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EC number: 946-797-7 | CAS number: -
Phototransformation in air
Administrative data
Link to relevant study record(s)
Description of key information
Phototransformation in air [hexamethydisiloxane]: Measured rate constant for reaction with OH radicles 1.4 E-12 cm3 molecule-1 second-1 at 24°C (half-life 11.5 days)
Phototransformation in air [octamethyltrisiloxane]: Rate constant for reaction with OH radicals: 1.2 E-12 cm³ / molecule.sec (half-life 13 days)
Phototransformation in air [decamethyltetrasiloxane]: Rate constant for reaction with OH radicals: 1.5 E-12 cm³ / molecule.sec (half-life 11 days)
Phototransformation in air [dodecamethylpentasiloxane]: Rate constant for reaction with OH radicals: 1.8 E-12 cm³ / molecule.sec (half-life 9 days)
Key value for chemical safety assessment
Additional information
The registration substance is a multi-constituent substance containing four constituents, decamethyltetrasiloxane (L4, CAS 141-62-8), dodecamethylpentasiloxane (L5, CAS 141-63-9), hexamethyldisiloxane (HMDS, CAS 107-46-0) and octamethyltrisiloxane (L3, CAS 107-51-7).
None of the constituents contain chromophores that would absorb visible or UV radiation so direct photolysis is not likely to be significant.
An experimental relative rates study with HMDS (Atkinson, 1991) found that the NO3 radical and O3 reactions are of no importance as tropospheric removal processes for this compound. The dominant gas-phase chemical loss process is by reaction with the OH radical, with measured rate constant of 1.4 x10-12cm³ molecule-1second-1at 24°C (calculated half-life 11.5 days, using tropospheric concentration of OH radicals 5x105molecule/cm3over 24-h period (EC, 2003)).
A further experimental relative rates study with HMDS (Sommerlade et al., 1993) found a reaction with the OH radical rate constant of 1.19 E-12 cm³ molecule-1second-1at 24°C (calculated half-life 13.5 days, using tropospheric concentration of OH radicals 5x105 molecule/cm3 over 24 -h period (EC, 2003)).
In another study with HMDS (Parlar, 1992) a reaction with OH radicles rate constant of 3.13x10-12cm³ molecule-1second-1(calculated half-life 5 days, using tropospheric concentration of OH radicals 5 x105molecule/ cm³ over 24 -h period (EC, 2003))
No measured data are available for the other constituents L3, L4 and L5. The AOPWIN program (v1.92, EPA 2010) has been used to obtain values of the rate constant kOH for reaction of these constituents 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, 2016).
The results are given in the table below:
Table: Results of photodegradation in air calculations
Parameter |
Result, hexamethyldisiloxane |
Result, octamethyltrisiloxane |
Result, decamethyltetrasiloxane |
Result, dodecamethylpentasiloxane |
kOH(cm3/ molecule.sec) |
0.90 x 10-12 |
1.2 x 10-12 |
1.5 x 10-12 |
1.8 x 10-12 |
kdegair (d-1) |
0.04 |
0.05 |
0.06 |
0.08 |
DT50(days) |
18 days |
13 |
11 |
9 |
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 constant for reaction with hydroxyl radicals (kOH(cm3/ molecule. sec)) |
Half-life (days) |
|
Tetramethylsilane |
1.28 x 10-12(Sommerlade,et al., 1993) 0.6 x 10-12(AOPWIN) 1.0 x 10-12(Atkinson, 1991) 8.5 x 10-13(Tuazonet al.,2000) |
13 27 16 19 |
|
Hexamethyldisiloxane |
1.19 x 10-12(Sommerlade,et al., 1993) 0.9 x 10-12(AOPWIN) 1.4 x 10-12(Atkinson, 1991) |
14 18 12 |
|
Octamethylcyclotetrasiloxane |
1.26 x 10-12(Sommerlade,et al., 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(Tuazonet al.,2000) |
2 20 |
|
Trimethylsilanol |
3.95 x 10-12(Sommerlade,et al., 1993) 3.9 x 10-12(AOPWIN) 7.2 x 10-13(Tuazon et al.,2000) |
4 4 22 |
The measured values from Sommerlade (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 (1993).
References:
EPA, 2010. US Environmental Protection Agency.AOPWIN program v1.92a (September, 2010)
ECHA (2016). European Chemicals Agency. Guidance on information requirements and chemical safety assessment Chapter R.16: Environmental Exposure Estimation. Version: 3.0 February 2016
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.
European Communities, 2003. European Union Technical Guidance Document on Risk Assessment for New and Existing Substances, Part II, Section 2.3.6.3.
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