Abstract:

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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 (t_P), reaction time (t_R), type of buffer used, molar mass of polymer, polymer concentration (C_p), 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.

Key Words: Silica, silicification, biosilica, poly(allylamine hydrochloride) (PAH), tetramethoxysilane (TMOS), sol-gel, supercritical carbon dioxide.