Use in Biological Research—Density Gradient

In separating blood cells in a colloidal silica medium some structural changes were observed (734). Addition of polyvinylpyrrolidone (PVP), as a competitive hydrogen-bonding agent for the silica surface apparently prevents interaction with the cells (735). 

The use of silica gel as a culture medium is described In Chapter 7 since such gels are made from other sources of silica as well as colloidal silicas. 

Colloidal silica is much more chemically reactive than the most finely pulverized sand, not only because the specific surface area is several hundred times greater, but also because amorphous silica is 10 times as soluble in water as crystalline quartz. 

Silica rapidly depolymerizes in the presence of strong alkali. Thus colloidal silica can be converted to a solution of sodium polysilicate containing from 4.2 to 6.0 moles of silica per mole of sodium oxide (738) or lithium polysilicate and lithium stabilized sols with 4-25 moles of silica oxide per mole of lithium oxide (739). These compositions cannot be obtained by dissolving a sodium or lithium silicate glass, nor by dissolving sand in the alkali because of the peculiar fact that lithium silicate is not soluble in hot water. For lower ratio silicates the advantage of starting with colloidal silica is only a matter of convenience because of the rapid reaction rate. 

Homogeneous glasses can be made by admixing the other Components intimately with colloidal silica (749). The chemical reactivity of colloidal silica plays a role in colored or conductive coatings on glass or refractory materials in which a vitrified bond is developed without damaging the substrate (558, 750). 

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