Small-angle X-ray scattering study of nanopore evolution of macroporous silica gel by solvent exchange

Abstract

Small-angle X-ray scattering (SAXS) together with nitrogen adsorption, have been employed to analyse the pore-structure evolution behaviour, on the nanometre scale, by varying solvent-exchange treatment for silica gels having bicontinuous interconnected macropores. For the gels prepared in the system containing poly(acrylic acid), the difference in SAXS profile between gels immersed in ethanol and 1 mol dm^–3 nitric acid was small in the wet state, but was considerably enlarged on drying. The dried gel immersed in ethanol exhibited a mass fractal feature, while that immersed in 1 mol dm^–3 nitric acid was non-fractal up to several nanometres. The evolution behaviour during heat-treatment also differed, leading to a smaller size and lower volumes of the pores in the former gel. These variations in pore structures are explained by the occurrence of swelling in ethanol, and microscopic phase separation in 1 mol dm^–3 nitric acid solution. However, for gels prepared in the system containing poly(ethylene oxide), the difference in profiles of similarly treated dried gels was not so evident, possibly because the progress of microscopic phase separation was restricted by the adsorbed organic polymer on silanols of the gel matrix. On the other hand, a significant increase in the average pore size, as well as the pore volume, was observed by solvent exchange in basic conditions, irrespective of the kind of polymer or catalyst used for gel preparation. This effect is interpreted as a coarsening process through a dissolution and reprecipitation mechanism. The time evolution of the Porod slope, as well as its dependence on the solution pH in the wet state, were monitored by in situ SAXS measurements. The steep increase of the Porod slope accompanied by the extension of the power-law region in the wet state indicated reorganization within the silica gel structures.