Isobutyldimethoxymethylsilane

Endpoint summary

Additional information

In contact with water, isobutyl(dimethoxy)methylsilane hydrolyses rapidly (half-life 1.7 hours at 25°C and pH 7) to produce isobutyl(methyl)silanediol and methanol according to the following equation:

 (CH₃)₂CHCH₂Si(OCH₃)₂(CH₃) + 2H₂O → (CH₃)₂CHCH₂Si(OH)₂(CH₃) + 2CH₃OH

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, isobutyl(methyl)silanediol, are assessed instead.

The substance may undergo condensation reactions in solution to give siloxane dimers, and linear and cyclic oligomers and highly cross-linked polymeric particles (a colloidal suspension of small solid particles known as sol) that may over time form an insoluble gel and a dynamic equilibrium is established. The overall rate and extent of condensation is dependent on nominal loading, temperature, and pH of the system, as well as what else is present in the solution.

The condensation reactions of silanediols may be modelled as an equilibrium between monomer, dimer, trimer and tetramer, with the linear tetramer cyclising to the thermodynamically stable cyclic tetramer. The reactions are reversible unless the cyclic tetramer concentration exceeds its solubility; in this case, the cyclic tetramer forms a separate phase, driving the equilibrium towards the tetramer. For isobutyl(methyl)silanediol, a solution at 100 mg/l (often the top loading used in ecotox tests) is predicted to contain >99% monomer, with small amounts of dimer, trimer, and cyclic tetramer. At loadings above about 100 mg/l the concentration of the cyclic tetramer of the silanol hydrolysis product is predicted to exceed its solubility, resulting in formation of a separate phase. In addition, the cyclic tetramer is expected to have a high volatility from water and this may cause losses from water under some conditions. Further information is given in a supporting report (PFA 2014am) attached in Section 13 of the IUCLID dataset.

The saturation concentration in water of the silanol hydrolysis product, isobutyl(methyl)silanediol, is therefore limited by condensation reactions to approximately 100 mg/l. However, it is hydrophilic (calculated solubility is 8.1E+04 mg/l at 20°C using a QSAR method) with a predicted log K_ow of 1.0. It is not surface active. The silanol hydrolysis product is much less volatile than the parent substance (predicted vapour pressure = 0.33 Pa at 25°C).

Isobutyl(dimethoxy)methylsilane is a liquid at standard temperature and pressure, with a measured melting point of <-100°C, and a measured boiling point of 144°C. It has a predicted density of 0.89 g/cm³ at 20°C and a predicted vapour pressure of 310 Pa at 25°C based on measured boiling point of 144°C.

The substance is classified for flammability on the basis of a measured flash point of 29.5°C. It has a measured auto-ignition temperature of 265°C, and is not explosive and not oxidising on the basis of structural examination.