Large-particle silica sols

Manufacturing of Large-Particle Silica Sols. A method for manufacturing silica sols consisting of large-diameter particles of 50-200... 

Figure 5. Flow chart of production of large-particle silica sols.

This is an effective and relatively simple method for characterizing silica sols and other colloids [75]. It has also been used to determine the particle size distributions of polymer lattices [76,77]. Separations are performed in a column packed with particles having pores substantially of the same size. A carrier liquid is passed through the column as a mixture of colloidal particles passes through the bed, the larger ones exit first since they are too large to sample the pore volume. Intermediate sized colloids enter the pores and are retained according to the volume that can be... 

High quality microporous membranes are almost exclusively reported for silica or for binary silica-titania or silica-zirconia systems [42,46]. This is due to the very fast hydrolysis and condensation rates of the metal organic precursor of the metals relevant for membrane synthesis (Ti, Zr, Sn, Al). This usually results in too large particles in the precursor solution. Though many authors claim to have produced microporous materials by sol-gel methods (see e.g. Section 8.2.3), only a few have shown the synthesis of membranes of these materials and a still smaller number has characterised them with appropriate separation properties to be reasonably defect free. Therefore in the remainder of Section 8.2.1 a focus will be given to silica-based membranes. 

The general theory of nucleation and polymerization in aqueous systems, in which silica shows some solubility, is discussed in detail in Iler s book (3). However, very little was known at the time the book was published (1979) about the polymerization of silica when Si(OH)4 is formed in nonaqueous systems. Progress made up to 1990 in the understanding of the hydrolysis and condensation of silicon alkoxides that leads to silica gels or to silica sols of large particle diameter are lucidly discussed by Brinker and Scherer (8). Brinker s chapter in this book (Chapter 18) includes a clear explanation of the difference between hydrolysis and condensation of aqueous silicates and silicon alkoxides. 

The Stober route (8) is a well-known method for providing submicro-meter- or micrometer-sized silica particles by hydrolysis and condensation of silicon alkoxide an excess of base and water is used in the reaction. Compared with this method, ours has quite different reaction conditions, namely, the use of a limited amount of water and a large amount of acid. In contrast to the reaction of silicon alkoxide with a large amount of water in basic conditions, Sakka and Kamiya (9) noticed from the measurement of the intrinsic viscosity of silica sols that linear particles or polymers, not round particles, are formed with acidic conditions and the addition of a small amount of water. Therefore, the reaction conditions for this method for producing round micrometer-sized particles is new, and the mechanism of formation of round particles is of interest. 

Silica sol samples with wide particle-size distributions also can show a bias toward particle sizes larger than actual, largely because turbidimetric detectors respond much more strongly to larger sol sizes. As a result, the apex of the sol peak appears to be at a smaller retention time than actually is the case. This phenomenon causes the calculated size value to be somewhat higher than actual for sols that do not have a narrow size distribution. 

Alkaline slurries of precipitated silicas are used as polishing agents for semiconductor silicon wafers. The mechanism of polishing is generally believed to be chemical-mechanical the high pH of the slurry leads to oxidation of the silicon surface, followed by mechanical removal of the oxidized layer under the action of the silica particles and the polishing pad. In recent years the use of precipitated silicas in this application has largely been supplanted by the use of silica sols. 

Catalyst 3, prepared from a highly stable silica sol of large average particle size (18 m/i), behaves very much like catalyst 2. The low temperature formation and the high concentration of gamma-alumina obtained (up to 25 %) indicate very little interaction between the silica and alumina during the preparation. 

Codeposition, which represents the concurrent processes of colloidal crystal template formation and simultaneous filling of the interstitial sites with the desired framework material, is usually achieved by the deposition of a mixture of the templating colloids with the matrix material precursor (for example, a sol-gel precursor or nanoparticles). For this purpose, a dispersion of large particles, which will constitute the template, is mixed with nanoparticles of the framework material, which have to be small enough to easily fit into the interstitial space without interfering with crystal formation. By this method porous silica [25,32,35] and titania [32] were fabricated. 

U.S. Pat. No. 3,668,088 to Her describes the preparation by electrodialysis at high temperature of aqueous sodium or potassium silicate, of dilute silica sols of large particle size and very dilute sols of small particle size larger than 10 nm which can be concentrated by evaporation. The patent recites that at lower temperatures only extremely small particles of colloidal silica are obtained and the sols can therefore not be concentrated without gelling (Column 2, lines 27-29). Again in this case, the number of nuclei was not kept constant during the heatup period so that in aU examples the final particle size is larger than 10 nm. 

The attractive van der Waals energy of interaction (Va) for spheres in the 10- to l(X3-nm size range for silica sols discussed here varies as the inverse of the separation distance, and at any separation Va is directly proportional to particle size. The Hamaker constant (A), which controls the magnitude of the variation of van der Waals attraction with particle radius (a) and separation (Hq) between surfaces, is for silica-water-silica not a large number. Further, the known hydration-polysilicic acid formation at silica-water interfaces will further reduce the overall Hamaker constant in the silica sol-water-silica sol system. 

Silica particles used in emulsion polymerization are of different origin and, consequently, their sizes and surface properties significantly vary. Although the large majority of works involve the use of anionic silica sols, cationic silica has also been used on some occasions. Silica particles are most often amorphous colloidal silicas of commercial origin with diameters in the range of 10-80 nm. Silica particles of larger diameters are prepared by controlled hydrolysis and precipitation... 

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