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EC number: 203-499-5 | CAS number: 107-52-8
- Life Cycle description
- Uses advised against
- Endpoint summary
- Appearance / physical state / colour
- Melting point / freezing point
- Boiling point
- Density
- Particle size distribution (Granulometry)
- Vapour pressure
- Partition coefficient
- Water solubility
- Solubility in organic solvents / fat solubility
- Surface tension
- Flash point
- Auto flammability
- Flammability
- Explosiveness
- Oxidising properties
- Oxidation reduction potential
- Stability in organic solvents and identity of relevant degradation products
- Storage stability and reactivity towards container material
- Stability: thermal, sunlight, metals
- pH
- Dissociation constant
- Viscosity
- Additional physico-chemical information
- Additional physico-chemical properties of nanomaterials
- Nanomaterial agglomeration / aggregation
- Nanomaterial crystalline phase
- Nanomaterial crystallite and grain size
- Nanomaterial aspect ratio / shape
- Nanomaterial specific surface area
- Nanomaterial Zeta potential
- Nanomaterial surface chemistry
- Nanomaterial dustiness
- Nanomaterial porosity
- Nanomaterial pour density
- Nanomaterial photocatalytic activity
- Nanomaterial radical formation potential
- Nanomaterial catalytic activity
- Endpoint summary
- Stability
- Biodegradation
- Bioaccumulation
- Transport and distribution
- Environmental data
- Additional information on environmental fate and behaviour
- Ecotoxicological Summary
- Aquatic toxicity
- Endpoint summary
- Short-term toxicity to fish
- Long-term toxicity to fish
- Short-term toxicity to aquatic invertebrates
- Long-term toxicity to aquatic invertebrates
- Toxicity to aquatic algae and cyanobacteria
- Toxicity to aquatic plants other than algae
- Toxicity to microorganisms
- Endocrine disrupter testing in aquatic vertebrates – in vivo
- Toxicity to other aquatic organisms
- Sediment toxicity
- Terrestrial toxicity
- Biological effects monitoring
- Biotransformation and kinetics
- Additional ecotoxological information
- Toxicological Summary
- Toxicokinetics, metabolism and distribution
- Acute Toxicity
- Irritation / corrosion
- Sensitisation
- Repeated dose toxicity
- Genetic toxicity
- Carcinogenicity
- Toxicity to reproduction
- Specific investigations
- Exposure related observations in humans
- Toxic effects on livestock and pets
- Additional toxicological data
Hydrolysis
Administrative data
Link to relevant study record(s)
- Endpoint:
- hydrolysis
- Type of information:
- (Q)SAR
- Adequacy of study:
- key study
- Reliability:
- 2 (reliable with restrictions)
- Rationale for reliability incl. deficiencies:
- results derived from a valid (Q)SAR model and falling into its applicability domain, with adequate and reliable documentation / justification
- Justification for type of information:
- Please refer to attached justification documents
- Principles of method if other than guideline:
- The result was obtained using an appropriate QSAR method (see attached QMRF and QPRF for details)
The model for hydrolysis at pH 7 has been developed for, and applies specifically to linear and cyclic siloxanes. It is a multiple linear regression based model with descriptors representing (i) ring strain, (ii) number of Si-O bond, and (iii) number of Si-H bond.
The models for hydrolysis at pH 4, 5 and 9 have been developed for, and apply specifically to organosilicon compounds. They are linear regression based models where the descriptor is the half-life at pH 7. - Transformation products:
- yes
- No.:
- #1
- No.:
- #2
- Key result
- pH:
- 4
- DT50:
- 12.1 h
- Remarks on result:
- other: 20-25°C
- Key result
- pH:
- 5
- DT50:
- 5.8 h
- Remarks on result:
- other: 20-25°C
- Key result
- pH:
- 7
- DT50:
- 6 300 h
- Remarks on result:
- other: 20-25°C
- Key result
- pH:
- 9
- DT50:
- 36.5 h
- Remarks on result:
- other: 20-25°C
- Conclusions:
- Hydrolysis half-life values at 20-25°C of 12.1 h at pH 4, 5.8 h at pH 5, 6300 h at pH 7 and 36.5 h at pH 9 were obtained using an accepted calculation method. The result is considered to be reliable.
