Registration Dossier
Registration Dossier
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EC number: 240-464-3 | CAS number: 16415-12-6
Endpoint summary
Administrative data
Link to relevant study record(s)
Description of key information
No studies are available. Based on molecular structure, molecular weight, water solubility, and octanol-water partition coefficient it can be expected that absorption of the substance via the gastrointestinal tract, the dermal or inhalation routes are expected to be low. Due to the limited water solubility, the test substance is not expected to be widely distributed in the body. Excretion via the bile is considered favoured, and test material deposited in the stratum corneum is expected to be sloughed off with the skin cells. Thus, bioaccumulation is expected to be low.
Key value for chemical safety assessment
- Bioaccumulation potential:
- low bioaccumulation potential
Additional information
There are no measured data on the toxicokinetics of hexadecyl(trimethoxy)silane.
The following summary has therefore been prepared based on the predicted and measured physicochemical properties of the registered substance (see Table below) and the available toxicology studies on the substance.
Hexadecyl(trimethoxy)silane hydrolyses slowly in contact with water (predicted half-life of > 14 d at pH between 4 and 10), generating hexadecylsilanetriol and methanol. Under physiological conditions (pH 2 and 37 °C), the hydrolysis might be faster.
Table: Physicochemical properties
Physicochemical properties
| Hexadecyl(trimethoxy)silane |
Water solubility | 0.002 mg/L at 20 °C (QSAR) |
Vapour pressure | 0.0001 Pa at 20°C (EU Method A.4) |
Log Kow | 8.1 at 20 °C (QSAR) |
Molecular weight (g/mol) | 346.6 |
Half-life | >14 d at pH 4-10 (OECD 111) |
Absorption
Oral
The absorption of highly lipophilic substances (log Kow of 4 or above) may be limited by the inability of such substances to dissolve into GI fluids and hence make contact with the mucosal surface. However, the absorption of such substances will be enhanced if they undergo micellular solubilisation by bile salts (Aungst and Shen, 1986). Substances absorbed as micelles (aggregate of surfactant molecules, lowering surface tension) enter the circulation via the lymphatic system, bypassing the liver (ECHA, 2017).
The oral absorption is suggested to be low for hexadecyl(trimethoxy)silane. The log Kow of 8.1 indicates that the test substance is highly lipophilic. Moreover, that substance is poorly soluble (<1 mg/L) and thus micellular solubilisation is of particular importance for hexadecyl(trimethoxy)silane to be absorbed orally. Signs of systemic toxicity were evident in the available the 90-day repeated dose toxicity study (BSL, 2021) and shows that absorption via the oral route is possible.
Dermal
The high log Kow (8.1), the low water solubility (0.002 mg/L) and the molecular weight (346.6 g/mol) of hexadecyl(trimethoxy)silane suggests that absorption via the dermal route is quite low, based on the limited transfer of the stratum corneum to the epidermis. QSAR based dermal permeability prediction (DERMWIN V2.00.2009) using molecular weight, log Kow and water solubility, calculated a dermal penetration rate of 0.00002 mg/cm²/h for hexadecyl(trimethoxy)silane. This shows that dermal penetration of the test substance is expected to be very low (10%).
Inhalation
The vapour pressure of the substance (0.0001 Pa) indicates that this substance has a low volatility, and therefore inhalation as a vapour is unlikely to occur. Absorption across the respiratory tract epithelium would only be possible by micellular solubilisation.
Distribution
The log Kow of 8.1 indicates that the substance is likely to be distributed into cells and therefore the intracellular concentration will be higher than the extracellular concentration.
It is generally the case that substances with high log Kow values have long biological half-lives. On this basis, daily exposure to a substance with a log Kow value of around 4 or higher could result in a build up of that substance within the body (ECHA, 2017).
Metabolism
Hexadecyl(trimethoxy)silane does not hydrolyse rapidly in contact with water. There are no data regarding the enzymatic metabolism. In the Ames test and in vitro chromosome aberration test no difference was observed in the tests performed with or without metabolic activation.
Excretion
Based on the molecular weight (> 300 g/mol) and low water solubility of the substance, excretion via bile seems to be favoured. Remaining substance in the stratum corneum may be sloughed off with the skin cells. Thus, the bioaccumulation potential is expected to be low.
References
ECHA (2017). Guidance on Information Requirements and Chemical Safety Assessment. Chapter R.7c: Endpoint specific guidance. Version 3.0. June 2017
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