Particles, monodisperse spherical silica

It can now be said that the microemulsion-mediated silicon alkoxide sol-gel process has come of age. The ability to form monodisperse spherical silica particles (20-39) and monolithic gels (40-53) by this method has been amply demonstrated. Recipes are available to prepare materials with predetermined characteristics, especially particle size and polydispersity. Potential applications of these microemulsion-derived... 

The Mainz group, specialised in chromatography, developed a reproducible and rugged synthesis of monodispersed, spherical silica particles that exhibit mesoporosity. 

Nishikawa, S. and Matijevic, E., Preparation of monodispersed spherical silica-alumina particles by hydrolysis of mixed alkoxides,/. Colloid Interf. Sci., 165, 141, 1994. 

Kosmulski, M. et al., Zeta potentials of monodispersed, spherical silica particles in mixed solvents as a function of cesium chloride concentration. Colloids Sutf. A, 162, 37, 2000. 

S. Nishikawa and E. Matijevic Preparation of Uniform Monodispersed Spherical Silica-Alumina Particles by Hydrolysis of Mixed Alkoxides. J. Colloid Interface Sci., 165, 141-147 (1994). 

An elegant method proposed by Stober et al. [61] for the preparation of monodisperse spherical silica particles with controlled size comprises the hydrolysis and condensation of aUcoxysilanes in mixtures of ammonia, water, and a lower alcohol. The reactions involved can be represented by ... 

Hiroshi Suzuki, Shigeyuki Takagi, Hideki Morimitsu, and Shin-ichi Hirano Microstructure control of porous silica glass with monodispersed spherical silica particles, J. Ceram. Soc. Jpn., 100 (1992) 284-287... 

Yoldas B.E. Alumina sol preparation from alkoxides. Am. Ceram. Soc. Bull. 1975 54 289-290 Yoshida M., Lai M., Kumar N.D., Prasad P.N. Ti02 nano-particle-dispersed polyimide composite optical waveguide materials through reverse micelles. J. Mater. Sd. 1997 32 4047-4051 Yoshizawa K., Sugoh Y., Ochi Y. Controlled growth of monodispersed spherical silica by a new synthetic process. In Science ofCeramics Vol. 14, D. Taylor, ed. Stoke-on-Trent, UK The Institute of Ceramics, 1988... 

In 1956 Gerhard Kolbe (1) published the first results that showed that spherical silica particles could be precipitated from tetraethoxysilane in alcohol solutions when ammonia was present as the catalyzing base. Several years later, in 1968, StOber, Fink, and Bohn (2) continued in this research area and published the frequently cited original article for the preparation of monodispersed silica particles form alkoxide solutions. StOber et al. improved the precipitation process and described the formation of exceptionally monodispersed silica particles. The final particle size could be controlled over a wide range from about 50 nm to 1 1/2 p,m. Variations of the particle size could be achieved by different means, e.g., temperature, water and ammonia concentration, type of alcohol (solvent), TEOS (tetraethoxysilane) concentration, or mixing conditions. 

In 1968, Stober et al. (18) reported that, under basic conditions, the hydrolytic reaction of tetraethoxysilane (TEOS) in alcoholic solutions can be controlled to produce monodisperse spherical particles of amorphous silica. Details of this silicon alkoxide sol-gel process, based on homogeneous alcoholic solutions, are presented in Chapter 2.1. The first attempt to extend the alkoxide sol-gel process to microemul-sion systems was reported by Yanagi et al. in 1986 (19). Since then, additional contributions have appeared (20-53), as summarized in Table 2.2.1. In the microe-mulsion-mediated sol-gel process, the microheterogeneous nature (i.e., the polar-nonpolar character) of the microemulsion fluid phase permits the simultaneous solubilization of the relatively hydrophobic alkoxide precursor and the reactant water molecules. The alkoxide molecules encounter water molecules in the polar domains of the microemulsions, and, as illustrated schematically in Figure 2.2.1, the resulting hydrolysis and condensation reactions can lead to the formation of nanosize silica particles. 

