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Comparative studies on the surface chemical modification of silica aerogels based on various organosilane compounds of the type RnSiX4−n
Journal of Non-Crystalline Solids 350 (2004) 216–223 www.elsevier.com/locate/jnoncrysol
Comparative studies on the surface chemical modification of silica aerogels based on various organosilane compounds of the type RnSiX4n A. Venkateswara Rao b
a,*
, G.M. Pajonk b, S.D. Bhagat a, Philippe Barboux
c
a Air Glass Laboratory, Department of Physics, Shivaji University, Kolhapur 416 004, Maharashtra State, India Laboratoire d’Application de la Chimie a l’Environnement, Universite´ Claude Bernard—Lyon 1, 69622 Villeurbanne Cedex, France c Laboratory for Condensed Matter Physics, E´cole Polytechnique, 91128, Palaiseau Cedex, France
Abstract We report the experimental results dealing with the surface chemical modification of silica aerogels using various precursors and co-precursors based on mono-, di-, tri- and tetrafunctional organosilane compounds of the type RnSiX4n (where R = alkyl or aryl groups, X = Cl or alkoxy groups, n = 0–4). The precursors exhibit either tetrafunctional or trifunctional chemical functions while the co-precursors have a number of functional groups varying from 1 to 3. The organosilane based on the methyltrimethoxysilane (MTMS) can be used as a precursor as well as co-precursor. The chemically modified aerogels have been produced by (i) co-precursor, and (ii) derivatization methods. The co-precursor method results in aerogels with higher contact angle (h 136) but the aerogels are opaque, whereas transparent (>80% optical transmission in the visible range) aerogels with lower contact angle (h 120) are obtained using the derivatization method. The hydrophobicity of the phenyl-modified aerogels has been found to be thermally stable up to a temperature of 520 C. Using the MTMS precursor, aerogels with contact angles as high as 175 have been obtained, but the aerogels are opaque. The aerogels obtained using TEOS precursor along with the trimethylethoxysilane (TMES) co-precursor, show negligible volume shrinkage. Water intrusion into the MTMS-modified aerogels at pressures greater than the Laplace pressure exhibits hysteresis, as shown in the pressure-volume curves. Water droplets placed on surfaces coated with superhydrophobic aerogel powder with 8 of inclination showed velocities as high as 0.4 ms1. The results are discussed with respect to the ratios of organic and inorganic components of the organosilane compounds. 2004 Elsevier B.V. All rights reserved. PACS: 82.33.Ln; 82.70.Uv; 81.70.Pg; 81.07.Pr; 68.08.Bc
1. Introduction Silica aerogels are exceptional in the field of materials science because they are highly porous (98% porosity) and simultaneously transparent (90% optical transmission) in the visible region [1]. This novel combination of properties is due to the fact that the aerogels have par*
Corresponding author. Tel.: +91 231 2690571; fax: +91 231 2691533. E-mail address: [email protected] (A.V. Rao). 0022-3093/$ - see front matter 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2004.06.034
ticle and pore sizes in the 1–100 nm range, with the particles linked together through a tenuous silica network. Hence, silica aerogels have several applications such as Cerenkov radiation detectors in nuclear reactors and high energy physics [2–5], lightweight thermal and acoustic insulating systems [6,7], and low dielectric constant materials for MOS devices [8,9]. However, the main problems with the aerogels are that they are: (i) very fragile; and (ii) absorb moisture from humid surroundings and deteriorate with time, which constrains their use in long-term technological applications
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[10,11]. An additional problem with the as-produced or less hydrophobic aerogels is that when used in thermal insulation solar panels, aerogel powder settles to the bottom of the panels leading to a decrease in light transmission of the solar panels. The aim of the present work is to attach organic groups to the silica clusters using various organosilane compounds of the type RnSiX4n, as co-precursors or derivatizing agents. Organic groups can introduce elastic as well as hydrophobic properties (water repelling), but these properties must be introduced into the aerogels without sacrificing optical transmittance (70–90%), density (0.02–0.2 gcm3), and porosity (90–98%). The application of organosilicon compounds RnSiX4n (n = 1–3, R = CH3, C2H5, or phenyl, etc., X = OCH3, OC2H5, or Cl, etc.) for the modification of surface properties of inorganic oxides is successfully used in many fields of technology [12]. In particular, this class of compounds has approved to be very efficient for preparation of coatings to protect natural and artificial materials against corrosive surface phenomena [13–15]. Further, covalent modification via coupling organosilicon agents (alkyl, aryl, vinyl, etc.) to oxide surfaces is often used in building materials [16,17]. Therefore, it is essential to combine the advantages of organic groups (which improve mechanical and hydrophobic properties) with silica aerogels. Two methods: (i) co-precursor [18]; and (ii) derivatization [19] were employed for this purpose using various precursors (three) and co-precursors (six) containing the (RnSiX4n) organic groups. The use of six different coprecursors in the preparation of silica aerogels facilitates the surface modification of the aerogels, which makes them hydrophobic as well as more elastic than the native aerogels [20]. In the co-precursor method, a hydrophobic reagent containing the organic groups was added to the alcosols prepared using either tetramethoxysilane (TMOS) or tetraethoxysilane (TEOS) and the resulting alcogels were dried supercritically from methanol. In the derivatization method, the alcogels of TMOS and TEOS were immersed in a chemical bath containing the hydrophobic reagent and a solvent (methanol or ethanol) for 12 h at 50 C and the gels were subsequently supercritically dried from methanol. In addition to these reagents, methyltrimethoxysilane (CH3–Si(OCH3)3, MTMS), which contains one non-hydrolyzable organic group and three hydrolyzable alkoxy groups, was found to be a useful precursor to synthesize silica aerogels. Methyltrimethoxysilane plays both roles at the same time, being the precursor as well as the co-precursor. Laplace pressure intrusion studies were performed to study the water penetration hysteresis in the hydrophobic aerogels. From the results of these experiments, various hydrophobic and physical properties of the aerogels prepared using TMOS, TEOS and MTMS precursors are compared.
