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 100% by using a specific TEOS volume ratio with respect to volumes of surfactant and oil. Decreasing the calcination temperature from 700 to 500 °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 μm 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.