Experimental deconvolution of depressurization from capillary shrinkage during drying of silica wet-gels with SCF CO2 why aerogels shrink?
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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.
The major part of the shrinkage during drying silica wet-gels to aerogels with SCF CO2 is associated with the depressurization phase of the drying process.
A part of the shrinkage equal to that reported as depressurization shrinkage (70–75%) has been transferred to the wet-gel phase of processing.
The remaining part of the drying shrinkage has been assigned to interfacial tension.
The practical significance of those findings is related to the fact that it is easier to control shrinkage at the wet-gel phase of processing.
From a theoretical perspective, drying with SCF CO2 is a two-stage moving boundary problem.
KeywordsSilica Wet-gel Aerogel Shrinkage Solvent exchange Toluene
We thank the NSF under award no. 1530603 for financial support. We also thank Prof. Marc Hodes of Tufts University for fruitful discussions and the Materials Research Center of the Missouri University of Science and Technology for support with materials characterization.
Compliance with ethical standards
Conflict of interest
The authors declare that they have no conflict of interest.
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