Dissolution of silica

The pores of the silica template can be filled by carbon from a gas or a liquid phase. One may consider an insertion of pyrolytic carbon from the thermal decomposition of propylene or by an aqueous solution of sucrose, which after elimination of water requires a carbonization step at 900°C. The carbon infiltration is followed by the dissolution of silica by HF. The main attribute of template carbons is their well sized pores defined by the wall thickness of the silica matrix. Application of such highly ordered materials allows an exact screening of pores adapted for efficient charging of the electrical double layer. The electrochemical performance of capacitor electrodes prepared from the various template carbons have been determined and are tentatively correlated with their structural and microtextural characteristics. 

A single technique that preserves all three nutrients (N03, P04, and Si(OH)4) in a single matrix would be most desirable. If this proves impossible, it will be necessary to use different approaches to preserve and store different nutrients. For example, glass containers cannot be used to store a reference material for Si(OH)4 due to the slow dissolution of silica (Zhang et al., 1999), while the polymerization of silicic acid upon freezing eliminates that option for preserving stable Si(OH)4 levels (Zhang and Ortner, 1998). 

Since the surface silanol groups react weakly acidic, neutralization with strong bases can be used for their direct determination. However, care must be taken that no dissolution of silica takes place. Greenberg (1ST) found that the adsorption of calcium hydroxide was roughly... 

The Ca(OH)2 adsorption was determined by conductometry. Superposed on the neutralization reaction is a slower formation of insoluble calcium silicates which is analogous to the dissolution of silica by sodium hydroxide. 

Sears 189) and Heston et al. 190) used the adsorption of sodium hydroxide for the determination of the surface area of colloidal silica. An empirical factor was used for the conversion of alkali consumption into surface area. This is permissible provided the packing density of surface silanols is constant. The determination was performed in concentrated sodium chloride solution in order to keep down the dissolution of silica. Using the same technique, it was found in my laboratory that all surface silanol groups as determined by other methods are neutralized at pH 9.0. At higher pH, siloxane bonds in the surface were opened. A maximum in the sorption of Na+ ions occurred usually at pH 10.5-10.6 which corresponded to a packing density of ca. 5 OH/100 A. On further addition of alkali, silicate ions H3Si04 went into solution. 

The existence of siloxane bonds on the surface of silica has been inferred mainly from the fact that the number of observed silanol groups is not sufficient for complete surface coverage. Practically no silanol groups are present in silicas heated to high temperatures. The siloxane bonds are quite unreactive. Actually, this is the cause of the inertia of fused silica vessels towards chemical attack. When siloxane bonds are opened, the process usually will not stop at the surface and dissolution of silica will take place. 

The surface chemistry of coesite and stishovite was studied by Stiiber (296). The packing density of hydroxyl groups was estimated from the water vapor adsorption. More adsorption sites per unit surface area were found with silica of higher density. Stishovite is especially interesting since it is not attacked by hydrofluoric acid. Coesite is dissolved slowly. The resistance of stishovite is ascribed to the fact that silicon already has a coordination number of six. Dissolution of silica to HaSiFg by hydrogen fluoride is a nucleophilic attack. It is not possible when the coordination sphere of silicon is filled completely. In contrast, stishovite dissolves with an appreciable rate in water buffered to pH 8.2. The surface chemistry of. stishovite should be similar to that of its analog, rutile. 

Organic groups were detected on the surface after grinding of silica in the presence of organic solvents. Silicon-carbon bonds are also formed by nucleophilic attack on the siloxane bonds with lithium organic compounds. This reaction is analogous to the dissolution of silica with alkali hydroxides. 

The Transition State Theory (TST) treatment of reaction kinetics accounts for the fact that the true (microscopic) behavior of a dissolution reaction may involve the production of an intermediate complex as an essential step of the bulk process. Consider, for instance, the dissolution of silica in water. The macroscopic reaction may be written as... 

Metal Content of Old and TEOS Type Silicas Determined by ICP/AES after Dissolution of Silica... 

The mechanism of action by which silane and siloxanes reduce expansion has been attributed to water repellence and air entrainment. Phosphate addition or coatings may interfere with the dissolution of silica gel and the formation of gel. It is also possible that phosphate reduces the osmotic potential and the swelling pressure in the gel. The manner in which air entrainment reduced expansion was attributed to the accommodation of alkali-silica gel in the air void system. For example, it was found that air-entrained concrete with 4% air voids could reduce AAR expansion by 40% [23]. 

TEOS/H.O, NH, HF DVB, EDMA Dissolution of silica from polymer/Si02 nanocomposites yields nanoporous polymer materials (39)... 

Column temperature affects the relative retention of different compounds and elevated temperature permits high-speed chromatography to be conducted.25 Figure 25-28 suggests a systematic procedure for method development in which solvent composition and temperature are the two independent variables.1 For elevated temperature operation, pH should be below 6 to retard dissolution of silica. Alternatively, zirconia-based stationary phases work up to at least 200°C. 

