Langmuir 1996,11, 136-142
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Chemical Vapor Surface Modification of Porous Glass with Fluoroalkyl-FunctionalizedSilanes. 2. Resistance to Water Hiroaki Tada* and Hirotsugu Nagayama Central Research Laboratory, Nippon Sheet Glass Co. Ltd., 1, Kaidoshita, Konoike, Itami, Hyogo 664, Japan Received November 18, 1993. In Final Form: October 17, 1994@ Smooth and porous glass surface substrates were modified with fluoroalkyltrichlorosilanes (CFdCFdnCH2CHZSiCl3, n = 0 , 3 , 7 , FAS) by means of chemical adsorption from the gas phase to give rise to highly hydrophobic surfaces. The hydrophobicity was found to increase with increasing molecular size of the adsorbates, accompanied by the more regular molecular orientation confirmed by the analysis of the X-ray photoelectron spectra (XPS).The resistance of the FAS layer to liquid H20 was examined by following the variations in the contact and critical tilting angles with time after immersion in boiling HzO. In all the samples tested, the contact angle (e,) decreases and the critical tilting angle (a)increases with time. For the same FAS treatment, the decreasing rate of Os is much smaller in the surface porous substrate, while the increasing rate of a is greater. These noticeable facts were rationalized in terms of the data of Fourier-transformedinfrared attenuated total reflection and XPS measurements. Also, the former data reveal that the FAS molecules are anchored via interfacial Si,-0-Si bonds and intermolecular Si,-0-Si, bonds in the monolayer formation process and partially desorbed by hydrolysis during immersion in HzO. Molecular orbital calculationsof the clusters employed as a model of the FAS molecule chemically bonding to the surface of Si02 were also carried out in order to obtain information on the electronic distribution and the reactivity with HzO, suggestingthat the crucial step of the deterioration is the nucleophilic attack of the interfacial Si atoms accompanied by cleavage of the intermolecular Si,-0-Si, bonds. On the basis of these results, the deterioration mechanism was discussed.
Introduction Basic investigations on the mechanism of the reaction on solid surfaces are of great importance in that they should contribute to a fundamental understanding of the heterogeneous catalytic reaction, surface modification,and corrosion. In an analytical sense, adsorbents with surface asperity have the advantage of smooth surfaced ones, since the former provide greater signal intensity of the adsorbates due to an increase in the surface area, while it becomes difficult to elucidate the structure characterizing the molecular assembly, particularly the orientation. In the field of the solid surface modification, the self-assembly method has usually been used for the preparation of robust functional ultrathin films on smooth and flat oxidel or gold2surfaces. More recently, it has been spectroscopically indicated that the monolayer of (fluoroalky1)trichlorosilanes (FAS) is formed not only on the external surface but on the internal surface of the pores of the poroussurfaced glass (PSG)having an excellent antireflectivity by the chemical vapor surface modification (CVSM) m e t h ~ d From . ~ the viewpoint of application, having high HzO and oil repellency due to the low surface free e n e r d and the surface asperity, the surface-treated glass would promise the improvement of the energy conversion efficiency of solar cells and solar collectors with its use as their front glass.5 The process of monolayer formation has been studied extensively to clarifythat the physisorbed HzO on the chemisorbed HzO, i.e., surface Si-OH (Sis@
Abstract published in Advance ACS Abstracts, December 15,
1994. (1)(a)Maoz, R.;Sagiv,J. J . Colloid Interface Sci. 1984,100,465. (b) Wasserman, S. R.; Tao, Y. T.; Whitesides, G . M. Langmuir 1989, 5 , 1074. (2) (a) Nuzzo, R.G.;Zegarski, B. R.; Dubois, L. H. J . Am. Chem. SOC. 1987,109, 733. (b) Bain, C. D.; Troughton, E. B.; Tao, Y. T.; Evall, J.; Whitesides, G . M.; Nuzzo, R. G. J . Am. Chem. SOC.1989, 111, 321. (3) Tada, H.; Nagayama, H. Langmuir 1994,10,1472. (4) Linder, E.; Arias, E. Lungmuir 1992, 8, 1195. ( 5 ) basad, A.; Balakrishnan, S.;Jain, S.EL;Jain, G. C. J . Electrochem. SOC.1982, 129,596.