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2. Experimental 2.1. Sample preparation Hydrophobic silica aerogels were prepared by two methods: (i) co-precursor; and (ii) derivatization. Various organosilane compounds of the type RnSiX4n (hydrophobic reagents, HR) such as methyltrimethoxysilane (MTMS), phenyltriethoxysilane (PTES), dimethylchlorosilane (DMCS), trimethylchlorosilane (TMCS), trimethylethoxysilane (TMES), and hexamethyldisilazane (HMDZ) were used either as co-precursors or derivatizing reagents along with the two precursors, namely, tetramethoxysilane (TMOS) and tetraethoxysilane (TEOS). Because MTMS contains three hydrolyzable groups and one non-hydrolyzable organic group (SiCH3), neither co-precursor nor derivatization methods were used. Instead, the hydrophobic silica aerogels were directly produced using MTMS as the precursor. The molar ratios of TMOS:methanol (MeOH):H2O (0.05 M NH4OH), and TEOS:ethanol (EtOH):H2O (0.001 M oxalic acid) were kept constant at 1:14:4 and 1:5:7, respectively, for both the co-precursor and derivatization methods. In the case of MTMS, the molar ratio of MTMS:MeOH:H2O (5 M NH4OH) was kept constant at 1:3.5:4. In the co-precursor method, the hydrophobic reagent was added to the sol. The molar ratio of HR/precursor was kept constant at 0.5 (higher values resulted in prolonged gelation time, cracked and opaque aerogels, etc.). In the case of the derivatization method, the TMOS- or TEOS-based alcogels were kept in a chemical bath containing 20 vol.% of the HR and 80 vol.% of solvent (EtOH for TEOS and MeOH for TMOS) for 12 h at 50 C in a Termaks oven (made in Norway) for the surface chemical treatment. After this step, the unreacted and trapped components were washed away by keeping the alcogels in the solvent bath for 12 h at 50 C. The alcogels were supercritically dried (at 265 C and 10 MPa) in an autoclave from methanol (critical temperature 243 C and critical pressure 7.0 MPa), as described previously [21]. 2.2. Characterization The bulk density of the aerogels was measured from the weight-to-volume ratio. The volume shrinkage was measured from the difference in the volumes of the alcogel and the aerogel. The contact angle (h) of the water droplet (3 mm in size, wherever explicitly not mentioned) on the aerogel surface was measured using the relation [22]: 1 2h h ¼ 2tan ð1Þ d
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Fig. 1. Schematic diagram of Laplace pressure water intrusion experiments. When applied pressure P is less than that of the Laplace pressure (PL), water cannot enter into the pores; when P > PL, water enters the pores.
where, h is the height and d the base width (touching the aerogel surface) of the droplet. The thermal stability of the aerogels in terms of retention of hydrophobicity was estimated from the thermogravimetric and differential thermal analyses (TGA–DTA) as well as by heating the aerogels in the furnace and then putting the heated samples on the water surface. The retention of hydrophobicity (water-repelling property) was judged from the absorption of water by the aerogels. The optical transmittance of the aerogels (sample thickness: 102 m) was measured at a wavelength of 750 nm (Systronics 119 spectrophotometer). The Laplace pressure for intrusion of water into the MTMS/TMOS-based hydrophobic aerogels was measured using volume–pressure curves. These studies were performed by keeping the powdered hydrophobic aerogel sample (0.1–0.2 g) in an evacuated chamber and allowing the water in the chamber to penetrate the aerogel pores by increasing the pressure using a piston, as shown in Fig. 1. The hysteresis between the volume of penetrated and recovered water was studied for various aerogels as a function of applied pressure in the range 1–100 MPa. In addition to the above studies, the velocity of a water droplet (3 mm in size) on an inclined surface, coated with MTMS-precursor-based hydrophobic aerogel powder, was measured using a specially built device interfaced with a personal computer.
3. Results In order to discern the most suitable combination of hydrophobic reagent and precursor, depending on the application, the physical and hydrophobic properties of the aerogels prepared using the various hydrophobic reagents and the two Si precursors are listed in Table 1 (TMOS) and Table 2 (TEOS). The bulk density is lower (0.1 gcm3) for the modified aerogels using hydropho-
bic reagents having higher R/Si ratios (Table 3) as compared to the HRs having lower R/Si ratios. Overall, the TEOS-based aerogels are denser and less transparent than the TMOS-based aerogels. Modification with organochlorosilane resulted in opaque, shrunk, and cracked aerogels. The MTMS-modified TMOS- and TEOS-based aerogels are more transparent than the other aerogels (88% and 50% optical transparency, respectively, for a 102 m thick sample at 750 nm). The thermal stability of the co-precursor-modified aerogel is greater and varies from 275 C for MTMS to 520 C for PTES co-precursors, respectively. The thermal stability was found to be nearly the same (275–285 C) for all the aerogels obtained by the derivatization method. Moreover, there is a difference in the TGA–DTA curves of the aerogels prepared using either the co-precursor or the derivatization method as can be seen from Fig. 2(a) and (b). The weight loss curve (TGA) for the derivatized aerogel shows a very sharp decline in weight, accompanied by a very strong and narrow exothermic peak (DTA) at 275–285 C, depending on the hydrophobic reagent, in contrast to a relatively steady loss curve accompanied by a small and wider exotherm for the co-precursor method. The HMDZ co-precursor modification resulted in higher contact angles (h 135) for the TMOS as well as for the TEOS precursor. The contact angle was found to be lower for the MTMS modification (for TMOS, h 95, and for TEOS, h 130). The TEOS-based aerogels modified by the TMES co-precursor showed negligible volume shrinkage ( < 1%). The derivatized aerogels are more transparent (Fig. 3(c) and (d)) (optical transmittance >88% for the TMOS-based aerogels and >50% for the TEOS-based aerogels) than the co-precursor-modified aerogels (Fig. 3(a) and (b)). The optical transmission of the derivatized aerogels is found to be almost independent of the hydrophobic reagent used, as can be seen in Fig. 3(c) and (d), with the exception of the organochlorosilane modification, which resulted in cracked and less transparent aerogels. The experimental results of the Laplace pressure intrusion studies of the aerogels prepared using MTMS/TMOS molar ratios (M) of 2 and 4 are shown in Fig. 4(a) and (b), respectively. Water intrusion is non-reversible and shows an open-loop hysteresis. As the M value increases, the water intrusion occurs at a lower pressure. Superhydrophobic aerogels with the highest contact angle (175, for 1 mm droplet) ever recorded, have been obtained by using MTMS as a precursor with the molar ratio of MTMS:MeOH:H2O:NH4OH at 1:3.5:4:3.5 · 101, respectively (Fig. 5). The highest previously reported contact angle is 150 for the 3,3,3-trifluoropropyltrimethoxysilane-modified TMOS-based aerogels [23]. The MTMS-based aerogels are opaque (optical transmittance 5%) and dense (bulk density
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Table 1 Physical and hydrophobic properties of TMOS-based aerogels prepared using different hydrophobic reagents Hydrophobic reagenta
Co-precursor method
*
Derivatization method
Remarks
Optical transparency (T%)
Contact angle, h ()
Bulk density, qb (gcm3)
Optical transparency (T%)
Contact angle, h ()
Bulk density, qb (gcm3)
MTMS PTES DMCS
88 70 8
95 132 98
0.13 0.15 0.19
90 85 86
121 121 115
0.14 0.11 0.12
TMCS TMES HMDZ
5 90 70
— 110 135
Powder 0.09 0.12
85 90 88
111 120 118
0.13 0.14 0.12
Higher optical transparency (88%) Highest thermal stability (520 C) Higher shrinkage (30%), cracked, and opaque Lower bulk density (0.09 gcm3) Higher contact angle (135)
*
Molar ratio of HR/TMOS = 0.5. a Methyltrimethoxysilane (MTMS), phenyltriethoxysilane (PTES), dimethylchlorosilane (DMCS), trimethylchlorosilane (TMCS), trimethylethoxysilane (TMES), and hexamethyldisilazane (HMDZ).
