Spontaneous formation of silica nanoparticles in basic solutions of small tetraalkylammonium cations

We have found that the spontaneous formation of silica nanoparticles is a general phenomenon in basic solutions of small tetraalkylammonium (TAA) cations. The nanoparticle formation and structure have been investigated using conductivity, pH, and small-angle scattering methods. The particles have a core-shell structure with silica at the core and the TAA cations at the shell. The particle core size is nearly independent of the size of the TAA cation but decreases with pH, suggesting electrostatic forces are a key element controlling their size and stability. The nanoparticle formation is a reversible process at low temperatures, in several ways similar to surfactant aggregation into micelles. These silica nanparticles may be a connection between the synthesis of zeolites and ordered mesoporous silicas such as MCM-41.