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EC number: 200-901-0 | CAS number: 75-78-5
Phototransformation in air
Administrative data
Link to relevant study record(s)
Description of key information
Phototransformation in air: Rate constant for reaction with OH radicals: Parent substance 3.0 x 10-13 cm3/molecule.sec (half-life 54 days), AOPWIN prediction; silanol hydrolysis product 8.1 x 10-13 cm3/molecule.sec (half-life 20 days), measured.
Key value for chemical safety assessment
Additional information
Dichloro(dimethyl)silane and its silanol hydrolysis product dimethylsilanediol contain no chromophores that would absorb visible or UV radiation, so direct photolysis is not likely to be significant. Indirect photolysis resulting from gas-phase reaction with photochemically-produced hydroxyl radicals may occur.
The AOPWIN program (v1.92, EPA 2010) has been used to obtain values of the rate constant kOHfor reaction of dichloro(dimethyl)silane and dimethylsilanediol 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 x 105OH molecules/ cm3
DT50= half-life
The concentration of hydroxyl radicals in air of 5 x 105OH 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, 2012).
The results are given in the table below:
Table: Results of photodegradation in air calculations
Parameter |
Result, |
Result, dimethylsilanediol |
kOH(cm3/ molecule.sec) |
3.0 x 10-13 |
7.2 x 10-12 |
kdegair(d-1) |
1.3 x 10-2 |
3.1 x 10-1 |
DT50 (days) |
54 |
2.2 |
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 Measured data and AOPWIN predictions for reaction with hydroxyl radicals in air.
Substance |
Rate constantfor reaction with hydroxyl radicals (kOH(cm3/ molecule. sec)) |
Half-life (days) |
Tetramethylsilane |
1.28 x 10-12(Sommerlade, 1993) 0.6 x 10-12(AOPWIN) 1.0 x 10-12(Atkinson, 1991) 8.5 x 10-13(Tuazon, 2000) |
13 27 16 19 |
Hexamethyldisiloxane |
1.19 x 10-12(Sommerlade, 1993) 0.9 x 10-12(AOPWIN) 1.4 x 10-12(Atkinson, 1991) |
14 18 12 |
Octamethylcyclotetrasiloxane |
1.26 x 10-12(Sommerlade, 1993) 1.2 x 10-12(AOPWIN) 1.0 x 10-12(Atkinson, 1991) |
13 13 16 |
Hexamethylcyclotrisiloxane |
0.9 x 10-12(AOPWIN) 0.5 x 10-12(Atkinson, 1991) |
18 31 |
Decamethylcyclopentasiloxane |
1.5 x 10-12(AOPWIN) 1.6 x 10-12(Atkinson, 1991) |
11 10 |
Dimethylsilanediol |
7.2 x 10-12(AOPWIN) 8.1 x 10-13(Tuazon, 2000) |
2 20 |
Trimethylsilanol |
3.95 x 10-12(Sommerlade, 1993) 3.9 x 10-12(AOPWIN) 7.2 x 10-13(Tuazon, 2000) |
4 4 22 |
The measured values from Sommerlade (1993) and Atkinson (1991) are roughly in 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 (1993).
The measured value for dimethylsilanediol (the silanol hydrolysis product of the registration substance) is used in the exposure assessment as a worst case.
References:
EPA, 2010. US Environmental Protection Agency.AOPWIN program v1.92a (September, 2010)
ECHA (2012). European Chemicals Agency. Guidance on information requirements and chemical safety assessment Chapter R.16: Environmental Exposure Estimation. Version: 2.1 October 2012. R.16.5.4.3. Photochemical reactions in the atmosphere
Sommerlade et al. (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, 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.
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