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Please be aware that this old REACH registration data factsheet is no longer maintained; it remains frozen as of 19th May 2023.

The new ECHA CHEM database has been released by ECHA, and it now contains all REACH registration data. There are more details on the transition of ECHA's published data to ECHA CHEM here.

Diss Factsheets

Administrative data

Hazard for aquatic organisms

Freshwater

Hazard assessment conclusion:
no hazard identified

Marine water

Hazard assessment conclusion:
no hazard identified

STP

Hazard assessment conclusion:
PNEC STP
PNEC value:
10 mg/L
Assessment factor:
100
Extrapolation method:
assessment factor

Sediment (freshwater)

Hazard assessment conclusion:
no hazard identified

Sediment (marine water)

Hazard assessment conclusion:
no hazard identified

Hazard for air

Air

Hazard assessment conclusion:
no hazard identified

Hazard for terrestrial organisms

Soil

Hazard assessment conclusion:
PNEC soil
PNEC value:
3.77 mg/kg soil dw
Assessment factor:
100
Extrapolation method:
assessment factor

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

The hydrolysis half-life of 1,1,1,3,5,5,5-heptamethyl-3-octyltrisiloxane (CAS 17955-88-3) is >13.7 days at pH 7 and 25⁰C. The water solubility of the substance is low (0.000028 mg/l, QSAR prediction) and the log Kow is high (9.0 at 20⁰C, QSAR prediction).

 

READ-ACROSS JUSTIFICATION

In order to reduce 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, 1,1,1,3,5,5,5-heptamethyl-3-octyltrisiloxane hydrolyses slowly in water (half-life >13.7 days at pH 7 and 25°C), therefore the chemical safety and hazard assessment will focus on the properties of the parent substance.

 

1,1,1,3,5,5,5 -Heptamethyl-3-octyltrisiloxane and the substances used as surrogate for read-across are members of the Reconsile Siloxanes Category. 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.

For substances with a log Kow of 8 and above no long-term toxicity effects are seen with aquatic organisms due to the substance low water solubility limiting the effects seen.

Data from several substances have been read across to the registered substance. Results from all the substances are consistent and there are no effects reported at the limit of solubility.

 

In the following paragraphs the category read-across approach for 1,1,1,3,5,5,5-heptamethyl-3-octyltrisiloxane is outlined, taking into account structure, hydrolysis rate and physico-chemical properties.

 

Additional information including application of the Read-Across Assessment Framework (RAAF) are found in the siloxanes category supporting report (PFA 2017at) attached in Section 13 of the IUCLID dossier.

 

Read-across from L4:

The registered substance, 1,1,1,3,5,5,5-heptamethyl-3-octyltrisiloxane (CAS 17955-88-3) and the surrogate substance decamethyltetrasiloxane (L4, CAS 141-62-8) are linear siloxanes with 3 and 4 silicon atoms respectively. 1,1,1,3,5,5,5-Heptamethyl-3-octyltrisiloxane and L4 possess similar physico-chemical properties: high molecular weight (334.73 g/mol and 310.69 g/mol, respectively), low water solubility (both insoluble, at 2.8E-05 mg/l and 6.7E-03 mg/l, respectively), high log Kow (9.0 at 20°C and 8.21 at 25.1°C, respectively) and high log Koc (6.0 and 5.16, respectively). Both substances have negligible biodegradability and slow hydrolysis rates at environmentally relevant temperature and pH.

 

No effects have been reported at the limit of solubility in short-term and long-term studies in fish, Daphnia and algae conducted with L4. Given the similar properties and structural similarities, it is considered valid to read-across data from decamethyltetrasiloxane (L4) to 1,1,1,3,5,5,5-heptamethyl-3-octyltrisiloxane.

 

L4 is used to read-across to the long-term toxicity to fish endpoint. A 60-day NOEC value of ≥7.9 µg/l for fish has been determined. Sediment data have also been read-across. No effects were reported.

 

Read-across from L5:

The registration substance, 1,1,1,3,5,5,5-heptamethyl-3-octyltrisiloxane (CAS 17955-88-3) and the surrogate substance dodecamethylpentasiloxane (L5, CAS 141-63-9) are linear siloxanes with 3 and 5 silicon atoms respectively,. 1,1,1,3,5,5,5 -Heptamethyl-3-octyltrisiloxane and L5 possess similar physico-chemical properties: high molecular weight (334.73 and 384.85 g/mol, respectively), low water solubility (both insoluble, at 2.8E-05 mg/l and 7.0E-05 mg/l, respectively), high log Kow (9.0 at 20°C and 9.41 at 25°C, respectively) and high log Koc (6.0 and 6.3, respectively). Both substances have negligible biodegradability and similar slow hydrolysis rates at environmentally relevant temperature and pH.

