Hydrophobic properties of TMOS/TMES-based silica aerogels

Materials Research Bulletin 37 (2002) 1667±1677

Hydrophobic properties of TMOS/TMES-based silica aerogels A. Venkateswara Rao*, Manish M. Kulkarni Air±Glass Laboratory, Department of Physics, Shivaji University, Kolhapur 416 004, Maharashtra, India (Refereed) Received 3 December 2001; accepted 23 April 2002

Abstract The hydrophobic properties of tetramethoxysilane (TMOS)-based silica aerogels by incorporating trimethylethoxysilane (TMES) as a synthesis component, are described. The molar ratio of TMES/TMOS (M) was varied from 0 to 4.0 by keeping the TMOS, methanol (MeOH), water (H2O) and ammonium hydroxide (NH4OH), molar ratio constant at 1:14:4:3:7  10 3 . The hydrophobic properties of the aerogels were studied using contact angle measurements, infrared spectroscopy and thermal analysis. The contact angle, y increased from 100 to 1408 for M ˆ 0:5 to 4. While the volume shrinkage of the aerogels increased whereas the bulk density decreased with increased M values. The hydrophobic aerogels are thermally stable up to a temperature of 3008C and above this temperature the aerogels become hydrophilic. # 2002 Elsevier Science Ltd. All rights reserved. Keywords: A. Oxides; A. Surfaces; B. Chemical synthesis; B. Sol±gel chemistry; C. Thermogravimetric analysis; D. Surface properties

1. Introduction Silica aerogels are exceptional materials that are highly porous (porosity > 98%), and transparent (optical transmission > 90% in the visible range), having low density (down to 3 kg/m3) [1,2], low refractive index (1.01) and surface area as large as 1600 m2/g [3,4]. Therefore, these materials have unique thermal, acoustic, optical, absorbing and catalytic properties and thus ®nd several potential scienti®c and technological applications [5]. However, the moisture sensitivity of the aerogels is *

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0025-5408/02/$ ± see front matter # 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 0 2 5 - 5 4 0 8 ( 0 2 ) 0 0 7 9 5 - X

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one of the major obstacles to several applications. The surface Si±OH groups present in the aerogel structure are the main source of hydrophilicity because they promote condensation reactions. The replacement of Si±OH groups by hydrolytically stable Si±R (R ˆ alkyl or aryl) groups inhibit the adsorption of water and, hence results in hydrophobic aerogels. In recent years, a few reports are available on the synthesis of hydrophobic silica aerogels using methyltrimethoxysilane (MTMS) as a co-precursor [6±8]. However, each MTMS monomer consists of only one alkyl (CH3) group and hence the hydrophobicity is limited. In the present work, we have used trimethylethoxysilane (TMES) as a co-precursor which consists of three methyl groups for each of the monomer, and produced hydrophobic silica aerogels. The in¯uence of TMES/TMOS (tetramethoxysilane) molar ratio (M) on bulk density, volume shrinkage and contact angle of the aerogels, is reported. The aerogels have been characterized by infrared (IR) spectroscopy, differential thermal analysis (DTA) and thermogravimetric analysis (TGA). 2. Experimental procedures Silica alcogels were prepared by sol±gel processing using tetramethoxysilane (TMOS), methanol (MeOH), water (H2O) and ammonium hydroxide (NH4OH) in the molar ratio of 1:14:4:3:7  10 3 . The molar ratio of TMES/TMOS was varied from 0 to 4.0. The chemicals used were of ``purum'' grade from Fluka, Switzerland. Double distilled water was used for the preparation of dilute NH4OH solution. The alcogels were aged for 1 day in MeOH bath at 258C. The alcogels were then kept in an autoclave of 2 l capacity (from Parr Instrument Company, USA), with an excess amount of methanol to surpass the critical temperature (Tc) and pressure (Pc) (i.e. Tc ˆ 2438C and Pc ˆ 79 bars) of methanol. The temperature of the autoclave was raised to 2658C at a rate of 488C/h and the corresponding pressure increased up to 100 bars. At this temperature and pressure, methanol gets transformed into a supercritical ¯uid which was taken out of the autoclave isothermally. The dried solid gel body (i.e. aerogel) was taken out of the autoclave after cooling it to the ambient. Further details of the drying procedures can be found from our earlier publication [9]. The bulk density (rb) of the aerogels was measured using a known volume of the aerogel and its weight (measured by a micro-balance, 10 5 g accuracy). The refractive index (RI) of the aerogels was calculated using the formula [10]: n ˆ 1 ‡ 0:19rb (1) where n is the refractive index and rb is the bulk density of the aerogel. The percentage of the volume shrinkage (Vs …%†) was determined from the change in the volume of the alcogel to the aerogel using the formula:   V  100 (2) Vs …%† ˆ 1 V1 where V is the volume of the aerogel and V1 is the volume of the alcogel. The hydrophobicity of the aerogels was tested by measuring the contact angle (y) of a

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Fig. 1. Calculation of the contact angle y from the dimensions of the water drop, `r' is the radius of the drop considered as a part of sphere, O the center of this sphere, D the base length of the drop and h is the height of the drop from the aerogel surface.

