Using supercritical CO2 for drying silica wet-gels should not cause any shrinkage, yet it does! This has enormous technological implications in the production of silica aerogel monoliths, for use, e.g., as thermal insulators. We have investigated the origin of that shrinkage, and we report that it lies with nanoscopic rearrangements within the nanoparticle aggregates that form the skeletal framework of the dry material (aerogels). This investigation employed silica wet-gels formed via base-catalyzed hydrolysis and polycondensation of tetramethylorthosilicate (TMOS: Si(OCH3)4). Typically, as-prepared silica wet-gels are aged (e.g., for 24 h) in order to reinforce their skeletal framework, and it is well-known that shrinkage observed during aging (syneresis) is due to dissolution and reprecipitation of silica (Ostwald ripening). For this study, the sol-gel process was monitored with liquid 29Si NMR and it was found that all monomer (TMOS) is incorporated into the developing solid framework within 2 h. Subsequently, in order to deconvolute drying shrinkage from syneresis, wet-gels were processed into aerogels at seven different stages during aging (2, 4, 8, 12, 16, 20 and 24 h). Importantly, we were able to emulate the drying shrinkage effect in the wet-gel stage by exchanging pore-filling gelation solvents with non H-bonding toluene and xylene. Such solvent-exchanged wet-gels did not show any drying shrinkage. It was concluded then that during a typical process, the nanostructure in the wet-gel state is stabilized by polar/H-bonding interactions between surface Si-OH groups and the usual solvents/reagents (methanol, acetone, water). During drying, the framework finds itself in a metastable state that seeks to achieve an energy minimum by increasing the interactions between the surface Si-OH groups, and that is what causes drying shrinkage. Characterization techniques for this project included solid-state 13C and 2Si NMR, IR, SEM, N2 sorption and Hg intrusion porosimetry, and small angle x-ray scattering (SAXS).

Structural reorganization of silica wet-gels upon drying: Why aerogels shrink?

256th National Meeting (2018)

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