(81a) A Lattice Model of Silica Polymerization

Authors: 
Khan, M. N., University of Massachusetts Amherst
Monson, P. A., University of Massachusetts Amherst
Auerbach, S. M., University of Massachusetts


We have developed a lattice model to describe the gelation transition in sol-gel synthesis and other process for silica material synthesis. In this model, we have used an atomic representation of silica monomer where the silica tetrahedra occupies a unit cell in the lattice with silicon atom located at the center and hydroxyl groups on the vertices. This lattice model has proved useful in understanding a great variety of materials formation processes such as silica polymerization at low pH, silica crystallization into microporous materials, and formation of surfactant-templated mesoporous materials. However, the transition from silica gel at low pH to nanoparticles at high pH remains poorly understood. To address this, we have adopted a coarse grained representation of the charge balancing species (cation) which occupy sites on the lattice with excluded volume. Silica condensation/hydrolysis is represented as the overlap of hydroxyl groups, from two different silica monomers. We have also included a simplified version of the electrostatic interaction of the cation with the silica molecule.

Such an approach has enabled us to study silica polymerization over the pH spectrum. At zero concentration of the cation, which represents the iso-electric point of silica (pH=2), we have obtain a gel. In this region large cluster percolates the system. Whereas at high cation concentration we have observed nanoparticles that have a core-shell structure with silica in the core surrounded by a shell of cations.
At intermediate values of the cation concentration, we have observed a gelation transition. This region is marked by decrease plateau regime in the average cluster size of the particles around 3 nm, in good agreement with experiment findings. During the transition, the cations start populating the surface of the gel, prohibiting further polymerization. As the concentration of the cations is increased further, larger particles break up into smaller particles. We are in the process of developing a structural descriptor that would be useful in studying the transition. Future work is required to determine the detailed structure of these core-shell nanoparticles, and how they transform to zeolites.