Colloidal solution of silica

Obtaining colloids of silica. The main laboratory methods of obtaining colloidal solutions of silica are condensational and come down to hydrolysis of soluble salts. The difference between obtaining colloids of Fe(OH)3 and Si02 is in the properties of the solutions of the original salts an acid reaction for iron and alkaline for silica. 

The effect of electrolytes on the stability of colloidal solutions of silica remains insufficiently clear despite the substantial number of pubhshed works. 

The stability of undialyzed colloidal solutions of silica in the absence of electrolytes (except NaCl) depends on the pH and concentration of the sol. Sols of all concentrations are quite stable in acid (pH = 2-3) and alkaline (pH = 8.5-10.0) environments. 

The colloidal stability of silica Suspensions in the present work was assessed by sediment volumes and from the optical coagulation rate constant. In the first method, 50 mg of silica was dispersed in 5 cm3 polymer solution (concentration 10-2 g cm 3) in a narrow tube and the sediment height found at equilibrium. Coagulation rates of the same systems were found by plotting reciprocal optical densities (500nm, 1cm cell) against time. When unstable dispersions were handled, the coagulation was followed in... 

In Zerrouki s experiments, the preparation of aqueous phases of identical clusters is performed in six steps. First, colloidal particles of silica, 1.2 pm in diameter, are synthesized. Next, the surface of the particles is made hydrophobic by chemical grafting. Then, an oil-in-water premix emulsion is made by adding an octane suspension of the colloids in an aqueous solution. Controlled shear of the premix in a Couette-type apparatus is subsequently performed to obtain a quasi-monodisperse... 

The easiest way to put a washcoat layer ort a mortolithic substrate is by using a colloidal solution of the coating material (60). For both silica and alumina, these colloidal solutions are readily available (and colloidal solutions of other materials are also available). Coating with colloidal solutions is a pore-filling method. 

The results obtained in studying colloidal forms of transport are less certain and not always reproducible. Therefore special experimental investigations of the stability of colloidal solutions of iron, silica, and mixtures of the two were conducted in conditions as close as possible to nature, in addition to summarizing and critically examining published data. 

Fig. 52. Coagulation of mixed colloidal solutions of iron and silica. Environment A—slightly acid B—slightly alkaline. Concentration of recent waters indicated at right. / = total precipitation of iron and silica with formation of banded sediments // = same, with formation of mixed sediments /// = total precipitation of iron and partial of silica / F= total precipitation of iron, silica in solution V = incomplete precipitation of iron, silica in solution VI— no precipitation observed.

Thus regardless of their origin, colloidal solutions of iron could hardly have accomplished the transport of a substantial amount of ore material. Coagulation of ferric iron colloids, even those stabilized by colloidal silica, already had occurred to a substantial extent in near-shore zones or even in rivers, under the influence of a relatively small increase in concentration of electrolytes. 

Yet some metallic surfaces with very small roughness could be reportedly produced outside of vacuum chambers [82], A layer of metal was consecutively polished with the colloidal slurries of silica and alumina. This procedure was followed by a multi-step cleaning from the slurry and residual metal particles by solutions of some chemicals. The roughness of a freshly deposited Pt surface characterized by grain sizes of 3 nm could be reduced to a rms of 0.1 nm. 

Sodium silicate, Si02 Na20, is commercially available as an aqueous (colloidal) solution. The silica/alkali ratio n is important in that ranges of 3 to 4 yield gels with adhesive properties, particularly suitable for grouting (beyond a ratio of 4, the silicate becomes unstable). Table 11.1 shows some of the related properties of commercially availablef sodium silicate solutions. 

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 common method for preparing a colloidal solution of hydrous ferric oxide, for example, has been to hydrolyze ferric chloride in solution by heat, and remove the hydrochloric acid by dialysis. Colloidal silica can be made in a similar way, that is, by dialyzing a solution of sodium silicate. One hydrolysis product—hydrochloric acid or sodium hydroxide—can pass through the membrane used for dialysis, whereas the colloidal particles of hydrous oxide cannot. The process is rather tedious and takes hours or even days. A much simpler method is to take out the acid or alkali with one of the solid ion exchangers or acid absorbers used in water purification. ... 

A study has been made of the aggregative stability of colloidal solutions of HDS as a function of concentration of a protein in the solution. It has been found that the presence of a protein at concentrations lower than 4 mg mL does not affect the stability of a colloidal solution of HDS because such a low amount of protein molecules is not sufficient for coalescence of silica particles into aggregates. At a high concentration of a protein (5 mg mL and more) the colloidal solution is stable owing to the colloid protection effect. The lower and upper limits of the dispersion stability make up an interval of concentrations (equivalence zone) suitable for flocculation of a colloidal solution of HDS to take place. In this case a colloidal solution of HDS possesses a higher proteinonektonic ability in comparison with an ordinary dispersion [5]. 

