NMR and IR spectroscopy of silica aerogels with different hydrophobic characteristics

doi:10.1016/j.jnoncrysol.2005.03.048

Journal of Non-Crystalline Solids

Volume 351, Issues 19–20, 1 July 2005, Pages 1603-1610

https://doi.org/10.1016/j.jnoncrysol.2005.03.048 Get rights and content

Abstract

Silica aerogels, prepared by the sol–gel method and dried by the CO₂ supercritical method, were produced from tetramethoxysilane (TMOS) and methyltrimethoxysilane (MTMS) in different proportions, in order to change the proportions of hydrophilic and hydrophobic surface groups. The aerogel hydrophobic or hydrophilic properties were related to their surface structural characteristics. The relative proportion of Si–OH, Si–OCH₃ or Si–CH₃ groups present on the aerogel pore surface and its silica network structure were determined by ¹H, ²⁹Si and ¹³C nuclear magnetic resonance (NMR). The results obtained were correlated with the infrared transmission spectra. From these data, a possible description of the mixed hydrophilic–hydrophobic nature of the surface of these materials is proposed.

Introduction

High-porosity silica aerogels became in the recent years an important research area for many scientific and technological applications, such as thermal and acoustic insulation [1], [2], Cerenkov radiation detectors [3], photoluminescent [4] and radioluminescent [5] devices, comet dust [6] and aerosol particles [7] collectors, adsorption and catalyst supports [8].

Synthesized via the hydrolysis of silicon alkoxides Si(OR)₄ followed by condensation to yield a polymeric oxo-bridged SiO₂ network by the sol–gel process, these solids are dried by the supercritical method, a technique which largely attenuates the capillary stresses and has a minor effect on the gel specific surface area. Silica aerogels are extremely porous materials with high specific surface areas (500–1000 m² g⁻¹), low bulk densities (0.003–0.35 g cm⁻³), low thermal conductivities (0.014 W m⁻¹ K⁻¹), and refractive indices between 1.008 and 1.4 [9].

Despite having interesting properties and applications, silica aerogels are deteriorated with time and their use is limited due to their sensitivity to atmospheric moisture and water [10]. The silanol polar groups Si–OH present in the aerogel structure are the main source of hydrophilicity because they can promote the adsorption of water. Therefore, with appropriate surface modification, such as the replacement of H from Si–OH by hydrolytically stable Si–R groups (R double bond CH₃ or C₂H₅), the surface of the aerogel can be rendered hydrophobic so that the water molecules will be repelled [11].

In the present paper, four types of silica aerogels with a specific surface area ranging between 388 and 837 m² g⁻¹, and a pore size between 2 and 13 nm [12] were synthesized. These aerogels were dried at low temperature under the CO₂ supercritical fluid (T ∼ 31 °C, P ∼ 75 bars) because they are used as biocatalyst supports for the entrapment of lipases [13], [14]. These aerogels were made from a combination of two types of silicon precursors, namely the tetramethoxysilane Si(OCH₃)₄ and the methyltrimethoxysilane Si(CH₃)(OCH₃)₃.

Section snippets

Experimental procedure

The chemicals used in this study were: tetramethoxysilane (TMOS, 98%) and methyltrimethoxysilane (MTMS, 98%) from Aldrich, aqueous ammonia solution (0.1 M) and methanol (for analysis 99.8%) from R.P. Normapur, polyvinyl alcohol (Molecular Weight 15 000) from Fluka, technical grade acetone, deionized and ultra-pure water prepared by a ELGA PURELAB UHQ water purification system.

The hydrophobic silica aerogels were prepared with different proportions of reactants according to the procedure described

Results

The ¹H NMR spectra of silica aerogel (Fig. 1), published in a previous paper [12], show the presence of four peaks which correspond to four chemical shifts or more precisely to four groups of shifts. The peaks at 6 ppm can be easily attributed to adsorbed water at the silica. The peaks between 3.3 and 3.6 ppm are attributed to Si–OCH₃. Peaks at 2 ppm and 0 ppm indicated the presence of Si–OH and Si–CH₃ in the silica matrix, respectively.

The ²⁹Si NMR spectra (Fig. 2) show an evolution of the peaks

Discussion

To quantify the ¹H NMR spectra, the intensities of the different peaks were compared by integrating them according to the techniques suggested by Gauss and Lorentz. The obtained areas are proportional to the density of the studied groups. Their relative importance was reported in Fig. 5. By comparing graphically the peaks intensity (integrated area) for each chemical shift, it can be noticed that the Si–CH₃ groups increases with MTMS and simultaneously the peak intensity corresponding to the

Conclusion

Silica aerogels made from mixtures of two silica precursors, TMOS and MTMS, and dried with supercritical CO₂, were studied by ¹H, ²⁹Si, ¹³C NMR and IR absorption techniques. This structural study shows that the Si–CH₃ groups increase with the $\frac{MTMS}{MTMS + TMOS}$ molar ratio, with an optimum molar ratio of 40% for a better surface modification. The two techniques showed a consistent trend with each other regarding the network structure and the surface modifications. In particular, these combined

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NMR and IR spectroscopy of silica aerogels with different hydrophobic characteristics

Abstract

Introduction

Section snippets

Experimental procedure

Results

Discussion

Conclusion

J. Non-Cryst. Solids

J. Nucl. Instrum. Meth.

J. Non-Cryst. Solids

Appl. Catal.

J. Supercrit. Fluids

J. Mol. Catal. B: Enzymatic

J. Non-Cryst. Solids

Appl. Catal.

J. Non-Cryst. Solids

J. Non-Cryst. Solids

J. Non-Cryst. Solids

Chem. Mater.