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Environmental fate & pathways

Hydrolysis

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Description of key information

Hydrolysis half-life: 0.4 h at pH 4, 0.4 h at pH 5, 8.8 h at pH 7, 0.1 h at pH 9 and 20-25°C (QSAR)

Key value for chemical safety assessment

Half-life for hydrolysis:
8.8 h
at the temperature of:
20 °C

Additional information

Hydrolysis half-lives at 20 - 25°C of 0.4 h at pH 4, 0.4 h at pH 5, 8.8 h at pH 7 and 0.1 h at pH 9 were determined for the substance using a validated QSAR estimation method. The results are considered to be 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 0.004 hours (approximately 14 seconds) at 20 - 25°C.

 

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 [2-(perfluorohexyl)ethyl]trimethoxysilane the hydrolysis half-life at 37.5ºC and pH 7 (relevant for lungs and blood) is approximately 3.2 hours. At 37.5ºC and pH 2 (relevant for conditions in the stomach following oral exposure), the half-life is 0.001 hours (approximately 4 seconds). 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 37.5°C is approximately 5 seconds.

 

The hydrolysis products in this case are [2-(perfluorohexyl)ethyl]silanetriol and methanol.

The hydrolysis half-lives of substances used for read-across in other areas are discussed below:

Hydrolysis of the read-across substance [2-(perfluorohexyl)ethyl]triethoxysilane (CAS No: 51851-37-7)

Data for the substance [2-(perfluorohexyl)ethyl]triethoxysilane (CAS No 51851-37-7) are read-across to the submission substance [2-(perfluorohexyl)ethyl]trimethoxysilane (CAS No 85857-16-5) for appropriate endpoints. The hydrolysis half-life and the silanol hydrolysis product of the two substances are relevant to this read-across, as discussed in the appropriate Sections for each endpoint.

 

For [2-(perfluorohexyl)ethyl]triethoxysilane, hydrolysis half-lives of 1.44 hours at 20°C, 0.9 hours at 30°C, 0.474 hours at 40°C and pH 4; 12.5 hours at 20°C, 5.25 hours at 30°C, 2.74 hours at 40°C and pH 7 and 5.18 hours at 20°C, 2.61 hours at 30°C, 1.38 hours at 40°C and pH 9 were determined in accordance with OECD 111 Test Guideline and in compliance with GLP (Lange 2016).

 

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 submission substance above. This gives a half-life of 0.0144 hours (approximately 52 seconds) at pH 2 and 20°C and 3.1 hours at pH 7 and 37.5°C. At pH 7 and 40°C, measured half-life is 2.7 hours. At pH 2 and 37.5°C, the half-life is 0.0036 hours (approximately 13 seconds).

 

The hydrolysis products are [2-(perfluorohexyl)ethyl]silanetriol and ethanol.

 

Hydrolysis of the read-across substance [2-(perfluorohexyl)ethyl]dichloro(methyl)silane  (CAS No: 73609-36-6)

Data for the substance [2-(perfluorohexyl)ethyl]dichloro(methyl)silane (CAS No 73609-36-6) are read-across to the submission substance [2-(perfluorohexyl)ethyl]trimethoxysilane (CAS No 85857-16-5) for appropriate endpoints.The hydrolysis half-life and the silanol hydrolysis product of the two substances are relevant to this read-across, as discussed in the appropriate sections for each endpoint.

 

For [2-(perfluorohexyl)ethyl]dichloro(methyl)silane, hydrolysis half-lives are read-across from analogous dichlorosilane substances.

Hydrolysis half-lives of 10 seconds at pH 4, 17 seconds at pH 7 and 7 seconds at pH 9 and 1.5°C were determined for dichloro(dimethyl)silane in accordance with OECD 111 (Dow Corning Corporation, 2001).

 

Hydrolysis half-lives of 8 seconds at pH 4, 12 seconds at pH 7 and 9 seconds at pH 9 and 1.5°C were determined for dichloromethyl(3,3,3-trifluoropropyl)silane in accordance with OECD 111 (Dow Corning Corporation, 2001).

 

Hydrolysis half-lives of 6 seconds at pH 4, 10 seconds at pH 7 and 8 seconds at pH 9 and 1.5°C were determined for dichloro(diphenyl)silane in accordance with OECD 111 (Dow Corning Corporation, 2001). Hydrolysis half-lives of <1 minute at pH 4, pH 7 and pH 9 and 1.5°C were determined in accordance with OECD 111 (Dow Corning Corporation, 2001).

 

Measured hydrolysis half-lives of <<27 mins at pH 4, pH 7 and pH 9 and 27°C were determined for dichlorocyclohexylmethylsilane in a study conducted according to generally acceptable scientific principles (Haas, 2012). Only a preliminary study was carried out and a more precise knowledge of the half-life would be needed for use in the chemical safety assessment.

 

Given the very rapid hydrolysis rates in water observed for all tested dichlorosilanes ( ≤17 seconds at 1.5°C and pH 4, 7 and 9)

, and the lack of significant variation in the half-lives for the different substances, it is considered appropriate to read-across this result to [2-(perfluorohexyl)ethyl]dichloro(methyl)silane.

 

Since rate of hydrolysis is faster at increased temperature, at ambient conditions (20 - 25°C), relevant to the environment, the hydrolysis half-lives are expected to be faster. Therefore, half-lives at pH 2 and 25°C, at pH 7 and 37.5°C and at pH 2 and 37.5°C, as a worst case considered to be approximately 5 seconds.

 

The hydrolysis products are [2-(perfluorohexyl)ethyl]methylsilanediol and hydrogen chloride.