Dimethoxydimethylsilane

Phototransformation in water

Link to relevant study record(s)

Reference

Reference 1

Endpoint:

phototransformation in water

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

% Degr.:

44

Sampling time:

30 d

Test condition:

natural sunlight

Remarks on result:

other: mole% conversion of dimethylsilanediol to methylsilanetriol and silicic acid

Transformation products:

yes

PHOTOCHEMICAL REACTIVITY

Since dimethylsilanediol exhibits no UV absorption in the region of 220 – 400 nm, chromophoric impurities were postulated to explain the observed photochemical reactivity. Thus, a series of dimethylsilanediol solutions containing potential inorganic chromophores were photolyzed to test this postulate. The results indicated that only the oxides of nitrogen significantly mediated the demethylation process by showing an evident correlation between NOx concentrations and the extent of demethylation. Since the enhancement of demethylation rates was even observed at part per billion levels of NOx chromophore species, the miniscule amounts of demethylation observed with distilled water and with sulfate, phosphate and iodide salts may also be attributable to trace NOx impurities. Furthermore, there are sufficient levels of nitrates and nitrites in inland lakes and waterways to anticipate significant photolytic oxidative demethylation of methylsiloxanols in these media. The data obtained for natural waters showed good correlation between chromophore concentration and extent of demethylation.

DEMETHYLATION WITH NATURAL SUNLIGHT

Demethylation with natural sunlight was also demonstrated for 60 µg/mL tetramethyl­disiloxanediol (dimer diol) in distilled water with 10 µg/mL added sodium nitrate after 30 d exposure to central Michigan mid-summer sunlight. After 30 d exposure, a 44 mole % conversion of dimethylsilanediol to methylsilanetriol and silicic acid in a mole ratio of about 3:1 was observed.

DEMETHYLATION PROCESS

The term “demethylation” in the context of nitrate-mediated photolytic degradations does not refer to a direct hydrolytic cleavage of Si-CH3 bond to form methane. On the contrary, the process was shown to be oxidative rather than hydrolytic by simply purging the solutions with N2 to remove most of the oxygen prior to photolysis. Furthermore, CO2 was conclusively identified by GLC, IR-MS analysis as the only gaseous product of photolytic demethylations. It is well established that the aqueous photolysis of NOx species in the presence of oxygen produces the highly reactive hydroxyl radical (OH) and it was presumed to be the responsible species for the observed “demethylations”.

DIATOM STUDIES

The effects of dimethylsilanediol and photolysis on diatom growth were investigated.

No significant growth was noted in media with no added silicate or photolyzed dimethylsilanediol showing that diatoms are not capable of metabolizing even simple water-soluble dimethyl substituted silicon species. Excellent growth curves were obtained in diatom medium containing Na2SiO3 and in Si-free diatom medium with dimethylsilanediol exposed to UV (18 d) prior to diatom inoculation, which contained photolytic degradation products from dimethylsilanediol as the only source of silicon-containing nutrient. Analytical results confirm the ecological acceptability of demethylated silicon moieties, demonstrate that the transformation products have no significant toxicity to diatoms and show that even monomethyl silicon moieties are not utilized as silicic acid sources by diatoms.

 

Conclusions:

An experimental study on photolytic oxidative demethylation of aqueous Dimethylsilanediol (source substance) resulted into a 44 mole % conversion of dimethylsilanediol to methylsilanetriol and silicic acid in a mole ratio of about 3:1 after 30 d of exposure to central Michigan mid-summer sunlight. DMSD is the hydrolysis product of the target substance. Since the hydrolysis half life is < 1 h the results of the hydrolysis product describes the expected results for the target substance Dimethoxydimethylsilane.

Reference 2

Endpoint:

phototransformation in water

Type of information:

experimental study

Adequacy of study:

key study

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

study well documented, meets generally accepted scientific principles, acceptable for assessment

Remarks:

The study was well documented and meets generally accepted scientific principles, but was not conducted according to OECD testing guidelines and GLP.

Study type:

indirect photolysis

Principles of method if other than guideline:

- Principle of test: The present study consists of two sets of experiments, namely a photolytic oxidative demethylation test and a diatom test. In the photolytic oxidative demethylation test, 50 to 100 ppm solutions of standard dimethyldisilanediol in distilled water were exposed to artificial radiation (sunlamps) and to natural sunlight and the reaction products were analyzed by GC-analysis for an accurate determination of dimethyl-, monomethyl- and non-methyl- containing silicon moieties. The accuracy of this method was ± 2% and > 90% of the initial diol was typically recovered as diol or degradation products. The exposure time for irradiation was 3 d to a few weeks. In the diatom test, the possible metabolism of simple methylsilanols by diatoms was assessed to determine the bio-equivalence of Q-species derived from polydimethylsiloxanes (PDMS) and natural silicic acid.
- Parameters analysed / observed: In the photolysis study, demethylation was assessed by measuring the concentration of D, T and Q species as a function of exposure time. In the diatom test, the effects of (demethylated) PDMS on the growth of diatom colonies was assessed.

GLP compliance:

no

Radiolabelling:

no

Analytical method:

gas chromatography

Details on sampling:

- Sampling intervals for the parent/transformation products: After 0, 7, 13 and 16 min exposure time

Light source:

other: sunlamps and natural sunlight

Light spectrum: wavelength in nm:

> 290

Details on light source:

LIGHT SOURCE 1: GE-275 RS sunlamps
- Emission wavelength spectrum: > 290 nm
- Filters used and their purpose: To further ensure the complete absence of short wavelength radiation, solutions were shielded by 2 - 3 mm thick Pyrex glass.
- Light intensity at sample and area irradiated: The exposure intensity at 365 nm was estimated to be approximately 5 times that of direct natural sunlight.
- Duration of light: 2 - 3 d exposure
- Test vessel for light exposure in the apparatus: Clear Teflon® flexible-walled bottles
- Other: Control solutions were treated identically except that the bottles were covered with aluminium foil to preclude exposure to UV radiation.

