Registration Dossier

Physical & Chemical properties

Endpoint summary

Administrative data

Description of key information

Trimethoxy(methyl)silane is a liquid at standard temperature and pressure, with a measured melting point of <-77°C, and a measured boiling point of 102°C. It has a predicted density of 0.95 g/cm3 at 20°C, a predicted kinematic viscosity of 0.5 mm2/s at 20°C and a measured vapour pressure of approximately 3000 Pa at 20°C and 15000 Pa at 50°C.

The substance is classified for flammability according to Regulation (EC) No. 1272/2008 on the basis of a measured flash point of 7.7°C and a measured boiling point of 102°C. It has a measured auto-ignition temperature of 238°C, and is not explosive and not oxidising on the basis of chemical structure.

In contact with water trimethoxy(methyl)silane reacts rapidly (half-life of 2.2 hours at pH 7, <0.033 hours at pH 4, 0.11 hours at pH 9 and 25°C) to produce methylsilanetriol and methanol according to the following equation:

CH3Si(OCH3)3 + 3H2O → CH3Si(OH)+ 3CH3OH


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, methylsilanetriol and methanol are assessed instead.

However, the key physicochemical properties of the submission substance has been predicted. The substance has a predicted log Kow of 0.7 at 20°C and predicted water solubility of 9.1E+04 mg/L at 20°C.

Methanol is miscible with water, has low log Kow (-0.82 to -0.64) and high vapour pressure (12790 Pa at 20°C) (OECD, 2004a: SIDS Initial Assessment Report for SIAM 19, Berlin, Germany, 18 - 20 October 2004, Methanol, CAS 67-56-1).


The saturation concentration in water of the silanol hydrolysis product, methylsilanetriol is limited by condensation reactions that can occur over time at loadings above approximately 1000 mg/l. However, it is very hydrophilic (calculated solubility is 1.0E+06 mg/l at 20°C using a QSAR method) and has a low log Kow (-2.4, predicted).

It is not expected to be surface active and is not expected to undergo significant dissociation within the environmentally-relevant range. The first dissociation constant of a structurally analogous silanetriol (phenylsilanetriol) has been reported to be around pKa of 10. It is much less volatile than the parent substance (vapour pressure = 0.05 Pa at 25°C, predicted). 


Additional information