Silica gels structures

The senior author first became interested in acid-base cements in 1964 when he undertook to examine the deficiencies of the dental silicate cement with a view to improving performance. At that time there was much concern by both dental surgeon and patient at the failure of this aesthetic material which was used to restore front teeth. Indeed, at the time, one correspondent commenting on this problem to a newspaper remarked that although mankind had solved the problem of nuclear energy the same could not be said of the restoration of front teeth. At the time it was supposed that the dental silicate cement was, as its name implied, a silicate cement which set by the formation of silica gel. Structural studies at the Laboratory of the Government Chemist (LGC) soon proved that this view was incorrect and that the cement set by formation of an amorphous aluminium phosphate salt. Thus we became aware of and intrigued by a class of materials that set by an acid-base reaction. It appeared that there was endless scope for the formulation of novel materials based on this concept. And so it proved. 

See also Alkenylsuccinic anhydrides production of, 17 726 Alkenylsuccinic anhydrides, 15 490 Alkoxidation, higher aliphatic alcohols, 2 5 Alkoxide catalysts, 10 683 Alkoxide-derived silica gels, structure of, 23 73... 

Compound 165 was reduced with SDMA and the product was hydrolyzed with acid to effect ring contraction, affording 4-deoxy-4-C-[(jR,S)-ethylphosphinyl]-o ,/ -D-ribo- and -L-lyxo-furanoses (166), which were characterized by conversion into the peracetates. After separation by chromatography on silica gel, structures 167-170 were established for these peracetates by 400-MHz, -n.m.r. spectroscopy and high-resolution mass spectrometry the structures of these products, their probable conformations, and the yields from 165 are summarized in Scheme 7. 

Activation energy function for amineborane and aminoborane formation on the silica gel structure... 

Monolithic silica columns can be prepared by bonding silica or ODS-silica particles together, by sintering or by embedding silica particles in a silica gel structure,... 

Pore diameter, specific pore volume. A controlled, macroporous silica can be obtained by hydrolytic polycondensation of polyethoxysiloxane, while silica gel structure modification results from thermal or rather hydrothermal treatment. When hydrothermal treatment is carried out with silica of a pore size of 100 A at 250 C and 50 atm for a period of 15-20 h, formation of silica gel with a homogeneous pore size of 900 A is possible. Of coarse, the increase of pore diameter reduced the surface area thereby the Rf. values are also generally increased in the case of adsorption type chromatography. El Rassi et al. 44] studied the effect of water and that of hydrothermal treatment on the activity of silica gel. 

The silica hydrogel is a compliant material whose network building-up is not completed when alumina is added. So, alumina precipitation can easily modify the silica gel structure and induce drastic texture modifications. 

Research of Soviet scientists is surveyed. Contributions to various aspects of the colloid chemistry of silica are examined preparation and stabilization of silica hydrosols preparation of silica gels structural characterization of silicas surface chemistry elucidation adsorption and ion-exchange property examination and geometric and chemical modification of silicas, silica coatings, and so forth. 

The mechanism of alteration of silica gel structure when silica gel is heated in water in an autoclave has been observed and summarized by Kiselev et al. (321) along the following lines. 

Silica gel structure is destroyed and the amorphous silica crystallizes at quite low temperature when 5 mole % of the alkali metal oxides is present. Lithium promotes crystallization to quartz, and sodium to cristobalite, at only 700 C. Potassium acts almost as rapidly at 700 C (354). The surface area of such alkali-containing gel drops almost to zero at 650-700 C (355). When the gel is taken at an intermediate stage as it shrinks, and is treated with acid, washed, and dried, it contains very wide pores, 300-1200 A diameter. Also there are micropores which are probably left after the sodium is extracted from the thin glassy layer on the.surface. Any surface area from 70 to 6 m g can be obtained by cooling the sample at the right point. 

Chexal K K, Handa B K, Rahman W 1970 Thin layer chromatography of biflavonyls on silica gel structure chromatographic behaviour correlations. J Chromatography 48 484-492... 

Himmel B., Gerber Th., Burger H. X-ray diffraction investigation of silica gel structures. J. Non-Cryst. Solids 1987 91 122-136... 

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