Silica migration

Hydrothermal solution containing appreciable amounts of dissolved silica migrates through andesitic volcanic rocks, accompanying Si02 precipitation. 

Khitarov, N.I., 1953. Experimental characteristics of quartz stability and silica migration under hydro-thermal conditions. Trudy IV Soveshchaniya po eksperimentalnoy mineralogii i petrografii (Acta of the Fourth Conference on Experimental Mineralogy and Petrography). Izd. Akad. Nauk S.S.S.R., Moscow, 2 180-186 (in Russian). 

Cj25 = 11.0%. Surprisingly, a remarkably uniformly distributed, very coarsely crystalline assemblage of silicates with a finely microcrystalline matrix results. The abundant "fluxing" constituents clearly promote silica migration during hard burning. 

Here GJo jpogj ion represents complex shear modulus of the blend without interfacial effects while GJ p is the interface contribution, which comprises the extra elasticity originating in drop deformability. The former contribution was computed from the Kemer model [equivalent to Eq. (2.23) with v 2 = 0], while the second was calculated from the Palierne model. The computed dependencies for PP with ethylene-vinyl acetate (EVAc) copolymer and Si02, hydrophilic or hydro-phobic, well represented the frequency dependencies of G, G", and rf. Also, the addition of 3wt% silica reduced EVAc droplet size from 2.2 to about 0.5 m. The hydrophilic silica migrated to EVAc, while hydrophobic silica migrated to the PP phase. 

Silicate apatites present several limitations for their potential use as SOFC electrolyte. One of the main drawbacks of apatite-type materials resides in the difficulty in preparing dense ceramic materials, which are needed for SOFC operation. On the other hand, degradation of the ionic transport properties with time in reducing atmospheres at high temperatures has been reported for these materials, which was ascribed to silica migration and volatilisation in the ceramic surface (Yaremchenko et al, 2004).The apatite structure allows a large number of cation substitutions. Among them, the partial substitution of Sb+ by AP+ seems to enhance the ionic conductivity and partially suppress silicon volatilisation (Shaula et a/., 2005,2006). 

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. 

Some manufacturers use the principle of the adsorption of a gas by a porous material such as silica gel or charcoal. Since the adsorbent is a solid and cannot migrate from the phial, these valves cannot suffer reversal of charge. 

Natural colloid particles in aqueous systems, such as clay particles, silica, etc. may serve as carriers of ionic species that are being sorbed on the particulates (pseudocolloids). It seems evident that the formation and transport properties of plutonium pseudocolloids can not yet be described in quantitative terms or be well predicted. This is an important area for further studies, since the pseudocolloidal transport might be the dominating plutonium migration mechanism in many environmental waters. 

In addition the role played by the sorbent on which the chromatography is carried out must not be neglected. For instance, it is only on aluminium oxide layers and not on silica gel that it is possible to detect caffeine and codeine by exposure to chlorine gas and treatment with potassium iodide — ben2idine [37]. The detection limits can also depend on the sorbent used. The detection limit is also a function of the h/ f value. The concentration of substance per chromatogram zone is greater when the migration distance is short than it is for components with high h/ f values. Hence, compounds with low h/ f values are more sensitively detected. 

Note Under the conditions employed emetine and cephaeline were not well separated but there was good resolution of the subsidiary alkaloids of the ipecacuanha tincture (Fig. 1). The separation and quantitative determination of the main alkaloids (Fig. 2) can be carried out under the following conditions Ascending, one-dimensional development in a trough chamber with chamber saturation layer HPTLC plates Silica gel 60 (Merck) mobile phase dichloromethane — methanol — ammonia solution (25%) (34+6+1) migration distance 6 cm running time 13 min h/ f cephaeline 65-70 emetine 75-80. 

A major metabolite was detected in the ethanol extract of 2,7-dichlo-rodioxin-treated soils. The metabolite was less mobile than DCDD in benzene-acetonitrile on TLC. The metabolite was eluted from the silica gel and methylated with diazomethane. The methylated metabolite was rechromatographed in benzene and migrated to the solvent front, suggesting a polar group on the non-methylated metabolite. 

