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EC number: 208-762-8
CAS number: 540-97-6
Phototransformation in air: Rate constant for reaction with OH radicals: Parent substance 2.62 E-12 cm3 / molecule. sec (half-life 6.12 days).
D6 is a member of the Reconsile Siloxane Category; siloxanes within the
Category do not contain 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 occurs.
The Category hypothesis is that the rate of photo-oxidation by hydroxyl
radicals of a compound is dependent on the constituent functional
groups. A reliable experimental relative rates study with D4 and
D5 (Atkinson, R., 1991) found that the NO3 radical and O3reactions
are of no importance as tropospheric removal processes for these
substances. The dominant gas-phase chemical loss process is by reaction
with the OH radical.
Measured data are available for D6; ameasured OH radical
rate constant (kOH) of 2.44 x 10-12cm3/
molecule.sec was determined byKim and Xu (2017), based on a
relative rate method. The value obtained by Safronet al. (2015) of2.80
x 10-12 cm3/ molecule.sec is
considered to be in good agreement.
The EPIWIN AOPWIN programme has also been used unadapted to obtain
values of the rate constant kOHfor reaction with hydroxyl
radicals. The overall half-life in air under default conditions of
hydroxyl radical concentration is calculated using the following
kdegair(d-1) = kOH(cm3/molecule.sec)
x OH Concair(molecules/cm3) x 24
DT50(d) = ln 2/ kdegair(d-1)
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 E+05 OH molecules/ cm3
The concentration of hydroxyl radicals in air of 5 E+05 OH
molecules/cm3, and the 24 hour photo period, are the values
specified in ECHA Guidance on Information requirements and chemical
safety assessment, Part R.16 Environmental exposure estimation
(R.220.127.116.11. Photochemical reactions in the atmosphere) (ECHA, 2016).
The results are given in the table below.
Table: Results of AOPWIN photodegradation in air calculations
1.8 x 10-12
7.8 x 10-02
Kim and Xu (2017) also determined rate constants for three other
cyclic siloxanes and four linear siloxanes, and compared the measured
mean values of OH radical rate constants with the literature values for
each substance. For D6, the measured values from the current study were
statistically similar to those of Safron et al. but apparently
greater than those of AOPWIN prediction. The measured values obtained by
Kim and Xu for other volatile methyl siloxanes (VMS) showed greater
variabilities with available literature values (see Table below). The
arithmetic mean of the measured rate constants is therefore considered
to give the best representation of the OH radical rate constant for the
relevant VMS species. For D6, the mean measured value of 2.62x 10-12 cm3/
molecule.sec is used as the key value for exposure assessment,
equivalent to a half-life in air of 6.1 days.
Table: Reconsile Siloxane Category: Measured data and AOPWIN predictions
for reaction with hydroxyl radicals in air.
Rate constant for reaction with hydroxyl radicals (kOH(cm3/ molecule. sec))
1.19 x 10-12(Sommerladeet al., 1993)
0.90 x 10-12(AOPWIN)
1.38 x 10-12(Atkinson, 1991)
1.32 x 10-12(Markgraf and Wells, 1997)
1.58 x 10-12(Kim and Xu, 2017)
1.83 x 10-12(Markgraf and Wells, 1997)
2.15 x 10-12(Kim and Xu, 2017)
1.20 x 10-12(AOPWIN)
2.66 x 10-12(Markgraf and Wells, 1997)
3.37 x 10-12(Kim and Xu, 2017)
1.50 x 10-12(AOPWIN)
4.03 x 10-12(Kim and Xu, 2017)
1.80 x 10-12(AOPWIN)
0.52 x 10-12(Atkinson, 1991)
1.84 x 10-12(Xiao et al. 2015)
0.91 x 10-12(Kim and Xu, 2017)
1.26 x 10-12(Sommerladeet al., 1993)
1.01 x 10-12(Atkinson, 1991)
1.90 x 10-12(Safron et al. 2015)
2.34 x 10-12(Xiao et al. 2015)
0.95 x 10-12(Kim and Xu, 2017)
1.55 x 10-12(Atkinson, 1991)
2.60 x 10-12(Safron et al. 2015)
2.46 x 10-12(Xiao et al. 2015)
1.46 x 10-12(Kim and Xu, 2017)
2.44 x 10-12(Kim and Xu, 2017)
2.80 x 10-12 (Safron et al. 2015)
1.8 x 10-12(AOPWIN)
Whelan et al. (2004) assessed the atmospheric fate of
volatile methyl siloxanes (VMS) and their degradation products. The
assessment used a simple equilibrium-partitioning model to investigate
