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Environmental fate & pathways

Hydrolysis

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Description of key information

Hydrolysis half-life: >329 h (13.7 d) at pH 7, >9.76 h at pH 9, >5.09 at pH 5 and 25°C (analogue read-across). 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:
13.7 d
at the temperature of:
25 °C

Additional information

No hydrolysis study is available for the submission substance, 1,3,5-trimethyl-1,1,3,5,5-pentaphenyltrisiloxane. Testing is waived as the substance is highly insoluble in water. However, a reliable study conducted in accordance with OECD 111 is available for a structurally related substance octamethyltrisiloxane (CAS 107-51-7, EC No. 203-497-4, L3).

Hydrolysis half-lives of 329 h (13.7 d) at pH 7, 9.76 h at pH 9, and 5.09 h at pH 5 and 25°C were determined for L3 using a method in accordance with OECD 111 and in compliance with GLP. The results are read-across to the submission substance. The results are considered to be reliable and are used for assessment purpose. The key study is supported by predicted half-lives of >1.9 h at pH 4, >200 h at pH 7 and >2.0 h at pH 9 and 20 -25°C for the submission substance.

Octamethyltrisiloxane (L3) a mono-constituent substance consisting of 98 -99% (typical ≥98%) of L3 and <2% identified impurities (typically silane and siloxane). The submission substance is also a mono-constituent substance consisting of >95% of 1,3,5-trimethyl-1,1,3,5,5-pentaphenyltrisiloxane and <5% identified impurities (phenyl-siloxane). Both substances are linear siloxane chains with three silicon atoms, connected by two oxygen atoms, in which the Si-O bonds are susceptible to hydrolysis. For L3, all silicon atoms present are fully substituted with methyl groups. For 1,3,5-trimethyl-1,1,3,5,5-pentaphenyltrisiloxane, each terminal silicon atom is substituted with one methyl and two phenyl groups, whilst the central silicon atom is substituted with one methyl and one phenyl group. The phenyl groups in the submission substance are expected to present steric differences between the Si centres in the two structures. Therefore, the reaction rate for the submission substance is expected to be slower than that of L3 and the hydrolysis half-lives for 1,3,5-trimethyl-1,1,3,5,5-pentaphenyltrisiloxane is reported as greater than that of L3. The ultimate end products of the hydrolytic reaction, for L3 is dimethylsilanediol and trimethylsilanol; for 1,3,5-trimethyl-1,1,3,5,5-pentaphenyltrisiloxane, the ultimate hydrolysis product is methyl(diphenyl)silanol and methyl(phenyl)silanediol. 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 methyl(phenyl)silanediol (1 mole) and methyl(diphenyl)silanol (2 moles) per mole of parent substance. The reaction pathway is expected to be the following:

k1

k2

Ph2(Me)Si(OSiMe(Ph))OSi(Me)Ph2

Ph2(Me)

Si(OSi(Me)Ph)OH

H(OSi(Me)Ph)OH

The following reaction pathway and rate constants were reported for L3:

k1

k2

Me3Si(OSiMe2)OSiMe3

Me3Si(OSiMe2)OH

H(OSiMe2)OH

Regression analysis (concentration versus reaction time) was performed on the experimental data for L3. The experiments conducted at pH 7 were more susceptible to recovery decreases by partitioning of L3 into vapour phase due to the extended durations of the study. Therefore, to explicitly account for this process, the pH 7 experiments were analysed using non-linear regression. Similarly, non-linear regression was applied to the data at pH 5 and pH 9, the rate of transfer of L3 from solution to the headspace was negligible compared to rate of hydrolysis.

The following estimates of the rate constants for hydrolysis of L3 and intermediate hydrolysis product at pH 7 and 10°C, 25°C and 35°C were obtained by non-linear regression of the measured results:

10°C k1= 4.7E-04 h-1,           25°C k1= 2.1E-03 h-1,           35°C k1= 4.9E-03 h-1

10°C k2= 3.9E-04 h-1,           25°C k2= 1.9E-03 h-1,           35°C k2= 5.3E-03 h-1

Predicted rates of hydrolysis for pH 7 based on these rate constants agreed with measured values to within 13%. This suggested that catalysis by hydronium and hydroxide accounts for the observed rates at neutral pH, with catalysis by hydronium dominating at lower temperature.

