Registration Dossier

Physical & Chemical properties

Endpoint summary

Administrative data

Description of key information

An overview of the physicochemical properties of the substance and the evaluation undertaken for each endpoint are stated 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 makes siloxane molecules flexible, which results in weak interactions between siloxane molecules. This is illustrated by the lower surface tension, viscosity and vapor 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's Law Constant). Furthermore, the electropositive nature of Si makes it amenable to hydrolysis, and siloxanes are more hydrolytically labile than carbon equivalents.

1,1,1,3,5,5,5-Heptamethyl-3-octyltrisiloxane is a liquid at ambient temperature and pressure, with a measured melting point of <-20°C and a measured boiling point of 263°C. It has a predicted density of 0.84 g/cm3 at 20°C and a predicted vapour pressure of 0.64 Pa at 25°C.

The substance is not classified for flammability in accordance with Regulation (EC) No. 1272/2008 on the basis of a measured flash point of 116°C and a measured boiling point of 263°C. It has a measured auto-ignition temperature of 241±3°C, and is not explosive and not oxidising on the basis of structural examination.

In contact with water, the substance reacts slowly with a half-life of >329 h (>13.7 days) at pH 7, >5.09 h at pH 5 and >9.76 h at pH 9 and 25°C. The stated half-life is for the removal of parent. Complete reaction to the ultimate end products, octylmethylsilanediol and trimethylsilanol will take longer. The equation for the reaction is stated below:

(CH3)3SiOSi(CH3)(C8H17)OSi(CH3)3+ 2H2O → (C8H17)(CH3)Si(OH)2+ 2(CH3)3SiOH

The substance has a predicted log Kow of 9.0 at 20°C and a predicted water solubility of 2.8E-05 mg/l at 20°C. The substance is not surface active based on structural examination.

Additional information