Electrostatic interactions in formation of particles from tetraethyl orthosilicate

The addition of electrolytes to tetraethyl orthosilicate (TEOS)-water-ethanol-ammonium solutions has been presented as a method of increasing particle sizes (Bogush and Zukoski, 1991; Blaaderen et al., 1992). Simulation results indicated that the addition of an electrolyte increases the size of the precipitated particles and leads to an increase in the final particle size. Previous investigators did not measure electrostatic surface potential of particles in the reaction systems, which is important in understanding the particle formation mechanism. Even small amounts of electrolytes added can influence the surface potential of colloidal silica. Thus, the particle formation mechanism during hydrolysis and condensation of TEOS was studied with in-situ measurements of particle size distribution, electric surface potential, and electric conductivity in the presence and absence of electrolytes. In the beginning phase of reaction where no TEOS hydrolysis occurred, the main ionic species were ammonia, and OH^- formed from water and ammonium. In-situ measurement of electric characteristics and size distributions showed that the reduction in the surface potential and the increase in the ionic strength increased the particle sizes with the addition of the electrolytes. Surface potential reduction was greater for KCl than for LiCl, and could be ascribed by adsorption of the electrolytes to the silica surface. The time-variation of the surface potential in the early reaction period was due to a change in the surface composition. The addition of KCl suppressed the formation of new particle in the seeds growth experiments, and allowed particle growth by 50-fold in volume. The electrostatic interaction between the particles was the dominant factor for particle size distributions in the particle formation.