Aqueous Dispersions of Silica

It has also long been knovm, that an acceleration of setting can be achieved by the addition of a finely divided amorphous silica to cementitious preparations (15). However, this additive was not used since initially the processability of the fresh concrete was greatly limited thereby. 

Recently, these problems could be overcome, and an aqueous dispersion has been described that contains water, precipitated silica, silicate, and a superplasticizer (16). The superplasticizer is a water soluble poly(carboxylate ether). The dispersion is free of binders. 

Precipitated silicas differ from pyrogenic silicas, which are also referred to as aerosils. The most preferred suitable plasticizers are Ugnosulfonate, sulfonated naphthalene-formaldehyde polycondensates, sulfonated melamine-formaldehyde polycondensates, and poly(carboxylate ether)s (16). 

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The oldest thermosetting vehicle used by man is porcelain enamel, which is an aqueous dispersion of silica, sand, and other components fused to produce a hard, ceramic finish. These high-quality finishes are still used extensively in bathtubs and household appliances such as range tops and hot water heaters where extreme... 

I begin this section by referring to the work on surfactants in solution and micellar and related systems. Popova and co-workers " reported relaxation times measurements for Na for sodium octanoate in aqueous solution and aqueous dispersions of silica. Concentration and... 

Dispersion of small colloidal particles can give rise to oscillatory forces. As one example, Klapp et al. [1448] measured the force between a silica microsphere (6.7 p.m diameter) and a silicon wafer in an aqueous dispersion of silica nanoparticles using an AFM. With a diameter of 26 2 nm, the dispersed nanoparticles were much smaller than the microsphere. All experiments were... 

Figure 11.15 Normalized force versus distance curves measured between a silica microsphere of 6.7(im diameter and a silica wafer in an aqueous dispersion of silica nanoparticles (26 nm diameter) with an AFM [1448]. The concentration of the nanoparticles was 2.0,4.5, and 13.2vol%. Results are vertically...

In a similar manner, OAV/O emulsions were prepared by first emulsifying oil into an aqueous dispersion of hydrophilic silica particles, and then gently reemulsifying the OAV emulsion so formed into an oil dispersion of hydrophobic silica particles. Figure 6.21 shows a typical microscopic image of a double emulsion with toluene as oil. 

The 3M Company manufactures a continuous polycrystalline alumina—silica—boria fiber (Nextel) by a sol process (17). Aluminum acetate is dissolved in water and mixed with an aqueous dispersion of colloidal silica and dimethylformamide. This mixture is concentrated in a Rotavapor flask and centrifuged. The viscous mixture is then extruded through spinnerettes at 100 kPa (1 atm) the filaments are collected on a conveyor and heat-treated at 870°C to convert them to metallic oxides. Further heating at 1000°C produces the 10-pm diameter aluminum borosilicate fibers, which are suitable for... 

TEX-WET colloidal silica is an aqueous colloidal dispersion of silica particles varying in solids content and particle size. 

Polymeric fibers are popular for reinforcing concrete matrices because of their low density (more number of fibers for a prescribed volume fraction), high tensile strength, ease of dispersion, relative resistance to chemicals, and relatively low cost compared to other kinds of fibers. Polypropylene and polyolefin fibers are typically hydrophobic, resulting in a relatively poor bond with concrete matrices compared to some other types of fibers. Treatment of polypropylene with an aqueous dispersion of colloidal alumina or silica and chlorinated polypropylene enhances the affinity of these fibers toward cement particles. Treatment of polypropylene fibers with a surface-active agent provides better dispersion of the fibers and a stronger bond between cement and fiber. The earlier attempts at surface treatments of polypropylene fibers have had only limited success and have not been commercially attractive. 

Vecchio C, Fabiani F, Gazzaniga A. Use of colloidal silica as a separating agent in film forming processes performed with aqueous dispersion of acrylic resins. Drug Dev Ind Pharm 1995 21(15) 1781-1787. 

Although the detailed nature of the surface of costal strips is not known, evidence from binding studies indicate that cations such as Co and Fe + are preferentially adsorbed onto these structures (9). Similar interactions have been shown with organic and colloidal materials (9). For example, phospholipid vesicles were shown to be closely attached to the surfaces of costal rods incubated in aqueous dispersions of phosphatidylcholine for 24 hr. Similar observations were made for costal rods incubated in solutions of colloidal silica. These results indicate that a range of interactions can take place on the surface of biogenic silica and such events may serve important functional roles, such as inhibition of dissolution and adhesion of components in the construction of microscopic structures. 

Cab-O-Sperse [Cabot], TM for aqueous dispersions of pyrogenic silica for use in the paper and textile industries. 

Laven, J. and Stein, H.N., The electroviscous behavior of aqueous dispersions of amorphous silica (Ludox), J. Colloid Interf. Sci., 238, 8, 2001. 

