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Physical & Chemical properties

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Physico-chemical properties

Triethoxy(phenyl)silane is a liquid at standard temperature and pressure, with a measured melting point of <-100 °C and a measured boiling point of 237.6 °C (at 1013 hPa). It has a measured density of 0.989 g/cm3at 20°C, a measured viscosity of 1.83 mm2/s at 20°C and a measured vapour pressure of 3E-02 Pa at 20°C. Based on the measured flash point of 53.5 °C the substance is a flammable liquid. It has a measured auto-ignition temperature of 248 °C, and is not explosive and not oxidising on the basis of chemical structure. On the basis of structure it is not expected to be surface active.

In contact with water, triethoxy(phenyl)silane reacts rapidly (half-life 1.5 hours at pH 7 at 20-25°C) to produce phenylsilanetriol and ethanol according to the following equation:

C6H5Si(OCH2CH3)3 + 3 H2O → C6H5Si(OH)3 + 3 CH3CH2OH

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 products, phenylsilanetriol 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).

Phenylsilanetriol is predicted to be very soluble in water (1,000,000 mg/l) with a measured low log Kowvalues (0.015, -0.021 and -0.419 at pH 4, 7 and 10). It is not surface active and does not undergo significant dissociation within the environmentally-relevant range. It is much less volatile than the parent substance (vapour pressure = 5.3E-05 Pa at 25°C, predicted).

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