Monodispersed spheres of silica were prepared with particular attention to the age of starting solutions, and a subtle irradiation (similar to mW power) of ultrasonic vibration, which we termed soft sonication, was applied to enhance the aging process. Aging effects on the precipitation behavior are as follows, (1) increase of the incubation period for precipitation, (2) increase of the final particle size, (3) improvement of the monodispersibility. The increment of the final particle size as a function of aging time is twice larger in the case of soft sonication than in the case of normal aging. We assume from these results that a starting solution, especially that including an organic solvent with a low dielectric constant, may be inhomogeneous at the microscopic level just after the preparation. In other words, the as-prepared solution may involve some clusters, which may cause unfavorable nucleation to degrade the monodispersibility. It is inferred that these "clusters" can be broken or dispersed by aging for days or weeks. Reduction of the aging period by soft sonication would be due to high frequency (similar to tens of kHz) and small amplitude (similar to nm) of the vibration.

Keywords: ultrasound, sol-gel process, solution structure, water-ethanol mixture