After an aqueous dispersion of monodispersed spherical colloids was injected into the cell, a positive pressure was applied through the glass tube to force the solvent (water) to flow through the channels. The beads were accumulated at the bottom of the cell, and crystallized into a three-dimensional opaline lattice under continuous sonication. So far, we have successfully applied this approach to assemble monodispersed colloids (both polystyrene beads and silica spheres) into ccp lattices over areas of several square centimeters. This method is relatively fast opaline lattices of a few square centimeters in area could be routinely obtained within several days. This method is also remarkable for its flexibility it could be directly employed to crystallize spherical colloids of various materials with diameters between 200 nm and 10 pm into three-dimensional opaline lattices. In addition, this procedure could be easily modified to crystalhze spherical colloids with diameters as small as 50 nm. ... 

Churaev, Nikologorodskaya, and co-workers (33) investigated the Brownian and electrophoretic motion of silica hydrosol particles in aqueous solutions of an electrolyte at different concentrations of poly(ethylene oxide) (PEO) in the disperse medium. The adsorption isotherms of PEO on the surface of silica particles were obtained. The thickness of the adsorption layers of PEO was determined as a function of the electrolyte concentration and the pH of the dispersed medium. The results can be used in an analysis of the flocculation and stabilization conditions for colloidal dispersions of silica (with non-ionogenic water-soluble polymers of the PEO type). 

The useful amorphous sihca are those having a particle size in the range from about 2 to 500 nm. In addition to the amorphous silica already present in aqueous solutions of high ratio alkah metal silicates, such sihcas can be obtained from silica sol (colloidal dispersions of silica in liquids), colloidal silica powders, or submicron particles of silica. The silica sols and colloidal silica powders, particularly the sols, are preferred in view of the shake-out properties of the binders made from them. 

Two thousand pounds of Portage No. 515 sand, AFS number 68 are charged to a batch muller. Forty pounds of pitch (Ashland Chemical Co. O grade) are thoroughly mixed with the sand over a period of three minutes. Twenty pounds of Cab-O-Sil M-5 pyrogenic silica powder, as a thick paste with 80 pounds of water, and 40 pounds of Du Font s Elvace 1873, a 55% aqueous dispersion of polyvinyl acetate/ethylene copolymer (13% ethylene) are then added to the mulled mixture over a period of two minutes. One hundred six pounds of Du Pont No. 20 sodium silicate are then added and the mixing continued for an additional two minutes. Half a... 

This phenomenon can be observed with many systems, for example, with aqueous electrostatically stabilized particles [126], with aqueous latexes containing particles with grafted polyoxyethylene chains [127] and with non-aqueous dispersions of coated silica particles [128] and sterically stabilized poly(methyl methacrylate) particles [129],... 

Alpha brass coating is used on steel to improve the adhesion of rubber to the steel. It reacts both with sulfur, producing zinc sulfide, and with the rubber. Organic cobalt compounds catalyze the reaction and necessitate high sulfur dosing. Resor-cin-formaldehyde-silica systems are an alternative. The brass layer is not needed if isocyanates are used as the adhesion promoter, although solvents are then required. Aqueous dispersions of chlorinated or sulfochlorinated polyethylenes cross-linked with polynitroso compounds offer an alternative [32]. 

Zhang et al. [63] prepared styrene-butadiene nanocomposites by dispersing an aqueous dispersion of montmoril-lonite and latex and flocculating the dispersion with acid. The performance of the rubber nanocomposites were compared with clay, carbon black, and silica rubber composites prepared by standard compotmding methods. The montmoriUonite loadings for the rubber nanocomposite were up to 60 phr. The morphology of the rubber nanocomposites by transmission electron microscopy appears to indicate intercalated structures. The mechanical properties of the rubber nanocomposites were superior to all of the other additives up to about 30 phr. However, rebound resistance was inferior to all of the additives except sUica. The state of cure was not evaluated. 

Clearly, the availability of the four-stepwise hierarchy in the structure of the nanosilica powder could cause some dependencies of the structural features and other characteristics of the colloidal dispersions on the pretreatment techniques and other conditions, which can influence the particles differently from different hierarchic levels. Therefore, several types of the sample preparation technique were applied. There were (a) ultrasonic (US) treatment of the freshly prepared suspensions of nanosilica (at different concentrations) at =1-12 h (Figures 1.89,1.90, and 1.95) (b) pretreatment of a dry silica powder in the ball mill for t cA = 1-24 h, then preparation of the aqueous suspensions sonicated for 1-9 h (c) MCA of the aqueous suspensions of silica in the ball mill for 0.1-24 h (d) MCA of the mixtures of the aqueous suspensions of silica and other compounds for 5-7 h and (e) addition of some compounds (polymers, biomacromolecules, surfactants, drugs) to the ball-milled suspensions of silica (vide infra). 

K. Shiratsuchi, H. Hokazono, Aqueous dispersion of core-shell type composite particles with colloidal silica as the cores and with organic polymers as the shells and production method thereof, US Patent 5,856,379 (1999). 

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