Highly porous silica xerogels without surface modification

• Silica xerogels with porosities up to 93% are observed.
• Neither surface modification nor forced ageing of the wet gels was applied.
• Oscillations in SAXS show a narrow particle size distribution.
• Low density correlates with predominant macroporous xerogels.

Silica xerogels with porosities up to 93% were prepared by a two-step process using tetraethoxysilane as silica source and a low water concentration in the stock solution. In the second step highly concentrated ammonia solutions were added (>0.5 mol/l). With these synthesis parameters, silica xerogels with densities down to 0.15 g/cm3 were prepared via ambient pressure drying at 50 °C. Depending on the synthesis parameters, the xerogels are either meso- or macroporous. Upon sample synthesis neither surface modification nor forced ageing of the wet gels was applied. Small angle X-ray scattering reveals oscillations in the Porod range of the scattering curves, indicating a narrow backbone particle size distribution. Differences between particle sizes determined via the two-phase model and the evaluation of the Porod-range can be explained by microporosity within the silica particles.

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