Registration Dossier
Registration Dossier
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EC number: 277-551-0 | CAS number: 73609-36-6
Endpoint summary
Administrative data
Description of key information
The substance, [2-(perfluorohexyl)ethyl]dichloro(methyl)silane, is not stable in water, which affects the approach to the determination of physicochemical properties.
[2-(Perfluorohexyl)ethyl]dichloro(methyl)silane is a liquid at standard temperature and pressure, with a measured melting point of 2 -2.5°C, and a measured boiling point of 197.1°C. It has a measured relative density of 1.564 at 20°C and predicted vapour pressure of 22 Pa at 25°C. The substance has a measured kinematic viscosity value of 3.6 mm2/s at 20°C.
The substance is not classified for flammability according to Regulation (EC) No. 1272/2008 on the basis of a measured flash point of 98.5°C and a measured boiling point of 197.1°C. It has a measured auto-ignition temperature of 350°C at 970 -973 hPa, and it is not explosive or oxidising on the basis of structural examination.
In contact with water under dilute conditions, [2-(perfluorohexyl)ethyl]dichloro(methyl)silane hydrolyses very rapidly (half-life <1 minute at pH 4, pH 7, pH 9 and 25°C) to produce [2-(perfluorohexyl)ethyl]methylsilanediol and hydrochloric acid according to the following equation:
C9H7Cl2F13Si + 2H2O → C9H9F13O2Si + 2HCl
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]methylsilanediol are assessed instead of the parent.
The silanol hydrolysis product, [2-(perfluorohexyl)ethyl]methylsilanediol has a predicted water solubility of 0.0048 mg/L at 20°C using a QSAR method and a predicted log Kow of 4.6.
The test for surface tension of [2-(perfluorohexyl)ethyl]dichloro(methyl)silane was waived because the submission substance hydrolyses very rapidly in contact with water to form [2-(perfluorohexyl)ethyl]methylsilanediol and hydrochloric acid. Instead, the surface tension of the silanol hydrolysis product, [2-(perfluorohexyl)ethyl]methylsilanediol, was determined from a study conducted in accordance with OECD Method 115 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. 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. The surface tension result is read-across to the silanol hydrolysis product of the submission substance, [2 -(perfluorohexyl)ethyl]methylsilanediol. [2-(Perfluorohexyl)ethyl]methylsilanediol has a predicted vapour pressure of 0.38 Pa at 25°C.
The silanol hydrolysis product, [2-(perfluorohexyl)ethyl]methylsilanediol, 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 2016am) attached in Section 13.
Silanediols may undergo condensation reactions to give siloxane dimers, oligomers and polymers, according to the equation:
RR’Si(OH)2↔RR’Si(OH)OSi(OH)RR’2↔RR’Si(OH)O[Si(RR’)O]nSi(OH)RR’
(where R is an alkyl or aryl side-chain)
The degree of condensation that will occur may vary with:
- Concentration of the silanol, the greater the initial concentration, the greater the degree of condensation.
- pH; the condensation reaction may be either acid or base catalysed
- Temperature
- Other species present
In the case of this substance, the R group (C9H7F13) contains a perfluorohexylethyl 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 silanediol 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 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 measured rates presented above 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 Feb 1982
PFA (2016am). Peter Fisk Associates, Silanols and aquatic systems, 404.105.003
Additional information
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