Trichloro(3,3,4,4,5,5,6,6,7,7,8,8,8-tridecafluorooctyl)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:

83.5 mg/L

Assessment factor:

1

Extrapolation method:

assessment factor

Sediment (freshwater)

Hazard assessment conclusion:

PNEC sediment (freshwater)

PNEC value:

0.078 mg/kg sediment dw

Extrapolation method:

equilibrium partitioning method

Sediment (marine water)

Hazard assessment conclusion:

PNEC sediment (marine water)

PNEC value:

0.008 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:

0.014 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 [2-(perfluorohexyl)ethyl]trichlorosilane (CAS 78560-45-9, EC 278-947-6) is approximately 5 seconds at pH 7 and 20-25°C based on read-across data. The substance will hydrolyse in contact with water and atmospheric moisture to [2-(perfluorohexyl)ethyl]silanetriol 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.

[2-(Perfluorohexyl)ethyl]silanetriol has a predicted water solubility of 0.58 mg/l. Results from the short-term aquatic toxicity tests show measured DOC concentrations above this predicted water solubility value. However, this value is subject to some uncertainty and the actual solubility may vary. An uncertainty of ±1.2 log units for the value of log (water solubility / mol/l) has been calculated using the regression statistics. Therefore, the measured DOC concentrations have been used for the calculation of PNEC. As described in Section 4.8 of IUCLID and 1.3 of the CSR, 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 VIII 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 of IUCLID and 1.3 of the CSR.  

The registration 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, [2-(perfluorohexyl)ethyl]triethoxysilane (CAS 51851-37-7, EC 257-473-3), hydrolyses to the same silanol as that of the registration substance, [2-(perfluorohexyl)ethyl]silanetriol. 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 [2-(perfluorohexyl)ethyl]trichlorosilane (CAS 78560-45-9) is assessed for the surrogate substance 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 registration and surrogate substances.

CAS Number	78560-45-9	51851-37-7
Chemical Name	[2-(perfluorohexyl)ethyl]trichlorosilane	[2-(Perfluorohexyl)ethyl]triethoxysilane
Si hydrolysis product	[2-(Perfluorohexyl)ethyl]silanetriol	[2-(Perfluorohexyl)ethyl]silanetriol
Molecular weight (parent)	481.54	510.37
Molecular weight (hydrolysis product)	426.21	426.21
log Kow (parent)	n/a (hydrolysis too rapid to measure parent)	7.2 (QSAR prediction)
log Kow (silanol hydrolysis product)	2.7 (QSAR prediction)	2.7 (QSAR prediction)
Water sol (parent)	n/a (hydrolysis too rapid to measure parent)	<2 mg/l measured, supported by 0.0000064 mg/l (QSAR prediction)
Water sol (silanol hydrolysis product)	0.58 mg/l (QSAR prediction)	0.58 mg/l (QSAR prediction)
Vapour pressure (parent)	30 Pa at 25°C	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	5 seconds (based on read-across data)	12.5 h (20°C)
Hydrolysis t1/2 at pH 4 and 25°C	5 seconds (based on read-across data)	1.44 h (20°C)
Hydrolysis t1/2 at pH 9 and 25°C	5 seconds (based on read-across data)	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 (CAS 51851-37-7) to [2-(perfluorohexyl)ethyl]trichlorosilane (CAS 78560-45-9):

The registration substance, [2-(perfluorohexyl)ethyl]trichlorosilane (CAS 78560-45-9), is a chlorosilane with a fluoroalkyl group attached to the silicon atom. In water under dilute conditions it hydrolyses rapidly (half-life approximately 5 seconds at 20-25°C and pH 7 based on analogue read-across) to [2-(perfluorohexyl)ethyl]silanetriol and hydrochloric acid.

The surrogate substance, [2-(perfluorohexyl)ethyl]triethoxysilane (CAS 51851-37-7), is 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).

The silanol hydrolysis product of the registration substance is the same silanol hydrolysis product as that of the surrogate substance, [2-(perfluorohexyl)ethyl]silanetriol. The parent substances and silanol hydrolysis product 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.

Due to the rapid hydrolysis rate of [2-(perfluorohexyl)ethyl]trichlorosilane the chemical safety assessment is based on its silanol hydrolysis product. Because the registration substance has such a rapid hydrolysis half-life, organisms are unlikely to be exposed to the parent substance. In the tests with the surrogate substance, organisms are likely to have been predominantly exposed to the parent substance. The parent substance is assumed to be more toxic than the hydrolysis products in terms of intrinsic narcosis effects, if expressed. It is therefore considered conservative to read across from [2-(perfluorohexyl)ethyl]triethoxysilane to [2-(perfluorohexyl)ethyl]trichlorosilane. However, 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 with the surrogate substance. 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. In addition, due to the properties of the substance including low solubility and high potential for adsorption, the aquatic compartment may not be the most relevant for chemical safety assessment. Therefore read-across from [2-(perfluorohexyl)ethyl]triethoxysilane to [2-(perfluorohexyl)ethyl]trichlorosilane is considered to be valid and a proposed biodegradation in soil stability study has also been read across from [2-(perfluorohexyl)ethyl]triethoxysilane (CAS 51851-37-7). Therefore read-across from [2-(perfluorohexyl)ethyl]triethoxysilane to [2-(perfluorohexyl)ethyl]trichlorosilane is considered to be valid.

[2-(Perfluorohexyl)ethyl]triethoxysilane is used to read-across to short-term toxicity to fish, invertebrates and algae endpoints. E(L)C₅₀ values of >3.1 mg/l, >9 mg/l and >7.1 mg/l, respectively, have been determined.

See Aquatic toxicity endpoint for further discussion.

Consideration of the non-silanol hydrolysis product hydrochloric acid:

Chloride ions occur naturally (typically at levels 40 – 160 mg/l in environmental fresh waters). Standard test media contain chloride salts at levels equivalent to approximately 20 – 64 mg Cl-/l.

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 No. 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. Physical hazards related to pH effects are considered in the risk management measures (e. g. neutralisation) for effluents/aqueous waste.

It is not appropriate for this substance to discuss the combined ecotoxicological potency of the silicon and non-silicon hydrolysis products because:

•            effects arising from exposure to HCl are related to changes in pH and not true chemical toxicity;

•            silanetriol and monosilicic acid have predicted first dissociation constants around 10 and so do not significantly affect the pH of an aqueous solution.

 

Consideration of the non-silanol hydrolysis product ethanol:

Ethanol is well-characterised in the public domain literature and is not hazardous at the concentrations relevant to the studies; the short-term EC₅₀ and LC₅₀ values are in excess of 1000 mg/l (OECD 2004b - SIDS for ethanol, CAS 64-17-5). Therefore, at the loading rates seen in these tests it is unlikely that the presence of ethanol as a hydrolysis product would significantly affect the results of the tests.

 

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 SIDS (2002) SIDS Initial Assessment Report for SIAM 15, Boston, USA, 22-25th October 2002, Hydrochloric acid, CAS 7647-01-0.

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

PFA, 2013y, Peter Fisk Associates, Analogue Report - Ecotoxicity of substances with ‘low-functionality’ side chains attached to silicon, PFA.300.003.004.