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Environmental fate & pathways

Hydrolysis

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Description of key information

In a non-guideline study to compare hydrolysis rates of four cyclic methylsiloxane compounds at pH7, a hydrolysis rate constant was established (reliability 2 study). A hydrolysis half-life of 2.2 minutes at pH 7 and 22.5°C was determined. The half-life refers to degradation of parent substance by ring-opening; full hydrolysis takes longer (approximately 1 day).

Key value for chemical safety assessment

Half-life for hydrolysis:
2.2 min
at the temperature of:
22.5 °C

Additional information

In a non-guideline, reliable study, the disappearance of the submission substance was measured from an aqueous solution buffered at pH 7, with a co-solvent to improve solubility. Hydrolytic degradation of the test substance is rapid even under pH 7 conditions, associated with the greatest stability of the positive control in an OECD 111 guideline study. The resulting half-life for hydrolysis is 2.2 minutes.

ICP analysis demonstrates that disappearance of test substance was degradation, not volatilization.

Quantitative evolution of hydrogen gas from hydrolysis of the Si-H bonds of the test substance further supports that mass balance was conserved under the test conditions, but was relatively slow compared to disappearance of the parent compound; this indicated that ring-opening hydrolysis of the cyclic siloxane was likely the initial mechanism of degradation, followed by a slower rate of hydrolysis of the SiH groups. Time to 80% conversion of all SiH groups was approximately 20 hours. The authors of the study concluded thatultimate degradation product of hydrolysis is very likely methylsilanetriol.

However, at higher concentrations the condensation of methylsilanetriol or the intermediate hydrolysis products may mean that polymeric species are formed. The hydrolysis half-lives at pH 7 and 20-25°C of the expected intermediate products of siloxane hydrolysis have been predicted using a validated QSAR method (PFA 2014). The substance is predicted to hydrolyse rapidly to methylsilanediol, which is then expected to hydrolyse further to methylsilanetriol.

-[Si(H)(Me)O]- + H2O → HOSi(H)(Me)[OSi(H)(Me)]2OSi(H)(Me)OH: 0.02 h (1 min)

HOSi(H)(Me)[OSi(H)(Me)]2OSi(H)(Me)OH + H2O → HOSi(H)(Me)OSi(H)(Me)OSi(H)(Me)OH: 0.2 h

HOSi(H)(Me)OSi(H)(Me)OSi(H)(Me)OH +H2O → HOSi(H)(Me)OSi(H)(Me)OH: 0.4 h

HOSi(H)(Me)OSi(H)(Me)OH + H2O → MeSi(H)(OH)2: 1 h

Hydrolysis of the read-across substance 2,4,6,8,10-pentamethylcyclopentasiloxane (CAS 6166-86-5; HD5)

Data for some endpoints are read-across from the analogue substance2,4,6,8,10-pentamethylcyclopentasiloxane (CAS 6166-86-5; HD5). The hydrolysis of the two substances is relevant to this read-across as discussed in the appropriate endpoint summaries. Both substances ultimately produce methylsilanetriol, with the intermediate hydrolysis products being siloxanediols and hydrogen being a by-product of the reaction.

In a non-guideline study (Miller JA, 2010) to compare hydrolysis rates of four cyclic methylsiloxane compounds at pH7, a hydrolysis rate constant for2,4,6,8,10-pentamethylcyclopentasiloxane (CAS 6166-86-5; HD5)was established (reliability 2 study). A hydrolysis half-life of 4.2 minutes at pH 7 and 22.5°C was determined. The half-life refers to degradation of parent substance by ring-opening; full hydrolysis takes longer (approximately 1 day).

The half-lives at other relevant temperatures and pH values may be calculated in the same way as for the submission substance above. This gives the following values:

Half-life at pH 7 and 22.5°C: 4.2 minutes

Half-life at pH 7 and 37.5°C: 1.3 minutes

Half-life at pH 4 and 22.5°C: 0.03 h (2 minutes)

Half-life at pH 9 and 22.5°C: 0.002 h (7 seconds)

Half-life at pH 2 and 22.5°C: 5 seconds

Half-life at pH 2 and 37.5°C: 5 seconds

Hydrolysis of the read-across substance dichloro(methyl)silane (CAS 75-54-7)

Data for some endpoints are read-across from the analogue substance dichloro(methyl)silane (CAS 75-54-7). The hydrolysis of the two substances is relevant to this read-across as discussed in the appropriate endpoint summaries.

The half-life at pH 4, 7 and 9 for dichloro(methyl)silane is <1 minute at pH 4, 7 and 9 and 25°C based on read-across within the category of chlorosilanes (PFA, 2013ab).

Dichloro(methyl)silane hydrolyses to methylsilanediol and hydrogen chloride and then reacts further to give methylsilanetriol and hydrogen.

Hydrolysis of the read-across substance trimethoxy(methyl)silane (CAS 1185-55-3)

Data for some endpoints are read-across from the analogue substance trimethoxy(methyl)silane (CAS 1185-55-3). The hydrolysis of the two substances is relevant to this read-across as discussed in the appropriate endpoint summaries.

Hydrolysis half-lives for trimethoxy(methyl)silane 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 in a reliable study conducted according to an appropriate test protocol, and in compliance with GLP (Miller JA 2004). The half-life 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 approximately 5 seconds.

Trimethoxy(methyl)silane hydrolyses to methylsilanetriol and methanol.