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Surface Characterization of Silica Aerogels with Different Proportions of Hydrophobic Groups, Dried by the CO2 Supercritical Method H. El Rassy,*,† P. Buisson,† B. Bouali,‡ A. Perrard,† and A. C. Pierre†,§ Institut de Recherches sur la Catalyse, UPR-CNRS 5401, 2 avenue Albert Einstein, 69626 Villeurbanne Cedex, France, IDEAL - Laboratoire de Chimiome´ trie ERT11, University Claude Bernard-Lyon I, CPE - Baˆ t. 308, 43 Bd du 11 Novembre 1918, 69622 Villeurbanne Cedex, France, and University Claude Bernard-Lyon I, 43 Bd du 11 Novembre 1918, 69622 Villeurbanne Cedex, France Received July 15, 2002. In Final Form: October 17, 2002 Silica aerogels, dried by the CO2 supercritical method, were made from tetramethoxysilane (TMOS) and methyltrimethoxylsilane (MTMS) in different proportions to change the proportions of hydrophilic and hydrophobic surface groups. The aerogel hydrophobic or hydrophilic properties were related to their textural and surface structural characteristics. In particular, water adsorption isotherms were recorded by successive equilibrium with saturated aqueous metal salt solutions. They were compared to nitrogen adsorption isotherms. The relative proportion of Si-OH, Si-OCH3, or Si-CH3 end groups present on the gel surface was determined by nuclear magnetic resonance (NMR) of 1H. The results obtained were correlated with measurements of the contact angle with water, after equilibration of the gels at various water activity. From these data, a possible description of the mixed hydrophilic-hydrophobic nature of the surface of these materials is finally proposed.
Introduction The synthesis of hydrophobic silica aerogels has been studied for several applications and different techniques have been applied. A first type of methods consists of making first a silica aerogel with precursors such as tetramethoxysilane (TMOS) or tetraethoxysilane (TEOS). Evacuation with respect to an organic solvent at high temperature (i.e., 260 °C), which is often methanol, makes them hydrophobic in a large sense. However, drying near room temperature gives hydrophilic materials. In this case, the hydrophobic character can be enhanced by further reaction with a precursor such as hexamethyldisilazane (HMDS) at temperatures about 200 °C1. A second type of methods to achieve hydrophobicity consists of gelling, or co-gelling with TMOS or TEOS, a silicon precursor which contains at least one nonpolar side group, such as SiCH3. Co-gelation of MTMS and TMOS has been studied, for instance, by Hu¨sing et al.2 and Venkatsewara Rao and Pajonk3, as part of a program to develop transparent aerogel thermal insulation tiles for use in windows. Another precursor combination was TMOS with trimethylethoxysilane (TMES).4 Other interesting applications concern the Cherenkov counter, for instance.5 In this case, supercritical drying was carried out near 260 °C, which helped to dehydrate the aerogels, as previously mentioned. The present study is actually tied to an application in biocatalysis. Hence, the techniques described so far cannot * Corresponding author. Telephone: + 33 4 72 44 53 28. Fax: +33 4 72 44 53 99. E-mail:
[email protected]. † Institut de Recherches sur la Catalyse. ‡ Laboratoire de Chimiome ´ trie ERT11,University Claude Bernard-Lyon I. § University Claude Bernard-Lyon I. (1) Sumiyoshi, T.; Adachi, I.; Enomoto, R.; Iijima, T.; Suda, R.; Yokoyama, M.; Yokogawa, H. J. Non-Cryst. Solids 1998, 225, 369-374. (2) Hu¨sing, N.; Schwertfeger, F.; Tappert, W.; Schubert, U. J. NonCryst. Solids 1995, 186, 37-43. (3) Venkateswara Rao, A.; Pajonk, G. M. J. Non-Cryst. Solids 2001, 285, 202-209. (4) Venkateswara Rao, A.; Kulkarni, M. M.; Pajonk, G. M.; Amalnerkar, D. P.; Seth, T. J. Sol.-Gel Sci. Technol. 2002, submitted.
