Trichloro(3,3,4,4,5,5,6,6,7,7,8,8,8-tridecafluorooctyl)silane

Water solubility

Link to relevant study record(s)

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 [2-(perfluorohexyl)ethyl]trichlorosilane]: not relevant

Water solubility [2-(perfluorohexyl)ethyl]silanetriol]: 0.58 mg/l at 20°C (QSAR)

Additional information

The requirement to conduct a water solubility study for the submission substance is waived on the basis that it hydrolyses very rapidly to [2-(perfluorohexyl)ethyl]silanetriol and hydrochloric acid. In accordance with ECHA decision number CCH-2120771164-56-01/F, a hydrolysis study is contracted with the registered substance. If the substance is demonstrated to decompose rapidly in contact with water, the water solubility endpoint can be waived in accordance with Column 2 of REACH Annex VII. Should this not be the case, a water solubility study according to OECD TG 105 will be attempted. The endpoint will be updated when the result of the hydrolysis study is available.

The silanol hydrolysis product, [2-(perfluorohexyl)ethyl]silanetriol, 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 and 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. This condensation may limit the saturation concentration in water below the estimated solubility of the silanetriol monomer. Further information is given in a supporting report (PFA 2016am) attached to Section 13.

The hydrolysis product, [2-(perfluorohexyl)ethyl]silanetriol, is slightly soluble in water based on a solubility of 0.58 mg/l calculated using a QSAR method. This QSAR method for water solubility cannot be validated for silanetriols because the saturation concentration of silanetriols in water is often limited by condensation reactions rather than lack of true solubility. The prediction is however considered to be valid for use in environmental exposure modelling and toxicokinetics modelling because it is considered to adequately describe the hydrophobicity of the substance and hence the partitioning behaviour.

The chemical structure contains a perfluorohexyl chain; fluorocarbons have very low solubility in water, relative to hydrocarbons of equivalent chain length (e.g. Horvath 1982), so it would be expected that a substance with this functional group would be of low solubility. The effective limits of solubility of both the parent substance and monomeric silanol hydrolysis products are very low. Condensation phenomena may limit the extent of hydrolysis under conditions at which the solubility of either parent or oligomer is exceeded. The predicted hydrolysis rates relate to dilute conditions. These factors are important in any consideration of aqueous behaviour and properties.

References:

Horvath A L (1982). Halogenated hydrocarbons: solubility-miscibility with water. CRC Press 26 February 1982.

PFA (2016am). Peter Fisk Associates, Silanols and aquatic systems, 404.105.003