Trimethoxy(3,3,4,4,5,5,6,6,7,7,8,8,8-tridecafluorooctyl)silane

Ecotoxicological Summary

Additional information

The hydrolysis half-life of [2-(perfluorohexyl)ethyl]trimethoxysilane (CAS 85857-16-5) is predicted to be 8.8 hours at pH 7, 20-25°C . The substance will hydrolyse in water under dilute conditions to [2-(perfluorohexyl)ethyl]silanetriol and methanol. REACH guidance (ECHA 2012, 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 2012) 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 4.8, the silanol hydrolysis products may be susceptible to condensation reactions.

READ-ACROSS JUSTIFICATION

In order to reduce testing read-across is proposed to fulfil up to REACH Annex VII requirements for the registration 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 registration substance and the substance 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 4.8.  

The registration substance is expected to hydrolyse rapidly in water, under dilute conditions, 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, [2-(perfluorohexyl)ethyl]triethoxysilane (CAS 51851-37-7), hydrolyses to the same silanol as the registration substance, [2-(perfluorohexyl)ethyl]silanetriol. Additional information is given in a supporting report (PFA, 2016y) attached in Section 13.

In the following paragraphs the read-across approach for [2-(perfluorohexyl)ethyl]trimethoxysilane is assessed for the surrogate substances taking into account structure, hydrolysis rate and physicochemical properties. Table 1 presents relevant physicochemical properties and the available ecotoxicological data.

Table 1: Summary of physicochemical and ecotoxicological properties of the registered and surrogate substances.

CAS Number	85857-16-5	51851-37-7
Chemical Name	[2-(Perfluorohexyl)ethyl]trimethoxysilane	[2-(Perfluorohexyl)ethyl]triethoxysilane
Si hydrolysis product	[2-(Perfluorohexyl)ethyl]silanetriol	[2-(Perfluorohexyl)ethyl]silanetriol
Molecular weight (parent)	468.28	510.37
Molecular weight (hydrolysis product)	426.21	426.21
log Kow (parent)	5.8 (QSAR prediction)	7.2 (QSAR prediction)
log Kow (silanol hydrolysis product)	2.7 (QSAR prediction)	2.7 (QSAR prediction)
Water sol (parent)	2.4E-04 mg/l (QSAR prediction)	6.4E-06 mg/l (QSAR prediction)
Water sol (silanol hydrolysis product)	0.58 mg/l (QSAR prediction)	0.58 mg/l (QSAR prediction)
Vapour pressure (parent)	13 Pa (25°C) (QSAR prediction)	No data
Vapour pressure (hydrolysis product)	6.9E-03 Pa (25°C) (QSAR prediction)	6.9E-03 Pa (25°C) (QSAR prediction)
Hydrolysis t1/2 at pH 7 and 25°C	8.8 hours (QSAR prediction)	12.5 h (20°C)
Hydrolysis t1/2 at pH 4 and 25°C	0.4 hours (QSAR prediction)	1.44 h (20°C)
Hydrolysis t1/2 at pH 9 and 25°C	0.1 hours (QSAR prediction)	5.18 h (20°C)
Short-term toxicity to fish (LC50)	n/a	>3.1 mg/l
Short-term toxicity to aquatic invertebrates (EC50)	n/a	>9 mg/l
Algal inhibition (ErC50 and NOEC)	n/a	EC50 >7.1 mg/l NOEC ≥7.1 mg/l
Long-term toxicity to fish (NOEC)	n/a	n/a
Long-term toxicity to aquatic invertebrates (NOEC)	n/a	n/a
Long-term sediment toxicity (NOEC)	n/a	n/a
Short-term terrestrial toxicity (L(E)C50)	n/a	n/a
Long-term terrestrial toxicity (NOEC)	n/a	n/a

 

Read-across from [2-(perfluorohexyl)ethyl]triethoxysilane to [2-(perfluorohexyl)ethyl]trimethoxysilane:

The registration substance, [2-(perfluorohexyl)ethyl]trimethoxysilane (CAS 85857-16-5), is a trialkoxysilane with a fluoroalkyl group attached to the silicon atom. In water under dilute conditions it hydrolyses rapidly (half-life 8.8 hours at 20-25°C and pH 7) to [2-(perfluorohexyl)ethyl]silanetriol and methanol.

The surrogate substance, [2-(perfluorohexyl)ethyl]triethoxysilane (CAS 51851-37-7), is also a trialkoxysilane with an alkyl and fluoro group attached to the silicon atom. In water under dilute conditions, [2-(perfluorohexyl)ethyl]triethoxysilane hydrolyses to [2-(perfluorohexyl)ethyl]silanetriol and ethanol (half-life of 12.5 hours at pH 7 and 20°C).

