Silica, near-surface

Once the molecules have been captured inside the protocell, the concept of a concentration gradient and semipermeable membrane becomes important. Near a negatively charged mineral surface, such as silica, the surface concentration... 

Under ideal circumstances, certain chemical processes which are relatively sluggish may possibly be used for water dating. Near-surface water which is low in dissolved silica, for example, might be undersaturated with respect to silica which in turn would suggest that the water is less than 10 years old and probably less than a few months old. Unfortunately, the large number of variables which control dissolution or precipitation of minerals in natural systems probably can never be defined with sufficient precision to enable more than the most general, qualitative dati ng. 

A surface peak effect has been observed during Rb and Sr diffusion in vitreous silica (13). Such large near-surface concentrations are postulated to result from the exposure at the glass surface of a greater number of interstices or defects over which diffusion can occur. This would lead to steep penetration curves observed in some XPS profiles of glass. 

The forms of SiC found in sediments and sedimentary rocks are quite varied but those which could be suspected of near surface origin are generally as follows quartz, chalcedony, opal, amorphous gels and ionic forms in solution. Natural occurrences indicate that the solid forms of silica precipitate which has crystallized after the time of initial deposition (Siever, 1962). 

It would appear from the above summary of natural occurrences that quartz is the most stable form of silica at near-surface conditions but that other metastable phases, representing initially poorly organized material, predominate in the natural occurrences or newly formed silica. Experiments demonstrate the persistence of metastable amorphous or cryptocrystalline hydrated Si02 at low temperature (Kittrick, 1969 Krauskopf, 1956, 1959) and slow conversion at higher temperatures (above 100 bars) (Frondel, 1962 Heydemann, 1964 Carr and Fyfe, 1958 Mlzutanl, 1970). 

The most intriguing fact is that this process is accompanied by a manifold, from 10 to 100 times (up to 1013 cm-2), increase in the concentration of PCs ((sSi-0-)3Si-0 and (=Si-)3C radicals) in the sample. As in the RSi samples, the latter are stabilized in the near-surface silica layers, whereas oxy radicals lie at the silica surface. The oxy radicals produce a characteristic red (620 nm) luminescence excited by the light with a wavelength of 280 nm [64], so that the samples thus prepared emit red light under UV irradiation. Due to a high concentration of the centers in fine-grained silica, this luminescence can be observed even in daylight. 

Figure 7.33a shows the optimized structure of the TS for the reaction of H abstraction from the hydrogen molecule by the F3Si-N -H radical as a low-molecular model of the =Si-N -H surface center. The =Si-N -Si= radicals that were stabilized on the surface of mechanically activated Si02(N) silica also reacted with H2 molecules. The process was accompanied by the chemisorptions of the gas in an amount that was comparable to the number of reacted radicals (recall that a portion of these radicals was stabilized in near-surface layers of the material, and they were inaccessible to molecules from a gas phase). The (=Si-0)2Si diamagnetic centers also occurred on the surface of the test sample, and the decay of radicals in an atmosphere of H2 was accompanied by the formation of new PCs > Si -H radicals. Thus, in this case, as well as in the reaction of =Si-N -H radicals with H2 molecules, the process occurred as follows ... 

Pinholes pose another threat. The effect of the presence of metals can be seen in Figure 7.14. Here silica based Xerogels were prepared with various amounts of metals. The near surface change in the 3-to-2 photon ratio was used to estimate the range of positronium. The same model as in the case for porous MSSQ was used. The range dropped from 1.55 0.05 mm in the plain silica sample to 0.15 0.07 mm in the case of tantalum and to 0.8 0.09 pm in the iron containing sample. 

The total amounts of adsorbate adsorbed show no large differences near saturation pressure at Pq = 73.3 hPa. All three samples base on fumed silica of surface area SA = 300 m g referring to a hydrophilic surface. In the high-pressure, multilayer and capillary condensation region the adsorption isotherm is dominated by particle size and aggregate interparticle voids in the mesopore range. But in the low-pressure submonolayer region of the isotherm at p p < 0.07, the adsorbate uptake is controlled by... 

An experimental setup similar to the Zimmerman photolysis cell was utilized by Lazare et al. (60) to determine the quantum yield for the photoreaction of a substrate adsorbed on silica gel. The photolysis cell consists of an aluminum dish for the powdered silica gel sample, which is covered by a double-walled hemispherical Pyrex cap filled with ferrioxalate actinometers solution. The sample is irradiated through a quartz light pipe, which enters the photolysis cell through a hole at the top of the cap. Thus, nearly all scattered light from the silica gel sample is absorbed by the surrounding actinometer solution, and the amount of light absorbed by the substrate (which is adsorbed on the silica gel surface) is determined by a similar subtraction method as described in the Zimmerman experiment (vide supra). 

Nelson D. A. and Goering J. J. (1977) Near-surface silica dissolution in the upwelling region off northwest Africa. Deep-Sea Res. 24, 65 -73. 

---