1,1,1,3,3,3-hexamethyldisilazane

Endpoint summary

Additional information

Data for 1,1,1,3,3,3-hexamethyldisilazane (CAS 999-97-3) are used to establish the classification and labelling of the registered substance due to the presence of the ammonia hydrolysis product, which drives toxicity. The PNECs for ammonia are derived based on literature data and the available measured data on trimethylsilanol (CAS 1066-40-6) are used to derive the PNEC for the silanol hydrolysis product. PNECs are not derived for the parent substance due to rapid hydrolysis.

1,1,1,3,3,3-Hexamethyldisilazane

Reliable short-term toxicity test results are available for the registration substance for freshwater fish (Brachydanio rerio), invertebrates (Daphnia magna) and algae (Scenedesmus subspicatus). The relevant short-term values are:

Fish: LC₅₀ (96 h): 88 mg/l (mean measured) (equivalent to 49 mg/l in terms of concentration of the silanol hydrolysis product, trimethylsilanol, and 9.3 mg/l in terms of NH₃),

Daphnia: EC₅₀ (48 h): 80 mg/l (nominal) (equivalent to 45 mg/l in terms of concentration of the silanol hydrolysis product, trimethylsilanol, and 8.4 mg/l in terms of NH₃),

Algae: EC₅₀ (72 h): 50 mg/l; NOEC: 7.5 mg/l (nominal) (equivalent to 28 and 4.2 mg/l respectively in terms of concentration of the silanol hydrolysis product, trimethylsilanol, and equivalent to 5.3 mg/l and 0.8 mg/l respectively in terms of NH₃).

The results from studies conducted with 1,1,1,3,3,3-hexamethyldisilazane have been expressed in terms of the silanol hydrolysis product, trimethylsilanol, using the following molecular weight correction: (MW of silanol = 90.2 / MW of parent = 161.4) * concentration of parent = concentration of trimethylsilanol. The following molecular weight correction was used to determine the concentrations in terms of the other hydrolysis product, ammonia: (MW of ammonia 17.03 / MW parent 161.4) * concentration of parent = concentration of ammonia. 

The very rapid hydrolysis of 1,1,1,3,3,3-hexamethyldisilazane (CAS 999-97-3) (half-life <<1 min at pH 7 and 20-25°C) means it is likely that the test organisms in each test were exposed primarily to the hydrolysis products, trimethylsilanol and ammonia. In the aqueous phase, trimethylsilanol (TMS) can react with itself in a condensation reaction to form hexamethyldisiloxane (HMDS, CAS 107-46-0) and water, which is discussed further below.

Trimethylsilanol (CAS 1066-40-6)

Reliable short-term toxicity tests results are available for freshwater fish (Oncorhynchus mykiss), invertebrates (Daphnia magna) and algae (Selenatrum capricornutum, new name: Pseudokirchneriella subcapitata) with trimethylsilanol (CAS 1066-40-6). The relevant short-term values are:

Fish: LC₅₀ (96 h): 271 mg/l (mean measured)

Daphnia: EC₅₀ (48 h): 124 mg/l (mean measured)

Algae: EC₅₀ (72 h): >684 mg/l; NOEC: 44 mg/l (initial measured)

It should be noted that trimethylsilanol (TMS) in the aqueous phase can react with itself in a condensation reaction to form hexamethyldisiloxane (HMDS, CAS 000107-46-0) and water. In an aqueous solution, an equilibrium is established between TMS (monomer) and HMDS (dimer). The equilibrium of the reaction is in favour of TMS, particularly at low concentrations. The condensation reactions of TMS are reversible unless the concentration of HMDS exceeds its solubility; in this case, HMDS forms a separate phase, driving the equilibrium towards HMDS. At a loading of 100 mg/l, >99.9% TMS is expected. At loadings above about 500-1000 mg/l the concentration of HMDS is predicted to exceed its solubility, resulting in formation of a separate phase.

Hexamethyldisiloxane (HMDS) is classified for the environment as Aquatic Acute 1 and Aquatic Chronic 2 according to Regulation (EC) No 1272/2008. Therefore it is possible that the outcome of the studies with trimethylsilanol were affected by the formation of HMDS in the test system and subsequent exposure of this to test organisms.

The data for trimethylsilanol are considered to be a reliable worst-case scenario.

Ammonia

Ecotoxicity data for ammonia has been reviewed by various regulatory programmes. Toxicity of ammonia is dependent on the degree of ionization and, therefore, is highly pH dependent and also dependent on temperature.

Reliable long and short-term test results are available for ammonia sourced from the Environment Agency Proposed EQS for Water Framework Directive Annex VIII Substances: Ammonia (un-ionized) 2007.

In the short-term tests the most sensitive species to ammonia was found to be fish (pink salmon), with an LC₅₀ of 0.083 mg un-ionised NH₃/l (Rice and Bailey, 1980). Long-term test results also showed fish as the most sensitive species with a NOEC of 0.014 mg un-ionised NH₃/l for rainbow trout (Solbe et al, 1989).

As can be seen from the results from the studies conducted with the parent substance, 1,1,1,3,3,3-hexamethyldisilazane, the toxic effects cannot solely be attributed to the silanol hydrolysis product, trimethylsilanol. The lower effect concentration values are however, in line with the expectation of the toxicity of the other hydrolysis product, ammonia.

Refer to IUCLID Section 6 endpoint summaries (CSR Section 7.0) for further discussion of the approach to chemical safety assessment.