GELS AND HYDRATED COLLOIDS

Silicic Adds. The behavior of silicate ions in solution, the dependence of various properties on pn, the nature of silica sols and gels, and the study of hydrated silicas constitute chapters in inorganic and colloid chemistry that go far beyond the scope of this review. Germane to the present subject, however, are certain observations on the formation of monosilicic acid and its stepwise polymerization. 

The SAXS diagrams show a very intricate transition hydrated solid gel in the Li-exchanged solids. Three phases are coexisting a hydrated solid with two water layer, a gel and a very disordered solid. This study should allow to elucidate the formation process of a colloidal structure in such systems. 

Synthesis of pure ZrSi04 is also recorded through a procedure in which zirconium oxychloride hydrate was dissolved in water and equimolar colloidal silica added to it. The composite sol was refluxed at 100°C for hydrolysis. The gel powders obtained through the process were calcined at 900-1400°C for 1-8 h for the synthesis ofZrSi04 powders. The primary particles had a size of about 20 nm. 

A. As-prepared films The particles present after deposition of the aqueous sol-gel-derived Ti02 are coUoids in suspension and each colloid is comprised of hydrated crystaUites and hydrated amorphous Ti02, in which the latter likely forms a gel structure that buffers the crystaUites. This results in a loosely packed structure that can confine pore water within it and this water is removed by evaporation after drying at low temperatures for a prolonged duration. 

Microscopic sheets of amorphous silica have been prepared in the laboratory by either (/) hydrolysis of gaseous SiCl or SiF to form monosilicic acid [10193-36-9] (orthosihcic acid), Si(OH)4, with simultaneous polymerisation in water of the monosilicic acid that is formed (7) (2) freesing of colloidal silica or polysilicic acid (8—10) (J) hydrolysis of HSiCl in ether, followed by solvent evaporation (11) or (4) coagulation of silica in the presence of cationic surfactants (12). Amorphous silica fibers are prepared by drying thin films of sols or oxidising silicon monoxide (13). Hydrated amorphous silica differs in solubility from anhydrous or surface-hydrated amorphous sdica forms (1) in that the former is generally stable up to 60°C, and water is not lost by evaporation at room temperature. Hydrated sdica gel can be prepared by reaction of hydrated sodium siUcate crystals and anhydrous acid, followed by polymerisation of the monosilicic acid that is formed into a dense state (14). This process can result in a water content of approximately one molecule of H2O for each sdanol group present. 

Silica gel is synthetic amorphous silica consisting of a compact network of spherical colloidal silica particles. Its surface area is typically between 300 and 850 m2/g. The predominant pore diameters are in the range 22-150 A. Silica gel is produced via the following procedure a sodium silicate solution reacts with a mineral acid, such as sulfuric acid, producing a concentrated dispersion of finely divided particles of hydrated Si02,... 

Hydrates of Niobium Pentoxide. Colloidal Niobium Pentoxide.— Niobium pentoxide does not combine directly with water to form acids of definite composition. Two hydrates of the oxide, namely, 3Nb20s.4H20 and 3Nb205.7H20, have been reported,8 but their existence is very improbable. The term nioUe acid, is applied to the more or less hydrated pentoxide. When niobium pentachloride or niobium oxytriehloride, NbOCla, is hydrolysed with excess of water there is produced a white, amorphous, hydrated gel, which can also be obtained by the action of sulphuric acid or hydrochloric acid on alkali niobates 10 the precipitate is redissolved by excess of acid. Similar solutions axe... 

---