Langmuir 1993,9, 3518-3522
3518
Effect of Fluoroalkyl Substituents on the Reaction of Alkylchlorosilanes with Silica Surfaces C. P. Tripp,' R. P. N. Veregin, and M. L. Hair Xerox Research Centre of Canada, 2660 Speakman Drive, Mississauga, Ontario, Canada L5K 2L1 Received August 13,199P
We have investigated the reaction of (fluoroalky1)chlorosilanes with silica at the solid/gas interface by infrared spectroscopy. In the absence of surface water, the (fluoroalky1)chlorosilanes react at room temperature to form a surface Si-0-Si bond. Under these same conditions, alkylchlorosilanesdo not react with the silica surface. This difference in reactivity is explained by an inductive effect of the fluoroalkyl groups on the gamma-carbon atom (7-C). The effect of surface water and adsorbed amines on the surface chemistry has also been examined. Surfacewater reacts with the (fluoroalky1)chlorosilanesto form silanols which then undergo a condensation reaction with the surface hydroxyl groups. Nitrogen-containing bases promote the reaction of the (fluoroalky1)chlorosilaneswith the surface and yield surface species that are identical to those obtained by direct reaction.
Introduction Organochlorosilanes find extensive use as surface coupling agents in the treatment of silica for diverse applications' such as in chromatography2and tribology? The behavior of the modified silica is dependent on the nature of the organo group and on the mode of attachment to the surface. The most widely used silanes are those that contain alkyl groups and these silanes can be directly attached by the high temperature (>300 "C) gas phase reaction with the surface hydroxyl groups.@
-
SiOH + R,SiCl,, SiOSiR,Cl,, + HC1 (1) Commercial hydrophobic fumed silicas such as Aerosil R 972 from Degussa' Corp. and Cab-0-Si1 TT-610 from Cabots Corp. are produced using dichlorodimethylsilane in this manner. However, it is common to perform the silanization reactions in solution because many alkylchlorosilanes do not possess sufficient vapor pressure for the above gas phase reaction. The reaction temperatures used in solution are much lower than are possible in the gas phase reaction and, without water or catalysts, there is no direct reaction with the hydroxyl groups at these lower temperatures. Thus, a mechanism other than the direct reaction depicted in eq 1 is occurring in solution. It is widely accepted"l3 that water (either on the surface or in solution)is a necessary ingredient to anchor self-assembled alkylchlorosilanes onto silica surfaces adsorbed from solution. Often, water is purposely added to the nonaqueous solvent to ensure reaction of the chlorosilane with the surface. The water is needed to hydrolyze the chlorosilane to silanols. A common mechanisms proposed a Abstract published in Advance ACS Abstracts, November 15, 1993. (1) Plueddemann, E. P. Silane Coupling Agents; Plenum Press: New York, 1982. (2) Nawrocki, J.; Buszewski, B. J. Chromotogr. Rev. 1988,449, 1. (3) Pilpel, N. Manuf. Chem. Aerosol News 1970,41, 19. (4) Hair, M. L.; Hertl, W. J. Phys. Chem. 1969, 73,2372. (5)Armiatead, C. G.; Tyler, A. J.; Hambleton, F. H.; Mitchell, S. A,; Hockey, J. A. J. Phys. Chem. 1969, 73,3947. (6) Tripp, C. P.; Hair, M. L. Langmuir 1991, 7,923. (7) Technical Bulletin Figments No. 18, Deguaaa AG, Frankfurt.
