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Hydrolysis

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Endpoint:
hydrolysis
Type of information:
(Q)SAR
Adequacy of study:
supporting study
Reliability:
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
results derived from a valid (Q)SAR model, but not (completely) falling into its applicability domain, with adequate and reliable documentation / justification
Remarks:
Using expert judgement, the substance was found to fall in the applicability domain of the model as defined in the corresponding QMRF because: i. The descriptor values for the substance fall within the descriptor ranges defined in Section 5.1 of the QMRF (Strain = 0.083 – 0.167; Si-O = 2 – 12; Si-H = 0 – 5; Observed t1/2 at pH 7 = 0.037 h – 9600 h; Predicted t1/2 at pH 7 = 0.018 h – 19700 h). ii. The structural type is represented in the training set. iii. The side-chain type or a related side-chain type is represented in the training set. The side-chain types represented in the training set include hydrogen, methyl and vinyl groups. Phenyl groups are not represented. The very low water solubility and high adsorption potential (high log Kow) of the constituents mean that hydrolysis in the environment and in vivo is likely to be very slow. However, the predictions are considered sufficient to indicate that the hydrolysis half-lives of this constituent is long and is therefore assigned reliability 2.
Justification for type of information:
See attached QMRFs/QPRFs
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
pH:
4
DT50:
> 1.9 h
Remarks on result:
other: 20-25°C
pH:
7
DT50:
> 200 h
Remarks on result:
other: 20-25°C
pH:
9
DT50:
> 2 h
Remarks on result:
other: 20-25°C
Conclusions:
Hydrolysis half-life values of >1.9 h at pH 4, >200 h at pH 7 and >2.0 h at pH 9 and 20-25°C were obtained using an accepted calculation method. The result is considered to be reliable.
Endpoint:
hydrolysis
Type of information:
(Q)SAR
Adequacy of study:
supporting study
Reliability:
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
results derived from a valid (Q)SAR model, but not (completely) falling into its applicability domain, with adequate and reliable documentation / justification
Remarks:
Using expert judgement, the substance was found to fall in the applicability domain of the model as defined in the corresponding QMRF because: i. The descriptor values for the substance fall within the descriptor ranges defined in Section 5.1 of the QMRF (Strain = 0.083 – 0.167; Si-O = 2 – 12; Si-H = 0 – 5; Observed t1/2 at pH 7 = 0.037 h – 9600 h; Predicted t1/2 at pH 7 = 0.018 h – 19700 h). ii. The structural type is represented in the training set. iii. The side-chain type or a related side-chain type is represented in the training set. The side-chain types represented in the training set include hydrogen, methyl and vinyl groups. Phenyl groups are not represented. The very low water solubility and high adsorption potential (high log Kow) of the constituents mean that hydrolysis in the environment and in vivo is likely to be very slow. However, the predictions are considered sufficient to indicate that the hydrolysis half-lives of this constituent is long and is therefore assigned reliability 2.
Justification for type of information:
See attached QMRFs/QPRFs
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.
pH:
4
DT50:
> 3.6 h
Remarks on result:
other: 20-25°C
pH:
7
DT50:
> 630 h
Remarks on result:
other: 20-25°C
pH:
9
DT50:
> 5.3 h
Remarks on result:
other: 20-25°C
Conclusions:
Hydrolysis half-life values at 20-25°C of >3.6 h at pH 4, >630 h at pH 7 and >5.3 h at pH 9 and 20-25°C were obtained using an accepted calculation method. The result is considered to be reliable.
Endpoint:
hydrolysis
Data waiving:
study technically not feasible
Justification for data waiving:
the study does not need to be conducted because the substance is highly insoluble in water
Endpoint:
hydrolysis
Type of information:
experimental study
Adequacy of study:
supporting study
Study period:
2004-07-09 to 2004-07-13
Reliability:
1 (reliable without restriction)
Rationale for reliability incl. deficiencies:
guideline study
Qualifier:
according to
Guideline:
OECD Guideline 111 (Hydrolysis as a Function of pH)
Qualifier:
according to
Guideline:
EU Method C.7 (Degradation: Abiotic Degradation: Hydrolysis as a Function of pH)
GLP compliance:
yes
Radiolabelling:
no
Analytical monitoring:
yes
Buffers:
Buffer pH 4, Biphthalate
Buffer pH 7, Phosphate
Buffer pH 9, Borate

