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The substance, [2-(perfluorohexyl)ethyl]trichlorosilane is not stable in water, which affects the approach to the determination of physicochemical properties. The significance of this for read-across is discussed in Section 1.5 of the CSR.

[2-(Perfluorohexyl)ethyl]trichlorosilane is a liquid at standard temperature and pressure, with a measured melting point of -21°C and a measured boiling point of 192.4°C. It has a predicted density of 1.6 g/cm3 at 20°C and a predicted vapour pressure of 30 Pa at 25°C.

The substance is not classified for flammability according to Regulation (EC) No. 1272/2008. In a flash point study, decomposition was observed at 138°C without ignition. It has a measured auto-ignition temperature of 380°C at 1015 -1020 hPa, and is not oxidising on the basis of chemical structure.

[2-(Perfluorohexyl)ethyl]trichlorosilane was not explosive under the conditions of an explosivity study in accordance with EU Method A.14.

In contact with water under dilute conditions, [2-(perfluorohexyl)ethyl]trichlorosilane reacts very rapidly (estimated half-life by read-across of <1 minute pH 4, pH 7 and pH 9 and 25°C) to produce [2-(perfluorohexyl)ethyl]silanetriol and hydrochloric acid according to the following equation:

Cl3SiCH2CH2(CH2)5CF3+ 3H2O → CH2CH2(CH2)5CF3Si(OH)+ 3HCl

Therefore, requirements for testing of water-based physicochemical properties for the substance are waived on the basis of instability in water. The properties of the silanol hydrolysis product, [2-(perfluorohexyl)ethyl]silanetriol are assessed instead of the parent.

The silanol hydrolysis product, [2-(perfluorohexyl)ethyl]silanetriol has a predicted water solubility of 0.58 mg/l at 20°C using a QSAR method and a predicted log Kow of 2.7.

The surface tension of[2-(perfluorohexyl)ethyl]trichlorosilane was waived because the substance hydrolyses very rapidly in contact with water to form [2-(perfluorohexyl)ethyl]silanetriol and hydrochloric acid. Instead, the surface tension of the silanol hydrolysis product, [2-(perfluorohexyl)ethyl]silanetriol was determined from a study conducted in accordance with EU Method A.5 using [2-(perfluorohexyl)ethyl]trimethoxysilane (CAS No. 85857-16-5) as the test substance. In contact with water, [2-(perfluorohexyl)ethyl]trimethoxysilane reacts rapidly (half-lives of 0.4 h at pH 4, 0.4 h at pH 5, 8.8 h at pH 7, 0.1 h at pH 9 and 20-25°C) to form [2-(perfluorohexyl)ethyl]silanetriol and methanol. The silanol hydrolysis product, [2-(perfluorohexyl)ethyl]silanetriol is considered to be surface active based on a measured surface tension value of <22 mN/m at 20°C and 1 g/l.[2-(Perfluorohexyl)ethyl]silanetriolis much less volatile than the parent substance (vapour pressure = 6.9E-03 Pa at 25°C, predicted).The silanol hydrolysis product, [2-(perfluorohexyl)ethyl]silanetriol, may undergo condensation reactions in solution to give siloxane dimers, linear and cyclic oligomers 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.

Silanetriols may undergo condensation reactions to give siloxane dimers, oligomers and polymers, according to the scheme:

RSi(OH)3    RSi(OH)2OSi(OH)2R    RSi(OH)2O[Si(R)(OH)O]nSi(OH)2R

(where R is an alkyl or aryl side-chain)

In an aqueous solution, a highly cross-linked gel may form. The degree of condensation that will occur may vary with:

  • pH; the condensation reaction may be either acid or base catalysed.
  • Temperature.
  • Other species present.  
  • The nature of the R group  
  • Timescale
  • The number of Si-OH groups; silanetriols condense more rapidly than silanediols

In the case of this substance, the R group (C8H4F13) 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 corresponding monomeric silanetriol is surface active based on measured and read-across data. The presence of fluorocarbon chains may mean that condensation products could have some surface activity. For these reasons, the effective limits of solubility of both the parent substance and monomeric silanol hydrolysis products are expected to be very low and all forms of the substance may have a tendency to enrich at surfaces rather than forming stable evenly dispersed solutions. These phenomena may limit the extent of hydrolysis under conditions at which the solubility of either parent or oligomer is exceeded. The predicted rates presented above relate to dilute conditions. These factors are important in any consideration of aqueous behaviour and properties.

Methanol is miscible with water, has low log Kow(-0.82 to -0.64) and high vapour pressure (12790 Pa at 20°C) (OECD, 2004a: 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.


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