Another important method for photonic crystal fabrication employs colloidal particle self-assembly. A colloidal system consists of two separate phases a dispersed phase and a continuous phase (dispersion medium). The dispersed phase particles are small solid nanoparticles with a typical size of 1-1000 nanometers. Colloidal crystals are three-dimensional periodic lattices assembled from monodispersed spherical colloids. The opals are a natural example of colloidal photonic crystals that diffract light in the visible and near-infrared (IR) spectral regions due to periodic modulation of the refractive index between the ordered monodispersed silica spheres and the surrounding matrix. 

Hematite was selected as a model colloid in this study due to its well understood a egation behaviour, the monodisperse, spherical nature of the colloids and the fact that the synthesis of colloids of various primary particle sizes (40 to 500 nm) is possible. While silica and clays may be more abundant in surface waters, hematite appears to be a good compromise between real systems and a simple model compound. 

The way to achieve HPLC is to return to packed columns, but to implement them with support particle and stationary-phase coating dimensions much smaller than those of packed column GC. Typically, one employs spherical silica particles of uniform diameters (monodisperse) in the range of 1.7-10 im. The interstitial spaces between the tightly packed spheres will be of similar dimension. Unlike the much larger support particles in packed colunm GC, these LC particles often... 

Dispersed colloidal silica particles of various sizes and colloidal silica crystals (opals) " have been used as templates of porous carbons, with spherical pores having narrow pore size distributions (PSDs). By coating monodisperse colloidal silica particles or crystals with a suitable carbon precursor, followed by carbonization and etching of the sUica, porous carbon particles can be obtained. The diameters of the mesopores are determined by the size of the silica particles. Because... 

A. van Blaaderen and A. Vrij Synthesis and characterization of colloid dispersions of fluorescent, monodispersed silica spheres, Langmuir, 8 (1992) 2921-2931 J.D. Wells, L.K. Koopal, and A. de Keizer Monodisperse, nonporous, spherical silica particles, Colloids Surf. A Physicochem. Eng. Asp., 166 (2000) 171-176 Howard A. Ketelson, Robert Pelton, and Michael A. Brook Surface and colloidal properties of hydrosilane-modifledStOber silica. Colloids Surf. A Physicochem. Eng. Asp., 132 (1998) 229-239... 

Using silica sols with the addition of PAA, films of ca. 25 fim in thickness are prepared on a stainless steel sheet with no cracks. SEM photographs of the surface of the silica thick films prepared without and with addition of 0.11 mass% PPA are shown in Figure 14-5. Both films are prepared under an applied voltage of 10 V for 10 mill. Micro-cracks are observed in the films prepared from the sol without PAA (Fig. 14-5(a)), whereas the films prepared with PAA have no cracks (Fig. 14-5(b)). SEM photographs of silica particles prepared with different amounts of PAA added are shown in Figure 14-6. Monodispersed spherical particles are prepared at all concentrations of PAA. The particles prepared with PAA are much smaller than those of the particles prepared without PAA. The addition... 

The films prepared by the electrophoretic sol-gel deposition are basically composed of monodispersed spherical particles, and have a lot of open spaces among these particles. If the open spaces are filled with some organic polymers, new type of inorganic-organic composite films with unique characteristics are expected to be obtained (Hasegawa, 1999). Silica particles are modified with 3-aminopropyltriethoxysilane (APS) and vinyltriethoxysilane (VTES). Smooth and crack-free films ca. 15 fim thick are obtainable when ASP-modified silica particles are used for cathodic deposition with addition of PEI. Thick films with reduced open spaces are obtained when VTES modified silica particles are co-deposited with polyethylene maleate. 

C., Sanchez, C, and Charleux, B. (2008) Elaboration of monodisperse spherical hollow particles with ordered mesoporous silica shells via dual latex/ surfrtctant templating radial orientation of mesopore channels. Langmuir, 24, 13132-13137. 

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