the relative rates of removal of two representative VMS (the linear
siloxane decamethyltetrasiloxane, L4, and the cyclic siloxane, D4) and
their hydroxyl-substituted degradation products by reaction and
atmospheric deposition. The modelling is based on the work of Atkinson,
R. (1991) and Sommerlade, R. et al.(1993), which demonstrates
that siloxanes break down in the atmosphere to form hydroxyl-substituted
degradation products by reaction with OH radicals. As substitution
proceeds the siloxanols and silanols become increasingly water-soluble
and less volatile, and so tend to be washed out of the atmosphere by wet
deposition. The silanols are also assumed to undergo hydrolysis
reactions when dissolved in liquid water droplets. Removal of the
silanols from the atmosphere by dry deposition is also accounted for.
The model indicated that L4 and D4 and the monohydroxy degradation
products occur mainly in the vapour phase, whereas the further
degradation products occur mainly in the dissolved and particulate
phases. Overall, it is concluded that >99% of L4 and D4 are removed from
the atmosphere as silanols in wet deposition and <1% are removed in dry
All the aforementioned studies are focused on homogenous
reactions. Although they definitely represent the major characteristics
of D6 degradation in the atmospheric environment, the real atmosphere is
much more complex. For example, the atmosphere contains a combination of
multiple oxidants such as O3, OH and other free radicals, as
well as UV radiation and aerosols. In addition, release of cyclic
volatile methyl siloxanes (cVMS) also follows distinguishable spatial
and temporal patterns: they are released mostly to the urban and
suburban atmosphere where the O3, OH radical and aerosol
concentrations may be much higher than the rural or remote regions. When
cVMS are transported from the source region through the air, they move
along with those oxidants, which may increase their exposure to the
intensified atmospheric degradation processes. The overall half-lives of
cVMS therefore may largely depend on the resident time of cVMS in urban
and suburban atmosphere.
In order to better understand the environmental fate of cVMS under
more realistic atmospheric conditions, several projects were initiated
both at the University of Iowa and Dow Corning Corporation. The specific
objectives of those studies are twofold: To determine the removal of
gas-phase cVMS by combination of multiple oxidants in the presence of UV
radiation and aerosols; and to determine the effects of the unique
release pattern of cVMS and any additional removal mechanism on the
overall half-lives of cVMS in the atmosphere. Data were obtained for the
analogous cVMS, D4 and D5, but the trends may be extrapolated to D6.
In the University of Iowa studies, the uptake of D4 and D5 vapours
(up to saturated vapour pressure) by carbon black and several types of
reactive mineral dust aerosols in the absence and presence of O3 and
hydroxyl radicals was simulated in an atmospheric chamber at room
temperature and monitored by FT-IR spectroscopy (Navea et al.,
2009a, b, c). It was found that the heterogeneous uptake (removal from
gas phase) of D4 and D5 by mineral aerosols such as kaolinite, hematite
and quartz was rapid and significant (Navea et al., 2009a). Under
dry conditions (< 1% RH) in the absence of O3and hydroxyl
radicals, removal of both D4 and D5 by hematite and kaolinite were
characterised by two processes. The initial fast removal process
completed within one minute accounts for 30 to 50% total removal for D4,
but 50~70% for D5. The subsequent slow removal process had a
pseudo-first order kinetics with rate constant k at room temperature
varying in the range of 0.5~0.9 x10-3s-1(corresponding
to half-lives from 24~13 minutes) for D4, and in a range of 0.2~4 x 10-3s-1(corresponding
to half-lives from 53 minutes to 12 minutes) for D5 for hematite and
kaolinite, respectively. The reactivity of various aerosols was in the
order, kaolinite > hematite > quartz, for both D4 and D5 after the
reaction rates were normalised to surface area of the aerosols.
