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Environmental fate & pathways

Hydrolysis

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Endpoint:
hydrolysis
Type of information:
(Q)SAR
Adequacy of study:
key study
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
Principles of method if other than guideline:
The result was obtained using an appropriate QSAR method (see attached QMRF and QPRF for details)
GLP compliance:
no
Transformation products:
yes
No.:
#1
No.:
#2
pH:
7
DT50:
5.7 h
Remarks on result:
other: 20-25°C
pH:
4
DT50:
0.3 h
Remarks on result:
other: 20-25 °C
pH:
9
DT50:
0.1 h
Remarks on result:
other: 20-25 °C
Conclusions:
A hydrolysis half life value of approximately 5.7 h at 20-25°C and pH 7 was obtained using an accepted calculation method. The result is considered to be reliable.
Endpoint:
hydrolysis
Type of information:
experimental study
Adequacy of study:
supporting study
Study period:
2017-01-12 to TBD
Reliability:
1 (reliable without restriction)
Rationale for reliability incl. deficiencies:
test procedure in accordance with generally accepted scientific standards and described in sufficient detail
Justification for type of information:
Please refer to the attached justification for grouping of substances provided in IUCLID Section 13.
Reason / purpose:
read-across source
Preliminary study:
The preliminary study was not required because methyltrimethoxysilane is known to be unstable in water.
Transformation products:
yes
No.:
#1
Details on hydrolysis and appearance of transformation product(s):
The decrease in concentration with time of methyltrimethoxysilane and the increase of concentration with time of methanol with time were followed (see Figure 1 and Figure 2). The anticipated reaction scheme is presented in the attachments section.
% Recovery:
>= 87.5 - <= 104
pH:
3
Temp.:
37 °C
Duration:
1 h
Remarks on result:
other: A molar recovery based on the quantity of methyltrimethoxysilane and molar equivalents of its hydrolysis product methanol (3 moles of methanol were assumed per mole of methyltrimethoxysilane) was between 87.5 and 104% at each time point
Key result
pH:
3
Temp.:
37 °C
Hydrolysis rate constant:
0.021 min-1
DT50:
33.3 min
Type:
(pseudo-)first order (= half-life)

Description of key information

Hydrolysis: half-life ca. 5.7 h at 20-25°C and pH 7 

Key value for chemical safety assessment

Additional information

A half-life value of approximately 5.7 h at 20-25°C and pH 7 was obtained using an accepted validated QSAR method (Peter Fisk Associates 2011a). The result is considered to be reliable and has been assigned as key study.

A QSAR that is currently being developed (Peter Fisk Associates 2011b) predicts half-lives at 20-25°C of 0.3 h at pH 4 and 0.1 h at pH 9. As the hydrolysis reaction may be acid or base catalysed, the rate of reaction is expected to be slowest at 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. This is supported by studies for various organosilicon compounds in which calculation of kH3O+ and kOH- from the experimental results at pH 4 and 9, respectively, resulted in reasonable estimates of the half-life at pH 7 (PFA, 2013a).

Therefore, at low pH:

kobs≈kH3O+[H3O+]

At pH 4 [H3O+]=10-4 mol dm-3 and at pH 2 [H3O+]=10-2 mol 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 trimethoxy(2,4,4-trimethylpentyl)silane at pH 2 is therefore 5 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 for trimethoxy(2,4,4-trimethylpentyl)silane 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) x e(0.08.(T-X))

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

Thus, for trimethoxy(2,4,4-trimethylpentyl)silane the hydrolysis half-life 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 are (2,4,4-trimethylpentyl)silanetriol and methanol.

In another supporting study (CRL, 2017), the hydrolysis of methyltrimethoxysilane dosed in corn oil was investigated under conditions design to mimic the rat stomach. The half-life for disappearance of methyltrimethoxysilane applied in corn oil to gastric simulation buffer was 33 mins at pH 3 and 37°C. The data suggest that hydrolysis occurs rapidly once methyltrimethoxysilane is in contact with the aqueous layer and the rate determining step is the transfer of the methyltrimethoxysilane from the corn oil to the water. Methanol content increased proportionally to the decrease in methyltrimethoxysilane and combined recoveries of methanol and methyltrimethoxysilane ranged from 87.5 to 104%. The study was conducted according to an appropriate test protocol and is considered reliable.