Trimethoxyvinylsilane

Hydrolysis

Link to relevant study record(s)

Reference

Reference 1

Endpoint:

hydrolysis

Type of information:

experimental study

Adequacy of study:

weight of evidence

Reliability:

4 (not assignable)

Rationale for reliability incl. deficiencies:

abstract

Remarks:

Only a summary report was available for review.

Principles of method if other than guideline:

Experimental measurement (information about the methodology is not available)

GLP compliance:

not specified

Transformation products:

not measured

Key result

pH:

7

Temp.:

23 °C

Hydrolysis rate constant:

2.9 h-1

DT50:

0.24 h

Key result

pH:

4

Temp.:

23 °C

Hydrolysis rate constant:

71.8 h-1

DT50:

0.58 min

Other kinetic parameters:

at pH9 t1/2 and k =not measurable.

Conclusions:

Hydrolysis half-lives of 0.58 minutes at pH 4 and 0.24 hours at pH 7 were reported for the substance in a summary report.

Reference 2

Endpoint:

hydrolysis

Type of information:

experimental study

Adequacy of study:

weight of evidence

Study period:

2001-07-24 to 2001-07-30

Reliability:

1 (reliable without restriction)

Rationale for reliability incl. deficiencies:

guideline study

Qualifier:

according to guideline

Guideline:

OECD Guideline 111 (Hydrolysis as a Function of pH)

Version / remarks:

May 12 1981

Qualifier:

according to guideline

Guideline:

EU Method C.7 (Degradation: Abiotic Degradation: Hydrolysis as a Function of pH)

Version / remarks:

EEC directive 92/69, December 1992

GLP compliance:

yes

Radiolabelling:

no

Analytical monitoring:

yes

Details on sampling:

Analysis was performed before incubation, after 2.4 hours and after 120 hours.

Buffers:

Buffer pH 4, Biphthalate, Baker Art. No 5657
Buffer pH 7, Phosphate, Baker Art. No 5656
Buffer pH 9, Borate/potassium chloride/NaOH, Baker Art. No 7145

The buffer solutions were sterilised for 25 minutes in an autoclave prior to first use. Nitrogen was passed through the buffer solutions for 5 minutes except when freshly sterilised.

Details on test conditions:

Reagents:
Water, RCC from Milli-Q supply
Acetonitrile, Baker, Art. No 9017

Equipment:
Glassware: All glassware which must be inert in the pH range applied, were rinsed with sterile buffer. The hydrolysis was carried out in flasks which were stoppered or sealed with an inert material (e.g. PTFE).
Water baths
Thermostatically controlled water bath, GFL 1086

Method:
The test item was dissolved in the buffer solutions and incubated at 50°C with the water bath kept constant at usually 0.1°C. The concentration of the test item was determined as a function of pH.

Test performance:
A preliminary test was performed at 50°C at each of pH 4, pH 7 and pH 9. Aliquots of each test solution were analysed in time intervals using the analytical method stated above, GC.

Preparation of test solutions:
pH 4: 100 mg of test substance was dissolved in 100 mL buffer solution (pH 4.0) to prepare a test solution of 1000 µg/mL. For complete dissolution, no further sample treatment was necessary. Two aliquots of this test solution of approximately 50 mL each were transferred into 50 mL Erlenmeyer flasks in order to perform duplicate test.

pH 7: 100.7 mg of test substance was dissolved in 100 mL buffer solution (pH 7.0) to prepare a test solution of 1007 µg/mL. For complete dissolution, no further sample treatment was necessary. Two aliquots of this test solution of approximately 50 mL each were transferred into 50 mL Erlenmeyer flasks in order to perform duplicate test.

pH 9: 105.8 mg of test substance was dissolved in 100 mL buffer solution (pH 9.0) to prepare a test solution of 1058 µg/mL. For complete dissolution, no further sample treatment was necessary. Two aliquots of this test solution of approximately 50 mL each were transferred into 50 mL Erlenmeyer flasks in order to perform duplicate test.

