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Water solubility

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In accordance with Column 2 of REACH Annex VII, the water solubility study does not need to be conducted as the substance has a hydrolytic half-life less than 12 hours at pH 4, 7 and 9. Water solubility of the initial and ultimate hydrolysis products, hexahydroxydisilane and monosilicic acid, is limited to approximately 100-150 mg SiO2/l, with condensation occurring at higher concentrations.

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Additional information

In accordance with Column 2 of REACH Annex VII, the water solubility study does not need to be conducted as the substance has a hydrolytic half-life less than 12 hours at pH 4, 7 and 9.

Hexachlorodisilane reacts very rapidly with water, generating hexahydroxydisilane and hydrochloric acid. 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 as SiO2equivalents), 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). Distinguishing Si-Si containing hydrolysis products (polyhydroxy-polysilanes) from (poly)silicic acid 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.

A water solubility of 1.0 E+06 mg/l was obtained for hexahydroxydisilane and monosilicic acid using an appropriate calculation method. The method has not been validated for inorganic substances and does not take into account the condensation reaction.

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