The buffer solutions were sterilised for 25 minutes in an autoclave prior to first use. Nitrogen was passed through the buffer solutions for 2 minutes
Transformation products:
not measured
pH:
4
Remarks on result:
not determinable because of methodological limitations
Remarks:
Due to the very low solubility of the test item in water, no further testing could be performed
pH:
7
Remarks on result:
not determinable because of methodological limitations
Remarks:
Due to the very low solubility of the test item in water, no further testing could be performed
pH:
9
Remarks on result:
not determinable because of methodological limitations
Remarks:
Due to the very low solubility of the test item in water, no further testing could be performed
Details on results:
The solubility of the test item in the buffer solutions at pH 4, pH 7 and pH 9 was very low. It was not possible to increase the solubility of the test item with the use of different solubilisers (acetonitrile and acetone). Peaks obtained, if any were too small to allow quantification or even to follow a degradation curve.

The test item shows no significant solubility in the different solvent systems. Therefore, no further testing could be performed with the test item at pH 4, pH 7 and pH 9.

A typical chromatogram of the smallest calibration point is given in Figure 1 (attached). Typical chromatograms of the pH 4, pH 7 and pH 9 test solutions with solubiliser and the respective buffer blanks are given in Figures 2 to 7 (attached). The linearity of the GC system was checked in the concentration range from 0.2616 µg/mL to 10.4225 µg/mL. The r2 fit was 1.000 (optimum).
Conclusions:
No hydrolysis half-lives at pH 4, pH 7 and pH 9 were determined due to the very low solubility of the test substance in water.

Description of key information

Hydrolysis half-life:

The stated half-lives below is for removal of parent. Complete reaction to the ultimate end products will take longer.

Constituent 1: >1.9 h at pH 4, >200 h at pH 7 and >2.0 h at pH 9 and 20-25°C.

Constituent 2: >3.6 h at pH 4, >630 h at pH 7 and >5.3 h at pH 9 and 20-25°C

Key value for chemical safety assessment

Additional information

The requirement to conduct a hydrolysis study for the substance is waived in accordance with Column 2 of REACH Annex VIII because the substance is highly insoluble in water. The constituents of the substance have predicted water solubility values of 2.8E-05 mg/L and 1.5E-10 mg/L for Constituent 1 and Constituent 2 respectively. In Weissenfield 2004, an attempt was made to conduct a hydrolysis study for the submission substance based on OECD Test Guideline 111; due to the very low solubility of the substance in the buffer solutions, it was not possible to increase the solubility of the test item even after the use of different solubilisers such as acetone and acetonitrile. The peaks obtained were too small to allow quantification or follow a degradation curve. Therefore, the hydrolysis study could not be conducted further.

The constituents of the submission substance are siloxanes with three to four silicon atoms connected by oxygen atoms. The Si-O bonds are susceptible to hydrolysis.

The hydrolysis half-lives for the constituents of the substance have been predicted using validated QSAR methods developed for siloxanes.

The constituents of the substance are hydrolytically unstable with a half-life for siloxane degradation of >200 hours at pH 7 and 20-25°C for Constituent 1 and >630 hours at pH 7 and 20-25°C for Constituent 2. The side-chain types represented in the QMRF training set include hydrogen, methyl and vinyl groups. Phenyl groups are not represented. The very low water solubility and high adsorption potential (high log Kow) of the constituents mean that hydrolysis in the environment and in vivo is likely to be very slow. The QSAR predictions are considered sufficient to indicate that the hydrolysis half-lives of these constituents are long, and hence the half-lives are reported as greater than values.

The reaction rate is faster under acidic or basic conditions and half-lives at 20-25°C of >1.9 hours and >3.6 hours at pH 4 and >2.0 hours and >5.3 hours at pH 9 are predicted for hydrolysis of the siloxane bond in Constituent 1 and Constituent 2 respectively. These half-lives refer to the hydrolysis of the first siloxane bond; complete hydrolysis to the ultimate hydrolysis products will take longer.

