Formation mechanism of amorphous Na2O–SiO2 spheres prepared by sol–gel and ion-exchange method

Mono-disperse Na2O–SiO2 (NS) amorphous spheres were synthesized by sol–gel and ion-exchange method, the latter involving ion exchange between alkali ion and proton of OH group that is formed by hydrolysis of metal alkoxide. The compositions, particle sizes and particle dispersion of NS spheres were investigated to clarify their formation mechanism. NS spheres containing sodium at the Na/Si ratio less that 0.9 were obtained without aggregation. The Na/Si ratios of NS spheres were controlled relatively easily employing various batch compositions, and the variation of Na/Si ratios of NS spheres with the batch compositions was almost independent of hydroxypropyl cellulose (HPC) species. The particle size of NS spheres was increased with an increase in amount of sodium included in NS spheres, and the rate of particle growth was increased with a decrease in molecular weight of HPC. HPC contributed not directly to the formation of NS spheres but to the dispersion of initial NS particles, and large HPC molecule inhibited the growth of NS spheres because of steric hindrance of HPC. Sodium played a role as a catalyst of condensation, which promotes the formation of Si–O–Si bonding.