Sheets silica

TLC plates or TLC aluminium sheets silica gel 60 Pz54 (Merck). 

FIGURE 5-45. Example of using TLC as a guide for HPLC separation of Azo dyes. Analytes shown in the figure, (a) TLC plate E Merck precoated TLC sheet, silica gel F-254. Solvent 10% methylene chloride in hexane. Development time 50 min. (b) Column Micropak Si-10 (silica, 10 /am), 2.4 mm ID x 15 cm. Mobile phase 10% methylene chloride in hexane. Flow rate 2.2 mL/min. Detector UV at 254 nm, 0.16 AUFS. (Reproduced from reference 21 with permission.)... 

Analytical TLC TLC-Aluminum sheets silica gel 60 P254 (Merck, Darmstadt, Germany)... 

In the tetrahedral sheet, silica (Si" " is sometimes partly replaced by aluminium (Al " ) in the octahedral sheet, aluminium (Al " ) can be replaced by magnesium (Mg " ) or other atoms (e.g. iron). Such a replacement by atoms of lower positive valence results in an excess of negative charge. This excess is compensated for by adsorption of cations (Na, Ca, Mg) from the adjacent water and an electric double layer is formed. The compensating cations on the layer surface can exchange with other cations. The number of the exchangeable cations is measured by the cation exchange capacity (CEC) . 

It was the idea of Winterton [2] to glue the otherwise fragile mica sheets onto polished silica discs to give them better mechanical stability, especially for friction experiments. The glue layer detemiines the final... 

Horn R, Smith D T and Haller W 1989 Surface forces and viscosity of water measured between silica sheets Chem. Rhys. Lett. 162 404-8... 

A procedure involving only the wall area and based on the cylindrical pore model was put forward by Pierce in 1953. Though simple in principle, it entails numerous arithmetical steps the nature of which will be gathered from Table 3.3 this table is an extract from a fuller work sheet based on the Pierce method as slightly recast by Orr and DallaValle, and applied to the desorption branch of the isotherm of a particular porous silica. 

Other Industrial Applications. High pressures are used industrially for many other specialized appHcations. Apart from mechanical uses in which hydrauhc pressure is used to supply power or to generate Hquid jets for mining minerals or cutting metal sheets and fabrics, most of these other operations are batch processes. Eor example, metallurgical appHcations include isostatic compaction, hot isostatic compaction (HIP), and the hydrostatic extmsion of metals. Other appHcations such as the hydrothermal synthesis of quartz (see Silica, synthetic quartz crystals), or the synthesis of industrial diamonds involve changing the phase of a substance under pressure. In the case of the synthesis of diamonds, conditions of 6 GPa (870,000 psi) and 1500°C are used (see Carbon, diamond, synthetic). 

Microamorphous silica can be divided into microparticulate silica, ie, microscopic sheets and fibers, and highly hydrated silica (1). The... 

Microscopic sheets of amorphous silica have been prepared in the laboratory by either (/) hydrolysis of gaseous SiCl or SiF to form monosilicic acid [10193-36-9] (orthosihcic acid), Si(OH)4, with simultaneous polymerisation in water of the monosilicic acid that is formed (7) (2) freesing of colloidal silica or polysilicic acid (8—10) (J) hydrolysis of HSiCl in ether, followed by solvent evaporation (11) or (4) coagulation of silica in the presence of cationic surfactants (12). Amorphous silica fibers are prepared by drying thin films of sols or oxidising silicon monoxide (13). Hydrated amorphous silica differs in solubility from anhydrous or surface-hydrated amorphous sdica forms (1) in that the former is generally stable up to 60°C, and water is not lost by evaporation at room temperature. Hydrated sdica gel can be prepared by reaction of hydrated sodium siUcate crystals and anhydrous acid, followed by polymerisation of the monosilicic acid that is formed into a dense state (14). This process can result in a water content of approximately one molecule of H2O for each sdanol group present. 

Fused Silica Code 7940 Data Sheet, Corning Glass Works, Coming, N.Y., 1992. 

Rigid Porous Media These are available in sheets or plates and tubes. Materials used include sintered stainless steel and other metals, graphite, aluminum oxide, silica, porcelain, and some plastics—a gamut that allows a wide range of chemical and temperature resistance. Most applications are for clarification. 

When the ratio MO/SiOj is a little less than 2/1, silica dimers form (Fig. 16.4b). One oxygen is shared between two tetrahedra it is called a bridging oxygen. This is the first step in the polymerisation of the monomer to give chains, sheets and networks. 

TLC aluminium sheets, TLC plates. Silica gel 60 F254 (Merck). 

A theoretical model whereby maximum peak capacity could be achieved by the use of 3-D planar chromatographic separation was proposed by Guiochon and coworkers (23-27). Unfortunately, until now, because of technical problems, this idea could not be realized in practice. Very recently, however, a special stationary phase, namely Empore silica TLC sheets, has now become available for realization of 3-D PC. This stationary phase, developed as a new separation medium for planar chromatography, contains silica entrapped in an inert matrix of polytetrafluoroethy-lene (PTFE) microfibrils. It has been established that the separating power is only ca. 60% of that of conventional TLC (28) this has been attributed to the very slow solvent migration velocity resulting from capillary action. 

Botz et al. (29) also demonstrated, by scanning electron microscopy, that application of overpressure increases the density of the layer, which could be one reason for the higher separation efficiency. These results showed that Empore silica TLC sheets enable extremely rapid separations (5-20 min) in one-dimensional OPLC, and gave good resolution. Theoretically, for a 3-D OPLC separations development times of 15-60 min would be required. The separation cube of sheets could be especially useful for micropreparative separations (30). 

L. Botz, Sz. Nyiredy, E. Wehrli and O. Shelter, Applicability of Empore TLC sheets for forced-flow planar cltromatography. I. Characterization of the silica sheets , 7. Liq. Chromatogr. 13 2809-2828 (1990). 

A mixture of powdered poly(vinyl chloride), cyclohexanone as solvent, silica, and water is extruded and rolled in a calender into a profiled separator material. The solvent is extracted by hot water, which is evaporated in an oven, and a semiflexible, microporous sheet of very high porosity ( 70 percent) is formed [19]. Further developments up to the 75 percent porosity have been reported [85,86], but these materials suffer increasingly from brittleness. The high porosity results in excellent values for acid displacement and electrical resistance. For profiles, the usual vertical or diagonal ribs on the positive side, and as an option low ribs on the negative side, are available [86],... 

---