Colloidal polysilicic acid

Stumm, Huper, and Champtin (195) reviewed the interaction of polysilicic acid as coagulants with other colloids. Polysilicic acids coagulate positive colloids at low concentration, but in excess can reverse the charge and restabilize the system. Specific interactions can outweigh electrostatic repulsion thus negative polysilicate ions can flocculate negative silver bromide sols. 

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. 

Hofmann and collaborators 168) were probably the first to postulate that the free valences of silicon atoms in the surface of silicates must be saturated with silanol groups. Carman 169) visualized the structure of a particle of colloidal silica as a network of interlinked Si04 tetrahedra with hydroxy groups attached to the surface, due to the tendency of silicon to complete tetrahedral coordination. Each particle of silica can be considered as a macromolecule of polysilicic acid. 

Fig. 2.21. Spray dried particles prepared from 50% colloidal silica (-20 nm diameter) and 10% silica derived from polysilicic acid [13].

In the case of the VPO catalyst for the butane oxidation process and the MCM catalyst for the acrylonitrile process, the preferred precursor of the peripheral hard phase is polysilicic acid (PSA). The term "polysilicic acid" is generally reserved for those "silicic acids that have been formed and partially polymerized in the pH range 1-4 and consist of ultimate silica particles generally smaller than 3-4 nm diameter" (4). Small, discrete particles of colloidal silica also migrate to the periphery of the droplet, but they do not coalesce as extensively as PSA in drying. The larger the particle size, the lower the mechanical strength of the coalesced dry product. 

In alkaline solutions, silica exists in the form of silicate ions e.g., Si03 ). In dilute solutions (up to 0.1 mg of Si per ml) between pH 1 and 8, water-soluble monomeric silicic acid is the stable form. In more concentrated solutions of the same acidity, monosilicic acid condenses to disilicic acid and polysilicic acids which can be transformed into colloidal species. 

Activated silica, niiich may have a favorable or an unfavorable effect on filtration, is composed of ionized micella formed by polysilicic acid-sodium polysilicate. This become negatively charged colloidal micella. The behavior of activated silicas depmds on the conditions of neutralization and the grade of the silicate used in the preparation of the material. Activated silica is a coagulant aid that contributes to coalescence of the particles. Hence, it brings about an improvement in the quality of settled or filtrated water, depending on the point at vdiich it is introduced. 

It remains in the monomeric state for long periods in water at 25 C. as long as the concentration is less than about 2 x 10M. but polymerizes, usually rapidly, at higher concentrations, initially forming polysilicic acids of low molecular weight and then larger polymeric species recognizable as colloidal particles. 

Silica sol. May refer broadly either to polysilicic acid or colloidal silica. 

Formation by freezing silica sols When a solution of colloidal silica or polysilicic acid is frozen, the growing ice crystals exclude the silica until it remains as a concentrated sol between the ice crystals and then polymerizes and forms a dense gel. When the ice is melted the silica is obtained usually as irregular flakes... 

The behavior of polysilicic acids, colloidal silica, and silica gels cannot be understood without taking into account the fact that the solubility of silica is higher when... 

Although monosilicic acid, Si(OH)4, is believed to be the material actually deposited, it is possible to use active silica in the form of low molecular weight polysilicic acids (including extremely small colloidal particles) as a source of silica. Such small particles are highly soluble and are in equilibrium with a concentration of Si(OH>4 that is highly supersaturated with respect to larger particles or a flat surface. 

Clear solutions of silica may contain polysilicic acids or small colloidal particles that will not react completely with molybdic acid. Thus before determining total silica, depolymerization to monomer is necessary. 

In 1954 it was discovered by Her (101) that concentrated stable solutions containing Si02 Li20 molar ratios from 4 1 to 25 1 could be obtained by adding LiOH to a solution of polysilicic acid, to a suspension of silica gel. or to a silica sol free from alkali metal or other cations. Since the mixtures thickened or immediately set to a gel, this approach appeared useless until it was found that after a few hours or a day or so at ordinary temperature, the mass spontaneously liquefied. When such mixtures were heated to 80-100 C to accelerate liquefaction, they remained gelled. The liquid compositions contained both ionic and colloidal silica. 

Since compositions of all Si02 Li20 ratios from 4 1 to 25 1 are soluble and stable, this system offered Her an opportunity to examine the relation between the silica to alkali ratio and the nature of the colloid species pressent. Lithium polysilicate solutions were made by mixing solutions of polysilicic acid, obtained by ion exchange from 3.25 ratio sodium silicate, and lithium hydroxide and aging the mixtures at 25 C for a week until clear liquids were obtained at equilibrium. The solutions contained 10% of SiO and the 8162 Li20 ratios ranged from 3 1 to 10 1. 

It will be noted that in these experiments the solutions were made from TMA hydroxide and colloidal silica. It is possible that if TMA had been added to a dilute solution of polysilicic acid of low molecular weight at a ratio of 3.3 and then vacuum evaporated to 10% silica, solubility equilibrium would become established and the distribution of silica species would be like that in sodium silicate solutions. 

From the above, it will be evident that colloidal silicates may vary from rather homogeneous colloidal aggregates of extremely small ultimate units of polysilicic acids and metal hydroxide, to heterogeneous masses in which either silica or the metal hydroxide is present as discrete colloidal units, held together by the other component. 

The ionization constants of disilicicic and polysilicic acids, colloidal silicas, and gels arc pertinent to the polymerization of monomer and so are considered here. 

It is not possible to discuss all the techniques used for measuring or characterizing silicic and polysilicic acids and small colloidal particles, but some of the methods, especially applicable to following the polymerization, are reviewed. 

The term active silica has sometimes been used in referring to polysilicic acid. For example, a distinction has been made by Rule (160) between active silica and other forms of polymeric or colloidal silica. Active silica is defined as any silica in molecular or colloidal aqueous solution, in such a state of polymerization that when diluted with sodium hydroxide solution to a pH of 12, and concentration of about 0.02 percent SiOj, at 30 C, the silica will be depolymerized substantially completely to monomer in not more than 100 minutes. The monomer is determined by the molybdic acid method. 

Polyvinyl alcohol (169) forms a. coacervate with particles of colloidal silica much larger than those in polysilicic acid. It is probable that no coacervate can be obtained with polysilicic acid because the PVA chain is much too long to coat a... 

In the following discussions mainly polysilicic acids rather than colloidal silicas aro onsidered. 

This particular form of silica must be discussed under the topic of polysilicic acid, since it bears little relation to colloidal silica of commerce. 

Most silica gels of the past have been formed from polysilicic acids or colloidal silica particles so small, generally less than 5 nm in diameter, that the nature and structure of the gel was long in doubt. Now that commercial sols of uniform known size are available, the mechanism of gelling is much better understood. 

It has been generally observed that in a mixture of a solution of a polyvalent metal salt and polysilicic acid or colloidal silica at low pH, coprecipitation occurs as the pH is raised to just below the pH at which the metal hydroxide is precipitated from the metal salt solution when no silica is present. This relation has been put on a quantitative basis by Schindler et al. (266b), who found a relation between the equilibrium constants for the adsorption of metal ions on silica and the equilibrium constants for the formation of the hydroxides (see Chapter 6 for details). 

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