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Trichloro(hexadecyl)silane (CAS No. 5894-60-0) reacts violently with water and hydrolyses very rapidly (DT50 << 2 min at pH 7 and room temperature) to the silanol hydrolysis product hexadecylsilanetriol and hydrochloric acid. Furthermore, it is characterized by a low vapour pressure of 7.0E-05 Pa at 20°C, indicating a low potential for volatility. Trichloro(hexadecyl)silane and its silanol hydrolysis product do 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 3.5 days. Therefore, photodegradation is not a relevant degradation pathway for the registered substance. However, for its silanol hydrolysis product a photodegradation half-life of 0.5 days was obtained. Based on the very low vapour pressure of hexadecylsilanetriol (< 1.0E-05 Pa at 25 °C) the evaporation into the air compartment is insignificant and therefore the photodegradation in air cannot be considered a relevant pathway for degradation of the silanol hydrolysis product.  

The substance is expected to be inherently biodegradable based on evidence from the structurally analogue read-across substance cloro(dimethyl)octadecylsilane (CAS No. 18643-08-8). Thus, biodegradation contributes significantly to its degradation in the aquatic environment and the registered substance does not persist in the environment.

 

Silanol hydrolysis product hexadecylsilanetriol 

The silanol hydrolysis product hexadecylsilanetriol is characterized by a moderate, estimated water solubility of 4.7 mg/L at 20 °C (QSAR). The silanol hydrolysis product is expected to be inherently biodegradable based on evidence from the structurally analogue read-across substance chloro(dimethyl)octadecylsilane (CAS No. 18643-08-8), which is the parent substance of a structurally very similar silanol hydrolysis product, octadecyldimethylsilanol. Thus, biodegradation contributes significantly to its degradation in the aquatic environment and hexadecylsilanetriol does not persist in the environment. Nevertheless, its potential for bioaccumulation and its affinity to soil and sediment might be high based on its estimated log Kow (5, QSAR) and its estimated log Koc value (5.6 (QSAR). Based on a low predicted vapour pressure of < 1.0E-05 Pa at 25 °C (QSAR) and a low calculated Henry’s Law constant of 5.9E-07 Pa m3mol-1, volatilization from the water phase into the atmosphere is negligible.

 

Second hydrolysis product hydrochloric acid (CAS No. 7647-01-0)

Hydrochloric acid is well characterized in the public domain. It has a high vapour pressure of 42200 hPa at 20 °C and a high solubility in water (823g/L at 0°C and 673 g/L at 30°C). In the environment, it is expected to distribute into the air and water. In water, it forms the aqueous solution hydrochloric acid.

In air, hydrogen chloride reacts with hydroxyl radicals into chloride free radicals and water (calculated half-life of 11 d). In water, hydrochloric acid readily dissociates into hydrated protons and chloride anions. Thus, it is ionised and neutralisation depends on the buffer capacity of the receiving water. Because hydrochloric acid is an inorganic substance, biodegradation is not a relevant degradation pathway. However, due to its high solubility and readily dissociation in water, it is not expected to accumulate in organisms. The aquatic hazard is mediated by the proton in a concentration-dependent manner (i.e. pH effect) and its potency strongly depends on the buffer capacity of the respective aquatic system. Toxicity only occurs when the buffering capacity of the receiving water is exceeded and pH values fall below pH 6. The pH in rivers and lakes fluctuates within a natural range. The natural pH range in aquatic systems is generally not expected to be perturbed to a relevant extent by anthropogenic emissions when appropriate risk control measures are in place. Variations in effect values of experimental studies can largely be explained by variations in the buffer capacity of the test media (OECD, 2002).

 

References:

OECD, 2002. Hydrogen Chloride - SIDS Initial Assessment Report for SIAM 15, Boston, USA: UNEP Publications.