Registration Dossier

Physical & Chemical properties

Endpoint summary

Administrative data

Description of key information

Additional information

An overview of the physicochemical properties of the substance and the evaluation undertaken for each endpoint are summarised below.

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 and substituent groups 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. Fundamental characteristics of the siloxanes, such as their large size (10 atoms per Me2SiO unit), low polarity /molecular polarizability, and only a moderate ability to accept hydrogen bonds, lead to key 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 and Koc). This is especially true for D4 where this unique combination of properties contributes to the distinct differences of the behaviour of D4 in the environment compared to other volatile hydrophobic chemicals (Xu et al., 2014).It is also important to note that often models do not take into consideration the unique properties of siloxanes and rely on the octanol-water partition coefficient to predict the other properties leading to false outcomes for siloxanes. For example, the Kow-Koc relationship is very different for siloxanes and this needs to be taken into account in the models. Therefore, these criteria and models often do not accurately characterise the environmental behaviour of siloxanes (Xu et al., 2013).

 

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

Octamethylcyclotetrasiloxane (D4) is a liquid at standard temperature and pressure, with a measured melting point of 17.7°C, and a measured boiling point of 175°C. It has a measured density of 0.95 g/cm3at 25°C and predicted kinematic viscosity of 1.6 mm2/s at 20°C. The substance has a measured vapour pressure of 132 Pa at 25°C.

The substance is classified as flammable according to Regulation (EC) No. 1272/2008 based on a flash point of 51 - 55°C and a measured boiling point of 175°C. It has a measured self-ignition temperature of 384 to 387°C, and is not explosive and is not oxidising on the basis of structural examination.

In contact with water, D4 will hydrolyse with a measured hydrolysis half-life of 69 - 144 h at pH7 and 25°C, according to the equation below. The product of hydrolysis is dimethylsilanediol.

[-Si(CH3)2O-]4 + 4H2O → 4(CH3)2Si(OH)2

D4 has a measured water solubility of 56 µg/l (0.056 mg/l) at 23°C and log measured log Kow of 6.49 at 25.1°C. Further testing for water-based physicochemical properties is waived due to the low solubility of the substance.