Decamethyltetrasiloxane

Hydrolysis

Description of key information

Hydrolysis half-life: 30.3 d (728 h) at pH 7, 14 h at pH 5 and 21.1 h at pH 9 and 25°C (measured data). The stated half-life is for removal of parent. Complete reaction to the ultimate end products will take longer.

Key value for chemical safety assessment

Half-life for hydrolysis:

30.3 d

at the temperature of:

25 °C

Additional information

Hydrolysis half-lives of 30.3 d (728 h) at pH 7 and 25°C, 14 h at pH 5, and 21.1 h at pH 9 were determined for the substance using a method in accordance to OECD 111 and in compliance with GLP. The result is considered to be reliable and was selected as key study.

Decamethyltetrasiloxane 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. The stated half-life is for removal of the registration substance due to hydrolysis. The products of this reaction are also unstable in water, and so further hydrolysis reactions will follow, the ultimate products being dimethylsilanediol (2 moles) and trimethylsilanol (2 moles) for each mole of parent substance.

Predicted half-lives of 3.6 h at pH 4, 2.3 h at pH 5, 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. The result is used as supporting data.

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.

k_obs= k₀+ k_H3O+[H₃O⁺] + k_OH^-[OH^-] + k_a[acid] + k_b[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:

k_obs≈k_H3O+[H₃O⁺]

 

At pH 4 [H₃O⁺] = 10^-4mol dm^-3 and at pH 2 [H₃O⁺] = 10^-2mol dm^-3; therefore, k_obs at pH 2 should be approximately 100 times greater than k_obs at pH 4.

The half-life of a substance at pH 2 is calculated based on:

t_1/2(pH 2) = t_1/2(pH 4) / 100

The calculated half-life of the substance at pH 2 is therefore 0.036 hours (approximately 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 L4 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).

The experiments at pH 7 were more susceptible to recovery decreases by partitioning of L4 into vapour phase due to the extended durations of the study. Therefore, to explicitly account for this process, the pH 7 experiments were analysed using non-linear regression. Similarly, non-linear regression was applied to the data at pH 5 and pH 9 experiments as confirmation of the linear regression results (k₁, _obs) and to obtain estimate for k₂, _obs and k₃, _obs.

The following estimates of the rate constants for hydrolysis of parent substance and intermediate hydrolysis product 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	→	HO(OSiMe2)2OH	→	HO(OSiMe2)OH

 

The rate of reaction of the intermediate hydrolysis products was faster than that of the parent substance.

 

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

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

Data for the substance octamethyltrisiloxane (CAS 107-51-7) are read-across to the submission substance decamethyltetrasiloxane for appropriate endpoints (see Section 1.4). The slow rate of hydrolysis 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 at pH 7 and 9.76 h at pH 9 were determined in accordance with OECD 111 (Dow Corning Corporation, 2007). 

The half-lives at pH 4 and 25°C 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 2.5 h at pH 4 and 25°C, 0.025 h at pH 2 and 25°C, 33 seconds at pH 2 and 37.5°C and 120 h at pH 7 and 37.5°C.

The ultimate products of hydrolysis are dimethylsilanediol (1 mole) and trimethylsilanol (2 moles).