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Partition coefficient

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Description of key information

Log Kow (hexachlorodisilane): Not relevant
Log Kow (hexahydroxydisilane): -4 at 20°C (QSAR)
Log Kow (monosilicic acid): -4 at 20°C (QSAR)

Key value for chemical safety assessment

Additional information

In accordance with Column 2 of REACH Annex VII, the partition coefficient study does not need to be conducted as hexachlorodisilane and its hydrolysis products are inorganic.

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 SiO2 equivalents), 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 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. 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 purpose of the log Kow study is to understand the interaction of the substance with water and octanol, which acts as a model for lipids in humans and organisms and for organic carbon in soils and sediments. On the basis of structure and supported by predictions, hexahydroxydisilane has a high affinity with water and low affinity for lipids and organic carbon. Log Kowvalues of -4 were determined for hexahydroxydisilane and monosilicic acid using a QSAR method. The method has not been validated for inorganic substances but gives an indication of very low log Kow. Neither hydrolysis product has been isolated and the predicted values are included for completeness only.