Silica from colloidal solutions

Migration and deposition of iron and silica from colloidal solutions... 

The pH gradient can be an important geochemical barrier controlling the deposition of silica from colloidal solutions. Such deposition would take... 

In some experiments and published reviews (Moore and Maynard, 1929 Krauskopf, 1956 Okamoto et al., 1957 Bien et al., 1958 Harder, 1965) the possibihty of deposition of silica from undersaturated solutions is examined. However, many of these experiments proved to be irreproducible, and in some cases the existence of colloids was not strictly proven or equilibrium was not attained. 

Very finely divided colloidal silica powders can also be obtained by treating certain silicate minerals such as clay or calcium silicate with acid, followed by suitable heat treatment in an alkaline medium. Similarly, finely divided colloidal silicas can be produced by precipitating silica from a solution of sodium silicate with carbon dioxide. Such precipitated silicas are commonly used as reinforcing fillers, for elastomers because they are extremely finely divided, and the ultimate particles are easily broken apart. Finely divided aerogels of silicas may be employed, such as those described by Kistler in U.S. Pat. Nos. 2,093,454 and 2,249,767. 

Muller, D., Malmsten, M., Tenodekaew, S. and Booth, C., Adsorption of diblock copolymers of poly(ethylene oxide) and polylactide at hydrophobized silica from aqueous solution, J. Colloid Interface Sci., 228, 326-334 (2000). 

The amounts oi adsorption of the polymer on latex and silica particles were measured as follows. Three milliliters of the polymer solution containing a known concentration was introduced into an adsorption tube(lO ml volume) which contained 2 ml of latex (C = l+.O wt %) and silica(C = 2.0 wt %) suspensions. After being rotated(l0 rpm) end-over-end for 1 hr in a water bath at a constant temperature, the colloid particles were separated from the solution by centrifugation(25000 G, 30 min.) under a controlled temperature. The polymer concentration that remained in the supernatant was measured colorimetrically, using sulfuric acid and phenol for the cellulose derivatives(12), and potassium iodide, iodine and boric acid for PVA(13). From these measurements, the number of milligrams of adsorbed polymer per square meter of the adsorbent surface was calculated using a calibration curve. 

It should be realized, at the outset, that colloidal solutions (unlike true solutions) will almost always be in a metastable state. That is, an electrostatic repulsion prevents the particles from combining into their most thermodynamically stable state, of aggregation into the macroscopic form, from which the colloidal dispersion was (artificially) created in the first place. On drying, colloidal particles will often remain separated by these repulsive forces, as illustrated by Figure 1.1, which shows a scanning electron microscope picture of mono-disperse silica colloids. 

Photonic crystals have only been studied in the laboratory for two decades, but naturally occurring examples exist, with the best known being the gemstone opal. Opals consist of tiny spheres of silica arranged in a face centred cubic structure. These are thought to have formed from colloidal silica solutions, and the colour depends on the size of the spheres. 

Colloid Stability as a Function of pH, Ct, and S. The effects of pertinent solution variables (pH, Al(III) dosage Ct, Al(III) dosage relative to surface area concentration of the dispersed phase S upon the collision efficiency, have been determined experimentally for silica dispersions and hydrolyzed Al(III). However, one cannot draw any conclusion from the experimental results with respect to the direct relationship between conditions in the solution phase and those on the colloid surface. It has been indicated by Sommerauer, Sussman, and Stumm (17) that large concentration gradients may exist at the solid solution interface which could lead to reactions that are not predictable from known solution parameters. 

The polymerization of basic metal ions will also interfere with crystallization. When a metal salt is mixed with a silicate, the increase in pH in the environment of the metal ion causes the formation of polymeric basic metal ions of colloidal metal hydroxides which are not likely to fit into a silicate crystal. Thus, the precipitation of a metal silicate from aqueous solution at normal temperature tends to produce a coagulation of positively charged colloidal metal hydroxide and negatively charged colloidal silica. 

Her also noted that in dilute solutions colloidal metal silicates precipitate at a pH slightly below that at which the metal hydroxide alone would be precipitated. The strong tendency for some hydroxides to react with silica is demonstrated by the fact that the addition of 300 p.p.m. of Mg (OH)2 to water will reduce the soluble silica content from 42 to 0.1 p.p.m. Aluminum oxide is capable of reducing the solubility of silica from 170 p.p.m. to 20 p.p.m. Cations capable of forming insoluble silicates will reduce the solubility of amorphous silica. Al3+ at a concentration of 100 p.p.m. reduces the solubility of silica from 120 p.p.m. to 1 p.p.m. at pH 8—9 (Okamoto et al., 1957). 

O. Olkhovyk, V. Antochshuk, and M. Jaroniec, Benzoylthiourea-modified MCM-48 mesoporous silica for mercury(II) adsorption from aqueous solutions, Colloids Surf. A 236, 69-72 (2004). 

Structure of silica colloids. Present ideas on the state of silica in aqueous solutions are examined in detail in the review by Sretenskaya (1970), where it is mentioned that one of the most typical features is the tendency to polymerize, i.e. to convert from a molecularly dispersed to a colloidally dispersed state. Polymerization is based on the process of condensation of silicol groups Si(OH)4 with liberation of water and as a result formation of the siloxane bonds Si-O-Si. 

The charge of a colloidal particle is explained by dissociation of the solid layer of the nucleus itself—the Si02 molecule—rather than by adsorption of ions from the solution. In a neutral environment a particle of colloidal silica... 

V. Electrolyte.—The nature of the anion often has a very important influence on the physical form of the deposited metal for example, lead from lead nitrate solution is rough, but smooth deposits are obtained from silicofluoride and borofluoride solutions. The valence state of the metal may affect the nature of the deposit thus, from plumbic solutions lead is deposited in a spongy form whereas relatively large crystals are formed in plumbous solutions. In an analogous manner, smooth deposits of tin are obtained from stannate baths, but from stannite solutions the deposits are of poor quality. The difference in the behavior of different electrolytes is sometimes due to the possibility of the formation of colloidal matter which serves to give a fine-grained deposit this may be the case in the deposition of lead from silicofluoride and borofluoride solutions where a certain amount of colloidal hydrous silica or boron trioxide may be formed by hydrolysis. 

Figure 2. Small-angle X-ray-scattering curves from silica polymers grown in alkaline solution with various ratios (R) of water to TEOS. The data are slit smeared and are plotted as log 1(h) versus log h, with the curves displaced vertically for clarity. The observed slopes are algebraically one greater than would be observed with pinhole geometry. The curve for R = 1 is consistent with a mass fractal structure with D/ = 2.84. The curves for R = 2-4 arise from colloids (Df = 3) with fractally rough surfaces with D, = 2.71, 2.51, and 2.45, respectively. (Reproduced with permission from reference 10. Copyright 1984 Elsevier.)...

Silica gel is jelly-like species formed from silicate solution under certain conditions, with properties between those of a solid and those of a liquid. It consists of a cross-linked network of SiC>2 colloidal particles in three-dimensional space. If it is an aqueous gel, the pores inside the gel will be filled with water or dilute solution. 

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