Silica sol, coagulated

The control of silica sol coagulation by pH and by addition of simple electrolytes is said to be anomalous in that it is not simply predicted by conventional Derjaguin-Landau-Verwey-Overbeck (DLVO) theory. This chapter describes a model based on the control of coagulation by surface steric barriers of polysilicate and bound cations. The model suggests new experimental directions. 

The coagulation-dispersion behavior of aqueous silica sols is central to almost all processes requiring their unique adsorption, dispersion, gelation, and sol-gel properties. Aqueous silica sols are of particular interest in colloid science because their coagulation-dispersion behavior is said to be anomalous , that is, their stability in terms of electrolyte-pH control does not follow the pattern followed by almost all other oxide and latex colloidal materials. This chapter examines aqueous silica sol coagulation effects in light of studies of macroscopic silica-water interfaces and in particular the electrical double layer at such interfaces. 

Hydration Effects. In this discussion of silica sols at pH 11, 8, 6, and 2, the cation effects or critical coagulation concentration effects are focused on Na+. As indicated in Figures 1 and 2, the silica sol coagulation behavior, with respect to the sequence Cs+ through Li+, is not simple. Thus, in general terms, Li+ behaves in a manner similar to Na+ in that the critical coagulation concentration increases as the pH moves from 11 to 6 ... 

Silica sol activated silica sol electrolytic coagulation waste treatment 4-6 1-20 ppm 1.50 as sodium silicate inorganic chemical manufacturers... 

In drop-coagulation to form beads of alumina hydroxide, the top of a column holding oil and 100°C is fed with a blend of a sol of aluminium oxychloride and hcxamcthylcnc tctraminc. Under the influence of the temperature, the amine is decomposed to liberate ammonia, which neutralizes the chloride ions. The product is then ripened, dried, calcined and can be used as a support for reforming catalysts or hydrodesulfurization catalysts [2], In the same way. a silica sol feeding the bottom of a column of trichloroethylene at around 75 °C permits obtaining silica beads at the top. 

Low-concentration silica sols (Cgjo 300 mg/1) are very stable and showed no signs of coagulation in the whole pH range studied (2-10) in the course of three months. 

The resistance of silica sols to the action of electrolytes is reduced as Si02 concentration increases. In low-concentration sols (Csjo < 150 mg/1) no coagulation is observed. 

The coagulating effect of electrolytes is produced by cations, the relative effectiveness of which increases in the order Na -Ca -Mg. Figure 51 shows as an example the dependence of completeness of coagulation of a silica sol in an almost neutral solution (pH = 6) on the concentra-... 

In an experimental study of mixed iron-silica colloids we found the opposite phenomenon, stabilization of colloids of iron hydroxide by silica colloids, which is manifested very clearly in undialyzed iron hydroxide sols of average concentration with a ratio of Fe203 SiOj = 1 3. Colloidal iron in mixed solutions proved to be more resistant to the action of electrolytes than in isolated sols of iron hydroxide. Only colloidal silica shows any stabilizing effect on sols of iron hydroxide dilute solutions undersaturated with Si(OH)4 are not stabilizers. It is characteristic that colloidal silica is capable of stabilizing colloidal iron in the same pH ranges in which pure silica sols are stable in acid (pH < 4) and alkaline (pH > 8) environments. In slightly acid environments (pH = 5-6) iron-silica sols are unstable and decompose to form mixed sediments, which sometimes are not uniform due to different rates of coagulation and deposition. 

As in the case of iron, the hydrodynamic regime of the water basin has a considerable effect on the processes of accumulation of siUca. In the case of simultaneous deposition of silica and iron, the sols of their hydroxides are usually converted to gels having the composition of ferruginous chlorites. In the case of coagulation of silica sol around submarine active springs with constant discharge, only small lenticular intercalations of spilite are formed. 

Figure 7.34 Influence of the polarity of the medium on the increase in viscosity attainable with Aerosil 200, a coagulated silica sol.

After the enzymatic treatment the juice can be clarified. Flocculation aids and fining aids (gelatine, silica sol, etc.) help to coagulate the cloudy substances and facilitate their separation by settling and filtration. Bentonites can be used to eliminate proteins and other cloudy substances. Filtration (Kieselgur precoat filtration with plate and frame filters, rotating vacuum filters, and sheet filters as police filters ) is used to produce a crystal clear juice. 

Silica sols lose their stability by aggregation of the colloidal particles. Colloidal silica particles can be linked together or aggregate by gelation, coagulation or flocculation, or coacervation. 

Healy (Chapter 7) and Dumont also prefer the first approach. Healy sets down a model based on the control of coagulation by surface steric barriers of polysilicate plus bound cations. Healy s electrosteric barrier model is designed to stimulate new experimental initiatives in the study of silica sol particles and their surface structure. Dumont believes that many particular aspects of the stability of silica hydrosols could be explained not only by the low value of the Hamaker constant but also by the relative importance of the static term of the Hamaker equation. 

Figure 1. Schematic view of the coagulation domains of Ludox silica sols observed by Allen and Matijevic. (Adapted from reference 3.)...

To construct a model for anomalous stability, observations of silica sols at pH 11 are used. Allen and Matijevic (19) observed a critical coagulation concentration of approximately 0.15 M for Na+ at pH 11.0. If they take this pH 11 sol to, for example, pH 8.0, it remains stable in salt concentrations much greater than 0.15 M, and the critical coagulation concentration increases even further as the pH is reduced from pH 8 to pH 6. 

At pH 11.0, if the silica sol is treated as a normal 30-nm-diameter silica (without adsorbed silicate layers), DLVO calculation generates a coagulation condition (Vi-max = 0) at just under 2-nm surface separation. The electrostatic repulsion is inside this separation and cannot overcome the van der Waals attraction. 

The origin of the coagulation behavior of Iler s silica sols is far from understood. The electrosteric barrier model herein proposed is designed to stimulate new experimental initiatives in the study of colloidal silica sol particles and their surface structure. The adsorbed steric layer, impregnated with bound (exchanged) cations, at the surface of 1-100-nm-diameter silica sol particles has the general properties needed to understand the anomalous coagulation behavior. The details await experimental and theoretical input. 

Mixtures of rubber latices or elastomer foams were modified with colloidal silica to give improved properties. Typical processes involved drying, gelling, or coagulating the colloidal silica within the elastomer system. Silica sols were used with phenolic, formaldehyde-based, melamine, polyester, acrylic, vinyl or styrene polymer-copolymer, polyamide, and styrene-butadiene rubber systems to provide strength to films and coatings. 

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