OH) groups, plays a vital role in the properties such as molecular orientation and packing density.6 In spite of the importance for the practical use in the outdoors, however, the investigation on the resistance of the molecular layer to HzO has been quite limited.' "his study is concerned with the effect of the surface roughness of the substrate and the molecular length of FAS on the stability of the FAS-treated glass in H20. "he deterioration mechanism was further discussed on the basis of the experimental and theoretical molecular orbital (MO) calculation results. Experimental Section PSG (ARglass, Nippon Sheet Glass Co.) and smooth-surfaced glass (SSG)substrates were soaked in an alkaline solution (pH = 13.7) for 30 s and subsequently rinsed by sonification in pure HzO (conductivity < 1pS cm-1) for 10 min. In order t o minimize the influence of alkali metal ions on the HzO resistivity of the FAS layer, a Si02 film (thickness = ca. 50 nm) was coated on soda-lime-silicateglass substrates in the SSG samples and most of alkali metal ions are leached out by an HzSiFG acid solution in the PSG sample.9 The surface compositions (0:Si:Na:F)of SSG and PSG were semiquantified by the XPS analyses to be 61.7:34.8:1.5:2.0and 63.8:33.9:1.5:0.9,respectively. The pore size of PSG was determined by the Cranston and Inkley method t o be distributed in a wide range between 1.8and 10 nm around a main peak of ca. 3.5 nm1.3After the substrates (38 mm x 40 mm)had been set in avacuum chamber (2.83 L),it was evacuated till the pressure reached ca. 10 Torr and heated to 80 "C. FAS of ca. 50 pL was dosed and allowed to react with the substrates for 1 h. Then the temperature was raised to 99 "C, with evacuationfor an additional 1h to remove the physically adsorbed HF"I'S. ('I'rifluoropropylXrichlorosilane (CF3CHzCHzSiC13,TFTS, > 98%, Shin-Etsu Chem.), (nonafluorohexy1)trichlorosilane (CF3(CFz)&HzCHzSiCl3,NFTS, >98%,Shin-EtsuChemical)and (6) Angst, D. L.; Simmons, G . W. Lungmuir 1991, 7, 2236. (7) Wei, M.; Bowman, R. S.; Wilson, J. L.; Morrow, N. R. J . Colloid Interface Sci. 1993, 157, 154. ( 8 )Nagayama, H.; Honda, H.; Kawahara, H. J.Electrochem. SOC. 1988,135, 2013. (9) Thomsem, S. M. RCA Reu. 1951, 143.
0743-7463/95/2411-0136$09.00/00 1995 American Chemical Society
Modification of Porous Glass with (Fluoroalky1)silanes (heptadecafluorodecy1)trichlorosilane(CF3(CF2)&H2CH2SiCl3, HFTS, > 98%,Toshiba Silicone) were used as (fluoroalky1)silane adsorbates. Adsorption-desorption isotherms of H20 were measured a t 20 and 30 “C by a computer-aided automatic gravimetric equipment with greaseless valves (BELSORP18,BEL Japan Inc.). Three pieces of PSG (5 mm x 99 mm) were put into a sample tube and preheated under Torr a t 50 “C for 3 h. The adsorbed amount of H20 was determined after the adsorption equilibrium was attained. Static contact angles (8,)were measured by using a contact angle meter (Model CA-D, Kyowa Interface Science Co.) a t room temperature (20 f 1 “C). H2O droplets with a diameter of approximately 2 mm were placed a t six positions for one sample, and the average was adopted as 8, (reproducibility within f 2.5%). Critical tilting angles (a), a t which H2O droplets on the substrate begin to slide, was determined in the same way as previously reported (reproducibility within f 3.5%). X-ray photoelectron spectra (XPS)were measured