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 acute and chronic aquatic toxicity data is available for the registered compound Trichloro(hexadecyl)silane (CAS No. 5894-60-0) or its silanol hydrolysis product. Therefore, good quality data was read-across from the structurally analogue substances trimethoxy(hexadecyl)silane (CAS No. 16415-12-6). The source substance hydrolyses slowly under environmental conditions (DT50 > 14 d at pH 7), but it forms the identical silanol hydrolysis product as the target compound and methanol as the second hydrolysis product instead of hydrochloric acid (target). Additional information on the source substance (CAS No. 16415-12-6) is given in a supporting read-across justification attached in IUCLID Section 13.

 

Short-term toxicity data is available for all three trophic levels (fish, algae, aquatic invertebrates) on the read-across source compound trimethoxy(hexadecyl)silane (CAS No. 16415-12-6) and chronic aquatic data is available for algae and aquatic invertebrates for the source compound. Due to the slow hydrolysis time (DT50 > 14 d at pH 7) of the source compound and the duration of the acute tests (48 to 96 hours), it is likely that the organisms were predominantly exposed to the parent source compound. Also in the chronic study on daphnia, the organisms were likely predominantly exposed to the parent source compound although due to the longer duration of the study (21 d) it is possible that the organisms were also exposed to the hydrolysis products. The second hydrolysis product formed by the source substance, methanol, is also well characterised in the public domain literature and is not hazardous at the concentrations relevant to the studies (OECD SIDS, 2004). Thus, methanol is not considered contributory to the overall aquatic toxicity of the source substance and can be disregarded in the assessment.

  

All aquatic studies were performed according to accepted standard guidelines and GLP criteria. No acute effects were observed towards the species of three trophic levels up to the highest tested concentrations. No chronic effects were observed in aquatic invertebrates with a 21-d NOEC of ≥10 mg/L and in algae 72-h NOEC ≥ 31.8 mg/L.

 

Table: Overview on aquatic effect values used for the registration of trichloro(hexadecyl)silane (CAS No. 5894-60-0).

	Parent	Silanol hydrolysis product	Source	Non-silanol hydrolysis products
CAS No.	5894-60-0		16415-12-6	7647-01-0	67-56-1
Chemical Name	trichloro(hexadecyl)silane	hexadecylsilanetriol	trimethoxy(hexadecyl)silane	Hydrochloric acid^a	Methanol^b*
Acute fish (96-h LC50)	RA CAS 16415-12-6	RA CAS 16415-12-6	> 1000 mg/L	4.92 mg/L (pH 4.3)	> 15000 mg/L
Acute Invertebrates (48-h EC50)	RA CAS 16415-12-6	RA CAS 16415-12-6	> 100 mg/L	0.492 mg/L (pH 5.3)	> 10000 mg/L
Algae (72-h ErC)	RA CAS 16415-12-6	RA CAS 16415-12-6	> 31.8 mg/L	0.492 mg/L (pH 5.3)	ca. 22000 mg/L
Chronic Invertebrates (21-d NOEC)	RA CAS 16415-12-6	RA CAS 16415-12-6	≥ 10 mg/L		No data
Algae (72-h NOErC50)	RA CAS 16415-12-6	RA CAS 16415-12-6	≥ 31.8 mg/L	0.097 mg/L (pH 6)	No data

^aSource = OECD SIDS (2002)

^bSource = OECD SIDS (2004)

* = hydrolysis product formed by the source substances

 

No effects on activated sludge microorganisms were recorded up to a concentration of 1000 mg/L with the registered substance itself. The 3-h EC50 and NOEC were found to be > 1000 mg/L and ≥ 1000 mg/L, respectively. Thus, no toxic effects were observed caused by the registered substance and the degradation process in sewage treatment plants is not considered to be inhibited.

 

References:

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

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