be used to encapsulate enzymes inside aerogels because the temperature at some step in the process is too high and would destroy the enzyme. In this case, it is necessary to apply a lower temperature supercritical drying process, such as with CO2. Indeed, silica xerogels and aerogels have recently been studied with some success, for the encapsulation of enzymes, more particularly lipases.6-9 For such an application, Reetz et al.6 showed that the hydrophobic-hydrophilic balance of the support was of great importance. Hence, it becomes necessary to analyze in more details the hydrophilic-hydrophobic nature of the surface of silica gels made in temperature conditions compatible with this kind of application. This was the aim of the present study, regarding silica aerogels made from mixtures of methyltrimethoxysilane (MTMS) and tetramethoxysilane (TMOS) and dried by the supercritical CO2 method, according to a synthesis technique derived from Schwertfeger et al.10 Experimental Procedure The materials used in this study were tetramethoxysilane (98%, MW 152.22, d. 1.033); methyltrimethoxysilane (98%, MW 136.2, d. 0.955) from Aldrich; aqueous ammonia solution (0.1 M) from R. P. Normapur-Prolabo; methanol (for analysis 99.8%, MW 32.04, d. 0.791) from R. P. Normapur; Poly(vinyl alcohol) (MW 15 000) from Fluka; metal salts from Fluka; technical grade acetone, distilled and ultrapure water prepared by a ELGA PURELAB UHQ water purification system. (5) Barnyakov, M. Y.; Bobrovnikov, V. S.; Buzykaev, A. R.; Danilyuk, A. F.; Ganzhur S. F.; Goldberg, I. I.; Kolachev, G. M.; Kononov, S. A.; Kravchenko, E. A.; Minakov, G. D.; Onuchin, A. P.; Savinov, G. A.; Tayursky, V. A. Nucl. Instrum. Methods Phys. Res., Sect. A 2000, 453, 326-330. (6) Reetz, M. T.; Zonta, A.; Simpelkamp, Biotechnol. Bioeng. 1996, 49, 527-534. (7) Gill, I.; Ballesteros, A. J. Am. Chem. Soc. 1988, 110, 8587-8598. (8) Pierre, M.; Buisson, P.; Fache, F.; Pierre, A. Biocatal. Biotransformation 1999, 18, 237-251. (9) Pierre, A. C.; Buisson, P. J. Mol. Catal., B: Enzymatic 2001, 11, 639-647. (10) Schwertfeger, F.; Hu¨sing, N.; Schubert, U. J. Sol.-Gel Sci. Technol. 1994, 2, 103-108.
10.1021/la020637r CCC: $25.00 © 2003 American Chemical Society Published on Web 12/20/2002
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Table 1. Proportions of Silicon Precursors, Solvent, Distilled Water, Additive (PVA Solution), and Gelation Catalyst (Aqueous Ammonia) for the Synthesis of Silica Aerogels MTMS content aqueous distilled PVA (molar) methanol TMOS MTMS NH3 0.1 M water solution 0% 20% 40% 60%
220 µl 220 µl 220 µl 220 µl
410 µl 328 µl 246 µl 164 µl
0 µl 82 µl 164 µl 246 µl
0 µl 0 µl 10 µl 20 µl
192 µl 192 µl 182 µl 172 µl
150 µl 150 µl 150 µl 150 µl
Table 2. Water Activity at 35 °C of Water-Saturated Hydrated Metal Salt Solutions saturated hydrated salt aqueous solution pure water H2O potassium nitrate KNO3 ammonium sulfate (NH4)2SO4 strontium chloride hexahydrate SrCl2‚6H2O magnesium nitrate hexahydrate Mg(NO3)2‚6H2O magnesium chloride hexahydrate MgCl2‚6H2O calcium chloride hexahydrate CaCl2‚6H2Oa lithium chloride hydrate LiCl‚H2O a
water activity aw
reference
1 0.893 0.798 0.682
11 11 12
0.506
11
0.325
11
0.26 at 29.4°C
13
0.117
11
Equilibrium CaCl2‚6H2O/CaCl2‚4H2O at 29.4°C.
