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Physical & Chemical properties

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The substance, trichloro(propyl)silane is not stable in water, which affects the approach to the determination of physicochemical properties.

Trichloro(propyl)silane is a liquid at standard temperature and pressure, with a measured melting point of <-100°C, and a measured boiling point of 124.4°C. It has a relative density of 1.198 at 20°C and measured vapour pressure values of 1500 Pa at 20°C and 7400 Pa at 50°C. The substance has a measured kinematic viscosity value of 0.585 mm2/s at 20°C.

The substance is classified as a flammable liquid in accordance with Regulation (EC) No. 1272/2008 on the basis of a measured flash point of 33°C at 1013 hPa and measured boiling point of 124.4°C at 1013 hPa. It has a measured auto-ignition temperature of 349°C at 959 to 971 hPa, and is not explosive and not oxidising on the basis of structural examination.

In contact with water, trichloro(propyl)silane hydrolyses very rapidly (half-life <1 minute at 25°C and pH 4, pH 7 and pH 9) to form propylsilanetriol and hydrochloric acid according to the following equation:

CH3CH2CH2SiCl3+ 3H2O → CH3CH2CH2Si(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, propylsilanetriol are assessed instead.

The silanol hydrolysis product, propylsilanetriol may undergo condensation reactions in solution to give siloxane dimers, linear and cyclic oligomers and highly cross-linked polymeric particles (a colloidal suspension of small solid particles known as a 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. At higher loadings, cross-linking reactions between the cyclic tetramers may occur. 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. At loadings below 500 mg/L of propylsilanetriol, the soluble monomer is expected to predominate in solution (>99%), with small amounts of dimer and oligomers. Condensation reactions are expected to become important at loadings above about 1000 mg/L causing the formation of insoluble polymeric particles and gels over time. Further information is given in a supporting report (PFA 2016am) attached in Section 13.

The saturation concentration in water of the silanol hydrolysis product, propylsilanetriol is therefore limited by condensation reactions to approximately 1000 mg/L. However, it is very hydrophilic (calculated solubility is 1E+06 mg/L at 20°C using a QSAR method) with a predicted low log Kow of -1.4. It is not surface active. The first dissociation constant of propylsilanetriol is predicted to be around pKa = 10. The silanol hydrolysis product is much less volatile than the parent substance (predicted vapour pressure = 4.6E-03 Pa at 25°C).

Reference:

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

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