Elsevier

Journal of Non-Crystalline Solids

Volume 350, 15 December 2004, Pages 216-223
Journal of Non-Crystalline Solids

Comparative studies on the surface chemical modification of silica aerogels based on various organosilane compounds of the type RnSiX4−n

https://doi.org/10.1016/j.jnoncrysol.2004.06.034Get rights and content

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 RnSiX4−n (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 (θ  136°) but the aerogels are opaque, whereas transparent (>80% optical transmission in the visible range) aerogels with lower contact angle (θ  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 ms−1. The results are discussed with respect to the ratios of organic and inorganic components of the organosilane compounds.

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 particle 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], [3], [4], [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 [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 RnSiX4−n, 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 gcm−3), and porosity (90–98%). The application of organosilicon compounds RnSiX4−n (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], [14], [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 (RnSiX4−n) organic groups. The use of six different co-precursors 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.

Section snippets

Sample preparation

Hydrophobic silica aerogels were prepared by two methods: (i) co-precursor; and (ii) derivatization. Various organosilane compounds of the type RnSiX4−n (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

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 gcm−3) for the modified aerogels using hydrophobic reagents having higher R/Si ratios (Table 3) as compared to the HRs having lower R/Si ratios. Overall, the TEOS-based

Discussion

From Table 1, Table 2, it is clear that the aerogels produced using TMOS as the precursor, are more transparent and less dense than the aerogels prepared by the TEOS precursor. The higher density (0.2–0.3 gcm−3) 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

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 RnSiX4−n. 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 highest-ever-reported water droplet

Acknowledgments

The authors are grateful to the Department of Science and Technology (DST), New Delhi for the research project on ‘Aerogels’ (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., École Polytechnique, Palaiseau, France) for help regarding the Laplace water intrusion experiments.

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