Reference
Description of key information
Hydrolysis half-life: 6300 h at pH 7, 12.1 h at pH 4, 36.5 h at pH 9 and 20-25°C (QSAR). The stated half-life is for removal of parent. Complete reaction to the ultimate end products will take longer.
Key value for chemical safety assessment
- Half-life for hydrolysis:
- 6 300 h
- at the temperature of:
- 20 °C
Additional information
Hydrolysis half-lives of 12.1 h at pH 4, 5.8 h at pH 5, 6300 h at pH 7 and 36.5 h at pH 9 and 20-25°C were determined for the substance using a validated QSAR estimation method. The result is considered to be reliable and was selected as key study.
Tetradecamethylhexasiloxane (L6) is a linear siloxane chain with six silicon atoms, connected by five oxygen atoms, in which the Si-O bonds are susceptible to hydrolysis. All silicon atoms present are fully substituted with methyl groups. The stated half-life is for removal of the registration substance due to hydrolysis. The products of this reaction are also unstable in water, and so further hydrolysis reactions will follow, the ultimate products being dimethylsilanediol (4 moles) and trimethylsilanol (2 moles) per mole of parent substance.
As the hydrolysis reaction may be acid or base catalysed, the rate of reaction is expected to be slowest at around pH 7 and increase as the pH is raised or lowered. For an acid-base catalysed reaction in buffered solution, the measured rate constant is a linear combination of terms describing contributions from the uncatalysed reaction as well as catalysis by hydronium, hydroxide, and general acids or bases.
kobs= k0+ kH3O+[H3O+] + kOH-[OH-] + ka[acid] + kb[base]
At extremes of pH and under standard hydrolysis test conditions, it is reasonable to suggest that the rate of hydrolysis is dominated by either the hydronium or hydroxide catalysed mechanism.
Therefore, at low pH:
kobs≈kH3O+[H3O+]
At pH 4 [H3O+] = 10-4 mol dm-3 and at pH 2 [H3O+] = 10-2mol dm-3; therefore, kobs at pH 2 should be approximately 100 times greater than kobs at pH 4.
The half-life of a substance at pH 2 is calculated based on:
t1/2(pH 2) = t1/2(pH 4) / 100
The calculated half-life of the substance at pH 2 is therefore 0.12 hours (7.2 minutes).
Reaction rate increases with temperature therefore hydrolysis will be faster at physiologically relevant temperatures compared to standard laboratory conditions. Under ideal conditions, hydrolysis rate can be recalculated according to the equation:
DT50(XºC) = DT50(T) * e(0.08.(T-X))
Where T = temperature for which data are available and X = target temperature.
Thus, for L6 the hydrolysis half-life at 37.5ºC and pH 7 (relevant for lungs and blood) is approximately 1600 hours. At 37.5ºC and pH 2 (relevant for conditions in the stomach following oral exposure), the hydrolysis half- life is calculated as 0.03 hours (1.8 minutes). At 37.5°C and pH 5.5 (relevant for dermal exposure), the hydrolysis half-life is expected to be between the values for pH 5 (1.4 h) and pH 7 (1600 h).
The ultimate products of hydrolysis are dimethylsilanediol and trimethylsilanol.
For the environmental exposures assessment, the parent will be considered as the half-life for hydrolysis of the parent is greater than 12 hours at pH 7.
The hydrolysis half-lives of substances used for read-across in other areas are discussed below
Hydrolysis of the read-across substance Octamethyltrisiloxane (L3, CAS No. 107-51-7)
Data for the substance octamethyltrisiloxane (L3, CAS No. 107-51-7) are read-across to the submission substance tetradecamethylhexasiloxane (L6) for appropriate endpoints (see Section 1.4 of the CSR).The hydrolysis half-lives and the silanol hydrolysis products of the two substances are relevant to this read-across, as discussed in the appropriate Sections of the CSR for each endpoint.
For octamethyltrisiloxane (L3), hydrolysis half-lives at 25°C of 5.09 h at pH 5, 329 h at pH 7 and 9.76 h at pH 9 were determined in accordance with OECD 111 (Dow Corning Corporation, 2007).
