Silica gels electron microscopic

FIGURE 3.3 Scanning electron microscope photograph of a PLC plate silica gel 60, layer... 

Figure 3.2 Electron microscopic photos of silica gels A, irregularly shaped B, ordinary spherical and C, high purity silica.

Transmission Electron Microscopy (TEM) micrographs were obtained using a BS 540 (Tesla) apparatus. Microscope samples were prepared using the platinum-carbon replication method, with evaporation of platinum and a low amount of carbon onto the adsorbents, then treated in HF to dissolve silica gel. 

Over the past 10 years, more crystallization systems supporting the solid-phase mechanism have been reported. Remarkable examples included Tsapatsis s study on the crystallization of zeolite L by using High Resolution Transmission Electron Microscope (HRTEM) technique in 1996 [23] Serrano s study on the crystallization of TS-1 by using a couple of spectral techniques in 1996[24] and on the crystallization of pure silica zeolite beta under the presence of F [25] and Uguina s study on the crystallization of TS-2 by using multiple techniques.[26] The commonly used dry-gel synthesis of zeolites in recent years (DGC and SAC, see Section 3.2.5, Sub-section Dry Gel Conversion in Chapter 3) partially confirms the rationality of the solid-phase mechanism. 

Practically all the internal surface and a large fraction of the pore volume of finely porous materials such as activated aluminas and silica gels are contained in pores smaller than 300 A diam. (micropores). The average diameter of the micropores is usually of the order of 50 A, so that pore-size distributions cannot be measured directly even using an electron microscope. Of the indirect approaches possible, low-temperature adsorption isotherms appear to provide the most complete data. 

The macroscopically different bulk structures of granulate and spray-dried silica gels are composed of small, almost spherically shaped particles, which can be seen in an enlarged transmission electron microscopic (TEM) micrograph (Figure 5). The latter... 

Alumina supports for metal particles were synthesized in the form of fibers and thin plates suitable both for catalyst studies and electron microscopic examination.(106) Light scattering, proton resonance, viscosity measurements, were used to study the formation of silica gels and monodisperse sols.(114) The same techniques were used to study synthesis of zeolites. 

The real structures of porous silica gels are unknown. However electron microscopic analysis revealed a smooth surface. Bonded silica gels made from pure silica are very chemically stable compared to ordinaiy silica gels. This means that the purer the silica, the more homogeneous the structure, which improves the physical strength and the reactivity of silanization. The basic structure may be based on rings of six Si02 units. 

Figure 1. Figures of electron microscope and the analysis of the spectrum of surface energy for pure silica gel and CMSP of mouse macrophage, (a) Pure silica gel carrier (magnified 3000 times) (b) CMSP of mouse macrophage (magnified 3000 times) (c) figure of surface energy for pure silica gel (d) CMSP of mouse macrophage.

Electron microscopic results on networks filled in situ by the sol-gel approach have shown that the filler particles typically have a narrow distribution of sizes, with most diameters in the range 200-250 A. TEM studies on the distribution of silica in situ generated within PDMS have shown that well-distributed particles can be obtained by using basic catalysts, thin samples, and long hydrolysis times, while the silica was found to precipitate mainly in the sample periphery in the case of acidic conditions, bulky... 

An electron microscopic study of fine-pored silica gel by Sugar and Guba (119) revealed that the structure indeed was made up of a threadlike or fiberlike network, but the fibers were made up of chains of spheres. The pore volume determined by adsorption could be correlated with the observed size and spacing of the chains of particles on the assumption that these were on the average cylindrical and that the pore space was the complementary space of a skeleton built up of the network of cylindrical elements. 

Formation of silica gel has been observed in electron microscope studies by Radezewski and Richter (227). Silica sols prepared by the hydrolysis of silicon tetrachloride and purified by electrodialysis initially contained particles less than 10 nm in diameter. On standing at pH 6.8, spongelike aggregates were observed to be formed from the individual particles. The polymeric silicas which were so obtained... 

The vapour pressure of the capillary water above the meniscus which is formed in the pores, is a function of the pore radius. It is not necessarily the same as the geometrical radius of the pores at the point where the meniscus is. A layer of water adheres to the pore waUs during dehydration of the pores [334]. C. Piebce [532] has made a compilation of experience in 4 his connection, with the help of which the radius calculated from the Kelvin equation can be converted into the efifective pore radius. The sizes of larger pores can be determined with the electron microscope. The average silica gel preparations have a mean pore radius of the order of 25 A. 

Material of similar appearance in the electron microscope may be prepared by the precipitation of gels of alumina and silica, van Reeuwuk [1967] has shown that spherical bodies similar to those seen in some natural allophane sometimes appear in synthetic gels, while Watanabe [1963] compares micrographs of allophane with those of siUca and alumina gels. Some synthetic alumina gels even have a fibrous appearance. 

FIGURE 5.4 Stages in sol-gel processing are captured by a new electron microscopy technique. (1) Spherical particles tens of nanometers across can be seen in a colloidal silica sol. (2) Addition of a concentrated salt solution initiates gelation. (3) The gelled sample, after drying under the electron beam of the microscope, shows a highly porous structure. Courtesy, J. R. Bellare, J. K. Bailey, and M. L. Mecartney, University of Minnesota. 

Although biogenic silica exhibits no long-range crystallographic order (as determined by ultra high resolution transmission electron microscopy and diffraction methods,morphological order exists at the microscopic level. The initial distinction that can be made is between gel, continuous, and particulate structures. In true gel-like phases such as costal strips from the... 

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