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Environmental fate & pathways

Hydrolysis

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Description of key information

Hydrolysis half-life: 40 - 90 h at pH 7, 0.8 - 1.2 h at pH 4 and 0.5 - 0.9 h at pH 9 and 20-25°C (QSAR)

Key value for chemical safety assessment

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

Additional information

A hydrolysis study is not considered to be technically feasible because the water solubility of the substance is low (≤ 1 mg/l) and analytical method of sufficient sensitivity is not available. Clariant 1998, reported that the substance has a very low water solubility; therefore, at concentrations required for the hydrolysis study, no suitable analytical method is available.

The hydrolysis half-life of the substance has been calculated using a validated QSAR estimation method to be 0.8 - 1.2 h at pH 4, 0.8 - 1.0 h at pH 5, 40 - 90 h at pH 7 and 0.5 - 0.9 h at pH 9 and 20-25°C. The result is 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 uncatalyzed 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:

kobskH3O+[H3O+]

At pH 4 [H3O+] =10-4 mol dm-3and at pH 2 [H3O+] =10-2mol dm-3; therefore, kobs at pH 2 should be approximately 100 times greater than kobs at 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 and 20 -25°C is therefore 0.008 - 0.012 hours (approximately 30 - 40 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) x e(0.08.(T-X))

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

Thus, for the substance, hydrolysis half-life at 37.5ºC and pH 7 (relevant for lungs and blood) is approximately 15 - 30 hours. At 37.5ºC and pH 2 (relevant for conditions in the stomach following oral exposure), the hydrolysis half-life is approximately 11 - 14 seconds.

The hydrolysis products are (3-{[3-(trihydroxysilyl)propyl]disulfanyl}propyl)silanetriol and ethanol. 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 Polysulfides, bis[3-(triethoxysilyl)propyl] (CAS Number: 211519-85-6)

Data for the substance polysulfides, bis[3-(triethoxysilyl)propyl] (CAS 211519-85-6) are read-across to the submission substance 4,4,13,13-tetraethoxy-3,14-dioxa-8,9-dithia-4,13-disilahexadecane for appropriate endpoints (see Section 1.4 of the CSR).

The properties of both substances 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. Polysulfides, bis[3-(triethoxysilyl)propyl] (CAS No: 211519-85-6) is also known as Reaction mass of 4,4,15,15-tetraethoxy-3,16-dioxa-8,9,10,11-tetrathia-4,15-disilaoctadecane and 4,4,14,14-tetraethoxy-3,15-dioxa-8,9,10-trithia-4,14-disilaheptadecane and 4,4,13,13-tetraethoxy-3,14-dioxa-8,9-dithia-4,13-disilahexadecane. It is a multiconstituent substance with three constituents. Each constituent is a bis[3-triethoxysilyl)propyl)polysulfide, where the number of sulfur atoms is 2 to 4. The constituents may be denoted as S2, S3 and S4 respectively.

For the three constituents S2, S3 and S4; hydrolysis half-lives at 2 0 - 25°C of 40 - 90 h, 40 - 110 h and 40 - 130 h at pH 7 respectively were estimated by QSAR . At pH 4 and 20 -25°C, half-lives are 0.8 – 1.2 h (S2), 0.8 – 1.4 h (S3) and 0.8 – 1.5 h (S4); Similarly, at pH 9, half-lives are 0.5 – 1 h (S2), 0.5 – 1.1 h (S3) and 0.5 – 1.4 h (S4). (See attached EPSR).

The half-lives at pH 2 and 20 - 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 half-lives of 0.008 – 0.012 h (approximately 30 - 40 seconds), 0.008 – 0.014 h (approximately 30 - 50 seconds) and 0.008 – 0.015 h (approximately 30 - 54 seconds) at pH 2 and 20 - 25°C for S2, S3 and S4 respectively. At pH 2 and 37.5°C, half-lives of S2, S3 and S4 are approximately 11 – 14 seconds, 11 – 18 seconds and 11 – 20 seconds respectively. Similarly, at pH 7 and 37.5°C, half-lives of S2, S3 and S4 are approximately 15- 30 h, 15- 40 h and 15 – 50 h respectively.

The hydrolysis products for S2, S3 and S4 respectively are (3-{[3-(trihydroxysilyl)propyl]disulfanyl}propyl)silanetriol, (3-{[3-(trihydroxysilyl)propyl]trisulfanyl}propyl)silanetriol, [3-({[3-(trihydroxysilyl)propyl]disulfanyl}disulfanyl)propyl]silanetriol and ethanol.