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Environmental fate & pathways

Hydrolysis

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Description of key information

Hydrolysis half-life: 1.8 h at pH 4, 69 - 144 h at pH 7, 0.9 - 1 h at pH 9, and 25°C (OECD 111). The average half-life for pH 7 at 25°C was calculated to be 3.9 days.

Key value for chemical safety assessment

Half-life for hydrolysis:
3.9 d
at the temperature of:
25 °C

Additional information

Hydrolysis half-lives at 25°C of 1.8h at pH4, 69-144h at pH 7 and 0.9-1h at pH 9 were determined in a reliable study conducted according to an appropriate test protocol, and in compliance with GLP. The average half-life for pH7 at 25°C was calculated to be 3.9 days. This experimental value is consistent with the value of 3.0 ± 0.6 (95% CI) days at pH 7.0 and 25.0°C, calculated from the experimental data for pH 4 and 9 between 10 and 35°C. Hydrolysis half-lives at 10°C of 4.8 h at pH 4, 540 h at pH 7 and 5.6-6.4 h at pH 9 were determined. Hydrolysis half-lives at 35°C of 0.9 h at pH 4, 25 h at pH 7 and 0.2 h at pH 9 were determined.

These values are supported by hydrolysis half-lives of 33 h at pH 5, 69 h at pH 7 and 0.56 h at pH 9 and 25°C determined in a non-regulated method development and preliminary assessment OECD Guideline 111 study.

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 and 35°C (relevant for conditions in the stomach following oral exposure) is therefore 0.009 hours (approximately 0.5 minutes).

The measured hydrolysis half-life of D4 at 35ºC and pH 7 (relevant for lungs and blood) is 25 hours. At 35ºC and pH 2 (relevant for conditions in the stomach following oral exposure), the hydrolysis half- life is calculated as 0.006 h (approximately 22 seconds). At 35°C and pH 5 (relevant for dermal exposure), the hydrolysis half-life is expected to be between the values for pH 4 (0.9 h) and pH 7 (25 h).

In the key hydrolysis study, the concentrations of parent, three hydrolysis intermediates (which are dimethylsiloxane-α,ω-diol oligomers, HO(Me2SiO)nH (n = 2-4) and the hydrolysis product (dimethylsilanediol, HO(Me2SiO)H) were measured using radio-HPLC.

The following mechanism was proposed for hydrolysis.

 

k1and k2

 

k3

 

k4

 

k5

 

D4

HOD4OH

HOD3OH

HOD2OH

HODOH

The first hydrolysis step is reversible however the extent of reversibility under the dilute conditions of the study is minor.

Non-linear regression analysis of the data was performed and the following rate constants at pH 7 and 25°C for the individual reactions were estimated.

k1≈ 8E-3 h-1

k2≈ 1E-3 h-1

k3≈ 4E-3 h-1

At pH 9, the following estimates were obtained:

k1≈ 0.9 h-1

k2≈ 0.1 h-1

k3≈ 0.4 h-1

k4≈ 0.3 h-1

k5≈ 0.4 h-1

The hydrolysis intermediates hydrolysed at rates that were 2 to 3 times slower than the parent substance.