Intrinsic surface areas and bond site concentrations of silica aerogels of different densities

• We investigated surface area and hydroxyl bond density in silica aerogel with different densities.
• Chlorosilanes with different molar masses were used to derive hydroxyl bond site density.
• Capacities for chemisorption and physisorption of volatile species in aerogel have been discerned.
• An optimum aerogel density for chemisorption has been identified.

Silica aerogels were investigated to determine the relationship of density, surface area, and the concentration of molecular absorption bonding sites. Aerogel surface areas were measured, by Brunauer–Emmett–Teller (BET), for densities ranging from 35 mg/cm3 to 200 mg/cm3. Chlorosilanes were chosen as the test adsorbates for this study, as they do not physisorb in multiple layers on the mesoporous silica surface. Trimethylchlorosilane (TMCS) was selected as the primary adsorbate because of its high vapor pressure at ambient conditions. Aerogel samples were exposed to TMCS vapors until saturation. Complete infiltration was verified by Raman spectroscopy of a cross-sectioned sample. The total amount of adsorbate at saturation was determined through the weight gain using a microbalance. The cumulative TMCS surface area at saturation, calculated from the footprint of the TMCS molecule and the total number of molecules derived from the weight gain, is significantly smaller than the intrinsic surface area measured by BET. This implies that chemisorption, and not physisorption, governs the saturation phenomenon. This hypothesis was verified with chlorosilanes with larger molecules – phenyldimethylchlorosilane and diphenylmethylchlorosilane. The gained weight per unit volume in the silica aerogels scaled as the molecular mass of the radicals, bonded to the silica network, and did not scale as the footprint area of the different chlorosilane molecules. This work provides fundamental information for understanding the capabilities of aerogels to collect and concentrate organic molecules for subsequent spectroscopic analysis.

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