Heat-treated silicas

London Component of the Surface Energy of Heated Treated Silicas. Surface energy is usually considered as the sum of two components the London component (y ), steming from London forces, and the specific component (y p), originating from all other types of forces (polar, H-bonding, metallic, etc). Two methods are commonly used for the measurement of surface energies wettability and adsorption techniques. 

Figure 6. Calculated and experimental (Ts(pH) curves of heat treated silica [34]. The experimental data are taken from Bolt, G.H., J. Phys. Chem. 61 166 (1957).

London Component of the Surface Free Energy of Heat-Treated Silicas. Figure 1 shows the evolution of 7sd for the different types of silicas versus heat-treatment temperature. The origin of the sample as well as the thermal treatments applied are important in determining 7sd. 

Specific Component of the Surface Free Energy of Heat-Treated Silicas. Specific interaction capacities of heat-treated silicas, that is, their ability to interact with polar molecules, were examined with chloroform (Lewis acid probe) and toluene and benzene (amphoteric molecules). Figure 2 provides examples of the evolution of the specific interaction parameter Zsp of the different silicas with chloroform as a probe. 

Silica-alumina has a very strong acidity of pK —8.2, but metal sulfates do not show such a strong acidity, at what ever temperature they are heat-treated. Silica-alumina, whose acidity (0.6 mmole/gm) at pK +3.3 is much larger than the acidity (0.1 mmole/gm) of nickel sulfate at the same acid strength, was found to be much less catalytically active than the latter for the depolymerization of paraldehyde (57) and for the esterification of anhydrous phthalic acid (11). This might be interpreted thus acid of moderate strength is effective but too strong acid does not act as catalyst for the reactions. 

Fig. 18. Effect of alumina content on specific gravity of heat-treated silica-alumina...

London Component of the Surface Free Energy of Heat-Treated Silicas. 381... 

Fig. 4.1 Non-binding of heat-treated aerogels with gelatin. Aqueous gelatin solution failed to bind with heat-treated silica aerogel granules but was instead absorbed by the aerogels is an indication of loss of hydrophobic properties...

As shown also by Yates and Healy, if attempts are made to obtain nonporous silica particles by heating to 800°C, much of the surface is dehydroxylated to siloxane groups. When this surface is then rehydroxylated there is no evidence that a gel layer is formed again. However, on such heat-treated silica and on pyrogenic silica particles, after hydration, the surface charge density in relation to pH is still much higher than observed, for example, at the classical Agl-solution interface. It was concluded that even on a nonporous surface of amorphous silica the charging ions must be able to penetrate the surface to some extent. 

The work was later extended to the study of adsorption of n-butanol from water and it was found that, again, as in the case of the adsorption from n-heptane, n-butanol forms closed packed monolayers but this time exclusively on the hydrophobic sites, which for graphitic carbons are basal planes. The work on carbons was reported by the author in 1987 [18], Subsequently the work on carbons was continued with other partly hydrophobic solids, such as heat treated silica gels, ZSM-5 zeolites, coals, and carbon blacks [28]. All these solids possessed various proportions of hydrophilic sites, the surface areas of which could be independently estimated from the integral heats of adsorption of n—butanol from water. 

For SiC>2, we have only considered sources for silica suspensions which were non-porous, such as Ludox (39), pyrogenic silica (40), heat-treated BDH silica (22), or ground quartz (41). The data from these sources at 0.1M concentration has been collected in Figure 7. The data of the various researchers is quite consistent, in spite of the differences in origin of the suspensions, and the different electrolytes used. The slope of the points above pH 7 shows that the adsorption capacitance for cations is very large for both sodium and potassium ions, around 200 pF/cm2. Such a capacitance corresponds to a distance of 0.25.X, when using the dielectric constant of immobilized water molecules. The equilibrium constant for adsorption is low, however, since both KNa+ and Kk+ lie between 0.1 and 0.01 dms/mol. A possible interpretation of these results is as follows there is little specific attraction between SiC>2 and alkali cations,... 

The HGMs produced were heat treated at 620 °C for 18 hours before they were leached in 4 molar HC1 solutions at 80 "C for 4 hours. Heat treatment produced phase separation which resulted in a silica-rich phase and a more soluble and interconnected, sodium borate phase. Leaching removed the soluble sodium borate phase of the glass and created porosity through the microsphere walls. The porous wall, hollow glass microspheres, sank to the bottom of the solution. These sinkers were collected, water washed and dried at 100 °C overnight. 

The 3M Company manufactures a continuous polycrystalline alumina—silica—boria fiber (Nextel) by a sol process (17). Aluminum acetate is dissolved in water and mixed with an aqueous dispersion of colloidal silica and dimethylformamide. This mixture is concentrated in a Rotavapor flask and centrifuged. The viscous mixture is then extruded through spinnerettes at 100 kPa (1 atm) the filaments are collected on a conveyor and heat-treated at 870°C to convert them to metallic oxides. Further heating at 1000°C produces the 10-pm diameter aluminum borosilicate fibers, which are suitable for... 

These were prepared by tethering Rh and Pt complexes to silica-supported metal catalysts (metal = Pd, Ni, Ru, Au). The catalysts are very active in the hydrogenation of benzene derivatives to the corresponding substituted cyclohexanes under mild conditions. The activities are higher than those of the separate homogeneous complexes, complexes just tethered to silica, or the silica-supported heterogeneous catalysts. When the sol-gel-entrapped [Rh2Co2(CO)12] complex was heat-treated at 100°C, immobilized metallic nanoparticles were formed.425 The catalyst thus prepared efficiently catalyzed substituted benzene derivatives. 

The carbon blacks consisted of Spheron 6 and Sterling FT (fine thermal) black, both non-heat-treated and 2700° C. heat-treated (obtained from the Godfrey L. Cabot Co., Boston, Mass.). Published surface areas for the four carbon blacks are 114, 84.1, 15.5, and 12.5 sq. meters per gram, respectively (31). The silica sand was supplied by the New Jersey Silica Sand Co., Millville, N. J., with the manufacturer s designation of No. 325 flour. Screen analysis by the supplier showed a considerable range in particle size about 60% was retained on a 90-mesh screen and 90% on a 200-mesh screen. Finally, a sample of asbestos was tested (obtained from the Canadian Johns-Manville Co., Port Credit, Toronto, with the designation 7R). Zettlemoyer (31) has studied this material. 

The usual preparation of supported micrycrystalline samples by the incipient wetness technique involves the impregnation of a support, e.g., silica gel or alumina, with a solution of a metal salt to form a thick slurry that is subsequently dried and sometimes heat-treated. 

Fubini B. et al., Relationship between surface properties and cellular responses to crystalline silica Studies with heat-treated cristobalite, Chem. Res. Toxicol., 12, 737, 1999. 

Recent surface force measurements revealed a similar trend (20). Comparing steam-treated to flame-treated silica sheets using site-dissociation/site-binding model, a decrease in silanol surface sites and apparent decrease in average pKa was observed upon heat treatment. Furthermore, a repulsive force other than double-layer and van der Waals forces was observed 15 A from the surface. This repulsion was attributed to hydration of the surface and was found to be independent of surface treatment and electrolyte concentration. In Bums treatment, an arbitrary plane of shear was introduced to provide a best model fit (l 3). A value of 9 A from the surface for the plane of shear was determined from electro-osmosis measurements. 

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