Registration Dossier

Physical & Chemical properties

Endpoint summary

Administrative data

Description of key information

Triethoxy(octyl)silane is a liquid at standard temperature and pressure, with a measured melting point of -75°C, and a measured boiling point of 259.6°C. It has a measured relative density of 0.878 at 20°C, a measured kinematic viscosity of 1.68 mm2/s at 20°C and predicted vapour pressure of 0.11 Pa at 25°C.

Triethoxy(octyl)silane has a water solubility value of <0.13 - 0.79 mg/l at 20°C and a measured log Kow of 6.41.

The presence of higher concentrations of volatile impurities such as ethanol; a hydrolysis product of the substance, can result in lower flash point values. The registered substance has flash point values ranging from 45°C to 100°C and measured boiling point of 259.6°C. Although, a sustained combustibility of >110°C at 1013 hPa was available for the substance, due to lack of the interpretation of the result in the study report; this study is not considered further. It has a measured auto-ignition temperature of 225°C at 1008.9 - 1020.8 hPa. The substance is not explosive and not oxidising on the basis of structural examination.

In contact with water, triethoxy(octyl)silane reacts moderately rapidly to form octylsilanetriol and ethanol (half-lives of 0.7 h at pH 4, 0.7 h at pH 5, 30 h at pH 7 and 0.4 h at pH 9 and at 20 - 25°C) according to the following equation:

CH3(CH2)7Si(OCH2CH3)3+ 3H2O → CH3(CH2)7Si(OH)3+ 3CH2CH3OH

Due to the moderate hydrolysis of triethoxy(octyl)silane in contact with water, the key physicochemical properties of the hydrolysis products are presented below.

Ethanol is miscible with water, has low log Kow (-0.3) and high vapour pressure (7910 Pa at 25°C).

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 surface tension study for triethoxy(octyl)silane was waived because the substance has a predicted water solubility of <1 mg/L. In contact with water, the substance will moderately hydrolyse to form octylsilanetriol and ethanol, 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. Octylsilanetriol is much less volatile than the parent substance (vapour pressure = 2.7E-05 Pa at 25°C, predicted).

 

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