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Environmental fate & pathways

Hydrolysis

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Description of key information

Hydrolysis half-lives

Constituent 1: <0.033 h at pH 4, 2.2 h at pH 7 and 0.11 h at pH 9 and 25°C (OECD 111)

Constituent 2: 0.2 h at pH 4, 2.6 h at pH 7, and 0.1 h at pH 9 and 20 -25°C (QSAR)

Constituent 3: 0.4 h at pH 4, 8 h at pH 7 and 0.1 h at pH 9 and 20-25°C (QSAR)

Constituent 4: 0.3 h at pH 4, 6 h at pH 7 and 0.1 h at pH 9 and 20-25°C (QSAR)

Key value for chemical safety assessment

Additional information

The regstration substance is a multi-constituent substance containing four constituents. Under the dilute conditions relevant for the environment, the hydrolysis of each constituent can be considered separately. The hydrolysis reactions are illustrated in the attached figure.

 

Constituent 1: Trimethoxy(methyl)silane

Hydrolysis half-lives 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 for the substance in accordance with OECD 111 and in compliance with GLP. The result is considered to be reliable.

In a supporting reliable study, the stability of the substance in aqueous media under physiological conditions was investigated. The rates of hydrolysis of 1000 ppm trimethoxy(methyl)silane were determined in water at pH 5.7, 0.15 molar (M) sodium-phosphate buffer (PBS), and 10% rat serum in 0.15M PBS at pH 7.4 and 37.4°C in soft glass reactors. In this study, the substance was hydrolysed in water, PBS, and PBS plus 10% rat serum at pH 7.4 and 37°C with half-lives of 24, 6.7 and 8.6 minutes respectively. This is also supported by a result in secondary literature of non-assignable reliability, which reports a half-life of 23 minutes at pH 5.7 and 37.4°C. In another supporting study (CRL 2017), the hydrolysis of trimethoxy(methyl)silane was investigated under conditions designed to mimic the rat stomach after dosing the substance in corn oil. The half-life for disappearance of trimethoxy(methyl)silane applied in corn oil to gastric simulation buffer was 33 mins at pH 3 and 37°C and appears to be determined by phase transfer. The data suggest that, in the investigated system, hydrolysis occurs rapidly once trimethoxy(methyl)silane comes into contact with the aqueous layer and the rate determining step is the transfer of the trimethoxy(methyl)silane from the corn oil to the water. Combined recoveries of trimethoxy(methyl)silane and methanol (in mole equivalents of trimethoxy(methyl)silane; 3 moles methanol to 1 mole trimethoxy(methyl)silane assumed) were 87.5 to 104% and methanol content increased proportionally to the decrease in trimethoxy(methyl)silane. The study was conducted according to an appropriate test protocol and is considered reliable.

 

The hydrolysis products are methylsilanetriol (hydrolysis product 1, 1 mole) and methanol (3 moles).

 

Constituent 2: 3 -(trimethoxysilyl)propylamine

The hydrolysis half-lives of 3-(trimethoxysilyl)propylamine have been predicted using a validated QSAR estimation method to be 0.2 h at pH 4, 0.3 h at pH 5, 2.6 h at pH 7, and 0.1 h at pH 9 and 20 -25°C. The result is considered to be reliable and selected as key study. In a secondary source to which reliability could not be assigned, a hydrolysis half-life of 17.8 minutes at 25°C was also reported; no information on pH was reported for the substance.

 

The hydrolysis products are (3-aminopropyl)silanetriol (hydrolysis product 2, 1 mole) and methanol (3 moles).

 

Constituent 3: 2,2-dimethoxy-8-{[3-(trimethoxysilyl)propoxy]methyl}-1,6,2-oxazasilocane

No measured data are available for the hydrolysis half-lives of constituent 3. Therefore, they have been predicted using a validated QSAR estimation method to be 0.4 h at pH 4, 8 h at pH 7 and 0.1 h at pH 9 and 20-25°C. The result is considered to be reliable and is selected as key study. These half-lives relate to loss of parent substance; subsequent hydrolysis steps are expected to have shorter half-lives.

