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Description of key information

Phototransformation in air.

Half-life for reaction with OH radicals:

Constituent 1: 4.8E-12 cm3/molecule.sec (half-life 3.3 days)

Constituent 2: 8.7E-12 cm3/molecule.sec (half-life 1.8 days)

Key value for chemical safety assessment

Additional information

No measured data are available for the submission substance.

The AOPWIN program (v1.92, EPA 2010) has been used to obtain values of the rate constant kOH for reaction of the constituents of the submission substance and their hydrolysis products with hydroxyl radicals. This prediction method has not been validated to assess applicability to organosilicon substances; therefore, there is uncertainty associated with the calculated values obtained.

The overall half-life in air under default conditions of hydroxyl radical concentration was calculated using the following expressions:

 

kdegair (d-1) = kOH (cm3/molecule.sec) x OH Concair (molecules/cm3) x 24 x 3600

 

DT50 (d) = ln 2/ kdegair (d-1)

 

Where:

kdegair = total rate constant for degradation in air

kOH = rate constant for reaction with hydroxyl radicals

OH Concair = concentration of hydroxyl radicals in air = 5 E+05 OH molecules/cm3

DT50 = half-life

 

The concentration of hydroxyl radicals in air of 5E+05 OH molecules/cm3, and the 24 hour photoperiod, are the values specified in ECHA Guidance on Information requirements and chemical safety assessment, Part R.16 Environmental exposure estimation (ECHA 2016).

 

The results are given in the table below for the constituents of the substance. 

Table 4.1.2: Results of photodegradation in air calculations

Parameter

Constituent 1- 3,3-Diphenylhexamethyltrisiloxane

Constituent 2 - 3,3,5,5-Tetraphenylhexamethyltetrasiloxane

kOH (cm3/molecule.sec)

4.8E-12

8.7E-12

Kdegair (d-1)

0.21

0.38

DT50 (days)

3.3

1.8

Available data for other organosilicon substances:

Measured data for reaction with hydroxyl radicals in air are available for some organosilanes. A summary of these measured data is in the table below.

AOPWIN predictions are also presented for comparison with the measured data.

Table 4.1.3. Measured data and AOPWIN predictions for reaction with hydroxyl radicals in air.

Substance

Rate constant for reaction with hydroxyl radicals (kOH (cm3/ molecule. sec))

Half-life (days)

Tetramethylsilane

1.28E-12 (Sommerlade et al., 1993)

0.6E-12 (AOPWIN)

1.0E-12 (Atkinson, 1991)

8.5E-13 (Tuazon, 2000)

13

27

16

19

Hexamethyldisiloxane

1.19E-12 (Sommerlade et al., 1993)

0.9E-12 (AOPWIN)

1.4E-12 (Atkinson, 1991)

14

18

12

Octamethylcyclotetrasiloxane

1.26E-12 (Sommerlade et al., 1993)

1.2E-12 (AOPWIN)

1.0E-12 (Atkinson, 1991)

13

13

16

Hexamethylcyclotrisiloxane

0.9E-12 (AOPWIN)

0.5E-12 (Atkinson, 1991)

18

31

Decamethylcyclopentasiloxane

1.5E-12 (AOPWIN)

1.6E-12 (Atkinson, 1991)

11

10

Dimethylsilanediol

7.2E-12 (AOPWIN)

8.1E-13 (Tuazon, 2000)

2

20

Trimethylsilanol

3.95E-12 (Sommerlade et al., 1993)

3.9E-12 (AOPWIN)

7.2E-13 (Tuazon, 2000)

4

4

22

   

The measured values from Sommerlade et al. (1993) and Atkinson (1991) are in sufficient agreement and correlate well with the predicted values. Indeed, the data from these two studies were used in the training set for the AOPWIN program.

 

The measured values from Tuazon (2000) indicate slightly lower rates of reaction for the silanols compared to the AOPWIN predictions and the measured value from Sommerlade et al. (1993).

References:

EPA, 2010. US Environmental Protection Agency. AOPWIN program v1.92a (September, 2010)

ECHA (2016). REACH Guidance on Information Requirements and Chemical Safety Assessment Chapter R16: Environmental Exposure Assessment Version: 3.0. February 2016. R.16-3.2.2 Photochemical reactions in the atmosphere

Sommerlade, R., Parlar, H., Wrobel, D. and Kochs, P.(1993). Product Analysis and Kinetics of the Gas-Phase Reactions of Selected Organosilicon Compounds with OH Radicals Using a Smog Chamber-Mass Spectrometer System. Environ. Sci. Technol. 1993, 27 (12), 2435-2440.

Tuazon E C, Aschmann S M and Atkinson R (2000) Atmospheric Degradation of Volatile Methyl-Silicon Compounds Environmental Science and Technology, Vol. 34, No. 10, 1970-1975

Atkinson R. 1991. Kinetics of the Gas-Phase Reactions of a Series of Organosilicon Compounds with OH and NO3 Radicals and O3 at 297 +/- 2 K. Environ. Sci. Technol. 25(5):863-866.