SYNTHESIS OF SILICA NANO AND MICROMETER SIZE STRUCTURES AT NEUTRAL pH AND
UNDER AMBIENT CONDITIONS
Author
PATWARDHAN, SIDDHARTH VIJAY
Degree
MS, University of Cincinnati, Engineering : Materials Science , 2002.
Pages
68p.
Abstract
The synthesis of silica particles by sol-gel processing and the chemistry
of colloidal silica are well known. With the recent advents in the
silicification process, at neutral pH and under modest conditions in diatoms,
it is of interest to develop an economical process using the above-mentioned
results and to understand the mechanism and the role of macromolecules in the
process in vivo and in vitro. To verify the specificity of polypeptides in
silicification and to attempt commercialization of silica synthesis at neutral
pH and under modest conditions, poly(allylamine hydrochloride) (PAH), a
cationically charged synthetic polymer was used in this research. PAH is
predicted to form hydrogen and ionic bonds with the intermediate negatively
charged growing species of the silica sol. To avoid the use of TEOS due to
solubility problems and slower kinetics, only tetramethoxysilane (TMOS) was
used throughout as the silica precursor. Dependence of silicification on
following process parameters was studied: TMOS pre-hydrolysis time (tP),
reaction time (tR), type of buffer used, molar mass of polymer, polymer
concentration (Cp), TMOS concentration, density and molecular environment of
active sites on the polymer and perturbation of the reaction mixture during
the course of the reaction. It was found that the TMOS pre-hydrolysis time was
an important parameter governing the resulting silica morphology along with
the reaction time and the TMOS concentration. Characterization was performed
using SEM, FTIR, EDS and XRD. The PAH, which acts as a catalyst / template,
was found to be incorporated into the silica particles. The synthesis of
elongated silica chains / rods of the nanometer size scale formed by the
orientation of a growing silica sol is also reported. The system was perturbed
by externally applied shear. It is proposed that the polymer orientation plays
an important role in the formation of such morphologies. It is also proposed
here that, using the results of the SCCO2 polymerizations and stabilization of
water - SCCO2 (W/C) emulsions, silica particles can be synthesized in W/C
emulsions. In this way, we can achieve controlled synthesis of silica
particles by controlling the size of the emulsion. In addition, we can
encapsulate water-soluble material in a silica shell.