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Physical & Chemical properties

Water solubility

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Description of key information

Water solubility [dichloro(dimethyl)silane] = not relevant

Water solubility [dimethylsilanediol]: 1E+06 mg/l at 20°C (QSAR)

Key value for chemical safety assessment

Additional information

The requirement to test the substance for water solubility is waived on the basis that it hydrolyses very rapidly to dimethylsilanediol and hydrochloric acid.

The submission substance hydrolyses very rapidly so the water solubility of the hydrolysis product is considered more relevant for regulatory purposes than the water solubility of the submission substance.

The silanol hydrolysis product, dimethylsilanediol, may undergo condensation reactions in solution to give siloxane dimers, and linear and cyclic oligomers. 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 dimethylsilanediol, 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 1000 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 2013am) attached in Section 13 of the IUCLID dataset.

The initial hydrolysis product, dimethylsilanediol, is very hydrophilic and hence the calculated solubility is 1E+06 mg/l using a QSAR method. However, the saturation concentration of dimethylsilanediol in water is limited by condensation reactions rather than lack of true solubility as discussed above. A prediction of 1E+06 mg/l or 1000 g/l is indicative but has no practical meaning. The prediction is however considered valid for use in calculation of the Henry's Law Constant and for environmental exposure modelling and toxicokinetics modelling because it is considered to adequately describe the hydrophilicity of the substance and hence the partitioning behaviour.