Chemistry of Silicates and Silicones

Chemistry of Silicates and Silicones

Earth's Crust Composition

• The Earth's crust is composed of approximately 95% silicate minerals, aluminosilicate clays, or silica.

• Silica makes up the bulk of rocks, sand, and their breakdown products (sand and clay).

• Building materials are mainly silicates, such as slate, granite, brick, cement, ceramics, and glass.

• Oxygen (O), silicon (Si), and aluminum (Al) are the three most abundant elements in the Earth's crust, making up 81% of it.

Soluble Silicates: Preparation

• Soluble silicates are prepared by fusing an alkali metal carbonate with sand in an electric furnace at approximately 1400 °C.

• $Na<em>2CO</em>3 + SiO<em>2 \rightarrow Na</em>4SiO<em>4 \text{ or } (Na</em>2SiO<em>3)</em>n$

• The product is a soluble glass of sodium or potassium silicate.

• It is dissolved in hot water under pressure.

Soluble Silicates: Uses and Limitations

• Used in detergent preparations to maintain a high pH, aiding in the removal of grease and fats by forming soap.

• Soluble silicates should not be used with hard water because they react with $Ca^{2+}$ to form insoluble calcium silicates.

• Sodium silicate is used as an adhesive, in asbestos roof tiles, in fireproof paint and putty, and in the production of silica gel.

Silicate Structures

• The majority of silicate minerals are insoluble due to their infinite ionic structure and the strong Si-O bond.

• Studying their structures is difficult.

• Structures are determined using X-ray crystallography.

• The electronegativity difference between O and Si (3.5 - 1.8 = 1.7) suggests that the bonds are approximately 50% ionic and 50% covalent.

Classification of Silicates

• Silicate minerals are classified based on how $(SiO_4)^{4-}$ tetrahedral units are linked together.

Orthosilicates (Neso-silicates)

• Contain discrete $(SiO_4)^{4-}$ tetrahedral units.

• General formula: $M<em>2^{II}[SiO</em>4]$ where M = Be, Mg, Fe, Mn, or Zn.

• Example: Zircon ($ZrSiO_4$) - used as a gemstone.

Pyrosilicates (Soro-silicates, Disilicates)

• Simplest of the condensed silicate ions, composed of two tetrahedral units joined by sharing an oxygen at one corner, forming the unit $(Si<em>2O</em>7)^{6-}$.

• Example: Thortveitite $(Sc<em>2[Si</em>2O_7])$.

Cyclic Silicates

• Made up of ring structures with the formula $(SiO<em>3)</em>n^{2n-}$.

• Examples:

  • $(Si<em>3O</em>9)^{6-}$ found in wollastonite $(Ca<em>3[Si</em>3O<em>9])$ and benitoite $(BaTi[Si</em>3O_9])$.

  • $(Si<em>6O</em>{18})$ found in beryl $(Be<em>3Al</em>2[Si<em>6O</em>{18}])$ and emerald (same formula as beryl but contains 1-2% Cr).

Chain Silicates

• Simple chain silicates or pyroxenes with the formula $(SiO<em>3)</em>n^{2n-}$.

Sheet Silicates (Phyllo-silicates)

• $(SiO<em>4)$ units share three corners, forming an infinite 2-D sheet with the empirical formula $(Si</em>2O<em>5)</em>n^{2n-}$.

• Strong bonds exist within the Si-O sheet, but weaker forces hold each sheet next to each other.

• Examples:

  • Clay minerals (kaolinite, pyrophyllite, talc)

  • White asbestos (chrysotile, biotite)

  • Micas (muscovite and margarite)

  • Montmorillonites (Fuller's earth, bentonite, and vermiculite)

3-D Silicates

• Examples: $SiO_2$ (quartz, tridymite, cristobalite, etc.).

• Generally contain no metal ions, but 3-D structures can form the basis of silicate structures if some $Si^{4+}$ is replaced by $Al^{3+}$.

• Zeolites (e.g., $Na<em>2[Al</em>2Si<em>3O</em>{10}]2H_2O$) are examples of 3-D silicates used as ion exchange materials and molecular sieves.

  • Molecular sieves have uniform pore sizes, allowing small molecules to be adsorbed while excluding larger ones (4 Angstrom).

Silicates in Technology

Alkali Silicates

• Refer to soluble silicates.

Cement

• Silicates are important components of both Portland cement and high alumina cement.

• High alumina cement is formed by calcining bauxite (an aluminum ore) and lime.