The silica aerogels were synthesized in the following way with proportions of reactants gathered in Table 1: In a first pill, methanol and either tetramethoxysilane (termed TMOS) and methyltrimethoxysilane (termed MTMS) or TMOS only for the hydrophilic gel and 0.1 M ammonia solution were mixed for 15 min. In another pill, distilled water was mixed with 4% polyvinyl acid (termed PVA) aqueous solution for 15 min. The two solutions were then mixed with each other for approximately 2 h, so that any emulsion eventually disappeared and the whole sample became homogeneous. After mixing, the solution was left at rest for about 24 h making it possible to transform to a gel. The gels formed were soaked in acetone for another 24 h, which allowed water to exchange for acetone inside the gels. This preparatory stage was necessary to further exchange acetone for liquid CO2 followed by drying in supercritical conditions (CO2 critical point temperature and pressure Tc ) 31.4 °C, Pc ) 7.37 bar). After drying, aerogels were obtained. The adsorption of water in the gels was studied by equilibrating the gels successively with a series of saturated metal salt aqueous solutions. Each type of metal salt solution was characterized by a fixed water activity aw. For this purpose, the gel samples were placed inside a covered crystallizing dish, close to water-saturated metal salt solutions. The adsorption temperature was set at ≈35 °C in a regulated oven after desorption under vacuum during 4 h at 180 °C. The mass evolution of each sample was recorded as a function of time. Once this mass remained constant, the gel was considered to have reached equilibrium at this water activity. The salts used in this study and their water activity at saturation at 35 °C are provided in Table 2.11,12,13 The pore texture of some samples was moreover characterized by their nitrogen adsorption isotherms after desorption under vacuum during 6 h at 180 °C. For all gel samples, it was clear that only surface end groups could contain H atoms possibly present in Si-CH3, Si-OCH3, Si-OH, or adsorbed water molecules. Hence, 1H MAS NMR appeared as an interesting technique to apply to surface study. These spectra were obtained on a Bruker DSX-400 spectrometer at 400 MHz. For this purpose, the samples were spun in the magic angle at ca. 10 kHz, with pulse interval time 2 s and pulse duration 2 µs. The chemical shifts were measured using Si(CH3)4 as a reference. To quantify the 1H NMR data, the integrated (11) Wexler, A.; Hasegawa, S. J. Res. Natl. Bur. Stand. 1954, 53 (1), 19-26. (12) Nyqvist, H. Int. J. Pharm. Technol. Prod. Mfr 1983, 4 (2), 4748. (13) Young, J. F. J. Appl. Chem. 1967, 17, 241-245.
Figure 1. Adsorption isotherms of water by aerogels made with an increasing proportion of MTMS, determined by equilibrium with saturated aqueous metal salt solutions, at 35 °C. peaks were fitted by the Gauss and Lorenz curves using WINNMR and XPLOT softwares. Contact angle measurements were carried out by dropping a drop of water (1 µl) on gel surface, after equilibrating the sample at different water thermodynamic activities aw. They were performed on a Digidrop apparatus from GBX-Instrumentation able to acquire 25 images per second.
Results Preequilibration at a given water activity, of material supported enzymes, is a common procedure. In the present study, reaching equilibrium between an aerogel and a given saturated metal salt solution took from 3 to 60 days. This was therefore a lengthy process to record the water adsorption curves for a series of gels made with increasing proportions of MTMS. As can be seen in Figure 1 where these isotherms are reported, the aerogels made with an increasing MTMS proportion were of similar shapes. However, the materials made with 40 and 60% MTMS were very close to each other and adsorbed very little water at saturation. These water isotherms differ sensibly from the nitrogen adsorption isotherms in Figure 2. Up to 40% MTMS, these isotherms were of type IV corresponding to mesoporous materials.14 Actually, the pore radius analysis by the Roberts method15,16 reported in Figure 3 shows the average pore radius decreased from ≈13 nm without MTMS to ≈9 nm plus a secondary maximum near ≈2.5 nm with 20% MTMS and then to a more widespread distribution with an extreme near the Kelvin limit for 40% MTMS. The situation was quite different for 60% MTMS, which shows a type I isotherm, corresponding to a microporous material as confirmed by Figure 3. Actually, the latter isotherm (14) Brunauer, S.; Deming, L. S.; Deming, W. S.; Teller, E. J. Am. Chem. Soc. 1940, 62, 1723. (15) Roberts, B. F. A procedure for estimating pore volume and area distributions from sorption isotherms, National Meeting of the American Chemical Society 1963. (16) Lowell, S.; Shields, J. E. Powder Surface Area and Porosity; Chapman and Hall: London, 1991; p 67.