In the presence of O3but no solar radiation, no
detectable change was observed in the concentration of the gas phase D4
or D5 after 50 minutes exposure (Navea et al., 2009b), consistent
with the low rate constant obtained for O3/D4 reactions in
the previous study (Atkinson, 1991). Under dry (< 1% RH) conditions, the
introduction of aerosols such as hematite and kaolinite triggered
immediate removal of both cVMS and O3(Navea et al.,
2009b). The kinetics of the gas phase removal apparently implied
multiple heterogeneous processes, significantly different than that
obtained with aerosols in the absence of O3. The major
differences were in two aspects: first, addition of O3 slowed
the uptake of the both D4 and D5 by the aerosols, while existence of D4
and D5 in the gas phase also slowed the decomposition of gas phase O3
relative to the aerosol-containing control with no cVMS,
suggesting the competition of cVMS with O3for surface sites
of the aerosols (Navea et al., 2009b). In addition, the constant
concentration profile observed at reaction times greater than 200
minutes in the cVMS/aerosol systems without O3was replaced
with a linear decline in D4 and D5 concentrations with increase of the
reaction time when O3was present. The disappearance of the
surface saturation characteristics (constant concentration profile as
time increases) increased the total removal over a longer time period
(Navea et al., 2009b).
Under simulated solar radiation and in the presence of O3,
hematite and kaolinite aerosols remove up to 50 to 70 % gas phase D4 and
60 to 90% gas phase D5 within 400 minutes under dry condition (< 1% RH)
(Navea et al., 2009c). An increase in humidity under those
conditions actually accelerates the removal presumably due to formation
of hydroxyl radicals through photolysis of O3in the presence
The University of Iowa’s studies were conducted under relatively
high concentrations of cVMS (>10 mg l-1) due to limitation
of the non-destructive analytical technique employed for cVMS analysis.
In follow-up studies conducted at Dow Corning Corporation (Kim et al.,
2009; Kim and Xu, 2009a, b), the mechanism for D4 sorption in the high
concentration range is verified as polymerisation of the sorbed D4
catalysed by clay surface. Modelling assessment results suggested that
aerosol effect on the overall D4 degradation in natural environment
should be relatively small by this polymerization mechanism (Navea et
al., 2010). However, D4 concentration in atmosphere is in nanogram
to micrograms per cubic meter, or 3 to 6 orders of magnitude lower than
those tested in Navea’s studies.
At low D4 concentration range, reactive adsorption via
depolymerization was observed on aerosol surface (Kim and Xu, 2009b).
Under those conditions (initial concentration of D4 (CD4)<
0.3 mg/l, RH 10~80%), 60% to 97% of sorbed D4 was not desorbable (Kim
and Xu, 2009a and b). Surface speciation analysis of the sorbed D4
revealed that almost all sorbed D4 in this low concentration range was
transformed to silanols within 2 hours at 28% RH, due to
surface-facilitated hydrolysis (or depolymerisation) (Kim and Xu,
2009b), similar to that found in dry soil (Xu 1999; Xu and Chandra,
Similar studies have been carried out with D5 (Kim and Xu, 2010
and 2011), and showed similar results, i. e. the mechanism for
adsorption at high concentration range is polymerisation of the sorbed
D5 by the clay surface, whereas at the low D5 concentration range,
reactive adsorption via depolymerisation was observed on the aerosol
surface (sorbed D5 being transformed to silanols and smaller cyclics (D3
and D4) as transient intermediates).
