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Physical & Chemical properties

Water solubility

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Description of key information

Water Solubility [2-(perfluorohexyl)ethyl]trimethoxysilane]: 2.4E-04 mg/l at 20°C (QSAR)
Water Solubility [2-(perfluorohexyl)ethyl]silanetriol]: 0.58 mg/l at 20°C (QSAR)
Water solubility (methanol) = Miscible at 20°C

Key value for chemical safety assessment

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

Additional information

The requirement to conduct a water solubility study for the submission substance is waived on the basis that it hydrolyses rapidly to [2-(perfluorohexyl)ethyl]silanetriol and methanol.

The water solubility of the parent [2-(perfluorohexyl)ethyl]trimethoxysilane has been calculated using a validated QSAR method to be 2.4E-04 mg/l at 20°C. A measured water solubility value in the range 0.6 -1.2 g/l at 20°C and pH 4.1 - 4.5 was obtained using a relevant test method. However, there are some uncertainties associated with the study as stated below:

- No preliminary study was conducted.

- At the pH (4 - 5) used during the study and given the equilibration time (72 - 10 h and 30°C), hydrolysis of the parent substance (half-life of 0.4 h at pH 4 and 20 -25°C) might have been almost complete.

- The method of analysis, ICP is unable to differentiate between the different forms of the substance that might have been present in solution.

However, at the loading rate used (0.5 g in 20 ml water), in a system where there are agglomerated structures and/or condensation products; the measured water solubility might be an indication of the limit of solubility of the parent submission substance and the hydrolysis product(s).

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 2013am) attached in Section 13.

The hydrolysis product, [2-(perfluorohexyl)ethyl]silanetriol, is slightly soluble in water based on the calculated solubility is 0.58 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 often limited by condensation reactions rather than lack of true solubility. The prediction is however considered to be 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 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 R-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.

 

Methanol is miscible in water at 20°C, based on experimental data (OECD 2004).

 

PFA, 2013am, Peter Fisk Associates, Silanols and aquatic systems, 350.001.003

 

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

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