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El Rassy et al. Table 3. Specific Surface Areas Determined from the Adsorption Isotherms of Water (Asp,w), from the Nitrogen Isotherms (Asp), and Contributions to Asp,Kel and to the Total Pore Volume Vsp,Kel of the Pores with a Radius above the Kelvin Limit nitrogen isotherms
water isotherms
%MTMS
Asp (m2/g)
C
Asp,Kel (%)
Vsp,Kel (%)
Asp,w (m2/g)
Cw
0 20 40 60b
837 826 737 388b
193 67 19 -66b
80 61 58 8b
94 84 86 19b
202 153 47 15b
58.5 7.9 4 single point
a These contributions were determined from the desorption hysteresis of the nitrogen isotherms by the Roberts method.15,16 b Data given as indications. Equilibrium could not be reached with nitrogen during adsorption. Data points did not at all align well in a BET plot with water, because the material initially contained more water than that required for equilibrium at low water activity (specific area indicated is by using single-point BET at medium water activity).
Figure 2. Adsorption-desorption isotherms of nitrogen by aerogels made with an increasing proportion of MTMS.
Figure 3. Pore radius distribution curves derived from the desorption branches of the nitrogen isotherms in Figure 2, by the Roberts method.15,16
only imperfectly describes the material texture, because nitrogen adsorption was very lengthy in this case and we did not have the material possibility to wait for longer times to reach equilibrium at each data point. In practice, data acquisition for each adsorption/desorption point had to be stopped after 3 h (several days overall for the full isotherm). Even during the supposedly desorption branch, the sample was still slowly adsorbing nitrogen. Such a behavior could possibly be explained by the presence of bottlenecked pores with a very narrow opening and also by the poor wetting ability of liquid nitrogen on this type of aerogel. Consequently, the data concerning the later aerogel are reported in italics in Table 3 because their meaning is very questionable in particular regarding the negative C constant.
The water adsorption isotherms can be used to derive an estimate of the specific surface area Asp,w according to the Brunauer, Emmett, and Teller (BET) method, assuming a cross-sectional area A ) 10.8 Å2 for water as quoted by Lowell and Shields.16 The value obtained and the corresponding BET C constants are reported in Table 3, together with corresponding data from the nitrogen adsorption isotherms. In this table, the percent contribution of pores with a radius higher than that for the Kelvin limit, to the total specific surface area Asp and pore volume Vsp, derived from the desorption branches of the nitrogen isotherms, are also reported. The 1H NMR spectra (Figure 4) show four well-marked chemical shifts or groups of chemical shifts, for which the experimental values are given in Table 4. The attribution of these chemical shifts to adsorbed water, Si-OH, SiOCH3, or SiCH3 was made by comparison of the present 1 H NMR spectra with Fourier transform infrared transmission (FTIR) spectra, as well as 13C and 29Si NMR data. In summary, the peak labeled A which corresponds to a chemical shift δH(A) ≈ 6 ppm, could easily by attributed to adsorbed water according to ref 17. The group B signals in Figure 4, which are in the range δH(B) ≈ 3.3-3.6 ppm, were attributed to Si-OCH3. It correlated very well with supplementary 13C NMR and 29Si NMR data showing that an increasing proportion of MTMS was poorly hydrolyzed at high MTMS content. Peak C at δH(C) ≈ 2 ppm was attributed to Si-OH. Its decreasing intensity correlated with the decreasing absorbance of the infrared data SiOH band at ≈950 cm-1, as well as with an increasing hydrophobic group content. These hydrophobic functions are Si-CH3, which correspond to peak D with chemical shift δH (D) ≈ 0 ppm. This is consistent with the fact that this peak is absent in the gel made from 100% TMOS. The contact angle for a given gel was checked as a function of time, so the kinetics of water drop spreading on the surface was performed (Figure 5). An average of three measurements at least were carried out for each data. The points at t ) 0 for each type of aerogel equilibrated at two different water activities (0.798 and 0.325) are reported in Figure 6 together with previous data from Venkatsewara Rao et al. on similar gels but supercritically dried in methanol at high temperature (260 °C).3 (17) Taylor, R. B.; Parbhoo, B.; Fillmore, D. M. Nuclear Magnetic Resonance. In The analytical Chemistry of Silicones; Smith, L. A., Ed.; John Wiley & Sons: New York, 1991; pp 347-419.