 

The final hydrolysis products under dilute conditions relevant for the environment are 5-oxa-9-aza-1,13-disilatridecane-1,1,1,7,13,13,13-heptol (hydrolysis product 3, 1 mole) and methanol (5 moles). The silicon-containing product will be in dynamic equilibrium with a cyclised product in which the secondary alcohol group has undergone a condensation reaction with one of the silanol (Si-OH) groups to form an 8 -membered ring (as is present in the parent substance). This species is named {3-[(2,2-dihydroxy-1,6,2-oxazasilocan-8-yl)methoxy]propyl}silanetriol. More concentrated conditions will favour the cyclised product. Under dilute conditions relevant for the environment, the linear species is expected to predominate and this species is used for assessment. The hydrolysis reaction is illustrated in the attached document.

 

Constituent 4: 9-{[dimethoxy(methyl)silyl]oxy}-3,3,15,15-tetramethoxy-2,7,16-trioxa-11-aza-3,15-disilaheptadecane

No measured data are available for the hydrolysis half-lives of constituent 4. Therefore, they have been predicted using a validated QSAR estimation method to be 0.3 h at pH 4, 6 h at pH 7 and 0.1 h at pH 9 and 20-25°C. The result is considered to be reliable and is selected as key study. These half-lives relate to loss of parent substance; subsequent hydrolysis steps are expected to have shorter half-lives.

 

The final hydrolysis products are 5-oxa-9-aza-1,13-disilatridecane-1,1,1,7,13,13,13-heptol (hydrolysis product 3, 1 mole), methylsilanetriol (hydrolysis product 1, 1 mole) and methanol (8 moles). The hydrolysis reaction is illustrated in the attached document.

 

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 trimethoxy(propyl)silane (CAS No. 1067-25-0)

The hydrolysis data for trimethoxy(propyl)silane (CAS No. 1067-25-0) are now discussed. The hydrolysis data for trimethoxy(propyl)silane (CAS No. 1067-25-0) are read-across to Constituent 1 [trimethoxy(methyl)silane] for appropriate endpoints (see Section 1.4 of the CSR). The silanol hydrolysis product 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.

For trimethoxy(propyl)silane, hydrolysis half-lives at 20-25°C of 0.2 h at pH 4, 2.6 h at pH 7 and 0.1 h at pH 9 were predicted for the substance using a validated QSAR estimation method.

The hydrolysis products are propylsilanetriol and methanol.

 

Hydrolysis of the read-across substance 3-aminopropyltriethoxysilane (CAS No. 919-30-2)

The hydrolysis data for 3-aminopropyltriethoxysilane (CAS No. 919-30-2) are now discussed. The hydrolysis data for 3-aminopropyltriethoxysilane (CAS No. 919-30-2) are read-across to Constituent 2 [3-(trimethoxysilyl)-1-Propanamine] of the submission substance for appropriate endpoints (see Section 1.4 of the CSR). The silanol hydrolysis product 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.

For 3-aminopropyltriethoxysilane, hydrolysis half-lives at 24.7°C of 0.8 h at pH 5, 8.5 h at pH 7 and 0.15 h at pH 9 were determined for the substance in accordance with OECD 111 (5.1.2.067, Dow Corning Corporation 2001). At pH 4, a predicted half-life of 0.4 h at 20-25°C was determined for the substance using validated QSAR estimation method.

The final hydrolysis products are 3-aminopropylsilanetriol and ethanol.

 

Hydrolysis of the read-across substance triethoxy(methyl)silane (CAS 2031-67-6)

The hydrolysis data for triethoxy(methyl)silane (CAS 2031-67-6) are now discussed. The hydrolysis data for triethoxy(methyl)silane(CAS 2031-67-6) are read-across to Constituent 1 trimethoxy(methyl)silane for appropriate endpoints (see Section 1.4).The silanol hydrolysis product of the two substances is relevant to this read-across, as discussed in the appropriate Sections of the CSR for each endpoint.

For triethoxy(methyl)silane, hydrolysis half-lives at 20-25°C of 0.3 h at pH 4, 5.5 h at pH 7 and 0.1 h at pH 9 were determined in using a validated QSAR estimation method.

The hydrolysis products are methylsilanetriol and ethanol.

