Density fractionated hollow silica microspheres with high-yield by non-polymeric sol-gel/emulsion route

Hollow silica microspheres were synthesized by non-polymeric sol-gel/emulsion technique using tetra ethyl orthosilicate (TEOS) as a source of silica. A sol mixture of TEOS, water, ethanol and acid was emulsified in a solution of light paraffin oil and surfactant (Span-80). Calcined spheres were density fractionated between density ranges: < 1.0, 1.0-1.594, 1.594-1.74 and > 1.74 g cm(-3). The samples were characterized by optical and scanning electron microscopy with energy dispersive X-ray analysis, Fourier transform infrared spectroscopy and laser diffraction size analyzer. Spheres of densities lower than 1.74 g cm(-3) were found to be hollow as observed from scanning electron microscopy (SEM) images and their yield was maximized to similar to 100% by using a specific TEOS volume ratio with respect to volumes of surfactant and oil. Decreasing the calcination temperature from 700 to 500 degrees C enhances the yield of hollow spheres emphasizing importance of slower diffusion kinetics at lower calcination temperature. Outer diameters of spheres were between 5 and 60 mu m with mean diameter expectedly increasing with increase in TEOS sol volume and with decrease in sphere density. It is proposed that silica shells form via hydrolysis and polycondensation at oil-water/ethanol interface in the water-in-oil emulsion, which subsequently form hollow spheres on removal of water-ethanol during calcination. (c) 2007 Elsevier B.V. All rights reserved.