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Figure 5. Variation of the water contact angle as a function of time for different types of aerogels equilibrated at aw ) 0.798.
Figure 4. 1H NMR spectra of aerogels made with increasing proportions of MTMS. Table 4. Chemical Shifts for the Observed 1H NMR Signals, as a Function of the Molar Ratio of MTMS Used as a Precursor molar ratio of MTMS
0
0.2
Signals Attributed to Si-OH2 δ(A) ppm 6.26 6.25
0.4
0.6
6.2
5.95
δ(B1) ppm δ(B2) ppm
Signals Attributed to Si-OCH3 3.62 3.60 3.53 3.39 3.38
δ(C1) ppm δ(C2) ppm
Signals Attributed to Si-OH 2.38 2.34 2.15 2.12 2.13
2.07
δ(D) ppm
Signal Attributed to Si-CH3 no signal 0.08 0.03
0.00
3.46
Figure 6. Water contact angle with aerogels synthesized with different molar fractions of MTMS.
Discussion The surface state of silica aerogels made from mixtures of TMOS and MTMS and dried at low temperature (≈35 °C) in supercritical CO2 is very different from that obtained from similar wet gels supercritically dried at high temperature (≈260 °C) in methanol by Venkatsewara Rao et al.3 Supercritical drying in methanol strongly dehydrates the silica surface and many residual surface groups SiOH are moreover esterified to produce Si-OMe groups.10 It is also well known that dehydration in these hightemperature conditions replaces terminal Si-OH by surface siloxane Si-O-Si bridges which are not easily rehydrated.18 Consequently, dehydration is largely ir(18) Brinker, C. J.; Scherer, G. W. Sol-Gel science. The Physics and Chemistry of Sol-Gel processing; Academic Press: New York, 1990; p 543 and 580-585.
reversible. The aerogels which are obtained are very hydrophobic and even a MTMS proportion as low as 20% is sufficient to produce monoliths which float indefinitely on the surface of water.3,10 This is not the case when drying in supercritical CO2. The proton NMR data show that a high proportion of SiOH remains in the gels made from pure TMOS and from mixtures of TMOS with 20% MTMS. These Si-OH can be vicinal, geminal, or isolated. It is not possible to relate simply the integrated area of each NMR peak to the population density of Si-OH versus Si-OCH3. Nevertheless, these relative integrated areas provide an estimate of the abundance of these surface groups. For instance, in Figure 7, the relative integrated area of the peak due to the chemical shift of Si-CH3 terminal groups increases linearly with the molar fraction of MTMS used to
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Figure 7. Evolution of the relative ratio of each terminal groups with a H atom, with the molar fraction of MTMS used as a silicon precursor. Table 5. Effective Cross Section σW,Eff of a Water Molecule and Equivalent Effective Density of Water Molecule nW,Eff per nm2. Difference Qw and QN between the Heat of Adsorption and the Heat of Desorption of Water at 30°C and Nitrogen at 77 K MTMS σw,eff mole % (nm2) 0 20 40 60
0.4465 0.5835 1.704 2.792
nw,eff molecule/nm2 2.4 1.7 0.59 0.36
Qw ) RTlnCw kJ QN ) RT mol-1 lnCN kJ mol-1 10.3 5.2 3.5
3.4 2.7 1.9
synthesize the gels and with a slope not far from 1, at least up to a molar ratio of 0.4. In these conditions, the Si-OCH3 and cumulated Si-OH + Si-OH2 terminal groups appear to be in relatively similar proportions in aerogels made from pure TMOS and up to a molar fraction of MTMS of 0.4. When the latter MTMS fraction increases to 0.6, we first observe a strong deviation from linearity regarding the area of the Si-CH3 peak. Analysis by other techniques, in particular 29Si NMR, showed this is due to the loss of about 20% MTMS during the supercritical process. Indeed, many MTMS molecules were not hydrolyzed and did not undertake condensation reactions with the neighbor precursor molecules. Hence, many MTMS monomers were lost during fluid evacuation. Moreover, many Si-OCH3 groups belonging to MTMS remained, as shown in Figure 7 where these terminal groups dominated largely over Si-OH, for a molar fraction of 0.6 of MTMS. For the latter aerogel, the major surface groups were actually Si-OCH3 and Si-CH3. To derive an estimate of the water coverage on the pore surface of the aerogels in the first adsorbed monolayer, it is possible to