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chloro silanes
Applied Surface Science 206 (2003) 262±270
Surface chemical modi®cation of silica aerogels using various alkyl-alkoxy/chloro silanes A. Venkateswara Raoa,*, Manish M. Kulkarnia, D.P. Amalnerkarb, Tanay Sethb a
Air Glass Laboratory, Department of Physics, Shivaji University, Kolhapur 416004, Maharashtra, India Centre for Materials for Electronics Technology, Off Pashan Road, Panchavati, Pune 411008, Maharashtra, India
b
Received 21 June 2002; received in revised form 17 September 2002; accepted 7 November 2002
Abstract The experimental results on the surface chemical modi®cation of silica aerogels using various alkyl-alkoxy/chloro silane (organosilane) compounds, are reported. Silica alcogels, prepared by keeping the molar ratio of tetramethoxysilane (TMOS), organosilane compound, methanol (MeOH), water (H2O) and ammonia (NH4OH) constant at 1:0.5:14:4:3:7 10 3 respectively, were dried supercritically to obtain the aerogels. In all, 10 organosilane compounds having zero to three functional groups were used. Large variations were observed in the hydrophobic and physical properties of the aerogels depending on the type of the organosilane incorporated in the gel. The contact angle (y) increased from 958 for a monoalkyl organosilane compound such as methyltrimethoxysilane (MTMS) to 1358 for trialkyl compound such as hexamethyldisilazane (HMDZ). Tetramethylsilane (TMS) modi®cation resulted in hydrophilic aerogels. While all the surface modi®ed aerogels were found to be thermally stable up to a temperature of 275 8C, the HMDZ and other trialkylorganosilane modi®ed aerogels showed higher thermal stability (300 8C). However, the long term water intake is slightly more for the trialkylorganosilane incorporated aerogels than the monoalkylsilane modi®ed aerogels, with the exception of HMDZ. The alkylchlorosilane modi®ed aerogels have higher volume shrinkage and density and lower optical transmission than the alkylalkoxysilane modi®ed aerogels. The alkylchlorosilane resulted in cracked aerogels and alkylalkoxysilane gave rise to monolithic aerogels. # 2002 Elsevier Science B.V. All rights reserved. PACS: 82.33.Ln; 78.55.Mb; 81.70.Pg; 68.37Hk; 78.30.Ly Keywords: Aerogels; Porous materials; TGA±DTA; SEM; IR
1. Introduction As the name implies, aerogels consist of air (as high as 99%) and a solid material (as low as 1%) having a density varying from 3 to 200 kg/m3 [1]. Silica aerogels have the unusual properties of high transparency *
Corresponding author. Tel.: 91-231-690571; fax: 91-231-691533. E-mail address: [email protected] (A.V. Rao).
(>90%) in the visible range, very low refractive index (1.01±1.1), large internal surface area (>1000 m2/g) and very low thermal conductivity (0.02 W/mK) [2,3]. Further, even though the aerogels are characterized by shape, they have the atomic density as low as for a gas (1026/m3). Therefore, the aerogels have various scienti®c and technological applications such as in shock wave studies at high pressures [4], Cerenkov radiation detectors [5,6], inertial con®nement fusion (ICF) targets [7], radioluminescent devices [8], containers for
0169-4332/02/$ ± see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 ( 0 2 ) 0 1 2 3 2 - 1
A.V. Rao et al. / Applied Surface Science 206 (2003) 262±270
liquid rocket propellants [9], and micrometeorites [10], light weight thermal and acoustic insulating systems [11,12], adsorption and catalyst supports [13]. However, the as produced silica aerogels, based on the tetraethoxysilane (TEOS) or tetramethoxysilane (TMOS) precursors, are hydrophilic and become wet with the atmospheric moisture and water. Hence, the aerogels get deteriorated with time due to the adsorption of water molecules from the humid surroundings. The reason for this is that the aerogels possess on their surface, polar OH groups that can take part in hydrogen bonding with H2O. Therefore, with appropriate surface chemical modi®cation, the surface of the aerogel can be rendered hydrophobic so that the water molecules will be repelled [14]. One way to achieve this is to substitute the hydrogens of the OH groups by attaching some organic groups (alkyl or aryl), thus hydrophobicizing the aerogel surface. Several hydrophobic agents can be used for this purpose [15]. In the present experiments, we used various hydrophobic agents having zero to three functional groups. We report in this paper the experimental results on the hydrophobic and physical properties of silica aerogels enhancing the stability of the aerogels and demonstrating the structure±property relationships. The aerogels have been characterized by scanning electron microscopy (SEM), contact angle, water adsorption, infrared spectroscopy, thermogravimetry (TGA) and differential thermal analyses (DTA). 2. Experimental 2.1. Sample preparation Silica alcogels were prepared by mixing tetramethoxysilane precursor, methanol (MeOH) solvent and water (H2O) containing ammonia (NH4OH) catalyst in the molar ratio of 1:14:4:3:7 10 3 respectively. In order to compare the various properties of the aerogels, the organosilane/TMOS molar ratio was kept constant at 0.5. The reason for choosing the value of 0.5 is that sols containing chloro compounds took long time (a few months) to set for higher organosilane/ TMOS molar ratio. Moreover, the sols containing HMDZ set while stirring for higher values of HMDZ/TMOS and even for lower values of H2O/ TMOS molar ratio. In all, 10 organosilane compounds
