# Silicon-Oxygen Bonded Molecules

• Contributed by Larry Coleman
• Engineering Consultant at Consultant on Demand

Silicon-oxygen bonded molecules occur naturally as silicates, in mineral formations like quartzite and sand, and can be extracted into soluble form by reaction with strong alkalis. However, by going thru a chlorosilane hydrolysis route as synthetically produced compounds, silicon-oxygen bonded molecules have found significant application in food-stuffs, paints, moisture absorption, and most notably as silicones.

## Unusual/Noteworthy Properties

1.In its natural form silicon-oxygen bonded compounds are referred to as silicas or silicates and exhibit strongly hydrophobic tendencies.The silicate ion ($$\ce{SiO_3^{-2}}$$) bonds ionicaly with alkali metals like sodium, potassium, and calcium. Water can be trapped within the interstices of silica solids, but the solids' surfaces are not truly wetted, nor will silica dissolve. Some solid silicate minerals can have complex molecular forms with water of hydration. An example is the sodium-alumina silicates known as zeolites.

The purified form of quartzite rock is known as quartz, which has a triagonal crystalline structure and is chemically written as SiO2. Fused quartz is commonly used as a structural material to make crucibles for holding molten silicon. As it is slowly wetted and dissolved over a period of days by molten silicon, its solute form is as silicon monoxide ($$SiO$$). The dissolved $$SiO$$ exhibits a significant vapor pressure , and when cooled will nucleate to form sub-micronic fibroids which remain hydrophobic. Other high temperature reactions with silicas will produce these fibroids, that when breathed will result in silicosis.

2. Synthetic forms of silicon-oxygen bonded molecules can be created by the liquid-phase reaction of chlorosilanes or organo-chlorosilanes with water or alcohols, or by gas-phase combustion of chlorosilanes with hydrogen. As simple monomers and dimers, these compounds are normally hydrophilic and referred to as siloxanes or silanols (the silicon analogs to ketones and alcohols). In polymeric form, these cross-linked silicon-oxygen bonded compounds are generally hydrophobic and referred to as silicones. Most silicones feature a sizeable organic content, so more is discussed in the module on silicon-carbon-oxygen bonded molecules.

When chlorosilanes are combusted with hydrogen and the flame is rapidly quenched in the presence of water vapor, the stable silicon oxide is silicon monoxide (SiO). This SiO will remain meta-stable at ambient temperature as a very light fluffy solid, with a bulk density of about 0.02 g/cc. This form of silica is known as fumed silica ( not to be confused with the silica fume that causes silicosis) and has most unusual properties. When mixed with water in a 1:5 ratio and whipped, it will form a gelatinous solid that resembles Jello. When this gelatinous form is partially dried, it forms beads known as silica gel. Fumed silica has been used for over 80 years as a thickener for both commercial purposes ( like paints and greases) and in edible applications like fast-food milk-shakes. See the below section of this module for more about its many uses.

When synthetic silicon-oxygen bonded compounds are formed by hydrolysis (i.e, reaction generically with any organic or inorganic compound that has one or more ionic OH- group), the pH of the hydrolyzing compound dictates the degree to which it is hydrophilic or hydrophobic. Chlorosilane acidic hydrolysis produces hydrophilic siloxanes. To the extent that the hydrolysis reaction is performed in a more alkaline environment, the siloxane will tend to be more hydrophobic.

Once the Si-O bonding type is formed (either hydrophobic or hydrophilic), subsequent acidification or alkaline adjustment does not alter the nature of the Si-O bonding.

## Main Applications/Uses

1. The main uses of naturally occurring silicon-oxygen bonded molecules like silica, silicates, and quartz are as:

1. Silica & Sand: Abrasives; as a concrete component; soil drainage enhancement; talcum and cosmetics
2. Silicates: High temperature refractories; detergent ingredient, surfactants, water glass ( after filling with fumed silica)
3. Quartzite & Quartz: Gemstones, acid & cut-proof countertops,raw material for producing crude and high purity silicon metal ( see the module on elemental silicon)

2. The main uses of synthetically produced silicon-oxygen bonded molecules are as:

a. Siloxanes

Building block intermediates in preparation of pharmaceuticals; wetting agents for organic chemicals