Table 2 Physical and hydrophobic properties of TEOS-based aerogels prepared using different hydrophobic reagents Hydrophobic reagent
Co-precursor method*
Derivatization method
Remarks
Optical transparency (T%)
Contact angle, h ()
Bulk density, qb (gcm3)
Optical transparency (T%)
Contact angle, h ()
Bulk density, qb (gcm3)
MTMS PTES DMCS
50 5 10
120 129 127
0.21 0.33 0.19
88 60 50
112 115 118
0.13 0.14 0.30
TMCS
5
132
50
102
0.26
TMES
5
136
Cracked pieces 0.11
55
119
0.16
HMDZ
5
134
0.10
70
109
0.12
*
Higher optical transparency (50%) Highest thermal stability (520 C) Higher shrinkage (30%), cracked, and opaque
Negligible volume shrinkage and higher contact angle (136) Lower bulk density (0.1 gcm3)
Molar ratio of HR/TEOS = 0.5.
Table 3 Units
Monofunctional
Difunctional
Trifunctional
Quadrifunctional
SiO0.5 3 TMES, TMCS, HMDZ
SiO1 2 DMCS
SiO1.5 1 MTMS, PTES
SiO2 0 TEOS, TMOS
Structure
Silicon oxide content R/Si ratio RnSiX4n
0.2 gcm3). The results of the water droplet velocity measurements as a function of the angle of inclination of the superhydrophobic aerogel powder coated surface are listed in Table 4. The droplet velocity increases from 0.4 to 1.44 ms1 as the angle of inclination, /, increases from 8 to 52.
4. Discussion From Tables 1 and 2, it is clear that the aerogels produced using TMOS as the precursor, are more transpar-
ent and less dense than the aerogels prepared by the TEOS precursor. The higher density (0.2–0.3 gcm3) of TEOS-based aerogels is the result of higher volume shrinkage (20–30%, with the exception of the TMES modification) due to the weaker network composed of smaller particles linked together via longer chains. Moreover, the silica content in the TEOS (28.8 wt%) is less than that of the TMOS (39.5%). When alkylchlorosilane modifiers such as DMCS or TMCS were used, the sol became acidic and resulted in longer gelation time and a very weak network that cracked upon drying, even in the case of the TMOS precursor. The gelation time of
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Fig. 4. Pressure-volume cycles of the Laplace pressure water intrusion studies for (a) MTMS/TMOS molar ratio = 2 and (b) MTMS/TMOS molar ratio = 4. Fig. 2. Thermogravimetric and differential thermal analyses (TGA– DTA) of PTES-modified TEOS-based silica aerogels prepared using (a) co-precursor, and (b) derivatization methods.
Fig. 3. TMOS-based silica aerogels prepared using PTES (a) coprecursor and (b) derivatization; and MTMS (c) co-precursor and (d) derivatization methods.
the unmodified TEOS-based sols is longer (3 days) [24] as compared to the TMOS-based sols (2 h). When the organic modifiers (HR) are added to the TMOS- and TEOS-based sols, the gelation time increases, due to the presence of non-hydrolyzable (Si-C) groups in the sol. The presence of Si-C groups also causes non-uniform growth of clusters and very large pore sizes, hence the opacity of the aerogels increases with an increase in the HR/precursor molar ratio [25]. This effect is amplified in the case of the TEOS-based aerogels prepared using the co-precursor method because of the very slug-
Fig. 5. Water droplet (1 mm diameter) placed on a superhydrophobic MTMS-based silica aerogel surface. The contact angle (h) is 175.
gish gelation time (P10 days) and non-uniform network formation. In the case of derivatization where the HR is not added to the pre-sol, the TMOS- or TEOS-based alcogel was kept in a mixture of 20 vol.% HR and 80 vol.% solvent (EtOH for TEOS and MeOH for TMOS gels) at 50 C for 12 h. Therefore, the alkyl or aryl groups graft to the clusters of the gel without affecting the gel network structure. The optical transmission of the derivatized aerogels is remarkably higher and almost independent of the HR used, in contrast to the coprecursor method, as shown in Fig. 3. However, the organochlorosilane-derivatized TEOS aerogels are less transparent due to the internal cracking of the network during the derivatization process. This cracking can be a
A.V. Rao et al. / Journal of Non-Crystalline Solids 350 (2004) 216–223 Table 4 Velocity of a water droplet (3 mm) on (MTMS-based) superhydrophobic aerogel-powder-coated surface as a function of the angle of inclination Angle of inclination, /()
Droplet velocity (ms1)
8 14 22 33 46 52
0.40 0.47 0.70 0.98 1.41 1.44
The molar ratio of MTMS:MeOH:H2O:NH4OH is 1:3.5:4:3.5 · 101, respectively.
result of the heat produced and released during the formation of HCl [26,27] that occurs when organochlorosilanes are used to derivatize the alcogels. The TGA–DTA curves show a gradual weight loss in the case of the co-precursor method, Fig. 2(a), because the HR is present in the sol and its hydrolytically stable groups reside in the inner silica clusters as well as at the surface. In the case of the derivatized aerogels, only the surface SiO2 clusters are modified, so there is a sharp decrease in the weight loss along with a strong exothermic peak when the temperature is raised (Fig. 2(b)). The thermal stability of the phenyl-modified aerogel is much higher (520 C) than that of all the other aerogels [28]. This stability is due to the higher electrophilic nature of the arylsiloxanes as compared to the alkylsiloxanes [29–31]. The TMES modification resulted in the lowest shrinkage values recorded for the whole series of the aerogels. We attribute this retention of pore volume upon drying to the large number of non-hydrolyzable organic groups (R/Si = 3) present in the modified gel, which gives rise to more elasticity and does not allow the pore walls to close in the course of drying, as in the case of the Ôspring-backÕ effect [32]. In the case of the TMOS-based aerogels, a large contact angle of 135 was obtained for the HMDZ modified aerogels prepared by the co-precursor method, whereas the MTMS modification resulted in a lower contact angle of 95. In the case of the TEOS-based aerogels prepared using the co-precursor method, a similar trend was observed where the MTMS modification resulted in a contact angle of 120 and the TMES- and HMDZmodified aerogels had a higher contact angle of 135. These results are expected, if one considers the R/Si ratio for each of the hydrophobic reagents. As shown in the Table 3, TMES and HMDZ have a R/Si ratio of 3 while MTMS, PTES (more C–H groups), etc. have a R/Si ratio of 1, thus the larger the R/Si ratio, the better is the hydrophobic covering yielding higher contact angles. However, in the case of the TEOS precursor, the difference in contact angle is only 15, which is due to the difference in the volume of the HR for the same HR/TEOS molar ratio.
