Octaphenylcyclotetrasiloxane

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:

no hazard identified

Sediment (freshwater)

Hazard assessment conclusion:

PNEC sediment (freshwater)

PNEC value:

13 mg/kg sediment dw

Assessment factor:

10

Extrapolation method:

assessment factor

Sediment (marine water)

Hazard assessment conclusion:

PNEC sediment (marine water)

PNEC value:

1.3 mg/kg sediment dw

Assessment factor:

100

Extrapolation method:

assessment factor

Hazard for air

Air

Hazard assessment conclusion:

no hazard identified

Hazard for terrestrial organisms

Soil

Hazard assessment conclusion:

no hazard identified

Hazard for predators

Secondary poisoning

Hazard assessment conclusion:

no potential to cause toxic effects if accumulated (in higher organisms) via the food chain

Additional information

Octaphenylcyclotetrasiloxane (CAS No. 546-56-5) has an extremely low estimated water solubility (<1E-05 mg/l at 20°C (insoluble)) and very high estimated log K_ow (9.0 at 20°C). It also has a large molecular size (molecular weight 793.19 g/mol) which can inhibit bioavailability by inhibiting the passage of the molecule through the cell membrane.

 

The substance is susceptible to hydrolysis, however the low water solubility and high adsorption potential mean that hydrolysis in the environment is likely to be slow. The final hydrolysis product is diphenylsilanediol (CAS 947-42-2).

The registration substance is a member 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 K_ow of 6.6 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 2017at) attached in Section 13.

 

On the basis that the substance is insoluble, data waivers are in place for short-term toxicity to fish, aquatic invertebrates and algae. The physicochemical properties of the registration substance indicate that maintaining stable aqueous test solutions will be extremely difficult. Furthermore, the substance will have the tendency to adsorb onto the glassware under experimental conditions. This contributes to technical difficulties making reliable experimental studies very difficult to conduct.

 

In addition:

- Above a log K_ow of 6.0 short-term toxicity effects are not expected to occur, because the very limited aqueous solubility prevents toxic concentrations being achieved in exposure media. Similarly, above log K_ow of 8, long-term toxicity effects are not expected.¹

 

- Regarding environmental exposure: the chemical properties indicate the substance will have the tendency to adsorb onto dissolved organic matter in the natural environment. Therefore, aqueous exposure is unlikely.

 

- The physicochemical properties and very high expected K_oc value means that if the substance were to enter a wastewater treatment plant in influent waste waters, it would partition mainly to sludges (a small proportion may be volatilised), with a negligible amount passing to water. The result is that exposure of freshwater or marine aquatic compartments is negligible.

 

¹The very high log K_ow (9.0) and very low limit of solubility in water mean the substance has very low bioavailability. For substances with these properties, chronic aquatic ecotoxicity is unlikely to be of concern. The principle of log K_ow-based cut-off values for toxic effects has been embraced by the ICCA and OECD high Production Volume (HPV) chemical programmes (e.g. UNEP, 2000). The US EPA has also accepted them in its Pollution Prevention Framework (http://www.epa.gov/oppt/p2framework/docs/casestu.htm). The ECOSAR QSAR applies a log K_ow cut-off value of 8.0 beyond which chronic toxicity of neutral organics would not be expected because of limitations in bioavailability and uptake of the substance (Clements, 1996). These cut-offs are empirically derived and based on experimental test data.

 

READ-ACROSS JUSTIFICATION

In order to reduce testing read-across is proposed to fulfil up to REACH Annex VIII requirements for the registered substance from substances that have similar structure and physicochemical properties in accordance with the approach set out in the Reconsile Siloxanes Category report (PFA, 2017at).

Substances in this category tend to have low water solubilities, high adsorption and partition coefficients and slow degradation. In the environment the substances will adsorb to particulate matter and will partition to soil and sediment compartments.

 

Sediment and terrestrial testing are not an Annex VIII requirement for the registration substance. However, the low water solubility, high log Kow and high log Koc of the substance indicate that it will adsorb to organic matter and partition to sludges in a waste water treatment plant, as mentioned above. As aquatic toxicity tests are not feasible, it may be more appropriate to assess the potential toxicity of the substance in the sediment and soil compartments.

Read-across of terrestrial effects data within the Siloxanes Category

The hypothesis for read across of terrestrial ecotoxicity evidence within the Siloxanes Category is based on structural considerations and physicochemical properties. To fulfil the requirements of REACH, a conservative approach is made by reading across on a nearest-neighbour basis the reliable data within the Category. Terrestrial toxicity data are read-across from the analogous substance, phenyl silsesquioxanes (EC No. 939-487-8).

The read-across substance, phenyl silsesquioxanes, is considered a multiconstituent substance, and is a reaction mass of the monomer, dimer and oligomers of phenyltris(trimethylsiloxy)silane. The linear oligomers of phenyl silsesquioxanes contain a siloxane chain, where the terminal Si atoms are fully methyl substituted, and the repeating unit contains a Si atom substituted with a phenyl and a trimethylsiloxy group. The major constituents comprise 70-80% of the linear oligomers n=1-5; the minor constituents comprise 10-20% cyclic (n=3-7) and linear (n=6-7) oligomers. The combined purity of both linear and cyclic oligomers is about 85-95%. The remaining impurities are higher polymerised material and alkoxy substituted oligomers. The registration substance, octaphenylcyclotetrasiloxane, is a cyclic siloxane, consisting of 4 silicon atoms connected by oxygen atoms; each silicon atom is substituted by two phenyl groups. Both phenyl silsesquioxanes and octaphenylcyclotetrasiloxane are high molecular weight, phenyl-substituted siloxanes, which have a low solubility, high log K_ow and high log K_oc. Both substances have negligible biodegradability and hydrolyse very slowly. The physicochemical parameters of the registration and read-across substance are presented in Table 7.0.1.