with a Shimadzu electron spectrometer (Model ESCA 750) using a Mg KaX-ray source (hv= 1253.6ev). The X-ray source was operated at 30 mA and 8 kV. The residual gas pressure in the spectrometer chamber during data acquisition was less than Torr. Incident and detected angles were fixed a t 90” and irradiation area was ca. 19.6 mm2. The binding energy scales for the fluorocarbonsamples were referenced by setting the hydrocarbon (CH,) peak maxima in the C1, spectra to 284.6 eV. The precision of the binding energy determined with respect to this standard value was within f 0.3 eV. Fourier-transformed infrared attenuated total reflection (FTIR-ATR)spectra were obtained using a FT-IR spectrophotometer (JIR-5500, Nihon Denshi Co.) equipped with a multiple ATR attachment. All spectra were measured in a sample compartment purged for 30 min with dry air. The ATR attachment was designed so that the nonpolarized incident light can be reflected between the KRS-5 plate (50mm x 20 mm x 3 mm in thickness) faces 17 times a t the average incident angle of 45”. Both sides of the internal reflection element plate were used. Spectra were recorded a t a resolution of 4 cm-l with 5 x lo3coadded scans and computed with triangular apodization. Molecular orbital calculations of model surface species were performed by the PM3 method using the program of MOPAC version 5.0.1° Figure 1shows the model clusters of the adsorbates bonding to the surface of Si02. Particular focus was on the effect of the substitution of X and Y by F atoms on the charge density distribution. In the cluster the dangling bonds of “surface (O(1,2,3))”and “intermolecular bonding (O(7)and O(8))”oxygen atoms are terminated by hydrogen atoms. The hydrogen atoms are thus used as pseudoatoms to replace the three-dimensional network (solid)and the overlaying two-dimensionalnetwork with a finite cluster model, respectively. This strategy for modeling of the surface species adsorbed chemically is based on the confirmation that the electronic state of the oxygens in Si-OH groups is comparable with that of the oxygen atoms in Si-0-Si bridges.11 Therefore, Si(4) and Si(6) express the “surface” and “(fluoroa1kyl)silane’s”Si atoms, respectively. The geometry of the cluster was optimized by the BFGS method.lO
Results and Discussion Figure 2A shows the H20 adsorption-desorption isotherm of the PSG substrate at 20 “C. It is a typical Brunauer’s type I1 isotherm where hysteresis is observed above 0.3 of PiPo. This can be explained by the capillary condensation due to the pores present on the surface.3 Figure 2B is the Brunauer-Emmett-Teller (BET) plot. Very good linearity (R = 0.999) indicates the multilayer formation of H2O. The surface area (SBET) and the monolayer volume ofH2O (V,) were determined to be 227.7 cm2g-l and 6.78 x mL g-l (stp), respectively. The (10) MOPAC Version 5.00 (QCPE No. 445), Stewart, J. J. P., QCPE Bull. 1989,9, 10. Hirano, T. JCPE Newslett. 1989, 1 (21, 36; Revised as Version 5.01 by Toyoda, J., for Apple Machintosh. (11)(a)Newton,M.D.; Gibbs,G. V. Phys. Chem. Miner. 1980,6,221. (b) Uchino, T.; Iwasaki, M.; Sakka, T.; Ogita,Y. J . Phys. Chem. 1991, 95, 5455.
Langmuir, Vol. 11,No. 1, 1995 137
Figure 1. A cluster (Si2F30&3H&Y2) modeling of the FAS molecule chemically adsorbed on the surface of Si02: cluster I, X=Y=H; cluster 11, X=H, Y=F; cluster 111, X=Y=F.
