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Physical & Chemical properties

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Hexachlorodisilane is a liquid at standard temperature and pressure, with a measured melting point of 3°C, and boiling point of 144°C at 980 mbar. A boiling point of 146°C at 1 bar is reported in a handbook of data that is peer-reviewed and in which the original sources of the data are cited. It has a predicted density of 1.6 g/cm³ at 20°C and a predicted vapour pressure of 280 Pa at 25°C, based on a boiling point of 146°C.

The substance is not classified for flammability on the basis of a measured flash point of 77°C. It has a measured auto-ignition temperature of 315°C, does not represent an explosion hazard and is not oxidising on the basis of structural examination.

Hexachlorodisilane reacts violently with water initially giving hexahydroxydisilane and HCl.

Further hydrolysis of the Si-Si bonds in hexahydroxydisilane is expected to happen rapidly and produces monosilicic acid. At concentrations above about 100-150 mg/l (measured asSiO2equivalents), condensation products of monosilicic acid can also form. At concentrations >100-150 mg/l of SiO2, monomeric monosilicic acid condenses into colloidal particles of polysilicic acid (silica sol) or a highly cross-linked network (silica gel). Hexahydroxydisilane is also likely to form condensation products (polyhydroxy-polysilanes) at similar concentrations (in terms of SiO2equivalents). The structure and predicted properties of Si-Si containing hydrolysis products (polyhydroxy-polysilanes) and (poly)silicic acid are very similar, and distinguishing between them would be very difficult analytically.

The condensation rate is dependent on temperature, concentration, and acidity/alkalinity (as in the pH) of the system. A dynamic equilibrium is established between monomeric monosilicic acid, oligomers and insoluble amorphous polysilicic acid. The composition of a solution is dependent upon conditions such as pH, temperature and the presence of ions.

Therefore, the requirements for testing of water based physicochemical properties for the substance are waived on the basis of instability in water. The properties of the initial and final hydrolysis products, hexahydroxydisilane and monosilicic acid, are assessed instead. Both substances have high predicted water solubilities, but in reality the solubility is limited by the condensation reactions as discussed above. Their volatility is expected to be low. They are not surface active and do not undergo significant dissociation within the environmentally relevant pH range; the first dissociation constant is approximately 10.