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Environmental fate & pathways

Phototransformation in air

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Endpoint:
phototransformation in air
Type of information:
read-across from supporting substance (structural analogue or surrogate)
Adequacy of study:
key study
Justification for type of information:
Please refer to the justification for grouping of substances provided in IUCLID Section 13.
Reason / purpose for cross-reference:
read-across source
Preliminary study:
Prior to the product study, an exploratory photolysis of an air mixutre of demethylsilanediol, CH3ONO and NO with initial concentration of 1.8 E+14, 2.4 E+14 and 1.8 E+14 molecules/cm³ was carried out.
Reaction with:
OH radicals
Rate constant:
0 cm³ molecule-1 s-1
Transformation products:
yes

KINETIC STUDY (IR Spectra and Dark Decay in the 5870 L chamber)

Vapor samples of dimethylsilanediol ((CH3)2Si(OH)2; 1.3 E+14 molecules/cm³) showed the initial presence and growth of another component with time. From the known amount of the sample and the differences between the spectra with time, the spectrum of the decay products at t = 61 min and of dimethylsilanediol at t = 0 were derived. The disappearance of (CH3)2Si(OH)2 was faster in the evacuated chamber than in the presence of 1 atm of air. In both cases, however, the decays could be described as first order in (CH3)2Si(OH)2, with observed decay rates of 0.015 min-¹ at reduced pressure (< 5 Torr N2 diluent) and 0.0022 min-¹ at 1 atm.

The observed decay rates of (CH3)2Si(OH)2 inits mixtures with (CH3)2O and O3 prior to injection of N2H4 were in the range of 0.6 – 1.3 E-²/min and these decay losses (due to self-reaction and/or wall losses) were significant fractions (48 – 62%) of the total losses after introduction of N2H4. Hence, only one injection of N2H4 per experiment was made, with the last spectral record of the “pre-reaction” decay measurement being used to establish the initial concentrations of (CH3)2Si(OH)2 and (CH3)2O and the final concentrations being determined from the next spectrum recorded after N2H4 injection and the mixing time of ca. 3 min duration. The elapsed time between these 2 spectra was 6 – 8 min. No measurable decays of the reference compound (CH3)2O in the reaction chamber prior to N2H4 injection were observed. The data was plotted and resulted in a slope for (k1/k2) of 0.285 ± 0.02 (where k1 and k2 are the rate constants for reactions 1 and 2 respectively, see below).

(1)   Organosilicon + OH -> products

(2)   (CH3)2O + OH -> products

 

The k1/k2 value can be converted to an absolute value by plugging in 2.84 E-12 cm3/molecule* s for k2, the rate constant for the (CH3)2O + OH reaction (reaction 2), resulting in a rate constant of 0.81 ± 0.10 E+12 x k1 (cm³ molecule-1s-1), corresponding to 8.1 ± 1.0 E-13 cm3/molecule*s.

TROPOSPHERIC LIFETIME

Based on the measured OH radical rate constant and a 24 h average OH radical concentration of 1.0 E+06 molecules/cm³, a tropospheric lifetime of 15 d was calculated for dimethylsilanediol.

PRODUCT STUDY

Product studies for dimethylsilanediol could not be carried out on pure samples due to its inherent tendency to form condensation products in the bulk sample or in the gaseous reaction mixtures. Thus, the investigation of the fates of the alkoxy radicals, formed after the initial OH radical and/or Cl atom abstraction of an H atom from a methyl group was complicated by formation and subsequent reaction of these impurities. For diemthylsilanediol, the possible interferences from a number of these products limited the analysis by FT-IR spectroscopy, although the product spectra provided evidence for the formation of formate ester intermediates. API-MS analysis indicated the formation of methylsilanetriol from dimethylsilanediol.

The findings provide support for the following tropospheric degradation scheme:

(CH3)2Si(OH)2 --> CH3Si(OH)2C•H2 (reaction with OH)

CH3Si(OH)2C•H2 --> CH3Si(OH)2OCH2O• (reaction with O2)

CH3Si(OH)2OCH2O --> CH3Si(OH)2OCHO (reaction with O2)

CH3Si(OH)2OCHO --> CH3Si(OH)3 (reaction with H2O)

Since dimethylsilanediol will be released into the atmosphere at concentrations where autocondensation will be negligible, incorporation into aerosols (and subsequent washout) could be significant removal pathways for dimethylsilanediol in the atmosphere in addition to reaction with OH radicals and wet deposition from the gas phase.

Conclusions:
An experimental rate study with the source substance Dimethylsilanediol (DMSD) found a reaction with the OH radical rate constant of 8.1E-13 cm³/molecule and second. DMSD is the hydrolysis product of the target substance. Therefore comparable results are expected with the target substance Dimethoxydimethylsilane.

Description of key information

Key value for chemical safety assessment

Additional information

Dimethoxydimethylsilane (CAS 1112-39-6) hydrolyses rapidly to dimethylsilanediol and methanol under environmental conditions (DT50 < 0.6 h at pH 7, pH 9 and 25 °C). Thus, the hydrolysis products rather than the parent substance are expected to be present in the environment after environmental release.

Direct photolysis is unlikely to be relevant for both hydrolysis products as they contain no chromophores that would absorb visible or UV radiation. However, indirect photolysis with hydroxyl radicals may occur.

Measured photodegradation rate constant data for indirect photodegradation with hydroxyl radicals are available for both dimethylsilanediol and methanol. A rate constant value for reactions with hydroxyl radicals of 8.1E-13 cm3/molecule*sec was determined for DMSD using a relevant test method (Tuazon et al. 2000). Using an average concentration of OH-radicals of 5E+5 mol cm-3, a half-life of 20 days can thus be estimated.

Methanol is degraded in the atmosphere by photochemical, hydroxyl-radical dependent reactions, with a rate constant of 0.932E-12 cm3/molecule*sec (Atkinson, 1989). A half-life in the troposphere of about 17 days can be estimated.

In addition, the AOPWIN program (v1.92, EPA 2010) has been used to obtain a value of the rate constant kOH for reaction of dimethoxydimethylsilane with hydroxyl radicals. The rate constant of 1.96E-12 cm3/molecule*sec, results in an estimated half-life of about 8 days for the parent substance. 

Thus both hydrolysis products and the parent substance dimethoxydimethylsilane are slowly degraded by photochemical processes in air.

Reference:

Atkinson R (1989). Kinetic and Mechanisms of the Gas-Phase Reactions of the Hydroxyl Radical with Organic Compounds. Journal of Physical and Chemical Reference Data. Monograph No. 1: 160