Dodecamethylcyclohexasiloxane

Hydrolysis

Description of key information

Hydrolysis half-life [dodecamethylcyclohexasiloxane]: >1 year at pH 7 and 25°C.

Key value for chemical safety assessment

Half-life for hydrolysis:

1 yr

at the temperature of:

25 °C

Additional information

The hydrolysis rate of dodecamethylcyclohexasiloxane (D6) has been estimated using two different methods (Dow Corning Corporation, 2009). In the first method, acid and alkali catalysed rate constants were extrapolated from reliable measured data for the related siloxanes hexamethylcyclotrisiloxane (D3), octamethylcyclotetrasiloxane (D4) and decamethylcyclopentasiloxane (D5). The rate constants at pH 4, 7 and 9 were then calculated using the equation:

k(pred)/h^-1= K_(H⁺)[H⁺] + k_(^-OH)[^-OH]

Hydrolysis half-lives of 42 h at pH 4, 401 d (approximately 9600 h) at pH 7 and 125 h at pH 9 and 25°C were determined for D6.

In the second method, screening experiments were carried out at elevated temperature and basic pH (40°C and pH 10, and 60°C and pH 9). A hydrolysis half-life of 4600-7500 h at pH 9 was determined; half-lives at pH 4 and 7 could not be determined using the data from this method.

The two results obtained by method 2 were in reasonable agreement with each other but differ significantly from the predicted value from method 1. The authors of the study could not explain the discrepancy or draw a conclusion as to which is more accurate. However, the data are sufficient to conclude that the hydrolysis half-life of D6 at pH 7 and 25°C will exceed 1 year. The half-life would be very difficult to measure due to the long half-lives, very low water solubility and relatively high volatility. Therefore, further testing is not considered appropriate. The results of this study are considered to be reliable and has been selected as key study.

The results from method 1 are supported by calculated half-lives at 20-25°C of 22.3 h at pH 4, 9.1 h at pH 5, approximately 20000 h (830 days) at pH 7 and 96 h at pH 9 which was obtained using validated QSAR estimation methods.

The half-life of the substance at pH 2 may be calculated using method 1 above as 0.42 hours and 25°C. 

 

For the environmental exposure assessment, the parent will be considered to be the relevant assessment entity, as the half-life for hydrolysis is greater than 12 hours at pH 7. 

The quoted half-lives refer to degradation of parent substance by ring-opening; full hydrolysis may take longer.

Hydrolysis half-life of the read-across substance decamethylcyclopentasiloxane, D5 (CAS 541-02-6)

Data for the substance decamethylcyclopentasiloxane (D5; CAS 541-02-6) are read-across to the submission substance D6 for the following endpoints: long-term toxicity to fish, toxicity to sediment, toxicity to soil microorganisms, toxicity to terrestrial arthropods and toxicity to terrestrial plants. The slow rate of hydrolysis is relevant to this read-across, as discussed in relevant sections for each endpoint.

For decamethylcyclopentasiloxane, hydrolysis half-lives at 25°C of 9.3 h at pH 4, 1590 h at pH 7 and 24.8 – 31.6 h at pH 9 were determined in accordance with OECD 111 (Dow Corning Corporation 2006).

The quoted half-lives refer to degradation of parent substance by ring opening; full hydrolysis may take longer.

The product of hydrolysis is dimethylsilanediol.