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Hydrolysis

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Description of key information

Hydrolysis: half-life: 6.6 h at pH 4, 2000 h at pH 7, 13.9 h at pH 9 and 20-25°C (QSAR).

Key value for chemical safety assessment

Half-life for hydrolysis:
2 000 h
at the temperature of:
25 °C

Additional information

No hydrolysis study is available for 1,1,5,5,5-hexamethyl-3,3-bis[(trimethylsilyl)oxy]trisiloxane (M4Q). Hydrolysis half-life values at 20-25°C of 1.9-6.6 h at pH 4, >200 - 2000 h at pH 7 and 2.0-13.9 h at pH 9 were obtained using an accepted estimation method. The result is considered to be reliable.

This hydrolysis half-life reflects the removal of parent substance. Rate constant information for the intermediate hydrolysis products of the structural analogue L4 is available from the standard OECD 111 study with that substance (Dow Corning Corporation, 2009a).

The following estimates of the rate constants for hydrolysis of decamethyltetrasiloxane (L4; CAS 141-62-8) and its intermediate hydrolysis products at pH 7 and 10°C, 25°C and 35°C were obtained:

10°C k1= 1.8E-04 h-1, 25°C k1= 9.5E-04 h-1, 35°C k1= 3.2E-03 h-1

10°C k2= 2.3E-04 h-1, 25°C k2= 1.9E-03 h-1, 35°C k2= 5.2E-03 h-1

10°C k3= 5.3E-04 h-1, 25°C k3= 9.2E-03 h-1, 35°C k3= 27E-03 h-1

k1

k2

k3

 

Me3Si(OSiMe2)2OSiMe3

Me3Si(OSiMe2)2OH

HOSi(Me2)OSi(Me­2)OH

HOSi(Me2)OH

At 25°C, these are equivalent to half-lives of 730 h, 365 h and 75 h.

The rates of reaction of the intermediate hydrolysis products were faster than that of the L4 parent substance;the same would be expected for M4Q, i.e. the rate limiting step is the initial hydrolysis of the parent substance. Under laboratory conditions, the final hydrolysis products of M4Q are monosilicic acid Si(OH)4 (1 mole) and trimethylsilanol (Me3SiOH) (4 moles). At concentrations above about 100-150 mg/l as SiO2, condensation products of monosilicic acid can also form.

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+]=10-4mol dm-3and at pH 2 [H3O+] =10-2mol dm-3; therefore, kobsat pH 2 should be approximately 100 times greater than kobsat 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 0.019 hours (approximately 68 seconds). 

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 M4Q the hydrolysis half-life at 37.5ºC and pH 7 (relevant for lungs and blood) is approximately 74 hours. At 37.5ºC and pH 2 (relevant for conditions in the stomach following oral exposure), the hydrolysis half- life is calculated as 25 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 4 (1.9 h) and pH 7 (74 h).

Hydrolysis of the read-across substance decamethyltetrasiloxane (L4, CAS 141-62-8)
     

Data for the substance decamethyltetrasiloxane (L4, CAS 141-62-8) are read-across to the submission substance M4Q for appropriate endpoints (see Section 1.4). The silanol hydrolysis product of the two substances is relevant to this read-across, as discussed in the appropriate summaries for each endpoint.

For decamethyltetrasiloxane, 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, 2009a).

A predicted half-life of 3.6 h at pH 4 and 20-25°C was determined for the substance using a validated QSAR estimation method (Peter Fisk Associates, 2014b).

The half-lives at 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.

The calculated half-life of the substance at pH 2 and 25°C is 0.036 hours (approximately 2.2 minutes). 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 approximately 50 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).

Under laboratory conditions, the final products of hydrolysis are dimethylsilanediol (2 moles) and trimethylsilanol (2 moles). 

Hydrolysis of the read-across substance dodecamethylpentasiloxane (CAS 141-63 -9)

Data for the substance dodecamethylpentasiloxane (CAS 141-63-9, L5) are read-across to the submission substance M4Q for appropriate endpoints. The hydrolysis half-lives of the two substances is relevant to this read-across, as discussed in the appropriate summaries for each endpoint.

The hydrolysis half-life of L5 (CAS 141 -63 -9) has been read-across from the structurally related substance L4 (CAS 141-62-8). Hydrolysis half-lives at 25°C of 30.3 d (728 h) at pH 7, 14 h at pH 5, and 21.1 h at pH 9 were determined at room temperature for L4 in accordance with OECD 111 (Dow Corning Corporation 2009).

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 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.

Half-life values of approximately 6.6 h at pH 4, 2000 h at pH 7 and 14 h at pH and 20-25°C were obtained for L5 using a validated QSAR estimation method.

Hydrolysis of the read-across substance 1,1,1,3,5,5,5-heptamethyl-3-[(trimethylsilyl)oxy]trisiloxane (M3T) (CAS 17928 -28 -8)
     

Data for the substance 1,1,1,3,5,5,5-heptamethyl-3-[(trimethylsilyl)oxy]trisiloxane (M3T) are read-across to the submission substance M4Q for appropriate endpoints (see Section 1.4).The silanol hydrolysis product of the two substances is relevant to this read-across, as discussed in the appropriate summaries for each endpoint.

For M3T, predicted half-lives of 3.6 h at pH 4, 630 h at pH 7 and 5.3 h at pH 9 and 20-25°C were determined for the substance using a validated QSAR estimation method (Peter Fisk Associates, 2014).

The half-lives at 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.

The calculated half-life of the substance at pH 2 and 25°C is 0.036 hours (approximately 2.2 minutes). The hydrolysis half-life at 37.5ºC and pH 7 (relevant for lungs and blood) is approximately 230 hours. At 37.5ºC and pH 2 (relevant for conditions in the stomach following oral exposure), the hydrolysis half- life is calculated as approximately 0.013 h (50 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 4 (1.3 h) and pH 7 (230 h).

Under laboratory conditions, the final hydrolysis products of M3T are methylsilanetriol (MeSi(OH)3) (1 mole) and trimethylsilanol (Me3SiOH) (3 moles).

Hydrolysis of the read-across substance 3,3-bis[(dimethylvinylsilyl)oxy]-1,1,5,5-tetramethyl-1,5-divinyltrisiloxane (CAS 60111-54-8 ViM4Q)

Data for the substance 3,3-bis[(dimethylvinylsilyl)oxy]-1,1,5,5-tetramethyl-1,5-divinyltrisiloxane (CAS 60111-54-8 ViM4Q) are read-across to the submission substance M4Q for appropriate endpoints. The hydrolysis half-lives of the two substances are relevant to this read-across, as discussed in the appropriate summaries for each endpoint.

Half-life values at 20-25°C of > 200 - 2000 h at pH 7, 1.9 – 6.6 h at pH 4, and 2.0 – 13.9 h at pH 9 were obtained for ViM4Q using accepted validated QSAR methods. The result is considered to be reliable and has been used for assessment purposes. The products of hydrolysis are dimethylvinylsilanol (4 moles) and silicic acid (1 mole).

The half-lives at 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. The calculated half-life of the substance at pH 2 and 25°C is 0.019 hours (approximately 70 seconds). The hydrolysis half-life at 37.5ºC and pH 7 (relevant for lungs and blood) is approximately 74 hours. At 37.5ºC and pH 2 (relevant for conditions in the stomach following oral exposure), the hydrolysis half- life is calculated as approximately 25 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 (1.9 h) and pH 7 (74 h).