A SAXS study of silica aerogels

doi:10.1016/0022-3093(86)90027-X

Journal of Non-Crystalline Solids

Volume 86, Issue 3, 1 October 1986, Pages 394-406

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Abstract

Aerogels produced by hypercritical drying of gels from hydrolysis of TMOS in various pH conditions and subjected to a densification process were studied by SAXS using the LURE synchrotron facility. The evaluation of scattering data combined with BET measurements leads to a model of aerogels consisting of a light density matrix in which meso- and macro-pores are embedded. No fractal features were observed for the gels, the Porod's limit having an exponent n = 4. This could mean that either these aerogels are not fractals or that the SAXS method suffers from an inherent ambiguity for fractal dimension D = 3.

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Aerogel production generally consists of three steps: wet gel making, washing/solvent exchange and drying. Classical studies on inorganic aerogels using scattering techniques had revealed that the initial gel making step is responsible for the formation of main porous skeletons (Craievich, Aegerter, dos Santos, Woignier, & Zarzycki, 1986; Hasmy et al., 1995; Hu, Littrel, Ji, Pickles, & Risen, 2001; Lours, Zarzycki, Craievich, & Aegerter, 1990; Pahl, Bonse, Pekala, & Kinney, 1991; Posselt, Pedersen, & Mortensen, 1992; Reidy, Allen, & Krueger, 2001; Rigacci et al., 2001; Woignier, Phalippou, Vacher, Pelous, & Courtens, 1990). Subsequent solvent exchange and drying steps make only minor modifications, such as internal primary particle formation in the skeletons (Perissinotto, Awano, Donatti, de Vicente, & Vollet, 2015) and necking through Ostwald ripening in supercritical alcohol drying (Yoda & Ohshima, 1999).
The functionality of biopolymer aerogels is inherently linked to its microstructure, which in turn depends on the synthesis protocol. Detailed investigations on the macroscopic size change and nanostructure formation during chitosan aerogel synthesis reveal a new aspect of biopolymer aerogels that increases process flexibility. Formaldehyde-cross-linked chitosan gels retain a significant fraction of their original volume after solvent exchange into methanol (50.3 %), ethanol (47.1 %) or isopropanol (26.7 %), but shrink dramatically during subsequent supercritical CO₂ processing (down to 4.9 %, 3.5 % and 3.7 %, respectively). In contrast, chitosan gels shrink more strongly upon exchange into n-heptane (7.2 %), a low affinity solvent, and retain this volume during CO₂ processing. Small-angle X-ray scattering confirms that the occurrence of the volumetric changes correlates with mesoporous network formation through physical coagulation in CO₂ or n-heptane. The structure formation step can be controlled by solvent–polymer and polymer–drying interactions, which would be a new tool to tailor the aerogel structure.
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In this work, silica gel networks were modified with a silylating agent dimethoxy-methyl (3,3,3-trifluoropropyl) silane (SiF₃) to obtain hydrophobic aerogels of various surface energy values. The baseline aerogels were synthesized from tetraethoxy silane (TEOS) using a two-step sol–gel process followed by supercritical drying in liquid carbon dioxide. The resultant aerogels were characterized using scanning electron microscopy, Instron tensile tester, contact angle goniometry, and nitrogen adsorption–desorption isotherms. Three modification methods were studied. In method 1, TEOS and SiF₃ were combined before gelation; in method 2, SiF₃ was added after TEOS was hydrolyzed and before its condensation, and in method 3, SiF₃ was added after the gels were produced from TEOS. It was found that method 3 produced the best results in terms of achieving high values of hydrophobicity and compressive properties. The data on solid state ¹³C and ²⁹Si NMR spectra revealed chemical reactions between the silylating agents and the silanol groups on silica surface. The bulk density and the fractal dimensions of silica networks gleaned from small angle X-ray scattering (SAXS) data showed weak dependence on the degree of silylation. The silylation process rendered the aerogels strongly hydrophobic and also doubled its compressive modulus.
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Structure formation during the sol-gel transition of resorcinol-formaldehyde (RF) solutions was traced by dynamic light scattering (DLS) and static light scattering (SLS) techniques. The decay time spectra obtained by DLS revealed that both the growth rates of colloidal particles formed during the early stage of the sol-gel transition and the time required for the colloidal particles to form a firm network structure could be related to the ratio of catalyst to water (C/W) of the starting RF solution. SLS results indicated that the molecular weight of colloidal particles increased with the progress of the sol-gel transition, the rate of which was also affected by the value of C/W. The mesoporosity of RF aerogels, which were prepared by drying RF hydrogels with supercritical carbon dioxide, was confirmed to depend on the size of colloidal particles estimated from the decay time spectrum collected at the last stage of the sol-gel transition.
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1998, Journal of Colloid and Interface Science
RF aerogels were prepared by sol-gel polycondensation of resorcinol with formaldehyde in a slightly basic aqueous solution and supercritical drying with carbon dioxide. The aerogels were characterized by nitrogen adsorption and density measurements. A small-angle X-ray scattering (SAXS) technique was applied to the gelation process of RF hydrogels. The structure formation of the hydrogels during the sol–gel transition was revealed by applying Guinier and power-law equations to the SAXS data. At the initial stage of the synthesis of RF hydrogels, small clusters of ca 2 nm consisting of branched polymeric species formed, showing a mass fractal dimension. Then the clusters aggregated and formed particles of ca 3–6 nm. These particles showed a surface fractal dimension. The hydrogel structure was fixed by gelation and the particles grew to ca 4–7 nm. Finally their surface became smooth by aging. The influence of the amount of resorcinol, basic catalyst, and water used in the polycondensation on the porous structures of the aerogels was explained by the structure formation model proposed.
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1997, Journal of Colloid and Interface Science
Silica–titania and silica–alumina aerogels were prepared by the two-step hydrolysis of metal alkoxides, the sol–gel polymerization and the supercritical drying with carbon dioxide. As a result of characterization by nitrogen adsorption, the aerogels were mesoporous materials with high surface areas and had few micropores. The experimental results suggested that the surface area, the mesopore volume, and the pore radius of the aerogel depended on the titanium (Ti) or aluminum (Al) content. Adsorption isotherms of ethane, ethylene, propane, and propylene were measured on the aerogels prepared. It was found that the adsorption capacities for these hydrocarbons were changed by mixing Ti or Al to the silica aerogel. In the ethane–ethylene or propane–propylene system, the adsorption selectivity of ethylene or propylene on the silica–titania aerogel was lower than that on the silica aerogel. On the other hand, the selectivity on the silica–alumina aerogel was higher than that on the silica aerogel. Hence, the silica–alumina aerogel was useful for separating ethylene from ethane or propylene from propane.

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A SAXS study of silica aerogels

Abstract

Nature

J. Chem. Phys.

J. Chem. Phys.

Nouv. J. Chem.

Phys. Rev. Lett.