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Environmental fate & pathways

Hydrolysis

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Description of key information

Hydrolysis half-lives: <0.033 h at pH 4, 2.2 h at pH 7 and 0.11 h at pH 9 and 25°C (OECD 111)

Key value for chemical safety assessment

Half-life for hydrolysis:
2.2 h
at the temperature of:
25 °C

Additional information

Hydrolysis half-lives at 25°C of <0.033 h at pH 4, 2.2 h at pH 7 and 0.11 h at pH 9 were determined for the substance in accordance with OECD 111 and in compliance with GLP. The result is considered to be reliable and selected as key study. In a supporting reliable study, the stability of the substance in aqueous  media under physiological conditions was investigated. The rates of hydrolysis of 1000 ppm trimethoxy(methyl)silane were determined in water at  pH 5.7, 0.15 molar (M) sodium-phosphate buffer (PBS), and 10% rat  serum in  0.15M PBS at pH 7.4 and 37.4°C in soft glass reactors. In this study, the substance was hydrolysed in water, PBS, and PBS plus 10% rat serum at pH 7.4 and 37°C with half-lives of 24, 6.7 and 8.6 minutes respectively. This is also supported by a result in secondary literature of non assignable reliability, which reports a half-life of 23 minutes at pH 5.7 and 37.4°C.

In another supporting study (CRL 2017), the hydrolysis of trimethoxy(methyl)silane was investigated under conditions designed to mimic the rat stomach after dosing the substance in corn oil. The half-life for disappearance of trimethoxy(methyl)silane applied in corn oil to gastric simulation buffer was 33 mins at pH 3 and 37°C and appears to be determined by phase transfer. The data suggest that, in the investigated system, hydrolysis occurs rapidly once trimethoxy(methyl)silane comes into contact with the aqueous layer and the rate determining step is the transfer of the trimethoxy(methyl)silane from the corn oil to the water. Combined recoveries of trimethoxy(methyl)silane and methanol (in mole equivalents of trimethoxy(methyl)silane; 3 moles methanol to 1 mole trimethoxy(methyl)silane assumed) were 87.5 to 104% and methanol content increased proportionally to the decrease in trimethoxy(methyl)silane. The study was conducted according to an appropriate test protocol and is considered reliable.

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 uncatalysed reaction as well as catalysis by hydronium, hydroxide, and general acids or bases.

kobs= k0+ kH3O+[H3O+] + kOH-[OH-] + ka[acid] + kb[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:

kobs≈kH3O+[H3O+]

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

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

t1/2(pH 2) = t1/2(pH 4) / 100

The calculated half-life of the substance at pH 2 is therefore less than 2 seconds at 25°C. However, it is likely that factors such as diffusion become rate-determining 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 room temperature 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°C) * e(0.08.(T-X))

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

Thus, for trimethyoxy(methyl)silane the hydrolysis half-life at 37.5ºC and pH 7 (relevant for lungs and blood) is approximately 0.8 hours. 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 methylsilanetriol and methanol.

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 trimethoxy(propyl)silane (CAS Number: 1067-25-0)

The hydrolysis data for substances used in this dossier for read-across purposes for other endpoints are now discussed. Data for the substance trimethoxy(propyl)silane (CAS 1067-25-0) are read-across to the submission substance trimethoxy(methyl)silane for appropriate endpoints (see Section 1.4 of the CSR). The silanol hydrolysis product and the rate of hydrolysis of the two substances are relevant to this read-across, as discussed in the appropriate Sections of the CSR for each endpoint.

For trimethoxy(propyl)silane, hydrolysis half-lives at 25°C of 0.2 h at pH 4, 2.6 h at pH 7 and 0.1 h at pH 9 were predicted for the substance using a validated QSAR estimation method (PFA 2012: QSAR Prediction Reporting Format (QPRF) for the hydrolysis of triemethyoxypropylsilane. Owner company: Reconsile).

The half-lives at pH 2 and 25°C, at pH 7 and 37.5°C and at pH 2 and 37.5°C may be calculated in the same way as for the registration substance above. This gives a half-life of 0.002 h (7.2 seconds) at pH 2 and 25°C, and approximately 1 hour at pH 7 and 37.5°C. It is likely that factors such as diffusion become rate-determining when the half-life is less than 5 - 10 seconds. As a worst-case, it can be considered that the half-life for the substance at pH 2 and 37.5°C is approximately 5 seconds.

The hydrolysis products are propylsilanetriol and methanol.

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(ethyl)silane (CAS Number: 115-21-9)

Data for the substance trichloro(ethyl)silane (CAS 115-21-9) are read-across to the submission substance trimethoxy(methyl)silane for appropriate endpoints (see Section 1.4 of the CSR). The silanol hydrolysis product and the rate of hydrolysis of the two substances are relevant to this read-across, as discussed in the appropriate Sections of the CSR for each endpoint.

For trichloro(ethyl)silane, hydrolysis half-lives at 1.5°C of <1 minute at pH 4, pH 7 and pH 9 were determined for the substance in accordance with OECD 111 test method (Miller, J. A.; Kozerski, G. E 2001. Hydrolysis Screening Studies of HPV Chlorosilanes. Testing laboratory: Dow Corning Corporation, Midland, USA. Owner company: SEHSC. Report number: 2001-I0000-50823. Report date: 2001-03-26).

The hydrolysis products are ethylsilanetriol and hydrochloric acid.

Hydrolysis of the read-across substance triethoxy(methyl)silane (CAS 2031 -67 -6)

Data for the substance triethoxy(methyl)silane(CAS 2031-67-6) are read-across to the submission substance trimethoxy(methyl)silane for appropriate endpoints (see Section 1.4).The silanol hydrolysis product of the two substances is relevant to this read-across, as discussed in the appropriate Sections of the CSR for each endpoint.

For triethoxy(methyl)silane, hydrolysis half-lives at 25°C of 0.3 h at pH 4, 5.5 h at pH 7 and 0.1 h at pH 9 were determined in using a validated QSAR estimation method.

The hydrolysis products are methylsilanetriol and ethanol.