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Silicon chemistry is fundamentally different from carbon chemistry. Silicon is one period lower than carbon in the periodic table of the elements; therefore silicon has a greater capacity than carbon to share electrons with oxygen. This difference is evidenced by the stronger bond (higher bond energies, higher bond angles, and shorter than expected bond lengths) associated with the silicon-oxygen bond as compared to the carbon-oxygen bond. The nature of the silicon-oxygen bond makes siloxane molecules flexible, which results in weak interactions between siloxane molecules. This is illustrated by the lower surface tension, lower viscosity and higher vapour pressure of siloxanes compared to hydrocarbons of similar molecular weight. Combined with their large size (10 atoms per Me2SiO unit) and only a moderate ability to accept hydrogen bonds, these fundamental characteristics of the siloxanes lead to differences in the ability of siloxanes to interact as solutes with environmental “solvents” or media such as water, organic carbon in soil/sediment, and lipids in biota, compared to traditional hydrophobic organic contaminants. Consequently, siloxanes possess a different combination of solubility and partitioning properties that influence their distribution and fate in the environment.For example, it is important to note that when assessing volatility of a substance, vapour pressure needs to be looked at together with other partition coefficients (e.g. Henry Law Constant).


Furthermore, the electropositive nature of Si makes it amenable to hydrolysis, and siloxanes are more hydrolytically labile than carbon equivalents.

Decamethylcyclopentasiloxane (D5) is a liquid at standard temperature and pressure, with a measured melting point of -38°C, and a measured boiling point of 210°C. It has a measured density of 0.959 g/cm3at 20°C and a predicted kinematic viscosity of 3.7 mm²/s (3.5 mpa.s dynamic) at 25°C. The substance has a measured vapour pressure of 33 Pa at 25°C.

The substance is not classified for flammability according to Regulation (EC) No 1272/2008 on the basis of measured flash point of 83°C and measured boiling point of 210°C. It has a measured self-ignition temperature of 372°C, and is not explosive and is not oxidising on the basis of structural examination.

In contact with water, D5 will hydrolyse with a measured hydrolysis half-life of 66 d at 25°C and pH 7. The product of hydrolysis is dimethylsilanediol.

[-Si(CH3)2O-]5+ 4H2O → 5(CH3)2Si(OH)2

D5 has a measured water solubility of 17 µg/l at 23°C and log Kow of 8.07. Further testing for water-based physico-chemical properties is waived due to the low solubility; the substance is expected not to dissociate but may have surface active properties.