The half-lives at pH 4 and 25°C pH 2 and 25°C, at pH 7 and 37.5°C and at pH 2 and 37.5°C may be calculated in the same way as for the registration substance above. This gives a half-life of 2.5 h at pH 4 and 25°C, 0.025 h at pH 2 and 25°C, 33 seconds at pH 2 and 37.5°C and 120 h at pH 7 and 37.5°C.
The ultimate products of hydrolysis are dimethylsilanediol (1 mole) and trimethylsilanol (2 moles).
Hydrolysis of the read-across substance decamethyltetrasiloxane (L4, CAS No. 141-62-8)
Data for the substance decamethyltetrasiloxane (L4, CAS No. 141-62-8) are read-across to the submission substance tetradecamethylhexasiloxane (L6) for appropriate endpoints (see Section 1.4 of the CSR).The hydrolysis half-lives and the silanol hydrolysis products of the two substances are relevant to this read-across, as discussed in the appropriate Sections of the CSR for each endpoint.
For decamethyltetrasiloxane (L4), hydrolysis half-lives at 25°C of 14 h at pH 5, 728 h (30.3 days) at pH 7 and 21.1 h at pH 9 were determined in accordance with OECD 111 (Dow Corning Corporation, 2009). The measured values are supported by predicted hydrolysis half-lives of 3.6 h at pH 4, 2.3 h at pH 5, 630 h at pH 7 and 5.3 h at pH 9 and 20 -25°C using a validated QSAR estimation method.
Thus, for L4 the hydrolysis half-life at 37.5ºC and pH 7 (relevant for lungs and blood) is approximately 270 hours. At 37.5ºC and pH 2 (relevant for conditions in the stomach following oral exposure), the hydrolysis half- life is calculated as 90 seconds. At 37.5°C and pH 5.5 (relevant for dermal exposure), the hydrolysis half-life is expected to be between the values for pH 5 (5 h) and pH 7 (270 h).
The half-lives at pH 4 and 25°C, pH 2 and 25°C, at pH 7 and 37.5°C and at pH 2 and 37.5°C may be calculated in the same way as for the registration substance above. This gives a half-life of 3.6 h at pH 4 and 25°C, 0.036 h at pH 2 and 25°C, 19 seconds at pH 2 and 37.5°C and 270 h at pH 7 and 37.5°C.
The ultimate products of hydrolysis are dimethylsilanediol and trimethylsilanol.
Hydrolysis of the read-across substance dodecamethylpentasiloxane (L5, CAS No. 141-63-9)
No hydrolysis study is available for the dodecamethylpentasiloxane (L5, CAS 141-63-9). However, the hydrolysis half-lives of L5 has been read-across from L4 (CAS No. 141 -62 -8) as stated above.
L4 is a linear siloxane chain with four silicon atoms, connected by three oxygen atoms, in which the Si-O bonds are susceptible to hydrolysis. All silicon atoms present are fully substituted with methyl groups. L5 is a structurally related linear siloxane, with five silicon atoms and four oxygen atoms.
As well as being structural analogues, both siloxanes have consistent physicochemical properties including high molecular weight (310 g/mol and 384 g/mol respectively), very high log Kow (above 8 for both substances) and very low solubility in water (7E-03 mg/l for L4 and 7E-05 mg/l for L5). The substances generally possess similar physicochemical properties. There are no significant steric differences between the Si centres in the two structures. Therefore, the rate of reaction at pH 7 is expected to be approximately the same. The ultimate end products of the hydrolytic reaction, dimethylsilanediol and trimethylsilanol, will be the same for both structures. The stated half-life is for removal of the registration substance due to hydrolysis.
For L5, half-life values 6.6 h at pH 4, 3.6 h at pH 5, approximately 2000 h at pH 7 and 14 h at pH 9 and 20-25°C were obtained using a validated QSAR estimation method.
Thus, for L5 (using the predicted half-lives as a worst case), the half-lives at pH 4 and 25°C, pH 2 and 25°C, at pH 7 and 37.5°C and at pH 2 and 37.5°C may be calculated in the same way as for the registration substance above. This gives a half-life of 0.066 h at pH 2 and 20-25°C, 0.016 h (approximately 60 seconds) at pH 2 and 37.5°C and approximately 490 h at pH 7 and 37.5°C.
The ultimate products of hydrolysis are dimethylsilanediol and trimethylsilanol.
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