 

No effects have been reported at the limit of solubility in short-term and long-term studies in fish, Daphnia and algae conducted with L5. Given the similar properties and structural similarities, it is considered valid to read-across data from dodecamethylpentasiloxane (L5) to 1,1,1,3,5,5,5-heptamethyl-3-octyltrisiloxane.

 

L5 is used to read-across to the long-term toxicity to invertebrates endpoint. A 21 day EC50 value of >47 ng/l and NOEC value of ≥47 ng/l have been determined.

Read-across from 3-hexyl heptamethyltrisiloxane (CAS 1873-90-1) to 1,1,1,3,5,5,5-heptamethyl-3-octyltrisiloxane (CAS 17955-88-3).

The registered substance, 1,1,1,3,5,5,5-heptamethyl-3-octyltrisiloxane (CAS 17955-88-3) and the surrogate substance 3-hexyl heptamethyltrisiloxane (CAS 1873-90-1) have a high degree of structural similarity. Both substances are linear siloxanes with three silicon atoms linked by two oxygen atoms. Each silicon atom is fully substituted by methyl groups, except for the central silicon atom which has a longer alkyl chain in place of one methyl group. This longer alkyl chain is octyl- for the target substance and hexyl- for the source substance. 1,1,1,3,5,5,5-Heptamethyl-3-octyltrisiloxane (CAS 17955-88-3) and 3-hexyl heptamethyltrisiloxane (CAS 1873-90-1) possess very similar physico-chemical properties: high molecular weight (334.73 g/mol and 306.66 g/mol, respectively), low water solubility (both insoluble, at 2.8E-05 mg/l and 3.0E-4 mg/l (QSAR), respectively), high log Kow (both 9.0 (QSAR)) and high log Koc (both 6.0). Both substances have negligible biodegradability and slow hydrolysis rates at environmentally relevant temperature and pH.

 

No effects have been reported at the limit of solubility in short-term studies in fish, Daphnia, algae and microorganisms conducted with 3-hexyl heptamethyltrisiloxane (CAS 1873-90-1). Given the similar properties and structural similarities, it is considered valid to read-across data from 3-hexyl heptamethyltrisiloxane to 1,1,1,3,5,5,5-heptamethyl-3-octyltrisiloxane.

 

3-Hexyl heptamethyltrisiloxane is used to read-across to short-term toxicity to fish and invertebrates and toxicity to algae and microorganisms endpoints. A 96-hour EC50 value of >49 μg/l for fish, 48-hour EC50 value of >41 μg/l for invertebrates, 72-hour ErC50 value of >21.6 μg/l and NOEC value of ≥21.6 μg/l for algae, and 3-hour EC50 value of >1000 mg/l for microorganisms, have been determined.  

 

Read-across from D5 (Terrestrial toxicity – Interim hazard and risk assessment)

The registration substance, 1,1,1,3,5,5,5-heptamethyl-3-octyltrisiloxane (CAS 17955-88-3) and the surrogate substance decamethylcyclopentasiloxane (D5, CAS 556-67-2) are siloxanes with 3 and 5 silicon atoms, respectively, having methyl branches and alternated by oxygen. The registered substance is a linear siloxane while the surrogate substance is a cyclic siloxane.1,1,1,3,5,5,5-Heptamethyl-3-octyltrisiloxane and D5 have similar physicochemical properties: high molecular weight (335 and 370 g/mol, respectively), low solubility (both insoluble at 2.8E-05 mg/l and 1.7E-03 mg/l, respectively), high log Kow (9.0 and 8.0, respectively) and high log Koc (6.0 and 6.03, respectively). Both substances have negligible biodegradability and hydrolysis rates. A comparison of the key physicochemical properties is presented in the table below.

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 data from D5 for an interim hazard and risk assessment.

The registered substance and the surrogate substance, D5, share similar physicochemical properties but are not close structural analogues (linear and cyclic siloxanes).

D5 is used to read-across to the toxicity to soil macroorganisms except arthropods, terrestrial arthropods and terrestrial plants endpoints. The lowest long-term result is for Folsomia candida, with a NOEC value of 377 mg/kg dry weight. This value is used to derive PNECsoil.

 

Table: Summary of aquatic ecotoxicological and physico-chemical properties for the registered substance, its hydrolysis products and the surrogate substances.

CAS Number

17955-88-3

141-63-9

141-62-8

  1873-90-1  

541-02-6- Interim approach

 

Chemical Name

1,1,1,3,5,5,5-heptamethyl-3-octyltrisiloxane

Dodecamethylpentasiloxane (L5)

Decamethyltetrasiloxane (L4)

 3-hexyl heptamethyltrisiloxane

Decamethylcyclopentasiloxane (D5)

Si hydrolysis product

Octylmethylsilanediol and Trimethylsilanol

(Note: the properties reported below in rows for ‘silanol hydrolysis product’ relate to octylmethylsilanediol. For properties of trimethylsilanol, see column for L4).