Number of replicates:

Duplicates

Positive controls:

no

Negative controls:

no

Transformation products:

not measured

Key result

pH:

4

Temp.:

50 °C

DT50:

< 10 min

Key result

pH:

7

Temp.:

50 °C

DT50:

< 2.4 h

Key result

pH:

9

Temp.:

50 °C

DT50:

< 10 min

Details on results:

When conducting this study two guidelines were followed, the OECD 111 and EC directive 92/69, C.7. The latter defines two criteria to stop the study after the preliminary test: "If the preliminary test indicates that 50% or more of the test substance has been hydrolysed in 2.4 hours at 50°C, or less than 10% has been hydrolysed after five days at each of the three pH values (4, 7 and 9) no further testing is necessary" At pH 7 more than 50% of the test item was actually hydrolysed after 2.4 hours, therefore no further testing was conducted. VTMS hydrolysis produces 3 moles of methanol for each mole of ethenyl silanetriol. Only the methoxy groups will be hydrolysed. The trisilanol condenses to siloxane oligomers; this condensation of silanols is affected by both concentration and pH, and since both change over time it is not feasible to isolate specific silanols for analysis (the structures continue to evolve until they either reach equilibrium or precipitate out of solution). This condensation to form highly cross-linked, high molecular weight polymers, further reduces the potential for exposure. If the VTMS is slowly released such that the concentration of the resulting ethenyl silanetriol is not high enough to result in polymerization, the ethenyl silanetriol will exist largely as a monomer.

The results of pH 4.0 and pH 9.0 show that the test substance was very unstable in aqueous test solutions and starts to hydrolyse immediately after being dissolved in the buffer (no test substance could be detected in the samples at the first time point; which was immediately after starting the test).

At pH 7.0 the analytical results indicate that the degradation of the test substance was more than 50% after 2.4 hours. It can be concluded that the estimated half-life times at pH 4.0, pH 7.0 and pH 9.0 are shorter than one day under representative environmental conditions (25°C). Therefore, no further testing was necessary at these pH values.

The results for individual measurements are in Table 1 below.

Table 1: Results of the preliminary test

Sample solution	Initial concentration [µg/mL]	Concentration measured after 2.4 hours incubation [µg/mL]	Concentration measured after 5 days incubation [µg/mL]	Hydrolysis reaction after 2.4 hours incubation [%]
pH 4	n.d. n.d.	n.d. n.d.	n.d. n.d.	- -
pH 7	298 256	n.d. n.d.	n.d. n.d.	>50 >50
pH 9	n.d. n.d.	n.d. n.d.	n.d. n.d.	- -

n.d. = not detected

The stability of the test substance in buffer solution pH 4 and pH 9 was very low. The test item could not be detected in any of the sample solutions.

Conclusions:

Hydrolysis half-lives of <10 minutes at pH 4, <2.4 hours at pH 7 and <10 minutes at pH 9 and 50°C were determined for the substance in a reliable preliminary study conducted according to OECD 111 and under GLP. Limited details are available.

Reference 3

Endpoint:

hydrolysis

Type of information:

(Q)SAR

Adequacy of study:

weight of evidence

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

results derived from a valid (Q)SAR model and falling into its applicability domain, with adequate and reliable documentation / justification

Justification for type of information:

See attached QMRFs/QPRFs

Principles of method if other than guideline:

The result was obtained using an appropriate QSAR method (see attached QMRF and QPRF for details).

The model for hydrolysis at pH 7 has been developed for, and applies specifically to di- and tri-alkoxysilanes. It is a multiple linear regression based model with descriptors representing (i) steric effects of the alkoxy group, (ii) steric effects of the side-chain(s), and (iii) electronic effects of the side-chain(s).

The models for hydrolysis at pH 4, pH 5 and pH 9 have been developed for, and apply specifically to organosilicon compounds. They are linear regression based models where the descriptor is the half-life at pH 7.

Transformation products:

yes

No.:

#1

No.:

#2

Key result

pH:

4

DT50:

0.04 h

Remarks on result:

other: 20 - 25°C

Key result

pH:

5

DT50:

0.1 h

Remarks on result:

other: 20 -25°C

Key result

pH:

7

DT50:

0.1 h

Remarks on result:

other: 20 - 25°C

Key result

pH:

9

DT50:

0.004 h

Remarks on result:

other: 20 -25°C

Conclusions:

Hydrolysis half-lives of 0.04 hours at pH 4, 0.1 h at pH 7 and 0.004 h at pH 9 and 20-25°C were obtained for the substance using an accepted calculation method. The result is considered to be reliable.