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. 

Colloidal suspensions of silica, primarily in the form of silicate ions, have been used commercially for over one hundred years [21]. Dent Glasser once called the silicate ions in solution the Cinderella anions because their chemistry was so intractable that they had little appeal to classical inorganic chemists [22]. They are generally added to systems to supply aqueous silica species, which through trial and error and experience are known to modify behavior hopefully in some desired manner. They are also used as raw materials to make silicon... 

High ratio sodium silicate solutions may be prepared by simply adding dilute silica aquasols (colloidal dispersions of silica in water) to dilute low ratio sodimn silicate solutions. In this case and until equilibrium is reached, average particle size of the colloidal silica fraction will be determined by time and silica particle size distribution of the original sol and the original silicate solution. 

However, Kondo [3 ] is of the opinion that the hydrolysis of the glass in water occurs and the calcium ions are released initially to the liquid phase. Simultaneously on the surface of slag grains an acid, colloidal shell of silica-alumina gel is formed. This shell has low permeability and hence the further slag reaction with water is hindered. In the presence of Ca(OH)2, added as alkaline activator, the siUcon and aluminum from the shell are released to the solution (Fig. 8.2). The solubiUty of aluminum compounds becomes considerably increased in the solution of pH higher than 12.5 because in this condition the Al(OH) ions are formed [4]. The concentration of aluminum in the liquid phase is increasing because the calcium aluminates crystallize a httle later, primarily the C-S-H (1) is formed. Simultaneously the solubility of hydrates formed in this condition is reduced. 

Silica gel is made from the coagulation of a colloidal solution of silicic acid. The term gel simply reflects the conditions of the material during the preparation step, not the nature of the final product. Silica gel is a hard glassy substance and is milky white in colour. This adsorbent is used in most industries for water removal due to its strong hydrophilicity of the silica gel surface towards water. Some of the applications of silica gel are... 

A similar technique was used for the preparation of polystyrene (PS)-Si02 nanohybrids, where colloidal silica solutions were mixed with PS solutions by means of ultrasonic homogenization [60]. Also, latex-silica nanohybrid films were synthesized upon mixing aqueous colloidal suspensions of silica and nanolatex polymer beads [61-63], Other silica-based nanohybrid systems with poly(ethylene oxide) (PEO) [64, 65], polyfvinyl alcohol) (PVA) [66], PS [67], polybutylacrylate [68], or PMMA [69] can be prepared by using the same suspension blending method. 

When silica is prepared by acidification of water glass (alkali solution of silica), polycondensation reactions occur between dissolved oligomeric silica species, resulting in (sub)colloidal particles [1]. These primary particles combine to very ramified aggregates, a process described by diffusion or reaction limited cluster-cluster aggregation with power-law dependent density (fractals) [2,3]. After gelation the fractal structure is still preserved at sub-micrometer scale, while at larger scale Euclidean behaviour is observed. 

Silicification of wood is commonly associated with volcanic ash, which is a rich source of readily available soluble silica (274). Correns (275) suggests that the silica may be precipitated from alkaline natural waters by the carbon dioxide evolved during decomposition of the wood. In this way, silica would be deposited immediately at the surface of the organic material, and as the organic portion dissolved away, it would be replaced by silica. This presupposes that the silica initially formed is amorphous and porous, permitting diffusion of solution through the specimen Since plant tissues contain membranes that can be penetrated by soluble silicic acid but not by colloidal particles of silica, Hellmers (276) believes that silicification occurs immediately after the soluble silica is liberated by decomposition of silicate minerals and before it can polymerize. 

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. 

Figure 2.4. Reaction of colloid species of silica in lithium polysilicate solutions of different ratios of SiO tLijO (indica d on curves) with molybdic acid reagent.

Cationic Silicon. The existence of a cationic form of monomeric silica is of course implied in the assumption that SKOH), has an iep. Colloidal particles of silica have been shown to carry a positive charge at low pH. but direct proof that silicon can exist as a cation has not been available. It is therefore interesting that in very dilute solution (66 ppm) monomeric silica has been shown to react with HCI to form the ion (H303Si(0H)3 CI according to Cherkinski and Knyaz kova (46). This was determined by the difference in precise conductivity measurements of 0.0025 N... 

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