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were used and they are: methyltrimethoxysilane (MTMS), dimethylchlorosilane (DMCS), dimethyldichlorosilane (DMDC), trimethylbromosilane (TMBS), trimethylchlorosilane (TMCS), trimethylsilylchloroacetate (TMSCA), trimethylmethoxysilane (TMMS), trimethylethoxysilane (TMES), tetramethylsilane (TMS) and hexamethyldisilazane (HMDZ). All the chemicals used were of ``purum'' grade from Fluka Company, Switzerland, except for the methanol, which was of GR grade (Glaxo Company, India). The silica alcosols were prepared in a 250 ml beaker and transferred in to 15 mm diameter test tubes as well as 30 mm diameter circular molds and they were tightly covered. After aging for one day, the gels were dried supercritically using methanol separately for each organosilane in order to avoid the in¯uence of pore ¯uid of one gel on the other. 2.2. Characterization Bulk density of the aerogel samples was calculated by measuring its weight to volume ratio. The porosity was calculated using the formula rb porosity % 1 100 (1) rs where rb is the bulk density and rs the skeletal density of the silica aerogels. rs was measured using helium pycnometry and its value was found to be 2.1 gm/cm3. The volume shrinkage of the samples was calculated by measuring the difference between the volumes of the alcogel and the aerogel. Aerogel samples of 10 mm thickness were used to measure the optical transmittance at 700 nm using Systronics 119 spectrophotometer. The contact angle (y) of the aerogel samples was measured by putting a water drop of 10 ml on the horizontal sample surface. From the measurements of base length (d) and the height (h) of the drop, `y' was calculated using the formula [16] 2h y 2 tan 1 (2) d `y' was also measured directly from the photograph of the drop. Unfortunately, the formula for the contact angle given in one of our earlier publications [17] is a mistake. The water adsorption studies were performed by putting the aerogel samples directly on the water surface and observing the weight increase of the sample with time using a microbalance having
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10 5 gm accuracy. The thermal stability of the aerogels was studied by thermogravimetric and differential thermal analyses (TG-DTA) using SDT 2960 TA Universal instruments, USA. Here the term thermal stability refers to the temperature up to which the aerogel retains its hydrophobicity. The surface modi®cation was con®rmed using infrared spectroscopy using Perkin-Elmer (model no. 783) IR spectrophotometer. The microstructure of the aerogels was observed using scanning electron microscope (SEM) analyzer (Model: Philips XL-30). 3. Results The results are divided into two sections, ®rst section describes the physical properties and second section describes the hydrophobic properties of the aerogels.
Table 1 shows the physical properties and Table 2 shows the hydrophobic and thermal stability of the silica aerogels prepared using various organosilanes. 3.1. Physical properties It can be seen from the Table 1 that when HMDZ was added to the sol, gelation occurred almost instantly. The pH of the sol was found to be 9, due to the amine groups present in the HMDZ, which increases the condensation rate and hence gelation takes place rapidly. The chlorine containing organosilanes like DMDC, TMCS, etc. react with the surface OH groups and water to yield HCl as per the following chemical reactions [18,19] BSi OH Clx Si R4 x ! BSi O Si R3 x Clx
1
HCl
(I)
Table 1 Physical properties of silica aerogels prepared using various organosilane compounds Hydrophobic compound
Gelation time Tg (h)
Bulk density, rb (gm/cm3)
Porosity P (%)
Transparency T (%)
Volume shrinkage Vs
Remarks
MTMS DMCS DMDC TMCS TMSCA TMBS TMMS TMES TMS HMDZ
2.5 3.7 12 72 5 months 70 1.6 2.1 2.1 0.1
0.1315 0.1833 0.1955 ± ± 0.2346 0.1391 0.09696 0.09674 0.1253
93.73 91.27 90.69 ± ± 88.82 93.37 95.38 95.39 94.03
88 10 8 5 5 7 80 90 91 70
12.39 28.90 32.77 ± ± 47.96 23.72 20.19 15.18 28.29
Monolithic Pieces Pieces Powder Powder Cracked pieces Monolithic Monolithic Monolithic Multiple cracks
Organosilane/TMOS molar ratio 0.5. Table 2 Hydrophobic and thermal properties of silica aerogels prepared using various organosilane compounds Hydrophobic compound
Abbreviation
Methyltrimethoxysilane Dimethylchlorosilane Dimethyldichlorosilane Trimethylchlorosilane Trimethylsilylchloroacetate Trimethylbromosilane Trimethylmethoxysiane Trimethylethoxysilane Tetramethylsilane Hexamethyldisilazane
MTMS DMCS DMDC TMCS TMSCA TMBS TMMS TMES TMS HMDZ
Contact angle y (8) Photographic method
Calculated
Weight increase in aerogel after keeping for 3 months in water
95 98 120 ± ± ± 125 102 0 135
94 100 119 ± ± ± 123 100 0 133
7% 13% 7% Hydrophobic powder Hydrophobic powder 9% 9% 12% >100% 5%
Thermal stability (8C) 277 275 298 290 300 300 295 300 <100 300
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R4 x SiClx H2 O ! R4 x SiClx 1 OH HCl (II) where x 1, 2 or 3 and R is an alkyl group. This HCl makes the sol acidic (pH 6), and hence gelation time increases [20]. Thus the gelation time for the sol with TMSCA was found to be about 5 months, but the sol containing HMDZ sets within a few minutes. The aerogels prepared using MTMS and TMS were found to shrink less (volume shrinkage (Vs) <15%), whereas the shrinkage for the aerogels prepared using TMBS (Vs 48%), and DMDC (Vs 32%) was more as compared to the other aerogels. The change in the volume shrinkage affects the bulk density and porosity of the aerogels. Thus the aerogels prepared using TMS and MTMS were found to have lower bulk density around 0.1 gm/cm3 and higher porosity around 95% and the aerogels prepared using TMBS had a bulk density of around 0.23 gm/cm3 and the porosity is around 88%. The optical transmittance measurements show that the aerogels prepared using organosilanes containing halides were almost opaque (transparency <8%) and the aerogels prepared using alkylalkoxysilanes were more transparent (>80%). Fig. 1(a±c) shows the SEM microstructure of some of the aerogels. It is clear from the ®gure that the DMDC modi®ed aerogels have a compact microstructure with smaller clusters but pores with non-uniform sizes, whereas bigger and round shaped clusters with almost uniform pore sizes are observable in the SEM of TMMS modi®ed aerogels. The microstructure of HMDZ is seen to be very porous and loosely packed with smaller clusters. The alkylalkoxysilane modi®ed aerogels are monolithic whereas the alkylchlorosilane modi®ed aerogels have been found to be cracked. In the case of TMSCA and TMCS, only powdered samples could be obtained. Addition of HMDZ resulted in internally cracked aerogel pieces of sizes 1±2 cm. 3.2. Hydrophobic and thermal properties Fig. 2(a±c) shows TG-DT analyses of some of the aerogel samples. It was observed that in the oxygen atmosphere the samples showed less thermal stability (300 8C, Fig. 2(a)) as compared to the inert (nitrogen) atmosphere where the aerogels were thermally
Fig. 1. SEM microstructure of aerogel samples prepared using different organosilanes: (a) DMDC; (b) TMMS; (c) HMDZ.
