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Description of key information

Phototransformation in air: Rate constant for reaction with OH radicals: Parent substance 0.9 E-12 cm3 / molecule.sec (half-life 18 days); silanol hydrolysis product 3.9 E-12 cm3 / molecule.sec (half-life 4.1 days); ammonia 1.5 E-13cm3/molecule.sec (half-life 34 days)

Key value for chemical safety assessment

Additional information

No measured data are available for hexamethyldisilazane.

Hexamethyldisilazane and its silanol hydrolysis product trimethylsilanol 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 hexamethyldisilazane and trimethylsilanol 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,
hexamethyldisilazane

Result, trimethylsilanol

kOH(cm3/ molecule.sec)

0.90 x 10-12

3.9 x 10-12

kdegair(d-1)

0.039

0.17

DT50 (days)

18

4.1

 

The other hydrolysis product of hexamethyldisilazane is ammonia. Ammonia may react with ozone, hydroxyl radicals, and atomic oxygen in air (OECD, 2007 citing NRC, 1979; Verschueren, 1996; Environment Canada, 1984). The primary reaction of ammonia is with hydroxyl radicals and is the rate-determining step in the atmospheric oxidation of ammonia. The rate constant is 1.5 x 10-13cm3molecule-1s-1at 25°C (OECD, 2007 citing Stuhl, 1973; Diau et al., 1990) with a corresponding lifetime of 34 days (OECD, 2007 citing Diau et al.,1990; Renard et al., 2004).

Measured data available for trimethylsilanol and other organosilanes

Measured rate constants and half-life data are available for four organosilicon compounds in a 20 L smog chamber coupled to a quadruple mass spectrometer (Sommerlade 1993). The rate constants for OH-radical reactions were 1.28 ± 0.46 x 10-12cm3molecule-1s-1for tetramethylsilane, 1.19 ± 0.30 x 10-12cm3molecule-1s-1for hexamethyldisiloxane, 3.95 ± 0.95 x 10-12cm3molecule-1s-1for trimethylsilanol and 1.26 ± 0.40 x 10-12cm3molecule-1s-1for octamethylcyclotetrasiloxane. Using the equation above, the corresponding half-lives are 13, 14, 4 and 13 days, respectively. The SRC AOPWIN calculated values for these substances are 0.60 x 10-12cm3molecule-1s-1, 0.90 x 10-12cm3molecule-1s-1, 3.9 x 10-12cm3molecule-1s-1and 1.2 x 10-12cm3molecule-1s-1, respectively.

Rate constants for the reactions of a range of organosilicon substances with OH and NO3radicals and O3have been determined using a 6400 -L all-Teflon chamber, GC-FID anlysis, and relative rate techniques (Atkinson, 1991). The NO3radical and O3reactions are calculated to be of no importance as tropospheric removal processes. The dominant gas-phase chemical loss process is by reaction with the OH radical.

The rate constants obtained for reaction with OH radicals were as follows: 1.0 x 10-12cm3molecule-1s-1for tetramethylsilane, 1.4 x 10-12cm3molecule-1s-1for hexamethyldisiloxane, 0.5 x 10-12cm3molecule-1s-1for hexamethylcyclotrisiloxane, 1.0 x 10-12cm3molecule-1s-1for octamethylcyclotetrasiloxane, 1.6 x 10-12cm3molecule-1s-1for decamethylcyclopentasiloxane. Using the equation above, the corresponding half-lives are 16, 12, 31, 16 and 10 days, respectively.

The AOPWIN calculated values for these substances are 0.6 x 10-12cm3molecule-1s-1, 0.9 x 10-12cm3molecule-1s-1, 0.9 x 10-12cm3molecule-1s-1, 1.2 x 10-12cm3molecule-1s-1and 1.5 x 10-12cm3molecule-1s-1, respectively.

The measured values from these two studies 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 kinetics of the gas-phase reactions of dimethylsilanediol, trimethylsilanol, and tetramethylsilane with the OH radical were measured using a relative rate method which employed the N2H4+ O3reaction as a nonphotolytic source of OH radicals, with analysis by Fourier transform infrared (FT-IR) spectroscopy in a 5870 -l chamber (Tuazon et al., 2000). The measured values of the OH radical reaction rate constants at 298±2 K are as follows: 8.1 ± 1.0 x 10-13cm3molecule-1s-1for dimethylsilanediol, 7.2 ± 0.8 x 10-13cm3molecule-1s-1for trimethylsilanol and 8.5 ± 0.9 x 10-13cm3molecule-1s-1tetramethylsilane. Using the equation above these rate constants correspond to half-lives of 20, 22 and 19 days. The AOPWIN calculated values for these substances are 7.2 x 10-12cm3molecule-1s-1, 3.9 x 10-12cm3molecule-1s-1and 0.6 x 10-12cm3molecule-1s-1, respectively. These data indicate lower rates of reaction for the silanols compared to the AOPWIN predictions and the two previous studies discussed above.

A summary of this measured data is in the table below:

TableResults of photodegradation in air calculations

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 data for trimethylsilanol (the silanol hydrolysis product of the registration substance) is in good agreement with the predicted data. The predicted value is used in the exposure assessment.

References:

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

OECD SIDS (2007). SIDS Initial Assessment Report for SIAM 24, Paris, France, 17-20 April 2007, Ammonia Category.

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.