Silica gels types

Since the literature cited did not reveal a significant effect of the differing pore systems of the various types of layer the aluminium oxide and silica gel types (60, 80, 100, etc.) are not specified. The same applies to binders, fluorescence indicators and trace impurities in the sorbents. 

Mean hydroxyl group density [1] (about 8 pmol/m independent of silica gel type)... 

Dehydration by the pressure swing process is largely the province of activated alumina or silica gel type adsorbents. Owing to the nature of their isotherms they demonstrate more favorable desorption characteristics for use in PSA. 

Thin layer chromatography is yet another means for determining identity of moxalactam materials. The mobility of the moxalactam sample must be identical to the mobility of the moxalactam reference standard which is run on the same TLC plate. The developing solvent for TLC consists of 42 parts ethyl acetate 14 parts glacial acetic acid 14 parts acetonitrile and 18 parts water. A silica gel type F plate is used and is viewed under short wavelength UV light to detect the position of the components on the developed plate. 

Phenyl-l,2-dithiole-3-thione (0.21 g, 1 mmol) in refluxing CCI4 (20 mL) was treated with an excess of ethyl azidoformate ( 0.5 mL) after 24 h further ethyl azidoformate (0.5 mL) was added, and reflux maintained. After the disappearance of the dark color of the starting thione, the solution was evaporated and the residue chromatographed (1 mm Camag silica gel type D.S.F.5, benzene/CHCl, 10 1). In order of elution the products were ethyl 5-phenyl-6-thioxo-l,2,3-dithiazine-3-carboxylatc (2) and ethyl (4-phenyl-5-thioxo-l,2-dithiol-3-yl)carbamate (10%). The products were further purified by recrystallization (benzene/petroleum ether 1 1) yield of 2 0.17 g (57%) mp 195 °C. The yield of the carbamate was 10% mp 131 132 °C. 

Figure 1. Typical Brunauer adsorption isotherms for (left) precipitated silica (Type II) and (right) silica gel (Type IV) (V, volume adsorbed P, pressure and P0, saturation pressure).

The reactions in this group comprise ring substitution and dealkylation of benzenoid compounds and syntheses with 1,3-dimethoxy compounds in the bicyclic and polycyclic series. The monosubstitution of polyalkoxy benzenes is generally not facile. Monobromination of 1,3-dimethoxybenzene in carbon tetrachloride was effected at 30°C during 2 hours with N-bromosuccinimide in the presence of acidic silica gel (type microbead 3A) in 99% yield without formation of dibromo compounds (ref.141). The evolution of HBr is avoided. 

Enhanced activity of zeolites with respect to alumina silica gel type of catalysts was ascribed to the enhanced mobility of the protons in the crystalline zeolite bringing about increased acid strength.(73) Thus,though individual catalytic sites may be equivalent, their ensembles of different size would have different catalytic activity.(75) Furthermore C-13 nuclear magnetic resonance of adsorbed molecules showed that strong electric forces present in the regular pores of crystalline zeolites polarize and activate adsorbed molecules (155). 

The isotherms at 25°C for some organic and inorganic materials are presented in Figure 4.14 [92]. In Figure 4.14, one can observe the various isotherm types, like type I for activated charcoal and silica gel, type II for leather, type III for soap, and so on. 

Commercially available TLC plates with 0.25 mm silica gel type 60 are cleaned by developing them in trichloromethane. After evaporation of the trichloromethane, the plate is immersed for 10 s in silver nitrate solution (20% m/v). After drying (0.5 h) in an oven at 50 C followed by 0.5 h in an oven at 110 °C, the plate is stored in a nitrogen-filled desiccator in the dark. 

These materials are suitable for RP-TLC owing to their lipophilic properties. The commonly used RP layers consist of dimethyl-(RP-2), octyl-(RP-8), octadecyl-(RP-18), and phenyl-bonded silica gel, type 60, with different mean particle sizes and particle size distributions. Recently, C30-modified silica gel was proposed for the separation of tocopherol homologues. 

Table 1 Rf Values of Substituted Phenols on Silica Gel (type 60) (E-254) and on Alumina (E-254)... 

Fig 2 6 Typical examples of adsorption isotherms of water vapor on Silica gel type A and B and active alumina... 

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