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EC number: 222-613-4 | CAS number: 3555-47-3
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:
1,1,1,5,5,5-hexamethyl-3,3-bis[(trimethylsilyl)oxy]trisiloxane: 1.79E-12cm3 / molecule.sec (half-life 9 days)
Key value for chemical safety assessment
- Half-life in air:
- 9 d
- Degradation rate constant with OH radicals:
- 0 cm³ molecule-1 s-1
Additional information
No measured data are available for 1,1,1,5,5,5-hexamethyl-3,3-bis[(trimethylsilyl)oxy]trisiloxane (M4Q, CAS 3555-47-3).
M4Q is a member of the Reconsile Siloxane Category; siloxanes within the Category do not contain 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 occurs.
The Category hypothesis is that the rate of photo-oxidation by hydroxyl radicals of a compound is dependent on the constituent functional groups. A reliable experimental relative rates study with D4 and D5 (Atkinson, R., 1991) found that the NO3 radical and O3 reactions are of no importance as tropospheric removal processes for these substances. The dominant gas-phase chemical loss process is by reaction with the OH radical.
The AOPWIN program (v1.92, EPA 2010) has been used to obtain values of the rate constant kOH for reaction of 1,1,1,5,5,5-hexamethyl-3,3-bis[(trimethylsilyl)oxy]trisiloxane 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 5 E+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:
Table: Results of photodegradation in air calculations
Parameter |
1,1,1,5,5,5-hexamethyl-3,3-bis[(trimethylsilyl)oxy]trisiloxane |
kOH(cm3/ molecule.sec) |
1.79 E-12 |
kdegair(d-1) |
0.08 |
DT50 (days) |
9 |
A summary of the measured data available for siloxanes 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 |
1.28 x 10-12(Sommerladeet al., 1993) 0.6 x 10-12(AOPWIN) 1.0 x 10-12(Atkinson, 1991) 8.5 x 10-13(Tuazonet al., 2000) |
12.5 26.7 16.0 18.9 |
Hexamethyldisiloxane (HMDS) |
1.19 x 10-12(Sommerladeet al., 1993) 0.90 x 10-12(AOPWIN) 1.38 x 10-12(Atkinson, 1991) 1.32 x 10-12(Markgraf and Wells, 1997) 1.58 x 10-12(Kim and Xu, 2017) |
13.5 17.8 11.6 12.2 10.2 |
Octamethyltrisiloxane (L3) |
1.83 x 10-12(Markgraf and Wells, 1997) 2.15 x 10-12(Kim and Xu, 2017) 1.20 x 10-12(AOPWIN) |
8.8 7.5 13.4 |
Decamethyltetrasiloxane (L4) |
2.66 x 10-12(Markgraf and Wells, 1997) 3.37 x 10-12(Kim and Xu, 2017) 1.50 x 10-12(AOPWIN) |
6.0 4.8 10.7 |
Dodecamethylpentasiloxane (L5) |
4.03 x 10-12(Kim and Xu, 2017) 1.80 x 10-12(AOPWIN) |
4.0 8.9 |
Hexamethylcyclotrisiloxane (D3) |
0.90 x 10-12(AOPWIN) 0.52 x 10-12(Atkinson, 1991) 1.84 x 10-12(Xiao et al. 2015) 0.91 x 10-12(Kim and Xu, 2017) |
17.8 30.9 8.7 17.6 |
Octamethylcyclotetrasiloxane (D4) |
1.26 x 10-12(Sommerladeet al., 1993) 1.20 x 10-12(AOPWIN) 1.01 x 10-12(Atkinson, 1991) 1.90 x 10-12(Safron et al. 2015) 2.34 x 10-12(Xiao et al. 2015) 0.95 x 10-12(Kim and Xu, 2017) |
12.7 13.4 15.9 8.4 6.9 16.9 |
Decamethylcyclopentasiloxane (D5) |
1.50 x 10-12(AOPWIN) 1.55 x 10-12(Atkinson, 1991) 2.60 x 10-12(Safron et al. 2015) 2.46 x 10-12(Xiao et al. 2015) 1.46 x 10-12(Kim and Xu, 2017) |
10.7 10.4 6.2 6.5 11.0 |
Dodecamethylcyclohexasiloxane (D6) |
2.44 x 10-12(Kim and Xu, 2017) 2.80 x 10-12 (Safron et al. 2015) 1.8 x 10-12(AOPWIN) |
6.6 5.7 8.9 |
Dimethylsilanediol |
7.2 x 10-12(AOPWIN) 8.1 x 10-13(Tuazonet al., 2000) |
2.2 19.8 |
Trimethylsilanol |
3.95 x 10-12(Sommerladeet al., 1993) 3.9 x 10-12(AOPWIN) 7.2 x 10-13(Tuazonet al., 2000) |
4.1 4.0 22.3 |
The measured values are in sufficient agreement, and correlate well with the predicted values. Consequently, the use of the predicted value for assessment of the registered substance is considered to be acceptable.
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, R.16, A.16-3.2.2 C) Photochemical reactions in the atmosphere
Kim J, Xu S. 2017. Quantitative structure-reactivity relationships of hydroxyl radical rate constants for linear and cyclic volatile methylsiloxanes. Environmental Toxicology and Chemistry 36:3240-3245.
Atkinson R. 1991. Kinetics of the gas-phase reactions of a series of organosilicon compounds with hydroxyl and nitrate(NO3) radicals and ozone at 297 .+-. 2 K. Environmental Science & Technology 25:863-866.
Xiao R, Zammit I, Wei Z, Hu W-P, MacLeod M, Spinney R. 2015. Kinetics and Mechanism of the Oxidation of Cyclic Methylsiloxanes by Hydroxyl Radical in the Gas Phase: An Experimental and Theoretical Study. Environmental Science & Technology 49:13322-13330.
Sommerlade R, Parlar H, Wrobel D, 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. Environmental Science & Technology 27:2435-2440.
Safron A, Strandell M, Kierkegaard A, Macleod M. 2015. Rate Constants and Activation Energies for Gas-Phase Reactions of Three Cyclic Volatile Methyl Siloxanes with the Hydroxyl Radical. International Journal of Chemical Kinetics 47:420-428.
Markgraf SJ, Wells JR. 1997. The hydroxyl radical reaction rate constants and atmospheric reaction products of three siloxanes. International Journal of Chemical Kinetics 29:445-451.
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