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A second variable affects the contact angle measured on the modified aerogels: if the HR/precursor molar ratio increases, the contact angle also increases. However, there is a limit to which the molar ratio can be increased without affecting the monolithicity and other physical properties of the aerogels. As observed in the case of the TMES/TMOS systems, this limiting value is 4 [33], whereas in the case of the TMES/TEOS aerogels it is only 0.6 [24]. Hence, to obtain aerogels with higher hydrophobicity, we used MTMS itself as a precursor, because it has only one organic group attached to each Si atom along with the three hydrolyzable –OCH3 groups that react to form a three-dimensional gel network. By keeping the molar ratio of MTMS:MeOH:H2O at 1:3.5:4, with a NH4OH catalyst concentration of 5M, superhydrophobic aerogels with contact angles as high as 175 were obtained, as shown in Fig. 5. The wettability of a solid surface with water depends on the relation between the interfacial energies of solid/liquid (csl), solid/air (csa) and liquid/air(cla), and is given by YoungÕs equation [34,35]: csl ¼ csa cla cosðhÞ
ð2Þ
where h is the contact angle of the water droplet with the aerogel surface. Therefore, a high-energy solid/liquid interface can be created for h 175 (csl > csa). The superhydrophobic aerogels are opaque (optical transmittance 5%) and dense (bulk density 0.2 gcm3). The surface coated with superhydrophobic aerogel powder and an angle of inclination of 52 resulted in the highest-ever-reported velocity (1.44 ms1) of the water droplet. As shown in Table 4, the velocity of the droplet was studied as a function of the angle of inclination of the aerogel-coated surface. Even for a small inclination, (8), a droplet velocity of 0.4 ms1 was observed, which is higher than that recently reported velocity for a liquid marble (0.01 ms1) [36]. The higher water droplet velocity indicates better hydrophobicity. The results of the Laplace water intrusion into the aerogels produced using MTMS/TMOS molar ratios of 2 and 4 are shown in Fig. 4(a) and (b), respectively. The hysteresis loop is open, which occurs because water intrusion is not completely reversible due to the trapping of some water inside the pores of the aerogels. This interpretation was confirmed from the second cycle that yields a straight line rather than a loop. The pore size increased with increased MTMS/TMOS molar ratio [37]. When the molar ratio of MTMS/TMOS increased from 2 to 4, the Laplace pressure intrusion of water decreased from 14 to 8 MPa for the same volume change (Fig. 4(a) and (b)) indicating larger pores for the higher MTMS/TMOS molar ratio (4). In other words, more Laplace pressure (PL) is required to fill the water in the smaller pores than the larger ones of the hydrophobic aerogels according to the following formula [38]:
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PL ¼
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2cLV cos h a
ð3Þ
where cLV is the interfacial energy of the liquid/vapor interface, h the contact angle of the liquid with the solid surface, and a the radius of the pore. The Laplace water intrusion method indirectly provides an idea about the pore sizes as well as the hydrophobicity because the Laplace pressure is inversely proportional to pore size and directly proportional to the contact angle.
5. Conclusion The organic modification of the silica aerogels derived from TMOS and TEOS precursors was carried out using the co-precursor and derivatization methods with various organosilane compounds of the type RnSiX4n. In addition, superhydrophobic silica aerogels with a contact angle as high as 175 were produced using MTMS as the precursor. A coating of the superhydrophobic aerogel powder resulted in a very effective water-repellent surface. On this surface, the highestever-reported water droplet velocity of 1.44 ms1 was observed for an inclination of 52. The Laplace pressure intrusion studies revealed the macroporosity of the hydrophobic aerogels and a shift to larger pore sizes as the MTMS/TMOS molar ratio is increased. The other major findings of these studies are: (1) The aerogels prepared using co-precursors such as HMDZ, TMES, PTES, etc. are more hydrophobic (h > 130), but less transparent than the corresponding derivatized aerogels (h 120). (2) Organosilane-modified aerogels with the highestever-reported thermal stability of 520 C (in terms of hydrophobicity) are obtained using the PTES coprecursor. (3) The organochlorosilane-modified aerogels are opaque and cracked. (4) The TEOS-based hydrophobic silica aerogels with TMES as a co-precursor have the lowest volume shrinkage (<1%) of all the aerogels prepared in this study. The contact angle in this case is around 136. (5) The organically modified TMOS-based aerogels show higher optical transmission (85%) than the TEOS-modified aerogels (60%).
Acknowledgments The authors are grateful to the Department of Science and Technology (DST), New Delhi for the research project on ÔAerogelsÕ (No. SR/S2/CMP-01/2002). The
authors are thankful to Dr A.P. Rao, Research Associate, Mr M.M. Kulkarni, Senior Research Fellow, and Mr R.R. Kalesh for their help in the experimental work. The authors are grateful to Dr Stela Sidis (formerly at Labo, P.M.C., E´cole Polytechnique, Palaiseau, France) for help regarding the Laplace water intrusion experiments.
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A.V. Rao et al. / Journal of Non-Crystalline Solids 350 (2004) 216–223 [29] X. Zhang, L. Li, S. Shi, Y. Lin, Polym. Degrad. Stabil. 20 (1988) 157. [30] D. Li, S.T. Hwang, J. Appl. Polym. Sci. 44 (1992) 1979. [31] D.A. Loy, K.J. Shea, Chem. Rev. 95 (1995) 1431. [32] S.S Prakash, C.J. Brinker, A.J. Hunt, S.M. Rao, Nature 374 (1995) 439. [33] A. Venkateswara Rao, M.M. Kulkarni, Mater. Res. Bull. 37 (2002) 1667.