(8) Technical Data Cab-0-Si1TS-610,Cabot Corp. Tuscola, IL. (9) Sagiv, J. J. Am. Chem. SOC.1980,102,92. (10) Tripp, C. P.; Hair, M. L. Langmuir 1992,8, 1120. (11)Anget, D. L.; Simmons, G. W. Langmuir 1991, 7, 2236. (12) Silberzan, P.; LBger, L.; AwerrB, D.; Benattar, J. J. Langmuir 1991, 7, 1647. (13) Tripp, C. P.; Hair, M. L. Langmuir 1992,8, 1961.
for surface attachment has the alkylsilanol adsorb on the surface hydroxyl groups via hydrogen bonding interactions followed by condensation to form Si-0-Si bonds with the surface. In an IR study,13 we have shown that the alkyltrichlorosilane is hydrolyzed to the trisilanol with the surface water but that the condensation of the trisilanol with the surface hydroxyl groups does not occur. At the solid/gas interface, the trisilanol adsorbs on the surface but does not undergo condensation or polymerization, whereas, at the solid/liquid interface, the trisilanol polymerizes in solution and adsorbs on the surface. However, triboelectric datal4 and friction datal5 from others suggest that the reaction of (fluoroalky1)chlorosilanes with silica proceed differently than their alkyl counterparts. Fluoroalkyl-containing silanes comprise another class of silanizing agents and are finding increased importance in industry.16 Much of this is due to the low surface energy associatedwith the fluorinated alkyl chains that, in turn, impart unique properties to the modified silica. Our triboelectric data suggested that the (fluoroalky1)chlorosilanesreact at room temperature in solution with the surface hydroxyl groups in the absence of water. A direct room temperature reaction between a silica surface and a silanizing agent has been noted by Owen and Williams.16 They showed that the reaction of (trifluoropropy1)dimethylsilyl-N-methylacetamide (TFSA) with silica occurs at room temperature under dry conditions and noted that this was contrary to the results published for alkylchlorosilanes. However, the silane used by Owen and Williams had a nitrogen-containingleaving group that may promote the reaction with the silica surface. Nitrogencontaining bases are often used to promote reaction of chlorosilanes with silica1' and nitrogen-containing silanes such as hexamethyldisilazane, (CH3)sSiNHSi(CH&, are well-known to react with silica at room temperature.ls Therefore, direct comparison of the N-methylacetamide to the chlorosilane may not be valid because the chemistry could be quite different. In this paper, we investigatethe reaction of (fluoroalky1)chlorosilanes with the surface of a fumed silica. We have targeted (fluoroalky1)chloroailanescontaining only trichlo(14) Tripp, C. P.; Veregin, R. P. N.; Hair, M. L. To be published in J. Vac. Sci. Technol. (15) Sharma, R. Private communication. (16) Owen, M. J.; Williams,I).E.J. Adhes. Sci. Technol. 1991,5,307. (17) Unger, K. K.; Kinkel, J. N. J. Chromotogr. 1984,316, 193. (18) Hertl, W.; Hair, M. L. J . Phys. Chem. 1971, 75, 2181.
Q743-7463/93/2409-3518$04.OO/Q0 1993 American Chemical Society
Langmuir, Vol. 9, NO.12,1993 3519
Reaction of Alkylchlorosilanes with Silica
rosilane headgroups because they can be used to effectively anchor the alkyleilane layer by cross-linking of adjacent alkyl chainss and because they can be used to build welldefiied multilayers.lg In particular, we have used infrared spectroscopic techniques developed from our studies on alkylchlorosilanessto provide direct spectral evidence of the nature of attachment for their fluorinated counterparts. The study was limited to the reaction of the (fluoroalky1)chlorosilanesat the solid/gas interface becasuse it enabled the use of a thin f i i techniqueq6 This allowed access to the spectral region containing the S i 4 and S i 4 1 modes that are needed to provide direct spectroscopic evidence of the mode of attachment to the surface. Strict control on the surface quality (particularly with respect to surface water) was achieved by limiting studies to the solid/gas interface. This latter condition allow us to examine the role of surface water in the adsorption process. Finally, we explore the role that nitrogen-containingbases play in promoting the surface reaction.
