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The substance, methylsilanetriyl triacetate, 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.

Methylsilanetriyl triacetateis a low melting crystalline solid at ambient temperature and pressure, however the substance is used in a non-solid form. It has a melting point of 40 -43°C, and a boiling point of 206 -220°C. The relative density of the substance is higher than that of water (1.17 at 20°C, measured). The requirement to conduct a viscosity study was waived because the substance is a solid under standard conditions. Methylsilanetriyl triacetate has a vapour pressure of 26 Pa at 20°C.

The substance is not classified for flammability on the basis of a measured flash point of 99°C at 1013 hPa and boiling point of 206 -220°C and is not explosive and not oxidising on the basis of structural examination.

In contact with water, methylsilanetriyl triacetate hydrolyses very rapidly (half-life <1 minute at 25°C and pH 4, pH 7 and pH 9) to produce methylsilanetriol and acetic acid according to the following equation:

(CH3C(=O)O)3SiCH3+ 3H2O ¿ CH3Si(OH)3+ 3CH3C(=O)OH

Therefore, requirements for testing of water-based physicochemical properties for the parent substance are waived on the basis of instability in water. The properties of the hydrolysis products, methylsilanetriol and acetic acid are assessed instead.

Acetic acid is miscible with water (O'Neil 2013), has low log Kow of -0.17 (Hansch et al. 1995) and vapour pressure of 2090 Pa at 25°C (Daubert and Danner 1985).

The silanol hydrolysis product, methylsilanetriol 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 methylsilanetriol, 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, methylsilanetriol, 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 -2.4 and it is not surface active. The first dissociation constant of methylsilanetriol is predicted to be around pKa = 10. The silanol hydrolysis product is much less volatile than the parent substance (predicted vapour pressure = 5.3E-02 Pa at 25°C).

References:

Daubert T E, Danner R P (1989): Physical and Thermodynamic Properties of Pure Chemicals Data compilation. Taylor and Francis, Washington D.C

Hansch. C., A. Leo and D. Hoekman.  1995.  Exploring QSAR. Hydrophobic, Electronic, and Steric Constants.  ACS Professional Reference Book. Washington, DC: American Chemical Society.

O'Neil M J (2013). The MERCK Index. An encyclopedia of chemicals, drugs, and biologicals. Fifteenth Edition. The Royal Society of Chemistry

PFA (2016am). Peter Fisk Associates, Analogue Report - Silanols and aquatic systems. Reference:404.105.003

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