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[34] P.G. de Gennes, Rev. Mod. Phys. 57 (3) (1985) 827. [35] P.G. de Gennes, F.B. Wyart, D. Quere, Capillarity and Wetting Phenomena, Springer, 2004. [36] P. Aussillous, D. Quere, Nature 411 (2001) 924. [37] A. Venkateswara Rao, G.M. Pajonk, D. Haranath, Mater. Sci. Technol. 17 (2001) 343. [38] C.J. Brinker, G.W. Scherer, Sol–Gel Science, Academic Press, San Diego, 1990, p. 414.
Comparative studies on the surface chemical modification of silica aerogels based on various organosilane compounds of the type RnSiX4n A. Venkateswara Rao b
a,*
, G.M. Pajonk b, S.D. Bhagat a, Philippe Barboux
c
a Air Glass Laboratory, Department of Physics, Shivaji University, Kolhapur 416 004, Maharashtra State, India Laboratoire d’Application de la Chimie a l’Environnement, Universite´ Claude Bernard—Lyon 1, 69622 Villeurbanne Cedex, France c Laboratory for Condensed Matter Physics, E´cole Polytechnique, 91128, Palaiseau Cedex, France
Abstract We report the experimental results dealing with the surface chemical modification of silica aerogels using various precursors and co-precursors based on mono-, di-, tri- and tetrafunctional organosilane compounds of the type RnSiX4n (where R = alkyl or aryl groups, X = Cl or alkoxy groups, n = 0–4). The precursors exhibit either tetrafunctional or trifunctional chemical functions while the co-precursors have a number of functional groups varying from 1 to 3. The organosilane based on the methyltrimethoxysilane (MTMS) can be used as a precursor as well as co-precursor. The chemically modified aerogels have been produced by (i) co-precursor, and (ii) derivatization methods. The co-precursor method results in aerogels with higher contact angle (h 136) but the aerogels are opaque, whereas transparent (>80% optical transmission in the visible range) aerogels with lower contact angle (h 120) are obtained using the derivatization method. The hydrophobicity of the phenyl-modified aerogels has been found to be thermally stable up to a temperature of 520 C. Using the MTMS precursor, aerogels with contact angles as high as 175 have been obtained, but the aerogels are opaque. The aerogels obtained using TEOS precursor along with the trimethylethoxysilane (TMES) co-precursor, show negligible volume shrinkage. Water intrusion into the MTMS-modified aerogels at pressures greater than the Laplace pressure exhibits hysteresis, as shown in the pressure-volume curves. Water droplets placed on surfaces coated with superhydrophobic aerogel powder with 8 of inclination showed velocities as high as 0.4 ms1. The results are discussed with respect to the ratios of organic and inorganic components of the organosilane compounds. 2004 Elsevier B.V. All rights reserved. PACS: 82.33.Ln; 82.70.Uv; 81.70.Pg; 81.07.Pr; 68.08.Bc
1. Introduction Silica aerogels are exceptional in the field of materials science because they are highly porous (98% porosity) and simultaneously transparent (90% optical transmission) in the visible region [1]. This novel combination of properties is due to the fact that the aerogels have par*
Corresponding author. Tel.: +91 231 2690571; fax: +91 231 2691533. E-mail address: [email protected] (A.V. Rao). 0022-3093/$ - see front matter 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2004.06.034
ticle and pore sizes in the 1–100 nm range, with the particles linked together through a tenuous silica network. Hence, silica aerogels have several applications such as Cerenkov radiation detectors in nuclear reactors and high energy physics [2–5], lightweight thermal and acoustic insulating systems [6,7], and low dielectric constant materials for MOS devices [8,9]. However, the main problems with the aerogels are that they are: (i) very fragile; and (ii) absorb moisture from humid surroundings and deteriorate with time, which constrains their use in long-term technological applications
A.V. Rao et al. / Journal of Non-Crystalline Solids 350 (2004) 216–223
[10,11]. An additional problem with the as-produced or less hydrophobic aerogels is that when used in thermal insulation solar panels, aerogel powder settles to the bottom of the panels leading to a decrease in light transmission of the solar panels. The aim of the present work is to attach organic groups to the silica clusters using various organosilane compounds of the type RnSiX4n, as co-precursors or derivatizing agents. Organic groups can introduce elastic as well as hydrophobic properties (water repelling), but these properties must be introduced into the aerogels without sacrificing optical transmittance (70–90%), density (0.02–0.2 gcm3), and porosity (90–98%). The application of organosilicon compounds RnSiX4n (n = 1–3, R = CH3, C2H5, or phenyl, etc., X = OCH3, OC2H5, or Cl, etc.) for the modification of surface properties of inorganic oxides is successfully used in many fields of technology [12]. In particular, this class of compounds has approved to be very efficient for preparation of coatings to protect natural and artificial materials against corrosive surface phenomena [13–15]. Further, covalent modification via coupling organosilicon agents (alkyl, aryl, vinyl, etc.) to oxide surfaces is often used in building materials [16,17]. Therefore, it is essential to combine the advantages of organic groups (which improve mechanical and hydrophobic properties) with silica aerogels. Two methods: (i) co-precursor [18]; and (ii) derivatization [19] were employed for this purpose using various precursors (three) and co-precursors (six) containing the (RnSiX4n) organic groups. The use of six different coprecursors in the preparation of silica aerogels facilitates the surface modification of the aerogels, which makes them hydrophobic as well as more elastic than the native aerogels [20]. In the co-precursor method, a hydrophobic reagent containing the organic groups was added to the alcosols prepared using either tetramethoxysilane (TMOS) or tetraethoxysilane (TEOS) and the resulting alcogels were dried supercritically from methanol. In the derivatization method, the alcogels of TMOS and TEOS were immersed in a chemical bath containing the hydrophobic reagent and a solvent (methanol or ethanol) for 12 h at 50 C and the gels were subsequently supercritically dried from methanol. In addition to these reagents, methyltrimethoxysilane (CH3–Si(OCH3)3, MTMS), which contains one non-hydrolyzable organic group and three hydrolyzable alkoxy groups, was found to be a useful precursor to synthesize silica aerogels. Methyltrimethoxysilane plays both roles at the same time, being the precursor as well as the co-precursor. Laplace pressure intrusion studies were performed to study the water penetration hysteresis in the hydrophobic aerogels. From the results of these experiments, various hydrophobic and physical properties of the aerogels prepared using TMOS, TEOS and MTMS precursors are compared.