Experimental Section A detailed description of the infrared cell, spectrometer, and thin film technique is given elsewhere.6 The fumed silica was Aerosil380 obtained from Degum A. G. and had a measured surface area of 375 m2gl.It was dispersed as a thin f i i (about 0.2 mg/cm2)on a CsI window. A hydrated silica was produced by a short evacuation (about 1 min) until the pressure in the vaccum line was about le2Torr. This procedure removed approximately 90% of the original amount of adsorbed water.10 A dehydrated silica was produced by evacuation (10-8 Torr) at 400 O C for 30 min followed by cooling to room temperature. This treatment gives rise to a silica containing only isolated/geminal hydroxyl groups at a density of about 1.4 O H / I ~ The ~ . ~ propyltrichloroeilane (CHaCHCHaSiCla,abbreviated as PTCS) and (3,3,3-trifluoropropyl)trichlorosilane(CFsCH2CH2SiC18,FPTCS) were obtained from Htik America, Inc. Triethylamine (TEA) was obtained from Aldrich. All reagents were transferred to glass bulbs and degassed severaltimes usingfreezethaw cycles. IR spectra of the reagents were recorded periodically to monitor purity. Conventional vacuum line techniques were used to transfer the gases to the infrared cell. Optimized geometries were obtained from semiempirical calculations using AM1 on MM+ optimized structures using Hyperchem (Release 2 for SGI). The optimized geometry parameters were used as input parameters to GAUSSIAN 90 for ab initio HartrwFockself-consistentfield frequencycalculations with a 6-31G* basis set.
h e u l t s and Di8CU88iOn For reasons of thermal and chemical stability, commercially available (fluoroalky1)chlorosilanes do not have the fluorine-bearingcarbon atoms directly attached to the silicon atom.21 Most often the fluorine-bearing carbon atoms are separated from the silicon atom by two methylene groups because a fluorinated substituent on the a-C o r b 4 weakens the S i 4 bondmaking it susceptible to nucleophilic attack and subsequent decomposition.22 Thus, for our purposes, the simplest (fluoroalky1)chlorosilane that has a sufficient vapor pressure for studies at the solid/gas interface is CFsCHzCH2SiC18, denoted as F-PTCS. Ita alkyl counterpart, CHsCH2CH2SiC4, is denoted as PTCS. Figure 1 shows the spectra obtained after addition, at room temperature, of F-PTCS to a dehydrated silica. These spectra are difference spectra produced by subtracting the spectrum of the silica before (19) Tripp, C. P.; Hair,M. L. J. Phys. Chem. 1993,97,5693. (20) Morrow, B. A.; McFarh, A. J. Langmuir 1991, 7,1696. (21) Silicon Compounds: Register and Reoiew; Anderson,R., Larson, G. L., Smith, C., Ede.; Hills America, Inc.: Picataway, NJ,1991;p 285. (22) Noll, W. Chemiatry and Technology of Silicones; Academic Press: New York, 1968; p 147.
,
181
.o 1
07."'
-
- 16
4000
3050
2700 CV-'
1150
200
Figure 1. Difference spectra of PTCS reacted (a) at room temperature and (b) at 400 "C and F-PTCS reacted (c) at room temperature and (d) at 400 OC to a dehydrated silica. Excess silane was added at the specified temperature for 10min followed by evacuation at the same temperature for 6 min. All spectra were recorded at room temperature.