Dimethylsilanediol and trimethylsilanol

(Note: the properties reported below in rows for ‘silanol hydrolysis product’ relate to dimethylsilanediol. For properties of trimethylsilanol, see column for L4).

Dimethylsilanediol and Trimethylsilanol

(Note: the properties reported below in rows for ‘silanol hydrolysis product’ relate to trimethylsilanol. For properties of dimethylsilanediol, see column for L5).

 Trimethylsilanol and hexyl(methyl)silanediol  

(Note: the properties reported below in rows for ‘silanol hydrolysis product’ relate to hexyl(methyl)silanediol. For properties of trimethylsilanol, see column for L4).

Dimethylsilanediol

 

Molecular weight (parent) g/mol

334.73

384.85

310.69

306.66

370.78

 

Molecular weight (hydrolysis product) g/mol

190.36

92.2

90.2

   no data

92.2

log Kow (parent)

9.0

9.41

8.21

  9.2 (limited to 9) (QSAR)

8.0

log Kow (silanol hydrolysis product)

3

-0.4

1.2

   no data

-0.4

 

Log Koc (parent)

6.0

6.3 (QSAR)

5.16

  6.0 (QSAR)

6.03

Water sol (parent)

2.8E-05 mg/l at 20°C

7.0E-05 mg/l at 23°C

6.7E-03 mg/l at 23°C

  3.0E-04 mg/l (QSAR)

0.017 mg/l

Water sol (silanol hydrolysis product)

4.86E+02 mg/l at 20°C

1.0E+06 mg/l at 20°C

995 mg/l

   no data

1.0E+06 mg/l at 20°C

 

Vapour pressure (parent)

 0.64 Pa at 25°C (QSAR prediction)

7.8 Pa at 25°C (QSAR prediction)

73 Pa at 25°C

  2 Pa (QSAR)

33.2 Pa

Vapour pressure (hydrolysis product) at 25°C

1.4E-02 Pa at 25°C

7 Pa at 25°C

1900 Pa

   no data

7 Pa at 25°C

Hydrolysis t1/2 at pH 7

>13.7 d (>329 h) at 20-25⁰C

30.3 d (728 h) at 25°C

30.3 d (728 h) at 25°C

 >13.7 d (>329 h) at 20-25⁰C

66 d (1590 h)  

at 25°C

Short-term toxicity to fish (LC50)

>110 mg/L

>75 ng/l

>6.3 μg/l

 >49 μg/l n/a

Short-term toxicity to aquatic invertebrates (EC50)

n/a

n/a

n/a

 >41 μg/l   n/a

Algal inhibition (ErC50 and NOEC)

n/a

n/a

EC50: >2.2 μg/l; NOEC: ≥2.2 μg/l

EC50: >21.6 μg/l;

NOEC: ≥21.6 μg/l 

ErC50: >2.0 μg/l; NOEC: ≥2.0 μg/l

Long-term toxicity to fish (NOEC)

n/a

≥39 ng/l (bioconcentration study as supporting data only - no reliable LT fish data available)

≥7.9 μg/l

  no data

≥4.4 μg/l

Long-term toxicity to aquatic invertebrates (NOEC)

n/a

≥47 ng/l

≥4.9 μg/l

  no data  

≥4.6 µg/l

 

Sediment toxicity (NOEC) (normalised to 5% OC)

n/a

n/a

92 mg/kg dwt, Hyalella azteca

  no data

110 mg/kg dwt, C. riparius;
≥1719 mg/kg dwt, L. variegatus;
135 mg/kg dwt, H. azteca

 

Short-term terrestrial toxicity (L(E)C50)

n/a

n/a

n/a

  no data

(IC50) 209 mg/kg dwt, H. vulgare;
>4054 mg/kg dwt,
T. pratense

 

Long-term terrestrial toxicity (NOEC)

n/a

n/a

n/a

  no data

≥4074 mg/kg dwt,
E. andrei
377 mg/kg dwt,
F. candida

 

Conclusion on classification

The substance has short-term E(L)C50 values of >110 mg/l in fish based on measured data, >0.041 mg/l in invertebrates and >0.0216 mg/l in algae based on read-across from structural analogues. It has a reliable NOEC of ≥0.0216 mg/l in algae and reliable long-term NOECs of ≥0.000047 mg/l in Daphnia and ≥0.0079 mg/l in fish, based on read-across from structural analogues.

 

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.