Unmodified silica

Silica with its constituents Si, O, and H may be regarded as one of the simplest supports. The huge number of varieties however shows that the architecture of the material is manifold and sensitively dependent on the preparation. Attempts to construct silica or just an amorphous Si02 by a computer program gave unsatisfactory results [9]. Since the basic work of Peri and Hensley [10], a simplified model of cristobalite planes is used to explain the properties of the silica surface, neglecting the aspect that the fractal dimension of the latter may reach almost the value three. The thermal behaviour of geminal, vicinal, or isolated OH 

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O. V. Olesen and B. Poulsen, On-line fully automated deteimination of clozapine and desmethylclozapine in human semm by solid-phase extraction on exchangeable car-ti idges and liquid cliromatography using a methanol buffer mobile phase on unmodified silica , 7. Chromatogr. 622 39-46 (1993). 

As mentioned above, V(0 Pr)3-modiricd silica treated with BuOOH shows no activity towards cyclohexene at room temperature. However, the material obtained by grafting Ti(0 Pr)4 directly on unmodified silica heated at 500°C, eq 4, does convert olefins to their epoxides upon treatment with BuOOH. The catalyst 4 also shows biphasic kinetic behavior towards cyclohexene, Figure lc. 

Fig. 3.1b lists some of the properties of two commercially available microparticulate packings. The first is a 5 /im unmodified silica, the second a 3 fim silica, chemically modified with octadecylsilane (ODS, C-18). 

The strength of the interaction between the adsorbent and the solute molecules increases as the polarity of the solute increases. Thus we can increase retention of our solute molecules (X) by decreasing the polarity of the mobile phase (S), which will shift the equilibrium above to the right. Polar solute molecules are strongly held on unmodified silica and tail badly, so the method is useful only for solutes having low or medium polarity. 

Fig. 3.4f shows an exclusion chromatogram on unmodified silica. The sample is an epoxy resin with an average Mr of 900. Fig. 3.4g shows the determination of pesticide residues in a sample of chicken fat, and is an example of how exclusion can be used to clean up complex samples. First, a pesticide-free sample of the fat is run as a blank, then the blank is spiked with the pesticides to determine their retention volumes. When the sample is injected, the eluent containing the pesticides is collected. The solvent is evaporated, the residue dissolved in acetonitrile and the pesticides are then separated on a reverse phase column. 

Adsorption chromatography uses unmodified silica with a relatively nonpolar mobile phase. It is used for the separation of solutes of relatively low polarity, although such separations are now often achieved more easily on bonded phases. 

Instead of blending them either by dispersing silica powders as fillers within PE, which is a well-developed technology with many applications, or by polymerizing ethylene within the pores of modified9 or unmodified silica,10 now the nanometric physical blending... 

The separation characteristics of unmodified silica, diol, cyano, cellulose and RP-18 stationary phases were compared for the analysis of ellagitannins, gallotannins and flavonoid glycosides in the aerial parts of Erodium cicutarum (Geraniaceae) (stork s bill). Its decoction has been used in cases of dysentery, fever, wounds and worm infections. The air-dried and powdered plant material (300 g) was extracted three times with water-acetone (1 1, v/v) at ambient temperature. The collected extract was filtered and concentrated. The aqueous phase was acidified to pH 3 with formic acid and separated in an ODS column. Polphenols... 

In this synthetic strategy, the macrocyclic antibiotic is covalently bonded to the silica matrix in two steps (1) chemical modification of the selector via reaction between suitable groups of the antibiotic and proper groups of the spacer, reacting also as a di- or trialkoxysilane (2) immobilization of the functionalized selector on unmodified silica particles. 

Covalently Anchored Organometallic Complexes on Unmodified Silica... 

Coi alently Anchored Orgonometollic Complexes on Unmodified Silica 169... 

Normal phases (unmodified silica gel) are rarely employed, except for the occasional separation of weakly polar flavonoid aglycones, polymethoxylated flavones, flavanones, or isoflavones. The polymethoxylated flavones present in citrus fruits can, for example, be separated on silica gel columns. The big drawback is that solvent gradients cannot normally be run with normal phases. 

Since MeReOs chemisorbs irreversibly on silica-alumina, we did not expect to be able to retrieve it as a soluble species upon SnMe4 treatment of perrhenate/silica-alumina. However, exposure of this material to SnMe4 resulted in an immediate color change from white to brown, similar to that observed upon treatment of unmodified silica-alumina with MeReOs. Evidence for the possible involvement of the in srto-generated MeReOs in olefin metathesis was then sought via reactivity studies. 

Bonded-micellar phases are porous unmodified silica supports with an ad-sorptively bonded detergent layer (119). In these phases, the hydrophilic sites of the detergent molecules are directed to the outside, which prevents the adsorption... 

Figure 2. (a) PSDs for 3.88 nm MCM-41 [22] calculated from nitrogen adsorption data at 77 K using i) the KJS approach with the BJH algorithm, ii) the hybrid DFT method, iii) the HK-based method with the relation between pore size and condensation pressure described by Eq. 2, and iv) the HK-based Saito-Foley method, (b) Comparison of PSDs for octyldimethylsilyl-bonded MCM-41 calculated from nitrogen adsorption using the KJS-calibrated BJH algorithm with t-curves for reference ODMS-modified silica and unmodified silica. 

Figure 3. (a) Nitrogen adsorption isotherm for uncalcined MCM-41. (b) Comparative plots for this sample calculated using reference data for ODMS-modified and unmodified silicas. 

The elution order of the standard FMOC amino acids on a CE unmodified silica capillary is usually Ala, Gly, Val, Leu, Phe. FMOH usually elutes after Phe. 

Treatment of 44e with 3 equivalents of AICI3 gave chlorosilane 47. Treatment of silica with 47 and pyridine in refluxing toluene gave modified silica (48) (equation 17). Its carbon content was 7.18%, suggesting that 0.21 molecule of Cgo is present per 100 A2 of silica surface. A mixture of C60 and C70 was well separated by means of modified silica (48). No separation occurred using unmodified silica as a stationary phase. 

Since the RSi= units cap the SiOx(OH)j,(OMe)z clusters formed during sol-gel processing of Si(OMe)4, a hydrophobic inner surface is created, and the resulting aerogels are permanently stable against moisture (unmodified silica aerogels are immediately destroyed... 

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