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

Endpoint summary

Description of key information

[2-(Perfluorohexyl)ethyl]trimethoxysilane is a liquid at standard temperature and pressure, with a measured melting point of <-150°C, and a measured boiling point of 207°C. It has a predicted density of 1.4 g/cm³at 20°C and a predicted vapour pressure of 13 Pa at 25°C.A surface tension value of 72.2 mN/m at 20°C and 10 mg/l was obtained based on read-across from the triethoxy analogue(CAS 51851-37-7).

The substance is not classified for flammability according to Regulation (EC) No. 1272/2008 on the basis of a measured flash point of 90°C and a measured boiling point of 207°C. It has a measured auto-ignition temperature of 330°C, and is not explosive and not oxidising on the basis of chemical structure.

In contact with water under dilute conditions, [2-(perfluorohexyl)ethyl]trimethoxysilane reacts rapidly (half-life of 8.8 hours at pH 7, 0.4 hours at pH 4, 0.1 hours at pH 9 and 20-25°C) to produce [2-(perfluorohexyl)ethyl]silanetriol and methanol according to the following equation:

C₁₁H₁₃F₁₃O₃Si + 3H₂O → C₈H₇F₁₃O₃Si + 3CH₃OH

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 and methanol are assessed instead of the parent.

However, the key physicochemical properties of the submission substance were predicted using a validated QSAR estimation method. [2-(Perfluorohexyl)ethyl]trimethoxysilane has a predicted water solubility value of 2.4E-04 mg/l at 20°C and a predicted log K_owvalue of 5.8 at 20°C.

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 K_owof 2.8.

The surface tension of[2-(perfluorohexyl)ethyl]trimethoxysilane was waived because the submission substance rapidly hydrolyses in contact with water. In addition, the submission substance has a predictedwater solubility of <1 mg/l at 20°C. The silanol hydrolysis product 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.Further information is given in a supporting report (PFA 2013am) attached in Section 13.

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)

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 (C₁₁H₁₃F₁₃) 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 expected to be 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 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 K_ow(-0.82 to -0.64) and high vapour pressure (12790 Pa at 20°C) (OECD 2004a).

References:

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

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

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