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Physical & Chemical properties

Endpoint summary

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Description of key information

The substance, dichloro(methyl)(phenyl)silane is not stable in water, which affects the approach to the determination of physicochemical properties.

Dichloro(methyl)(phenyl)silane is a liquid at standard temperature and pressure, with a measured melting point of -49.4°C, a predicted boiling point of 204°C. It has a predicted density of 1.12 g/cm3 at 20°C, a predicted viscosity of 2.2 mm2/s at 20°C and a predicted vapour pressure of 32 Pa at 25°C.

The substance is not classified for flammability on the basis of a measured flash point of 79.7°C at 1013 hPa and the predicted boiling point of 204°C. It has a measured auto-ignition temperature of 485°C at 1013 hPa and is not explosive and not oxidising on the basis of chemical structure.

In contact with water, dichloro(methyl)(phenyl)silane reacts very rapidly (half-life of <1 minute at pH 4, pH 7 and pH 9 and 25°C, based on read across from dichloro(dimethyl)silane) to produce methylphenylsilanediol and hydrochloric acid according to the following equation:

C6H5Si(CH3)Cl2 + 2H2O → C6H5(CH3)Si(OH)2 + 2HCl

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, methylphenylsilanediol are assessed instead.

The silanol hydrolysis product, methylphenylsilanediol, may undergo condensation reactions in solution to give siloxane dimers, linear and cyclic oligomers 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 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 as long as the concentrations are low and all species remain soluble; thus, a dynamic equilibrium is established. At higher concentrations one or more of the oligomers may exceed their solubility limit and form a separate phase, driving the equilibrium towards the oligomer/polymers. At loadings below 1000 mg/L of methylphenylsilanediol, the soluble monomer is expected to predominate in solution (>99%), with small amounts of dimer and oligomers. Condensation reactions are expected to become important at loadings above about 30 mg/L (or lower under some circumstances) causing the formation of insoluble polymeric particles and gels over time. Further information is given in a supporting report (PFA 2016am) attached in Section 13.

The saturation concentration in water of the silanol hydrolysis product, methylphenylsilanediol, is therefore limited by condensation reactions to approximately 30 mg/L. However, it is very hydrophilic (calculated solubility is 2.9E+04 mg/L at 20°C using a QSAR method) with a predicted low log Kow of 0.8. It is not surface active and the silanol hydrolysis product is much less volatile than the parent substance (predicted vapour pressure 0.0036 Pa at 25°C). The hydrolysis product (methylphenylsilanediol) is not expected to dissociate within the relevant environmental range.

Reference:

PFA (2016am). Peter Fisk Associates, Silanols and aquatic systems, 404.105.003

Additional information