attempt an analysis by using the adsorption isotherms of nitrogen and water. The specific areas tabulated in Table 3 were determined using the usual value σN ) 0.162 nm2 for the cross section of an adsorbed N2 molecule while a cross section σw ) 0.108 nm2 was used for water, both cross sections being taken from reference 16. If one considers that the correct specific surface areas were determined with the nitrogen adsorption isotherms, it is possible to calculate an effective area σw,eff occupied by a water molecule in the first adsorbed monolayer. The values obtained are much larger than σw. They are reported in Table 5, together with the equivalent effective number nw,eff of H2O molecules per nm2. This procedure is equivalent to calculating the ratio Asp,w/Asp as done by Takei et al. in a comparison of nitrogen and
Figure 8. Correlation between the effective density of water molecules nw,eff calculated from the adsorption isotherms of nitrogen and water (Table 5), and the relative area of the 1H NMR peak attributed to Si-OH.
water adsorption isotherms on porous glasses.19 The heat difference Q between the adsorption heat in the contact layer with the solid (E1) and the condensation heat of the gas (EL)
Q ) E1 - EL
(1)
can be estimated from the BET constants C and the temperature T where adsorption was studied, by the following formula where R is the perfect gas constant.
Q ) RT ln C
(2)
The results are also reported in Table 5 for water (Qw) and nitrogen (QN). Obviously, water adsorption is more energetic than nitrogen adsorption. This can be understood if it occurs by the establishment of O‚‚‚H-O hydrogen bonds, which are more energetic than N‚‚‚H-O or N‚‚‚ H-C. One water molecule can also bind by two hydrogen bonds to two vicinal Si-OH or only by one hydrogen bond to one geminal Si-OH, hence a possible range of heat adsorption. It was also interesting to examine whether the effective number nw,eff of water molecules adsorbed in the first monolayer during water adsorption (determined from water and nitrogen adsorption isotherms) would correlate with the relative abundance of some Si terminal functionality, estimated from the 1H NMR data. Actually, an excellent correlation was only found with the relative abundance of silanols Si-OH, as shown in Figure 8, at the exclusion of the Si-OH2, Si-OCH3, and Si-CH3 or any combination of the four types of groups. This finding is consistent with the fact that the water molecules already present before water adsorption do not participate in the adsorption process to complete the first monolayer and that this monolayer adsorption only occurs on Si-OH sites. Moreover, the residual water already present in the gels before performing water adsorption, or measuring the contact angles, was more abundant in the gels made from a high proportion of TMOS. This may explain that up to a MTMS content of 20%, the contact angle of water did not appear to depend on the gel composition. Nevertheless, it depended on the water activity aw at which the gel had been equilibrated. Indeed, preequilibrium with a water activity of 0.33 was expected to leave a lower SiOH content than for aw ) 0.80, hence a more hydrophobic (19) Takei, T.; Yamazaki, A.; Watanabe, T.; Chikazawa, M. J. Colloid Interface Sci. 1997, 188, 409-414.
Surface Characterization of Silica Aerogels
surface at aw ) 0.33 resulting in a higher contact angle, as this was experimentally found. Conclusions The surface of silica aerogels made from mixtures of two silica precursors, TMOS and MTMS, dried with supercritical CO2, was significantly different from that known to exist after supercritical drying in methanol at higher temperature. Besides Si-CH3, Si-OH, Si-OH2,
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and Si-OCH3 functions are present in significant amount in the low-temperature aerogels. Comparison of the nitrogen adsorption isotherms, water adsorption isotherms, and proton NMR showed an excellent correlation with each other, showing that hydrophilicity was first due to Si-OH functions. The contact angle with water also depended, in the most hydrophilic aerogels, on the water activity at which they had been preequilibrated. LA020637R