ratio of the SBET value to the apparent surface area gives a roughness factor (RF) of 25.6. The BET C constant of 31.3, increasing with an increase in the adsorption energy of the first layer, is near to the values for adsorption on porous Si02(27-29).12 Figure 2C shows the isosteric heat of adsorption (Q,,), calculated using the ClausiusClapeyron equation, as a function of coverage (8)(=VI V,,,). In the range of 8 < 1, the value of Qstdecreases with increasing 8. The adsorption of H20 is thought to occur preferentially from the siteswith larger adsorption energy. Above 8 = 1, the Qst value increases from 25 to 43 k J mot1, that is in good agreement with the heat of vaporization of H20 to the equilibrium vapor at 20 “C (44 This is consistent with the multilayer kJ formation of H2O. Figure 3A shows the CI,-XPS spectrum of the PSG substrate treated with HFTS. The profiles of the spectra of both the SSG and PSG substrates were similar. Five peaks are observed at the binding energies of 284.6,286.7, 288.3,291.6, and 294.0 eV, which are characteristic of the groups CH, (x = 1 , 2 , 3 , 4 ) , CH20, C=O, CF2, and CF3, re~pective1y.l~ Since the peaks of CF2 and CF3 are shifted toward higher binding energy with respect to those of the adsorbed hydrocarbon due to the great electronegativity of F atoms (XF = 4.01, the former peaks are clearly distinguishable from the latter ones. The atomic percent ofthevariouskindsofcarbonare31.96(C&), 7.80(CH20), 4.27 (C=O), 44.66 (CFz), and 11.31% (CF3), while the values for the HFTS molecule are 20 (CHZ), 70 (CF2), and 10% (CF3). The fact that the ratio of CH, is greater and that of CF2 in the HFTS layer is indicative of the adsorption of hydrocarbons on the surface. Also, the CH20 and C=O groups seem to belong to oily contaminations from the apparatus. For all the samples analyzed, chlorine con(12) Naono, H.; Hakuman,M. J . Colloid Interface Sci. 1991, 145, 405.
(13) Barrow, G. M. In Physical Chemistry; McGraw-Hill, New York, 1961. (14) Castner, D. G.; Lewis, K. B., Jr.; Fischer, D. A.; Ratner, B. D.; Grand, J. L. Langmuir 1993,9, 537.
Tada and Nagayama
138 Langmuir, Vol. 11, No. 1, 1995 0.04
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Table 1. Results of the Analyses of ClSSpectra for the FAS-Treated Smooth- and Porous-Surfaced Glass Samples before and after Immersion in Boiling Ha0 for 10 h
(B)
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8 (C) Figure 2. (A) HzO adsorption (closed circle)-desorption (open circle) isotherm of the PSG substrate at 20 "C; (B)BET plot; (C)isosteric heat of adsorption as a function of 8.
tained in the raw material, HFTS, could not be detected. Although it is not clear whether or not the reaction proceeds via an intermediate of the hydrolyzed HFTS, the HFTS molecules react with the isolated Si,-OH groups on the substrate surface to be immobilized through Si,0-Si bonds, as will be discussed in detail later. Table 1 shows XPS signal intensity ratios of Cls(F3)/ Cl,(Fz), Cla(F3)/Sizp,and Cls(FZ)/Sizpfor SSG and PSG substrates treated with HFTS and for the PSG substrates treated with NFTS and TFTS. The ratios of CF$CFz are
0.225 0.063 (28.0%)
0.888 0.4 (45.0%)
Before or after 10 h-boiling.
almost equal for both the samples treated with HFTS (before boiling). The values are 1.78 times greater than for the HFTS molecule (0.143),which must be approximate to that for the orientation with the molecular chain parallel to the surface plane. This fact suggests a preferred surface normal orientation of the HFTS molecules with the CF3 groups as the outermost layer. In the NFTS-treated sample, the ratio of 0.387 is 1.16 times as large as that of the NFTS molecule (0.333). Since the increase in regularity of the molecular orientation should increase the ratio of (CFdCFz (layer))/(CF$CFz (molecule)), the HFTS molecules are considered to be more highly oriented and ordered than the NFTS molecules; this can be attributed to the difference in the molecular cohesive energy the increase of which raises the packing density of the m01ecule.l~It is also noteworthy that the ratios of (15)(a) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E.D. J.Am. Chem. SOC.1987, 109, 3559. (b) Tao, Y. T. J. Am. Chem. SOC. 1993,115,4350.
