Colloidal silicas

A detailed study by Grieser and co-workers [169] of the forces between a gold-coated colloidal silica sphere and a gold surface reveals the preferential adsorption of citrate ions over chloride to alter the electrostatic interaction. 

Comparison of particle diameter of colloidal silica by electron microscopy (cf,). by nitrogen adsorption (d ) and by light scattering (d,)... 

Photomultipliers are used to measure the intensity of the scattered light. The output is compared to that of a second photocell located in the light trap which measures the intensity of the incident beam. In this way the ratio [J q is measured directly with built-in compensation for any variations in the source. When filters are used for measuring depolarization, their effect on the sensitivity of the photomultiplier and its output must also be considered. Instrument calibration can be accomplished using well-characterized polymer solutions, dispersions of colloidal silica, or opalescent glass as standards. 

Collimation Collimators (+)-CoUinusin S. collinus Tb 365 Colloidal particles Colloidal silica Colloidal silver Colloidal state Colloidal sulfur Colloid mills Colloids... 

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. 

Microscopic sheets of amorphous silica have been prepared in the laboratory by either (/) hydrolysis of gaseous SiCl or SiF to form monosilicic acid [10193-36-9] (orthosihcic acid), Si(OH)4, with simultaneous polymerisation in water of the monosilicic acid that is formed (7) (2) freesing of colloidal silica or polysilicic acid (8—10) (J) hydrolysis of HSiCl in ether, followed by solvent evaporation (11) or (4) coagulation of silica in the presence of cationic surfactants (12). Amorphous silica fibers are prepared by drying thin films of sols or oxidising silicon monoxide (13). Hydrated amorphous silica differs in solubility from anhydrous or surface-hydrated amorphous sdica forms (1) in that the former is generally stable up to 60°C, and water is not lost by evaporation at room temperature. Hydrated sdica gel can be prepared by reaction of hydrated sodium siUcate crystals and anhydrous acid, followed by polymerisation of the monosilicic acid that is formed into a dense state (14). This process can result in a water content of approximately one molecule of H2O for each sdanol group present. 

Luzenac America, 237 LYDOX , colloidal silica, 84 Lyondell Chemical Company, 237... 

Synthon AcMaRi Cheniie GmbH Co. KG, 166 SYTON , colloidal silica, 124... 

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). 

Ionic silica is not totally removable by DI. Colloidal silica is difficult to remove by both DI and reverse osmosis (RO) it may cause some resin fouling as well as leaking into the treated water. Where the cation effluent is maintained at a pH of 2.0 to 3.0, however, silica tends to both depolymerize and ionize thus enabling its effective removal in strongly basic, anion resin beds. 

Also, basic factors such as the transport of materials, residual hardness, ion leakage, soluble iron, colloidal silica and clays, and other contaminants, which can produce scales and deposits in the FW lines and other parts of the pre-boiler section, may also produce similar detrimental effects in the boiler section. In the boiler itself, however, the buildup rate may be quicker and the results may be more devastating. 

The development of magnesium hydroxide [Mg(OH)2] at the heat transfer surfaces, especially at cathodic sites where localized pH may exceed 10. The Mg(OH)2 then reacts with colloidal silica from the bulk water to form silicate scale. 

Ground-water composition would be altered by the glass, either by two-phase chemical reactions or, more likely, by leaching of glass components. In particular, relatively high concentrations of dissolved and colloidal silica would be expected. 

Values of are obtained partly by previous calibration using a series of standard light scatterers whose Rayleigh ratios have been precisely determined. Typical standards used in practice are poly(methyl methacrylate) blocks, colloidal silica suspensions, or tungsto-silicic acid, H4SiW 2O40-... 

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 6.2. (a). Colloidal silica network on the surface of spores from Isoetes pantii (quill wort). Scale = 20 pm. (b). Polystyrene networks and foams produced as a biproduct of colloidal latex formation. Both types of colloidal system are typical of the diversity of patterns that can be derived from the interactions of minute particles. Scale (in (a)) = 50pm. 

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... 

The fabrication of colloidal silica and optical glasses by the sol-gel process has attracted a great deal of attention (8). The process relies on the hydrolytic polycondensation reactions of alkoxysilanes, usually (EtO)4Si, in which the reactive silanols (EtO)4 Si(OH)n (n = 1-4) are formed. These then undergo acid- or base-catalyzed condensation with both water and alcohol formation, as shown in Scheme 2. 

Note Colloidal silica (glidant) and sodium starch glycolate (disin-tegrant) were fixed at 0.1% and 5%, respectively, in all batches. Source. Ref. 100. 

Oleaginous bases consist of vegetable oil thickened with agents such as aluminum monostearate, colloidal silica, and xanthan gums. The lubricant properties of the oil make these formulations less adhesive than water bases. 

NH4VO3 solution + colloidal silica + 10%WO3/TiO2 optional + pore building agent => Suspension... 

Formation permeability damage caused by precipitation of dissolved minerals such as colloidal silica, aluminum hydroxide, and aluminum fluoride can reduce the benefits of acidizing (132-134). Careful treatment design, particularly in the concentration and amount of HF used is needed to minimize this problem. Hydrofluoric acid initially reacts with clays and feldspars to form silicon and aluminum fluorides. These species can react with additional clays and feldspars depositing hydrated silica in rock flow channels (106). This usually occurs before the spent acid can be recovered from the formation. However, some workers have concluded that permeability damage due to silica precipitation is much less than previously thought (135). 

Coradin, T., Durupthy, O. and Livage, J. (2002) Interactions of amino-containing peptides with sodium silicate and colloidal silica A biomimetic approach to silicification. Langmuir, 18, 2331-2336. 

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