217
2. Experimental 2.1. Sample preparation Hydrophobic silica aerogels were prepared by two methods: (i) co-precursor; and (ii) derivatization. Various organosilane compounds of the type RnSiX4n (hydrophobic reagents, HR) such as methyltrimethoxysilane (MTMS), phenyltriethoxysilane (PTES), dimethylchlorosilane (DMCS), trimethylchlorosilane (TMCS), trimethylethoxysilane (TMES), and hexamethyldisilazane (HMDZ) were used either as co-precursors or derivatizing reagents along with the two precursors, namely, tetramethoxysilane (TMOS) and tetraethoxysilane (TEOS). Because MTMS contains three hydrolyzable groups and one non-hydrolyzable organic group (SiCH3), neither co-precursor nor derivatization methods were used. Instead, the hydrophobic silica aerogels were directly produced using MTMS as the precursor. The molar ratios of TMOS:methanol (MeOH):H2O (0.05 M NH4OH), and TEOS:ethanol (EtOH):H2O (0.001 M oxalic acid) were kept constant at 1:14:4 and 1:5:7, respectively, for both the co-precursor and derivatization methods. In the case of MTMS, the molar ratio of MTMS:MeOH:H2O (5 M NH4OH) was kept constant at 1:3.5:4. In the co-precursor method, the hydrophobic reagent was added to the sol. The molar ratio of HR/precursor was kept constant at 0.5 (higher values resulted in prolonged gelation time, cracked and opaque aerogels, etc.). In the case of the derivatization method, the TMOS- or TEOS-based alcogels were kept in a chemical bath containing 20 vol.% of the HR and 80 vol.% of solvent (EtOH for TEOS and MeOH for TMOS) for 12 h at 50 C in a Termaks oven (made in Norway) for the surface chemical treatment. After this step, the unreacted and trapped components were washed away by keeping the alcogels in the solvent bath for 12 h at 50 C. The alcogels were supercritically dried (at 265 C and 10 MPa) in an autoclave from methanol (critical temperature 243 C and critical pressure 7.0 MPa), as described previously [21]. 2.2. Characterization The bulk density of the aerogels was measured from the weight-to-volume ratio. The volume shrinkage was measured from the difference in the volumes of the alcogel and the aerogel. The contact angle (h) of the water droplet (3 mm in size, wherever explicitly not mentioned) on the aerogel surface was measured using the relation [22]: 1 2h h ¼ 2tan ð1Þ d
218
A.V. Rao et al. / Journal of Non-Crystalline Solids 350 (2004) 216–223
Fig. 1. Schematic diagram of Laplace pressure water intrusion experiments. When applied pressure P is less than that of the Laplace pressure (PL), water cannot enter into the pores; when P > PL, water enters the pores.
where, h is the height and d the base width (touching the aerogel surface) of the droplet. The thermal stability of the aerogels in terms of retention of hydrophobicity was estimated from the thermogravimetric and differential thermal analyses (TGA–DTA) as well as by heating the aerogels in the furnace and then putting the heated samples on the water surface. The retention of hydrophobicity (water-repelling property) was judged from the absorption of water by the aerogels. The optical transmittance of the aerogels (sample thickness: 102 m) was measured at a wavelength of 750 nm (Systronics 119 spectrophotometer). The Laplace pressure for intrusion of water into the MTMS/TMOS-based hydrophobic aerogels was measured using volume–pressure curves. These studies were performed by keeping the powdered hydrophobic aerogel sample (0.1–0.2 g) in an evacuated chamber and allowing the water in the chamber to penetrate the aerogel pores by increasing the pressure using a piston, as shown in Fig. 1. The hysteresis between the volume of penetrated and recovered water was studied for various aerogels as a function of applied pressure in the range 1–100 MPa. In addition to the above studies, the velocity of a water droplet (3 mm in size) on an inclined surface, coated with MTMS-precursor-based hydrophobic aerogel powder, was measured using a specially built device interfaced with a personal computer.
3. Results In order to discern the most suitable combination of hydrophobic reagent and precursor, depending on the application, the physical and hydrophobic properties of the aerogels prepared using the various hydrophobic reagents and the two Si precursors are listed in Table 1 (TMOS) and Table 2 (TEOS). The bulk density is lower (0.1 gcm3) for the modified aerogels using hydropho-
bic reagents having higher R/Si ratios (Table 3) as compared to the HRs having lower R/Si ratios. Overall, the TEOS-based aerogels are denser and less transparent than the TMOS-based aerogels. Modification with organochlorosilane resulted in opaque, shrunk, and cracked aerogels. The MTMS-modified TMOS- and TEOS-based aerogels are more transparent than the other aerogels (88% and 50% optical transparency, respectively, for a 102 m thick sample at 750 nm). The thermal stability of the co-precursor-modified aerogel is greater and varies from 275 C for MTMS to 520 C for PTES co-precursors, respectively. The thermal stability was found to be nearly the same (275–285 C) for all the aerogels obtained by the derivatization method. Moreover, there is a difference in the TGA–DTA curves of the aerogels prepared using either the co-precursor or the derivatization method as can be seen from Fig. 2(a) and (b). The weight loss curve (TGA) for the derivatized aerogel shows a very sharp decline in weight, accompanied by a very strong and narrow exothermic peak (DTA) at 275–285 C, depending on the hydrophobic reagent, in contrast to a relatively steady loss curve accompanied by a small and wider exotherm for the co-precursor method. The HMDZ co-precursor modification resulted in higher contact angles (h 135) for the TMOS as well as for the TEOS precursor. The contact angle was found to be lower for the MTMS modification (for TMOS, h 95, and for TEOS, h 130). The TEOS-based aerogels modified by the TMES co-precursor showed negligible volume shrinkage ( < 1%). The derivatized aerogels are more transparent (Fig. 3(c) and (d)) (optical transmittance >88% for the TMOS-based aerogels and >50% for the TEOS-based aerogels) than the co-precursor-modified aerogels (Fig. 3(a) and (b)). The optical transmission of the derivatized aerogels is found to be almost independent of the hydrophobic reagent used, as can be seen in Fig. 3(c) and (d), with the exception of the organochlorosilane modification, which resulted in cracked and less transparent aerogels. The experimental results of the Laplace pressure intrusion studies of the aerogels prepared using MTMS/TMOS molar ratios (M) of 2 and 4 are shown in Fig. 4(a) and (b), respectively. Water intrusion is non-reversible and shows an open-loop hysteresis. As the M value increases, the water intrusion occurs at a lower pressure. Superhydrophobic aerogels with the highest contact angle (175, for 1 mm droplet) ever recorded, have been obtained by using MTMS as a precursor with the molar ratio of MTMS:MeOH:H2O:NH4OH at 1:3.5:4:3.5 · 101, respectively (Fig. 5). The highest previously reported contact angle is 150 for the 3,3,3-trifluoropropyltrimethoxysilane-modified TMOS-based aerogels [23]. The MTMS-based aerogels are opaque (optical transmittance 5%) and dense (bulk density
A.V. Rao et al. / Journal of Non-Crystalline Solids 350 (2004) 216–223
219
Table 1 Physical and hydrophobic properties of TMOS-based aerogels prepared using different hydrophobic reagents Hydrophobic reagenta
Co-precursor method
*
Derivatization method
Remarks
Optical transparency (T%)
Contact angle, h ()
Bulk density, qb (gcm3)
Optical transparency (T%)
Contact angle, h ()
Bulk density, qb (gcm3)
MTMS PTES DMCS
88 70 8
95 132 98
0.13 0.15 0.19
90 85 86
121 121 115
0.14 0.11 0.12
TMCS TMES HMDZ
5 90 70
— 110 135
Powder 0.09 0.12
85 90 88
111 120 118
0.13 0.14 0.12
Higher optical transparency (88%) Highest thermal stability (520 C) Higher shrinkage (30%), cracked, and opaque Lower bulk density (0.09 gcm3) Higher contact angle (135)
*
Molar ratio of HR/TMOS = 0.5. a Methyltrimethoxysilane (MTMS), phenyltriethoxysilane (PTES), dimethylchlorosilane (DMCS), trimethylchlorosilane (TMCS), trimethylethoxysilane (TMES), and hexamethyldisilazane (HMDZ).
