Experimental Deconvolution of Depressurization from Capillary Shrinkage during Drying of Silica Wet-Gels with SCF CO₂ Why Aerogels Shrink?
Abstract
Silica aerogels are prepared by drying wet-gels under conditions that eliminate surface tension forces, typically by exchanging the pore-filling solvent with liquid or supercritical fluid (SCF) CO2 that is vented off like a gas. Thereby, silica wet-gels should not shrink during drying, but they do. According to the literature, most shrinkage (~71%) happens during depressurization of the autoclave. Here, based on prior literature, and working with wet-gels obtained via base-catalyzed gelation of tetramethylorthosilicate (TMOS), the basic hypothesis was that depressurization shrinkage takes place at the primary/secondary particle level. For this to happen there has to be available space to accommodate merging secondary particles, and a driving force. Secondary particles are mass fractals (by SAXS) and their empty space can accommodate primary particles from neighboring assemblies. The driving force was assumed to be H-bonding developing between surface silanols as soon as all fluids are removed from the pores. That hypothesis was put to test by replacing gelation solvents with nonhydrogen bonding toluene or xylene. Indeed, while the total drying shrinkage of toluene- or xylene-filled wet-gels was equal to that observed with aerogels obtained from acetone-filled wet-gels (~8-9%), the major part of that shrinkage (~74%) was transferred to the wet-gel stage. The remaining shrinkage (~26%) was assigned to interfacial tension forces between the pore-filling solvent and liquid or SCF CO2. Having transferred the major part of drying shrinkage to the wet-gel stage has technological implications, because it is easier to manipulate gels at that stage. Furthermore, our results underline that optimization of the drying process should take into account the fact that drying of silica wet-gels into aerogels is a two-stage moving boundary problem.
Recommended Citation
C. Mandal et al., "Experimental Deconvolution of Depressurization from Capillary Shrinkage during Drying of Silica Wet-Gels with SCF CO₂ Why Aerogels Shrink?," Journal of Sol-Gel Science and Technology, vol. 92, no. 3, pp. 662 - 680, Springer New York LLC, Dec 2019.
The definitive version is available at https://doi.org/10.1007/s10971-019-05124-x
Department(s)
Chemistry
Keywords and Phrases
Aerogel; Shrinkage; Silica; Solvent exchange; Toluene; Wet-gel
International Standard Serial Number (ISSN)
0928-0707; 1573-4846
Document Type
Article - Journal
Document Version
Citation
File Type
text
Language(s)
English
Rights
© 2019 Springer New York LLC, All rights reserved.
Publication Date
01 Dec 2019
Comments
The authors thank the NSF under award no. 1530603 for financial support and the Materials Research Center of the Missouri University of Science and Technology for support with materials characterization.