stable nearly up to 500 8C (Fig. 2(b)). The TGA and DTA studies in oxygen environment revealed that all the modi®ed aerogels, except for TMS, are thermally stable up to a temperature of at least 275 8C. However, the TMS modi®ed aerogels are hydrophilic even at ambient temperature. IR spectroscopic investigations shown in Fig. 3 indicate that apart from the Si±O±Si absorption band at 1100 cm 1 and O±H absorption bands observed at
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Fig. 2. Thermogravimetric and differential thermal analyses (TGA±DTA) of the aerogel samples modi®ed using: (a) DMDC in oxygen atmosphere; (b) DMDC in nitrogen atmosphere; (c) HMDZ in oxygen atmosphere.
around 1600 and 3500 cm 1, for unmodi®ed aerogel samples, there are additional absorption bands at 2900 and 1450 cm 1 related to C±H bonds and at 840 cm 1 related to Si±C bonds for all the modi®ed aerogels except for TMS [21,22]. It can be seen that as the number of alkyl groups in the organosilane modi®er increases, the intensity of the absorption bands due to Si±C and C±H increases indicating a better surface modi®cation. These facts are also seen to be re¯ected
in the contact angle studies depicted in Table 2 and Figs. 4 and 5(a±c). The contact angle increases with increase in the number of alkyl groups present in the organosilane. Correspondingly, the water adsorption studies (Fig. 6) showed that the initial weight increase (3%) is less for trialkylorganosilane modi®ed aerogels. But when exposed to water for a very long time (>2 months), the trialkylorganosilane modi®ed aerogels absorb more water and hence the weight increase
A.V. Rao et al. / Applied Surface Science 206 (2003) 262±270
267
Fig. 3. Infrared (IR) absorption spectra of the aerogels modi®ed using: (a) DMDC (b) TMMS; (c) HMDZ, co-precursors.
Fig. 4. Photograph showing a water droplet placed on a hydrophobic silica aerogel sample modi®ed by TMMS.
(>9%) is more than that of monoalkyl organosilane modi®ed aerogels, which is contrary to the expectation and the contact angle results. 4. Discussion It has been observed that hydrolysis and condensation rate of most of the organosilanes like MTMS, DMDC or TMMS is slower than that of TMOS [23]. Hence initially during the gelation, the cluster surface
Fig. 5. Photographs of water droplets on aerogel surfaces modi®ed using different co-precursors: (a) MTMS, (y 95 ); (b) TMMS, (y 125 ); (c) HMDZ, (y 135 ).
formation would occur by hydrolysis and condensation of TMOS species. The surface being mostly covered by OH groups, formed during the hydrolysis, is hydrophilic [24]. All the organosilane modi®ers used in the present studies have at least one BSi± OR or BSi±Cl group, except for TMS. Although this group itself is hydrolyzable, when it reacts with the
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Fig. 6. Water adsorption observed in aerogels modi®ed using various organosilanes when kept directly on water surface as a function of time.
surface OH groups of the silica clusters, it forms a layer on the surface which is non-hydrolyzable and hydrophobic as shown below
(III) where R, R0 are the organic groups. Thus it can be seen that, bulkier the organic group attached, better will be the hydrophobic covering. Also as the number of alkyl groups attached to the surface increases, hydrophobicity of the surface increases as observed from the increased intensity of the Si±C absorption band in IR spectra. Thus, trialkylsilane modi®ed aerogels show higher contact angle than that of mono or dialkylsilane compounds. In the case of HMDZ, each monomer of HMDZ consists of two trialkylsilane groups, which gets attached to the surface as discussed by Yokogawa and Yokoyama [25]. Hence HMDZ modi®ed aerogels show the highest contact angle (1358) due to its better hydrophobic covering.
