Hydrous gels

Sn02, cassiterite, is the main ore of tin and it crystallizes with a rutile-type structure (p. 961). It is insoluble in water and dilute acids or alkalis but dissolves readily in fused alkali hydroxides to form stannates M Sn(OH)6. Conversely, aqueous solutions of tin(IV) salts hydrolyse to give a white precipitate of hydrous tin(IV) oxide which is readily soluble in both acids and alkalis thereby demonstrating the amphoteric nature of tin(IV). Sn(OH)4 itself is not known, but a reproducible product of empirical formula Sn02.H20 can be obtained by drying the hydrous gel at 110°, and further dehydration... 

Kinniburgh D.G., Jackson M.L., Syers J.K. Adsorption of alkaline earth, transition and heavy metal cations by hydrous gels of iron and aluminium. Soil Sci Soc Am J 1976 40 796-799. 

Zeolites are generally prepared under hydrothermal conditions in the presence of alkali (Barrer, 1982). The alkali, the source of silicon and the source of aluminium are mixed in appropriate proportions and heated (often below 370 K). A common reactant mixture is a hydrous gel composed of an alkali (alkali or alkaline-earth metal... 

Gelatinous surface layers are formed due to the presence of some impurity in the liquid that interacts with the liquid to form a hydrous gel structure, extending from the surface to a depth of about 900 A. The liquid makes up the main portion of the surface film (at least 97 percent by weight). The liquid is oriented and partly immobilized in an icelike configuration which acts as the linkage between solute molecules. 

Lignites contain water and can be considered to be hydrous gels. Removing the water causes irreversible changes in their structure. The water is an integral part... 

Barium titanate powders were produced using either an amorphous hydrous Ti gel or anatase precursor in a barium hydroxide (Ba(OH)2) solution via a hydrothermal technique in order to discern the nucleation and formation mechanisms of BaTi03 as a function of Ti precursor characteristics. Isothermal reaction of the amorphous Ti hydrous gel and Ba(OH)2 suspension is believed to be limited by a phase boundary chemicd interaction. In contrast, the proposed BaTi03 formation mechanism from the anatase and Ba(OH>2 mixture entails a dissolution and recrystallization process. BaTi03 crystallite nucleation, studied using high resolution transmission electron microscopy, was observed at relatively low temperatures (38°C) in the amorphous hydrous Ti gel and Ba(OH)2 mixture. Additional solution conditions required to form phase pure crystallites include a C02-free environment, temperature >70°C and solution pH >13.4. Analysis of reaction kinetics at 75°C was performed using Hancock and Sharp s modification of the Johnson-Mehl-Avrami approach to compare observed microstructural evolution by transmission electron microscopy (7). 

Ti Precursor Characteristics. The phase of the as-received hydrous Ti gel was both XRD and TEM amorphous. Heat treatment of the as-received gel at 130°C produced a mixture of an amorphous phase with anatase. Increase in the reaction temperature to the 150°C hydrothermal treatment produced anatase crystallites, whereas calcination at 600°C produced a rutile phase. Further investigation proceeded with only the as-received gel and hydrothermally treated precursor, hereafter referred to as the anatase precursor. Observation of the precursor morphology by TEM indicated a fine gel structure and a lOnm granular nature for the as-received hydrous gel and anatase precursor, respectively. 

As predicted, the hydrothermal treatment of the amorphous Ti gel at pH 7 resulted in the formation of nanosize granular anatase. However, the heat treatment and calcination of the as-received hydrous gel produced different phases as a function of treatment temperature. Results of TGA/DTA confirm the transformation from the amorphous to the metastable anatase and eventually to the stable rutile phase. 

Nakamura, K., Koga, Y. Sericin-containing polymeric hydrous gel and method for producing the same. Japan Patent, 2001,106794A. 

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