Description of key information

Hydrolysis: 0.04 h at pH 4, 0.1 h at pH 7 and 0.004 h at pH 9 and 20-25°C (QSAR)

Additional information

Hydrolysis half-lives at 20 -25°C of 0.04 h at pH 4, 0.1 h at pH 7 and 0.004 h at pH 9 were determined for the substance using a validated QSAR estimation method. Also, in a preliminary study conducted in accordance with OECD 111, hydrolysis half-lives of <10 minutes at pH 4, <2.4 h at pH 7 and <10 minutes at pH 9 and 50°C were determined for the substance. Both results were considered to be reliable and used as weight of evidence. A half-life of 0.24 h at pH 7 and 23°C, from a study of non-assignable reliability adds further weight of evidence to the very rapid hydrolysis potential of the submission substance.

As the hydrolysis reaction may be acid or base catalysed, the rate of reaction is expected to be slowest at around pH 7 and increase as the pH is raised or lowered. For an acid-base catalysed reaction in buffered solution, the measured rate constant is a linear combination of terms describing contributions from the uncatalyzed reaction as well as catalysis by hydronium, hydroxide, and general acids or bases.

k_obs = k₀ + k_H3O+[H3O⁺] + k_OH^-[OH^-] + k_a[acid] + k_b[base]

At extremes of pH and under standard hydrolysis test conditions, it is reasonable to suggest that the rate of hydrolysis is dominated by either the hydronium or hydroxide catalysed mechanism.

Therefore, at low pH:

k_obs ˜ k_H3O+[H3O⁺]

At pH 4 [H₃O⁺] = 10^-4 mol dm^-3 and at pH 2 [H₃O⁺] = 10^-2 mol dm^-3; therefore, k_obs at pH 2 should be approximately 100 times greater than k_obs at pH 4.

The half-life of a substance at pH 2 is calculated based on:

t_1/2(pH 2) = t_1/2(pH 4) / 100

The calculated half-life of the substance at pH 2 is therefore less than 2 seconds. However, it is not appropriate or necessary to attempt to predict accurately when the half-life is less than 5-10 seconds. As a worst-case it can therefore be considered that the half-life of the substance at pH 2 and 20-25°C is approximately 5 seconds.

Reaction rate increases with temperature therefore hydrolysis will be faster at physiologically relevant temperatures compared to standard laboratory conditions. Under ideal conditions, hydrolysis rate can be recalculated according to the equation:

DT₅₀(XºC) = DT₅₀(T) * e^(0.08.(T-X))

Where T = temperature for which data are available and X = target temperature.

Thus, for trimethoxy(vinyl)silane the hydrolysis half-life at 37.5ºC and pH 7 (relevant for lungs and blood) is approximately 2 minutes. At 37.5ºC and pH 2 (relevant for conditions in the stomach following oral exposure), it is not appropriate to apply any further correction for temperature to the limit value and the hydrolysis half -life is therefore approximately 5 seconds.

The hydrolysis products in this case are vinylsilanetriol (CAS 143-48-6) and methanol (CAS 67-56-1; EC No. 200-659-6).

The hydrolysis data for substances used in this dossier for read-across purposes for other endpoints are now discussed.

Hydrolysis of the read-across substance trichloro(vinyl)silane (CAS 75-94-5; EC No. 200-917-8)

Data for the substance, trichloro(vinyl)silane (CAS 75-94-5) are read-across to the submission substance trimethoxy(vinyl)silane for toxicity to microorganisms endpoint. The silanol hydrolysis product and the rate of hydrolysis of the two substances are relevant to this read-across, as discussed in the appropriate section for the endpoint.

For trichloro(vinyl)silane, hydrolysis half-lives at 1.5°C of <1 minute at pH 4, pH 7 and pH 9 were read-across from trichloro(methyl)silane in accordance with OECD Test Guideline 111 (Dow Corning Corporation 2001).

The hydrolysis products are vinylsilanetriol and hydrochloric acid.

Hydrolysis of the read-across substance diethoxy(dimethyl)silane (CAS 78-62-6; EC No. 201-127-6)

Data for the substance, diethoxy(dimethyl)silane (CAS 78-62-6) are read-across to the submission substance trimethoxy(vinyl)silane for basic toxicokinetics endpoint. The rate of hydrolysis of the two substances are relevant to this read-across, as discussed in the appropriate section for the endpoint.

For diethoxy(dimethyl)silane, hydrolysis half-lives at 20-25°C of 0.3 hours at pH 4, 5.5 hours at pH 7 and 0.3 hours at pH 9 were determined using validated QSAR estimation methods.

The hydrolysis products are dimethylsilanediol and ethanol.