4,4,13,13-tetraethoxy-3,14-dioxa-8,9-dithia-4,13-disilahexadecane

Phototransformation in air

Description of key information

Phototransformation in air:

Phototransformation in air: Rate constant for reaction with OH radicals:

4,4,13,13-Tetraethoxy-3,14-dioxa-8,9-dithia-4,13-disilahexadecane: 2.9E-10 cm³/molecule.sec (half-life 0.1 days)

 

(3-{[3-(Trihydroxysilyl)propyl]disulfanyl}propyl)silanetriol: 2.7E-10 cm³/molecule.sec (half-life 0.1 days)

Key value for chemical safety assessment

Half-life in air:

0.1 d

Degradation rate constant with OH radicals:

0 cm³ molecule-1 s-1

Additional information

No measured data are available for 4,4,13,13-tetraethoxy-3,14-dioxa-8,9-dithia-4,13-disilahexadecane (S2).

 

4,4,13,13 -Tetraethoxy-3,14-dioxa-8,9-dithia-4,13-disilahexadecane (S2) and its hydrolysis products (3-{[3-(trihydroxysilyl)propyl]disulfanyl}propyl)silanetriol and ethanol 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 constants k_OH for the reactions of 4,4,13,13-tetraethoxy-3,14-dioxa-8,9-dithia-4,13-disilahexadecane (S2) and its hydrolysis product, (3-{[3-(trihydroxysilyl)propyl]disulfanyl}propyl)silanetriol, 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:

 

kdeg_air(d^-1) = k_OH(cm³/molecule.sec) x OH Conc_air(molecules/cm³) x 24 x 3600

 

DT₅₀(d) = ln 2/ kdeg_air(d^-1)

 

Where:

kdeg_air= total rate constant for degradation in air

k_OH= rate constant for reaction with hydroxyl radicals

OH Conc_air= concentration of hydroxyl radicals in air = 5E05 OH molecules/ cm³

DT₅₀= half-life

 

The concentration of hydroxyl radicals in air of 5E+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	Result, S2	Result, silanol hydrolysis product
kOH (cm^3/ molecule.sec)	2.9E-10	2.7E-10
kdegair (d^-1)	12.5	11.7
DT50 (days)	0.1	0.1

 

S2 and its silanol hydrolysis product may undergo indirect phototransformation; calculated half-lives for the reaction with hydroxyl radicals in air are 0.1 days for both S2 and the silanol hydrolysis product.

 

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.

 

Measured data for other organosilane substances:

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 (cm^3/ 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.