Ultrasound driven aggregation and surface silanol modification in amorphous silica microspheres

Post formed, silica submicrospheres synthesized by Stober's method have been subjected to a high intensity ultrasound radiation (20 kHz, 100 W/cm²) and their size, morphology, and surface silanol structure modified in situ. The processed silica powders have been characterized by a variety of techniques, such as powder x-ray diffraction (XRD), transmission electron microscopy (TEM), dynamic light scattering (DLS), BET nitrogen adsorption, and FT-IR spectroscopy. The silica microspheres formed through an irreversible sol-gel transition have been shown to aggregate by the condensation of interparticle silanols to larger particles under the influence of the shock waves emanating from an imploding cavity. The particle size as a function of sonication time passes through a maximum, suggesting the disintegration of the aggregates on longer exposure to ultrasound radiation. The sonication of dried silica microspheres in an inert dispersant decalin also led to the aggregation of microspheres to a lesser degree, suggesting the deactivation of surface silanols. Infrared spectroscopic investigations suggest a disruption of the hydrogen bonded network of surface silanols. The observed morphological changes have been discussed in terms of direct effect of cavitation on well-formed spheres rather than changes in growth mechanism and capture of primary particles.