silane addition from the spectrum recorded after silane treatment. In this representation, bands that appear positive are due to groups that have formed on the surface, while bands that appear negative are due to groups that have been removed from the surface. Parts a and c of Figure 1clearly show a differencein the reactivity of these two silanes with silica In the control experiment using PTCS, there was no change in the difference spectrum. Thus there is no reaction with the surfacehydroxyl groups, a result which agrees with our fiidings for methylchlor0~ilanes.l~In contrast, the treatment with F-PTCS showed many spectral changes. A decrease in the bands at 3747 cm-l (SiO-H) and at 978 cm-1 (Si-OH) and the appearance of a broad band centered at 1060 cm-l (Si&Si) is direct evidence of a chemical reaction between F-PTCS and the surface hydroxyl groups. HCl was also produced in the gas phase (spectrum not shown). Other bands located between 1450and 700 cm-' are due to various CF and CH modes. Two bands at 588 and 490 cm-l are S i 4 1 modes of the attached silanee6 These bands disappear when the sample is exposed to water vapor and are replaced with SiO-H bands. The amount of reaction of F-PTCS with silica was measured using the decrease in the integrated band intensity at 3747 cm-' (SiO-H). After 1 min contact time at room temperature with F-PTCS, 20% of the surface hydroxyl groups had reacted. After 10 min, this value increased to 64%. We note that a reaction of this extent with the alkyl PTCS can only be achieved at temperatures of 400 OC (see Figure lb) or by using a base to promote the reaction. We leave the discussion of bases to the section entitled Base Promoted. While there is a major difference in the spectra generated for the PTCS reaction at room temperature versus 400 OC, the spectra obtained under the same conditions for F-PTCS are very similar. All available surface hydroxyl groups reacted almost instantaneously with the F-PTCS at 400 OC compared with 64% which are reacted after 10 min contact time at room temperature. Thus, the same reaction occurs at 400 "C and at room temperature when using F-PTCS. Effect of Surface Water. It is now well established that surface adsorbed water on silica is a necessary component for adsorption of alkylchlorosilaneaat the solid/ liquid i n t e r f a ~ e . ~The ' ~ effect of adsorbed water on the reaction a t the solid/gasinterfaceis shown by addingPTCS and F-PTCS to a hydrated silica at room temperature. For PTCS (see Figure 2a), the behavior is, again,similar to that of the methylchlorosilanes reported earlier.13 Reaction with the surface water is shown by the negative band at 1620 cm-l (H20 deformation mode) and complete
Tripp et al.
3620 Langmuir, Vol. 9, No. 12,1993 .23
I
9350cd'
.I 3
.03
- 1,-. 4000
I
3050
''50
21 00 CAJ-.
200
4
Figure2. Differencespectra of (a)PTCS and (b)F-PTCSadded to a hydrated silicaat room temperature. Excess silanewas added for 1min followed by evacuation for 6 min.
hydrolysis of the adsorbed PTCS has occurred because there are no S i 4 1 modes. Although the spectrum shown in Figure 2a was obtained after evacuation of the excess PTCS, we noted that the negative band at 1620 cm-l occurred before this evacuation step. Thus the decrease in surface water is due to reaction with the PTCS and not the prolonged evacuation. The broad feature centered at 3350 cm-l (SiO-H stretching) and the band at 910 cm-l (Si-OH stretching) are due to a trisilanol adsorbed on the surface. The band which is evident at 1016 cm-l requires more explanation. It is not due to a surfaceSi-0-Si linkage because this would produce a broad band at 1060cm-l (for example, see Figure lb). In Figures l b and 2a the negative SiOH bands at 3747 cm-l have about the same intensity. If the decrease in Si-OH groups shown in Figure 2a is due to the formation of Si-0-Si surface linkages, then this should produce a strong band at 1060 cm-l. The absence of a band at 1060cm-l in Figure 2a means that few (ifany) SiOH groups condense and form a surface Si-0-Si bond. More likely, the trisilanol is strongly physisorbed to the surface through hydrogen bonding interactions with the surface hydroxyl groups. This would cause a shift to lower frequency in the band at 3747 cm-l. Thus, the broad band at 3350 cm-l is, in part, due to these hydrogen bonded surface hydroxyl groups. The band at 1016 cm-l could be an artifact arising from the superimposition of the negative Si-OH band at 978 cm-l on a broad positive band in the spectrum but it is more likely due to an Si-0-Si mode arising from polymerization of the trisilanol which is physisorbed on the surface. We will show in the Base Promoted section that an Si-0-Si network extending outward from the surface gives rise to a similar narrow band in this region. It is noted that the amout of PTCS or F-PTCS adsorbed on a hydrated silica (measured from the CH and CF band intensities) is about twice the amount obtained for the reaction at 400 "C with a dehydrated silica. Thus, a multilayer is formed on the hydrated silica and this is consistent with the assignment of the band at 1016 cm-l to a Si-0-Si network extended outward from the surface. Most importantly, however, Figure 2a shows no evidence of a broad band at 1060cm-l due to a surface Si-0-Si mode and this shows that condensation with the surface hydroxyl groups has not happened. For F-PTCS (Figure 2b), conversion to the trisilanol by reaction with the surface water also occurs and this is evidenced by the appearance of bands at 3350 and 910 cm-l, the absence of bands due to S i 4 1 modes, and the disappearance of the HzO deformation mode at 1620cm-1. However, in this case an additional band at 1060 cm-l is present showing that a Si-0-Si surface bond has formed.
