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Reference
Endpoint:
hydrolysis
Type of information:
(Q)SAR
Adequacy of study:
key study
Study period:
2011
Reliability:
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
other: The result was obtained by an appropriate predictive method.
Justification for type of information:
QSAR prediction: migrated from IUCLID 5.6
Principles of method if other than guideline:
The result was obtained using an appropriate QSAR method (see attached QMRF and QPRF for details).
Transformation products:
yes
No.:
#1
No.:
#2
pH:
7
DT50:
0.7 h
Remarks on result:
other: 20 - 25°C
pH:
4
DT50:
0.1 h
Remarks on result:
other: 20 - 25°C
pH:
9
DT50:
0.02 h
Remarks on result:
other: 20 - 25°C
Conclusions:
A hydrolysis half life of 1 hours was obtained for the substance using an appropriate calculation method. The result is considered to be reliable.

Description of key information

Half-life for hydrolysis: approximately 0.7 h at 20 - 25°C

Key value for chemical safety assessment

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

Additional information

A half-life of 0.7 hours for hydrolysis of the triethoxy group was obtained for the substance using an appropriate calculation method. The result is considered to be reliable.

The initial hydrolysis products are silanetriol and ethanol.The Si-H bond may also be unstable, resulting in further hydrolysis toSi(OH)4.Silanols may undergo condensation reactions to form siloxane dimers, oligomers and polymers. In the case of triethoxysilane, where no organic side-chains are present, rapid condensation to inorganic siloxanes and finally silica is expected. The precise rate of this condensation and the species present are dependent on conditions.

A QSAR that is currently being developed (Peter Fisk Associates 2012c) predicts half-lives at 20-25°C of 0.1 h at pH 4, 0.2 h at pH 5 and 0.02 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.

Therefore, at low pH:

kobs≈kH3O+[H3O+]

At pH 4 [H3O+]=10-4mol dm-3and at pH2 [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

As a worst-case it can therefore be considered that the half-life for triethoxysilane 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.

For triethoxysilane the hydrolysis half-life at 37.5ºC and pH2 (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 initial hydrolysis products are silanetriol and ethanol.