Registration Dossier

Environmental fate & pathways

Hydrolysis

Currently viewing:

Administrative data

Link to relevant study record(s)

Description of key information

Hydrolysis half-life: 3.7 h at pH 5, 5.8 days (139 h) at pH 7 and 2.6 h at pH 9 and 25°C (OECD 111)

Key value for chemical safety assessment

Half-life for hydrolysis:
5.8 d
at the temperature of:
25 °C

Additional information

Hydrolysis half-lives of 3.7 h at pH 5, 5.8 days (139 h) at pH 7 and 2.6 h at pH 9 and 25°C were determined for the substance in accordance with OECD 111 and in compliance with GLP. The result is considered to be reliable and selected as key study.

The substance was found to hydrolyse according to the following scheme:

Me2(Vi)Si-O-Si(Vi)Me2+ H2O→ 2Me2(Vi)SiOH

The pH dependence of the hydrolysis kinetics was investigated, by carrying out experiments at pH values of 5, 7 and 9. The slowest reaction was observed at pH 7, with faster hydrolysis at pH 5 and 9. Estimates of kH3O+ (the hydroxonium ion catalysed rate constant) and kOH- (the hydroxide ion catalysed rate constant) were obtained from the data measured at pH 5 and pH 9. These can be used to estimate the reaction rate at any pH for zero buffer concentration. Predicted hydrolysis rates at pH 7 based on these catalytic constants agreed with measured values to within 28%.

The temperature dependence of the hydrolysis kinetic was investigated by carrying out experiments at 10, 25 and 35°C. The reaction rate was found to increase with temperature, to a greater extent for the hydronium catalysed reaction than the hydroxide catalysed reaction. Arrhenius parameters were calculated for the hydronium and hydroxide calculated reactions and these can be used to estimate the reaction rate at any temperature.

The authors of this summary have used the catalytic constants and Arrhenius parameters quoted in the study report (and reported in the Endpoint Study Record) to calculate the half-lives at toxicologically relevant temperature and pH values. The half-lives at 37.5°C and pH 7 (relevant for lungs and blood), 5.5 (relevant for skin) and 2 (relevant for the stomach) are 84 h, 5.8 h and 7 s, respectively.

The measurements in the study were taken at pH 5.0 - 9.0 and 10 - 35°C; therefore, the estimates at 37.5°C and pH 2 represent extrapolations. The difference between 35°C and 37.5°C is very small, so this extrapolation is not considered to add significant uncertainty to the results. The estimated result at pH 2 does represent an extrapolation significantly outside the range of pH values studied (5.0 - 9.0). However, the study results are supportive of a hydroxonium ion catalysed reaction being dominant at acid pH and the reaction rate increasing as the hydroxonium ion concentration increases (pH decreases).

The hydrolysis product in this case is dimethylvinylsilanol (2 moles per mole of parent).

For the environmental exposure assessment, the parent will be considered as the half-life for hydrolysis is greater than 12 hours at pH 7. 

The hydrolysis half-lives of other substances used for read-across in other endpoints are discussed below.

Hydrolysis of the read-across substance octamethyltrisiloxane (L3, CAS No. 107-51-7)

Data for the substance octamethyltrisiloxane (L3, CAS No. 107-51-7), are read-across to the submission substance 1,1,3,3 -tetramethyl-1,3 -divinyldisiloxane (Vi2 -L2) for appropriate endpoints (see CSR Section 1.4 of the CSR). The slow rate of hydrolysis is relevant to this read-across, as discussed in the appropriate Sections of the CSR for each endpoint.

For L3, hydrolysis half-lives at 25°C of 5.09 h at pH 5, 329 h at pH 7 and 9.76 h at pH 9 were determined in accordance with OECD 111 (Dow Corning Corporation 2007). 

The ultimate products of hydrolysis are dimethylsilanediol (one mole) and trimethylsilanol (2 moles). 

Hydrolysis of the read-across substance hexamethyldisiloxane (L2, CAS No. 107-46-0)

Data for the substance hexamethyldisiloxane (L2, CAS No. 107-46-0) are read-across to the submission substance 1,1,3,3-tetramethyl-1,3-divinyldisiloxane (Vi2 -L2) for appropriate endpoints (see CSR 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 hexamethyldisiloxane, hydrolysis half-lives of 1.4 h at pH 5, 116 h at pH 7 and 12.4 h at pH 9 at 25°C were determined in accordance with OECD 111 (Dow Corning Corporation 2006a).

The half-lives at pH 7, pH 5.5 and pH 2 and 37.5°C have been calculated as for the target substance above. This gives half-lives at 37.5°C of 57 h at pH 7, 2.2 h at pH 5.5 and 5 s at pH 2.

The ultimate hydrolysis product is trimethylsilanol (2 moles per mole of parent).