Modification of Porous Glass with (Fluoroalky1)silanes
Langmuir, Vol. 11, No. 1, 1995 139 120
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CFJSi and CFdSi for the PSG sample are 1.026 and 1.024 times larger than for the SSG sample, respectively. This seems to be due to the larger surface area of the former sample (RF = 25.6). Figure 4 presents the dependence of the wetting properties on the FAS molecular length calculated by the CPK model (HFTS, 1.83 nm; NFTS, 1.26 nm; TFTS, 0.89 nm). Evidently, the 6, value increases and the a value conversely decreases with increasing molecular length. The 8, value is asymptotic to a limiting value of ca. 115" when the number of carbons is more than six. A datum of the self-assembled monolayer of CF,(CFZ)&H&H2SiCl3 on Si wafer (105", 1.58 nm) reported by DePalma and Tillman is also plotted in Figure 3.16 The difference in the surface geometry of the substrates may be mainly responsible for the discrepancy. Ohtake et al. reported a similar trend in the alkylsilane-glass system: the 8, value increases with an increase in the molecular size, reaching a plateau (76"for an aqueous solution of ethylene glycol monomethyl ether with a surface tension of 54 mN/ m) above eight carb0ns.l' Also, it was previously demonstrated in the flat and smooth substrate system that the increase in the interfacial interaction between HzO and the surface of the substrate leads to a decrease in 6, and a simultaneous increase in a.18 The variations of 6, and a with increasing molecular length suggest a decreasing trend in the interfacial interaction. The dependences of 8, and a on the molecular length can be explained in terms of a decrease in surface free energy of the substrate, which results from an increase in the molecular ordering and packing density with increasing molecular length. This is consistent with the findings of the XPS analyses described above. For the purpose of examining the resistance against liquid HzO, the SSG and PSG samples treated with HFTS were simultaneously immersed in boiling HzO. Figure 5 shows variations of 6, and a with soak time. The initial difference of 6, between PSG and SSG(-11") is ascribable to that in the surface morphology; its influence on 6, was discussed on the basis of a modified Cassie-Baxter theory in the previous papere3In both the samples, the value of 6, decreases and the value of a increases monotonously as a function of time. The similar tendency was observed Si in the 1,3,5,7-tetramethylcyclotetrasiloxane-treated wafer.18 Noticeably, the decreasing rate of 6, is much smaller in the PSG sample, while its increasing rate of a is greater. Figure 3B shows the Cl,-XPS spectrum of the PSG sample boiled in HzO for 10 h. The atomic percent (16)DePalma, V.;Tillman, N. Langmuir 1989,5, 868. (17)Ohtake, T.; Mino, N.; Ogawa, K. Langmuir 1992,8, 2081. (18)Tada, H.; Nagayama, H. J.Electrochem. SOC.1995,140,L140.
60 0
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.
ofthe various carbons are 51.82 (CH,), 17.16 (CHZO),4.58 (C=O), 22.21 (CFZ),and 4.23%(CF3). In comparison with the original values, the signal intensities of CFZand CF3 remarkably decrease and the ratios of CH,, CHzO and C-0 due to adsorbed hydrocarbon relatively increase. In the Sizp-XPSspectra [Figure 3, parts C (before boiling) and D (after boiling)], the peak due to the Si,-OH groups cannot be resolved from those of Si02 and no significant change is observed before and afier 10-h immersion in
HzO. The XPS signal intensity ratios of the SSG and PSG samples boiled in HzO for 10 h are also listed in Table 1. All the ratios decrease compared to the initial values in both the samples; however, the decrement in the PSG sample is much smaller. This result explains its smaller decreasing rate of 6, with immersion in HzO. The remarkable decrease in the ratios of CFJSi (45.0%) and CFdSi (28.0%)for the SSG sample clearly indicates the partial desorption of the FAS molecules from the adsorbent. On the other hand, in the PSG sample, 72.4% of the CFdSi ratio is maintained after 10-h boiling, whereas the CFdSi ratio decreases to 58% of the initial value. Figure 6A shows FT-IR-ATR spectrum of the untreated PSG substrate. A sharp peak at 3743 cm-l is assigned t o the stretching vibration of the isolated Si,-OH groups. In the SSG substrate, the signal intensity at 3743 cm-l did not exceed the noise level. As the calculated penetration depth of 0.46pm at 3740 cm-l under the present conditions is about 5 times larger than that of the pores on the substrate, the increasing quantity of the isolated Si,-OH groups in the PSG substrate is thought to result from its larger surface area. Assuming that the area per HFTS molecule (u)on the SSG substrate is approximate to that in a close-packing state (u = 0.33 nm2 one can estimate the surface concentration of the grafted molecules. From the RF value of the PSG substrate and the intensity ratio of the vas(CFz)IR band of the PSG sample to that of the SSG sample ( ~ 5 . 1 1it, ~is calculated to be 0.7 nm2 molecule-l. Also, the coverage of the PSG surface by HFTS molecules was estimated to be ca. 0.3 by analyzing the visible reflection spectra.2O This yields a value of 1nm2molecule-' as the surface concentration. The average surface concentration of the HFTS molecule seems to be in the range from 0.7 to 1nm2 molecule-l. The fairly low values can (19)(a) Nakahama, H.; Miyata, S.; Wang,T. T.; Tasaka, S. Thin Solid Films. 1986,141,165.(b)The validity of the assumption may be partly justified by the fact that the value of Os for the HFTS-treated SSG sample (107.6")is approximate to that for the self-assembled monolayer of CF3(CFz)&HzCH2SiC13on Si (105"). (20)Tada, H. Chem. Lett. 1994,1271.
