3_6 Silicates and Silicones: Structures, Preparation and Uses

Silica is a network of

\mathrm{SiO_4}

tetrahedra sharing every corner. If some corners carry a negative charge instead of being shared, the result is a silicate. Silicates make up most rocks, soils and clays. If the outer positions carry organic groups instead, the result is a silicone, a man-made polymer with a silicon–oxygen backbone. Both families rest on the same strong, flexible Si–O–Si linkage.

How Silicate Structures Are Built

The building block is the orthosilicate ion,

\mathrm{SiO_4^{4-}}

, a silicon atom with four oxygens at the corners of a tetrahedron. Tetrahedra can join by sharing oxygen atoms. Each shared oxygen removes one negative charge and builds a larger structure, and metal cations sit between the units to balance the remaining charge:

Main classes of silicate

Corners shared per tetrahedron	Structure	Repeat unit	Example
0	Isolated tetrahedra	SiO₄⁴⁻	Zircon, ZrSiO₄
2	Single chains or rings	SiO₃²⁻	Pyroxenes; asbestos-type fibres
3	Sheets	Si₂O₅²⁻	Talc, mica, clays
4	3D framework	SiO₂ (Al replaces some Si in feldspars)	Quartz, feldspar, zeolites

The structure explains the physical properties. Sheet silicates such as talc and mica split into thin flakes, and talc feels greasy because its sheets slide over one another. Chain silicates can form fibres, which is the case for asbestos minerals. Asbestos was once used for fireproof fabrics and boards, but its fibres cause serious lung disease, so its use is now banned or tightly restricted. Framework silicates, in which aluminium replaces some silicon, include the feldspars and the cage-like zeolites used as ion exchangers and catalysts.

Water Glass and Clay

Sodium silicate, often written

\mathrm{Na_2SiO_3}

, is made by fusing sand with sodium carbonate in a furnace:

\mathrm{Na_2CO_3 + SiO_2 \xrightarrow{\ \text{fuse}\ } Na_2SiO_3 + CO_2}

It dissolves in water to give a thick, strongly alkaline solution called water glass. Its uses include soap and detergent fillers, adhesives, fireproofing of fabric and wood, and sealing cement. Crystals of coloured metal salts dropped into water glass sprout coloured, plant-like growths of insoluble metal silicate, known as a chemical garden.

Clay forms when feldspar-rich rocks weather. Over long periods, water and carbon dioxide break potassium feldspar down into soluble potassium carbonate, sand and hydrated aluminium silicate. Pure white clay is kaolin, used for porcelain and china. Ordinary clays contain iron oxides that turn red on firing and lower the melting point, which makes them good for bricks, tiles and stoneware. Wet clay is plastic because water slips between its layers. Firing drives the water out and fuses the particles into a hard, rock-like mass.

Making Silicones

Silicones are made from organochlorosilanes. The key starting material is dimethyldichlorosilane,

\mathrm{(CH_3)_2SiCl_2}

. Water rapidly replaces its Si–Cl bonds with Si–OH groups, releasing hydrogen chloride:

C[Si](C)(Cl)Cl.O.O>>C[Si](C)(O)O.Cl.Cl

The resulting silanediol is unstable. Its OH groups condense with those of neighbouring molecules, losing water each time and joining silicon atoms through oxygen bridges. Repeating this builds a long chain,

\mathrm{-[Si(CH_3)_2-O]_n-}

, called polydimethylsiloxane, the most common silicone:

C[Si](C)(O)O.C[Si](C)(O)O>>C[Si](C)(O)O[Si](C)(C)O.O

• \mathrm{(CH_3)_3SiCl}

  has one reactive site, so it caps chain ends and controls chain length.

• \mathrm{(CH_3)_2SiCl_2}

  has two reactive sites and builds linear chains, giving oils and rubbers.

• \mathrm{CH_3SiCl_3}

  has three reactive sites, so it cross-links chains into rigid, three-dimensional resins.

Properties and Uses of Silicones

The Si–O backbone is strong and flexible, and the outer methyl groups make the surface oily and water-repellent. Silicones are therefore heat-stable, chemically inert and non-toxic. Their viscosity also changes remarkably little with temperature. Cooling from 100 °C to 0 °C can make a petroleum oil about a hundred times thicker, but a silicone oil becomes less than four times thicker.

• Oils and greases: lubricants and hydraulic fluids that work over a wide temperature range.
• Rubbers: sealants, gaskets, tubing, bakeware and medical implants that stay flexible from very low to high temperatures.
• Resins: heat-resistant electrical insulation and protective coatings.
• Water-repellent films: treating fabric, glass, masonry, paper or ceramic insulators with silicone makes water bead up and run off.

Tips for remembering

• Everything is built from SiO₄ tetrahedra: silicates differ only in how many corners each tetrahedron shares, from none (isolated ions) to all four (silica).
• Silicone = Si–O backbone + organic side groups: replace the outer oxygens of a silicate chain with methyl groups and you get a silicone.
• Two Cl give chains, three give cross-links, one caps the end: the number of chlorines on each silicon decides whether a silicone is an oil, a rubber or a resin.