Dichloro(dimethyl)silane

Ecotoxicological Summary

Hazard for aquatic organisms

Freshwater

Hazard assessment conclusion:

no hazard identified

Marine water

Hazard assessment conclusion:

no hazard identified

STP

Hazard assessment conclusion:

PNEC STP

PNEC value:

6.9 mg/L

Assessment factor:

10

Extrapolation method:

assessment factor

Sediment (freshwater)

Hazard assessment conclusion:

PNEC sediment (freshwater)

PNEC value:

5.54 mg/kg sediment dw

Extrapolation method:

equilibrium partitioning method

Sediment (marine water)

Hazard assessment conclusion:

PNEC sediment (marine water)

PNEC value:

0.554 mg/kg sediment dw

Extrapolation method:

equilibrium partitioning method

Hazard for air

Air

Hazard assessment conclusion:

no hazard identified

Hazard for terrestrial organisms

Soil

Hazard assessment conclusion:

PNEC soil

PNEC value:

0.346 mg/kg soil dw

Extrapolation method:

equilibrium partitioning method

Hazard for predators

Secondary poisoning

Hazard assessment conclusion:

no potential for bioaccumulation

Additional information

The hydrolysis half-life of dichloro(dimethyl)silane is <<1 min at pH 4, 7 and 9, 25⁰C. The registered substance will hydrolyse in contact with water and atmospheric moisture to dimethylsilanediol (CAS 1066-42 -8) and hydrochloric acid. REACH guidance (ECHA 2016, R.16) states that “for substances where hydrolytic DT₅₀ is less than 12 hours, environmental effects are likely to be attributed to the hydrolysis product rather than to the parent itself”. TGD and ECHA guidance, (EC 2003, ECHA 2016) also suggest that when the hydrolysis half-life is less than 12 hours, the breakdown products, rather than the parent substance, should be evaluated for aquatic toxicity. Therefore, the environmental hazard assessment, including sediment and soil compartments due to water and moisture being present, is based on the properties of the silanol hydrolysis product, in accordance with REACH guidance. As described in Section 1.3, the silanol hydrolysis products may be susceptible to condensation reactions.

READ-ACROSS JUSTIFICATION

In order to reduce animal testing read-across is proposed to fulfil up to REACH Annex X requirements for the registered substance from substances that have similar structure and physicochemical properties. Ecotoxicological studies are conducted in aquatic medium or in moist environments; therefore the hydrolysis rate of the substance is particularly important, because after hydrolysis occurs the resulting product has different structural features, physicochemical properties and behaviour.

 

The registered substance and the substances used as surrogate for read-across are part of a class of low-functionality compounds acting via a non-polar narcosis mechanism of toxicity. The group of organosilicon substances in this group contain alkyl, aryl, alkoxy or hydroxy groups attached to the silicon atom when present in aqueous solution. Secondary features may be present in the alkyl chain (e.g. halogen, nitrile, unsaturated bonds) that do not affect the toxicity of the substances.The silanol hydrolysis products may be susceptible to condensation reactions, see Section 1.3.

 

The registered substance hydrolyses rapidly in water and therefore the selection of surrogate substances is based on log K_ow of the resulting silanols and the chemical groups present in them. The surrogate substance, dimethysilanediol, is the hydrolysis product of the registered substance. The second surrogate substance, trichloro(ethyl)silane (CAS 115-21-9), rapidly hydrolyses to a similar silanol, ethylsilanetriol. The third surrogate substance, trimethoxy(methyl)silane (CAS 1185-55-3), will rapidly hydrolyse to a similar silanol,methylsilanetriol (CAS 2445-53-6).

Additional information is given in a supporting report (PFA 2016y) attached in Section 13 of the IUCLID dossier.

 

In the following paragraphs the read-across approach for dichloro(dimethyl)silane is assessed for the surrogate substances taking into account structure, hydrolysis rate and physico-chemical properties, presented in Table 7.0.1.

