Structural features of silicas prepared in n-heptane/water/ethanol/sodium dodecylsulfate microemulsions

Abstract

Silica aerogels were prepared from hydrolysis of tetraethoxysilane (TEOS) in n-heptane/water/ethanol/sodium dodecylsulfate (SDS) microemulsions and ambient pressure drying (APD). APD was carried out after silylation of the wet gels surface with trimethylchlorosilane (TMCS). The samples were studied by small-angle X-ray scattering (SAXS) and nitrogen adsorption. The structure of the wet gels can be described as a mass-fractal with fractal dimension D=2.2, independently of the oil phase quantity, in a length scale which extrapolates the experimental range 0.4nm<1/q<13nm probed by SAXS. The fractality range is shortened with silylation and APD process in a mechanism in which share of the porosity is eliminated and the silica particles increase. Typical values for surface area and pore volume obtained for APD aerogels are comparable with those obtained by supercritical drying. The pore volume increases and the pore size distribution shifts toward the macropore region with increasing the oil phase. The mass-fractal features obtained from nitrogen adsorption are in good agreement with the SAXS results.