water droplet with the aerogel surface, as shown in Fig. 1, using the formula [11]:   y 2h tan ˆ (3) 2 D where h is the height of the water droplet and D is the width (i.e. base) of the droplet touching the aerogel surface. In addition, the water droplet on the aerogel surface was photographed and y was measured from the enlargement directly. In order to know the thermal stability of the aerogels both the DTA and TGA were made using SDT 2960 TA universal instruments, made in USA. The surface chemical modi®cation of the aerogels was studied using infrared spectroscopy, Perkin-Elmer (model no. 783), USA, which gave the information about the various chemical bonds such as O±H, Si±C, C±H and Si±O±Si. 3. Results 3.1. Bulk density and shrinkage of the aerogels Fig. 2 illustrates the effect of TMES/TMOS molar ratio (M) on the bulk density of the aerogels. As the M value increased, the bulk density of the aerogels decreased. The effect of M value on the percentage of relative volume shrinkage of the aerogels is shown in Fig. 3. The volume shrinkage of aerogels with M ˆ 0, was found to be negligible and is taken as zero. The volume shrinkage increased as a function of M value. 3.2. Infrared spectra (IR) of the aerogels Fig. 4 shows the IR spectra of the aerogels as a function of wave number for different M values. The absorption bands observed at around 2900 and 1500 cm 1 are due to stretching and bending of C±H bonds and the peak observed around 840 cm 1 is due to the Si±C bonds [12]. The peak at 1600 cm 1 and the broad absorption band due to ±OH groups at around 3500 cm 1 are due to the adsorbed water. The peaks at

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Fig. 2. Bulk density as a function of TMES/TMOS molar ratio.

Fig. 3. Relative volume shrinkage of the aerogels as a function of TMES/TMOS molar ratio.

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Fig. 4. IR spectra of the aerogels prepared using different molar ratios of TMES/TMOS (M) (a) M ˆ 0, (b) M ˆ 0:75, (c) M ˆ 1:4.

1080, around 800 and 465 cm 1 are due the to asymmetric, symmetric and bending modes of silicon dioxide, respectively, [13]. The residual Si±OH groups are the main source of hydrophilicity of the aerogels [6]. With an increase in M value, the intensity of the peak at 1600 cm 1 and the broad OH absorption band at 3500 cm 1 decreased, whereas the intensities of the C±H absorption peak at around 3000 cm 1 and Si±C absorption peak at around 840 cm 1 increased. 3.3. Contact angle measurements In order to study the hydrophobicity of the aerogels, the contact angle (y) of a water droplet with surfaces of the aerogels prepared with various M values have been measured. It was found that y increased with increased M values. Table 1 gives the Table 1 Contact angle (y) values of the hydrophobic silica aerogels prepared using different M values Serial number

TMES/TMOS molar ratio (M)

Calculated y (8)

Directly measured y (photographic method) (8)

1 2 3 4

0.5 1 2 4

97 121 127 141

100 120 130 140

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Fig. 5. Photographs showing a water droplet on the surface of silica aerogels prepared using different molar ratios of TMES/TMOS (M) (a) M ˆ 0:5, y ˆ 1008, (b) M ˆ 2, y ˆ 1308, (c) M ˆ 4, y ˆ 1408, where y is the contact angle.

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Fig. 6. Water droplet on a transparent, hydrophobic silica aerogel.

results of y values measured using Eq. (3) [11] and directly obtained from the photograph following the method of Hrubesh et al. [14]. Fig. 5a±c demonstrates the contact angle for low (M ˆ 0:5), medium (M ˆ 2) and high (M ˆ 4) M values of the aerogels. It is clear from the ®gures that the sphericity of the water droplet

Fig. 7. TG-DT analysis for hydrophilic silica aerogel (a) TGA, (b) DTA.

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Fig. 8. TG-DT analysis for hydrophobic (TMES modi®ed) silica aerogel (a) TGA, (b) DTA.

increased with increased M values. Fig. 6 shows a transparent monolithic hydrophobic silica aerogel with a water droplet on its surface. 3.4. Thermal analysis The thermal stability of the hydrophilic and hydrophobic aerogels was tested using differential thermal analysis (DTA) and thermogravimetric analysis (TGA) (Figs. 7 and 8). In the case of the hydrophilic aerogels (Fig. 7) the increase in weight loss observed for the aerogels is very rapid from around 50 to 1008C indicating the evaporation of H2O and alcoholic groups, produced from condensation reactions between Si±OH groups. On the other hand, for hydrophobic aerogels (Fig. 8), the percent of weight loss is negligible up to around 3008C. There is a sharp exothermic peak at 3008C corresponding to the oxidation of surface (CH3)3 groups and the residual organic groups [15]. No such exothermic peak was observed in the case of the hydrophilic aerogels. 4. Discussion The as-produced silica aerogels contain a large number of hydroxyl and alkoxy groups which are responsible for the hydrophilicity of the aerogels [16]. However,

