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Physical & Chemical properties

Water solubility

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Reference
Endpoint:
water solubility
Data waiving:
study scientifically not necessary / other information available
Justification for data waiving:
the study does not need to be conducted because the substance is hydrolytically unstable at pH 4, 7 and 9 (half-life less than 12 hours)

Description of key information

Water solubility [3-(trimethoxysilyl)propiononitrile]: Waived due to rapid hydrolysis in contact with water

Water solubility [3-(trihydroxysilyl)propiononitrile]: Above approximately 1000 mg/L condensation reactions can occur over time, limiting the concentration dissolved in water. The calculated solubility is 1.0E+06 mg/L (QSAR)

Water Solubility [methanol]: miscible at 20°C

Key value for chemical safety assessment

Additional information

The requirement to conduct a water solubility study for the submission substance is waived on the basis that it hydrolyses rapidly to form 3-(trihydroxysilyl)propiononitrile and methanol.

However, the water solubility of 3-(trimethoxysilyl)propiononitrile has been calculated using a validated QSAR method to be 1.3E+05 mg/L.

The silanol hydrolysis product, 3-(trihydroxysilyl)propiononitrile, may undergo condensation reactions in solution to give siloxane dimers, linear and cyclic oligomers and highly cross-linked polymeric particles (sol) that may over time form an insoluble gel. 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 solution.

The condensation reactions of silanetriols may be modelled as an equilibrium between monomer, dimer, trimer and tetramer, with the linear tetramer cyclising to the thermodynamically stable cyclic tetramer. At higher loadings, cross-linking reactions may occur. The reactions are reversible unless the cyclic tetramer concentration exceeds its solubility, in this case, the cyclic tetramer forms a separate phase, driving equilibrium towards the tetramer. At loadings below 500 mg/L of 3-(trihydroxysilyl)propiononitrile, 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 causing the formation of insoluble polymeric particles (sols) and gels over time. Further information is given in a supporting report (PFA 2016am) attached in Section 13.

The hydrolysis product, 3-(trihydroxysilyl)propiononitrile, is very hydrophilic and hence the calculated solubility is 1.0E+06 mg/L using a QSAR method. This QSAR method for water solubility cannot be validated for silanetriols because the saturation concentration of silanetriols in water is limited by condensation reaction rather than lack of true solubility as discussed above. A prediction of 1E+06 mg/L is indicative but has no practical meaning. The prediction is however considered valid for use in environmental exposure modelling and toxicokinetics modelling because it is considered to adequately describe the hydrophilicity of the substance and hence the partitioning behaviour.

Methanol is reported to be miscible in water at 20°C (OECD 2004).

Reference:

OECD (2004): SIDS Initial Assessment Report for SIAM 19, Berlin, Germany, 18-20 October 2004, Methanol, CAS 67-56-1.

PFA (2016am). Peter Fisk Associates, Analogue Report - Silanols and aquatic systems. Reference:404.105.003