Octaphenylcyclotetrasiloxane

Toxicity to microorganisms

Description of key information

Toxicity to microorganisms: 3 h EC₅₀ >1000 mg/l; NOEC >=1000 mg/l (OECD 209)

Key value for chemical safety assessment

EC50 for microorganisms:

1 000 mg/L

EC10 or NOEC for microorganisms:

1 000 mg/L

Additional information

There are no microorganism toxicity data available for octaphenylcyclotetrasiloxane, therefore good quality data for the structurally-related substance phenyl silsesquioxanes (CAS 70131-69-0), have been read across.

Octaphenylcyclotetrasiloxane (CAS No. 546-56-5) and phenyl silsesquioxanes (CAS No. 70131-69-0) are structurally-similar substances, both are siloxanes with phenyl functionality. Octaphenylcyclotetrasiloxane is a cyclic tetrasiloxane with two phenyl substituents at each silicon. Phenyl silsesquioxanes is a reaction mass of the monomer, dimer and oligomers of phenyltris(trimethylsiloxy)silane. Phenyltris(trimethylsiloxy)silane is a branched tetrasiloxane with a phenyl substituent on the central silicon atom and three methyl groups on the other three silicons. The other major constituents of phenyl silsesquioxanes are also branched siloxanes with phenyl and methyl substituents.The target substance and all constituents of the source substance have a very low water solubility, very high low K_ow and log Koc, high molecular weight and very low vapour pressure.

Both octaphenylcyclotetrasiloxane and silsesquioxanes, phenyl are within the Reconsile Siloxanes Category. Substances in this category tend to have low water solubility, high adsorption and partition coefficients and slow degradation rates, and category member substances exhibit no evidence of significant toxicity to sewage treatment plant microorganisms.

The table below presents microorganism toxicity data for the siloxanes category member substances. It is therefore considered valid to read-across the results for phenyl silsesquioxanes to fill the data gap for the registered substance. Additional information is given in a supporting report (PFA, 2017) attached in Section 13.

 

Table: Microorganism toxicity data for substances within the siloxanes category

CAS	Name	Result: E(I) C50(mg/l)	Result: NOEC (or EC10/ EC20) (mg/l)	Guideline Number	Duration (h)	Reliability
540-97-6	Dodecamethylcyclohexasiloxane	>100 mg/l		OECD 209	3	1
541-05-9	Hexamethylcyclotrisiloxane	>100 mg/l	>100 mg/l	OECD 209	3	1
541-02-6	Decamethylcyclopentasiloxane	>2000 mg/l		C11	3	1
556-67-2	Octamethylcyclotetrasiloxane	>10 000 mg/l		ISO 8192 (Test for Inhibition of Oxygen Consumption by Activated Sludge)	3	4 (WoE)
556-67-2	Octamethylcyclotetrasiloxane	little or no toxicity (24 h): 0.1 g/g solution test mat. based on: reduction in number of viable bacteria		Toxicity toEscherichia coliandStaphylococcus aureusat 37°C for 24 hours	24	4 (WoE)
107-46-0	Hexamethyldisiloxane	>100	≥100	OECD 209	3	1
141-62-8	Decamethyltetrasiloxane	>100 mg/l		OECD 209	3	1
352230-22-9	Siloxanes and Silicones, di-Ph, Ph (trimethylsilyl) oxy	>1000 mg/l	≥1000 mg/l	OECD 209	3	1
2627-95-4	1,1,3,3-Tetramethyl-1,3-divinyldisiloxane	>1000 mg/l	≥1000 mg/l Nitrification control indicated no heterotrophic inhibition of respiration rate	OECD 209	3	1
2554-06-5	2,4,6,8-Tetramethyl-2,4,6,8-tetravinylcyclotetrasiloxane	>1000 mg/l	≥1000 mg/l Nitrification control indicated no heterotrophic inhibition of respiration rate	OECD 209	3	1
70131-69-0	Silsesquioxanes, Phenyl	>1000	≥1000 Nitrification control indicated no heterotrophic inhibition of respiration rate	OECD 209	3	1

 

Consideration of low functionality side chains and reactive groups

Phenyl group: The ecotoxicity of phenyls is related to non-polar narcosis. The ecotoxicity of substances with phenyl groups increases with increasing log K_ow. For example, trichlorophenylsilane (CAS 98-13-5) has a very fast hydrolysis rate and the log K_ow of the hydrolysis product is -0.2. Short-term toxicity data with trichlorophenylsilane determine no effects up to 100 mg/l (Springborn Smithers 2009a, b and c).

Conclusion

In the study with phenyl silsesquioxanes, an activated sludge respiration inhibition 3 hour NOEC value of ≥1000 mg/l (based on loading rate) was reported.