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using TMES as a co-precursor in the sol±gel processing stage, the OH groups on the silica clusters are replaced by hydrolytically stable O±Si±(CH3)3 groups as per the following chemical reactions: (4)

(5) In this way, the Si±(CH3)3 groups are attached to the silica clusters resulting in hydrophobic silica aerogel. At lower TMES/TMOS molar ratios (M values), the cluster surfaces are covered with fewer silicon alkyl groups, leading to less hydrophobicity and hence resulting in lower contact angle y (Fig. 5a). However, as the M value increases, more Si±(CH3)3 groups get attached to the SiO2 clusters and hence larger y values (Fig. 5b and c). It has been found that as the M value increases, the bulk density decreases (Fig. 2) which is due to less silica content in the aerogel. Due to steric crowding of non-hydrolyzable methyl groups attached to the silica clusters, the cross-linkage between the clusters becomes less and the network becomes weak and therefore it gets shrunk during the supercritical drying process causing an increase in volume shrinkage (Fig. 3) at higher M values. This statement was supported by the fact that for very high M values (>4), monolithic silica aerogels could not be obtained. It is interesting to note that despite the large volume shrinkage, the bulk density decreases with increasing M values. This seems to be a contradictory result but is consistent with the results obtained by Schwertfeger et al. [6]. This anomalous behavior is due to the fact that the increase in bulk density due to shrinkage is over compensated by decrease in the bulk density due to less number of methoxy groups available for polymerization reactions. The IR spectra of aerogel samples prepared using three different M values: (a) M ˆ 0, (b) M ˆ 0:75 and (c) M ˆ 1:4 are shown in Fig. 4. It is seen from the ®gure that as M value increased, the intensity of the OH absorption peaks at 3500 and 1600 cm 1 decreased, while the intensity of the absorption peaks around 840 cm 1due to Si±C and the C±H absorption bands at around 2900 and 1500 cm 1 increased, clearly indicating the enhancement in the surface modi®cation and hence hydrophobicity with M values. The optical transmission of the aerogels in the visible range at 750 nm decreased with increased M values (Table 2) which is due to larger pores and particles of non-uniform sizes as observed from the SEM observations (not shown here). The decrease in refractive index (Table 2) with an increase in M value is due to higher porosity of the aerogels. Comparison of the TGA/DTA curves of hydrophilic and hydrophobic (TMES modi®ed) aerogels (Figs. 7 and 8) clearly demonstrate the surface chemical

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Table 2 Some physical properties of silica aerogels for various M values Serial number

TMES/TMOS molar ratio (M)

Gelation time, Tg

Optical transmittance (%)

Refraction index (RI)

1 2 3 4 5 6

0 0.5 1 2 3 4

2h 2.2 h 2.5 h 5h 15 h 2 days

93 90 85 20 5 5

1.016 1.015 1.014 1.014 1.013 1.012

modi®cation of the aerogels is due to the TMES co-precursor. The sharp decrease in weight of the unmodi®ed aerogels below 1008C is due to the evaporation of alcohol and H2O produced from the condensation reactions and above 1008C it is due to the evaporation of trapped organic (OCH3) and H2O molecules. The unmodi®ed aerogels showed a 4% weight loss in the temperature range of 20±2808C, whereas the aerogels with Si±(CH3)3 surface modi®cation showed a weight loss of less than 1% in this range. The hydrophobicity of the aerogels retained up to a temperature of 3008C and above this the aerogels became hydrophilic. That means, most of the Si±(CH3)3 groups were oxidized into OH groups above 3008C [17]. This statement looks contradictory because the Si±CH3 groups in Si±(CH3)4 do not decompose even after many days at 5008C [18]. The silicon tetra alkyls are air stable materials which are not hydrolyzed under neutral conditions. However, electrophiles cleave Si±CH3 bonds in the presence of a Lewis acid [18]. But, the surface byproducts on the aerogels do not contain any electrophiles or Lewis acid. Therefore, the other possibility can be that at higher temperatures (>3008C), the trapped OH and H2O molecules cleave the surface Si±O±Si (CH3)3 groups into Si±OH and HO±Si(CH3)3 and the latter groups can react with residual ROH and form TMES groups and these groups evaporate at and above 3008C leading to hydrophilic aerogels. 5. Conclusions Monolithic, hydrophobic silica aerogels based on TMOS precursor with TMES as a co-precursor were prepared. Silica aerogels with contact angle (y) as high as 1408 have been produced by increasing the TMES/TMOS molar ratio (M) to 4. As the M value increased from 0.5 to 4, the bulk density of the aerogels decreased from 0.085 to 0.05 g/cm3, even though the shrinkage increased from 2 to 10%. The IR spectra showed an increase in the intensity of Si±C and C±H peaks and decrease in the intensity of O±H peaks with increase in M value, clearly indicating the increased aerogel surface chemical modi®cation by the organic groups. The DTA and TGA studies indicated that the hydrophobic aerogels are thermally stable up to 3008C.

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Acknowledgments The project grant received from University Grants Commission (UGC project, on Aerogels, no. F.10-68/2001 (SR-I)), New Delhi, Government of India, is gratefully acknowledged. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18]

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