Hexamethyldisiloxane

Hydrolysis

Description of key information

Hydrolysis half-life: 1.4, 116 and 12.4 h at pH 5, 7 and 9, respectively, at 24.8°C (OECD 111)

Key value for chemical safety assessment

Half-life for hydrolysis:

116 h

at the temperature of:

24.8 °C

Additional information

Hydrolysis half-lives of 116 h (5 d) at pH 7, 12.4 h at pH 9, and 1.4 h at pH 5 and 24.8°C were determined for the substance using a method in accordance with OECD 111 and in compliance with GLP. The result is considered to be reliable and was selected as key study.

As the hydrolysis reaction may be acid or base catalysed, the rate of reaction is expected to be slowest at around 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.

k_obs= k₀+ k_H3O+[H₃O⁺] + k_OH^-[OH^-] + k_a[acid] + k_b[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. The following hydrolysis half-lives were obtained:

At pH 5: 4.5 h at 9.8°C, 1.4 h at 24.8°C, and 0.83 h at 34°C.

At pH 7: 418.0 h at 9.8°C, 116 h at 24.8°C, and 53.4 h at 34°C.

At pH 9: 86.0 h at 9.8°C, 12.4 h at 24.8°C, and 4.07 h at 34°C.

For the environmental exposures assessment, the parent will be considered as the half-life for hydrolysis of the parent is greater than 12 hours at pH 7.  The hydrolysis product is trimethylsilanol (CAS number: 1066-40-6).

The effect of partitioning to dissolved organic carbon (DOC) on the hydrolysis half-life of HMDS in water has been considered (Kozerski and Kim, 2018, supporting report attached to the IUCLID dossier). It is demonstrated that the effective rate constant for degradation of HMDS in water containing DOC may be calculated for any combination of DOC-water partition coefficient (K_DOC) and concentration of DOC in water ([DOC]). This allows the exploration of a range of hypothetical K_DOC values and DOC concentrations, to take account of the variety of dissolved organic matter types and concentrations in the environment. It is concluded that, at typical DOC concentrations encountered in natural waters, the effect of sorption by dissolved organic matter on the half-life of L2 in water will be small, and it is highly unlikely that the half-life at pH 7 and 12°C would exceed 40 days (persistence criteria (P) in fresh water) in any realistic situation.  

Hydrolysis of the read-across substance Octamethyltrisiloxane (CAS 107-51-7)

Data for the substance octamethyltrisiloxane (CAS 107-51-7) are read-across to the submission substance hexamethyldisiloxane for appropriate endpoints (see 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 octamethyltrisiloxane, 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 (Mosey 2007).  The half-lives relate to degradation of the parent substance.

The half-lives at pH 4 and 25°C 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 registration substance above. This gives a half-life of 2.5 h at pH 4 and 25°C, 0.025 h at pH 2 and 25°C, 33 seconds at pH 2 and 37.5°C and 120 h at pH 7 and 37.5°C.

The ultimate products of hydrolysis are dimethylsilanediol (1 mole) and trimethylsilanol (2 moles). 

Reference

Mosey J L (2007). Hydrolysis of Octamethyltrisiloxane (MDM). Testing laboratory: Dow Corning Corporation Health and Environmental Sciences (HES) 2200 W. Salzburg Road Auburn, MI 48611. Owner company: Silicones Environmental, Health and Safety Council (SEHSC) 2325 Dulles Corner Blvd. Suite 500 Herndon, VA 20171. Study number: 10390-102.