Silica glass, dissolution

Finally, our observations regarding the longterm impact of alkali ion exchange on glass dissolution now provide a mechanistic basis for the empirical residual rate of reaction appended to the TST rate law articulated by Grambow (1985). The residual rate was appended to prevent calculated glass dissolution rates from dropping to zero under silica-saturated conditions, which is not in accord with experimental observations. 

If the aluminate ion is a component in the chemical affinity term, then the activity of dissolved silica, by itself, cannot describe the change in rate with changes in chemistry of the contacting fluid. A number of attempts have been made to explicitly include Al activity in the chemical affinity term. Gin (1996) suggested that glass dissolution could be modelled using a mixed Si/Al term for the ion activity product ((2) ... 

Figure 10. Mechanisms of glass corrosion for a soda-silica glass. Conditions a, t = 0, pH = 7 b (stage 1), t > 0, pH < 9, selective Na dissolution and c (stage 2), t 0, pH 9 total dissolution. (Reproduced, with permission, from Ref. 1. Copyright 1979, Books for Industry.)...

The reaction of acids with glass may be either a leaching process or a complete dissolution process. Acids such as hydrofluoric acid attack silica glasses by dissolving the silica network. Other acids such as hydrochloric acid or nitric acid may react by dissolving certain glasses. However, the reaction mechanism is by selective extraction of alkali and the substitution of protons in a diffusion-controlled process. 

All silica glasses are particularly susceptible to dissolution by solutions above pH 9 due to the nucleophilic attack of the hydroxyl ion on the silicon-oxygen bond J), This is relevant in our particular discussion on bricks, as these are set in masonry with mortar, from which lime may be leached out such that solutions reaching the neighbouring pores could well reach pH 11 or 12. 

The main features of basalt glass dissolution are illustrated on Figure 1. For all elements one can observe after an ephemeral period of fast dissolution a linear sleady-state dissolution period. Looking at the stoichiometry of cation release with respect to silica, we can recognize two different groups of cations ... 

Figure 1. (a) Examples of basalt glass dissolution at pH 2.5 plotting silica release in solution us ft function of time and temperature, (b) Rx of Na and Mg to Si at 50"C versus solution pH and time,... 

The radius of curvature of the silica-water interface is of critical importance even with a porous silica solid. Charles (170) found that the rate of dissolution of porous high silica glass could be explained on the basis of a high local solubility of the silica surface owing to its small radius of curvature. 

Unfortunately the addition of large amounts of alkali oxides to silica leads to a reduction of the chemical durability of the resulting glass. Although commercial alkali alkaline earth silicate glasses are used as bottles and other containers for liquids, these glasses are compared with silica glass rather susceptible to dissolution in water and chemical reactions with acids and alkaline lyes. 

Vitreous silica is susceptible to attack by alkaline solutions, especially at higher concentrations and temperatures. For 5% NaOH at 95°C, although craving may be evident, surface corrosion is only 10 p.m after 24 h (87). For 45 wt % NaOH at 200°C, dissolution proceeds at 0.54 mm /h (88). The corrosion rates in other alkaline solutions are Hsted in Table 3. Alkaline-earth ions inhibit alkaline solution attack on vitreous siUca. Their presence leads to the formation of hydrated metal siUcate films which protect the glass surface (90). 

Studies on hot water tank enamelsin media of varying pH demonstrate a minimum corrosion rate at pH value of 4. In citric acid (pH 2), IR measurements indicate that ion exchange is the principal mode of corrosion. Distilled water (pH 7) showed evidence of a bulk dissolution mechanism with no silica enrichment of the surface layer. In neutral solutions, the first stage of attack is leaching of alkali ions, raising the pH of solution, which subsequently breaks down the glass network of the acidic oxides. 

In the specific case of silica nanoparticles-pH EMA hybrid materials, the synthesis relies on obtaining a fine dispersion of silica nanoparticles (with a mean diameter of 7nm) in HEMA monomers (liquid phase). When a homogeneous solution is obtained, a free radical initiator is added at a concentration based on the weight of the monomer mixture. After the initiator dissolution, the solution can be poured into molds or between two glass plates to obtain monoliths or uniform films, respectively, after being cured at temperatures around 60-85 °C for several hours. 

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). 

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