A recent study (Kim, J. and Xu, S., 2016) investigated the
sorption and desorption behaviours of airborne VMS (including D5) on
nine major primary and secondary atmospheric aerosols (RH 30%). It was
found that sorption and desorption of VMS took place via a two-phase
process, which included an initial rapid step, followed by a
slower subsequent step. The initial rapid step was favoured especially
at low concentrations. Some aerosols such as carbon black and sea salts
interacted reversibly with D5 whereas other aerosols such as kaolinite
and sulphates showed highly irreversible sorption for the VMS,
especially at low concentrations. Values of apparent aerosol-air
partition coefficients ranged from 2.1 to 284 L/m(2) for D5, with carbon
black having the largest values.
These results suggest that the heterogeneous interaction of D5
with mineral aerosols, therefore, can be an important mechanism in
reducing the concentrations and transport of this volatile siloxane
compound in the environment. The exact effects from this
depolymerisation by aerosol on the half-life of airborne D5 could not be
estimated at this juncture. Nevertheless, the actual half-life of D5 in
air should be shorter than that calculated based solely on the
homogenous reaction rate.
More recent work using actual field monitoring data has tested
this hypothesis (Xu et al., 2017). Evidence is presented there
which shows that on the basis of measurements of the concentration of D6
in the northern hemisphere, the degradation rate of D6 in air could be
significantly faster than the rate based on the rate constants reported
above, indicating that there may be additional removal processes for
airborne D6 currently not accounted for in the determination of
half-life in air based on photodegradation alone.
Kim J, Xu S. 2017. Quantitative structure-reactivity relationships of
hydroxyl radical rate constants for linear and cyclic volatile
methylsiloxanes. Environmental Toxicology and Chemistry 36:3240-3245.
Atkinson R. 1991. Kinetics of the gas-phase reactions of a series of
organosilicon compounds with hydroxyl and nitrate(NO3) radicals and
ozone at 297 .+-. 2 K. Environmental Science & Technology 25:863-866.
Xiao R, Zammit I, Wei Z, Hu W-P, MacLeod M, Spinney R. 2015. Kinetics
and Mechanism of the Oxidation of Cyclic Methylsiloxanes by Hydroxyl
Radical in the Gas Phase: An Experimental and Theoretical Study.
Environmental Science & Technology 49:13322-13330.
Sommerlade R, Parlar H, Wrobel D, 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.
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Safron A, Strandell M, Kierkegaard A, Macleod M. 2015. Rate Constants
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Markgraf SJ, Wells JR. 1997. The hydroxyl radical reaction rate
constants and atmospheric reaction products of three siloxanes.
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D.(2017).Long-range transport potential and atmospheric persistence of
cyclic volatile methylsiloxanes based on global measurements.
Chemosphere. Volume 228, August 2019, Pages 460-468
Navea, J. G., Xu, S., Stanier, C. O., Young, M. A. and Grassian,V.
H. (2009a). Heterogeneous uptake of octamethylcyclotetrasiloxane (D4)
and decamethylcyclopentasiloxane (D5) onto mineral dust aerosol under
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V. H. (2009b). Effect of ozone and relative humidity on the heterogenous
uptake of octamethylcyclotetrasiloxane and decamethylcyclopentasiloxane
on model mineral dust aerosol components. J. Phys. Chem. A. 113:
Navea, J. G., Stanier, C. O., Young, M. A. and Grassian, V.
H. (2009c). A Laboratory and Modeling Study at the University of Iowa
Designed to Better Understand the Atmospheric Fate of D4 and D5. Final
Report (August 2006 – July 2007) Centre Européen des Silicones (CES).
Navea, J., Young, M. A., Xu, S., Grassian, V. H. and Stanier, C.
O. (2010). The atmospheric lifetimes and concentrations of cyclic
methylsiloxanes octamethylcyclotetrasiloxane (D4) and
decamethylcyclopentasiloxane (D5) and the influence of heterogeneous
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ECHA. (2016). European Chemicals Agency. Guidance on information
requirements and chemical safety assessment Chapter R.16: Environmental
Exposure Estimation. Version: 3.0 February 2016. A.16-3.2.2 Degradation
rates in the environment
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