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Environmental fate & pathways

Hydrolysis

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Description of key information

Hydrolysis half-life: 11 min at pH 7 and 25°C (modified OECD 111)

Key value for chemical safety assessment

Half-life for hydrolysis:
11 min
at the temperature of:
25 °C

Additional information

A hydrolysis half-life of 11.3 min at pH 7 and 25°C was determined for the substance in a modified study conducted according to OECD 111; extra co-solvent was added because the substance has low solubility in water (13 mg/l at 25°C). The half-life refers to disappearance of parent substance. The study also measured the half-life of a reference substance, hexamethyldisiloxane, for which a standard OECD 111 study is available. The half-lives for this substance at pH 7 and 25°C under standard and non-standard (10% acetonitrile co-solvent) conditions were 5 d and 11.5 d, respectively. This suggests that the half-life for the registration substance, 1,1,3,3-tetramethyldisiloxane, under standard conditions could be less than that measured in the non-standard study. Therefore, a half-life of ≤11 minutes at pH 7 and 25°C under the standard conditions of the OECD 111 study is estimated. The chemical safety assessment is not sensitive to variations in the hydrolysis half-life within this range and the measured half-life of 11 minutes is used as the key value.

1,1,3,3-Tetramethyldisiloxane (H2-L2) may theoretically undergo two consecutive reactions in water: hydrolysis of the siloxane (Si-O-Si) group and hydrolysis of the Si-H bond. The latter reaction would produced hydrogen gas as a co-product of hydrolysis. The formation of gas was monitored by head-space analysis. Very little hydrogen was evolved on the time-scale of the degradation of the parent substance. A half-life of 2.5 days was obtained for the degradation of the intermediate hydrolysis product.

Therefore, it is postulated that:

- Initial removal of parent substance is associated with breaking of the siloxane bond to give dimethylsilanol: H(CH3)2SiOSi(CH3)2H → 2 H(CH3)2SiOH. This reaction has a half-life at pH 7 and 25°C of 11.3 minutes.

- The Si-H bond then hydrolyses more slowly (half-life 2.5 days) to give dimethylsilanediol and hydrogen: H(CH3)2SiOH → (CH3)2Si(OH)2

In the absence of more specific information, the well-defined half-life for removal of parent compound is applied for general use in the chemical safety assessment, along with the apparent half-life for the second step of reaction where needed.

A half-life value of approximately 1 h at 20 - 25°C and pH 7 was obtained for 1,1,3,3-tetramethyldisiloxane using an accepted validated QSAR method (PFA 2012a).

A QSAR that is currently being developed (PFA 2012c) predicts half-lives at 20-25°C of 0.1 h at pH 4, 0.2 h at pH 5 and 0.02 h at pH 9.

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]

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, 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 0.001 hours (3.6 seconds).However, it is likely that factors such as diffusion become rate-determining when the half-life is less than 5-10 seconds. As a worst-case it can therefore be considered that the half-life for1,1,3,3-tetramethyldisiloxaneat pH 2 and 20-25°C is approximately 5 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 1,1,3,3-tetramethyldisiloxane, the hydrolysis half-life at 37.5ºC and pH 7 (relevant for lungs and blood) is approximately 4 minutes. At 37.5ºC and pH2 (relevant for conditions in the stomach following oral exposure),it is not appropriate to apply any further correction for temperature to the limit value and the hydrolysis half-life is therefore approximately 5 seconds.

Hydrolysis of the read-across substance hexamethyldisiloxane (CAS 107-46-0)

Data for the substance hexamethyldisiloxane(CAS 107-46-0) are read-across to the submission substance 1,1,3,3-tetramethyldisiloxane 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 Sections of the CSR for each endpoint.

Forhexamethyldisiloxane, hydrolysis half-lives at 25°C of 1.4 h at pH 5, 120 h at pH 7 and 12.4 h at pH 9 were determined in accordance with OECD 111 (Miller 2006).

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. This gives a half-life of 0.014 h (50 seconds) at pH 2 and 25°C, and 44 h at pH 7 and 37.5°C. At pH 2 and 37.5°C, the hydrolysis half-life is 18 seconds.

The ultimate hydrolysis product is trimethylsilanol.  

Hydrolysis of the read-across substance 2,4,6,8-tetramethylcyclotetrasiloxane (CAS 2370-88-9)

Data for the substance2,4,6,8-tetramethylcyclotetrasiloxane(CAS 107-46-0) are read-across to the submission substance 1,1,3,3-tetramethyldisiloxane 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 Sections of the CSR for each endpoint.

For 2,4,6,8-tetramethylcyclotetrasiloxane, hydolysis half-life at 22.5°C of approximately 2.2 minutes at pH 7 was determined using a relevant test method (Miller 2010). This is supported by predicted half-lives of 0.001 h at pH 4; 0.004 h at pH 7 and 0.0008 h at pH 9 were determined using validated QSAR estimation method (PFA 2012c).

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. This gives a half-life of approximately 5 seconds at pH 2 and 25°C, pH 7 and 37.5°C and pH 2 and 37.5°C.

The ultimate hydrolysis product is methylsilanetriol and hydrogen.   

Hydrolysis of the read-across substancetrimethoxysilane(CAS 2487-90-3)

Data for the substance trimethoxysilane(CAS 2487-90-3) are read-across to the submission substance 1,1,3,3-tetramethyldisiloxane for appropriate endpoints (see Section 1.4).The presence of Si-H reactive groups in the the two substances is relevant to this read-across, as discussed in the appropriate Sections of the CSR for each endpoint.

Fortrimethoxysilane, hydrolysis half-lives at 2°C of ≤0.2 minutes at pH 4, ≤0.3 minutes at pH 7 and ≤0.2 minutes at pH 9 were determined in accordance with OECD 111 (DCC 2002).

Since rate of hydrolysis is faster at increased temperature, at ambient conditions (20 - 25°C), relevant to the environment, the hydrolysis half-lives are expected to be faster.

The ultimate hydrolysis products are silicic acid and methanol.   

Hydrolysis of the read-across substance dichloro(methyl)silane (CAS 75-54-7)

Data for the substance dichloro(methyl)silane(CAS 75-54-7) are read-across to the submission substance 1,1,3,3-tetramethyldisiloxane 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 Sections of the CSR for each endpoint.

For dichloro(methyl)silane, hydrolysis half-lives at 1.5°C of approximately 0.2 minutes at pH 4, approximately 0.3 minutes at pH 7 and approximately 0.1 minutes at pH 9 were determined in accordance with a relevant test method (Miller 2001).

The ultimate hydrolysis products are methylsilanetriol and hydrochloric acid.