However, water adsorption studies showed that upon longer exposure to water, trialkylorganosilane (except HMDZ) modi®ed aerogels absorb more water than for mono or dialkylorganosilane. This discrepancy in the contact angle and water adsorption studies indicates that, the bridging oxygen atoms may play an important role in keeping the organic groups attached to the surface. More the bridging oxygen's between organosilane and the silica cluster surface, the better the withstanding power of hydrophobic coverage against water. The water penetration in the pores due to capillary action, is resisted by the surface alkyl groups. Hence the weight increase in the case of trialkylsilane modi®ed aerogels is lesser than that of for monoalkyl silanes, initially. But some water molecules might penetrate the aerogel surface from randomly ``uncovered'' surface and attack the Si±O±Si bonds rearward as shown below
(IV)
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(V) Thus it can be seen from this model that even though the probability of water attacking the surface Si±O bonds is less in the case of trialkylsilane modi®ed aerogels, but once it cleaves the Si±O bonds, the surface protecting Si±(CH3)3 groups get completely detached from the surface rendering it hydrophilic and hence more permeable to the water molecules. This causes rapid increase in the weight of the aerogel upon longer time exposure to water. But in the case of monoalkylsilane modi®cation, whilst the penetrating capability of the water is more due to less coverage of the cluster surface, this protective cover does not get detached completely in a single step as in the case of trialkylsilane. Thus the weight increase in the case of monoalkylsilane modi®ed aerogels is almost linear with time. The role of bridging oxygens can be further understood in the case of TMS. As the TMS does not contain any OR group, hydrophobic methyl groups cannot be attached to the cluster surface according to the reaction (I) as explained earlier and leaving it hydrophilic as observed from the IR, TGA±DTA, contact angle and water adsorption studies. The TMS modi®ed aerogels do not show any adsorption band corresponding to the Si±C or C±H bonds and the contact angle was found to be zero. The TGA±DTA curves (Fig. 2(a) and (c)) show an exotherm at around 300 8C, which is connected with the oxidation of the surface alkyl groups, along with a sudden weight loss for all the aerogels except for TMS which showed a rapid weight loss below 100 8C indicating evaporation of water and other organic groups produced during condensation reactions [26,27]. The higher thermal stability of the aerogels under inert atmosphere (Fig. 2(b)) is due to the lack of oxygen. The aerogels prepared using MTMS and TMS were found to be less shrunk and hence less dense
269
whereas shrinkage for the aerogels prepared using halide containing organosilanes was more. This shows that pH is the determining factor in the case of shrinkage. For that, acidic sols led to produce more shrunk and cracked aerogels is because, in acidic sol, only polymeric chains entangled in each other, as seen from the SEM of the DMDC modi®ed aerogel shown in (Fig. 1(a)), would form a gel [28]. Earlier it was observed that addition of mineral acids like HCl to the sol led to cracked aerogels [20,29]. Low permeability of the aerogels due to the smaller particle and pore radii, causes the cracking of the aerogels [30]. According to the Cramen-Kozeny equation [31] D / 1
rr 3
(3)
where D is permeability of the aerogel. For a ®xed value of gel density r, lower radius (r) values decrease the permeability. It can also be seen that as the pore sizes are non-uniform, chlorosilane modi®ed aerogels are less transparent. Conversely, in basic sols with pH around 8, the condensation takes place faster and bigger particles linked to each other by `neck' formation leads to gelation [24]. The SEM of the TMMS modi®ed aerogel, shown in (Fig. 1(b)) represents such network. This type of network does not shrink easily. A further increase in pH leads to rapid gelation but weakened network, since the particle connectivity decreases and the particle size remains small, resulting in cracked aerogels. This is because the effective neck growth do not take place due to the very small negative curvatures between the clusters causing decrease in the dissolution and re-precipitation rates. Such a network is seen in the SEM microstructure of HMDZ modi®ed aerogels shown in (Fig. 1(c)). The more uniformity in the pore sizes of the alkylalkoxysilane and HMDZ modi®ed aerogels is responsible for the higher transparency of the aerogels. 5. Conclusions Hydrophobic silica aerogels were prepared using various alkyl-alkoxy/chloro silane modi®ers. It was observed that the pH of the sol affects the properties like gelation time and shrinkage of the aerogels. Addition of chlorosilane compounds led to acidic sols and cracked aerogels with higher shrinkage. Monolithic,
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low density, higher transparent hydrophobic aerogels could be obtained using alkylalkoxysilane modi®ers. The results of TMS surface modi®cation clearly shows that at least one functional group is essential for any organosilane compound to have the aerogel surface modi®cation. The thermal stability of all the modi®ed aerogels, except for TMS, was found to be above 275 8C. It was observed that the contact angle increases with the number of alkyl groups covering the surface. But, at the same time, as the number of bridging oxygens holding these alkyl groups decreases, the weight increase, in the case of aerogels having trialkyl covering, when exposed to water for a longer time (>2 months), is more. However, further investigations of the hydrophobic structure are necessary to understand the exact role of the bridging oxygens in holding the hydrophobic groups on the surface. Acknowledgements The project grant received from the University Grants Commission (UGC project, on Aerogels, No. F.10-68/2001 (SR-I)), and the Departmental Research Scheme under Special Assistance Program (UGCDRS (SAP)), Delhi, Government of India, is gratefully acknowledged. References [1] L. Kocon, F. Despetis, J. Phalippou, J. Non-Cryst. Solids 225 (1998) 96. [2] A.R. Buzykaev, A.F. Danilyuk, S.F. Ganzhur, E.A. Kravchenko, A.P. Onuchin, Nuclear Instrum. Methods Phys. Res. A 433 (1999) 396. [3] J. Fricke, T. Tillotson, Thin Solid Films 297 (1997) 212. [4] N.C. Holmes, H.B. Radousky, M.J. Moss, W.J. Nellis, S. Henning, Appl. Phys. Lett. 45 (6) (1984) 626. [5] P.J. Carlson, K.E. Johansson, J.K. Norrby, O. Pingot, S. Tavernier, F. Van Den Bogert, L. Van Lancker, Nucl. Intrum. Methods 160 (1979) 407.
