Silica sol

ALUMDIUMCOMPOUNDS - ALUMINIUMOXIDE(ALUMINA) - HYDRATED] (Vol 2) Alumina-coated silica sols... 

Silica sols are often called colloidal silicas, although other amorphous forms also exhibit colloidal properties owing to high surface areas. Sols are stable dispersions of amorphous siUca particles in a Hquid, almost always water. Commercial products contain siUca particles having diameters of about 3—100 nm, specific surface areas of 50—270 m /g, and siUca contents of 15—50 wt %. These contain small (<1 wt%) amounts of stabilizers, most commonly sodium ions. The discrete particles are prevented from aggregating by mutually repulsive negative charges. 

Zorbax PSM particles are made from small (80-2000 A), extremely uniform colloidal silica sol beads. In a patented polymerization process, these beads are agglutinated to form spherical particles. The size of the Zorbax PSM particles is controlled by the polymerization process, and the pore size is determined by the size of the silica sol beads. After polymerization, the silica is heated to remove the organic polymer and sinter the particles. The result is a spherical, porous, mechanically stable, pure silica particle that provides excellent chromatographic performance (Pig. 3.1). 

FIGURE 5.4 Stages in sol-gel processing are captured by a new electron microscopy technique. (1) Spherical particles tens of nanometers across can be seen in a colloidal silica sol. (2) Addition of a concentrated salt solution initiates gelation. (3) The gelled sample, after drying under the electron beam of the microscope, shows a highly porous structure. Courtesy, J. R. Bellare, J. K. Bailey, and M. L. Mecartney, University of Minnesota. 

Figure 4. SEC calibration curve for silica sol separation (hard sphere particles, single pore size column) (1), Column PSM-1500 (8.9 fim), 30 X 0.78 cm mobile phase O.IU Na.HPO -NaH.PO, pH 8.0...

Colloids embedded in a silica sol-gel matrix were prepared by using fully alloyed Pd-Au particles. The Mossbauer data have yielded evidence that alloying Pd with Au in bimetallic colloids leads to enhanced catalytic hydrogenation and also to improved selectivity [426]. 

Silica sol-gel inunobihzed La(OTf)3 (Scheme 48.2B) previously used in the acylation of a series of alcohols and activated aromatic compounds using acetic anhydride as acylating agent, showed a poor activity compared with other various sihca sol-gel inunobihzed triflate derivatives (tert-butyl-dimethylsilyl-trifluoromethane-sulfonate (BDMST), or trifhc acid (HOTf)). Acylation at the aromatic ring occurred over the BDMST and HOTf catalysts, while the La(OTl)3 catalysts only led to O-acetylated products [22]. Such behavior is characteristic... 

Spherical microparticles are more difficult to manufacture and can be prepared by several methods. One method prepares silica hydrogel beads by emulsification of a silica sol in an immiscible organic liquid [20,21,24,25]. To promote gelling a silica hydrosol, prepared as before, is dispersed into small droplets in a iater immiscible liquid and the temperature, pH, and/or electrolyte concentration adjusted to promote solidification. Over time the liquid droplets become increasingly viscous and solidify as a coherent assembly of particles in bead form. The hydrogel beads are then dehydrated to porous, spherical, silica beads. An alternative approach is based on the agglutination of a silica sol by coacervation [25-27], Urea and formaldehyde are polymerized at low pH in the presence of colloidal silica. Coacervatec liquid... 

Furthermore, the same sol-gel matrices have been used in a system where acid and base catalysis occur in the same pot without quenching either catalyst [29]. In this case, the acids were either entrapped Nafion (perfluorinated resin sulfonic super acid, a3) or entrapped molybdic acid (M03-Si02, a2), while the bases were two ORMOSILs (organically modified silica sol-gel materials), one with H2N (CH2)2NH(CH2)3 groups (bi) and the other guanidine base residues (b2) (Scheme 5.12). 

Fig. 1.3 Schematic representation of the entrapped enzyme in a silica matrix (left side). Enzymatic activity, under extreme alkaline conditions, of acid phosphatase (A) immobilized in silica sol-gel matrices with or without CTAB, or (B) in solution. Reprinted with permission from [56]. Copyright 2005, American Chemical Society.