Again this band is distinct from the narrower band at 1016 cm-l (also present in this spectrum). The chemical attachment of the F-PTCS through a surface Si-0-Si linkage could occur either by condensation of the (fluoroalky1)trisilanolorby direct reaction of the F-PTCS with the surface hydroxyl groups. The dehydrated silica used in these experiments contains approximately 1.4 OH groups/nm2. When it is reacted with F-PTCS, 20% of the reaction (as measured by the decrease of the integrated intensity of the band at 3747 cm-l) occurs within 1 min of contact time. For the hydrated silica, however, under the same experimental conditions, 80% reaction occurs within 1min. Aa the ratios of the band intensities 3747 to 1060 cm-l are identical in both cases, this means that 80% of the isolated SiOH groups on the hydrated silica are converted to surface S i 4 S i bonds within 1min of contact time. In essence, the presence of adsorbed water and subsequentconversion of F-PTCS toa silanol incrsaeee the rate of formation of the surface Si-0-Si bond, most likely by the condensation of the fluoroalkylsilanol with the surface SiOH groups. Base Promoted. It is a common practice in the preparation of alkylchlorosilane-treatedsilica surfaces to add a nitrogen-containingbase to the solution to promote the chemical attachment of the silane to the surface." In homogeneous solutions,= it has been suggested that this reaction proceeds by a one-step nucleophilic mechanism through the formation of a pentacoordinate intermediate where Nu is a nucleophile.
R,SiCl
-
+ Nu
L
Nu
-l
The S i 4 1bonds are weakened in the intermediate making it more susceptibleto react with a second nucleophile (i.e., hydroxyl groups on the silica surface). There is a problem in controlling the reaction because polymerization in solution is also promoted (water being the nucleophile in this case). In a previous work, we have shown that solution polymerization can be suppressed by performing the basepromoted reaction using a two-step process.l9 This twostep process unfolds from the realization of a second mechanism which is alternative to that shown above. ,,.*NOW,
a
,,*E a* 0
o \
+")a
-wjl,l
a111111111
In this case, the base forms a strong hydrogen bond to the surfacehydroxyl groups rendering the S i 4 group more nucleophilic and leading to a reaction with the incoming chlorosilane. Attachment of the base to the surface rather than to the silicon atom of the chlorosilane means that the base can be preadsorbed prior to addition of the chlorosilane. Removal of the excess base before chlorosilane treatment ensures that reaction occurs with the surface and that solutionpolymerizationis avoided. By extending the two-step process, we have shown from infrared (23)Corriu, J. P.;Guerin, C. J. Organomet. Chem. 1980, 198, 231.
Reaction of Alkylchlorosilanes with Silica
Langmuir, Vol. 9,No. 12,1993 3521
-1
(b)PTCS
b -.16C
-.3d
-.44
I 3050 2100 1150
4000
200.
