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Additional information

No reliable test data are available for [2-(perfluorohexyl)ethyl]trimethoxysilane (CAS 85857-16-5). Reliable acute aquatic toxicity test results are however available from the read-across substance [2-(perfluorohexyl)ethyl]triethoxysilane (CAS 51851-37-7) for aquatic invertebrates (Daphnia magna) and algae (Scenedesmus subspicatus). The relevant short-term values are:

Daphnia: EC50 (48 h): >9 mg/l; NOEC: ≥9 mg/l (greater than the limit of solubility) (based on mobility) (highest concentration tested) (EC50 >7.5 expressed in terms of concentration of the hydrolysis product, [2-(perfluorohexyl)ethyl]silanetriol).

Algae: EC50 (72 h): >7.1 mg/l; NOEC: ≥7.1 mg/l (greater than the limit of solubility) (based on growth rate) (highest concentration tested) (EC50 >5.9 mg/l and NOEC ≥5.9 mg/l expressed in terms of concentration of the hydrolysis product, [2-(perfluorohexyl)ethyl]silanetriol).

Test solution preparation for the invertebrate and algal tests, for the surrogate substance [2-(perfluorohexyl)ethyl]triethoxysilane, involved making a stock solution and stirring this for 18 hours prior to adding the test organisms. The pH during the invertebrate test was measured as 7.9 to 8.0. The pH at the start of the algae test was 7.6 to 8.0. By the end of the algae test the pH had risen to 9.4 to 9.6. The surrogate substance will hydrolyse in water under dilute conditions, although rates are slightly slower than that of the registration substance (half-life rates of the surrogate substance have been measured at 20°C as 1.44 hours at pH 4, 12.5 hours at pH 7 and 5.18 hours at pH 9). Given that the rate of hydrolysis is expected to be slowest at around pH 7 and increase as the pH is raised or lowered, it is likely that the test organisms will have been exposed to a mixture of [2-(perfluorohexyl)ethyl]triethoxysilane and its silanol hydrolysis product, [2-(perfluorohexyl)ethyl]silanetriol, at the very beginning of the tests. After this time, it is likely that the test organisms were exposed mainly to the hydrolysis products. The amount of silanol present in the test solution is dependent on how much of the parent substance was dissolved. The water solubility of the parent is extremely low and the test results show values above the limit of solubility. Measurements of the test substance in solution were made by dissolved organic carbon analysis (DOC). With DOC it is not possible to differentiate dissolved forms of the test substance and it is therefore unclear how much of the parent substance, silanol hydrolysis product or oligomer were dissolved in the test solution. However, there were no effects seen during the tests and so it can be concluded that no effects were seen at the limit of solubility of the test substance.

The presence of fluorocarbon chains means that the parent substance, silanol and condensation products have the potential for some surface activity. This is proven for the silanol based on measured and read-across data. The effective limits of solubility of both the parent substance and monomeric silanol hydrolysis products are very low because of the tendency to condense to oligomers which are of very low solubility. All forms of the substance may have a tendency to enrich at surfaces rather than forming stable evenly dispersed solutions. These phenomena may limit the extent of hydrolysis under conditions at which the solubility of either parent or oligomer is exceeded. The surrogate substance is extremely difficult to test and so best use has been made of the existing studies. Therefore read-across is considered to be valid.