicon framework can cause the formation of highly acid Bronsted sites. These would be stronger nucleophiles and would accelerate many surface reactions. The introductionof the chemically similar boron into the silica surface has been shown to affect the kinetics of gas phase reactions between surface hydroxyls and chl~rosilanes.~~ LA9400565 (30) Hair, M. L.;Hertl, W.J.Phys. Chem. 1973, 77, 1965.