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Physical & Chemical properties

Endpoint summary

Administrative data

Description of key information

Additional information

Dichloro(dimethyl)silane is a liquid at standard temperature and pressure, with a measured melting point of -76.1°C, and a measured boiling point of 70.2°C. It has a predicted density of 1.1 g/cm³ at 20°C, a predicted kinematic viscosity of 0.35 mm²/s at 20°C and a measured vapour pressure of 14600 Pa at 20°C and 22800 Pa at 30°C. An extrapolated vapour pressure of 18300 Pa at 25°C was obtained from the regression reported in the study report.

The substance is classified as a highly flammable liquid in accordance with EC Regulation 1272/2008 on the basis of a measured flash point of 1°C and a measured boiling point of 70.2°C. It has a measured auto-ignition temperature of 425°C, and is not explosive and not oxidising on the basis of chemical structure.

In contact with water, dichloro(dimethyl)silane reacts very rapidly (half-life <1 minute at 25°C and pH 4, 7 and 9) to produce dimethylsilanediol and hydrochloric acid according to the following equation:

Cl2Si(CH3)2 + 2H2O → (CH3)2Si(OH)2 + 2HCl

Therefore, requirements for testing of water-based physicochemical properties for the substance are waived on the basis of instability in water. The properties of the silanol hydrolysis product, dimethylsilanediol, are assessed instead. The saturation concentration in water of the silanol hydrolysis product, dimethylsilanediol, is limited by condensation reactions to approximately 1000 mg/l. However, it is very hydrophilic (calculated solubility is 1E+06 mg/l at 20°C using a QSAR method) with measured low log Kow of -0.38 at 20°C. It is not surface active.

The first dissociation constant of dimethylsilanediol has been reported to be around pKa= 11.9. The hydrolysis product is much less volatile than the parent substance (predicted vapour pressure = 7 Pa at 25°C).

Silanediols may undergo condensation reactions to give siloxane dimers, oligomers and polymers, according to the scheme:

R2Si(OH)2   R2Si(OH)OSi(OH)R2     R2Si(OH)O[Si(R2)O]nSi(OH)R2

 

(where R is an alkyl or aryl side-chain)

 

The degree of condensation that will occur may vary with:

  • Concentration of the silanol; the greater the initial concentration, the greater the degree of condensation. Significant condensation is not expected at concentrations less than approximately 1000 mg/l for this substance, but is dependent on specific conditions.
  • pH; the condensation reaction may be either acid or base catalysed.
  • Temperature.
  • Other species present.
  • Timescale
  • The nature of the R group