[6] J. Pinto da Cunha, F. Neves, M.I. Lopes, Nuclear Instrum. Methods Phys. Res. A 452 (2000) 401. [7] K. Kim, K.Y. Jang, R.S. Upadhye, J. Am. Ceram. Soc. 74 (1991) 1987. [8] S.T. Reed, C.S. Ashley, C.J. Brinker, R.J. Walko, R. Ellefsoon, J. Gill, SPIE 1328 (1990) 220. [9] G.M. Pajonk, S.J. Teichner, in: J. Fricke (Ed.), Proceedings of the First International Symposium on Aerogels, Wurzburg, Germany, 23±25 September, 1985 p. 193. [10] L.W. Hrubesh, Report UCRL-21234, LLNL, Livermore, USA, 1989. [11] R. Caps, J. Fricke, Sol. Energy 26 (1986) 361. [12] D. Haranath, P.B. Wagh, G.M. Pajonk, A. Venkateswara Rao, Mater. Res. Bull. 32 (8) (1997) 1079. [13] G.M. Pajonk, Appl. Catal. 72 (1991) 217. [14] F. Schwertfeger, A. Emmerling, J. Gross, U. Schubert, J. Fricke, in: Y.A. Attia (Ed.), Sol±Gel Processing and Applications, Plenum Press, New York, 1994, p. 343. [15] U. Schubert, F. Schwertfeger, N. HuÈsing, E. Seyfried, Mat. Res. Soc. Symp. Proc. 346 (1994) 151. [16] J.J. Bikerman, in: Surface Chemistry: Theory and Applications, 2nd ed., Academic Press, New York, 1958, p. 343. [17] A. Venkateswara Rao, G.M. Pajonk, D. Haranath, Mater. Sci. Technol. 17 (2001) 343. [18] F. Schwertfeger, D. Frank, M. Schmidt, J. Non-Cryst. Solids 225 (1998) 24±25. [19] R.J. Field, E.W. Olson, J. Non-Cryst. Solids 285 (2001) 194. [20] A. Venkateswara Rao, G.M. Pajonk, N.N. Parvathy, E. Elaloui, in: Y.A. Attia, Sol±Gel Processing and Applications, Plenum Press, New York, 1994, p. 237. [21] B.E. Yoldas, J. Non-Cryst. Solids 63 (1984) 145. [22] N. Hering, K. Schriber, R. Riedel, O. Lichtenberger, J. Woltersodorf, Appl. Organomet. Chem. 15 (2001) 879. [23] F. Schwertfeger, W. Glaubitt, U. Schubert, J. Non-Cryst. Solids 145 (1992) 85. [24] R.K. Iler, in: The Chemistry of Silica, Wiley, New York, 1979. [25] H. Yokogawa, M. Yokoyama, J. Non-Cryst. Solids 186 (1995) 23. [26] A.-Y. Jeong, S.-M. Goo, D.-P. Kim, J. Sol±Gel. Sci. Technol. 19 (2000) 483. [27] S.-K. Kang, S.-Y. Choi, J. Mater. Sci. 35 (2000) 4971. [28] C.J. Brinker, G.W. Scherer, in: Sol±Gel Science Academic Press, San Diego, 1990, p. 203. [29] G.M. Pajonk, A. Venkateswara Rao, P.B. Wagh, D. Haranath, J. Mater. Synth. Process 5 (6) (1997) 403. [30] G.W. Scherer, J. Non-Cryst. Solids 145 (1992) 33. [31] A.E. Scheidegger, in: The Physics of Flow Through Porous Media, 3rd ed., University of Toronto Press, Toronto, 1974.
Surface chemical modi®cation of silica aerogels using various alkyl-alkoxy/chloro silanes A. Venkateswara Raoa,*, Manish M. Kulkarnia, D.P. Amalnerkarb, Tanay Sethb a
Air Glass Laboratory, Department of Physics, Shivaji University, Kolhapur 416004, Maharashtra, India Centre for Materials for Electronics Technology, Off Pashan Road, Panchavati, Pune 411008, Maharashtra, India
b
Received 21 June 2002; received in revised form 17 September 2002; accepted 7 November 2002
Abstract The experimental results on the surface chemical modi®cation of silica aerogels using various alkyl-alkoxy/chloro silane (organosilane) compounds, are reported. Silica alcogels, prepared by keeping the molar ratio of tetramethoxysilane (TMOS), organosilane compound, methanol (MeOH), water (H2O) and ammonia (NH4OH) constant at 1:0.5:14:4:3:7 10 3 respectively, were dried supercritically to obtain the aerogels. In all, 10 organosilane compounds having zero to three functional groups were used. Large variations were observed in the hydrophobic and physical properties of the aerogels depending on the type of the organosilane incorporated in the gel. The contact angle (y) increased from 958 for a monoalkyl organosilane compound such as methyltrimethoxysilane (MTMS) to 1358 for trialkyl compound such as hexamethyldisilazane (HMDZ). Tetramethylsilane (TMS) modi®cation resulted in hydrophilic aerogels. While all the surface modi®ed aerogels were found to be thermally stable up to a temperature of 275 8C, the HMDZ and other trialkylorganosilane modi®ed aerogels showed higher thermal stability (300 8C). However, the long term water intake is slightly more for the trialkylorganosilane incorporated aerogels than the monoalkylsilane modi®ed aerogels, with the exception of HMDZ. The alkylchlorosilane modi®ed aerogels have higher volume shrinkage and density and lower optical transmission than the alkylalkoxysilane modi®ed aerogels. The alkylchlorosilane resulted in cracked aerogels and alkylalkoxysilane gave rise to monolithic aerogels. # 2002 Elsevier Science B.V. All rights reserved. PACS: 82.33.Ln; 78.55.Mb; 81.70.Pg; 68.37Hk; 78.30.Ly Keywords: Aerogels; Porous materials; TGA±DTA; SEM; IR
1. Introduction As the name implies, aerogels consist of air (as high as 99%) and a solid material (as low as 1%) having a density varying from 3 to 200 kg/m3 [1]. Silica aerogels have the unusual properties of high transparency *
Corresponding author. Tel.: 91-231-690571; fax: 91-231-691533. E-mail address: [email protected] (A.V. Rao).
