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Physical & Chemical properties

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Physico-chemical properties              

Dichloro(diphenyl)silane is a liquid at standard temperature and pressure, with a measured melting point of <-20°C, and a measured boiling point of 305°C. It has a measured density of 1.22 g/cm³ at 20°C, a predicted viscosity of 8.6 mm²/s at 20°C and a predicted vapour pressure of 0.078 Pa at 25°C.

The substance is not classified as flammable on the basis of a measured flash point of 162°C and the boiling point of 305°C. It has a measured auto-ignition temperature of >500°C, and is not explosive and not oxidising on the basis of chemical structure.

In contact with water, dichloro(diphenyl)silane is predicted to react very rapidly (half-life 10 seconds at 25°C and pH 7) to produce diphenylsilanediol and hydrochloric acid according to the following equation:

Cl2Si(C6H5)2 + 2 H2O → (C6H5)2Si(OH)2 + 2 HCl

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, diphenylsilanediol,are assessed instead. The hydrolysis product is predicted to be very soluble in water, with low log Kow(2). It is not surface active. The first dissociation constant of a structurally analogous silanediol (dimethylsilanediol) has been reported to be around pKa= 11.9. The hydrolysis product is much less volatile than the parent substance (predicted vapour pressure = 9.9E-06 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 100 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