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Physico-chemical properties

Trimethoxyphenylsilane is a liquid at standard temperature and pressure, with a measured melting point of < -20°C and a measured boiling point of 226°C. It has a calculated density of 0.99 g/cm3 at 20°C, a predicted viscosity of 2.1 mm2/s at 20°C and a measured vapour pressure of 18.2 Pa at 25°C.

 

Flash point values of 50°C and 94.3°C have been recorded in reliable studies. Presumably the flash point decreases with an increase of the low boiling impurities, like methanol, a hydrolysis product of the substance. Trimethoxyphenylsilane has a measured auto-ignition temperature of 394°C, and is not explosive and not oxidising on the basis of chemical structure.

 

In contact with water, trimethoxyphenylsilane reacts rapidly (half-life 0.4 hours at pH 7 at 25°C) to produce phenylsilanetriol and methanol according to the following equation:

 

C6H5Si(OCH3)3 + 3 H2O → C6H5Si(OH)3 + 3 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 silanol hydrolysis product, phenylsilanetriol, are assessed instead.

Methanol is miscible with water, has low log Kow(-0.82 to -0.64) and high vapour pressure (12790 Pa at 20°C).

Phenylsilanetriol is predicted to be very soluble in water (> 10000 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 = 0.000053 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, for example pH (the condensation reaction may be either acid or base catalysed), temperature and other present species.