Registration Dossier

Physical & Chemical properties

Water solubility

Currently viewing:

Administrative data

Link to relevant study record(s)

Description of key information

Water solubility (1,1,1,3,3,3-hexamethyldisilazane): Not relevant
Water solubility (trimethylsilanol): 20000 mg/L at 20°C (EU Method A.6); at concentrations above 1000 mg/l condensations may occur over time, limiting the concentration dissolved in water.
Water solubility (ammonia): 510000-531000 mg/l at 20°C

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 form trimethylsilanol and ammonia.

The water solubility of the hydrolysis product is relevant for the purpose of chemical safety assessment due to the very rapid hydrolysis of the submission substance.

The silanol hydrolysis product, trimethylsilanol may undergo condensation reactions in solution to give the siloxane dimer (hexamethyldisiloxane; CAS 107-46-0). A dynamic equilibrium is established. The overall rate of condensation is dependent on nominal loading, temperature, and pH of the system, as well as what else is present in solution.

The condensation reactions of monosilanols may be modelled as an equilibrium between monomer and dimer. The reaction is reversible unless the dimer concentration exceeds its solubility; in this case, the dimer forms a separate phase, driving the equilibrium towards the dimer. For trimethysilanol, a solution at 100 mg/l (the highest concentration often used in ecotoxicity tests) is predicted to contain >99.9% monomer. At loadings above about 500 - 1000 mg/l the concentration of the dimer is predicted to exceed its solubility, resulting in formation of a separate phase. In addition, the dimer 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 water solubility of the silanol hydrolysis product, trimethylsilanol has been calculated using a validated QSAR method to be 11000 mg/l. Similarly, a water solubility value of 20000 ± 700 mg/l at 20°C using OECD 105 test method was determined for trimethylsilanol. However, the saturation concentration may be in reality limited by the formation of less soluble dimer rather than the solubility of the monomer itself. This statement is supported by a measured solubility of 995 mg/l at 20°C determined for trimethylsilanol in accordance with OECD 105. This is however considered not to reflect the true solubility of trimethylsilanol but its potential to undergo condensation reactions to form the less soluble hexamethyldisiloxane. Therefore, for environmental exposure assessment, calculation of Henry's Law Constant and toxicokinetics modelling, the water solubility value of 20000 mg/l will be used. This value is considered to adequately describe the hydrophilicity of the substance and hence partitioning behaviour.

The different measurements obtained in the two different studies may reflect the fact that condensation reactions do not take place immediately.

Ammonia has a measured water solubility of 510,000 - 531,000 mg/l at 20°C [OECD SIDS (2007). SIDS Initial Assessment Report for SIAM 24, Paris, France, 17-20 April 2007, Ammonia Category].