Dimethoxydimethylsilane

Endpoint summary

Description of key information

Dimethoxydimethylsilane (CAS No. 1112-39-6) is neither toxic nor hazardous to aquatic organisms.

Additional information

Dimethoxy(dimethyl)silane (CAS No. 1112-39-6) hydrolyses rapidly in contact with water (DT50 < 0.6 h), to dimethylsilanediol (DMSD) and methanol. 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 case 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 aquatic toxicity was assessed based on read-across to the silanol hydrolysis product dimethylsilanediol (DMSD, CAS No. 1066-42-8). For the assessment of the toxicity to activated sludge microorganism read-across to dichloro(dimethyl)silane (CAS No. 75-78-5) was performed, which very rapidly forms the same silanol hydrolysis product dimethylsilanediol (DMSD, CAS No. 1066-42-8), just like the registered substance. Additional information on the read-across approach is provided in a supporting report attached in IUCLID Section 13.

Dimethylsilanediol (DMSD, CAS No. 1066-42-8)
Acute aquatic toxicity studies evaluating the short-term toxicity of the silanol hydrolysis product DMSD to aquatic organisms are available for all three trophic levels (Table 1). All studies were performed according to accepted standard guidelines and GLP. No acute effects were observed towards the tested organisms of all three trophic levels up to the highest concentrations tested (126 mg/L, 117 mg/L and 118 mg/L for fish, aquatic invertebrates and algae, respectively). 
Chronic aquatic toxicity studies are available for all three trophic levels (Table 1). All studies were performed according to accepted standard guidelines and GLP. No effects were observed towards the tested aquatic organisms of all three trophic levels up to the highest concentrations tested (12 mg/L, 12.6 mg/L and 118 mg/L, for fish, aquatic invertebrates and algae, respectively).
For the toxicity of the substance toward microorganisms, one experimental study is available by means of read-across from the analogue substance dichloro(dimethyl)silane (CAS No. 75-78-5) for which a study according to OECD guideline 209 and GLP was performed. No effects were observed up to the highest concentration tested (1000 mg/L) and the degradation process in sewage treatment plants is not expected to be inhibited.

Methanol (CAS No. 67-56-1)
The second hydrolysis product methanol is not considered to be hazardous to aquatic organisms (Table 1). The effect concentrations derived for methanol in standard guideline studies are all well above 100 mg/L (OECD SIDS, 2004). Thus, methanol is not considered to contribute to the overall aquatic toxicity of the target substance. In consequence, data for methanol are not presented specifically for each environmental toxicity endpoint within this dossier. 

 

Table 1: Overview of aquatic toxicity effect values for dimethoxydimethylsilane (CAS No. 1112-39-6) and its hydrolysis products

	Parent	Silanol hydrolysis product	Alcohol hydrolysis product
CAS	1112-39-6	1066-42-8	67-56-1
	Dimethoxydimethylsilane	Dimethylsilanediol	Methanol*
Acute Fish (LC50)	RA CAS 1066-42-8	> 126 mg/L	> 15000 mg/L
Acute Invertebrates (EC50)	RA CAS 1066-42-8 > 100 mg/L	> 117 mg/L	> 10000 mg/L
Acute Algae (ErC50)	RA CAS 1066-42-8	> 118 mg/L	> 28 000 mg/L
Chronic fish (EC10)	RA CAS 1066-42-8	> 12 mg/L	No data
Chronic Invertebrates (NOErC)	RA CAS 1066-42-8	≥ 12.6 mg/L	No data
Algae (NOErC)	RA CAS 1066-42-8	≥ 118 mg/L	No data

* Source: OECD (2004). SIDS Methanol.