LIGHT SOURCE 2: natural sunlight (central Michigan mid-summer sunlight)
- Test vessel: The test solutions were contained in clear Teflon® bags.

Details on test conditions:

TEST SYSTEM
- Type, material and volume of test apparatus/vessels: Clear Teflon® flexible-walled bottles and Teflon® bags
- Indication of test material adsorbing to the walls of test apparatus: No
- Irradiation: λ = 365 nm

TEST MEDIUM
- Kind and purity of water: Distilled water
- Test media: Natural waters obtained at various locations (Lake Huron, Pt. AuGres, MI; Tittabawassee River, Emerson Park, Midland, MI; and the Gulf of Mexico, Rampa Bay, FL, USA). Silicon-free diatom medium and Silicon-containing diatom medium
- Preparation of test medium: Silicon-free diatom medium: 1250 mg/L KNO3, 696 mg/L K2HPO4 and 122 mg/L MgSO4.7H2O in distilled water and the trace nutrients 140 mg/L Ca(NO3)2.4H20, 110 mg/L FeSO4, 75 mg/L MnSO4.1H2O, 50 mg/L ZnSO4.7H2O, 50 mg/L H3BO3 and 1.5 mg/L CuSO3.5H2O. Silicon-containing diatom medium: 104 mg/L Na2SiO3.9H20 was added to the silicon-free medium.

REPLICATION
- No. of replicates (irradiated): Duplicate or triplicate vessels were used for every test.

Duration:

20 d

Dark controls:

yes

Computational methods:

Computer modeling of PDMS utilizing the "Unit World" Level I environmental distribution approach of Neely and Mackay [Ann Arbor Science, 1982] for environmental fate and partitioning.

% Degr.:

44

Sampling time:

30 d

Test condition:

natural sunlight

Remarks on result:

other: mole% conversion of dimethylsilanediol to methylsilanetriol and silicic acid

Transformation products:

yes

PHOTOCHEMICAL REACTIVITY

Since dimethylsilanediol exhibits no UV absorption in the region of 220 – 400 nm, chromophoric impurities were postulated to explain the observed photochemical reactivity. Thus, a series of dimethylsilanediol solutions containing potential inorganic chromophores were photolyzed to test this postulate. The results indicated that only the oxides of nitrogen significantly mediated the demethylation process by showing an evident correlation between NOx concentrations and the extent of demethylation. Since the enhancement of demethylation rates was even observed at part per billion levels of NOx chromophore species, the miniscule amounts of demethylation observed with distilled water and with sulfate, phosphate and iodide salts may also be attributable to trace NOx impurities. Furthermore, there are sufficient levels of nitrates and nitrites in inland lakes and waterways to anticipate significant photolytic oxidative demethylation of methylsiloxanols in these media. The data obtained for natural waters showed good correlation between chromophore concentration and extent of demethylation.

DEMETHYLATION WITH NATURAL SUNLIGHT

Demethylation with natural sunlight was also demonstrated for 60 µg/mL tetramethyl­disiloxanediol (dimer diol) in distilled water with 10 µg/mL added sodium nitrate after 30 d exposure to central Michigan mid-summer sunlight. After 30 d exposure, a 44 mole % conversion of dimethylsilanediol to methylsilanetriol and silicic acid in a mole ratio of about 3:1 was observed.

DEMETHYLATION PROCESS

The term “demethylation” in the context of nitrate-mediated photolytic degradations does not refer to a direct hydrolytic cleavage of Si-CH3 bond to form methane. On the contrary, the process was shown to be oxidative rather than hydrolytic by simply purging the solutions with N2 to remove most of the oxygen prior to photolysis. Furthermore, CO2 was conclusively identified by GLC, IR-MS analysis as the only gaseous product of photolytic demethylations. It is well established that the aqueous photolysis of NOx species in the presence of oxygen produces the highly reactive hydroxyl radical (OH) and it was presumed to be the responsible species for the observed “demethylations”.

DIATOM STUDIES

The effects of dimethylsilanediol and photolysis on diatom growth were investigated.

No significant growth was noted in media with no added silicate or photolyzed dimethylsilanediol showing that diatoms are not capable of metabolizing even simple water-soluble dimethyl substituted silicon species. Excellent growth curves were obtained in diatom medium containing Na2SiO3 and in Si-free diatom medium with dimethylsilanediol exposed to UV (18 d) prior to diatom inoculation, which contained photolytic degradation products from dimethylsilanediol as the only source of silicon-containing nutrient. Analytical results confirm the ecological acceptability of demethylated silicon moieties, demonstrate that the transformation products have no significant toxicity to diatoms and show that even monomethyl silicon moieties are not utilized as silicic acid sources by diatoms.

 

Validity criteria fulfilled:

not applicable

Conclusions:

An experimental study on photolytic oxidative demethylation of aqueous Dimethylsilanediol resulted into a 44 mole % conversion of dimethylsilanediol to methylsilanetriol and silicic acid in a mole ratio of about 3:1 after 30 d of exposure to central Michigan mid-summer sunlight.

Additional information

Dimethoxydimethylsilane (CAS No. 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, experiments carried out for dimethylsilanediol indicate that aqueous photolytic oxidative demethylation with tropospheric ultraviolet radiation in the presence of suitable chromophores, such as nitrogen oxides, are possible (Buch et al. 1984).