Microamorphous hydrated silica

The most common matrix particle is kieselguhr (microamorphous hydrated silica) that has a pore size of 1 pm and a surface area of 20 m g , which can be made smaller by heat treatment ... 

Microamorphous silica can be divided into microparticulate silica, ie, microscopic sheets and fibers, and highly hydrated silica (1). The... 

Amorphous silica, ie, silicon dioxide [7631-86-9] Si02, does not have a crystalline stmcture as defined by x-ray diffraction measurements. Amorphous silica, which can be naturally occurring or synthetic, can be either surface-hydrated or anhydrous. Synthetic amorphous silica can be broadly divided into two categories of stable materials (1) vitreous silica or glass (qv), which is made by fusing quart2 at temperatures greater than approximately 1700°C (see Silica, vitreous silica), and microamorphous silica, which is discussed herein. 

Jorgensen suggested that in the presence of salt a uniform, reproducible, ordered layer of silica hydrate is developed on the solid surface, and this is less soluble than other forms of microamorphous silica. However, the lower solubility may be explained instead by the larger radius of curvature of the surface of the final aged amorphous silica. 

When such gels are dried, the pores become so small that even nitrogen molecules cannot penetrate, and the specific surface area by the BET method becomes very small. It would therefore not be surprising if the rate of dissolution should also be very low. However, there is no reason to believe that at equilibrium this form of silica should be any less soluble than microamorphous or vitreous forms. In fact, because of its hydration, since many of the silicon atoms arc linked to hydroxyl groups, the true solubility might be expected to be higher than the other forms. 

Lithium hydroxide differs from NaOH and KOH in the reaction of concentrated solutions with silica (194). In 1 solutions all three bases dissolve microamorphous silica at about the same rates, but in 2.2 N solutions LiOH does not react. Alsoi the addition of LiOH to strong solutions of the other alkalis retards the rate of dissolution of silica, apparently by the formation of an insoluble film of lithium silicate on the silica surface. It should be pointed out that lithium silicate is soluble in water below about 60 C, and the system acts much like the other alkali silicates, but at higher temperature lithium silicate is insoluble, and comes out of solution as a gel or precipitate. It appears that the less hydrated form of the lithium ion behaves more like calcium does.. ... 

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