Both the registration and surrogate substances share similar physicochemical properties: both have log K_ow >5 (5.8 and 8.4, respectively), similar molecular weight (468.28 and 510.37, respectively) and have very low predicted water solubility (2.4E-04 mg/l and 6.4E-06 mg/l, respectively). They both have a chemical structure that contains a perfluorohexyl chain; fluorocarbons have very low solubility in water, relative to hydrocarbons of equivalent chain length, (e.g. Horvath 1982), so it would be expected that these substances would be of low solubility. The effective limits of solubility of both the parent substances and monomeric silanol hydrolysis products are very low. Condensation phenomena may limit the extent of hydrolysis under conditions at which the solubility of either parent or oligomer is exceeded. The rates of hydrolysis for the registration substance relate to dilute conditions. These factors are important in any consideration of aqueous behaviour and properties.

[2-(Perfluorohexyl)ethyl]silanetriol, the silanol hydrolysis product of both the registration substance and the surrogate substance, is the substance on which the chemical safety assessment is based. The hydrolysis of [2-(perfluorohexyl)ethyl]triethoxysilane is slower than that of [2-(perfluorohexyl)ethyl]trimethoxysilane. However, during the tests carried out with [2-(perfluorohexyl)ethyl]triethoxysilane, it is likely that the test organisms were exposed to a mixture of the test substance and the hydrolysis products at the beginning of the tests, after this it is likely to be predominantly the hydrolysis products. See Section 6.1 Aquatic toxicity for further discussion.

Due to the differences in hydrolysis rates it is considered to be conservative to read-across from [2-(perfluorohexyl)ethyl]triethoxysilane but read-across is justified for the following reasons: the registration substance is extremely difficult to test. It has very low water solubility and the presence of fluorocarbon chains indicates the potential for some surface activity. The amount of silanol in test solution depends on the amount of dissolved parent substance and condensation phenomena may limit the extent of hydrolysis under conditions at which the solubility of either parent or oligomer is exceeded. The short-term toxicity values from the studies of the surrogate substance show measured EC₅₀ values in excess of the solubility of the substance; however, best use has been made of the existing studies for [2-(perfluorohexyl)ethyl]triethoxysilane, although it is not clear how much parent or silanol hydrolysis product is in solution. No effects were seen in any of the tests. New studies are not recommended due to difficulties in testing the substance and avoiding physical effects associated with condensation/precipitation of insoluble oligomers, and surface activity related effects. Therefore read-across from [2-(perfluorohexyl)ethyl]triethoxysilane to [2-(perfluorohexyl)ethyl]trimethoxysilane is considered to be valid.

Consideration of the non-silanol hydrolysis products:

Methanol and ethanol are well-characterised in the public domain literature and are not hazardous at the concentrations relevant to the studies; the short-term EC₅₀ and LC₅₀ values for the substances are in excess of 1000 mg/l (OECD (2004a): SIDS Initial Assessment Report for SIAM 19, Berlin, Germany, 18-20 October 2004, Methanol, CAS 67-56-1 and OECD (2004b): SIDS Initial Assessment Report for SIAM 19, Berlin, Germany, 19-22 October 2004, Ethanol, CAS 64-17-5).

References:

EC 2003: European Union Technical Guidance Document on Risk Assessment for New and Existing Substances, Part II, European Chemicals Bureau, 2003.

ECHA 2012: European Chemicals Agency. Guidance on information requirements and chemical safety assessment Chapter R.16: Environmental Exposure Estimation. Version: 2.1, October 2012 (Section R.16.5.4.1. Hydrolysis).

Horvath, A.L. (1982) Halogenated Hydrocarbons. Solubility-Miscibility with Water, Marcel Dekker, New York, 1982.

OECD (2004a): SIDS Initial Assessment Report for SIAM 19, Berlin, Germany, 18-20 October 2004, Methanol, CAS 67-56-1.

OECD (2004b): SIDS Initial Assessment Report for SIAM 19, Berlin, Germany, 19-22 October 2004, Ethanol, CAS 64-17-5.

PFA, 2016y, Peter Fisk Associates, Analogue Report - Ecotoxicity of substances with ‘low-functionality’ side chains attached to silicon, PFA. 404.003.004.R1

 

Conclusion on classification

The substance has reliable short-term EC₅₀ values of >9 mg/l in invertebrates and >7.1 mg/l in algae based on read-across data from the structural analogue [2-(perfluorohexyl)ethyl]triethoxysilane (CAS 51851-37-7) (although these values exceed the solubility of the substance). It has a reliable NOEC value of >=7.1 mg/l in algae. The available aquatic toxicity data indicate that there are no effects on aquatic organisms at the limit of solubility of the substance in water.

The registration substance hydrolyses rapidly to [2-(perfluorohexyl)ethyl]silanetriol, which is not readily biodegradable, based on read-across data, and is poorly water soluble, which could mean it is likely to persist in the environment. However the log K_ow of the silanetriol is <4 and so the 'safety net' criteria for classification does not apply.

These data are consistent with the following classification under Regulation (EC) No 1272/2008 (as amended) (CLP):

Acute toxicity: Not classified.

Chronic toxicity: Not classified   

Methanol is included in the United Nations Environment Programme (UNEP) Chemicals Branch, OECD SIDS list of harmonised substances and does not require classification for the environment under the CLP Regulation (Regulation (EC) No. 1272/2008).