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Physico-chemical Properties

Trimethoxy(2,4,4-trimethylpentyl)silane is a liquid at standard temperature and pressure, with a measured melting point of <-100°C, and a measured boiling point of 210.8°C. It has a measured relative density of 0.9 at 20°C, a measured static viscosity of 1.88 mm2/s at 20°C and a measured vapour pressure of 0.2 hPa at 20°C.

The substance is classified as a flammable liquid on the basis of a measured flash point of 60°C and the boiling point of 210.8°C. It has a measured self-ignition temperature of 302°C, and is not explosive and not oxidising on the basis of chemical structure.

In contact with water trimethoxy(2,4,4-trimethylpentyl)silane reacts rapidly (half-life approximately 5 hours at pH 7 and 20 - 25°C) to produce (2,4,4-trimethylpentyl)silanetriol and methanol according to the following equation:

           (CH3)3CCH2CH(CH3)CH2Si(OCH3)3+ 3H2O → (CH3)3CCH2CH(CH3)CH2Si(OH)3+ 3CH3OH

Therefore, requirements for testing of the water-based physicochemical properties for the substance and measurement of the surface tension are waived on the basis of instability in water. The properties of the hydrolysis products are assessed instead.

The silanol hydrolysis product, (2,4,4-trimethylpentyl)silanetriol, is predicted to be very soluble in water 20°C (2.4E+05 mg/l, predicted) and has moderate log Kow (0.9, predicted). It is less volatile than the parent substance (vapour pressure1.2 E-04 Pa at 25°C, predicted). The first dissociation constant of a structurally analogous silanetriol (phenylsilanetriol) has been reported to be around pKa= 10.

Methanol is miscible with water, has low log Kow (-0.82 to -0.64) and high vapour pressure of 12790 Pa at 20°C (OECD, 2004).

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.  
  • The nature of the R group