Registration Dossier

Diss Factsheets

Environmental fate & pathways

Endpoint summary

Administrative data

Description of key information

Additional information

Trimethoxy(2,4,4-trimethylpentyl)silane (CAS No. 34396-03-7) has an estimated water solubility of 110 mg/L at 20-25°C (QSAR) and a predicted log Kow of 4.0 (QSAR). Furthermore, it is characterized by a vapour pressure of 0.2 hPa at 20 °C (measured), indicating a potential for volatility. Trimethoxy(2,4,4-trimethylpentyl)silane does not contain chromophores that would absorb visible or UV radiation, so direct photolysis is not likely to be significant. Indirect photolysis resulting from gas-phase reaction with photochemically-produced hydroxyl radicals may occur. The calculated photodegradation half-life based on the reaction with hydroxyl radicals is 1.8 days. Therefore, photodegradation is a relevant degradation pathway for the registered substance.

The substance is not readily biodegradable. However, once in the water phase or when exposed to humid environments, the substance hydrolyses rapidly (DT50 = 5.7 h at pH 7 and 20-25 °C, QSAR), which contributes significantly to its degradation in the aquatic environment. The resulting transformation products are the silanol hydrolysis product (2,4,4-trimethylpentyl)silanetriol and the alcohol hydrolysis product methanol (CAS No. 67-56-1).

The ECHA guidance R.16 states that “for substances where hydrolytic DT50 is less than 12 h, environmental effects are likely to be attributed to the hydrolysis product rather than to the parent itself” (ECHA, 2016). The ECHA guidance R.16 also suggests that in cases where the hydrolysis half-life is less than 12 h, the breakdown products, rather than the parent substance, should be evaluated for aquatic toxicity."

Thus, the environmental fate assessment in this dossier is based on the hydrolysis products (2,4,4-trimethylpentyl)silanetriol and methanol.


Silanol hydrolysis product (2,4,4-trimethylpentyl)silanetriol 

The silanol hydrolysis product (2,4,4-trimethylpentyl)silanetriol  is characterized by a high, estimated water solubility of 2.4E05 mg/L (QSAR). There is no evidence of any significant biodegradation. However, its potential for bioaccumulation and its affinity to soil and sediment is low based on its low log Kow (0.9, QSAR) and its low, estimated log Koc (1.48, calculation method), respectively. Based on a low predicted vapour pressure of 1.2E-04 Pa at 25 °C (QSAR) and a low calculated Henry’s Law constant of 1.95E-06 Pa m3mol-1, volatilization from the water phase into the atmosphere is not significant.


Alcohol hydrolysis product methanol

The alcohol hydrolysis product methanol is well characterized in the public domain. It is fully water miscible at ambient temperatures and is characterized by a low partition coefficient (log Kow = -0.64 to -0.82, measured) and a low adsorption coefficient (Koc = 1) and is considered to have a low overall environmental hazard profile (OECD SIDS, 2004). Based on its high vapour pressure of 127.9 hPa at 20 °C (OECD SIDS, 2004) and a Henry’s Law constant of 0.461 Pa m3mol-1, air is expected to be a major compartment for the distribution of methanol. 



OECD SIDS, 2004. Methanol - SIDS Initial Assessment Report For SIAM 19, Berlin, Germany: UNEP Publications.