-.06
CM-I
Figure 3. Difference spectra of (a)F-PTCS and (b) HC1 added to a dehydrated silica predoped with TEA. Excess TEA was
added at room temperaturefor 1min, followed by evacuation for 5 min at 1o-B Torr then by addition of excess F-PTCS or HCl for 1 min followed again by evacuation at 1o-B Torr for 5 min. (c) b - a and (d) same as Figure IC.
spectroscopic evidencelgthat a multilayered silane can be tailored on the surface in a well-defined manner. Figure 3 showsthe spectrum obtained for the reaction of F-PTCS with the silica surface using triethylamine as the base in such a two-step process. The curve shown in Figure 3a includes the bands due to the triethylammonium chloride produced by the combination of HC1 and the adsorbed TEA. The HC1is generated as a byproduct of the reaction of the chlorosilane with the hydroxyl groups. In a separate experiment, HC1 was added to a TEA-treated silica and the spectrum shown in Figure 3b was obtained. Figure 3c is the result of subtracting Figure 3b from Figure 3a and (after the NRsHCl is removed) is identical to the spectrum produced by direct reaction at room temperature (replotted as Figure 3d). The NR3HC1 salt can be removed by sublimation at elevated temperature or by washing with methanol. Although we have shown that (fluoroalky1)chlorosilanes react directly with the surface hydroxyl groups, the use of a base to promote the reaction offers some advantages. Not surprisingly, we find that the two-step process results in an instantaneous reaction with the surface hydroxyl groups, and this is to be compared with the 647% conversion that was obtained after 10-min contact time in the direct reaction at room temperature. A fast reaction did occur with surface water, but this was accompanied by multilayer buildup. The fast reaction of the two-step base promoted process, however can be used to build a multilayered silane coating of the (fluoroalky1)chlorosilane that extends from the surface in a well-defined manner. The details of the multistep process are described in detail elsewhere.19 After the initial application of the two-step process the silanized surface is then exposed to TEA/H20 (50:l)to hydrolyze the residual Si-C1 bonds to Si-OH groups. These new Si-OH groups can now participate in further reactions with the chlorosilanes. The two-step process can then be repeated to produce multiple silane layers extending from the surface. This is shown in Figure 4 for F-PTCS and PTCS. The uptake of silane is shown by the increase in CH and CF modes in the 1500-1200 cm-’ region. In the initial silane treatment a broad band at 1060 cm-1 (Si0-Si surface bond) is produced. With subsequent treatments there is an increase in an Si-0-Si mode at 1010 cm-I that is consistent with the formation of a polysiloxane network. Role of Fluoroalkyl Group. It is clear from these results that substitution of a fluoroalkyl group for an alkyl
!
I
1325
1600
1050
775
500
CM-1
Figure 4. Difference spectra the multistep base promoted reaction of (a) F-PTCS and (b) PTCS with a dehydrated silica. The fiist spectrum in each series was recorded after addition of excess TEA, evacuation for 5 min at 1o-BTorr, and then addition of excess silane. Thie was followed by addition of a 1:50 H?O/
TEA mixture,evacuation for 5min at lOBTorr, and then addition of excess silane and further evauation at 1o-BTorr for 5 min to generatethe second spectrumin each series. Subsequentspectra were generated by repeating the steps outlined for the second spectrum. The spectralcomponents of the salt have been digitally eliminated from the spectra. Table 1. SiC1 Bond Lengths (A)
CHxSiCLq
PTCS
F-PTCS
2.061 2.061 2.061
2.062 2.062 2.065
2.058 2.068 2.056
~
group on the y-C greatly affects the reactivity of the chlorosilane with the silica surface. Steric effects cannot account for this difference in reactivity because CFsCH2CH2 is about the same size as CH~CH~CHZ. Nor can the effect of the fluoroalkyl group be explained from a comparison to the role of an amine in promoting the reaction. We recall that amines can promote the reaction in two ways. One way stems from the hydrogen bonding interaction between the amine and the surface hydroxyl groups which leads to a more nucleophilic Si-OH. The strong hydrogen bond produces a large shift in frequency in the band due to surface hydroxyl groups. For example, the adsorption of TEA shifts the infrared band at 3747 cm-I about 900 cm-l to lower frequency. In contrast, adsorption of F-PTCS or PTCS shifts the band at 3747 cm-l only about 100 cm-l to lower frequency. This weak physisorption cannot accout for the different reactivity of PTCS and F-PTCS. The second way for promotion of the reaction arises from the attachment of the amine to the silicon atom of the chlorosilane and the formation of a pentacoordinate intermediate. This results in a weaker Si-C1 bond enabling a reaction with the surface hydroxyl groups. However, an opposite inductive effect is expected for a fluoroalkyl group; the CFSCHZCHZis electron withdrawing with respect to a C H ~ C H ~ C group H Z and this would result in a shorter S i 4 1 bond in the (fluoroalky1)chlorosilane. Semiempirical (AM1) calculations confirm this shorter Si-C1 bond for F-PTCS. (See Table 1.) A shorter Si-C1 bond in F-PTCS is also consistent with the measured and calculated S i 4 1 frequencies. These are given in Table 2. Methyltrichlorosilane belongs to point group C30 (all three Si-Cl bonds are 2.061 A) resulting in a double degenerate Si-C1 asymmetric band at 577 cm-l and a nondegenerate symmetric S i 4 1 band a t 458 cm-l. For F-PTCS and PTCS, the Si-C1 bonds are not equal in length and thus the degeneracy arising from the CgU
3522 Langmuir, Vol. 9,No. 12,1993
Tripp et al.