Tada and Nagayama
140 Langmuir, Vol. 11, No. 1, 1995
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be attributed to the presence of the micropores inaccessible to the HFTS molecule. Brinen reported that the presence of the micropores on the supports reduces the XPS catalyst/ support intensity ratios when the weight percent of the catalyst is constant, in the RWC and RldAl systems.21 These findings were ascribed to the insufficiency of the amount of the catalyst for the complete coverage of the surface of the porous support. The difference between the PSG and the SSG samples in the XPS signal intensity ratios (CFdSi and CFdSi) is much smaller than that of the absorbance of the &-OH groups in the FT-IR-ATR spectra; this seems to be partly due to the smaller penetration depth of XPS and partly due to the size exclusion of HFTS molecules from the micropores.20Since H2O cannot penetrate into the pores with hydrophobic internal surface in the PSG ample,^^^^ the decrease in the contact area would suppress the attack by H2O. The superior HzO resistance can be accounted for by "the air trapping effect". Figures 6, parts B and C, show the FT-IR-ATRdifference spectra of the PSG substrate before and after HFTS treatment. In spectrum B, a negative peak a t 3743 cm-l (v(Si,-OH)) is clearly observed. In spectrum C, intense positive peaks a t 1236,1203,1145,1134, and 1113 cm-l and two weak and broad positive peaks at 1060 and 1010 cm-l appear. The former absorption peaks are due to the stretching vibrations of CF2(v(CFz)) and C(F2)-C(F& (21) Brinen, J.S. InAppZied Surface Analysis;ASTM Philadelphia, 1980. (22) Adamson, A. W. In Physical ChemistryofSurfaces, 4thed.;Wiley; New York, 1982; p 341.
(v(C-C)) bonds.23 The strong bulk mode of the PSG substrate is present below 1240 cm-'; however, it was confirmed that the difference spectrum of separate PSG substrates produced no apparent peaks in the 1100-1000 cm-l region. According to Tripp et al., the latter peaks can be assigned to the stretching vibrations of the interfacial Si,-0-Si bonds (v(Si,-O-Si)) and the intermolecular Si-0-Si bonds (v(Si,-O-SiJ), r e ~ p e c t i v e l y . ~ ~ The absorbance of the v(Si,-0-Si,) band is greater than the v(Si,-0-Si) band, which also agrees with their result. This seems to be reasonable because the ratio of the number of the Si,-0-Si, bonds to that of the Si,-0-Si bonds becomes 3 for the ideal monolayer. There is another possibility: the peaks are due to deposits which form via polymerization of HFTS in the liquid or gas phase containing H2O. However, in Figure 6D, the peak at 1035 cm-l, which was assigned to the polymer,25 is not observed. Also, the CA value of the monolayer on the SSG substrate (107.6")is comparable to that of the self-assembly monolayer with a well-ordered orientation (see Figure 3a). If the polymer is present on the surface, the CA value should remarkably decrease because of the increase in the surface free energy accompanied by the molecular disordering. Further, Wasserman et al. reported that the self-assembly film of the alkyltrichlorosilane becomes cloudy when the polymer is formed.26All the present monolayers prepared by means of the CVSM method were completely transparent. The reflectance spectrum ofthe PSG substrate has a minimum at approximately 490 nm and the wavelength was found to undergo a red-shift after the treatment with (fluoroalky1)~ilanes.