Vycor-glass

This material also has a porous structure by virtue of its being made by etching a phase-separated borosilicate glass. PS chains have also been introduced into the pores of this type of glass [146], but the only results to date involve mechanical property measurements, and scattering investigations. 

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In the case of Ru(2,2 -bipyridine)3 adsorbed on porous Vycor glass, it was inferred that structural perturbation occurs in the excited state, R, but not in the ground state [209]. 

Similar, very detailed studies were made by Ebert [112] on water adsorbed on alumina with similar conclusions. Water adsorbed on zeolites showed a dielectric constant of only 14-21, indicating greatly reduced mobility of the water dipoles [113]. Similar results were found for ammonia adsorbed in Vycor glass [114]. Klier and Zettlemoyer [114a] have reviewed a number of aspects of the molecular structure and dynamics of water at the surface of an inorganic material. 

Recently, many experiments have been performed on the structure and dynamics of liquids in porous glasses [175-190]. These studies are difficult to interpret because of the inhomogeneity of the sample. Simulations of water in a cylindrical cavity inside a block of hydrophilic Vycor glass have recently been performed [24,191,192] to facilitate the analysis of experimental results. Water molecules interact with Vycor atoms, using an empirical potential model which consists of (12-6) Lennard-Jones and Coulomb interactions. All atoms in the Vycor block are immobile. For details see Ref. 191. We have simulated samples at room temperature, which are filled with water to between 19 and 96 percent of the maximum possible amount. Because of the hydrophilicity of the glass, water molecules cover the surface already in nearly empty pores no molecules are found in the pore center in this case, although the density distribution is rather wide. When the amount of water increases, the center of the pore fills. Only in the case of 96 percent filling, a continuous aqueous phase without a cavity in the center of the pore is observed. 

Vycor glass (Corning, 7930) is porous silica processed in the same way as CPG except that the last step of base etching is missing. Therefore the surface is rough. Only a 40-A pore is available. When porous silica with a pore size smaller than 75 A is needed, Vycor glass is the choice. Bulk pieces of Vycor glass are available commercially and need to be crushed into small particles before use. 

For special purposes, Corning Vycor glass (96 per cent silica) may be used. It has great resistance to heat and equally great resistance to thermal shock, and is unusually stable to acids (except hydrofluoric acid), water, and various solutions. 

Coming Vycor glass, containing 96 per cent of silica, is usually suitable. 

Fig. 15. Fluorescence spectra of porous Vycor glass after heating (a) in air at 100°, and (b) in oxygen at 550°. The spectra were run under the same conditions except that the amplification for (b) was ten times higher than for (a) (85).

For other centrosymmetric adsorbates such as C02 on zeolites X and Y (1) and ethene on porous Vycor glass (3), only marginal changes in line position were observed. 

A 0.40g sample was sulfated by running 3h SOj oxidation at 704°C in a down-flow Vycor glass reactor with flowing Nj (126 ml/min) containing 9.50 vol.% Oj and 0.6000 vol.% SOj. The sample was cooled in flowing Nj prior to discharge. 

We have also obtained measurements for the relaxation time of octafluorocyclo-butane (c-C4F8) gas in porous Vycor glass. Vycor (Corning Glass, -96.5 wt-% Si02) has a specific surface area of approximately 120 m2 g-1, and was cleaned using H202 and evacuated to 10-8 Torr before all measurements. The Vycor was placed in... 

Fig. 3.5.3 Spin-lattice (Tt) and spin-spin (72) relaxation times as a function of pressure for c-C4F8 gas in porous Vycor glass at 291 K.

Fig. 3.5.8 Schematic and NMR image of C4F8 gas at 80 kPa in a hybrid phantom containing Vycor glass, a nanoparticulate AI2O3 powder, a nanoparticulate ZnO powder and sintered ceramics made from each of these powders. Dashed boxes indicate the regions of interest (ROIs) from which the isotherms in Figure 3.5.9 were extracted. Adapted from Ref. [21].

In an exceptionally clear-cut experiment methyl radicals have been formed on silica (Vycor glass) by using 2537-A radiation to decompose... 

Solid surfaces nature of the surface of colloidal silica, clays, zeolites, silica gels, porous Vycor glasses, alumina rigidity, polarity and modification of surfaces... 

Rgure 2.1. Pore radius distribution curve for a porous Vycor glass heat-treated at 500 C for 5 h (McMillan 1980). 

Surface diffusion has been extensively studied in literature. An overview of experimental data is given in Table 6.1. Okazaki, Tamon and Toei (1981), for example, measured the transport of propane through Vycor glass with a pore radius of 3.5 nm at 303 K and variable pressure (see Table 6.1). The corrected gas phase permeability was 0.69 m -m/m -h-bar, while the surface permeability was 0.55 m -m/m -h-bar, and so almost as large as the gas phase permeability (Table 6.1). It is clear from Table 6.1, that the effects of surface diffusion, especially at moderate temperatures, can be pronounced. At higher temperatures, adsorption decreases and it can be expected that surface diffusion will become less pronounced. 

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