The rate of reaction of the intermediate hydrolysis product was as fast, or faster, than that of the parent substance.

In a further study report (Dow Corning Corporation 2016), hydrolysis half-lives over a range of pH at 25°C, 12°C and 9°C were calculated using data from the key study for L3 (Dow Corning Corporation 2007). 12°C and 9°C are considered broadly representative of freshwater and marine environments, respectively. The Arrhenius parameters were derived by linear regression analysis of the original experimental data, and subsequently rate constants were calculated for temperatures and pH range of interest.

 

Half-life at 9°C (days)

Half-life at 12°C (days)

Half-life at 25°C (days)

pH 6

7 

5.5 

2.1 

pH 7

58 

44.1 

13.5 

pH 8

31.9 

21

3.9

 

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 very low water solubility and very high log Kow(very high absorption potential) for the registration substance mean that hydrolysis in the environment is expected to be limited.

Hydrolysis of the read-across substance 3-[(dimethyl(vinyl)silyl)oxy]-1,1,5,5-tetramethyl-3-phenyl-1,5-divinyltrisiloxane (CAS 60111-47-9)

Data for the substance 3-[(dimethyl(vinyl)silyl)oxy]-1,1,5,5-tetramethyl-3-phenyl-1,5-divinyltrisiloxane (CAS 60111-47-9) are read-across to the submission substance 1,3,5-trimethyl-1,1,3,5,5-pentaphenyltrisiloxane for appropriate endpoints (see Section 1.4 of the CSR). No measured hydrolysis data is available; predicted 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.

The products of hydrolysis are dimethyvinylsilanol and phenylsilanetriol.

Hydrolysis of the read-across substance Silsequioxanes, Phenyl (CAS 70131-69-0)

Data for the substance Silsesquioxanes, Phenyl (CAS 70131-69-0) are read-across to the submission substance 1,3,5-trimethyl-1,1,3,5,5-pentaphenyltrisiloxane for appropriate endpoints (see Section 1.4 of the CSR). No measured hydrolysis data is available; predicted 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 1,3,3,5 -tetramethyl-1,1,5,5 -tetraphenyltrisiloxane (CAS 3982-82-9)

Data for the substance 1,3,3,5 -tetramethyl-1,1,5,5 -tetraphenyltrisiloxane (CAS 3982-82-9) are read-across to the submission substance 1,3,5-trimethyl-1,1,3,5,5-pentaphenyltrisiloxane for appropriate endpoints (see Section 1.4 of the CSR). The silanol hydrolysis product of the two substances is relevant to this read-across, as discussed in the appropriate Sections of the CSR for each endpoint.

No hydrolysis study is available for 1,3,3,5 -tetramethyl-1,1,5,5 -tetraphenyltrisiloxane, CAS 3982-82-9. However, a reliable study conducted in accordance with OECD 111 is available for a structurally related substance octamethyltrisiloxane (CAS 107-51-7, EC No. 203-497-4, L3).

 

Hydrolysis half-lives of 329 h (13.7 d) at pH 7, 9.76 h at pH 9, and 5.09 h at pH 5 and 25°C were determined for L3 using a method in accordance with OECD 111 and in compliance with GLP. The results are read-across to 1,3,3,5 -tetramethyl-1,1,5,5 -tetraphenyltrisiloxane. The results are considered to be reliable and are used for assessment purpose. The key study is supported by predicted half-lives of >1.9 h at pH 4, >200 h at pH 7 and >2.0 h at pH 9 and 20 -25°C for 1,3,3,5 -tetramethyl-1,1,5,5 -tetraphenyltrisiloxane.

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

Hydrolysis of the read-across substance 1,1,1,5,5,5-hexamethyl-3-phenyl-3-[(trimethylsilyl)oxy]trisiloxane (CAS 2116-84-9)

Data for the substance 1,1,1,5,5,5-hexamethyl-3-phenyl-3-[(trimethylsilyl)oxy]trisiloxane (CAS 2116-84-9)

are read-across to the submission substance 1,3,5-trimethyl-1,1,3,5,5-pentaphenyltrisiloxane for appropriate endpoints (see Section 1.4 of the CSR). 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 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 phenylsilanetriol (1 mole) and trimethylsilanol (3 moles) per mole of parent substance.