Monodisperse colloidal silica spheres with radii in the range 10–500 nm were prepared by hydrolysis and condensation of tetraethoxysilane (TES) in a mixture of water, ammonia and a lower alcohol at several temperatures. It was attempted to establish a relation between the morphology and the chemical microstructure of the particle. Particle morphologies were examined with transmission electron microscopy and static and dynamic light scattering. The microstructure of the spheres was studied with quantitative direct excitation 29Si nuclear magnetic resonance (NMR) spectroscopy and a combination of qualitative cross-polarization 13C NMR and elemental analysis. A comparison was made between these particles and particles prepared from TES in an ammonia/water in cyclohexane microemulsion and also with Ludox® silica particles. The siloxane microstructure was found to show almost no variation as a function of concentration of reagents, catalyst, co-solvent and temperature. Around 65% of the silicon nuclei was bonded through siloxane bonds with four other silicons, approximately 30% was bonded with three other silicons and a few percent with only two. It was shown that under most experimental conditions several percent of the ethoxy groups never leave the TES molecule and end up inside the silica. The Ludox particles were found to consist of a more condensed silicon structure as compared with particles synthesized in alcohol, ammonia, water mixtures, whereas the spheres prepared in the microemulsion were less condensed and contained more alkoxy groups. The differences in particle morphologies — ranging from irregularly shaped rough particles to perfect, smooth spheres — are not caused by differences in siloxane and ethoxy microstructure. It is proposed instead, that a smooth and spherical particle shape is the result of the growth by monomers or small oligomers, and that a rough, irregular shape is the result of growth by larger silicon structures.

Particle morphology and chemical microstructure of colloidal silica spheres made from alkoxysilanes

The ultrastructural control of materials through sol-gel processes offers significant promise for the achievement of reliable performance in ceramics, glass and composites. Several examples of new structural, optical and electromagnetic materials with superior and unique properties are presented based on maximal homogeneity or heterogeneity attained through understanding of the fundamental chemistry. New concepts such as ceramic molecular composites and optically active gels have been derived through polymer, physical and synthetic chemistry. Scaling calculations based on molecular orbital calculations for prediction of the silanol polymerization mechanism, and hierarchical clustering predictions for the sol-gel derived ultrastructures are presented. The application of polymeric network theory to the design of gel ultrastructures is presented.

Better ceramics through chemistry

Phase behavior studies of microemulsions containing the monomers methyl methacrylate (MMA), acrylic acid (AA), and tetraethoxysilane (TEOS), the cross-linking agent ethylene glycol dimethacrylate (EGDMA), and water (W), stabilized by the surfactant cetyltrimethylammonium bromide (CTAB), were carried out. An analagous surfactant-free system was also studied and compared with the surfactant-based system. Macroscopically single-phase, transparent microemulsions were formed over a considerable portion of the composition domain for the surfactant-based system. The effects of hydrochloric acid (HCl) and base (NH4OH) on the gel times were examined. Selected compositions in the surfactant-based system were chosen to examine the effect of the water to TEOS ratio (R) on the thermal stability and pore morphology of the polymer composite formed from the hydrolysis and condensation reactions of TEOS and the polymerization of organic-monomer-containing microemulsions. The polymer composites thus obtained exhibit high t...

Polymer Composites Obtained by Polymerization of Microemulsions Formed with Inorganic and Organic Monomers

The hydrolysis of tetraethoxysilane was studied in an ethanol-water system with ammonium hydroxide catalyst. Reactions were carried out with water concentrations of 5M and 10M and ammonia concentrations up to 1.7M, and the products examined by electron optical methods. Variation of reagent concentrations lead to changes in the product morphology from small irregular particles, through roughly spherical and spherical particles to large fused particles. Within the range of particulate material the variations in size can be related to the effect of reagent concentrations by consideration of the reactions occurring. The effect on morphology of replacing ammonia with ethylamine was investigated and resembles that of the use of much higher ammonia concentrations.

Jeffrey Adams

Papers

An Electron Microscopy Study of the Alkaline Hydrolysis Products of Tetraethoxysilane