Polysilicic acid sol

Monosilicic acid is stable in aqueous solution only at low pH and very low concentration (Her, 1979, p. 209). The rate of condensation-polymerization of silicic acid is dependent on pH, concentration and temperature. At a certain stage, the polysilicic acid sol is converted into either a precipitate (i.e. a flocculated system) or a hydrogel. 

The primary polysilicic acid sol (or alumosilicic sol) coagulates and forms gel, the primary coagulation structure. In the presence of a supersaturated dispersion medium gel particles bridge forming phase... 

Figure 3.36. Changes in reduced viscosity of polysilicic acid sol due to particle growth and aggregation /I, single particles, diameter in nanometers aggregates, molecular weight in dalions. [From Acker (133a),]...

Polysilicic acid sols Mixing aqueous sodium silicate solution with 1 1 hydrochloric acid. Dialysis... 

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. 

Alcoa s rehydratable CP alumina powders can be used effectively to improve a viable FCC catalyst formulation. Well-formed microspheres which have superior attrition resistance can be fabricated by controlling the pH and viscosity of the FCC slurry. At this time, the preferred formulation uses CP-2 as the free alumina source and a silica sol which has aged at conditions conducive to the formation of chains of polysilicic acid aggregates. The addition of the rehydratable alumina can also have a beneficial effect on the cracking activity of the catalyst. The conversion and selectivity of a CP-2 formulated sample were comparable to a commercial grade catalyst and an experimental reference, which was alumina-free. After heavy metals poisoning, the CP-2 material had activity which was superior to the reference formulation. 

Ryabenko et al. (66) developed a method for synthesizing highly pure silica by heterogeneous hydrolysis of tetraethoxysilane followed by the concentration of the sol of polysilicic acid and thermal treatment. 

The attractive van der Waals energy of interaction (Va) for spheres in the 10- to l(X3-nm size range for silica sols discussed here varies as the inverse of the separation distance, and at any separation Va is directly proportional to particle size. The Hamaker constant (A), which controls the magnitude of the variation of van der Waals attraction with particle radius (a) and separation (Hq) between surfaces, is for silica-water-silica not a large number. Further, the known hydration-polysilicic acid formation at silica-water interfaces will further reduce the overall Hamaker constant in the silica sol-water-silica sol system. 

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... 

Based on what is now known of this system, as is discussed in Chapter 3, it is likely, that the sol made by hydrolyzing ethyl silicate contains polysilicic acid or silica particles that are so small that a substantial fraction of the silicon atoms are at the surface of the particles and bear OH groups. 

Gels can be made with pores so small that nitrogen molecules cannot enter, tier has prepared gels from polysilicic acid obtained by hydrolyzing ethyl silicate in an alcohol-water mixture with HCl catalyst at 25 C, and then diluting to % SiOj at pH 2. The particle size was calculated to be 19 A based on the specific surface area of 1405 m g by a titration procedure. Part of the sol was vacuum-dried and the glass-clear silica gel had a specific surface area by nitrogen adsorption of only 45 m ... 

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. 

The relation between the isoelectric point of polysilicic acid and the stability of sols, rate of gelling, and properties of resulting gels has been summarized by Klimentova, Kirichenko, and Vysotskii (54). This behavior can be summed up by saying that all the phenomena observed involve the formation and hydrolysis of Si-O-Si bonds, and that the rates of these reactions depend on a catalytic effect which is at a minimum at pH 1.5-2.0 in the presence of anions of strong acids and the minimum becomes greater at higher pH in the presence of anions of weaker acids. 

When polysilicic acid is formed at low pH at 0 C, polymer particles are formed, but the inner SiOH groups arc not all fully condensed. However, when such a sol is warmed and/or the pH is raised above 2, further internal condensation occurs. It is therefore logical to expect that if the sol is diluted at 0 C, only depolymerization will occur. However, if the temperature is raised, or if the pH is also raised, two processes occur. The particle starts to depolymerize or dissolve, but also it condenses internally with formation or more siloxane bonds that must later be broken to form monomer for reaction with molybdic acid. Thus the reaction rate with molybdic acid is greatly reduced. 

Diethylaniline adsorbed as an oriented monolayer on silica occupies an area of around 50 A. If it is adsorbed as a monolayer on spherical silica particles, in two experiments the ratio of amine to SiOj corresponded to particles about 1-2 nm in diameter. This is about the size of freshly formed polysilicic acid particles as judged from other studies. When the sol was aged 2 hr so that microgel began to be formed, the coacervate was a paste containing emulsified brine. 

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. 

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 is well known that sols of polysilicic acid in which the silica particles or polymer units are only 20-30 A in diameter can form gels with silica concentrations as low as 0.5-1.0% by weight, yet conventional silica sols of commerce, with particles 50-300 A diameter, will not form gels at such low concentrations. One reason is that the rate of gel formation is so extremely slow that such gels are seldom observed. 

This can be done in three general ways (1) deposition of additional silica on the gel structure (2) addition of active silica or polysilicic acid to a sol (particles larger than 5 nm or so) at the time of gelling or (3) heat>aging wet gel to a limited extent to increase coalescence of the particles. 

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