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Description of key information

Key value for chemical safety assessment

Bioaccumulation potential:
no bioaccumulation potential

Additional information

There are no in vivo or in vitro data on the toxicokinetics of hexachlorodisilane. The following summary has therefore been prepared based on physicochemical properties of the substance itself and its hydrolysis products. Hexachlorodisilane is an inorganic, moisture-sensitive liquid that hydrolyses very rapidly in contact with water (half-life approximately 5 seconds at 25°C and pH 4, 7 and 9 (analogue read-across)). This half-life relates to hydrolysis of the Si-Cl bonds to give hexahydroxydisilane and hydrochloric acid. Further hydrolysis of the Si-Si bonds in hexahydroxydisilane is expected to occur rapidly, producing monosilicic acid (Si(OH)4), which condenses at concentrations above approximately 100-150 mg/l as SiO2 to give insoluble amorphous polysilicic acid (SAS). Hexahydroxydisilane is also likely to form condensation products (polyhydroxy-polysilanes) at similar concentrations (in terms of SiO2 equivalents). The structure and predicted properties of the Si-Si containing hydrolysis products (polyhydroxy-polysilanes) and (poly)silicic acid are very similar, and distinguishing between them would be very difficult analytically.

Most, if not all of this hydrolysis and condensation will have occurred before absorption into the body, therefore relevant systemic exposure is limited to the hydrolysis products.

Human exposure can occur via the inhalation or dermal routes. Relevant inhalation exposure would be to the hydrolysis products (hydrolysis would occur rapidly when inhaled, even if a mixture of parent and hydrolysis products were present in air). The substance would also hydrolyse rapidly in contact with moist skin. The resulting HCl hydrolysis product would be severely irritating or corrosive.

Absorption

Oral

Significant oral exposure is not expected for this corrosive substance. Should it occur then gastrointestinal absorption of insoluble condensation products will be insignificant as compared to the absorption of the soluble species (Carlisle, 1986).

Inhalation

The high water solubility of hexahydroxydisilane and monosilicic acid might lead to some of this hydrolysis product being retained in the mucous of the lungs. Damage to membranes caused by the corrosive nature of the HCl hydrolysis product might enhance the uptake. Absorption of the insoluble condensation products is not expected.

Dermal

The molecular weights of the parent and hydrolysis products favour absorption across the skin. However, the very high water solubility ( 1E+06 mg/l; which is a theoretical value and does not take into account the condensation reaction) and low predicted log Kow (-4) of the silanol hydrolysis products, hexahydroxydisilane and monosilicic acid, suggest that they are too hydrophilic to cross the lipid rich stratum corneum. Since the other hydrolysis product, HCl is corrosive to the skin, damage to the skin might increase penetration. Absorption of the insoluble condensation products is not expected.

There are no reliable studies to check for signs of dermal toxicity.

Distribution

All absorbed material is likely to be in the form of the hydrolysis products, hexahydroxydisilane, monosilicic acid and hydrogen chloride. Hexahydroxydisilane and monosilicic acid are small molecules, and therefore have potential to be widely distributed, but their hydrophilic nature will limit their diffusion across membranes (including the blood-brain and blood-testes barriers) and their accumulation in fatty tissues. Human blood contains 1 mg SiO2/l of monosilicic acid (Iler, 1979). Hydrogen and chloride ions will enter the body's natural homeostatic processes.

Metabolism

Hexachlorodisilane is rapidly hydrolysed generating HCl and hexahydroxydisilane, which will then further hydrolyse to monosilicic acid, both will then condense to give an amorphous polydydroxy-polysilane precipitate.There are no data regarding the metabolism of hexahydroxydisilane. Silicon is an essential trace element participating in the normal metabolism of higher animals. It is required in bone, cartilage and connective tissue formation as well as participating in other important metabolic processes. The silicon is present almost entirely as free soluble monosilicic acid (Carlisle 1986).

Excretion

The low molecular weight and high water solubility of hexahydroxydisilane and monosilicic acid suggest that they are likely to be rapidly eliminated via the kidneys in urine. There is therefore no evidence to suggest that this substance will accumulate in the body.

Carlisle E.M. (1986) Silicon is an essential trace element in animal nutrition in Silicon Biochemistry, Ciba Foundation Symposium 121, Wiley, p123.

Iler, R.K. (1979) The Chemistry of Silica: Solubility, Polymerization, Colloid and Surface Properties and Biochemistry of Silica, Wiley, p. 13.