Table 2 Physical and hydrophobic properties of TEOS-based aerogels prepared using different hydrophobic reagents Hydrophobic reagent
Co-precursor method*
Derivatization method
Remarks
Optical transparency (T%)
Contact angle, h ()
Bulk density, qb (gcm3)
Optical transparency (T%)
Contact angle, h ()
Bulk density, qb (gcm3)
MTMS PTES DMCS
50 5 10
120 129 127
0.21 0.33 0.19
88 60 50
112 115 118
0.13 0.14 0.30
TMCS
5
132
50
102
0.26
TMES
5
136
Cracked pieces 0.11
55
119
0.16
HMDZ
5
134
0.10
70
109
0.12
*
Higher optical transparency (50%) Highest thermal stability (520 C) Higher shrinkage (30%), cracked, and opaque
Negligible volume shrinkage and higher contact angle (136) Lower bulk density (0.1 gcm3)
Molar ratio of HR/TEOS = 0.5.
Table 3 Units
Monofunctional
Difunctional
Trifunctional
Quadrifunctional
SiO0.5 3 TMES, TMCS, HMDZ
SiO1 2 DMCS
SiO1.5 1 MTMS, PTES
SiO2 0 TEOS, TMOS
Structure
Silicon oxide content R/Si ratio RnSiX4n
0.2 gcm3). The results of the water droplet velocity measurements as a function of the angle of inclination of the superhydrophobic aerogel powder coated surface are listed in Table 4. The droplet velocity increases from 0.4 to 1.44 ms1 as the angle of inclination, /, increases from 8 to 52.
4. Discussion From Tables 1 and 2, it is clear that the aerogels produced using TMOS as the precursor, are more transpar-
ent and less dense than the aerogels prepared by the TEOS precursor. The higher density (0.2–0.3 gcm3) of TEOS-based aerogels is the result of higher volume shrinkage (20–30%, with the exception of the TMES modification) due to the weaker network composed of smaller particles linked together via longer chains. Moreover, the silica content in the TEOS (28.8 wt%) is less than that of the TMOS (39.5%). When alkylchlorosilane modifiers such as DMCS or TMCS were used, the sol became acidic and resulted in longer gelation time and a very weak network that cracked upon drying, even in the case of the TMOS precursor. The gelation time of
220
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Fig. 4. Pressure-volume cycles of the Laplace pressure water intrusion studies for (a) MTMS/TMOS molar ratio = 2 and (b) MTMS/TMOS molar ratio = 4. Fig. 2. Thermogravimetric and differential thermal analyses (TGA– DTA) of PTES-modified TEOS-based silica aerogels prepared using (a) co-precursor, and (b) derivatization methods.
Fig. 3. TMOS-based silica aerogels prepared using PTES (a) coprecursor and (b) derivatization; and MTMS (c) co-precursor and (d) derivatization methods.
the unmodified TEOS-based sols is longer (3 days) [24] as compared to the TMOS-based sols (2 h). When the organic modifiers (HR) are added to the TMOS- and TEOS-based sols, the gelation time increases, due to the presence of non-hydrolyzable (Si-C) groups in the sol. The presence of Si-C groups also causes non-uniform growth of clusters and very large pore sizes, hence the opacity of the aerogels increases with an increase in the HR/precursor molar ratio [25]. This effect is amplified in the case of the TEOS-based aerogels prepared using the co-precursor method because of the very slug-
Fig. 5. Water droplet (1 mm diameter) placed on a superhydrophobic MTMS-based silica aerogel surface. The contact angle (h) is 175.
gish gelation time (P10 days) and non-uniform network formation. In the case of derivatization where the HR is not added to the pre-sol, the TMOS- or TEOS-based alcogel was kept in a mixture of 20 vol.% HR and 80 vol.% solvent (EtOH for TEOS and MeOH for TMOS gels) at 50 C for 12 h. Therefore, the alkyl or aryl groups graft to the clusters of the gel without affecting the gel network structure. The optical transmission of the derivatized aerogels is remarkably higher and almost independent of the HR used, in contrast to the coprecursor method, as shown in Fig. 3. However, the organochlorosilane-derivatized TEOS aerogels are less transparent due to the internal cracking of the network during the derivatization process. This cracking can be a
A.V. Rao et al. / Journal of Non-Crystalline Solids 350 (2004) 216–223 Table 4 Velocity of a water droplet (3 mm) on (MTMS-based) superhydrophobic aerogel-powder-coated surface as a function of the angle of inclination Angle of inclination, /()
Droplet velocity (ms1)
8 14 22 33 46 52
0.40 0.47 0.70 0.98 1.41 1.44
The molar ratio of MTMS:MeOH:H2O:NH4OH is 1:3.5:4:3.5 · 101, respectively.
