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Hydrolysis

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Description of key information

Hydrolysis: half-life 3.6 h at pH 4, 630 h at pH 7, 5.3 h at pH 9 and 20-25°C (QSAR).

Key value for chemical safety assessment

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

Additional information

No hydrolysis study is available for 1,1,1,3,5,5,5-heptamethyl-3-[(trimethylsilyl)oxy]trisiloxane (M3T). Hydrolysis half-life values of 3.6 h at pH 4, 630 h at pH 7, 5.3 h at pH 9 and 20-25°C were obtained using an accepted calculation method. The results are considered to be reliable.

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

The following estimates of the rate constants for hydrolysis of the 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

HO-Si(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 M3T, i.e. the rate limiting step is the initial hydrolysis of the parent substance. Under laboratory conditions, the final hydrolysis products of M3Tare methylsilanetriol (MeSi(OH)3) and trimethylsilanol (Me3SiOH).

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 -2 mol 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 0.036 hours (2.2 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 1,1,1,3,5,5,5-heptamethyl-3-[(trimethylsilyl)oxy]trisiloxane 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 0.013 hours (48 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 (3.6 h) and pH 7 (630 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 M3T for appropriate endpoints (see CSR Section 1.4).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.

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, 2009).

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, 2014a).

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 likely final products of hydrolysis are dimethylsilanediol and trimethylsilanol. 

 

Hydrolysis of the read-across substance silsesquioxanes, phenyl (CAS 70131-69-0)

Data for the substance Silsesquioxanes, Phenyl(CAS 70131-69-0)are read-across to the submission substance M3T for appropriate endpoints (see Section 1.4).The structural similarity and the siloxane hydrolysis half-life of the read-across substance to the submission substance is relevant to this read-across, as discussed in the appropriate Sections of the CSR for each endpoint.

No measured hydrolysis data is available for Silsesquioxanes, Phenyl(CAS 70131-69-0); 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.

Under laboratory conditions, the likely final products of hydrolysis are phenylsilanetriol and trimethylsilanol. 

Hydrolysis of the read-across substance octamethyltrisiloxane (L3, CAS 107-51-7)

Data for the substance octamethyltrisiloxane (L3, CAS 107-51-7) are read-across to the submission substance M3T for appropriate endpoints (see CSR Section 1.4). The structural similarity and the siloxane hydrolysis half-life of the read-across substance to the submission substance is relevant to this read-across, as discussed in the appropriate Sections of the CSR for each endpoint.

For octamethyltrisiloxane, hydrolysis half-lives at 25°C of 5.09 h at pH 5, 329 h (13.7 days) at pH 7 and 9.76 h at pH 9 were determined in accordance with OECD 111 (Dow Corning Corporation, 2007).

A predicted half-life of 2.5 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 0.025 hours (approximately 1.5 minutes). The hydrolysis half-life at 37.5ºC and pH 7 (relevant for lungs and blood) is approximately 121 hours. At 37.5ºC and pH 2 (relevant for conditions in the stomach following oral exposure), the hydrolysis half-life is calculated as approximately 32 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 (2 h) and pH 7 (121 h).

Under laboratory conditions, the likely final products of hydrolysis are dimethylsilanediol and trimethylsilanol. 

Hydrolysis of the read-across substance 1,1,1,3,5,5,5-heptamethyltrisiloxane (H-L3, CAS 1873-88-7)

Data for the substance1,1,1,3,5,5,5-heptamethyltrisiloxane (H-L3, CAS 1873-88-7) are read-across to the submission substance M3T for appropriate endpoints (see CSR Section 1.4). The structural similarity and the siloxane hydrolysis half-life of the read-across substance to the submission substance is relevant to this read-across, as discussed in the appropriate Sections of the CSR for each endpoint.

A hydrolysis half-life of 2.2 days at pH 7 and 25°C was determined for the substance in a study conducted according to OECD 111 but with significant modification. It was concluded that degradation of parent substance was due to a combination of Si-O and Si-H bond hydrolysis.

Predicted half-lives at 20-25°C of 0.7 h at pH 4, 0.6 h at pH 5,

and 0.4 h at pH 9 were 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 0.007 hours (approximately 0.4 minutes). The hydrolysis half-life at 37.5ºC and pH 7 (relevant for lungs and blood) is approximately 19 hours. At 37.5ºC and pH 2 (relevant for conditions in the stomach following oral exposure), the hydrolysis half- life is calculated as approximately 9 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 (0.2 h) and pH 7 (19 h).

Under laboratory conditions, the likely final products of hydrolysis are methylsilanetriol and trimethylsilanol.