The ultimate hydrolysis products of the reaction for both constituents are diphenylsilanediol, CAS No. 947-42-2 (1 mole for Constituent 1, 2 moles for Constituent 2) and trimethylsilanol, CAS No. 1066-40-6 (2 moles for both constituents).

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 uncatalyzed reaction as well as catalysis by hydronium, hydroxide, and general acids or bases.

kobs= k0+ kH3O+[H3O+] + kOH-[OH-] + ka[acid] + kb[base]

The hydrolysis half-life of other substances used as read-across in other areas are discussed below:

Hydrolysis of the read-across substance 1,3,5-trimethyl-1,1,3,5,5-pentaphenyltrisiloxane (CAS No. 3390-61-2)

Data for the substance 1,3,5-trimethyl-1,1,3,5,5-pentaphenyltrisiloxane(CAS No. 3390-61-2) are read across to the submission substance for appropriate endpoints. The hydrolysis half-lives of the two substances are relevant to this read-across, as discussed in the appropriate sections for each endpoint.

For 1,3,5-trimethyl-1,1,3,5,5-pentaphenyltrisiloxane, hydrolysis half-lives at 20-25°C of >2.0 hours at pH 4, >200 hours at pH 7 and >2.0 hours at pH 9 were determined using a validated QSAR estimation method.

The ultimate products of hydrolysis are methyl(diphenyl)silanol and methyl(phenyl)silanediol.

Hydrolysis of the read-across substance Silsequioxanes, Phenyl (CAS No. 70131-69-0)

Data for the substance Silsesquioxanes, Phenyl (CAS No. 70131-69-0) are read-across to the submission substance for appropriate endpoints. The hydrolysis half-lives of the two substances are relevant to this read-across, as discussed in the appropriate sections for each endpoint.

For Silsequioxanes, Phenyl, hydrolysis half-lives of 1.9 -3.3 h at pH 4, 200 - 630 h at pH 7 and 2.0 - 5.3 h at pH 9 and 20-25°C are calculated for this substance.

Hydrolysis of the read-across substance 1,1,1,5,5,5-hexamethyl-3-phenyl-3-[(trimethylsilyl)oxy]trisiloxane (CAS No. 2116-84-9)

Data for the substance 1,1,1,5,5,5-hexamethyl-3-phenyl-3-[(trimethylsilyl)oxy]trisiloxane (CAS No. 2116-84-9) are read across to the submission substance for appropriate endpoints.

For 1,1,1,5,5,5-hexamethyl-3-phenyl-3-[(trimethylsilyl)oxy]trisiloxane, hydrolysis half-lives of 1.9 -3.6 h at pH 4, 200 - 630 h at pH 7 and 2.0 -5.3 h at pH 9 and 20-25°C were determined for the substance using a validated QSAR estimation method. The stated half-life is for removal of the 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 phenylsilanetriol (1 mole) and trimethylsilanol (3 moles) per mole of parent substance.

Hydrolysis of the read-across substance decamethylcyclopentaxasiloxane (CAS No. 541-02-6)

Data for the substance decamethylcyclopentaxasiloxane (CAS No. 541-02-6) are read-across to the submission substance for appropriate endpoints. The slow rate of hydrolysis of the two substances is relevant to this read-across.

For decamethylcyclopentasiloxane, hydrolysis half-lives at 25°C of 9.3 h at pH 4, 1590 h at pH 7 and 24.8 – 31.6 h at pH 9 were determined in accordance with OECD 111 (Dow Corning Corporation, 2006). 

The ultimate products of hydrolysis are dimethylsilanediol and trimethylsilanol. 

Hydrolysis of the read-across substance dodecamethylcyclohexasiloxane (CAS No. 540-97-6)

Data for the substance dodecamethylcyclohexasiloxanee (CAS No. 540-97-6) are read-across to the submission substance for appropriate endpoints. The slow rate of hydrolysis of the two substances is relevant to this read-across.

For dodecamethylcyclohexasiloxane, hydrolysis half-lives at 25°C of 42 hours at pH 4, 401 days at pH 7 and 125 hours at pH 9 were determined in accordance with OECD 111 (Dow Corning Corporation, 2009). 

The ultimate product of hydrolysis is dimethylsilanediol.