Volume fraction dependent morphological transition of silica particles derived from sodium silicate

The volume fraction dependent morphological transition of droplets during the evaporation of colloidal silica solution was investigated using the spray-drying method. The colloidal solution was prepared from sodium silicate using the sol-gel method. Spray drying with a tubular reactor was used in the experiment, with the volume fraction of the colloidal silica varying from 15 to 2%. It was demonstrated that a morphological transition from a sphere shape to a donut-like shape takes place at a colloidal volume fraction of between 4% and 2%, even when the drying rate remains moderate and is not extremely fast. A spherical silica particle shape was found at a colloidal volume fraction of between 15% and 8%. The morphological transition depends strongly on the volume fraction of the colloids in the droplets. Further, the ζ-potential of the particles in the droplet also affects the transition by applying an interparticle electrostatic force. The same high surface charge of sol silica provides a repulsive interaction between the sol particles inside the droplet. The transition is hindered when the colloid volume fraction is increased because of the inherent spatial constraint. The Fourier-transform infrared (FT-IR) spectra of both the spherical and donut-like particles confirm the chemical bonding of the powder silica product. Moreover, X-ray diffractometry (XRD) analysis revealed an amorphous phase of the silica particles produced from spray drying. These preliminary results open up a new path for controlling the formation of a wide variety of silica particles using the spray-drying method. In addition, the different silica particle morphologies enable a variety of particle applications.