~ It is noteworthy that the quantity of the red-shift is below approximately 5 nm, while it attains 80 nm when a large amount of the polymer is adsorbed. All these facts deny the possibility of polymer formation. In contrast to the liquid phase deposition, the reaction must occur selectivelyon the surface of the substrate under the present experimental conditions. All the HFTS molecules would have a tendency to adsorb on the substrate with the orientation of their hydrolyzable SiC1 groups toward the surface because of the advantage of energy owing to the resultant small surface free energy and the interfacial chemical bonds (Si,-O-Si).27 It follows that the cross-linking between the HFTS molecules adsorbed chemically rather than the polymerization of HFTS molecules in the bulk takes place preferentially. Figures 6, parts D and E, show FT-IR-ATR difference spectra of the HFTS-treated PSG substrate before and after 5-h immersion in boilingH20. Spectrum D indicates the reproduction of the partial isolated &,-OH groups consumed by the reaction with HFTS. Spectrum E also reveals decreases in the absorbances due to the Si,-0-Si and the Si,-0-Si, bonds. These findings above suggest the formation of the interfacial Si,-0-Si bonds and the intermolecular Si,-0-Si, bonds in the CVSM process and the cleavage of a part of them by hydrolysis during immersion in boiling H2O. Table 2 summarizes the results of the PM3-MO calculations. The optimized bond lengths are in good (23) (a) Peacock, C. J.; Hendra, P. J.; Willis, H. A.; Cudby, M. E. A. J. Chem. SOC.A 1970,2943. (b) Cho, H. G.;Straws, H. L.;Synder, R. G . J . Phys. Chem. 1992,96, 5290. (24) Tripp, C. P.; Veregin, R. P. N.; Hair, M. L. Langmuir 1993,9, 3518. (25) Tripp, C. P.;Hair, M. L. J . Phys. Chem. 1993,97, 5693. (26)Wasserman, S.R.; Tao, Y-T.;Whitesides, G. M. Langmuir 1989, 5. 1074. (27) Tada, H.;Nakamura, K.;Nagayama, H. J . Phys. Chem. In press.
Modification of Porous Glass with (Fluoroalky1)silanes
(ii) Process-11
Langmuir, Vol. 11,No. 1, 1995 141
(iv) procesS-IV
Figure 7. A proposed scheme of the deterioration by HzO of the sample with chemically adsorbed (fluoroalky1)silanes. Table 2. Results of the PM3-MO Calculations on the Clusters I-III cluster I cluster I1 cluster I11 X(20,21) = H X(20,21) = H X(20,21) = F param Y(22,23) = H WW3) = F Y(22,23) F Si(4)-0( 1) Si(4)-0(5) Si(6)-0(5) Si(6)-0(7) Si(6)-C(9) C(9)-C(lO) C(lO)-C( 11) Si(6)-C(9)C(lO)-C( 11) O(1) Si(4) O(5) Si(6) O(7) C(9) X(21) C(10) all) Y(22)
Interatomic Distances 1.626 1.626 1.626 1.626 1.626 1.626 1.626 1.626 1.880 1.880 1.523 1.523 1.523 1.523 Dihedral Angle -178.9 65.1 Atomic Charge -0.502 -0.507 1.230 1.242 -0.636 -0.620 1.072 1.120 -0.511 -0.502 -0.276 -0.363 -0.071 0.098 -0.099 0.256 0.315 0.281 -0.143 -0.120
1.626 1.626 1.625 1.626 1.880 1.523 1.523 60.8
-0.493 1.245 -0.612 1.204 -0.486 -0.058 -0.138 0.151 0.319 -0.118
agreement with the experimental values for si-0(1.634),28 C-C (1.53), and Si-C (1.89),29 whereas the data of the dihedral angle was not available for comparison. All the bond lengths in the molecular chain are almost invariant, regardless of the replacement of H(20,21) and H(22,23) by F atoms. On the other hand, the dihedral angle of Si(G)-c(g)-C( 10)-c( 11) is drastically changed from -178.9" (cluster I) to 65.1" (cluster 11)and 60.8" (cluster (28)Berton, T. J.; Boudjouk, P. In Silicon-Based Polymer Science A Comprehensive Resource; Zeigler, J. M.; Fearon, F. W. G.Eds.; ACS, Washington, DC, 1990; p 8. (29)Pauling,L. In The Nature of The Chemical Bond; Cornell University Press: New York, 1948.