The overall pathway for the conversion of the unsaturated azido ether 281 to 2,5-dihydrooxazoles 282 involves first formation of the dipolar cycloaddition product 287, which thermolyzes to oxazoline 282 or is converted by silica gel to oxazolinoaziridine 288. While thermolysis or acid-catalyzed decomposition of triazolines to a mixture of imine and aziridine is well-documented [71,73], this chemoselective decomposition, depending on whether thermolysis or exposure to silica gel is used, is unprecedented. It is postulated that acidic surface sites on silica catalyze the triazoline decomposition via an intermediate resembling 289, which prefers to close to an aziridine 288. On the other hand, thermolysis of 287 may proceed via 290 (or the corresponding diradical) in which hydrogen migration is favored over ring closure. 

Chromatographic Characterization of TTXs. The vast majority of reports have identified TTX and anhydro-TTX in bacterial cultures using HPLC, TLC, and GC-MS. Yasumoto et al. (30) showed that TTX-like substances extracted from a Pseudomonas sp. culture could bind to activated charcoal at pH 5.5 and be eluted with 20% ethanol in 1% acetic acid. In addition, HPLC analysis demonstrated TTX and anhydro-TTX-like fluorophors following strong base treatment. These compounds migrated on silica gel comparably to TTX and anhydro-TTX. Furthermore, when analyzed by electron ionization (EI)-MS and fast atom... 

The use of t.l.c. for separating organophosphorus compounds has been reviewed. In a study of phosphoryl compounds it was shown that / m values i.e. log [(1 - Rv)lRy on silica gel using hexane-acetone as eluant, depend upon the polarity of the P=0 group.The migration order found was... 

Capillary electrophoresis (CE) has several unique advantages compared to HPLC, snch as higher efficiency dne to non-parabolic fronting, shorter analytical time, prodnction of no or much smaller amounts of organic solvents, and lower cost for capillary zone electrophoresis (CZE) and fused-silica capillary techniques. However, in CZE, the most popular separation mode for CE, the analytes are separated on the basis of differences in charge and molecular sizes, and therefore neutral compounds snch as carotenoids do not migrate and all co-elute with the electro-osmotic flow. 

A large increase In dispersion following pretreatment was explained by considering the migration of the metal precursor from the catalyst to the additional silica support during pretreatment. 

TLC analysis of oligomers was performed on Silica gel 60 aluminium sheet (Merk) using buthanol and formic acid (1 1.5) as solvent [26]. The dye reagent was prepared by dissolving 9.5 mg of 1,3 dihydroxynaphtalin (Aldrich) in 5 mL of ethanol/H2S04 mixture (1 0.05, V/V). The migration front (Rf) of each spot was measured and expressed as Rm, were ... 

This interpretation of the experimental data is supported by the differences observed in the deactivation patterns and carbon contents after test, since one notorious effect of Hjp is the capacity to diminish the deactivation caused by coke deposition on the active sites [21,22]. This is supposed to be due to a reaction with the coke precursors, very likely a hydrogenolysis. In pure silica-aluminas, where no source of spillover is present, no special protection against deactivation should be observed. Indeed, the silica-aluminas lose most of their activity (about 80%) before reaching the steady-state and present the highest carbon contents after catalytic test. On the other hand, in the case of the mechanical mixtures, where spillover hydrogen is continuously produced by the CoMo/Si02 phase and can migrate to the silica-alumina surface, the predicted protection effect is noticed. The relative losses of activity are much lower... 

These findings support the view that during the reaction ions are extracted from the surface of the glass particles, migrate to the aqueous phase where they form the matrix, and leave a silica gel relict. This explains why the glass particles appear to be unattacked when examined under the microscope. The presence of both A1 and P in the cementing matrix and the... 

This alternative hypothesis may explain the observational differences between different workers. Brune Smith (1982), unlike Wilson et al. (1972), found Si distributed throughout the cement but were uncertain whether it was due to Si in the matrix or the degradation of fine particles to silica gel. But Brune Smith (1982) used a normal glass powder while Wilson et al. (1972) removed fine particles to improve resolution. These differing observations are reconciled if the silicic acid which is formed migrates slightly before condensing to silica gel. 

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