(>90%) in the visible range, very low refractive index (1.01±1.1), large internal surface area (>1000 m2/g) and very low thermal conductivity (0.02 W/mK) [2,3]. Further, even though the aerogels are characterized by shape, they have the atomic density as low as for a gas (1026/m3). Therefore, the aerogels have various scienti®c and technological applications such as in shock wave studies at high pressures [4], Cerenkov radiation detectors [5,6], inertial con®nement fusion (ICF) targets [7], radioluminescent devices [8], containers for
0169-4332/02/$ ± see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 ( 0 2 ) 0 1 2 3 2 - 1
A.V. Rao et al. / Applied Surface Science 206 (2003) 262±270
liquid rocket propellants [9], and micrometeorites [10], light weight thermal and acoustic insulating systems [11,12], adsorption and catalyst supports [13]. However, the as produced silica aerogels, based on the tetraethoxysilane (TEOS) or tetramethoxysilane (TMOS) precursors, are hydrophilic and become wet with the atmospheric moisture and water. Hence, the aerogels get deteriorated with time due to the adsorption of water molecules from the humid surroundings. The reason for this is that the aerogels possess on their surface, polar OH groups that can take part in hydrogen bonding with H2O. Therefore, with appropriate surface chemical modi®cation, the surface of the aerogel can be rendered hydrophobic so that the water molecules will be repelled [14]. One way to achieve this is to substitute the hydrogens of the OH groups by attaching some organic groups (alkyl or aryl), thus hydrophobicizing the aerogel surface. Several hydrophobic agents can be used for this purpose [15]. In the present experiments, we used various hydrophobic agents having zero to three functional groups. We report in this paper the experimental results on the hydrophobic and physical properties of silica aerogels enhancing the stability of the aerogels and demonstrating the structure±property relationships. The aerogels have been characterized by scanning electron microscopy (SEM), contact angle, water adsorption, infrared spectroscopy, thermogravimetry (TGA) and differential thermal analyses (DTA). 2. Experimental 2.1. Sample preparation Silica alcogels were prepared by mixing tetramethoxysilane precursor, methanol (MeOH) solvent and water (H2O) containing ammonia (NH4OH) catalyst in the molar ratio of 1:14:4:3:7 10 3 respectively. In order to compare the various properties of the aerogels, the organosilane/TMOS molar ratio was kept constant at 0.5. The reason for choosing the value of 0.5 is that sols containing chloro compounds took long time (a few months) to set for higher organosilane/ TMOS molar ratio. Moreover, the sols containing HMDZ set while stirring for higher values of HMDZ/TMOS and even for lower values of H2O/ TMOS molar ratio. In all, 10 organosilane compounds
263
were used and they are: methyltrimethoxysilane (MTMS), dimethylchlorosilane (DMCS), dimethyldichlorosilane (DMDC), trimethylbromosilane (TMBS), trimethylchlorosilane (TMCS), trimethylsilylchloroacetate (TMSCA), trimethylmethoxysilane (TMMS), trimethylethoxysilane (TMES), tetramethylsilane (TMS) and hexamethyldisilazane (HMDZ). All the chemicals used were of ``purum'' grade from Fluka Company, Switzerland, except for the methanol, which was of GR grade (Glaxo Company, India). The silica alcosols were prepared in a 250 ml beaker and transferred in to 15 mm diameter test tubes as well as 30 mm diameter circular molds and they were tightly covered. After aging for one day, the gels were dried supercritically using methanol separately for each organosilane in order to avoid the in¯uence of pore ¯uid of one gel on the other. 2.2. Characterization Bulk density of the aerogel samples was calculated by measuring its weight to volume ratio. The porosity was calculated using the formula rb porosity % 1 100 (1) rs where rb is the bulk density and rs the skeletal density of the silica aerogels. rs was measured using helium pycnometry and its value was found to be 2.1 gm/cm3. The volume shrinkage of the samples was calculated by measuring the difference between the volumes of the alcogel and the aerogel. Aerogel samples of 10 mm thickness were used to measure the optical transmittance at 700 nm using Systronics 119 spectrophotometer. The contact angle (y) of the aerogel samples was measured by putting a water drop of 10 ml on the horizontal sample surface. From the measurements of base length (d) and the height (h) of the drop, `y' was calculated using the formula [16] 2h y 2 tan 1 (2) d `y' was also measured directly from the photograph of the drop. Unfortunately, the formula for the contact angle given in one of our earlier publications [17] is a mistake. The water adsorption studies were performed by putting the aerogel samples directly on the water surface and observing the weight increase of the sample with time using a microbalance having
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10 5 gm accuracy. The thermal stability of the aerogels was studied by thermogravimetric and differential thermal analyses (TG-DTA) using SDT 2960 TA Universal instruments, USA. Here the term thermal stability refers to the temperature up to which the aerogel retains its hydrophobicity. The surface modi®cation was con®rmed using infrared spectroscopy using Perkin-Elmer (model no. 783) IR spectrophotometer. The microstructure of the aerogels was observed using scanning electron microscope (SEM) analyzer (Model: Philips XL-30). 3. Results The results are divided into two sections, ®rst section describes the physical properties and second section describes the hydrophobic properties of the aerogels.
Table 1 shows the physical properties and Table 2 shows the hydrophobic and thermal stability of the silica aerogels prepared using various organosilanes. 3.1. Physical properties It can be seen from the Table 1 that when HMDZ was added to the sol, gelation occurred almost instantly. The pH of the sol was found to be 9, due to the amine groups present in the HMDZ, which increases the condensation rate and hence gelation takes place rapidly. The chlorine containing organosilanes like DMDC, TMCS, etc. react with the surface OH groups and water to yield HCl as per the following chemical reactions [18,19] BSi OH Clx Si R4 x ! BSi O Si R3 x Clx
1
HCl
(I)
Table 1 Physical properties of silica aerogels prepared using various organosilane compounds Hydrophobic compound
Gelation time Tg (h)
Bulk density, rb (gm/cm3)
Porosity P (%)
Transparency T (%)
Volume shrinkage Vs
Remarks
MTMS DMCS DMDC TMCS TMSCA TMBS TMMS TMES TMS HMDZ
2.5 3.7 12 72 5 months 70 1.6 2.1 2.1 0.1
0.1315 0.1833 0.1955 ± ± 0.2346 0.1391 0.09696 0.09674 0.1253
93.73 91.27 90.69 ± ± 88.82 93.37 95.38 95.39 94.03
88 10 8 5 5 7 80 90 91 70
12.39 28.90 32.77 ± ± 47.96 23.72 20.19 15.18 28.29
