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Physical & Chemical properties

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Trimethoxy(octyl)silane is a liquid at standard temperature and pressure, with a measured melting point of -66°C at 1013 hPa and a measured boiling point of 227°C at 1013 hPa. It has a measured relative density of 0.907 at 20°C and a predicted kinematic viscosity of 2.4 mm2/s at 20°C. The substance also has measured vapour pressure values of 2.1 Pa at 20°C and 30 Pa at 50°C.

The substance is not classified for flammability according to Regulation (EC) No 1272/2008 on the basis of a measured flash point of 95°C and a measured boiling point of 227°C at 1013 hPa. It has a measured auto-ignition temperature of 225°C at 1018.5 -1022.0 hPa, it is not explosive and not oxidising on the basis of chemical structure.

In contact with water, trimethoxy(octyl)silane reacts rapidly (half-life = 4 hours at pH 7, 0.2 hours at pH 4, 0.3 hours at pH 5, 0.1 hours at pH 9 and 20-25°C) to produce octylsilanetriol and methanol according to the following equation:

CH3(CH2)7Si(OCH3)3+ 3H2O → CH3(CH2)7Si(OH)3+ 3CH3OH

However, the water-based physicochemical properties of the submission substance have been determined. It has a measured log Kow value of 3.9 ±0.2 at 24°C ±1 and measured water solubility value of 13.3 ± 5.6 mg/L at 20°C. Since, the hydrolysis half-life of the submission substance is <12 hours; the physicochemical properties of the hydrolysis products are also discussed.

Methanol is miscible with water, has low log Kow (-0.82 to -0.64) and high vapour pressure (12,790 Pa at 20°C) (OECD, 2004a).

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 (1.1, predicted).

The surface tension study of trimethoxy(octyl)silane was waived because in contact with water, the substance rapidly hydrolyses to form octylsilanetriol and methanol. Instead, the surface tension of the hydrolysis product was determined from a study conducted in accordance with EU Method A.5, using octyltrichlorosilane as the test substance. In contact with water, octyltrichlorosilane reacts very rapidly (half-life less than one minute) to form octylsilanetriol and hydrochloric acid. 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). 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.

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. It is much less volatile than the parent substance (vapour pressure = 2.7E-05 Pa at 25°C, predicted).

Reference:

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

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