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Diss Factsheets

Physical & Chemical properties

Water solubility

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Description of key information

Water solubility (hexamethylcyclotrisiloxane): ca. 1.6 mg/l at 23°C

Water solubility (L3-diol): 1700 mg/l at 25°C

Water solubility (DMSD): 1,000,000 mg/l at 25°C

Key value for chemical safety assessment

Water solubility:
1.6 mg/L
at the temperature of:
23 °C

Additional information

The water solubility study is not required under Column 2 of REACH Annex VII because the hydrolytic half-life of the substance at pH 4, 7 and 9 is <12 hours. However, data are available that indicate that the parent substance has a solubility of approximately 1.6 mg/l at 23°C. The hydrolytic instability of the substance means this value can only be considered an approximation to the solubility. The water solubility of the hydrolysis products is also relevant because of the rapid hydrolysis.

A water solubility of 1700 mg/L was determined for the intermediate hydrolysis product (L3-diol) of the submission substance using a validated estimation method. The result is considered to be reliable.


The final 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 (see discussion below). 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. A collection of reliable data is also available that indicates that DMSD is miscible with water at 25°C.


The intermediate and final silanol hydrolysis products are silanediols; these 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. Condensation of the intermediate silanol hydrolysis products is also possible, but unlikely to be significant below 1000 mg/l. Further information is given in a supporting report (PFA 2016am) attached in Section 13 of the IUCLID dataset.

Reference: PFA (2016am). Peter Fisk Associates, Silanols and aquatic systems. Report no: PFA.404.105.003. Owner company: Reconsile. Report date: Feb 16, 2016