Silicates and colloidal silica

March 11th, 2010

Geopolymer chemistry is a member of the Sol-gel process of material processing. The Sol-gel method comes directly from colloidal silicate chemistry, such as the precipitation of silica gels and the growing of quartz. Colloidal chemistry and the sol-gel process are much broader than just colloidal silica, but the art of gelling silicates is old and well understood, making it an easy entrance into other earthen chemistries, including gelling of alumino-silicates (GP).

I have been reading “The Colloid Chemistry of Silica and Silicates” by Ralph Iler, published in 1955 by Cornell University, as part of their non-resident lectureship in chemistry series. It is a very informative book, and for those that do not have the time or access to read it, I would like to share bits of what I have learned.

Silica exists anhydrously as SiO2, in quartz, glass, opal, etc. However, Silica also has an important reaction with water, where it becomes hydrated to Si(OH)4: silicic acid. In all situations where water and silica come into contact, water exerts its solvating power and releases some amount of silica in this non-ionic, dissolved form.

The amount of silicic acid in water is usually very small (several hundredths of a percent), though micro-organisms and plants make much use of it. The exact amount depends upon two factors, the state of the mother silica and the temperature of water. pH, surprisingly does not play much of a role in the formation of silicic acid, but has other effects which I will discuss later.

In the mother silica, it seems that the amount of surface area of silica per volume is the significant factor. (actually, it is surface energy). On a macroscopic scale, this corresponds to silica particle size. On a microscopic scale, this corresponds to the crystal structure of the silica. Tightly packed silica (quartz), is much less soluble than more loosely packed amorphous silica (glass, biological silica, opal). Though amorphous silica exists with silicic acid in water at just above 0C, Quartz only begins to give it at 150C.

The rate of formation of silicic acid is very slow, and it takes days for finely ground amorphous silica to reach equilibrium with water. Quartz is likely more slow. The existence of alkali salts in solution, such as potassium carbonate, accelerates this process. I do not know by how much.

Because this reaction is so slow, solutions of silicic acid easily over saturate when temperature or pH is changing, or when silicic acid is being concentrated because its solvent is evaporating. If super saturation is kept minimal (ie, conditions are changed slowly), silicic acid will condense as anhydrous SiO2. If the solution is allowed to super saturate more, silica will precipitate out as an amorphous gel with both Si-O-Si (siloxo) and Si – O – H (silanol).

This condensation could take place on an existing silica surface, or as a particle formation within the solution. Mono-silicic acid polymerizes to di-silicic acid (OH)3 SiOSi (OH)3, and so on to high molecular weight poly silicic acids, up to colloidal size and monolithic silica gels / quartz. In fact, any silica in the presence of water has a silanol surface, and can be thought of as a polysilic acid of very high molecular weight and a high ratio of siloxo bonds to silanol. Mono-silicic acid is very unstable, tending to polymerize, and (to my knowledge) has not been isolated in quantity or for very long.

The existence of alkali ions such as sodium, potassium and lithium, make the existence of a soluble ionic silicate possible. So, though the amount of silcic acid in solution is hardly affected by pH, alkali removes silicic acid from solution to form silicate ions, and more mother silica can turn to silicic acid in solution to replace it. The role of the silicate ion becomes significant at around pH 9 or 10. Titrating a solution of alkali silicate with an acid below this point quickly over saturates the solution and a silica gel coagulates.

Aluminium in solution reacts with silicic acid to form a highly insoluble alumino-silicate. When this reaction is allowed to happen on the surface of the mother silica, the surface area available for forming silicic acid is greatly reduced and the equilibrium concentration of silcic acid is lowered. In this way, aluminium ions decrease the solubility of silica. Other multivalent metal ions, such as Cu++ and Ca++ have a similar effect.

For this reason, copper and calcium salts are used as accelerators to “set” sodium silicate. Also, sodium silicate is used in conjunction with portland cement because of its reactions with calcium. As a sealer for portland cement, sodium silicate reacts with the free lime produced by the setting reaction of calcium silicate.

The rest of Dr. Iler’s book is equally interesting. He discusses in great detail the formation and use of silica gels and colloidal silica. Topics include the use of alcohols and ketones to modify the surface silanol groups to be hydrophobic, hydrophilic and hydrophobic silica adsorbants, the inclusion of modified silica in lubricants and paints, and details of the role of silica in organic life.