Solution silicic acid

Precipitates Made by the precipitation of the silicic acid solution. 

Other than accelerating the formation of spherical nanoparticles of silica from metastable silicic acid solutions, the silaffins appear to have no structure-directing activity. If they are responsible for silica formation in the living diatom, as seems quite likely, control of the higher order architecture of the resulting silica apparently must be determined by the pre-formed shape of the silica deposition vesicle (the envelope within which the silica grows) serving as a complex three-dimensional mold. 

Effects of diluting silicate solutions Before proceeding to the main experiments involving silicic acid solutions made from silicates by methods already described, some preliminary tests were made by injecting samples of the higher ratio silicates directly into the molybdic acid reagent. In this case very small samples of one to ten microliter volumes were injected suddenly into rapidly stirred molybdic acid solution and the development of the yellow color recorded. 

Silicic acid solutions prepared from silicates... 

It is believed that the fresh dilute silicic acid solution at pH 1.7, prepared by the described method, represents the silicate species found in the silicate solution from which it was made. In the following experiments concentrated silicate solutions were compared with diluted and aged solutions as sources of silicic acids. Also silicic acids prepared at two different concentrations were compared. As a matter of side interest, the changes that were observed in silicic acid solutions as they were aged at 25 C were also noted. 

Figure 7. Classic light scattering results ho against time (h) for a 1,5 M mono-silicic acid solution at 2O C,...

In Method A-1, a fixed amount of this liquid is first charged in an evaporator that is heated from the outside with steam. Simultaneous to the beginning of solution concentration under reduced or normal pressure, aqueous active silicic acid solution is continuously metered into the evaporator to carry out particle growth and concentration at the same time. 

Forming a slurry comprised of catalyst, catalyst precursor or catalyst support particles dispersed in an aqueous silicic acid solution equivalent to a weight of Si02 not exceeding about 6 wt%, the relative amounts of the particles and silicic acid chosen so that the weight of the Si02... 

The aqueous silicic acid solution that is useful in this invention contains silica of the proper particle size, that is, no greater than 5 nm, and provides a solution of sufficient stability to allow the formation of the slurry and subsequent spray drying. The silicic acid can be in the form of a monomer or in the form of low molecular weight polymeric units. For a review of the characteristics of silicic acid, see R. K. Her, The Chemistry of Silica, John Wiley and Sons, NY., 1979. Monomeric silicic acid Si(OH)4 has never been isolated. It is a very weak acid... 

Replacement of sodium ions Na" by protons H by means of an ion exchange resin in its acid form, thus forming a silicic acid solution and initiating gelation by addition of a Lewis base (F ) or Brpnsted base (OH ). 

A general theory of polycondensation equilibrium in silicic acid solutions was proposed by Stbber (94). From it he deduced the concentration of monomer in equilibrium with polymers of different degrees of condensation. Further data are needed to check the validity of the complex equations that were developed. 

The value of the pH for optimum stability of silicic acid depends on what impurities are present in the solution. Traces of aluminum ions and to a lesser extent, iron, thorium, and beryllium ions tend to offset the effect of fluoride ion by forming complex fluorides and thus retard polymerization in this pH range. Depending on the purity of the solution, the pH of optimum stability may range from 1 to as high as 3-3.5. In silicic acid. solutions free from aluminum impurity, as little as 1 ppm of fluorine has a marked effect on the rate of polymerization in acid solution. 

The complex of tetraethylurea was prepared as follows. To 500 parts of a silicic acid solution prepared as above, aged for about 2 hr. were added 45 parts of tetraethylurea and 155 parts of sodium chloride. The mixture was stirred for 5 min and then centrifuged. A layer lighter than the aqueous phase formed and collected at the upper part of the solution 43 parts of this liquid layer was recovered, to which was added 12.5 parts of 95% alcohol. This resulting solution was analyzed and found to contain 21.4% SiOj by weight. The liquid complex was soluble in alcohol and in excess tetraethylurea. and as originally separated must have contained about 28% SiO,. 

To increase silica concentration without particle growth beyond 5-10 nm size, a silicic acid solution is alkalinized with sodium silicate and then more silicic acid is added while water is removed by vacuum distillation. Under these circumstances the low temperature avoids excessive particle growth in spite of the addition of active silica to the system (33). An earlier approach was to form the particles to the desired size in dilute solution and then concentrate by evaporating water and adding more dilute sol. Since the added particles are the same size as those in the evaporator, no buildup occurs (34). 

Marotta (40) made 5-10 nm particles by adding silicic acid solution to a dilute solution of sodium silicate over a period of hours at 25-50 C, then more at 50-90 C, and still more at 70-100"C while evaporating water. This apparently involved a combination of additional nucleation and particle buildup. 

Work of Formation of Contact Nuclei, and of 2D Nuclei, Aj, for Gypsum and Silica at Various Supersaturations, a, of Calcium Sulfate and Silicic Acid Solutions (f = 10 dyn)... 

The polysilicic acid globules in silica sols are very small, which does not allow one to conduct the experiments described with single particles. Hence, the experiments were conducted with glass threads (rods) modified with silica particles. These threads were brought into cross-contact in supersaturated silicic acid solutions. 

FIGURE 6.10 Histograms showing the differential distribution of the contact strength, p, between silica-coated rods in supersaturated silicic acid solutions at pH 7, with the compressive strength/= 10 dyn, and for... 

Akahane, M., and H. Yano, 1959. Silicic acid solutions II Si02-NaOH-MgCl2 aqueous solutions. Kogyo Kagaku Zasshi 62 366. 

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