Structure and diffuse-boundary in hydrophobic and sodium dodecyl sulfate-modified silica aerogels

• Hydrophobic ambient pressure drying aerogels were prepared from surfactant-modified silica gels.
• A mixture of hexamethyldisiloxane and trimethylchlorosilane was used as silylating agent.
• Silytation and ambient pressure drying modified the mass-fractal structure of the wet gels.
• The silica particles in the aerogels exhibited interfacial diffuse-boundary and internal inhomogeneity.

Small-angle X-ray scattering (SAXS) and nitrogen adsorption were used to study ambient pressure drying (APD) silica aerogels prepared from hydrolysis of tetraethoxysilane (TEOS) with additions of sodium dodecyl sulfate (SDS). The surfactant-extracted precursor wet gels have presented mass-fractal structure with typical fractal dimension 2.25 in a SAXS characteristic length scale from ∼10 nm to ∼0.35 nm. Hydrophobic APD aerogels with typical specific surface of 800 m2/g and bulk density of 0.20 g/cm3 were obtained after silylation of the precursor wet gels with a mixture of hexamethyldisiloxane (HMDSO) and trimethylchlorosilane (TMCS). The pore volume and the mean pore size of the APD aerogels increased with increasing the SDS quantity. APD aerogels presented most of the mass-fractal characteristics of the precursor wet gels at large length scales. The radius of gyration of the clusters of the APD aerogels (typically 17 nm) increased with increasing the SDS quantity, while the radius of the silica primary particles (typically 2.0 nm) increased at first with the addition of SDS (with respect to the sample without SDS) and decreased regularly afterward with increasing the SDS quantity. The primary particles presented yet some internal inhomogeneity and a diffuse-boundary interface with thickness of about 0.7 nm, according to a linear-gradient model for the diffuse boundary.

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