Monolithic Pieces Pieces Powder Powder Cracked pieces Monolithic Monolithic Monolithic Multiple cracks
Organosilane/TMOS molar ratio 0.5. Table 2 Hydrophobic and thermal properties of silica aerogels prepared using various organosilane compounds Hydrophobic compound
Abbreviation
Methyltrimethoxysilane Dimethylchlorosilane Dimethyldichlorosilane Trimethylchlorosilane Trimethylsilylchloroacetate Trimethylbromosilane Trimethylmethoxysiane Trimethylethoxysilane Tetramethylsilane Hexamethyldisilazane
MTMS DMCS DMDC TMCS TMSCA TMBS TMMS TMES TMS HMDZ
Contact angle y (8) Photographic method
Calculated
Weight increase in aerogel after keeping for 3 months in water
95 98 120 ± ± ± 125 102 0 135
94 100 119 ± ± ± 123 100 0 133
7% 13% 7% Hydrophobic powder Hydrophobic powder 9% 9% 12% >100% 5%
Thermal stability (8C) 277 275 298 290 300 300 295 300 <100 300
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R4 x SiClx H2 O ! R4 x SiClx 1 OH HCl (II) where x 1, 2 or 3 and R is an alkyl group. This HCl makes the sol acidic (pH 6), and hence gelation time increases [20]. Thus the gelation time for the sol with TMSCA was found to be about 5 months, but the sol containing HMDZ sets within a few minutes. The aerogels prepared using MTMS and TMS were found to shrink less (volume shrinkage (Vs) <15%), whereas the shrinkage for the aerogels prepared using TMBS (Vs 48%), and DMDC (Vs 32%) was more as compared to the other aerogels. The change in the volume shrinkage affects the bulk density and porosity of the aerogels. Thus the aerogels prepared using TMS and MTMS were found to have lower bulk density around 0.1 gm/cm3 and higher porosity around 95% and the aerogels prepared using TMBS had a bulk density of around 0.23 gm/cm3 and the porosity is around 88%. The optical transmittance measurements show that the aerogels prepared using organosilanes containing halides were almost opaque (transparency <8%) and the aerogels prepared using alkylalkoxysilanes were more transparent (>80%). Fig. 1(a±c) shows the SEM microstructure of some of the aerogels. It is clear from the ®gure that the DMDC modi®ed aerogels have a compact microstructure with smaller clusters but pores with non-uniform sizes, whereas bigger and round shaped clusters with almost uniform pore sizes are observable in the SEM of TMMS modi®ed aerogels. The microstructure of HMDZ is seen to be very porous and loosely packed with smaller clusters. The alkylalkoxysilane modi®ed aerogels are monolithic whereas the alkylchlorosilane modi®ed aerogels have been found to be cracked. In the case of TMSCA and TMCS, only powdered samples could be obtained. Addition of HMDZ resulted in internally cracked aerogel pieces of sizes 1±2 cm. 3.2. Hydrophobic and thermal properties Fig. 2(a±c) shows TG-DT analyses of some of the aerogel samples. It was observed that in the oxygen atmosphere the samples showed less thermal stability (300 8C, Fig. 2(a)) as compared to the inert (nitrogen) atmosphere where the aerogels were thermally
Fig. 1. SEM microstructure of aerogel samples prepared using different organosilanes: (a) DMDC; (b) TMMS; (c) HMDZ.
stable nearly up to 500 8C (Fig. 2(b)). The TGA and DTA studies in oxygen environment revealed that all the modi®ed aerogels, except for TMS, are thermally stable up to a temperature of at least 275 8C. However, the TMS modi®ed aerogels are hydrophilic even at ambient temperature. IR spectroscopic investigations shown in Fig. 3 indicate that apart from the Si±O±Si absorption band at 1100 cm 1 and O±H absorption bands observed at
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Fig. 2. Thermogravimetric and differential thermal analyses (TGA±DTA) of the aerogel samples modi®ed using: (a) DMDC in oxygen atmosphere; (b) DMDC in nitrogen atmosphere; (c) HMDZ in oxygen atmosphere.
around 1600 and 3500 cm 1, for unmodi®ed aerogel samples, there are additional absorption bands at 2900 and 1450 cm 1 related to C±H bonds and at 840 cm 1 related to Si±C bonds for all the modi®ed aerogels except for TMS [21,22]. It can be seen that as the number of alkyl groups in the organosilane modi®er increases, the intensity of the absorption bands due to Si±C and C±H increases indicating a better surface modi®cation. These facts are also seen to be re¯ected
in the contact angle studies depicted in Table 2 and Figs. 4 and 5(a±c). The contact angle increases with increase in the number of alkyl groups present in the organosilane. Correspondingly, the water adsorption studies (Fig. 6) showed that the initial weight increase (3%) is less for trialkylorganosilane modi®ed aerogels. But when exposed to water for a very long time (>2 months), the trialkylorganosilane modi®ed aerogels absorb more water and hence the weight increase
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Fig. 3. Infrared (IR) absorption spectra of the aerogels modi®ed using: (a) DMDC (b) TMMS; (c) HMDZ, co-precursors.
Fig. 4. Photograph showing a water droplet placed on a hydrophobic silica aerogel sample modi®ed by TMMS.
(>9%) is more than that of monoalkyl organosilane modi®ed aerogels, which is contrary to the expectation and the contact angle results. 4. Discussion It has been observed that hydrolysis and condensation rate of most of the organosilanes like MTMS, DMDC or TMMS is slower than that of TMOS [23]. Hence initially during the gelation, the cluster surface
Fig. 5. Photographs of water droplets on aerogel surfaces modi®ed using different co-precursors: (a) MTMS, (y 95 ); (b) TMMS, (y 125 ); (c) HMDZ, (y 135 ).
formation would occur by hydrolysis and condensation of TMOS species. The surface being mostly covered by OH groups, formed during the hydrolysis, is hydrophilic [24]. All the organosilane modi®ers used in the present studies have at least one BSi± OR or BSi±Cl group, except for TMS. Although this group itself is hydrolyzable, when it reacts with the
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Fig. 6. Water adsorption observed in aerogels modi®ed using various organosilanes when kept directly on water surface as a function of time.
surface OH groups of the silica clusters, it forms a layer on the surface which is non-hydrolyzable and hydrophobic as shown below
(III) where R, R0 are the organic groups. Thus it can be seen that, bulkier the organic group attached, better will be the hydrophobic covering. Also as the number of alkyl groups attached to the surface increases, hydrophobicity of the surface increases as observed from the increased intensity of the Si±C absorption band in IR spectra. Thus, trialkylsilane modi®ed aerogels show higher contact angle than that of mono or dialkylsilane compounds. In the case of HMDZ, each monomer of HMDZ consists of two trialkylsilane groups, which gets attached to the surface as discussed by Yokogawa and Yokoyama [25]. Hence HMDZ modi®ed aerogels show the highest contact angle (1358) due to its better hydrophobic covering.