Lewin, J. C. The dissolution of silica from diatom cell walls. Geochim. Cosmochim. Acta 21, 182-189 (1961). 

Sarex (1) [Saccharide extraction] A version of the Sorbex process, for separating fructose from mixtures of fructose and glucose. The usual feed is com syrup. The adsorbent is either a proprietary zeolite or an ion-exchange resin. Unlike all the other Sorbex processes, the solvent is water. The process depends on the tendency of calcium and magnesium ions to complex with fructose. The patents describe several methods for minimizing the dissolution of silica from the zeolite. The process is intended for use with a glucose isomerization unit, so that the sole product from com syrup is fructose. Invented by UOP in 1976 by 2003, five plants had been licensed. 

Thus, it can be assumed that in the presence of small amounts of water, the interaction between magnesium hydroxide and silica, finely ground and thoroughly mixed, proceeds via the dissolution of silica in the presence of magnesium hydroxide. This followed by the interaction of silica with magnesium hydroxide resulting in the formation of insoluble magnesium silicate. 

At pH greater than 7, phosphate buffers accelerates the dissolution of silica and severely shortens the lifetime of silica-based HPLC columns. If possible, organic buffers should be used at pH greater than 7. Ammonium bicarbonate buffers usually are prone to pH changes and are usually stable for only 24 to 48 hours. The pH of this mobile phase tends to become more basic due to the release of carbon dioxide. 

The type of pH modifier to make a desired mobile phase pH also has an effect on the column stability, and this is indirectly related to the peak efficiency and the retention of the analyte. As an increasing number of column volumes of the mobile phase are traversed through the column, the stability of the packing material could be comprised. Rearrangement of the packing bead leads to the loss of efficiency, dissolution of silica leads to loss in efficiency and retention, and hydrolytic decomposition of the bonded phase could impact the peak shape and retention. Different compounds, such as neutral compounds, acidic compounds, and basic compounds, could show different behaviors. 

Different types of buffers at the same ionic strength and wpH can have a significant impact on the dissolution of silica. The dissolution of silica is usually measured by the silicomolybdate colorimetric method [41]. When determining the bonded-phase stability using different run buffers (effect of buffer counteranion or countercation), the same H must be used. The H values (pH of the mobile phase aqueous -i- organic) may be different from the aqueous portion of the mobile phase and may obscure if the dissolution of the silica is directly related to the type of anion/cation and/or the pH. Generally, with the addition of organic solvents the pH of the mobile phase decreases for basic buffers and increases for acidic buffers (see Section 4.5 for more details). 

Generally, the lower the concentration of the buffer, the slower the dissolution rate of the silica. The rate of the dissolution of silica or the increase of... 

Tiemann (T8) studied the dissolution of silica from a siliceous iron ore by sintering the ore with sodium carbonate followed by leaching the sodium silicate with water. The reaction rates were found to be low after sintering for 4 hr at 1450°F. The residual concentrate was analyzed to be 56% iron, corresponding to 88% dissolution of the silica. Partial to complete fusion resulted when the temperature was increased. 

Exposures of some metal oxide membranes, both dense and porous, to extreme pH conditions (e.g., pH less than 2 or greater than 12) can cause structural degradations, particularly with extended contact time. The extent of degradation depends on the specific phase of the material, porosity, and temperature. Steam can also be deleterious to some metal oxide and Vycor glass membranes. For example, as mentioned earlier, porous glass membranes undergo slow structural changes upon exposure to water due to partial dissolution of silica. 

Polster W. (1994) Hydrothermal precipitation and dissolution of silica Part I. Conditions in geothermal fields and sedimentary basins Part 2. Experimental evaluation of kinetics. PhD, Pennsylvania State University. 

At the first stage of dissolution of silica in a saturated solution of catechol, the nucleophilic substitution occurs on the silicon atom. 

The rate of dissolution of marble (CaC03) in hydrochloric acid is an example of a chemically controlled reaction. The rate can be measured from the evolution of carbon dioxide. Palmer and Clark found that the rate of dissolution of silica in hydrofluoric acid was proportional to surface area and acid concentration. The rate of reaction was measured by noting the increase in conductivity of the solution... 

Calculation of the distribution of the species as a function of pH is shown in Fig. 4.32. The rate of dissolution of silica is then determined by the number of reactive species. 

In alkaline solutions, according to Hooley, the dissolution of silica follows two consecutive reactions. First, an adsorption of water occurs followed by reaction with hydroxyl ions to produce soluble products ... 

The dissolution rate equation for silica in nonflnoride solntions may also be expressed according to the surface complexation model described by Eq. (4.3) and Fig. 4.32 considering the contributions of different surface complexes sSiOHJ, sSiOH, =SiO-Na% and =SiO. The rate equation for the dissolution of silica in NaCl solutions at pH 2-13 at 25 °C can be described as ... 

In strong acidic solutions, with addition of HQ, the dissolution of silica is first order with respect to HF concentration. ... 

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