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The substance, 1,2-bis[dichloro(methyl)silyl]ethaneis not stable in water, which affects the approach to the determination of physicochemical properties. The significance of this for read-across is discussed in Section 1.4.1 of the CSR.

1,2-Bis[dichloro(methyl)silyl]ethane is a liquid at standard temperature and pressure, with a melting point of 9°C, and a boiling point of 211.4°C. It has a relative density of 1.242 at 20°C and a predicted vapour pressure of 11 Pa at 25°C.

The substance is not classified for flammability according to Regulation (EC) No. 1272/2008 on the basis of a measured flash point of 96.5°C and a measured boiling point of 211.4°C. It has a measured auto-ignition temperature of 309.0°C, and is not explosive and not oxidising on the basis of chemical structure.

In contact with water,1,2-bis[dichloro(methyl)silyl]ethanehydrolyses very rapidly (half-life <1 min at 25°C and pH 4, 7 and 9) to produce 1,2-bis[dihydroxy(methyl)silyl]ethane and hydrogen chloride according to the following equation:

C4H10Cl4Si2+ 4H2O → C4H14O4Si2+ 4HCl

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, 1,2-bis[dihydroxy(methyl)silyl]ethane are assessed instead.

The silanol hydrolysis product, 1,2-bis[dihydroxy(methyl)silyl]ethane, 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, or insoluble cross-linked polymers may form, driving the equilibrium towards the oligomer/polymers. At loadings below 1000 mg/l of 1,2-bis[dihydroxy(methyl)silyl]ethane, 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 1000 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 of the IUCLID dataset.

The saturation concentration in water of the silanol hydrolysis product, 1,2-bis[dihydroxy(methyl)silyl]ethane, is therefore limited by condensation reactions to approximately 1000 mg/l. However, it is very hydrophilic (calculated solubility is 1E+06 mg/l at 20°C using a QSAR method) with a predicted low log Kowof -1.6. It is not surface active. The silanol hydrolysis product is much less volatile than the parent substance (predicted vapour pressure =2.4E-05 Pa at 25°C).