Silica particles applications

Hydrophobic silica defoamers work on a basis which may not be chemical at all. They are basically finely divided sohd sihca particles dispersed in a hydrocarbon or silicone oil which sei ves as a spreading vehicle. Kulkarni [Ind. Eng. Chem. Fundam., 16, 472 (1977)] theorizes that this mixture defoams by the penetration of the silica particle into the bubble and the rupture of the wall. Table 14-23 hsts major types of defoamers and typical applications. 

In 1972, Kirkland at E. I. du Pont de Nemours patented porous silica microspheres (PSM) specifically for high-performance liquid chromatography (HPLC) applications (3). Prior to this development, silica particles used for chromatographic applications were simply adapted from some other use. In the 1970s, Kirkland showed that porous silica particles could be used for size-... 

The latest innovation is the introduction of ultra-thin silica layers. These layers are only 10 xm thick (compared to 200-250 pm in conventional plates) and are not based on granular adsorbents but consist of monolithic silica. Ultra-thin layer chromatography (UTLC) plates offer a unique combination of short migration distances, fast development times and extremely low solvent consumption. The absence of silica particles allows UTLC silica gel layers to be manufactured without any sort of binders, that are normally needed to stabilise silica particles at the glass support surface. UTLC plates will significantly reduce analysis time, solvent consumption and increase sensitivity in both qualitative and quantitative applications (Table 4.35). Miniaturised planar chromatography will rival other microanalytical techniques. 

The application of polymer monoliths in 2D separations, however, is very attractive in that polymer-based packing materials can provide a high performance, chemically stable stationary phase, and better recovery of biological molecules, namely proteins and peptides, even in comparison with C18 phases on silica particles with wide mesopores (Tanaka et al., 1990). Microchip fabrication for 2D HPLC has been disclosed in a recent patent, based on polymer monoliths (Corso et al., 2003). This separation system consists of stacked separation blocks, namely, the first block for ion exchange (strong cation exchange) and the second block for reversed-phase separation. This layered separation chip device also contains an electrospray interface microfabricated on chip (a polymer monolith/... 

Butts, M. D. Genovese, S. E. Glaser, P. B. Williams, D. S., Hollow silica particles and methods for making same, US Patent Application 20070036705 2007... 

A RAM column functions through a size exclusion mechanism. Large biomolecules such as proteins are restricted from the adsorptive surfaces inside silica particles. Small analyte molecules are able to penetrate into the inner surfaces of the particles. As a result, protein molecules pass through the column rapidly and analytes of interest are retained on the adsorptive sites. Depending on the application, the analyte molecules are directed to MS for detection or transferred onto an analytical column for separation prior to MS detection. Detailed applications are discussed in a recent review.8... 

Figure 2.5 A broken, spherical silica particle entrapping an API has 85% free volume. Such particles are used in formulations such as Eusolex UV-Pearls that reduce dermal uptake compared to free UV filters thus they do not irritate the skin while they make new application possibilities for hydrophobic UV filters. (Photo courtesy of Sol-Gel Technologies Ltd.)... 

Silica particles surface-imprinted with a TSA of a-chymotrypsin were applied for the enantio-selective hydrolyzation of amides. Surprisingly, the particles showed reverse enantio-selectivity, i. e., the sol-gel imprinted with the L-isomer of the enzyme s TSA showed a higher selectivity for the D-isomer of the substrate [125]. Also Ti02 gels have been imprinted, e.g., with 4-(4-propyloxypheny-lazo)benzoic acid. QCM coated with ultrathin films of this gel were prepared by an immersion process and showed selective binding of the template [ 126]. These examples demonstrate once more the broad applicability of the concept of molecular imprinting. 

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... 

Fine silica particles are incorporated in the surface of plastic films to prevent the adhesion of two sheets in contact with one another. The application is called antiblocking. The relatively low refractive index of the silica particles makes them more difficult to detect. 

Two more recent applications for amorphous silicas are expected to grow to large volumes. Precipitated silicas are used in the manufacture of separator sheets placed between cells in automotive batteries. Their function is to provide a controlled path for the migration of conductive ions as a result of the porosity of the silica particles. Additionally, both precipitated silicas and aerogels are being developed for use in low temperature insulation, where the low thermal conductivity of the dry silica powders makes them useful in consumer products such as refrigerators (83). 

Extraction discs (0.5 mm thick, 25 to 90 mm diameter) constitute a variation of column-based SPE. These discs allow rapid extraction of large volumes of sample, which is not possible using a small column. The discs are made of bonded-phase silica particles, a few micrometres in diameter, trapped in a porous Teflon or glass fibre matrix. The discs are operated in a similar way to a paper filter on a vacuum flask. After extraction, the analyte is recovered by percolating a solvent through the filter. The major application of this technique is the isolation of trace amounts of compound dispersed in an aqueous medium. 

Y. Gong and H. K. Lee, Application of Cyclam-Capped (5-Cyclodextrin-Bonded Silica Particles as a Chiral Stationary Phase in Capillary Electrochromatography for Enantiomeric Separations, Anal. Chem. 2003, 75,... 

A novel synthesis route was developed to produce spherical silica particles. The synthesis is based on a modified Stoeber method and the room-temperature synthesis of MCM 41S-materials applying tetraethoxysilane, alcohol, water, ammonia and homopolymers as template. The specific surface area, the specific pore volume and the average pore diameter were varied in the following ranges 5 - 1,000 m2/g 0.1 — 1.0 cm3/g and 2-50 nm. With respect to catalytic applications hetero-atoms e.g. Al were incorporated into the silica framework. 

Preparation of spherical micrometric MSU-X mesoporous silica particles for chromatography applications... 

For sub-micron silica particles with grafted PDMS (up to 22 K), a different result was obtained (Yates and Johnston, 1999). The particles were unstable and flocculated well above the UCSD of the PDMS-C02 binary system. These results may suggest that it is necessary to raise the density to the UCSD for PDMS at infinite molecular weight (theta density). Another possibility is that the parameters used in the theory and simulation are not applicable to PDMS, since the polymer-polymer interactions are far stronger than the polymer-C02 interaction, unlike the case for PFOA. 

A direct application to chemical process technology of the principle of electric wind is in electrostatic precipitators (Leonard et al.,1983) and electrocyclones for size separation of particles in powder technology (Nenu et al., 2009). Electrostatic precipitators applied to exhaust gas cleaning have recently been reviewed 0aworek et al., 2007). A particularly interesting development is that of a small electrocyclone with a diameter of 75 mm (Shrimpton and Crane, 2001). With this device it was shown that the separation quality of the smallest size particles with a diameter below 38 pm doubled upon application of the electric wind. Later experiments performed with submicron silica particles demonstrated that classification of such particles is possible by use of an electrical hydrocyclone (Nenu et al., 2009). 

A main feature of interest in this work is the porosity of the amorphous silica forms. Porosity introduces a large surface area inside the silica particles. As interphase processes require a large surface/mass ratio, amorphous silicas are far more interesting for chemical and physical applications than their crystalline counterparts. 

---