The sol-gel synthesis can be obtained from metallo-organic compounds using alkoxides as precursors. Alkoxides with a M(OR)n structure, where M is a metal or a metalloid and R is an alkyl group (R = CH3, C2H5, …). Such chemicals are dispersed in a solvent (usually organic, ethyl alcohol) and react in an pH controlled medium. In this post, the effects of pH in the sol-gel synthesis are presented.
In the pH range 2.5–4.5, gelation time decreases with increasing temperature, pH and water:TEOS molar ratio. Aging alcogels in ammonia produces mesoporous xerogels because of a cross-linking reaction that occurs in basic media. In the synthesis of xerogels in a basic medium, the first hydrolysis of the alkoxide groups is the rate limiting step. Once the first alkoxide group is hydrolyzed, the remaining groups hydrolyze fast, and small nuclei are created from fully hydrolyzed species .
Alkaline media favours dissociation of silanol (surface) groups, which increases surface charge density and stabilizes colloidal suspensions. Consequently, gelation often requires the addition of electrolytes to destabilize the sol and obtain the alcogel . The following figures show the corresponding hydrolysis reactions that occur in acid and alkaline media. As it can be seen, a basic medium promotes the formation of a higher cross-linked than an acid medium
Fig. Solution pH influences the sol-gel process. Right, acid catalysis forms linear chains with length affected by pH. Left, basic catalysis forms branched chains .
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 Musgo, J., et al., Ammonia-catalyzed silica xerogels: Simultaneous effects of pH, synthesis temperature, and ethanol:TEOS and water:TEOS molar ratios on textural and structural properties. Microporous and Mesoporous Materials, 2009. 118(1–3): p. 280-287.
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Adapted from: Redin, Maeztu, Juan Deyo. “Design and study of nanostructured coatings for the improvement of the corrosion resistance of aluminium alloys.” (2016), Universidad Pública de Navarra
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