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EC number: 205-491-7 | CAS number: 141-62-8
The hydrolysis half-life of decamethyltetrasiloxane (L4) is 30.3 d (728 h) at pH 7 and 25°C (measured data). The water solubility of the substance is low (6.7 μg/l) and the log Kow is high (8.21). It is therefore likely that, under the flow-through exposure conditions of the fish and invertebrate tests that the test organisms will have been predominantly exposed to very low concentrations of the registered substance.
In order to reduce animal testing, read-across is proposed to fulfil up to REACH Annex IX 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 since after hydrolysis occurs the resulting product has different physicochemical properties and structure.
In aqueous media, decamethyltetrasiloxane hydrolyses very slowly in water (half-life 728 hours at pH 7 and 25°C).
The registration substance, decamethyltetrasiloxane (L4, CAS 141-62-8) and the substance used as surrogate for read-across are members of the Reconsile Siloxanes Category. Substances in this category tend to have low water solubility, high adsorption and partition coefficients and slow degradation rates in the sediment compartment. For substances with a log Kow of 6 and above no effects are seen with aquatic organisms due to the substance low water solubility limiting the effects seen. In the environment the substances will adsorb to particulate matter and will partition to soil and sediment compartments.
Additional information is given in a supporting report (PFA 2017) attached in Section 13 of the IUCLID 6 dossier.
In the following paragraphs the category read-across approach for decamethyltetrasiloxane is outlined, taking into account structure and physicochemical properties.
Sediment toxicity: Read-across from L3 to L4
The registration substance, L4, and the surrogate substance octamethyltrisiloxane (L3, CAS 107-51-7) are linear siloxanes four silicon atoms and three silicon atoms respectively, alternated by oxygen atoms. In both L3 and L4, the Si atoms are fully methyl substituted. L3 and L4 possess similar physicochemical properties. A comparison of the key physicochemical properties is presented in table 7.0.1 below. Both substances have negligible biodegradability and similar moderate hydrolysis rates.
No effects at the limit of solubility have been reported in short-term and long-term studies in other trophic levels conducted with both substances.
Given the similar properties and structural similarities, it is considered valid to read across sediment toxicity data from L3 to L4.
Table 1: Summary of ecotoxicological and physicochemical properties for the registered substance and the sediment and terrestrial toxicity data surrogate substances.
Si hydrolysis product
Trimethylsilanol and dimethylsilanediol
Note: the properties reported below in rows for ‘silanol hydrolysis product’ relate to trimethylsilanol. For properties of dimethylsilanediol, see data reported for D5 hydrolysis product in next column
Note: the properties reported below in rows for ‘silanol hydrolysis product’ relate to trimethylsilanol. For properties of dimethylsilanediol, see data reported for D5 hydrolysis product in previous column
Molecular weight (parent)
Molecular weight (hydrolysis product)
log Kow(silanol hydrolysis product)
Water sol (parent)
Water sol (silanol hydrolysis product))
995 mg/l (trimethylsilanol)
1.0E+06 mg/l (dimethylsilanediol)
Vapour pressure (parent)
Vapour pressure (hydrolysis product)
1900 Pa (trimethylsilanol)
7 Pa (dimethylsilanediol)
Hydrolysis t1/2at pH 7 and 25°C
Short-term toxicity to fish (LC50)
Short-term toxicity to aquatic invertebrates (EC50)
Algal inhibition (ErC50and NOEC)
ErC50: >2.2 μg/l; NOEC: ≥2.2 μg/l
EC50: >9.4 μg/l; NOEC: ≥9.4 μg/l
Long-term toxicity to fish (NOEC)
Long-term toxicity to aquatic invertebrates (NOEC)
Sediment toxicity (NOEC)
(normalised to 5% OC)
92 mg/kg dwtH. azteca
89 mg/kg dwt ,C. riparius61 mg/kg dwtL. variegatus95 mg/kg dwt, H. azteca
Short-term terrestrial toxicity (L(EC)50)
Long-term terrestrial toxicity (NOEC)
The substance has reliable short-term E(L)C50 values of >6.3 μg/l in fish and >2.2 μg/l in algae. It has reliable NOEC 2.2 μg/l in algae and reliable long-term NOEC of 7.9 μg/l in Daphnia and 6.3 μg/l in fish.
The available short- and long-term aquatic toxicity data indicate that there are no effects on aquatic organisms at the limit of solubility of the substance in water. The substance hydrolyses slowly in water and is not readily biodegradable.
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.
Information on Registered Substances comes from registration dossiers which have been assigned a registration number. The assignment of a registration number does however not guarantee that the information in the dossier is correct or that the dossier is compliant with Regulation (EC) No 1907/2006 (the REACH Regulation). This information has not been reviewed or verified by the Agency or any other authority. The content is subject to change without prior notice.Reproduction or further distribution of this information may be subject to copyright protection. Use of the information without obtaining the permission from the owner(s) of the respective information might violate the rights of the owner.
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