2. Literature Review
2.3. Silica Surface 1. Surface Species
2.3.3. Silica and Water
The discussion of silica surface hydroxyls leads directly into how silica behaves in water. A fully hydroxylated silica surface is hydrophilic and is able to absorb water in multiple layers.69,89,92 The Si−OH groups present on surfaces are the main source of hydrophilicity because they promote the adsorption of water.89,97 The removal of these Si−OH groups through heat treating above 850°C or by replacement of these Si−OH groups by hydrolytically stable Si−R (where R is CH3) groups inhibits the adsorption of water and results in hydrophobic surfaces that will be unaffected by water.97
Silica undergoes chemical reactions in water. Namely SiO2 reacts with 2 H2O to form Si(OH)4 groups.98 This reaction is considered the dissolution,
depolymerization or hydrolysis reaction.27,99 The solubility of silica is dependent on the particle size, temperature, and pH of the water.27,100 Further reaction will create physisorbed water in a multi-layer, as previously shown in Figure 2.3.1.3. Certain silica containing glasses, such as sodium silicate or soda-lime silicate have poor chemical durability compared to silica, worse so for sodium silicate. In addition to the reaction of silica with water forming monosilicic acid groups, sodium leaches out of the structure and molecular water is able to enter. This exchange creates what is known as the silica “gel layer” and is very similar to a gel created through wet chemical means. Pure silica forms however do not lend themselves to the creation of a gel layer due to the absence of an exchanging species. However, Yaminsky et al. have stated otherwise, that the surface of fused quartz swells under water to form these layers of silica gel.92 In the same article, Yaminsky et al.
discuss many surface phenomena of silica including the history dependence of the state of the surface, effect of solution condition, and even evidence of “cold fusion”.92 At elevated temperatures water will etch pure silica through a dissolution reaction:
≡Si−O−Si≡ + (R+ + OH−) ≡Si−OH + RO−Si≡ (1) where R can be either H for water or any alkali ion for a basic solution.17 This reaction explains why silica is less durable in basic solutions of high pH.
In any case, the rate of dissolution is a thermally activated process such that the natural log of the amount of material extracted has a linear relationship to the temperature with a slope of activation energy.17
Again, to study the interaction of silica surfaces with water, high surface area samples are practically required, although one study has suggested otherwise.101 Silica xerogels were studied in aqueous media as described by Okkerse.80 He investigated the effect of various solutions at 80°C for a minimum of 14 hours on the texture of silica xerogel. The porous texture of xerogels is modified after treatment in water or electrolyte solutions. There is a decrease in specific surface area but the pore volume
remains constant. Water is essential for the changes to take place since immersion in both sulfuric acid and ethanol do not produce any effects.80 Salt and acid solutions appeared to be more effective in increasing mean pore radius and the particle diameter than just pure water.80 It was also found in the pore size distribution data that large pores grow in number while the small pores disappeared. This change was explained by the dissolution and reprecipitation of “silica”. Since the same volume of small particles and large particles will have more surface area in the small particles, and every system strives to minimize surface area/energy, the larger particles will grow at the expense of smaller ones.80 The same is true for pores.
The influence of pH on this system was also studied by Okkerse. He found that the rate of the dissolution-reprecipitation reaction (and specific surface area change) is a minimum in buffered solutions at pH 2, the isoelectric point of silica. The change in specific surface area increases above and below a pH of 2.80 A plot of dissolved silica versus time in buffered solutions with pHs of 2,4,6 and 8 kept at 60°C was put forth by Okkerse. It was deduced that the rate of dissolution is minimal at pH 2 and increases with increasing pH. Therefore, pH is a very important factor in the stability of silica in aqueous solutions. A compilation of data was done by Iler 10 years after Okkerse’s work and very different results were shown.27 However, this data was obtained at much lower temperatures and the water was not said to be buffered at each pH. Iler presented a plot of solubility versus pH and the solubility appears to be constant (possibly decreasing slightly) from low pH to a pH of 8 and then increases drastically at pH 8 and above.27 The effects of pH on silica systems is a difficult subject because of variations in the form of the silica and the type of solutions in which it is placed.
Okkerse and Iler also discussed the effect of electrolyte concentration of the solution on silica stability.27,80 A submicroporous silica was treated in a sodium chloride buffered solution of varying concentration of sodium chloride by Okkerse. It was observed that the change in salt concentration
changes the specific surface area of the silica to a small extent. This change was much smaller than the pH effect.80 Iler also reported from the data he compiled that the electrolyte concentration essentially has no effect on the solubility of silica.27
The diffusion of water into silica and its analysis by infrared spectrometry is a field in which Tomozawa excels. Although it was Doremus that first proposed that molecular water is the diffusing species through silica and that it will penetrate until it confronts an agreeable site with which to react; H2O + Si−O−Si 2 Si−OH.102 Tomozawa et al. found that water accelerates structural changes in silica during isothermal hydration heat- treatments.103,104 There is an acceleration of structural relaxation. Also, when the rate of structural relaxation is slower than the rate of hydroxyl diffusion at a temperature less than 850°C, the hydroxyl surface concentration and the hydroxyl content in the glass is a maximum.103 Tomozawa et al. proved that Doremus’ proposal was indeed correct at high temperatures.103
2.3.4. Surface Treatments