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Physical & Chemical properties

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Description of key information

The substance, trichloro(octyl)silane 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 relevant sections.

Trichloro(octyl)silane is a liquid at standard temperature and pressure; it has a measured melting point of <-20°C at 1013 hPa and a measured boiling point of 233°C at 998.2 hPa. It has a measured relative density of 1.0769 at 20°C, a viscosity value 2.18 mm2/s at 20°C and a predicted vapour pressure of 3.5 Pa at 25°C.

The substance is not classified for flammability on the basis of a measured flash point of approximately 85°C at 1013 hPa and the boiling point of 233°C at 998.2 hPa. It has a measured auto-ignition temperature of 213°C at 966.7 to 972.5 hPa. The substance is not oxidising on the basis of structural examination. Similarly, trichloro(octyl)silane is not explosive based on structural examination.

In contact with water, trichloro(octyl)silane is expected to react very rapidly (based on read-across from related substance; half-life <1 min at 25°C and pH 4, pH 7 and pH 9) according to the following equation:

CH3(CH2)7SiCl3 + 3H2O → CH3(CH2)7Si(OH)3 + 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, octylsilanetriol, are assessed instead.

The silanol hydrolysis product, octylsilanetriol, 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.

 

The condensation reactions of silanetriols may be modelled as an equilibrium between monomer, dimer, trimer and tetramer, with the linear tetramer cyclising to the thermodynamically stable cyclic tetramer. The reactions are reversible unless the cyclic tetramer concentration exceeds its solubility; in this case, the cyclic tetramer forms a separate phase, driving the equilibrium towards the tetramer. For octylsilanetriol, at loadings above about 100 mg/L the concentration of the cyclic tetramer of the silanol hydrolysis product is predicted to exceed its solubility, resulting in formation of a separate phase. Further information is given in a supporting report (PFA 2016am) attached in Section 13.

 

The saturation concentration in water of the silanol hydrolysis product, octylsilanetriol is limited by condensation reactions that can occur over time at loadings about 100 mg/L. However, it is very hydrophilic (calculated solubility is 5.9E+04 mg/L at 20°C using a QSAR method) and has a low log Kow of 1.1 (predicted). The silanol hydrolysis product is not expected to undergo significant dissociation within the environmentally-relevant range. The first dissociation constant of a structurally analogous silanetriol (phenylsilanetriol) has been reported to be around pKa of 10. Octylsilanetriol is much less volatile than the parent substance (vapour pressure = 2.7E-05 Pa at 25°C, predicted).

The surface tension of trichloro(octyl)silane was waived because in contact with water, the substance hydrolyses very rapidly to form octhylsilanetriol and hydrochloric acid. In a study conducted in accordance with EU Method A.5, using trichloro(octyl)silane as test substance, rapid hydrolysis was observed with the formation of white precipitate. The hydrolysis product was determined to be potentially surface active (surface tension of 53 mN/m) using EU Method A.5 test method. Significant condensation of the test substance was visually observed in water and following chemical analysis there was no indication of the test substance or an organic group as product of hydrolysis in aqueous solution, thus the result is difficult to interpret. However, the surface activity observed was assumed to be as a result of a very small amount of the hydrolysis product which remained on the surface of the aqueous solution.

Reference:

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

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