Bulk silica

Table 3.1 lists commercially available bulk silica gels for the manual preparation of PLC layers snitable for straight phase chromatography. These bulk silica gels are produced by different manufacturers, and in some cases they are offered with binding additives and fluorescent indicators. The data summarized in this table are traceable to product information from the concerned manufacturers. 

In prineiple, the bulk silica sorbents and the preeoated plates for partition PLC are the same types as those used in straight phase adsorption ehromatography (see Table... 

The use of small polyhedral silsesquioxanes as molecular models of silica surfaces has been studied increasingly in recent years, because catalysts, etc., attached to such species are relatively easy to handle and characterize when compared to bulk silica. The preparation and structures of silsesquioxanes used in this type of work are described later in Sections II,C, IV,E, and IV,F. Further recent examples may be found in Ref. 5. 

In a bulk silica matrix that differs from the silica nanomatrix regarding only the matrix size but has a similar network structure of silica, several kinetic parameters have been studied and the results demonstrated a diffusion controlled mechanism for penetration of other species into the silica matrix [89-93]. When the silica is used as a catalyst matrix in the liquid phase, slow diffusion of reactants to the catalytic sites within the silica rendered the reaction diffusion controlled [90]. It was also reported that the reduction rate of encapsulated ferricytochrome by sodium dithionite decreased in a bulk silica matrix by an order of magnitude compared to its original reaction rate in a homogeneous solution [89], In gas-phase reactions in the silica matrix, diffusion limitations were observed occasionally [93],... 

PMMA-impregnated sol—gel-derived silica gels have also been examined (54). Long-wave uv illumination was employed in addition to benzoyl peroxide for PMMA polymerization. This method prohibited the degradation of the silica xerogel from moisture adsorption and desorption. Overall the material behaved more like bulk PMMA than bulk silica, with the exception of hardness. 

At a fairly late stage in 1966 optical fibers were first considered seriously to be a possible transmission medium by Kao and Hockham1. In 1969 Jones and Kao2 demonstrated that minimum attenuation losses in bulk silica can be as low as 5 db/km. The actual breakthrough was achieved in 1970 with the preparation of fibers with optical losses below 20 db/km3. Since then rapid progress has been made in this field all over the world. 

Fig. 4.2. Model Si(OX)4 cluster (X = OSiH3) simulating the defect-less bulk silica structure.

In the Si(OX)3 cluster, which simulates a surface E -center in bulk silica, two lowest in energy electronic transitions occur from MOs combined of oxygen lone pairs to the singly occupied sp3-hybrid-type orbital of Si. In this case, the TD-B3LYP result (4.66 eV) underestimates the experimental value by 1.5eV, while the OVGF excitation energy (6.36 eV) is close to the MRDCI value (6.21 eV) and experimental data (6.2 eV). 

Why optical (photoabsorption and PL) properties of Si02 and Ge02 nanomaterials can dramatically differ from the properties of bulk silica... 

The amount of carbon introduced into the stationary phase by the functional group is referred to as the carbon load, and it is measured as a weight percentage of the bulk silica packing. The carbon load is altered by... 

One of the first examples of the covalent imprinting of bulk silica was described by Katz and Davis.58 In this study, an imprint molecule containing an aromatic core and protected 3-aminopropyltrimethoxysilane groups was synthesized. When hydrolyzed and condensed with TEOS followed by removal of the aromatic core, a shape-selective base catalyst was prepared.58 Katz and Defreese also used a similar approach to create proline-amide imprints in bulk microporous silica.59 Additional... 

As-synthesized AFS zeolites do not contain extraframework aluminum as evidenced by Al NMR. As-synthesized USY zeolites contain appreciable amounts of extraframework material as seen by comparing framework and bulk silica-alumina ratios and by examining 27A1 spectra. Upon calcination both AFS and USY materials contain extraframework aluminum. The amount of extraframework aluminum in both AFS and USY materials increases on steaming. 

Proton donor ability of surface silanols is believed to be the source of peak tailing for analytes with proton acceptor functionaUty (usually basic analytes). The presence of impurities such as iron, boron, and aluminum [70] in bulk silica decreases the silanol pK and decrease the hydrolytic stability of bonded phases. 

Bass JD, Katz A (2003) Thermolytic synthesis of imprinted amines in bulk silica. Chem Mater 15 2757... 

The building blocks of bulk silica, both crystalline and amorphous, are SiC>4 tetrahedra, which are linked by sharing comers, while each O is bonded to two Si. However, in the amorphous variety, the three-dimensional network of the tetrahedra lacks the symmetry and periodicity characteristic of the crystalline forms. The Si-O distances are similar to other varieties, about 0.16nm, but the Si-O-Si angles deviate by as much as 20° from the mean 153° [42]. Naturally, the structures on the surface are even more complex and variable. 

A similar simulation technique was used in Ref. [19] to obtain a model cylindrical pore in porous silica. Again a bulk silica atomic structure was simulated and the surface of a pore was created by removing silicon and oxygen atoms from the interior of a cylinder inscribed around a fixed axes. The simulation technique was essentially the same as in Refs. [17, 18], the only difference being that in the former case a silica surface was created by boring a pore from bulk silica while in the latter case it was created by cutting out a sphere from the bulk. 

Physico-chemical properties of Ti02/Si02 samples and those of bulk silica and titania Sample Titanium amount N2 adsorption... 

Silica-titania may also be cogelled in the presence of chromium to form a "tergel" of Cr203, Si02, and Ti02. However, Cr2C>3 does not fit into the silica lattice, and exists only as a separate but highly dispersed phase. Upon activation, the Cr(III) is oxidized to Cr(VI) and it then resides on the surface as in other Cr/silica catalysts. There is no evidence that chromium in either oxidation state ever becomes part of the bulk silica structure. It can be easily removed from the catalyst by dissolution. 

Fig. 8-4 Molecular imprinting in bulk silica gel a sol-gel hydrolysis and polycondensation (pH 2.0), b capping of surface silanols...

In this chapter, recent progress in several key areas is reviewed. These areas are catagorized by material classification rather than by end-use application (1) bulk silica optics (2) optically active doped silica glasses (3) gel-polymer composites (4) organically modified silicates (ormosils) and (5) gradient-index glasses. These represent the five most significant developments in the area of bulk optical materials by the sol-gel process to date. 

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