Table 2. Measured and Calculated S i c 1 Frequencies (cm-1) PTCS F-PTCS ~~
S i - C b . Itr.
Si-Cb,. Itr.
IR
calc
IR
calc
596 518 480
610 601 512
605 582 410
618 606 489
symmetry is removed. The Si-C1 asymmetric stretching mode is split and appears as two bands a t 605 and 580 cm-1 in F-PTCS and a t 598 and 576 cm-l in PTCS. The corresponding symmetric stretching modes are located at 470and 480 cm-I, respectively. Smithz4has reported that an increase in the electron-withdrawing ability of a group attached to the silicon atom of a trichlorosilane produces a shift to higher frequency in the Si-C1 asymmetric stretching mode and a lower frequency in the symmetric stretching mode. This trend agrees with the observed SiC1frequenciesand those from Hartree-Fock self-consistent field frequency calculations shown in Table 2. Although the shorter Si-C1 bond is clear evidence of an inductive effect of the fluoroalkyl group, it does not explain the difference in reactivity of PTCS and F-PTCS. However, a possible explanation can be derived from earlier work of Stewart and Pierce.z6 In the acid-catalyzed hydrolysis of alkylsilanes they found that a CF3 substitution on the y-C still exerts a large inductive effect on the silicon atom. (24) Smith, A. L. J. Chem. Phys. 1953,21, 1997. (25) Steward,0. W.; Pierce, 0. R. J. Am. Chem. SOC.1959,81, 1983.
They reported that the rate of hydrolysis of R(CH3)zSiH was 59 times higher when R = CF~CHZCHZand 8.3times as compared to when R = when R = CF~CHZCHZCHZCH~CHZCHZ-.Stewart and Pierce explained this difference in terms of a polarization of the silicon by highly polar substituents. This is a plausible explanation for the reaction of F-PTCS with silica. We suggest that the electron-withdrawing effect of the fluoro group makes the incoming Si atom a better electrophile for nucleophilic attack by the oxygen atom of the surface SiOH group. Summary In contrast to alkylchlorosilanes, y-substituted (fluoroalky1)chlorosilanes react with surface hydroxyl groups at room temperature to form a surface Si-0-Si bond (eq 1). This is explained in terms of an inductive effect of fluorinated substituent on the 7-C. The presence of surface water causes hydrolysis of the fluoroalkylsilane to silanols which then undergo a condensation reaction with the surface hydroxyl groups. Nitrogen-containing bases can be used to promote the reaction of the y-substituted (fluoroalky1)silane with the surface and can be used repeatedly to build a multilayered silane network extending out from the surface. Acknowledgment. C.P.T. is indebted to T. Kavassalis, J. Bareman, A. Elmer, and F. Torres for help along the way in performing the calculations. Special thanks to P. Kazmaier for his helpful discussions, his patience, and guidance through the software.