111). Clearly, the bond of Si(6)-C(9)-C(lO)-C( 11)takes a trans-configuration in cluster I, while helical structure is stabilized in clusters I1 and 111; this is ascribable to the large repulsion between F(22,23) atoms and H(20,21) (or F(20,21))atoms. Nakahama et al. indicated experimentally that even short fluoroalkyl chains can form a helical structure in the Langmuir-Blodgett film of C10F21COOH.~~ It is well-known that the silane molecules having F atoms on a-and/or B-C atoms (C(9) and C(l0) in model I, respectively) are very susceptible to h y d r o l y ~ i s .Note ~~ that the positive charges on Si(4) and Si(6) increase and the negative charge on O(5) and O(7) decrease with the F atom substitution of X and Y. The increment of the positive charge on Si(6) is greater than that on Si(4) by a factor of 8.8, while the positive charges of Si(4)are greater than Si(6) for clusters 1-111. Also, in cluster I11 the negative charge on O(5) is 1.26 times as large as that on O(7). Since the pH of H20 used was in the range from 6 to 7, comparable amounts of OH- and H+ are present as ionic species. Consequently, the hydrolysis can be considered as occurring via the protonation of O(5) and the concurrent nucleophilic attack of Si(6) by OH- ions, Le., the deterioration of the FAS layer probably starts from the cleavage of the interfacial Si,-0-Si bonds between the molecule and the oxide surface. When Si(4)is attacked by OH- ions, the Si,-0-Si bonds are broken as well. The fact that the Silane layers are fairly resistant t0 an acid resistant to an solution, while very solution (data is not presented here), is consistent with the the discussion above, which importance of the cross-linking between the molecules for the H20 durabilit~.~' The general conclusion induced from the MO calculations of the clusters modeling the FAS molecule attached on the Si02 surface should be the (30)Noll, N. In Chemistry and Technology of Silicones; Academic Press: New York, 1968. (31)Plueddemann, E. P. In Silane Coupling Agents; Plenum Press: New York, 1982.
Tada and Nagayama
142 Langmuir, Vol. 11, No. 1, 1995 case for the present HFTS-glass system, Le., the interfacial Si,-0-Si bonds would be more easily attacked and hydrolyzed than the interfacial Si,-0-Si, bonds. It should be noted in Figure 6C-b that, despite the greater original absorbance of the 43,-0-Si,) band, the decrement in its intensity after 5-h boiling in HzO is smaller than that of the v(Si,-0-Si) band. This can also be accounted for by the conclusion of the MO calculation.
Conclusions The deterioration scheme presumed on the basis of the results described above is shown in Figure 7. HzO molecules could penetrate into the micropores incompletely covered with FAS and further into the spacing between the molecules without affecting their packing density and orientation (process I).6 Because of the capillary force, this process causes an increase in a. HzO molecules that reach the adsorbate-adsorbent interface nucleophilically attack the Si atoms participating in the intermolecular Si,-0-Si, bonds and/or the Si, atoms (process 11). The interfacial Si,-0-Si bonds would be preferentially broken (process 111). This process is borne out by the experimental (the difference FT-IR-ATR
spectrum before and after hydrolysis) and the MO calculation results. As a result, the disordering of the molecular orientation takes place and the hydrolysis further proceeds by the acceleration of the diffusion of HzO molecules to the interface. The XPS data and the decrease in 0, support the certainty of the process. Apart of the molecules that are completely hydrolyzed desorbs from the adsorbent and are replaced with HzO molecules, and the region changed to a hydrophilic one (process IV). A remarkable decrease in 8, after long-term immersion is due to this process. By the repetition ofprocesses I-IV, the FAS layer is thought to be degraded. In light of the large difference in the durability of the alkylsilane layer with different degrees of intermolecular condensation, process IV can be considered as being the crucial step for the deterioration.
Acknowledgment. The authors express their sincere gratitude to Dr. S. Tsuchihashi, K. Mitani, H. Yamamoto, K. Kinugawa, N. Hirayama, and K. Shimoda (Nippon Sheet Glass Co.) for helpful discussions and experimental support. LA930668+
