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Reference
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, but not (completely) 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
Estimation method (if used):
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:
2.6 h
Type:
(pseudo-)first order (= half-life)
Remarks on result:
other: 20-25 °C
pH:
4
DT50:
0.2 h
Type:
(pseudo-)first order (= half-life)
Remarks on result:
other: 20-25 °C
pH:
5
DT50:
0.3 h
Type:
(pseudo-)first order (= half-life)
Remarks on result:
other: 20-25 °C
pH:
9
DT50:
0.1 h
Type:
(pseudo-)first order (= half-life)
Remarks on result:
other: 20-25 °C

The result was obtained using an appropriate QSAR method (see attached QMRF and QPRF for details).

Description of key information

Dimethoxymethyloctadecylsilane CAS 70851-50-2: half-life approx. 2.6 h at 20-25 °C and pH 7 (QSAR)

Key value for chemical safety assessment

Half-life for hydrolysis:
2.6 h

Additional information

Once released into the water phase, hydrolysis contributes significantly to the degradation of this substance. A QSAR that has been developed (Peter Fisk Associates, 2014) predicts half-lives of 2.6 h at pH 7, 0.2 h at pH 4 and 0.1 h at pH 9 (20-25 °C). 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 extreme 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 (Peter Fisk Associates, 2014).

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

The calculated half-life of dimethoxymethyloctadecylsilane at pH 2 is therefore 0.002 h. However, it is likely that factors such as diffusion become rate-determining when the half-life is less than 5-10 seconds. Reaction rate increases with temperature therefore hydrolysis will be faster at physiologically relevant temperatures compared to standard laboratory conditions and the half-lives at 37.5 °C (relevant for in vivo studies) are expected to be faster than those at 20-25°C. Under ideal conditions, hydrolysis rate can be recalculated according to the equation:

DT50(X°C) = DT50(T) x e(0.08*(T-X))

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

Thus, for dimethoxymethyloctadecylsilane the hydrolysis half-life at 37.5 °C and pH 2 is very fast (relevant for conditions in the stomach following oral exposure) and 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 of dimethoxymethyloctadecylsilane (CAS 70851-50-2) are octadecylmethylsilanediol and methanol.