Table 7.0.1 Summary of physico-chemical and ecotoxicological properties of the registered and surrogate substances.

 

CAS Number	75-78-5	1066-42-8	115-21-9	1185-55-3
Chemical Name	Dichloro(dimethyl)silane	Dimethylsilanediol (DMSD)	Trichloro(ethyl)silane	Trimethoxy(methyl)silane
Si hydrolysis product	Dimethylsilanediol	n/a	Ethylsilanetriol	Methylsilanetriol
Molecular weight (parent)	129.06	92.17	163.51	136.22
Molecular weight (silanol hydrolysis product)	92.17	n/a	108.17	94.14
log Kow(parent)	n/a	-0.38	n/a	0.7 (at 20°C, QSAR prediction)
log Kow(silanol hydrolysis product)	-0.38	n/a	-1.87	-2.4 ( at 20°C, QSAR prediction)
Water sol (parent)	n/a	1E+06 mg/l (QSAR prediction) Limted to about 1000 mg/l by condensation reactions	n/a	29000 mg/l at 20°C (QSAR prediction)
Water sol (silanol hydrolysis product)	1E+06 mg/l(QSAR prediction) Limted to about 1000 mg/l by condensation reactions	n/a	1000000 mg/l (QSAR prediction) This may be limited by condensation reactions	1E+06 mg/l  at 20°C (QSAR prediction)
Vapour pressure (parent)	14600 Pa at 20°C	7 Pa	4780 Pa	4600 Pa at 25°C (QSAR prediction)
Vapour pressure (hydrolysis product)	7 Pa (QSAR prediction)	n/a	0.02 Pa (QSAR prediction)	0.05 Pa at  25°C (QSAR prediction)
Hydrolysis t1/2at pH 7 and 25°C	0.3 m	n/a	n/a	2.2 hrs
Hydrolysis t1/2at pH 4 and 25°C	0.2 m	n/a	n/a	0.033 hrs
Hydrolysis t1/2at pH 9 and 25°C	0.1 m	n/a	n/a	0.11 hrs
Short-term toxicity to fish (LC50)	n/a	>126 mg/l	n/a	>110 mg/l
Short-term toxicity to aquatic invertebrates (EC50)	n/a	>117 mg/l	n/a	>122 mg/l
Algal inhibition (ErC50and NOEC)	n/a	>118 mg/l and ≥118 mg/l	n/a	EC50 >120 mg/l NOEC >= 120 mg/l
Long-term toxicity to fish (NOEC)	n/a	n/a	n/a	n/a
Long-term toxicity to aquatic invertebrates (NOEC)	n/a	n/a	≥100 mg/l	n/a
Long-term sediment toxicity (NOEC)	n/a	n/a	n/a	n/a
Short-term terrestrial toxicity (L(E)C50)	n/a	n/a	n/a	n/a
Long-term terrestrial toxicity (NOEC)	n/a	n/a	n/a	n/a
Toxicity to microorganisms (3-hour EC10)	n/a	n/a	n/a	>100 mg/l

 

Read-across from dimethylsilanediol to dichloro(dimethyl)silane

Dichloro(dimethyl)silane (CAS 75 -78 -5) hydrolyses rapidly in contact with water to form dimethylsilanediol (CAS 1066-42 -8) and hydrochloric acid (half-life <<1 minute at pH 7, 25°C). The surrogate substance is dimethylsilanediol (CAS 1066-42 -8). Dimethylsilanediol is a silanediol containing two methyl groups bound to silicon and is not subject to further hydrolysis. The aquatic data available for this substance indicates that it is of low short-term toxicity to aquatic organisms, with L(E)C₅₀values all >100 mg/l , all short-term endpoints data are as read-across.

