Native silicas

Nowadays, almost all commercially available HPLC stationary phases are also applicable to planar chromatography. In addition to the polar hydroxyl groups present on the surface of native silica, other polar functional groups attached to the silica skeleton can also enter into adsorptive interactions with suitable sample molecules (34). Silica with hydrophilic polar ligands, such as amino, cyano, and diol functions, attached to the silica skeleton by alkyl chains, all of which have been well proven in HPLC, have also been developed for TLC (34). 

Unfortunately, silica gel suffers from a severe disadvantage in that it is slightly soluble in water. This means that the native silica cannot be used in conjunction with aqueous mobile phases. Consequently, silica is precluded from use in the separation of those substances that are strongly polar and require aqueous solvents to render them soluble and stable. For this reason the relatively new, micro-reticular resins are now used in the separation of strongly polar substances by exclusion chromatography. 

Although the melting point of silicon is about 1410 °C, because of the need to remove the native oxide layer coating the SiC so that the silicon wets the grains, the infiltration is usually carried out at temperatures >2000°C when a-SiC forms. The removal of the native silica layer depends upon the ambient and in neutral atmospheres reaction-bonding can be effected at lower temperatures forming /f-SiC. 

Banerjee, S., Das, J., and Santra, S. 2009. Native silica nanoparticle catalyzed anti-Markovnikov addition of thiols to inactivated alkenes and alkynes A new route to linear and vinyl thioethers. Tetrahedron Letters, 50 124—27. 

HPLC enantiomeric separation of Co(acac)3 was also performed with monolithic silica columns containing a covalently attached 3,5-dimethylphenylcarbamate derivative of cellulose . Covalent immobilization of 3,5-dimethylphenylcarbamate derivative of cellulose was performed in situ onto native silica monoliths cladded in a polyether HPLC column. The covalent attachment of cellulose derivative in the range of 16-19% (w/w) was performed via an epoxide moiety. The covalent attachment of the cellulose derivative... 

Independently, Erlenmeyer and Bartels performed similar studies using starch as the immobilising agent [56]. They demonstrated that imprinted silicas could be used to distinguish between dimethyl- and diethyl-aniline, a distinction that the native silica was unable to make in the solvent mixture used (ethyl acetate/methanol/ HO Ac (5 M) 60 30 10). 

Table 3.8 gives the corresponding data on the pore structural parameters. With increasing silane chain length the specific surface area, as well as the specific pore volume, is evidently reduced by almost a factor of two for the Qg modification compared with the native silica sample. In the same fashion, the initial pore diameter of... 

In normal-phase chromatography the native, non-modified adsorbent is employed with organic solvent mixtures as eluents. Normal phase chromatography was the classical chromatography mode performed with native silica or alumina, i.e. the adsorbent s surface is hydrophilic and the interaction with the solutes takes place via the hydroxy] groups on the surface. As an example the surface of silica consists of... 

It has been found experimentally that PS ageing in air stabilizes the PS surface [6]. This is associated with the native silica on the PS surface. With RTP, high temperatures should enhance the silica growth providing rapid passivation of the PS surface with oxygen. 

Figure 3.48 Simplified procedure to establish the start and end composition for a preparative gradient elution on native silica.

Native silicas are employed In SEC of polar synthetic poly-... 

Although native silicas have not been employed for SEC of biopolymers, and are also little used in SEC of polar synthetic polymers, it is worthwhile to survey those silicas which are parent materials for bonded SEC silica packings. The products... 

These aerogels offer unprecedented hydrophobic as well as mechanical character unlike native silica aerogels. Let us discuss in detail their interactions with water and hydrocarbons, and also the response under mechanical loading. 

The Y value of the as-prepared aerogels was found to be 9.6 x 10 and 2.6 x 10 times smaller than those of Iron and Indium, respectively. The Y values of these aerogels are closer to those of rubber when compared with the metals mentioned above. It is only 2.6 X 10 times smaller than the Y value of rubber which is quite rarely observed in case of silica aerogels. This improvement in elastic modulus of superhydrophobic flexible aerogels with those of native silica aerogels is roughly two orders of magnitude. 

Flguro 13.2. Scanning electron micrographs iSEM) at random spots in the interior of a riactuied native silica aerogel monolith (A., pb = 0.169 gem ) and after the skeletal network was coated (crosslinked) widi Desmodur N3200-derived polyurea (B., Pb = 0.380 g cm ). 

Figure 13.13. Compressive stress-strain curves of native silica (SiOx and X-SiOx) and polyurea-crosslinked vanadia (VO and X-VOx) aerogels. Bulk densities SiOx - 0.213 gcm X-SiOx- 0.548 gcm VOx-0.121 gcm and X-VOx 0.430 gcm . A. Quasi-static compression of SiOx at a strain rate of 9 x 10 " s at 23°C X-SiOx at 5 x 10 s at 23°C and X-SiOx at 5 x 10 s at —196°C inset same curves plotted using a different scale. B. Quasi-static compression of yO iat a strain rate of 9 x 10 s at 23°C X-VOx nt 5 x 10 s at 23°C and X-VOx at 5 x 10 s at — 196°C inset up to 10% compressive strain.

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