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Environmental fate & pathways

Phototransformation in air

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Description of key information

Phototransformation in air:

Rate constant for reaction with OH radicals:

Trichloro(octyl)silane: 9.6E-12 cm3/molecule.sec (half-life 1.7 days)

Octylsilanetriol: 2.0E-11 cm3/molecule.sec (half-life 0.8 days)

Key value for chemical safety assessment

Half-life in air:
1.7 d
Degradation rate constant with OH radicals:
0 cm³ molecule-1 s-1

Additional information

No measured data are available for trichloro(octyl)silane.

Trichloro(octyl)silane and its hydrolysis product octylsilanetriol 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 kOH for reaction of trichloro(octyl)silane and octylsilanetriol 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 = 5E05 OH molecules/ cm3

DT50 = half-life

 

The concentration of hydroxyl radicals in air of 5E05 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 4.1.2: Results of photodegradation in air calculations

Parameter

Result, trichloro(octyl)silane

Result, octylsilanetriol

kOH(cm3/ molecule.sec)

9.6E-12

2.0E-11

kdegair (d-1)

0.41

0.86

DT50(days)

1.7

0.8

The performance of the predictive method (AOPWIN, v1.92) is validated up to a reaction rate of 1E-10 cm3/mol.sec (equivalent to a half-life 0.16 day under European conditions of typical atmospheric hydroxyl radical concentration, as defined in ECHA Guidance R16 (ECHA 2016). The reaction rate is therefore acceptable for use in environmental exposure assessment.

Trichloro(octyl)silane is hydrolytically unstable. Therefore, reaction with water vapour rather than photodegradation is expected to be the primary degradation process in air.

This interpretation is supported by a simulated nose-only exposure study (Dow Corning Corporation 2013) with a chlorosilane substance (see Section IUCLID 5.1.2, CSR Section 4.1.1). The study was conducted to determine the hydrolytic stability of dimethyldichlorosilane (CAS 75-78-5) under conditions typical of nose-only vapour inhalation exposures. The estimated hydrolysis half-life at pH 7, 22°C and 57% RH was in the range 3-11 seconds (half-life calculated from the data by the reviewer of the report for REACH technical dossier). 57% RH is within the typical range of RH of indoor/outdoor air. This half-life is similar to the measured half-life for dimethyldichlorosilane in water: 1-5 seconds at 22°C (result extrapolated to 22°C from 1.5°C by the reviewer of the report for REACH technical dossier).

Measured phototransformation in air for other organosilanes:

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). European Chemicals Agency. Guidance on information requirements and chemical safety assessment Chapter R.16: Environmental Exposure Estimation. Version: 3.0 February 2016. R.16 -3.2.2C. 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.

Dow Corning Corporation (2013). Hydrolytic stability of Dimethyldichlorosilane Under Conditions Typical of Nose-Only Vapor Inhalation Exposures. Dow Corning Corporation Health and Environmental Sciences Technical Report. Study Number 12356-116.