Silica sols, aggregation

Silica sols are often called colloidal silicas, although other amorphous forms also exhibit colloidal properties owing to high surface areas. Sols are stable dispersions of amorphous siUca particles in a Hquid, almost always water. Commercial products contain siUca particles having diameters of about 3—100 nm, specific surface areas of 50—270 m /g, and siUca contents of 15—50 wt %. These contain small (<1 wt%) amounts of stabilizers, most commonly sodium ions. The discrete particles are prevented from aggregating by mutually repulsive negative charges. 

Amorphous silica exists also in a variety of forms that are composed of small particles, possibly aggregated. Commonly encountered products include silica sols, silica gels, precipitated silica, and pyrogenic silica (9,73). These products differ in their modes of manufacture and the way in which the primary particles aggregate (Fig. 8). Amorphous silicas are characterized by small ultimate particle size and high specific surface area. Their surfaces may be substantially anhydrous or may contain silanol, —SiOH, groups. These silicas are frequendy viewed as condensation polymers of silicic acid, Si(OH)4. 

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. 

FIGURE 1034 Electron micrographs of (a) a gold sol with a radius of 7.2 0.8 run [74], (b) silica sols with a radius of 2.7 nm [75] aggregated by Browninan motion showing fractal geometry. Photo taken from Russel et al. [3, p. 282-283]. 

Recently, Boilot et al. utilized sol-gel method as the subsequent annealing treatment for the preparation of YV04 Eu " NPs (Mialon et al., 2008). At room temperature, water-phase precipitation always produced inorganic NPs with low crystallinity, so the author re-dispersed the crude YV04 Eu + NPs into a polymeric silica sol for sol-gel thermal annealing. The silica matrix could prevent the aggregation and growth of NPs even... 

The texture of precipitated silica-alumina depends on the texture of the silica when silica results from a sol-gel transition. The condensation of silicic acids leads to the formation of primary spherical particles (sol) which aggregate in defined conditions, forming the tridimensional network of the gel [1]. In the gel framework each primary particle of silica is connected to two or three particles [1] and the gel pores are the cavities existing between these particles [2], The size of the particles, conjugated to their connectivity, defines the surface area, the volume and diameter of the gel pores. Thus, the silica texture could be controlled by mastering the size and the packing of the silica particles [3], characteristics which depend on the conditions of preparation of silica sol and gel. 

The pore texture of an adsorbent is a measure of how the pore system is built. The pore texture of a monolith is a coherent macropore system with mesopores as primary pores that are highly connected or accessible through the macropores. Inorganic adsorbents often show a corpuscular structure cross-linked polymers show a network structure of inter-linked hydrocarbon chains with distinct domain sizes. Porous silicas made by agglutination or solidification of silica sols in a two-phase system are aggregates of chemically bound colloidal particles (Fig. 3.25). 

Many productive methods have been developed for the preparation of silica sol including acidification/121 electrolysation-electrodialysis,[13] ion-exchange,[14] peptization/111 and hydrolysis of silicon compounds/101 which can be grouped into two main types. One is called the aggregation method that contains two steps the polymerization of silicate ions and the aggregation of these polysilicate anions via condensation reaction between the hydroxy groups of the particles. The other one is called the peptization method, i.e., dispersal of a precipitate of Si02 to form colloid. The acidification method will be discussed in detail below. 

Silica sol, silica gel, and amorphous Si02 are spherical Si02 colloidal particles with different aggregation states. As shown in Figure 5.9, silica sol is a dispersed colloid, while silica gel is a continuous solid containing a cross-linked network of colloidal particles in three-dimensional space. The amorphous Si02 is basically fragments of silica gel. 

Silica gel could be prepared via the gelation of silica sol. The process for the formation of water-containing uniform gel from spherical silical colloidal particles is very fast. It is known that there is adhesive force on the surface of spherical silica collodial particles, which could lead to the aggregation of these particles. This process could be described as below. 

Silica sol which is a stable sol of monodispersed particles is made by the decomposition of sodium silicate at low concentration under controlled pH and surface treatment and then evaporation of water. A stable floccular aggregates of silica particles are made by the peptization of dilute hydrogels under controlled pH. The former material is used as fiber and paper sizing, binder and the latter is used for water treatment. 

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. 

The classic description of the structure and mechanisms of formation of silica sols by the hydrolysis and condensation of silicates in aqueous media was given by Iler in 1979 (I). According to Iler, polymerization may occur in essentially three stages (1) the polymerization of monomers to oligomers and then to primary particles, (2) growth of particles, and (3) particle aggregation to form networks that eventually give rise to a gel... 

Selection of the mobile phase is critical in the characterization of silica sols by SEC. As with the other separation methods, pH should be slightly basic, and low ionic strength must be used to prevent particle aggregation. In addition, the mobile phase must interact with the surface of the packing-particle pores to neutralize undesirable charge effects. Negatively charged surfaces within the pore can result in ion-exclusion effects whereby... 

The Future, Two types of silica sol products are needed in the 1990s specialty products and organosols. Specialty products are used in high-technology areas. Price is generally unimportant if they work. Typical examples of specialty products are monodisperse sols [i.e., one particle size, low sodium, and low metal (aluminum, iron, etc.) concentrations, and no aggregation]. 

Frolov, Shabanova, and co-workers (37-39) studied the transition of a sol into a gel and the aggregate stability of colloidal silica. Their aim was to develop a technology for the production of highly-concentrated silica sols and to use them as binders, catalyst supports, polymer fillers, adsorbents, and so forth. Kinetic studies were made of polycondensation and gel formation in aqueous solutions of silicic acids. At the stage of particle growth, poly condensation proceeds in the diffusion-kinetic region. With changes in pH, temperature, concentration, and the nature of electrolytes,... 

It is particularly good for colloids, emulsions and particles. Samples separated thus far include lattices, latex aggregates, silica sols, alkyd resins, perfluorocarbon emulsions, milk, carbon particles, hematite, clay, water-borne particles, liposomes, subcellular particles, viruses, and polymerized proteins. 

Chiral phospholipid molecules aggregate spontaneously to form tubes with diameters of 500 nm and lengths of 50-100 pm. Diacetylenic phosphatidylcholine structures were first coated in 1993 by Baral and Schoen [72] with silica nanoparticles. The tubule dispersion was mixed with Ludox (a silica sol with a particle diameter of 10-15 nm and negative surface charge at pH 8.2) and allowed to stand for up to 9 days, during which time a white precipitate formed. TEM analysis of the collected precipitate showed a film with a thickness of about 50 nm, composed of silica particles, on the hollow cylindrical templates. The adsorption of the nanoparticles to the headgroups of the phospholipid is believed... 

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