On the Role of Cross-Linking Density of Surfactants on the Stability of Silica-Templated Structures

Silica-templated microstructures were synthesized using surfactants with different cross-linking densities, i.e., the ionic cetyl trimethylammonium bromide (CTAB) and nonionic Triton X-100. The textural properties of the final solids, including the wall thickness between pores (h w), were determined by N2 adsorption at 78 K. For Triton/SiO2 ratios of less than 2 (i.e., TXaD, TXaE, TXaF) the X-ray diffraction (XRD) measurements show periodicities of about 40 Å, in contrast to materials having Triton/SiO2 ratios greater than 2 (i.e., TXaG), which showed larger periodicities of 91 and 140 Å. After the materials were calcined at 500 °C, new periodicities appear in the XRD patterns, i.e., 90, 284, and 980 Å, which indicates the formation of larger cavities, probably arising from the relative weakness of the pore walls caused by the presence of structural defects. Also, this series showed an h w between 10.5 and 61.8 Å. In contrast to this behavior, the materials synthesized from the CTAB/TEOS system showed a consistent periodicity of about 35 Å, before and after calcination at 540 °C, with h w values between 11.7 and 16.2 Å. From these results it seems that structural stability should depend less on the wall thickness than on the cross-linking density associated with each surfactant, i.e., Triton X-100, and CTAB.