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Environmental fate & pathways

Hydrolysis

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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.