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Environmental fate & pathways

Hydrolysis

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Description of key information

Half-life in well water: about 3 d at 24.7°C

Key value for chemical safety assessment

Additional information

Trimethylsilane reacts with water. The mechanism of the reaction is unknown; it may not be a true hydrolysis reaction. Potential reaction mechanisms are discussed in the document attached to Section 13 of IUCLID (PFA 2014: The aquatic chemistry of inorganic silicic acid generators, PFA Report Number: 404.001.001). However, it is a chemical reaction with water and is referred to as hydrolysis in the remainder of this CSA.

The stability in water study with trimethylsilane was carried out using a non-standard method. The study was performed using well water at a test substance concentration of 100 µg/l under static conditions for a period of 14 days. The sample analysis was performed using Gas Chromatography (GC) and the reaction product, trimethylsilanol, was monitored using GC-MS. The studies were performed in triplicate and the disappearance of the parent trimethylsilane was monitored as well as the formation of the reaction product, trimethylsilanol. A half-life of 4.2 d was reported by the study author. However, a further review of the study indicates that the rate increased during the study, with a half-life of around 3 days obtained towards the end of the study. This value is therefore used for the chemical safety assessment. There was no information on the statistical analysis of the degradation rate included in the study report. On the other hand, a mass balance of 100% was achieved for the degradation of the parent trimethylsilane and formation of the hydrolysis product, trimethylsilanol.

The study is not in accordance with the standard guidelines (for example OECD 111) for hydrolysis testing. The test solution was not buffered and no information is available in the study report on the pH. However, trimethylsilane is a gas under standard conditions of temperature and pressure; the use of a standard hydrolysis method is considered not technically feasible.

The level of dissolved oxygen in the test water is not stated in the study report. If the reaction is assumed to be oxidation, this parameter could be relevant to the rate of reaction observed. No steps to artificially reduce the dissolved oxygen level (e.g. nitrogen purging) are referred to and it is therefore assumed that oxygen levels are likely to have been normal in the well water at the start of the test. 

 

It is noted that if the dissolved oxygen content had been low, this would have had various implications for interpretation:

i) Rate of reaction to trimethylsilanol in oxygen-rich waters in the environment could potentially be faster than indicated by this study.

ii) There might be implications for technical feasibility to conduct the various aquatic studies required in the REACH data set, specifically it might not be possible to maintain appropriate concentrations of the registration substance in aquatic test media due to oxidation as well as volatilisation processes, particularly if artificial aeration was required to maintain dissolved oxygen levels (e.g. in aquatic ecotoxicity studies) or in open systems.

 

Hydrolysis of the read-across substance triethylsilane (CAS No: 617-86-7)

Data for the substance triethylsilane (CAS No 617-86-7) are read across to the submission substance trimethylsilane for appropriate endpoints. The structural similarities and the silanol hydrolysis products of the two substances are relevant to this read-across, as discussed in the appropriate sections for each endpoint.

For triethylsilane, hydrolysis half-lives were determined in accordance with OECD Test Guideline 111 and in compliance with GLP (Lange 2016). The half-lives are:

pH 4.0 = 53.8 hours (at time period 0-53.8 hours), 250 hours (at time period 0-269 hours) at 20°C; 26.3 hours (at time period 0-30.9 hours), 199 hours (at time period 0-245 hours) at 30°C and 16.6 hours (at time period 0-30 hours), 94.8 hours (at time period 0-221 hours) at 50°C; 218 hours at 25°C.

 

pH 7.0 = 377 hours at 20°C, 314 hours at 25°C, 279 hours at 30°C and 60.7 hours at 50°C

 

pH 9.0 = 56.8 hours at 20°C, 39.9 hours at 30°C and 8.67 hours at 50°C

 

The half-lives at pH 2 and 25°C, at pH 7 and 37.5°C and at pH 2 and 37.5°C may be calculated in the same way as for the submission substance above. This gives a half-life of 0.538 hours (at the time period 0 -53.8 hours), 2.5 hours (at the time period 0 -269 hours) at pH 2 and 20°C. At pH 2 and 37.5°C, the half-life is 0.13 hours. The calculated half-life at pH 7 and 37.5°C is 93 hours.

At 37.5°C and pH 5.5 (relevant for dermal exposure), the hydrolysis half-life is expected to be between the values for pH 4 (13.3 hours) and pH 7 (93 hours).

The hydrolysis products are triethylsilanol and hydrogen.

 

Hydrolysis of the read-across substance tetramethylsilane (CAS No: 75-76 -3)

Data for the substance tetramethylsilane (CAS No: 75-76-3) are read-across to the submission substance trimethylsilane for appropriate endpoints. The structural similarities of the two substances are relevant to this read-across as discussed in appropriate sections for each endpoint.

Tetramethylsilane is a silane with four methyl groups attached to the silicon atom. Tetramethylsilane contains only Si-C and C-H bonds which are stable to hydrolysis. Trimethylsilane is a structurally related silane, with three methyl groups (Si-C) and one hydrogen atom (Si-H) attached to the silicon atom.

As well as being structural analogues, the two silanes have consistent physicochemical properties including low molecular weight (88 g/mol and 74 g/mol respectively), boiling point close to ambient temperature (27°C and 6.7°C respectively), and high vapour pressures (79000 Pa and >1E+05 Pa respectively). Both substances have low predicted log Kow (2.7 and 2.2 respectively). The predicted water solubilities of the substances are 90 mg/l and 1400 mg/l at 20°C, respectively; however, given the very high vapour pressures of the substances, these concentrations in water are not expected to be achieved in practice.