Triethoxyhexadecylsilane

Endpoint summary

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 the submission substance is unlikely to be absorbed via the oral, dermal, and inhalation routes. Hydrolysis is expected to occur rapidly, and based on the physico-chemical parameters of the silanol containing degradation product, absorption via the gastrointestinal tract, but not via the dermal or inhalation routes are expected. Due to the limited water solubility, the hydrolysis product 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 studies available in which the toxicokinetic properties of Triethoxyhexadecylsilane (CAS 16415-13-7) have been investigated. Therefore, the toxicokinetic behaviour assessment of the substance and its hydrolysis products was estimated by its physico-chemical properties and the available toxicology studies on the substance itself.  

Triethoxyhexadecylsilane hydrolyses rapidly in contact with water (predicted half-life 58.4 h at pH 7 and 5 s at pH 2 and 37.5°C), generating ethanol and hexadecylsilanetriol. This suggests that systemic exposure to both the parent and to the hydrolysis products is possible. Hence, this toxicokinetic behaviour assessment will try to predict the behaviour of these substances. The toxicokinetics of ethanol is discussed elsewhere and is not included in this summary.

The molecular weight of Triethoxyhexadecylsilane is 388.34 g/mol. In contrast, the molecular weight of the hydrolysis product hexadecylsilanetriol is 304.6 g/mol. The hydrolysis product is smaller in size and is more water soluble (4.74 mg/L) than the parent substance (5.6E-05 mg/L) and, thereby suggests that it will have greater potential to be absorbed through biological membranes than the parent substance. Furthermore, predicted log Kow of 8.9 (QSAR) for the parent substance and log Kow of 5.0 (QSAR) for the hydrolysis product indicate that these substances are lipophilic and could therefore be taken up by micellular solubilisation, as the water solubility is also low for the substance and its hydrolysis product.

 

Absorption

Oral:

In an acute toxicity study performed according to OECD TG 423 with the analogue substance Hexadecyltrimethoxysilane (CAS 16415-12-6) no signs of systemic toxicity were observed and an acute oral LD50 value of >2000 mg/kg bw was derived, thus no prediction of systemic availability is possible. If ingestion occurs, the hydrolysis of the parent substance in the low pH of the stomach will be rapid, so any absorption of the parent substance is expected to be minimal and it is more likely to be the hydrolysis product that is absorbed.

The log Kow of 8.9 for the parent substance Triethoxyhexadecylsilane (CAS 16415-13-7) and 5.0 for the hydrolysis product hexadecylsilanetriol indicate that these substances are lipophilic and would therefore suggest low absorption except they undergo micellular solubilisation.

 

Inhalation

The vapour pressure of the parent substance (0.014 Pa) and the boiling point (352°C) indicate that inhalation of the registered substance as a vapour is unlikely. 

 

Dermal

The high log Kow (8.9 and 5.0), the low water solubility (5.6E-05 and 4.74 mg/L) and the molecular weights (388.34 and 304.6 g/mol) of Triethoxyhexadecylsilane and its hydrolysis product hexadecylsilanetriol suggest 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 (DERWIN V2.02.2012) using molecular weight, log Kow and water solubility, calculated a dermal penetration rate of 5.28E-3 µg/cm²/h for the parent substance Triethoxyhexadecylsilane and 1.97 µg/cm²/h for the hydrolysis product hexadecylsilanetriol, respectively. This shows that dermal penetration is very low (1%) for the parent substance and medium (40%) for the hydrolysis product, respectively.

In support of this an acute dermal toxicity study performed according to OECD TG 402 is available with Triethoxyhexadecylsilane indicating no signs of systemic toxicity. Moreover, neither erythema nor edema formation was recorded during the 14-day observation period. Thus, an acute dermal LD50 value of >2000 mg/kg bw was derived.

Metabolism

Triethoxyhexadecylsilane hydrolyses rapidly in contact with water, generating ethanol and hexadecylsilanetriol. There are no data regarding the enzymatic metabolism of Triethoxyhexadecylsilane.

Distribution

For blood:tissue partitioning a QSPR algorithm has been developed by De Jongh et al. (1997) in which the distribution of compound between blood and human body tissues as a function of water and lipid content of tissues and the n-octanol: water partition coefficient (Kow) is described. Using this value for Triethoxyhexadecylsilane predicts that it will distribute into the main body compartments as follows: fat >> brain > liver ≈ kidney > muscle with tissue: blood partition coefficients of 142.5 for fat and 5.3 to 23.4 for the remaining tissues. For the hydrolysis product, distribution would be similar with blood partition coefficients of 142.3 for fat and 5.3 to 15.1 for the remaining tissues. In comparison to the parent product, distribution of the hydrolysis product hexadecylsilanetriol would be comparable to fat and for the remaining tissues.

 

Table 1: Tissue:blood partition coefficiens

	Log Kow	Kow	Liver	Muscle	Fat	Brain	Kidney
Triethoxyhexadecylsilane	8.9	7.9E+08	8.4	5.3	142.5	23.4	9.3
Hexadecylsilanetriol	5	1.0E+05	8.3	5.3	142.3	15.1	7.8

Any absorbed test substance is likely to be in the form of the hydrolysis product, hexadecylsilanetriol. The molecular weight (304.6 g/mol) and low water solubility (4.74 mg/L) of the hydrolysis product suggests it will be taken up by micellular solubilisation and will not diffuse through aqueous channels, pores and will not be widely distributed in the body.

Metabolism

No data are available describing the metabolism of Triethoxyhexadecylsilane. However, metabolism of the target substance is considered negligible, since abiotic and enzyme independent hydrolysis is the prominent degradation reaction, leading to the highly water soluble products ethanol and hexadecylsilanetriol.

 

Excretion

Based on the molecular weight (>300 g/mol) and low water solubility of the parent substance and the hydrolysis product 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.