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Physical & Chemical properties

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Physico-chemical properties

Triethoxy(methyl)silane is a liquid at standard temperature and pressure, with a predicted melting point of < -20°C and a predicted boiling point of 152°C. It has a predicted density of 0.93 g/cm3 at 25°C, a predicted viscosity of 0.8 mm2/s at 20°C and a predicted vapour pressure of 100 Pa at 25°C.

 

The substance is classified as a flammable liquid (Cat. 3) on the basis of a measured flash point of 34°C and the boiling point of 152°C. It has a measured auto-ignition temperature of 221°C, and is not explosive and not oxidising on the basis of chemical structure.

 

In contact with water, triethoxy(methyl)silane is predicted to rapidly hydrolyse (half-life of approximately 4 hours at 20 - 25°C and pH 7) to produce methylsilanetriol and ethanol according to the following equation:

 

CH3Si(OCH2CH3)3+ 3H2O → CH3Si(OH)3+ 3CH3CH2OH

 

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 initial silanol hydrolysis products, methylsilanetriol and ethanol, are assessed instead.

Ethanol is miscible with water, has low log Kow (-0.3) and high vapour pressure (7910 Pa at 25°C).

Methylsilanetriol is very soluble in water (predicted solubility in water of 1.0 E+06 mg/l at 20°C), has low log Kow (-2.4 predicted).It is not surface active and it is much less volatile than the parent substance (vapour pressure = 0.052 Pa at 25°C, predicted). The first dissociation constant of a structurally analogous silanetriol (phenylsilanetriol) has been reported to be around pKa=10.

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

RSi(OH)3     RSi(OH)2OSi(OH)2R     RSi(OH)2O[Si(R)(OH)O]nSi(OH)2R

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

 

A highly cross-linked gel may form. 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.