result of the heat produced and released during the formation of HCl [26,27] that occurs when organochlorosilanes are used to derivatize the alcogels. The TGA–DTA curves show a gradual weight loss in the case of the co-precursor method, Fig. 2(a), because the HR is present in the sol and its hydrolytically stable groups reside in the inner silica clusters as well as at the surface. In the case of the derivatized aerogels, only the surface SiO2 clusters are modified, so there is a sharp decrease in the weight loss along with a strong exothermic peak when the temperature is raised (Fig. 2(b)). The thermal stability of the phenyl-modified aerogel is much higher (520 C) than that of all the other aerogels [28]. This stability is due to the higher electrophilic nature of the arylsiloxanes as compared to the alkylsiloxanes [29–31]. The TMES modification resulted in the lowest shrinkage values recorded for the whole series of the aerogels. We attribute this retention of pore volume upon drying to the large number of non-hydrolyzable organic groups (R/Si = 3) present in the modified gel, which gives rise to more elasticity and does not allow the pore walls to close in the course of drying, as in the case of the Ôspring-backÕ effect [32]. In the case of the TMOS-based aerogels, a large contact angle of 135 was obtained for the HMDZ modified aerogels prepared by the co-precursor method, whereas the MTMS modification resulted in a lower contact angle of 95. In the case of the TEOS-based aerogels prepared using the co-precursor method, a similar trend was observed where the MTMS modification resulted in a contact angle of 120 and the TMES- and HMDZmodified aerogels had a higher contact angle of 135. These results are expected, if one considers the R/Si ratio for each of the hydrophobic reagents. As shown in the Table 3, TMES and HMDZ have a R/Si ratio of 3 while MTMS, PTES (more C–H groups), etc. have a R/Si ratio of 1, thus the larger the R/Si ratio, the better is the hydrophobic covering yielding higher contact angles. However, in the case of the TEOS precursor, the difference in contact angle is only 15, which is due to the difference in the volume of the HR for the same HR/TEOS molar ratio.
221
A second variable affects the contact angle measured on the modified aerogels: if the HR/precursor molar ratio increases, the contact angle also increases. However, there is a limit to which the molar ratio can be increased without affecting the monolithicity and other physical properties of the aerogels. As observed in the case of the TMES/TMOS systems, this limiting value is 4 [33], whereas in the case of the TMES/TEOS aerogels it is only 0.6 [24]. Hence, to obtain aerogels with higher hydrophobicity, we used MTMS itself as a precursor, because it has only one organic group attached to each Si atom along with the three hydrolyzable –OCH3 groups that react to form a three-dimensional gel network. By keeping the molar ratio of MTMS:MeOH:H2O at 1:3.5:4, with a NH4OH catalyst concentration of 5M, superhydrophobic aerogels with contact angles as high as 175 were obtained, as shown in Fig. 5. The wettability of a solid surface with water depends on the relation between the interfacial energies of solid/liquid (csl), solid/air (csa) and liquid/air(cla), and is given by YoungÕs equation [34,35]: csl ¼ csa cla cosðhÞ
ð2Þ
where h is the contact angle of the water droplet with the aerogel surface. Therefore, a high-energy solid/liquid interface can be created for h 175 (csl > csa). The superhydrophobic aerogels are opaque (optical transmittance 5%) and dense (bulk density 0.2 gcm3). The surface coated with superhydrophobic aerogel powder and an angle of inclination of 52 resulted in the highest-ever-reported velocity (1.44 ms1) of the water droplet. As shown in Table 4, the velocity of the droplet was studied as a function of the angle of inclination of the aerogel-coated surface. Even for a small inclination, (8), a droplet velocity of 0.4 ms1 was observed, which is higher than that recently reported velocity for a liquid marble (0.01 ms1) [36]. The higher water droplet velocity indicates better hydrophobicity. The results of the Laplace water intrusion into the aerogels produced using MTMS/TMOS molar ratios of 2 and 4 are shown in Fig. 4(a) and (b), respectively. The hysteresis loop is open, which occurs because water intrusion is not completely reversible due to the trapping of some water inside the pores of the aerogels. This interpretation was confirmed from the second cycle that yields a straight line rather than a loop. The pore size increased with increased MTMS/TMOS molar ratio [37]. When the molar ratio of MTMS/TMOS increased from 2 to 4, the Laplace pressure intrusion of water decreased from 14 to 8 MPa for the same volume change (Fig. 4(a) and (b)) indicating larger pores for the higher MTMS/TMOS molar ratio (4). In other words, more Laplace pressure (PL) is required to fill the water in the smaller pores than the larger ones of the hydrophobic aerogels according to the following formula [38]:
222
PL ¼
A.V. Rao et al. / Journal of Non-Crystalline Solids 350 (2004) 216–223
2cLV cos h a
ð3Þ
where cLV is the interfacial energy of the liquid/vapor interface, h the contact angle of the liquid with the solid surface, and a the radius of the pore. The Laplace water intrusion method indirectly provides an idea about the pore sizes as well as the hydrophobicity because the Laplace pressure is inversely proportional to pore size and directly proportional to the contact angle.
5. Conclusion The organic modification of the silica aerogels derived from TMOS and TEOS precursors was carried out using the co-precursor and derivatization methods with various organosilane compounds of the type RnSiX4n. In addition, superhydrophobic silica aerogels with a contact angle as high as 175 were produced using MTMS as the precursor. A coating of the superhydrophobic aerogel powder resulted in a very effective water-repellent surface. On this surface, the highestever-reported water droplet velocity of 1.44 ms1 was observed for an inclination of 52. The Laplace pressure intrusion studies revealed the macroporosity of the hydrophobic aerogels and a shift to larger pore sizes as the MTMS/TMOS molar ratio is increased. The other major findings of these studies are: (1) The aerogels prepared using co-precursors such as HMDZ, TMES, PTES, etc. are more hydrophobic (h > 130), but less transparent than the corresponding derivatized aerogels (h 120). (2) Organosilane-modified aerogels with the highestever-reported thermal stability of 520 C (in terms of hydrophobicity) are obtained using the PTES coprecursor. (3) The organochlorosilane-modified aerogels are opaque and cracked. (4) The TEOS-based hydrophobic silica aerogels with TMES as a co-precursor have the lowest volume shrinkage (<1%) of all the aerogels prepared in this study. The contact angle in this case is around 136. (5) The organically modified TMOS-based aerogels show higher optical transmission (85%) than the TEOS-modified aerogels (60%).
Acknowledgments The authors are grateful to the Department of Science and Technology (DST), New Delhi for the research project on ÔAerogelsÕ (No. SR/S2/CMP-01/2002). The
authors are thankful to Dr A.P. Rao, Research Associate, Mr M.M. Kulkarni, Senior Research Fellow, and Mr R.R. Kalesh for their help in the experimental work. The authors are grateful to Dr Stela Sidis (formerly at Labo, P.M.C., E´cole Polytechnique, Palaiseau, France) for help regarding the Laplace water intrusion experiments.
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