Fujii and coworkers reported the synthesis and detailed structural analyses of alkylammonium/magnesium phyllosilicate hybrids [88], which were prepared by hydrothermal reaction from a mixture ofoctadecyldimethyl(3-trimethoxysilylpropyl)-ammonium chloride, silica sol, and magnesium hydroxide Mg(OH)2. The structure of the hybrid compound was studied by XRD, TEM, electron diffraction, high-resolution solid-state NMR, TG-DTA/MS, and elemental analysis. The resulting analytical information confirmed the unit structure, which consists of a 2 1... 

Preparation and physisorption characterization of D-glucose-templated mesoporous silica sol-gel materials. Chemistry of Materials, 11, 2023-2029. 

Beta/montmorillonite composite was prepared under dynamic hydrothermal conditions. Firstly, montmorillonite calcined at 800 °C were added to a diluted solution of sodium hydroxide, potassium chloride and TEAOH in distilled water and the resulting mixture was vigorously stirred for 1 h secondly, silica sol was added into the above uniform mixture to allow at least 3 h stirring finally, the gel was moved into stainless steel autoclaves (1L) and heated at 413 K for 48 h. The samples were characterized by XRD, N2 adsorption-desorption, FT-IR and SEM-EDS. The catalytic assessment experiments were carried out in a flowing-type apparatus designed for continuous operation. 

Silica sol-gel glasses are porous materials, which means some kind of solid material that has holes in it. These holes are also called pores. To call a material a porous material, there is usually some kind of implicit assumption of homogenization and length scale. The holes must be smaller enough, compared to the typical size of the piece of material that is considered, so that it is reasonable to consider the material as a mixture of solid framework and pores28. 

Reliable measurements of L-lactate are of great interest in clinical chemistry, the dairy and vine industry, biotechnology, or sport medicine. In particular, blood lactate levels are indicative of various pathological states, including shock, respiratory insufficiencies, and heart and liver diseases. Silica sol-gel encapsulation of the lactate dehydrogenase and its cofactor was employed as a disposable sensor for L-lactate51. The sensor utilized the changes in absorbance or fluorescence from reduced cofactor nicotinamide adenine dinucleotide (NADH) upon exposure to L-lactate. 

American scientists prepared the organo-silica sol-gel membranes60 and demonstrated in a single layer format for pH measurement and multiple-layer format for both C02 and NH3. The sensors used a hydroxypyrenetrisulfonic acid (HPTS) as the indicator immobilizes in a base-catalyzed sol-gel containing poly(dimethyl)siloxane, aminopropyl-triethoxysilane (APTES) and tetraethylorthosilicate (TEOS). This indicator gel was over coated with a hydrophobic sol-gel to reduce cross reactivity to pH when either carbon dioxide or ammonia were examined. 

Optical pH sensors with linear responses over different pH ranges can be designed by using different groups of indicators. A silica sol-gel glass pH sensor was developed by co-entrapping three indicators (bromocresol green, bromocresol purple, and phenol red)64, which produced a linear response over 3.5 pH units from pH 6.3 to 9.8 with a standard deviation of pH equal to 0.03. 

The two water-soluble porphyrins, 5,10,15,20-tetra(4-N-methyl-pyridiniumyl) porphyrin (H2TMPyP4+) and 5,10,15,20-tetra(4-sulfonato) porphyrin (H2TPPS4 ), were immobilized in the silica sol-gel matrix and served as the pH sensors as well65. The sol-gels doped with porphyrin were... 

The introduction of 2-[4-(dimethylamino)phenylazo]benzoic acid into a silica sol allows the preparation of pH-sensitive doped coatings upon glass substrates. The behavior of this system was evaluated as the function of pH changes in liquid and gas media68. Optical absorption and sensitivity against pH were monitored by Vis spectroscopy. Chemical and mechanical stability tests carried out with coatings demonstrated that they were resistant enough to be use in sensor devices for pH measurements in laboratories. 

Over the last several years, the number of studies on application of artificial neural network for solving modeling problems in analytical chemistry and especially in optical fibre chemical sensor technology, has increase substantially69. The constructed sensors (e.g. the optical fibre pH sensor based on bromophenol blue immobilized in silica sol-gel film) are evaluated with respect to prediction of error of the artificial neural network, reproducibility, repeatability, photostability, response time and effect of ionic strength of the buffer solution on the sensor response. 

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