Ceramics

• Inorganic materials can be made into a paste, shaped at normal temperatures, and then fired at high temperatures.

• Carbides, oxides, and clays are treated this way to make bricks, tiles, and pottery.

• Heating kaolinite $(Al<em>2(OH)</em>4[Si<em>2O</em>5])$ leads to water loss at 500-600 °C, forming $Al<em>2O</em>3 <br>cdot 2SiO<em>2$ and at about 950 °C forms a solid solution mullite $(3Al</em>2O<em>3 cdot 2SiO</em>2)$ and $SiO_2$.

Glass

• A small amount of glass is made of silica, which has excellent properties but requires very high temperatures to produce.

• Silica glass is too expensive for general use.

• Oxides used in making silica glass: $Na<em>2O$, $K</em>2O$, MgO, CaO, BaO, $B<em>2O</em>3$, $Al<em>2O</em>3$, PbO, and ZnO.

• If only $Na<em>2O$ and $K</em>2O$ were used, the glass would be water-soluble.

• Normal domestic glass for windows is a calcium alkali silicate glass made by fusing alkali metal carbonate, $CaCO<em>3$, and $SiO</em>2$.

• If $Na<em>2CO</em>3$ is used, soda glass is obtained; $K<em>2CO</em>3$ yields potash glass.

• Replacing most of the CaO with PbO produces lead glass, which has a higher refractive index and is used for optical parts and glass ornaments.

• If $Al<em>2O</em>3$ is used, $Al^{3+}$ may be present in the structure as a free metal ion or replace $Si^{4+}$ in $SiO_4$ tetrahedra.

• If $B<em>2O</em>3$ is used, $B^{3+}$ replaces some $Si^{4+}$ to form borosilicate glass.

• Borosilicate glasses have a low coefficient of expansion and can withstand heat changes without cracking.

• Such glassware contains less alkali, is less prone to chemical attack, and is used as laboratory equipment like Pyrex glassware.

Organosilicon Compounds and Silicones

Organosilicon Compounds

• Si-C bonds are almost as strong as C-C bonds.

• SiC is extremely hard and stable.

• Many organosilicon compounds are inert and stable to heat.

Preparation of Organosilicon Compounds

• Grignard Reaction: $SiCl<em>4 + CH</em>3MgCl \rightarrow CH<em>3SiCl</em>3 + MgCl_2$

• Using an organolithium compound: $4LiR + SiCl<em>4 \rightarrow SiR</em>4 + 4LiCl$ (R = alkyl or aryl substituent)

• Rochow 'Direct Process: $Si + 2CH<em>3Cl \xrightarrow{\text{Cu catalyst, 280-300°C}} (CH</em>3)<em>2SiCl</em>2$

• Catalytic addition of Si-H to an alkene.

  • This is a useful general method but not applicable to making methyl and phenyl silanes required by the silicone industry.

Silicones

• A group of organosilicon polymers.

• Commercial uses include fluids, oils, elastomers (rubbers), and resins.

• Complete hydrolysis of $SiCl<em>4$ yields $SiO</em>2$, which has a very stable 3-D structure.

• Research by F.S. Kipping on the hydrolysis of alkyl-substituted chlorosilanes led to long-chain polymers called silicones instead of the expected silicon compound analogous to a ketone.

• Silicones are water repellent because a silicone chain is surrounded by organic side groups, resembling an alkane on the outside.

Silicone Polymers

• Straight chain polymers of 20-500 units are used as silicone fluids.

• Silicone rubbers are made of long, straight chain polymers (dimethylpolysiloxanes) between 6000 and 600,000 Si units long, mixed with fillers, typically finely divided $SiO_2$ or occasionally graphite.

Uses of Silicones

• Water repellents for treating masonry and buildings, glassware, and fabrics.

• Included in car and shoe polish.

• Silicone fluids are non-toxic and have a low surface tension. Used in car polish and shoe polish. Silicone fluids are non-toxic and have a low surface tension. Silicones are water repellent. Straight chain polymers of 20 -500 units are used as silicone fluids. Silicone rubbers are made of long, straight chain polymers (dimethylpolysiloxanes) between 6000 and 600 000 Si units long, mixed with fillers normally finely divided SiO2 or occasionally graphite. • Uses of silicones: water repellents for treating masonry and buildings, glassware and fabrics. They are also included in car polish and shoe polish. Silicone fluids are non-toxic and have a low surface tension.