Read-across of sediment effects data within the Siloxanes Category

The hypothesis for read across of sediment ecotoxicity evidence within the Siloxanes Category is that no structure-based or property-based pattern is evident from the Category data set of existing studies, although patterns are identifiable associated with extrinsic aspects of test design to which effects may be attributed. A single overall interpretation is made across the Category. To fulfil the requirements of REACH, a conservative approach is made by reading across on a nearest-neighbour basis the reliable data within the Category. For octaphenylcyclotetrasiloxane, the nearest neighbour category members with reliable existing data are considered to be decamethylcyclopentasiloxane (D5) and dodecamethylcyclohexasiloxane (D6) based on molecular weight and log Kow value as well as structural considerations (PFA, 2017at). The physicochemical parameters of the registration and read-across substance are presented in Table 7.0.1.

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_owof 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).

Table7.0.1Physicochemical parameters and ecotoxicity data for the registration and surrogate substances

CAS Number	546-56-5	EC 939-487-8	541-02-6	540-97-6
Chemical Name	Octaphenylcyclotetrasiloxane	Phenyl silsesquioxanes	Decamethylcyclopentasiloxane (D5)	Dodecamethylcyclohexasiloxane (D6)
Si hydrolysis product	diphenylsilanediol	Phenylsilanetriol and trimethylsilanol	Dimethylsilanediol	Dimethylsilanediol
Molecular weight (parent)	793.19	372.8 to 1635.1	370.78	444.93
log Kow(parent)	9.0	9.0	8.0	8.9
Water sol (parent)	<1E-05 mg/l at 20°C (QSAR)	0.0066 mg/l to 4.94E-51 mg/l	0.017 mg/l	0.005 mg/l
Vapour pressure (parent)	<1E-06 Pa at 25°C (QSAR)	0.23 Pa at 20°C	33.2 Pa	5 Pa
Hydrolysis t1/2at pH 7 and 25°C	n/a	630 – >63 000 h	1590 h	>1 year
Short-term toxicity to fish (LC50)	n/a	n/a	>16 µg/l	n/a
Short-term toxicity to aquatic invertebrates (EC50)	n/a	n/a	>2.9 µg/l	n/a
Algal inhibition (ErC50and NOEC)	n/a	n/a	ErC50: >12 μg/l; NOEC: ≥12 μg/l	EC50: >2.0 μg/l; NOEC: ≥2.0 μg/l
Long-term toxicity to fish (NOEC)	n/a	≥0.05 mg/l (nominal); ≥0.11 mg/l (mean measured)	≥14 μg/l	n/a
Long-term toxicity to aquatic invertebrates (NOEC)	n/a	≥980 mg/l (loading rate WAF); ≥0.00605 mg/l (geometric mean measured)	≥15 µg/l	≥4.6 µg/l
Long-term sediment toxicity (NOEC)	n/a	n/a	70 mg/kg dwt,C. riparius ≥1272 mg/kg dwtL. variegatus 130 mg/kg dwt,H. azteca	≥420 mg/kg dwt, L. variegatus; ≥620 mg/kg dwt, C. riparius
Short-term terrestrial toxicity (L/EC50)	n/a	n/a	(IC50) 2050 mg/kg dwt,H. vulgare; >4054 mg/kg dwt, T. pratense	n/a
Long-term terrestrial toxicity (NOEC)	n/a	≥1000 mg/kg dwt, Eisenia fetida; EC25 and EC50 >100 mg/kg dwt, soil microorganisms	≥4074 mg/kg dwt, E. Andrei 377 mg/kg dwt, F. candida	>1000 mg/kg dw,Eisenia fetida; >100 mg/kg dw, soil microflora

 

 

 

References:

Clements (1996). Estimating toxicity of industrial chemicals to aquatic organisms using structure-activity relationships. Edited by: Richard G. Clements. Contributors: R.G. Clements, J.V. Nabholz, M. Zeeman, Environmental Effects Branch, Health and Environmental Review Division, Office of Pollution Prevention and Toxics, U.S. Environmental Protection Agency, Washington, DC 20460, August 30, 1996.

 

UNEP (2000). OECD SIDS Alfa Olefins (CAS N°:592-41-6, 111-66-0, 872-05-9,112-41-4, 1120-36-1). UNEP Publications.

Conclusion on classification

The substance has no reliable ecotoxicity data: testing is waived due to the insolubility of the test substance meaning it is technically not feasible to perform the studies and toxic effects are not likely to be expressed at the limit of solubility.

For low solubility, highly adsorbing substances, the safety net classification Category Chronic 4 may apply. However, in this instance the bioavailability of the substance is expected to be limited by the large molecular size (molecular weight 793.19) inhibiting the passage of the molecule through the cell membrane. For the purposes of assessment, the log K_ow of the substance is limited to 9.0, however it is highly likely that the log K_ow >10. The very high log K_ow and very low limit of solubility in water mean the substance has very low bioavailability. For substances with these properties, chronic aquatic ecotoxicity is unlikely to be of concern.

An aquatic / sediment steady-state BCF value of 284 l/kg has been read across from a structural analogue. The substance does not meet the screening criteria nor the definitive criteria for bioaccumulation.

 

It is therefore proposed that octaphenylcyclotetrasiloxane (CAS 546-56-5) should not be classified in the EU for acute or chronic toxicity.

 

According to Regulation (EC) No 1272/2008 (CLP):

Aquatic Acute: Not classified

Aquatic Chronic: Not Classified