 

Hydrolysis of the read-across substance bis(trimethoxysilylpropyl)amine (CAS 82985-35-1)

The hydrolysis data for bis(trimethoxysilylpropyl)amine (CAS 82985-35-1) are now discussed. Hydrolysis data for bis(trimethoxysilylpropyl)amine (CAS 82985-35-1) need to be taken into account when interpreting ecotoxicity data read-across to the silanol hydrolysis product of Constituents 3 and 4 in the submission substance for appropriate endpoints (see Section 1.4). The hydrolysis half-lives of the substances are relevant to this read-across, as discussed in the appropriate Sections of the CSR for each endpoint.

It should be noted that although QSAR would predict half-lives at pH7 could be up to 6.9 h respectively, this half-life is not reliable as experimental evidence demonstrates that the reaction is considerably faster under a range of conditions. This type of structure is outside the applicability domain of the standard QSAR model for pH7.

Bis(trimethoxysilylpropyl)amine (CAS 82985-35-1) hydrolyses in contact with water and tend to generate basic solutions. Hydrolysis half-lives are <12 h across the pH range at ambient temperatures.

The hydrolysis products are (3-{[3-(trihydroxysilyl)propyl]amino}propyl)silanetriol and methanol.

Hydrolysis of the read-across substance N,N-bis(3-triethoxysilylpropyl)amine (CAS 13497-18-2)

The hydrolysis data for N,N-bis(3-triethoxysilylpropyl)amine (CAS 13497-18-2) are now discussed. Hydrolysis data for N,N-bis(3-triethoxysilylpropyl)amine (CAS 13497-18-2) need to be taken into account when interpreting ecotoxicity data read-across to Constituents 3 and 4 in the submission substance for appropriate endpoints (see Section 1.4). The hydrolysis half-lives of the substances are relevant to this read-across, as discussed in the appropriate Sections of the CSR for each endpoint.

It should be noted that although QSAR would predict half-lives at pH7 could be up to 66 h, this half-life is not reliable as experimental evidence demonstrates that the reaction is considerably faster under a range of conditions. This type of structure is outside the applicability domain of the standard QSAR model for pH7.

N,N-bis(3-triethoxysilylpropyl)amine (CAS 13497-18-2) hydrolyses in contact with water and tend to generate basic solutions. Hydrolysis half-lives are <12 h across the pH range at ambient temperatures.

The hydrolysis products are (3-{[3-(trihydroxysilyl)propyl]amino}propyl)silanetriol and ethanol.

Hydrolysis of the read-across substance triethoxy(3-isocyanatopropyl)silane (CAS No. 24801-88-5)

The hydrolysis data for triethoxy(3-isocyanatopropyl)silane (CAS No. 24801-88-5) are now discussed. The hydrolysis data for triethoxy(3-isocyanatopropyl)silane (CAS No. 24801-88-5) are read-across to Constituent 2 [3-(trimethoxysilyl)-1-Propanamine] of the submission substance for appropriate endpoints (see Section 1.4 of the CSR). The silanol hydrolysis product 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.

Triethoxy(3-isocyanatopropyl)silane, has two hydrolysable groups, trimethoxy (-OCH3) and isocyanate (-N=C=O). The isocyanate group is expected to hydrolyse very rapidly, for example the hydrolysis half-lives of 2,2,4(or 2,4,4)-trimethylhexane-1,6-diisocyanate were measured in accordance with OECD 111 test method and in compliance with GLP. Very rapid hydrolysis following pseudo-first order kinetics with the following half-lives was determined:

pH 4 - 8.08 min at 10°C, 3.81 min at 20°C and 2.51 min at 30°C

pH 7 - 12.1 min at 10°C, 4.88 min at 20°C and 2.15 min at pH 9

pH 9 - 5.78 min at 10°C, 1.93 min at 20°C and 0.74 min at 30°C

For triethoxy(3-isocyanatopropyl)silane, this means very rapid hydrolysis to form 3-aminopropyltriethoxysilane (CAS No. 919-30-2) as an intermediate hydrolysis product and carbon dioxide. The hydrolysis half-lives of 3-aminopropyltriethoxysilane are discussed above.

The ultimate product of the hydrolysis reaction under dilute condition is 3-aminopropylsilanetriol. The other hydrolysis product is ethanol and carbon dioxide.