Tetramethyl orthosilicate

Phototransformation in air

Key value for chemical safety assessment

Half-life in air:

4.9 d

Degradation rate constant with OH radicals:

3.3 cm³ molecule-1 s-1

Additional information

The SRC AOPWIN program (v1.92) has been used to obtain values of the rate constant k_OHfor reaction oftetramethyl orthosilicatewith hydroxyl radicals. This prediction method has not been validated to assess applicability to silanes and silanols; therefore, there is uncertainty associated with the calculated values obtained.Silicic acidcannot be further oxidised by OH radicals.

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 = 5 x 10⁵OH molecules/ cm³

DT₅₀= half-life

The results are given in Table 4.1.2.

Table 4.1.2 Results of photodegradation in air calculations

Parameter	Result, tetramethyl orthosilicate
kOH(cm^3/ molecule.sec)	3.3 x 10^-12
kdegair (d-^1)	0.14
DT50(days)	4.9

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 and13 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 have 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, 1.4x 10^-12cm³molecule^-1s^-1for hexamethyldisiloxane, 0.5 x 10^-12cm³molecule^-1s^-1for hexamethylcyclotrisiloxane, 1.0

x 10^-12cm³molecule^-1s^-1for octamethylcyclotetrasiloxane, 1.6 for x 10^-12cm³molecule^-1s^-1fordecamethylcyclopentasiloxane. Using the equation above, the corresponding half-lives are 16, 12, 31, 16 and 10 days, respectively.

The SRC 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 dimethylsilanetriol, 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 SRC 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 slightly lower rates of reaction to the AOPWIN predictions and the two previous studies above.

References:

EC (2003a) European Union Technical Guidance Document on Risk Assessment for New and Existing Substances, Part II, European Chemicals Bureau, 2003.

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.