Stability of sol-gel derived porous silica monolith to solvents

Abstract

The mechanical stability of a dried silica gel monolith (with micropores ã16 nm average diameter, prepared from an Si(OCH₃)₄-(CH3₂)NCHO-CH₃OH-H₂O-NH₄OH solution) on exposure to vapour and subsequent immersion in liquid, was examined using various solvents. The surface tension of the test solvent ranged from 17.1 dyn cm⁻¹ for diethyl ether, to 72.8 dyn cm⁻¹ for water. It was found that there is a distinct critical surface tension of the solvent in causing cracks in the gel on exposure to vapour and immersion in liquid for a particular shape of gel monolith. It has also been shown that the critical surface tension shifts to lower values when the gel is directly immersed in liquid, without prior exposure to solvent vapour, and that the critical surface tension also changes with the shape of the gel. The stability of the dried gel monolith to crack formation on exposure to solvent vapour and immersion in liquid solvent may be caused by the capillary force of the solvent filling pores on pore walls consisting of silica skeletons. The capillary force may be largely determined by the surface tension of the solvent. This may also explain the role of dimethylformamide as a component of the starting solution in producing dried gel rods used in this study without crack formation during the drying process.