 

Read-across from trichloro(ethyl)silane to dichloro(dimethyl)silane

The registered substance, dichloro(dimethyl)silane (CAS 75 -78 -5) is a dichlorosilane with two methyl groups bound to silicon. In aqueous media, dichloro(dimethyl)silane hydrolyses rapidly (half-life <<1 minute at 25°C and pH 7) to dimethylsilanediol (CAS 1066 -42 -8) and hydrochloric acid. Trichloro(ethyl)silane (CAS 115 -21 -9) is a trichlorosilane with one ethyl group bound to silicon. Trichloro(ethyl)silane reacts very rapidly in water (half-life <<1 minute at 25°C and pH 7) to produce ethylsilanetriol and chlorides. Dimethylsilanediol, the hydrolysis product of the registered substance on which the chemical safety assessment is based, and ethylsilanetriol are both silanes with hydroxyl groups and small alkyl groups bound to silicon, low MW (92.17 and 108.17 respectively) and very low log Kow values (-0.38 and -1.87 respectively). Therefore read-across from trichloro(ethyl)silane to dichloro(dimethyl)silane is considered valid. The substance is used to read-across to long-term invertebrates endpoint, for which a 21-d NOEC value of ≥100 mg/L has been determined.

Read-across from trimethoxy(methyl)silane to dichloro(dimethyl)silane

The registered substance,dichloro(dimethyl)silane (CAS 75-78-5) is a dichlorosilane with two methyl groups bound to silicon. In aqueous media, dichloro(dimethyl)silane hydrolyses very rapidly (half-life <<1 minute at 25°C and pH 7) to dimethylsilanediol (CAS 1066-42-8) and hydrochloric acid. Trimethoxy(methyl)silane (CAS 1185-55-3) is a trimethoxysilane with one methyl group bound to silicon. Trimethoxy(methyl)silane will rapidly hydrolyse in aqueous media (half-life 2.2 hrs at 25°C and pH 7) to methylsilanetriol (CAS 2445-53-6) and methanol.

Dimethylsilanediol, the hydrolysis product of the registered substance on which the chemical safety assessment is based, and methylsilanetriol are both silanes with hydroxyl groups and small alkyl groups bound to silicon, low MW (92.17 and 94.14, respectively) and very low log K_ow values (-0.38 and -2.4 respectively). Therefore read-across from trimethoxy(methyl)silane to dichloro(dimethyl)silane is considered valid. The data available with trimethoxy(methyl)silane, where the organisms were exposed to its hydrolysis products, indicate that the substance does not induce short-term toxicity to aquatic organisms (i.e. EC/LC₅₀s are >100 mg/L), nor toxicity to aquatic microorganisms (EC₁₀ >100 mg/l).

The available data for dimethylsilanediol and trimethoxy(methyl)silane indicate that both substances are not toxic to aquatic organisms when tested in short-term toxicity studies (i.e. EC/LC₅₀s are >100 mg/L, see Table 7.0.1) and are not toxic to aquatic microorganisms (EC₁₀ >100 mg/l for trimethoxy(methyl)silane, see Table 7.0.1).

 

Consideration of the non-silanol hydrolysis product hydrochloric acid

Aquatic ecosystems are characterised by, among other factors, their pH; the organisms of the ecosystem 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 No. 7647-01-0). Effects on aquatic organisms arising from exposure to hydrogen chloride 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.

It is not considered to be appropriate or useful to derive a single aquatic PNEC for hydrogen chloride 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. In addition, neutralisation of waste water is typically part of waste water treatment of effluents containing acids. This significantly removes or reduces the pH effects of HCl released to the environment.

Conclusion on classification

According to Regulation (EC) No 1272/2008 (CLP):

Aquatic Acute Not classified.

Aquatic Chronic Not Classified

According to Directive 67/548/EEC:

Not classified

It is proposed that dichloro(dimethyl)silane should not be classified in the EU for acute or chronic toxicity on the grounds that it rapidly hydrolyses and the silanol hydrolysis product is not toxic at an initial loading rate of the substance of 100 mg/l. The silanol hydrolysis product has a low log K_ow and high water solubility.

Hydrochloric acid is listed on Annex VI of Regulation (EC) No 1272/2008 (CLP) and in Annex I of Directive 67/548/EEC and does not require classification for the environment.