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Physico-chemical properties

Dimethoxydimethylsilane is a liquid at standard temperature and pressure, with a measured melting point of < -20°C and a measured boiling point of 81.5°C at 101.09 kPa. It has a predicted density of 0.91 g/cm³ and a measured density of 0.86 g/cm³ at 25°C, a predicted viscosity of 0.5 mm2/s at 20°C and a predicted vapour pressure of 7400 Pa at 25°C.

The substance is classified as a highly flammable liquid on the basis of a measured flash point of -10.5°C and the boiling point of 81.5°C. It has a measured auto-ignition temperature of 324°C, and is not explosive and not oxidising on the basis of chemical structure.

In contact with water, dimethoxydimethylsilane is predicted to rapidly hydrolyse (half-life of 0.6 hour at 25°C and pH 7) to produce dimethylsilanediol and methanol according to the following equation:

(CH3)2Si(OCH3)2  +  2 H2O  →  (CH3)2Si(OH)2  +  2 CH3OH

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 hydrolysis product, dimethylsilanediol are assessed instead. Dimethylsilanediol is very soluble in water (predicted solubility of 1000000 mg/l at 20°C), has low log Kow (-0.4 predicted) and low vapour pressure (7 Pa at 25°C). Dimethylsilanediol is not surface active and the volatility is expected to be very low. It is not expected to undergo significant dissociation within an environmentally-relevant range. The first dissociation constant has been reported to be around pKa= 11.9.

Methanol is miscible with water, has low log Kow(-0.82 to -0.64) and high vapour pressure (12790 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 thanapproximately100 mg/l, 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