However, water adsorption studies showed that upon longer exposure to water, trialkylorganosilane (except HMDZ) modi®ed aerogels absorb more water than for mono or dialkylorganosilane. This discrepancy in the contact angle and water adsorption studies indicates that, the bridging oxygen atoms may play an important role in keeping the organic groups attached to the surface. More the bridging oxygen's between organosilane and the silica cluster surface, the better the withstanding power of hydrophobic coverage against water. The water penetration in the pores due to capillary action, is resisted by the surface alkyl groups. Hence the weight increase in the case of trialkylsilane modi®ed aerogels is lesser than that of for monoalkyl silanes, initially. But some water molecules might penetrate the aerogel surface from randomly ``uncovered'' surface and attack the Si±O±Si bonds rearward as shown below
(IV)
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(V) Thus it can be seen from this model that even though the probability of water attacking the surface Si±O bonds is less in the case of trialkylsilane modi®ed aerogels, but once it cleaves the Si±O bonds, the surface protecting Si±(CH3)3 groups get completely detached from the surface rendering it hydrophilic and hence more permeable to the water molecules. This causes rapid increase in the weight of the aerogel upon longer time exposure to water. But in the case of monoalkylsilane modi®cation, whilst the penetrating capability of the water is more due to less coverage of the cluster surface, this protective cover does not get detached completely in a single step as in the case of trialkylsilane. Thus the weight increase in the case of monoalkylsilane modi®ed aerogels is almost linear with time. The role of bridging oxygens can be further understood in the case of TMS. As the TMS does not contain any OR group, hydrophobic methyl groups cannot be attached to the cluster surface according to the reaction (I) as explained earlier and leaving it hydrophilic as observed from the IR, TGA±DTA, contact angle and water adsorption studies. The TMS modi®ed aerogels do not show any adsorption band corresponding to the Si±C or C±H bonds and the contact angle was found to be zero. The TGA±DTA curves (Fig. 2(a) and (c)) show an exotherm at around 300 8C, which is connected with the oxidation of the surface alkyl groups, along with a sudden weight loss for all the aerogels except for TMS which showed a rapid weight loss below 100 8C indicating evaporation of water and other organic groups produced during condensation reactions [26,27]. The higher thermal stability of the aerogels under inert atmosphere (Fig. 2(b)) is due to the lack of oxygen. The aerogels prepared using MTMS and TMS were found to be less shrunk and hence less dense
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whereas shrinkage for the aerogels prepared using halide containing organosilanes was more. This shows that pH is the determining factor in the case of shrinkage. For that, acidic sols led to produce more shrunk and cracked aerogels is because, in acidic sol, only polymeric chains entangled in each other, as seen from the SEM of the DMDC modi®ed aerogel shown in (Fig. 1(a)), would form a gel [28]. Earlier it was observed that addition of mineral acids like HCl to the sol led to cracked aerogels [20,29]. Low permeability of the aerogels due to the smaller particle and pore radii, causes the cracking of the aerogels [30]. According to the Cramen-Kozeny equation [31] D / 1
rr 3
(3)
where D is permeability of the aerogel. For a ®xed value of gel density r, lower radius (r) values decrease the permeability. It can also be seen that as the pore sizes are non-uniform, chlorosilane modi®ed aerogels are less transparent. Conversely, in basic sols with pH around 8, the condensation takes place faster and bigger particles linked to each other by `neck' formation leads to gelation [24]. The SEM of the TMMS modi®ed aerogel, shown in (Fig. 1(b)) represents such network. This type of network does not shrink easily. A further increase in pH leads to rapid gelation but weakened network, since the particle connectivity decreases and the particle size remains small, resulting in cracked aerogels. This is because the effective neck growth do not take place due to the very small negative curvatures between the clusters causing decrease in the dissolution and re-precipitation rates. Such a network is seen in the SEM microstructure of HMDZ modi®ed aerogels shown in (Fig. 1(c)). The more uniformity in the pore sizes of the alkylalkoxysilane and HMDZ modi®ed aerogels is responsible for the higher transparency of the aerogels. 5. Conclusions Hydrophobic silica aerogels were prepared using various alkyl-alkoxy/chloro silane modi®ers. It was observed that the pH of the sol affects the properties like gelation time and shrinkage of the aerogels. Addition of chlorosilane compounds led to acidic sols and cracked aerogels with higher shrinkage. Monolithic,
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low density, higher transparent hydrophobic aerogels could be obtained using alkylalkoxysilane modi®ers. The results of TMS surface modi®cation clearly shows that at least one functional group is essential for any organosilane compound to have the aerogel surface modi®cation. The thermal stability of all the modi®ed aerogels, except for TMS, was found to be above 275 8C. It was observed that the contact angle increases with the number of alkyl groups covering the surface. But, at the same time, as the number of bridging oxygens holding these alkyl groups decreases, the weight increase, in the case of aerogels having trialkyl covering, when exposed to water for a longer time (>2 months), is more. However, further investigations of the hydrophobic structure are necessary to understand the exact role of the bridging oxygens in holding the hydrophobic groups on the surface. Acknowledgements The project grant received from the University Grants Commission (UGC project, on Aerogels, No. F.10-68/2001 (SR-I)), and the Departmental Research Scheme under Special Assistance Program (UGCDRS (SAP)), Delhi, Government of India, is gratefully acknowledged. References [1] L. Kocon, F. Despetis, J. Phalippou, J. Non-Cryst. Solids 225 (1998) 96. [2] A.R. Buzykaev, A.F. Danilyuk, S.F. Ganzhur, E.A. Kravchenko, A.P. Onuchin, Nuclear Instrum. Methods Phys. Res. A 433 (1999) 396. [3] J. Fricke, T. Tillotson, Thin Solid Films 297 (1997) 212. [4] N.C. Holmes, H.B. Radousky, M.J. Moss, W.J. Nellis, S. Henning, Appl. Phys. Lett. 45 (6) (1984) 626. [5] P.J. Carlson, K.E. Johansson, J.K. Norrby, O. Pingot, S. Tavernier, F. Van Den Bogert, L. Van Lancker, Nucl. Intrum. Methods 160 (1979) 407.
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