Trichloro(hexadecyl)silane

Endpoint summary

Additional information

The registered substance trichloro(hexadecyl)silane (CAS No. 5894-60-0) hydrolyses very rapidly in contact with water (DT50 << 2 min at pH 7 and room temperature, EU Method C.7) to hexadecylsilanetriol and hydrochloric acid. REACH 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). TGD (EC, 2003) and REACH guidance R.16 (ECHA, 2016) also suggest that when the hydrolysis half-life is less than 12 h, the breakdown products, rather than the parent substance, should be evaluated for aquatic toxicity testing. Therefore, the hazard and risk assessment is based on the silanol hydrolysis product hexadecylsilanetriol. The second hydrolysis product hydrochloric acid is well characterised in the public domain literature and effects on aquatic organisms arising from exposure to hydrochloric acid are thought to result from a reduction in the pH of the ambient environment (arising from an increase in the H+ concentration) to a level below their tolerable range. Aquatic ecosystems are characterized by their ambient conditions, including the pH, and resident organisms are adapted to these conditions. The pH of aquatic habitats can range from 6 in poorly-buffered ‘soft’ waters to 9 in well-buffered ‘hard’ waters. The tolerance of aquatic ecosystems to natural variations in pH is well understood and has been quantified and reported extensively in ecological publications and handbooks (e.g. OECD SIDS for CAS 7647-01-0, hydrochloric acid). It is not considered appropriate or useful to derive a single aquatic PNEC for hydrochloric acid because any effects will not be a consequence of true chemical toxicity and will be a function of, and dependent on, the buffering capacity of the environment. Generally the changes in pH of the receiving water should stay within the natural range of the pH, and for this reason, adverse effects on the aquatic environment are not expected due to anthropogenic or naturally occurring hydrochloric acid (OECD SIDS, 2002). Thus, hydrochloric acid is not considered to contribute to the overall aquatic toxicity of the registered substance. Therefore, hydrochloric acid can be disregarded in the environmental hazard assessment. In consequence, data for hydrochloric acid is not presented specifically for each environmental toxicity endpoint within this dossier.

No data on terrestrial toxicity is available on the parent substance trichloro(hexadecyl)silane (CAS No. 5894-60-0) or its silanol hydrolysis product hexadecylsilanetriol. The registered substance has a very limited use pattern with only intermediate and professional uses and therefore exposure to the terrestrial environment can be excluded. However, in order to estimate the possible effects of the substance and its silanol hydrolysis product as a precautionary measure data was read-across from the structurally similar analogue substance trimethoxy(hexadecyl)silane (CAS No. 16415-12-6). Both substances hydrolyse under environmental conditions but with very different hydrolysis rates (DT50 << 2 min at room temperature and pH 7 (target); DT50 > 14 d at pH 7 (source)). However, both substances are structurally very similar and form the same silanol hydrolysis product hexadecylsilanetriol. The second hydrolysis product of the source substance is methanol instead of hydrochloric acid (target). However, methanol is well known in the public domain literature and is not expected to cause any environmental effects up to the concentrations that are relevant in the studies (OECD SIDS, 2004).

Terrestrial toxicity studies with hexadecyl(trimethoxy)silane (CAS No. 16415-12-6) are available for soil macroorganisms (acute and chronic), terrestrial plants (chronic), and soil microorganisms (chronic).

The available acute toxicity study to earthworms (OECD 207) resulted in an LC50 (14 d) of > 1000 mg/kg soil dw whereas in the chronic toxicity study with earthworm (OECD 222) effects on biomass and reproduction were observed (NOEC: 500 mg/kg soil dw).

In a study testing the toxicity to six terrestrial plant species according to OECD 208 in a seedling emergence and seedling growth test, no significant differences for shoot height, number of emerged seedlings and survival of plants were observed for all tested plant species. Oats and lettuce were the most sensitive plant species and showed a statistically reduced shoot fresh weight after 21 days. For these species the NOEC is 250 mg/kg dw.

One GLP study is available testing the effects of the test substance on nitrogen tranformation in soil according to OECD 216. A significant inhibition of the microbial nitrate transformation was observed for test item concentrations ≥ 100 mg/kg soil dw. The corresponding no-observed effect concentration (NOEC) was 31.6 mg/kg soil dw which is the lowest effect value derived among terrestrial species. This effect value has been used to derive the PNEC soil (see IUCLID section 6).

References:

OECD SIDS, 2002. Hydrochloric acid - SIDS Initial Assessment Report For SIAM 19, Berlin, Germany: UNEP Publications.

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