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Water solubility

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Description of key information

Water solubility [trimethylsilane]: predicted water solubility of 1400 mg/l at 20°C was obtained using a validated QSAR.

Water solubility [trimethylsilanol]: at concentrations above 995 mg/L at 20°C (EU Method A.6); condensation may occur over time, limiting the concentration dissolved in water.

Key value for chemical safety assessment

Water solubility:
1 400 mg/L
at the temperature of:
20 °C

Additional information

Trimethylsilane is a gas with a boiling point of 6.7°C at standard atmospheric pressure; the vapour pressure at standard temperature and pressure is expected to be above the limit of measurement (>100000 Pa). Therefore, conducting a water solubility study is considered technically not feasible or necessary.

However, the water solubility of the substance has been estimated to be 1400 mg/l at 20°C using a validated QSAR estimation method. In practice this concentration in water is not expected to be achieved due to the very high volatility of the substance.

In contact with water, trimethylsilane hydrolyses at a slow rate to form trimethylsilanol and hydrogen.

The silanol hydrolysis product, trimethylsilanol may undergo condensation reactions in solution to give the siloxane dimer (hexamethyldisiloxane; CAS No: 107-46-0) and 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 trimethylsilanol, 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.

There are two measured water solubility results for trimethylsilanol. In the first study (Wieser 2005), a water solubility value of 995 mg/l at 20°C was obtained for trimethylsilanol using the flask method in accordance with OECD 105. In this first water solubility study, 1H-NMR was used for the determination of the concentration of trimethylsilanol in water. This analytical method is specific and it is expected to give the dissolved concentration of trimethylsilanol in water. In addition, in the NMR spectrum attached to the study report, the formation of the dimer (hexamethyldisiloxane) was observed.

In the second study (Dow Corning 1999), a water solubility value of 20000 ± 700 mg/l at 20°C was obtained for trimethylsilanol using the flask method in accordance with OECD 105 test method. However, the reliability of this second result is unassignable due to deficiencies in the analytical method used (see reviewer's comment in Endpoint study record of Dow Corning 1999 for further details).

Therefore, for environmental exposure assessment, calculation of Henry's Law Constant and toxicokinetic modelling, the water solubility value of 995 mg/l will be used for trimethylsilanol.

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